An Approach to Alternative Energy Solutions for Heating Swimming
Pools
by
Matthew Ryan Harwell
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Sudhangshu Bose, Engineering Project Advisor
Rensselaer Polytechnic Institute
Hartford, Connecticut
December 2012
© Copyright 2012
by
Matthew Ryan Harwell
All Rights Reserved
ii
CONTENTS
An Approach to Alternative Energy Solutions for Heating Swimming Pools ................... i
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
ACKNOWLEDGMENT .................................................................................................. vi
ABSTRACT .................................................................................................................... vii
1. INTRODUCTION AND BACKGROUND ................................................................ 1
2. THEORY AND METHODOLOGY ........................................................................... 2
2.1
SOLAR WATER HEATER DESIGNS ............................................................. 2
2.1.1
THERMOSYPHON SOLAR WATER HEATER ................................. 3
2.1.2
FORCED CIRCULATION SOLAR WATER HEATER ...................... 4
2.1.3
COLLECTORS ...................................................................................... 5
2.1.4
METHODOLOGY FOR SELECTING SOLAR COLLECTOR .......... 8
2.1.5
METHODOLOGY TO OBTAIN DATA FOR HEATING A POOL . 10
2.1.6
SELECTION OF EVACUATED TUBE COLLECTOR MODEL ..... 14
2.1.7
FINAL DESIGN OF SOLAR POOL HEATING SYSTEM ............... 16
3. RESULTS .................................................................................................................. 20
3.1
COST ANALYSIS OF SOLAR HEATING SYSTEM ................................... 20
3.2
COST OF OPERATING COMMERCIAL POOL HEATER ......................... 21
3.3
ENVIRONMENTAL IMPACTS ..................................................................... 23
4. CONCLUSION.......................................................................................................... 26
REFERENCES ................................................................................................................ 27
iii
LIST OF TABLES
Table 1: SRCC Values for Various 30 Tube Evacuated Heater Assemblies [11] ........... 16
Table 2: Cost Breakdown of System ............................................................................... 20
Table 3: Cost and Performance Difference Due to Number of Collectors ...................... 20
Table 4: Cost of Operation for Commercial Pool Heaters [16] ....................................... 21
Table 5: Cost Analysis, Pay Back Period ........................................................................ 23
iv
LIST OF FIGURES
Figure 1: Thermosyphon Solar Water Heater [4] .............................................................. 3
Figure 2: Active, Closed Loop Solar Water Heater [4] ..................................................... 4
Figure 3: Flat Plate Collector [6] ....................................................................................... 5
Figure 4: Flat Plate Collector Construction [6] ................................................................. 6
Figure 5: Evacuated Tube Heater Design [6] .................................................................... 7
Figure 6: Evacuated Tube Collector Example [7] ............................................................. 8
Figure 7: Rainy Day – very low and fluctuating solar radiation levels [8] ....................... 8
Figure 8: Hot summers day – high peak solar radiation levels [8] .................................... 9
Figure 9: Affect of solar insolation due to angle of sun[9].............................................. 10
Figure 10: Optimum tilt angle[9] ..................................................................................... 11
Figure 11: Solar Insolation Map of USA [9] ................................................................... 12
Figure 12: Solar insolation incident on Earth's atmosphere compared to Earth's surface 13
Figure 13: Required energy inputs to raise temperature of pool ..................................... 14
Figure 14: Sample SRCC Certification [11] .................................................................... 15
Figure 15: Pool Water Heating System ........................................................................... 18
Figure 16: CO2 Emissions from Electricity Generation[17] ........................................... 24
Figure 17: CO2 Emissions from Heat Pump Electricity Requirements[17].................... 25
v
ACKNOWLEDGMENT
I would like to take this opportunity to thank the faculty and staff at Rensselaer
Polytechnic Institute for their time and support, especially Professor Sudhangshu Bose
for his advice.
vi
ABSTRACT
The global energy consumption in the last 50 years has increased drastically and
will continue to do so for the next 50 years. With the increasing population of the world
and the demanding increase for a higher standard of living, the energy requirements
needed to sustain the world’s needs are steadily increasing.
The reserves of
nonrenewable energy resources are shrinking; in order to avoid a worldwide energy
crisis, renewable resources must be implemented in every application possible.
Of the various renewable forms of energy, solar energy proves to be amongst the
most diverse and effective renewable energies. One of the most successful applications
of solar energy has been in the use of heating water. In this project, a home swimming
pool heating system will be developed for use in Southeastern Connecticut (SE CT).
