目前，熱存儲系統(tǒng)已經(jīng)成為一種低成本的溫控解決方案，在溫暖氣候下，可以在發(fā)動機怠速時維持車輛的制冷功能。最近的建模研究與實驗測試顯示，這種技術(shù)也同樣可以作為一種平價、高效的解決方案，為電動車的車艙供暖，從而解決電動車冬季續(xù)航里程減少的問題。SAE 2016全球汽車年會期間，來自國際領(lǐng)先汽車與發(fā)動機制造商馬勒(Mahle)公司與美國橡樹嶺國家實驗室的研究人員展示了他們的最新研究結(jié)果。
通常來講，電動車在寒冷天氣下續(xù)航里程可能會減少60%。這是由于在天氣寒冷時，車輛必須耗費大量電力為車艙供暖，而電動車動力電子元件釋放的少量熱量又很難收集利用。至今為止，電動車的動力電池仍然是車輛供暖系統(tǒng)的唯一電源，除此之外，只有一些從制冷/制熱反向循環(huán)中收集的能量可供利用。
46分鐘通勤駕駛所需熱量
對于駕駛員而言，由于需要供暖，在寒冷的季節(jié)里駕駛電動車上班可能并不舒服，行駛距離更長時，甚至還會讓他們產(chǎn)生里程憂慮。
現(xiàn)在大家都已了解到，為了保證電動車的取暖需求，而為車輛加裝額外的電池，從而為車上PTC（正溫度系數(shù)）或電阻加熱器供電，成本實在太高了。提前預(yù)熱車艙可以在一定時間內(nèi)保證車內(nèi)溫度，加熱座椅和加熱方向盤等功能也能讓乘客感覺更溫暖。然而，根據(jù)一項蓋洛普(Gallup)調(diào)查，美國人平均每天的往返通勤時間為46分鐘。在這樣的背景下，研發(fā)人員知道他們必須采取更多措施，緩解車主的冬日電動車“里程焦慮”，或者降低電動車在工作日的充電需求。
馬勒公司與橡樹嶺國家實驗室的研究人員表示，如果能夠通過相變材料(PCM)儲存充足的熱量，滿足車主正常通勤途中的取暖需求，則可以提高電動車在一些終年寒冷地區(qū)市場中的銷量。PCM材料是指一種可以隨溫度變化而改變物質(zhì)狀態(tài)（可以由固體變?yōu)橐后w，或由液體變?yōu)楣腆w），并能夠在相對常溫的條件下吸收或釋放潛熱的物質(zhì)。
如今，市面上已經(jīng)有一些使用內(nèi)燃機的傳統(tǒng)汽車開始利用PCM材料，維持空調(diào)在發(fā)動機怠速時的運轉(zhuǎn)。通常來說，由于發(fā)動機怠速最多不會超過1分鐘，因此PCM材料在這種情況下的應(yīng)用成本較低，封裝也很簡單。駕駛員也總是可以在有必要時，重新啟動發(fā)動機和空調(diào)，保持車艙內(nèi)的舒適環(huán)境。內(nèi)燃機車型一般將PCM材料存儲在蒸發(fā)器內(nèi)，可以在空調(diào)壓縮機停止工作后繼續(xù)提供冷空氣。
在供暖方面，PCM材料一般存儲在發(fā)動機蓋下的熱交換器艙室中，冷卻液流向儀表盤后方加熱器核心區(qū)域的回路內(nèi)。這種PCM材料屬于固體石蠟族，與輔助冷卻系統(tǒng)中的材料很類似，但加工流程存在一定差異，最終導致該材料可以與空氣和水分發(fā)生反應(yīng)。因此，這種PCM材料必須小心儲存，在向PCM熱交換器艙室中加入這種材料之前，必須先排空其中所有的氣體與水分。接著，再使用特殊設(shè)備充入PCM材料，然后密封熱交換器的艙室。
研究人員評估了8種備選的PCM材料，結(jié)果發(fā)現(xiàn)：DPT 83的綜合表現(xiàn)最佳，可以作為首選材料，其熔點（即相變溫度）為83°C (181°F)，與電動車冷卻液的85°C (185°F)非常接近。因此，這種材料可以提供與PTC加熱系統(tǒng)類似的加熱器冷卻溫度。DPT 83是8種材料中潛熱容量最高的一種，高達348J/g，遠遠超過之前PCM材料200J/g的潛熱容量，而更高的潛熱容量有助于縮小系統(tǒng)的封裝體積。
另一種PCM材料DPT 68的性能略低于DPT 83，但同樣也能滿足需求，其相變溫度為68ºC (154ºF)，潛熱容量為342 J/g。
由于空調(diào)的怠速需求，PCM材料必須能在空調(diào)-發(fā)動機運轉(zhuǎn)期間快速儲存熱量，并為下一次怠速做好準備。但如果要滿足寒冷氣候下(-10°C/14°F)的車艙取暖需求，PCM系統(tǒng)需要面臨一個更為關(guān)鍵的挑戰(zhàn)，那就是如何保證系統(tǒng)不會在車輛長時間停在戶外時，損失存儲熱量。為了解決這一問題，研發(fā)人員為PCM系統(tǒng)采用了真空絕熱面板材料的輕質(zhì)隔熱外殼。如果一個工作日按八小時計算，預(yù)計這種PCM系統(tǒng)可以將熱量損失控制在20%之內(nèi)。
However, the engineering target is 90% heat retention and the Mahle and ORNL researchers believe that target can be met with improved construction.
然而，現(xiàn)階段的目標，是將熱量損失控制在10%之內(nèi)。不過，雖然要求更高，但馬勒公司與橡樹嶺國家實驗室的研究人員認為，通過結(jié)構(gòu)優(yōu)化，10%的目標并非難以企及。
重量的代價
