Sustainable Energy for Drying

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Introduction

As the world population increases and with it our dietary needs, the conservation of agricultural produces and the reduction of post-harvest losses are pivotal to food security. While drying food has been practiced for centuries, traditional methods such as open-air drying or other unimproved drying processes should be replaced by modern solar drying in order to meet the standards of an increasingly competitive globalized market. This not only increases the produce quality but also farmers’ revenues, supporting income generation. By promoting food-processing technologies like solar dryers, food quality becomes better, guaranteeing export suitability for different types of food throughout the different regions, improving incomes and food security of small-scale farmers in remote areas.

Technologies

Perishable products like fruits, vegetables, tubers or even meat and fish can be saved from spoilage by using the energy of the sun. Especially in countries where industrial technologies for conservation are not available, simple solutions like solar drying hold high potential.

Solar drying consists of accumulating the sun’s energy inside a heat collection device, and then leading the hot air flow through natural or forced convection to the products. When passing the food, the warm dry air removes moisture which is led outside through a chimney device at the other end. Depending on the requirements of the final product, the sophistication levels of solar drying may vary. While traditional solar dryers use the natural convection processes of hot air, innovative approaches include a fan that runs on photovoltaic energy, moving the air inside the dryer artificially and increasing its efficiency. As opposed to conventional sun drying, solar drying usually takes place inside a closed system, protecting the commodities from outside impurities. Different types of solar dryers can be of a variable complexity: direct, indirect, mixed or hybrid drying are the main options for different needs. Read more…

Solar Box Dryer

The solar box dryer consists of a box with a glass cover on top, inclined at an angle to allow maximum solar radiation to enter. The inner walls of the box are covered with an aluminium sheet with black coating to absorb the radiation entering through the transparent top. The products to be dried are spread on three trays made of stainless-steel wire mesh inside the box. At the lower part of the construction, a rectangular opening at the front wall allows the entrance of air, which through convection enters the box, dries the products, and leaves with the extracted moisture through a chimney made of galvanized iron sheets at the top. It has a small capacity and the drying rates are relatively slow, leading to discoloration of the products, which makes this simple technology suitable for domestic but not commercial use. Read more…

Solar Cabinet Dryer

A little more complex than the solar box dryer is the relatively more expensive solar cabinet dryer. It consists of two parts: a collector to heat the incoming ambient air using solar radiation and a drying chamber in which food to be dried is spread on a number of trays on different layers. Using glass wool for insulation and aluminium and galvanized iron for heat conduction, the dryer allows indirect heating, which is recommended for drying herbal products, usually sensitive to direct sunlight. In contrast to the solar box, the cabinet dryer is recommended for community use and small-scale income generating industries.

Solar-Biomass Hybrid Cabinet Dryer

The hybrid biomass-solar version includes a biomass stove installed adjacent to the collector system of the basic solar cabinet dryer. Using a supplementary fuel like biomass can enhance the drying capacity of the simple solar cabinet dryer, allowing higher drying temperatures, recommendable for drying fish and meat products.

Solar Tunnel Dryer

While the above-mentioned technologies use air circulation entirely from natural convection, the solar tunnel dryer includes a small blower running on photovoltaic energy to force air circulation through the solar collector and the drying chambers. Arranged in the form of a tunnel, dryer boxes and solar collectors capture solar energy and heat the product on the trays, while the air forced through the tunnel removes the moisture even under unfavourable conditions. These dryers are recommended for large-scale drying for commercial uses.

Actors & Innovations

Different innovators have shown the potential held in renewable energies applied in drying. Creative approaches span from solar photovoltaic systems to biogas-powered models, facilitating access to the poorest in rural areas without electricity. As solar radiation is usually high and agricultural by-products, which cannot be sold on the market, abundantly available in the Global South, solar and biomass are often the only source of energy available in these regions. The use of renewable resources means lower costs for food conservation, leading to less post-harvest losses and higher incomes.

Biogas Powered Infrared Coffee Dryer

The Horn of Africa Regional Environment Centre and Network from the Addis Ababa University in Ethiopia and partners have developed and implemented a clean energy solution for drying coffee beans. Using the coffee husk and pulp waste for biogas production, an efficient infrared coffee dryer can be powered, reducing the coffee cherry drying time and minimizing post-harvest losses that occur with conventional sun drying. This enhances working conditions for women and children, who are primarily responsible for the drying process, which requires all-day exposure to the sun. Read more...

Solar Bubble Dryer

Another innovation allowing safe and efficient drying conditions using solar energy is the solar bubble dryer, designed by the University of Hohenheim (Germany) and the International Rice Research Institute (IRRI). The technology consists of a 15 to 26 metres long plastic tube where the rice is laid out. The transparent upper side of the tube allows the sun’s rays to penetrate, building up heat inside and drying the product. The heat is distributed uniformly by solar-powered fans that make the air flow, removing the moisture. For optimized drying, the rice is turned regularly using a rolling bar. Being currently optimized energetically and trialled in different countries, the bubble dryer can cost between € 1,200 and € 3,400. Read more ...

