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SOLAR ENERGY SYSTEM FOR DRYING AGRICULTURAL BY-PRODUCTS

Ziel

A great problem in Sardinian sheep and goat farming is the continuous availability of feed all year round. The project aims to produce from agricultural by-products dry pellets which can be stocked and fed during the dry season especially in summer and autumn. The drying process requires a lot of energy which should be provided through a solar installation. This installation should generate thermal as well as electrical energy. Solar collectors are used for hot air production 60 m2 and a concentrating Fresnel-Lens collector 48 m2.
Many of the technical problems encountered were attributed to the fact that the two main vendors (Jacorossi and Telec), having their headquarters in Rome, could not be present on-site on a continual basis. The local suppliers had no specific experience in this sector. The main difficulties met concerned :
- connection of Northrup concentrating collectors (on water service) and relevant commissioning;
- erection of the Rankine cycle generator (manufactured by Gemmindustria).
Collector efficiency was satisfactory, with measured mean efficiency values during the demonstration of greater than 50%. Data collected show that the range can be wide, particularly in autumn and spring making efficiencies of 0.35 and 0.75 in daytime quite natural. During July and August the efficiency curve of collectors was more regular.
Transparency of the glass element used at the collector was highly influential. For example, one month after washing, the efficiency of the collectos was 5% less thantheir recorded efficiency immediately after washing. Three days after washing efficiency had decreased by 2-3%.
Efficiency is satisfactory even when collector surfaces are not perfectly clean.
The Northrup collectors were unsatisfactory and failed to reach the required temperature of + 100 deg. C. Concerning collector orientation the heat carrier temperature did not exceed 70 deg. C. With regard to optimum heat carrier flow-rate, several methods were adopted and the recommended flow-rate of 4 m3/h was verified. The concentrating panel, though satisfactory from a standpoint of dynamic performance, is not in a position to feed the heat carrier to 130 deg. C.
During the trial year, in no less than three collectors, buses broke:
this also affected the Rankine cycle motor, which could not be operated.
Starting with the biomass of less than 30% humidity it takes 0.8-1.33 hrs to produce 100 Kg of dried by-product. If the humidity content is +- 40%, drying time exceeds 2-5 h/q, with loading and unloading expenditive essentially unchanged. The quantity of dried by-product obtainable annually ranges from 200 to 250 tons.
Commercial feasibility depends on the granting of special credits and the sale of low-cost by-products, particularly to livestock farmers in the Mediterranean, as feed.
The solar part of the plant can be divided in two parts:
1. Thermal energy production.
Flat plate collectors (60 m2) for the hot air supply of the dryer. (Aluminium absorber with a single glazed non-selective surface). The hot air can be conducted directly to the dryer or to a pebble stone bed (13 m3) capable of storing a four hour heat supply for the dryer. The dryer capacity is about 70-100 kg of by-products. Before entering the collectors the air is preheated by the condensor of the tetrachlorethylene steam turbine. The temperature at the collector's outlet reaches 40-80 deg C.
A fan blows the air from below through the ground by-products
2. Electrical energy production.
Concentrating linear Fresnel-Lens collectors (48 m2) heat up the hot water circuit which is feeding a 6 m3 storage tank. The Fresnel-Lens collectors are one-axis-sun-tracked. Average outlet temperature of the collectors is 150 deg C. From the storage tank, heat is transferred to an evaporator (Tetrachlorethylene). Evaporation temperature of the thermodynamic cycle is maximum 80 deg C - 90 deg C (Rankine cycle generator).
Condensation temperature of the thermodynamic cycle is 30 deg C and electric power of the generator designed to supply all the plant's demand of electric energy, e.. dryer fan, ventilators, pumps, the motor of the pellets extruding machine, control system etc, is 5.5 kW.
The drying chamber has three special features:
- the special diffuser plate with directional vanes which swirl the air up through the material at an angle, in parallel streams;
- the air enters at an angle, in the same direction of the vanes, so causing the effect described above;
- a group of mixing paddles whose purpose is to eliminate "bridging" and thus to ensure that the fluidization bed is uniform.
This fluidized bed has a better drying efficiency, since the drying air covers a greater distance in the mass (at an angle as well as vertically) for a smaller installed capacity, since the circulationof the air is facilitated.

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Università degli Studi di Sassari
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