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Phase change materials for energy storage and thermal comfort in buildings
16
Phase change materials for energy storage and thermal comfort in buildings
M. M. F a r i d and A. S h e r r i f, University of Auckland, New Zealand
Abstract: This chapter reports on recent research conducted on the use of phase change materials (PCM) for thermal comfort and heating and cooling peak load shifting in buildings. The objective here is to show experimentally and through a computer simulation that PCM impregnated in building materials can provide thermal energy storage benefits. Paraffin (RT20) has been used as the PCM because of its desirable thermal and physical attributes including its melting temperature of 20–22 °C, which is close to human comfort temperature. The PCM was impregnated into gypsum wallboards to produce an efficient thermal storage medium (PCMGW), which consists of 26%-wt PCM impregnated in gypsum boards. This PCMgypsum wallboard structure was tested in-situ in an office size building. In parallel, a thermal building simulation code (SUNREL) was used to simulate the performance of the office size rooms. Measured and simulated results in summer showed that the use of PCMGW effectively reduced diurnal daily fluctuations of indoor air temperatures and maintained the indoor temperature at the desired comfort level for a longer period of time. A major benefit of thermal energy storage in winter is to reduce electricity demand charges by limiting the need to run electrically operated heating and airconditioning devices during peak load periods. This study reveals that this application of PCM storage in buildings can lead to healthier interior spaces, and more efficient energy use in terms of demand charge reduction and use of favourable off-peak rates. Key words: PCM, peak load, comfort in buildings, paraffin wax, gypsum wallboard.
16.1
Introduction
The likelihood of a shortfall in future availability of non-renewable energy, which powers much of modern society, is increasing. It is advisable to consider the use of building materials that minimize the need for space conditioning while maintaining comfortable and healthy internal environments [1]. The fluctuation of indoor air temperature, which is a consequence of the low thermal mass of lightweight buildings, plays a special role among other factors influencing thermal comfort. It can be considered as the single largest cause of complaints from occupants in modern buildings constructed using lightweight building materials [2]. 384 © Woodhead Publishing Limited, 2010
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Thermal energy storage in buildings, which has received an increasing attention, seems to provide better thermal comfort for occupants and ensure efficient energy use. The need for air-conditioning could be minimized through an effective incorporation of phase change materials (PCM) into building materials, which also enables strategies for electricity peak-load shifting. The use of PCM will also allow the passive capture of solar energy when it is available, which can then be stored and used later when there is a demand for it. The work presented in this chapter shows how well PCM building materials could perform in real buildings in New Zealand. Full-scale office size buildings constructed using normal and PCM-impregnated gypsum wallboard are used for that purpose.
16.2
Background
Research on PCMs and methods of thermal energy storage (TES) in buildings was initiated and encouraged by the US Department of Energy in 1982 to demonstrate the benefits of utilizing latent heat of PCMs in term of reducing variation in indoor temperature (thermal inertia) in lightweight constructions [3]. Later, many researchers confirmed the effectiveness of PCMs in improving energy efficiency and indoor thermal comfort of buildings based on numerical simulations and experimental studies [4]. In particular, Athienitis et al. [5] and Schossig and Hennin [6] carried out extensive studies in full-scale office size rooms, which were constructed using PCM building materials. It was shown that these materials could function efficiently as a thermal storage medium. Recently, Cabeza et al. [7] used microencapsulated PCM in a real size concrete cubicle and compared its performance with a control concrete cubicle containing no PCM. They showed an improved thermal inertia and efficient energy use in the PCM cubicle. Comprehensive reviews on the PCM thermal energy storage in buildings have been adequately discussed in the literature [8]. Another application of TES is peak-load shifting to take advantage of the off-peak electricity tariffs. There is a growing interest in the use of daily TES for electrical load management in both new and existing buildings. The TES technology has matured and is now accepted internationally as a proven energy conservation technology [9]. A simplified sketch in Fig. 16.1 is given by Dincer [10] to show different strategies for charging and discharging TES to meet cooling demands during peak demand hours. The full-storage strategy, which is sketched in Fig. 16.1(a), shifts the entire peak load to off-peak hours. This strategy is most attractive and likely to be economically advantageous only when spikes in the peak load curve are of short duration [11]. In the partial-storage strategy (Fig. 16.1(b)), equipment operates to meet part of the peak-load and the rest is met by drawing energy from storage. On mild
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Tons
Reduced on-peak demand
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24 hours daily cycle A: storage meets load B: chiller meets load directly C: chiller charging storage (a)
24 hours daily cycle Dotted line: chiller on Solid line: load (b)
16.1 Operating strategies: (a) full storage; (b) partial-storage [10].
spring or autumn days the partial-storage system designed for space heating at winter design temperatures may function as a full-storage system with a full peak demand shift. Promising results of peak-load shifting by using the concepts of TES have encouraged investigators to put more effort in energy efficiency and conservation.
