Home-Size Hydro Power

A Practical Case Study

by Joseph T. Kohler, Ph.D., P.E.

Adam Stockton gets all the household electricity he needs from a small stream powering a microhydro setup. His private power plant puts out 700 kilowatts a month during three-quarters of the year (less during the summer months). This is enough to run energy-efficient appliances, provide domestic hot water, and supplement passive solar and wood energy for space heating. Here's a look at how the microhydro setup was designed and built, and how it has performed technically and economically since it began tapping the stream for power in the fall of 1980.	Off the grid, the Stockton house uses only microhydro for its electrical needs.
What's the resource? Power output (P) from a stream in kilowatts (kW) is equal to the flow (Q) in cubic feet per second (cfs) times the head (h) in feet times the fractional efficiency (n/100) of the turbine-generator, all divided by a conversion factor of 11.8. P = (Q * h * (n/100)) / 11.8 Well-designed microhydro systems operate at efficiencies from 40 to 50 percent. If the system is not connected to the utility power grid, the required battery bank and inverter will decrease efficiency up to another 30 percent or so. The monthly energy production in kWh's is then calculated by multiplying the power output (P) by 720, the number of hours per month. If the flowrate changes from month to month, monthly energy production will also change. The head of the stream is the easiest parameter to measure. It is the vertical height that the water drops from the head water inlet of the penstock (pipeline) to the tail water outlet below the turbine. The water flow in cubic feet per second is harder to measure. The best way is to construct a weir, a small dam with a rectangular overflow notch (see "Flow and Head Measurement"). Flow data should be collected for a year at least. If the system is connected to a utility grid, you are most concerned about the flows in high water months since this will determine the turbine size. If the system is to stand alone, you must know the minimum flow to find out how much head will be needed to supply the electricity you need. If the year turns out to be unusually wet or dry, you ought to monitor the stream longer. The power output can be calculated by plugging the head and flow data into the formula. P is then compared to the needs of the owner. At Adam's house, the flow varied from 0.1 cfs in the later summer to more than 2.9 cfs (898 gallons per minute) during the spring runoff. The average flow from October through June was about 0.5 cfs (225 gallons per minute - about equal to the water provided by 45 garden hoses going full blast). Because Adam wanted energy efficiency, we needed enough head so that the power output would meet the minimum required monthly electrical use even at the lowest flowrate. "Minimum required" meant an energy-efficient refrigerator, a few lights, and a water pump, which altogether consume 75 kWh per month. Working backwards with the formula, assuming a flow of 0.1 cfs, and guessing a total system efficiency of 20 percent, a head of at least 60 feet was necessary. A survey of the site reveled it would be possible to get 72 feet of head by tapping into the stream at a small natural pool with a 1300-foot pipeline. Tough Choices Grid-connected systems use energy more efficiently. Any energy produced nut not needed in the household can be sold back to the utility. Since the utility provides supplemental electricity during dry spells, storage batteries are eliminated and the cost of the system is lower. Most new grid-connected installations use induction generators (Solar Age, 4-1982). Induction generators do not require expensive governors. (in AC systems using alternators, governors are required to ensure that the electrical output matches the demand.) However, grid-connected systems usually depend on the utility-line power. When utility power goes down, the microhydro system goes down with it. Stand-alone systems are used when the owner is farther from the grid or wants to be electrically independent. The trick is to design a system capable of producing enough power all year. Very small systems - less than about 2 kW - are usually designed to produce DC power. Since AC cannot be stored in batteries, a larger AC system would be needed to satisfy the peak load demand. The DC system would use batteries to store energy for peak demand. Most often the batteries would charge and discharge on a daily cycle. The DC power can be converted to