Refrigerators and freezers can operate on electricity or on heat derived from the burning of a liquid fuel or gas.

Electricity may be supplied either by a mains power system, by an electric generator, or from renewable energy sources, typically solar power supplied from a photovoltaic array. Most electric refrigerators are vapor-compression cycle units, of which the ice-lined refrigerator (ILR) is a sub-type.

The table below summarizes the advantages and disadvantages of each refrigeration technology and energy source.

Table 1: Refrigeration options

Compression refrigerators and freezers are powered by electricity; they cannot operate on gas or kerosene. The electrical supply may come from the main power grid, from a generator or from a renewable energy source such as solar power. Compared to absorption options their great advantage is that compression refrigeration provides powerful and more energy efficient cooling. In addition, compression units have better temperature control in most conditions and less refrigerator maintenance is required.

All new PQS prequalified compression refrigerators listed in the data sheets in this section have vaccine storage compartments that are designed not to freeze if used in accordance with the manufacturer’s instructions. In addition, they now have non- adjustable thermostats. However, if vaccines are placed against the internal lining without baskets there is still a risk of freezing. In 2016 WHO IMD-PQS introduced a grading scale: A, B and C. Appliances graded A are certified as low vaccine freezing risk even if baskets are not used (see paragraph on freeze-protection). This new approach should eliminate accidental freezing due to improper user interventions, such as thermostat adjustment or misplacement of vaccines in the refrigerator compartment.

Low-energy chest freezers: These units are used to store oral poliovirus (OPV) vaccines in bulk. Typically, they are used in smaller primary stores where a freezer room is not justified, or at intermediate healthcare levels. They can also be used to freeze water-packs and/or to store frozen ice-packs.

E003.2.1 Mains-powered refrigerators and freezers

E003.2.1.1 Compression refrigerators and freezers with no additional lining

These are standard refrigerators that have been tested to ensure that acceptable vaccine storage temperatures between +2°C and +8°C are maintained in the vaccine storage areas. No freezing (i.e. temperature of 0°C or less) is accepted. If power is interrupted, these refrigerators will remain below +8°C for at least four hours in continuous hot-zone temperature.

E003.2.1.2 Ice-lined refrigerators (ILR)

Most ILRs can maintain acceptable temperatures below +8°C on as little as eight hours of electricity per 24 hours, day after day. Ice-lined refrigerators are a very good choice wherever there is at least eight hours electricity a day, but no standby power supply. SDD appliances may also be an appropriate option.

Cooling during power cuts is maintained by an internal lining surrounding the vaccine storage compartment. The lining is made up of ice or cold water filled compartments or frozen ice-packs. When the electricity supply fails, this ice or cold-water store keeps the vaccine cool for at least 20 hours. When the power is restored, the compressor runs until the lining is refrozen or re-cooled.

Other new ILR-like products make novel use of chilled water combined with ice. These refrigerators can operate on fewer hours of power per day and provide from 20 hours to over 120 hours of acceptable temperature control without electricity.

Temperature control is inherently stable and potentially extends the reach of the ILR approach to areas with as little as 28 hours of power per week or an average of 4 hours per day.

WARNING: During the refreezing process some areas of an ILR can fall below 0°C – e.g. the bottom of the compartment. For non-grade A appliances freeze- sensitive vaccines should NEVER be stored within 150 mm of the base of these models. Always respect the manufacturer’s storage limit lines inside the cabinet, indicating the extent of the hazardous zone and always use the vaccine storage baskets provided by the manufacturer.

In order to avoid the freezing problem, some programmes adjust the thermostats on older ILRs so that the water-filled lining remains liquid. This strategy reduces the appliance’s cold-life but does eliminate the freezing risk. It should be adopted only after a carefully conducted study using data loggers to monitor vaccine temperature in the cold chain. Refer to the WHO Study Protocol for Temperature Monitoring in the Vaccine Cold Chain for further guidance.

