Successful thermal management with liquid cooling
Thermal management is essential for charging stations and electric vehicles.
The increasingly more effective lithium-ion batteries in hybrid and electric vehicles also require high-performance charging stations to supply the vehicles with power. All components in these charging stations have to maintain an optimal temperature level – firstly, because rapid charging processes massively heat the entire system, and secondly, in order to reduce negative effects on the range of the electric car and the life of the batteries. Efficient thermal management on the basis of liquid cooling is an ideal solution here. A tight cooling system made of plastic with matching conduits and connectors that can also be equipped with sensors ensures utmost safety.
Electric and hybrid vehicles are getting increasingly popular, and the associated charging infrastructure with great coverage is installed. The range of electrically driven vehicles – i.e., how far they can go on one charge – is decisive for their acceptance. To ensure its sustainable increase and to remain flexible while traveling, the power density of lithium-ion batteries is going up while the charging duration of modern electric and hybrid vehicles continues to decrease. At the same time, shorter charging times for electric and hybrid vehicles are becoming more important. Yet, rapid charging processes develop a great deal of heat losses leading to massive waste heat flows. To ensure a continuously high charging cycle efficient thermal management is required.
All components generating heat losses need to be considered.
The service life as well as the performance and safety of lithium-ion batteries greatly depend on the operating temperature and temperature fluctuations that occur inside each individual cell. Batteries or energy storage systems in principle have different temperature requirements: For instance, the batteries and their cells must not exceed resp. undercut the average temperature of 15°C to 35 °C to ensure a maximum service life. Thermal management systems help to keep lithium-ion batteries at an optimal thermal degree, and minimize temperature differences in the cells.
Yet along with the battery cooling that has been primarily considered so far, it is also essential to cool the increasingly more potent systems as well as the entire thermal circuit. This includes, for instance, converters and radiators in electrically powered vehicles, or the cable and charging system with reservoirs, pumps and heaters in charging stations. This is because all components potentially emitting heat have an impact on the temperature and functionality of the entire system.
Liquid cooling is an ideal solution for energy stores.
The tasks of cooling systems include constant temperature reduction and, at the same time, heat transfer. To achieve this, the heat needs to be transported away from the emitting components, and possibly activated where heat is required. There are two variants available for cooling – with air or water. Air is characterized by low thermal capacity, i.e., it quickly absorbs heat making heat transfer hard to implement. Significant noise level and lots of space required are further disadvantages of air-based heat management.
Since modern systems can store increasingly more energy, and there is often only little construction space available for thermal management, liquid-based cooling has the ever-growing potential – both for charging stations and inside the hybrid and electric cars themselves. Water absorbs heat slower than air, which leads to a lower heat transfer coefficient. Due to this, more heat can be absorbed with water-based cooling. To achieve similar cooling performance with air, a significantly greater volumetric air flow is required due to the lower thermal capacity.
Constant flow is lower with water.
Another important aspect for the assessment of both variants is that of flow rate: To get the battery cells to the ideal average temperature of 35 °C, a constant flow of some 13 °C is required in an air-cooled system. A water-cooled system already operates at a far higher temperature of 32 °C. Thus, to achieve the same cooling rate with air, the flow rate must be significantly higher than with liquid. This suggests that water cooling systems can have a more space-saving design, and at the same time enabling the technical advantages of even heat distribution. A battery - whether for vehicles, trucks, buses or energy storage devices - can be temperature controlled directly on the cooling plate and connected to the entire liquid cooling cycle.
Reliable conduit system is crucial for water-based cooling.
Different components are required to successfully implement heat transfer in liquid cooling. Each water cooling system features, e.g., sensors to measure the temperature of the medium. When used in very cold regions a heating element is normally integrated to balance too low temperatures. Conduits also play a significant role in charging stations – after all, they contain the water that is routed through sensitive cable systems and connects them to reservoirs, pumps and heaters.
Since the construction space tends to get smaller and smaller, flexible and at the same time mechanically robust corrugated and smooth plastic conduits are best suited for cooling systems. They must allow high flow rates and withstand operating pressure of up to four bar. The advantages as compared with rubber or metal solutions are higher performance, easy installation, weight reduction and flow optimization to name but a few. Conduit dimensions in nominal diameters between 8 and 37 can be used in a water-cooled system.
Due to the flexible design of conduit systems that can be adapted to the individual requirements, for example, by means of rigid expansion elements of oval shape, liquid can be routed through narrow spaces without any difficulty. What matters here is that the conduits do not change their mechanical strength through thermal deformation. They do not necessarily have to be extruded from polyamide (PA) 12; multi-layer conduits can also be used that require different design depending on the application and system pressure.
A harmonized system ensures utmost safety.
In general, systems for liquid cooling should be maintenance-free. In exceptional cases, special connectors (shut-off) are used which enable drip-free maintenance with their dripless function. Since liquid cooling systems work continuously - and keep the batteries at the optimum temperature even when the vehicle is at a standstill - the quality and workmanship of the heavily used components are crucial for a long service life. Perfectly matched, customized components from one source have been developed, for instance, by the supplier of thermal management solutions FRÄNKISCHE Industrial Pipes (FIP).
The conduits and connectors must be based on the common connection standards. It means, for example, that an SAE standard can be used for a battery connection. For energy stores or charging stations, a connector based on the VDA standard can often be used as a counterpiece connection in the interface itself.
Higher efficiency for charging stations and e-cars goes with water cooling.
To summarize the above, it can be stated that water cooling allows effective transfer of large volumes of heat with relatively small flow rates. For this reason, it achieves better continuous cooling performance as compared to air. Given that, the water-based approach is particularly well suited for the cooling of systems with hight energy storage requirement, such as charging stations and electrically driven vehicles themselves. To ensure sustainable heat transfer, a well-matched system solution consisting of flexible and at the same time stable plastic conduits coupled with reliable connectors is the best choice.