Effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator

This study reports on a numerical investigation of the effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator. The performance of a thermoacoustic refrigerator is evaluated based on the cooling power, coefficient of performance (COP), and the entropy generation rate within the device. The effect of the variation of the working fluid is observed by changing the Prandtl number (Pr) between 0.7 and 0.28. The operating conditions investigated are drive ratio (DR), stack plate spacing (y₀), and mean pressure (p_m). The present research shows that lowering the Pr of the working fluid does not improve the performance of a thermoacoustic refrigerator for all of the selected operating conditions. COP increases 78% by reducing the Pr from 0.7 to 0.28 at y₀ = 3.33δ_k, at atmospheric pressure and a DR of 1.7%. While the COP decreases by reducing the Pr from 0.7 to 0.28 at y₀ = 1.0δ_k, at atmospheric pressure, and a DR of 1.7%. The results are compared with the available experimental data and found good agreement.     

Influence of a magnetic field on the energy,work, and heat flux of a multi-plate thermoacoustic system

Research on clean and efficient energy conversion is extremely important to mitigate the high price of fossil fuel and its adverse effects on the environment. Thermoacoustic is a clean energy conversion technology that uses the conversion of acoustic to thermal energy and vice versa. However, the efficient conversion of acoustic to thermal energy using thermoacoustic systems (e.g., engine, refrigerator, or heat pump) demands research on working fluids, operational, and geometric parameters. The present study is a contribution to improve the efficiency of a thermoacoustic heat system by introducing a magnetic field perpendicular to the direction of the oscillating fluid. The major focus of this study is to examine the effect of a magnetic field on three important performance parameters: energy, heat, and work fluxes of a multi-plate thermoacoustic system. Initially, analytical expressions for the fluctuating velocity and temperature are derived from the governing continuity, momentum, and energy equations by applying the first order perturbation technique and solving these equations. The derived first order analytical equations for the fluctuating velocity and temperature enable us to calculate the energy, heat, and work fluxes and are expressed in terms of dimensionless Hartmann number (Ha_δ), temperature gradient ratio (Γ₀), Swift number (S_w), Prandlt number (Pr), and modified Rott's and Swift's parameters (f_v and f_k). It is observed that the normalized energy flux density increases with increasing Ha_δ and Γ₀ when S_w < 1.5. The heat flux and work flux densities also increase with increasing Ha_δ and Γ₀ when S_w < 1.5 and decrease when Ha_δ > 1.5. The findings of this research will provide useful information to thermoacoustic system's designers for the devloepment of effieicnt magnetic thermoacoustic heat pumps.     

Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

A thermoelectric domestic refrigerator has been developed, with a single compartment of 0.225 m³, for food preservation at 5 °C. The cooling system is made up of two equal thermoelectric devices, each composed of a Peltier module (50 W) with its hot side in contact with a two-phase and natural convection thermosyphon (TSV) and a two-phase and capillary lift thermosyphon (TPM), in contact with the cold side.The entire refrigerator has been simulated and designed using a computational model, based on the finite difference method. Subsequently an experimental optimization phase of the thermosyphons was carried out, until thermal resistance values of R_TSV = 0.256 K/W and R_TPM = 0.323 K/W were obtained. These values were lower than those obtained with finned heat sinks.Finally, a functional prototype of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal drop (Ambient Temperature–Inside Temperature) of 19 °C. The electric power consumption at nominal conditions was 45 W, reaching a COP value of 0.45. The study demonstrated that by incorporating these two-phase devices into thermoelectric refrigeration increases the COP by 66%, compared with those which use finned heat sinks.     

Experimental and numerical investigations of an orifice type cryogenic pulse tube refrigerator

A helium filled orifice type pulse tube refrigerator (OPTR) was designed, built and operated to provide cryogenic cooling. The OTPR is a traveling wave thermoacoustic refrigerator that operates on a modified reverse Stirling cycle. The experimental studies are carried out to characterize the performance of the OPTR at various values of the mean pressure of helium (0.35 MPa–2.2 MPa), amplitudes of pressure oscillations, frequencies of operation and sizes of orifice opening. A detailed time-dependent axisymmetric computational fluid dynamic (CFD) model of the OPTR is also developed to predict its performance. In the CFD model, the continuity, momentum and energy equations are solved for both the refrigerant gas (helium) and the porous media regions (the regenerator and the three heat-exchangers) in the OPTR. An improved representation of heat transfer in the porous media is achieved by employing a thermal non-equilibrium model to couple the gas and solid (porous media) energy equations. The computational results are compared with specific experimental results to validate the numerical model. The model predictions show better temporal comparisons with the experimental results when the effects of wall thicknesses and natural convective losses of the various components of the OPTR to the surroundings are included in the model.     

