Theoretical performance analysis of heat pump water heaters using carbon dioxide as refrigerant

The aim of this paper is to simulate the performance of an air source heat pump water heater using carbon dioxide (CO₂) as a working fluid. The heat pump water heating system consists of a compressor, a gas cooler, an expansion device and an evaporator. The computer simulation model has been developed by using the heat transfer data and the thermodynamic properties of CO₂. The effects on the heat pump performance by the operating parameters such as the compressor rotational speed, the inlet water temperature at the gas cooler, the inlet air temperature at the evaporator and the mass flow rate ratio of water to refrigerant were presented. For rated capacities of a 4 kW compressor with a 10 kW gas cooler and a 6 kW evaporator, the coefficient of performance is found to be between 2.0 and 3.0. The mass flow rate ratio of water and CO₂ between 1.2 and 2.2 is the most suitable value for generating hot water temperature above 60°C at 15–25°C ambient air temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigation of the effects of vapour quality and oil concentration on performance of a swash plate compressor

The main objective of the present study is to experimentally investigate the effects of vapour quality and oil concentration on the performance of a swash plate compressor for automotive air conditioning systems. R‐134a is used as refrigerant. The compressor used is a typical automotive swash‐plate‐type compressor driven by a 10 hp variable‐speed electric motor and lubricated by polyalkylene glycol (PAG) oil. The variables measured during the experiment are pressure, temperature, oil concentration, total mass flow rate and vapour mass flow at the inlet and outlet of the compressor. The experiment was performed at varying compressor speeds, compression ratios and vapour quality. The results revealed some unknown aspects of the compression process in an automotive air conditioning system. The vapour quality does not affect volumetric efficiency, but influences isentropic efficiency of the compressor. In the vapour quality range of 80–90%, isentropic efficiency decreases with increasing vapour quality. During the compression process, only a portion of the liquid refrigerant evaporates. However, at the outlet of the compressor, refrigerant/oil mixture never reaches steady state. The evaporation ratio decreases with increasing compressor speed, and with increasing vapour quality as well. Copyright © 2005 John Wiley & Sons, Ltd.     

Absorption of organic fluid mixtures in plate heat exchangers

It is well known that the absorber is the key component in energy conversion systems that are based on absorption cycles. This paper describes an experimental investigation into the absorption process of organic fluid mixtures in an absorption system which has a spray and a plate heat exchanger. The absorber consists of an adiabatic mixing chamber with a spray, where the solution that is weak in refrigerant is sprayed into the refrigerant vapour. A two-phase mixture is formed and enters a plate heat exchanger, where the solution is cooled to complete the absorption process.We carried out experiments with different types of spray nozzles using the organic fluid mixtures methanol–tetraethyleneglycol dimethylether (TEGDME) and trifluoroethanol (TFE)–TEGDME. We analyse how the solution mass flow rate, absorber pressure and cooling water temperature affected the absorber performance and we discuss the results in terms of the absorber load, absorbed mass flux, degree of subcooling of the solution at the absorber outlet, solution film heat and mass transfer coefficients.The results indicate that the absorption system proposed is suitable for relatively low pressures. For water temperatures of 30 °C and absorber pressures between 2 and 6 kPa, the absorption rates for TFE–TEGDME were 1 to 2.5 g·s⁻¹·m⁻². The corresponding values for methanol–TEGDME with absorber pressures between 10 and 15 kPa were 0.4 to 1.2 g·s⁻¹·m⁻².     

Performance characteristics of a high temperature water-to-water heat pump

A simulation model is utilized to predict the performance of a high temperature water-to-water heat pump, running on Refrigerant 11, over a range of evaporator and condenser water temperature (10 to 40°C and 40 to 70°C) and compressor speeds (500 to 3000 r.p.m.). It is shown that heat pump power output can be effectively controlled by varying compressor speed. Effects of compressor speed, heat source and heat sink (end-use) temperature on the heat pump efficiency are presented. Special attention is devoted to the values of predicted refrigerant temperature at the compressor discharge. These are compared with the thermal limit of the refrigerant. Modifications to the system, to reduce refrigerant maximum temperature, are also discussed.     

