Nested Cascade Refrigeration for Ultra-Low Temperature Transport
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Solution Overview
Problem
Current transport refrigeration units struggle to maintain extremely low temperatures, such as below −50° C., required for distributing COVID-19 vaccines and other perishable goods at vast scales, while managing heat transfer efficiently and maintaining acceptable compressor discharge temperatures.
Innovation Solution
A nested cooling arrangement is implemented, featuring an outer refrigeration cycle and an inner cascade refrigeration cycle with an inter-stage heat exchanger, using low-temperature refrigerants like R-23 or R-469A, and a controller to manage thermodynamic conditions, ensuring the inner container reaches temperatures as low as −70° C. without excessive compressor discharge temperatures and maintaining efficient heat transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a single-stage refrigeration cycle is used to achieve ultra-low temperatures below -50°C, then the target temperature is reached, but the compressor discharge temperature becomes excessively high
Solution Approach 1:
The refrigeration system is divided into two separate stages: a first stage cycle that operates at moderate temperatures (rejecting heat to ambient) and a second stage cycle that achieves ultra-low temperatures. This segmentation allows each compressor to operate within safe temperature limits while collectively achieving the desired -70°C or lower cargo space temperature.
Solution Approach 2:
An inter-stage heat exchanger serves as a mediator between the first and second stage cycles. It receives heat rejection from the second stage cycle and transfers it to the first stage cycle, enabling the second stage to achieve ultra-low temperatures without directly discharging excessive heat to the ambient environment.
2Object-affected harmful factors
If a cascade refrigeration cycle with two stages is implemented, then compressor discharge temperature is controlled, but system complexity increases
Solution Approach 1:
The inter-stage heat exchanger performs multiple functions: it serves as the condenser for the second stage cycle, the evaporator for the first stage cycle, and a heat transfer interface between the two stages. This multi-functionality reduces the number of separate components needed and simplifies the overall system architecture.
Solution Approach 2:
The system merges the first stage and second stage refrigeration cycles into a single integrated cascade system where components serve dual purposes. The common refrigeration equipment and shared heat exchanger pathways combine the two cycles into a cohesive unit that manages complexity through functional integration.
3Use of energy by moving object
If heat is rejected from the inner container directly to ambient air, then cooling efficiency is reduced, but system simplicity is maintained
Solution Approach 1:
The first stage cycle acts as an intermediary heat rejection system. Instead of the second stage cycle rejecting heat directly to ambient air (which would be inefficient due to the large temperature difference), the first stage cycle serves as an intermediate heat sink, receiving heat from the second stage and rejecting it to ambient conditions more efficiently.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution allows for sustainable low-temperature maintenance within the container, optimizing heat transfer and reducing the compressor discharge temperature, thereby enhancing the refrigeration unit's efficiency and capacity to handle ultra-low temperatures effectively.
Implementation Method 1
an outer refrigeration cycle configured to transfer heat from the cargo space exteriorly of the cargo space, to cool the cargo space
Implementation Method 2
an inner refrigeration cycle configured to transfer heat from an interior of the container, to cool the interior of the container
Implementation Method 3
An inter-stage heat exchanger may be provided between the first stage cycle and the second stage cycle. The inter-stage heat exchanger may serve as a condenser heat exchanger in the second stage cycle and may serve as an evaporator heat exchanger in the first stage cycle
Data Source
AI summary
There is disclosed a transport refrigeration unit comprising an enclosed cargo space, an outer refrigeration cycle configured to transfer heat from the cargo space exteriorly of the cargo space, to cool the cargo space; a container installed within the cargo space; and an inner refrigeration cycle configured to transfer heat from an interior of the container, to cool the interior of the container. There is also disclosed a portable refrigeration device for use in such nested cooling arrangements, methods of adapting a transport refrigeration unit and/or portable refrigeration device for such use, a refrigeration module for such use, and an associated method of operating a transport refrigeration unit.