Comparative study of the efficiency of earth/air heat
exhangers versus direct solar floors systems for residential
buildings in Belgium
M. H. Benzaama1, *, M. C. Lekhal2, A. Mokhtari2, S. Attia3
1Ecole des Mines de Douai, IA, F-59500 Douai, France
2LMST Laboratory, University of Sciences and Technology, Mohamed Boudiaf, Oran, Algeria
3Sustainable Building Design Lab, Department ArGEnCo, Faculty of Applied Sciences, Univeristé de Liège, Belgium
Corresponding author: mohammed-hichem.benzaama@imt-lille-douai.fr
Abstract. Net-zero-energy building concept is a solution for high-performance buildings that integrates passive design and energy efficiency measures with renewable energy systems. Given the specific technical characteristics of net-zero energy buildings, two technological solutions are investigated, in this experimental study: Earth/Air Heat Exchangers and Direct Solar Floors. To carry out this study, an experimental cell composed of two rooms equipped with two energy systems was constructed. One room is coupled to a solar thermal collector with a surface area of 4.60 m2 and the second room is coupled to an air ground heat exchanger (with a 20m conduct length, 0.012 m diameter and 2 m burial depth). A data acquisition system was set up to measure the air temperature at the inlet-outlet of the air/ground heat exchanger, solar thermal collector and indoor air temperature. The obtained experimental results were compared with those resulting from the simulation under the TRNSYS building performance simulation environment. Good agreement between the simulated results and the experimental data was achieved through model calibration. After the validation of the models, a parametric study was conducted to identify the energy contribution of each system under Belgian climate conditions. A control strategy is proposed to improve the performance of each system. The results of this study help building designers to better dimension and control the operation of renewable systems to ensure higher building energy autonomy.
1. Introduction
The reduction and control of energy consumption and optimal use of energy continues to be the major concern in all areas of research. For high performance buildings, low temperature heating is one of the solutions to reduce energy consumption and heating energy needss in the airtight and highly insulated buildings. Therefore, the research aims is to apply direct solar floor solution and the air/ground heat exchanger to meet the heating needs of buildings in the Belgian context.
The air/ground heat exchanger or earth/air heat exchanger (EAHE) is a geothermal system used for pre-conditioning the air in winter. The development of geothermal energy in both of electricity and the
heat sectors until December 2016, shows growth in all areas, but notes that there is a great deal of untapped potential. Over the last decade, a substantial number of projects have been developed throughout Europe, and geothermal energy is on its way to become a key player in the European energy market. Among various renewable energy and waste heat sources geothermal energy has been regarded as efficient heat source for heating and cooling purposes [1].
For central Europe climate, C.O. Popiel et al., [2] show that quite stable ground temperature occurs at a depth of about 2 m and varies from 4 °C to 10 °C depending on the ground thermal properties (mainly thermal diffusivity), ground cover and season.
For Western Europe climate, Arvind Chel et al., [3] presented an analyze of the energy performance of a nearly zero energy house built with the passive house standard and integrated with an air-air heat exchanger (AAHE), an earth-water heat exchanger (EWHE) and a water-air heat exchanger (WAHE), for the climatic conditions of Belgium. The performance assessment of the present house is investigated using a thermal model of the house, which integrates all the energy efficiency systems mentioned above. This thermal model is simulated using TRNSYS 17 building simulation software. Simulation results are obtained with various modeling assumptions for the designed central air heating capacity and ventilation airflow rates in day and night during winter and summer. The results demonstrate the role of each of the installed systems along with their recommended control strategies for minimizing annual heating consumption and maximizing summer comfort.
The method for data evaluation of EAHE in operation presented in [4] for the climatic conditions of Germany give some noteworthy considerations for design and operation of EAHE: the influence of earth parameters (soil and surface) and of the building on the earth temperature is as important as the pipe diameter on the thermal efficiency. In operation, the control strategy plays a decisive role for the actually usable energy supply by the EAHE. A temperature control is important to prevent unwanted heating in summer and cooling in winter.
In order to assess the energy and environmental performance of a passive house in Greater Paris area (France), a model has been developed for innovative ventilation systems and integrated in a thermal simulation tool [5]. The model, including the main phenomena occurring in the EAHE, has been validated against experimental results. Simulations have shown substantial reduction of energy consumption and summer discomfort for the passive building compared to a standard building. Thermal comfort can be achieved most of the time using appropriate measures (solar protection, ventilation and possible ETAHE, thermal mass).
