Air Conditioning Applications Research Articles

Collector selection for solar ejector cooling system

The performance of a solar ejector cooling system is simulated using three different collectors: a conventional flat plate collector, a high efficiency flat plate collector and a vacuum-tube collector. It is shown that with the proper selection of the generating temperature an optimum COP can be achieved. The solar ejector cooling system using the single-glazed solar collector with selective surface and an enhanced air insulating layer can be most economical when operated at the optimum generating temperature of the ejector cooling machine. In this case, the solar system cost is around 1 USD per watt of cooling capacity for air conditioning applications.

D101 自然作動冷媒新技術の動向(オーガナイズドセッション12 : 冷凍空調システムにおける新世代技術)

In the present paper, current states of the art of natural working fluids for heat pumps are overviewed. The coefficients of performance of ammonia and propane are as high as those of HCFC22 and HFC alternatives. Because of their flammability and toxicity, the use of these refrigerants is restricted for limited applications. Carbon dioxide has inherent disadvantage of low COP for air conditioning application. A high-performance water heater, in which carbon dioxide was used for a working fluid, was put on the market. Possibility of other natural refrigerants is discussed.

Analysis and modelling of a phase change storage system for air conditioning applications

A phase-change energy storage system consisting of sections of different materials with different melting temperatures is proposed for air conditioning applications. The Phase Change Materials (PCMs) are placed in thin flat containers and air is passed through gaps between them. A semi-analytical model has been developed and calculations carried out using the finite elements method. Results obtained from three models with different assumptions are compared. The effect of the design parameters such as, PCM slab thickness and fluid passage gap on the storage performance is also investigated. The system is being developed for energy storage in solar heating and energy efficient space heating and cooling applications.

Theoretical Performance Comparison of CO2 Transcritical Cycle Technology Versus HCFC-22 Technology for a Military Packaged Air Conditioner Application

The world-wide agreement to restrict the use of chlorofluorocarbons (CFCs) and hydrofluorocarbons has prompted research into the possibilities of replacement refrigerants, particularly those occurring naturally in the environment. One such natural refrigerant, carbon dioxide, has been considered for certain refrigeration and air conditioning applications. In order to evaluate the potential performance of a refrigeration cycle based on carbon dioxide, a cycle model has been developed which can simulate the operation of a carbon dioxide based refrigerator/air conditioner. This model is called ACCO2 and accounts for the realistic effects of the thermo-physical properties of carbon dioxide. The model simulates the operation of a carbon dioxide based air conditioner that uses air as the heat source and heat sink. A parametric study has been conducted using ACCO2 as well as a validated cycle model which simulates the operation of an HCFC-22 based packaged air conditioner designed for use by the U.S. Army in a combat environment. The size and weight constraints of this application render HCFC-22 based units less efficient than typical HCFC-22 unitary air conditioners of the same capacity. It is hypothesized that these same constraints will not affect the performance of a carbon dioxide based packaged air conditioner as severely. The evaporative capacity, compressor power, and COP were calculated for each cycle over a range of indoor and outdoor temperatures, and used as a basis for comparing the two cycles.

Solar radiation for sorption cooling in Australiasia

Psychrometry and solar radiation of Perth in western Australia, and Shanghai in China, are examined in some detail. The aim is to provide assistance in choosing collector technologies and sorption cycles for air conditioning applications for climates prevalent in Australiasia.

Cooling and heating rate limits of a reversed reciprocating ericsson cycle at steady state

