Ground Source Heat Pumps

1. INTRODUCTION
Ground source heat pumps (GSHP) gain importance world-wide with respect to energy efficiency in heating and cooling operation. The ground acting as a seasonal store offers the possibility of damping the effects of the outside air temperature fluctuations, in colder climates it enables monovalent heating operation of the heat pump, and for utilities it is – compared with outside air operated heat pumps – a tool for demand side management measures.

In the last decades, heat pumps have acquired fundamental shares in markets such as Japan and the United States; the market development in Europe started later. Since the Eighties heat pump units and the components used like advanced compressors and flat plate heat exchangers, respectively, have been improved significantly. The development of heat source systems and heat sink systems on the one side and the system approach on the other side took much more time, and the development in the direction of highly efficient systems is still going on. The first heat sink /heat source for cooling/heating operation was outside air, and outside air is still dominating heat pump systems for both heating and cooling. However, in the case of continental climates outside air has disadvantages. The characteristics of ground water are completely different compared with outside air; ground water has, depending of the depth of the groundwater table, a more or less constant temperature, which means that heat pumps for cooling as well as heating can be operated at much better heat source/heat sink conditions. The problem is the limitation of the availability. A heat sink/heat source, which is, similar to outside air, almost not limited by availability, is the ground.

2. APPLICATION OF GROUND SOURCE HEAT PUMPS
Ground-source heat pumps can be applied for different climates, different ground properties, for small and large systems, and for heating-only as well as heating and cooling applications.

Climate: The climate has a strong influence on the ground temperature available and on the operating conditions of a heat pump systems (cold, moderate and hot climate, hot an humid climate, oceanic climate with small temperature fluctuations or continental climate with large temperature fluctuations).

Ground properties: Ground properties are responsible for the type of ground utilisation and the heat disposal/heat extraction method, i.e. open or closed loop system. In the case of a closed loop system they are also responsible for the ground heat exchanger type used.

Small systems: The common characteristic of small systems is natural ground recovery, mainly by solar radiation and precipitation collected by the ground surface. Small systems are in use for heating as well as heating and cooling, they can be used, depending on the climate and the distribution system, for direct cooling (without heat pump operation), at least at the beginning of the cooling season.

Large systems: For large system recovery of the ground has to happen by heat removal and heat extraction. Sometimes additional systems for recharging the store have to be provided. Heat removal can happen by direct cooling (without heat pump operation) and active cooling (with heat pump operation).

3. MATRIX OF GROUND-SOURCE HEAT PUMPS
Ground source heat pumps cover a variety of heat source/heat sink systems:
• Ground-coupled (heat is directly extracted from/removed to the ground)
• Groundwater (heat is extracted from/removed to groundwater)
• Surface water (heat is extracted from/removed to rivers, ponds, lakes, or the sea) Ground source heat pumps (GSHPs) provide heating, cooling and hot water.
• The ground dampens temperature variations and leads to high GSHP efficiencies
• The initial costs of GSHP systems are most commonly higher, but operating and maintenance costs are lower (Climates requiring heating and cooling are most promising)

A ground-coupled heat pump uses the shallow ground as a source of heat, thus taking advantage of its seasonally moderate temperatures. In the summer, the process can be reversed so the heat pump extracts heat from the building and transfers it to the ground. Transferring heat to a cooler space takes less energy, so the cooling efficiency of the heat pump gain benefits from the lower ground temperatures. Shallow horizontal heat exchangers experience seasonal temperature cycles due to solar gains and transmission losses to ambient air at ground level. These temperature cycles lag behind the seasons because of thermal inertia, so the heat exchanger can harvest heat deposited by the sun several months earlier. Deep vertical systems rely heavily on migration of heat from surrounding geology. In the case of heating-only operation recharging of the ground has to happen by natural effects, in the case of heating and cooling operation recharging can happen by the exhaust heat from cooling operation; a possible mismatch between heat extraction and heat removal can happen by natural effects. Ground-coupled heat pumps must have a heat exchanger in contact with the ground to extract or dissipate heat. This component accounts for a third to a half of the total system cost. Several major design options are available for these, which are classified by fluid and layout:

• closed loop direct exchange systems (circulate refrigerant underground)
• closed loop systems (use most commonly a mixture of anti-freeze and water)
• open loop systems

3.1 Closed Loop Systems
The direct exchange ground-coupled heat pump (also called direct expansion, direct evaporation ground-coupled heat pump) is the oldest type. The ground-coupling is achieved through a loop circulating refrigerant in direct thermal contact with the ground. Direct exchange systems are slightly more efficient and have potentially lower installation costs than closed loop water systems. The main reasons for the higher efficiency are the elimination of the secondary fluid circulation pump (which uses electricity), and the elimination of the water heat exchanger (which is a source of thermal and temperature losses). Most installed closed loop systems have two loops on the ground side: the primary refrigerant loop is contained in the appliance cabinet where it exchanges heat with a secondary water and water-antifreeze, respectively, loop that is buried underground. A horizontal closed loop field is composed of pipes that run horizontally in the ground. A long horizontal trench, deeper than the frost line, is dug and U-shaped or slinky coils are placed horizontally inside the same trench. Excavation for horizontal loop fields is significantly cheaper than vertical drilling, so this is the most common layout used wherever there is adequate land available.

A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground, typically 20–120 (240) m deep. Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole. The borehole is commonly filled with a bentonite grout, thermally enhanced grouts are available to improve this heat transfer. Vertical loop fields are typically used when there is a limited area of land available. Bore holes are spaced 5–6 m apart and the depth depends on ground and building characteristics.