Various solar heating methods will be reviewed and the solar technology most
appropriate for the subject application will be determined. Subsequent to selection of
the technology, a specific solar collector manufacturer and model will be selected based
on performance ratings.
The heating requirements for a typical swimming pool will be analyzed and the
specified collector will be sized in accordance with the subject heating requirements.
Then, the general function, components, operating parameters and costs of the solar
heating system will be established.
The cost of operation of the most common type of commercial swimming pool
heater will be compared to the chosen solar collector system. Based on operation of the
commercial heater, breakeven points of the various solar heating arrays will be analyzed.
From the data gathered, a final conclusion will be discussed to detail how well the
solar technologies are able to cost effectively perform against conventional energy
technologies. All comparisons will utilize data gathered from consistent initial
conditions.
After completion of the analysis, it was evident pool owners can utilize renewable
energy in an affordable manner and relieve their dependence on nonrenewable fuel
resources to heat swimming pools.
vii
1. INTRODUCTION AND BACKGROUND
Energy to be delivered to residential areas is expected to increase from 11.6
quadrillion Btu in 2003 to 14.1 quadrillion Btu in 2025; this is consistent with population
growth rates and household formation. The fastest growth in residential energy demand
in the above projection is considered to be for electricity used to power computers,
electronic equipment, and appliances [1]. 11% of the world’s power is supplied by
biomass, while 85% is derived from fossil fuels. If nonrenewable energy resources are
utilized as a main source of electricity, a time will come when the subject resources can
no longer meet the demand of the population.
For this reason, renewable energy
resources must become more abundant in residential areas, such as solar thermal energy.
The average solar power incident on the earth is approximately 1000W/m 2 or
about 100,000 TW. This energy source is much greater than the current world power
consumption of approximately 15 TW [2]. Solar energy is abundant to everyone in the
world; small steps must be taken to harness the power to fuel energy needs.
As stated earlier, one of the most successful applications of solar energy has been
in the use of heating water. This process can be applied to many uses in residential
areas; this paper will discuss and evaluate the use of solar energy applied specifically in
heating a typical swimming pool. Two basic solar heating systems will be reviewed
including a thermosyphon solar water heating system and a forced circulation water
heating system. Furthermore, the basic types of solar collectors will be reviewed and
compared to determine which will be sufficient for the subject application. The heating
requirements of the pool will be determined and a heating system capable of meeting
these requirements will be detailed. The cost of heating a pool with a commercial
heating system will be evaluated and compared to that of the solar heating design.
This project will verify that pool owners can utilize renewable energy in an
affordable manner and relieve their dependence on nonrenewable fuel resources to heat
their swimming pools.
1
2. THEORY AND METHODOLOGY
The methodology and approach that will be followed in this project includes
reviewing basic solar water heating designs, comparing different solar collector designs,
performing an engineering analysis of the heating requirements, creating a solar heating
system sized sufficiently for the subject application, determining operating costs of a
commercial pool heater, and finally performing a cost analysis of the solar heating
system.
As stated, the basic solar water heating designs will be reviewed including a
thermosyphon heating system and a forced circulation heating system. A thermosyphon
heating system relies on natural convection to circulate the working fluid through the
solar collector and heat exchanger.
A forced circulation heating system relies on
mechanical pumps to transfer the working fluid through the collector and heat
exchanger. Next, the different types of solar collectors will be compared, including flat
plate collectors and evacuated tube collectors. Existing testing data will be reviewed to
determine which collector will perform most efficiently provided the climate conditions
of SE CT.
Next, an engineering analysis will be performed to understand solar
insolation and how it varies geographically and seasonally.
Subsequently, the heating requirements for a typical pool will be determined. After
the heating requirements are understood, the specific collector will be chosen and a
functioning heating system will be developed to meet the heating requirements of the
pool.
Then, the cost of operating commercial heating units will be analyzed and
compared to the cost of the solar heating system.
2.1 SOLAR WATER HEATER DESIGNS
The various solar water heaters that are going to be discussed are broken down into
two different categories: thermosyphons and forced circulation. Thermosyphons are
considered passive systems since they function on the natural circulation of water.
Forced circulation water heaters are classified as active systems due to the incorporation
of an external means to circulate the water, such as an electric pump. Solar water
heating systems transfer heat in two different ways, directly and indirectly. In a direct
system, the solar collector transfers the heat to the water; an indirect system utilizes a
2
heat transfer fluid, circulating in the collector in a closed loop, to transfer the heat to
water via a heat exchanger [3].
