Abstract
The continuous energy transformation processes in heating, ventilation, and air conditioning systems of buildings are responsible for 36% of global final energy consumption. Tighter thermal insulation requirements for buildings have significantly reduced heat transfer losses. Unfortunately, this has little effect on energy demand for ventilation. On the basis of the First and the Second Law of Thermodynamics, the concepts of entropy and exergy are applied to the analysis of ventilation air handling unit (AHU) with a heat pump, in this paper. This study aims to develop a consistent approach for this purpose, taking into account the variations of reference temperature and temperatures of working fluids. An analytical investigation on entropy generation and exergy analysis are used, when exergy is determined by calculating coenthalpies and evaluating exergy flows and their directions. The results show that each component of the AHU has its individual character of generated entropy, destroyed exergy, and exergy efficiency variation. However, the evaporator of the heat pump and fans have unabated quantities of exergy destruction. The exergy efficiency of AHU decreases from 45–55% to 12–15% when outdoor air temperature is within the range of −30 to +10 °C, respectively. This helps to determine the conditions and components of improving the exergy efficiency of the AHU at variable real-world local climate conditions. The presented methodological approach could be used in the dynamic modelling software and contribute to a wider application of the Second Law of Thermodynamics in practice.
Highlights
Buildings are one of the largest global energy consumers, using about 36% of final energy and responsible for nearly 40% of energy-related CO2 emissions [1]
This paper presents the thermodynamic analysis of an air handling unit of a modern HVAC system
On the basis of the previously described methodical foundation we can obtain air handling unit (AHU) indicators relevant to HVAC design that are the focus of dynamic modelling, such as COPAHU (Equation (7)) or exergy efficiency ηex (Equation (8))
Summary
Buildings are one of the largest global energy consumers, using about 36% of final energy and responsible for nearly 40% of energy-related CO2 emissions [1]. When applying the SLT, several values that define degradation of energy are encountered, for example, entropy and exergy [4,5], and, more rarely, entransy [6,7] These indicators allow to demonstrate the true potential of a thermal system in terms of performance, they become important when analyzing and comparing energy systems that use various types of energy [8]. The use of this thermodynamic indicator still causes a lot of discussion and there exists several definitions of it [29,30,31] Both exergy analysis and entropy generation analysis allow to determine the most efficient process [14,18].
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