Is That a Toyota Prius in Your Refrigerator?
by Steve McCarney, PATH
Solar refrigeration has been a promising technology for rural health outposts in developing countries for two decades, but it has one fatal flaw: the batteries tend to die long before the rest of the system is ready to give up. Solar modules themselves have a tremendous life span of 20-25 years, and refrigerators can last 10 years or more. But batteries tend to die within five to eight years. In small rural health posts where cotton swabs and soap are in scant supply, replacement batteries are next to impossible to find, fund, and deliver.
To overcome this challenge, Optimize has been working on two fronts. One is to develop a battery-free solar refrigerator that uses an ice bank to cool the system through nights and cloudy days. The other is to build a better battery—perhaps even one that lasts a lifetime.
Battery technology has come a long way in the last 20 years, and the new nickel metal hydride (NiMH) battery has made a mark in products like the Toyota Prius while the lithium ion family of batteries is safely propelling the Segway personal transporter. Working with two battery manufacturers, PATH will introduce versions of these long-life, maintenance-free batteries into solar-powered vaccine refrigerators. In Vietnam we are introducing both lithium-ion and NiMH batteries into two identical solar-powered fridges. These batteries are to be installed in two nearly identical health centers which will have nearly identical, challenging remote locations. Early field testing will help determine how close these products are to entering the market and, hopefully, solving the battery problem. Time will tell the rest of the story, but prospects are promising as battery makers compete fiercely to lower price in a bid to win the race to supply next generation hybrid and electric vehicles. A positive side effect is the expected benefit to solar technologies.
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From the simulations we have carried out in the past we have found that the number of days of autonomy required in the tropics is dependent on the size of the solar array. A larger array reduces the number of days of autonomy required. Will the design tool being developed by PATH consider the interactive nature of array size and the size of the required battery bank?
Considering the difficulty in obtaining good quality deep-cycle replacement batteries it may be desirable to reduce the number of days of autonomy required by initially increasing the size of the array. This would reduce the depth of discharge of the batteries and increase the life of a lower quality replacement battery.
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