Improved Mechanical Design and Thermal Testing of MIT Solarclave
by
Louise E. van den Heuvel
___
___
MASSACHUSETS INSTM'UTE
OF TECHNOLOGY
JUL 3 0 2014
Submitted to the
Department of Mechanical Engineering
in Partial Fulfillment of the Requirements for the Degree of
LIBRARIES
Bachelor of Science in Mechanical Engineering
at the
Massachusetts Institute of Technology
June 2014
@ 2014 Massachusetts Institute of Technology. All rights reserved.
Signature redacted
Signature of Author:
Department of Mechanical Engineering
May 9, 2013
Signature redacted
Certified by:
Sanjay E. Sarma
Professor of Mechanical Engineering
Thesis Supervisor
Signature redacted
Accepted by:
AnetteHosoi
Professor of Mechanical Engineering
Undergraduate Officer
Improved Mechanical Design and Thermal Testing of MIT Solarclave
by
Louise E. van den Heuvel
Submitted to the Department of Mechanical Engineering
on May 9, 2014 in Partial Fulfillment of the
Requirements for the Degree of
Bachelor of Science in Mechanical Engineering
ABSTRACT
Solarclave is a solar-powered autoclave designed for use in rural health clinics in
developing countries. The autoclave must sufficiently sterilize medical instruments to
ensure that they can safely be used in providing patient care. The medical instruments are
sterilized in a pressure cooker that is heated by concentrated sunlight from a parabolic
reflective surface. Previous iterations and testing of the Solarclave proved that sunlight and
a pressure cooker could sufficiently sterilize equipment. However, usability problems and
cost constraints require that the design be further improved before dissemination can
occur. Critical design decisions that this work makes include updating the reflective
structure, the pressure cooker, and the pressure cooker insulation. The combination of
choices must meet user needs and provide sufficient sterilization at a minimum cost.
A reflective structure was selected based on its low cost and listed ability to meet existing
power consumption needs. A mathematical model was created to estimate the actual
usable power output of the selected reflective structure based on its dimensions and
reflectivity, as well as expected intensity of solar radiation. Furthermore, a thermal circuit
model was developed to predict the temperature over time inside the pressure cooker as a
function of input power, pressure cooker dimensions, and insulation material choice. The
mathematical model was evaluated by measuring the temperature outside the pressure
cooker over time, recording all relevant parameters, and comparing the results to those
predicted by the model. The results indicated that the theoretical input power estimate was
too high, but that the thermal circuit was an appropriate approach to modeling the heat
loss of the system. The usability of the system showed major improvements in terms of
ease-of-use, but needs further design in terms of its aesthetics.
Thesis Supervisor: Sanjay E. Sarma
Title: Professor of Mechanical Engineering
3
ACKNOWLEDGEMENTS
I would like to acknowledge Dr. Stephen Ho for helping in the development of this thesis, as
well as Professor Sanjay Sarma for being my faculty advisor for this thesis.
I would like to thank current leaders of the Solarclave project, Anna Young and Josd G6mezMdrquez, and past contributing members to project. I must especially thank Anna Young
for organizing and accompanying me on the trip to Nicaragua to perform testing for this
thesis. I thank Dr. Miguel Orozco, Director of CIES-UNAN, and other members of CIES for
facilitating our stay in Managua. I would also like to thank Kenneth McAneney for
introducing me to the project in late 2012.
Lastly, I must also thank my friends and family for providing support along the way.
Contents
List of Figures.............................................................................................................................................................
9
List of Tables ...........................................................................................................................................................
11
Introduction...................................................................................................................................................
12
1
2
3
1.1
Solarclave M otivation........................................................................................................................
12
1.2
Sun as a Source of Energy................................................................................................................
13
1.3
Sterilization by High Temperature Steam Using a Pressure Cooker ..........
14
1.4
User Needs..............................................................................................................................................15
1.5
M IT Solarclave History......................................................................................................................
16
New Solarclave Design..............................................................................................................................
18
2.1
Concentrator Selection......................................................................................................................
18
2.2
Pressure Cooker Selection...............................................................................................................
20
2.3
Therm al Insulation .............................................................................................................................
21
Theoretical M odel of Solarclave............................................................................................................
22
Pow er of Parabolic Solar Concentrator ...............................................................................
3.1
3.1.1
Pow er from Sun...........................................................................................................................
22
3.1.2
Input Pow er to Pressure Cooker......................................................................................
23
3.1.3
Limitations of Concentrator M odel................................................................................
25
Therm al Analysis of Pressure Cooker ..................................................................................
26
3.2
4
22
3.2.1
Sim plified Physical M odel of Pressure Cooker ..........................................................
26
3.2.2
Temperature over Time Given Power Input: Heating Phase .............................
27
3.2.3
Temperature over Time Given Power Input: Holding Phase .............................
30
3.2.4
Iterative Im plem entation of M odel................................................................................
30
3.2.5
Lim itations of Pressure Cooker M odel..........................................................................
31
Tem perature M easurem ent....................................................................................................................
6
32
4.1
Fagor Cooker Insulation Assem bly................................................................................
33
4.1.2
Tramontina Cooker Insulation Assembly ....................................................................
35
4.1.3
Sterilization Packet Preparation .....................................................................................
. 36
Experim ental Procedure ..................................................................................................................
37
R esults & D iscussion ..................................................................................................................................
5.1
Comparison of Temperature Results with Model...........................................................
39
40
5.1.1
T rial 1.1 ...........................................................................................................................................
40
5.1.2
T rial 1.2 ...........................................................................................................................................
42
5.1.3
T rial 2.2 ...........................................................................................................................................
43
5.1.4
T rial 2.4 ........................................................................................................................................
45
Oth er Tem perature R esults from Trials....................................................................................
47
5.2
6
33
4.1.1
4.2
5
Assem bly of Solarclave Com ponents.....................................................................................
5.2.1
T rial 2.1 ...........................................................................................................................................
47
5.2.2
T rial 2. ...........................................................................................................................................
48
5.2.3
Trial 2.4b ........................................................................................................................................
49
5.2.4
Trial 2.5a and Trial 2.5b ...................................................................................................
5.2.5
Trial 3.1 ...........................................................................................................................................
51
5.2.6
Trial 3.2 ...........................................................................................................................................
52
5.3
R eliability of A ssessing Sterilization ....................................................................................
. 53
5.4
Usability: Comparison of Concentrators ............................................................................
54
5.5
Suggestions for Future W ork ........................................................................................................
56
. 50
5.5.1
Physical System Design Improvements and Considerations............ 56
5.5.2
Further Development of Theoretical Model..............................................................
58
5.5.3
Im prov ed T esting .......................................................................................................................
58
Conclusion ......................................................................................................................................................
7
59
60
7
Appendix A: Sterilization Times for Varying Steam Temperatures [4]...........................
9
Appendix B: MATLAB for Calculating Maximum Projected Area of Cylinder .............. 61
10
Appendix C: Iterative MATLAB CODE .........................................................................................
62
12
Appendix D: Weather Data [7] during Testing.......................................................................
64
13
B ibliography ...............................................................................................................................................
65
8
List of Figures
Figure 1-1: World insolation map shows the expected number of full sun-hours one can
expect across the globe, averaged over a year. 1 sun-hour equals 1 kWh per square meter
per day. (Source: BP, 2009) ..............................................................................................................................
13
Figure 1-2: Iteration of Solarclave from 2012. The concentrator adjustment system was
particularly unrefined and difficult for a single user to operate.................................................
17
Figure 2-1: The two collectors are pictured.........................................................................................
19
Figure 3-1: A parabolic mirror can be oriented such that all of the incoming rays reflect off
the mirror toward the same point, which is the focal point of the parabola.......................... 24
Figure 3-2: The right graphic shows a cross-sectional view of the simplified model of the
pressure cooker used in this work, with critical dimensions listed. The "mass" includes the
m edical instrum ents and trivet.......................................................................................................................
26
Figure 3-3: Thermal resistance network between the pressure cooker at temperature opc
and the ambient environment at temperature eamb..........................................................................
28
Figure 3-4: Plot shows the theoretical temperature over time with expected conditions
during testing. The model with loss appears linear, but is not, which can be seen when
com paring to the no-loss m odel.....................................................................................................................
31
Figure 4-1: Picture of Fagor cooker shows layers of insulation and attachment of
Temperature Probe A. Temperature Probe B was similarly attached on the opposite side of
th e cook er.................................................................................................................................................................
34
Figure 4-2: Picture of Fagor cooker shows position of Temperature Probe C and initial
layer of ceram ic fiber on cooker lid...............................................................................................................
34
Figure 4-3: Fagor cooker shown with full insulation.......................................................................
35
Figure 4-4: Tramontina cooker shown with insulating sleeve. The photo is from the third
day of testing, which used two temperature probes on the Tramontina cooker................. 36
Figure 4-5: An example of the contents in a sterilization packet. Two medical instruments
and one Vapor Line integrator were inside, and an additional integrator was taped to the
outsid e.........................................................................................................................................................................
36
Figure 4-6: Solarclave shown with light concentrated on bottom of cooker........................
38
9
Figure 5-1: Temperature results from Trial 1.1, plotted with the theoretical prediction... 41
Figure 5-2: The three sterilization packets of Trial 1.1 are pictured. None of the steam
integrators passed, though some progress was made toward achieving sterilizing.......... 42
Figure 5-3: Temperature data from Trial 1.2 is compared to the theoretical model. Trial 1.2
reached a standard boil and was allowed to cool down while the Solarclave was turned
away from the sun.................................................................................................................................................
43
Figure 5-4: The data from Trial 2.2 compared with the theoretical model is shown. There
may not have been enough water added to the cooker, which caused the temperature to
clim b after the w ater ran out...........................................................................................................................
44
Figure 5-5: The three steam integrators from Trial 2.2 are pictured. One of the integrators
(center in image) showed evidence of burning, but all three passed.......................................
45
Figure 5-6: The data for Trial 2.4a is shown in comparison with the theoretical model
prediction. The model clearly overestimates the rate of heating...............................................
46
Figure 5-7: Example of clouds overhead during testing. The scattered higher clouds did not
impact testing progress, but the lower clouds caused drops in temperature readings......... 47
Figure 5-8: Temperature data from Trial 2.1 is shown.................................................................
48
Figure 5-9: The plot of Trial 2.4a (6-petal and Fagor) and 2.4a (24-petal and Tramontina)
are shown. The heating time of the former combination is significantly faster than the
latter............................................................................................................................................................................4
9
Figure 5-10: Temperature data from Trials 2.5a (6-petal and Tramontina) and 2.5b (24petal and Fagor) are shown..............................................................................................................................
50
Figure 5-11: Temperature data from both probes of Trial 1 on Day 3 are shown. The
uninsulated probe had much more variability in its readings....................................................
51
Figure 5-12: A 2x4 array of integrators was placed in the cooker for Trial 3.2. All passed,
but the results from the chemical indicator cause varying degrees of certainty in the results.
.......................................................................................................................................................................................
10
53
List of Tables
Table 1: Trials are listing with the time of the trial, combination of cooker and
concentrator, as well as which probe positions were used..........................................................
39
Table 2: This Pugh chart compares the previous mirror assembly to the two collectors
tested in this work. A "+" indicated that the product does well on that characteristic
compared to the previous Solarclave iteration as a baseline, while a "-" indicated the
opposite and blank is neutral...........................................................................................................................
