SCIENCE
SOLAR THERMAL COOLING TECHNOLOGIES
Robert E. Critoph1
1. School of Engineering, University of Warwick, CV4 7AL, UK, R.E.Critoph@warwick.ac.uk
ABSTRACT
The demand for cooling, whether for comfort or food preservation is a major consumer of energy across the world and is growing.
The use of solar energy to meet that demand has a natural synergy, but the technical and economic challenges are large. Some of the
different available technologies are compared and contrasted, including absorption, adsorption, desiccant wheels etc. and the state of
the art as reviewed in the IEA Solar Heating and Cooling Task 38 is outlined. Adsorption cooling, the technology under development
at Warwick, is explained in more detail and progress is charted with reference to different projects: one on car air conditioning and a
new one to develop a 5m3 solar thermal cold store.
INTRODUCTION
Cooling is a major energy demand in many areas
of the world and not only in warm climates. Even in the
UK, air conditioning accounts for 2 Mt CO2 emissions
per year with a rapid growth rate and refrigeration
accounts for 11% of electricity use. Naturally there
is considerable interest in the use of renewable energies
for cooling and this has lead to the establishment of the
International Energy Agency Solar Heating and Cooling
Task 38 – Solar Air Conditioning and Refrigeration.
The main objective of the Task is the implementation of
measures for an accelerated market introduction of solar
air conditioning and refrigeration with focus
on improved components and system concepts. The
market introduction will be supported through activities
in development and testing of cooling equipment for the
residential and small commercial sector [1].
There are many technological routes/types
of thermodynamic cycle that can be used to achieve
solar powered cooling:
· A standard mechanical vapour compression cycle,
requiring
an
electrical
input
to a hermetically sealed compressor. The electricity is
generated by photovoltaic panels. This has the
advantage of using off-the-shelf technology, but the
disadvantages of high cost and the probable need for
an electricity storage sub-system. There are many
systems for vaccine storage marketed and some
demonstration projects for air conditioning, but the
technology is very costly for the latter application.
· A continuous absorption cycle (e.g. Ammonia – water
as manufactures by Pink or LiBr systems) with an
electrically
driven
feed
pump
may
be used. For an autonomous application the use of a
small
amount
of
photovoltaic
electricity
to drive a feed pump might be justified.
· Intermittent absorption cycles are thermodynamically
identical to continuous systems but avoid the use of a
feed pump and other electrical power. Typically the
pair
used
is ammonia-water, but ammonia-NaSCN, methanolpolska energetyka słoneczna | 14
LiBr and other pairs have been used experimentally.
One of the most successful demonstrations was that of
a 100kg per day solar icemaker built in the 1980’s by
Exell, [2].
· The Platen-Munters diffusion absorption cycle
is continuous and does not use a mechanical pump. It
is
used
successfully
in
small
gas
or kerosene refrigerators and freezers but has proved
difficult
to adapt
to
larger
sizes
and
to irregular heat sources such as solar energy.
However, in more recent years purpose-built
machines have been developed by for example Jakob
and Eicher [3]
· Solid desiccant wheels. The most common
arrangement of desiccant system is the desiccant
rotor. A desiccant rotor consists of a honeycomb
support which has been impregnated with
a finely divided desiccant. As air flows axially
through the narrow honeycomb channels, moisture is
absorbed by the desiccant. The design of the rotor
gives
a
large
surface
area
of contact between air and desiccant. As the air
stream passes through the rotor, moisture
is absorbed and the heat of absorption, almost equal
to
the
latent
heat
of
condensation,
is released. The resulting air stream is therefore
warmer but drier. The latent enthalpy contained in the
moisture vapour is effectively exchanged for sensible
enthalpy in the temperature of the resulting air.
· Liquid desiccant systems. Liquids such as glycol,
sulphuric acid or lithium bromide solution can
be used in a similar fashion to solid desiccants but
with packed bed or crossflow sorbers. Saman et al
have developed systems in which the desorber section
is integrated with a flat plate solar collector [4].
· Intermittent adsorption cycles rely on the adsorption
of a refrigerant gas into an adsorbent at low pressure
and subsequent desorption by heating. The adsorbent
acts as a ‘chemical compressor’ driven by heat. In its
simplest form an adsorption refrigerator consist of two
linked vessels, one of which contains adsorbent and
both of which contain refrigerant as shown in Fig. 1.
