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研究背景

熔融(rong)(rong)鹽是(shi)太陽能(neng)光(guang)熱(re)電(dian)站中(zhong)(zhong)蓄(xu)熱(re)系(xi)統的(de)(de)重要材料(liao)，但是(shi)由于其(qi)導(dao)熱(re)性能(neng)差、比(bi)熱(re)容(rong)相對較低等(deng)缺點使(shi)得(de)在蓄(xu)熱(re)系(xi)統中(zhong)(zhong)的(de)(de)應(ying)用受到限制(zhi)，故添加膨脹(zhang)石墨[1]、泡沫金屬[2]、納米顆(ke)粒(li)或石墨烯[3]等(deng)具有(you)良好導(dao)熱(re)性材料(liao)，成為(wei)提高熔融(rong)(rong)鹽蓄(xu)熱(re)性能(neng)的(de)(de)有(you)效(xiao)方式。

目前對純熔(rong)融鹽(yan)的熔(rong)化(hua)特(te)性(xing)研究(jiu)(jiu)(jiu)不充分(fen)，以(yi)多(duo)孔(kong)介質為基(ji)材(cai)和相(xiang)變(bian)材(cai)料為母(mu)體的復合相(xiang)變(bian)材(cai)料在儲/放能過程中的相(xiang)關流動與傳熱(re)過程的研究(jiu)(jiu)(jiu)尚不深入(ru)，納(na)米材(cai)料強化(hua)熔(rong)融鹽(yan)的特(te)性(xing)與機(ji)理尚未闡明，多(duo)孔(kong)基(ji)材(cai)和納(na)米顆(ke)粒添加入(ru)在系統層面缺(que)乏一些實驗驗證(zheng)。本文(wen)重點介紹肖(xiao)鑫副教(jiao)授等在多(duo)孔(kong)基(ji)納(na)米熔(rong)融鹽(yan)熱(re)物(wu)性(xing)及儲/放能特(te)性(xing)方面的研究(jiu)(jiu)(jiu)進(jin)展。

研究成果

本研(yan)究(jiu)首先兼顧納(na)米顆粒和(he)泡沫金(jin)屬的(de)優點(dian)，制備100~250℃溫區(qu)對應的(de)以熔(rong)融鹽(yan)為母體的(de)熔(rong)鹽(yan)/泡沫金(jin)屬/石墨(mo)烯復合(he)相變儲能(neng)材料，其(qi)吸(xi)熱(re)系(xi)數可增(zeng)大360%（圖(tu)1(c)）。并且(qie)多次循(xun)環(huan)之后，仍能(neng)維持其(qi)相變特征，即合(he)適的(de)相變點(dian)和(he)相變潛(qian)熱(re)（圖(tu)1(b)）。

圖1納米熔融鹽/泡沫金屬復合相變材料的制備、循環穩定性和吸熱系數

分(fen)子動(dong)力學模(mo)(mo)擬(ni)方法作(zuo)為一種(zhong)應用廣泛的計算(suan)(suan)(suan)(suan)機模(mo)(mo)擬(ni)手段(duan)，可(ke)以(yi)(yi)對新材料的研(yan)制(zhi)起到預測(ce)和指導作(zuo)用，同時可(ke)以(yi)(yi)微觀尺度(du)探索(suo)物(wu)(wu)質性能，揭示相應的微觀機理。本研(yan)究建(jian)立了(le)太陽鹽納米流體模(mo)(mo)型(xing)，并探究加入不(bu)(bu)(bu)同質量分(fen)數納米顆(ke)粒對熔融(rong)鹽熱(re)物(wu)(wu)性的影響。發現使(shi)用分(fen)子動(dong)力學模(mo)(mo)擬(ni)計算(suan)(suan)(suan)(suan)物(wu)(wu)質的粘(zhan)度(du)、比熱(re)容、均方位移等特性時不(bu)(bu)(bu)會因為模(mo)(mo)型(xing)的大(da)小不(bu)(bu)(bu)同而(er)產生(sheng)尺寸效應；但是用非平衡態(tai)法計算(suan)(suan)(suan)(suan)熱(re)導率(lv)時，會受到尺寸效應的影響，應該使(shi)用大(da)小一致的模(mo)(mo)型(xing)。熔融(rong)鹽納米流體的粘(zhan)度(du)隨著納米顆(ke)粒的添加而(er)不(bu)(bu)(bu)斷增(zeng)大(da)，通過對體系(xi)的徑向分(fen)布函數計算(suan)(suan)(suan)(suan)，可(ke)以(yi)(yi)推測(ce)粘(zhan)度(du)的增(zeng)大(da)是由(you)于納米顆(ke)粒的加入使(shi)得基液(ye)中陰陽離(li)子之間相互作(zuo)用增(zeng)大(da)，從而(er)限制(zhi)了(le)基液(ye)的擴散(san)運(yun)動(dong)。

