february 2004 • tec 6 • SuMMary

a N IN trODuct ION tO G rO u N D S O u rc e Heat P uMP SyS teM S chris arkins
This note TEC 6, originally published in February 1999, was reviewed by Chris Arkins in January 2004. This summary page includes recent updates to the topic since publication.

SUMMARY OF

actIONS tOwarDS SuStaINable OutcOMeS
Introduction
Alternative low energy air conditioning solutions are now commonly sought in preference to typical air conditioning systems for both residential and commercial applications. The industry has seen a growing emergence of ground source heat pump (GSHP) installations throughout Australia over the last five years. A broad spectrum of facilities ranging from domestic housing, hospitals, education facilities, commercial offices and civic buildings to name a few, are now realising the environmental benefits offered by GSHP systems over more commonly used air conditioning systems. This summary note provides a brief overview of the previous note and provides an update on changes that have occurred since.

basic Strategies
Heat rejection is fundamental to all air conditioning systems. Typically, unsightly roof mounted air cooled condensers and cooling towers are by far the most commonly used method for rejecting heat from a building. Ground source heat pumps are somewhat different to the norm. Basically GSHP are refrigeration machines that provide heating and cooling by using ground water and the earth as a medium to reject and/or absorb heat and as such do not require air cooled condensers or cooling towers. This is made possible because ground temperatures are stable, remaining relatively constant throughout the year. During summer when space cooling is required, heat is removed from the building and transferred to the ground. In winter the reverse occurs, with heat being removed from the ground and supplied to the building.

Environmental Benefits
• Water Efficiency – Ground heat exchangers require no make up water, hence significant water savings are achieved when compared to cooling tower systems that rely on the evaporation of water and the subsequent cooling effect to reject heat from the water. Low maintenance – Ground heat exchangers require no regular chemical dosing or make up water. Energy efficiency – Ground source heat pumps achieve greater efficiencies due to constant return water temperatures from the ground. With air cooled equipment, efficiency varies with changes in ambient air temperature. On hot days, air cooled systems are less efficient as more energy is required to achieve the same cooling effect. Flexibility – Ground source heat pumps can adapt to residential and commercial buildings. They can be placed in new buildings or used as retrofits in existing buildings. Carbon dioxide emissions – Use of fossil fuels is reduced due to the energy efficient operation. Energy costs – energy costs are reduced by 10 to 30%. Aesthetics – Noise and visual exposure associated with roof top equipment is eliminated. Legionnaires’ control – Since cooling towers do not form part of the system, the risk of Legionnaires’ disease is eliminated. Ground source heat pumps are therefore particularly attractive for health care facilities.

• •

• • • • •

changes in the Industry
The principles of GSHPs have not changed, however the following indicative costs reflect current market costs compared to those published in the previous note. Installation costs vary according to site conditions, site accessibility, ground structure, size of heat exchanger and appointed contractor. The following unit rates are suitable for preliminary costing purposes. Final costing should obviously be confirmed by a qualified professional for each specific project. For drilling, piping and grouting of vertical bore holes, horizontal header pipework and horizontal trenching from the building line to the ground heat exchanger field, allow around $45 to $50 per meter of pipe.

february 1999 • tec 6 • PaGe 1

a N IN trODuct ION tO G rO u N D S O u rc e Heat P uMP SyS teM S chris arkins
Alternative low energy air conditioning solutions are now commonly sought in preference to typical air conditioning systems for both residential and commercial applications. Whilst still a developing market in Australia, ground source heat pumps (GSHP) offer environmental advantages over more commonly used air conditioning systems. This Note introduces the GSHP and provides an overview of applications, benefits and system types.

1.0 INtrODuctION
Heat rejection is fundamental to all air conditioning systems. Whilst ground heat rejection, or geothermal heat rejection as it is commonly called, is seldom used in Australia, it is not a new technology and has been used for many years in other countries. Ground source heat pumps have often been misunderstood or misapplied. This has led to reservations by some practitioners to specify such systems. Such systems can, however, be used successfully. The new Nursing Faculty building at the University of Newcastle is a recent example within Australia of where geothermal technology has been successfully employed. The building has since won the 1998 Ecologically Sustainable Development (ESD) Award of the NSW Chapter of the RAIA. This note attempts to dispel the myths and provide guidance to encourage application of ground source heat pumps in both residential and commercial buildings.

