PV for UAV: a High-End Photovoltaic Application
Powering unmanned aerial vehicles (UAVs) with solar cells is one of my earlier projects that I submitted to DoD in 2005 for funding. The focus of the project was to significantly enhance UAVs by adding photovoltaic modules and an on-board automaton for multi-source and multi-load energy management. Firefox UAV model, built by Advanced Ceramic Research Inc., was an excellent UAV platform for the purpose of my project. A large PV module was integrated with the UAV to enhance its mission and the operational capabilities, through powering more electronic instruments and increasing the mission duration. I referred to the then new technology as "PV for UAV". My research group and I have focused on maximizing the harvested energy during the UAV mission, and optimizing the energy usage of the on board sensors of the mid-size Firefox UAV. A follow-up project funded by the Army Research Lab (ARL) was also carried out; it was titled: "Third Generation Photovoltaic Cells for Autonomous Sensors".
The then new technology had broader impacts that span all autonomous vehicles (humanoids, unmanned rovers, unmanned sea vehicles, space vehicles,...) and now is being utilized for ordinary vehicles. Such critical impacts made the PV for UAV even more attractive for the sponsors at the time the project was launched. The idea of extending UAV autonomy using photovoltaic cells leading to un-interrupted UAV mission was then new, but difficult to achieve, as the harvested energy was insufficient, even with the most efficient and expensive tandem cells. These tremendous constrains and extenuating limitations that led the UAV developers to re-think the design of the UAV platform.
High Altitude Long Endurance (HALE) UAV (Here the PHASA-35™ model developed by BAE Systems, that acquired Advanced Ceramic Research Inc. Tucson, AR)
One of the goals of HALE is to replace satellites, once the UAV could stay in near outer space months, or years. Inspired by the US Air Force and NASA programs, Chinese companies-built solar drones similar to HALE. 'Rainbow' (next picture) is the largest solar powered HALE-like drone designed to fly at 20,000m altitude.
The PV for UAV technology involved various research aspects, from improving efficiency of solar cells to attain the highest performances and robustness, to encapsulation of these solar cells on curved surfaces, to energy management, to Smarter Flight(C) and other innovative topics. The proposal to DoD brought $750,000 funds from NAVAIR, and then $1,920,000 from ARL. The NASA Pathfinder and Helios had some similarities, but our projects had different goals. The fact that both Pathfinder and Helios were extremely light-weight had non-limiting surfaces exposed to the sun (over the large wing spans they had) eliminates the challenges we faced with powering a much smaller UAV. Also the photovoltaic powered plane Solar Impulse developed three years later in Switzerland, and had become popular two years later, had different settings and different technology problems.
In addition to the technology concerns associated with the PV for UAV, we had defined stringent scientific goals, including developing lightweight high efficiency silicon solar cells, and researching materials issues related to the encapsulant. The photovoltaic aspect focused on third generation thin silicon solar cells that have an enhanced efficiency in the IR. As for the encapsulation, our research led to the then new "Glassless Encapsulation of Silicon based Solar Cells(C) technology" and special materials that allowed conformal encapsulation of silicon solar cells on curved UAV wings and fuselage. The energy management dealt with optimization of the usage of energy, considering variable multi-source and multi-load system. The energy management was achieved by an on-board automaton.
Even after twelve years, the PV for UAV research realm still presents scientific and technology challenges. It has been leading the development of many useful high-tech applications. Just recently, the company Alta Devices, announced an excellent performance. The researchers have produced single-junction thin film GaAs based solar cell with nearly 29% efficiency. The cell has the highest efficiency over weight ratio, which makes it particularly useful for UAVs. The announcement is worth giving special attention for its relevance not only to the PV for UAV technology, but also to many other PV applications. Scientists have been focusing on solving materials and device issues that limit the efficiency of solar cells, and precisely they have been pushing the efficiency of single junction cells towards the Shockley–Queisser limit (33.7% for GaAs).
The new record produced by single-junction GaAs cell is indeed excellent, but possibly there is still a room for enhancing this new design. For instance, by combining the simple junction cell with another thin film junction made of a semiconductor that absorbs in a different part of the solar spectrum. One can think of making the GaAs cell on ultra-thin silicon cell for UAVs. But, matching GaAs crystals with silicon has been a long time challenging research problem, and scientists have not given-up solving that challenge. If successful, more efficiency points will be added, which will increase the efficiency to above the Shockley–Queisser limit.
The reported high PV performance is a big win for photovoltaics for UAV, but space solar cells based on III-V are very expensive and not desirable for terrestrial photovoltaics, because of the high cost and the scarcity of the material. The latter is a strong negative argument that does not support photovotlaics over very large scale, contrary to silicon PV technology. That is one of the reasons for our research interest in silicon cells. Succeeding in the more robust thin silicon materials for UAV technology will be also beneficial for powering homes, portable computers, electric cars, autonomous machines... Some believe that silicon has reached its performance limits, that is absolutely not true. Even though silicon cell improvement have been a very long incremental journey, the material still offers benefits for the higher performance goal, such as enhancing its absorption in the UV and in the IR parts of the solar spectrum. Several other areas where photon and carrier losses are easier to tackle are being looked at by industry research labs. In accademia, more difficult areas interest the scientists, such as nucleation and growth of defect in the silicon, alloying silicon with other materials, engineering defects and dopants, use of nanoscale materials options, modeling materials to understand fine phenomena many of which involve quantum mechanics effects, energy levels and mulit-band optical transitions, involvement of quasi-particles,... Also a prominent research area is emerging aiming at correcting light absoprtion by involving other materials (GaAS, Ge, Perovskite,... ) whether by modifying silicon, or adding an absorbing layer, or a light converter layer, or a material window optical layer,... Certainly, there is still a long way before reaching the 32% Shockley–Queisser limit for single junction design. For instance, my team and many others are working on improving silicon IR response by modifying silicon.
In closing, not all solid state mechanisms at play in silicon solar cells have been addressed by the community, as some of the involved physics is still not fully understood, and research in these areas is progressing incrementally and slowly. But, the real challenge for scientists is, funding silicon research which is very scarce. While federal funding agencies assume that industry should take care of silicon research, companies in the field have not paid attention in investment in mid-term and long term research as they are busy with keeping their books afloat. The case of PV-UAV and the PV for Electric Planes, promoted recently by NASA, are excellent high end applications and future technologies that will drive new funding programs for advanced research on silicon cells. It is important to aim at reaching extreme conversion efficiencies and solar cell robustness, despite the weight limitation constrain.
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I hope a useful debate is engage with colleagues in the field, and we can discuss possible cooperation opportunities.
Prof. A. Karoui
A dissemination article published by PV-Tech reported these findings to the large public. Since the article is extremely relevant to my project, I am reproducing it here in case the paper will no longer be available in PV-Tech website, the article copyright belongs to PV-Tech.