11
55
1
Introduction
Solarclave is a solar-powered autoclave, which is a device used to sterilize medical
equipment. It was designed by the MIT Little Devices Lab as a way to address sterilization
needs at off-grid rural health clinics in developing countries. The project was piloted in
Nicaragua in 2012. During the pilot testing, several functional models of the Solarclave
showed that concentrating thermal energy on the bottom of an insulated pressure cooker is
a viable method for sufficiently achieving sterilization conditions. However, usability and
cost of the Solarclave was wanting. This work aims to improve the previous design of the
Solarclave by addressing several usability concerns, while maintaining functionality of the
system and satisfying cost constraints. The design of a sustainable autoclave for developing
countries has the potential to provide safer care to a substantial patient population [1].
The two major systems of the Solarclave are the solar collectors and the insulated pressure
cooker. In redesigning the Solarclave, new components were selected for both systems (see
Section 2). A thermal model was created of the resulting Solarclave to estimate the ability
of the system to achieve sterilization conditions given the expected input solar energy (see
Section 3). The primary variable of interest is the temperature inside the pressure cooker
over time. The physical system was tested in Nicaragua (see Section 4). The Solarclave was
run while several temperature probes were used to measure the temperature of the
outside of the pressure cooker over time. This parameter is also estimated by the
theoretical model, which allows for comparison of the model to the physical test (see
Section 5.1). Achievement of sterilization conditions is assessed based on measured
temperatures as well as chemical indicators used during testing (see Section 5.3). Further
recommendations for the Solarclave are made based on the experimental results (see
Section 5.5).
1.1 Solarciave Motivation
The Solarclave project was started as an attempt to address the lack of standard and
sufficient sterilization methods in rural health clinics where elevated post-surgical
infection rates are observed. Some clinics receive a donation of a first-world electricallypowered autoclave, a device that sterilizes medical equipment through high pressure
12
saturated steam. However, the benefit of such a donation quickly subsides if the autoclave
breaks down, because of a lack of a local maintenance infrastructure necessary to repair
the machine. Furthermore, a majority of the clinics lack the electricity required to run a
first-world autoclave [1]. Thus, the Solarclave project aims to create an autoclave that
would be sustainable in the limited-resource environment of rural health clinics.
1.2
Sun as a Source of Energy
In 2012, the Solarclave project was piloted in the developing country of Nicaragua. Like
many other developing countries, Nicaragua is located in a region where one source of
energy is particularly abundant and renewable: solar power. The sun is a clean energy
source and, perhaps more importantly, it is also a free energy source. Figure 1-1 is a map of
the solar insolation of the world, averaged over a year. Nicaragua is located in an area that
experiences 4.0 to 5.9 full sun-hours per day, averaged over a year. One sun-hour is
equivalent to 1 kWh/m 2 [2]. This average range of hours suggests that there is significant
opportunity for utilizing the sun to power medical devices at rural health clinics.
NicarIA
Figure 1-1: World insolation map shows the expected number of full sun-hours one can expect across
the globe, averaged over a year. 1 sun-hour equals 1 kWh per square meter per day. (Source: BP, 2009)
Two primary forms of harvesting power from the sun include photovoltaics, which convert
solar radiation into electricity, and concentrating solar power, which concentrates a large
area of sunlight onto a smaller one using lenses or mirrors. Electricity from a photovoltaic
system could be used to power an electric autoclave, but the power consumption required
to reach the temperatures necessary for sterilization is high and economically impractical.
13
In areas with good solar insolation, it is more efficient to directly sterilize with
concentrated thermal energy using a solar thermal collector system, which can be very
low-cost [3]. Solar thermal collector systems are also more durable than their photovoltaic
counterpart, and less technologically complex, both desirable properties for the Solarclave.
Hence, concentrating solar power was selected as the energy source for the Solarclave
project.
1.3 Sterilization by High Temperature Steam Using a Pressure Cooker
Sterilization is the process of using a killing agent to significantly reduce the bioburden on
an object. By standards of the International Organization for Standardization (ISO), an
object is considered sterile if the chance that there are viable microorganisms on it is less
than 1 to a million. One of the safest and most comment agents for sterilization of medical
supplies in health facilities is high temperature steam. High temperature is achieved by
pressurizing the steam. Steam is easily made, clean, not toxic, and not corrosive.
Furthermore, due to its effective transfer of heat during condensation, steam is also able to
kill living organisms at relatively low temperatures in a short amount of time [3].
Sterilization can be achieved using steam at any temperature, but the required sterilization
time is longer for lower temperatures. Standard sterilization conditions using high
temperature steam are 121 *C for 15 minutes or 134 *C for 3 minutes (see Section 7,
Appendix A, for full range of temperatures and corresponding sterilization times).
An autoclave is a device that uses high-temperature steam as a killing agent for
sterilization. The process of sterilizing medical instruments using an autoclave is known as
autoclaving [4]. In order to create and maintain high temperature steam, the basic
requirement for an autoclave is that it be able to sustain the corresponding high pressures.
A simple device that satisfies this requirement is a standard pressure cooker. A pressure
cooker is constructed as a pressure vessel with a lid and closing mechanism and a gasket
between them. It also has a pressure control valve, as well as a safety valve in case the
pressure valve fails.
To use a pressure cooker as an autoclave, a layer of water is added to pressure cooker pot.
A metal shelf, called a trivet, is placed inside the pot. This allows the instruments to be
14
sterilized to be placed inside the cooker in a way that prevents the instruments from
becoming wet by contact with the water rather than the steam. The pressure cooker then
requires a heat source, which is concentrated sunlight in the case of the Solarclave. The
water in the pressure cooker will heat up until it turns into steam. The steam will escape
noticeably from the pressure valve when the cooker's set pressure (and corresponding
sterilization temperature) is reached. The escape of steam signifies when timing of the
sterilization should begin. After sufficient time has passed based on the conditions of the
pressure cooker, it can be removed from the heat source and allowed to cool down and
depressurize, which completes the sterilization cycle.
1.4 User Needs
A successfully designed Solarclave must satisfy a number of user needs. The essential
requirement is the ability to adequately sterilize medical equipment while powered by a
locally available source. In this case, the sun is used as the power source. The Solarclave
will be used in rural environments of developing countries. It should be durable and be
able to operate while exposed to wind, dirt, and of course, high temperature.
In order to encourage dissemination and continued use of the system, there are a number
of additional requirements driven by user needs. The cost of the Solarclave should be
minimized. The target sum of the bill of materials is $150-$200, based on past discussion
between the Solarclave group and with several Nicaraguan health organizations.
The expected users of the Solarclave are the female nurses that work at the health clinics.
To save valuable time of the nurses, the ideal Solarclave should be able to be operated by a
single nurse. Thus, any actions required to operate the system should be reasonable for a
single, female user. The time to complete sterilization as well as active time for the user
should be minimized. The Solarclave should be intuitive and safe to use. Additionally, it
should be possible for repairs to happen locally in the event of a component failure.
In the early stages of the Solarclave, it was considered a high priority to construct the
Solarclave from locally manufactured materials. However, potential buyer demands
changed and allowed this to be a low priority item. Foreign manufactured components
could be considered if they were more cost efficient than the potential local counterparts.
15
However, long-term use and possible repairs should be considered in the analysis of cost
efficiency.
Finally, the Solarclave should also have a number of aesthetic characteristics. It is desired
that the system looks "cool" and easy to use. It must also look like reliable medical
equipment. However, it must also be designed in a way that discourages theft. Any
components that would be left unattended should not appear valuable for other purposes.
1.5 MIT Solarciave History
The Solarclave has had several past iterations. As mentioned in Section 1.4, preliminary
designs required the design to be built using materials available in Nicaragua. Figure 1-2
shows an example of an iteration from 2012. The mirror array was arranged on top of a
structure comprised of wooden slats with cuts that were angled to allow the mirrors to
concentrate the sunlight.
The Solarclave requires a minimum of two modes of adjustment to align the concentrated
sunlight with the bottom of the pressure cooker: one for azimuth adjustment and the other
for elevation adjustment to account for the travel of the sun in the sky. The Solarclave
shown in Figure 1-2 addressed those two necessary modes with a total of four adjustment
areas, three of which are highlighted in the figure. The discrete rungs of the ramp allow
angling of the concentrator box from the ground to adjust for the elevation of the sun. The
rungs along the vertical rails allow discrete adjustment of the height of the cooker, also for
elevation changes. Finally, the sliding clamp between the vertical rails and the concentrator
box are another joint that allows adjustment for changes in the elevation of the sun. The
second necessary mode of adjustment is to respond to changes in the azimuth of the sun.
This iteration of the Solarclave required the.operator to rotate the entire system by lifting
and turning it. The concentrator box was cumbersome to move: it was heavy, lacked a
convenient turning point, and did not have features to help grip the box. Initial testing
revealed that it was difficult for one person to make all of the necessary adjustments
without receiving additional help.
The pressure cooker part of the system also had a few issues. The attachment to the
adjustment system was precarious; the cooker would swing if there was wind present. The
16
insulated lid interfered with the pressure valve, which was centrally located on top of the
pressure cooker. The pressure cooker was placed inside a modified 5-gallon plastic bucket
filled with insulating fiberglass. The fiberglass had to be carefully handled to avoid skin
irritation. The plastic burned in the concentrated sunlight and was aesthetically
unfavorable because the obvious bucket form made the system look cheap, or worse,
unreliable. The bucket was wrapped in tin foil to improve both issues, but the bucket still
experienced melting and burning and the appearance was still lacking.
multiple
adjustment
areas
concentrator
box
Figure 1-2: Iteration of Solarclave from 2012. The concentrator adjustment system was particularly
unrefined and difficult for a single user to operate.
Regarding the effectiveness of the reflective system, the use of flat mirrors compounded
with imperfect alignment caused the focal point of the concentrated light to be significantly
dispersed. This is an important problem to address, because it not only makes it harder to
assess if the system is properly aligned, but also evidences significant losses of input
power. The loss of power slows down the heating process, which exacerbates other issues.
For example, with a longer process time, the sun travels farther in the sky, and the
Solarclave will require adjustment in order to maintain enough power to complete
sterilization.
The use of flat mirrors was initially considered desirable because the mirrors are cheaper
and easier to replace than a single perfectly parabolic mirror, which would be ideal. The
mirror assembly was easily made with accurate angles with the use of a laser cutter.
However, accurately constructing the same design without the aid of a laser cutter would
17
likely be an expensive manufacturing step due to the need for many parts and accurate
angles. Furthermore, the flat mirrors and the additional structures needed to hold each
mirror to a specific angle contributed significantly to the overall weight of the system.
Thus, finding an alternative reflective structure was considered necessary.
2
New Solarclave Design
New components were selected for the Solarclave to address a number of the usability
issues associated with the previous design (see Section 1.4 and Section 1.5). Though
previous designs had focused on constructing the Solarclave from parts that could be
locally sourced in Nicaragua, new buyer input allowed for a shift from this limitation. Due
to time constraints related to the timing of the trip to Nicaragua for testing, new parts were
ordered as soon as possible. Because the previous system had proved to be reasonably
functional, it was deemed prudent to retain the critical dimensions of the main
components, such as the size of the pressure cooker and effective area of reflector, while
addressing a maximal number of the raised usability concerns. The general layout of the
Solarclave was maintained, but both the reflective structure and the insulated pressure
cooker were replaced with newly selected components.
