Refrigerant carbon dioxide (R-744): It is not widely used in conventional systems primarily because of its __________ power requirements per tonne of refrigeration and high operating pressures.

Introduction / Context:CO₂ (R-744) has attractive environmental credentials (ODP = 0, very low GWP for transcritical venting applications), but its thermodynamic characteristics impose design and efficiency challenges compared to many halocarbon refrigerants in traditional cycles.

Given Data / Assumptions:

• CO₂ operates at very high pressures, especially in transcritical cycles.
• Comparison is to typical HFC/HFO cycles at similar temperature lifts.
• Metric of interest: power per tonne of refrigeration (kW/TR).

Concept / Approach:At common ambient temperatures, CO₂ systems often run transcritically with a gas cooler rather than a condenser. The absence of a condensation plateau and high pressure levels lead to higher compressor discharge temperatures and significant compressor power for a given cooling capacity.

Step-by-Step Solution:Consider a medium-temperature application at 35–40 °C ambient.CO₂ discharge pressures can exceed 90 bar; optimal gas-cooler outlet temperature control is critical.For the same cooling load, compressor input tends to be higher than many conventional refrigerants unless advanced cycles (parallel compression, ejectors) are used.

Verification / Alternative check:Performance maps show improved efficiency in cooler climates or with system enhancements; however, baseline systems typically exhibit higher kW/TR than legacy HFCs at warm ambients.

Why Other Options Are Wrong:

• Low/moderate/negligible/variable: Do not match the general baseline behavior; “high” is the key reason cited alongside high operating pressures.

Common Pitfalls:Assuming CO₂ is always inefficient; in cold climates or with modern architectures, efficiency can be competitive. The question asks about general non-use historically.

Final Answer:high