Harvesting heat from below: Groundwater's secret power

Thursday, November 2, 2023

Energy control centre for the Quartier Champagne district in Biel uses new low-GWP compressors from Bock

With its new semi-hermetic HG compressors, Bock offers the right solution for environmentally friendly HFO refrigerants. The compressors, which are marked with the suffix LG (for Low-GWP), are available from the autumn of 2020. They have proved their suitability for operation with HFO refrigerants in a field test in the control centre of the local heat supply for the Quartier Champagne district of Biel.

Resource-saving and environmentally friendly

“When selecting the heat pumps, the operators placed great emphasis not only on low power consumption but also on environmentally friendly technology,” explains Claudio Müller. He is the Head of Technology Air Conditioning/Refrigeration at CTA AG (Münsingen) and, together with his team, he planned and installed the heat pumps. They chose R1234ze, a hydrofluoroolefin (HFO), as the refrigerant. “This refrigerant has an ozone depletion potential of zero and a low warming potential of seven. It is an environmentally friendly alternative to previously used conventional refrigerants such as R134a, and we have already experienced good results with it on other projects.”

So that the heat pumps can cover the greater part of the heating requirement, each of them offers a capacity of up to 650 kW. This is sufficient to provide four fifths of the heat quantity, which the network consumers require on average. It is only on very cold winter days, that a natural gas boiler (approx. 1000 kW) has to cover the peak load. When compared to heating with natural gas, the heat pumps provide a reduction in CO2 output of some 620 tons per year.

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Heat pumps with Bock compressors in the energy control centre for the Quartier Champagne district extract heat from the ground water, thereby providing heating energy for dwellings, local shops and a school. (Picture: ESB)

Energy city Biel commits to reducing its own energy consumption

Biel (Switzerland) has been an Energy City since 2008, and it has committed itself to promoting renewable energy and reducing its own energy consumption. This also includes solutions for environmentally friendly heating - such as the Quartier Champagne Heating Network, which has been in operation since 2018. The operators, Energie Service Biel (ESB) and BKW/AEK Contracting AG (BAC), offer with this new heating network a very ecological solution for providing heat to an area covering some 10 hectares. The energy for heating comes primarily from ground water, which is abundantly available there from a depth of around 16 metres.

In order to provide the heat from the water, up to 6000 litres per minute flow into an underground control centre. There two heat pumps extract thermal energy from the water at 10-12 °C and raise the temperature to a very usable 50-63 °C. The water then cools down by max. 4 K before being fed back again into the ground.

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Two heat pumps, each with four Bock HG88e LG compressors, provide up to 650 kW each and deliver around 80 % of the heat quantity, which the network consumers require. (Picture: ESB)

The heart of the system are semi-hermetic compressors from Bock

The heart of the heat pumps are four HG88e semi-hermetic, reciprocating compressors per heat pump, all manufactured by Bock in Frickenhausen. The high-performance eight-cylinder compressors are designed for 4 °C evaporation temperature and 57-64 °C condensing temperature, so that heat up to 63 °C can be provided in the local heating network. (Higher temperatures up to 75 °C can be achieved by using the natural gas boiler, or by reheating in the objects supplied – for example for sanitising the particular hot water network at over 70 °C.)

The Bock compressors were equipped by CTA with frequency converters for adjusting the output, so that they can be used incrementally from partial load operation to peak load at a high level of efficiency. The COP (Coefficient of Performance) of the heat pumps is 3.45, which is a very good value for this temperature lift. “The efficiency level and COP achieved correspond roughly to that of a heat pump with conventional refrigerants, except that in this case the chosen refrigerant has a very much lower global warming potential.”

Optimised for use with HFO refrigerants

Since the refrigerant affects the viscosity of the oil and therefore also has an effect on lubrication, the designers at Bock have optimised the driving gear. These measures improve the emergency running properties and increase the service life of the compressor, and they have been very successful. “The compressors have operated faultlessly from the start and, since the heat pumps cover the base load, they have already completed a large number of operating hours,” says Claudio Müller. “We have of course paid particular attention to the degradation of the oil, so that we could get an indication of the state of the compressors,” he adds. “But the condition of the oil has never given us any grounds for concern.” In order to be able to always monitor the oil, CTA also ordered oil pressure and oil temperature monitors in addition to the oil sump heater, which was recommended by Bock.

Low-GWP compressors are ready for introduction to the market

The successful field test of the modified, semi-hermetic compressors is good reason for Bock to offer them as a full-scale product from the autumn of 2020. The relevant models of the HG series bear the abbreviation "LG" at the end of the type designation, and this stands for Low-GWP (refrigerant with a GWP below 150). The new HG compressors will soon perhaps be installed in a third heat pump in the energy control centre for the Quartier Champagne district. Space is already provided, since in the coming years ESB and BAC want to gain new customers for their heating network.