It is standard practice to include an antifreeze agent in the collector for obvious reasons. The antifreeze agents are toxic, expensive, have a poor ability to carry heat and can change viscosity which can cause leaks. To avoid the need for antifreeze, some systems automatically drain off when there is not enough solar gain. Another approach would be to design an absorber which can accommodate the expansion accompanying freezing. It could be that the time taken to melt the ice as the sun rises would be compensated for by the higher efficiency of a system without antifreeze.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.