Renewable energy sources have one thing in common; they all existed before man appeared on this planet. Wind, wave, hydro, solar, geothermal and tidal power are all forces of nature and are mostly intermittent energy sources, geothermal is the only consistent phenomenon. Geothermal renewable energy sources where probably the first to be fully utilised by man. Early civilisations tapped this heat to cook, fire clay pottery, create baths and spas and even heat their homes. Roman villas had under floor heating from natural hot springs over 2000 years ago.

Shallow geothermal resources (<400 m depth by governmental definition in several countries) are omnipresent. Below 15-20 m depth, everything is geothermal. Figure 1 show a summary of the soil thermal properties.

Figure 1: Measured thermal properties for different soils

The temperature difference between the ground and the fluid in the ground heat exchanger drives the heat transfer so it is important to determine the ground temperature. The temperature field is governed by terrestrial heat flow and the local ground thermal conductivity structure (groundwater flow). In some countries, all energy stored in form of heat beneath the earth surface is per definition perceived as geothermal energy. The same approach is used in North America. The ubiquitous heat content of shallow resources can be made accessible either by extraction of groundwater or, more frequent, by artificial circulation like the borehole heat exchanger (BHE) system. This means, the heat extraction occurs–in most cases–by pure conduction; there is no formation fluids required. The most popular BHE heating system with one of more boreholes typically 50-200 m deep is a closed circuit, heat pump coupled system, ideally suited to supply heat to smaller, de-central objects like single family or multi-family dwellings (Figure 2).

Figure 2. Typical application of a borehole heat exchanger (BHE) heat pump system in a central European home, typical BHE length = 100 m

The heat exchangers (mostly double U-tube plastic pipes in grouted boreholes) work efficiently in nearly all kinds of geologic media (except in material with low thermal conductivity like dry sand or dry gravel). This means to tap the ground as a shallow heat source comprise:

  • Groundwater wells (“open” systems),
  • Borehole heat exchangers (BHE),
  • Horizontal heat exchanger pipes (including compact systems with trenches, spirals, etc.), and
  • “Geo-structures” (foundation piles equipped with heat exchangers).

A common feature of these ground-coupled systems is a heat pump, attached to a low-temperature heating system like floor panels/slab heating. They are all termed “ground-source heat pumps” (GSHP) systems. In general, these systems can be tailored in a highly flexible way to meet locally varying demands. Experimental and theoretical investigations (field measurement campaigns and numerical model simulations) have been conducted over several years to elaborate a solid base for the design and for performance evaluation of BHE systems. While in the 80s, theoretical thermal analysis of BHE systems prevailed in Sweden monitoring and simulation was done in Switzerland, and measurements of heat transport in the ground were made on a test site in Germany.

In the German test system at Schöffengrund-Schwalbach near Frankfurt/Main, a 50-m BHE was surrounded by a total of 9 monitoring boreholes at 2.5, 5 and 10 m distance, also 50 m deep. Temperatures in each hole and at the BHE itself were measured with 24 sensors at 2 m vertical distance, resulting in a total of 240 observation locations in the underground. This layout allowed investigating the temperature distribution in the vicinity of the BHE. The influence from the surface is visible in the uppermost approximately 10 m (Figure 1), as well as the temperature decrease around the BHE at the end of the heating season. Measurements from this system were used to validate a numerical model for convective and conductive heat transport in the ground. Starting in 1986, an extensive measurement campaign has been performed at a commercially delivered BHE installation in Elgg near Zurich. The object of the campaigns is a single, coaxial, 10 m long BHE in use since its installation in a single-family house. The BHE supplies a peak thermal power of about 70 W per m of length.

The ground temperature results are highly informative with respect to the long-term performance. Atmospheric influences are clearly visible in the depth range 0-15 m, and below 15 m, the geothermal heat flux dominates. The results show that in the near field around the BHE, the ground coils down in the first 2-3 years of operation. However, the temperature deficit decreases from year to year until a new stable thermal equilibrium is established between BHE and ground, at temperatures that are some 1-2 K lower than originally. Thus, a “thermal collapse” (i.e., sudden drop of heat extraction efficiency) will not happen. After calibration of a numerical model with the data from the Elgg system, the extrapolation for an operation over a 30-year period as well as the thermal recovery for 25 years following the end of the operation period has been simulated. Temperature close to the BHE in winter drops quickly in the first years, only to stay more or less stable over the next years. In summer time, initial temperatures are not achieved again, but the temperature drop is decreasing from year to year. After termination of the operation, a rapid thermal recovery can be seen in the first spring, followed by a slowing down of the recovery process due to the decreasing temperature gradients. In the numerical simulation, a complete recovery will occur only after an indefinitely long time period; nevertheless, the remaining temperature deficit 25 years after the operation is stopped, is only in the order of 0.1 K. The long-term reliability of BHE-equipped heat pump systems, along with economic and ecological incentives, led to rapid market penetration. This was accomplished by the development of design standards (e.g., and easy-to-use design tools.

