# Practical problem - fill 1000 liter water tank with pump using solar power

#### mario_avatar

Hi all. This is my first post so please forgive me if I write anything wrong or post on wrong forum.
In the attachment below I have a problem. I need to buy a water pump (PUMPA in img) that works on 12 or 24 volt DC current being powered with solar panels.
This pump must be able to fill a water tank (REZERVAR in img) in 10 hours 1000l (1 m^3) of water (VODA in img).
Distance the water must travel is around 40 meters and has to climb 6 meters in air.

I have found some formulas online and one looked interesting:
density of water * g * height => 1000*9,81*6 = 58872 Pa or N/m^2 ~ 8.54 Psi

now the pump i've found on ebay: pump
has 100 PSI. It is 12v DC powered, and has 5A amp draw.

I wanted your opinion on this problem, If I have the right formula, are the calculations ok, is this pump more than enough to fill the tank in 10 hours, how big must me the solar panel system, how strong must be the car battery to hold that much energy, if you have some cheaper or more convenient solutions, what are the potential risks and problems I did not think of, and any other sugestions you may have.

http://img703.imageshack.us/img703/9431/imgwdw.jpg [Broken]

Thank you,
Mario

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#### sophiecentaur

Gold Member
This is so much a practical problem that I would be inclined to look at the actual spec of the pump for the answer. I expect that information would be available - head of water and flow rate at maximum power. This is not something that can be calculated without more knowledge.

But the energy required to lift 103kg of water through 6m will be
E=103 X 10 X 6 = 6X104J.
(assuming g=10)
If you have 10 hours to do it, then this is an average power of 6X104/3.6 X 104 W = less than 2W !!!

The pump can supply plenty enough pressure for the 6m lift (your calculations), so that's not a problem.
The question is how big an area of solar cells would you need to give you an average of 2W AT 12V. That's 1/6A!! So you then need to thing in terms of actual efficiency. Be pessimistic and think only about 10% efficiency. That is about 1.5A, or nearly 20W worth of panel (cross-check the 2W figure above for average power requirement). These are pretty cheap (less than £100) and I would have thought that the best thing would be to buy just one panel and try it - going for another one if it's inadequate.

I don't see why you think you would need a storage battery as you may as well store the energy from the solar cells directly in the form of water at 6m, rather than as chemical energy in a battery (along with the associated efficiency of the charge / discharge cycle). The only reason for having a battery would be if your water tank was not big enough to hold a day's worth of water or if your sunlight supply was so marginal that you couldn't guarantee to fill the tank every day. (But, hell, a bigger water tank is cheaper than a battery and needs no maintenance)

P.S. I think the pump would probably work better if placed near the supply water level.
P.P.S The wider bore pipe the better - for efficiency.
P.P.P.S. A water level sensor / cut-off switch would be a good idea as I suspect you may have way enough capacity here.

#### CWatters

Homework Helper
Gold Member
now the pump i've found on ebay: pump
has 100 PSI. It is 12v DC powered, and has 5A amp draw
The spec for that pump says it can deliver a pressure of 100PSI and a flow rate of 5L/min. Unfortunately it doesn't say if it can do both at the same time. It's not allways safe to assume that. The manufacturer should be able to provide a graph of flow rate vs pressure.

You need to know what flow rate it can deliver at a pressure equal to roughly a 6m head (about 8.5 psi) plus some losses in the pipework. Perhaps ask the seller if he has info on the flow rate at say 10-20psi.

If he says it can do 5L/min at 100psi the pump should do what you want just fine.

One other issue to consider is the duty cycle/pump design life. Some pumps are intended for intermitent use such as on a spray tank for weedkiller, others are much more robust and intended for continuous use. Depending on your application that might be a factor.

#### CWatters

Homework Helper
Gold Member
I've just noticed that you show the pump at the top rather than the bottom of the rise. You need to check the pump will work in such a position. For example the pressure at the input to the pump is negative. You may also need to use a rigid pipe to prevent it collapsing.

Note that the maximium height any pump can lift from the top is about 10 meters. Any higher and a vacuum will form in the pipe.

Can you put the pump at the bottom?

#### Dickfore

you need to raise 1000 kg of water (1 l weighs 1 kg) for 6 meters in 10 hours. The useful power is:
$$\frac{1000 \, \mathrm{kg} \times 9.81 \, \frac{\mathrm{m}}{\mathrm{s}^{2}} \times 6 \, \mathrm{m}}{10 \, \mathrm{h} \times \frac{3600 \, \mathrm{s}}{1 \, \mathrm{h}}} = 1.6 \, \mathrm{W}$$

This is quite a low power and you can safely use a low voltage DC motor.

Just be careful to make sure you have enough sunlight for 10 hours.

Edit:
The current drawn by the motor is:
$$\frac{1.6 \, \mathrm{W}}{12 \, \mathrm{V}} = 0.13 \, \mathrm{A} = 130 \, \mathrm{mA}$$

This means you need a charge of 1300 mAh.

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#### sophiecentaur

Gold Member
I don't know your exact circs. but if you can chop and change your system then just build a minimal one with all the above comments considered. Then buy an extra solar panel or add a tank in parallel to cope with fluctuations in supply if necessary.
Go for it NOW. ;-)

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