Experimental study on transcritical Rankine cycle (TRC) using CO2/R134a mixtures with various composition ratios for waste heat recovery from diesel engines

A carbon dioxide (CO2) based mixture was investigated as a promising solution to improve system performance and expand the condensation temperature range of a CO2 transcritical Rankine cycle (C-TRC). An experimental study of TRC using CO2/R134a mixtures was performed to recover waste heat of engine coolant and exhaust gas from a heavy-duty diesel engine. The main purpose of this study was to investigate experimentally the effect of the composition ratio of CO2/R134a mixtures on system performance. Four CO2/R134a mixtures with mass composition ratios of 0.85/0.15, 0.7/0.3, 0.6/0.4 and 0.4/0.6 were selected. The high temperature working fluid was expanded through an expansion valve and then no power was produced. Thus, current research focused on the analysis of measured operating parameters and heat exchanger performance. Heat transfer coefficients of various heat exchangers using supercritical CO2/R134a mixtures were provided and discussed. These data may provide useful reference for cycle optimization and heat exchanger design in application of CO2 mixtures. Finally, the potential of power output was estimated numerically. Assuming an expander efficiency of 0.7, the maximum estimations of net power output using CO2/R134a (0.85/0.15), CO2/R134a (0.7/0.3), CO2/R134a (0.6/0.4) and CO2/R134a (0.4/0.6) are 5.07 kW, 5.45 kW, 5.30 kW, and 4.41 kW, respectively. Along with the increase of R134a composition, the estimation of net power output, thermal efficiency and exergy efficiency increased at first and then decreased. CO2/R134a (0.7/0.3) achieved the maximum net power output at a high expansion inlet pressure, while CO2/R134a (0.6/0.4) behaves better at low pressure

Preliminary experimental comparison and feasibility analysis of CO2/R134a mixture in Organic Rankine Cycle for waste heat recovery from diesel engines

This paper presents results of a preliminary experimental study of the Organic Rankine Cycle (ORC) using CO2/R134a mixture based on an expansion valve. The goal of the research was to examine the feasibility and effectiveness of using CO2 mixtures to improve system performance and expand the range of condensation temperature for ORC system. The mixture of CO2/R134a (0.6/0.4) on a mass basis was selected for comparison with pure CO2 in both the preheating ORC (P-ORC) and the preheating regenerative ORC (PR-ORC). Then, the feasibility and application potential of CO2/R134a (0.6/0.4) mixture for waste heat recovery from engines was tested under ambient cooling conditions. Preliminary experimental results using an expansion valve indicate that CO2/R134a (0.6/0.4) mixture exhibits better system performance than pure CO2. For PR-ORC using CO2/R134a (0.6/0.4) mixture, assuming a turbine isentropic efficiency of 0.7, the net power output estimation, thermal efficiency and exergy efficiency reached up to 5.30 kW, 10.14% and 24.34%, respectively. For the fitting value at an expansion inlet pressure of 10 MPa, the net power output estimation, thermal efficiency and exergy efficiency using CO2/R134a (0.6/0.4) mixture achieved increases of 23.3%, 16.4% and 23.7%, respectively, versus results using pure CO2 as the working fluid. Finally, experiments showed that the ORC system using CO2/R134a (0.6/0.4) mixture is capable of operating stably under ambient cooling conditions (25.2–31.5 °C), demonstrating that CO2/R134a mixture can expand the range of condensation temperature and alleviate the low-temperature condensation issue encountered with CO2. Under the ambient cooling source, it is expected that ORC using CO2/R134a (0.6/0.4) mixture will improve the thermal efficiency of a diesel engine by 1.9%

Investigation of Adiabatic Refrigerant Pressure Drop and Flow Visualization in Flat Plate Evaporators

