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This research appraises comparative analysis between single diode and double diode model of photovoltaic (PV) solar cells to enhance the conversion efficiency of power engendering PV solar systems. Single diode model is simple and easy to implement, whereas double diode model has better accuracy which acquiesces for more precise forecast of PV systems performance. Exploration is done on the basis of simulation results and MATLAB tool is used to serve this purpose. Simulations are performed by varying distinct model parameters such as solar irradiance, temperature, value of parasitic resistances, ideality factor of diode and number of series and parallel connected solar cells used to assemble PV array. Conspicuous demonstration is executed to analyze effects of these specifications on the efficiency curve and power vs. voltage output characteristics of PV cell for specified models.

The globe gains an unbelievable supply of solar energy. The sun is an average star, a fusion reactor. It has been lighting over 4 billion years. It contributes sufficient energy in one minute to supply the world’s energy demands for one year [

Global energy requirement and environmental issues are the compelling force for use of sustainable, alternative, and clean energy resources [_{2} [

A solar/PV cell is formed by fabricating a p-n junction in a thin wafer of semiconductor. These cells depend on photovoltaic effect for converting solar radiation into electricity [

Recently a new era for PV cell material has been launched with the study of perovskite, which is a mineral found in the Earth’s mantle. Researchers claim that it could be more efficient than conventional cell material by placing it on the top of traditional silicon cells. As well, it presents an economy friendly process. However, stability tests are needed to observe the water and temperature sensitivity of the material. Nevertheless, search for efficient and cost effective material for solar cell is going on [

Typically, Silicon is used to assemble solar cells and an inadequate amount of power is produced by the silicon cell, because of low conversion efficiency [

For various commercial operations, distinct types of photovoltaic (PV) cell technologies have been used. These cell technologies can be classified as multicrystalline, mono-crystalline and thin film. Single and double diode PV models have been widely used for modelling the output characteristic of a PV module [

Single diode model is the simplest as it has a current source in parallel to a diode. This model is upgraded by the inclusion of one series resistance, R_{s} [_{p} [_{oc}). Two-diode model (consisting R_{p} and R_{s}) shown in

・ I_{ph} is the current generated by the incident light.

・ I_{D}_{1} is the Shockley diode equation due to diffusion.

・ I_{D}_{2} is the Shockley diode equation due to charge recombination mechanisms.

・ I is the output Current of PV cell.

・ I_{01}, I_{02} [A] are the reverse saturation current of the diodes D_{1} and D_{2} respectively.

・ q is the electron charge [1.60217646*10^{−19} C].

・ k is the Boltzmann constant [1.3806503*10^{−23} J/K].

・ T [K] is the temperature of the p-n junction.

・ a_{1} and a_{2} are ideality factor of the diodes D_{1} and D_{2} respectively for two diode model.

・ a is ideality factor of diode for one diode model.

・ V_{T} is the thermal voltage of the module.

Proportion of output energy of the solar cell to input energy from the sun is described as efficiency. Simultaneously reflecting the capability of the solar cell itself, the efficiency relies upon the spectrum and intensity of the incident sunlight and the temperature of the solar cell [_{oc}) and short circuit current (I_{sc}).

・ V_{OC} is open circuit voltage & I_{SC} is the short circuit current and

・ G_{n} is the irradiance, T_{n} is the temperature, all at standard test conditions.

・ K_{V} is the open circuit voltage temperature coefficient & K_{I} is the short circuit current temperature coefficient. η is efficiency.

The dominant phenomena that confine cell efficiency are [

Ø Reflection from the cell’s exterior.

Ø Light that is not enough dynamic to isolate electrons from their atomic bonds.

Ø Light that has excess energy beyond that required to isolate electrons from bonds.

Ø Light-produced electrons and holes (empty bonds) that casually collide with each other and recombine before they can promote to cell performance.

Ø Light-produced electrons and holes that are brought together by exterior and material blemishes in the cell.

Ø Resistance to current movement.

Ø Self-shading ensuing from upper-surface electric contacts.

Ø Performance degradation at non optimal (high or low) conducting temperatures.

In order to analyze the behavior of both PV model, simulation is operated in MATLAB environment [

Input Power | 260 W |
---|---|

Open Circuit Voltage (V_{oc}) | 37.92 V |

Short Circuit Current (I_{sc}) | 8.67 A |

Temperature Coefficient of V_{oc} | −0.33%/˚C |

Temperature Coefficient of I_{sc} | 0.06%/˚C |

Reference Temperature | 25˚C |

The efficiency of a PV appliance is contingent on the spectral distribution of the solar radiation. The Sun is a source of light and its radiation spectrum may be examined with the spectrum of a blackbody near 6000 K. Radiation of electro magnet in all wavelengths are absorbed and emitted by a black body [

