# Performance Analysis of the Milk Pasteurization Process Using a Flat Plate Solar Collector

Authors: Surafel Tigabe; Addisu Bekele; Vivek Pandey
Journal: Journal of Engineering
 (2022)

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## Abstract

A large share of Ethiopia’s milk production comes from its rural areas. However, this milk does not undergo further processing into dairy products on a large scale due to lack of conventional energy sources such as electricity and gas. Hence, the country is not utilizing its milk resources to the maximum possible. In this study, the possible use of solar energy for the milk pasteurization process is investigated by using a flat plate solar collector (FPSC) with and without reflectors integrated. Reflectors added to the FPSC have augmented the incident solar radiation on the collector from 0.27–0.91 kW/m2 to 0.36–1.18 kW/m2. The thermal efficiency obtained for the FPSC with reflectors integrated is 51.8% whereas that of the collector without reflectors integrated is 46.2%. Likewise, the exergy efficiency is found to be 5.43% for the collector integrated with reflectors and 2.53% for the collector without reflectors. Also, an increase in the daily processed milk production of 18% is observed in September and 16% in November by using the solar collector with reflectors integrated. The quality of pasteurized milk is checked by using the methylene blue reduction test (MBRT) method, and it indicates a better quality of pasteurized milk when compared with raw milk. Therefore, this study shows that the use of solar energy for milk pasteurization has great potential to increase the shelf life of milk, and it can be used as an instrument to improve the quality of life of people living in rural areas.


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## Body

## 1. Introduction

In Ethiopia, the traditional milk production system accounts for about 97-98% of the total annual milk production [1, 2]. From the estimated amount of collected milk, households consume 85%, while 8% of milk is processed in order to increase the product shelf life and 7% is sold [3]. This indicates that milk productivity and commercialization are low because of the small number of dairy processing units, traditional techniques of milk production in rural areas, and transportation difficulties from production areas to markets.Due to the lack of infrastructure, milk is an important component of the rural economy, but only 7% is marketed out of the rural areas of production [4]. This is the great gap in the country, which results in a shortage in the supply of dairy products to urban areas. Another concern is related to the safety of dairy products concerning foodborne disease, especially in developing countries where the production of milk and various dairy products takes place under hygienically poor production systems [5].The best-known heat treatment in a milk processing plant is pasteurization, which is carried out between 60 and 75°C. Pasteurization extends the shelf life of milk, improves milk quality, and minimizes the risk of milk poisoning [6]. Furthermore, Ethiopia is ranked as one of the top 20 countries with a lack of access to electricity. The majority of people in Ethiopia are living in rural areas, where modern energy services are unavailable [7]. Fortunately, Ethiopia is blessed with a large amount of solar radiation for a maximum number of hours, and this abundant resource can be used as the primary source of energy for heating procedures [8]. Recently, various solar thermal collector designs have been proposed, and these may be classified into two general types based on concentration ratio, namely, flat plate collectors and concentric collectors [9].Mihret et al. [10] reviewed the dairy sector in Ethiopia and identified the causes of its underdeveloped state such as low milk productivity, low milk quality, poor organization of development actions in the subsector and the chain, lack of market-oriented producers, and lack of infrastructures.Broadly, pasteurization can be classified as low-temperature or high-temperature pasteurization methods either by the batch or continuous process. Low-temperature pasteurization aims at killing pathogenic microorganism and reducing spoilage. High-temperature pasteurization aims at killing vegetative pathogenic and spoilage bacteria [11]. During pasteurization, the rate of destruction of pathogens is dependent on both the temperature and time at which milk is held. Pasteurization temperature and holding time recommendations of the U.S. Public Health Service/Food and Drug Administration as per the Standards of Pasteurized Milk Ordinance (PMO) are listed in Table 1.Table 1
Milk pasteurization standards [12].
Temperature (°C)Holding time6330 min7215 s8910 s900.5 s940.1 s960.05 s1001.1 sNielsen and Pedersen [13] designed a control system for solar-based milk pasteurization for villages in Tanzania. They have found that 71°C is the best temperature for pasteurization. The major problem in their system was the attainment of a constant temperature during startup and normal production conditions. Wayua et al. [14] designed and analyzed a flat plate solar milk pasteurizer for arid pastoral areas of Kenya. They could get an optimum quantity of milk after pasteurization at 69.1°C. Atia et al. [15] designed and analyzed a pasteurization system using a flat plate collector in remote areas and village communities who are deprived of the availability of electricity and gas. In their study, liquid milk was pasteurized by using a 1.2 m2 flat plate solar collector. Franco et al. [16] fabricated a low-cost solar concentrator without sun-tracking mechanism for pasteurizing goat milk for cheese production. The system used a Fresnel-type concentrator with a focal distance of 20 cm to reduce sensitivity in sunlight tracking. This device had no automatic tracking system. Atia et al. [17] fabricated a solar milk pasteurization device with a dual-axis, closed-loop control, and sun-tracking device. The device operation was based on the contrast of sunlight levels using light-dependent resistors (LDRs) and an op-amp chip. Degefa [18] studied the performance analysis of a solar energy system for small-scale milk pasteurization with a coil heat exchanger fully immersed within a milk storage tank. Mallett [19] developed a low-cost parabolic solar reflector unit for the pasteurization of raw milk in rural areas of Southern Africa for those lacking adequate infrastructure, including electricity. The parabolic trough system was tested by heating water up to the necessary pasteurization temperatures. In each of the receiver tubes, solar absorber coatings were used, and temperatures above 75°C were achieved with all of them. The system was then tested using milk, and the same temperature ranges were obtained. This allowed milk to be successfully pasteurized.The performances of most FPSC devices are based on energy analysis which essentially accounts only for energy entering and exiting the system. An additional analysis, which is called exergy analysis, can identify the cause, location, and magnitude of system inefficiencies and gives a better measure of performance in terms of quality of energy. Therefore, it is essential to consider both energy and exergy analysis for estimating the thermal performance and optimizing the working parameters of the FPSC [20].The use of solar energy paired with pasteurization of milk may be a good alternative to solve the energy problem caused by the scarcity of fuel or electricity and to ensure a healthy milk supply. However, there is no experimental study conducted yet on the use of milk pasteurization using solar energy in Ethiopia. Likewise, improving the performance of the flat plate solar collector for milk pasteurization done by previous researchers using various techniques is very vital for implementing the technology in the rural areas of Ethiopia.Hence, the aim of this study is to develop a prototype of a milk pasteurization system using a flat plate solar collector and analyze its performance in detail. This study will benefit the community by decreasing the wastage of milk production in rural areas, increasing the shelf life of milk, and increasing the marketing potential and hence improve the economy of the farmers.


