# Experimental analysis of thermal behavior of latent heat thermal energy storage coupled to opaque ventilated façade

T Colinart, M Batard, Adrien Fuentes, H Noël, Patrick Glouannec

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## To cite this version:

T Colinart, M Batard, Adrien Fuentes, H Noël, Patrick Glouannec. Experimental analysis of thermal behavior of latent heat thermal energy storage coupled to opaque ventilated façade. 13th Nordic Symposium on Building Physics, Jun 2023, Aalborg, Denmark. pp.012113, 10.1088/1742-6596/2654/1/012113. [hal-05028523](https://hal-05028523)

HAL Id: hal-05028523

https://ubs.hal.science/hal-05028523v1

Submitted on 23 Apr 2025

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PAPER • OPEN ACCESS

Experimental analysis of thermal behavior of latent
heat thermal energy storage coupled to opaque
ventilated façade

To cite this article: T Colinart et al 2023 J. Phys.: Conf. Ser. 2654 012113

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# Experimental analysis of thermal behavior of latent heat thermal energy storage coupled to opaque ventilated façade

T Colinart¹, M Batard¹, A Fuentes¹, H Noël¹, P Glouannec¹

¹ Univ. Bretagne-Sud, UMR CNRS 6027, IRDL, F-56100 Lorient, France

thibaut.colinart@univ-ubs.fr

**Abstract.** Opaque ventilated façades are increasingly used in building envelopes due to their potential to reduce summer overheating. During cold seasons, such façades can be used to preheat air before indoor blowing. However, to overcome the mismatch between energy supply and demand, using thermal energy storage may be appropriate. This work investigates the experimental thermal behavior of opaque ventilated façade coupled to latent heat thermal energy storage. The experimental set-up includes a mobile and tilting façade, an air/PCM heat exchanger and a ventilation unit. This system is instrumented with numerous thermocouples, heat flux sensors, mass flow sensors and anemometers. The system is tested in the campus of Université Bretagne Sud (Lorient, France) under various weather and operating conditions. The measurements are analyzed in detail to assess the thermal performance of the system, particularly the energy collected by the façade and the energy stored by the system.

## 1. Introduction

In the last decades, there was a growing interest in preheating air with solar air collector (SAC) to reduce building heating energy consumption and the associated greenhouse gas emissions [1]. However, solar energy faces two major concerns: the fluctuational and discontinuous availability and the mismatch between the availability of the energy and the actual demand. To overcome these issues, a solution consists in coupling SAC with a separate thermal energy storage (TES) system: solar energy can be stored in the storage medium and then released on demand. Among the TES, latent heat thermal energy storage (LHTES) systems have several advantages: compactness, heat storage at (almost) constant temperature and high heat storage capacity [2]. Sharma et al. [3] reviewed recent works dealing with SAC coupled to LHTES, highlighting the great diversity of designs of SAC and LHTES, and of applications (among which comfort heating).

Gholamibozanjani and Farid [4][5] tested a 1m² SAC with LHTES with a storage capacity of 2.2 MJ (9.5 kg of paraffin RT25H) to preheat air of a small experimental hut in New Zealand. In comparison with a reference hut, their experimental results obtained for 24 days showed that up to 47% of daily energy-saving could be achieved with a corresponding 65% of electricity cost-saving. Besides, the authors mentioned that the heating and cooling energy saving could be improved by optimizing the melting temperature of phase change material (PCM). Therefore, further works focused only on SAC coupled to LHTES. Whereas there are a lot of numerical studies aiming to evaluate heating and cooling performance of such a system [6-9], there are a few experimental studies for building heating purpose. Stritih et al. [10] tested a 1.7 m² SAC coupled to a LHTES with a storage capacity of 5.8 MJ (approx. 29 kg of paraffin RT22HC) during six sunny days in the spring in an office building in Slovenia. The LHTES could store heat up to 1 kWh and release up to 0.72 kWh

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during a day. Besides, they developed and validated a Trnsys model to evaluate monthly energy and cost savings. Note, however, that air temperature reached 65 °C at LHTES outlet, which is too high to be blown directly in the room. Poole et al. [11] evaluated the performance of a transpired SAC coupled to one or two LHTES with a storage capacity of 7.6 and 7.4 MJ respectively (40 kg of salt hydrate HS29P and 40 kg of salt hydrate HS22P respectively) for different airflow rates. Over one week in the spring, 34 % of solar energy could be recovered by LHTES. Particularly, they found no influence of the airflow rate on the stored and released energy.

All above mentioned results were mainly obtained under favorable sunny conditions and for constant airflow rate. In this study, we aim to investigate the performance of SAC coupled with LHTES under more miscellaneous conditions of weather or of airflow rate and to provide a large experimental database.