2.1.1
THERMOSYPHON SOLAR WATER HEATER
A thermosyphon solar water heater functions due to the principle that hot water has a
lesser density than cold water, and will rise above the colder, denser water. For this
reason, the collector in the system is mounted below the water storage tank; the cold
water will always have a descending path from the storage tank to the collector. The
purpose of the collector is to absorb the solar radiation and transfer this energy to the
water. As water absorbs the energy, the temperature will increase, and the density will
decrease; this induces a flow of hot water up, from the collector into the storage tank,
and cold water down, from the storage tank to the collector, see Figure 1. The storage
tank is located well above the collector, at least 300 mm above the top of the collector, to
preclude the chances of the cycle running backwards during the night, cooling down all
the heated water. This system is susceptible to having water in the collectors freeze
during the winter months; however, this can be mitigated with the addition of an antifreeze solution to the water and the addition of a heater in the storage tank [3].
Figure 1: Thermosyphon Solar Water Heater [4]
3
2.1.2
FORCED CIRCULATION SOLAR WATER HEATER
A forced circulation solar water heater is considered an active system due to the
incorporation of an electric pump to move the fluid from the storage tank, through the
collectors, and back to the storage tank. Two temperature detectors constantly monitor
the fluid temperatures within the collector and storage tank. Additionally, a controller is
incorporated into the system to start the pump when the collector reaches a set
temperature, approximately 5 – 10oC above the storage tank temperature. The activated
pump moves the heat transfer fluid in the solar cycle; however, when the temperature
difference reaches a set low limit, the controller shuts the pump off. An advantage of
the subject system is the ability to install the collector and storage tank independently of
each other; there is no height difference requirement between the tank and collector [3].
Figure 2 provides an example of the subject style of water heater.
Figure 2: Active, Closed Loop Solar Water Heater [4]
Water or antifreeze is used as the heat transfer fluid and is stored in the solar storage
tank. Water from the pool is pumped in through the cold water supply side, heated in the
storage tank, and circulated back to the pool via the hot water to house outlet. This
4
heating system will transfer heat to the heat exchanger and pool water much faster than
the thermosyphon system; therefore, the forced circulation water heating system will be
considered for the pool heating design.
2.1.3
COLLECTORS
The purpose of the solar thermal collector is to absorb the solar radiation and transfer
the energy to water in the storage tank. The two main types of solar collectors are flat
plate and evacuated tube collectors.
2.1.3.1
FLAT PLATE COLLECTOR
Figure 3: Flat Plate Collector [6]
The flat plate collector includes an absorber, a transparent cover, a frame and
insulation. Typically, a transparent cover consisting of an iron-poor solar safety glass is
used; this transmits a large amount of short wave light of the solar spectrum. Due to the
traditional greenhouse effect, minimal heat emitted by the absorber escapes the cover.
Furthermore, the transparent cover precludes heat transfer due to convection when the
collector is exposed to wind. The frame and cover protect the absorber from being
exposed to potential damaging weather conditions. The materials most commonly used
in construction of the frame include aluminum, galvanized steel and fiberglass
reinforced plastic.
Conduction heat losses through the side walls and back of the
collector is mitigated by lining the subject areas with polyurethane foam insulation [6],
5
see Figure 3 and Figure 4 for an example of a flat plate collector and a detailed view of
the construction, respectively.
Figure 4: Flat Plate Collector Construction [6]
2.1.3.2
EVACUATED TUBE COLLECTOR
The second type of collector to be discussed is the evacuated tube collector, also
known as a vacuum collector. This collector incorporates an absorber strip within an
evacuated and pressure proof glass tube. The heat transfer fluid flows through the
absorber in two possible arrangements:
1. Directly in a U-tube arrangement
2. Counter current in a tube-in-tube arrangement
The evacuated tube solar collector is constructed of multiple single tubes, serially
interconnected, or tubes connected to each other via a manifold. A heat pipe collector
contains a unique heat transfer fluid that vaporizes at low temperatures. When the heat
transfer fluid absorbs heat it vaporizes. This vaporized fluid then rises to the top of the
heat pipe due to the change in density and condenses at the top. The top of the heat pipe
extends into the storage tank and acts as a heat exchanger. The vaporized fluid transfers
6
the collected energy to the storage water and thereby condenses and flows back down to
the bottom of the heat tube [6].
