Determination of Correction Factors for Heat Losses at High Temperatures in Parabolic Trough Fields

In former and current parabolic trough solar thermal power plants, thermal oil is typically used as heat transfer medium. The thermal oil enters the irradiated absorber tube at temperatures around 300 °C and leaves the solar field with temperatures near 400 °C. The upper temperature is limited by the decomposition temperature of the oil. Due to the fact that the selective coating of the absorber tubes improved strongly, within this temperature range the total heat loss of the solar field can be calculated by simplified equations, e.g. using the arithmetic mean of the loop temperatures m=0.5 (in + out). This simplification leads to an underestimation of the heat losses in a magnitude below 3 %. For medium quality yield estimations this error is tolerated [1] in favor of calculation speed. New parabolic trough systems with different heat transfer media endeavor to higher outlet temperatures. Since radiative heat losses become more and more significant at higher temperatures, the heat loss curve converges to a characteristic corresponding to the temperature to the 4th power. In consequence, using the simplified equations in high temperature systems up to 580 °C leads to errors higher than 15 %. Additionally, medium quality yield estimations typically do not take into account the part load behavior of the solar field’s temperature distribution (see figure below). During times of low solar irradiation the main temperature rise will take place near the inlet of the solar field, thus, the mean temperature of the solar field will be raised. This leads to a further underestimation of the solar field’s heat losses such that the error cannot be tolerated anymore in future high temperature parabolic trough plants. The proposed paper will address this problem and will show how correction factors depending on heat losses, solar field temperatures and part load behavior can be defined in order to achieve both fast and accurate calculations for high-temperature parabolic trough systems.