Article
Numerical evaluation of a phase change material–shutter using solar energy for winter nighttime indoor heating
Journal of Building Physics 2014, Vol. 37(4) 367–394 Ó The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1744259113496388 jen.sagepub.com
Nelson Soares1,2,3, Jose´ J Costa2, Anto´nio Samagaio4 and Romeu Vicente5
Abstract The incorporation of phase change materials in movable structural cells of shading elements associated with southward-facade windows is evaluated in this article. The proposed phase change material–shutter is a thermal energy storage system designed to take advantage of solar energy for winter nighttime indoor heating. A two-dimensional phase-change heat diffusion model based on the enthalpy formulation was considered. The numerical model follows the finite-volume method with a fully implicit formulation and allows the alternating melting and solidification of a phase change material submitted to cyclical thermal boundary conditions. Parametric investigations were carried out about the effects of thermophysical properties of the phase change material and temperature and convection heat transfer boundary conditions on the charge/discharge rates of energy. Due to the low thermal diffusivity of the phase change material, an aluminum fin arrangement was considered as a heat transfer enhancement technique. The distance between fins is directly proportional to the daily energy storage/release capacity of the system. The solar radiation flux has a strong effect on the charging/melting
1
MIT-Portugal Program, Energy for Sustainability Initiative, University of Coimbra, Coimbra, Portugal
2
ADAI-LAETA, Mechanical Engineering Department, University of Coimbra, Coimbra, Portugal
3
ISISE, Civil Engineering Department, University of Coimbra, Coimbra, Portugal
4
Environment and Planning Department, University of Aveiro, Aveiro, Portugal
5
Civil Engineering Department, University of Aveiro, Aveiro, Portugal Corresponding author: Nelson Soares, Departamento de Engenharia Mecaˆnica, Faculdade de Cieˆncias e Tecnologia, Universidade de Coimbra, Rua Luı´s Reis Santos—Po´lo II, 3030-788 Coimbra, Portugal. Email:
[email protected]
368
Journal of Building Physics 37(4)
processes during the day. The indoor temperature and the interior convection heat transfer coefficient have a major influence on the discharging/freezing processes during the night. The design of the phase change material–shutter depends strongly on the thermophysical properties of the phase change material and on the interior and exterior boundary conditions considered. Keywords Phase change materials, solar energy, thermal energy storage, latent heat, heat transfer, enthalpy formulation
Introduction Thermal energy storage (TES) is used to reduce buildings’ dependency on fossil fuels and to supply heat reliably. The best known method of TES in buildings involves sensible heat storage. It is used for the storage and release of thermal energy in a passive way, but in comparison with latent heat storage (by changing the phase of a storage material), a much larger volume of material is required to store the same amount of energy. Nowadays, an effective way to improve the thermal performance of buildings is by incorporating phase change materials (PCMs) in passive latent heat thermal energy storage (LHTES) systems of buildings’ walls, windows, ceilings, and floors. In their work about ‘‘The path to the high performance thermal building insulation materials and solutions of tomorrow,’’ Jelle et al. (2010) identified PCMs as one of the materials of tomorrow in the context of high-performance building envelopes. PCMs provide a large heat capacity over a limited temperature range, and they can act like an almost isothermal reservoir of heat. When the temperature rises and reaches Tm, the melting temperature of the PCM, the material changes phase from solid to liquid and absorbs latent heat maintaining the temperature equal to Tm. When the temperature decreases and reaches Ts, the solidification temperature of the PCM, the material solidifies and releases stored latent heat at a constant temperature Ts. During the last decade, more than 20 extensive review articles about the benefits of integrating PCMs in buildings were published allowing to conclude that interest in the subject is rising (Soares et al., 2013). For example, the surveys carried out by Soares et al. (2013), Kuznik et al. (2011), Zhou et al. (2012), Osterman et al. (2012), Rodriguez-Ubinas et al. (2012), Baetens et al. (2010), and Tyagi and Buddhi (2007) show how construction solutions with PCMs are related to buildings’ energy performance. Some PCMs have been identified for integration in passive LHTES systems such as PCM Trombe walls; PCM-shutters and window blinds (Weinlaeder et al., 2011); translucent PCM walls (Bontemps et al., 2011; Manz et al., 1997); PCM bricks (Alawadhi, 2008; Castell et al., 2010; Silva et al., 2012); PCM-enhanced wallboards (Kuznik and Virgone, 2009; Kuznik et al., 2008a, 2008b); PCM-enhanced concrete systems and mortars (Cabeza et al., 2007;
Soares et al.
