Available online at www.sciencedirect.com ScienceDirect Energy Procedia 78 (2015 ) 459 464 6th International Building Physics Conference, IBPC 2015 Applications of Active Hollow Core Slabs and Insulated Concrete Foam Walls as Thermal Storage in Cold Climate Residential Buildings Navid Ekrami a *, Raghad S. Kamel a, Anais Garat b, Afarin Amirirad a, Alan S. Fung a a Ryerson University, 350 Victoria Street, Toronto, Ontario, M5B 2K3, Canada b Institut Catholique des Arts et Métiers, 6 Rue Auber,59800 Lille, France Abstract A test facility is designed and is under construction to experimentally verify the effect of thermal energy storage systems in overall performance of a coupled Building Integrated PhotoVoltaic / Thermal (BIPV/T) and Air Source Heat Pump (ASHP). This study shows how the loads for the test facility were adjusted by a regular size single family residential building. Moreover, the article explains different unique options of storing thermal energy in the test facility using the thermal mass of the building itself. Numerical models of Insulated Concrete Form (ICF) wall and Ventilated Concrete Slab (VCS) were developed using SolidWorks software Flow Simulation module and ANSYS Fluent software. 2015 The Authors. Published by by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license Peer-review (http://creativecommons.org/licenses/by-nc-nd/4.0/). under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL. Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL Keywords: Ventilated Concrete Slab (VCS); Thermal Energy Storage (TES); Insulated Concrete Form (ICF) 1. Introduction Despite the fact that solar energy is freely available source of thermal and electrical energy in buildings, mismatch between supply and demand periods is the major obstacle of maximizing solar utilization in buildings. The peak demand of thermal energy often happens at nights or early mornings in winter season when the sun is not available, * Corresponding author. Tel.: +1-416-979-5000 Ext. 7833 E-mail address: navid.ekrami@ryerson.ca 1876-6102 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL doi:10.1016/j.egypro.2015.11.698
460 Navid Ekrami et al. / Energy Procedia 78 ( 2015 ) 459 464 while the maximum solar irradiation occurs during the day in summer when the heating is not required. Therefore, storing the energy during the day and releasing it upon demand would be a wiser choice considering that a welldesigned sustainable building must also satisfy the thermal comfort of occupants. On the other hand, the thermal efficiency values of Photovoltaic/Thermal (PVT) systems lie commonly within the range of 30-40% while the electrical efficiency values range is 10-20% [1, 2]. In other words, the amount of thermal energy production from a PVT system is up to four times of electrical power generation [3] and converting solar irradiation to thermal energy instead of electricity may become more economically viable. The difference between supply and demand of solar generated thermal energy can be compensated by a short term thermal energy storage (TES) system such as building s façade / thermal mass [4]. On the other hand, the collected thermal energy by a solar system may not be warm enough for direct heating purposes in the winter. However, it could be a useful source for an Air Source Heat Pump (ASHP) [5, 6]. Coupling the TES to the space heating operator, potentially can enhance the overall performance of the building s integrated system. In an integrated system, replacing the outdoor air by the solar heated air as an inlet to the ASHP would increase the Coefficient of Performance (COP) of the heat pump. It means TES improves the thermal performance of the system and consequently electrical consumption of the heat pump will decrease. As a result, the combined heating system would operate more economically [3]. In general, thermal storage is preferred to be included in the solar assisted heat pump system to avoid the harm effect of non-constant solar radiation intensity [7, 8]. Hence, the COP of the heat pump is higher and electricity saving is enhanced when Thermal Energy Storage (TES) unit is linked to the solar air heater-heat pump system. Additionally, TES could decrease the temperature fluctuation in the building. Building integrated thermal energy storage (BITES) systems is a potential efficient solution to manage the energy consumption in buildings and requires to be investigated in more details. Effectiveness of BITES in different areas are highly recommended in the literature. Rad et al. [9] and Wang et al. [10] have shown that solar thermal energy storage in the ground could reduce the length of ground heat exchanger by 15%. Chen et al. [11] designed and modeled a building integrated photovoltaic/thermal (BIPV/T) system in a near zero energy building in cold climate of Canada. Accordingly, they have studied the effect of integrating a ventilated concrete slab (VCS) with BIPV/T system as an active TES system. Since most of the times temperature output from the BIPV/T is not high enough to be fed into the VCS, it is recommended to consider and include an air source heat pump in the integrated system. Moreover, Kamel and Fung [12] studied a BIPV/T system integrated with air source heat pump. It has been shown that the combined system will enhance the performance of the overall system and it is recommended that an appropriate BITES system will increase the overall efficiency of the entire integrated system further [12]. Accordingly, Ekrami et al, [13] showed the potential effectiveness of a concrete based BITES. 