Limits to solar thermal energy set by intermittency and low DNI: Implications from meteorological data

Hourly DNI data from the Australian Bureau of Meteorology over 8 years have enabled analysis of implications for solar thermal power generation systems. Six sites were selected, mostly in central Australia and the occurrence and duration of gaps in the availability of energy inputs to solar thermal generation were tallied. In a three month period late in 2010 12 periods of three or more days with an overall average DNI of 2.3 kWh/m2/day occurred. The relationship between DNI and solar thermal generation efficiency was examined and this indicated that on many more days power output would have been very low or zero. The relation between daily total DNI and hourly average DNI was also found to be important, as a high total might be made up of many hours in which DNI was too low for significant generation. These two factors show that there is a significant problem of intermittency for solar thermal systems. Although the annual output of each plant may be commercially viable a solar thermal system might not be capable of meeting demand reliably.