17-08-2012, 01:12 PM
High efficiency photovoltaic systems
[attachment=31841]
INTRODUCTION
High efficiency photovoltaic systems available in the market are composed of a concentrating optical system, a monolithically stacked solar cell, a tracking system and a cooling system. The overall conversion efficiency depends mainly on the efficiency of the solar cell. Efficiencies as high as 41.6 % have been measured on devices consisting of a stack of three junctions made of InGaP/InGaAs/Ge . On double-junction devices, an efficiency of 32.6 has been measured on a device made of InGaP/GaAs under 1000 suns. These high efficiencies have been achieved by using Ill-V expensive materials. The cells are also monolithically stacked which is the cause of their high cost.
In concentrating photovoltaic systems, the cost of the solar cell represents around 18% of the cost of the entire system. The high cost of the cells involved in CPV is the reason why the total capacity of CPV installed and under-construction worldwide is less than 300 MW, and this is preventing it from expanding widely in the near future unless the installation cost is brought down or the efficiency is increased further. For this reason, multi-junction Solar Cells [2] are used mainly for space applications where the cost of the generated power is not a major concern. For terrestrial applications, concentrating Solar Cells [2] have the highest efficiency comparatively with all of the other existing technologies; however, their share in the PV market is less than 1%. It is also interesting to know that high concentration photovoltaic systems (HCPV) convert only the normal direct irradiance of the incident solar flux, which is about 75% of the overall incident energy on the ground .
Based on the above, an increase in the share of CPV systems in the energy market can only be achieved by improving the CPV systems efficiency and/or by lowering their cost.
Our approach for lowering the cost of CPV systems is to use cheap optical components for splitting the sunbeam before reaching the Solar Cells [2], as an alternative to stacking the cells monolithically and concentrating the incident sun beam on them. In addition, we attempt to use cheap abundant materials like Si instead of Ill-V expensive semiconductors.
Photovoltaic Effect
The collection of light-generated carriers does not by itself give rise to power generation. In order to generate power, a voltage must be generated as well as a current. Voltage is generated in a solar cell by a process known as the “photovoltaic effect". The collection of light-generated carriers by the p-n junction causes a movement of electrons to the n-type p-type side of the junction. Under short circuit conditions, there is no buildup of charge, as the carriers exit the device as light-generated current.
I-V CURVE
The IV curve of a solar cell is the superposition of the IV curve of the solar cell diode in the dark with the light-generated current.[1] The light has the effect of shifting the IV curve down into the fourth quadrant where power can be extracted from the diode. Illuminating a cell adds to the normal "dark" currents in the diode so that the diode law becomes:
POWER DELIVERED
Power delivered to from a solar cell is the product of its current and voltage.We can observe from 1.4, that the current remains almost constant till a particular voltage and then begins to drop. The constant current is called short circuit current ISC (refer section 4.2), and the voltage at which the current begins to drop is VMP . Till VMP is reached current is constant and hence power is propotional to V. Power linearly increases with voltage having a slope equal to ISC. After VMP current varies according to the equation 1.2 and so is the Power.
Maximum Power Point
Let’s have a look into the P-V curve. For the solar cell to work efficiently the cell should be configured at the operating point at which Power obtained is maximum. From the figure 1.5 it can be seen that power is maximum niether at IMAX nor at VMAX, because at IMAX (VMAX) V (I) is zero and hence power is zero. PMAX is obtained where the peak occurs in a P-V curve. This is the point where the slope becomes zero. The P-V and I-V curve can change due to the change of
• Atmospheric conditions
• Insolation: power of radiation at a particular area can change. This is measured by the parameter called Insolation (W=mm2).
• Temperature:Temperature can significantly affect the power.
• Geography:The radiation may vary at different positions throughout the Solar pannel. It’s due to these challenges that it we require an efficient tracking algorithm. The subsequent chapters of this report discuss on selected efficient and popular algorithms, its pros and cons.
[attachment=31841]
INTRODUCTION
High efficiency photovoltaic systems available in the market are composed of a concentrating optical system, a monolithically stacked solar cell, a tracking system and a cooling system. The overall conversion efficiency depends mainly on the efficiency of the solar cell. Efficiencies as high as 41.6 % have been measured on devices consisting of a stack of three junctions made of InGaP/InGaAs/Ge . On double-junction devices, an efficiency of 32.6 has been measured on a device made of InGaP/GaAs under 1000 suns. These high efficiencies have been achieved by using Ill-V expensive materials. The cells are also monolithically stacked which is the cause of their high cost.
In concentrating photovoltaic systems, the cost of the solar cell represents around 18% of the cost of the entire system. The high cost of the cells involved in CPV is the reason why the total capacity of CPV installed and under-construction worldwide is less than 300 MW, and this is preventing it from expanding widely in the near future unless the installation cost is brought down or the efficiency is increased further. For this reason, multi-junction Solar Cells [2] are used mainly for space applications where the cost of the generated power is not a major concern. For terrestrial applications, concentrating Solar Cells [2] have the highest efficiency comparatively with all of the other existing technologies; however, their share in the PV market is less than 1%. It is also interesting to know that high concentration photovoltaic systems (HCPV) convert only the normal direct irradiance of the incident solar flux, which is about 75% of the overall incident energy on the ground .
Based on the above, an increase in the share of CPV systems in the energy market can only be achieved by improving the CPV systems efficiency and/or by lowering their cost.
Our approach for lowering the cost of CPV systems is to use cheap optical components for splitting the sunbeam before reaching the Solar Cells [2], as an alternative to stacking the cells monolithically and concentrating the incident sun beam on them. In addition, we attempt to use cheap abundant materials like Si instead of Ill-V expensive semiconductors.
Photovoltaic Effect
The collection of light-generated carriers does not by itself give rise to power generation. In order to generate power, a voltage must be generated as well as a current. Voltage is generated in a solar cell by a process known as the “photovoltaic effect". The collection of light-generated carriers by the p-n junction causes a movement of electrons to the n-type p-type side of the junction. Under short circuit conditions, there is no buildup of charge, as the carriers exit the device as light-generated current.
I-V CURVE
The IV curve of a solar cell is the superposition of the IV curve of the solar cell diode in the dark with the light-generated current.[1] The light has the effect of shifting the IV curve down into the fourth quadrant where power can be extracted from the diode. Illuminating a cell adds to the normal "dark" currents in the diode so that the diode law becomes:
POWER DELIVERED
Power delivered to from a solar cell is the product of its current and voltage.We can observe from 1.4, that the current remains almost constant till a particular voltage and then begins to drop. The constant current is called short circuit current ISC (refer section 4.2), and the voltage at which the current begins to drop is VMP . Till VMP is reached current is constant and hence power is propotional to V. Power linearly increases with voltage having a slope equal to ISC. After VMP current varies according to the equation 1.2 and so is the Power.
Maximum Power Point
Let’s have a look into the P-V curve. For the solar cell to work efficiently the cell should be configured at the operating point at which Power obtained is maximum. From the figure 1.5 it can be seen that power is maximum niether at IMAX nor at VMAX, because at IMAX (VMAX) V (I) is zero and hence power is zero. PMAX is obtained where the peak occurs in a P-V curve. This is the point where the slope becomes zero. The P-V and I-V curve can change due to the change of
• Atmospheric conditions
• Insolation: power of radiation at a particular area can change. This is measured by the parameter called Insolation (W=mm2).
• Temperature:Temperature can significantly affect the power.
• Geography:The radiation may vary at different positions throughout the Solar pannel. It’s due to these challenges that it we require an efficient tracking algorithm. The subsequent chapters of this report discuss on selected efficient and popular algorithms, its pros and cons.