Micro-Hydro Electric Power

Probably the least common of the three renewable energy sources, micro hydro offers the greatest potential to produce the most power, if you have the right site, on a continuous basis. Having said that, however, it should also be mentioned that micro hydro systems are perhaps the most complicated to install and maintain.

We categorize micro hydro systems, as producing less than 22.5 kW. The most widely recognized type is the Pelton wheel, where water flow is directed by nozzles towards a series of cups mounted on a hub causing the turbine to spin. Other types include the turgo, crossflow and various axial flow turbines where the shaft through the centre of the turbine runs in the same direction as the water flow.

How much energy can be produced?

Generally there are two types of hydro systems, flow of river and storage type. Two factors determine the gross power output of either micro hydro system, water pressure and water volume.

The pressure is generated by the "head", or the vertical distance (in meters or feet) from the water take-off point, down to the turbine. The water volume is simply measured as the flow rate (in litres/second, or GPM) of the water. To get an idea of the available power in watts for a particular site multiply the head (in feet) times the flow (in GPM) times 0.18 times efficiency. Turbine efficiencies range from 25 to 50% with the higher efficiencies being achieved at the higher heads (we suggest you use 50% as a rule of thumb).

(Head (ft) x Flow (GPM) x 0.18 x Efficiency) / 1000 = Maximum Power Output (kW)
Example: (10 x 300 x .18 x .5) / 1000 = 0.270 kW

Useful information: 1 cubic foot of fresh water weighs 62.4 lbs 1 cubic foot of water contains 7.48 gals 1 foot of head is 0.433 PSI 1 PSI is equal to 2.31 feet of head 1 gallon of water is 0.13368 cubic feet 1 GPM is equal to 3.785 litres per minute 1 HP (horsepower) is 745.7 watts

Flow measurement

Knowing the true, minimum flow of water is critical to accurately determining the size and output of the turbine. It is best to conduct these measurements during low season for conservative estimation.

If the stream is relatively small a measured container such as a 55 gal drum or garbage can may be used. Simply divert the water as directly (no restrictions, turns etc.) into the reservoir and measure the time it takes to fill it to a predetermined level. If you use a 55 gal drum that took 126 seconds to fill the flow was 26.19 GPM or 3.5 CFS (cubic feet per second).

GPM = Capacity x (60/Time) or 55 x (60/126) = 26.19

For storage type hydro systems a more accurate method for determining the flow of streams over 1 CFS is the use of a weir that is simply a dam with a slot of specific width and depth cut into it.

A wall is set up parallel and exactly four feet upstream from the weir. A board is then laid perfectly level on top of the weir and the upstream wall. Take a very accurate measurement of distance between this level board and the water. The water will be shallower at the weir end than four feet upstream and this difference combined with the width of the slot in the weir and the factor taken from a Weir chart will indicate the flow of water.

When the construction of weir is not required, in the case of smaller river flow type systems, for example, you may calculate the CFS of the entire river or stream. Note that this is the total flow capacity, not how much you may actually use, that will be determined by the diameter of the pipe or penstock selected.

Start by marking a very strong cable at predetermined intervals such as six inches or one foot etc. Stretch this cable across a proposed section (distance) of water at the end of a relatively straight section of the river that has a relatively uniform from to it. At each interval marked on the cable measure the exact depth of the water in order to plot the contour of the riverbed. Using a bit of trigonometry you can now determine the cross sectional area of the river.

Next, upstream of this cable use a float and weight to measure the time it takes the float to travel the preset distance to the cable you have set across the body of water. Take a number of measurements of both the depth and speed over a period of time to obtain a good set of realistic data.

CFS = (Cross sectional area x Distance) / Time

The importance of the Penstock

Hydro turbines operate from the pressure at the end of a pipeline called a penstock that is directly related to the head or vertical height.

The pressure at the lowest point of a pipeline is equal to 0.433 times the total head. This pressure determines not only the power available but also the type of pipe required. Polyethylene pipe may be used for pipe up to 100 PSI, PVC to 160 and 350 PSI and some steel pipe may be specified to 1000 PSI.

Pipe diameter is also important since the smaller the pipe the greater the effect of friction. The maximum power a pipeline will produce is achieved when the dynamic pressure, the pressure of the running water through the nozzle (s), is about 2/3 of the static pressure or the pressure measured at the bottom of the pipe when no water is flowing. In short, go for the biggest pipe you can afford, then go one size bigger, it will be worth it.

Waterhammering - Remember the rattling sound you hear in the wall when someone shuts the water off abruptly in your house - that is waterhammering. It is caused by the rapid loss of momentum of the water in the pipe and this kinetic energy must go somewhere so the pipe contorts, expands and bangs against the interior of the walls to absorb the stress. The same affect will take place in a penstock with long runs with a high rate of flow so ensure the pipe and gate valves are able to withstand these forces.

Points to consider

Environmental - We recommend that no more than 50%, 20% is even better, of the water in a river or stream be diverted for energy production to preserve the local environment. Seasonality - Each season brings its own challenges to micro hydro systems. If under designed, spring floods may overflow the banks or weir flooding the turbine or debris may damage the components or block the intake to the penstock. In other words plan your installation for the historical high water potential and base your power output on the median water measurements.

Distance - As with wind turbines, hydro systems are seldom located adjacent to the site where the power will be needed. The choice of alternating or direct current output must be measured against the wire transmission costs, penstock cost and the cost of the conversion electronics. The AC direct system is simpler in overall design and transmission of the power is less costly, however, it must be sized to handle the largest of all of the power requirements at one time, usually at least 2 - 3 kW.

For example, incandescent lights will use 10 times their running power to turn on, while induction motors used in water pumps and furnaces will require five to seven times their operating currents in order to start. These cumulative instantaneous loads may be quite large thus high voltage DC systems in combination with batteries and inverters often are selected. A micro hydro system offering 300 watts continuous DC output coupled with inverters and batteries will meet the needs of most remote homes. The DC output will be fed into the batteries, thereafter the inverters will convert the power to 120 or 240 VAC for transmission.

The use of an induction generator and tuneable converter is the ideal system for very long transmission runs to optimize system efficiency at the lowest cost.

 

 

The information contained herein is subject to change without notice.