The inlet, canal and charger site are very important points if the charger will be installed there permanently. However, when it is just for testing or a demonstration of a few hours to a few months, a lot of short cuts are possible:

• When it is for testing only, an easy way of testing the alternator and the electrical circuit is by driving the charger with an electric drill. For this, a drill is needed with electronic speed control and a maximum speed of 2000 RPM or above. The easiest way of driving the alternator is by driving the bolt that fixes the runner on the shaft. If this is an Allen bolt, grind off the bent of an old, 5 mm Allen key and put the long, straight hexagonal part in the head of the drill. If it is a normal bolt, get the 10 mm socket from a set of socked keys and somehow fix this in the drill head (use the extension piece, if it fits into the drill head, or make a kind of extension piece from a long bolt. Try out this make-shift drive construction carefully at low speed and try to keep the drill well-aligned to the shaft. Also take care not to overheat the drill motor since the alternator can take up much more mechanical power than the drill could deliver.
  Apart from the turbine and the water splashing around, testing this way is the same as testing in the field. See par. 4.11.3 for the necessary preparations.
• If it is for testing the charger or for demonstrating a few hours only and there is no time to make a more realistic test setup, it is possible to connect the charger directly to a pump that provides the necessary head and flow. A pump with a ca. 1 kW (1.4 horsepower) engine and capable of pumping up water to 10 m high, should do. In this way, also the costs for a penstock pipe are saved since only a few m. of hose are needed to connect the charger. The head can be measured by connecting a transparant hose to the charger inlet and the flow can be measured by using a measuring weir (see annex 0).
• A more realistic setup can be made by pumping up the water to a reservoir at the appropriate height, and connecting the charger with penstock pipe to this reservoir. In this case, a penstock pipe is needed and an equally long pipe or hose for the pump.
• If irrigation canals are around, probably farmers will not object if a canal is used for demonstrating the charger for a few hours or a day.
• Maybe there is a waterfall that offers a perfect place, but is too far away from the houses to be a permanent site, can be used for demonstrating a few hours.
• When the charger should be demonstrated for a few months, things are still a lot easier that for a permanent charger:
  • When the demonstration period is in dry season, there is very little risc of a major flood. So a temporary weir built in a creek that will not dry up completely, will do. Then there is no need to build the first stretch of canal very strong since the creek will not rise much higher. Also there is no danger that water running down the hillside will erode the canal.
  • When the demonstration period is in wet season, a small creek that will dry up in dry season can be used.
• Generally, during demonstrations there is no need that the charger functions very reliably and this makes that further short cuts are allowable. It is important however that local people will understand what went wrong and how such a problem could have been prevented. When the charger stops because the creek went dry, they will know that the charger itself is still reliable and that the problem can be solved by choosing a more reliable water source.

In the subsequent paragraphs, the inlet, canal and charger site are dealt with as if the charger will be installed permanently. This has been done because such permanent solutions will be needed in case the firefly system will be introduced for real after a demonstration project. Please bear in mind that cheaper and less time-consuming options can be used for demonstration purposes.

With respect to the inlet and the canal, it makes a lot of difference whether there are irrigation canals in the area and local people clearly have the skills to plan, build and maintain them. If irrigation canal technology is known in the area, local people probably know much better how to plan, build and maintain an inlet and canal that will function in their environment. Then there is no need for an outsider to tell how or where to build an inlet or canal, so you could skip the corresponding paragraphs. However, don't expect that the charger can be connected to any suitable irrigation canal because farmers will claim the water in such a canal, see next paragraph.

If irrigation canal technology is not known in the area, the firefly project will have to bring this in as well, see par. 4.10.3 and par. 4.10.4. In these paragraphs, only major points are discussed. It would go too far to discuss in detail all subjects related to estimating whether a creek has enough water in dry season and planning the inlet, canal and site. Literature is available on these subjects, e.g. HARVEY, 1993. Do not underestimate the importance and complexity of these issues since they could easily lead to a disappointing performance of the whole project. A charger connected to a dry creek, with a washed-out inlet structure or a silted canal will just not work.

The first problem that arises when there is no irrigation technology around, is choosing the creek or river that will be used as a source. Of course local people should be asked, but their memory might prove unreliable once it comes to estimating what is the lowest level in dry season, whether a creek will fall dry every dry season or only in exeptionally dry years etc. Local people might also give the answers you want to hear so badly just to please you or because they want the project to push through. Generally, people who use the water in a creek, will know more about it, so maybe ask the women who use it for washing, or the children who go there for a swim.

