Heating and cooling account for 30–40 per cent of power consumption within a typical building, and are an increasing influence on peak electrical demand.

The International Energy Agency has identified district heating and cooling (DHC) systems as an important tool in reducing energy use and greenhouse gas emissions, while also providing a demand-side management tool for utilities to reduce peak loads.

This ‘utility’ or ‘infrastructure’ approach to heating and cooling using geothermal energy is applied with similar technical, financial and management models to other utility services. A utility benefits through the reduction of peak loads.

Building owners benefit through:

  • The attraction of high-value government and corporate tenants that mandate ‘green’ buildings
  • Reduced maintenance
  • The freeing up of space that was formerly allocated to plant rooms
  • Greater flexibility with building design
  • Increased long-term property values.

A building occupant benefits through lower costs – and the environment benefits, thanks to reduced greenhouse gas emissions.

Geothermal heating and cooling

These systems utilise stable, moderate temperatures within the top 100 metres or so of the earth’s surface. These temperatures result from solar radiation, and thus are not limited to areas of unique geology. In Australia, these temperatures range from 10-12° Celsius in Tasmania to 30-34° Celsius in the Far North.

The ground heat exchanger (GHX) is the closed system of pipes that transfers water through the ground to access these stable, moderate temperatures. GHXs can be horizontal trenches, vertical boreholes, within building piles or located in water bodies such as harbours, lakes and dams. Open loops that utilise either aquifers or surface water bodies are also common applications.

The GHX operates by using the ground as a heat source in winter and as a heat sink in summer. It is these seasonally stable, moderate temperatures (in comparison with seasonally fluctuating ambient air temperatures) that result in reduced energy usage and reduced peak electrical demand.

Ground source heating pumps (GSHPs) deliver heating and cooling to a building. GSHPs are typically located either centrally in each building as a ‘reversible chiller’, or are distributed throughout each building where they service their local zones via forced air.

The central reversible chiller option can supply heated or chilled water concurrently throughout the building to either hydronics such as radiators or floor coils, or – for air delivery – fan coils. The distributed air systems share a common water loop around the building, and the GSHPs will supply either heating or cooling to their respective zones as required.

Recent estimates indicate the presence of over 2 million geoexchange systems worldwide, providing over 15 gigawatts of thermal capacity. The majority of installations are in North America and Europe, although there has been rapid uptake of the technology in countries such as China and Korea, with systems also present throughout Australia, Asia, the Middle East, Africa and South America.

Applying geoexchange systems

A district geoexchange system is applied over multiple buildings, on applications such as school or university campuses, military bases, aged care facilities, business parks and subdivisions. Installed and managed as project infrastructure in the same way as water, sewerage and power, they use either individual GHXs for each building or a common GHX that is shared across multiple buildings.

The GHX transports either heated or chilled water and has a typical operating range in Australia of 0-40° Celsius. There is no requirement for insulated piping to avoid distribution losses, nor for separate chilled and hot water networks between buildings.

Increased load diversity is a significant benefit of DHC, whether it be conventional or geoexchange. Applying load diversity both within buildings and across multiple buildings provides a reduction in overall system size, as it is unlikely that all users will require the full capacity of the system at any given time.

An example is a mix of commercial and residential buildings on a common GHX. Commercial peak load would occur on weekday afternoons, whereas residential peak loads would occur in the early evening
or weekends.

Load sharing or ‘balancing’ is one of the greatest values offered by the geoexchange infrastructure approach. It applies when mixed heating and cooling loads occur either simultaneously or over the course of a day, season or year.

Simultaneous load sharing occurs when different heating and cooling requirements enable a simple heat transfer from a cooling zone to a heating zone. The simplest example is where heat rejected from air conditioning is transferred into a local hot water service or swimming pool.

The GHX also enables this to occur annually, whereby heat rejected into the ground in summer is used to warm the building in winter. Underground thermal energy storage is a term commonly applied to these applications.

District geoexchange heating and cooling systems are an infrastructure approach that can assist with the reduction of greenhouse gas emissions, energy use and peak electrical demand within the built environment. Increasing load diversity and load sharing results in smaller, more efficient GHXs that improve environmental, energy and financial outcomes.