Sizing a BESS for demand charges

Hi there, 

An international user here, thank you for the tool and really appreciate if you can help with the request. I have attached the file with
solar and battery models.

For the solar module, I have nominated a feed-in tariff of4c/kWh. 

When stimulated, for monthly data, I can see excess generation $ credit ( $/mo) but when I try to see the cumulative kWh/mo that actually earned these $/mo credits the data table shows nil values. 

Why is this and how can I see the excess energy exported by the system for each month? 

This takes me to the second and most important section.

I then try to size a BESS for demand charge savings. In this example, we have a flat monthly demand charge and ToU demand charge.

I picked a 100W/100kWh BESS size for this example but understand that I cannot use ( international user) the automatic size and dispatch option in the ‘system design’ section. 

As such I jumped on to REopt lite online version and proceeded to make a new tariff to get the solar and BESS optimization and BESS
dispatch schedule. 

However, REopt output (attached)suggested a larger solar system size and no BESS system. It did produce the BESS Dispatch spreadsheet (attached).

The attached sheet shows excess solar exported to the grid and all BESS charge/discharge fields are empty. I notice that the solar exported can easily fill a 100KW/100kWh BESS, to then reduce demand charges.

I am a bit confused as to why the optimization cant pick on-demand charge savings for the BESS.

Could you please help and advice as to what I should be doing. I have spent a lot of time on this, fixed other issues with this as an exception.

Appreciate your help. I am not concerned about the financial and taxation section just yet.

Hi Ash,

I'll answer your first question about SAM, but don't have an answer on REopt. I'll see if I can get someone to help with that questions.

In your SAM file, you chose the "Buy all /sell all" metering option, which means that all of the power to meet the load is purchased from the grid at the appropriate buy rates, and all of the power generated by the system is sold to the grid at the appropriate sell rates. There is no "excess energy" for this option. The net energy metering and net billing options calculate excess energy and credits. Please see the Help topic for the Electricity Rates page for more details.

Also, if your system is in the southern hemisphere and the array is facing north, you should set the azimuth angle on the System Design page to zero (instead of 180 degrees).

Best regards,
Paul.

Hi Ash,

Please email my colleague This email address is being protected from spambots. You need JavaScript enabled to view it. with your REopt question.

Best regards,
Paul.

Thanks Paul,

Appreciate your time and hope things are well at your end.

As suggested, I re-read the energy rates and corrected azimuth.
For background, I am trying to calculate an electricity bill for a scenario where a feed-in tariff (c/kWh) is provided i.e. customer gets paid a $/month for excess energy exported.

So to correct myself, this will not be a buy all/sell all scenario since any on-site generation will first feed the load, any excess goes to the grid. The measurements are done over interval time periods, in our attached file case it’s 30mins. So, the generation and loads are at 30min intervals.

To that end, I believe ‘Net Billing’ perhaps may represent this scenario better. However, could you comment on what this means, from the Net Billing help menu?

‘Excess generation is the sum of differences between generation and load in each simulation time step over the month’.

Is the simulation time step reference to the customer load and generation time steps? If so that’s good, then please read further.

I simulated the attached file ( solar case only).  I can see excess generation kWh credit earned/ month.

However, the excess generation $ credit earned/month is still 0.

That does not make sense, the excess generation kWh/mo should be multiplied by the sell rate to provide excess generation $ credit/month.

What are your thoughts? 

Thank you Paul, I have now emailed Emma.

Hi Ash,

There are two issues: First is with your inputs, and second I think is a bug in how SAM reports the credit amounts.

I think the Net Billing option is the correct one for your application. For that option, SAM calculates excess generation in each time step (in your case, excess = generation minus load in each 30-minute time step), and then at the end of the month adds up the total excess to determine the $ credit amount. That is different from net metering, where the excess generation is calculated as the total monthly generation minus total monthly load.

For the Net Billing options, you should assign both a buy rate and a sell rate for each time of use (TOU) period. The buy rate is the retail price the system owner pays for electricity to meet the load, and the sell rate is the rate the owner receives for excess generation delivered to the grid. In your file, the sell rate is zero, so the monthly credit is also zero.

As for the bug, it looks like SAM is not reporting the monthly credit amount "Excess generation $ credit earned/applied" for the Net Billing option. However, the monthly bill calculation is correct. You can verify that using the attached SAM file (based on your file) as follows:

1. In the "sell rate = 0" case, show the monthly "Electricity bill with system" and "Excess generation kWh credit earned" output variables. The latter shows the total excess generation for each month.



2. Show the same outputs for the "sell rate = 0.03" case. To simplify things, I set the sell rate for all TOU periods to $0.03/kWh, so all excess generation in a given month is credited to that month's bill at $0.03/kWh.



3. Note that the bill difference between the two cases is equal to 0.03 * excess generation, so the credit is being applied to the monthly bill. For example, in January, for sell rate = 0, the monthly bill is $6957.44, and for sell rate = $0.03/kWh the bill is $6640.10. The difference is $317.34. The excess generation for January is 10,578 kWh, and $0.03/kWh * 10,578 kWh = $317.34.

We will fix this issue for the next version of SAM. See github.com/NREL/SAM/issues/372 to track our progress.

Best regards,
Paul.