Butane Heat Pipes for Stagnation Temperature Reduction of Solar Thermal Collectors

Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry out limit. Typically water, pentane or acetone are used as heat transfer media in collector heat pipes. Butane is very suitable to reach a high temperature gradient of the dry out even if the maximum temperature in the fluid circuit should be designed to 120 ̊C or below. The paper presents experimental results with butane heat pipes that operate up to a maximum temperature of 120 ̊C with a high temperature gradient in the dry-out region. This ensures that the collector performance in the operating range (typically up to 100 ̊C) is not affected negatively by the dry-out. Different approaches to increase the thermal conductance of butane heat pipes by enhancing the inner surface of the condenser or of both, the condenser and the evaporator are experimentally assessed and discussed. Measurement results report an increase of the heat pipes’ thermal conductance from 3 W/K (standard geometry) to 23 W/K.