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SCIENCE
Fig. 1. Idea of an adsorption refrigerator
Initially the whole assembly is at low pressure and
temperature, the adsorbent contains a large
concentration of refrigerant within it and the other
vessel contains refrigerant gas (a). The adsorbent vessel
(generator) is then heated, driving out the refrigerant
and raising the system pressure. The desorbed
refrigerant condenses as a liquid in the second vessel,
rejecting heat (b). Finally the generator is cooled back
to ambient temperature, readsorbing the refrigerant and
reducing the pressure. Because the liquid in the second
vessel is depressurised and boils, it takes in heat and
produces the required refrigeration effect. The cycle
is discontinuous since useful cooling only occurs for
one half of the cycle. Two such systems can be operated
out of phase to provide continuous cooling. Such
an arrangement has a comparatively low Coefficient of
Performance (COP = Cooling / Heat Input). Also, the
thermal conductivity of the bed is generally poor so the
time taken for a cycle could be an hour or more and the
cooling power per mass of adsorbent could be as low as
10 W/kg. This is not a problem with solar powered
vaccine refrigerators which produce a few kg of ice
each day and operate on a diurnal cycle. However,
a refrigerator producing one tonne of ice in a diurnal
cycle would need 5 tonnes of carbon and contain 1.5
tonnes of ammonia. When contemplating larger
icemakers it is obviously necessary to use a much faster
acting cycle in order to reduce the mass of adsorbent
and the cost of the system. Two beds, similar to the one
shown above, can be heated and cooled out of phase
to provide continuous cooling Good heat transfer
is required to reduce the cycle time to a few minutes
and thereby increase the specific cooling power (SCP)
to the order of 1 kW/kg of adsorbent. We can also
achieve a higher COP (Coefficient of Performance =
Cooling power / high temperature heat input)
by maximising the quantity of heat regenerated. The
heat rejected by one bed when adsorbing can provide
a large part of the heat required for desorbing in other
bed. This also requires good heat transfer.
Warwick specialises in cycles that utilise an active
carbon adsorbent with ammonia refrigerant and has
recently developed compact plate sorption reactors that
can achieve well in excess of 1 kW/kg.
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COMPACT SORPTION SYSTEMS USING PLATE
GENERATORS
The first theoretical predictions of the performance
of a carbon – ammonia plate generator design were
made by Critoph and Metcalf [5] in which it was
implied that power densities two orders of magnitude
higher that the then state-of-the-art were possible. The
first application of the concept was in the EUTOPMACS project under the grant TST4-CT-2005012394. This is aimed at developing a car air
conditioning system driven by the waste heat of its
engine. The technology has recently been refined and
is now being applied to a solar powered refrigerator
of about 5m3 volume, used for food storage.
TOPMACS car air conditioning system
The novel sorption generator is a nickel brazed
stainless steel design with 29 layers of active carbon
adsorbent each 4 mm thick. By incorporating the
carbon adsorbent in thin layers, conduction path
lengths through the material are reduced and the area
for fluid heat transfer is increased which enables rapid
temperature cycling and thereby a high SCP. The
separating stainless steel plates are constructed from
chemically etched shims with 0.5 mm square water
flow channels on a 1 mm pitch. These channels give
a high heat transfer coefficient and a large heat transfer
area, further improving heat transfer performance. The
square design ensures equal flow path lengths in every
channel and therefore even heating and cooling of the
adsorbent. The internal pressure (up to 20 bar when
condensing at 50°C) is withheld by the stainless steel
shims which act as supporting webs to the outer wall,
which only needs to be 3 mm thick despite being
straight. The open end of the front face as shown
in Fig. 2 is used to insert and remove the adsorbent
in order that a range of adsorbents can be tested. Fig 3
is a photograph of the unit fitted with water manifolds
and pressure flanges prior to testing. The top and
bottom ‘ammonia flanges’ are necessary due to the
open face and would be unnecessary in an eventual
completely enclosed unit. The end pressure flanges are
necessary to prevent deformation of the ends of the
unit, but could be replaced by lighter domed ends.
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Fig. 2. Plate heat exchanger generator design
Fig. 3. Novel compact sorption generator Experimental prototype
The laboratory cooling system is designed
to simulate a mobile air conditioning system (MACS)
for a Class C passenger vehicle (such as a Ford Focus
or Fiat Stilo) with a 1.9 litre turbo diesel engine. This
is a demanding application requiring high efficiency
with limited waste heat availability. The engine coolant
is to be used to provide the heat input at a temperature
of 90°C and a nominal flow rate of 24 litre/min.
