Sunday, September 14, 2008

Cogeneration, CHP, Heat Recovery

A typical coal plant has an efficiency in the low 30% range, meaning 65% or more of the energy is wasted. CHP can improve the plant efficiency to the 60%-80% range._Source
Excess heat production in US industry could potentially provide up to 20% of the nation's electrical power. For now, most of that energy goes into the atmosphere and is lost. With increased heat recovery methods, that heat will be converted to useful power. Some new plants, such as a Hormel meat processing plant in Texas, and other new plants are beginning to salvage waste heat for useful purposes--including electric power.
More than half of the energy potential in traditional power generation goes up the stack as waste heat. In contrast, the UTC Power fuel cell power system converts heat exhaust into heating and cooling, turning potential waste into useable energy. While central powerplants achieve conversion percentages in the lower 30s, the PureCell® system can attain energy conversion efficiencies up to 90 percent. High system efficiencies translate into greater fuel utilization, thereby conserving natural resources and energy. _Source
Here is more on co-generation, or Combined Heat and Power (CHP):
Any place energy is wasted, there's a chance to capture it and do useful work. The scale of waste heat in a steel mill, cement plant or silicon plant makes the potential obvious, but there are a few other types that take a sharper eye.

The heat from a power plant, instead of being lost in a cooling tower or surrendered to the atmosphere, can be used for local heating via underground hot water or steam pipes to nearby businesses, homes or industry. There's a limit on how far the heat can travel, hence the name, district heating. Once more common, today in the US this is mostly limited to college campuses and a few old downtown neighborhoods.

Anytime there is a pressure drop in a pipe, a backpressure turbine generator can capture the lost energy. For example, long distance natural gas pipelines operate at high pressure and when the pressure is reduced for local distribution, some of the significant energy originally used to pressurize the pipe can be recovered. This is sort of like regenerative braking for gas lines. An investment of $8 to $10 billion could capture 6.5 GW, another bargain at $1,250 to $1,500/kW. Steam pipelines are more numerous and have even more potential. The college campuses with district heating mentioned above could also be producing some fuel-free power where ever the steam pressure is reduced from transmission pressures to the pressure used in buildings.

Many industrial processes have leftover gas or create some low quality gas that can be burned. Quite often, this is simply flared ( that is, burned ) at the top of a smokestack. I watched flaring gas coming off steel mill blast furnaces for years as a kid in Gary, Indiana without knowing what it was. In any event, I was awed by 15 foot high tongues of flame dancing on top of a 300 foot high stack. Other sources are oil refineries, auto painting plants, carbon black plants and ethanol refiners.

One more advantage of CHP is that the electricity usually doesn't have to travel far and rarely requires new transmission lines. Unlike many large utility plants sited far away from population centers, most CHP installations are already where there are people and power demand. _Co-Generation
Technically, co-generation is the combined generation of useful heat and electricity. CHP is an equivalent term. Heat recovery, on the other hand, can refer to the retrofitting of heat recovery technology to previously wasted process heat, to yield electric power and/or other productive energy from waste heat. The distinction may seem too fine after the fact, but is quite meaningful at the design stage.

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Blogger miggs said...

I'm associated with Recycled Energy Development, the company mentioned in your source article. And yes, we could slash greenhouse emissions by 20% according to EPA and DoE studies if we recycled more energy in the U.S. Meanwhile, we'd save about $70 billion a year in power costs. This is the single biggest way to combat climate change in the short term, yet barely anyone is talking about it. So why isn't more being done? Simple: regulations favoring monopoly utilities.

9:25 AM  

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