Thermodynamic Analysis Of HVAC Systems Based On Exergy Method

Energy saving and emission reduction has become common understanding ofmany countries, classes and groups as the relations between human being andresources and environment are getting tension. It is considered that development andapplication of new energy saving technology in HVAC systems which take up a greatproportion in building energy consumption is very important. Generally, HVACsystems are studied by energy analysis method, which is simple and easy tounderstand but is not an effecitive method to describe different irreversible losses.Thus, the exergy analysis method has attracted more and more attentions in nowadaysand obtained many achievements. However, the past researches on this filed areemphasized in concret systems, the performance of the HVAC systems in the meaningof thermodynamic limit is negnected and have some deficienceies in the application ofexergy analysis. Therefore, this paper conducted thorough research intobuoyance-driven ventilation, evaporative cooling process, air conditioning exergy load,ideal liquid dessicant cycles, ideal absorption refrigeration/heatpump cycles and idealabsorption-compression refrigeration/heat pump cycles in respects of exergy transfer,running conditions and impact factors of exergy efficiency by exergy analysis methodand empasized on the research in the meaning of thermodynamic limit so as tohighlight the university of results and provides a theoretical guidance for assessmentand improvement of real systems. The main research work and its conclusions areintroduced as follows:(1) The basic concepts of exergy analysis are introduced, as well as the dynamicdifferential equations in energy and exergy transfer process and their physicalmeanings, it introduced the general energy and exergy transfer equations, mainlydiscussing the differences and connections, to elucidate the theoretical foundation ofexergy analysis method used for HVAC systems.(2) A model of buoyancy-driven ventilation in a high building with an atrim isanalyzed in respects of exergy transfer, driving mechanism and exergy performance byexergy balance equations. It expouds the differences in buoyancy-driven ventilationand mechanical ventilation from energy and exergy concepts, analyzes the effect oftemperature difference between the atrim and the environment on exergy efficiency,points out the exergy optimization opportunity. Moreover, the ventilation systems driven by photothermal conversion and photoelectric conversion of solar energy arecompared on exergy efficiency with a result that the photoelectric conversion is moreefficient, providing a theoretical guidance for rational utilazation of solar-drivenventilation systems.(3) By analyzing the exergy compositions and physical meanings of moist airand water, it puts forward an expression of water exergy which involves water vaporexergy and latent heat exergy of water for relating the water exergy calculation withthe phase change process in air conditioning systems. Moreover, the relations betweenexergy of water, water vapor and latent heat on different temperature are studied andthree feature regions are outlined with different characteristics of exergy transfer.After analyzing the ideal direct and indirect evaporation cooling systems, it shows thatthere exist exergy optimization opportunities in both systems. The exergy transfer andthe effects of water-to-air ratio and thermal efficiency on exergy efficiency are alsodiscussed in detail by theoretical calculation for the counter-flow cooling tower.(4) The compositions and physical meanings of various air conditioning exergyloads have been investigated through exergy analysis, and the variations of differentair conditioning exergy load are studyed by dynamic energy-consuming simulationmethod. The differences and reasons between exergy load and air conditioning loadare analyzed, and the effects of air cooled mode and water cooled mode on airconditioning exergy loads are made. The method to evaluate the air conditioningprocess by exergy analysis based on independent temperature and humidity contry isput forward, and a primary return air system in summer condition is analyzed forexample, showing the application value of this independent evaluation method,promoting the development of temperature and humidity control technology.(5) An ideal liquid desiccant dehumidification system has been built based on theideal liquid desiccant cycle, in which the running conditions are divided, and theoperation characteristics and the exergy transfer under various running conditions arestudied which have realistic guiding significance to the operation of practical systems.It discusses the effects of dessicant temperature, the environment temperature and thewater vapor pressure on the exergy efficiency, and indicates that the efficiency ofliquid dessicant system has relative advantage only under certain conditions whencomparing with the condensation dessicant systems. A double-effect liquid dessicantsystem is further proposed, and corresponding exergy analysis is done. It shows thatthe newly presented system is always superiorious over the original system, providingpreliminary theoretical foundation for application of the double-effect system. (6) The ideal absorption refrigeration system, the ideal typeⅠ/typeⅡabsorptionheat pump systems based on the ideal liquid cycle are studied by exergy analysis, theexergy transfer, exergy balance and running conditions of these systems discussed, aswell as the effects of operation parameters, the exergy performance of these systemsare discussed from from the view of thermodynamic limit with a guiding significancefor further understanding, evaluating and improving real systems.(7) A study focusing the absorption-compression refrigeration/heat pump systemsin respects of exergy balance, running conditions and impact factors is carried out,which is expected to be universally significant for further understanding andevaluating practical systems. The typeⅡ absorption-compression double effect heatpump system, typeⅡ absorption-compression double temperature heat pump systemand typeⅡ absorption-compression double-temperature double effect heat pumpsystem are presented and analyzed from exergy concept, these systems enrich thetypes of the absorption-compression systems and the discusses lay a theritical basis forpractical applications.