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2 uses of solar energy
1) solar thermal energy
2) solar photovoltaics
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Photovoltaics advantages
- reliability
- low operating costs
- modularity
- low construction costs
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photovoltaics disadvantages
- high initial cost
- variability
- lack of infrastructure
- increased storage costs
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Photovoltaic systems convert energy into electricity on a _____-_____device
solid-state
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PV materials
- silicon
- gallium arsenite crystals
- polycrystalline copper
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Semiconductor
material with electrical conductivity
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Doping
- performance increase (electrical behavior)
- impurities
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p-type semiconductor
- doped with boron
- deficit of electrons (holes)
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n-type semiconductor
- doped with phosphorus
- surplus of electrons
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semiconductor band structure
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extrinsic semiconductor
doped
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Band (energy) gap
-energy difference between the top of valence band to bottom of conduction band
-energy required to promote electrons into conduction band
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Isc and Voc
- Short circuit current
- open current voltage
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silicon solar cell structure
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photodiode
photodetector capable of converting light into current or voltage
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Photocurrent is a combination of:
dark current and light current
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Fermi level
highest occupied molecular orbit in valence band
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Diode
A 2 terminal electrical component
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I0
measure of recombination in a device
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I-V characteristics of a p-n junction diode (graph)
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Fill Factor
-measure of sharpness of knee in I_V curve
-Indicates 1) how well junction was made in the cell 2) how low the series resistance is
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Calculate Pmax
Pmax=Imax*Vmax
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Calculate max efficiency
η=Pmax/Pin
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Calculate cell area
A=Preq/Pmax
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Latitude
Angular distance north or south of earth's equator
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Glazing purpose
allows solar radiation to enter the collector but not reflect out
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Components of a solar panel
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Heat loss (conduction) equation
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Solar Ponds
- -bottom has high optical absorption
- -absorbs solar energy
- -vertical salinity gradient
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Active solar heating
uses solar energy to heat a fluid, then transfer the heat directly to the space
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passive solar heating
-walls, windows, and floors are made to collect store and distribute solar energy
-no mechanical or electrical devices
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Conservatory
energy store is the building itself (sunspace)
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Trombe wall
solar radiation warms the store and is radiated into the house
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direct gain
if the building is massive enough, and the heating system is responsible, the gains are useful
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wind energy physics
the kinetic energy of the air in motion
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wind turbine naming comvention
named on the basis of the way the wind flows over airfoils and their geometrical configuration
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solidity
the fraction of the swept area that is solid
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measure of turbine rotation speed
- -rotation speed [rpm], N
- -angular velocity [rad/s], ω
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Tip speed
tangental velocity of the rotor at the tip of blades, U [m/s]
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Power extraction efficiency decrease
- -blades too close together
- -blades rotating too fast
- -> blades in turbulent air
- vice-verse
- -> air passes without interfering with a blade
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Betz limit
maximum fraction of the power in the wind that can be extracted
=16/27=59.3%
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Blade pitch angle
equal to the relative wind angle minus the angle of attack (β)
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Zero set pitch
the setting angle of the blade causes the chord to be in line with a tangent to the circular path of rotation
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wind energy disadvantages
- -variable
- -low density source
- -> high initial cost
- -distance from area to land center
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wind
- -flow of gases on a large scale
- -caused by pressure difference
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Earth's atmospheric circulation diagram
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Pressure weather effects
- -isobars -> equal P
- -high P -> fine weather, little wind
- -low P -> changeable windy weather, precipitation
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Sea breeze vs land breeze
- sea breeze:
- -daytime
- -land heats up quickly
- land breeze:
- -night
- -land cools off quickly
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Lenticular clouds
stationary lens shaped clouds, perpendicular to wind direction
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Mountain valley winds
-hills and valleys distort airflow by increasing friction (between atmosphere and land)
-wind deflection occurs (barrier jet)
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Mountain valley winds diagram
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Sun
- -ball of plasma
- -fusion reactor
- -92% hydrogen, 8% helium
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electromagnetic spectrum
range of EM radiation from short λ to long λ
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Insolation
solar radiation at the surface of the earth
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direct vs diffuse radiation
- direct:
- -amount of radiation on a surface perpendicular to the sun
diffuse:
-indirect radiation from sky & from other surface objects
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Insolation measurement
-Pyronometer (solarimeter)
-voltage proportional to total incident light energy is produced
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Rooftop solar water heater
- 1. Pumped
- -needs heat exchanger
2. Thermosyphon
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Heat exchanger
uses heat transfer to transfer heat from fluid on one side of a barrier to a fluid on the other side
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Pumped solar water heater diagram
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Solar collectors
1. Heat pipe evacuated tube
2. Line-focus collectors
3. Parabolic dish collectors
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Wind mill
reduces a solid or coarse substance into minute grains by grinding
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How is the electric field generated in p-n junctions?
The dissimilar semiconductors at the junction. The electrons move in one direction and the protons in another.
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Bias
An external voltage applied to a P-N junction
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Equilibrium band structure
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Reverse biased band structure
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Forward biased band structure
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Thermosyphon solar heater
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Components of a solar panel
-black painted steel plate bonded with copper tubing
-glass
-insulation
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Solar collector application
- -domestic water heating
- -domestic space heating
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Heat pipe evacuated tube collectors
-the vacuum enables the liquid to boil at a much lower temperature
-when the solar radiation hits the surface of the absorber, the liquid turns to hot vapor and rises to the top
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line-focus collectors
-a parabolic trough focuses to a higher energy focal line rather that a focal point
-water flows through center
-generates steam for electricity generation
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parabolic dish collectors
- -focus the suns energy on a small (focal) point
- -dish has mirror like absorbers at the focal point
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Solar power tower
- -sunlight is concentrated from the mirrors
- -molten salt is pumped through the receiver and heated
- -pumped through the steam generator and creates steam
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Solar chimney
- -passive solar energy
- -sun heats chamber and warm air rises at high velocity through turbines
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Stall
a loss of lift and torque and the stalled regions of the blades
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Axial interference factor
the factor that takes into account the wind being slowed down as a result of power extraction
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measure of performance of air foils
lift to drag ratio
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airfoil pressure distribution
low pressure is created on the upper surface and high pressure on the lower surface. The pressure differences causes aerodynamic lift
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Comparing wind turbines of different characteristics
- using tip speed ratio
- λ=U/V=wr/V
- λ changes between 6 to 20
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VAWT and HAWT comparison (solidity, rotors, loading, sizing)
- -VAWT have greater solidity
- -VAWT have heavier and more expensive rotor
- -HAWT have shorter lifetime
- -VAWT don't experience gravitational loads
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Best places for wind farms
- -coastal areas
- -tops of rounded hills
- -open plains and gaps in mountains
- -average wind speed of 25 km/h
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