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Post-project monitoring proves worth for hybrid heat pump facility

The new Earth Sciences Building at the University of Oxford used ground-source heat pumps backed by conventional systems, but post-occupancy monitoring was vital in understanding the system’s true performance.

When the team behind the University of Oxford’s Department of Earth Sciences began planning its new building, it wanted ground-source heat pumps to play a major role in heating and cooling it.

The department places an emphasis on understanding the fundamental principles of geological processes, so it was only natural that its new home would interact closely with the ground.

The £38m development, commissioned in 2010 and opened in May 2011, provides the department with a new teaching and research facility, with the space to accommodate 400 students and staff. Prior to this, it had been housed in a mixture of buildings unsuitable for modern-day scientific research.

Client drive to use ground-source

Hoare Lea acted as consultant on the project, and had worked with the university before.

“We built a temporary home for the Department of Earth Sciences before, but they really needed new facilities to match the level of research they were doing,” says Ian Durbin, partner at Hoare Lea.

GI Energy specialises in the installation of ground-source heat pumps and was approached by the department and Hoare Lea to work on the new building.

“The faculty spoke to us as they were considering using ground-source due to the nature of their research and the fact they wanted the building to be as sustainable as possible,” says GI Energy managing director Karl Drage.

But before it won the contract, the firm’s geology credentials were tested.

“The final tender interview we had was with the head of the department and he quizzed us on geology and our capabilities,” Mr Drage explains. “We passed the exam and won the contract.”

Installing ground-source heat pumps

Laing O’Rourke was appointed principal contractor for the project. The building includes a number of metal-free ‘clean’ laboratories, lecture theatres, conference rooms, offices and a library.

Before work on this could begin, however, the ground-source heat pumps were installed. A total of 63 boreholes were drilled down to a depth of 64 m to house the pumps.

Ground-source heat pumps work in a similar way to a household refrigerator, Mr Drage explains, except on a much larger scale.

“Ground-source heat pumps work just like your fridge at home, but on a larger scale”

Karl Drage, GI Energy

“Your fridge at home reduces the temperature of what’s inside and then displaces the heat from those objects into the space behind it,” he says. “This raises the temperature behind the fridge by a very small, usually indiscernible, amount.

“The ground-source heat pumps do the same: they refrigerate a large area of ground by a very small amount and then move the displaced heat into the building where it is expelled via fan coils, radiators, underfloor heating or other methods.”

The system works the other way, too, providing either heating or cooling depending on what is required.

Site challenges prompt hybrid approach

The constraints of the site soon threw up several challenges for the project team. “One of the peculiarities of the site was the presence of a high-pressure artesian aquifer underneath,” Mr Drage says.

“This was an additional factor to consider on top of everything else and meant that, were we to drill down as normal, we would have had to manage construction very carefully and then plug the well afterwards.”

The firm consulted the Environment Agency, and after careful consideration decided to avoid penetrating the aquifer.

“This meant we could only drill down 64 m, whereas we usually go down to around 100 m,” Mr Drage says. “This immediately limited the heat exchange available.”

“We could not drill down as far as we usually would, which immediately limited the heat exchange available”

Karl Drage, GI Energy

The second problem was down to the size of the site itself. The building was constructed on a confined site within the university’s Science Area, in close proximity to a number of listed buildings.

Boreholes are often sunk horizontally for ground-source heat pumps but there was not enough room for that in this instance, so the team drilled vertically instead. The reduced depth also meant the system couldn’t support peak heating and cooling.

“We worked closely with the university, the consultant Hoare Lea and the contractor Laing O’Rourke to develop an optimised solution,” Mr Drage says. This took the form of a hybrid system, where the ground-source heat pumps are backed up by gas boilers and conventional cooling methods.

Mixing ground-source and conventional systems

Even though the system is a hybrid, GI Energy’s brief was to provide as much heating and cooling as possible using ground-source.

“The building’s requirements are quite complex, and the laboratories inside mean that the building requires both heating and cooling all year round,” Mr Drage says.