369
Sa´ et al., 2012); shape-stabilized PCM-enhanced elements (Zhou et al., 2008, 2011; Zhu et al., 2011); and other PCM walls. According to Soares et al. (2013), PCM passive LHTES systems can contribute to (1) increase indoor thermal comfort (air temperature peak reduction, decrease of daily temperature swing, changing in the surface temperature); (2) improve buildings’ envelope performance and to increase systems’ efficiency (insulation capacity, change in heat flow through them, enhancing the thermal capacity); (3) decrease the conditioning power needed (reduction of the heating and cooling peak loads); (4) reduce energy consumption; (5) take advantage of off-peak energy savings; (6) take advantage of renewable sources like solar thermal energy; (7) save money during the operational phase of the building; and (8) contribute for the reduction of CO2 emissions associated to heating and cooling. In Mediterranean climates, during clear winter days when the temperature is low, solar energy is an abundant resource. To take advantage of this, new passive TES systems should be designed to store solar thermal energy during the day via PCM phase change from solid to liquid and to release the stored energy indoors during the night via PCM phase change from liquid to solid. This could be seen as a new approach of the indirect solar gain technique associated to southward building facades. The huge sensitive thermal mass of the traditional systems and the big amount of materials could be somehow replaced by the latent heat loads from the PCM phase change processes, and less quantity of materials would be necessary to store the same amount of energy. The main problem lies on how to incorporate PCMs into building components to store solar thermal energy during the day and to release it indoors during the night. This includes the major design parameters, namely, the PCMs melting/freezing temperature, its mass quantity, and its position within the TES systems. Moreover, such parameters need to be specified for given indoor loads and specific exterior climatic conditions. The main goal of this work is to evaluate the potential of incorporating PCMs in movable structural cells of shading elements associated with southward-facade windows. A new kind of PCM-shutter system is proposed and numerically evaluated to take advantage of solar energy for winter nighttime indoor heating in Coimbra, Portugal. The new TES system with PCMs is designed to store solar energy during the day and to release the stored thermal energy indoors during the night. A two-dimensional phase-change heat diffusion model based on the enthalpy formulation was developed. The numerical model follows the finite-volume method with a fully implicit formulation and allows the alternating melting and solidification of a PCM submitted to cyclical thermal boundary conditions. The numerical model was used for the parametric assessment of the influence of the thermophysical properties of the PCM on the charge/discharge daily rates of energy. This study also aims to evaluate the influence of the imposed exterior and interior temperatures and convection heat transfer coefficients on the performance of the system.
370
Journal of Building Physics 37(4)
Figure 1. (a) Configuration of the PCM-shutter system. Schematic representation of the functioning of the system: (b) during a winter day—the system stores thermal energy when it is opened (PCM changes phase from solid to liquid), (c) during a winter night—the TES system releases the stored latent heat when it is closed (PCM changes phase from liquid to solid). PCM: phase change material.
Methodology System description The architectural configuration of the PCM-shutter system is described by Soares et al. (2011), and it is schematically represented in Figure 1(a). It consists of two shutter panels, each 0.5 m 3 1.5 m, composed of a stack of aluminum rectangular cavities filled with the PCM. The horizontal walls of the cavities are supposed to act as a metal fin arrangement to enhance the rate of heat transfer to the PCM. The use of metal fins in LHTES systems to compensate the low thermal conductivity of PCMs may be found in works by Costa et al. (1998), Chen and Sharma (2006), Sharma et al. (2006), Chen et al. (2008b), Stritih (2003), Jegadheeswaran and Pohekar (2009), and Shatikian et al. (2005). The insulation layer on the back surface of the shutter is essential to enable the control of the direction of the heat flow, especially during the thermal discharge of the system. The rationale of the system operation during a winter daily cycle is sketched in Figure 1(b) and (c). The system must operate cyclically, reflecting the ongoing daily cycles of 24 h. Similarly, the cyclical operation of the system should enable the fusion of the PCM mass during the day and its solidification during the night, enabling the daily cyclic storage and release of thermal energy. The system is to be opened during the day to maximize the solar direct gains indoors through the glass window and, simultaneously, to allow its charging—PCM melting. According to Santos et al. (2010), solar heat gains through the windows and door glazing have a considerable impact on the thermal performance of buildings, particularly in facades exposed south and west (northern hemisphere). During the night, the system must be closed to minimize the heat losses through the window area and to allow its discharging by releasing the thermal energy indoors—PCM solidification. During the night, the
Soares et al.
