SITE SURVEYS

With the exception of the 1.8 metre battery charging machine, the design of each water current turbine has to be tailored to suit the whole range of conditions at the site where it will operate. To estimate the MINIMUM output of electricity or pumped water from the turbine an accurate site survey at the LOWEST WATER SPEED from which power is to be extracted is therefore essential before a machine can be ordered. To estimate whether additional sets of blades are required the MAXIMUM WATER SPEED likely to be encountered at the site must also be measured. The minimum information required is the maximum and minimum water speed and depth at the point where the turbine will be installed. For water pumping turbines the MAXIMUM static head (i.e. vertical distance between the LOWEST WATER LEVEL and the delivery point) and the pipe length and diameter are also required.

Tools required;

*If no current meter is available several pieces of light coloured wood and a stop watch or wrist watch with a second hand.

1) Measure static head (water pumping turbines only);
Stand the 1.5m long piece of wood at the water's edge. Hold the spirit level on top of the wood so that it is horizontal and look along the top of it towards the bank. Get your colleague to mark the spot level with the top of the wooden post, move the post to this spot and repeat the process until the spirit level is aligned with the irrigation channel. Work out the static head to the nearest 0.5m.

2) Measure water depth:
Using the boat and the 6m long weighted rope find the depth within 12m of the bank (use the 24m long rope to check your distance from the bank). If you do not find at least 2.5m then move out further, up to a maximum of 24m. Note the depth and the distance from the bank.

3) Measure water speed with current meter:
Assemble the current meter taking great care of the propeller. At the same place that the depth was measured, position the meter 1.5 m below the surface and take at least three readings of the number of counts in 100 seconds. If there is more than 10% difference between any two readings take another three readings. Average all the readings and find the current speed from the table.

4) Measure water speed without current meter:
Wait for a calm day or period before taking this measurement. Wind will spoil the accuracy of the result. Use your 24m rope to measure 24m upstream and 24m downstream from the proposed turbine position. Mark the two points clearly and throw a piece of wood or other float into the water 12m away from the bank well upstream of the first mark. Start the stop watch when the float passes the first mark and stop it when it reaches the second one. If you take the readings when the sun is low in the sky across the water from you, then you can greatly improve the accuracy of the timing by starting and stopping the watch when the float crosses the line of reflected light on the water. Time at least five floats and calculate the average water speed. (The water speed in metres/second is 48 divided by the average number of seconds it took the float to travel between the two marks).

 

5) Check viability of site:
Plot your water speed and depth readings on the Site Assessment Chart. If at none of the positions where you have measured do the readings fall within the hatched area on the drawing then take further readings up to 100 metres up or down stream from where you started. If your readings are within the hatched area then note which size of blades are required and refer to the relevant electrical or water output graphs. Note that the water discharge graphs assume a delivery pipe of not more than 25 metres of of 75mm bore polythene pipe; a longer pipe will reduce the discharge.

6) EXAMPLE:
Suppose that next to a farmer's irrigation channel you measured a static head of 6m and took the following readings from the boat;

a) current speed of 0.7m/s and a depth of 2.5m at a distance of 12m from the bank,

b) Current speed of 0.65m/s and a depth of 4m at a distance of 24m from the bank,

c) current speed of 0.8m/s and a depth of 3m at a distance of 12m from the bank at a position 50m upstream from the irrigation channel,

Looking at the site assessment chart;

a) is outside the hatched area and therefore not viable (except for battery charging),

b) would suit the long blades and from the long blades pump output graph would give a discharge of 2.3l/s,

c) would suit the medium blades and from the relevant graph would give a discharge of 3.6l/s.

7) Consult user:
Explain to the user what you have done and tell him what the electrical output or water discharge will be. To give the water pumping figures some meaning; the number of litres per second is the approximate number of hectares the turbine can irrigate if it is run for 15 hours per day seven days per week (this figure is based on an average crop requirement for desert conditions of 54 cubic metres/hectare/day). Also, compare the expected discharge with a 3" diesel or petrol powered pump set which delivers about 10 litres/second (at heads of 6-10metres). It must also be explained to the farmer that the turbine can be run for 24 hours per day continuously without a full time attendant and thus irrigate an even larger area.

8) EXAMPLE:
Thus in our examples above, at position b) the turbine would pump enough water for a farm of 2.3 hectares if operated for 15 hours every day. At position c) the turbine could irrigate a 3.6 hectare farm (or irrigate a 1.2 hectare farm in 5 hours per day).

Position b) would require an additional float and an additional 12m pipeline as compared to the standard machine and would thus increase the capital cost by about 20%. Position c) would give 50% more water but involve rebuilding the irrigation channel (or the additional capital cost of buying 50m of 3" pipe). Position c) might also involve the machine being on a neighbour's land and this would have to be negotiated by the farmer before an order was placed.

Relating the discharge at position c) to other pumps, the minimum water output of the turbine would be just over one third that of the 3" diesel pump.

9) Mark position of mooring post:
Measure 40m along the river bank in the upstream direction and mark the position of the mooring post far enough from the water's edge to be above the normal flood level. If an 18m pipeline is required then this distance should be increased to 55m and again increased to 70m for a 24m pipeline. If the post will have to be on a neighbour's land explain to the farmer that he must negotiate with the owner and get his permission before installation day. Note this fact on the survey form. At some sites the mooring post position may be in a house or other impossible place. In this case measure until the next possible place and note the TOTAL length of mooring cable needed.

10) Complete site survey form
If you would like a quote for a turbine to suit the site you have surveyed please email your details to .

 

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