This piece on Solar without the Panels points out an interesting trade-off in solar thermal power station design:
In fact, the capacity to store energy is critical to the economics of the solar thermal plant. Without storage, a solar thermal plant would need a turbine large enough to handle peak steam production, when the sun is brightest, but which would otherwise be underutilized. Stored heat means that a plant can use a smaller, cheaper steam turbine that can be kept running steadily for more hours of the day. While adding storage would substantially increase the cost of the energy produced by a photovoltaic array or wind farm, it actually reduces the cost per kilowatt of the energy produced by solar thermal plants.
The amount of storage included in a plant — expressed as the number of hours that it can keep the turbine running full tilt — will vary according to capital costs and the needs of a given utility. “There is an optimal point that could be three hours of storage or six hours of storage, where the cents per kilowatt-hour is the lowest,” says Fred Morse, senior advisor for U.S. operations with Abengoa Solar. Morse says that the company’s 280-megawatt plant in Arizona, set to begin operation by 2011, will have six hours of storage, while other recent projects promise seven to eight.