Introduction to Solar-Pumping System Design Considerations

This introductory information is primarily focused on solar-powered water pumping systems for tenant farmers on rented ground and for systems designed for use in freeze-free months. The information and demonstration systems are to help enhance pasture management options, and exclude livestock from surface streams, where permanent watering system installation is not currently a possibility for a location.

Where possible, a properly designed permanent watering system is generally recommended, and needed, for year-round usage.

Please Contact Your Local USDA Service Center to explore options for permanent installations

Step 1:
Determine Your Water Requirements

The first step in designing a solar-powered livestock watering system is to estimate the daily water requirements for the livestock. Different species, breeds, and age ranges of animals have different water requirements, and water requirements can also fluctuate based on season, site among other factors.

Informational resources, like the table here, can help with estimating ranges of water intake requirements. However, your observations and experiences are key to adjusting these estimates to reflect reality in order to provide adequate water and to meet actual needs.

Cattle Accessing Livestock Waterer Stock Tank from System Demonstration Site

More on Livestock Water Quantity References...

For more information on Water Quantity Guidelines for Various Livestock, please see Appendix A-2 of the Virginia Engineering Design Note 614 (DN-614) Watering Facility from the Virginia NRCS. The summary table from this resource is also copied below for reference.

Step 2:
Evaluate the Water Source

Next, it is important to evaluate the water source you plan to use. Is it a natural or manmade water source? Is it free-flowing, or standing? Does the volume or flow rate fluctuate throughout the year? Is there enough water available (i.e., is the rate of recharge greater than the rate of your withdrawals)? Is there sand, silt, or organic matter? Do you need to consider using more than one source? Is there a risk of high-water events, and could they potentially damage your equipment? Will low water events affect your water quality? Finally, you should be aware of national, state, and local regulations applicable to your use of groundwater types and in your particular context.

Sample Collection from a Surface Water Source

More on Livestock Water Quality References...

For more information on Water Quality Guidelines for Livestock, please refer to pages 1-18 and 1-19 from the "Part 651: Agricultural Waste Management Field Handbook: Chapter 1 Laws, Regulations, Policy," summary tables 1-7 and 1-8 from this section of this source are copied below for reference.

The Virginia Department of Health maintains a list of water testing laboratories, available at this link.

Step 3:
Consider the System Layout

Your third step is designing your system layout. You need to consider the location of the major elements to your system, including:

Water Source
Pump
Solar PV Array
Reservoir
Stock Tanks
Routing of Pipelines

One major consideration for this step is to analyze different potential configurations and system layout, taking into account the topography of the land, distances covered, and elevation changes from water source to reservoir to troughs- along with the length of any slopes you need to consider. Typically, the smaller footprint for a layout the less materials needed which can help improve system economics and system installation time.

The freely available Google Earth Pro mapping tools can be an excellent resource for this kind of site layout planning and evaluation. Please see this link for more information on measuring distances in Google Earth. Note, the option "clamped to ground" is selected to display slope length measurements based on estimates of surface distances corresponding to terrain features along the selected path. More details on Altitude settings is available at this link.

Example Water System Layout from
NRCS Technical Note No. 28, Appendix C
(pg. C-23)

Screenshot from the Free Google Earth Pro Software Tool Showing the Elevation Profile Between Two Points

Some Considerations for Trailer-Mounted Designs...

Some system components can be configured for relocation and even transportability. These features can be helpful when a tenant lease is not renewed or even for in-season relocation as part of pasture management strategy. For trailer-mounted systems it is important to keep in mind load restrictions of trailer running gear. While certain system components are relatively light (e.g., PV panels, controller, etc.), water itself is very dense. For example, one gallon of water at 70F weighs about 8.3 lbs (1 cubic foot = 62.3 lbs). A single 330-gallon IBC tote (i.e., "cage tank") would have approximately 2,739 pounds of water when full. Be sure to double check weight ratings for any trailer-mounted options, especially when considering storing water onboard.

Step 4:
Determine Water Storage Requirements

Determine your daily water storage requirements. Solar PV systems require sunshine to function, since it is not always sunny its recommended to have some amount of storage within the watering system. This additional storage can also be helpful when there are equipment and maintenance issues by permitting gravity flow from storage while system issues are addressed.

A common "rule of thumb" is to plan for plan for three days of water storage. However, more or less storage may be warranted depending on availability of site-specific alternative back-up options. This can be calculated by taking your estimated daily water requirement for your particular livestock situation (from STEP 1) and multiplying that number by a factor of three. Some system operators have explored using batteries to store energy for later use in pumping water (e.g., during cloudy days, etc.), which may be an effective option for some installations. However, often storage in the form of "already pumped water" (i.e., into a reservoir uphill) can be more viable by avoiding the potential operation and maintenance costs associated with battery banks.

Overhead View of Two Plastic Reservoir Tanks

Host Farmer Demonstrating Reservoir Storage at System High Spot to Enable Gravity Flow to Stock Tanks

Step 5:
Determine the Solar Resource

The next step is to determine the solar resource for your site. Solar resource capacity, or site solar insolation analysis, is defined as the amount of the sun’s energy that can reach your solar photovoltaic (PV) system. This information is needed to calculate the energy that can be generated to run the system. Solar insolation varies depending on latitude, elevation, time of year, site factors (e.g., slope aspect, shading from terrain trees, buildings, etc.), among other factors.

