Review on air and water thermal energy storage of …[email protected] Review on air and...

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Review on air and water thermal energy storage of buildings with phase change materials

Yin Ma1,2, Yilin Luo1,2, Hongxiang Xu1,2, Ruiqing Du1,2, Yong Wang1,2 ()

1. National Centre for International Research of Low-Carbon and Green Buildings, Ministry of Science & Technology, Chongqing University, Chongqing 400045, China

2. Joint International Research Laboratory of Green Buildings and Built Environments, Ministry of Education, Chongqing University, Chongqing 400045, China

Abstract With high energy consumption in buildings, the emissions of greenhouse gases are also increasing.

It leads to some environmental problems. To realize resource conservation and environmental protection target, latent heat thermal energy storage systems (LHTES) are introduced into all kinds of buildings. A variety of air-LHTES and water-LHTES are analyzed in this study based on the

heat transfer fluid medium adopted. The results of this study indicate that the air-LHTES uses the low-temperature ambient air to store cold during nighttime and releases cold during the daytime in summer vice versa in winter with auxiliary heat sources. The water-LHTES stores the cold and

heat generated by various natural sources (solar energy, nighttime sky radiation, air conditioning condensate) through the water, and then releases the cold and heat to the buildings to reduce the energy consumption of the buildings. However, for some regions with extremely hot climate,

the ambient temperature is still high during nighttime in summer. It is difficult to achieve cold storage of ambient air. Accordingly, other natural cold sources should be adopted for cooling in air-LHTES. Due to the cooling effect of nighttime sky radiation, water temperature in water-LHTES

could be lower enough for cold storage. Thus, a combination system of water-LHTES and air-LHTES is recommended. In this system, cold storage is achieved by collecting low-temperature, and released by supplying cooling air. The proposed system can also achieve heat storage in winter

by collecting solar energy, and release heat by supplying heating air.

Keywords phase change material

air-latent heat thermal energy storage

system

water-latent heat thermal energy

storage system

heat transfer enhancement

Article History Received: 31 January 2020

Revised: 16 March 2020

Accepted: 20 March 2020

Review Article © Tsinghua University Press 2020

1 Introduction

Energy plays a vital role in human survival, economic development, and social progress (Cruz-Peragon et al., 2012). According to the world energy outlook 2014 published by the international energy agency (IEA), global energy demand will increase by 1/3 between 2011 and 2035 (Cronshaw, 2015). Meanwhile, 80% of the world’s energy supply comes from traditional fossil sources, mainly coal, petroleum, and natural gas. However, with the continuous exploitation, the exhaustion of fossil sources is inevitable that leads to a series of environmental problems (global warming, ozone layer destruction) (Xu et al., 2015; Souayfane et al., 2016). Widespread use of renewable energy will play an important role in a low-carbon society.

With the development of the social economy, the

proportion of building energy consumption has reached 30%–40% of the total energy consumption (Pérez-Lombard et al., 2008). The major building energy consumption is dedicated to heating, cooling, and ventilation systems (Pérez- Lombard et al., 2008). Hence, it is very important to utilize various thermal energy storage (TES) systems to reduce building energy consumption. Generally, TES is divided into sensible heat thermal energy storage systems (SHTES), thermochemical thermal energy storage systems (TTES), and latent heat thermal energy storage systems (LHTES) (Sharma et al., 2009; Xu et al., 2014; Seddegh et al., 2015b). SHTES mainly depends on the temperature change to achieve thermal energy storage, in which the heat storage density is small and the device volume is large; TTES stores thermal energy by the reversible physio-chemical reaction, whose technology is complex (N’Tsoukpoe et al., 2009). LHTES

Vol. 3, No. 2, 2021, 77–99Experimental and Computational Multiphase Flow https://doi.org/10.1007/s42757-020-0064-4

Y. Ma, Y. Luo, H. Xu, et al.

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mainly relies on phase change material (PCM) to store/release latent heat of fusion, during charging and discharging process (about 10 times thermal energy from SHTES at the same temperature difference, and the volume of storage device is smaller than SHTES (Liu et al., 2016)). Thus the potential value of LHTES is significant in buildings (Regin et al., 2008; Fernandes et al., 2012).

PCM is widely utilized in both active and passive systems. The melting and solidification of PCM is usually involved in the active system as the temperature of the heat transfer fluid varies only within a certain range. It means that the gas–liquid phase transition process and the flow process between the liquid PCM and PCM vapor are rare in the active system. For the passive systems, PCMs are widely incorporated into all kinds of building envelopes, such as roof (Chung and Park, 2016), wall (Lv et al., 2006), and floor (Yun et al., 2019). Moreover, in partial passive structures, not only the phase change process of PCM is included, but also the two-phase flow process of PCM is involved. The literature review (Wu and Lei, 2016) introduced a passive solar heating system proposed by Rice (1984). This passive system used PCM in heat pipes to absorb solar energy and then transfer it to the water wall. In this system, the liquid PCM was at the evaporator end of the heat pipe. When the liquid PCM absorbed the heat of solar energy, it evaporated into PCM vapor and PCM vapor flowed to the condenser end of the heat pipe. At the condenser end of the heat pipe, the latent heat of PCM vapor was transferred to the water wall. The PCM vapor was ultimately into liquid PCM. Then

the liquid PCM flowed back to the evaporator end of the heat pipe by gravity. Through the phase change process of PCM and the gas–liquid two-phase flow process, the whole passive thermal energy storage cycle was completed. This paper will focus on reviewing the LHTES system integrated with active system.

In this paper, the LHTES is divided into air-LHTES and water-LHTES based on the heat transfer fluid medium adopted. These two LHTES are discussed and studied comprehensively. The air-LHTES is also known as the free cooling system in summer, which stores ambient cold during the nighttime, and releases cold during the daytime, to decrease the indoor temperature and maintain the indoor temperature within the indoor thermal comfort for a long time (Turnpenny et al., 2000, 2001). However, for heating purposes in winter, the air-LHTES usually needs auxiliary heat sources such as heat pumps to heat the air (Iten et al., 2016). Hence, the heat is not available freely during the winter. The air-LHTES can not only be integrated in the air conditioning system (Chaiyat, 2015) but also be installed independently in the floor (Nagano et al., 2006) and ceiling (Kang et al., 2003; Butala and Stritih, 2009) of the buildings respectively, shown in Fig. 1. Therefore, this technology can be combined with the building to realize the use of free ambient cold to reduce the building’s cooling load flexibly.

The water-LHTES combines water heat exchangers and PCM so that thermal energy from various free cold and heat sources (solar energy (Wu et al., 2020), nighttime sky radiation (Meng, 2009; Wang et al., 2019), air conditioning

Fig. 1 Storage device installment for free cooling of building: (a) in ceiling (Butala et al., 2009, reproduced with permission © Elsevier B.V.2008; Kang et al., 2003, reproduced with permission © Elsevier Science B.V. 2002), (b) under floor (Nagano et al., 2006; reproduced with permission © Elsevier B.V. 2005), (c) combined with air conditioner (Chaiyat, 2015; reproduced with permission © Elsevier Ltd. 2015).


