Performance enhancement of a household refrigerator by addition of latent heat storage

This paper studies the effect of adding a phase change material (PCM) slab on the outside face of a refrigerator evaporator. A dynamic model of the vapour compression cycle including the presence of the phase change material and its experimental validation is presented. The simulation results of the system with PCM show that the addition of thermal inertia globally enhances heat transfer from the evaporator and allows a higher evaporating temperature, which increases the energy efficiency of the system. The energy stored in the PCM is yielded to the refrigerator cell during the off cycle and allows for several hours of continuous operation without power supply.     

A study of accurate latent heat measurement for a PCM with a low melting temperature using T-history method

When the latent heat of a phase change material (PCM) with a lower melting point than ambient temperature was assessed according to the standard T-history method using a vertically oriented test tube, a temperature gradient occurred in the longitudinal direction of the tube due to natural convection. This led to a decrease in the accuracy of the latent heat of fusion measurement. In this study, the accuracy of the measurement with the original T-history method was improved without decreasing the test's simplicity and convenience by setting the test tube horizontally. The heat transfer to the vapor-layer of the tube under volume change during melting was assumed to be negligible and the results were calculated using the two inflection points of temperature as the start and end of latent heat period. Under these assumptions, the results agree closely with other reference data. And, the new method proposed in this study showed a remarkable reduction in data scattering.     

Airflow blockage and guide technology on energy saving for spiral quick-freezer

The effect of airflow blockage and guide technology on energy saving for spiral quick-freezers were investigated by simulating and analyzing the airflow field and measuring of the velocity distribution in the freezing zone for different designs. The k– turbulence model was used. The velocities and temperatures of the air in the freezing zone for different designs of airflow blockage and guide boards were measured. The study shows that the airflow pattern plays a key role on energy efficiency, freezing time, and production rate. In the study case, through the optimization of the airflow blocking boards and the guide boards, the average air velocity in the freezing zone would be enhanced to 2.5–2.7 times compared with the original design. Correspondingly for bean curds in a stationary condition, the freezing time would be shortened by 78–85%, energy efficiency and the production rate would be increased by approximately 18–28% individually.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 1: System design and modeling

The advanced energy storage technology proposed and patented by authors can be applied for cooling, heating, dehumidifying, combined cooling and heating, and so on. It is also called the variable mass energy transformation and storage (VMETS) technology in which the masses in one or two storage tanks change continuously during the energy charging and discharging processes. This paper presents an advanced energy storage system using aqueous lithium bromide (H₂O–LiBr) as working fluid. As one of VMETS systems, this system is a closed system using two storage tanks. It is used to shift electrical load and store energy for cooling, heating or combined cooling and heating. It is environmental friendly because the water is used as refrigerant in the system. Its working principle and process of energy transformation and storage are totally different from those of the traditional thermal energy storage (TES) systems. The electric energy in off-peak time is mostly transformed into the chemical potential of the working fluid and stored in the system firstly. And then the potential is transformed into cold or heat energy by absorption refrigeration or heat pump mode when the consumers need the cold or heat energy. The key to the system is to regulate the chemical potential by controlling the absorbent (LiBr) mass fraction or concentration in the working fluid with respect to time. As a result, by using a solution storage tank and a water storage tank, the energy transformation and storage can be carried out at the desirable time to shift electric load efficiently. Since the concentration of the working solution in the VMETS cycle varies continuously, the working process of the VMETS system is dynamic. As the first part of our study, the working principle and flow of the VMETS system were introduced first, and then the system dynamic models were developed. To investigate the system characteristics and performances under full-storage and partial-storage strategies, the numerical simulation will be performed in the subsequent paper. The simulation results will be very helpful for guiding the actual system and device design.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 2: System simulation and analysis

This paper is the second part of our study on the advanced energy storage system using H₂O–LiBr as working fluid. In the first part, the system working principle has been introduced, and the system dynamic models in the operation process have also been developed. Based on the previous research, this paper focuses on the numerical simulation to investigate the system dynamic characteristics and performances when it works to provide combined air-conditioning and hot water supplying for a hotel located near by Yangzi River in China. The system operation conditions were set as follows: the outdoor temperature was between 29 °C and 38 °C, the maximum air-conditioning load was 1450 kW, the total air-conditioning capacity was 19,890 kWh and the 50 °C hot water capacity for showering was 20 tons which needed heat about 721 kWh on a given day. Under these conditions, the system operation characteristics were simulated under the full- and partial-storage strategies. The simulation results predicted the dynamic characteristics and performances of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The results also showed that the integrated coefficient of performances (COP_int) of the system were 3.09 and 3.26, respectively, under the two storage strategies while the isentropic efficiency of water vapor compressor was 0.6. The simulation results are very helpful for understanding and evaluating the system as well as for system design, operation and control, and device design or selection in detail.     

