Although site optimization is generally thought of as a practice that takes place at a distinct point within a solar project’s lifecycle, developers and EPCs are discovering that significant cost savings can be realized during several key project phases.

When ClearPath Energy, a Northeast community solar project developer, began work on a 38.25-MW portfolio of six New York projects, developers knew they needed an experienced design and engineering firm to provide site optimization to address the challenges these projects face. From meeting National Grid’s requirements in a short time frame to ensuring the inverters would integrate smoothly with future energy storage additions, these projects posed some unique obstacles.

Knowing that Castillo Engineering had over 300 utility-scale projects under its belt, including many projects in New York and Massachusetts, ClearPath shortlisted the firm to provide design and engineering support on these projects. ClearPath ultimately selected Castillo due to its track record and reputation for being adaptable, providing a range of options rather than forcing clients to stick with standard, non-customized designs.

After agreeing to work on this project portfolio, Castillo had to overcome several key site optimization challenges, which took place at various project stages — from interconnection application to extended post-design.

Challenge #1: Reducing excess equipment during utility interconnect application

Castillo has completed over 1 GW of solar projects since it was founded in 1998. From that experience, its team of senior engineers knew that one way to reduce costs when designing a utility interconnect application is to reduce excess equipment. For example, for each of these six ground-mount projects — all of which had overhead distribution lines at the common coupling point to the utility — Castillo noticed excesses in equipment, such as sets of surge arresters on every pole, extra disconnect switches and duplicate metering.

Oftentimes, a gang-operated air break (GOAB) switch, a commonly used switch in smaller electrical substations, is located two electrical poles down from a disconnect switch. It’s unlikely that anyone will use the disconnect switch since they can open and lock at the GOAB from the ground. Operating disconnect switches can require additional live line tools which will add costs. They’re considered to be difficult to use and subject to OSHA-required testing every two years, which results in further avoidable costs.

Typical overhead distribution lines at the point of common coupling

Castillo determined that there was no reason to spend money on these excess devices since they were functionally redundant or wouldn’t be used at all. Instead, Castillo advised that these items should be removed from the bill of materials so that the funds could be saved or invested in other aspects of the six projects.

Challenge #2: Meeting the utility’s interconnection requirements in a short time frame

National Grid is one of the most demanding utilities to interconnect with in the United States. Unlike its previous National Grid projects, Castillo began work on these six arrays at an early stage, which required quickly producing a large amount of documentation to meet its “Phase B” requirements earlier in the design process to avoid delays.

Besides the usual drawings and equipment cut sheets, Castillo had to include setting files for the recloser relays for each project’s point of interconnection and maintenance, telecommunications, system energization and sequence of operation documentation early in the design process. National Grid is also particular about utility pole details, requiring full details, exact clearances above grade and detailed elevations showing the line from the distribution system’s connection point with the power line.

Challenge #3: Eliminating redundancies during equipment procurement

After interconnection was approved, ClearPath requested that Castillo review, assess and provide feedback on its balance of plant equipment quotes. These items included a 15 kV recloser, grounding transformer, combiner boxes, metering cluster, GOAB switch, fuse cutouts, data acquisition system, auxiliary power transformer and panel, surge arresters and secondary metering.

Castillo identified hardware savings of over $200,000 for each of these six projects. Most these savings were derived through the elimination of redundant equipment (an additional GOAB switch, surge arresters on each overhead pole, customer revenue and instrument transformers) and the reduction of overspecifications on other equipment.

Challenge #4: Mitigating cable cost while considering O&M costs

Once equipment procurement was complete and Castillo started designing the six projects, it was tasked with reducing future operations and maintenance (O&M) costs. Castillo worked with the contractor early on to determine equipment preferences.

Castillo focused on design considerations like determining the maximum desired size of cable runs, the preferred locations of combiner boxes, whether the combiner boxes will be provided with pre-installed mounting rails and what adequate widths for access roads and vehicle turn-around areas. There are trade-offs with choices like combiner box locations because ease of access for O&M work might come at the expense of longer string conductors or combiner box output conductors.

The final design that Castillo produced for each of these six projects required several iterations to find the compromise between the original design and the contractor’s recommended design. The example below from one of ClearPath’s solar tracker projects shows combiner box locations and trenching as originally designed to optimize cable lengths, sizes and voltage drop in green and contractor recommended locations to minimize trenching in red.

Project design for combiner box locations and trenching as originally designed in green (left) and contractor recommended design in red (right) for one of ClearPath’s recent solar tracker projects in New York.

While trenching and length of aluminum combiner box output conductors and conduit is minimized by the contractor recommendations, the table indicates the increase in length for the copper wiring, along with an increase in average direct current voltage drop from 1.5% to 1.7%. Despite the varying recommendations for Castillo’s assessed equipment, it determined the pros and cons of each design to find the most cost-effective solution.   

Challenge #5: Maintaining flexibility despite module unavailability

Castillo has learned from its 25 years of experience in the solar industry that flexibility is the key because there will always be design alterations as each project progresses. Specifically, module unavailability resulted in two project sites switching from Heliene to Longi modules. The modules were relatively similar electrically, but it still required redoing system calculations for operating voltage, currents and voltage drop.

Castillo was able to adapt to these design alterations and ensured that they did not impact project schedule or budget.

Challenge #6: Ensuring that the inverters would integrate seamlessly with mismatched step up transformers as well as future energy storage additions

Each of the six community solar projects needed the option to add future energy storage. That meant using SMA inverters that could seamlessly integrate with retrofitted batteries.

Additionally, step-up transformers for most of these project sites were procured early on to ensure the project was safe-harbored. Unfortunately, this meant that the low-voltage transformer interface with SMA’s standard inverter connections was a mismatch.

Castillo had to make sure the correct parameters were specified for array polarity, ensuring proper integration with future energy storage. There needed to be an ungrounded array input to the inverter rather than a typical grounded array input. To address the step-up transformer mismatch, Castillo provided several solutions, like a custom bus, cables in ducts and above-grade cables.

Despite product unavailability from supply chain constraints, equipment integration challenges, National Grid’s interconnection requirements and a brief design schedule, Castillo was able to adapt to and overcome these challenges. From its experience in site optimization, the firm saved ClearPath $1.2 million on the entire portfolio.

In total, the six projects in this portfolio will generate enough clean energy to power 8,700 homes per year and will support New York in achieving its renewable energy goals, including a target to have 70% of the state’s energy produced from renewable resources by 2030.

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