Abstract: A heat storage pond, of the type having a lower heat storage layer of water covered by an insulating layer, has a partition that extends from a location on the periphery of the pond toward an opposite peripheral location, but terminates short thereof, for dividing at least the heat storage layer into two interconnecting portions. Water from the heat storage layer in one of the portions is drawn off and passed through a heat exchanger for extracting heat from the water. The water passed through the heat exchanger is then returned to the heat storage layer in the other of the two portions thereof, the partition effecting a physical separation between the intake and discharge. This arrangement sets up a circulation pattern in the lower region of the heat storage layer, while water in the other portion flows away from the point at which the cooler water is discharged into the heat storage layer, the only interconnection between the portions being in the region at the free end of the partition.