## 2. Methodology

This study was conducted at Gondar, Ethiopia, located at 12.6° N latitude and 34.446°E longitude in the northern hemisphere with an elevation of 2133 meters above sea level. The climate is mild and generally warm temperate. The average maximum ambient temperature is 29.7°C and the average minimum ambient temperature is 14.3°C.In this study, both theoretical and experimental approaches are used. The theoretical approach applies governing equations for the analysis of each component to study the overall system performance of milk pasteurization by using FPSC. The prototype of the collector is developed after completing the sizing and selection of materials for each component. For designing the FPSC, the solar potential of Gondar is predicated based on the sunshine hours data collected from the Ethiopian Meteorological Agency (EMA). Finally, the theoretical and numerical formulations leading to the energy and exergy efficiencies of the collector are analyzed.Several methods relate the global radiation to climatic conditions such as sunshine hours, temperature, humidity, cloudiness, and precipitation [21]. Among these, sunshine hour is the most widely and commonly used parameter for study. The first empirical method for estimating global radiation based on sunshine duration is the model presented by Prescott [22]. They calculated the monthly average global radiation based on extraterrestrial radiation on a horizontal surface. Figure 1 shows the values of five-year average sunshine hours (2015–2019), accessed from the Ethiopian Meteorological Agency (EMA).Figure 1
Monthly variation of sunshine hours.The detail specifications of the FPSC used in the experimental setup are summarized in Table2.Table 2
Specifications of FPSC.
SpecificationsValueGross area1.19 m2Area of the collector1.18 m2Length of the tube11 mOuter diameter of the tube12 mmThickness of the tube0.5 mmLength of the absorber plate1.28 mWidth of absorber plate0.92 mThickness of the glass4 mmInsulation thickness5 mmAir gap between the plate and glass30 mm


### 2.1. Energy Analysis of the Flat Plate Solar Collector

The performance of a solar collector is identified using energy balance that indicates the distribution of incident solar energy into useful energy gain, thermal loss, and optical loss. An essential notation for energy analysis of any thermal system is only related to conservation of energy, which can be evaluated through energy balance calculation under steady state condition.The useful energy gain in terms of the fluid inlet and outlet temperature is determined using the following expression:(1)Qu=m˙cpTo−Ti.The useful heat gain by the working fluid from the collectorQuc can also be analyzed in terms of the fluid inlet temperature Tin as(2)Quc=AcFRS−UlTin−Ta,where FR represents the collector heat removal factor, S=ταIT represents the absorbed solar radiation per unit time, τ represents the transmissivity of the glass cover, α represents the absorptivity of the absorber plate, and IT represents the incident solar energy per unit area.The heat removal factor is the ratio of useful energy gained by the collector to the energy gained by the collector where it is entirely at the fluid inlet temperature. Determining the heat removal factor for a serpentine collector is more difficult than for a conventional header-riser flat plate collector because they have different performance in the internal heat transfer coefficient. Unlike the analysis of header-riser tube configuration of a flat plate collector where the fins between the tubes are assumed adiabatic at the center of the tube spacing, there is heat transfer between the tubes for a serpentine collector.There are many formulations and equations developed and modified for serpentine flows, and the last modified equation is used for heat removal factor calculations with serpentines of arbitrary geometry, and the number of bends is given by [23](3)FR=m˙CpAcUl1−expAcUlF1F2−1m˙Cp,where F1 and F2 are the parameters for determining collector heat removal factor which is a function of physical design parameters [23]:(4)F1=NκLUlAcκR1+γ2−1−γ−κRκR1+γ−12−κR2,F2=1κR1+γ2−1−γ−κR,κ=KδmW−Disinhm,γ=−2coshm−DUlκ,m=W−DUlKδ,m=W−DUlKδ,where Di represents the tube inner diameter, and δ represents the plate thickness.The thermal efficiencyηth of the solar collector is the ratio of the output useful energy to the absorbed solar energy, and it is expressed as(5)ηth=QuAcταIT.The overall heat lost coefficient from the collector is the sum of the heat lost from the top, the bottom, and the side heat losses, and it is expressed as follows:(6)Ul=Ut+Ub+Us,where Ut,UbandUs are the heat lost coefficients from the top, bottom, and side, respectively.The top loss coefficient is expressed empirically as the following equation:(7)Ut=Upg+Uga.The heat transfer coefficient from the absorber plate to the glass coverUpg includes the thermal energy loss by convection and radiation.The convective heat transfer coefficienth1c of the top heat loss from the collector plate to the cover which is tilted at some angles is related to three dimensionless parameters, Nusselt number, Nu, the Rayleigh number, Ra, and the Prandtl number, Pr, expressed as follows:(8)h1c=NuKL,Nu=1+1.441−1708Racosβ+1−1708sin1.8β1.6Racosβ+Racosβ58301/3+,where “+” exponent means only positive value in square bracket is considered and zero is to be used for negative value.The Rayleigh number is expressed as(9)Ra=gβ′ΔTL3Prυ2.The volumetric expansion coefficientβ′ is given by(10)1β′=Tp+Tg2,where K represents the is plate thermal conductivity, g represents the gravitational constant, L represents the length between plate and glass, ΔT is the temperature difference between absorber plate and the glass cover, and υ represents the kinematic viscosity of air.The radiation heat transfer coefficienth1r from the absorber plate to the glass cover is given by(11)h1r=εeffσTp+2734−Tg+2734Tp−Tg.The effective emissivityεeff of the plate-glazing system is given by(12)εeff=1εp+1εg−1−1.Hence, the total heat transfer coefficient from the absorber plate to the glass coverUpg is simplified as(13)Upg=NuKL+εeffσTp+2734−Tg+2734Tp−Tg.The heat transfer coefficient from the glass cover to the ambientUga includes the thermal energy loss by convection and radiation.The convective heat transfer coefficienth2c from the glass cover to the ambient is calculated by(14)h2c=2.8+3Vw,where Vw is the wind speed.The radiation heat transfer coefficienth2r from the glass cover to the ambient is calculated from the following equation:(15)h2r=εgσTg+2734−Tsky+2734Tg−Ta.The effective temperature of the skyTsky is given as(16)Tsky=Ta−6.Hence, the total heat transfer coefficient from the glass cover to ambientUga is simplified as(17)Uga=2.8+3Vw+εgσTg+2734−Tsky+2734Tg−Ta.The bottom loss coefficientUb from the absorber plate to the ambient in the downward direction is by conduction through the insulation and convection through the bottom of the collector. It is assumed that the flow of heat is one dimensional and steady state condition, and the thickness of insulation provided that the thermal resistance associated with conduction dominates.Therefore, by neglecting convective resistance at the bottom surface, the bottom loss coefficient can be expressed as(18)Ub=Kiδb,where Ki represents the thermal conductivity of insulation, and δb represents the back insulation thickness.The heat loss coefficientUs at the side of the collector is expressed as(19)Us=KiδeAsAc,where δe represents the edge insulation thickness, As-p∗de, P represents the perimeter of the absorber plate, and de represents the height of the edge.


