*2.4. Drying Process*

In a typical medium-sized Togolese company, around 1.5 t/d of fresh pineapples are processed, of which roughly 900 kg/d are dried at 50–60 ◦C for 20 h. The remaining 600 kg/d leave the production process as waste. For the conventional drying process, the daily butane consumption is around 49.5 kg which corresponds to a daily cost of 39.20 EUR [55]. Assuming a calorific value of butane of 12.72 kWh/kg at 25 ◦C, the daily energy consumption required for drying is 629.6 kWh or 0.7 kWh per kilogram of fresh fruit.

For the experiments, the indoor test facility was switched on and the drying chamber was preheated without the trays for 15 min. Then, six prepared pineapple slices were placed at different locations on each tray, and the trays were placed into the drying chamber. Subsequently, continuous drying tests were run with the parameters being set according to Table 2. The temperature and relative humidity of air were measured and recorded every two minutes during the entire experimental period. Afterwards, the pineapple slices were crushed, and their moisture content was determined. The moisture content in the product to be dried was expressed on total material basis as *W* = *<sup>m</sup>*W/(*<sup>m</sup>*DM + *<sup>m</sup>*W), where *m*W is the mass of water contained in the pineapples and *m*DM is the respective dry solid mass. The moisture content can also be expressed on the dry basis,

$$X = \frac{m\_W}{m\_{\rm DM}},$$

where the mass of the dry solid, *m*DM, remains constant during the entire drying process. Both values of product moisture can be converted between each other using *X* = *W*/(*W* − <sup>1</sup>).

The ambient air entering the solar collector was characterized by the temperature of 25 ◦C, relative humidity of 30 wt %, and pressure of 1.012 bar. The simplified equation,

$$
\dot{m}\_{\text{air}} = \frac{\Delta \dot{m}\_{\text{W}}}{\chi\_{\text{out}} - \chi\_{\text{in}}},
\tag{3}
$$

assuming a continuous, steady-state dryer operation, can be used for the calculation of drying air consumption. Here, the moisture removed from a product to be dried is defined asΔ .*m*W = .*<sup>m</sup>*V,out − .*<sup>m</sup>*V,in and the absolute air humidity is defined as *x* = *<sup>m</sup>*V/*<sup>m</sup>*A. In these equations, .*<sup>m</sup>*V,in and .*<sup>m</sup>*V,out denote

the vapor inlet and outlet mass flow rates, respectively, *m*V is the mass of vapor, and *m* A is the mass of dry air. The enthalpy of moist air is then given by the following equation:

$$h = h\_{\mathcal{A}} + \mathbf{x}h\_{\mathcal{V}} = c\_{\mathbb{P},\mathcal{A}}t + \mathbf{x}(L + c\_{\mathbb{P},\mathcal{V}}t)\_{\prime} \tag{4}$$

where *h*A denotes the enthalpy of dry air, *h*V is the enthalpy of vapor, *<sup>c</sup>*p,<sup>A</sup> and *<sup>c</sup>*p,<sup>V</sup> are the specific heat capacities of dry air and vapor, respectively, *t* is the temperature, and *L* is the specific heat of vaporization. The corresponding absolute humidity can be calculated using the following equation:

$$\mathbf{x} = \frac{R\_{\rm A}}{R\_{\rm V}} \left( \frac{q p\_{\rm S}}{p - q p\_{\rm S}} \right) \tag{5}$$

where ϕ denotes the relative air humidity, *R* A = 0.2871 kJ/(mol K) and *R*V = 0.4614 kJ/(mol K) are the specific gas constants of dry air and vapor, respectively, *p*S is the saturation pressure, and *p* is the actual pressure. The energy required by the drying process is then calculated as . *Q*D = . *<sup>m</sup>*air(*h*in − *h*out) = . *<sup>m</sup>*air·*h*D, where *h*in and *h*out denote the enthalpies of air at drying chamber inlet and outlet, respectively, and Δ*h*D is the change in air enthalpy in the drying chamber.

