*Article* **Development and Experimental Study of Smart Solar Assisted Yogurt Processing Unit for Decentralized Dairy Value Chain**

**Syed Nabeel Husnain 1,2,\* , Waseem Amjad <sup>2</sup> , Anjum Munir <sup>2</sup> and Oliver Hensel <sup>1</sup>**


**\*** Correspondence: nabeel.husnain@uaf.edu.pk

**Abstract:** Yogurt production at the farm level is important for adding value to milk. In this study, a solar-assisted yogurt processing unit capable of performing the three processes of heating, fermentation, and cooling in a single unit was developed. It consisted of a circular chamber surrounded by a coil for heating by a solar vacuum tube collector and a pillow plate for cooling by a solar PV-powered chiller unit. Experiments were performed using 50, 40 and 30 L of raw milk under a constant water circulation rate of 50 L per minute for heating followed by a cooling process under 36, 18 and 6 rpm of stirrer speeds. The heat absorption rates of the milk were 5.48–0.31, 4.75–0.16 and 4.14–0.24 kW, and the heat removal rates from water ranged from 6.28–0.49, 5.58–0.49 and 4.88–0.69 kW for 50, 40 and 30 L of milk volume, respectively. The overall heat transfer efficiency was above 80% during the heating process. A stirring speed of 18 rpm was found to be optimal in terms of cooling speed and consistency of the yogurt. The total energy consumed was calculated to be 6.732, 5.559 and 4.207 kWh for a 50, 40 and 30 L batch capacity, respectively. The study offers a sustainable energy solution for the decentralized processing of raw milk, particularly in remote areas of the developing countries where access to electricity is limited.

**Keywords:** yogurt processing; solar energy; solar-based heating and cooling; thermal analysis

#### **1. Introduction**

Milk and its products are considered to be a good medium for the infectious growth of bacteria and other pathogens which grow faster at ambient temperature. Raw milk and yogurt are spoiled due to an increase in temperature, and these losses are significant in developing countries due to the non-availability of processing facilities at the farm level. Therefore, producers have to sell their products at low prices. Pakistan is ranked as the fourth largest milk-producing country in the world after the USA, China and India [1] by producing about 42 billion liters of milk annually [2], while the majority of producers are small-scale farmers (>80%). Unfortunately, only 5% of this milk is processed while the remainder is handled by milkmen which are mostly unhygienic and pose high health risks. About 15–19% of the total milk produced in the country is wasted due to a lack of processing facilities while the rest is handled improperly [3]. Yogurt is one of the popular dairy products in the Indo-Pak subcontinent. In Pakistan, the yogurt share is about 70% of total fermented dairy products [4], but diminutive attention is given to the fermentation of milk to increase its shelf life, aroma and nutritional value.

About 70% of dairy farms have limited access to the market, forcing milk producers to sell raw milk at a low price to middlemen and depriving them of a reasonable profit. Moreover, lack of handling and processing facilities at the farm level, poor financial support to farmers, and importantly, frequent interruption of the power used for farm processing are major hindrances to processing raw milk. There is a need not only to handle the raw milk (pasteurization), but also to convert it into a highly demandable and value-increased

**Citation:** Husnain, S.N.; Amjad, W.; Munir, A.; Hensel, O. Development and Experimental Study of Smart Solar Assisted Yogurt Processing Unit for Decentralized Dairy Value Chain. *Sustainability* **2022**, *14*, 4285. https://doi.org/10.3390/su14074285

Academic Editor: Andrea Pezzuolo

Received: 15 March 2022 Accepted: 2 April 2022 Published: 4 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

byproduct, i.e., yogurt and milk powder [5]. Although few commercial brands of yogurt are available in Pakistan, their prices are almost double those at the local market. Due to an increase in inflation, the majority of consumers are price conscious and buy yogurt from local shops which are exposed to dust and flies and possess no refrigeration facility, thus compromising on shelf life and the quality of the yogurt.

