Battery Not Dispatching fully

Good evening,

Thanks for the questions, I have examined your project file and think I can explain the behavior you see.

The peak-shaving dispatch model seeks to reduce peak power demand (kW) from the grid as much as possible over 24-hours, but it does not care about reducing energy (kWh). It also considers the peak demand so far in the month and doesn't bother discharging if the daily peak is less than the monthly peak in order to preserve battery life. This, with the assumption that reducing demand charges rather than energy charges will be the most economic use of the battery system. What this really points out is that SAM does not currently perform a cost-based optimization when choosing the dispatch strategy, rather, it uses a set of heuristics focused on reducing grid-power.

I'm not sure what you mean by changing the dispatch model to look at rates, do you mean performing multiple dispatches considering different utility rate structures? If so, you can certainly do this by changing the rate on the "Electricity Rates" page, but we don't have a way for you to do this easily in parametric form currently.

If you want to examine energy charges specifically, it will be better to use the manual dispatch controller as you suggested you tried, where you schedule to discharge the battery at specific times. In your case however, the most expensive energy is only $0.08/kWh, while the most expensive demand is $21/kW, so I don't believe it would be more economic to focus on reducing kWh, and you result in cycling the battery much more.

In general, energy arbitrage between peak and offpeak periods is not a very lucrative endeavor for a battery system due to round-trip efficiency losses and wear-and-tear on the battery, unless the difference in price between these periods is very high (perhaps >$0.20-$.30/kWh). The manual dispatch strategy isn't effective in this case, because you probably don't reduce the peak demand (kW) as much as the automated controller.

So, unfortunately SAM doesn't currently have a way to co-optimize battery usage to reduce demand and energy charges as much as possible, but can give you insight on what to do. If you have very high demand charges, I'd use the Peak Shaving control. If you don't have demand charges, you can use the manual dispatch controller. Also, consider that your battery dispatch will be dependent on the controls avaiable, and will likely not be operating optimally at all times, considering that PV production and electric load are variable quantities.

Hopefully that helped give some insight into what the model is doing. Please let us know if you have additional questions. I'd also encourage you to check out the following reports: www.nrel.gov/docs/fy15osti/64641.pdf , www.nrel.gov/docs/fy16osti/64987.pdf .

Best regards,
Nick

Dear Shannon,

Regarding operational systems predicting peak demand, I unfortunately do not know too much. There are advanced controllers certainly, and there are methods of forecasting demand, I just don't know how it fits together. When we model the dispatch, we are making several assumptions, including 1.) that a controller exists that could be programmed with this kind of dispatch 2.) that the user has access or ability to accurately predict their PV production and electric load. Those are clearly two large assumptions, illustrating that modeled results should be carefully examined and compared to real-world capabilities. I believe that a peak shaving model will almost always be more economic that a battery which discharges over an entire period, simply because the value of demand charges is so much greater than the value of energy arbitrage. Of course, this depends heavily on the utility rate structure you are working with.

To modify the manual dispatch control to only dispatch in the summer months, select the months/hours you'd like to change and then press the keyboard number that corresponds to the period. For example, I've attached a file that changes the dispatch to period 4 for all months except June-September, and programmed period 4 to do nothing but charge the battery from excess PV.


Regarding the demand charges, I believe the issue is in the non-peak periods. In August, it looks like there are three periods as you have it programmed:

- Period 1: $21.03/kW
- Period 2: $6.1/kW
- Period 3: $3.2/kW

And you have no facility demand charge programmed, which should be under "Demand Rates by Month with Optional Tiers" The way SAM calculates the demand charge would then be:

[Peak kW in Period 1]*[$21.03/kW] + [Peak kW in Period 2]*[$6.1/kW] + [Peak KW in Period 3]*[$3.2/kW] + [Peak kW in month]*[Facility demand charge $/kW].

I.e, we don't do a rolling 12-month maximum on the facility demand charge to my knowledge. I believe as outputs we should also consider adding the peak kW in each period/tier to make this easier to reproduce. Does this calculation make sense? I can take a closer look at the load profile and utility rate tariff if needed. Please send me an updated SAM file if you're looking at something newer than what I attached.

Thanks,
Nick