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[en] Highlights: • The energy output characteristics of the solar hybrid CCHP system are defined in a clear perspective. • The particle swarm optimization (PSO) algorithm is adopted to find the optimum design parameters. • The design features and the performance of solar hybrid systems under five different operation strategies are analyzed. • The comparison between the hybrid system and the conventional system is given. - Abstract: The hybridization between conventional combined cooling heating and power (CCHP) systems and solar systems has been considered as a good solution to the urgent energy and environment issues. This study develops the mathematical model of a CCHP system hybridized with PV panels and solar thermal collectors. The particle swarm optimization (PSO) algorithm is adopted to find the optimum values of design parameters. Based on the energy output characteristic of the solar hybrid CCHP system, five operation strategies of the conventional CCHP system are adjusted and applied for the solar hybrid CCHP system. The simulation work of the hybrid CCHP systems based upon a hotel building in Atlanta is carried out to find an appropriate design scheme. The results show that the hybrid CCHP system under the FEL-ECR mode is the best choice. Besides, its PESR, CO2ERR and ATCSR can reach 36.15%, 53.73% and 4.16%, respectively. Compared with a conventional CCHP system, the hybrid CCHP system achieves better energy-saving and CO2 reduction performance. However, the hybrid CCHP system consumes more annual total costs because of its high initial investment.
[en] Highlights: • A 3D model couples flow and heat transfer processes of DHE, wellbore and reservoir. • The model is validated against experimental data with a maximum error of 8.3%. • The entire temperature and flow fields of DHE system is analyzed comprehensively. • Performances of single U-tube, double U-tube and spiral tube are compared. • Effects of key factors on heat extraction performance of DHE system are studied. - Abstract: The downhole heat exchanger (DHE) geothermal system is commonly used to exploit geothermal energy for space heating. In this paper, a 3D unsteady state numerical model is established to couple fluid flow and heat transfer processes of DHE system. The model is validated by field experimental data. Temperature and velocity fields are analyzed to understand thermal process of DHE system. Heat extraction performances of three different DHE structures, including single U-tube, double U-tube and spiral tube, are compared. Subsequently, cases are studied to investigate how key parameters affect DHE performance. Simulation results depict that spiral-tube has the best heat extraction performance. As working fluid mass flow rate rises, outlet temperature declines and thermal power increases. When inlet temperature ascends, outlet temperature rises while thermal power decreases. Effects of reservoir porosity and tube wall heat conductivity on DHE performance are minor. Higher subsurface water velocity and larger rock heat conductivity can improve DHE performance, but the former has a more significant influence. Besides, subsurface water flow direction has neglected influence on performances of single and double U-tube, but appreciable impact on that of spiral tube. Key findings of this work are beneficial for optimal design and optimization of DHE geothermal system.
[en] Highlights: • We develop a direct passage arrangement method for multistream plate fin heat exchangers. • An improvement of passage arrangement with good synergistic effect is proposed. • A symmetry arrangement method is first presented to arrange passages directly. • Effectiveness of this method is validated by three evaluation means. • This method performs better than the existing optimization methods. - Abstract: Passage arrangement quality significantly affects the performance of multistream plate fin heat exchanger (MPFHEs) because a bad arrangement may result in uneven temperature difference field and pressure field between passages and thus reduce the thermal efficiency. However, it is very difficult to design an effective passage arrangement owing to large numbers of possible passage arrangement patterns and complex heat transfer processes between the passages in a MPFHE. This work develops a direct passage arrangement method for MPFHEs to address this problem. In this method, an improvement of passage arrangement with good synergistic heat transfer effect is first proposed. The determination of passage quantity for each fluid is suggested to be proportional to its design heat load. Next, based on the results of the checking calculation, the fin and heat exchanger structure parameters should be adjusted until the constraints including thermal effectiveness, length deviation and pressure drops have been satisfied. Afterwards, the passages are arranged directly by a symmetry arrangement method. To evaluate the effectiveness of this method, three different industrial cases are performed and compared with the existing optimization designs by three evaluation means. The validation results indicate that this method performs better than the complex optimization methods.
