New research suggests that using geothermal heat pumps (GHP) is the best solution when it comes to the energy performance of heating and cooling systems.
More than two years of data has been collected and analyzed at the headquarters of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) in Atlanta, where they have set up a living laboratory.
A research team comprising the Geothermal Exchange Organization, Oklahoma State University and Oak Ridge National Laboratory they have conducted experiments in collaboration with to assess the relative performance of GHP and variable refrigerant flow (VRF) systems within the commercial building.
The building was originally built in the 1960s but then renovated and enlarged to incorporate the different systems for comparison:
- Variable refrigerant flow system serving the first floor
- Ground source heat pump system serving the second floor
- Dedicated outdoor air-system serving both floors
More than 1,600 sensors were then placed throughout the renovation project. The raw data indicated that the VRF system used twice as much energy as geothermal, and when differences in heating and cooling loads were normalized, the GHP system averaged 44 per cent less energy use than VRF.
“Though the control strategy for the VRF system resulted in longer run times compared to the geothermal system, it is clear that the ground loop water supply temperatures were more favorable than ambient air temperatures for heat pump operation. This allows the geothermal equipment to operate at higher efficiencies,” said Jeff Spitler, regents professor of mechanical engineering at Oklahoma State University and one of the researchers on the project.
The use of geothermal systems is certainly continuing to gain in popularity, and offers various benefits:
- The air temperature can be lower than in normal ducted systems, as long as you have warm surfaces around you.
- Maintaining these levels is cheaper than maintaining air temperature levels and also results in a better energy efficiency outcome.
- It is estimated that the system requires approximately 50 per cent lower maintenance than a traditional ducted system. Virtually none is required in the ground loop and, as it is more passive, it requires less user intervention.
Radiant systems are the most natural heating/cooling system. Early humans lived in caves for a reason. They have a greater thermal stability because the temperature of rocks naturally meets the human comfort zone.
In a wine cellar on a hot day, you feel cool and comfortable because of the cool walled surfaces surrounding you. If you pass a construction site on a warm day, you will feel cooler as you are exposed to concrete surfaces that have been cooled at night.
Geothermal systems have the potential to be used more widely than simply for HVAC. They could be part of a broader energy efficiency strategy for industrial sectors requiring process applications as they offer a thermal storage solution. The dairy industry, wineries, aquaculture, mushroom farms, chicken farms and the like could all benefit from the approach.
There are few climatic limitations but the system is more commercially viable in temperate climates, rather than in extreme hot or cold regions. It may not be as appropriate in Darwin, for example, although there are still examples of the system being used. It may be appropriate for mining towns in the desert because of the large diurnal temperature differences; it just requires more pipes in the ground to work.
It is also better in certain soil types for the installation of the ground loops. Sandy soil is more difficult for effective drilling and more expensive, but this type of soil is good for heat transfer. There is usually a trade-off between soil performance and installation costs driven by soil type.
Geothermal is not a new concept but it is still relatively new in Australia. There are a small number of geothermal projects here, including the Birdsville Organic Rankine Cycle Geothermal Power Station (Birdsville Plant) which produces 80 kilowatts, which is enough energy to power the town of Birdsville. The Plant is Australia’s only Hot Sedimentary Aquifer project currently producing electricity.
The Peninsula Hot Springs Bath House and Spa Centre in Victoria, meanwhile, is a tourist destination where natural hot water flows from aquifers more than 600 metres below the surface. The water temperature within the pools is between 37 and 43 degrees Celsius and the pools are reputed to have healing properties.
The ACT government has also been looking at plans to heat a cordoned-off area of Lake Burley Griffin to create a year-round 'beach.' The possible urban beach development in West Basin is part of the ACT government's new City to the lake project. It would feature a swimming area which, while part of Lake Burley Griffin, would be separated by a barrier and treated with chemicals like a standard swimming pool. It would also be heated, potentially using geothermal electricity.
At RACV Healesville in Victoria, on-site ornamental lakes are used as low grade heat source/heat sink for heating and cooling. The system also uses direct cooling from the lake during suitable lake water temperature conditions via a by-pass arrangement.
The sustainable strategy at the resort is supplemented by a hydronic floor heating and cooling system which provides well balanced temperature conditions supplemented by positive displacement ventilation system for appropriate fresh air supply and air change. Fully automated controls and monitoring ensure the optimum system performance under different operational conditions.
Solar boosters supplement the domestic hot water production. A perimeter lighting system with daylight compensation ensures the minimization of artificial lighting which works in tandem with motorised blinds to reduce direct sun penetration.
The design achieved dramatic cost savings and reduction in energy use: the lifecycle costs were $4,504 compared with $5,923 for traditional systems; annual energy consumption was 56kWh/m2 compared with 126kWh/m2; and C02 emissions were 577 tons compared with 873 tons.
With all of Australia’s major cities on the ocean there are other opportunities to use water to heat and cool buildings as they do in Toronto, Canada, which in this case uses the coldness of the water of Lake Ontario. The water is transferred through 18 pairs of stainless steel heat exchangers to a closed-loop chilled water supply distribution network to provide a number of environmental benefits.
- Reduces electricity usage by 90 per cent compared to a conventional cooling system.
- Frees up more than 61 megawatts of electricity for Ontario’s and Toronto’s electrical grids
- 79,000 tonnes of carbon dioxide are removed from the air annually (based on the displacement of coal and at full system build out) – equivalent to taking 15,800 cars off the road
- Over 1,880 litres per second of lake water cooling demand avoided due to reduction in electricity generation
- Removes 145 tonnes of nitrogen oxide and 318 tonnes of sulphur dioxide from the atmosphere relative to the use of coal-fired electricity
- Reduces the need for cooling towers, thus relieving valuable commercial office space for other uses and saving some 714 million litres of fresh potable drinking water