Figure 5: Evacuated Tube Heater Design [6]
In order to function properly, the pipes must be installed at a specific angle in order
to promote the vaporizing and condensing processes. See Figure 5 for an example of a
installed evacuated tube collector system. There are two types of arrangements used to
connect the pipes to the storage tank. The first is called a wet connection where the heat
exchanger penetrates directly into the manifold. The second connection is called a dry
connection where the heat pipe is connected to the manifold via a heat conducting
material. The advantage of having a dry connection is to prevent draining the system
when exchanging or removing pipes from the system [6].
7
Figure 6: Evacuated Tube Collector Example [7]
2.1.4
METHODOLOGY FOR SELECTING SOLAR COLLECTOR
The following study was referenced in order to evaluate which type of collector
would provide the greatest output of energy when installed in a SE CT climate.
Figure 7: Rainy Day – very low and fluctuating solar radiation levels [8]
Figure 7 illustrates the results of a test where the performance of an evacuated tube
collector and a flat plate collector were compared during rainy conditions. The test
8
compared the energy output of the collectors by monitoring the temperature of a tank of
water (geyser temperature) each collector provided heated water to. Given the low
radiation levels, the evacuated tube collector was able to compensate for normal heat
losses from the tank and provide enough heat to raise the temperature of the tank volume
by 7oC. It was determined the evacuated tube collector provided approximately 830W/h
more than the flat plate during this test. The flat plate collector was only able to
compensate for the heat losses of the tank and no temperature change was found within
the tank. One of the advantages of the evacuated tube collector is the ability to deliver
energy during overcast and rainy periods while the flat plate collector provides
negligible energy outputs [8].
Figure 8: Hot summers day – high peak solar radiation levels [8]
The next test completed was a comparison between a flat plate collector and an
evacuated tube collector during a hot summer day. The test was set up equivalent to the
test discussed previously. The performance of the flat plate collector is considerably
better than the evacuated tube collector as can be seen by the steeper slope of the flat
plate collector’s geyser temperature in Figure 8. This is due to the much larger surface
area of the flat plate collector compared to the surface area of the evacuated tube
collector assembly. However, the evacuated tube collector maintained a steady heating
9
curve, beginning to heat much earlier than the flat plate collector and maintaining well
past the flat plate collector slows. The final results proved the flat plate collector
performed slightly better than the evacuated tube collector, heating the tank of water by
27oC versus the evacuated tube collector heating its tank 26oC.
With the above results, it was determined an evacuated tube collector would be
the most efficient solar collector to be used in the subject application due the collector’s
performance under rainy conditions. The evacuated tube collector would perform well
in the unpredictable weather conditions of the region where a mix of sunshine, clouds
and rain is prevalent. The flat plate collector would be much more desirable when
installed in areas such as the southern midwest regions of the USA where little rain
occurs, and the majority of days consist of sunshine.
2.1.5
METHODOLOGY TO OBTAIN DATA FOR HEATING A POOL
The type of collector to be used in the solar heating system has been selected; however,
the magnitude of solar insolation incident in the subject location must be understood.
Solar insolation is defined as the measure of solar radiation energy received on a given
surface area. The most common units used to express the subject quantity is watts per
square meter (W/m2). The amount of insolation incident on the Earth’s surface is
dependent on the angle of the sun, the state of the atmosphere, altitude and geographic
location [9].
Figure 9: Affect of solar insolation due to angle of sun[9]
10
As can be seen in Figure 9, the solar insolation is highest when the sun’s rays are
directed perpendicular to the exposed surface. However, as the sun’s angle decreases
from 90 degrees, the insolation incident on the same surface is reduced in proportion to
the cosine of the sun’s angle. This effect explains the large difference in temperature
and climate throughout the equatorial, temperate and polar regions. The polar regions
annually receive less insolation than the equator due to the small angle between the sun’s
rays and polar surface.[10] When installing solar collectors, they must be installed at the
most optimum angle to receive direct sunlight, see Figure 10.
Figure 10: Optimum tilt angle[9]
Solar insolation levels are used to determine what size solar collector is required to
efficiently provide sufficient quantities of hot water. Regions with high solar insolation
levels require smaller collectors than those with small insolation levels. Per Figure 11,
the southeastern regions of the USA obtain average insolation levels of 5.5 kWh/m2 per
day compared to an average of 2.5 kWh/m2 per day in the New England area. The
southeastern regions will require less collector area to heat up a given amount of water
compared to areas in New England where a much larger collector area will be required
to obtain comparable temperature changes for the same volume of water.
