# Defining excess sink in Oemof

Hey everyone,

I have modelled the following simple energy system in Oemof (capacities of the components are part off the optimization problem) with

• pv: connected to dc-bus ,
• diesel gen: connected to ac-bus ,
• battery: connected to dc-bus,
• 3 ac loads: connected ac-bus,
• inverter: interface between the two buses, and
• sink excess

I did two simulations where the excess is differently connected.

• Simulation 1: excess connected to the dc-bus
• Simulation 2: excess connected to the ac-bus

In the two simulation the optimized capacities of all components have remained the same except of the inverter’s optimized capacity which has increased when it is connected to ac-bus. However I was wondering why the amount of the excess energy is not same or how is the right way to connect the excess ac-bus or dc-bus ? in simulation 2 the excess has fallen about -30% in respect to the first simulation.

Does the inverter have an efficiency of 0.7?

sorry I’ve absolutely forgotten to mention that! the efficiency is 0.95. I noticed that the fuel consumption of they diesel has increased however very slightly

The easiest thing is to sum up all flows, create a balance and see where the energy is lost. In both cases the balance should be 0 if all losses are included.

hi Uwe,
I did the two simulations again and I did calculate all the inflows and the outflows in the ac-bus and dc-bus. the Delta in the two simulations of flows in the two buses is almost null is almost null, I got values as 0.00003454 kWh or something, if I use round() it’s null. I guess the raison why is that not absolutely a null, lies in the solver performance right?

Anyway the problem why the amount of energy excess is greater wenn the excess sink connected to dc_bus, still be not solved. I noticed that there is a big difference in Power_in Power_out of the battery, wenn the excess are differently connected. I’ve made a graph of the two simulations (in quick and dirty way) it’s shown that:

• the power_in of the battery (BA) wenn the excess connected to ac-bus (red colour) is higher than the power_in of the battery wenn the excess connected to dc-bus. The same behaviour of power_out is observed in the other graph.

I would be grateful for help to have an understanding about the relation between excess and battery performance depending on the excess bus connection.

Hi jess,

do you have operational parameters for your components (for example O&M costs)?

It could be more expensive to operate the inverter than to use the battery slightly more.
Based on what you described,

However, I think your excess is on the DC side (from the PV), so a component with losses (inverter) is between your excess generation and the sink in Sim 2.

Best, Matthias