Concentrating Solar Power and Desalination: An Ideal Fit

Solar power and desalination fit well together for geographic, technological and economic reasons.

Geographically, dry desert areas with powerful sun are ideal for generating concentrating solar power. They also tend to be where water shortages are severe.
Technologically, concentrating solar power generates heat. This heat can be turned into electricity to power reverse osmosis desalination. Or it can be used directly as thermal energy in multi-effects desalination. This increases flexibility.
Economically, water stressed areas results have high water provision costs. This makes desalination attractive for those areas, spurring takeup and long-term economies of scale.

Geographically

The International Water Management Institute forecasts that China, India and the Middle East, along with southern Africa will suffer some of the world's worst water scarcity problems by 2025. These are also areas with huge and growing populations. This will put increasing stress on natural water supplies.

Those areas also have some of the world's most powerful sunlight, which is ideal for generating concentrating solar power. Other favorable areas with very strong solar resources include the southwest United States, a region suffering water problems due to urban growth and an arid climate. These regions are where most of the world's current desalination capacity has either been installed or is being planned.

The global map of anticipated water stress areas in the year 2005... ...matches very closely areas of high direct normal radiation, a
key criteria in selecting favorable concentrating solar power plants
Source: International Water Management Institute Source: Schott Solar
The Middle East and China are building and planning
huge amounts of desalination capacity
   

Technologically

power and water infrastructure are often located together. Indeed, some of the world's largest desalination plants, located in the Middle East, are located alongside massive electricity-generating power plants. This saves on infrastructure costs and allows waste heat to be used as an energy input to desalination. That's because desalination can be powered either by electrical or thermal energy.

There are two forms of desalination:
Thermal desalination: this involves vaporising seawater in a series of heated vacuum chambers and recondensing the water vapor
Reverse osmosis desalination: this involves forcing seawater through fine membranes that trap salt

Both processes take alot of energy. With concentrating solar power, which concentrates the sun's rays to create heat of 400C and above, this energy can provided in several ways
1. It can be used to flash steam to drive a steam turbine, creating electricity that can be used to power reverse osmosis desalination
2. It can be used to directly flash seawater into vapour, powering multi-effects desalination
3. The thermal energy created from concentrating solar power can be stored in specialised salts, thus creating a "battery" function allowing the energy to be used at different times and not just when the sun is shining.

Below is a diagram that shows how all this can work. The diagram includes a fossil-fuel cogeneration plant as part of the configuration to provide a backup source of power. The inherent synergies between the technologies can create a 1+1=3 outcome that lower the greenhouse gas emissions and financial costs of desalinating seawater.

Concentrating solar power, solar thermal storage, electricity cogeneration and
reverse osmosis and multi-effects desalination are a bundle of technologies with inherent synergies.
 

By using concentrating solar power, operational price volatility is reduced since solar energy is a flat price resource. This reduces the financial risk of a desalination plant because energy consumption generally accounts for more than half the average plant's ongoing costs (see graphic, below left). Using renewable energy also reduces greenhouse emissions. While wind is another renewable energy resource that can be used for desalination, it lacks the flexibility of concentrating solar power. That's because wind, like solar photovoltaics, only generates electricity. By contrast, concentrating solar power creates heat, which can either be turned into electricity to power reverse osmosis or used directly just as heat to power multi-effects desalination. The city of Perth, Western Australia, is powering a large municipal desalination plant through wind, albeit indirectly. The actual plant is drawing electricity from the local municipal grid, which is largely coal fired. But the desalination plant is paying a premium to purchase wind power from a wind farm located several hundred kilometers north of the city, netting out its consumption financially. This system offers a handy template to desalination plants elsewhere seeking to reduce their greenhouse footprint even if they don't have renewable energy sources right at the plant site. A solution like this offers the double benefit of ensuring a desalination plant doesn't add to overall greenhouse-gas emissions and stimulates investment in renewable energy by creating long term customers for renewable power.