AC using an inverter. Battery-storage DC systems are less efficient than grid-connected systems due to battery and inverter losses. When more than 2 or 3 kW can be produced, AC systems make more sense because peak load can be met directly without the need for expensive and inefficient batteries and inverters. In stand-alone systems like Adam Stockton's, all the power should be used for maximum cost effectiveness. Excess power cannot be stored efficiently for more than a couple of days without oversizing the battery bank. Notwithstanding the cost of inverters and battery storage, the investment in the pipeline is the largest single concern. It is absolutely imperative that the pipeline be sized correctly to transport the maximum flow of water to the turbine without undue loss in head due to friction. Table 1 shows the head loss per thousand feet of pipe. In the Stockton installation, the maximum flow for the single-jet turbine is about 0.5 cfs. The choice was between a 6-inch and a 8-inch pipe. From Table 1 a 1300-foot, 6-inch pipe loses 6.5 [(1300 / 1000) x 5] feet of head while and 8-inch pipe loses only 1.6 [(1300 / 1000) x 1.25] . A 6-inch pipe was chosen because it cost less, even though it lost more head. Table 1 Cu.ft./ second 0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 Pipe Size Head loss per 1000 feet of pipe 4" ½' 2' 7' 15' 26' 40' 56' 74' 95' 118' 144' 172' 201' 230' 268' 6" 0 ¼' 1' 2' 4' 5' 7' 10' 13' 16' 19' 23' 27' 30' 36' 8" 0.00 0.00 0.25' 0.25' 0.50' 1.25' 1.75' 2.33' 3.00' 3.75' 4.50' 5.33' 6.33' 7.30' 8.50' Most microhydro installations have PVC pipelines because it is relatively inexpensive and easy to handle in the field. PVC pipe is manufactured in different classifications. SDR-26, rated at 160 psi, is often used for water mains. SDR-35, rated at 100 psi, is used for irrigation systems. Because it was available in the area, SDR-26 was used, although SDR-35 would have been less expensive. PVC pipe should be buried to protect it from sunlight, freezing, and vandalism. Burying the pipe in the woods alongside a stream is no simple task. In this case a bulldozer made a road, a backhoe dug the trench, and a loader dumped a protective layer of sand over the pipe before backfilling. The work was back-breaking. All power equipment is contained in a small house located next to the stream. The first dam, constructed of earth and stone with a stone spillway, proved to be no match for flood waters. It required constant repairs with sandbags. The replacement concrete dam is simply a 30-inch-high 15-foot-long barrier across the stream to divert water into the pipeline. Excess water goes over the top of the dam. Very little water is actually impounded. The Final Design Since one of the goals of this project was to demonstrate energy self-sufficiency at Adam's house, we were forced to choose a relatively expensive stand-alone system. We chose a 6-inch, single-jet Pelton wheel, manufactured by Small Hydroelectric Systems and Equipment. The Pelton wheel works well with the available 70-foot head and has the ability to operate over the wide range of flowrates. The brass wheel mounted on heavy duty bearings in a 1/4-inch-thick steel housing should last a lifetime with an occasional bearing replacement. The only maintenance is a shot of grease every couple of weeks. The wheel drives a 24-volt Electrodyne 80SC DC alternator. This brushless, heavy duty truck alternator was chosen for it's high efficiency (about 70 percent) and long-life bearings. Operating at about 3600 rpm, 8760 hours per year, the alternator should run about 3 years before the bearings need rebuilding. Ordinary automobile alternators have a service life of only a few thousand hours, and are only about 50 to 55 percent efficient. The alternator charges two 12-volt Surette HD8 extra heavy-duty marine batteries wired in series to provide 210 amp hours at 24 volts, or about 5 kWh of storage. This relatively small amount of storage is satisfactory because the turbine operates 24 hours per day. The batteries serve mostly to average out short peak demand, such as when the water pump, toaster or iron are on. The batteries are rarely discharged deeply and should have a life of 5 to 10 years. A Best 2500-watt BR 2499 inverter converts the DC power to AC at an efficiency of about 85 percent over most of it's operating range. It has proven it's ability to operate normal energy-efficient appliances including lights, TV, stereo equipment refrigerator, washing machine, 1/3-hp water pump, and even a 1-hp table saw. The