Small multi-utility wind-diesel system

The modelled system (Fig. 1) consists of four subsystems, corresponding to four physical domains, and is centered around an electrical minigrid of three busbars (electrical subsystem, light red)

Fig. 1: Schematic view of the demonstration system.

Busbar 1 is connected to a mechanical subsystem (orange) through two induction generators, each of which receives mechanical power from a wind turbine. Three sets of diesel generators feed into busbar 2, where a dump load is connected as well. The diesels’ speed governors are set to operate in droop mode, controlling the system frequency together with the dump load, which has frequency dependent characteristics. The droop curves determine the ratio of active power sharing between the diesel units. Each generator provides a regulated output voltage as a function of active and reactive power output, which facilitates reactive power sharing. The third busbar feeds a time-variable consumer load and the pump of a reverse osmosis desalination plant. Busbars 1-2 and 2-3 are interconnected through non-ideal transmission lines.

Surplus heat from diesel engines and dump load is collected in a heat subsystem (yellow) and exchanged with a cooling unit and a second desalination plant which works after the evaporation principle. Combined freshwater production of both desalinators is balanced against a time-variable water demand in a water subsystem (light blue).

The thermal and freshwater circuits operate on a policy that attempts to maximize the utilisation of waste heat from diesel gensets and dumpload: The cooler only dissipates excess heat which can not be used for desalination, and the heat-driven desalination unit gets priority over the electricity-driven (reverse osmosis) one. This establishes a weak feedback loop across the thermal, water and electrical domains, as a decrease in diesel loading will reduce the amount of heat available for desalination. To meet water demand, the throughput of the electric desalination unit will have to be raised, resulting in higher electricity demand and thus higher diesel loading.

Dispatching of the diesel generators is handled by a system controller, implemented in a controller module.