Phase change materials for solar thermal energy storage in residential buildings in cold climate

Phase change materials for solar thermal energy storage in residential buildings in cold climate

Renewable and Sustainable Energy Reviews 48 (2015) 692–703 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journa...

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Renewable and Sustainable Energy Reviews 48 (2015) 692–703

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Phase change materials for solar thermal energy storage in residential buildings in cold climate Zhihua Zhou a, Zhiming Zhang a, Jian Zuo b,n, Ke Huang a, Liying Zhang c a

School of Environmental Science and Engineering, Tianjin University, China School of Natural and Built Environments, University of South Australia, Adelaide 5000, Australia c Baoding Institute of Architecture Design Co. Ltd., Hebei, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 21 July 2014 Received in revised form 2 December 2014 Accepted 9 April 2015

Heating accounts for a large proportion of energy consumption in residential buildings located in cold climate. Solar energy plays an important role in responding to the growing demand of energy as well as dealing with pressing climate change and air pollution issues. Solar energy is featured with low-density and intermittency, therefore an appropriate storage method is required. This paper reports a critical review of existing studies on thermal storage systems that employ various methods. Latent heat storage using phase change materials (PCMs) is one of the most effective methods to store thermal energy, and it can significantly reduce area for solar collector. During the application of PCM, the solid–liquid phase change can be used to store a large quantity of energy where the selection of the PCM is most critical. A numerical study is presented in this study to explore the effectiveness of NH4Al(SO4)2  12H2O as a new inorganic phase change material (PCM). Its characteristics and heat transfer patterns were studied by means of both experiment and simulation. The results show that heat absorption and storage are more efficient when temperature of heat source is 26.5 1C greater than the phase transition temperature. According to heat transfer characteristics at both radial and axial directions, it is suggested to set up some small exchangers so that solar energy can be stored unit by unit in practice. Such system is more effective in low density residential buildings. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Solar thermal energy storage NH4Al(SO4)2  12H2O Phase change material Sustainability

Contents 1. 2. 3.

4.

5.

6.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693 Solar energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693 Selection of PCM and its heat transfer characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695 3.1. Selection of phase change temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695 3.2. The determination of PCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695 Simulation to optimize the heat source temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695 4.1. Mathematical model of PCM heat transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695 4.2. Boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 696 4.3. Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 697 4.3.1. Simulation of heat charging process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 697 4.3.2. The effects of heat source temperature on the heat charging rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 697 The design of phase change heat transfer experimental system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 698 5.1. Heat source and thermostat system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 5.2. Phase change heat transfer device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 5.3. Temperature measurement system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 5.4. Flow measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 6.1. Heat transfer process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 699 6.2. Temperature variation in radial direction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 700

Corresponding author. Tel.: þ 61 883021914. E-mail address: [email protected] (J. Zuo).

http://dx.doi.org/10.1016/j.rser.2015.04.048 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

Z. Zhou et al. / Renewable and Sustainable Energy Reviews 48 (2015) 692–703

6.3. PCM temperature variation in axial direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction There are significant environmental impacts associated with construction activities. Globally, the building stock is responsible for a high proportion of the total primary energy use [1,2]. In many countries, this proportion is as high as 40% [3]. The energy consumption in the residential sector accounted for more than 70% of the total amount of energy consumed by buildings worldwide [4]. This is compounded by the rapid urbanization associated with massive demands of resources including energy in order to satisfy needs of human beings' activities [5,6]. It was projected that building energy use will account for more than 35% of the national energy consumption in 2020 in China of which 65% are consumed by heating, ventilation and air-conditioning systems (HVAC) [7–9]. Furthermore, it is well recognized that space heating and water heating consume a large quantity of the energy in residential buildings [10–12]. This is particularly the case in those regions located in cold climate such as North China with a prolonged period of extreme cold weather. District heating has been introduced to China as one of building energy efficiency measures to satisfy the heating demands during the winter season [13,14]. As a result, significant efforts have been made to investigate the deployment of renewable energies in buildings, either large scale or community scale [15–17]. The integration of renewable energies with buildings helps not only satisfy growing energy demand but also deal with pressing environmental pollution issues. As an inexhaustible clean energy resource, solar energy plays a crucial role in dealing with pressing climate change and global warming issues [18]. Indeed, the direct solar radiation has become one of the most effective ways of utilizing renewable energies [19,20]. A large amount of radiation energy is released by sun to its surroundings [21,22]. However, due to its nature of low-density and intermittency, solar energy needs to be collected and stored efficiently [23,24]. There are necessary conditions for utilizing solar energy, i.e. sufficient amount of solar energy resources and adequate area for solar collector [25,26]. In addition, there should be substantial amount of demand for using solar energy such as water heating, space heating and refrigeration and airconditioning [27–30]. These external factors significantly affect the utilizing of solar energy in buildings. China has five climate zones according to the climatic conditions of each geographic area, i.e. Cold; Very cold; Hot summer and cold winter; Hot summer and warm winter; and Moderate [16]. Regions located in cold climate zone (mainly North China) have significant amount of heating demands with sufficient solar energy resources. Therefore, these regions are very suitable for utilizing solar thermal energy storage. This paper presents a critical review of literature related to solar thermal energy storage particularly the selection of phase change materials. This is followed by a numerical study to test the new application of this way of utilizing solar energy. The focus is placed on the solar thermal energy storage and heating in North China. PCM NH4Al(SO4)2  12H2O, a new inorganic material is tested in experiments. The melting temperature of this material can satisfy the corresponding heating requirements. Consequently its phase change mechanism and heat transfer in the heat reservoir are analyzed. The findings provide useful references for

