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.

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.

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).

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.

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: 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

The 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 to 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.

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.

Solar Power for Subs: Part 1

Harness the power of the sun; over time, that age-old dream has become a reality. With solar technology, we can use the sun’s energy to do work, to move things and drive machines. Today several manufacturers, including Franklin Electric, offer solar pumping systems that harness the power of the sun to get water from the ground—even where there’s no power grid.

Over the next several weeks, Franklin AID will examine in detail how these systems work, their sizing, their proper application and set-up, and their advantages and disadvantages. By the end of this series, you will have a very complete understanding of where solar pumping fits into your product portfolio, along with how to install one of these systems.

Why Solar?

To begin with, why use a solar pumping system? There are two simple answers: 1) because there’s no power where the water well is located, and 2) because the power is free. But there can be other reasons to use a solar pumping system that may be less obvious. Perhaps power is available where you need water, but it’s unreliable. If that location is remote, the end user may not even be aware when the power is off. Couple that with a critical water supply and the simple reliability of solar pumping can make a strong case for it. A prime example here is open range livestock.

In other cases, there may be power nearby, but the well is several hundred feet away from the power source. The cost of trenching and running copper to the wellhead could often outweigh the cost of a solar pumping system. In other areas, electrical power may available but very expensive. Since the sun shines for free, solar simply makes the economic sense.

Finally, some people want to use greener products as much as possible and nothing fits the bill better than solar. Marketing reasons can even enter the picture. For example, organic farm operations that market themselves as such may want to use solar pumping for irrigation in order to be able to promote this to their customer base. In such cases, electricity from the grid may be readily available.

Why Not Solar?

Of course, solar pumping by definition has a big drawback: it needs the sun to operate.  As a result, pumping systems that are strictly solar can’t deliver water 24 hours a day. But irrigation normally isn’t required 24 days a day, and in other applications, a storage tank or cistern may store more than enough water when the sun is down or the weather doesn’t cooperate.

In a residence or business, however, we do want water to be available under pressure 24 hours per day. So although stand-alone solar pumping systems aren’t likely to completely replace conventional residential systems, that doesn’t mean they can’t be combined with battery-powered systems or traditional grid systems. We’ll cover these options later.

In summary, where solar systems really come into their own are in applications such as livestock watering, irrigation, vineyards, and anywhere a grid power is unreliable or nonexistent. That’s a pretty exciting development for our industry.

Stay tuned for the next post in this series, where we’ll cover the starting point of all solar pumping systems: the panels.