SemiSerious

The reason that utilities prefer solar thermal today is simple. Power engineers have a deep appreciation for rotating mass. Think about it. Every major power plant in the world uses its preferred source of energy to spin a turbine that’s attached to an electric generator. Hydro has it’s channeled waterfall. More commonly, heat generated by uranium, coal or natural gas turns water into steam that drives turbine blades. Solar thermal just uses the sun as the heat source. Electricity comes from a rotating machine. In fact, the one great proven renewable energy source installed on any significant scale is an icon of the turbine, obvious to just about anyone. I’m talking about the wind mill.  In fact images of wind power farms have come to represent much more than just the simple turbine. They are the preferred clip art, photo, or what-have-you for the much broader field of renewable energy.

All those big rotating machines are pretty good at storing energy too. have you ever thought about how long it takes a 100MW generating unit to stop spinning after you shut it down?

But what was the point of this again? Right. Solar power is a great technology that’s actually starting to be installed in significant quantities. It just so happens that for power plant installations, the preferred form is concentrators that focus the sun to heat water or another liquid. Various mechanisms use that heat energy to eventually spin a turbine driving a very traditional electric generator. On the other side of the solar power industry, there are the photovoltaics, known to clean tech hipsters as “PV.” If you haven’t heard, these are the “solar cell” devices that have been around for 170 years or so. They use a semiconductor material to convert photons from the sun directly into electricity. Before those electrons get onto the power grid, though, they need to jump through some hoops. Due to that grand old man of electric power generation, the rotating generator, electric utility grids the world over depend on alternating current or AC. Unfortunately (or fortunately if you are a supplier of power electronics IC’s) the PV panel produces a direct current or DC supply of power. PV panels require  a device known as an inverter to create grid-friendly AC from the DC supplied by the panel.

Electrical power utilities even store energy in rotating mass. “Spinning reserves” are used on peaking units used to quickly supply power when required. Storing energy in flywheels also allows some electricity generated slightly before peak times to be shifted out toward periods of peak demand. The maximum PV production usually coincides very well with peak demand, so this “load-shifting” is better suited to traditional coal-fired base load power plants since friction in traditional flywheel systems kills the momentum relatively quickly. For PV, the trick is obviously to store energy until after the sun goes down which is much longer than the typical big wheel can keep on spinnin’. But advanced flywheel research might change that. It actually sounds more exotic than the photovoltaics themselves.

It was a bit surprising to hear that California (or maybe the US in general) is not doing that much on the solar thermal front. As Rick Merritt reported in EETimes, Sue Kateley, executive director of the California Solar Energy Industries Association, was disappointed in the gap that had developed between California and places like Germany and Korea.

Solar power is a great source of renewable energy but an even better source of catchy headlines. In Days getting sunnier for solar in Silicon Valley, Sheila Riley reported on some more optimistic views of the industry in California. Rick and Sheila’s articles are definitely worth a read.

But the most natural way to store energy in a solar thermal system is just to keep the heat on tap until you need it. This “load-shifting” was one of many things George Leopold discussed in another recent EETimes article. Thermal storage is just one more reason to take a serious look at solar thermal for large power plants.