I have in my hybrid system two types of loads, a critical load (so a telecommunication load) and a non-critical load (a non telecommunication load). In reality, this non-critical load would be disconnected from the supply whenever not enough electricity (low voltage in batteries) would be available for both types of loads. Moreover, the low voltage threshold for the non-critical load would different and so higher than the low voltage threshold for battery disconnection. This strategy would help the system to keep a higher SOC in the batteries (as we disconnect the non-critical loads from the supply before we get to the disconnection of the batteries and so we then use some of the solar energy to charge the batteries again).

Our aim is to have a zero % of capacity shortage for the critical load and to allow a certain percentage (we still do not know this value) of capacity shortage for the non-critical load without going over a certain value of litres of diesel per year consumption and at the same time keeping a high SOC in the batteries. It would be ideal if the non-critical load would be fed with the excess energy (from the PV panels instantaneously, from the excess charging in the batteries (if we want to keep a certain autonomy or a certain SOC in the batteries, the rest of the energy in the batteries is excess energy) or from the diesel if we have not reached the limit of fuel consumption).

As I do not know the profile of the non-critical load (it is a random profile) and I just have a rough number of the energy that it consumes during the day, I have thought of two different ways of simulating this in HOMER, but I do not know if they are the right ways.

Which of these two strategies do you think is the best? Or would you have any other suggestions (homer tricks, homer functions...) and ways of doing this?

By default in the built-in Controllers in HOMER Pro, HOMER will serve Electric Load #1 first, and then Electric Load #2. It will then serve the Deferrable Load. HOMER considers the deferrable load as less urgent than the primary load because it has some flexibility as to when it meets the deferrable load. However, HOMER does not distinguish between unmet Electric Load #1, unmet Electric Load #2, or unmet Deferrable Load.  So you can’t tell HOMER that you will accept some unmet load on one of those, but no unmet load in the other loads.

Also, you don't specify the Deferrable Load to the same degree of accuracy that you specify the primary load.  HOMER assumes that energy leaves the deferrable load ‘tank’ at a constant rate.  You can specify that rate separately for each month, but you can’t specify it any more accurately than that.

For those reasons, I don’t think you can use the deferrable load module to model your non-critical load. If you have a system with a generator, you could use it to ensure that you have enough power and reserves to serve the Electric Load #1, but this would serve Electric Load #2 and the Deferrable Load when Electric Load #1 is met and there's still available capacity in the generator.

You have two options:

1. If you want to use the built-in Controllers in HOMER Pro, you can specify both your critical and non-critical loads as primary loads, and then do some post-processing of the results to determine how much of non-critical load gets served.  To do this, I would first model the system with only the critical load, to make sure that the system can serve the critical load without any unmet load or capacity shortage.  Then I would add the non-critical load as a second primary load, and allow some capacity shortage.

From the Simulation Results window, I would export the time series simulation results and then bring that data into a spreadsheet.  In each time step, I would subtract the critical load from the total load served, which gives the non-critical load served.  (That should always be positive, since you made sure that the system could always meet the critical load.)

2. You can use the MATLAB Link controller to develop your own dispatch that will behave in the way you prefer. The MATLAB Link controller enables you to write the rules and algorithms that the controller uses in MATLAB script, and HOMER Pro will communicate with MATLAB to follow the dispatch algorithm that you write. More information about MATLAB Link can be found in HOMER Pro's built-in help, and you can find examples of MATLAB Link scripts that others have written in the HOMER Community group.