Until recently, kerosene and bottled gas refrigerators were the only practical means of storing vaccines. Now a newer power system technology—Photovoltaics—offers an alternative. Photovoltaic-generated electricity is used to power conventional vapor-compression refrigerators, the type most of us have used all of our lives, units familiar to refrigeration mechanics all over the world. The benefits of photovoltaic-powered units are causing decision makers to reconsider their use of kerosene and gas-powered absorption refrigerators.

This document provides information on which to base a decision about procuring vaccine refrigerators. Photovoltaic systems make good sense where refrigeration is required and there is no access to reliable grid electricity, and where previous use of kerosene refrigeration has proven to be troublesome because the fuel is of poor quality or there are problems with delivery.

About Photovoltaics

Photovoltaic modules convert sunlight directly into electricity in a process that is both elegant and simple. The cells in the modules are made from silicon, the most abundant element on earth after oxygen. Photovoltaic modules have no moving parts, which makes them inherently more reliable than other energy sources. They are appropriate for many applications, especially where conventional electric utility service is not available.

Rural Health Clinic Needs

Despite the advantages of Photovoltaics, a number of factors are slowing the rate at which the technology is being used. They include:

• Lack of knowledge about and familiarity with photovoltaic technology and its advanced state of development for many applications.
• Difficulties and uncertainties associated with procuring photovoltaic systems, including sizing and specification of appropriate components.
• Fear of problems developing after the systems have been purchased and uncertainty concerning their resolution.
• High initial cost, which is often considered more heavily than the extremely low operating cost.
• Problems that occurred when photovoltaic refrigeration systems were a new technology; most of them have been eliminated.

Sandia National Laboratories, through its Photovoltaic Systems Design Assistance Center, offers technical support to minimize the risk associated with procurements and to help ensure that only reliable and long-lasting systems are purchased. The Design Assistance Center is part of the U.S. Department of Energy’s National Photovoltaic Program.  

A rural health clinic often has needs for electricity other than vaccine refrigeration, and they can be met by photovoltaic systems. Lighting and communications are commonly powered by photovoltaic systems because they are the most economic for small energy needs. Photovoltaic lighting systems supply reliable, high-quality light without the soot and possible danger of kerosene or other flame-based illumination. Batteries can be counted on to provide required energy for emergency communications when they are kept charged by a photovoltaic system. In addition, a health clinic could use photovoltaic-generated electricity to power a television set and videocassette recorder for health-related educational programs.

It is easy to envision the situation in which electricity supplied by a photovoltaic system to a rural health clinic could dramatically increase the importance of the clinic in the community, enhancing its ability to perform its function. In such a case, a photovoltaic- powered vaccine refrigerator would simply be part of the health clinic power usage.  The system’s initial capacity to generate power can be increased simply by adding modules and batteries.

Once a vaccine has made the often lengthy journey to a village health clinic, it is imperative that the refrigerator maintain the vaccine at the proper temperature to ensure its viability. Spoiled vaccines not only are useless—they can undermine local confidence in the entire immunization program. Thus, refrigerators used for vaccine storage must be able to maintain a constant temperature. The ability to control the temperature precisely distinguishes a refrigerator powered by Photovoltaics from one powered by kerosene or bottled gas. 

Photovoltaic systems use vapor-compression refrigerators, and their temperature is thermostatically controlled. This means that their internal temperature is maintained constant no matter how the external temperature changes. The vapor-compression process also responds rapidly when conditions change inside the refrigerator, for example when the door is opened, or items are added to the shelves. The time required to compensate for most changes is about 10 minutes. 

The absorption refrigerators used with kerosene and bottled gas do not at present have thermostatically controlled interior temperatures. This means that as external temperatures change, so do the internal temperatures. The internal temperature is adjusted by altering the intensity of the flame. Typically, the user monitors the internal temperature with a thermometer—a good practice for any kind of vaccine refrigerator—and then adjusts the flame accordingly. In practice, this usually means turning the flame down in the evening and up in the morning. In addition, the time required for the refrigerator to respond to door openings and added internal load is significantly longer than for vapor-compression units. 