Estimation of heat transfer coefficients in oscillating flows: The thermoacoustic case

The classical linear thermoacoustic theory is integrated through a numerical calculus with a simple energy conservation model to allow estimates of the optimal length of thermoacoustic heat exchangers and of the magnitude of the related heat transfer coefficients between gas and solid walls. This information results from the analysis of the temperature and heat flux density distributions inside a thermally isolated thermoacoustic stack. The effects of acoustic amplitude, plate spacing, plate thickness and Reynolds number on the heat transfer characteristics are examined. The results indicate that a net heat exchange between the acoustically oscillating gas and the solid boundary takes place only within a limited distance from the stack edges. This distance is found to be an increasing function of the plate spacing in the range (0 ⩽ y₀/δ_κ ⩽ 2), becoming constant for y₀/δ_κ ⩾ 2. The calculated dimensionless convective heat transfer coefficients, the Nusselt numbers, between gas and solid wall are comparable to those evaluated from classical correlations for steady laminar flow revised under the “Time-Average Steady-Flow Equivalent” (TASFE) and “root-mean-square Reynolds number” (RMSRe) models. Numerical results agree with measurements of the heat transfer coefficient found in literature to within 20%.     

Performance of hybrid refrigeration system using ammonia

This paper investigates the performance of a hybrid refrigeration system that combines sorption–conventional vapour compression refrigeration machine driven by dual source (heat and/or electricity). The dual source makes the system highly flexible and energy efficient. The ammonia refrigerant (R717) is used in both adsorption and associated conventional refrigeration cycles. The model of thermal compressor corresponds to a multiple pair of compact adsorption generators operating out of phase with both heat and mass recovery for continuous cooling production and better efficiency. Each generator is based on a plate heat exchanger concept using the activated carbon–ammonia pair. The model of conventional vapour compressor is a reciprocating compressor from Frigopol. The hybrid refrigeration performances are presented mainly for ice making and air conditioning applications (T_C = 40 °C, −5 °C < T_E < 20 °C). The exhaust temperature of the compressor (driving temperature for thermal compressor) varies from 90 °C to 250 °C. The results show a cooling production ranging from 4 kW to 12 kW with back-up mode (both cycles not operating simultaneously) and from 8 kW to 24 kW with complementary mode (both cycles operating simultaneously). The effective overall COP based on the total equivalent heat rate input varies from 0.24 to 0.76.     

Optimum packing factor of the stack in a standing‐wave thermoacoustic prime mover

A bench consisting of a pulse tube refrigerator driven by a standing‐wave thermoacoustic prime mover has been set up to study the relationship among stack, regenerator and working fluids. The stack of the thermoacoustic prime mover is packed with dense‐mesh wire screens because of their low cost and easy manufacture. The effect of the packing factor in the stack on onset temperature, refrigeration temperature and input power is explored. The optimum packing factor of 1.15 pieces per millimeter has been found experimentally, which supplies an empirical value to satisfy a compromise for enhancing thermoacoustic effect, decreasing heat conduction and fluid‐friction losses along the stack. The pulse tube cooler driven by the thermoacoustic prime mover is able to obtain refrigeration temperatures as low as 138 and 196K with helium and nitrogen, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Optimization of the Stack Unit in a Thermoacoustic Refrigerator

ABSTRACT

A thermoacoustic refrigerator is a device that uses acoustic power to pump heat in the absence of harmful refrigerants with no or few moving parts. However, the performance of the thermoacoustic refrigerator, particularly the standing wave types, is currently not competitive compared to its counterpart, the conventional vapor-compression refrigerator. Presently, thermoacoustic refrigeration prototypes only achieved 0.1–0.2 relative coefficient of performance, compared with that of 0.33–0.5 for the conventional vapor-compression refrigerators. Past optimization efforts had been completed based on parametric studies where individual parameters are discretely varied and the final optimized outcome was based on the limited series of numerical/experimental tests. This paper discusses the initial investigation of the optimization of the thermoacoustic refrigerator stack parameters using a multi-objective genetic algorithm. The desired outputs, the maximization of the cooling load and the minimization of the acoustic power at the stack, are obtained with the parameters to be optimized set within some range of values. The stack length and center position are then optimized simultaneously. The optimized results showed that the coefficient of performance of the thermoacoustic refrigerator improves from the published value of 1.3 to 1.37.     

