Making seawater into drinking water with the help of the sun
Source: http://www.rio12.com/rio3/proceedings/RIO3_281_K_Schwarzer.pdf
In Morocco, a big new solar-powered plant for turning
seawater into drinking water is being built. Is the business case for
renewable-energy-powered desalination technology becoming strong enough
to unleash a boom?
This summer, the Spanish firm Abengoa announced it had signed an
agreement with a Moroccan government agency to forge ahead with the
first phase of a project to build the world's largest renewable
energy-driven seawater desalination plant.
Abengoa will undertake
the engineering, construction, operation and maintenance of the plant
for 27 years. The project will produce 275,000 cubic meters (m3) of
desalinated seawater daily, to supply 150,000 m3 water for drinking
as well as 125,000 m3 for irrigation of 13,600 hectares of farmland
near Agadir, a coastal town in western Morocco. The contract provides
for a possible future capacity expansion up to 450,000 cubic meters a
day.
According to the Moroccan government, the electricity to
power the plant will come in by planned new high-tension wires from the Noor Ouarzazate solar power plant (pictured at top) nearly 400 kilometers (249 miles) east of Agadir.
But is this new plant is a harbinger of a boom in desalination plants
powered by sun or wind? Is there an emerging business case for
unsubsidized renewable-energy-driven desalination plants?
Desalination by the numbers
At
present, less than 1 percent of the world's population depends on
desalinated seawater for its daily fresh-water supply. There are around
21,000 large desalination plants in operation; most are in the Middle
East.
While the Agadir plant will draw seawater from the ocean
and turn it into fresh water, "only about half the desalination plants
in the world do that. The rest process water from other impure sources,
such as brackish groundwater or polluted river water," according to
Klemens Schwarzer, a scientist at the Jülich Solar Institute in western
Germany, 60 kilometers west of Cologne.Can scarce fresh water be made abundant?
In theory, the potential for increasing global freshwater supplies
by using desalination technologies is enormous. About 97.5 percent of
the 1,385 million cubic kilometers of water on Earth is salty seawater.
The remaining 2.5 percent is freshwater, but around 90 percent of that
freshwater is locked into the ice caps of Antarctica, Greenland, or
other glaciers. Humanity's total annual water use is, in turn, a small
fraction of the remainder.
Given these numbers, could
desalination plants drawing on seawater turn the world's deserts and
semi-arid drylands into thriving green plantations?
The short
answer is: In theory yes, but in practice, not easily - because "it
takes a lot of energy and equipment to make fresh water from seawater,"
Schwarzer said. "That means it's expensive."
The cost of
desalination always must be compared with the cost of piping or trucking
fresh water from somewhere it can be obtained without needing
desalination - i.e. from lakes, rivers, or freshwater aquifers.Technologies for desalination
A bewildering
variety of desalination equipment exists, but are only two main kinds of
process for turning saltwater into fresh, drinkable water: Thermal
desalination and "reverse osmosis" (RO) desalination. Both are
energy-intensive.
Thermal desalination works by causing water to
evaporate, leaving behind salt and other impurities. RO works by using a
multi-stage filtration process culminating in the use of high-pressure
pumps to force salty water through a membrane whose mesh is so fine that
water molecules can pass through, but salt and other impurities cannot.
Abengoa's Agadir plant will use RO.
According to Schwarzer, thermal desalination tends to generate purer water than RO desalination.
On
average, each human being directly or indirectly uses 3.8 cubic meters
of water each day, when everything from washing and drinking through
agriculture and industrial water use is counted. That means Abengoa's
Agadir plant, once complete, will produce enough to cover the needs of
about 72,500 average global citizens.Since there are about 7.5 billion people in the world, a
back-of-the-envelope calculation shows that it would take nearly 104,000
plants the size of the one being built in Agadir to provide freshwater for everyone on Earth.
The business case for solar desalination depends on location
Most of
the large desalination plants in oil-rich countries like Saudi Arabia
are thermal rather than RO plants. They use waste heat generated as a
byproduct in oil-fired electricity generating plants, Schwarzer
explained. That means the energy input needed for desalination is nearly
free-of-charge.
