王春艷,田 磊,俞 敏,劉 毅*

電力行業(yè)水-能耦合關系研究綜述

王春艷1,田 磊2,俞 敏3,劉 毅1*

(1.清華大學環(huán)境學院,北京 100084；2.國家發(fā)展和改革委員會能源所,北京 100038；3.國務院發(fā)展研究中心資源與環(huán)境政策研究所,北京 100010)

電力行業(yè)消耗了全球約8%的水資源,電力生產(chǎn)與輸配過程中消耗的能源與水資源之間的相關關系被定義為電力行業(yè)水-能耦合關系.本文從電力行業(yè)水耗和節(jié)水潛力研究、電力生產(chǎn)與水資源空間分布匹配研究、電力行業(yè)水-能耦合關系與其他環(huán)境問題的關系研究三個角度對國際上相關文獻的研究內(nèi)容、研究方法和主要結論進行梳理.研究結果表明:冷卻技術選擇對電力行業(yè)的水-能耦合關系影響較大,電力生產(chǎn)的需水量與各地水資源稟賦在空間上不匹配,電力行業(yè)水-能耦合系統(tǒng)管理體系尚未建立且面臨迫切的實際需求.

電力生產(chǎn)；水-能耦合關系；水耗預測；環(huán)境問題；綜合管理

初級能源開采[1]、生物質能種植[2-3]、能源加工[4-6]、能源使用[7]等能源部門的生產(chǎn)活動需要大量的水資源投入.電力生產(chǎn)的需水量在能源部門中占比最高,約為25%~80%[4].火力發(fā)電的取水量可占全社會用水量的40%以上[8-10].近年來,能源部門,特別是電力部門水耗研究成為了研究熱點.學者們將電力生產(chǎn)過程中能源與水資源的相關關系定義為“電力行業(yè)的水-能耦合關系(Water-Energy Nexus of the Electricity Sector)”.已有關于電力行業(yè)水-能耦合關系的綜述總結了特定區(qū)域電力行業(yè)的水耗估算方法、不同電力生產(chǎn)方式的水耗差異比較等,但尚未對電力生產(chǎn)與水資源空間分布不匹配、電力行業(yè)水-能-環(huán)境耦合關系等方面的研究進展進行總結和分析[4,11-14].因此,有必要進一步梳理電力行業(yè)水-能耦合關系研究領域的重點和難點,為后續(xù)研究提供基礎.

1 電力生產(chǎn)與水資源消耗關系研究

在研究電力生產(chǎn)過程的水耗時,學者們往往考慮以下幾個方面:①發(fā)電方式(如煤電、核電、太陽能發(fā)電、風電、水電、生物質能發(fā)電等);②冷卻方式(如循環(huán)冷卻、直流冷卻、空冷、海水冷卻等);③水源(如地表水、地下水、海水等);④環(huán)境影響(如溫室氣體排放、大氣污染物排放、生態(tài)影響等).

1.1 電力生產(chǎn)水耗現(xiàn)狀分析

對電力生產(chǎn)水耗分析的相關文獻從估算方法、數(shù)據(jù)獲取方式、研究對象、水耗影響因素等幾個方面分別進行歸類總結.

電力行業(yè)水-能耦合關系研究中常用的方法包括全生命周期評估方法(LCA)[15-16]、物質流分析方法(MFA)[17-19]、投入產(chǎn)出分析方法(I-O)[5,15-16]、基于過程的分析方法(Process-based Analysis)[17-19].其中,基于過程的分析方法通常以鍋爐、冷卻塔、污染物處理設施等生產(chǎn)環(huán)節(jié)為基本單元,運用MFA對電力生產(chǎn)中的水資源流和能源流進行量化和匡算[17-19].此外,部分學者將上述方法加以混合使用,如IO-LCA方法[15-16].

電力行業(yè)水-能耦合關系研究的數(shù)據(jù)主要來源于兩方面:LCA模型或者I-O模型中的水耗理論估計值、大量的電力企業(yè)一手調(diào)研數(shù)據(jù)[20].其中,通過調(diào)研獲取的水耗參數(shù)來源(如企業(yè)上報、理論估算等)不同,可能會導致數(shù)據(jù)質量可信度較低[21].