2.1 Concentrator Selection
The previous reflector design had an approximate effective area of 1 square meter, thus a
similar dimension was desired for the new structure to ensure that the Solarclave could be
sufficiently powered. The dimension, which corresponds with a standard power rating of
1,000 Watts (see Section 3.1.2), is relatively typical for reflective structures known as
"solar cookers," used for outdoor cooking. Such solar concentrators sold in the U.S. are
found at prices upwards of $150, which is too high for the Solarclave. However,
internationally-sourced concentrators could be found with the desired power rating as well
as a satisfactorily low price. Searches on global trading Web sites led to two reflectors,
which satisfied the basic cost and power requirements for the Solarclave. In the interest of
time, both options were pursued.
18
The first concentrator, referred to as the "24-petal concentrator," was ordered from China
(see Figure 2-1). Discussion with the seller indicated that a bulk order in the future would
bring the price of the collector to a satisfactory $69 per collector, though the initial
purchase of a single collector for testing was more expensive. The collector was advertised
as having a power output of 1,500 Watts, concentrated using curved bright anodized
aluminum sheets, which have a claimed reflectivity of 95%. The structure clearly included
two simple pivots to allow for both the azimuthal and elevation adjustments. One notable
drawback of this design is that it has several hundred parts, due to the fastening method of
the twenty-four petals, which will make assembly slow. With a diameter of 1.4 meters, the
24-petal concentrator is also relatively large and requires a sizeable reach to access an item
placed centrally on its heating rack.
Figure 2-1: The two collectors are pictured.
Marked at just $25 per collector for bulk orders, the 6-petal collector (Figure 2-1) showed
particularly promising pricing through a global trading Web site. Unfortunately,
correspondence with the sellers was too delayed to order from them for this work. A
presumably identical model was purchased from an American company with faster
shipping, but at a significantly higher price, but it is believed that the cheaper international
purchase would be possible if this collector was ultimately chosen for the Solarclave.
The 6-petal collector also has a claimed power rating of 1,500 Watts. Its parabolic surface is
made from six thin, molded steel panels coated with a layer of aluminum reflective film,
which has a claimed reflectivity of 87%. Though similar in diameter to the 24-petal
19
collector, the 6-petal collector has a focal point farther from the largest diameter of the
reflectors, which allows for easier access to the heating rack. The azimuth adjustment is
performed by swiveling the system on its caster wheels, while adjustments to the elevation
are made by spinning a circular crank handle.
Both products require only the minimum of two separate motions necessary to align the
parabola with the sun. Because of their designs, neither collector requires that the pressure
cooker position to be adjusted. Critically, each model can realistically be operated by a
single user. Usability testing with both products would allow comparison between the two
to help decide which concentrator was ultimately more suitable for use in the Solarclave.
2.2 Pressure Cooker Selection
The pressure cooker chosen for the Solarclave was a 4-quart stainless steel Fagor Splendid.
The pressure cookers used with the previous Solarclave iteration were typically 4- or 6quart pressure cookers, and thus this parameter was maintained for the new pressure
cooker choice. A 4-quart cooker could hold approximately eight to ten instrument
sterilization packets; assuming that the sterilization process takes under an hour, this is an
acceptable process rate. The 4-quart cooker is also a reasonably manageable weight and
size. A larger cooker, which would fit more sterilization packets, requires more power to
heat, yet blocks more of the incoming sunlight, two factors that work against each other.
One of the most important parameters of the cooker is the gauge pressure it is capable of
producing. Fortunately, modern pressure cookers often have a standard high pressure
setting of 15 psi, which corresponds with the standard sterilization temperature of 121 *C.
The Fagor cooker has a low pressure setting of 8 psi and a high pressure setting of the
desired 15 psi. Note that the absolute pressure at a gauge pressure of 15 psi is 29.7 psi at
sea level, which converts to 205 kPa [4].
A majority of pressure cookers are sold with the pressure release valve located centrally on
the lid. The central location causes a large area to be left without insulation to
accommodate for the sputter of the cooker. The Fagor cooker features a pressure valve
located on the edge of the lid, which facilitates easier insulation coverage of the lid.
Centrally located valves are usually weight-based, which means that pressure builds inside
20
the cooker until the pressure is high enough to lift the weight and release steam. Weightbased release can be dangerous. If the pressure cooker is slightly tilted, the weight can
become jammed, allowing a much higher pressure to build up, which is why commercial
pressure cookers include a back-up over-pressure release, often in the form of a rubber
stopper. The Fagor cooker features a spring-based pressure valve, which can operate
properly even if the pressure cooker tilted.
Another important characteristic of the Fagor cooker is its 3-ply diffuser base. The pot's
base features a layer of conductive aluminum that helps distribute the heat applied to the
bottom of the cooker, which is extremely important when dealing with concentrated
sunlight. If the concentration is done accurately, a relatively small area of the cooker must
absorb an enormous amount of heat; there is higher risk for melting or burning if the heat
is not quickly distributed. Thus, the 3-ply base is yet another important characteristic that
led to selecting the Fagor cooker over other options. It can be purchased for $60 for a single
unit, but there is potential to lower the price in bulk order. With the low cost of the
concentrator and remaining materials, it was a satisfactory price.
2.3 Thermal Insulation
Due to time constraints, the insulation was selected from materials already available in the
laboratory. Fiberglass had been used in previous iterations. It is an extremely good
insulator and is suitable for high temperature applications. However, dealing with
fiberglass requires safety precautions that could be avoided by selecting a more userfriendly material. Furthermore, another problem experienced in previous testing with the
fiberglass is that it requires additional structuring to affix it around the pressure cooker.
This additional structure posed problems, because it needed to be aesthetically appealing
in addition to being safe for use in a high temperature application. The 5-gallon bucket
used in the previous design (see Figure 1-2) was susceptible to melting or scorching when
accidentally subjected to the concentrated sunlight. To address both issues, extreme
temperature ceramic insulating strips were chosen as the insulating material (see Section
4.1.1, Figure 4-1, for more details on the insulation).
21
The material is nominally 80% more conductive than fiberglass. However, the decrease in
overall effectiveness of the insulation could be compensated for by adding insulation layers
if necessary. The strips can be wrapped around the pressure cooker as a sleeve for the pot.
They can also be shaped by cutting in order to insulate the lid without blocking the
pressure valve. High temperature tape was suitable for fastening the strips to each other
and the pot. A fiberglass cloth tape was used around the outside of the sleeve to cinch the
layers around the pot; paired with the white ceramic strips, the overall aesthetic was
appropriately "clean" and, based on initial observation, an improvement over the previous
iteration.
3
Theoretical Model of Solarclave
The Solarclave has two major systems: the solar concentrator system and the insulated
pressure cooker. The concentrator receives input power from the sun, and in turn acts as
the power source for the solar cooker. The desired outcome is high pressure saturated
steam in the pressure cooker. The goal of this model is to predict the temperature inside
the Solarclave over time given the updated design choices.
3.1 Power of Parabolic Solar Concentrator
The input power from the solar concentrator to the insulated pressure cooker can be
predicted based on solar radiation, movement of the sun, and the concentrator system
geometry and materials. Due to the inherent, high variability of the actual power due to
unpredictable conditions such as cloud coverage, simple estimations were deemed
appropriate for further calculations. For this reason, this work does not address the
movement of the sun. It is anticipated that the Solarclave will at least initially be used
primarily in Nicaragua; estimations are based on this expectation.
3.1.1 Power from Sun
The average power density of solar radiation, called solar irradiation, outside the Earth's
atmosphere is 1366 W/m 2 . However, due to scattering and absorption, when the sun is at
the zenith, solar radiation is reduced by at least 22%, even on a perfectly clear day. The
standard accepted value for peak solar irradiance reaching the Earth's surface on a given
22
day is 1 kW/m 2 [2]. The actual peak daily value varies depending on location and time of
year and is also greatly affected by changing factors, such as climate characteristics,
particularly cloud coverage. Solar irradiance has been measured globally over time and
Figure 1-1 shows the average number of sun-hours a given location receives throughout
the year. Solar radiation measurements are typically collected using measurement devices
that receives and measures both direct and diffuse sunlight. Even on a clear day, diffuse
radiation accounts for 15% of the measured radiation. Thus, for an application such as the
autoclave, which strictly utilizes direct sunlight, a more appropriate general estimate for
.
usable solar irradiance is 0.85 kW/m 2
Based on a known latitude coordinate and date of year, one could estimate the expected
solar irradiance for a given time of day, but actual values can vary greatly. Extreme cloud
coverage can completely eliminate the presence of direct sunlight, even during the day.
However, because the Solarclave could simply not operate in such conditions, this model
will simplify calculations by using the estimated average input radiation of (0.85
kW/m 2, where the uncertainty of
0.10)
0.10 kW/m 2 is an estimation made by the writer.
Latitudinally, Nicaragua is located near the equator, which is favorable for this assumption
of relatively high solar irradiance, because the sun is not far from its zenith throughout the
majority of daylight hours. Furthermore, the expected peak irradiance assuming clear
conditions varies minimally throughout the year [5].
3.1.2 Input Power to Pressure Cooker
An ideal parabolic mirror can be oriented toward the position of the sun such that all of the
rays reflect off the mirror toward a single point, the focal point of the parabola (see Figure
3-1). This alignment is achieved by aligning the mirror such that its major axis is aligned
directly toward the sun.
23
parabolic
mirror
focal point
Figure 3-1: A parabolic mirror can be oriented such that all of the incoming rays reflect off the mirror toward
the same point, which is the focal point of the parabola.
When a perfectly parabolic mirror is properly oriented toward the sun, the maximum
potential power, Pmax, collected is a product of its projected area, Ap, on a surface normal to
the rays of the sun, which are effectively parallel from the perspective of the Earth's
surface, and the solar irradiance, q:
Pmax = Ap-q.
(1)
For the parabolic structure used in the Solarclave, the projected area is given by the
formula for the area of a circle:
(D=7r
2(2)
where Dr is the maximum diameter of the parabolic mirror. The actual input power to the
pressure cooker system is limited by materials properties, such as the reflectivity of the
reflector surface, p, and absorptivity of the incident surface, a, as well as the physical
geometry during operation. An area, A, of the mirror is shaded during operation due to the
pressure cooker's position between the sun and the mirror. The updated estimate for input
power, Pi, is given by:
Pi =a-p-(Ap-AJ)-q.
(3)
The shaded area depends on the dimensions of the insulated cooker and varies as the angle
of the sun's rays change with respect to the orientation of the cooker. The projected area of
the cylinder depends on the angle of elevation of the sun, Oe, according to the equation:
24
D2
As = - -r- sin0,, ++D-h-cos,
(4)
4
where D and h, are the diameter and height of the insulated cooker, respectively, and an
angle of elevation of 900 corresponds with the sun rays being directly overhead the cooker.
To simplify the model, As is assumed to be constant such that As is maximal, which would
be the worst-case scenario (greatest power loss) that is possible during Solarclave
operation. Appendix B (Section 8) shows the MATLAB code used to calculate the maximum
for the experimental geometry. The shaded area due to the pressure cooker heating stand
and the small opening near the center of the collector are considered to contribute
negligible losses of power.
Based on the geometry of the components selected for the Solarclave as described in
Section 2, the expected power input to the insulated pressure cooker is (1.08
Reflectivity was assumed to be 0.87
.22) kW.
0.05 based on the reported product value. The
uncertainty of the value is an estimate. Absorptivity was estimated as 0.90
diameter of the concentrator was measured with a measuring tape as (1.47
the diameter and height of the pressure cooker were (28
0.05. The
0.01) m, and
0.01) m and (19
0.01) m,
respectively. Uncertainty of the final result is dominated by the uncertainty of the power,
reflectivity, and absorptivity.
