屋頂上的發電機

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2011 / 6月

文‧張瓊方 圖‧藍春曉


日本311大地震後,災區大停電或因供電吃緊而限電,大阪、東京地區的業者紛紛向清大化工系講座教授馬振基求援,請求台灣提供簡易拆裝的小型風力發電機,以解決災民急迫的民生用電需求。

然而,因交通受阻,台灣各式小型風力發電機無法在第一時間送抵災區,解救他們的燃眉之急,但已引發各界對台灣中小型風力發電機的注意。

在核能安全堪憂、能源吃緊的時代裡,中小型風力機能不能成為讓民間普遍使用的新興產業?


 

高美濕地、新竹漁港岸邊,矗立的白色巨大風車讓人印象深刻,但一般人往往忽略了生活周遭還有許多中小型風力機,正在擷取風能,努力發電……

無論是古樸的淡水老街、國道一號桃園路段,或竹圍及淡水國小、台科大、清華大學等校園裡,都可以發現小型風力發電機的曼妙身影。

步入清華大學,在右邊路旁就能看到第一座仰賴風力和太陽能發電的「風光綠能」路燈,燈桿頂端三、四片隨風轉動的葉片,加上一小面太陽能板、一盞LED燈,就大功告成。因體積不若大型風力發電機般龐大顯著,經過時也沒有風車轉動的呼嘯聲,很容易讓人忽視它的存在。

風來去空空,留下萬古功

說到風力,大家立刻聯想到風車,腦海裡就自然浮現「風車之國」荷蘭的美麗畫面。

12世紀,荷蘭發明了世界上第一座驅動磨坊的動力風車;19世紀,丹麥人建立了第一座風力發電站;20世紀,美國人開始將風力發電商業化,法國人則發明了第一台100∼300千瓦(kW)的小型風力發電機,正式宣告小風機時代的來臨。

小風機先鋒

5年前,台灣開始投入中小型風機的研發,清華大學的「風光綠能研發團隊」是其中的佼佼者。

所謂的「風光綠能」,是指結合風力、太陽能與蓄電設施「三合一」的發電設施。清華大學由動力機械工程系特聘教授葉銘泉、化工系講座教授馬振基及電機系教授潘晴財組成的「清大風光綠能研發團隊」,結合了奈米材料、材料力學與電網等技術,致力於開發「垂直軸小型風力發電系統」。

「風是取之不盡、用之不竭,又不必付費的能源,」馬振基表示,風力發電對於二氧化碳排放減量的效果顯著,生產一度電可以減少0.64公斤的二氧化碳排放量。

運用風力發電,成本也不高。馬振基比較,風力發電平均每度電不到2元,比起燃煤發電約3元、柴油發電約11元,可以說相對廉價。

只是,「風力不穩定,忽大忽小,是目前最大的局限,」馬振基指出,並不是每個地方都適合發展風力發電。

拿台灣來說,符合國際認定具發展風力發電經濟效益──每秒5公尺以上的風力(可吹揚塵土及碎紙,樹木分枝搖動)一年達2,500小時以上──的「好風區」,只有北起新竹到竹南一帶、雲林麥寮,以及澎湖的中屯。

沒有風不行,風太大卻也不行。馬振基指出,每秒風速超過20公尺,相當於9級風力的颱風(屋頂瓦片會被吹翻),發電機會自動煞車,讓風車空轉,免得瞬間發電量過大,發生危險。

由於風力的不穩定因素難以控制,因此,加上太陽能輔助的「風光互補」型發電,便成為一種綠能新趨勢。

風與太陽交互運用

俗話說:「風和日麗」,一般而言,太陽大時風小,風大時通常沒有太陽,風力與太陽很難兩全,但卻可以互補,「風光互補」型發電機就能確保在有風或有太陽時都能發電。

以裝設在清大校園裡的風光互補型路燈為例,雖然位居「風城」,但新竹的九降風大約從10月吹到隔年4月,在炎炎夏日,常常半點風都沒有,風車葉片「文風不動」,只能靠太陽能板來收集電力。但一整年平均的發電量還是以風力為主,風力與太陽能比例約為四比一。

風力大於太陽能的主要原因在於轉換效率,風能轉換成發電效率大約30~40%,遠大於太陽能的轉換率(10~20%),以清大校園裡300瓦(W)的風光互補型路燈為例,風車轉一天,就可以存下三、四天的電力。

風光互補發電原理,簡而言之就是將風力與太陽能經過「穩壓」,再將直流電轉變為交流電,最後將電力儲存在儲電系統裡。其電力可以獨立使用,也可以加裝併網系統後與市電併連使用(先用完自己發的電,再用台電的電)。

