Multi-temperature Zone Air Conditioning System and Optimization

多溫區空調系統及優化

Student thesis: Doctoral Thesis

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Award date28 Sep 2021

Abstract

Heating, ventilation and air conditioning (HVAC) systems account for approximately 25% of the total primary energy consumption worldwide. Therefore, the energy-saving of the HVAC system has drawn increasing attention. The HVAC system includes three sub-systems: heat source/sink, air handling unit (AHU), and terminal system. The heat source/sink consumes the largest part of the energy for the HVAC system. Conventional heat source/sink usually operates under a high-grade heat source/sink mode of a single-temperature zone, which results in the low efficiency of these systems. Reducing the demand for a high-grade heat source/sink is an efficient method for improving the system's performance. The AHU also plays an important role in the HVAC system. Conventional AHU works based on a high-grade heat source/sink of a single-temperature zone considering dehumidification and sanitary demand. The demand for the high-grade heat source/sink leads to low heat source/sink efficiency and large exergy destruction during the air handling process in the AHU. The terminal system determines the air distribution pattern, which is the connection between the indoor environment and the HVAC equipment, significantly affecting indoor environment quality and the system energy efficiency. Conventional terminal systems, such as the radiator, ceiling/floor radiator, split air conditioner, and fan coils, are hard to reach a balance between power consumption, thermal comfort, and indoor air quality. Based on the research gaps mentioned before, the novel HVAC system is worthy of investigation.

This thesis proposes an air conditioning system with multi-temperature zones, which satisfies heating demand in winter and cooling demand in summer, considers indoor air quality, thermal comfort, and minimizes system power consumption simultaneously.

The proposed system includes three parts. The first part is the multi-temperature zones heat source/sink. An ejector-compression multi-temperature air source heat pump (EDHP) system is developed in this part, which provides two kinds of heat sources/sinks at different temperatures under heating or cooling conditions. Theoretical models are established to analyze the energetic, exergetic, economic, and environmental (4E) performance of the proposed system. Comparisons with a conventional air-source compression heat pump (CDHP) are conducted. The effect of different parameter variations on the system's performance is conducted. The results indicate that the EDHP has significant advantages over the CDHP regarding the 4E indicators. The second part is the AHU. The conditioned air segmented handling process based on the multi-temperature heat source/sink is proposed in this part. The exergy loss and exergy efficiency in comparison with the conventional air handling process are conducted under different conditions. The result shows that the multi-temperature air handling process outperforms the conventional air handling process both for the cooling and heating mode. The third part is the terminal system significantly. The terminal system is the connection between the indoor room environment and HVAC equipment, which has various types. The terminal type presents a critical effect on the HVAC system. Under heating mode, two steps are conducted to investigate the performance of different terminal systems. The first step, the floor and ceiling heating (FH/CH), wall radiator heating (WRH), and stratum ventilation heating (SVH), are compared based on a typical sleeping environment. Performance indicators, including energy consumption, exergy consumption, energy-saving ratio, and exergy-saving ratio are analyzed under different conditions. The results show that SVH performs better than the conventional FH, CH, and WRH in terms of energy and exergy consumptions. The second step, for improving the heating performance, three kinds of terminal systems based on the multi-temperature zones heat source (warm air heating (WA), radiator heating with fresh air supply (RA + FA), and floor heating with fresh air supply (FL + FA) are investigated and compared. The detailed technical models of these heating systems are developed to investigate the system performance under different conditions. Under cooling mode, three ventilation methods are compared based on a sleeping environment, including mixing ventilation (MV), displacement ventilation (DV), stratum ventilation (SV). Based on computational fluid dynamics (CFD) technology, the energy and exergy efficiency of the three air distribution methods are conducted and compared. The results show that SV provides a nonuniform environment in both the vertical and horizontal directions in a typical sleeping environment with a higher energy and exergy efficiency than MV and DV. The results show that SV has a high energy-saving potential.

Based on the previous works on each part of these systems, the proposed system operation performance under different conditions is conducted. The performance indicators, including energy efficiency, exergy loss and efficiency, economic index, and environmental effect, are investigated and compared with those of a conventional system. To investigate the applied potential of the proposed system further, various ecofriendly refrigerants and yearly performance in different climate zones are conducted. The results demonstrate that the proposed system outperforms the conventional system under different conditions in the selected climate zones.

    Research areas

  • Multi-temperature zone, Air conditioning system, 4E performance, Performance optimization, Ejector- compression, Multi-temperature heat source/sink.