熱泵讓車輛的氣候控制系統(tǒng)的成本上升了50%，但是捷豹最新車型和電動校車的應(yīng)用案例表明，如果能改善設(shè)計，熱泵產(chǎn)生的效益完全可以彌補這些額外成本。
氣溫低時，搭載了熱泵的純電動汽車是否能以合理的系統(tǒng)成本，既減少顯著的能量損失，又保持座艙的舒適溫暖呢？這個問題在2018年SAE熱管理系統(tǒng)研討會上引發(fā)了熱烈討論。盡管針對量產(chǎn)純電動車型的有效解決方案尚未成型，但純電動汽車的熱管理已經(jīng)取得了重大進步。
如果采用目前通行的熱敏電阻（PTC）加熱的方法，純電動汽車的續(xù)航里程在冬季會縮水大約40%。對此，車企普遍表示不滿。根據(jù)Hanon Systems在2017年所做的測試，按照50公里通勤距離和30分鐘通勤時間算，如果座艙已經(jīng)預(yù)熱完畢，那么一個簡易的制冷劑循環(huán)余熱收集回路從電子元件、電機和電池包中采集的熱量，對座艙制熱而言已經(jīng)足夠了。
Hanon的研究表明，這種回路設(shè)計的經(jīng)濟效益和效率都遠遠高出熱泵。盡管如此，隨著續(xù)航里程更長的純電動汽車進入市場，熱泵還是會占有一席之地。尤其是對于將由柴油轉(zhuǎn)為電力驅(qū)動的校車而言，熱泵比制冷劑回路更為合適。
蒸氣注入法
捷豹將于2019年推出的全新I-Pace四驅(qū)車型在瑞典阿爾耶普盧格的冬季測試中排名第一，該款車型搭載了一個90千瓦時的電池包，額定續(xù)航里程高達240分鐘/384公里。和日產(chǎn)聆風(fēng)相比，I-Pace著眼于高端市場，定價更加靈活。據(jù)稱，該款純電動車型在極寒天氣下的續(xù)航里程幾乎不會衰減，其采用的熱泵系統(tǒng)也融入了多項創(chuàng)新設(shè)計。
作為此次SAE熱管理研討會的演講嘉賓之一，捷豹路虎熱管理工程師Nilabza Dutta在報告中指出，雖然I-Pace的HVAC系統(tǒng)增加了“30%-50%”的成本，但如果通過增大電池包的尺寸來彌補使用熱敏電阻造成的續(xù)航里程損失，成本將上升150-450%，除此之外，電池的充電還將產(chǎn)生80%到300%的額外成本。
因此，捷豹I-Pace選用了熱泵系統(tǒng)，并大力對其進行了優(yōu)化。首先，HVAC的空氣分配有三種模式：僅駕駛座、駕駛座加副駕、前排加后排。其次，還部署了吸收電子器件熱量的制冷劑回路。為了實現(xiàn)最佳制熱性能，I-Pace搭載了一個注氣式HVAC熱泵系統(tǒng)，不過該系統(tǒng)并不包含目前很多純電動車型所采用的油門啟動式再生制動系統(tǒng)。
去年，電裝在普銳斯Prime插電式混合動力車型上引入了注氣式熱泵，其中包含一個液氣分離器。這種從商用熱泵系統(tǒng)中借鑒而來的設(shè)計，在低溫條件下往壓縮機里注入熱蒸氣產(chǎn)熱，有效克服了熱泵在氣溫低至0℃/32℉時因為制冷劑的質(zhì)量流率降低而失效的問題。
豐田稱，Prime的最低工作溫度是14℉/-10℃。Dutta表示，搭載了吸熱回路的I-Pace系統(tǒng)可以回收高達4.5 千瓦的能量。他在研討會上指出，I-Pace的吸熱回路的功率為0.3千瓦，在0℃（32℉）時，可以從電子器件和電機中分別收集2.5 千瓦和1.9千瓦的能量，凈收益總共為3.8千瓦。當氣溫降到-8℃(18℉)時，制冷劑回路系統(tǒng)也可以收集3.5千瓦的能量。
Hanon的研究表明，捷豹I-Pace的座艙預(yù)熱的作用已然十分顯著。如果沒有座艙預(yù)熱，熱泵系統(tǒng)只能節(jié)約17%的能量。有了座艙預(yù)熱后，節(jié)能效果高達83%。就算關(guān)閉熱泵，搭載了座艙預(yù)熱的I-Pace依然能節(jié)約49%的能量。其續(xù)航里程效益更是令人驚嘆：在零下10℃到0℃（14-32℉）的條件下可以增加30-45公里（18-27公里）。
此外，為了優(yōu)化續(xù)航里程，I-Pace還具備電池控制模塊的空中升級軟件性能。升級軟件和所有Alexa語音控制設(shè)備兼容，還能升級車載娛樂系統(tǒng)。
校車熱泵項目
一年中最熱的幾個月，學(xué)校通常是在放暑假，因此校車座艙設(shè)計一般重點關(guān)注寒冷天氣下的制熱問題，制熱部件一般放置在座椅、駕駛員和階梯下方。在熱能管理研討會上，來自艾默生環(huán)境優(yōu)化技術(shù)有限公司的Shawn Vehr表示，有些地區(qū)的校車的A/C裝配率大約只有25%。
如果要在校車上部署制冷劑回路，那就要從車頭繞到車尾，線路會拉得很長，這也是為什么很多校車沒有安裝A/C系統(tǒng)。不過，盡管以柴油驅(qū)動的校車仍占主導(dǎo)，很多汽車制造商也在著手研發(fā)電動校車。對此，Vehr提議采用四季通用的雙電動密閉式注氣熱泵組件。
