solar energy - sfu.ca · solar energy •direct photovoltaic generation (solar cells) •solar...

14
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:

Upload: others

Post on 30-Apr-2020

10 views

Category:

Documents


2 download

TRANSCRIPT

Page 1: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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:

Page 2: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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:

Page 3: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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

Page 4: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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)

Page 5: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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).

Page 6: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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

Page 8: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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)

Page 9: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

“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

Page 10: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

3/26/2012 10

Page 11: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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

Page 12: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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)

Page 13: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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

Page 14: Solar Energy - SFU.ca · Solar Energy •Direct photovoltaic generation (solar cells) •Solar powered heat engines (solar thermal) •Solar biomass (ethanol, wood chips …) •Passive

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