Performance of a single‐duct portable propane air conditioning system under different refrigerant charge levels

The effects of the refrigerant charge on the performance of a portable propane air conditioning system have been evaluated and compared to nonportable systems in which the surrounding temperatures of the evaporator and condenser are not equal. This study aims to determine the similarities and differences in the performance of the two types of propane air conditioners under different charge levels, and to serve as a source of reference for future designs of portable air conditioners. The refrigerant charge was changed from ?12.3% to +30% of its normal charge at several room temperatures ranging from 20 °C to 35 °C. The performance parameters include the refrigerant temperature, mass flow rate, maximum velocity of refrigerant, maximum pressure, cooling capacity, compressor work, specific cooling capacity, and coefficient of performance of the system. It has been found that an increase in charge level was found to increase the cooling capacity, coefficient of performance, and maximum velocity of refrigerant in the system while decreasing specific cooling capacity. The increase in the charge caused a relatively insignificant rise in the maximum pressure of the system and useful work of the compressor.     

Experimental model of a variable capacity compressor

The refrigeration and heat pump systems are lately characterized by a remarkable evolution period. The principal reasons that have determined changes are the substitution of environmentally unfriendly refrigerants and the energy saving necessity. As fundamental component of a vapour compression plant, the compressor is an object of optimizations; the variation of the compressor speed, obtained regulating the supply current frequency of the compressor motor, allows to obtain energy savings. The principal aim of this paper is the determination of an experimental model that represents the variable speed reciprocating compressor working. In particular, equations that allow to get the refrigerant mass flow rate, the compressor input power and the cooling capacity in terms only of the frequency are obtained. The experimental model allows to determine the optimum frequency for each working condition and then the related energy saving. Copyright © 2008 John Wiley & Sons, Ltd.     

Study on a direct‐expansion solar‐assisted heat pump water heating system

Analytical and experimental studies were performed on a direct‐expansion solar‐assisted heat pump (DX‐SAHP) water heating system, in which a 2 m² bare flat collector acts as a source as well as an evaporator for the refrigerant. A simulation model was developed to predict the long‐term thermal performance of the system approximately. The monthly averaged COP was found to vary between 4 and 6, while the collector efficiency ranged from 40 to 60%. The simulated results were used to obtain an optimum design of the system and to determinate a proper strategy for system operating control. The effect of various parameters, including solar insolation, ambient temperature, collector area, storage volume and speed of compressor, had been investigated on the thermal performance of the DX‐SAHP system, and the results had indicated that the system performance is governed strongly by the change of solar insolation, collector area and speed of compressor. The experimental results obtained under winter climate conditions were shown to agree reasonably with the computer simulation. Copyright © 2003 John Wiley & Sons, Ltd.     

Numerical analysis on the effects of refrigerant injection on the scroll compressor

Refrigerant injection is an effective method to improve the performance of the scroll compressor and its system under high compression ratio working conditions. This paper intends to find the exhaustive relationship between the injection parameters and the compressor’s performance. Based on a thermodynamic model, the effects of various parameters of refrigerant injection on general performance and inner compression process of scroll compressor have been investigated. As a result, it is found that the injected scroll compressor will get the maximum indicated efficiency when the ratio of inner compression ratio and outer compression ratio is a right value. The right value is 1 for the isentropic compression process, and smaller than 1 for a real compression process. Finally, the effects of all the injection factors on the compression work, refrigerant mass flow rate, p–h diagram, volumetric efficiency, and indicated efficiency are investigated detailedly.     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

Simplified steady-state modeling for variable speed compressor

This paper presents a detailed analysis of semi-empirical methods to calculate mass flow rate, shaft power and discharge temperature for three types of variable speed compressors: reciprocating, scroll and piston rotary. The proposed methods are an integration of physical-based models for constant speed compressor and the physical characteristics of volumetric efficiency and isentropic efficiency between different speeds. The physical-based models were first validated with good agreement with experimental data from publication for the three types of constant speed compressors. The comparison between modeling results and experimental data from publication for the three types of variable speed compressors shows the RMS errors are less than 3%, 3% and 3 °C for refrigerant mass flow rate, compressor power input and discharge temperature, respectively. The model of variable speed compressor will allow the reduction of the number of experimental data required to characterize variable speed compressor behavior in the modeling of refrigeration systems because of its physical mechanisms.     