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condensate (Cao, 2019a, 2019b), and waste heat (Pandiyarajan et al., 2011)) can be stored in the storage devices by heat transfer fluid, water. Then the water-LHTES releases the thermal energy to reduce the energy consumption of the building significantly. Meanwhile, the LHTES can also help to solve the mismatch between energy demand and supply (Sharma et al., 2009).

Most of the free cooling systems are applied in all kinds of buildings in the Europe (Waqas and Ud Din, 2013). Whereas for the hot desert climate (Pasupathy and Velraj, 2008), the nighttime is short and ambient temperature is high about 30 °C in summer. In these areas, free cooling applications will have lower thermal performance or even cannot complete the solidification of PCM. However, since the sky in these areas is clear at night, the nighttime sky radiation is abundant. Hence, the cold produced by the nighttime sky radiation can be stored in the storage devices by water, thus realizing the charging process. Furthermore, as the temperature of air conditioning condensate is low and stable between 10 and 15 °C and the flow rate is abundant, this cold source can also be stored in the storage devices through the water. In winter, the free heat generated by solar radiation can also be stored in the storage devices by water, and then the heat can be used to preheat the fresh air. In conclusion, by combining air and water heat exchangers with PCM, sustainable energy can be used maximum throughout the year and energy consumption of buildings can be reduced significantly.

2 PCM used in LHTES

Materials used to store energy in LHTES are known as phase change materials (PCMs) (Farid et al., 2004). Phase change materials realize the charging and discharging processes in a constant temperature or narrow temperature range (Agyenim et al., 2010b; Riffat et al., 2015). Because of the advantages of PCM: high thermal storage density, phase change with constant temperature, and small volumetric change, the LHTES is more valuable than the SHTES (Xu et al., 2016). PCMs are divided into organic, inorganic, and eutectics (Zalba et al., 2003; Sharma and Sagara, 2005; Kenisarin and Mahkamov, 2007; Liu et al., 2012), shown in Fig. 2, and the advantages and disadvantages of organic and

Fig. 2 Classification of PCMs.

inorganic PCMs are shown in Table 1. The properties of PCM (phase change temperature,

thermal conductivity, specific heat capacity, and latent heat of fusion) play a vital role in thermal performance of the LHTES. Therefore, PCM must be chosen according to different applications in LHTES. The PCM for free cooling of buildings must be able to realize the charging process (solidification of PCM) during nighttime and discharging process (melting of PCM). Generally, the phase change temperature (PCT) range of such PCM for free cooling is basically 15–30 °C (Raj and Velraj, 2010). The properties of some PCMs are shown in Tables 2–5.

Table 1 Advantages and disadvantages of organic and inorganic PCMs

Advantages Disadvantages

Organic PCMs

No subcooling Long-term thermal stability No corrosive

Flammable Lower thermal conductivity compared to inorganic

Inorganic PCMs

Higher thermal conductivity compared to organic Small volume change

Subcooling Corrosive

Table 2 Properties of inorganic PCMs (Zalba et al., 2003)

Compound Melting

temperature (°C)

Heat of fusion (kJ/kg)

Thermal conductivity (W/(m·K))

Density (kg/m3)

LiClO3·3H2O 8.1 253 — 1720

KF·4H2O 18.5 231 — 1480

Mn(NO3)2·6H2O 25.8 125.9 — 1738 (liquid)1795 (solid)

CaCl2·6H2O 29 190.8 0.540 1562 (liquid)Na2SO4·10H2O 32.4 254 0.544 1485 Na2S2O3·5H2O 48 201 — 1600

Na(CH3COO)·3H2O 58 264 — 1450

NaOH 64.3 227.6 — 1690

Ba(OH)2·8H2O 78 265.7 0.653 1937 (liquid)2070 (solid)

Mg(NO3)2·6H2O 89 162.8 0.490 1550 (liquid)1636 (solid)

Table 3 Properties of paraffin (Zalba et al., 2003)

Compound Melting

temperature (°C)



Density (kg/m3)

Paraffin C14 4.5 165 — —

Paraffin C15-C16 8 153 — —

Paraffin C16-C18 20–22 152 — —

Paraffin C13-C24 22–24 189 0.21 760 (liquid)900 (solid)

Paraffin C18 28 244 0.148 (liquid)0.15 (solid)

774 (liquid)814 (solid)


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(Continued)

Compound Melting

temperature (°C)



Density (kg/m3)

Paraffin C16-C28 42–44 189 0.21 (solid) 765 (liquid)910 (solid)



Paraffin wax 64 173.6 0.167 (liquid)0.346 (solid)

790 (liquid)916 (solid)


Table 4 Properties of fatty acids (Zalba et al., 2003)

Compound Melting

temperature (°C)



Density (kg/m3)

Propyl palmitate 10 186 — —

Caprylic acid 16 148.5 0.149 (liquid) 901 (liquid)981 (solid)

Vinyl stearate 27–29 122 — —

Capric acid 32 152.7 0.153 (liquid) 878 (liquid)1004 (solid)

Myristic acid 49–51 204.5 — 861 (liquid)990 (solid)

Stearic acid 69 202.5 0.172 (liquid) 848 (liquid)965 (solid)

Table 5 Properties of commercial PCMs (Zalba et al., 2003)

Name Melting temperature (°C)


Density (kg/m3)

RT5 9 205 —

ClimSel C23 23 148 1480

RT25 26 132 —

STL27 27 213 1090

RT30 28 206 —

RT40 43 181 —

STL47 47 221 1340

STL52 52 201 1300

RT50 54 195 —

RT65 64 207 —

3 Air-LHTES

In summer, the ambient temperature during nighttime is lower than that in daytime in many areas. If the low- temperature ambient air is introduced into the room to reduce the indoor temperature, the cooling load of the building will be greatly reduced or the mechanical ventilation

refrigeration equipment will be substituted completely (Givoni, 1991; Geros et al., 2005). The concept of free cooling which stores ambient cold during the nighttime and releases during the daytime was first proposed in 2000 by Turnpenny et al. (2000), shown in Fig. 3. In the free cooling system, air is used as heat transfer fluid and forced moving by fans. Moreover, the system works better in areas where the diurnal temperature variation is greater than 15 °C (Raj and Velraj, 2010). Section 3.1 states the principle of free cooling. Section 3.2 introduces the theory and research on free cooling, and main parameters are studied including air flow rate, inlet air temperature, PCM encapsulation, phase change temperature.

In winter, for indoor heating purposes, the air-LHTES usually needs auxiliary heat sources such as heat pumps or solar air collector to heat the air. Vakilaltojjar (2000) designed an air-LHTES with two PCMs. This air-LHTES was used to indoor cooling during summer and indoor heating during winter. Chaiyat (2015) integrated PCM bed into air duct and investigated the use of PCM to improve the efficiency of an air conditioner under local climate, shown in Fig. 1(c). The result showed that the electrical power of this system could save around 9% compared to the normal air conditioner. Extensive efforts have been made to apply air-LHTES to solar air collector. Osterman et al. (2015) used solar air collector to heat the air and then the high temperature air flowed through the storage unit to store the heat during daytime in winter. Furthermore, this system could store cold by ambient air during nighttime in summer. The experimental and numerical investigations showed that the largest savings were in March in winter and in July, August in summer.

Fig. 3 Schematic of proposed heat pipe/PCM installation (Turnpenny et al., 2000; reproduced with permission © Elsevier Science Ltd. 2000).