Heat Pumps

Heat pumps work on a similar principle to domestic refrigerators, extracting heat from one source and transferring it to another. A key ingredient in the heat pump is the refrigerant in its coils, usually a substance called Freon, which vaporises into a gas at a boiling point far lower than the 100ºC that water requires to boil. When the refrigerant boils, it changes from a liquid to a gas, absorbing heat from its surroundings. As the refrigerant changes back into liquid form it gives up its heat to the surrounding atmosphere. An expansion valve and an electric compressor control this process of transformation from liquid to gas and back again.

Earth energy (EE) heat pump is one of the most efficient means available to provide space heating/cooling for homes and offices (Figure 3).

Figure 3: Earth energy budget

It transfers the heat located immediately under the earth’s surface (or in a body of water) into a building in winter, using the same principle as a refrigerator that extracts heat from food and rejects into a kitchen. A heat pump takes heat from its source at low temperature and discharges it at a higher temperature, allowing the unit to supply more heat than the equivalent energy supplied to the heat pump.  An Earth Energy system relies on the 51% of solar energy that is absorbed by the land and water.

Terminology

Due to the large demand for EE as cooling devices, the earth energy industry uses the term ‘ton’ to describe a unit that will provide approximately 12,000 Btu of cooling capacity. On average, a typical 2,000 square-feet new residence would require a 4-ton unit for sufficient heat. Within the full swing of heat pump applications in Europe, ground-coupled heat pumps play a significant role. The development started around 1980 when the first BHE coupled heat pump systems were built in Germany and Switzerland. Following a larger number of new units installed during the oil price crises and a subsequent low (except for Switzerland), the number of new installations is again increasing in the 90s.

Airflow

EE units work efficiently because they provide a small temperature rise, but this means that the air coming through the register on the floor is not as hot as the air from a gas or oil furnace. A unit must heat more air to supply the same amount of heat to the houses, and duct sizes must be larger than those used for combustion furnaces to accommodate the higher CFM (cubic feet per minute) air flow. The major advantage of an EE system is that the heat obtained from the ground (via the condenser) is much greater than the electrical energy that is required to drive the various components of the system. The efficiency of a unit is the ratio of heat energy provided versus the electrical energy consumed to obtain that heat, and it is called its Coefficient of Performance (COP). EE units must exceed 3.0 (i.e., for every kilowatt of electricity needed to operate the system, the heat pump provides three kilowatts of heat energy).

Soil Type

Loose dry soil traps air and is less effective for the heat transfer required in EE technology than moist packed soil. Each manufacturer provides specifications on the relative merits of soil type; low-conductive soil may require as much as 50% more loop than a quality high-conductive soil.

Auxiliary Heat

When the outdoor air temperature drops below the design balance point, the EE unit cannot meet the full heating demand inside the house (for units sized to 100% of heat loss, this is not an issue). The difference in heat demand is provided by the supplementary or auxiliary heat source, usually an electric resistance element positioned in the unit’s plenum. Like a baseboard heater, the COP of this auxiliary heater is 1.0; so excessive use of backup heat decreases the overall efficiency of the system and increases operating costs for the homeowner.

Balance Point

The outdoor temperature at which an EE system can fully satisfy the indoor heating requirement is referred to as the balance point, and is usually -10°C in most regions of North Europe. At outdoor air temperatures above this balance point, the unit cycles on and off to satisfy the demand for heat indoors. At temperatures below this point, the unit runs almost continuously, and also turns on the auxiliary heater (called second stage heat) to meet the demand.

Heat Transfer Fluids

Closed-loop units can circulate any approved fluid inside the pipe, depending on the performance characteristics desired. Each manufacturer must specify which fluids are acceptable to any particular unit, with the most common being denatured ethanol or methanol (the latter is not approved for use).

Loop Depth

EE technology relies on stable underground (or underwater) temperature to function efficiently. In most cases, the deeper the loop is buried, the more efficient it will be. A vertical borehole is the most efficient configuration, but this type of digging can be very expensive.

