Energy Storage Strategies

Abstract

This report describes the development of energy storage strategies for short term Thermal Energy Storage in residential buildings with an air-source heat pump. Short term TES allows advanced integration of renewables because the associated mismatch between demand and availability is solved. The aim of the research was to develop control strategies which define a control sequence of heat pump operations with the purpose of minimization of primary energy input for space heating. This is achieved by more frequent utilization of free, low quality energy input. Exergy principles were used to assess the quality of energy. More free input will minimize the amount of work (high quality input) that is additionally required for the heat pump to generate the heating energy. In a conventional heat pump energy system, the installation is controlled without notion of exergetic optimal operation. This reference control strategy was compared to three optimization control strategies that were developed in this research, in combination with different storage capacities. The most advanced optimization strategy involves Greedy Optimal Control. This strategy defines optimal control of the installation based on estimates of future heat loads and future conditions for generation. First, a numerical MATLAB model was constructed in order to explore and compare the energetic potential of the strategies. This model showed that the optimization strategies result in significant primary energy saving when applied to large storage volumes that can only be realized within dwellings with latent TES. In latent TES, the high storage density during the phase change allows more compact storage. Secondly, the most potential storage configurations were translated into six use cases. The performance in terms of energy and exergy of these uses cases in combination with the most optimal control strategy was further simulated in a detailed TRNSYS model, and compared with the conventional control strategy. The aim of this model was to assess the influence of dynamic behavior of the heating emission system and temperature control on the performance of both strategies. It includes transient simulation of latent storage (macroencapsulated hydrated salt modules in a TES tank). This study has shown that the control strategy that optimizes operation and storage according to exergy principles, results in maximum 10% reduction of primary energy consumption for space heating compared to the reference situation.