Abstract
The paper presents the steady state performance of two types of commercially available evacuated-tube heat-pipe solar collector that were tested under the meteorological conditions of Singapore. The tests were made in arrays of collectors in contrast with the normal tests performed under controlled laboratory conditions. A theoretical model is also presented to predict the collector efficiency as well as the collectable energy during a steady state test. Within the range of expected coolant temperatures, the efficiency and the heat loss coefficient of collectors are measured and a linear expression can adequately depict their characteristics.