When information of local people is not conclusive, you'll have to find out yourself. There are several handy methods of measuring the flow of a creek at a one moment (see e.g. HARVEY, 1993) but the problem is to estimate how the flow will vary over the year. That is why estimation methods based on rainfall data, soil type etc. could prove even more useful. Also there are handy methods to process flow data into power output for a certain type of micro hydro plant, and evaluate what capacity plant would be most advantageous. In general terms, this issue was discussed already in par. 3.1. Not only the low flow figures during dry season should be known, but also the highest water level that is reached during floods. The strength of floods determine what type of inlet is most appropriate, what is a safe site for the charger etc.

If there are irrigation canals in the area, maybe the charger can be connected to an existing irrigation canal and then the inlet and canal are virtually ready for use. However, problems are to be expected with the farmers using this canal. A logical solution would be that the firefly project guarantees that the capacity of the first stretch of canal will be increased so much that the farmers further along the canal will always get as much water as before or even more (of course this only makes sense when it is the capacity of the canal that is limiting the water supply, and not a too low dry season flow in the creek it gets its water from). Then the maintenance work of the first stretch should be shared equally by all water users.

However, farmers could demand an exorbitantly high compensation or just want this thing off their canal. Take the following issues into account:

• Water rights are likely to be very heavy issues in a community that depends on irrigated agriculture. They are comparable to land rights in unirrigated areas, but land is still easier to mark off than a water supply so water rights are even more likely to cause serious problems. Even a slight change in how water is used and divided which seems beneficial for all concerned, could upset a delicate balance in the community and cause disputes.
• If the firefly project is supported by a development project or the government, farmers might try to use their ownership over the canal as a way to get as much money out of it as possible.
• When it is just for demonstrating the technology, farmers might be curious and probably don't mind. But when the charger is there to stay, they could react completely different.
• When farmers realize that in the long run many more chargers might be installed if the first one proves to be succesful, they might become afraid that the first charger will set a precedent and oppose violently against it.
• Farmers might react differently depending on the agricultural season. When they don't need irrigation water, they probably don't mind a charger using it. But when dry season comes, they might just demand all the water and want the charger shut down for a couple of months (seen from the point of view of a firefly project, this is unacceptable because batteries that are left discharged for a few months, will deteriorate badly due to sulphatizing). They will argue that food has a higher priority than lighting.
• In principle, the tailwater from the charger could still be used for irrigation, but only at least 5 meters lower than the canal. In practice, it is probably quite difficult to use this water for irrigation. Only in rare cases, there might be a canal lower along the slope that has a lack of water, while the higher canal has plenty.
• It is not only water rights but also maintenance needs that have to be settled. Farmers might argue that every user has to maintain a portion of the canal, so firefly users have to maintain the inlet and the first stretch of the canal. This would mean that the farmers get rid of a lot of maintenance work while the firefly users bear an unproportionally high part of the maintenance work.
• Negociations would be easier if the farmers that are affected, would also benefit from the firefly project. If there is another canal that is used mainly by firefly users or their direct family, it might make sense to choose this canal.

If it is about a permanent site for the charger, preferably the firefly user group should negociate directly with the affected farmers. Having a representative from a development project around at such a meeting could be counterproductive because then the easiest way out for all concerned is to ask more money from the development project, irrespective of whether the demands of the farmers are fair. Such a request would probably cause delay (it has to be decided upon and if granted, the money has to be found) and if it is refused, the negociations have to start all over again. When the request would be granted, it sets a precedent and with subsequent problems, users might come again to the development project for a solution. Also other user groups that want to start later on, might expect a similar amount of money from the development project.

The issues mentioned above could make it practically impossible to use an existing irrigation canal as a source. Then options are:

• Rehabilitating an old, dilapidated canal that will be used exclusively for the firefly charger.
• Digging a new canal. Quite likely land rights will be a big issue there since now a new canal might pass through the land of some or more owners. But with digging a new canal, there is more freedom to choose where it will be built. Maybe there is a convenient place where the land is owned by an enthousiastic prospective firefly user, or maybe there is a place where the land is used only extensively and seen as less valuable for agriculture.