E003.2.2 Solar refrigerators and freezers

Solar powered vaccine refrigeration systems are now widely used. They are sometimes the only available solution in areas where no reliable conventional energy supply is available. Solar powered appliances must pass the relevant appliance tests and must also include a solar power system certified by the appliance manufacturer to be in compliance with the WHO IMD-PQS performance specification E003/PV01: Solar power system for vaccine refrigerator or combined vaccine refrigerator and water-pack freezer. These solar power systems are intended to be delivered as a standard kit and several kit options may be offered.

WHO, PATH and others have conducted several in-depth reviews of solar vaccine refrigeration programmes launched in Africa, the Americas and in South-East Asia. These have shown that:

• The technology can be reliable and improve the quality of the vaccine cold chain when compared with the use of absorption refrigerators.
• The 10-year life cycle cost of solar direct drive (SDD) refrigerators is estimated to be equal or lower than gas-powered refrigeration systems in settings where bottled gas is readily available and its supply is reliable. See Figure 1 below.
• The 10-year life cycle cost of solar battery powered systems remains relatively high in comparison with gas-powered refrigeration systems. Maintenance and replacement of batteries and regulators remains the major problem. Field reports historically indicate that on average, replacement is necessary after five years and these systems are often located in remote, isolated areas making maintenance and battery replacement challenging.
• Solar modules and batteries (when used) must be physically secure to prevent theft of these valuable items. (Due to the cost and maintenance of batteries, SDD systems have become more prevalent.)
• It is essential to ensure that a qualified servicing network is in place to make an on-site assessment, install the systems and provide long-term system maintenance. Replacement of parts such as batteries, battery charge regulators and refrigerator components must be anticipated and fully funded.

Fig 1: TCO comparison between SDD, solar battery and gas refrigerators from the latest PATH TCO tool (2019 July)

Both direct-drive and battery powered models currently provide vaccine storage at +2°C to +8°C. The flow chart under section E003.5 provides guidance on when to select solar refrigeration. It is essential to ensure that there is adequate sunlight throughout the year at all the proposed sites and a solar site evaluation must be conducted before the equipment is installed on site.

There are now two types of solar refrigeration. The first type consists of a compression- cycle refrigerator or freezer or combined refrigerator/freezer powered by a battery-pack which is charged by a photovoltaic array. The second type is a compression-cycle refrigerator or freezer or combined refrigerator/freezer directly driven by the solar array, with energy stored in a sealed thermal mass inside the equipment to maintain cool temperatures overnight. No batteries are needed to operate the compressor. A sub- category of this type uses an ancillary battery to operate a fan.

Field reports indicated that solar-powered refrigeration systems can sometimes achieve a 10+ year service life. This is only possible if the installation is properly planned, correctly designed, competently installed and then supported by a regular maintenance program with adequate, ongoing funding.

Solar refrigeration is a mature technology that can be used with confidence provided certain fundamental conditions are met. These conditions are:

1. Maintenance and repair: All solar equipment requires maintenance or it will fail, especially power systems with a battery. Plan for maintenance in accordance with maintenance tools for users and technicians; provide adequate funding in advance and carry out the work in a timely manner. Only install solar refrigerators at sites which can be reached consistently by a qualified service network throughout the year and in a reasonable time.

2. Equipment selection: A purchase should not be committed until the solar site evaluation has been completed. Then only refrigeration equipment that has been prequalified under WHO IMD-PQS and which is suitable for the climatic conditions at the selected site(s) should be considered. Qualified suppliers should provide a solar power system that is matched to the refrigeration equipment, meets IMD-PQS equipment specifications and is suitable for the site-specific climate. Before buying a large quantity of equipment, a pilot study including one or a few systems must be conducted in order to assess the installation feasibility and the good working conditions in country.

3. Solar site evaluation: Subject to condition 1, the system supplier/installer must carry out a site evaluation and must confirm in writing that:

