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Solar efficiency

Solar efficiency

The efficiency of photovoltaic cells is one of the elements that determine the production of a photovoltaic solar energy installation. The other factors that determine the performance of a solar plant are latitude and climate.

The conversion efficiency value of a photovoltaic cell depends on several factors. When we refer to conversion efficiency, we refer implicitly to the thermodynamic efficiency, to the separation efficiency of the load carrier, to the reflectance efficiency and to the values ​​of conduction efficiency. These parameters are difficult to measure directly, so other parameters are measured instead, including quantum efficiency, the open circuit voltage ratio and the fill factor.

Technical methods to improve solar efficiency

Radiative cooling

For each degree centigrade that increases the temperature of the photovoltaic solar cell the solar efficiency decreases around 0.45%. To avoid the decrease in solar efficiency due to heating, a layer of visibly transparent silica glass can be applied to a photovoltaic solar panel. The silica glass layer acts as a thermal black body that emits heat in the form of infrared radiation to space. With this action you can get down the temperature of the photovoltaic cell up to 13 degrees Celsius.

Promoting the dispersion of light in the visible spectrum

By coating the light receiving surface of the cell with nanometric-sized metal poles, the efficiency of the cell can be substantially increased, since solar radiation is reflected in these poles at an oblique angle to the cell. This change in direction causes an increase in the length of the path taken by the light through the solar cell. Consequently, the increase in the path increases the number of photons absorbed by the cell, and also the amount of direct current generated.

The main materials used for nano-studs are silver, gold and aluminum, to name a few. Aluminum is able to increase the efficiency of the cell up to 22% (under laboratory conditions). Aluminum, on the other hand, absorbs only ultraviolet radiation, and reflects both visible and infrared light, so that energy loss is minimized on that front.

However, gold and silver are not very efficient, since they absorb a large part of the light in the visible spectrum, which contains most of the energy present in sunlight, reducing the amount of solar radiation that reaches the photocell

Choose the optimal transparent conductor

The illuminated side of some types of solar cells, the thin films, have a transparent conductive film that allows light to enter the active material and collect the generated charge carriers.

In general terms, films with high transmittance and high electrical conductance, such as indium and tin oxide, conductive polymers or conductive nanowire networks, are used for this purpose. There is a tradeoff between high transmittance and electrical conductance, so the optimal density of the conductive nanowires or the structure of the conductive network must be chosen to achieve high efficiency.

Antireflective coatings and textures

Anti-reflective coatings could result in more destructive interference from light waves incident on the sun. Therefore, all the sunlight would be transmitted to the photovoltaic system.

In addition, another technique used to reduce reflection is texturing, in which the surface of a solar cell is altered so that the reflected light hits the surface again. These surfaces can be created by engraving or by lithography. Adding a flat back surface in addition to texturing the front surface helps trap the light inside the cell for a longer optical path.

Thin film materials

In terms of low costs and adaptability to existing structures and structures in the technology, thin film materials are a very good option for photovoltaic cells.

However, because the materials are so thin, they lack the optical absorption that solar cells have of bulk material. While attempts to correct this problem have been attempted, the most important thing is the focus on the recombination of the surface of the thin film.

Since this is the dominant recombination process of nano-scale thin-film solar cells, it is crucial to its solar efficiency. Adding a thin passivating layer of silicon dioxide could reduce recombination.

Passivation of the rear surface

While many improvements have been made to the front of photovoltaic cells for the mass production of solar energy, the aluminum rear surface slows down efficiency improvements.

The efficiency of many solar cells has benefited by creating so-called passive and passive emitting cells. The chemical deposition of a stack of dielectric passivation layers of the back surface that is also made of a thin film of silica or aluminum oxide covered with a silicon nitride film helps to improve the efficiency of the silicon solar cells in more than 1%

This helps increase the cell's solar efficiency for the commercial Cz-Si wafer material at 20.2% and the efficiency of the cell for almost mono-Si at a record of 19.9%.

Factors that affect energy conversion efficiency

To analyze the factors that influence solar efficiency, we can refer to energy factors that affect the energy conversion efficiency that William Shockley and Hans Queisser presented in a historical article in 1961.

Filling factor

Another defining term in the general behavior of a solar cell is the fill factor. This factor is a measure of the quality of a solar cell. This is the power available at the maximum power point divided by the open circuit voltage and the short circuit current.

The fill factor is directly affected by cell series values, bypass resistors and diode losses. Increasing the resistance of the shunt and decreasing the series resistance leads to a higher fill factor, which results in higher efficiency and brings the output power of the cell closer to its theoretical maximum.

Typical fill factors range from 50% to 82%. The filling factor for a normal silicon photovoltaic cell is 80%.

Limit of thermodynamic efficiency and limit of infinite pile

If one has a source of heat at the temperature Ts and a cooler heat sink at the temperature Tc, the theoretically maximum possible value for the working ratio (or electrical power) obtained from the supplied heat is 1- Tc / Ts, given by a thermal engine of Carnot.

If we take 6000 Kelvin for the temperature of the sun and 300 Kelvin for the environmental conditions on Earth, this reaches 95%. In 1981, Alexis de Vos and Herman Pauwels showed that this can be achieved with a stack of an infinite number of cells with band intervals ranging from infinity (the first cells found by the incoming photons) to zero, with a voltage in each cell very close to the open circuit voltage, equal to 95% of the band range of that cell, and with 6000 kelvin of blackbody radiation coming from all directions.

Maximum power point

A solar cell can operate in a wide range of voltages (V) and current intensities (I). By increasing the resistive load in a cell irradiated continuously from zero (a short circuit) to a very high value (an open circuit), the maximum power point can be determined, the point that maximizes V × I; that is, the load for which the cell can deliver the maximum electrical power at that level of irradiation.

The maximum power point of a photovoltaic varies with the incident lighting. For example, the accumulation of dust in photovoltaic panels reduces the point of maximum power. For systems large enough to justify the additional expense, a maximum power point tracker tracks the instantaneous power by continuously measuring the voltage and current (and, therefore, the power transfer), and uses this information to dynamically adjust the load so that the maximum power is always transferred, regardless of the variation in lighting.

Maximum efficiency

However, normal photovoltaic systems only have a pn junction and, therefore, are subject to a lower efficiency limit, called "maximum efficiency" by Shockley and Queisser. Photons with an energy below the band gap of the absorbing material can not generate a pair of electron holes, so their energy does not become useful output, and only generates heat if absorbed. For photons with an energy above the energy band, only a fraction of the energy above the band can be converted into useful output. When a higher energy photon is absorbed, the excess energy on the band is converted into kinetic energy from the carrier combination. The excess kinetic energy is converted into heat through the phonon. Interactions as the kinetic energy of the carriers decreases at the equilibrium speed. Traditional single-junction cells with an optimal band for the solar spectrum have a maximum theoretical efficiency of 33.16%, the Shockley-Queisser limit.

Solar cells with multiple band separation absorbent materials improve efficiency by dividing the solar spectrum into smaller deposits, where the limit of thermodynamic efficiency is greater for each container.

Quantum efficiency

When a photon is absorbed by a photovoltaic solar cell, it can produce a pair of electron holes. One of the transporters can reach the pn junction and contribute to the current produced by the solar cell; it is said that such carrier is collected. Or, the carriers recombine without a net contribution to the cellular current.

Quantum efficiency refers to the percentage of photons that become electrical current (ie, harvested carriers) when the cell is operated under short-circuit conditions. The "external" quantum efficiency of a silicon solar cell includes the effect of optical losses such as transmission and reflection.

In particular, some measures can be taken to reduce these losses. Reflection losses, which can represent up to 10% of the total incident energy, can be drastically reduced by using a technique called texturing, a method of light trapping that modifies the average light path.

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Last review: February 8, 2019

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