Numerical simulation of enhanced heat transfer of PVT unit under condensation and cooling conditions

Abstract

In order to cope with energy related sustainability issues, it is urgent to reduce building energy consumption. Developing and using clean energy is an effective way to solve this problem. In recent years, solar photovoltaic thermal (PVT) heat pump system is being developed at the rapid pace. It can provide a variety of energy and resources such as heat, electricity, cold and domestic hot water, while realizing the conversion and utilization of a variety of renewable energy. With research advancement, a novel of Photovoltaic Thermal (PVT) heat pump system that can operate both during day and night in summer is currently under development. In the daytime, PVT heat pump system uses PVT unit as evaporator, that can not only generate electricity and heat, but also effectively cool the solar cells, so that the temperature of the solar cells can be maintained in the range of high power generation and therefore, improvement in the power generation efficiency of the system is achieved. At night, the PVT unit will act as a condenser to release the condensation heat of the heat pump system in the form of long wave cold radiation and natural convection of the surrounding air. The heat pump system will realize the night cooling process to produce the chilled water or ice (ice storage) required by the building. Specifically, the PVT unit is composed of a roll bond heat exchange plate added behind the PV unit. A serpentine refrigerant channel is arranged inside the inflatable heat exchange plate. In daytime, the refrigerant vapor-liquid two-phase fluid flows into the inflatable heat exchanger plate, evaporates in its serpentine channel and then flows out. At night, the refrigerant vapor flows into the inflatable heat exchanger plate, cools, condenses and releases condensation heat in its serpentine channel, and then flows out. In the process, the condensation heat of the PVT heat pump system is continuously released to the ambient and the sky by convection, radiation and conduction through different layer materials of the PVT unit. Therefore, the heat released by the PVT unit under condensation condition will directly affect the cooling capacity of PVT heat pump system at night. The condensation and heat release characteristics of PVT unit are closely related to the shape of serpentine channel structure, the shape of surface structure, the flow and heat transfer state of refrigerant in channel, the thermal conductivity between the plate and PV unit, and the sky effective temperature. In order to improve the night cooling performance of PVT heat pump system, it is necessary to study the condensation and heat transfer characteristics of PVT units at night in summer. In addition, PVT units can participate in the heat exchange of the system in the form of the above condenser at night in summer, and can also realize the natural cooling operation with the sky and air in the form of radiation heat exchange cooler alone. At this time, the working medium flowing through the PVT channel can be water, glycol solution and nano fluid. This sensible heat Numerical Simulation of Enhanced Heat Transfer of PVT Unit Under Condensation and Cooling Conditions - II - exothermic cooling technology has been applied in building roof cooling systems, its cooling and heat transfer performance also needs to be further improved. Therefore, this thesis uses numerical simulation method to study the condensation and cooling enhanced heat transfer characteristics of PVT unit at night in summer, in order to optimize the design of PVT unit channel and improve its heat transfer performance. In order to achieve this goal, this paper has carried out numerical modeling and simulation research in ANSYS fluent 18 on four types of PVT units: the existing serpentine channel PVT unit condenser (model 1), the serpentine channel PVT unit condenser with fins (model 2), the PVT unit condenser with hexagonal grid channel coupled with fluid channel with serpentine arrangement (model 3), and the PVT unit with water-Al3O2/Ag as the fluid medium (model 4). The purpose of this thesis is to develop and propose alternative methods to enhance the heat transfer characteristics of PVT units. Therefore, firstly, the existing system experimental for studying PVT unit condenser is introduced in details and then followed the description of other used representative numerical models of PVT units. For the sake of simplicity and less computational time, the models are scaled down before being solved. In addition, the mathematical equations of fluid heat transfer process in PVT unit materials are described in details. In the research process, the refrigerant used in the existing serpentine channel PVT unit condenser and the serpentine channel PVT unit condenser with fins is R407C, and the refrigerant used in the PVT unit condenser with hexagonal grid channel coupled with fluid channel with serpentine arrangement is R134a. Then, simulation assumptions and model verification are carried out to better solve mathematical equations. First, before simulation, this thesis studies and sets up appropriate model solving methods, operating conditions, assumptions and grid quality that can capture the solution. The accuracy of the model is determined by comparing the simulation results of this paper with the existing research results. In this process, the main parameters compared are performance coefficient, heat dissipation, outlet temperature, condenser surface temperature, temperature, speed and vapor volume fraction. Secondly, in the process of simulation, this thesis verifies the model. The verification model of the existing serpentine channel PVT unit condenser is helpful to determine the optimal size of the component channel structure and optimize the heat transfer characteristics of the PVT unit condenser. The influence of fins on the heat transfer characteristics and overall performance coefficient of PVT unit condenser is realized through the validation model of PVT unit with fins. In order to further find an alternative method to improve