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Solar Cooling and Hybrid Systems with Heating and Hot Water

Solar cooling technologies transform solar radiation to provide space cooling and refrigeration services. Air conditioning in buildings has traditionally been provided by air conditioners using electrically driven vapour compression chillers. These are responsible not only for GHG emissions, but also use CFCs and HCFCs and related compounds as refrigerant fluids, which also contribute to climate change and are known to deplete the ozone layer.

Introduction top

Solar cooling technologies transform solar radiation to provide space cooling and refrigeration services. Air conditioning in buildings has traditionally been provided by air conditioners using electrically driven vapour compression chillers. These are responsible not only for GHG emissions, but also use CFCs and HCFCs and related compounds as refrigerant fluids, which also contribute to climate change and are known to deplete the ozone layer.

Other methods of cooling include natural ventilation in the design of buildings coupled to design to minimise solar gain and heat transfer. These design and material considerations should always be incorporated anyway into buildings so that the mechanical cooling load is less and requires fewer resources to deal with it. Chilled beams and other building design features can also provide a comfortable cool atmosphere

At its simplest solar cooling collects solar thermal energy via a working fluid such as water or water plus antifreeze and then uses an interface such as a heat exchanger or absorption chiller to convert the energy in a refrigeration cycle. An absorption system uses a refrigerant which expands from a condenser through a throttle as in a conventional system. The absorbent absorbs the refrigerant vapour from the evaporator at low pressure and desorbs into the condenser at high pressure when heat is supplied to the desorber. It represents a heat driven heat pump where the heat can come from solar or waste or other source of heat.

There are of course many possible ways of achieving the same service and Henning (2007) provides an overview of the possible configurations with an indication of those commercially available and under development as shown below.

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Figure 1: Solar radiation

For air conditioning/cooling the technologies can be categorised in the following way:

  • Absorption chillers,
  • Desiccant cooling techniques ( adsorption chillers) with solid desiccants, powered by solar collectors for air or water heating,
  • New desiccant cooling cycles with liquid sorbents,
  • Closed cycle solid sorption (mainly adsorption), and
  • Advanced combined systems (e.g. solar dehumidification with conventional temperature control).

These can be used for cooling and dehumidification applications and have the potential to reduce energy requirements by 92-95% (Philibert, 2005). In humid areas liquid dessicants would be preferred.

The basic system involves coupling together a solar thermal collector and a thermally driven chiller. This is not simple and requires control systems if they are to operate at maximum efficiency. The coefficient of performance of the systems (produced cold per unit of driving heat, COP) depends on the radiation input, the temperature of the chilled water and that of the cooling water for the chiller. The main components of a solar assisted cooling and air conditioning system are given by Balaras et al. (2007) and are reproduced below.

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Figure 2: Solar cooling load

The technologies based on heat transformation are the most developed and two main types are distinguished:

  1. Thermally driven chillers produce chilled water which can be used in any type of air conditioning equipment, and
  2. Open cycles or dessicant cooling systems for direct treatment of air in ventilation systems.

The operation of absorption chillers is described in ASHRAE (1988). They are available in a wide range of sizes and have been designed for a range of applications. Only a few systems are available below 30 kW and most are above 100 kW. They depend on a cheap or ‘free’ source of heat. For air conditioning, the refrigerant/absorbent pair is usually water/liBr. These operate with a driving temperature of the heat source of 80-100°C. Single-effect machines have an evaporator and a generator but double effect machines are available where two generators working at different temperatures are operated in series. This gives a higher COP but these require a driving temperature of 140-160°C and are only available in sizes >100 kW (Yattara et al., 2002).

Solid adsorbents (rather than liquid absorbents) are also available and typically commercial systems use a driving heat temperature of 80°C with water/silica gel as the refrigerant/adsorbent pair. Other pairs are water/zeolite and ammonia/activated carbon (Afonso, 2006). Metal Hybrid systems developed by Japanese companies for cold storage of -30°C uses the hydrogen absorption and desorption of metal hybrid alloys. It has low noise with no moving elements, is CFC-free, safe and only uses pumps for circulating water and brine.

Open-cycle systems produce the conditioned air directly rather than indirectly through chilled water as in the thermally driven chillers. They operate by evaporative cooling and then air dehumidification by dessicant. The dessicants in common use are silica gel and LiCl. The systems require only the standard components normally used in air conditioning systems and are usually used in temperate zones. Operating at Mediterranean temperatures is possible using additional components and the different possibilities are described in detail in Henning (2007).

