Thermoeconomic Analysis of a Solar Enhanced Energy Storage Concept Based on Thermodynamic Cycles

Large scale energy storage may play an increasingly important role in the power generation and distribution sector, especially when large shares of renewable energies will have to be integrated into the electrical grid. Up to now pumped-hydro is the only technology that has been widely used for that purpose. However the spread of this technology is limited by geographic constraints. In the present work, a particular implementation of the storage concept based on thermodynamic cycles, as introduced by ABB Switzerland ltd. Corporate Research, has been analysed thermoeconomically. This concept can be scaled up to a GWh range of stored energy, and is site independent. The underlying idea is to use electricity to power a heat pump transferring heat from a cold source to a hot source (charging mode), while storing the heat and the cold at both sources. Subsequently the energy that has been stored/removed to/from the hot/cold sources can be used to run a heat engine, hence re-delivering the electricity when it is needed (discharging mode). In the implementation presented here, using the ambient temperature as the hot source for both the heat pump and the heat engine allows taking benefit from daily variations of the atmospheric temperature. Indeed, the heat-pump can operate with a higher Coefficient of Performance (CoP) during the night and the heat-engine at higher efficiency during the day. This results in an effective increase of the ratio between the redelivered electricity (discharge) and the stored electricity (charging), compared to a situation where the hot source of both cycles are at the same temperature. The use of solar collectors is a further way to enhance this ratio by increasing the hot source temperature above the ambient temperature during the day. In charging mode, the system consists in an ammonia based Rankine heat pump interfaced at the evaporator with a latent heat cold storage, and at the condenser to the ambient through aircoolers. In discharging mode an ammonia based Rankine heat-engine is interfaced at the condenser with the latent heat cold storage, and the evaporator(boiler) of the thermal engine is interfaced with hot water produced by a field of flat plate solar collectors. A steady state multiobjective optimization of a 50 MW power output plant was carried out, with the objectives of minimizing the investment costs, and maximizing the ratio between electricity output and input. Several types of cold storage substances have been implemented in the formulation and two different types of solar collector were investigated. A storage efficiency of 57% at a cost of 1200 USD/kW was calculated for an optimized plant using solar energy. Finally, a computation of the behaviour of the plant along the year was carried out, reaching a yearly availability of 84.4%.