Battery Energy Storage (BES)

The BES technology represents a battery electric storage system connected to the electric grid. It can be either a small scale battery, a swarm of small scale batteries or grid scale batteries. The definition of the technology is not defining this, but only maintenance and capital cost.

It can be charged by all the technologies that are defined via the connections parameters in the input file. The battery has a given maximum capacity that can be set by the optimizer if optimization is activated. When charging and discharging, a fixed loss rate is applied. Furthermore, the storage loses energy at a constant rate. The electric energy stored in batteries can be discharged to any consumers in the local district.

Using the parameter force_asynchronous_prod_con, the optimizer can be forced to never charge and discharge the storage at the same time. This can be useful to prevent the energy system from disposing of electric energy using the charging and discharging losses of the storage.

The initial charge of the storage is usually set to be optimized. Then, the constraint is that the initial and final storage level have to be equal. Alternatively, a fixed initial state of charge can be defined.

Attribute	Description	Standard value	Unit	Data type
deployment	If set to ‘true’, the technology will be considered in the energy system model (this does not necessarily mean it will be used). Only relevant for optimisation.	False	—	bool
force_asynchronous_prod_con	If set to ‘true’, the tes cannot be charged and discharged simultaneously	True	—	bool
eta_chg_dchg	Charging and discharging efficiency (fixed). Roundtrip-efficiency is calculated as eta_chg_dchg*eta_chg_dchg.	0.95	—	float
bes_gamma	Loss rate: fraction of electricity lost during one timestep (e.g. 1 hour)	0.001	1/timestep	float
capacity_kWh	Storage capacity.	inf	kWh	float
chg_dchg_per_cap_max	Max. charge/discharge (kW) per storage cap (kWh) per timestep.	0.1	1/timestep	float
initial_charge	Initial charge of battery (fraction of total storage capacity)	0	—	float
optimized_initial_charge	If True, initial_charge is determined within the optimization s.t. the initial charge and the final charge are the same	True	—	bool
co2_intensity	Carbon-dioxide intensity of technology output (annual average value).	0	kg CO2/kWh	float
lifetime	Expected lifetime of technology before replacement is required.	10	years	int
interest_rate	Interest rate for computing levelised costs (if required).	0.025	—	float
capex	CAPEX cost of technology per unit of capacity.	500	CHF/kWh
maintenance_cost	OPEX of the technology	2	CHF/kWh/year	float

The variables of the

The relationship between q_e_bes(t) and q_e_bes(t+1) is given by

\[\mathtt{q\_e\_bes}(t+1) = \mathtt{q\_e\_bes}(t) - \gamma \mathtt{q\_e\_bes}(t) + \eta \mathtt{u\_e\_bes}(t+1) - (1/\eta) \mathtt{v\_e\_bes}(t+1)\]

where we refer to bes_gamma as \(\gamma\) and to eta_chg_dchg as \(\eta\). Importantly, the charging and discharging powers are measured at the system connection, not directly at the storage. Therefore, the amount of energy charged into the storage is smaller than the charging power u_e_bes and the amount energy discharged from the storage is larger than v_e_bes, both by a factor eta_chg_dchg.

Importantly, the value given in q_e_bes is the stored energy at the end of a given time period.

The symbols and names of the flows are