16.3
Selection of phase change materials (pcm) and fabrication of pcm-gypsum wallboards (pcmgw)
A comprehensive list of possible material that may be used for latent heat storage are reported by Abhat [12]. Readers who are interested in such information are referred to the papers by Lorsh et al. [13], Lane et al. [14], Humphries and Griggs [15] and more recently by Khudhair and Farid [8] who have reported a large number of possible candidates for latent heat storage covering a wide range of temperatures. Paraffin compounds undergo negligible supercooling, are chemically inert and stable with no phase segregation. Pure paraffin waxes are very expensive; therefore only technical-grade paraffin should be used. Paraffin waxes are essentially available as linear alkyl hydrocarbons at almost any temperature. Furthermore, the paraffin waxes are non-polar materials; they are not subject to hydrogen bonding with other polar components of the building materials. Fatty acids and their esters can also be used as more environmentally friendly PCMs; however, they have lower latent heat than paraffin. Feldman and Shapiro [16] have analysed the thermal properties of fatty acids (capric, lauric, palmitic and stearic acid) and their binary mixtures. The results have shown that they are attractive candidates for latent heat thermal storage in space heating applications. The melting range of the fatty acids was found to
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vary from 30 °C to 65 °C, while their latent heat of transition was observed to vary from 153 to 182 kJ/kg. Hydrated salts are attractive materials for use in thermal storage due to their high volumetric storage density (~350 MJ/m3), relatively high thermal conductivity (~0.5 W/m °C) and moderate costs compared to paraffin waxes, with few exceptions [8]. However, the high storage density of these materials is difficult to maintain and usually decreases with cycling. This is because most hydrated salts melt congruently with the formation of lower hydrated salt, making the process irreversible, leading to the continuous decline in their storage efficiency. Subcooling is another serious problem associated with all hydrated salts. A number of companies have successfully succeeded in solving these problems and commercialized CaCl2·6H2O encapsulated in different forms of containments to be used as a PCM. To allow safe and straightforward application of PCMs in different environments (e.g. within plasterboard of residential housing) they can be microencapsulated as micron-sized particles with a polymer shell as a coating [17]. This eliminates any possibility of leakage and diffusion of the liquid PCM or evaporation of volatile components over time. The microencapsulation also reduces the flammability of the PCM. The microcapsules can be easily incorporated into many systems to reduce energy consumption and maintain temperatures at a comfortable level. Prime examples include heat transfer media in heating and cooling systems or heat storage media in insulating and building materials. The commercial paraffin, RT20, was selected as a suitable PCM in this study mainly due to its melting temperature, which is close to human comfort temperature. Paraffin RT20 is stable, chemically inert and possesses a good level of latent heat. Thermal energy storage characteristics of PCMs are most accurately evaluated by using differential scanning calorimetry (DSC) analysis. DSC graphs in Fig. 16.2 show that paraffin RT20 has a melting range of 18.1–21.5 °C and a reasonably high latent heat of fusion of ~162.5 kJ/kg. RT20 melts and solidifies within a narrow temperature range so that the stored heat may be released over a limited temperature swing that must be maintained inside buildings. Repeated 100 melting/freezing cycles were conducted on samples of RT20 to study the changes in the thermal properties. The results showed that this commercial paraffin has good thermal properties and undergoes congruent melting and freezing with no supercooling. A most flexible process whereby the PCM can be incorporated into gypsum wallboards is the immersion process, which does not interfere with the manufacturing processes of the gypsum wallboards. It involved dipping the stocks of the gypsum wallboards in molten PCM. The process was found easy to control and susceptible to a wide variation of process conditions such as PCM temperature and immersion time. Impregnation of
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10th Cycle: Peak 18 °C, Heat 162.5 kJ/kg 50th Cycle: Peak 17.75 °C, Heat 161 kJ/kg 100th Cycle: Peak 18.15 °C, Heat 162 kJ/kg
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16.3 Effects of immersion time and PCM temperatures on the PCM absorptivity into 13 mm thick gypsum wallboards.
the gypsum wallboard with 24–26% by weight of RT20 was achieved by immersing ordinary gypsum wallboards (60 ¥ 60 ¥ 1.3 cm) for 10 minutes in a bath filled with molten RT20 at 70–80 °C. Figure 16.3 shows that the rate of paraffin uptake into the gypsum wallboards increases with PCM temperature. The rate of paraffin uptake was very high during the first three
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minutes but diminished gradually after 10 minutes of immersion time. The alternative to this method is to microencapsulate the paraffin with a polymer. The microencapsulated product can then be mixed with the gypsum during manufacturing. This will prevent any possible migration of the paraffin from the board but will add cost that has prevented its commercial use up to now.
16.4
Full-scale testing facilities
A full-scale facility consisting of two identical office size constructions were built at the Tamaki Campus of the University of Auckland, New Zealand with the view to conduct long-term thermal performance involving monitoring and modelling work. A schematic plan view of the test room is shown in Fig. 16.4. The interior walls and ceiling of the first office (ORD) were finished with ordinary gypsum wallboards while the interior walls and ceiling of the second office (PCM) were finished using PCMGW. Each office in the test facility is a single-storey design of a typical lightweight construction. They measure 2.6 ¥ 2.6 ¥ 2.6 m giving a floor area of 5.76 m2 each. Wooden frames made of 9.8 ¥ 6.3 cm dressed pine timber were used in the construction. The interior coverings were sets of either gypsum wallboards or PCMGW panels (60 ¥ 60 ¥ 1.3 cm) mounted
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16.4 Schematic plan view of the test room.
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on the wooden frame. The exterior walls were 1.25 cm thick sheets of plywood. The wall cavities were filled with fibreglass thermal insulation. The insulation is installed with no gaps and no folds so as to achieve high thermal resistance to heat flow. The thickness of the insulation is 9.4 cm for both the walls and ceilings. The test facility faces north to maximize sun exposure, as this is a key aspect of energy-efficient building design. Each office was supplied with one window facing north. The two constructions were situated in a large open area with a 4.2 m distance between them, free from any shading. The test facility was provided with a computer-controlled data acquisition system with 20 data channels. The system consisted of a data recorder connected to a modem for data remote downloading. In addition to the temperature measurements inside and outside the offices, relative humidity, wind speed and solar radiation were also continuously measured and recorded.
16.5
Benefits of applying thermal energy storage
16.5.1 Thermal comfort in summer season For the sake of clarity, only the results obtained over a seven-day period in January 2005 are presented here for discussion. Figure 16.5 shows measurements of solar radiation and wind speed in Auckland, New Zealand. The solar radiation exceeds 1000 W/m2 during the peak sunshine hours on all days. A large portion of New Zealand has at least 2000 hours of sunshine a year. The two offices were not supplied with any active cooling in order to Solar radiation Wind speed
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accurately compare the performance of ordinary gypsum wallboards and PCMGW and their effects on the fluctuation of the indoor air temperatures in a real, albeit somewhat controlled, situation. Figure 16.6 shows that, in the PCM room, as the indoor air temperature rises passively to within the solid–liquid phase change temperature, the PCM begins to melt by absorbing heat from the room and storing it principally as a latent heat of melting. Thus, the PCMGW acts as a cooling medium. On the other hand, in the ORD room, the temperature rises much more steeply and reaches a higher level because only sensible heat storage is available. When the indoor air temperature falls below the PCM transition temperature, the PCM begins to solidify in the PCMGW, partially or completely, releasing the stored latent heat. The thermal behaviour of the wallboards affects significantly the indoor air temperatures of the room over the seven-day period. The temperatures in the PCM room rises at a lower rate during the day compared to that of the ORD room. At night, the temperature in the PCM room is higher than that in the ORD room. The ORD room maintained a wider range of weekly-averaged indoor air temperature compared to the PCM room, as shown in Table 16.1. It appears that the need for mechanically assisted cooling in the summer season in Auckland could be eliminated or reduced to a large extent through the use of PCMGW. The weekly-averaged indoor air temperature in the ORD room varied by 10.6 °C, while the corresponding variation was only 5.1 °C in the PCM room. Thus, the stored thermal energy in the form of latent heat has effectively 30
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16.6 Summer ambient and indoor temperatures in the two rooms.