The system installed by GI Energy monitors the building’s current energy demands at any one time and looks at how to meet them in the most carbon-efficient way possible.

This control unit is computerised and connected to the building remotely, and can switch each of the three heat pumps individually between heating and cooling, as well as telling the building management system when to switch on the conventional back-ups.

“The tie-in between the ground source and the building’s internal system was very complex,” says Mr Durbin.

“You choose to optimise the system for carbon savings or monetary savings; unusually, the university chose maximum carbon savings”

Karl Drage, GI Energy

There is a choice with the system about whether to optimise it for carbon savings or monetary savings, as Mr Drage explains: “Depending on which you choose, the balance between ground-source and conventional heating changes. Unusually, the university opted for maximum carbon savings.”

The efficiency of the heat pumps can change depending on the condition of the ground, too. If the ground is particularly hot then cooling will be more efficient for the ground-source heat pumps, and any heating requirements may be met through more conventional methods instead to maximise carbon efficiency.

Post-occupancy evaluation vital to understanding

Following practical completion on the building, it seemed initially that the ground-source system was performing inefficiently.

“The amount of energy being used was a lot higher than anticipated in the design,” Mr Drage says. “But the client was very proactive.

“They were intent on ensuring that what we had demonstrated the system could do was correct, so they contracted us for the environmental conditioning of the building.”

In the end, the team discovered that the building’s heating and cooling demands were far higher than the design suggested: 100 MWhr of heat per week on average, compared with the original estimate of 50 MWhr.

The cooling load average was also higher at 25 to 35 MWhr per week – well above the initial estimate of 5 to 10 MWhr.

“If you didn’t know the building was operating out of bounds then our system looked inefficient. It took 12 months and lots of analysis to conclude this”

Karl Drage, GI Energy

A full year’s-worth of data showed that, for ever kW of power used, either 3.6 kW of heating or 4.7 kW of cooling was produced.

If only a conventional heating system was used, the amount of power used for the same heating and cooling would have been far higher. CO2 emissions are also 120 tonnes a year lower than anticipated – a 17 per cent reduction.

“It took 12 months to conclude that the building was operating out of bounds and that the system was operating well,” Mr Drage explains. “If you didn’t know this then it would’ve seemed that the ground-source was operating inefficiently.”

GI Energy encourages remote monitoring and operation of its ground-source heat systems, and Mr Drage believes clients should engage with the system installers after the project is finished.

“A building doesn’t get into ‘normal’ mode for some time after it has been finished,” he says. “Now, if the building is right, we even offer to fund the initial installation and then take an annual fee from the client instead.

“The fee should be covered by the savings generated by the system, and we’re seeing uptake for this option.

Mr Durbin believes that the project provided some important lessons for his firm and others.

“It proved to us all that buildings with fairly complex technology would take time to settle down,” he explains. “It’s not a position of weakness to say that.”

“For me, this was one of the most interesting projects of the last five years and proves that as the complexity of building increases, it will take time for them to bed in and work properly.”

Beyond just ground-source

The building was constructed using a passive design, with laboratories separated from the offices by an atrium ‘hinge’ so that the less highly serviced spaces are naturally ventilated.

The thermal mass of the building is used for cooling in summer, with solar gain reduced through user-controlled external blinds.

Recycled rainwater is being used for flushing toilets and a green roof was also installed. Low-energy lighting was used throughout, and external blinds on the facade can be moved as required.

During the construction process, a post-tensioned concrete slab system was used, which reduced the volume of concrete by 10 per cent; ground granulated blast-furnaced slag cement replacement was also selected for the structural elements.

Columns, stairs and cladding were precast offsite to minimise waste and improve the programme delivery times.

Laing O’Rourke collaborated with the professor of stratigraphy on the building’s façade, working with him to select the most durable stone.

The final design contained geometric bands of Clipsham, Jura and Purbeck. This created a ‘narrative wall’ to frame the entrance, with the idea being to advertise the nature of the faculty’s work to those viewing the facility from outside.

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