從研究對象來看,學者主要關注不同發(fā)電方式、不同冷卻方式的取水量和耗水量差異(表1).電力生產(chǎn)的取水量一般高于耗水量,直流冷的取水量甚至可高于耗水量100倍以上[22].以采用循環(huán)冷卻的煤炭發(fā)電為例,其取水量中約有80%-90%在蒸發(fā)過程中損失,20%-15%的水量轉移到了可銷售的固體副產(chǎn)品中,剩余5%在經(jīng)過廢水處理后外排,即耗水量約為取水量的95%[20].此外,已有研究多關注全生命周期階段和運行階段的水耗.例如,中國風力發(fā)電耗水量為0.6L/kWh,其中上游關聯(lián)產(chǎn)業(yè)的耗水量約占風力發(fā)電耗水量的50%[15].需要注意的是,水力發(fā)電的水耗研究仍有一定的爭議.水力發(fā)電,特別是蓄洪式水電站,會使得水體表面蒸發(fā)量增大[23].但水電站除具有發(fā)電功能外,還有防洪、灌溉等多種功能,在計算水力發(fā)電水耗時如何將蒸發(fā)水耗分配到各個功能上尚缺乏統(tǒng)一認識[24-25].

表1 不同類型發(fā)電方式的取水量和耗水量比較(L/kWh)

此外,學者們還對發(fā)電水耗的影響因素進行了分析.除了能源品種、冷卻方式外,外界環(huán)境(如溫度、濕度等)也會影響發(fā)電水耗.例如,火力發(fā)電的夏季耗水量比年平均耗水量高15%以上,而冬季耗水量則低12%以上[20].生物質能源作物的耕種方式對其水耗也有較為重要的影響[3].

1.2 電力生產(chǎn)水耗預測

電力是經(jīng)濟發(fā)展的基礎.電力生產(chǎn)與水資源消耗之間關系密切,越來越多的學者開始關注電力生產(chǎn)水耗預測問題.已有研究通常以發(fā)電水耗參數(shù)估計和電力生產(chǎn)結構及產(chǎn)量預測為基礎,分析未來電力生產(chǎn)的水耗情況,預測的時間跨度(2030年、2050年、2095年等)和空間范圍(全球、美國、中國、歐洲等)均較大.表2對其中代表性文獻的研究方法、時空尺度以及主要目的和結論進行了總結.目前的研究從微觀企業(yè)的水耗數(shù)據(jù)估算到宏觀區(qū)域層面的能源生產(chǎn)、調(diào)配及水耗預測都有所涉及,并多以2030、2050年為節(jié)點.研究內(nèi)容上,已有研究主要分析電力生產(chǎn)結構、水資源供給方式、冷卻技術選擇等的變化對區(qū)域(省級、流域、或電網(wǎng)等)電力行業(yè)水-能耦合關系的影響,以及電力生產(chǎn)與其他經(jīng)濟活動之間的權衡關系.總結發(fā)現(xiàn),電力生產(chǎn)水耗預測方面的研究尚未建立統(tǒng)一的研究方法和框架.雖然研究方法不盡相同,但研究結果均表明冷卻技術選擇對電力系統(tǒng)的水耗影響較大,水耗和發(fā)電量之間存在一定的權衡關系.

表2 電力預測及水耗研究進展

根據(jù)研究地區(qū)進行分類,全球主要國家的相關研究結果如下:

(1)中國

學者們分析了電力結構[8,53]、冷卻技術[8,53]等方面的變化對中國2030-2050年的電力生產(chǎn)水耗的影響,結果發(fā)現(xiàn)電力生產(chǎn)水耗將依然集中在北方和沿海地區(qū),能源效率提高、電力結構調(diào)整和冷卻技術均有助于節(jié)水,其中冷卻技術的節(jié)水效果更顯著.Li等[15]還發(fā)現(xiàn)2020年中國風電的推廣應用可以帶來23%的碳強度減排,同時節(jié)約8億m3水資源,相當于1120萬家庭用水的需求.

(2)美國

學者們分別預測了美國2050年[27]和2095年[55]的電力生產(chǎn)的水耗,結果發(fā)現(xiàn)冷卻技術變化帶來的部分地區(qū)(如加州)電力系統(tǒng)取水量和耗水量之間的折中關系尤為突出[55].

(3)英國

學者們預估了2030和2050年英國電力生產(chǎn)的水耗變化情況[28,31,52],結果表明到2030年,電力生產(chǎn)水耗將有所降低,但到2050年,若將溫室氣體減排作為約束條件,電力生產(chǎn)水耗會由于高水耗的低碳技術的推廣應用而增加[31].

(4)歐洲

Behrens等[51]分析了歐洲熱電生產(chǎn)與水資源之間的匹配關系,發(fā)現(xiàn)到2030年部分流域范圍內(nèi)的熱電企業(yè)會因為水資源量的減少而降低發(fā)電量.

(5)全球及其他地區(qū)

全球來看,通過提升能源生產(chǎn)技術的用水效率,特別是可再生能源的用水效率,能源系統(tǒng)水耗到2030年可以降低37%~66%(相對于2012年)[58].

