The 34th informatory note on refrigeration technologies published by the International Institute of Refrigeration (IIR) in 2017 looks at solar cooling as a promising, environmentally friendly technology that can help satisfy the increasing world demand for space cooling.
Solar cooling can be obtained by various technologies. The two main commercial options are photovoltaic (PV) driven vapour compression chillers and heat driven cooling machines fed by solar collectors.
Thermally driven cooling equipment can be coupled with various types of solar collectors with different efficiencies and costs. Overall system efficiencies of PV driven, and solar thermal driven plants may have not so different values. The economic analysis indicates the investment cost for the PV solution to be at least half of the other systems.
Solar cooling may have a very positive environmental impact reducing the usage of fossil fuel, and the technology is almost mature to compete with conventional cooling equipment.
*This Informatory Note was prepared by Renato Lazzarin, president of IIR Section E, with the assistance of the IIR head office and was reviewed by several experts from the IIR network.
A large proportion of global energy demand is represented by space heating and cooling. Whereas the energy demand for space heating is currently greater than the energy required by space cooling, several reasons suggest a forecast of a decreasing demand for space heating and increasing for space cooling. They are:
- The economic growth in developing countries results in higher comfort standards and a higher demand for space cooling;
- It is easier to insulate a building from outside cold conditions, while it is more difficult to limit the incoming solar radiation particularly for a building largely using glass;
- The increasing use of electric appliances in homes and offices and other plug loads increase internal gains;
- Global warming must also be considered; and
- IEA gives an energy use for space cooling of 3.5 EJ /y (exajoules, 1018J) and forecasts a more than twice as high demand of 9 EJy-1 in 2050.
As the demand for cooling depends on solar radiation intensity, it is not surprising that many studies have been devoted to solar cooling since the first energy crisis of 1973. Some pilot plants were soon built and experimented with and a variety of solar cooling technologies was developed.
A solar cooling system features a part devoted to collecting solar radiation (the solar section) and turning it into heat or electricity, and equipment that uses heat or electricity to produce cooling. Therefore, solar cooling systems development is strictly linked to the efficiency improvement and cost reduction of the solar section. The rapid improvement of thermal solar collectors initially favoured heat driven cooling equipment, whereas the enhanced efficiency and impressive cost reduction of photovoltaic cells now tend to favour electrically driven cooling equipment.
For a proper understanding of the current situation and prospects, a review of the main alternative routes from solar energy to cooling is presented.
Passive solar cooling technologies are not included in this presentation. These technologies comprise cooling effects by evaporation, natural ventilation and other heat dissipation techniques together with solar and heat control and heat amortisation.
Passive solar cooling should always be considered in the design or in the refurbishment of a building.