A Solar Solution For Long-Term Refrigeration
Swaziland is a country facing many challenges. The nation of 1.2 million people located in Southern Africa has massive economic and health-care troubles. Most people are subsistence farmers or livestock herders; nearly 70% of the population lives in poverty. Health-care problems are equally drastic: more than one in four adults are infected with HIV; tuberculosis is widespread and there is an 18% mortality rate. Life expectancy is one of the lowest in the world at 31.88 years.
The country is primarily rural, and getting medicine to the population is difficult. Storing it after it gets to the desired location adds to the difficulty, especially since the electrical grid in the country is spotty at best and large areas remain underserved.
To address this problem, a local manufacturer working to World Health Organization (WHO) specifications has developed the Solar Therm line of refrigerators powered directly by solar energy. Models include a commercial unit, a residential unit and a unit designed for use in off-grid medical clinics.
The Fridge Factory, originally established as Palfridge, is based Matsapha, Swaziland. The company is principally a niche manufacturer of off-road camping and outdoors products.
Instead, the refrigerators needed to meet WHO specifications, which are more stringent. For instance, the medical unit refrigerator needs to maintain internal temperatures of between 2-deg. C and 8-deg. C for a minimum of five days when ambient temperatures are 42-deg. C.
In addition to meeting these internal temperature specs, Tish Foster, marketing director at Fridge Factory, said that the company was challenged to manufacture a unit that runs solely on solar power. There could be no battery backup as a power source, she explained, even in the event of low or no sunshine. Batteries, said Foster, contain lead, mercury and other materials that are not environmentally friendly. They can also be a target for thieves, and are costly to replace and maintain.
Simple DesignWhile powering refrigerators and other electronics with solar energy is not a new concept, the simplicity of the Fridge Factory’s design sets these units apart.
Most solar refrigerators work by converting solar energy into direct current (DC) electrical energy via a photovoltaic array, said Roy Singh, technical director at Fridge Factory. The current typically ranges from 12 VDC to 48 VDC.
An inverter is required to convert the DC energy into alternating current (AC) to power a refrigerator. Battery banks are used to store energy. A controller regulates voltages and charge levels. This concept, while used extensively and successfully, has endemic considerations that the Fridge Factory wanted to avoid such as conversion loss that can occur as the light energy passes to the photovoltaic array, the battery, inverters, compressors and so on.
The Solar Therm units feature two 90-W monocrystalline solar modules that convert the solar radiation into 12 VDC. The power passes directly to a Danfoss DC compressor without any required interfaces. The connection between the solar panels and the compressor is a simple male/female hella plug.
The compressor drives the refrigerant throughout the refrigeration system and cools the unit. Each Solar Therm model features separate freezer and refrigerated compartments. The freezer compartment contains a ballast load that is full of water that freezes, and the ice thermally stores the energy, said Singh. A damper located between the two compartments regulates the flow of energy from the freezer to the refrigerated section. The damper shuts off automatically when the temperature in the refrigeration compartment reaches specs.
In addition, the compressor shuts off when temperature specifications are met and also at night. The ballast load keeps the unit cold overnight and during low-light conditions. The next day, the process continues.
Another key to maintaining the internal temperatures, especially during periods of low or no sunlight, is an extra thick layer of rigid polyurethane foam insulation from Bayer MaterialScience. A conventional unit has an insulation thickness of approximately 60 mm for the freezer, and 40 mm for the refrigerator. The Solar Therm unit has an insulation thickness of 120 mm. The extra thickness results in lower energy consumption and helps maintain internal temperatures, Singh said.
Efficient ConstructionIn addition to the solar panels and thicker insulation, the company has designed its refrigerator units to more efficiently cool its cabinets. The refrigerators feature galvanized steel piping around the freezer compartment, which is attached to the freezer tanks using thermal conducting paste and thermal tape. This helps ensure maximum contact to the unit and maximizes freezing potential-resulting in even freezing, improved temperature conservation and electrical savings through reduced component run time, as well as reduced wear and tear on components.
The compressors are stored behind a removable grate, which increases airflow to the unit’s working parts. The airflow places less demand on the working components, saves electricity and extends the working life of the units.
This construction has “proven to be more robust and suitable” for the African market as they are less likely to collect dust and grime that can reduce the cooling effectiveness and efficiency, according to McCullough.
Environmentally FriendlyThe company has converted many of its production processes and materials to ensure a more environmentally friendly product and a factory with a lower carbon footprint. The company undertook what it calls the GreenFreeze Factory upgrade program. As part of the program, the Fridge Factory converted its foam expansion blowing agents and refrigerants to more environmentally friendly materials.
The company replaced hydro chlorofluorocarbon (HCFC) R-141b blowing agents with cyclopentane, an organic compound that has a zero global warming potential (GWP) rating. Cyclopentane features low foam density, uniform cell size and high R-values. The foam expansion agent is high-pressure foam injected at 200 Bars.
In addition, the company converted its refrigerants from R-134a-a hydrofluorocarbon with a GWP of 1,430-to natural refrigerants including R290 Butane and R600 Isobutane gases that have zero GWPs. McCullough said these refrigerants are, by far, more efficient cooling agents than 134a.
Efficiency is key for a refrigerator used in a country with little electrical infrastructure. The Solar Therm refrigerator runs completely on solar power in remote locations, and the local population will depend on its ability to keep medicine and food safe. Swaziland is an old culture with many challenges, but few resources. One resource is sunshine, and the Fridge Factory’s Solar Therm refrigerators and freezers utilize that resource in an effort to solve, or at least alleviate, some of the nation’s troubles.