2.0 WHat are GrOuND SOurce Heat PuMPS?
In elementary terms, a ‘heat pump’ is a device which pumps heat from a lower temperature to a higher temperature level. This applies for all refrigeration machines. However, the label ‘heat pump’ has evolved to define those refrigeration machines which are configured to provide both cooling and heating, commonly referred to as ‘reverse cycle’. The term is unfortunate, as every refrigeration machine pumps heat even if it is in one direction. Ground source heat pumps, as the name implies, are refrigeration machines that provide heating and cooling by using ground water and earth as a medium to reject or absorb heat. This is made possible because ground temperatures are stable, remaining relatively constant throughout the year. During summer when space cooling is required, heat is removed from the building and transferred to the ground. In winter the reverse occurs, with heat being removed from the ground and supplied to the building.

Figure 1. Soil temperature profile

PaGe 2 • tec 6 • february 1999

EnvironmEnt DEsign guiDE

Ground source heat pumps use water as the medium in which to transfer heat between the building and ground. The heat pumps are essentially water cooled package units which can either be diversely located as smaller units throughout a building or housed collectively with a lesser number of larger units in central plantrooms. The system configuration used will depend on building function and use. The interface used to transfer/absorb heat with the ground is referred to as a ground heat exchanger. Ground heat exchangers are configured either as a closed loop circuit or as an open circuit. These different types of ground heat exchangers are discussed in more detail in the following section.

+

figure 3. Heat pump operation heating mode

3.0 DeScrIPtION Of Heat PuMP OPeratION
When in cooling mode, the heat pump removes heat from the space and transfers it to the ground via circulating water. The water temperature leaving the heat pump is in the range of 30–38°C. When this warm water passes through the ground heat exchanger it is cooled by the ground which would be approximately 19°C , (the actual temperature is dependent on geographic location). The heat from the water is dissipated by the earth and, if present, ground water aquifers. This is in contrast with conventional air conditioning systems which, in cooling mode, reject waste heat to the air via air cooled condensers or cooling towers.

4.0 tyPeS Of GrOuND Heat eXcHaNGerS
4.1 closed loop
The most commonly used ground heat exchanger is a closed loop system. This is comprised of high density polyethylene (HDPE) pipe distributed in the ground. The pipe can be buried horizontally in trenches below the ground, sunk in vertical boreholes, or placed on the bottom of a large pond or lake. Most sites can accommodate one of these three closed loop designs. The attributes of each system type are discussed below.
Vertical

Vertical loops provide the most economical use of land and are an ideal choice when available land surface area is limited. The vertical bore holes are usually arranged in a 4.5m x 4.5m grid pattern. Rectangular shaped fields are preferred over square fields as they provide a greater area of exposure to the earth mass and can result in a reduced field size. Drilling equipment is used to bore holes usually 150mm in diameter to a depth of between 50 to 100 metres.

figure 2. Heat pump operation cooling mode

When in heating mode the heat pump transfers heat from the ground to the building. Simplistically, the heat pump now operates in reverse (reverse cycle), cooling the earth and heating the building. The ground is now used to provide a ‘dummy’ heating load for the heat pump. (In cooling mode this heat load is supplied by the building.) In the reverse of the cooling cycle, waste heat from the heat pump is now used to heat the building. This is in contrast with conventional air conditioning systems, where heating is achieved via hot water boilers, reverse cycle or electric heater units.

figure 4. Vertical closed loop ground heat exchanger

Once the required number of holes is drilled, two HDPE pipes are fed down each bore hole. The bottom ends of these pipes are fusion welded together with a U-bend to close the circulation loop. The pipework is usually 20–25mm in diameter. When all vertical

EnvironmEnt DEsign guiDE

february 1999 • tec 6 • PaGe 3

pipes have been interconnected with horizontal header pipework the system as a whole is pressure tested for around 24 hours to identify any leaks that may be present in the system. After testing, each bore hole is pressure filled from the bottom up with a grout slurry made from bentonite clay. The grout plays a critical role in ensuring that a good thermal junction is made between the earth and pipework. Any air cavities will act as insulation barriers thus reducing the performance of the system.

figure 6. Pond closed loops

Pond

figure 5. Horizontal closed loop ground heat exchanger

Pond (lake) loops are very economical to install when a body of water is available, as excavation costs are virtually eliminated. Coils of pipe are simply placed in banks on the bottom of the pond or lake. Evaporation and exposure of the water to the lake floor will help to regulate the pond temperature thus providing a stable temperature for the coils. If the pond or lake is susceptible to large changes in water volumes, for example during times of drought, then this type of system is not appropriate.

