The objective of this EU JOULE project was to discover how to use each renewable energy source to its best advantage within a hybrid system and whether such a system could provide an economic and reliable electrical power supply.
Wind Turbine
A Vergnet stall regulated upwind free yaw turbine with a 25kW induction generator mounted on a 24m high tubular tower, held upright by 12 steel guy wires. The rotor consists of two wooden blades each of five metre radius, with the rotor speed controlled by a mechanical governor operated by two pitch regulation bars. The wind turbine was able to be operated in both grid and autonomous mode.
Anaerobic Digester
A below ground insulated rectangular concrete tank with a hydraulic capacity of about 120m3. Feedstock was introduced via a hopper and motor driven auger and the digested waste was removed by a pump. Heating of the waste was provided by two water filled plate heat exchangers, using hot water supplied by the waste heat from the CHP units, with provision for electrical heating for start-up purposes. Mixing of the waste was by biogas recirculation, using a biogas rotary vane compressor, a rotary valve, and twelve pipes with outlets at the bottom of the digester tank. A 50m3 butyl rubber gas bag provided two hours of energy storage, whilst the digested waste from the anaerobic digester was pumped 35m underground to a 700m3 capacity slurry storage tank, where it was held until required for spraying onto arable land.
Gasifier
A downdraught moving bed design with a biomass consumption of 50kg/h. Biomass for the gasifier was supplied from either locally grown short rotation coppice or waste wood. The producer gas generated by the gasifier was designed for use as a fuel for the CHP units.
CHP Units
Two Waukesha six cylinder spark ignition gas engines. One was connected to a three phase synchronous generator for autonomous operation and the other to a three phase induction generator for grid connection. Waste heat from the engine (cooling water, oil, exhaust and generator casing) was used to heat the anaerobic digester.
The gas engine of the CHP unit could be fuelled by either biogas or gas generated by the gasifier. The difference between the manner and rate of supply of these two gases was significant to the versatility of the system. Biogas was generated by the anaerobic digester and stored in the biogas storage bag. This biogas was then used to meet any demand from the gas engine over short periods of time. The gasifier, on the other hand, generated combustible gas only when primary combustion produced a very high temperature. It could thus be operated only over a range of 70% to 100% of full power.
These different characteristics suggested the following general operating protocol for the combined system:
1 Whenever there was sufficient wind for the wind turbine to be generating electricity, the wind turbine power output was connected to the load.
2 The anaerobic digester supplied biogas to a storage bag that could be connected instantly to the gas engine of the CHP unit. Consequently the engine, operating between idling and full load supplied the difference between the load demand and the wind turbine power output.
3 When the biogas stored in the storage bag became low, the wood gasifier would start up so that the wood gas could take over from biogas as the engine fuel. The biogas from the anaerobic digester was then diverted back to the storage bag.
Though designed primarily for the agricultural sector, it has become evident that this type of hybrid system has a role to play not only in the management of agricultural wastes but also in food manufacturing and processing wastes.
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A project where we acted as project co-ordinator was the development of a hybrid wind-biomass system for rural electricity generation (funded in part by the European Commission JOULE III programme). This hybrid system