Solar cell driving a load a. A Si solar cell of area 2.5 cm x 2.5 cm is connected to drive a load R as in Figure 6.42a. It has the -V characteristics in Figure 6.41. Suppose that the load is 20 and it is used under a light intensity of 800 W m². What are the current and voltage in the curcuit? What is the power delivered to the load? What is the efficiency of the solar cell in this circuit? Figure 6.41 Typical /-V characteristics of a Si solar cell. The short circuit current is /ph and the open circuit voltage is Voc. The I-V curves for positive current require an external bias voltage. Photovoltaic operation is always in the negative current region. 1 (mA) 100- Dark 0 0.1 0.2 0.3 0.4 0.5 0.65 I = 400 W m2 -100 OC -200 I = 800 W m² Figure 6.42 (a) When a solar cell drives a load R, R has the same voltage as the solar cell but the current through it is in the opposite direction to the convention that current flows from high to low potential. (b) The current /' and voltage V in the circuit of (a) can be found from a load line construction. Point P is the operating point (I", V). The load line is for R = 30. 1 (mA) 0.1 0 Voc 0.2 0.3 0.4 0.5 0.6 مسلسل سلسلسلسسلسل سلسلس I-V for a solar cell under an illumination of 700 W m² -100 Slope - R Operating point 1 R sc-ph (a) (b) The load line for R=30 (I-V for the load) b. What should the load be to obtain maximum power transfer from the solar cell to the load at 800 W m² illumination? What is this load at 400 W m²? c. Consider using a number of such solar cells to drive a calculator that needs a minimum of 3 V and draws 3 mA at 3-4 V. It is to be used at a light intensity of about 400 W m². How many solar cells would you need and how would you connect them?