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Experimental research on a double-layer radiant floor system with phase change material under heating mode
Applied Thermal Engineering 96 (2016) 600–606
Contents lists available at ScienceDirect
Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Experimental research on a double-layer radiant floor system with phase change material under heating mode Yi Xia a,b,*, Xiao-Song Zhang a a b
School of Energy and Environment, Southeast University, Nanjing 210096, China School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210096, China
H I G H L I G H T S
• • • • •
A new double-layer radiant floor system with phase change material is developed. An experimental system was built and the temperature field of the system was studied. The experimental results show that the new system can meet the thermal need of users. Compared to box A, box B has higher air temperature and air temperature change. The increase in supplied water temperature will decrease the thermal storage time.
A R T I C L E
I N F O
Article history: Received 13 April 2015 Accepted 21 November 2015 Available online 15 December 2015 Keywords: Energy-saving Radiant floor Phase change material Double-layer Experimental study
A B S T R A C T
In this paper, a new double-layer radiant floor system with phase change material (organics) is proposed, which can store thermal or cold energy in the off-peak period and use it in the peak period. The new system can work in both summer and winter, and the system does not need an additional refrigeration system, which only works in summer to supply cold air. By this way, the initial cost of the system will be decreased. An experimental setup was developed to study the performance of the new system under heating mode. The experimental results show that the double-layer radiant floor system with phase change material can meet the thermal need of users under heating mode. As the temperature rise in the thermal storage phase change material happens in the thermal storage process, the energy consumption of the system is lowered during thermal storage process. Under the same condition, experimental box B has the higher air temperature, but the air temperature change in box B is also bigger than box A. The floor temperature in box A in the heat release process changes little, which has a temperature difference of 2 °C during the whole heat release process. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The electrical power used in buildings varies greatly during day and night, which causes power companies in many countries to have difficulties in keeping up with the demand in peak periods. Therefore, it is meaningful to look for ways to shift the electrical consumption from the peak period to the off-peak period. At present, phase change materials (PCMs) are recognized as one of the effective ways [1–3]. With the development of social economy and the improvement of personal living standards, the use of air-conditioners is widespread in the society. Among all air conditioning technologies, the radiant floor heating system has been widely used in many countries [4–8]. The radiant floor heating system is a low-temperature
* Corresponding author. Tel.: +0086 13912999465; fax: +0086 13912999465. E-mail address: [email protected] (Y. Xia). http://dx.doi.org/10.1016/j.applthermaleng.2015.11.133 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
heating system that can make efficient use of building space and needs no requirements for cleaning. The radiant floor heating system does not produce noise, cause drafts or use ducts. Moreover, the system provides uniform temperature distribution, which is suitable for improving personal living standards. However, the radiant floor heating system usually uses electric power among the whole working time, which is harmful to the energy power supplying and the environment. Therefore, the radiant floor heating system with phase change material has been studied by many researchers in recent years [9–14]. Mazo et al. [11] described a model developed to simulate a radiant floor system with PCM in simple building types. The PCM radiant floor model was based on a one-dimensional finite difference scheme where the effective capacity method is used for PCM simulation. Ansuini et al. [12] reported the development of a lightweight piped radiant floor prototype with an integrated PCM layer aimed at buffering internal gains at constant temperature during summer cooling regimes without affecting its winter warming capacity. Zhou and He [13]
Y. Xia, X.-S. Zhang/Applied Thermal Engineering 96 (2016) 600–606
investigated the performance of a low-temperature radiant floor heating system with different heat storage materials (sand and phase change material) and heating pipes (polyethylene coils and capillary mat) in the floor structure of a test room experimentally. The results indicated the advantages of using a PCM–capillary mat combination for low-temperature floor panel typical of solar hotwater heating systems. The radiant floor heating system combined with phase change material can store thermal energy in the phase change material by using valley electricity at night, and the difference between the peak and valley power load will be smaller. However, the system can only work in winter and will be idle in summer. As a result, the maintenance consumption of the system will be improved and people need an additional refrigeration system, which only works in summer to supply cold air. By this way, the initial cost and operational cost will be improved. In order to reduce the cost of the radiant floor heating system combined with phase change material, Jin and Zhang [14] developed a new double layer phase change material (PCM) floor, which can work for people in both winter and summer. The system is used to store thermal or cold energy in the off-peak period and release them in the peak period. Compared to the radiant floor without PCM, the energy released by the floor with PCM in the peak period will be increased by 41.1% and 37.9% during heating and cooling when the heat of fusion of PCM is 150 kJ/kg. In order to improve the heat transfer performance of the double layer phase change material (PCM) floor, a new double-layer radiant floor system with phase change material is proposed in this paper. In this system, the heat exchange pipes are placed in the thermal storage phase change material and the cold storage phase change material directly. In this paper, an experimental investigation on the performance of the double-layer radiant floor system with phase change material under heating mode is reported. The thermal storage performance and heat release characteristic of the new system are analyzed. Moreover, the energy consumption in the thermal storage process of the new system is studied. The results will not only support the theoretical analysis but also inspire new ways to optimize the performance of radiant floor systems with phase change material. 2. Double-layer radiant floor system with phase change material In order to improve the heat transfer performance of the double layer phase change material (PCM) floor, a new double-layer radiant floor system with phase change material is proposed. The structure of the new system is shown in Fig. 1. The new system consisted of a floor, leveling layer, thermal storage phase change material, cold storage phase change material, reflecting film, insulation layer, building, hot water pipe and cold water pipe. The hot water pipe is filled in the thermal storage phase change material, and the cold water pipe is filled in the cold storage phase change material. Under different climate conditions, the new system has two operational modes. 1. Heating mode. Under this operational mode, the new system uses cheap electric power at night to heat the water. The hot water flows into the hot water pipe and heats the thermal storage phase change material. By this way, the temperature of the thermal storage phase change material is improved and the thermal energy is stored. In the daytime, when people need a thermal environment, the thermal storage phase change material will release the thermal energy to heat the air in the room. As a result, the temperature of the building room will be maintained in the suitable zone. Under this operational mode, the thermal energy storage process happens at night and the heat release process happens in the daytime.
