Functional model and second law analysis method for energy efficient process design: applications in HVAC systems design

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Harutunian, Vigain

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A method is developed for the conceptual design phase of Heating, Ventilating and Air Conditioning (HVAC) systems design. The main objectives of the method are to document HVAC process flow decisions without bias from equipment offerings, to evaluate the potential energy efficiency of HVAC process flow decisions independent of equipment selection, and to develop guidelines to increase the energy efficiency (reduce required work input) of HVAC systems. Various functional and physical modeling methods are evaluated and requirements are generated for a new HVAC process flow model. A functional flow diagram is developed to document HVAC system process decisions, including intended component functions and the physical interactions between these functions independent of equipment selection. Process flows are quantified and exergy (2nd Law of Thermodynamics) analysis is invoked to calculate the maximum efficiency (minimum work input) of each alternate functional flow diagram independent of equipment selection. The method assumes the equipment has zero exergy destruction (no internal losses unless so prescribed by flow conditions) and thus does not include losses dependent on equipment selection. Thus, the method distinguishes between the losses associated with the functional flow (process) decisions and those associated with equipment selection. Equipment selection hence proceeds with the functional flow diagram having the highest energy efficiency potential. Furthermore, the equipment independent functional flow efficiency serves as an efficiency benchmark to evaluate alternate equipment selections for each functional flow diagram. Guidelines are developed based on exergy principles to increase HVAC system efficiency. The guidelines incorporate principles from exergy based design guidelines and current energy efficiency standards in engineering practice, such as ASHRAE 90.1 and others. Examples of heating, cooling and dehumidification systems are presented to illustrate the method. The examples illustrate the development of exergy flow schedules, exergy functional schedules, the evaluation of alternate designs and the implementation of the proposed exergy guidelines to improve system efficiency. Opportunities and future work required to apply the method in current HVAC design practice are discussed. Applications of this method to systems outside HVAC design are presented.