在冬季，如果采用傳統的電采暖系統，電動汽車的續航里程可能“縮水”60%，在極度嚴寒的情況下甚至更為嚴重。因此，豐田 (Toyota) 聯手日本電裝(Denso)，專門設計了一款適用于電動汽車的先進熱泵系統。據了解，這款熱泵已經安裝于豐田普銳斯Prime (Toyota Prius Prime) 插電式混合動力車上。與傳統的電加熱系統以及日產聆風(Nissan Leaf) 和起亞Soul (Kia Soul) 等電動車型采用的熱泵（相對高效，但最低工作溫度通常無法低于0°C/32°F）相比，這款先進熱泵可以提供明顯的性能優化。
在WCX17 – SAE 2017全球汽車年會期間，豐田和電裝的研究人員在一項展示環節中表示，與傳統熱泵相比，豐田普銳斯Prime采用的熱泵可節省高達63%的能量，預計可將車輛的續航里程提高21%。
普銳斯Prime PHEV的熱泵設計借鑒了一個來自固定式商用熱泵的概念，即采用制冷劑液氣分離器 (refrigerant liquid-gas separator) 和制冷劑注氣回路的設計，從而將熱泵有效工作的最低溫度極限降低至-10°C/14°F。一般來說，大型固定式熱泵液氣分離器幾乎很難裝進車輛的發動機艙內。但據稱普銳斯Prime所采用的液氣分離器設計緊湊，大小幾乎與汽車恒溫膨脹閥相差無幾。
電動汽車專用熱泵？
電動模式下，普銳斯Prime混合動力車型的汽油發動機也可能會啟動運行，為車輛提供采暖和除濕功能，但這顯然會影響車輛的燃油經濟性。為了避免損失，混合動力車型將在電動模式下關閉發動機，由熱泵、精密的控制閥系統及液氣分離器實現這部分功能。我們不難想象，普銳斯Prime混合動力車采用的這款熱泵也同樣適用于純電動車型。
簡單來說，熱泵性能受溫度限制的關鍵原因之一在于：制冷劑的質量流率 (mass-flow rate)將隨著環境溫度的變化而變化。與其他汽車熱泵一樣，Prime熱泵也在車輛儀表盤下配置了一款內部冷凝器，作為車艙熱量的釋放源。Prime熱泵選擇了電裝的渦旋式壓縮機單元搭配一個制冷劑氣體注入口的設計。在采暖模式下，高溫的制冷劑氣體將從壓縮機排出至內部冷凝器，而后在內部冷凝器中完成部分冷凝，并在這個過程中為車艙提供熱量。這點與前置式外部冷凝器有較大不同——在空調 (A/C) 模式下，前置式外部冷凝器將向車外釋放熱量，以實現制冷劑的冷凝。
隨后，經過內部冷凝器部分冷凝的氣體將流經一個電子膨脹閥，進行進一步降壓，而后流入Prime熱泵的超緊湊液氣分離器，這也是制冷劑注氣系統的關鍵部分。在采暖模式下，這種設計可以提高熱泵回路的性能。
此后，氣體將離開液氣分離器，進入壓縮機進氣口。此時，液體制冷劑將流經一條節流通道實現進一步冷凝，而后流入外部冷凝器。在采暖模式下，制冷劑將吸收周圍空氣的熱量，而后流入內部冷凝器（位于儀表盤下方），進一步提升車艙內的溫度。
濕度控制挑戰
普銳斯Prime的前擋風窗上安裝了一個濕度/溫度傳感器，可以測量玻璃及車艙內的空氣溫度和濕度。然而，由于在電動模式下，車輛的發動機冷卻系統或排氣系統并不會產生余熱，與壓縮機運行產生的熱量相結合，因此，實際的濕度控制需求也將給熱泵帶來挑戰。
不過，豐田—電裝聯合研發的熱泵系統還可以提供另一種濕度控制解決方案，而且無需啟動發動機。事實上，豐田普銳斯Prime熱泵系統擁有串聯和并聯兩套除濕回路，每套回路對應一段特定的溫度范圍（并聯：32-40° F/0-4° C；串聯：40-60° F/4-16° C）。其中串聯回路可以降低重新加熱的需求。
兩套回路都采用了蒸發器的設計。蒸發器可以基于位于散熱片之間的溫度傳感器信號，蒸發任何液體制冷劑，并從流經熱交換機的空氣中吸收熱量。此外，這兩套除濕回路均可以與車艙采暖回路同時工作。
為了支持車輛空調系統的六種模式，即車艙制冷、采暖、串聯除濕、并聯除濕、外部熱交換機除霜、熱泵采暖（用于周邊環境溫度低時），系統還專門配置了一款濕度傳感器。
The loss of winter driving range in electric vehicles equipped with conventional electric heating systems is well-documented—range being reduced by up to 60% and even more in severe cold. The advanced heat pump system in the Toyota Prius Prime plug-in (PHEV), developed in conjunction with Denso, is designed to deliver improvements compared with conventional electric heating and the heat pumps in the Nissan Leaf and Kia Soul EV. Their automotive heat pumps are efficient, but typically operate down only to 0° C/32° F.
