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Author: Thomas Jenkins, Professor, Department of Engineering Technology and Surveying Engineering, tjenkins@nmsu.edu, New Mexico State University. (Print friendly PDF)

Introduction

This publication provides step-by-step instructions for using a Microsoft Excel spreadsheet (available at http://aces.nmsu.edu/pubs/_circulars/CR671/CR671.xlsx) to design a solar-powered water pumping system, and to help decide if it is feasible to implement such a system to meet your needs. The spreadsheet allows you to enter values such as location, number and type of animals, other daily water requirements, depth of well, etc. By accepting different values, you can investigate a variety of scenarios before you make a final decision.

There are NO expressed guarantees with this spreadsheet, nor is it intended to replace professional expertise. It is also important to note that the spreadsheet is designed around a small set of submersible DC pumps and a PV module as a teaching example ONLY; other pumps and PV modules are available, and there are no implied endorsements. This educational version has theoretical design limits of flow rates less than 4 gpm and a pressure head of no more than 230 ft.

Before beginning, it is useful to have an understanding of how the spreadsheet is designed, the calculations used, and the recommendations for proper operation. A companion publication, Circular 670, Designing Solar Water Pumping Systems for Livestock (http://aces.nmsu.edu/pubs/_circulars/CR670.pdf), explains these issues in depth. The spreadsheet consists of eight different "sheets" or pages that are indexed via tabs at the bottom of each window. There is a title sheet, six sheets that represent the six main steps in the design methodology, and a conclusion sheet. Italicized text (e.g., Daily Water Requirement) refers to sheets in the spreadsheet; sheets are accessed using the tabs at the bottom of the spreadsheet window. Figure 1 identifies these for you utilizing the example Daily Water Requirement sheet.

Screen capture of different sheet tabs within the spreadsheet.

Figure 1. Different sheet tabs within the spreadsheet.

These sheets will be referenced continuously throughout this manual. Looking at Figure 1 again, you will notice different shaded boxes or cells. There are blue and light purple shaded cells, which are instructions and user notes that should be read when using each sheet. The green cells are where information can be entered. To enter a value, move the cursor and click on the cell (green box), enter your value in that green cell (typically a number), then press the "enter" or "return" key. The orange cells display intermediate values that are automatically calculated by the spreadsheet. The yellow cells show the final calculations on each sheet. The values in the yellow cells will be used to generate the final design specifications.

Illustration of basic schematic of a pumping system from a well to a storage tank.

Figure 2. Basic schematic of a pumping system from a well to a storage tank.

User NOTE: Some cells have a red triangle in the top right corner. These cells have a "pop-up" note that gives an additional short explanation related to the information in that cell. When you place the cursor on the cell, this informational note will be displayed. Some input cells are restricted in their allowable entries. For example, the multiplier cell in Figure 1 will only accept entries between 0 and 20. If you enter a value outside of this range, an error message will display and you must re-enter a valid value in this cell before continuing.

Step-by-Step Design Procedure

Step 1

Read the accompanying paper 1 and skim through this publication to get a basic understanding of the spreadsheet and general overview of solar water pumping. You should have a fundamental understanding of water wells, and some experience with computers and spreadsheets is also helpful.

Using Figure 2 as an example, sketch out some basic parameters of your well. The key values needed are the discharge elevation, well water level, total length of pipe and its nominal inside diameter, and fittings like elbows and valves. Units of length are feet.

Water level is defined as the lowest depth of the water in the well including any draw-down levels and seasonal variations—not the pump set level, which is the depth of the pump below the water level. Elevation is the distance from ground level to the discharge point. Total length of pipe is for all pipe from the pump to the discharge point (including any horizontal pipe, such as from the well head to the storage tank). There is an example in the spreadsheet that is color-coded for your convenience.

Once you have sketched out the basic layout of your piping system and identified valves and fittings, move on to Step 2.

Step 2

Open the spreadsheet, click on the Title Page sheet (the left-most tab at the bottom of each page), and read the material and look at the Internet links. Then proceed to the first design sheet by clicking on the Daily Water Requirement tab at the bottom of the page. You should see a similar view as Figure 1. Make sure when transitioning to each new sheet that you begin at the top of each page and read the user notes and instructions.

