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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, methanol-

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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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.

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 EU-

TOPMACS project under the grant TST4-CT-2005-

012394. 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

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

Fig. 3. Novel compact sorption generator -

Experimental prototype

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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.

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

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. 7. Evaporator / ice-bank

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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 2m2 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.

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.

Fig. 8. Simulation of cooling power v insolation with

optimal and simplified control strategies

Cooling Power

Insolation (W m-2

)

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Fig. 9. Initial laboratory test results

Fig. 10. Field testing in Tucson, March

2010

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.

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.

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.

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