Unlike BESS (Battery Energy Storage Systems), solar energy systems come in a wide variety of visually apparent, unique flavors: fixed tilt ground mount, tracker, rooftop, carport, floating, mixed use agricultural, and space-borne arrays. BESS, by contrast, are predominantly grids of conex boxes and step-up transformers, appearing very similar on a surface level. You would therefore be forgiven for assuming that there is not much electrical difference between each BESS. However, when you dig deeper, you will discover a plethora of critical subtle and hidden differences in the balance of plant electrical design that must be taken into consideration to ensure a well-integrated, high performing and cost effective BESS project. Listed below are ten of the key design considerations that our team has encountered in its efforts to produce code-compliant, reliable, and economically buildable BESS designs.
1. High energy density and conductors
Energy densities, particularly with the most popular lithium-ion technology batteries, are extremely high within the relatively small BESS footprint. This means that somewhere on site, if not most places on site, there are going to be multiple, very large sets of conductors connecting items of equipment. These conductor sets will most likely be located underground, usually in conduit. As a result, it is essential to determine and implement techniques that can minimize the overall cross-sectional area of the conductor to reduce installation costs while also creating a design where the necessary number of conduits can actually stub up within the designated floor access panel. This task can be especially challenging with floor access panels that appear to have been dimensioned arbitrarily small, making it difficult to fit large circles within this small square.
2. Maximum battery string voltage and central inverters
The maximum battery string voltage allowed by the technology is often not an economical match for large central inverter dc input voltages and requires a dc/dc converter. In these instances, it’s important to determine whether the dc/dc converter is packaged within individual conexes or if it is a separate, large device for multiple conexes. Similarly, it’s necessary to determine if the dc/ac inverters are packaged within the conexes or if they are a separate, larger device for multiple conexes. Depending on the number of building blocks in the system architecture, you may need to double or triple the amount of large, parallel cable runs, with the accompanying doubling and tripling of the number of cable lugs to attach and bolts to fasten.
3. Auxiliary loads
There are a number of factors to consider when it comes to auxiliary loads in order to minimize project costs. First, you need to determine if your auxiliary power for cooling and heating the BESS is an independent feed from the utility so that the HVAC can be provided even when the utility recloser is opened. Second, you will need to establish whether the auxiliary power loads are clearly defined or if they appear to be an ultra-high, overly cautious value, thereby requiring costly oversized equipment. Additionally, you need to determine, out of that large list of auxiliary loads, which of these truly require an external circuit from the balance of plant auxiliary power panel and which ones are actually internally-fed auxiliary loads that can be ignored. This is important to determine because battery designers often don’t differentiate between internal and external power sources, as they often lump all of these together under the general category of auxiliary loads.
4. Communications and cabling
One common BESS design challenge is that communications diagrams often do not indicate what sort of cabling should go between the various points. For the bi-directional power flow, it is necessary to establish what are the maximum full load amps in each direction based on the site-specific number of battery racks at a given operating current.
5. Determining applicable codes
If you can decipher early on what codes apply and whether the AHJ and Owner’s Engineer are up to speed on the latest BESS designs, this will help save you time and energy in the long run. When it comes to determining applicable codes, there can sometimes be conflicts or gaps between NEC Article 480, UL 1788, UL 1973, UL 9540, NFPA 101, NFPA 855, NFPA 5000, IFC Chapter 12, NFPA 1 Chapter 52, and locally adopted more restrictive requirements. Additionally, it will be essential to know how many code rules change if your project involves a hybrid BESS plus PV installation. Learning who the one-stop expert in the jurisdiction is that can give a clear answer as to which codes apply will also go a long way toward ensuring the project is executed as efficiently and cost effectively as possible.
6. Cut sheets, design guides and installation manuals
BESS designs are evolving so fast that the BESS cut sheets, design guides, and installation manuals often have outdated, conflicting or missing information, or a combination of all three. Do not be surprised if multiple RFIs and document revisions fail to clarify things. It is a fortunate, but now rare, circumstance if the answers make pragmatic, experience-based adjudications of the information; provide numerous explanatory notes regarding design elements as required to provide clarity; and don’t increase the level of confusion.
7. Gray NEC code areas
The NEC has spent much of its evolution dealing with less sophisticated, non-automated equipment that does not have on-board supercomputer-type processing capability. As a result, there are several gray code areas that the NEC does not properly address. For example, suppose that the BESS has been elegantly designed with multiple fail-safe protection levels in order to prevent an overcurrent from occurring, and it has also passed exhaustive UL testing and been certified. Will the AHJ, Owner’s Engineer, or another third-party codebook lawyer demand that a 3000 amp external fused disconnect be installed with a 60-week lead time? We are probably at least two NEC code cycles away from any language on this gray area.
8. Lack of clarity regarding terminology
Also as a result of rapid BESS growth and continual technological innovations, there is still a great deal of confusion, misunderstanding and lack of agreement with regard to specific terminology. Here are some recent examples from projects we have worked on:
“We don’t consider that a battery, that’s just a cell.”
“We don’t connect those cables to the battery rack, we need to connect them at the end of the string of interconnected battery racks.”
“That conex over there just has batteries in it and only needs dc power output cables connected to it, while the identical looking conex right next to it contains an inverter and only half the number of battery racks, so it gets ac power conductors. Was there a reason that they placed the one with the ac power output conductors the furthest from the step-up transformer and utility point of interconnection?”
9. Increase in underground conduits and standpipes
There are going to be a lot of large electrical conduits underground with BESS, but these conduits are also starting to have some non-electrical company. Many conexes come equipped with a standpipe at either end so that firefighters can attach a hose and flood the compartment with water, should that be required. Depending on the intensity of the fire, there probably won’t be many volunteers to step up and attach that hose, so BESS installation manuals are starting to recommend that the standpipe connections be made at some distance from the conex. Therefore, not only has the number of large underground pipes drastically increased, but you must now take into consideration whether those water pipes be run under or over the electrical ducts, and at what separation distance.
10. Lack of terminals
Assuming that you are fortunate enough to have a great BESS product with excellent documentation and well-labeled terminations installed without any major issues, there are still issues that may present themselves in later stages to keep in mind. For example, don’t be surprised if your lead electrician comes to you because all she has left to terminate is the main BESS ground in the master controller, and it appears that no terminals have been provided for it.
The next time you have a BESS project, you will now have a solid idea of how to address each of these design considerations. Do you have utility-scale BESS design and engineering questions that you need help with? Get in touch with one of our experts today!