• Energy Research

Abstract The accurate prediction of heat transfer deterioration (HTD) is important to ensure the safe operation of scCO2 cycles driven by various heat sources. Here, the scCO2 heat transfer experiment is performed in a 10 mm diameter vertical tube, covering the ranges of pressures 7.51–21.1 MPa, mass fluxes 488–1500 kg/m2s and heat fluxes 43.7–488 kW/m2. Both uniform heating and non-uniform heating cases are dealt with, but more attention is paid on non-uniform heating. We show that non-uniform heating displays strong circumference angles dependent heat transfer characteristic. Normal heat transfer (NHT) displays gentle rise of wall temperatures along flow length, but for HTD, wall temperature peak is detected ahead of pseudo-critical point. Pseudo-boiling is introduced to deal with scCO2 heat transfer. Heat added to scCO2 is decoupled into a temperature rise part and a phase change part. Flow structure includes a vapor-like fluid near tube wall and a liquid-like fluid in tube core. The analogy between subcritical boiling and supercritical heat transfer results in a supercritical-boiling-number SBO to govern the vapor layer thickness. Sudden change from NHT to HTD is found when crossing a critical SBOcr, which is 5.126 × 10−4 for uniform heating based on our experimental data and other data in the literature, but becomes 8.908 × 10−4 for non-uniform heating using our experimental data. Compared to uniform heating, non-uniform heating is found to delay the occurrence of HTD. The criterion presented here is useful to avoid the occurrence of HTD in the design and operation of scCO2 cycles.

Abstract In this study, a chance-constrained two-stage fractional optimization (CTFO) method is proposed for planning regional energy systems in the province of British Columbia, Canada. Through simultaneously integrating two-stage stochastic programming (TSP), chance-constrained programming (CCP), and mixed-integer linear programming (MILP) techniques into a linear fractional programming (LFP) framework, CTFO can effectively tackle multiobjective and capacity-expansion issues, as well as uncertainties described as probability distributions in the constraints and objectives. Based on the developed CTFO method, a chance-constrained two-stage fractional regional energy model (CTFO-REM) is developed in this study for supporting energy management in the province of British Columbia. Conflicts between environmental protection that maximizes the renewable energy resource utilization and economic development that minimizes the system cost can be effectively addressed through the CTFO-REM model without setting a factor for each objective. The results also indicate that the CTFO-REM model can facilitate dynamic analysis of the interactions among efficiency, policy scenarios, economic cost, and system reliability.

Abstract Experiments of carbon dioxide flowing in a helical pipe at near-critical pressure were performed at constant heat flux boundary condition. The helical curvature diameter, helical pitch and tube diameter were 283.0 mm, 32.0 mm and 9.0 mm, respectively. The inlet Reynolds number was larger than 104 to ensure the turbulent flow. The renormalization group RNG k–e model simulated the three-dimensional turbulent heat transfer of CO2 in the helical pipe. Much attention was paid to the combined effects of the centrifugal force and buoyancy force on the heat transfer. The RNG k–e model reasonably simulates the complicated heat transfer. The wall temperatures near the tube exit were slightly over-predicted, due to the suppression of the increased wall temperatures near the tube exit by axial thermal conduction in the experiment. Before and near the pseudocritical temperature region, the varied physical properties caused significantly non-uniform velocity and temperature distributions over the tube cross section. The larger axial velocities appear at the outer-bottom location, and the higher wall temperatures appear at the inner-top location. Thus, the outer-bottom locations hold larger heat transfer coefficients. The turbulent kinetic energies are increased along the axial angles and larger in the inner-top region of the tube cross section. The effective viscosities are decreased along the axial angles, and the larger effective viscosities are shifted to the tube center with the axial flow development. Beyond the pseudocritical temperature region, the decreased buoyancy force suppressed the non-uniformity of the heat transfer coefficients over the tube circumference.

Abstract The test and analysis of an Organic Rankine Cycle (ORC) with R123 as the working fluid were presented in this paper. A scroll expander was integrated in the system to generate work. The expander was connected with an AC dynamometer unit, which was used to control and measure the expander shaft torque and rotating speed. The conductive oil simulated the low grade heat source. Operation characteristics were compared between the heat source temperatures of 140 °C and 160 °C. The experiments were conducted by adjusting two independent parameters: the pumping frequency of the R123 pump and the shaft torque of the expander. The former parameter was directly related to the R123 mass flow rate and the later to the external load. The optimum system performance can be determined by these two parameters. The maximum measured shaft power and thermal efficiency were 2.35 kW and 6.39% at the heat source temperature of 140 °C, but they were 3.25 kW and 5.12% at the heat source temperature of 160 °C. This study identified that the measured shaft power was about 15–20% lower than the enthalpy determined values, and the pumping power of the organic fluid was 2–4 times higher than the enthalpy determined values. The enthalpy determined values were based on the local pressure and temperature sensor measurements.

Abstract Metallic nanoparticle chains exhibit strong plasmon coupling in periodic arrays that provides numerous opportunities for manipulating heating at nanoscale. The energy transport in a chain is sensitive to the interparticle plasmon coupling. A better understanding of optical and heating properties of the chain would provide useful guidance for future design of the novel devices. In this work, we use the organization of gold nanoparticles as linear assemblies and perform a detailed analysis of the heating features as a function of the number of particle, interparticle spacing and particle size in the chains using General Mie Theory. The results show that the heat generation of particle presents an exponential decay with increasing of the interparticle spacing. As the number of particle increases, the particle spacing increases and radius decreases, the boundary effect is gradually diminished to the center particle. When the particle spacing is 1 nm, the number of particle is seven and the radius is 10 nm, the heat production of center particle is 1.65 times than that of a single particle. The large particle size effectively increases the cross-section of absorption as well as the range of electromagnetic radiation, and thus more light energy can be absorbed and converted into heat as the radius increases. This study provides an original understanding on the plasmon heating behavior of one-dimensional nanoparticle chains and may be generalized to other types of aggregates formed by large numbers of interacting particles.

