Why Are Solar Panels So Inefficient?

  1. Oct 18, 2010 #1
    Hey guys, I am a complete beginner in the field of electrical engineering so please bare with me in this discussion.

    It is my understanding that;
    1) The Earth absorbs 3,850,000 exajoules in solar energy every year, which means that if we could harness even 50% of the suns power for just one day that it would be enough to power Earth for many many years.

    2) Current photovoltaic panel technology captures and converts about 15% of the solar waves it receives.

    3) The majority of this energy loss is due to band gap loss. The optimal band gap when balancing spectrum and voltage is about 1.4 eV.

    So here's my question:
    Why does less band gap result in lower voltage? Is there not some way to regulate or keep voltage constant while allowing the capture and conversion of a wider band of light?

    Thanks for any help!
  2. jcsd
  3. Oct 18, 2010 #2
    The band gap dictates the maximum voltage a solar panel can provide, they are inseparable. Thats why if you change the band gap you change the voltage. As to exactly why - you'd need to read up about semiconductors and PN junctions because I don't know off my head. Maybe someone else here does. The aim of a solar panel is not to provide the max voltage, but the max power P=IV.

    If you made the band gap (voltage) too big then photons from the sun would not have enough energy to excite electrons and you'd lose efficiency. On the other hand, if you made the band gap too small, you'd get almost no power because of the tiny voltage.
  4. Oct 18, 2010 #3
    Companies such as Spectrolab can make cells that are 40% and higher efficiency. They do this by layering different semiconductor materials which have different bandgaps. This allows them to use more of the solar spectrum. The cost so far is very high, which makes them only suitable for space applications: http://www.spectrolab.com/space.htm
  5. Oct 19, 2010 #4


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    This is an instance where 'efficiency' (in terms of energy out / energy in) may not be the most relevant factor, in engineering terms. There aren't many places (on Earth) where actual panelarea is a real limitation. What is often more important is the power output and lifetime (i.e. total energy produced) for a given cost.
  6. Oct 19, 2010 #5
    Hey =).

    If you want to make more efficient solar cells should you study physics or electrical engineering?
  7. Oct 19, 2010 #6
    not all the light falling on a solar panel (photovoltaic cell) are turned into electronics, or are turned into electrons without enough energy to jump the bandgap.

    But 25% or 40% is amazing efficiency, compare that with a motor car petrol engine that will top out at mabey 25%.

    And if you want to make more efficient solar cells, study both physics AND engineering :)

    There are also other things that limit the efficiency of PV's like if they are wired in series, you have kirchoff's law, limiting the current through a series circuit to the current of the lowest source in that circuit.

    So shading of some cells will drop the efficiency of large chunks of an array.

    Also the regulation system to provide enough current to charge batteries (not a 100% efficient process), IR losses in wires, reflection of light. Cells in darkness or shaddow robbing power from the system.

    There is some performance curve called a FILL FACTOR, that from memory is a ratio of open circuit voltage and short circuit current that is a determination of the 'goodness' of the cell.
  8. Feb 8, 2011 #7
    Photovoltaic panels are EXTREMELY efficient. The best multijunction solar cells (2011) have energetic efficiencies over 40%. In comparison, a biological organism (i.e. photosynthesis of a plant) has an energetic efficiency of less that 1%.

    That said, what you have to remember is that efficiency is meaningless on its own, since sunlight is "free". The only important factor is COST. Of course, efficiency affects the cost (~ inversely proportional). But efficiency is ONLY important in the cost calculation, nowhere else.
  9. Feb 8, 2011 #8
    I should clarify, when I say only COST is important, I mean with respect to global energy demand, not with respect to specialist applications, such as space travel, where efficiency is much more important due to weight restrictions.
  10. Feb 8, 2011 #9
    One of the limitations is that the basic process is the photoelectric effect; a visible light photon hits an electron in the silicon (valence band) and raises it up across the band gap into the conduction band. Although the band gap is 1.4 volts, a blue photon (4000 Angstroms) is 3 eV, so the rest of the photon energy (1.6 eV) is wasted as heat. Also, IR photons >8500 Angstroms don't have enough energy.

    Bob S
  11. Feb 9, 2011 #10

    I assume you realize this thread has been dormant for almost four months?

    Anyway, the biggest problem with solar is that it is not currently economically viable, and is not likely to ever be viable as the sole source of energy unless a means of economical energy storage emerges. But, it seems you already know that :-)

  12. Feb 9, 2011 #11
    Oops, I didn't realize that, sorry.

    Actually, solar is the only ongoing source of energy on the Earth, (I guess you could count Nuclear potential as non-solar), so the question of cost is rather moot once we run out of fossil fuels.
  13. Feb 9, 2011 #12
    Eeek, people are going to be upset. I forgot that Tidal energy comes from gravitational forces, not from the sun. Sorry. But anyway the Tidal resource is rather limited.
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