The Prius Prime system uses 63% less energy than conventional heating and should extend driving range up to 21%, Toyota and Denso researchers reported in a presentation at the 2017 SAE World Congress (WCX17).
The Prime PHEV incorporates a concept from some static-mount commercial heat pumps: a refrigerant liquid-gas separator and refrigerant gas injection circuit to provide heat pump efficiency down to -10° C/14° F. The large static-mount heat pump separators would be impractical to package underhood. However, the Prime unit was described as similarly compact to an automotive thermostatic expansion valve.
Heat pump coming for EVs?
The Prime gasoline engine could be started and run to provide heat and dehumidification. But this would affect PHEV fuel economy, so the engine is kept off and the heat pump and a complex system of control valves and the liquid-gas separator is used in EV mode. Which indicates Toyota plans this system also for EVs.
A key reason heat pump performance is thermally-limited is because refrigerant mass-flow rate drops with ambient temperature. Like other automotive heat pumps, the Prime's has an under-dashboard internal condenser (the discharge source for all cabin heat). The compressor is a scroll-type Denso unit with a refrigerant gas injection port added. In heating mode the hot, high-temperature refrigerant gas is discharged from the compressor into the internal condenser, where it provides some cabin heat, condensing partly in the process. This contrasts with the front-mounted outer condenser, which in A/C mode condenses refrigerant by giving up heat to the ambient air.
The partly-condensed gas from the internal condenser then flows through an electric expansion valve for further decompression,and into the ultra-compact liquid-gas separator that is a key part of the refrigerant gas injection system. This gives this heat pump circuit extra performance in the heating mode.
From the separator, the gas goes to the compressor inlet. The liquid refrigerant condenses further as it goes through a throttle passage, and flows through to the outer condenser, where in heat mode it absorbs heat from the ambient air and flows to the internal condenser to contribute further to the passenger compartment warming.
Humidity control challenge
The cabin has a humidity/temperature sensor mounted to the windshield (measuring glass and adjacent interior air temperature, plus cabin humidity). However, actual control of dehumidification is another challenge for a heat pump system, because in EV mode there is no waste heat (from engine cooling system or exhaust system) to combine with heat from compressor operation.
However, the Toyota-Denso heat pump system also provides a way without having to start the engine. In fact, there are two circuits, a series and a parallel, each intended for a specific ambient temperature range (32-40° F/0-4° C for parallel circuit, 40-60° F/4-16° C for serial), the latter for the reduced amount of reheating necessary.
Both include a flow path through the evaporator, which vaporizes any liquid refrigerant and absorbs heat from the air going through that heat exchanger, based on signals from a temperature sensor between the fins. And both dehumidification circuits are able to operate in conjunction with cabin heating.
The humidity sensor also is part of the system configuration that provides operation in six different HVAC modes (cabin cooling, cabin heating, serial dehumidification for the cabin (with heating), parallel dehumidification for the cabin (with heating), defrosting the outer heat exchanger, and heat pump-generated cabin heating when ambient temperature is low.
Author: Paul Weissler
Source: SAE Automotive Engineering Magazine
在注氣模式下，豐田普銳斯Prime熱泵的回路示意圖：外部冷凝器在左，內部冷凝器在右。其中，外部冷凝器可以作為一個普通冷凝器，或在有需要時吸收熱量。
制冷劑液氣分離器的工作示意圖：左邊為其他模式，右邊為采暖模式。“中壓電磁閥(Middle Pressure Magnetic Valve)”將在采暖模式下關閉，推動液體從孔口 (Orifice) 流向外部冷凝器。
