Exemption eligibility: eligible units may claim area based excise exemption even if option is exercised after sunset. Eligible new or substantially expanded units that commenced commercial production on or before the sunset date remain entitled to area based excise exemption for goods even where excise levy was imposed subsequently. The sunset clause is an eligibility cutoff only; exercising the written option may occur after the sunset, and the ten year concession period is computed from commencement of production, not from the date the option is exercised.