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Should cogeneration be considered a green energy technology?

Posted by Alex Barrett on 22 June 2017 at 3:01 pm

We consume energy in two main ways, as electricity and as heat. These are produced, distributed and used in very different ways. Most electricity is generated by centralised power stations, with a relatively small fraction being produced at the point of use. It is then distributed, potentially over long distances, and used to power everything from lighting to computers.

Heat is generally generated at the point of use, whether in domestic boilers or industrial heating systems. Heat is generally radiated immediately, or transferred through hot water, which cannot be piped over long distances. Consequently heat has to be produced either in the building where it is to be used, or in a nearby facility, as in the case of district heating systems [1].

Generating electricity usually produces heat as well, power stations produce vast amounts of thermal energy, but this isn’t often put to use. Electricity and heat are usually generated separately even though this is much less efficient than producing them both at the same time. The Association for Decentralised Energy report that up to two thirds of the energy consumed by a power station is wasted in the form of lost heat. If captured this wasted heat can be used to supply the needs of numerous industrial processes, and can provide domestic heating for houses in the vicinity.

A Cogeneration, or Combined Heat and Power (CHP) system captures the waste heat and puts it to use [2, 3]. CHP is usually cited as a green energy technology but it isn’t necessarily generated from a renewable source of power. Rather it helps us to make the most of the fossil fuels or biomass we burn [4]. Making use of waste heat reduces the need to burn additional fossil fuels, so emissions are reduced overall. The emissions of the power station will still be high, but more useable energy is being produced for the same emissions.

Domestic CHP systems work along similar principles, but with the opposite purpose. Rather than capturing heat from an electric generator they replace a household’s boiler. Hot water is produces as usual, and a heat engine coverts the waste heat produced by the system into electricity.

CHP has a lot of advantages from an energy efficiency standpoint. Every time energy is transformed from one form to another there will be conversion losses, usually in the form of heat. Electricity is generally more valuable than heat, it is harder to make, and it can be transmitted over long distances, converted into work more easily and is essential to keep our technology running. Consequently it is inefficient to convert electricity back into heat rather than using the waste heat directly. Government guidelines suggest CHP systems can be up to 30% more efficient than a regular boiler or power station. Householders could save as much as 20% on energy bills and reduce their carbon emissions by up to 30% [3].

There are a variety of downsides to CHP, particularly in domestic heating. CHP boilers are often larger than regular ones, and of course are far more expensive. One major issue is that the system cannot generate electricity all of the time, only when the boiler is running [5, 6]. In the winter when lots of heating is required there will likely be an excess of electricity. During the warmer months the system may not generate enough.

This shortfall could be overcome by using energy storage systems, or by combining domestic CHP with renewable energy sources. CHP synergises well with solar panels, since domestic heating is generally used the most at times when solar panels are least effective.

There are concerns that large scale adoption of CHP could prevent the uptake of more sustainable options [5]. CHP will always be dependent on burning some sort of fuel so it is not sustainable in the long term. Similar systems could be built to take advantage of the heat generated by biomass boilers, or solar thermal systems but these are less common. CHP in its current form will never allow us to create a zero carbon world, but while fossil fuels remain the dominant source of heat and electricity it remains a very useful technique to get the largest energy return from the emissions we produce.
 

More information about Combined Heat and Power (CHP) on YouGen.

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Image credit: Frank Hebbert.

References

1.        Association for Decentralised Energy: District Heating Systems http://www.theade.co.uk/more-about-district-heating_3592.html

2.       Association for Decentralised Energy: Combined Heat and Power http://www.theade.co.uk/what-is-combined-heat-and-power_15.html

3.       UK Government: Combined Heat and Power technology https://www.gov.uk/government/publications/combined-heat-and-power-chp-technology

4.      The Energy Saving Trust: Domestic CHP http://www.energysavingtrust.org.uk/domestic/micro-chp

5.       Triple Pundit: Combined Heat and Power Pros and Cons. http://www.triplepundit.com/special/energy-options-pros-and-cons/combined-heat-power-pros-cons/#

6.      Greenspec: Micro CHP http://www.greenspec.co.uk/building-design/micro-chp/

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