Though it is difficult to be definitive, the specific installed collector area m2/kW for water chillers both absorption and adsorption is higher for dessicant systems at 3 m2/kW compared to 1.5 m2/kW for dessicant systems. Cooling requirements are usually in line times of high solar radiation, thus matching the solar assisted thermal energy availability with the cooling load cycle. 

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Figure 3: Solar cooling house comparison (Source: Heliocol)

Feasibility of technology and operational necessities top

As discussed above, there are many different possible configurations for solar thermal cooling and many are commercially available while some new developments such as liquid sorption materials are close to market and others are still in the R&D stage. The bias is at the larger capacity end of the market with a capacity of 100 kW and greater, but a few systems are available with a capacity below 30 kW. 

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Video 1

Absorption chillers predominate at 59%, adsorption chillers in 11% and 23% are dessicant cooling systems. 6% of all installations use liquid dessicants, so this is a less developed technology. Absorption chillers, however, are the main technology in terms of cooling capacity as this technology is most suitable at the larger end of the market. The SACE database also gives details of 54 projects compared on a common basis and these are listed in the Appendix. The COP depends on the technology type and the system driving temperature determines the type of solar collector which can be used. 

According to the SACE project, the main barriers to the introduction of these technologies is the high cost coupled with the lack of familiarity and practical experience of design, control and operation of the technology for architects, planners and builders (Balaras et al, 2007).

Further research and development is also recommended to enable market integration of the new technologies and reduce the costs. The IEA Solar Heating and Cooling subgroup provides practical user guidelines for application of the technologies and provides a conduit for information for the technology dissemination (IEA SHC, 2002, 2009). However, more demonstration projects are required.

Status of the technology and its future market potential top

According to Henning (2007), 70 systems amounting to 6.3 MW and collector area 17 500 m2 have been installed in Europe, and these are mainly in Germany or Spain though there are plants in Greece, Portugal, Italy, Austria, France, the Netherlands, Israel, Turkey, and Serbia. Absorption chillers are mainly manufactured in Japan, China and South Korea and some in North America. However, these are mainly using waste heat or in combination with CHP or are gas-fired. In Japan they are actively encouraged by the government because although they are more expensive, they reduce gas imports and summer electrical loads (IPCC, 2003). A typical solar cooling installation in Spain is presented in the Figure below

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Figure 4: Solar cooling in Seville, Spain (Source: Medium and Small Scale Concentrated Solar Thermal Power Platform)

The EU SACE (Solar Air conditioning in Europe) programme and the IEA solar heating and cooling programme have provided a portfolio of projects (IEA SHC, 2002).

Dessicant cooling systems are useful for centralised ventilation systems, and thermally driven chillers producing cold water can be added into most air conditioning equipment. 70 installations exist in Europe. An example is in Freiburg in Germany where the university hospital uses it to air condition a laboratory. An absorption chiller of 70 MW capacity and evacuated tube collector of 170 m2 is used. Over four years, the system was monitored and showed that the solar collector works well and the system can be optimised to give a reasonable COP but electricity consumption on the cooling tower cycle was too high.

Another air conditioning application in Freiburg, in the Chamber of Commerce, has provided satisfactory performance using a silica gel dessicant cooling system, though the COP was lower than expected.

A dessicant (sorption) wheel with two cooling coils in the return air stream has been built in Palermo, Italy. In this case, a compression chiller is used for the cold water giving a high COP. This plant is in combination with a CHP system which provides the heat to regenerate the dessicant.

In developing countries it has been more difficult to find example applications of solar cooling for air conditioning. Solar refrigeration through a variety of technologies seems to be more widely used for ice production and food storage. China has been involved in research in this area for at least 35 years. Many solar absorption air conditioning systems have been developed. In 1999, in Rushan in Shandong province a 100 kW solar thermal absorption system with 540 m2 collector area was built. This was a hybrid system, also providing hot water to improve overall energy use and efficiency. A solar ice maker has also been produced in China and a hybrid system for heating and cooling has been developed. Work is also in progress to integrate solar energy technologies into building design and fabric.