3.1.3 Limitations of Concentrator Model
The model does not account for the variation in input solar power that would result in
testing in a different location farther from the equator, a different day of the year, or even a
specific time of day. The temperature values estimated by the model are based on an
assumption of generally good conditions: clear skies and the sun being near its zenith.
These assumptions are not unreasonable, because the conditions would be the suggested
conditions for operation of the product. To accurately model the expected conditions in the
cooker for a given test, one would need the actual input power, which was not measured.
Cloud conditions were noted during testing, but not used to modify estimates used in the
model. The results of this work can only suggest that worse conditions, such as a higher
latitude location with cloud coverage, would result in lower input power from the sun,
which ultimately leads to a slower rise in the temperature of the pressure cooker system.
25
The maximum input power is based on an assumption that the reflector system is perfectly
parabolic and aligned toward the sun, which will not be true. However, this work does not
attempt to assess the accuracy of the purchased reflector assembly's geometry or accuracy
of the alignment of the reflector assembly with the sun, and will thus not account for either
condition.
3.2 Thermal Analysis of Pressure Cooker
The process of achieving sterilization involves two main phases: heating and holding. After
the pressure cooker is prepared for operation, the cooker is given input heat by aligning
the concentrator system properly with the sun. When the cooker reaches its maximum
pressure setting, a noticeable hiss of steam is steadily released from the pressure valve,
signifying when timing of the holding phase can begin. During the holding phase, the
pressure cooker maintains the maximum pressure and corresponding sterilization
temperature.
3.2.1
Simplified Physical Model of Pressure Cooker
For the thermal model of the Solarclave, the pressure cooker was modeled as a hollow
cylinder, with inner diameter, Di, height, H, and constant stainless steel thickness wall, t,.
The insulation shares the outer diameter of the pot as its inner diameter and has thickness
ti. Figure 3-2 shows a cross sectional view of the simplified pressure cooker model.
insulation lid (Ac, ti)
air
cooker wall
( Di, tw)
insulation sleeve
(D, h, ti)
-wat
incoming heat
cooker base,
3-ply
Figure 3-2: The right graphic shows a cross-sectional view of the simplified model of the pressure cooker
used in this work, with critical dimensions listed. The "mass" includes the medical instruments and trivet.
26
The cooker base of the Fagor cooker is actually 3-ply, with a layer of aluminum between
two layers of stainless steel. The aluminum, which is more than 10 times more conductive
than stainless steel, allows the water to be heated evenly even if the incident radiation is
not focused evenly along the bottom of the pot. Because the exact thicknesses of the layers
are unknown, the 3-ply design of the bottom layer is only considered for the assumption of
even heating; in thermal calculations, the thickness of the bottom layer is assumed to be the
same as the sides. Because of the relatively slow process rate as well as good conduction
inside the cooker due to water vapor, a major assumption this model makes is that the
cooker and its contents are a lumped mass, which includes the cooker and the water, air,
medical instruments, and trivet inside it.
3.2.2
Temperature over Time Given Power Input: Heating Phase
The only input to the system is assumed to be the power from the reflector system as
calculated in Section 3.1.2. This power is in the form of heat energy over time and causes
the increase temperature of the system. The power is assumed to be completely absorbed
by the pressure cooker and its contents. The equation relating change in temperature, AG,
to heat energy, Q, is
Q= C-A1,
(5)
where C is effective thermal capacity given by
m
*(6) =
M
Ij=1
given the mass, m, and corresponding specific heat, c, of n components. This work
considers the contribution of the air within the pressure cooker as negligible.
The heat energy,
Q, is the product of input power, P, and the period of time of duration
Q =P-dt.
dt:
(7)
Combining Equations 5-7 with the parameters of the Solarclave allows calculation of the
temperature rise of the solar cooker after time dt has passed. However, such a calculation
would not account for the heat loss that is certainly present.
27
To consider the reduction in temperature of the pressure cooker due to heat loss, the
system is model as a thermal resistance network shown in Figure 3-3. The resistance
network has three main paths between the pressure cooker temperature, Opc, and the
ambient temperature, 8 amb. The top path in the figure represents the resistive sum of the
conduction through the insulation on top of the pressure cooker lid, Rtop,
cond,
and the
convection between the insulation surface and the ambient environment, Rtop, cony. The
middle path represents the resistive sum of the conduction through the insulation on the
side of the pressure cooker pot,
Rside, cond,
and the ambient environment,
and the convection between the insulation surface
Rside, con.
The bottom path represents the convection
between the uninsulated bottom of the pressure cooker and the ambient environment.
Rtop,conv
Rtopcond
Rottom,
conv
Figure 3-3: Thermal resistance network between the pressure cooker at temperature Opc and the ambient
environment at temperature Oamb.
Rtop, cond
is approximated as the thermal resistance through a slab, given by the equation
-
Rtop,cond =
(8)
Ac is the circular area of the top of the lid, ki is the conductivity of the insulation, and ti is the
thickness of the insulation [6]. The magnitude of conductivity varies slightly with
temperature, but the change is negligible for this work. All conductivity values are taken to
be constant at the level of conductivity for that material at the ambient temperature.
Rtop, cony is convection or radiation resistance given by
Rtopconv
28
1
(9)
where hoo is the heat transfer coefficient from the insulation to the environment [6]. The
heat transfer coefficient roughly depends on the temperature differences and the
orientation of surface, according to Equation 10 [5].
hoo = K(AO).2 5
(10)
Where K is 2.5 for a flat surface, and 1.77 for a vertical surface, but can also vary with other
factors such as wind. In air, typical value is 10
, but might be closer to 20 w in wind.
Because wind is not uncommon in the region of testing in Nicaragua, a constant value of 15
was chosen for the model.
For the resistance values of the middle branch,
Rside, cond is
approximated using the equation
for thermal resistance through a cylinder, which is given by
-
D.
-
=2
(
1n
R side,cond
where D is the outer diameter of the cooker wall, or equivalently the inner diameter of the
insulation sleeve, and Di is the inner diameter of the cooker wall [6]. The convection term,
Rtop, cony, can be calculated using Equation 9, except that the circular area, Ac should be
replaced by the surface area of the side of the pot, which is the product of its circumference,
D - ir, and height, h. Lastly, Rside, cond is also a convection resistance also given by Equation 9,
because the top and bottom of the cooker are assumed to have equivalent areas.
The equivalent thermal resistance,
Req =
Req,
for the system is given by
12
+
+
Rtopcond+Rtopconv
Rside,cond+Rside,conv
Rbottom,conv
This equivalent resistance is useful in the lumped parameter model solution for the heat
diffusion equation for unsteady heat transfer, given as follows:
epc
= 8 amb
t(-dt\(3
[6].
+ (6 pcj - Oamb)exp
(13)
Opcj is the temperature of the pressure cooker at time dt = 0. Equation 13 predicts the
temperature of the pressure cooker over time if no energy was added to the system.
29
3.23
Temperature over Time Given Power Input: Holding Phase
If the input power is enough to achieve the sterilization pressure and corresponding
temperature, the internal temperature of the pressure cooker reaches a steady state,
because the temperature of the water inside the pressure cooker cannot increase unless all
of the water has become steam. The energy contributed to the pressure cooker converts
the water to steam, which is released by the pressure valve. Thus, a constant temperature
is maintained as long as there is still water in the pressure cooker and the heat intake from
the concentrator continues to exceed the heat loss. Heat loss might exceed heat intake if the
concentrator was shaded, which would reduce the power output. For the Fagor cookers
high pressure setting of 15 psi, the corresponding holding time is 15 minutes (see Section
1.3).
3.2.4 Iterative Implementation ofModel
MATLAB was used to calculate the temperature by combining the aforementioned
equations and conditions. The heating phase was modeled by calculating the infinitesimal
increase in temperature due to the input power and the infinitesimal decrease in
temperature due to heat loss. In actuality, the two components happen simultaneously, but
they happen sequentially in the MATLAB code (see Section 10, Appendix C). Once the
temperature of the cooker reaches the temperature corresponding to the pressure setting
of the pressure cooker, the holding phase is initiated, which keeps temperature constant,
based on the assumption that the input is unchanged and that the water in the pressure
cooker does not run out.
The dotted blue line in Figure 3-4 shows the predicted temperature of the pressure cooker
over time based on the newly selected components (Section 2). The predicted temperature
during the heating phase of the Solarclave appears linear over time, but actually
increasingly strays below the linear case, shown as the solid red line in the plot. The linear
case would represent the pressure cooker's temperature over time if it was perfectly
insulated. The graph predicts that heat loss of the Solarclave does slow down the rate of
temperature rise, but not significantly.
30
holding
heating
- - - -- - -
120
- - - - - - - - -
-
140
O100
80E
/
/
15 60
40
- -- Theoretical w/ loss
20
-
00
0
5
Theoretical no loss or cut-off
10
Time(mins)
15
20
Figure 3-4: Plot shows the theoretical temperature of the cooker over time based on expected conditions
during testing (black dash lined). The model, which includes heat loss, appears linear, but is not. This can be
seen by comparing it to the exactly linear case of a perfectly insulated pressure cooker (red solid line).
Because the peak temperature of the model is determined by the pressure setting of the
physical pressure cooker, the validity of the model is more appropriately assessed by
comparing the rate of heating predicted by the model to that of the actual test. The data can
be compared qualitatively, but also quantitatively by comparing the total time it takes to
complete the heating phase, theat, in each case.
3.2.5 Limitations of Pressure Cooker Model
The model makes a number of assumptions to simplify calculations for estimating the
temperature
over time inside the pressure
cooker.
These assumptions include
implementing the lumped mass model, assuming a constant heat transfer coefficient,
modeling the holding phase as a constant temperature, assuming negligible water loss, and
approximating geometries. Some of the assumptions make the solution less accurate, while
others limit the ability of the model to respond to measurable inputs and provide a
comprehensive temperature profile of the Solarclave.
The significant assumption of a lumped mass predicts that there is no temperature gradient
across the pressure cooker, which is not true. While the assumption may allow for
reasonably accurate predictions of temperature inside the cooker over time, it does not
31
distinguish between different positions along the pressure cooker that would in reality
have different temperatures. The model estimates the temperature inside the pressure
cooker, while the temperature was measured outside of the cooker during testing.
The model assumes a constant heat transfer coefficient, but the coefficient would be
changing over time. Wind throughout testing changed significantly, but it was beyond the
scope of this work to attempt to accurately account for change in the heat transfer
coefficient over time.
During the holding phase, the model predicts a constant temperature. In reality, the
temperature would likely oscillate slightly, because the opening of the pressure valve
would allow a sudden outburst of steam and corresponding drop of pressure, lowering the
boiling temperature inside the cooker. However, the actual physics of the pressure valve
are not assessed in this model.
The model assumes a negligible loss of water over time. However, through working with
the cooker, it was clear that for extended periods of cooking, water loss could be significant.
If the pressure cooker is taking in heat energy that is extremely excessive for maintaining
its high pressure setting, the energy continues to go toward converting the water to steam,
which would rapidly exit the cooker.
The simplified physical model of the system models the pressure cooker as a single, solid
cylinder. It is generally cylindrical, but it comprised of two parts and has rounded edges
and a somewhat rounded lid. This would change the resistive estimates. The insulation
around the actual cooker was certainly not a perfect open-ended cylinder as modeled.
There were gaps such that the bottom of the cooker was not the only area exposed directly
to the ambient atmosphere. Accounting for such gaps would allow for a more accurate
resistive estimation.
