Not all wind turbines, inverters, and their use together in a given system will produce similar results when it comes to producing energy. In our system, on days where wind speeds repeatedly exceed 28 mph, our Xantrex GridTek 10 inverter spends much of its time in standby mode. The turbine’s power output increases as blade speed increases to 300 rpm—where the nominal maximum output of 10 KW is produced. In winds exceeding 28 mph, the turbine’s rpm continues to slowly increase. At 420 rpm (12.4 KW output), the inverter goes into standby mode to protect it from being overloaded. The inverter is designed to stay in standby for five minutes before turning on again. It will attempt to go back online regardless of how fast the blades are spinning.
But if the inverter tries to reconnect when the turbine is spinning faster than 460 rpm, it goes offline and stays that way until I notice it and do a manual reset. At times, there may be hours or days of lost production if I’m traveling or don’t notice that the inverter is offline. This issue isn’t unique to my unit: Testing at the National Renewable Energy Laboratory (NREL) documented this behavior back in 2002. Because I only recently added anemometers to the tower for measuring wind speed, I can’t yet be certain how much energy production is lost due to this issue, but I estimate between 8% and 15%.
Considering what was to follow, the inverter performance issue was minor. In February 2006, the wind generator threw a blade. This caused the turbine to vibrate so violently that it broke the pivot point connecting the turbine mount to the tower and the turbine fell—my $23,000 wind generator was toast. Fortunately, the 1,100-pound turbine did not hit any guy wires on its way down—otherwise it could have taken down the tower with it. A batch of defective blades was the cause of the failure. The system sat idle for almost six months until we received a new turbine in August 2006. The five-year warranty covered all the costs, and Bergey generously compensated us for the value of our lost electrical production—above and beyond the warranty terms.
In the spring of 2005, while we waited for our replacement wind turbine to arrive, construction of a new barn was underway. The barn faces toward true south with 1,000 square feet of roof area for our 10 KW PV array. We contracted Vermont Solar Engineering to design and install a grid-tied solar-electric system with about 60 KWH (at 80% depth of discharge) of battery backup for critical loads. An array of 80 BP Solar 125-watt modules would feed five OutBack MX60 charge controllers. But before we got very far into the project, we hit a few more regulatory stumbling blocks.
NYSEG rules did not allow both a wind and a PV net-metered system on the same residential line. A call to NYSERDA confirmed this, but they said they were in the process of redrafting rules. NYSERDA contacted all power companies in New York and found that Niagara Mohawk would allow that combination. This gave the NYSERDA the leverage they needed to rule that all New York power companies must follow the policy allowed by Niagara Mohawk.
That good news came with caveats, though. Due to the state’s 10 KW cap on residential net-metered PV systems, NYSEG would not approve the use of more than 10 KW of rated inverter capacity. This restricted our inverter choices and ultimately resulted in a less-than-optimal system. To meet code, we ended up undersizing the inverter capacity to 8 KW and selecting older-model Xantrex SW inverters. It was a clear case of poor regulations dragging technology down with them, as a variety of more efficient battery-based inverters are currently on the market.
Setting aside the regulatory walls we hit, our PV system has been operating without a serious glitch since November 2006. We were pleased with the job Vermont Solar did on our installation, and they promptly corrected a couple of minor problems.