*3.4. Cooling Process of Yogurt*

A one-ton refrigeration system (using R410a refrigerant) energized by a solar hybrid photovoltaic system was used for the cooling of yogurt below 8 ◦C in order to store it without quality deterioration. In the current study, the temperature of yogurt was lowered to 4 ◦C by considering the effect of stirrer speed on the cooling rate and consistency (thickness) of yogurt. For this, three different stirrer speeds (36, 18 and 6 rpm) were selected using a variable-speed motor to cool down the product of three different capacities (50, 40 and 30 L), and the outcomes are shown in Figure 9. It can be observed that the time required to cool 50 L of yogurt from 43 ◦C to 4 ◦C was 64, 73 and 103 min with the stirrer speeds of 36, 18 and 6 rpm, respectively. Similarly, for 40 and 30 L, the time required for cooling was observed to be 59, 64, 97 min and 48, 54 and 65 min, respectively, at 36, 18 and 6 rpm stirrer speeds. It can be noted that the higher speed of the stirrer resulted in a higher cooling rate. Importantly, greater and lesser stirrer speeds caused rupturing of yogurt texture and non-uniform cooling, respectively. It was observed that at 36 and 18 rpm stirrer speeds, the cooling uniformity was satisfactory, but at 6 rpm, a non-uniform cooling behavior was observed for all volumes of product. Although stirring at 36 rpm gave a good cooling rate, the consistency of the yogurt was found to be on the higher side compared to stirring at 18 rpm. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 18 of 29

**Figure 9.** Effect of stirrer speeds on cooling rates for different volumes of yogurt. **Figure 9.** Effect of stirrer speeds on cooling rates for different volumes of yogurt.

To evaluate the cooling efficiency of the system, the coefficient of performance (COP) of the cooling unit was calculated after every 10 min throughout the process for 50, 40 and 30 L of yogurt. In this context, the output is the amount of heat removed from the yogurt. Under all operating conditions, it was found that the COP was higher at the beginning of the cooling process and lowered as the process proceeded to the end, which is in accordance with the second law of thermodynamics [29]. As discussed earlier, variation in stirrer speed affects the rate of cooling, so COP was also calculated using three different stirrer To evaluate the cooling efficiency of the system, the coefficient of performance (COP) of the cooling unit was calculated after every 10 min throughout the process for 50, 40 and 30 L of yogurt. In this context, the output is the amount of heat removed from the yogurt. Under all operating conditions, it was found that the COP was higher at the beginning of the cooling process and lowered as the process proceeded to the end, which is in accordance with the second law of thermodynamics [29]. As discussed earlier, variation in stirrer speed affects the rate of cooling, so COP was also calculated using three different stirrer speeds

speeds (36, 18, 6 rpm) as shown in Figure 10. Although the fluctuation of the COP followed an almost similar trend throughout the process under 36 and 18 rpm, the maximum value

30 L, the maximum COP was calculated to be 2.41 (Figure 10b) and 2.75 (Figure 10c) under 36 and 18 rpm, respectively. Taking into account the cooling rate and consistency of yo-

gurt, a stirrer speed of 18 rpm was found to be optimum.

(36, 18, 6 rpm) as shown in Figure 10. Although the fluctuation of the COP followed an almost similar trend throughout the process under 36 and 18 rpm, the maximum value of COP (3.36) was achieved at 18 rpm for 50 L yogurt (Figure 10a). In the cases of 40 and 30 L, the maximum COP was calculated to be 2.41 (Figure 10b) and 2.75 (Figure 10c) under 36 and 18 rpm, respectively. Taking into account the cooling rate and consistency of yogurt, a stirrer speed of 18 rpm was found to be optimum. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 19 of 29

(**b**)

**Figure 10.** *Cont.*

**Figure 10.** Effect of stirring speed on the COP of the refrigeration unit for 50 L (**a**), 40 L (**b**) and 30 L (**c**) yogurt volumes. **Figure 10.** Effect of stirring speed on the COP of the refrigeration unit for 50 L (**a**), 40 L (**b**) and 30 L (**c**) yogurt volumes.

