Energy in Agriculture, 4 (1985) 285--304 285 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands A SOLAR COLLECTOR AND A COGENERATOR FOR LOW-TEMPERATURE AIR HEATING GIOVANNI RIVA and FABRIZIO MAZZETTO Institute of Agricultural Engineering, University of Milan, Via Celoria 2, 1-20133 Milan (Italy) (Accepted 24 July 1985) ABSTRACT Riva, G. and Mazzetto, F., 1985. A solar collector and a cogenerator for low-tempera- ture air heating. Energy Agric., 4: 285--304. This paper reports on the results obtained with a simplified air-type collector of 154 m 2 feeding a forage dryer. Air was delivered by a fan driven by a 10-kW diesel engine with waste-heat recovery. The experiment was run in northern Italy (45°N) by the Institute of Agricultural Engineering of Milan. The aim of this research is to compare the results of Bliss's equation applied to an open system with those actually obtained by means of the experimental air-heating collector. Concerning the solar plant, the average efficiency of the collector (specifically 0.45) coincides with the instantaneous value under average temperature and solar radiation conditions. This significantly sim- plifies dimensioning of the collector surface. For engine waste heat recovery it was found that the air temperature increased some 1.5--2°C, depending on the quality of the equipment and on forage stacking height. A mathematical expression for the deter- mination of this increase is given. In the system tested, cogeneration contributed from 30 to 70% of the total energy supplied to the air, under good and poor weather con- ditions, respectively. INTRODUCTION This paper reports on the results of 2 years' tests with an innovative system for air heating, feeding an experimental forage dryer of 30 t/year capacity. It is well known that hay-making by barn-drying reduces nutrient losses between green and dried product. In addition to the better quality, hay yield is also greatly improved (Bonsembiante and Parigi-Bini, 1968; Cavalchini, 1981; Rees, 1982). On the other hand, barn-drying has high thermal needs for air heating. In order to reduce oil consumption, the possibility of using solar radiation by means of simplified flat air-heating collectors has been tested for about 6 years by several authors (Nydegger et al., 1981; Nydegger, 1982; Riva, 1984). Solar radiation has been found suitable for this purpose for the following reasons: barn-drying requires low working temperatures (under 45°C); and the working period (May-- September) coincides with the maximum solar radiation availability. 0167-5826]85/$03.30 © 1985 Elsevier Science Publishers B.V.