The source of heat in the solar thermal collector was the radiation generated by the lamps in the indoor test facility. The input power is usually the solar radiation received by the surface of the collector, absorbed and transferred to the drying air. This must be heated from ambient conditions at around 25 ◦C to conditions required at the inlet of the drying chamber (at least 55 ◦C). The heat flux to the drying air is therefore given by the following equation:

.

$$
\dot{Q}\_{\text{C}} = \dot{m}\_{\text{air}} (h\_{\text{in}} - h\_{\text{amb}}) = \dot{m}\_{\text{air}} \cdot h\_{\text{C}\_{\text{A}}} \tag{6}
$$

where *h*amb denotes the enthalpy of ambient air and Δ*h*C is the change in air enthalpy in the solar thermal collector. The same heat flux can be written in terms of quantities representing the energy irradiated to the thermal collector and the losses, . *Q*loss, as . *Q*C = η0*EA*C − . *Q*loss, where η0 denotes the optical efficiency of the solar thermal collector and *A*C is its area. The overall e fficiency of the solar thermal collector, including optical and thermal losses, can then be obtained using the following equation:

$$
\eta\_{\rm C} = \eta\_0 - \frac{a(t\_{\rm C} - t\_{\rm amb})}{E},
\tag{7}
$$

where η0 and *a* are constants to be evaluated either analytically or experimentally, while *t*C denotes the mean collector temperature and *t*amb is the ambient temperature. According to [63], the typical experimental coe fficients in e fficiency correlations for air collectors operating between 20 ◦C and 50 ◦C are η0 = (0.75–0.80) and *a* = (8–30) W/(m<sup>2</sup> K). The required collector area then follows from the general definition of the overall e fficiency of the solar thermal collector, that is,

.

$$A\_{\mathbb{C}} = \frac{Q\_{\mathbb{C}}}{\eta\_{\mathbb{C}}E}.\tag{8}$$

### **3. Results and Discussion**

The overall performance and e fficiency of the solar collector used to provide hot air to the drying process were considered first. With respect to the proposed experimental design, the residual moisture of dried pineapple fruit was used as the metric. The drying kinetics were determined for both slice thicknesses. Finally, the impact of local climatic conditions on the drying process performance was estimated together with the expected fossil fuel savings, and the design of the solar thermal collector was improved.

### *3.1. Solar Thermal Collector Performance*

Initially, the solar thermal collector was designed for the worst-case scenario, that is, the required drying time of 4 h and low radiation intensity of 650 <sup>W</sup>/m2, obtained via Equations (3) and (5) for a minimum air flow rate of 33.4 m<sup>3</sup>/h. Under these assumptions, the collector heat flux calculated using Equations (4)–(6) was 280.12 W. The necessary solar thermal collector area, *A*C, was then determined using Equations (7) and (8) to be 1.49 m<sup>2</sup> for η0 = 0.75, *a* = 20 W/(m<sup>2</sup> K), and (*<sup>t</sup>*C − *t*amb) = 15 K.

To determine the overall performance and efficiency of the solar thermal collector for each test, temperature and relative humidity of air at the collector inlet and outlet were recorded. In total, 16 tests at radiation intensities of 650 <sup>W</sup>/m<sup>2</sup> and 1100 <sup>W</sup>/m<sup>2</sup> were carried out. Figure 5 shows an example of typical measured values at the high solar radiation intensity of 1100 <sup>W</sup>/m<sup>2</sup> over the period of 480 min. For further consideration, the respective average temperatures and relative humidities after the warm-up phase of 50 min were used.

Table 4 summarizes for each test the increases in drying air temperature, *t*in − *t*amb. Using the corresponding mean value, the air temperature at the solar collector outlet was 46.8 ◦C at 650 <sup>W</sup>/m2, whereas at 1100 <sup>W</sup>/m<sup>2</sup> it was 56.8 ◦C. This means that the required temperature of about 55 ◦C at the drying chamber inlet can only be guaranteed at the high solar radiation intensity level.

**Table 4.** Air temperature changes, *t*in − *t*amb (K), between the solar collector inlet and outlet.


Given the air flow rate of 42.1 m<sup>3</sup>/<sup>h</sup> and mean air density of 1.12 kg/m3, the overall efficiency of the solar thermal collector was calculated using Equations (4)–(6) and (8). The results are listed in Table 5.

