# Development and Performance Evaluation of a Hybrid Solar Dryer with Latent Heat Storage and Dehumidification Systems

Alain Tossa, Said Aballo, Daniel Sabi Takou, Loubatou Djindi, Guy

Semassou, Odette Fokapu

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Alain Tossa, Said Aballo, Daniel Sabi Takou, Loubatou Djindi, Guy Semassou, et al. Development and Performance Evaluation of a Hybrid Solar Dryer with Latent Heat Storage and Dehumidification Systems. Current Journal of Applied Science and Technology, 2025, 44 (2), pp.69-81. 10.9734/cjast/2025/v44i24485. hal-05047840

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# Development and Performance Evaluation of a Hybrid Solar Dryer with Latent Heat Storage and Dehumidification Systems

Alain K. TOSSA<sup>a*</sup>, Said I. A. ABALLO<sup>b</sup>, Daniel Sabi Takou<sup>a</sup>, Loubatou DJINDI<sup>a,b</sup>, Guy C. SEMASSOU<sup>a</sup> and Odette FOKAPU<sup>c</sup>

## Authors' contributions

This work was carried out in collaboration among all authors. All authors read and approved the final manuscript.

## Article Information

DOI: https://doi.org/10.9734/cjast/2025/v44i24485

**Open Peer Review History:**

This journal follows the Advanced Open Peer Review policy. Identity of the Reviewers, Editor(s) and additional Reviewers, peer review comments, different versions of the manuscript, comments of the editors, etc are available here: https://pr.sdiarticle5.com/review-history/130619

## Original Research Article

Received: 28/11/2024

Accepted: 31/01/2025

Published: 04/02/2025

## ABSTRACT

Drying agricultural products has become an essential technique for preservation, but current drying methods often prove to be energy-intensive, costly, and unsuitable for the needs of local small and medium-sized enterprises (PME/PMI). In this study a hybrid solar dryer has been designed

<footnote><sup>a</sup> Laboratoire d'Énergétique et de Mécanique Appliquée (LEMA), École Polytechnique d'Abomey-Calavi (EPAC), Benin.</footnote>

<footnote><sup>b</sup> Takaz-Eng, 01 BP 2197, Cotonou, Bénin.</footnote>

<footnote><sup>c</sup> Diasporeines Africa Association, UMR CNRS 7338 Biomécanique et Bioingénierie, Université de Technologie de Compiègne, Paris, France.</footnote>

<footnote>*Corresponding author: E-mail: toskalain@gmail.com;</footnote>
specifically to meet the needs of PME/PMI. The current limits to the development of solar drying in Africa are low efficiency and low autonomy. To remove these bottlenecks, thermal storage and air dehumidification systems are often used.

This article firstly, presents a brief review, not only of thermal storage materials but also of dehumidification systems. This review led to the choice of paraffin and silica gel respectively for thermal storage and dehumidification of drying air.

The selected materials were subsequently implemented in an indirect hybrid dryer with a drying capacity of up to 50 kg of fruits and vegetables. It is made up of an aluminum absorber painted in matt black, 1.5 mm thick. Its exterior structure is made of steel separated from the interior face made of stainless steel, by a layer of glass wool, 30 mm thick. Performance tests are carried out on the classic dryer, then with the dryer integrating silica gel on the one hand and on the other hand, the dryer with paraffin. The tests were conducted in Cotonou (latitude: 6°21'50"N; longitude: 2°26'32"E) during the month of November with an average irradiation of 137.23 kWh/m² received on the horizontal plane with an average temperature of 27.4°C. The experiments are carried out over several days so as to be able to extract, for each of the improved dryers, 03 days similar, from the point of view of daily irradiation, to the tests on conventional dryers. Tests carried out on the dryer with the new latent heat storage system reveal an increase in efficiency of 3% compared to the classic dryer. The proposed dehumidification solution showed a good dehumidification rate of 16% which helps to improve the overall efficiency of the dryer. In practice, this improvement in performance will result in a reduction in drying time and energy consumption. These first promising experimental results will be validated with other tests which will be carried out over several consecutive days, also varying the quantities of paraffin and silica gel.

Keywords: Solar dryer; latent thermal storage; dehumidification.