A schematic diagram of the system is shown
in Fig. 4. The engine coolant is alternately passed
through the two generator beds in order to heat them.
A second pumped coolant loop is used to recover heat
between the two beds. An air-to-water heat exchanger
placed in front of the vehicle radiator (labeled
adsorption heat exchanger) is used to cool the
generator
beds
to
ambient
temperature.
An interconnecting pipe with a valve is also
incorporated which enables the ammonia side of the
two generators to be connected for mass recovery
purposes. In this process, the heated high pressure bed
is connected to the cooled low pressure bed and
ammonia is transferred from the high pressure to the
low. This increases the concentration change in the
adsorbent during the cycle, thereby increasing SCP and
COP. Check valves are used to control the flow
of ammonia between the generators and the condenser
and evaporator, which are as per a conventional
system. One key difference however is the use
of an indirect evaporator with an intermediate chilled
water glycol loop – this prevents leakage of toxic
ammonia into the cabin, which could occur with
a direct evaporator. Fig. 6 shows the laboratory scale
MACS.
Fig. 4. Schematic diagram of Laboratory MACS
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Fig. 5. Lab MACS: Generators and condenser view
With a driving temperature of 90°C, the cooling
power was 1.6 kW, exceeding the 1.2 kW target
by 33%. The COP was 0.22, which is close to the
target value of 0.24. The decreased COP obtained with
higher driving temperature is due to the fact that the
cycle time was not optimised for each condition. The
power density was 114 W/litre based on generator
volume and 77 W/litre based on total system volume.
The specific cooling power SCP is about 0.800 kW/kg.
The variation of cooling power over a cycle (160s)
is shown in Fig 6.
Solar powered refrigerator
There is a requirement for maintaining chilled food
at 0-5°C in transportable containers in remote areas
away from grid electricity. The conventional
technology solution is to use vapour compression
refrigeration powered from motor-generator sets.
volume of 4.7m3. It is normally cooled by a
conventional vapour compression chiller, rated at about
2kW cooling at 2°C. It is required to maintain normal
use at ambient temperatures of 40°C using a solar
thermal cooling system.
Naturally, any solar powered system requires
thermal storage and it has been decided to use an ice
bank integrated with the flooded evaporator of the
refrigerator. Approximately 50 kg of ice is needed and
this is incorporated into a vertical wall within the
container. The wall has enough fins extending into the
cold space so that cooling within the container
is achieved by natural convection. Figure 7 shows the
complete evaporator/ice-bank assembly.
Fig 6. Laboratory MACS cooling power v. time
The University and Advanced Technology
Materials Inc. are collaborating in the development of a
solar thermal powered system, which will have
parasitic power for controls etc. delivered by PV’s..
The standard container, manufactured by CMCI has
external dimensions 2.4 m x 1.5 m x 2.1 m and internal
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Fig. 7. Evaporator / ice-bank
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SCIENCE
The evaporator consists of approx. 40 vertical half
inch tubes with a large reservoir above and parallel
feed below. Later versions will have direct expansion
evaporators which will have the advantages of lower
mass, lower cost and reduced refrigerant charge, but
the overwhelming advantage of a flooded evaporator is
that it requires less development and is comparatively
risk free. Each of the vertical tubes fits tightly between
the fins of an aluminium extrusion that forms part of
the ice-bank. Without this heat transfer enhancement,
towards the end of the process of freezing the water,
the evaporating temperature would drop significantly
as heat from the freezing front had to be conducted
through an increasing thickness of ice, thereby
reducing the system COP (cooling power / driving heat
input). The same aluminium extrusion is used on the
outside
of the ice tank to transfer heat to the cold space by
natural convection.
The refrigeration system is based on very
similar generators to the TOPMACS unit. In the
original generator, granular carbon in 4mm thick layers
was sandwiched between stainless steel shims
containing numerous water channels for heating and
cooling the carbon. The new design utilises a more
highly conductive carbon developed by ATMI which
enables the use of 12mm carbon layers, reducing cost,
complexity and thermal mass. The two beds will
be operated in a simple cycle with both mass and heat
recovery, with typical cycle times of 2 minutes.
The original design for car air conditioning was
heated or cooled by unpressurised water. The solar
collectors are expected to operate at well above 100°C
and so the choice had to be made between using a heat
transfer fluid or pressurised water. The heat transfer
properties of water are so superior that pressurised
water was selected.