## 2. Materials and methods

### 2.1. *Experimental set-up*

The experimental set-up shown in figure 1 consists in a mobile tiltable opaque ventilated façade (OVF) coupled to one or two LHTES modules. Here, OVF acts as SAC to preheat air. Even if the efficiency is not optimized compared to other SAC, OVF is a component already included in the building project: the capital cost of SAC is therefore limited which is advantageous for the economical rentability of the system [12]. This OVF with large realistic dimensions (3.5 m height x 1.9 m wide) is made of black metal cladding with horizontal trapezoidal profile. The inner side of the wall is made of 10 cm mineral wool insulation covered by 6 mm radiant barrier (acting also as airtight rainscreen). Air warms up in three 4 cm-thick ventilated cavities and is collected in a plenum sealed on the top of the OVF. This OVF is instrumented with numerous sensors, and its thermal behavior was analyzed in a previous study [13].

<figure><img src="image_7.png" /><figcaption>**Figure 1.** View of the experimental set-up (left) and schematic drawing of the cross section of OVF and LHTES including the instrumentation (right).</figcaption></figure>

The plenum outlet is connected to a multi-channel LHTES by a duct. One module of LHTES is made of 10 horizontally oriented aluminum panels (Rubitherm CSM panels with dimensions of 0.45 m × 0.30 m × 0.015 m) with 6 mm air space between them. Each panel is filled with 1.21 kg H-THERM23 IP (provided by MCI Technologies) with melting temperature range of 23–27 °C and heat of fusion of 230 kJ/kg. By cross checking the specifications of PCM using differential scanning calorimetry (DSC), we found melting temperature range of 20–27.5 °C and heat of fusion of 190 kJ/kg. Therefore, the storage capacity of one LHTES module ranges between 2.3 and 2.8 MJ. The panels are inserted in an insulated box (50 mm of PVC/NBR black rubber foam) to limit heat
exchange from the environment. The external unit dimensions are 0.3 m x 0.4 m x 0.83 m. A fan is connected to LHTES outlet providing a constant or variable airflow rate.

Numerous K-thermocouples are used to monitor air temperature at OVF and LHTES inlet and outlet and surface temperatures of different panels at position x = 5 cm, 22 cm and 40 cm in the direction of air flow. Additional thermocouples are inserted within the panels through the cap to directly measure the temperature of PCM. Volumetric airflow rate $q_v^{air}$ is measured between OVF and LHTES with a differential pressure flow meter. A local home-made weather station measures outdoor conditions at the wall vicinity, namely the outdoor temperature, the global solar irradiance received by the vertical wall, the wind velocity, and the sky temperature. All sensor signals are recorded by dataloggers with one minute time step.

The experimental set-up is tested in the campus of Université Bretagne Sud (Lorient, France). The OVF is oriented in SSE direction. Several experiments are performed during separate days between April 2021 and October 2021.

## 2.2. Thermal performance indicators

Considering the opaque ventilated façade as solar air collector, numerous performance indicators are evaluated from experimental data. Heat collected by air in the cavity $Q_{col}$ is given by:

$$Q_{col} = q_v^{air} \rho^{air} c_p^{air} (T_{OVF}^{out} - T_{OVF}^{in}) \quad (1)$$

with $T_{OVF}^{out}$ and $T_{OVF}^{in}$ outlet and inlet OVF temperature, $\rho^{air}$ air density and $c_p^{air}$ air specific heat. Collected energy $E_{col}$ is calculated by integrating collected heat $Q_{col}$ over time. Collector efficiency $\eta_{OVF}$ is defined as:

$$\eta_{OVF} = \frac{\int_{E_s}^{Q_{col}}}{\int_{E_s}^{A}} \quad (2)$$

with $E_s$ the incoming total solar irradiance and A the effective area of the façade.

Heat stored by LHTES $Q_{stor}$ is given by:

$$Q_{stor} = q_v^{air} \rho^{air} c_p^{air} (T_{LHTES}^{out} - T_{LHTES}^{in}) - U_{LHTES} (T_{LHTES}^{mean} - T_{ext}) \quad (3)$$

with $T_{LHTES}^{out}$ and $T_{LHTES}^{in}$ outlet and inlet LHTES temperature, $T_{ext}$ the exterior temperature and $U_{LHTES}$ the heat loss coefficient. The later was evaluated in lab to 0.8 W.K$^{-1}$. Stored energy $E_{stor}$ is calculated by integrating stored heat $Q_{stor}$ over time. Storage efficiency $\eta_{LHTES}$ is defined as:

$$\eta_{LHTES} = \frac{E_{stor}}{E_{col}} \quad (4)$$

# 3. Results and discussion

## 3.1. Detailed analysis of one experiment

The detailed experiment is performed in June 2021 from 9:30am to 5:00pm under sunny conditions (with a maximal incoming global solar irradiance of 450 W.m$^{-2}$). During the experiment, the exterior temperature varies slightly between 20 and 25 °C with a mean value of 22.9 °C. The OVF is coupled with one LHTES module and a constant airflow rate $q_v^{air}$ of 220 m$^3$.h$^{-1}$ is set.

