Scientists from Carnegie Institution for Science and the Polytechnic University of Milan attempted to understand what if the humankind seriously places a bet on water desalination as a universal solution in the environment of growing deficit of fresh water. Their analysis gives a visceral sense of the scale of future challenge and shows how high and uneven may the price of such option be for different countries in the global climate change situation.
Today the fresh water shortage already touches upon over 4 bn people, and it will grow as and when the climate gets warmer. Droughts are occurring more often, evaporation grows, and water consumption in agriculture, industry and energy sector continues to increase. Against this background desalination is often perceived as a logical way out: there is enough of saline water, and the process of converting it into fresh water is believed to be relatively energy-efficient. However, the scientists proposed to understand the price of such solution from the standpoint of energy, money and CO₂ emissions.
Their analysis was mainly based on the reverse osmosis method, which is used in approximately 70% of all water desalination facilities globally. This principle is well-known: saline or slightly saline water is put through the membranes under high pressure, and such membranes capture salts and admixtures. However, this process has a strict physical limitation. The higher is the salinity of the input water, the more pressure is required to overcome the osmosis pressure, and hence, the more electricity is needed. This is the fundamental principle of thermodynamics.
The calculations show that when slightly saline water is converted into fresh water, e.g., from slightly saline subsoil aquifers, about 1.8 KW·h of electricity is needed on average to produce one cubic meter of fresh water. If we are talking about marine water with salinity almost three times higher, the electricity consumption grows approximately up 3 KW·h per one cubic meter. Hence, the transition from slightly saline water to marine water automatically means 70-75% growth of energy costs.
The scientists combined the physics of this process with the climate forecasts. Using five different climate models, they evaluated the so-called water gaps – the periods and amounts, when water consumption exceeds available resources of rivers and subsoil waters. The calculations were made both for the contemporary climate and for two scenarios of future warming – approximately by 1.5 and 3 °C. This allowed for obtaining not just one average estimate, but a wide range of possible future loads.
It turned out that in order to cover the expected fresh water shortage by desalination the world will need from 800 to 1,600 TW·h of electricity per annum depending on the climate scenario and the level of salinity of the processed water. This is compatible with annual consumption of major industrial economies and makes circa 1% of the overall global energy consumption. And there is another thing of crucial importance: this energy needs to be generated in the particular locality with fresh water shortage, and often those are regions with vulnerable and carbon-intense energy systems.
Considering the real global generation mix, desalination is such amounts may result in emitting up to 1 bn tons of CO₂ per annum, i.e., 2-3% of total global emissions. In the end of the day the technology called for adjusting to the global climate change starts to increase the climatic load noticeably. It is especially acute in the countries, where electricity generation is still based on fossil fuels: each cubic meter of desalinated water in such countries is accompanied by hundreds and sometimes even by thousands of grams of carbon dioxide emissions.
The economic aspect also looks pretty tough. Even if we account only for the electricity costs without construction and infrastructure CAPEX, global water desalination costs required for covering the fresh water shortage may exceed USD 100 bn per annum. For poor regions with fresh water deficit such costs may reach several per cent of GDP, and in combination with the growing energy consumption it makes water desalination unaffordable without external financial and engineering support.
Eventually the scientists came to the conclusion that water desalination cannot be viewed as a universal solution for the fresh water crisis. It may be efficient in the countries having access to cheap low-carbon energy, sustainable energy systems and sufficient financial resources. In other cases placing a bet exclusively on desalination bears the risk of converting fresh water shortage into energy shortage and climate difficulties.
