Study on the effectiveness of a solar cell with a holographic concentrator

1 Introduction

The solar technology uses solar cells based on single-crystal silicon, the efficiency of which reaches 20% at best, but along with this the photoelectric cells run hot. Thermal energy from sunlight heats solar panels to an average temperature of about 55 °C. It is commonly known that with an increase of a photovoltaic cell temperature by 1 °C, its efficiency decreases by 0.5%, but this dependence is not linear and an increase in the temperature by 10 °C leads to almost double decrease in efficiency. Active elements of cooling systems (fans or pumps) pumping cooling fluid consume a significant amount of energy, require periodic maintenance and reduce the reliability of the entire system. Passive cooling systems have very low performance and cannot cope with the task of cooling solar-cell arrays (Szondy 2014).

One of the ways to solve this problem is to develop a system of converting solar radiation into electrical energy using a holographic concentrator (Ferrara et al. 2019; Khan and Yadav 2022). The method of converting solar energy into electrical power using semiconductor solar cells (SC) is currently the most developed scientifically and practically (Niyazbekova et al. 2021). It is widely used in power supply systems for spacecraft and is increasingly being used to provide electricity to autonomous consumers (portable equipment, beacons, automatic weather stations, etc.) (Niyazbekova et al. 2022).

The possibility of using holographic structures to concentrate solar radiation was considered back in the 1980s. Holographic optics is a generalised name for various optical elements, the effect of which on the wavefront is based on the radiation diffraction. A holographic micro-optical element is a class of holographic optical elements, the geometry of structure (period, relief depth) of which does not exceed a micrometre. A holographic micro-optical element is most often a flat glass substrate with a microstructure on one of the sides, which introduces a phase delay due to modulation of the transmittance or the height of the micropattern. The simplest examples of holographic micro-optical elements are a diffraction grating and a Fresnel-zone plate. The papers by N.S. Buktukov and M. Buktukov and Aitkulov (2018) “Efficiency of new generation solar photoelectric batteries” and N.S. Buktukov and K.A. Vasin “Experimental research of new generation solar cells” describes the operational principle of a photovoltaic panel with a holographic concentrator under the patent RK No. 31,796. The diagram of this solar panel is shown in the Figure 1.

Figure 1:

Diagram of a solar-cell array with holographic concentrator.

Number 2 is a holographic concentrator that disperses solar radiation in spectral composition and concentrates it along the optical axis. The infrared spectral range extends to the red spectral range. Photovoltaic cells are located on the optical axis in the visible part of the spectrum. Thus, the installed solar cells will not run hot and lose power during operation. To increase the energy generation by solar cells, solar radiation concentrators are used, they increase the radiation flux per unit area of solar modules. There are different types of solar concentrators, sometimes holographic solar concentrators in the form of a film are used in production. The main differences between holographic concentrators and other concentrators are low cost and simplicity. Currently, there are systems with solar cells and holographic concentrators, systems with other types of concentrators and water cooling. There are systems with solar cells and water cooling, however, the systems with solar cells, holographic concentrators and water cooling at the same time are unknown. Gallium arsenide solar cells (Concentrator solar gallium … 2020a) are used to convert concentrated solar energy into electrical power. Based on the structure of solar panels, a solar cell 5 × 5 mm in size was chosen. To determine the volt-ampere parameters of the photovoltaic panel, a measuring system was developed based on a microprocessor controller and digital sensors. The purpose of the study is to determine the electrical characteristics of the solar cell inside the solar cells.

2 Literature review

The cost of solar photovoltaic cells is high, primarily due to the synthesis of materials used in production of photovoltaic panels. This hinders the widespread use of solar photovoltaic energy as an alternative to non-renewable energy sources compared to other renewable energy sources in the current energy production scenario. One attractive way to reduce the cost factor is to concentrate light on solar cells, reducing the required cell area for a given output power (Butler et al. 2011; Solanki et al. 2008; Yupeng et al. 2015). This concentration can be achieved with linear glass mirrors, lenses, holographic and other optical reflectors that are very cheap compared to solar grade silicon materials (Chi-Feng et al. 2010; Chemisana et al. 2013; Yun et al. 2014).