For Eastern Europe climate, the experimentally obtained flow characteristics of multi-pipe earth-to-air heat exchangers [6] were used to validate the EAHE flow performance numerical model prepared by means of CFD software Ansys Fluent for the climatic conditions of Poland. The results show that airflow in each pipe of the multi-pipe EAHE structures is not equal. Usage of CFD for a designing process of EAHEs can be helpful for HVAC engineers (Heating Ventilation and Air Conditioning). It can also be helpful for optimizing the geometrical structure of multi-pipe EAHEs in order to save the energy and decrease operational costs of low-energy buildings.
The floor heating occupies a special place in technology and provides optimal heat distribution in the occupied area, it is part of low temperature systems whose use can reduce energy consumption. Since the development of direct solar floor heating (DSF) technology, several installations were carried out, especially in Mediterranean climates. As an example, an analysis of the experimental data made by Menhoudj et al., [7] showed that solar coverage is over 67% for a capture ratio of 0.2 for Mediterranean climates.
The DSF is very often used in Europe (Papillon, 1992 [8]; IEA, 2000 [9], Fraisse 2007 [10]). The recovery of heat is privileged and the control strategy is the same as in a traditional installation equipped with a thermal solar collector [10]. It is based on the starting and the stopping of the circulating pumps according to the temperature discrepancy between the collector outlet and either the return of the floor loop or the domestic hot water tank. For this purpose, this article presents the application of DSF for the climatic conditions of Belgium. This country is an industrialized nation
with one of the largest ecological footprints in the world. Each household built in Belgium the average energy consumption is 200 kWh / m2) [11]. However, without the use of renewable technology such as PV systems, solar panels and the geothermal installations the building could not achieve a zero impact [11].
Lekhal et al., [12], have studied the combination between the SDF and EAHE for the climatic conditions of Algeria. The results show that the heating energy requirements of the house integrated with the DSF and the EAHE subject to the control strategy reduced by 70%, whereas they are only reduced by 46% with the DSF, by 42% with the EAHE and by 42% using the combined system without the control strategy. Due to the different performances of the EAHE and direct solar floor (DSF) heating systems, the main objective of this study is to investigate these two systems in Belgium. Indeed, these two renewable energy systems are not enough investigated in the Belgian context. A numerical model on TRNSYS, developed based on experimental data from an experimental cell implanted in Algeria, was used. The study focuses on the analysis of the energy performance of two systems such as the coverage rate of each system for the climatic conditions in Belgium.
2. Solar and geothermal potential in Belgium
Bertrand et al., [13], presented the daily solar surface irradiation over the Belgian territory. As shown in Figure 1, the South East to North West positive gradient is well apparent as well as some of the regional specificities. For instance, the Gaume region (area in the southeast of Belgium) located on the south side of the Ardenne and that enjoys longer sunshine time appears clearly on the mapping, as shown in Figure 1.
Figure 1. Daily spatial distribution of surface solar global irradiation over Belgium [13].
Figure 2. Potentiel géothermique en Belgique [14].
A diversified geological underground characterizes Belgium. Figure 2 shows the different areas with high geothermal potential in Belgium according to the geological structure of the soil. Areas in Flemish Kempen consist of limestone aquifers with good permeability. This allows direct heat applications by drilling over 500 m depth and thus reaching a temperature of about 25 °C. Temperatures more than 40 °C are still available in Kempen but also under the coalfields of Hainaut. Nevertheless, a very low temperature geothermal system linked to a heat pump can be a good alternative to exploit the heat of the soil in the South of Belgium.
3. Description of the experimental setting
The experimental cell occupies an area of approximately 40 m2, which is oriented East-West with windows on the south facade and consists of two identical rooms: the room 1 is equipped with an
EAHE and the room 2 is equipped with a DSF. The internal dimension of each room is 4.7 m in length, 3.7 m in width and 2.8 m in height, as seen in Figure 3.
The floor heating system under study consists of a concrete slab of 0.12 m (upper side); serpentine pipes of cross-linked polyethylene with 0.018/0.020 m diameter and 0.20 m spacing; an insulation layer of 4 cm; and a second concrete layer of 0.10 m (lower side). The floor heating is directly connected to 6 thermal collectors installed on the roof with a south-facing orientation and a slope of 45°. Furthermore, the EAHE system is composed of a 20 m of pipe, buried 2 m from the ground (2% slopes).
Figure 3. The test cell integrated with the DSF and the EAHE [12]. 4. Simulation under TRNSYS
5. Model development
The simulations of the studied system are made under TRNSYS software, which is designed to simulate the energy performances of dynamic systems. The test cell is modelled using Type 56 (Multizone Building Modeling) via TRNBuild. This component provides a more efficient way to calculate the interaction between two or more zones by solving the coupled differential equations. The DSF and EAHE system model are developed based on the types and techniques offered by the TRNSYS software. The main component of the DSF model is a solar thermal collector, which is modeled using the Type 73. The Type 3b ensures the fluid circulation inside the solar network and the Type 2b manages the differential temperature regulation. The developed DSF model is illustrated in Figure 4.