[en] Highlights: • A new design of hybrid loop heat pipe using pump assistance. • Pump was only activated when the dry-out will took place. • When the pump turned on, it will successful to prevent the dry-out. - Abstract: A loop heat pipe (LHP) is one of the two-phase cooling technologies used in passive cooling systems. The LHP is an efficient heat transfer device, but its extreme power density can cause dry-out at the evaporator. Many researchers have predicted that passive devices will not be able to meet future cooling challenges because of this limitation. The objective of this research is to design a modified LHP that overcomes the dry-out problem by adding a diaphragm pump to accelerate fluid transportation (called a hybrid loop heat pipe, HLHP). The pump installed on the liquid line is coupled with a reservoir. The developed HLHP works passively using wick capillary pressure when there is no sign of dry-out. When dry-out occurs, the pump is activated via diaphragm pumping and has a temperature controller. Thus, the working fluid is circulated by both the capillary force and driving force of the diaphragm pump during the heat-transfer process. The operating characteristics of the HLHP under a variety of heat load supply and low-power start-up conditions have been investigated. The experimental results indicate that the installation of a diaphragm pump in a modified LHP system can prevent the occurrence of dry-out in the evaporator and significantly reduce the evaporator temperature.
[en] Highlights: • Possibility of integrating thermoelectric generators with Kalina cycle is investigated. • The proposed system performance is compared with the conventional Kalina cycle. • Performance enhancement of 7.3% is indicated at a typical operating condition. • Economic evaluation indicated the conditions under which the system is profitable. - Abstract: The Kalina Cycle (KC) is one of the most promising options for power generation from renewable energy and low temperature heat sources such as geothermal energy. Also, employing thermoelectric generators (TEGs) is widely developed recently to convert heat into electricity directly. The possibility of employing thermoelectric generators to utilize the waste heat of a Kalina cycle is investigated in the present paper. The proposed system performance is modeled, analyzed and compared with the conventional Kalina cycle performance. To assess the systems’ performances, thermodynamic and economic models are developed and a parametric study is carried out. The results indicated an enhancement of around 7.3% for net output power and energy and exergy efficiencies for the proposed system as compared to the conventional Kalina cycle, at a typical operating condition. In addition, an economic evaluation of integrating thermoelectric generators with the Kalina cycle is conducted and the conditions are indicated under which the proposed system is profitable.
[en] Highlights: • Designed three lab-scale sensible heat storage (SHS) prototypes of 15 MJ capacity. • Heat transfer augmentation for concrete SHS prototypes was implemented. • Hi-tech Therm 60 was used as heat transfer fluid. • Thermo-physical properties of the selected concrete mixture were estimated. • Performance tests on the prototypes were conducted at various operating conditions. - Abstract: This paper presents the performance tests on lab-scale sensible heat storage (SHS) prototypes made up of cast steel and concrete. Thermal storage performances of the prototypes in terms of charging/discharging times and energy storage/discharge rates have been estimated at various operating temperatures and heat transfer fluid (HTF) flow rates. These prototypes were designed in the form of a shell-and-tube type heat exchanger with a heat storage capacity of 15 MJ. Five different concrete mix designs were studied and the mix design M30 was selected for thermal storage, as they possess high compressive strength-cost ratio. Heat transfer enhancement in the concrete prototypes was incorporated by welding longitudinal fins on the HTF tubes. Hi-tech Therm 60 was used as heat transfer fluid. The charging and discharging times of cast steel (M1) prototype in the temperature range of 353–413 K were 1263 and1803 s, respectively. The effective charging/discharging time of the concrete prototype with copper tubes (M2) and concrete prototype with MS tubes (M3) prototypes in the temperature range of 353–433 K were 5210/6297 s and 7160/7780 s, respectively. The storage performance of the system highly depends on the operating temperature range due to the temperature dependence of the thermo-physical properties of the SHS materials and the HTF.
