The risk of energy crisis and climate change motivate researchers to improve the energy system’s operation. The short-term mitigation can be made by oil well recovery [1] and utilizing municipal solid waste for producing solid fuel [2]. Meanwhile, a notable outcome is achieved in the renewable energy sector. It can be seen in the development of wind and solar renewable energy facilities. The deployment in wind energy facilities is achieved through extensive effort in design optimization, improving research methods and advanced manufacturing systems [3]. As a result, a commercial-stage is achieved in the wind energy sector [4]. Also, solar power, as a renewable source, can be harvested for electric production. Moreover, it also can be applied as a thermal substitution for heating applications [5].

The heating demand account for 76% of the total consumed energy in the building sector [6]. The heat can be supplied from solar power as a renewable source using solar collectors and solar electric heaters. The net carbon cycle for the heating sector can be reduced significantly since the heat can be supplied from solar energy as a renewable source. Heat energy storage is implemented to overcome the intermittent nature of solar systems [7]. It is a critical technology to ensure sustainability in the solar heating system. It reduces the cost of operation and increases the dispatchability of the system [8].

Heat energy storage increases the effectiveness and cycling efficiency of the solar heat storage system (SHSS) [9]. The conventional operation of the SHSS uses water as the storage medium, which operates as a sensible SHSS [10]. It limits the operation of the SHSS due to dependency on the mass and volume of water. The storage density can be increased using solid-liquid phase change material (PCM) as a heat storage material for SHSS [11]. The compositing process can eliminate the low heat transfer rate for PCM [[12], [13], [14]]. The operation of PCM as heat storage material is also optimized using a suitable heat exchanger [[15], [16], [17]] and nanofluid [[18], [19], [20]], which significantly accelerate the heat transfer process during the operation. It makes PCM a suitable material for the SHSS that allows the system to operate optimally for storing solar energy and reduces the backup power source from non-renewable energy.

Two common PCMs are employed for the storage material in SHSS: fatty acids and hydrocarbon wax [21]. However, the melting temperature for those PCMs is under 100 °C which limits the maximum temperature operation of the storage system. Another alternative PCM for storage material in SHSS that can be considered is a salt mixture [22]. The nitrate-based salt mixture (NBSM) is commonly applied as a heat transport medium in concentrated solar power (CSP) [23]. The melting temperature of NBSM is between 240 °C (for binary mixture) and 142 °C (for ternary mixture) [24], which makes it suitable in the SHSS for water heating applications. The melting temperature of the NBSM is above the boiling point of water, which potentially increases the storage capacity of SHSS and reduces the buffer time during the operation.

The main drawback of NBSM is a relatively lower enthalpy of fusion compared to organic PCM. It makes the storage capacity of NBSM tank is unfavorable for SHSS application. To encounter the issue, the storage system is modified by using several storage tanks or cascaded tank. The cascaded tank increases the storage density and promote a better operation of the thermal energy storage (TES) system [25]. For example, the overall efficiency of the TES system can be improved more than 85% by using cascaded sensible and latent tank [26]. The usage of different melting temperature of PCM for three-stage cascaded tank increases the power effect during discharging process up to 18.2% [27]. Another study reported the increment on solar fraction around 30% for water heater application by utilizing cascaded storage tank [28].

The utilization of cascaded storage tank contributes positively for the operation of TES, especially for SHSS application. Unfortunately, using multiple tanks require an extensive control system which make the operation is more complex [29]. As a result, the system needs some further adjustment, particularly for high temperature operation [30]. The utilization of direct contact cascaded system by using capsulated PCM can be taken as an alternative approach. The capsulated PCM is cascaded in a single storage tank which makes a direct contact with the working fluid [31]. However, it changes the volumetric ratio between the PCM and storage tank which reduces the effective storage density [32].

Direct contact cascaded PCM also can be utilized using annular containment. It allows to maintain the effective volumetric ratio of the storage tank [33]. The solid interface between each PCM compartment can be removed by using immiscible layer. The immiscible fluid can be used to provide direct contact heat transfer [34], particularly for solid-liquid PCM which experiences phase transition during the operation as SHSS. The immiscible layer can be obtained using two type PCM that has different density. As a result, both PCM remains separated in a single container without using solid interface during the charge and discharge process of TES system.

The direct contact cascaded immiscible PCM can be taken as a novel concept. Thus, preliminary characterization for the proposed concept is performed in this work. The present study uses two type PCM: ternary salt mixture (TSM) and polymeric PCM (high-density polyethylene/HDPE). Both PCM has melting temperature above 100 °C which is suitable for SHSS in water heating application. Static thermal profiling is performed to observe the charge/discharge profile of the mixtures. The finding from this work is expected to provide fundamental basis for developing direct contact cascaded immiscible PCM to improve the storage density of SHSS in water heating application.

