Patent Application
20250035365
The disclosure relates generally to temperature-controlled environments.
A solid-state refrigeration system uses a heat exchanger, referred to as an “accept”, to transfer heat from the air to be conditioned and the thermoelectric cooling unit. The air being conditioned in a freezer tote can accumulate moisture from the outside ambient air, or from the food stored in the tote. Because the accept heat exchanger in a freezer tote is almost always colder than the dew point of ambient air or air that holds moisture evaporated from foods, moisture will naturally condense and then freeze on the accept heat exchanger.
A defrost heater and operating cycle is required to melt frost that gradually accumulates on the accept heat exchanger of the freezer tote during normal operation. This is helpful because the accept heat exchanger becomes significantly less efficient at higher levels of frost.
Under normal tote operation, humidity inside the tote condenses and freezes on the surface of the accept heat exchanger. This humidity comes from many sources, such as lid-openings, gasket leaks, and water-rich foods being stored in the tote.
At low levels of frost (mass <50 g), there is negligible impact on tote performance. However, once the mass of frost exceeds this level it begins to act as an insulative cover on the heat exchanger. This drastically decreases the coefficient of performance of the refrigeration system and eventually leads to an inability of the tote to maintain set point temperature even at maximum power input to the thermoelectric module.
At some point, the accept heat exchanger becomes completely blocked with frost and air flow across the heat exchanger is reduced, preventing proper cooling performance of the tote.
Systems and methods for automatic defrosting of an actively cooled tote are provided. In some embodiments, a method of operating an actively cooled container includes: determining to initiate a defrost cycle for the actively cooled container; deactivating a cooling system of the actively cooled container; activating a defrost heater of the actively cooled container; determining to deactivate the defrost heater of the actively cooled container; setting the defrost heater to maintain a temperature of a thermal accept system at a threshold; waiting a set duration of time to allow for melted frost to drip into a collection tray; and activating the cooling system of the actively cooled container.
In some embodiments, the method also includes: deactivating one or more fans of the actively cooled container. In some embodiments, the method also includes: after waiting the set duration of time: activating one or more fans of the actively cooled container. In some embodiments, activating the one or more fans comprises: in response to determining that the temperature of the thermal accept system is at or below a set threshold temperature, activating one or more fans of the thermal accept system. In some embodiments, the set threshold temperature is zero Celsius.
In some embodiments, determining to deactivate the defrost heater comprises one or more of: determining that the defrost heater has been activated for a predetermined amount of time; and deactivating that a temperature of a thermal accept system crosses above a set threshold.
In some embodiments, determining to initiate a defrost cycle comprises one or more of: reaching an elapsed time since prior defrost; determining that a difference between an accept temperature and a chamber temperature exceeds a specified value; determining that a lid open duration and subsequent full-power operation ratio exceeds a specified value; determining a relative humidity of the actively cooled container; and determining a mass of frost currently on the heat exchanger.
In some embodiments, the method also includes: activating an alarm indicating that frost may have built up. In some embodiments, the method also includes: determining that a threshold number of defrost cycles have been performed. In some embodiments, the method also includes: ensuring that the temperature of the actively cooled container does not rise above an allowable limit.
Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.
The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.
The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.
Last mile delivery of food requires temperature-controlled transport of perishable food items using transit vans or similar vehicles. For temperature control, refrigerated or freezer totes can be used which are installed in the van (e.g., a cargo van) or a box truck.
These totes use an active heat pump to pull heat from an enclosed chamber and reject it to surrounding ambient air. The hot air must be removed from the van to ensure optimum operation of the totes.
These totes require power while in transit maintain food safety requirements for perishable consumption. The electrical system needs to reach (and/or maintain) the correct temperature must be met for operation of the totes.
For more details, the interested reader is directed to U.S. Provisional Patent Application Ser. No. 62/953,771, entitled THERMOELECTRIC REFRIGERATED/FROZEN PRODUCT STORAGE AND TRANSPORTATION COOLER; U.S. patent application Ser. No. 17/135,420, entitled THERMOELECTRIC REFRIGERATED/FROZEN PRODUCT STORAGE AND TRANSPORTATION COOLER, now U.S. Patent Application Publication No. 2021/0199353 A1; and International Patent Application No. PCT/US2020/067172, entitled THERMOELECTRIC REFRIGERATED/FROZEN PRODUCT STORAGE AND TRANSPORTATION COOLER, now International Patent Publication No. WO 2021/134068. These applications are hereby incorporated herein by reference in their entirety.
