Solar Power (PV) Part 1 Flashcards
What are the operating principles of the solar concentrators?
- Sunlight hits the linear reflectors
- It is reflected onto the collector where it heats the water
- The steam produced is at high pressure & drives a turbine
- The steam is condensed and recycled
- The turbine is connected to a generator which produces electricity
- The electricity is converted to the correct voltage and supplied onto the national grid
What are the operating principles of the solar chimneys?
The heated air rises, and due to the slight slope of the collector it moves towards the central chimney.
The hot air rising up the chimney generates suction which draws cooler air into the collector to subsequently be heated.
The warm air rising up the chimney drives the wind turbines to produce electricity.
What are the operating principles of freshwater solar ponds?
The pond is filled from a storage tank during hours of sunlight.
The water is trapped between a black absorbing layer at the bottom of the pond and various transparent insulating layers on top.
Operated in batch or continuous mode
In the continuous mode, the shallow solar pond can be compared to a flat-plate collector in which the water is flowing continuously through the system and distributing the thermal energy evenly.
In the batch mode, the water is pumped into the pond during daylight hours and at night is passed into a storage chamber.
What are the operating principles of salt gradient solar ponds? ( layers, process and hot water vs salt water)
Three distinct layers naturally form :
The surface layer (fresh water)
The non-convective layer (medium salinity)
The storage layer (saturated salt layer)
Hot water can contain more dissolved salt
Salt water is more dense than fresh water.
Black plastic on lining the pond absorbs sunlight.
The storage layer is warmed up and can rise to approximately 90°C causing a salinity difference and hence density difference between the surface layer and the storage layer.
The salt gradient layer acts as a thermal barrier inhibiting convection.
The hot water can then be siphoned off and passed through an organic Rankine cycle engine.
What are the operating principles of closed cycle OTEC?
Warm water from the ocean surface is pumped into the evaporator which is a heat exchanger
The heat exchanger evaporates a transfer fluid such as NH3 which drives a gas turbine and generator to produce power.
The transfer fluid is recycled in a second heat exchanger where it is re-condensed using cold deep sea water.
The transfer fluid is then pumped (can be gravity fed) back into the evaporator.
What are the operating principles of open cycle OTEC?
The warm surface seawater is placed under a partial vacuum, which results in a low pressure steam.
The low pressure steam then drives a turbine and generator to produce power.
The water vapour is then re-condensed by mixing with cold sea water or in a heat exchanger as in the closed cycle OTEC.
The condensed water can be siphoned off as fresh water.
Describe the electronic processes in a solar cell?
A solar cell works by firstly absorbing sunlight.
Therefore it should be made from a material that readily absorbs light at many wavelengths of the spectrum
The absorbing material needs to have a band gap to prevent the excited electron rapicly relaxing back into the ground state from which it was excited.
Semiconductors have suitable band gap energies.
Only photons with energy above the band gap energy can be absorbed. Once excited above the band gap the electrons (and holes) rapidly relax to the band edges.
Once at the band edges the charge carriers (e & h) must be separated by some sort of asymmetry within the device.
The charge carriers must be transported to their respective electrodes.
Electrons to the cathode and holes to the anode.
Once at the electrodes the charges build up to generate an electrical potential which can drive a current through an external circuit before returning to the opposite electrode.
What is Jsc, Voc, Jmax and Vmax?
Jsc- short circuit current density
Voc- open current voltage
Jmax- maximum power point
Vmax- maximum voltage point
What is the formula for solar cell efficiency?
Eta= Power Density Out/Power Density in=Jm.Vm/Ps=Jsc.Voc.FF/Ps
What are the different materials used to make solar cells?
Metal-Readily conducts electricity and heat since many electrons are available for conduction
Semimetal- Reasonably high conductivity since at room temperature some electrons are available for conduction
Semiconductor- Small conductivity in the dark at room temperature since only a very small number of electrons are available for conduction
Insulator- Negligible conductivity at room temperature since essentially no electrons are available for conduction
What are the standard test conditions for solar cells\?
Air Mass Spectrum, incident power density of 1000 Wm^-2 at 25 deg