A change in thermostat setting will be realized in 10 or 15 minutes in a vapor-compression refrigerator—a similar change in flame for an absorption refrigerator may take 8 to 10 hours to achieve.

Characteristics of Vapor-Compression Refrigeration

• The precise and stable temperature control of vapor compression refrigerators makes them particularly well suited for vaccine storage, provided reliable electricity can be supplied.

• A compressor drives the vapor-compression process and requires either electrical or mechanical power input. 

• Although mechanical power can be used, as in automotive air conditioners, an electric motor is the most common device to power the compressor. 

• Electricity for the motor can be supplied from an electric utility company, photovoltaic modules, batteries, or other electric generators.

• Electrically powered vapor-compression refrigerators are by far the most common type refrigerator in the world; they are also the most reliable, durable, and easiest to maintain.

Characteristics of Absorption Refrigeration

• The control of heat transfer processes is less precise than electrical control, and it takes much longer to produce a desired temperature change within an absorption refrigerator.

• Maintaining precise and stable temperatures requires user attention at least twice daily.

• Absorption systems rely on heat transfer, chemical processes, and gravity flow of fluids rather than on electrical or mechanical power.

• Although bottled gas, solar thermal, or electric power can be used, the heat for absorption systems is most often supplied from burning kerosene. 

In summary, vapor-compression refrigeration has the advantages of control and response time over absorption refrigeration for all small refrigeration applications, including vaccine storage. Absorption refrigeration should only be used when a reliable power source for a vapor-compression system is unavailable, or when it can be shown that costs of absorption systems are lower than vapor-compression alternatives and the absorption system can provide an equivalent degree of cooling for the vaccines.

Photovoltaic-Powered Vaccine Refrigeration Systems

A photovoltaic-powered vaccine refrigeration system has four major components, as illustrated.

The photovoltaic modules in the array convert sunlight directly into electricity to power the compressor of the vaccine refrigerator. Because sunlight is not available at nighttime or during periods of poor weather, rechargeable batteries are used to store electric energy. A properly designed and maintained battery subsystem will allow the refrigerator to operate a week or more without sunlight.

The charge controller regulates the flow of electricity to protect the batteries from overcharging and over-discharging, either of which will shorten the lifetime of the battery.

Vaccine Refrigerator Powered by a Photovoltaic System

Temperature Control

• The temperature in a system powered by photovoltaics is automatically controlled by a thermostat and is precise and stable.

Preventive Maintenance

• Photovoltaic systems require little preventive maintenance and demand little of the user’s time. As with all systems, however, it is important to perform a minimal amount of maintenance to ensure the system will function properly.  

• Daily maintenance for photovoltaic systems involves checking the refrigerator’s temperature and operating lights; no adjustments are normally required.

• Weekly maintenance for photovoltaic systems involves checking for excessive ice buildup and defrosting if necessary. Usually defrosting is only necessary several times a year.

• Monthly maintenance for photovoltaic systems involves checking for dirt buildup on the array and refrigerator condenser, checking the level of electrolyte in the battery, and checking the battery terminals for corrosion.  The above items only require action when a problem is noted, and the maintenance required is simple.

Reliability

• Photovoltaic systems are inherently reliable, but as with anything, improper use can reduce their reliability, It is important that users be educated about their systems and trained in their use. This will allow them to realize the high level of reliability that is possible with these systems.

Installation, Service and Repair

• The skills involved in installation, service and repair are few and can be learned readily with appropriate education and training programs.  Once the systems are installed, the amount of time the photovoltaic refrigerators are not running because of planned or unplanned service and repair is small, as long as repairmen with adequate training are available.

• When possible, materials used in the system’s design should be available locally, or at least in the region. This will probably include mounting hardware and wiring devices. The design should not be too rigid and should allow for possible substitutions for specified components when necessary.  The vapor-compression refrigerator is known world-wide and in most places can be serviced locally.