### 2.2. Exergy Analysis of the Flat Plate Solar Collector

Exergy is defined as the maximum output of work that can be achieved relative to the environment, which is based on the second law of thermodynamics. The exergy analysis to the flat plate collector helps to achieve an optimum design and gives the direction to decrease exergy loss.Considering a flat plate collector exergy analysis is performed based on the general form of exergy equation which is expressed as follows:(21)Exin−Exout=Exd+Exl,where Exin,Exout,Exl,andExd are the inlet, outlet, leakage (loss), and the destroyed exergy rates, respectively.The inlet exergy rateExinf due to the fluid flow can be calculated by(22)Exinf=m˙CpTi−Ta−TalnTiTa.The inlet exergy due to radiationExinr can be calculated by(23)Exinr=ITA1−TaTs,where Ts is the surface temperature of sun which is equal to 5777 K.Therefore, the total inlet exergy rate of the solar collector can be calculated as follows:(24)Exin=Exinf+Exinr.The outlet exergy accounts for the exergy rate of outlet fluid which can be calculated as follows:(25)Exoutf=−mCpTo−Ta−TalnToTa.The destructed exergy compromises of three terms. The first term is caused due to the heat leakage from absorber plate to the ambient, and it is calculated as(26)Edleak=ApUgaTp−Ta1−TaTp.The second part is caused by the temperature difference between the absorber plate and the sun which is calculated by(27)Edp−s=ITAcη0TaTp−TaTs,where Ts is the apparent sun temperature as exergy source which is assumed to be 5700 K.The third part is caused by the temperature difference between the absorber plate and the working fluid, which is calculated by(28)Edp−f=mCpTalnT0TiT0−TiTp.Therefore, the total distracted energy is calculated by(29)Ed=Edp−s+Edp−f+Edleak.The exergy efficiency is the ratio of the increase in working fluid flow exergy to the inlet exergy by solar radiation. Hence, the exergy efficiency can be evaluated as(30)ηex=Exoutf−ExinfExinr.


## 2.1. Energy Analysis of the Flat Plate Solar Collector

The performance of a solar collector is identified using energy balance that indicates the distribution of incident solar energy into useful energy gain, thermal loss, and optical loss. An essential notation for energy analysis of any thermal system is only related to conservation of energy, which can be evaluated through energy balance calculation under steady state condition.The useful energy gain in terms of the fluid inlet and outlet temperature is determined using the following expression:(1)Qu=m˙cpTo−Ti.The useful heat gain by the working fluid from the collectorQuc can also be analyzed in terms of the fluid inlet temperature Tin as(2)Quc=AcFRS−UlTin−Ta,where FR represents the collector heat removal factor, S=ταIT represents the absorbed solar radiation per unit time, τ represents the transmissivity of the glass cover, α represents the absorptivity of the absorber plate, and IT represents the incident solar energy per unit area.The heat removal factor is the ratio of useful energy gained by the collector to the energy gained by the collector where it is entirely at the fluid inlet temperature. Determining the heat removal factor for a serpentine collector is more difficult than for a conventional header-riser flat plate collector because they have different performance in the internal heat transfer coefficient. Unlike the analysis of header-riser tube configuration of a flat plate collector where the fins between the tubes are assumed adiabatic at the center of the tube spacing, there is heat transfer between the tubes for a serpentine collector.There are many formulations and equations developed and modified for serpentine flows, and the last modified equation is used for heat removal factor calculations with serpentines of arbitrary geometry, and the number of bends is given by [23](3)FR=m˙CpAcUl1−expAcUlF1F2−1m˙Cp,where F1 and F2 are the parameters for determining collector heat removal factor which is a function of physical design parameters [23]:(4)F1=NκLUlAcκR1+γ2−1−γ−κRκR1+γ−12−κR2,F2=1κR1+γ2−1−γ−κR,κ=KδmW−Disinhm,γ=−2coshm−DUlκ,m=W−DUlKδ,m=W−DUlKδ,where Di represents the tube inner diameter, and δ represents the plate thickness.The thermal efficiencyηth of the solar collector is the ratio of the output useful energy to the absorbed solar energy, and it is expressed as(5)ηth=QuAcταIT.The overall heat lost coefficient from the collector is the sum of the heat lost from the top, the bottom, and the side heat losses, and it is expressed as follows:(6)Ul=Ut+Ub+Us,where Ut,UbandUs are the heat lost coefficients from the top, bottom, and side, respectively.The top loss coefficient is expressed empirically as the following equation:(7)Ut=Upg+Uga.The heat transfer coefficient from the absorber plate to the glass coverUpg includes the thermal energy loss by convection and radiation.The convective heat transfer coefficienth1c of the top heat loss from the collector plate to the cover which is tilted at some angles is related to three dimensionless parameters, Nusselt number, Nu, the Rayleigh number, Ra, and the Prandtl number, Pr, expressed as follows:(8)h1c=NuKL,Nu=1+1.441−1708Racosβ+1−1708sin1.8β1.6Racosβ+Racosβ58301/3+,where “+” exponent means only positive value in square bracket is considered and zero is to be used for negative value.The Rayleigh number is expressed as(9)Ra=gβ′ΔTL3Prυ2.The volumetric expansion coefficientβ′ is given by(10)1β′=Tp+Tg2,where K represents the is plate thermal conductivity, g represents the gravitational constant, L represents the length between plate and glass, ΔT is the temperature difference between absorber plate and the glass cover, and υ represents the kinematic viscosity of air.The radiation heat transfer coefficienth1r from the absorber plate to the glass cover is given by(11)h1r=εeffσTp+2734−Tg+2734Tp−Tg.The effective emissivityεeff of the plate-glazing system is given by(12)εeff=1εp+1εg−1−1.Hence, the total heat transfer coefficient from the absorber plate to the glass coverUpg is simplified as(13)Upg=NuKL+εeffσTp+2734−Tg+2734Tp−Tg.The heat transfer coefficient from the glass cover to the ambientUga includes the thermal energy loss by convection and radiation.The convective heat transfer coefficienth2c from the glass cover to the ambient is calculated by(14)h2c=2.8+3Vw,where Vw is the wind speed.The radiation heat transfer coefficienth2r from the glass cover to the ambient is calculated from the following equation:(15)h2r=εgσTg+2734−Tsky+2734Tg−Ta.The effective temperature of the skyTsky is given as(16)Tsky=Ta−6.Hence, the total heat transfer coefficient from the glass cover to ambientUga is simplified as(17)Uga=2.8+3Vw+εgσTg+2734−Tsky+2734Tg−Ta.The bottom loss coefficientUb from the absorber plate to the ambient in the downward direction is by conduction through the insulation and convection through the bottom of the collector. It is assumed that the flow of heat is one dimensional and steady state condition, and the thickness of insulation provided that the thermal resistance associated with conduction dominates.Therefore, by neglecting convective resistance at the bottom surface, the bottom loss coefficient can be expressed as(18)Ub=Kiδb,where Ki represents the thermal conductivity of insulation, and δb represents the back insulation thickness.The heat loss coefficientUs at the side of the collector is expressed as(19)Us=KiδeAsAc,where δe represents the edge insulation thickness, As-p∗de, P represents the perimeter of the absorber plate, and de represents the height of the edge.