Performance evaluation of ecofriendly R1234ze(E) refrigerant in an automotive air conditioning system

A mathematical model for an air conditioning system used in five-seater cars is developed with R1234ze(E) and R134a refrigerants, consisting of real system geometry like an evaporator, compressor, condenser, and thermostatic expansion valve. The mathematical model includes refrigerant properties, heat transfer, and pressure loss correlations for two-phase and single-phase regions. The performance parameters of a system like evaporator cooling duty, condenser heat loss, compressor power, refrigerant flow rate, and compressor volumetric efficiency obtained from a mathematical model are validated with the results of an experimental facility developed with R134a. The uncertainty analysis performed for the testing facility showed below 11% deviation. The simulation and experimental results showed an overall 10%–15% difference. It is found that the experimental cooling capacity with R134a and numerical cooling capacity with R134a show a 4%–12% variation, experimental cooling capacity with R134a and numerical cooling capacity with R1234ze(E) show a 7%–20% variation, and numerical cooling capacity with R134a and numerical cooling capacity with R1234ze(E) show a 5%–15% variation for the given range of compressor speed (500–1500 rpm), and condensing temperature (26–45°C). The study concluded that R1234ze(E) could potentially replace R134a because it has similar thermophysical properties and an average performance difference of up to 10% with R134a. Due to the limitations of the electric motor used to drive the compressor, tests in the current study were conducted at modest compressor speeds (500–1500 rpm). Future research will focus on experiments with high compressor speed (in the range of 1500–4000 rpm) and R1234yf and R1234ze(E) refrigerants for performance evaluation of automobile air conditioning systems.     

A seawater freeze desalination prototype system utilizing LNG cold energy

The freeze desalination method is not being used widely, since it needs refrigeration system that consumes much electricity. On the other hand, liquefied natural gas (LNG) releases a lot of cold energy during its vaporization process. Thus, combining the two processes of LNG vaporization and seawater freezing may produce freshwater in an economical and environment-friendly way. In this paper, a seawater freeze desalination prototype system is designed and manufactured. In this system, R410A is chosen as the secondary refrigerant to transfer cold energy from LNG to seawater, and a flake ice-maker is adopted to produce ice. Experiments are conducted with the prototype system, with liquid nitrogen as the cold source. The results show that the system is able to reach the designed fresh water capacity of 150 L h^?1, with the converted cold energy efficiency above 2 kg (fresh water)·kg (LNG)^?1. The salt removal rate of the system is about 50%, indicating that one cycle of the freeze desalination is not enough for producing drinking water. The influences of some key factors, such as refrigerant evaporating temperature, number of spraying nozzles at the water distributing disk, and seawater flowrate, on the salinity of the formed ice are also tested.     

Year round test of a solar adsorption ice maker in Kunming,China

A solar adsorption ice maker with activated carbon–methanol adsorption pair was developed for a practical application. Its main features include utilization of a water cooled condenser and removing all valves in the refrigerant circuit except the one that is necessary for refrigerant charging. Year round performance tests of the solar ice maker were performed in Kunming, Yunnan Province, China. Test results show that the COP (coefficient of performance) of the solar ice maker is about 0.083–0.127, and its daily ice production varies within the range of 3.2–6.5 kg/m² under the climatic conditions of daily solar radiation on the surface of the adsorbent bed being about 15–23 MJ/m² and the daily average ambient temperature being within 7.7–21.1 °C. The suitable daily solar radiation under which the solar ice maker can run effectively in Kunming is above 16 MJ/m².     

