Modeling an Autonomous Island Microgrid (Karpathos, Greece) - Problems

Hello,

My name is Nikolaos Bampaliaris, writing on behalf of a senior class in the Department of Electrical & Computer Engineering at the Aristotle University of Thessaloniki, Greece. We are working on a project for our "Modern Power Systems and Smart Grids" course. Our goal is to design a fully autonomous/isolated microgrid for the island of Karpathos. We are using the following configuration:

• Template: Hybrid (Custom Generation + PVWatts + Wind + Battery)
• Financial Model: Distributed (Third Party - Host / Developer)

Data Inputs:

• Load: 8760 hourly entries for the island.
• Solar: SAM weather data.
• Wind: We generated a .srw file by extrapolating 10m wind speed data using the Power Law: WSx​=WS10​*(x/10)^0.14.
• Custom Generation: Used as a proxy for the existing Diesel generators, following the load profile with a specific multiplier (e.g., Load*0.3).

The problems we are facing:

1. Battery Dispatch Issues: Even when we have significant excess generation (Peak generation reaches ~9.6 MW for a ~6.5 MW peak load), the battery never charges. The "Battery State of Charge" results show NaN throughout the year. We have already checked "Allow charging from system" in the Dispatch tab. Does the "Third Party" financial model require specific "Grid Charging" or "PPA" logic to trigger battery activity?

2. Achieving True "Islanded" Operation: We want the Developer to sell energy only to the Karpathos load. We need to prevent the model from "selling" excess energy to an infinite grid.

• We tried setting the Interconnection Limit to 0 kW in the Grid tab, but the model still reports zero "Energy loss from curtailment" even when generation exceeds load, and it doesn't force the energy into the battery.
• Is there a way to strictly limit the system to the local load so we can accurately measure Unmet Load?

We truly appreciate the open-source nature of SAM, as it is a vital tool for our studies, given the current funding challenges in Greek higher education.

Thank you in advance for your time and help!

Best regards,
Nikolaos Bampaliaris Aristotle University of Thessaloniki

Hi Nikolaos,

When the battery charges and discharges depends on the options on the Battery Dispatch page, power available from the generating equipment in the system, and the electric load. For some of the dispatch options, the battery may not be required when there is sufficient power from the diesel generator, wind turbines, and solar array to meet the load. Please attach your .sam file if you would like help troubleshooting.

You should use the "eenergy loss from interconnection" variables in the results to see the effect of the interconnection limit. The "energy loss due to curtailment" variables are determined by the grid curtailment inputs on the Grid Limits page. Curtailment and interconnection limits are applied separately.

Best regards,
Paul.

I'm sorry, I made a mistake in the attachments while thinking that I had uploaded the .sam file. I just edited my original post, but I will upload the file in this answer as well.

Hi Nicolaos,

I think the problem is with the electric load scaling factor on the Electric Load page: The scaling factor is 1000 which makes the load much bigger than the system. I tried changing the scaling factor to 1, which results in an annual peak load of 10.3 MW, which seems more reasonable for the 15 MW hybrid system.

Changing the load scaling factor to 1 results in the battery charging and discharging daily.

Best regards,
Paul. 

Thank you very much! 

The IRR was very high before (reaching close to 30%), while the 8.84% now is very realistic! 

There are also three questions related to the Grid that are bugging me.

1) How does the Grid work in SAM? Is it like an infinite feeder, from where the host can buy as much power as needed?

2) How is the developer selling the power to the host? Directly or through the Grid?

3) Is the PPA price only the price of the energy that the host pays the developer? If the developer does not produce enough energy to cover the load in one year, does SAM (the host) buy the rest of the energy from the Grid with the same or a different price (I'm not sure if there is a relevant value in the Financial tab for buying EUE - Expected Unserved Energy)?

For example, my results now show:

Total generation in 1 year = 33.23GWh. The energy that is needed according to the load is about 38.27GWh.
Does SAM know this and buy the rest of the energy 38.27 - 33.23 = 5.04GWh from somewhere else, affecting the economics?

I do not want this to happen, and I suspect that using Grid Interconnection Limit = 0 is what I need. The annual energy lost from interconnection limit = 4.33GWh, which is very close to my calculation of 5.04GWh, although this confuses me because it's "lost energy" and not expected unserved energy. Like the project generates more energy than the load needs (33.23GWh), and we throw it away.

Thank you again!

Hi Nickolaos,

1. SAM treats the grid as an infinite source of electricity for electricity purchases. The interconnection limit or curtailment on the Grid Limits page limit exports to the grid, but not imports from the grid, as explained in the Grid Limits help topic: samrepo.nrelcloud.org/help/grid_limits.html . We may implement limits for grid imports in a future version of SAM, depending on the availability of funding: github.com/NatLabRockies/SAM/issues/1957 . 

2. The developer earns revenue by selling the power generated by the system at the specified PPA price(s). The assumption is that the host purchases that electricity at the PPA price, but if you ignore the host portion of the cash flow and the host financial metrics, you could assume that any entity purchases the electricity.

3. See SAM's Help topic for the Financial Parameters page for a description of how this works: samrepo.nrelcloud.org/help/fin_tpo_host_developer.html . The host purchases the electricity generated by the system at the PPA price. That electricity goes to meet the load and thereby reduces the amount of electricity the host purcases from the electric utility at the buy rate(s) on the Electricity Rates page. The PPA agreeement with the developer and electricity rates from the utility are separate.

Best regards,
Paul.