CHP comes to Milton Keynes

What could be the largest such CHP installation in the UK, with an electrical output of 3.2 MW, is up and running in Milton Keynes.While electricity is a very efficient fuel to use, its generation exploits less than half the energy content of the primary fuel. Combined heat and power uses primary fuel more completely, which is why it is well regarded by the Government as a measure to reduce carbon-dioxide emissions. A major scheme in Milton Keynes will achieve just this. At the heart of the scheme is a GE Jenbacher CHP unit with a 20-cylinder engine. It is fuelled by natural gas and has an electrical output of 3.2 MW, the largest in the company’s range of reciprocating units. 3.0 MW of heat is recovered form the water jacket, oil cooler and exhaust system. There is a Jenbacher Danstoker waste-heat boiler on the exhaust system. The installation is just off Avebury Boulevard in Central Milton Keynes at the rear of CBX II. It will supply electricity and heat, via a district-heating network, to The Hub, a development comprising hotels, apartments and offices, and will also supply electricity and heat to a proposed new residential area in the west end of Central Milton Keynes. The area served has a radius of some 1.5 km with a private-wire network to distribute electricity. The 11 kV off the engine is converted to 415 V by an air-cooled transformer. Back up The CHP unit is supported by a 10 MW Dunphy gas-fired boiler. Why such a big back-up boiler? The answer. explains Frank Martin of Xergi Services, which designed and installed the project as a £6.5 million contract with Thameswey Central Milton Keynes, is that a future phase will see the addition of another CHP unit with the same outputs, and this back-up boiler already has sufficient capacity. The back-up boiler is run once a month to prove its availability. When required, it kicks in automatically. Readers who think the Thameswey name seems familiar would be quite right. There is a direct link with Thameswey at Woking, and Thameswey Central Milton Keynes is owned by Thameswey Energy, which is owned by Woking Borough Council. Quite naturally, the Milton Keynes project benefits from the years of experience with CHP in Woking town centre. The environmental benefits of CHP are explained by Frank Martin in terms of the utilisation of primary fuel. Normal power stations convert only 40% of the energy content of primary fuel into electricity, and huge quantities of heat are rejected. CHP makes use of much of that rejected heat, so that over 70% of the potential of the primary fuel is utilised. District heating A means of using the rejected heat is needed, which is often, as at Milton Keynes, a district-heating network. Such a district-heating network is most effective in a mixed-use area that includes, for example, residential, schools/colleges and businesses. Whereas an excess of electricity can readily be disposed of to the National Grid, and a shortfall can be met by the Grid, managing the heat balance is quite a different problem. A district-heating system itself must be able to meet the entire demand for heat at any time, and it is wasteful simply to reject heat. At Milton Keynes, this requirement is met by a large volume of thermal storage. There are three tanks, each with a capacity of 120 m3, but operated as a single thermal-storage unit. Water is stored at 85°C, and the storage capacity of 12.6 MWh of heat can provide a 3-day buffer. One of them can been seem in front of the plant-room exhaust in the photograph above. The CHP unit runs for 14 h a day, during office hours, and the thermal store can meet the demand for heat when the engine is not running. The district-heating system is designed for a flow of 85°C, with return temperatures of 75 to 78°C. Commercial and residential buildings in the area covered by the scheme, do not have boilers. Instead, they are linked to the district-heating system by plate heat exchangers, so that systems in the buildings are effectively separated from the district-heating pipework. The condition of water in the DH pipework, which is maintained by filters and inhibitors, is thus not compromised. Apartments One building connected to the scheme is the Amethyst House development of Barratt Homes. It comprises 307 high-specification apartments with individual systems for heating and hot water via interface units supplied by Elson. These units are made by APV and are effectively 37 kW combi boilers, but using energy drawn from the district-heating system rather than burning gas directly. These units are fitted with heat meters to monitor energy consumption, which is radioed to a central point. As is normal with any utility services, there is a requirement for these meters to be read once a year. The plate heat exchanger in these units has separate channels for heating and hot water, and each channel has its own thermostatic control. They will be serviced once a year by Xergi to satisfy the requirement of the utility companies. There is very little to service, however, and when the same units are installed in Denmark, they are not serviced regularly. Features of these APV units include a very quick response to a demand for hot water, which is provided by instantaneously heating the incoming cold water to the apartment. The circulating pump for the space heating is inverter controlled to provide good control of the heating; the pump also runs more quietly and uses 40% less energy than a fix-speed pump. This scheme in Milton Keynes currently serves two development areas. Future plans include building distribution systems to a further two developments in the area, which will require an increase in the capacity of the central plant. This is a long-term project, And Xergi will be involved with running the plant for 25 years via Xergi Services, a company set up by Xergi and Thameswey and 80% owned by Xergi. The generator is fully expected to last that long, and the 60 000 h intervals between major overhauls will occur about every 10 years.

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