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
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