Loop Length

The longer the amount of piping used in an outdoor loop, the more heat that can be extracted from the ground (or water) for transfer to the house. Installing fewer loops than specified by the manufacturer will result in lower indoor temperature, and more strain on the system as it operates longer to compensate for the demand. However, excessive piping can also create a different set of problems, as well as additional cost. Each manufacturer provides specifications for the amount of pipe required. As a broad rule of thumb, an EE system requires 400 feet of horizontal loop or 300 feet of vertical loop to provide heat for each ton of unit size.

Loop Spacing

When the distances between buried loops are greater then, the efficiency is higher. Industry guidelines suggest that there should be 3 m (10 feet) between sections of buried loop, in order to allow the pipe to collect heat from the surrounding earth without interference from the neighbouring loop. This spacing can be reduced under certain conditions.

Loop Configuration

Closed loops generally are installed either in a vertical or in a horizontal configuration, depending on the land available and a number of other factors. Earth Energy ground pipe comes in two common diameters: 0.75″ and 1.25″. Two coiled loops (commonly called the Svec Spiral and the Slinky) require less trenching than conventional straight pipe. As a result, the lower trenching costs and the savings in property disruption offset the higher cost of the coiled pipe.

Varieties of Heat Pumps

Air conditioning systems are an example of an air-to-air heat pump. They are becoming increasingly prevalent, particularly, because new cars are often fitted with air conditioning systems and people are beginning to ask for more controlled internal environments. However in the UK, the need for air conditioning is often a result of overheating because of unsatisfactory shading and poor natural ventilation. Every attempt should be made to design buildings, which do not require air conditioning, because of the additional energy load required.

In addition to air-to-air heat pumps there are air to water heat pumps and water to air systems. These can draw water from a well or pond and expel the used water to a discharge well. Because the source of heat is fairly constant (about 10ºC) the heat pumps are more efficient than air-to-air systems. Water to water heat pumps are even more efficient, taking the energy from geothermal supplies which are at a constant year round temperature and transferring heat to about 53°C.

Because heat pumps do not produce very high temperatures, they work best when heating well insulated houses, which are designed to be heated by low temperature systems. Traditional radiators, which are oversized, will give a larger area to dissipate heat and so work at lower surface temperatures. Underfloor water based heating systems are ideal as they work on the radiant heating principle which creates a comfortable environment at a lower temperature.

The heating loads for a house will vary considerable over the year. At the coldest time of the year, the energy requirements will be greatest. If the design to these levels of maximum load, the heat pumps size can get very big, and as a result costly. It is thought best to design the heat pumps to only cover about 50 to 70% of the annual heating demand, and where demand peaks over a smaller period, to provide supplementary direct electrical heating (or alternatives) to meet this demand.

Types of Geothermal Systems

There are a number of different methods to heat a building using geothermal energy:

  • Groundwater GSHP, of which there are two variations, open loop and closed loop. An open loop groundwater GSHP supplies ground water directly to each heat pump and then returns the well water to the source. This system is normally not recommended because of fouling and corrosion concerns. The closed loop uses an isolation plate and frame heat exchanger between the ground water and the building water loop.
  • Surface-water GSHP, which uses multiple heat exchangers made from spooled plastic pipe submerged in a body of surface water and connected to the building heat pumps.
  • Ground heat exchanger GSHP, which relies on a ground-coupled heat exchanger installed either horizontally in trenches or as “U” tubes in vertical bores.

The heat exchangers are connected together in parallel, and run-outs are tied to the building’s water loop. The selection of a particular design depends on the available land area. Table 1 provides the guidelines on the surface-area requirements for horizontal/vertical configurations. The decision to use any of the above systems depends on the results of geotechnical / hydrogeological investigations.

Water Discharge Quality

There are environmental regulations, which govern how the water used in an open-loop system can be returned to the ground. A return well is acceptable, as long as the water is returned to the same aquifer or level of water table. A discharge pit is also acceptable, as long as certain conditions are followed.

Open water systems depend on a source of water that is adequate in temperature, flow rate and mineral content. EE units are rated under the nation performance standard (CSA C446) based on their efficiency when the entering water temperature is 10°C (0°C for closed loop units), but this efficiency drops considerably if the temperature of water is lower when it comes from the lake or well. Each model has a specified flow rate of water that is required, and its efficiency drops if this rate is reduced. The CSA installation standard demands an official water well log to quantify a sustainable water yield. Water for open-loop systems must be free of many contaminants such as chlorides and metals, which can damage the heat exchanger of a unit.