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Materials for energy efficiency and thermal comfort in buildings Table 16.1 A comparison of measured weekly-averaged indoor air temperatures in the test rooms ORD room
DTORD
PCM room
DTPCM
28.1 °C–17.5 °C
10.6 °C
25.7 °C–20.6 °C
5.1 °C
reduced the variation of the indoor air temperature within the PCM room. The general notion is that if PCM is integrated into the walls and ceilings, it will act as a thermal reservoir and prevent excessive rise or fall of the indoor air temperature. As a result, it leads to a reduction in the overheating hours and hence in the cooling loads in summer, and yet provides a comfortable indoor atmosphere.
16.5.2 Peak-load shifting in winter season On sunny days in winter, the use of PCM in the wall and ceilings of buildings will allow the capture of solar radiation during the day for later use at night. However, on predominantly cloudy or foggy days, the indoor air temperature in both rooms did not reach the melting range of the selected PCM. Consequently, there was no stored energy and hence no major benefit was observed from using the PCMGW. When an internal heating system is supplied, there is potential for use of the high storage density of the PCM in the walls and ceiling for electrical peak-load shifting. This will allow some reduction in the cost of electricity by shifting electrical heating (in winter) and cooling (in summer) demands to periods when electricity prices are lower, for instance during the night. Thus the main application of thermal energy storage in winter would be to capture solar energy and reduce electrical demand charges by peak-load shifting. The indoor and ambient temperatures were monitored, collected and analysed during the month of July 2006, which is usually the coldest month in Auckland. For the sake of clarity, only the results obtained over two specific days (18 and 19 July), which are representative of typical cloudy and cold days in Auckland, are presented. Figure 16.7 shows the measured solar radiation and wind speed. A 5-fin oil radiator electrical heater with a power setting of 1 kW was used in each room. The heaters were programmed to turn on from 1:00 am to 7:00 am (off-peak period) every night, using a digital timer. In the PCM room, when the indoor air temperature rose to within the solid–liquid phase change temperature, the PCM melted in the PCMGW by absorbing heat from the room. At 7 am the heaters in both test rooms were turned off. Subsequently, the indoor air temperature fell below the PCM transition temperature, the PCM solidified in the PCMGW, completely or partially, and the stored latent heat was released. The heating load needed during the day
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in winter was reduced while the thermal load was met with the released heat from the PCM. Figure 16.8 shows that the variation in the indoor temperature was reduced significantly by the application of PCM. Figure 16.8 shows that thermal storage is often advantageous in facilities where there is a limited number of peak demand hours every day. Office
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buildings with no heating demands overnight and high heating demands in the morning hours, exhibit the optimum profile for this application of thermal energy storage.
16.6 Computer simulation 16.6.1 Thermal building model It was necessary to conduct a computer simulation of the buildings so that the idea of using PCM could be tested in future at any location worldwide. SUNREL version 1.04 is technical software used for building energy simulations based on finite difference approaches to model active or passive building elements. It is an upgrade to SERIRES version 1.0 that was written under the guidance of the Solar Energy Research Institute (SERI), now the National Renewable Energy Laboratory (NREL) at Golden, CO, USA. The upgrade of SERIRES to SUNREL was completed by Colorado State University and NREL. SUNREL has been tested satisfactorily through experimentation using the procedure of the International Energy Association [18]. A development of thermal models for buildings is required to arrive at optimal design parameters especially with regard to the required thermal mass. It is well known that thermal models of buildings depend in a complex way upon many interrelated factors. But using an appropriate level of detail in the SUNREL program depends primarily on the nature of the desired outputs and the applications under study. The basic descriptive constructs provided by SUNREL for developing the thermal models will be created within the constraints of the program. Given the correct input parameters that cover different aspects of the building size, construction, and location, SUNREL is, therefore, able to internally convert them to a mathematical form suitable for numerical solutions. The SUNREL simulation software uses the concept of ‘thermal zone’ to define thermal properties necessary for the simulation of a specific area. The thermal zone is either a single room or a group of rooms. Usually, a building is represented as one or more thermal zones with thermal communications (heat flow) between them and with the ambient including solar radiation. The most common paths of thermal communications are windows and walls including those walls with special constructions such as layers of PCMs. The wall construction consists of up to 10 layers from inside to outside. The layers of the walls are usually composed of different building materials and insulations in the ‘ORD’ case, which does not contain PCM materials. In the ‘PCM’ case, the layers of the walls include PCM in addition to the building materials and insulations. Within the SUNREL program, the most important requirements are the building materials of the wall. Figure 16.9 shows the main configuration used to construct the walls and the ceiling in
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Siding
Insulation
External side
Internal side
Gypsum boards
Wood
16.9 A schematic of hypothetical four layers of the walls in the ‘ORD’ case. Table 16.2 Thermo-physical properties of the mass types used in the ‘ORD’ case Mass name
Conductivity (W/m K)
Density (kg/m3)
Specific heat (kJ/kg K)
Thickness (m)
Board Insulation Wood Siding
0.25 0.038 0.12 0.094
670 32 510 640
1.089 0.835 1.38 1.17
0.013 0.075 0.025 0.01
Table 16.3 Thermo-physical properties of the PCM used in the ‘PCM’ case Conductivity (W/m K)
Density (kg/m3)
Specific heat (kJ/kg K)
Latent heat (kJ/kg)
Melting point (°C)
Thickness (m)
0.2
810
2.1
172
20
0.004
the simulations of the ‘ORD’ case. Four layers have been used to construct the walls and the ceiling. From the internal to the external side, the layers are gypsum wallboards, insulation, wood and siding. The physical properties of each material, used to make the wall construction, have been defined. The thermal conductivity, density, specific heat and thickness of each layer are listed in Table 16.2. The PCM type has been defined based on its heat of fusion and melting point in addition to the foregoing properties as listed in Table 16.3. Composite walls, such as a typical wood framed wall with stud and insulation may be modelled either as two separate walls belonging to the same exterior surface or two consecutive layers in one wall, as is the case here. In the PCM case, the same configuration of the construction of the walls has been used with the addition of a PCM layer that is placed between two
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layers of the gypsum wallboards. The two layers of the gypsum wallboards are identical in all physical and thermal properties. The combined thickness of the two layers of the gypsum wallboards used in the PCM case is equal to that of the gypsum wallboards used in the ORD case.