# 1. INTRODUCTION

Post-harvest losses of fruits and vegetables, recorded each year in sub-Saharan African countries are enormous. About 1/3 of food production is lost or wasted each year, emitting the equivalent amount of CO2 as 1.5 million cars (Rwubatse et al., 2014) For example, in Benin, post-harvest losses for the pineapple sector are estimated at around 70% throughout the value chain (AgriProFocus & Postharvest Network.Com, 2017) However, in Benin, according to the National Institute of Statistics and Economic Analysis (INSAE), in August 2017, 42.9% of the population was estimated to be food insecure, including 9.6% (i.e. 80,000 people) who were food insecure. To prevent the deterioration of perishable products such as fruit and vegetables, several preservation techniques exist, including drying. Drying agricultural products has become an essential technique, but current drying methods are often energy-intensive, expensive, and unsuitable for the needs of local small and medium-sized enterprises (SMEs).

Our team previously designed (Nounangnon et al., 2023) a hybrid and connected solar dryer, specially to meet the needs of SMEs. The dryer is intended for drying fruits and vegetables: such as pineapple, mango, banana, spirulina etc. It operates through the hybridization of four energy

sources, namely solar thermal, solar photovoltaic, gas and conventional electricity (public grid). The dryer currently has low efficiencies and is very affected by fluctuations in solar radiation. To optimize its efficiency and increase its autonomy, we aim to equip it with an improved thermal storage system and an air dehumidification system.

Firstly, a state of the art is carried out not only on existing storage systems but also on-air dehumidification systems generally used in drying. Material choices were then made for thermal storage and dehumidification. The initial tests and results are subsequently presented on the dryer after adding storage and a dehumidification bed.

# 2. STATE OF THE ART

## 2.1 Brief Review on Thermal Storage

To overcome the problem of intermittent solar energy, it is possible to use thermal storage to extend drying at a constant temperature. Thermal storage works by cycle in which we can observe three phases: charging, storage, discharge (Gatellier, 2018). There are three modes of thermal storage, namely:

* Sensitive storage

* Latent storage

* Thermochemical storage
Kant et al., (2016) have made a classification of the several types of thermal storage presented below:

<figure><img src="image_3.png" /><figcaption>Fig. 1. Classification of the different types of thermal energy storage</figcaption></figure>

**Sensible heat storage materials:** A list of sensitive heat storage materials has been identified. The Table below presents those available in Africa sub-Saharan context (Dinker et al., 2017).

<table><caption>Table 1. Africa sub-Saharan available sensible heat storage materials</caption><thead><tr><th>Storage Materials</th><th>Thermal conductivity (W.m<sup>-1</sup>. K<sup>-1</sup>)</th><th>Density (kg/m<sup>3</sup>)</th><th>Heat capacity by weight (kJ.kg<sup>-1</sup>. K<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Roche Granite</td><td>1.47</td><td>1468.11</td><td>0.627</td></tr><tr><td>Concrete</td><td>1.5</td><td>2200</td><td>0.85</td></tr><tr><td>Sand</td><td>1.21</td><td>1605.08</td><td>0.546</td></tr><tr><td>Water</td><td>0.58</td><td>1000(liquid)</td><td>4.184</td></tr><tr><td>Laterite</td><td>1.07</td><td>1093.44</td><td>0.499</td></tr><tr><td>Laterite rock</td><td>1.17</td><td>1142.13</td><td>0.559</td></tr><tr><td>Clay</td><td>2.16</td><td>1187.70</td><td>1.398</td></tr></tbody></table>

**Latent heat storage materials:** Reddy et al., (2018) classified storage materials by latent heat.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Classification of Phase Change Materials</figcaption></figure>

The following table compares organic and inorganic PCMs:
<table><caption>Table 2. Main characteristics of organic and inorganic PCMs</caption><thead><tr><th rowspan="2">Properties</th><th colspan="2">Organic</th><th colspan="2">Inorganic</th></tr><tr><th>Paraffin wax</th><th>Fatty acids</th><th>Salt Hydrates</th><th>Metals</th></tr></thead><tbody><tr><td>Corrosion resistant Cost</td><td>Non applicable Low</td><td>Non applicable Medium</td><td>Non applicable High</td><td>No resistant High</td></tr><tr><td>Latent Phase Change Heat (J/g)</td><td>Medium (200 - 220)</td><td>Medium (146-218)</td><td>High (184-264)</td><td>High</td></tr><tr><td>Thermal Conductivity (W/m. K)</td><td>Low (0.17-0.35)</td><td>Medium (0.15-1.6)</td><td>High (0.46-5)</td><td>High (8.3 - 29.3)</td></tr></tbody></table>