The heat driving the desalination process could
also be provided by large arrays of sunlight-concentrating mirrors - but
it's expensive to make and install those mirrors.
That's one
reason why the answer to the question posed at the top – "is the
business case for solar desalination technology finally strong enough to
unleash a boom?" – is no, not in countries like Saudi Arabia where natural freshwater is scarce, but oil and gas are cheap, seawater is readily accessible, and waste heat from fossil-fueled power plants is abundant.Gas-rich Qatar has built a number of gas-fired thermal desalination
plants - with money it got from selling gas to Europe. In early 2017 it
completed its first major reverse osmosis seawater desalination plant,
Ras Abu Fontas A3, big enough to supply water to a million people in
Doha
But what about regions where both freshwater and fossil fuels are scarce and expensive?
"In
those contexts, solar desalination may make sense," Schwarzer said.
"Morocco imports most of its fossil fuels. It has a great deal of solar
and wind energy that can be tapped, and it's beginning to do that."
Africa's financial challenge
There
is an enormous unmet need for clean freshwater in sub-Saharan Africa
too, Schwarzer added. With abundant sunshine, solar desalination makes
technical sense for the continent, at least near the coasts. It could
also be used to purify polluted river water. But most Africans can't
afford desalination equipment.People need at least 5 liters a day of water for personal use.
Schwarzer's group at Jülich Solar Institute has developed small,
decentralized multi-stage thermal distillation units that produce 10
cubic meters of freshwater per day - or 10,000 liters – at a cost of
about two euro cents per liter. It's enough to provide drinking and
cooking water for a village of about 2,000 people.
Two cents per
liter sounds, to European ears, like almost nothing, "but it's actually
quite a lot," Schwarzer said. "Consider a family of eight people - two
adults, six kids. That's 40 liters, or 80 cents a day. For an African
family earning just a couple of euros a day, it's too much."
The
solution, he said, will have to entail some form of partial subsidy to
cover the cost of building and providing desalination equipment.
Source: http://www.dw.com/en/making-seawater-into-drinking-water-with-the-help-of-the-sun/a-39924334
Source: http://www.dlr.de/sf/en/desktopdefault.aspx/tabid-8560/15527_read-44867
