Building a Grid-Tied Solar Electric System

This shows how I built a solar electric system in New Hampshire. This grid interactive photovoltaic system is also sometimes called grid-tied, grid-interactive, or net-metered. It consists of an array of solar panels, a charge controller, an inverter, a battery array, and circuit wiring and protection.

I have had a stand-alone battery system for about two years. One problem I noticed was that my batteries would charge fully by late morning on sunny days. This meant that energy production for the second half of each day was wasted. My thinking was that the electric grid in a grid-tied system would act like a giant battery that would never fill up. It would allow me to make electricity at full capacity all day long.

The downside of such a system is that many grid-interactive systems shut off automatically whenever the grid goes down. This is necessary for safety purposes. If utility power was cut for some reason and your solar panels were still putting out 120 volts at 60 hz AC out onto the lines, you could electrocute a lineman who unknowingly works with live wires. 

Fortunately there is at least one grid-interactive inverter that allows the use of a battery storage system and it powers dedicated loads that are isolated from the main electrical service. You need to run separate circuits, but these will stay on even after your utility power shuts off. The inverter I chose to accomplish this was the Outback GVFX3648. This is a 3.6 kw inverter that uses a 48 volt dc battery system.

People looking to install their own grid-tied systems have to follow rules promulgated by their local state authorities. These regulate how to apply for permission to generate electricity, how the system is wired, electrical testing, and other issues. The rules for New Hampshire residents say that installations must be constructed according to local building and electrical codes, which for me, means the National Electrical Code (NEC). Work will need to be inspected. I called upon my friend, Mike Meehan, a licensed electrician, who kindly supervised my work, answered questions I had about code compliance, and inspected my installation. Public Service Company of New Hampshire, the state electric utility, offers a summary and guidelines for the application process. PSNH was extremely helpful and responsive throughout my application and construction process.

An essential resource titled,  Photovoltaic Systems and the National Electrical Code: Suggested Practices, by John Wiles, is available as a download from New Mexico State University. It is important to note that at this time, there are not many people who are knowledgeable in the area of building grid-tied photovoltaic systems. Licensed electricians, who know a great deal about conventional residential and other electrical applications, will likely not have a great deal of experience installing solar electric applications, especially in the cloudy Northeast. The suggested practices given above is valuable because it draws on the knowledge of people experienced in both solar installations and the NEC.

Solar Panels

An early consideration is what is needed in the way of solar panels. I have experimented with making panels from scratch, that is, obtaining a number of silicon cells. soldering them into an array and mounting them on a glass covered foundation. This is a valuable educational experience, and one can make a sizeable electric system this way, but there are a few big hitches. First, it is expensive and nearly impossible to "roll-yer-own" for less than it costs to buy factory manufactured panels. Second, it is very hard to match the quality of panels manufactured with an EVA/Tedlar encapsulation process. Third is the guarantee. Who pays for replacement of panels you make yourself? Most manufactured panels have a 20 year replacement warranty that covers any panels that fail to produce 80% of their rated power. 

To explain just a little about EVA and Tedlar encapsulation, these are types of plastic. EVA, or ethylene vinyl acetate,  is a clear high transmission plastic resistant to degredation from ultraviolet radiation. Tedlar is a durable polyvinyl fluoride plastic. In the encapsulation process, the silicon cells are wired together and sandwiched between thin layers of these two materials. They are baked in an oven and vacuum sealed forming a laminate, impervious to weather and moisture.

The panels used in this project are a combination of two different brands, The six panels seen on the left are Matrix Photowatt 1650 165 watt 12/24 volt panels. The four on the right side of the photo below are Alps ATI1650 165 watt 12/24 volt panels. Electrically, all the panels are equivalent. For those interested, AlpsTechnology panels are available from Altenergyweb.com. We have a Solar Panel Estimator that will help you decide how many panels are needed for an installation to meet a given demand.

Maybe you can see a slight difference in appearance of the two brands of panels. This is because they use different types of silicon cells. The Photowatts are made using polycrystalline cells that are a crystalline sheet of silicon. The AlpsTechnology cells are made using monocrystalline cells that are thin cross slices of a single large silicon crystal. Monocrystalline cells typically have slightly higher output than polycrystalline cells. In the real world, I've observed that these Photowatt panels have slightly higher output in cloudy conditions; the Alps have slightly higher output in sunny conditions, "slightly" being the operative word. There really is not much electrical difference. The difference is in the looks, as can be seen in the close-ups.

Polycrystalline Photowatt Cells (above)

 Monocrystalline AlpsTechnology Cells (below)

Wind is the enemy when mounting solar panels. You must build a solid mount to protect the sizeable investment. I opted for a ground-mount rather than a roof-mount because I was concerned about snow removal. I have a broom with a long handle for clearing the panels of snow. The job would be considerably more difficult if the panels were mounted up on a roof. I have talked with one owner in southern New Hampshire (where they don't get as much snow as I do) who says that snow accumulation is not a problem for his roof-mounted solar panels. He told me that snow melts quickly off them. The issue of roof-mount versus ground-mount in snowy locales deserves more investigation. Design and construction details for a ground solar panel mount can be found by following the link.

