# Experimental Evaluation of a Pilot-Scale Thermocline Thermal Energy Storage Combining Latent and Sensible Materials

Muhammad Asaad Keilany, Gilles Flamant, Mathieu Milhé, Quentin Falcoz, Jean-Jacques Bézian

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Muhammad Asaad Keilany, Gilles Flamant, Mathieu Milhé, Quentin Falcoz, Jean-Jacques Bézian. Experimental Evaluation of a Pilot-Scale Thermocline Thermal Energy Storage Combining Latent and Sensible Materials. SolarPACES 2022 - 28th International Conference on Concentrating Solar Power and Chemical Energy Systems, Sep 2022, Albuquerque, United States. 9 p., 10.52825/solarpaces.v1i.620. hal-04770115

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# Experimental Evaluation of a Pilot-Scale Thermo-cline Thermal Energy Storage Combining Latent and Sensible Materials

Muhammad Asaad Keilany<fnref n="1" />,<fnref n="2" />[https://orcid.org/0000-0003-0046-4393], Gilles Flamant<fnref n="2" />[https://orcid.org/0000-0003-4562-8515], Mathieu Milhé<fnref n="1" />[https://orcid.org/0000-0001-6947-0623], Quentin Falcoz<fnref n="2" />, and Jean-Jacques Bézian<fnref n="1" />[https://orcid.org/0000-0001-5667-2488]

**Abstract.** This work presents the experimental evaluation of pilot-scale thermocline that integrates a layer of phase change material (PCM) at the top of a sensible heat storage material in a thermocline thermal energy storage (TES) tank. The TES is integrated to the MicroSol-R parabolic trough pilot plant at the PROMES research facility in Odeillo, France. The tank is filled with alumina spheres as sensible heat storage. The PCM is NaNO₃ encapsulated in stainless steel horizontal tubes that fill about 5.5% of the tank volume. The charge is evaluated at three mass flow rates 2600, 3000, and 3900 kg/h at two different operating temperature ranges 285-315 °C and 295-330 °C. The discharge is studied at three mass flow rates 1600, 2000, and 3000 kg/h from 315 to 220 °C and 330 to 225 °C. The performance of the TES is analyzed with two main indicators: the thermocline thickness and the efficiency during the charge and discharge. The results indicate that lower mass flow rates during the charging process result in smaller thermocline thickness. Similarly, during discharge, the thermocline thickness reduces with lower discharge rates. Efficiency evaluation during discharge suggests that an optimal flow rate could be achieved.

**Keywords:** Thermocline Combined Thermal Energy Storage, Latent-Sensible, Concentrated Solar Power

## 1. Introduction

Thermocline thermal energy storage (TES) is an economically viable solution for concentrated solar power plants (CSP), Desai et al. [1]. When solid filler is introduced within the tank to increase its thermal capacity, a layer of thermal gradient appears at the frontier between hot and cold heat transfer fluid (HTF). This layer is known as thermocline thickness and it is attributed to additional thermal diffusion within the tank due to the heat exchange between the HTF and the solid filler, Keilany et al. [2]. The stored energy in this layer is a low grade energy, thus thermocline thickness negatively influences the overall performance of the TES. One of the solutions to reduce the thermocline thickness is to combine latent and sensible materials in the same tank, which allows extending the thermal capacity as well as discharge duration, while keeping a relatively constant outlet temperature during the discharge process. The advantage of such combination is using the constant temperature heat release of the PCM during the melting/solidification phase due to its latent heat of fusion.

<footnote n="1">¹ Centre RAPSODEE, Université de Toulouse ; Ecole des Mines d'Albi, CNRS, Albi F-81013, France.</footnote>

<footnote n="2">² CNRS-PROMES, 7 Rue du Four Solaire, 66120 Font Romeu-Odeillo, France.</footnote>
Ahmed et al. [3] used one-dimensional Schumann model to compare three configurations of thermocline TES. Namely, (1) a reference tank filled only with solid rod as sensible heat storage, (2) a thermocline that is filled only with PCM spheres, and (3) a combination between the two. The thermocline operates between 135 – 195 °C, while the fusion temperature of the PCM is 165 °C. They suggested that a combined solution provides a competitive cost reduction of the fully-filled PCM thermocline while it performs better than the sensible heat TES.

A numerical model was developed by Hernández et al. [4] of a combined PCM layer of AlSi at the top of steel slag (sensible heat storage) in a thermocline TES. The operating conditions of the HTF are 27-597 °C while the fusion temperature is 576 °C. They studied various combinations of PCM ratio of the TES 0%, 1%, 2.5%, 5%, 10%, and 20%, and concluded that 5% is the recommended ratio.

