The CLFR/cavern 2010 proposal of 54% CF at US$1784 per kWe, offers costs well below 2020 estimates for both troughs at 56% CF (2225 — 3220 $/kWe) contained in a NREL report (NREL, 2003) which use Hitec salt storage at up to 500°C. The CLFR/cavern proposals at 68% and 81% offer costs (2118 and 2486 $/kWe) much below 2018 ‘base case’ solar tower plants at 73% (3591 $/kWe) and comparable to the revised Sunlab reference case of $2340 for the year 2018. It should be mentioned that the CLFR/cavern 2010 proposal is far from optimised; Tanner (2003) suggests cavern storage at 350°C would be cheaper, and a US nuclear turbines or modern Kalina cycle turbine operating at close to 300°C would offer a 10% efficiency increase, but this would have to be compared against turbine cost.

Conclusions

The potential cost advantage gained by low temperature operation derives from an unusual combination of large low cost low temperature turbines developed for the nuclear industry, and an inexpensive storage concept which suits that particular temperature range. Should both options be applicable, then this is likely to be the most

cost-effective and simple solar thermal electricity development path, using simple solar collector technology already being installed, and a proven turbine from the nuclear industry.

Cavern storage cannot be taken higher than about 360°C and still has some developmental uncertainty ahead of it, but two reports have now identified it as potentially the lowest cost storage concept. Recent discussions that the authors have had with geologists and mining companies suggest the concept is in the realm of current mining technology and can be widely applied; suitable rock structures are common. If suitable geological structures are not available, Caloria oil storage with a CLFR array is a low risk option available for a cost which is still below the trough collector systems. Environmentally, however, cavern storage would be safer than either molten salt or oil solutions.

The electricity wholesale cost for the unoptimised CLFR/cavern in 2010 (the earliest that one can be finished is about 2009) at 68% capacity factor, without the use of any Green support mechanisms, is comparable to the cost of some current conventional pulverised coal-fired (PC) generation in the USA. The cost advantage of coal appears at high capacity factor, but even at a coal CF of 90%, the advantage is only about US$5 per MWhe.

The CLFR/cavern approach is unoptimised and may benefit from slightly higher operational temperatures should a suitable turbine be available. Such turbines may be available in the USA or Europe. The coal fired plant referenced also has a larger turbine than the solar 240 MWe. According to NREL, 2003, a 400 MWe power block should be 25% cheaper per kWh delivered than a 240 MWe equivalent, which reduces cost by about US$3 per MWhe. Furthermore, David and Herzog (2003) suggest that pulverised coal plants could incur an additional cost of US$30 per MWhe for long term cost carbon sequestration.

This brief discussion needs extensive elaboration and more detailed work within the scope of a real project structure. The authors have begun site investigations for a 240 MWe plant of the type described, assisted by Australia’s largest utility.