Numerical And Experimental Study On Fresh Air Supply System With PVT Heat Pump For Buildings

Solar energy is seen as the most efficient source of renewable energy in the form of a photovoltaic(PV)and photo thermal(PT).In recent era,the photovoltaic thermal(PVT)heat pump system involves photovoltaic integration with a direct-expansion heat pump system and has a higher performance coefficient in water heating applications than conventional energy resources due to its high efficiency and energy-saving.Compared with other sectors,the building sector consumes a large portion of energy.Air conditioning consumes more than half of the total energy consumption of the buildings.The building envelope is considered a critical component in building energy reduction which takes about 20-50%of heating,ventilation,and air-conditioning energy.Ambient fresh air is an essential part of people's health and quality of air.The required energy for fresh air is 20-40%of heating/cooling and ventilation loads.The integration of the PVT heat pump technology into the building fa(?)ade,to full utilization of the PT generation and realization of the precooling/preheating of the ambient fresh air to reduce the traditional energy consumption of the building and ensure fresh air supply,is the key problem that needs to be solved by the deep development and utilization of solar energy which has important academic and engineering application value.Based on the above research background,this dissertation puts forward the fresh air supply system with PVT heat pump,the integrated design of the PVT unit and the building fa(?)ade constitutes the PVT fresh air chamber,and makes full use of the fresh air cooling and heating generated by the PVT unit during the operation of the fresh air supply system with PVT heat pump due to the evaporation or condensation heat transfer of the working refrigerant,to realize the cooling of fresh air to the building through proposed new PVT fresh air chamber.At the same time,the fresh air supply system with a PVT heat pump can also be used in a multi-energy generation for heating/cooling of fresh air,photovoltaic power generation,and hot water generation under different seasonal weather conditions throughout the year.To this end,the following research work is carried out by using the combination of CFD simulation and experimental research.First of all,this dissertation proposes and establishes the physical model of the fresh air supply system with PVT heat pump and the physical model of the new PVT fresh air chamber which combines the integrated PVT fresh air of the PVT unit and the building facade system.in which the refrigerant evaporation and condensation effect inside the PVT fresh air chamber to fully achieve the cooling and heating process of the airflow and heat transfer processes in the new PVT fresh air chamber.To further study the physical and structural layout of the PVT fresh air chamber,this study establishes a mathematical model of the PVT fresh air chamber for the CFD simulation to facilitate the simulation analysis and given boundary conditions of the inner surface of the PVT fresh air chamber to the characteristics of the refrigerant heat transfer process.Furthermore,a numerical simulation study is carried out on flow and heat transfer characteristics.The natural cooling and heating of the fresh air in the PVT fresh air chamber is analyzed through the influence of the different inner surface temperature of the PVT unit under natural and forced convection mode on the airflow and heat exchange processes.The ambient fresh air enters the PVT fresh air chamber through the openings at the lower ends of the chamber,enables the cooling of fresh air in summer and heating in winter.For photovoltaic condenser part of a fresh air system,1,the flow rate ranged between 0.07 kg/s and 0.08 kg/s,and the temperature rise was from 6.3-8.4?.2,for the photovoltaic evaporator part of a fresh air system,the flow rate varied from 0.048 kg/s to 0.061 kg/s,and the fall of temperature was from 7.1-4.5?.3,forced mass flowrate,the temperature fall varies from 6-9?,while the temperature rise varies from 4-10?.Moreover,the proper chamber layout of the PVT fresh air chamber is determined.The above results not only provide the characteristics of the airflow heat exchange in PVT fresh air chamber in winter and summer,but also provide an important reference for the design and implementation of the fresh air supply system of the PVT heat pump for the proposed building,and the development and utilization of the PVT fresh air chamber with a heat pump as cooling and heating of ambient fresh air through fa(?)ade of the building.Secondly,based on the results of the above numerical simulation,this dissertation proposes the first of its kind a fresh air roll-bond photovoltaic thermal panels cooled by a heat pump system at building fa(?)ade to generate the cooled fresh air.A supply fan was