11
Figure 11: Solar Insolation Map of USA [9]
Figure 12 details the average insolation levels SE CT is exposed to annually. The
insolation incident on the earth’s outer atmosphere is much higher than what the earth’s
surface is exposed to. This is due to a majority of incident radiation being reflected back
into space and absorbed by the earth’s atmosphere.
Therefore, the average solar
insolation incident on the earth’s surface is approximately 57% of the insolation incident
on the earth’s atmosphere [10].
12
Solar Insolation of Southeastern CT
Solar Insolation (W/m2)
475
425
375
325
Incident on Earth
Atmosphhere
275
Incident on Earth
Surface
225
175
125
75
0
30
60
90
120
150
180
210
240
270
300
330
360
Days of the Year
Figure 12: Solar insolation incident on Earth's atmosphere compared to Earth's surface
In order to size a solar water heater adequate to heat a pool, the heat requirements of
the pool must be understood.
The subject pool to be heated has a volume of
approximately 26,000 gallons. The following equation is used to calculate the heat
required to produce a change in temperature of the given pool volume.
Q = cp m dT
or
J = (J/kg*oC)*(kg)*( oC)
The amount of heat, Q (Joules, J), is dependent on the specific heat of the water, Cp. The
specific heat is defined as the energy required to raise the temperature of a unit mass of a
substance by one degree. For water in this case, the value is approximately 4200
J/kg*oC. Furthermore, additional variables influencing the heat requirements include the
size of the mass, m (kilograms, kg), being heated and the change in temperature
required, dT (degrees Celsius, oC). The following graph shows the heat requirements,
expressed in kWh, for various changes in temperature of the subject pool water:
13
Energy Requirements to Heat Pool
700
Energy (kWh)
600
500
400
300
200
100
0
1
2
3
4
5
6
7
8
9
10
Change in Water Temperature of Pool (oF)
Figure 13: Required energy inputs to raise temperature of pool
2.1.6
SELECTION OF EVACUATED TUBE COLLECTOR MODEL
The standard for rating solar water heating collectors is performed by the Solar
Rating & Certification Corporation (SRCC)*.
The SRCC performs a number of
durability, efficiency and panel heat output tests on various collectors and rates the
subject collectors based on their performance. Certain ratings will be used to compare
various types of evacuated tube heaters in order to make a final selection based on panel
heat output. The following figure provides a page from a sample rating for a specific
evacuated tube collector.
*© Solar Rating & Certification CorporationTM
400 High Point Drive, Suite 400
Cocoa, Florida 32926
14
Figure 14: Sample SRCC Certification [11]
The results in Category A provide the service rating for heating a pool in a warm
climate. This service covers pool heater climates where the ambient temperature is
15
typically higher than the pool water going through the collector, Tinlet – Tambient = -9oF.
This is the most likely scenario for heating a pool in the subject location during the
summer. Three output ratings are provided; outputs for a clear day, a mildly cloudy day,
and a cloudy day. The test for a clear day exposes the collector to 23 MJ/m2 day, while
the mildly cloudy and cloudy day test exposes the collector to 17 MJ/m2*day and 11
MJ/m2*day, respectively. Various types of evacuated tube collectors will be compared
in order to make a final decision based on performance [11]. The following table lists
various evacuated tube collectors and their resultant ratings:
Table 1: SRCC Values for Various 30 Tube Evacuated Heater Assemblies [11]
Manufacturer
High
Model
Radiation Medium Radiation Low
(6.3 kWh/m².day) (4.7 kWh/m².day)
Zhejiang Hurras Solar Energy Technology Co., Ltd. DIYI-C01-26
Radiation
(3.1 kWh/m².day)
11
8.3
5.6
SunMaxx Solar LLC
ThermoPower UDF30
11.7
8.8
5.9
SunMaxx Solar LLC
ThermoPower VHP30
14.2
10.7
7.1
Zhejiang Gaodele New Energy Co., Ltd.
GDL-SP58-1800-30
12.4
9.3
6.2
Hubei Huayang Solar Group Co., Ltd.
GM65HP-30
14.1
10.6
7.2
Westech Solar Technology Wuxi Co., Ltd.
WT-B58-30
12.9
9.8
6.6
Jiangsu Sunrain Solar Energy Co. Ltd.
TZ47/1500-30U
11.7
8.8
5.9
Zhejiang Eammar Solar Energy Co., Ltd.
