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Latent Heat Storage for Heat Pump and Solar Systems
Phase Change Materials (PCM) for Heating Systems
Modern heating systems for buildings need a supply temperature of approximately 35 °C. In order to use such supply temperatures more efficiently, the standard heat storage systems must be redesigned. The integration of a latent heat storage system in a modern heating system ought to enhance the overall system performance. Therefore a latent heat storage plate design for cooling applications has been adapted to the requirements of a modern heating system. The new storage system can be linked to a heat pump or a thermal solar system supplying a typical residential building, e.g. floor and ceiling heating system.
Figure 1: Comparison of stored energy for a small temperature range.
Latent Heat Storage Devices
Latent heat storage systems use a phase-changing process to store energy at small temperature gradients. Special paraffin and salt hydrates can offer a phase-changing temperature in the range of 5 – 50 °C. Within this temperature range, latent heat storage systems are superior to standard water systems augmenting the sensible heat by the latent heat. The aim of this work is to combine the new latent heat storage devices to a standard heat pump and a solar heating system. Figure 2 illustrates the simulation setup for the combination with a heat pump.
Figure 2: Simulation setup for the combination with a heat pump.
Data of an example study
For this study a building, which exists of two floors and each floor has four rooms with an area of 24 m², has been chosen. Each room has a floor heating system. The residential building is insulated and is connected to the weather model. The heat pump used is an air to water heat pump. The buffer storage has a volume of 780 l. The latent heat storage has a mass of 100 kg PCM (phase change temperature of about 35 °C and melting enthalpy of 150 kJ/(kgK)) and a volume of 150 l of water, which surrounds the PCM. The number of solar collectors is varied.
Figure 3: Simulation set-up for the combination with PCM and solar thermal collectors.
Results of the Combination with a Heat Pump, PCM and Collectors
Figure 4 shows the results of a study of four different systems – System1: 2x4 collectors and PCM; System2: 4x4 collectors and PCM; System3: 4x8 collectors and PCM and System4: only PCM. Simulations show that it is possible to reduce the primary energy demand in all systems in comparison to the reference case. The amount of solar thermal energy is 9.5 % for the first system, 12.6 % for the second system and 14.4 % for the last system. Figure 5 shows that with a higher number of thermal solar collectors the amount of the reduction of primary energy demand increases.
Figure 4: Results of the combination of heat pump, buffer storage / PCM and thermal solar collectors.
Experimental Set-Up of the Test Bench for PCM storages
A temperature-controlled water circuit test bench has been built in the experimental hall of the institute. The measured data are implemented in a numerical model of the storage system. The results are compared to the simulation results of the existing latent heat storage model, so that the model can be improved and validated. Combining this model with models for the heat pump, solar heating system and dynamic building simulation models provides a closed simulation model to study major influences on the overall system performance.
Figure 5: The temperature-controlled water circuit test bench with two latent heat storage devices.
Grateful acknowledgement is made to E.ON gGmbh for providing financial support.
Associate: Corinna Leonhardt