07/02/2013

1

Photovoltaics:Solar Power for Electricity

Week 2

Renewable Generation & Control

Solar ResourcePV cell functionPV cell typesPV equivalent circuitGrid connection and MPPT control

Sun

Winter in North,

Summer in South

(December)

Summer in North,

Winter in South

(June)

Tilt of Axis: 23.5º

Orbit radius:150 million km Sun’s radius:

696,000 km

Radiation at the Sun’s surface: 63 MW/m2

What is the radiation level at Earth’s radius?

Temperature at the Sun’s surface: ~5800 K

2.1 Variations in Solar Radiation

All electromagnetic fields follow “Inverse Square” Law

07/02/2013

2

2.2 Solar Spectrum at edge of atmosphere

UV Infra-Red

The sun’s raysare reduced instrength by theatmosphere:

especially whensun is at a lowerangle

Visible:

380 – 780 nm

Ref: Quaschning

2.3 Solar Spectrum through atmosphere

07/02/2013

3

2.4 Solar Spectrum at Earth’s surface

The sun’s rays areabsorbed andscattered by theatmosphere:

UV + others

Approx. 26% of solar radiation is lost.

What is the radiation level at Earth’s surface?

2.5 p-n junction Diode: Energy bands

-

+

Applied Voltage (c. 0.7 V) increases energy ofn-type conduction band relative to p-type.

=> Current increases

Note: electron flow isopposite toconventional current

Animation

1exp0

kT

qVII

07/02/2013

4

2.6 PV cell: p-n junction Energy bands

Photon withenergy hf

+

Photon Energy releases an electron:

Current flows from p side of pn junction

h (Planck’s Constant) = 6.62 x 10-34 Js

2.7 Solar Spectrum at Earth’s surface

What is longest wavelength that can beabsorbed by a-Si PV cell – Eg = 1.7 eV?

UV Infra-Red

Visible:

380 – 780 nm

07/02/2013

5

2.8 Solar Spectrum at Earth’s surface

What is longest wavelength that can beabsorbed by a-Si PV cell – Eg = 1.7 eV?

2.9 PV module – crystalline silicon

The diagramright shows

methods used toimprove Poly-crystalline PVperformance.

(Mitsubishi)

Energy falling on panel= EApanel (J/s)

E = solar irradiance(W/m2)

07/02/2013

6

2.10 Amorphous silicon module

Layers are all “thin film”: lower cost thancrystalline cells; lower efficiency, too.

New thin film technologies under development

07/02/2013

7

2.12 Modelling a PV module

Vout

Rs

Load orBatteryRp

Currentsource

2.13 Solar PV cell characteristic…

Short-circuit Current, Isc

Open-circuit Voltage,VocFill factor is ratio of areas: Pmax/ (Voc x Isc)

Maximum V x I

Voc x Isc

07/02/2013

8

2.14 Solar PV characteristic

If max. cell efficiency is 18%

What is the area of the cell?

2.15 Solar PV example

Example: How many cells in series and parallel will berequired to produce a current suitable for charging anominal 24 V battery at up to 12 A?

Assume battery open circuit voltage is 26 V and internalresistance is 0.15 Ω.

0.15 Ω

26 VVout

Rs

Rp

07/02/2013

9

2.16 Temperature Effects on PV output

If max. efficiency is 18% at 25 C,what is efficiency at 50 C? …at 0 C?

2.17 Connecting the PV system

Typicaldomesticsystemmayhave 20modulesx 30 cells

Uses DC-DC converter with control, then an inverter to produce a.c.

07/02/2013

10

Optimum output varies withoperating conditions:

Most PV systems(and many wind generators)use MPPT(Maximum Power PointTracking ) to optimise output.

0

2

4

6

8

10

12

0 5 10 15 20 25Voltage (V)

Cu

rre

nt

(A)

Power T=25C Power T=50C Power T=0C Power at 0.1E

Perturb & Observe method -controller checks:

• Current (I) & Voltage (V)• Calculates dP/dV• Adjusts operating voltage

Results in oscillations

2.18 Maximum Power Point Tracking(MPPT)

2.19 MPPT -> adjusts operating voltage

0

2

4

6

8

10

12

0 5 10 15 20 25

Voltage (V)

Cu

rren

t(A

)

MPP0

dV

dP

V

I

dV

dI

dV

dI

At MPP

But, since P = IV;dP = IdV + VdI

is known as the incremental conductance.

Again need to measure changes in current andvoltage: use this to decide control strategy.

07/02/2013

11

2.20 MPPT -> a simpler control method

MPP

.constV

V

oc

mpp

An alternativemethod:

Uses the factthat the ratio:

Thus, by checking the value of Voc it is possible to determine Vmpp.

Need to measure only voltage: but when current is interrupted

0

2

4

6

8

10

12

0 5 10 15 20 25

Voltage (V)

Cu

rren

t(A

)

2011:$1.50/Wp

SOURCE: EPIA/Fraunhöfer

2012:<$1.00/Wp

07/02/2013

12

2.22 Concentrated PV – high efficiency solar

Concentrated PV – various technologies;

Fresnel lens for lower concentration

Parabolic mirrors for upto 500x

High efficiency (30-40%)<– but needs cooling

Solar Power: photovoltaics (PV)

• Converts sunlight directly into electricity

• High investment costs – but decreasing

• Well suited for distributed generation e.g. domestic.

• This will be main contribution: building-integrated

• At large powers, used for “peak shaving” projects

1MW station, California

Solar panels onoffice building