盡管PCM系統(tǒng)性能有所提升，但該系統(tǒng)的確也會同時增加車輛的重量。但與通過增加電池組解決取暖問題相比，PCM系統(tǒng)增加的重量不會增加，甚至還可能減少。研究人員最終選定的PCM系統(tǒng)重量約33 kg (73 lb)，內(nèi)含重約21 kg (46 lb)的PCM材料，外加重約12 kg (26 lb)的PCM熱交換器（溫度上限為120°C/248°F），可至少將車輛的續(xù)航里程延長20%。
PCM系統(tǒng)也能用于預(yù)熱車艙，從而將車輛行駛過程中的供暖需求降至最低，也就是溫度保持狀態(tài)。
在制冷中，冷空氣在經(jīng)過儀表盤后的蒸發(fā)器時，可以使液態(tài)石蠟PCM材料凝固成固體。當車輛怠速時，空調(diào)壓縮機停止工作，此時熱空氣將穿過處于冷卻固體狀態(tài)中的PCM材料，降低溫度后進入車艙，實現(xiàn)制冷功能。由于熱空氣會在這個冷卻過程中向PCM材料釋放熱量，此時PCM材料將從固態(tài)轉(zhuǎn)為液態(tài)，直到空調(diào)重新開始工作，或固態(tài)PCM材料全部變?yōu)橐簯B(tài)。
在用于車內(nèi)取暖時，PCM系統(tǒng)的功能與車輛怠速時正好相反。目前，這種系統(tǒng)被命名為ePATHS，即電動PCM協(xié)助熱采暖系統(tǒng)(electric PCM-Assisted Thermal Heating System)。ePATHS系統(tǒng)內(nèi)含兩個熱交換器：PCM熱交換器和儀表盤后方的車艙加熱器，以及連接這兩個設(shè)備之間的管道和控制閥。此外，系統(tǒng)還配置了用于輔助常規(guī)PTC加熱器的電路。值得注意的是，這一系統(tǒng)中的PCM材料在常溫下為固態(tài)。
該系統(tǒng)的工作方式是，首先車輛通過插電式供電系統(tǒng)進行預(yù)熱，可利用電力驅(qū)動PCM熱交換器中的加熱元件提升車艙的溫度，并同時加熱PCM材料使其儲存熱量，變?yōu)橐簯B(tài)。
電動泵回路
車輛運行時，車內(nèi)的電動泵回路（持續(xù)運行和/或脈寬調(diào)制模式）可以將水和乙二醇的混合物送入PCM熱交換器，吸收PCM材料的熱量，再流入儀表盤后方的車艙加熱器。內(nèi)燃機車型也會將經(jīng)過發(fā)動機加熱的冷卻液送入車艙供暖裝置，采用的原理與電動車類似。
這種解決方案可以將現(xiàn)有電動車在寒冷天氣下的續(xù)航里程提高20%，輕松超過原定目標。如果進入PCM系統(tǒng)的外部空氣有70%溫度在-10°C/14°F之間，則系統(tǒng)能夠運行40分鐘。
整個PCM加熱系統(tǒng)的功率輸出相當于3.3kW·h。不過，PCM系統(tǒng)還可以利用一些其他熱量：當PCM材料已經(jīng)釋放了絕大部分潛熱時，系統(tǒng)內(nèi)還會殘留一些熱量，原因是這部分熱量的溫度無法達到60°C (140°F)，這是PCM系統(tǒng)發(fā)揮加熱功能的溫度需求。
即使如此，這部分熱量還有其他可以發(fā)揮作用的地方，系統(tǒng)的雙加熱器核心就可能從中受益。此時， PCM熱交換器將可以繼續(xù)循環(huán)從核心前方流入的經(jīng)過加熱的乙二醇/水混合物，對流入的氣流進行預(yù)熱，這個過程可以將PCM系統(tǒng)的工作時間延長10分鐘，也就是說此時PCM系統(tǒng)總共可以運行50分鐘。接著，當經(jīng)過加熱后的乙二醇/水混合物流入核心后方區(qū)域時，車輛的PTC加熱器將被激活，開始進行供暖。
作者：Paul Weissler
來源：SAE《汽車工程》雜志
翻譯：SAE上海辦公室
2016 SAE Congress: Thermal storage is solution to EV winter range loss
Thermal storage, now coming into use as a low-cost method to maintain A/C cooling during idle stops in warm weather, has been identified by modeling studies and laboratory tests as also offering an inexpensive and effective solution for heating the cabin of electric vehicles. The study results were presented by researchers from Mahle and the Oak Ridge National Laboratory during the 2016SAE World Congress.
Electric vehicles typically lose up to 60% of their operating range during cold weather operation due to the power required to heat the passenger compartment, while the small amount of heat rejected by EV power electronics would be difficult to collect efficiently. To now, the vehicle's traction batteries have been the only power source for conventional heating, with some added efficiency from reverse-cycling the A/C into heat pump operation.
