Rivers, canals, lakes, and even the sea are sources of thermal energy. In the summer, water warms up, allowing you to harvest heat. In the winter, water cools down, enabling you to extract cold. This is called surface water energy. The heat and cold can then be stored in an ATES or BTES system for use in heating or cooling buildings in the next season. Surface water energy is also often used to thermally balance UTES installations. The challenge with this technique is to construct an effective intake and outlet that can self-clean.