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.     

Regional report Europe: “heat pumps — status and trends”

By 1997 about 90 million heat pumps have been installed worldwide, only less than 5% are located in Europe, historically the cradle of this “thermodynamic heating and cooling process”. The majority of the approximately 4 million installed heat pumps are imported reversible air-to-air systems in southern Europe and only 30% represent the typical European-made heating only electric driven compression systems for space and water heating in buildings in central and northern Europe. The first and second oil crises has been the main cause for a first European heat pump “boom” at the end of the seventies. Consequently the following drop in energy prices negatively influenced the market in some countries. The new renaissance in Europe in the middle of the nineties was initiated by the understanding of sustainable development for a more efficient energy use and the related protection of the environment.     

Application of thermal battery in the ice storage air-conditioning system as a subcooler

This article experimentally investigates the thermal performance of a thermal battery used in the ice storage air-conditioning system as a subcooler. The thermal battery utilizes the superior heat transfer characteristics of two-phase closed thermosyphon and eliminates the drawbacks found in convectional energy storage systems. Experimental investigations are first conducted to study the thermal behavior of thermal battery under different charge temperatures (−5 °C to −9 °C) in which water is used as the energy storage material. This study also examines the thermal performance of the subcooled ice storage air conditioner under different cooling loads. Experimental data of temperature variation of water, ice fraction, refrigerant mass flow rate and coefficient of performance (COP) are obtained. The results show that supercooling phenomenon appears in the water and it can be ended when the charge temperature is lower than −6 °C. The system gives 28% more cooling capacity and 8% higher COP by the contribution of the thermal battery used as a subcooler.     

Modelling and simulation results for a domestic exhaust-air heat pump heating system

Energy consumption in residential buildings has gained an increasing interest the latest years due to the rising demand for efficient energy use and higher comfort standards. In tight building constructions with controlled ventilation, heat recovery with exhaust-air heat pump connected to floor heating is regarded as energy efficient heating system that optimises the energy use in buildings while maintaining an acceptable level of thermal comfort. In this study, we use the computational tools TRNSYS and EES to model and analyse the performance of a residential house, its ventilation system and its floor heating system based on an exhaust air heat pump. The system analysis focuses particularly on the influence of internal and solar gains on the operation of the heating system and the thermal comfort of the house. Furthermore, the way that gains influences the performance of the floor heating system is examined. Overall, the results bring to light the impact of factors that are not easy to predict on the indoor climate and the thermal comfort.     

Experimental results of a solar powered cooling system at low temperature

A solar thermochemical prototype producing low-temperature cold has been built and tested during the summer and autumn 2005 in Perpignan, France. It cools a 560 L cold box down to about −25 °C using only low-grade heat produced by two simple flat plate solar collectors. The process involves two cascaded thermochemical systems using BaCl₂ salt reacting with ammonia. Its working mode is discontinuous, as it alternates between one decomposition mode at high pressure (daytime) and one cold production mode at low pressure (nighttime). Experimental results prove the feasibility of this new concept of solar cold production, with temperatures as low as −30 °C, demonstrate its potential use in housing, by the acceptable size and weight of the system and show the system performances during the sunniest months of the year, with a rough solar coefficient of performance (COP) of about 0.031 over the test period. The major meteorological parameters influencing the process efficiency are the solar irradiation and the outside temperature.     