Adiabatic pressure drop and flow visualization in chevron plate, 1:1 aspect ratio bumpy plate, and 2:1 aspect ratio bumpy plate heat exchangers were investigated for vertical upward flow with R134a. Qualities ranging from subcooled liquid to superheated vapor were investigated. Mass fluxes ranged from 16 kg/m2-s (for superheated vapor) to approximately 300 kg/m2-s (for sub-cooled liquid). The pressure drop experiments were conducted for 10o C and 20o C inlet temperatures. The flow visualization experiments were conducted at a 10o C inlet temperature. The following is the order of highest to lowest pressure drop geometries on both a mass flux and mass flow bases: chevron plate, 1:1 aspect ratio bumpy plate, and 2:1 aspect ratio bumpy plate. These trends are more pronounced on a mass flow basis. Four flow regimes were observed for the flat plate geometries investigated and are mapped out on a mass flux versus quality basis for each geometry. The chevron geometry was seen to undergo flow transitions at lower qualities and mass fluxes than the bumpy plate geometries. The kinetic energy per unit volume of the flow was found to have a strong linear relationship with pressure drop for both single-phase and two-phase flow, suggesting that inertial effects are the dominant mode of pressure drop in flat plate heat exchangers. Vapor pressure drop prediction models based on the kinetic energy of the flow are presented, which predict pressure drop within 20%. A two-phase pressure drop model is developed, also based on kinetic energy per unit volume of the flow. A pseudo void fraction is defined in order to correlate the two-phase pressure drop to the single-phase pressure drop. The two-phase pressure drop model predicts two-phase pressure drop to within 15% of experimental measurements. A description of and modifications to the experimental test facilities are provided. In addition, the geometries and construction of the plates are provided.Air Conditioning and Refrigeration Project 12

Dynamic control strategy of a distillation system for a composition-adjustable organic Rankine cycle

Using zeotropic mixtures as working fluids can improve the thermal efficiency of Organic Rankine cycle (ORC) power plants for utilising geothermal energy. However, currently, such ORC systems cannot regulate the composition of zeotropic mixtures when their operating conditions change. A composition-adjustable ORC system could potentially improve the thermal efficiency by closely matching the cycle to the changing ambient conditions provided that the composition of the working fluid mixture can be adjusted in an economic way. In this paper, a dynamic composition control strategy has been proposed and analysed for such a composition-adjustable ORC system. This method employs a distillation column to separate the two components of the mixture, which can then be pumped back to the main ORC system to adjust the composition of the zeotropic mixture to the required level according to the ambient temperature. The dynamic composition control strategy is simulated using an optimisation algorithm. The design method of the distillation column is presented and its dynamic response characteristics have been analysed using Aspen Plus Dynamics. The results indicate that the average power output can be significantly improved using a composition-adjustable ORC system when the ambient temperature decreases. The size of the distillation system is relatively small and its energy (mainly thermal) consumption is only around 1 percent of the system’s input heat. The research results also show that the dynamic response characteristics of the distillation system can satisfy the requirements of the ORC system

Investigation of an R134A Refrigerant/Iso 32 Polyol Ester Oil Mixture in Condensation

Air Conditioning and Refrigeration Project 12

Subcritical and supercritical fluid extraction a critical review of its analytical usefulness

Subcritical R134a is suggested as a low-pressure alternative to supercritical CO2 in the supercritical fluid extraction technology in particular of palm oil application. Therefore, a measurement of solubility of palm oil in subcritical Rl34a will be carried out at temperatures of 40, 60, 70 and 80°C and pressures up to 300 bar. The solubility of carotene are also will be measured using UV Spectrophotometer. Results obtained from this study will be compared with the previous work and for the first time, simulation for the SFE process of palm oil will be performed using Artificial Neural Network (ANN) and it will be implemented in comparisons as well when the operating conditions of the previous findings are different from this study. It is expected that the solubility of the palm oil in subcritical Rl34a is much higher than SC-C02, and it is expected that R134a could be a viable alternative solvent to supercritical carbon dioxide as R134a could be perform well at a lower pressure used whereas can achieved a higher solubility compared to SC-C0

