November 8, 2021

Effective Thermal Management for Lithium-Ion Batteries: The Case for Liquid vs Air for Cooling and Heating

Doug Pooler - Product Testing Engineer, Hotstart Thermal Management

This is a guest blog post from Hotstart Thermal Management. Connect with Hotstart at #ESACon21 in Phoenix, AZ, December 1-3. Registration is open!

Stationary energy storage is estimated to become an $8.9 billion annual market by 2026 with particular growth in lithium-ion energy storage systems (ESS) for short duration applications. A primary concern with lithium-ion ESS operation is effective thermal management, namely removing the significant heat generated during battery charge and discharge and maintaining the batteries above the minimum charging temperature in cold environments. While conditioning the air within ESS containers has been typical for most deployments to date, it is not sufficiently effective. Liquid-based thermal management provides a more effective method for controlling and optimizing battery temperature in both warm and cold environments.

Effective thermal management of lithium-ion ESS must both remove heat during battery charge/discharge to minimize thermal gradients that cause internal battery cell damage and maintain the temperature of the cells above a minimum threshold for safe operation in cold environments. Conditioning of the air within an ESS container has been the most common solution as it seems the least technically complex and the least costly. However, the limitations inherent in air cooling and heating have significant impacts on battery performance, energy availability, battery degradation and lifetime, and total cost of ownership.

HVAC systems in an ESS condition the air within the container to lower the overall ambient temperature, which is then circulated around and within the battery modules by natural or forced convection. The specific heat of air and airflow limitations limit the amount of cooling that can be provided within the battery module. Overall energy density of an air-cooled ESS is reduced as air-cooled battery module designs require more pathways within the module and container to accommodate forced airflow.

Air-cooled systems also have a low thermal transfer rate and are slow to react to battery charge/discharge-driven temperature changes. Battery management systems (BMS) typically de-rate the ESS in response, including limiting the depth of discharge and the zenith of charge. The resulting decreased charge/discharge rates reduce ESS usefulness as a power infrastructure asset. HVAC systems also typically do not provide heating, which limits the effectiveness of the system in cold environments where charging should not occur when battery cell temperatures are below the battery manufacturer’s specified minimum charging temperature.

In contrast, liquid-based thermal management systems (TMS) can directly cool and heat battery modules to maintain battery cells in their optimal temperature range. The use of heat exchanger plates in contact with the battery module’s exterior surface allows for a direct heat transfer to cells through thermal paths integrated in the module design. While operating during charge/discharge states, the liquid TMS cools and circulates a water-glycol mixture to the heat exchangers. In cold environments, the system circulates heated water-glycol, maintaining lithium-ion battery modules and cells within their ideal temperature range so they can safely charge.

Liquid-based thermal management has several advantages over air systems. Since the heat exchange fluid has a much higher heat capacity than air, module and container energy density can be increased due to the smaller fluid paths necessary for liquid TMS. The high thermal transfer rate of the integrated design improves cell temperature uniformity and temperature control, reducing the need to de-rate the system through the BMS due to spikes in individual cell temperatures. Cell temperature uniformity within the ideal temperature range can improve battery state of health, extending the productivity of the system. Overall parasitic load is reduced when liquid TMS is integrated into the ESS. Traditional air-conditioning parasitic load is reported between 5-10%. Third-party testing has demonstrated that the liquid TMS parasitic load is less than half of a comparable HVAC system during continuous charge and discharge cycling.

Lithium-ion ESS installations benefit from liquid thermal management in both the short and long term. A scalable liquid TMS minimizes the conditions that cause cell degradation and loss of capacity, increasing the installed useful life of an ESS and maximizing its value as a power infrastructure asset. Expanding the range of lithium-ion ESS usefulness into both hotter and colder environments with the use of a single thermal management system allows for energy storage opportunities that would be limited otherwise.

Energy storage deployment growth will be significant between now and 2026, adding over 150 GWh capacity to the US power landscape. Effective liquid thermal management integrated into lithium-ion ESS modules will help maximize those future investments and provide reliable energy delivery to the evolving power landscape.

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