Any use conducted outside the confines of a building, except as specifically permitted. This prohibition shall include outdoor storage or display of any kind, except for limited and temporary storage of refuse and recyclables accessory to the use. Rooftop or at-grade equipment designed as part of the heating, ventilating, air-conditioning, emergency power or similar systems for the principal use shall be excluded from the foregoing prohibition, provided that such equipment is properly screened and that environmental impacts from the equipment are appropriately mitigated.