Characteristics of an air source heat pump with novel photoelectric sensors during periodic frost–defrost cycles

To avoid mal-defrost phenomenon, an innovative photoelectric sensor is developed and presented in this paper. It is referred to as “Tube Encircled Photoelectric Sensor” (TEPS). Experiments are carried out in a controlled environmental chamber under standard frosting conditions. Ten TEPSs in 4 different models are tested on a commercial size air source heat pump with the nominal heating capacity of 60 kW. The characteristics of the air source heat pump, together with the performance of the TEPSs are investigated during 9 periodic frost–defrost cycles. Compared with the original defrosting control strategy equipped by the manufacturer, the proposed TEPS sensor reveals its potential ability to accurately control the defrosting process. Experimental results demonstrate that TEPSs can substantially prolong defrost intervals from 28.8 min to 52 min under the experimental conditions, and the number of defrost cycles can be reduced from 9 to 5. The performance improvement is found to be 6% to the heating efficiency, and 5% to the COP.     

Residential solar air conditioning: Energy and exergy analyses of an ammonia–water absorption cooling system

Large scale heat-driven absorption cooling systems are available in the marketplace for industrial applications but the concept of a solar driven absorption chiller for air-conditioning applications is relatively new. Absorption chillers have a lower efficiency than compression refrigeration systems, when used for small scale applications and this restrains the absorption cooling system from air conditioning applications in residential buildings. The potential of a solar driven ammonia–water absorption chiller for residential air conditioning application is discussed and analyzed in this paper. A thermodynamic model has been developed based on a 10 kW air cooled ammonia–water absorption chiller driven by solar thermal energy. Both energy and exergy analyses have been conducted to evaluate the performance of this residential scale cooling system. The analyses uncovered that the absorber is where the most exergy loss occurs (63%) followed by the generator (13%) and the condenser (11%). Furthermore, the exergy loss of the condenser and absorber greatly increase with temperature, the generator less so, and the exergy loss in the evaporator is the least sensitive to increasing temperature.     

A mathematical model with experiments of single effect absorption heat pump using LiBr–H2O

A mathematical model of a single effect, LiBr–H₂O absorption heat pump operated at steady conditions is presented. This model took into consideration of crosscurrent flow of fluids for heat and mass exchangers, two-dimensional distribution of temperature and concentration fields, local values of heat and mass transfer coefficients, thermal parameter dependent physical properties of working fluids and operation limits due to the danger of the LiBr aqueous solution hydrates and crystallization. Improvements of the calculation method make this simulation much more convenient and efficient. An improved absorber experiment set-up and a complete absorption heat pump were built and tested for further study. It was found that the mass flux of vapor increased with the increase of absorber pressure, coolant flow rate, spray density of LiBr solution and decrease of coolant and input temperature of solution. And the vapor mass flux increased almost linearly with the increase of absorber pressure. Results derived from this model show agreement within 7% with experimental values.     

Numerical analysis of convective heat loss from a cylindrical–hemispherical receiver using a glass cover and an air curtain

It is imperative to mitigate the convective heat loss from the receiver to improve the overall efficiency of the parabolic dish concentrator. In this study, the reductions of convective heat loss from the cylindrical-hemispherical receiver are numerically analyzed and the model was validated by the experimental data from literature. In the first case, the impact of the glass cover on convective heat loss is examined under conditions of both natural and forced convections at various receiver orientations (γ = 0°, 30°, 60°, and 90°). Numerical results clearly demonstrate that the use of a glass cover significantly reduces the intrusion of surrounding air into the receiver cavity which leads to an enhancement of the stagnation zone inside the cavity and, as a consequence, a noticeable reduction in convective heat loss is observed. To perform analysis of the receiver with glass cover under forced convective condition, the wind velocities over the receiver are considered in the range of 1–6 m/s. The maximum reduction of convective heat loss using the glass cover is achieved to be 58.44% with wind velocity of 5 m/s at γ = 60°. In the second case, the influence of air curtain at the receiver aperture under natural convective heat loss conditions is analyzed. The analysis incorporates three variables: receiver orientation (γ = 0°–60°), nozzle width ( $L_{\mathrm{noz}}$ = 0.002–0.004 m), and nozzle outlet velocity ( $V_{\mathrm{noz}}$ = 0.5–3.5 m/s). The results show that the air curtain minimizes the outflow of receiver inside air and results in an improvement in the stagnation zone inside the cavity. The maximum effectiveness of the air curtain is found to be 43.2% at nozzle width of $L_{\mathrm{noz}}$ = 0.004 m and nozzle velocity of $V_{\mathrm{noz}}$ = 1.5 m/s at receiver orientation of 60°. It is also noteworthy that the optimal nozzle velocity decreases with the increase of nozzle widths.     

Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: A case study of TEG–water

In this research, the critical heat flux (CHF) due to flooding limit of thermosyphon heat pipe using triethylene glycol (TEG)–water mixture has been investigated. From the experiment it is found that, use of TEG–water mixture can extend the heat transport limitation compared with pure water and higher heat transfer is obtained compared with pure TEG at high temperature applications. Moreover it is found that ESDU equation is appropriate to predict the CHF of the thermosyphon in case of TEG–water mixture.For thermosyphon air preheater at high temperature applications, it is found that with selected mixture content of TEG–water in each row of the thermosyphon the performance of the system could be increased approximately 30–80% compared with pure TEG for parallel flow and 60–115% for counter flow configurations. The performances also increase approximately 80–160% for parallel flow and 140–220% for counter flow compared with those of pure dowtherm A which is the common working fluid at high temperature applications.     

Numerical study of chemical reaction and heat transfer of MHD slip flow with Joule heating and Soret–Dufour effect over an exponentially stretching sheet

A study of Soret–Dufour effects along with chemical reaction, viscous dissipation combining on MHD Joule heating for viscous incompressible flow is presented. It is assumed that fluid is flowing past an angled stretching sheet saturated in porous means. The slip conditions of velocity, concentration, and temperature are accounted for at the boundary. The mathematical expression of the problem contains highly nonlinear interconnected partial differential equations. To convert governing equations into ordinary differential equations, appropriate similarity transformations were utilized. These differential equations with boundary constraints are resolved by homotopy analysis method. Expression for velocity, concentration, and temperature are derived in the form of series. Effects of numerous physical parameters, for example, Schmidt number, Soret number, buoyancy ratio parameter, slip parameter, and so forth, on various flow characteristics are presented through graphs. Numerous values of velocity, concentration, and temperature gradient are tabulated against different parameters. Results show that the fluid velocity increases by enhancing the Soret number, Dufour number, or permeability parameter. The fluid's concentration rises as the Soret number increases, while it falls as the Dufour number, chemical reaction parameter, or permeability parameter increases.     

Techno-economic analysis of H2 production processes using fluidized bed heat exchangers with steam reforming – Part 1: Oxygen carrier aided combustion

This work presents the techno-economic assessment for a new process where a fluidized bed heat exchanger (FBHE) is used as heat source for steam reforming in a hydrogen production plant. This suggested process configuration is compared with a reference case representing a conventional steam methane reforming (SMR) large-scale hydrogen production plant. The use of a FBHE as a heat source for the endothermic reforming is an advantage because of the high heat transfer coefficient to the reformer tubes. The suggested process configuration utilizes oxygen carrier particles as bed material and a bubbling fluidized bed reactor with immersed reformer tubes to ensure sufficient heat production for the reforming and improved heat transfer to the reformer tubes compared a conventional plant. The results include a comparison of hydrogen production efficiency and levelized production costs (LCOH) of the two plants where the production efficiency is more than 11% higher and the LCOH is more than 7% lower for the suggested process configuration.     

