Solar maximum power point

FIG. 60. Current-voltage characteristics of a solar cell made at 65 MHz and 42 mW/cm-, The dashed line indicates the maximum-power point. 

When cells are delivering power to electrical loads under real-world conditions, the intensity of solar radiation often varies over time. Many systems use a maximum power point circuit that automatically varies the load seen by the cell in such a way that it is always operating around the maximum power point and so delivering maximum power to the load. 

Here AVmax = Vsave(max) represents the difference in voltages at the semiconductor electrode and metal electrode at the maximum power conversion point. For example, in their experiment using a p-type InP photocathode, Heller and Vadimsky [120] obtained a current 23.5 mA/cm at maximum power point. A voltage of 0.1 IV vs SCE was applied in the case of InP electrode and -0.33V vs SCE in the case of platinum electrode, to obtain this current. Thus, the maximum saved voltage AVmax= 0.11( 0.33) V= 0.43V. Therefore, Psaved=0.43 V X 23.5 mA/cm = lO.lmW/cm. As they used a solar illumination of 84.7 mW/cm, the efficiency is 11.9%. 

In order to get an estimate of the solar-to-electrical conversion efficiency on layered compounds, sample D has been measured in sunlight. The result, obtained at 92.5mW/cm2 insolation is shown in Fig. 8. The maximum power point is at 0.33V and 10.7mA/cm2, with a resulting solar conversion efficiency of 3.7%. As is evident from Fig. 7, some samples show better overall performance than sample D. The best of these, sample G, the surface of which was accidentally damaged before being measured in the sun, had a maximum power output which exceeded that of sample D by a factor of 1.4 bringing the estimated solar conversion efficiency to 5.2%. 

For solar cells, the fill factor FF determines the position of the maximum power point in the 4th I/V quadrant of the illuminated diode and is therefore a quality sign of the photodiode. Besides the increased efficiency, the FF of a photodiode is also important when evaluating the proper function of the diode. High FF values are expected only for diodes with a strict selection principle for the separation of positive and negative carriers. There are several loss mechanisms for photodiodes that can reduce the FF in a photodiode ... 

The principal method of characterizing solar-cell performance is the measurement of conversion efficiency while the cell is exposed to 1 sun illumination (—100 mW cm-2). The conversion efficiency is determined by measuring the current - voltage characteristic (see Fig- 8), locating the maximum power point (Pm = JmVm), and also measuring the solar insolation... 

We have used a simplified approach to check whether the superposition principle is approximately valid. We apply Eq. (5.1) directly to the experimental results obtained with the solar cells. We choose three values of the voltage, V = V0c, V = Vp (Vp is the voltage at the maximum power point) and V = 0.25 Foe- We read Id and /sc for each voltage from Fig. 5.18. We do this for each composition. We then plot 81 = Id — Isc versus composition. We also plot I(V) read from Fig. 5.18. The difference of 81 from I(V) is a measure of the deviation from the superposition principle. For open circuit... 

Photoelectrochemical cells for solar photon conversion are usually designed to produce either electric power or solar fuels this book focuses on the latter. Power-producing solar cells are designed to be operated at their maximum-power point to produce electric power at the energy conversion efficiency... 

Figure 1.8 Cell schematics for a regenerative solar cell based on (a) an n-type photoelectrode (b) ap-type photoelectrode. The top diagrams show the cell reactions under illumination, the middle diagrams the electronic energy levels and band bending, and the bottom diagrams the cell current-voltage (I-U) characteristics with the photoelectrode and counter electrode (CE) currents shown in the same quadrant. The maximum power point is located at the point on the current-voltage curve at which the rectangle of maximum area may be inscribed in this quadrant. The photovoltage V, the electron and hole quasi-Fermi levels E and fip and the solution Fermi level f o.R, the open-circuit potential Ugc of the photoelectrode and the standard redox potential 17 ° of the 0,R redox couple are also shown.

Power Conversion Efficiency (rjmax) The power conversion efficiency is the ratio of the power produced by the cell at its maximum power point to the solar power incident on the device. 

Jayadevaiah 44 45 has also discussed the use of semiconductor-electrolyte interfaces for solar energy conversion, and Figure 7 gives the cell characteristic for his cell, which contains a silicon electrode. It is clear from the form of this that the internal resistance of the cell is rather high. The power conversion efficiency at the maximum power point is 2.7%. The stability of the Si is not discussed, but is almost certainly poor. 

Fig. 1 Matching of current-voltage characteristics of solar cell and electrolyzer (a) at constant light power density 1, 2, 3 - characteristics of solar cell at Ng = 1,2, and 4 (at Sj = const) 4 - characteristic of electrolyzer (Ng = 1) o - maximum power point (MPP) dashed line shows the locus of maximum cell output power at the given radiation power density (b) at varying radiation power density 1 , 2, 3, -characteristics of solar cell dashed line - locus of MPP hatched area - variations of MPP in the most probable limits of variation of the light power density and temperature 4 - characteristic of electrolyzer.

Fig. 7. 1, 2 - Characteristics of solar array and electrolyzer, respectively, for the plant described in [25] 3 - maximum power curve for the solar array 4 - the dependence of I Ng on V/Ng corresponding to the optimum system (Ng = 9, Ng = 2) - the maximum power points for characteristics 1 and 4 [24],...

It should be emphasized that the plant must operate under optimum conditions the solar array is at the maximum power point, and the voltage at each battery module is v, for which purpose the optimum values of design parameters r and t should be maintained. 

G. L. Araujo, and E. Sanchez, Analytical expressions for the determination of the maximum power point and the fill factor of a solar cell, Solar Cells 5 (1982) 377-386. 

C. M. Singal, Analytical expressions for the series-resistance-dependent maximum power point and curve factor for solar cells, Solar Cells 3 (1981) 163-177. 

The maximum power point of efficient solar cells is located close to the open circuit voltage (see Figures 2.12 and 2.89). For p-type semiconductors, the open circuit condition is the most anodic potential at which the photocathode is operated and anodic dark currents compensate the cathodic photocurrent at this potential. 

Figure 12.3 Typical photocurrent-voltage characteristics in a solar cell.Jsc is the short-circuit current density, Voc is the open-circuit voltage, and /mpp and Vmpp are the current and voltage at the maximum power point, respectively.

Figure 18.2 Typical current-voltage characteristics for dark and light current in solar cells and the main photovoltaic parameters that define the device quality. Tc, short circuit current Voc, open circuit voltage FF, fill factor l ax nd V ax, the current and voltage values at the maximum power point rj, photoconversion...

---