Battery Energy Storage Unit – Computational Fluid Dynamic (CFD) Case Study
In Today’s World of Renewable energy, Whether it’s solar wind or photovoltaic, the means to collect the energy is only part of the story. The collection of sunlight or the harnessing of wind is only useful while the sun is shining or the wind is blowing. That’s why the ability to store solar energy for later use is vitally important. The delicate balance between electricity generation and the demand for electricity use is better managed with the use of an energy storage system.
Many utilities and industrial organizations are incorporating alternative means to generate electricity. with the increase in the use of renewable energy, comes the increase in need for battery energy storage systems. There are unique challenges that come with incorporating a battery storage system that require careful design consideration. The low prescribed battery operating temperature (68° to 77°F), requires a refrigeration cooling system rather than direct ambient air cooling. The narrow allowable temperature variation, no more than 5°F between hottest and coldest battery, requires near perfect air distribution. The rapid changes in power with time require tight control. Without proper thermal management, overheating cells will degrade, malfunction or even catch fire.
ECF Engineering Consultants was tasked with analyzing a battery storage system to be utilized within a wind energy farm in the North East United States. The battery storage system was installed within a standard 42 ft. long shipping container. ECF Engineer’s verified the capacity and pull-down capabilities so the battery storage system thermal control will result in the desired performance and extend the life for the batteries. An under-sized cooling system could lead to battery overheating while an over-sized cooling system could lead to short cycling of the cooling system and large container temperature swings during operation.
The battery container analyzed by ECF had several pieces of equipment that generated heat within the container. The largest heat generator within the container are the batteries and therefore the most important heat generator to predict. Heat from the batteries results from reaction entropy and activation energy losses, electrical and ionic resistance, and chemical transport. The heat generated is typically flat between 20% and 80% state of charge (SoC). Heat generation increases significantly as a discharge approaches 0 SoC and when the charge approaches 100% (see Figure 1 Below).
Figure 1
State of Charge
The verification of the total heat load internal and the heat that also comes in from the external environment using Commercial HVAC load software accurately calculated the environmental heat load for the battery container.
After understanding the factors of heat generation within the battery container, it was important for ECF Engineering to also analyze how the air shall be distributed throughout the battery enclosure. This is a key aspect of the cooling system to ensure the batteries are within the allowable operating temperature variance. Computational Fluid Dynamic (CFD) is the best means at calculating spatial values for temperature, air velocity and air flow directions within the battery energy storage system. The CFD analysis performed by ECF helped make critical decisions in the application of the battery container such as:
· Verify sufficient cross-section area for air flow throughout the enclosure.
· Select locations for air circulators.
· Determine optimum size, shape and register locations.
· Provide direction for optimizing the vane positions in supply registers.
· Determine significance and location for air barriers between or around equipment. · Determine the effectiveness of additional fans/blowers in the enclosure to augment the HVAC.
A CFD analysis was used by ECF to analyze air flow and the resulting temperatures for the battery container with Ceiling-mounted HVAC units. The analysis used battery data and HVAC data from the respective manufacturers. Figure 2 below displays graphical results of the temperature profiles of three planes in the x, y and z axis. These profiles detail the air flow through the enclosure and the resulting temperatures. It is easy to see how the cold air (in blue) is introduced and how it distributes throughout the container. This information was used to help improve the HVAC design in this project.
Batteries generate heat like other electrical equipment, however, Battery manufacturer performance warranties require a low temperature and a very narrow window in which the batteries can operate. Although designing the thermal management system for a battery energy storage enclosure presents unique challenges, ECF Engineering Consultants utilized the tools presented in this case study to provide our client A reliable solution to their engineering needs that lead to overall project success.