ASHRAE OR-16-C074-2016 Using Building's Thermal Mass as Short Term Integrated Energy Storage.pdf

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1、 Navid Ekrami and Raghad S.Kamel are PhD candidates, Kajen Ethirveerasingham is an undergraduate student and Alan S. Fung is associate professor in the Department of Mechanical Engineering, Ryerson University, Toronto, Ontario, Canada. Camille Lecherf is visiting research student from Institut Catho

2、lique des Arts et Mtiers, Lille, France. Using Buildings Thermal Mass as Short Term Integrated Energy Storage Navid Ekrami Raghad S. Kamel Camille Lecherf Student Member ASHRAE Student Member ASHRAE Kajen Ethirveerasingham Alan S. Fung, PhD, PE Student Member ASHRAE Member ASHRAE ABSTRACT Building i

3、ntegrated thermal energy storage (BITES) systems specifically ventilated concrete slabs (VCS) has been discussed in this article. Additionally, a simplified three dimensional model of the VCS was developed in SolidWorks Flow Simulation software. The model was used to better understand the charging a

4、nd discharging process of the VCS as a thermal energy storage. It was tested for multiple charging and discharging scenarios such as being connected to the building integrated photovoltaic and thermal (BIPV/T) system or using an air source heat pump (ASHP) as the heat source. Preliminary results sho

5、w VCS is able to store up to 30% of the generated energy by BIPV/T and charging process is even more effective with an ASHP. Moreover, it was shown a properly charged VCS would warm up the cold outdoor air by approximately 5C (9F) to be used as an inlet for the heat pump. INTRODUCTION Building Integ

6、rated Thermal Energy Storage (BITES) systems use the buildings thermal mass to store the thermal energy inside the buildings for short periods. Implementing BITES techniques is beneficial to better managing building energy consumption. These systems have potential effectiveness in both active and pa

7、ssive ways for cooling/heating purposes when they are integrated into the building (Park and Krarti, 2015). Traditionally, storage tanks (with or without PCM) are known as active thermal storages and buildings walls and slabs as passive thermal storages. However, ventilated facades, floors, foundati

8、ons, and walls are new concepts of active systems. A ventilated thermal mass can be used for both storage purposes and heating/cooling processes. These systems work similar to a hydronic/in-floor heating approach but the working fluid is air instead of water. Studies on ventilated slabs as passive t

9、hermal storage showed that they can significantly reduce the energy consumption in buildings (Candanedo et al, 2010; Park and Krarti, 2015). However, most of the studies are specifically focused on the passive cooling use of ventilated thermal masses (Henze et al. 2008; Bilgen and Richard 2002; Dinc

10、er 2012). However, Ventilated Thermo-Active Foundations (VTAF) are also unique features for newly constructed buildings that may potentially improve the energy use (Ekrami et al, 2015). Previously, thermal piles were studied as thermo-active foundation which work as heat exchangers to transfer therm

11、al energy between building structures and the ground (Brandl 2013; Almanza Huerta and Krarti 2015). Nevertheless, the VTAF can be actively charged and discharged as thermal energy storages by forced air. There has been limited studies on VTAF systems. Additionally, implementing Insulated Concrete Fo

12、rm (ICF) walls into residential and commercial buildings not only reduces the construction time and increases the strength of the structures, but can also be used as an integrated thermal energy storage for buildings (Arthur and Ribando 2004). Having an embedded hydronic system inside the concrete w

13、ill upgrade the structure from a wall to an active thermal storage system. All these options will allow a building to store thermal energy inside itself without additional discrete storage systems such as water tanks. They enable buildings to store the excess energy and release it when there is heat

14、ing demand. Applying these techniques will reduce energy costs and save living space inside buildings. The excess thermal energy is normally provided by solar collectors. Despite the fact that solar energy is freely available and could be the source of thermal and electrical energy in buildings, mis

15、match between supply and demand periods is the major obstacle of maximizing solar energy utilization in buildings. The peak demand of thermal energy often happens at nights or early mornings during the winter season when the solar radiation is not available, while maximum solar irradiation occurs du

16、ring 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 well-designed sustainable building must also satisfy the thermal comfort of the occupants. The difference between the supply

