Sensible and latent heat energy storage systems for concentrated solar power plants, exergy efficiency comparison

摘要

Different options of sensible and latent heat storage systems comprising different types of heat transfer fluids, heat storage media or phase change materials (PCMs) have been compared considering the upstream and downstream requirements of a concentrated solar power cycle. For an optimal system, analysis of both the energy efficiency and exergy recovery of the storage system and the whole cycle of heat to power is necessary. This study provides this analysis, comparing a two-tank sensible storage system to latent heat shell and tube storage system, using an analytical method, epsilon-NTU. For the case of molten salt as the PCM and sCO(2) as the heat transfer fluid, results show 4.02% higher exergy recovery for the two-tank system at a working temperature range 450-700 degrees C compared to 450-660 degrees C for the latent heat storage system where the upper limit is the PCM melting temperature. Compared to the Carnot cycle efficiency (the highest rate of useful energy (exergy) recovery for an isentropic process) of 70%, the overall efficiency of the two-tank system, the base case of molten salt/sCO(2), molten salt/Na, and molten metal/sCO(2) are 57.41%, 53.39%, 55.21%, and 53.55%, respectively. The gap in efficiency between the sensible and latent heat storage systems decreases by lowering the thermal resistance of heat transfer fluid side and/or the PCM side. Specifically, the gap in efficiency decreases 45% by using liquid Na instead of sCO(2) and a 4% decrease is observed when replacing molten salt with a molten metal like aluminium as the PCM. Using PCMs with higher melting temperature is preferred with liquids with high thermal conductivity like Sodium. For a heat transfer fluid such as sCO(2) with low thermal conductivity, a higher inlet fluid temperature and/or a low temperature PCM can provide high exergy efficiency. For a specific combination of PCM and heat transfer fluid, an optimal melting temperature exists depending on the inlet temperatures during charging/discharging and the thermophysical properties of the media. The study shows that PCM storage provides opportunities to minimise causes of irreversibilities to achieve simultaneous high volumetric energy density and high exergy recovery for CSP application in comparison with the conventional two-tank sensible storage.