18 October 2005

Wind Powered Fuel Cells.

One of the principal difficulties with wind power is that wind is characteristically ephemeral. "Most deployed turbines produce electricity about 25% of the time (load factor 25%), but some reach 35%." It comes and goes with far less regularity than require for a source of base power for the grid. This is fine when you have a large battery of conventional power plants whose output can be adjusted to balance the electrical supply from wind, but a purely wind powered grid wouldn't work.

This makes Xcel Energy's research project which will mate a hydrogen fuel cell to a wind turbine significant. The $1.75 million dollar research investment for a single system large enough to power 20-25 homes isn't huge. And, the basic ideas behind the technology are hardly revolutionary. Wind mills were used hundreds of years before fossil fuels kick started the industrial revolution. And, who hasn't performed electrolysis in a junior high school physics laboratory? As fuel cells and wind mills have both been around for a long time, the real aim of this project is simply to make the combined system more efficient -- in the real world, efficiency and price matter. But, the marriage could very well be a perfect match.

The attraction of wind power is that it is non-polluting, non-toxic and renewable. You build it and no fuel has to be combusted at all. The vast majority of the structure is simply a tall metal tower. The footprint is fairly small, so turbines can be placed in farmer's fields, over the horizon in coastal areas, or even conceivably in the middle of a suburb, without materially disrupting surface uses. The only meaningful environmental impact is that birds occasionally fly into the blades, which is bad for the bird, but they are working on ways to keep birds away and this impact, to the extent that it does exist, is petty compared to the massive impact of fossil fuel power plants, the nuclear waste disposal issues associated with nuclear power, and even the large footprint and comparatively toxic fabrication processes involved in photovoltaic panels (which are also far more expensive per kilowatt hour of power generated using current technologies). Wind is also actually cheaper than "conventionally generated" power in Colorado at the moment (although part of this is a product of tax incentives for renewable energy).

Hydrogen fuel cells that produce hydrogen through electrolysis, the kind the article appears to indicate will be used here, are also very environmentally friendly. The hydrogen is produced, basically, by dipping electrodes into water producing hydrogen gas and oxygen. When the hydrogen is combusted, to run a turbine to generate electricity, the only byproduct is water vapor -- something that any farmer hosting wind turbines would welcome.

One of the major downsides of a hydrogen fuel cell in powering vehicles, its low energy density compared to many other hydrocarbon fuels. As the Department of Energy explains:

A significant barrier to using these fuel cells in vehicles is hydrogen storage. Most fuel cell vehicles (FCVs) powered by pure hydrogen must store the hydrogen onboard as a compressed gas in pressurized tanks. Due to the low energy density of hydrogen, it is difficult to store enough hydrogen onboard to allow vehicles to travel the same distance as gasoline-powered vehicles before refueling, typically 300-400 miles. Higher-density liquid fuels such as methanol, ethanol, natural gas, liquefied petroleum gas, and gasoline can be used for fuel, but the vehicles must have an onboard fuel processor to reform the methanol to hydrogen. This increases costs and maintenance requirements. The reformer also releases carbon dioxide (a greenhouse gas), though less than that emitted from current gasoline-powered engines.

But, this obviously isn't a concern in a stationary energy storage device for a wind turbine, where keeping cost and maintenance down by not having a fuel processor is desirable, keeping carbon dioxide emissions to near zero is important, and neither weight or volume is a serious concern.

One of the other main energy storage options called "pumped hydro" that has been discussed for wind powered grids is to have a two reservoir system, one above a hydroelectric dam, and the other below it, with peak wind power used to pump water from the lower reservoir to the higher one. Water then runs through the dam during down times to provide energy. Usually, the idea with those systems is that a whole grid of wind turbines feed into a single reservoir system. But, this kind of system is not trivial to construct on the arid plains where wind resources are greatest, and if an entire grid powers the pump, a considerable amount of energy may be lost in the transmission lines. Storing power in a fuel cell right at the base of a wind turbine, in contrast, produces almost no loss of power in transmission to the energy storage unit.

Another energy storage option that is being considered in Iowa is compressing air into geological formations. A third is to use flywheels. Conventional batteries, supercapacitors and superconducting coils have also been considered.

We pay scientists and engineers the big bucks to figure out which of these energy storage systems is going to be the cheapest way to store wind power for off peak use. But, finding some way to do it at a reasonable price with few collateral consequences like pollution, is a vital piece of the larger problems of reducing pollution, finding sustainable sources of energy.

1 comment:

Andrew Oh-Willeke said...

Pictures of windfarms can be found here.