17、and demand of solar generated thermal energy can be compensated by a short term thermal energy storage (TES) system such as buildings faade/thermal mass. On the other hand, the thermal energy collected by a solar system may not be enough for direct heating purposes during the winter. However, it cou

18、ld be a useful source for an Air Source Heat Pump (ASHP) (Chen et al, 2010). Coupling the TES to the space heating operator, can potentially enhance the overall performance of the buildings integrated system. In an integrated system, replacing the outdoor air by a solar heated air as a source to an

19、ASHP would increase the Coefficient of Performance (COP) of the heat pump. This means the 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 (Pinel et al,

20、 2011). In general, thermal storage is preferred to be included in the solar assisted heat pump system to avoid cases of irregular solar radiation intensity (Dincer and Rosen, 2011; Chen et al, 2010). Hence, the COP of the heat pump is higher and electricity savings are enhanced when the TES unit is

21、 linked to the solar collector/heat pump system. Additionally, the TES could act as a buffer and decrease the temperature fluctuation in the building. DESIGN OF THE TEST FACILITY In order to investigate the effectiveness of BITES systems on the overall performance of buildings, a full scale (30ft25f

22、t)(9.1m7.6m) test facility, equipped with the combined VCS, VTAF, ICF, multiple heat pump systems, and BIPV/T systems is designed and currently under construction at Toronto and Region Conservation Authority (TRCA) Kortright Centre in Vaughan, Ontario, Canada. The ASHP is integrated with roof based

23、BIPV/T panels to improve the performance of the system (Kamel and Fung, 2014) and is designed to produce hot air/water. All ICF walls and the concrete floor are designed to be used as BITES. Stored thermal energy can be used later for space heating and/or domestic hot water use. The test facility is

24、 smaller than a single family house. Therefore, in order to test the integrated system under real life conditions for a regular size residential house, calculated heating demand of the building using the TRNSYS software was scaled up to a two story house located at TRCA, House A of Archetype Sustain

25、able House (Zhang et al, 2011), with available data and specifications (Ekrami et al, 2015). A proper design of a combined system cannot be achieved without knowing the detailed working process and the advantages and disadvantages of each component. As shown in Figure 1-left, a 6.5 kWp (22179 Btu/Hr

26、) building integrated photovoltaic/thermal (BIPV/T) system pre-heats the air in order to either feed to the ASHP or to the thermal energy storage systems. Directly providing preheated air to the heat pump has shown to effectively enhance the overall COP of the system (Safa et al, 2014; Kamel and Fun

27、g, 2014). However, during cold sunny days, an alternative option can be storing the thermal energy provided by the BIPV/T system in the building and then release it at night. The TES can be discharged passively to the conditioned space above it or actively into the heat pump. Using excess thermal en

28、ergy to preheat the air for night time operation is expected to improve the performance of the combined system. As mentioned above the test facility is designed for a VTAF system as well as a VCS and ICF thermal storages. The focus of this article is on the charging and discharging of the VCS. A det

29、ailed analysis of ICF systems has been previously studied by Ekrami et al (2015). The test facility will also benefit from the ventilated concrete slabs for both zones. Above the foundation, a rigid insulation panel separates the VCS from the VTAF. The VCS is made of a 4 inch concrete layer topped w

30、ith a corrugated steel deck. Then there is another concrete layer poured on top of the steel deck. The voids between bottom layer concrete and the corrugated steel work as air channels. A schematic of the VCS is provided in Figure 1-right. A theoretical analysis of the VCS performance has been studi

31、ed previously (Ekrami et al, 2015). In order to investigate the behavior of the VCS during the charging and discharging periods under different conditions a simplified three dimensional simulated model of the slab was developed using the Solidworks Flow Simulation software. Figure 1: Schematics of t

32、he test facility (Left) and Ventilated Concrete Slab (Right) VCS MODEL In order to reduce the computational time and due to the complicated nature of the full model, a fundamental three dimentional model was created. To accompolish this, advantage was taken of the symmetry in the geometry of the des

33、igned slab and a simplified model of the system was developed as shown in Figure 2. Since the geometry repeats a single channel from sides in parallel, the simplified model would retain the key features needed to be studied. The main feature, which needed to be investigated, is how well the concrete

