Pumped heat electricity storage (PHES) has been recently suggested as a potential solution to the large-scale energy storage problem. PHES requires neither underground caverns as compressed air energy storage (CAES) nor kilometer-sized water reservoirs like pumped hydrostorage and can therefore be constructed anywhere in the world. However, since no large PHES system exists yet, and theoretical predictions are scarce, the efficiency of such systems is unknown.
André Thess of the Ilmenau University of Technology has formulated a simple thermodynamic model that predicts the efficiency of PHES as a function of the temperature of the thermal energy storage at maximum output power. This latest research could help boost both the energy and cost efficiencies of these storage systems. This theory predicts that for storage temperatures above 400C PHES has a higher efficiency than existing CAES and that PHES can even compete with the efficiencies predicted for advanced-adiabatic CAES.
Renewable energy sources such as wind and solar do not produce energy at a constant rate and as a result engineers are developing large-scale energy-storage methods that can hold excess energy for use when the wind is not blowing or when the sun is not shining. The integration of intermittent sources of electric power from renewable energies into the future energy infrastructure requires suitable storage capacities. There are currently two nonchemical techniques—pumped hydrostorage (PHS) and compressed air energy storage (CAES)—that could potentially solve this large-scale energy storage problem.
However, PHS requires water reservoirs with volumes on the order of 107 m3 and CAES requires underground caverns with 106 m3 size. Hence the location of both PHS and CAES cannot be chosen freely and is constrained by geographical and geological limitations.
PHES is much simpler - electricity from a source such as a solar or wind farm is used to run a heat pump. The pump heats water stored in a large tank (normally about 100,000 cubic metres in volume) and then, when needed, the heated water is sent to a heat engine and electricity is produced. A heat pump, rather than an electric heater, is used to heat the water because it makes the whole process much more efficient. Heat pumps are designed to move thermal energy in the direction opposite to that of spontaneous heat flow and so use much less energy than would be needed to generate the heat with an electrical heater.
In his model, Thess assumes that the heat engine is optimized for maximum power - meaning that it produces electricity as quickly as it can - but not at maximum efficiency. By doing so, the efficiency of an entire cycle of storing and retrieving energy can be described by the ratio of the storage temperature to the ambient temperature of the surroundings. So, for example, a PHES system that heats water at 20°C to 60°C would have an efficiency of about 38%. Thess says that the efficiency could be increased by increasing the storage temperature - which would involve using storage fluids other than water. However, he points out that water-based systems would be cheaper to build. With regard to established technologies, Thess's analysis suggests that for storage temperatures above 400°C, PHES would be more efficient than CAES.
Sources: lmenau University of Technology and Physics World
Top image of André Thess: lmenau University of Technology