Introduction

With the Efficiency guideline for PV-battery

systems (available in German at bves.de)

standardized performance evaluation test

procedures are proposed. By means of the

experimental results obtained from the

laboratory tests, simulation models of PV-

battery systems can be parametrized and

use case-specific simulation tests can be

conducted. An appropriate performance

index can be derived from the simulation

tests and an efficiency label for PV-battery

systems could be established on its basis in

the future. In this way, the comparability

with regard to the overall performance of

different products is improved. Fig. 1: Measures to increase the comparability

between grid-connected PV-battery systems.

An appropriate performance index for grid-

connected PV-battery systems has to fulfill

several requirements. The index should …

• assess the overall system performance

including all system components.

• enable the comparability between PV-

battery systems of different sizes and

different system topologies.

• take all loss mechanisms into account.

• characterize the system performance

over a period of at least one year.

• enable the economic assessment of grid-

connected PV-battery systems.

Open source simulation model for AC-coupled PV-battery systems

PerModAC enables the simulation of energy

flows in AC-coupled PV-battery systems

with a temporal resolution of one second

over a period of one year. The model takes

sizing, conversion, energy management,

control and standby losses into account. Its

parameters can be derived from laboratory

measurements according to the efficiency

guideline. Several performance metrics can

be obtained from the simulation. The Matlab

source code including an exemplary input

data set is freely available at:

http://pvspeicher.htw-berlin.de/permod

Fig. 2: Simplified structure of PerModAC and order of the modelled loss mechanisms.

System Performance Index (SPI) – a novel efficiency indicator for PV-battery systems

With the System Performance Index (SPI) a

new efficiency indicator was devised. The

SPI is defined as the ratio of the grid

electricity cost savings of the real PV-

battery system to the grid electricity cost

saving potential of an identical lossless

system. With detailed loss analyses, the

relevance of the various loss mechanisms

can be extracted from the SPI-reduction.

Fig. 3: Grid electricity costs (left) and savings

(right) of an exemplary PV-battery system (PV

output 5 kWp, battery capacity 3.7 kWh, load

demand 5010 kWh/a, retail price 0.28 €/kWh,

feed-in tariff 0.12 €/kWh).

The main scope of application of the SPI is

the comparison of different PV-battery

systems. Moreover, the impact of distinct

system parameters on the overall efficiency

can be assessed. Hence, the SPI allows

improving the overall system performance

of grid-connected PV-battery systems and

can be used for market reviews and system

optimization purposes.

Fig. 4: Left: Impact of the battery capacity on

the grid electricity cost savings and the SPI of

an ideal and real PV-battery system. Right: SPI

for varying system parameters.

Model-based performance evaluation of

grid-connected PV-battery systems

Johannes Weniger, Tjarko Tjaden, Volker Quaschning

HTW Berlin - University of Applied Sciences

Research group: solar storage systems

Standardized data sheet specifications

Label

Efficiency index

Laboratory test according to the efficiency guideline

Model-based simulation test

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HTW Berlin - University of Applied Sciences | Email: pvspeicher@htw-berlin.de | Web: pvspeicher.htw-berlin.de

SPI = Cost savings of the real system

Cost savings of the ideal system

PerMod AC

Battery system

Inp

ut t

ime s

erie

s

PV ystem s

MPP-output of thePV generator

Conversion lossesof the PV inverter

Residualpower

Dead time of thecontrol unit

Steady state errorof the control unit

Max. power restriction

Settling time of the control unit

End of chargemode

Conversion lossesof the battery converter

Conversion lossesof the battery unit

Change in the state of energyStandby lossesElectrical load