Possible Pitfalls

• The subsystem for energy storage—the battery and charge controller—is critical to the proper operation of the photovoltaic refrigeration subsystem.  Without energy storage, the refrigerator will not function at night or in poor weather. Unfortunately, this subsystem is often taken for granted, with the thought that a battery is just a battery. Consequently, battery problems are the most frequent cause of failure in a photovoltaic system.

• The battery must be designed for many cycles of deep discharge. The current EPI recommendation is for 1000 cycles to 50% discharge, which will provide a life for the battery of roughly two to three years. A requirement more consistent with the lifetime of the array and the refrigerator is 1500 cycles to 80% discharge, which would provide a battery life of eight to ten years. The cost of such a battery is only slightly more than for the former type. 

• It is important that the charge controller be designed for the specific battery being used, because various types of batteries have different requirements regarding the upper and lower voltage limits and the charge rate as the battery approaches full charge. A properly designed and maintained energy storage subsystem will provide years of trouble-free operation—but its design and maintenance are crucial to this operation.

Kerosene-Powered Vaccine Refrigeration Systems

A kerosene-powered vaccine refrigeration system has the components illustrated.

A wick inside the burner is in contact with kerosene in the fuel tank. The flame from the wick provides heat energy which drives the absorption process. The exhaust gases and particulates from the burning process pass through the flue and are exhausted in the surrounding air (often inside the health clinic). 

The winder rod is used to adjust the flame height. A person controls the temperature within the refrigerator by manually raising and lowering the height of the flame. The rate of heat transfer from the burner to the generator, which determines the amount of cooling produced, is affected by the condition of the burner, wick, flue and kerosene. As soot builds up on the generator, heat transfer through the generator is slowed, and it becomes increasingly difficult to maintain temperatures within the prescribed 00C to 8~C range.

Temperature Control

• The temperature in a kerosene system is manually controlled by adjustment of the flame. Temperature control is not precise, and the time for the system to respond to a change in setting is several hours.

Preventive Maintenance

• Kerosene systems require frequent preventive maintenance.

• Daily maintenance for kerosene systems involves adjusting the flame setting to maintain proper temperatures, usually twice a day.

• Weekly maintenance for absorption systems involves checking for excessive ice buildup and defrosting if necessary, filling the fuel tank, cleaning the flue and baffle,  disassembling and cleaning the burner, and trimming the wick. 

• Monthly maintenance for kerosene systems involves checking for dirt buildup on the condenser and absorber and cleaning them when necessary, and thoroughly cleaning the fuel tank.

Reliability

• Because kerosene refrigerators have no thermostat, control of the interior temperature is dependent on inspecting a thermometer and manually adjusting the temperature. Control of the temperature is, therefore, less reliable than in thermostatically controlled vapor-compression refrigerators. Also, the reliability of a kerosene refrigerator is strongly affected by the availability of fuel—as well as its quality—and by how the refrigerator is used.

Installation, Service and Repair

• Installation of kerosene systems is straightforward. However, downtime for both scheduled and unscheduled service is more frequent than for photovoltaic systems.

Economic Comparison of Photovoltaic and Kerosene Systems

In procuring a vaccine refrigerator, the primary goal is long-term reliable cold storage of vaccines so that increased immunization coverage and improved health care can be attained. Another goal is to do this at the least cost over the life of the refrigeration system. Before considering the economics of photovoltaic refrigeration systems, examine the following flowchart to help in making a decision about this technology.

Many simple “yes/no” decisions may clearly favor photovoltaics or kerosene.  The following discussion is for those who determine that examining economic trade-offs makes sense.

Because the characteristics of the two types of systems are so different, economic comparisons are often confusing. For example, the lifetime of a kerosene-powered vaccine refrigerator is typically about five years. For a photovoltaic system, on the other hand, the array has an expected lifetime of more than 20 years, and the vapor-compression refrigerator has an expected lifetime of about ten years. Using current EPI guidelines, batteries will last only two to three years, but this can be extended at little cost to as much as ten years by specifying batteries designed for long cycle life and deep discharge.