• The refrigerator can be located in a level, secure and safe position out of direct sunlight and moisture.
• The proposed site has sufficient solar radiation throughout the year to power the chosen equipment (verified by officially-sourced regional multi-year weather data) and that system performance will not be compromised as a result of persistent dust storms, fog, air pollution or other more localized atmospheric effects.
• A solar site analysis has quantified the times of and the amount of shading on the proposed solar array location.
• The system autonomy (amount of energy stored for periods of low solar radiation) has been assessed and is matched to the site-specific autonomy requirement as determined by WHO IMD-PQS specification E003 PV01.
• The site identified to install the solar array is not shaded by trees, vegetation, poles, cables, adjoining structures or buildings. Even something as small as an overhanging cable can seriously affect the efficiency of some solar modules.
• There is a suitable place to mount the solar array so that it cannot easily be vandalized, stolen or accidentally damaged.
• Safe access can be provided to clean and maintain the solar array. Note: this is especially important where snow or dust storms are common.
• Details are specified for any roof-strengthening required and clear guidance is provided on the installation of access ladders and/or safety ropes.
• All cable lengths are estimated as accurately as possible in advance.
• If batteries are used, advice is provided on a suitable and secure location for a battery enclosure (a locking, ventilated box) close to the vaccine refrigerator/freezer

4. Site preparation: The solar site evaluation may show that preparatory building work is required to make the site safe and suitable. If this is so, make arrangements to carry out this work before the equipment is delivered.

5. Servicing network: There must be a competent, trained and affordable solar servicing network in the country. This service network must offer installation and long-term support with a reliable supply of spare parts for the refrigerator, the solar array and battery system (if used).

6. Installation: It is essential that the equipment be installed and commissioned in accordance with the relevant IMD-PQS Quality Assurance protocol: IMD-PQS PV01-VP2 Solar power system for compression-cycle refrigerator or combined refrigerator/water-pack freezer.

7. User training: Users must be trained in the operation and maintenance of the system. They should also be provided with clear instructions on whom to inform in case of maintenance requests or system failure.

WARNING: The freezer compartments of IMD-PQS prequalified solar refrigerators are not suitable for storing or freezing vaccines. Freezer compartments should only be used to freeze water-packs.

WARNING: Some single compressor solar refrigerators may not be able to freeze icepacks when the ambient temperature is permanently below +15°C to +20°C. If you plan to use a solar refrigerator in a mountainous area or an area where the ambient temperature is permanently or seasonally low, you should select a system with two independent compressors or a model that has shown good performance at low temperatures.

WARNING: The photovoltaic panels and battery packs (whenn used) which power the vaccine refrigerator must be used for this purpose alone. If there is a need for additional photovoltaic power on the site – e.g. for health facility lighting, for small-scale laboratory equipment and the like – this must be supplied from a separate system. If other equipment is supplied from the refrigerator battery pack, its use may be difficult to control and this will drain power and could compromise the vaccine.

IMD-PQS has published Solar Energy Harvest Controller (EHC) specifications to open up the possibilities to use some extra energy supplied by solar arrays, while securing the power to the appliance. Note: this applies only for SDDs approved for use with specific EHCs.

The solar direct drive refrigerators listed in this catalogue are laboratory tested over a specific ambient temperature range, using a specific solar diurnal power cycle.

Establishing the correct size of the solar array for a solar direct drive refrigerator installation cannot be done by the purchasing agency. Instead, predetermined system kits with an SDD appliance and matched solar power system are provided. Laboratory testing and the amount of cold storage required by IMD-PQS are based on relatively poor solar radiation conditions to assure adequate solar electricity is generated in most tropical climates.

WARNING: IMD-PQS prequalified equipment may only be used within the limits stated on the relevant IMD-PQS product data sheet. Manufacturers are required to nominate a solar array size at the time of testing. The output from that solar array is used to determine the performance of the refrigerators. Only the nominated array size that is stated on the IMD-PQS product data sheet, or a larger array, may safely be used.

Absorption refrigerators and water-pack freezers do not perform as well as their compressor-driven equivalents. They may require constant attention to ensure adequate performance for the vaccine cold chain.

Furthermore, models that combine vaccine storage with water-pack freezing within the same insulated chamber do not control temperatures or freeze water-packs as well as models that have separate freezing and vaccine storage areas.

WARNING: Operating costs can be higher with absorption refrigeration than with most other options. For this reason, SDD systems have become more prevalent for locations with unreliable mains power.

