Enphase Power pushes solar inverter, battery expertise to extremes

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This sixth-generation Enphase micro-inverter is built to last 25 years, work with battery storage devices and send sophisticated data to distant servers in the cloud. (James Dunn / North Bay Business Journal, Feb. 9, 2017)

Enphase’s new storage units, touted as “AC batteries”, are based on lithium iron phosphate cells.

Like other batteries, they store electricity as direct current. The 1.2 kilowatt-hour batteries cost about $ 2,200, weigh 53 pounds, and can discharge up to about 7,300 cycles and up to zero percent of capacity without damage. The modular structure enables expansion to optimize energy consumption in relation to system costs.

With four batteries, a typical household can store the energy it needs for a rainy day or clouds with minimal solar energy. To survive a rainy week without the sun without mains electricity, around 20 batteries would be required – prohibitively expensive. With typical use, the batteries will last about 10 years.

Raghu Belur, engineer and co-founder of Enphase (Nasdaq: ENPH), worked for Cerent, a Petaluma optical equipment telecommunications company that Cisco Systems acquired in 1999 for $ 7.2 billion. Years ago, an efficient inverter in off-grid systems would lose 10 percent of its power when switched from direct current to alternating current, Belur said. Enphase increased the inverter’s efficiency to almost 97 percent.

If each panel is equipped with its own inverter and immediately switched to alternating current, fire protection is increased and the failure of one panel does not have a major impact on the entire system.

“You get huge benefits from keeping them separate,” Belur said, and for switching to AC.

“We put so much intelligence into these devices,” said Belur.

Each panel and each battery act as an Internet of Things appliance and transmit power generation and temperature statistics to the cloud. “It is involved in the reliability of the network.” The system can optimize usage by drawing from the battery during times when utility power is expensive and from the grid when that source is cheaper.

Since solar collectors last around 25 years, the inverters need a comparable service life. The micro-inverters are attached to the back of the panels and are exposed to high temperature, moisture and wind loads.

Laboratories send new versions of the micro-inverter through extreme temperatures, as well as through high humidity and ultraviolet radiation. The inverters sit about an inch below the back of the solar panels on the roof. The inverter housing is filled with a substance that dissipates heat from sensitive electronics and keeps moisture out. The outside of the case has angled surfaces to reduce ice build-up.

‘The failure rate must be very low. We put these devices under 15 feet of water, ”he said with a pressure cooker. The devices operate at minus 60 degrees Celsius (minus 76 Fahrenheit) to 100 degrees Celsius (212 Fahrenheit) with antifreeze added to the liquid. The test lasts 21 days while driving at full power.

“You can’t test them for 25 years. They speed up the stress to simulate long-term, Belur said.

An app on his mobile phone shows him how solar panels on his roof cover or fall below the energy requirements of his house. If there is a net energy deficit, he or his wife may decide to run the washing machine later or they may reduce the winter thermostat setting by a few degrees.

Conventional inverters are called string inverters. Photovoltaic modules are connected in series, then electricity is sent through the inverter to power a home or business. A faulty module – covered in dirt or leaves – can affect the performance of the entire string and the entire system suffers.

With Enphase, each solar panel is independent of others. Everyone maximizes their production with the help of the micro-inverter.

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