GROUND SOURCE HEAT PUMPS CASE STUDY
used by the heat pumps and ground-loop circulation pump and uses an algorithm to apportion this load between heating and cooling.
Oxford, we have a problem… 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
Schematic diagram showing the heating and cooling systems to each wing of the building.
CO2 savings generated by the
GSES before and after remedial/ optimisation works
100,000 80,000 60,000 40,000 20,000 0
the contribution from the area of ground available under the building. The result is a system that comprises
72,800 -67%
Heating Cooling
23,700 8,300 Estimated optimisation works were complete
100,000 80,000 60,000 40,000 20,000 0
23.5% 59,094
Heating Cooling
73,423
6,600 Actual
Actual CO2 savings generated by the GSES, compared to predicted, in a six-month period before the remedial/
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
15,665 Estimated remedial/optimisation work 18,881 Actual
Actual CO2 savings generated by the GSES, compared to the predicted, in the 8½ months after completion of the
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
38 CIBSE Journal August 2013
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.
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