This journal article compares the performance of two types of solar dryers, in use with African indigenous vegetables and small peppers. The article was published January 2017 in the Journal of Food Processing and Preservation. The article's authors are Emmanuel Ayua, Violet Mugalavai, James Simon, Stephen Weller, Pamela Obura and Naman Nyabinda.

Below are the article's abstract and a summary of additional highlights from the article. 

Abstract

A mixed modes solar dryer was modified from a Horticultural Innovation Lab design and its performance compared to a direct mode solar dryer to process horticultural crops. The mixed modes and the direct mode solar dryers dried vegetables at different rates with the mixed modes being faster for all products (p ≤ .001). Leaves and flowers of the spider plant (Cleome gynandra) had the shortest drying time (270 ± 6 min) while the fruit of the African bird's eye chili (Capsicum frutescens) took the longest time (3,866 ± 31 min). Moisture content of the dried vegetables met the industry standard for dehydrated fruits and vegetables (≤8.4%) and of chilies which had a moisture content of (9.5 ± 0.15%). This low cost mixed modes dryer is suitable for small‐scale vegetable processors and farmers as it does not require grid electricity to operate.

Solar dryer types: Mixed modes versus direct mode

Both mixed modes and direct mode solar dryers contain glass or polythene enclosures where the food is placed and where heat builds up to dry the food. This is considered direct mode solar drying.

What makes the mixed modes design different is another enclosure — a solar radiation collector — that is empty and exists for the sole purpose of preheating the air before it passes through the main enclosure that holds the food. Heating up air to dry product is considered indirect solar drying. The combination of both direct and indirect types of drying is what makes this design a mixed modes solar dryer. This design also incorporates a chimney to facilitate air flow through the drying chamber.

Mixed modes solar dryer outperforms direct mode solar dryer

Using African indigenous vegetables the researchers confirmed that the mixed modes solar dryer is capable of dehydrating vegetables to a moisture content below 10 percent, the point at which otherwise perishable products are considered stable. Additionally the mixed modes was able to dry fruits and leafy greens faster than the direct method and to concentrate hot air more efficiently than the direct mode dryer.

Mixed modes solar dryer: effective, affordable, and full of potential

The combination of affordability, versatility, simplicity, and energy independence make this mixed modes solar dryer an attractive option for horticulturalists in Africa and elsewhere.  The unit costs approximately $75 to build, it does not use electricity, and it can be constructed with relative ease from readily available materials. Additionally, the authors estimate an average return on investment period of about 2.2 months.