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Combined Heat and Power

Category: Systems of interest September 30th, 2010 by mbc

Combined Heat and Power (CHP) systems generate electrical power and heat simultaneously.

There are three main technologies utilised to provide CHP systems – External combustion, Internal combustion and Fuel Cells.

External combustion
Central to the generation of electricity is the Stirling engine. A Stirling engine operates by being heated externally and so is described as an external combustion engine (ECE).

Fuel Cells
A fuel cell is an electrochemical cell that converts an input fuel into a number of outputs – an electrical current, heat and waste water or carbon dioxide. The primary function of a fuel cell is usually generating electricity but they run hot and so provide heat as a ‘bonus’. A wide range of fuels, both fossil based (gas, coal, oil) and renewable (biomass) can be utilised.

Internal combustion
Commonly used in automotive applications the fuel burning internal combustion engine (ICE) that we are all familiar with is applicable to CHP applications although the noise, emissions and need for servicing make the other options more generally suitable.

The Green Angle
Greenpeace claim that:

 ‘On average, our large, centralised power stations throw away two thirds of the energy they generate.’

(greenpeace.org.uk/~/combined-heat-and-power-chp).

In contrast the overall efficiency of CHP systems can be in excess of 80% at the point of generation.

With CHP power & heat generation is moved closer to the consumer. Being close to the consumer has two main benefits:

  1. With its dual function – heat and electrical power – the CHP facility avoids the inefficiency of it’s single purpose cousins such as the traditional coal electric power station. In such traditional power plants, generated heat is often treated as a waste product and merely vented or otherwise disposed of.
  2. Power does not need to be transferred over distance to remote consumers and so losses during transmission and distribution of electricity through the national and local distribution networks are avoided.

The Combined Heat and Power Association website has more detail on the advantages of CHP over traditional power plants.

Sizes
Large CHP installations are sized to provide for the needs of large consumers of heat and power such as industrial sites or large hospitals. Mini installations can be sufficient for the needs of a group of dwellings. Micro CHP is applicable to the individual householder.

Cooling
CHP techology can be further evolved to incorporate cooling. Through cooling generated heat using absorption chillers a CHP plant can add the ability to provide cooling and become a Combined Cooling, Heat and Power (CCHP) plant.

Micro CHP ~ Now?
The first widely available micro CHP unit in the UK is the Baxi Ecogen micro-CHP unit. This is basically a replacement for a traditional gas boiler that contains a sterling engine and so is able to generate electricity as well as heat water. It can generate 3.5kW to 6kW of heat that can be boosted to 10kW or 24kW if necessary – thus allowing a relatively standard heating range of between 3.5 and 24kW. The Ecogen is capable of producing up to 1 kWh of electricity. It can be grid connected, with excess electricity sold back to the grid operator. In the UK these micro CHP units can be included in the Government’s Feed-in Tariff Scheme providing owners with an additional payment on top of the standard price for any power they sell back to the grid. See my recent article on Feed-in Tariffs for more details.

Worcester (a part of the Bosch group) have the Greenstar CDi DualGen in development. It has a similar footprint and specification to the Baxi Ecogen unit. It utilises a stirling engine that can generate up to 7kW of heat (that can be boosted higher) and 1kWh of electricity. The Greenstar unit is due for full release in 2012 following a europe wide field trial.

Summary
So, is now (September 2010) the time to consider CHP as a viable technology?

I think the answer to the question depends on your specific project. Let’s start with this advice from the Worcester-Bosch website:

‘Micro CHP is most suited to older buildings that are poorly insulated for example, with sash windows and no cavity wall insulation.’

worcester-bosch.co.uk/homeowner/products/micro-chp~

That quotation causes a couple of issues. Firstly, for the sake of sustainability and common sense the fabric of the building itself should be addressed. Insulation installed and leaky windows renovated or replaced. Then secondly, once the building has been sorted out, what are the heating & energy requirements – is a CHP system appropriate and can it be made to pay? That’s a pretty straight-forward analysis based on:

    Costs:

  • The cost of installing the CHP system.
  • It’s cost of running in terms of how much fuel it will consume.
  • Cost of servicing.
    Benefits:

  • How much it will save you financially in comparison to other forms of heating.
  • How much will be generated electricity save from your electricity bill.
  • Feed-in tariffs confuse things somewhat, suffice to say that the electricity generated will be have a value to you beyond the current 13 pence or so that you’ll pay per kWh on a standard tariff.

Also, bear in mind that the systems I’ve been discussing here are gas powered (the Ecogen has an LPG version), so you’ll need a mains connection or the ability to juggle LPG bottles.

To me this seems an immature technology, one perhaps for early adopters who aren’t too sensitive to the raw economics of these systems in comparison to the heating only alternatives such as modern condensing gas boilers, heat pumps and wood-burners. These systems also seem more suitable for replacing old gas boilers than for installing in ‘green-field’ sites such conversions or new builds with no mains gas.

Further References

http://www.envocare.co.uk/combined_heat_and_power.htm

http://www.worcester-bosch.co.uk/homeowner/products/micro-chp?gclid=CKunl4Owk6MCFQT92AodS1z5qA

http://www.baxi.co.uk/ecogen

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