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3.1 Principle of free cooling

The free cooling system can create a comfortable indoor environment and reduce building energy consumption through two processes: charging process (solidification of PCM) and discharging process (melting of PCM) (Hed and Bellander, 2006), shown in Fig. 4.

Charging process (solidification of PCM) (Butala and Stritih, 2009)

When the ambient temperature is lower than the indoor temperature at nighttime in summer, the fan starts operating to force the air circulation. The cold ambient air flows through the storage unit and takes away the heat of the PCM that gradually changes from liquid to solid, shown in Fig. 4(a). Through solidification of PCM, ambient cold is stored in the storage unit through the air. The charging process does not stop until the PCM has completely solidified or the nighttime is over.

Discharging process (melting of PCM) (Stritih and Butala, 2007):

When the indoor temperature exceeds the thermal comfort range during the daytime, the fan starts to operate, shown in Fig. 4(b). Hot air flows through the storage unit and transfers heat to the PCM. The air temperature is decreased and then the air re-enters the room, thus keeping the indoor temperature within the indoor thermal comfort. As the PCM absorbs the heat of the air, it gradually changes from a liquid state to a solid state. The discharging process does not stop until the PCM is completely melted or the daytime is over.

3.2 Parameters influencing the thermal performance of free cooling during charging and discharging process

The thermal performance of the free cooling system is influenced by a number of parameters, such as the property of PCM, PCM encapsulation, air flow rate, inlet air tem-perature. The impacts of these parameters are discussed

in detail below.

3.2.1 Phase change material

PCM plays an important role in free cooling of buildings. Hence, selecting the suitable PCM for free cooling is vital to get a better thermal performance. PCM suitable for free cooling must be selected according to the local climate (Takeda et al., 2004; Arkar et al., 2007; Xu et al., 2015) and the properties of PCM. PCM suitable for free cooling should have the following properties:

(1) the PCT should meet to complete the charging process, and the air outlet temperature should be within the indoor thermal comfort when cold is released during the daytime (Nicol and Humphreys, 2002);

(2) it should have higher thermal conductivity, to accelerate the charging process during the nighttime, and store more ambient cold in the storage device;

(3) it should have higher latent enthalpy, which can greatly reduce the volume of the storage device;

(4) it should be non-toxic, non-flammable, and non- corrosive to the storage device;

(5) it should not have the property of sub-cooling, so the extra time caused by subcooling can be eliminated.

Based on the above properties, and according to the advantages and disadvantages of organic and inorganic PCMs, shown in Table 1, many researchers choose paraffin as PCM. Firstly, paraffin has no sub-cooling property; the charging time is shorter than that of the salt hydrate. Secondly, paraffin is non-corrosive, so it will not react with the encapsulation materials, which will not result in paraffin leakage. However, paraffin has a common disadvantage: low thermal conductivity. This disadvantage is a great challenge to use paraffin as PCM (Aadmi et al., 2014).

Inorganic PCMs have higher thermal conductivity and latent heat of fusion than organic PCMs and are non- flammable. But inorganic PCMs are corrosive and will react with encapsulation materials, thus causing leakage (Cárdenas and León, 2013). The biggest disadvantage of using inorganic

Fig. 4 Free cooling principle: (a) charging process, (b) discharging process (Hed and Bellander, 2006; reproduced with permission © Elsevier B.V. 2005).


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PCMs is the property of sub-cooling in which the tem-perature of the materials must be lower than the PCT before the charging process. Because of the sub-cooling property, the charging process takes a longer time than that of organic PCMs. When Waqas and Kumar (2011a) used salt hydrate as PCM for the study of free cooling, they found that the phenomenon of sub-cooling occurred at 1.5–2.2 °C during the charging process and prolonged the charging time by 0.5–2.3 h, shown in Fig. 5. Therefore, the sub-cooling property of PCM will greatly affect the charging process of free cooling.

3.2.2 Phase change temperature

The PCT of PCM plays a significant role in the design of energy storage device (Arkar et al., 2007). The appropriate PCT should not only make the solidification of PCMs completely during limited nighttime but also enable the air outlet temperature from the storage device within indoor thermal comfort during the daytime (de Dear and Brager, 2002; Kang et al., 2003). As the ambient temperature fluctuates with time, PCM with fixed PCT cannot satisfy all meteorological parameters. Hence, PCM with a PCT range can better adapt to the fluctuant ambient climate (Arkar and Medved, 2007).

Different researchers have different criteria for determining the PCT of PCM. For hot climate regions in summer, PCM should have a higher melting point to make the solidification of PCM maximum during nighttime. Waqas and Kumar (2011b) conducted experimental using PCM SP27 with melting point of 27 °C. Nicol (2004) found that the PCT of the PCM should be equal to the indoor comfort temperature during the hottest month in summer in the dry and hot desert climate. According to Medved and Arkar (2008), the appropriate PCT of PCM for free

cooling applications is Ta + 2 K, where Ta is the average ambient temperature.

To adapt ambient temperature fluctuation, it is also feasible to use multiple-PCMs with different PCT in the free cooling system (Pasupathy and Velraj, 2008; Mosaffa et al., 2013; Peiró et al., 2015). Liu et al. (2017) used a multiple- PCM approach. The impact of different combination of RT18 (PCT range of 16–18 °C), RT20 (PCT range of 18–20 °C), RT25 (PCT range of 23–25 °C) on the air outlet temperature was carefully investigated during the discharging process of free cooling, shown in Figs. 6 and 7.

3.2.3 PCM encapsulation

In free cooling, the PCMs change from liquid to solid, thus realizing the charging process and vice versa. Both processes require storage containers to encapsulate PCMs to avoid leaking of liquid PCM (Cárdenas and León, 2013). Then, to ensure the volume expansion caused by the melting process of PCM, the storage vessels with the extra 10% volume must be considered (Sharma et al., 2009).

PCM encapsulations are also used as heat exchangers. During the charging/discharging process, thermal energy is exchanged between storage containers and heat transfer fluid. Therefore, the design and dimension of encapsulations will have a great influence on thermal performance. There are different PCM encapsulations: flat plate, shell and tube, PCM balls, aluminum panel and pouch, shown in Fig. 8. Detailed technology about PCM encapsulations can refer to Pasupathy et al. (2008) and Raj and Velraj (2010).

Morovat et al. (2019) used flat plate encapsulation to carry out experimental and numerical investigations, shown in Fig. 8(a). The effect of plate thickness and length on air outlet temperature of charging process was studied, as shown in Fig. 9. When the flat plate is 2.4 m long and 5.2 mm

Fig. 5 (a) Air outlet temperature during charging process; (b) side effect of sub-cooling (Waqas and Kumar, 2011a; reproduced with permission © Elsevier B.V. 2011).


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Fig. 7 Effect of different combination of PCMs on air outlet temperature (Liu et al., 2017; reproduced with permission © Elsevier B.V. 2017).

thick, the free cooling system can obtain the best thermal performance, and the charging and discharging process can be completed within 8 h.

Medved and Arkar (2008) used a cylinder as the heat exchanger filled with PCM encapsulated in spheres, shown in Fig. 8(c). The authors first designed two different spheres with different diameters 50 mm and 37.6 mm, respectively. The diameter of the cylinder is 0.34 m and the height is 0.525 m. Through experimental and numerical investigation, when the diameter of the sphere is 50 mm, the experimental model can better agree with the numerical model.

