Does anyone know about pumping water?

[balbecdaze]

I'd appreciate any help,

I'm quoting on a solar powered water pump to provide water from a borehole via a tank to troughs distributed throughout an allotment.

The borehole is 300m from the tank which is 20m above it. The tank has a capacity of 3000l, it (gravity) feeds 10 ~300l troughs which each service ~6 allotments. The borehole seems to replenish at about 1000l/h, the maximum demand would be about 6000l/day. Sorry if I've mixed up my measurements, this area is new to me but it's for my father-in-law so I'd like to do it right.

I need to quote on the panels, mounting, controller and pump.

So far, I reckon a couple of ET Solar (i.e. cheapish but OK) 250W in parallel plus larch frame plus Lorentz PS150 C Solar Pump (and associated controller) will do the job given a 50mm pipe from pump to standing tank.

At well over £2000 it seems expensive!

My questions:

Why does it cost so much?
How can I figure out which pump to buy?

I really hope someone reading this forum can give me some pointers.

 

[Marvo]

I can give you some advice about the pump side of things. With a borehole pump you need to select it according to the depth of the borehole. If you use a borehole pump that's rated to operate at a depth of 200 meters for example but your borehole is only 50 meters then it cause excessive wear due to impellor lift.

 

[jason121]

Try Caprari pumps they will size it correctly

 

[solarfred]

Does the price include the batteries?

 

[Marvo]

The borehole is 300m from the tank......The borehole seems to replenish at about 1000l/h, ......will do the job given a 50mm pipe from pump to standing tank.

I just noticed your specified pipe size. A 50mm pipe for this application sounds very large (even at that distance). How did you calculate the required size to be 50mm at 16-17 litres per minute? What size pipe fitting is on the pump? I doubt you'd need to go any larger than the fitting it comes with.

*Edit* I just looked at the pump specs in the manufacturers brochure and I'm more confused than I was to start with. It's only got a 20m head capability. You say there's a 20 meter height difference between the borehole and the tanks which are 300m away. I would assume you're talking about the the top of the borehole and the tanks but how deep is you bore hole that you're dropping the pump down?

 

[balbecdaze]

Thanks for the replies, I'll try to answer your questions/address your comments:

Marvo - I've not explained myself well enough, the water comes from a "well" created by drilling through the bedrock, the pump will sit in the well, not in the borehole. Sorry for any confusion, my fault.

Jason121 - cheers will look into it

Solarfred - no, I'm not providing a quote for a system with a charge controller and batteries. There are a few reasons for this: they only have a one-off chance to bid for funds and it can't include maintenance/replacement; cost; the system (I think) will effectively store the energy in the pumped-to tank which then gravity feeds the troughs which provide the water to the users. Basically I suggested batteries, they were dubious and I think it can work without them.

Marvo#2 - this is more serious, I hope I haven't messed up! The 20m is the vertical distance the water has to travel from pump (in well) to tank. The 50mm internal diameter is (I thought) a more cost effective way to use a lower powered pump - I'm going on quotes from companies who have inputed my data into their software, my days of using the Bernoulli equation are long gone!

Please ask more questions, I'm not a water-pumper by nature!

 

[balbecdaze]

@Marvo, that is I didn't calculate the 50mm myself, it was suggested by a third party (is it ok to say who here? They seem very respectable)

 

[Marvo]

Yep it's fine to say, you can mention manufacturers and suppliers, we just don't allow blatant spammed advertising.

 

[solarfred]

Couple of glaringly obvious issues
Are we talking the UK or sub Sahara Africa? (Allotments suggests UK?)
- the head of 20m is the limit for the pump so what is the flow rate at this maximum? It says 4m3 =4000L per hour but that is at maximum power and unlikely to be true in practice as there is a long pipe run with friction losses. You will need to calculate all this to find out the refill rate at maximum power and likely average power.
You need to refill the system each day to maintain the 6000L which will take 1.5 hours at maximum power.(notwithstanding system losses). If it is dull or worse you will not refill the system. If there is insufficient "power" ie. watts then I assume the pump will not work? You will have volts output but no amps.
You therefore have no reserve in the system and without sun every day it will not work.
A battery system will have estimated reserve capacity built in to cover the load demand for average local weather and "black days".
Once you know the solar power available you can start to calculate the system losses and requirements.

 

[balbecdaze]

Thanks Solarfred,

It will be in Bristol, UK.

Yes, I've spent some time discussing this with the allotment chairman - he is of the opinion that on a dull day there is much less need for water, similarly in the winter their is very little call for water for irrigation (they will still have 1 mains-fed tap). The solar power available (average values) is fine - there should be sufficient power to fully replenish the system from apr-sep - the friction losses are included in this calculation which isn't mine but from the pump suppliers - Energy Development Co-operative, using I believe, Lorentz software.

The allotment chairman believes (he's been running the allotments for over 15 years) that the full troughs alone are adequate to meet the demands of a heavy days watering, and so the tank is in effect providing a full days backup.

I know I haven't fully answered your comments - I'm still working on it!

On another point, the pump suppliers recommend a tilt angle of 51deg, to better meet demand all year round, wouldn't I be better designing in an angle of 36deg to maximise power at the times water is needed (and slightly improve performance on dull days)?

 

[solarfred]

Your array angle is well down the list of current issues to be honest.Your demand is not a long annual average but a requirement for a daily delivery. Once you establish what you need each day then you can begin to design the power requirement. Using renewable energy in this scenario you can advise the customer how much capacity they have on average but if they exceed the agreed demand the the system may not be able to keep up. In summer when demand is high you have peak potential power but this is not available on a daily basis it needs to be harvested when available and stored for those dark days which we just had in August. It may not actually rain as much as it did so water would still be in demand but you need a reserve of power to keep the reservoir full as you only have one days worth in reserve. If you have stored 3 days worth of "power" during that one sunny day then the system becomes viable.I would target a weeks reserve of power.
Hope this helps with design ideas.
I wonder if a cheaper pump set up which trickle feeds constantly would be a much better and cheaper option. You may need to consider a bigger cistern which is a cheap part of the system.??
A whopping great pump is good when the power is there. We are in the UK.

 

[balbecdaze]

This makes a lot of sense to me, I will need to talk to the allotment chairman, my common-law-FIL about this. There is enough sun, we may not have a large enough reserve capacity - he did mention a possibility of expanding this so ...

On a positive note, they have submitted their bid for funding using my quote, so if they get the money, I get the job. If a trickle feed pump into a larger capacity tank was a better solution how would I figure this out? If you are a supplier or would rather not discuss this publicly then please PM me.