On a utility scale, which of these two technologies is cheaper: photovoltaics (PV) or concentrating solar thermal (CST)? -- Lisa B, Denver, Colorado
The short answer would be that on an apples-to-apples comparison in the southwestern United States, concentrating solar thermal parabolic troughs (CST) have a lower levelized cost of energy (LCOE) per kilowatt-hour (kWh) than photovoltaics (PV) today. Outside of the broad southwest (which includes 7-8 states), PV is really the only game in town (at least for now, a CST project in Florida not-withstanding).
Looking forward, PV probably has a faster cost reduction curve than CST and their LCOE will cross paths within the next five years, i.e. PV will become cheaper. Concentrating photovoltaics (CPV) is the wild card that could also reach parity with CST over the next ten years.
The next obvious question is that given cost parity between the three technologies, which one is preferred by utilities?
The cost of energy is not the only criteria for utilities seeking renewable energy. The ability to generate during periods of high demand, including into the evening, makes any renewable technology much more valuable. Additionally, minute-to-minute operating characteristics on the grid, such as the potential for rapid changes in output, are also important, especially at the utility scale.
CST projects can offer both better peak capacity characteristics, with 6-8 hour thermal storage, as well as a smoother short-term fluctuations. Intermittent clouds going over a PV system will cause output to spike widely. But with a CST plant, which has thermal inertia in the heat transfer fluids, an operator can have some warning of the clouds, slow the fluid flow rate, which increases the fluid temperatures, and ride through short-term cloud events. (For reference, a wind turbine probably falls somewhere between the two technologies-there is kinetic inertia in the wind and the turbine blades, which provides a somewhat better short-term profile than PV.)
Of course, utility scale storage technologies for PV are being developed, and in some cases demonstrated. Initially these will provide similar ride-through capabilities during short-term cloud events, but as the storage technologies scale-up, they could potentially offer hourly storage into the evening. But right now in the southwest, CST has the cost and storage advantage, but PV essentially has a geographic monopoly everywhere else.