Heating - Biomass system - School / United Kingdom

Case study assignment

A primary school is being built close to an existing secondary school. The county council wants to evaluate the feasibility of combining the schools' heating systems and installing a wood-fired boiler for the two. The biomass boiler in the new primary school would be linked with the oil-fired boiler in the existing high school by a hot water loop. You have been hired by the county council to determine whether this is financially viable.

Site information

The schools are located in the United Kingdom, about 20 km south of Birmingham. The nearest weather data is from London.

The primary school, intended for 210 students, will have a floor area of 2,800 m². It will be 200 m from a secondary school of 6,200 m² floor area. The secondary school has 650 students. It was built in 1960 and is presently heated by a 750 kW oil-fired boiler with a medium-temperature hot water distribution system.

Only a limited amount of wood is available but the community is considering planting a short rotation energy forest.

Financial information

Typical financial figures for the analysis are provided by the council: inflation at 2%, debt ratio of 80%, debt interest rate of 8%, discount rate of 9% and a debt term of 10 years. The heating system is assumed to last 25 years. The average price of electricity is $0.12/kWh and oil is delivered for $0.60/L. The cost of energy is expected to escalate at the same rate as inflation. The price of wood is estimated at $25/tonne.

Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.

Solution

The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.

Teacher's notes

• When the type of wood waste is unknown, assuming a medium heating value and about 40% moisture content is often reasonable. Here we have assumed wood that is slightly wetter than average, since it would come directly from a plantation.
• The new primary school is assumed to be well insulated and the old secondary school is assumed to have moderate to poor levels of insulation.
• The total peak heating load for the primary school is estimated to be 126 kW. A credit of 126 kW x $100/kW = $12,600 has been entered under renewable energy equipment to account for the oil-fired boiler that would have been installed in the primary school in the base case.
• Balance of plant costs have been set to $0 since these costs would be the same regardless of whether a biomass or oil-fired plant were installed.

Real project

Results

Weobley School, near Birmingham, was the first school in the United Kingdom to have a purpose-built wood-fired heating system. A hot water loop permits the biomass system in this primary school to heat the adjacent secondary school; the oil-fired boiler in the secondary school is used for peaking and back-up heat. The system went online in April of 1998.

In 1998 the school won the inaugural "Built Environment Innovation Award" of the UK National Engineering Council. The school was designed for the Hereford and Worcester Council by ESD Ltd. The boiler system was supplied by Nordist and installed by Dantrim Ltd. It has been fuelled with wood chips produced from a local wood yard and woodland thinnings; in the future chips will be available from a nearby dedicated short rotation coppice plantation. The wood supply process has generated the equivalent of one full-time job.

The initiative for the Weobley school project stems from the Hereford and Worcester County Council's progressive policy of improving the energy efficiency of their buildings and investing in sustainable technologies. Weobley School is expected to encourage more wood-fired heating systems in the area. The European Commission provided supplementary capital for this project.

System description

The Weobley School wood-fired heating system was installed over the winter of 1997-1998. A 350 kW wood-fired boiler meets the primary school's base load heating needs, as well as those of the secondary school on the adjoining site. An oil-fired 750 kW system sited in the secondary school provides back-up and peaking power. During most weather conditions the biomass boiler is able to heat both schools without back-up from the oil-fired boiler. The oil-fired boiler at the secondary school is required to meet the load of both schools when the wood-burning boiler is unavailable.

The primary school has a maximum design heat load of 115 kW. The primary school uses an underfloor heating system.

Weobley School is supplied with about 300 tonnes of locally-grown dry wood chips per year. On arrival at the school, the chips are stored in a concrete silo built to receive the twice-weekly deliveries.

Lessons learned

• The small amount of ash produced by combustion and by the exhaust gas cleaning process can be collected in an ash bin for use as fertiliser on the school garden.
• Renewable energy systems that are installed in educational buildings not only supply energy, but can help students understand energy issues and may raise awareness of environmental concerns.

The big picture

Using wood from a plantation to provide heat through a biomass system makes environmental and financial sense. Because the wood used by the system is grown sustainably, the system is carbon-neutral and does not contribute to global warming. Biomass heating systems generate a market for fuel that can be supplied locally, creating employment in the community. The cost of fossil fuels is expected to rise, while wood fuel costs will likely fall, resulting in increased cost savings.

Photo

School - Boiler - Biomass system, Herefordshire, United Kingdom

References

• Department of Trade & Industry (UK), Website: http://www.dti.gov.uk/NewReview/nr37/html/back_page.html.
• Iain, Paul, "Personal communication," Worcestershire County Council, 2000.
• Ziegler, Urban, "Personal communication," PEMtec, 2000.