Achieved specific power is typically about a tenth of the powers listed here. In a well designed structure, however, the structural mass should be able to be decreased roughly proportionately to the cell mass. As a rule of thumb, the array structural mass is generally roughly equal to the (covered) cell mass. (The rest of the power system—batteries, power conditioners and controllers, etc.—contributes an additional mass element which is nearly independent of the array.) Specific power is not the only concern in solar array design. Other criteria include high array stiffness (i.e. resistance to bending during acceleration), high resonant frequency, and low moment of inertia in order to minimize force required for orientation. For all these parameters higher specific power, by reducing the mass of the solar cells, increases the relevant performance; while lower efficiency, by increasing the size, decreases it. In general, for these parameters the relevant figure of merit scales as product of the specific power and the efficiency [49], The fact that thin-film arrays typically use monolithic interconnection (i.e. the interconnections between cells are manufactured as the cell is made) means that the mass of additional wiring and interconnection will be much lower for a thin-film array compared to conventional arrays. Low Earth orbit provides a special case, where the drag area is a criterion. For these orbits, efficiency takes on increased importance. Price may also be a concern for some applications, especially applications which require extremely large arrays. Cost has not been discussed in this review, however, thin-film arrays are expected to be considerably lower cost than conventional arrays, both due to the lower cost of the materials and deposition process, and due to the fact that the interconnections are integral to the cell manufacturing, thus requiring less additional hand labour. However, for many, and perhaps even most missions, these concerns are secondary compared to the array mass. In this case achieving maximum specific power is the dominant factor in the choice of technology. System Applications and Missions The important applications for thin-film solar cells are to missions where specific power is a concern or where significant radiation exposure occurs during the course of the mission. While most spacecraft can benefit from increased specific power and radiation tolerance, specific missions for which thin-film photovoltaic arrays may be an enabling technology are solar electric propulsion, a manned Mars mission, and lunar exploration and manufacturing. For solar electric propulsion, the system performance is directly proportional to the specific power. Accurate pointing is not important during the thrust. One proposed mission for solar electric propulsion is for a low-thrust vehicle to transfer satellites from low Earth orbit to geosynchronous orbit, or from low Earth orbit to lunar transfer. In both cases the orbit is a slowly rising spiral which spends a long time in the radiation belts, and for these missions the potential radiation hardness of thin-film cells may be very important. For a Mars unmanned cargo ship, required power levels could be very high (megawatts), and specific power very important. A manned Mars mission would require up to 1 MW of power, both for the spacecraft during the journey, and to power the surface base [50]. For the baseline mission, the transportation cost is extremely high, and specific power becomes the dominant concern, with efficiency of little importance. This makes thin-film cells a
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