Performance evaluation method of solar-assisted heat pump water heater

The present study derives a simple linear correlation for the performance evaluation of different solar-assisted heat pump water heater (ISAHP). The correlation was derived from the principle of energy conservation with some simplifications. The correlation is then verified using the long-term outdoor field test data of four different ISAHP. The problems of seasonal repeatability and method of data scattering were examined. From that, a standard performance test method is proposed. The test method suggests that only the measurement of instantaneous solar incident radiation on horizontal surface, ambient temperature, hot water temperature in the storage tank, total mass of water in the storage tank and total power input to the ISAHP are required. It is suggested to select the value of COP at T_f − T_a,ave = 15 °C as the characteristic COP for performance comparison of ISAHP. It is found from the test results that the same performance correlation holds for ISAHP operating with single or dual energy source.     

Effect of heat transfer law on the finite-time exergoeconomic performance of a Carnot refrigerator

The operation of a Carnot refrigerator is viewed as a production process with exergy as its output. The economic optimization of the endoreversible refrigerator is carried out in this paper. The Coefficient of Performance (COP) of the refrigerator is a secondary consideration of the practical engineering effort of maximizing cooling rate and exergy whose goodness is constrained by economical considerations. Therefore, the profit of the refrigerator is taken as the optimization objective. Using the method of finite-time exergoeconomic analysis, which emphasizes the compromise optimization between economics (profit) and the appropriate energy utilization factor (Coefficient of Performance, COP) for finite-time (endoreversible) thermodynamic cycles, this paper derives the relation between optimal profit and COP of an endoreversible Carnot refrigerator based on a relatively general heat transfer law q∝Δ(Tⁿ). The COP at the maximum profit is also obtained. The results obtained involve those for three common heat transfer laws: Newton's law (n=1), the linear phenomenological law in irreversible thermodynamics (n=−1), and the radiative heat transfer law (n=4).     

Optimum operating conditions for a water purification process integrated to a heat transformer with energy recycling using neural network inverse

Artificial neural network inverse (ANNi) is applied to calculate the optimal operating conditions on the coefficient of performance (COP) for a water purification process integrated to an absorption heat transformer with energy recycling. An artificial neural network (ANN) model is developed to predict the COP which was increased with energy recycling. This ANN model takes into account the input and output temperatures for each one of the four components (absorber, generator, evaporator, and condenser), as well as two pressures and LiBr + H₂O concentrations. For the network, a feedforward with one hidden layer, a Levenberg–Marquardt learning algorithm, a hyperbolic tangent sigmoid transfer function and a linear transfer function were used. The best fitting training data set was obtained with three neurons in the hidden layer. On the validation data set, simulations and experimental data test were in good agreement (R > 0.99). This ANN model can be used to predict the COP when the input variables (operating conditions) are well known. However, to control the COP in the system, we developed a strategy to estimate the optimal input variables when a COP is required from ANNi. An optimization method (the Nelder–Mead simplex method) is used to fit the unknown input variable resulted from the ANNi. This methodology can be applied to control on-line the performance of the system.     

Analysis of crossed van't Hoff metal hydride based heat pump

In this paper, the operating feasibility of a single-stage metal based hydride heat pump (SS-MHHP) working on the principle of crossed van't Hoff line concept is presented. The performance of the system is predicted by solving the unsteady, two-dimensional mathematical model in an annular cylindrical configuration employing two different hydride alloy pairs, namely, V_0.846Ti_0.104Fe_0.05/Fe_0.9Mn_0.1Ti and V_0.855Ti_0.095Fe_0.05/MmNi_4.7Al_0.3 (regeneration alloy/refrigeration alloy). The influences of heat source (T_H) and refrigeration (T_C) temperatures on the amount of hydrogen transferred between the paired reactors, coefficient of performance (COP) and specific cooling power (SCP) of the crossed van't Hoff SS-MHHP system are studied. Within the selected ranges of operating temperatures, the COP of the crossed van't Hoff SS-MHHP is about 60% higher than the conventional single-stage MHHP. The optimum operating temperatures of V_0.846Ti_0.104Fe_0.05/Fe_0.9Mn_0.1Ti and V_0.855Ti_0.095Fe_0.05/MmNi_4.7Al_0.3 combinations are found to be 373/303/291 K and 400/303/283 K (heat source/heat sink/refrigeration temperatures), respectively. At the optimum operating temperatures, the COP and SCP of the V_0.846Ti_0.104Fe_0.05/Fe_0.9Mn_0.1Ti and V_0.855Ti_0.095Fe_0.05/MmNi_4.7Al_0.3 combinations are 0.89 and 30.8 W/kg of total mass and 0.86 and 30.3 W/kg of total mass, respectively.     