Solar holographic optics can be used in combination with photovoltaic cells or solar heat absorbers to generate electricity or heat, respectively. Among the different types of holograms, thin holograms, both in amplitude and in phase, are not suitable for white-light applications (sunlight) because the problem of very low efficiency arises with maximum values (after the bleaching process) of 33%. Although they have greater chromatic and angular selectivity than thin holograms, almost all elements used in holographic solar energy applications are volume and phase holograms (Collados et al. 2016). With this type of hologram, efficiencies of up to 100% can be achieved for certain wavelengths and incident directions of light. Reflection and transmission elements can be found on top of various solutions using volume holograms. By their mode of behaviour, they can be divided into elements that are not concentrators (they only change the direction of sunlight, but do not concentrate it) and concentrating elements. Holographic elements can exist alone or in combination with other holographic elements, refractive elements and/or reflective or light guide systems.

Researchers have long agreed that economic rather than technical constraints now dictate the path to large-scale use of photovoltaic power generation. Other renewable or traditional options remain more cost effective. The closest alternative for cost-effective solar power generation based on a new technology for concentrating sunlight is the integrated high-concentration photovoltaic systems (Garhoushian 1997).

3 Materials and methods

Microprocessor-based measurement and control systems are popular in their respective domains in different countries (Donzellini et al. 2022; Isembergenov et al. 2019; Sinha 1986). To implement a system for measuring the electrical parameters of a solar cell inside a photovoltaic panel, digital sensors HC-SR04, INA219 and the “Arduino Nano” microprocessor controller were used. Figure 2 shows an electrical circuit of a measuring system based on a microprocessor controller and digital sensors. The HC-SR04 sensor is used to measure the distance from the holographic concentrator. The INA219 sensor is used to measure the current voltage of a photocell. The microprocessor controller “Arduino Nano” is used to read measured values from digital sensors and transfer them through the USB (Universal Serial Bus) serial port to a laptop, where the received data are processed using the “Excel” program. The HC-SR04 digital distance sensor uses acoustic ultrasonic radiation to determine the distance to an object. This non-contact sensor provides high measurement accuracy and stability. The measuring range: from 2 cm to 400 cm. The sensor readings are practically unaffected by solar radiation and electromagnetic noise. The HC-SR04 distance sensor is connected to digital ports D2 and D3, which are responsible for TRG (trigger signal) and ECHO (echo signal), respectively (Module HC – SR04 … 2017).

Figure 2:

Circuit diagram of the measuring system based on a microprocessor controller and digital sensors.

The INA219 digital sensor is designed to measure the electrical parameters of direct-current – currents and voltages. The INA219 module – a current and voltage meter with zero drift and has small dimensions and weight with very large capabilities and high measurement accuracy. The microcircuit measures the voltage across the shunt (at the Vin+ and Vin−contacts), a low-ohmic resistor, and at the Vin-contact, alternately with respect to the GND (Ground Contact) contact. The results of calculations are written into registers, then they are transmitted to the microcontroller via the I2C communication bus. The board has a shunt with a resistance of 0.1 Ohm. The voltage in the microcircuit is measured by an analogue-digital converter ADC (Analog-to-digital converter) (INA219 Current … 2011). The INA219 sensor is connected via a two-wire I2C interface, and power is supplied from +5 V. The I2C interface in the “Arduino Nano” boards is implemented on ports A4 and A5, which are responsible for SDA (data bus) and SCL (clock bus), respectively (Arduino Nano – ATmega328P … 2018).

The program code, written into the microprocessor controller using a laptop, provides for the initialisation of digital sensors, reading the measured parameters from them and transferring the data to the laptop for storage and analysis. The obtained data are recorded into the “Excel” spreadsheet processor, where they are analysed and graphs of the solar cell parameters – voltage, current and power of the generated electrical energy – are plotted. Figure 3 (a) shows the measuring system during the measurement. Figure 3 (b) shows a microprocessor measurement system mounted on a wiring board.

Figure 3:

Measuring system based on microprocessor controller and digital sensors. (a) Measuring system during measuring operation. (b) Microprocessor measuring system.

4 Results and discussion

Using a microprocessor measuring system, the electrical performance of the solar cell inside the photovoltaic panel was recorded. The measurements were taken on July 3, 2020 at 12:00 in the city of Almaty, Republic of Kazakhstan. Figure 4 shows line graphs of the electrical characteristics of a GaAs solar cell (Gallium arsenide) depending on the distance along the optical axis of the holographic concentrator. The straight lines show the readings of the INA219 sensor in direct sunlight without holography. From the graphs in Figure 4, it can be seen that the generated current and output power of a GaAs solar cell illuminated with concentrated radiation of different spectrum exceeds the values obtained by the INA219 sensor under direct sunlight without a holographic concentrator. Ten solar cells were installed according to the diagram in Figure 4 given in (Buktukov and Vasin 2019). Table 1 shows the power values for GaAs solar cells in a photovoltaic panel model.

