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Financial & Energy Analysis of Mechanical Refrigeration vs. Alternatives

Advanced Processing Technology

Financial & Energy Analysis of Mechanical Refrigeration vs. Alternatives

Using liquid nitrogen or dry ice for processing is a major consumable operating cost. Although using these consumables is perceived as a simpler solution it is very inefficient, and comes with asphyxiation hazards and logistical overheard.

Analysis comparing mechanical refrigeration systems vs. open loop refrigeration via liquid nitrogen and liquid CO2 can be found in the article cited[1]. As shown in the analysis for a meat freezing application mechanical refrigeration even accounting for energy, maintenance and capital costs is approximately 3-8 times more efficient per unit of mass to be frozen. Further analysis and projections for the refrigeration systems offered by GPS in specific processing scenarios are offered below.

Dry Ice(CO2) vs. Mechanical Refrigeration: Butane Case

Specific comparisons can be drawn for butane based on the following assumption 1:7 biomass to butane solvent mass ratio and 2:1 mass ratio of dry ice consumed per unit mass of butane chilled. These assumptions are based on a general survey of lab operators reporting their rates of dry ice usage.

Figure 1: Mass of butane solvent chilled per hour (left side), vs. estimated equivalent dry ice consumption rate (right side)
Figure 2: Monthly costs of dry ice consumption(@ 0.20 cents/lb) vs. refrigeration system costs (loan payments, interest, power, maintenance and depreciation).
Figure 3: Estimated time to pay off compared to dry ice for various systems and configurations. 

The findings in figure 3 are slightly skewed as the 42 TR system includes an additional cool and heating loop(3 loops total) which is not accounted for in this analysis, whereas the 2.5 & 4 TR systems only include a one cooling loop, and the 14.5 and 15 TR systems include one cooling loop and one heat recycling loop. If the economic effects of the additional cooling and heating loops were taken into effect the results would appear slightly more favorable for the 14.5, 15, and especially 42 TR model(s).

Figure 4: Hourly operating costs for the 2.5 & 4 TR models are shown vs. liquid nitrogen and dry ice. Waste heat recycling is offered as an additional option on the  2.5 and 4 TR models, waste heat effects are not included in the analysis above.
Figure 5: Hourly operating costs for the 14.5 & 15 TR models are shown vs. liquid nitrogen and dry ice. Both models have waste heat recycling. The 14.5 TR piston model operates at a -40 Deg C saturated suction temperature(SST) while the 15 TR screw compressor model operates at a much lower -54 Deg C SST.
Figure 6: Hourly operating costs for the 42 TR model is shown, this model has (2) cooling and (1) heat recycling loop. Several configurations are possible with different heat exchangers which will impact the overall efficiency.
Figure 7: A total cost breakdown for the 42 TR model is shown, use of waste heat recycling substantially affects the overall economics of the system.

Note assumptions and simplifications were made in this analysis which is only intended to represent overall trends not exact results. Contact GPS for more detailed analysis of your specific use case. 

It is our contention based on first order analysis that once total ownership and consumable costs are compared mechanical refrigeration can pay off within 1-3 years or less depending on implementation and provide many advantages over other cooling methods.

[1] Cryogenics VS Mechanical Freezing Systems 

[2] The Future of Ethanol Cooling: Pinnacle Stainless Ahead of the Curve

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