Thermal analysis of a direct-gain room with shape-stabilized PCM plates

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1 Renewable Energy 33 () 36 Thermal analysis of a direct-gain room with shape-stabilized PCM plates Guobing Zhou, Yinping Zhang, Kunping Lin, Wei Xiao Department of Building Science, School of Architecture, Tsinghua University, Beijing 4, PR China Received 1 February 7; accepted June 7 Available online August 7 Abstract The thermal performance of a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) plates has been analysed using an enthalpy model. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM (melting temperature, heat of fusion and thermal conductivity), inner surface convective heat transfer coefficient, location and thickness of the SSPCM plate, wall structure (external thermal insulation and wallboard material) etc. The results show that: (1) for the present conditions, the optimal melting temperature is about 1C and the heat of fusion should not be less than 9 kj kg 1 ; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation of the exterior wall obviously influences the operating effect and period of the SSPCM plates and the indoor temperature in winter; (5) the SSPCM plates create a heavyweight response to lightweight constructions with an increase of the minimum room temperature at night by up to 3 1C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime. r 7 Elsevier Ltd. All rights reserved. Keywords: Passive solar house; Energy storage; Shape-stabilized PCM; Thermal performance; Simulation 1. Introduction Latent heat storage in buildings is becoming more and more attractive due to the large energy storage density and nearly isothermal nature of the storage process when compared with the sensible heat storage. For a passive solar design, such as a direct-gain room, solar energy can be stored by a phase change material (PCM) which undergoes phase change from solid to liquid during daytime and later released passively to the room air when the PCM goes from liquid to solid. This can not only address the problem of intermittent and variable character of solar energy but also lower the indoor air temperature fluctuation and improve the indoor thermal comfort degree, particularly for large glazed-area rooms [1]. As walls, ceilings and floors etc. of a building offer large areas for passive heat transfers, thermal energy storage Corresponding author. Tel.: ; fax: address: zhangyp@mail.tsinghua.edu.cn (Y. Zhang). may be enhanced by incorporating PCMs with these building envelope components. Over the past decade extensive investigations have been made on the thermal performance of PCM wallboard systems [2 7]. In these studies, potential of energy savings and reduction in heating loads has been demonstrated for these PCM building components. Also, to maximize the thermal energy storage, selection of PCM should be optimized. For instance, Peippo et al. s [2] simulation results indicated that the optimal diurnal heat storage occurs with a melting temperature 1 3 1C above the average room temperature. Most of the above PCM composites are prepared by immersion of wallboard into PCM or by direct incorporation at the mixing stage of wallboard production. These two methods are flexible and economical. However, as Schossig et al. [] have pointed out, the leakage may be a problem over the lifetime of many years for these methods, and this also limits the amount of PCM in the wallboard. 96-1/$ - see front matter r 7 Elsevier Ltd. All rights reserved. doi:./j.renene.7.6.