Parkinson等[59]以成本、水資源可持續(xù)性、和電力部門的CO2為約束,分析了沙特阿拉伯地區(qū)電力生產(chǎn)結構(如燃氣發(fā)電、太陽能發(fā)電、煤電等)、水資源供給結構(地下水、海水淡化、廢水循環(huán)等)、冷卻方式等因素變化對水-能耦合關系的影響. Antipova等[56]對錫爾河流域的水力發(fā)電和農(nóng)業(yè)灌溉用水進行優(yōu)化分析,尋求該地區(qū)水電電力供應和灌溉水量之間的最佳平衡方式.

1.3 電力生產(chǎn)中節(jié)水潛力分析

學者們對電力生產(chǎn)的節(jié)水潛力進行了量化分析,包括電力結構的轉變帶來的節(jié)水效果[42,60]、電力系統(tǒng)節(jié)能措施帶來的協(xié)同節(jié)水效果[61-63]、電力系統(tǒng)節(jié)能成本等[64].例如Taxes地區(qū)從燃煤發(fā)電到燃氣發(fā)電的轉化帶來的節(jié)水量相當于現(xiàn)狀煤電水耗的60%[42];Tucson地區(qū)光伏發(fā)電量增加15%,其電力系統(tǒng)的水耗可以減少17%,同時還可以減少13%的電力輸送損耗[60].中國2007~2012能源部門節(jié)能措施的協(xié)同節(jié)水效果主要來自于電力行業(yè),其直接的節(jié)水量約為5.6億m3,上下游相關產(chǎn)業(yè)的節(jié)水效果為12.5億m3[62].除對節(jié)水效果絕對量的考量外,學者們選取單位節(jié)水量的經(jīng)濟成本作為指標,分析電力行業(yè)的節(jié)水潛力[64].

2 電力生產(chǎn)與水資源的空間匹配性研究

學者們注意到了區(qū)域水資源稟賦與電力生產(chǎn)水耗之間的緊密關系.研究的空間尺度包括:行政區(qū)域[26,65]、流域層面[66]、電網(wǎng)層面[39].例如APEC經(jīng)濟體中,約55%(437條)的因能源生產(chǎn)帶來的高水資源風險的流域與熱電有關,主要分布于美國東部、中國東北部、澳大利亞、俄羅斯西部等地區(qū)[66].學者對典型國家的電力生產(chǎn)與水資源壓力的空間匹配進行了更詳細的分析.例如中國煤電耗水量占全國總工業(yè)耗水量的11%,且約75%來自于極度缺水和長期缺水的地區(qū)[26].美國東部40%以上的電廠冷卻水耗都取自于缺水地區(qū)[67],到2035年美國10%~19%的新增熱電企業(yè)有可能會建在地表/地下水資源缺乏的地區(qū)[65,67].這種水-能矛盾還具有兩個主要特征:①季節(jié)性差異,一方面是由于季節(jié)性降水差異引起,另一方面是由于冬季結冰導致可用水量降低,水資源壓力增大[8];②可傳輸性,電力生產(chǎn)和水資源壓力之間的匹配關系會隨著電力輸送而產(chǎn)生一定的空間變化,例如中國水資源較匱乏的東北電網(wǎng)、北方電網(wǎng)、西北電網(wǎng)和中部電網(wǎng)由于電力輸出引起的虛擬水輸出加劇了當?shù)氐乃Y源壓力[39,68].總體來看,全球各個國家和地區(qū)均存在一定程度的水資源和電力生產(chǎn)空間不匹配的問題,從水-能耦合關系的角度出發(fā),綜合考慮和評估未來電力生產(chǎn)布局和水資源稟賦之間的關系顯得尤為重要.

3 水-能耦合關系與其他環(huán)境問題

電力行業(yè)是主要的溫室氣體(GHG)排放者,相關研究主要包括:核算不同能源類型下發(fā)電帶來的GHG排放[15,69-71]、碳捕獲與封存技術的使用與發(fā)電水耗之間的折中關系[42]、GHG減排壓力下發(fā)電結構的調(diào)整路徑[59,72]等.除GHG外,電力行業(yè)水-能耦合系統(tǒng)引起的環(huán)境問題還包括冷卻廢水外排時還會帶來熱污染問題[1],水力發(fā)電引起的生態(tài)環(huán)境問題[73-74],發(fā)電過程產(chǎn)生的SO2、NO等大氣污染物[19,75].

此外,生物質能源的種植占用了大量的土地資源,對糧食的生產(chǎn)、農(nóng)業(yè)灌溉等均有一定程度的影響[2,76-80].據(jù)估算,全球生物質種植消耗了2%~3%的農(nóng)業(yè)用水和用地,相當于30%的營養(yǎng)不良人口(Malnourished Population)的資源消耗量[2].