16.6.2 Computer predictions using the SUNREL software SUNREL thermal simulations have some limitations and these are related to the methods of calculating the effect of actual thermal mass in the interior environment and random behaviours of real occupants. Thermal mass is not only present in the building structure, but also in occupants and furniture within the building. Although SUNREL deals with these details in a systematic manner and allows estimating these thermal masses in a certain way, there are generally large uncertainties in the description of the real situation in the simulation input files. Other uncertainties include the method SUNREL uses to calculate the effect of latent heat of the PCM that melts and solidifies at a fixed temperature, while commercial PCMs melt and solidify within a range of temperatures (generally ~5 °C or higher). Also, SUNREL deals with the PCM as a layer placed between multi-layers of building materials, which does not represent the actual situation when the PCM is impregnated and is uniformly distributed within the pores of the building materials. It is, therefore, important to examine the validity of the SUNREL model against real experimental measurements. This can be accomplished by making a comparison between the measured data of the real test rooms and simulation results of the thermal zones, representing the test room, for at least a oneday period, and this was selected randomly as 15 February 2006. The main climatic measurements of hourly solar radiation, ambient temperature and wind speed of that day were used as input data for SUNREL. The results of the simulations were compared with those measured for both thermal zones as shown in Fig. 16.10(a-b). It can be seen that the simulation results (Fig. 16.10(b)) of the indoor temperature in both the ORD case and the PCM case are in a reasonable agreement with the measured data (Fig. 16.10(a)). This agreement between the theoretical and measured indoor temperatures shows that the latent heat of the PCM is well accounted for by the SUNREL simulation program.
16.7
Conclusions
The study presented in this chapter examined the benefits of thermal energy storage in buildings by means of incorporating PCM throughout the building materials. Paraffin RT20, as a PCM, was successfully impregnated into gypsum wallboards to form PCMGW of a significant thermal energy storage
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16.10 Profiles of temperature of the ORD and PCM cases: (a) experimental results, and (b) simulation results.
effect. These PCMGW walls act as a heat storage or heat sink for trapping heat from the sun during daylight hours for using it later at night. In summer, measured thermal performances of the full-scale room that was constructed using PCMGW as interior surfaces, showed a significant reduction in the daily fluctuation of indoor air temperature (~5.5 °C), providing thermal comfort and healthier interior spaces compared to the control room containing no PCMGW. In winter, this application of PCM storage in buildings can lead to a significant improvement in the energy efficiency of the buildings in terms of use of favourable off-peak rates. Similar observations were obtained through the computer simulation conducted using SUNREL software.
16.8
References
1. R. Vale, ‘The role of insulation and thermal mass in the design of zero-heating homes’, PCM2003 Workshop, The University of Auckland, New Zealand, 2003. 2. P. O. Fanger, ‘Strategies to avoid indoor climate complaints’, ICBEM 1987, Polytechnique Romandes Press, Lausanne, Switzerland. 3. I. O. Salyer, ‘Thermal energy storage’, The DOE Energy Storage Research Activities Conference, 1989, New Orleans, pp. 97–110. 4. D. W. Hawes, D. Banu, D. Feldman, ‘Latent heat storage in concrete’, Solar Energy Materials, 1989, pp. 335–348. 5. A. K. Athienitis, C. Liu, D. Hawes, D. Banu, D. Feldman, ‘Investigation of the thermal performance of a passive solar test-room with wall latent heat storage’, Building and Environment, 1997, pp. 405–410.
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6. P. Schossig, H. M. Hennin, ‘Encapsulated phase change materials integrated into construction materials’, Futurestock ‘2003’, Warsaw, Poland. 7. L. F. Cabeza, C. Castellón, M. Nogués, M. Medrano, R. Leppers, O. Zubillaga, ‘Use of microencapsulated PCM in concrete walls for energy savings’, Energy and Buildings, 2007, 39, 113–119. 8. A. M. Khudhair, M. M. Farid, ‘A review on energy conservation in building application with thermal storage by latent heat using phase change materials’, Energy Conversion & Management, 2004, pp. 263–275. 9. S. D. White, D. J. Cleland, R. Fraser, ‘Demand Side Response from HVAC&R’, AIRAH Energy Efficient Design Conference, November 2002, Sydney, Australia. 10. I. Dincer, ‘On Thermal Energy Storage Systems and Applications in Buildings’, Energy and Buildings, 34, 2002, pp. 377–388. 11. I. Dincer, M. A. Rosen, Thermal Energy Storage Systems and Applications, Wiley Inc., London, 2002. 12. A. Abhat, ‘Low temperature latent heat thermal energy storage; heat storage materials’, Solar Energy, 30, 1983, pp. 313–332. 13. H. G. Lorsh, K. W. Kauffman, J. C. Denton, ‘Thermal energy storage for heating and air conditioning: future energy production system’, Heat and Mass Transfer Processes, 1, 1976, pp. 69–85. 14. G. A. Lane, D. N. Glew, E. C. Clark, H. E. Rossow, S. W. Quigley, S. S. Drake, J. S. Best, ‘Heat of fusion system for solar energy storage subsystems for the heating and cooling of building’. Chalottesville, Virginia, USA, 1975. 15. W. R. Humphries, E. I. Griggs, ‘A designing handbook for phase change thermal control and energy storage devices’. NASA Technical Paper, 1977. 16. D. Feldman, M. M. Shapiro, ‘Fatty acids and their mixtures as phase-change materials for thermal energy storage’, Solar Energy Materials, 18, 1989, pp. 201–216. 17. M. C. Smith, M. M. Farid, A. J. Easteal, ‘Review of microencapsulated phase change materials for thermal energy storage applications’, IIR/IRHACE Conference, 16–18 February 2006, Auckland, New Zealand. 18. M. Deru, ‘BESTEST Results and SUNREL’, 1997, NREL, Version 1.0, Golden, CO, USA.