After analyzing this table, we chose paraffin wax because of its availability, low cost compared to other materials. The paraffin has also the highest latent phase change heat and an acceptable conductivity. Also, Paraffin has remarkable long-term thermal and chemical stability, retaining its properties after thousands of melting and solidification cycles (Sharma et al., 2016). Paraffins based thermal energy storage system have shown their effectiveness for small-scale thermal energy storage especially for air conditioning in the built environment (Lin et al., 2020; IRENA, 2020). This explains why, paraffin wax is the most widely used PCM (Kavya & Ramana, 2024; Lingayat et al., 2023) especially in solar dryers (Vijayrakesh et al., 2021; Manirathnam et al., 2021; Lingayat et al., 2020). A study (Rahman et al., 2023) shows that the paraffin based thermal storage system efficiency decreases 12.7% after 10,000 fast charging cycles (10°C/min). The use of fatty acids is not widespread, as it costs more than paraffin and is flammable (Kavya & Ramana, 2024).

**Thermal storage system:** In this section, we present some of the thermal storage systems identified.

A study (Mohanraj & Chandrasekar, 2009) experimented with a solar dryer incorporating heat-sensitive thermal storage using sand and aluminum scrap as thermal storage materials. They observe not only an increased drying time by about 4 hours per day after sunset but also, a drying of chili pepper from 72.8% to approximately 9.2% and 9.7% in the bottom and top trays respectively. The thermal efficiency of the solar dryer has been estimated to be around 21%.

Khaldi, (2018) did a study on a solar dryer incorporating a gravel thermal bed. For a thermal bed thickness of 0.05m, a maximum temperature of 50.35°C is obtained in the drying chamber and for a thermal bed thickness of 0.15m, a maximum temperature of 45.85°C is obtained in

the drying chamber for an efficiency of 44.59%. The drying time is estimated at 21 hours. A higher thickness promotes storage and decreases the maximum temperature.

Chaouch et al., (2018) worked on a natural convection solar dryer with thermal storage in which rock pebbles were used as material.

In this case, the storage of sensible heat by the pebble bed improved the daytime thermal efficiency of the direct drying chamber by 11.8% compared to use without storage. 28% improvement in solar collector efficiency (Chaouch et al., 2018).

A solar dryer integrating a latent heat storage system was evaluated by (Rabha & Muthukumar, 2018). Paraffin wax was used as a thermal storage material. The moisture content of the dried sample in the dryer increased from the initial moisture content of 83.4% to the final moisture content of 6.7% in 45 h, while the sun-dried sample took 80 h (Rabha & Muthukumar, 2018).

A study (Azaizia et al., 2020) experimented with a solar greenhouse integrating a latent heat storage system with paraffin wax as a thermal storage material. The experimental results show that the air temperature inside the solar greenhouse with PCM is higher than that of other dryers by about 7.5°C during the entire night period. The relative humidity in the drying chamber with PCM, is reduced by 95% in 30 hours in the dryer with PCM, whereas it took 55 hours in the dryer without PCM and 75 hours in full sun.

An experiment on forced convection solar air heaters using a finned absorber plate lined with latent heat storage materials (paraffin wax) and another flat absorber plate, both absorbers are painted black (Arul Kumar et al., 2016).

The results showed that the outlet temperature increased by 5°C and the heat storage time was
increased up to 3 h by increasing the energy efficiency was from 2 to 15%.

## 2.2 Brief Review on Air Dehumidification Systems

In dehumidification-assisted drying, the driving force of moisture mass transfer can be enhanced by lowering the relative humidity of the drying medium, resulting in a more efficient drying system. A study (Wang et al., 2013) indicate that five (05) types of air dehumidification systems are developed and applied nowadays:

* Condensation dehumidification

* Dehumidification by heating

* Membrane dehumidification

* Absorption dehumidification

* Adsorption dehumidification

They (Wang et al., 2013) reviewed the strengths and weaknesses of different air dehumidification methods. It shows that air dehumidification using a solid adsorbent is more interesting in terms of operability, efficiency, investment, and maintenance.