In Morocco, a big new solar-powered plant for turning seawater into drinking water is being built. Is the business case for renewable-energy-powered desalination technology becoming strong enough to unleash a boom?
太陽能與海水淡化組合是否經濟可行？取決於這些因素
Source: Google map
近日，西班牙Abengoa公司宣佈與摩洛哥政府機構簽署協議，將在位於摩洛哥西部沿岸的Agadir建設海水淡化廠，該專案將採用反滲透海水淡化技術，項目建成後或將成為全球最大的太陽能海水淡化設施。
據瞭解，Abengoa將負責該項目的設計、建設、運行及維護，整個合作週期長達27年。該專案投運後將實現日產淡水275000m³，其中150000
m³淡水將作為Agadir居民的飲用水，其餘的125000m³淡水將作為當地13600公頃農業用地的灌溉水。此外，這項協議還提到，未來該海水淡化項目產能或可增至450000m³/天。同時，摩洛哥政府表示，該海水淡化設施運營所需電力將從位於Agadir東部的Noor Ouarzazate太陽能發電園區輸入，並架設長約400千米的高壓輸電線路。截止目前，全球有不到1%的人口依賴淡化海水作為日常的飲用水源，共計約21000座在運營的大型海水淡化設施，而這些海水淡化設施大都位於中東地帶。
來自德國Jülich太陽能研究機構(位於德國科隆西部60公里處)的科學家Klemens
Schwarzer表示，儘管Agadir海水淡化設施是將海水轉變為淡水，但是全球僅有約一半的海水淡化項目採取同樣的做法，而其餘的水處理設施的源水並非潔淨水源，而是地下咸水或被污染的河水。
海水淡化易實現 但耗費能源成本高
從理論上講，利用海水淡化技術來補充淡水資源的潛力是巨大的。地球上水資源總量約13.85億m3，其中97.5 %是海水，剩餘2.5%是淡水，但是淡水中約90%的水量又儲存於南極洲、格陵蘭島或其它冰層。目前，人類可飲用淡水僅為剩餘水源中的一小部分。鑒於淡水資源如此匱乏，那麼大規模建設海水淡化設施能將全球荒漠及半乾旱土地變成可利用的農業用地嗎?對於這個問題，Schwarzer表示，理論上可以實現，但實際上並不容易。因為與從湖泊、河流或地下蓄水層直接獲取淡水的方式相比，開發海水淡化專案需要投入太多的能源和設備，這意味著其成本會很高。
目前主要海水淡化技術有兩種: 熱法和反滲透法(RO)
目前有多種海水淡化技術，但是主要的技術路線有兩種：熱法和反滲透法(RO), 這兩種技術對於能源的需求都很大。
熱法海水淡化是通過加熱海水使之沸騰汽化，再把蒸汽冷凝成淡水的方法。反滲透法採用多級過濾工藝，利用滲透膜在壓力下允許水透過而使鹽份和雜質截留的技術。據Schwarzer介紹，採用熱法海水淡化產生的淡水比反滲透法產生的淡水更純。
一般來說，如果將洗滌用水、飲用水以及工農業用水的一切都計算在內，那麼每個人每天將直接或間接使用3.8m3水。按此計算，Abengoa建設的Agadir海水淡化專案投運後，其生產的淡水將滿足全球約72500名普通公民的用水需求。目前，全球大約有75億人口，大約需要建設104000座與Agadir同等規模的海水淡化設施就能滿足所有人的淡水需求。
哪些區域更適合發展光熱海水淡化技術?
在沙烏地阿拉伯等石油資源豐富的國家，絕大多數海水淡化設施採用的是熱法而不是反滲透法。對此，Schwarzer解釋道，這些國家燃油發電廠產生的廢熱作為副產品利用於海水淡化項目，這意味著其幾乎不需要在脫鹽所需能源方面投入額外的資金。同樣，人們也可以採用光熱電站大型集熱陣列聚集的熱量進行海水淡化，但製造和安裝這些鏡子的成本目前仍非常昂貴。
因此，太陽能海水淡化技術的商業應用是否能夠實現爆發?答案似乎是否定的，因為在沙烏地阿拉伯等天然淡水稀少而近海的國家，石油和天然氣豐富且廉價，化石燃料發電廠產生熱量也很多，因此太陽能海水淡化技術在這類國家不會實現繁榮發展。例如，天然氣豐富的卡塔爾通過向歐洲輸送天然氣獲得資金，而後建成一批天然氣熱能海水淡化廠。2017年初，卡塔爾首個反滲透海水淡化廠—Ras Abu Fontas A3建成投運，該項目為杜哈一百萬人提供淡水。
然而，如果在淡水和化石燃料稀缺且昂貴的地區，情況又如何呢?Schwarzer 認為:“在這種情況下，太陽能海水淡化的優勢或將凸顯出來。此前，摩洛哥大量進口化石燃料，而現在其正在利用自身豐富的太陽能和風能資源，並開始建設太陽能海水淡化項目。”
非洲居民暫承擔不起
Schwarzer補充道，撒哈拉以南的非洲對淡水的需求量相當大。由於當地充足的太陽能資源，位於沿岸地區的海水淡化設施多少會受益。同樣地，太陽能也可以用來淨化被污染的河水。然而，大多數非洲人並不贊同建設海水淡化項目。一般地，每個人每天自身至少需使用5升水。而Schwarzer的團隊已經開發出一種小型的、分散的多級熱法蒸餾系統，它每天可以生產10m³(10000L)淡水，其成本為2歐分/升。如此計算，該系統每天生產的淡水將滿足約2000名農村居民的飲用水和烹調用水需求。
Schwarzer說：“對於歐洲人來說，2歐分或許一文不值，但是對於非洲人來說，它意味著很多。你可以想像一下，在上述情況下，在一個有8口人的家庭裡(2個成年人，6個小孩)，40L水或者說80歐分對他們意味著什麼(一個非洲家庭一天僅能賺到幾歐元)。針對非洲資金和水源匱乏的現狀，最好的解決辦法就是政府能夠給予公正的補貼來建設海水淡化設施，並提供相關設備。
轉載自:
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