2 G. Zhou et al. / Renewable Energy 33 () 36 Nomenclature A w,i area of wall, ceiling or floor surface, i ¼ 1, 2,y,N (m 2 ) A win window area (m 2 ) ACH air change per hour (h 1 ) c p specific heat (J kg 1 1C 1 ) Gr h Grashof number convective heat transfer coefficient (W m 2 1C 1 ) H enthalpy (kj kg 1 ) H m heat of fusion (kj kg 1 ) k thermal conductivity (W m 1 1C 1 ) L thickness (mm) Nu Nusselt number Pr Prandtl number q heat flux (W m 2 ) Q w,i Q L Q sc the convection heat transfer rate between air and inner surfaces of the room, i ¼ 1, 2,y,N (W) heat transfer rate by air leakage (W) the convection heat transfer rate from the indoor heat sources (W) Q win heat transfer rate through window (W) q r,in indoor thermal radiation heat fluxes from indoor thermal disturbance and the other inner surfaces of building envelopes (W m 2 ) q r,out t outdoor thermal radiation heat fluxes, mainly from the sun shine (W m 2 ) temperature (1C) U overall heat transfer coefficient (W m 2 1C 1 ) V R volume of the room (m 3 ) Greek letters r the density (kg m 3 ) t time (s) Subscripts a air b brick layer f floor gap air gap i insulation layer in indoor init initial l liquid state of PCM L air leak from the room m phase transition state of PCM out outdoor p PCM layer r radiation R room s solid state of PCM SSPCM shape-stabilized phase change material un under surface up upper surface w wall win window In recent years, a kind of novel compound PCM, the socalled shape-stabilized PCM (SSPCM) has been attracting the interests of the researchers [9 ]. Fig. 1 shows the picture of this PCM plate. It consists of paraffin as dispersed PCM and high-density polyethylene (HDPE) or other materials as supporting material. Since the mass percentage of paraffin can be as much as % or so, the total stored energy is comparable with that of traditional PCMs. As long as the operating temperature is below the melting point of the supporting material, the compound material can keep its shape even when the PCM changes from solid to liquid []. This reduces the liquid PCM leakage danger and it can be used for thermal storage in buildings without encapsulation. As pointed out by Ibanez et al. [], the thermal improvements in a building due to the inclusion of PCMs depend not only on the climate, design and orientation of the construction, but also on the amount and type of PCM. Effect of shape-stabilized PCM application in direct-gain passive solar houses is rarely studied and needs to be optimized. Therefore, the purpose of the present work is to perform a numerical analysis on the thermal effect of shape-stabilized PCM plates as inner linings on the indoor air temperature of a direct-gain room in winter. The following effecting factors are considered: (1) different melting temperature and heat of fusion of SSPCM; (2) thermal conductivity of SSPCM and convective heat transfer coefficients between the inner surface and indoor air; (3) SSPCM plates located in different parts of the envelope, i.e. four walls, ceiling and floor and different thicknesses of the PCM plates; (4) wall structure (with or without external thermal insulation and different wallboard materials). 2. Mathematical model A verified enthalpy model [] is extended from floor to walls and ceiling and applied for this simulation. For convenience, it is redescribed here. The following assumptions are made: (1) heat transfer through walls, ceiling and floor is one-dimensional; (2) thermophysical properties of building materials are constant except the specific heat of the PCM; (3) natural convection of the PCM during melting process and the super-cooling effect during freezing process can be ignored; (4) the solar gains into the room are hypothesized to be averagely distributed on the four walls, the roof and the floor by area.