Zanganeh et al. [5] simulated three ratios of 0.67%, 1.33%, and 2.67% PCM with three different PCMs in the same thermocline TES for HTF running between 590 and 650 °C. They indicated that 1.33% PCM ratio has the best discharge efficiency at all three tested PCMs. Then Zanganeh et al. [6] developed an experimental setup of 1.33% AlSi₁₂ (PCM) with rocks. The PCM is encapsulated within a bundle of stainless-steel tubes placed in staggered positions. They found that the discharge lasted for about 28.5% longer with this combination compared to the sensible heat TES only, with a stabilized outlet temperature at about the PCM melting point.

While there are only few studies that only numerically evaluated such solution, experimental data are scarce. This work presents the experimental evaluation of pilot-scale thermocline that integrates a layer of phase change material (PCM) at the top of sensible heat storage material in the thermocline.

## 2. Experiment

The thermocline storage for applied research (TSAR) is integrated to the MicroSol-R facility at the PROMES-CNRS research center in Odeillo (France), **Figure 1**.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Schematic of MicroSol-R pilot plant.</figcaption></figure>

The thermocline is a 3.3 m³ tank filled with 4.66 tons of alumina spheres as sensible heat storage, and 337 kg of NaNO₃ as PCM. The PCM is encapsulated in a stainless steel horizontal tube that fills about 5.5% of the thermocline volume, Keilany et al. [7].
The sodium nitrate NaNO₃ melts at 306 °C [8], [9] and latent heat of fusion is about 169 kJ/kg. It is commonly used in agriculture application with purity >99% and it has a relatively low cost [10]. This PCM was found compatible to be used with stainless steel 304L encapsulation [11]. Furthermore, this stainless steel has an acceptable decay rate of 6-15 µm/ year at 570°C when experimented with NaNO₃ [12], which is higher than the operating condition of this work.

Materials thermo-physical properties variation with temperature are given Table 1.

<table><caption>**Table 1.** Temperature dependence thermos-physical properties of storage materials</caption><thead><tr><th>Property</th><th>Jarytherm® oil</th><th>Alumina Spheres [13][14]</th><th>NaNO<sub>3</sub> [15]</th><th>SS304</th></tr></thead><tbody><tr><td>Density ρ [kg/m<sup>3</sup>]</td><td>1261.569 - 0.7419173T</td><td>1000 (3.9853 - (7.158<sup>-5</sup>(T-273.15)) - (3.035<sup>-8</sup>(T-273.15)<sup>2</sup>) + (7.232<sup>-12</sup>(T-273.15)<sup>3</sup>))</td><td>Solid phase: 2160<br>Liquid phase: 1908</td><td>8030</td></tr><tr><td>Heat Capacity Cp [J/kg.K]</td><td>649.84 + 3.1872180451T</td><td>1117 + 0.14T - 411exp(-0.006T)</td><td>444.53 + 2.18T</td><td>502.48</td></tr><tr><td>Thermal conductivity k [W/m.K]</td><td>0.1521663 - 8.2406015038<sup>-5</sup> T</td><td>(-2.469<sup>-8</sup> T<sup>3</sup>) + (9.509<sup>-5</sup> T<sup>2</sup>) - (0.124T) +61.76</td><td>0.3057 + 4.47<sup>-4</sup> T</td><td>16</td></tr><tr><td>Dynamic viscosity μ [kg/m.sec]</td><td>exp(19.75102[ln(T)]<sup>4</sup> - 492.2114[ln(T)]<sup>3</sup> + 4602.039[ln(T)]<sup>2</sup> - 19136.34[ln(T)] +29858.54)</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Latent heat of fusion [kJ/kg]</td><td>-</td><td>-</td><td>169</td><td>-</td></tr></tbody></table>

The charge is evaluated at three mass flow rates 2700, 3100, and 3900 kg/h at two different operating temperatures 285-315 °C and 295-330 °C. The discharge is studied at three mass flow rates 1600, 2000, and 3000 kg/h from 315 to 220 °C and from 330 to 225 °C, respectively.

### 3. Performance parameters

This work evaluates the performance of a TES based on two criteria: thermocline thickness and charge and discharge efficiency. To define these parameters, the charge and discharge threshold values must be explained as follow.