2. Motivation There is an interest in using the building's concrete slab as TES. Since concrete slab is common in the basement or garage of North American houses, designing and utilizing a Ventilated Concrete Slab (VCS) with proper control strategy is viable and does not increase the construction costs [14]. Also, it can improve the thermal performance of the house without employing water or other thermal storage systems/materials [9, 10]. The energy collected by the solar collector can be stored in the TES when space heating is not required. The cold outdoor air during the night can be tempered by the TES and enhance the ASHP performance [12]. Additionally, Insulated Concrete Form (ICF) walls are becoming more popular in low rise and high performance residential construction industry because of their strength and energy efficiency. The ICF wall is basically made with two Expanded PolyStyrene (EPS) side panels and concrete in between. The R value of the EPS panels can be as high as 30 ft 2.ºF.h/Btu (5.3 K.m 2 /W) which makes ICF a perfect insulated wall for buildings [15]. Meanwhile, the concrete inside the ICF can be used as thermal storage. The wall can be constructed with embedded pipes inside the concrete for the purpose of thermally charging and discharging it with preheated water. However, the thermal behaviour of the slab and ICF walls in heating applications have to be well understood and quantified in order to realize successful designs and operations in terms of efficiency and cost. 3. Test Facility To serve above mentioned purposes and investigate the effect of a BITES on the overall performance of the integrated system, a full scale (30ft 25ft) test facility, equipped with the combined VCS, multiple heat pump systems,
Navid Ekrami et al. / Energy Procedia 78 ( 2015 ) 459 464 461 and BIPV/T systems, is designed and currently under construction at Toronto and Region Conservation Authority (TRCA) Kortright Centre in Vaughan, Ontario, Canada. The Air Source Heat Pump (ASHP) is integrated with roof based BIPV/T panels to improve the performance of the system [15]. The ASHP can produce hot air/water. All ICF walls and the concrete floor are designed to be used as Building Integrated Thermal Energy Storage (BITES). Stored thermal energy can be used later for space heating and/or domestic hot water use. This configuration is expected to enhance the overall performance of the integrated system by implementing the TES. Considering the geometry and material of the test facility, a detailed model of the building was developed in TRNSYS software. The model provides the hourly heating and cooling demand of the building for a whole year. Maximum calculated thermal demand of the test facility based on the simulated model is 3.77 kw. Figure 1 illustrates the heating/cooling demand of the test facility for every hour in 365 days using Toronto weather library data. Since the size of the test facility is smaller than a single family house, the maximum thermal demand of the building is also relatively lower than a residential house. The objective of this research is to test and validate the feasibility of mechanical systems such as heat pumps and thermal energy storages under real life conditions which fits a regular size building. Therefore, all mechanical systems were designed/sized to produce enough thermal energy for House Figure 1 - Hourly Heating / Cooling Demand of the Test Facility A of Archetype Sustainable Houses (ASH) in TRCA. House A is a semidetached two story building, which is built to test different sustainable technologies for residential housing market [16, 17]. House A represent a regular single family building in Canada. Hence, the house model was developed in TRNSYS by Safa et al. [18] and the result was used for the design of the test facility s mechanical systems. The hourly demand of the House A in Toronto weather is illustrated in Figure 2. Comparing the loads of the test facility and House A, shows a higher cooling demand (in summer time) for the test facility. It is because of large openings/windows on south, east, and west side of the building resulting in high solar gain and consequently higher cooling demand. Also, for the same reason the thermal demand of the house in winter decreases during sunny hours. Figure 3 is generated in order to show the trend of thermal demand for both test facility and House A based on outdoor temperature. As expected, thermal demands are higher when the outdoor temperature is lower but the demands for test facility is more scattered due to high solar radiation gain. The simulated thermal load of buildings found to be linear relative to the outdoor temperature and defined as follows: Figure 2 - Hourly Heating / Cooling Demand of the House A (1) (2) The mechanical systems are designed based on House A thermal demand. However, they will be installed in the test facility. Therefore, the correlation between two buildings loads becomes important, which is calculated as follows: (3)
462 Navid Ekrami et al. / Energy Procedia 78 ( 2015 ) 459 464 4. Integrated Thermal Energy Storage Systems Designing an integrated thermal energy storage system requires a detail understanding of all other parts of the system. Each part plays a key role in overall performance of the coupled system. Also, they have to be connected in a way that not to forfeit the living space of the building and also not to sacrifice comfort of the occupants. In that note, the entire building has to be design as multiple efficient thermal Figure 3 Thermal Demand of both Buildings versus Outdoor Temperature storage systems along with the heat pumps and BIPV/T systems. The integrated system is designed to store energy inside the test facility as much as possible from the sun during the day. Hence, there are three major parts included in the building as thermal energy storage which are explained in the following sections. 