employed to control the air velocity at the outlet of a PVT fresh air chamber.The experiment facility mainly consists of the PVT fresh air chamber,heat pump unit,air distribution system to building,and data monitoring system.The air chamber was designed to supply fresh air to 12 person in the building of 36.4 m2 which requires 20-30 m3/h of fresh air to be provided to one person(design air volume of 360 m3/h).The photovoltaic thermal system was built on the south fa(?)ade of an office building at 6th floor of Institute of Building Energy in Dalian University of Technology,China,having an air chamber at the rear of the photovoltaic thermal panels.The experiment shows the variation in cooling fresh air temperature under varying air velocity at the outlet of the PVT fresh air chamber.1,the temperature fall was about 4.0-10.27? at 0.6 m/s of air velocity(corresponds to air volume of 345 m3/h).2,the average fresh air COP during a typical summer test day was 0.88.3,the average air velocity of 0.4 m/s tends to increase the temperature fall but decreases the flowrate.4,the average electrical efficiency is 10.16%during sunny testing day while during cloudy period the electrical efficiency decreases to about 9.6%.Moreover,the cooling exchange efficiency of cooling fresh air through proposed air chamber varies from 50-91%.While,the maximum power consumption of compressor and fan during July testing period were 0.99 kW and 0.02 kW,respectively.Thirdly,this dissertation focuses on the photovoltaic thermal unit works as evaporator was used externally on an office building's southern fa(?)ade in a multi-energy generation.In this chapter,the dissertation extends the application of roll-bond PVT unit assisted by heat pump system that serves the function of precooled fresh air,hot water,and power generation during the summer.The heat balance based on the energy conservation principle for each component of PVT fresh air chamber is proposed.1,the average temperature of external PVT skin and rear PVT skin was 26? and 24.4?.The average air chamber temperature was 21.7? and the inner panel temperature was 23.2?.2,the average electrical efficiency of the PVT was 8.27%.For cooling the fresh air:the average value of useful cooling gain was 291 W during a typical test day along with temperature fall of 4-5?.3,for heating the water,the system takes about 30 minutes to heat the 60 L water tank up to 55? and its maximum COP was found 6.0.The average power consumption for the fan and compressor during summer were 0.02 kW and 0.78 kW,respectively.Fourthly,this dissertation focuses on the photovoltaic thermal panels for the preheating of fresh air and hot water production during winter during heat pump operation.1,the average temperature of external PVT skin and rear PVT surface temperatures were 34? and 33.4?.The average air temperature in the PVT fresh air chamber was 15.2?.While the inner panel temperature was 13.7?.2,the average thermal efficiency and PV efficiency during a typical test day were about 26%and 8.1%,respectively.The heating gain of fresh air is 2 kW while the temperature rise is 11.7-22.6?.3,as the outermost skin,the PVT unit works as evaporator and was used for water heating purpose:it was found that the maximum COP was 5.8 and the system takes about 92 minutes to heat the 60 L water tank up to 55?.The average power consumption for the fan and compressor during winter were 0.012 kW and 0.21 kW,respectively.This dissertation presents the use of wasted cooling gain from the roll-bond PVT units.Firstly,a numerical analysis considered the effect of condensing and evaporating temperatures on the flow rate and heat transfer phenomena in the configuration of double skin facades to heat or cool fresh ambient air under the natural ventilation scheme.Secondly,this study constructed the PVT units cooled by a refrigeration system with an air chamber to pre-cool ambient fresh air and produce PV electricity.Thirdly,the experimental investigation of a photovoltaic thermal fresh air heat pump system in a multi-energy generation was carried out via our proposed photovoltaic thermal fresh air heat pump system in terms of pre-cooled fresh air in the summer,hot water and PV electricity.Fourthly,the proposed system was utilized in multi-energy generation during winter for pre-heated fresh air,hot water generation,and PV electricity.The photovoltaic heat pump distributed fresh air system proposed in this dissertation provides a new idea for the deep development and utilization of renewable energy in the field of buildings to reduce the cold fresh air load and to realize the multi-energy co-production of heat,electricity,cooling and fresh air,in the hot areas of office and residential building fresh air pre-cooling and PV power generation performance,as well as in the cold areas of the building fresh air pre-heating and hot water production,etc.,it has a broad application ct in engineering.