693

700 700 701 701 701

future development of PCMs for thermal energy storage in those regions located in cold regions.

2. Solar energy storage Solar energy is an attractive substitute for conventional fossil fuels for heating applications [38]. There are many ways of utilizing solar energy such as solar photovoltaic, solar hot water and solar lighting [39–41]. The solar water heating technologies are comparatively mature. By contrast, solar space heating has comparatively limited application due to the instable thermal energy storage and the significant demand of the collector area. There are two mechanisms for solar thermal energy storage, i.e. sensible heat storage and latent heat storage. Sensible storage has disadvantages such as small thermal storage density and large heating loss [31]. On contrary, by using PCMs, latent heat storage is one of the most effective methods to store thermal energy due to higher heat storage capacity as well as more isothermal behavior during charging and discharging [32–34]. As a result, phase change materials have been widely applied in practice with an aim to enhance the storage capacity of various thermal energy systems [35–37]. As an important technology to deal with the time-discrepancy issue associated with the solar energy utilization, latent heat storage is a challenging key technology for space heating and can significantly increase the solar fraction [42–44]. Therefore, solar storage has attracted considerable attention from both industry and academics for its various applications especially for solar heating systems. A typical system of solar thermal energy system is shown in Fig. 1. North China is very suitable for utilizing solar thermal energy because: (1) there are massive heating demands due to its cold climatic condition; and (2) there are abundant solar resources available in the local region. For example, in Tianjin, a typical city of North China, district heat supply will be provided 135 days every year. Many regions of North China have 3200 annual sunshine hours and annual solar radiation of 160 kJ/cm2 on average [45]. According to the thermal storage temperature, solar energy storage can be classified into two methods, i.e. low temperature and high temperature. It is very common that low temperature thermal storage system is integrated with ground source heat pump in order to increase the temperature for space heating purpose [46–49]. When the storage temperature reaches 60 1C, it is suitable to utilize the direct heating. Recently, latent heat storage technology has mainly been applied in space heating and domestic hot water (DHW) supply for which the required temperature ranges from 40 to 80 1C [50]. Such storage systems must have on the one hand the lower heat losses during storing, and on the other hand, the smaller volume i.e. the highest energy density [51–53]. Materials to be used for phase change thermal energy storage must have a large latent heat and high thermal conductivity [54,55]. They should have a melting temperature lying in the practical range of operation, melt congruently with minimum subcooling and be chemically stable, low in cost, nontoxic and non-corrosive [56–58]. There are many types of materials that could be utilized for solar thermal energy storage. There materials can be broadly classified into organic and inorganic substances. Organic PCMs include Lauric acid [59–62] (melting temperature of 41–43 1C with a latent heat of

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Nomenclature β T t Tsolidus Tliquidus ρ μ v μ Cp p

liquid fraction temperature (1C or K) time (min) solidus temperature (1C or K) liquidus temperature (1C or K) density (kg/m3) fluid velocity in the radial direction (m/s) fluid velocity in the axial direction (m/s) dynamic viscosity (kg/m K) specific heat at constant pressure (J/kg K) pressure (Pa)

Fig. 1. Solar thermal energy system.

H h ΔH ref href tref ρref Tref ε Amush α L

enthalpy (J) sensible enthalpy (J/kg) latent heat enthalpy (J/kg) reference reference enthalpy (J/kg) reference time (min) density corresponding to tref (kg/m3) reference temperature (K) a small computational constant morphology constant cubic expansion coefficient (K  1) latent heat capacity (J/kg)

211.6 kJ/kg), Stearic acid [62–65] (melting temperature of 41–43 1C with a latent heat of 211.6 kJ/kg) and paraffin waxes [66–71]. Although the melting temperature range of paraffin with a latent heat of 128–198 kJ/kg can be 12–71 1C, its low thermal conductivity means a large surface area is required which limits its application in practice. In solar thermal energy storage (TES), inorganic salt hydrate was used as PCM, such as CaCl2  6H2O [72–77] (melting temperature of 29 1C), NaSO4  10H2O [78](melting temperature of 32.4 1C), and Na2HPO4  12H2O [79] (melting temperature of 35 1C). The melting temperature of these materials is too low to be used for heating directly in residential buildings. MgCl2  6H2O (melting temperature of 116.7 1C, with a latent heat of 168.6 kJ/kg) [80,81] is a good alternative. Last decades witnessed significant amount of studies been undertaken to improve the drawbacks of materials used in TES

Fig. 2. Selection criteria for latent heat storage materials.

