The chilling power of sunlight
The sun is already being used to power air-conditioning systems so it seems a natural progression to apply it to refrigerate perishable foodstuffs - a huge consumer of fossil fuel-based energy. Scientists from the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany, are taking this task to heart with two solar refrigeration trials in the Mediterranean region – one at a Tunisian winery and the other at a dairy in Morocco.
The MEDISCO project (MEDiterranean food and agro Industry applications of Solar COoling technologies) utilizes solar plants for refrigerating milk and wine.
“Our method is ideal for countries which have many days of sunshine and in remote areas where there are no conventional means of refrigeration owing to a lack of water and non-existent or unreliable energy sources. It is environmentally-friendly and reduces the use of expensive electricity for conventional refrigerators to a minimum,” says Dr Tomas Núñez, scientist at the ISE.
Solar-powered refrigeration has two-fold benefits because the refrigeration is always available when the sun shines, which also equates to when refrigeration is needed most, says Dr Núñez.
The scientists have installed concentrating collectors which direct the sunlight onto an absorber via a reflector, making it possible to convert the solar radiation into hot water with a temperature of 200°C.
“This extreme water temperature is necessary in order to drive the absorption refrigeration machine for the high external temperatures that prevail there. We do not use electricity to provide the refrigeration, we use heat. The result is the same in both cases: refrigeration in the form of cold water or – in our case – a water-glycol mixture,” he says.
As the absorption refrigeration machine produces temperatures of 0°C, so the experts use the mixture to prevent the water from freezing (the same principle as in car radiators). The water-glycol solution is collected in cold accumulators and then pumped through a heat exchanger, which cools the milk.
“We use a slightly different system for wine, with the refrigerant flowing through coiled pipes in the wine tanks,” says Dr Núñez.
“MEDISCO is a demonstration project. The system is not yet ready for the market, but I am certain it will be possible in future to use solar refrigeration on farms and in the chemical and cosmetics industries,” he says.
The project funded by the European Commission is run by the Polytechnic University of Milan.