熔融(rong)鹽(yan)納(na)米(mi)流體的比熱(re)容隨著納(na)米(mi)顆(ke)粒(li)質(zhi)量分(fen)(fen)數(shu)的增(zeng)大呈(cheng)現(xian)先增(zeng)大后減(jian)小(xiao)的趨勢(shi)，在加入2%質(zhi)量分(fen)(fen)數(shu)納(na)米(mi)顆(ke)粒(li)時達到最大值，相比純太陽鹽(yan)，增(zeng)大了2.05%（圖(tu)2(c)）。通(tong)過(guo)分(fen)(fen)子(zi)動力(li)學(xue)模擬計算(suan)發現(xian)在納(na)米(mi)顆(ke)粒(li)表(biao)面(mian)存在K+壓縮層，并推測這是比熱(re)容增(zeng)強的微觀機(ji)理（圖(tu)2(e)）。納(na)米(mi)顆(ke)粒(li)質(zhi)量分(fen)(fen)數(shu)的增(zeng)大也(ye)使得熔融(rong)鹽(yan)體系(xi)的熱(re)導(dao)率(lv)不斷增(zeng)大，通(tong)過(guo)對體系(xi)能(neng)量分(fen)(fen)析可以推測是離(li)子(zi)的碰撞被(bei)強化導(dao)致熱(re)導(dao)率(lv)的增(zeng)強（圖(tu)2(d)）。

圖2太陽鹽熱物性的分子動力學模擬研究

熔(rong)(rong)融(rong)鹽(yan)(yan)以及(ji)其在多(duo)孔介質(zhi)中(zhong)(zhong)的(de)相變特性對于指導(dao)熔(rong)(rong)融(rong)鹽(yan)(yan)蓄(xu)能(neng)有重要意(yi)義。采用VOF和(he)(he)(he)焓-多(duo)孔介質(zhi)模型(xing)耦合求解，數值(zhi)研(yan)究(jiu)了(le)熔(rong)(rong)融(rong)鹽(yan)(yan)熔(rong)(rong)化過程(cheng)(cheng)熔(rong)(rong)鹽(yan)(yan)/空(kong)氣界(jie)面的(de)上升(sheng)和(he)(he)(he)固/液界(jie)面的(de)變化。發現(xian)由體積(ji)膨脹引起的(de)熔(rong)(rong)融(rong)鹽(yan)(yan)/空(kong)氣界(jie)面在熔(rong)(rong)化過程(cheng)(cheng)中(zhong)(zhong)逐(zhu)漸上升(sheng)，而由體積(ji)收縮引起的(de)熔(rong)(rong)融(rong)鹽(yan)(yan)/空(kong)氣界(jie)面在凝固過程(cheng)(cheng)中(zhong)(zhong)逐(zhu)漸下(xia)降（圖3(b)）。受到自然(ran)對流(liu)和(he)(he)(he)密度差影響，固態熔(rong)(rong)融(rong)鹽(yan)(yan)會出現(xian)明(ming)顯的(de)下(xia)沉現(xian)象，這(zhe)為(wei)蓄(xu)能(neng)系(xi)統封裝過程(cheng)(cheng)提供了(le)重要的(de)理(li)論指導(dao)。