When discussing a firefly scheme with local people, it might be difficult to explain what it is about. Some people might think that the charger just needs running water and that it can be thrown in the river to have it work. They will not object to this since water down in a river is useless for them. But water in an irrigation canal is a totally different matter and they might not like it at all when foreigners come with ideas that interfere with this water.

Local people know all about height differences and amounts of water, but not in terms of meters of head and liters per second of flow. So to explain what the charger needs, more familiar terms have to be used:

• The head could be explained by telling that the charger needs the power of a waterfall of a certain height.
• The flow can be explained by indicating with your hands the diameter of the penstock pipe and telling that the water goes `fast'. Alternatively, a flow can be calculated back into the area of agricultural land that can be irrigated with it. A flow of 8 l/s (the maximum flow for the standard charger) is enough to irrigate roughly 5 ha of wet rice or 10 ha of normal crops.

The inlet site, the course of the canal and the site of the charger are mutually linked. One could start with an advantageous inlet site, plot out the course of the canal with the desired slope and where the canal is high enough above the flood level of the creek or river, the penstock starts and the charger can be placed down there. Or one could choose a good site for the charger, plot out the penstock and the canal, and the inlet will be there where the canal reaches the creek. In reality, one could work out several different alternative plans (inlet, canal and charger site) and choose the most advantageous one.

Important aspects for choosing the inlet site are:

• Natural features in the streambed. For instance there might be natural weir formed by the bedrock with a pool of water behind it. Or there might be protruding rocks with only gaps to fill for making a weir.
• Generally, a stretch where the creek or river is wide, is more advantageous since floods will be less destructive there.
• Generally, an inlet at a straight stretch of river poses less problems. An inlet at the outer side of a bend is more prone to flood damage while an inlet at the inner side will be silted more easily.
• With the inlet site, the first stretch of the canal is determined. The canal has to `climb out' of the riverbed from the inlet. If the creek or river banks are steep and consist of hard rock, it will be very hard to construct this first stretch.
• The slope of the river downstreams. If there is a rapid downstream, a short length of canal will do to have enough head for the charger.

Generally, an inlet consists of:

• A weir crossing the the creek or river. This is to raise the water level upstream of the weir to a high and fairly constant height. Otherwise the canal should continue until the deepest point in the river bed to catch any water when the creek is almost dry.
• An intake structure. This could be a gate in a concrete wall from where the canal starts, or just the canal that ends in the river.
• A flow limiting structure. Often the first stretch of canal has such a low wall that it acts as a spillway. Any excess water will flow over the wall back to the creek (to prevent erosion, sometimes the area below the spillway must be paved with stones). At the end of the spillway, there is sometimes a blockbrigde: A bridge over the canal that just allows the desired flow to pass underneath without touching it. When the water level is very high, the blockbridge stops the top layer of the water and the flow underneath it will not increase that much.
• A trashrack or a skimmer, to prevent rubbish from entering the canal. For a firefly scheme, these are not necessary.

There is also the option of using temporary weirs that are likely to be washed out by floods in wet season, but that are cheap to rebuild using locally available materials and labour.

Important aspects for the canal are:

• The land issue: Who own the land that the canal is going to pass through and will they allow that such a canal is constructed and used.
• Difficult stretches to cross: Steep sections with hard rock, a gully that will erode away the canal during heavy rain, stretches that are prone to landslides.
• Seepage losses. These could amount from 20 to 30 mm per hour (20 to 30 liters per m² of wet canal surface per hour!) for a sandy loam soil to only 1 liter per m² per hour for a clay soil. With the very low flow the charger needs, a large fraction of the water that enters the canal might be lost along the way. If clay is available nearby, it could be used for lining the canal.

Designing a canal involves hydraulic calculations that are too specialistic to discuss here, see HARVEY, 1993. Only a simple comparison will be given.

An important feature of the canal is its slope. In irrigation schemes or Micro Hydro schemes, generally the slope is chosen such that the water velocity in the canal will be a compromise between two evils:

• At a low velocity, the canal gets silted more easily. Therefor the water velocity should be above 0.3 m/s.
• At a high velocity, the canal walls and bottom could be eroded (unless they are protected by concrete, masonry etc.). This depends on the type of soil the canal walls are made of. A sandy loam soil can stand only 0.4 m/s while clay could stand 0.8 m/s.

