Estimate Site Output

How to Estimate your sites output

Step 1:  Determine Static Head. The most accurate way to measure this is with surveying equipment if the slope is relatively free of vegetation. My favorite is an old altimeter from a small airplane. Garden hoses strung together with an adaptor on the end for a pressure gauge works well but purge the air from the hose before attaching the gauge for accuracy. Work up the creek incrementally. You will also need to know the penstock length to determine pipe loss, now would be a good time to chart that as well. You may also want to consider different points of diversion and hydro placements because of pipe and wire issues.

Step 2:  Determine Flow Rate. Many streams vary in flow rate usually due to rock stratification barriers underground. It is a good idea to measure several possible diversion points. For a small spring, one method is to use a container of known volume such as a cut off milk jug, paint can, or 5 gallon pale etc. Dig a hole and fill containers and factor for one minute reducing the stream flow to exactly zero. It may be easier to build a small flume and fill buckets from there. Another method is to build a small makeshift dam with a pipe sealed thru it instead of the flume. If it takes 15 seconds to fill a 5 gallon bucket, the flow rate is 20 gpm. Somewhat larger streams will require a different method. Weir charts are available but generally too large for our purposes. Another method is to measure the square area of a stream and then measure the distance traveled by a bobber. The average flow rate will be about 70-80% of that. One gallon is roughly 6.25” cubed and one cubic foot is equal to approximately 7.5 gallon. If our hypothetical creek has a width of 3 ft and an average depth of 1 foot we have a cross sectional area of 3 square feet. If the bobber traveled 1 foot in 10 seconds or 6 ft. in a minute we have 3 X 6 X 70% = 12.6 cubic feet per minute. 7.5 X 12.6 = 94.5 gpm. Admittedly this is not a very accurate way to measure stream flow but it is good enough for our purposes. Ballpark figures are workable especially considering most water sources vary significantly during the year. We do need figures that are close and within certain ranges to select the right turbine for your needs however.

Step 3:  Determine Pipe Loss and Dynamic Head. Static head pressure is reduced by friction from the pipe and it is a product of pipe diameter, pipe length, and volume of water flowing in the pipe. In addition to friction losses there are also turbulence losses associated with elbows or turns in the penstock and harmonic losses similar to a standing wave in a musical instrument. The pipeline is the engine behind the turbine and the actual source of the power. The pipeline can exhibit certain behaviors and is almost a living thing. Pipe loss on low head systems is critically important and should be minimized. A well designed penstock should not have more than about 15% pipe loss and a maximum flow velocity of 5 ft./sec. The chart has figures for flow velocity as well as flow rate. 3 ft./sec. or less is preferred. Up to 5 ft./sec. is acceptable for hydro. Figures above 5 ft/sec. are useful for irrigation purposes, but not for generating power. Pipe loss varies with the type of pipe used but will be close to the figures in the table. Turbulence and harmonic losses are close to impossible to fix after the fact if you exceed 5 ft/sec. Temperature and salinity or dissolved minerals and dissolved air in the water can also play major roles but are difficult to calculate. Let us try a typical example. Our theoretical stream has 25 gpm flow rate during the lowest time and 60 ft. of static head and a run of 700 ft. of pipe. The 2” pipe column shows 1.3 ft. of head loss per hundred ft. of pipe   1.3 X 7 = 9.1 ft. of head loss or 50.9 ft. of dynamic head. If we used 3” pipe instead the figures would be .18 X 7 = 1.26 ft of head loss or 58.74 ft. of dynamic head which will probably yield about 15% more energy harvest on the same amount of water.

Step 4:  We now have the nuts and bolts from the site and we can estimate the probable power output. First a little academic stuff.

Flow in gpm   X Head in ft.

