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Lecture 19

 

Alternative Energy – Solar

–        The solar Flux (Fig. 1)

–        Types of solar energy

–        Direct use of solar energy

•         Heat based systems

•         Photoelectric systems

–        Indirect use of solar energy

•         Photosynthesis

•         Biomass

 

Solar flux

•         Solar flux at top of Atmosphere:

–        1370 W/m2

•         Solar flux at sea level: 100 ΰ 250 W/m2

•         Average Flux at UR: ~ 170 W/m2

 

ΰ    The main problem of solar flux is the low power density ΰ Concentration needed to convert into ‘useful’ energy

ΰ    Direct concentration

ΰ    Indirect concentration

ΰ    The solar energy system (Fig. 2)

 

Direct uses of solar radiation

•         Heat

–        Space and water heating (Fig. 3)

–        Conversion into electricity (Fig. 4)

•         Light

–        Lighting

–        Photoelectric systems

 

Photoelectric systems

•         Definitions:

–        Valence band: electrons are tied to specific atom

–        Conduction band: electrons are shared, free to move

–        Energy gap: ‘forbidden’ energy between valence and conduction band (Fig. 5)

•         Electric conductivity

–        Insulator: Conduction band empty; energy gap very large ΰ no electrons free to move

–        Conductor (mostly metals): conduction band populated ΰ electrons can follow field potential

–        Semiconductor (Si; GaAs): conduction band empty, but gap small enough to allow electrons to get from valence to conduction band

•         Addition of ‘impurities’ can change the width of the energy gap

–        n-type (donor, e.g. P) ΰ lowers conduction band

–        p-type (acceptor, e.g. B) ΰ raises valence band

–        Junction: combination of n- and p-type semicondutors

 

Principles of photovoltaic systems

•         Photons add energy to electrons ΰ e- jump from valence band into conduction band ΰ production of electron-hole pair

•         Near junction between n and p semiconductor, charge separation can occur

•         Electrons remain in conduction band ΰ flow of electric current (Fig. 6)

 

Efficiency

•         Theoretical efficiency fx of energy gap and spectrum

–        Si 22 %  GaAs 24 %

•         Losses

–        Recombination

–        Transparency

•         Practical efficiency ~ 10ΰ15%

 

Considerations on photovoltaic systems

•         Gap between valence band and conduction band right for electron jump

•         p-n junction provides charge separation

•         Gap is correct only for one specific wavelength ΰ efficiency for photovoltaic systems ~ 20 % (practical ~ 10 %)

•         Efficiency can be improved with multiple layers of different photovoltaic layers

 

Current use of Photovoltaic systems (Fig. 7)

 

Photosynthesis

•         Break-up of water by photons

•         Charge separation (membranes)

•         Electrons react with molecules (NADP) and reach higher energy levels – chlorophyll molecules (two steps) ΰ NADPH2

•         Molecules react with CO2 ΰ CO2 assimilation and formation of carbohydrates (Fig. 8)

•         Efficiency is low (Fig. 9)

 

Use of Photosynthesis

•         Annual net photosynthesis production ~ 10 times total energy consumption

•         Food: Food production uses 0.5 % of photosynthesis; needs energy input ΰ 2 to 3 % of total energy

•         Biomass as energy source

–        Direct use (combustion etc.)

–        Conversion to fuels (ethanol; biodiesel etc.)

 

Biomass as energy source

 

•         Energy extraction:

–        Combustion

•         Large plants can burn waste together with coal and gas

•         Small plants in rural areas (cow dung + crop residue + wood)

–        Other methods:

•         Hydrogenation: Waste is treated with steam and CO under high pressure ΰ oil ~ 60% efficiency

•         Pyrolysis: Heating organic material in absence of air ΰ ‘charcoal’, gases, liquids ~ 65% efficiency

•         Biogasification: Breakdown of organic matter by anaerobic bacteria ΰ methane

Ethanol production (Fig. 10)