Yang et al. (2019) used a shell and tube heat exchanger to encapsulate PCM in a cylinder. Air flowed through the center tube to achieve the charging and discharging process, shown in Fig. 8(b). In this paper, the authors investigated the effects of different outer diameters that equaled to thicknesses of PCM, and different cylinder lengths on the air outlet temperature and liquid fraction of PCM, shown in Fig. 10. Through numerical simulations, the heat exchanger with internal diameter 0.4 m, external diameter 0.6 m, and

length 20 m, was selected finally. Based on the special ambient temperature of Chongqing, the maximum temperature difference between the inlet and outlet was 5.4 °C.

The PCM encapsulations above are also used by other researchers. A flat encapsulation PCM container was used by Jiang et al. (2000). The authors carried out some experimental investigations under different parameters. The results showed that the heat transfer performance of the simulation was in agreement with the experimental investigation. Mosaffa et al. (2013) and Zalba et al. (2003) also used flat plate PCM encapsulations. The former studied the influence of different length (1.2 m, 1.3 m, 1.4 m) and thickness (8 mm, 9 mm, 10 mm) of a flat plate on the air outlet temperature in free cooling. Mosaffa et al. (2013) obtained the optimal PCM encapsulation with 1.3 m long and 10 mm thick. In the latter paper, when the thickness of the air channel is 15 mm, it had a suitable charging time of 4 h and a discharging time of 6 h. When Dolado et al. (2011) used flat plate encapsulation, the authors found that the charging time during nighttime would be greatly reduced with the decrease of thickness. On the contrary, when the thickness of the flat plate increased, the air outlet temperature would decrease, and the charging time would increase correspondingly, and the energy consumption of the fan would increase too.

In shell and tube heat exchangers, the heat transfer between air and PCM is not only in the axial but also in the radial direction; hence the heat transfer increases corres-pondingly. Lazaro et al. (2009a, 2009b) investigated the effects of aluminum panel and pouch shown in Figs. 8(d) and 8(e). Results showed that aluminum panel had a better thermal performance.

Based on researches above, most researchers use flat plate encapsulation, and it may be because of the simple encapsulation structure, symmetrical layout. And the number of the channel can be changed easily in the flat plate encapsulation to change the air flow rate. However, it does

Fig. 6 Schematic of the multiple-PCM system: (a) charging process, (b) discharging process (Liu et al., 2017; reproduced with permission © Elsevier B.V. 2017).


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Fig. 8 PCM encapsulations: (a) flat plate (Morovat et al., 2019; reproduced with permission © Elsevier B.V. 2019), (b) shell and tube (Yang et al., 2019), (c) PCM balls (Medved and Arkar, 2008; reproduced with permission © Elsevier B.V. 2007), (d) aluminum panel (Lazaro et al., 2009a, 2009b; reproduced with permission © Elsevier Ltd. 2008), (e) aluminum pouch (Lazaro et al., 2009a, 2009b; reproduced with permission © Elsevier Ltd. 2008), (f) PCM slab (Mosaffa et al., 2013; reproduced with permission © Elsevier Ltd. 2012).

Fig. 9 (a) Effect of PCM layer thickness and (b) PCM-HX length on air outlet temperature (Morovat et al., 2019; reproduced with permission © Elsevier B.V. 2019).

Fig. 10 Different outlet air temperature, inner surface temperature, and liquid fraction under different PAHE external diameters (Yang et al., 2019a; reproduced with permission © Elsevier Ltd. 2019).


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not mean that flat plate encapsulation is most suitable for free cooling; the appropriate PCM encapsulation needs to be designed for the actual application.

3.2.4 Inlet air temperature

The effect of inlet air temperature on the thermal performance of free cooling is mainly caused by the temperature difference between inlet air and PCM. For the charging process during nighttime, the large temperature difference can accelerate the charging process. And for the discharging process during the daytime, the large temperature difference can shorten the melting time of PCMs. As the actual free cooling application takes the local ambient temperature as the inlet air temperature, this also determines the PCT of PCM in free cooling of buildings.

In flat plate encapsulation, Waqas and Kumar (2011a) used SP29 with PCT range of 28–29 °C as PCM in the hot and dry desert climate. In this paper, the inlet air temperature for the charging process during nighttime was 20 °C, 22 °C,

and 24 °C respectively, and the time of complete solidification was 10 h, 13 h, and 19 h, shown in Fig. 11. During the discharging process in the daytime, the inlet air temperature was 36 °C, 38 °C, and 40 °C respectively, and the outlet temperature via time obtained was shown in Fig. 12. When the inlet air temperature for discharging and discharging process was 20 °C and 36 °C respectively, the free cooling system could have the best thermal performance. In Liu et al. (2017), a multiple-PCM approach was used. As the PCT range of RT18 was 16–18 °C, RT18 did not fall below 16 °C after 10 h when the authors combined RT18 and RT20. Therefore, this kind of combination was not suitable for free cooling. Hence, inlet air temperature plays a vital role in the free cooling system. Similarly, in Zalba et al. (2004), the lower inlet temperature used during the charging process reduced the charging time. While the lower inlet air temperature used in the discharging process kept the indoor temperature within the indoor thermal comfort for a longer time.

Fig. 11 (a) Air inlet and outlet temperature; (b) cold accumulated in PCM (Waqas and Kumar, 2011a; reproduced with permission © Elsevier B.V. 2011).

Fig. 12 Air outlet temperature during discharging process: (a) discharging air temperature 36 °C; (b) discharging air temperature 38 °C(Waqas and Kumar, 2011a; reproduced with permission © Elsevier B.V. 2011).


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3.2.5 Air flow rate

Different airflow rate will result in different Nu, which will generate different heat transfer coefficient. With the large heat transfer coefficient, the heat transferred between air and PCM is more. During the solidification of PCM from liquid to solid during nighttime in free cooling, the con-duction of PCM is dominant. Subsequently the air velocity must be increased to strengthen the convective heat transfer between air and PCM, thus accelerating the solidification process during the limited nighttime (Zalba et al., 2004; Saman et al., 2005). The melting process is accelerated due to natural convection during the transition from solid to liquid of PCM. So in the discharging process, the air flow rate can be appropriately reduce during the daytime (Antony Aroul Raj and Velraj, 2011).

Liu et al. (2017) investigated the effects of different air flow rates on air outlet temperature during the discharging process through numerical simulation. The results showed that the discharging time decreased with the increase of air flow rate. Whereas the lower air flow rate increased the time required to maintain the indoor thermal comfort. Therefore, a lower air flow rate is beneficial to the discharging process in free cooling. On the contrary, in the literature (Zalba et al., 2004), the authors studied the effect of air flow rate on charging time in free cooling during nighttime. The inlet air temperature was 16 °C and the flat plate thickness was 15 mm. The charging process could be completed within 3.2 h and 4.05 h when the air flow rate was 150 m3/h and 100 m3/h respectively, shown in Fig. 14. In the literature (Waqas and Kumar, 2011a), the flat plate container was made of galvanized steel with length, width, and height of 1.5 m × 0.5 m × 0.01 m. The authors compared the effects of air flow rate of 4 m3/h/kg.PCM and 5 m3/h/kg.PCM on the thermal performance of free cooling when the charging temperature was 20 °C and 22 °C respectively, shown in Fig. 15. Experimental results showed that 5 m3/h/kg.PCM is more effective for cold storage for the particular container.

