SOLAR THERMAL COOLING TECHNOLOGIES Robert E. Critoph 1 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 5m 3 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 CO 2 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. polska energetyka słoneczna 14 1-4/2009 1/2010
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 5m 3 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. 1-4/2009 1/2010 polska energetyka słoneczna 15
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 polska energetyka słoneczna 16 1-4/2009 1/2010
SCIENCE 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 1-4/2009 1/2010 Fig. 7. Evaporator / ice-bank polska energetyka słoneczna 17
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 10m 2 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. Cooling Power Insolation (W m -2 ) Fig. 8. Simulation of cooling power v insolation with optimal and simplified control strategies polska energetyka słoneczna 18 1-4/2009 1/2010
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. Fig. 10. Field testing in Tucson, March 2010 ACKNOWLEDGEMENTS 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. 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 1-4/2009 1/2010 polska energetyka słoneczna 19