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Phase change materials for energy storage and thermal comfort in buildings
M. M. F a r i d and A. S h e r r i f, University of Auckland, New Zealand
Abstract: This chapter reports on recent research conducted on the use of phase change materials (PCM) for thermal comfort and heating and cooling peak load shifting in buildings. The objective here is to show experimentally and through a computer simulation that PCM impregnated in building materials can provide thermal energy storage benefits. Paraffin (RT20) has been used as the PCM because of its desirable thermal and physical attributes including its melting temperature of 20–22 °C, which is close to human comfort temperature. The PCM was impregnated into gypsum wallboards to produce an efficient thermal storage medium (PCMGW), which consists of 26%-wt PCM impregnated in gypsum boards. This PCMgypsum wallboard structure was tested in-situ in an office size building. In parallel, a thermal building simulation code (SUNREL) was used to simulate the performance of the office size rooms. Measured and simulated results in summer showed that the use of PCMGW effectively reduced diurnal daily fluctuations of indoor air temperatures and maintained the indoor temperature at the desired comfort level for a longer period of time. A major benefit of thermal energy storage in winter is to reduce electricity demand charges by limiting the need to run electrically operated heating and airconditioning devices during peak load periods. This study reveals that this application of PCM storage in buildings can lead to healthier interior spaces, and more efficient energy use in terms of demand charge reduction and use of favourable off-peak rates. Key words: PCM, peak load, comfort in buildings, paraffin wax, gypsum wallboard.
16.1
Introduction
The likelihood of a shortfall in future availability of non-renewable energy, which powers much of modern society, is increasing. It is advisable to consider the use of building materials that minimize the need for space conditioning while maintaining comfortable and healthy internal environments [1]. The fluctuation of indoor air temperature, which is a consequence of the low thermal mass of lightweight buildings, plays a special role among other factors influencing thermal comfort. It can be considered as the single largest cause of complaints from occupants in modern buildings constructed using lightweight building materials [2]. 384 © Woodhead Publishing Limited, 2010
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Thermal energy storage in buildings, which has received an increasing attention, seems to provide better thermal comfort for occupants and ensure efficient energy use. The need for air-conditioning could be minimized through an effective incorporation of phase change materials (PCM) into building materials, which also enables strategies for electricity peak-load shifting. The use of PCM will also allow the passive capture of solar energy when it is available, which can then be stored and used later when there is a demand for it. The work presented in this chapter shows how well PCM building materials could perform in real buildings in New Zealand. Full-scale office size buildings constructed using normal and PCM-impregnated gypsum wallboard are used for that purpose.
16.2
Background
Research on PCMs and methods of thermal energy storage (TES) in buildings was initiated and encouraged by the US Department of Energy in 1982 to demonstrate the benefits of utilizing latent heat of PCMs in term of reducing variation in indoor temperature (thermal inertia) in lightweight constructions [3]. Later, many researchers confirmed the effectiveness of PCMs in improving energy efficiency and indoor thermal comfort of buildings based on numerical simulations and experimental studies [4]. In particular, Athienitis et al. [5] and Schossig and Hennin [6] carried out extensive studies in full-scale office size rooms, which were constructed using PCM building materials. It was shown that these materials could function efficiently as a thermal storage medium. Recently, Cabeza et al. [7] used microencapsulated PCM in a real size concrete cubicle and compared its performance with a control concrete cubicle containing no PCM. They showed an improved thermal inertia and efficient energy use in the PCM cubicle. Comprehensive reviews on the PCM thermal energy storage in buildings have been adequately discussed in the literature [8]. Another application of TES is peak-load shifting to take advantage of the off-peak electricity tariffs. There is a growing interest in the use of daily TES for electrical load management in both new and existing buildings. The TES technology has matured and is now accepted internationally as a proven energy conservation technology [9]. A simplified sketch in Fig. 16.1 is given by Dincer [10] to show different strategies for charging and discharging TES to meet cooling demands during peak demand hours. The full-storage strategy, which is sketched in Fig. 16.1(a), shifts the entire peak load to off-peak hours. This strategy is most attractive and likely to be economically advantageous only when spikes in the peak load curve are of short duration [11]. In the partial-storage strategy (Fig. 16.1(b)), equipment operates to meet part of the peak-load and the rest is met by drawing energy from storage. On mild
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Tons
Reduced on-peak demand
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24 hours daily cycle A: storage meets load B: chiller meets load directly C: chiller charging storage (a)
24 hours daily cycle Dotted line: chiller on Solid line: load (b)
16.1 Operating strategies: (a) full storage; (b) partial-storage [10].
spring or autumn days the partial-storage system designed for space heating at winter design temperatures may function as a full-storage system with a full peak demand shift. Promising results of peak-load shifting by using the concepts of TES have encouraged investigators to put more effort in energy efficiency and conservation.
