Sun + Water = Fuel

從鹽水作物中提取生物燃料

Sun + Water = Fuel

With catalysts created by an MIT chemist, sunlight can turn water into hydrogen. If the process can scale up, it could make solar power a dominant source of energy.
  • NOVEMBER/DECEMBER 2008, BY KEVIN BULLIS
燃料作物:北非國家厄立特裏亞一片實驗田裏的耐鹽水作物海蓬子
"I'm going to show you something I haven't showed anybody yet," said Daniel Nocera, a professor of chemistry at MIT, speaking this May to an auditorium filled with scientists and U.S. government energy officials. He asked the house manager to lower the lights. Then he started a video. "Can you see that?" he asked excitedly, pointing to the bubbles rising from a strip of material immersed in water. "Oxygen is pouring off of this electrode." Then he added, somewhat cryptically, "This is the future. We've got the leaf."
What Nocera was demonstrating was a reaction that generates oxygen from water much as green plants do during photosynthesis--an achievement that could have profound implications for the energy debate. Carried out with the help of a catalyst he developed, the reaction is the first and most difficult step in splitting water to make hydrogen gas. And efficiently generating hydrogen from water, Nocera believes, will help surmount one of the main obstacles preventing solar power from becoming a dominant source of electricity: there's no cost-effective way to store the energy collected by solar panels so that it can be used at night or during cloudy days.
Solar power has a unique potential to generate vast amounts of clean energy that doesn't contribute to global warming. But without a cheap means to store this energy, solar power can't replace fossil fuels on a large scale. In Nocera's scenario, sunlight would split water to produce versatile, easy-to-store hydrogen fuel that could later be burned in an internal-combustion generator or recombined with oxygen in a fuel cell. Even more ambitious, the reaction could be used to split seawater; in that case, running the hydrogen through a fuel cell would yield fresh water as well as electricity.
Storing energy from the sun by mimicking photosynthesis is something scientists have been trying to do since the early 1970s. In particular, they have tried to replicate the way green plants break down water. Chemists, of course, can already split water. But the process has required high temperatures, harsh alkaline solutions, or rare and expensive catalysts such as platinum. What Nocera has devised is an inexpensive catalyst that produces oxygen from water at room temperature and without caustic chemicals--the same benign conditions found in plants. Several other promising catalysts, including another that Nocera developed, could be used to complete the process and produce hydrogen gas.
