Energy, Society, and the Environment Unit 6: Solar Energy

Solar Energy is not a New Concept Solar cells on a satellite Solar cells on a rooftop Cookies baking in a solar oven

How do we use sun’s energy? 1. Passive solar: For example, heat your home with south-facing windows

How do we use sun’s energy? 2. Solar-Thermal: Use the heat from the sun to boil water A solar power plant in Australia

How do we use sun’s energy? 3. Photovoltaic Cell: Directly produce electricity from sunlight using “semi-conductors”

How do we use sun’s energy? 4. The Cheapest Way: The plants know how to Efficiency: 0.3 % We can’t meet the world’s energy demands in this way

Why Solar Energy? We need ~ 30 TW of power, the sun gives us 120,000 TW. Solar cells are safe and have few non-desirable environmental impacts. The sun shines when we need energy the most. Using solar cells instead of burning coal to generate electricity is a easy way to reduce carbon emissions.

The Sun

What is Sunlight? Light is made of waves

Sun Light (Waves) Frequency: How frequent the peaks are Wavelength: How far apart the peaks are Speed = frequency x wavelength

What is Sunlight? ALL LIGHT WAVES TRAVEL AT THE SAME SPEED Speed = frequency x wavelength = c

Composition of Sunlight

Solar Spectrum Our atmosphere preferentially absorbs some wavelengths Most UV and some infrared is blocked See the handout.

Visible Light It is not a coincidence that human/animal eyes see “visible” light, which the sun produces the most of (the peak of its “spectrum”) The size of human/animal eyes (lenses) are set by the visible wavelength Had we evolved on a planet around a different type of star, we would most likely see a different wavelength!

Photovoltaic Cells Concept: How does it work? Makes use of materials called semi- conductors Aluminum wire: carries electricity -- a conductor Plastic: Does not let electricity through -- an insulator A (doped) Silicon crystal: Can conduct electricity if enough voltage is provided -- a semi-conductor

Photovoltaic Cells p-n junction is the basis for all photovoltaic devices When the two types are brought into contact, electrons (n) and holes (p) are exchanged over a short region, the interface, known as depletion or junction region

Photovoltaic Cells The junction region “band-gap” determines what wavelengths of light the solar cell is sensitive to. The bigger part of the solar spectrum it is sensitive to, the higher its “efficiency” e-e- usable photo- voltage (qV) Energy e-e- n-type p-type η max = 32% heat loss hνhν h+h+ Jenny Nelson, The Physics of Solar Cells, 2003.

Photovoltaic Cells When Light energy ≥ band gap energy electrons will gain enough energy to move and electricity is generated See the handout for the wavelengths that Si crystal is sensitive to.

Photovoltaic Cells The efficiency of the solar cells drop with increasing temperature: have to cool them well to keep electricity production high A Si solar cell operating at 0 C has a maximum possible efficiency of 24% At 100 C, it drops to 14%

Multi-Junction Cells Grid p p p p p n n n p n p p n n ++ p n p n ++ Ge Substrate (0.67 eV) GaAs (1.42 eV) GaInP (1.90 eV) AlInP AlGaInP GaInP n GaAs GaAs:N:Bi (1.05 eV) GaAs:N:Bi n n Slide credit: McGehee, Stanford U. SpectroLab has achieved 37 % efficiency Costs are estimated at $50,000/m 2, so concentrators must be used.

Thin Film Cells Slide credit: McGehee, Stanford U.

A Balancing Act Research in solar cells tries to optimize efficiency (achieve highest possible) while making it as cheap as possible Other concerns are durability, availability of materials, production speed Module CostAmortized Capital Expense Material YieldSubstratePV Efficiency Crystalline Silicon$2/W$0.16/W<50%Rigid15% Vacuum Thin Film$1/W$0.32/W70%Rigid10% Nano$0.3/W$0.04/W95%Flexible 3%*

Solar-Thermal Energy Use the heat of sunlight to boil water and operate a steam engine Remember the demonstration Dr. Angel talked about: when sunlight is concentrated by a mirror, can melt 1/4 inch thick steel in 15 seconds!!

Solar-Thermal Energy A very old new technology!

Solar Energy: Insolation Map

Solar Energy Is it feasible? Can provide all of the U.S. electricity needs with solar energy (100 square miles in AZ or Nevada covered with 20% efficient solar cells can do it: e.g., Turner, 1999, Science, 285, 687) Currently, the biggest challenge is the cost of photovoltaic cells; focus is on decreasing cost Global solar industry has grown 20% every year for the past 10 years (but is still in its infancy) 30% annual growth in the U.S.

PV Land Area Requirement (U.S.)

Solar Concentrators

Solar Energy and Demand Solar Energy provides most of the demand, can be supplemented in evening hours

Energy Storage Pumped water hydroelectric storage Compressed air energy storage

Energy Storage

Energy policy is hard, finally, because there is no technological “silver bullet”: No known energy option is free of liabilities oil and gas: not enough resources coal: not enough atmosphere biomass: not enough land hydropower & wind: not enough sites nuclear fission: too unforgiving nuclear fusion: too difficult photovoltaics: too expensive, intermittent end-use efficiency: high consumer discount rate, needs users to pay attention, inherently diffuse industry hydrogen:not a primary source Holdren, John P. (2006) ENR302 Energy Technology, Markets, and Policy. Lecture 1, February 2, 2006