Because the firefly charger needs such a low flow, the canal will have a small cross-sectional area and even for a velocity of only 0.3 m/s, the slope will have to be some 3 %. This would be acceptable only if the slope of the main river is high as well so that the head loss in the canal is still low compared to the head at the charger site.

In cases where the slope of the creek or river itself is only a few %, this is not practical. Too much of the available head will be lost in the canal, therefor the canal must be made longer for obtaining the head the charger needs, this in turn makes tha seepage losses increase considerably etc. By reducing the velocity to 0.2 m/s, a slope of only 1 % is needed. This means that the canal will get silted and needs digging out regularly. Since the flow is low, also the amount of silt carried in by it will be low and this should not be such a problem. Anyway it saves the costs of building a large forebay tank that serves as a silt basin and emptying that regularly.

Another consequence of choosing a low velocity is that the canal needs to be wider and deeper, see table below:

When a canal has to be designed and plotted out in the field, it is worthwhile to get proper literature (for instance HARVEY, 1993) and make such calculations yourself. This would also make it possible to compare alternatives properly. Or ask advice from someone who is familiar with designing canals, e.g. an engineer from the dept. of irrigation, a civil engineer. Also it could be worthwhile to visit an area where irrigation canals are functioning. Topographical maps (if available), landscape pictures and details from inlet sites, charger sites, the terrain the canal has to cross etc., are helpful for documenting and for explaining to other people about the plans.

The following could serve as a rough guideline:

• Start constructing the canal from the inlet site onward with a slope of 1 % and the dimensions for a velocity of 0.2 m/s. After a stretch of canal is dug, try out whether it functions properly. The correct slope can be set out using a spirit level or a transparant hose with water in it.
• If it turns out that a lot of water is lost through seepage because the soil type turns out to be very coarse, the canal must be lined with clay, cement, stones etc.
• If the canal just does not conduct enough water, it has to be made wider and deeper.
• For difficult stretches along steep rocks, a slope of 3.3 % could be used so that the canal dimensions can be chosen smaller and that stretch will not silt easily.
• For very difficult stretches along rocks or for crossing a gully, a length of penstock pipe could be used, if the money is available.
• Avoid causing landslides: Do not dig deeper into the hillside than necessary and finish the uphill wall with a safe slope.

Important aspects for choosing the charger site are:

• The charger should be reasonably safe from floods. The alternator will be seriously damaged when water comes in, especially when this water carries silt (bearings!) and other impurities. For Micro Hydro schemes, it is advised to place the charger above the 20-year flood level (the highest level that is reached on average once every 20 year). This would mean that the charger should be placed quite high above the normal water level in the creek or river, so that a substantial portion of the available head is lost. With a firefly charger, it is allowable to place the charger below the 20 year flood level because:
  • The charger can be moved fast if rains are such that a flood is likely to come (with a small creek as a water source, the catchment area is nearby and it is less likely that a flood comes without warning). Then of course the charger should be easy to move without using tools, so it should not be fixed with bolts on a foundation or heavy logs.
  • The consequences are not too bad when the charger would be flooded. Once the alternator is dry, it will probably work for a few days at least before the bearings are badly worn out. Only when the charger or switchboard is washed away completely, things are getting serious.
• The place should be accessible. This especially goes for where the switchboard stands since this is where the batteries have to be carried to (the switchboard could stand a couple of meters uphill from the charger itself).
• Preferably, it should be near the house of the operator so that it takes little time to check it while it is charging. If this is not possible, it is important that at least someone who is interested in the firefly scheme, can keep an eye on it. This to keep away children who might very much like to play with it, to prevent that unauthorised people will try to charge their batteries for free and to prevent theft.
• The place of the charger practically determines where the forebay tank should come. So there should be enough room for a forebay tank and it is handy if there is a stable gully nearby that can act as a spillway for excess water.
• The place of the charger also determines the length of penstock pipe that is needed. A short length of penstock is important in two ways:
  • The penstock forms a major part of the total charging costs, so a lot can be saved if less length of pipe is needed.
  • A short pipe means less friction losses (see par. 4.10.8). In an unlucky case, a longer pipe should also have a larger diameter in order to keep friction losses within limits.

  The penstock pipe can be short if the forebay tank is close to the charger site, so if the river bank at that point is quite steep.

