Deutsche Version  ACT
Electrical Storage
Analysis 24.9.2015

The figure shows how the battery could be used in the perfect case. