High Efficiency Photovoltaic Systems - Engineering Seminar Report

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 are used mainly for space applications where the cost of the generated power is not a major concern. For terrestrial applications, concentrating Solar Cells  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, 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.

SolarCell Electrical Parameters

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.

However, if the light-generated carriers are prevented from leaving the solar cell, then the collection of light-generated carriers causes an increase n the number of electrons on the n-type side of the p-n junction and a similar increase in holes in the p-type material. This separation of charge creates an electric field at the junction which is in opposition to that already existing at the junction, thereby reducing the net electric field Since the electric field represents a barrier to the flow of the forward bias diffusion current, the reduction of the electric field increases the diffusion current. A new equilibrium is reached in which a voltage exists across the p-n junction. The current from the solar cell is the difference between IL and the forward bias current. Under open circuit conditions, the forward bias of the junction.

Increases to a point where the light-generated current is exactly balanced by the forward bias diffusion current, and the net current is zero. The voltage required to cause these two currents to balance is called the "open-circuit voltage". The following animation shows the carrier flows at short-circuit and open-circuit conditions. The P-N junction solar cell under equilibrium. Short circuited and open circuited conditions are shown in figure1.1 (a),(b) and (c) respectively.

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.

P = ISC * V      V <VMP                         (2.3)

P = IL - I0(e(qV=nkT) - 1) *V                 (2.4)

The variation is depicted in 1.5. The curve in the region 'a' is governed byequation 1.3 and in region 'b' is governed by equation 1.4.

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.

FUNDAMENTALS OF PHOTOVOLTAICS

The solar spectrum in outer space resembles the theoretical radiation provided by a black body of 5900 K [20]. As the light passes through the atmosphere, some of the light is absorbed or reflected by gasses such as water vapour and the ozone. The spectrum of the sun’s light that reaches Earth’s upper atmosphere ranges from the ultraviolet to the near  peak region (48%) from 400 Photovoltaic cells can be defined as p-i-n photodiodes, which are operated under forwardbias. They are designed to capture photons from the solar spectrum by exciting electrons across the band gap of a semiconductor, which creates electron-hole pairs that are then charge separated, typically by p-n junctions introduced by doping. The space charge at the p-n junction interface drives electrons and holes in opposite directions, creating at the external electrodes a potential difference equal to the bandgap (Figure 2) . A semiconductor can only convert photons with the energy of the bandgap with good efficiency. Photons with lower energy are not absorbed and those with higher energy arereduced to gap energy by thermalization of the photogenerated carriers. Behaviour of a solar cell is represented by current versus voltage curves on Figure 3.1. 

For each point on the graph, the voltage and current can be multiplied to calculate power.Most moderate resistive loss mechanisms, the short-circuit current and the light-generated current are identical. Therefore, the short-circuit current is the largest current which may be drawn from the solar cell the point at which a curve intersects the horizontal axis is known as the open circuit condition. The open-circuit voltage, oc V, is the maximum voltage available from a solar cell, and this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current. Voc depends on the saturation current of the solar cell and the light-generated current. Open-circuit voltage is then a measure of the amount of recombination in the device.

Maximum power point

Maximum power point is the point on the I-V curve of a solar cell corresponding to themaximum output electrical power, max max P [Watts] V I m Maximizing total power is the goal of solar cell’s design. Multi-junction Photovoltaics[1], ascompared to single-junction cells, have reduced currents, because fixed total number of photons is distributed over increasing number of cell layers, so that the amount available for electron promotion in any one layer is decreased. At the same time, the electrons excited are more energetic  and have a greater electric potential, so the reduction of currents is compensated for by increase in voltages, and the overall power of the cell is greater. Moreover, multi-junction design is advantageous, because resistive losses, which are proportional to the square of the current, can be significantly reduced Another defining term in the overall behaviour of a solar cell is the fill factor, FF. This is the ratio that describes how close the I-V curve of a solar cell resembles a perfect rectangle.

Quantum efficiency

Quantum efficiency is a term intrinsic to the light absorbing material and not the cell as a whole; it refers to the percentage of absorbed photons that produce electron-hole pairs. WhereasEnergy conversion efficiency, is the percentage of incident electromagnetic radiation that is converted to electrical power, when a solar cell is connected to an electrical circuit.

This overall efficiency depends on many factors including the temperature, amount of incident radiation and the surface area of the solar cell this overall efficiency depends on many factors including the temperature, amount of incident radiation and the surface area of the solar cell

BANGAPS

In order to optimize conversion efficiency of a photovoltaic cell, the solar cell should absorb as much of the spectrum as possible, and so band gaps should cover a wide range. Besides, band gaps of adjacent layers should differ by as small amount as possible, because the amount of excess energy from light converted to heat is equal to the difference between the photon energy and the band gap of the absorbing material.

Dependency of the conversion efficiency on the semiconductor bandgap is shown on Figure.5 GaAs has nearly the optimal band gap (1.4 eV) for solar energy conversion in a conventional solar cell design, which is inherently limited to efficiencies of about 25% or less at one-sun concentration.

Current matching

The serial architecture of monolithically-grown multi-junction Solar Cells [2] makes matching of currents a desirable characteristic. The output current of the multijunction solar cell is limited to the smallest of the currents produced by any of the individual junctions. If this is the case, the currents through each of the subcells are constrained to have the same value. The current is proportional to the number of incident photons exceeding the semiconductor’s bandgap, and the absorption constant of the material. A layer must be made thinner if the photons that exceed the bandgap are in abundance. At the same time, a layer with a low absorption constant must be made thicker, since on average a photon must pass through more of the material before being absorbed. After materials are selected with desired bandgaps and lattice [4] constants, the thickness of each layer must be determined based on the material’s absorption constant and the number of incident photons with a given energy Figure Absorption coefficient versus wavelength for various semiconductor materials.

LATTICS CONSTANT

To produce optical transparency and maximum current conductivity in monolithic multijunctionSolar Cells, where different semiconductor layers are grown directly on top of the other layers using the same substrate, all layers must have similar crystal structure. The lattice constant describes the spacing of the atom locations in a crystal structure. Mismatch in the crystal lattice constants of different layers creates dislocations in the lattice of the cell layers and significantly deteriorates the efficiency of the solar cell. NREL showed that a lattice mismatch as small as 0.01% significantly decreases the current produced by the solar cell . The lattice constants and bandgap energies of common semiconductor materials are shown on Figure 6. Lines between different materials represent semiconductors that can be created by combining different amounts of the two materials. Ge, GaAs and AlAs have roughly the same lattice constant with different bandgaps.

OPTICAL COMPONENTS

Parabolic Mirror and collector assembly

Other known photovoltaic systems converts only direct irradiance of incident solar flux. Thus 25% of the radiation is loss.This is taken care by a parabolic mirror. It makes the radiations coming at any angle of incidence available to the cell.

OPTICAL FILTER

Optical filters selectively transmit light in a particular range of wavelengths, that is, colours, while blocking the remainder. They can usually pass long wavelengths only (longpass), short wavelengths only (shortpass), or a band of wavelengths, blocking both longer and shorter wavelengths (bandpass). The passband may be narrower or wider; the transition or cutoff between maximal and minimal transmission can be sharp or gradual. There are filters with more complex transmission characteristic, for example with two peaks rather than a single band[1]; these are more usually older designs traditionally used for photography; filters with more regular characteristics are used for scientific and technical work.

Dichroic filter

Alternately, dichroic filters (also called "reflective" or "thin film" or "interference" filters) can be made by coating a glass substrate with a series of optical coatings. Dichroic filters usually reflect the unwanted portion of the light and transmit the remainder. Dichroic filters use the principle of interference. Their layers form a sequential series of reflective cavities that resonate with the desired wave lengths. Other wavelengths destructively cancel or reflect as the peaks and troughs of the waves overlap. Dichroic filters are particularly suited for precise scientific work, since their exact colour range can be controlled by the thickness and sequence of the coatings. They are usually much more expensive and delicate than absorption filters.

BEAM SPLITTING

Schematic representation of a beam splitter cube

1 - Incident light

2 - 50% Transmitted light

3 - 50% Reflected light Definition:

A beam splitter (or beam splitter, power splitter) is an optical device which can split an incident light beam (e.g. a laser beam) into two or more beams, which may or may not have the same optical power. Different types of beam splitters exist, as described in the following, and are used for very different purposes. 

Higher efficiency is attained only if we stack the cells. This increases the cost significantly as mentioned previously. Beam splitting enables us to use two cells Beams are split and given to two separate receivers at two ends (therefore avoided stacking). Both of them absorbs lights of wavelength corresponding to its band gap.We got the effect of stacking without a physical stacking.

DESCRIPTION AND MODELING OF THE OPTICAL SYSTEM

The system is composed of a parabolic mirror, a lens, an optical filter and two Solar Cells made of Si and AlGaAs. The designed CPV system is depicted in figure 1, and its optical concentration ratio is 63 suns. The incident sun beam is concentrated by the mirror on the lens coated with the optical filter. The filter reflects one part of the beam towards the Si cell, and transmits the other part of the beam to the A1GaAs cell.

The beam splitter is a multi-layer dichroic filter. The filter consists of a stack of a low-pass filter and two band-pass filters. The transition band of the coating is at 780 nm (see figure 2).

This paper presents the photovoltaic system and the modeling approach. The modeling procedure consists of three steps, which are: modeling the optical system, generating the spectrum incident on each cell, using the obtained spectrum files for modeling the Solar Cells. The three steps are demonstrated on the proposed CPV system.

The system was built and optimized in TracePro Expert, and the light source was simulated to have the spectral distribution of the ASTM 173D ranging between 280 nm and 4000 nm. Figure 3 shows the spectrum incident on each cell.

