The issue of whether renewable energy projects should pay for ancillary services such as spinning reserve is addressed on this page. Ancillary services may be a big costs that really matters or they may be not very important because the ancillary services are a by-product of normal disptch. Batteries and hydro can provide ancillary services and an issue is whether installation of batteries provides more benefits than costs.
Background on Ancillary Services
I am not an engineer and I do not claim to be an expert in this subject. But maybe this helps as the subject of ancillary services can quicly get very mysterious. In particular the question is how much is the cost of this stuff really,
The way you can think about ancillary services has been explained to me by a good engineer in Mlaysia named Adzhari. If a 1,000 MW coal plant suddenly has a forced outage meaning it is out of service, a couple of things can happen. If other plants such as natural gas plants cannot ramp up fast enough, the frequency in hertz will decline and if the frequency goes low enough, the whole system will fail. If the system is large, 1000 MW may reduce frequency for a while but this will not be enough to cause an outage.
If there is a hydro it can be running as a motor meaning it is spinning. There can be a circuit that automatically changes the hydro from running as a motor to running as a generator. If there is enough hydro on the system to cover the 1,000 MW plant that goes out, the 1,000 plant suddenly going can be easily covered. Depending on the level of demand, this 1,000 MW could be covered even without the hydro plant and in a couple of minutes if there is enough spinning reserve from gas plants, the plant outage would just reduce frequency a bit.
Ancillary Service and Wind Power Changes
If there is a small amount of wind on the system or if there are a lot of hydro resources, the cost of ancillary services may not be very much. Think about a sudden increase or decrease in the wind speed. If the wind speed change can be predicted, then the dispatcher can adjust the capacity of natural gas plants to meet the change in the wind plants. In this case there is probably no or little cost that should be attributed to the wind farms.