| Energy is one of the foundations of human socioeconomic development, but the energy problem becomes more and more prominent along with the society development. As one of the most important energy consumption departments, building energy consumption accounts for about a third of world primary energy consumption, building energy conservation is therefore imperative. It is reported that the energy use of heating ventilating and air conditioning(HVAC) system is an important part for the building energy consumption. Thus, the energy conservation of HVAC system is of importance for saving energy for buildings. From the point of exergy, saving energy is to reduce the use of the available part of energy-exergy. In this dissertation, recent research of building energy system is reviewed and summarized first. And then, the indoor environment is proposed as the reference environment for the exergy analysis of building energy system. The thermodynamic model of building envelope is developed accordingly. The impact of main building envelope parameters is studied to provide theoretical guidance for building design, especially for passive building energy-saving technology. Based on the analysis of thermal load of the fresh air, the energy recovery ventilator based on the thermoelectric effect is proposed to reduce its thermal load. The thermodynamic model is developed. And then, the experiments are conducted for the thermoelectric ventilator. The advanced exergy analysis method is introduced to analyze the thermodynamic performance of HVAC system. A general model is proposed for advanced exergy analysis of HVAC system. The HVAC system of a reference office building is studied with the advanced exergy analysis method to optimize the HVAC system. Some key points come as follows:(1)The concepts and methods of conventional exergy analysis are discussed in details followed by the regular method for the exergy analysis of building energy system. Then, the limitations of the conventional exergy analysis are discussed. The advanced exergy analysis method is introduced by providing its concept and method. These methods and concepts are the theoretical foundation of the thermodynamic analysis of building energy system.(2)For the exergy analysis of building energy system, the outdoor environment is usually selected as the reference environment. However, there are the limitations, which somehow block the development of exergy analysis. The exergy of moist air is taken as an example to illustrate the impact of time and location on the results. Moreover, a typical air heating process is analyzed with the outdoor environment as the reference environment to illustrate the limitations. The results show that the outdoor environment varies with time and location, which leads to the results complex and incomparable. Based on the analysis of energy flows of building energy system, the indoor environment can be taken as the final state for all the energy flows of building energy system. Considering its relative stability, the indoor environment is proposed to be taken as the reference environment for the exergy analysis of building energy system.(3)Based on the energy and exergy analysis method, the thermodynamic analysis model for building envelopes is developed. A reference building located in Changsha is taken as an example to study the thermodynamic performance. The main parameters, e.g., heat transfer coefficient of the external wall and window, window wall ratio, are studied to evaluate their impacts on the building thermal performance. Then, the reference building is assumed as the buildings in the five typical cities in the main climate zones to evaluate the impacts of climate condition on the building thermal performance. The results show that the exergy value of the building thermal load is relatively low compared to its energy value. This means the actual exergy demand of the building thermal load is small. However, it is always met by HVAC system by consuming the high quality energy resources. The low quality energy resources should be introduced in order to improve the overall system efficiency. The parameters of building envelope have great impacts on the building thermal performance, which should be optimized considering both the annual heating and cooling load. For the components of building thermal load, large exergy destruction is found for the thermal disturbance into and out of the building envelope, indicating the availability of the thermal disturbances. The solar radiation has the largest exergy destruction, suggesting that it should be utilized properly. The latent exergy load cannot be disregarded for exergy analysis of building thermal load and energy flows, especially for the building in humid climate. The energy and exergy analysis of thermodynamic performance of building envelope provides the theoretical guidance of building design and analysis.(4)The ventilation load of the reference buildings in different typical cities in the main climate zones is analyzed to explore the impact of ventilation load on the HVAC system load. It is conclude that the ventilation load is important for the HVAC system load for all the cities. To reduce the ventilation load, a novel ventilator based on the thermoelectric effect, especially for the small size HVAC system, is proposed in this dissertation. The thermodynamic model is first developed for the thermoelectric ventilator. And then, a series of experiments are conducted to test its performance. The results show that the thermoelectric ventilator is able to provide sufficient energy for fresh air handling and heat recovery from exhaust air. From the energy analysis perspective, it works efficiently. However, the maximum exergy efficiency is 11.04% for the summer mode and 5.06% for the winter mode, respectively. According to the exergy analysis, the largest exergy destruction occurs during the process of energy transfer inside the thermoelectric modules(TEMs). Further study should focus on reducing the working current, improving the performance of TEMs, and increasing heat transfer efficiency of the heat exchangers, especially for the hot side of TEMs. It is concluded that the thermoelectric ventilator is adequate for the mild weather, e.g., mild region in China, transition seasons.(5)The general model for advance exergy analysis of HVAC system is developed by introducing the advanced exergy analysis method. The exergy match ratio(M) is proposed to evaluate the exergy mismatch between building thermal load and HVAC system energy use. An office building is taken as an example to evaluate its HVAC system. The exergy match ratio is 10.75 for the heating season and 54.15 for the cooling season, respectively. The mismatch between building thermal load and HVAC energy use is very large, and some low quality energy resource should be introduced to improve the exergy match ratio. From the conventional exergy analysis, the system exergy efficiency is 6.18% for the heating season and 3.19% for the cooling season, respectively. The largest exergy destruction occurs in the air-cooled heat pump, which is 15.11 MWh for the heating season and 129.23 MWh for the cooling season, respectively. However, the advanced exergy analysis results suggest that a large part of the largest exergy destruction in the heat pump is unavoidable exergy destruction. The avoidable exergy destruction of the heat pump is 5.52 MWh for the heating season and 38.67 MWh for the cooling season. Moreover, the avoidable exergy destruction should be further divided into endogenous and exogenous avoidable parts to address the source of exergy destruction. The results also indicate that the exogenous available part of exergy destruction is larger than the endogenous available exergy destruction. Attentions should be paid both air source heat pump and other parts of the HVAC system simultaneously to improve the overall energy use efficiency. The modified exergy efficiency of the HVAC system is 20.47% for the heating season and 13.80% for the cooling season, which reveal the real efficiency of the system. |
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