Launch Slideshow

Teaming PV with Heat Pumps

Teaming PV with Heat Pumps

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    ReVision Energy project manager Brandon Bernard explains the control center of the rooftop solar array his crew is installing. Bernard’s hand is on the direct current disconnect switch, which sits between the high-voltage direct current feed from the roof to the inverter. Above that switch is the inverter itself, which converts the incoming DC power to alternating current and feeds the AC power into the electrical sub-panel at right. In case of a utility power outage, this model of inverter, a transformerless SMA Sunny Boy 4000TL, can also supply the two-plug receptacle mounted below it with as much as 1,500 watts of power (depending on available sunlight). When power from the street fails, however, the inverter will only supply this dedicated outlet—it will not feed any power to rest of the house or back to the grid, for the safety of utility line workers who may be working on downed wires

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    On the roof, electrician apprentice Mel Janarelli assembles a run of PVC conduit that will hold the grounding cable and direct-current conductors from two “strings” of photovoltaic modules (nine panels per string, for a total rated output of roughly 4,000 watts).

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    With the outdoor portion of the conduit run fully assembled, Janarelli and Bernard pull a 50-foot run of wire through the conduit—enough to reach from the rooftop solar array to the inverter inside the garage. The bundle of wires includes two sets of conductors (one for each string of panels), along with a heavy bare copper grounding cable that must be bonded to every mounting bracket on the roof.

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    On the roof, Janarelli tightens down hardware that bonds the copper ground wire to an aluminum mounting bracket for the solar panel array. All six mounting brackets must be bonded to the grounding cable, which will run continuously back to the direct current disconnect switch and be connected to the copper grounding system for the main house.

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    Before the crew begins to mount panels, Janarelli clips the “home run” conductors for the panel arrays into a groove on the aluminum mounting bracket. Each panel comes pre-wired with plug-in connectors so that the panels can be connected in series to create a “string.” When the strings are assembled, the home-run conductors will be connected at the beginning and end of each string, and DC power will begin flowing to the inverter in the garage.

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    At the junction box on the roof, Bernard connects the home-run cables for the 2 strings of modules to the conductors that feed the inverter on the ground. These wires will be closed into the junction box before the panels are set; current won’t start to flow through this connection until the home-run cables are connected to the panels after all other wiring is complete. (The modules themselves, however, are energized as soon as the sun hits them.)

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    With the connection made between the solar array’s home-run cabling and the wiring that will bring the DC current to the inverter, Bernard and Janarelli begin setting panels on the mounting rails. As each new module is added to the string, they connect it to the last module using the plug-in connections preinstalled on each module.

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    Bernard and Janarelli lower a module onto the mounting rail.

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    While the panels are being mounted on the roof, electrician Ted Houghton completes the indoor portion of the conduit run from the roof, using metallic conduit. When all the conduit is connected, the conductors and ground wire serving the roof array will get pulled through and connected to the DC disconnect switch at the inverter.

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    Electrician Ted Houghton prepares to connect a metallic conduit run to the DC disconnect switch.

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    While the electrical crew is installing the photovoltaic array and running conduit, two more ReVision technicians are installing the twin Fujitsu heat pumps that will use the power provided by the PV panels. Here, Dave Ragsdale purges and pressure-tests the system’s refrigerant lines and Andy Melrose connects the compressor to power.

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    ReVision Energy master plumber Dave Ragsdale examines pressure gauges connected to the refrigerant lines of a 12,000-Btu Fujitsu mini-split heat pump. The refrigerant lines operate at about 400 psi of pressure; Ragsdale is testing the system at 600 psi.

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    With the conduit run fully connected, the electrical crew continues snaking the wires that connect the rooftop solar array to the inverter. Here, Mel Janarelli feeds the conductors and ground wire into the conduit from outside the garage.

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    Inside the garage, project manager Brandon Bernard pulls the conductors through the wall as Janarelli feeds them in from the outside.

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    Electrician Ted Houghton and project manager Brandon Bernard pull wire through the final leg of conduit, preparing to complete the circuit from the rooftop panels to the inverter mounted on the wall.

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    Electrician Ted Houghton connects conductors to terminals in the direct current disconnect switch serving the system’s inverter.

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    With the circuit run to the disconnect switch at the inverter in the garage completed, project manager Brandon Bernard makes the final link, plugging the last module in the rooftop string to the home-run conductor on the roof. Now the circuit is energized and DC voltage will begin to flow to the inverter, which will make AC power for the house.

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    At the inverter disconnect switch in the garage, Bernard measures the DC voltage provided by the panels: 302 volts on string 1, and 305 volts on string 2.

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    Once Bernard has flipped the disconnect switch to energize the inverter, and flipped the breaker in the house electrical panel to connect the system to the AC house power, the inverter goes through its start-up check routine and begins to send power to the house. The green readout on the inverter indicates that the system has begun supplying 3,200 watts (about 3 kilowatts) to the house.

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    Bernard demonstrates the emergency-power capability of the Sunny Boy inverter by throwing the breaker serving the inverter at the house electrical panel, simulating a power outage. After switching on the emergency backup outlet, he plugs in a fluorescent job light—which works. With the panel putting out 3,000 watts on this sunny day, the emergency outlet could likely supply its maximum output of 1,500 watts as long as there continued to be sunshine.

Portland, Maine, contractor ReVision Energy started out as a solar energy company, installing solar water heaters and photovoltaic power arrays on rooftops in northern New England. But in the last five years, ReVision has opened up a new niche: along with the solar power, ReVision installs high-efficiency air-source heat pumps that can use the solar-generated juice to heat the house—with a three-to-one multiplier effect.

This month, JLC went to a jobsite in Portland to see a ReVision crew install a 4,000-watt solar array on the south-facing roof, and a pair of one-ton (12,000-Btu) Fujitsu mini-split heat pumps.

The solution depends on a "grid-tied" setup: An inverter in the home's garage converts direct current (DC) from the PV panels on the roof into alternating current (AC) synchronized with the power supplied to the house from utility power lines on the street. On long summer days when the solar panels make more power than the house needs, the house feeds power to the grid. During the winter, the utility supplies the home with power to run its heat pumps and meet other domestic needs. In effect, the house is using the electrical grid as a storage battery. The solar array is sized so that on a net annual basis, the PV supplies about the same amount of current that the heat pumps will use every year, which means that, aside from financing the installation cost of the equipment, the owners get the output of the heat pumps for free. Once the system is paid for, the house will be harvesting free heat from the roof for as long as the heat pumps and the solar panels last.

ReVision sent a crew of five technicians to the house: three to set and wire the solar panels, and two to install the heat pumps, requisite wiring, and refrigerant lines. It took two days for the crew to equip the house.