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Fig. 1. The structure of double-layer radiant floor system with phase change material.
2. Cooling mode. Under this operational mode, the new system uses cheap electric power at night to cool the water. The cold water flows into the cold water pipe and cools the cold storage phase change material. By this way, the temperature of the cold storage phase change material is decreased and the cold energy is stored. In the daytime, when people need a cold environment, the cold storage phase change material will release the cold energy to cool the air in the building. As a result, the temperature of the building room will be maintained in the suitable zone. Under this operational mode, the cold energy storage process happens at night and the cold energy release process happens in the daytime. The new system has many advantages. Firstly, the new system can use cheap electric power at night to heat or cool water, and stores the thermal energy or cold energy which should be used in the daytime. By this way, the cost of the system will be decreased. Secondly, the hot water pipe is filled in the thermal storage phase change material, and the cold water pipe is filled in the cold storage phase change material. By this way, the heat transfer resistance of the system will be decreased and the heat transfer efficiency of the system will be improved. Finally, the system can work in both summer and winter, and the system does not need an additional refrigeration system, which only works in summer to supply cold air. By this way, the initial cost of the system will be decreased. 3. Experiment setup 3.1. Experimental material The phase change energy storage material in the experimental system is the organic fatty acid. The specific parameters of the thermal and cold storage phase change materials are shown in Table 1. 3.2. Experimental system In the experiment, two different experimental boxes (shown in Fig. 2) were established to investigate the effect of different floor structures on the performance of the double-layer radiant floor system with phase change material. The two experimental boxes were built in a temperature and humidity control room. By this way,
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Y. Xia, X.-S. Zhang/Applied Thermal Engineering 96 (2016) 600–606
Table 1 Specific parameters of phase change materials. Name
Thermal storage material
Cold storage material
Analysis
Main components Phase-transition temperature (°C) Super-cooling temperature (°C) Latent heat (kJ/kg) Flash point (°C)
Alcohols 34–35 – 170 >120
Lipids 19 – 140 >130
– T-history T-history DSC –
the environment temperature of the two experimental boxes could be controlled. Comparing experimental box A with B, only the place of the thermal storage phase change material and the cold storage phase change material are different. In the experimental box A (shown in Fig. 3), the thermal storage phase change material is placed below the cold storage phase change material. In the experimental box B (shown in Fig. 4), the cold storage phase change material is placed
below the thermal storage phase change material. In both experimental boxes A and B, the thickness of floor is 10 mm, the thickness of leveling layer is 30 mm, the thickness of the thermal storage phase change material is 25 mm, and the thickness of the cold storage phase change material is 30 mm. 3.3. Measuring instruments In the experiment, the temperatures of the thermal storage phase change material, the cold storage phase change material, the leveling layer, the floor, the air in the experimental box and the air out of the experimental box are tested by a T-type thermocouple, which has an accuracy of 0.1 °C. The T-type thermocouples are placed in the thermal storage phase change material and cold storage phase change material and upon the leveling layer and floor. Moreover, the T-type thermocouples are also placed in the air in the experimental box and out of the experimental box. In the experiment, the temperature readings are scanned and recorded into the computer by a data acquisition instrument (Agilent 13970A). In the experiment, the flow rate of hot water is tested by LC-WB micro flow meter, which has an accuracy of 0.1 L/h. 3.4. Experimental protocol
Fig. 2. Experimental box.
The applied flow rate of hot water in the experiment was 120 L/h, and the environment temperature of the two experimental boxes was 8 °C. In the experiment, the hot water was acquired from the constant temperature water bath and the experiment was carried out under 3 operational conditions. In condition 1, the temperature of hot water was 40 °C. In condition 2, the temperature of hot water was 45 °C. In condition 3, the temperature of hot water was 52 °C. In the experiment, the double-layer radiant floor systems with phase change material in both box A and box B were operated in continuous operation mode. In the first 8 hours, the double-layer radiant floor system with phase change material was under thermal energy storage mode. In this mode the feed hot water was recycled from the constant temperature water bath. In the next 16 hours, the double-layer radiant floor system with phase change material was under heat release mode. In this mode, there was no hot water flowing into the double-layer radiant floor system with phase change material and the thermal energy in the thermal storage phase change material could exchange to the air in the experimental box (both A and B). In the experiment, the temperatures were scanned at time intervals (thirty minutes).
Fig. 3. Floor structure of box A.
Fig. 4. Floor structure of box B.
