Potential power output from a river.
I am posting this question on behalf of David Lloyd-Jones, a thermal engineer in UK.
While in Kenya i was asked to estimate potential power output from a river. We have around 80mtr head but the water flow/volume is throwing me. I found a flat weir upstream with a flat pond behind and measured 30cm depth across 2.6 width - ie .78M2.(ie 780 Litre) My logic tells that the flow rate must be pretty close to 780 litres'sec. The only advice i can find online, tells me to measure cross sectional area x flow rate as measured with say a floating tennis ball covering 10mtrs in say 10 seconds. 1/ I didnt take this measurement and am now back in UK 2/ I can understand the need for a flow rate on a sloping water body (ie river) BUT when you have a flat body (pond) with a flat topped weir then the M2 over weir MUST correlate with volume/flow rate yes??? - PS i dont need to dead accurate - plus/minus 20% would be good enough. If 780 L'sec then x say 80mtr head and lets say i only take 20% of river x 60% to allow for losses will give around 110kw?? (+-20%) And i think around a 10 or 12" steel penstock pipe Am i anywhere near with the above numbers??
Briefly, so my analysis may be too rushed here:
Flow figures used will be conservative because floating balls on the surface run at a lower speed than the body of the water so you are on the safe side with your assumptions.
Rule of thumb: litres/second x head (m) x 10 /2 = Power (W) The /2 aspect is for efficiency at 50%. 780 x 80 x 5 = 62400 x 5 = 312,000 W Take 20% of the flow so 312,000/5 = 60,000W = 60kW roughly.
Watch out for seasonal variation but using only 20% may guard you from this?
Other issues: Ask if you should only take 20% - if the fall is very steep then will you be causing any ecological impact ? But if taking a bigger factor there may be a need to seriously examine environmental impact depending on seasonal variations. Impact of other works? Land ownership - compensation - security - potential for disputes? Political economic risk factors: power structures - who loses, who gains?
At 80 m I would use a Pelton wheel form Gilbert Gilkes and Gordon up in the Lake District (UK). Probably a 7inch and use a 4 pole machine (1500rpm) directly mounted.
You should be able to find a nice old cast squirrel cage 3 phase motor of around 10kW and use this with an Induction generator Controller from Nigel Smith who runs Sustainable Control (https://www.sustainablecontrol.com/) who makes IGCs and knows all about setting up the electronic control side of these systems.
Such an arrangement is very reliable PROVIDED lightning strikes are catered for.
I installed a micro-hydro system like this in the DRC in 1993. It has been mechanically overhauled once since then but the biggest nuisance is from lightning strikes. Don't despair if the 60kW figure is correct compared to the 110kW - the system I installed only produces around 2kW but has been a real life-line for the staff at the hospital where it is installed. Sounds tiny but having cheap power for lighting, and refrigeration for over 20 years, 24 hours a day - that's worthwhile . You can do a huge amount with 60kW.
Penstock depends on length but 156 l/s through a 12" ID pipe looks like 2.2m/s which is getting towards the limiting speeds to avoid wear according to https://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.html For a proper analysis you will need to know the pipe material so as to get a roughness factor.
You may also want to know about how to limit load consumption at intakes so as to avoid people plugging cookers and heaters in ......
If you want any more info feel free to PM me but Adam Harvey and particularly Andy Brown
authors of the Micro-Hydro Design manual: https://www.amazon.co.uk/Micro-hydro-Design-Manual-Small-scale-Schemes/dp/1853391034
both ex-ITDG would be best placed to help.
Exciting stuff - always wish I could do more of this!
Assuming your weir behaves close to one of a few classic designs then it’s fairly straightforward to get a close-enough approximately of flow. Bear in mind that these “classics” are those specifically designed for flow measurement, so may well not be perfect for other types. For flat weirs (ie where the crest of the weir is horizontal), check out the following estimations:
(i) Broad crested weir (ie one that has a flat surface to the weir crest): Flow [m3s-1] = 2.1 x crest width [m] x (water depth over crest [m] ^ 1.5) (ii) Sharp crested weir (ie normally a metal plate): Flow [m3s-1] = 1.75 x crest width [m] x (water depth over crest [m] ^ 1.5)
So if I understand your blurb correctly and we assume you have a broad crested weir, then flow at that moment = 0.9m3s-1 or 900 litres per second.
If you have an 80m head, and you take 20% of the water that you saw available that day, at an efficiency of 60%, multiply by gravity, then available power capacity = 85kW.
60% is quite low, so you will probably find that 70% is more appropriate unless you’re planned scheme is very ‘agricultural’. Modern schemes in the UK can easily achieve efficiencies of over 80%.
20% of available water is also very low. Was the day you took the measurements part of a very wet day? If not then you could easily up this depending on how it compared to an average day and how much water you need to leave behind for river ecology and any downstream uses.
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