Ground source heat pumps

Annex 8 of the IEA Geothermal Implementing Agreement57 reports on geothermal (ground source) heat pumps (GHPs) in detail, linked with other issues relating to the direct use of geothermal heat. Small-to medium-scale GHPs can be used virtually anywhere for both heat supply in winter and cooling in summer (Box C) as well as to provide hot water. They use the heat storage capacity of the ground as an earth-heat sink since the temperature at depths between 15 and 200 m remains fairly constant all year round at around 12 to 14oC. Vertical bores enable heat to be drawn out in the winter and brought to the necessary temperature by a heat pump. The ground normally cools to below 10oC as a result. Circulating water in summer is initially sufficient to provide the desired cooling with the heat pump being operated in reverse if more cooling is later required. The ground stores the extracted heat, rising to an average temperature of around 20oC. The cost of drilling bores remains a high proportion of the total so shallow horizontal pipes around 1-2 m depth can be an alternative system but with lower efficiencies.

A large-scale GHP project is at the Pfizer GmbH factory in Freiburg where 19 double-U geothermic probes have been installed at 6 m intervals over an area of 0.12 ha in 15 cm diameter boreholes drilled to around 130 m depth. After the pipe probes were inserted, the holes were refilled with a bentonite mortar solution to prevent connection of the probes with any water-conducting fissures. The total heating capacity is 135 kW and cooling, 100 kW, with the compressor having a coefficient of performance (COP) of well over 4. After one season, and following extensive energy efficiency measures, it is claimed that cost savings were around USD 440 000 per year compared with conventional heating and cooling systems (gas and electricity) and 1 200 tonnes of CO2 emissions were avoided.58

Other examples include a large project at Zurich Airport, with 58 000 m2 of floor area heated (2 120 MWh/yr heating load) and cooled (1 240 MWh/yr) using 300 geothermal energy piles drilled to 30 m depth, and the renovated Hotel Dolder in Zurich, which uses 72 borehole heat exchangers installed beneath the spa at 150 m depth for heating, cooling and domestic hot water (1000 MWh/yr). The largest GHP district heating scheme is being developed in Milan, Italy that will provide heating for some 250 000 inhabitants (Sparacino, et al , 2007).

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Box C * Ground Source Heat Pump, Three Rivers District Council, Rickmansworth, England

Three Rivers Council, servicing a population of 180 000, has its offices located in Rickmansworth, Hertfordshire to the north west of London just inside the M25 motorway. The three-storey council building constructed in 1990 has public meeting rooms on the top floor with the main room where council meetings are held accommodating up to 150 people. This room is served by a mechanical ventilation system comprising an air handling system to provide cooling, tempered air heating, and humidification. Energy saving measures are provided by a plate heat exchanger and motorised recirculation dampers. Air quality is constantly monitored to maintain optimum air supply into the room space.

Due to the age and reliability of the air condensing units, and the R22 refrigerant used (which is to be banned by the European Union by 2010), replacement of the units was investigated. After rejecting maintaining the current system or replacing it with a similar new system, advice was sought from the Ground Source Heat Pump Association and various heat pump manufacturers whether to install a cooling-only ground source heat pump (GHP) or a reverse cycle GHP system to provide both cooling and heating to the public room. The latter was chosen and installed, thereby integrating a new renewable energy technology with the existing engineering installation in the building.

Since the small garden area in front of the council offices was in need of an upgrade, the opportunity was taken to use this area to install the ground heat energy transfer system. The heat loss and heat gain required for the meeting room were calculated and gave the basis for the design of a horizontal, closed loop pipe acting as the heat exchanger. A 300 m snaking trench, 2 m deep and 1.6 m wide was dug to accommodate the polyethylene pipe that was then connected to an inverter-controlled circulation pump. The pipe was filled with a water glycol solution and a Mitsubishi WR2 (25 kW heating and 22 kW cooling capacity) heat pump was connected between the ground source and the existing air handling unit supply duct. To achieve optimum performance from the unit, the pressure and extractor fan motors were replaced with inverter-driven units to match the supply air with the air conditioning requirements. The GHP system was integrated into the building management system for optimum control and monitoring is to be conducted on-line to measure the energy consumption. Since commissioning in early summer 2009, the cooling objectives have been successfully met, but the cost effectiveness has not been analysed to date.

Millions of domestic scale GHPs have been installed (IEA, 2007a). Sweden leads the way with almost half a million, as well as around 600 large-scale installations for district heating schemes.59 Around 70% of new houses in Sweden and 30% in Switzwerland have a GHP installed.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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