The results of a seven month trial just published by Mitsubishi
Electric have demonstrated an average Coefficient of Performance
(COP) of 6.27 for a ground source heat pump air conditioning system
and an impressive COP of 7.14 when the efficiency is maximised
through heat recovery during the months when there is a mixed demand
for heating and cooling.
The ground source system was designed to provide heating and cooling to the Hoddesdon headquarters of civil engineering and building contractor, VolkerFitzpatrick. It was installed by sustainable energy solutions provider, Cool Planet in conjunction with VolkerFitzpatrick and incorporates seven Mitsubishi Electric City Multi WR2 condensing units in conjunction with a closed loop ground array of 36 bore holes. These were arranged at a spacing of 5.5 metres and at a depth of 100 metres.
VolkerFitzpatrick designed its new building to replace outdated facilities, enhance the company’s corporate identity and showcase its sustainable credentials. The company’s Building Division carried out the design and build of the new office space, with the equipment installed to provide 100% of the heating and cooling demand for the building. VolkerFitzpatrick’s in-house building services department specified a range of energy efficient solutions to suit the overall building design.
A ground source heat recovery system was installed in conjunction with other technologies including PQFY heat pump boilers, solar thermal hot water preheating, an Ecodan® hot water generator, ground air technology, low energy light fittings, daylight saving lighting controls and photo-voltaic brise soleil.
Cool Planet was contracted by VolkerFitzpatrick to design and install a closed loop geothermal system to provide 100% of the heating and cooling load for its new corporate headquarters building. The location on the outskirts of Hoddesdon in Hertfordshire and the amount of surrounding land made it ideal for a ground source heat pump solution.
“First we carried out a feasibility study to determine the scale of the ground works required based on indicative peak heating and cooling loads,” explained Cool Planet’s Steve Gray. “We also carried out a thermal model of the building to ensure the correct sizing of the system to minimise energy consumption and capital costs.”
Both trench and vertical closed loop boreholes were considered, with boreholes preferred because they could be confined to particular areas of the site and in comparison to their trench counterparts impinged less on the construction works. In addition, there is a more consistent COP due to more stable ground temperatures.
“This was the regeneration of a brownfield plant compound and VolkerFitzpatrick’s plan was to use realistic and practical low energy design to reduce carbon emissions, both in construction and in use,” added Gray. “The pioneering three-storey office design had to utilise current best practice and sensible sustainable solutions with the company aiming to achieve a BREEAM Excellent rating.”
From the seven months of analysis it is clear that the ground source system was highly efficient and capable of dealing with the building’s heating and cooling requirements in an environmentally friendly way. It is also clear that system efficiency is maximised when heat recovery takes place during months when there was a mixed demand for heating and cooling. However, even when heat recovery was not being used, the ground source system was still able to produce very high COP’s.
“When we compare the results with alternative technologies and contrast expected efficiencies we estimated a 46% reduction in running costs compared to a chiller and gas boiler system as well as a 53% reduction in CO2 emissions,” added Gray. “Taking this a step further, the cost per m2 to heat and cool the building using the ground source system over the 7 months was Ł1.29/m2.
From the results it is clear that the building is actually performing better than envisaged at design stage, highlighting the efficiency of the Mitsubishi Electric ground source units and therefore enabling VolkerFitzpatrick to heat and cool their building in a highly environmentally friendly manner.
“We had anticipated monthly COP’s of between 4.0 and 6.0 at the design stage and the resulting 5.54 and 7.14 are a significant improvement which demonstrates the advantages of a heat recovery system,” said Gray.