(**c**)

All the calculated data (temperature change in storage tank water, milk, heating rates of circulating water and milk, net heating rate effect, cumulative heat energy released by water and absorbed by milk and cooling rate of milk) with respect to process time were modeled using sigma plot-12, and it was found that polynomial cubic model fitted best to all the data obtained under the prevailing operating conditions. It helps to interpolate new, invisible data points. All the values of respective model constants for the parameters calculated are tabulated in Table 2. All the calculated data (temperature change in storage tank water, milk, heating rates of circulating water and milk, net heating rate effect, cumulative heat energy released by water and absorbed by milk and cooling rate of milk) with respect to process time were modeled using sigma plot-12, and it was found that polynomial cubic model fitted best to all the data obtained under the prevailing operating conditions. It helps to interpolate new, invisible data points. All the values of respective model constants for the parameters calculated are tabulated in Table 2.

**Parameter Milk Quantity Y0 A b C R2 Table 2.** The parameters of fitted models (polynomial cubic) for the investigated parameters.

**Table 2.** The parameters of fitted models (polynomial cubic) for the investigated parameters.



**Table 2.** *Cont.*

#### *3.5. Thermal Profile of the Entire System*

The complete yogurt-making process consists of heating, cooling the milk, fermentation and cooling the yogurt, exchanging thermal energy. In order to verify that either system is capable of harvesting enough solar energy for all the processes to be completed effectively and economically, complete thermal profiles were developed for three different milk volumes with an optimized stirrer speed (18 rpm) as shown in Figure 11. It can be observed that the heating of 50 L of milk took 140 min to reach 80 ◦C, and after that its temperature fell down to 43 ◦C in 14 min using tap water (Figure 11a). This showed that the system was able to maintain the temperature of inoculated milk governed through a thermostat valve at 43 ◦C for five hours followed by the rapid cooling of yogurt in 73 min when the stirrer rpm was 18. Milk was converted to yogurt by lowering its pH value to 4.45 during those five hours. In order to estimate the amount of heat extracted and absorbed from the product during the entire process, heating and cooling rate trends were also developed. It was also noted that the product heating rate started from 5.09 kW and it reduced to 0.29 kW at the end of the heating process. After that, the cooling process of milk started which resulted in a maximum decline in heating rate of −8.68 kW followed by the fermentation process with an average heating rate of 0.129 kW. Negative values of the heating rate during the fermentation process were due to the heat loss when the thermostat valve was closed and hot water circulation was shut down. So, no transfer of heat energy would take place under this condition. Finally, the process of yogurt cooling occurred with a maximum heat extraction rate of −3.047 which reduced −0.1945 kW at the end of the cooling process of yogurt. The total process time was calculated to be 540 min (9 h).

Similar trends were found for the cases of 40 L and 30 L. However, due to having less volume than 50 L, it took 110 and 80 min to complete the heating process for 40 and 30 L, respectively. The heating rate was about 4.40 kW, 3.85 kW at the start and 0.15 kW, 0.23 kW at the end of the heating process for 40 and 30 L, respectively. During the milk cooling process, the heating rate was reduced to −6.48 kW and −4.94 kW, and at the end of the fermentation process, it was 0.104 kW and 0.077 kW for 40 and 30 L, respectively. The total process time was estimated to be 5.7% and 13.2% less than that of 50 L in the case of 40 and 30 L, respectively.

**Figure 11.** *Cont.*

**Figure 11.** Thermal profile of the yogurt-making process for 50 L (**a**), 40 L (**b**) and 30 L (**c**) yogurt under a constant stirrer speed of 18 rpm. **Figure 11.** Thermal profile of the yogurt-making process for 50 L (**a**), 40 L (**b**) and 30 L (**c**) yogurt under a constant stirrer speed of 18 rpm.