———————————–   =   Theoretical horsepower


For example:     39.6 gpm X 100 ft of head

——————————-   = 1 t hp = 746 watts


To complicate the issue we have to figure efficiencies of the nozzle, Pelton wheel, water box, drive system belts, shafts and bearings, alternator, cooling, system voltage and there is more. To simplify…actually oversimplify, nearly everyone in the business uses the formulae of gpm X Head in ft. divided by 10 to get to a best case installed scenario of 53% efficiency. This is a figure that was possible with many of the wound field units and easier to get with the PM’s commonly found today. Working within the confines of some streams may force some decisions that will make this figure unattainable. A typical owner installed mountain stream hydro ends up around 45% efficient which is a divider of 13 instead of 10 and is the more conservative and safer estimate if you are going to use a formulae. One of our PM 1800’s is capable of 74.5% efficiency at one specific head and flow rate but most of the rest will fall between 50-60% and that includes our competition, contrary to many of their claims. Most applications will need a good overall efficiency across a wide range of water flow and the unit with the highest peak efficiency at high flow is usually not the best choice overall. We prefer to use a test data book with actually test figures for more realistic and honest estimates. There is 192 possible versions of the PM 1800 alone to satisfy the highly site specific nature of home scale hydro. The theoretical horsepower formulae above is shown, because many of the water permits will require stream potential in t hp. Hydro power potential, flow charts, and pipe loss are figured on a standard water temperature of 68.4 F which is warmer than most mountain streams in the winter. My own hydro works on an average stream temperature of 41 F which is about a 20% power penalty. Temperatures approaching 32 F are about a 50% penalty. Warm water springs or hot wells can offer an increase up to about 15% but that also depends on the amount of dissolved gases and other factors. Dissolved air will also reduce power levels and can be a serious issue exacerbating freeze up in very cold environments.

Step 5:  Output wiring.  Very similar to static and dynamic pipe losses, electrical transmission suffers the same costs in efficiency. Similarly, long distances and small wire sizes increase the resistance. Voltage loss in wiring is called voltage drop. This can be figured by using a formulae based on ohms/ft./amp but it is easier to use a chart. Copper wire is the most commonly used. Aluminum has 1.6 times the resistance as copper and is more subject to corrosion. The price difference is attractive but almost always a mistake. Basic Ohms law goes like this: 12 volts thru 4 ohms of resistance will allow a current of 3 amps to flow. To transmit low voltages long distance requires very low resistance. The problem is most pronounced with a 12 volt system. For example: if our theoretical hydro can produce 360 watts that would be 30 amps at 12 volt or 15 amps at 24 volt. The wire run for the 12 volt system will need to be larger and of course more expensive to go the same distance. A judgment call needs to be made concerning an acceptable loss of efficiency for electrical transmission and cost of wire large enough. Heat wire or charge batteries ? The chart for wire loss that is shown is a little different than most. It is altered to reflect a “hydro” reality whereby a fairly high amount of loss is deemed acceptable at lower voltages. For longer distances we offer the option of high voltage units used with a step down transformer rectifier. These units are single or 3-phase AC transmission but wild frequency, so they can’t be used directly to feed the AC side of the system except to a dedicated fixed resistive load to heat a room or water.

Step 6: Controls.  This doesn’t have much to do with siting but will need some thought. The turbine does not use integral voltage regulation but rather is allowed to run full tilt. Any surplus is diverted to a load, usually air heaters or water heaters at the system voltage using a diversion control unit like the Morningstar TS 45. There are many other options for diversion including voltage sensing relays either stand-alone or part of many inverters aux function. The voltage sensing relay can also be set up to turn off/on a solenoid or geared water valves sometimes in conjunction with timers and float switches. Sometimes this can get a little complex so it is always a good idea to have a back-up fail safe DC diversion in case something goes wrong… Murphy’s law always applies.

Step 7: Costs.  Aside from the stuff already listed, some thought should be directed at constructing a point of diversion. Something inexpensive like a pipe stuck in a creek can work but it can be a maintenance issue. Self cleaning screens are available but expensive. Dams, bulges and ponds are best but not always allowed. The bulge is a mini pond that acts as a forbay where the dissolved air has a chance to come out of suspension. Buried pipe is always better but is definitely more work at least initially. Water permits can be easy or a nightmare depending on the state in which you live and the quality of people working in those government agencies. Hydro is a non-consumptive water use. There are issues between the point of diversion and discharge. The potential exists to dry out the creek and possibly create hardships for plants and animals, which includes any neighbors. Natural law ( common sense ) dictates… do no harm. In over 30 years of doing this, I have encountered only two people that would do harm and they came around after only a few discussions. I think we could do this ourselves as a people but unfortunately the permit process is governed by many different political agendas that are not always based on good science but rather control and greed which flies in the face of the intent behind our constitution and our birthright. Originally mineral and water rights were granted to the people. Unfortunately the 16th amendment opened up gray areas by which government could enact any law they wanted. I listed these concerns under costs because you may experience mental costs dealing with red tape and you should be prepared. I would estimate that only about 5% of people go thru the legal process, the rest hide it the best they can and hope for the best. Obviously I can’t recommend that. To push my radical conservative libertarian/constitutionalist agenda, I highly recommend a very accurate book on the US constitution by Michael Holler titled The Constitution Made Easy. I also recommend another book by Joel Salatin, a rightfully angry permaculture farmer entitled “Everything I want to do is Illegal”. Water rights vary by state and sometimes in different areas of a state, so it might be a good idea to visit your states website and read for yourself what the laws actually are. Many times these departments are just bullying their way along because they want to do…. what they want to do.