Myths and Legends of Microinverters and Other DC Power Converters for Solar Energy

Principal Solar Institute

Dan LepinskiNorth Texas Renewable Energy EngineeringDan has been professionally involved in solar energy for more than 40 years. He founded and for 15 years ran a unique business involving several cutting-edge technologies: telecommunications, computers, and solar energy. He also spent more than a decade as a Senior Engineer designing inverters for off-grid and grid-tied solar applications.

Dan now consults in the solar energy and energy conservation industries.

Myths and Legends of Microinverters&

Other DC Power Converters for Solar Energy

Questions and Discussion

Dan LepinskiNorth Texas Renewable Energy [email protected]

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Questions and Discussion

Introduction:

UL 1741 / IEEE 1547 Standards Technical Panel

Solar America Board for Codes and Standards ("Solar ABCs")

Electric Power Research Institute Smart-Grid Development Committee

Solar Industry National Electric Code Work Group (NFPA-70) (Sections 690 and 705)

Honoree - Texas Solar Energy Industries Association Special Achievement Award for Excellence and Accomplishments in the field of Solar Energy.

Questions and Discussion

Topics Today:

}Microinverters

AC Modules

DC Optimizers

String Inverters

Central Inverters

We'll discuss a few ...

Facts. Myths. Legends.

Costs. Choices. Options.

… Present - and Future.

Questions and Discussion

1. Basics:Microinverter: < 400W (typ). Stand-alone inverter, usually attached to the PV frame or mounting rack. AC output.

AC Module: PV-embedded microinverter. Pre-attached to the PV by the manufacturer. AC output.

Optimizer: Connects to PV, similar to a microinverter. Adjusts individual PV output current to match the array. DC output.

String Inverter: 2 - 30 kW (typ). AC output.

Central Inverter: Large! "Utility-scale". 100kW - 1.5 MW+. AC output (typically 3 phase).

Questions and Discussion

Microinverters AC Modules

General Product Applications

Incrementally-sized systems. Area unsuitable for string system. Fill in empty space. Customer wants them.

Same as micro-inverters .. but .. ... generally less labor involved. No concern for perfect PV match.

Questions and Discussion

Because ...shade happens.

(See the birds?)

There are also other causes…

Why Use Microinverters, AC Modules, and Optimizers?

Questions and Discussion

Multiple string inverters can connect to a circuit.

String Inverters .. One … or many:

Questions and Discussion

Large systems. Multiple PV arrays connected. Three-phase.

Central Inverters:

Questions and Discussion

Some what you hear about inverters, AC Modules, and optimizers is marketing hype.

2. Myths .. Legends .. and Facts.

“Truth” or “Fiction” Also Depends on Whom You Ask.

Questions and Discussion

#1: Will electrolytic capacitors fail prematurely?

Other manufacturers say “yes”, and tell us “film” capacitors are superior.

Some manufacturers say “no”.

First, what is meant by “failed”?There’s a difference between “failed” and “worn out”.

Myths .. Legends .. and Facts.

Questions and Discussion

Fact: A quality electrolytic capacitor utilized within its ratings and specifications will work in rooftop inverter and converter applications up to 70 years in Phoenix, Arizona heat. (A “worst-case” environment!)

Electrolytic capacitors are also much smaller and MUCH lower cost than “film” capacitors.

Myths .. Legends .. and Facts.

Questions and Discussion

Electrolytic

22 uF, 400VHeight 22 mmDiameter 16 mm

Volume 4,423 cu mmCost* $0.36 ea (5K)

Film

20 uF, 400VHeight 37 mmWidth 22 mmLength 32 mmVolume 26,048 cu mmCost* $10.69 ea (5K) * Source: Digi-Key, Apr 02, 2014. www.digikey.com

Myths .. Legends .. and Facts.

Questions and Discussion

Fact: In systems without shade, and all PV facing the same direction and angle, the performance difference among products is limited to the worst-case tolerance in the photovoltaic modules.

#2: Microinverters and AC Modules Are More Efficient Than String Inverters.

Myths .. Legends .. and Facts.

Questions and Discussion

#2: Microinverters and AC Modules Are More Efficient Than String Inverters.

Systems that experience unavoidable shade may see up to 30% increased output, depending on the severity of the shade. (Try to avoid shade in the first place!)

Myths .. Legends .. and Facts.

Questions and Discussion

#4: Microinverter systems are more efficient than string systems.

#3: Optimizers with string inverters are more efficient than microinverters.

#5: Microinverters have a 25 year warranty. This means they are more reliable than string inverters.

Myths .. Legends .. and Facts.

#6: Microinverters and/or AC Modules can completely replace string inverters and central inverters.

Questions and Discussion

Problem is, we have a paradox:

Downward price pressureon inverters and other BOSproducts, with simultaneousupward pressure on life expectancy,product performance, and warranty.

2014 National Electric Code changes will also add cost.

3. Decreasing the Cost…

Questions and Discussion

In the average residential system, inverters represent only 5.7% of the total cost.

Still .. any cost reduction helps.

So what can be done?

Decreasing the Cost…

Graphics copyright by and reprinted here by permission of Rocky Mountain Institute, www.rmi.org Data from 2012.

Questions and Discussion

#1: Manufacture high-voltage (250V+) PV for microinverters and AC Modules. Very do-able in film PV. (Difficult to achieve in a cost-effective manner in crystalline PV.)

This would eliminate the DC -> HV circuit, which represents 50% of the total parts, and includes some of the more expensive components in microinverter / AC Module products.

Net benefit: more than 50% decrease in cost, with increased efficiencies up to 98.5% or better. This may also make possible use of plastics for enclosures, such as are used by optimizers.

Decreasing the Cost…

Questions and Discussion

#2: Make the BOS hardware as “NEC Code-friendly” and “installer-friendly” as possible.

#3: Consider designs that eliminate the PV junction box, bypass diodes, and cables, and use the inverter in its place. At least one optimizer company is already doing this. Inverter manufacturers can do likewise.

These components represent approximately 7% of PV cost today. In the world of manufacturing, this is significant!

Decreasing the Cost…

Questions and Discussion

What can we expect from future inverter and optimizer products? My “crystal ball” shows…

Neighborhood “Solar Gardens”. Active power-factor correction. Energy storage for surge peak abatement and more. Intelligent load control by energy-producing devices.

5. Looking Ahead…

Questions and Discussion

“Smarter” use of the power we generate and use. Improved energy storage efficiencies. Hybrid on-grid / off-grid systems. Intelligent load control by energy-producing devices. Feed-in tariff rates matching value to time-of-day energy

production by PV systems.

Looking Ahead…

Questions and Discussion

Questions?

Dan LepinskiNorth Texas Renewable Energy [email protected]

Thank You.