Saturday, August 24, 2013
Chemical batteries are the first energy storage technologies that leap to mind, but they are far from being the only show in town.
Heat storage is a big player too.
In our seventh installment on the issue of energy storage for intermittent renewable energy resources, we will look at the little-used but increasingly important issue of storing energy as heat.
One cool concept actively in research is heat batteries for cars. As you run the heater or air conditioned in the car, the car's fuel efficiency can suffer as much as 30% in some high fuel efficiency cars.
That that makes it hard to meet stringent fuel efficiency standards, which are soon going to be measured with the A/C running—so you’ll see automobile fuel efficiency numbers dropping.
Manufacturers are quietly looking at a separate system for cooling and heating in automobiles—a system not tied to the engine, and thus a system that won’t reduce a car’s fuel economy.
The idea is a heat battery. The stored heat can go to a condenser or an evaporator, depending on whether you want heating or cooling in the vehicle.
You could charge up your heat battery by plugging it in. Where today you might look for a shady parking spot to keep a car cool, this could change the approach entirely. You could power up your air conditioner energy system by parking your car in the hot sun and charging your heat battery.
Heat has other applications, including utility scale energy storage.
There are numerous variants of systems that use mirrors to transfer the sun's heat into a storage medium, and then from there into steam that turns a turbine and makes electricity. This normally goes by the generic name solar thermal as opposed to solar photovoltaic.
A lot of the current research is on storage media—when the mirror focuses the sun’s energy on a target, what’s that storage target made of? Some ideas include using liquids for lower temperatures, ceramics at super high temperatures, but also molten glass, molten aluminum, plain gravel, concrete, even metal and ceramic-encapsulated phase change materials. (More on those in a later installment.)
A German researcher suggests you could use nitrate salts, which don’t degrade, and when you dismantle the plant after 30 years, you can use the stuff for fertilizer.
The scientists at the Massive Energy Storage conference referenced in earlier stories in this series spent considerable time on the subject of heat storage. They conceded that the big price drop in photovoltaic panels, driven in part by lots of capital investment and tax credits, have left heat storage the high-priced alternative, but they are convinced that research will bring down prices and make them competitive again.
One of the selling points for solar thermal, compared to solar photovoltaic, is that the storage is built into the system. Concentrated solar heating projects are likely to benefit from economies of scale. Indeed, they are anticipated to be players in the energy world only in a pretty large format.
In the most common application, the heated storage medium is used to make steam, which can then turn a turbine to make electricity.
There is also work underway in converting high temperature solar heat to storable liquid fuel, which could then be used for either utility or transportation purposes. You can use the heat to make hydrogen, or if you have a source of carbon dioxide, you can make syngas, which then can be made into a number of fuels.
Liquid fuels are extremely versatile. They can be used in cars and trucks, in aircraft, in stationary power plants and in fuel cells.
“Numerous storage solutions are being pursued, but the chemical storage of solar energy as a (liquid) fuel is a superior concept due to the high energy density and the existing global infrastructure for fuel transport and storage,” said James Klausner, of ARPA-E and an engineering professor at the University of Florida.
Next: phase change materials.
© Jan TenBruggencate 2013