A novel experimental investigation of a solar cooling system in Madrid

This paper reports novel experimental results derived through field testing of a part load solar energized cooling system for typical Spanish houses in Madrid during the summer period of 2003. Solar hot water was delivered by means of a 49.9 m² array of flat-plate collectors to drive a single-effect (LiBr/H₂O) absorption chiller of 35 kW nominal cooling capacity. Thermal energy was stored in a 2 m³ stratified hot water storage tank during hours of bright sunshine. Chilled water produced at the evaporator was supplied to a row of fan coil units and the heat of condensation and absorption was rejected by means of a forced draft cooling tower. Instantaneous, daily and period energy flows and energy balance in the installation is presented. System and absorption machine temperature profiles are given for a clear, hot and dry day's operation. Daily and period system efficiencies are given. Peak insolation of 969 W m⁻² (at 12:30 solar time on 08/08/03) produced 5.13 kW of cooling at a solar to cooling conversion efficiency of 11%. Maximum cooling capacity was 7.5 kW. Cooling was provided for 8.67 h and the chiller required a threshold insolation of 711 W m⁻² for start-up and 373 W m⁻² for shut-down. A minimum hot water inlet temperature to the generator of 65 °C was required to commence cold generation, whereas at 81 °C, 6.4 kW of cooling (18.3% of nominal capacity) was produced. The absorption refrigeration machine operated within the generation and absorption temperature ranges of 57–67 and 32–36 °C, respectively. The measured maximum instantaneous, daily average and period average COP were 0.60 (at maximum capacity), 0.42 and 0.34, respectively. Energy flows in the system are represented on a novel area diagram. The results clearly demonstrate that the technology works best in dry and hot climatic conditions where large daily variations in relative humidity and dry bulb temperature prevail. This case study provides benchmark data for the assessment of other similar prototypes and for the validation of mathematical models.     

Effect of the airflow rate of a ceiling type air-conditioner on ventilation effectiveness in a lecture room

We performed a study on the effect of the discharge airflow rate of the ceiling type air-conditioner on ventilation performance in the lecture room with the mixing ventilation. The experiments and CFD were conducted for analyzing ventilation performance. The concepts of mean air age and indoor CO₂ concentration were used for evaluating ventilation performance. We made the CO₂ generation model in the simulation and calculated a lot of cases with respect to the airflow rate of air-conditioner and the mechanical ventilation rate. And the selected experimental measurements were performed in the lecture room of the same layout as the numerical one for verifying simulation results. Mean air age is gradually increased, but CO₂ concentration is oppositely decreased in the occupied zone with the increment of the discharge airflow rate of the ceiling type air-conditioner. This result shows that both mean air age and residual life time must be considered for evaluating ventilation performance when the contaminants are generated indoors. And the increment of discharge airflow of the ceiling type air-conditioner can induce the piston effect and push the contaminants out of the occupied zone. From this result, it is found out that ventilation performance can be increased when the momentum source like an air-conditioner is used in the room with the mixing ventilation.     

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.     

Optimization of finned-tube condensers using an intelligent system

The refrigerant circuitry influences a heat exchanger's attainable capacity. Typically, a design engineer specifies a circuitry and validates it using a simulation model or laboratory test. The circuitry optimization process can be improved by using intelligent search techniques. This paper presents experiments with a novel intelligent optimization module, ISHED (Intelligent System for Heat Exchanger Design), applied to maximize capacity through circuitry design of finned-tube condensers. The module operates in a semi-Darwinian mode and seeks refrigerant circuitry designs that maximize the condenser capacity for specified operating conditions and condenser slab design constraints. Examples of optimization runs for six different refrigerants are included. ISHED demonstrated the ability to generate circuitry architectures with capacities equal to or superior to those prepared manually, particularly for cases involving non-uniform air distribution.     

Methodology for uncertainty calculation of net total cooling effect estimation for rating room air conditioners and packaged terminal air conditioners

This article presents the general procedure for uncertainty calculation of net total cooling effect estimation for rating room air conditioners and packaged terminal air conditioners, by means of measurements carried out in a test bench specially designed for this purpose. The uncertainty analysis presented in this work looks for establishing a confidence degree or certainty of experimental results. It is particularly important considering that international standards related to this type of analysis are too ambiguous when treating this subject. The uncertainty analysis is on the other hand an indispensable requirement to international standard ISO 17025 [ISO, 2005. International Standard. 17025. General Requirement to Test and Calibration Laboratories Competences. International Organization for Standardization, Geneva.], which must be applied to obtain the required quality levels according to the Word Trade Organization WTO.     