Figure 4:

Electrical characteristics of GaAs solar cell.

The total output power of GaAs solar cells in the solar panel model was 37.29 mW. The power of one GaAs solar cell in normal sunlight is 2.87 mW, therefore, the power of 10 cells is 28.7 mW. The area of the holographic concentrator is (diameter 7 cm) 38.47 cm². The efficiency of a solar panel model with a holographic concentrator is 37.29/28.7 = 1.3 times higher. The efficiency of converting solar energy into electrical energy by GaAs solar cell is equal to 30% (Concentrator solar gallium … 2020a). This means that the power of the luminous flux illuminating this solar cell was 2.87*100/30 = 9.56 mW. The solar cell area is 0.25 cm². Then the power of solar radiation at the time of the experiment is 9.56/0.25 = 38.27 mW/cm². The area of the holographic concentrator is 38.47 cm². From this it follows that it converted solar energy with a capacity of 38.27*38.47 = 1472 mW = 1.47 W. Hence, the efficiency coefficient of the holographic concentrator will be 37.29*100/1472 mW = 2.53%.

As can be seen, a very low-quality holographic concentrator with an efficiency coefficient of 2.53% was used in the experiment. In other words, when using 2.53% dispersed and focused solar radiation, the efficiency of conversion into electricity is 1.3 times higher than direct sunlight on a photoelectric cell. Consequently, with the efficiency of a holographic concentrator, for example, 50%, the efficiency of converting solar energy into electrical energy will increase 50/2.53 = 19.76 times.

The prototype solar panel contains a spherical mirror 60 cm in diameter, which concentrates solar energy on a holographic concentrator. Then the power of the solar panel 60 cm in diameter will be 2826/38.47*37.29 = 2739 mW = 2.74 W with the efficiency of the holographic concentrator of 2.53% and 2.74·19.76 = 54.14 W with the efficiency equal to 50%. The considered GaAs solar cells are designed to work with concentrated solar radiation up to 1000 times. With the cost of ten GaAs solar cells (Concentrator solar gallium … 2020a) $ 9.9·10 = $99, the cost of converting one watt of electricity will be 99/54.14 = 1.82 $/W. At the same time, the very low cost of a spherical mirror 60 cm in diameter and two collimator lenses with a diameter of 7 cm were not taken into account.

In addition, it should be remembered that increasing the area of received solar radiation, the number and parameters of photocells and collimator lenses remain constant, and only the diameter of the spherical mirror increases. For comparison, let us take 18 pieces of single-crystal silicon solar panels (Mono-crystalline silicon solar … 2020b) with a total area of 2826 cm², equal to the area of the considered solar panel with a spherical mirror. The specified area will have a solar energy power equal to 2826*38.27 = 108,141 mW = 108.14 W. The maximum conversion efficiency of the solar energy into electrical energy by single-crystal silicon solar panels is 18.8% at a temperature of 25 degrees Celsius (Mono-crystalline silicon solar … 2020b). The output power will be: P₂₅ = 108.14*18.8/100 = 20.33 W. Considering that an increase in the temperature by 10 °C leads to almost double decrease in efficiency, the output power at 35 degrees Celsius will be 20.33/2 = 10.16 W, at 45 °С – 5.08 W, at 55 °С – 2.54 W, and at 65 °С – 1.27 W. At the cost of 18 pieces of single-crystal silicon solar panels (Mono-crystalline silicon solar … 2020b) $ 0.42*18 = $ 7.56, the cost of one watt of electricity will be 7.56/1.27 = 5.95 $/W.

5 Conclusions

The use of holograms as concentrators opens up new possibilities in the design of photovoltaic panels. A holographic concentrator is a better option than lenses and mirrors, allowing, in the same manner, to reduce of silicon consumption per panel of a given power. The concentrator allows for maintaining the thickness, lightness and structural simplicity of conventional solar panels, eliminating the need for Sun trackers. It allows the selection of the most efficient range of solar radiation for conversion into electrical energy and cut off thermal radiation, which necessitates the abstract of heat from silicon photo-integrated panels.

The experimental work conducted and the processing of the results showed a high potential for using the proposed design of the solar panel. With the help of a holographic concentrator, it is possible to increase the efficiency of converting solar energy into electrical power by 19.76 times. Moreover, the HC technology is cheaper than alternative options. Thus, the potential advantage of a photovoltaic panel with a holographic concentrator compared to traditional solar energy based on single-crystal silicon has been revealed. The practical value of the study lies in the fact that with the help of a holographic concentrator, it is possible to increase the efficiency of solar energy conversion.