Thermal blankets:

Thermal blankets consist of plastic sheets containing lots of air bubbles and are used to trap solar heat in outdoor swimming pools. The blankets allow solar heat rays to pass into the water during the day, yet provide insula­tion against heat loss at night by trapping the heat in the pool.

These pool covers are highly effective and are economical in many situ­ations. You can find more information by searching the internet, using the keywords “solar blanket” and “thermal blanket”.