Solar cooling technology has not yet been fully implemented in industrialised countries, but there is no reason why transfer to developing countries should not happen in parallel. Skills development, quality control issues as well as technology development experiences can be shared. Developing countries like Tunisia experience peak loads in summer for electricity for air conditioning and it has been found that absorption solar air conditioning systems would be suitable for the country and would help minimise fossil fuel based energy use by reducing demand. According to Balghouthi et al. (2005), a pilot solar cooling installation is being built in Tunisia.They were also keen to remove the use of CFCs.

R&D activities on thermally driven water chillers have focussed on the low range of cooling capacities from <50kW down to <5kW using different technologies such as liquid sorption materials LiBr/water, Ammonia/water, and solids such as silica gel/water, zeolite/water or salt/water systems. Afonso (2006) also describes a number of modifications to the absorption cycle involving replacement of the liquid pump by hydrogen (Platen-Munters system). Another modification is the steam ejector recompression for enhancing the concentration process and an electrochemical absorption refrigeration system (Newell, 2000). Another area is the development of open cooling cycles using liquid dessicants. These allow the separation of the absorption and regeneration in time, so that liquid sorption material can be used as a chemical storage . Liquid dessicants are also easier to cool.

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Figure 5: Solar cooling with airconditioning (Source:

An advanced solid sorption process is also being developed. A prototype small adsorption heat pump for solar heating, hot water and cooling is being developed.

Another key development area is in the dessicant cooling systems using rotary wheels which have several disadvantages. A novel dessicant concept called ‘Indirect Evaporative Cooled Sorptive heat exchanger’ (ECOS) is being developed for tropical and Mediterranean climates where the ambient air humidity is high. This design is also simpler.

According to Afonso (2006), besides the thermally operated systems described above, hybrid systems are also being developed which optimise different energy sources to provide greater efficiency. In addition, a thermoacoustic refrigeration system can cool without refrigerants.

Another such technology is the Hybrid geothermal heat pump with thermal solar collectors for space heating,cooling and hot water. Trillat-Berdal, et al. (2007) report on a new hybrid for delivering a range of energy services including cooling. They use a reversible geothermal heat pump with solar thermal collectors and have already installed this in a 180 m2 private residence which has been subject to monitoring since 2004. The proposed process is called GEOSOL and is still under development.

Finally, a quite known innovative technology applied in some countries are Cool Roofs. Cool roofs are roofs with high solar reflectance and high thermal emittance which tend to stay cool in the sun. They decrease cooling electricity demand, cooling power demand and cooling equipment capacity requirements but increase slightly heating energy requirements. They can also lower summer air temperatures in cities slowing ozone formation (Levinson et al., 2005). They also lead to energy savings and cost savings which can offset the cost of providing the cool roof. Recommendations are made for incorporation into the building codes.

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Figure 6: Cool Roof concept (McGraw Hill Construction)

The IEA in its Task 25 report (IEA SHC, 2002) suggested that the development of small size chillers and the possible integration of heat and electrically driven systems would be beneficial. Since then many developments have occurred and the range of technologies and sizes have been reported above.

China is active in developing technologies and foresees an internal market for them. All developing countries have a requirement for air conditioning and the demand for an environmentally benign solution is high. However the costs are still too high to be feasible in this context at present. Local development as in China could speed up the process.

How the technology could contribute to socio-economic development and environmental protection top

The number of air conditioning systems in Europe with cooling capacity over 12 kW has increased by a factor of 5 from 1980 to 2000. The implication of this increase in electrically driven vapour compression systems on primary energy demand in Europe is that annual energy use has risen from 6 TJ in 1990 to 40 TJ in 1996 and is estimated to reach 160 TJ in 2010 (Balaras et al., 2007). For southern European and Mediterranean areas these technologies can provide energy savings in the range 40-50%. The related cost savings are €0.07/kWh for the best technologies.

The increasing demand for cooling is driven by the change in climate as well as higher standards and expectations in working and living conditions. However, conventional electricity driven equipment means that to meet this demand there is a significant electricity load which, depending on the climate, can be equivalent to or greater than the heating load in winter.

Conventional cooling systems have all the disbenefits of increased grid electricity generation plus the problems of the effect of the CFCs, HCFCs and substitutes used as refrigerants on the ozone layer. Solar thermal systems on the other hand have much less electricity load and use the ‘free’ heat from the sun to drive the system thus avoiding the GHG loads and the CFC and HCFC refrigerant loads from conventional air conditioners.

However, the solar assisted air conditioning can only achieve these savings in primary energy if the systems are properly designed with sufficient collector size and energy storage. Supplying other services such as hot water maximises the use of the solar energy and optimises the economics.

