• Energy Research

Abstract The structure of photoreactors plays a key role in the photocatalysis by affecting the catalyst loading, mass and light transport properties. Nowadays it has become increasingly important to investigate the hydrodynamic and photocatalytic performances of photoreactors by computational fluid dynamic (CFD) method, thanks to its less cost and more steerable conditions. However, only a limited number of reports about CFD modeling of the packing bed photoreactors were presented because the unstructured substrates are difficult to reconstruct. In this paper, the bed generation technology and relevant hydrodynamic model, reaction kinetic model, and irradiation transport model of packing bed photoreactors are systematically reviewed and analyzed. The deficiency and the required modification of different CFD coupling strategies with the simulations of packing bed photoreactors are presented. The effects of temperature and local light intensity on the kinetic models and modified kinetic equations are summarized. This work may bridge the knowledge gap between the unstructured geometry generation technologies and the CFD simulations of packing bed photoreactors to promote the development/commercialization of the photocatalysis radically.

Abstract The structure of photoreactors plays a key role in the photocatalysis by affecting the catalyst loading, mass and light transport properties. Nowadays it has become increasingly important to investigate the hydrodynamic and photocatalytic performances of photoreactors by computational fluid dynamic (CFD) method, thanks to its less cost and more steerable conditions. However, only a limited number of reports about CFD modeling of the packing bed photoreactors were presented because the unstructured substrates are difficult to reconstruct. In this paper, the bed generation technology and relevant hydrodynamic model, reaction kinetic model, and irradiation transport model of packing bed photoreactors are systematically reviewed and analyzed. The deficiency and the required modification of different CFD coupling strategies with the simulations of packing bed photoreactors are presented. The effects of temperature and local light intensity on the kinetic models and modified kinetic equations are summarized. This work may bridge the knowledge gap between the unstructured geometry generation technologies and the CFD simulations of packing bed photoreactors to promote the development/commercialization of the photocatalysis radically.

Abstract Natural draft dry cooling system (NDDCs) of thermal power plants is susceptibly affected by ambient conditions. Moreover, the flexibility of cold-end system must be improved to satisfy load fluctuations in power grids caused by the rapid spread of power generation capacity from unpredictable renewable energy sources. To overcome the shortage of traditional cooling systems, a natural draft hybrid cooling system (NDHCs) with airside in series is proposed. Based on the control equations of flow, heat and mass transfer, an iterative algorithm is developed to predict the performance of NDHCs. Compared with the natural draft dry cooling system (NDDCs), natural draft wet cooling system (NDWCs) and recently proposed Pre-cooled NDDCs, the annual performances of NDHCs are investigated based on the practical weather data. The results indicate that NDHCs shows more priority in the areas rich in coal but short of water. Compared with the NDDCs, the operation cost of NDHCs drops 46.89 $/h. In terms of requirements for flexible operation, NDHCs can improve the dispatchability of power plants with varying the water mass flow rate ratio. With increasing water of the wet section, heat load varies about 48%. Moreover, NDHCs can avoid visible plumes and further pollution caused by the plumes effectively, since the saturated air is warmed by dry section and within the sub-saturated region.

Abstract Natural draft dry cooling system (NDDCs) of thermal power plants is susceptibly affected by ambient conditions. Moreover, the flexibility of cold-end system must be improved to satisfy load fluctuations in power grids caused by the rapid spread of power generation capacity from unpredictable renewable energy sources. To overcome the shortage of traditional cooling systems, a natural draft hybrid cooling system (NDHCs) with airside in series is proposed. Based on the control equations of flow, heat and mass transfer, an iterative algorithm is developed to predict the performance of NDHCs. Compared with the natural draft dry cooling system (NDDCs), natural draft wet cooling system (NDWCs) and recently proposed Pre-cooled NDDCs, the annual performances of NDHCs are investigated based on the practical weather data. The results indicate that NDHCs shows more priority in the areas rich in coal but short of water. Compared with the NDDCs, the operation cost of NDHCs drops 46.89 $/h. In terms of requirements for flexible operation, NDHCs can improve the dispatchability of power plants with varying the water mass flow rate ratio. With increasing water of the wet section, heat load varies about 48%. Moreover, NDHCs can avoid visible plumes and further pollution caused by the plumes effectively, since the saturated air is warmed by dry section and within the sub-saturated region.

