Control strategies and performance analyses of a central solar heating plant with seasonal storage
The concept of seasonal heat storage is based on storing sensible heat during the season of low heat demand to be used during the cold season, when the need for heating is higher. Combining this concept with solar thermal energy collection produces what is known as: Central Solar Heating Plant with Seasonal Storage (CSHPSS).
Thermal energy is stored by injecting heat into the ground by circulating hot water through a matrix of U-tubes inserted 10 to 30 meters deep into the soil. While the CSHPSS technology is becoming increasingly attractive the heat transfer process of seasonal storage remains complex and only very few modeling tools are available.
In an effort to properly design a CSHPSS system for the University of Massachusetts at Amherst, several comprehensive design and performance analyses were conducted. Thus, a complete and detailed CSHPSS system model using TRNSYS is presented in this dissertation. The U-tube seasonal storage is modeled based on a validated ground heat storage module, DST. The storage performance given by this model was compared with a finite element model and experimental results. The simulations yielded positive comparisons and a procedure for modeling U-tube storage system was established.
The CSHPSS model developed in this work features a unique control strategy that operates on a seasonal basis. This control strategy is based on four modes of operation and is found to contribute to the simulation stability. This strategy can also provide greater solar contribution to the load as opposed to the conventional ON/OFF controllers.
The heat collected by solar collectors is transferred to a storage or a building side through a shell and tube heat exchanger. A heat exchanger model that takes into account variations in the heat transfer coefficient and effectiveness caused by variable flow rates is developed to provide a realistic heat exchanger performance.
A thermal analysis of the seasonal storage and the heat exchanger was performed from the first and second law analysis viewpoints. Meaningful interpretations of the heat transfer process in the CSHPSS system can be made from these analyses. Finally, recommendations for improving CSHPSS simulations and performance are discussed.