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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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.