Theoretical analysis and experimental research on transcritical CO2 two stage compression cycle with two gas coolers (TSCC + TG) and the cycle with intercooler (TSCC + IC)

As one of the natural refrigerants, CO₂ is a potential substitute for synthesized refrigerants with favorable environmental properties. In order to improve the performance of the CO₂ transcritical compression cycle, the performance of the two stage compression cycle with two gas coolers (TSCC + TG) and the two stage compression cycle with intercooler (TSCC + IC) were analyzed, respectively. Under the given calculation condition, the optimum intermediate pressure of the cycle TSCC + TG and the TSCC + IC are 7.09 MPa and 5.89 MPa, and the maximal COP are 2.77 and 3.08, respectively. Range of the given evaporating temperature and outlet temperature of gas cooler, the experimental testing shows that the performance of cycle TSCC + IC are 11.88% and 10.87% better than that of the cycle TSCC + TG, respectively. Range of the given inlet temperature and cooling water volume flow of gas cooler, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.97% and 4.39% higher than that of the cycle TSCC + TG. Range of the given inlet temperature and chilled water volume flow of evaporator, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.71% and 3.67% higher than that of the cycle TSCC + TG, respectively. The error between theoretical calculation and experimental testing is not exceeds 20%.     

Performance investigation of double-stage metal hydride based heat pump

In this paper, the performance investigation of a Double-Stage Double-Effect Metal Hydride Heat Pump (DSDE-MHHP) working with LaNi_4.1Al_0.52Mn_0.38, LmNi_4.91Sn_0.15 and Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 hydride alloys is presented. The effects of half cycle time (θ), hydride mass ratio (M_R), sensible heat exchange factor (?) and operating temperatures, viz. heat source (T_D), heat sink (T_M), and refrigeration (T_C) temperatures on the amount of hydrogen transferred between the paired reactors, coefficient of performance (COP) and specific cooling power (SCP) of the DSDE-MHHP system are investigated. For the present analysis the heat rejection temperature (T_H) is maintained constant at 373 K. Numerically predicted hydride bed temperatures are compared with experimental data and a good agreement is observed between them. It is observed from the numerical results that the COP and SCP of the DSDE-MHHP system increase with heat source and refrigeration temperatures, and decrease with heat sink temperature. For operating temperatures of 578, 373, 298 and 283 K (T_D, T_H, T_M and T_C), the average COP and SCP of the system are found to be 0.81 and 48.1 W/kg of total alloy mass, respectively.     

Optimal COP prediction of a solar intermittent refrigeration system for ice production by means of direct and inverse artificial neural networks

A direct and inverse artificial neural network (ANN and ANNi) approach were developed to predict the required coefficient of performance (COP) of a solar intermittent refrigeration system for ice production under various experimental conditions. Ammonia/lithium nitrate was used as a working fluid considering different solution concentrations. The configuration 6-6-1 (6 inputs, 6 hidden and 1 output neurons) presented an excellent agreement (R > 0.986) between experimental and simulated values. The used inputs parameters were: the solution concentration, the cooling water temperature, the generation temperature, the ambient temperature, the generation pressure and the solar radiation. The sensitivity analysis showed that all studied input variables have effect on the COP prediction but the generation pressure is the most influential parameter on the COP, while the rest of input parameters were less significant. COP performance was also determined by inverting ANN to calculate the unknown input parameter from a required COP. Because of the high accuracy and short computing time makes this methodology useful to simulate and to optimize the solar refrigerator system.     

Experimental evaluation of prototype thermoelectric domestic-refrigerators

A number of prototype thermoelectric refrigerators are investigated and their cooling performances evaluated in terms of the coefficient-of-performance, heat-pumping capacity and cooling-down rate. The coefficient-of-performance of a thermoelectric refrigerator is found to be around 0.3–0.5 for a typical operating temperature at 5 °C with ambient at 25 °C. The potential improvement in the cooling performance of a thermoelectric refrigerator is also investigated employing a realistic model, with experimental data obtained from this work. The results show that an increase in its COP is possible through improvements in module contact resistances, thermal interfaces and the effectiveness of heat exchangers.     

