Zero price for over-supplied commodities

[tsmbland]

I've noticed in the two_outputs model that a couple of commodities have a price of zero for some timeslices (actually -0.0, to be precise):

• DIESEL has a price of zero in winter 2020
• GASPRD has a price of zero in summer 2020

Both of these are produced by assets with more than one output commodity, and in both cases the price of zero coincides with the commodity being over-supplied across these seasons, which is necessary for the asset to meet demand of the other commodity it outputs.

In this model it seems to only happen in the base year (perhaps a sign of poor calibration), but I see no reason why it couldn't happen in later years where MUSE is choosing the investment decisions.

I see this was predicted as a potential issue here, but don't think this has been followed up on.
@ahawkes Do you have any ideas how we might address this?

I've also come across zero prices while working on #986, where hydrogen appears free, which is at least one reason you can end up in an infinite loop. That might be unrelated though

[ahawkes]

Thanks, yes this is expected. Shadow prices will be zero (or even negative?) in cases where relaxing the commodity balance constraint for that time slice results in no change in the objective function. Rough interpretation is that the system doesn't want any more of that commodity at that time, so is not willing to pay anything for it.

I think we should do nothing about this for the moment. Let's keep an eye on it though.

Once flexible processes are implemented it might help with this (if users actually use them). Also on prices, the case where users have an option to choose the market pricing method for each commodity - to allow shadows prices, or prices that include CAPEX and FIXOM, etc. These future developments will helpfully mitigate this issue, where zero or negative prices are no a real market feature (note they indeed are a real market feature in UK and European electricity markets at the moment!).