Selecting A GSHP

GSHPs are very similar to conventional heat pumps. Their specifications differ from conventional water-source heat pumps (WSHP) only in the following areas:

  1. GSHPs operate over a very wide range of entering water temperatures from source (ground), typically, 20°F to 110°F, whereas the conventional WSHP operates over a very narrow range (60 to 90°F). This requires the use of an extended-range heat pump to preserve the ability of the system to operate at low ground-water temperatures. Table 2gives the typical temperature ranges for the water loop of GSHPs.
  2. GSHPs with the ground as a heat exchanger must be rated under ARI 330 or CSA 446 closed-loop conditions. GSHPs are to be rated under ARI 325 or CSA 446 open-loop conditions. Conventional heat pumps are rated under ARI 325 or CSA 656 conditions.
  3. GSHPs usually use a thermal-expansion valve as opposed to the capillary expansion device used in WSHPs.
  4. GSHPs typically encounter low suction temperatures and, therefore, need to be specified with low-temperature/pressure controls for freeze protection.
  5. GSHPs usually employ larger liquid side and airside heat exchangers and insulated internal components to prevent internal condensation.
  6. In conventional WSHPs, the insulation on the loop piping is not required because the loop temperatures are always maintained above 45°F. GSHP system piping will require insulation, and, in some cases, antifreeze solutions will be required to prevent freeze up.
  7. Specify copper heat exchangers for heat pumps on closed-loop ground source, groundwater, or surface-water applications. Use only cupronickel heat exchangers for open ground-water systems.
  8. While calculating the loads for the ground-source heat pumps, it is necessary to perform the calculations with an hour-by-hour and month-by-month simulation program because these calculations will be required to design the well field.

Selection and Pre-Installation Considerations

The ground source heat pump (GSHP) system represents the natural evolution of a traditional water loop heat pump (WLHP) system. The GSHP system offers all the advantages of the WLHP system, combined with considerable reductions in building operating costs. The beauty of this system is that it can perform both heating and cooling without the use of separate boilers/furnaces and A/C systems.

A GSHP system does not create heat; it moves heat from one area to another. GSHP systems use the ground (earth, ground water, or surface water) as heat sink in the summer and a heat source in the winter. This system is considered the most energy-efficient, environmentally safe, and cost-effective system available. Among the many components of a GSHP system, the most important is the heat pump itself.

Heat Pump Accessories & Controls

Considerations for heat pump:

  • Heat pumps, whether water or ground source, should not be used to handle large outdoor air loads. These outdoor air loads should be handled through separate A/C units, preferably with heat-recovery capabilities and conditioned outdoor air ducted to each heat pump.
  • Heat-pump sizing is very critical. It doesn’t need to oversize heat pumps. In general, size at no less than 95 percent for adequate latent-heat capacity. Do not size greater than 125 percent of the zone peak sensible-cooling load unless the heat pump has multi-speed fan/compressor and automatic means of adjusting flow.
  • Pay special attention to the specifications for the on/off automatic valve in the source water-supply connection to the heat pump, which is interlocked with the compressor to permit compressor operation only after it is fully open. Though seemingly a small component in the overall system, this is prone to frequent failures if it is not of good quality. Its failure will lead to expensive compressor failures.
  • Heat-pump schedules must include the minimum acceptable coefficient of performance for heating performance and energy efficiency ratio for the cooling performance to take advantage of the most efficient heat pumps available on the market.

Geothermal Heating Systems

Geothermal energy is a natural resource, which can be used in conjunction with heat pumps to provide energy for heating and hot water. CO2 emissions are much lower than gas fired boilers or electric heating systems. Geothermal heating is more expensive to install initially, than electrical or gas fired heating systems. However, it is cheaper to run, has lower maintenance costs, and is cleaner in use than other sources of heating.

The temperature of the earth under 2 metres of the surface is a fairly constant 10ºC throughout the year. At a depth of about 100 metres, the temperature of any water or rock is at about 12°C throughout the year. The heat stored at this depth comes largely from the sun, the earth acting as a large solar collector. For very deep wells, in excess of about 170 metres, there is an added component of heat from the core of the earth. As an approximation, one can add 3°C of heat gain for every 100 metres of depth drilled into the earth.

