New Design Trends in Photovoltaic Converters

Alex Van den Bossche Ghent University,

EELAB Electrical Energy Laboratory

Sint –Pietersnieuwstraat 41 Gent Belgium

Outline

Introduction: Solar Energy: Concurrent Technologies?

PV Inverter overview of improvements

cost improvements

active components

approximate efficiencies

improve efficiency

topology

topology for large power

inductive components

system efficiency

suggestions for efficiency improvements

efficiency benchmark

improve reliability

direct DC use in vehicles

Solar Trackers

Conclusion

Solar Energy:concurrent

technologies?

Concentrated solar (CSP)

PV

Grid connected

inverters

PV

Stand alone use

AC or DC use?

converters

Solar thermal

-Water

-Air

-Absorption cooling

Stored

- Thermal

- Biomass

- Methanol

(With C02+ PV)

Introduction

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CSP Concentrated solar power

+ Classical thermal

technology

+ possiblestorage in

molten salts

- Sunlight:

70% direct,30% scattered

- CSP cost: 4.8$/ peak

watt?PV 1.5 $?

Fig. 1. A project in California, “catching the sun“

177 000 mirrors, 377 MW

Introduction

4/30

European Supergrid

Introduction

“The Medgrid together with Desertec would serve as the backbone of the European Supergrid”

DESERTEC

en.wikipedia.org/wiki/Medgrid 5/30

Surface use in PV desert area (Tamanrasset), horizontal.

200W/m2 [*]= 487kWh/m2/year

For 50% used in PV panels at 14% efficiency (PV + converter)

200*8760*10^4*0.14/3600/1000*0.5=

=341 MWh/ha/year

Surface use of energy from Biomass in fertile areas1ha, 19 ton max. dry material

(miscanthus, sugar beet)70 MWh thermal/year/ha

25 MWh/year/ha at 35% efficiency to electricityStill a high price for the possibility to store energy!

Introduction

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* http://www.umc.edu.dz/vf/proceeding/sigcle-2010/pages/themes/energies_renouvelables/Session%20III/35%20Fekih%20Sess%203.pdf

PV Inverter: overview of improvements

PV inverters,

converters,

Improve component efficiency?

Improve converter topology?

Improve reliability?

MTBF

MTTR

DC use?

Cost?

now 250-500 €/kW peak

Lifetime 10 years compared to 30 for PV panels

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PV Inverter: cost improvements

Cost of inverter = major part?

now 250-500 €/kW peak

Lifetime 10 years compared to 30 for PV panels

PV panel

Cost of ownership

1000

[Euro/kWp]

2000

Inverter

0 10 20 30time [Years]

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PV Inverters, active componentsTable 1. Three “well chosen” components

SiC MOSFETSCH2080KEC+ internal SBD

Si MOSFETIXFK64N50P

Si IGBTSGL160N60UFD

Ron [ ]25°C150°C

0.0800.150

<0.0850.216

0.0100.010 (125°C)

Vdrop [V] 0 0 1.1 (25°C-125°C)Vmax [V] 1200 500 600Qrr 25°C

150°C@Idcref

60nC 60nC*@ 10A

600nC2400nC**@25A

112nC420nC@25A

Etot [mJ]@ 25°C@125°C

I1.1

0.036 ***0.036 ***@300V, 10A

I1

0.360 ***1.44 *** @300V, 25A

I1.4

0.57 @25A0.725 @125°C@300V, 40A

Diode dropdiff R@100°C

0.9 V 0.06

0.6V0.004

0.8V @100°C0.02

Price/unit@100

25.75$ 9.19$ 7.23$

* the temperature influence of Qrr in SBD is almost negligible [10]** calculated on 4x more at 150°C*** calculated using (1) with K=2 and Vdc=300V

dcref

dc

dcref

dctotreftot

V

I

V

VEE

9/30

PV Inverters, active componentsTable 1. Three well chosen components

0 10 20 300

20

40

60

80

100

W

Psc I f( )

Ps I f( )

Pig I f( )

I

0 10 20 300

20

40

60

80

100

W

Psc I f( )

Ps I f( )

Pig I f( )

I

Fig. 3 Component losses at 25 kHz Fig. 4 Component losses at 125 kHz

Fast IGBT and ultrafast diode.