16.3
Selection of phase change materials (pcm) and fabrication of pcm-gypsum wallboards (pcmgw)
A comprehensive list of possible material that may be used for latent heat storage are reported by Abhat [12]. Readers who are interested in such information are referred to the papers by Lorsh et al. [13], Lane et al. [14], Humphries and Griggs [15] and more recently by Khudhair and Farid [8] who have reported a large number of possible candidates for latent heat storage covering a wide range of temperatures. Paraffin compounds undergo negligible supercooling, are chemically inert and stable with no phase segregation. Pure paraffin waxes are very expensive; therefore only technical-grade paraffin should be used. Paraffin waxes are essentially available as linear alkyl hydrocarbons at almost any temperature. Furthermore, the paraffin waxes are non-polar materials; they are not subject to hydrogen bonding with other polar components of the building materials. Fatty acids and their esters can also be used as more environmentally friendly PCMs; however, they have lower latent heat than paraffin. Feldman and Shapiro [16] have analysed the thermal properties of fatty acids (capric, lauric, palmitic and stearic acid) and their binary mixtures. The results have shown that they are attractive candidates for latent heat thermal storage in space heating applications. The melting range of the fatty acids was found to
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vary from 30 °C to 65 °C, while their latent heat of transition was observed to vary from 153 to 182 kJ/kg. Hydrated salts are attractive materials for use in thermal storage due to their high volumetric storage density (~350 MJ/m3), relatively high thermal conductivity (~0.5 W/m °C) and moderate costs compared to paraffin waxes, with few exceptions [8]. However, the high storage density of these materials is difficult to maintain and usually decreases with cycling. This is because most hydrated salts melt congruently with the formation of lower hydrated salt, making the process irreversible, leading to the continuous decline in their storage efficiency. Subcooling is another serious problem associated with all hydrated salts. A number of companies have successfully succeeded in solving these problems and commercialized CaCl2·6H2O encapsulated in different forms of containments to be used as a PCM. To allow safe and straightforward application of PCMs in different environments (e.g. within plasterboard of residential housing) they can be microencapsulated as micron-sized particles with a polymer shell as a coating [17]. This eliminates any possibility of leakage and diffusion of the liquid PCM or evaporation of volatile components over time. The microencapsulation also reduces the flammability of the PCM. The microcapsules can be easily incorporated into many systems to reduce energy consumption and maintain temperatures at a comfortable level. Prime examples include heat transfer media in heating and cooling systems or heat storage media in insulating and building materials. The commercial paraffin, RT20, was selected as a suitable PCM in this study mainly due to its melting temperature, which is close to human comfort temperature. Paraffin RT20 is stable, chemically inert and possesses a good level of latent heat. Thermal energy storage characteristics of PCMs are most accurately evaluated by using differential scanning calorimetry (DSC) analysis. DSC graphs in Fig. 16.2 show that paraffin RT20 has a melting range of 18.1–21.5 °C and a reasonably high latent heat of fusion of ~162.5 kJ/kg. RT20 melts and solidifies within a narrow temperature range so that the stored heat may be released over a limited temperature swing that must be maintained inside buildings. Repeated 100 melting/freezing cycles were conducted on samples of RT20 to study the changes in the thermal properties. The results showed that this commercial paraffin has good thermal properties and undergoes congruent melting and freezing with no supercooling. A most flexible process whereby the PCM can be incorporated into gypsum wallboards is the immersion process, which does not interfere with the manufacturing processes of the gypsum wallboards. It involved dipping the stocks of the gypsum wallboards in molten PCM. The process was found easy to control and susceptible to a wide variation of process conditions such as PCM temperature and immersion time. Impregnation of
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10th Cycle: Peak 18 °C, Heat 162.5 kJ/kg 50th Cycle: Peak 17.75 °C, Heat 161 kJ/kg 100th Cycle: Peak 18.15 °C, Heat 162 kJ/kg
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the gypsum wallboard with 24–26% by weight of RT20 was achieved by immersing ordinary gypsum wallboards (60 ¥ 60 ¥ 1.3 cm) for 10 minutes in a bath filled with molten RT20 at 70–80 °C. Figure 16.3 shows that the rate of paraffin uptake into the gypsum wallboards increases with PCM temperature. The rate of paraffin uptake was very high during the first three
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minutes but diminished gradually after 10 minutes of immersion time. The alternative to this method is to microencapsulate the paraffin with a polymer. The microencapsulated product can then be mixed with the gypsum during manufacturing. This will prevent any possible migration of the paraffin from the board but will add cost that has prevented its commercial use up to now.
16.4
Full-scale testing facilities
A full-scale facility consisting of two identical office size constructions were built at the Tamaki Campus of the University of Auckland, New Zealand with the view to conduct long-term thermal performance involving monitoring and modelling work. A schematic plan view of the test room is shown in Fig. 16.4. The interior walls and ceiling of the first office (ORD) were finished with ordinary gypsum wallboards while the interior walls and ceiling of the second office (PCM) were finished using PCMGW. Each office in the test facility is a single-storey design of a typical lightweight construction. They measure 2.6 ¥ 2.6 ¥ 2.6 m giving a floor area of 5.76 m2 each. Wooden frames made of 9.8 ¥ 6.3 cm dressed pine timber were used in the construction. The interior coverings were sets of either gypsum wallboards or PCMGW panels (60 ¥ 60 ¥ 1.3 cm) mounted
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16.4 Schematic plan view of the test room.
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on the wooden frame. The exterior walls were 1.25 cm thick sheets of plywood. The wall cavities were filled with fibreglass thermal insulation. The insulation is installed with no gaps and no folds so as to achieve high thermal resistance to heat flow. The thickness of the insulation is 9.4 cm for both the walls and ceilings. The test facility faces north to maximize sun exposure, as this is a key aspect of energy-efficient building design. Each office was supplied with one window facing north. The two constructions were situated in a large open area with a 4.2 m distance between them, free from any shading. The test facility was provided with a computer-controlled data acquisition system with 20 data channels. The system consisted of a data recorder connected to a modem for data remote downloading. In addition to the temperature measurements inside and outside the offices, relative humidity, wind speed and solar radiation were also continuously measured and recorded.
16.5
Benefits of applying thermal energy storage
16.5.1 Thermal comfort in summer season For the sake of clarity, only the results obtained over a seven-day period in January 2005 are presented here for discussion. Figure 16.5 shows measurements of solar radiation and wind speed in Auckland, New Zealand. The solar radiation exceeds 1000 W/m2 during the peak sunshine hours on all days. A large portion of New Zealand has at least 2000 hours of sunshine a year. The two offices were not supplied with any active cooling in order to Solar radiation Wind speed
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accurately compare the performance of ordinary gypsum wallboards and PCMGW and their effects on the fluctuation of the indoor air temperatures in a real, albeit somewhat controlled, situation. Figure 16.6 shows that, in the PCM room, as the indoor air temperature rises passively to within the solid–liquid phase change temperature, the PCM begins to melt by absorbing heat from the room and storing it principally as a latent heat of melting. Thus, the PCMGW acts as a cooling medium. On the other hand, in the ORD room, the temperature rises much more steeply and reaches a higher level because only sensible heat storage is available. When the indoor air temperature falls below the PCM transition temperature, the PCM begins to solidify in the PCMGW, partially or completely, releasing the stored latent heat. The thermal behaviour of the wallboards affects significantly the indoor air temperatures of the room over the seven-day period. The temperatures in the PCM room rises at a lower rate during the day compared to that of the ORD room. At night, the temperature in the PCM room is higher than that in the ORD room. The ORD room maintained a wider range of weekly-averaged indoor air temperature compared to the PCM room, as shown in Table 16.1. It appears that the need for mechanically assisted cooling in the summer season in Auckland could be eliminated or reduced to a large extent through the use of PCMGW. The weekly-averaged indoor air temperature in the ORD room varied by 10.6 °C, while the corresponding variation was only 5.1 °C in the PCM room. Thus, the stored thermal energy in the form of latent heat has effectively 30
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16.6 Summer ambient and indoor temperatures in the two rooms.
