Solar Power for Subs: Panel Connections

In this edition of Franklin AID, we continue our series on solar pumping systems, specifically the configuration and wiring of solar panels. Though this step is relatively straightforward, it is a critical one. A system could have all the right panels, but if they are not wired together correctly, the controller/motor won’t receive the voltage and/or current needed to fulfill the system’s water requirements.

Solar panels are DC devices. That is, just like batteries, they produce a direct current. Also just like batteries, they can be wired together to produce the exact combination of voltage and current required. There are two key points to remember when connecting multiple solar panels:

1)     When panels are connected in series, the total voltage delivered is the sum of the voltage produced by each panel. However, the amount of current (amperage) available will only be the current produced by a single panel. This is very similar to what happens when two or more pumps are connected in series. The total head produced will be the sum of the pressures produced by each pump, but the flow produced will remain equal to the flow of one of the single pumps.

2)     When solar panels are connected in parallel, results are flipped. The currents become cumulative, but not the voltages. Once again, this is very similar to connecting two or more pumps in parallel. If the intakes and discharges of two or more pumps are connected, we produce more flow, but the pressure generated will only be the pressure generated by a single pump.

So how are solar panels connected in series and in parallel? Using our pump analogy again, think about connecting two or more pumps in series. The discharge of the first is connected to the intake of the second and so forth. Likewise, to connect two or more solar panels in series, the positive terminal of one solar panel is connected to the negative terminal of the next. The positive connection of the panel can even be thought of as the “output” and the negative terminal as the “input”. This is shown in the diagram below.

panels in series

To wire two or more solar panels in parallel, all of the positive terminals are simply connected together and the all of the negative terminals are connected as seen below.

panels in parallel

To summarize, when panels are connected in series:

  • Total voltage is the sum of each panel in the series
  • Current (Amps) remains the same as a single panel in the series
  • Power (watts) is the sum of each panel in the series (since power = voltage x current, this makes sense) 

When panels are connected in parallel:

  • Voltage remains the same as a single panel in the parallel connection
  • Current (Amps) is the sum of each panel in the parallel connection
  • Wattage is the sum of each panel in the parallel connection

What about a combination of panels in which some are wired in series and others in parallel? The same rules apply, of course. Voltage will add up for those panels that connected in series and current will add up for those panels connected in parallel. In some installations, a combination of connections may be needed to produce the voltage and current required.

The good news is that Franklin Electric’s SolarPAK Selector provides the needed panel array configuration for any given installation. In that segment of the Selector, note that in the first column of the Panel Array Configuration box, the term “String” denotes how many panels should be wired together in series.  The second column indicates how many strings (groups wired in series) should be wired in parallel. Looking at the example we used in the last post, the SolarPAK Selector tells us that we need ten of the panels we have specified and that they need to be configured in one parallel string (one string of ten wired in series).

 Solar Screenshot-frank

Once again, we’ve only provided three pieces of information – location, water requirements, and the panel characteristics supplied by the manufacturer, and Franklin’s Solar Selector has done the rest, even how to connect our panels.

Solar Power for Subs: More Sizing Info

In the last edition of Franklin AID, we demonstrated how to enter information into Franklin Electric’s Solar Selector. We selected Amarillo, Texas as the location and in this fictional example, the end user initially specified 900 gallons per day. However, let’s say that he has since come back and said that he actually needs 9000 gallons per day. No problem, right? We can easily input the new requirement into the Solar Selector.

SolarPAK example updated

Just given that amount of data, the Solar Selector automatically provides a wealth of information, starting with the average amount of useable sunlight that location receives each month. In our Amarillo example, we see that Amarillo receives an average of 5.35 hours of useable sunlight each day over the course of a year. Of course, this value is an average; it doesn’t account for specific weather conditions or real-time trends. But we can also see a monthly average. For example, Amarillo receives 4.65 hours in February and 6.31 hours in July. (Click the image above for a larger view in a new window.)

Although this information is interesting, what we really want to know is which SolarPak system is needed. The Solar Selector figures this out automatically. In this case, the Selector has specified model 25SDSP-3.0HP. Although you actually don’t need to make the translation, this means a 25 GPM unit attached to a 3 horsepower motor.

Below this you can see the monthly performance of the system. For example, in October this installation is projected to be able to deliver 9102 gallons per day, based on the system capability and the average solar hours.

In summary, all we’ve done is provide the location and water requirements. The Solar Selector has done the rest, telling us which SolarPak product is right for this installation.

There’s one more piece of information that the Solar Selector needs: the electrical characteristics of your panel. These values are specified by the panel manufacturer and in this example, the panel manufacturer tells us the each panel delivers 250 watts at its maximum power point (Wmpp), its open-circuit voltage (Voc) is 37, and the voltage at the its maximum power point (Vmpp) is 30. Note that these are for each individual panel. (For an explanation of these terms, see the post Solar Power for Subs: The Panels.) Given this information, the Solar Selector tells us how many panels we need and in what configuration. In this example, we need a total of 10 panels connected in a single parallel string.

There you have it. From three pieces of information – location, water requirements, and panel characteristics – Franklin’s Solar Selector has done all the work to spec our SolarPak and our array. In the next post, we’ll discuss connecting panels in parallel versus connecting them in series.

Solar Power for Subs: Sizing the System

In the third post in our series on solar pumping systems, we’ll start our discussion with sizing a submersible solar pumping system. With today’s web tools, sizing is usually remarkably easy, as most solar manufacturers offer some type of online sizing tool. For our example, we’ll use Franklin Electric’s SolarPak Selector. To find it, go to www.franklin-electric.com/solar, which will take you to Franklin’s solar home page.