The purpose of the Daily Water Requirement sheet is to calculate the amount of water you need per day based on the number and type of animals—plus any other water needs. Keeping in mind that you only enter data into the green cells, input the number of each of the animal types you want to water. If you do not have any of a type, just leave that cell blank (or put in 0). If something is not listed, input the amount of gallons that item requires per day under the category of "Other Water Requirements." The amount of water for each animal is calculated by multiplying the quantity of animals (five horses for example) by the amount of water each animal needs per day (15 gal/day). The amount of water each animal needs varies due to animal size, season, and location (desert vs. mountain), etc. 2 Generally, you would use a worst-case value—typically a summer value when water needs are highest. This sheet shows common water requirements for various animals, but you can change these default values by entering values in the green cells in the "Water requirement for animals" section at the left side of this sheet (Figure 3). Gray cells indicate a typical range of recommended values. Using the values in the green cells multiplied by the number of each animal, the system will calculate a total daily water requirement for each animal and then sum all these to get a total daily water requirement, which is displayed in yellow (see Figure 1).

You may also enter a percentage multiplier (0–20%), which will add an additional percentage to the final calculation. This can be used to offset evaporation, refill a storage tank, or adjust for anticipated growth in water requirements. The new adjusted total is also displayed in yellow. Under this is the required water in gal/day.

Similarly, you can enter a number of days for storage 3 . This will only be used to provide a general size of a storage tank that might be used with the system. It has no other use in the design. For New Mexico, a normal storage value is three days but may be up to 10.

Step 3

Navigate to the sheet titled Solar Resource by clicking on that sheet tab name at the bottom of the display. The purpose of this sheet is to calculate the total daily insolation (sunlight) falling on the well's location. In the "Latitude" cell, enter the nearest latitudinal coordinate to your well location; coordinates will be between 31° and 37°N (the latitudes of NM). A green highlight will appear on the map according to the latitude—see example in Figure 4 for 31° latitude entry.

User NOTE: The "Latitude" cell, like several others, has a drop-down menu associated with it, a menu that can be accessed by placing the cursor on the cell and clicking on it. Opening the drop-down menu displays valid entries for this cell. Just click on any menu entry to make it the cell input.

Screen capture of

Figure 3. In the "Water requirement for animals" section, you can adjust the daily water requirement for different types of animals.

Design NOTE: The solar insolation is calculated assuming that the PV modules will be fixed and pointing south at a tilted angle equal to the chosen latitude value (31–37°) from the ground.

Next, you may choose a season ("Winter," "Summer," or "Yr. Avg") during which your animals may be watering/grazing. It is strongly recommended to use "Winter" so that the system uses the shortest day of the year or a "worst case" sunlight value. This will help ensure that enough sun exists at this location year-round to run the pump efficiently. Choosing "Summer" might be appropriate if you plan to only graze/water livestock during the summer months, while "Yr. Avg" (a yearly solar average) would be used for only spring or fall times (which is rare).

Step 4

Navigate to the next sheet titled Total Dynamic Head (Figure 5). This sheet determines how hard the pump must work to move water from the well through the total pipe length and all fittings to a discharge level at a certain speed or flow rate. This value is used to later determine the pump size and PV modules required for this system.

Take the values (elevation, water level, etc.) that you pre-determined in Step 1 and enter them into their associated green cells in the Total Dynamic Head sheet. Use the color-coded example drawing on the left side of the sheet as an aid. These values are used to calculate the total vertical lift of the system plus the friction loss in the pipe and fittings used to transport the water from the well to the discharge point, giving the Total Dynamic Head (TDH).

Water level is the lowest depth of water in the well including any draw-down and seasonal variations—this is not the depth of the pump, which is located deeper (submerged underwater). Pump level is how far below the water level the pump is—this is used to determine total pipe length. Total length of pipe is for all pipe, including length in the well to the pump and above-ground and horizontal runs.

Screen capture of the Solar Resource sheet.

Figure 4. The Solar Resource sheet.

Screen capture of the Total Dynamic Head sheet.

Figure 5. The Total Dynamic Head sheet.

Screen capture of the Pump Selection sheet.

Figure 6. The Pump Selection sheet.