Characterization of Two-Phase Flow in Microchannels

Aluminum multi-port microchannel tubes are currently utilized in automotive air conditioners for refrigerant condensation. Recent research activities are directed toward developing other air conditioning and refrigeration systems with microchannel condensers and evaporators. Three parameters are necessary to analyze a heat exchanger performance: heat transfer, pressure drop, and void fraction. The purpose of this investigation is the experimental investigation of void fraction and frictional pressure drop in microchannels. A flow visualization analysis is another important goal for two-phase flow behavior understanding and experimental analysis. Experiments were performed with a 6-port and a 14-port microchannel with hydraulic diameters of 1.54 mm and 1.02 mm, respectively. Mass fluxes from 50 to 300 kg/s.m2 (range of most typical automotive applications) are operated, with quality ranging from 0% to 100% for two-phase flow experiments. R410A, R134a, and air-water mixtures are used as primary fluids. The results from the flow visualization studies indicate that several flow configurations may exist in multi-port microchannel tubes at the same time while constant mass flux and quality flow conditions are maintained. Flow mapping of the fluid regimes is accomplished by developing functions that describe the fraction of time or the probability that the fluid exists in an observed flow configuration. Experimental analysis and flow observations suggest that pressure drop and void fraction in microchannel is dependent on the most probable flow regime at which the two-phase mixture is flowing. In general, correlations for void fraction and pressure drop predictions are based in a separated flow model and do not predict the experimental results in the range of conditions investigated. A flow regime based model is developed for pressure drop and void fraction predictions in microchannels.Air Conditioning and Refrigeration Project 10

CFD modelling of a two-phase closed thermosyphon charged with R134a and R404a

This paper examines the application of CFD modelling to simulate the two-phase heat transfer mechanisms in a wickless heat pipe, also called a thermosyphon. Two refrigerants, R134a and R404a, were selected as the working fluids of the investigated thermosyphon. A CFD model was built to simulate the details of the two-phase flow and heat transfer phenomena during the start-up and steady-state operation of the thermosyphon. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data, thus confirming that the CFD model was successful in reproducing the heat and mass transfer processes in the R134a and R404a charged thermosyphon, including the pool boiling in the evaporator section and the liquid film in the condenser section

Tribological Studies on Scuffing Due to the Influence of Carbon Dioxide Used as a Refrigerant in Compressors

The refrigeration and air conditioning industry has expressed a great interest in the use of carbon dioxide (CO2) as a refrigerant. CO2 is anticipated to replace HFC refrigerants, which are known to have a negative effect on the environment. The reason behind the interest in CO2 is the fact that it is a natural refrigerant, thus environmentally acceptable. Of course, such a replacement raises concerns regarding design criteria and performance due to the different thermodynamic properties of CO2 and the very different range of pressures required for the CO2 refrigeration cycle. So far, work related to CO2 has been done from a thermodynamics point of view and researchers have made significant progress developing automotive and portable air-conditioning systems that use the environmentally friendly carbon dioxide as a refrigerant. The purpose of this work is to develop an understanding of how CO2 plays a role from a tribology standpoint. More specifically, the goal of this work is to gain an understanding on how CO2 influences friction, lubrication, wear and scuffing of tribological pairs used in compressors. Work in the area of tribology related to CO2 is very limited. Preliminary work by Cusano and coworkers showed that consistent data for tests using CO2 could not be acquired nor could a satisfactory explanation be offered for the inconsistency. Their results triggered the initiation of the work presented here. In this first attempt to understand the tribological behavior of CO2 several problems were encountered. During this work we noted that its behavior, unlike conventional refrigerants, could not always be predicted. We believe that this can be attributed to the thermodynamic properties of CO2, which cannot be ignored when studying its tribological behavior. Thermodynamic Properties such as miscibility are very important when tribological testing is performed. A limiting factor with our tester was that it was not designed for CO2 testing, but for other conventional refrigerants and therefore made previously developed testing protocols non-applicable with CO2. Through a different approach and some modifications to our tester we were able to establish a protocol for testing under the presence of CO2. CO2 was then compared to R134a and the experimental results showed that it performs equally well.Air Conditioning and Refrigeration Project 13