Similar trends were found for the cases of 40 L and 30 L. However, due to having less volume than 50 L, it took 110 and 80 min to complete the heating process for 40 and 30 L, respectively. The heating rate was about 4.40 kW, 3.85 kW at the start and 0.15 kW, 0.23 kW at the end of the heating process for 40 and 30 L, respectively. During the milk cooling process, the heating rate was reduced to −6.48 kW and −4.94 kW, and at the end of the fermentation process, it was 0.104 kW and 0.077 kW for 40 and 30 L, respectively. The total process time was estimated to be 5.7% and 13.2% less than that of 50 L in the case of 40 and 30 L, respectively. The energy consumption pattern for the heating, fermentation and cooling processes is shown in Figure 12. The energy required to complete the yogurt-making process was provided by solar thermal and solar photovoltaic systems. The total energy consumed was calculated to be 6.732 kWh, 5.559 kWh and 4.207 kWh for 50, 40 and 30 L batches, respectively. It can be observed that, in all cases, a large part of the total energy was consumed during the heating process of the milk, followed by the cooling process of yogurt. The heating process was found to account for 40% of the total energy consumed during the 50 L process (Figure 12a), which was 39% of the respective total energies for the 40 L (Figure 12b) and 30 L cases. Similarly, the cooling of the yogurt process consumed 2.107 kWh (31% of total), 1.686 kWh (30% of the total) and 1.264 kWh (30% of the total) for the cases of 50, 40 and 30 L, respectively. Although a significant part of the energy is consumed during the cooling process of the milk, it is also important to note that this energy was neither provided by the solar photovoltaic nor by the solar thermal system or utility but through the thermal energy gained by circulating tap water. In addition, the specific product energy (SPE) was calculated to be 485 kJ/kg, 500 kJ/kg and 505 kJ/kg for 50, 40 and 30 L batches, respectively. In the case of 40 and 30 L milk processing, the contact area The energy consumption pattern for the heating, fermentation and cooling processes is shown in Figure 12. The energy required to complete the yogurt-making process was provided by solar thermal and solar photovoltaic systems. The total energy consumed was calculated to be 6.732 kWh, 5.559 kWh and 4.207 kWh for 50, 40 and 30 L batches, respectively. It can be observed that, in all cases, a large part of the total energy was consumed during the heating process of the milk, followed by the cooling process of yogurt. The heating process was found to account for 40% of the total energy consumed during the 50 L process (Figure 12a), which was 39% of the respective total energies for the 40 L (Figure 12b) and 30 L cases. Similarly, the cooling of the yogurt process consumed 2.107 kWh (31% of total), 1.686 kWh (30% of the total) and 1.264 kWh (30% of the total) for the cases of 50, 40 and 30 L, respectively. Although a significant part of the energy is consumed during the cooling process of the milk, it is also important to note that this energy was neither provided by the solar photovoltaic nor by the solar thermal system or utility but through the thermal energy gained by circulating tap water. In addition, the specific product energy (SPE) was calculated to be 485 kJ/kg, 500 kJ/kg and 505 kJ/kg for 50, 40 and 30 L batches, respectively. In the case of 40 and 30 L milk processing, the contact area of the heating coil was not fully exposed to the product, resulting in higher heat loss, which is why the value of SPE is high. Although the process design, product quantity to be processed and operating conditions vary and effect the rates of energy usage, the process can be compared with other somehow similar work. It can be observed that the solar yogurt processing unit gave quite good results in terms of thermal analysis when compared with results reported by Yaseen et al. (2019) [20] who worked on milk pasteurization (100 L milk was heated up to 63 ◦C) and cooling (4 ◦C in the chiller within 90 min). In the current study, milk was pasteurized at 80 ◦C and it can also be noted that the system can easily be used for low-temperature pasteurization (63 ◦C) as well for 50 L of milk, making it feasible in terms of yogurt making and pasteurization. For cooling of the yogurt from 43 ◦C to 4 ◦C, the energy-utilization pattern showed similar rates as reported by Yaseen et al. (2019) [20] and Khan et al. (2020) [13] for milk cooling using rotary compressor.

sor.

