When it comes to air conditioning for buildings and in particular when it comes to air conditioning using air cooled direct expansion systems, every one degree increase in ambient temperature has a significant impact on power demand and efficiency of the unit.

The key reason for this is that like any heat transfer device, an air-cooled air conditioning unit requires a temperature differential  between the ambient air and the heat transfer medium, the refrigerant gas. During air conditioning cooling mode operation, the larger the differential between the hot refrigerant gas and the ambient air temperature, the easier it is to transfer the waste heat, extracted from the building interior, to the outside air.

With climate research indicating that temperatures are gradually rising on average and the frequency of days with above average temperatures increasing, this is a significant issue.

The norm:

Typically an air-cooled air conditioner needs to compress its refrigerant gas to generate gas temperatures of 45 to 55 degrees Celsius in order to ensure it can reject sufficient heat during summer air temperatures of 35 to 45 degrees Celsius and thereby provide the required cooling effect to the building interior.

Air Conditioner

A number of measures can be applied to reduce the refrigerant gas temperatures required to reject heat. These primarily involve the use of water to assist with the heat rejection process. The two key applications are:

1. A condenser water system which is connected to either an open or closed loop cooling tower. The condenser water flows through a refrigerant to a water heat exchanger at the air conditioning unit and is pumped to a cooling tower, where evaporation of the condenser water is used to remove the heat from the loop.

This system is normally applied to larger refrigeration loads as it requires significant additional plant, additional controls and dedicated plant space for the cooling tower(s). For open cooling tower systems, it also requires ongoing water monitoring and treatment of the condenser water system.

Water System

Water System

2. Evaporative cooling pads applied to the air conditioning air cooled condensers. This retains the basic air cooled heat rejection setup but utilises evaporation via wet pads to lower the incoming ambient air temperature around the condenser. It achieves the same effect as an evaporative cooler, lowering the incoming outside air and providing this to the building interior.

Cooling Pads

The evaporative pads are only required to lower the incoming ambient temperature to the condenser on days with elevated temperatures. Outside of these periods, they are not required. However, the pads do create additional air resistance and therefore the units require additional power to push air through the condenser coils.

What about looking down at the earth beneath our feet?

Using the ground as a means to reject heat or extract heat has been successfully applied for many years in Europe and North America.

It is important to make clear the distinction between the term geothermal plant and geo-exchange plant. Whilst the terms are sometimes interchanged, the common definition is that a geothermal plant is a plant that taps into hot rock or heated water in the ground to generate heat energy (as in New Zealand where it is used to generate electrical power). A geo-exchange plant on the other hand, uses the earth’s stable temperature as a medium to reject or absorb heat.

Geo-exchange systems can be used as a low-energy, high-efficiency alternative to air-cooled air conditioning plant. In simple terms, a geo-exchange system uses buried piping, usually in bores, to transfer the heat extracted from the building interior to the ground. In most systems, this is achieved by running a condenser water loop from the air conditioning condenser to the ground loop.

The Earth Connection

The efficiency is achieved because the ground temperature is normally within a much closer range to the building control temperature when compared to the summer and winter ambient air temperatures. Ground temperatures in Melbourne have been measured at around 15 degrees Celsius a few metres below the surface. During cooling mode operation, an air conditioning system rejecting heat to a 15 degree Celsius earth medium compared to 30 or 40 degree Celsius ambient air will be much more efficient.  Similarly, during heating mode, an air conditioning system absorbing heat from a 15 degree Celsius earth medium compared to five or 10 degree Celsius ambient air will be more efficient.

Whilst geo-exchange systems are gaining popularity in Australia, they are still relatively unknown in the industry. Part of this is due to additional expertise required to install these types of systems, including bore drilling expertise. The systems also need additional consideration regarding suitability as they will not be appropriate for all buildings and applications.

