I have a good flow of water but not much vertical drop, can I still install micro hydro?

The ‘head’ of water(drop in level) available to you on a site is important for power output. You can harness much more energy from a flow of water that is also dropping. The smaller the head is, the harder and more expensive it is to harness energy.

Low Head

Where there is only a small drop of water, perhaps of only a few metres, then some sort of 'low head' turbine could be used. Low head turbines tend to be much larger, as they have to hold a large volume of water flow to compensate for the low head. Examples include crossflow turbines, but a common choice nowdays is an Archimedes Screw turbine, which can be good for sites with heavy debris. For a very small-scale setup, there is the PowerSpout LH turbine, available from a few companies in the UK.

See the question about converting old mill sites for more advice on low head sites, including some links for Archimedes Screw turbines.

Zero Head

Where there is no drop at all, then it can still be possible to place a ‘zero head’ turbine in a river, or tow one behind a boat. The output will tend to be very low. It may be possible to produce a small amount of electricity for trickle-charging a 12 volt battery.

The relevant calculation for finding the power contained in a given flow through a rotor is:

Power (in watts) = 0.5 x p x A x V3 (Where p is the density - about 1000 kg per cubic metre for water, A is the area swept by the rotor, and V is the speed of water flow in metres per second)

The efficiency of the turbine will then be a big factor in how much of that power can actually be converted into electricity (or mechanical power, for an undershot waterwheel).

For fixed or towed turbines, the efficiency tends to be quite low. For example, the Ampair UW100 underwater turbine gives an output of about 100 watts in a flow of 4 metres per second (m/s). It has a swept area of about 0.076m2, so in a flow of 4 m/s (so V3 is 64) the maximum power available is about 0.5 x 1000 x 0.076 x 64 = about 2400 watts. So the efficiency is about 5% (100/2240). A standard micro hydro system (where water is channelled in a pipe) should have at least 50% overall efficiency (after all losses).

The flow rate passing through a 0.076m2 swept area at a speed of 4m/s will be about 0.3 cubic metres per second, or 300 litres per second. In comparison, a small low-head (such as the PowerSpout LH) or Archimedes screw turbine setup could generate about 1 kilowatt (1000 watts) from a flow of 100 litres per second dropping through 2 metres. So much more energy from a smaller flow.

It will not simply be a matter of placing a propeller-type turbine into a stream. There will be various issues that need to be addressed to ensure a long operating life:

  1. Debris coming down the stream when it is in spate, and damaging the turbine or mountings.
  2. Some ground work may have to be done in small streams to make sure there that there is sufficient depth to submerge the turbine at low flows.
  3. At 12 or 24 volts DC output, heavy cable will be needed to run from the generator to the house, and this can be costly (e.g. if more than 20 metres).
  4. The turbine should be easily accessed to remove weed build up.
  5. Flow rates will be very seasonal, so you may well need other power sources to meet electricity needs in the drier months.

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