Having worked in and around comemrcial and industrial pwoerplant for the past 35 years, I know that pumping power back into the grid is not practical with a home plant. Utilities, righfully,r equire all manner of protective relaying and transfer trip circuits on independent power plants pumping power into the grid. This is to insure that if the grid goes down for whatever reason, the independent plants (like home cogen plants) do not keep pumping power into the grid. In addition, the utilities require some sophisticated revenue metering equipment. The end result is the economies of scale do not favor a small independent generating plant selling surplus power into the grid.
The Power Authority I work for built a 3 unit smallhydro plant in 1981. I was consturction super on that job and took it thru startup. It consists of three low-head Francis type hydro turbines with 1 Megawatt induction generators on each turbine. An induction generator is nothing more than an induction motor driven by the turbine. To start up, the turbine is started rolling and gets the motor to nearly synchronous or rated speed. The line breaker is closed in. Since the motor is already roling, it has little inrush currnet. Not being a synchronous type generator, it simply "slips" into synch with the grid. At that point, the induction generator is motorized, but since the turbine is already turning it, it uses little additional power. As soon as that occurs, the turbine gates are opened wider and the unit begins producing power into the grid. This is about as simple a powerplant as it gets. This plant is interconnected into the local utility distribution grid. As a result, there is alineup of cabiets int he plant with protective relaying, transfer trip circuitry and revenue metering. This lineup of cabinets looks like it belong sin a plant with several hundred megawatts of generating capacity rather than a paltry 3 Mw. For this reason, I do not think very small cogenerating plants or very small hydro plants can be sell power into the grid economically. The portective relaying, transfer trip circuits and metering are the killer. It comes down to what the local utility requires for protecting their grid from the small, independent generators. Some federal regulations may have been enacted requiring the utilities to purchase the power and relaxing the amount of protective relaying and similar that used to be required. Induction generators really do make life a lot simpler if a person is going to try to pump power back into the grid.
Induction generators are about as simple a generator as can be had. Plenty of people do use old induction motors for the purporse. This type generator uses power from the grid to excite and regulate, so really is not a true "stand alone generator". Smaller inducation generators have been made from motors and have been built as "stand alone" unit using capacitor circuits.
Years ago, I did design two small instalations using induction generators. One was driven by a "fitz" type waterwheel and wasused for park/municiapl ballfield lighting. Since it was an induction generator, it was tied into the grid for excitation and regulation, so pumped excess power into the grid. This was about a 25 Kw installation. The other induction generator went into a dowel mil up in Maine. They had an old Ames horizontal unaflow steam engine which had driven the lineshafting. The boilers were fired on waste wood. When OSHA made it difficult, if not impossible, to use lineshaft driven machinery, they went to individual electric motor deives and shut down the steam engine. After the 1973 oil embargo and a few other spikes in electric prices, the mill owners reconsidered. They wanted to produce their own power and heat the mill with the exhaust steam, using the waste wood fuel (shavings and sawdust) to fire the boiler. As it was, they were giving away the shavings and sawdust to farmers. I calculated the available HP of the engine and theoretical KW output based n some losses thru the lineshafting. The result was about 150 Kw of generating capacity. The mill owners got a big, used induction motor of about 200 HP. We designed a drive to get from the lineshaft, which turned at about 350 rpm, on up to 1800 rpm. What was used wa smodern belting and sheaves. The Ames engine drove a chunk of the original lineshaft, and that, in turn drove a jackshaft at about 900 rpm, which then drove the generator. Governing of the Unaflow was by a flywheel-mounted cutoff governor. The only way to adjust engine speed was to shut down and get a wrench and adjust the governor springs in the flywheel. This didn't work for the induction generator as it had to be put on line at less than synch speed and then load increased by raising the "speed" of the governor. The mill owners took a small gearhead motor and drive from an old hospital bed and put it to working the speeded spring tension adjustment in the flywheel. They then rigged sliprings and burshes so they culd get power to the gearhead motor as it was now turning inside the rim of the flyhweel. The speed and load control on that engine was nothing more than the pendant control with "raise-lower" pushbuttons that the hospital patients had used to raise and lower the bed. It worked out well as the local utility was reasonable to deal with and did not make matters overly complicated or expensive. Minimal cricuitry in the form of a transfer trip circuit (so the mil generator would be automaticaly tripped off line if there were a problem in the grid) was mainly all that the utility required. A "two way" meter was installed and the millowners began pumping power back into the grid and heating their mill.
I am working out details for a small home cogen plant of my own. It will be a stand-alone plant. I am planning on using a Lister-clone diesel built in India, a 12 HP/2 cylinder engine running at about 650 rpm. Generator will be a 10 Kw 1200 or 180 rpm 2 bearing synchronous generator. I am deliberating using a Harley-davidson final drive belt and Harley rear wheel pulley in this drive rather than vee belts.
Waste heat will be recovered from the jacket water as well as frm the exhaust gasses. The engine jacket and exhaust heat exchanger will be pipe thru a coil in a stone-lined water storage tank. A bypass thermostat and radiator will allow for dumping excess heat from the engine. The stone lined tank will be piped into my home heating system. I do not expect it to entirely take over my home heating, but it should take quite a chunk off what my oil-fired heating boiler currently does. In addition, we burn wood for supplemental heat (barrel stove and heat-a-lator insert), so I am hoping to reduce my fuel oil and electric bills. I am designing my own waste heat recovery "boiler" for the exhaust gas. It is to be a shell-and tube type heat exchanger with larger tubes and "turbulators" in them. It will be able to be easily taken apart for running a shotgun brush thru the tubes as I anticipate some carbon soot fouling.
I figure the plant will be more of an experimental platform at first. I am using the Indian-Lister engine as it turns slow enough that I can work over to burning veggie diesel. I will have to come up with an electric starting motor for the Lister engine, as it is hand crank start.
We get a number of power outages around our home area each year. I think that even if the plant is marginaly economical, it will be good standby power for prolonged running. I also know that I have a habit of designing and building stuff as if I were working ona real powerplant or steam locomtoive, so whatever I put in my home plant wil liekly be "overkill" to most people.
I think the main reason for putting in this type of plant, for me, is to have a neat "old time" engine running, kind of a "fun" reason. At the same time, if this little plant get built by me and piped into my home heating loops, it should be able to do soemthing towards taking a bite out of fuel and electric bills as well as providing good backup power for long outages.
Joe Michaels
