solar water pumping basics
Fundamentals of Solar Water Pumping Systems
Designing and building water collection systems for use in agriculture and for consumption is a practice as old as civilization. One of the fundamental questions for operating off-grid water pumping systems is where the power will come from.
One of the best options for powering water pumps in remote and off-grid applications is through solar energy. Solar works as an excellent compliment to water pumping because typically the sun is brightest, and thus the pump flow highest, when water resources are needed (during the mid portions of the day).
This page will help explain the fundamentals necessary to design and select the right solar water pumping system and equipment for your application and needs.
Figure 1: Basic submersible Solar Water Pumping System
As we can see from Figure 1 above, most simple solar water pumping systems contain the following major components. There may be small or large design differences between systems - consult with a SunWize engineer if questions.
Pumping Rate & Well Design
Water pumps are typically specified as having a specific Gallons per Minute (GPM) production given a specific solar array size and TDH depth. This value is multiplied by 60 (minutes in an hour), and then multiplied again by the number of sun hours for your project location. Multiplying the hourly production rate with the number of hours the sun shines at full power will give you the total gallons of water pumped per day (Gallons per Day). Typically the Gallons per Day metric is what is used to size and design the solar water pumping system. It's the responsibility of the user or designer to determine how much water is required for the specific application. It's always helpful to be conservative when estimating.
Finally, make sure that the total well casing diameter is sufficient to support the pump you plan on using. Consult the pump manufacturers specification sheets for details on minimum well casing diameter.
Solar Resources & Site Location
One of the most important considerations when designing and selecting an off-grid solar water pumping system is the solar resources that are available at the project location. Since solar energy is the sole energy source in many systems the sun resources available at the site will dictate the amount of water that is produced. In certain cases, such as with the Franklin Electric SubDrive pumps, alternate AC input for generators may be available.
Additionally, seasons significantly affect solar energy. Therefore, if specific flow rates are required at different times of year, it may be necessary to design and account for the worst case season (whether winter or a different season). In some cases and systems, poor solar resources can be compensated for by adding additional solar panels to the system. However, in other cases, the pump will limit the maximum amount of solar that can be used, which may limit the total production that can be achieved with one pump.
The below solar resources map can be used as a guideline for determining approximate solar resources. Please call and discuss your project specifications with a SunWize engineer for an exact system quote.
Total Dynamic Head (TDH) Calculations
One of the most critical steps in designing ANY water pumping system is determining Total Dynamic Head (TDH). All of the SunWize Water Pumping Kit lookup tables require TDH to select the appropriate pump for your project. TDH is comprised of (3) major components: elevation head, friction head loss, and pressure head.
Figure 2: Basic Total Dynamic Head (TDH) Calculations
Elevation Head - 65 ft
Elevation head refers to the distance between the water level in the well (not the pump) and the maximum height the water is pumped to at the water storage tank. In the Figure 2 example above the total elevation head is 65 ft. Remember to only count the vertical distance that the water has to travel to fight gravity.
Pressure Head - 46.2 ft
The pressure head represents the amount of water pressure at the outlet of your water pipe. This could be an open pipe into the water storage tank or a water hose or other pressurized outlet. If no pressure head is present then this value can be ignore or set to zero. In cases where pressure head is present, simply multiply the pressure at the outlet in PSI by the equivalent pressure of a column of water, which is equal to 2.31 ft / psi. In our example above, an outlet PSI of 20 would equate 46.2 ft of additional head in our total dynamic head calculation.
Friction Loss - 30 ft
Calculating friction losses in water piping systems is the most challenging aspect of determining your total dynamic head (TDH). We recommend utilizing any number of free online tools to help you calculate what your total friction losses are.
Follow these general steps to calculate your piping system's friction losses.
- Determine the total length of pipe that the water will travel through. This includes both horizontal and vertical distances, as well as any pipe sections at an angle.
- Go to http://www.freecalc.com/fricfram.htm or other similar sites to calculate friction losses given piping specifications.
- Enter piping specifications, including pipe size, pipe schedule, piping material, flow rate, temperatures, and how many valves and fittings are present.
- Press calculate and use the total friction loss value in ft of head given.
Following on from our example above, let's imagine we have the following specifications:
- 4" Nominal Pipe Diameter
- Sch. 40 Pipe Size
- Clean Steel Pipe
- Flow Rate of 300 Gallons per Minute
- Piping Length = 581 ft
- (4) 90° Elbows
- (4) 45° Elbows
- (1) Pipe Entrance
Summing Total Dynamic Head (TDH)
Now that we have completed the calculations for our (3) major components of Total Dynamic Head, we simply add them together to determine what our TDH for this example, as shown in Figure 2, is 141.2 ft.
Elevation Head (65 ft) + Pressure Head (46.2 ft) + Friction Loss (30 ft) = 141.2 ft