Small Hydropower & Micro Hydropower

Here’s some questions from Ben, one of our  Small & Micro Hydropower Newsletter readers. He asked these questions regarding turbine sizes, efficiency and penstock design.

Sincerely,
Jess

Hydro System Design Questions:

——– Original Message ——–
Subject: turbine size
From: Ben
Date: Thu, June 11, 2009 8:16 am To: jess.blog@smallhydro.com  

Jess,

1. Was wondering, does a smaller hydro propeller, ( not sure of terminology) add rpm’s thus giving more electric production??  If so, what is a general increase?? 
2. Also,  in the formula you sent me about converting to psi, how can you increase the effeciency rating of 72%, in general terms???  
3. Finally, on the penstock, how much do you typically decrease the intake from the main water line??  In other words,  with a 4″ line is the penstock intake 4″ or does it decrease to 3″, and what if you decreased it to 2″ penstock intake??  Will that increase the flow pressure and what impact does it have on spinning the turbine??    I still don’t have specifics but working on it.

Thank you,
 Ben 

Hi Ben,

Looking at your questions I’ll try and answer them the best I can.  Turbine deisgn and choice is complex and efficiency of hydro design is also.  Mixing both in one response is a great simplification. With that said here are my Answers.

Jess

Hydro System Design Answers:

1. Choosing Smaller or larger propeller or water wheels is not the issue. Fluid flow over the hydro generator propulsion device gives rise to rotational motion. Rotor specific speed is the issue you refer to and different designs have higher or lower specific speeds due to the physics involved with that wheels geometry. That is the rotational speed of a rotor of one design compared to others will be higher or lower depending on the actual design or geometry involved regardless of scale. Specific speed is a comparative “unitless” number much like a Mach number is used for supersonic flow.  Specific speed often stated something like this “ηs= η/η0″. Efficiency at a given flow/pressure regime depends on this number a lot. Enough on that topic for now. What a smaller diameter wheel (using a pelton wheel for example) does is imply that for a given water jet velocity they must spin at a faster rate to produce their optimum power output. Max Pelton efficiency is at about 47% of the water jet velocity. This speed maximizes power output for given a pelton impulse design. So with all other factors being fixed the wheels pitch or diameter will increase rotational speed to keep the center line of water impact moving at the same 47% linear speed of the water jet. Power will be about the same, but generator frequency will change with rotational speed which may make a speed reducer/increaser a requirement.
2. Increasing hydro system %efficiency is a complex art, starting with intake and Penstock friction, then turbine choice & performance curve optimization, combined with inlet valve and draft tube design if required, then any speed transmission or reducer friction, next will be generator/alternator efficiency, finally the transmission losses including wires, transformer/inverter systems. The 72% efficiency example I give is a very simple average of a model 85% turbine x 85% geneator/transmission output.  Mileage for your site and turbine choices will vary. The more accurate you make your hydropower model the better the result will be. You will be modeling power production and loss until the first water flows through the system, then you get your course grade based on how close your “Real/Model” score approaches 100% of actual results.
3. The last part of your request has more to do with jet design and penstock friction. Typically the penstock will be much larger diameter than the jet intake. The pipe friction means that there is a large dynamic head loss over long distance flows. The jet or turbine intake design then takes the pressure drop and accelerates water through the intake throat, a much smaller distance.   These cross sectional intake areas are much smaller on higher head units like pelton and turgo units vs. lower head pressure ones like Francis, PAT and Kaplan/Popeller ones.  That’s part of why the low head turbine prices rise so rapidly with flow rate, there’s a large opening to a much larger volume of machine for the same output power. Generally you should seek to keep penstock intakes and penstocks free of friction loss, then reducers at the turbine may be possible if required. For example a PAT I know of takes a >30inch diameter main and necks it down to 10inches right at the PAT input valve. The overall system efficiency is just a part of the equation, remember those big penstocks cost big bucks so make sure you balance the equation for optimum water power system ROI$.

I hope this helps clarify things,

Sincerely,
Jess
DoradoVista, Inc.
PS.  I plan to post this set of question/answers on the Blog OK? Generic name & content only.

Ben replied,

Use the question, so far I am 2 for 2 in getting my questions in the blog. Batting a thousand here.
Thank you for the answers.
Ben

You’re welcome!
Jess

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When it comes to hydropower one of the first steps in hydro site evaluation is to perform a site energy survey.  Basically regardless of whether its small or micro hydro the power, a needs survey will help you to tell just how much value there is for the hydro electric energy potential that your site has.

Generally, the Micro Hydro sites will be easier to evaluate since more is known about the site power usage.  A simple checklist of power loads and maximum wattage will suffice to get the initial data.

For Small Hydro sites that plan to export power to the grid, you will need quite a bit more information about grid interconnects and local vs. remote loads, energy price structures and various generation vs. transmission charges associated with commercial power. Frankly this part of project evaluation can be quite intimidating for some energy markets.  Let us know if you need help performing this type of survey and we’ll see what can be done.

Regardless of hydro system size, keep your focus on answering the following question; “What is the market going to pay for this power?”  Later you will need to evaluate “What is the power generation going to cost?”

For a simple appliance load calculation form see ABS Alaskan’s:  Simple Energy Survey Form
Note:  This application does not save data, so fill it and print it out.

One problem with energy surveys is getting a handle on peak power usage. This peak energy need will determine the required maximum hydro generator or battery-inverter capacity. It can be tempting to simply take the sum of all devices and use that for power estimation, don’t do it.  In reality the overall system load capacity will limit out at the maximum rating for the home, farm or cabin’s maximum service panel rating (the big breaker or fuse feeding the whole system.)

Also, it may be worthwhile to count Heating loads as fed by gas or other sources since these can require much larger peak energy loads than is practical.   Hint: Hydropower Load dumps can supply part of this stored heat for a more system savings.

These calculations are designed to compensate for battery inverter conversion losses. Remember your system may have a different conversion efficiency factor. Also remember the hydro turbine and power generator have their efficiency factors as well.

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