4 Temperature Measurement
The Solarclave is ideally tested in an environment most similar to its end-case use, thus
testing was performed when the opportunity arose to do so during a 3-day trip to
Nicaragua. The primary purpose of testing was to characterize the temperature profile of a
32
typical use case of the current Solarclave. A secondary goal was to assess the usability of
the Solarclave. Two models of solar concentrators and two different pressure cookers were
tested. Temperature data was collected for eight total trials with the 6-petal concentrator
and two trials with the 24-petal concentrator. Trials were performed at different times
during daylight hours. A trial consisted of preparing the pressure cooker and placing it on
the solar concentrator, and then aiming the concentrator and allowing it to heat the cooker
until desired conditions of time and temperature were achieved. Each trial included steam
sterilization integrators inside the pressure cooker to provide a binary assessment of
achievement of sterilization. A steam sterilization integrator is a chemically based indicator
that has a dark colored bar that migrates toward a "PASS" zone as sterilization conditions
are met. The physical set-up and experimental procedure are further detailed in Section 4.1
and Section 4.2, respectively.
4.1 Assembly of Solarciave Components
A given test used one solar concentrator and one insulated pressure cooker. The two
concentrators described in Section 2.1 were both fully assembled according to their
instruction manuals. The original intention was to test with only a Fagor brand pressure
cooker purchased in Cambridge. However, after a crack developed in the plastic handle of
the Fagor cooker, which initially had unknown consequences on the effectiveness of the
cooker, a Tramontina brand pressure cooker was purchased from a local hardware store.
Due to material constraints and a need to make a more visually appealing product for the
user, there were significant variations in the insulation of each cooker. The temperature
data-logger was an Extech SD200 with logging for three Type-K temperature channels. The
logger was set to record temperature every 5 seconds.
4.1.1
Fagor Cooker Insulation Assembly
The bottom of the Fagor cooker was spray-painted black using Rust-Oleum High Heat
Enamel Spray in Bar-B-Q Black. The cooker was insulated with four layers of ceramic fiber
(see Figure 4-1).
33
Figure 4-1: Picture of Fagor cooker shows layers of insulation and attachment of Temperature Probe A.
Temperature Probe B was similarly attached on the opposite side of the cooker.
Figure 4-2: Picture of Fagor cooker shows position of Temperature Probe C and initial layer of
ceramic fiber on cooker lid.
The ceramic fiber was cut and bent to wrap around the shape of the lid and pot. Separate
pieces and layers were fastened to each other using high temperature tape. The insulation
was not tightly fixed to the pot. The first two trials included three K-type temperature
probes. One was taped centrally on top of the lid and the other two were taped on opposite
sides of the side of the pot, as shown in Figure 4-2 and Figure 4-1, respectively.
Figure 4-3 shows the cooker with complete insulation. The darker material was a ceramic
fiber braid used for its adhesive backing and aesthetic properties.
34
Figure 4-3: Fagor cooker shown with full insulation.
4.1.2
Tramontina Cooker Insulation Assembly
Out of prudence for the limited time window of testing, a second cooker was purchased
from a local hardware store after the Fagor cooker developed a crack in its handle during
the second trial, which was also the last trial of the first day of testing. The cause of the
crack and the consequences on the sterilization system were unknown. The purchased
cooker was an aluminum 4.5L Tramontina cooker, model 20575020. It was also insulated
with the ceramic fiber, but there was not enough material to wrap the cooker in an
identical manner to the first. The sides of the pot were wrapped in a 3-layer, 3-inch sleeve
of the ceramic fiber. The lid included a working pressure valve in the center, which did not
lend well to insulating it.
However, preliminary results from the first day of testing
indicated that the cooker would still be able to reach the temperature corresponding to its
maximum pressure setting, thus the lid was left without insulation.
One of the drawbacks noted about using the ceramic fiber was that it browned in the areas
where sunlight was focused on it, which was aesthetically not appealing. Colorful glass tiles
were added around the Tramontina cooker, to create a more finished-looking surface.
Testing began with the Tramontina cooker on the second day. Only one temperature probe,
Probe B, was attached to the side of the cooker, because the Probe A was used to
simultaneously collect data from the Fagor cooker, and Probe C had broken. On the third
day of testing, Temperature Probe A was attached to the top of the cooker to observe the
variation on the lid due to lack of insulation.
35
Figure 4-4: Tramontina cooker shown with insulating sleeve. The photo is from the third day of testing,
which used two temperature probes on the Tramontina cooker.
4.1.3
Sterilization Packet Preparation
Packets of medical equipment were added inside the cooker for each trial. A binary test for
achievement of sterilization was performed using Vapor Line Steam Sterilization
Integrators. As per standard practice of using chemical indicators for testing sufficient
sterilization in an autoclave, an indicator was included inside each packet of medical
equipment as well as taped to the outside of each packet [4].
Figure 4-5: An example of the contents in a sterilization packet. Two medical instruments and one Vapor
Line integrator were inside, and an additional integrator was taped to the outside.
The packets were constructed of white paper and masking tape. Approximately three
packets were used in each test, based on a limited number of medical instruments.
36
4.2
Experimental Procedure
Testing was performed during the daylight hours of three consecutive days in Nicaragua.
The testing for the first two days was performed on the campus of the Centro de
Investigaciones y Estudios de la Salud (CIES) in Managua, Nicaragua. Testing on the third
day was performed on the campus of UNAN FAREM Matagalpa, Matagalpa, Nicaragua.
Weather conditions were recorded from data provided by online METAR reports.
Eleven trials were performed. A trial consisted of an attempt to achieve sterilization of
medical instruments with a solar concentrator and pressure cooker combination. First, the
pressure cooker was prepared. The gasket was washed with water and mild soap. Then,
(350
50) mL were added to the cooker pot. A steam tray was placed inside the cooker,
and the prepared sterilization packets were arbitrarily placed on top of it. The pressure
cooker was then closed and placed on the concentrator stand, which was not yet facing the
sun. The thermocouple wires were attached to the data collector, taped underneath the
concentrator. Then, the concentrator was turned toward the sun. Azimuth was adjusted
first; altitude was adjusted second. Adjustment was stopped once a concentration of light
could be seen on the center of the bottom of the pressure cooker, allowing the cooker to
begin heating up. Data collection was started, and time of day was noted. During testing,
notes were taken regarding the usability of the system, and any problems that arose during
the test that may cause inconsistent results, whether due to outside conditions or operator
error.
37
Figure 4-6: Solarclave shown with light concentrated on bottom of cooker.
Once the pressure cooker started steaming, a stopwatch was started. If the system was able
to continuously steam for the next 20 minutes, the holding phase was stopped by turning
the concentrator away from the sun. Data collection was also stopped. If there was a break
in the steam, it was up to operator discretion to continue, depending on the trial conditions.
If there was a sudden rise in temperature above the steady-state, as observed from the data
collector, the heating was stopped, because this indicated a lack of water inside the cooker.
After heating was stopped, the cooker was removed from the collector. After releasing
excess pressure, the cooker was opened access the steam integrators. A picture was taken
to document the results of the integrator.
Table 1 shows a number of key parameters for the eleven trials. The original intention was
to begin a trial every hour, on the hour, collecting data only from the 6-petal concentrator
and Fagor cooker combination. However, due to various complications, further described
in Section 5, testing was simply performed as soon as a system was ready to go on a given
day. The timing of the testing is listed in the table. The Tramontina cooker was used for
several tests on Day 2, because it seemed practical to also collect data using another cooker
given that the Fagor cooker had a crack. Data was collected using the 24-petal concentrator
only when it was possible to simultaneously run a trial using the 6-petal concentrator. Day
3 testing used only the Tramontina cooker, because the Solarclave was being shown at a
38
school in Matagalpa, and the insulated Tramontina cooker was visibly in better condition.
Not all trials involved multiple thermocouples, thus the table shows which thermocouple
probe location collected data for each set. Refer to Section 4.1.1 and Section 4.1.2 for the
thermocouple probe placement on the Fagor cooker and Tramontina cooker, respectively.
Table 1: Trials are listed with the time of the trial, combination of cooker and concentrator, as well as which
probe positions were used.
5
Trial
Day
1.1
Cooker
1
Time
( 1 min)
12:00 PM
Fagor
6-petal
1.2
1
1:55 PM
Fagor
2.1
2
9:43 AM
2.2
2
2.3
Concentrator Probe A
Probe B
Probe C
yes
yes
yes
6-petal
yes
yes
yes
Fagor
6-petal
yes
no
yes
11:30 AM
Fagor
6-petal
yes
no
yes
2
11:48 AM
Tramontina
24-petal
no
no
no
2.4a
2
1:30 PM
Fagor
6-petal
no
no
yes
2.4b
2
1:30 PM
Tramontina
24-petal
no
yes
no
2.5a
2
2:23 PM
Fagor
24-petal
no
no
yes
2.5b
2
2:23 PM
Tramontina
6-petal
no
yes
no
3.1
3
12:00 PM
Tramontina
6-petal
yes
yes
no
3.2
3
1:04 PM
Tramontina
6-petal
yes
yes
no
Results & Discussion
The performance of the Solarclave, measured as temperature outside the cooker over time,
was tested. The ultimate goal was sufficient sterilization of medical instruments.
Depending of test conditions, several of the results can be compared to the theoretical
thermal model (Section 3) to assess the validity of the model for future predictions.
39
5.1 Comparison of Temperature Results with Model
The thermal model was developed for the Fagor cooker and 6-petal concentrator
combination. Thus, only trials 1.1, 1.2, 2.2, and 2.4a are compared to the model. Each is
compared separately, because of significant changes in the conditions for each trial caused
significant changes in the thermal model. Though data from several probes was collected,
the data from all three probes is only shown for Trial 1.1, because the additional data was
not particularly revealing beyond conclusions that can be drawn from the first trial and the
values are fairly redundant. The probe chosen for comparison is whichever achieved the
generally higher temperatures, because it is assumed to be closer to the actual internal
temperature of the pressure cooker, which is the primary concern.
5.1.1
Trial 1.1
Trial 1.1 was performed starting at noon on December 16 (see Section 7, Appendix D for
corresponding weather data). The starting temperature of all materials was assumed to be
the outside temperature of (29
1) *C. However, data collection did not start at the
beginning of the test, and thus the initial temperature used for the code was the
approximate starting point of the data, 60 *C.
The pressure cooker was mistakenly set on its lower pressure setting, which according to
the manual corresponds with an internal pressure of 8 psi, or a sterilization temperature of
112.6 *C. The model predicts a heating time, theat, mode, of (120
40) s. Figure 5-1 shows the
rise in temperature over time of the three sensors in comparison with the temperature
predicted by the model. The results are somewhat sporadic during the heating phase, but
the three sensors achieve fairly similar values as the water approaches boiling. Data from
Probe B was used for calculation. The heating time,
theat, actual, was
(190
15) s, which does
not fall within the uncertainty of the model. The model appears to overestimate the heating
rate for this trial. The actual heating time was 58% longer than the predicted time. The
holding temperature was (109.6
.1) *C with a 95% confidence interval. It is possible that
the pressure cooker is pressurized to a slightly lower pressure than 8 psi, which would
correspond with a slightly lower holding temperature. However, if the pressure cooker is
assumed to accurately reach a pressure of 8 psi, then this data suggests that the
40
temperature on the outside of the cooker may actually be 3 *C lower than the pressure in
the cooker, which makes sense but is not accounted for in the model.
