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DOI: 10.32604/ee.2022.019200

REVIEW

Prospects and Challenges of Utilizing Solar Energy for the COVID-19 Vaccine Cold Storage in Remote Clinics of Tropical Countries: Review

Indra Mamad Gandidi1,*, Apri Wiyono1, Nugroho Agung Pambudi2,*, Rahmat Romadhon3 and Alfan Sarifudin2

1Department of Mechanical Engineering Education, Universitas Pendidikan Indonesia, Bandung, 40154, Indonesia
2Department of Mechanical Engineering Education, Universitas Sebelas Maret, Surakarta, 57126, Indonesia
3Department of Mechanical Engineering, Institute Teknologi Bandung, Bandung, 40116, Indonesia
*Corresponding Authors: Indra Mamad Gandidi. Email: indra.gandidi@upi.edu; Nugroho Agung Pambudi. Email: agung.pambudi@staff.uns.ac.id
Received: 08 September 2021; Accepted: 10 February 2022

Abstract: The rapid spread of COVID-19 pandemic has forced several countries in the world to store vaccines in cold storage towards ensuring their protection from being damaged and to maintain their stability. However, most remote clinics, especially those in the equator and islands, are faced with the challenges of hot climates and the inability to afford electricity resources needed to power the cold storage facility. Meanwhile, the hot equatorial region has abundant solar energy to power the vaccine cold storage but previous studies showed that several field workers do not have the ability to maintain the vaccine storage temperature as indicated by the manufacturer’s recommendations. Therefore, this literature review study examines the prospects and challenges of implementing solar-powered cold storage to provide cooling space for remote clinics. This is expected to contribute significantly to cold chain vaccine management technology. The findings showed that four technology integration schemes including Cold Storage Ice Maker, Cold Storage Ice Maker-PCM, Refrigerator-Ice Maker and, Absorbing Cooling-PV-Ice Maker have the potential to be applied in clinics situated in certain tropical regions.

Keywords: COVID-19; vaccine; cold storage; solar power; remote clinic

Nomenclature

COP   coefficient of performance
Cp   specific heat at constant pressure (Jkg1K1)
f   circulation ratio
h   specific enthalpy (Jkg1)
H2O   water
LiBr   lithium bromide
LiNO3   lithium nitrate
m˙   mass flow rate
NaSCN   sodium thiocyanate
NH3   ammonia
P   pressure (Pa)
Q˙   heat transfer (W)
S   entropy (Jkg1K1)
T   temperature (K)
W   mechanical power (W)
x   mass fraction (%)

Subscripts

1   first stage
2   second stage
AB   absorber
CD   condenser
COM   compressor
ECH   inter-exchanger solution
Ent   input
EV   evaporator
FF   refrigerant
GE   generator
L   liquid
p   weak solution
r   rich solution
v   vapor

Abbreviation

AC   Alternating Current
COP   Coefficient of Performance
COVID-19   Coronavirus Disease of 2019
DC   Direct Current
PCM   Phase Change Materials
PV   Photovoltaics
UK   United Kingdom
USA   United States of America
WHO   World Health Organization

1  Introduction

The world is currently focused on solving the COVID-19 pandemic problem which has caused human and economic losses [1] with several countries reported to have implemented lockdowns and restrictions on social interaction to prevent wider spread of the virus [2,3]. This lockdown policy was found to have led to a reduction in the spread rate [46] but posed a threat of worsening the previous economic downturn [79].

Another strategy used by governments to minimize the mortality rate was through the provision of vaccines [1012] but several challenges are associated with the efforts towards reaching the minimum number of vaccinations targeted [13]. An example of this is the maintenance of the stability of the vaccine storage according to the manufacturer’s recommendations [14]. Meanwhile, the data on the vaccines with distribution permits are presented in Table 1.

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The storage of these vaccines is becoming increasingly difficult in tropical countries due to hot environmental temperatures as indicated in Fig. 1 where the average value is 29°C [24] and this is higher than the values in other regions. Sunlight is normally stable for 12 h/day in the tropics and countries with island regions have additional challenges due to the fact that there is no easy access to electrical energy. Moreover, there is need to transport vaccines across a wide ocean in archipelagic areas in addition to the lack of on-grid access to electricity. This makes it difficult to provide effective cold storage which does not depend on on-grid energy sources for vaccines in hot tropical areas. This is particularly important because of the need to position vaccine supply chain resilience in a multi-network context [25]. The process of distributing these vaccines is, however, presented in the following Fig. 1.

images

Figure 1: Global distribution of annual mean temperature in °C [24]

To meet the need for vaccines, the government needs to provide standard cold storage units classified into 5 parts, namely national vaccine store, provincial vaccine store, district vaccine store, health center and outreach based on Fig. 2 [17]. This is necessary to provide a clear description of vaccine delivery methods to remote areas, infrastructure implementation, and necessary vaccine delivery modifications [25]. The examples of some vaccine cold storage are shown in Fig. 3.

