Simulation and evaluation of stratified discharging and charging devices in combined solar thermal systems

Abstract This simulation study investigates different discharging and charging strategies and their effects on the performance of a combined solar thermal system in a single family house. Based on the system according to Task 32 of the Solar Heating and Cooling Program of the International Energy Agency (IEA SHC) the system layout and in specific the storage connections are modified to investigate different stratified charging and discharging concepts realized by external valves or devices within the storage tank. A broad literature overview shows that a comprehensive comparison of charging and discharging strategies under the same boundary conditions and after optimization of the storage connection heights is not available. The results of this study show that a good thermal stratification within the storage and thus higher energy savings can be reached by both a stratified charging and discharging. Depending on the system size and the design of charging and discharging connections the stratified discharging leads to the same or even higher energy savings than a stratified charging. Already one single four-way mixing valve in the space heating flow (i.e. two tapping points) leads on the one hand to more than 80% of the advantage of an idealized discharging with seven tapping points, and on the other hand in all cases to significantly higher energy savings if compared to two charging devices with three-way valves. The relative energy savings increase with increasing solar fraction, e.g. with larger dimensions and better insulated buildings. The best option with the highest benefit depends on the system design like storage in- and outlet positions, system size and load conditions. Therefore, the decision for the best suited strategy can only be determined by simulations representing the respective system. The results presented in this paper, however, allow deriving general dependencies.