The two main things a charger needs are a head H and a flow Q. There are minimum requirements for these (see par. 4.1), but once the head is above 7 m, the charger becomes quite flexible. It can generate the same electrical power with a relatively high head and low flow, or with a lower head and higher flow. In this paragraph, some general guidelines will be derived regarding the planning of inlet, canal and charger site. In this, many of the site-specific issues that were mentioned in the previous paragraphs, are ignored. Also guidelines that are valid in all cases (e.g. high slope of river S_r, low flood allowance F, low seepage losses constant C_s are favourable in all situations) are not mentioned again.

To work things out, the normal water level at the canal inlet can be used as a reference height. Clearly the head H the charger experiences, is the difference between height h_s of the charger site itself and height h_f of the forebay tank in the canal just above the charger (the forebay is the place where the water enters the penstock pipe, see par. 4.10.7):

H = h_f - h_s

From the canal inlet (the reference height, so height = 0), both the river and the canal go downhill so to keep things logic, for both the slope of the canal S_c and the slope of the river S_r, negative figures are used. Also the height of the forebay h_f and the height of the charger site h_s are negative compared to the reference height of the inlet.

The slope S_c of the canal is much smaller (less negative) than the slope S_r of the river and that is why downstream of the inlet site, the height difference between the canal and the river becomes bigger and bigger. In practice, the charger must be protected against floods by placing it well above the normal water level of the river at that place: The flood level allowance F.

Then:

h_s = S_r * L_r + F

And:

h_f = S_c * L_c

So:

H = h_f - h_s = S_c * L_c - S_r * L_r - F

With:

h_f = Height of forebay (negative)
h_s = Height of charger site (negative)
S_c = Average slope of the canal (negative)
S_r = Average slope of the creek or river (negative)
L_c = Length of the canal
L_r = Length of river between inlet site and charger site
F = The `flood level' allowance, the height of the charger above normal water level in the river at that site

Along the canal, part of the flow Q_i that entered the canal, is lost as seepage losses Q_s. Roughly speaking, these seepage losses Q_s will be proportional to the length of the canal L_c. So:

Q = Q_i - Q_s = Q_i - C_s * L_c

With:

Q = Flow at charger
Q_i = Flow at inlet
Q_s = Flow lost as seepage losses
C_s = Constant representing seepage loss per m of canal length, C_s depends on soil type and wetted part of canal cross-section, which in turn depends on the slope S_c of the canal and the flow Q_l at that point.

In reality, seepage loss per m canal C_s is not constant, but depends a.o on the flow Q_l at that point in the canal, which in turn depends on the seepage losses from the inlet up to that point. By taking a constant value for C_s, this effect is neglected.

Using these formulas, some general guidelines can be derived for planning the inlet, canal and charger site, see table 4.3. Depending on what criteria one sees as most important, different guidelines will come out.

The standard charger is quite flexible in different ways:

• It can operate under a wide range of heads, requiring only that the blocking timber has the right size for the head at the site.
• Compared to most micro hydro schemes, the firefly charger needs only a small flow so it can use even small creeks as a source (of course also the electrical power output is low compared to other M.H. schemes).
• Charger and penstock are easy to move to another site. Only the inlet, canal and forebay tank cannot be moved.
• Only 10 users living within 1 to 2 km from the charger are needed to form a complete user group. Even with only a couple of users, investment costs per house are still acceptable.
• The capacity of the charger can be increased easily if more users join in later (see above).

This flexibility can be used to make the most out of the opportunities that are offered by the natural environment in the area. Also with regard to organisational aspects, this flexibility can be used. A small group of users can start, charger capacity could be increased if more people join in, eventually a number of users living a bit far from the charger might split off and start with their own charger etc. This all could be done without writing off the investment in the original charger and the penstock.

First, an expensive, permanent design will be discussed. See the end of this paragraph for a cheap, makeshift solution.

Before the water enters the penstock pipe, it should pass a screen with about 5 mm mesh (of galvanised iron wire, or stiff plastic). This is to protect the charger against pieces of wood or any other foreign object from entering the pipe and getting stuck between the nozzle and the runner. Without a screen, playing children might even let pebbles be sucked into the pipe. The larger the surface area of the screen, the longer it can work before it gets blocked by water plants, dead leaves etc and needs cleaning. Of course this also depends on how much water plants etc. are carried along by the water. A screen area of 0.1 m² per liter/second of flow worked well in the Cambulo project, but it still needed cleaning every day. So for the standard charger, a screen area of about 0.8 m² should do.