The optimal cooling and heating rates for the reversed reciprocating Ericsson cycle with ideal regeneration are determined for heat pump operations. These limiting rates are based on the upper and lower thermal reservoir temperature bounds and are obtained using time and entropy minimization procedures from irreversible thermodynamics. Use is made of time symmetry (a second law constraint) to minimize cycle time. This optimally allocates the thermal capacitances of the cycle and minimizes internal cycle entropy generation. Although primarily a theoretical work, a very practical and extensive parametric study using several environmentally friendly working fluids (neon, nitrogen and helium) is included. This study evaluates the relative contributions of various system parameters to rate-optimized design. The coefficient of performance (COP), and thus the quantity of cooling or heating for a given energy input, is the traditional focus; instead this work aims at the rate of cooling or heating in heat pumps under steady state conditions and using ideal gases as their working substances. The results obtained provide additional criteria for use in the study, design and performance evaluation of employing Ericsson cycles in refrigeration, air conditioning and heat pump applications. They give direct insight into what is required in designing a reversed Ericsson heat pump to achieve maximum heating and cooling rates. The choices of working fluids and pressure ratios were found to be very significant design parameters, together with selection of regenerator and source—sink heat transfer parameters. The parameter most influencing both the heating and cooling mode COPs and the heat transfer rates was found to be the heat conductance of the thermal sink.

A comparison of the predicted and experimental heat transfer performance of a finned tube evaporator

Experimental data have been generated for a single-circuit, multi-pass finned tube heat exchanger representative of an element of a typical packaged air conditioning unit evaporator. The data have been used to validate a cross flow heat exchanger computer program (ACOL5), developed originally for large scale steam plant, for air conditioning applications. The tests were conducted for a wide range of spatially uniform air flow conditions on to the coil, together with a variety of R22 refrigerant entry dryness fractions. The correspondence between the predicted and experimental heat transfer performance was good, thus suggesting that the program could be used with a degree of confidence in the design of air conditioning and refrigerant equipment. A particular application is that of prediction of the effects of maldistribution of air flow through heat exchangers, a common cause of loss of efficiency in air conditioning and refrigeration units; this topic is addressed in a companion paper.

A Two-Dimensional Numerical Study of Heat Transfer in a Finned Tube Assembly During Axisymmetric Dehumidification

This paper presents the analysis of two-dimensional heat transfer in an annular finned tube assembly during the process of dehumidification. All possible fin surface conditions, namely, dry, fully wet, and partially wet, have been studied and fin efficiency under these conditions have been modeled. Computations have been carried out using a control volume-based finite-difference method and compared with past one-dimensional analytical studies and available experimental data. The parameters that influenced the heat transfer rate in the finned tube structure are ratio of fin and wall thermal conductivities, ratio of fin thickness to fin pitch, ratio of wall thickness to fin pitch, ratio of fin length to fin pitch, cold fiuid Biot number, ambient Blot number, the relative humidity and dry bulb temperature of the incoming air, and the cold fluid temperature inside the coil. It was found that the heat transfer increased with increment in both dry bulb temperature and the relative humidity of the air. The fin efficiency changed rapidly with relative humidity under partially wet condition. The results suggest that coil performance can be very significantly altered by the condensation phenomenon on the fin surface and designs with dry fin data may not be adequate. The present results are expected to be very useful for the design of dehumidifier (cooling) coils for air conditioning applications.

Chemical dehumidification by liquid desiccants: theory and experiment

Chemical dehumidification of air by a liquid desiccant in a packed tower has been investigated both theoretically and experimentally for air conditioning and industrial applications. A computer model of a packed tower, able to determine heat and mass transfer between air and desiccant, has been developed and a parametrical study was carried out considering the solutions H 2O/LiBr and H 2O/CaCl 2 to determine the optimum operative conditions. An experimental apparatus including a packed tower and a desiccant regenerator has been described together with experimental results: a set of 70 experimental runs with H 2O/LiBr. Data have been reported and compared against the results of the computer code simulations.

Power Moves

This article discusses developments in electric motors and drives that are boosting efficiency, simplifying setup, and cutting costs. New drives offer compact designs and simplified installation, and start-up. An inexpensive controller for single-phase ac induction motors has been developed by Anacon Systems Inc. of Springfield, Montana. The controller is targeted to a huge installed base of fan, pump, and compressor motors. The company also has introduced a Smartcom series of motor controllers specifically designed for residential and light commercial Heating, ventilation, and air conditioning (HVAC) applications. New products currently under development by Anacon Systems will extend the range of offerings to cover power ranges up to 2 hp. The company also plans to offer controllers for three-phase motors. A new energy-efficient integrated ac drive/motor, combining plug-and-play installation and compact design, has been introduced by ABB Drives & Power Products of New Berlin, Wisconsin. The new drive has power cables at the bottom rather than on the side. The drive's compact size allows it to fit into many retrofit spaces and frees up panel space that would be occupied by a larger drive, according to ABB.