**Figure 5.** Typical dependence of air temperatures at the inlet and outlet of the solar thermal collector, and the corresponding air relative humidities, on drying time at the solar radiation intensity of 1100 <sup>W</sup>/m2.


**Table 5.** Overall performance and efficiency of the solar thermal collector.

### *3.2. Results of the Experiments*

The experiments were compared on the basis of moisture content in the pineapple fruit before and after the drying process. The eight tests of full factorial design as given in Tables 1 and 2 carried out twice yielded 16 moisture content values shown in Table 6. The data indicate that the best drying performance was reached in test number 4 with the drying time of 480 min, radiation intensity of 1100 <sup>W</sup>/m2, and slice thickness of 6–8 mm (Figure 6a). Significant shrinkage was apparent in the case of the corresponding dried pineapple slice. The second-best result was obtained in test number 2. In contrast, test number 5 with the drying time of 270 min, solar radiation intensity of 650 <sup>W</sup>/m2, and slice thickness of 12–14 mm showed the least satisfactory outcome with the highest residual moisture content (Figure 6b). Compared to a fresh pineapple slice, here there was only a slight change in appearance due to the removal of moisture just from the surface.

**Table 6.** Residual relative moisture content (wt %) for the eight tests comprising the full factorial design.


**Figure 6.** Visual comparison of the dried pineapple slices obtained using (**a**) the best combination of input factors and (**b**) the worst combination of input factors.

Using the factor signs from Table 3 and the means in Table 6, the means for the two levels, (−) and (+), can be calculated by column for each input factor and each combination of input factors. The results are shown in Figure 7. The greater the deviation between the two means, (−) and (+) (i.e., the steeper the line connecting both means), the greater the influence of a factor or a factor interaction on the drying process. It is obvious from Figure 7 that the moisture content in the dried pineapple fruit is most affected by slice thickness (factor C) and drying time (factor A). In comparison, solar radiation intensity (factor B), as well as all interactions of individual factors, influence the process output significantly less. Furthermore, the effects of the individual input factors as well as of the combinations of factors can be expressed quantitatively using Equation (1), as shown in Table 7.

**Figure 7.** Effects of individual input factors, A, B, and C, and their interactions, AB, AC, BC, and ABC.



The widths of the overall 95%, 99%, and 99.9% confidence intervals obtained using the data in Table 6 and other relevant values were ±4.26 wt %, ±6.20 wt %, and ±9.31 wt %, respectively. These are plotted in Figure 8 against the corresponding factor effects from Table 7. It is obvious that factors A and C are statistically highly significant, factor B and interaction AC are statistically significant, and the remaining interactions are statistically insignificant.

**Figure 8.** The overall 95%, 99%, and 99.9% confidence intervals (CI) plotted against the effects of the individual factors, A, B, and C, and their interactions, AB, AC, BC, and ABC.

The conclusions of the drying experiments are that the slice thickness of prepared fresh pineapples should not exceed 8 mm. The available daily sunshine duration limited by the climatic conditions should be utilized as much as possible. The solar drying process is affected by the variation of the annual solar radiation intensity to a lesser degree and, therefore, the solar thermal dryer can be used in the respective locality over the entire year. Performance of the drying process, however, also depends on the local climatic conditions, particularly on the temperature and humidity of the ambient air. In the case of Togo, the performance was verified as shown in Table 8. Table 9 then lists for Togo the air enthalpies, *h*in, from Equation (6), temperatures, *t*in, from Equation (4), and absolute humidities, *x*in, from Equation (5) at the solar collector outlet corresponding to the two solar radiation intensity levels (i.e., the constant collector outputs of 270 W and 391 W).

In any case, the best drying result, characterized by a product moisture content of 29.4 wt %, still does not meet the quality of the reference product having the residual moisture content of 13.7 wt %. The solar thermal drying process must, therefore, be extended for a further 10–12 h period or another suitable post-solar drying procedure must be implemented.


**Table 8.** Climatic conditions in the laboratory and in Togo (average).

**Table 9.** Calculated data for the average climatic conditions in Togo.

