Exploiting the sun’s heat to power cooling systems offers clear benefits. It is a clean, sustainable technology with a small carbon footprint thus mitigating climate change; it reduces peak electrical demand on the grid andhas the additional benefit of working better on hot days when more cooling is needed. Of course, it’s not a panacea. For instance, the sun doesn’t always shine when cooling is required. While we can circumvent this by installing solar energy storage systems, it adds another layer of complexity to the system.
The additional demand for air conditioning systems required by our rapidly warming planet is likely to raise city temperatures by several degrees. Currently,there are around 1.6 billion air conditioners worldwide; according to the International Energy Agency,this could skyrocket to 5.6 billion by 2050, consuming as much electricity as the whole of China does today; a spiral that will further drive global warming.
In many hot countries people don’t have access to a reliable electricity supply; 1.3 billion people globally with 600 million in sub-Saharan Africa and 300 million in India. For these, conventional air conditioning is impossible.
The case for solar cooling has never been more apparent. Here we will look at how solar cooling works, some of the options, its history, and its possible future.
What is solar cooling?
Solar-drivencooling works on similar principles to conventional refrigeration: cooling is produced by the evaporation of a liquid refrigerant, a process that absorbs heat and cools its surroundings. The refrigerant vapouris converted back to a liquid, a process that requires energy. While a conventional cooler uses electric power from the grid, a solar cooleruses solar energy.
Types of solar coolers
Solar coolers fall into one of two categories: those powered by solar generated electrical energy collected by photovoltaic cells; and solar thermal energy coolers powered by heat collectedfrom the sun’s rays by a solar thermal collector, flat plate collectors being the most common type.
Solar electrical energy coolers
Solar electrical energy coolers work in a similar manner to conventional coolers. Heat is removed as the refrigerant evaporates, and the refrigerant gas is mechanically compressed, increasing its temperature and pressure. It then passes through a heat exchangeror condenser, andheat is expelled into the ambient environment. The high-pressurerefrigerant next passes through an expansion valve, reducing the pressure and initialisingevaporation.
Solar thermal energy coolers
Our primaryfocus is on coolersdriven by solar thermal energy. Essentially, such coolers replace the compression stage of the cooling cycle described above with a chemical process.
There are two major types: adsorption coolers and absorption coolers. In both of these, the refrigerant is usually water.Cooling is provided by evaporation of the refrigerant, and the refrigerant vapouris converted back liquid by one of two processes:
- Absorption cooler – here the refrigerant vapouris converted to liquidrefrigerant by absorbing it into a highly active absorbent. Many different refrigerant/absorbent combinations are possible, but typically the refrigerantis water and the absorbent an aqueous solution of lithium bromide or ammonia. The resulting solution passes to a generator where heat, provided by solar energy, causes the refrigerant to desorb from the solution as a vapour. These can operate with low-temperatureheat sources, typically around 70 o We describea typicalthermal absorption cooler in more detail below.
- Adsorption cooler – in an adsorption cooler the refrigerantvapouris adsorbedinto the surface of a powerful adsorbent such as silica, zeolite or carbon-ammonia. The refrigerant is released from the adsorbent using solar thermal energy. These have advantages in specific applications; in particular,they can accommodate high-temperatureheat sources.
A lithium bromide cooler
Let’s look in more detail how a solar lithium bromide absorption cooler works. As mentioned, lithium bromide (LiBr) is the absorbent and water (H2O) the refrigerant. The four essentialcomponents of the system are:
- Generator: receives heat input from the solar collector and heats the LiBr-H2O solution causing it to boil. Water vaporises: the vapourpasses tothe condenser, andthe remaining concentrated LiBr solution returns to the absorber via the heat exchanger.
- Absorber: contains LiBr-H2O, the absorbent-refrigerant solution. This is pumped to the generator via a heat exchanger which increases its temperature. The concentration of LiBr is in this solution is relatively low (typically 52% LiBr)
- Condenser: condenses the water vapor generating heat which is expelled externally, typicallyusing an evaporative cooling tower. The condensate routes to the evaporator via an expansion valve, thus reducing its pressure.
- Evaporator: evaporation of the low-pressurerefrigerant absorbs the heat providing the cooling effect. Water vapourflows back to the absorberand mixes with dilute LiBr solution (60% LiBr). Temperature is around -6 C.
Many alternative designs are possible, though they all use the same fundamental principles.
The past present and future of solar cooling – Nothing new under the sun!
Although concerns over global warming highlight the attractiveness of solar cooling, it is far from being a new technology. It dates back to the 1960sand the realisationthat flat plate and similar solar collectors may be used for cooling in the summer and heating in the winter, considerably increasing their justification. The real question though is that, given almost a half-centuryof development, why solar coolers remain a niche rather than mainstream market sector.
The reason appears to boil down to cost. Throughout the developed world, electricity is still relatively cheap. It is cheaper to power an air conditioning system with electricity than it is to invest in the solar thermal collector and other components needed for a solar cooler. Even when combined with solar heating, the payback period remains uneconomic compared with the cost of fossil fuel generated electricity.
But perhaps that is about to change.
The air conditioning market as a whole is growing rapidly, particularly in hotter countries. Globally, as mentioned above, the number of air conditioning units may reach 5.6 billion by 2050 compared with the current estimate of 1.6 billion. However, very few of these units are solar powered.
Even though the solar cooling market currently remains fairly niche, the demand is increasing, particularly in countries such as Italy, Spain, andthe Middle East. In the long term, electricity prices are likely to increase, tipping the balance further in the direction of solar cooling.
Globally there are several governmentand commercial initiatives that target solar air conditioning as a way of reducing carbon emissions. New solar cooling kits are coming to market with the potential of reducing capital costs significantly. These kits, usually produced for low capacity applications, include all the main system components pre-engineered, making installation and maintenance easy and low cost.
Solar cooling would appear to be waiting in the wings, but, driven by the need to reduce carbon emissions, could be set to take centrestage in the future, particularly if the world continues to get hotter.
This article is written by Simon Lamberton-Pine of DPAC UK Ltd
DPAC UK operate nationally through their network of agents and specifier’s,.
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