How to deal with back up sources in the heat network?

[DorinevanderVlies]

As mentioned in #1567 a heat source is often combined with a back up source.

My general question is: how can we deal with a back up source in the heat network?

I can imagine a situation where the back up sources is estimated to run for example 5% of the time. So the main source supplies 95% of the heat load.

Modelling the investment and O&M costs of this back up source is currently possible.
Modelling the marginal costs associated with the use of this back up source however is currently not possible.

Curious about your opinions
@antw @grdw @AlexanderWirtz