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.
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.
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.
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.
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 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 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.