除電力生產(chǎn)引起環(huán)境問題外,外界環(huán)境也會對水-能耦合關系有影響.例如干旱不僅會造成水力發(fā)電的減少,還會引起地下水使用的增加,從而需要更多的能源(如電力等)提取地下水[81].

現(xiàn)有研究表明,電力行業(yè)水-能耦合系統(tǒng)與環(huán)境問題具有密切的相互作用關系,但如何量化評估該作用關系,進一步增加對耦合系統(tǒng)的認識仍面臨一定的挑戰(zhàn).

4 電力行業(yè)水資源和能源的綜合管理

水、能以及其他環(huán)境要素之間的緊密關聯(lián)關系使得單要素的環(huán)境管理措施可能出現(xiàn)偏差.如若維持現(xiàn)有的冷卻技術不變,中國東部電網(wǎng)的電力行業(yè)水耗將會超過“三條紅線”規(guī)定的水耗要求[8].此外,節(jié)能措施的應用也可能帶來一定的節(jié)水效果[64].因而多個要素的綜合管理和核算十分必要[55,82].

學術界在水、能綜合管理的方法和框架方面已經(jīng)有初步進展.例如以水、能安全為主要目標的歐盟COST(European Cooperation in Science and Technology)框架可協(xié)助綜合管理水-能耦合關系,并制定相應的政策制度[83].在模型方面,部分研究試圖將水資源管理和能源管理的相關模型進行結合,如以電廠水耗數(shù)據(jù)為基礎的ReEDS模型結合水資源管理模型WEAP,從流域層面評估發(fā)電對當?shù)厮Y源的影響[84].考慮到水-能耦合關系具有較高的區(qū)域性特點,因此有必要實施空間差異化管控[85].

5 結語

對電力行業(yè)水-能耦合關系的國際研究進展進行了綜述.目前電力行業(yè)水耗分析的研究方法多樣,傳統(tǒng)發(fā)電方式以及可再生能源發(fā)電方式均有關注.學者們在不同的時空尺度下,分別模擬和預測了電力生產(chǎn)及其水資源消耗情況,并指出了水資源與發(fā)電量之間的權衡關系,量化了電力行業(yè)的節(jié)水潛力.現(xiàn)狀和未來的電力生產(chǎn)和水資源壓力均存在一定的空間不匹配問題,遠距離輸電更加劇了這種水-能矛盾.此外,電力行業(yè)水-能耦合關系與溫室氣體排放、大氣污染物排放、生態(tài)系統(tǒng)等有著密切的聯(lián)系.因此,迫切需要綜合評估水-能耦合關系與環(huán)境問題,提出空間差異化的水-能耦合系統(tǒng)管控方案.

對未來關于電力行業(yè)水-能耦合系統(tǒng)的研究提出以下幾方面建議:①結合一手數(shù)據(jù),研究水-能耦合關系的區(qū)域特征;②構建可推廣的水-能耦合關系量化分析方法,提供決策支撐;③將水-能耦合關系研究內(nèi)容延伸到更廣泛的環(huán)境問題上,如水資源匱乏、大氣污染物排放等.

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Review of the studies on the water-energy nexus of the electricity sector.

WANG Chun-yan1, TIAN Lei2, YU Min3, LIU Yi1*

(1.School of Environment, Tsinghua University, Beijing 100084, China；2.Energy Research Institute, National Development and Reform Commission, Beijing 100038, China；3.Institute for Resources and Environmental Policies, Development Research Center of the State council, Beijing 100010, China)., 2018,38(12)：4742~4748

The electricity generation consumes around 8% of global water use. The water use for the electricity generation and transmission is defined as “the water for energy” or “the water-energy nexus of the electricity sector”. This study went through the literatures that are relevant to this topic from the following aspects: the quantification of the water consumption and withdrawal by various electricity generation types; the analysis of the mismatch between electricity generation/transmission and water resources; and other related environmental issues. This study concluded that: the cooling technologies would have significant influence on the water consumption and withdrawal for the electricity generation; there are tremendous spatial disparities of local water resources and electricity generation; comprehensive management of the water and energy is still lacking and urgently needed.

electricity generation；water-energy nexus；water consumption prediction；environmental issues；integrate management

X703.5

A

1000-6923(2018)12-4742-07

王春艷(1991-),女,河南濮陽人,博士后,主要研究方向為水-能耦合系統(tǒng)分析、環(huán)境系統(tǒng)分析、產(chǎn)業(yè)生態(tài)學.發(fā)表論文3篇.

2018-05-24

國家自然科學基金資助項目(71774096);國家重點研發(fā)計劃項目(2017YFC0404602)

* 責任作者, 教授, yi.liu@mail.tsinghua.edu.cn