Because of solar irradiance, air flowing in the cavities of OVF warms up to 38 °C: the collected heat $Q_{col}$ increases from 200 to 1000 W until maximal incoming global solar irradiance is reached and then decreases slightly at the end of the experiment (see Figure 2a). The measurement uncertainty of $Q_{col}$ is evaluated to 5 %. For the entire experiment, the collected energy $E_{col}$ is 4.7 kWh and the collector efficiency is 29 %. Once preheated, air flows to LHTES through the duct. In general, the difference between temperatures measured at OVF outlet ($T_{OVF}^{out}$) and LHTES inlet ($T_{LHTES}^{in}$) is less than 1 °C, meaning that the heat losses through the duct are less than 100 W.
Figure 2b shows the temperature variations measured at LHTES inlet ($T_{LHTES}^{in}$) and outlet ($T_{LHTES}^{out}$) and at the surface of a panel located in the middle of the module. $T_{LHTES}^{in}$ increases from 20 to 37 °C during the first 3 hours of experiment, is almost constant for 2 hours and then decreases progressively as solar irradiance declines. Once air enters the LHTES, it exchanges heat with the panels and $T_{LHTES}^{out} < T_{LHTES}^{in}$ throughout the experiment, meaning that heat is stored in LHTES. Consequently, surface temperatures increase whatever the position within the LHTES and PCM should melt within the panel. At x = 5 cm, surface temperature depends mainly on inlet conditions. At x = 22 cm and x = 40 cm, surface temperatures increase slightly until 3.5h due to PCM melting, and then sharply until 4.25h due to sensible heating. After 4.25h, all surface temperatures vary like $T_{LHTES}^{in}$.

Evaluating the state of charge (SOC) of LHTES is essential, but difficult for several reasons. First, the temperature measured by the thermocouple inserted through the panel cap was not usable. Indeed, the exact position of the sensor within the panel thickness remains unknown and measured temperature variations are not logical compared to other measured temperatures. Therefore, it is not possible to evaluate locally the liquid fraction of PCM (or SOC) during melting. However, preliminary experiments performed in lab have shown that surface temperature presents similar behavior than PCM temperature and particularly that the overall melting time predicted by both temperatures are identical. In the present case, the temperature increase is sharper close to the inlet (x = 5 cm) than in the middle (x = 22 cm) or near the outlet (x = 40 cm) and the melting time at these positions are 2h, 2.9h and 3.2h respectively. Similar behavior is observed for other panels located in the middle of the module, but not for the two panels close to the module box for two reasons: first, as heat loss coefficient $U_{LHTES}$ is not negligible, there are additional heat transfer with the exterior air; second, the incoming airflow is unevenly distributed in the channels between the panels: local airflow rate and, thus, convective heat transfer is much lower near the panels close to the module box. Consequently, the melting time of these panels is longer, explaining why $T_{LHTES}^{out}$ converges toward $T_{LHTES}^{in}$ only after 6.5h (and not 4.25h like all surface temperatures). After all, SOC of LHTES vary locally and is difficult to be assessed by local temperature measurements, which is in line with previous observations [14]. Alternatively, the time to charge the LHTES module could be evaluated by comparing the stored energy (see figure 2a) to the storage capacity of the LHTES module (2.3 MJ = 0.64 kWh). Since LHTES initial temperatures are close to the PCM solidus temperature, charging starts from the beginning of the experiment and a charging time of 2.8h is found. Nevertheless, heat is unevenly distributed in LHTES module and sensible heat can also be stored in the module: this charging time is lower than the melting time.

Last, the stored energy in LHTES module $E_{stor}$ is 1.63 kWh, corresponding to 35 % of the collected energy by OVH.

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Heat collected by OVH and heat and energy stored by LHTES (left) and temperature variations measured at different locations within the LHTES module (right) for a sunny day of June.</figcaption></figure>
## 3.2. Synthesis of all experiments

