Combined heat and power CHP

From time to time reference has been made in the text to CHP. Since there seems to be strong EU and UK government support for this system of energy distribution it calls for special consideration.

First, electricity generation in the UK is the largest single contributor to CO2 emissions, amounting to 26% of the total. Many buildings are responsible for more CO2 from the electricity they use than from the fuel for the boilers providing hot water and space heating. At the same time the heat from power stations is ejected to cooling towers and thence to rivers and the sea. The River Trent in the UK is almost suitable for tropical fish.

Combined heat and power can be scaled to meet almost any level of demand from a single home to a whole city. Already some cities like Sheffield receive heat and power from the incineration of municipal waste. However, this is not an ideal heat and power source for the future on account of its emissions. Importantly the infrastructure exists to be converted in the future to more environmentally benign producers of heat and power.

The European Commission aims to double the use of CHP by 2010. The UK government has a target of 10 GWe of CHP with associated carbon savings of six million tonnes per year, which represents 25% of its declared target savings or 20% by 2010.

A basic division in providing CHP is between factory-produced units up to 1 MWe and custom-built site-specific plants producing up to hundreds of megawatts. Rapid developments in the technology of small gas turbines and gas engines is helping to create favourable conditions for CHP which have an efficiency exceeding 70%. According to

David Green, Director of the CHP Association, a typical payback time for CHP is three to five years with an operating life of 15-20 years.

As stated earlier there is already an alpha-type Stirling engine on the market in the UK known as WhisperGen, producing 5 kW as heat and 800 W of electricity. On the verge of being market-ready is a 1 kWe, 15 kWth home CHP package based on a 'free-piston' Stirling engine under the name 'Microgen' (pp. 104 and 106).

In Chapter 9 there was reference to a domestic CHP package centred on a 1 kWe solid oxide fuel cell that can operate with natural gas. Ceres Power plans to market the system in 2007.

Community CHP systems using turbines powered by natural gas or, preferably, biogas from anaerobic digestion plants are a viable system now and could take off with the right government support. Housing estates now being built throughout the UK should be provided at the outset with an insulated hot water pipe infrastructure even though the system may not be operational initially. It is much more cost-effective to use a common trench at the construction stage than retrofit when the external works are complete.

The ultimate opportunity is for city-wide CHP. A genuine commitment to a sustainable future would require a government to promote and subsidise city-wide CHP using large, low-temperature water grids as used successfully in Denmark.

One option being proposed is to exploit the waste heat from large coal/oil fired or nuclear power stations. Nuclear power is enormously wasteful of heat, producing much more excess heat per unit of power than conventional power stations. Plants of this size are usually some distance from conurbations. However, Denmark again provides the prototype with the city of Aarhus receiving heat from a coal-fired plant 30 km distant. The rate of flow is around 3 m/second through large-diameter pipes.

However, with pressure increasing for the electricity infrastructure to evolve into a much more fragmented, distributed system, this would not be a wise investment. The capital commitment would inhibit conversion to a fundamentally different distribution system. This would be especially alarming in the case of nuclear power since it would establish a cost-effective argument (however flawed) for replacing the ageing nuclear plants with a new generation of nuclear power stations. The route ahead is for near zero carbon CHP plants fuelled by wood from rapid rotation crops or biogas from farm and municipal waste and the anaerobic conversion of sewage The ultimate CHP technology at city scale must be high-temperature fuel cells fed with direct piped hydrogen, but that is decades in the future.

CHP has advantages when used in conjunction with solar energy and wind power. Large plants between 100 and 300 MW of electrical capacity are flexible, having the ability to change their mode of operation from producing electricity alone to delivering both electricity and heat in varying proportions. This means that it can adjust its mode according to how much electricity is being provided from renewable sources. CHP is reliably available at times of system peak, being immune to the vagaries of wind or sun. It is therefore ideal for complementing renewable technologies.

There is a final twist to CHP. It can also provide cooling in so-called 'trigeneration' mode. Most trigeneration schemes are to be found in cities in the USA where air conditioning accounts for considerable electricity consumption. Piped chilled water can significantly reduce the electricity demand from air conditioning plants in large office complexes. Even a number of financial institutions in the City of London enjoy this facility.

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