## 2.2. Exergy Analysis of the Flat Plate Solar Collector

Exergy is defined as the maximum output of work that can be achieved relative to the environment, which is based on the second law of thermodynamics. The exergy analysis to the flat plate collector helps to achieve an optimum design and gives the direction to decrease exergy loss.Considering a flat plate collector exergy analysis is performed based on the general form of exergy equation which is expressed as follows:(21)Exin−Exout=Exd+Exl,where Exin,Exout,Exl,andExd are the inlet, outlet, leakage (loss), and the destroyed exergy rates, respectively.The inlet exergy rateExinf due to the fluid flow can be calculated by(22)Exinf=m˙CpTi−Ta−TalnTiTa.The inlet exergy due to radiationExinr can be calculated by(23)Exinr=ITA1−TaTs,where Ts is the surface temperature of sun which is equal to 5777 K.Therefore, the total inlet exergy rate of the solar collector can be calculated as follows:(24)Exin=Exinf+Exinr.The outlet exergy accounts for the exergy rate of outlet fluid which can be calculated as follows:(25)Exoutf=−mCpTo−Ta−TalnToTa.The destructed exergy compromises of three terms. The first term is caused due to the heat leakage from absorber plate to the ambient, and it is calculated as(26)Edleak=ApUgaTp−Ta1−TaTp.The second part is caused by the temperature difference between the absorber plate and the sun which is calculated by(27)Edp−s=ITAcη0TaTp−TaTs,where Ts is the apparent sun temperature as exergy source which is assumed to be 5700 K.The third part is caused by the temperature difference between the absorber plate and the working fluid, which is calculated by(28)Edp−f=mCpTalnT0TiT0−TiTp.Therefore, the total distracted energy is calculated by(29)Ed=Edp−s+Edp−f+Edleak.The exergy efficiency is the ratio of the increase in working fluid flow exergy to the inlet exergy by solar radiation. Hence, the exergy efficiency can be evaluated as(30)ηex=Exoutf−ExinfExinr.


## 3. Experimental Setup

The experimental setup was constructed and tested at Gondar city of Ethiopia (12.6°N latitude and 34.446°E longitude). All the experiments are performed during the months of September and November, between 8 a.m and 4 p.m. The FPSC consists of a single glass cover, absorber plate, insulation, serpentine shaped copper tube, controlling valve, and fresh milk tank. It is mounted on a wooden frame built to support the collector at the optimum tilt angle (β) of 27.6°. The photographs of the experimental setup including FPSC without and with reflectors integrated and schematic diagram are shown in Figures 2(a), 2(b), and 2(c), respectively.Figure 2
Experimental setup: (a) FPSC without reflector, (b) FPSC with reflector integrated, and (c) schematic of FPSC with reflector integrated.
(a)(b)(c)A digital thermometer with a range of −55°C to + 148°C (Figure3(a)) is used to measure the ambient temperature, absorber plate temperature, glass temperature, and the inlet and outlet milk temperatures. For controlling the outlet temperature of milk, a 12 V relay dual LED digital thermostat temperature controller is installed in the outlet pipe near to the outlet controlling valve as shown in Figure 3(b), and the flow of milk outlet is controlled by an opening valve.Figure 3
(a) Digital thermometer. (b) 12 V relay dual LED digital thermostat temperature controller.
(a)(b)


## 4. Experimental Procedure

The flat plate solar collector was designed for milk pasteurization at a constant milk flow rate of 0.01 kg/s. The first experiment was conducted to study the performance of solar thermal collectors with and without reflectors by assuming that heat transfer and fluid flow rate are in steady state conditions. The second experiment is performed to ascertain the production rate of pasteurized milk by retaining the milk flow in the collector for the recommended pasteurization time at a given temperature.The quantity of pasteurized milk for given a pasteurization time and temperature can be calculated by(31)qb=V∗t,where qb is the quantity of milk per batch, and V is the volume flow rate of milk.The daily quantity of pasteurized milk per dayqday can be calculated by(32)qday=∑i=1nqbi.