The following Table 3 shows the properties of three (03) solid adsorbents at room temperature of 27°C:

Silica gel is the well-known and widely used sorption material for dehumidification in direct and indirect forced convection solar dryers (Dake et al., 2021). Silica gel is particularly effective for solar drying applications compared to other solid adsorbents like activated alumina and activated charcoal, as it demonstrates higher adsorption rates and retains moisture for longer periods, making it ideal for low-temperature solar regeneration systems (Yadav & Bajpai, 2012). In a study (Lim et al., 2019) comparing the performance of three adsorbents (silica gel, zeolite 13X and zeolite 4A) in an adsorption thermal energy storage system, zeolite 13X offers the best storage density per unit mass, while silica gel excels in storage density per unit volume and reaction time, thanks to its high thermal conductivity and superior water adsorption capacity, making it more effective for solar drying applications. Some solar dryers with dehumidification systems are shown below:

<table><caption>Table 3. Properties of some solid adsorbents at 27°C (Wang et al., 2013)</caption><thead><tr><th>Adsorbents<br>solids</th><th>Relative humidity of<br>the medium (%)</th><th>Adsorption capacity (g<br>water/g adsorbent)</th><th>Regeneration<br>Temperature (°C)</th></tr></thead><tbody><tr><td rowspan="2">Silica gel</td><td>60</td><td>0.08 – 0.36</td><td rowspan="2">80 - 150</td></tr><tr><td>7</td><td>0.02 – 0.05</td></tr><tr><td rowspan="2">Zeolites</td><td>60</td><td>0.02-0.30</td><td rowspan="2">250-350</td></tr><tr><td>7</td><td>0-0.25</td></tr><tr><td rowspan="2">Activated<br>carbon</td><td>60</td><td>0.18-0.29</td><td rowspan="2">70</td></tr><tr><td>7</td><td>0-0.4</td></tr></tbody></table>

A dehumidification system working with the desiccant wheel is shown below:

<figure><img src="image_5.png" /><figcaption>Fig. 3. Dehumidification system with desiccant wheel</figcaption></figure>
Thanks to the desiccant wheel, the air temperature increased from 65°C to 82°C and the air humidity level increased from 15g of water/kg to 8.8g of water/kg (Misha et al., 2012).

### 3. MATERIALS AND METHODS

The solar dryer used for the tests is an indirect hybrid dryer with a drying capacity of up to 50 kg of fruits and vegetables. It is made up of an aluminum absorber painted in matt black, 1.5 mm thick. Its exterior structure is made of steel separated from the interior face made of stainless steel, by a layer of glass wool, 30 mm thick.

Roughly, the solar dryer is made up of four (04) main parts:

i. **The solar collector:** It is in this unit that the air receives the solar thermal energy necessary to dry the product. It consists of (1) an "absorber" made of aluminum to which we have added a "matte black" coating to reinforce the absorption coefficient; (2) glass wool "insulation" to limit heat loss and (3) a transparent "glass" cover.

ii. **The drying chamber:** It is inside this unit that the products are dried. It consists of an inner stainless-steel carcass and an outer steel carcass; the two carcasses are separated by glass wool insulation to limit

losses; There are also three (03) stainless steel racks and a chimney for the evacuation of air from the interior.

iii. **The funnel:** This is a duct that connects the sensor to the drying chamber. It is made of steel and insulated from the outside environment by glass wool.

Table 3 shows the materials and dimensions of the dryer.

Fig. 4a shows the dryer with the paraffin introduced into cylindrical profiles, welded to the absorber Fig. 4b shows, at the top, the bed of silica gel installed at the level of the funnel, on the drying air flow. Below the silica gel bed, it is presented the positioning of the sensors for measuring temperature and humidity before and after the silica gel bed.

Experimental tests were conducted to verify the performance of the solar dryer with thermal storage. These are the following tests: i. Solar Mode Test (Without Thermal Storage), ii. Solar test (with thermal storage).

