A shallow hole is dug and lined with concrete. A number of large black plastic bags, each filled with several centimeters of water, are placed in the hole and then covered with fiberglass insulation panels. The panels let sunlight in but keep most of the heat stored in the water during the daytime from being lost to the atmosphere. This heat can then be used to heat water and space. It cannot be used to generate electricity. This technology is called:

Freshwater solar ponds or freshwater floating collector type solar pond.

Explanation:

A shallow hole is dug and lined with concrete. A number of large black plastic bags, each filled with several centimeters of water, are placed in the hole and then covered with fiberglass insulation panels. The panels let sunlight in but keep most of the heat stored in the water during the daytime from being lost to the atmosphere. This heat can then be used to heat water and space. It cannot be used to generate electricity. This technology is called freshwater solar ponds or freshwater floating collector type solar pond.

This type of solar pond technology is introduced to provide a cheap, renewable heat source which is used mainly for space - heating purposes. The freshwater solar pond is covered with floating collectors / fiberglass insulation panels which are made of thin plastic sheet, insulation board and it also requires a little constructional work. In summary, the collector also provide thermal and evaporation insulation so that fresh water may be used when the pond gives reduction of mean collector and pond temperature and ensures high efficiency through forced flow in collector layer of pond.