Research on heat transfer performance of passive solar collector-storage wall system with phase change materials

Accepted Manuscript Title: Research on heat transfer performance of passive solar collector-storage wall system with phase change materials Author: Dan Sun Lijiu Wang PII: DOI: Reference:

S0378-7788(16)30202-X http://dx.doi.org/doi:10.1016/j.enbuild.2016.03.048 ENB 6521

To appear in:

ENB

Received date: Revised date: Accepted date:

3-6-2015 3-10-2015 19-3-2016

Please cite this article as: Dan Sun, Lijiu Wang, Research on heat transfer performance of passive solar collector-storage wall system with phase change materials, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2016.03.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Research

on

heat

transfer

performance

of

passive

solar

collector-storage wall system with phase change materials

Dan Sun*, Lijiu Wang

Faculty of Infrastructure Engineering, Dalian University of Technology, Dalian, 116024, China

* Corresponding author Faculty of Infrastructure Engineering, Dalian University of Technology, Dalian, 116024, China E-mail address: [email protected] Tel.:+86 0411 84706297

Highlights: 

A new type of passive solar collector-storage wall system with PCMs was proposed.



Energy balance equations are established to describe the heat transfer process.



Passive solar collector-storage wall system with PCMs can save energy conservation and improve indoor thermal comfort.

Abstract Heat transfer performance on the wall has a great influence on energy conservation and indoor thermal comfort. In this paper, a new type of passive solar energy utilization technology, phase change materials (PCMs) added into passive solar collector-storage wall system was proposed. Heat transfer performance and energy saving characteristics were investigated theoretically and experimentally. Energy balance equations that including sunlight board, air in the channel, collector mortar layer and inner surface of wall were established to describe the heat transfer process of system. Experimental room was used to studied the energy saving characteristics in winter. The results indicate that the new passive solar collector-storage wall system with PCMs can promote indoor air thermal circulation and decrease indoor air temperature fluctuations. Its good heat storage capacity can apparently improve indoor thermal environment. Therefore, this new passive solar collector-storage wall system with PCMs can be popularized in engineering applications.

Keywords: Passive solar collector-storage wall; Phase change materials; Heat transfer; Energy-saving characteristic

Nomenclature

1 transmissivity of sunlight board

 absorptivity of sunlight board to sun

 absorptivity of collect mortar layer to sun  a channel thickness ( m )

i

each layer thickness of wall ( m )

 w collector mortar layer thickness ( m )  'w inner surface of wall thickness ( m )

 s emissivity of sunlight board  w emissivity of collector mortar layer

i each layer thermal conductivity of wall materials ( W m  K )



mass percentages of PCM in collect mortar layer(%)

'

mass percentages of PCM in wall(%)

c p specific heat of air( J kg  K ) c p , s , c p ,m , c p ,l specific heat of PCM in process of solid, solid-liquid, liquid( J kg  K )

m mass flow rate of air in channel ( kg ) mm mass of collect mortar layer( kg )

mm' mass of inner surface mortar layer ( kg )

 density of air ( kg m3 ) 

wind velocity ( m

s

)

hso convective heat transfer coefficient between sunlight board and outside air ( W m2  K ) hsa convective

heat transfer coefficient between sunlight board and air in channel ( W

m2  K )

（ has =hsa ）

hwa convective heat transfer coefficient between collector mortar layer and air in channel ( W m2  K )

（ hwa =haw ）

hsw radiative heat transfer coefficient between sunlight board and collector mortar layer ( W m2  K ) （ hws =hsw ）

As

2

sunlight board area ( m )

Aw outer surface area of collector mortar layer ( m 2 ) Aw ' inner surface area of wall ( m 2 ) Dh hydraulic diameter of flow interlayer( m )

H channel width ( m ) I c solar radiation intensity ( W m2 ) Ta air temperature in channel ( K ) Tin indoor temperature ( K ) Tout ambient temperature ( K ) Tt top vent temperature ( K ) Tb bottom vent temperature ( K ) Ts sunlight board surface temperature ( K ) Tn inner surface temperature of collect mortar layer ( K ) Tn ' inner surface temperature of wall ( K ) Tw outer surface temperature of collect mortar layer ( K ) Tw ' outer surface temperature of wall ( K ) T1 phase change temperature of PCM in collect mortar layer( K ) T2 phase change temperature of PCM in wall( K ) Re Reynolds Pr prandtl number

