Branch lines to Weobley

Think of non-fossil fuels, and you think of the massive potential of wind, wave and solar power. While these sources of energy will play an increasing role in meeting the nation's energy needs, there is another rather less glamorous renewable energy fuel quite capable of contributing to the drive towards clean power generation: biomass.

Biomass energy results from the conversion of indigenous renewal resources, wastes and residues to useful biofuels and chemicals. Such organic fuel can be derived from animal waste, agriculture crops and residues, biogas and forest products. Plant extracts and oils also fall into the biofuel category.

The use of biofuels is not new. Historically, wood was the dominant energy source until the exploitation of coal in the 19th Century, and the decline accelerated as the use of oil and gas increased. By the 1960s, wood had almost disappeared from the world scene as a major energy resource.

The energy crises of the 1970s brought about a revival, and year-on-year the residential use of wood for space heating has increased to the stage where the forest products industry is now approaching self-sufficiency in its generation of process heat and electricity. With the global warming imperative, more mainstream and conventional buildings are beginning to rediscover the virtues of biofuels such as wood.

Not surprisingly, rural regions tend to be best placed to take advantage of readily available sources of wood fuel. With the demands of Agenda 21, local authorities are considering biomass fuels as a sustainable alternative to gas and oil-fired heating in their buildings.

The starting point is the need for many farmers, in these days of disappearing subsidies, to find alternative sources of income. Farmers have a constant need to control the growth of saplings and woodlands – effectively an overhead. The possibility of turning such overhead into income makes, potentially, the cost of wood fuel very competitive. What the fuel user needs, of course, is a reliable supply of fuel of a consistent quality and at a known cost. Precisely the situation which existed when Hereford & Worcester County Council came to design a new primary school in the village of Weobley.

Weobley Primary School

The deciding factor in using wood as a space heating fuel was the 7Y Machinery Ring based in Herefordshire. This is a co-operative of over 300 local farmers which owns coppicing machinery, a woodchipper and the means to transport the chips to potential clients.

The woodchipping business is well organised. The farmers cut timber to specification and leave it to dry for about two years. The chipper moves around the farms transforming the dried timber into chips which are then transported to the users (about two large farm trailer loads per week for the Weobley primary school). The co-operative sells the chips and pays the farmers.

The user has to have a boiler that will burn the chips, a fuel store and a fuel handling system suitable for the chips. This is what the Weobley Primary School has invested in; partly as a demonstration project, and partly to satisfy local political demands.

Weobley had a typical 1960s glass and system-build secondary school adjacent to a very dilapidated 1930s primary school. Rebuilding the 240 pupil primary school had been an objective for many years and went live in the mid-1990s. Hereford and Worcester County Council has a track record of low energy schools and it made sense for the new primary school to follow the genre. At the same time there was a developing political thrust to encourage new rural industries.

The two objectives came together with a decision to make the new primary school a wood-burner – although it must be said that this was motivated more by being seen to be green rather than by hardnosed financial logic.

Consultancy came from the Energy Technology Support Unit (ETSU) and Energy for Sustainable Development. A company called LRZ Bio-Energy Systems advised on the availability and specification of the fuel. Further down the line, wood heating specialists Nordist were appointed to supply, install and commission the boiler and control system.

The school itself is a simple design based on linear accommodation, which itself reflects the flow from preschool intake through to the pre-secondary year. The entrance/office building abuts the teaching areas, and a hall has also been provided for out-of-hours community use. The ground plan provides an enclosed courtyard on one side of the classroom block and a playground on the other.

The school was built within normal budget requirements but with extra funding coming from European sources for assisting agricultural regeneration. Much of the extra cost was due to the building costs of the silo and boilerhouse.

The building has Warmcell (recycled paper) insulated breathable walls, locally made bricks, maximum use of timber and minimum use of concrete. Efforts were also made to reduce the school's energy consumption. As hot water use is low, the building relies on electrical point-of-use heaters. The space heating is also underfloor.

The architecture is classic passive solar design. The building is cross-ventilated through openable windows with the distinctive waveform roof and clerestory windows aiding wind-induced and buoyancy-driven extract. Daylighting is well thought out and artificial lighting has been minimised.

The biomass boiler

The environmental argument for wood as the space heating fuel is that it is CO₂ neutral. This means that the CO₂ produced by the burning process is equal to that absorbed during the wood's growth.

At first sight the boilerhouse looks very much like a coal-fired installation. There is a large enclosed space containing a single 350 kW boiler, burning chips with a moisture content of 35%. The firing is controlled by the water temperature and a pause-firing technique used to maintain the fire bed.

Chips are raised up from a silo by use of hydraulic rams, a screw lift and a stoker. The silo has been constructed as a semi-basement so that the farm trailers can tip at ground level. A requirement of the school designers was that chips would be delivered out of school hours and that the boiler and silo should be close to the main vehicular access for the school site.

Exhaust gases pass through a grit arrestor to a very unassuming flue outlet. There is almost complete combustion so the ash collection from the boiler and grit arrestor, although automated, is of very low quantity and is simply put into an ash bin.

The installation has been linked to the adjoining secondary school, with its much higher heating load. The woodchip boiler now forms the base-load supply for the secondary school with the existing oil-fired boiler now used as top-up. What is clearly missing from the installation is a large, constant load, such as a swimming pool or 24-h use that would best suit a constant level of heat production.

The boiler is now in operation for its first winter season. Performance of all aspects is being monitored and the results will be published. Already, feedback is coming through – including a suggestion that existing coal-fired buildings might well benefit from a switch at the next refurbishment.

At £145 000, the capital costs for the woodchip heating system were significantly higher than the £93 500 budget for a conventional heating system. The fuel store and sump was the biggest cost at £31 000, followed by the Dantrim boiler at £34 400 and the scraper floor at £24 250.

These additional costs were considered an acceptable penalty for providing both the existing school and the new primary school with a (sustainable) fuel source with low environmental impact. In any case the costs were offset by grants from ETSU (£23 000) and the Ministry of Agriculture, Fisheries and Food (£79 000).

The Rural District Commission also stumped up £32 000, and Hereford and Worcester County Council £55 000. The total project cost, including the reboilering of the adjacent secondary school, was £1 080 150.

The success of the Weobley School project has not gone unnoticed by the local authority engineers. Iain Paul, design director at Hereford and Worcester County Council, believes that such schemes have a wider role to play beyond meeting the CO₂ demanded by Agenda 21. "Local biomass fuel schemes are of economic and social importance," said Paul. "Short rotation coppice is not only an environmentally sound fuel but also a cash crop which supports the local economy." Hereford and Worcester, along with other local authorities, is considering how to collaborate to extend the use of woodchip heating systems in order to create a network of woodchip-fuelled schools. Iain Paul has identified around ten sites with old heating systems ripe for conversion.

There is still the issue of fuel cost. Natural gas would be the cheapest option, but at 2.65 – 2.98 p/GJ, woodchip fuel compares favourably with oil at 2.82 – 2.89 p/GJ.

The main question mark is over the supply and consistency of the timber. If the supply of chips can be shown to be reliable, then what has been done at Weobley could well turn out to be a demonstration that wood fuel is as effective as conventional fossil fuels. 7Y Machinery Ring itself is confident that the Weobley schools are the first of a small but significant group of users in its area that will provide the base-load for a good business.