A Combined Heat and Power system, or CHP, is an energy machine that makes electricity and useful heat from the same fuel source. In a normal power plant, much of the fuel energy becomes waste heat and is released to the air or water. CHP captures that heat and sends it to buildings, factories, greenhouses, or water-heating systems.
This matters because using the same fuel twice can greatly reduce fuel use and carbon emissions.
Key Facts
- CHP produces electricity and useful heat from one fuel input.
- Electrical efficiency = useful electrical energy output / fuel energy input.
- Total CHP efficiency = (electricity output + useful heat output) / fuel energy input.
- A typical separate power plant may waste 50% to 65% of fuel energy as heat.
- CHP systems often reach total efficiencies of 70% to 90% when heat is used nearby.
- Energy conservation for a CHP system: fuel energy in = electricity out + useful heat out + losses.
Vocabulary
- Combined Heat and Power
- A system that generates electricity and captures the leftover heat for useful heating.
- Waste Heat
- Thermal energy produced by a machine or process that would normally be released unused.
- Generator
- A device that converts mechanical energy into electrical energy using electromagnetic induction.
- Heat Exchanger
- A device that transfers heat from one fluid or gas to another without mixing them.
- Efficiency
- The fraction of input energy that becomes useful output energy.
Common Mistakes to Avoid
- Counting waste heat as useful heat when it is not captured, which is wrong because only heat delivered to a real heating need improves CHP efficiency.
- Assuming CHP is automatically renewable, which is wrong because CHP can use fossil fuels or renewable fuels such as biogas, biomass, or renewable hydrogen.
- Adding electrical efficiency and heat efficiency without using the same fuel input, which is wrong because total efficiency must compare all useful outputs to one shared input.
- Ignoring distance between the CHP unit and heat users, which is wrong because heat is difficult to transport far without losses.
Practice Questions
- 1 A CHP unit uses 1000 kWh of fuel energy and produces 320 kWh of electricity plus 480 kWh of useful heat. What is its total efficiency?
- 2 A separate power plant produces 350 kWh of electricity from 1000 kWh of fuel, and a boiler produces 500 kWh of heat using 600 kWh of fuel. How much fuel is used in total, and how does that compare with a CHP unit that provides the same outputs using 1000 kWh of fuel?
- 3 A school wants to install a CHP system, but its heating demand is very low during summer. Explain why this affects the system's usefulness and total efficiency.