120
110
LA
100
Ii
I
90
Probe C
80
Probe B
-
70
E
Probe A
60
Model
----
50
40
0
2
4
6
8
10
Time (mins)
Figure 5-1: Temperature results from Trial 1.1, plotted with the theoretical prediction.
Sterilization was not achieved with the conditions in this trial. None of the steam
integrators passed, which makes sense because of the lower pressure setting. In order to
pass, the black bar on the indicator must migrate beyond the red "fail" zone and into the
green "pass" zone. An indicator that undergoes no sterilization conditions would have a
white indicator area with no black bar. The chemical indicators from this trial did show
that some progress had been made toward achieving sterilization, evidenced by the
presence of a short black bar on each indicator. Figure 5-2 shows the three sterilization
packets removed from the cooker after the trial. The steam integrators inside the packet
showed similar progress to those pictured outside of the packets.
41
Figure 5-2: The three sterilization packets of Trial 1.1 are pictured. None of the steam integrators passed,
though some progress was made toward achieving sterilizing.
5.1.2
Trial 1.2
Trial 1.2 was performed starting at 9:43 AM on December 17 (see Section 7, Appendix A for
corresponding weather data). The cooker did not pressurize. It was at first assumed that a
crack that was noticed during Trial 1.1 may have compromised the ability of the cooker to
develop pressure. The heating process was stopped after 43 minutes, but the data
collection was left on to allow comparison of the heat loss over time with that predicted by
the model, as shown in Figure 5-3. The data for the theoretical model was calculated by
inputting the starting temperature of 100 'C, and then setting the input power to zero. The
model shows excellent agreement with the experimental data, which provides validation
for the heat loss calculations. The cause behind lack of pressure build-up was initially
unknown. However, a similar test issue during Trial 2.1 revealed that the likely problem
was that the pressure cooker was never in its fully locked position.
42
120
100
) 80
U
60
(U
E-
40
*Probe Data
20
-
-
Theoretical Model
0
0
20
40
60
Time (mins)
Figure 5-3: Temperature data from Trial 1.2 is compared to the theoretical model. Trial 1.2 reached a
standard boil and was allowed to cool down while the Solarclave was turned away from the sun.
As expected, sterilization was not achieved with the conditions from this trial. The
temperature, pressure, and timing of the trial were such that even less achievement of
sterilization was expected compared to Trial 1.1. The steam integrators showed no
evidence of progress toward sterilization.
5.1.3
Trial 2.2
Trial 2.2 was started at 11:30 AM on December 17 (see Section 7, Appendix A for
corresponding weather data). The cooker was locked and correctly set to the higher
pressure setting of 15 psi. At 11:38 AM, the temperature reading from Probe A reached a
steady-state value of (116.5
0.2) *C, which shows moderate agreement with the value
corresponding to the pressure, 121 *C. Figure 5-4 shows the rise in temperature over time
of the two sensors in comparison with the temperatures predicted by the model. The model
predicts a heating time, theat, model, of (180
40) s. The actual heating time was again longer,
this time by approximately 100%. The heating process and data collection was stopped
after a total of 45 minutes had passed. It was only noted after the trial ended that the
pressure cooker had run out of water. There may not have been enough new water added
to the system after the preceding trial.
43
160
140
L 120
-
-
100
80
u
E
$
60
M
40
-Probe
20
- --
Data
Theoretical Model
0
0
10
20
30
40
50
Time (mins)
Figure 5-4: The data from Trial 2.2 compared with the theoretical model is shown. There may not have been
enough water added to the cooker, which caused the temperature to climb after the water ran out.
The temperature consistently registered a temperature above 113 'C for 30 minutes. With
these conditions, considering that the temperature inside the pressure cooker is several
degrees higher that the temperature on the outside, it was expected that all steam
integrators would pass and that was the case. However, it is important to note that this trial
also included much higher temperature, hitting a maximum of 140.1 *C before the trial was
stopped. Through the duration of the abnormally high temperature conditions was brief, it
may have affected the results. It can be seen in Figure 5-5 that one of the steam integrators
has evidence of burning from the dry heat. Based on the other two integrators, the burn
does not appear to have modified the functionality of the steam integrator; the progress of
the chemical indicator is not significantly different among the three integrators.
44
YAPONaj.
VAPOR L
c,
Figure 5-5: The three steam integrators from Trial 2.2 are pictured. One of the integrators (center in image)
showed evidence of burning, but all three passed.
5.1.4
Trial 2.4a
Trial 2.4a was started at 1:30 PM on December 17 (see Section 7, Appendix A for
corresponding weather data). The cooker was locked and correctly set to the higher
pressure setting of 15 psi. Only Temperature Probe C, located on the lid of the insulated
Fagor cooker, collected data, because data was simultaneously being collected on the other
cooker and concentrator combination. Figure 5-6 shows the results compared with those
predicted by the model, given the initial temperature of the system. The holding process
was stopped after 20 minutes. Once again, the model overestimates the rate of heating by a
significant factor. The holding temperature from the trial was (115.9
below the expected temperature, similar to Trial 1.1 and Trial 2.2.
45
0.1) 'C, which is
140
2
-- - - - - - -- - - - - - -
-
120120
100
80
2
60
40
Probe Data
20
Theoretical Model
0
0
10
20
30
Time (mins)
Figure 5-6: The data for Trial 2.4a is shown in comparison with the theoretical model prediction. The model
clearly overestimates the rate of heating.
The temperature had begun to drop in the final minutes of the trial. This was explained by
an increasing amount of overhead clouds. Figure 5-7 shows an example of the overhead
clouds experienced during testing. According to real-time METAR data, there were
"scattered clouds" at 2133 m, and "few" at 670 m. The presence of scattered high clouds
was fairly constant throughout the test and did not seem to cause any variability in the test.
However, when one of the lower-lying cumulus clouds passed in front of the sun, causing a
significant reduction of the input power for several minutes, the temperature readings
began to slowly decrease. The change in input power is not accounted for in the model,
which explains the overestimate of the model when the actual steady-state temperature
dropped. Despite the drop in temperature at the end, the two steam integrators in the trial
passed. The fact that they passed despite only having a sterilization time of 20 minutes
suggests that the internal pressure was actually at least 118 *C, if not the expected 121 'C.
46
Figure 5-7: Example of clouds overhead during testing. The scattered higher clouds did not impact testing
progress, but the lower clouds caused drops in temperature readings.
5.2 Other Temperature Results from Trials
The following sections describe trials that were performed, but not compared to a
theoretical model. The theoretical model accounts for the 6-petal concentrator and Fagor
cooker combination. Trial 2.1 was performed with the 6-petal concentrator and Fagor
cooker, but is not compared to the theoretical model due to critical testing issue that
affected the results. Trials 2.3, 2.4b, 2.5b, 3.1, and 3.2 were performed with the Tramontina
cooker, which was purchased in Nicaragua after Day 1 testing revealed a crack in the Fagor
cooker. Trials 2.3, 2.4b, and 2.5a were performed using the 24-petal collector.
5.2.1
Trial 2.1
Trial 2.1 was performed starting at 9:43 AM on December 17 (see Section 7, Appendix A for
corresponding weather data) with the Fagor cooker and 6-petal concentrator. The cooker
reached a temperature plateau of (98.2
0.1) *C, because the higher pressure setting was
not engaged. The cooker was not in its locked position, thus did not build pressure. It was
at first assumed that a crack that was noticed during Trial 1.1 may have compromised the
ability of the cooker to develop pressure. However, after it was noticed that the pressure
cooker was not in its locked position, the cooker was closed and the testing was continued.
The probe temperature readings started to rise, but this time continued to rise much higher
than expected until a cloud significantly blocked the incoming sun. The readings dropped,
as shown in Figure 5-8. However, once the clouds moved, the readings once again climbed
quickly, and after 82 minutes the heating process was ended and data collection was
47
turned off. After opening the cooker, it was realized that all of the water had evaporated,
which explained the uncharacteristic thermal readings from the cooker. Although this trial
included the desirable Fagor cooker and 6-petal concentrator combination, the results are
not compared to the model, because of the lack of water complication. Running out of water
is highly undesirable and should not happen in a typical use case, and thus was not
modeled.
140
120
100
80
0
S60
E
40
20
0
0
40
20
60
80
Time (minutes)
Figure 5-8: Temperature data from Trial 2.1 is shown.
5.2.2
Trial 2.3
Trial 2.3 involved no temperature probes, because it was started while Trial 2.2 was still in
progress. This trial used the Tramontina cooker on the 24-petal concentrator. It was the
first time that the 24-petal concentrator was being used; the trial was performed to ensure
that the system could in fact reach boiling conditions that would be worthy of data
collection for future trials. The test was started at 11:48 AM. The cooker began to steam
mildly at 12:04 PM and more rapidly at 12:13 PM. At 12:33PM, twenty minutes after the
steam indicated that the cooker was pressurized at its maximum level, the heating process
was stopped.
All three steam integrators in the Tramontina cooker passed, indicating that sterilization
conditions could be sufficiently achieved with the cooker and collector combination.
48
However, it did take nearly 25 minutes for the cooker to reach its maximum pressure
conditions. At the time of the test, the longer rise time compared to the Fagor cooker and 6petal concentrator combination could not be specifically attributed to either the cooker or
the concentrator, because both variables were changed.
5.2.3
Trial 2.4b
Trial 2.4b happened concurrently with Trial 2.4a and used the Tramontina cooker and 24petal concentrator combination. Figure 5-9 shows the temperature over time of Probe B,
which was located on the side of the cooker, under the insulation. The figure also shows the
temperature readings from the probe attached to the Fagor cooker (Section 5.1.4) to allow
qualitative comparison of the two systems. The Fagor cooker on the 6-petal concentrator
clearly heats up faster than the Tramontina cooker on the 24-petal concentrator. The
Tramontina cooker does eventually reach a plateau that is similar to that of the Fagor
cooker, but a few degrees lower. Checking the owner's manual of the Tramontina cooker
later revealed that its maximum pressure setting is 12 psi, which corresponds with a
sterilization temperature of 117.6 *C and sterilization time of 24 minutes. This indicates
that the single pressure setting of the Tramontina cooker is similar to the high pressure
level setting of the Fagor cooker. The single steam integrator in this trial passed.
140
120
100
80
60
-6-petal and Fagor
40
24-petal and
Tramontina
20
0
0
20
10
30
Time (mins)
Figure 5-9: The plot of Trial 2.4a (6-petal and Fagor) and 2.4a (24-petal and Tramontina) are shown. The
heating time of the former combination is significantly faster than the latter.
49
5.2.4
Trial 2.5a and Trial 2.5b
Trial 2.5a and Trial 2.5b were run concurrently. Trial 2.4a used a Fagor cooker with the 24petal concentrator, while 2.5a used the Tramontina cooker with the 6-petal concentrator.
After the temperature readings in Trial 2.5a stopped rising at a temperature of
approximately 97*C, it was noted that the handle of the Tramontina cooker was slightly
open. The handle was closed 14 minutes into the test, and Figure 5-10 shows that the
temperature rose to the usual equilibrium around 116'C. At the same time, the readings of
Trial 2.4a were rising at a much slower rate. The temperature plateaued at 98*C, once again
indicating a problem with the pressurization of the cooker. Twenty-two minutes into the
test, and again twenty-four minutes into the test, the sensor from Trial 2.5a became
unattached to the cooker, causing jumps in the data. Approximately 30 minutes into the
test, there was too made shade to continue testing and the heating process was stopped for
both trials. None of the steam integrators passed, as would be expected from the
conditions.