A good resource for latitude and seasonal solar insolation analysis is the free online calculator PV Watts maintained by the US Department of Energy. For consideration of site shading analysis, a Solar Pathfinder device can also be used to assess shade for a given location. The combined data from these two sources are used to help you make solar panel selections for the system you need.

PV Watts is a Free Tool to Assess Solar Resource for a Site

A Series of Brief Educational Videos on Using PV Watts

Step 6:
Determine the Design
Flowrate for Pump

The next step is to determine the design flowrate, in gallons per minute (GPM) for the pump. This is defined as daily water needs, in gallons per day (GPD), divided by the peak sun hours, for the specified time of year, from STEP 1 and STEP 5 above.

A sample calculation for this analysis is shown in the formula below, and interactive calculator to the right.

Example Flowrate Calculation

Flow (Gallons per minute/ GPM)
=
[Daily Water Requirements (in Gallons)/ Day )] / [Peak sun hours (KWh/ m2/ day) * 60 minutes/ hour

Example:

1080 gallons per day is needed, and it was determined that the site receives 6 hours of peak sunlight per day. 1080 gallons/ 6 peak hours multiplied by 60 minutes/ hour indicates that the pump will need to supply 3 gallons per minute (GPM) to meet the watering needs.

Step 7:
Determine Total Dynamic Head

Now determine the Total Dynamic Head (TDH) of your system. This is the pressure required to move the water from its source to the desired location.

These three factors are key to determining the Total Dynamic Head:

Vertical Lift
The difference in elevation between water source and delivery point
Friction Loss
The loss in pressure due to the friction of the flowing water along the internal walls of the pipeline, as well as from presence and type of pipe elbows, valves, fittings, etc.
Pressure Head
Any pressure requirements at system delivery point

A sample calculation for this analysis is shown using the formula below, and interactive calculator to the right. This material is provided as an initial introduction to TDH calculations, a more detailed analysis is needed to properly account for pipe elbows, valves, and fittings, among other site/system-specific factors.

Example Total Dynamic Head Calculation

As mentioned in STEP 4, Google Earth Pro can be a useful tool in exploring site layout options and for this step too, for estimating Vertical Lift and the total pipe length for the Friction Loss calculations using the Elevation Profile in Google Earth for the selected layout configuration.

Total Dynamic Head (feet) = Vertical Lift (feet) + Friction Losses (feet) + Pressure Head (feet)

Vertical Lift (feet)
=
Elevation of Water Surface at Delivery Point - Elevation of Water Surface at Source

Vertical Lift = 71' = 1323' (delivery elevation) - 1252' (source elevation)

Pressure Head (feet)

Required System Pressure at Delivery Point (PSI) X 2.31 (feet of head/PSI)

Example

Pressure Head = 11.55 = 5 psi x 2.31

Friction Losses (feet)

Is Based on the Hazen-Williams Equation below, where:

hf = friction loss per 100' of pipe

Q = flowrate in gallons per minute

C = friction coefficient of pipe material

d = inside pipe diameter in inches

Example

hf = friction loss per 100' of pipe

Q = 5 GPM

C = 140 (PE Plastic Pipe)

d = 1.049 inches (Nominal 1" PE Plastic Pipe)

hf = 1.74 feet of head loss due to friction per 100' of pipe

For more information on calculating friction losses, please see the resources on this page, and Section 801.17 of the Midwest Plan Service. Structures and Environment Subcommittee. (1983). Structures and environment handbook (11th ed.). Midwest Plan Service.

Step 8:
Pump Curve Analysis & Determine System Power Requirement

A Pump Curve Analysis is performed to select an appropriate pump for the system design and also to help determine the power requirements of the pump motor and system controls. These requirements are primarily based on the following three factors:

Design Flowrate, in gallons per minute (GPM) as determined in STEP 6 above
Total Dynamic Head, as determined in STEP 7 above, and
Pump Curve, a description of a pump's performance (e.g., varying head and flowrates, etc.), typically provided by the manufacturer

Reading a Pump Curve

For more information on Pump Curves, please see the resources on this page, and this Pump Curves, LSU Extension Fact Sheet and the Grundfos Ecademy Video Module "How to Read a Pump Curve"

Step 9:
Determine Solar Array Components and Configuration

The next step is to specify your solar panel array. You select the panels based on your calculated system requirements. The general recommendation is to oversize your PV panels by a factor of 125% , to ensure that the system will have sufficient power. Then verify that voltage and current are sufficient for operating all of the system components, including controllers, pump motors, etc.

Step 10:
Verify System Pressure and Flow Rates at Delivery Points

Verify that system pressure and flow rates are sufficient to provide adequate water at your delivery points, such as watering troughs. Make sure that there is adequate water pressure to operate any valves or float switches in the systems. If you are using a gravity fed system to bring water from your reservoir to the troughs, you need to allow for sufficient fall to meet pressure requirements of float valves. Periodically check the system to ensure that it is working as it should be, accumulated algae growth, among other issues, can restrict flowrates and impact float valve function.

Additional Resources

Introductory Video to the “Watering Facility: Virginia Engineering Design Note 614 (DN-614)” USDA, Natural Resources Conservation Service (NRCS), Virginia

Acknowledgements

Infographic Design by Ashley Yanego
Videography and Video Narration by Becky Szarzynski
Equipment Video Demonstration by Felicity ZImmerman
Copy editing by Rachael Shenyo

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