1. Introduction Passive solar collector-storage wall system is widely used in engineering applications [1-4]. It consists of a glazing wall, an air channel ,a massive wall and an insulating wall. In winter, solar radiation is absorbed and stored in massive wall when bottom and top vents opened at daytime, the heated air in channel flow into room to improve indoor temperature. At night, the energy that stored in wall is released into the room when all vents closed. In summer, by opening bottom vent of massive wall and top vent of glazing wall, air in channel rises to outdoor under heat pressure, ventilation and cooling take place [5]. Numerous studies have been focused on the heat transfer performance of passive solar collector-storage wall system. Khalifa [6] simulated the heat transfer characteristics of the thermal storage wall. Chel et al. [7] estimated the passive heating potential of Trombe wall for a honey storage building by TRNSYS. The thermal storage wall can enhance the availability of the solar energy, but the outside surface temperature of external wall rises slowly in winter with a long lag time. Chen et al. [8] analyzed the variation of flow and temperature field in the composite solar wall and the effects of the particle size, the porous within porous layer, the material and the position of porous absorber on heating, the result show that all factors should be taken into account for a better design of the heating system. Zhang et al. [9] investigated the heat transfer performance and energy-saving characteristic of wall implanted with heat pipes, the result shows that the new system can apparently reduce the heating load and improve thermal environment.

However, passive solar collector-storage wall system cannot both meet heating in winter and ventilated/cooling in summer. To solve this problem, phase change materials (PCMs) are used in building envelope. Phase change materials can not only decrease the building energy consumption，but also improve the indoor thermal comfort by enhancing thermal energy storage capacity [10-14]. The application of PCMs has been focused on brick, concrete, mortar and other wall-covering materials. Principi et al. [15] discussed the possibility of increasing energy performance of thermal brick through integration with PCM. Zalewski et al. [16] presents an experimental study of a small-scale Trombe composite solar wall. PCM was inserted into the wall in the form of a brick-shaped package and energy performance of wall from heat flux measurement and enthalpy balances also presented. Haurie et al. [17] studied the effect of different flame retardants on the fire behavior and mechanical properties of epoxy mortars. The results prove the effectiveness of the tested flame retardants on the improvement of the fire properties of the epoxy mortars without a significant decrease on their mechanical properties. Kong et al. [18] established a mathematical model of two new PCM systems combined in the building envelope and studied their thermal performance. It is found that the numerical simulation achieved good validity. Ling et al. [19] studied the active heat storage performance of the active-passive triple phase change material wall and solar concentrators. This new system can improve the heat storage capacity. In the present study, we have explored a new system: passive solar collector-storage wall contained PCMs on both sides surface. The heat transfer

performance and energy saving characteristics were investigated theoretically and experimentally. 2. Description of new wall system Passive solar collector-storage wall system with PCMs locates on the south side of room. From outside to inside, the system is made up of a 6-mm-thick sunlight board, a 15-mm-thick collector mortar layer, a 40-mm-thick extruded board, a 390mm×190mm×190mm concrete block and a 15-mm-thick phase change mortar layer. The air gap between sunlight board and collector mortar layer is 100mm. Four vents with dimension of 200mm×200mm are located at the top and bottom position of wall. Paraffin/expanded perlite/graphite PCMs that prepared in our previous word [20] are added into collector mortar layer and interior mortar layer to storage energy. Phase change temperature and latent heat are 19.45℃ and 128.46J/g, respectively. The illustration of passive solar collector-storage wall system with PCMs is shown in Fig.1. A part of solar energy transmit into the room through air channel and vents to improve indoor temperature, a part of solar energy store in the collector mortar layer through PCMs and the rest solar energy is conducted slowly through massive wall to the room by radiation and convection. The advantage of passive solar collector-storage wall system with PCMs is storing more heat from sun during the day and releasing it into the building during the night.

3. Theoretical analysis The heat transfer process of passive solar collector-storage wall system with PCMs is very complex. In order to investigate the process theoretically, the following assumptions have been made while establishing equations: (1) Long wave radiation is negligible. (2) The effect of thermal capacity of sunlight board is negligible. (3) Sunlight board is considered to be opaque for infrared radiation. (4) Reflections of sunlight board and collector mortar layer are assumed to be neglected. Heat transfer process of passive solar collector-storage wall system with PCMs is expressed by energy balance equations including sunlight board, air in the channel, collector mortar layer and inner surface of wall. 3.1.Sunlight board energy balance equation Energy balance suggests that the amount of energy must be equal to the useful energy gain at a given time. Therefore, sunlight board energy balance equation can be written as: solar radiation by sunlight board from sun + radiation heat between sunlight board and outer surface of collector mortar layer = convective heat between sunlight board and outdoor ambience + convective heat between sunlight board and air in the channel:

 I c As  hsw Aw Tw    Ts    hso As Ts    Tout    hsa As Ts    Ta  

(1)

Where the radiation heat transfer coefficient between sunlight board and outer surface of collector mortar layer hsw is given below [21]

hsw 

 Ts2    Tw2    Ts    Tw    1 1     1  s w 

(2)

