*2.5. Sizing of Evacuated Tube Collector (ETC)*

For the thermal application of the yogurt processing unit, an evacuated tube collector (ETC) was used. The ETC area was calculated by using the following formula [26]

$$\mathbf{A}\_{\mathbf{c}} = \frac{\mathbf{m} \mathbf{C}\_{\mathbf{p}} \Delta \mathbf{T}}{\mathbf{I}\_{\mathbf{b}} \tau \alpha \mathbf{t}} \tag{6}$$

where A<sup>c</sup> is the area of the receiver tube exposed to radiation (m<sup>2</sup> ); m is the mass of water (kg); CpC<sup>p</sup> is the specific heat capacity of the receiver tube (kJkg−<sup>1</sup> K −1 ); ∆T is the change in temperature (K); I<sup>b</sup> is the solar irradiance (W/m<sup>2</sup> ); τ is the transmission coefficient; α is the absorption coefficient and t is the time in seconds.

The amount of heat energy required to increase the temperature of milk (m = 50 kg, specific heat: 3.89 kJkg−<sup>1</sup> K −1 ) up to 50 ◦C (30 ◦C to 80 ◦C) in two hours' time (t = 7200 s) using a vacuum tube collector (transmission coefficient ττ = 0.95, absorption coefficient α = 0.95) was calculated for a tropical region like Faisalabad (31.4303◦ N, 73.0672◦ E) lying in the solar belt having an average global horizontal irradiance (P) of 0.8 kWm−<sup>2</sup> . Substituting these values into Equation (6), the area required is calculated to be 1.87 m<sup>2</sup> . The absorber area of one tube is calculated by multiplying the circumference of the absorber plate and calculated to be 0.164 m<sup>2</sup> , while the number of tubes is calculated by dividing the total absorber area by the absorber area of one tube and was calculated to be 11.4 ≈ 12 tubes. Therefore, an evacuated tube collector of 15 tubes was selected considering thermal losses in the system as shown in Figure 3. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 8 of 29 tubes. Therefore, an evacuated tube collector of 15 tubes was selected considering thermal losses in the system as shown in Figure 3.

**Figure 3.** Pictorial view of the installed solar-assisted yogurt processing unit. **Figure 3.** Pictorial view of the installed solar-assisted yogurt processing unit.

#### *2.6. Experiments 2.6. Experiments*

Trials were conducted to assess the heating and cooling performances using three different volumes of raw cow's milk (50, 40 and 30 L). Before conducting an experiment, the heating component of the system was turned on in order to raise the water temperature of the storage tank up to 90 °C, which normally took 2–3 h depending upon solar radiation. After that, raw milk is poured into the fermentation tank and the circulation of hot water from the storage tank to the fermentation chamber is turned on. The heating of Trials were conducted to assess the heating and cooling performances using three different volumes of raw cow's milk (50, 40 and 30 L). Before conducting an experiment, the heating component of the system was turned on in order to raise the water temperature of the storage tank up to 90 ◦C, which normally took 2–3 h depending upon solar radiation. After that, raw milk is poured into the fermentation tank and the circulation of hot water from the storage tank to the fermentation chamber is turned on. The heating of raw milk up

raw milk up to 80 °C took about 140 min, and it continuously stirred at a speed of 36 rpm. To bring down the temperature of heated milk to 43 °C, which is recommended for fer-

by manually operated valves. The inoculation of starter culture was carried out (2–3%) at this temperature, and it was maintained by a solenoid valve controlling the water circulation for a period of 5–6 h until the required pH (4.85–4.5) was attained. After that process, the cooling process of yogurt was started by turning on the refrigeration unit to bring down the temperature of yogurt below 8 °C to reduce the bacterial activities which normally took 48–103 min depending upon the quantity of the processed milk and stirrer speed. For this, three different speeds (36, 18 and 6 rpm) of the stirrer were selected using a variable speed motor. Each experiment was replicated thrice, and values were averaged for each volume of milk. For data acquisition, a controller with resistance temperature detector (RTD)-based temperature sensors was used to measure temperature at the inlet and outlet of the ETC, the top and bottom of the hot water storage tank, and inside the fermentation chamber. A portable pH meter (ML1010) was used to measure the pH of milk during the fermentation process. To access the performance of the installed PV sys-

tem, a clamp meter (Fluke 345PQ), and pyranometer (METEON) were used.

