An enthalpy-based energy savings estimation method targeting thermal comfort level in naturally ventilated buildings in hot-humid summer zones
Introduction
Due to the poor quality of the thermal environment in hot-humid climate zones, there is an increased demand for improvement of indoor thermal conditions [1]. Artificial cooling always results in increased energy usage and increased electricity cost to the occupants in a hot and humid climate [2], [3]. Moreover, many studies reveal a statistically significant relationship in which buildings with air-conditioning, with or without humidification, are consistently associated with 30–200% higher incidences of sick building syndrome symptoms, compared to naturally ventilated buildings [4], [5]. Considering such issues, natural ventilation in summer and transition seasons is a commonly used passive cooling technology to improve the indoor thermal comfort and air quality in such climate zones [6], [7]. Determining whether natural ventilation would remain an acceptable means of indoor thermal comfort control requires prediction of the indoor conditions in the spaces when they are naturally ventilated. Consequently, the energy performance of natural ventilation should be assessed associated with the relation of climate, building design, and thermal comfort. A number of studies on cooling energy savings of using natural ventilation have considered the ventilation parameters, thermal comfort and energy savings together for various building types and climate zones [8], [9], [10]. Some research aimed at the natural ventilation performance of residential building designs by taking thermal comfort into account [11], [12], [13], [14] while some were focused on hybrid ventilated buildings [15], [16], [17]. Some research discussed the link between the natural ventilation performance and urban environments [18]. Studies also covered various climates such as warm conditions [19], warm-humid conditions [20], and hot-dry summer climate [21], [22], [23].
Almost all of the aforementioned research took thermal comfort into account. The assessment of thermal comfort under naturally ventilated environment is mainly based on either PMV model [24], [25], [26] or adaptive models [27]. However, the PMV method, which is based on steady-state, human body heat balance theory [28] has been challenged by adaptive models [29]. The adaptive models adopted in existing methods are mainly based on four well-known and most widely accepted thermal comfort international standards: American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard-55 [30], the International Organization for Standardization (ISO) standard-7730 [31], the European standard EN15251 [32], and the Chartered Institution of Building Services Engineers(CIBSE) [33]. These international comfort standards are almost all based on North American and northern European subjects [31], [34]. It has not been clarified in the aforementioned standards whether they are applicable to different environmental conditions [29]. Taking hot-humid summer zones of China as an example, studies broadly covering different types of buildings found that adaptive models adopted in aforementioned standards, such as the ASHRAE and ISO, are not suitable due to the different climatic characteristics, significant sensation variations and human adaptations in China [35], [36], [37], [38]. These studies generally indicate that there are discrepancies between people’s real thermal sensation and the predicted thermal sensation, especially for naturally ventilated buildings. Taking into account factors such as climate, social, psychological and behavioural adaptations of these regions, studies have been conducted within the context of thermal comfort, and Adaptive Predicted Mean Vote (APMV) model for China conditions has been proposed [29], [39], [40].
According to the China “Standard of climatic regionalization for architecture (GB 50178)” [41], hot-humid zones in China refer to the hot summer and cold winter climate zone, and the hot summer and warm winter climate zone. In other words, it contains the Yangtze River basin and Southern coastal regions. These areas are of important to the development of China due to the growth in population and the economy [42]. Natural ventilation is highly recommended in hot-humid zones in China [43], [44]. Therefore, the objective of this work was to review existing methods for rating energy saving using natural ventilation, and then propose a new method to rate energy use in naturally ventilated buildings based on energy balance and thermal comfort in the hot-humid climate regions of China.
Section snippets
Rating energy savings with a consideration of thermal comfort using natural ventilation
To improve the estimation of energy benefits when deploying natural ventilation to achieve thermal comfort in a hot and humid environment, it is crucial to use a robust heat transfer model with appropriate thermal comfort criteria.
Energy embedded in the flowing material consists of internal energy, macroscopic kinetic energy, gravitational potential energy, and flow work. The internal energy and flow work embedded in the flowing material are regarded as “enthalpy” in thermodynamics because they
Methodology
The energy savings of a naturally ventilated room could be analysed by using the first law of thermodynamics and the following steps were used in the proposed method:
- (1)
Identifying the thermodynamic system;
- (2)
Analysing the energy flow to the system in regard to natural ventilation;
- (3)
Building the energy balance model of the thermodynamic system;
- (4)
Assessing the energy saving potential of natural ventilation targeting thermal comfort.
Given those steps, this section consisted of two parts, namely analysing
Case conditions
To discuss the energy saving potential of natural ventilation by using the proposed energy flow model and evaluation method, three typical cities within these area of China, namely Chongqing (E106°45′, N30°), Changsha (E113, N28°11′) and Guangzhou (E113°3′, N23°2′), were selected to do the further case study. A two-level free-running office building was set out in each of the city and occupied from 8:00 a.m. to 6:00 p.m.
Compared to the conventional rating model
A contrast comparison was carried out in order to investigate the distinction between the new method which integrates Chinese indoor thermal comfort standard with total heat balance model and the common used method which couples adaptive ASHRAE standard-55 thermal comfort and sensible heat balance model. The common used method has been integrated into the EnergyPlus simulation tool [12]. By using EnergyPlus, energy saving potential of natural ventilation for each city in the typical year can be
Conclusions
Considering the climate features of hot-humid zones, the distinctions of thermal comfort assessment between naturally ventilated buildings and air-conditioning buildings and also between China and North American and northern European subjects, a new approach with Eqs. (5), (6), (7), (8), (9), (10), (11), (12), (13), (14), (15), (16), (17), (18), (19), (20) could be used to determine energy savings in naturally ventilated buildings in hot-humid summer zones like Yangzi River Basin and Southern
Acknowledgments
This work was supported by the China NSFC International Cooperation Program [grant numbers 51561135002].
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