Fig. 13 PCM average temperature during the charging process for RT25+RT20 (Liu et al., 2017; reproduced with permission © Elsevier B.V. 2017).

Fig. 14 Effect of different air flow rates on solidification time (Zalba et al., 2004; reproduced with permission © Elsevier Ltd and IIR 2004).

Fig. 15 Cold accumulated in PCM at varying air flow rates: (a) charging air temperature 20 °C, (b) charging air temperature 22 °C (Waqas and Kumar, 2011a; reproduced with permission © Elsevier B.V. 2011).

Therefore, it can be concluded that a larger air flow rate contributes to the charging process in free cooling within a certain range. When the flow rate reaches a certain level, the heat exchange will not be increased, but the energy consumption will be increased (Wang et al., 2017).

In free cooling, it is proposed in the literature (Arkar and Medved, 2007; Arkar et al., 2007; Medved and Arkar, 2008) that the thermal performance was optimal when the air flow rate during charging process was 3–4 times of that during discharging process. In the literature (Medved and Arkar, 2008), the authors proposed that the optimal ratio of PCM mass to air flow rate was 1–1.5 kg.PCM/m3/h, for this specific mass of PCM could ensure 95% of the CDH maximum value for specific storage device and climate conditions.


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4 Water-LHTES

In water-LHTES, water is used as a heat transfer fluid. When hot water flows through storage devices, heat is transferred from the water to PCM. The PCM absorbs heat and changes from solid to liquid. And this process is called charging process (melting of PCM). When cold water flows through the storage devices, the heat of the PCM is transferred to the water. The PCM changes from liquid to solid. This process is called discharging process (solidification of PCM). Because the specific heat capacity of water is larger than that of air and water with various temperature range is available easily, application of the water-LHTES is wider. The water-LHTES can be used for solar energy (Lin et al., 2020), waste heat recovery (Pandiyarajan et al., 2011), air conditioning (Oró et al., 2012). It can provide auxiliary cold and heat for buildings when the building load is highest in summer or winter, thus playing a vital role in peaking load and reducing building energy consumption (Ismail et al., 2009).

4.1 Different heat exchangers

As water-LHTES is widely used in many applications, a variety of PCMs are also produced accordingly, shown in Tables 2–5. It is also important to select suitable PCM for various applications. Once PCM is selected, the storage device used to encapsulate PCM must be selected. Because shell and tube heat exchangers have a small heat loss, most researchers choose shell and tube heat exchangers as encapsulation (Agyenim et al., 2010b). According to different kinds of heat exchange tube in shell and tube heat exchangers,

a variety of shell and tube LHTES are produced. Different heat exchange tube includes a single tube (Pahamli et al., 2018), multiple tubes (Esapour et al., 2016), coiled tube (Yang et al., 2019b), and spiral tube (Sun et al., 2017), shown in Fig. 16.

The heat exchanger used by Pahamli et al. (2018) was shown in Fig. 16(a). Heat transfer fluid water flowed through the central tube, and PCM was filled within the shell. The PCT range of PCM was 318–324 K. The influence of inlet water temperature and water flow rate on the charging process was studied through numerical investigation, shown in Fig. 17. The inlet water temperature played a much more important role than that of the water flow rate in the charging process. When the inlet water temperature increased from 70 to 75 °C and from 70 to 80 °C, the melting time decreased by 16% and 27%, respectively. Finally, the authors also studied the influence of different positions of water tubes on the charging process. When the water tube was moved down from the central position by 7.5 mm, 15 mm, and 22.5 mm, the total melting time was reduced by 33%, 57%, and 64%, respectively. This was mainly because the PCM at the bottom of the container was the most difficult to melt. Due to the impact of natural convection, with the water tube closing to the bottom of the container, the melting time became short.

To compare the effect of heat transfer rate between the multi-tube heat exchanger and single-tube heat exchanger, Esapour et al. (2016) investigated the impact of tube numbers (1, 2, 3, 4) on the charging process, shown in Fig. 16(b). Firstly, the authors compared the calculated results of model with the experimental results of Mat et al. (2013), and the

Fig. 16 Different heat exchangers: (a) single tube (Pahamli et al., 2018; reproduced with permission © Elsevier Inc. 2017), (b) multiple tube (Esapour et al., 2016; reproduced with permission © Elsevier Ltd. 2016), (c) coiled tube (Yang et al., 2019b; reproduced with permission © The Author(s)), (d) spiral tube (Sun et al., 2017; reproduced with permission © Fluid Machinery 2017).


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two results were in good agreement. Then, the authors obtained the diagram of liquid fraction versus time of four different heat exchangers through numerical investigation, shown in Fig. 18. It can be seen from the figure that the heat exchanger containing four water tubes had the shortest melting time. The layout of the four tubes was then studied, and the results showed when the tubes were closer to the bottom of the container, the PCM melted faster. The influence of inlet water temperature, water flow rate, and arrangement of water tubes on the charging process was also studied. Through numerical investigation, the following conclusions were obtained: when inlet water temperature and flow rate increased, the charging time would decrease significantly, but the effect of inlet water temperature on the charging process was more significant.

Yang et al. (2019b) carried out careful numerical and experimental investigation on a shell and tube LHTES. The system consisted of a cubic shell with length, width, and height of 2 m × 1 m × 2 m and 23 coiled tubes with PCM filled in the shell, shown in Fig. 16(c). The effect of coiled tube

Fig. 18 Melting time for different MTHX cases (Esapour et al., 2016; reproduced with permission © Elsevier Ltd. 2016).

diameter, inlet water temperature, and natural convection on the charging/discharging process was studied, shown in Fig. 19. It can be shown from Fig. 19 that when the tube diameter was 20 mm, the charging process was the fastest. Although the inlet water temperature difference between the two models was only 5 °C, the higher the inlet water temperature was, the faster the liquid fraction changed. Figure 20(a) shows that due to the natural convection, the charging process was accelerated. And during the charging process, the liquid PCM transferred heat to solid PCM in both vertical and horizontal direction. On the contrary, it can be seen from Fig. 20(b) that natural convection played a very small role in discharging process, whereas conduction was dominant. Similarly, Seddegh et al. (2015a) compared the pure conduction and conduction-natural convection models in the charging/discharging process. The authors obtained that natural convection played a dominant role in the charging process, but a faint role in the discharging process.

Sun et al. (2017) designed a water-LHTES with a spiral tube heat exchanger, shown in Fig. 16(d). By establishing the numerical model of the spiral tube heat exchanger, the charging process and discharging process were numerically investigated. With different spiral tube diameters (80 mm, 120 mm, 160 mm) and two different pitch (30 mm, 40 mm) designed, the thermal performance of five different models was compared and analyzed. When the inlet water tem-perature was 323 K, the water outlet temperature of each model was studied. It concluded that the outlet temperature was lower when the diameter of the spiral tube was larger and the pitch was smaller. This was mainly because the larger the diameter and the smaller the pitch, the larger the heat exchange area with PCM. Furthermore, the effect of natural convection on the charging process was also studied. In the spiral tube heat exchangers, the heat transfer rate between PCM and heat transfer fluid was enhanced due to the secondary flow (Janssen and Hoogendoorn, 1978).