Air conditioning in developing countries is usually reserved for the rich and for tourism and some offices. However, as tourism is a key industry in many countries and the electricity demands are significant, all of the above benefits applying in the EU also apply to developing countries. In addition, there may be an accompanying benefit from decreased imports of fossil fuels depending on the electricity supply mix in the country. The technology and skills transfer involved in this technology could also stimulate other solar thermal applications.

Contribution of the technology to economic development (including energy market support) top

Energy consumption in commercial and residential buildings represents about 40% of Europe’s energy demand. In the EU there are real benefits to be gained by decreasing electricity demand for energy services. The projections for the growth in demand from air conditioning given above for the EU alone show that this is a significant area to be targeted for improvement. By decreasing electricity demand, resource requirements for new power stations are avoided and the resulting pollutant emissions and other detriments from such new buildings are also prevented. In addition, there is no need for refrigerants which are detrimental to the ozone layer. New industries are created with skills and with accompanying jobs. Savings are also made in monetary terms with decreased electricity bills.The increase in electrically driven air conditioning systems means that annual energy use of room air conditioners has risen from 6 TJ in 1990 to 40 TJ in 1996 and is estimated to reach 160 TJ in 2010 in Europe (Balaras et al., 2007). To tackle this problem the solar assisted cooling technologies can provide an alternative to the conventional systems with an increase in the overall electricity security of supply as a result and secure provision of the cooling service.

In the projects studied in the SACE project the most efficient systems were Water/LiBr, with adsorption systems performing less well. The least preferred were the Ammonia/water diffusion systems (Hisajima et al., 2007). The electricity demand for fans and pumps was 225W/kW cooling capacity on average. The water consumption was between 4 and 6 kg/h per kW cooling capacity. Energy savings can also provide € 0.07/kWh to offset against initial high costs (SACE, 2003).

Contribution of the technology to protection of the environment top

As indicated above, the systems available fit in with existing building systems but have a much lower environmental footprint as they use the energy from the sun to drive the system. They avoid GHG emissions which would otherwise be released in electricity generation, i.e. mainly CO2 but also CFCs or HCFCs. These refrigerants and their substitutes come under the Montreal Protocol as they also have a detrimental effect on the Ozone Layer. Local air pollution from fossil fuel fired power stations is also avoided. Thus, avoidance of the conventional approach and substitution with solar thermal systems delivers many environmental advantages. Nevertheless, we still have absorbents or dessicants which have to be carefully recycled or disposed of when the equipment reaches the end of its life. It is not clear that these considerations have so far been dealt with. 

Climate top

For the EU the increase in demand associated with electrically driven air conditioners will result in an expected increase in CO2 emissions in the EU of 20 times that of 1990 by 2010. It also means that electricity demand is barely being met in the EU at peak loads (Balaras et al., 2007).

Financial requirements and costs top

The initial cost of the technology depends on the solar collector area per unit of installed cooling capacity and is expressed as cost per specific collector area. The 54 projects evaluated in the SACE project had costs ranging from € 1,286 to 8,420/kW depending on the cooling capacity and solar collector type as well as the type of device and stage of development (SACE, 2003). The SACE project examined the value of the primary energy saving and showed that the best performance is with a flat plate solar collector driving a single effect absorption chiller. The best energy-cost performance relates to a collector area of 3 m2/kW cooling capacity of chiller and heat buffer of 12 m3. Savings of € 6.8/kWh are expected.The EU has funded the SACE project with the aims of examining the state of the art of the technology, the potential of the different technologies, identify future needs and provide an overview in Europe and advantages and disadvantages of different technologies.

The IEA have a solar heating and cooling programme of activities (see above) and under this they provide a range of enabling materials to promote the technologies. IEA has initiated a programme directed towards the development and promotion of solar heating and cooling technologies with demonstration projects. An example is the decision scheme for the selection of the appropriate technology using solar thermal air conditioning. Also the handbook “Solar Assisted Air-Conditioning of Buildings: a handbook for planners” has been published by IEA and is available on their website (Henning and Albers, 2004). Task 38 continues this work but only started in September 2006 so that there are as yet no outputs from this task (IEA SHC, 2002). Indicatively, a solar cooling system in the USA costs around 630 $/m2, while with a learning rate of 10% it should need 450 $/m2 to reach commercialization (IEA, 2008). China has been actively developing solar-assisted air conditioning as described earlier. Existing manufacturing capability seems to be in Asia but not predominantly using solar thermal, but gas fired and waste heat configurations. No facilitating programmes were found.