The reliable simple one-dimensional 64-CPWTR model applied to the two-dimensional heat transfer problem of an insulated rectangular duct in an air conditioning or refrigeration system

The heat-transfer characteristics of an insulated long rectangular or square duct are analyzed by using the one-dimensional plane wedge thermal resistance (PWTR) model and plate thermal resistance (PTR) model in this study. It is found that the errors generated by the PWTR model are all positive and the errors generated by the PTR model are all negative. Thus, the combined plate wedge thermal resistance (CPWTR) model generated by paralleling PWTR and PTR models with the proportion factors of α=0.6 vs. β=0.4 (64-CPWTR model) can neutralize the positive and negative errors and obtain very accurate results in comparison with the two-dimensional numerical solutions analyzed by the CFD software. The errors generated by the one-dimensional 64-CPWTR model are within 1% for practical sizes and practical insulated thickness in air conditioning and refrigeration systems. Thus, the engineer can obtain very reliable heat transfer results when applying the one-dimensional 64-CPWTR model to an insulated rectangular duct.     

Temperature and energy performance of refrigerated retail display and commercial catering cabinets under test conditions

An analysis of the performance of well freezers, chest freezers, frozen and chilled door cabinets (solid or glass door) and open fronted chilled cabinets under EN441 test conditions demonstrated that maximum temperatures in cabinets were generally in the most exposed (to ambient) areas and that minimum temperatures were located in the least exposed areas. Detailed positions of maximum and minimum temperature varied between cabinet types. In chest freezers 95% of the maximum temperature positions were located in the top layer and 95% of the minimum temperature positions were located in the middle layer of the cabinets. In full door frozen cabinets the maximum temperature position was in the majority of cases on the top shelf (64%) with most maximum packs being at the front of the top shelf (53%). In the chilled full door cabinets 94% of the maximum temperature packs were situated at the front of the cabinet. In open fronted cabinets the majority of maximum temperature packs (97%) were located at the front of the cabinet, the largest number (60%) being at the front of the base of the cabinet. In well cabinets the majority of maximum temperature packs (81%) were located in the top layer of the cabinet and the majority (91%) of minimum temperature packs were located in the bottom of the cabinet.Large differences in energy consumed by cabinets of similar size and temperature performance were found indicating that large reductions in energy and CO₂ emissions could be achieved by selection of the most efficient cabinets.     

Efficiency and optimization of an annular fin with combined heat and mass transfer – An analytical solution

An analysis was carried out to study the efficiency of annular fin when subjected to simultaneous heat and mass transfer mechanisms. The temperature and humidity ratio differences are the driving forces for the heat and mass transfer, respectively. Analytical solutions are obtained for the temperature distribution over the fin surface when the fin is fully wet. The effect of the atmospheric pressure on the fin efficiency was also studied, in addition to fin optimum dimensions. It is demonstrated that the closed-form solutions for a dry-fin case presented in many text books are special cases for the solutions presented in this paper.     

Air-side heat transfer enhancement of a refrigerator evaporator using vortex generation

In most domestic and commercial refrigeration systems, frost forms on the air-side surface of the air-to-refrigerant heat exchanger. Frost-tolerant designs typically employ a large fin spacing in order to delay the need for a defrost cycle. Unfortunately, this approach does not allow for a very high air-side heat transfer coefficient, and the performance of these heat exchangers is often air-side limited. Longitudinal vortex generation is a proven and effective technique for thinning the thermal boundary layer and enhancing heat transfer, but its efficacy in a frosting environment is essentially unknown. In this study, an array of delta-wing vortex generators is applied to a plain-fin-and-tube heat exchanger with a fin spacing of 8.5 mm. Heat transfer and pressure drop performance are measured to determine the effectiveness of the vortex generator under frosting conditions. For air-side Reynolds numbers between 500 and 1300, the air-side thermal resistance is reduced by 35–42% when vortex generation is used. Correspondingly, the heat transfer coefficient is observed to range from 33 to 53 W m⁻² K⁻¹ for the enhanced heat exchanger and from 18 to 26 W m⁻² K⁻¹ for the baseline heat exchanger.     

Prediction of the heat transfer rate of a single layer wire-on-tube type heat exchanger using ANFIS

In this paper, we applied an Adaptive Neuro-Fuzzy Inference System (ANFIS) model for prediction of the heat transfer rate of the wire-on-tube type heat exchanger. Limited experimental data was used for training and testing ANFIS configuration with the help of hybrid learning algorithm consisting of backpropagation and least-squares estimation. The predicted values are found to be in good agreement with the actual values from the experiments with mean relative error less than 2.55%. Also, we compared the proposed ANFIS model to an ANN approach. Results show that the ANFIS model has more accuracy in comparison to ANN approach. Therefore, we can use ANFIS model to predict the performances of thermal systems in engineering applications, such as modeling heat exchangers for heat transfer analysis.