How to size a solar pump system

 Sizing a Solar Water Pumping System

In this post we’re going to show you how to size and determine the various basic components that make up a submersible solar water pumping system:

  1. Solar powered submersible pump that goes in your well
  2. Holding tank or cistern to store a reserve of water
  3. Solar panel array to power the pump
  4. Linear current booster that optimizes the power from the solar panels to the pump
  5. Booster pump to increase the pressure of the water to usable household levels (if your cistern isn’t high enough)

We will be specifically covering solar pumping systems where you are drawing water from a well .

When determining what pump(s) to get, you need to evaluate your pumping needs.  How much water a day do you need, how many days of storage, how high and far does it need to be pumped, and what is the source?  One hundred litres per day (GPD) per person is common for domestic use. Your local weather patterns will help determine the amount of storage; do you have long stretches without sun, or is it only an occasional cloudy day?

Example Solar Pumping System Design

Let’s use an example of a family of 4 in Kisumu to supply their off-grid house with domestic water. They need to store enough water for a week of bad weather. They have a well 800ft away from the house, running through ¾in plastic pipe. The tank is 80ft higher than the water level, and 25ft higher than the house.

A solar water pumping system that includes a submersible solar pump and a booster pump to increase water pressure for household use.

Sizing the Water Storage Tank or Cistern for a Solar Pumping System

To figure out how much water they need to store in case there is little sun during rainy days we will take the amount of water they need per day per person, multiply it by the number of people and then how many days you can expect (worse case) to would would go without sunlight. So, in this example, they would need a tank that can store:

 100 litres x 4 people x 7 days = 2,800 litres storage

Sizing the Submersible Pump for the Well

There are two pieces of information we need to select a solar water submersible pump to meet their needs – they are:

  1. Pumping Rate: How many litres per minute will the pump need to move water when it’s powered by the sun?
  2. Effective Dynamic Head: How high, vertically, will the pump have to move the water up? Plus, how much effective head is added due to friction loss in the pipe?

Determining How Many litres per Minute the Pump Needs to be Capable of

To calculate the needed litres per minute their pump will have to do per minute we will first make an assumption of how many hours per day the pump can be reasonably run of the solar panels. Then we will simply divide the litres they need per day by the total number of minutes the pump would be running. So in this example, a submersible pump would need to provide 400 litres of water a day (100 litres of water used per person x 4 people = 400 litres), plus extra for stretches of bad weather, so let’s say 500 gallons of water a day. If we figure most of the pumping will be from 9AM to 3PM, that’s 6 hours of pumping, or 360 minutes.  And so we can determine how many gallons per minute we need their pump to be able to do minimally:

500 litres ➗ 360 minutes = 1.39 litres per minute

Determine Total Dynamic Head

We know that the pump has to raise the water vertically 80 feet. But it also needs to push it 800 feet horizontally. Even if the 800ft is completely horizontal, there will be friction loss from flowing through the pipe. The rate of flow will determine how much Friction Head Loss you will get.

Friction head loss is a way to take into account the friction of the water moving through the pipe and incorporate the additional load on the pump as if it were part of the vertical pumping distance (i.e. dynamic head) the pump has to get the water up to. Fortunately, there are handy tables that can tell us how much effective dynamic head we have to add due to every foot of pipe for different pipe sizes and rates of pumping the water through them.

In the chart below, each 100 feet of pipe length will be equal to the corresponding number of vertical feet of head. In our example, ¾in pipe flowing 2Litres/minute though it will be equal to 1ft vertical head per 100ft. So 800ft of horizontal pumping will equal 8 additional feet of head, for a total of 88ft of head (80ft of actual vertical head + 8ft friction head loss).

Total Dynamic Head = Vertical Head + Effective Additional Head Due to Pipe Friction
                   =      80Ft      + (1ft effective vertical head / 100ft) x 800ft
                   =      80ft      +  8'
                   =      88ft
Friction Head Loss  (in feet per 100ft plastic pipe)
Nominal Pipe Diameter
GPM
3/8”
½”
¾”
1”
1 ¼”
1 ½”
1
3.3
1.1
0.3
x
x
x
2
11.8
3.8
1.0
0.3
0.1
x
3
42.5
13.7
3.5
1.1
0.3
0.1
4
62.2
20.7
5.3
1.6
0.4
0.2
5
x
29.0
7.4
2.3
0.6
0.3
6
x
49.5
12.6
3.9
1.0
0.5
8
x
74.5
19.0
5.9
1.6
0.7
10
x
x
68.6
21.2
5.6
2.6
20
x
x
x
x
11.8
5.6
30
x
x
x
x
20.1
9.5
40
x
x
x
x
x
9.5
50
x
x
x
x
x
14.4

 

Now to find the Sumbersible Solar Pump that Fits the Bill

So we now know that this family in Kisumu will need a submersible pump that is capable of pumping at least 1.39 litres per minite and up 88ft of head. With just those two pieces of data we can go and drill down to any of the submersible solar pumps specification pages and there will be a chart for each pump that will tell us what each model can do for that pump rate (some will be in Liters Per Minute (LPM), others will show an hourly Litres Per Hour (LPH)) and dynamic head. We then just need to narrow down which one will do at least as much pumping in LPM as we will need for a total of 88ft of dynamic head.

This is usually done using the catalogues from the manufactures. Look at the catalogue to see if one of their pumps would work with the specifications you have arrived at.  The specification can be online or in hard copy.

 

Determining How Many Solar Panels You’ll Need to Power the Pump

Once you have figured which models of pumps could work for our application the next step is to figure out how many watts of solar panels we will need to power the pump and ensure it provides us with the water we need.

 

Figuring the Voltage of the Solar Panel we Should Use

For example according to the catalogue the motor that is up to your task is 30 volts DC, and recommends at least a solar panel of 120 watts (W). Since a nominal 24V solar panel has a Vmp (i.e. Maximum Power Voltage is what voltage at which panel produces the most power) of about 36V, we can either use a single 24V nominal solar panel or two 12V panels wired in series would work.

Determining the Wattage of the Solar Panel for the Pump

To calculate the minimum wattage solar panel we should use we oversize the wattage rating of the pump by 30%: For example if the wattage of the recommended solar Dc pump is 120 Watts.

120W x 1.3 oversizing = 156W of solar panels or greater

We could use one  24V 200W panel, or two 12V solar panels that are half the wattage, like the 12V 80W solar panels in series for 24V nominal 160W total.

 

Don’t Forget the Controller between the Pump and the Solar Panels

To optimize the amount water you can pump in a day it is important to use a pump controller, which are often referred to as a Linear Current Booster . The controller will get your pump to turn on earlier in the morning and stay on later into the day.  Each manufacturer has a pump controller they recommend for their series of pumps but there are a number of other linear current boosters that are from other manufacturers that provide other features that may work with the pump you choose, such as a float switch or water level switch.

 

Our Solar Pumping System’s Final Diagram

The drawing below shows the solar water pumping solution for our domestic water use example. Please contact us at the altE Store to help you find the right solution for your water pumping needs.

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Otieno Christopher

Otieno Christopher has been in the solar industry since 2008. He’s been a Technician, sales rep, an instructor, and an all around solar evangelist, sharing his passion for solar around the world. When not at work, He’s either dancing or hiking, depending on the season, but odds are good he’s still talking about solar in the dance floor or on the slopes.

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