The shape of the screen does not matter much as long as it is easy accessible for cleaning. It could be fitted directly onto the penstock pipe. Then it could be like a long cylinder, or have a square or rectangular cross section, see fig. 4.32. It could also be a flat screen fitted to a wall of the forebay tank, with the penstock starting from the cavity behind the screen.

Fig. 4.32: An expensive, permanent forebay tank.

If the screen needs cleaning very often, it could make sense to have a removable screen in front of it. With a screen like in fig. 4.32, one could tie a bag of woven plastic (like the ones for rice etc.) around it, take it off once it gets clogged and shake off all dirt.

Apart from the screen, the forebay construction of fig. 4.23 has the following features:

• It serves as a silt basin. That is why there is a gradual increase in depth so that turbulence is reduced.
• The valve at the bottom serves two functions:
  • When opened, any silt collected at the bottom of the tank will be flushed out.
  • It acts as a gate for the charger: When closed, the water level in the forebay tank will rise so that the entrance of the penstock pipe is submerged and water starts to flow through the turbine. For this, it is handy to fix a rope to the valve and lead it through a pulley down to where the charger stands so that the valve can be operated from there.
• The wall at the end of the tank serves as an overflow (normally the canal will supply more water than the charger can handle).
• The canal for excess water. This should divert excess water (or all water in case the charger is not running) without causing erosion. When the canal is not used for irrigation, it could be a spillway canal running straight down the slope to the river or to a nearby stable gully. In this case, also silt flushed out when the valve is opened, can be drained through this.
  When the canal is used for irrigation, the excess water canal is just the continuation of the canal. Then any silt flushed into it will make that this canal becomes silted faster and needs more maintenance. If one would like to avoid this, a separate valve for flushing out silt and a spillway could be made in the downhill side of the forebay tank.

In fig. 4.32, it looks like the penstock pipe leaves the forebay tank above the canal for excess water. In reality, it should be fitted besides the canal for excess water, so that it can bend downwards to the charger.

The top end of the penstock pipe should be inclined 5 to 15° downwards. If it would be completely horizontal or even inclined upwards, a large bubble of air gets trapped in the penstock pipe. Then the charger might still get quite some water, but not enough head (= water pressure) since there is not enough weight of water in the pipe to produce this head. With the top end slightly inclined downwards, the air can come out at the forebay end.

A pipe going straight down from the forebay tank to the turbine is no good either since this might cause a `water hammer' effect. Once the valve is closed, the pipe might be filled so fast that a mere `wall of water' starts falling down the pipe. Once the water reaches the nozzle, it cannot escape that easily and the whole column of water is forced to slow down suddenly. As a result, there is a pressure surge that makes the charger and pipe jerk. It could damage the pipe, the turbine or knock the charger off its foundations. This is why the top end should not be inclined too much, 15° seems a safe value.

A penstock pipe filled with water is quite heavy so it should be fixed properly:

• Fix it to the end wall of the forebay tank. Cement does not stick to plastic so having the pipe plastered through the end wall is not enough. Either have the piece of steel pipe from the screen pass through the end wall and connect the penstock pipe to this, or fit some tight windings of iron wire around the pipe where it passes through the end wall.
• Make sure the weight of the part that goes straight down the slope, is supported. Without this, the downward bend might change into a hitch. This could be done by tying a nylon rope to the pipe, just after the downward bend, and fixing this to a pole, large rock or whatever. The rope itself should be in line with the part of the pipe that hangs down. To prevent that the rope will slip over the pipe, have the rope make 2 or 3 windings around the pipe before knotting the ends together.

The length of the rear leg of the charger can be adjusted, so that it is well-aligned with how the pipe comes in. If the turbine is not stable enough when it is just standing on its legs, fix the legs on boards or logs (but make sure that the charger can easily be moved to a safe place when the place where it stands could be flooded). If the soil is soft, this might also be necessary to prevent that soil is washed away by the water from the turbine or that the whole site changing into a mudhole.

