Compare energy using accepted liquid output, a common equipment boundary and actual operating conditions. Motor ratings alone cannot establish consumption.
Start with the denominator
Specific electricity consumption is metered kWh divided by tonnes of accepted product over the same period. State whether the product is liquid leaving the liquefier, liquid transferred to storage or liquid delivered to a customer. Account for inventory changes if the production and dispatch meters are at different boundaries. Off-spec product and recycled CO₂ should not be counted repeatedly as new saleable tonnes.
Build a complete power boundary
| Load | Question for the offer |
|---|---|
| CO₂ compressor and auxiliaries | Are intercooling pumps and lubrication auxiliaries included? |
| Refrigeration | Is the figure compressor-only or the full refrigeration package? |
| Cooling and regeneration | Are tower fans, water pumps and electric heaters counted? |
| Standby and start-up | Are these loads excluded from a steady-state guarantee but included in annual economics? |
A refrigeration-only number can be useful for a refrigeration purchase. It becomes misleading when compared directly with another supplier’s whole-plant number. We ask for separate subtotals and a clearly stated total so different offers can be normalized.
Worked comparison
During an illustrative eight-hour test, a refrigeration package uses 800 kWh and the rest of the agreed plant boundary uses 480 kWh. Accepted liquid production is 8 tonnes. Refrigeration-only consumption is 100 kWh/t; whole-boundary consumption is 160 kWh/t. Both calculations are correct, but they answer different questions. Neither is a performance claim for our equipment.
At a reduced load, suppose the same boundary consumes 760 kWh while producing 4 tonnes. Consumption is now 190 kWh/t, even though total energy is lower. A single full-load test therefore cannot establish annual cost for a plant that spends much of its time at reduced load.
Conditions that must accompany the number
Record inlet composition, pressure and temperature, required product state, cooling conditions, ambient conditions, operating load, product quality and recovery losses. Compression and refrigeration duties respond to these conditions. CO₂ phase behavior also matters: cooling alone does not produce stable liquid CO₂ at atmospheric pressure. Use validated properties for the selected pressure and composition; the pure-CO₂ data in NIST are a reference, not a mixed-gas process simulation.
Turn the test into an annual estimate
Divide the year into realistic full-load, reduced-load, start-up and standby periods. Calculate electricity for each, then divide the annual total by annual accepted tonnes. List uncertainty in operating hours, summer cooling and rejected product. Meter actual operation against the same boundary after handover so a change in kWh/t can be traced to load, conditions or equipment performance.
Sources and scope
We use these sources for the physical principles or guidance identified in this article. Our examples and decision tables are explanatory calculations, not measured plant results. Industry guidance does not replace the edition, customer specification and local requirements agreed for your project. Publication listings are cited for their public scope; we do not claim to reproduce their full standards.
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