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6.4 Power Beaming-Assisted CO2 Reduction 45 Although this is certainly a large mass to consider sending to Mars, the appropriate comparison to make would be to the nuclear reactors that are noted in some roadmaps for providing power for Mars ISRU operations. The mass of these planned nuclear reactors to provide the requisite 26 kW is 7.8 mt to the surface. With a higher cost of mass to the surface versus mass to orbit of approximately two times, there is actually a significant saving in sending 6.6 mt to Martian orbit. A rectenna array is needed on the surface to collect the beamed power, but rough calculations estimate the mass of this array to be about 1000 kg to harness up to 50 kW of power. A commercially available microwave frequency of 94 GHz is chosen for this example; the higher frequency gives a lower (27%) electric-to-microwave conversion but will generate a smaller spot size on the surface scaling inversely with the frequency, making it possible to send a smaller rectenna array. The microwave beam spot size is approximately 220 m square. There is a cosine latitude dependence on the power density, so 45◦ was used as a worst case scenario. At this latitude, the power density on the ground is 50 W/m2 (for comparison, at the equator the power density would be 71 W/m2). While the spot size is quite large, in order to collect 50 kW (the amount to be supplied by the proposed nuclear reactors), a rectenna array of only 31.5 m square would be required (this shrinks to 26.5 m square at the equator). If we assume a mass of 20 g/m2, the mass to surface for the rectenna would be 20 kg, with considerable savings over the nuclear reactors. Once the solar energy has been beamed to the surface of Mars and converted into DC electrical power by the rectenna array, this power can be used to drive the ISRU EC conversions described earlier. None of these systems have been used on a mission yet, however the technology is well-characterized, well-understood, and engineered to high Technology Readiness Level (TRL) for use on Earth. This power source could be used for any chosen EC conversion technology, such as the solid oxide conversion technology proposed on the Mars 2020 mission, MOXIE, or others. In order to consider these techniques for use on the surface of Mars, light-weighting options would be advantageous, along with environmental (such as thermal management), power, efficiency, and lifetime requirements. Tying all of these individual pieces together into a mission to synthesize oxygen and fuels using ISRU on the surface of Mars could solve the problem of the ascent launch of a manned mission to the surface of Mars (Figure 6.5). Beyond this, a power system of this type could have significant implications for later missions. A continuous supply of power, via solar electric or chemical feedstocks, would be in place for a mission with an extended stay and eventually for a permanent base on Mars, supplying ascent and vehicle fuels and life support. More surface area rectenna array coverage can scale to up to more than 2 MW for the described solar array. Redundancy in power supply for the scientific instruments enables more flexibility and capability in science experiments. It is feasible that this power supply methodology could eventually replace other options as a primary power source for all systems, eliminating the need to deliver fuels and power supplies from Earth. While not the primary motivation, it is important to note the consequencesPDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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