Inverters for solar, storage, and EVs: Same genus, different species

Aug 09, 2016

Inverters for solar, storage, and EVs: Same genus, different species

By Martin Fornage, CTO, Enphase Energy

With recent news about the potential Tesla Motors takeover of SolarCity, there’s been a lot of discussion about whether they’ll design and manufacture inverters as part of the vertical integration strategy. Business strategy aside, it’s raised a number of questions about how similar or different inverters for solar plus storage are compared to inverters for electric vehicles.

I’d argue that they are different enough that having motor drive expertise creates little or no advantage, and the engineering expertise required to pull this off means that a whole new team will be required, regardless of whether they go for a string, string with optimizer or certainly for a microinverter topology.

The most fundamental difference is in the mission of the two inverters: The mission of a motor drive is, simply, to drive a motor; a solar inverter’s mission is to drive a component of the grid, so it needs filters to make sure that the current is clean.

These different missions drive a different component set that leads to fundamentally different design. An EV has six or more switches (insulated-gate bipolar transistors or IGBTs), many hundreds of amps of input current, and a relatively simple controller. A solar inverter is more complex, despite having fewer IGBTs and lower amperage input: it needs boosters and filters and that changes the underlying design. An overview of some of the other differences in the table below shows that it’s not a simple case of expertise in one type of inverter translating to expertise in the other.

 

Motor drives

Residential solar inverters

Mission

Drive a motor

Inject energy on the grid

Cost

Less important

Critical

Efficiency

Important (>90%)

Critical (>96.5%)

Volume

15,000/quarter

15,000/week (string) or 15,000/day (microinverters/optimizers)

Reliability

Important (10 years)

Important (10-25 years)

Power

100-300kW

1-10kW

Components

More switches

No input booster

No filter

Standard chips

Fewer switches

Booster needed

Input and output filters

Grid-decoupling devices

Custom semiconductors for MLPE

Control parameters

Motor torque and RPM

Output current, distortion

Density

Very important

Not important

Interface

DC bus (battery) and motor, ECU

PV (MPPT), grid, potentially storage, monitoring

Cooling

Liquid

Convection

Operating condition

Very harsh, high temps and vibration

Not as harsh, no ongoing vibration

Duty cycle

Little amount of time is spent at full power

A lot of time is spent at full power

Certification

Radiated emissions, whole vehicle assessed on performance

Safety (UL), grid codes, radiated and conducted emissions

 

It’s an exciting time to be in the inverter industry. The emergence of storage and advent of various energy management technologies are creating the need for even smarter systems of systems, and the inverter is central to this energy solution’s success. Doing it well is no small feat, and it will be interesting to see how the inverter world grows as EVs are added to the mix.

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