The Weekly Geek: combined heat and power (CHP)

Posted by bex — 7 March 2008 at 1:22pm - Comments

The ROCA 3 CHP plant in Rotterdam provides electricty and heat to 400,000 homes

The ROCA 3 CHP plant in Rotterdam provides electricty and heat to 400,000 homes

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Back in 1882, Thomas Edison built the United States' first electric power plant. Pearl Street Station, which supplied the good folks of Lower Manhattan with electricity for lighting and steam for manufacturing, was around 50 per cent efficient.

125 years on, the typical UK power plant is just 38 per cent efficient. But those modern power plants that have been built on the same principles as Edison's are reaching efficiency levels of up to 95 per cent.

So how did Edison do it? And where are we going so wrong?

In this week's slightly tardy Weekly Geek, we're looking at combined heat and power (CHP): the system Edison was using, and the heart of any truly clean and efficient decentralised energy system. (Those who read the first Weekly Geek on decentralised energy may notice a fair bit of crossover.)


A conventional energy system

I'm starting to feel like this is becoming my catchphrase but it's worth saying again: the UK's biggest source of greenhouse gas emissions comes from heat. Not electricity. Not transport. Not agriculture. Heat, for our homes, offices, hot water and industrial processes.

For most of the UK, the heating system is completely separate from electricity system. Space and water heating mostly comes from burning natural gas in boilers. Heat intensive industrial processes (like smelting, say) require vast quantities of fossil fuels to be burned in blast furnaces, smelters and the like.

Electricity is largely generated by burning more fossil fuels, this time in power plants. But, in this electricity generation process, heat is produced as a by-product - and just thrown away up the cooling dowers, or dumped in local water courses.

It's pretty obvious that this is a grossly inefficient way of using fuel; burning one load of fossil fuels to produce heat, and another to produce electricity and just throwing away heat. On average, our conventional power stations throw away two thirds of the energy they generate. Together, UK power plants throw away enough heat to provide hot water and heating for every building in the UK.


A CHP-based energy system

With his Pearl Street Station, Edison was essentially ‘recycling' energy. Aware that one of the by-products of electricity generation is waste heat, Edison captured that heat and delivered it to where it was needed - in this case as steam, to be used in industrial processes.

Edison was able to do this because, in the kind of DC system he pioneered, electricity proved to be poor at travelling long distances. So Pearl Street Station was built in an urban area, meaning that both the electricity and the steam could be delivered to nearby buildings. Today, we would call it a CHP plant as it combined the generation of both heat and power.

CHP plants carried on being common at the start of the twentieth century. It was just common sense: economical, efficient, local energy. But as coal became the fuel of choice, coal dust and particulate emissions became a public annoyance, and power plants were increasingly relegated to rural areas. This meant that the capture and transmission of heat became impractical and uneconomical.

Bar a few minor resurgences (after World War II, for example, newly built residential estates in London were supplied with waste heat from Battersea Power Station), electricity generation has been relegated to large remote power stations, and heat generation has been hived off into a separate system.

In the 1970s, the ‘energy crises' sparked a new interest in CHP but, by then, the large utility companies had become expert at protecting their interests. Today, although CHP is well established in countries like Denmark and the Netherlands, the UK's legislative and regulatory framework still favours large, centralised power plants over smaller, cleaner more efficient ones.


How it works

CHP is the most efficient way possible to burn both fossil fuels (usually natural gas) and renewable fuels (including biomass and biogas).

Pretty much any organic matter can be used to produce biogas; we could be reaping energy from farm waste, and from all of the organic waste - like uneaten food - that makes up about half of our landfill.

With gas-fired CHP, biogas or natural gas is burned in a combustion chamber. This produces a flow of hot air, which drives a turbine. A generator converts this rotational energy into electricity.

After the hot air is used to power the turbine, it's captured in a heat recovery boiler where it heats water which is pumped out through insulated pipes, to provide space and water heating for local buildings.

Low grade heat is also captured from the system and used to drive a steam turbine. This turbine boosts the efficiency of the system by producing yet more electricity.

Sometimes cooling is also produced (‘trigeneration'). Here, some of the heat drives an absorption chiller, producing cold air for air conditioning:


CHP in the UK

Whereas countries like the Netherlands and Denmark get up to half of their energy from ultra efficient CHP, in the UK today it's still a tiny fraction. But there are some pioneering examples (visit EfficienCity for more):

  • Southampton district energy scheme is one of the largest commercially developed community heating and cooling networks in the UK. The scheme now serves thousands of customers - including residential properties, office buildings, hotels, a hospital and a university.
  • Woking Borough Council is at the forefront of the decentralised energy revolution in the UK. By decentralising its energy, Woking Council has slashed its energy use by nearly half, and its CO2 emissions by a massive 77 per cent since 1990.
  • The Natural History Museum has reduced its CO2 emissions by 18000 tonnes a year following the installation of a tri-generation system (electricity, heating and cooling).
  • Scottish & Newcastle is installing a combined heat and power (CHP) plant in their Manchester brewery, which will be fuelled entirely by biomass - partly from spent grain produced in the brewing process.
  • Birmingham City Council is embracing large scale CHP, and believes that as well as saving 2,800 tonnes of carbon in its first year, the new system will cut bills for the organisations involved.
  • The industrial CHP plant at Immingham supplies two refineries in Humberside with heat, steam and power, and is about to be expanded to reach the same electricity generating capacity as the UK's flagship nuclear power station, Sizewell B.


The future

But there's still vast untapped potential for CHP in the UK. CHP plants can either be built in communities to supply building and space heating, or on industrial sites for industrial processes.

If we combine the potential for CHP on industrial sites and in communities, according to government figures, we could generate more than double the expected output of useful energy from the proposed nuclear programme. And the heat, which nuclear can't do.

CHP alone is not the complete solution - it still often uses fossil fuels. But, because it's the most efficient way possible to use these fuels, it cuts emissions and reduces fuel dependency immediately.

Moreover, some CHP plants - so-called flexi-fuel plants - can use diverse fuels in the same boilers. This means that, as more greener fuels like biomass (straw bales, for example, or waste wood pellets or certain specially grown crops) become available, they can be used in the CHP plants instead - with no need to refit the equipment, but with an immediate reduction in CO2 emissions and with the knowledge that these precious green fuels are also being used in the most efficient way possible.

If we combined the efficiencies of CHP with improved efficiencies in the home (proper insulation and minimum efficiency standards for appliances, say), we'd practically eliminate the profligate wastage of our current system.

For more on CHP and decentralised energy, visit EfficienCity or watch our 2006 film, What Are We Waiting For?:


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