Low-tech Magazine: Water eats energy: desalination

If we fail to reduce water use, we may safely double predictions on future energy consumption.

Desalination – the process of turning seawater into fresh water - is increasingly becoming the world's solution to a growing water shortage problem. But if we count on the oceans to fulfill our future need, we have to find an extra 30.000 terawatt-hours of energy - twice the current global electricity production figure. (Picture: Nicolas Metzl)

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"Water production will consume all available energy, and energy production will consume all available water"

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We are running out of fresh water. In arid and semi-arid regions, rivers run dry before they reach the sea. In many parts of the world, groundwater tables are falling rapidly due to the over pumping of water. Agriculture, the main culprit, can be accountable for more than 70 percent of total water use worldwide.

Virtual water

In rich nations, people use 100 to 200 litres of tap water a day. But this number becomes almost insignificant when compared to the hidden ('virtual') water consumption required to feed (and dress) every one of us. Around 300 litres of water is needed for the production of a mere two eggs, one bag of potato crisps or two cups of coffee.

A cotton t-shirt or a 300g steak demands a hefty 5.000 litres of water. Installing a water saving showerhead is not going to save us from trouble. World population is growing, and as more people become richer, the consumption of meat and dairy products increases.

According to the 'International Water Management Institute', by 2050, the withdrawal of water by the global agricultural sector alone will grow by 75 percent. This projected growth would mean that we would have to find another 5.000 cubic kilometres of fresh water to produce the world’s meal demand.

Fossil water

Extracting that supplement from groundwater and rivers is a recipe for disaster. In many arid regions, agriculture is dependant on 'fossil water' – like fossil fuels, this water source is non renewable. It was captured in underground reservoirs as a result of melting glaciers thousands of years ago. These so-called aquifers are depleting rapidly. Even the renewable groundwater is being pumped up faster than nature can compensate.

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"Some basic calculations demonstrate that desalination will get us nowhere, since it replaces the water shortage by an energy shortage problem"

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In areas with a concentration of human activity (urbanisation, agriculture, industry) groundwater tables fall by 2 to 10 metres a year. At this rate our most important fresh water sources will be used up faster than the remaining fossil fuel reserves. Around 2 billion people are dependent on groundwater for drinking, while more than 1 billion people eat food that is cultivated by it.

According to the 'World Water Council', a vegetarian diet consumes only half as much water as a typical meat diet. Collecting rainfall would also prove to be a low-tech solution. But unfortunately, harvesting the vast reserves of seawater is a solution that fits our way of thinking better.

Making water

Desalination has become the 'solution' of countries facing water shortage problems: Australia, Spain and China, to name a few. Even England has plans to build a water factory on the banks of the river Thames. Some basic calculations, however, demonstrate that desalination will get us nowhere, since it replaces the water shortage by an energy shortage problem.

Most scientific sources report a power consumption of around 6 kilowatt-hours for producing 1 cubic metre of fresh water out of seawater. If we choose to 'create' the 5.000 km³ of water for agriculture by desalination, we need 30.000 terawatt-hours of energy. That is twice the worldwide electricity production today. Not to mention the exponentially increasing amount of water used by industrial processes (when people get richer, they also buy more cars and computers).

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"In the United States, energy production already competes with agriculture for available fresh water"

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Taking into account that the world is already facing an energy crisis to which no one has an answer, desalination comes to a dead end. Even without a massive effort in desalination technology, the 'International Energy Association' is expecting a doubling of electricity use by 2030.

Cooling water

Over and above this, energy production in itself is also water intensive (that consumption is included in the fast growing amount of industrial fresh water use). For every kilowatt-hour of electricity, an energy plant (fossil or nuclear) consumes 140 to 180 litres of cooling water. Which means that the desalination of 1 cubic metre of water asks 1 cubic metre of water ( 6x140/180 = 840/1080 litres = 0,84/1,08m³).

In other words, we should use salt water as cooling water. If not, water production will consume all available energy, and energy production will consume all available water.

The amount of cooling water withdrawn by energy plants is not completely lost. Only a small portion evaporates. The remaining water flows back into the rivers or lakes. The problem lies in the obtainability of the water. In the United States for example, energy production already competes with agriculture for available fresh water.

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"Plants dump salt from the filtered water back into the ocean, and the higher the waters salt content, the more energy it takes to turn it into fresh water"

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Desalination will get more efficient – around 4 percent a year, according to the industry. But with the projected 3 kilowatt-hour per m³ (by late twenties), we are still on track to doubling electricity consumption.

Dead seas

Desalination is not a sustainable solution. Plants dump salt from the filtered water back into the ocean, and the higher the waters salt content, the more energy it takes to turn it into fresh water. The cycle continues.

Drilling for new fossil groundwater reserves or recycling waste water – the latter option uses the same technology as desalination – are more energy efficient alternatives. According to the 'World Wildlife Fund', who in june introduced a report on the ecological consequences of the technology for marine life, both strategies only need one eighth of the energy used by desalinating seawater.

It would, however, still boil down to the production of an extra 3.750 terawatt-hour of electricity – a doubling of the worldwide nuclear energy capacity.

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