Solar radiative heat-driven Sakiadis flow of a dusty nanoliquid with Brownian motion and an exponential space-based heat source: Koo–Kleinstreuer–Li (KKL) model

The advancement of heat transportation is a significant phenomenon in nuclear reactors, solar collectors, heat exchangers, and electronic coolers; and it can be accomplished by choosing a nanofluid as the functional fluid. Nanofluids have improved thermophysical properties, due to their great progress in engineering and industrial applications. Therefore here, the significance of exponential space-related heat source (ESHS) on radiative heat motivated Sakiadis two-phase flow over a moving plate is analyzed for a particulate nanoliquid (CuO–H₂O). The impact of the haphazard motion of nanoparticles is analyzed through the Koo–Kleinstreuer–Li model. On applying a similarity transformation to the governing equations, a set of ordinary differential equations is obtained and numerically solved. Through the perception of graphs, the behavior of the velocity and temperature constraints for diverse values of effective parameters is decoded. The results show that the temperature of both phases (dust and fluid) improves with the ESHS aspect. Also, the heat transport rate/friction factor enhances/declines with the concentration of dust particles.     

Zero-dimensional single zone engine modeling of an SI engine fuelled with methane and methane-hydrogen blend using single and double Wiebe Function: A comparative study

Combustion modeling plays a key role in an engine simulation to predict in-cylinder pressure development and engine performance with a high level accuracy. Wiebe function, representing mass fraction burned (MFB) as a function of crank angle position, is widely used to predict the combustion process. The work presents a predictive zero-dimensional (Zero-D) single zone engine modeling of an SI engine fuelled with methane and methane-hydrogen blend. In this work, the single and double forms of Wiebe function were used to estimate the combustion process in the modeling. For this purpose, the single and double-Wiebe functions' parameters were calculated using the least squares method by fitting to the MFB curves calculated from experimental pressure data. These Wiebe functions were, then, introduced to the Zero-D single zone engine model developed for the methane and methane-hydrogen blend fueled SI engine to obtain in-cylinder pressure development and gross indicated mean effective pressure (GIMEP) for the engine performance prediction. The results show that the model with double-Wiebe Function fit better than that with single-Wiebe function. In addition, the fitted double-Wiebe function has a significant improvement in the GIMEP prediction for methane-hydrogen blend fueled SI engine modeling rather than the methane-fueled modeling.     

Hydrolysis of CuCl2 in the Cu–Cl thermochemical cycle for hydrogen production: Experimental studies using a spray reactor with an ultrasonic atomizer

The Cu–Cl thermochemical cycle is being developed as a hydrogen production method. Prior proof-of-concept experimental work has shown that the chemistry is viable while preliminary modeling has shown that the efficiency and cost of hydrogen production have the potential to meet DOE's targets. However, the mechanisms of CuCl₂ hydrolysis, an important step in the Cu–Cl cycle, are not fully understood. Although the stoichiometry of the hydrolysis reaction, 2CuCl₂ + H₂O ↔ Cu₂OCl₂ + 2HCl, indicates a necessary steam-to-CuCl₂ molar ratio of 0.5, a ratio as high as 23 has been typically required to obtain near 100% conversion of the CuCl₂ to the desired products at atmospheric pressure. It is highly desirable to conduct this reaction with less excess steam to improve the process efficiency. Per Le Chatelier's Principle and according to the available equilibrium-based model, the needed amount of steam can be decreased by conducting the hydrolysis reaction at a reduced pressure. In the present work, the experimental setup was modified to allow CuCl₂ hydrolysis in the pressure range of 0.4–1 atm. Chemical and XRD analyses of the product compositions revealed the optimal steam-to-CuCl₂ molar ratio to be 20–23 at 1 atm pressure. The experiments at 0.4 atm and 0.7 atm showed that it is possible to lower the steam-to-CuCl₂ molar ratio to 15, while still obtaining good yields of the desired products. An important effect of running the reaction at reduced pressure is the significant decrease of CuCl concentration in the solid products, which was not predicted by prior modeling. Possible explanations based on kinetics and residence times are suggested.