### 4.1. Microbiological Quality Test of Raw and Pasteurized Milk

In order to determine the microbiological quality of raw and pasteurized milk, there are several tests commonly used, including the standard plate count (SPC), psychotropic bacteria count (PBC), coliform count (CC), somatic cell count (SCC), and methylene blue reduction test (MBRT) [24]. Among these, MBRT was selected to assess the microbiological quality of pasteurized and raw milk due to its rapid, simple, and inexpensive methodology. The main principle of MBRT is to determine the quality of milk based on observing the change in color of milk by adding methylene blue. The color disappears more or less quickly [25]. Methylene blue is a blue colored redox indicator dye, and it loses its color when in contact with oxygen and harmful substances from microbial metabolism within milk [26]. The rate of reduction of methylene blue is, therefore, directly related to the bacterial load in milk. The grading of milk as per MBRT is given in Table 3 [27].Table 3
Grading of milk on the basis of MBRT.
Quality of milkDecolourization timeExcellentMore than 8 hoursGoodBetween 6 and 8 hoursFairBetween 2 and 6 hoursPoorLess than 2 hoursFor the quantitative MBRT, 10 mL of raw milk sample is incubated at 37°C in a waterbath with 0.5 mL of methylene blue, as shown in Figure4. The time (in hours) for the blue coloration to disappear is measured according to the technique described in ISO 4833-2:2013 [28].Figure 4
(a) Milk samples (raw milk, heat-treated milk, and pasteurized milk). (b) Materials and equipment used for MBRT.
(a)(b)


## 4.1. Microbiological Quality Test of Raw and Pasteurized Milk

In order to determine the microbiological quality of raw and pasteurized milk, there are several tests commonly used, including the standard plate count (SPC), psychotropic bacteria count (PBC), coliform count (CC), somatic cell count (SCC), and methylene blue reduction test (MBRT) [24]. Among these, MBRT was selected to assess the microbiological quality of pasteurized and raw milk due to its rapid, simple, and inexpensive methodology. The main principle of MBRT is to determine the quality of milk based on observing the change in color of milk by adding methylene blue. The color disappears more or less quickly [25]. Methylene blue is a blue colored redox indicator dye, and it loses its color when in contact with oxygen and harmful substances from microbial metabolism within milk [26]. The rate of reduction of methylene blue is, therefore, directly related to the bacterial load in milk. The grading of milk as per MBRT is given in Table 3 [27].Table 3
Grading of milk on the basis of MBRT.
Quality of milkDecolourization timeExcellentMore than 8 hoursGoodBetween 6 and 8 hoursFairBetween 2 and 6 hoursPoorLess than 2 hoursFor the quantitative MBRT, 10 mL of raw milk sample is incubated at 37°C in a waterbath with 0.5 mL of methylene blue, as shown in Figure4. The time (in hours) for the blue coloration to disappear is measured according to the technique described in ISO 4833-2:2013 [28].Figure 4
(a) Milk samples (raw milk, heat-treated milk, and pasteurized milk). (b) Materials and equipment used for MBRT.
(a)(b)


## 5. Results and Discussion

The ambient air temperature, inlet temperature, and outlet temperature of milk are recorded from 8:00 hr to 16:00 hr. The estimated hourly variation of global, beam and diffuse radiation, and radiation falling on the surface of the collector with and without reflectors, which is tilted at an angle of 27.6° during the months of September and November, is shown in Figure5.Figure 5
Estimated hourly variation of global solar radiation in Gondar on the month of September and November.


### 5.1. Hourly Variation of Measured Temperature Difference

The hourly variations of the inlet temperature of milk, outlet temperature of milk, and ambient temperature for a collector without and with reflectors are shown in Figures6 and 7, respectively. It is observed that the outlet temperature of milk increases in proportion to the incident solar radiation intensity. The maximum outlet temperature for a collector without reflectors is 44.9°C and 49.4°C in September and November, respectively. Similarly, the maximum outlet temperature for a collector with a reflector is 49.2°C and 52.2°C in September and November, respectively. This shows that the outlet temperature of milk increases with an increase in solar radiation. Therefore, addition of the reflectors to the solar collectors aids in increasing the solar intensity and, thereby, the outlet temperature of the milk.Figure 6
Hourly variation of ambient temperature, milk inlet temperature, and milk outlet temperature for a collector without reflector.Figure 7
Hourly variation of ambient temperature, milk inlet temperature, and milk outlet temperature for a collector with reflector.


### 5.2. Hourly Useful Heat Gain

The temporal variations of useful heat gain of the solar collector with and without reflectors integrated are shown in Figure8. The useful heat gain of the collector increases in proportion to the solar radiation intensity. The maximum useful heat gain obtained from FPSC integrated with reflectors is 750.63 W in September and 809.14 W in November. The minimum useful heat gain obtained from FPSC without reflectors integrated is 581.64 W in September and 679.89 W in November. Therefore, the useful heat gain achieved by FPSC with reflectors integrated is 29% higher for the month of September and 19% higher for the month of November when compared with that of FPSC without reflectors integrated.Figure 8
Variation of useful heat gain of the collector with time.


### 5.3. Collector Heat Loss Coefficient

Figure9 shows the effects of the difference in collector plate temperature and ambient temperature on the collector heat loss coefficient. It is observed that the overall heat loss coefficient increases when the inlet temperature increases and also as the temperature gradient between the absorber plate and the ambient increases. The heat loss coefficient of the collector varies from 6.68 to 6.94 W/km2 when the temperature difference is 15–55°C. The average heat loss coefficient of the collector is 6.85 W/km2 and 6.91 W/km2 for FPSC integrated with and without reflectors, respectively.Figure 9
Effects of the temperature difference between the absorber plate and ambient on the overall heat loss coefficient of the collector.


### 5.4. Thermal Efficiency of the Collector

Figure10 shows the hourly variation of the thermal efficiency of the FPSC integrated with and without reflectors. The thermal efficiency is directly proportional to the outlet temperature of milk. The results show that the outlet temperature of milk starts increasing from 08:00 hr and attains maximum at 13:00 hr. Therefore, the maximum thermal efficiency occurs at around noon when the outlet temperature and solar radiation are at their peak. The maximum thermal efficiency is 51.8% for FPSC integrated with reflectors and 46.2% for FPSC without reflectors in November. These results show that integrating reflectors into FPSC increases the thermal efficiency of the collector by 5%. Atia et al. [22] reported similar results for pasteurization of milk using a FPSC.Figure 10
Hourly variation of thermal efficiency with and without reflectors.