4.1. Ventilated Sand / Gravel Bed There are two separate zones/rooms and a mechanical room in the test facility. The foundation of the building is considered to be deeper than regular residential houses to facilitate enough thermal mass for the sand and gravel thermal storage systems. As illustrated in Figure 4, a four feet deep fine sand bed (zone 1) is considered to be ventilated by air pipes. The sand bed is located beneath a rigid insulation layer which separates the sand bed from concrete slab. Diameter of embedded pipes are designed to be 4 inches each and they are connected to a 16 inches manifold to distribute the air evenly between the pipes. Heated air is supplied by the BIPV/T system and is fed to the sand bed. Thermal energy will be transferred and stored into the sand during the sunny hours and can be extracted for later time when there is a demand. The stored thermal energy can be used to preheat the inlet air to the heat pump system during cold nights. This process will enhance the coefficient of performance (COP) of the heat pump. Then the heat pump can be used for space heating and/or producing domestic hot water. Furthermore, the foundation of zone 2 is designed for another type of air based thermal storage. A gravel bed or Figure 4 - Ventilated Sand Bed recycled concrete aggregate bed is used instead of fine sand. Since the air can passes through the voids of gravel/aggregate, there is no need for piping system. However, in order to distribute air evenly inside the bed a header/manifold is used for each of the inlet and outlet. This system simplifies the construction by eliminating embedded pipes and reduces the capital cost as well. Having these two systems side by side allows a detail comprehensive comparative test under equal condition. 4.2. Ventilated Concrete Slab (VCS) In addition to the sand bed and gravel bed storages, the slab of each zone will also be used as thermal storage. Above the rigid insulation panel that separates the foundation from the slab, there is a layer of concrete with 4 inches thickness. A corrugated steel deck will be placed on top of this concrete layer during the construction. Then another concrete layer will be poured on top of the steel deck. The voids between bottom layer concrete and the corrugated steel can work as an air channel. Figure 5, shows a schematic of the concrete slab. Heated air from BIPV/T+ASHP
Navid Ekrami et al. / Energy Procedia 78 ( 2015 ) 459 464 463 system can alternatively pass through the air channels and warm up the concrete. In other words, the concrete slab as a thermal storage can be charged by the air during the day and discharged at night. A portion of the stored thermal energy also will be transferred to the room as well. However, in this study the goal is to store the maximum thermal energy and use it to preheat the heat pump source air rather than releasing it to the room. The floor is also equipped with infloor/radiant heating system. This system will use hot water for space heating and is not a storage system. A simplified 3D model of the ventilated concrete slab was simulated in ANSYS Fluent program to study the heat transfer phenomena between air and Figure 5 -Ventilated Concrete Slab concrete. This model helps designers to fully understand the effects and relationships between different variables, such as inlet temperature, inlet velocity, heat transfer rate inside the channel, percentage of wasted thermal energy, etc. A dense concrete with density of 2100 kg/m 3, specific heat of 840 j/(kg*k), and thermal conductivity of 1.4 W/(m*K) was selected for the simulation. In order to find the optimal heat transfer rate inside the channels variety of air velocities were numerically tested. Also the percentage of missed/rejected thermal energy for each case was calculated. A set of sample air outlet temperatures for few velocities is shown in Table 1. As expected, for low air velocities outlet temperature drops more than higher air velocities. This is because of higher residence time of the fluid in contact with the solid. It was observed that the overall heat transfer rate between air and concrete is higher when the velocity is higher. However, higher heat transfer rate does not necessary mean a better configuration Table 1 - Sample results of Outlet Air Temperature with Different Inlet Temperature since the ratio of heat transfer between air and concrete and total input heat transfer rate (the rate that thermal energy is added to the system by heated air) is the key variable. Also the total amount of transferred thermal energy is another important item in the design. The optimum air velocity depends on all other components of the integrated system as well such as number of PV/T panels, size of the heat pump, floor area, and concrete thickness. 4.3. Insulated Concrete Form (ICF) Wall Flow Simulation module of Solidworks program was used to investigate the heat transfer process of the embedded hot water pipes inside the ICF wall. A simplified three dimensional model of the ICF wall with a unit length (1 m) of PVC pipe was created. All pipes are parallel to Table 2 - Sample Results of ICF Simulation each other and located in a fixed distance of 20 inches apart. Insulations (EPS boards) were assumed to be perfect. Various water temperatures for inlet was numerically tested to address the associated changes in the heat transfer rate and temperature distribution in concrete. A sample set of simulation results for three inlet temperatures is presented in Table 2. Also rate of heat transfer for different configurations were calculated. The heat transfer rate in Table 2 is the rate of transferred thermal energy between the hot water inside the embedded pipe and concrete. Preliminary thermal analysis of the ICF walls shows that it could be a possible option to be integrated as a TES with BIPV/T+ASHP system.
