Briefing: Wind turbines and ground sourced energy at AMRC

The advanced manufacturing research centre (AMRC) is an environmentally innovative facility that will be the UK's first carbon-neutral building of its type—that is, the building's operation will cause no net emissions of carbon dioxide to the atmosphere.

The AMRC building was completed in December 2007 and provides a world class facility where research, design, manufacture and study interact effectively. This building, on the outskirts of Sheffield, has been designed as an exemplar ‘factory of the future’, embracing renewable technologies and many sustainable features.

Buro Happold's Leeds office was responsible for the design of the ground engineering, civil engineering, structural engineering and building services design, and provided specialist sustainability consulting.

Reclaimed mine land forms the bulk of the AMRC site. The 4600 m² facility, funded by Yorkshire Forward and EU Objective 1, provides a mixture of flexible workshops, laboratories and offices, to support the University of Sheffield's work in the field of innovative manufacturing techniques for the aviation industry. The accommodation is good for students and staff but it is designed as carbon neutral in use.

At the heart of the AMRC's energy strategy are ground source heat pumps (GSHPs) and on-site renewable electricity generation. Moreover, this is designed to be a financially viable, repeatable solution.

The most environmentally responsible means of applying GSHPs is when the motive energy can be renewably supplied on-site. At AMRC, the design team combined the ground-sourced systems with two 30 m diameter wind turbine generators to help deliver a carbon-neutral building. It is now capable of generating its entire annual energy consumption.

GSHPs, linked to a closed loop network in a system of boreholes, provide the building heating, cooling and hot water loads. All the heating and cooling energy for the centre is thus provided by the heat pumps driven, indirectly, by the wind.

This building will provide occupants with a high quality internal environment. Workshops and laboratories are closely temperature controlled, primarily to maintain equipment calibration. In contrast, the offices offer a comfortable, naturally ventilated environment.

During the earliest stage of the project, the engineering designer applied a ‘carbon mitigation’ design strategy to the design process. This involved focusing on reducing energy consumption initially through good building form and fabric design. Only once the energy saving contribution of the building's form and fabric had been fully exploited did the design team move on to developing the use of energy efficient services in detail, of which GSHPs formed a key element. Applying this ordered design process enabled the designer to minimise the scale of the heat pump installation thereby maximising its positive contributions by reducing the energy consumption and system costs.

A combination of good structural and building design with the use of GSHPs, natural ventilation, daylighting and, of course, the wind turbines make economic as well as environmental sense. One of the most important aspects of this project is that carbon neutrality was achieved cost effectively with well-proven technology.

A vital part of the carbon mitigation design process was to ensure that the technologies applied to the project complement one another. Hence, when considering the fourth stage of the carbon mitigation design process, the introduction of renewable technologies, a key element was to ensure that the renewable technology was compatible with the heat pumps.

The wind turbines, which will generate around 600 000 kWh of electricity per annum, ideally complement the GSHP installation by supplying all the system's power needs and the entire building's electrical demand. It should be noted that during periods of low demand, excess electricity is exported to the national grid.

The site is exposed and has reasonable average wind speeds (5·3 m/s). The turbines selected (WES 30 mk 1 from Wind Energy Solutions BV) have a nominal rating of 250 kW. The 30 m diameter turbines are well proven with hundreds already working in the Netherlands and other mainland European countries. Twin-bladed turbines are mounted on conical tubular steel towers at a hub height of 40 m. Their expected lifetime is in excess of 20 years.

A GSHP system provides the entire cooling load for the AMRC building. The heating load for the offices, laboratories and ancillary spaces is also provided by the heat pumps. The heating and cooling requirements are supplied by four reverse cycle heat pumps, each providing 45 kW of heating and 38 kW of cooling. Care was taken to balance the heating and cooling loads so as to achieve the highest efficiencies from the heat pumps, to ensure the overall installation is as economically feasible as possible and to maintain the boreholes in the long term.

Closed loop GSHP systems can be sized to meet dissimilar heating and cooling loads. However, it is possible to make considerable savings if these parameters can be equated. Heat is abstracted from the ground in heating mode and rejected to it in cooling mode. If there is net heat abstraction from the ground over the year, the potential to ‘recharge’ the borehole is reduced and the length of the ground loop must be increased to ensure continued long term performance.

The entire ground loop system is located beneath the site car parking area adjacent to the building. The car park is provided with a permeable surface to prevent the soil surface drying out which would decrease the ground loop heat transfer capability. A total of 20 bore holes have been sunk at a depth of 100 m each.

Heat pump hydraulic circuits are arranged to achieve free cooling from the ground loop whenever possible. The heat pumps are only activated in cooling mode once this free cooling capacity is exceeded. The control system ensures that when in cooling mode any normally wasted heat from the heat pumps first ‘looks’ for either a domestic hot water (DHW) or heating circuit load before being rejected back to the ground circuit. This significantly increases the efficiency of the system during any periods when simultaneous heating and cooling are required.

Cooling energy is transferred to the distribution system via a plate heat exchanger. A buffer vessel maintained at the required distribution temperatures ensures chilled water is always available.

The heat pumps are arranged with a lead unit that provides higher temperature water to a domestic hot water cylinder. A distinct and separate ‘hot gas’ circuit through the heat pumps provides additional heat recovery from each unit when they are in operation. The heat pumps contain an extra integral heat exchanger to recover all available heat from the refrigerant gas before it enters the expansion side of the system. The harder the heat pump units work the more secondary heat is available for recovery. Flow in this heating circuit is varied to achieve the slightly higher flow temperature required to heat the DHW cylinder.

Cooling is coupled with efficient displacement-ventilation systems which raise the flow and return temperatures and reduce the demand for chilled water. Displacement-ventilation supply-air temperatures are in the region of 19°C instead of the 12–14°C that would be required from a traditional mixing ventilation system. Increasing the supply-air temperature significantly reduces the amount of cooling that is required, especially considering the fact that latent cooling is not required. Increased water flow-and-return temperatures of 11–15°C instead of the more conventional 6–12°C have been used to further reduce the energy requirements to generate chilled water.

Heating distribution is via wet underfloor heating circuits throughout. This allows low flow-and-return temperatures of 40–30°C to be used for the low temperature hot water circuit. These temperatures are chosen in order to achieve a good coefficient of performance from the heat pumps.

The final stage of the carbon mitigation design process, ‘operation’, embraces the need to ensure that the building services operate as the designer intended. Only by continually monitoring the energy and usage characteristics can the low carbon credentials of a building be fully proven and potentially improved upon.

The operation and monitoring of the AMRC has only just begun. At least a full year of heating and cooling will be needed before any valid conclusions can be drawn. However, modelling and experience suggests that a carbon neutral building is a realistic achievement.

The factory of the future has been designed as a prototype but the concept is applicable to many energy-hungry facilities. Funders Yorkshire Forward, the University of Sheffield itself and Buro Happold are currently putting in place the post-occupancy analysis programme to ensure that this ground breaking project is a success. In addition, it has been provided with targets and methods to improve the monitored performance continually.

The heat pump installation has been provided with sufficient monitoring equipment to enable the seasonal coefficient of performance to be measured. To achieve this, the heat pump's heating and cooling generation will be metered separately along with the power consumption of the heat pumps in both modes.

This project is now almost complete following the installation of the wind turbines in April 2008. Buro Happold and the University of Sheffield are confident that the subsequent monitoring and evaluation will reinforce the value of the AMRC as a learning facility that will help teach the construction industry the way to achieve carbon neutral industrial buildings.