A low-temperature glide cycle for pumped thermal energy storage

Pumped thermal energy storage is seen as a possible alternative to pumped-hydro schemes for storing electricity at large scale and facilitating increased integration of renewable sources. This paper presents a novel form of pumped thermal energy storage in which the thermodynamic cycle exploits the temperature glide exhibited by zeotropic mixtures. The working fluid is a blend of linear alkanes, optimised so as to obtain a near-constant effective heat capacity in the two-phase region. This enables heat exchange with the storage fluid in a manner that incurs very low exergetic losses whilst also achieving a high cycle work ratio. These two features allow the cycle to attain a respectable round-trip efficiency whilst operating at low temperature (0-100 degrees C). The analysis presented constitutes a preliminary thermodynamic design; further improvements to performance may be possible with comprehensive optimisation. Nonetheless, the results show that an overall (electricity-to-electricity) round-trip efficiency of around 50% should be achievable with unpressurised water as the storage fluid. Initial cost estimates have also been undertaken, showing marginal energy (capital) costs in the range 15-45 $/kWh(e), depending on the type of containment. Due to the low power density and high heat-to-work ratio of low-temperature storage, the estimated marginal capital cost per unit power capacity is less favourable (1,300-2,900$/kW) implying the system is best-suited to long-duration discharge.