Forestry and Land Scotland Glentress Zero Carbon Heating

The project was realised in a manner sensitive to the engaging natural environment created by FLS at Glentress and has significantly improved user comfort and resilience at the site, with the added benefit of simpler and reduced system maintenance.

The visitor complex at Glentress, between Peebles and Innerleithen in the Scottish Borders, serves as a gateway for numerous activities within the wider Glentress Forest estate, which is owned and operated by Forestry and Land Scotland.  A mecca for mountain bikers from far afield, the Glentress hub and hill tracks are also used for marathons and other wild trail events.

Construction of the buildings at the complex was originally completed in 2009 with a focus on sustainable practice.  The original designers specified extensive use of natural construction materials and a district heating network was included fed from a centralised biomass boiler.  The individual buildings at the site drew heat from the district loop using heat interface units to provide space heating and domestic hot water.

FLS had identified a need for a biomass boiler replacement since the original unit was approaching the end of its useful life, suffering from parts obsolescence and an increased rate of breakdowns.  In recognition of the forthcoming national targets to deliver zero-carbon buildings, there was a need to assess zero carbon heating system options to replace the woodchip biomass boiler.

Baseline: Develop an energy model of the site taking account of as-built information, survey findings and occupancy data to estimate how the site currently performs.  Apply local historic weather data to estimate annual consumption and assign carbon dioxide figures to each energy source to develop a carbon footprint for the site.

Be Lean: Compare modelled versus actual data to identify distinct behaviour patterns that are energy intensive or very carbon-heavy.  From the findings, recommend fabric and building investigations and improvements.

Zero Carbon: Identify suitable low or zero carbon sources which can meet the energy demands of the site.  Assess their feasibility and the associated engineering modifications required.  Review off-site means that may also contribute to project success.

On completion of the initial zero carbon heating study, an outline design package and cost appraisal was developed centred on the option of high-temperature air source heat pumps to replace the biomass boiler.  The key factors in the selection of the air source heat pumps as the preferred solution were the high co-efficient of performance, relatively simple retrofit, low maintenance, and critically, the fact that FLS could procure renewable electricity under their electricity supply agreements for the site.  The original boiler was installed as a single unit and had suffered from lack of redundancy during periods of maintenance, therefore, the replacement system design included for N+1 redundancy at the heat pumps.

The transfer away from biomass to an air source heat pump scheme meant that, to ensure optimal efficiency, the district heating system flow temperatures would be set lower than those previously available from the biomass boiler.  This effected a need to also replace the heat interface units within each building to maintain optimal heat transfer and system performance.  Domestic hot water storage was also provided with the option of electric boost backup for areas with higher instantaneous visitor loads.  Some legacy issues with the existing building management system and user controls were also be addressed as part of the project.

The transfer of heat load from biomass woodchip fuel onto the electrical supply of the air source heat pumps required our review of existing spare capacity in the electrical supply serving the site.  A maximum demand assessment was undertaken for the site including metered data where available, and a modest upgrade was taken forward to utilise spare network capacity in the existing SSE transformer and low-voltage mains cables.