3 3 ARTICLE IN PRESS G. Zhou et al. / Renewable Energy 33 () 36 Fig. 1. The photos of the shape-stabilized PCM: (a) photo of the PCM plate and (b) electronic microscopic picture by scanning electric microscope (SEM) HITACHI S-45 []. thermal radiation q r,out outdoor I II III thermal radiation q r,in indoor x 1 x 2 indoor reinforced concrete SSPCM plate heat convection h c heat convection h out heat convection h in thermal radiation q r,c Fig. 3. Schematic of the ceiling s heat transfer. x 1 x 2 x 3 x Fig. 2. Schematic of exterior wall s heat transfer (I, insulation layer; II, hollow brick; III, SSPCM plate) Heat transfer model of SSPCM wall and ceiling The schematic of heat transfer through the exterior wall is shown in Fig. 2. The transient enthalpy equation is qh r j qt ¼ k q 2 t j qx 2, (1) where for SSPCM, H ¼ R t 1 t c p;s dt þ R t 2 t 1 c p;m dt þ R t t 2 c p;l dt; for the insulation layer and the hollow brick layer, H ¼ R t t c p;j dt. The temperature range of phase transition is from t 1 to t 2. The equivalent specific heat capacity during the phase transition process for SSPCM is also assumed to be uniform; r j, k j and c p,j are as follows: r j ¼ r i ; k j ¼ k i ; c p;j ¼ c p;i ; pxox 1 ; >< r j ¼ r b ; k j ¼ k b ; c p;j ¼ c p;b ; x 1 pxox 2 ; >: r j ¼ r p ; k j ¼ k p ; x 2 pxox 3 : The initial condition is tðx; tþj t¼ ¼ t init. (2) For surfaces exposed to the outside and inside air, the boundary conditions are qt q r;out þ h out ðt out t i;out Þ¼ k i qx, (3) x¼ qt q r;in þ h in ðt in t p;in Þ¼k p qx. (4) x¼x3 Forexteriorwall,q r, in and q r,out are indoor and outdoor radiation heat flux, respectively (Fig. 2). q r, in is mainly from indoor heat source such as equipments, lights and occupants and the other building envelopes, and q r,out is mainly from the sunshine and the ground. The empirical values of convective coefficients h out and h in are calculated according to the ASHRAE handbook []. The above equations are also applicable to interior walls and the ceiling. For interior walls, the central planes are assumed insulated, thus h out and q r,out are zero. For the ceiling (Fig. 3), the surface at x ¼ is assumed insulated and the inner surface corresponds to convective heat transfer coefficient h c and thermal radiation q r,c. Thermal radiations among the internal surfaces of walls, floor and ceiling are calculated by thermal radiation network method. More detailed description can be seen from Oppenheim [] Heat transfer model of SSPCM floor Based on a practical consideration, SSPCM is not at the surface of the floor but incorporated in the floor construction shown in Fig. 4. Again, the transient enthalpy equation is written as qh r j qt ¼ k q 2 t j qy 2. (5) Similarly, for SSPCM, H ¼ R t 1 t c p;s dt þ R t 2 R t 1 c p;m dtþ t t 2 c p;l dt; for the insulation layer and wood floor,

4 G. Zhou et al. / Renewable Energy 33 () y y 4 y 3 y 2 y 1 heat convection thermal radiation wood floor air layer PCM plate insulation layer where V R represents volume of the room, Q w,i the convection heat transfer rate between air and inner surfaces of the room, Q s,c the convection heat transfer rate from the indoor heat source which is assumed 7% of the total energy from the heat source [], Q L the heat transfer rate by air leakage and Q win the heat transfer rate through the window. Q w,i, Q L and Q win are calculated by the following equations: Q w;i ¼ h in ðt w;i t in ÞA w;i, (11) H ¼ R t 1 t c p;j dt. r j, k j and c p,j are as follows: r j ¼ r i ; k j ¼ k i ; c p;j ¼ c p;i ; pyoy 1 ; >< r j ¼ r p ; k j ¼ k p ; y 1 pyoy 2 ; r j ¼ r a ; k j ¼ k a ; c p;j ¼ c p;a ; y 2 pyoy 3 ; >: r j ¼ r f ; k j ¼ k f ; c p;j ¼ c p;f ; y 3 pyoy 4 : The initial condition is tðy; tþj t¼ ¼ t init. (6) The boundary conditions are qt k i qy ¼ ; y ¼ ; y¼ q gap þ sðt 4 f;un t4 p;up Þ¼k p qt >< qy ; y ¼ y 2 ; y¼y2 q gap þ sðt 4 p;up t4 f;un Þ¼k f qt (7) qy ; y ¼ y 3 ; y¼y3 qt >: q f;up þ h f ðt in t f;up Þ¼k f ; y ¼ y 4 ; y¼y4 qy where q f,up is the radiation heat flux from the walls and ceiling to the wood floor and h f the natural convective heat transfer coefficient between the indoor air and the wood floor upper surface and is also calculated according to ASHRAE Handbook []. For the air-gap, heat flux q gap can be calculated by the following equation []: q gap ¼ Nu k a ðt p;up t f;un Þ, () L gap where ( Nu ¼ :2ðGr LPrÞ 1=4 ; Gr L ¼ 1 4 4:6 5 ; :61ðGr L PrÞ 1=3 ; Gr L 44:6 5 : (9) When Gr L is less than 4, only thermal conductivity is considered Model of the indoor air The energy conservation equation for the indoor air is c p;a r a V R dt a dt ¼ XN Fig. 4. Schematic of the floor s heat transfer. i¼1 Q w;i þ Q s;c þ Q L þ Q win, () Q L ¼ c p;a r a V R ACHðt out t in Þ=36, () Q win ¼ U win ðt out t in ÞA win, () where t w,i is the inner surface temperature of the wall, ceiling and floor while A w,i is its area. U win and A win are the overall heat transfer coefficient and the area of the window, respectively. The aforementioned equations are solved numerically using the Gauss Seidel method. A fully implicit finitedifference scheme was applied and the number of grids was checked to ensure accuracy and to eliminate initial errors. 