Case Study: The long roof challenge

Not all roofs are created equal, which was the challenge faced by Tecta Solarwhen they installed a large PV system on a distribution centre in Whitinsville, Massachusetts (US).  What was unusual was the 3000-foot run of roof, which was made up of four different heights and design. The challenge, according to Brett Chapman, Tecta’s Sales Director, was accommodating the long runs.

Fortunately Tecta Solar has the advantage of the experience of parent corporation, Tecta America., a national commercial roofing contractor. Prior to the installation of the solar facility, Tecta America’s Roofing Division repaired and replaced the roof so a Total System Warranty could be issued that covered all roof and solar work. They chose Suntech polycrystalline modules primarily because the high quality makes them very bankable, according to Chapman. Tecta also chose a ballasted mounting system from Sunlink because, as Chapman stated, “they are a large [company] with a proven track record and a product that is quick to assemble, requires zero roof penetrations and is economically priced.” Furthermore, he said they provided a stamped, professionally engineered racking design that met the local wind and snow loads.

In its first year of operation, the 1,869 kWp rooftop photovoltaic solar facility is projected to generate 2,178 MWh of energy.

With its commitment to using local labour and materials as much as possible, the 11 inverters (10 500kW and 1 300kW) came from Massachusetts-based Solectria Renewables. Chapman said that while modules rarely have problems, inverters may need maintenance and the owners wanted to have service readily available with a local provider.

The owner of the system provided the financing, and the project qualified for the Massachusetts Solar Carve Out, which means the project can produce Solar Renewable Energy Credits (SRECs) that are purchased by the local electric utilities to meet the state’s Renewable Portfolio Standard (RPS) requirements.

In its first year of operation, the 1,869 kWp rooftop photovoltaic solar facility is projected to generate 2,178MWh of energy, or the equivalent to the energy needs of 272 average Massachusetts homes in one year.  The project is net metered, so the two occupants that are the two major users of the power, Blackcomb Solar LLC and Core-Mark, will use some of the electricity and some will be virtual net-metered to other customers.  Through virtual Net Metering, Blackcomb sold the net metering credits to numerous private companies in the same utility zone.

Exceeding expectations

This table shows the production data (in kWh) collected on a monthly basis for this system:

The expected generation is based off of values obtained from using PV Watts to model system generation, which is based off of Typical Meteorological Year 3 (TMY3) data, which is obtained from NREL.  The actual meteorological data is compared to actual data measured at the site. This system has produced considerably well since it was officially interconnected, particularly compared to the irradiation data collected over the same period of time.  Each month, the system has outperformed its expected production when considering the average solar radiation measured during that month.

Written by: Anne Fischer, Solar Novus Today