Thermodynamic Analysis for Efficient Refrigeration System

to 80 ◦C took about 140 min, and it continuously stirred at a speed of 36 rpm. To bring down the temperature of heated milk to 43 ◦C, which is recommended for fermentation of milk, tap water passed through the heating coil in an open loop controlled by manually operated valves. The inoculation of starter culture was carried out (2–3%) at this temperature, and it was maintained by a solenoid valve controlling the water circulation for a period of 5–6 h until the required pH (4.85–4.5) was attained. After that process, the cooling process of yogurt was started by turning on the refrigeration unit to bring down the temperature of yogurt below 8 ◦C to reduce the bacterial activities which normally took 48–103 min depending upon the quantity of the processed milk and stirrer speed. For this, three different speeds (36, 18 and 6 rpm) of the stirrer were selected using a variable speed motor. Each experiment was replicated thrice, and values were averaged for each volume of milk. For data acquisition, a controller with resistance temperature detector (RTD)-based temperature sensors was used to measure temperature at the inlet and outlet of the ETC, the top and bottom of the hot water storage tank, and inside the fermentation chamber. A portable pH meter (ML1010) was used to measure the pH of milk during the fermentation process. To access the performance of the installed PV system, a clamp meter (Fluke 345PQ), and pyranometer (METEON) were used.

#### Thermodynamic Analysis for Efficient Refrigeration System

While selecting a cooing machine, the coefficient of performance (COP) is the key factor to be considered for maximum cooling output with the lowest possible input energy requirement. Therefore, a comprehensive thermodynamic analysis is mandatory for evaluation and cooling system optimization for the best application. For this purpose, the system was connected with temperature sensors (thermocouples: K type, error: <0.1%) to record the temperature differentials of the circulating refrigerant in order to monitor the phase changes during the complete refrigeration cycle. The design and selection of the compressor was carried out with the aim that it must be capable of superheating the refrigerant to provide a reasonable degree of superheat to provide a high enthalpy drop during the expansion process (process (irreversible adiabatic process) with the help of a capillary tube for maximum cooling efficiency during the evaporation process to chill the milk/yogurt. Therefore, the rotary compressor was used to decrease the torque load and increase the pressure to achieve the required degree of superheat from the refrigerant.

R410a refrigerant was used in the compressor and the flow rate of the refrigerant was maintained as 1.42 L/min, having a specific volume of 0.0009 m<sup>3</sup> kg−<sup>1</sup> (m0 = 1.58 kg/min). The temperature after compression was recorded to be 347 K, while the temperature before compressing was noted as 261 K, and this line is represented on the T-S diagram by a vertical line showing an isentropic process owing to the high speed of the compressor. It has been noted that the temperature after the compression process is 347 K, and this temperature is well above the saturation temperature of the refrigerant (318 K at 2758 kPa), thus providing a degree of superheat as 29 K. The heat is removed at constant pressure in the condenser: the process first removes sensible heat from superheated vapor (347 K) to saturated vapor (318 K), i.e., a process from 2 to 20 , then in the form of latent heat from dry saturated vapor to the saturated liquid line (process from 20 to 30 ) and then sensible heat from the saturated liquid line to 312 K as a sub-cooling process (30 to 3). This high heat dissipation rate was due to the high heat transfer coefficient of copper coiling used and the efficient design of the condenser as well as the addition of two fans to dissipate heat at a faster rate. Therefore, the condensation process takes place partially in the superheated region and predominantly inside the saturation region to change the phase of the refrigerant from gas to liquid and then follows a sub-cooling line to decrease the refrigerant temperature. The expansion takes place through the capillary tube and undergoes an irreversible adiabatic (constant enthalpy) process as shown in process 3–4 in Figure 4. Process 4-1 is accomplished at constant pressure which evaporates the refrigerant once again at saturation temperature to complete the vapor compression refrigeration cycle using R410a refrigerant.

cle using R410a refrigerant.

