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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
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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
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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
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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
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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)
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2.10 Amorphous silicon module
Layers are all “thin film”: lower cost thancrystalline cells; lower efficiency, too.
New thin film technologies under development
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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
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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
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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.
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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.
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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
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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