During the operation of the TES, $T_{high}$ indicates the highest temperature of the HTF and $T_{low}$ the lowest one. The charge threshold $T_{thr,c,k_c}$ represents the maximum HTF temperature at the TES outlet that could be flowed back to the solar field loop (charging loop). Above this temperature, the charge should be stopped to avoid damaging the solar receiver [2]. It is by equation (1), where the charge factor $k_c$ is usually chosen based on the specifications of the solar field, for benchmarking it is selected at 20% according to Fasquelle et al. [16]
$$T_{thr,c,k_c} = T_{low} + k_c(T_{high} - T_{low}) \quad (1)$$

During discharge, the lowest HTF temperature at the TES outlet that the downstream thermal process can use is identified by equation (2) as $T_{thr,d,k_d}$. The discharge factor $k_d$ is related to the operation of the downstream process, such as the steam generator temperature, and could be also considered for evaluation purposes at 20% Fasquelle et al. [16].

$$T_{thr,d,k_d} = T_{high} - k_d(T_{high} - T_{low}) \quad (2)$$

## 1.1 Thermocline thickness

The thermocline thickness size is the height of the zone inside the tank that is bounded by the two threshold temperatures during charge and discharge, equation (3).

$$Th = H(T_{thr,d,20\%}) - H(T_{thr,c,20\%}) \quad (3)$$

The relative thermocline thickness is the ratio between the thermocline thickness-size to the total tank height, equation (4).

$$\delta = \frac{Th}{H_{Tank}} \quad (4)$$

The thermocline thickness is required to be as small as possible because large thermocline thicknesses characterize low TES efficiency as suggested by Bonanos et al. [17].

## 1.2 Charge efficiency

The charge efficiency is the ratio between the accumulated energy and the maximum potential stored energy in the tank, equations (5)

$$\eta_{charge}(t) = \frac{E_{acc}(t)}{E_{max}} = \frac{E_{acc}(t) + E_{acc}(t)_p + E_{acc}(t)_{PCM}}{E_{max} + E_{max}_p + E_{max}_{PCM}} \quad (5)$$

The accumulated energy is calculated for each filler material in accordance with its related thermo-physical properties porosity and volume, equation (6)

$$E_{acc}(t) = \int_{0}^{H} (A_{int}\varepsilon(\rho \cdot Cp) \cdot (T_{(z,t)} - T_{low}) \cdot dz \quad (6)$$

The maximum potential energy is calculated by equation (7).

$$E_{max} = V(1 - \varepsilon)(\rho \cdot Cp)(T_{high} - T_{low}) \quad (7)$$

The accumulated energy and the maximum potential energy of PCM are calculated from equations (8) and (9), respectively.

$$E_{acc}(t)_{PCM} = \int_{0}^{H_{PCM}} A_{int}(1 - \varepsilon_{PCM})\rho_{PCM} \cdot L_{fus} dz \quad (8)$$
$$E_{max,PCM} = V_{tank,PCM}(1 - \varepsilon_{PCM})\rho_{PCM}L_{fus} \quad (9)$$

## 1.3 Discharge efficiency

The discharge efficiency is the ratio of discharged energy to the maximum energy stored in the system at the initial stage, equation (10).

$$\eta_{discharge}(t) = \frac{E_{discharge}(t)}{E_{max}} \quad (10)$$

Where

$$E_{discharge}(t) = \int_{0}^{t} m_{f} \cdot C p_{f} \cdot (T_{(outlet,t)} - T_{low}) \cdot dt \quad (11)$$

# 4. Results and Discussion

## 4.1 Thermocline thickness

Figure 2 plots the thermocline thickness evolution in time during charges (a) from 285 – 315 °C and (b) from 295 -330°C at the three tested HTF mass flow rates. The results indicate that lowest mass flow rate is favorable during the charging process because it results in the smallest thermocline thickness for both temperature differences.

Moreover, Figure 2 (a) and (b) reflects that increasing the inlet temperature reduces the thermocline thickness at the two lowest mass flow rate of 2600 kg/h and 3000 kg/h, while at the highest mass flow of 3900 kg/h the thermocline thickness increases with the inlet temperature.

This suggest that for the latent-sensible heat TES configuration, a low charge rate and relatively low inlet temperature (but higher than the melting point of the PCM) is recommended. Similar findings was indicated by Bédécarrats et al. [18] for a TES filled with encapsulated PCM.

