def temperature_trans(air_in, fuel_in, ex_out):
    """
    Convert degree celsius to kelvin.
    Optional keyword arguments:
    ---------------------------
    :param air_in: temperature of air coming in to fuel cell(FC)
    :param fuel_in: temperature of fuel coming into (FC)/
                    temperature of reformer
    :param ex_out: temperature of exhaust coming out of FC
    :return: return the degree of kelvin
    """
    T_FC_air_in = air_in + 273.15
    T_FC_fuel_in = fuel_in + 273.15
    T_FC_ex_out = ex_out + 273.15

    return [T_FC_air_in, T_FC_fuel_in, T_FC_ex_out]