3. Model validation Although the model was just extended from floor to walls and ceiling, it is reverified by the experimental data in a cabin in Beijing without auxiliary heating supply. The dimensions of the cabin are 3 m (depth) 2 m (width) 2m (height). There is a 1.6 m 1.5 m double-glazed window facing south. The roof and the walls were made of -mmthick polystyrene boards. The floor was composed of -mm-thick wood floor, -mm-thick air layer, -mmthick SSPCM and a 5-mm-thick polystyrene layer. The heat of fusion of the SSPCM is 1 kj kg 1 and the melting temperature range is from to 1C. The experiments were performed during January 2 6, 5. Air change per hour (ACH) was obtained by the tracer-gas technique with methane and was about.5 h 1. For more details on the test room and instrumentations refer to Lin et al. []. The measured outdoor temperature, solar radiation and ACH were recorded to be the data source for simulating the experimental house. Fig. 5 shows the measured and simulated results. It is seen that the indoor air temperature line simulated by the present model agrees well with the measured data. 4. Description of model room and climate conditions A typical south-facing middle room in a multi-layer building in Beijing, China, is considered for simulation, which has only one exterior wall the south wall, and others are all interior envelopes (room A shown in Fig. 6). The dimension of the room is taken as 3 m 3m 3 m, i.e., all inner surfaces have the same areas, which is convenient for comparing the effect of SSPCM plate position on the thermal behaviour of the room. There is a 2 m 1.5 m

5 32 ARTICLE IN PRESS G. Zhou et al. / Renewable Energy 33 () 36 double-glazed window in the south wall and a.9 m 2m wood door in the north wall which is adjacent to another room or the corridor. SSPCM plates can be attached to the inner surfaces of four walls and the ceiling or can become a layer in floor construction shown in Figs Thermophysical properties of the SSPCM and materials of the building envelopes are shown in Table 1. The phase transition temperature range of SSPCM is assumed to be 1 1C. The ACH is assumed to be 1. h 1. The software Medpha (Meteorological Data producer for HVAC Analysis) can provide the climate data of 3 regions in China based on the measured results of the past 3 years. It was used to generate the winter climate data (including outdoor temperature and solar radiation) for simulation. The calculation was performed from October to March of the next year covering the whole heating season. Simulated results January 1 5 are used for the analysis. Since indoor air temperature is the parameter of interest, it is selected for comparison. Before discussion, the hourly variation of outdoor temperature and solar radiation on the south wall are shown in Fig Results and discussion To maximize the benefits from the thermal storage by SSPCM plates, effecting factors such as PCM thermophysical properties, inner surface convection, location and thickness of SSPCM plates, wall structure, etc. need to be analysed. The present model is applied to perform a parametric study. In each parametric analysis, only one specific parameter is changed, whereas others are kept constant when the simulation is carried out. temperature ( C) outdoor indoor, measured indoor, simulated outdoor temperature ( C) solar radiation outdoor temperature Beijing January 1 to solar radiation on the south wall (W m -2 ) Fig. 5. Comparison of the simulated and measured results of the experimental house. Fig. 7. Hourly variation of outdoor temperature and solar radiation on the south wall. ceiling SSPCM north wall A window door floor Fig. 6. Schematic of the simulated room: (a) location of the simulated room A in the building and (b) profile of room A with SSPCM. Table 1 Envelope materials and their thermophysical properties Materials r (kg m 3 ) c p (kj kg 1 1C 1 ) k (W m 1 1C 1 ) U (W m 2 1C 1 ) SSPCM Hollow brick Reinforced concrete Insulation (EPS) Wood board Window 3.1 Door.75

