# Operating reserve in PV penetration systems

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I've been working on high penetration PV-hybrid systems and have come across the same problem in several projects:

As I understand from the manual, Homer calculates the operating reserve as a percentage of the current PV output plus a percentage of the current load (or peak load). In high penetration PV systems where the PV output is close to or higher than the load, this might lead to a required operating reserve that is larger than the load, which in turn leads to an oversized battery or generators running unnecessarily. In reality, the maximum PV output would be as high as the load and everything above that would be curtailed. Homer does not know curtailment and just counts the unneeded energy as excess energy, which is okay to understand the overall energy balance. But the operating reserve calculation is still based on the total PV output, which would be much smaller in reality.

Consider the following example:
1,200 kWp PV
Operating reserve:
10% of load in current time step
80% of PV output (which is a realistic drop of PV generation in 1 minute)
Current time step:
1000 kW PV output

The operating reserve requirement would be: 0.1 x 600kW + 0.8 x 1000kW = 860kW

This is larger than the load and would require a larger battery or more running generator capacity than necessary.

Is there a way to simulate more realistic behavior in Homer?

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Hi Niklas,

In high penetration PV systems, the variability is also higher. Hence, the trade off between excess electricity and capacity shortage should be considered. When you consider such high drop (80%) in PV, increasing the capacity of battery/generator is one viable option. You can probably vary the % dependency of operating reserve on load(increase) and PV(decrease) so that the operating reserve is more dependent on load than PV.

When the PV production exceeds the load, the difference in power is considered as excess electricity as you had mentioned. If you want to custom-define the operating reserve based on PV penetration in your system, you can always write a custom dispatch strategy using the MATLAB Link option in HOMER Pro. Here you get more flexibility in defining your dispatch algorithm and your operating reserve requirements as well. Hope this helps.

Best,

Dhiwaakar Rajasekaran

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Hi Dhiwaakar,

Thanks for the quick answer. Just to confirm my understanding of the standard Homer dispatch strategy is correct:
With high PV penetration and a high percentage of PV output as operating reserve requirement, is it possible for the operating reserve requirement to be larger than the load (like in the above example)?

Thanks!

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Hi Niklas,

Yes, it is possible for the operating reserve requirement to be larger than the load when the PV penetration is high. It depends on the % dependency of the operating reserve on the PV generation that you set really. I have attached a file that I built based on your example and checked out the result. I set the operating reserve to its default constraints. I have attached the file with this message. I hope I replicated your example. If you look into the time series results table after simulating it, look at instances when PV production exceeds the load. Hope this helps.

Best,

Dhiwaakar Rajasekaran

Edited: 9 months  ago

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Does HOMER still use the above calculations for spinning reserves in grid tied systems?

Hi Kurt,

Yes, HOMER uses similar calculations for grid-tied systems as well.

Best,

Dhiwaakar Purusothaman

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Thanks Dhiwaakar,

I was curious if the same amount of reserves would be required in a grid tied system as I was hoping that the grid would help in reducing or totally eliminating the requirement for the spinning reserve.

Hi Kurt,

There are two numbers we're talking about here. The operating reserve required and the operating reserve served. The required operating capacity would still be based on the load and % dependency on PV generation while the operating capacity served is from the grid. The grid eliminates the need for additional spinning reserve as the operating capacity served is essentially the grid power capacity. Hope this helps.

Best,

Dhiwaakar Rajasekaran

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