風能大小則與風葉掃撂面積及風速的3次方成正比,換言之,風車的風葉面積越大、裝得越高(越往高空風力越大),產生的風能越高。例如1.65百萬瓦的大型風力發電機,機身高度78公尺(約22層樓高),葉片旋轉的直徑82公尺,一年發電量430萬度,可供應970戶家庭用電。

不過,如此大型的風力發電機座有可能影響航空安全,地震與颱風來襲時也可能造成損害,再加上噪音危害,必須裝置在離岸或空曠地區,方圓200公尺內都不得住人,還得通過環境影響評估。但小型風機裝置地點的彈性較大,無須環評,甚至還可以配合造景,進駐社區大樓、公園、農場。

雖說場地的限制小,但馬振基表示,樹木、建築物都會產生擾流,在都會區,小風機仍以裝設在屋頂為宜。因此,稱其為「屋頂上的發電機」再貼切不過。

由39家業者組成的台灣中小型風力機發展協會,副秘書長蘇美惠指出,由於電廠、電網等基礎建設的成本太高,因此許多國家紛紛在偏遠電網密度低的地區補助設置分散式電力系統,國際組織援助非洲的「光明計畫」,就是補助設立分散式的能源系統。

小風機、大市場

而繼太陽能光電之後,從2009年開始,中小風機也加入補助之列,各國政府補助額度不一,從美國的30%到韓國的60%都有。

台灣則從去年開始收購綠能電費,太陽能每度電收購價11元,小風機的風力發電則每度收購價7元。

蘇美惠指出,台灣小風機(400W~150kW)正值萌芽階段,這3年業績大躍進,出貨量從2009年的四千多部,到今年預估量四萬七千多部,業績大幅成長十倍以上;產值也從2009年的台幣1億多元,躍升到17億多元。

「小風機的市場非常大,」蘇美惠指出,中小型風機在應用方面不斷推陳出新,除了偏遠地區、家庭用電、交通號誌、路燈外,新近又擴展到遊艇、漁船上,目前越南和中國大陸很多湖泊上的漁船遊艇都裝設小風機,用以節省柴油的耗費。

台灣「垂直軸式」技術有優勢

由於大型風力發電機的關鍵技術仍掌握在歐美大廠手中,台灣的大型風力發電機組多需仰賴進口。但中小型風機的自主性高,無論是複合材料葉片或電控系統、基座等相關產業,台灣都很強,發展潛力與立基相當大,再加上有堅強的研發團隊作為後盾,前途一片大好。

以清大開發的小型風機為例,特色在於「葉片」材質與「垂直軸」設計。

不同於過去「水平軸式」的風力機,葉片旋轉主軸與立桿平行;「垂直軸式」風力機,葉片旋轉主軸與立桿垂直。

水平軸式風力機有很吵(葉尖端形成的噪音與風車轉速的五次方成正比)、會打到飛鳥、吃不到風就不動等缺點,因此美國已有22個州立法禁止水平軸式小風機裝設在住宅區或都會區。

垂直軸式風機則具有安靜、安全、好安裝等優點,且上風、下風各方向的風都「吃」(收集)得到。

其次,葉片材質也是影響風速和風機壽命的關鍵。早期國內外業者多使用鋁或不鏽鋼金屬的葉片,截面積小,在大陸內蒙地區測試3年就被沙塵、冰雪鏽蝕了。

清大研發的風機葉片採用高分子複合材料(玻璃纖維或碳纖維和耐腐蝕性的聚乙烯基樹脂),安全性較高,架設方便,無論是都會區大樓的樓頂平台、休閒遊樂區、農場、學校、社區、公園都能裝設,號稱是一種「可攜式電力」。

這種安全葉片材質較輕(約為傳統材質的五分之一重量),比較好吹動。一般小型風機只要在「樹葉及小枝搖動不息、旌旗飄展」的微風(每秒3.4∼5.4公尺的風)下即可啟動,而清大研發的垂直軸小風機大約只要每秒2公尺左右的風速即可轉動。

更值得推崇的是,其葉片設計與強度,可延長使用年限至15~17年。

至於小風機所使用的蓄電池,也是沒有污染問題、可回收的磷酸鋰鐵蓄電池。目前清大正與北京清大合作開發第三、四代的環保電池(全釩液電池)。馬振基表示,台灣發展中小型風機才五年,中國大陸已發展了近二十年,但他們在材料開發技術上一直無法突破,直到與台灣合作,才開始有長足的進展。