Vehr表示，根據(jù)臺架測試結(jié)果，對于一輛搭載了600伏特、440千瓦時的電池包的電動客車而言，如果單獨使用熱敏電阻加熱，續(xù)航里程會從原先的305英里/488公里降到177英里/283公里，而如果搭載了注氣熱泵系統(tǒng)，續(xù)航里程可以維持在230到257英里/368-411公里之間。
該熱泵系統(tǒng)可以按照美國國家運輸安全委員會（NTSB）的標準，把座艙的溫度從0℉提高到68℉（-18℃到20℃）。根據(jù)NTSB的規(guī)定，熱泵系統(tǒng)應(yīng)該在30分鐘內(nèi)將環(huán)境溫度從80℉加熱到100℉（27℃到38℃），這比NTSB的降溫標準寬松一些。
They add 50 percent to climate-system outlay, but improved designs indicate costs can be recovered, for Jags or an electric school bus.
Can a heat pump-equipped battery electric vehicle (BEV) overcome major energy losses for cabin heating comfort in cold ambient conditions –and at reasonable system cost? These questions were addressed at the 2018 SAE Thermal Management Systems Symposium, and although definitive answers may not all be production ready, promising work has been done.
The industry recognizes as unacceptable the BEV battery pack range losses of up to 40%+ from winter operation with PTC (positive temperature coefficient) heating, which is the conventional approach. Testing by Hanon Systems in 2017 showed simple, coolant-circulating waste heat-gleaning circuits from power electronics, electric motor and battery pack would produce sufficient thermal storage during a commute-length drive (50 km/30 mi) in a pre-conditioned cabin (as part of a battery pack charge).
The Hanon research indicated this approach is far more cost-effective and efficient than a heat pump. However, as BEVs with greater range enter the market, the heat pump comes back into play. It also is being considered for such other uses as school buses, now typically diesel-powered, but being evaluated for electrification.
Vapor-injection approach
Jaguar’s new 2019 I-Pace AWD model (top, winter testing in Arjeplog, Sweden) features a 90 kW·h battery pack and up-to-240-mi/384 km rated range. It’s a premium-market vehicle with more flexibility in its cost/pricing structure than a Nissan Leaf, for example. The electric Jag also is expected to deliver near its rated range in very cold weather. And although it includes a heat pump, there is much more to the system.