Nocera sees two ways to take advantage of his breakthrough. In the first, a conventional solar panel would capture sunlight to produce electricity; in turn, that electricity would power a device called an electrolyzer, which would use his catalysts to split water. The second approach would employ a system that more closely mimics the structure of a leaf. The catalysts would be deployed side by side with special dye molecules designed to absorb sunlight; the energy captured by the dyes would drive the water-splitting reaction. Either way, solar energy would be converted into hydrogen fuel that could be easily stored and used at night--or whenever it's needed.
Nocera's audacious claims for the importance of his advance are the kind that academic chemists are usually loath to make in front of their peers. Indeed, a number of experts have questioned how well his system can be scaled up and how economical it will be. But Nocera shows no signs of backing down. "With this discovery, I totally change the dialogue," he told the audience in May. "All of the old arguments go out the window."
一個研究項目將能生產可用於噴氣式發動機的生物燃料,而又不必消耗淡水和耕地
--作者 Kevin Bullis
在中東,有一個研究項目致力於從生長在沙漠中的耐鹽水作物中制造可用於噴氣式發動機的燃料。阿聯酋的馬斯達爾學院的研究者們在兩平方公裏的一個示範農場的內 種植海蓬子,海蓬子的種子含有大量的油,可以被轉化成燃料。同時研究人員還把種植海蓬子和魚蝦養殖、培育紅樹林結合了起來。 燃料作物:在北非國家厄立特裏亞的實驗地裏種植著耐鹽水作物海蓬子
領導這個研究項目的馬斯達爾學院助理教授司各特肯尼迪說,傳統的生物燃料有兩大缺點:一是它占用了大量的本可以用來種植糧食作物的土地,二是耗費大量的淡 水,而他的這項研究的目標是既生產生物燃料,又避免這兩大弊端。這個計劃還得到了波音、阿提哈德航空(阿拉伯聯合酋長國國家航空公司)等大企業的資助, UOP霍尼韋爾公司提供轉化化學前物質和燃料的技術。瑪斯達爾學院位置處於阿聯酋最大的酋長國阿布紮比的一個零排放城市內。
肯尼迪和他的 同事們將要對一個被稱作整體型海水農業的技術進行改進。一開始要從海裏開鑿一個灌溉水渠,水渠把海水引向這個工程中的幾個部分。首先,把海水抽到水塘裏, 或者讓海水流過養著魚蝦的籠子。通常情況下這樣的水產養殖會造成為“環境災難”。肯尼迪說,比如,養殖使用後的廢水含有大量的渣滓、廢物,導致危險的赤 潮。但在馬斯達爾的實驗系統中,研究人員將利用池塘裏流出的廢水來給海蓬子提供養料。海蓬子生長在用海水灌溉的田地裏,成熟後可以象稻麥一樣收割。灌溉完 海蓬子的水變得鹽度更高,同時還攜帶著來自養魚塘的廢物,和灌溉渠裏的水一起流到一片紅樹林裏,而紅樹林是可以生長在鹽度更高的水裏的。紅樹林形成了一個 屏障,使得養魚塘裏的水不會直接排到海裏,樹葉還能作為魚蝦的飼料被利用起來。
富含油分的海蓬子種子可以使用和壓榨其他油料作物種子,如向日葵一樣的工藝榨油。榨出的油使用UOP霍尼韋爾的專利工藝進行精煉,使之可以添加入噴氣發動機油。植物剩下的部分可以被進一步利用,生產液體燃料,或者用來燃燒,產生蒸汽發電。
在生產生物燃料的過程中中,肥料的生產和使用通常是一個巨大的碳排放源,而魚塘既可以成為人們的收入來源,又能提供肥料,這樣還能降低整體碳排量。紅樹林的 的根系可以起到隔絕碳的作用。大多數生物燃料頂多達到碳平衡,即他們生長時吸收的二氧化碳與被制成生物燃料和燃燒後釋放的二氧化碳等量。馬斯達爾研究項目 的一個重要部分就是確定到底有多少碳可以被商業性地隔離開來。
海水基金會主席,也是其創始人卡爾霍奇斯已經在北非國家厄立特裏亞示範這個 項目體系。(他現在也是馬斯達爾項目的特別顧問)。在厄立特裏亞的項目中,海蓬子及紅樹林的樹葉被用來當作動物飼料。海蓬子果實的一部分油可以被轉化成生 物燃料。霍奇斯先生說他的項目因這個國家的政治動亂而終止,但是它還是表明了這種綜合的方式是可以起效的。
堪薩斯州立大學生物和農業工程 教授馬克施洛克說,不耗費淡水,不占用種植糧食的土地來生產生物燃料的舉措應該得到贊揚。但是他還說盡快研制出收割海蓬子的機械設備也很重要。亞利桑那大 學旱地研究室韋恩柯茲教授說,盡管可以利用現有的設備來收割海蓬子,但是這種植物的鹽分很高,可能會損壞這些機器。
這種作物還將要與其他 的生物燃料作物相競爭。每英畝海蓬子的油料收成與 大豆相當(與非油料作物相比,能產生額外的經濟價值),但是與棕櫚相比,它的收獲只是棕櫚的八分之一。然而海水種植體系有不需要昂貴的土地和淡水的優點, 而且還能自身生產肥料。肯尼迪說初步估計從海蓬子中提取的燃料可以和石油燃料媲美,但他同時還警告說這些還需要更細致的研究。

留言