EM-C01-30
11.4
8.8
6.1
Sunflower Energy
SETE-30
11.6
8.8
6
All the collectors are tested under standard laboratory conditions; the performance
ratings shown above relate the energy output of the collector during the test with the
parameters indicated. The SRCC performance ratings establish a means to compare the
relative performance of various collectors under laboratory conditions. Therefore, based
on the above information, the ThermoPower VHP30, manufactured by SunMaxx Solar
LLC, has the highest rating and will be the collector chosen for the subject pool heating
system.
2.1.7
FINAL DESIGN OF SOLAR POOL HEATING SYSTEM
The chosen collector model must be sized to provide sufficient energy to heat the
pool at a timely rate. Optimal sizing of a solar thermal system requires an overall solar
fraction of 60-70%. The solar fraction is the percentage of the overall load that is
supplied by the system over a specific period of time [12]. This is completed by
matching the load heating requirement to the output of the solar array on a clear summer
16
day. The advantage of sizing the system to this requirement is to create a design that
operates at maximum performance without reaching the stagnation temperature of the
collector. The stagnation temperature is the maximum achievable collector temperature
that can be reached with no flow within the system and can be detrimental to system
components [13]. The output of the ThermoPower-VHP30 collector is approximately
15MJ/m2 on a clear summer day [14].
From the previous energy requirements
established, approximately 229 MJ are required to heat the entire pool volume by 1oF in
one day. The following equation is provided to estimate the collector area required to
heat a single kilogram of water:
Sizing Ratio = 1.15 * Cp * (X –Y) / Z [14]
The constant 1.15 is used to oversize the required collector area by 15% to account for
efficiency losses in various components throughout the heating system.
C p is the
specific heat of water, approximately 4200 J/kg*oC, which is the energy required to raise
the temperature of a unit mass of substance by one degree [15]. X and Y terms provides
the temperature difference between the heated and unheated fluid. The Z term is the
power flux rating of the solar collector. The sizing ratio yielded an area of 1.75E-4
m2/kg of water. Provided 90,850 kg of pool water to be heated, a net aperture area of
15.89m2 is required. The aperture area is the surface of the collector absorbing the solar
energy. The subject collector has an aperture area of 2.983m2. Dividing the net aperture
area required by the aperture area of a single collector yields six collectors required to
heat the total pool volume by 1oF in one day. [14]
The purpose of the pool’s heating system is to transfer the energy collected by
evacuated tube collector to the pool water. This energy will be used to maintain a
desired pool water temperature. The following figure details the basic solar heating
system components:
17
3.
Solar Collector
2.
1.
Heat
Exchanger
5.
6.
8.
7.
7.
4.
Pool Water Reservoir
Figure 15: Pool Water Heating System
1. Working Fluid Transfer Pump
2. Cooled Working Fluid
3. Heated Working Fluid
4. Heated Pool Water
5. Pool Water
6. Pool Water Circulation Pump
7. Temperature Controller
8. Three-way temperature control valve
The purpose of the system above is to capture solar energy via the collector and transfer
the energy to the pool water within the heat exchanger. The temperature controller
energizes the components of the system to heat the pool when required or dump heat to
the pool to maintain desirable operation temperature within the collector. The desired
pool water temperature is input into the temperature controller.
The temperature
controller detects the temperature of the pool water and energizes both the pool water
pump and working fluid transfer pump when the temperature of the pool water falls
18
below the input temperature. Additionally, the three-way temperature control valve is
signaled to direct flow to the heat exchanger in lieu of circulating pool water back to the
pool. Once the temperature of the pool water is reached, the two pumps are shut down
and the three-way temperature control valve is signaled to stop directing flow to the heat
exchanger. The second function of the temperature controller is to sense the temperature
of the working fluid of the collector. If the temperature of the working fluid exceeds the
design temperature of the collector, the two pumps are activated to dump excess heat to
the pool water until the working fluid is below a set operating temperature. Furthermore
a secondary function of the subject system would be to dump excess heat to a home hot
water heater for preheating boiler feed water. This would reduce heating requirements
and hot water heating costs.
19
3. RESULTS
The following results detail the final design of the solar pool heating system, a cost
analysis of the subject system and a comparison to operation costs of a commercial pool
heating system.
