Mission Design for the CubeSat OUFTI-1

CHAPTER 9We have now all the parameters to connect the nodes: we only need thesurfaces parameters and the conductivity.If the structure’s and coating’s parameters are fixed, the solar cells opticalparameters vary as a function of the temperature.The goal of a solar cell is in fact to collect the sun’s flux and to convert it intoelectrical power: not all the energy collected becomes heat. The absorptionfactor of a solar cell varies following the relation:α(T ) = α 0 (1 − η(T )) (9.14)where α 0 is the absorption at 28 ◦ C and η(T ) the efficiency.This formula shows an important conclusion: higher is the efficiency, and so thepower produced, lower is the percent of collected energy converted into heat.We have so a double interest in having an efficiency as higher as possible.Then, the efficiency varies in function of temperature:η(T ) = η 0 + dηdT (T − 28◦ C) (9.15)where η 0 is the efficiency at 28 ◦ C.As the efficiency is defined as the maximum percent of incident power convertedinto electrical energy (see formula 8.10 ), we define its derivate respectto temperature as:dηdT = ddT( )Pmax= 1 ()dI maxV maxC s S C s S dT+ I dV maxmaxdT(9.16)Using the values indicated in table 8.2, we can calculate the absorption factorof the solar cells.Concerning the infrared emissivity ɛ, we need to point out that each solarcell is usually covered by a transparent tape and by a so-called cover glass whichis transparent for the visible wavelength but determine the infrared emissivity:ɛ is usually between 0.8 and 0.85. We used ɛ = 0.8.The dimension of solar cells are indicated in table 8.1.Concerning the structure, it’s an aluminium alloy with the following properties:Table 9.3: Structure propertiesConductivity k [ W mK ] 138Thickness [mm] 1.27Galli Stefania 92 University of Liège