The yogurt-making process (heating and cooling) from raw milk is an energy-intensive process, which is why dairy is considered the fifth most energy-consuming industry [6]. Major energy demand in the dairy processing sector is still being met using fossil fuels [7] which not only increases operational cost, but also leads to environmental pollution [8]. Therefore, on-farm yogurt processing using renewable energy (solar energy with a potential average value of 5.3 kWh/m2/d) can help to overcome such issues [9]. For solar thermal heating at low–medium temperature range, Michael et al. (2016) [10] highlighted the significance of using flat plate and evacuated tube collectors and reported their contribution as about 30%, while parabolic troughs, dishes and Fresnel collectors contribute 22% of industrial segments. Amjad et al. (2021) [11] reported the use of solar evacuated tube collectors for the decentralized processing (drying) of agricultural produce. A thirty-tube evacuated-tube solar collector was attached with a water–air heat exchanger to warm up the drying medium. It was reported that proper integration of the evacuated tube collector performs well in terms of meeting low temperature demand (up to 100 ◦C). Ismail et al. (2021) [12] also reported the potential of solar thermal applications in the food industry to meet low temperature demand (up to 100 ◦C). In this study, the use of flat plate collectors in the food industry has been estimated to be 38% for pre-heating, pasteurization and cleaning processes. Similarly, in the dairy sector, providing decentralized, energy-efficient and low-cost milk handling and yogurt processing units can play a vital role in uplifting the rural economy through value addition and income generation.

Various studies reported the use of solar energy in the dairy industry. Khawer et al. (2020) [13] developed a solar-based milk chilling system comprising a one-ton vapor compression refrigeration unit and a 2 kW<sup>p</sup> PV system to process 200 L of milk using less than 1 kW power. Desai et al. (2013) [14] conducted a study to highlight the need for solar energy to assist vapor absorption cooling systems in maintaining cold storage conditions for milk handling in India and concluded its high scope in the dairy industry. Mekhilef et al. (2011) [15] comprehensively reviewed the use of solar thermal and PV systems for industrial applications. It is concluded that the greatest efficiency can be achieved through proper systems integration and suitable selection of solar collectors for water heating, solar refrigeration and steam generation. Anderson and Duke (2007) [16] reported the potential of solar thermal applications employing flat plate and evacuated tubes collectors for heating and cooling in the dairy industry. Zahira et al., 2009 [17] investigated the potential of solar energy to pasteurize raw milk at a temperature range of 65 ◦C to 75 ◦C while ambient air temperature was 40 ◦C. It was found that a pasteurizer unit fabricated from shipping cardboard easily attained the required temperature. Atia, 2011 [18] reported the working of a milk pasteurizer connected with a solar flat plate collector. It was found that 73.9 L of raw milk was successfully pasteurized at 63 ◦C, but fluctuation in solar radiation showed direct impact on the performance of the flat plate collector. Similarly, Wayua et al., 2013 [19] developed a milk pasteurizer made of a stainless steel cylindrical vessel having a jacket for the circulation of water being heated with a flat plate collector. Although the capacity of the container was 80 L, it was found that 40 L milk can be pasteurized optimally. In all these reported studies, it can be assessed that use of a flat plate collector cannot provide consistent energy and possess low energy efficiency. For this to get better energy efficiency, Yaseen et al. 2019 [20] reported the use of a vacuum tube collector to pasteurize 200 L milk using steam. Milk was pasteurized at 63 ◦C followed by cooling up to 30 ◦C with tap water which further cooled down to 4 ◦C in a PV-powered rotary compressor chiller. This study reported the use of a vacuum tube collector only for milk pasteurization. A single-glazed flat plate solar collector and water in a glass evacuated tube solar water heater were primarily used [21]. However, the use of solar collectors,

especially evacuated tube collectors (ETC), is limited to water heating only [22,23] and none of the studies reported its use for yogurt fermentation. Moreover, at the industrial scale, for cooling and heating processes involved in yogurt production, the raw milk is transferred in separate containers requiring more infrastructure and clean-in-place (CIP) cost, which could be viable at large scale, but would not be good practice for handling a small quantity of milk (less than 100 L). There is no study about a system capable of performing all the processes required for yogurt making in a single unit/system using solar energy.

