تجزیه و تحلیل سیستم لوله های فشرده به خاک سپرده شده برای تهویه هوای داخل ساختمان

Given the international state of affairs in what concerns the heating of buildings and the necessary reduction of costs in the heating and cooling energy consumption, it is imperative to study and develop passive methods of heat transfer including heat exchange through buried pipes. Common configurations of a heat exchanger usually consider one single layer of pipes requiring a large installation area. This major drawback can be overcome using a multiple layer configuration. This paper presents a study considering the use of a heat exchanger with a multiple layer configuration, namely, comparing it with a single layer of pipes and describing the major performance differences. A parametric analysis was also performed to better understand the effect of the main input parameters on the heat exchanger power output. It was concluded that the heat exchanger power increases with the layers depth until 3 m and that the more efficient distance between layers should be kept at 1.5 m. The heat exchanger layout is also described as well as the implementation of the numerical model and the corresponding application to a real case study.

About 40% of the energy consumption in Europe is delivered to the building sector [1]. Current trend on building design and European legislations, like the EPBD recast of 2010 [2], are pushing buildings to low energy designs with the creation of the “near zero energy buildings” concept. More than 50% of this consumption could be reduced through energy efficiency measures, leading to a possible annual reduction of 400 millions of tons of CO2 – nearly the total commitment of the EU to the Kyoto Protocol target. In light of this, it is clear that a major potential for the implementation of low energy consumption systems like earth-to-air heat exchangers (buried pipes) can be found in the household and service sectors. A wide diversity of ground cooling/heating systems has been already used and applied [3], from closed to open system, and both analytical and discrete models have been studied and compared to predict the performance of buried pipe systems. For example, Santamouris et al. [4] compared eight different models to study their sensitivity to change the main operation parameters of an open horizontal earth-to-air heat exchanger. The analysis of buried pipe systems must consider the study of the soil's thermal behaviour. It includes formulating the proper energy balance equations at the soil surface level [5] and developing the temperature profiles of the soil due such energy balance [6]. It was noted that the soil energy balance is affected by the type of cover, for example, grass covered or bare, and the humidity ratio [7] and [8]. A sophisticated model describing the complex mechanisms of simultaneous heat and mass transfer occurring around an earth tube has also been developed and integrated into TRNSYS by Mihalakakou et al. [9]. In order to describe the performance of the buried pipes system theoretical and numerical models were developed. Frequent cases found on literature are discrete numerical models applied to earth-to-air heat exchangers with a single layer of pipes [3], [10], [11] and [12]. Another approach not so frequent is the analytical modelling of the heat exchanger [13] and [14]. Other features of this technology have already been studied, such as economic feasibility and integration into a building [15], [16] and [17]. In the Middle Eastern Europe a similar technology on buried ducts cooling air into buildings is also being carried out. However, the major limitation for the implementation of this kind of ground-coupled heat exchanger is the area restrictions particularly in dense urban areas, where large buildings leave few spaces in the surroundings to install these systems. The major advance of the model described in this article is its capacity to considerably reduce the installation area of the system, while maintaining the high performance characteristics of this technology. The system under study consists of a compact multi-layer earth-to-air heat exchanger. The exchanger consists of layers of horizontal tubes buried underground next to each other in each layer, in which air will flow from the outside into the building and will be cooled or heated while it crosses along the buried pipes in an open circuit. The system will be installed in a green area besides the building instead of beneath the building. However, horizontal exchangers have a serious disadvantage of requiring large horizontal area to install the pipes, thus, in this paper the effectiveness of a horizontal exchanger on two and three levels without saturating the soil's heat exchange capacity was explored. This system can be thought for retrofit and new buildings. However, for retrofit buildings some problems can arise up affecting the foundations. The implementation should take into account safety, economical and technical issues. For performance analysis of the multiple layer heat exchangers it is important to bear in mind that the gap from the surface and between layers both need to be dimensioned, otherwise the pipes can saturate quickly the soil's thermal capacity, thereby reducing the overall system performance. Multiple layer horizontal heat exchangers have an advantage when compared to single layer heat exchangers because of the considerably reduced horizontal area needed for the system installation. However, the decreased area should not sacrifice significantly the heat exchanger performance. The analysis of both configurations performance will be studied in the present paper. The approach followed in this paper starts with a description of the Portuguese soil as well as climatic factors, in order to establish their characteristics regarding the heat transfer capacity. Then, a physical model of a heat exchanger with two and three levels was developed with the corresponding boundary conditions and assumptions. Next, the most promising numerical models were analysed, namely, analytical and discrete (one and two-dimensional) with the corresponding validation. The one-dimensional model was selected as the best option to model this heat exchanger, under an accuracy and computational effort perspective. A parametric study was also carried out to check all the variables of the heat exchanger system and the corresponding optimal values for each parameter were obtained. Finally, the developed model was applied to a heat exchanger in a real case study in Portugal. The systems were designed for cooling purposes where the target indoor temperature was 25 °C defined by the national Portuguese regulation, RCCTE [18]. During the winter season the soil is able to gather about 260 kWh of useful energy and deliver about 870 kWh during the summer season.

The work reported here aimed to expand the applications of this technology to residential buildings where available space is limited, considering the feasibility of compact exchangers that can meet the requirements of such buildings. The thermal behaviour of the soil is the basic principle of operation considering the heat exchanger. This behaviour is controlled by the corresponding thermal properties, namely, thermal conductivity and thermal capacity but also by soil density. In turn, the characteristics mentioned above are strongly influenced by soil porosity and moisture. It was found that a one-dimensional discrete model can respond satisfactorily to a performance analysis of compact buried pipes systems, producing results significantly faster than the bi-dimensional model, within a good accuracy. When the multiple layer configuration is adopted, it was verified that maintaining similar transfer areas and velocity flows incurs a decrease of 3–6% in the duty delivered by heat exchangers with two and three layers respectively, when compared to heat exchangers with a single layer. However, this corresponds to a decrease of 50% and 67% in the area needed, respectively. This factor is very attractive given the current limitations in urban spaces. Analysed parameters showed that the heat exchanger duty increases with the floor depth until 3 m and that the optimum space between layers is 1.5 m, but distances up to 2 m are also efficient. This value allows making a good compromise between the installation area and the intensity of the usage. Economic aspects of the installation are the constraints to the total depth of the system. To maximize the duty of this system, high speeds should be maintained to ensure a turbulent flow, but given the noise issues it is recommended to limit the maximum speed up to 5 m s−1. However, this value could be adapted to each case study. The creation of high speeds requires a selection of appropriate diameters and numbers of tubes. Small diameters and high numbers of tubes should be aimed for, instead of fewer tubes and bigger pipes. This fact makes it possible to consider the installation of compact systems in limited urban residential and office building areas.