5.1 Variability

The North American Electric Reliability Corporation (NERC) completed a report [28] looking into the accommodation by the electrical grid of high levels of variable generation. Figure 50 shows the power output of a 64 MW parabolic trough solar thermal plant for a sunny day (top plot) and a partly cloudy day (bottom plot). Figure 51 shows similar plots, but for a 10 MW PV plant located in Nevada. A comparison of the sunny day output profiles shows the PV plant is more variable, but with only small movements. A significant difference is evident when looking at the partly cloudy day output profiles, where the variability of the PV plant is far greater than for the CST plant. Parabolic trough type CST plants utilise a ‘working fluid’ such as water or oil to transfer heat from the receiver to a heat exchanger. The working fluid has a thermal inertia and drops in irradiance do not result in an immediate drop in the temperature of this working fluid. This contributes to significant reduction in variability. Not all CST plants however utilise a working fluid. Parabolic dish type CST plants do not utilise water but their inherent thermal storage acts as a buffer to rapid ramp rates of their power output due to passing clouds. Similarly, ‘Solar Tower’ type CST plants, such as the one installed at CSIRO Newcastle, which have thermal storage (e.g. molten-salt as the working fluid), are expected to exhibit a 75% capacity factor and with minimal variability in minute to minute output [28].

Figure 50 Output for 64 MW parabolic trough 10-sec sample rate. Top: sunny day, bottom: partly cloudy [28]

Figure 51 Output for 10 MW PV plant 10-sec sample rate. Top: sunny day, bottom: partly cloudy [28]