Carnot battery, Electrical storage, Reversible heat pump/organic rankine cycle, waste heat recovery, experimental investigation
The growth of renewable energy requires flexible, low-cost and efficient electrical storage systems to balance the mismatch between energy supply and demand. The Carnot battery (or Pumped Thermal Energy Storage) converts electric energy to thermal energy with a heat pump (HP) when electricity production is greater than demand; when electricity demand outstrips production, the Carnot battery generates power from two thermal storage reservoirs (Rankine mode). Classical Carnot batteries architectures do not achieve more than 60% roundtrip electric efficiency. However, innovative architectures, using waste heat recovery (thermally integrated Carnot batteries) are able to reach electrical power production of the power cycle larger than the electrical power consumption of the heat pump (power-to-power-ratio), increasing the value of the technology. It can be shown that the optimization of such a technology is a trade-off between the maximization of the power and the power-to-power ratio (depending on electricity prices among others). In this paper, the full development of a prototype of thermally integrated Carnot battery using a reversible heat pump/organic Rankine cycle (HP/ORC) is described. It includes the selection of the nominal design point, the architecture, the components and the sizing. This first experimental campaign showed a roundtrip electrical energy ratio of 72.5% with ORC efficiency of 5% (temperature lift is equal to 49 K) and COP of HP of 14.4 (temperature lift is equal to 8 K). These results are very encouraging because the performance can easily be improved (probably up to 100% roundtrip electrical energy ratio) by optimizing the volumetric machine, working at larger scale, optimizing the control and thermal insulation. Also, the performance of the main components (volumetric machine and heat exchangers) is analyzed.