2. Materials and method

Industrial grade NaNO₃ (7 wt%), NaNO₂ (40 wt%), and KNO₃ (53 wt%) with an average purity level 97.4% were chosen for preparing the ternary salt mixture (TSM). The TSM is generally called «hitec salt«, with a melting point of around 142 °C [35]. The TSM was pretreated to remove the water content. Two-stage heating treatments were performed under inert atmosphere (argon) for each salt constituent to remove the water content. The first heat treatment was conducted for 5 h at 150 °C, continue with second heat treatment at 250 °C for 8 h.

Three mixtures of immiscible PCM (TSM/HDPE) were prepared according to the mass ratio: TSM₇₅/HDPE₂₅, TSM₅₀/HDPE₅₀, and TSM₂₅/HDPE₇₅. The mixing process was performed in a sealed glove box under an argon atmosphere. The thermal stability of each sample was evaluated using the thermogravimetric method (Mettler-Toledo) with heating rate 10 °C/min. The enthalpy of fusion, melting and freezing temperature was evaluated through the calorimetry method (DSC). The heating rate was set at 5 °C/min to ensure smooth phase transition for the mixture, particularly for the TSM which generally experiences phase separation [36].

The key performance of a heat storage system is the ability to absorb thermal energy during the operation. Thus, the heating profile for each sample was evaluated to analyze the charging performance. The heat absorption test was conducted using the static charge method (Fig. 1a). Each sample was located within the test container. Two heaters (coil and band) were used to supply heat inside the test container. The present work uses the energy rate to conduct the heat absorption test rather than the temperature rate. The heater regulator adjusted the heating rate at 20 Watts by means of the electric energy supplied to the heater. The energy supplied to the heater was recorded by the energy meter. The heat absorption test was done until the sample reached 170 °C. After that, the heater was set off. A slow discharge process then released the stored heat. The sample temperature was recorded by three thermometers installed at the bottom, center and upper region. The mass of each sample was 300 g.

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Fig. 1. a) Apparatus for charge/discharge evaluation, and b) detailed dimension of the container for each sample.

The high variation of the density between TSM and HDPE required further adjustment of the container for charge/discharge evaluation. Before performing charge evaluation, the mixture was heated to 200 °C and cooled back to room temperature. The slow cooling process caused the mixture to separated as illustrated in Fig. 1b. The treatment was repeated ten times to ensure phase separation between HDPE and TSM. In addition, the inside diameter of the container was adjusted according to the total volume for each sample (Fig. 1b).

3. Results and discussion

Morphology observation using a scanning electron microscope (SEM) is conducted for the TSM₅₀/HDPE₅₀ after ten times initial preheating stage before charge/discharge test. Fig. 2a shows that the dispersed salt particle is stacked on the surface of HDPE. It implies the immiscible blend between TSM and HDPE due to different physiochemical properties. The interfacial area displays a hairy HDPE structure, allowing the diffusion of the salt particle during solidification. It can be observed as various salt dispersion (red circle in Fig. 2b) trapped within the solid HDPE structure.

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Fig. 2. Microscope observation for composite TSM/HDPE.

The temperature transition of TSM and HDPE indicates a high supercooling degree expressed by a high-temperature gradient between the melting and solidification point (Fig. 3a). The addition of HDPE for TSM promotes a better transition temperature, which reduces the supercooling degree. The lowest temperature gradient is obtained by TSM₅₀/HDPE₅₀ (less than 1 °C). Also, the solid-liquid transition is decreased at higher HDPE content while the crystallization temperature remains high. It can be said that the rapid solidification nature of HDPE accelerates the freezing process for TSM. Moreover, the dispersed salt particle on the HDPE surface (Fig. 2) causes local crystallization.

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Fig. 3. (a) Enthalpy of fusion and (b) phase change temperature of evaluated samples.

The TSM has a lower enthalpy of fusion, making it unfavorable for the SHSS application (Fig. 3a). The high enthalpy of fusion of HDPE influences the TSM/HDPE positively. It promotes a higher enthalpy of fusion for the mixtures. It can be considered a favorable outcome since adding HDPE generally reduces the enthalpy of fusion for wax-based PCM [[37], [38], [39]]. The increment in the enthalpy of fusion for TSM/HDPE mixture is also affected by a relatively close melting temperature (Fig. 3b) and the immiscibility nature between TSM and HDPE (Fig. 2).

The decomposition temperature for TSM starts above 600 °C (Fig. 4), which is highly desirable for medium-temperature applications such as SHSS. In contrast, the low molecular structure of HDPE causes a direct decomposition between 400°C-520 °C [40]. The TSM/HDPE mixtures show two contrasting decomposition profiles. It signifies that each constituent has an immiscible blend. Despite that, TSM and HDPE decompose at elevated temperatures which is suitable for the operating temperature of typical SHSS.