The SiC would like components in parallel, but at an even higher cost.

MOSFETS are good in soft switch topologies

Slow IGBTs (not shown) good slow switching multilevel topologies.

Vertical: Power in Watt, horizontal: current in A, 50% duty ratio, 300Vdc

sc = SiC MOSFET s= Si MOSFET ig=Si IGBT

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0

PV Inverters, approximate efficiencies

Fig. 5. Approximate efficiencies of parts in the converter for a total of 6% loss

An example of total losses = 6%, often 3 converters in cascade. from grid to PV:

1.5 % DC-DC conversion: adapting voltage

2 % galvanic separation (soft switch, but transistors with high Vpeak and Ipeak

2.5% DC-AC conversion (PWM switching loss and EMC filters)

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PV Inverters, improve efficiency?

Still improve active component efficiency?

Fig. 6. RCD (residual current device) to protect against leakage currents for safety.

- Less losses than galvanic separation

- Also ”Galvanic separation” still has some capacitor coupling

RCD

Inverter

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PV Inverters, improve efficiency?

Still improve active component efficiency?

Fig. 7. Using a combination of MOSFETs and IGBTs to reduce

switching losses,

- Fast MOSFETS with slow diodes

- Fast diodes in IGBT

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Without galvanic separation?

“Advanced ferrite material for photovoltaic systems”

http://www.powersystemsdesign.com/inverterefficiency1?a=1&c=4257

Problem in case of fire, risk of radio disturbance and residual current detectors

PV Inverters, improve efficiency?

Fig. 8. Converter to single phase without galvanic separation

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PV Inverters: topology

Fig. 9. Filling in converter parts in a scheme (for simplicity drawn in single phase)

11

2

3

1

http://solar.smps.us/grid-tie-inverter-schematic.html 15/30

PV Inverters: topology for large power

Fig. 10. 1.5MW Multilevel DC/AC with in large PV

Fig. 13. Line to line voltage before filtering

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PV Inverters: topology for large power

Fig. 11. 1.5MW Multilevel converter with boost converter in wind energy

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PV Inverters: inductive components

Better design of inductive components?

http://www.powersystemsdesign.com/inverterefficiency1?a=1&c=4257

Ferrite core components are good at high

switching frequencies sized at 3kW

109 × 55 × 115 mm³. (689cc) http://www.powersystemsdesign.com/inv

erterefficiency1?a=1&c=4257

18/30

PV Inverters: inductive components

Better design of inductive components?

Similar purpose, but amorphous cut core for input and for output

inductor sized at 3kW, the saturation inductance is much higher.

71 x 45.5 x 101.3 mm³ (327cc) half of the volume

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PV Inverters: system efficiency

The losses of wires, cabling, monitoring?

Use lower current/mm2 than in normal cabling. Typical 4mm2

up to 10A. One can compare investment in copper or more PV.

Avoid normal computers for measurements or monitoring

50W = 1.2kWh/day, = about 500W of solar panel installed for

nothing….

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PV Inverters: suggestions for efficiency improvements

Overview of improving efficiency?

Better design of inductive components

Better materials Nanocrystalline, ferrites, better understanding of excess losses (eddy current loss,

DC bias, gaps, mechanical strain etc…)

Lower switching losses?

MOSFET IGBT combination, SiC or GaAs diodes

Without galvanic separation?

= one conversion less, but less safe, EMC problems

Without DC-AC stage?

AC is not necessary for storage in batteries of

electrical vehicles, PV at parking places

Without DC-AC stage?

Information technology,

Communication, computers are moving

from 48Vdc to 300Vdc.

More availability? Better/more HR needed in research and application.

