Solar Collection and Energy Transport.

Solar Energy: Collection, Energy Generation and Heat Transfer

How Solar Energy is Used:

Solar Thermal Power:

Photons into Electrons: PhotoVoltaic Devices

Charge Generation Photoelectric Effect

  • When photons strike a metal, their energy is used to liberate loosely bound electrons and therefore induce a current.

  • Efficiency of this process depends upon the material

  • This is the principle behind many digital cameras, otherwise known as CCD cameras . These kinds of cameras are used in astronomy to take digital pictures . Incoming photons are converted into units of electric charge and stored at individual pixel locations. Different amounts of charge represent different intensity levels. This encoded information is a digital image.

    To make use of the photoelectric effect, we need material that is a good conductor of electricity and which can be manufactured in bulk at reasonable cost. This conditions strongly constrain the available choices. For most practical aspects, Silicon is the material of choice.

    Silicon:

    Schematic structure of energy bands in Silicon:

    Hence, if a silicon atom receives at least 1.11 Electron Volts from some source, a valence electron will move to the conduction band. Once an electron is in the conduction band, the material can carry a current and the material is now a conductor.

    So much energy is 1.11 Electron Volts?

    This does not mean that the efficiency of silicon in converting solar photons to electrons is 77%!

    Energy Losses:

    The efficiency is strongly temperature dependent. As the temperature is raised, the internal resistance of the material increases and the electrical conductivity decreases.

    The fundamental physical limitation in production photovoltaic cells is then this decrease in efficiency as the temperature of the cell increases. Because of this, for a material like silicon, the operating efficiency of a photovolatic array will probably never be higher than 20% and will most likely be between 5 and 15%.

    This doesn't mean that production is not possible. It does mean that relatively large collection areas must be obtained which means high capital costs. If those costs can not be subsidized, then PV arrays can never be competitive in the commercial energy market place.

    To have a production photovoltaic cell, one must mix impurities into silicon (like boron). This will create an internal electric field which will allow the liberated electrons to move down the material.

    Over the last 40 years, the effort has gone into increasing the efficiency of PV cells and bringing down the manufacturing costs.

    Recent Advances:

    Advances in Amorphous Silicon technology has led to continuous thin-film deposition process.

    Can increase efficiency by using solar cells in conjunction with focussed systems (parabolic collectors).

    BUT

    Current Economics:

    Consumer cost for energy from newly constructed coal-fired plant in the US ranges from 8-20 cents per KWH

    PV power generation would cost the consumer 25-50 cents per KWH.

    Costs might be equivalent when pollution from coal is also considered but still, the structure is not here for the consumer to pay the true cost of energy

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