Wednesday, 25 May 2011

The key to staying cool when the heat is on

16 May 2011, Page: 29

THE sun is obviously the world's greatest source of heat but, paradoxically, it is also being harnessed for cooling buildings, important on the hottest days when air conditioners can overwhelm the electricity supply, and institutions such as hospitals are taking advantage of the evolving technology.

Sustainability Victoria, which is working with the CSIRO and the Energy Resources Institute in India to develop small scale solar coolers, says solar cooling systems are either closed cycle systems or open cycle systems. Closed cycle systems produce chilled water that can supply any type of air conditioning equipment.

Open cycle systems, or desiccant systems, are a combination of sorptive dehumidification and evaporative cooling, providing cool, dry air to a building. Heat for thermally driven cooling is gathered by solar collectors typically used in home solar hot water systems. Using sorption, it is then converted into cold, which is delivered in the shape of chilled water or dry, cool air.

The Victorian body is also a partner in the installation of solar coolers at Echuca hospital, where it has part funded the replacement of two electric air conditioning units. The cooler has a solar field of 102 evacuated tube collectors over 422m² that feeds hot water to an absorption chiller, cutting the hospital's natural gas consumption.

"When the chiller is not in use, the hot water from the collector field is used for the domestic hot water demand or stored in hot water tanks for later use", Sustainability Victoria's Sonja Ott says. The solar cooling system was installed in March and is now under commissioning. "It is expected that the solar cooling system will save 1400 tonnes CO₂ equivalent and $60,000 in energy bills a year", Ott says.

CSIRO research team leader Stephen White says supply and demand problems with solar cooling are the subject of further research. "In a conventional air conditioning system, you use the electricity grid, which you hope will supply you with energy as and when it is needed to keep your building cold. "But once you have a solar air conditioning process, not only is the call for air conditioning variable over the day but also the supply of driving heat is variable".

The hard part is "trying to match that variable supply of power with a variable demand for cold". "In the last five to 10 years researchers around the world have been getting that integration working well to bring down the cost and improve the solar fraction", White says. Many conventional sorption units are powered by gas fired heat or by waste heat from an engine. "That's become a lot more popular in the past few years and now were looking at adding in a solar component that doesn't require the fossil fuels", he says.

White says the technology could be used in two ways: large systems with economies of scale for buildings such as hospitals or schools, or smaller plug and play systems for houses. "Absorption chillers are most cost effective at a large scale, in the MW range. To get economies of scale, go larger; but to go smaller, companies need to come up with a mass produced plug and play device". White says a solar cooling system is more efficient than a conventional electric system.

The focus for the next five to 10 years is for manufacturers to develop their products and get them on the market. "There's a bright future but a lot of work to do in terms of product development and also getting legislation so solar cooling is treated the same way as other renewables", White says. "It is not explicitly mentioned in renewable energy targets, [yet] solar cooling has the potential to address peak demand. The 200 member Solar Cooling Interest Group, of which White is chairman, is working with Standards Australia "to recognise the performance and benefits of solar cooling" as a precursor to lobbying government to have the technology recognised as a renewable energy device.