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4.7. Sundrying of fodder: application to other grass products

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- M.A. Salcedo



Sundrying is still nowadays an important way of conserving agricultural produce. The increase in energy prices has an obvious impact on this technology.

Solar energy is a very suitable low-temperature drying technique for fodder (which is the main topic of this paper) or other products such as cereals, Boys, sunflower seeds, or plants for medical use.

In order to collect the solar energy' simple technologies can easily be applied such as the transformation of roofs of agricultural buildings or green houses used for cash cropping.

The choice of the type and size of solar collectors depends on the specific drying conditions of each product.

Nevertheless, one must keep in mind that this new technology has to be included in the production line as a whole. This document will discuss the drying of fodder in barns with solar energy' carried out by SOLAGRO and IDRR.

SOLAGRO is an association that deals with the study and disseminatation of the utilization of renewable energy in the field. of agriculture.

From time immemorial the technique of sundrying has been used for the conservation of agricultural food products. The artificial drying -in this case the drying of grass products in barns- stemmed from the desire of the farmers to obtain a better use of their crops.

Through sun-drying a better quality of fodder could be obtained and, when harvested at the right moment, it could conserve the best nutritive qualities and reduce the poet-harvest losses to a maximum (a 30% increase in comparison to grass dried in the fields), besides protecting the farmers against the consequences of adverse climatic conditions. This will constitute an element of security for the farmers, that cannot be overlooked.



The traditional way of haymaking, by drying in the field, was most commonly used in of drying led to considerable loss of dried grass, the more the product was exposed to adverse climatic conditions.

This technique consists of harvesting the grass when it is still wet (50% loss in weight after 12-36 hours of being spread out in the fields) and then spreading it out on planks for ventilation in the storage rooms (see Figure 1. Drying of grass spread out on planks in storage rooms).

The air is heated by fossil energy (oil, fuel) but the increase of energy prices has induced the farmers to apply ventilation with cold air. The quality of the product has suffered from this and slowly this technique has been disregarded. It must be remembered that for the drying of agricultural products, the flow of the ventilating air and its temperature have to be precisely regulated in order to obtain a product of good quality and free of moisture and other alterations. This technique could also be adapted to the drying of medicinal plants by using trays, stacked one above the other. The size of the installation and the electric power used, will be less the lower the water content is.



The finding of alternative sources of energy became a crucial question. What kind of energy is requested in order to produce a small but noticeable increase (3 to 4 average in 24 hours in our climate) to create a considerable air flow (20 000 to 40 000 m /hour depending on the products and the quantities to be dried, from 25 to 50 T.D.M.). Therefore it would be a waste to use energy (fuel, gas, electricity) that can produce high levels of temperatures, if solar energy can be used.

Solar energy, received through a collector, is well adapted to these demands and can be used in very dispersed locations.

Various types of collectors can be used for drying.

  1. Greenhouses, of the classical type of marketgardening greenhouses (tunnel greenhouse), which can also be used outside the drying season and this enhances the investment (Figure 2. Drying in a greenhouse).
  2. The roofs of agricultural buildings constitute important surfaces for sun exposure; the transformation of the roofs into sun collectors started 3-4 years ago. There are two types, depending on the temperature required:
  1. these roofs are mainly used for the drying of cereals under a transparent cover (Figures 3 and 4. Drying on roof with transparent coverings). They considerably increase the temperature (up to 20 during 12 hours of sunshine) and are less sensitive to windcooling;
  2. driers without a transparent cover (Figure 5. Drying on roof without transparent covering) and with a simpler and less costly construction; these sun roofs are used for the drying of grass products.

These types of sun collectors have the advantage of being simple and can be manufactured by the farmer himself. To obtain the proper dimension, it is, however, necessary to calculate the ratio between the various elements: the air flow used (which depends on the volume of the product), the surface of the collector and the space in which the air circulates (these calculations can be made on mini computers).

The collector can also be used for other products, as well as the heating or air-conditioning of livestock buildings. They could be used in a collective way, if production is spread over a period of time. However? the compatibility between the dried products, the airflow and the temperature must be checked, if one wants to keep the nutritive qualities for an ultimate storage.

Moreover, this type of procedure needs electric power (for instance 6 kw for an installation drying 25 TDM) and it will be possible to establish a ratio between the total energy that is recuperated and the electric power used. This ratio, that could be evaluated in the installations showed a variation of 3 to 24. This clearly indicates the importance of suitable measurements and size of the installations.



Two sun-drying installations are considered, one for grass, the other one for cereals, both located in the southern Pyrenees. It has not been possible to carry out a precise economic analysis of these units, because of their recent installation and therefore, the figures have only an indicative value for France.


Specific solar investment (sunroof 320 m², ventilator sheath' perforated floor) 94 000 (tax free)
Other investment 120 000 " "
Operating cost (1F/quintal of mais, dried to 23 - 15%) 2 400 " "
Cost in case maize had been dried in an agricultural cooperative 13 880 " "

The above mentioned operating cost are only valid for the 1983 campaign. That year was an exception because neither sunflower nor soya were dried (construction work not completed). On the other hand, maize could be entirely dried with the sundrying technique because its moisture was only 23% at harvest time.

In case of more humid years, a drier -operated with fuelis planned for drying maize; which was not calculated in the initial investment.



Here again, we have only one year of experience. However, a prospective study has been carried out on the financial implications of this installation.

This study underlines the necessity for the farmer to adapt his entire production system to the new conditions created by the drier. In such circumstances, the investment (solar roof 11 500 FF, ventilator, planking 53 300 FF) could be returned in 7 years; it will take 13 years if the production system remains the same.

It is not feasible to try and make a comparison with a drying system run with fuel' which has almost entirely disappeared because of the excessively high cost.

In conclusion' this technique can also be used for the drying of medicinal herbs, as is done in the Alps, in thick layers' with 1.50 m of trays, one above another. This technique gives the best use of a dispersed and free solar energy.

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