小風機外銷看漲

小風機好處多多,但目前台灣生產的小風機仍以外銷為主,根據台灣中小型風力機發展協會統計,去年台灣中小型風機出口比例高達79%,其中又以中國大陸的64%為最大宗。

馬振基指出,中國大陸積極發展風光綠能,內蒙呼和浩特市附近的浩瀚大草原上,可以看見各國風力發電機的身影,當然也有台灣的風光互補型小風機。內蒙現在趕羊不騎馬,而是騎「充電式」的電動摩托車。家家戶戶的蒙古包上都裝設太陽能板,甚至連雜貨店都販售太陽能板。

反觀台灣,何以不見新竹家家戶戶屋頂都裝設小風機和太陽能板呢?主要原因是,台灣電價低廉,再加上小風機目前造價不斐,缺乏誘因。

身兼清華大學「低碳綠能校園」召集人的副校長葉銘泉指出,一座風光互補路燈的造價將近20萬,電能回收成本至少需要10年以上,受限於經費,尚難大規模的裝設,目前清華大學裡6座路燈皆為研究用途。

然而,為了推廣,清大與新竹市政府合作設置「中小型風力機系統示範與測試平台」,先後將在青草湖畔以及年底即將要開館的「台灣世博館」園區,裝設一整排「風光互補型」路燈。

只是,示範歸示範,要推廣到學校、社區、一般民宅,價格誘因仍是關鍵因素。

目前300瓦(W)的小風機售價約台幣10~12萬元,而一般家用功率3千瓦(kW)、農場5千瓦(kW),造價則高達四、五十萬元台幣,若非電網無法到達的偏遠地區,現階段仍缺乏誘因。

期待「綠能風光」

放眼未來,家家戶戶屋頂上風機轉動的風光,可不可能變成台灣另一番風景?

蘇美惠表示,台灣電費太便宜,一般住家用電耗電量在330度以下,每度僅2.68元,加上台灣各地區的風況不同,很難普遍推廣,現階段多用於路燈、農場、魚塭、學校、公園等戶外或公有設施。

馬振基則相當看好未來市場,只要產量增加,價格自然會下降,他預估,三、五年內,當300W小風機價格降到10萬以下;3kW價格降到二、三十萬以下,約10年內可回本,政府若能進一步便捷收購申請程序,台灣屋頂的「綠能風光」指日可待。

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EN

Rooftop Power Plants

Chang Chiung-fang /photos courtesy of Lan Chun-hsiao /tr. by Scott Williams

Massive blackouts and brownouts followed Japan's March 11 Tohoku earthquake. Businesspeople in the Tokyo and Osaka areas immediately turned for help to Ma Chen-chi, a professor of chemical engineering at Taiwan's National Tsing Hua University (NTHU). They were seeking easily installed and removed small-scale wind-driven turbines to provide urgently needed electrical power to disaster victims.

Transportation difficulties prevented any of Taiwan's small wind turbines from quickly getting to the disaster zone, but they attracted attention nonetheless.

Given the tightness of contemporary energy supplies and worries about the safety of nuclear power, will we see widespread use of small and medium-sized wind turbines? Will the industry take off?


Most people are familiar with the giant white turbines that tower over the Gao-mei Wetlands and the Hsin-chu-Harbor-area coastline. But they often overlook the many small and medium-sized wind turbines generating power that surround them in their everyday lives.

Such turbines really are everywhere. You'll find them on the quaint old streets of Danshui, along the Taoyuan stretch of National Highway 1, at elementary schools in Zhuwei and Danshui, and on the campus of National Taiwan Univer-sity of Science and Technology and NTHU.

For example, there's a streetlamp with its own hybrid wind and solar power system on the right side of the road as you enter the NTHU campus. The installation consists of three or four turbine blades spinning atop the lamp pole, a small solar panel, and an LED light fixture. Because the turbine is so much smaller and quieter than a large-scale windmill, it's easy to miss.

Images of windmills and idyllic scenes of the Netherlands tend to leap to mind at the mention of wind power.

The world's first wind-driven grain mill was built in the Netherlands in the 12th century. The Danes developed a wind-powered electrical generator in the 19th century. The Americans began to commercialize wind-generated electricity in the 20th century, but it was the French who developed the first small-scale (100-300 kilowatt) wind-driven turbine and kicked off the era of small wind.

The leading edge of small wind

Taiwan began developing small and medium-sized turbines five years ago, and NTHU's "Green Energy Development Team" has done stellar work in the field.