The initial cost for installed kerosene systems is about $1500 (late 1988). The installed cost for a system powered by photovoltaics is higher. A complete photovoltaic-powered system with a 30-liter refrigerator and no ice-making capability currently sells for less than $1800. The WHO/UNICEF EPI Technical Series indicates that a volume of 25 liters is sufficient to store 100,000 doses of vaccine. A larger photovoltaic system with a 30-liter freezer and a 50-liter refrigerator currently sells for about $3400. Because fuel, service, repair, and replacement costs are high for kerosene systems, the cost per delivered dose of vaccine over the life of the system may still be less for the system powered by photovoltaics than for one using kerosene.

Criteria for Making Decisions

A number of criteria may be used to make procurement decisions involving vaccine refrigerators. Representative criteria include the following:

• Lowest first cost. Decisions based on this criterion use a short-term perspective. Unfortunately, the short-term nature of budget cycles, the partitioning of budgets into restrictive categories, and procurement policies often result in the use of this criterion only.

• Lowest life-cycle cost. This may be expressed as either total cost over the system’s lifetime, or cost per effective dose of vaccine over the same time period. All costs must be included, especially an estimate of the value of lost vaccines.

• Greatest reliability. For vaccine refrigeration systems, reliability means a minimum of downtime, ability to continuously maintain stable temperatures, and components with long lifetimes. 

• Operating convenience and productive use of time. The more simple and convenient the unit is to operate and maintain, the more likely it is that maintenance will be performed and the more time the health professional can devote to health-related matters.

• Other factors. Other criteria that might be considered include susceptibility to energy shortages and reductions in operating budgets, environ mental concerns, etc.

Of the criteria discussed above, the only one that typically favors a kerosene system procurement is lowest first cost. First costs are often poor measures of longer term cost effectiveness. Consequently, procurement officers are encouraged to give consideration to the other criteria, especially as they apply to improving and sustaining the cold chain.

Procurement Strategies

The WHO/UNICEF EPI Technical Series provides documents in English, French, and Spanish covering

* EPI product information sheets

* Guides on the implementation of solar energy for the EPI

* EPI training information sheets.

These documents provide information on available hardware and suggestions on designing systems and specifying components for procurement actions. They can be obtained from

UNICEF-United Nations Children’s Fund
United Nations
New York, NY 10017
USA
Telephone 212-754-1234
Telex 234292 UNICEF New York

Consideration should be given to options for specifications in the EPI Technical Series.

These suggestions are offered to avoid problems in the field. The systems chosen following them will be low in maintenance and cost-effective. A recommendation for all refrigerators, and the first step in solar design, is to minimize the power requirements. Over sizing increases fuel requirements, the size of the power system, and maintenance. The hints for saving energy and minimizing the size and cost of a vaccine storage refrigerator are the same for all types: buy only the size that is required for a given use, do not make ice unless it is needed, open the refrigerator as infrequently as possible, and only store essential items in the unit.

As an aid to the design specification, we have constructed a one-page form that contains much of the information specific to photovoltaics. It is included at the end of this report.

Technical and Procurement Assistance Available from Sandia

Sandia National Laboratories, through its Photovoltaic Systems Design Assistance Center, is now offering unique support services that smooth the procurement process, minimize any risk, and help ensure the reliability of photovoltaic-powered vaccine refrigeration systems.

The support services are offered with the realization that there have been some less than optimal experiences with photovoltaic systems in the past. Most of these problems have been minor from a technical standpoint, but major from that of the user. Through these support services, it is Sandia’s goal to avoid such problems with future procurements. In this way, buyers can take advantage of the excellent match that exists between photovoltaic technology and vaccine refrigeration needs throughout the world. 

The specific technical support and procurement services offered by Sandia include:

• Assisting in the use of appropriate technical specifications, including the proper combination of functional requirements for the system and design requirements for components.

• Disseminating results of on-going laboratory evaluations of both photovoltaic and kerosene-powered vaccine refrigeration equipment.

• Recommending education and training programs, materials, and instructors for target groups.

• Making recommendations on the appropriateness of photovoltaics for specific applications and conditions.

• Distributing information and publications on the use of photovoltaics for water pumping, water purification, lighting and rural electrification.

• Offering additional technical assistance for projects with high impact potential.