Solar Energy and Hydrogen Economy (Photovoltaic Cells (Photovoltaic Effect…
Solar Energy and Hydrogen Economy
What is it?
Direct extraction of useful energy in dif ways
~ 3.8mil exajoules (EJ) energy is available to use on earth from the sun - 10,000x current FF consumption rate
In 2,000 FF consumption rate amounted to ~ 360EJ
Solar thermal energyy:
Active solar heating - discrete solar collector, usually mounted on the roof of a building. Mostly for heating domestic hot water.(see slide 3 of 30 for schematic)
Passive solar heating - Absorption of solar energy directly into a building or space heating. Integrated, low energy building design (slide 4)
Solar Thermal engines - An extension of active solar heating employing more complex collectors to produce temperatures high enough to drive steam turbines.
Effective use of solar radiation often requires the radiation (light) to be focused to give a higher intensity beam.
Point focus. A parabolic dish or concentrating lens, possibly combined with a heliostat are used to concentrate light at a point (the focus). At the focus there might be placed a high concentration of photovoltaic cells (solar cells) or a thermal energy 'receiver'.
Line focus. A parabolic trough or a series of long narrow mirrors are used to concentrate light along a line. The SEGS systems in California are an example of this type of system.
Silicon is the most common semiconductor, but germanium is also used. Semiconductor more easily controlled than something like copper or silver. Silicon has four valence electrons.
Silicon mixed w two other elements. Electrons move to produce current.
Sunlight hits, excites electrons, and because of the other elements, they have somewhere to move in an orderly fashion to produce a current.
Germanium more expensive
Doping: Mixing elements - Si is a semiconductor (only conducts electricity under certain conditions), can be manipulated.
Boron has only three valence electrons - can easily accept electrons. Phosphorous has five valence electrons, can easily lose electron.
suitable doping materials in photovoltaic cells because of their ability to give up or accept electrons.
Added in small quantities to a semiconductor to create an imbalance, of sorts, in electron distribution.
more direct method of generating electricity from solar energy.
most important feature of semiconductor from electronic perspective is ability to control its resistivity
silicon, involves “doping” semiconductor with either group V element (P, or As) or group III element (Al or B).
These atoms provide either a surplus of free electrons (Group V) or a deficit of free electrons (Group III), generating n-type and p-type silicon,respectively.
important parameter in a semiconductor is the band-gap which is an energy gap between allowed electron energy levels in the semiconductor material.
p-n junction has a built-in electric field which results in the separation of electrons and holes generated by the incident light. These move in opposite directions and contribute to current flow.
band-gap of silicon is 1.1eV = 1.76 x 10-19J. If incident light has energy greater than this, it can generate electron-hole pairs. This energy corresponds to a wavelength given by: E = hc/λ
E = 1.76 x 10-19J the wavelength is 1130nm which is in the infrared region of the spectrum
all of the wavelengths in the visible region of the spectrum have sufficient energy to generate electron-hole pairs in silicon.
voltage generated by a single solar cell is no more that 0.5V and a maximum current of 3 Amps giving a potential power output of 1.5W. This is why they are grouped together in arrays.
PV Array components: PV cells, modules, arrays
To improve overall efficiency cells can be layered on top of each other, each layer extracting energy from a particular portion of the spectrum of incoming light.
E.G. the band gap of amorphous silicon can be increased by alloying the material with carbon so that the resulting material responds better to light at the blue end of the spectrum.
GaAs based multijunction device Gallium Arsenide has a higher efficiency in terms of light absorption than silicon.
wider band-gap which: more suited to absorbing visible light
Can operate at higher temperature without a significant drop in efficiency
considerably more expensive and is used in applications where cost isn’t a issue – e.g. satellite.
: In Africa - Solar park proposed to power Europe but silica can't withstand the high temps. GaAs can
Conversion Efficiency: proportion of sunlight energy that the cell converts into electrical energy.
important bc improving efficiency is vital to making PV energy competitive w more trad sources of energy, such as FFs.
first PV cells were converting light to electricity at 1-2% efficiency. Today convert up to 17% of radiant energy that strikes them into electric energy.
ideal for remote applications where other power sources impractical/unavailable, like Swiss Alps or navigational buoys.
not practical to connect these applications to an electric grid. Also used to power small, independently placed objects e.g. billboards and road signs.
Advantages and Disadvantages
• No need for steam - v efficient, env. stable
• Free (excluding material)
• Provide electricity to remote places
•Less efficient and costly equipment (improving)
•Part Time (daylight)
•Storage needed, can't currently store enough for city
•Reliability Depends On Location
•Environmental Impact of PV Cell Production
Applications: 1997 "Sojourner" began exploring Mars. High efficiency PV cells located on top of vehicle generated 16 watts of power at noon on Mars, enough to carry out a day's mission.
~1/3 of annual energy usage goes into transportation
Petrol v convenient but heat engine efficiency only 20%
Natural gas very bulky (and will run out)
Problems with ethanol and biodiesel production
Solar cars are impractical at the moment, at 1–2 horsepower
Electric cars need batteries (but could at least use solar/wind as source of electricity)
Desperately need a replacement for portable gasoline
H carries ~32 Calories/g. thru simple reaction: 2H2 + O2 2H2O
No natural pockets of hydrogen gas
not a fuel
for our future
Very bulky in gaseous form
11,000x less energy-dense than petrol
3 times more bulky than natural gas, energy-wise
can be extracted from naturally occurring substances (like H2O)
H in H2O is “post-reaction” H - endproduct of energetic reaction
To get it back, must put energy in, running reaction backwards
hydrogen = way to
energy derived from other sources
pass electric current thru H2O and dissociate H from O
H forms on negative terminal, O on positive terminal
bubbles collected for use
twice as much H2 forms as O2
if the H is produced by FFs, all advantage is lost
have to deal with plant efficiency (33%) times electrolysis
efficiency (65%) times engine efficiency (optimistic 65% fuel cell??) = 14%
better off burning FF directly in your car, and getting 20%
efficiency: less CO2 would be emitted this way
best to use solar or wind power to run the electrolysis
near zero-emission more importantly, it’s not then diminishing a resource
When the sun is not shining, and the wind is not blowing, use H2 stored energy
need to greatly expand our electricity production far beyond today’s levels because all transportation would have to come from this resource - exacerbated by inefficiencies of process
Doesn’t make sense to pursue hydrogen until we increase non-fossil electricity production
Water availability must be considered - Using salt water adds an energy cost bc of desalination. Freshwater is a limited resource
H Fuel Cells:
Run electrolysis backwards, gaining electrical energy from bonding of H gas with O gas.
Theoretical efficiency 83%, but practical efficiency can be <65%
Much better than heat engine at 20-25%
H Fuel Cell - GM made H fuel cell car prototype, currently $1M
CAN be done and would be cheaper if mass produced