PhD Thesis - Energy Systems Research Unit - University of Strathclyde

Figure 7.30 Following Electricity and Heat Demand (2)
When intermittent supplies are also used, these are considered before the
auxiliary supplies. This is seen in Figure 7.31 where the output from a wind
turbine is included, which reduces the required load on the engine from that
shown in Figure 7.29. This is the case no matter how many intermittent supplies
are used.
Where more than one auxiliary is used, they are applied in the order in which
they are chosen. This can be checked by analysing the load characteristics of
the different auxiliary supplies. For example, in Figures 7.32 and 7.33, two
engines are specified – the first is set to follow the heat demand, and the second
is set to follow the electricity demand. Figure 7.32 shows that the first engine
chosen follows the heat demand exactly. As the heat demand is lower than the
electricity demand, this still leaves a substantial residual electricity demand.
The second engine then runs only when necessary to meet the residual
electricity demand, as shown in Figure 7.33. If the engine order is reversed, the
first engine that is following the electricity demand also provides most of the
heat required due to the higher electricity demand. The second engine,
therefore, does not require to run to any great extent, and cannot supply all the
244