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Fig. 4. Thermal decomposition characteristic of TSM, HDPE and TSM/HDPE mixtures.

The heating profile for PCM is an essential parameter for the actual operation in the SHSS. As presented in Fig. 5a, none of TSM and HDPE demonstrate a plateau profile during phase transition. A transient temperature increment accompanies the solid-liquid transition. It implies the partial melting process for each PCM [[41], [42], [43]]. The gradient temperature for TSM is relatively lower than HDPE since it has a lower enthalpy of fusion. It causes a rapid phase transition for the TSM before entering the liquid sensible region. The HDPE experiences a longer phase transition duration (38 min) which is affected by the high enthalpy of fusion.

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Fig. 5. Temperature evolution profile for the TSM and HDPE (a) and TSM/HDPE mixtures (b).

The actual performance for the TSM/HDPE mixture can be observed according to its heating profile (Fig. 5b). The phase transition profile for TSM₇₅/HDPE₂₅ implies no deceleration. It is affected by a higher TSM content and lower enthalpy of fusion (Fig. 3b), making the phase transition occur without a slow temperature increment. The melting transition for TSM₇₅/HDPE₂₅ takes a shorter duration (red circle) at a temperature range of 136.2°C-138.1 °C, in good agreement with its melting transition (Fig. 3a).

The heating characteristic for a higher HDPE content causes a significant profile change. It can be seen in the phase transition profile of TSM₅₀/HDPE₅₀ and TSM₂₅/HDPE₇₅, which demonstrate two consecutive melting transitions. The melting transition for TSM₅₀/HDPE₅₀ occur between 105.4°C-122.4 °C and 137.4°C-139.6 °C. The first transition is defined as the initial melting process of HDPE. It indicates a shorter melting process of HDPE, which is affected by the presence of TSM content in the mixture. The same phenomenon is observed for TSM₂₅/HDPE₇₅ where the first melting transition occurs between 110.2°C-120 °C. The second transition can be taken as the region where the TSM melts.

The stored heat is released through discharging process. The temperature profile from the discharging process is plotted in Fig. 6. All samples indicate one particular characteristic where the discharging process takes more time to reach the initial temperature. Moreover, none of the samples demonstrate a plateau line. Therefore, the non-isothermal behavior occurred during the liquid-solid transition for all samples [44].

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Fig. 6. Heat releasement profile for the TSM and HDPE (a) and TSM/HDPE mixtures (b).

Fig. 6a presents the cooling profile for TSM and HDPE in pure form. The TSM exhibits multistep crystallization between temperatures 138.5 °C-114.7 °C. It causes two distinguish supercooling regions (red circle), potentially reducing the average heat release process. It is also responsible for the increment of volume fraction after solidification, which is undesirable for the closed storage tank. The liquid-solid transition for HDPE occurs between 116.6 °C-102.7 °C with immediate step solidification. There is a small supercooling region at a temperature 119.5 °C which is close to its solidification temperature (Fig. 3a). It can be seen that the supercooling for HDPE occurs at the initial crystal growth. It corresponds to the stable crystalline structure of HDPE [45], which allows the liquid-solid transition occurs steadily with a low-temperature gradient.

The TSM and HDPE mixture demonstrate different solidification profiles (Fig. 6b). The supercooling region increases as the HDPE content rises within the mixture. Despite that, adding HDPE reduces the temperature gradient during the freezing process significantly. Compared to pure TSM, the temperature gradient is decreased between 52.9% and 79.4%. It makes the heat can be discharged adequately, promoting a better heat release process.

4. Conclusion

Improvement on the operation of SHSS is proposed using direct contact cascaded immiscible PCM. Preliminary characterization for the proposed method is evaluated using ternary salt mixture (TSM) and high-density polyethylene (HDPE). Several key outcomes can be taken from the present works, namely:

• •

  The mixing process can be done in a relatively simple process, which potentially reduces the processing cost

• •

  The immiscible blend between TSM and HDPE maintains its nature, which allows the mixture to promote a higher storage capacity and suitable phase transition temperature

• •

  The decomposition temperature occurs above 400 °C (for HDPE) and 600 °C (for TSM), indicating a suitable thermal stability as SHSS, which operates between 100 °C–200 °C

• •

  Multistep phase transition is observed for TSM₅₀/HDPE₅₀ and TSM₂₅/HDPE₇₅, which ensures a stable heat transfer process during the solid-liquid transition

The finding indicates that compositing TSM with HDPE improves the operational aspect as heat storage material. It can be considered a suitable approach for applying direct contact cascaded immiscible PCM using TSM/HDPE mixture for SHSS. The working temperature of the storage material is above 100 °C, which makes it more thermodynamically favorable and allows for a higher storage capacity. Hence, the operational and storage cost can be minimized, making the operation of SHSS more effective to reduce the heating demand in the building sector.