A new concept for analyzing the energy efficiency of air-conditioning systems
Highlights
► The characteristics of heat sources and humidity sources are explored. ► An ideal air-conditioning system is established. ► The gap between ideal system and actual system is concluded into three imperfections. ► Some improvement directions are proposed based on actual influencing factors.
Introduction
In the conventional air conditioning system, the control strategy of indoor environment is close to form a uniform temperature and humidity environment by mixing the supply air with the indoor air completely. On the other hand, the heat sources of the indoor environment, including solar radiation, human beings, computer, light, envelope, etc., cover a large temperature range, which indicates the character of indoor environment is non-uniform and provides the possibility of using cooling sources in different temperature level. Moreover, cooling and dehumidifying are two major requirements of indoor environment control, but are different in essence. The difference of cooling and dehumidifying exists as the requirement of cooling grade. For example in condensing dehumidification the temperature of chilled water should be lower than the dew point of air to be treated, but for cooling the temperature of chilled water should be lower than air temperature.
Based on the character of non-uniform indoor heat sources and the essence of cooling and dehumidifying, considerable researches have been conducted in recent years. Relevant researches include the application of air terminal devices such as personalized ventilation [1] and displacement ventilation [2], which avoid the defects of traditional mixing air supply method. And radiant terminals such as radiant ceiling, radiant floor are also used to remove sensible cooling load, which aims at reducing fan and chiller energy consumption. The system of combined cooled ceiling and displacement ventilation [3] can provide better indoor air quality and thermal comfort with proper design, and the potential of reducing energy consumption depends on the supply air temperature, outdoor airflow rate, and cooling load. Dedicated outdoor air systems (DOAS) integrated with ceiling radiant cooling panels as a parallel sensible cooling system are being considered as an alternative to conventional variable air volume (VAV) systems for commercial buildings [4]. Hourly energy simulation predicts that the annual electrical energy consumption of the pilot DOAS/radiant panel cooling system is 42% less than that of the conventional VAV system with air-side economizer. Compared to a full air system a 30% reduction of energy consumption of radiant floor system was predicted during the design [5]. And radiant floor cooling system integrated with dehumidified ventilation can solve the problem of condensation on a floor surface by lowering the dew-point temperature [6]. The natural cooling strategies and efficiency are studied with application double-skin facade in office buildings [7], which studies the natural night ventilation possibility in related to the double skin orientation and the speed and orientation wind. Moreover, free-cooling is understood as a means to store outdoors coolness during the night, to supply indoors cooling during the day [8]. The use of PCMs is suitable because of the small temperature difference between day indoors and night outdoors. Free-cooling systems can generate significant savings [9], however, the amount of potential energy savings available depends almost totally on the overall system design and on the selection of equipment for use in the system.
This paper sets the viewpoint of air-conditioning system from heat sources and moisture sources, especially focusing on the temperature distribution of heat sources and how it influences the efficiency of air-conditioning system. Actually the energy-saving methods mentioned above are rooted in the non-uniform character of heat sources. The objective of this paper is to establish the ideal air-conditioning system of cooling and dehumidifying, based on the ideal treatment of heat sources, and humidity sources. The results of the work are most useful in hot and humid climate, where cooling and dehumidification are both important for air-conditioning system. Then the difference between ideal system and actual system will be explored and concluded in order to analyze the route from ideal to actual and the influences of actual factors. In the last, according to the actual influencing factors, some improving methods are proposed to improve the current system correspondingly.
Section snippets
Characteristics of indoor heat sources and moisture sources
The requirement of air-conditioning system is mainly consisted of two parts: removing indoor cooling load and moisture load, and handling outdoor air to meet the fresh air requirement of occupants. Indoor cooling load includes solar radiation, heat transferred from envelope, indoor equipments and devices, lights, occupants and etc. Indoor moisture load is mainly from occupants, and can from plants, open water, or infiltration air if there is any. For the handling of outdoor air, cooling and
Ideal energy efficiency of cooling and dehumidification
In the ideal cooling and dehumidifying process, the sensible cooling load Qc and humidity load Qd are treated by two systems, as shown in Fig. 2. Moreover, all the heat sources are treated separately, the detail process of which will be discussed in Section 3.1. And the ideal process of dehumidifying and fresh air handling will be introduced in Sections 3.2 Ideal energy efficiency of dehumidification, 3.3 Ideal cooling and dehumidification of fresh air. Since the focus of this chapter is to
Case study for ideal cooling and dehumidifying process
To study the concept of ideal cooling and dehumidifying process, a calculated case is analyzed, including the test results of heat sources and humidity sources, the calculation of ideal cooling and dehumidifying efficiency, and comparison with the traditional cooling and dehumidifying system.
The test of temperature of heat sources and humidity sources is conducted in a room in Beijing, China during summer period. The positions and information of test points are shown in Fig. 5 and Table 1. The
Difference between ideal system and actual HVAC system
The gap between actual system and ideal system can be concluded into four main imperfections: (1) limited flow rates of air and water in real system, in consideration of energy consumption by fans and pumps; (2) limited heat and mass transfer area and coefficient in actual system, in consideration of initial cost; (3) the performance of actual chiller, due to manufacture limit; and (4) energy consumption by transportation system (fans and pumps) in real system, while only energy consumption by
Improving direction for air-conditioning system
From the result of chapter 5, there is a large distance between ideal system and actual system, which establishes an ultimate goal for air-conditioning system and a direction for improvement.
Conclusions
From the analysis and calculation above, the conclusions are listed as:
- (1)
The temperatures of indoor heat sources cover a relatively large range, and solar radiation is distinguished from other heat sources for its high temperature and intensity. Moisture load is different from cooling load for its physical essence.
- (2)
Based on the non-uniform characters of heat sources and essence of humidity sources, the ideal air-conditioning system of separated treatment of cooling and dehumidifying is established.
Acknowledgements
The research described in this paper was supported by National Natural Science Foundation of China (No. 51006058) and the foundation for the author of National Excellent Doctoral Dissertation of China.
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