In 1978, my wife and I bought a 190-acre farm near Ithaca, New York, consisting of an 1840s-era farmhouse, a barn, pastures for our horses, lots of room for vegetable gardens, and a sizable woodlot for our wood heater, which backs up an oil-fired boiler and gas furnace. The farm was a great place to raise our kids; gives us a sense of security in an uncertain world; and allows us to be closer to the land and its resources.
Weaving Wind into the Energy Mix
Our loads were fairly typical for an older home. While we did not do a comprehensive energy audit and efficiency upgrades, we focused on the biggest energy load—heating. I redesigned our heating system to provide comfortable temperatures to fewer rooms. And we added a lot of insulation and then hired a local firm to add even more insulation to replace the vermiculite in the walls that had settled.
A few years ago, Art Weaver, an installer and builder of wind turbines, came to our place. We sat on our deck and watched the leaves blow in the wind as he told us about the benefits of wind energy. We seemed to have more wind than most of our neighbors because of our hilltop location.
Our home is grid-tied, but I was concerned about the security and robustness of our nation’s electricity grid. I wanted to be as prepared as possible for outages, and installing a wind-electric system seemed a good way to diversify our energy sources for greater protection against utility failures.
Art studied the wind, topography, and surrounding trees, on which he based the tower height. A tower height of 120 feet was chosen, to raise the turbine 30 to 40 feet above mature trees in the area. This height gets the turbine above the most turbulent boundary layer and into much smoother winds. Smoother winds mean less turbine yawing—with less yawing you increase energy production and decrease wear.
In June 2013, the Weaver 5 wind turbine began producing energy. I read the meter faithfully and could see that it was going to fall short of our consumption, which hovers around 20 kWh per day. The daily wind fluctuations didn’t surprise me as much as the seasonal changes did. I decided to add solar-electric modules and batteries to the system to further decrease dependence on fossil fuels—a key goal.
Pairing the PV Array
I wanted enough renewable electricity to be self-sufficient in the event of a lengthy grid failure, without having to resort to fossil fuels. This meant an allocation of at least 5 kW of PV to supplement wind for household consumption, and a little more than 5 kW of additional PV for the charging our electric car, a Nissan Leaf.
The wind system was originally a batteryless grid-tied system, with 3 kW and 7 kW SMA Windy Boy inverters. The 3 kW inverter is used at lower wind speeds (lower start voltage) and the 7 kW is used at higher wind speeds (higher start voltage). The addition of an 11.4 kW PV system, along with whole-house battery backup, required that the wind system also be able to charge the new 15.4 kWh lithium-ion battery. The solution was to AC-couple the SMA wind inverters to the AC load port of a 10 kW Princeton Power Systems DRI-10 inverter, which is able to accommodate a high-voltage lithium-ion battery. The PPS has four bidirectional ports—two DC ports (used for PV and battery) and two AC ports (used for grid and load). The Weaver 5 wind controller was also upgraded (by the manufacturer) to sense battery and load status in addition to grid status, and to safely control wind turbine output according to the availability of these loads.
We still need to do some “stress” testing on the system, by pulling energy out of the batteries under controlled conditions—such as intentional grid loss with and without PV and wind inputs. We want to test how well the inverter responds to changing generation levels and house loads, including EV charging. Art’s team is developing Web page interfaces for both the Li-ion battery management system (BMS) and DRI inverter so we can monitor and tweak the systems. The ultimate goal is to simplify and make the whole system as robust, reliable, and manageable as possible.
Selecting & Using Lithium Batteries
Although lead-acid batteries are typical for battery-based RE systems, I wanted to try lithium-ion batteries. My research indicated that they can be superior in many ways if properly cared for, offering about twice the energy density of a lead-acid battery; four times the storage capacity in the same footprint; a longer cycle life; and half of the weight. Lithium-ion battery management systems continuously monitor every cell’s voltage and temperature and actively manage each cell’s state of charge (SOC) with a “balancing” circuit.
Our 15.4 kWh GBS battery appears modest compared to the estimated daily energy use of 20 kWh. However, having more than 16 kW of generating capacity guarantees that the battery bank will not be stressed by daily deep-cycling—even if the utility grid goes down for an extended period. I prefer to invest in ample generating capacity rather than massive storage capacity. If generation cannot keep a large battery bank charged, then the batteries will degrade faster, requiring premature replacement. Wind and solar energy are complementary—there’s wind at night and in winter, and sun during the day and in summer. This characteristic allows a smaller battery without sacrificing energy reliability.
We expect battery maintenance to be essentially zero. We also expect them to last two to three times longer than an equivalent lead-acid battery. The GBS lithium-ion batteries require no “watering” or electrolyte monitoring. If a cell in the series string goes bad (there are 120 cells), it will require replacement, costing less than $100. Individual cell monitoring lets us know if a problem develops in any cell.
The DRI-10’s high-voltage battery input is attractive for the same reason that high-voltage PV strings are attractive—smaller wire can be used without increasing voltage loss. We used #8 AWG to handle up to 40 A at 400 V (16 kW), which is surely more attractive (and cheaper) than conventional bulky 2/0 or 4/0 low-voltage battery cables.
An Added Load
A day after the PV system was in, Art showed up in a 2103 Nissan Leaf electric vehicle (EV) and let me drive it. I was in need of a new car, and I liked the idea of fueling my car with energy from the wind and sun, so I decided to buy one!
I routinely charge my car’s battery after each trip, even though I could easily make two trips. However, if the battery is fully charged when I set out from my home atop a hill, I cannot capture the benefits from regenerative braking. This means I’m using electricity to charge the battery, and then using the conventional braking, which adds wear and tear to the car. Instead, I charge the battery to 80% so I can use the regen braking, and possibly extend the life of the battery and the car’s conventional brakes.
Energy use for the Leaf is typically 3.5 to 4.0 miles per kWh. So if I drive 12,000 miles annually, that means it consumes 3,000 to 3,429 kWh each year, which is about equal to the wind generator’s output. As such, the PV system’s output—estimated to be about 14,000 kWh annually—covers all other household loads, about 20 kWh/day or 7,300 kWh per year. This leaves some surplus energy (about 6,700 kWh per year) that we could use, perhaps for electric space heating to displace some of the fuel we’re currently burning.
We’ve never done a load analysis in our home, but by adding solar, we’re producing more than our usage—even with charging the EV. Should the grid go down, I probably won’t charge the Leaf until I verify that I have ample energy from the PV and wind systems. I have driven the Leaf 3,491 miles in the 17 months since I bought it, and all of those were within 15 miles of the center of Ithaca. So my driving habits fit very nicely with the Leaf’s specs.
Cost & Motivation
The cost—$127,000—for the wind, PV array, and battery systems seemed reasonable considering the system’s novelty and complexity. The 30% federal tax credit applied to both the wind and PV systems. And a New York State Energy Research and Development Authority (NYSERDA) wind incentive of $11,525 reduced the upfront cost of the wind system. (There is also an NYSERDA incentive for PV systems. However, there were going to be delays in getting the DRI-10 approved for the NYSERDA incentive, so we sacrificed receiving this incentive to expedite the project.)
I’ve been asked how long the payback is, and what effect it has on my monthly bills. I don’t know, because saving money was not my main motive, although I certainly have no objections to it.
“Adding Battery Backup to Your PV System with AC-Coupling” by Justine Sanchez • homepower.com/168.38