Keeping in view the aforementioned facts, in this study a sustainable energy solution for the decentralized handling and processing of raw milk, especially in remote areas of the developing countries where access to electricity is reduced, has been presented. A solar-assisted three-in-one (heating, fermentation and cooling) yogurt processing unit for the value addition of raw milk has been developed. Integration of solar evacuated tube collectors (for heating) and a solar PV system (for cooling) with the yogurt processing unit was evaluated not only to pasteurize raw milk but also to make yogurt in a single unit. The salient feature of the system is the design of a single container capable of performing both heating and cooling processes which not only reduce capital cost but also make it user friendly.

#### **2. Material and Methods**

The complete unit for yogurt processing was developed and fabricated in the Department of Energy Systems Engineering Workshop, Faculty of Agricultural Engineering and Technology at the University of Agriculture Faisalabad (UAF) Pakistan in collaboration with the International Center for Development and Decent Work (ICDD, University of Kassel, Germany and Dairy Industries, Okara-Pakistan).

#### *2.1. System Description*

The design and selection of the yogurt processing unit was largely based on some basic parameters such as energy efficiency and maintenance, and especially on the product life cycle and environmental sustainability. Figure 1 shows a solar-assisted yogurt processing unit designed to process raw milk and its fermentation into yogurt in a timely manner at the production site. It consists of a cylindrically shape fermentation chamber (560 mm diameter and 230 mm depth) made of stainless steel (food grade SS 304) having a capacity of 50 L which is surrounded by a heating coil (3.5 m long, 40 mm wide and 12.5 mm high). The walls and bottom of the fermentation chamber were insulated by 100 mm thick PU (Polyurethane) material, so that heat loss through conduction and convection can be reduced. A variable frequency drive (VFD) electric motor was installed on the top of the chamber to rotate a stirrer for maintaining uniform temperature in the processed product. For cooling purposes, the bottom surface of the chamber was fabricated by a pillow plate which itself acts as an evaporator. The use of a pillow plate heat exchanger not only reduces the size and cost of the unit but also provides a higher heat transfer coefficient in comparison to the conventional coil heat exchanger. For cooling of yogurt, one ton of rotary compressor compatible with R-410A (environmentally friendly) gas was installed employing an inverter kit to reduce torque load to run on 2 kW<sup>p</sup> PV modules.

For the heating of raw milk, the yogurt processing unit was connected with a hot water storage tank (100 L capacity) which receives heat from a solar evacuated tube collector (2.46 m<sup>2</sup> ) having a connection through polyvinyl chloride (PVC) pipe fittings as shown in Figure 2. The outer and inner diameters of the ETC tubes were Ø58 mm ± 0.7 mm and Ø47 mm ± 0.7 mm, respectively. The glass tube length was 1800 mm ± 5 mm and the vacuum was P < 5 <sup>×</sup> <sup>10</sup>−<sup>3</sup> Pa. The thermal energy absorbed by the collector is transferred to 100 L of water in the storage tank to raise the water temperature to 90 ◦C. A centrifugal pump (Wilo-SP106) was installed between the hot water storage tank and the evacuated tube collector for the circulation of propylene glycol solution (50% by volume). The pump can be operated at three variable speeds (600 L/h, 900 L/h and 1100 L/h), and it required 80 W power at maximum speed. The current research was conducted at a flow rate of

600 L/h. Heated glycol solution entered the storage tank and transferred its heat to water while passing through the helix-type heat exchanger present in the storage tank. In order to transfer heat from the storage tank to the yogurt processing unit, another water circulation pump (stainless steel centrifugal, WB50/025D, 50 L/min) was installed between the outlet of the hot water storage tank and the inlet of the yogurt processing unit to circulate the hot water through the square spiral coil heat exchanger to increase the temperature of milk up to 80 ◦C. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 4 of 29

**Figure 1.** Three-dimensional (3D) layout of solar-assisted yogurt processing unit. **Figure 1.** Three-dimensional (3D) layout of solar-assisted yogurt processing unit.

