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September 17, 2007

More on energy storage – Flywheel Batteries

Category: Flywheels – Dan 8:03 pm

Energy storage is the least talked about, and most challenging problem facing green energy today. There’s plenty of potential power from either wind or solar, but these sources are intermittent. What’s needed is reliable storage.

There are four broad general ways to store energy[1]:

  • Chemical, with batteries being the most popular form. But in truth all fossil fuels are, in effect, chemical energy stores, and storing hydrogen is another effective chemical means of storing energy.
  • Mechanical, which might be water pumped uphill, compressed gas, or kinetic energy in a flywheel.
  • Electrical, either capacitors or inductors.
  • Thermal, which is not particularly efficient but if your source is thermal (as with CSP), this might be a cost effective approach.

There’s clearly a lot of research going on around batteries, especially since they are suitable for any mobile application. Electric or hybrid cars rely on batteries, and alternatives (such as flywheels or compressed hydrogen) so far do not look nearly as promising. There’s also considerable research going into storing hydrogen.

One option that gets far less press are flywheels, which store energy in a disk spinning at high speeds. These are commonly called “Flywheel Batteries”. Currently traditional lead-acid batteries today have an advantage in terms of cost, but flywheels have far longer service lives (>20 years vs. 3 or 4 for lead-acid batteries), a wider range of operating temperatures, and high power density.[1]

25 kWh Flywheel from Beacon Power

Imagine for a moment the potential of combining a solar PV system with a modest flywheel battery with a 25 KWHr capacity. This combination installed in a small energy-efficient home would have the potential of providing nearly all of the power needed over a 24 hour period. Properly sized, the PV panels could provide sufficient power during the day to both power lights and electric appliances, and spin up the flywheel in the basement. Peak AC requirements would closely coincide with peak PV generation. When the sun goes down, load could be pulled from the flywheel battery. A grid connection covers any under or over generation during the course of the day.

What’s the advantage of such a system? There are several:

  • Near continuous power – The flywheel battery effectively extends the power generating time of the PV system, providing a nearly continuous power source.
  • Long service life – Both the PV system and the flywheel battery are virtually maintenance free and have >20 year service life given today’s technology.
  • Modular design – Both the PV panels and the flywheel batteries are produced as manufactured components that can easily be shipped to a site and installed. Need more capacity — add more units.
  • Local source – There are no transmission losses, because most of the power generated on site is used on site.
  • Grid Stability – From the grid connection’s perspective, the system is relatively stable. The combination of flywheel battery and PV panels means that the house can handle much of it’s own power regulation. Could these systems work off of the grid? Certainly, but having them on the grid provides advantages with local power regulation.

Flywheel batteries are manufactured today, but are primarily sold as short-term high current uninteruptable power supply sources to even out fluctuations in the grid. Some manufacturers include Tribology Systems, Powercorp, and Beacon Power.

What’s not to like? Cost is the primary objection, but as with PV systems, the prices are falling as quantity ramps up. 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