The head (measured in metres(m)) is the vertical drop from the top of system (where the water enters your pipe/penstock) to the bottom (where it is released). Head should not be confused with the distance between the top and bottom of the system (which should be kept to a minimum). It is the difference in height, the drop, that counts. The greater the head, the higher the potential power. There are several ways you might measure the head. You could check an ordinance survey map for contour lines. You could measure the straight line distance between top and bottom and the angle between the two and use trigonometry. You could even borrow an altimeter (some gimmicky watches have them).

The flow (measured in litres per second (l/s)), is the volume of water which flows past any given point in the system within the period of one second. It needs to be measured before the turbine is in place in order to find the potential power because the flow will be reduced when it is fitted. The greater the flow, the higher the potential power. There are several ways to measure flow but the simplest is to use a bucket of a known capacity and time how long it takes to fill. Eg. If it take 5 seconds to fill a 10 litre bucket you have 2 litres per second.

Calculating the power (measured in watts (W)), is done by multiplying the head and flow by the force of gravity (around 9.8 m/s/s which you can't easily change significantly without leaving the planet). For ease of calculation you could use 10 m/s/s as the force of gravity - no one will notice (unless they are very unfit).

To calculate the potential power.. head (m) x flow (l/s) x gravity (10) = power available (w) For example, if you have just 0.25 l/s and a head of 20 metres, the power available would be 20 x 0.25 x 10 = 50 watts Note: What you consider practical or a useful amounts of power, may well differ from other peoples ideas so bear that in mind when you see figures sometimes quoted for minimum usable head and flow. If all you want is to provide is a trickle charge to keep a battery toped up when not in use for long periods of time, you might be happy with a fraction of a watt from a home made system rather than buy expensive solar panels or have to keep buying new batteries.

The main parts of a hydro system are the penstock and the turbine. The penstock is any artificial method by which you get water to your turbine from the highest point in your system with the minimum of obstruction and resistance. Penstock may be in the form of pipes or an artificial channel but sometimes the landscape may have provided a natural solution.

Other factors that effect the amount of power available include the resistance experienced by the water while on its way to your turbine which is directly proportional to the length of your penstock. If you are using pipe it is important to use a large enough diameter and keep it as short as possible while still providing the amount of head you want. Avoid any sharp bends or joins etc.

The type of turbine you use will dictate how much of the available power you can actually provide to the generator and the efficiency of the generator will dictate the amount of electrical power provided. You will probably get less than half the available power. Your losses don't end there. If your turbine is located a great distance from the site you plan to use you power you may also have transmission losses in the cable. But what the hell, even with the losses, something is better than nothing.

This page will be updated to include;

• Information on the use of pumps as turbines,
  Our first system used a rotary pump as a turbine. These are fairly easy and cheap to obtain and can provide quite high efficiencies. Information about suitable types of pump can be obtain from a book produced by Intermediate Technology (see below).
• Possible designs for building turbines from scratch
  We have build a pelton wheel from flat plate aluminium. Other ideas include using teaspoons or casting individual pelton cups and fitting them onto a hub. If you want to chat about these ideas, please do get in touch.
• The reuse of impellers and small propellers from model boats etc.
  Just an ideal but it should work. There is a name for this but I can't recall it. Coaxial / In flow ?
  Basically you would place a propeller like turbine inside a curved section of pipe so that the shaft could exit the pipe and be attached to a generator.
• How to build a filter
  Our first system failed fairly often due to a poorly designed filter. A system using a pump as an turbine will need a good filter. Our latest system doesn't suffer from the same problem since small object can not jam the turbine.
• The use of DC motors as generator
  Permanent DC motors are a cheap and easy source of generators. They work in the same way as a dynamo. There are more efficient ways of doing this but this is certainly the easiest. DC motors can be found in scraped cars (fan motor etc.). Turn them fast enough and you've got power.
  
• How to generate AC and why you might want to
  There are reasons why you might want to generate AC even if you only use 12v DC systems. High voltage AC can be transferred long distances in thin cable while much thicker cable would be required for low voltage DC. AC can be stepped up or down easily and efficiently using transformers. The same is not true of DC. For these reasons it often makes sense to start with AC. Induction motors are a very common form of AC motor that can be obtained very cheaply. They come in a variety of sizes and can be made to act as generators. See the book from Intermediate Technology to find out how to do this (and then let us know).

For more detailed information on the subject of small scale hydro power you might like to consult these outside links:

The Micro Hydro Centre - Research, information and development projects.
Pico hydro website - Articles, newsletter, useful guides.
Hydropower literature - A comprehensive list of books on the subject. We can't recommend any specifically because we haven't got read any yet, however we are sure that the books published by Intermediate Technology on the use of pump as turbines and the use of induction motors as generators both sound very useful and would be a great addition to our library (hint hint).

Note: If you are planning on setting up a hydro power system in the United Kingdom you may require a water extraction license under the Water Resources Act 1991. There are exceptions from the annual fee based on the power generating capacity of the system and also on the level of water use and type of installation. To find out more, contact the Environment Agency. Similar legal considerations may apply in other countries.

See also: General information about hydro electric power
Our own micro hydro system at Steward Wood