Patent Application
20250116449
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 active vaporization of moisture in a Thermoelectric Module (TEM) or subsystem are provided. In some embodiments, a method of method of operating a TEM includes: determining to initiate a drying cycle for the TEM; and activating the drying cycle for the TEM. In this way, moisture can be removed from the TEM. This can increase the efficiency and/or longevity of the TEM. This can lead to decreased maintenance times and cost savings.
In some embodiments, activating the drying cycle for the TEM comprises: reversing a polarity of a Direct Current (DC) power supply to the TEM. In some embodiments, activating the drying cycle for the TEM comprises: providing an Alternating Current (AC) power supply to the TEM. In some embodiments, activating the drying cycle for the TEM comprises: repeatedly switching the polarity of the DC power supply to the TEM. In some embodiments, repeatedly switching the polarity comprises: repeatedly switching the polarity of the DC power supply to the TEM using an H-bridge.
In some embodiments, the method also includes, after activating the drying cycle for the TEM, removing vaporized moisture by one or more of: natural convection; and active assistance from one or more fans. In some embodiments, activating the drying cycle for the TEM is in conjunction with activation of a defrost cycle. In some embodiments, determining to initiate a drying cycle for the TEM is controlled by system logic controller/firmware.
In some embodiments, determining to initiate a drying cycle for the TEM is at a regular timed interval, based on elapsed time since last vaporization cycle. In some embodiments, determining to initiate a drying cycle for the TEM is based on measurement of relative humidity. In some embodiments, determining to initiate a drying cycle for the TEM is via manual control from an Active Cooling System dashboard and/or control interface.
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. Additional details can be found in International Patent Application PCT/US2024/039387 filed on Jul. 24, 2024. That application is included herein by reference in its entirety. The three goals include:
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 critical 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:
k would be a tunable factor based on how impactful lid openings are determined to be in testing. The number of days, X, would also be tuned depending on how quickly frost is observed to accumulate.
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.
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.
Thermoelectric modules (“TEMs”) or Peltier modules, whether single, in parallel, series, or combinations of the two are powered by a DC voltage source which produces heat on one side of the TEM/system and cold on the other side of the TEM/system. Due to moisture in the environment, moisture/water will accumulate on the cold side header of the TEM when powered. Corrosion of the thermoelectric legs leading to high AC resistance (ACR) is the eventual failure mode due to this moisture. Methods to minimize or slow the ingress of moisture involve physical barriers such as sealants or housings. Once the moisture penetrates these physical barriers, water vapor begins to accumulate inside, on the subsystem's cold side. As the beads of water increase, they begin to spill over the cold side header(s) and run downwards across the TEMs legs. Depending on the TEMs orientation, most water ends up hanging from the bottom of the TEMs in droplet form, saturating the lower legs. These water droplets grow from additional water forming from above, and eventually the drip falls further downward. These lower legs are wet the longest, since they are the lowest legs before the drip falls off the bottom of the TEM (if mounted vertically). This is the point where most of the corrosion begins to form, though it can be present on any legs. This water causes corrosion of the legs, high ACR, low performance, and eventual failure of the TEM. See
Periodic removal of the water from the TEM legs would extend the life of the thermoelectric module or subsystem. The first level of moisture prevention is the physical barriers to keep moisture out, but once moisture is in, improved ways to remove the water are needed. TEM devices are meant to be powered by DC voltages. Applying a DC voltage will provide one side hot and one side cold where the cold side will accumulate moisture from the air. Reversing the polarity of the DC supply will change the cold side of the TEM to the hot side and the corresponding hot side to the cold side. This vaporizes the water from the cold side legs and moves it to accumulate on the previous hot side legs and header. Though this method does dry the cold side legs/header, and may be effective, a preferred embodiment is to remove the DC source and apply an AC voltage to the input of the TEM(s). An AC voltage views the thermoelectric module as a resistive element, not allowing either side to get cold, but instead it just produces heat. Moisture is thereby vaporized from both sides of the TEM, returning the TEM's legs back to a dry state.
An alternate method to AC would be alternating polarity DC to vaporize the water. This would rapidly switch the DC source's polarity back and forth, thereby simulating AC. This is accomplished by putting a DC voltage across the TEM/subsystem in one direction, and then switching the polarity of that voltage source to the other direction, repeating this back and forth and a particular frequency. Switching or ramping the voltage in a manner which reverses polarity very quickly and repetitively would cause both sides of the thermoelectric device to produce heat. In some embodiments, DC-polarity switching is accomplished using an H-bridge or comparable electronic circuit.
Upon completion of the vaporization cycle, a means of removal of the vaporized moisture from the system can be used. This may be achieved with either natural convection, or with active assistance from fans, moving the moisture out of the system via an exhaust vent or port.
The active vaporization cycle is controlled to occur in conjunction with the auto defrost cycle, when implemented in an Actively Cooled Tote or comparable refrigeration product containing an auto-defrost feature.
In an alternate embodiment of the control scheme, the active vaporization cycle is controlled to occur independently of an active defrost cycle. In certain system applications, the defrost cycle may not be required or present, leaving the active vaporization cycle in place to operate independently. In this embodiment, the active vaporization cycle is initiated by one or more of the following methods:
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 and the claims that follow.
This application claims the benefit of provisional patent application Ser. No. 63/588,004, filed Oct. 5, 2023, the disclosure of which is hereby incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63588004 | Oct 2023 | US |