Geo-Exchange Click to enlarge

Geo-exchange systems require sufficient space for bores or horizontal ground loops, and can generate significant site management issues due to spoil generated by the boring or horizontal ground loop works.  The ground conditions need to be suitable to provide sufficient heat transfer as some ground conditions have poor heat transfer conductivity and therefore require significantly more ground loop piping to achieve sufficient heat rejection/absorption.

Unlike air or water, which will naturally displace due to thermal buoyancy, the ground can become saturated with excess heat (or loss of heat) and may require a recovery period where the air conditioning system is not running. Therefore, applications where air conditioning use is limited to certain periods of the day has advantages over applications where either cooling or heating is required for extended periods.

Why consider geo-exchange systems?

Geo-exchange systems provide a viable alternative to conventional air conditioning systems for buildings and an energy efficient response to the challenges that rising global temperatures are creating.  These systems provide significant benefits including:

  • Lower energy demands with efficiencies two to three times better than air cooled systems during peak cooling times
  • Carbon reduction of up to 75 per cent compared to air cooled systems
  • Smaller electrical infrastructure due to lower peak power demand
  • Can be coupled with other energy transfer systems such as solar collectors, night time sky cooling loops and water body (i.e. pond , stream) heat transfer loops
  • Lower noise to neighbouring properties due to deletion of air cooled condenser/cooling towers
  • Longer life of the air conditioning system due to lower gas refrigerant pressures in the refrigeration circuit

The future of geo-exchange systems

There are a growing number of buildings in Australia which have installed geo-exchange air conditioning systems, and feedback has been generally positive, with significant savings being achieved in energy use. These include residential buildings, educational facilities, civic buildings and commercial buildings.

I recently had the opportunity to meet with the Geotechnical Research Group of the Department of Infrastructure Engineering at the University of Melbourne. They are undertaking significant research on geo-exchange systems and have a number of operational systems which provide ongoing operational data on ground conditions, ground loop water temperatures and system operational efficiencies. One such site is the Elizabeth Blackburn School of Sciences in Parkville, Melbourne.

The group’s ongoing research indicates that depending on site conditions, shallower ground loop bores can be used to reduce time and cost of boring, which is one of the most significant costs related to installation of these types of geo-exchange systems.

With growing industry knowledge on geo-exchange and further development of standards to set guidelines for these systems, there is potential for significant increase in take up of this technology to address some of the challenges we are facing with the rise in global average temperatures.

  • Awesome invention… This project will help us to maintain a stable green house effect which might protect our earth. But it looks like a power plant based model and the cost will be more than a Air conditioners price.

  • I have read a few articles regarding geo-exchange systems with the general consensus being that they are very effective but installation is also expensive. This expense makes it not an attractive to the general public to implement for residential purposes. Like all emerging environmentally friendly energy efficient building systems, I feel more investment is required develop cost effective methods of implementation. Do we need to lobby our government for more research and investment?

  • Viable?

    Over all system efficiency is not what makes a geothermal / ground source heat exchange system viable.

    In my experience this depends on what your priorities are, where you are installing it, what regulatory requirements are in force, and if there are government concessions or funding arrangements are in place.

    Priorities might include: capital expenditure, ROI, operating costs/savings, efficiency, environmental impact, total system footprint, electrical demand/site capacity, and so forth.

    Deployment of the GSHEx system depends on the geology, and whether you can go open or closed loop depends on how much money you have to spend on drilling your bore field (and maintaining them into the future) or excavation of your trenches. This is the make or break part because in comparison to the air exchange system, it's the civil works and extra components that are needed in a geothermal systems that are the significant extra overheads – water cooled dx systems and heat pumps are equivalent in cost to air cooled.

    Other considerations may include regulatory frameworks: are there risks of aquifer cross contamination, or changes to water quality? Is there site conditions that need m

  • For residential in Australia (with our mild climate) it seems the latest air-based systems with COPs of 5.8 (ex' the Daikin Sarara 2.5kW cool/3.5kW heat – would be hard to beat. But for the larger building commercial size the ground-source may work out better in the long run?

