Economic, efficient and reliable thermal storage systems are a key need of solar thermal power plants in order to smooth out insolation changes, to permit operation during the night period and to provide an output management tool for shifting output production to periods with high revenues. Nevertheless, thermal storage development has become almost a stepchild within the international solar thermal programs. This is due to the fact, that thermal storage is less a technological problem - technologically sophisticated storage concepts have been tested successfully in the past - but a problem of system optimization and cost minimization. The very limited storage research conducted for solar thermal power 0 plants concentrated mostly on high temperature storage, starting at 400 C and 0 reaching beyond 1000 C, as required by central tower technology. On the lower end of the temperature scale, a large variety of storage projects has been conducted for house 0 heating and house cooling at temperatures ranging from ambient to 100 C and for 0 0 concentrating collectors at temperatures between 180 C and 300 C. In the meantime, commercial solar thermal plant technology has made considerable technological progress since the last R&D project on solar thermal storage had ended in 1985: Since then, more than 300 MWe of solar plants with parabolic trough collectors were installed in California and their operating temperature was increased from the 0 0 former 300 C threshold to almost 400 C.
