Thermoelectric energy storage with geothermal heat integration - Exergy and exergo-economic analysis

The proposed Thermo-Electric Energy Storage (TEES) system addresses the need for peak-load support (1-2 daily hours of operation) for small-distributed users who are often owners of small/medium size PV systems (4 to 50 kWe) and wish to introduce a reliable storage system able to compensate the productivity/load mismatch. The proposed thermoelectric system relies on sensible heat storage: a warm resource at 120/160 degrees C (a hot water reservoir system), and a cold resource at - 10 / - 20 degrees C (a cold reservoir system containing water and ethylene glycol). The power cycle operates through a trans-critical CO2 scheme including recuperation; in the storage mode, a supercritical heat pump restores heat to the hot reservoir, while a cooling cycle (using a suitable refrigerant) cools the cold reservoir. The power cycle and the heat pump benefit from geothermal heat integration at low-medium temperatures (80-120 degrees C), thereby allowing to achieve a marginal round-trip efficiency (electric-to-electric) in the range from 50 to 75% (not considering geothermal heat integration). The three systems are analyzed with different resource conditions and parameters setting (hot storage temperature, pressure levels for all cycles, ambient temperature...); exergy and exergo-economic analyses are performed to evaluate the economic competitiveness and in order to identify the critical items in the system. A sensitivity analysis on the main parameters affecting the produced power cost of the system per unit electric energy is carried out.