An AD as an integral component of a HRES is able to provide a reliable source of both electrical and thermal energy. However the energy output from the HRES is a function of all of the system components working interdependently. However if the AD is at operating temperature it needs to be diverted to a heat dump. Consequently the energy value of an AD as part of a HRES is different to that of a typical decentralised AD, which is simply linked to a either a gas engine or boiler system. In this instance the CHP is only being supplied biogas from the AD and not woodgas from the gasifier.
Vegetable waste consists of an assortment of brassicas and discarded leaf matter.
The CHP was able to be supplied biogas directly from the AD or from the gasbag. The flexibility of the biogas production rate, a function of the quantity and type of feedstock, was maximised by selecting as large an AD volume as possible. In this way, the biogas production rate could be increased by increasing the feedstock rate without making the retention time unduly short. Taking into account all the energy conversion losses within the CHP, this equates to about 10kW e and 23kW t . Consequently the driving function of the AD, and the other system components of the HRES, was one of technology implementation rather than financial return.