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Top 5 Battery Energy Storage System (BESS) Design Essentials

With the price of lithium battery cell prices having fallen by 97% over the past three decades, and standalone utility-scale storage prices having fallen 13% between 2020 and 2021 alone, demand for energy storage continues to rapidly rise. The increase in extreme weather and power outages also continue to contribute to growing demand for battery energy storage systems (BESS). As a result, we have been receiving a large number of questions about sizing and optimizing BESS to provide either energy, grid ancillary services, and/or site backup and blackstart capability. Below we cover the top five BESS design essentials you need to know about: auxiliary power design, site layout, cable sizing, grounding system design, and site communications design.

1. Auxiliary Power Design

Without a doubt, this tends to be the number one engineering design topic we receive questions about at Castillo Engineering. Auxiliary power is electric power that is needed for HVAC for the battery stacks as well as control and communications. This sounds deceptively simple for equipment that has no moving parts, yet it is often a moving target, as BESS vendors continue to morph their designs after an order is placed. Therefore, when it comes to auxiliary design, you must be able to answer the following questions:

  • Do you want a separately metered utility service for the auxiliary power so the cooling still works if the utility trips their recloser to the BESS? This has been a real-world problem for some sites.

  • What voltages are needed – three phase 480 V and 208 V, single phase 120/240 V, or does the equipment happen to come with European-style three phase 400 V and 230 V equipment?

  • What is the real design load – sum of all HVAC nameplates?

  • How much margin is really in those HVAC nameplate values?

  • Is there a smart controller to prevent concurrent start of all HVAC compressors?

  • If high hydrogen is sensed and ventilation is triggered to go full on, are the fans staggered by a few seconds to avoid concurrent inrush currents, and does it include compressors?

  • For northern located projects, is heating load the peak design issue?

  • For the typically large uncertainty, should the aux transformer(s) be provided with fan kits as insurance if the loads end up being larger?

  • Should the aux transformer(s) be provided with a fan rating anyway just so that they can be economically downsized?

  • What about the little things like lighting and security power?

These are just a few examples of the many questions that must be answered prior to embarking on the auxiliary design. We are happy to help you determine the answers to these questions and provide additional design support and guidance as needed.

2. Site Layout

BESS site layouts can be easy or complicated, depending on the site location, the site owner’s preferences or requirements, and the BESS itself. Some of the main questions to consider for the site layout are:

  • Does the BESS vendor have a minimum spacing requirement?

  • Does the Owner have a minimum spacing requirement?

  • How much of a lip do you want on the pad the containers sit on – 6 inches, 2 feet?

  • Do you want to be able to fully open both doors on back-to-back containers?

  • How do you make the cable runs short and simple?

3. Realistic Cable Sizing

BESS systems usually involve short, high ampacity underground runs from the battery rack containers to the inverters or Dc/Dc converters. In order to avoid excessive cable derates and resulting in larger cables and costs for short underground runs, you will need to consider:

  • Will the Owner’s Engineer require a design per NEC tables, or is a site-specific ampacity calculation allowable?

  • Is the owner open to alternative cabling methods compared to multiple runs of conductors in large conduits?

  • Has the vendor clearly identified where power and communications cables enter the enclosures?

  • Does the container just take one large feed, or is it split into two separate feeds that land on dual fused disconnect switches inside the enclosure?

  • What about future expansion/augmentation battery segments, and how do these get connected?

4. Grounding System Design

Another key topic within BESS system design is grounding system design. Many BESS sites are relatively small, and substation grounding design methods don’t really apply. So you need to ask yourself: what is the design goal? Depending on how the design is approached, a site can either end up with an expensive mesh grid or a reasonable design to connect all of the equipment, and both will be equally safe.

Additionally, are you concerned about possible Radio Frequency Interference being generated by the BESS? Some vendors require additional methods, such as six additional grounding connections per inverter. Is the BESS located at a 911 Call Center or in a remote area? Answering questions like this will help your design and installation process go as smoothly as possible.

5. Site Communications Design

The last of the top five topics customers most frequently come to us with questions about is BESS site communications design. Communications are an integral part of BESS design, as it allows for remote data monitoring and/or management, and for the BESS system to communicate with the power grid as well as connect to peripheral components. You should be able to answer the following site communications design questions in order to properly design this aspect of a BESS:

  • What items need to be connected?

  • Is it a radial connection between containers like string inverters, or does it need an extra conduit run to make a communications loop?

  • What sort of data monitoring is there, and how does it communicate back to the owner?

  • Does the site Point of Interconnection control device, such as a recloser, need to be monitored/controlled as well?

  • How is the required metering being accomplished, and what does it need to talk to?

  • Is a utility connection required, and with what communication method?

The next time you have a BESS project, you will now have a solid idea of how to address each of these design areas. Do you have utility-scale BESS design and engineering questions that you need help with? Get in touch with one of our experts today.


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