Y. Xia, X.-S. Zhang/Applied Thermal Engineering 96 (2016) 600–606
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3.5. Analytical methods Under heating mode, the energy consumption of the doublelayer radiant floor system with phase change material mainly relies on the thermal energy consumption of the water heater. The energy consumption of the water pump is neglected in the following analysis as it is much less than the energy consumption of the water heater. As a result, the ideal energy consumption of the doublelayer radiant floor system with phase change material can be acquired:
P = cm (T 2 − T1 )
(1)
where: P is the ideal energy consumption of the double-layer radiant floor system with phase change material; c is the specific heat capacity of hot water; m is the mass flow rate of the hot water; T 2 is the temperature of hot water at the exit of the water heater; and T1 is the temperature of hot water at the entrance of the water heater. 4. Results and discussion In the experiment, the temperature field of the double-layer radiant floor system with phase change material in the two experimental boxes under different conditions was studied. Moreover, the energy consumption of the system and the air temperature in the two experimental boxes were also researched. The temperature field of the double-layer radiant floor system with phase change material in the experimental box B under condition 2 is shown in Fig. 5. Fig. 5 shows that the environment temperature of experimental box B during the experimental process is in the range of 8 °C and 10 °C. In the first 8 hours (thermal storage process), the temperature of the thermal storage phase change material changes from 7 °C to 43 °C, which has the biggest speed of temperature rise. The temperature rise of the thermal storage phase change material leads to the temperature rise of the cold storage phase change material and leveling layer. The temperature of the cold storage phase change material changes from 7 °C to 20 °C in the thermal storage process, and the temperature of the leveling layer changes from 7 °C to 29 °C. The temperature rise of leveling layer leads to the temperature rise in the floor, which changes from 7 °C to 25 °C in the thermal storage process. Finally, the temperature of air in experimental box B changes from 7 °C to 20 °C since the temperature rise in the floor. In the next 16 hours (heat release process), there’s no hot water flows into the double-layer radiant floor system with phase change material. As a result, the thermal energy in the thermal storage phase change material mainly
Fig. 5. Temperature field of the system in box B under condition 2.
Fig. 6. Energy consumption and temperature of thermal storage layer in thermal storage process.
releases to the leveling layer, which leads to the temperature drop in the thermal storage phase change material. In the end of heat release process, the temperature of the thermal storage phase change material is 25 °C. As the cold storage phase change material is among the thermal storage phase change material and insulation layer, the temperature of the cold storage phase change material is close to 21 °C in the heat release process. The temperature of leveling layer changes from 29 °C to 18 °C, which leads to a decrease in the temperature of the floor to 18 °C in the end of heat release process. In the heat release process, the air temperature finally decreases to 14 °C, which shows that the double-layer radiant floor system with phase change material can meet the thermal need of users under condition 2. The energy consumption of the double-layer radiant floor system with phase change material and the temperature of thermal storage phase change material in the experimental box B under condition 2 during the thermal storage process are shown in Fig. 6. Fig. 6 shows that the energy consumption of the system is biggest at the start of thermal storage process, since the temperature of thermal storage phase change material is lowest in the start of thermal storage process. As the temperature increases in thermal storage phase change material, the energy consumption of the system is decreased during thermal storage process. At the start of thermal storage process, the maximum energy consumption of the system is 450 W. The air temperature in experimental box B and the environment temperature in box B under the 3conditions are shown in Fig. 7.
Fig. 7. Air temperature in and out of box B under different conditions.
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Y. Xia, X.-S. Zhang/Applied Thermal Engineering 96 (2016) 600–606
Fig. 7 shows that the air in box B has the biggest temperature under condition 3, which is 26 °C at the end of thermal storage process. The air in box B has the lowest temperature under condition 3, which is 17 °C at the end of thermal storage process. Compared to the big temperature difference among three conditions at the end of thermal storage process, the temperature difference among the three conditions at the end of the heat release process is 5 °C. It is indicated that with the same time of thermal storage process, the increase of water temperature supplied to the double-layer radiant floor system with phase change material will increase the air temperature variation during the heat release process, which could lead to the discomfort of users. On the other hand, if the air temperature at the end of the thermal storage process is the same under the three conditions, the increase in the supplied water temperature will decrease the time of the thermal storage process and ensure the comfort of users. Fig. 7 also shows that, since the air temperature in box B under condition 3 is much higher than other two conditions, the environment temperature in box B under condition 3 is higher than the other two conditions. The air temperature in the different experimental boxes and the environment temperature under condition 2 are shown in Fig. 8. Fig. 8 shows that, under condition 2, the air temperature in box B at the end of thermal storage process is much higher than that in box A. However, the air temperature in box B at the end of the heat release process is close to that in box A (the temperature difference at the end of heat release process is only 2 °C). In box B, the air temperature is 20 °C at the end of the thermal storage process and 14 °C at the end of the heat release process. It is indicated that, under the same condition, box B has the higher air temperature, but the air temperature change in box B is also bigger than that in box A. The temperature of the thermal storage phase change material in box B under different conditions is shown in Fig. 9. Fig. 9 shows that the thermal storage phase change material in box B has the biggest temperature under condition 3. It is indicated that the temperature of the thermal storage phase change material under condition 3 is much higher than that under other two conditions at the end of the thermal storage process. However, the temperature of thermal storage phase change material under condition 3 is closed to that under other two conditions in the end of heat release process. The temperatures of the thermal storage phase change material and the cold storage phase change material in the different boxes under condition 2 are shown in Fig. 10. Fig. 10 shows that the temperature of thermal storage phase change material at the end of heat release process in box B is lower than that in box A, which illustrates
Fig. 8. Air temperature in and out of box A and B under condition 2.
