The electro-adsorption chiller: a miniaturized cooling cycle with applications to micro-electronics

A novel modular and miniature chiller is proposed that symbiotically combines adsorption and thermoelectric cooling devices. The seemingly low efficiency of each cycle individually is overcome by an amalgamation with the other. This electro-adsorption chiller incorporates solely existing technologies. It can attain large cooling densities at high efficiency, yet is free of moving parts and comprises harmless materials. The governing physical processes are primarily surface rather than bulk effects, or involve electron rather than fluid flow. This insensitivity to scale creates promising applications in cooling personal computers and other microelectronic appliances.     

New absorption chiller and control strategy for the solar assisted cooling system at the German federal environment agency

Typically the cooling capacity of absorption chillers is controlled by adjusting the driving hot water temperature according to the load. Meanwhile the cooling water temperature is controlled to a constant set value. In order to increase the solar cooling fraction and/or to decrease the operating costs of solar assisted cooling systems (SAC-systems) a new control strategy has been developed which controls hot and cooling water temperature simultaneously. Hereby the specific cost of cold – generated from solar or conventional heat – can be reduced. The basic concept of the strategy is explained and results are shown for the SAC-system at the Federal Environment Agency in Dessau, Germany. Here a recently developed absorption chiller is now used instead of a former adsorption chiller. With the new absorption chiller and the control strategy the seasonal energy efficiency ratio SEER is above 0.75, electric efficiency is 35% higher and water consumption is reduced by 70%.     

Thermodynamic modeling of an ammonia–water absorption chiller

This article develops a general thermodynamic framework for the modeling of an irreversible absorption chiller at the design point, with application to a single-stage ammonia–water absorption chiller. Component models of the chiller have been assembled so as to quantify the internal entropy production and thermal conductance (UA) in a thermodynamically rigorous formalism, which is in agreement with the simultaneous heat-and-mass transfer processes occurring within the exchangers. Local thermodynamic balance (viz. energy, entropy, and mass balance) and consistency within the components is respected, in addition to the overall thermodynamic balance as determined by the inlet and outlet states of the components. For the absorbers, Colburn-and-Drew mass transfer equations are incorporated to describe the absorption process. Furthermore, the impact of various irreversibilities on the performance of chiller is also evaluated through the use of a general macroscopic equation.     

Performance analysis of a supersonic ejector cycle working with R245fa

A supersonic ejector chiller for industrial use is currently being developed and tested as part of a project cooperation between Frigel s.p.a and DIEF (Department of Industrial Engineering, University of Florence). The refrigerator was built following a “ready to market” setup criterion and is intended for applications on the industrial refrigeration market or in air conditioning. The plant has a nominal cooling power of 40 kW and is powered by low temperature heat (from 90 up to 100 °C). The ejector is equipped with a movable primary nozzle and 9 static pressure probes along the mixing chamber/diffuser duct. The working fluid is R245fa. An extensive numerical campaign was performed to analyze the internal dynamics of the ejector. All the simulations were carried out by accounting for the real gas properties of the refrigerant. Comparison with experimental data resulted in close agreement both in terms of global and local parameters. Analyses showed that in order to achieve an accurate matching with the experimental data, it is necessary to correctly account for the surface roughness of the ejector. This is especially true for off-design operating conditions.     

Experimental thermal performance study of an inclined heat pipe heat exchanger operating in high humid tropical HVAC systems

In an earlier paper [Y.H. Yau, Application of a heat pipe heat exchanger to dehumidification enhancement in tropical HVAC systems – a baseline performance characteristics study, International Journal of Thermal Sciences 46 (2) (2007) 164–171], the author had established the baseline performance characteristics of the eight-row wickless heat pipe heat exchanger (HPHX) for a vertical configuration under a range of conditions appropriate for a tropical climate. Now, the same basic experimental set-up was to be used in the present research with the HPHX tilted 30°. In this configuration, the gravitational force would be expected to enhance drainage of any condensation forming on the extended fin surfaces of the HPHX evaporator section, and therefore, the effectiveness of the HPHX could be anticipated to be better than the vertical configuration, particularly when processing inlet air with high RH. The investigation has been carried out for 32 experiments with typically high RH and the results are presented in this paper. The results suggested that the possibly adverse influence of condensate forming on the fins of the HPHX was negligible, and therefore the HPHX in a typically-used vertical configuration could perform equally as well as it would if the HPHX was installed in an inclined position.     

