Ground-source heat pumps deliver over 20% renewable energy for Stockport school
Being underneath the flight path for Matthew Airport necessitated this new Stockport Academy building being sealed — making mechanical ventilation and cooling necessary. Both cooling and heating are provided by a large ground-source heat-pump installation.
With a heating/cooling capacity of 360 kW, the ground-source heat-pump system at the new Stockport Academy is the largest such system designed by Buro Happold. James Dickinson, David Bell and Elaine Bissell outline the scheme and associated plant.From an early stage in the Stockport Academy project ground-sourced heat pumps (GSHPs) were at the forefront of the technologies considered due to the nature of the building and its location. Design decisions were driven by site-specific factors such as the proximity to Manchester Airport as well as more general considerations such as Part L of the Building Regulations. The site of the building, adjacent to the existing Avondale High School, is on the flight path for aircraft to Manchester Airport. This meant it was essential to provide a sealed building due to acoustic restrictions. This decision, in turn, led to building-services systems with mechanical ventilation and heat recovery for all occupied areas. Ground-source heating and cooling is being used to provide the required cooling for all the ICT areas, cooling to air-handling units as well as low-grade heat to the air handlers and underfloor heating systems. The requirement for a sealed building meant that the building must be mechanically ventilated, so cooling became a necessity. The application of a GSHP was partly driven by new Part L Conservation of Fuel Building Regulations (2006), which requires a 28% reduction in carbon-dioxide emissions on a notional compliant building (2002). Local-authority planning requirements also require a renewable energy content of greater than 20%. Additionally there was an aspiration to reduce the operational costs and maintenance in the building, wherever possible. The inclusion of GSHP was part of a process of carbon reduction, which included passive approaches with close attention to the fabric of the building and the use of natural ventilation where possible — as well as the specification of low-energy building-services components. The carbon savings and payback period on this system justified a Department for Children, Schools & Families (DCSF) grant covering the capital costs. However, to hit the target for carbon emissions, additional low-carbon technologies needed to be reviewed. The GSHP was considered to be the best of these technologies; it is an efficient substitute for more conventional heating and cooling plant (gas boilers and electric chillers) which are commonly used in educational buildings. The grant from the Department for Children, Schools & Families (DCSF) of £300 000 covered the cost of the ground-source heat-pump system and ancillary plant. Other low-carbon options such as biomass boilers were also investigated, but the year-round cooling load, due to building being sealed, meant that there would always be a need for heat rejection. This requirement added weight to the case for the ground-source option. The total budget for the project was about £27 million, of which around £5 million was for building services. The building’s total area is 10 000 m2. Most appropriate technology
As the best alternative technology for this building, the GSHP system chosen comprises an array of 45 vertical boreholes sited next to the building. These boreholes are connected to a series of four heat pumps, which are used to step up or bring down the temperature to a more usable level for the space-heating and cooling circuits. The advantages of using a GSHP can be outlined as follows. • Proven low-carbon technology.
• Very reliable and low-maintenance system. • Makes use of free energy from the ground.
• In addition to reducing the carbon footprint of the building, the GSHP provides a particularly efficient method of heating and cooling, which can lead to operational cost savings.
• The GSHP is also an effective way of helping achieve compliance with the recently published Part L of the Building Regulations.
• The system will add resilience to future fluctuations in energy prices, in particular gas.
• Eliminates the need for roof-top heat rejection. For Stockport Academy the peak heating and cooling loads have been assessed as 1200 kW and 360 kW, respectively. In addition, hourly analysis of the building heating and cooling loads was completed to optimise the size and integration of the GSHP. A system with a capacity of 360kW capacity was chosen as the most cost-effective size by avoiding the need for other heat-rejection plant. During the winter conventional gas fired boilers are required to cope with infrequent peak heat loads. Ground energy
The ground absorbs solar energy throughout the year. Below a depth of about 10 m the temperature remains fairly constant at the mean ambient-air temperature, regardless of the time of year. Depending on the location and exact depth this temperature can vary, typically, from 7 to 13ºC in the UK. Using ground energy to provide heating and cooling in building requires equipment (heat pumps) to raise the temperature of a process medium from a low to a higher more useful temperature level using additional energy. The energy is delivered to the pumps using a ground heat exchanger, which usually comprises a number of pipe loops (vertical or horizontal) with a primary process medium of water or ,more normally, a glycol solution to eliminate the possibility of freezing. The energy in the ground used by the heat pump will, if sized accordingly, be replenished by solar irradiation, rain and, for deeper vertical collector systems, underground water flow. More importantly, a balance between the heating and cooling energy exchanged throughout the year can help to ensure the long-term performance of the system. Site restrictions at Stockport meant a vertical closed-loop ground heat exchange system was chosen. A vertical closed-loop system utilises vertical ground heat exchangers or probes that are inserted into specially drilled boreholes up to 150 m deep. Extraction rates generally vary from 20 to 110 W/m. The useable energy depends on similar factors to the horizontal system, although more specialist geologist analysis is sometimes needed, including deep test-bores to ascertain: • type and depth of each soil and rock layer;
• heat-transfer potential for the different layers of soil and rock over the depth of the borehole;
• presence and height of water table. This type of system lends itself to larger applications where the relatively expensive mobilisation of drilling plant can be justified and where there is limited space. The GSHP system was compared with a conventional gas-fired heating and electric chiller system. This analysis showed that in year 10, taking into account predicted relative increases in both gas and electricity prices, the operational savings for the GSHP system would be £17 888. The estimated carbon-dioxide savings would be 67 014kg (11.5% of the building operational total). The GSHP would provide 30.6% of the total building energy, 23% of which is renewable.