**Figure 4.** Temperature–entropy (T-s) and pressure–enthalpy (p-h) diagrams of the optimized compressor of the yogurt processing system.

While selecting a cooing machine, the coefficient of performance (COP) is the key factor to be considered for maximum cooling output with the lowest possible input energy requirement. Therefore, a comprehensive thermodynamic analysis is mandatory for evaluation and cooling system optimization for the best application. For this purpose, the system was connected with temperature sensors (thermocouples: K type, error: <0.1%) to record the temperature differentials of the circulating refrigerant in order to monitor the phase changes during the complete refrigeration cycle. The design and selection of the compressor was carried out with the aim that it must be capable of superheating the refrigerant to provide a reasonable degree of superheat to provide a high enthalpy drop during the expansion process (process (irreversible adiabatic process) with the help of a capillary tube for maximum cooling efficiency during the evaporation process to chill the milk/yogurt. Therefore, the rotary compressor was used to decrease the torque load and increase the pressure to achieve the required degree of superheat from the refrigerant.

R410a refrigerant was used in the compressor and the flow rate of the refrigerant was maintained as 1.42 L/min, having a specific volume of 0.0009 m3 kg−1 (m′ = 1.58 kg/min). The temperature after compression was recorded to be 347 K, while the temperature before compressing was noted as 261 K, and this line is represented on the T-S diagram by a vertical line showing an isentropic process owing to the high speed of the compressor. It has been noted that the temperature after the compression process is 347 K, and this temperature is well above the saturation temperature of the refrigerant (318 K at 2758 kPa), thus providing a degree of superheat as 29 K. The heat is removed at constant pressure in the condenser: the process first removes sensible heat from superheated vapor (347 K) to saturated vapor (318 K), i.e., a process from 2 to 2′, then in the form of latent heat from dry saturated vapor to the saturated liquid line (process from 2′ to 3′) and then sensible heat from the saturated liquid line to 312 K as a sub-cooling process (3′ to 3). This high heat dissipation rate was due to the high heat transfer coefficient of copper coiling used and the efficient design of the condenser as well as the addition of two fans to dissipate heat at a faster rate. Therefore, the condensation process takes place partially in the superheated region and predominantly inside the saturation region to change the phase of the refrigerant from gas to liquid and then follows a sub-cooling line to decrease the refrigerant temperature. The expansion takes place through the capillary tube and undergoes an irreversible adiabatic (constant enthalpy) process as shown in process 3–4 in Figure 4. Process 4-1 is accomplished at constant pressure which evaporates the refrigerant once again at saturation temperature to complete the vapor compression refrigeration cy-

The COP of the refrigerant is calculated using Equation (7)

$$\text{COP} = \frac{\text{h}\_1 - \text{h}\_{\text{f3}}}{\text{h}\_2 - \text{h}\_1} \tag{7}$$

where h1, h<sup>2</sup> and hf3 are the enthalpies of the refrigerant used at the compressor inlet, compressor outlets and liquid enthalpy after the condenser outlet.

The values of dry saturated vapor before the compressor and after the compressor were found to be 418.8 and 425 kJ/kg at temperatures of 261 and 318 K, respectively (at 535 and 2758 kPa), while the value of the dry liquid line was found to be 277 kJ/kg at a temperature of 312 K and 2758 kPa pressure. The total enthalpy of actual refrigeration points after compression 'h2' was calculated by using Equation (8):

$$\mathbf{h\_2 = h\_{2'} + C\_{pg}(T\_2 - T\_{2'})} \tag{8}$$

where h<sup>2</sup> is the actual enthalpy of the refrigerant after the compression process, h<sup>2</sup> 0 is the enthalpy of the refrigerant at the dry saturated steam line after the compression process and is taken from the enthalpy table, Cpg is the specific heat capacity of the refrigerant at constant pressure in gaseous state (0.84 kJ kg−<sup>1</sup> K−<sup>1</sup> ), and T<sup>2</sup> and T<sup>2</sup> 0 are the actual temperatures recorded of the refrigerant and saturation temperature at the compressor outlet, respectively.

