Solar Cogeneration and Flow Boiling

Energy From the Sun

The sun produces an enormous amount of energy. Gravitational forces acting on the Sun's mass causes very high pressures in the core. These extreme pressures cause the fusion of hydrogen atoms to make helium atoms. The Sun gives off approximately 63 million W/m2 of energy.[12] Approximately 1369 W/m2 reach the earth and only a fraction of that reaches the Earth's surface due to absorption and reflection in the Earth's atmosphere. The Earth's surface recieves approximately 168 W/m2 which is still a very substantial amount of energy.[11] The total energy aborption of the earth is 2000 times more energy that that is needed by the entire global population.

Harnessing Solar Energy

There are four major ways of harnessing the energy of the sun. One way is by converting solar radiation directly into electricity using photovoltaic cells. Photons from the solar radiation hit the surface of the PV semiconductor knocking loose electrons. These electrons are then guided to contact via an electric field which then create an electric current. Another way to harness solar energy is through solar thermal collection where solar radiation is absorbed as heat and the heated fluid can then be used for other purposes. Solar power plants use the same principle just on a much larger scale. A solar power plant uses mirrors or solar concentrators to produce very high temperatures. These temperatures are then absorbed by a working fluid which is used in some electricity generation system, usually a turbine. Solar powerplants produce large quantities of energy. Finally, solar energy can be used to produce hydrogen from water in solar chemical plants. The hydrogen can then be used for many applications.

PV Cell Types and Efficiencies

There are many different types of photovoltaic cells all having pros and cons. Currently the highest efficiency reached in a solar cell is 40.7%, using a multijunction cell and a solar concentrator. The issue with multijunction cells are that they are very expensive. On the opposite side of the spectrum are organic solar cells. These cells are currently only about 5% efficient but are cheap to manufacture, very thin and flexible.[5] Organic cells can be used in many different applications, such as in clothing.

Solar Concentrators

There are four major types of solar concnetrators. One type of concentrator uses optical lenses to focus light. Another type of concentrator uses parabolic shaped troughs to reflect light to a specific point. A dish concentrator uses a dish shape to reflect light. Finally a power tower uses many mirrors that are controlled to reflect the light to a single point creating very high temperatures.

PV Cooling Systems

The efficiency of a solar cell decreases approximately 0.08% with every increase in temperature of 1 degree celcius, therefore it is imporatant to cool some types of cells.[16] Cooling systems can be either passive or active both having advantages and disadvantages. A passive system does not require input energy to cool the system, but has a very limited cooling capability. On the other hand an active system must have a control system and an input to control the pumping, but has a much higher cooling capability and can be precisely controlled.

Sun Tracking Systems

Like cooling systems, sun tracking systems can be either passive or active. A passive system uses fluid pressure differences and gravitational forces to control the direction of the system. This type of system does not require an energy input, but is less accurate than an active system. An active system is usually motorized and either uses predetermined sun location data in order to point the system in the correct direction or it uses a solar sensor to continuosly track the sun and adjust the system orientation.

Flow Boiling Correlations

When cooling a high temperature PV module there is a significant possibility that the fluid being used will change phases. When a fluid changes phases its heat transfer characteristics change drastically and so too does the fluid characteristics. To account for the phase change and the different flow regimes a correlation can be implemented. A flow correlation is a great tool to find the heat transfer coefficient or nusselt number of a working fluid anywhere within the phase transition from liquid to gas. The most popular flow boiling correaltion is the Chen Correlation and other popular correlations are those of Shah and of Gungor and Winterton.[4]

Solar Power Plants

Solar Power Plants use the heat created by solar energy in an engine cycle to create electricity. There are many possible cycles that can be used such as the rankine, stirling, and Brayton cycles. These engine efficiencies increase with increased temperature. On the other hand the efficiency of a solar collector decreases with increasing temperature. Because solar plants use solar collectors as well as engine cycles, an optimum operating temperature can be found for each different solar plant setup.[11]

References:

  1. C. J. Winter, R. L. Sizmann, L.L. Vant-Hull, Solar Power Plants, p.137, 1991
  2. F. P. Incropera, D. P. DeWitt, Introduction to Heat Transfer 4th ed., 2002
  3. G. F. Hewitt, G.L. Shires, T. R. Bott, Process Heat Transfer, 1994
  4. K. E. Gungor, R. H. S. Winterton, Simplified General Correlation for Saturated Flow Boiling and Comparisons of Correlations with Data, Chem. Eng. Res. Des., Vol. 11, 1987
  5. M. A. Green, K. Emery, D. L. King, Y. Hishikawa, W. Warta, Solar Efficiency Tables (Version 29), Progress in Photovoltaics: Research and Applications. 15, p.35-40 (2007).
  6. R. Leutz, A. Suzuki, A. Akisawa, T. Kashiwagi, Design of a Nonimaging Fresnel Lens for Solar Concentrators, Solar Energy. 65(6), p.379-387 (1999).
  7. N. H. Helwa, A. B. G. Bahgat, A. M. R. El Shafee, E. T. El Shanawy, Maximum Collectable Solar Energy by Different Solar Tracking Systems, Energy Sources Part A: Recovery Utilization and Environmental Effects. 22(1) p.23-34 (2000)
  8. B. S. Petukhov, Heat transfer and friction in turbulent pipe flow with variable physical properties, Adv. Heat Transfer 6, p.503-565 (1970).
  9. H. D. Baehr, K. Stephan, Warme und Stoffubertragung, p.389 (2003)
  10. http://www.nrel.gov/pv/thin_film/docs/kaz_best_research_cells.ppt
  11. http://powerfromthesun.net/book.htm
  12. http://home.iprimus.com.au/nielsens/solrad.html
  13. http://www.eoearth.org/article/Solar_radiation
  14. http://www.ukerc.ac.uk/Downloads/PDF/07/0704SustainableHydrogen/0704HydroProdFusionSattler.pdf
  15. http://www.ez2c.de/ml/solar_land_area
  16. http://alumni.media.mit.edu/~nate/AES/PV_Theory_II.pdf
  17. http://home.iprimus.com.au/nielsens/#
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