A unique residential PV system

In May 2010 a 10kW solar system was installed at a residence in the Philadelphia region.  Oftentimes, residential PV systems are installed on the owner’s roof top.  In this case, the owner did not have a lot of space on the roof, but had lots of land.  So, the system was ground mounted.  Additionally, the owner wanted to maximize the output of the system while minimizing the installation cost.  The system was designed so the tilt could be adjusted several times each year to capture more sunshine.

The construction started with digging eight holes that were each 6 feet deep and 2 feet in diameter.  Six-inch diameter steel pipes were placed in the holes and then set in concrete and backfilled.   The tops of the pipes were then cut to the proper height and a backbone was attached across the top.  Twenty-two rails were then bolted to the backbone using brackets that can swivel on the backbone.  This allows the owner to adjust the tilt of the system several times a year to follow the sun.  In the summer the sun is high in the sky so the panels are tilted toward the sky.  In the winter the sun is lower in the sky so the panels are tilted toward the horizon.

Next forty-four Solon 225 watt solar panels were attached to the rails.  Then the electrical connections between all the panels were made and ground wires were added.  Wires carrying the DC current were run underground from the PV array to an existing shed where the control system was installed.  The shed houses the inverters, combiner box, subpanel, disconnects, and production meter.

The resulting AC current from the inverters runs underground from the shed to the owner’s garage.  The wires are tied into the existing circuit breaker box that services the house.  When the sun shines and the house is using fewer killowats than the PV system is using, the electric meter spins backwards!

I spoke to the owner recently.  He reports the system is running well.  It has already generated 8 megawatt hours (MWh).  The system should produce about 12 MWh a year which is worth 12 Renewable Energy Credits (REC).  There is a market for RECs.  The price varies daily.  A PV owner can sell them for the daily price or sign an agreement with a third party that will lock in the price for a number of years.  For example,  a REC might sell for $300 today or you could lock in a price of $265 for the next 5 years.