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Materials for energy efficiency and thermal comfort in buildings Table 16.1 A comparison of measured weekly-averaged indoor air temperatures in the test rooms ORD room
DTORD
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reduced the variation of the indoor air temperature within the PCM room. The general notion is that if PCM is integrated into the walls and ceilings, it will act as a thermal reservoir and prevent excessive rise or fall of the indoor air temperature. As a result, it leads to a reduction in the overheating hours and hence in the cooling loads in summer, and yet provides a comfortable indoor atmosphere.
16.5.2 Peak-load shifting in winter season On sunny days in winter, the use of PCM in the wall and ceilings of buildings will allow the capture of solar radiation during the day for later use at night. However, on predominantly cloudy or foggy days, the indoor air temperature in both rooms did not reach the melting range of the selected PCM. Consequently, there was no stored energy and hence no major benefit was observed from using the PCMGW. When an internal heating system is supplied, there is potential for use of the high storage density of the PCM in the walls and ceiling for electrical peak-load shifting. This will allow some reduction in the cost of electricity by shifting electrical heating (in winter) and cooling (in summer) demands to periods when electricity prices are lower, for instance during the night. Thus the main application of thermal energy storage in winter would be to capture solar energy and reduce electrical demand charges by peak-load shifting. The indoor and ambient temperatures were monitored, collected and analysed during the month of July 2006, which is usually the coldest month in Auckland. For the sake of clarity, only the results obtained over two specific days (18 and 19 July), which are representative of typical cloudy and cold days in Auckland, are presented. Figure 16.7 shows the measured solar radiation and wind speed. A 5-fin oil radiator electrical heater with a power setting of 1 kW was used in each room. The heaters were programmed to turn on from 1:00 am to 7:00 am (off-peak period) every night, using a digital timer. In the PCM room, when the indoor air temperature rose to within the solid–liquid phase change temperature, the PCM melted in the PCMGW by absorbing heat from the room. At 7 am the heaters in both test rooms were turned off. Subsequently, the indoor air temperature fell below the PCM transition temperature, the PCM solidified in the PCMGW, completely or partially, and the stored latent heat was released. The heating load needed during the day
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in winter was reduced while the thermal load was met with the released heat from the PCM. Figure 16.8 shows that the variation in the indoor temperature was reduced significantly by the application of PCM. Figure 16.8 shows that thermal storage is often advantageous in facilities where there is a limited number of peak demand hours every day. Office
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buildings with no heating demands overnight and high heating demands in the morning hours, exhibit the optimum profile for this application of thermal energy storage.
16.6 Computer simulation 16.6.1 Thermal building model It was necessary to conduct a computer simulation of the buildings so that the idea of using PCM could be tested in future at any location worldwide. SUNREL version 1.04 is technical software used for building energy simulations based on finite difference approaches to model active or passive building elements. It is an upgrade to SERIRES version 1.0 that was written under the guidance of the Solar Energy Research Institute (SERI), now the National Renewable Energy Laboratory (NREL) at Golden, CO, USA. The upgrade of SERIRES to SUNREL was completed by Colorado State University and NREL. SUNREL has been tested satisfactorily through experimentation using the procedure of the International Energy Association [18]. A development of thermal models for buildings is required to arrive at optimal design parameters especially with regard to the required thermal mass. It is well known that thermal models of buildings depend in a complex way upon many interrelated factors. But using an appropriate level of detail in the SUNREL program depends primarily on the nature of the desired outputs and the applications under study. The basic descriptive constructs provided by SUNREL for developing the thermal models will be created within the constraints of the program. Given the correct input parameters that cover different aspects of the building size, construction, and location, SUNREL is, therefore, able to internally convert them to a mathematical form suitable for numerical solutions. The SUNREL simulation software uses the concept of ‘thermal zone’ to define thermal properties necessary for the simulation of a specific area. The thermal zone is either a single room or a group of rooms. Usually, a building is represented as one or more thermal zones with thermal communications (heat flow) between them and with the ambient including solar radiation. The most common paths of thermal communications are windows and walls including those walls with special constructions such as layers of PCMs. The wall construction consists of up to 10 layers from inside to outside. The layers of the walls are usually composed of different building materials and insulations in the ‘ORD’ case, which does not contain PCM materials. In the ‘PCM’ case, the layers of the walls include PCM in addition to the building materials and insulations. Within the SUNREL program, the most important requirements are the building materials of the wall. Figure 16.9 shows the main configuration used to construct the walls and the ceiling in
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Siding
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16.9 A schematic of hypothetical four layers of the walls in the ‘ORD’ case. Table 16.2 Thermo-physical properties of the mass types used in the ‘ORD’ case Mass name
Conductivity (W/m K)
Density (kg/m3)
Specific heat (kJ/kg K)
Thickness (m)
Board Insulation Wood Siding
0.25 0.038 0.12 0.094
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0.013 0.075 0.025 0.01
Table 16.3 Thermo-physical properties of the PCM used in the ‘PCM’ case Conductivity (W/m K)
Density (kg/m3)
Specific heat (kJ/kg K)
Latent heat (kJ/kg)
Melting point (°C)
Thickness (m)
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the simulations of the ‘ORD’ case. Four layers have been used to construct the walls and the ceiling. From the internal to the external side, the layers are gypsum wallboards, insulation, wood and siding. The physical properties of each material, used to make the wall construction, have been defined. The thermal conductivity, density, specific heat and thickness of each layer are listed in Table 16.2. The PCM type has been defined based on its heat of fusion and melting point in addition to the foregoing properties as listed in Table 16.3. Composite walls, such as a typical wood framed wall with stud and insulation may be modelled either as two separate walls belonging to the same exterior surface or two consecutive layers in one wall, as is the case here. In the PCM case, the same configuration of the construction of the walls has been used with the addition of a PCM layer that is placed between two
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layers of the gypsum wallboards. The two layers of the gypsum wallboards are identical in all physical and thermal properties. The combined thickness of the two layers of the gypsum wallboards used in the PCM case is equal to that of the gypsum wallboards used in the ORD case.
