solar energy - sfu.ca · solar energy •direct photovoltaic generation (solar cells) •solar...
TRANSCRIPT
Solar Energy
•Direct photovoltaic generation (solar cells)
•Solar powered heat engines (solar thermal)
•Solar biomass (ethanol, wood chips …)
•Passive solar heating
•Ground/air/water based heat pumps
3/26/2012 1
Some examples:
Seattle WA: 147 W/m2
Average Solar Fluxes Worldwide
McKay, page 6
Cairo237 W/m2Munich DE:
124 W/m2
World average solar constant (above atmosphere): I0 = 1369W/m2
Net average solar flux (above atmosphere): I0/4 = 342 W/m2
Looks promising compared with e.g. offshore wind: 3W/m2
3/26/2012 2
Actual numbers by location:
Semiconductor Physics: Energy Bands
Filled energy states
Empty energy states
•Energy gap depends on chemical constituents of crystal
•As a general rule, smaller atoms give large energy gaps (e.g. AlN~5eV)
•Larger atoms give smaller gaps, e.g. InSb: 0.18eV
•Once the electron is in the conduction band, its energy can be recovered in an external circuit
EG
“Conduction band”
“Valence” band
e
h
Photon absorption
EC
EV
“Energy bandgap”
3/26/2012 3
Semiconductor optical absorption
•If a photon has energy > Eg, it will promote an electron from the valence band to the conduction band (photon is absorbed, ~100% absorption).
•This results in a free carrier in the conduction band and an empty electron state in the valence band, each of which can be used for electrical current.
•If light has energy below Eg, it passes through the semiconductor (~100% transmission).
gEhf
gEhf
absorption
no absorption
3/26/2012 4
Semiconductor Bandgap (eV)
Cutoff Wavelength: hc/Eg(µm)
Si 1.12 1.1 (infrared)
GaAs 1.43 0.87 (infrared)
GaP 2.26 0.55 (visible)
ZnO 3.37 0.37 (UV)
3/26/2012 5
Example: The band gap energy of Si is 1.12eV. What range of wavelengths will be absorbed in Si?
gphoton EhfE gphoton Ec
hhfE
gEc
h
Therefore:
m
eVJeV
smJs
E
ch
g
6
19
834 101.1
)/106.1(12.1
)/103()1062.6(
Therefore wavelengths shorter than 1.1 μm will be absorbed (shorter wavelength= higher energy).
Longer wavelengths will pass through (longer wavelengths= lower energy).
3/26/2012 6
•Think of a solar cell as a device that can generate a potential of the order of the bandgap energy: •large bandgap = large voltage, small bandgap = small voltage•If a photon has E>Eg, it can result in work ~ Eg per electron
light
Rloadp-n junction:
Photovoltaic Device (solar cell)
e.g. silicon solar cell
Max voltage across resistor ~ Eg = bandgap of the semiconductor
“Photo current”
cross section view
3/26/2012 7
http://visual.merriam-webster.com/energy/solar-energy/solar-cell.php
Solar Cell Schematic
Limits to solar cell efficiency
Semiconductors have a bandgap
•This means that light below a certain energy is not used. Why not use a semiconductor with a narrow bandgap?
•The problem is that when the light energy is much greater than the bandgap, the excess energy goes into heat, not more charge carriers.
Maximum theoretical efficiency for a single junction solar cell:
31% (Shockley-Queisser limit)
Solar spectrum
Shaded region represents the energy captured by a Si photovoltaic device3/26/2012 8
Pow
er d
en
sity
(W
/m2/e
V)
“The Basic Physics and Design of III-V Multijunction Solar Cells “ NREL publication
Solution: multijunction solar cells
Ge
GaInP2
GaAs
•3 photovoltaic devices stacked in series on top of each other
•Like three batteries in seriesRload
light
Max theoretical efficiency ~60%. World record so far: 43.5%3/26/2012 9
•Each layer uses a different region of the spectrum to generate photo-current
•Wide gap layer on top takes out highest energy photons
•Successive layers take out lower and lower energy photons
Drawback: these cells are much more expensive than silicon
3/26/2012 10
Solar Cell efficiency comparison
Material Number of junctions
HighestReported Efficiency
Cost
Si amorphous 1 11% $
Si multicrystalline 1 18% $$
Si crystalline 1 25% $$
CdTe 1 16% $$
CuInSe 1 20.3% $$
III-V multijunctionconcentrator
3 43.5% $$$$$
3/26/2012 11
3/26/2012 12
Example:
Day4 energy (Burnaby BC). Multi crystalline Si solar cells.
Model 60 MC-I
Dimensions: 1.65 m x 1.01 m = 1.67 m2
Rated power (at 1000 W/m2 illumination) : 250 W
Power per unit area: 250 W/ 1.67 m2 = 150 W/m2
Efficiency: (150 W/m2)/(1000 W/m2) = 15%
Rated power assumes 1000 W/m2 solar fluxThe average daily solar flux is much lower than this, even in Libya
Current prices in volume are around $2/W (or lower) of power at 1000W/m2 illumination
In Vancouver this works out to ~$20/W at 100W/m
Also need to account for cost of electrical infrastructure (batteries, inverters etc)
Flux of solar radiation at different latitudes
McKay page 38
Effect of latitude/seasonal fluctuations
Power flux (W/m2) is reduced by a factor of cos(θ) where θ = latitude
For Vancouver cos(θ)=cos(49°)=0.65
Sample Calculation
What area of solar panels would supply the typical BC house with 11,000 kWh/year?
Yearly average solar energy flux in Vancouver ~147 W/m2.Let’s assume 100 W/m2 (to account for improper alignment of the panels etc.)
Payback time = (525,000kWh)/(11,000kWh/year)= 48 years
11,000 kWh/year = (11,000kWh/year)x(1year/365x24 h) = 1.26 kW
Assuming $500 /m2 , Cost = (84 m2)x($500 /m2)= $42,000
At BC hydro rate of ~$0.08/kWh this works out to $42,000/(0.08$/kWh)=525,000 kWh
How much hydro would this buy?
Day4 cells generate 150W/m2 for 1000 W/m2 of solar flux (15%)
Therefore Day4 cells generate ~ 15W/m2 in Vancouver (averaged day/night all year)
Therefore total area of cells required is: (1.26kW)/(.015kW/m2) = 84 m2