A hybrid solar-assisted adsorption cooling unit for vaccine storage

A concept of a hybrid adsorption cooling unit for vaccine storage utilizing solar energy as a main power supply and a gas burner as an alternative power supply has been developed. The components of the cooling unit have been designed to work under the weathering conditions of Burkina Faso, West coast of Africa according to the requirements of the World Health Organization. For the first adsorber, which is driven by a gas burner, zeolite-13X has been selected. For the second adsorber to be driven by solar energy selective water sorbent SWS-2L has been applied. Water is selected as a refrigerant for both adsorbents. Theoretical investigations of the expected performance of the designed cooling unit have shown a coefficient of performance (COP) of 0.28 for the solar-operated system based on the heat input to the adsorption unit, at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=122 °C. For the gas-heated system, also a COP of 0.28 has been estimated at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=280 °C. The variations of COP, cooling capacity and the heating power required to operate both systems have been estimated for a broad range of desorption temperatures. It turns out that the SWS-2L/water system is much more sensitive to the operating conditions than the zeolite-13X/water system. The obtained results should serve in designing both control and heating components of the cooling unit.     

Isothermal sorption characteristics of the BaCl2–NH3 pair in a vermiculite host matrix

Chemisorption could be useful in adsorption systems due to the large concentration change compared with physisorption. Equilibrium concentration characteristics of ammonia with a composite adsorbent material (BaCl₂ impregnated into a vermiculite matrix) are investigated: the maximum concentration is about 0.4 kg ammonia/kg adsorbent. Hysteresis was observed between the synthesis and the decomposition reactions. The analysis of the data suggests that the hysteresis could be due to the dimensional changes of the solid during the reactions. The bi-variant behaviour observed was contrary to the mono-variant behaviour anticipated and the reasons are discussed. The COP of a basic adsorption cycle for typical ice-making and air-conditioning applications utilizing ammonia and the composite material were calculated. The results show that the material could be used for air-conditioning or other refrigeration applications. The COP could reach up to 0.6 at typical conditions (T_ev = 15 °C, T_con = 35 °C).     

A complete analysis of the effects of transfer phenomenons and reaction heats on sono-hydrogen production from reacting bubbles: Impact of ambient bubble size

Several experimental and computational works have been focused on the production of hydrogen by using ultrasonic irradiation. However, the effects of the different ultrasonic conditions have been analyzed by considering a single value for the ambient bubble radius R₀ (mean value), which is not the true case as the size of active bubbles in sonicating medium is an interval rather than a sole value. In the present paper, the impacts of mass transport, heat exchange and chemical reactions heat on the sono-production of hydrogen are examined over a range of ambient bubble radii. These effects are shown for various ultrasonic frequencies of 355, 500 and 1000 kHz and under a range of acoustic amplitudes, from 1.5 to 3 atm. The numerical simulations results demonstrated that the increase of the production rate of hydrogen (around R₀ of the maximal production rate) is amortized (for all models) for the wave frequencies of 355 and 500 kHz at higher amplitude (i.e. 3 atm). On the other hand, the total production rate (around R₀ of the maximal response) is increased proportionally with the reduction of ultrasonic frequency or if the acoustic amplitude is increased. The effect of heat exchange mechanism (on H₂ and the total production rate) was found to be dominant whatever the acoustic amplitude or the wave frequency (on all the range of R₀). It has been demonstrated that at the acoustic amplitudes >1.5 atm (for f = 355 and 500 kHz) and >2 atm (for f = 1000 kHz), the impacts of chemical reactions heat and mass transport are clear compared to the normal model throughout a range of bubble sizes. The ambient bubble size (R₀) of the maximal response (maximal production rate) is shifted toward lower values when the ultrasound frequency or the acoustic amplitude is raised. In addition, it is observed that the increase in the wave frequency or the decrease in acoustic amplitude cause a narrowing in the range of active bubbles.     

Transient analysis and performance prediction of a solid adsorption solar refrigerator

The transient analysis and performance prediction of a solid adsorption solar refrigerator, using activated carbon/methanol adsorbent/adsorbate pair are presented. The mathematical model is based on the thermodynamics of the adsorption process, heat transfer in the collector plate/tube arrangement, and heat and mass transfers within the adsorbent/adsorbate pair. Its numerical model developed from finite element transformation of the resulting equations computes the collector plate and tube temperatures to within 5 °C. The condensate yield and coefficient of performance, COP, were predicted to within 5% and 9%, respectively. The resulting evaporator water temperature was also predicted to within 5 °C. Thus the model is considered a useful design tool for the refrigerator to avoid costly experimentation.