Low temperature glide cycles for energy storage applications : thermodynamic and capital cost analysis

Among the technologies being researched for grid-scale electricity storage, pumped thermal energy storage consists of storing electricity as thermal energy, with the conversions being done by a reversible heat pump/heat engine system. Advantages include the use of abundant and cheap materials, conventional components from the power industry, as well as full flexibility in siting. Various thermodynamic cycles can be used for the energy conversion, including the Joule-Brayton, transcritical, and glide (also known as Kalina) cycles. The latter has not been extensively studied yet for PTES and is the focus of the present work. Akin to a Rankine cycle, it differs by employing a zeotropic mixture for the working fluid. As a result, evaporation and condensation occur non-isothermally over a temperature "glide", making this cycle a good match for sensible heat storage. The mixture used for the working fluid must be chosen carefully. Its effective heat capacity during phase change (a well defined property due to the temperature glide) can exhibit extremely high variability, which would incur substantial pinch-point problems and thus inefficiency in the heat transfer processes integral to PTES. In the present work, an alkane mixture is therefore optimised to achieve near-constant heat capacity. This alkane mixture is then used as the working fluid in a model of a low temperature glide cycle that uses water for both the hot and cold stores. Despite the low temperatures involved, the computed cycle round-trip efficiency reaches roughly 55% under realistic assumptions for isentropic efficiency and heat exchanger effectiveness. This is largely due to the efficient heat transfer enabled by optimising the working fluid. Energy density is relatively low at 2.2 kWh.m⁻³ compared to other PTES cycles, though still several times higher than for pumped hydro storage. Capital cost is estimated at 15-45 $/kWh_e for the specific energy cost and 1300-2900 $/kW for the specific power cost, making this competitive with batteries for long duration storage even under conservative assumptions. Capital cost is then investigated further in order to analyse the physical drivers of cost in PTES systems in general, both in terms of power and energy. The nature of the working fluid influences the specific power cost via the turbomachinery as well as the heat exchangers. Operating conditions like pressure and temperature also have a major effect; in particular, increasing temperature lowers the heat-to-work ratio of the cycle and therefore lowers heat exchanger cost, although beyond a certain point material limits force a switch to more expensive materials and cause a discrete jump in cost. In order to improve efficiency, energy density, and potentially reduce cost, a higher temperature glide cycle is then modelled, after the optimisation of a new working fluid mixture. To that end, many fluids are examined according to several criteria, before deciding on a final mixture that is also based on alkanes. Pressurised water is used for the hot store at up to 180 °C while unpressurised water is again used for the cold store. Performance is characterised by parametric studies which suggest round-trip efficiency can reach 60%, with an energy density of around 5 kWh.m⁻³. The specific power cost is (conservatively) estimated at 1600 $/kW, an improvement on the low-temperature cycle thanks to the lower heat-to-work ratio. However, the specific energy cost is higher at around 120 $/kWh_e, due to the expense of the pressure vessel for the hot store. Consequently, the PTES plant's storage duration determines which of the low and high temperature glide is most cost-effective.