Abstract Latent heat thermal energy storage is a promising option for efficient utilization of intermitted and instable energy. However, the intrinsically low thermal conductivity of phase-change materials (PCMs) is the major shortage, leading to low energy charging and discharging rate. An experimental setup was designed to investigate the dynamic thermal behavior of a shell-and-tube latent heat thermal storage unit. Paraffin wax was employed as the PCM, of which the thermal properties, including that of melting temperature and latent heat, were detected by Differential Scanning Calorimetry. In order to improve the heat transfer performance, copper foam and a bottom fin were compounded to the PCM. High temperature water flowed through the copper tube as the heat transfer fluid (HTF). The temperature variations of the selected detected points inside PCM and the solid–liquid interface evolution in axial plane of symmetry for three samples, including that of pure paraffin, paraffin–copper foam composites without and with the bottom fin, were recorded under the different heating temperatures and flow rates of HTF. The experimental results indicate that completely melting the PCM in composite takes over 1/3 less time than that of pure paraffin under the same operating conditions. The total charging time consumption of the composite with a bottom fin is the least, as well as the heat transfer rate is the largest among the three samples. The influence of HTF temperature on the charging process is more significant than that of the HTF flow rate.

Abstract Latent heat thermal energy storage is a promising option for efficient utilization of intermitted and instable energy. However, the intrinsically low thermal conductivity of phase-change materials (PCMs) is the major shortage, leading to low energy charging and discharging rate. An experimental setup was designed to investigate the dynamic thermal behavior of a shell-and-tube latent heat thermal storage unit. Paraffin wax was employed as the PCM, of which the thermal properties, including that of melting temperature and latent heat, were detected by Differential Scanning Calorimetry. In order to improve the heat transfer performance, copper foam and a bottom fin were compounded to the PCM. High temperature water flowed through the copper tube as the heat transfer fluid (HTF). The temperature variations of the selected detected points inside PCM and the solid–liquid interface evolution in axial plane of symmetry for three samples, including that of pure paraffin, paraffin–copper foam composites without and with the bottom fin, were recorded under the different heating temperatures and flow rates of HTF. The experimental results indicate that completely melting the PCM in composite takes over 1/3 less time than that of pure paraffin under the same operating conditions. The total charging time consumption of the composite with a bottom fin is the least, as well as the heat transfer rate is the largest among the three samples. The influence of HTF temperature on the charging process is more significant than that of the HTF flow rate.

The natural wind plays disadvantageous roles in the operation of air-cooled steam condensers in power plant. It is of use to take various measures against the adverse effect of wind for the performance improvement of air-cooled condensers. Based on representative 2×600 MW direct air-cooled power plant, three ways that can arrange and optimize the flow field of cooling air thus enhance the heat transfer of air-cooled condensers were proposed. The physical and mathematical models of air-cooled condensers with various flow leading measures were presented and the flow and temperature fields of cooling air were obtained by CFD simulation. The back pressures of turbine were calculated for different measures on the basis of the heat transfer model of air-cooled condensers. The results show that the performance of air-cooled condensers is improved thus the back pressure of turbine is lowered to some extent by taking measures against the adverse impact of natural wind.

The natural wind plays disadvantageous roles in the operation of air-cooled steam condensers in power plant. It is of use to take various measures against the adverse effect of wind for the performance improvement of air-cooled condensers. Based on representative 2×600 MW direct air-cooled power plant, three ways that can arrange and optimize the flow field of cooling air thus enhance the heat transfer of air-cooled condensers were proposed. The physical and mathematical models of air-cooled condensers with various flow leading measures were presented and the flow and temperature fields of cooling air were obtained by CFD simulation. The back pressures of turbine were calculated for different measures on the basis of the heat transfer model of air-cooled condensers. The results show that the performance of air-cooled condensers is improved thus the back pressure of turbine is lowered to some extent by taking measures against the adverse impact of natural wind.

Abstract The thermo-flow performances of conventional air-cooled condenser (ACC) using mechanical ventilation are basically susceptible to ambient winds due to its geometrical flaws, so more attentions have been paid to weakening such unfavorable effects, but the hybrid ventilation has never been considered. Based on representative 2 × 600 MW power generating units, two types of hybrid ventilation direct dry cooling systems (HVDDCS) utilizing the buoyancy force from the cooling tower, circular-type and rectangular-type, are developed. Furthermore, the thermo-flow performances in three wind directions of 0°, 45° and 90° are presented and compared with the conventional ACCs. The results show that the hot plume recirculation of the peripheral condenser cells for HVDDCS can be avoided, thus the inlet air temperature of air-cooled condensers is reduced. For circular HVDDCS, the reversed flows in upwind condenser cells are much weakened, leading to increased heat rejection and improved cooling performance in any case. In the wind direction of 0°, the rectangular HVDDCS shows a superior performance to those in the wind directions of 45° and 90°, so it is applicable to the region with a prevailing wind direction. The HVDDCS could be recommended for the potential engineering application thanks to its more energy efficient performance.