For the heating of raw milk, the yogurt processing unit was connected with a hot

Being a closed cycle, an expansion vessel (12 Liter) was also provided to avoid highpressure build-up in the system. The controller turns on the circulation pump (Wilo-SP106) when the temperature differential between the glycol solution leaving the collector and the water in the lower portion of the storage tank exceeds 5 °C and turns the pump off when the differential is below 5 °C or when the water temperature of the storage tank Being a closed cycle, an expansion vessel (12 Liter) was also provided to avoid highpressure build-up in the system. The controller turns on the circulation pump (Wilo-SP106) when the temperature differential between the glycol solution leaving the collector and the water in the lower portion of the storage tank exceeds 5 ◦C and turns the pump off when the differential is below 5 ◦C or when the water temperature of the storage tank exceeds to 90 ◦C.

processed. Considering a milk capacity of 50 L, the chamber was designed by keeping the diameter to depth ratio in such a way that it could not only get enough exposure to the pillow plate fabricated at the bottom of the chamber but also to increase the surface area of surrounding heating coils. Moreover, the calculation was made for 56.6 L to provide space for the stirrer, stirrer shaft and spacing required during the shaking phenomenon. Normally, semi-circle type chambers are used which are suitable only for the cooling of milk. To perform both heating and cooling in the same chamber, a cylindrical-type chamber was designed having the provision of installing a heating coil around the chamber.

Height =

where *V* is the volume of the chamber (m3), and d is the diameter of the fermentation

The height and diameter of the chamber were calculated to be 230 mm and 560 mm, respectively. A 13.2% provision was given in the total volume of the chamber for air and

The size of a refrigeration system depends on the cooling load of the fermentation chamber, which further depends on the mass of milk or yogurt and the time required to

Heat transfer through the walls and bottom can be calculated using the following

 (πdଶ

4ൗ ) (1)

exceeds to 90 °C.

chamber (m).

equation [24]:

*2.3. Sizing of Refrigeration System* 

reach the chilling temperature (4 to 8 °C).

stirrer.

*2.2. Sizing of Fermentation Chamber* 

#### *2.2. Sizing of Fermentation Chamber*

The size of the fermentation chamber depends on the quantity of the product to be processed. Considering a milk capacity of 50 L, the chamber was designed by keeping the diameter to depth ratio in such a way that it could not only get enough exposure to the pillow plate fabricated at the bottom of the chamber but also to increase the surface area of surrounding heating coils. Moreover, the calculation was made for 56.6 L to provide space for the stirrer, stirrer shaft and spacing required during the shaking phenomenon. Normally, semi-circle type chambers are used which are suitable only for the cooling of milk. To perform both heating and cooling in the same chamber, a cylindrical-type chamber was designed having the provision of installing a heating coil around the chamber.

$$\text{Height} = \frac{V}{\left(\pi \text{d}^2/\text{4}\right)}\tag{1}$$

where *V* is the volume of the chamber (m<sup>3</sup> ), and d is the diameter of the fermentation chamber (m).

The height and diameter of the chamber were calculated to be 230 mm and 560 mm, respectively. A 13.2% provision was given in the total volume of the chamber for air and stirrer.

#### *2.3. Sizing of Refrigeration System*

The size of a refrigeration system depends on the cooling load of the fermentation chamber, which further depends on the mass of milk or yogurt and the time required to reach the chilling temperature (4 to 8 ◦C).