6 G. Zhou et al. / Renewable Energy 33 () indoor temperature( C) H m = 1kJ kg -1 L PCM = mm t m = C t m = C t m = C t m = C Fig.. Hourly indoor temperature by using different melting temperatures of PCM Melting temperature and heat of fusion One most important factor influencing the thermal performance of the SSPCM building is the melting temperature of PCM, t m, which should be in the desired range. Its exact value should be optimized. By using SSPCM plates attached to all surfaces of the four walls, ceiling and in the floor structure of the simulated room, the indoor air temperature is calculated with different PCM melting points. Fig. shows the simulated results for January 1 5. It can be seen that the optimal value of t m is about 1C for the present case, which keeps the daily minimum temperature higher than other melting points. This is due to the fact that if the melting temperature is too high, the quantity of solar radiation heat stored by SSPCM plates is too little []; if the melting temperature is too low, the resultant air temperature would be even lower at night time. This is because the PCM wallboard goes from liquid to solid and releases heat into the room air at night time. To keep the heat transfer progress, there must exist some difference of temperature between the PCM wallboard surface and the indoor air. Therefore, if the melting temperature of PCM, which is equal or close to the inner surface temperature, is too low, the resultant air temperature would be even lower and deviates far from the comfortable level. Fig. 9 shows the comparison of hourly inner surface temperature and indoor temperature by using different melting temperatures of PCM. It shows that for PCM with melting temperature of 1C, the inner surface temperature curve is relatively flat and remains at the phase transition point for most of the time. Also, the indoor temperatures are always lower than the inner surface temperatures. This is due to the large quantity of the PCM being used and also the low outdoor temperature on these days. To keep the indoor air temperature in a relatively comfortable range for a long time without heating load, the heat of fusion of PCM, H m, should be high enough so as to make the wall s inner surface at the melting temperature temperature ( C) 2 26 H m = 1kJ kg -1 L PCM = mm air temperature (t m = C) inner surface temperature (t m = C) air temperature (t m = C) inner surface temperature (t m = C) Fig. 9. Comparison of hourly inner surface temperature and indoor temperature by using different melting temperatures of PCM. indoor temperature ( C) t m = C L PCM = mm H m = 3 H m = 6 H m = 9 H m = 1 H m = Fig.. Hourly indoor temperature for various heat fusion of PCM (H m,kjkg 1 ). over a whole day. Fig. indicates that when the heat fusion is higher than 9 kj kg 1, the indoor temperature is almost not influenced by it. This implies that, to keep the PCM in the phase transition region for a long time, the heat of fusion should not be smaller than 9 kj kg 1 for the case discussed SSPCM thermal conductivity and convective heat transfer coefficient of inner surface This section attempts to discuss the influence of internal conduction of SSPCM plates and inner surface convection on energy storage. Fig. 11 shows that thermal conductivity has no obvious effect on indoor temperature in the heat discharging process, but influences indoor temperature during the heat charging process. When the conductivity is above.5 W m 1 1C 1, no obvious influence appears for both charging and discharging processes. A similar result

7 34 G. Zhou et al. / Renewable Energy 33 () 36 was also obtained by Xu et al. [] for the condition of south-facing glass curtain wall. Fig. shows the effect of convective heat transfer coefficient between the inner surface and air on indoor temperature. It is indicated that the convective heat t m = C k =.1 k = 1. k = 2. H m = 1kJ kg -1 k =.2 L PCM = mm k = Fig. 11. Hourly indoor temperature for various thermal conductivity of SSPCM (k, Wm 1 1C 1 ). natural convection formula h in = 2.5 k =.2 h in = 5.6 k =.2 h in =.7 k =.2 h in =.7 k = 2. t m = C H m = 1kJ kg Fig.. Hourly indoor temperature for various convective coefficients (h in,wm 2 1C 1 ). transfer coefficient calculated with natural convective formula in ASHRAE handbook [] is much lower (about 2 W m 2 1 C 1 ) which limits the availability of the latent storage. Higher convective heat transfer coefficient improves the indoor temperature level due to the fact that the increased convection enhances the exchange