Adsorption cooling system for cold storage using methanol/silicagel

For the cold storage of agricultural products at temperatures of 2–4°C in India, a solar-hybrid cooling system has been developed, using solar energy from flat plate collectors and the waste heat of a genset, operated with producer gas. A commercially available low temperature (80–90°C) adsorption cooling system for air-conditioning application had to be modified for operation at cooling temperatures below the freezing point of water. Methanol instead of water was investigated as a refrigerant. Because of the inferior thermodynamic properties of methanol and the lower operation temperature, the efficiency is reduced. Calculations showed that the COP for a commercial adsorption cooling system is about 30% when operating the system with methanol/silicagel at a chilled water temperature of T 0=−2°C, a heating water temperature of T h=85°C and a condenser temperature of T c=30°C. A laboratory test facility with heating and cooling capacities of about 2 kW was set up. By comparative experiments with water and methanol, the calculations could be validated, though the specific power density is only 1/10 of the commercial system.

Review on sustainable thermal energy storage technologies, Part II: cool thermal storage

This paper describes the inherent pros and cons of the two common (i.e. chilled water and ice storage) commercially available thermal energy storage (TES) technologies for off-peak air conditioning applications. Case studies on cool thermal storage have demonstrated not only savings in energy and other operation and maintenance costs but also significant savings in initial capital costs. This paper also examines the use of cool thermal storage equipment for gas turbine inlet air cooling, which can positively enhance its efficiency. It has been observed that the application of TES has been predominantly in North America. However, in the mid-1990 s, applications have begun to appear in Asia, Australia, Europe and South America. Finally, prospects of TES technologies for electric load management in Saudi Arabia are also elucidated.

Conduite automobile et lésion médullaire. Le permis de conduire passé à l'hôpital est-il utilisé ?

Aluminum brazed heat exchangers are used widely in automobile air conditioning systems. Usually, this kind of heat exchanger is made of a flat tube with several independent passages in the cross-section, and formed into a serpentine or a parallel flow geometry. In these heat exchangers, a multitude of corrugated fins with louvers are inserted into the gaps between flat tubes. As a result of their high performance, some companies are starting to use extruded brazed aluminum heat exchangers in heating, ventilating and air conditioning applications. The purpose of this investigation is to provide the basic condensing heat transfer data for the extruded aluminum flat tube. Four different flat tubes and one micro fin tube were employed in this series of experimental studies. HFC-134a and HCFC-22 were the working fluids used. Our results were closest to Shah's correlation in the low quality region.

Condensing heat transfer characteristics of aluminum flat tube

Aluminum brazed heat exchangers are used widely in automobile air conditioning systems. Usually, this kind of heat exchanger is made of a flat tube with several independent passages in the cross-section, and formed into a serpentine or a parallel flow geometry. In these heat exchangers, a multitude of corrugated fins with louvers are inserted into the gaps between flat tubes. As a result of their high performance, some companies are starting to use extruded brazed aluminum heat exchangers in heating, ventilating and air conditioning applications. The purpose of this investigation is to provide the basic condensing heat transfer data for the extruded aluminum flat tube. Four different flat tubes and one micro fin tube were employed in this series of experimental studies. HFC-134a and HCFC-22 were the working fluids used. Our results were closest to Shah's correlation in the low quality region.

Impact of cool-storage air-conditioning on power-system planning in Thailand

An assessment is presented of the impact on system load shape of the application of cool-storage air-conditioning in the commercial sector of Thailand. A computer tool for least-cost planning is used to formulate an alternative power-development plan. Features of the resultant plan include an 8% reduction in the required capacity addition compared to a reference plan.

