5 Comparison between concentrating solar thermal (CST) and PV power generation - Intermittency

The key difference between CST and PV is that generation from CST plants can be considered to have better capability of riding through cloud transients, i.e. smoother output with less variability, due to some storage inherent in all CST systems, simply through the thermal mass of the working fluid and broader system, even if there is no major storage element included. This is an important difference between CST and solar PV - CST’s relative controllability moves the discussion away from a comparison based on variability. It may resolve itself into a discussion on how these two technologies complement each other and would best contribute to the move to meeting the majority of our energy needs through variable renewable generation.

The key differences between CST and other variable renewable energy generation, including PV, is summarised in a report by BrightSource [27], a developer and manufacturer of solar thermal power systems in the US:

  • Solar thermal projects are inherently capable of more accurate forecasts as they are more independent of the variability of weather conditions. This independence is due to the output being moderated by the thermal mass of Heat Transfer Fluid (HTF). This makes them less susceptible to short-term fluctuations in output due to passing clouds. See section 5.1 for more on variability of CST systems.
  • Solar thermal plants are able to utilise backup fossil fuel generation (hybridisation, see Figure 49) and thermal energy storage (TES) to increase their capacity factor. Forms of storage are obviously possible for PV plants but they are not as cost-effective or efficient as CST with TES. See Section 5.2 for some details on storage capabilities of CST systems.
  • Solar thermal plants are able to respond to system operator instructions in a similar manner to conventional units and are more able than PV to provide ancillary services.

Figure 49 SEGS IV hybrid parabolic trough (yellow) and gas generation (green) [28]

These statements are generally supported by existing literature. At large scales, CST also has a better MW/km2 than PV. Finally, CST is considered a more ‘mature’ technology than PV (at a very large scale) with a greater number of large-scale systems operational for longer periods. One of the earliest large-scale PV plants is the 4.6 MW system in Springerville, Arizona, USA, which has been operating since 2003. The SEGS I (13.8 MW) CST system has been operational since 1985 and includes 3 hours of TES.

Some advantages of PV systems compared with CST systems are:

  • PV systems do not necessarily require new land for their installation and can utilise building rooftops. This enables PV to be installed in already built up areas, with little impact on the surrounds and without the need to add new electrical infrastructure
  • Distributed PV can be located close to loads, resulting in reduced line losses. There may also be the capacity to defer required upgrades of electrical infrastructure
  • The process of installing a small-scale PV system does not require a large injection of capital or extensive studies to determine its feasibility, unlike as a CST system. This increases the likelihood of the installation going ahead, reducing the barriers to greater levels of renewable generation.