Performance enhancement of a split air conditioner using porous material inside the evaporator pipes

In this experimental study, a porous material is used inside the pipes of the evaporator as the main heat exchanging device in the air conditioning cycle. The used porous material consists of stainless steel balls of different diameters. As a case study, refrigerant R454B, which is a drop-in replacement to refrigerant R410A, is used as a working fluid in the air conditioner thermodynamic cycle. Four different porosities were used during the experimental tests; 100% (empty tube), 46%, 40%, and 33%. This study investigated the influence of variation of porosity as well as outside air temperature and refrigerant evaporation temperature on the cycle coefficient of performance, evaporation capacity, pressure drop, and power consumption during the compression process. Measured evaporation temperatures and indoor temperatures during tests were in the range of 1.5–12°C and 18–25°C, respectively. The use of porous material in the evaporation heat exchanger resulted in a considerable increase in the cycle evaporation capacity and coefficient of performance. Varying porosity from 100% to 33% resulted in an average percent increase of cycle evaporation capacity and coefficient of performance by 48.8% and 84.3%, respectively. Also, decreasing porosity from 100% to 33% resulted in an average percent increase in power consumption during the compression process by about 27%. An average percent increase of power consumption of compressor by about 25.9% is also reported, when evaporation temperature increased from 1.5°C to 12°C. Increasing outside air temperature from 27.1°C to 39.5°C resulted in decreasing evaporation capacity and coefficient of performance by 35.2% and 34.5%, respectively, and in increasing compressor power consumption by about 14.3%. A considerable pressure drop was recorded during the evaporation process when using porous material. The volumetric evaporation capacity, as well as compressor discharge temperature, are increased by increasing evaporating temperature and by decreasing evaporator porosity. The increase in air ambient temperature resulted in a considerable increase in refrigerant mass flow rate.     

Absorption of water vapour in the falling film of water–lithium bromide inside a vertical tube at air-cooling thermal conditions

In the absorbers of air-cooled water–lithium bromide absorption chillers, the absorption process usually takes place inside vertical tubes with external fins. In this paper we have carried out an experimental study of the absorption of water vapour over a wavy laminar falling film of water–lithium bromide on the inner wall of a smooth vertical tube. The control variables for the experimental study were; absorber pressure, solution mass flow rate, solution concentration and cooling water temperature. Relatively high cooling water temperatures were selected to simulate air-cooling thermal conditions. The parameters considered to assess the performance of the absorber were; the mass absorption flux, the outlet solution degree of subcooling and the falling film heat transfer coefficient. The results indicate that in water cooling thermal conditions the mass absorption fluxes are in the range 0.001–0.0015 kg·m⁻²·s⁻¹, whereas in air-cooling thermal conditions the range of mass absorption values decreases to 0.00030–0.00075 kg·m⁻²·s⁻¹.     

The performance of a solar assisted heat pump water heating system

Analytical and experimental studies were performed on a solar assisted heat pump water heating system, where unglazed, flat plate solar collectors acted as an evaporator for the refrigerant R-134a. The system was designed and fabricated locally, and operated under meteorological conditions of Singapore. The results obtained from simulation are used for the optimum design of the system and enable determination of compressor work, solar fraction and auxiliary energy required for a particular application. To ensure proper matching between the collector/evaporator load and compressor capacity, a variable speed compressor was used. Due to high ambient temperature in Singapore, evaporator can be operated at a higher temperature, without exceeding the desired design pressure limit of the compressor, resulting in an improved thermal performance of the system. Results show that, when water temperature in the condenser tank increases with time, the condensing temperature, also, increases, and the corresponding COP and collector efficiency values decline. Average values of COP ranged from about 4 to 9 and solar collector efficiency was found to vary between 40% and 75% for water temperatures in the condenser tank varying between 30°C and 50°C. A simulation model has been developed to analyse the thermal performance of the system. A series of numerical experiments have been performed to identify important variables. These results are compared with experimental values and a good agreement between predicted and experimental results has been found. Results indicate that the performance of the system is influenced significantly by collector area, speed of the compressor, and solar irradiation. An economic analysis indicates a minimum payback period of about two years for the system.     