Inflatable solar bubble dryer (©GIZ/Schock)

GrainSafeTM Dry (GSD) Development

In collaboration with the International Rice Research Institute (IRRI) and the University of Hohenheim, GrainPro, Inc. have designed the GrainSafeTM Dry (GSD). The GSD combines in-store drying with hermetic grain storage. In-store drying aims to control the relative humidity of the drying air, so that all grain layers in the deep bed reach equilibrium moisture content. This is possible as a blower that runs on solar power, pushes warm air at the bottom of the device into the grain bulk until the desired humidity level is reached. In hermetic storage the grains are enclosed in an airtight container made from material with very low oxygen permeability, protecting the grains from insects and water reabsorption. Combining the in-store dryer with the hermetic storage properties allows drying and storing food in a protected environment. Including a drying controller allows increasing energy efficiency adapting the blower speed to the relative humidity. With a capacity of 1 to 5 tons of rice, and anticipated system costs of $ 1,100, the GSD still needs to be tested and optimized before a commercial prototype can be developed.

Case Studies

Adapting the solar dryer to meet the specific needs of every agricultural value chain has allowed the implementation of this technology in many parts of the world under very different operational conditions. The here presented case studies are only a fraction of the applicational possibilities of drying technologies and showcase how the innovative drying approaches could increase product quality and thereby farmers’ incomes. With examples from different value chains from across the globe, this technology reveals a high adaptability and a great potential for livelihood improvement.

Modern Solar Drying in Afghanistan

Food drying is a very common way of preserving edibles in Afghanistan. However, the traditional drying practices, which consist of placing the food on flat grounds such as rooftops, are vulnerable against dust, dirt and insects. Therefore, the Modern Solar Drying project, in collaboration with the Afghan Bedmoschk Solar Center e.V. have adapted the Hohenheim Solar Tunnel Dryer to smaller versions, that enables farmers to test and evaluate the technology in a non-expensive way. Despite the positive outcomes of the technology, the higher end prices of the dried products will require a marketing campaign in order to reach wealthier end-consumers and provide higher revenues to the farmers. Read more…

Coffee Processing with Solar Dryers in Peru

Energising Development Peru promotes solar dryers among smallholder coffee farmers for the first drying period, where the humidity of the beans is reduced to around 25 percent. The dryer filters UV radiation and reduces the relative humidity of the air with constant and natural ventilation. As coffee can only be stored and exported at a lower level of humidity, a second drying phase is required to get the beans down to 12 percent humidity. For this, a second solar dryer is employed which has a capacity of 2 tonnes of coffee and is managed by farmers’ associations. The implementation of this solar dryer also provided by EnDev has increased farmers’ incomes by up to 30 percent per year. Read more…

Drying Oregano with Solar Dryers in Peru

Also in Peru, in Candarave, oregano has been dried traditionally for many years. However, the residents have tried to improve their improvised driers without success. After adapting the coffee dryer (see above) to the needs of the product in order to keep its characteristic green colour despite the drying process (adjustment of level of solar radiation, degree of hydration, positioning and air flow), the quality of the product has increased notably, meeting export standards and reaching a larger market. This showcases the broad versatility EnDev’s solar dryer has, allowing its use for many different product types, reaching from fruits as pineapples and bananas to vegetables and tubers as potatoes. Read more…

Drying Peaches with Solar Dryers in Bolivia

Especially countries with a high variety of agricultural products can profit from the adaptability of the solar dryer. Another implementation example of EnDev’s solar dryer can be found in rural Bolivia, a country with a high geographical diversity, where one third of the population relies on agriculture for their main livelihood. EnDev supports two kinds of dryers: the manufacturer either delivers a complete dryer for 150 USD, or provides a simpler version that can be constructed by the farmer using local materials such as wood and bamboo, which also encourages the technical understanding and keeps maintenance costs low. The association AFRUCH dries fruits to make them more durable. Peaches, for example, are dried for conservation and preparation of the traditional soft drink “mocochinchi”, which consists of dried peaches boiled with cinnamon and clove. After the acquisition of the solar dryer, the association were able to increase their income by 60 percent over the last three years. Read more…

Drying Chili Peppers with Solar Dryers in Peru

An example of how solar dryers can be adapted to the producers’ needs could be found in Inclán, Peru, where the development of the dryer took place as a participatory process. Involving the farmers, who provided the necessary information about the product requirements, and the technical provider, which offered assistance and helped to modify the technology, the solar dryer for chili drying was developed. The main advantages were the reduced contamination of the product, which normally is dried on the ground, guaranteeing a uniform product quality, and saving enormous amounts of time. This allowed the product to enter a quality certification process and to become part of other food value chains, where the purity and adequate management of the product were required. Allowing the product to reach a higher position in the markets, the solar dryer helped generating a higher economic benefit for the Peruvian farmers of Inclán. Read more…

Publications

How Access to Energy can Influence Food Losses

The FAO Report “How Access to Energy can Influence Food Losses” highlights the crucial interlinkages between access to energy and food losses in developing countries. It identifies the main stages of the food value chain where increasing access to energy can play a dominant role in reducing food losses directly, by making food processing possible, as well as indirectly by acting as the main enabling factor affecting the rate at which cooling technologies are adopted. It outlines low cost and off-grid post-harvest technologies such as cooling and solar drying that can be made available in developing countries. Most importantly, it assesses the technical and economic feasibility since access to capital can be a significant barrier hindering its implementation in the developing world. Read more...