In an article recently published in the journal Solar Energy, researchers discussed the enhancements of photovoltaic (PV) receivers in laser wireless power transmission (LWPT) by using non-imaging optics.
Study: Improvements of PV receiver in laser wireless power transmission by non-imaging optics. Image Credit: Bk87/Shutterstock.com
Laser Wireless Power Transmission (LWPT)
In the area of wireless energy transmissions, such as space control in orbit, spacecraft sensor networks, satellite-to-satellite communication and power transmission, ground-to-ground, ground-to-air, ground to unmanned aerial vehicle (UAV), etc., LWPT systems have a wide range of potential applications. Until now, concentrated photovoltaics (CPV) has been employed in solar energy as a way to reduce photovoltaic size while still increasing efficiency. Lenses and concentrators are crucial to reshaping and guiding the laser beam towards the photovoltaics, and in case of misalignment, laser beam spots often display a Gaussian distribution, and the laser somehow must match the size of the PV cell or array.
The use of renewable energy sources to replace coal plants and achieve zero emissions has raised interest in electrical energy in recent years. The concept of beamed power transmission, also known as LWPT, describes how to transmit energy from one location to another without using physical conduits like copper cables or optical fibers and only employing light amplification using laser technology. Depending on its intensity, a laser beam can travel large distances before losing coherence and eventually losing its fringes. High energy flux density, improved performance in specific directivity, increased conversion efficiency, longer transmission distance, etc. are all benefits of LWPT.
The key technical challenge for LWPT systems is the development of highly effective receivers employing particular PV panels, which are frequently paired with optical concentrators to increase optical efficiency after the laser passes through the atmosphere. Low laser beam reception efficiency is another challenge for LWPT systems, thus it relies on photovoltaic technology and the advancements it brings.
Cross Compound Parabolic Concentrator in LWPT
In this study, the authors used a common non-imaging optical device called a cross-compound parabolic concentrator (CCPC), which helped increase the transmission of laser beams from various incident directions, to improve the output performance of LWPT receivers. Based on parameters from non-linear regression and experimental study, the multi-field characteristics of the CPV module under laser power were solved. Similar to LWPT receivers, the multi-field performances of CPV modules and single solar cells were also examined. Important references for LWPT system design and optimization were also provided.
The team proposed a novel non-imaging optical device to enhance the optical efficiency of LWPT receivers at various laser beam incidences since the receiving radiation and vectors substantially fluctuate during long-distance beam transmission. An experimental platform was created to extract coefficients from a multivariable parameter regression model of a single diode PV cell, which was used to solve the I-V characteristics of the CPV module under irregular receiving laser irradiance. Then, the optical-heat-electrical performances of the CPV module and the single PV cell were thoroughly examined and compared.
The effects of crucial structural factors for the LWPT system, such as the transmission distance, divergence half-angle, rotation angle, and misalignment distance, were also studied. Using computational and experimental methodologies, the current study examined the multi-field performance of CPV receivers for LWPT applications. Multivariable parameter regression was carried out using the LWPT experimental platform to extract many parameters from a PV cell with a single diode equivalent circuit.
Experimental Observations and Results
As per the observations, the most crucial variables for multi-field performances and LWPT conversion are the transmission distance, divergence half-angle, rotation angle, and misalignment distance. The effectiveness of the system was impacted by any laser beam reflection or obstacle, such as clouds, water drops, and dust, among others. If there was a misalignment between the laser and the receiver for whatever reason, the divergence of the laser beam, with no adequate collimation and huge distances, could influence the energy conversion. In the worst-case scenario, the efficiency could go to zero.
The efficiency of PV panels could be increased and the amount of energy produced by each PV array could be multiplied by several orders of magnitude with the help of CPV technology. The regression parameters used to fit the I-V data, which ranged in intensity from 2365.08 W/m2 to 3468.79 W/m2, closely matched the test data. The fitting's root mean square value was only 0.00432, which demonstrated the effectiveness of the simulation modeling approach. In comparison to the most widely utilized convex lenses in front of the PV cells, CCPC achieved higher optical efficiency that was more stable over a wide range of rotation angles. At all possible transmission distances and rotation angles, it was discovered that the optical efficiency of a CPV module using CCPC was significantly higher than that of a bare PV cell.
Due to the PV cell's bus bar and emitter's asymmetrical structure, which served as a key component for energy harvesting in LWPT systems, the varying trends of optical efficiency were not the same. Also, the misalignment distance was examined, which demonstrated how misalignment would undoubtedly affect the received optical power. According to the heat transfer data, the temperature distribution directly complied with the optical irradiance trend. The rays' convergence was made possible by the CCPC's internal reflections, which also increased the local peak temperature. Due to the great thermal conductivity of solid materials, temperature changes in all circumstances were quite small.
Both the voltage/current distribution and the I-V curves were examined in relation to electrical performance. It was discovered that the non-uniformity of optical irradiance, which was based on a linear and a log-linear relationship by semiconductor photoelectric response, had a significant impact on the distribution of current and voltage. Without the concentrator, when the distance or rotation angle increased as more rays radiate outside, the short current and conversion power decreased. Short-circuit current (Isc), power (Pm), and fill factor (FF) significantly improved for a CPV receiver, especially when the rotation angle was not greater than 30°.
In conclusion, this study elucidated a helpful guide for the CPV receiver in the LWPT system when deciding on an appropriate transmission distance, rotation angle, misalignment, etc.
The authors mentioned that the multi-field coupling properties of various geometric dimensions and focusing modules, as well as series-parallel complicated CPV receivers, would be studied in more detail in the future, and they would be applied to the load.
Xian-long, M., Yi-Chao, H., Bei, L., et al. Improvements of PV receiver in laser wireless power transmission by non-imaging optics. Solar Energy, 255, 157-170 (2023). https://doi.org/10.1016/j.solener.2023.03.016