A study on effective use of heat transfer additives in the process of steam condensation

It is well known that the additives in absorption chillers play a significant role in increasing absorber performance. Realizing that the additives in absorption chillers circulate throughout the system including the condensers, we investigated the effect of additives in the condenser. Reported herein are the results of the experimental and theoretical investigations done by using effective heat transfer additives for enhancing heat transfer coefficient in condensation of steam over a horizontal copper (99.9% Cu, 0.1% P) tube surface. By using effective additives, the condensation heat transfer coefficient can be enhanced as much as 1.47 times when compared to filmwise condensation. The steam condensation, which occurred in our experiments while using effective additives, was mostly pseudo-dropwise like. In our experiments, we noted that the use of heat transfer additive such as 2-ethoxyethanol for steam condensation was highly effective. This increase in heat transfer coefficient can be attributed to concept of Marangoni effect. It is understood that this surface convection is caused by local variations in the interfacial tension. So far there has been very little noted literature available on the theoretical aspect of surface tension effect on enhancing heat transfer rate in steam condensation. In the current research we try to explain the surface tension effect for enhancing heat transfer rate in steam condensation using effective heat transfer additives.     

A vapour compression chiller fault detection technique based on adaptative algorithms. Application to on-line refrigerant leakage detection

The existence of faults in vapour compression chillers plays a significant role in terms of energy efficiency loss, performance degradations, and even environmental implications. In this paper, a dynamic model-based fault detection technique suitable for real-time implementation is proposed. The main objective is to obtain a reliable and automated tool for fault detection in vapour compression chillers, which can be applied in steady-state or transient operation. The fault detection methodology is based on comparing actual and expected performance using an adaptative model and operating variables dynamic thresholds. The technique has been successful applied for on-line refrigerant leakage detection with experimental tests involving the artificial introduction of the fault in a laboratory vapour compression plant, showing the results its capability of detecting incipient leakage failure conditions avoiding false alarms.     

CoilDesigner: a general-purpose simulation and design tool for air-to-refrigerant heat exchangers

A simulation and design tool to improve effectiveness and efficiency in design, and analysis of air to refrigerant heat exchangers, CoilDesigner, is introduced. A network viewpoint was adopted to establish the general-purpose solver and allow for analysis of arbitrary tube circuitry and mal-distribution of fluid flow inside the tube circuits. A segment-by-segment approach within each tube was implemented, to account for two-dimensional non-uniformity of air distribution across the heat exchanger, and heterogeneous refrigerant flow patterns through a tube. Coupled heat exchangers with multiple fluids inside different subsets of tubes can be modeled and analyzed simultaneously. A further sub-dividing-segment model was developed in order to address the significant change of properties and heat transfer coefficients in the single-phase and two-phase regime when a segment experiences flow regime change. Object-oriented programming techniques were applied in developing the program to facilitate a modular, highly flexible and customizable design platform and in building a graphic user-friendly interface. A wide variety of working fluids and correlations of heat transfer and pressure drop are available at the user's choice. The model prediction with CoilDesigner was verified against experimentally determined data collected from a number of sources.     

A dynamic simulation model for transient absorption chiller performance. Part II: Numerical results and experimental verification

This paper is the second paper out of two which present the development of a dynamic model for single-effect LiBr/water absorption chillers. The first part describes the model in detail with respect to the heat and mass balances as well as the dynamic terms. This second part presents a more detailed investigation of the model performance, including performance analysis, sensitivity checks and a comparison to experimental data. General model functionality is demonstrated.A sensitivity analysis gives results which agree very well to fundamental expectations: it shows that an increase in both external and internal thermal mass results in a slower response to the step change but also in smaller heat flow oscillations during the transient period. Also, the thermal mass has been found to influence the heat flow transients more significantly if allocated internally. The time shift in the solution cycle has been found to influence both the time to reach steady-state and the transients and oscillations of the heat flow. A smaller time shift leads to significantly faster response.A comparison with experimental data shows that the dynamic agreement between experiment and simulation is very good with dynamic temperature deviations between 10 and 25 s. The total time to achieve a new steady-state in hot water temperature after a 10 K input temperature step amounts to approximately 15 min. Compared to this, the present dynamic deviations are in the magnitude of approximately 1–3%.     

Use of hydrocarbons as drop-in replacements for HCFC-22 in on-farm milk cooling equipment

Many refrigeration systems on New Zealand dairy farms use HCFC-22 which is being phased out by 2015. Both laboratory and on-farm trials were undertaken to investigate hydrocarbons as drop-in replacements to HCFC-22 in milk silo refrigeration systems. A mixture of propane and ethane (Care-50) reduced energy use by 6–8%, and had similar system cooling capacity relative to HCFC-22. With propane (Care-40), energy use decreased by 5% but cooling capacity was 9% lower. The retrofits were simple and low cost because no alterations to the systems other than change in refrigerant and appropriate safety labelling and documentation were made. For most farms, the outside refrigeration system location and small charge mean that hydrocarbons could meet NZ standards for safe use of refrigerants. The low retrofit cost, improved energy efficiency, low environmental impact, mineral oil compatibility, similar cooling capacity and controllable flammability risks mean that the propane–ethane mixture is an attractive replacement for HCFC-22 on NZ dairy farms.