CIBSE Case Study: Innovative Ground- Source Energy System in University
CIBSE Case Study: Innovative Ground- Source Energy System in University
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Article from the August 2013 edition of the CIBSE Journal written by Keith Horsley of Hoare Lea.
When an innovative ground-source energy system at the Department of Earth Sciences in Oxford failed to live up to expectations, the original project team were on hand to sort it out. Unless we all adopt this approach, these technologies are bound to fail.
Low-carbon technologies such as ground-source heat pumps have the potential to make a real dent in the UK's carbon reduction targets. However, if these systems are to fulfil that potential, the industry's attitude to their specification, commissioning and operation needs to change.
If carbon-saving technologies are simply "fitted and forgotten", the chances are they will not perform as expected, and the result will be more "turned off and forgotten". Experience from one prestigious university project in Oxford shows the importance of continued involvement from the design and construction team during the postoccupancy stage - and the significant rewards that can be gained through perseverance.
The Department of Earth Sciences at the University of Oxford (OUES) is a leading centre for geological research designed by Wilkinson Eyre Architects with Hoare Lea as mechanical and electrical consultant. The development, which had a £29.5m construction cost, comprises a 7,100m2 building with a five-storey office and teaching wing and a four-storey specialist laboratory wing, linked by an atrium entrance.
In some of the laboratories, environmental control is of critical importance to the experiments conducted. The building also houses a server room, and a number of hub rooms, where 24-hour cooling is required.
Energy efficiency was a key part of the brief and a 20% renewable energy contribution was a planning requirement of Oxford City Council. Early in the design, Hoare Lea carried out a comparison of low and zero carbon (LZC) technologies, and concluded that a ground-source energy system (GSES) was the most appropriate solution. OUES is one of the first laboratory buildings in the UK to use a GSES and the principles of the technology fitted neatly with the climate change and geological research interests of the university department.
The financial payback of the system - at current energy prices - was longer than would normally be considered viable, but became more attractive when fuel cost inflation was factored in.
Also, Oxford University was attracted by the predicted carbon savings and instructed the designers to push beyond planning requirements and maximise the contribution from the area of ground available under the building.
The result is a system that comprises 63 closed-loop boreholes, each about 65m deep, with three reversible heat pump units connected in a "sliding header" arrangement. Hoare Lea worked with ground-source energy specialist GI Energy on the initial design prior to being novated to main contractor Laing O'Rourke. Laing O'Rourke appointed GI Energy to carry out the detailed design and installation of the system.
The control strategy ensures that the GSES provides heating, cooling or both, depending on what is most carbon-efficient at any particular time. The main low temperature hot water (LTHW) system operates at flow and return temperatures of 45° C and 35° C to maximise the efficiency of the heat pumps. A separate LTHW system, with its own boilers, generates higher temperature water for domestic hot water and conventional radiators.
Heat meters measure how the heating load is shared between the heat pumps and the boilers and also how the cooling load is shared between the heat pumps and the chillers. Heat meters on the secondary circuits identify where in the building the heating and cooling is used. The GSES control system measures the electricity used by the heat pumps and ground-loop circulation pump and uses an algorithm to apportion this load between heating and cooling.
In the spring of 2011, just after the building was fully occupied, it became clear that all was not well with the system. A number of reliability issues had led the client to lose confidence in the GSES and it was not operating as efficiently as had been predicted. Although no formal appointment extension was agreed, all parties involved were keen to uncover what had gone wrong and what could be done to improve it. Hoare Lea, GI Energy, mechanical and electrical contractor Crown
House Technologies, and its controls specialist Matrix, returned to the table to resolve the problems one by one.
The first issue was that three of the heat pump compressors and three of the actuators on the valves in the sliding header failed. Such early-life equipment failures could just as easily have befallen a conventional system, and the faulty components were replaced free of charge. Yet this did not help the client gain confidence in what was, for them, an unfamiliar system.