Next, enter the nominal pipe diameter 4 (inside diameter in inches) and type of pipe (PVC, poly, or metal) 5. Using the schematic from Step 1 and the example diagram on the Total Dynamic Head sheet, input the number of each type of fitting you think you might use in your concept design of the piping system in the "Quantity" column. If you do not have or plan to use some of the fittings listed, leave the cells empty or enter 0.

The TDH is calculated and displayed in yellow in units of feet (with equivalent meters right above and psi right below).

User NOTE: Under the example well diagram on the left ("Typical Solar Water Pumping Layout," Figure 5), there is a calculated "Hydraulic Workload" value. This value can be used to determine if the system, as designed so far, is a candidate for solar power. If the value is less than 1,500 it is a good candidate, if between 1,500 and 2,000 it is marginal, and if greater than 2,000 other pumping options should be considered for this scenario.

Step 5

If the "Hydraulic Workload" is acceptable for solar power, you can continue to the sheet titled Pump Selection (Figure 6), whose function is to determine an acceptable pump, the voltage at which to run the pump, and the solar panel wattage needed to power this pump. This is all determined by two important parameters: the previously calculated flow rate (Q) and TDH. For this version of the spreadsheet, with its limited choices for pumps, only flow rates less than 4 gpm and TDH less than 230 ft are permitted.

You must look at a couple of supplied pump curve charts to determine pump voltage and wattage required. Each pump can operate only at a max flow rate for a certain TDH. For example, the SDS-D-228 can pump water up to a calculated maximum equivalent TDH of 231 ft, but only at flow rates less than 1 gpm. If we run a SDS-D-128 at 15 V, it can only pump at a rate of 0.54 gpm (from this depth). If we run this same pump at 30 V, we could pump at 1.25 gpm at this same TDH. If we run the pump/motor at 15 V and our TDH is 92 ft, we can pump only at a max rate of around 0.71 gpm.

Screen capture of flow rate and TDH calculation.

Figure 7. Pumping at the required flow rate and TDH.

So, if our calculated flow rate is about 0.95 gpm and our calculated TDH is about 100 ft, then several pumps would work at several different operating voltages. We would want to pick a pump that we could run at its lowest "Pumps Motor Voltage" while requiring the lowest "Peak Panel Wattage (W)." By using the smallest values, we minimize the size and number of PV modules required, which lowers costs.

Therefore, for this sheet, you must look at the different pumps (SDS-D-128 for example) and find one that can meet the calculated TDH and at the required flow rate. Once you determine which pump to use, note the "Motor Voltage" and the "Peak Panel Wattage (W)" from that pump's chart, which is below the graph, and then manually enter these values into the green cells labeled with these names at the top of the sheet.

Each of the pump tables has color-coded cells to help you select the proper pump. Green cells indicate the pump meets or exceeds TDH requirements, while the orange cells indicate the pump meets or exceeds the required flow rate. The user is aided in this choice by the spreadsheet coloring values in the charts that fit (greater than or equal to) the flow rate and TDH calculations. Finding a pump where both colors are active in a given row will indicate this pump can operate at the required flow rate and TDH. For example, if the calculated required flow rate is 3.7 gpm and the TDH is 114 ft (34.7 m), then the SDS-Q-135 pump would work only at 30 V and require 230 W of power to operate. See where the green and orange highlited colors meet in Figure 7.

Screen capture of efficiency calculations to double-check the proper PV power input for the system.

Figure 8. Efficiency calculations to double-check the proper PV power input for the system.

So, using the pump's chart, locate a pump model that will meet (or exceed) the requirements of both flow rate and TDH. Input the selected model name into the green box next to "Pump Choice." Using the pump chart (like the one in Figure 7), find the values for "Motor Voltage (V)" and "Peak Panel Wattage (W)" that are closest to the calculated flow rate and TDH for your system. Enter those values into their respective cells located below the "Pump Choice" input cell at the top of the sheet.

If a pump has multiple acceptable combinations of flow rate and TDH (i.e., where both colors are active together in the table), you should choose the one with the smallest pump voltage (e.g., 15 V over 30 V) and the smaller panel wattage values. By choosing smaller values, we will minimize the number of PV panels required.

