Polly Hill presentation plugs solar panels
Photo courtesy of Polly Hill Aboretum
About three dozen persons curious about solar power gathered last week at the maintenance building of the Polly Hill Arboretum to enjoy snacks and a talk by Rob Meyers of South Mountain Company (SMC) on photovoltaic (PV) generation of electricity. SMC has recently installed arrays of PV cells on the building and an attached garage, which are providing about 30 percent of the electric power needs of the Polly Hill campus.
Thirty-two shiny black PV panels, each about the size of a poster, are installed on the two roofs and are producing a total of 10.4 kilowatts. The two roofs are serviced by separate inverters, machines that convert the direct current generated by the cells into the alternating current used by nearly all devices from light bulbs to computers. The inverters are connected to the NSTAR power grid in such a way that during sunny days, no power is drawn from NSTAR. When the PV arrays produce more power than Polly Hill needs, the electric meter runs backwards, which creates a kw credit used when the meter runs forward, at night and on cloudy days. Since the 2008 Green Communities Act, Massachusetts requires that power companies "buy" kw at nearly the same rate customers pay for grid power. This on-grid process is called net metering. SMC's affordable housing homes at Eliakim's Way are all-electric homes with the potential to operate for a year at zero net cost for energy, and two of them did just that in a recent year.
A cynic might say that what SMC and Polly Hill presented last week was a two-hour infomercial for SMC and SunPower, the manufacturer of PV cells favored by SMC. However, Mr. Meyers's formal presentation was only 15 minutes long. Most of the morning was a lively discussion and question-and-answer session. By their questions and later comments to The Times, almost all those in attendance revealed that they were home-owners who had been interested in solar power, sometimes for many years, and were seriously considering installing it here on the Vineyard or in off-Island homes. The snacks were delicious, but this group was hungry for information.
Peter Cabana, Dukes County board member of the Cape Light Compact, provided additional insight.
The questions were wide-ranging, covering matters from optimal installation and cost to fire safety and foreign manufacture.
Mr. Meyers was understandably cagey about the cost, explaining that it depends on the power needs of the home, the lot, and the house itself. Homes under construction are always easier to equip for solar power. For older homes, the condition of the roof, engineering problems, routing wires, and other factors come into play. There is at the moment a 30 percent federal tax credit, which would dramatically reduce the startup cost, as well as Massachusetts tax rebates. However, he offered that for most homes a 5 to 10 kw system would be adequate, and would pay for itself in energy savings in 5 to 7 years, assuming a net installation cost of about $35,000. Mr. Meyers announced that for members of the Polly Hill Arboretum, SMC will donate one percent of the cost to Polly Hill.
SunPower guarantees the cells for 25 years, which is an indication of their life expectancy. However, the inverters, which are machines and have moving parts, need to be replaced more often.
Several attendees asked questions about optimal installations. Mr. Meyers said that SMC recommends a clear view of the southern sky (actually, 190 degrees magnetic). The "correct" angle to the horizon changes from summer (when the sun is higher in the sky) to winter. Ground installations are usually adjusted manually in spring and fall. Roof installations usually are not adjustable. Systems that track the sun have so far not been cost effective. The two roofs at Polly HIll maintenance have different pitches — the main building is steeper than the garage, which means that the main building is slightly more efficient in the winter; the garage, in summer. This is part of the reason for using a separate inverter for each roof. There was a discussion about trees that shade roofs, particularly when the trees belong to a neighbor. Vineyarders used to hassles about views will find this problem familiar. Mr. Meyers said that there is no legal recourse if a neighbor's tree shades your PV panels.
One man asked about the safety of firefighters working at a house that has PV panels. When the sun is shining, the cells are producing electricity, even if the system is shut down. Mr. Meyers answered that when the inverters are shut off, the cells produce volts but not current, and are not dangerous. Others in the crowd questioned whether a connection between the cells and a ground might allow current to flow through a fire fighter. The dispute was not resolved.
Attendee Taffy Rodell described her Island home as the last off-grid holdout at Quansoo. She is now interested in using solar power. Mr. Meyers explained that an off-grid installation is very different from the sorts of homes SMC usually works with. The big difference is the need for large and expensive batteries to store energy generated while the sun shines for use at night or on cloudy days. Few homeowners would want electric power only while the sun shines. On-grid installations use NStar as the "battery." However, off-grid is possible. The Times knows of at least one year-round Aquinnah home with PV and battery storage. Mr. Meyers commented that the technology is improving. One day the Vineyard may have a "smart grid," which would store energy in batteries such as those in electric cars.
Mr. Meyers explained the SMC recommends PV cells made by SunPower because they are more efficient than conventional cells. Conventional solar cells have wires printed on the face of the cell which carry the power from the cell to wherever it's going. Even though the wires are very thin, they shade the working part of the cell (silicon), reducing efficiency. SunPower cells have no wires on the front, but instead have a copper backing which collects the power. SunPower cells are 18 to 21 percent efficient, as opposed to about 12 percent for conventional cells. The copper backing also makes the cells stronger and less likely to break. He demonstrated by snapping one of each design. The conventional cell fell to small pieces. The SunPower cell only cracked, and Mr. Meyers claimed it would still work.
In response to a question, Mr. Meyers said that SunPower cells are manufactured in the Philippines. The cells are assembled into panels at plants in the Philippines, Mexico, and California. It is possible to specify SunPower panels assembled in the U.S.
The Times asked Mr. Meyers what percentage of the attendees he guessed would buy a PV system. He replied that he had no figures from other, similar presentations, but he volunteered that it is SMC's experience that of those who go as far as to get site estimates at their homes, about 75 percent go ahead and have a system installed.