Experimental data were recorded from 8:00 a.m. in the morning to 6:00 p.m. in the evening.

To conduct the measurements of the various parameters, we used the equipment presented in the following Table 5.

<table><caption>Table 4. Dimensions and materials of manufacture of the dryer</caption><thead><tr><th rowspan="2">Component</th><th rowspan="2">Hardware</th><th rowspan="2">Size (mm)<br>(L*W*h)</th><th colspan="3">Material properties</th></tr><tr><th>Thermal<br>conductivity<br>(W.m<sup>-1</sup>.K<sup>-1</sup>)</th><th>Density<br>(kg.m<sup>-3</sup>)</th><th>Specific<br>heat (J.kg<sup>-1</sup>.K<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Absorber</td><td>Aluminum</td><td>1 500 * 740 * 1.5</td><td>237</td><td>2700</td><td>897</td></tr><tr><td>Sensor casing</td><td>Steel</td><td>1 500 * 740 * 150<br>Thickness 1.5</td><td>80.2</td><td>7870</td><td>447</td></tr><tr><td>Fins (air<br>opening)</td><td></td><td>Thickness 1.5</td><td>80.2</td><td>7870</td><td>447</td></tr><tr><td>Carcass<br>drying<br>chamber</td><td></td><td>Diameter 50</td><td>80.2</td><td>7870</td><td>447</td></tr><tr><td>Glass lid</td><td>Glass</td><td>800 * 800 * 600<br>Thickness 30</td><td>1.05</td><td>2500</td><td>670</td></tr><tr><td>Side<br>Insulation</td><td>Glass<br>wool</td><td>1 500 * 740<br>Thickness 4</td><td>0.037</td><td>25</td><td>1030</td></tr><tr><td>Bottom<br>insulation</td><td></td><td>Thickness 30</td><td>0.037</td><td>25</td><td>1030</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Fig. 4. The solar dryer with the positioning of paraffin and silica gel bed</figcaption></figure>

<table><caption>Table 5. Measuring devices</caption><thead><tr><td>Measuring devices</td><td>Characteristics</td></tr></thead><tbody><tr><td>Uni-T UT330A Data Logger</td><td>Range: 80°C; Accuracy: 0.5°C</td></tr><tr><td>Anemometer PM6252B</td><td>Range: 30m/s; 60°C; 100% RH</td></tr><tr><td>TES 1333 Solarimeter</td><td>Range: 2000W/m²; Accuracy: 0.1 W/m². Spectral sensitivity: 400~1100nm</td></tr></tbody></table>

The efficiency of the solar collector (is defined as the ratio between the heat received by the drying air and the irradiation incident on the collector in each time interval. It is calculated from equation 1: $\eta_c$)

$v_t$ Air velocity at the solar collector outlet (m/s)

$$ \eta_{CS} = \frac{1}{36} \times \sum_{t=1}^{36} \frac{\dot{m}_t c_p (T_{out\_t} - T_{in\_t})}{I_t S} \\ \dot{m}_t = \rho S_2 v_t \quad (1) $$

# 4. RESULTS AND DISCUSSION

## 4.1 Experimentation and Characterization of the Dryer with Latent Storage

$\dot{m}_t$ Air Mass Flow Rate (kg/s)

$C_p$ Heat capacity of air (J/K.kg)

$T_{out\_t}$ Solar collector outlet air temperature (°C)

$T_{in\_t}$ Air temperature at the inlet of the solar collector (°C)

$I_t$ Solar Irradiance (W/m²)

$S$ Solar collector area (m²)

$\rho$ Air density (kg/m³)

$S_2$ Air opening section at solar collector outlet (m²)

The tests were conducted in Cotonou (latitude: 6°21'50"N; longitude: 2°26'32"E) during the month of November with an average irradiation of 137.23 kWh/m² received on the horizontal plane with an average temperature of 27.4°C.