Fig. 9. Temperature of thermal storage layer in box B under different conditions.
Fig. 10. Temperatures of the thermal storage layer and the cold storage layer in different boxes.
that the thermal storage phase change material in box B can exchange its heat to air better than that of the thermal storage phase change material in box A. Fig. 10 also shows that the temperature of the cold storage phase change material at the end of heat release process in box B is lower than that in box A, which indicates that less heat is stored in the cold storage phase change material in box B, and the thermal storage phase change material in box B can exchange its heat to air better than that of the thermal storage phase change material in box A. The temperature of the floor in experimental box B under different conditions is shown in Fig. 11. Fig. 11 shows that the floor temperature under condition 3 is higher than that under the other two conditions. It is indicated that the floor temperature at the end of the thermal storage process under condition 3 is much higher than that under the other two conditions, and the floor temperature at the end of heat release process under condition 3 is close to that under the other two conditions. The temperatures of floor in different experimental boxes under condition 2 are shown in Fig. 12. Fig. 12 shows that the floor temperature in box B is higher than that in box A. Fig. 12 shows that the floor temperature at the end of thermal storage process in box B is much higher than that in box A, and the floor temperature at the end of the heat release process in box B is close to that in box A. In the heat release process, the floor temperature in box B changes greatly, which has a temperature difference of 10 °C. However, the
Y. Xia, X.-S. Zhang/Applied Thermal Engineering 96 (2016) 600–606
Fig. 11. Temperature of the floor in box B under different conditions.
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2. The double-layer radiant floor system with phase change material can meet the thermal need of users. As the temperature rise in the thermal storage phase change material happens in the thermal storage process, the energy consumption of the system is lowered during the thermal storage process. At the start of thermal energy storage process, the maximum energy consumption of the system is 450 W. 3. Under the same condition, experimental box B has the higher air temperature, but the air temperature change in box B is also bigger than box A. The floor temperature in box A in the heat release process changes little, which has a temperature difference of 2 °C during the whole heat release process. 4. It is indicated that with the same time of thermal energy storage process, the increase in the water temperature supplied to the system will increase the air temperature variation during the heat release process, which could lead to the discomfort of users. On the other hand, if the air temperature at the end of thermal energy storage process is the same under the three conditions, the increase in the supplied water temperature will decrease the time of the thermal energy storage process and ensure the comfort of users. Acknowledgements This research work reported in this paper was supported by the Project from the National Science and Technology Pillar Program in the 12th Five-Year Plan Period (No. 2011BAJ03B14) and the Natural Science Foundation of China (No. 51376044). Nomenclature c m P
Fig. 12. Temperatures of the floor in different boxes under condition 2.
t T T1 T2
floor temperature in box A in the heat release process changes little, which has a temperature difference of 2 °C. This is because the thermal resistance between the thermal storage phase change material and the floor in box A is bigger than that in box B, so the heat from the thermal storage phase change material to the floor will be later in box A. 5. Conclusions In this paper, a new double-layer radiant floor system with phase change material (organics) is proposed, which can store thermal or cold energy in the off-peak period and use it in the peak period. The new system can work in both summer and winter, and the system does not need an additional refrigeration system which only works in summer to supply cold air. By this way, the initial cost of the system will be decreased. 1. An experimental setup was developed to study the performance of a double-layer radiant floor system with phase change material under heating mode. In the experiment, the temperature field of the double-layer radiant floor system with phase change material in the two experimental boxes under different conditions was studied. Moreover, the energy consumption of the system and the air temperature in the two experimental boxes were also researched.
Specific heat capacity of hot water (kJ/(kg·°C)) Mass flow rate of the hot water (kg/s) Ideal energy consumption of the double-layer radiant floor system with phase change material (kW) Time (h) Temperature (°C) Temperature of hot water at the entrance of the water heater (°C) Temperature of hot water at the exit of the water heater (°C)
References [1] B. Zalba, J.M. Marín, L.F. Cabeza, H. Mehling, Free-cooling of buildings with phase change materials, Int. J. Refrig. 27 (2004) 839–849. [2] C.K. Halford, R.F. Boehm, Modeling of phase change material peak load shifting, Energy Build. 39 (2007) 298–305. [3] A. Castell, I. Martorell, M. Medrano, G. Pérez, L.F. Cabeza, Experimental study of using PCM in brick constructive solutions for passive cooling, Energy Build. 42 (2010) 534–540. [4] D. Song, T. Kim, S. Song, et al., Performance evaluation of a radiant floor cooling system integrated with dehumidified ventilation, Appl. Therm. Eng. 28 (2008) 1299–1311. [5] J. Seo, J. Jeon, J.-H. Lee, et al., Thermal performance analysis according to wood flooring structure for energy conservation in radiant floor heating systems, Energy Build. 43 (2011) 2039–2042. [6] D. Zhang, N. Cai, Z. Wang, Experimental and numerical analysis of lightweight radiant floor heating system, Energy Build. 61 (2013) 260–266. [7] L. Fontana, Thermal performance of radiant heating floors in furnished enclosed spaces, Appl. Therm. Eng. 31 (2011) 1547–1555. [8] M. Maerefat, A. Zolfaghari, A. Omidvar, On the conformity of floor heating systems with sleeping in the eastern-style beds; physiological responses and thermal comfort assessment, Build. Environ. 47 (2012) 322–329. [9] L. Haurie, J. Mazo, M. Delgado, et al., Fire behaviour of a mortar with different mass fractions of phase change material for use in radiant floor systems, Energy Build. 84 (2014) 86–93. [10] J. Jeon, S.-G. Jeong, J.-H. Lee, et al., High thermal performance composite PCMs loading xGnP for application to building using radiant floor heating system, Solar Energy Mater. Solar Cells 101 (2012) 51–56. [11] J. Mazo, M. Delgado, J.M. Marin, et al., Modeling a radiant floor system with phase change material (PCM) integrated into a building simulation tool: analysis
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of a case study of a floor heating system coupled to a heat pump, Energy Build. 47 (2012) 458–466. [12] R. Ansuini, R. Larghetti, A. Giretti, et al., Radiant floors integrated with PCM for indoor temperature control, Energy Build. 43 (2011) 3019– 3026.
