Solar Air-Conditioning: The Next Big Step for Solar Energy

solar air conditioning

Air-Conditioning (AC) has become an essential part of modern society as it enables a productive and comfortable life style in hot and humid climates. The amount of installed AC systems is expected to dramatically increase in the coming decades, largely driven by economic growth in developing countries. Since many of these countries, such as China, India, Indonesia and Brazil are in hot climates their AC use will be larger than in most Western countries [1]. Out of the 30 metropolitan areas with the highest demand for AC, all but two are in developing countries[2].

Air-conditioning is a very energy intensive activity. In the USA 5% of the electricity produced is used for AC, and up to 40% of the electricity demand for households in warm climates is due to AC. The rise of air-conditioning will cause a significant increase of fossil fuels.

It is difficult for a power grid to handle large amounts of air-conditioning. These power systems are characterizes by large peaks during summer days. In Israel, AC consumption requires 40% of the nation’s electricity during peaks while hardly any AC is required in the winter. Similarly, during peaks in Qatar 65% of the nation`s electricity goes to AC.  It will be very difficult for poorly developed grids, common in many developing economies, to handle the large increase of peak electricity associated with a rise of AC.

A good solution for decreasing the demand of fossil fuels and removing electricity peaks is to use thermal solar AC. This technology makes excellent use of solar power by directly utilizing solar heat to produce cooling and drying, instead of attempting to convert it to electricity. AC demands and solar power supply coincide forming a perfect combination of demand and availability. As opposed to PV solar cells, an AC using thermal solar power could harness up to 60% of the sun’s radiated energy.

Solar thermal AC can be categorized into three technologies: absorption systems, desiccant systems and vapor compression systems.

Absorption Cooling.

Absorption cooling was invented already in 1858 and is today a common technology. The energy input is usually natural gas or exhaust heat from cogeneration plants. Single effect absorption systems require a heat source of about 75° C and will have a COP of 0.7. Double effect absorption requires a warmer heat source at about 130°C but will also have a higher COP of 1.4. Absorption systems include low pressure chambers and are complex and expensive to construct. This, in combination with the cost of solar collectors, makes solar absorption expensive. It is nevertheless the most popular form of solar thermal AC today. Many traditional companies within the absorption cooling segment can today supply systems that work well will solar heat. There are also new companies, such as Climate Well, that have developed novel types of absorption technology specifically for solar use.

Vapor Compression

Conventional air conditioning uses a vapor compression cycle for cooling. Several new types of vapor compression systems have recently been developed which utilizes solar heat. Some of these systems rely primarily on electricity and merely uses solar energy to boost the AC. Other systems rely on solar heat to power the compressor. As opposed to solar absorption cooling, most of the companies supplying solar vapor compression are new and have developed technologies specifically designed for solar power.

Desiccate Cooling

In air-conditioning both cooling and dehumidification are important. Desiccant systems are a technology that chemically dehumidifies the air. Both solid and liquid desiccant systems have been used for solar AC purposes. An advantage of these systems is that they can utilize low grade heat at 50°C – 75°C. Several hybrid desiccant systems have been developed that incooperates either vapor compression or absorption technology.

Small incentives for Solar Cooling

There is a great demand for solar cooling and also several good technological solutions. In spite of this, there are very few installed systems. One of the main reasons for this is the lack of political incentives for this technology. In both the US and in Europe there are many effective incentives for solar power (solar energy used for electricity generation). Solar thermal technologies, such as solar thermal cooling and solar heating, do not however enjoy the same policies. Reports from the US[3] and Europe[4] have shown that the incentives for solar thermal energy are both to low and accompanied with complicated bureaucracy. Countries that have effective policies for solar thermal energy, such as Greece, Cyprus and Israel, are today world leading in solar energy use. Solar energy supplies 3% of Israel’s primary energy. This could be compared to the world leader in solar power – Germany, in which solar energy only supplies 1% of the country’s primary energy.

With the rapid increase in air-conditioning, solar cooling can be expected to become more important. It is unfortunate that this technology has not enjoyed the same rapid development as solar power, as it has the potential to both significantly decreases fossil fuels and stabilize electricity grids.

[1]Lucas.W Dawis and Paul G Werler (2015) . Contribution of air conditioning adoption to future energy use under global warming. Proceeding of the National Academy of Science, Vol 112 no.19, 5962-5967

[2] Sivak, M. (2009). Potential energy demand for cooling in the 50 largest metropolitan areas of the world: Implications for developing countries. Energy Policy, 37(4), 1382–1384

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