Geothermal Desalination

Direct use and power generation based on geothermal resources are increasing at a steady pace around the world. Although geothermal energy sources are abundantly available in large parts of the world, they are still underutilized for desalination applications. Geothermal resources have the potential to serve as excellent heat sources for thermal desalination processes. Since thermal desalination processes require large amounts of heat sources, a geothermal-based energy source represents a viable, sustainable and environmentally friendly option. The advantage of geothermal sources is that they can act as both a heat source and a storage medium for the use of process energy. If these water sources have a high dissolved solids content, they can serve as feed water for the desalination process. Since external energy consumption is minimized, except for mechanical energy requirements, geothermal desalination processes could have less environmental impact compared to other non-renewable energy-driven desalination processes. Cogeneration programs for simultaneous water and power production are also possible with geothermal resources, as is polygeneration with multiple process advantages in terms of cooling and heating applications.

Drinking Water Requirements

The demand for fresh water is increasing due to the growth of the world's population, which brings with it a growing demand for fresh water for agricultural and industrial purposes. However, the availability of fresh water on Earth is limited and many countries are experiencing severe water shortages. Water desalination could be a possible solution to this problem. Of the many existing desalination technologies, three are particularly promising: reverse osmosis (RO), multi-stage flash (MSF), and multi-effect distillation (MED). The energy consumption of desalination processes is determined by factors such as the capacity of the desalination plant (small, medium, large), the energy source (electricity vs. thermal), the type of feedwater (brackish (BW) vs. seawater (SW)), the desalination method (thermal vs. membrane), the use of renewable energy sources (solar, wind, geothermal), and the need for pretreatment of the feedwater (mechanical and/or chemical). Field studies indicate that membrane technologies are the least energy intensive. BW RO of medium and large scale requires 1.9 kW h/m3. This is followed by SW RO of medium and SW RO of large scale with 4.3 and 4.4 kW h/m3 energy consumption. The thermal desalination techniques, mainly MSF and MED have a much higher energy footprint, than the membrane techniques. They consume 17.1 and 11.9 kW h/m3 respectively, but the thermal technologies are more efficient for desalination of highly saline waters. Nevertheless, because of their less energy-intensive nature and small footprint, membrane-based desalination methods have become more popular than the thermal technologies, and significant effects have been observed in the integration of RO with renewable energy sources, mainly wind and solar power. The energy footprint of this type of desalination technologies is in between that of the membrane and thermal routes. The energy consumption of desalination plants powered by renewable energy sources ranges from 1.5 to 21.1 kW h/m3. Their main disadvantage is their small capacity, which makes them uncompetitive with conventionally powered plants. It could be said that globally mankind consumes 7 kW h of energy for the desalination of 1 m3 of water.

Desalination processes require different types of energy, either thermal or electrical. Membrane technologies do not require thermal energy, all process steps are done using electricity. For RO plants, energy is required to generate high pressure to force water through the membrane. ED plants require electrical energy to create an electric potential difference. Thermal technologies require both electrical and thermal energy to evaporate seawater to separate the salts in the seawater. Desalination technologies, such as MSF and MED, use thermal energy as a primary source and electricity to drive the associated pumps as a secondary source. Thermal methods are considered more expensive than membrane methods because traditionally large amounts of fossil fuel are rare to evaporate salt water. However, they are more efficient for desalination of highly saline waters than membrane technologies. This proposal aims to replace the fossil heating process by using geothermal energy. Thermal energy could be obtained from fossil fuel-fired boilers, waste heat from power plants, renewable energy sources, and industrial waste heat sources. Basically, high energy consumption is a critical factor affecting the economics of desalination. It is necessary to make desalination processes as energy efficient as possible by improving technology and equipment.

Energy consumption in desalination

Energy Solutions

The addition of geothermal energy offers two different possibilities in energy solutions:

1. Electricity production. In this possibility, the geothermal energy would be used to produce steam that is used to drive a turbine to produce electricity

2. Steam production. As discussed, the most efficient way to desalinate (especially when the seawater has a high salt content) is provided by the methods of vapor compression, multi-stage flash or multi-effect distillation. These methods have in common that the seawater must be heated. This can be done directly by using the geothermal brine (via heat exchangers/vaporizer)