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Figure 2: Cold chain vaccine management [17]

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Figure 3: Classification of vaccine cold storage [2629]

The most important part of cold chain vaccine management is maintaining the temperature until the usage time but several field workers have not been able to ensure adequate maintenance of the required temperature. For example, Hanson et al. [30] summarized that several health workers lack the knowledge on vaccine freeze-damage and its effects on temperature monitoring and the same was also reported by Falcón et al. [31] that staff prioritize protecting vaccines from heat damage, thereby, leading to frequent exposure to freezing temperatures. To maintain the stability of the vaccination process, the government needs to carry out a national stock of COVID-19 vaccines by providing a large number of cold storage rooms.

Previous studies also showed that several officers do not maintain the vaccine temperature according to the manufacturer’s recommendations [31,32]. Therefore, this study was conducted to review the findings of most of these studies on the technology in order to determine those with potential to be applied on vaccine storage in the study area. Furthermore, this research examines the prospects and challenges of implementing a solar-powered cooling system to build vaccine cold storage in remote areas. The result is expected to provide a recommendation related to solar-powered cold storage for COVID-19 vaccines in accordance with existing regional conditions. Furthermore, the results of this recommendation also need to be studied experimentally in the laboratory for actual performance testing.

2  Method

This research is a literature review by processing data obtained from international reputable journals and literature published by official health institutions such as WHO and the ministry of health. This study also uses report data and general guidelines published by vaccine manufacturers. The reference limit used is the last 5 years.

The keywords used in the reference search focus on cold storage vaccine innovations by utilizing solar power such as absorption cooling system, solar PV cooling system, and intermittent cooling system. Furthermore, this study also reviews the potential for integration of existing solar PV technology with the findings of other cold storage technologies such as ice makers, phase change materials (PCM), and refrigerators. In the end, it is hoped that this research will provide some potential for technology development to meet the supply chain problems of cold storage vaccines.

3  Result and Discussion

3.1 Current Solar-Powered Cooling Technology

The solar cooling system is an important part of the technology development for storing vaccines because its presence is intended to increase the efficiency of electricity. Conventional cooling systems drain electrical energy to meet the associated load. It is estimated that cooling systems from households and commercial buildings consume between 38% and 65% of the electrical energy used [33].

The solar cooling technology discussed in this study can is classified based on the source of energy used to operate the technology, such as the absorption, solar PV, and intermittent cooling system [33]. These three cooling system models are used to design and manufacture cold storage for the COVID-19 vaccine.

3.1.1 Absorption Cooling System

The absorption cooling system is similar to the vapor compression cycle. The main difference between the two is the force that causes the pressure difference between the vaporization and condensation process, which allows the steam to move from the low-pressure to the high-pressure region.

In the vapor compression cooling system, a compressor is used, while an absorber and generator are used in the absorption. Low-pressure steam is absorbed with an increase in pressure by the pump while the generator supplies heat. Therefore, the absorber and generator have the ability to replace the compressor function through mechanical and energy inputs. The vapor-compression and absorption cycles are often referred to as work and heat-operated cycles [34].

Heat is provided in various ways, such as using solar collectors, biomass energy, waste, and boilers. The absorption cycle uses two types of substances that are generally different in physical and chemical properties, namely lithium-air (LiBr/H2O)pairs. The first substance is called the absorber, while the second is the refrigerant (absorbate). Based on Fig. 4, an aqueous solution of a mixture of absorbent and refrigerant at low absorbent concentration enters the generator at high pressure. Furthermore, heat from a high-temperature source, such as diesel and exhaust heat, is evaporated. The solution flows into the absorber, and the refrigerant vapor flows into the condenser. In addition, a choke valve is used to maintain the pressure difference between the generator and the absorber. The cooling effect occurs due to the combination of condensation and evaporation of the two substances at both pressure levels, which are similar to the compression cycle. The solar absorbing cooling system method is used to provide the room with a temperature below the ambient temperature [3538]. The amount of COP generated from this type of cooling system is dependent on the material pair, heat source, cycle configuration, etc. [39].

