Slough Heat and Power is the largest dedicated biomass CHP plant in the UK. Tracey Colley reviews the project in the aftermath of its sale to Scottish and Southern Energy and highlights some of the knowledge gained from its operation.
Slough Trading Estate (STE) began life in the 1920s as a storage site for military vehicles. Since then it has metamorphosed to become Europe’s largest trading estate in single ownership, with around 400 companies, 17,000 employees and 600 buildings on its 500-acre site in the Thames Valley.
The companies on the site are a mix of commercial and industrial businesses, all of which are supplied with electricity, heat and water from utility company Slough Heat and Power (SH&P). Over 2000 nearby homes are also supplied with their electricity from SH&P via 100 km of underground cable as part of a private electricity network. Nearly 30% of the site has been rebuilt since 1990.
STE, where the SH&P plant is located, has a number of famous associations – it was the world’s first trading estate; the world’s first Mars Bar was made here in 1932; the original Ford GT 40 was developed here in the 1960s, and Gerry Anderson developed and filmed his world-famous Thunderbirds TV series here. More recently, it has achieved a certain notoriety as the location where the award-winning British comedy series The Office is set. STE aims to ‘help business to thrive by offering a unique combination of quality properties, lifestyle-oriented facilities and supportive customer care in an unrivalled location’ and the site includes a combination of offices, factories, warehouses, retail outlets (shops, banks, post office, gym), community facilities (child care centre, children’s activity centre) and business clusters, such as food, automotive and information technology.
Current site tenants include large international companies such as Ferrari, O2, Black & Decker and LG Electronics, Fujitsu, GE Medical, Mars Confectionery, ICI Paints, Sara Lee, as well as smaller businesses. STE is 10 miles (16 km) from Heathrow airport, 20 minutes from London Paddington Station and within 20 minutes’ drive for one million people.
The secure power supply, proximity to London and information technology (IT) infrastructure has meant that the STE is a prime location for IT-based businesses such as data centres. As some large companies are making carbon neutral goals, renewable energy supply also takes on an added priority when selecting a site.
Cost competitive energy
The first CHP station at the site was built at the very start of STE’s life in the 1920s, to ensure that businesses located on the estate could be supplied with a continuous, reliable supply of steam and electricity.
The current station has more than 100% redundancy, meaning that it can supply the estate more than twice over. Excess power is sold to the grid and electricity is imported to balance site requirements if and when required.
The security of electricity supply at STE is many times better than the average across the UK, meaning that businesses located on the estate often do not require standby power generation. There is an extensive underground electrical distribution network, so the electricity supply is protected from potential storm damage caused by high winds, falling trees and lightening. The electricity network can be backed up by a connection to the local grid, but can operate in ‘island mode’ if there is a problem with the external electricity supply network.
Average interruptions are around one tenth of those suffered by the south-east England electricity supply grid; for example during 2005 there were no interruptions, whereas the regional average was 84 minutes.
Centralized provision of steam and hot water to estate businesses also means that they do not need to divert resources and money to operating and maintaining energy infrastructure such as boilers.
STE aims to be very cost competitive in the supply of energy and, to ensure transparency in decision making, retail tariffs can be downloaded from the STE website.
Only industrial heat users are supplied with low pressure steam at 14 bar and 315°C and specific heat exchangers are required to use the heat for their particular applications.
For residential consumers, prices for water and electricity were guaranteed by SH&P to be cheaper than alternatives available locally, even though they are, in effect, a captive market. Many of the residential consumers have prepayment meters, which can be topped up using token cards that can be purchased from local retail outlets such as the Post Office and newsagents. This enables residential consumers to monitor the cost of their electricity consumption, rather than receiving a bill well after the consumption.
Since the sale of SH&P to Scottish and Southern Energy (SSE), SH&P no longer has any residential customers directly. As part of the regulatory requirements of the sale this was subsumed into SSE’s local network.
Switching to renewables
A strategic decision was made to convert SH&P to renewable fuel in about 2001 at a cost of approximately £30 million (US $60 million). After conversion, the plant capacity was about 40 MW, with all the heat being used on STE, mostly by the Mars factory, and excess electricity being exported to the grid after the Renewables Obligation Order came into force in April 2002.
This led to the development of an innovative, integrated supply chain for secure fuel supply in partnership with local suppliers such as Thames Valley Energy, a not-for-profit renewable energy agency. The security of fuel supply for the required 130,000 tonnes of biomass annually, particularly during winter, was a risk factor at the start of the project, but this has been overcome by developing co-operative arrangements with suppliers.
Site tenants at STE can direct waste wood, non-recyclable paper, packaging and cardboard to the SH&P plant, thus reducing waste disposal costs and diverting a reusable resource from consuming landfill space. Fibre Fuel Limited, a subsidiary company of SH&P, coverts this non-recyclable fibre, such as waxed cardboard, laminates, photographic paper and mixed papers, into fuel cubes. To improve the calorific value of the fibre fuel, non-PVC plastics are added, up to a volume of 15%. The Fibre Fuel cubes are then used in the SH&P plant, a process which avoids 100,000 tonnes of material being disposed of in landfill.
More than one quarter of SH&P’s wood fuel is sourced from within 50 miles (80 km), which returns £3 million ($6 million) each year into the rural economy. Suppliers of large biomass (forestry and tree surgery waste) or wood waste (sawmill offcuts) deal directly with SH&P.