Z. Zhou et al. / Renewable and Sustainable Energy Reviews 48 (2015) 692–703

systems such as low thermal conductivity and phase segregation [82-84]. Two of most critical factors during the application of PCMs are the selection of the phase change material as heat transfer mechanisms [85–87]. Various applications of PCM have been studied in domestic hot water tanks, space heating and cooling of buildings, peak load shifting, solar energy applications, and seasonal storage [32,35,36,63,87–95].

3. Selection of PCM and its heat transfer characteristics The selection of PCM is crucial for the solar thermal energy storage. There are different ways to classify PCMs such as phase change temperature, material properties etc. Common factors during the selection of PCMs include: thermodynamic properties, kinetic properties, chemical properties and economic properties [96] (see Fig. 2).The phase change temperature is one of the most important selection criteria as it affects the entire heat transfer process [97,98]. 3.1. Selection of phase change temperature The heating temperature in buildings varies according to types of heating devices. In China, the ordinary heaters usually use 95 1C or 85 1C of water temperature. Fan coil system usually use water with a temperature of 65 1C or cooler. For radiant floor heating, it is usually about 45 1C. Theoretically, PCM works as long as its phase change temperature is 5–10 1C greater than the temperature of the supply water. In practice, PCM with higher phase change temperature cannot only satisfy various types of heating devices but also store more Table 1 Physical properties of NH4Al(SO4)2  12H2O. Molar mass

Melting temperature

Thermal conductivity

Solid density

Liquid density

Volume expansion

g/mol 453.33

1C 93.5

W/(m K) 0.55

kg/m³ 1639.2

kg/m³ 1527.2

% 6.84

695

sensible heat. However, the higher outlet temperature, the lower efficiency is the solar collector. Therefore, the selection of PCMs should take end users' demand into consideration. In this study, focus is placed on heaters with supply water temperature of about 85 1C and the phase change temperature of 90–95 1C. 3.2. The determination of PCM There are many types of materials with a phase change temperature between 90 and 95 1C, including organic and inorganic substances. Disadvantages of organic PCMs include low thermal conductivity, low intensity, flammable and low heat storage capacity [99]. Inorganic substances include crystallized salt hydrate, molten salt, metal and alloy. As the most typical inorganic PCM, crystallized salt hydrate has a number of advantages such as low cost, high volumetric latent thermal storage capacity, high heat of melting, fixed melting point, and high thermal conductivity. However, phase separation or super-cooling could happen during the practice [63,93]. NH4Al(SO4)2  12H2O is chosen due to its no phase separation phenomenon, good thermal stability and affordability. The effects of super-cooling during the crystallization can be dealt with by means of adding nucleating agent [98]. The physical properties of NH4Al(SO4)2  12H2O are shown in Table 1 [100].

4. Simulation to optimize the heat source temperature 4.1. Mathematical model of PCM heat transfer The liquid fraction of PCM can be calculated in the following equation [101]: 8 0 T o T solidus > > < T T solidus ð1Þ β ¼ T liquidus  T solidus T solidus o T o T liquidus > > :1 T 4 T liquidus The value of β depends on the T, Tsolidus and Tliquidus. The continuity, momentum and thermal energy equations of PCM heat transfer are described as below [102].

Fig. 3. The temperature measurement points.

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Continuity equation: ∂ρ ∂ðρuÞ ∂ðρυÞ þ þ ¼0 ∂t ∂x ∂y

Table 3 The corresponding boundary conditions of heat transfer fluid.

ð2Þ

Momentum equation:    2  ∂u ∂u ∂u ∂ u ∂2 u ∂p þu þυ ¼μ þ  þ Su ρ ∂t ∂x ∂y ∂x ∂x2 ∂y2

ð3Þ

   2  ∂υ ∂υ ∂υ ∂ υ ∂2 υ ∂p þu þυ ¼μ þ 2  þSυ ρ 2 ∂t ∂x ∂y ∂y ∂x ∂y

Properties

Settings

Initial temperature (melting) Inlet condition Inlet velocity Outlet condition

40 1C velocity-inlet 2.1 L/min Outflow

ð4Þ 90.0

Thermal energy equation:     ∂H ∂H ∂H k ∂2 H ∂2 H þu þυ ¼ þ 2 þ Sh ρ 2 ∂t ∂x ∂y cp ∂x ∂y

