Combined heat and power (CHP) can be defined as the generation of electricity and the production of needed heat. This brief focuses on CHP for end users and not utility power generation.
Combined heat and power can take one of many forms:
- Back-pressure turbines – Large steam distribution systems operate at relatively high pressure/temperature to serve loads requiring the highest temperature. Many systems served require and operate at substantially lower steam pressure. Steam pressure from the distribution system is lowered for these systems by throttling. This wastes the potential to convert less expensive thermal energy into more valuable electrical energy. A back pressure turbine could be used rather than a throttling valve to generate electricity. There are many thousands of such opportunities in the U.S.
- Topping cycle – Consider a situation in which steam at 100 psig is needed for a manufacturing or large commercial complex such as a healthcare or college campus. Rather than simply generating 100 psig steam, higher pressure steam could be generated and from that electricity can be generated with the use of a steam turbine, the exhaust from which would be the needed 100 psig steam.
- Bottoming cycle – Some processes require high pressure, high temperature steam only. The returning steam or condensate has a great deal of energy that could be extracted and converted to electricity. Like the back pressure turbine, a bottoming cycle converts relatively cheap thermal energy into more valuable electrical energy. Note however, that this opportunity is not nearly as prevalent as the previous two.
- Waste heat recovery – Some manufacturing processes dump enormous amounts of heat to the atmosphere. Typical heat recovery would include steam generation or directing heat back into the process by preheating combustion air. Another alternative is using the organic Rankine cycle to generate electricity. Conventional steam power plants (coal, nuclear) use the Rankine cycle or “engine”. Organic simply means the cycle uses refrigerant rather than water/steam. Due to relatively low temperatures, the efficiency of these systems is poor, but the heat is free.
- Conventional CHP – This is possibly what most people think of for combined heat and power. It involves a gas turbine (jet engine) or an internal combustion engine using natural gas or number 1 or number 2 fuel oil (jet fuel or diesel fuel). Waste heat in the form of steam or hot water is used for processes.
The D in DSM, or demand side management (energy efficiency) programs implies the programs are to reduce demand or consumption of energy. Many programs do not include CHP because regulators do not authorize incentives for CHP as it is considered a supply side, not demand side technology. There is also a fuel switching argument that electric rate payers and utilities for that matter could be promoting the use of natural gas or other fuels.
However, the line between DSM and CHP is wide with many shades of gray. These issues will be expanded upon in a future brief. The case for CHP is strong.