16.6.2 Computer predictions using the SUNREL software SUNREL thermal simulations have some limitations and these are related to the methods of calculating the effect of actual thermal mass in the interior environment and random behaviours of real occupants. Thermal mass is not only present in the building structure, but also in occupants and furniture within the building. Although SUNREL deals with these details in a systematic manner and allows estimating these thermal masses in a certain way, there are generally large uncertainties in the description of the real situation in the simulation input files. Other uncertainties include the method SUNREL uses to calculate the effect of latent heat of the PCM that melts and solidifies at a fixed temperature, while commercial PCMs melt and solidify within a range of temperatures (generally ~5 °C or higher). Also, SUNREL deals with the PCM as a layer placed between multi-layers of building materials, which does not represent the actual situation when the PCM is impregnated and is uniformly distributed within the pores of the building materials. It is, therefore, important to examine the validity of the SUNREL model against real experimental measurements. This can be accomplished by making a comparison between the measured data of the real test rooms and simulation results of the thermal zones, representing the test room, for at least a oneday period, and this was selected randomly as 15 February 2006. The main climatic measurements of hourly solar radiation, ambient temperature and wind speed of that day were used as input data for SUNREL. The results of the simulations were compared with those measured for both thermal zones as shown in Fig. 16.10(a-b). It can be seen that the simulation results (Fig. 16.10(b)) of the indoor temperature in both the ORD case and the PCM case are in a reasonable agreement with the measured data (Fig. 16.10(a)). This agreement between the theoretical and measured indoor temperatures shows that the latent heat of the PCM is well accounted for by the SUNREL simulation program.
16.7
Conclusions
The study presented in this chapter examined the benefits of thermal energy storage in buildings by means of incorporating PCM throughout the building materials. Paraffin RT20, as a PCM, was successfully impregnated into gypsum wallboards to form PCMGW of a significant thermal energy storage
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16.10 Profiles of temperature of the ORD and PCM cases: (a) experimental results, and (b) simulation results.
effect. These PCMGW walls act as a heat storage or heat sink for trapping heat from the sun during daylight hours for using it later at night. In summer, measured thermal performances of the full-scale room that was constructed using PCMGW as interior surfaces, showed a significant reduction in the daily fluctuation of indoor air temperature (~5.5 °C), providing thermal comfort and healthier interior spaces compared to the control room containing no PCMGW. In winter, this application of PCM storage in buildings can lead to a significant improvement in the energy efficiency of the buildings in terms of use of favourable off-peak rates. Similar observations were obtained through the computer simulation conducted using SUNREL software.
16.8
References
1. R. Vale, ‘The role of insulation and thermal mass in the design of zero-heating homes’, PCM2003 Workshop, The University of Auckland, New Zealand, 2003. 2. P. O. Fanger, ‘Strategies to avoid indoor climate complaints’, ICBEM 1987, Polytechnique Romandes Press, Lausanne, Switzerland. 3. I. O. Salyer, ‘Thermal energy storage’, The DOE Energy Storage Research Activities Conference, 1989, New Orleans, pp. 97–110. 4. D. W. Hawes, D. Banu, D. Feldman, ‘Latent heat storage in concrete’, Solar Energy Materials, 1989, pp. 335–348. 5. A. K. Athienitis, C. Liu, D. Hawes, D. Banu, D. Feldman, ‘Investigation of the thermal performance of a passive solar test-room with wall latent heat storage’, Building and Environment, 1997, pp. 405–410.
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6. P. Schossig, H. M. Hennin, ‘Encapsulated phase change materials integrated into construction materials’, Futurestock ‘2003’, Warsaw, Poland. 7. L. F. Cabeza, C. Castellón, M. Nogués, M. Medrano, R. Leppers, O. Zubillaga, ‘Use of microencapsulated PCM in concrete walls for energy savings’, Energy and Buildings, 2007, 39, 113–119. 8. A. M. Khudhair, M. M. Farid, ‘A review on energy conservation in building application with thermal storage by latent heat using phase change materials’, Energy Conversion & Management, 2004, pp. 263–275. 9. S. D. White, D. J. Cleland, R. Fraser, ‘Demand Side Response from HVAC&R’, AIRAH Energy Efficient Design Conference, November 2002, Sydney, Australia. 10. I. Dincer, ‘On Thermal Energy Storage Systems and Applications in Buildings’, Energy and Buildings, 34, 2002, pp. 377–388. 11. I. Dincer, M. A. Rosen, Thermal Energy Storage Systems and Applications, Wiley Inc., London, 2002. 12. A. Abhat, ‘Low temperature latent heat thermal energy storage; heat storage materials’, Solar Energy, 30, 1983, pp. 313–332. 13. H. G. Lorsh, K. W. Kauffman, J. C. Denton, ‘Thermal energy storage for heating and air conditioning: future energy production system’, Heat and Mass Transfer Processes, 1, 1976, pp. 69–85. 14. G. A. Lane, D. N. Glew, E. C. Clark, H. E. Rossow, S. W. Quigley, S. S. Drake, J. S. Best, ‘Heat of fusion system for solar energy storage subsystems for the heating and cooling of building’. Chalottesville, Virginia, USA, 1975. 15. W. R. Humphries, E. I. Griggs, ‘A designing handbook for phase change thermal control and energy storage devices’. NASA Technical Paper, 1977. 16. D. Feldman, M. M. Shapiro, ‘Fatty acids and their mixtures as phase-change materials for thermal energy storage’, Solar Energy Materials, 18, 1989, pp. 201–216. 17. M. C. Smith, M. M. Farid, A. J. Easteal, ‘Review of microencapsulated phase change materials for thermal energy storage applications’, IIR/IRHACE Conference, 16–18 February 2006, Auckland, New Zealand. 18. M. Deru, ‘BESTEST Results and SUNREL’, 1997, NREL, Version 1.0, Golden, CO, USA.
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