A closed loop system takes the heat gained from the bedrock itself. In a vertical system a borehole of a diameter of about 150mm is drilled, depth varies between 32 and 180 metres but will depend on the energy requirements. Multiple boreholes can be drilled. A pair of pipes with a special U-bend assembly at the bottom is inserted into the borehole and the void between pipe and hole backfilled with a special grout solution so that the pipe is in close contact with the rock strata or earth. Fluid (referred to as ‘brine’ is then circulated through this loop and is heated up by the bedrock. Different rock types will give different results. In some cases a number of boreholes will be made (for example, over a car park) to provide sufficient energy for the heat pump supply. If the ground is not suitable, horizontal loops can be laid or even trench filled ‘slinky’ loops, which are very simple to install. However, trench filled systems and horizontal systems require much more ground than vertical systems. If one has a pond or lake nearby, then can lay a closed loop at the base of the pond (it needs to be about 2 metres deep), or simply extract the water directly out of the lake at low level and re-distribute it elsewhere in the lake.

Heat pumps can be cheaper to operate than other heating systems because, by tapping into free heat in the outdoor air, ground or water supply, they give back more energy-in the form of heat-than the equivalent amount of electrical energy they consume. For example, in heating mode, a highly efficient heat pump could extract energy from the earth and transfer it into a building. For every 1 KWh of electrical energy used to drive the heat pump, around 3 to 4 kWh of thermal energy will be produced. In cooling mode, the heat pump works in reverse and heat can be extracted from a building and dissipated into the earth. Heat pumps which work in a heating mode are given a ‘coefficient of performance’ or ‘COP’ calculated by dividing the input kWh into the output kWh. This will give a COP figure, which varies with the input temperature and is the ratio of energy in to energy out. In cooling mode, the ratio is called the ‘energy efficiency ratio’ or ‘EER’. When the EER and COP ratios higher, the more efficient the unit. Geothermal/GSHPs are self-contained systems. The heat pump unit is housed entirely within the building and connected to the outside-buried ground loop.

Conclusion

The direct expansion (DX) ground source heat pump (GSHP) systems have been identified as one of the best sustainable energy technologies for space heating and cooling in residential and commercial buildings. The GSHPs for building heating and cooling are extendable to more comprehensive applications and can be combined with the ground heat exchanger in foundation piles as well as seasonal thermal energy storage from solar thermal collectors. Heat pump technology can be used for heating only, or for cooling only, or be ‘reversible’ and used for heating and cooling depending on the demand. Reversible heat pumps generally have lower COPs than heating only heat pumps. They will, therefore, result in higher running costs and emissions and are not recommended as an energy-efficient heating option. The GSHP system can provide 91.7% of the total heating requirement of the building and 55.3% of the domestic water-heating requirement, although only sized to meet half the design-heating load. The heat pump can operate reliably and its performance appears to be at least as good as its specification. The system has a measured annual performance factor of 3.16. The heat pump system for domestic applications could be mounted in a cupboard under the stairs and does not reduce the useful space in the house, and there are no visible signs of the installation externally (no flue, vents, etc.).

The performance of the heat pump system could also be improved by eliminating unnecessary running of the integral distribution pump. It is estimated that reducing the running time of the pump, which currently runs virtually continuously, would increase the overall performance factor to 3.43. This would improve both the economics and the environmental performance of the system. More generally, there is still potential for improvement in the performance of heat pumps, and seasonal efficiencies for ground source heat pumps of 4.0 are being achieved. It is also likely the unit costs will fall as production volumes increase. By comparison, there is little scope to further improve the efficiency of gas- or oil-fired boilers.

Nomenclatures

ACH Air changes per hour
GSHP Ground source heat pump
HRV Heat recovery ventilator
DC Direct current
HSPF Heating season performance factor
SEER Seasonal energy efficiency ratio
Btu British thermal unit
EER Energy efficiency rating
DX Direct expansion
GS Ground source
EPA Environmental Protection Agency
HVAC Heating, ventilating and air conditioning
DETR Department of the Environment Transport and the Regions
DTI Department of Trade and Industry
AFUE Annual fuel utilisation efficiency rating
ARI The Air-conditioning and Refrigeration Institute
COP Coefficient of performance (%)
GHP Geothermal heat pump
GL Ground loop
HP Heat pump
N Air change per hour (ACH) (h-1)
P Pressure (Pa) (kPa)
Q Heat (thermal energy) (J)
Qc Capacity (thermal power) (W)

AUTHORS CREDIT & PHOTOGRAPH

Abdeen Mustafa Omer
Energy Research Institute (ERI)
Nottingham, UK