Solar assisted biogas plants III: Energy balance of fixed dome biogas plants and enhancement of production in winters

Abstract

This paper presents a thermal model of a fixed dome biogas plant, with or without hot charging and having the ground surface above the plant, blackened and glazed. The rate of energy transfer, between the ground and the plant has been determined by electrical simulation experiments, using a small scale model of the plant (made of copper), suspended in copper sulphate solution. For hot charging a shallow solar pond, built on the ground has been considered. The monthly variation of the slurry temperature, corresponding to 1, 4, 8, 15 and 30 m3 plants have been evaluated for Madras, New Delhi and Srinagar, which correspond to typical hot, composite and cold climates; the following cases have been considered: 1 Bare plant, without hot charging and blackening/glazing of the ground, above the plant. 2 Plant, charged with hot slurry from a shallow solar pond. 3 Plant, with ground above blackened and glazed. 4 Plant, with ground above blackened and double glazed and also provision for heating the slurry by burning a part of the produced biogas. It is seen that blackening and single glazing of the ground above the plant is able to maintain the slurry temperature between 28 and 35°C throughout the year in composite climate such as Delhi; in cold climates even double glazing alone is not enough for that purpose. Hot charging from a shallow solar pond alone is not sufficient to maintain satisfactory slurry temperatures in winters of Delhi and of course Srinagar. In hot climates the slurry temperature is sufficiently high, except for a couple of months, when either hot charging from a shallow solar pond or blackening/glazing of the ground above is sufficient to have the slurry temperature in the desirable range. In cold climates such as Srinagar, the desired slurry temperatures can be maintained if a part of the gas produced is burnt for heating the slurry, in addition to blackening and double glazing of the ground above the dome. Theoretical predictions of slurry temperatures have been compared with the field data recorded for an 8 m3 digester at the Energy Complex, Masoodpur, Delhi; the agreement is satisfactory.