371
glazing frame must be opened to ensure the air circulation in the air cavity between the window glass and the PCM-shutter, promoting the heat release indoors. If the PCM mass is overestimated, the time needed for the heat to penetrate the PCM could become larger than the sunshine period, and the storage process (melting process) cannot be completed. Likewise, if the PCM mass is overestimated, the time needed for the heat to be released indoors could become larger than the period when the system is closed, and the solidification process cannot be completed. Therefore, the optimization process is to increase the storage/release capacity using as little PCM mass as possible. The ideal behavior of the system under cyclic conditions is obtained when both the mass of solid PCM is totally melted during the charging period (storing the maximum potential energy) and the mass of liquid PCM is totally solidified during the discharging process (releasing the maximum energy stored). Several previous works have been devoted to idealize and design this kind of system. Costa et al. (1998) and Brousseau and Lacroix (1998) proposed and studied two different LHTES systems designed to store the off-peak electrical energy in the form of thermal energy via PCM phase change processes. Using off-peak electricity, a PCM can be melted as to store electrical energy in the form of latent heat thermal energy, and the heat is then available when needed for indoor heating during the period when the electricity is most expensive. The PCM-shutter system aims to take advantage of the solar energy for melting the PCM instead of the electrical energy. As regards the appearance of the buildings facades, the system described above has the same architectural appearance of the traditional Portuguese exterior shutter systems. Therefore, the PCM-shutter could be particularly interesting for the thermal refurbishment of existing buildings, when the improvement measures cannot change the visual appearance of the buildings facades, and for the design of more energy efficient buildings.
Boundary conditions Figure 2 shows the reference outdoor conditions to be used in the model in which Te,ref (°C) is the monthly average of the hourly external air temperature and qrad,ref (Wm22) is the monthly average of the hourly solar radiation flux on a vertical south-facing wall during January in Coimbra, Portugal. Both variables are calculated using the national climatic data stored in SOLTERMÒ. The external convection heat transfer coefficient considered is he = 25 Wm22K21. The indoor air temperature, Ti, is influenced by outdoor conditions, by the internal loads, and by the building envelope constitution. However, in this study, during the time when the system is to be closed, the average indoor air temperature is assumed to be constant. Under comfort conditions in winter, the indoor temperature at night should not fall below 18°C (which is the lowest category of thermal comfort in EN 15251:2007 (2007)). However, for the analysis of the heat diffusion with phase change and for the parametric assessment of the system’s behavior considering different boundary conditions in the numerical model, temperatures lower
372
Journal of Building Physics 37(4)
(a)
(b)
Figure 2. Reference outdoor conditions during the time when the PCM-shutter is opened— sinusoidal evolution of the (a) air temperature, Te,ref, and (b) solar radiation calculated for a vertical south facade wall, qrad,ref. Climate data determined from SOLTERM—conditions for Coimbra in January. PCM: phase change material.
than 18°C were considered. The low indoor temperature during winter is one of the problems in Portuguese nonefficient existing buildings, and it was the background reality for the research idea presented in this article. During the last years, the energy efficiency of the Portuguese building stock has increased a lot due to recent regulations and new requirements of the inhabitants. However, several studies have introduced the concept of fuel poverty and the effect of poor housing conditions on health in southern-European countries including Portugal. As suggested by Healy (2004), the ability to heat the home adequately is a fundamental aspiration and households which declare an inability to do this may be considered fuel-poor. Results show that an alarming 74.4% of households in Portugal declare this inability at the time of the study carried out by Healy (2004). The internal convection heat transfer coefficient, hi, can be estimated by appropriate empirical correlations; its value can vary significantly according to the free or forced convection regime or the flow velocity. It is important to evaluate how hi affects the overall performance of the system. During the night, three different models will be considered for the evaluation of hi according to the system’s configuration: (1) when the lower and upper parts of the window frame are kept open, the natural convection heat transfer over the vertical hot plate is considered (Figure 3(a)); (2) when the window frame is totally closed during the night, the natural convection in a vertical rectangular air cavity is considered (Figure 3(b)); and (3) when the lower and upper parts of the window frame are opened and a ventilator is adjoined in the enclosure, forced convection conditions are ensured and considered. For the natural convection heat transfer over the vertical hot plate model, the simple empirical correlation for the average Nusselt number, Nu, is
Soares et al.
373
Figure 3. Natural convection models considered. Natural convection (a) over vertical hot surface and (b) in a vertical rectangular cavity. PCM: phase change material.
Nu =
hLc = C ðGrL PrÞn = CRanL k
ð1Þ
where the Rayleigh number, Ra, is the product of the Grashof and Prandtl numbers (for air Pr ’ 0.7) RaL = GrL Pr =
gbðTs T‘i ÞL3c Pr n2
ð2Þ
The values of the constants C and n depend on the geometry of the surface and the flow regime, which is characterized by the range of Ra. Considering a vertical plate with characteristic length, Lc, equal to the plate height (1.5 m) Nu = 0:59Ra(1=4) , if Ra
