Ministry of Innovation —

Energy storage to smooth green power sources

With solar only working when the sun's out, and wind power limited by the …

As datacenter power needs grow, many datacenter builders are looking to locate new facilities in areas that contain sources of renewable energy. But with the exception of hydroelectric power, most sources of renewable energy—solar, wind, and tidal power—have a pretty significant limitation: the energy source is variable. The sun isn't always up, the wind doesn't always blow, and the tides come and go, all of which leaves those running the energy grid with the challenge of evening out the power supply. Fortunately, this isn't actually a new problem, and there are many options being pursued for storing power from renewable sources.

Power companies have always faced the problem that demand is uneven, peaking during daylight hours and tailing off at night. The simple solution—build enough generating capacity to meet peak demand—isn't appealing, as generating facilities require a large outlay of cash up-front, so it is hard to justify for a facility that's intended to be active only part of the time. Solar generation has the fortunate property of peaking during the times of highest demand, although the same is not true for tidal or wind power. Still, if the country is planning on removing some of the fossil-fuel burning plants from the grid to meet targeted emissions reductions, a way of using solar power overnight will be necessary.

All of this makes storing energy produced off peak for use during peak hours a more appealing option. At least one of the more obvious energy storage solutions has actually been in use for a long time. Many hydroelectric systems include the capacity to pump energy into reservoirs during off-peak hours, and then use that water for electricity generation once demand climbs. Not every area of the grid has access to water storage facilities, though, and fresh water is becoming an increasingly scarce resource for much of the country (events in Iowa notwithstanding). So let's take a quick tour of some of the other technologies that may come into play for energy storage.

Charge Storage: It's possible to store charge directly, but the two mechanisms for doing so, supercapacitors and high-temperature superconductors, remain firmly in the experimental stage. The first attempts at commercial supercapacitors are currently being targeted as replacements in places where batteries are used now, such as small devices or cars, while high-temperature superconductors are being tested purely for transmitting electricity. Until they prove themselves in either of these contexts, it's difficult to judge whether supercapacitors or superconductors might be capable of providing the features needed for a high-capacity storage facility.

Chemical Storage: We use chemical storage every time we unplug our laptop, but there are a number of problems with scaling it. Batteries are expensive, take up a lot of space, have finite life spans, and generally involve the use of toxic chemicals. One alternative to regular batteries is hydrogen, which can be produced from water by electrolysis and then fed into a fuel cell later to produce power. The Department of Energy was already looking into this in 2004 (PDF) as part of the much-hyped hydrogen economy. Unfortunately, the process relies on having a source of water that won't destroy the system, which typically means fresh water, which will likely mean problems.

Compressed Air: The idea behind compressed air as an energy storage mechanism is to find a geologic formation that's relatively airtight and then use excess generating capacity to pump air into the formation. At times of high demand, the air can be bled out and used to push standard turbines, producing energy in the process. A single plant is in operation in Alabama, but there appears to be little information available on it; a second is slated to open in Iowa in 2011. It's unclear how widespread the suitable geology is, but otherwise, the technology for these facilities is available now.

Mechanical Energy: Mechanical energy storage is already on the market in the form of flywheels. Beacon POWER offers both portable trailers with capacity in the Megawatt range, and will build 20 Megawatt facilities designed for smoothing demand on the electric grid. The primary downside of these flywheels is the precision manufacturing needed to fit the tolerances demanded of something that has to spin fast enough to store significant amounts of energy. It's possible that economies of scale can drive these costs down.

Thermal Storage: Converting electricity to heat isn't always done efficiently, but insulation for heat storage is well understood, already mass produced, and doesn't have any moving parts to wear out. Best yet, for at least some renewable energy sources, the conversion to heat won't be necessary. The National Solar Thermal Test Facility is using mirrors to heat molten salt, which then transfers the heat to a conventional steam generator. With proper insulation, the molten salt can easily retain the heat for generation well after the sun has set.

Although a breakthrough may skew the economic equation in favor of any one of these options, it's entirely possible that a mixture of these technologies will eventually enter the electrical grid. If I were tasked with locating a new datacenter, I wouldn't place a long-term bet on any of the energy options that require a source of fresh water, but the rest all seem to be promising.

Further reading:

  • A 2005 report (PDF) prepared by the state of California.

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