The team works on "three-in-one" hybrid power systems that combine wind and solar power with storage. The university formed the team around Yip Ming-chuen, a professor in the Department of Power Mechanical Engineering, Ma Chen-chi, a chaired professor in the Department of Chemical Engineering, and Pan Ching-tsai, a professor in the Department of Electrical Engineering. The team, which brings together expertise in the fields of nanomaterials, mechanics of materials, and electrical networks, has been working diligently to develop better small vertical-axis wind power systems.

It costs little to generate electricity from wind power. According to Ma, wind generates electricity at a cost of less than NT$2 per kilowatt-hour, versus roughly NT$3 per kWh for coal and NT$11 per kWh for diesel.

"But wind is variable, sometimes strong, sometimes weak," says Ma. "Currently, that's its biggest constraint." And not every location can be exploited for wind power.

Taiwan's prime wind sites-those that meet the internationally recognized economic viability standard of having winds of at least five meters per second for at least 2,500 hours per year-consist of only the stretch between Hsin-chu and Zhu-nan, Mai-liao Township in Yun-lin County, and the Zhongtun area of -Penghu.

Given the variability of the wind, the new trend in green energy has been to use wind and solar power in a complementary fashion.

Wind and solar

Typically, sunny days mean little wind, and windy days mean little sun. Neither sun nor wind is a perfect solution by itself, but they complement one another very well.

Take the wind-solar hybrid streetlamp on the NTHU campus, for example. NTHU may be located in the windy city, but the area's strong monsoonal winds blow only from October to April. There's hardly a breath of breeze to stir the turbine blades in the summer, leaving the lamp dependent on its solar panel for power. Even so, over the year as a whole, the lamp gets four times as much power from the wind as it does from the sun.

The main reason that wind generates more electricity than the sun has to do with the conversion ratio. The conversion ratio for wind energy to electrical energy is in the 30-40% range, while that for solar energy is just 10-20%. In the case of the 300-watt wind-solar streetlamp on the NTHU campus, one day's good wind can power the lamp for three to four days.

The hybrid generating system routes wind and solar power through a voltage regulator, then converts the DC current into AC, and stores the power in a storage system. This power can be used where generated or the system can be linked to the grid and the power used as part of a household's electrical supply. (Current is drawn from Taipower only after that generated by the system is exhausted.)

The rated power of a turbine is related to the turbine's swept area and the cube of wind speed. In other words, the bigger the blades and the taller the turbine's tower (wind speeds are higher at altitude), the more power produced. For example, a 1.65 megawatt wind turbine mounted atop a 78-meter tower has a blade diameter of 82 meters and generates about 4.3 megawatt-hours per year, enough to power about 970 homes.

But such large wind power generators may impact air safety, could damage their surroundings in an earthquake or typhoon, and are a noise hazard. Since no one can safely live within 200 meters of them, they must be built in remote locations or even offshore. In addition, wind power projects of this size are subject to environmental impact reviews. But small wind is more flexible in terms of its siting and doesn't require an environmental impact assessment. It can even be integrated into the built environment, cohabiting with buildings, parks, and farms.

But Ma points out that while small wind is less constrained by location, buildings and trees interfere with airflow. In urban areas, it is best suited to rooftops. Given that limitation, it might be more aptly called "rooftop power."

Small wind, big market

The Taiwan Small and Medium Wind Turbine Association (TSMWTA) currently has 39 members. According to Su Mei-hui, the group's deputy secretary-general, the high price of infrastructure such as power plants and power grids is encouraging many nations to install distributed generation systems in remote areas beyond the reach of their power grids.

Following the trail blazed by solar power, small wind has been a supplemental power option since 2009. Government subsidies vary from nation to nation, ranging from 30% in the US to 60% in Korea.

Taiwan began purchasing green power last year at a rate of NT$11 per kWh of solar power and NT$7 per kWh of small wind.

Su says that Taiwan's small (400 W to 150 kW) wind turbine industry is beginning to grow rapidly. The last three years have seen companies leaping into the business, and shipments have soared from over 4,000 units in 2009 to a projected 47,000 units in 2011. The industry's production value has also skyrocketed from something over NT$100 million in 2009 to a projected NT$1.7 billion this year.

"There's a huge market for small wind," says Su, explaining that people keep coming up with new applications for small and medium-sized turbines. In addition to providing power to remote areas, for household use, and for traffic signals and lights, such turbines are now also being deployed on yachts and fishing boats, particularly on the lakes of mainland China and Vietnam, where small wind is reducing diesel expenses.

Taiwan's vertical-axis advantage

Moreover, since the key technologies for large wind power systems are controlled by American and European firms, Taiwan must import most of the components of such systems. But Taiwan can go it alone on small wind systems. We have a solid foundation in all the technologies related to its production, from the manufacture of composite blades to that of the electrical control systems and the bases. Taiwanese small wind is well positioned to grow and create its own niches. Given that these efforts are being supported by outstanding R&D teams, the future looks bright.

