Solar Fruit Drying Project

Solar fruit drying is becoming an attractive technology for small farmers in the developing world.  Its low upfront cost, zero electricity requirements and simplicity make it an easy way for farmers to avoid post harvest losses and increase profits.  As part of the 2011 D-lab 2 group, Dominic La Marche, Marco Pritoni, and Blake Ringeisen were asked if reflecting the sun’s energy onto the dryers using some kind of solar concentration device could potentially improve the performance of these solar fruit dryers in the African nation of Tanzania, where hazy and cloudy conditions frequently disrupt the drying process.

Motivation for Solar Drying in Tanzania 

Fresh tomatoes in Tanzania sell for approximately $0.13 per kg in the high season and $0.50 per kg in the low season. Dried tomatoes are generally $3 per kg, but are only one-tenth the weight of a fresh tomato.  Tomatoes can be a very large source of income for rural farmers in Tanzania, who make $200-$400 per year on average. However, due to drastic price instability in Tanzania, many crops are often left to rot in the fields when the market price drops.  Additionally, in developing countries, post harvest losses of fruits and vegetables generally range from 20-50%.  Being able to dry and preserve tomatoes during the high season and delay their sale until the more valuable low season could add a significant amount to farmers’ incomes.

Take an average Tanzanian farmer on a ¼ hectare plot of land producing 1000 kg of tomatoes and earning $300 annually. In an attempt to reduce post-harvest losses, he decides to dry 350 kg during the high season which produce 35 kg of dried fruit. Several months later during the low season, he sells the dried fruit at $3 per kg.  This produces an additional $105, which is a 35% increase in the farmer’s income.

Why it is so difficult to dry tomatoes in hazy conditions?

Well, everyone can probably imagine the reason; there is little sun out there to remove the moisture from the fruit. But what are the factors influencing the drying process? Drying is a complicated process and it depends upon many factors such as: type of tomatoes, maturity, percent of moisture, humidity and temperature of the air, and solar radiation, but also the convective effect of the air, radiative exchange with the sky and the surroundings.

How can we improve the drying rate in these conditions?

D-Lab team tried to address the problem using solar concentrators. They built two identical dryers and they tested them with various concentrators, measuring temperature and relative humidity inside the dryer, air speed, irradiance inside the dryer, as well as ambient weather conditions. These variables will be used to interpret the drying time and to guide potential improvements of the system.

With two similar dryers built and calibrated, the team was able to test their concentrator design against a control.  One dryer was set up with a prototype solar concentration; the other was set up without one.  Data such as temperature, relative humidity and solar radiation were taken at multiple locations in each of the dryers and outside in ambient conditions.

Preliminary results were encouraging so the group decided to test a larger size concentrator.The team constructed a replica of the first concentrator nearly doubled in size as their second prototype. Results were not as encouraging; It could be that other geometric factors are more important than size. They decided to try a completely different design.

 

The third design was to use concentrated sunlight to increase the temperature of an elevated black pipe to induce airflow through the dryer.The designs were all simultaneously tested against a control dryer with no concentration device. The DLab team is continuing to iterate their designs and experiment with new ones.

Work will continue throughout the summer and after some additional testing and modeling, they will hopefully be able to determine a viable solution to the problem of solar fruit drying in hazy conditions. They will continue testing these dryers during the summer while working on a mathematical model of the process. To watch the team in action, check out the video.