ð5Þ

80.0 70.0

For the density change during the phase change process, Solidification/Melting model can be supposed as below:

60.0 50.0

Based on the aforementioned assumptions, the density in the gravity related momentum equation can be calculated as: ρ ¼ ρref ½1  βðT T ref Þ

ð6Þ

Source terms in both momentum equation and thermal energy equation in the FLUENTs software were adjusted considering the effects of natural convection due to the density change of PCM during the phase change process: Source terms in the thermal energy equation: Sh ¼

ρ ∂ðΔH Þ cp ∂t

ð7Þ

Source terms in the momentum equation: ð1  β Þ2 Su ¼  3 Amush u β þε

ð8Þ

ð1  βÞ2 Sυ ¼  3 Amush υ þ ρref gαðt t ref Þ β þε H ¼ h þ ΔH h ¼ href þ

ð10Þ T T ref

cp dT; ΔH ¼ βL

ð11Þ

4.2. Boundary conditions The physical model of the concentric tube heat exchanger shown in Fig. 3a can be simplified as a two-dimensional model. Gambit2.3 software is adopted to develop grid. The grid spacing ratio was set as 2 and there were a total of 17,000 grids. Every Table 2 Thermal physical properties and corresponding boundary conditions of NH4Al (SO4)2  12H2O. Properties

Settings

Phase transition temperature Latent heat of phase change Density Heat conductivity coefficient Initial temperature

93.5 1C 300 kJ/kg ρ¼ 1640–1.6Tn(kg/m3) 0.5 W/(m K) 34 1C

Tn is temperature, K.

30.0 0 10 20 30 40 50 60 70 80 90 100110120130140150160170180 time/minute experimental data simulated values Fig. 4. The comparison of average temperature between experiment and simulation.

130.0 120.0 110.0 100.0 t 90.0 / 80.0 70.0 60.0 50.0 40.0 30.0

2mm

ð9Þ

H in the thermal energy equation can be expressed as: Z

40.0

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

(1) The viscosity dissipation of fluid is ignored. (2) All physical properties are constant except density. (3) Density change is only considered in those related to volume in the momentum equation.

time/minute 15mm

27mm

130.0 120.0 110.0 100.0 t 90.0 / 80.0 70.0 60.0 50.0 40.0 30.0 0 10 20 30 40 50 60 70 80 90 100110120130140150160170180 time/minute 0.1m 0.3m 0.5m 0.7m 0.9m 1.1m Fig. 5. The temperature variations pattern when the flow inlet temperature is 120 1C.

single line and surface was defined, such as heat transfer fluid, PCMs, inlet and outlet of heat transfer fluid, and the heat exchanger tube wall, etc. Grid files were imported into the FLUENT6.3 software and twodimension double precision solver was chosen. Solver was defined as segregated, 2D, unsteady, and implicit. K-epsilon (Eq. (2)) was chosen to model viscosity. Solidification/Melting model was

Z. Zhou et al. / Renewable and Sustainable Energy Reviews 48 (2015) 692–703

697

Fig. 6. The melting rate variation cloud diagram when flow inlet temperature is 120 1C.

activated and the energy equation was automatically enabled. Thermal physical properties and corresponding boundary conditions of the heat transfer fluid and NH4Al(SO4)2  12H2O are shown in Tables 2 and 3. During the simulation, nine lines of temperature measurement were set up in the heat storage area of phase change collector in order to monitor the average temperature in different places of PCM (see Fig. 3b). Therein, 6 lines were in the axial direction (0.1–1.1 m), 0.1 m, 0.3 m, 0.5 m, 0.7 m, 0.9 m, and 0.9 m from the top respectively. There are three lines in the radial direction, distant from the internal tube wall of 2 mm, 15 mm, and 27 mm respectively. Simulation results and experimental data for a period of 180 min are shown in Fig. 4. A similar changing pattern emerged.

at the location of the first column of measurement points all melt into liquid, whereas NH4Al(SO4)2  12H2O melt in 100th minute and 125th minute at the second column and the third column of measurement points respectively. The phase of PCM no longer changed and the temperature of all PCM was close to 120 1C. (2) Heat transfer in the axial direction As shown in Fig. 5b, the simulation results are some different from with experimental results. During the same period of time, the temperature dropped along the flow in simulation, whereas the temperature was higher in the middle than both ends in experiment. This further illustrates that the low temperature in two ends of PCM in the axial direction is due to the poor insulation.

