Case study

Knowle Manor, Somerset

195 kW Wood Chip District Heating Scheme

Key facts

Commissioned

December 2010

Scheme

Supply of hot water and heating to Knowle Manor, including a swimming pool and two cottages.

Boiler and plant

195 kW ETA Hack woodchip boiler installed in a converted outbuilding, with a 5,000 litre accumulator tank.  This is connected to the heated properties by 260m of pre-insulated district heating main.

Fuel

The boiler is expected to use approximately 120 tonnes of wood chip per year to produce around 260,000 kWh of heat. A close relationship with a local timber yard allows Knowle to produce wood chips from bulk purchases of slab wood. Bulk deliveries of slab wood are received approximately every 6 months and chipped on arrival directly into the bulk store.

Wood chip storage

A large redundant agricultural building was converted into a fuel store, hopper and boiler room. The fuel store is sufficiently large to contain six months’ worth of chips. A tractor is used to push chips over the dividing wall, from the store into the hopper, which holds 20 tonnes – sufficient for at least a month’s supply of wood fuel. Woodchip is transferred directly from the hopper to the boiler by 6m auger and rotary arm agitator.

Grant / Funding

The system received a 37% grant from the Bioenergy Capital Grant Scheme but the property holders intended to repay this in order to qualify for the Renewable Heat Incentive.

Savings / Investments

The previous oil and LPG heating systems were generating bills of around £18,000 per year, compared to £6,500 for wood chip. Payback with the capital grants received is expected to take 6 years. This should be substantially reduced by the RHI, with a net saving of around £32,000 per annum.

CO2 saving

Estimated at 100 tonnes per annum.

Knowle Manor is a substantial house with 19 en-suite bedrooms and an indoor swimming pool which is available to rent for large groups and corporate events. Prior to this installation, it was heated by a combination of oil and LPG boilers, the cost of which was proving prohibitive. The owners decided to utilise a large, redundant, agricultural building as a wood chip store and boiler room; it was also decided to provide heat through a ‘district heating system’, not only to the Manor, but also to two cottages. Additional capacity has been retained in order to provide heat for two barns which may be converted in the future.

Fuel supply and chip handling

Seasoned waste timber is purchased from a local sawmill and delivered in bulk to the estate where it is chipped by a contractor directly into the bulk chip store. The size and layout of the store mean this operation is only required about every six months. A tractor and loader is then used to push chip from the bulk store over an internal wall into the large hopper where it is fed into the boiler by a 6m spring arm agitator. The size of the hopper means that this latter operation is undertaken approximately monthly.

Case Study: Knowle Manor 01

System design and installation

The system was designed for 100% of heat and hot water to be provided by biomass fuel. The heat is transferred from the boiler room to the Manor and adjacent cottages by 260m of pre-insulated district heating main. The 5,000 litre accumulator tank stores enough heat to allow peak demand to substantially exceed the 195Kw rating of the boiler; it also allows the boiler to operate efficiently by burning for long periods rather than ‘short cycling’.

Case Study: Knowle Manor 02a

Benefits

In addition to dramatically reducing the heating bills for the properties involved, the project has further contributed to the ‘green’ image of Knowle Manor.

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