Utility companies are doing a great job but one must understand that anticipating the load demand is a critical task. Complexity comes in with the increasing production of electricity from solar energy. Curve shown below represents the load demand curve of electricity in California on 31 March 2012. The blue line in the curve represents the actual load demand in California on 31 March 2012. Least power is required over midnight. During the morning, curve ramps up when offices and schools get started, during evening time curve ramps up to the peak load and then again goes down. 

Duck Curve illustration by CAISO (California Independent System Operator) with "Net Load" versus "Time of Day" plot. Predictions for Net Load are made for different Years.

             As electricity from solar is produced only during the day time and grid gets flooded with solar energy, a dip is observed in the demand curve (red lines) during the day time with an increase in solar energy production. With further increment in solar energy production, dip observed in the curve goes on lowering down during the day time (red lines 2014-2020). The graph curve resembles a "Duck" so it was given the name "Duck Curve" by CAISO (California Independent System Operator).
              In the graph with increased solar capacity, it looks like demand has been dropped during the day time. Now the problem arises due to intense ramps in the new graph. As the sun sets, utilities have to suddenly ramp up the demand to meet the peak load which is not possible as it destroys the economics of power plant. Nuclear and Coal power plants are baseload power plants, so they need to be turned on 24X7 and can't be turned on and off every day for economic load dispatch with solar power plant. To avoid this problem curtailment of solar energy must not be considered as a solution.

Some of the solutions are:
  • Integrating solar energy with energy storage technologies (batteries) could possibly eliminate the risk of overgeneration of electricity during day time.
  • Switching to electric vehicles will enhance the usage of solar energy.
  • Encouraging the use of innovative solutions that are cost effective such as Tesla Powerwall.
  • Using Machine Learning to predict load demands much more accurately leading to lower losses.
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These are essentially thin film solar cells. So, thin film of CIGS is used as primary absorber. These are essentially called chalcopyrite based solar cells. CuInSe2 ( Copper Indium Selenide ), CuInS2 ( Copper Indium Sulphide ) and CuGaSe2 ( Copper Gallium Selenide ) with band gap of 1 eV, 1.5 eV and 1.7 eV are the major chalcopyrite compounds that are being used for solar cell applications. With  a band gap of 1 eV CuInSe2 ( Copper Indium Selenide ) provides the heighest efficiency and its efficiency can be further increased by addition of Gallium, thus making it Cu(InGa)Se2 (Copper Indium Gallium Selenide) with a band gap of 1.15 eV.

Note :
Band gap of 1.45 eV  is most appropriate for solar cell applications because wavelength corresponding to 1.45 eV comes out to be 855 nm (λ = hc/E = 1240/1.45 = 855 nm). Thus, whole visible region will be absorbed and wavelength above 855 nm will not be absorbed.

Absorption coefficient of chalcopyrites is quite high; therefore, a few micron thick active layer is required for the absorption of visible region of sunlight. CIGS is an interesting material as by varying the ratio of Indium to Gallium, one can change the bandgap. As we can vary this ratio with large tolerance CIGS band gap is tuned efficiently. Commercially bandgap of CIGS is between 1.01 eV to 1 eV. CIGS is used as P-type semiconductor and CdS is used as N-type semiconductor leading to the formation of junction and depletion region. Band gap of CIGS and CdS is 1.1 eV and 2.42 eV. As the major portion of the solar spectrum is absorbed by CIGS, CdS is mainly used as partner layer to form the cell and is referred as "Buffer Layer".

   CIGS Solar Cell Parameters

Efficiency (η) : 20 %
Open Circuit voltage (VOC) : 0..69 V
Short Circuit Current Density (Jsc) : 35.5 mA/cm2
Fill Factor (FF) : 81 %
Mobility of Electrons (μe) : 90-900 cm2 / Vs
Mobility of Holes (μh) : 5-50 cm2 / Vs
Melting Temperature : 1259 K
Density : 5.75 g / cm3


Fabrication


For substrate, Mo coated glass or Soda Lime glass is used because sodium helps in surface passivation by controlling dangling bond density. Moreover coefficient of thermal expansion of soda lime glass matches with that of CIGS leading to lower thermal stress and prevents cracking in thin film. CIGS is the heart of this solar cell. CIGS is commonly deposited with co-evaporation technique. In this process temperature of around 550 degree celsius is required and all the elements ( Copper, Indium, Gallium, Arsenide ) are evaporated together. CdS is deposited using Chemical Bath Deposition (CBD) method. In CIGS solar cell substrate configuration is used instead of superstrate configuration as in CdTe solar cell. Since CIGS deposition requires high temperature conditions so, CIGS is deposited before CdS.


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