When you choose a refrigerator or freezer consider the following points (see glossary for definitions):

1. Temperature zone: Assess climate-specific and location-specific factors

• Climate: Identify the appropriate temperature zone by checking the mean maximum temperature during the hottest month and the mean minimum during the coldest month. Bear in mind that some parts of the country, e.g. mountain regions, may require equipment with a different temperature rating than other parts.
• Setting: Choose potential appliance types to suit the setting using the table below. There are two factors to consider:
  • The mean maximum temperature in the hottest month in the area where the appliance will be located: ignore air-conditioning because it is very likely to be off at weekends and during power cuts and cold chain equipment is often located in store rooms or similar locations.
  • The lowest winter temperature in the room where the appliance will be located: in cold climates, the winter temperature inside a health facility can drop below freezing; e.g. where heating is absent, unreliable, or is turned off at night and/or at weekends. In such cases, choose equipment that is able to operate at low ambient temperatures. Currently there are no models available with active low-temperature protection which can operate at temperatures below +5°C

2. Temperature zone selection

Table 2: Temperature zone selection

3. Vaccine storage capacity: In each setting (primary store, sub-national store or health facility) assess how much vaccine must be stored at +2°C to +8°C and how much at -20°C.

4. Water-pack freezing capacity: If icepacks are required, assess how many water- packs need to be frozen per 24 hours in each setting. Once frozen, the icepacks can be stored in any locally-available chest freezer with low power consumption. If high throughput is not a major concern, chest freezers can be used on their own. Where cold water-packs are used, lower cost, locally available refrigerators are perfectly adequate, so long as they are never used to store vaccine.

5. Energy source: Continuous refrigeration is required for vaccine storage and this requires a reliable source of power. In each setting, assess which power sources are available and which type of refrigeration is most suitable. The available sources are mains electricity, generator produced electricity, renewable energy (e.g. solar electricity), and bottled gas. Solar, gas and long-term passive devices are currently the only alternatives where mains electricity is absent, or is available for less than eight hours per day. The flowchart below outlines the decision process:

Figure 2: Selecting a suitable energy source

6. Associated supplies: Make sure you order the necessary associated supplies at the same time as the refrigerator or freezer. These include:

• Spare parts: The data sheets in this section list the spare parts recommended for the first five years of use. To avoid shortages later, order these spares with the equipment. Before purchasing equipment check that repair facilities and spare parts are available, whether from a competent local service agent or in-house.
• Voltage stabilizer: Unless mains electricity is highly reliable, order a voltage regulator for all mains or generator powered equipment – see Section E001 Cold Room and Freezer Rooms and Section E007 Cold Chain Accessories of the IMD-PQS Catalogue for more information. Note that some suppliers include a voltage regulator either integrated in the device or bundled with the device.

• Temperature monitoring equipment: IMD-PQS specifications require all vaccine refrigerators and freezers, whatever the power source, to be supplied with a Level 1 Electronic Monitoring Device (EMD) datalogger. In addition, for reliable continuous temperature monitoring, every vaccine refrigerator should include an external reading thermometer and integrated Level 2 EMD, integrated Level 3 EMD, or a 30-day electronic refrigerator temperature logger. See section E006 Temperature Monitoring Devices of the IMD-PQS Devices Catalogue for more information. Note: that refrigerator temperature loggers will need to be replaced approximately every two years when the battery runs out. Ensure that adequate recurrent funding is available to purchase these consumable items in future years. 

  Table 2: EMS configurations

• Manuals: Order user and service manuals in the appropriate language.
• Energy harvest: Energy harvest is a new technology that provides a limited amount of electricity to health facilities with unreliable electricity (weak-grid) or no electrical service (off-grid). Energy harvest is only available for some solar direct drive refrigerators at the current time. See Section E007 Cold Chain Accessories of the IMD-PQS Devices Catalogue for more information.

7. Training: Ensure that users and maintenance technicians are properly trained to use the specific equipment that you order. All solar systems require on-site installation by a competent technician specifically trained in solar installation work. It is also essential that adequate servicing facilities are available after the equipment has been installed. The importance of training is often underestimated and under-budgeted. A cold chain with good equipment but inadequately trained staff can seriously hamper an immunization programme.

8. Cost: Select the best available refrigerator from the point of view of reliable temperature control and low maintenance. Do not buy solely on the basis of purchase cost. This can lead to expensive loss of vaccine and/or higher lifetime costs due to greater running and maintenance costs.