Abstract The integration temperature difference Δ T i considers the heat transfer routes, linking the heat transfer process with the thermodynamic behavior of heat exchangers. The first and second non-dimensional integration temperature differences are defined as Δ T i, h ∗ = Δ T i / T h, i and Δ T i, s ∗ = Δ T i / ( T h, i - T 0 ) respectively, where T h,i is the heat source temperature and T 0 is the environment temperature. This paper is the first to experimentally verify the significance of the non-dimensional integration temperature differences on organic Rankine cycle (ORC) systems. The first non-dimensional temperature difference is shown to have linear relationship with the revised entropy generation numbers ( N s ). With increases of the second non-dimensional integration temperature difference, the expander powers, system thermal and exergy efficiencies had parabola distributions. They simultaneously reached maximum at Δ T i, s ∗ = 0.282 , under which the vapor cavitation in the expander disappears and the exergy losses of heat exchangers are acceptable to elevate the expander efficiency. Beyond the optimal point, the ORC performance is worsened either due to the vapor cavitation in the expander, or due to the poor thermal matches in the evaporator and condenser. The second non-dimensional integration temperature difference comprehensively reflects the effects of heat source temperatures, heating powers and organic fluid flow rates and pressures, etc. It balances exergy destructions of various components to optimize the system. Thus, it can be an important parameter index to maximize the power or electricity output for a specific heat source. The usefulness of the integration temperature difference and the future work are discussed in the end of this paper.

Abstract CO2 power cycle is one of the promising sustainable technologies that generate power from fossil fuel, nuclear, solar, and various waste heat energy. The heat transfer process plays a key role in influencing the thermodynamic or thermo-economic performance of CO2 power cycle. The drastic variation of CO2 thermo-physical properties results in great difficulty in modeling and optimizing the heat exchangers and CO2 cycle configurations. In the present study, a mathematical programming method is proposed for the simulation and optimization of simple, regenerative, and recompression super-critical CO2 power systems. An accurate equation of state is applied to calculate CO2 properties, thereby enabling the performance indicator a continuous function of system parameters. A multi-objective non-linear programming model is formulated in GAMS for the automatic heat exchange pinch locating and system optimization. The simulation and optimization models are validated by comparison with REFPROP-based method. The maximum and average simulation errors are 0.064% and 0.015%. The thermal efficiency values achieved using the developed optimization method are improved by 6.14–10.85% compared with previous solutions. Then, a case study of a 573.15 K waste heat driven super-critical CO2 power system optimization is elaborated. Results show that pinch locating of all heat exchangers and system optimization can be simultaneously conducted within a short time. The solution optimality is validated by comparing the traditional pinch assignment concept and optimization method. Sensitivity analyses of heat source inlet temperature and heat sink temperature rise are also conducted. For both objectives of maximizing thermal efficiency and net power output, the recompression super-critical CO2 cycle features highest in thermal efficiency and the regenerative super-critical CO2 cycle features highest in net power output. Finally, a multi-objective optimization is conducted to achieve the Pareto Front for the studied three super-critical CO2 power systems.

This paper reports condensation heat transfer in a horizontal tube with a 14.81 mm inside diameter and a 1200 mm heat transfer length, by using the phase separation concept. The hollow mesh cylinder was formed by packaging two layers of mesh screen surface. The outer layer had the pore size of 76 μm. This invention constructs a multiscale condenser tube. The modulation of stratified flow and annular flow was focused on. Liquid was held within the mesh cylinder to increase the vapor covered tube wall surface to enhance the heat transfer for stratified flows. For annular flows, liquid droplets in the vapor core were captured by the mesh screen surface to increase the vapor void fractions near the tube wall, enhancing the hat transfer. The measurements showed that the heat transfer coefficients could be more than two times of those in the bare tube. The total thermal resistance was decreased by 45.6%, maximally. The heat transfer enhancement ratios were increased with the vapor mass fluxes, Gxin, for the condensation flow along the whole tube length. A pressure analysis explained the increased heat transfer trend. The increment of heat transfer enhancement ratios was suppressed with a liquid flow part in the tube.

The design, construction, and characterization of a solar simulator are reported. The solar simulator consists of an optical system, a power source system, an air cooling system, a control system, and a calibration system. Seven xenon short-arc lamps were used, each consuming 10 kW electricity. The lamps were aligned at the reflector ellipsoidal axis. The stochastic Monte Carlo method analyzed the interactions between light rays and reflector surfaces as well as participating media. The seven lamps have a common focal plane. The focal plane diameters can be changed in the range of 60–120 mm with the lamp module traveling the distance in a range of 0–300 mm. The calibration process established a linear relationship between irradiant fluxes and grayscale values. The measures to reduce irradiant flux error and fluctuations were described. The irradiant flux distribution can be changed by varying the power capacities and/or moving the focal plane locations. The peak fluxes are 1.92, 3.16, and 3.91 MW/m2 for 25%, 50%, and 75% of the full power capacity. The peak flux and temperature exceed 4 MW/m2 and 2300 K, respectively, for the full power capacity. A 8 cm thick refractory brick can be melt in 2 min with the melting temperature of about 2300 K when the solar simulator is operating at 70% of the maximum power capacity.