Hydropower Cost and Output

more info please.Is the 1200 feet elevation above the supply source or below it.Is the supply provided by a pump or gravity feed?How long and what size is the discharge pipe?What will be the heighth of the discharge pipe at exit end?

What is your pressure at maximum flow rate?

The 1200' is the source, above where outlet pipe is. It's gravity fed. I don't know what the pressure is at max flow rate, but will use every bit of the 500k gallons/day. That gives me a flow rate of 5.8 gallons/sec. I don't know what friction and impedence the 6" pipe will put up, but there must be a handbook somewhere that gives this info. I just don't have one.

The discharge pipe is at the river and I only need a ballpark figure for cost and hp output. This is very preliminary and only feasibilty is desired right now. Thanks.

Sloopster -- I reread your comments here and I am wondering about that 171 psi number you gave in the opening question for this topic. Is that from a pressure gauge on the system? Exactly where was the gauge located? Is the gauge known to be accurate? What was the flow rate in the system when the gauge was read? These questions of mine are somewhat rhetorical at this point given your level of interest in pursuing an actual project so answers are primarily of value to one who might review the situation at a later date. Another engineer is likely to ask the same questions as myself.

One of the major factors here, and I can tell you are aware of it, is the friction losses in the pipe. These will vary with the flow rate. If the pressure is read at the right location and a known flow rate can be measured at the same time then we can fairly well predict friction loss factors. Engineers use a popular chart of what we call Darcy Weisbach friction factors to help calculate pressure losses in water piping. With one or preferably two accurate readings of pressure and flow we can draw a reasonable graph of calculated pipe friction losses across a wide range of flow rates through your pipeline. This also gives to the experienced water system engineer a good approximation of the internal condition of the pipe and some insight into its remaining life. I have not worked closely with municipal water supply systems so when you need to make such a detailed determination you ought to seek out a civil engineer who has had that experience. My experience has been in the industrial area where piping systems tend to be fairly new and clean inside.

The problem with obtaining good data on pressure and flow rates is in measuring the actual flow rate at the time pressure reading are taken. This is where direct on site involvement of an engineer or an experienced water system technician (there are some good ones) is probably necessary. There are other measurement schemes such as averaging cumulative readings over a period of time that can produce good data. I suspect you are not quite at that stage yet.

Ed Weldon

Are you the same "Ted" who stars in the Dilbert cartoon series? Your tactics of getting someone else to do your engineering for you fits. OK, you'll have to make your problem definition a bit clearer.

Is the 171 psi reading what you have just up stream of the valve with a setting that allows the 500,000 gallons per day flow?

If that's the case then your flow at the end is 347 gpm at 171 psi. So the water horsepower to be harvested at that condition is 347 x 171 divided by 1715 = 34.6 hp or 25.8 kw. I'll leave the commitment of how efficient the water turbine/generator set and auxiliaries like support machinery and controls up to whoever wants to quote the job. Seems to me you are looking at netting somewhere between 15 and 20 KW electric output. At 10 cents/kwh that's a return of $12K - $16K per year on your investment.

Important to note that 1200 feet elevation change should produce ideally 519 psi with the valve closed. That means that at 500K gallons per day (348 gpm) 348 psi is being lost to friction from the flow in the pipe. This may not be the best condition to produce the greatest revenue or return on investment. That you need to work out yourself. The company that might sell you the equipment new should be able to provide the engineering analysis to help you make the right selection.

Ed Weldon

I am, how did you know? I am looking for free engineering work and you are looking to give it away, or you wouldn't go to this site!

The 6" pipe is 8 miles long and goes up and down hills and I'm sure loses a lot to friction. The pipe is also 100+ years old and probably closed in a bit here and there. This is our town's old potable water system. I am on a public works committe, have no engineering background, just a vivid imagination, thinking that with all that elevation and volume, a viable generation facility could be born. I just have no idea about feasibility or cost.

Thanks for your input. At $12k/year, I don't think we'd go for it. Our retail is 8.3cents per kW in town from our little municipal hydro power company.

Bravo, Sloopster for your efforts on behalf of your community. I apologize for prejudging you. I am delighted to be of assistance to you. You are exactly the type of inquirer that I want to help.

We do see a good number of inquiries from trained engineers asking for very simple calculations that they could well take the time to do themselves. I'm not excusing my cynicism and clumsy attempt at humor. Just telling you my own shortcomings.

Supporting your position that the payback may not be adequete for your community water system I'd like to add that I made no mention of the cost of ongoing maintenance and operator attention to running the power generating equipment, a necessary component in your case. This will further discount the financial payback.

In the case of an individual homeowner this aspect often becomes a tolerable almost hobby like activity and expense. It is such situations that often give rise to systems cobbled up from used and surplus materials.

Such an approach is incompatible with the operation of a even a small public utility except perhaps in the most dire emergency situations. I think you are headed toward the right decision. But were I in your position I would still take the time to engage a supplier of new equipment for advice and some preliminary cost estimates. This information may be helpful to have on hand if at some time in the future you folks are faced with major construction to improve or replace that old pipeline or your water system. In such a circumstance the potential for power generation may be more interesting and could well soften the financial load of a large project. It will be handy to have the numbers at hand.

Ed Weldon

Thanks Ed, cynical humor is always okay by me. Even though I am now a math and former physics teacher, I don't have access to an engineer's handbook or I would do this myself.

The 500k gal/day is a figure that was given to me by our water works engineer as to what our watershed produces. I would assume that this goes up and down with the weather, but is a minimum as far as I know. It seems the idea is to use this much water a day, restricting flow either by friction or head pressure on the shaft.

Your input has been most helpful, and I think that going to a generating company and asking their engineers to do the calculations is a great idea.

Ted