In total, 22 experiments have been performed. The experiment duration $t_{exp}$ varies between 4 and 7.5h. Mean solar irradiance ranges between 270 and 620 W.m$^{-2}$ (the highest value being measured for experiment performed in autumn when the sun height is lower), while mean air temperature varies between 15 °C (in spring) and 35 °C (in summer). Last, airflow rate is set to values between 110 and 230 m$^3$.h$^{-1}$. For all experiments, OVF collector efficiency $\eta_{OVF}$ and collected energy $E_{col}$ have been evaluated. As shown in figure 3a, $\eta_{OVF}$ increases with increasing airflow rate. Indeed, collected heat increases with airflow rate, while OVF outlet temperature $T_{OVF}^{out}$ is less influenced by airflow rate [13]. Such behavior agrees with the one observed by Paya-Marin et al. for transpired SAC [15]. On the other hand, maximal value reached by $T_{OVF}^{out}$ ranges from 30 to 54 °C and tends to increase logically with increasing solar irradiance. Consequently, $E_{col}$ increases also with increasing solar irradiance: $E_{col}$ ranges between 0.59 and 5.4 kWh for all experiments. When divided by the experiment duration $t_{exp}$, the collected energy varies between 0.4 and 1 kWh.h$^{-1}$, as shown in figure 3b.

<figure><img src="image_9.png" /><figcaption>**Figure 3.** Influence of airflow rate $q_v^{air}$ on OVH collector efficiency $\eta_{OVF}$ (left) and mean collected energy $E_{col}$ per hour of experiment as function of mean solar irradiance (right).</figcaption></figure>

The OVF is coupled to one LHTES module for 10 experiments (from April 2021 to July 2021) and to two LHTES modules for 12 experiments (from July 2021 to October 2021). For all experiments, $T_{LHTES}^{in}$ is higher than PCM solidus and liquidus: PCM could melt completely and LHTES could store a high amount of energy. Figure 4a shows the evolution of $E_{stor}$ as function of mean solar irradiance. $E_{stor}$ increases from 0.4 to 3.2 kWh with increasing solar irradiance, as the amount of available energy ($E_{col}$) is higher. $E_{stor}$ is globally higher than the LHTES storage capacity (except for one point), meaning that $E_{stor}$ includes latent and sensible energy. Regarding latent energy, melting depends mainly on LHTES inlet conditions. Even if evaluating charging and SOC is difficult (see previous section), the melting time measured at $x = 40$ cm of the first module can be considered as representative indicator. As shown in figure 4b, this melting time decreases logically with increasing mean $T_{LHTES}^{in}$ and is lower than experiment duration. Once PCM is melt, additional sensible heat can be stored since $T_{LHTES}^{in}$ exceeds PCM liquidus to large extend. Finally, maximal $T_{LHTES}^{out}$ of the first LHTES module ranges between 23.5 and 42 °C, meaning that high amount of energy is still available and can be stored for instance in a second LHTES module. In this case, more energy $E_{stor}$ can be obviously stored (see figure 4a). For the tested conditions, the mean inlet temperature of the second LHTES module is also slightly higher than PCM liquidus, meaning that PCM can be also completely melt. Nevertheless, the melting time of the second LHTES module is longer than the one of the first LHTES (see figure 4b). For the tested conditions, maximal $T_{LHTES}^{out}$ of the second LHTES module ranges between 25 and 37 °C. All these observations agree with the conclusions of Sharma et al. [3].
<figure><img src="image_10.png" /><figcaption>**Figure 4.** Stored energy $E_{stor}$ as function of mean solar irradiance (left) and melting time measured at $x = 40$ cm as function of mean temperature at the inlet of LHTES (right).</figcaption></figure>

Finally, figure 5 presents the storage efficiency $\eta_{LHTES}$ as function of experiment time. Here, $\eta_{LHTES}$ ranges between 23 and 45 % when one LHTES module is used and between 50 and 70 % when two LHTES modules are used. $\eta_{LHTES}$ tends to decrease with experimental time, as the storage capacity of LHTES is limited (latent or sensible energy), while the collected energy mainly increases with experimental time. One possibility to increase this efficiency could consist in adding another LHTES module or in using a PCM with different melting/solidification temperatures and latent heat [16].

<figure><img src="image_11.png" /><figcaption>**Figure 5.** Storage efficiency $\eta_{LHTES}$ as function of experimental time.</figcaption></figure>

## 4. Conclusions

This work investigated the experimental thermal behavior of latent heat thermal energy storage (LHTES) module coupled to mobile tiltable opaque ventilated façade (OVF). The set-up was tested from spring to autumn in Lorient, France. The main conclusions of this study are:

* OVF acted as SAC to preheat air, with a collector efficiency varying between 7 and 30 % with airflow rate. The amount of collected energy increases with increasing solar irradiance and experiment time.

* Part of heat collected can be stored in LHTES module in latent form and in sensible form. Particularly, the melting of phase change material is not homogeneous throughout LHTES module.

* Stored energy increases with increasing solar irradiance and experiment time, while melting time decreases with increasing temperature.

* Finally, up to 70 % the energy collected by OVF could be stored in LHTES modules
These conclusions still have to be extended to other conditions, particularly in winter when inlet temperature is much lower, but when solar irradiance can be high for a shorter time.

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