of the heating coil was not fully exposed to the product, resulting in higher heat loss, which is why the value of SPE is high. Although the process design, product quantity to be processed and operating conditions vary and effect the rates of energy usage, the process can be compared with other somehow similar work. It can be observed that the solar yogurt processing unit gave quite good results in terms of thermal analysis when compared with results reported by Yaseen et al. (2019) [20] who worked on milk pasteurization (100 L milk was heated up to 63 °C) and cooling (4 °C in the chiller within 90 min). In the current study, milk was pasteurized at 80 °C and it can also be noted that the system can easily be used for low-temperature pasteurization (63 °C) as well for 50 L of milk, making it feasible in terms of yogurt making and pasteurization. For cooling of the yogurt from 43 °C to 4 °C, the energy-utilization pattern showed similar rates as reported by Yaseen et al. (2019) [20] and Khan et al. (2020) [13] for milk cooling using rotary compres-

**Figure 12.** Energy consumption pattern during the yogurt-making process under 50 (**a**), 40 (**b**) and **Figure 12.** Energy consumption pattern during the yogurt-making process under 50 ( 30 (**c**) liters. **a**), 40 (**b**) and 30 (**c**) L.

#### **4. Conclusions**

In the current study, a solar-assisted decentralized solution for yogurt processing was presented, capable of performing all required processes (heating, fermentation and cooling) in a single chamber. The systems consisted of a cylindrically shape chamber surrounded by a squared helix coil heat exchanger for heating and a pillow plate at its bottom for cooling. Experiments were performed using raw milk from cows at different volumes and stirrer speeds. The total energy consumed was calculated to be 6.732 kWh, 5.559 kWh and 4.207 kWh for the cases of 50, 40 and 30 L, respectively. This shows that approximately 2020, 1668, and 1262 electrical units (kWh) can be saved annually to process 50, 40 and 30 L, respectively, by considering three hundred sunny days (one batch per day) throughout the year in Faisalabad, Pakistan. During milk heating and cooling, the optimized stirrer speed was found to be 36 rpm, while for the cooling of yogurt it was 18 rpm. It was found that a majority of the total energy was consumed by the heating processes followed by the cooling process. This supports the integration of solar evacuated tube collectors for heating purposes instead of converting electrical energy into thermal energy which can increase the operating and capital costs of the system. The use of solar thermal energy (for heating) and PV modules (for cooling) not only played a significant role in reducing the operational energy cost, but also facilitated its decentralized applications. The study provides a basis for design optimization to process any milk volume. Being a batch system, the integrated solar system (both PV and thermal) can be used for farm electrification and heating purposes in the event of non-operational periods.

#### **5. Directions for Further Research**

Once processing a large quantity of raw milk, the effect of the flow rates of the circulating hot water need to be investigated. Similarly, impact of heat transfer fluids, i.e., silicon fluids and paraffin oil, on the heating efficiency of the evacuated tube collector can also be considered for further research. Thirdly, use of the Scheffler fixed focus concentrator as a heat source can also be compared.

**Author Contributions:** Conceptualization, S.N.H. and O.H.; methodology, S.N.H. and W.A.; software, S.N.H. and W.A.; validation, S.N.H., W.A. and O.H.; formal analysis, S.N.H. and W.A.; investigation, S.N.H., W.A., A.M.; resources, O.H. and A.M.; data curation, S.N.H. and W.A.; writing—original draft preparation, S.N.H.; writing—review and editing, S.N.H., W.A., A.M. and O.H.; visualization, S.N.H. and W.A.; supervision, O.H. and A.M.; project administration, A.M. and O.H.; funding acquisition, A.M. and O.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** The development cost of the system was provided by the International Center for Development and Decent Work (ICDD) Germany.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The author thanks the Higher Education Commission (HEC) of Pakistan and the German Academic Exchange Service (DAAD) for providing subsistence/research costs for the current study. The Author would also like to thank the University of Kassel, Germany for providing funds for publishing the work.

**Conflicts of Interest:** The authors declare no conflict of interest.

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