Solar sizing1

From here, click on “Solar Selector”. You should see the following page:

Solar sizing2

There are three things we need to specify on this page in order to properly size our solar pumping system:

1. Where is the installation located? (This will help determine how much sun is available.)
2. How much water is needed in terms of pressure and volume? (Just like a conventional system.)
3. What are the electrical characteristics of my solar panels?

In the first step, the system calculates how much sun should be available based on the latitude and longitude of the installation. Chances are, you don’t have those numbers handy, but that’s not a problem. Although you can enter the latitude and longitude of the installation directly, an easier way is to select “Look up Your Latitude and Longitude”. With this option, a map will pop up and give you two options: 1) move the crosshairs on the map to the location or 2) simply enter the name of your location in the box at the top of the page and the SolarPak Selector will do the rest. If you do enter the latitude and longitude directly, don’t forget that in the Western Hemisphere, longitude is expressed as a negative number. You also have the option to use the device’s location (iPad, laptop, etc.). As soon as you open the SolarPak Selector page, you should see a pop-up menu that asks for permission to track your physical location, as you can see on the bottom of the screen shot above. If you allow this, your coordinates will automatically load into the SolarPak Selector. Of course, your device should be somewhat close to the installation’s location for this to work properly.

If you choose to use the map to look up your coordinates, you should see this screen:

Solar sizing3

For our example, we’ll use Amarillo, Texas, which we would enter into the box at the top left of the screen.

Solar sizing4

The SolarPak Selector calculates that on average, Amarillo receives 5.35 hours of usable sunlight each day for our solar pumping system.

Solar sizing5

Step 2 (and actually, the order here doesn’t matter) is to specify how much water we need per day and at what total dynamic head. Note that our volume requirement per day can be expressed in cubic meters (m3), gallons, or liters. The drop down box allows you to select your unit of measure. Similarly, Total Dynamic Head can be expressed in meters, feet, or PSI, again specified with the adjacent drop down box.

For our example in Amarillo, we’ll say that on average, we need 900 gallons per day at 200 feet of head. We enter this into the Output Requirements section on the left of the SolarPak Selector page.

Solar sizing5

Once we enter this information, the SolarPak Selector goes to work and provides us with a wealth of information, including which Franklin system is recommended for this installation.

The only input we haven’t covered is solar panels. Simply enter the manufacturer-listed Wmpp, Vmpp, and Voc values (covered in our last post) into the boxes provided in the section called Solar Panel Characteristics. The SolarPak Selector will do the rest, helping you define your array.

That’s all the information the Solar Selector requires to size the system. In our next post, we’ll move on to what information the Solar Selector does with that information.

Solar Power for Subs: The Panels

When it comes to solar-powered pumping systems, they all start with the panel. After all, it’s the panel that captures the sunlight needed to run the system.

How it works

Solar-cell2The electricity to run the pump and motor in any solar-powered system resides in the property of certain types of silicon crystals to produce a small amount of DC voltage when exposed 111022-N-OH262-322to light. This is called the photovoltaic effect, and it is often just abbreviated PV. The term photovoltaic or PV system simply refers to a solar system that generates electricity using this property.

When silicon crystals are connected together, they can generate useful amounts of electricity. One unit of these connected crystals is called a solar cell, and dozens of cells are contained in a single solar (photovoltaic) panel.

How to spec a solar panel

Several different variables are associated with solar panels, but for a pumping system, we only need to consider four values: Voc, Vmpp, Impp, and Wmpp.  Voc stands for open circuit voltage and is exactly what the name implies. That is, with no load (zero current being delivered), the array will generate this amount of DC voltage. This is similar to measuring AC voltage from the power company in a conventional water system when the motor is not running.

Unlike AC power from the power company, however, once we start to pull current (amperage) from the array (to drive a motor for example), the amount of voltage produced will start to fall off as the amount of current increases. This is remarkably similar to a pump curve. That is, Voc can be thought of as the shut-off head.

As we move down the pump curve delivering water (GPM), the pressure drops accordingly. At a point about midway on the curve, the pump will deliver its maximum horsepower. In the case of a solar cell, the amount of power being delivered will simply be the voltage multiplied by amperage:

power (in watts) = voltage X amperage

This point where the most power is delivered is denoted by Wmpp, or maximum power point watts. It’s also sometimes called just Pmax or maximum power. The voltage and current at this point are called Vmpp and Impp for maximum power point voltage and maximum power point current, respectively.

All panel manufacturers provide the values Voc, Vmpp, Impp, and Wmpp for each of their panels. For example, in the data sheet excerpt below, Vmpp is 32.1 volts and Impp is 8.42 amps. Notice that when those two are multiplied (32.1 x 8.90), it equals 270 watts.

SolarWorld Sunmodule™ solar panel 270 watt mono black data she

The values above are at standard conditions. Since the amount of energy produced by a solar panel is dependent on the amount of light striking it as well as the ambient temperature, the industry has defined a standard set of conditions to ensure that different panels from different manufacturers can be compared side-by-side. In real-life conditions, the actual values will be somewhat more or less than listed by the manufacturer.

Today, dozens and dozens of companies manufacture solar panels. In most cases, your local distributor can probably make some recommendations.

What’s next?

Although the values above are for a single panel, most solar pumping systems will require more than one. The question then becomes, “for my given water requirements, how many panels do I need?” With today’s web tools, calculating this is quite easy. We’ll cover this topic in the next post of Franklin AID.