### 5.5. Exergy Analysis

In order to make a detailed analysis of the exergetic efficiency of the collector, the leakage exergy and destructed exergy owing to the collector components are taken into consideration. As shown in Figures11 and 12 with an increase in the ambient temperature, the exergy also increases. All maximum values were found to occur in November. In September, the inlet exergy of the working fluid and absorbed solar radiation are in the range of 1023–386 W and 458–1094 W for the FPSC integrated with and without reflectors, respectively. The outlet exergy of the working fluid is between 3.52–22.8 W and 2.94–18.25 W for the FPSC integrated with and without reflectors, respectively. The destructed exergy is between 272–777.14 W and 245.64–914.2 W for the FPSC integrated with and without the reflectors, respectively. All maximum exergy values occur at midday when the solar radiation value is at its peak. In November, the inlet exergy of the working fluid and absorbed solar radiation is in the range 1154–431 W and 1023–409.21 W for the FPSC integrated with and without reflectors, respectively. The outlet exergy value is between 4.32–20.4 W and 5.36–26.8 W for the FPSC integrated with and without reflectors. The destructed exergy is between 283.14–784.14 W and 256.64–916.2 W for the FPSC integrated with and without reflectors, respectively.Figure 11
Exergy of the collector without and with reflector based on time of the day in September.Figure 12
Exergy of the collector without and with reflector based on time of the day in November.


### 5.6. Comparison of Destructed Exergy Rate with and without Reflectors

The destructed exergy rate due to heat transfer from the absorber plate to the environment increases with an increase in the ambient and plate temperatures. As shown in Figure13, as the ambient temperature increases, the exergy leakage increases, but the increment in ambient temperature is very slow compared to the plate temperature. Therefore, with equivalent ambient temperature variation, the collector integrated with reflectors has a high leakage exergy rate compared to the collector without reflectors. This is due to high plate temperature of the collector when integrated with reflectors.Figure 13
Variation of exergy loss related to ambient temperature for FPSC integrated with and without reflectors.The exergy loss rate due to the temperature difference between solar radiation and absorber plate increases with the increment of ambient temperature and incident solar radiation. The increasing tendency of absorber plate temperature has a great effect on exergy destruction. As shown in Figure14, the value of destructed exergy due to the temperature difference between solar radiation and absorber plate is small in the collector integrated with reflectors compared to the collector without reflectors. Therefore, as shown in Figure 14, the largest exergy loss is caused by the temperature difference between the absorber plate and the sun, which is about 89.21% and 89.08% of the total inlet exergy rate for FPSC integrated with and without reflectors, respectively.Figure 14
Comparison of exergy destruction (a) with and (b) without reflectors.
(a)(b)It is observed from the result that the increasing absorber plate temperature effect is higher on the collector integrated with reflectors. This shows that the destructed exergy is more affected by solar radiation when the ambient temperature is constant as shown in Figure15.Figure 15
Variation of destructed exergy caused by temperature difference between sun and absorber plate versus time.The overall heat loss coefficient and exergy destruction increase as the temperature difference between the collector plate and ambient temperature increases as shown in Figure16.Figure 16
Effects of the temperature difference between the absorber plate and environment on overall heat loss coefficients and exergy destructions for the collector.


### 5.7. Exergetic Efficiency

Figure17 shows the variation of exergy efficiency versus time. The results indicate that the exergy efficiency of the collector without reflectors starts increasing from 0.17% at 08:00 hr and reaches a maximum value of 2.53% at 12:00 hr, and thereafter, it starts decreasing as the solar radiation decreases. Additionally, the exergy efficiency of the collector starts increasing from 0.49% and reaches a maximum value of 4.71% in September. Similarly, in November, the exergy efficiency reaches a maximum of 5.43%, starting from 0.52%. This shows that with an increase in solar radiation, there is an increase in the heat gain and exergy efficiency.Figure 17
The variation of exergy efficiency versus time.


### 5.8. Time to Attain Pasteurization Temperature

Figure18 shows the time to reach pasteurization temperature (72°C) between 09:00 hr and 16:00 hr. The result shows that the collector with a reflector needs a shorter time to attain pasteurization temperature compared to the collector without reflectors. The minimum time to attain pasteurization temperature is at noon in September, which is 4.2 min and 6 min for FPSC integrated with and without reflectors, respectively. Also, in November, it is 3.9 min and 5.72 min for FPSC integrated with and without reflectors, respectively.Figure 18
Time to attain pasteurization temperature.


### 5.9. Quantity of Solar-Pasteurized Milk

Figure19 shows the quantity of pasteurized milk with respect to time of day. The maximum quantity of pasteurized milk per day is achieved at noon due to peak solar radiation at noon. The daily pasteurized milk in September is 69.19 l/day and 58.59 l/day, and in November, it is 70.2 l/day and 60.24 l/day for FPSC integrated with and without the reflectors, respectively. Therefore, by adding the reflectors to the collector, the quantity of pasteurized milk increases by 9 l/day.Figure 19
The quantity of pasteurized milk with respect to the time of the day.As shown in Table4, due to the addition of reflectors on both sides of the collectors, the thermal efficiency and the quantity of pasteurized milk is higher by 6.9% and 9%, respectively, compared to the results of Atia et al. [17].Table 4
Comparison of thermal efficiency and quantity of pasteurized milk (liter per day) with the present study with literature for FPSC without reflectors.
ParameterPresent studyAtia et al. [22]Maximumηth46.2%43%Quantity of pasteurization per day60.24 l/day55.6 l/day


### 5.10. Detection of Raw Milk, Heat-Treated Milk, and Pasteurized Milk Using the Methylene Blue Reduction Test (MBRT)

The quality of milk is tested using MBRT, and the time required for methylene blue to become colorless is termed as methylene blue reduction time, which is an indicator of the milk quality. A smaller value of methylene blue reduction time implies a lower quality of milk and vice versa. Figure20 shows that the raw milk starts changing its color after 30 min from the base time, and this indicates the quality of raw milk is very poor. The grading of milk samples with reduction time on the basis of MBRT is shown in Table 5.Figure 20
Result of MBRT after 30 min.Table 5
Decolorization time and grading of milk samples.
Milk samplesDecolorization time (h)Quality of milkRaw milk1.10PoorHeat-treated milk4.30FairPasteurized milk6.25GoodThe result shows that the methylene blue reduction time for raw milk is 1.10 h, which means that the quality is poor. However, after heating milk at pasteurization temperature, the methylene blue reduction time is increased to 4.30 h and the quality of milk is fair. Finally, the pasteurized milk (after heat treating milk and storing it in a refrigerator at 4°C) shows good quality due to destruction of contaminating microorganisms.


