Solar Air Conditioning – Cool Technology!

A recent Sustainability Assessment job took me to the corner of Victoria to the town of Mildura.  A town of around 30,000 people, Mildura borders NSW on the Murray River.  In summer the maximum temperature averages over 30 degrees and naturally there is a high demand for air conditioning and water.  Conventional air conditioning systems contribute substantially to greenhouse gas emissions through the consumption of electricity (predominantly generated from burning coal) and water, for cooling towers and evaporative cooling systems.

The idea of solar air conditioning was mentioned, which sounds great, but does it exist and is it practical?

After doing some reading, yes, solar air conditioning does exist, but despite Australia’s sunny climate there does not seem to be much awareness, knowledge and skills in this area.  So how does it work and why isn’t it widespread in Australia?

From my understanding, there are essentially two categories of solar air conditioning.

1. Ventilation based systems that use photovoltaic power to power fans via heat exchangers or  through desiccant filters  that remove moisture from the air and improve thermal comfort and;
2. Solar thermal systems that harness the sun’s thermal energy (heat) to drive the cooling system.

Photovoltaic powered ventilation based systems are very much dependent on climatic conditions and building design.  These kinds of systems take advantage of fresh air intake to a building and remove unwanted levels of humidity, improving thermal comfort and reducing cooling (heating) requirements.  These kinds of systems have generally been adopted for smaller-scale applications, including the residential market.  This form of solar cooling does not actively cool the air supplied to a building space and is therefore limited to the right environmental conditions to work effectively.

Solar active thermal systems are expensive, more complex and generally too big for smaller applications such as residential housing.  The common system uses an absorption chiller, which compared to a conventional compressor based chiller, does not use electricity to power the system, but instead uses solar collectors to heat water that is then used to produce chilled water through interaction with a refrigerant or desiccant solution.  Such systems can also be used to generate hot water for heating requirements and general hot water needs.  Such systems make good sense where ample sunlight is available and for large-scale or remote applications.

Source: http://greensource.construction.com/products/images/0704_11.jpg

Australia has a high demand for air conditioning and therefore a high demand for electrical energy through use of conventional compressor driven coolers.  The high demand of air conditioning in summer puts a huge strain on the electricity network and contributes substantially to global warming through burning fossil fuels and use of refrigerants (which have an extremely high global warming potential) in air conditioning systems.

Why then is solar air conditioning not widespread, at least for large-scale applications?  The main barriers to solar cooling include:

• The high cost of solar collectors
• Lack of skills in solar cooling technology and increased complexity
• Abundant, cheap energy with no environmental cost
• Minimum incentive to reduce peak power consumption

Overcoming these barriers really requires government intervention through subsidies, research funding, introduction of interval metering to pass on peak costs to electricity users and introducing a carbon tax or trading scheme to factor in an environmental cost on energy.

The Australian National University (ANU) is currently developing a hybrid solar air conditioning that is aimed at replacing conventional residential air conditioners.  The design employs a solar thermal powered compressor that can provide cooling and heating just like a reverse cycle air conditioner. The break-through is the ability to generate cooling like an absorption chiller, but without the need for a bulky and complex system.

The system employs what is called an ‘ejector pump’, which is capable of generating low pressure (necessary for refrigeration) and is low cost to manufacture.  A refrigerant is pumped to high pressure and then heated by solar collectors (the same used for solar hot water systems).  The ejector creates a drop in pressure (the cooling effect) by converting the energy in the refrigerant into kinetic (movement) energy by firing fluid into the ejector at supersonic speed.

Source: http://solar-thermal.anu.edu.au/low_temp/solarac/image071.jpg

Amazingly, the only electricity needed to power the ejector unit is 150 watts or 1/10 the power minimum of a conventional air conditioner.  This is set to revolutionise air conditioning as we know it and the project is moving closer to commercial viability.  With the onset of a carbon trading scheme, the rollout of smart meters beginning, energy rates set to increase and summers increasing in temperature, the economic case for this upcoming technology holds great promise.   I certainly can’t wait for this ‘cool’ technology!