Introduction to Micro-CHP Systems
United Trades Exclusive
By Dale Yelnich
The newest tech HVAC system, if it can be classified as that, is called a Micro-CHP combined heat and power system, which in the industry is known as cogeneration. It is a unique idea that has been used for years in commercial industry applications, but the newest models, featuring cutting edge technology, have downsized the large industrial units into the “micro” units that are adapted to use for residential applications. The hyphenated CHP designation simply means “C”ogeneration, “H”eat and “P”ower, or CHP for short.
A micro-CHP is simply a small heat engine that provides both electrical power and heat at the same time. It does this by use of a fuel powered electrical generator, and the resulting heat, gained from the internal combustion of the fuel, is bled off and used as a heat source. This type of heat engine essentially kills two birds with one stone. As the fuel burns to turn the generator, which supplies electricity, water captures the waste heat from the combustion process. The heated water is piped into a residential building, much as in any regular hydronic heating system. That’s where the term cogeneration comes from; both of these technologies are linked and used together cooperatively.
On the face of it, this is an incredibly efficient means of using electricity while capturing the high temperatures of a combustion engine. Regular industrial heat engines are never more than 60-percent efficient, and many types fall into the 30 and 40-percent efficiency range. A micro-CHP heat system, on the other hand, can achieve efficiency ratings of around 90-percent. That type of efficiency is some serious mojo, right there.
The concept is actually fairly simple, but making it into a residential heating system has been a problem. For example, although it is classified as a heating system, it is really an electrical generator that gives off heat as a byproduct. So every time you turn on your thermostat and call for heat, the internal combustion engine comes on to power the generator. Because the generator always needs to comes on when you call for heat, you are going to have more electricity being generated than what you can use.
If you think about it, you can understand why this technology is extremely useful in a business setting. Commercial buildings, like factories, manufacturing plants and even large office buildings, will always be able to use the energy generated from an electrical generator. They are so electrically intensive that there won’t be any excess electricity, no matter how often or long the generator is run to produce the heat needed to keep the building warm.
But it doesn’t work that way in a residential setting. If you need heat only a few times per day, as soon as the generator comes on, you’ll get a massive dose of electricity running through your lines for as long as you call for heat. Then just as suddenly, when the correct temperature is reached, the generator will shut down and so will its electrical production. In essence, you’ll never really be able to run a home on these spikes of electricity, so you must always be hooked into the grid as your primary source of power. And that’s where the overall high efficiency of the system comes to play.
By being hooked into the grid, you can resell your excess electricity through “net metering” to your local electrical company. Your electric bill will be greatly reduced, your home will be heated, and the only thing you’ll have to worry about is the fuel to power the system. And with that, it’s all beginning to make sense.
But what about the fuel to use? The reality is that the fuel is going to be your largest expense to power the cogeneration system, but once again, modern technology comes to the fore.
The most common way to power the generator is by burning natural gas. Natural gas is abundant, it is relatively inexpensive, it burns clean and many residential households already have gas piping available for use. For most residential applications, in the foreseeable future, natural gas will be the fuel of choice.
But modifications are already taking place within the micro-CHP industry that will allow an array of different fuels to be burned. Alternate fuels like vegetable oil, diesel and even hydrogen powered units are being tested for their viability and consumer appeal. Although none of these cogeneration systems are available to the public yet, the buzz of being able to use a virtually unlimited fuel type like hydrogen for electrical and heat generation, is well on its way to being a practical fuel source.
The final link in this equation is the internal combustion power plant that generates both the heat and the power. Both reciprocating and gas turbine internal combustion engines have been used successfully, with the gas turbine models being the most efficient. Unlike reciprocating engines, which give off heat through both the exhaust and the cooling system, most of the waste heat from a gas turbine engine is in the exhaust, and using exhaust gas is the most effective way to heat the water in a cogeneration system.
External combustion engines, like a Stirling engine or steam engines, are also being adapted for use in micro-CHP applications, and such technologies as a combined heat and power solar unit, called CHAPS, are photovoltaic panels that produce both heat and electricity at the same time.
It is safe to say that although micro-CHP technology is an extremely efficient way to generate heat and electricity, the initial start-up cost, with installation, will be prohibitive for most residential applications. Right now, this technology works best for large residences that are difficult to heat, or applications where almost constant heated water is needed, like say to heat up a swimming pool or outbuildings.
But as prices come down and the technology becomes perfected, the new wave of micro-CHP HVAC systems may soon be the “furnace” of choice for all of your heating, electrical and energy saving needs.