Fig. 17 Variation of liquid fraction versus time for: (a) different HTF inlet temperatures, (b) different HTF mass flow rates (Pahamli et al., 2018; reproduced with permission © Elsevier Inc. 2017).


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Bezyan et al. (2015) took the vertical spiral tube heat exchanger as the buried tube heat exchanger and studied its heat transfer performance. Through numerical calculation, the heat transfer performance of the spiral tube heat exchanger was better than that of the U-tube heat exchanger. Yang (2017) carried out a detailed experimental and numerical investigation on the heat transfer performance of an efficient storage unit with a spiral tube heat exchanger and a composite PCM, with the parameters including inlet water temperature, water flow rate, and natural convection.

Although the types of heat exchangers above are different, they reach the same conclusions: when inlet water temperature and flow rate increase, the melting time decreases, but inlet water temperature has a more significant influence; natural convection plays an important role in the charging process; the heat transfer performance of a multi- tube heat exchanger is better than that of a single-tube heat exchanger; the different arrangements of the tube can have a distinct thermal performance of LHTES. Especially when the tube is closer to the bottom of the container, the melting time is shorter.

4.2 Parameters influencing the thermal performance of water-LHTES during charging and discharging process

This subsection investigates various parameters (natural convection, geometric structure, inlet water temperature, and inlet water flow rate) on thermal performance of water-LHTES.

4.2.1 Natural convection

Natural convection is mainly caused by the buoyancy of liquid PCM. At the initial stage of the charging process, PCM is solid and mainly relys on conduction. When the PCM absorbs the heat from the hot water, partial PCM begins to melt and flows to the top of the container, while solid PCM moves to the bottom, which results in natural convection and accelerates the charging process (Tan et al., 2009; Ezan et al., 2011; Jmal and Baccar, 2015). Through numerical investigation, the researcher found that natural convection was stronger when the container was placed vertically than that was placed horizontally (Xiong, 2017). Murray and Groulx (2014) experimentally studied the

Fig. 19 Variation of liquid fraction with time using different diameters of tube at (a) 373.15 K, (b) 378.15 K inlet temperaturerespectively (Yang et al., 2019b; reproduced with permission © The Author(s)).

Fig. 20 Variation of the liquid fraction with time in: (a) charging process, (b) discharging process (Yang et al., 2019b; reproduced with permission © The Author(s)).


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charging/discharging process of lauric acid under various parameters. The results showed that the natural convection promoted the charging process, but it could be almost ignored during the discharging process, and this conclusion was the same as that of Yang et al. (2019b). Fornarelli et al. (2016) investigated the influence of natural convection on the thermal performance of the system through numerical calculation. The results showed that natural convection could increase heat flux and thus reduce charging time by 30%. Hence, it is significant to consider the natural con-vection effect for the charging process during the numerical investigation (Yang et al., 2017).

4.2.2 Geometric structure

In LHTES, the heat transfer performance of the multi-tube heat exchanger is better than that of the single-tube heat exchanger due to greater heat transfer area. The tube layout in the container can also have a significant impact on the thermal performance. Meanwhile, other influencing parameters such as pressure drop and fouling should also be considered when selecting the tube layout (Abdelkader et al., 2019). In the shell-tube LHTES, Luo et al. (2015) compared the effects of different number of tubes (1, 4, 9) on the charging process and obtained the heat exchanger with 9 tubes had the best heat transfer performance. Then, through numerical investigation, the authors studied the influence of different arrangements of 9 tubes on the charging process. The results showed that the centrosymmetric arrangement had a better heat transfer performance than

the inline and staggered arrangement, shown in Fig. 21. Similarly, Agyenim et al. (2010a) studied the influence of different arrangements of multi-tube in shell and tube heat exchangers on heat transfer performance through experiments. Trp (2005) and Xiao et al. (2015a, 2015b) also analyzed the heat transfer process of multi-tube heat exchangers through experimental and numerical investigations. Bechiri and Mansouri (2015) analyzed the heat transfer process of multi-tube heat exchangers through theoretical investigation. For shell and tube LHTES, the more tube layout can be found in the literature (Zhang et al., 2017).

4.2.3 Inlet water temperature and water flow rate

The impact of inlet water temperature on thermal per-formance is mainly caused by the temperature difference between water and PCM. Hosseini et al. (2014) taking paraffin RT50 as PCM, experimentally studied the heat transfer characteristics of the charging and discharging process in shell and tube heat exchangers. The experimental results showed that when the inlet water temperature increased from 70 to 80 °C, the total charging time decreased by 37%, and the theoretical efficiency of the charging process and the discharging process increased to 88.4% and 81.4%, respectively. While increasing the inlet water temperature will shorten the melting time, it is not a linear relationship. Rahimi et al. (2014a, 2014b) in the finned tube heat exchanger studied heat transfer characteristics of the charging process through experiments. When the inlet water temperature increased from 50 to 60 °C, the total

Fig. 21 Schematics of the shell and tube heat exchanger: (a) with different numbers of tubes: one, four, and nine tubes; (b) with different layouts: centrosymmetric, inline, and staggered (Luo et al., 2015; reproduced with permission © Elsevier Ltd. 2015).


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charging time reduced more than that increased from 60 to 70 °C. The authors studied the effect of the fin, and the results showed that in both laminar and turbulent area, the temperature of the PCM rised faster than that without fin. To accelerate the charging process, Al-Abidi et al. (2014) not only increased the inlet water temperature but also increased the water flow rate. The experimental results showed that when the inlet water temperature and flow rate increased, the charging time is reduced by 86% and 58%, respectively. Hence, inlet water temperature played a more important role in the charging process than water flow rate, shown in Fig. 22. Through experimental and numerical investigations, Kibria et al. (2014) and Ismail et al. (2014) also obtained that inlet water temperature has a greater influence on the charging process and discharging process than water flow rate. Hence, inlet water temperature plays a significant role in the charging process in a certain range. However, when the inlet water temperature reaches a certain level and the melting time varies little, increasing the inlet water temperature will reduce the efficiency of the heat source equipment or increase the cost. Therefore, it is necessary to choose the heat source temperature reasonably to achieve the best cost-efficiency ratio. Due to limited heat transfer area, the flow rate can only change the temperature difference between inlet and outlet water, but the impact on charging time is not obvious. What is worse, it can increase the energy consumption of circulating pumps, which is unnecessary.

5 Challenges and approaches in LHTES

For air-LHTES and water-LHTES, PCM is used as a thermal energy storage medium, mainly because of their advantages of high thermal storage density and phase change with a constant temperature. Inorganic PCM is widely used in LHTES because of the properties: non-subcooling, non- corrosive, and long-term thermal stable. However, inorganic PCM has a big disadvantage: low thermal conductivity

usually by 0.2 W/(m·K) (Fang et al., 2014) which hinders the charging/discharging process. This disadvantage may lead to incomplete solidification of PCM in air-LHTES during nighttime. In order to improve the heat transfer performance of LHTES, finned tubes, nanoparticles, metal matrix (Mesalhy et al., 2005; Li and Wu, 2014), metal foam (Tian and Zhao., 2011; Chen et al., 2014), porous materials, carbon fibers (Fukai et al., 2000; Frusteri et al., 2006), PCM-graphite composite and micro-encapsulation are often used, shown in Fig. 23.

