Solar Drying

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Introduction

Preservation of agricultural produce is one of the central problems faced by developing countries. And as time goes on, these problems will be aggravated by the growing dietary needs of the ever increasing population of these countries . In many developing countries large quantities of fruits and vegetables spoil due to inadequate infrastructure, insufficient processing capacities, and growing marketing difficulties caused by intensifying competition and protectionism in the worldwide agricultural markets. Drying these products can help solve these problems, while also making an important contribution to improving the population's income and supply situation.

Drying is an important form of food preservation that is often carried out at farm level right after harvest, or especially with highly perishable crops, at peak harvest time when local markets are saturated. Drying vegetables, fruits and meat with thermal energy enables longer storage times and easier transportation (by decreasing the moisture content). If properly done, drying can also help to preserve the nutritional value of the food and reduce the growth of harmful pathogens/bacteria in the food.

Drying is an energy-intensive process and the drying rate depends on a number of factors such as type of produce (moisture content), shape and size of the produce (how thick is each piece), how are they spread on the drying rack/try (what is the thickness of each drying layer) and how often are they turned during the drying process. For optimal drying, it is important to not to overcrowd the drying track and to turn the produce regularly to dry evenly.

Agricultural products can be dried open-air directly in the sun or with dryers powered by biomass or solar energy. This article outlines the different drying methods:

Open-Drying

Open-air drying takes place when food is exposed to the sun and wind by placing it in trays, on racks, or on the ground. The advantage of drying products directly open-air is that almost no costs for fuel and appliances have to be spent by the farmer. However, the dried products are often of lower quality due to varying temperature levels and contamination of the products with dust, vermin’s and leafs. Up to 70 per cent of agricultural products spoil during the traditional process of open-air drying, especially in tropical and subtropical regions. Open-drying is also limited to certain times of the day when the sun is shining and is mostly suitable for locations where the humidity is below 60%. Open drying is popular for drying grains like rice, wheat in many Asian and African countries. For more information about open drying see here.

Solar Dryers

Schematic view of a solar dryer

Solar dryers require a certain investment for the set-up of the appliance, but no expenditures for the fuel. The basic function of a solar dryer is to heat air to a constant temperature with solar energy, which facilitates extraction of humidity from crops inside a drying chamber. Ventilation is enabled at a constant rate through defined air inlets and outlets, small solar ventilators or temperature difference, either due to exposition or vertical height. In direct sun driers the food is put in boxes with a transparent lid. Additionally, the temperature in the drier is raised due to the greenhouse effect and the air exchange is regulated by vents. The food is not exposed to direct sunlight in indirect sun driers as the fresh air is heated separately from the food chamber. This method is preferable for drying foods which lose nutritional value when exposed to direct sunlight. Hybrid driers combine solar energy with a fossil fuel or biomass fuel (Green and Schwarz, 2001a).

A first step when considering solar drying is to compare the different drying options available. Solar drying will only be successful, when it shows tangible benefits in comparison to existing drying methods. In comparison to the traditional way of drying outside in an open field, solar dryers prevent contamination of produce by dust, insects, etc., thereby ensuring quality. They allow small-scale farmers to transform their harvest into storable and tradable goods, which they can sell off-season at higher prices. The constant temperature and ventilation allows a consistent drying process which results in better product quality and higher prices. However, the investments costs of solar dryers vary highly depending on the size of the solar dryer, locally available materials and environmental conditions, such as slope and exposition of the side, rainy seasons.

Solar drying in Bolivia (Source: N. Pereya, GIZ EnDev Bolivia).

There are different types of solar dryers, such as:

• direct drying (solar box dryer),
• indirect drying (solar cabinet dryer),
• mixed mode drying (solar tunnel dryer) or
• hybrid drying (hybrid solar/biomass cabinet dryer).

Small-scale solar box and cabinet dryers are based on natural air convection, while solar tunnel dryer is based on forced convection (air circulation fan necessary).

