Use of solar electricity in tandem with diesel

Advice given by Lyndon Frearson, from the Alice Springs think tank Ekistica, a company operating throughout Australia and south-east Asia. He writes:


The key issue at hand is that you have worked out the total cost of generation ($ to run the enterprise / MWh delivered). However a chunk of the Enterprise expenditure is actually fixed. The comparison really needs to be between the marginal cost of generation, ie the cost associated with generating one more (or one less) MWh. This is important as the extra MWh won’t necessarily increase the fixed costs of operation.


Rule of thumb is that diesel costs around $1-1.2/l delivered (after excise rebates), and average around 4kWh/l. On that basis 1 extra MWh (1000kWh) should be around $250 (25c/kWh). This obviously closes the gap considerably with solar. Now, not all MWhs are equal. As you reduce, or increase, the number of MWh delivered, certain thresholds are reached. Maintenance and operation of generators is generally based on the number of hours of operation more so than MWh delivered. To that end a generator running at 80% loading will have the same maintenance costs (roughly) as a generator that has run at 40% loading.


Consequently, at certain levels of renewable contribution, the only savings are to do with diesel consumption reductions, but as you progress with increased levels of solar penetration the opportunity exists to shut down one generator, or maybe use a smaller one more often. At this point you are able to get the benefit of the marginal cost reductions as well as some fixed cost reductions. The amounts, though, are not normally huge. Further to this, as you increase the renewable contribution, the more you rely on the generators to spin up quickly if clouds come over, or to spin down quickly once the clouds leave. This is where it starts to get a bit tricky.


It is understood that PWC have worked to optimise the operation of the diesels such that the fraction of peak power contributed by renewables (RPF, the Renewable Power Fraction) is regularly as high as 30%, (this is the maximum renewable amount relative to the total demand) resulting in the fraction of Energy provided by renewables being around 15%.


To progress further than these percentages, it is then necessary to start considering installation of batteries. The challenge with batteries is that they are a sunk cost and produce no energy, so their only value is in the ability to either avoid starting up a genset for a period of time (saving runtime and diesel) or to limit loss of solar energy when more is produced than is required.




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