34、 slab preserved heat that was extracted from the air flowing through the channels. The model excludes the losses in the connecting air ducts from BIPV/T system to the floor. The combined, radiative and convective, heat transfer coefficient between the slab surface and indoor room conditions was set

35、to 9 W/m2/K (1.58 Btu/hr/ft2/F) according to the ASHRAE Handbook (AHRAE, 2005). The model used dense concrete for the slabs which was chosen primarily due to it having a higher conductivity, 1.9 W/m/K (1.10 Btu/hr/ft/F), and better heat transfer; this was supported by the VCS study performed by Y. C

36、hen et al (2010). Since the floor is located on top of a thick and rigid insulation foam layer, the bottom surface is assumed to be perfectly insulated. This model implements average values and make reasonable assumptions for the flow and boundary conditions. However, under real conditions each chan

37、nel has slightly different inputs with distance from the duct inlet and outlet of the system. Keeping that in mind, this simulation is able to provide a fairly accurate trend of how the system would function under certain conditions. The single channel has three key dimensions, the length, width and

38、 thickness of the concrete slab. The channel has a length of 4.271 m (168 in), a width of 0.144 m (6 in) and a concrete slab thickness of 0.1016 m (4 in). These measurements are for one of the 27 channels in the entire ventilated concrete slab of each zone. The applied boundary conditions took into

39、account the number of channels to create an average volume flow rate, different scenarios to define the indoor temperatures as conditioned or unconditioned, and an atmospheric boundary condition for the outlet where the air is discharged. The flow rate through each individual channel was the total f

40、low, 0.51 m3/s (1081 CFM), divided evenly among the 27 channels for the sake of simplicity, which resulted in a flow rate of approximately 0.019 m3/s (40 CFM). The convective heat transfer modeling within the channel however, was solved using the softwares default settings, where a turbulent boundar

41、y layer was used at an intensity of 2% and length of 1.44 mm (0.06 in) for the turbulence parameters. Figure 2: A simplified geometry of a single channel of VCS This simplified model was developed in order to have a better understanding of the VCS system at its core components. The key goals that we

42、re studied in the simulation included the heat transfer rate from air to concrete, the inlet and outlet temperature changes and the average temperature of the concrete. These goals are to help understand how well the concrete can extract and retain energy, i.e. charging the VCS. In order to keep the

43、 model simple, yet increase the accuracy of the values obtained from the goals, a simplified duct was added to the inlet of the system. This was done in order to fully capture the direction of the flow entering the channel, affecting the turbulence, and hence the heat transfer rates. However, there

44、were no simple methods to implement the flow direction correctly to the channel, thus the resulting simulation was not as accurate as the initial model. Therefore, the single channel simulation with a normal flow through the inlet was used. RESULTS There were five scenarios that were modeled. Two of

45、 them being unconditioned spaces above the slab (such as a garage space) strictly using the generated thermal energy from PV/T system, and the other three settings are based on charging the slab with an ASHP at different outlet temperatures. The data for the first two simulations of unconditioned sp

46、ace were taken from a BIPV/T system model using the TRNSYS software (Kamel et al, 2014). Using the Toronto metropolitan weather library data, two sunny cold winter days were chosen to show how much thermal energy can be generated by the BIPV/T system and also how well the energy can be stored inside

47、 the TES. The purpose of these samples was to show the differences that may occur in two different climate conditions. Since the system is designed to be charged at day time and discharged at night, there is no benefit of charging the slab for few continuous days. Therefore a sunny and a partly clou

48、dy winter day was selected as samples to represent the real cold climate conditions in order to study the charging behavior of the VCS. These days happened to be January 21st and 22nd, where the 21st showed to have high solar radiation, i.e. a bright sunny day which is the most optimal condition, an

49、d the 22nd had a notable change in its heating cycle with couple of cloudy hours (lower radiation), i.e. a partly cloudy day which is a more common condition, in midday. Accordingly, since the space is assumed to be unconditioned, the outdoor ambient temperature was selected to be the initial concrete and room temperature. The second reason for starting at an ambient temperature is that the system is considered to be fully discharged after a whole night in operation. This was done so the simulation would