of energy between the SSPCM plates and the air. Stovall and Tomlinson [3] also got a similar conclusion under the condition of no solar radiation. It is seen from Fig. that h in ¼ 5.6 and.7 W m 2 1C 1 have a similar effect on the indoor temperature. However, as Stovall and Tomlinson pointed out, occupants may be unwilling to accept such high air flows. Fig. again confirms that thermal conductivity has no obvious influence on indoor temperature Location and thickness of SSPCM plates To minimize the quantity of PCM used, and therefore the production cost of the plates, the location and thickness of SSPCM plates need to be considered. For convenience of comparison, the difference between the minimum daily temperature with and without SSPCM plates is calculated. The average of these differences during the studied period (January 1 5) is considered to indicate the effects of PCM plate locations. Table 2 presents some of the most interesting results calculated by the model. It is seen that SSPCM plates located on the south wall or the floor are unfavourable. This is because south wall is the exterior wall for which the inner surface temperature is lower than that of interior walls such as the west, east, north and ceiling. Also, the net area of the south wall is smaller due to the large window included. These two aspects decreased the heat flow rate from the SSPCM plates to the indoor air at night time. For the case of the floor, the heat resistance of the wood board and the air layer may account for the lower value of temperature difference since the SSPCM plates can only be placed under the wood board for practical consideration. From this table, it is also indicated that for the same amount of PCM, a thin SSPCM plate with large surface area is superior to a thick one with small surface area. For instance, if the SSPCM plate is only located at the west wall surface with a thickness of 4 mm, the average difference of Table 2 Average of the difference between minimum daily temperatures for several alternatives Location East South West North Ceiling Floor Dt Location SN NW SC SF EW W(L PCM ¼ 4) Dt Location EWN NWC ESWNCF (L PCM ¼ ) ESWN ESWNC ESWNCF Dt Note: ESWNCF means that SSPCM plates are located at all the inner surfaces of East wall, South wall, West wall, North wall, Ceiling and in the floor construction. The rest may be deduced by analogy; Dt is the average of the difference between minimum daily temperatures with and without SSPCM plates on corresponding inner surfaces of the envelope in 1C. The thickness of all SSPCM plates (L PCM, mm) is mm unless indicated otherwise.

8 G. Zhou et al. / Renewable Energy 33 () minimum daily temperature concerned is.6 1C. While the SSPCM plate of the same PCM amount is located at both the east and west wall surface with a thickness of mm, the average difference of minimum daily temperature is C. This can be explained by the fact that large surface area increases the heat charge/discharge rate of SSPCM, particularly for the factual conditions of low thermal conductivity of PCM and small convective heat transfer coefficient at the surface, which has been discussed in Section 5.2. A similar result is drawn from the case of NWC (L PCM ¼ mm) and ESWNCF (L PCM ¼ mm) with Dt of 1.57 and 1.9 1C, respectively. To gain an insight into the appropriate amount of SSPCM, the effect of the thickness of SSPCM plates located at all inner surfaces of the envelope (ESWNCF) is considered. Fig. shows the daily indoor temperature for various thicknesses of SSPCM plates during January 1 9. It is clear that for the first 4 days there are no obvious differences of minimum daily indoor temperature among various thicknesses of SSPCM plates. However, in the later 5 days with continuously descending outdoor temperatures, SSPCM plates with thickness of and mm lead to much lower indoor temperatures than others due to the smaller heat storage. It seems that L PCM ¼ mm may be a more appropriate thickness of SSPCM plates for practical design under the conditions described in this paper External thermal insulation and wallboard material Fig. presents hourly variation of indoor temperature with and without external thermal insulation (for exterior wall in Fig. 2) and inner surface SSPCM plates during January 1 5. It can be seen that without external thermal insulation the indoor air temperature is much lower than that