[13] G. Zhou, J. He, Thermal performance of a radiant floor heating system with different heat storage materials and heating pipes, Appl. Energy 138 (2015) 648–660. [14] X. Jin, X. Zhang, Thermal analysis of a double layer phase change material floor, Appl. Therm. Eng. 31 (2011) 1576–1581.
Contents lists available at ScienceDirect
Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Experimental research on a double-layer radiant floor system with phase change material under heating mode Yi Xia a,b,*, Xiao-Song Zhang a a b
School of Energy and Environment, Southeast University, Nanjing 210096, China School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210096, China
H I G H L I G H T S
• • • • •
A new double-layer radiant floor system with phase change material is developed. An experimental system was built and the temperature field of the system was studied. The experimental results show that the new system can meet the thermal need of users. Compared to box A, box B has higher air temperature and air temperature change. The increase in supplied water temperature will decrease the thermal storage time.
A R T I C L E
I N F O
Article history: Received 13 April 2015 Accepted 21 November 2015 Available online 15 December 2015 Keywords: Energy-saving Radiant floor Phase change material Double-layer Experimental study
A B S T R A C T
In this paper, a new double-layer radiant floor system with phase change material (organics) is proposed, which can store thermal or cold energy in the off-peak period and use it in the peak period. The new system can work in both summer and winter, and the system does not need an additional refrigeration system, which only works in summer to supply cold air. By this way, the initial cost of the system will be decreased. An experimental setup was developed to study the performance of the new system under heating mode. The experimental results show that the double-layer radiant floor system with phase change material can meet the thermal need of users under heating mode. As the temperature rise in the thermal storage phase change material happens in the thermal storage process, the energy consumption of the system is lowered during thermal storage process. Under the same condition, experimental box B has the higher air temperature, but the air temperature change in box B is also bigger than box A. The floor temperature in box A in the heat release process changes little, which has a temperature difference of 2 °C during the whole heat release process. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The electrical power used in buildings varies greatly during day and night, which causes power companies in many countries to have difficulties in keeping up with the demand in peak periods. Therefore, it is meaningful to look for ways to shift the electrical consumption from the peak period to the off-peak period. At present, phase change materials (PCMs) are recognized as one of the effective ways [1–3]. With the development of social economy and the improvement of personal living standards, the use of air-conditioners is widespread in the society. Among all air conditioning technologies, the radiant floor heating system has been widely used in many countries [4–8]. The radiant floor heating system is a low-temperature
* Corresponding author. Tel.: +0086 13912999465; fax: +0086 13912999465. E-mail address: [email protected] (Y. Xia). http://dx.doi.org/10.1016/j.applthermaleng.2015.11.133 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
heating system that can make efficient use of building space and needs no requirements for cleaning. The radiant floor heating system does not produce noise, cause drafts or use ducts. Moreover, the system provides uniform temperature distribution, which is suitable for improving personal living standards. However, the radiant floor heating system usually uses electric power among the whole working time, which is harmful to the energy power supplying and the environment. Therefore, the radiant floor heating system with phase change material has been studied by many researchers in recent years [9–14]. Mazo et al. [11] described a model developed to simulate a radiant floor system with PCM in simple building types. The PCM radiant floor model was based on a one-dimensional finite difference scheme where the effective capacity method is used for PCM simulation. Ansuini et al. [12] reported the development of a lightweight piped radiant floor prototype with an integrated PCM layer aimed at buffering internal gains at constant temperature during summer cooling regimes without affecting its winter warming capacity. Zhou and He [13]
Y. Xia, X.-S. Zhang/Applied Thermal Engineering 96 (2016) 600–606
investigated the performance of a low-temperature radiant floor heating system with different heat storage materials (sand and phase change material) and heating pipes (polyethylene coils and capillary mat) in the floor structure of a test room experimentally. The results indicated the advantages of using a PCM–capillary mat combination for low-temperature floor panel typical of solar hotwater heating systems. The radiant floor heating system combined with phase change material can store thermal energy in the phase change material by using valley electricity at night, and the difference between the peak and valley power load will be smaller. However, the system can only work in winter and will be idle in summer. As a result, the maintenance consumption of the system will be improved and people need an additional refrigeration system, which only works in summer to supply cold air. By this way, the initial cost and operational cost will be improved. In order to reduce the cost of the radiant floor heating system combined with phase change material, Jin and Zhang [14] developed a new double layer phase change material (PCM) floor, which can work for people in both winter and summer. The system is used to store thermal or cold energy in the off-peak period and release them in the peak period. Compared to the radiant floor without PCM, the energy released by the floor with PCM in the peak period will be increased by 41.1% and 37.9% during heating and cooling when the heat of fusion of PCM is 150 kJ/kg. In order to improve the heat transfer performance of the double layer phase change material (PCM) floor, a new double-layer radiant floor system with phase change material is proposed in this paper. In this system, the heat exchange pipes are placed in the thermal storage phase change material and the cold storage phase change material directly. In this paper, an experimental investigation on the performance of the double-layer radiant floor system with phase change material under heating mode is reported. The thermal storage performance and heat release characteristic of the new system are analyzed. Moreover, the energy consumption in the thermal storage process of the new system is studied. The results will not only support the theoretical analysis but also inspire new ways to optimize the performance of radiant floor systems with phase change material. 