Heat exchangers are used in both cooling and heating processes. A Stirling engine can function in reverse as a heat pump for heating or cooling. The cabin has a recirculation fan which aids in cooling and heating. Heat generated from the cooling process is dissipated into the hoistway. All physical systems are capable of heating or cooling others. The effectiveness of evaporative cooling depends upon humidity. Air cooling and water cooling are usually combined. These systems often work in combination with engine cooling and exhaust systems. Nuclear power plants were sited for availability of cooling water. The oculus also serves as a cooling and ventilation method. They were large and required plenty of cooling air. Hafnium has excellent properties for reactors using water for both moderation and cooling. The plants are watered by water collected from the building's cooling system. Other applications include cooling and ice storage. Vacuum tubes with silica envelopes allowed for radiation cooling by incandescent anodes.