Research On The Building Heating And Cooling System With Exergy Analysis

Productivity development and human life level improving are based on energy sources, theshortage of energy sources prompt people to be aware of the importance of energy efficiency.But answering what is energy efficiency is not so simple. The first law of thermodynamicscan not answer this question. The energy efficiency is essentially "exergy" efficiency from thevalue of energy usage perspective. The method based on exergy analysis is proposed from thebuilding envelope to the energy conversion system of the building energy system. Thesensitivity of the exergy analysis results with environment reference state is discussed indetail. The main content is outlined as follows.First, based on the meteorological data of27cities in China, the exergy analysis resultswith the dynamic and static environment reference state is researched. The results show thatthe chemical exergy of moist air can not be ignored in hot and humid climate of the airconditioning period through exergy analysis of air cooling process. The exergy result errorsbetween the static and dynamic state is small. So the average outdoor temperature andhumidity can be used to calculate. For most of the cities in cooling season, the chemicalexergy errors between dynamic and static state is small. So the chemical exergy of moist aircan be calculated with average outdoor humidity. In heating period, the exergy analysis resulterror between the two environment states is small. So the outdoor average temperature can beused to calculate instead of dynamic hourly temperature as environment reference state.Second, the exergy flow rate is calculated from the power plant to the building envelopewith hourly outdoor temperature and humidity as reference state. The chemical exergy ofroom air is considered in cooling mode. Exergy method is applied to analyze the heating andcooling system of an example building. Results show that building envelop insulation isnecessary in the hot summer and cold winter area of China in reducing the building exergyconsumption. On the premise of envelop necessary insulation, the more effective energyefficiency solution of the building is to increase the air conditioner efficiency rather thanincrease the envelop insulation layer. The total year larger exergy loss rates take place in theprimary energy transformation and heating/cooling production, which is about80%of thesystem total exergy loss.Next, according to the energy flowing process of building heating and cooling system,including the building envelope, indoor air, terminal equipment, pipe network system, coldand heat sources and primary energy conversion, the exergy analysis theoretical model and various evaluation index in every stage are established. A case of an office building isanalyzed with the exergy theoretical model from the building envelope to energy conversion.The exergy flow rate, exergy loss, exergy loss rate and exergy efficiency in evergy stage arecalculated to evaluate the pros and cons of the different cases. The results show that the mostexergy loss stage is the primary energy conversion. The exergy loss rate of the cold sourcecases are from64.8%to92.2%, and the heat source cases are from64.6%to84.3%.In addition, the principle of cascade utilization energy sources should be considered. Thegreater the energy level difference, the greater the exergy loss. An example of1KJ cold andheat energy are analyzed. The results show that it should use the lower temperature hot waterin heating season, and minimize the temperature difference between supply and return water.It should be use the higher cold water in cooling season, and maximize the supply and returnwater temperature. So the efficiency and energy level equilibrium coefficient are optimal.Therefore, it should be use the cooling and heating medium close to the room temperature inthe terminal equipment system.Finally, a kind of exergy analysis combined with life cycle assessment method is presentedto evaluate the effect of the building envelope energy efficiency retrofit. A typical case of theexisting building envelope energy efficiency retrofit is analyzed. The results show that thecumulative saved energy consumption in the use stage is roughly1.75times larger than thetotal embodied energy of the newly added insulation materials in the product stage. Thecumulative saved exergy consumption in the use stage is only about1/7of the embodiedexergy of the newly added insulation materials in the product stage. For the existing buildingwith age, it should be prudent to take any retrofit measures; and it should take the energy andexergy efficiency and environment impact of the retrofit measures as a whole to analyze.