Wednesday, 2 December

Two Queensland towns, Dalby and Maleny, are running dry says the ABC Online Bulletin. The Western Downs Regional Council warned Dalby only had eight hours of water reserves left because the residents had been using too much. With the town's weir and the Condamine River dry, only two megalitres are left in storage, while the daily consumption is six megalitres per day, or about 420 litres/day (Dalby has a population of close to 12000). In Maleny, the town's only source of water, Obi Obi Creek, is on the brink of running dry because of a lack of rain. Is it about time Queensland should seriously start thinking about geothermal desalination? Queensland has substantial geothermal resources ranging from high-temperature Hot Fractured Rock (HFR) resources of Cooper and Eromanga and possibly Drummond Basins to Hot Sedimentary Aquifers (HSA) through and around the Great Artesian Basin. Some of these resources may not be hot enough for electricity generation but would be a perfect fit for thermal desalination of brackish aquifers. I do not know the quantity of salty or brackish water resources that are suitable for desalination but let us assume that there is enough of them accessible at a negligible cost.

Australian emphasis so far has been on large-scale desalination plants serving metropolitan cities. As water shortage is becoming a serious problem for country towns, it is probably the time to rethink. An 2008 analysis of desalination costs (Withtholz et al, Desalination, 229:10-20, 2008) concluded that for plants in the range of 1 to 100 ML/day, thermal desalination technologies are more suitable if there is a cheap supply of heat. A geothermally-powered multi-effect distillation(MED) or multi-stage flash (MSF) desalination unit can easily provide the entire fresh water needs for an outback city at the cost of around $1.00-$1.40/kL. This is a rough but conservative cost estimate based on the following parameters:

• Cost of thermal desalination without considering the cost of the heat = $0.55-$0.95/kL (from the GHD Report commissioned by the QLD Department of Natural Resources and Mines, 2001)
• Heat input required = 10-20 kWh/kL (the same reference)
• Cost of geothermal heat (assume $0.03/kWh)

The cost of $1.00-$1.40/kL compares very well against the 2010/2011 bulk water prices of $1.00 to $2.00 per kL listed on the Queensland Water Commission web site. While this shows that large-scale geothermal desalination should be of interest, small-scale applications are even more interesting especially when combined with agricultural usage.

A recent paper (Mahmoudi et al, Renewable and Sustainable Energy reviews, 14:512-517), is proposing the use of geothermal heat and the sea/brackish greenhouse water desalination process to produce water in arid regions. This is a clever combination where desalination function is coupled with a plant-growing function. It may not be the solution for large-scale desalination like for Brisbane but should have significant contribution in smaller towns. A schematic of the process is shown on the left. The brackish water is pumped and filtered from a well and sent into a ground heat exchanger where it absorbs heat from a geothermal fluid. This heat exchanger can be built of polyethylene to conserve costs. The heated brackish water is then fed in a cascade to the first evaporator then to the second evaporator. The brine can be circulated in the circuit several times until its concentration increases over an acceptable dissolved salt concentration. The concentrated brine is finally collected in a tank, where it is stored for later treatment or processing or reinjection. The evaporator is the entire front wall of the greenhouse structure. It consists of a cardboard honeycomb lattice and faces the prevailing wind. Hot brackish water trickles down over this lattice, heating and humidifying the ambient cooler air passing through into the planting area and contributing to the heating of the greenhouse. Fans draw the air through the greenhouse. Air passes through a second evaporator and is further humidified to saturation point. Air leaving the evaporator is nearly saturated and passes over the passive cooling system with a condenser (IC) immersed in a water basin. The fresh water condensing from the humid air is piped for irrigation or other purposes. This design can be scaled up to provide 10-20 kL/day while also helping greenhouse plant growing.

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