Abstract
Ventilation and air conditioning systems are emerging as the major energy consumers in low energy buildings. The objective of this paper is to present new methodology for assessment of Air Handling Units (AHUs) taking into account the variations of reference temperature. The methodology using the concept of coenthalpy, developed for heat exchangers and published by the authors previously has been used. Four AHUs that comprise energy transfer devices, such as: Water-to-Air Heater (WAH), Heat Recovery Exchanger (HRE) and Heat Pump (HP) have been investigated. Thermodynamic parameters including Coefficient of Performance (COP), universal and functional exergy efficiencies have been used to compare AHUs and to calculate the exergy destruction in AHU components at variable environment temperature −30 °C… + 10 °C. The results of this study show that using HRE the COP and exergy efficiencies are significantly better compared with AHU without HRE. Using the HRE of higher effectiveness, the thermodynamic indicators can be improved considerably. The study shows that AHUs equipped with HP with advanced control method and HRE are more advantageous compared with other investigated AHUs.The presented methodology could have practical application for evaluating of energy and exergy efficiency of AHUs at different reference temperatures when designing HVAC systems and implementing optimum control methods.
Highlights
In recent decades, due to stricter energy efficiency requirements the thermal performance of buildings has been rapidly improving
From equation 1 it is seen that the Coefficient of Performance (COP) of Air Handling Units (AHUs) is calculated as the ratio of the heat flow rate and the exergy flow rate supplied to AHU components
In the second part of this section the exergy-based indicators: universal exergy efficiency and functional exergy efficiency are presented (Table 2). These indicators depend on the environment temperature, and one of the aims of this study is to present the methodology suitable for evaluation of AHUs using variable environment temperature
Summary
Due to stricter energy efficiency requirements the thermal performance of buildings has been rapidly improving. To save thermal energy in airtight and well insulated buildings the ventilation systems with heat recovery units are often recommended. Improvement of those systems is of vital importance in order to achieve better indoor climate and to reduce energy consumption in buildings [4,5]. To investigate Heating Ventilation and Air Conditioning (HVAC) systems, which can consist of different subsystems, a variety of methods can be used One of those methods is thermodynamic or exergy analysis. In an important study on low exergy buildings it “is recommended to use the (current) surrounding outdoor air as the reference environment for the exergy analysis of buildings and their energy supply systems” [12]. There is no recommendation in this study how to conduct the exergy analysis when the reference temperature is variable
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