solid-state inverter requires no maintenance and is equipped with an option to shut it off automatically when there is no current draw. This conserves power by reducing standby losses during low water periods. When batteries reach full charge, a Natural Power Dynamic Loading Switch shunts DC to an electric water heater fitted with a special 400-watt heating element, In addition to providing domestic hot water, the switch keeps a load on the turbine at all times, minimizing the freewheeling phenomenon. A problem arose when the system first started up. When the load on the alternator exceeded the turbine output, the alternator stalled the turbine, and the power output stopped. The problem was solved by wiring a rheostat (variable resistor) in series with the alternator's field winding to tune alternator output to turbine output. Because the turbine spins at 1200 rpm at full load, and the alternator needs to turn at a minimum of about 2600 rpm, a three to one speed increase is needed. This required a 3-inch pulley on the alternator and a 9-inch pulley on the turbine. Our first chain drive failed prematurely (one week) due to repeated stresses that occurred when sudden loads slowed the alternator. The final solution is a Dayco Gold Medal 1/2-inch V-belt drive transmitting power from the turbine to the alternator. The V-belt can provide over 95-percent power transmission efficiency. With no load, a Pelton freewheels at about 1.9 times it's loaded speed (aka: runaway speed). Although this happens very rarely, it is important for the alternator to be able to withstand the high rpm. The three-to-one pulley ratio was chosen so that the alternator would operate at the low end of it's design range when loaded, and at the high end during freewheeling. A better, but more expensive, method would be to include a speed limiting device to divert water from the nozzle under partial load conditions. Performance And Costs The electrical output of the turbine depends on the amount of water supplied through the jet. Flow, in turn, is regulated by the diameter of the nozzle. Nozzles ranging from 9/16-inch (0.1 cfs) to 1 1/8-inch (0.5 cfs) are changed seasonally to correspond with the available flows. The measured output and efficiency are shown in Figure 1. The costs (1980) of the components are shown in Table 2 totals over $9,000. Contributed labor and equipment probably were worth another $6,000 to $9,000. There are no tax credits for residential microhydro installations unfortunately, and unjustly. As in many projects, costs have a way pf creeping upward as the project goes from the early stages to actual construction. Unexpected needs and oversights, and many small items, such as gauges, wiring, and small fittings add up. Inflation also takes its toll. A spare alternator, purchased one year after the original alternator, cost 30 percent more than the first one. All in all, the costs of this project are probably quite typical of what to expect for a stand-alone microhydro installation on the residential scale. A grid-connected system might cost a few thousand dollars less. The pipeline is the biggest variable. Its cost depends on the length of the pipeline and on the terrain. In this project, it devoured two-thirds of the total installed cost (labor and equipment). Performance can vary widely and is very site dependent. Double the flow would give double the power, at much less than double the cost. Similarly, twice the head would be double the power. A site with twice the head and flow would payback four times as fast. The system delivers about 6000 kWh of electricity annually, which if fully used is worth perhaps $600 per year, resulting in a simple payback of about 30 years. This ignores the foregone interest on the money spent and the inevitable maintenance costs, but also ignores inflation in electric rates, which will be substantial. A utility hookup was available if we had desired one - for roughly $6,000.

Table 2: Costs (in 1980 dollars)
Generating Equipment
Turbine, nozzles, shipping	$ 1568.00
Alternator and spare	$ 860.00
Drive	$ 230.00
Generating Equipment Total	$ 2658.00
Electrical
Inverter and load demand	$ 1600.00
Load switch	$ 84.00
Gauges, lighting protection and miscellaneous electrical equip.	$ 204.00
Batteries	$ 362.00
Electrical Total	$ 2250.00
Pipeline
Pipe	$ 3236.00
Equipment rental	$ 663.00
Sand	$ 280.00
Pipeline Total	$ 4179.00
Powerhouse & dam materials	$ 100.00
Subtotal	$ 9187.00
Other debits
Contributed equipment	$ 3000.00
Contributed labor @ $10/hr	$ 3200.00
Grand Total	$ 15,387.00