4.3. Simulation As shown in the experiment, the heat charging process is slow when the heat source temperature is 99 1C for PCM with a phase transition temperature of 93.5 1C. Both the heat exchange intensity and the heat source temperature need to be enhanced. Therefore, the heat charging process and heat transfer characteristics at 120 1C of heat source temperature are simulated. Consequently, the heat charging characteristics at different temperatures of heat source were compared in order to determine the best heat source temperature in terms of heat charging NH4Al(SO4)2  12H2O. 4.3.1. Simulation of heat charging process Heat source with a temperature of 120 1C was selected to transfer heat with PCM and boundary conditions are set as shown in Table 3 with the physical model as shown in Fig. 4. The simulation of heat charging process is shown as below. (1) Heat transfer in the radial direction As shown in Fig. 5a, the average temperature dropped in sequence according to its distance from the heat source. In addition, the average temperature difference between groups of temperature measurement points increased in the first instance before descending. Since the 80th minute, NH4Al(SO4)2  12H2O

As shown in Fig. 6, the heat transfer intensity is the biggest near the inlet of heat transfer fluid where solid/liquid phase change occurs first. Consequently, the melting area gradually extends to the radial and axial direction, shown as T distribution in Fig. 6. The phase change material at the bottom of tube melted last. In particular, the PCM most distant from inner sleeve experienced difficulties to be melted.

4.3.2. The effects of heat source temperature on the heat charging rate During the experiment, when the temperature difference between heat source and PCM was 5.5 1C, the heat transfer and the melting process took a very long period of time. During the simulation, when temperature difference between heat source and PCM was 26.5 1C, the PCM melt in 3 h. Simulation was conducted to investigate the effects of inlet temperature on the melting process of PCM where other conditions are kept the same. The inlet water temperature is set as 96.5 1C, 120 1C and 150 1C respectively. Other boundary conditions were: heat transfer liquid flowing top down out; the initial average temperature of PCM was 34 1C; the heat transfer fluid flow was 2.1 L/min.

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1.2

100.0

1.0 t /

0.8 60.0 0.6 40.0

0.4 0.2

0.0

0.0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

20.0

melting rate

80.0

time /minute average temperature

melting rate

1.2 120.0 0.8

80.0

0.6

60.0 40.0

0.4

20.0

0.2

0.0

0.0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

t /

melting rate

1.0

100.0

tmie/minute average temperature

melting rate

160.0

1.2

140.0

1.0 0.8

t 100.0 / 80.0

0.6

60.0

0.4

melting rate

120.0

40.0 0.2

20.0

0.0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

0.0 time/minute average temperature

melting rate

Fig. 7. The temperature and melting rate change pattern.

Fig. 8. Experimental system.

(1) Inlet temperature of heat source is 96.5 1C As shown in Fig. 7a, solid/liquid phase change did not occur until 120th minute. Before the 120th minute mark, the melting rate is 0 and there is only sensible heat charging. During this

process, the average temperature of PCM increased dramatically. Since then, PCM started to melt, where sensible and latent heat storage processes coexist. The heat transfer method was gradually changed from thermal conduct to thermal convection. The average temperature of PCM still rose; however the increase rate became slower. PCM melted quickly when the melting rate is smaller than 0.8. However, when PCM started to melt, its thermal conductivity is reduced. Furthermore, the temperature of PCMs increased to bridge the gap with the temperature of heat transfer liquid. This weakened the intensity of subsequent heat transfer. It is a very slow process that the melting rate increased from 0.8 to 1, taking 1270 min, accounting for 70% of the total period of time. The entire solid/liquid phase change process of PCM took 1800 min. Therefore, the inlet temperature of heat transfer liquid needs to be increased if NH4Al(SO4)2  12H2O is selected as PCM. (2) The heat source temperature is120 1C PCM started to melt from the 10th minute, accompany with both sensible and latent heat storage. The average temperature of PCM and melting rate increased constantly. At the 100th minute, the average temperature reached 114 1C and the melting rate was 0.94. Since then, as almost PCM became liquid, the thermal conductivity was reduced. Furthermore, the temperature of PCM increased to bridge the gap with the inlet temperature (see Fig. 7b).When the inlet temperature was 120 1C, the entire phase change process took 160 min, and the temperature of PCM was 119.5 1C. In next 20 min, PCM stored heat through the sensible method; however the temperature increased very slowly. The final PCM temperature was 119.6 1C at the 180th minute. Therefore, when the inlet temperature of heat transfer fluid increased from 96.5 1C to 120 1C, the heat transfer intensity enhanced significantly. The entire period of time of solid/liquid phase change was reduced by 91.1%, from 1800 to 160 min. (3) Heat source temperature is 150 1C As shown in Fig. 7c, the temperature of PCM increased dramatically where the melting process started from the fourth minute. PCM melted completely after 60 min where the average temperature of PCM was 148.6 1C. Since then, the heat transfer intensity reduced as there is little difference between temperature of PCM and that of heat transfer fluid. The average temperature of PCM reached 149.7 1C at the 160th minute. When the inlet temperature increased from 120 1C to 150 1C, the solid/liquid phase change time was reduced by 62.5%, from 160 min to 60 min. However, the increase of inlet temperature has some cost-related implications such as high temperature resistant performance of system, the selection of heat transfer fluid, and the type of solar collector. A thorough consideration of all these factors is required when determining the inlet temperature. This study found that it is more efficient to use heat source with a temperature of 120 1C if NH4Al(SO4)2  12H2O is selected as PCM for solar energy thermal storage. The entire charging process is less than 3 h. If the heat storage device can be divided into several small units, the charging and discharging process will be more efficient.