The Hohenheim ‖-type solar tunnel dryer combines simple construction, use of renewable energy and easy handling. The model is equipped with a photovoltaic (PV) driving fan with a power requirement of 20-40 W. The thermal energy gained from solar radiation is up to 60 kWh/day (equivalent to 15 kg of firewood). As a cooperative development between the Institute for Agricultural Engineering in the Tropics and Subtropics of Hohenheim University and INNOTECH Engineering Ltd., solar tunnel dryers are in commercial operation in more than 60 countries all over the world.There is a guidance paper on how to install a solar tunnel dryer with explanations and pictures, available at: http://www.ats.uni-hohenheim.de/_deutsche_site/projekte/_pdf/Guidance.pdf.

A simple overview about different types of solar dryers can be found here. Also see: Sustainable Energy for Drying.

Some considerations for solar dryers:

• Align the panels for receive optimal irradiation and also to allow the rainwater to run off the panels.
• Avoid shading on the panels to increase the dryer efficiency.
• The air moving through the dryer needs to be hot and dry.
• Avoid overcrowding of the dryer rack for uniform drying

Choosing a Right Dryer

Below are few guiding questions for choosing a right dryer:

1. What kind of produce is being dyed and in what quantity?
2. Is the produce being dried for household use or for selling in the market? In case of sale in the market, is there enough demand/revenue generated to spend on a mechanical dryer like solar or biomass dyer?
3. What is the climatic profile of the location? Is there enough resources such as sunshine or biomass for solar and biomass dryer respectively?
4. Who is going to pay for the dryer and is it affordable for the target community? If not, what financial support (grants, loans) is needed to make it affordable?
5. Preferred method of drying i.e for direct exposure to solar, box dryers are the better option and for indirect exposure to sun, solar cabinet dryers are a better option.

Project examples

Coffee processing with solar dryers in Peru

Energising Development Peru promotes solar dryers among individual smallholder coffee farmers for the first drying period, during which the humidity of the beans is reduced to around 25 per cent. The solar dryer improves the drying process by filtering UV radiation, concentrating heat, reducing the relative humidity of the air and thus drying the beans with constant and natural ventilation. However, coffee can only be stored and exported at a lower level of humidity. A second drying phase is therefore required, that reduces the humidity of the beans to around 12 per cent; this phase takes place in a bigger solar dryer with a capacity for up to 2 tonnes of coffee, which is managed by farmers’ associations. Victoria Esteban Fuentes has a farm of two hectares in Santa Anita, Satipo, Peru. Before the adoption of the new solar dryer, approximately 70 per cent of Victoria’s coffee harvest met export standards. The solar dryer has increased this rate by eight per cent. Victoria’s income has therefore increased by PEN 2,400 (USD 885) or 30 per cent per annum. The investment in an efficient solar dryer certainly paid off for her. Please find further information at http://www.produse.org/imglib/downloads/energy_sources/PRODUSE-Factsheet-Peru.pdf

Drying within a solar dryer protects the yield against spoiling by rain or wind and animals (Source: J. C. Quiroz, GIZ EnDev Peru).

The tent out of the protective plastic film increases the drying process extremly (Source: J. C. Quiroz, GIZ EnDev Peru).

Drying peaches with solar dryers in Bolivia

One third of the population in rural Bolivia relies on agriculture for their main livelihood. Food preservation with thermal energy technologies such as solar dryers, can help improve the income and food security of small farmers in remote areas. Instead of merely drying the products without protection from other environmental influences or weather risks, using a solar dryer allows for a faster and more hygienic drying process. The result is a product with a higher economic value in the market. Dry fruits are also a successful export product for Bolivia: 67.5 tonnes of dry fruits (banana, peaches, mango, pineapple, green tea and anise) were exported in 2011. Energising Development (EnDev) promotes food processing technologies such as solar dryers among small and medium enterprises (SME) and farmers. The demand is increasing. Throughout the different regions of the Altiplano (Andean dry highlands), at least 40 solar dryers have been constructed and installed for the production of Andean products such as sweet potato, oca, maca, medicinal plants and meat; in the more humid valleys, peanuts, peaches and chillies are grown; and in the tropical region, cocoa, coffee, stevia and jatata leaves are produced. EnDev supports two kinds of solar dryers: one is completely constructed and delivered by the manufacturer. The cost depends on the size of the solar dryer and the materials used, but is at least USD 150. The other solar dryer model has a simpler design and can be constructed by the farmer using local materials such as wood and bamboo. Farmers thereby develop a good technical understanding and working knowledge of the solar dryer, which keeps maintenance
costs low. Further information are available at: http://www.produse.org/imglib/downloads/energy_sources/PRODUSE-Factsheet-Bolivia.pdf

