What is Battery storage thermal management?
By Tyler Van Dooren, PE – Mechanical Engineer | Seattle Office
Lithium-ion Energy Storage Systems (ESS) are increasingly being deployed in the US and globally for a wide range of applications, and installed ESS capacity is projected to rise from 25GWh to 150GWh within the next 20 years. Typical system configurations include battery-based buildings, outdoor enclosures, and custom containers ranging in size from a few hundred kWh to massive facilities with GWh energy capacities.
As lithium-ion battery storage has increased in scale and importance in today’s renewable energy projects, proper thermal management of batteries has become critical to the reliability and economic feasibility of these facilities. Thermal management ensures compliance with battery manufacturer warranty requirements. Without thermal management, facilities can experience costly downtimes or have their capacity derated throughout their lifespan. The following information explains basic concepts of properly cooling lithium-ion batteries.
Battery Manufacturer Warranty Requirements
The main driver for HVAC design of a battery facility is the mandatory warranty requirements issued by the battery manufacturer. These requirements state the allowable temperatures, humidity, and dust levels. Humidity and dust can be mitigated relatively easily with proper dehumidification and filters, but properly meeting temperature requirements can be more complex.
Temperature requirements generally have three main aspects to them. The first is an overall allowable temperature range, requiring all warranty monitoring temperature probes stay within a specified range. The second is a daily average temperature that the warranty probes cannot exceed. The third and generally most challenging are temperature uniformity requirements. These requirements stipulate a maximum allowable difference between the warranty probe reading the lowest and highest temperatures within a battery string or cold aisle.
Proper HVAC Unit Design
Although most battery manufacturer warranty requirements look similar, they can apply in consequentially different ways depending on the manufacturer. The biggest difference is whether these requirements apply to the battery room’s cold aisle or hot aisle. If applied in the cold aisle, supply air temperature control becomes critical. Because warranty temperature ranges are tight, the supply air temperature out of the HVAC unit cannot fluctuate as the HVAC unit stages up and down during cooling, heating, or dehumidification modes. To complicate things, the allowable warranty temperature lower threshold can often be significantly higher than the temperature HVAC units typically supply. This challenge necessitates using capacity control techniques such as variable speed compressors, return air bypass, or hot gas reheat to achieve proper supply air temperatures. Even if the battery manufacturer requires the warranty temperatures to be measured at the hot aisle and the HVAC unit can supply more typical temperatures, a steady supply air temperature control is still important to maintaining the warranty requirements.
Another aspect that becomes crucial in HVAC unit design is achieving proper airflow. Many air-cooled battery module designs feature an internal fan to help push cool air through the battery modules. There can be dozens, if not hundreds, of these fans within a battery room. If the HVAC unit cannot match, or ideally slightly exceeds the total battery fan airflow, the result will be a negatively pressurized cold aisle. This can cause hot air to be sucked from the hot aisle back into the cold aisle, which could result in warranty temperatures not being achieved. The airflow per ton of HVAC units can be as high as 600-800 CFM/ton, significantly above the normal 350-400 CFM/ton. If the battery modules do not contain an internal fan, the design team will need to ensure the cold aisle has a consistent level of pressurization throughout so that a similar volume of air flows through all battery modules.
The Necessity of CFD (Computational Fluid Dynamics)
With tight enclosure construction, custom duct designs, and strict temperature requirements, one way to get a sense that a design works as intended is to use computational fluid dynamics (CFD) modeling. For air-cooled battery systems, this may include an analysis of the enclosure and the inside of the ductwork to ensure the air distribution system can be properly balanced. CFD is the preferred method to inform the design team where hot and cold aisle barriers need to be placed, whether cooling is evenly distributed throughout the enclosure, where hot spots appear, and what the overall enclosure pressure drop will be.
Whether the warranty is measured from the hot or cold aisle, CFD is equally important in determining if the HVAC design will be successful. When temperatures are measured on the hot aisle side, CFD will show if there is uniform pressure in the cold aisle so that air pushes evenly through the battery racks. When temperatures are measured on the cold aisle side, CFD will show how efficiently the supply air gets through tight spaces to each battery module. Without CFD, there is a high probability that a battery facility will run into warranty temperature issues during commissioning and after the battery facility is operational. The cost to fix temperature issues at the late stages of a project are often significantly higher than the upfront investment in CFD or similar methods to verify the HVAC design before construction starts.