Info

www.sunelco.com

100 Skeels St. PO Box 787 Hamilton, MT 59840

Sunelco

with a Solar Powered Lawn

Christopher Zach

©2003 Christopher Zach

Christopher Zach

©2003 Christopher Zach

The author's children playing "farmer" on the Elec-Trak lawn tractor (safely turned off, of course).

It all started back in the summer of 2000 when the power line to my work shed shorted out. Replacing the line would have cost well over US$1,000, along with the inconvenience of tearing up my driveway and yard. It was at this point that I thought: "Now would be a great time to try solar electricity!"

So I installed my first solar-electric panel, got a few batteries, ran some wires, threw the disconnect switch, and lit up the shed. At that point, I knew I was hooked.

Making a Difference

One of the things I quickly noticed about solar electricity was its ability to make a difference. Here I was with a single solar-electric panel, two batteries, and an inverter, and I had enough energy to light my shed and use some small power tools as well. Every time I flipped the light switch, I felt a sense of pride in the fact that this was energy I had generated cleanly, and it was truly mine to use.

I began to think about other uses for all that energy coming down from the sky. What I really wanted was to use the solar-electric panels to make the biggest dent possible in my pollution output, while spending a reasonable amount of money, and providing a solution that would clearly display to anyone the benefits of solar energy. While I was pondering this, I went outside and sat down on the biggest, loudest, most pollution-generating device I owned—my Craftsman 16 horsepower lawn tractor.

The Craftsman

Small, gasoline powered yard tools are notorious for the amount of noise and extreme amounts of pollution that they generate. This is partially due to the small sizes of the engines, but mostly to a complete lack of anything resembling pollution control. Garden tool motors typically have single-stage carburetors, so they run very rich. They also have limited (if any) mufflers; and no air injection, fuel injection, or any of the clean air features found on even the most basic automobile engines. The gas tanks are typically

The author's children playing "farmer" on the Elec-Trak lawn tractor (safely turned off, of course).

vented to open air, and the crankcase fumes aren't reburned.

According to an article from the CNN Web site, lawn mowers in the Los Angeles basin put out more pollution than all the aircraft in that area. In fact, per hour of use, a lawn mower puts out more pollution than 73 automobiles.

Thus, my choice was clear. The gasoline powered lawn tractor was the biggest polluter I owned. But I use it a lot. It mows the lawn, it lugs around loads, and it's a very useful tool to have. What could I possibly replace it with?

The Elec-Trak

In browsing past issues of Home Power magazine, I read about the General Electric Elec-Trak line of lawn tractors in HP70. These units were built in the early 1970s, and instead of using a gasoline engine, they were powered by electric motors run from six golf cart batteries.

The solar-electric charging system hooks up to the Elec-Trak's 36 volt accessory port.

The solar-electric charging system hooks up to the Elec-Trak's 36 volt accessory port.

My first thought was, "Wow, I've got to get one of those." I could plug it into the wall and get rid of the tractor. Then the second thought hit me: "Why not take this idea to the limit and run an Elec-Trak on solar electricity?"

The idea of mowing a lawn with solar electricity is not quite a new one. At least one manufacturer makes a tiny solar powered lawn mower. But the Elec-Trak isn't a little plastic contraption. It's a 900 pound (408 kg) steel tractor with a 42 inch (107 cm) cutting deck—a difference in scale to be sure. My next steps were to find an Elec-Trak, and then figure out a way to run it on solar electricity.

Buying an Elec-Trak was actually not too difficult. General Electric made a lot of them, and they were built extremely well, so many still survive. I checked on eBay for a few weeks, and found an E20 model (the biggest they made) for the low price of US$520. The batteries were shot, and it had some surface rust, but the basic mechanics were solid. I rented a small pickup truck, drove from my home in Maryland up to New Jersey, and hauled it back.

It took a bit of work to get it running, but thanks to lots of help from the Elec-Trak Web site (www.elec-trak.org) and a very helpful e-mail list, I was able to find local people who could help me bring it back to operation. So now that I had the tractor, it was time to build a solar powered charger.

