By Todd Karin, Materials Project Scientist/Engineer; and Anubhav Jain, Chemist Staff Scientist/Engineer, Berkeley Lab
String size in solar PV power plants has increased over the years because longer strings drive down costs. As 1,500-V-rated systems become more common, there’s a question of how the industry can push up string size even further.
Up until the 2014 version of the National Electrical Code (NEC), the maximum allowable string size was determined by a module’s open-circuit voltage when exposed to full sun irradiance and the minimum ambient temperature. However, such a scenario doesn’t occur in practice because (i) high irradiance conditions do not occur at the same time as minimum temperatures, and (ii) PV cells run hotter than ambient temperature. Also, solar PV systems spend most of the time at the maximum power point rather than at open-circuit conditions, further lowering voltage.
Fortunately, the 2017 NEC has provided a new method for sizing strings in standard 690.7(a)(3). With the recent adoption of the 2017 NEC in multiple states, it’s time to re-examine string sizing methods.
The NEC 690.7(a)(3) allows a licensed engineer to use an “industry standard method” to choose string sizes, but does not further specify the details of such a method. Most commonly, an engineer will model the expected open-circuit voltage (Voc) of the module over time and use this information to determine string length. However, the details of this procedure may vary between engineers, who must further justify their design to an AHJ.
To simplify this calculation, we at Lawrence Berkeley National Lab have developed an easy-to-use web tool for sizing strings according to NEC 690.7(a)(3). The user provides simple installation details such as the intended location, module type and mounting type. The tool uses this information to model the expected voltage of the module over time and guides the user in determining an appropriate string length based on this calculation.
Averaged over the continental United States, we have found that string sizes can be increased by approximately 10% under the newer guidelines. This increase in string size is expected to result in a 1.6% reduction in capital costs due to reductions in electrical balance-of-systems costs, installation labor and structural balance-of-systems costs. Using standard financial models, this translates into a 1.2% reduction in levelized cost of electricity.
With this easy-to-use tool now available, we encourage engineers and developers to take advantage of the newer NEC guidelines and adopt a site-specific modeling approach to determine string sizes.
|Max Module Voltage||Safety Factor||Max String Size|
|690.7(A)(1)-ASHRAE. Traditional design using minimum temperatures||48.8 V||0.0%||29|
|690.7(A)(3)-P99.5. Recommended site-specific modeling approach||45.2 V||3.3%||32|
|690.7(A)(3)-P100. Conservative site-specific modeling approach||46.2 V||3.3%||31|
Table: Improvement in string size using site-specific modeling. Simulations performed using a 72-cell poly-Si module located in Berkeley, California, with an open-circuit voltage of 45.7 V and open-circuit voltage temperature coefficient of -0.137 A/C, using open-rack polymer-back temperature coefficients and a maximum design voltage of 1,500 V.
This work was supported by the Durable Modules Consortium, an Energy Materials Network Consortium funded by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Office. The Lawrence Berkeley National Laboratory was supported by the DOE under Award DE-AC02-05CH11231.