Have you ever stopped to think about waste heat? Fortunately, engineers have. A process called cogeneration puts it to good use. It’s also called combined heat and power (CHP).
According to the laws of thermodynamics, heat results when any work is done by anything. That’s true whether the work is done by our bodies or our machines.
More often than not, the heat just dissipates into the environment. And about 70% of the energy humanity produces gets lost as waste heat.
We can find ways to generate less heat. LED bulbs, for example, shed much less heat than equivalent incandescent light bulbs. But the heat our gadgets produce is the greatest source of energy on the planet.
There are plenty of reasons for us to create heat, which requires fuel. Cogeneration uses waste heat instead to accomplish the same purpose.
It’s nothing new. Thomas Edison built the first commercial power plant in 1882. Steam ran the generator, and he sold that steam to heat nearby buildings. In other words, he generated and sold both electricity and heat.
Even before Edison built his plant, many businesses operated their own power plants. Many of them used cogeneration. In fact, cogeneration systems produced almost 60% of all onsite power generation in industrial plants. By 1974, it accounted for only 4%. The availability and lower cost of electricity from the grid made privately owned onsite power plants less attractive for factories.
That low point in off-grid CHP plants coincides with skyrocketing fuel prices and increased awareness of the harmful effects of air pollution. Companies started exploring cogeneration again.
How does cogeneration work?
A cogeneration plant needs a prime mover, that is, a power source to create mechanical energy. It can be a steam turbine (which is the most efficient), a combustion turbine, a microturbine, a reciprocating engine, or a fuel cell. A fuel cell can be the least efficient prime mover, but at best, it can approach the efficiency of a steam turbine and exceed that of other turbines.
An electrical generator converts the mechanical energy to electricity. A heat recovery system captures waste heat from the prime mover. A heat exchanger puts it to work.
Cogeneration can use a variety of fuels. Coal, diesel, and gasoline are the dirtiest, but most operate on natural gas. More and more operate on biofuels, including methane captured from landfills and burning organic wastes.
Some CHP systems start by generating electricity and capturing the waste heat (called topping cycle plants). Others start by generating heat and using it to make steam to run a turbine (bottom cycling plants). Bottoming cycle plants are less common, but they work well for industries such as steel mills that operate furnaces at very high temperatures.
Advantages and disadvantages of cogeneration
Most fossil fuel plants today are about 33% efficient. That is, producing 30 units of electricity requires burning 90 units of fuel and produces 60 units of waste heat. After all, once the steam has moved past the turbine, it’s still hot. As it cools, the heat dissipates into the air.
Waste heat recovery makes CHP plants typically 60-80% efficient. Some even approach 90% efficiency. Office buildings typically get electricity from the grid and have a boiler to provide heat. Cogeneration would enable a building to get the same heat without needing a boiler. Therefore, it gets the same amount of energy while burning much less fuel.
Power grids send electricity long distances for transmission and distribution. About 5% of electricity is lost in the process. Transmission and distribution losses increase in very high temperatures or when the grid is otherwise strained. Onsite cogeneration plants avoid these losses.
Cogeneration reduces fuel costs by as much as 20%. It is also among the most cost-effective ways to reduce emission of greenhouse gases. Less air pollution exposes machinery to less particulate matter, which in turn produces less wear and tear on it. So cogeneration indirectly lowers maintenance costs. It also avoids the capital costs of building maintaining a separate boiler.
And it’s off grid. A company that operates its own CHP plant can continue to operate if the grid goes down. Otherwise, it can always buy electricity from the grid at times it needs more than its plant can produce.
Some applications of cogeneration plants require the electricity more than the heat. Others require more heat than electricity. In either case, they can sell the excess to a utility.
Utilities, by the way, can operate CHP plants, provided that they are near a large enough population to provide customers for the heat.
Both power plants and boilers emit greenhouse gases. CHP eliminates the boiler’s share.
Cogeneration requires less fuel for each unit of energy output. Less fuel means less generation of greenhouse gases and other air pollutants such as sulfur dioxide and nitrogen oxides.
A 5 megawatt combined heat and power plant burning natural gas might emit 23 thousand tons of pollutants every year. Comparable conventional generation would produce total emissions of 45 thousand tons.
Waste heat is not a direct driver of global warming, and eliminating it would have little impact on climate change. But little impact isn’t the same as zero impact. Every little bit helps.
Why, then, are not all power plants CHP?
Cogeneration is expensive to build and maintain. It only works in places that need both electricity and hot water at consistently high levels.
What’s more, heat can’t be sent long distances.
CHP isn’t really an energy source. It makes burning fuels more efficient. Even though those fuels can include such renewables as biomass or renewable natural gas, it still requires incineration, and therefore emission of air pollutants. Some critics worry that it could hinder development of other more eco-friendly energy sources.
Cogeneration plants at factories can have output measured in megawatts. On the other hand, small-scale cogeneration, often called micro CHP, refers to plants with an output of no more than 50 kilowatts.
Micro CHP installations work well for residences and small commercial buildings. So far, the US hasn’t installed very many systems this size. Europe and Japan, on the other hand, have used them extensively.
Waste heat recovery on this scale offers all the same advantages as the largest installations. Greater use of micro CHP in the US can have a tremendous impact. Residences consume 21% of our electric output. Residential cogeneration, then, can lower national energy consumption and protect more people from power outages. They occupy about the same space as traditional boilers.
Here are two examples of successful small-scale cogeneration installations, both in Rexburg, Idaho:
The AmericInn Lodge & Suites installed a 38-kilowatt system in 2014 when its leaky boilers could no longer supply all the heat the hotel needed. The unit now supplies all of the hotel’s heat (including the pool) and about a third of its electricity. As a result, the hotel cut its annual energy costs by about $9,600 and will recoup the cost of the system in about eight years.
When the Towers II student housing complex was built near the Rexburg campus of Brigham Young University, the owners wanted to keep rents as low as possible. It’s 15 kilowatt micro CHP system provides electricity for common areas and heats water for showers. It also provides a snowmelt system for the complex’s walkways. It saves more than $15,000 in energy costs and has a five-year payback.
Combined heat and power (CHP) Partnership: CHP benefits / US Environmental Protection Agency
Micro-CHP: unleashing the benefits of cogeneration in the residential sector / David Landolfi, CHP Alliance. April 7, 2021
Waste heat: innovators turn to an overlooked renewable resource / Nicola Jones. Yale Environment 360. May 29, 2018
What is a cogeneration plant? An intro to CHP systems / Vista Projects. May 25, 2020