HOMER Energy Support

Storage FAQs

Last Updated: Nov 16, 2018 03:38PM MST
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The battery models are not easy to understand. How can I understand what parameters define HOMER's various battery models?


The Generic 1 kWh Li-Ion, Generic 1 kWh Li-Ion [ASM], Generic 1 kWh Lead Acid, and Generic 1 kWh Lead Acid [ASM] are a good place to start. You can find these in the HOMER Pro library (and in HOMER Grid as of the next release, 1.3.0). Make a copy and play with the inputs. Also refer to the help: https://www.homerenergy.com/products/pro/docs/3.12/storage_library.html
How does the simple model account for variations of battery voltage as a function of SOC and charging/discharging current?
We don't model changes in voltage with different states of charge. This would increase the model complexity a lot for not that much change in the economics (usually).
How is the CYCLE defined?​
The modified kinetic battery model uses the "rainflow method" for counting cycles (defined in ASTM E1049 − 85).


Is battery calendar aging related to its SOE level?

The calendar aging only depends on the temperature of the battery. The "cycling degradation" is based on the battery SOC history.

Does the idealized model adjust lifetime based on no. of cycles?​

The idealized model calculates lifetime based on years and/or throughput in kWh.

Is the idealized model the best model to be used with Li-Ion Batteries?

Yes. Later in the design process (once an approximate system design is already chosen), one could also use the modified kinetic model in combination with a multiyear model (HOMER Pro only for now) to get more detail on the impact of battery degradation over the project lifetime.

Are losses/decay also modeled in the idealized model and/or the kinetic model?​

The idealized and kinetic models include round-trip efficiency only. You need to use the modified kinetic model (requires the advanced storage module) to model rate-dependent losses (ESR) and degradation/decay.
We got unrealistic results when modeling the temperature effect in HOMER. Why is that happening?
Make sure you set up the temperature resource correctly under the resources menu. Also make sure you picked a reasonable effective series resistance for the battery model (in the library, Functional Model tab). Finally, check the values in the Thermal tab. You can see the battery temperature in the time series plot -- adjust the thermal parameters until you are happy with the temperature profile.
How do you model batteries that are put into a room with AC and there is no temperature effect? ​
You can use the idealized battery model, which doesn't worry about temperature effects. You can also use the modified kinetic model and check the box for "Ignore capacity changes with temperature" in the "Temperature vs. Capacity" tab and check the box for "use constant shelf life regardless of temperature" in the "Temperature vs. Lifetime" tab in the library view.


Does the modified kinetic battery model model the temperature increasing due to performing in container without cooling?
Yes. In the Thermal tab in the library, you can set a low "conductance to ambient" which represents your set up.
Does the Battery model apply reduction in efficiency as the battery ages, particularly in the second half of its useful life?
Yes, for the modified kinetic model only. The series resistance increases with the degradation amount. So a 20% degradation level would increase the ESR by 20% (and decrease the maximum capacity by 20%). In order to model these effects accurately, you should use the Multi-Year functionality, to properly represent the changes in performance from year to year.
What are the I, h, k variables?
Some other help articles to look at:

https://www.homerenergy.com/products/pro/docs/3.12/kinetic_battery_model.html
https://www.homerenergy.com/products/pro/docs/3.12/modified_kinetic_battery_model.html
https://www.homerenergy.com/products/pro/docs/3.12/creating_an_idealized_storage_component.html
https://www.homerenergy.com/products/pro/docs/3.12/creating_a_kinetic_storage_component.html

https://www.homerenergy.com/products/pro/docs/3.12/creating_a_modified_kinetic_storage_component.html

Are RTEs (Round Trip Efficiency) specified in battery specification sheets, or do they need to be computed?​
The RTE's may need to be computed depending on your configuration. Most battery data sheets will include an ESR. The roundtrip efficiency would be that of the charge controller/inverter, wiring, and etc. If you don't want to include that, you can set the RTE to 100% and just use the effective series resistance alone (modified kinetic model only).


For the battery economic optimization study discussion and $/kWh LCOE, what is the approximate cost of storage used as $/kW or $/kWh assumed?

Users of HOMER Pro or HOMER Grid have the ability to set the cost per kWh or cost per kW. It is relatively easy to set cost per kWh by using the Generic 1kWh Li-Ion battery. For the most accurate modeling, I highly encourage that you set your own prices. But, to help you get started, the Generic components in HOMER have default pricing. Generally, this pricing comes from NREL industry reports.

How can I model hours of autonomy in HOMER? All I have been able to find is a % of time with power...somewhat of a plant factor.
Hours of autonomy is listed in the detailed results (double click on a row in the bottom table in the results screen) under the tab for the storage component.

Can HOMER Pro come up with proposed time and source of charging the battery or the time and amount of discharge for most cost effective solution or should those be done manually?

HOMER Pro will simulate hundreds to thousands of systems. For a given system, HOMER calculates timestep-by-timestep how the given generation sources will serve the energy demand. So, yes, HOMER Pro will propose time and source of charging the battery, in the most cost effective solution. This is not done manually, unless you want to create a dispatch in MATLAB yourself.

Do you have Flywheel models?

We have a flywheel model as well, but more information can be found in the help: https://www.homerenergy.com/products/pro/docs/3.12/flywheel.html

Can we model PV and waste-to-energy for an on-grid system? If so, what are the data required to initiate modeling?
If I understand you correctly, you'd like to model a system that has a PV array and a biomass generator. In HOMER, you can use the Biomass resource with a generator that is set to burn Biomass as the fuel. This requires the biomass module. You'll need data on the amount of biomass that is available.


HOMER seems focused on economics like COE, but slowing down climate change should also be added to the analysis. Is there any criteria in your software for emissions?

You can set limits and penalties on emissions. This is described in the help: https://www.homerenergy.com/products/pro/docs/3.12/emissions.html



 
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