Heat needed for 46-min commute
For EV drivers, the winter-heating demand can make a routine commute uncomfortable and even dreadful over longer distances.
Adding enough extra battery capacity for a commute-length source of conventional warmth from PTC (positive temperature coefficient) or resistance heaters has been deemed too costly. Preheating the cabin provides a short period of warmth, and features such as heated seats and steering wheel can improve occupants' perception of warmth. However, the length of the average U.S. round-trip commute is 46 minutes, according to a Gallup survey. Engineers recognize that more has to be done to make the winter commute free of range anxiety – or the need to recharge while the car is parked during the workday.
Sufficient thermal storage in Phase Change Material (PCM) for a complete commute could improve the EV's prospects as year-round transportation in cold-winter markets, according to the Mahle and ORNL experts. PCM is a class of materials that go through a phase change (solid-to-liquid or vice versa) while absorbing or releasing a large amount of latent heat at a relatively constant temperature.
The use of PCM to maintain A/C cooling during an idle stop, already available for some internal-combustion vehicles, is a relatively low-cost, easily packaged answer, as the total required cooling interval typically is less than one minute. And if necessary, the engine and A/C system can be restarted to maintain comfort. For IC vehicles, the PCM is held in storage areas built into the evaporator, chilling the airflow when the compressor has been stopped.
For heating, the PCM is stored in chambers in an underhood heat exchanger that is in a loop with coolant flow to the under-dash heater core. The material is in the paraffin wax family - similar to that used for supplemental cooling, but it's processed differently and as a result would react with air and moisture. Therefore, the material must be stored carefully and prior to installation in the PCM Heat Exchanger (Hx) chamber, the chamber must be evacuated to remove any air and moisture. Using special equipment, the Hx then is charged with PCM and the chamber is sealed.
