by Joanie Robertson, PATH, and Steve McCarney, Solar Electric Light Fund
Field-tests conducted in the cloudy north of Vietnam suggest there may be a bright future for battery-free solar refrigerators in low-light conditions.
As reported in the Op.ti.mize October 2011 issue, project Optimize is collaborating with Vietnam’s National Expanded Programme on Immunization to evaluate the effectiveness of battery-free solar-powered vaccine refrigerators. One of the key questions being asked is, how well do these refrigerators perform under relatively poor solar conditions?
For years, solar refrigeration has helped developing countries to increase their cold chain capacity while decreasing energy costs and consumption. However, problems with battery maintenance and cost have made solar refrigeration a challenge. To address these problems, manufacturers have created a new type of refrigerator that eliminates the need for a battery. But without a battery to store solar energy, how can solar refrigerators continue to keep cool in low-light conditions?
“Insolation” refers to incoming solar radiation at a particular time and place. If a location is typically cloudy for long periods, then it would be said to have low insolation, and would not usually be considered an appropriate location for solar-powered equipment. We decided to test this assumption with the True Energy BLF 100 DC Sure Chill® vaccine refrigerator .
Periods of low insolation are commonly encountered from December to February in northern Vietnam, which can often go for several weeks without a sunny day. However, in the sunnier south of the country, year-round high insolation is more common. We installed and evaluated the Sure Chill vaccine refrigerator in both parts of the country—Phu Tho province in the north, and Ben Tre province in the south.
The Sure Chill vaccine refrigerator installed in northern Vietnam showed constant temperature performance into mid-February 2012. This was particularly impressive as the period being measured included 15 consecutive cloudy days. Figure 1 charts the daily insolation values in this critical time frame of very low-light conditions.
The insolation for this entire period trends below the average daily insolation of 3.0 kWh/m2 for the full monitoring period from May 2011 to February 2012. For comparison, the data from the site in the sunnier south of Vietnam are also charted, along with the average daily insolation there of 5.0 kWh/m2 from October 2011 to February 2012.
From December 23, 2011 to February 14, 2012, the refrigerator in the south performed well, as expected given the good solar conditions. But how well did the vaccine refrigerator in the cloudy north of the country perform during this period? The data show that the temperature in the middle of the refrigerator was maintained at a steady average of 4.4°C with no excursions to less than 3.9°C or greater than 7.0°C. Perfect performance.
You may wonder how the Sure Chill vaccine refrigerator can cope so well under these conditions. First of all, it should be noted that the cloudy conditions in northern Vietnam correspond with generally cool ambient temperatures. From December 23, 2011 to February 14, 2012, the ambient temperature in the room where the refrigerator was located averaged around 17°C, with a minimum of 15°C and a maximum of 21°C. Compare these temperatures to the summer temperatures from May 12, 2011 (when monitoring started) to September 30, 2011. During this time, inside ambient temperatures ranged from 26°C to 32°C, with an average of 30°C. In effect, the vaccine refrigerator did not need to work as hard to keep cool in the winter.
Another design feature that seems to serve this technology well is the low starting voltage. The Sure Chill vaccine refrigerator uses a specially designed compressor that requires a lower voltage to start than some other refrigerators, so it can start earlier in the morning and continue until later in the day, when the solar energy is still low. During cloudy days, the compressor is able to start with the small amount of solar energy coming through the clouds, whereas another system might never have enough energy to get going.
The thermal storage system of the Sure Chill vaccine refrigerator has been shown to have adequate capacity to cover a long period of low energy input. (“Thermal storage” refers to the frozen liquid that is generally used in battery-free refrigerators to store energy for cooling, in place of the battery used in traditional solar systems.) These results indicate that battery-free solar refrigerators could be reliably placed in more locations than previously thought, and that batteries are not necessarily the only way to manage low-insolation conditions for vaccine refrigerators. This may be good news for immunization programs in many countries.
Optimize will publish an evaluation of the battery-free solar refrigerator testing in Vietnam later this year. For more information, please email Joanie Robertson ([email protected]).
See related post: In Pictures: Installing/testing battery-free solar fridges in Vietnam
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The insolation for this entire period trends below the average daily insolation of 3.0 kWh/m2 for the full monitoring period from May 2011 to February 2012. For comparison, the data from the site in the sunnier south of Vietnam are also charted, along with the average daily insolation there of 5.0 kWh/m2 from October 2011 to February 2012.
From December 23, 2011 to February 14, 2012, the refrigerator in the south performed well, as expected given the good solar conditions. But how well did the vaccine refrigerator in the cloudy north of the country perform during this period? The data show that the temperature in the middle of the refrigerator was maintained at a steady average of 4.4°C with no excursions to less than 3.9°C or greater than 7.0°C. Perfect performance.
You may wonder how the Sure Chill vaccine refrigerator can cope so well under these conditions. First of all, it should be noted that the cloudy conditions in northern Vietnam correspond with generally cool ambient temperatures. From December 23, 2011 to February 14, 2012, the ambient temperature in the room where the refrigerator was located averaged around 17°C, with a minimum of 15°C and a maximum of 21°C. Compare these temperatures to the summer temperatures from May 12, 2011 (when monitoring started) to September 30, 2011. During this time, inside ambient temperatures ranged from 26°C to 32°C, with an average of 30°C. In effect, the vaccine refrigerator did not need to work as hard to keep cool in the winter.
Another design feature that seems to serve this technology well is the low starting voltage. The Sure Chill vaccine refrigerator uses a specially designed compressor that requires a lower voltage to start than some other refrigerators, so it can start earlier in the morning and continue until later in the day, when the solar energy is still low. During cloudy days, the compressor is able to start with the small amount of solar energy coming through the clouds, whereas another system might never have enough energy to get going.
The thermal storage system of the Sure Chill vaccine refrigerator has been shown to have adequate capacity to cover a long period of low energy input. (“Thermal storage” refers to the frozen liquid that is generally used in battery-free refrigerators to store energy for cooling, in place of the battery used in traditional solar systems.) These results indicate that battery-free solar refrigerators could be reliably placed in more locations than previously thought, and that batteries are not necessarily the only way to manage low-insolation conditions for vaccine refrigerators. This may be good news for immunization programs in many countries.
Optimize will publish an evaluation of the battery-free solar refrigerator testing in Vietnam later this year. For more information, please email Joanie Robertson ([email protected]).
See related post: In Pictures: Installing/testing battery-free solar fridges in Vietnam
To comment, make sure you are logged in and click Reply. 
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