Clean Development Mechanism market status top

[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group]

Project developers of solar thermal energy projects in the CDM pipeline mainly apply the following methodologies:

ACM2 “Consolidated baseline methodology for grid-connected electricity generation from renewable sources”
AMS- I.C. “Thermal energy production with or without electricity”
Further information on these metholodogies can be found here.

As of 1 February 2010, there are 10 solar thermal projects in the CDM pipeline - 5 areregistered and 5 are at the validation stage.

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Figure 7: Overview of solar projects in the CDM (Source: UNEP Risoe CDM/JI Pipeline Analysis and Database, February 1st 2010)

Example CDM project:
Title: Federal Intertrade Pengyang Solar Cooker Project (CDM Ref. No. 2307)
The Federal Intertrade Pengyang Solar Cooker Project is located on the dry land of southern Ningxia in northwestern China. Implemented by Ningxia Federal Intertrade Co., the proposed project will install 17,000 solar cookers for the poor rural residents in mountainous areas with a rural population of 92,331 (or 20,341 households). The project will cover 83.6% of the households in the project region. The rating power of each solar cooker is 773.5 W and the total capacity of the proposed project is 13.1 MW. The proposed project will enable the rural residents to efficiently substitute solar energy for the fossil fuel (coal) used in daily cooking and water boiling - avoiding CO2 emission that would be generated by fossil fuel consumption.
Project investment: USD 900,000 million
Project CO2 reduction over a crediting period of 10 years: 357,230 tCO2e
Expected CER revenue (USD 10/CER): USD 3,572,300
References top

Afonso, C.F.A., 2006. Recent advances in building air conditioning systems. Applied Thermal Engineering, 26 (16), pp. 1961-1971.

ASHRAE, 1988. ASHRAE handbook, Absorption cooling, Heating and Refrigeration Equipment.

Balaras, C.A., Grossman, G., Henning, H.M., Infante Ferreira, C.A., Podesser, E., Wang, L. and Wiemken, E., 2007. Renewable and Sustainable Energy Reviews, 11, pp. 299-314.

Balghouthi, M., Chahbani, M.H. and Guizani, A., 2005. Solar Powered air conditioning as a solution to reduce environmental pollution in Tunisia. Elsevier.

Henning, H.M., 2007. Solar Assisted Air conditioning of Buildings: An overview, Applied Thermal engineering 27, pp. 1734-1749.

Henning, H.M. and Albers, J., 2004. Decision Scheme for the selection of the appropriate technology using solar thermal air conditioning, Task 25, IEA Solar heating and Cooling Programme.

Hisajima, D., Kawamura, H. and Oouchi, T., 2007. Determination of thermal properties of dilute LiBr-water solutions. International Journal of Thermophysics, 18(2), pp. 397-406.

IEA, 2008. Energy Technology Perspectives – Scenarios and Strategies to 2050.

IEA SHC, 2002. Ongoing research relevant to Solar Assisted air conditioning systems, Task 25, IEA SHC technical report.

IEA SHC, 2009. Solar Heat Worldwide – Markets and Contribution to the Energy Supply 2007, IEA SHC.

IPCC, 2003. Safeguarding the Ozone Layer and the Global Climate System, Residential and Commercial Air Conditioning and Heating, p. 269. Available at:

Levinson R., Akbari, H., Konopacki, S. and Betz, S. 2005. Inclusion of cool roofs in nonresidential Title.

24 prescriptive requirements. Energy policy, 33 (2), pp. 151-70.

Newell, A., 2000. Thermodynamic analysis of an electrochemical refrigeration cycle, International journal of Energy Research, 24, pp. 443-453.

Philibert, C., 2005. The present and future use of solar thermal energy as a primary source of energy, IEA, Paris, France.

SACE, 2003. Evaluation report.

Trillat-berdal, V., Souyri, B. and Achard, G., 2007. Coupling of Geothermal heat pumps with thermal solar collectors, Applied Thermal Engineering.

Yattara, A., Zhu, Y. and Mosa Ali, M., 2002. Comparison between solar single-effect and single-effect double-lift absorption machines (Part I). Applied Thermal Engineering, 23(15), pp. 1981- 1992.