New Control Strategy for Solar Thermal Systems with Several Heat Sinks
Abstract
Most solar thermal systems have only one heat sink, usually a storage tank. In most cases such systems are equipped with a controller, which compares the current collector temperature with a sensor representing the heat sink conditions, e.g. a temperature sensor in the lower part of the storage tank. However, solar thermal systems may have more than one heat sink to use the solar heat on different temperature levels, thus in- creasing the solar benefit. The control of such a system needs to permanently determine an independent de- mand signal for the different heat sinks. The paper introduces control variants for a solar thermal system operating on different heat sinks. A more theoretical control approach uses the ambient and operating conditions to determine the potential collector temperatures of each heat sink. In a second approach, the collector pump of one defined heat sink is switched on periodically to get the current collector status. The controller uses this information to determine the po- tential temperatures for all other heat sinks. A detailed simulation study shows that the second, less complex and lower-priced variant leads to a slightly lower system performance than the theoretic approach which can be defined as the benchmark. Apart from the collector temperature the control has to decide which heat sink should be used in case of more than one positive demand signal. Simulations show that the best results occur if this decision is based on the potential thermal power, taking into account an optimized predefined weighting factor.