Large energy storage systems for utilities

In addition to the capacity on line, electric networks usually need 8-10 per cent of their installed capacity readily available to handle load variations. Thermal storage of energy as pressurised saturated hot water has done this job for years, but only on a small scale because of cost limitation of the steel pressure vessel. This paper shows that steel lined cavities deep underground, using the rock to provide containment, are economical and practical in large capacities for this energy storage. By reducing the cavity pressure, steam is flashed from the hot water and used to drive peaking turbines when needed; at low load periods surplus steam is condensed in the water to recharge the vault. The saturation pressure of the hot water is borne by the overburden pressure of the rock formation in which the storage vault is prepared. At usual steam plant saturation temperatures the effective storage density is in the range of 18-21 electric kWh/m3 of storage volume, which is 20-50 times the capacity of the usual pumped hydro systems. Recovery of stored energy ranges from 75-90 per cent. The cost of a facility to deliver 1000 MW of peaking power for 10 h would fall in the range of $250-350 million, including indirect costs, interest, etc. The underground facilities represent about 40 per cent of the cost: the balance is for conventional generating and steam plant equipment.