Not just ‘hunting’: make studies research and job attractive

COST

EFFICI ENCY

COMPLEXITY

Fig. 15. The control compromise

21/30

PV Inverters: efficiency benchmark

Reported efficiencies

Remarks:

- With or without transformer = With or without insulation

- Parallel tandem working for higher efficiency at low power

http://labs.ti.bfh.ch/fileadmin/user_upload/lab1/pv/publikationen/wrt_dresden_06.pdf 22/30Source: Berner fachhochshule Switzerland, site not active any more

PV Inverters: improve reliability

Overview of improving reliability, causes of failure?

Not perfect design of undervoltage lockout of internal supplies.

Overcurrent protection, fast, also active if the processor hangs?

Surges or overvoltages on the grid due to switch operation or short circuit

surges in the grid.

Current/voltage surges by lightning,

Short circuits from the load to the ground.

ESD in manufacturing and handling

Not enough back up protection if a processor hangs.

Vandalism, sabotage, stealing

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10 kW peak installed can generate 1750 kWh/year in a lot

of sunny regions in Africa.

A new generation economic electric car could use 12.5

kWh/100km which corresponds to 14 000 km year.

Ultra light vehicles need rather 2.5 kWh/ 100km and

would need only 2kW peak installed to reach a similar

distance, see

With bad roads, airco needs and storage losses and

retrofit vehicles one would need rather 25 kWh/100km,

which would correspond to 7 000 km per year for 10kW

peak installed, but it is still possible.

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PV Inverters: direct DC use in vehicles?

Ultra light electrical vehicle

“ELBEV” concept

Ecologic low budget electric vehicle

Solar Trackers

Solar tracker? Project UGent electromechanical engineering by

3rd year students 2012 “interdisciplinary project

Diffuse light yield increases low inclination.

Morning and evening yield increases by rotating towards the sun.

The yield increase by trackers is sensitive to the utilized model:

Absorption by atmosphere, pollution, diffuse light model, weathers statistics.

10% increase in Belgium up to 20% subtropical regions?

Single axis Trackers could already get 90% of dual axis trackers

Vertical axis two axes polar axis

+ correction

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Solar Trackers

Solar tracker? Project by 3rd year students 2012,

1D control reduced panel of 15W, simulating forces of 1m2 panel.

Control: 6….30V with PWM controlled DC motor, good for “12V” panel

Stand by current of the control:10mA , so very low own consumption.

Bill of materials <50 Euro for control, motor and gear is possible for 1D

May be economic for about 2m2

Control PCB 40mm*80mm, single side,

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Solar Trackers

Solar tracker? Project by 3rd year students 2012, projects under

construction

Fig. 16. Solar tracker as a third year student project at EELAB UGENT

27/30

Solar Trackers

Solar tracker:

Could be discussed for lifetime and cost effectiveness

But generates more in the morning and evening

Or some panels eastwards and some westwards oriented?

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Conclusion Cost effectiveness of solar inverters?

PV is more cost effective than solar thermodynamic and biomass if storage is not needed

Converters may cost more than the PV panels in total cost of ownership

Good active components exist, also in Si-IGBT types

Magnetic materials : amorphous and nano-crystalline materials can reduce cost and losses

Human are resources needed for improvement of MTBF and MTTR?

Solar tracker systems?

The cost has to be compared with the increased yield and own consumption

Student projects are used to generate modeling and mechanical arrangement ideas

Topology

Low power without galvanic separation has a higher efficiency but needs RCD for protection

MOSFETS for soft switch, fast IGBT for hard switch

Slow IGBTs for large power plants with multilevel

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Thank you

for

your attention!

???

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REFERENCES

[1] Mark Crawford, “Catching the sun” , Mechanical engineering March

2013.p33-37, https://www.asme.org/getmedia/44edaee0-d607-4ec4-b241-

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http://www.solar-trackers.com/specific-pv-applications/high-temperature-pv-

panels

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