140
120
100
80
60
40
6-petal andTramontina
20
24-petal and Fagor
-
0
0
10
20
30
Time (mins)
Figure 5-10: Temperature data from Trials 2.5a (6-petal and Tramontina) and 2.5b (24-petal and Fagor) are
shown.
Based on Trials 2.4a, 2.4b, 2.5a, and 2.5b, the 6-petal collector is likely a significantly better
power source than the 24-petal collector. In both cases, the 6-petal cooker appeared to heat
up at a rate nearly 3 times faster than the 24-petal cooker.
50
5.2.5
Trial 3.1
Trial 3.1 was started at 12:00PM on Day 3. Testing was performed in Matagalpa. The
outside temperature, To, was (22
1) 'C (see Section 7, Appendix A for more weather
data). Two temperature probes were used. As described in Section 4.1.2, Probe A and B
were located on the Tramontina cooker on top of the uninsulated lid and under the
insulation on the side, respectively. Figure 5-11 shows the temperature readings for the
trial, which was run for 30 minutes. Probe B shows relatively smooth readings while Probe
A has much higher variability. Due to being exposed on the surface of the lid, the
fluctuations of the Probe A readings were likely caused by the surrounding outside
conditions, and are a less accurate representation of internal conditions. It is unclear is this
was due to the lack of the insulation on the lid, or simply the exposed positioning of the
probe. and not internal temperature variations as Probe B shows comparatively.
140
120
100
80
60
E
Uninsulated (Probe A)
40
-
Insulated (Probe B)
20
0
20
10
30
Time (mins)
Figure 5-11: Temperature data from both probes of Trial 1 on Day 3 are shown. The uninsulated probe had
much more variability in its readings.
Of the two steam integrators placed in the cooker, one of the chemical strips had reached
the end of the pass zone, which showed thorough sterilization, while the other had only just
reached the pass zone, considered as a failure to sterilize. This trial was run for 30 minutes
to experiment with the idea of leaving the system "on" (equivalent to allowing the heating
process to continue) for a certain amount of time. Thirty minutes was chosen based on the
rise times experienced in previous trials. However, the rise time of Trial 3.1 was slower
51
than the rise time from previous trials with the Fagor cooker and 6-petal concentrator. The
cooker was only at high pressure conditions for 20 minutes, which was not enough time to
have expected the steam integrators to pass, because of the Tramontina cooker's need for a
longer holding time at a slightly lower pressure. It is strongly believed that both steam
integrators would have passed if the heating process was continued for 5 additional
minutes.
It should have been considered that the lack of insulation on top of the Tramontina cooker
would cause a slower temperature rise rate. Similarly, the lower outside temperature
experienced on the third day of test would also be a source of slowed heating.
5.2.6
Trial 3.2
The second trial on Day 3 was started at 1:04 PM. Conditions were initially clear, but large
groups of clouds began to block the sun for several minutes for several minutes at a time.
Given the temperature readings of the trial, it was not expect that the integrators would
indicate that sterilization was achieved. The requirements for sterilization, as described in
Section 1.3, calls for continuous high temperature steam conditions for the duration
corresponding to the given steam conditions. The conditions inside the cooker clearly
dropped below that value for a significant amount of time several times during the trial.
Because of the multiple drops in temperature, this trial was run for an extended period. It
was hoped that the clouds would clear and allow for twenty continuous minutes of
sterilization conditions. However, the cooker ran out of water after 64 minutes and the
temperature inside the cooker began to climb rapidly.
Because of the inconsistent steam integrator results from the last trial, a 2x4 array of
integrators had been used in this trial to determine if position of the integrator correlated
with a specific extent of sterilization. All of the steam integrators passed, but with varying
degrees of certainty. Figure 5-12 shows the arrangement of integrators, which all reached
the pass zone. There is no clear indicator of position corresponding with the extent that the
chemical indicator penetrated the pass zone. It is not known if this variability is due to a
property of the integrator, or of the actual trial conditions, especially because this trial ran
out of water, which would cause much more inconsistent heating.
52
Figure 5-12: A 2x4 array of integrators was placed in the cooker for Trial 3.2. All passed, but the results from
the chemical indicator cause varying degrees of certainty in the results.
5.3 Reliability of Assessing Sterilization
It is clear that the new Solarclave is fully capable of achieving sterilization conditions.
Because the data logger showed readily accessible real-time temperature readings, it was
possible to note when the cooker had reached its steady-state maximum temperature
condition, and then allow the cooker to continue sterilizing for another 20 minutes.
However, future operators of the Solarclave will not have the data logger, thus this is not a
method they can use. Fortunately, the high temperature conditions corresponded with
visible and audible signs from both the Fagor and Tramontina cooker. For the Fagor
cooker, a steady flow of steam was visible from its steam release valve once the high
temperature condition was achieved. It was accompanied by a steady hissing noise. For the
Tramontina cooker, some steam was visible as soon as the cooker achieved boiling
conditions. However, the highest temperature condition corresponded with a significantly
more intense release of steam. The release valve (see Figure 4-4) would often begin to
oscillate, causing audible and visible spurts of steam release, rather than a continuous
output.
Careful observance of either cooker could allow an operator to determine when the cooker
has achieved its maximum temperature condition. Similarly, it is also possible to observe or
hear when the pressure cooker drops beneath those conditions. Furthermore, the steam
integrators generally showed results that agreed with the observed conditions.
53
In Trials 2.2, 2.4a, 2.4b, and 2.3, the cooker being tested reached its maximum pressure
condition and approximately maintained that condition for at least 20 minutes before the
heating process was stopped. These conditions were expected to sufficiently achieve
sterilization, and in all of those trials, the steam integrators passed. Though the sample size
is small, the results suggest that with uninterrupted sterilization conditions, the Solarclave
can reliably achieve sterilization conditions.
Conversely, in Trials 2.1 2.5a, and 2.5b, high temperature conditions had not been achieved
to a significant degree or duration, and the steam integrators correctly showed that the
sterilization had failed. Thus, in seven out of the nine trials, the steam integrators provided
results that were expected based on the collected data. The exceptions were Trial 3.1 and
Trial 3.2. Both cases showed at least one "false positive" for successful sterilization. In Trial
3.1, it is believed that the rise in temperature at the end due to lack of water may have
caused uneven heating that caused one of the integrator to pass. In Trial 3.2, all integrators
passed, despite the fact that the sterilization conditions were only achieved intermittently.
It is believed that the steam integrators are not sensitive to the desired condition of 20
continuous minutes of high temperature steam, and instead show sufficient sterilization
provided that the total amount of time at sterilization conditions sums to the proper total;
this is not technically acceptable. Potential design changes to prevent false positives are
included in Section 5.5.1.
5.4 Usability: Assessment of New Components
The two solar concentrators had a number of positive and negative features that were
discernible from seeing pictures and specifications of each product online. However,
actually using the products quickly gave a much better understanding of their benefits and
drawbacks. Table 2 is a Pugh chart comparing the positive characteristics desired of the
collector; it uses the previous Solarclave as the baseline option.
54
Table 2: This Pugh chart compares the previous mirror assembly to the two collectors tested in this work. A
"+" indicated that the product does well on that characteristic compared to the previous Solarclave iteration
as a baseline, while a "-" indicated the opposite and blank is neutral.
6-petal collector
Previous Iteration,
24-petal collector
flat mirror
assembly
+
Powerful
Operable by single
person
+
Easy initial location of
sun
+
+
Easy adjustment to
track sun
+
+
Locally repairable
+
Durable
Easy assembly
+
Minimal active time
Low-cost In long-term
+
+
Aesthetically pleasing
In general, both models are superior to the previous Solarclave iteration. However, the 6petal collector out-performed the 24-petal collector in more categories and would be the
recommended collector for future models. The fact that it was clearly the most powerful
model is significant for a number of reasons. Naturally, it ensures that the pressure cooker
can easily achieve sterilization conditions. However, because of the rapidness of the
heating, it has the gratuitous additional characteristic that it can perform sterilization in a
short enough amount of time that adjustment of the concentrator during the sterilization
cycle is not necessary to successfully complete the process; this characteristic was unique
to the 6-petal cooker, because of its significantly faster heating time.
Regarding the pressure cooker selection, the Fagor cooker met all functional expectations
despite having a crack in it. The Tramontina cooker could also be used for sterilization, but
its lower high pressure setting requires more than five extra minutes to complete
sterilization, which is less desirable. It is recommended that the pressure cooker used in
55
the Solarclave has a gauge pressure setting equivalent to 15 psi, which corresponds with
the standard sterilization time of 15 minutes.
5.5 Suggestions for Future Work
In general, the current Solarclave showed improved usability over previous iterations.
Notably, the improved power collected from the solar concentrator allowed for a single,
initial adjustment of the concentrator orientation, rather than multiple adjustments over
time. While the mechanical system achieved needs not addressed by previous iterations,
there are still a number of necessary improvements before dissemination can occur,
(Section 5.5.1). Improvements to theoretical model could help inform optimal operation of
system (Section 5.5.2). Lastly, a number of aspects of the testing could be improved
(Section 5.5.3).
5.5.1 Physical System Design Improvements and Considerations
As suggested by testing issues and the usability results (Section 5.4), a number of design
changes could improve the product. The 6-petal solar concentrator is unquestionably a
sufficient power source for the size of pressure cooker tested in this work. However, it
could be desirable to increase the amount of medical instruments that can be sterilized in a
given use by increasing the size of the cooker. However, a larger cooker may cause
additional issues such as being less manageable for the user or taking longer to complete a
run. These considerations should be carefully weighed in choosing the cooker for the final
product.
The crack that formed in the Fagor cooker was an obvious cause for concern for continued
use that model. However, its ability to continue performing without issue even with the
crack allows it to still be considered for future iterations. It is possible that the travel to
Nicaragua caused the fault, rather than the testing conditions. It is desirable to find a
similar model that is available for purchase in Nicaragua.
Although the 24-petal concentrator generally proved inferior to the 6-petal concentrator
based on this work's test of its usability and functionality, there were comments from local
workers that the 24-petal concentrator seemed more feasibly manufactured locally.
Though the Nicaraguan health officials did not specifically request that this product be able
56
to be locally manufactured, the ability to do so is still an objective of D-lab. Thus,
consideration of the design of the 24-petal cooker may be useful for future iterations if
further testing can prove that the power it provides is sufficient for powering the autoclave
portion of the system. Alternatively, more research of the manufacturing capabilities of
Nicaragua could show that the 6-petal design can also feasibly be manufactured locally.
The pressure cooker ceramic insulation wrap functioned well, but needs a more standard
fabrication method for each cooker, as well as some form of protection for the wrap from
concentrated light to prevent browning of the material. Once the pressure cooker for the
final product has been selected, a possible solution is to design a hard ceramic outer sleeve
to house the cooker and protect the insulating material. Glass and color could also be
incorporated into the sleeve to improve the aesthetics of the Solarclave.
The concentrator alignment process was generally satisfactory with regard to its ease.
However, there is still a safety concern. It would be ideal to develop a method that reduces
the temptation to look at the sun when making adjustments, even if the user is wearing
sunglasses. Careful considerations should be made to ensure that the user always operates
the system while wearing sun protection. The system was operated without any special
protective clothing except sunglasses, but it would be better to take extra sun safety
precautions, especially when dealing with concentrated sunlight.