464 Navid Ekrami et al. / Energy Procedia 78 ( 2015 ) 459 464 Remarks Designing an integrated mechanical system for residential buildings requires a detail analysis of each part. Coupling these technologies together, will enhance the overall performance of the system but also make the system much more complicated than conventional systems. Numerical studies of the VCS and ICF walls shows that each of these thermal storage systems could be a possible option to be integrated as a TES with BIPV/T+ASHP system. However, the performance of these systems change if the defined initial conditions vary. The work presented in this article describes a summary of the numerical analysis of TES systems and authors will provide further details in future publications. Acknowledgements The authors would like to acknowledge the funding support for this project from the Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grant (DG) and Smart Net-Zero Energy Building Research Network (SNEBRN), Toronto Atmospheric Fund (TAF), MITACS/Acclerate Ontario, ASHRAE (GIA) and Ontario Graduate Scholarship (OGS). References [1] Sarhaddi, F., Farahat, S., Ajam, H., Behzadmehr, A., and Mahdavi Adeli, M. (2010). An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector. Applied Energy, 87(7), 2328-2339. [2] Tonui, J. K. and Tripanagnostopoulos, Y. (2007). Air-cooled PV/T solar collectors with low cost performance improvements. Solar Energy, 81(4), 498-511. [3] Pinel, P., Cruickshank, C. A., Beausoleil-Morrison, I., and Wills, A. (2011). A review of available methods for seasonal storage of solar thermal energy in residential applications. Renewable and Sustainable Energy Reviews, 15(7), 3341-3359. [4] Khalifa, A. J. N., and Abbas, E. F. (2009). A comparative performance study of some thermal storage materials used for solar space heating. Energy and Buildings, 41(4), 407-415. [5] Koca, A., Oztop, H. F., Koyun, T., and Varol, Y. (2008). Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector. Renewable Energy, 33(4), 567-574. [6] Kousksou, T., Bruel, P., Jamil, A., El Rhafiki, T., and Zeraouli, Y. (2014). Energy storage: Applications and challenges. Solar Energy Materials and Solar Cells, 120(PART A), 59-80. [7] Chen, Y., Galal, K., and Athienitis, A. K. (2010). Modeling, design and thermal performance of a BIPV/T system thermally coupled with a ventilated concrete slab in a low energy solar house: Part 2, ventilated concrete slab. Solar Energy, 84(11), 1908-1919. [8] Dinçer, I. E., and Rosen, M. A. (2011). Thermal energy storage: Systems and applications (2nd ed.). Hoboken, N.J.: Wiley. [9] Rad, F. M., Fung, A. S., and Leong, W. H. (2013). Feasibility of combined solar thermal and ground source heat pump systems in cold climate, Canada. Energy and Buildings, 61(0), 224-232. [10] Wang, E., Fung, A. S., Qi, C., & Leong, W. H. (2012). Performance prediction of a hybrid solar ground-source heat pump system. Energy and Buildings, 47, 600-611. [11] Chen, Y., Galal, K., and Athienitis, A. K. (2010). Modeling, design and thermal performance of a BIPV/T system thermally coupled with a ventilated concrete slab in a low energy solar house: Part 2, ventilated concrete slab. Solar Energy, 84(11), 1908-1919. [12] Kamel, R. S., and Fung, A. S. (2014). Modeling, simulation and feasibility analysis of residential BIPV/T+ASHP system in cold climate-canada. Energy and Buildings, 82, 758-770 [13] Ekrami, N., Kamel, R. S., and Fung, A. S. (2014). Effectiveness of a ventilated concrete slab on an air source heat pump performance in cold climate. In Proceeding of esim Conference, Ottawa. [14] Zmeureanu, R., and Fazio, P. (1988). Thermal performance of a hollow core concrete floor system for passive cooling. Building and Environment, 23(3), 243-252. [15] Amvic Building System. http://www.amvicsystem.com [16] A. Dembo, A.S. Fung, K.L.R. Ng, A. Pyrka, The archetype sustainable house: investigating its potentials to achieving the netzero energy status based on the results of a detailed energy audit, in: Proceedings of the 1st International High Performance Buildings Conference, 2010, pp. 1 8, 3247. [17] D.H. Zhang, R. Barua, A.S. Fung, TRCA-BILD archetype sustainable house overview of monitoring system and preliminary results for mechanical systems, ASHRAE Trans. 117 (2) (2011) 597 612. [18] Safa AA, Kumar R, Fung AS. Performance of two-stage variable capacity air source heat pump: Field performance results and TRNSYS simulation. Energy and Buildings, 2014.