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Top 10 energy storage design considerations

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Unlike battery energy storage systems (BESS), solar systems come in a wide variety of visually apparent, unique flavors. BESS are grids of container boxes and step-up transformers that are very similar on a surface level. It’s likely that there isn’t much difference between each BESS. A plethora of critical subtle and hidden differences in the balance of plant electrical design are discovered that must be considered to ensure a well-integrated, high-performing and cost-effective BESS project. The Castillo Engineering team has encountered a number of key design considerations in its efforts to produce code- compliant, reliable and economically buildable BESS designs.

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Credit: Castillo Engineering
  1. High energy density and conductors.

    Within the relatively small BESS footprint, energy densities with the most popular batteries are very high. This means that there will be a lot of large sets of conductors on site, most likely underground. To reduce installation costs and create a design where the necessary number of conduits can actually be found within the designated floor access panel, it’s essential to determine and implement techniques that can minimize the overall cross-sectional area of the conductor. It can be difficult to fit large circles within a small square with floor access panels that appear to be small.

    2. There is a maximum battery-string voltage.

    The technology’s maximum battery-string voltage is often not an economical match for large central inverter DC input voltages and requires a DC/DC conversion. It’s important to determine if the DC/DC converter is packaged within individual containers or a separate, large device for multiple containers. If the DC/AC inverters are a separate, larger device for multiple containers, it’s necessary to determine if they’re packaged within the containers. Depending on the number of building blocks in the system architecture, the amount of large, parallel cable runs may need doubled with the accompanying doubling and tripling of the number of cable lugs to attach and bolts tofasten.

    3. Auxiliary loads.

    When it comes to auxiliary loads, there are a number of factors to consider. Is the auxiliary power for cooling and heating the BESS an independent feed from the utility so the heating can be provided even when the utility reopens? Second, it should be established if the auxiliary power loads are clearly defined or if they are an overly cautious value that requires expensive equipment. Out of the large list of auxiliary loads, which ones need an external circuit from the balance of the power panel and which ones can be ignored, needs to be determined. External power sources are often lumped together with internal power sources under the general category of auxiliary loads.

    4. Communications and cabling.

    BESS design challenges include communications diagrams that don’t indicate what sort of cabling should go between points. The maximum load in each direction is determined by the number of battery racks at a given operating current.

    5. Determining applicable codes.

    There are sometimes conflicts or gaps between codes when it comes to determining applicable codes. Chapter 52 was adopted locally. It’s important to know how many code rules change if the project involves a hybrid BESS plus solar photovoltaic installation. Finding the one-stop expert in the jurisdiction who can give a clear answer as to which codes apply will also go a long way towards ensuring the project is executed as efficiently and cost effectively as possible.
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Credit: Castillo Engineering
  1. 6. There are cut sheets and design guides.

    BESS designs are evolving so fast that the cut sheets, design guides and installation manual often have outdated, conflicting or missing information. RFIs and document revisions can fail to clarify things. 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, it’s a lucky situation.

    7. Gray NEC code areas.

    The NEC has spent a lot of its evolution dealing with less sophisticated, non-automated equipment that doesn’t have on-board processing capability. There are gray code areas that the NEC doesn’t properly address. The BESS has been designed with multiple fail-safe protection levels in mind in order to prevent an overcurrent from occurring, and it has also passed extensive testing and been certified. Will the owner’s engineer or another third-party codebook lawyer demand that a 3,000-amp external fused connect be installed with a 60-week lead time? There are at least two NEC code cycles away from this gray area.

    8. Lack of clarity regarding terminology.

    As a result of rapid BESS growth and continual technological innovations, there’s still a lot of confusion, misunderstanding and lack of agreement about specific terminology. Castillo Engineering worked on some recent projects. We don’t consider that a battery, that’s just a cell. The cables are not connected to the battery rack. We need to connect them at the end of the string. The container over there has batteries in it and only needs DC power output cables connected to it, while the identical looking container next to it only has half the number of battery racks, so it gets AC power conductors.

    The one with the AC power output conductors was placed furthest from the step-up transformer and utility point of interconnection.

    9. An 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. The standpipe at either end of the container makes it possible for firefighters to attach a hose and flood the compartment with water. Depending on the intensity of the fire, the BESS installation manual recommends that the standpipe connections be made at a distance from the container. Water pipes are running under and over electrical ducts as a result of the increased number of underground pipes.

    10. Lack of terminals.

    There are still issues that may present themselves in later stages if one is installing a BESS product with excellent documentation. It appears that no terminals have been provided for the main BESS ground in the master controller, so don’t be surprised if the lead electrician terminates it.