Exergetic Evaluation of Solar Controller Using Software-in-the-Loop Method

Abstract

Solar thermal systems without appropriate control strategies are often working inefficiently despite high quality components and suitable hydraulic concepts. In this paper different control strategies for solar cooling systems are compared and evaluated according to their exergetic efficiency. Therefore, a Software-In-The-Loop test bench is used. The original controller of a solar thermal system is connected to a building simulation model. The simulation model is created with the modeling language Modelica using the simulation environment Dymola. This enables testing and monitoring of various control strategies. These controlling strategies are evaluated according to the energetic as well as exergetic output. Models for different parts of the hydraulic system are set up and validated. These different components are combined to a virtual solar test bench. The virtual test bench is connected to the system controller using Ethernet connection and an adapting program which has been created in C- Code (Software-In-The-Loop). Thus it is possible to use the advantages of a simulation environment, like performing repeatable tests with specified weather conditions, without implementing the control algorithms of the devices into Dymola. In a test scenario, two different control strategies for a solar cooling system have been implemented. First, a strategy to maximize the energetic output of the solar collectors is applied (Maximum Yield Strategy). Thereby the flow temperature of the solar system is kept as low as possible in order to increase the collectors’ efficiency. Second, the flow temperature is adapted to the required temperature of the absorption chiller, which leads to a lower energetic output of the collector and to a higher temperature level of storage (High Temperature Strategy). This paper shows that Software-In-The-Loop tests can be used for the optimization of control strategies. In the test scenario, the High Temperature Strategy leads to lower energy yields due to lower solar collector efficiency, whilst the exergy output and the output of cooling energy of the absorption chiller can be significantly increased.