The convective heat transfer coefficient between sunlight board and outdoor ambience hso is given by [22], includes convection by wind, radiation heat transfer from sunlight board to sky and conduction through sunlight board.

hso  hwind  haw  hs

(3)

Here, the convective heat transfer coefficient due to wind is defined by [23] (4)

hwind =2.8+3.0v

The radiation heat transfer coefficient from outer sunlight board surface to sky referred to ambient can be found by

haw 

2  s Ts2    Tsky   Ts    Tsky   Ts    Tsky  

Ts    Tout   

(5)

Where the sky temperature is [21] 1.5 Tsky   =0.0552Tout  

(6)

The convective heat transfer coefficient between sunlight board and air in the channel hsa may be obtained from [24] hsa =  0.0158R e0.8 

a

a

(7)

3.2.Air in the channel energy balance equation Air in the channel energy balance equation can be written as: heat from air in the channel = convective heat between air in the channel and sunlight board + convective heat between air in the channel and outer surface of collector mortar layer: mc p Tt   - Tb    has As Ts   - Ta    haw Aw Ta   - Tw  

(8)

The mass flow rate of air in the channel can be calculated according to Bansal et al [25] and Anderson [26] m

0.57    A0 1  Ar2



2 gL T  a  Tin   

(9)

Tin  

Here, 0.57 is the coefficient of discharge of air channel inlet [27], Ao and Ai are outlet and inlet areas, respectively, Ar 

Ao

Ai . Ta   =

Tt    Tb   2

The convective heat transfer coefficient between outer surface of collector mortar layer and air in the channel haw may be obtained from [24] hsa =  0.0158R e0.8 

a

w

(10)

3.3.Collector mortar layer energy balance equation The energy balance equation of collector mortar layer can be written as: solar radiation by collector mortar layer from sun = convective heat between collector mortar layer and air in the channel+ radiation heat between collector mortar layer and sunlight board + heat from collector mortar layer to inner surface of wall+ heat absorb of collector mortar layer.

1 I c Aw  hwa Aw Tw   - Ta    hws Aw Tw   - Ts    qwn  qw '

(11)

Here, heat from collector mortar layer to inner surface of wall qwn is calculated by Fourier’s Law, and the equation is

qwn =

Tn '    Tw '   3

i

 i 1

i

For phase change material, the heat storage can be expressed by

(12)

q1  mm

H 

(13)

In the above equation, H is the enthalpy of phase change material that just correlating with temperature and specific heat of material and it can be found by T1

T2

T

T0

T1

T2

H =  c p , s dT   c p ,m dT   c p ,l dT

(14)

For cement mortar, the heat storage can be given by q2  mm (1   )

Tw    Tn  

(15)

w

Therefore, the heat storage of collector mortar layer be written as Tw    Tn    mm (1   ) w  qw '  q1  q2   m (1   ) Tw    Tn    m  H m  m w  

Tw    T1

(16) Tw    T1

3.4.Inner surface of wall energy balance equation The energy balance equation of inner surface of wall can be written as: heat from inner surface of wall = convective heat between inner surface of wall and indoor air + radiation heat between inner surface of wall and other surface of room + heat absorb of inner surface of wall + heat from inner surface of wall to collector mortar layer. q f  hw' a Aw ' Tw '    Tin    qno  qw ''

(17)

Radiation heat between inner surface of wall and other surface of room qno is neglected. The heat storage of inner surface of wall qw '' be written as Tw'    Tn'    ' mm' (1   )  w'  qw ''  q1  q2   m (1   ' ) Tw'    Tn'    m  ' H m'  m'  w'  

4. Experimental study

Tw'    T2 Tw'    T2

(18)

The experimental room is located at the city of Jilin in China, which represents a typical warm temperate semi-humid continental monsoon climate, with cold winters and hot and relatively dry summers. The dimension of experimental room is 5000mm×1600mm×2200mm. Extruded board with 240mm width divides the experimental room into an ordinary room and a passive solar phase change room (PSPCR). Two doors with the dimensions of 800mm×1800mm are installed side of experimental room. Fig.2 shows the external view of experimental room and Fig.3 shows the structures of walls. The properties of building envelope materials are shown in Table 1 and Table 2, respectively. Intelligent multi-channel temperature logging devices ( Range:＞4℃) and K-type thermocouples (copper-constantan, 5mm diameter and temperature accuracy ±0.1℃) are used to monitor the temperature variation at different points. These points are located on/ in the top and bottom vents, the center of air channel and experimental room.