5. The design of phase change heat transfer experimental system To study the heat exchange properties of PCMs, an experimental system was set up. The system (see Fig. 8) consisted of solar collectors, water tank with constant temperature, circulating pump, phase change heat exchanger, etc.

Z. Zhou et al. / Renewable and Sustainable Energy Reviews 48 (2015) 692–703

699

5.1. Heat source and thermostat system 90.0

Glass vacuum tube collectors were selected for heat collection. The solar outlet temperature lacks stability or continuity. Therefore, this system used electric heater for auxiliary heating. Furthermore, thermostat system was designed to maintain the water temperature at about 99 1C. Heating system was composed of water tank (600 mm  400 mm  400 mm), electric heater (3 kW), and the power controller. In order to reduce heat loss, the water tank was wrapped by 50 mm of silicate insulation board.

80.0 70.0 t / 60.0 50.0 40.0

5.2. Phase change heat transfer device 1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221 231 241 251 261 271 281 291 301 311 321 331 341

30.0

NH4Al(SO4)2  12H2O has little corrosive effects on copper. The thermal conductivity of copper is 398 W/(m K) [103], ranking the second in metal materials, only behind silver. Therefore, concentric tube with a height of 1.2 m was adopted as the phase change heat exchanger in the experimental system. The diameter of internal tube was 12 mm with a wall thickness of 1.5 mm. Hot water flowed in the internal tube to exchange heat with PCM. The diameter of external tube was 80 mm with a thickness of 5 mm. PCM was placed between internal tube and external tube. In order to enhance the efficiency of heat transfer, copper wires were distributed in PCM evenly. The distance of copper wires from the top of tube was 0.1 m, 0.3 m, 0.5 m, 0.7 m, 0.9 m, and 1.1 m respectively (see Fig. 3).

time/minute 2mm

15mm

27mm

90.0 80.0 70.0 t / 60.0 50.0 40.0

5.3. Temperature measurement system Temperature sensors were installed to measure the temperature of PCM in concentric tube, inlet/outlet water temperature of phase change transfer, water tank and environment. PT1000 platinum resistor was selected as thermometer as NH4Al(SO4)2  12H2O has certain corrosive effects to metal in its melting state. The temperature ranged from 20 1C to 100 1C with 70.1 1C accuracy. The sensors were distributed with the assistance of copper wire mesh. There were six groups of sensors in the axial direction, each with 3 measurement points. The first group of measurement points were 2 mm away from the internal tube wall (marked as 1, 4, 7, 10, 13, and 16). The second group of sensors were 15 mm away from the internal tube wall ( marked as 2, 5, 8, 11, 14, and 17).The third set of measurement points were 27 mm away from the internal tube wall (marked as 3,6, 9, 12, 15, and 18) (see Fig. 3). Temperature sensor with acquisition module converted analog signals to digital signals that were displayed and stored in computer. 5.4. Flow measurement

100.0

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

90.0 80.0 70.0 60.0 50.0 40.0 30.0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340

t /

heat flow/kW

Turbine flowmeter was utilized to measure the water flow which can be adjusted by valves. The circulating flow rate was set as 2.0 L/min (0.12 m3/h), where the water velocity was 1.06 m/s. PT1000 platinum resistors were used to measure inlet and outlet water temperature. The flow and temperature signal was connected

time/minute supply water

return water

average temperature

Fig. 9. Heat transfer changes trend.

heat flow

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340

30.0 time/minute 0.1m

0.3m

0.5m

0.7m

0.9m

1.1m

Fig. 10. The temperature variation pattern.

to the integrating heat meter through wires and then upload to computer via the communication interface. This system helped to measure and record flow rate, temperature, heat transfer rate, cumulative flow, and cumulative heat volume instantly.