The first-choice PCM, designated DPT 83, has a melting point (phase change temperature) of 83°C (181°F), sufficiently close to the EV coolant temperature specification of 85°C (185°F). This should enable it to provide heater coolant temperature approximately equivalent to what a PTC (Positive Temperature Coefficient) heater system would deliver. Latent heat capacity is 348 joules/gram, highest of eight PCMs evaluated and well above the 200 J/g of older PCMs, which helps minimize package size.
Slightly lower in performance but also potentially suitable is DPT 68, which has a phase change temperature of 68ºC (154ºF) and a latent heat rating of 342 J/g.
With A/C idle stop, it is important for the PCM to recharge quickly from the A/C-engine restart and be ready for the next idle stop. But for cabin heating, the more critical demand is ensuring remaining PCM heat storage isn't lost through the workday in cold weather (-10°C/14°F) while the EV is parked outdoors. Through an eight-hour workday, the PCM heat exchanger is projected to retain 80% of the remaining latent heat by employing a lightweight insulation package made with vacuum-insulated panels.
However, the engineering target is 90% heat retention and the Mahle and ORNL researchers believe that target can be met with improved construction.
Weight penalty
The improved PCM notwithstanding, the PCM system does add weight. But (depending on the lithium-ion battery pack used) the overall mass increase is no more, or even less, than increasing the size of the battery pack. The researchers decided on a total package of 33 kg (73 lb), including 21 kg (46 lb) of PCM plus 12 kg/ 26 lb for the PCM heat exchanger (charged to 120°C/248°F) to provide at least 20% range extension.
The PCM system also assumes a pre-heated cabin, which reduces the vehicle-in-operation demand to steady-state heating.
For A/C operation, cold air passing through the under-dashboard evaporator freezes the liquid wax-type PCM to form a cold solid. During idle stop, the compressor stops and warm air passes over the solidified PCM to provide cooled cabin airflow. As the air gives up heat to the PCM, the PCM changes from solid to liquid until the A/C restarts or the cold storage is exhausted.
When the PCM is used for heating the cabin, it functions opposite of the idle-stop process. This configuration was named ePATHS—electric PCM-Assisted Thermal Heating System. It consists of two heat exchangers: the PCM Hx and the under-dash passenger compartment heater, with tubing and control valves to connect them. In addition, there is an electrical loop that adds the conventional PTC heater. The PCM is a solid at ambient temperature.
The pre-charge starts with the EV plug-in system that uses electricity to charge the EV battery pack and, via electric heating elements inside the PCM Hx, also heats the PCM to a liquid (while also operating the PTC heaters to pre-warm the cabin).
Electric pump circuit
In vehicle operation, an electric pump circuit (operating continuously and/or in pulse-width-modulated mode) runs a water-glycol mixture through the PCM Hx, absorbing heat from the molten PCM and flowing to the under-dash heater. This is somewhat similar to the IC-engine setup that flows engine-heated coolant to the cabin heater.
The proposed system is projected to comfortably exceed the targeted 20% increase in EV range. Using 70% outside air at -10°C/14°F, it would operate in PCM-only mode for 40 min.
Total PCM heating is equivalent to 3.3 kW·h. However, there's more heat available: when most of the PCM latent heat is exhausted, there still is some residual heat, even if at about 60°C (140°F) it is at too low a temperature for full PCM heating function.
That's energy worth using, and a dual-heater core is projected to take advantage. The PCM Hx continues to circulate the heated glycol/water mixture through the forward section of the core to preheat the incoming airflow, adding an additional 10 minutes to PCM heating, for a total of 50 min. The PTC heater is activated to complete the heating of the glycol/water solution as it flows through the rear section of the core.
Author: Paul Weissler
Source: SAE Automotive Engineering Magazine
ePATHS示意圖：左邊主要是PCM熱交換器，右邊則是兩步式加熱器核心，車艙PTC加熱器則位于背面。 
熱容量與電功耗數(shù)據(jù)，都可以用于衡量在特定行駛時間內(nèi)，通過儲熱系統(tǒng)節(jié)省的能力。
如圖所示，所需PCM材料的質(zhì)量與行程時長基本成線性增長關(guān)系。