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Classical refrigeration using vapor compression has been widely applied over the past decades to large-scale industrial systems, with few known applications to the microelectronic cooling field, due to the small size limitation. The present study proposes an efficient mechanical refrigeration system to actively cool the electronic components populating a printed circuit board in high-power microelectronic system. The proposed system includes several miniaturized components-compressor; evaporator; and condenser-part of a refrigeration system designed to fit the smaller scale power electronics. The system is thermally optimized to reach high coefficients of performance (COPs). An array of microchannels is used for the evaporator/condenser units. A previous study indicated that the R-134s refrigerant provides the best COP/feasibility ratio, while being the most suitable for microelectronic applications (Phelan, et al., "Designing a Mesoscale Vapor Compression Refrigerator for Cooling High-Power Microelectronics," Proceedings of the ITHERM'04, Las Vegas, NV). The present study develops an analytical model of the proposed small scale vapor-compression refrigerator using the R-134a refrigerant. The refrigeration system is thermally optimized for cooling powers ranging from 20 W to 100 W, with the COP of the system reaching values up to 4.5. The advantages of the proposed system are highlighted, establishing a baseline performance versus size relationship for vapor-compression refrigerators, to serve as the basis for comparison for future miniaturized refrigeration systems. [ABSTRACT FROM AUTHOR]

The efficient operation of a solar cooling system strongly depends on the chiller behavior under part load conditions, since driving energy and cooling load are never constant. For this reason, the performance of a single stage, hot water driven 30 kW H2O/LiBr-absorption chiller employed in a solar cooling system with a field of 350 m² evacuated tube collector has been analyzed under part load conditions with both simulations and experiments. A simulation model has been developed for the whole absorption chiller (Type Yazaki WFC-10), where all internal mass and energy balances are solved. The connection to the external heat reservoirs of hot, chilled, and cooling water is done by lumped and distributed UA values for the main heat exchangers. In addition to an analytical evaporator model-which is described in detail-experimental correlations for UA values have been used for the condenser, generator, and solution heat exchanger. For the absorber, a basic model based on the Nusselt theory has been employed. The evaporator model was developed, taking into account the distribution of refrigerant on the tube bundle, as well as the change in operation from a partially dry to an overflowing evaporator. A linear model is derived to calculate the wetted area. The influence of these effects on cooling capacity and coefficient of performance (COP) is calculated for three different combinations of hot and cooling water temperature. The comparison to experimental data shows a good agreement in the various operational modes of the evaporator. The model is able to predict the transition from partially dry to an overflowing evaporator quite well. The present deviations in the domain with high refrigerant overflow can be attributed to the simple absorber model and the linear wetted area model. Nevertheless, the results of this investigation can be used to improve control strategies for new and existing solar cooling systems. [ABSTRACT FROM AUTHOR]

Presented research is an experimental study of the performance of a standard plate heat exchanger evaporator, both with and without a novel nano- and microporous copper structure, used to enhance the boiling heat transfer mechanism in the refrigerant channel. Various distance frames in the refrigerant channel were also employed to study the influence of the refrigerant mass flux on two-phase flow heat transfer. The tests were conducted at heat fluxes ranging between 4.5 kW/m2 and 17 kW/m2 with 134a as refrigerant. Pool boiling tests of the enhancement structure, under similar conditions and at various surface inclination angles, were also performed for reasons of comparison. The plate heat exchanger with the enhancement structure displayed up to ten times enhanced heat transfer coefficient in the refrigerant channel, resulting in an improvement in the overall heat transfer coefficient with over 100%. This significant boiling enhancement is in agreement with previous pool boiling experiments and confirms that the enhancement structure may be used to enhance the performance of plate heat exchangers. A simple superposition model was used to evaluate the results, and it was found that, primarily, the convective boiling mechanism was affected by the distance frames in the standard heat exchanger. On the other hand, with the enhanced boiling structure, variations in hydraulic diameter in the refrigerant channel caused a significant change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance. [ABSTRACT FROM AUTHOR]

Vapor compression cycle systems using accumulators and receivers inherently operate at or near a transition point involving changes of phase at the heat exchanger outlets. This work introduces a condenser/receiver model and an evaporator/accumulator model developed in the moving-boundary framework. These models use a novel extension of physical variable definitions to account for variations in refrigerant exit phase. System-level model validation results, which demonstrate the validity of the new models, are presented. The model accuracy is improved by recognizing the sensitivity of the models to refrigerant mass flow rate. The approach developed and the validated models provide a valuable tool for dynamic analysis and control design for vapor compression cycle systems. [ABSTRACT FROM AUTHOR]

The paper describes the successful amalgamation of the thermoelectric and the adsorption cycles into a combined electro-adsorption chiller (EAC). The symbiotic union produces an efficiency or COP (coefficient of performance) more than threefold when compared with their individual cycles. The experiments conducted on the bench-scale prototype show that it can meet high cooling loads, typically 120 W with an evaporator foot print of 25 cm², that is 5 W/cm² at the heated surface temperature of 22°C, which is well below that of the room temperature. The COPs of the EAC chiller vary from 0.7 to 0.8, which is comparable to the theoretical maximum of about 1.1 at the same operating conditions. With a copper-foam cladded evaporator, the high cooling rates have been achieved with a low temperature difference. In addition to meeting high cooling rates, the EAC is unique as (i) it has almost no moving parts and hence has silent operation, (ii) it is environmentally friendly as it uses a nonharmful adsorbent (silica gel), and (iii) water is used as the refrigerant. [ABSTRACT FROM AUTHOR]