Average heat rejection in cooling mode is approximately 1 kW per 15m length of bore hole and can be as high as 1kW per 20m. The capacity and efficiency of the soil and rock to absorb and diffuse heat obviously varies for each site. It is recommended that a geotechnical consultant is engaged to advise site suitability, conductivity of soil and other parameters that effect the design of the system. These issues are discussed in more detail below. Installation costs vary according to site conditions, ground structure, size of heat exchanger and appointed contractor. However, the following unit rates are suitable for preliminary costing purposes. Drilling, piping and grouting for vertical bore holes is in the order of $22–$25 per vertical metre. Allow $80–$90 per bore hole for the horizontal header pipework and $50–$65 per metre of pipe for the horizontal trenching from the building line to the ground heat exchanger field.
Horizontal

figure 7. Pond open loop

4.2 Open loop
Open loop systems utilise ground water as a direct energy source. In ideal conditions, an open loop application can be the most economical type of geothermal system. The water that circulates in an open loop system is sourced from a lake, river, or well. Rather than being recirculated as with closed loop systems, the water is returned to its source or to another acceptable discharge point. Open loop systems can only be used at sites that have a plentiful supply of water, and where local councils do not prohibit it. A similar concept to open loop heat exchange is used in Sydney with the Opera House and Power House Museum which both use the harbour water as a heat sink enabling simultaneous heating and cooling. Similarly, in Hong Kong, buildings reject heat to the harbour in lieu of cooling towers, due to shortage and cost of providing fresh water to cooling tower systems.

Horizontal loops are often considered when adequate land surface is available, for example a car park, park or playing field. Horizontal installations are simpler to construct as the HDPE pipes are placed in shallow trenches at a depth of 1.5–2 metres. However, because the earth temperature at shallow depths varies, longer lengths of pipe are required to overcome the variations in soil temperature and moisture content. Horizontal closed loop systems are more appropriate for domestic applications due to the simplicity of installation and lower cost. Horizontal fields will limit future development opportunities on the site, as the field cannot be easily built over and may need to be fully or partially relocated.

PaGe 4 • tec 6 • february 1999

EnvironmEnt DEsign guiDE

Flexibility – Ground source heat pumps can adapt to residential and commercial buildings. They can be placed in new buildings or used as retrofits in existing buildings. Carbon dioxide emissions – Use of fossil fuels are reduced due to the energy efficient operation. Energy costs – Energy costs are reduced by 10 to 30%. Aesthetics – Noise and visual exposure associated with roof top equipment is eliminated. Legionnaires’ control – Since cooling towers do not form part of the system, the risk of Legionnaires’ disease is eliminated. Ground source heat pumps are therefore particularly attractive for health care facilities.

6.0 SuItabILIty Of SIte
Site suitability is a function of many variables and extends to include: figure 8. Well open loop

• • • • • •

thermal conductivity (ability to diffuse heat) of the soil and rock allowable area of field (existing/future structures) including space for future capacity drilling conditions (slope of site, trees) species of flora with aggressive root systems should be removed from the proposed site quantity and diversity of heat to be rejected to the field when considering suitability of the site, all underground and above ground services, both present and future, need to be taken into account an allowance should be made at the plantroom header for connecting a cooling tower at a later date, should extra capacity be required and where additional field loop is unable to be laid.

4.3 Hybrid
Hybrid systems can be used in large commercial buildings where the cooling load is much greater than the heating load. With such a system, the ground heat exchanger is supplemented with a conventional cooling tower to cope with peak cooling periods. This allows a reduction in the initial installation cost of the system, by reducing the amount of ground heat exchanger required.

5.0 WHat beNefItS DO GrOuND SOurce Heat PuMPS PrOVIDe?
Diversity of use - Each heat pump operates only when the zone it serves is occupied. Simultaneous heating and cooling is possible for different parts of the building. Spacial planning – Plantroom sizes are reduced by 2050% over more traditional systems, allowing an increased net lettable area or a reduction in the size of the building. Mechanical equipment such as chillers, boilers and cooling towers are no longer required. Durability – Ground source heat pumps last longer than conventional systems as they are protected from the weather. The unit is housed indoors and the loop underground. Cooling towers have a economic life of 10 to 25 years, air cooled package units have an economic life of 10 to 15 years, while the ground heat exchanger has an expected life of over 50 years. Low maintenance – Ground heat exchangers require no regular chemical dosing or make up water. Energy efficiency – Ground source heat pumps achieve greater efficiencies due to constant return water temperatures from the ground. With air cooled equipment, efficiency varies with changes in ambient air temperature. On hot days, air cooled systems are less efficient as more energy is required to achieve the same level of cooling.