Pahamli et al. (2017) added nanoparticles to PCM in shell and tube LHTES, and obtained that when the volume fraction of nanoparticles was 0.05, the melting time decreased by 14.6% through numerical calculation. Du et al. (2019), Ho and Gao (2013), and Motahar et al. (2014) also adopted nanoparticles to improve the thermal conductivity of PCM. Mat et al. (2013) reduced the melting time of PCM by 43.3% with internal and external finned tubes. Velraj et al. (1997) inserted longitudinal fins into the inner tube. Through experimental and numerical investigations, it was found that the complete solidification time of PCM was about 1/n that without fins, where n was the number of fins. Other finned enhanced heat transfer techniques can be referred to (Tao et al., 2012; Yang et al., 2015). Combining graphite with PCM also can enhance heat transfer. Marín et al. (2005) added porous graphite to the PCM in the air-LHTES with flat plate encapsulation, and through experimental and numerical investigations, it was found that the charging time could be reduced by half during nighttime. Sarı (2004), Sarı and Karaipekli (2007), and Zhong et al. (2010) also used compound graphite with PCM to enhance the thermal conductivity of PCM. The addition of porous materials to PCM can also enhance the thermal conductivity to improve the thermal performance of the LHTES (Wang et al., 2009; Srivatsa et al., 2014). Finally, micro-encapsulation of PCM can increase the heat transfer area and reduce the volume change of PCM. Özonur et al. (2006) adopted micro- encapsulation technology and compared the heat transfer

Fig. 22 (a) HTF mass flow rate and (b) HTF inlet temperature effect on the PCM melting time (Al-Abidi et al., 2014; reproduced with permission © Elsevier B.V. 2013).


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performance with traditional PCM. The results showed that the charging performance of micro-encapsulation of PCM was better than that of traditional PCM. For LHTES, more comprehensive enhanced heat transfer methods can be obtained (Zalba et al., 2003; Ibrahim et al., 2017).

Inorganic PCMs have relatively high thermal conductivity compared with organic PCMs. The thermal conductivity of inorganic PCM is generally 0.5 W/(m·K) (Iten and Liu, 2014). However, inorganic PCMs have the disadvantages of subcooling and corrosivity, so they are limited by many applications. Due to the subcooling property, the solidification of PCM requires more time (Waqas and Kumar, 2011a). To decrease the solidification time caused by the subcooling property, auxiliary materials can be added to PCM (Günther et al., 2007a, 2007b).

In addition to the above heat transfer enhancements, it is also feasible to apply the shape-stabilized PCMs to buildings. The shape-stabilized PCM has the high thermal conductivity; it was investigated in detail in Li et al. (2017). However, it is difficult to use the shape-stabilized PCMs in two kinds of storage devices based on air-LHTES or water-LHTES.

6 Summary of the LHTES study

In this paper, LHTES is divided into air-LHTES and water- LHTES according to the different heat transfer fluid, and two kinds of LHTES are studied comprehensively. As different authors have different research methods, the different research methods are shown in Tables 6 and 7.

In the air-LHTES, the system applied to buildings can get a better thermal performance in areas where the diurnal temperature variation is greater than 15 °C. The charging

process can be accelerated during the limited nighttime by the following methods: increasing the air flow rate, reducing the inlet air temperature, reducing the thickness of the plate, reducing the plate length, and increasing the air channel. The air outlet temperature of the storage device can be maintained within the indoor thermal comfort for a long time during the daytime by the following methods: reducing the inlet air temperature and flow rate, and increasing the thickness and length of the plate. The temperature difference between inlet air and PCM has a significant influence on the thermal performance. Hence the PCT must be selected according to the actual meteorological parameters. Suitable PCT should allow the PCM to solidify completely during limited nighttime, especially in hot and dry desert climate. The air flow rate in the charging process should be 3–4 times higher than that in discharging process.

In the water-LHTES, the charging/discharging process can be promoted by the following methods: increasing the inlet water temperature and water flow rate, using multi- tube heat exchangers, and moving the water tube to near the bottom of the container. Natural convection has a greater influence on the melting process than the solidification process, and the solidification process is dominated by conduction. The inlet water temperature has a more significant influence on the charging/discharging process than the water flow rate in a certain range. Different types of heat exchangers and arrangements of water tube have a great influence on thermal performance.

Furthermore, the PCM used in the LHTES has low thermal conductivity, so suitable enhanced heat transfer technologies can be adopted to enhance the thermal conductivity, thus improving the thermal performance of LHTES.

Fig. 23 Heat transfer enhancement techniques (Zalba et al., 2003, reproduced with permission © Elsevier Science Ltd. 2002; Ibrahim et al.,2017, reproduced with permission © Elsevier Ltd. 2017).


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7 Future research direction

The free cooling system applied to buildings can get a better thermal performance in areas where the diurnal temperature variation is greater than 15 °C. However, in areas where the ambient temperature is still very high in summer night (generally around 30 °C), the thermal performance of free cooling will greatly decrease or it is impossible to complete the charging process during nighttime. Therefore, it is necessary to find other cold sources to provide cooling for the storage device. The nighttime sky radiation technology (Tang et al., 2004; Amir and van Hout, 2019) is used to produce low-temperature water as the heat transfer fluid which transfers the cold to the PCM. The temperature of air conditioning condensate is generally 10–15 °C (Al- Farayedhi et al., 2014), and the flow rate is abundant, which

is discharged to the outdoor directly. With the development of science and technology, some researchers have taken air conditioning condensate as sanitary water, a supplement for cooling, landscape water (Algarni et al., 2018). But air conditioning condensate has not been used for LHTES. Because the temperature of condensate is low, and the flow rate is abundant, the air conditioning condensate can be collected as a cold source, to provide cold for LHTES. Meanwhile, solar energy is also a free heat source in winter, so the hot water generated by the solar collector can also be collected to provide heat for LHTES. To maximize the utilization of sustainable and renewable energy throughout the year.

The above two LHTES are integrated into a new LHTES which contains air heat exchanger and water heat exchanger, shown in Fig. 24. Meanwhile, PCMs with suitable PCT are

Table 6 Investigations of air-LHTES

Researcher Configuration PCM PCT Inlet temperatures Important results

Waqas and Kumar (2011a)

Flat plate encapsulation

Salt hydrate SP29 28–29 °C melt 36–38–40 °C

solid 20–22–24 °C

The lower ambient temperature and higher air flow rate are favorable for the charging process during the short nighttime. The subcooling property of PCM will prolong the solidification time of PCM.

Medved and Arkar (2008)

Packed bed (spherical encapsulation) paraffin RT20 20–22 °C Ambient air

temperature

The optimal PCT should be equal to the average ambient temperature in the hottest month. For local climatic conditions, 1–1.5 kg.PCM/m3/h is suitable for free cooling.

Turnpenny et al. (2000, 2001)

Heat pipe embedded into storage device Na2SO4·10H2O 20–22 °C melt 45 °C

solid 13 °C The larger the difference between the inlet air and PCM, the stronger the heat transfer of air-LHTES.