Drying oregano with solar dryers in Peru

In Candarave, they have dried oregano traditionally

The solar oregano dryers permit a uniform drying in all the seasons of the year (Source: C. Bertello, GIZ EnDev Peru).

Solar drying of oregano allows the entrance
of the product into quality certified commercial chains (Source: C. Bertello, GIZ EnDev Peru).

for many years. However, the residents have also tried to produce improvised driers (with sheets of mica), but without obtaining the searched for results.

Starting with a coffee dryer, we are seeking to develop a similar model for oregano. We are conducting this process jointly with the farmers of the Central Association of Candarave Province Oregano Producers (CEAPRO). In this context, the model is being adjusted to the needs of the zone and the product, such as the degree of hydration, the level of solar radiation, the positioning, and the air flow, besides using and maintaining it properly. The improvement of the product is taken from the proposals and needs of those same agriculturists.

It must be taken into account, for example, that in order to obtain an optimum drying, in which the oregano doesn't lose its characteristic green color, at a certain temperature with a certain degree of ventilation, so that the oregano will not get dehydrated. In this manner, a product of quality, competitive in the market, is attained.

In the same manner, by not being in contact with the soil, the product stays free of impurities and its contamination is avoided. Additionally, advantage can be taken of practically all of the production: with traditional drying a decrease of around 15% was obtained, while with the current process it only reaches 5%.

Once the benefits of these dryers are proven their acquisition will be recommended, a proposal that could be presented to the AGROIDEAS program, of the Ministry of Agriculture, an entity that could help with the financing.

Furthermore, it is also possible to define strategic collection points for the drying of the oregano, to where the producers can bring their product and – according to a schedule of turns – take advantage of the dryers in the most efficient manner, besides remaining in permanent contact with the provider of the equipment, in a manner that optimizes the good use and maintenance of the product.

All of this influences in bettering the quality of the product and opens up its commercial possibilities.The exporters consider the degree of impurities present in the product, as well as the color, the aroma, and the size of the cut, that's to say, the elements that permit access to a market that generates greater earnings for the local producer.

Drying chili peppers with solar dryers in Peru

The technologies for the drying of chili peppers are developed in conjunction with local producers (Source: C. Bertello, GIZ EnDev Peru).

The Association of Agricultural Producers of Chili Peppers of Inclán is looking to enter into the program AGROIDEAS in order to finance technologies that would permit it to elevate its productivity. This process is participatory and seeks to generate a change in the manner in which the challenges of agricultural production are affronted. Looking to the future on the large scale is what we need.

The principal advantage of using a dryer is that the product is not contaminated. Normally, the drying process is carried out on the soil and the product is exposed to the impurities of the earth and to the birds and other animals that loiter around the product. Additionally, with the dryer, time is saved and a uniform product quality can be obtained.

The dryer permits the product to enter into a quality certification process or to ingress as prime material into other productive chains in which it is necessary to guarantee the purity and adequate management of the product. In other words, the product would have an added value that would permit it to position itself in the high quality markets, which in turn would generate a greater economic benefit for the farmers of Inclán.

We are working to provide technologies that can be adapted to the productive chain that the agriculturists manage, reason for which the dryer is being developed from a participative process with them.