The panels are secured to treated wood cross members from the front and the back. Around the edges of the aluminum frames, welding nuts are used as clips over the edge of the frames. Between the bottom and top rows of panels, 3/8 inch bolts tighten down on large fender washers, which hold the panels to the members. One must use care to tighten these fasteners snugly, but not too tight, which increases the risk of bending the aluminum frames or even breaking the glass covering. 

Welding nut serves as a clip

Bolts and Fender Washers 

1/4 inch bolts attach the panels from the rear. One bolt in each corner.

Panel Interconnections

The Alps ATI1650 and Photowatt 1650 panels can be configured to put out 12 volts or 24 volts. Jumper wires (the red ones in the photo below) are placed accordingly inside the junction box, also called the J-Box, on the back sides of the panels. It is hard to tell from the photo, but the J-Box has a hinged cover that is swung open to give access to the inside terminals. After all connections have been tightened the cover is screwed shut, making a waterproof seal.

You might be able to pick out the four schottky diodes above the screws in the J-box. The purpose of these diodes is to prevent backfeeding through rows of cells, which can occur under conditions of shading. Even a shadow from a fallen leaf or a person standing in front of the panels can reduce output and allow other illuminated cells that are still putting out full current to feed through the darkened cells. Depending on the amount of current, this can cause overheating of the thin ribbon conductors. It can get hot enough to melt solder and burn the plastic encapsulant. The Schottky diodes, which have very low voltage drop, prevent this damage from occurring.

It is necessary to choose which voltage you wish to use in your system. Generally, higher is better because the higher the voltage, the lower the current will be in the conductors between the panels and the inverter. You want to keep current low to limit line losses as much as possible.  You are limited by the voltage rating of your equipment, however. The Outback GVFX uses the MX60 MPPT charge controller, and it's maximum allowable open circuit input voltage is 150 vdc.

I chose to configure my ten panels for 12 volts and wire them in two strings of five. With five panels wired in series the nominal operating voltage would be 60 volts. The open circuit voltage would be about 108 volts. The diagram below shows how the panels were wired.

The wire used to interconnect the panels was #10 AWG copper RHW-2. This is a single conductor stranded service entrance wire. The insulation is sunlight resistant, and is temperature rated to 90 degrees C., and can be used in wet or dry conditions. RHW-2 also has fire retardant properties that permit its use in buildings as well as outdoors. An equipment ground conductor of #6 AWG stranded bare copper wire is used to connect the aluminum panel frames and other equipment to an 8 foot grounding electrode that is driven into the earth. 

This grounding electrode is driven 8 feet into the ground.

UL Listed CU/AL lugs are used to fasten the ground wire to the panel frames. A lug is bolted to the back of each frame. A set screw clamps the ground wire, which is threaded through each lug.

To keep the cables from flopping in the breeze, possibly damaging their insulation, adhesive backed cable-ties were used. 

An outdoor rated electrical box, called a combiner box is use to combine the separate conductors from each string of five panels. The positive conductors from each string has a 15 amp dc rated circuit breaker. The two lines are connected together after the breakers and #6 AWG RHW-2 copper conductors carry the current to the residence.

The Midnite Solar combiner box connects the two strings of five panels together. It has two dc rated 15 amp circuit breakers within. The object attached to the left side of the box is a Delta LA302DC lightning arrestor. Exiting the box through the rigid PVC conduit at the bottom are three conductors: the bare ground conductor, the positive conductor and the negative conductor from the PV array. These last two are RHW-2 stranded copper. All are AWG #6.  These wires lead to a switch mounted on the side of the residence near the electric meter. This switch serves as the main PV disconnect where the conductors enter the building.

Notices and warning labels are a requirement and must be affixed on the meter, the disconnect and on the side of the building near the meter. The purpose is to warn electrical workers and emergency personnel, most notably firefighters, that disconnecting the meter will not shut off all power within the residence.

Because the RHW-2 wire has fire resistant properties, it can be used inside buildings as well as outside, so from the main PV disconnect switch, rigid PVC conduit ran the conductors through a crawl space into the cellar where the inverter and batteries are located.

To the right is the Outback GVFX3648 inverter mounted on a Midnite Solar Epanel. The Epanel is a box that has dc and ac breakers and terminal lugs that facilitate a code compliant connection of panels, batteries, inverter, the main ac service panel, and dedicated ac loads that will remain on even when utility power is lost.