自(zi)(zi)然(ran)(ran)對流(liu)在熔(rong)鹽融(rong)化過程中占據主(zhu)導，可分為出現、發展、消退三個階段；熔(rong)鹽熔(rong)化過程中的溫差和固/液界面的位置也影響了自(zi)(zi)然(ran)(ran)對流(liu)的發展，熔(rong)化后期自(zi)(zi)然(ran)(ran)對流(liu)顯著地削弱。與(yu)沒有泡沫金屬的純熔(rong)融(rong)鹽相(xiang)比，泡沫金屬的加(jia)入可以有效地提高熔(rong)融(rong)鹽的熔(rong)融(rong)速率(lv)，但(dan)對自(zi)(zi)然(ran)(ran)對流(liu)有抑(yi)制(zhi)作用。

圖3純太陽鹽的熔化/凝固特性以及其在多孔介質中的熔化特性

接著(zhu)(zhu)(zhu)，以該共熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)、熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)/泡沫(mo)(mo)(mo)銅(tong)復合物(wu)和(he)(he)熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)/泡沫(mo)(mo)(mo)鎳復合物(wu)作為蓄存(cun)介質(zhi)，在(zai)圓柱(zhu)(zhu)形潛(qian)熱(re)(re)(re)(re)蓄熱(re)(re)(re)(re)單(dan)元(yuan)內完成了(le)(le)其儲(chu)/放(fang)能實驗。構(gou)建(jian)了(le)(le)一(yi)個包(bao)括焓-多孔介質(zhi)項、非(fei)達西(xi)效應項、考慮熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)和(he)(he)泡沫(mo)(mo)(mo)金(jin)(jin)(jin)(jin)屬(shu)間熱(re)(re)(re)(re)非(fei)平(ping)衡的(de)(de)雙(shuang)溫(wen)度(du)能量(liang)(liang)方程的(de)(de)三維模型來進一(yi)步(bu)數值研究(jiu)該蓄熱(re)(re)(re)(re)單(dan)元(yuan)的(de)(de)傳(chuan)熱(re)(re)(re)(re)特性。通過(guo)圓柱(zhu)(zhu)繞流的(de)(de)方式(shi)構(gou)建(jian)了(le)(le)熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)相變材料和(he)(he)泡沫(mo)(mo)(mo)金(jin)(jin)(jin)(jin)屬(shu)的(de)(de)雙(shuang)溫(wen)度(du)能量(liang)(liang)方程，發現(xian)由(you)于(yu)泡沫(mo)(mo)(mo)金(jin)(jin)(jin)(jin)屬(shu)的(de)(de)流動阻力，對(dui)于(yu)熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)/泡沫(mo)(mo)(mo)金(jin)(jin)(jin)(jin)屬(shu)復合物(wu)，熔(rong)(rong)化過(guo)程自(zi)然(ran)對(dui)流有所(suo)削(xue)弱。但由(you)于(yu)熱(re)(re)(re)(re)導率顯(xian)(xian)著(zhu)(zhu)(zhu)增(zeng)強，由(you)導熱(re)(re)(re)(re)主導的(de)(de)放(fang)能過(guo)程顯(xian)(xian)著(zhu)(zhu)(zhu)加快。此外(wai)，發現(xian)了(le)(le)熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)和(he)(he)泡沫(mo)(mo)(mo)金(jin)(jin)(jin)(jin)屬(shu)間的(de)(de)熱(re)(re)(re)(re)非(fei)平(ping)衡特性，由(you)于(yu)金(jin)(jin)(jin)(jin)屬(shu)骨架(jia)高的(de)(de)熱(re)(re)(re)(re)導率，熔(rong)(rong)融(rong)(rong)鹽(yan)(yan)(yan)(yan)和(he)(he)泡沫(mo)(mo)(mo)金(jin)(jin)(jin)(jin)屬(shu)間存(cun)在(zai)很明顯(xian)(xian)的(de)(de)溫(wen)差（圖4(c)），比如：儲(chu)能過(guo)程中(zhong)(zhong)熔(rong)(rong)鹽(yan)(yan)(yan)(yan)和(he)(he)銅(tong)骨架(jia)的(de)(de)最大溫(wen)差為6.8°C，而熔(rong)(rong)鹽(yan)(yan)(yan)(yan)和(he)(he)鎳骨架(jia)的(de)(de)最大溫(wen)差為4.4°C。這提出了(le)(le)在(zai)構(gou)建(jian)多孔蓄熱(re)(re)(re)(re)介質(zhi)中(zhong)(zhong)的(de)(de)傳(chuan)熱(re)(re)(re)(re)模型時需考慮熱(re)(re)(re)(re)非(fei)平(ping)衡現(xian)象。