The EAHE system was modelled using the Type 711. Type 711 models a buried heat exchanger consisting of a horizontal pipe that interacts thermally with the ground. A fan (Type 3c) ensures the air circulation through the pipe. Figure 5 shows the established EAHE model.
Figure 4. The direct solar floor heating model [12]
Figure 5. The earth/air heat exhanger model
6. Validation
The validation of the models was done using the experimental data of the cell presented, in the section 3. Figure 6 shows the evolution of the numerical and experimental results for the indoor air temperature. A good agreement was obtained between experience and simulation. The recorded temperature of the indoor air varies between 17 and 19°C, while the numerical one varies between 16.5 and 18.5 °C for an outdoor air temperature varying between 9 and 20°C.
Figure 6. Comparison between experimental and simulated results of the indoor air temperature of the zone equipped with the DSF
The air temperature at the outlet of the EAHE was also validated (Figure 7). From the results, it was noted that the two curves (experimental and numerical) have the same tendencies. The recorded temperature of the air temperature at the outlet of the EAHE varies between 19 and 22°C, while the numerical one varies between 19 and 23°C.
Figure 7. Comparison between experimental and simulated results of the air temperature at the outlet of the EAHE.
7. Efficiency of DSF and EAHE systems in the Belgian context
The DSF was simulated for floor heating and the EAHE for air preheating under Liège city climate. Figure 8 shows the heating requirements evaluated by numerical simulation of the test cell. We note that the energy need is important during the winter season and especially the months of January and December. In January, and for a set temperature of 18°C, the heating requirements for the both rooms are 180 kWh.
Figure 8. Heating requirements of rooms.
The objective of this study is to present the impact of the DSF and EAHE systems on the reduction of heating energy needs for a high performance building located in Liège city. Keeping the same configuration presented in Section 4, the results in Figure 9 show that for the months of January, February, November and December, the DSF system reduces heating energy requirements by 20 kWh. On the other hand, for the same months, the EAHE system is less efficient than DSF.
The results in Figure 9 show that the DSF system is more efficient during the months March, April and October with a maximum reduction in heating energy requirements of 40 kWh, due to the solar power received in this period.
Figure 9. Impact of the DSF and EAHE systems for the reduction of heating requirements.
The thermal behaviour of the DSF and EAHE systems are influenced by several factors. As a result, we have varied the burial depth of the EAHE system for depths (Z=3 m and Z=4 m) and solar collector surface (S=6.6m2 and S=8.6m2). This step shows the reduce heating energy requirements for different depths and solar collector surfaces, which allows the designer to choose the optimal configuration for the best system operation. The results in Figure 10 show the effect of the solar collector surface on reducing heating requirements. By increasing the area until 8.6 m2, heating energy requirements are reduced with 20 kWh compared to the reference case. In contrast, the effect of the burial depth for the EAHE system is not as important.
Figure 10. Parametric study.
8. Conclusion
The thermal performance of an experimental cell equipped with a Direct Solar Floor (DSF) and an Earth-Air Heat Exchanger (EAHE) has been analysed. The thermal model of the cell with the integrated systems under weather conditions of Liège city has been developed using TRNSYS. The models used in the simulation are validated using experimental data from a test cell integrated with renewable energy systems located in Algeria.
The developed model is used to investigate the thermal performance of two different systems and their impact on heating energy needs. However, our study findings are initial and future work should explore more in detail the design parameters of DSF heating systems and couple them to thermal comfort.
Finally, a comparative study is conducted in order to evaluate the contribution the effect of the solar collector surface and the burial depth of the EAHE on the systems performance. The results of this study show the interest of DSF compared to the EAHE especially for the months of March, April and October for the climatic conditions of Liege. Hence, future works will be directed to test the combination of these two systems with a control strategy in order to synergize the benefits for both heating for several cities in Belgium.
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![Figure 1. Daily spatial distribution of surface solar global irradiation over Belgium [13].](https://thumb-eu.123doks.com/thumbv2/123doknet/6273531.163781/3.892.103.775.573.886/figure-daily-spatial-distribution-surface-global-irradiation-belgium.webp)
![Figure 3. The test cell integrated with the DSF and the EAHE [12].](https://thumb-eu.123doks.com/thumbv2/123doknet/6273531.163781/4.892.210.706.330.581/figure-test-cell-integrated-dsf-eahe.webp)