The Solar Charging System

The first step in designing any solar-electric system is to answer these three questions:

1. How much do your loads draw?

2. How much time do your loads run?

3. How good is your solar resource?

The first question was answered with a bit of basic research: I estimated that the Elec-Trak would draw approximately 60 amps at 36 volts while cutting my lawn.

The second question was the key to designing this system. My 1 acre lawn takes about an hour to mow. So the electrical load would be 60 amps at 36 volts for one hour, or about 2,160 watt-hours. I mow my lawn about once a week. In the hot summer months, I might only need to mow the lawn every other week. The rest of the time, the tractor sits idle. So the solar-electric array for my electric tractor needed to provide 2,160 watt-hours once every week or two weeks.

The third question has to do with the environment the system is installed in. I live in Maryland, on the south side of a nice hill, and have a heavily wooded backyard around my shed. During the summer, when I use the tractor most, I can typically get only about four useful hours of sun per day because of the number of the trees in my backyard. Due to clouds, I can count on about five days of sun a week, or about twenty hours of peak sun per week.

When I take into account the decrease of PV output as temperature increases, the losses inherent in charging batteries, and the losses in the wiring, I figure that about 80 percent of the PV array's rated output is available for charging the tractor's batteries.

Fill 'er up! The tractor getting topped off with solar electricity.

Given these assumptions, I made the following calculations: A solar-electric array that is rated at 180 watts can probably charge the batteries at about 80 percent or 144 watts. Multiplying this by an average of four peak sun hours a day, I have about 575 watt-hours (WH) of daily energy production.

So in five sunny days, I can expect 2,880 WH of energy production. If I get a full week's worth of sun (all seven days) I'll get more than 4,000 WH. A 180 W rated solar-electric array would provide more than enough energy to mow my lawn every other week, and might provide just enough to mow my lawn every week (depending on the amount of sun).

A great deal of assistance in the design process was provided by Kirk Mulligan from Atlantic Solar Products. In addition to providing the equipment, he was quite helpful in determining the right products to use and answering my questions.

Building the Solar Charger

Building the solar-electric system was not as complicated as I anticipated. I mounted the panels to my shed's roof with zinc plated steel girders. The girders are mounted to the panels with standard 5/16 inch (8 mm) bolts, and screwed into the shed's roof. The roof of my shed consists of 1 by 12 lumber planking, nailed to 2 by 4s, and covered with 20 year shingles, so I am rather confident that the rack will not come apart in a windstorm.

I first used one BP Solarex MSX120 panel configured for 24 volts, series connected to an MSX60 panel configured for 12 volts. (These PVs are field configurable for 6, 12, or 24 volts.) This provides a combined output voltage of 36 volts at 180 W rated, which matches the power requirements of the tractor. The outputs of the panels are connected via #10 (5 mm2) wires to a junction box, which is connected via conduit to a weatherproof Cutler-Hammer 30 amp disconnect.

The disconnect serves two functions. First, it allows me to turn off the panels when I want to work on the downstream wiring. I make it a personal policy to ensure that all connections from a solar-electric panel can be disconnected with the simple throw of a switch.

Boost the Power

The 180 W rated solution I installed was working well. But I wanted to decrease the time it took to fully charge the tractor battery, so I could mow my lawn weekly, and have power to spare to drive my tractor around. So I went out to Atlantic Solar and picked up two more 120 watt panels. Installing them on the roof was a simple matter of building a mount and wiring them in.

For the 360 watt solution, I configured the three BP Solarex MSX120 panels to output 12 volts nominal.

Then I wired the three modules in series. This gave me a 360 watt output at 36 volts. The original MSX60 is now being used to charge spare batteries.

The three MSX120s are providing the tractor with more than 7.5 amps of charge at 36 volts nominal. With this much solar electricity, I can run the batteries down and bring them up in three days. It turns out that this is more than I need, so I'm starting to reconfigure my system again so I can sell some of the surplus to the utility.

36C Watt, 36 Volt Charging System

Photovoltaics: Three BP Solarex MSX120, 120 W each, individual modules configured for 12 VDC output, wired for 360 W total at 36 VDC

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