PV Flashcards
(105 cards)
1
Q
Top 10 problems in the world
A
- Energy
- Water
- Food
- environment
- Poverty
- War
- Diseases
- Education
- Democracy
- Population
2
Q
“Solar is the new _”
A
King
3
Q
What is Irradiance
A
Radiant power flow [W/m^2]
4
Q
What is Irradiation
A
integral of irradiation flux [Wh/m^2*h]
Beam of direct + Diffuse
5
Q
What is the clearness index
A
Global radiation/ extraterrestial radiation
6
Q
What is the Solar cte?
A
1367 W/m^2 (= irradiance of solar beam)
7
Q
What are the 3 sky conditions?
A
Cloudless, partially clouded, overcast
8
Q
What are the 2 important angles
A
Solar altitude
Solar Azimuth
9
Q
What is the solar altitude
A
angular elevation of centre of the solar disc above horizontal plane (vertical)
10
Q
What is Solar Azimuth
A
horizontal angle between vertical plane containing centre of solar disc and vertical plane in N-S direction (horizontal)
11
Q
What is air mass (AM)
A
it describes the atmsophere path - path lenght which light takes through the atm - normalized to shortest possible (= when sun is overhead)
AM = 1/ cos (theta)
12
Q
AM0
AM1
AM1 41
A
AM0 = black body/ extraterrestrial (absorbs all incident electromagnetic radiatio) , outside Earth’s atmosphere
AM1 = sun is direct
AM1 41 = 45° zenith
13
Q
Cloudless sky global radiation is a function of what?
A
latitude of sky, clarity of atm
14
Q
What is the Linke Turbidity factor
A
describes the clarity of sky: dust, humidity, pollution, decrease in beam of irradiation, diffusion
15
Q
Describe the following L Turbidity factors
<2.5
3
4.5
>6
A
at sea leavel
cloudless sky, 10-20% diffuse
large cities
extremes of dust in atm ex: desert
16
Q
Explain partially clouded conditions
A
direct beam of irradiance is very variable
short term variations small, drastic changes in seconds
17
Q
Explain overcast sky conditions
A
NO DIRECT BEAM
influenced by height, type and depth of clouds
less short term variations
18
Q
Explain a photovoltaic simple mechanism
A
it is a direct conversion of solar light into electricity
no moving parts
modular (large scale)
sun light –> solar cell/PV –> elec + heat
19
Q
What is a solar cell
A
basic building block
20
Q
module
A
smallest unit
21
Q
string/array
A
of modules connected in series
22
Q
What is a PV - system
A
modules + BoS (balencing of systems)
23
Q
What is a BoS
A
electronic storage + wiiring + support structures + Labour
24
Q
What is the efficiency of cells, modules, systems?
A
10-20%
24
What is the performance ratio?
average ac efficiency / Standard test condition dc efficiency
24
What is the capacity factor
#hours ac peak power / year * hours in a year = 9%
25
What are PV factors affecting the design? (6)
irradiation density < 1000 W/m^2
spectrum AM /= 1.5
T /= 25°C
thetat not 90
cable losses
DC/AC conversion
26
Which factor from LCOE can we take out
Fuel factor
27
Which are the most important costs for PV in LCOE calculations
investment cost, maintenance cost, marginal
28
What is the avg lifetime of PV
25 years --> need more than 35
29
What are the 3 steps of PV process?
1. absorbtion of light
2. Separation of excess carriers
3. Transport
29
Explain the first step of PV process : Absorbtion of light
energy of photon absorbed = excitation of é = free é + holes
30
Explain step 2 of PV process: Seperation of excess carriers
p-type has an extra hole, n-type has an extra é = electric field at junction = seperation of charges =
31
Explain step 3 : Transport
Metal in contact of solar cell = collects seperated charges, flow of é = electric current
32
Explain Diode in the dark
Dark = no power generation = no sunlight
33
Explain diode in the dark at V =0
no light = no photon
0V = no external voltage (open or short-circuit)
diode is off, no current flow, no external driving force
thermal equilibrium
few major carriers diffuse from n --> and are balanced thermally
= gen of minority carrier that drift from p --> n
34
***** Explain forward voltage + diode in the dark
p-type connected to positive, n-type connected to -
1. reduced barrier = applied forward V drops at the PN junction --> é from n and holes from p crosse the junction (majority carrier diffuse across barrier)
2. Carrier minority injection: é injected from n-type to p-type and hole injected from p-type to n-type diffuse in quasi-neutral region under the influence of concentration gradiant
35
***** Explain reverse voltage + diode in the dark
System: p-type is connected to -, n-type is connected to +
1. Depletion regions widen:
2. Minority carrier drift
small current flows : é-hole pairs = thermal gen, é in the p-type, holes in the n-type = swept accross the junction by strong electric field
36
Explain Diode under illumination
light = solar cell behavior
carrier generation by photon or thermal energy
no voltage applied
37
Explain short-circuit condition
V = 0 across diode
no external resistance
1. photons generate electron - hole pairs
2. They are seperated by electric field in depletion region : é --> to n-type side, holes --> to p-type side = w/ a short circuit = carriers flow out through external wire = current max, V=0, no power output
38
Explain the effect of T on solar cell
if T rises, small increase of short-circuit current (Isc), V open-cicuit drops = band gap lowers, more photons have enough energy to generate carriers
efficiency losses
P = V x I --> V losses are greater than I gains = loss of P = loss of efficiency
39
what is ni^2
intrinsic carrier concentration :
# of free é or holes in pure semi-conductor at T eq
n = p = ni
40
What does ni^2 strongly influences, what value
Io
ni^2 = 2 mV/°C
41
What is a hetero-junction?