Given the high collector temperatures, the only
commercially available options are evacuated tubes.
The collectors used are Thermomax DF100 2m 2 panels
which feature direct flow of the fluid (water
at up to 6 bar pressure) through the tubes. It is expected
that up to 10m2 will be used to obtain a peak cooling
power of up to 2 kW at an ambient temperature of 40°C
in a desert environment.
A critical area of design is the waste heat rejection
from the condenser and adsorbers. This is done using
conventional fan coils and with attention being paid
to minimising the fan power. The design compromises
are critical. A small compact heat exchanger may have
higher temperature differences which lead to lower
COP and hence more heat to be rejected. It may also
need more fan power and since parasitic electrical
power will be met from PV, this must be minimised.
Conversely, very large heat exchangers could be both
impractical and costly. The compromise chosen uses
a direct condenser measuring 650 h x 900 l x 570 w and
with a 66W fan motor and a cooler measuring
650 h x 900 l x 470 w with a 102W fan motor.
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Simulation of solar powered refrigerator
The operation of the complete system has been
modelled in Matlab to assist the design. The operation
of the chiller has to be modelled at a timestep of about
0.001 s which is obviously impractical for modelling
several days of operation. This problem has been
overcome by deriving a pseudo-dynamic model
in which the chiller is assumed to respond much more
quickly (within minutes) than changes in the load
or ambient conditions.
An example of the approach is given in Figure 8
in which each point (derived from detailed simulation
every 0.001 s) corresponds to a balance between the
heat input from the collectors at the particular insolation
and ambient temperature, together with a particular
cycle time (control parameter) and evaporating
temperature (corresponding to the state of the load). The
envelope of the points (linear fit in red, quadratic
in blue) gives the instantaneous cooling power
corresponding to the best control strategy for that
particular evaporating temperature and ambient
temperature for the full range of insolation.
A set of these correlations for a range of evaporating
and ambient temperatures may be combined empirically
to yield a polynomial function for cooling power under
any conditions which can act as input to the model
of the ice-bank and cold box. This work is not yet
complete, but preliminary examination implies that the
initial ice bank should normally form within the first
day of operation. A near to optimal control strategy has
been developed to determine the cycle time for
maximising the cooling power. The cycle time may
be made a function only of insolation levels and
neglecting the effect of ambient temperature
or evaporating temperature may be justified.
Cooling Power
-2
Insolation (W m )
Fig. 8. Simulation of cooling power v insolation with
optimal and simplified control strategies
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Progress in construction of solar powered
refrigerator
The system was completed in late 2009, tested
in the UK (Figure 9)and shipped to Tucson, USA for
testing in the field. Figure 10 shows the machine under
test.
Fig. 10. Field testing in Tucson, March
2010
ACKNOWLEDGEMENTS
This research is supported by EU-TOPMACS project
under the grant TST4-CT-2005-012394.and by ATMI
under US DoD contract W911QY-07-0073.
Fig. 9. Initial laboratory test results
Mechanically the unit functioned perfectly, with
regular cycles, heat and mass recovery etc. The
electronic expansion valve finally used worked very well
even at low cooling powers which means that in future
work a direct expansion evaporator may be used instead
of a flooded one. Unfortunately the heat transfer in the
new 12mm layered generator and the decreased porosity
of the monolithic carbon combined led to a disappointing
cooling power, approximately 25% of that hoped for.
Any future work will use new designs of generator
recently developed for heat pump applications and a
direct evaporator. and there is no reason to suppose that
the original target cooling power of 2kW cannot be met.
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REFERENCES
http://www.iea-shc.org/task38/index.html
Critoph R. E. and Metcalf S. J., Specific cooling power
intensification limits in carbon-ammonia adsorption
refrigeration systems, Applied Thermal Engineering,
24 (5-6), 661-679 (2004).
Exell, R.H.B., Kornsakoo, S., Oeapipatanakul, S., A
village size solar refrigerator, Asian Institute of
Technology Report No. 173, Bangkok, Thailand,
1987.
Jakob U. and Eicher U., Solar cooling with diffusion
absortion principle, proc WREC VII, Cologne, 2002.
Saman W. Y. and Alizadeh S., Experimental study of a
cross-flow type plate heat exchanger for
dehumidification/cooling, Solar Energy Vol. 73, NO. 1,
pp 59-71, 2002
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