Solar Energy (Photovolatics)
Prior to NEC 2017, the common practice for PV string sizing is to calculate the voltage gain caused by ambient temperature gradient from the rated 25oC. The calculation starts from the PV module rated open circuit voltage which is measured at 1,000 W/m2 and 25oC cell temperature. In addition, the calculation of short circuit current, which has a direct impact on DC cable and combiner box sizing, has a safety factor of 1.25 to account for irradiance greater than full sun. NEC 2017 introduces alternative methods in calculating the open circuit voltage and short circuit current that consider both ambient temperature and irradiance. In many cases, these alternative methods result in lower string Voc and Isc, bringing cost savings in balance of system equipment and bulk material. This paper discusses key considerations when selecting the irradiance and coincident ambient temperature profile to ensure safe and cost effective plant design. The paper will also present two case studies in different locations to contrast the design improvements and cost saving opportunities when designing solar plant using NEC 2017, Article 690 versus the previous code revision.