[en] Highlights: • A design model is built for a radial-inflow turbine used for geothermal power generation. • Preliminary design with heat loss models is used with 3-D numerical simulation for optimization. • A scheme based on a genetic algorithm is devised to optimize turbine geometry. • Flow separation tends to disappear with increasing specific velocity. • Optimizing specific velocity and blade shape can increase power output up to 3.6%. - Abstract: A numerical model associated with preliminary design of a turbine for utilization of geothermal energy is developed. Considering the geothermal water flow at low/moderate temperature of the target area, Yilan (Taiwan), a radial-inflow turbine with a refrigerant, R134a, which has high density has been adopted in the study. A preliminary design method based on theoretical formulations, namely the mean-line approach, and an optimization scheme based on a genetic algorithm are used to create the optimal geometry of the turbine. This provides an input for three-dimensional (3-D) simulation of the flow field, in terms of the commercial software ANSYS CFX. The results of various physical features are compared with that of the preliminary design in order to identify the sources of some disagreement that have not been clarified for general 1-D analyses. Specifically, by changing the specific speed, we have found that there is a minimum value of total entropy increase throughout the passages. This enables us to conclude that the model of incidence loss in the preliminary analysis underestimates the loss that causes flow separation, which can be identified in the numerical simulation, therefore leading to their disagreement. The integration of the geometrical optimization via the preliminary design and the numerical simulation for the detailed flow properties can help us attain superior analysis and design. Consequently, it resulted in an increase of turbine power about 3.6% when the specific speed and blade shape were optimized in the tested ranges.
[en] Highlights: • A transient TLC technique is used to measure detailed Nusselt number distributions on the concave target surface. • The effects of the jet nozzle position (E/d) at different jet Reynolds number (Re) are investigated. • The mechanism of impingement heat transfer for different E/d is analyzed in detail. - Abstract: In this study, the flow and heat transfer characteristics of jet impingement on the leading edge of a turbine blade are experimentally and numerically investigated. A transient TLC technique is employed to acquire detailed heat transfer coefficient distributions on the concave target surface. The effect of tangential jet impingement on the impingement heat transfer is investigated by adjusting the jet nozzle position (E/d) off the centreline of the cavity. The jet Reynolds number (Re) varies from 12,000 to 20,000, and three different jet nozzle positions (E/d = 0, 0.5, 1.0) are adopted. The flow pattern is also discussed in detail to analyse the impingement heat transfer mechanism. The results show that the heat transfer enhancement of the tangential jet impingement depends on the jet nozzle position and jet Reynolds number. When the Re is less than 15,000, the tangential jet impingement can enhance the heat transfer on the target surface. The highest impingement heat transfer is obtained by E/d = 0.5. However, for Re = 20,000, the tangential jet impingement heat transfer decreases with the increase of E/d because the turbulent kinetic energy near the target surface decreases significantly.
[en] Highlights: • Latent energy storge in impure phase change material is studied. • Non-orthogonal curvilinear coordinate system is used. • A modified enthalpy-porosity approach is implemented. • The concentric oblate-shaped inner tube stores the maximum energy in LHTES unit. • Lower eccentric inner circular tube is more thermally efficient. - Abstract: For the storage of latent energy in an arbitrary-shaped double-pipe heat exchanger is considered in this study. The heat exchanger is numerically modeled considering the convective heat transfer boundary condition on the inner tube. The horizontal heat exchanger is composed of an insulated outer hexagonal tube and an inner tube. The commercial grade PCM which melts over a temperature range of 8.7 °C is placed in the annular gap between the two tubes. The flow rate and the inlet temperature of the heat transfer fluid (HTF) flowing through the inner tube are varied for the low-temperature solar energy storage. The cross-sectional shapes of the inner tube as well as the vertical position of the circular tube are varied, keeping the cross-sectional area of the annular gap constant. In order to correctly account for the arbitrary-shaped boundaries, a non-orthogonal boundary-fitted coordinate technique on a staggered grid system is used. A control volume based finite difference scheme is employed to solve the non-dimensional set of equations. The predicted results of the velocity and temperature fields, the surface-averaged Nusselt number on the heat transfer surface of the inner tube, the complete and total melt fractions, and the latent and total cumulative energy stored, all as a function of time are presented and discussed. The results show that the effect of the change of the bulk temperature is much more prominent on the storage of energy compared to the change in the mass flow rate of the HTF. For the identical conditions, the oblate-shaped inner tube stores the maximum amount of energy irrespective of the studied shapes and the vertical positions of the inner tube.