Junction between 2 different semiconductors
band offset = enhance performance
$$$
42
What is a PN junction
depletion layer very small at interface, used in diode, LEDs, solar cells
diffusion length = collecting
[p]---[n]
43
PIN junction
Intrinsic, undoped layer = widder depletion layer = strong internal electric field
thicker intrinsic layer = higher internal resistance = lower fill factor
43
What is an exciton?
bound pair, Coulombic interaction between excess hole and electron is strong --> they move together. Need additional energy to seperate (electric field), dissociation at hetero-interface
44
What are the steps of an excitonic solar cell?
1. Absorbtion of light
2. Separation of excess carriers = move in different directions
3. Transport
45
Explain the 1st step of excitonic cell
Absorbtion of light:
photons from sun absorbed by active layer
absrobtion energy --> generates excitons
dissociation of exciton, diffusion at heterojunction interface
46
Explain the 2nd step of excitonic cell
Seperation of excess carriers = move in different directions, exciton seperates into free é and holes because of energy level offset
47
Explain the 3rd step of excitonic cell
Free é moves through acceptor material --> cathode
Free hole move through donor material --> anode
all collected at respective electrode = current flows in external circuit
48
What is a Fill Factor (FF)?
It measures how "square" the current–voltage (I–V) curve is
Indicates how efficiently the solar cell converts available power from light into usable electrical power
49
What is the Voc effect?
The bandgap becomes smaller as T increases
(typically ~–2 mV/°C per cell), -0.0015/°C for Si
50
What is usually the max power output?
0.4% / °C
51
What are the 4 main factors for efficiency calculations?
Long-wavelenght losses
Excess energy losses
Voltage factor
Fill Factor
52
What are the 2 different recombination mechanisms?
Radiative recombination
Auger recombination
53
Explain the Auger recombination
energy of é-hole recombination transferred to another carrier
54
Explain radiative recombination
Energy of recombination é-hole = emmits a photon
55
What are the assumptions for the Shockley-Queisser model?
1. Cell = black body in T equilibrium (AM1.5 or AM0)
2. Isotropic irradiation
3. Radiative recombination only
4. All photons above bandgap are absorbed hν>E
g
5. Only one electron–hole pair per photon
6. No resistive or optical losses
7. Cell behaves as ideal diode
55
What is the S-Q limit?
Max efficiency for single junction (AM1.5) = 33%, optimal band gap = 1.34 eV
for AM0 = 41%
if efficiency higher: more than one bandgap/ semiconductor
56
What happens if Vg is smaller than 1,34 eV?
Voc and FF drops, absorbs more photons but efficiency drops
57
What happens if Vg is higher than 1,34 eV?
absorbs less photons, Voc and FF rises, efficiency drops
58
Why use Si for a crystalline Si-solar celle
Si is 2nd most abundant element
cheap
59
What is MG-Si
Metallurgical grade Si --> carbothermic reduction of Si
Must undergoe a purification step in a fluidized bed reacting with HCl
60
What is Czochralski process
Cristal growth process of MG-Si
Produces monocrystalline silicon wafers, which are:
Highly efficient (20–26% in solar cells)
Used in premium photovoltaic modules
61
What is an Industrial Screen-Printed Crystalline Silicon (c-Si) Solar Cell?
most widely manufactured type of photovoltaic cell today. It uses crystalline silicon (mono or multi) and a screen printing technique to apply metallic contacts for current collection.
62
What are the 5 steps for Industrial screen printed c-Si ?
1. Saw dammage removal
2. Phosphorus diffusion
3. ARC (anti-reflecting coating)
4. Metallization (screen-printing)
5. Firing treatment
63
Explain step 1 of c-Si
Purpose: Remove surface defects caused by wafer slicing.
+ Etchange treatment using KOH or NaOH = texture surface to reduce reflexion and ^efficiency
64
Explain step 2 of c-Si
Phosphorus diffusion
Purpose: Form the p-n junction by doping the surface with phosphorus.
T= 750-850°C
Treatment in furnace in POCl3 --> phosphorus diffuses in Si, Cl reacts w/ impurities= cleans wafer
65
Explain step 3 of c-Si
ARC (Anti-Reflective Coating) Deposition
Purpose: Reduce light reflection and passivate the surface. drop in surface recombination velocity = favorises electric field
silicon nitride (SiNx) is deposited
66
Explain step 4 of c-Si
Metallization (Screen Printing)
Purpose: Apply metal contacts to collect and conduct current.
Silver on front side
Al in rear side
67
Explain step 5 of c-Si
Firing treatment
Purpose: Activate the contacts and form the back-surface field (BSF).