Attached to the lower left of the Epanel is an Outback MX60 MPPT charge controller. This device takes the dc power from the solar panels and converts it down to match the voltage needed by the batteries. MPPT stands for maximum power point tracking, and is a highly efficient way to utilize solar panels for battery charging. A MPPT controller periodically "sweeps" across a range of operating voltages to determine the optimum voltage at which the solar panels will deliver the greatest number of watts of power. The controller then steps the voltage down to match the voltage needed by the battery bank. It does this while maintaining approximately the same wattage as the input, and this results in a current boost. The panels may be delivering ten amps of current, but the controller is converting it to 20 or even more amps that it delivers to the batteries. In general, you will see the most boost when you can maximize the difference in voltage between the solar panels and the battery array. 

Above the MX60 is an Outback Hub 4. This device is like a computer network hub to which the GVFX, the MX60, and the Outback Mate is attached. The Mate is an electronic display that shows the various modes and states in which the inverter operates. It allows settings to be made and reports error conditions.

Outback inverters are designed to be used with a bank of batteries. Other brands of grid-tie inverters are available that are batteryless, that is, they will convert the solar dc directly into 120vac for grid consumption. This batteryless design is slightly more efficient than a battery-inverter system, I understand, but there is at least one drawback. That is, when the grid goes down, you go down -- even if the sun is shining brightly.

The four 12 volt marine batteries in the photo above are wired together in series to make an array of 48 volts, the voltage needed by this particular model of inverter. The batteries are nothing special. They are 125 amp hour Everstarts, the brand sold at Walmart. 2/0 copper THHN cable was used for the battery connections. Terminal lugs were crimped and soldered. These UL listed lugs were obtained at a local NAPA auto parts store.

The cables enter rigid PVC conduit that leads to the Epanel where the positive conductor is connected to a 175 amp dc disconnect breaker and the negative conductor is connected to a shunt and eventually to the negative terminal of the inverter. 

The stand on which these batteries sit is one area I want to improve upon. Batteries can be very dangerous. They give off explosive hydrogen and oxygen gas during charging; they are filled with concentrated sulphuric acid that can cause injury andpermeate the air around the batteries, causing corrosion; they can discharge thousands of amps of current in a short circuit, causing cables or tools to melt, literally spewing molten metal.

The battery terminals need to be covered and the batteries enclosed to prevent accidental short circuits across the terminals. The enclosure needs to be vented to allow hydrogen to dissipate. Hydrogen is lighter than air and it will dissipate readily if given a chance. When designing and building battery enclosures, keep in mind that conduit openings should be lower than the tops of the batteries to prevent hydrogen from traveling up into the conduit and on to the electrical equipment. In the same vein, the battery enclosure should not be directly beneath the inverter and the associated electrical panel. This not only prevents the chance of fire, but also of corrosion because hydrogen from batteries can carry droplets of battery acid with it.

RANDOM THOUGHTS - a list of thoughts on construction of a grid-tied solar electric system. I'll add thoughts pertaining to construction as they come to me or as readers write and ask about the project.

1. Wire used to interconnect panels can be single conductor service entrance cable (type USE or RHW) that is sunlight resistant, or type UF underground cable. It should have a temperature rating of 90deg. C. 

2. The NEC says one conductor needs to be grounded. Usually the negative conductor is the one that is grounded. The connection of this conductor to the ground should be at one single point.

3. Use standard color code for the conductors that run between the panels and the inverter: Black is positive, white is negative. A bare or green insulated ground conductor should also be run. The ground conductor should be the same size as the current carrying conductors.

4. When a GVFX3648 inverter is used, four batteries are wired in series to make an array with an output of 48 vdc. Make cables with 2/0 THHN copper wire and UL listed crimped battery terminals. These terminals are fairly easy to find in better auto parts stores. Cables made in this way will meet the code requirements only if a proper crimping tool is used. I found a local machine shop that was able to crimp the terminals onto the ends of the cables for me. 

5. A load break test is required by the utility company prior to permanent operation. To conduct this test, wait for a day when the inverter is selling electricity constantly. Place a volt meter at your service entrance panel on the breaker terminal to which the AC HOT IN wire from the inverter is attached. Open the service entrance panel main breaker to disconnect all power from the building. To meet NH regulations the inverter must shut down within two seconds of the main breaker disconnect. The measured AC voltage at the terminal should therefore drop to zero volts within two seconds. If the voltage does not drop, it means the inverter is energizing the service entrance panel and electric power would go out onto the grid if the main breaker were closed. Either something is wired incorrectly or the inverter is malfunctioning. Either way, it poses a safety risk for linemen who unknowingly work with energized powerlines. 

6. If you set up a GFX/MX/Hub/Mate system and you have troubles with the Mate not displaying according to the manual, or reporting ac voltages that seem really wrong, check that the inverter is plugged into port #1 of the Hub and the MX is plugged into port #2. I had mine reversed initially and it led to crazy readings. 

 7. If you need solar panels, click here .

(to be continued) 

tags: solar  grid tied  net metering  

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