圖4儲能過程多孔基熔融鹽的溫度云圖和固/液界面、儲/放熱溫升/降特性、復合物中熔鹽和金屬骨架的溫差

熔(rong)(rong)(rong)融(rong)(rong)鹽(yan)儲/放(fang)熱(re)(re)(re)的(de)(de)實際系(xi)統(tong)報道很少，采用多孔介質強化純熔(rong)(rong)(rong)鹽(yan)的(de)(de)復(fu)合相(xiang)(xiang)變(bian)材料(liao)(liao)系(xi)統(tong)的(de)(de)運行特性更是鮮有報道。在對材料(liao)(liao)的(de)(de)熱(re)(re)(re)物(wu)性研究之后，開展了(le)多孔基納(na)米(mi)復(fu)合熔(rong)(rong)(rong)融(rong)(rong)鹽(yan)的(de)(de)儲/放(fang)熱(re)(re)(re)特性研究。整個蓄(xu)能系(xi)統(tong)填充了(le)純熔(rong)(rong)(rong)融(rong)(rong)鹽(yan)、納(na)米(mi)熔(rong)(rong)(rong)鹽(yan)（含2 wt.%Al2O3）和(he)(he)納(na)米(mi)熔(rong)(rong)(rong)鹽(yan)/泡沫(mo)銅(tong)復(fu)合物(wu)。然(ran)后在不(bu)同加熱(re)(re)(re)溫(wen)度(du)下對純熔(rong)(rong)(rong)鹽(yan)和(he)(he)復(fu)合相(xiang)(xiang)變(bian)材料(liao)(liao)進行了(le)蓄(xu)能系(xi)統(tong)的(de)(de)儲/放(fang)熱(re)(re)(re)試驗，測(ce)量了(le)不(bu)同位(wei)置(zhi)（包括(kuo)徑向位(wei)置(zhi)、角向位(wei)置(zhi)和(he)(he)軸向位(wei)置(zhi)）的(de)(de)溫(wen)度(du)變(bian)化和(he)(he)分布。結果表明，填充納(na)米(mi)熔(rong)(rong)(rong)鹽(yan)/泡沫(mo)銅(tong)復(fu)合相(xiang)(xiang)變(bian)材料(liao)(liao)的(de)(de)系(xi)統(tong)得到(dao)大幅度(du)的(de)(de)強化提(ti)(ti)高，例如(ru)：與純HITEC熔(rong)(rong)(rong)鹽(yan)相(xiang)(xiang)比，在160℃的(de)(de)加熱(re)(re)(re)溫(wen)度(du)下的(de)(de)蓄(xu)熱(re)(re)(re)時(shi)間可縮短約58.5%。納(na)米(mi)熔(rong)(rong)(rong)鹽(yan)/泡沫(mo)銅(tong)復(fu)合相(xiang)(xiang)變(bian)材料(liao)(liao)在加熱(re)(re)(re)溫(wen)度(du)為(wei)180℃時(shi)的(de)(de)平均(jun)蓄(xu)熱(re)(re)(re)功率為(wei)109.32kW/m3，較純HITEC鹽(yan)的(de)(de)53.01 kW/m3提(ti)(ti)高了(le)近100%（圖5(c)）。

