Ozone

1
Global primary energy use
1 W 1J/sec ? 14 TW (1 TW 1012 watts)
1 EJ 1018 J
(per year)

• 80 of energy from fossil fuels
• Three-quarters of Renewables are from
  developing countries biomass
• Solar wind together account for less than 1
• It is most likely that developing countries will
  switch from biomass to
• fossil fuels as economies develop

2
Passive vs. active solar heating
Active system

• Passive relies on building design greenhouses,
  thick masonry, dark color
• Active some other source of energy besides solar
  is also required
• water circulation to pick up solar energy for
  heat and hot water
• efficiency improvements for water and energy
  savings

3
Solar thermal electricity

• Various designs are used for concentrating solar
  energy into a receiver.
• Mirrors provide up to 5000-fold concentration of
  photons
• High temperatures are produced
• eg, oil flowing in a closed receiver pipeline is
  heated ?
• heat transfer to steam electrical generator
• Can be coupled to insulated storage systems to
  deal with intermittency
• Dish-engine system is most efficient T up to
  800C
• Still more expensive than fossil fuels

4
Light reactions
C-assimilation reactions
CO2 IN
electron transfer reactions
NET FIXED CARBON OUT
Photosynthesis uses light photons to fix carbon
and generate oxygen
oxygen generated
photosynthetic reaction centers
5
Charge separation in photosynthesis is analogous
to the process that occurs in photovoltaic
cells (PV cells)
6
Photovoltaic (PV) cells
Based on semiconductor materials where the energy
band gap is comparable to the energy of
solar photons silicon Contrast insulators
(very large band gap) and conductors (no band
gap) Silicon band gap 124 kJ/mol (l 1140 nm)
insulator
conductor
7
PV cells
Ga 3 valence electrons (p)
As 5 valence electrons (n)
The Si semiconductor must be doped with foreign
atoms that have either a greater or fewer
number of valence electrons this creates an
intrinsic charge asymmetry Arsenic has an
additional valence electron while gallium has a
hole The additional As electron does not fit
and has a higher energy nearer to the
conduction band, to which it is easily
promoted Promoting the As electron leaves a fixed
charge on the As atom This is an n-type
semiconductor negative mobile carriers (e-)
in the conduction band
8
PV cells
Ga 3 valence electrons (p)
As 5 valence electrons (n)
The Si semiconductor must be doped with foreign
atoms that have either a greater or fewer
number of valence electrons this creates an
intrinsic charge asymmetry Arsenic has an
additional valence electron while gallium has a
hole Doping with Ga the missing electron is
supplied by adjacent atoms The hole in the
valence (lower energy) band moves There is a
fixed negative charge on the Ga atom while the
charge moves This is a p-type semiconductor
with positive mobile carriers in the valence
band
9
Energy level diagrams n and p semiconductors
10
junction
mobile charge
p-n junction
fixed charge
Ga
As
p fixed (-) charge mobile () charge
n fixed () charge mobile () charge
In a PV cell the p-n junction creates the
required charge asymmetry Note the mobile charges
on each side do not migrate to the other side,
because of the fixed charges of the same sign
there. This creates an electrical potential
at the junction. When a photon is absorbed in
the p-type semiconductor, the promoted
electron can be accelerated across to the n side
(fixed charges) before recombining with a
hole Photon absorption in the n-type
semiconductor would cause the hole created in
the valence band to accelerate toward the fixed
(-) charges on the p side The directed movement
produces current in an external circuit
11

• Efficiency of solar cell - limitations
• 1/3 of solar photons are in lower-energy IR,
  below the Si band gap energy
• Extra energy in higher-energy photons, above
  band gap, is not used
• Some photons are absorbed by the surrounding
  material of the cell
• Best achievable is about 28 efficiency with very
  pure Si
• Amorphous Si is much less efficient, but cheaper

12
Solar energy

• Useful in remote areas where there is high cost
  to extend electrical grid
• Useful to provide small amounts of power locally
• Diffuse low energy density per surface area
  about 1 kW per m2
• Intermittency efficient electricity storage and
  transmission needed
• Produce direct current which needs to be
  converted to AC
• Current annual world capacity is about 3 GW
  (compare 14 TW total)
• Very substantial cost decreases but still not
  competitive with fossil fuel
• Important input to the hydrogen economy

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14
Wind power
Total world capacity 50 GW Fastest-growing and
cheapest form of renewable energy Rotor drives a
turbine to generate electricity Power density
(W/m2) increases with height and rotor
size Problem in US most capacity located far
from sites of greatest need Very strong
potential to provide large amounts of
energy European targets are for 10 of
electrical demand by 2020 Important connection to
hydrogen economy Downsides intermittency,
NIMBY, birds
15
Hydroelectric power

• change in river elevation
• gravity flow

• much of the US capacity is already being
  exploited
• potential for growth in Russia, some developing
  countries
• 20 of world electricity already comes form
  hydroelectric power
• Issues river management (silt buildup, dead
  zones, migratory fish)

16

• Ocean Energy
• Use of tides to generate electricity
• trap rising ocean water behind a dam
• release the trapped water through a turbine
• Other possibilities (not exploited)
• wave power on the open ocean
• vertical thermal gradient in the ocean water
• Geothermal energy
• Hot springs and volcanoes channel heat from the
  earths interior
• The steam escapes through drilling into a
  geothermal reservoir
• Hot steam is used to generate electrical power
  through a steam turbine