In his SAE TMSS presentation, Nilabza Dutta, a Jaguar Land Rover (JLR) thermal-management engineer, cited “30% to 50%” as the extra cost of the I-Pace’s HVAC system. However, he said, there would be added cost of 150-450% for covering the range shortfall of using PTC by increasing the size of the battery pack. There would be an additional cost over that of the battery pack: recharging it from the grid, which Dutta said was in the range of 80-300%.
The Jaguar approach, therefore, was “do whatever is necessary” within the alternative cost framework. First, the HVAC air distribution is three-zone: driver only, driver and a front passenger and both front and rear. Next, there is a heat-gleaning coolant circuit for the electronics. It is integrated with a vapor-injection HVAC heat pump system for maximum heating season performance. It does not cover the regenerative braking – a “one-pedal” control system that is based off the accelerator and similar to those used on various BEVs.
The vapor-injection heat-pump design, introduced on motor vehicles last year by Denso on the Prius Prime plug-in hybrid, incorporates a liquid-gas separator. This approach, adapted from commercial heat pump systems, injects the hot vapor into the compressor to produce system heat at low ambient temperatures. In this arrangement the heat pump, which normally is ineffective at 0°C/32°F because of the slowdown in refrigerant mass flow in low temperatures, remains operational.
Toyota cites 14° F/-10°C as the lower end for the Prime. The I-Pace system, with its integrated heat gleaning, can pull up to 4.5 kW, Dutta said. At 0°C (32° F), it can draw 2.5 kW from the electronics/motor system with a collection loss of 0.3 kW, for a net gain of 2.2; and 1.9 kW with a loss of 0.3, for a total of 3.8 kW, he told the TMSS session. Even at -8°C (18°F), the system can pull a total of 3.5 kW, he added.
The effect of cabin preconditioning, as demonstrated by the Hanon research, is enormous even for the Jaguar I-Pace. Without it, the heat pump saves only 17%; with it, the saving jumps to 83%. And even with the heat pump turned off, the preconditioned car saves 49%. Range extension on the I-Pace as equipped is an impressive 30-45 km (18-27 mi), in operation from minus 10 C to 0 C (14-32 F).
Additionally, the I-Pace has an over-the-air software update feature for the battery control module, to optimize range. It works with any Alexa voice communication device, and at this time also can update the infotainment system.
School bus heat pump project
Schools typically are closed for vacation during the hottest weather months, so bus cabin comfort typically focuses on heat in cold weather, with under-seat and driver/stairwell units. There are geographical exceptions, however, and A/C is used in perhaps 25% of school buses, according to a TMSS presentation by Shawn Vehr of Emerson Climate Technologies.
But the need for long refrigerant lines to the rear of the vehicle has been a deterrent to A/C installations. Although diesel-powered school buses are predominant, work is underway to develop electrically powered buses. Emerson’s proposal is for an all-electric bus using two electrically powered self-contained heat pump assemblies with vapor injection. This would permit year-round use.
According to Vehr, off-vehicle testing indicated a severe loss with PTC heating alone for an electric bus with a 600-volt, 440 kW·h battery pack, with a projected range of 305 mi/488 km diminishing to 177 mi/283 km. With a vapor-injected heat pump system, however, the range would be 230-257 mi/368-411 km, he said.
The heat pump was demonstrated to be capable of pulling cabin temperature from a cold soak at 0°F up to 68°F (-18°C to 20°C). It also would meet the NTSB (National Traffic Safety Bureau) standard for hot soak pulldown to 80°F from 100°F (27°C to 38°C) in 30 minutes, though falling slightly short of high- performance cooling standards.
Author: PAUL WEISSLER
Source: SAE Automotive Engineering Magazine
在Jaguar I-Pace中，該圖表顯示了熱泵冷卻操作的優(yōu)勢，包括四區(qū)和雙區(qū)（下方的虛線）。
Emerson提出的電動校車將使用兩個獨立的熱泵系統(tǒng)，全年為車廂提供舒適的環(huán)境。(ECT)
在3.5℉（零下15.8℃）的高性能測試中，Emerson圖表顯示校車熱泵依然達到標準冷卻需求。