Exhaust air from the classrooms passes through transfer grilles to the central atrium. It is then extracted at high level and passed through thermal wheels prior to exhausting to atmosphere.
Although the GSHP capital investment per kilowatt of heat output is greater than a conventional system, the cost savings from the optimised GSHP system mean it will be economically favourable in overall terms. It should be noted that the operational savings and payback are very sensitive to the relative prices between gas and electricity. However, sensitivity analysis on the most likely future energy-price scenarios suggests a simple payback of 11.5 years. This analysis takes into account the displaced conventional system, but does not allow for possible increases in cooling and decreases in heating demand due to the effects of global warming. Meters are to be installed to enable the system performance to be analysed by the school and students through the school intranet. To assure best value, competitive tendering against performance specified requirements was used to select the specialist GSHP contractor (Geothermal International). Key factors in this selection were robustness of design and previous experience. Holistic design approach
Other aspects of the design of the services reflect the need to alter the flow and return temperatures to optimise the use of the GSHP system. The underfloor heating system serves 40 to 50% of the building’s total floor area and has a flow/return temperature of 45/35°C. The chilled-water flow/ return temperatures of 14/18°C are well suited to the chilled beams used in the IT and other cooled areas. There are four heat pumps each of 90 kW. With basic sequencing control of cooling and heating, they can simultaneously provide 90 to 360kW. For heating loads above 360 kW, gas-fired boilers provide a separate injection of heat to the circuit from the gas fired boilers. The systems were designed so the cooling load is entirely served by the ground-sourced system. Effectively the heat pumps are controlled on a cooling-led strategy in sequence. There was originally some redundancy in the cooling systems, but changes to the building now mean the GSHP cooling provision of 360 kW almost exactly matches the building’s peak cooling load. In fact, as the cooling loads have gone up there is now a need for a couple of additional direct-expansion units for the server room. The higher flow/ return temperatures from the boilers (80/60°C) enable the system to supply and store DHW at the correct temperatures. Structural design on this steel-framed building was also by Buro Happold. The use of castellated beams enables services to run through the voids. The building-services engineers integrated their design with the structure, particularly when considering the air flow through the building. The air supply through the classrooms is passed via transfer grilles to the central atrium, from where it is extracted at high level and passed through thermal wheels prior to exhaust. The thermal wheels in the roof-mounted air-handling units recover the vast majority of the heat, and variable-speed drives on all the main drives to the fans and pumps further reduce wastage. Conclusion
With the largest Buro Happold closed-loop GSHP now well underway, the practice is learning invaluable lessons in the application of this emerging technology. Indeed, with a number of projects completed or about to be completed in North America and Europe, Buro Happold is one of the industry leaders in ground-source heating and cooling. Much of this experience is now documented in the recently published ‘Patterns on ground source energy’, showcasing the practice’s expertise. Stockport Academy is due for completion in summer 2008. The 3-storey building will become a secondary school and sixth-form college for 1150 students. Project team:
Main contractor: Bowmer and Kirkland
M&E contractor: NG Bailey
QS: Parker Browne
Project manager: Townson Associates
Buro Happold provided structural-engineering and M&E consulting services Equipment
Ground-source specialist: Geothermal
Boilers: Ideal Pumps: Grundfos
AHUs: Air Handlers
Chilled beams and grilles: Halton
Underfloor heating: Warmafloor