Balancing the ‘Dominos’ in EV Thermal Management
Excessive internal valve leakage is an inefficiency that worsens upstream system inefficiencies. Effective valve design is the solution.
Electric vehicle thermal management requires OEMs and suppliers to control many variables that potentially create excessive heat. Designing thermal-management systems (TMS) to effectively control these variables is critical to overall EV performance. Internal leakage and other coolant-valve performance criteria are among the variables affecting EV performance, most notably driving range. Valve design is fundamental to a robust, thermally efficient and leak-free system, according to engineers at Robertshaw Controls, a specialist in plastic coolant-valve innovation and TMS design.
The delicate balance
Excessive heat generation around multiple EV systems represents an inefficiency that prevents them from operating at optimal temperatures. To manage this inefficiency, the TMS is tasked with simultaneously maintaining optimal temperatures in and around multiple components and systems. Lithium-ion battery temperature control receives a lot of attention, but it is one of many components and systems to consider. It is also critical to protect the “brains” of the vehicle — the ECU or ECM — from extreme temperatures. Accordingly, some ECUs and ECMs are equipped with their own chiller. Typically, the vehicle gearbox also has a cooling loop. Finally, the cabin often has a cooling loop with a coolant valve for cooling the air.
The TMS must simultaneously maintain optimal temperatures in multiple systems — a delicate balance. If one system’s operating temperature increases significantly above optimal, the TMS compensates by diverting colder fluid from other systems’ cooling loops to the “hot” system’s loop. Although this process immediately lowers the hot system’s temperature, during a trip of sizable distance it can push operating temperatures higher around other systems that have experienced the cooling loop diversion. This thermal “domino effect” compounds the original inefficiency of the upstream system’s excessive heat production and eventually impacts EV performance.
As an example, consider how the thermal domino effect impacts the battery — and, ultimately, driving range. When the battery temperature rises above the optimal 21° C (70° F), its cooling loop needs colder fluid from another loop. Meanwhile, the pumps and compressor that feed the battery’s cooling loop with colder fluid need to work harder, increasing the energy demand on the battery. If the TMS does not respond with peak efficiency (measured by the time it takes to lower the battery’s temperature to optimal), the larger and more abrupt the stored-energy loss from the battery’s most recent charge.
Ultimately, TMS inefficiency reduces the vehicle’s driving range compared with its potential range. Over the long term, inefficient TMS performance also diminishes battery life.
Maintaining the optimal battery operating temperature becomes more critical, not to mention challenging, in extreme outside temperatures. A 2019 AAA study revealed the potential impact of outside temperatures on battery performance. The study indicated that an outside temperature of 35° C (95° F) decreased driving range in five different EV makes and models, on average, by 4%, and by 17% with HVAC system operation. Note that when temperatures around EV components and systems are below optimal, TMS efficiency is equally or more important.
The challenge in a cold operating environment is not excessive heat production, but too little heat production. In the AAA study, when the vehicles were operated in outside temperatures of 7° C (20° F), the result was an average 12% decline in driving range – 41% when HVAC systems were operated.
Internal valve leakage
How rapidly or gradually the thermal domino effect occurs depends on overall TMS operating efficiency, which hinges on variables such as internal leakage in coolant valves. In contrast to external leakage (i.e., physical loss of coolant), internal leakage means diversion of coolant from one loop to another. Excessive internal valve leakage is an inefficiency that worsens upstream inefficiencies.
Among its TMS design solutions, Robertshaw offers Ranco electronically-controlled multi-port valves (typically 2, 3, 4 and 5 ports) that move coolant throughout cooling loops with minimal energy consumption and maximum flow efficiency. They are designed and manufactured to limit internal leakage — impossible to eliminate completely — to well less than 10cc.
Additionally, these valves are designed to maintain low fluid pressure at the inlet point and in line with overall system pressure. Optimal fluid pressure aids TMS performance in a couple of ways.
Minimizing fluid pressure flowing through a valve reduces the energy demand on fluid pumps. In addition to the energy savings, it might be possible for the TMS designer to reduce pump size and realize space and cost savings.
Also, minimizing fluid pressure prevents steep, abrupt coolant Delta-Ps (pressure drops) – the enemy of high performance in critical EV systems because they mitigate a necessarily steady coolant flow. Preventing abrupt pressure drops makes it easier to maintain desired temperatures in an often narrow range in and around these systems. Tight control of coolant pressure in a typically intricate TMS design is critical; even small decreases in flow rates from precipitous coolant pressure drops can produce increases in cooling air temperatures outside of specified ranges. This adversely impacts system and vehicle performance.
Other key considerations
OEMs should also prioritize several other coolant valve properties to make their TMS designs flexible, space efficient and operationally reliable:
- A modular electrical connection platform to accommodate different pin shapes and standards
- Coolant hose connection modularity: VDA, quick connector, hose barb, etc.
- Space and weight optimization
- Voltage compatibility: 12V, 24V and 48V
- Durability: At least 100,000 cycles in 10 years
Coolant valves are critical components within a thermal-management system that keeps critical electric vehicle systems operating at peak efficiency. Robertshaw advises OEMs to take valve performance properties into account in TMS design. Overall EV performance depends on it.
Steven Hokky is CTO for Robertshaw Controls, a global design, engineering and manufacturing company that supplies parts and subassemblies to electric vehicle OEMs.