تجزیه و تحلیل حساسیت از عملکرد حرارتی از پایانه های تابشی و همرفتی برای ساختمان های خنک کننده

Heating and cooling terminals can be classified in two main categories: convective terminals (e.g active chilled beam, air conditioning) and radiant terminals. The mode of heat transfer of the two emitters is different: the first one is mainly based on convection, whereas the second one is based on both radiation and convection. In order to characterise the advantages and drawbacks of the different terminals, steady-state simulations of a typical office room have been performed using four types of terminals (active chilled beam, radiant floor, wall and ceiling). A sensitivity analysis has been conducted to determine the parameters influencing their thermal performance the most. The air change rate, the outdoor temperature and the air temperature stratification have the largest effect on the cooling need (maintaining a constant operative temperature). For air change rates higher than 0.5 ACH, differences between terminals can be observed. Due to their higher dependency on the air change rate and outdoor temperature, convective terminals are generally less energy effective than radiant terminals. The global comfort level achieved by the different systems is always within the recommended range, but differences have been observed in the uniformity of comfort.

Differences can be observed between offices built nowadays and the ones built in the eighties or before. First of all, the level of insulation and air tightness of buildings has increased due to strengthening of the different building regulations. A better treatment of daylight by architects and the development of new products have led to an increase of the glazed area of buildings; fully glazed façades are becoming more widely installed. The use of buildings has also changed with the emergence of computers and other electronic devices, thus increasing internal heat loads. For these reasons and also due to a raised focus on thermal comfort, more cooling systems are installed in offices. In the European Union, the cooled area in non-residential buildings has increased by 45% between 2000 and 2010 and it led to an electricity consumption of 95 TWh for the EU-15 members [2]. This situation creates serious supply difficulties during peak load periods, especially in southern European countries such as Spain or Italy [3]. Convective terminals are the most widely installed cooling system, despite their high initial costs, high energy use and often unacceptable indoor climate. Occupants sometimes complain about the noise or the draught of this type of system [4]. Switzerland and the state of Hamburg in Germany have even restricted the installation of full air conditioning systems for buildings [5]. Radiant technology is an alternative to air-based emitters. Contrary to convective terminals, which transfer heat mainly by convection, radiant terminals transfer heat partly by radiation to (or from) the neighbouring surfaces, and partly by convection to (or from) the indoor air [6]. The first radiant cooling system was installed after the First World War, in the Bank of England [7]. In the 1990s, European offices were increasingly equipped with cooled radiant ceilings because of longer overheated periods during summer time [8]. In 2004, a cooled radiant floor was installed in a humid climate, in Bangkok airport [9]. More recently, radiant walls have been introduced to the market. Most of the studies comparing radiant and air-based systems conclude to the lower energy use of radiant systems [10], [11], [12], [13], [14], [15] and [16]. Radiant systems are an efficient way of transporting energy [4], mainly due to the higher heat capacity of water and the reduced fan usage. Moreover, the large surface of exchange of radiant systems allows the use of source temperature closer to the room temperature, increasing the efficiency of production systems. The total energy savings oscillate between 10 up to 60%, depending on the climate, the source considered, the area of the radiant system and the efficiency of the different components. Fabrizio et al. [16] have compared numerically the performance of radiant floor and ceiling systems versus all-air and fan coil systems. Dynamic simulations of a typical office building showed that the cooling energy use is greatly reduced for warm climates, whereas the reduction is smaller for cold climates. In addition to the total energy use, some studies compare the energy need in the space. Differences in the heat balance are noted in several publications [11], [15], [16], [17], [18], [19] and [20], but the effect on the cooling need is not clearly defined: some studies show a higher demand [17] for radiant terminals, whereas some others conclude to a lower [18] and [19] or similar demand [11] and [16]. As stated by Djunaedy et al. [20] and Feng et al. [17], “no research can be found that fundamentally studies the differences of the heat transfer process in zones conditioned by an air and a radiant system”. In most of the studies, the dynamic simulations do not highlight the sensitivity of one specific parameter on the cooling need. Parameter variation is needed to emphasize the influencing factors. In this paper, four terminals have been selected (active chilled beam, radiant floor, radiant wall and radiant ceiling) and their thermal performances have been compared. This paper focusses on describing the heat transfer within the space. Therefore, the source and the type of energy used to remove heat have not been taken into consideration. Humidity control has also not been considered, as the problem of humidification or dehumidification has to be treated in the plant, before the air enters the space. The main objective is to identify the case(s) in which the different technologies achieve the best performance in maintaining a constant operative temperature of 26 °C. The robustness of the different cooling systems will be evaluated by performing sensitivity analyses and parameter variations. The parameters varied are related to the outdoor conditions (outdoor temperature, part of direct to total solar radiation), the type of ventilation system (air change rate, air temperature gradient, convective flow in the room), the room properties (emissivity and absorptivity of the internal surfaces) and the position of the person/sensor in the room. A typical European office building has been chosen as the base case and numerical simulations have been performed under steady state conditions.

Different cooling strategies have been applied in a typical office room in order to evaluate the influence of the terminal type on the cooling power needed to achieve a fixed operative temperature. The thermal performances of four terminals (active chilled beam, radiant floor, radiant wall and radiant ceiling) have been compared in term of delivered energy for different boundary conditions. The most influencing parameters have been identified by performing a sensitivity analysis. The objective of this analysis is to identify the advantages and drawbacks of the different technologies and also find out the parameters that are important to include in the evaluation of terminals. It has been observed that the interaction between the ventilation system and the terminal (e.g. air change rate, outdoor temperature, air temperature stratification) plays an important role in the zone heat balance. At low air change rates (lower than 0.5 ACH), the performances of the different terminals are similar. But differences between the terminals can be observed at higher air change rates. The air temperature is warmer with radiant cooling terminals, resulting in higher air temperature, thus increasing the ventilation losses and decreasing the cooling need. The higher the air change rate and the warmer the outdoor air, the larger the savings achieved with a radiant cooling terminal. It has also been observed that the effectiveness of the active chilled beam is dependant of the type of flow in the room, i.e. on the design of the cooling system. Finally, the positive effect of a vertical air temperature gradient has been observed. At 1 ACH, the cooling need of the terminal decreases by 10% if a temperature gradient of 1.5 K/m is achieved. Such a temperature gradient can be achieved by a cooled floor or wall or by using displacement ventilation. Among radiant cooling terminals, the cooled floor has the lowest cooling need due to the large view factor between a sitting person and the activated surface. When considering people standing, the three terminals have a similar effectiveness for the considered geometry. These conclusions are valid for well-insulated buildings (R > 5 m2.K/W) and also for multi-storey buildings. For single-storey buildings with a low level of insulation, air-based terminals might be more energy-efficient compared to radiant terminals due to the lower conduction losses. Additionally, the variation of comfort with the different terminals has been evaluated. The radiant ceiling achieves the most uniform comfort conditions in the space, whereas the least uniform conditions were obtained with the cooled floor. The active chilled beam achieves uniform comfort conditions in theory, but specific studies (e.g. CFD simulations) are needed to validate these results and account for the draught risk. Finally, it has been observed that the cooling power from a radiant wall is not limited by the risk of down-draught from the cold surface.