6. Experiment There are three kinds of physical phenomenon during the phase change process, i.e. phase change, thermal conduct and thermal convection. Thermal conduct and thermal convection predominately takes place in solid phase and liquid phase respectively [104]. During the process of phase change, there is a moving interface between solid and liquid phase which is intrinsically highly nonlinear [105–107]. 6.1. Heat transfer process Prior to the experiment, water with temperature of 40 1C was circulated in the loop until the temperature is stabilized. The measurement showed that the initial temperature of PCM distant from external tube wall of 2 mm, 15 mm, and 27 mm is 38 1C, 34 1C and 32 1C respectively. Consequently, PCM was heated continuously by using water with a temperature of 99 1C. The computer automatically collected and recorded the inlet and outlet water temperature, temperature of PCM, and heating transfer volume in the system (see Fig. 9). The experiment lasted 340 min, and the average temperature of the heat storage material reached 80 1C, which is lower than the phase transition temperature. It is worth noting that inlet and outlet water temperature, PCM temperature and heat flow transfer all tended to stabilize during the late period of experiment from 200th to 340th minutes. The temperature curve showed:

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(1) At the beginning of experiment, there was a large gap between the temperature of heat source and that of PCM. PCM was in solid phase; therefore the heat transfer was mainly via thermal conduct. The heat transfer process of PCM was very efficient where the temperature rose very quickly. (2) Since then, the temperature of NH4Al(SO4)2  12H2O close to the internal tube wall increased, which reduced the temperature gap of heat transfer process. In addition, solid/liquid phase change took place in the PCM close to the internal tube wall. Heat transfer during this period was through thermal conduct and thermal convection. Thermal conductivity decreased, so as the power of heat exchange. (3) During the late period of experiment, the phase change amount of PCM close to internal tube wall increased gradually. As a result, PCM transferred heat to the distance when it absorbed heat simultaneously. The efficiency of heat transfer and the temperature of return water remain stable. (4) The phase transition temperature of NH4Al(SO4)2  12H2O was 93.5 1C. It was difficult to change its phase when using heat source with a temperature of 99 1C for heat charging.

6.2. Temperature variation in radial direction During the process of heat charging, hot water exchanged heat with PCM near the tube through the inner sleeve in the first instance, and then to the distance in sequence. The variations of average temperature of each part of PCM are shown in Fig. 10a. In the first 60 min, the temperature rose very quickly, and then slowed down. During the running period of more than 6 h, the average temperature of the first, second and third columns of measurement points rose from 38 1C, 34 1C, 32 1C, to 83 1C, 80 1C and 76 1C respectively. Within the radius of 25 mm, the maximum temperature difference was 7 1C. The lowest temperature is 23 1C lower than the water temperature, and 17.5 1C lower than the phase transition temperature of NH4Al(SO4)2  12H2O. It indicated that the thermal conductivity of heat storage materials is low. According to the temperature variation curve, the relationship between phase of PCM and time can be calculated by the following equation: T ¼  10  8 t 4 þ 10  5 t 3  0:0039t 2 þ 0:63t þ 12:5x  10:33

ð12Þ

where T stands for temperature, 1C; t stands for time, minute; x stands for the distance between heat storage material and source of heat, mm. At the first 100 min of experiment, the average temperature variation between the first column and the third column of measurement points ranged from 7 to 11 1C. Since then, the average temperature difference is 5.4–6.9 1C. It shows that, to meet the radial heat transfer of 30 mm, the temperature of the heat source needs to be 11 1C higher than the phase transition temperature. 6.3. PCM temperature variation in axial direction In this experiment, the hot liquid flowed from top to bottom in axial direction. As shown in Figs. 3 and 6 sets of sensors were set up, each with 3 measurement points. The average radial temperature of three measurement points was taken as this group's average temperature. The axial average temperature variation pattern of six groups is shown in Fig. 10b. The average temperature of heat storage material gradually decreases along with the direction of heat liquid flow. The average temperature of last group of measurement points is lower, and its temperature variation is greater than other groups. This is most likely due to the poor thermal insulation performance of the sealing material at the bottom of heater. As a result, most of the heat was transferred to the external environment with lower temperature via sealing material at the bottom apart

from throughout the radial direction. This part of data does not represent the overall heat transfer process therefore no further analysis is undertaken. Temperature variation pattern of other groups of measurement points was similar to that of the radial direction, i.e. rising quickly and then slowing down. During the whole period of 340 min, from the inlet to outlet, in every 0.2 m of distance, the average temperature of PCM was 73.1 1C, 74.6 1C, 76.3 1C, 71.6 1C and 72.1 1C respectively. The lowest temperature is 28.6 1C lower than the water temperature, and 21.9 1C lower than the phase transition temperature of NH4Al(SO4)2  12H2O. It can be observed that the temperature is higher in the middle than the inlet and outlet temperature. From the perspective of heat transfer, it is mainly due to the insulation performance of the heat exchanger on both ends, which leads to heat loss. The relationship between the overall average temperature of heat storage material and the time can be calculated by the following formula: T ¼  10  8 t 4 þ 10  5 t 3  0:0041t 2 þ 0:642t þ 39:2

ð13Þ

In axial direction, the temperature difference ranged from 6.7 1C to 3.7 1C, which was lower than that in radial direction. Therefore, when setting the heat exchanger, the axial distance can be much longer than the radial distance. In addition, considering the heat transfer of PCM in the end of both axial and radial directions, the temperature of heat source temperature shall be 22 1C higher than that of phase transition point.

