A system comprising a structure (1) defining a volume for containing or receiving a body of water. The system further comprises a solar energy system for heating a body of water. The system comprises a solar radiation receiving unit (2) configured to receive solar radiation and configured to convert said solar radiation into heat energy. The system also comprises a barrier means (3) of varying solar radiation transmittance arranged over said solar radiation receiving unit (2). The barrier means (3) is configured to varyingly control the solar radiation receivable by said solar radiation receiving unit (2).