Large biomass suppliers have contracts, but there is no ‘take or pay’ provision and generally the contracts are for a few years only. They are based on the fact that most of the suppliers are smaller companies and they would not wish to take on the liability of a take or pay contract and might wish to revisit contract arrangements on a regular basis to meet their changing business needs. This flexibility on the part of SH&P means that large suppliers are not locked into long-term contracts.
Thames Valley Energy (TVE) is one of two companies that acts as resource aggregator for small and medium producers and has a contract to provide SH&P with 10,000 tonnes of woodchip per year for 15 years starting in 2006. TVE liaises with local sources of wood such as tree surgeons, woodland clearance operations, foresters, contractors and farmers with dedicated short rotation coppice willows.
Initially, most of the material will be clean chipped wood from tree surgeons, but TVE hopes that by 2021, 70% of the supply with be from short rotation coppicing (SRC) of willows. Farmers growing coppice are offered a guaranteed minimum price for dry wood chips delivered to SH&P, with a possible premium. The growers group established by TVE currently has 14 members growing 110 hectares and the organization is seeking to increase its membership and the numbers of farmers growing willow coppice, although current high world grain prices are providing competition. Farmers who plant SRC on set-aside land may qualify for a planting grant – worth £1000/hectare ($2000/ha) in 2006 – and still receive their Single Farm Payments. ‘Our fifteen year short rotation coppice contract with Slough Heat and Power offers local farmers a fantastic opportunity to diversify into energy crop production, thereby allowing them to save greenhouse gas emissions and improve local wildlife,’ said Dr Gillian Alker, of TV Bioenergy Coppice Limited, a company growing willow coppice with local farmers.
The advantage of the SH&P plant for local suppliers is that, due to the design of the plant, it can accept wood waste with a range of water content of up to 55% for virgin wood and particle size of up to 50mm. This is up to and including the water content of wood that is newly harvested, as long as it is free from green leaves and needles, which has meant that suppliers do not need large storage areas for wood or wood chip. Recycled wood or wood waste derived fuel has slightly different quality standards: less than 25% moisture but up to 20% of particles below 5mm are accommodated, compared to less than 5% for virgin wood.
The six boilers and turbines at the SH&P energy centre can operate on a wide variety of fuels, ranging from renewable sources such as wood and fibre through to natural gas, coal and distillate.
A small amount of natural gas and coal is used to control the boilers. As part of the plant, steam from two wood-fired fluidized bed boilers is directed to a 35 MW pass out steam turbine. A new multi-fuelled vibrating grate boiler dedicated to burning Fibre Fuel and wood chip, was commissioned based on a contract under the 4th Non-Fossil Fuel Obligation scheme. No climate change levy is payable on the hot water and steam resulting from burning wood.
The total electrical output of the combined system is 50 MWe of which 40 MWe comes from wood chips and 10 MWe from waste derived fuel (Fibre Fuel) in addition to 20 MWth.
The equivalent energy value of 8 tonnes of biomass or 6 tonnes of recycled wood is 3 tonnes of coal, so the feed systems to the existing boilers were modified to accept up to 70 tonnes/hr of wood when they were converted from coal.
Changes to the fuel handling system to reduce contamination from oversized materials, metals such as nails and grit/dirt from forestry operations through screening and other controls has solved some of the operational problems encountered during the first years of operation.
In 2004 when the plant started burning wood chips, it encountered corrosion problems which were countered by using a small amount of coal in the feed mix. The plant also has a gas turbine which can be run in open cycle to produce 22 MWe and there is one other steam turbine that can produce 8 MWe.
Although there is a strong preference for utilizing renewable bioenergy, SH&P can still use traditional fossil fuels such as gas, oil and coal.
‘We burn about 1000 tonnes of coal a month, primarily for the chemistry,’ said John Watson, the facility’s fuel manager. He adds: ‘It assists us with corrosion properties. The plant uses about 35,000 tonnes a month of wood-chip fuel, which is both wood and biomass, used for the main part of the power station. We are at the moment about 87% green energy.’
Performance where it counts
The SH&P plant has a potential generating capacity of 101 MW, and a current generating capacity of 80 MW. It generates about 270 GWh of electricity per year, some 200 GWh of which qualifies for Renewable Obligation Certificates (ROCs). As such it is the largest dedicated biomass combined heat and power plant in the UK.
Recently, on 2 January 2008, STE’s parent company, Slough Estates Group (SEGRO) sold its shares in SH&P to Scottish and Southern Energy (SSE), one of the UK’s largest energy companies. The transaction will allow SEGRO to concentrate on its core business by divesting its energy asset to an energy company. The sale will end the previous monopoly supply of electricity, which was requested by occupiers. This is largely as a result of a desire on the part of companies with multiple sites nationally to integrate the supply of utility services throughout their UK operations with a single contract.
The sale value was £49.25 million ($100 million) and SEGRO is giving SSE a 45-year lease on the site. SEGRO negotiated an ‘Energy Availability Agreement’ for providing core SH&P services to SEGRO customers on the Slough Trading Estate.
As Dr Mike Nicholas of the Environment Agency says: ‘What SH&P has demonstrated is that use of renewable resources not only reduces emissions of greenhouse gases, but has additional benefits which contribute to cleaner air and reduction of waste. This has been an excellent example of a company that has adapted its operations for the benefit of the environment as a whole.’
Tracey Colley is working with Sustaining Australia, Stockton, New South Wales, Australia.
The author would like to thank Dr Gillian Alker of TV Energy and Dr Andrew Ellis for their assistance in developing this article. This article also appeared in COSPP magazine.