During testing, the visible temperature on the data logger allowed the tester to monitor the
temperature level and end a test after an appropriate amount of time at an appropriate
temperature had passed. It was a relatively subjective process to determine if testing
should continue after a large cloud had passed and caused a drop in the temperature. It is
undesirable for the end project to cause the user to make such a decision. The Solarclave
should include a user interface that alerts a user when sterilization conditions have been
achieved, or if the given run has been deemed insufficient. Another useful indication would
be of the water level in the cooker, because it is important that the water does not run out
to prevent excessive overheating. However, adding electronics to the product could be
difficult due to the high temperature application.
57
5.5.2 FurtherDevelopment of Theoretical Model
The main flaw of the theoretical model was that it overestimated the power received from
the solar concentration. It is likely that the geometry of the system is less ideal than
expected and that not all sunlight is concentrated onto the bottom of the pressure cooker.
In fact, it was observed during testing that some of the concentrated light was scattered.
The percentage of light that actually focuses on the bottom of the pot when the
concentrator is properly aligned could be considered the efficiency of concentrator, which
could be another reduction factor in Equation 5. It is also possible that the estimate for
absorptivity was too high. It does not seem as though the decision to ignore the travel of
the sun was a cause for the error. Because the rise in temperature over time was effectively
linear, the data suggests that there was not a significant change in power over time, as long
as no shadows were cast on the system.
The results from Trial 1.2 (Section 5.1.2) offer validation for the heat loss model and
consequently also for a majority of the pressure cooker system parameter estimation. By
adjusting the power estimate and otherwise not modifying the model, a fairly usable model
for predictable the temperature inside the pressure cooker could result. However, the
limitation of the model is that it does not provide a complete temperature profile of
insulated pressure cooker, which might be desirable to have a more in-depth
understanding of the system.
5.5.3 Improved Testing
Due to the brevity of the trip, testing was performed in a manner that maximized the
amount of data collected, and not necessarily repeatability. A sizable number of trials were
performed, but the number of changed variables between each test was less than ideal to
allow for comparison of results. To increase the confidence of the results, it would ideal to
perform a multitude of tests with as many of the same outside conditions as possible. These
conditions would include time of day, cloud coverage, air temperature, humidity, and wind
speed, among other possible factors. Thus, it would be ideal to simultaneously test two or
more identical systems, which was not possible with the available materials on the trip.
58
In some of the trials, several sterilization integrators clearly passed the tests, while others
marginally failed. The packets were not placed in specific positions inside the cooker.
Future testing may benefit from intentionally planning and recording the positions of the
packets in the cooker in an effort to identify is there are any correlation between the packet
placement and achievement of sterilization.
6
Conclusion
The newly designed Solarclave offers a significant number of improvements over the old
design. It can be operated by one person and has a faster over process time than the
previous model. The theoretical model of the concentrator overestimated the power
collected by the solar collector, which was likely a return of assuming too much ideal
conditions. However, the actual power was still more than sufficient for achieving
sterilization conditions with a 4-quart pressure cooker. The thermal model was
satisfactorily able to predict the heat loss of the pressure cooker over time and could be
used to further improve the design of the insulation of the pressure cooker, which is still
wanting, especially as it pertains to the aesthetic of the product. One of the other major
obstacles that exists for the Solarclave is ensuring that a user can easily assess whether or
not sterilization has been successfully completed. However, the system works reliably
enough that a beneficial next step would be having potential users can test device and
provide feedback to help prioritize the remaining usability concerns.
59
Appendix A: Sterilization Times for Varying Steam Temperatures [4]
Pressure
Storilisator
Absoute
Terperature
Gauge
Pressure
Pressure
_C_
110
111
112
113
114
115
116
117
118
119
120
122
123
124
125
126
127
128
129
130
131
132
133
135
136
137
138
139
140
[kPaj
143.26
148.15
153.15
158.31
163.62
189.05
174.64
180.38
186.27
[Bar..]I I1 43
1 48
0.43
0,48
153
1.58
163
1 69
053
058
1886
1.92
196.53
1.98
211.48
2,11
2 18
2.25
2.32
2.39
2.46
2.54
2.62
2.70
2.78
2.86
2-95
218.16
225.02
232.10
239.33
246.76
254.36
262.16
270.13
278.30
286.70
295,26
312.94
322.15
331.73
341.38
351.28
361.42
063
069
1 74
1 80
192.33
I
[Bar.]
-
7
074
080
0.86
092
0.98
Hoking bme
[Minutes]
5854
51 73
4670
40-38
35.68
31.53
27.86
2461
21.75
19.21
16-98
111
13.25
1.18
1 25
1.32
11.71
10-35
8.14
1 39
1.46
1.54
1.62
1.70
8.06
1 78
7 14
631
5 57
4.92
435
1.86
3.84
1-95
3-40
3 12
3.22
3.31
2.12
2.22
2.31
3.41
285
234
2 07
2,41
183
162
3.51
3.61
-
2.51
2.61
I
143
9
Appendix B: MATLAB for Calculating Maximum Projected Area of
Cylinder
%projected
area
of cylinder
for dif
ferent
angles
of
i ncident
rays
d=[diameter measured from insulated cooker]; %%need to be filled in
h=[height measure from insulated];
need to be filled in
areas=zeros(91,1);
for x=0:90
areaofellipse=d^2/4*pi*sin(degtorad(x));
areaofrectangle=d*h*cos(degtorad(x));
areas(x+1,1)=areaofellipse + areaofrectangle;
end
areas
maxarea=max(areas)
61
10 Appendix C: Iterative MATLAB CODE
%%Temperature
Over Time
..Input Parameters
min=6;
tnumber of minutes to plot
dt=5;
"infinitesimal" time segment
P=0; %%power as determined by model
T inf=30;
'initial
temperature
T i=60; '%start of measurement,
2
all
metric
in standard
m_pot=1.7;
in seconds
in celsius
to line
up
(outside
estimates
temperature)
units
mass of pressure cooker
c_pot=457;
%-specific heat of pressure cooker, stainless
m_water=.35;
kmass of water added to cooker
cwater=4.186; :'specific
heat of water
m mass=. 3;
mass of trivet
and instruments in cooker
cmass=cpot;
'specific heat of instruments
steel
C=mpot*cpot+mwater*cwater+mmass*c mass;
t
i= 0.0127
;
% insulation
thickness
t w= 0.00225; %cooker wall thickness
h= 20; %%height of cooker
k i= 0.0721; ''conductivity of insulation
D= .23;
Di=D-t w;
outer
diameter of pressure
cooker
-inner diameter of pressure cooker
Ac= pi*D^2/4;
%%area of lid
hinf = 15; %"heat transfer
coefficient
As = pi*D*h;% area of side
Rtopcond = t i/(k i*Ac);
Rtopconv
=
1/(hinf*Ac);
Rsidecond = log(D/Di)/(2*pi*k_i*t_i);
Rsideconv = 1/(hinf*As);
Rbottomconv =1/(hinf*Ac);
Req=(1/(Rtopconv+Rtopcond)+1/(Rsideconv+Rsidecond)+(1/Rbottomconv)
;tHeating
Phase
%%Heat Rise
dQ=P*dt;
%%heat added
to system over
time
dTheat=dQ/C;
tau=C*Req;
t=0:dt:(60*min+dt);
62
dt
)^-1
T=zeros(60*min/dt+2,1);
T(1)=T i;
for n=1:(60*min/dt+1)
T(n+1,1)=T(n)+dTheat;
T(n+1,1)=T inf+(T(n+1)-Tinf)*exp(-dt/tau);
end
%Holding phase
for n=l:(60*min/dt+2)
if T(n)>121
T(n)=121;
end
end
figure(1)
plot (t/60,T, 'k')
hold on
no loss scenario
Tnl=zeros(60*min/dt+2,1);
Tnl(1)=T i;
for n=l:(60*min/dt+l)
Tnl (n+l, 1) =Tnl (n) +dTheat;
end
"-"Holding phase
for n=: (60kmin/dt+2)
if
Tni(n)>l21
Tnl(n)=121;
end
end
plot (t/60,Tnl, 'r')
63
12 Appendix D: Weather Data [7] during Testing
D
Time
(CST)
Te
T
12:00
PM
1:00
PM
2:00
PM
3:00
PM
29.0
ew
Poin
(P
H
umi
dity
Pressure
Visibi
lity
Wind
Dir
Wind
Speed
Prec
ip.ons
12/16/ 2013, Managua
0
C
30.00
C
20
.0 *C
20
.00 C
58
%
55
%
29.002
C
29.00
C
1011 hPa
100ha
100hPa
1010 h
.0 0 C
21
.00 C
%
62
NE
km
10.0
km
k
km
1009 hPa
%
EE
ENE
E
ENE
km
11.1 km/
h/ 3.1 m/s
22.2 kinm
h / 6.2 m/s
11.1 k
h / 3.1 m/s
14.8 km/
h / 4.1 m/
N/A
N/
N/A
N
N/A
Scattered
Clouds
Scattered
Clouds
Scattered
Clouds
Scattered
Clouds
12/17/ 2013, Managua
9:00
AM
10:00
AM
11:00
AM
12:00
PM
1:00
PM
2:00
PM
3:00
PM
12:00
PM
1:00
PM
29.00
C
30.0
C
31.00
C
31.0 *
C
32.0 *
C
32.00
C
32.00
C
22.0
C
23. 00
C
21
.0 C
20
.0 C
20
0
.0 C
20
0 *C
20
.0 C
19
.0 C
20
.0 C
19
.0 C
19
.0 C
62
%
55
100
1013 hPa
1013 hPa
%
52
%
1012 hPa
52
49
1011 hPa
1010 hPa
46
1009hP
49
1008 hPa
%
%
%
%
km
10 0
km
10.0
km
10.0
km
10.0
km
10.0
km
10.0
km
ENE
ENE
East
ENE
NNE
East
NNE
22.2 km/
h / 6.2 m/s
29.6 kin
h / 8.2 m/s
14.8 km/
h /4.1 m/s
22.2 km/
h / 6.2 m/s
25.9 km/
h /7.2 m/s
22.2 km/
h /6.2 m/s
14.8 km/
h / 4.1 m/s
12/18/2013, Jinotega (30km North of Matagalpa)
83
7.0 k
7.4 km/h
%
1012 hPa
/North
2.1 m/s
78
1011 hPa
3.0 k
North
11.1 km/
h / 3.1 m/s
%
n
64
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
Partly
Cloudy
Partly
Cloudy
Scattered
Clouds
Scattered
Clouds
Scattered
Clouds
Scattered
Clouds
Scattered
Clouds
Mostly
Cloudy
Drizzle
13 Bibliography
[1]
Little Devices Lab, "Solarclave/Nicaragua," D-Lab, 2012. [Online]. Available: http://dlab.mit.edu/scale-ups/solarclave. [Accessed 15 1 2014].
[2] C. J. Chen, Physics of Solar Energy, Hoboken, New Jersey: John Wiley & Sons, 2011.
[3] A. C. Jimenez and K. Olson, "Renewable Energy for Rural Health Clinics," September
1998. [Online]. Available: www.nrel.gov. [Accessed 29 April 2014].
[4] J. Huys, Sterilization of Medical Supplies by Steam, Renkum, The Netherlands: HEART
Consultancy, 1996.
[5] S. Wieder, An Introduction to Solar Energy for Scientists and Engineers, New York:
John Wiley & Sons, 1982.
[6] E. G. Cravalho, J. Joseph L. Smith, J. G. B. II and G. H. McKinley, Thermal-Fluids
Engineering, Cambridge, MA: Oxford University Press, 2005.
[7] [Online]. Available: http://wunderground.com.