Heat transfer through the walls and bottom can be calculated using the following equation [24]:

$$\mathbf{Q\_{t}} = \frac{\mathbf{U} \mathbf{A} (\mathbf{T\_{h}} - \mathbf{T\_{c}}) \times \mathbf{24}}{1000} \tag{2}$$

where Q<sup>t</sup> stands for the total heat transfer through the walls and bottom (kWh/d), U is the overall heat transfer coefficient (W/m<sup>2</sup> ◦C), A is the heat transfer area (m<sup>2</sup> ), T<sup>h</sup> is the hot face temperature of the fermentation chamber (◦C) and T<sup>c</sup> is the cold face temperature of fermentation chamber (◦C).

Similarly, using the same equation, heat transfer through the walls (Qt1) and heat transfer through the top of the chamber (Qt2) were also calculated to be 0.162 kWh/d and 0.2179 kWh/d using U values of 0.28 and 0.95, respectively.

The cooling load of the product can be calculated by using Equation (3)

$$\mathbf{Q\_R} = \frac{\mathbf{mC\_p\Delta T}}{3600} \tag{3}$$

where Q<sup>R</sup> stands for the cooling load of the product (2.10 kWh/d), m is the mass of the product (kg) and C<sup>p</sup> is the specific heat of the milk (3.89 kJ/kg K). Adding the outcomes from Equations 2 and 3, the total heat load was calculated to be 2.4799 kWh/d.

The required refrigeration capacity was calculated by dividing the total heat load by the specified time of cooling (2 h in this case), and it was found to be 0.709 TR. Considering the 20% factor of safety, the required refrigeration capacity was calculated to be 0.851 TR. Keeping in view other losses and the availability of standard size, one ton of refrigeration system was used for yogurt processing.

#### *2.4. Photovoltaic System Design*

The size of the solar system was selected as per the load requirements of the developed yogurt processing unit, and it can be estimated for any size of the yogurt processing unit. The size of the photovoltaic (PV) system based on the peak power (Pp) in kWp required

to operate the compressor, stirrer motor and water pumps can be estimated by using the following equation [25].

$$\mathbf{P\_P} = \frac{\mathbf{L\_e I\_b}}{\mathbf{H\_{avg}} \eta\_{\rm inv} \ \eta\_{\rm bat} \ \mathbf{T\_{CF}}} \tag{4}$$

where P<sup>p</sup> is the peak power of the solar system (kWp); L<sup>e</sup> is the electric load (kWh/d) which is the product of the power required to run appliances and the time of operation (hours per day); I<sup>b</sup> is the solar irradiance (kW/m<sup>2</sup> ) and 1 is taken as its peak value for calculation; Havg is the average global horizontal irradiance (kWh/(m<sup>2</sup> d)) and its value lies between 5 and 6 kWh/(m<sup>2</sup> d) for Faisalabad, Pakistan; and minimum value taken for calculation, ηinv is the efficiency of the inverter (95–98%); ηbat is the efficiency of battery (85–95%); and TCF is the temperature correction factor obtained by subtracting the product of the loss factor (0.4% per ◦C) and the change in the PV temperature from unity, and for the current study it was 0.92. This means that the power will reduce by 0.4% per degree rise in temperature from its optimum value, i.e., 25 ◦C at standard testing conditions. Solving Equation (4), the peak power was calculated to be almost 2 kWp. So, eight PV panels (polycrystalline, each 250 Wp) were installed with a 3 kW inverter. Here the surge factor has been taken as 1 due to the presence of inverter technology which stats the compressor with zero torque load.

In order to maintain the required load for the cooling system, a battery bank was used to charge and discharge with the varying solar intensity throughout the day. The size of the battery bank was calculated by the following equation [25]

$$\mathbf{C\_{Bat}} = \frac{\mathbf{N\_{ccd}} \mathbf{L\_e}}{\mathbf{D\_d} \eta\_{\text{bat}} \mathbf{V\_{bat}}} \tag{5}$$

where CBat is the capacity of the battery bank (Ah), Nccd is the autonomy and taken as the number of continuous cloudy days, D<sup>d</sup> is the depth of discharge in fraction, and Vbat is the nominal voltage of the battery. All of the specifications of the solar system used in the current study have been tabulated in Table 1.