97/03154 Air conditioning applications and design

97/03154 Air conditioning applications and design

High-efficiency recovery for air-conditioning applications in a mild climate: a case study

Indoor comfort requirements often involve strong ventilation in buildings, but they clash with the necessity to limit energetic and economic costs connected with supply air handling. The introduction of efficient thermal recovery from return air is therefore fundamental. In air-conditioning applications, also in the presence of the traditional solution which uses cooling coils for air dehumidification, it is possible to increase the reuse of return air thermohygrometric content if compared with simple sensible heat recovery between the two air streams. The opportunities connected with evaporative precooling of return air are analysed here in a frequent application case. The resulting advantages are superior to those ones offered by enthalpy recuperators.

The performance of an optimized thermoacoustic air conditioner

Up to 25 thermoacoustic cooler design parameters were optimized to provide high COP predictions in selected air conditioning applications. Capacities ranging between 300 and 1500 W were considered. Two different configurations were investigated: (1) a single stack with a Helmholtz resonator termination, and (2) two drivers and two stacks. The optimization scheme utilized the Simplex algorithm, and the analysis was performed by DELTAE coupled with an original heat exchanger modeling tool which incorporated conventional straight fin analysis and semiempirical flow correlations. Various requirements were imposed for the external cooling and heat rejection temperatures, ranging between 270 and 287 K on the cold side and between 310 and 320 K on the hot side. Finned tube heat exchangers with water as the secondary fluid were considered. The predicted values of COP were greater than 3.8 (0.36, relative to the Carnot value) for an application analogous to a small window air conditioner. The performance and characteristics of the two different configurations were compared, and the loss distribution in each case was calculated. Furthermore, the impact of various constraints was examined. The performance predictions are encouraging and suggest that thermoacoustic air conditioning may be a viable proposition.

Performance assessment of multistage absorption cycles

The performance potential of 11 multistage, multi-effect absorption cycles is evaluated. They include water-lithium bromide, ammonia-water and cascade configurations. All evaluations are based on air-conditioning applications assuming a 4°C evaporator temperature and a 35°C condenser and absorber temperature. The sensitivity of the performance to the approach temperature in the heat exchanger was studied. Eight cycles were selected for a more detailed simulation. The highest COP at zero approach temperature was obtained for a three-stage water-lithium bromide cycle cascaded with two single-stage ammonia water cycles, while for approach temperatures of 5 K the best COP was obtained for the three-stage water-lithium bromide cycle.

An analysis of the heat transfer rate and efficiency of TE (thermoelectric) cooling systems

The heat transfer rate and efficiency of TE (thermoelectric) cooling systems were investigated. The emphasis of the present study is focused on the use of large-scale TE refrigerators for air conditioning applications. A one-dimensional heat transfer analysis was performed to determine the cooling power and electricity consumption of the TE elements. The constant-property results are in good agreement with the variable-property solutions for TE materials and temperatures typical for air conditioning applications. A heat transfer analysis was also carried out for TE refrigerators equipped with a heat exchanger. Both parallel- and counter-flow heat exchangers were considered. Fluid temperature variations of these two flow arrangements were found to be quite different, but the efficiencies and cold fluid exit temperatures differed only slightly when a uniform current was used for all TE elements. If the length of the heat exchanger exceeds an optimal value, the cold fluid temperature begins to rise and the efficiency drops for both parallel- and counter-flow arrangements. The second law of thermodynamics was applied to the optimization of TE refrigerators operating between two constant-temperature reservoirs and between two flowing fluids. It was found that if a TE cooling system incorporates a heat exchanger, a nonuniform current distribution should be used to achieve the maximum efficiency and the lowest cold fluid temperature. The optimization results for TE refrigerators operating between two constant-temperature reservoirs are not applicable to TE cooling systems between two flowing fluids. The most energy-efficient current distribution for the parallel-flow arrangement is the one which increase in the direction of the cold fluid.