2. Double-layer radiant floor system with phase change material In order to improve the heat transfer performance of the double layer phase change material (PCM) floor, a new double-layer radiant floor system with phase change material is proposed. The structure of the new system is shown in Fig. 1. The new system consisted of a floor, leveling layer, thermal storage phase change material, cold storage phase change material, reflecting film, insulation layer, building, hot water pipe and cold water pipe. The hot water pipe is filled in the thermal storage phase change material, and the cold water pipe is filled in the cold storage phase change material. Under different climate conditions, the new system has two operational modes. 1. Heating mode. Under this operational mode, the new system uses cheap electric power at night to heat the water. The hot water flows into the hot water pipe and heats the thermal storage phase change material. By this way, the temperature of the thermal storage phase change material is improved and the thermal energy is stored. In the daytime, when people need a thermal environment, the thermal storage phase change material will release the thermal energy to heat the air in the room. As a result, the temperature of the building room will be maintained in the suitable zone. Under this operational mode, the thermal energy storage process happens at night and the heat release process happens in the daytime.
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Fig. 1. The structure of double-layer radiant floor system with phase change material.
2. Cooling mode. Under this operational mode, the new system uses cheap electric power at night to cool the water. The cold water flows into the cold water pipe and cools the cold storage phase change material. By this way, the temperature of the cold storage phase change material is decreased and the cold energy is stored. In the daytime, when people need a cold environment, the cold storage phase change material will release the cold energy to cool the air in the building. As a result, the temperature of the building room will be maintained in the suitable zone. Under this operational mode, the cold energy storage process happens at night and the cold energy release process happens in the daytime. The new system has many advantages. Firstly, the new system can use cheap electric power at night to heat or cool water, and stores the thermal energy or cold energy which should be used in the daytime. By this way, the cost of the system will be decreased. Secondly, the hot water pipe is filled in the thermal storage phase change material, and the cold water pipe is filled in the cold storage phase change material. By this way, the heat transfer resistance of the system will be decreased and the heat transfer efficiency of the system will be improved. Finally, the system can work in both summer and winter, and the system does not need an additional refrigeration system, which only works in summer to supply cold air. By this way, the initial cost of the system will be decreased. 3. Experiment setup 3.1. Experimental material The phase change energy storage material in the experimental system is the organic fatty acid. The specific parameters of the thermal and cold storage phase change materials are shown in Table 1. 3.2. Experimental system In the experiment, two different experimental boxes (shown in Fig. 2) were established to investigate the effect of different floor structures on the performance of the double-layer radiant floor system with phase change material. The two experimental boxes were built in a temperature and humidity control room. By this way,
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Table 1 Specific parameters of phase change materials. Name
Thermal storage material
Cold storage material
Analysis
Main components Phase-transition temperature (°C) Super-cooling temperature (°C) Latent heat (kJ/kg) Flash point (°C)
Alcohols 34–35 – 170 >120
Lipids 19 – 140 >130
– T-history T-history DSC –
the environment temperature of the two experimental boxes could be controlled. Comparing experimental box A with B, only the place of the thermal storage phase change material and the cold storage phase change material are different. In the experimental box A (shown in Fig. 3), the thermal storage phase change material is placed below the cold storage phase change material. In the experimental box B (shown in Fig. 4), the cold storage phase change material is placed
below the thermal storage phase change material. In both experimental boxes A and B, the thickness of floor is 10 mm, the thickness of leveling layer is 30 mm, the thickness of the thermal storage phase change material is 25 mm, and the thickness of the cold storage phase change material is 30 mm. 3.3. Measuring instruments In the experiment, the temperatures of the thermal storage phase change material, the cold storage phase change material, the leveling layer, the floor, the air in the experimental box and the air out of the experimental box are tested by a T-type thermocouple, which has an accuracy of 0.1 °C. The T-type thermocouples are placed in the thermal storage phase change material and cold storage phase change material and upon the leveling layer and floor. Moreover, the T-type thermocouples are also placed in the air in the experimental box and out of the experimental box. In the experiment, the temperature readings are scanned and recorded into the computer by a data acquisition instrument (Agilent 13970A). In the experiment, the flow rate of hot water is tested by LC-WB micro flow meter, which has an accuracy of 0.1 L/h. 3.4. Experimental protocol
Fig. 2. Experimental box.
