ASK THE EXPERTS: Generator Grid-Tie

Intermediate
Lister Diesel Engine
Lister diesel engine.

I have an old 6 hp, 600 rpm Lister diesel engine that’s been running on waste veggie oil for about six months. It drives a 3-kilowatt, 240 VAC, 60 Hz generator, which powers various equipment in my shop.

I’m now wondering if I might convert it to back-feed the grid to take advantage of net metering. Aside from any legal issues, what would I need technically? Can the AC generator work with some kind of grid-tie inverter to back-feed the grid, or would I need a DC generator? If this could work, what type, voltage, and brand of grid-tie inverter would work best? I would like to run the generator for eight to 10 hours a day to offset some of our electrical loads. Though it will not be enough to run the whole farm, it could offset a good portion of our electricity use.

George Berz • Fresno, California

Congratulations, George. You’ve made an important piece of internal combustion history come alive! Diesel-fueled stationary engines from the U.K.-based R.A. Lister Company were first produced in 1929. They are legendary for their low fuel consumption and tolerance of varied fuels, plus quiet, reliable, and low-rpm operation. Lister clones are still produced in India, and are widely used there for pumping water and generating electricity in remote areas.

However, using this engine and generator to offset a portion of your electricity use may not be cost-effective here in the United States. To determine if it’s a wise idea, try connecting a steady load of about 1 kW to your generator through a kWh meter (like the inexpensive Kill A Watt unit) and measure exactly how much fuel the generator consumes to produce 1 kWh. Compare the utility’s retail electricity rate that you pay per kWh, the wholesale rate that they will pay you for electricity you generate, and your cost per kWh for bringing the fuel to your Lister. Even with your “free” fuel source, you’ll still have to consider the costs of procuring, transporting, and processing the vegetable oil, as well as engine wear, maintenance, and your time costs. The profit margin will be slim at best, and most likely negative.

If you somehow still find the math favorable, consult with a renewable energy dealer for advice on which battery-based grid-tie inverters to consider and the cost of the balance-of-system components, wiring, permits, and inspections you’ll need. It’s unlikely that any “direct” batteryless grid-tie inverter would be guaranteed to work properly with the output of your generator, even if you were to convert it to produce DC directly. These inverters are intended for either the DC output of a photovoltaic system or the wild three-phase AC output of a small wind or hydro turbine rectified to DC.

Once you have a grid-tie system cost estimate from a dealer, you can predict how long it would take you to pay off the investment—I would guess many years, if ever. Instead, consider adding a solar-electric array with battery backup to your grid-tie system instead. Then, in the dire case of a grid blackout with no solar input, you can listen to the gentle “putt-putt-putt” of your veggie-oil-powered Lister while your inverter powers your home from the battery bank.

Dan Finkwww.otherpower.com

Comments (3)

o-town sparky's picture

You cannot legally bring an AC generator onto the grid unless your sine wave is in phase with the grid AND you are at EXACTLY 60 cycles per second. No home generator can do that without a phase synchronizer. (Only power plant generators have phase synchronizers)
An inverter is the wrong equipment for the job since it only converts DC power to AC. Grid tie inverters use the grid's sine wave to synchronize its output power with the grid.
The only feasible way of connecting an AC generator to the grid is to use a pulse-width modulator (or thyristor) to convert the AC power to DC power (a standard diode rectifier will not work due to the DC sine wave), then use a grid tie inverter to convert the DC power back to AC power. You will also need a power loss shunt to cut the generator from the grid during a power out.
Last but definitely not least, In the event that someone was to go to the expense of purchasing the correct equipment to make this work properly, it must still pass inspection. The odds of finding a building inspector who will pass this mini power house is slim to none, and a lot closer to none.
The best chance for passing an inspection is to add a battery bank between the rectifier and the inverter (the less expensive diode rectifier will work here). That will remove the generator from any direct tie to the grid. The generator will charge the batteries and the batteries will supply power to the grid tie inverter.
On the financial side, with the renewable energy tax credits, it may be more cost effective to install a solar power array.

N R Rao's picture

Mr Fresno,Lister diesel engine. can I run using my dairy bi product, bio gas, if yes pl tell me how to go further.

Thank you

nstojadinovic's picture

Clearly missing the bit about "running on vegetable oil" which I'm assuming that George is getting for free.

There are many inverters that will do the job, the main one that comes to mind is the "Windy Boy" (made by the Sunny Boy crowd). I can't remember offhand if the Windy Boys needs AC or DC but either is not a problem.

AC generators are easy to get in any size you like while DC permanent magnet motors were used in industry for many years and are readily available on the second hand market, generally in 90 or 180 volt operating voltages. I have a 1.5KW @180V version that makes a very nice generator and, by the way, will directly drive all sorts of loads like power drills, electric stoves, lights, etc etc.

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