with external insulation, and the SSPCM plates do not level the temperature swing but even sharpen it. This is due 11 9 t m = C H m = 1kJ kg -1 k =.2W m -1 C -1 L PCM = L PCM = L PCM = L PCM = 25 L PCM = time (day) Fig.. Daily indoor temperature for various thicknesses of SSPCM plates (L PCM, mm) t m = C H m = 1kJ kg -1 k =.2W m -1 C -1,without insulation with ESWNCF PCM,without insulation,with insulation with ESWNCF PCM,with insulation Fig.. Hourly indoor temperature with and without external insulation and inner surface PCM. Table 3 Thermophysical properties of solid brick, hollow brick and foam concrete Materials r (kg m 3 ) c p (kj kg 1 1C 1 ) k (W m 1 1C 1 ) Solid brick Hollow brick Foam concrete to the fact that if the wall is not externally insulated, the indoor temperature during the considered period would be so low that the PCM plate is in fully solid state without phase transition and it functions just as a single-phase layer with low thermal conductivity at the inner surface. However, in the case with external insulation, the PCM plate works well. It really stores energy during daytime and improves the indoor comfort level. This confirms the importance of external thermal insulation. The wallboard material used in above sections is hollow brick (see Fig. 2). In this section, solid brick and foam concrete are also considered to understand the effect of SSPCM plate application in heavy or light weight constructions. The thermophysical properties of the three materials are listed in Table 3. Fig. compares the hourly indoor temperature for the three wallboard materials with and without SSPCM plates. It is indicated that,, lightweight wallboard material such as foam concrete leads to higher indoor temperature fluctuation than heavyweight material (solid brick). However, with SSPCM plates attached to the wall inner surfaces, indoor temperature swing is shaved for both lightweight and heavyweight wallboard materials and there is no obvious difference between their minimum daily temperatures. This confirms that SSPCM plates really create a heavyweight response to lightweight constructions. It is shown that the minimum room temperature at night can be increased by about 3 1C with SSPCM plates for foam concrete wallboard.

9 36 G. Zhou et al. / Renewable Energy 33 () Conclusions In this paper an enthalpy model is applied to simulate the thermal performance of a middle direct-gain room with SSPCM plates as inner linings in a multi-layer building in Beijing, China, in winter. Several influencing factors are analysed and the results supported that the PCM plates are really advantageous in direct-gain passive solar houses. To maximize the benefit from the energy storage using SSPCM plates, (1) the melting temperature and heat of fusion should be optimized and for the conditions in this paper, the appropriate melting temperature is about 1C and the heat of fusion should not be less than 9 kj kg 1 ; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) thin PCM plates with large areas are advantageous and the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation obviously influences the operating effect and period of the SSPCM and the indoor temperature in winter; (5) SSPCM plates create a heavyweight response to lightweight constructions with a increase of the minimum room temperature at night by up to 3 1C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime. The presented analysis is expected to be helpful for the design and application of SSPCM plates in passive solar buildings. Acknowledgements foam concrete, foam concrete,eswncf PCM hollow brick, hollow brick,eswncf PCM solid brick, solid brick,eswncf PCM t m = C H m = 1kJ kg -1 k =.2W m -1 C -1 Fig.. Hourly indoor temperature with various wallboard materials and inner surface PCM. This work was supported by the National Key Basic Research Special Funds Project of China, no. 1CB496 and The National High Technology Research and Development Program of China, no. 2AA55. References [1] Khudhair AM, Farid MM. A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. Energy Convers Manage 4;45(2): [2] Peippo K, Kauranen P, Lund PD. Multicomponent PCM wall optimized for passive solar heating. Energy Build 91;(4): [3] Stovall TK, Tomlinson JJ. What are the potential benefits of including latent storage in common wallboard? 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