Melting of ice slurry in a tube‐in‐tube heat exchanger

One of the main components of a closed ice slurry system is the heat exchanger in which ice slurry absorbs heat resulting in the melting ice crystals. Design calculations of heat exchangers are mainly based on heat transfer coefficient and pressure drop data. But experiments presented in this paper show the effect of ice slurry mass flux on heat transfer rate and heat transfer coefficient during melting. For the experiments, ice slurry was made from 6.5% ethylene glycol–water solution, flowing through a 16.91mm internal diameter, 1500mm long horizontal copper tube. The ice slurry was heated by hot water circulated at the annulus gap of the heat exchanger. Experiments of the melting process were conducted with changing the ice slurry mass flux and the ice fraction from 800 to 3500kgm^?2s^?1 and 0 to 25%, respectively. During the experiment, it was found that the measured heat transfer rates increase with the mass flow rate and ice fraction; however, the effect of ice fraction appears not to be significant at high mass flow rate. At the region of low mass flow rates, a sharp increase in the heat transfer coefficient was observed when the ice fraction was more than a certain value. Experiments were also conducted to investigate the effect of hot water temperature on the heat transfer coefficient. Copyright © 2006 John Wiley & Sons, Ltd.     

Experimental study on a monovalent inverter-driven water-to-water heat pump with a desuperheater for low energy houses

In this study, a novel monovalent inverter-driven water-to-water heat pump with a desuperheater was developed. In this unique system, domestic hot water is produced at a constant temperature controlled by a variable flow rate and stored in a tank. The heat demand is constantly matched by the system through the use of an inverter-driven compressor, which eliminates the need for a buffer tank. Three heating configurations of the system were examined with respect to variable climate conditions and two space heating target temperatures: space heating (mode 1), domestic hot water production (mode 2) and a combination of both (mode 3). Mappings of the performance variables per frequency were constructed for mode 3. For the other modes, the highest COP was identified for each examined climate condition. The difference between modes 1 and 3 was less than 5% for every variable. The space heating target temperature had a strong influence on both modes, showing an average difference of 29% in the COP between 35 °C and 45 °C. Mode 2 exhibited a considerably reduced COP compared to the other modes, as well as the lowest refrigerant mass flow rate and highest compression ratio among the three modes. From the previous results and the examination of the compressor, the compression ratio presents itself as a key parameter that can help to increase the COP if maintained at low values. The results of this research could be applied to the design of a control methodology for monovalent heat pumps.     

Defrost improvement by heat pump refrigerant charge compensating

During winters, the air-source heat pump often operates with substantial frost formation on the outdoor heat exchanger, and the frost layer has to be melted away periodically to keep a high heat pump coefficient of performance (COP). Otherwise, the unmelted frost layer and water will become high density frost or ice layer in heating mode. However, it is difficult to melt the frost layer in the defrosting cycle, where the effective defrosting time plays an important role in improving the defrosting ability. Generally, the defrosting time can be decreased by the following ways: increasing the refrigerant flow rate effectively, and rapidly establishing the suction pressure, discharge pressure, and the compressor power. A new heat pump defrost system with a refrigerant charge compensator, instead of the accumulator which is a key component for the frosting cycle performance, is developed in this paper. Furthermore, test results showed that the improved frost system with the compensator worked as expected, and its suction and discharge pressures and the power of the compressor during the defrosting were much larger than before.     

Experimental analysis of a transfer function for an air cooled evaporator

A transfer function to model a direct expansion air cooled evaporator, inserted in a vapor compression refrigeration plant, is deduced by means of experimental analysis. For inlet air temperatures onto the evaporator and refrigerant mass flow rate variable in appropriate ranges, the evaporator dynamic behavior is simulated by a linear model with delay. The results of transfer function are compared with experimental data, obtained by applying both step inputs and periodic changes to the refrigerant mass flow rate. The influence of the hunting, typical of a thermostatic expansion valve, is also estimated experimentally and then validated by the transfer function, obtaining a good agreement. These results could be applied to obtain a control algorithm for the refrigerant mass flow rate feeding the evaporator, by varying the speed of the compressor motor.