Concept

System boundary

Everything crossing is an input, an output, or an emission. Inside the closed control surface, operations are modeled in detail; outside enters only through the streams that cross. Inward is a feed taken as given — a feedstock or energy cost; outward is a product or byproduct carrying revenue or a credit, or a waste/emission. Nothing wholly inside appears in the external accounting. The same region is the control volume for a mass balance (in = out + accumulation, zero at steady state) — physical, cost, and emissions ledgers all kept over it.

Where it’s placed — and that it’s a choice. Boundaries nest: a unit operation; the process units; the whole plant (“gate-to-gate”); “cradle-to-gate,” reaching upstream to producing the feedstocks and energy; “cradle-to-grave,” through use and end-of-life. Each step outward turns a purchased input into an internally-produced stream. Where the line falls is chosen, not given: one analysis buys hydrogen at the fence, another makes it inside, and both report legitimately different numbers. It’s distinct from the battery limits (the narrower on-site capital fence); an unstated boundary is the hidden assumption behind a levelized cost or a carbon-intensity figure.

Limits & typical error

• Often the single largest lever on the headline number — larger than any choice inside it. For a feedstock-dominated process, whether the feedstock is purchased across the line or produced inside can move levelized cost by tens of percent — more than refining any equipment size within it.
• Mismatched boundaries aren’t comparable. A gate-to-gate figure beside a cradle-to-gate one compares unlike systems; the gap can be pure boundary artifact — and the boundary is usually unstated.
• Drawn too narrowly, it silently truncates real costs and emissions just outside the line — a gate-to-gate boundary omitting upstream feedstock understates the delivered product’s true cost and carbon.
• The cost, physical, and carbon boundaries can diverge — pricing a feed across the fence while not counting its upstream carbon makes cost-per-unit and carbon-per-unit describe different systems. And where two analyses abut, the crossing stream must be booked exactly once: a cost to one side, revenue to the other.

See also

• Mass balance — the conservation check over the boundary as control volume.
• Accounting (capex, opex & revenue) — the cost ledger for the inputs and outputs that cross the line, including the narrower ISBL/OSBL capital fence.
• Levelized cost — the headline cost whose meaning depends on the “gate.”