<figure><img src="image_3.png" /><figcaption>Figure 2. Thermocline thickness during charge.</figcaption></figure>
During the discharge, Figure 3 evaluates the thermocline thickness at three discharge HTF mass flow rates 1600, 2000, and 3000kg/h with two temperature differences (a) 315 - 220°C and (b) 330 - 225°C. Similar to the charge, during the discharge process the thermocline thickness reduces with the increase in the discharge rate. However, comparing Figure 3 (a) and (b) suggests that the thermocline thickness has a similar behavior when the inlet temperature difference increases.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Thermocline thickness during discharge.</figcaption></figure>

The comparison of Figure 2 and Figure 3 shows that the thermocline thickness is smaller during discharge process than charge process. This could be attributed to the PCM phase-change influence, as illustrated in Figure 4.

(1) The thermocline starts to form and moves up with the HTF direction.

(2) The thermocline expands, and the hot thermal front moves faster than the cold thermal front [2].

(3) The hot thermal front reaches the PCM layer at the tank top, where the PCM stops it from moving up due to the phase-change.

(4) The hot thermal front remains still, or progresses slowly due to the phase-change, while the cold thermal front moves up quickly to the PCM zone, and thus, the thickness is reduced.

(5) The phase change is finished, so both the hot and cold thermal front move up further until the end of the process.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Depiction of thermocline thickness development during discharge.</figcaption></figure>
## 4.2 Charge efficiency

Figure 5 plots the charge efficiency against time for the two evaluated temperature differences (a) 285 – 315 °C and (b) 295 -330°C. The figure illustrates that the charge efficiency does not exhibit significant changes in response to the rate change nor the temperature difference.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Charge efficiency.</figcaption></figure>

## 4.3 Discharge efficiency

Figure 6 compares the efficiency during discharge with the three tested HTF mass flow rates 1600, 2000, and 3000 kg/h at two temperature differences (a) 315 - 220°C and (b) 330 - 225°C. The figure indicates that the maximum discharge efficiency could be obtained at about the discharge rate of 2000 kg/h. This suggests that an optimal mass flow rate could be achieved for operating the TES unit. A similar finding was found by Hoffmann et al. [19].

Increasing the maximum operating temperature during discharge does not influence the order of magnitude of the discharge efficiency.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Discharge efficiency.</figcaption></figure>

## 5. Conclusions

This work provides an experimental evaluation of the performance of a thermocline TES combining a 5.5% of NaNO₃ as PCM, to an alumina spheres. It is found that low mass flow rates are favorable during the charging and discharging because they result in reducing the thermocline thickness. Moreover, thermocline thickness during discharge is smaller than during charge, which suggests that the solidification of the PCM helped in reducing the thermocline thickness during discharge. While the efficiency of charging process does not show a signifi-
cant influence of the tested operating temperature nor the charge rates, the discharge effi-
ciency improves with low discharge rates. Furthermore, the operating temperature has no sig-
nificant influence on the discharge efficiency in the tested ranges.

**Data availability statement**

Not applicable.

**Underlying and related material.**

Not applicable.

**Author contributions**

Conceptualization by G. FLAMANT and J-J. BÉZIAN. Data curation by M. A. KEILANY. For-
mal analysis M. A. KEILANY, G. FLAMANT, M. MILHÉ, Q. FALCOZ, and J-J. BÉZIAN. Funding
acquisition G. FLAMANT and J-J. BÉZIAN. Investigation M. A. KEILANY. Methodology M. A.
KEILANY, G. FLAMANT, M. MILHÉ, Q. FALCOZ, and J-J. BÉZIAN. Project administration G.
FLAMANT, J-J. BÉZIAN. Resources M. A. KEILANY and Q. FALCOZ. Software M.A. KEI-
LANY and M. MILHÉ. Supervision M. MILHÉ, Q. FALCOZ. Validation M. A. KEILANY, G. FLA-
MANT, M. MILHÉ, Q. FALCOZ, and J-J. BÉZIAN. Visualization M. A. KEILANY. Writing - orig-
inal draft M. A. KEILANY. Writing - review & editing M. A. KEILANY, G. FLAMANT, M. MILHÉ,
Q. FALCOZ.

**Competing interests**

The authors declare no competing interests.

**Funding**

This work was supported by the French "Investments for the Future" program managed by the National Agency for Research under contract ANR-10-LABX-22-01 (LABEX SOLSTICE) and ANR-10-EQPX-49-(Equipex SOCRATE).

**Acknowledgement**

The authors acknowledge Ségolène Vannerem, Celine Boachon and Nicolas Boullet for their
technical assistance during tests.

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