The new IMD-PQS specifications have introduced a number of technical changes. The following are the most significant.

E003.5.1: Electronic Monitoring System (EMS)

A Level 1 Electronic Monitoring Device (EMD) is now required for all vaccine refrigerators. Additionally, an integrate Level 2 EMD or 30-DTR is also required.

E003.5.2: Humidity control

Although humidity control is not required at this time, it is encouraged and is a testing protocol is provided in the Verification Protocol.

A number of interesting new developments have been pursued by WHO and its partners:

• Solar direct drive freezers have now been prequalfiied.
• Basic USB EHCs are now common on most SDD appliances: however, these devices cannot take advantage of the full energy from the solar array, which prioritizes the refrigeration load over all others. Therefore EHC options are desirable for health centers that lack access to electricity.
• Transportable, powered vaccine storage appliances are now available. The corresponding IMD-PQS specification is WHO/PQS/E003/TS01. These appliances use batteries to provide "independence" time by actively cooling the vaccines. Thermal insulation provides a further "hold-over" time. 
• Phase change materials that freeze at positive temperatures (e.g. +5°C) may be used instead of an ice-lining if permanently contained and affixed into the equipment to prevent vaccine freezing. This technology will provide more cooling capacity than a lining consisting of unfrozen cool water-packs. IMD-PQS has published a specification for containment and use of non-water PCMs: WHO/PQS/E005/PCMC0.1 – Phase change material containers.

Performance specifications

PQS performance specification E003/RF06.4: Refrigerator or combined refrigerator and water-pack freezer: Solar direct drive with ancillary rechargeable battery

PQS performance specification E003/RF04.4: Refrigerator or combined refrigerator and water-pack freezer: Solar powered with rechargeable battery

PQS performance specification E003/RF01.4: Refrigerator or combined refrigerator and water-pack freezer: minimal holdover mains powered, compression cycle

PQS performance specification E003/FZ04.1: Vaccine freezer or combined vaccine and water-pack freezer: Solar-direct drive

PQS performance specification E003/TS01.1: Transportable, powered vaccine storage appliances

PQS performance specification E003/RF05.7: Refrigerator or combined refrigerator and water-pack freezer: Solar direct drive without battery storage

PQS performance specification E003/RF03.6: Refrigerator or combined refrigerator and water-pack freezer: intermittent mains powered, compression cycle

PQS performance specification E003/ULT01.1: Vaccine ultra-low temperature freezer compression cycle

PQS performance specification E003/POW01.0: Power systems for ultra-low temperature systems

PQS performance specification E003/BC01.1: Global asset identification

PQS performance specification E003/RF02.3: Refrigerator or combined refrigerator and water-pack freezer: absorption cycle

PQS performance specification E003/RF06.3: Refrigerator or combined refrigerator and water-pack freezer: solar direct drive with ancillary rechargeable battery

PQS performance specification E003/PV01.4: Solar power system for vaccine refrigerator or combined vaccine refrigerator and water-pack freezer

PQS performance specification E003/FZ01.2: Vaccine freezer or combined vaccine and water-pack freezer: compression cycle

PQS performance specification E003/FZ02.2: Water-pack freezer: absorption cycle

PQS performance specification E003/FZ03.3: Water-pack freezer: solar direct drive without battery storage

Verification protocols

Target product profile

Guidance for manufacturers/suppliers

Generic field evaluation checklist for IMD-PQS reviewers

Generic guide for the field evaluation of new technologies for WHO PQS prequalification GENERIC/GUIDE 1.1

PQS solar autonomy calculation method

IMD-PQS Guidelines for Prequalification Holders

Supporting information for users

How to calculate vaccine volumes and cold chain capacity requirements

How to develop and repair a maintenance system for cold chain equipment

Introducing solar-powered vaccine refrigerator and freezer systems – A guide for managers in national immunization programmes

Evidence brief: Solar direct-drive vaccine refrigerators and freezers

Procurement guidelines: solar direct drive refrigerators and freezers

Study protocol for temperature monitoring in the vaccine cold chain