T= 700-800°C
Front side: Silver goes through Silicone nitride = contact to n+ Si
Rear-side: Al reacts with Si, diffuses in p+-Si = flattening effect , ohmic contact
68
What is the goal of industrial screening c-Si? what is the desired efficiency?
Reducing effect of defects and recombination
efficicency = 20-30%
69
What is thin-film solar cell technology Using Amorphous Silicon (a-Si)?
Amorphous silicon (non-crystalline form of Si)
PIN structure
High absrobtion coefficicent
thin layers
not stable
low efficicency (6-7%)
70
Why use a tripple junction for the a-Si?
try to improve efficicency (10-14%
71
What material is best for triple junction?
Ge, but expensive
|a-Si:H| a-SiGe:H| a-SiGe:H|
72
What is CdTe polychristaline + what is the value of the bandgap?
thin-film solar technology that uses a polycrystalline CdTe layer as the light-absorbing material
Most versatile technology
High absorption efficiency
Bandgap = 1.5eV
Direct absorption
73
What is a multijunction cell?
stacks two or more different semiconductor materials, each designed to absorb a different part of the solar spectrum
achieve much higher efficiencies
* a single junction is limited to SQ limit = 30% efficicency
74
Explain which cell absorbs what in a multijunction cell (3 layers)
Top cell: Absorbs blue and green light (short wavelengths)
Middle cell: Absorbs yellow and red
Bottom cell: Absorbs infrared and remaining photons
75
What is a high concentration solar cell? What is its efficicency for Si? What is its max efficicency
Uses Lenses or mirrors = cheaper
For direct sunlight
efficicency = 26.5% for Si
multijunction (MJ) cells, made from III-V materials like GaAs, InGaP, and Ge
Max eff = 50%
76
Name the 2 geometrical losses in a cell-to-module performance
1. border loss
2. spacing loss
77
Name the 5 optical losses in a cell-to-module performance
air-glass reflexion
glass absorption
glass-encapsulant reflexion
encapsulant abs
ribbon shading
78
name the 2 optical gains
coupling front
coupling backsheet
79
Name the 3 electrical losses (ressistance)
Cell interconnection
String interconnection
Cabling
80
What is the PV-system MPPT?
Max power point tracking
81
What are the main features of MPPT
simple, cheap
Steady state error
Dynamic response
Digital implementation
Sensing
82
What are the MPPT main methods?
1. Open-circuit, Vmax=cte = 0.76, k=0
2. Cte V = not accurate
3. Short-circuit
4. Thermal methode + measure of incoming radiance
5. Perturbe and observe (PandO) = algorithm, fail
6. Sensing
83
What are the benefits of bifacial PV?
higher electricity output
2x light collection
84
What are the 5 ways light is captured in bifacial PV?
1. Direct (front)
2. Sky diffuse (front)
3. Sky diffuse (back)
4. Ground-reflexion sky diffuse
5. Groud-reflected sky direct
85
What are some problems of bifacial PV?
if non-uniforme = mis match = less rad received
Structre frames
Shaded vs unshaded ground
# of PV rows
86
Is the gain and albedo correlated
yes, but other scattering factors
87
Does T influences behavior?
Yes, clear link with the electrical model + all physicial inputs are measurable
88
Do you want a higher or lower T for increase in efficicency?
lower T
89
Does glass-glass (transparent) module have a higher or lower rear heat generation than white blacksheet?
higher
90
How many modules does a 100MW PV plant have?
300 000
91
How can you optimize bifacial PV?
higher elevation
30° tilt
longer row spacing / horizontal / vertical spacing
higher PV area (m^2)
92
Why use façade BIPV?
better for
Climate
legislations
higher CAPEX, lower OPEX = rentable
image of building
roof space
higher efficicency
Security / flexibility
93
What are the main challenges of façade BIPV (6)
integrating module-level converter (MLC)
increasing lifetime over 25 years = maintenance
compactness
$ control
Works at high T
94
What does LVDC mean, why is it used?
(Low Voltage Direct Current)
DC (direct current) electrical system operating at low voltages, typically:
Reduced energy loss
Improved efficiency at small scale
less components = more compact
Controlability
V e [48-1500V]
95
What is the methodology for LVDC? (6 steps)
1. input profile Vpv, Ipv, IR as a fct of time
2. BIPV output (power)
3. MLC derating factor evaluated w/ measured data
4. Cabling
5. Voltage balencing conversion
6. Bipolar LVDC grid in building
96
What is the main conclusion of different PV techs compared?
1. The PV technology influences the efficicency, Voltage, I, Power
2. Lower DC bus tension (<380V) = higher efficicency
3. Majority of losses occur in MLC --> if drop Voltage = shifts losses into Cabling, doesn't heat up the components and need of a smaller converter = compactness
97
What are the losses in the MLC?
P_L, Pswitch, Pdiode, Pcapacitor
98
What is the system efficicency
Calculated at every data point
99
What is peak efficiency
max efficicency of system over the whole time span
100
What is the total efficiency
Egrid / Epv (energy gen by PV module)