  • Disclaimer: I work in the geoexchange industry.

    I read this article with interest and was intrigued by the use of the term viable. However, as the article advises there are many benefits of a geoexchange system and thus in many ways – viability is in the eye of the beholder, whether homeowner or commercial building owner.

    Firstly, comparing a geoexchange system to a split unit of 2-3 kW capacity is not relevant to the vast majority of geoexchange residential installations – just very different markets. The article indicates that geoexchange is not suitable in all instances and this is part of the feasibility / design process.

    A full understanding of the owners requirements and expectations as well as the building itself is what will make any system viable – geoexchange or other. In terms of simple paybacks, we have systems that range from 2-3 years to 10+ years. The longer payback projects still regularly proceed because of the other benefits listed.

    Our approach is to leave the definition of 'viability' to the home/ building owner once they have a full understanding of the pros and cons of the various systems available.

    Education is key and enables good viable decisions to be made.

    • Well pointed out Yale. You can't Compare geo-exchange to a split unit. The medium of heat transfer is different in both with splits using air and geo-exchange using water.

      Firstly, the heading is a little misleading in this article: "Is Geo-exchange more Viable than Air-based Air Conditioning?". I would have preferred "Is geo-exchange with water more viable than air-based direct expansion air conditioning?" as throughout his article the author tries to compare direct expansion air cooled XYZ with water cooled bores buried under buildings.

      Secondly, the reason behind XYZ is the following: This article is aimed at buildings with "Air-based Air Conditioning" so naturally I would have thought he was talking about air cooled chillers but no where in the article does he mention this or even use the word chiller. Does he then imply that buildings are using direct expansion air cooled condensers? Very rare that you would find a building with so much refrigerant gas in it.

    • Thirdly, I would be reluctant to go with bores buried under buildings as the maintenance of water in the systems would be a nightmare. Working in the commercial building industry I come across so many ill maintained water cooled systems. The corrosion and calcium deposits that builds up on check valves, actuator valves and so forth is time consuming and expensive to replace/maintain. If there was a leak in a conventional water cooled tower it would be easy to identify (most importantly) and fix (important as well) compared to buried bores.

      Lastly, we all know we mostly use copper as a heat exchanger but the author doesn't tell us what material is the buried bores or their construction. After all this article is all about better heat exchange efficiency.

      Maybe Yale Carden can enlighten us?

  • Perhaps just keep simple to start and compare conventional heat pumps (reverse cycle a/c units) with ground source (using either water or direct medium)?

  • All good comments and thoughts. Good to see the article has sparked interest and debate in this often overlooked alternative to the conventional air conditioing systems used in Australia.
    I would like to clarify a number of items regarding the article and comments recieved to date (I will keep it brief).
    Yes, the article is comparing air cooled DX to water based (ground side loop) geo-exchange systems. On the load side, the system can either be heating and cooling water or variable refrigerant per conventional VRF air conditioning units.
    Due to the past issues with legonella control (which have now mostly been resolved), uptake of water cooled equipment is not as common as it used to be and is primarily limited to larger capacity commenrcial installations. Hence my comparison with air cooled systems.
    The ground loops I have referred to are not open to ground water. They comprise plastic tubing in a closed circuit which is not exposed to calcium deposits and does not increase risk of ground water seepage from a bore.
    Yes, an options study is recommended to understand the full cost for the system, the in-ground conductivity and the space available for the ground loops etc.

  • Another point to consider is that the ground sourced heat pump equipment will normally have a longer life compared to an air cooled system ie. 15 – 20 years compared to 10-15 years for air cooled. Equipment replacement will factor in any whole of life comparison of the systems. The longer life is primarily due to the much reduced refrigerant high and low pressure band the equipment needs to work with. Geo-exchange equipment typically sees mild in-ground temperature compared with the temperature extremes of the outside air an air cooled systems (particulary summer temperature extremes).

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