The following Fig. 5 shows the variation of solar irradiation over one year on the site:

The figs. below show on the right the tests carried out with the dryer without paraffin and on the right the equivalent tests (near daily irradiation) with the dryer containing paraffin.
<figure><img src="image_7.png" /><figcaption>Fig. 5. Variation in irradiation at the drying site (PVGIS, 2023)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Variation of the temperature at the outlet of the solar collector, the ambient environment, and the solar irradiance test n°1</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>(a) Paraffin-free solar dryer</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>(b) Solar dryer with paraffin</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(a) Paraffin-free solar dryer</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>(b) Solar dryer with paraffin</figcaption></figure>

<caption>Fig. 8. Variation of the temperature at the outlet of the solar collector, the ambient environment, and the solar irradiance test n°3</caption>
<table><caption>Table 6. Test results and performance indicators</caption><thead><tr><th>Types de tests</th><th>Paraffin wax quantity (kg)</th><th>Average daily irradiation (Wh)</th><th>Solar Collector Efficiency %</th></tr></thead><tbody><tr><td rowspan="3">Solar Collector + Thermal Storage Tests</td><td rowspan="3">1.74</td><td>4226.40</td><td>28.79</td></tr><tr><td>3577.73</td><td>28.23</td></tr><tr><td>5303.74</td><td>22.27</td></tr><tr><td rowspan="3">Solar Collector Test</td><td rowspan="3">-</td><td>4228.09</td><td>21.25</td></tr><tr><td>4907.30</td><td>26.77</td></tr><tr><td>5122.49</td><td>23.81</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>Fig. 9. Variation in relative humidity before and after silica gel</figcaption></figure>

The Table 6 shows the calculated dryer efficiencies for each of the six tests.

These tests reveal efficiencies of 23.94 and 26.43% respectively for the solar mode and the solar + thermal storage mode.

From the analysis of the curves and the table, it appears that the addition of paraffin does not cause a significant variation in the temperature of the dryer at the end of the day. However, we note a positive effect of latent storage on efficiency which shows a slight increase of 3%. This reflects a greater quantity of energy absorbed due to the presence of storage.

## 4.2 Air Dehumidification Using Silica Gel

Several tests were carried out on the dryer with or without silica gel in order to assess the effectiveness of the dehumidification of the air at the entrance to the drying chamber. The Fig. 9 shows the results of one of these tests. It shows the change in relative humidity before and after silica gel.

Several tests were carried out on the dryer with or without silica gel in order to assess the

effectiveness of the dehumidification of the air at the entrance to the drying chamber. The previous Fig. 9 the results of one of these tests. Overall, the addition of silica gel made it possible to reduce the humidity of the drying air by around 16%. This will undoubtedly result in a reduction in drying time and a concomitant increase in efficiency. These first experimental results will be validated with other tests which will be carried out over several consecutive days, also varying the quantities of paraffin and silica gel.

## 5. CONCLUSION

This paper presented the results obtained from the implementation of the new latent heat storage system in the solar collector. These results reveal a 3% better efficiency of the sensor thanks to thermal storage. This improvement in efficiency can result in concrete benefits for solar drying, such as a reduction in drying time or an increase in drying capacity. To go further, it would be relevant to evaluate the impact of this latent storage on the overall performance of the solar dryer. This evaluation should consider the efficiency of the complete drying system and the final quality of the dried products.
The proposed dehumidification solution is simple to install and use. The test conducted with this dehumidification device showed a dehumidification rate of 16%. For the next steps, we recommend additional dehumidification tests to achieve an average dehumidification rate as well as drying tests to evaluate the effect of dehumidification on the drying of the products.

In summary, this study demonstrates the potential of latent heat thermal storage to improve the efficiency of solar dryers. The silica gel dehumidification solution is also interesting for optimizing drying. These technologies combined pave the way for more efficient and environmentally friendly solar drying.

## DISCLAIMER (ARTIFICIAL INTELLIGENCE)

Authors hereby declare that NO generative AI technologies such as Large Language Models (ChatGPT, COPILOT, etc.) and text-to-image generators have been used during the writing or editing of this manuscript.

## ACKNOWLEDGMENT

We would like to express our deep gratitude to the Diasporeines Africa association for believing in us so far and for their financial support which has allowed us to make considerable progress in improving the performance of the hybrid solar dryer design to achieve these first results. We would also like to thank the company Takaz-Eng, our host company in which the work took place.

## COMPETING INTERESTS

Authors have declared that they have no known competing financial interests or non-financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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