5. Results and discussion For a better analysis of thermal performance of passive solar collector-storage wall system with PCMs in winter, we made a series of experiments. The results are analyzed as follows: As shown in Fig.4, the temperature near the bottom is lower, in contrast, the temperature neat the up of passive solar collector-storage wall is higher, and the temperature gets to rise from bottom to top. This can be explained by “chimney

effect”. Passive solar collector-storage wall absorbs solar radiation to heat the air in channel. Then, the heated air rises and flows into the room through top vents to improve indoor temperature, while the low temperature air in the room flows into the channel through bottom vents. This result indicates that passive solar collector-storage wall system with PCMs can promote air thermal circulation of room to improve indoor temperature. Fig.5 compares the indoor temperature of ordinary room and PSPCR. In the experiment, auxiliary heat source is used to guarantee indoor temperature greater than 4℃and lower than phase change temperature of PCMs. The opening time of vents is 10:00-16:00. As shown in the figure, the indoor temperature of PSPCR is higher than ordinary room. In the period of 16:00 to 9:00, the sun is not shining and the indoor temperature of two rooms decrease gradually. Due to the indoor temperature of PSPCR is lower than phase change temperature of PCMs in internal mortar layer, PCMs do not occur phase change process. Therefore, the influence of PCMs is discussed in the following article. In addition, the indoor temperature of two rooms all reaches the minimum temperature at 6:00. The temperature difference is up to 1.13℃. In the period of 9:00 to 14:00, with the increasing of solar radiation, the indoor temperature of two rooms increases quickly. Moreover, the indoor temperature of PSPCR rises faster than ordinary room because of passive solar collector-storage wall system. At 14:00, the indoor temperature of two rooms reaches maximum temperature, and the temperature difference is up to 5.38℃. In the period of 14:00 to 16:00, the indoor temperature of two rooms decreases with solar radiation drop. In addition, there is a time lag of about 30min relative to maximum temperature, which is due to heat energy release of collector mortar layer. Therefore, it can be concluded that passive solar collector-storage wall system with PCMs can improve indoor thermal comfort.

In order to identify the effect of PCMs, cooling curves of ordinary room and PSPCR are investigated. In the experiment, the two rooms are heated to same temperature that exceeds phase change temperature of PCMs and all vents of PSPCR closed. From Fig.6, it can be seen that the temperature of ordinary room decrease to 3.5℃ while PSPCR' dropped to 10.13℃ during the same time. Moreover, the temperatures of PSPCR decrease slower than ordinary room. When the temperature is 15-20℃, the cooling curve of PSPCR changed slightly. The reason is that PCMs releases thermal energy when indoor temperature is lower than phase change temperature. This is beneficial to reduce indoor temperature fluctuation and improve thermal comfort in winter. Thereby, the use of PCMs improve energy-saving characteristic for occupancy because of its good heat storage capacity.

Conclusion To solve the problem of meeting heating in winter and ventilated/cooling in summer, a new type of passive solar collector-storage wall system with PCMs was put forward. Heat transfer performance and energy-saving characteristic were investigated theoretically and experimentally. Energy balance equations are established to express heat transfer process of passive solar collector-storage wall system with PCMs. The experimental results show that the indoor temperature of PSPCR is higher than ordinary room. Passive solar collector-storage wall system with PCMs can promote air thermal circulation of room to improve indoor temperature and good heat storage capacity of PSPCR improve energy-saving characteristic for occupancy.

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collector motar layer

collector motar layer

air channel

air channel

sunlight board

sunlight board

Indoor

(a) Winter

Indoor

(b) Summer

Fig.1 Illustration of passive solar collector-storage wall system with PCMs

Fig.2 External view of experimental room

1 2

3

(c)

(b)

(a)

1

45 6 2

3

7

1 2

3

7

1: cement mortar layer, 2: extruded board, 3: concrete block, 4: sunlight board, 5: air channel, 6: collector mortar layer, 7: inner surface mortar layer of PSPCR

Fig.3 Constructions of walls (a) wall of ordinary room, (b) south wall of PSPCR, (c) other walls of PSPCR

Fig.4 Temperature distribution in the channel

Fig.5 Indoor temperature of experimental room

Fig.6 Cooling curves of ordinary room and PSPCR

Table 1 Thermal physical properties of building envelope materials Thickness

Density

Thermal conductivity

mm

kg/m3

W/(m·k)

cement mortar

15

1800

0.93

phase change mortar1

15

1680

0.52

phase change mortar2

15

1680

0.52

concrete block

190

2400

1.40

extruded board

40

32

0.028

Name

Note：phase change mortar 1: outer surfer of collector mortar layer of phase change room phase change mortar 2: inner surfer of phase change room

Table 2 Properties of sunlight board Minimum Thickness

Density

Transmittance

Name

bend radius mm

3

kg/m

% mm

sunlight board

6

1.20

81

1050