3.2 Open Loop Systems
In an open loop system (also called a groundwater heat pump), the secondary loop pumps natural water from a well or body of water into a heat exchanger inside the heat pump. Heat is either extracted or added by the primary refrigerant loop, and the water is returned to a separate injection well. Irrigation trench, tile field or body of water are very often prohibited by law. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source. Since the water chemistry is not controlled, the appliance may need to be protected from corrosion by using special metals in the heat exchanger and pump. Limescale may foul the system over time and require periodic acid cleaning. If the water contains high levels of salt, minerals or hydrogen sulfide, a closed loop system is usually preferable. Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling. Open loop systems using ground water are usually more efficient than closed systems because they are better coupled with ground temperatures. Closed loop systems, in comparison, have to transfer heat across extra layers of pipe wall and dirt. A standing column well system is a specialized type of open loop system. Water is drawn from the bottom of a deep rock well, passed through a heat pump, and returned to the top of the well, where travelling downwards it exchanges heat with the surrounding bedrock.

3.3 Seasonal Thermal Energy Storage
The efficiency of ground source heat pumps can be improved by designing a seasonal thermal storage. If heat loss from the ground source is sufficiently low, the heat pumped out of the building in the summer can be retrieved in the winter. Heat storage efficiency increases with scale, so this advantage is most significant in commercial or district heating systems. Possibilities for a seasonal thermal store are:

• Heating and cooling operation with a balanced heat extraction/heat removal into the store or
• A hybrid heating and cooling system where the balance is achieved by additional cooling of the store by a cooling tower ore additional charging of the store by solar energy.

The seasonal thermal storage can be formed as aquifer thermal energy store, multiple standing column well systems, borehole thermal energy storage in the ground, or using the building foundation as a storage.

Aquifer Thermal Energy Stores: An aquifer is an underground layer of permeable rock, sediment (usually sand or gravel), or soil that yields water. The pore spaces in aquifers are filled with water and are interconnected, so that water can flow through them (Fig. 4).

Sandstones, unconsolidated gravels, and porous limestones make the best aquifers. Two wells (typically) on either side with hydraulic coupling are necessary. One well is for the warm water and the other one is for the cold. In winter water is extracted from the warm side and removed at the cold side, in summer the process is reversed. The only problem is that this system can only be used in areas with no or negligible natural water flow.

Borehole Energy Stores: In such a case the store is made accessible by boreholes. The store is used as heat source when operating in heating mode, with a fluid (usually water or a water–antifreeze mixture) as the medium that transfers the heat from the store to the evaporator of the heat pump, thus utilizing geothermal energy. In cooling mode, the store is used as a heat sink. With Borehole Heat Exchangers (BHE), ground-source heat pumps can offer both heating and cooling at virtually any location, with great flexibility to meet any demands.

The building foundation as energy store:High-rise buildings require proper foundations, often consisting of a bed plate and piles, and such piles can form a store similar to a borehole store (Halozan 2010): but this store is more or less for free: only the piles or membrane walls have to be equipped with coils and connected to a header (see Fig. 5). All these stores work properly if heat extraction and heat removal are more or less balanced; if not the store will get through the years an increasing or decreasing temperature, which means disadvantages in heating or cooling operation. In such a case a hybrid system has to be designed with an additional heat source/heat sink (i.e. cooling tower, solar thermal system).

4. CONCLUSIONS
The following conclusions can be drawn:

• Ground-source heat pumps are presently dominating the heating-only heat pump market in Europe, they have been also identified as energy efficient solution for the heating and cooling market in North America, but also Japan and China.

• New developments like variable-speed heat pump units or heat pumps combined with heat pipe based vertical probes with CO2 as heat carrier show that there is still room for new ideas for being competitive and successful in the future.

• Direct-exchange ground-source heat pumps already achieve SPFs between 4 and 6 !, if building standards are kept and the overall system design has been made carefully.

• The choice of refrigerants presently in use – R-407C, R-410A and propane, for large units additionally R-134a and ammonia, is motivated by efficiency, reliability, environmental considerations, safety and regulations.

• Air conditioning systems have the task to compensate external and internal loads and to provide hygienic conditions and year round comfort for the customers. By means of heat pumps additionally shifting heat from spaces, which have to be cooled to spaces, which have to be heated at the same time is possible.

• Using the ground and/or the foundation as a store heat and cold can be stored and used for direct cooling, and for increasing the heat source temperature for heating; with low-ex systems these effects can be further increased.

• Besides new buildings ground-source heat pumps offer the possibility to retrofit existing buildings from energy wasting to highly efficient systems.

The potential for reducing world-wide CO2 emissions assuming a 30% share of heat pumps in the building sector using technology presently available is about 6% of the total world-wide CO2 emissions. With advanced future technologies in power generation, in heat pumps and in integrated control strategies up to 16% seem to be possible (Gilli and Halozan, 2001). Therefore, heat pumps, and especially ground-source heat pumps, are one of the key technologies for energy conservation, increasing the efficiency, increasing the share of renewables and reducing CO2 emissions.

REFERENCES
1. Gilli, P.V., Halozan, H. (2001), Heat Pumps for Different World Regions – Now and in the Future, Proc. 18th WEC Congress, Buenos Aires, Argentina.
2. Halozan, H. (2010) Limits of Heat Pumps in LowEx Design, Proceedings ECBCS Annex 49 Conference The Future for Sustainable Built Environments with High Performance Energy Systems, 19th – 21st October 2010, Munich, Germany
3. IEA HPC (2010), Annex 29 Ground Source Heat Pumps – Overcoming Market and Technical Barriers, IEA HPC, Sittard, Netherlands, 2010