Designing Grid-Connected Solar Systems
Solar system design is a multi-layered process that draws from several technical and non-technical disciplines. Engineers and designers integrate elements of electrical engineering, structural and/or geotechnical engineering, investment metrics and state policy to arrive at an optimal solution for the client.
The most apparent factor that affects the design is the physical area available for the solar system. In a rooftop installation, areas with HVAC units or surface fans cannot have modules. Areas prone to shading will also typically not have panels. Additionally, it is considered best practice to have larger, more contiguous arrays, as they will be easier to wire and put less strain on the roof.
Another consideration for roof mounts is structural loading. A roof must have enough support to withstand the added weight of the panels. The design of the roof and supporting structure, along with the climate, proximity to urban areas, geographical location and design of the solar system itself are all necessary to determine a roof’s structural feasibility. For example, various areas, such as the St. Louis area and much of California, require roof designs to withstand seismic events, whereas the northeast has wind and snowfall factors that need consideration.
Ground mounted systems require different screenings to determine suitable areas. Typically, ground mounts cannot be constructed on a federally or state delineated wetland, in a 100 year flood plain or any smaller interval, or on a grade steeper than 15%. The material under the ground mount also plays a role in how the system is designed. Solar panels on a ground mounted system are usually placed on a rack that penetrates roughly 7 feet below the surface, and additional measures may be necessary if the soil is particularly loose or rocky. Ground mounted solar arrays must also conform to local zoning codes. Most municipalities will have required setbacks from the property lines for any structure within a given zone (industrial, small commercial, multi-family residential, etc.), and an increasing number of municipalities have solar specific by-laws that complement or supersede the existing zoning codes. Solar arrays can be installed on top of impervious surfaces such as concrete or stone, but in that case, the rack will be secured by ballast blocks. Ballasted ground mounts are also used to install solar over decommissioned landfills because they do not breach the protective cap.
In addition to physical constraints, each jurisdiction will adopt safety standards that have various applications to solar systems. The two that are the most applicable for the design of solar systems are the National Electric Code (NEC) and the International Fire Code (IFC), both of which govern safety standards in the (unlikely) event of a fire. The National Electric Code is a uniform standard that is released every three years, with the most recent version released in January 2017. The standards are nationally recognized, yet states and/or municipalities are free to adopt different versions of the code as they see fit. Most states are either on NEC 2011 or NEC 2014. These codes have standards that govern details, such as how to install solar modules and supporting equipment, the location of electrical equipment relative to the array and how to interconnect the system based on the existing electrical service. By contrast, fire code standards relate mainly to the physical layout of the site. Solar-specific fire codes are in place to ensure that firefighters can safely extinguish a fire with the electrical equipment present on the roof. Standard provisions for fire access typically include minimum setbacks from the roof edge, access to all roof top units and a readily accessible disconnect to shut off the solar array.
While the physical site and relevant locally-adopted codes define the space available for panels and other electrical equipment, there are plenty of opportunities to optimize the design within the remaining spaces. System designers will often balance technical and financial considerations at this stage to provide the best solution for the client. For example, a manufacturing facility with more consumption than a solar system can independently offset, might opt to maximize the energy density (energy per unit area of panels) rather than the energy efficiency of a system. On the other hand, a client interested in a third party procurement option might seek to increase the efficiency of the system in order to obtain the lowest price possible for the energy produced.
State regulations and government and utility incentives can also drive system design based on the type of incentive and rate structure present. For example, various utilities in California have time-of-use rates that result in higher energy costs during the afternoon. By orienting an array to face west rather than south, the solar system can offset consumption when the rates are higher, even if the production on an annual basis might not be as high.
Altogether, there are a plethora of optimizations that engineers and system designers take into account when designing a solar system. They must consider the technical and financial factors to propose a system that best meets the client’s needs and offers the most lucrative return on investment. If you are interested in learning more about going solar and/or have questions about the viability of your property for solar, please contact us here or give us a call at 888-225-0270.