Combined heat and power (CHP) cogeneration
by Chris Woodford. Last updated: April 2, 2018.
The world's running out of oil, coal, and natural gas, but that's probably a good thing on balance because these dirty old "fossil fuels" are accelerating the problem of global warming and threatening major change to the climate. Unfortunately, we can't just stop using fossil fuels overnight: something like 80–90 percent of world energy still comes from them. Until renewable energy, such as solar and wind power, comes fully on stream, what can we do instead? One solution is to swap some of our power plants over to a different system called combined heat and power (CHP), also known as cogeneration. CHP plants make better use of the fuel we put into them, saving something like 15–40 percent of the energy in total. They're good for our pockets and good for the planet. So let's take a closer look at how they work!
Photo: A modern CHP power generator. Photo by Dennis Jones courtesy of US Department of Energy/NREL.
How does CHP work?
Photo: A typical fossil-fuel power plant in Didcot, England. This one burns natural gas; a previous plant on the same site burned coal and oil.
A conventional power plant makes electricity by a fairly inefficient process. A fossil fuel such as oil, coal, or natural gas is burned in a giant furnace to release heat energy. The heat is used to boil water and make steam, the steam drives a turbine, the turbine drives a generator, and the generator makes electricity. (You can find out more in our main article on power plants.)
The trouble with this is that energy is wasted in every step of the process—sometimes quite spectacularly. For example, the water that's boiled into steam to drive the steam turbines has to be cooled back down using giant cooling towers in the open air, wasting huge amounts of energy—much of which literally disappears into thin air! Now a fuel-driven power plant has to work by heating and cooling—that's what the laws of physics say—but surely we don't have to waste quite so much energy in the process?
Instead of letting heat escape uselessly up cooling towers, why not simply pipe it as hot water to homes and offices instead? That's essentially the idea behind CHP: to capture the heat that would normally be wasted in electricity generation and supply it to local buildings as well. Where a conventional power plant makes electricity and wastes the heat it makes as a byproduct, a CHP power plant makes both electricity and hot water and supplies both to consumers. Cogeneration (the alternative name for CHP) simply means that the electricity and heat are made at the same time.
Artwork: How much more efficient is combined heat and power? A conventional plant is at best about 60 percent efficient (for every 100 tons of coal or other fuel it burns, 40 tons is completely wasted); a CHP plant can be 90 percent efficient or even more. That gives a huge saving in fuel and operating costs and a major environmental benefit.
Types of CHP
Photo: A micro-CHP turbine produced by Capstone Turbine Corporation. It's powered by LPG (in this case, propane) supplied from the white tank at the back. Photo by Jim Yost courtesy of US Department of Energy/NREL.
The actual efficiency of a CHP plant depends on how well it supplies the heat it produces. Since the heat is generally carried as hot water, the efficiency is greatest when the power plant is closest to the buildings it's serving. In other words, CHP works best as a decentralized form of energy supply with more and smaller power plants built very close to local communities. Cutting the distance between power plants and consumers also makes the electricity supply more efficient: since the electrical power has to travel down shorter lengths of wire, less energy is lost due to resistance. Taking decentralization to its logical conclusion, it can even work out efficient for offices, schools, hotels, and apartment buildings to have their own mini or micro CHP power plant producing their electricity and hot water where it is consumed and sending any unwanted electricity to the power grid for other people to use.
In theory, you could make a CHP plant simply by sending the waste hot water from a conventional plant to local buildings. In practice, CHP plants make energy in completely different ways using entirely different heat engines (the machines that burn fuels to release heat). Smaller CHP plants often use what are essentially internal combustion engines (similar to gasoline engines in cars and diesel engines in trucks) to drive electricity generators, with heat exchangers recovering waste heat in hot water. Larger plants use very efficient gas and steam turbine engines. In future, CHP plants are likely to use fuel cells burning hydrogen gas.
None of this means CHP is a new or untested idea. The world's first proper power plant (built at Pearl Street in New York City by Thomas Edison in 1882) was essentially a CHP design: it supplied both heat and power to nearby buildings in Manhattan. CHP was a brilliant idea we somehow lost in the decades that followed, largely in the rush to create huge power plants that burned inexpensive coal. It's an idea we urgently need to rediscover in these environmentally challenging times.
Photo: A CHP power unit that runs on woodchips. The chips are loaded in at one end, converted to a gas inside the machine, and then the gas is burned to fuel the heat and power engine. The photo on the right shows the internals of the unit, including the electricity generating unit (colored orange) made by Generac. Both photos by Jim Yost courtesy of US Department of Energy/NREL.