If the charger will be installed only for demonstrating for a few hours to a few months, there is no need to build an expensive, permanent, concrete forebay tank. The only thing that is needed is a cylindrical screen that fits directly on the penstock pipe. For connecting the charger to a small, existing canal, choose a stretch where the downhill canal wall is wide and strong. Place the pipe with screen over the canal wall with its opening upstream. Maybe dig out a bit from the wall where the pipe crosses it, so that the pipe will be a little lower there (take care to repair this properly when the charger is removed later, it might start to leak and erode away the soil underneath). Tie the pipe to a tree or a pole, so that it won't slide down once it is filled with water. Then build up the canal wall upstream from where the pipe crosses it with soil, sods, stones etc. The wall should be so high that the water can be raised until the screen is completely submerged. Finally build a dam in the canal right behind the screen so that the water level upstream of it rises, the screen is submerged and the charger starts to run. This dam will serve as an overflow for excess water so it should be made of stones or sods and not of mere soil. If conditions at the site are favourable and there are some people to help, the charger can be installed in half an hour.

If a canal is dug especially for the charger, a similar simple forebay structure can be made of soil, sods etc. Then take care that excess water does not erode away stable soil that should serve later as a foundation to the canal should continue. It might stimulate people to see the charger run already before the canal is finished. Once the head is 3 m or higher, the charger should deliver a little bit of electricity.

As a penstock pipe, PE (Poly-Ethylene, a kind of plastic) pipe can be used. This is the black plastic pipe that is often used for potable water systems. For one outer diameter, pipes with a different pressure rating could be sold. These have a different wall thickness and consequently also a different inner diameter. Normally, even the pipe with the thinnest wall thickness should do since pressures in a potable water system usually are higher than in a firefly system.

PE pipes are light and slightly flexible: In the Philippines, diameters up to 2.5" were sold on a large roll like a hose. When it is bent too much, it produces a sharp hitch where the pipe is almost flattened. Such a hitch is difficult to get straightened out again and it might be necessary to cut it out. PE pipes can be joined by heating the contact surfaces and then holding them together, but proper tools and experience are needed for this. Connections can also be made by making a piece of steel pipe that fits inside the PE pipe and then securing the PE pipe with iron wire or a hose clamp.

What diameter pipe is needed, depends on the flow the charger will consume:

One would expect that plastic pipe would be as smooth at the inside as at the outside. However, in the Philippines, PE pipes were sold that were definitely not smooth at the inside. That is why also values for less smooth pipes are included.

The calculation of the maximum flow ratings are based on the following:

• A total power loss due to friction in the pipe of 20 % is seen as the limit of what is acceptable. Then the friction losses in the pipe make that the pressure at the nozzle is 14 % less than the actual head, which in turn makes that also the flow will be 7 % less. Friction losses are proportional to the square of the flow. So for reducing to 10 % at most, the maximum flow ratings should be multiplied by 1/ square root of 2 = 0.71. And to reduce friction losses to 5 % at most, maximum flow ratings should be multiplied by 1/2.
• It is assumed that the length of the pipe is twice the head. If the pipe is longer or shorter that this, the maximum flow ratings should be corrected:

If these maximum flow ratings are compared with the flow the charger will consume at different heads (see par. 4.1) it follows that, roughly speaking, the following pipe diameters will be needed for different heads:

These values are valid in case such a blocking timber is fitted that power output will be 165 W. If one chooses a smaller blocking timber so that the charger will produce more power, the flow will be higher and also a larger diameter pipe is needed.

The above guideline is rather rough. It is better to inspect the site, measure the head and the length of pipe that is needed, check whether the inside of the locally available PE pipe is smooth, and make your own calculations.

It is not adviseable to go above the maximum flow ratings that are calculated this way since then friction losses increase very fast and power output could become disappointingly low. In case the actual flow is coming close to the maximum flow for a certain pipe diameter, length and roughness or even surpasses it a little, consider the following options:

• Maybe fit a slightly larger blocking timber so that the charger needs a little less flow. Of course power output will be reduced accordingly, but at least the charger will consume less flow and become more efficient.
• Maybe buy the next higher pipe diameter if this bigger size pipe is still affordable. This would reduce the friction losses from around 20 % to less than 8 %, so that power output will increase by 12 % while flow hardly increases.
• Try to find such a charger site that the pipe can be made somewhat shorter.