## 5.1. Hourly Variation of Measured Temperature Difference

The hourly variations of the inlet temperature of milk, outlet temperature of milk, and ambient temperature for a collector without and with reflectors are shown in Figures6 and 7, respectively. It is observed that the outlet temperature of milk increases in proportion to the incident solar radiation intensity. The maximum outlet temperature for a collector without reflectors is 44.9°C and 49.4°C in September and November, respectively. Similarly, the maximum outlet temperature for a collector with a reflector is 49.2°C and 52.2°C in September and November, respectively. This shows that the outlet temperature of milk increases with an increase in solar radiation. Therefore, addition of the reflectors to the solar collectors aids in increasing the solar intensity and, thereby, the outlet temperature of the milk.Figure 6
Hourly variation of ambient temperature, milk inlet temperature, and milk outlet temperature for a collector without reflector.Figure 7
Hourly variation of ambient temperature, milk inlet temperature, and milk outlet temperature for a collector with reflector.


## 5.2. Hourly Useful Heat Gain

The temporal variations of useful heat gain of the solar collector with and without reflectors integrated are shown in Figure8. The useful heat gain of the collector increases in proportion to the solar radiation intensity. The maximum useful heat gain obtained from FPSC integrated with reflectors is 750.63 W in September and 809.14 W in November. The minimum useful heat gain obtained from FPSC without reflectors integrated is 581.64 W in September and 679.89 W in November. Therefore, the useful heat gain achieved by FPSC with reflectors integrated is 29% higher for the month of September and 19% higher for the month of November when compared with that of FPSC without reflectors integrated.Figure 8
Variation of useful heat gain of the collector with time.


## 5.3. Collector Heat Loss Coefficient

Figure9 shows the effects of the difference in collector plate temperature and ambient temperature on the collector heat loss coefficient. It is observed that the overall heat loss coefficient increases when the inlet temperature increases and also as the temperature gradient between the absorber plate and the ambient increases. The heat loss coefficient of the collector varies from 6.68 to 6.94 W/km2 when the temperature difference is 15–55°C. The average heat loss coefficient of the collector is 6.85 W/km2 and 6.91 W/km2 for FPSC integrated with and without reflectors, respectively.Figure 9
Effects of the temperature difference between the absorber plate and ambient on the overall heat loss coefficient of the collector.


## 5.4. Thermal Efficiency of the Collector

Figure10 shows the hourly variation of the thermal efficiency of the FPSC integrated with and without reflectors. The thermal efficiency is directly proportional to the outlet temperature of milk. The results show that the outlet temperature of milk starts increasing from 08:00 hr and attains maximum at 13:00 hr. Therefore, the maximum thermal efficiency occurs at around noon when the outlet temperature and solar radiation are at their peak. The maximum thermal efficiency is 51.8% for FPSC integrated with reflectors and 46.2% for FPSC without reflectors in November. These results show that integrating reflectors into FPSC increases the thermal efficiency of the collector by 5%. Atia et al. [22] reported similar results for pasteurization of milk using a FPSC.Figure 10
Hourly variation of thermal efficiency with and without reflectors.


## 5.5. Exergy Analysis

In order to make a detailed analysis of the exergetic efficiency of the collector, the leakage exergy and destructed exergy owing to the collector components are taken into consideration. As shown in Figures11 and 12 with an increase in the ambient temperature, the exergy also increases. All maximum values were found to occur in November. In September, the inlet exergy of the working fluid and absorbed solar radiation are in the range of 1023–386 W and 458–1094 W for the FPSC integrated with and without reflectors, respectively. The outlet exergy of the working fluid is between 3.52–22.8 W and 2.94–18.25 W for the FPSC integrated with and without reflectors, respectively. The destructed exergy is between 272–777.14 W and 245.64–914.2 W for the FPSC integrated with and without the reflectors, respectively. All maximum exergy values occur at midday when the solar radiation value is at its peak. In November, the inlet exergy of the working fluid and absorbed solar radiation is in the range 1154–431 W and 1023–409.21 W for the FPSC integrated with and without reflectors, respectively. The outlet exergy value is between 4.32–20.4 W and 5.36–26.8 W for the FPSC integrated with and without reflectors. The destructed exergy is between 283.14–784.14 W and 256.64–916.2 W for the FPSC integrated with and without reflectors, respectively.Figure 11
Exergy of the collector without and with reflector based on time of the day in September.Figure 12
Exergy of the collector without and with reflector based on time of the day in November.


## 5.6. Comparison of Destructed Exergy Rate with and without Reflectors

The destructed exergy rate due to heat transfer from the absorber plate to the environment increases with an increase in the ambient and plate temperatures. As shown in Figure13, as the ambient temperature increases, the exergy leakage increases, but the increment in ambient temperature is very slow compared to the plate temperature. Therefore, with equivalent ambient temperature variation, the collector integrated with reflectors has a high leakage exergy rate compared to the collector without reflectors. This is due to high plate temperature of the collector when integrated with reflectors.Figure 13
Variation of exergy loss related to ambient temperature for FPSC integrated with and without reflectors.The exergy loss rate due to the temperature difference between solar radiation and absorber plate increases with the increment of ambient temperature and incident solar radiation. The increasing tendency of absorber plate temperature has a great effect on exergy destruction. As shown in Figure14, the value of destructed exergy due to the temperature difference between solar radiation and absorber plate is small in the collector integrated with reflectors compared to the collector without reflectors. Therefore, as shown in Figure 14, the largest exergy loss is caused by the temperature difference between the absorber plate and the sun, which is about 89.21% and 89.08% of the total inlet exergy rate for FPSC integrated with and without reflectors, respectively.Figure 14
Comparison of exergy destruction (a) with and (b) without reflectors.
(a)(b)It is observed from the result that the increasing absorber plate temperature effect is higher on the collector integrated with reflectors. This shows that the destructed exergy is more affected by solar radiation when the ambient temperature is constant as shown in Figure15.Figure 15
Variation of destructed exergy caused by temperature difference between sun and absorber plate versus time.The overall heat loss coefficient and exergy destruction increase as the temperature difference between the collector plate and ambient temperature increases as shown in Figure16.Figure 16
Effects of the temperature difference between the absorber plate and environment on overall heat loss coefficients and exergy destructions for the collector.


