An introduction to heat transfer

Introduction to heat transfer

Personal thermal management using portable thermoelectrics for potential building energy saving

Personal comfort systems: A review on comfort, energy, and economics

Review on solar thermal energy storage technologies and their geometrical configurations

Personal cooling devices reduce energy loads by allowing buildings to operate with elevated setpoint temperatures, without compromising on the occupant comfort. One such novel technology called the Roving Comforter (RoCo) uses a compact R134a based vapor compression system for cooling. Following its cooling operation, during which waste heat from the condensing refrigerant is stored in a phase change material (PCM), a two-phase loop thermosiphon is used to discharge (solidify) the PCM to enable its next operation. The transient operation of this thermosiphon is the focus of the present article. Use of a PCM as the storage medium provides high energy density due to the ability to store thermal energy as latent heat during the phase transition; however, the discharge rate is limited by the low thermal conductivity of the PCM. Insertion of a graphite foam within the PCM can increase the rate of discharge and decrease the downtime of the cooling device. Since graphite enhancement involves a tradeoff between improving the discharge time at the expense of PCM volumetric latent heat, the impact of graphite foam density on the PCM discharge rate is investigated by using a Modelica-based transient model of the thermosiphon. The semi-empirical model, which uses relevant heat transfer coefficient and pressure drop correlations for both refrigerant and airside heat transfer, captures the complex phenomena involving simultaneous phase change of the refrigerant and the PCM. The graphite enhanced PCM selected from this analysis results in a 51% reduction in the discharge time with addition of only 5% to the thermal storage weight, without compromising the required cooling time.

Enhancing the thermosiphon-driven discharge of a latent heat thermal storage system used in a personal cooling device

Improving system performance of a personal conditioning system integrated with thermal storage

Portable personal conditioning systems: Transient modeling and system analysis

Despite otherwise uncomfortable conditions in a surrounding environment, a customizable microenvironment can be created around a user to maintain a comfortable temperature and/or humidity level using a comfort unit. For example, the environment may be an office building where conditions are out of the comfortable range to save on energy or for other reasons, a factory/shop environment that is poorly conditioned, or an outdoor location with little to no conditioning. A sensing unit can monitor biometric and environmental data and can determine a comfort level of the user. The comfort unit can then dynamically respond to the determined comfort level and adjust the microenvironment to improve the user's comfort level. The comfort unit can follow the user as the user moves within the macro-environment, or can otherwise move within the macro-environment to achieve certain functions, such as recharging or spatial shifting of thermal load within the overall macro-environment.

Comfort units and systems, methods, and devices for use thereof

The Roving Comforter (RoCo) is an innovative personal thermal management technology currently being developed at the University of Maryland as part of Advanced Research Projects Agency – Energy (ARPA-e) project. Among several system configurations of RoCo, the paper focuses on a miniature battery-powered vapor compression cycle (VCC) system fitted on a remote-controlled robotic platform. The heat rejected from the condenser during its operation is stored in a compact phase change material (PCM) based heat storage device. Once the PCM storage reaches full capacity, it needs to be recharged. The PCM heat exchanger needs to be designed so that the melting PCM captures the heat generated during the VCC operation and then re-solidifies again with minimal energy usage during the its recharge mode. The thermosiphon operates through the same refrigerant circuit by bypassing certain components like the compressor to reduce flow resistance. Thus the circuit is designed to operate as a VCC during the cooling mode and as thermosiphon during the recharge mode. The VCC needs to have a high coefficient of performance (COP) while the thermosiphon needs to solidify PCM material at highest possible rate. To capture the time-dependent behavior of solidification, transient modeling of thermosiphon is desired. The prototype developed has a COP of 2.85 and needs roughly 8 hours to recharge the thermosiphon. Comparison of modeling results with the experimental data have been provided to validate the model. Several cases of RoCo thermosiphon are then simulated for different PCM using the model to select the most suitable PCM.

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Rohit Dhumane

Papers

Enhancing the thermosiphon-driven discharge of a latent heat thermal storage system used in a personal cooling device

Improving system performance of a personal conditioning system integrated with thermal storage

Portable personal conditioning systems: Transient modeling and system analysis

Comfort units and systems, methods, and devices for use thereof

Transient Modeling of a Thermosiphon based Air Conditioner with Compact Thermal Storage: Modeling and Validation