The applied flow rate of hot water in the experiment was 120 L/h, and the environment temperature of the two experimental boxes was 8 °C. In the experiment, the hot water was acquired from the constant temperature water bath and the experiment was carried out under 3 operational conditions. In condition 1, the temperature of hot water was 40 °C. In condition 2, the temperature of hot water was 45 °C. In condition 3, the temperature of hot water was 52 °C. In the experiment, the double-layer radiant floor systems with phase change material in both box A and box B were operated in continuous operation mode. In the first 8 hours, the double-layer radiant floor system with phase change material was under thermal energy storage mode. In this mode the feed hot water was recycled from the constant temperature water bath. In the next 16 hours, the double-layer radiant floor system with phase change material was under heat release mode. In this mode, there was no hot water flowing into the double-layer radiant floor system with phase change material and the thermal energy in the thermal storage phase change material could exchange to the air in the experimental box (both A and B). In the experiment, the temperatures were scanned at time intervals (thirty minutes).
Fig. 3. Floor structure of box A.
Fig. 4. Floor structure of box B.
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3.5. Analytical methods Under heating mode, the energy consumption of the doublelayer radiant floor system with phase change material mainly relies on the thermal energy consumption of the water heater. The energy consumption of the water pump is neglected in the following analysis as it is much less than the energy consumption of the water heater. As a result, the ideal energy consumption of the doublelayer radiant floor system with phase change material can be acquired:
P = cm (T 2 − T1 )
(1)
where: P is the ideal energy consumption of the double-layer radiant floor system with phase change material; c is the specific heat capacity of hot water; m is the mass flow rate of the hot water; T 2 is the temperature of hot water at the exit of the water heater; and T1 is the temperature of hot water at the entrance of the water heater. 4. Results and discussion In the experiment, the temperature field of the double-layer radiant floor system with phase change material in the two experimental boxes under different conditions was studied. Moreover, the energy consumption of the system and the air temperature in the two experimental boxes were also researched. The temperature field of the double-layer radiant floor system with phase change material in the experimental box B under condition 2 is shown in Fig. 5. Fig. 5 shows that the environment temperature of experimental box B during the experimental process is in the range of 8 °C and 10 °C. In the first 8 hours (thermal storage process), the temperature of the thermal storage phase change material changes from 7 °C to 43 °C, which has the biggest speed of temperature rise. The temperature rise of the thermal storage phase change material leads to the temperature rise of the cold storage phase change material and leveling layer. The temperature of the cold storage phase change material changes from 7 °C to 20 °C in the thermal storage process, and the temperature of the leveling layer changes from 7 °C to 29 °C. The temperature rise of leveling layer leads to the temperature rise in the floor, which changes from 7 °C to 25 °C in the thermal storage process. Finally, the temperature of air in experimental box B changes from 7 °C to 20 °C since the temperature rise in the floor. In the next 16 hours (heat release process), there’s no hot water flows into the double-layer radiant floor system with phase change material. As a result, the thermal energy in the thermal storage phase change material mainly
Fig. 5. Temperature field of the system in box B under condition 2.
Fig. 6. Energy consumption and temperature of thermal storage layer in thermal storage process.
releases to the leveling layer, which leads to the temperature drop in the thermal storage phase change material. In the end of heat release process, the temperature of the thermal storage phase change material is 25 °C. As the cold storage phase change material is among the thermal storage phase change material and insulation layer, the temperature of the cold storage phase change material is close to 21 °C in the heat release process. The temperature of leveling layer changes from 29 °C to 18 °C, which leads to a decrease in the temperature of the floor to 18 °C in the end of heat release process. In the heat release process, the air temperature finally decreases to 14 °C, which shows that the double-layer radiant floor system with phase change material can meet the thermal need of users under condition 2. The energy consumption of the double-layer radiant floor system with phase change material and the temperature of thermal storage phase change material in the experimental box B under condition 2 during the thermal storage process are shown in Fig. 6. Fig. 6 shows that the energy consumption of the system is biggest at the start of thermal storage process, since the temperature of thermal storage phase change material is lowest in the start of thermal storage process. As the temperature increases in thermal storage phase change material, the energy consumption of the system is decreased during thermal storage process. At the start of thermal storage process, the maximum energy consumption of the system is 450 W. The air temperature in experimental box B and the environment temperature in box B under the 3conditions are shown in Fig. 7.
Fig. 7. Air temperature in and out of box B under different conditions.