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Figure 4: The cycle of absorption cooling system

3.1.2 Solar PV Cooling System

In this solar photovoltaic cooling system, solar energy is captured through the photovoltaic (PV) process, which converts the heat into DC electrical energy, used to operate a conventional vapor compression cooling system. The cycle is shown in Fig. 5 [4042].

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Figure 5: Solar photovoltaic cooling system

The main components of the cooler with this solar panel system are PV, batteries, and a DC to AC converter. The PV module is the most considered part because changes in its surface temperature tend to affect the solar cells. For instance, the higher the temperature, the lower the voltage output. Therefore, this technology is also equipped with a cooling system [43]. This system can be easily integrated with those that are chiller to provide a cooling room for vaccine storage that saves electricity [44,45].

3.1.3 Intermittent Cooling System

This intermittent cooling system is often also called a solar ice maker because it produces ice cubes used for cooling. Unlike the types discussed above, where absorbing cooling still requires external electrical energy for fluid pumping, this type of refrigerator does not require electrical energy to operate because the cooling process occurs naturally in the evaporator, as shown in Fig. 6 [46,47].

images

Figure 6: Solar ice maker [46]

The working principle of this technology uses solar heat with a pair of solid-liquid refrigerant absorber materials placed in the pipe, such as methanol-activated carbon. This heating causes a chemical reaction and separates the pair material. Methanol, in the form of gas, flows into the condenser and changes the phase into liquid before it is evaporated and placed in a box filled with water. This process takes place throughout the day. Meanwhile, the chemical reaction is reversed at night, and the solid (carbon) absorber sucks the liquid refrigerant back into the collector. Movement occurs by natural convection, without pumps and other devices. In order to extend the autonomy time, the ice mass in storage becomes the most promising parameter to consider [48].

In the process, the liquid refrigerant evaporates, thereby making the evaporator very cold. This, in turn freezes the water that touches the outside of the evaporator into an ice cube, which is used to create a cold room to meet storage needs [4952]. Pairs of materials that can be used in the intermittent cooling process are shown in Table 2.

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3.2 Mathematical Equation of Thermal Cooling System Absorbance

The input data required to simulate the overall the processes are absorption cycle capacity, hourly ambient temperature, hourly wind velocity, hourly solar radiation intensity are shown in Fig. 7, generator, condenser, evaporator, and absorber temperatures range and heat exchanger effectiveness.

3.2.1 Generator

The energy balance of the generator can be described as follows:

Qg= m˙1,h1,w+ m˙8,ss h8,ss  m˙7,ws h7,ws(1)

where m˙1,is the mass flow rate of working fluid (water), m˙8,ss is the mass flow rate of water at saturation condition and h1,w is the water enthalpy, and h7,ws is the enthalpy at saturation condition.

3.2.2 Condenser

The energy balance of the condenser can be described as follows [53]:

Qc= m˙1,h1,w  m˙2,h2,w(2)

3.2.3 Expansion Valve

Energy balance of the expansion valve can be described as follows [53]:

h2,w= h3,w(3)

3.2.4 Evaporator

The energy balance for the evaporator can be described as follows [53]:

Qe= m˙4,h4,w  m˙3,h3,w (4)

3.2.5 Pump Solution

wp= m˙6,ws h6,ws  m˙5,ws h5,ws(5)

3.2.6 Absorber

The energy balance for the absorber can be described as follows [53]:

Qabs= m˙4,h4,w + m˙10,ss h9,ss  m˙5,ws h6,ws(6)

And the total energy balance is obtained as [53]

Qg+Qe= Qc+Qabs(7)

3.2.7 Heat Exchanger

The heat balance in the heat exchanger can be written as follows [53]:

Qhx=m˙8,ss (h8,ssh9,ss)=m˙7,ws (h7,ws h6,ws)(8)

3.2.8 Coefficient of Performance (COP)

The coefficient of performance of the refrigeration system is determined as [53,54]

COP= QeQg=QeW(9)

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Figure 7: Global mean solar irradiance [55,56]

3.3 New Idea

3.3.1 Scheme 1: Cold Storage Ice Maker

The cold storage ice maker is suitable to be applied in very isolated areas that do not have electrical energy. This technology is designed in the form of a vaccine box where an evaporator is placed on the wall connected to the collector to produce ice cubes at night and used during the day to maintain its room temperature in standard conditions. The vaccine box needs to be insulated to prevent thermal loss during the day.