The study of the effect of the solar radiation on PV devices is difficult because the spectrum of the sunlight on the Earth’s outward is affected by components such as the variation of temperature on the solar disc and the impact of the ambient [

As demonstrate in ^{2}, efficiency of two diode models is 2.4% higher than one diode model. Consequently when irradiance is 250 watt/m^{2}, efficiency of two diode models is 3.4% higher than one diode model. Hence

Changing Parameter | Two Diode Model | One Diode Model | ||
---|---|---|---|---|

Irradiance (watt/m^{2}) | P_{max} (watt) | Efficiency (%) | P_{max} (watt) | Efficiency (%) |

250 | 65.45 | 25.1733 | 56.59 | 21.7667 |

230 | 59.92 | 23.0467 | 51.69 | 19.8822 |

210 | 54.39 | 20.9229 | 46.81 | 18.0038 |

190 | 48.88 | 18.8008 | 41.94 | 16.1340 |

170 | 43.37 | 16.6818 | 37.11 | 14.2731 |

Increasing temperature increases the intrinsic carrier concentration. This urges the Fermi level adjacent to the intrinsic Fermi level (the middle of the band gap). Inequality between Fermi-levels of the p-type and n-type regions determines the built-in potential of a diode. As temperature increases, the Fermi level in each region shifts closer to the center of the gap, hence the built-in potential is decreased [

Changing Parameter | Two Diode Model | One Diode Model | ||
---|---|---|---|---|

Temperature (deg. Celsius) | P_{max} (watt) | Efficiency (%) | P_{max} (watt) | Efficiency (%) |

15 | 61.52 | 23.6634 | 60.55 | 23.2895 |

25 | 61.04 | 23.4774 | 58.56 | 19.8822 |

35 | 60.55 | 23.2922 | 56.59 | 21.7667 |

45 | 60.08 | 23.1077 | 54.64 | 21.0170 |

55 | 59.60 | 22.9239 | 52.71 | 20.2749 |

65 | 59.12 | 22.7407 | 50.80 | 19.5407 |

Diode’s built-in potential is relevant with the conducting voltage of a solar cell. As a solar cell gets hot, the voltage is reduced, and therefore the output power and efficiency both are reduced. Thus the performance of solar cells decreases at high temperatures.

Power dissipation across internal resistances affects efficiency as well as maximum output power of solar cells. These parasitic resistances can be modelled as a parallel shunt resistance (R_{p}) and series resistance (R_{S}) [_{p} would be infinite and would not provide an alternate path for current to flow, while R_{S} would be zero, resulting in no further voltage drop before the load.

As shunt resistance declines, current passed through it increases for a given level of junction voltage. Consequence is that the voltage-controlled portion of the I-V curve begins to sag far from the origin, producing a remarkable devaluation in the terminal current I and a minor reduction in V_{OC}. Hence output power is reduced. Very inferior amount of R_{p}_{ }will attain a significant deflation in V_{OC}. Much as in the case of a large value of series resistance, a poorly shunted solar cell will take on operating attributes analogous to those of a resistor [

Changing Parameter | Two Diode Model | One Diode Model | ||||
---|---|---|---|---|---|---|

Shunt Resistance (R_{p}) | P_{max} (watt) | Efficiency (%) | P_{max} (watt) | Efficiency (%) | ||

200 | 58.18 | 22.3772 | 54.47 | 20.9507 | ||

250 | 59.09 | 22.7291 | 55.28 | 21.2642 | ||

300 | 59.70 | 22.9642 | 55.83 | 21.4744 | ||

350 | 60.14 | 23.1329 | 56.22 | 21.6247 | ||

400 | 60.47 | 23.2594 | 56.51 | 21.7374 | ||

450 | 60.73 | 23.3578 | 56.74 | 21.8251 | ||

PV cell efficiency as well as maximum power is increased with increasing value of shunt resistance.

For the same amount of current, the voltage drop between the junction voltage and the terminal voltage becomes greater as series resistance increases [_{SC}, the short-circuit current. Tremendous values of R_{S} will also generate a significant reduction in I_{SC}; in these regimes, series resistance governs and the behavior of the solar cell resembles that of a resistor [

Inspecting

Multiple numbers of solar cells are connected to form panels. Therefore panels can be connected in series string

Changing Parameter | Two Diode Model | One Diode Model | |||
---|---|---|---|---|---|

Series Resistance (R_{s}) | P_{max} (watt) | Efficiency (%) | P_{max} (watt) | Efficiency (%) | |

0.2 | 60.64 | 23.3241 | 56.67 | 21.7978 | |

0.4 | 59.84 | 23.0182 | 55.89 | 21.4989 | |

0.8 | 58.26 | 22.4085 | 54.34 | 20.9034 | |

1.2 | 56.68 | 21.8026 | 52.81 | 20.3122 | |

1.4 | 55.90 | 21.5015 | 52.04 | 20.0189 | |

to increase the voltage level and in parallel to increase the current level or in a consolidation of the two. The accurate configuration depends on the current and voltage load prerequisites. Efficiency of the array can be maximized by coordinating interconnected panels in respect of their outputs [

Imbalance in the short-circuit current of series connected solar cells can, contingent upon the conducting point of the module and the degree of conflict, have a severe repercussion on the PV module.