the night cooling performance of PVT unit condenser, a PVT unit condenser model integrating hexagonal grid channel coupled with fluid channel with serpentine arrangement is proposed and verified in this thesis. Furthermore, a model of nanofluid based PVT unit is solved to study the influence of using new fluid medium of nanofluid on the heat transfer characteristics of PVT unit under cooling condition. Dalian University of Technology Doctoral Dissertation - III - On the basis of validated models, this thesis conducts numerical simulation for the specific application conditions of various PVT unit condensers. First, in order to design and optimize the PVT unit condenser, model 1 is established and verified, and then the PVT unit condenser is designed and optimized. In the PVT unit condenser, the heat flux of the outer surface largely depends on the heat conduction through the material. Increasing/decreasing heat transfer through conduction will increase/decrease the heat dissipation flux, while the Nusselt number will decrease/increase. In this case, through the correlation between the maximum heat dissipation flux and the average fluid Nusselt number, the optimal size of the PVT unit can be obtained. The calculation results of the model show that, except the dimensions of inner diameter and outer diameter which increase by 0.36mm and 0.35mm respectively, the rest of structural parameters of PVT unit show reduction in size. Second, compared with the existing serpentine channel PVT unit condenser, the optimal size of the two channel spacing, transverse distance and longitudinal distance are reduced by 5.3mm, 137.76mm and 6 3mm. Secondly, in consideration to the new size, a representative model of PVT unit condenser with a size of 160.56mm x 388.56mm is established and solved, and the effects of different internal and external factors on heat transfer characteristics are analyzed. The calculation results of the model show that the heat dissipation flux of the optimized model is 12.5% higher than that of the existing model. The above research has important theoretical significance for determining the structure size of PVT unit condenser and predicting the heat transfer characteristics and overall performance of PVT unit condenser under different time lengths and specific weather conditions. In order to further improve the heat transfer characteristics and overall performance coefficient of PVT module condenser, fins and new channel structure are adopted in the existing serpentine channel PVT unit condenser. Under the experimental average condition, the numerical model of PVT unit condenser with four kinds of fins and ribs, including straight rectangle, straight triangle, rectangular pin and parabolic pin, was solved. For comparative analysis, this thesis derives from the results of other related research, and determines the reference cases of fin size and fin position parameters (width, height and angle), in which the fin structure parameters depend on the size and type of radiator. First of all, a comparative study of temperature distribution and heat dissipation is carried out, in order to determine the corresponding results as compared with those of the PVT unit condenser without finned serpentine channel, so as to determine the improvement effect of heat transfer characteristics. Secondly, based on the PVT unit condenser model with fins of straight rectangular and triangular, the heat dissipation and overall fin efficiency under different fin numbers are studied to determine the most suitable fin number. Finally, based on the optimal working condition model of PVT unit condenser with fins of straight rectangular and triangular, the effects of fin Numerical Simulation of Enhanced Heat Transfer of PVT Unit Under Condensation and Cooling Conditions - IV - size and fin position on the coefficient of performance, heat dissipation, total fin efficiency and pressure drop are studied. The results show that the heat dissipation and performance of the PVT unit condenser can be improved to 5.079 by installing heat sinks on the outer surface of the flow channel. In order to solve the PVT unit condenser model with hexagonal grid channel coupled with fluid channel with serpentine arrangement, volume of fluid (VOF) and k-𝜺 models are used to simulate the two-phase refrigerant R134a and deal with the turbulence problem during the phase change at different temperatures, respectively. The maximum coefficient of performance obtained is 4.66, which is 0.419 smaller than that obtained by the PVT unit condenser with fins. The above results show that the PVT unit condenser with fins and new channel structure can speed up the cooling process and meet the cooling load demand in a short time. In order to determine the influence of other fluids other than refrigerant on heat transfer characteristics, this thesis also uses ANSYS FLUENT software to solve a PVT unit cooler model with nanofluid under cooling conditions. Firstly, it is assumed that the two kinds of nanofluids in the tube are single-phase, incompressible laminar fluids. Secondly, PVT unit models of Al3O2water and Ag- water nanofluids were solved in steady state. Thirdly, under different nanoparticle fractions, weather conditions and flow coefficients, the research results are discussed in terms of Nusselt number, entropy generation (𝛥s), outlet temperature and coefficient of performance (COP). When the PVT unit operates with Ag-water as the fluid medium, the maximum coefficient of performance can reach 3. This means that in addition to the existing common working fluids, using nanofluids, especially Ag-water, has a great potential to improve the heat transfer characteristics and overall performance coefficient of PVT units under night cooling conditions. The research in this thesis provides numerical simulation methods and specific enhanced heat transfer methods for improving the condensation heat transfer and sensible heat cooling heat transfer performance of PVT units, which has important theoretical significance. It provides theory, method and data support for the development of new PVT units, and has important practical value.