Solar cooling, or solar air conditioning, is a relatively new technology that has a strong potential to break into Australia’s residential market. Phil Kreveld explains how it works.

Currently, solar energy is used primarily in photovoltaic electricity generation and solar hot water applications. However, if we were to use solar energy for cooling a great power demand pressure would be taken off the national electricity grids and there would be signifi cant energy savings.

Households have a high use of air conditioners, estimated at about 70% penetration. It is estimated that approximately 120 million square metres of office space in Australia have climate control provided by mechanical services – an estimate for commercial building total energy use is about 50TWh annually. National electrical energy usage annually is 230TWh and it is estimated that 20% (46TWh) is consumed for air conditioning, commercially and domestically.

Projections by the Australian Bureau of Agricultural and Resource Economics (ABARE) have Australia’s overall energy demand growing by an aggregate of 40% by 2020. Given the forecast growth figures and the continuing cost increases in electricity, the scene would appear attractive for solar cooling.

The reality is that electrical (vapour compression) air conditioning is causing great summer and even winter peaks (in reversible systems), with summer peaks sometimes stressing grids to breaking point. However the cost of solar systems, particularly for domestic use, is prohibitive. In commercial applications, it can be less expensive on a wholeof- life basis when all maintenance and operating costs are taken into account – although still signifi cantly above vapour compression systems. With rising costs in electricity the situation may well change. The ANU’s Centre for Sustainable Energy Systems projects that at +50 cents per kWh, solar air conditioning will be more economical than traditional vapour compression. There is no argument when it comes to greenhouse gas emissions – solar contribution is minimal!

The basic parameters of cooling

Conventional air conditioning makes use of vapour compression for the operation cycle. Cooling happens through the evaporation caused by throttling the compressed refrigerant, having removed the heat of compression externally.

The cycle repeats itself by compressing the heat absorbing refrigerant vapour back to the liquid phase. Mechanical work is necessary – in other words there has to be a compressor motor. The cooling process is governed by the physics of the Carnot cycle.

The Carnot cycle is nature’s most efficient thermodynamic cycle whether it is for heating, cooling, or for extracting mechanical energy as in the case of thermal electricity generation. When cooling is the desired process, the Carnot cycle tells us that the best efficiency we can expect is given by a simple formula: Q/W = T2/(T1 – T2).

Q is the heat extracted (in units such as calories) by the work W (measured in units like joules, watt-seconds, etc) from the cold area at temperature T2 and dumped in the external area at temperature T1. The temperatures are the absolute thermodynamic ones, i.e. temperature in degrees Celsius plus 273 degrees Celsius. The symbol = indicates ‘equal or less’. The ratio Q/W is the Coefficient Of Performance (COP) of a cooling device (refrigerator, air cooling system).

Vapour compression cycle machines, whether driven from the grid, or powered from a photovoltaic panel-inverter typically have a COP of three to four, whereas for the heat-driven cycle the COP is less then unity.

By using a photovoltaic method, a conventional vapour compression cycle can be used, with the inverter powering the compressor. The purely solar system, on the other hand, does not use a compression stage. Instead, it supplies heat rather than mechanical energy as happens in conventional cooling, though some mechanical work is still necessary by way of pumps, for example. The cooling process uses the ability of certain materials to absorb or adsorb refrigerant vapour (thus causing a cooling action).

Using a source of heat, such as provided by a solar collector, the absorbent or adsorbent is made to release the refrigerant by heating so that the cycle can be repeated.

The difference between absorbents and adsorbents is that the former allows the refrigerant to go into solution whereas in the latter case, the refrigerant ‘sticks’ to the surface of the adsorbent agent.

There is nothing novel about the basics. Vapour absorption chillers work on the same basis – in this case an absorbent, often lithium bromide, and a refrigerant, usually water. The absorber acts basically like the suction side of a compressor drawing in water vapour and consequently cooling the surroundings. Vapour absorption chillers are a standard feature of tristate generators.

In that case waste heat from the electricity generation is used to provide heat to the absorbent so that it releases the water vapour in order that the cycle can continue. Some mechanical energy is needed to pump the absorber to a heat exchanger but the total mechanical energy use is 90% less than where a compression cycle is used.

There are a number of heat-driven cooling processes. They are listed below: • Heat for driving these processes is derived from solar energy collectors (e.g. parabolic reflectors, evacuated glass collectors, etc).

• Absorption cycle.
• Adsorption cycle.
• Desiccant cycle.
• Ejector cycle.

Absorption cycle

Refrigerant vapour from the evaporator is dissolved in an absorber, the solution thus becoming refrigerant-loaded. This is pumped to a generator where it is heated to give up the refrigerant. The only mechanical energy used is for the pump to bring the absorbent solution to the heat exchanger. The refrigerant is condensed and passes through an expansion valve to bring it to evaporation pressure.

Adsorption cycle

The refrigerant is adsorbed by a solid with a very high microscopic porosity. A single adsorbent is required for a basic cycle. The number of adsorbent stages can be increased to enhance the efficiency (multi-stage), and is easily adapted to a continuous process.

Desiccant cycle

Water is commonly used as a refrigerant. This cycle is basically a drying process, followed by heat exchange and humidifying at atmospheric pressure. Drying is achieved with a desiccant wheel using solid desiccant (silica, zeolite). Heat exchange to ‘revive’ the desiccant wheel is achieved by a heat exchanger wheel.

Ejector cycle

The major components comprise of an ejector, a condenser, generator, evaporator, an expansion device as well as a pump. The vapour from the low temperature evaporator is sucked into the high velocity vapour stream in the ejector resulting in the vapour being sucked from the low temperature evaporator and because of very low pressure in the narrowest section of the ejector, reaching close to sonic speed.

Looking ahead

The future for solar cooling and climate control would appear bright, given that sunlight costs nothing. However as is pointed out in the paper Solar Cooling in Australia: The Future of Air Conditioning? (P. Kohlenbach and M. Dennis, ANU Centre for Sustainable Energy), there are enormous price barriers.

Domestic air conditioners cost perhaps 10 times that of conventional units on an identical kilowatt equivalent basis, the authors claim. Commercial systems have much lower cost ratios but even on a whole-of-life basis (allowing for the replacement of compressors half way through a 20-year span) electricity process would have to double at least if solar were to be the preferred option. Currently all solar cooling technology is imported – an interesting reflection on a country with one of the highest insolation.

Architectural design focused on wholeof- life costs will result in lower energy use buildings and homes. However the roadblocks are universal ones: we tend to differentiate projects based on initial construction or acquisition costs and discount the effect of future expenditures whether on maintenance, repairs, energy usage, etc. Irrespective of climate change considerations, conservation of energy should be a very high priority.