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Fig. 7 shows that the air in box B has the biggest temperature under condition 3, which is 26 °C at the end of thermal storage process. The air in box B has the lowest temperature under condition 3, which is 17 °C at the end of thermal storage process. Compared to the big temperature difference among three conditions at the end of thermal storage process, the temperature difference among the three conditions at the end of the heat release process is 5 °C. It is indicated that with the same time of thermal storage process, the increase of water temperature supplied to the double-layer radiant floor system with phase change material will increase the air temperature variation during the heat release process, which could lead to the discomfort of users. On the other hand, if the air temperature at the end of the thermal storage process is the same under the three conditions, the increase in the supplied water temperature will decrease the time of the thermal storage process and ensure the comfort of users. Fig. 7 also shows that, since the air temperature in box B under condition 3 is much higher than other two conditions, the environment temperature in box B under condition 3 is higher than the other two conditions. The air temperature in the different experimental boxes and the environment temperature under condition 2 are shown in Fig. 8. Fig. 8 shows that, under condition 2, the air temperature in box B at the end of thermal storage process is much higher than that in box A. However, the air temperature in box B at the end of the heat release process is close to that in box A (the temperature difference at the end of heat release process is only 2 °C). In box B, the air temperature is 20 °C at the end of the thermal storage process and 14 °C at the end of the heat release process. It is indicated that, under the same condition, box B has the higher air temperature, but the air temperature change in box B is also bigger than that in box A. The temperature of the thermal storage phase change material in box B under different conditions is shown in Fig. 9. Fig. 9 shows that the thermal storage phase change material in box B has the biggest temperature under condition 3. It is indicated that the temperature of the thermal storage phase change material under condition 3 is much higher than that under other two conditions at the end of the thermal storage process. However, the temperature of thermal storage phase change material under condition 3 is closed to that under other two conditions in the end of heat release process. The temperatures of the thermal storage phase change material and the cold storage phase change material in the different boxes under condition 2 are shown in Fig. 10. Fig. 10 shows that the temperature of thermal storage phase change material at the end of heat release process in box B is lower than that in box A, which illustrates
Fig. 8. Air temperature in and out of box A and B under condition 2.
Fig. 9. Temperature of thermal storage layer in box B under different conditions.
Fig. 10. Temperatures of the thermal storage layer and the cold storage layer in different boxes.
that the thermal storage phase change material in box B can exchange its heat to air better than that of the thermal storage phase change material in box A. Fig. 10 also shows that the temperature of the cold storage phase change material at the end of heat release process in box B is lower than that in box A, which indicates that less heat is stored in the cold storage phase change material in box B, and the thermal storage phase change material in box B can exchange its heat to air better than that of the thermal storage phase change material in box A. The temperature of the floor in experimental box B under different conditions is shown in Fig. 11. Fig. 11 shows that the floor temperature under condition 3 is higher than that under the other two conditions. It is indicated that the floor temperature at the end of the thermal storage process under condition 3 is much higher than that under the other two conditions, and the floor temperature at the end of heat release process under condition 3 is close to that under the other two conditions. The temperatures of floor in different experimental boxes under condition 2 are shown in Fig. 12. Fig. 12 shows that the floor temperature in box B is higher than that in box A. Fig. 12 shows that the floor temperature at the end of thermal storage process in box B is much higher than that in box A, and the floor temperature at the end of the heat release process in box B is close to that in box A. In the heat release process, the floor temperature in box B changes greatly, which has a temperature difference of 10 °C. However, the
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Fig. 11. Temperature of the floor in box B under different conditions.
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2. The double-layer radiant floor system with phase change material can meet the thermal need of users. As the temperature rise in the thermal storage phase change material happens in the thermal storage process, the energy consumption of the system is lowered during the thermal storage process. At the start of thermal energy storage process, the maximum energy consumption of the system is 450 W. 3. Under the same condition, experimental box B has the higher air temperature, but the air temperature change in box B is also bigger than box A. The floor temperature in box A in the heat release process changes little, which has a temperature difference of 2 °C during the whole heat release process. 4. It is indicated that with the same time of thermal energy storage process, the increase in the water temperature supplied to the system will increase the air temperature variation during the heat release process, which could lead to the discomfort of users. On the other hand, if the air temperature at the end of thermal energy storage process is the same under the three conditions, the increase in the supplied water temperature will decrease the time of the thermal energy storage process and ensure the comfort of users. Acknowledgements This research work reported in this paper was supported by the Project from the National Science and Technology Pillar Program in the 12th Five-Year Plan Period (No. 2011BAJ03B14) and the Natural Science Foundation of China (No. 51376044). Nomenclature c m P
Fig. 12. Temperatures of the floor in different boxes under condition 2.
t T T1 T2
floor temperature in box A in the heat release process changes little, which has a temperature difference of 2 °C. This is because the thermal resistance between the thermal storage phase change material and the floor in box A is bigger than that in box B, so the heat from the thermal storage phase change material to the floor will be later in box A. 5. Conclusions In this paper, a new double-layer radiant floor system with phase change material (organics) is proposed, which can store thermal or cold energy in the off-peak period and use it in the peak period. The new system can work in both summer and winter, and the system does not need an additional refrigeration system which only works in summer to supply cold air. By this way, the initial cost of the system will be decreased. 1. An experimental setup was developed to study the performance of a double-layer radiant floor system with phase change material under heating mode. In the experiment, the temperature field of the double-layer radiant floor system with phase change material in the two experimental boxes under different conditions was studied. Moreover, the energy consumption of the system and the air temperature in the two experimental boxes were also researched.
Specific heat capacity of hot water (kJ/(kg·°C)) Mass flow rate of the hot water (kg/s) Ideal energy consumption of the double-layer radiant floor system with phase change material (kW) Time (h) Temperature (°C) Temperature of hot water at the entrance of the water heater (°C) Temperature of hot water at the exit of the water heater (°C)
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