Most modern industrial transport, agricultural and industrial systems depend on the relative low cost and high availability of fossil fuel. The point in time when the maximum rate of global petroleum extraction is reached is known as peak oil. Predictions vary greatly as to the year when this peak oil will occur. However, product life cycles in aviation industry typically range from 40 to 60 years. Given these long life cycles it is necessary to think of alternative fuels for aviation today, even when the exact year of peak oil is not known. One possible solution is the development of substitutes for petroleum, e.g. biofuels. A second aspect when thinking of the future of aviation is the environmental impact. Recently, also the aviation industry has come into focus when debating climate change and global warming. Therefore, a more radical approach to the future of aviation seems necessary.
The development of electric propulsive systems for aircraft seems to be a promising long term approach. Thereby, the aircraft runs on electrical power rather than internal combustion engines, with electricity coming from fuel cells, solar cells, ultracapacitors, power beaming and/or batteries. A major factor when designing an aircraft is its weight. Considering fuel cells, ultracapacitors or batteries it seems likely that there will be no major savings in weight. The effectiveness of aircraft-mounted solar cells, on the other hand, would be limited by cloud coverage, the available fuselage and wing area and solar power availability. Power beaming is another possibility of providing an aircraft with the necessary electrical power. With this concept, only a small amount of energy needs to be stored onboard for take-off and landing. During cruise, energy is provided by a power beaming mechanism. An almost unlimited source for this power beam is given by the sun. Space based solar power is a concept that is explored since the 1970s. There, large solar panels reside on a satellite in orbit, being unaffected by weather, season or the filtering effect of Earth´s atmosphere.
Space-based solar power consisting of three parts: a means of collecting solar power in space, for example via solar cells or a heat engine, a means of transmitting power to earth, for example via microwave or laser and a means of receiving power on earth, for example via a microwave antennas (rectenna) Source: NASA
The proposed concept of solar energy conversion in space and wireless power transfer to aircraft can be segmented as follows. First, sunlight needs to be converted to electric energy. Recent research describes either thin-film solar cells or solar dynamic power systems as two possible approaches to this . The next step concerns the transmission of this electric energy. The two main mechanisms for this are either microwave or laser power transmission. Based on the mode of transmission, the electric energy needs to be converted accordingly. Lastly the transmitted energy has to be converted back to electric energy. Along this process aspects such as conversion efficiency, beam quality, transmission losses, beam steering or pointing quality need to be taken into account.
Benefits to the Air Transport System
As the finite availability of oil is inevitable, other forms of energy sources become necessary. A more radical approach to this need is the development of a system that directly beams space solar power to aircraft, driving their electrical engines.
Implementation of the proposed idea would result in three main benefits. First, an emission-free aircraft could be realized. That is, there will be no more emissions into the upper layers of the atmosphere. Second, weight could be saved as only a small amount of energy relative to the total mission energy demand needs to be carried along. Although additional equipment would be needed for power reception and conversion, weight savings should be substantial. Another potential benefit might be cost savings. In the long run, cost for satellite infrastructure should be less compared to cost for fuel and fuel-related infrastructural needs, especially facing increasing oil production cost.
Likelihood of Public Acceptance
The stigma of travelling by air polluting aircraft may be remedied. Furthermore, an overall decrease in travel prices is conceivable. Therefore, the acceptance by the travelling as well as the non-travelling public should be high. Impediments on the other hand could arise from the satellite infrastructure in orbit and the beaming mechanism. Depending on the nature of their orbits (low orbit, geostationary orbit) the satellites could be distracting sights on the sky or could be seen as potential threats when it comes to unforeseen de-orbiting in case of failure. Thinking of today’s discussions related to electromagnetic “pollution” caused by the mobile phone infrastructure the envisioned wireless power transfer mechanism could also be subject to such discussions. Furthermore, the satellite infrastructure could be perceived as a potential weapon system when thinking of the highly focused energy beams. Possible safety concerns related to dangers connected to the power beam have to be addressed.
With the proposed concept a completely new power system will be introduced to air transport. Although electrical propulsion systems are already tested in small aircraft, the adoption in commercial aircraft still has to occur. Radical changes will be necessary in the equipment of the aircraft with respect to the reception and conversion of the beamed energy as well as to the electrical engines. An additional radical impact is of course given by the satellite infrastructure and the related energy beaming mechanism.
Credibility of the Physics
The general applicability of space-based solar energy collection and wireless transfer to Earth has been investigated in the SERT NASA project and also been proven in a number of laboratory scale experiments. Therefore, the basic concept must be considered technologically viable. From a physical point of view the overall concept should therefore be realizable. The physical and technical analysis of the proposed concept for powering aircraft can be divided into the following aspects: space solar satellite, energy conversion, wireless energy transfer and electric aircraft. The transmission of energy to an electric model aircraft has also been shown by NASA. However, there is no true-scale prototypical implementation yet.
Criteria for Successful Incubation
Providing access to an almost unlimited source of energy and saving weight and potentially cost are the main purposes of the proposed concept. Impediments may arise concerning energy conversion and transmission efficiency, beam quality, beam steering or pointing quality. The following questions have been identified for investigation during the incubation phase:
• Which sunlight and energy conversion and transmission mechanisms make for the best efficiency in combination with reasonable deployment cost?
• Which wavelengths are to be chosen for transmission?
• Which will be the best cruise altitude for aircraft for minimized transmission losses?
• Will there any protective devices be necessary for passengers?
• Which is the best aircraft size for such a concept?
• Are living organisms in danger when in or near the power beam?
• What amount of energy storage will be necessary for take-off and landing?
• How many aircraft are necessary for a minimum profitable system?
• Which will be the optimal orbit for the satellites?
A 20 panel array to supply earth based aaplications with solar power - source: NASA
Timescale for Incubation
Investigating the questions given above will require a period of 2 to 3 years.
Budget required for Incubation
A budget of 500.000€ for two university researchers (three years) might be necessary for these investigations. An additional budget will be need for experiments.
Partnering needs for Incubation
Successful incubation of the proposed concept will require input from several different fields. Aeronautical engineers will be needed for an optimal design of the aircraft. Physicists and electrical engineers will be needed for developing the conversion and transmission mechanisms and devices. Space engineers will be needed for the design of the space solar power satellites and their deployment into orbit.
Related and Supporting Capabilities
The proposed concept requires changes to aircraft and airport systems as well as a completely new satellite infrastructure. Aircraft need to be equipped with energy reception and conversion devices as well as electric engines. For this, aircraft manufacturers need to design the aircraft accordingly. The reception and conversion devices need to be developed. Airport systems need to be adapted as far as charging onboard batteries for take-off and landing is concerned. Furthermore, the satellite infrastructure needs to be transported into space. For this, it has to be investigated whether existing transport capabilities are sufficient or whether new vehicles need to be developed.
Scalability of the Idea
A limiting factor to scalability is the number of aircraft that can be fed by a single given space solar power satellite. This number is limited by the maximum useful size of a single space solar satellite. Furthermore, an increasing number of aircraft would also require an increasing number of space solar power satellites. On the other hand, a certain amount of respectively equipped aircraft would be needed in order to create positive cash flow for a minimum space solar power satellite infrastructure.
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