3.1 COST ANALYSIS OF SOLAR HEATING SYSTEM
The following tables detail the costs of the major components of the subject system
and the change in cost and performance due to the number of collectors used in the
system:
Table 2: Cost Breakdown of System
Componenent
6 ThermoPower-VHP30 Collectors*
Working Fluid Transfer Pump**
Hot Water Storage Tank*
Price
$8,553.60
$363.00
$1,738.00
3 Way Temperature Control Valve***
$593.00
Temperature Controller*
$377.45
Total Cost
*
$11,625.05
From http://www.sunmaxxsolar.com
** From www.houseneeds.com
*** From www.ecomfort.com
Table 3: Cost and Performance Difference Due to Number of Collectors
# of Collectors
Heating Capacity per Day
Total Cost of System
1
.17oF
$4,497.05
2
.33oF
$5,922.65
3
.50oF
$7,348.25
4
.67oF
$8,773.85
5
.83oF
$10,199.45
6
1oF
$11,625.05
* Assumed linear relationship between heating capacity and # of collectors
As can be seen from Tables 2 and 3 above, the cost difference between a pool heating
system containing one collector and six collectors is approximately $7,000. How fast
the pool owner wants to heat the pool and how much money he or she is willing to spend
will dictate which system will best suit the owner’s pool heating needs. The median
20
heating system costs $7,348.25, which provides three evacuated tube collector arrays
each containing 30 evacuated tubes. This system would be capable of heating the
subject pool 0.50oF per day provided an output value of approximately 15MJ/m2*day is
available. It is recommended, however, to have a maximum of 120 evacuated tubes in
series; therefore, any system containing four collectors or less will be most efficient
provided the cost [14].
3.2 COST OF OPERATING COMMERCIAL POOL HEATER
The most efficient way of heating a swimming pool using non-renewable resources
is with a heat pump. A heat pump operates with a coefficient of performance of three to
six, which converts to an efficiency of 300% to 600%. Due to the high efficiency of heat
pumps, the cost of operation is relatively low when compared to other non-renewable
heating sources such as propane. The typical heat pump is capable of providing $1.00 of
heat for every $0.20 of electric input [16]. The following table lists and compares the
cost of heating the subject volume of water using a commercial pool heater provided an
electric rate of 16.352 cents per kWh.
Table 4: Cost of Operation for Commercial Pool Heaters [16]
Change in Temp oF
Energy Required kWh
Heat Pump Cost
1
63.79074074
$2.09
2
127.5814815
$4.17
3
191.3722222
$6.26
4
255.162963
$8.34
5
318.9537037
$10.43
6
382.7444444
$12.52
7
446.5351852
$14.60
8
510.3259259
$16.69
9
574.1166667
$18.78
10
637.9074074
$20.86
As can be seen in Table 4, it is expensive to heat the subject pool using electricity,
costing up to $20.86 per day to heat the subject pool volume 10oF, provided no heat
losses to the environment. Table 5 details the pay back period of the various systems of
21
Table 3. It is assumed the commercial heater is used 150 days of each year, at a rate of
1oF per day.
It is also assumed that no heat is lost during the heating period.
Additionally, it is assumed that the cost to operate the working fluid transfer pump is
negligible compared to the cost of running the heat pump.
Provided the above
conditions are met, it would take approximately 15 years to pay off a solar heating
system consisting of one evacuated tube collector array and approximately 29 years to
payback the most efficient solar heating system containing four evacuated tube collector
arrays. The manufacturer of the subject model evacuated tube collector provides a tenyear warranty when a certified installer installs the solar array; however, it is assumed
the subject collectors outlast their warranty provided the collectors are maintained and
used properly.
22
Table 5: Cost Analysis, Pay Back Period
Years Heating (150 Heating Days @ 1oF per day)
Cost of Operating Heat Pump
1
$312.75
2
$625.50
15
$4,691.25
17
$5,316.75
18
$5,629.50
19
$5,942.25
22
$6,880.50
23
$7,193.25
24
$7,506.00
27
$8,444.25
28
$8,757.00
29
$9,069.75
31
$9,695.25
32
$10,008.00
33
$10,320.75
36
$11,259.00
37
$11,571.75
38
$11,884.50
Key
1 Collector System Break Even Point
2 Collector System Break Even Point
3 Collector System Break Even Point
4 Collector System Break Even Point
5 Collector System Break Even Point
6 Collector System Break Even Point
3.3 ENVIRONMENTAL IMPACTS
Electrical power generation is one of the largest contributors to CO2 emissions in
the world. Coal is presently the largest source of electricity in the world and in 2002, the
power sector accounted for 40% of all CO2 emissions.[1] Emissions can be reduced by
the use of renewable energy, such as through the use of solar collectors. The following
23
figure quantifies the emission of CO2 from various sources of electricity generation
including coal, oil and gas in North America:
CO2 Emissions
(grammes/kW-h)
CO2 Emissions from Electricity Generation
1 000
900
800
700
600
500
400
300
200
1990
Electricity Generation from
Coal/Peat
Electricity Generation from
Oil
Electricity Generation from
Gas
1995
2000
2005
2010
Year
Figure 16: CO2 Emissions from Electricity Generation[17]
As can be seen in Table 4, approximately 64 kW-hr are required to raise the temperature
of the subject pool by 1oF. In 2010, the production of 1 kW-hr of electricity from coal
caused 886 grams of CO2 to be generated in North America [17].
