The previous post described the highly depleted state of the major reservoirs of the hydroelectric system of Tasmania. In this post I propose a simple water consumption rule that is (almost) guaranteed to produce a recovery in the Lake Gordon reservoir, the one that is most depleted:
The change in level of a reservoir is the net effect of water-in (rain/snow) and water-out (electricity generation and evaporation). Water-in has been below average over the past year, but water-out was not reduced in order to maintain the already low reservoir level, in fact water consumption for electricity generation has been mostly high, as shown in the following figure (blue curve) for daily average MW generation over the past 9 months (the figure also shows the data for the generators associated with the Esk and King basins):
Note: The Gordon MW figures are relative to the blue reference line (500 MW), and the Esk MW figures are relative to the black reference line (150 MW).
The result of the high generation of electricity from the Gordon Power Station can be seen in the data for water storage history, as shown in the following figure (mauve curve). The figure also shows the effect of the cumulative energy consumption of the Gordon Power Station on a store of 1250 Gwh on 31st July 2015 (black curve):
The two curves in the figure above have a very similar shape, being mostly parallel apart from three periods, probably reflecting periods of rainfall (I’ve not yet fully checked this hypothesis, but will assume it to be true in what follows). In August 2015 the water consumption by the power station was roughly compensated by some water inflow (the BoM reported snowfall on 3rd/5th/17th August). In late November 2015 and again in April 2016 there were periods of rising water storage, thanks to the combination of relatively low power generation and some water inflow.
To restore the water level in the Gordon reservoir its minimum value in 2017 must be higher than its minimum value in 2016, and similarly for several years to come. There is a simple water consumption rule that guarantees this outcome, as long as there is some net hydrological gain (water-in minus evaporation). The rule is based on the notion that a large reservoir like Lake Gordon should be mainly used for electricity generation during the “dry” season (spring/summer). In the “wet” season (autumn/winter) most electricity should come from small reservoir or run-of-river generators, allowing the large reservoirs to be replenished, giving them an annual variation of water level. This annual variation of large reservoir levels allows the proposed water consumption rule to be stated as follows:
To recover a large depleted reservoir the maximum amount of electrical energy that can be generated in the “dry” season is the gain in energy storage relative to the most recent minimum.
The rule is illustrated in the following figure, showing the recent storage history for the Gordon reservoir (mauve segment), together with a simplified version of its future levels (black curve):
The figure above shows the case where some rainfall during the spring/summer period has given the desired result of a higher minimum in 2017 than in 2016.
It is likely that this water consumption rule, applied to all the large reservoirs, will impose significant constraints on the electricity system of Tasmania, and these constraints will be discussed in future posts.