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Hydro-geothermal energy

Hot rock technology works great in volcanically active areas like New Zealand where high temperatures occur close to the surface, but it’s less practical in cooler tectonically continents like Australia. In Australia, lower temperature geothermal energy can still be harvested efficiently by pumping naturally occurring heated groundwater in a process known as hydrogeothermal power. Hydrogeothermal projects involve extracting groundwater from bores; harvesting its thermal benefits through a heat exchange system, then reinjecting the groundwater back into the aquifer through separate re-injection bores, with no net change to the water balance. We spoke to Don Scott, Managing Director of a leading hydro-geothermal and groundwater specialist company, Pennington Scott, about the state of hydro-geothermal power development in Australia.

Don Scott, Managing Director of Pennington Scott, points out that hydrogeothermal energy is one of the only ‘green’ energy sources in Australia that can compete on a level playing field with fossil fuels and win without government subsidies or penalties such as grants, carbon taxes or feed-in tariffs. The “Payback periods are very short, allowing users to make the case for these technologies purely on economic grounds, with the green credentials an added bonus.”

Shallow ground source heat pumps use the natural insulating properties of the ground to dramatically improve the efficiency of conventional reverse-cycle air conditioning systems. A conventional reversecycle air conditioner uses a compressor to pump heat from the air outdoors to the air in the building in the winter and the opposite in summer. In the southern capital cities, the system needs to work against air temperatures ranging from close to freezing in the winter to 40ºC or higher in the summer. By contrast, a hydro-geothermal heat pump uses natural groundwater as the heat sink and source, with a relatively constant temperature of about 20ºC, reducing the electricity requirements by over 50%. Because the system is heating in winter and cooling in summer, net heat change in the groundwater is close to zero, with minimal environmental impact. The method is so effective that the groundwater under a site can be used on a large scale to service numerous buildings, allowing the capital costs to be shared among many users. For example, Pennington Scott has recently completed the largest shallow hydrothermal project in the southern hemisphere for the Australian Fine China development in Subiaco, Western Australia. This district heating and cooling system, consisting of five production bores and six injection bores in the shallow sand aquifer, services over 300 residences and 37,500 sqm of commercial space.

Intermediate hydro-geothermal projects use heated groundwater to directly to heat air or water via a heat exchanger. These systems require groundwater of at least 40ºC, generally from about 1000m depth, and are most useful for projects with a significant heating requirement such as swimming pools or hospitals. The Perth Basin, with deep, permeable aquifers at the target depth is perfect for this technology. Seven of these systems have been built in Perth, but the project that Pennington Scott has just successfully completed at the Cannington Multipurpose Leisure Centre is the deepest and hottest intermediate hydro-geothermal in Western Australia so far. The production bore draws water at 49ºC from 1104m depth which is passed through a cascading heat exchange system on the surface to harvest as much of the energy as possible; first for the spa with the highest temperature requirement, followed by the swimming pool, and the building heating.

Going even deeper to depths below 2000m and temperatures above 60ºC, hydro-geothermal energy can be used to completely eliminate the need for electricity for air conditioning, using sorption chiller technology. Sorption chillers operate on the sample principle as conventional compressor driven heat pumps; that is by cycling the phase of a fluid between liquid and gas. However instead of using an electrically-driven mechanical compressor to drive the phase change, they rely on the use of heat. The most common example of sorption technology that most people would be familiar with is the propane fridge. Early next year, Pennington Scott will be installing the deepest sorption chiller bore in the southern hemisphere at Kagara Ltd’s Admiral Bay zinc mine near Broome, Western Australia. The underground mine will be 1500m below ground with background temperatures of over 50ºC necessitating a major cooling system to provide a safe working environment. Rather than burning costly diesel, Pennington Scott is planning to construct a groundwater bore to about 2000m to drive a sorption chiller system. As Don Scott points out, hydro-geothermal energy in this case “is an elegant way of turning the problem into the solution, by converting heat from the ground into the refrigeration needed to allow miners to work underground.”

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