To the left of the pump selection graph, there are some calculated values (Figure 8). These calculations and assumptions (pump efficiency) are used primarily as a validity check for the PV power required to run the pump. These values should be close to the pump table value, which you select from the charts. If not, there is an error in the choice or in the system and all values should be rechecked.

Design NOTE: The calculations assume a submersible pump with an efficiency of 35% and PV modules with an 85% de-rating value for temperature and soiling effects.

Step 6

Navigate to the sheet titled Array Sizing (Figure 9), whose function is to determine the amount of PV modules required in series (called a string) to provide enough energy (wattage) for the pump to operate at the calculated flow rate and TDH, thereby meeting the daily water requirements specified earlier. It also calculates the amount of photovoltaic strings that will be needed in parallel, which means parallel rows of modules in series. You are not required to input any data within this specific sheet.

Design NOTE: This spreadsheet version only uses 50-W Kyocera PV modules, but many, many other models and sizes exist.

In this example, six 50-W solar panels (6 × 50 = 300 W total) are needed to generate the required power to run the pump. There will be three strings of two panels in series connected together in parallel. Each string will be two panels in series (providing the 30 V required) and the current to power the pump. This system will generate more energy than required by the pump and therefore will pump more water than calculated—this is generally not an issue, especially if pumping to a storage tank with a float switch.

Step 7

Navigate to the sheet titled Design Specifications (Figure 10), which summarizes the previous sheets and generates a design specifications sheet for the direct-coupled solar pumping system scenario. The sheet includes many of the materials required and specific descriptions of each—for example, size and model names, the quantity of each item or material, an itemized total for each item or material, and an approximate grand total price for the entire system minus labor, well, construction materials, and other minor costs. The pipe is assumed to be PVC and the wire is priced by a 250-ft roll with length calculated from the pipe length plus 25%. You do not have to input any data within this sheet as well. Example prices are from 2012 and from a random supplier—again, no endorsement of this supplier is implied and prices always change and should only be used as a guide.

Screen capture of the Array Sizing sheet.

Figure 9. The Array Sizing sheet.

Screen capture of the Design Specifications sheet showing the grand total for the system as designed.

Figure 10. The Design Specifications sheet showing the grand total for the system as designed.

Step 8

Print out the Design Specifications sheet and go to the Conclusion sheet before closing the spreadsheet.

Upon completion of Step 8, you have gone through the basic step-by-step method to design a direct-coupled solar water pumping system for your specific application. The parts and materials can be purchased through many different suppliers, many of which also provide professional installation and consulting. Remember that this is an educational tool and should be used only in that manner. If you are in need of further assistance, you can contact most solar pump manufactures, the NMSU College of Engineering Engineering NM program, or your local NMSU county Extension agent

Footnotes

1Circular 670, Designing Solar Water Pumping Systems for Livestock (http://aces.nmsu.edu/pubs/_circulars/CR670.pdf).

2Circular 670, Designing Solar Water Pumping Systems for Livestock, gives information on water needs, or you may contact your Extension agent for specific values for your location, season, conditions, and animal types.

3Solar (like windmills) often pumps extra water into a storage tank to supply water to a drinker at night or during periods of low light.

4It is best to use the smallest feasible pipe diameter since low flow rates (1–5 gpm) will not have sufficient water velocity through a large pipe to keep suspended solids from settling into the bottom of the piping. A common mistake is to use too big of a pipe.

5PVC is recommended.

Acknowledgments

The author wishes to express extreme gratitude for the assistance of the following:

College of Engineering and Engineering NM: http://engr.nmsu.edu/index.shtml

NMSU Cooperative Extension Service: http://extension.nmsu.edu/

New Mexico Space Grant Consortium: http://www.nmspacegrant.com/

Southwest Technology Development Institut: http://www.nmsu.edu/~tdi/

Department of Engineering Technology and Surveying Engineering: http://et.nmsu.edu/

 


Disclaimer: This document is intended for educational purposes only in providing a basic understanding of terminology and concepts of design. It is not intended to be used beyond this purpose. The authors incorporated good analysis, but all situations are different and you should consult a professional in all cases.


Photo of Thomas Jenkins.


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Printed and electronically distributed October 2013 Las Cruces, NM