Combined Heat and Power: Achieving the Right Balance
- CHP systems help to improve energy efficiency and reduce environmental impact.
- CHP installations must balance steam and electric load requirements with reliability needs.
- CHP systems produce on-site power using the waste heat to produce thermal or mechanical energy.
Combined heat and power (CHP) or cogeneration is the simultaneous production of electrical and thermal energy. CHP is not new; it has been used for years in industrial facilities to increase control and reliability. The recent growth in CHP systems has been driven more by the need to cut operating costs through improved energy efficiency and to reduce environmental impact through lower greenhouse gas emissions. A successful CHP installation, however, requires properly balancing steam and electric loads with economic and reliability requirements.
What are CHP systems?
CHP systems produce on-site power, electricity and steam from a single fuel source, using the waste heat to produce thermal or mechanical energy. Thermal energy from CHP systems is typically used for space or processing heating, drying, air conditioning or process cooling. Mechanical energy is most often used to drive a generator for producing electricity. Components of a CHP system include: heat engine or prime mover, generator, heat exchanger and electrical interconnection.
The prime mover or heat engine defines the system and includes the following:
Reciprocating engines come in two basic types: spark ignition and compression ignition. Spark ignition engines typically use natural gas, although they can run on propane, gasoline or landfill gas. Compression ignition engines operate on diesel fuel, heavy oil or a fuel mixture of diesel and natural gas. Natural gas engines offer low initial cost, fast start-up, proven reliability when properly maintained, excellent load-following characteristics and significant heat recovery potential.
Microturbines are small combustion turbines fueled by natural gas; the high-speed rotation generated is converted into electricity. In CHP systems, a heat exchanger transfers thermal energy from the microturbine's exhaust to a hot water system. The exhaust heat can be used for a number of applications; including potable hot water, absorption chillers, desiccant dehumidification and process heating.
Gas turbines are used in a variety of CHP configurations. A heat exchanger recovers exhaust heat from the gas turbine, which is converted to useful thermal energy in the form of steam or hot water. Gas turbines produce high-quality exhaust heat; achieve overall system efficiencies up to 80 percent; and are environmentally friendly, with low emissions of nitrous oxides and carbon dioxide.
Fuel cells are powered by hydrogen, normally derived from natural gas. They use an electrochemical process to convert the chemical energy of hydrogen into water or electricity. Several types are commercially available. In CHP applications, heat is recovered in the form of hot water or low-pressure steam. Generally, the heat recovered is appropriate for low-temperature process heating, space heating and potable water heating.
The following table compares the size, cost and efficiency of CHP systems:
| Summary of CHP Technologies
|| Reciprocating Engines
| 100 kW to 6 MW
| 30 kW–350 kW
|| 500 kW–40 MW
|| 200 kW–250 kW
|Cost per kW, $
|Energy Efficiency, percent
| 70 to 80
| 50 to 70
| 70 to 80
| 60 to 80
CHP has the potential to reduce your operating costs and your impact on the environment. Benefits include:
- Increased energy efficiency—Conventional power plants are roughly 33 percent efficient, according to the U.S. Environmental Protection Agency. Because CHP systems recycle waste energy, efficiencies of up to 80 percent are achieved.
- Improved power reliability—CHP systems are located on site, providing reliable and high-quality power and heat to the facility. CHP systems also reduce congestion on the grid by removing or reducing load, thereby increasing the reliability of power supplied to all energy users.
- Reduced environmental impact—Cogeneration systems help reduce emissions of carbon dioxide and air pollutants such as nitrogen oxide, sulfur dioxide and volatile organic compounds. CHP systems also require less fossil fuels than traditional power generation systems.
- Improved indoor environmental quality—Traditional heating, ventilation and air conditioning systems often do not provide adequate humidity control. CHP systems, in combination with a desiccant dehumidifier, improve humidity control and reduce the potential for mold and bacteria growth.
Potential drawbacks of CHP systems include high installation costs and complex mechanical systems, but these benefits make them an effective option in many applications.
Implementing CHP systems
With increasing demand for electricity and aging steam systems, opportunities for CHP systems continue to grow. In evaluating the installation of a CHP plant, there are a number of factors to consider: steam load versus electricity load, reliability requirements, local utility rates and fuel availability.
A good understanding of the steam and electricity demands of the facility is required before installing any CHP system. The goal is to balance the demands and variability of the steam supply with those of electricity generation from an economic perspective. The ideal situation is to match electricity generation with the process steam demand.
However, most facilities have some variability in steam and electricity demand, which not only causes thermal inefficiencies, but can drive up capital costs as well. Reliability is also important; most facilities require a backup source of electricity, typically the local utility grid. These factors drive system design to meet average loads for lowest capital cost while utilizing secondary sources for remaining demands.
Overall, CHP plants are an efficient means to meeting electricity and steam requirements from an economical and environmentally friendly perspective. They are best suited for loads where there is a significant steam or heat requirement, and where the electric and steam loads are well balanced.
Check with your utility to see if they offer discounted rates for CHP. The U.S. Environmental Protection Agency also offers an online database where you can search for CHP polices and incentives by state or at the federal level.