DISTRICT HEATING SCHEMES
District heating – an overview
District heating systems comprise a network of insulated pipes delivering heat in the form of water or steam from a central heat source to end users.
Energy sources vary greatly and include natural gas, biomass, geothermal and domestic and/or commercial waste. In some cases, only heat is produced, but in others, both heat and electricity are produced (Combined Heat and Power plant, or CHP).
District heating systems are used in a wide range of developments, proving their versatility. They have been used successfully across hospitals, housing developments (from single tower blocks to developments consisting of many units), mixed-use areas and, for example in Southampton, across many kilometres of the city connecting a huge range of end users to a single geothermal heat source.
Case study – Derwenthorpe housing development
Derwenthorpe is a housing development in York which, at completion, will consist of 540 units. The initial phase of the development has been constructed for the Joseph Rowntree Housing Trust (“JRHT”) as affordable housing with the remainder under construction by David Wilson Homes. As part of the development, JRHT wanted the energy supply to be beneficial environmentally but also to draw the community together. A large and impressive boiler house was constructed towards the entrance of the site housing a biomass boiler capable of servicing all 540 units at completion. The boiler house serves the primary purpose of providing heat to the development but also comprises several meeting rooms and facilities for the use of residents.
The boiler is fuelled by sustainable and locally-sourced wood chip, delivered to the site by lorry and stored in secure, sealed and watertight underground fuel stores. It is essential for the fuel to be kept dry and therefore at its most efficient. The fuel is transferred into the main boiler which then serves to heat the water pipe network running around the site.
The pipe network is pre-insulated; main pipes are constructed from steel with the networks serving individual properties in plastic.
The energy boiler needs to meet requirements for clean air. ‘Scrubbers’ are attached to the chimney flues which remove the particulates from the air before it is diverted up the flue. The particulates can in fact be used themselves, particularly as fertiliser for plants. Any filter used for such a boiler must be matched to the size of the facility and so costs can vary greatly.
Residents of the JRHT properties (and others, once constructed) will contract with JRHT as their energy supplier and JRHT operate the biomass facility.
The fuel stores, when running at full capacity, will store around 3-4 days’ supply. In the event of a sudden surge or peak in demand, if the biomass store cannot cope gas is connected as a back-up. However, it is not expected that this will need to be used and the system has been designed to be as reliable as possible. JRHT, with Econergy who developed the system, invested in good-quality materials and believe that this will make the difference to ensure that the scheme is reliable, efficient and straightforward to operate. Feedback and buy-in from residents so far has been positive. JRHT did not disclose details of their funding arrangements for the project and whether they needed to borrow. An indication of cost for a system was given as £300,000-£400,000 based on an example of a simple biomass plant serving approximately 120 flats.
Econergy maintain the system under an ongoing maintenance contract following installation. JRHT deal with invoicing end users themselves, however. When undertaking a project such as this, it is important from the outset to provide for operation and maintenance long-term, both for certainty of cost and energy supply.
Frequently asked questions
How is fuel stored?
Fuel is stored in sealed, underground fuel storage tanks. The tanks are constructed from steel and must be watertight to ensure that fuel is kept dry. The fuel supplier for Derwenthorpe carries out a drying process on the product before delivery to ensure that the fuel is as dry as possible before entering the boiler. Any water present in the fuel makes the burn less efficient.
A fuel auger in the tank delivers the wood chip from the storage tank to the boiler’s fuel receptor above ground.
Is there a back-up in case of shortage of fuel?
Gas is used as a back-up at Derwenthorpe in the event of demand surges.
What are the alternatives to wood chip? Why might these be preferred?
Wood chip is cheap which makes the fuel appealing to users. An alternative may be wood pellets, which are cleaner and have a higher density therefore making them more efficient. However, pellets are more expensive. Pellets may be preferred for example in high-density, inner-city areas where it is more difficult to deliver and/or store fuel; less is required due to the higher density of the pellets. If less storage space is available, pellets make the most efficient use of this.
JRHT agreed a price for their fuel supply, although details of this were not given. Cost and security of fuel supply is one of the biggest concerns for biomass plants and district heating systems and it is important that this is addressed right from the outset of any such project to ensure that it is viable to run and cost-effective to end users.
Other biomass-fuelled district heating schemes have used residential or commercial waste, or a combination of the two. The emissions would need to be treated to ensure that harmful particulates are removed but this presents an opportunity for waste to be used more positively than landfill.
How are end users charged?
Each property houses a meter and heat exchange system which is similar to a standard combi-boiler. Heat is transferred to the property’s radiators or under-floor heating system from the heat exchange box. End users contract with their landlord, developer or other operator of the scheme’s heat source (in the case of Derwenthorpe this is JRHT) who invoices as per the meter usage. At Derwenthorpe, residents of JRHT’s properties may only obtain their heat from the biomass source. It is important that there is a guaranteed load on such a system as this in order to ensure cost-effectiveness to both the operator and the end users. If residents were able to go elsewhere for their energy, the costs of the district heating scheme would rise accordingly for those users left.
What happens to surplus energy?
A large thermal heat store in the boiler house stores surplus heat for future use.
How flexible is the biomass system and does this allow for future growth?
The heating scheme is designed with some ‘futureproofing’ in mind, particularly in terms of potential growth of the development. As technologies advance it will be possible for boiler improvements to be made and even for additional heat sources such as a geothermal supply to be ‘bolted on’ to the system. However, the system is designed to the best specifications possible at installation to ensure that it is as efficient to run as possible from the outset. At Derwenthorpe, it is presently intended to use just biomass as a heat source but in future it is possible to add others if necessary.
How are fuel deliveries made and do these impact on development?
Fuel deliveries are made to the energy centre at Derwenthorpe by lorry. The impact of this was considered at the planning stage. A large turning/car parking area is sited at the front of the centre along with the underground fuel storage tanks. The centre itself has been sited near to the entrance to the development meaning that delivery lorries do not need to travel through residential areas.
Even at full strength it is expected that fuel deliveries will only need to be made every 3 to 4 days; JRHT do not expect there to be a constant stream of deliveries to site.
Who owns the pipe infrastructure?
This remains in private ownership and is not adopted or maintained at public expense. The local authority liaises with the energy provider as to maintenance requiring the opening up of adopted highways.
What weaknesses does the system present?
Electricity is used to pump the heated water around the site through the pipe network. In the event of a power cut, the whole power to the estate goes down therefore meaning that the heat supply stops. The provider cannot plan for this and is aware that this presents a fault in the reliability of the system.