Morovat et al. (2019)

Flat plate encapsulation Paraffin wax 22.3 °C melt 28 °C

solid 10 °C

Although increasing the length of the plate can improve the outlet temperature, it will also lead to the higher energy consumption of the fan. Increasing the air channel from 1 to 6 will reduce the charging and discharging time by 64%.

Mosaffa et al. (2013)


CaCl2·6H2O Paraffin C18 Paraffin RT25

29 °C 27.5 °C 26.6 °C

melt 36 °C solid 25 °C

With the shorter and thinner of the plate, the COP of the system can be higher. The combination of PCMs with different PCT can obtain a better thermal performance.

Zalba et al. (2004)


molecular alloyParaffin RT25

19.5–22.2 °C20–24 °C

melt 28–30 °C solid 16–18 °C

Reducing the air flow rate and inlet air temperature will increase the solidification and melting time of PCM. Increasing the plate thickness leads to a decrease in heat transfer rate.

Liu et al. (2017)


Paraffin RT18Paraffin RT20Paraffin RT25

16–18 °C 18–20 °C 23–25 °C

melt 38 °C solid 16 °C

Improper matching of PCMS will lead to incomplete solidification of PCMs during the charging process. For specific climatic conditions, combining RT20 and RT25 can not only completely realize the solidification process but also realize the melting process. The optimal air flow rate for the charging and discharging process is 2 kg/s and 0.25 kg/s, respectively.

Yang et al. (2019a)

Cylindrical annulus filled with PCM Paraffin OP24 23–24 °C ambient air

temperature

The annual fluctuation of ambient temperature can promote the repeated solidification and melting of PCMs. The effects of six months of ambient meteorological parameters on air-LHTES in Chongqing were studied. With the lower air flow rate, the outlet temperature changes little.


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Table 7 Investigations of water-LHTES

Researcher Configuration PCM Flow rate Inlet temperatures Important results

Pahamli et al. (2018)

Double pipe heat exchanger

Paraffin RT50

0.023 kg/s 0.0345 kg/s 0.046 kg/s

Charging 70 °C, 75 °C, 80 °C

In the initial stage of the charging process, conduction is dominant; whereas in the later stage, natural convection dominates due to the influence of buoyancy. The closer the tube is to the bottom, the shorter the melting time is. Charging time can be reduced by increasing inlet water temperature and flow rate, but inlet water temperature has greater influence.

Yang et al. (2019b)

Coiler tube heat exchanger

Salt hydrate 0.0314 kg/s

Charging 373.15 K, 378.15 K Discharging 333.15 K

The model of pure conduction is compared with the conduction-natural convection model. Natural convection plays a leading role in the charging process, whereas conduction plays a leading role in the discharging process. PCM at the bottom of storage containers is the most difficult to melt.

Esapour et al. (2016)

Multi-tube heat exchanger

Paraffin RT35

0.024 kg/s 0.032 kg/s 0.04 kg/s

50 °C, 60 °C, 70 °C

The heat transfer performance of a multi-tube heat exchanger is better than that of a single-tube heat exchanger. The melting time decreases with the increase of inlet water temperature and flow rate. The different arrangement of tube results in different heat transfer performance, and the closer the water tube is to the bottom of the container, the faster the charging process is.

Sun et al. (2017)

Spiral tube heat exchanger

Paraffin RT6 0.06 kg/s 322 K

The heat transfer increases with the increase of spiral tube diameter in a certain range. The smaller the pitch, the stronger the secondary flow and the stronger the heat transfer. In the discharging process, natural convection affects the distribution of liquid PCM, but it does not affect the discharging period of the storage tank.

Xiong (2017)

Spiral tube heat exchanger

Paraffin wax

Charging 0.25 m3/h, 0.50 m3/h, 0.75 m3/h, 1 m3/h Discharging 0.05 m3/h,0.1 m3/h

Charging 55 °C, 60 °C, 65 °C, 70 °C Discharging 31.5 °C

Firstly, the PCM wrapped around the helical tube gradually melts. When the total liquid fraction reaches a certain proportion, natural convection plays a dominant role. The thermal storage unit under vertical installation has better performance than horizontal installation, but the difference between them is only reflected in the later stage of the charging process, which has less influence on the total charging time. In the numerical calculation of the charging process, the results of the conduction-natural convection model are more accurate than the pure conduction model.

Seddegh et al. (2015a)

Vertical con-centric tube heat exchanger

Paraffin wax 0.08 kg/s Charging 358 K

Discharging 301 K

Comparing the pure conduction model with the conduction-natural convection model, the results show that the latter is more agreement with the experimental data. Natural convection accelerates the charging process but has little effect on the discharging process.

Hosseini et al. (2014)

Horizontal tube heat exchanger

Paraffin RT50 1 L/min

Charging 70 °C, 75 °C, 80 °C Discharging 25 °C

The higher the inlet water temperature, the stronger the heat transfer and the more energy storage. Natural convection plays a major role in the melting process of PCM.

Al-Abidi et al. (2014)

Triplex tube heat exchanger

Paraffin RT82 4, 8, 16 kg/min

Charging 85 °C, 90 °C, 95 °C, 100 °C Discharging 68 °C

The influence of inlet water temperature on the charging process is more significant than that of flow rate.


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filled in the shell. Later, a series of experimental investigations on this system will be carried out. Water is used as the heat transfer fluid in the charging process in winter and summer, and the air is used as the heat transfer fluid in the dis-charging process in the new LHTES. As the specific heat capacity of water is about 4 times larger than that of air, using water as the heat transfer fluid will greatly accelerate the charging process, which improves the thermal per-formance of LHTES.

8 Conclusions

This paper introduces various LHTES of buildings in detail, including air-LHTES and water-LHTES. In this paper, various parameters that affect the thermal performance of the LHTES are carefully investigated, including inlet tem-perature of heat transfer fluid, heat transfer fluid flow rate, properties of PCMs, PCT, PCM encapsulation, geometric parameters, and water tube layout. Due to the low thermal conductivity of PCM, the paper also introduces various enhanced heat transfer technologies. Furthermore, if there is little difference in charging time in the LHTES, the temperature of the heat source should be controlled within a certain range.

This paper also discusses the climatic conditions for air-LHTES. It is found that the higher ambient temperature will lead to incomplete solidification of PCM during the limited nighttime in air-LHTES. Even in some extreme climate, when the ambient temperature is higher than PCT in the summer night, the ambient cool cannot be utilized for air-LHTES. Water-LHTES is mostly used in solar heating and waste heat recovery but seldom used in the fresh air system. Therefore, in the end of this paper, it is recommended to combine the air heat exchanger with the water heat exchanger in the new LHTES. Using the low-temperature water produced by the nighttime sky radiation or the air conditioning condensate with 10–15 °C to provide cool for the energy storage device. Furthermore, the free heat of solar energy can be supplied to the energy storage device in winter. Subsequently, an appropriate structural arrangement and PCT of PCM can maximize the

utilization of renewable energy. Finally, the preheating and precooling of fresh air can

be realized all the year through renewable energy com-bined with new LHTES, thus reducing building energy consumption.

Acknowledgements

The authors would like to express gratitude to the support provided by the Science and Technology Ministry of China (SQ2019YFE011560).

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