While reviewing some of the metering data, Hoare Lea noticed that there was a large mismatch between the heat meters on the primary and secondary circuits. Further investigations revealed that one of the flow meters had seized and there had been inconsistencies in the way the meters had been "zeroed" at the time of handover. There was also a problem with the algorithm that allocated the heat pump electricity use to either heating or cooling.
However, the most serious problems for the department were those connected with the control systems. The GSES control system was designed to identify when the heat pumps were unable to meet the heating or cooling load alone and to send a signal to the main BMS requesting assistance from the building's boilers or chillers.
This had appeared to work during initial commissioning - but, in practice, there were times when these signals did not appear to be getting through. The result was a disruptive loss of temperature control in the critical labs and IT rooms. It was at this stage that the department took the decision to turn the GSES off and rely on the conventional systems.
By April 2012, all these issues appeared to be resolved. Some additional tweaks to the control software were also made to improve efficiency, but a further period of close monitoring was needed to ensure the problems had been rectified.
Since this date, the system has operated without any significant fault, and has actually been shown to deliver greater carbon savings than originally predicted. While on the face of it this seems a positive result, in fact part of the reason for this is that the annual heating and cooling loads are significantly higher than predicted.
This is, perhaps, unsurprising in a building with variable volume laboratory ventilation systems, the use of which is largely occupant-dependent and almost impossible to predict. It does however raise interesting questions about the specification of GSESs and the difficulty in establishing a criterion at design stage against which performance can be measured.
The key lesson from the project is that a significant amount of effort is necessary to ensure that the potential savings of LZC technologies are realised. At OUES, it was fortunate that the client and the design and construction team had a long-standing relationship and shared a commitment to proving the system's reliability and performance. It is easy to imagine many buildings with LZC systems installed where there is not the will or the resources to resolve the issues and where, instead, the systems are turned off and forgotten.
The carbon reduction performance of LZC systems is, quite rightly, placed under the microscope. These systems should be made to justify the claims made on their behalf. Equally, it must be accepted that optimum performance is unlikely to be achieved without effort. If we are serious about closing the "performance gap" we must allocate time and resources to finetuning during the early months and years of operation. CJ
When installing the ground loop, the designers were limited to the footprint of the building, writes Roger Macklin of GI Energy. This constrained the GSES's total energy exchange capacity and peak output, so conventional heating and cooling systems were installed as part of the original design. This led to the development of a unique control philosophy in which the GSES could choose whether to provide heating or cooling on the basis of optimising carbon savings, rather than to satisfy a specific load.
The overall conditioning is, therefore, led by the GSES control system, which selects the technology to use. GI Energy modelled the performance of the selected heat pumps to identify, for a given ground-loop temperature, how carbon savings would be maximised, taking into account the anticipated performance of the conventional systems. Further development of the control system added stability and incorporated detailed hourly performance monitoring, observable from GI Energy's office.
Our first major GSES has provided us with a series of valuable lessons in managing low-carbon technology, writes Steve Pearson, head of building services at the University of Oxford's Estates Services. Thermal models are fundamental to GSES design, but it is all too easy to trust the output of a model without understanding the assumptions on which they are built, and how they affect performance.
It is all the more important for a GSES because the performance in any given period is heavily dependent on how the system was used in the preceding period.
There is a requirement for sensitivity analysis on our next project and a "sign off" of the model assumptions. Getting the control systems communicating effectively and the metering system accurate was a struggle.
It would be naive to think that all of this can be perfect before handover as the system cannot be optimised without in-use data.
Much has been talked of "soft landings" but if ever there was a system that required attention post-PC it is GSES. We are fortunate that we were able to devote this attention to it and now have a GSES that is performing significantly better than the specification.
- Client: Estates Services, University of Oxford; University of Oxford Department of Earth Sciences
- M&E consulting engineer: Hoare Lea
- Ground-source energy system designer and installer: GI Energy
- Main contractor: Laing O'Rourke
- M&E contractor: Crown House Technologies
- Project manager: RBDML
- Architect: Wilkinson Eyre Architects
- Cost consultant: EC Harris
- Civil/structural engineer: Pell Frischmann