The maximum allowable flow values for a pipe with a certain diameter, roughness and length ratio were calculated based on the assumption that friction losses are 20 % of total power. Since friction losses increase with the square of the flow, it is possible to estimate friction losses by comparing the actual flow with the maximum flow for a certain pipe:

P_f = (Q_a / Q_m)² * 20 %

With:

P_f = friction losses, as a % of expected power without friction
Q_a = actual flow through pipe
Q_m = maximum flow for such a pipe

Friction losses were not included in the calculation of expected power output in par. 4.1.

Pipes are classified by their diameter in inches but this inch rating gives little information on the actual dimensions of the pipe. Even the inner diameter can be 3 % larger than the rated diameter in inches. Pipes for high pressure could have the same outside diameter than low pressure pipes, but a greater wall thickness. Consequently different types of pipes with the same inch rating could have slightly different inner diameters. So for making the steel connection pipes, you will have to measure the inner diameter yourself once you have found a suitable pipe. Often PE pipe is not completely round so measure the largest and smallest diameter and take the average of the two as the real diameter. See par. 4.6 for how to calculate and build steel pipes that fit tightly into plastic penstock pipe.

Connection pipes are needed for:

• Connecting the PE pipe to the socket that fits on the nozzle. This has been dealt with in par. 4.6 already.
• Connecting lengths of PE pipe to one another. In this case, PE pipe must be fitted on both ends of the connection pipe so the connection pipe must be twice as long. Therefor the sheet should be some 200 mm wide instead of 100 mm.
• Connecting a cylindrical screen on the top end of the pipe, see par. 4.10.7.

The penstock pipe forms a major part of the charger costs so it is worthwhile to try to economise on it. When local hardware shops don't have the size in stock that is needed, or don't have any large diameter PE pipe at all, also delivery time becomes an issue. And finally, it might be impossible to buy just the length you need since hardware shops could insist on selling complete lengths and complete rolls of pipe or ask exorbitant high prices if you want to buy per meter. It might be possible to economise on penstock costs by:

• Moving the charger site a little so that penstock length fits well with the standard length of pipes (usually 6 m).
• Choosing a site with a high head so that a large blocking timber can be used and a smaller pipe diameter will do.
• If the size you need is not in stock, consider fitting 2 pipes of the next smaller size. Then at the charger, the two pipes should join into one socket. For this, a 2.5" T-piece could be fitted onto the nozzle. Then on the side connection, a 90° bend should be fitted so that the pipe fitted to this end, will be in line with the one that goes straight on. Or make a piece of connection pipe of about 0.5 m and cut this in two equal parts with a long, slanted cut. Then the two parts can be welded together again in such a way that they make only a small angle with one another. Now cut off so much from pointed tip of the joined pipes, that they fit well on the 2.5" socked and weld them on.
  At the forebay tank, each of the two pipes could have its own screen.

There are also other kinds of pipes that can be used for the penstock:

• Galvanised iron pipes. Normally, it will be more expensive and also require more fitting work on the site. Iron pipes are much stronger against cuts or blows than PE pipes.
• `Fire-brigade hose'. This is easy to transport (light, it can be rolled up flat), flexible, U.V. resistant and its pressure rating is way above what is needed for a firefly penstock. But it could be damaged by cuts or punctures and is quite expensive (ƒ 24 per m for a 3" hose in Holland). Maybe some hoses that were declared unfit by a fire brigade, can be bought cheaply.
  In Holland, a kind of flexible hose with a pressure rating of only 3 bar is available. This is still high enough for a firefly penstock and it is much cheaper (ƒ 3.05 per m for a 3" hose). It is not U.V. resistant but a supplier thought it would still last a few years. Probably it would not last this long under tropical conditions, so it might need to be buried or covered in one way or another. For testing, it would be ideal.
• PVC. This is often used in Micro Hydro schemes. Joints are made using sockets and a special glue. PVC pipe must be buried so that it is supported over its whole length and because it cannot stand U.V. light.
• Bamboo. The partinitoning walls inside have to be removed as much as possible and even then, friction losses will be relatively high so a larger diameter will be needed to conduct the same flow as a PE pipe. Joints could be made by sticking the bamboo into pieces of steel pipe, PE pipe or firehose. Maybe parts of an inner tire tube of a suitable size with canvas tied around it for strength, can also be used as joints. Bamboo would be very attractive if it is available locally, but experimenting is needed before a reliable pipe can be made. Therefor it seems less suitable for a demonstration project.

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