Therefore,
approximately 56.7 kg of CO2 are produced from generation of electricity (from oil)
required to run the heat pump to heat the subject pool 1oF. Figure 17 details the
equivalent emissions of CO2, due to electricity production from various fuels, to meet
the electrical demands of the heat pump to heat the subject pool volume 1oF per day, 150
days per year. After 10 years of heating, approximately 415,645 kg of CO2 is released
due to production of the equivalent electricity from coal. The use of renewable energy
resources such as solar energy will reduce the need to obtain electricity produced from
nonrenewable fuels and therefore reduce harmful CO2 emissions.
24
CO2 Emissions from Heat Pump Electricity Requirements
1600000.00
Production of CO2 (kg)
1400000.00
1200000.00
1000000.00
Electricity Produced from Coal
800000.00
Electricity Produced from Oil
Electricity Produced from Gas
600000.00
400000.00
200000.00
0.00
0
5
10
15
20
25
30
35
40
o
Years of Heating (150 Heating Days @ 1 F)
Figure 17: CO2 Emissions from Heat Pump Electricity Requirements[17]
25
4. CONCLUSION
From review of multiple solar heating options, it was determined the evacuated tube
collector would perform most efficiently in the climate conditions of SE CT. The energy
required to heat the pool to a specified temperature was determined; then the size of the
collector array was calculated based on the rated capacity of the subject collector and the
daily heating requirements. These results could be misleading to a potential customer
since the output rating of the solar collector is based on the collector being exposed to 23
MJ/m2*day, which is the energy flux per day the collector is exposed to for clear day
SRCC rating. The average solar flux Southeastern CT is exposed to between the months
of May and September is approximately 11MJ/m2*day provided 12 hours of direct
insolation exposure. Therefore, the actual output of the solar collector array would be
approximately half of the commercial rating when used in Southeastern CT.
After the solar collector was chosen, the basic solar pool heating system design was
determined. The system’s function is to monitor the temperature of the pool water. The
system would be capable of energizing system pumps to transfer heat to the pool via the
solar collector or dump heat to the pool when the water temperature within the solar
collector becomes too high.
Finally, the cost and CO2 emissions from operating an electric heater were analyzed.
It was determined to take approximately 15-30 years to payback the most efficient solar
heating system designs depending on the size of the system. Additionally, after 15-30
years of heating, production of equivalent electricity from various fuels could cause up
to 1.2 million kg of CO2 to be released to the atmosphere. Provided the collected data, it
has been proven pool owners can utilize renewable energy in an affordable manner and
relieve their dependence on nonrenewable fuel resources to heat swimming pools.
26
REFERENCES
[1] Kreith, Frank; Goswami, Yogi D.: Handbook of Energy Efficiency and Renewable
Energ, CRC Press, 2007
[2] Andrews, John; Jelly, Nick: Energy Science principles, technologies, and impacts,
Oxford University Press, United Kingdom, 2007.
[3] Asif, M .; Muneer, T.: Encyclopedia of Energy Engineering and Technology – 3
Volume Set, CRC Press, 2007
[4] http://gogreenheatsolutions.co.za/project-type/solar-water-heating/thermosyphonsystem
[5] http://www.solar-energy-at-home.com
[6] http://www.greenspec.co.uk/solar-collectors.php
[7] http://www.solar-energy-for-home.com/solar-collectors.html
[8] http://gogreenheatsolutions.co.za/category/project-type/solar
[9] http://solarinsolation.org/
[10] http://aom.giss.nasa.gov/srlocat.html
[11] https://secure.solar-rating.org
[12] http://www.solarhotwater-systems.com
[13] http://en.wikipedia.org/wiki/Stagnation_temperature
[14] http://www.sunmaxxsolar.com
[15] Boles, Michael A.; Cengel, Yunus A.: Thermodynamics An Engineering Approach,
Fourth Edition, McGraw Hill, 2002
[16] http://www.solardirect.com
[17] http://www.iea.org/publications/freepublications/publication/name,4010,en.html
27