The enthalpy of actual the refrigeration points after the condensation process 'h3' was calculated by using Equation (9):

$$\mathbf{h\_{\{3\}}} = \mathbf{h\_{\{3'\}}} + \mathbf{C\_{p1}(T\_{\{3'\}} - T\_{\{3\}})} \tag{9}$$

where h<sup>3</sup> is the enthalpy of the refrigerant after the condensation process and is noted physically, h<sup>3</sup> <sup>0</sup> is the enthalpy of the refrigerant at the water line after condensation and is taken from the enthalpy table, Cpl is the specific heat capacity of the refrigerant at constant pressure in the liquid state (1.8 kJ kg−<sup>1</sup> K <sup>−</sup><sup>1</sup> ), T<sup>3</sup> is the actual value of the temperature of the refrigerant after condenser and T<sup>3</sup> 0 is the saturation temperature at the liquid line after the condensation process, respectively.

By using Equations (8) and (9), the values of enthalpies h<sup>2</sup> and h<sup>3</sup> are calculated to be 454 kJ/kg and 266.2 kJ/kg, respectively. It is worth mentioning here that the value h<sup>3</sup> is the total enthalpy at point 3, and as there is no latent heat consideration as the refrigerant is in a liquid state after condensation in the condenser, this enthalpy h<sup>3</sup> is equal to hf3 (liquid enthalpy), as the refrigerant at the inlet of the compressor is in a dry saturation condition and enthalpy at point 1 is equal to 418.8 kJ/kg.

Using these values in Equation (7), the COP of the system was found to be 4.33. In fact, this is the thermodynamic value for the COP. However, the actual value of the refrigeration system for the cooling application is a little lower, as these values include the thermal losses and heat transfer losses through the pillow plate of the chiller unit where processing is carried out.

The capacity of refrigeration (TR), i.e., the heat extraction rate, is calculated using the following equation

$$\text{TR} = \frac{\text{m}\_{\text{r}}^{\prime} (\text{h}\_{1} - \text{h}\_{\text{f3}})}{3.5} \tag{10}$$

where m0 <sup>r</sup> is the mass flow rate of the refrigerant (1.58 kg/min), h<sup>1</sup> is the enthalpy of the refrigerant used at the compressor inlet (kJ), and hf3 is the liquid enthalpy after the condenser outlet (kJ).

The capacity of refrigeration (TR) is calculated to be 1.15.

The power required to run the system is calculated by using Equation (11)

$$\mathbf{P} = \mathbf{m}\_{\mathbf{r}}'(\mathbf{h}\_2 - \mathbf{h}\_1) \tag{11}$$

where P is the power, m0 <sup>r</sup> is the mass flow rate of the refrigerant (1.58 kg/min), h<sup>2</sup> is the enthalpy of the refrigerant at the compressor outlet (kJ), and h<sup>1</sup> is the enthalpy of the refrigerant used at the compressor inlet (kJ)

The power required is calculated to be 0.93 kW; however, a 1 kW compressor motor was used considering electro-mechanical losses. The research depicts that through proper design and optimization of the refrigeration system, less than 1 kW of power is required to achieve one ton of cooling effect (3.5 kW/210 KJ/min or 12,000 BTU/h), which is in accordance with the findings investigated by [13,20,27,28] for high-performance cooling machines. The actual COP results were also conducted under field conditions using milk/yogurt by direct method, and the details of these trials are given in Section 3.4.

