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44 Chapter 6. System-Level Considerations due to reflections lower the power that would be expected to successfully propagate to the surface during a storm. Microwaves, while more challenging to focus into a narrow beam, are expected to penetrate through these storms without significant attenuation. Furthermore, because of the closer stationary orbit position (17,000 km, versus 36,000 km on Earth), the technology should be even more feasible. A mid-level or low orbit (to less than 7,000 km) might even be acceptable. With frequencies in the range of 1–10 GHz, the electric to microwave conversion is expected to be high, conservatively on the order of 70% or more. Experiments done so far, albeit over significantly shorter distances, indicate high conversion of the beamed microwave power back into electrical energy (> 80%). Further research is certainly needed to make this method succeed, but with the state-of-the art technologies combined, it appears close to reality. One of the shortcomings of this type of power delivery is the extreme size of the solar arrays required to collect the solar energy, resulting in large mass and cost of launch and delivery. However, significant improvements have been made in deployable structures, making possible lightweight, high energy capture structures. In fact, research being done at the California Institute of Technology has resulted in the design of a deployable solar array that measures 60 m by 60 m square, with a density of only 80 g/m2 (Arya et al., 2016). Furthermore, that array was designed to be scalable such that multiple individual deployments can be attached together to make a larger structure or several separate ones can fly in constellation form. The array solar-to-electric efficiency is on the order of 30%, and research continues to advance these experimentally realized values. On Earth, current estimates show that wireless microwave power delivery requires extremely large surface area rectenna arrays, which might make this a less attractive approach. Due to the lower orbit possibilities on Mars, the area required for the rectenna array would be 1/4–1/2 the size of one required on Earth. Additionally, the coverage of the Martian surface with such an array will not carry the same concerns as on Earth, and in fact might be used to aid in the collection of feedstock, such as removal of water from the regolith. Progress has been made in all relevant areas, including conversion of electrical to microwave power, the focusing of microwave beams, and in the rectenna array receivers. It is apparent how many variables can be modified with this design in order to accommodate the power requirements on the surface. The size of the solar array in space affects both the amount of solar energy harnessed and the resulting power beam spot size on the Martian surface; the frequency of microwaves used simultaneously affects the electric-to-microwave efficiency and the beam spot size; and the chosen orbit for the solar collector affects the power profile throughout the Martian day, as well as the spot size. Many options exist for this design and several were calculated to determine the viability of this concept. For example, assume that the solar array is put into Mars areosynchronous orbit, which can generate a continuous power supply with the appropriate targeting. Using the mass estimated for the solar array designed for use in Earth orbit, a 300-m-square array should weigh 9 mt, not including a surface rectenna and power converter.PDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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