In some embodiments, an auto-defrost system is implemented in the Actively Cooled Tote, accomplishing three key goals in mitigating the described frost-up problem:
Defrost of the accept heat exchanger is achieved by powering a resistive heater wire (or array of wires) attached to the back surface of the accept heat sink. When powered, this heater raises the temperature of the heat sink to well above freezing to melt accumulated frost.
A summary of the properties of the defrost heater are listed below. Ideally a nichrome 80 alloy (4:1 ratio of nickel to chromium) should be used to achieve the listed resistivity. 60 watts is targeted as the power level because it provides a good balance between minimizing two factors: the time required to run a defrost cycle and the thermal losses associated with using higher levels of power.
The primary control scheme/algorithm for a defrost cycle includes one or more of:
The temperature of air around the accept fan, and control of the fan, is important in minimizing temperature rise of food in the tote. If the fan were turned on immediately after defrost heating, warm air would be blown over the food. If the fan were left on for a long duration the food would warm up due to the lack of cool air circulation.
Further refinement of the defrost control scheme may be achieved by monitoring key operational parameters and adjusting the defrost cycle steps accordingly to match the real-time operating conditions of an ACT in the field. These parameters include, but are not limited to:
Monitor difference between Taccept (Accept temperature) and Tcontrol (Chamber temperature). When this difference exceeds a specified value, the risk of impending frost-up is high, and could be used to trigger an immediate defrost cycle rather than waiting for the next regularly scheduled cycle. An alarm may also be issued to the customer or customer service that manual intervention is required to empty the ice tray or mitigate excessively high-moisture use/conditions.
Monitor lid open duration and subsequent full-power operation ratio. When this ratio exceeds a specified value, the risk of impending frost-up is high, and could be used to trigger an immediate defrost cycle rather than waiting for the next regularly scheduled cycle. An alarm may also be issued to the customer or customer service that manual intervention is required to empty the ice tray or mitigate excessively high-moisture use/conditions.
Frost mitigation “limp mode” could be triggered by the above warning parameters to reduce thermoelectric power and raise the temperature setpoint of the tote to a higher-than-normal value (but not exceeding maximum food safety value) and maintain this temperature until the risk of frost-up decreases. This mode augments the regular defrost cycle operation as an additional protection against complete frost-up, particularly in use cases where excessive moisture buildup occurs. In extreme use cases where a frost-up does occur, this mode also brings the temperature and power of the tote under control until manual mitigation can be completed, preventing out of control operation with unpredictable power and temperature oscillations.
With capability to measure Relative Humidity (RH) added to the control system, an alternative embodiment of the control algorithm could be:
Note that a control scheme based on direct sensing of the mass of frost on the heat exchanger has been dismissed due to excessive cost and complexity.
To prevent disruption to cold-chain fulfillment operations, defrost cycles can and should be run while food is still in the tote. To ensure that the temperature of food does not rise above the allowable limit (−16° C., t>1 hour), short and regular defrost intervals are defined in the control scheme described above.
Removing Re-Frozen Ice from Tote
When a defrost cycle is run, frost on the accept heat exchanger melts and drips down, collecting in a well (or trough) at the bottom of the tote internal chamber. During normal operation of the tote, the water will refreeze into a block of ice in this well. After some amount of defrost cycles have occurred, it will become necessary to remove this ice.
The water/ice collection well is placed under the accept heat exchanger to catch the melted water. The well is smooth and cleanable in accordance with food safety requirements. Inside the well will fit a removable ice tray to retain this water/ice, prevent it from spreading onto the floor of the tote chamber, and allow convenient removal of the ice from the tote. After the limiting number of defrost cycles is reached, the control in the tote may create a notice or alarm that informs a maintenance worker to manually remove the tray from the tote and dispose of the ice. The alert may be through an onboard IoT system or with a light or indication on the tote display itself. The worker will remove the ice from the tray and then replace the tray in the tote for further defrost cycles.
The tray may be made of a flexible material, such as silicone rubber, which can be peeled away from the re-frozen ice. Alternatively, a harder plastic may be used that allows twisting to release the ice from the tray.
The ice tray may be configured with a feature that contacts a switch or sensor to indicate its presence in the tote and allow or disallow operation with or without the tray.
Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein.
This application claims the benefit of provisional patent application Ser. No. 63/528,475, filed Jul. 24, 2023, the disclosure of which is hereby incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63528475 | Jul 2023 | US |