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

## *3.1. Heating of Raw Milk*

Temperatures changes of hot water and milk during the heating process for three different product volumes (50, 40, 30 L) are shown in Figure 5. It can be observed that the initial temperatures of the milk and water storage tanks were 30 ◦C and 90 ◦C, respectively. During the first thirty minutes of the heating process, the temperature of the hot water dropped to 72 ◦C due to the high heat transfer rate, resulting in milk temperature increases of 62 ◦C, 66 ◦C and 70 ◦C for 50, 40 and 30 L of milk capacities, respectively. Thereafter, it was observed that the rate of temperature change was slower for all processed capacities. However, based on the quantity of the product to be processed, the time required to achieve the desired temperature (80 ◦C) was found to be 140, 110 and 80 min for 50, 40 and 30 L of milk capacities, respectively. In the case of a hot water storage tank, after thirty minutes, the temperature started to increase due to the continuous addition of thermal energy from ETC and a significantly lower rate of heat transfer to milk. This shows that the system can achieve the temperature (80 ◦C) required to denature and unfold most of the milk protein "lactoglobulin". This allows lactoglobulin to bind with some of the other proteins in milk, called caseins, thus forming a thick and well-structured yogurt. In addition to this, it can also be noted that the system can easily be used for the low-temperature pasteurization (63 ◦C) of up to 50 L of milk in approximately thirty minutes, which makes the system state of the art in terms of yogurt making and pasteurization.

respectively.

**Figure 5.** Relationship between change in the hot water of the storage tank and milk temperatures **Figure 5.** Relationship between change in the hot water of the storage tank and milk temperatures under different loading capacities.

under different loading capacities. The heating rates of milk in response to the heating rate of circulation water during the entire heating process under different product capacities have been shown in Figure 6. The solar irradiance and the total solar power available at ETC were also recorded and calculated during the experiments conducted on 2 July 2020, 3 July 2020, and 14 July 2020 for 50, 40 and 30 L of batch capacities, respectively by using a pyranometer (METEON, Accuracy ± 0.1%). These values were found in the ranges of 850–896 W/m2, 840–891 W/m2, 840–881 W/m2 and 2.16–2.23 kW, 2.10–2.22 kW, 2.10–2.2 kW for 50, 40 and 30 L of milk capacities, respectively. At the early stages of the heating process, the rate of heat extrac-The heating rates of milk in response to the heating rate of circulation water during the entire heating process under different product capacities have been shown in Figure 6. The solar irradiance and the total solar power available at ETC were also recorded and calculated during the experiments conducted on 2 July 2020, 3 July 2020, and 14 July 2020 for 50, 40 and 30 L of batch capacities, respectively by using a pyranometer (METEON, Accuracy ± 0.1%). These values were found in the ranges of 850–896 W/m<sup>2</sup> , 840–891 W/m<sup>2</sup> , 840–881 W/m<sup>2</sup> and 2.16–2.23 kW, 2.10–2.22 kW, 2.10–2.2 kW for 50, 40 and 30 L of milk capacities, respectively. At the early stages of the heating process, the rate of heat extraction from hot water was higher than that of heat addition, and it reduced as soon as milk temperature increased. It can be observed that the heating rate of milk is higher than the thermal energy provided by ETC due to the energy provided by the hot water of the storage tank at the beginning of the heating process. The heating rate of milk and the heat extraction rate from hot water was calculated at 10 min intervals throughout the heating process, and their values were found in ranges of 5.48–0.31 kW, 4.75–0.16 kW, 4.14–0,24 kW and 6.28–0.49 kW, 5.58–0.49 kW, 4.88–0.69 kW for 50, 40 and 30 L of milk capacities, respectively.

tion from hot water was higher than that of heat addition, and it reduced as soon as milk

temperature increased. It can be observed that the heating rate of milk is higher than the

thermal energy provided by ETC due to the energy provided by the hot water of the stor-

age tank at the beginning of the heating process. The heating rate of milk and the heat

extraction rate from hot water was calculated at 10 min intervals throughout the heating

process, and their values were found in ranges of 5.48–0.31 kW, 4.75–0.16 kW, 4.14–0,24

**Figure 6.** *Cont.*

**Figure 6.** Effect of solar irradiance on the performance parameters of the system (heating rate of circulation water, milk and net heating rate) during the heating process of milk at 50 L (**a**), 40 L (**b**) and 30 L (**c**) of milk. **Figure 6.** Effect of solar irradiance on the performance parameters of the system (heating rate of circulation water, milk and net heating rate) during the heating process of milk at 50 L (**a**), 40 L (**b**) and 30 L (**c**) of milk.