•

The number and depth of bore holes required will depend on the type of soil or rock and their formations below the proposed site as well as the peak heat capacity to be rejected. A check list for what should be performed in the geotechnical survey includes: • • a check to determine if underground aquifers exist and to what depth standing water will rise any fault lines or unusual geological formations should be noted along with any reasons why drilling on the proposed site may be inadvisable. Any other unsuitable conditions which may effect drilling or installing the proposed field should be highlighted local ground stability, over time advice as to what depth conventional trenching equipment can reach without requiring blasting (the trench for header pipes should preferably be dug without blasting) the potential variability of the geological conditions over the entire proposed field should be noted and the advisability or necessity of drilling further test holes to gain a more reliable estimate, stated

• •

•

EnvironmEnt DEsign guiDE

february 1999 • tec 6 • PaGe 5

•

site gradient, soil surface conditions and weight bearing capacity (to determine site suitability for supporting drilling machinery) soil temperature and moisture content at 5 metre intervals.

•

International Ground Source Heat Pump Association 1989, Soil and Rock Classification for the Design of Ground Coupled Heat Pump Systems: Field Manual, Oklahoma State University.

7.0 DeSIGN PrecautIONS
Ground heat exchanger installation - The installation of the ground loop heat exchanger should be carried out by a specialist contractor who is suitably qualified in the following areas: • • • boring of vertical holes, horizontal trenching and back fill heat fusion of the high density polyethylene pipework pressure grouting of vertical bores.

bIOGraPHy
Chris Arkins, BE (Mech), an Associate of Steensen Varming, is responsible for the development and coordination of Steensen Varming’s ability to provide specialist environmental and sustainable design services. Chris has completed a variety of challenging projects encompassing the design of innovative, low energy mechanical and passive systems. He has developed specialised fields of competence in geothermal heat pump systems, solar slab heating, natural ventilation, daylighting and mixed mode ventilation systems. Chris has specialised in ESD design for the past ten years and has participated in the following key projects: • The Richardson Wing, University of Newcastle, which won the RAIA New South Wales Chapter Ecologically Sustainable Development Award 1998 and the Master Builders Association National Energy Award – Commercial. Life Sciences Building, University of Newcastle, which won the 2001 Sulman Award. Interactive Learning Centre, Charles Sturt University, Dubbo which won the RAlA Environmental Architecture Award 2002. Gold Medal, The Francis Greenway Society, 2002 Green Building Award, Fry Street Residential Development. Gold Medal, The Francis Greenway Society, 2003 Green Building Award, Masterplan for Martin Bright Steel.

Preferably, the contractor should have current International Ground Source Heat Pump Association (IGSHPA) certification or equivalent. Site management – During the drilling of vertical bore holes, a significant amount of water can be expelled (particularly if aquifers are present). Allowance should be made for the capture, treatment and removal of this waste-water. Treatment of this waste-water usually entails provision of silt traps and pump stations to prevent silt entering local stormwater drains. Drilling equipment can generate excessive noise levels which may cause noise pollution problems, both on site and for adjoining properties. Thermal changes – Geothermal heat pump systems are not suitable for 24 hour operation over extended periods, unless during that period heating and cooling is performed. Continued heat extraction or dumping can result in a sustained change in the temperature of the field, and associated loss in system performance. To avoid this the field needs time to regain thermal equilibrium with the surrounding earth. An annual heat balance is desirable. That is, during summer the earth is heated while during winter the earth is cooled, in this way the net thermal balance is maintained. If the earth is just continually heated its capacity to act as a heat sink will progressively reduce over time. This will vary depending on geographic location and heat load diversities.

• •

•

•

refereNceS aND furtHer reaDING
Commonwealth of Pennsylvania Department of Environmental Protection 1996, Ground Source Heat Pump Manual. International Ground Source Heat Pump Association 1988: Closed-Loop/Ground Source Heat Pump Systems: Installation Guide, Oklahoma State University. International Ground Source Heat Pump Association 1997, Closed-Loop/Ground Source Heat Pump Systems: Design and Installation Standards, Oklahoma State University.

The views expressed in this Note are the views of the author(s) only and not necessarily those of the Australian Council of Building Design Professions Ltd (BDP), The Royal Australian Institute of Architects (RAIA) or any other person or entity. This Note is published by the RAIA for BDP and provides information regarding the subject matter covered only, without the assumption of a duty of care by BDP, the RAIA or any other person or entity. This Note is not intended to be, nor should be, relied upon as a substitute for specific professional advice. Copyright in this Note is owned by The Royal Australian Institute of Architects.