The volume of cold storage depends on the amount of refrigerant supplied from the collector to the evaporator. Therefore, the size of the collector needs to be designed to accommodate the volume of material pairs proportional to the evaporator. The cold box ice maker concept schematic is shown in Fig. 8.

images

Figure 8: Cold box ice maker

However, one of the disadvantages of this technology is that it cannot be used to produce ice cubes during the day. Consequently, at night, cold box temperature instability becomes possible. Therefore, the addition of PCM and a good insulator is expected to keep the ice cube frozen at night. It is also advisable to use an elliptical pipe [57] and Epsom salt (MgSO47H2O) or salt hydrate as thermochemical energy storage [58,59] in the Ice Maker to improve its performance.

3.3.2 Scheme 2: Cold Storage Ice Maker-PCM

Scheme-2 is model-1 which is enhanced by adding phase change material (PCM) to the walls of the vaccine’s box to minimize the entry of heat energy. This process is carried out by absorbing heat that passes through the box, thereby maintaining the temperature stability for 24 h and avoiding the ice cube melting process [60]. The PCM should be tightly sealed in a barrier containment, preferably in small microcapsules where PVA-Na2SO4·10H2O and PVA-CaCl2·6H2O are used, to maximize its performance [61].

3.3.3 Scheme 3: PV Refrigerator-Ice Maker

This technology works by assigning the PVC to generate electrical energy, which is then used to operate the cold storage refrigerator installed in the vaccine storage room. The amount of electrical energy that needs to be supplied depends on the compressor power required to meet the load of the cooling chamber wall. The cold storage volume of 3–4 m3 needs ≈600 W of electrical energy [62], equivalent to 19 panels of 100 WP with 12 energy-saving batteries. However, the price is relatively high for small cold storage, closely related to the required cooling load.

Therefore, the work of the refrigerator compressor is to reduce the cooling load on the vaccine box by decreasing the heat energy entering the vaccine box. Insulators and the addition of PCM have the ability to increase the electrical efficiency of the system [6366].

Furthermore, to achieve super-efficient thermal conditions, the addition of a solar ice maker is recommended to reduce the cooling load by the refrigerator and PV. This tends to have an impact on the use of batteries at night to operate cold storage and cheaper system prices.

3.3.4 Scheme 4 Absorbing Cooling-PV-Ice Maker

The combination of 3 methods in a solar cooling system can be used to produce medium-sized cold storage of 5--10 m3 for vaccines. An ice maker also provides this additional cooling load energy mounted on the wall of the storage. Solar ice makers can also be installed at the input of the evaporator to make it works optimally and increases the Coefficient of Performance (COP) of cold storage. The PV in this system is used to generate electricity to power the pump for circulating the cooling cycle and the fan on the evaporator. The pump and fan power are assumed to be approximately 300 W or the equivalent of 10 solar panels and 6 energy-saving batteries.

Schemes 4 and 3 are suitable for clinics that require a lot of vaccines but their selection is limited by different environmental conditions and infrastructure such as the difficulties in the determination of COP results and prices considering the different clinical conditions. Moreover, Scheme 3 is recommended for clinics in areas facing direct sunlight. This scheme is also observed to have more potential compared to Scheme 4 due to the availability and stability of an existing power source.

4  Conclusion

This study offered 4 schemes and the potential to be used in procuring vaccine cold storage in remote clinics. Schemes Cold Storage Ice Maker and Cold Storage Ice Maker-PCM are more suitable for very remote clinics with little need for vaccines, while schemes PV Refrigerator-Ice Maker and Absorbing Cooling-PV-Ice Maker has more potential to be used in clinics requiring many vaccines. However, scheme PV Refrigerator-Ice Maker is not suitable for areas with little need for vaccines because it is quite expensive. Interestingly, from all the schemes above, the fifth is produced, which is an application of a solar ice maker on cold storage chiller to reduce cooling, thereby reducing the need for electrical power. Cold storage chillers are widely used in hospitals, commercial buildings, and drug and food companies. In this case, experimental research is being conducted on a combination of the ice maker and cold storage chiller to enhance the chiller’s thermal performance.

Acknowledgement: The authors are grateful to the Research Institutions and Community Service (LPPM) of the Universitas Pendidikan Indonesia, Laboratory of Refrigeration and Air Conditioning, Mr. Nugroho Agung Pambudi as the host of the PPKI research, and the Directorate General of Higher Education (DIKTI) for funding this research.

Funding Statement: This work was supported by the Program Penelitian Kolaborasi Indonesia (PPKI), LPPM Universitas Pendidikan Indonesia, Ministry of Education and Culture of Indonesia.

Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.

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