Figure 7 shows the cumulative heat energy released by hot circulating water, the heat energy absorbed by milk and the corresponding heat transfer efficiency. It was found that during the first 40 min of the heating process, hot water released 66%, 69%, and 74% of its total energy for 50, 40 and 30 L of milk capacities. The rest of the energy was transferred to milk in the next 100, 70 and 40 min for 50, 40 and 30 L of milk. Contrarily, the milk absorbed 69%, 81% and 84% of the total required energy to reach the desired temperature of 80 °C. Therefore, the quantity of milk plays an important role in energy consumption. The heat transfer efficiency was found to be above 80% during the whole heating process because milk received energy from the storage tank where any fluctuation was compensated continuously by ETC. Figure 7 shows the cumulative heat energy released by hot circulating water, the heat energy absorbed by milk and the corresponding heat transfer efficiency. It was found that during the first 40 min of the heating process, hot water released 66%, 69%, and 74% of its total energy for 50, 40 and 30 L of milk capacities. The rest of the energy was transferred to milk in the next 100, 70 and 40 min for 50, 40 and 30 L of milk. Contrarily, the milk absorbed 69%, 81% and 84% of the total required energy to reach the desired temperature of 80 ◦C. Therefore, the quantity of milk plays an important role in energy consumption. The heat transfer efficiency was found to be above 80% during the whole heating process because milk received energy from the storage tank where any fluctuation was compensated continuously by ETC.

#### *3.2. Cooling Process of Milk*

The cooling rates of heated milk in response to the circulation of tap water through the coils under different product capacities have been shown in Figure 8. Tap water was passed through the coils of the fermentation chamber in the opposite direction of the stirrer in order to reduce the temperature rapidly. The temperature and flow rate of tap water was 29.8 ◦C and 32.25 L per minute, respectively. The time required to drop the temperature from 80 ◦C to 43 ◦C was found to be 19, 16 and 14 min for 50, 40 and 30 L of volume, respectively. Similar trend lines of heat extraction for all the milk volumes were observed with a slight difference in time taken to complete the process.

(**a**)

(**b**)

**Figure 7.** *Cont.*

(**c**) *Sustainability* **2022**, *14*, x FOR PEER REVIEW 17 of 29

*3.2. Cooling Process of Milk* 

**Figure 7.** Total heat energy released by water and absorbed by milk during the processing of 50 L (**a**), 40 L (**b**) and 30 L (**c**) milk. **Figure 7.** Total heat energy released by water and absorbed by milk during the processing of 50 L (**a**), 40 L (**b**) and 30 L (**c**) milk.

**Figure 8.** Time required to cool down the temperature of heated milk of different capacities up to **Figure 8.** Time required to cool down the temperature of heated milk of different capacities up to 43 ◦C using tap water.

and the stirrer was kept off during that period. It took five hours to complete the fermentation process. The pH value of milk was found to be 6.45, which changed to 4.35 after being made into yogurt. At this stage, the temperature was maintained between 43 °C to 44 °C, being controlled using a thermostatic valve and water circulation pump. The thermostatic valve opened automatically, letting the hot water flow through the coils when the temperature of the milk falls below than 43 °C and closing once it rises above 44 °C.

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 com-

43 °C using tap water.

*3.3. Fermentation Process* 

*3.4. Cooling Process of Yogurt* 

pared to stirring at 18 rpm.

## *3.3. Fermentation Process*

For the fermentation process, 2% of starter culture was added to milk to make yogurt and the stirrer was kept off during that period. It took five hours to complete the fermentation process. The pH value of milk was found to be 6.45, which changed to 4.35 after being made into yogurt. At this stage, the temperature was maintained between 43 ◦C to 44 ◦C, being controlled using a thermostatic valve and water circulation pump. The thermostatic valve opened automatically, letting the hot water flow through the coils when the temperature of the milk falls below than 43 ◦C and closing once it rises above 44 ◦C.