Take the small wind system developed by NTHU, which utilizes composite blades and a vertical-axis design.

Vertical-axis turbines differ from horizontal-axis systems in the direction of the axis of rotation of the turbine's blades.

Horizontal-axis systems are very loud. (The noise created by the blade tips is in direct relation to the fifth power of the speed at which the turbine rotates.) They also kill birds and won't spin unless they are facing into the wind. As a result, some 22 US states bar the installation of horizontal-axis small wind turbines in urban and residential areas.

Vertical-axis systems, on the other hand, are quiet, safe, easy to install, and turn no matter what the direction of the wind.

Crucial to a system's longevity is the material from which its blades are made.

Manufacturers in Taiwan and abroad used to use stainless steel or aluminum for the blades. But the blades, which had relatively small cross sections, corroded badly in three years of testing in the icy, sandy conditions of Inner Mongolia.

NTHU developed a turbine blade made from a polymer composite consisting of glass or carbon fibers and corrosion-resistant polyvinyl resin. The blades are safe and easy to install virtually anywhere, providing portable power whether sited atop a commercial highrise, in an amusement park, neighborhood or park, or on a farm or school campus.

Blades made from the composite material are only about one-fifth the weight of those made from older materials, and are more easily turned by the wind. Typical small wind systems need only light winds (3.4-5.4 meters per second) to generate power, but the NTHU vertical system works in as little as two meters per second.

Even more amazing is that the strength and design of the blades gives them a functional life of 15-17 years.

Small wind systems currently utilize recyclable lithium iron phosphate batteries to store power, but NTHU and Bei-jing's Tsing-hua University are now working together on the development of third- and fourth-generation environmentally friendly all-vanadium redox flow batteries.

Outlook for exports

Small wind has numerous strengths, but most of the systems presently manufactured in Taiwan are destined for export. According to the TSMWTA, Taiwan exported some 79% of the small wind systems it produced in 2010, with 64% going to mainland China.

Ma says that the mainland is making an aggressive move into wind and solar power and that wind turbines from around the world dot the broad Hoh-hot Plains of Inner Mongolia, hybrid wind-solar systems from Taiwan among them. Nowadays, the shepherds of Inner Mongolia don't ride horses, they ride electric motorcycles. They are also installing solar panels on the outside of their families' yurts. Nowadays, even convenience stores are selling solar systems.

Why don't we see small wind systems and solar panels on every rooftop in Hsin-chu? The main reason is that power in Taiwan is cheap and small wind systems aren't. There's simply no incentive for people to buy them.

NTHU vice president Yip Ming-chuen, the force behind the school's "low-carbon, green-energy campus" program, says it takes at least a decade to earn back a hybrid wind-solar streetlamp's nearly NT$200,000 cost through energy savings. Such high costs have prevented large-scale installation of the devices. As a result, the school has just six that it is using for research purposes.

NTHU and the Hsin-chu City Government have now created the "Small and Medium-sized Wind Power System Demonstration and Testing Platform" to promote the lamps. Under the program, a row of the hybrid street lamps will be set up along the shore of Qing-cao Lake and alongside the Taiwan Pavilion (relocated from the Shang-hai World Expo, and due to open in Hsin-chu at the end of the year).

But demonstration projects are only worth so much. If we hope to deploy the technology on campuses, in communities, and around ordinary homes, discounts are going to be crucial.

Right now, a 300 W small wind system costs NT$100-120,000. The kind of system necessary to provide the 3 kW a typical home uses or the 5 kW consumed by a farm runs NT$400-500,000. Consequently, there's just no incentive to buy small wind.

Going green

Will our built landscape someday incorporate small wind systems turning atop every home? Su argues that electricity is just too cheap right now. Most homes use less than 330 kWh per month at a cost of only NT$2.68 per kWh. Moreover, wind conditions vary greatly across the island, making it unlikely that small wind will be implemented islandwide. At the present time, its use is generally lim-ited to outdoor facilities and public spaces, such as streetlamps and fishponds, on farms and schools, and in parks.

Ma, on the other hand, is very positive on the market's outlook, arguing that prices will fall if production capacity grows. He forecasts that in three to five years the price of a 300 W system will fall below NT$100,000 and that of a 3 kW system will drop to NT$200-300,000. At those prices, buyers would earn back their costs in about 10 years. He believes that if we simplify the process for having Taipower purchase green energy from producers, rooftop power generation could have a very bright future in Taiwan.

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