Australasian Science: Australia's authority on science since 1938

Fresh Water Using Geothermal Heat

By Hal GurGenci

Geothermal heat can provide cheap fresh water to homesteads and small townships in the outback by removing salt from brackish aquifers.

Water is essential for life. Living in the driest continent on Earth, Australians probably know this better than anyone else. Most of our large cities are currently subject to water restrictions. Large-scale desalination plants are already operating in Perth and Brisbane; Sydney is building one bigger than those two and Melbourne has plans to build the biggest.

These plants use reverse osmosis technology. This essentially involves pumping the salty water through membrane filters that trap the salt and pass only freshwater.

Reverse osmosis is popular but it is not the only way to desalinate water. In fact, only 40% of the world’s desalinated water comes from reverse osmosis plants. About 50% is produced by thermal desalination and the remaining 10% by other means or hybrid systems.

Thermal desalination is preferred wherever there is access to cheap heat. The world’s largest desalination plants are thermal desalination plants using waste heat from large combined-cycle gas turbine facilities built in the oil-producing countries of the Middle East, such as Saudi Arabia’s Shuaiba III desalination plant (Fig. 1).

Recent studies indicate that Queensland has ample geothermal resources. There are two types of resources:

• the deep hot fractured rock resources of the Cooper, Eromanga and Drummond basins; and

• the hot sedimentary aquifers through the entire Great Artesian Basin.

While the main focus is on electricity generation, some of these resources may not be hot enough for electricity generation but they may still be useful for other purposes, such as desalination.

For example, a geothermal water supply at about 100–130°C may not be feasible for electricity generation but it would be perfect for desalination of salty or brackish waters that may be present in the same area. There are different thermal desalination technologies available but two would be most appropriate for geothermal desalination:

• multi-stage flash plants to supply freshwater to small towns; and

• smaller greenhouse desalination units generating about 10–20 kg/day for homesteads and cattle stations.

How Does a Multi-Stage Flash Plant Work?
Geothermal desalination for a town would be best using a multi-stage flash plant. This is not the most efficient thermal desalination technology but it is the most reliable one, and it is currently providing about 40% of the world’s desalination output.

Figure 2 shows the essentials of a multi-stage flash facility. Hot fluid from the geothermal reservoir (90–120°C) heats the brackish/salty water flowing inside the tubes of a heat exchanger. The geothermal fluid is sent back to the reservoir while the hot salty water in the tube is introduced into a tank kept at a lower pressure. When the pressure drops, some of the water evaporates immediately in a process called “flashing”.

The flashed vapour is pure H2O vapour, and it rises towards the top of the flash tank. Near the top of the tank it comes into contact with the walls of pipes bringing cooler salty water. When the vapour touches the cold pipes, it condenses and the condensate, which is freshwater, is collected in pans and taken out. The unflashed liquid is then passed to the next flash tank, which is at a lower pressure, and the same process is repeated in that tank.

In a real system, a series of tanks is used, with each tank producing more freshwater. The heat of condensation is passed onto the incoming salty water so that its temperature keeps increasing as it moves through the flash tanks leftwards as shown at the top of Figure 2.

The only heat input to the process is from the geothermal fluid in the first flash tank. Depending on its temperature, flow rate and chemistry, the geothermal fluid can be introduced as dry steam after being through a flash tank or mixed with the salty water in a direct-contact heat exchanger instead of the shell-and-tube heat exchanger shown in Figure 2.

A salient question is what to do with the concentrated brine that comes out as the waste product. It would not be environmentally sensible to let it run off.

One possibility is to reinject it back into the geothermal reservoir if the reservoir fluid already has a high salt content, as is usually the case. Another possibility would be to inject it into a shallower but salty aquifer. This would add to the cost but would help to maintain the water table pressure in the basin.

Greenhouse Desalination
There are many outback properties with access to hot borewater and also brackish or salty aquifers (Fig. 3), but the freshwater demand for an isolated farm or cattle station will not be large enough to justify building a multi-stage flash desalination plant.

Instead, humidify–dehumidify technology would be most appropriate in such instances. Moreover, such a facility can easily be built and maintained entirely using local resources, and would provide the dual function of producing freshwater and humid air for growing plants inside and immediately outside a greenhouse.

For greenhouse desalination, geo-thermal hot water heats brackish water in a heat exchanger, which can be buried in the ground or built as a surface tank. This heat exchanger can be cheaply built using high density polyethylene tubing that carries the brackish water with the geo-thermal fluid flowing outside the tubes.

The heated brackish water is then sent to the greenhouse where it trickles down the first evaporator. The “evaporator” is basically a honeycomb lattice built using materials such as recycled cardboard or golf or table tennis balls restrained in a frame and chicken mesh. However it is built, the evaporator covers the entire front wall of the greenhouse, which faces the prevailing wind direction.

Fans suck the outside air in and, as the air passes through the evaporator, some of the hot brackish water trickling down evaporates and humidifies the air. This provides hot and humid air for plants growing inside the greenhouse.

In the original concept, which was introduced in the Mediterranean, the “greenhouse” is a proper greenhouse with glass walls to provide additional heating from the sun. Such additional heating is optional in the outback, and can be prevented by using non-transparent walls.

Before exiting the building, the air is further humidified by passing through a second evaporator. The hot humid air coming out of the second evaporator, which is saturated with moisture, is made to pass through a condenser coil that is cooled by the brackish water. Freshwater condenses on the coils and is collected in a bottom pan and pumped to the storage tank.

Further use can be made of the air, which will still be humid, by growing plants in its flow path. The size of this outside garden depends on the size of the greenhouse and local climate conditions. Overhead shading can be employed if necessary.

Such greenhouse desalination installations are common in north African and Mediterranean countries, but there they use heat from a solar collector field. Solar heating is also a possibility for Australia but it is cheaper to use geothermal heat in locations where it is available.

Conclusion
Two technologies can be used for geothermal desalination.

• Multi-stage flash desalination can produce freshwater at a scale large enough for a small country town; and

• Greenhouse desalination can produce freshwater for a farm or cattle station while providing a greenhouse environment to help grow plants such as lettuce and cucumber.

Hal Gurgenci is a Professor of Mechanical Engineering at the University of Queensland, and the Director of the Queensland Geothermal Energy Centre of Excellence, which was established last year by a $15 million grant from the Queensland government.