def capacity_cost_rule(mod, prj, prd):
    """
    The capacity cost of projects of the *fuel_spec* capacity type is a
    pre-specified number with the following costs:
    * fuel production capacity x fuel production fixed cost;
    * fuel release capacity x fuel release fixed cost;
    * fuel storage capacity x fuel storage fixed cost.
    """
    return (
        mod.fuel_production_capacity_fuelunitperhour[prj, prd]
        * mod.fuel_production_capacity_fixed_cost_per_fuelunitperhour_yr[prj, prd]
        + mod.fuel_release_capacity_fuelunitperhour[prj, prd]
        * mod.fuel_release_capacity_fixed_cost_per_fuelunitperhour_yr[prj, prd]
        + mod.fuel_storage_capacity_fuelunit[prj, prd]
        * mod.fuel_storage_capacity_fixed_cost_per_fuelunit_yr[prj, prd]
    )