Merging Wind, PV & Batteries for RE & Independence: Page 2 of 3

Intermediate

Inside this Article

William Bassett with the hybrid system that makes his 19th century farmstead energy independent.
A Weaver 5 wind turbine with 210 square feet of swept area and 11.4 kW of SolarWorld PV modules provide more than 100% of the grid-tied home’s year-round energy use.
An insulated battery box houses 120 Elite Power Solutions GBS-LFMP40AH, lithium-iron-phosphate battery cells. Provides 40 Ah at 384 VDC.
LiFeMnPO4 batteries require a battery management system (BMS) for each 3.2 V cell.
At the base of the tower are the short-circuit brake switch (left) and tail motor disconnect (right), both fitted with lightning arrestors. The box below the tail motor switch houses connection to additional wired anemometers on the tower.
With bidirectional DC and AC ports, the multimode DRI-10 inverter handles either PV or wind DC input and provides MPPT charge control for PV. It can be used with high-voltage battery banks, and can be connected to the utility grid, an AC generator, or neither.
The DRI-10 inverter’s touch screen displaying that there’s 7.3 kW of PV power on the PV port and 5.0 kW of wind power coming in via the load port—the arrows indicate direction of current flow.
William with his Nissan Leaf electric car, which is powered entirely by the sun and wind.

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.

Comments (2)

Robert Pollock_2's picture

Very nice and timely. We're planning a Net Zero house in Ontario, only a few miles North of where you are, with less snow, actually.
My own design/calculations using SketchUp and energy analysis software was pointing at a similar system. You pointed out a few fairly sophisticated approaches however, being in 'smooth air' for one, and the business about DC Voltage. Thanks a lot.
We'll be putting the solar array on the ground with a single axis tracker, depending on what a dual axis costs and how much trouble it might be. Also a Passive Institute design, not that I copied them, we just ended up at the same place, and a ground source heat pump. (mini-split) Burning wood intrigues me. I think you can burn it fairly cleanly by letting the stove run wide open and mitigating the excess heat with a masonry thermal mass surround, maybe ten tons. When you live in the woods it seems so silly to not be able to burn wood. Really dry and burned at 1400 F or so, what gases and particulates are emitted and how much?

ideas2014's picture

is it possible to share the single line wirring diagram with us , if i send u my email
thanx

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