Typical costs for a very large seawater thermal desalination plant Global installed desalination capacity is rising very rapidly

Water Pumping

Once water is desalinated, pumping the water to consumption centers adds to the electricity consumed. If the desalination plant is located in a coastal city, the pumping costs will be limited roughly to those necessary for piping the water through the municipal water network. But if the water must be piped over long distances, more electricity will be used with the exact number dependent upon a number of factors, chief among them whether the water must be pumped uphill, which increases the electricity load, and the diameter of the pipe. However, long distances have not stopped pipe construction to carry water long distances. One of the longest pipelines is the roughly 600-kilometer Perth-Kalgoorlie pipeline in Western Australia. One of the highest volume pipelines are those in California which bring Colorado River water to the thirsty cities of southern California, making the California water system one of the largest electricity consumers in the state. If built, the Sana'a Project would entail desalinating seawater from the Red Sea and pumping it 135 kilometers inland and 2,500 meters up in elevation to serve Yemen's capital city of two million people.

Brine Waste

Desalinating seawater results in two different liquids: drinking water and brine in roughly equal measure. Brine is hypersaline water roughly twice as salty as the sea. Dumped back into a still marine environment, the heavy brine can sink to the sea floor and smother the life there under high levels of salt. Dumped back into a dynamic marine environment with a lot of mixing, experts are mixed on the impact. One alternative, depending on the space available, is to take this brine and harvest it on land into commercial grade salt by allowing it to evaporate in large ponds and then scraping up the resulting salt. This is one solution to the brine pollution problem, but it requires land which can be at a premium in seafront locations. However, one company that is building a solar salt field to harvest brine from desalination is Acquasol Pty Ltd. an Australian company.

Economically

Desalination is becoming an increasingly cost-effective solution to society's water needs.

In the past 100 years, water extraction rates from natural sources have been rising due to growing populations, higher per capita consumption and more intensive water use by industry. This is putting huge stress on natural sources of water such as waterways and acquifers. Just below, the water industry trade publication Global Water Intelligence offers two charts outlining the rising cost of marginal water extraction, but also on the continually falling costs of desalination. An extrapolation of its figures indicate desalinated water supplies could be economically competitive with natural water supply extraction as early as 2020. The charts below that indicate that, while their absolute prices may vary, the Western Australian government also found that desalination would be fully competitive with natural water supplies by 2020, and so did the German Aerospace Center in studying the cost of solar desalinated water for desert areas of the Middle East and North Africa.

 
Global Marginal water extraction costs are rising ..... ...at the same as desalination costs are falling
Source: Global Water Markets 2005-2015, Global Water Intelligence Source: Global Water Markets 2005-2015, Global Water Intelligence
This implies a crossover point around 2020
Source: Acquasol, from data above
   
   
   

 

The Western Australian government similarly found that desalination in that state should be fully competitive with natural water sources between 2020 and 2030 Cost of Water in the Middle East/North Africa area desalted by CSP in cogeneration with MED for 4, 9 and 14% rate of return, electricity cost US 4ct/kWh. 8,000 full load hours per year, annual irradiance 2500 kWh/m²/y.
Source: Concentrating Solar Power for the Mediterranean Region, Final Report,
German Aerospace Center (DLR), 2006, page 154

CSP Can Also Be Used To Refurbish Arid Landscapes Or Reintroduce Agriculture

In its study of the potential for concentrating solar power in the Middle East and North Africa, the German Aerospace Center also hit on the idea that concentrating solar power could be one leg of a larger strategy to, literally, "make the desert bloom." Below is an excerpt from the center's landmark study Concentrating Solar Power for the Mediterranean Region, page 154 (bold type added for emphasis)

In the world's arid regions, such (CSP) plants could become the nucleus of a totally new social paradigm: the conservation and recuperation of land endangered by desertification, comparable to the conservation and recuperation of flooded land in the Netherlands. Providing power, water, shadow and foreign exchange from the export of green power and revived agriculture, such plants can provide all what is needed to effectively combat desertification and to regain land for human settlement and agriculture that otherwise would be lost to the desert.
Arable land resources in MENA and world wide are disappearing at a speed of several hectares per minute. Concentrating solar multipurpose plants in the margins of the desert could generate solar electricity for domestic use and export, freshwater from seawater desalination and provide shade for agriculture and other human activities. Such plants could turn waste land into arable land and create labour opportunities in the agriculture and food sector. Tourism and other industries could follow. Desertification could be stopped, solar energy and saltwater are unlimited resources if used in a way compatible with environmental and socioeconomic constraints. The economic figures of most renewable energies indicate clearly that within a manageable time span they will become much more cost effective than fossil fuels. Renewable energies are the least cost option for energy and water security in the Middle East and North Africa (MENA). With increasing electricity intensity in a developing world, their importance will steadily grow, being only limited by demand, not by resources.