SOLAR ENERGY

Projected global population and economic growth will more than double the energy consumption rate by the middle of 21th, and Photovoltaics is expected to make a sizeable contribution, to world electricity production, reaching 65% portion in 2100. At present, the most efficient photovoltaic cells use multiple III-V-semiconductor materials with band gaps spanning the solar spectrum. Today, commercially available multi-junction photovoltaic devices are triple-junction Solar Cells made of GaInP, GaAs, and Ge layers that achieve typical conversion efficiencies above 30%. At present, only Emcore and Spectrolab in US and Aixtron in Germany are producing this type of cells in relatively large volumes. One exciting aspect of multi-junction Photovoltaics is that there are still many possibilities to explore. A current record efficiency of 40.7%, achieved with a triple-junction version of the cell, corresponds to less than a half of the maximum theoretical limit efficiency of 86.8% . By the contrast, efficiencies of single-junction Solar Cells are almost reached their potential limits. It was shown in this project, that the current design of multi-junction Photovoltaics can be improved by design optimization of each layer, by increasing the number of junctions in a photovoltaic structure, or by inclusion of semiconductor quantum dots, which offer the potential for high conversion efficiency by tailoring the material properties of existing materials. Prospectively, the gap between the ideal and real values of the conversion efficiency is expected to decrease due to fundamental advances in understanding of materials behavior. New approaches and concepts, relying on phenomena allowed by nanotechnologies, may also revolutionize multiple junction devices by allowing control over band structure, growth, and defects. Besides, in order to apply multi-junction Photovoltaics widely, it is necessary to develop a large-area, cost-effective, and highly reproducible fabrication processes.

Solar electricity market installations reached a record high of 1,744 megawatts in 2006 .Photovoltaic industry is growing >40% per year, and high-efficiency multijunction Solar Cells will help the solar industry grow even faster. On April 26, 2007, the Canadian Press  has announced that California “OptiSolar” plans to will build Canada’s largest solar farm near Sarnia, Ontario, installing more than 1 million photovoltaic panels to generate 40 megawatts of power. This company will be paid 42 cents per kilowatt-hour for the solar power. Though solar electricity is currently expensive, it is expected that the use of high-efficient multijunction Solar Cells.

DISCUSSION OF THE RESULTS

The system shows an optical efficiency equal to 89.46 %, and 46.55% of the collected energy is received at the top surface of the Si cell. The other 42.91 % cell of the collected energy is received at the top surface of the AlGaAs solar cell. This means that optical losses result in losing only about 10% of the solar flux received by the collector. This adds up to the diffuse solar rays that are not concentrated on the Solar Cells , which are estimated at 23.3% of direct and diffuse sunlight. This value was obtained after subtracting the overall flux in the files AM 1.5 G and AM 1.5 D. The Silicon solar cell used in the model is the one available in the package PC1D.

The obtained spectrum files were used as input parameters in modeling the Solar Cells  under one sun. Modeling of the two cells gave the output parameters shown in table 

The results shown in figure 7.1 and figure 7.2 represent the response of the cells under the spectrum of sunlight after splitting and under no concentration. The efficiency of the A1GaAs solar cell in the designed CPV system was estimated at 17.42 %, while the efficiency of the Silicon solar cell was estimated at 5.8 %. The overall efficiency of the system is 23.22 %. Modeling results show that the system converts 23.22 % of the collected solar energy into electricity. The proposed optical design enables re-sizing the cells to have an area that enables a certain amount of current That ultimately enables connection of the cells in series. In this study, ohmic losses and losses due to non-uniform distribution of solar flux as well distribution of solar flux as well as other minor losses not taken into account. 

In a previous study conducted by the author, an A1GaAs on Si double-junction solar cell was optimized by using PC1D under AM 1.5 G. In this study, the two cells are separated and the A1GaAs optimized in that study is used in our model.

ADVANTAGES

1. Easy to manufacture: no need of stacking( As there is no stacking, we don’t have any restriction on lattice [4] constant. Hence, a choice of wide range of materials)

2. Cost is cheap as the optical components used are very cheap

3. High maintainability.

4. Less thermal management constraints,

5. Possibility of integrating up to four cells

CONCLUSION

In this work, we proposed a novel concentrating photovoltaic system with beam-splitting features and two single-junction Solar Cells: an AlGaAs single- junction cell with 1.817 eV energy bandg-gap and a Si solar cell with 1.124 eV energy band-gap. This configuration was adopted to achieve a high conversion efficiency at a low cost.

The modeling approach for assessing the multi-receiver CPV system was demonstrated through three steps: modeling the optical system, generating the spectrum of the two sub-beams that result after splitting the sunbeam, and modeling the Solar Cells  under the spectrum of light incident on them.

The CPV system showed an optical efficiency of 89.46%. The efficiency of the A1GaAs and Si Solar Cells in the system was estimated at 17.42 %, and 5.8 % respectively. The overall conversion efficiency of the system is then 23.22 %. In this work, we have shown that an efficiency of 23% can be achieved on a low cost system that involves cheap Solar Cells  (Si) and low-cost optical components that prevent stacking the cells together.