HOMER Energy Support

Energy storage question

Last Updated: Jul 29, 2016 12:19PM MDT

In our model, we have set the converter parameters to have 0% Capacity relative to Inverter and an Efficiency of 1%.  Essentially, we only want a converter to act as an inverter, and not a rectifier.  (We aren't experts, but from what we can understand, the inverters on most PV systems cannot act as rectifiers).  When the converter is set to only act as an inverter, it never charges. In the file you sent, the batteries are recharging immediately after discharge by purchasing power from the grid.  So that is where our problem is occuring. I was wondering if you had any thoughts on how we might achieve this?

 

If you mean to model a stand-alone converter that you would buy from someone like Xantrex, into which you could plug both the PV array and the battery, then it probably can both invert and rectify.  If you mean to model a dedicated inverter that will be married to the PV array, then it probably cannot rectify, but in that case you should really model the PV array and its dedicated inverter as an AC PV array.  That said, I’m not an expert on converters either.

In any case, there is no problem modeling a uni-directional converter like this.  Unless you want the grid to charge the battery, there is no need for the system to rectify any power.  (Although if I could buy power for 15 cents/kWh and then sell it for $5/kWh, I would.)

The more important issue is that HOMER is not operating your system the way you would like.  Here is your system:


                                                      
 

I think the way you want it to operate is as follows:  During off-peak hours, the priority for the PV power should be first to charge the battery, second to serve the load, and third to sell to the grid.  During peak hours, the priority should be for the PV and battery to serve the load and sell to the grid.

The problem is that during off-peak hours, HOMER is not using the PV power to charge the battery, but rather to serve the load and minimize grid purchases.  To do otherwise would require the ability to look ahead and see that the PV power would be more valuable if stored in the battery until the peak period.  HOMER does not look ahead in this way.  It sees only the current time step, and decides that serving the load with “free” PV power is cheaper than buying $0.15/kWh grid power.

The Advanced tab of the Grid Inputs window lets you specify several threshold parameters that control the buying and selling of grid power based on the power price and the sellback rate, but unfortunately there is no way to force HOMER to store PV power in the battery during off-peak hours. 

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