圖5多孔基納米熔融鹽儲/放熱曲線和蓄熱功率

最(zui)后，實(shi)驗(yan)研(yan)究了梯(ti)(ti)級蓄(xu)(xu)(xu)(xu)熱(re)(re)裝置的(de)(de)特性(xing)，制備并(bing)填(tian)充(chong)(chong)了相變(bian)溫度分(fen)別(bie)為120℃(Ca(NO3)2-KNO3-NaNO3),142℃(NaNO2-KNO3-NaNO3),155℃(Ca(NO3)2-KNO3-NaNO3)的(de)(de)3種(zhong)共(gong)熔鹽，在(zai)加熱(re)(re)溫度分(fen)別(bie)為180℃、200℃和220℃下，研(yan)究了填(tian)充(chong)(chong)任意兩種(zhong)熔融鹽的(de)(de)梯(ti)(ti)級蓄(xu)(xu)(xu)(xu)熱(re)(re)特性(xing)。結(jie)合(he)ε-NTU分(fen)析方(fang)法，并(bing)與單(dan)一HITEC鹽（142℃）的(de)(de)蓄(xu)(xu)(xu)(xu)熱(re)(re)特性(xing)比較。發現(xian)梯(ti)(ti)級蓄(xu)(xu)(xu)(xu)熱(re)(re)的(de)(de)方(fang)式(shi)可(ke)將總蓄(xu)(xu)(xu)(xu)熱(re)(re)時長縮短10%左右，潛熱(re)(re)蓄(xu)(xu)(xu)(xu)熱(re)(re)時長提(ti)(ti)升(sheng)13%（圖6(b)）；梯(ti)(ti)級蓄(xu)(xu)(xu)(xu)熱(re)(re)的(de)(de)方(fang)式(shi)也可(ke)以將蓄(xu)(xu)(xu)(xu)熱(re)(re)系統的(de)(de)有效度由0.06提(ti)(ti)升(sheng)至0.14（圖6(d)）。當換熱(re)(re)流體(ti)為200℃時，120~155℃組合(he)的(de)(de)梯(ti)(ti)級儲熱(re)(re)裝置具有最(zui)佳的(de)(de)性(xing)能。

圖6梯級熔鹽儲存裝置蓄熱時長及有效度

結論與展望

近些年(nian)，肖鑫副教(jiao)授課題(ti)組從熔(rong)(rong)融(rong)鹽(yan)(yan)(yan)/納米(mi)顆粒/泡沫金屬復合物(wu)的(de)(de)結構(gou)特性(xing)和(he)(he)(he)熱(re)物(wu)性(xing)出發(fa)，實驗和(he)(he)(he)模擬剖析(xi)了熔(rong)(rong)融(rong)鹽(yan)(yan)(yan)熔(rong)(rong)化特性(xing)（界面(mian)上升(sheng)、重力下沉），理論分(fen)析(xi)揭示納米(mi)顆粒和(he)(he)(he)熔(rong)(rong)融(rong)鹽(yan)(yan)(yan)的(de)(de)界面(mian)尺(chi)寸(cun)效應，以及多(duo)孔介質和(he)(he)(he)熔(rong)(rong)鹽(yan)(yan)(yan)間的(de)(de)熱(re)非平衡(heng)特性(xing)。最(zui)后研究了填(tian)充復合熔(rong)(rong)融(rong)鹽(yan)(yan)(yan)的(de)(de)蓄能(neng)裝(zhuang)置的(de)(de)儲/放(fang)熱(re)特性(xing)，以及梯級蓄能(neng)方式的(de)(de)優化。相關研究有望為熔(rong)(rong)融(rong)鹽(yan)(yan)(yan)在太陽(yang)能(neng)集熱(re)發(fa)電的(de)(de)高效應用(yong)中發(fa)揮(hui)重要作用(yong)。未來可從耐腐蝕(shi)性(xing)、成本、相容(rong)性(xing)、封裝(zhuang)等(deng)上開展(zhan)工作，從而推動熔(rong)(rong)融(rong)鹽(yan)(yan)(yan)蓄熱(re)的(de)(de)發(fa)展(zhan)。

論文信息

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