7. Discussion Focusing on low-density and intermittency features of solar energy, this paper critically analysed the advantages and disadvantages associated with sensible heat storage and the latent heat storage methods. In those regions with sufficient solar energy sources and large heating demand, latent storage is an effective approach for space heating in buildings. This is an environmentally friendly approach which reduces the primary energy consumption. Latent storage cannot only improve the application of solar energy but also overcome the drawback of low utilization rate of solar energy in previous studies where solar energy is used directly or by sensible heat storage in domestic hot water or space heating. Compared to the traditional methods of heat storage, the approach proposed in this study helps to achieve the optimal energy storage for the solar thermal energy storage. Furthermore, compared to sensible heat storage, the latent heat storage cannot only store more energy but also lose little energy. From an economic point of view, the initial investment and operation cost is low for latent heat storage, which makes it suitable for solar thermal energy storage and applicable for heating buildings. Solar thermal energy storage for space heating in buildings can effectively reduce the collector areas therefore suitable for low density regions (both urban and rural areas) with sufficient roof area. By contrast, the high rise apartment buildings are not suitable for this kind of solar energy utilization due to limited collector areas and obstruction to sunlight. This paper selected NH4Al(SO4)2  12H2O as PCM for solar thermal energy storage from the solar energy which can be applied in suburbs and rural areas where the building density is lower than that in downtown and cities with an excessive period of very cold winter season. Results of experiment and simulation showed that the charging process is most efficient when the water temperature in solar collector is 120 1C, 26.5 1C higher than the phase change temperature. In order to produce hot water with 120 1C of temperature for thermal storage, it is necessary to use storage absorber such as vacuum tubes. Similarly, the water pressure of the system has to be higher than the saturation

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pressure corresponding to the temperature of 120 1C to avoid vaporization of water. Therefore, it is recommended to use media with higher vaporization temperature and better thermal conductivity such as ethylene glycol, propylene glycol, and silicone oil. In practical sense, if NH4Al(SO4)2  12H2O is selected as PCM, it is better to charge heat with hot water produced by the parabolic solar collector. As the phase transition temperature is 93.5 1C, the phase change heat storage system is suitable for all types of domestic radiators. In order to charge and discharge more effectively, the heat storage device can be divided into several sets of small heat exchange units. By means of automatic control, each unit can be fully charged in 1–2 days when there is sufficient solar energy. This helps to enhance the effects of heat transfer and to shorten the charging process.

8. Conclusion In this study, NH4Al(SO4)2  12H2O was selected as PCM for solar thermal energy storage from the solar energy which can be applied in suburbs and rural areas where the building density is lower than that in downtown and cities with an excessive period of very cold winter season. Through the experiment and simulation, the conclusions are: (1) Phase change heat storage material absorbs the solar radiation from solar collector during the period of spring, summer and autumn, and store thermal energy in the form of latent heat. This energy can be discharged to meet the heating demand during the winter. This mechanism improves the efficiency of utilizing solar energy and is suitable for heating the scattered small scale buildings in winter. (2) The heat charging efficiency is optimized when the temperature of heat source is 26.5 1C higher than the phase transition temperature. This helps to satisfy the demand of storage and usage of solar thermal energy. (3) During the charging process, the heat transfer from the heat source is slow in the radial direction while the temperature in the axial direction changes very slowly. In practice, this pattern can be used as a reference when distributing heat transfer units. There are policy implications from these findings. Policies should be developed to motivate the utilization of solar thermal energy storage system in buildings with consideration of various external factors such as solar energy resources, building types, density and economic viability. This contributes to fulfill the national target of achieving 20% of primary energy consumption from non-fossil fuel resources as specified in the U.S.-China Joint Announcement on Climate Change in November 2014. Demonstration projects will be helpful to showcase the benefits and effectiveness of this way of utilizing solar thermal energy. Similarly, these findings provide useful inputs to building design. According to the local conditions, construction authority could endorse the solar thermal energy storage as the main space heating method for those buildings with low density. During the design process, attention is required to the building integrated solar energy heating. During the operation stage, auditing should be conducted to monitor the performance of solar heating system. Alternatively, solar thermal energy storage is one of options for the energy retrofitting of existing buildings.

Acknowledgements This research is supported by the Ministry of Science and Technology of the People’s Republic of China (Project No. 2013BAJ09B01 and

701

2015BAJ01B01), and the Tianjin Municipal Science and Technology Commission (Project No. 14ZCDGGX00795). We acknowledge their support and assistance in this research.

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