Therefore, it is necessary that both parties collaborate, as much the provider of the technology, who must be completely available in order to give technical assistance and be disposed to modify the product, as the farmers, who must furnish the necessary information in order to develop a product that adequately covers their needs, while also using the dryer in the proper way.

Moreover, from this experience, it will be possible to validate a technology that can be included in the business plan that the association is developing for the AGROIDEAS program.

Further Information

• Axtell, Barrie/ Swetman, Tony (2002): Small-scale Drying Technologies, Published by Practical Action. This Technical Brief describes improved drying systems that suitable for use by small-scale producers.
• Azam Ali, Sue (2002): Drying of Foods, published by Practical Action. This Technical Brief describes some of the requirements for food drying, as well as summarising information on the various drying equipment.
• Buchinger, Josef/ Weiss, Wender (???): Solar drying. Training course within the scope of the project: Establishment of a prodution, sales and consulting infrastructure for solar thermal plants in Zimbabwe, Arbeitsgemeinschaft ERNEUERBARE ENERGIE, Institute for Sustainable Technologies: Gleisdorf, Austria.
• Häuser, M./ Ankila, O. (1997): Solar Drying in Morocco - Experience with Solar Drying in Marocco from the GTZ Special Energy Programme (SEP); Overview on technologies, drying techniques, and economics.
• Food and Agriculture Organization of the United Nations, and Technical Centre for Agricultural and Rural Cooperation (Ede, Netherlands) (2011): Rural structures in the tropics: design and development. Rome: Food and Agriculture Organization of the United Nations.
• GIZ HERA and EUEI PDF (2013): Use of Thermal Energy_Overview.pdf Use of Thermal Energy_Overview.pdf Use of Thermal Energy_Overview.pdf Productive Use of Thermal Energy - An Overview of Technology Options and Approaches for Promotion.
• GIZ-HERA (2011): Modern Energy Services for Modern Agriculture - A Review of Smallholder Farming in Developing Countries.
• Green, Matthew G./ Schwarz, Dishna (2001): Solar Drier Plans: PGCP Coconut Drier and Kenya Black Box Drier; GTZ-GATE. 2001
• Green, Matthew G./ Schwarz, Dishna (2001): Solar Drying Technology for Food Preservation; GTZ-GATE 2001. This publication presents possibilities of solar drying with a focus on technical needs, classification of driers and selection criteria´s. Moreover the publication provides information on moisture content of foods, drier components, the drying process, and the capabilities of solar driers.
• Green, Matthew G./ Schwarz, Dishna (2001): Solar Drying Equipment GATE 2001; GTZ-GATE.
• Swetman, Tony (2012): Séchage Solaire, published by Practical Action. La chaleur du soleil, associée au vent, est utilisée pour sécher les cultures vivrières, dans le but de leur conservation, depuis des milliers d'années.
• Noble, Neil (2002): A Simple Solar Dryer, published by Practical Action. This dryer is a mud brick construction built close to the ground using low-cost materials for all its parts.
• Russell, A. (1998): DRYIT Batch Tray Dryer, published by Practical Action. A double chamber batch dryer used for drying foods and herbs.
• Solari, Giannina (2011): A rebell with a cause; Solar drying in Peru, published by Practical Action. Bruno Caracchini has followed up on his idea of producing and marketing dehydrated fruit. His business is in the pilot stage.
• Weiss, Werner and Buchinger, Josef (2001?): Solar Drying, published by the AEE - Institute for Sustainable Technologies. Trainings course on the production and sale of solar thermal plants in Zimbabwe.
• Overview about Solar Energy in Agriculture
  

References

This article was originally published by GIZ HERA and is based on the publication "Productive Use of Thermal Energy - An Overview of Technology Options and Approaches for Promotion" published by GIZ Programme "Poverty-oriented Basic Energy Services" and European Union Energy Initiative Partnership Dialogue Facility.

Information on drying oregano and chili peppers in Peru was originally published by EnDev Peru in the third issue of the Amaray magazine published in April 2013 (Amaray N°3).

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