The proposed plant uses the concept of Underground Thermal Energy Storage (UTES), which we will refer to in this paper as ‘cavern storage’. Pressurised water cavern storage appears to have been first proposed by R&D Associates in 1977, but the original reference is no longer available. The oldest extant major analysis is a 1983 report (Copeland and Ullman, 1983; Dubberly et al, 1983) from the Solar Energy Research Institute SERI (which later became NREL). The SERI report was a study of different storage options prepared for the U. S. Department of Energy (DOE) in the early 1980’s. Cavern storage involves storage of water under pressure in deep metal lined caverns where the pressure is contained by the rock and the overburden weight. There are no heat exchangers, and a low cost makeup water tank is provided on the surface. The array supplies steam to the cavern water, and steam is flashed directly from the cavern into the turbine, in a very similar manner as steam is evaporated from a nuclear boiler vessel into a nuclear turbine. Fourteen organizations were involved in deriving the comparative rankings, which indicated quite definitively that UTES for a large system was the cheapest storage method.

Because costs have changed greatly in some areas, Tanner (2003) has produced, at the suggestion of one of the authors, an engineering thesis report on cavern storage applied to the case of the CLFR. This study investigates, using estimates supplied by experienced engineering and excavation companies, the current costs of a steel lined caverns at depths of 200m and 400m using modern excavation techniques. This report indicates that cavern storage is now much cheaper than other currently proposed storage methods at installed costs under US$3 per kWht. This report is being rewritten for publication. With low cost storage, there is a tendency for total system delivered electricity costs to be reduced as the capacity factor increases.