Series connections increase output power because voltage output is increased whereas output current remains almost constant.

Parallel mismatch is not an issue for small modules, because in these cases cells are connected in series. Large arrays are generated by combining modules in parallel. So conflict mostly contributes at a module level rather than at a cell level.

Changing Parameter | Two Diode Model | One Diode Model | ||
---|---|---|---|---|

Number of Series Connected Cells (N_{s}) | P_{max} (watt) | Efficiency (%) | P_{max} (watt) | Efficiency (%) |

45 | 46.37 | 17.8366 | 42.64 | 16.4025 |

50 | 51.05 | 19.6374 | 47.32 | 18.2015 |

55 | 56.09 | 21.5768 | 51.97 | 19.9895 |

60 | 60.75 | 23.3656 | 56.59 | 21.7667 |

65 | 65.70 | 25.2705 | 61.18 | 23.5331 |

Expanded number of parallel connected cells causes the output current to increase and the horizontal part of the I-V curve moves upward. Along with this,

Changing Parameter | Two Diode Model | One Diode Model | |||
---|---|---|---|---|---|

Parallel Connected Cells (N_{p}) | P_{max} (watt) | Efficiency (%) | P_{max} (watt) | Efficiency (%) | |

1 | 60.75 | 23.3656 | 56.59 | 21.7667 | |

2 | 122.12 | 46.9694 | 113.39 | 43.6139 | |

3 | 181.30 | 69.7340 | 168.38 | 64.7632 | |

Ideality factor of a diode is an assessment of how intimately the diode pursues the conceptual diode equation. It is also evaluate the junction feature and the type of recombination in a solar cell [

A constant value for the ideality factor is assumed for single diode equation. Practically, ideality factor is a function of voltage across the device. At high voltage, when surfaces and the bulk provinces command the recombination in the device, the ideality factor (a_{1}) is approximately one. However the ideality factor (a_{2}) approaches two when recombination in the junction dominates at lower voltages. The junction recombination is designed by including a second diode in parallel with the first and locating the ideality factor typically to two [

A superior value of diode ideality factor degrades the FF and efficiency for a single diode model. However it usually signals high recombination and gives low open-circuit voltages [

Diode ideality factors a_{1} and a_{2} respectively represent the diffusion and recombination current components for a double diode model. In accordance with Shockley’s diffusion theory, the diffusion current, a1 must be unity [_{2} is malleable. _{max} and efficiency are almost constant after a_{2} reaches the value of (1.2). Hence this will be the appropriate ideality actor for diode (D_{2}) to have maximum cell efficiency.

Changing Parameter | Two Diode Model | One Diode Model | ||
---|---|---|---|---|

Diode Ideality factor (“a” for one diode model and “a_{2}” for two diode model | P_{max} (watt) | Efficiency (%) | P_{max} (watt) | Efficiency (%) |

1 | 58.55 | 22.5226 | 59.15 | 22.7537 |

1.2 | 60.55 | 23.2922 | 56.59 | 21.7667 |

1.4 | 60.66 | 23.3333 | 54.16 | 20.8342 |

1.6 | 60.67 | 23.3371 | 51.86 | 19.9493 |

1.8 | 60.67 | 23.3377 | 49.68 | 19.1082 |

2 | 60.67 | 23.3379 | 47.59 | 18.3066 |

Because of the large expenditure of PV modules, optimal utilization of the accessible solar energy has to be assured in PV power generation. This desires an authentic, detailed, dependable and extensive investigation of the designed scheme prior to initiation. Inclusion of the additional diode for double diode model increases model parameters. To achieve desired performance, prime challenge is to compute the values of all the model specifications. Using this MATLAB simulation-based comparative analysis, double diode model is found to contribute superior performance compared to single diode model. Accordingly selected model could be effective for professionals who require easy, understandable and accurate PV models with most desired performance to design their system. Influence of air pollutants, dirt and many other climate factors are not considered in this research. It will be appealing to investigate how these components will affect the entire energy delivered from the Sun. Additional approach specifies two-diode model by inspecting its physical attributes such as the electron diffusion coefficient, minority carrier’s lifetime, intrinsic carrier density and other semiconductor properties.

Tanvir Ahmad,Sharmin Sobhan,Md. Faysal Nayan, (2016) Comparative Analysis between Single Diode and Double Diode Model of PV Cell: Concentrate Different Parameters Effect on Its Efficiency. Journal of Power and Energy Engineering,04,31-46. doi: 10.4236/jpee.2016.43004