## 5.7. Exergetic Efficiency

Figure17 shows the variation of exergy efficiency versus time. The results indicate that the exergy efficiency of the collector without reflectors starts increasing from 0.17% at 08:00 hr and reaches a maximum value of 2.53% at 12:00 hr, and thereafter, it starts decreasing as the solar radiation decreases. Additionally, the exergy efficiency of the collector starts increasing from 0.49% and reaches a maximum value of 4.71% in September. Similarly, in November, the exergy efficiency reaches a maximum of 5.43%, starting from 0.52%. This shows that with an increase in solar radiation, there is an increase in the heat gain and exergy efficiency.Figure 17
The variation of exergy efficiency versus time.


## 5.8. Time to Attain Pasteurization Temperature

Figure18 shows the time to reach pasteurization temperature (72°C) between 09:00 hr and 16:00 hr. The result shows that the collector with a reflector needs a shorter time to attain pasteurization temperature compared to the collector without reflectors. The minimum time to attain pasteurization temperature is at noon in September, which is 4.2 min and 6 min for FPSC integrated with and without reflectors, respectively. Also, in November, it is 3.9 min and 5.72 min for FPSC integrated with and without reflectors, respectively.Figure 18
Time to attain pasteurization temperature.


## 5.9. Quantity of Solar-Pasteurized Milk

Figure19 shows the quantity of pasteurized milk with respect to time of day. The maximum quantity of pasteurized milk per day is achieved at noon due to peak solar radiation at noon. The daily pasteurized milk in September is 69.19 l/day and 58.59 l/day, and in November, it is 70.2 l/day and 60.24 l/day for FPSC integrated with and without the reflectors, respectively. Therefore, by adding the reflectors to the collector, the quantity of pasteurized milk increases by 9 l/day.Figure 19
The quantity of pasteurized milk with respect to the time of the day.As shown in Table4, due to the addition of reflectors on both sides of the collectors, the thermal efficiency and the quantity of pasteurized milk is higher by 6.9% and 9%, respectively, compared to the results of Atia et al. [17].Table 4
Comparison of thermal efficiency and quantity of pasteurized milk (liter per day) with the present study with literature for FPSC without reflectors.
ParameterPresent studyAtia et al. [22]Maximumηth46.2%43%Quantity of pasteurization per day60.24 l/day55.6 l/day


## 5.10. Detection of Raw Milk, Heat-Treated Milk, and Pasteurized Milk Using the Methylene Blue Reduction Test (MBRT)

The quality of milk is tested using MBRT, and the time required for methylene blue to become colorless is termed as methylene blue reduction time, which is an indicator of the milk quality. A smaller value of methylene blue reduction time implies a lower quality of milk and vice versa. Figure20 shows that the raw milk starts changing its color after 30 min from the base time, and this indicates the quality of raw milk is very poor. The grading of milk samples with reduction time on the basis of MBRT is shown in Table 5.Figure 20
Result of MBRT after 30 min.Table 5
Decolorization time and grading of milk samples.
Milk samplesDecolorization time (h)Quality of milkRaw milk1.10PoorHeat-treated milk4.30FairPasteurized milk6.25GoodThe result shows that the methylene blue reduction time for raw milk is 1.10 h, which means that the quality is poor. However, after heating milk at pasteurization temperature, the methylene blue reduction time is increased to 4.30 h and the quality of milk is fair. Finally, the pasteurized milk (after heat treating milk and storing it in a refrigerator at 4°C) shows good quality due to destruction of contaminating microorganisms.


## 6. Conclusions

In this study, a small-scale flat plate solar collector is designed, manufactured from locally available materials and tested for pasteurization of milk. The system performance is evaluated by using a flat plate solar collector with and without reflectors. By adding the reflectors on the collector, the incident solar radiation is found to increase from 0.27–0.91 kW/m2 to 0.36–1.18 kW/m2.The experimental investigation onto the performance of flat plate solar collectors integrated with and without reflectors using energy and exergy analysis was carried out at a constant milk flow rate of 0.01 kg/s and the following results are highlighted:(i)
Thermal efficiency is directly proportional to the outlet temperature of milk, and the maximum efficiency obtained is 51.8% for FPSC integrated with the reflector and 46.2% without a reflector in November, adding a reflector onto the collector results in an efficiency increment of 5%.(ii)
The experimental results show that the maximum exergy efficiency of the flat plate solar collector without reflector is 2.53% and with reflector is 5.43% in NovemberThe ability of the FPSC to pasteurize milk is tested experimentally with and without the reflector integrated. The results may be summarized as follows:(i)
The minimum time required to attain pasteurization in September is at noon and it is 4.2 min and 6 min for FPSC integrated with and without reflector, respectively. Furthermore, in November, the minimum time required to attain pasteurization is at noon and is 3.9 min and 5.72 min for FPSC integrated with and without reflector.(ii)
The daily pasteurized milk production in September is 58.59 l/day and 69.19 l/day for FPSC integrated with and without reflector, respectively. Also, the daily pasteurized milk production in November is 60.2 l/day and 70.2 l/day for FPSC integrated with and without reflector, respectively. Due to the integration of the reflector, an increment of pasteurized milk production of 9 l/day is achieved.(iii)
The system successfully attains pasteurization temperature along with acceptable milk quality (which is tested using the methylene blue reduction test method)(iv)
This device avails the use of solar energy for milk pasteurization has great potential for the rural populations that rely on milk as a primary source of income by extending the shelf life of milk.



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*Source: 6214470-2022-09-29.xml*