Combined Heat and Power Systems

Using energy efficiently has become a goal across industries in the past decade. Rising energy prices, an increasingly competitive marketplace, and environmental regulation of harmful pollutant emissions have all incited commercial and industrial energy users to search out the most efficient and cleanest ways to use energy.

One innovation that is finding applications in commercial, industrial, and even residential settings is what is known as Combined Heat and Power (CHP) systems. Essentially, this type of system takes the waste heat from the burning of fossil fuels and applies it to power another process. For example, a basic CHP system might generate electricity through a large gas-fired turbine. The generation of this electricity would produce a great amount of waste heat. A CHP system might apply it to heating an industrial boiler instead of allowing this heat to escape. In this way, more of the energy contained in the natural gas is used than with a simple gas turbine. This increases energy efficiency, which implies that less energy is needed to begin with (costing the user less), and fewer emissions are generated because a smaller amount of natural gas is used. Typically, a CHP system produces a given amount of electricity and usable heat with 10 to 30 percent less fuel than would be needed if the two functions were separate. A typical electric generation facility may achieve up to 45 percent efficiency in the generation process, but with the addition of a waste heat recovery unit, can achieve energy efficiencies in excess of 80 percent.

Energy Efficiency in a Regular Electric Generation Facility


Energy Efficiency in a Combined Heat and Power Generation Facility 

CHP systems can be implemented to produce as much as 300 megawatts (MW) of electricity, to as little as 20 kilowatts (kW) of electricity, depending on the electrical and usable heat needs of the facility. It is not uncommon for larger cogeneration units to be installed in a facility that has very high space and water heating requirements, but lower electricity requirements. Under this scenario, the excess electricity is easily sold to the local electric utility.

Types of Combined Heat and Power Systems

A typical CHP system consists of an electric generator, which can take the form of a gas turbine, steam turbine, or combustion engine. In addition to this electric generator, a waste heat exchanger is installed, which recovers the excess heat or exhaust gas from the electric generator to in turn generate steam or hot water.

There are two basic types of CHP systems. The first is known as a ‘topping cycle’ system, where the system generates electricity first, and the waste heat or exhaust is used in an alternate process. Four types of topping cycle systems exist. The first, known as a combined-cycle topping system, burns fuel in a gas turbine or engine to generate electricity. The exhaust from this turbine or engine can either provide usable heat, or go to a heat recovery system to generate steam, which then may drive a secondary steam turbine.

The second type of topping cycle systems is known as a steam-turbine topping system. This system burns fuel to produce steam, which generates power through a steam turbine. The exhaust (left over steam) can be used as low-pressure process steam, to heat water for example.

The third type of topping cycle systems consists of an electric generator in which the engine jacket cooling water (the water that absorbs the excess emitted heat from an engine) is run through a heat recovery system to generate steam or hot water for space heating. The last type of topping cycle system is known as a gas turbine topping system. This system consists of a natural gas fired turbine, which drives a generator to produce electricity. The exhaust gas flows through a heat recovery boiler, which can convert the exhaust energy into steam, or usable heat.

While topping cycle systems are the most commonly used CHP systems, there is another type of CHP system known as ‘bottoming cycle’ systems. This type of system is the reverse of the above systems in that excess heat from a manufacturing process is used to generate steam, which then produces electricity. These types of systems are common in industries that use very high temperature furnaces, such as the glass or metals industries. Excess energy from the industrial application is generated first, and then used to power an electric generator second.

In addition to these two types of systems, fuel cells may also be used in a CHP system. Fuel cells can produce electricity using natural gas, without combustion or burning of the gas. However, fuel cells also produce heat along with electricity. Although fuel cell CHP systems are still in their infancy, it is expected that these applications will increase as the technology develops. To learn more about fuel cells, click here.

A Fuel Cell Cogeneration Plant at Miramar Naval Air Station

Combined Heat and Power Applications

CHP systems have applications both in large centralized power plants, and in distributed generation settings. Cogeneration systems have applications in centralized power plants, large industrial settings, large and medium sized commercial settings, and even smaller residential or commercial sites. The key determinant of whether or not combined heat and power technology would be of use is the nearby need or purpose for the captured waste heat. While electricity may be transferred reasonably efficiently across great distances, steam and hot water are not as transportable.

Heat that is generated from cogeneration plants has many uses, the most common of which include industrial processes and space and water heating. Those facilities that require both electricity and high temperature steam are best suited for CHP systems, as the system can operate at peak efficiency. There are many industries that require both electricity and steam, for example the pulp and paper industry is a major user of CHP systems. Electricity is required for lighting and operating machines, while the steam is useful in the manufacturing of paper.

Many commercial establishments also benefit from CHP systems. Universities, hospitals, condominiums, and office buildings all require electricity for lighting and electronic devices. These facilities also have high space and water heating requirements, making cogeneration a logical choice. For example, the University of Florida has an on-campus 42 MW gas turbine cogeneration facility that produces electricity and space and water heating for the campus. For more information on this cogeneration system, click here.

CHP systems are also available to serve smaller sized facilities. In this type of facility, these smaller, ‘modular’ cogeneration units can generate anywhere from 20 kW to 650 kW, and produce hot water from engine waste heat. It is most common to install a system based on the hot water needs of the establishment. For facilities like restaurants or medical facilities, which require hot water year-round, cogeneration makes an economic and environmentally friendly option. In terms of household sized CHP systems, it is possible to install a small system that can generate up to 10 kW, and fulfill all of the household heating requirements of an average home. However, these types of systems are not common. Fuel cell manufacturers are expected to target these small sized cogeneration units once the technology is perfected and it is economical for a household to install such a unit.

To learn more about CHP systems and explore other internet resources, visit the United States Combined Heat and Power association here.