Solar ponds store solar thermal energy by

A solar pond stores heat by maintaining a salt concentration gradient that stops warm bottom water from rising.

Photovoltaic cells produce electricity from light and are not how a solar pond stores heat.

Solar thermal collectors are separate devices, not the storage mechanism of a pond.

Covering the pond with plastic sheets is not what traps the heat.

The dense, salty bottom layer holds the absorbed heat while the fresher top layer insulates it.

Such ponds can store heat for months and drive low-temperature power or heating.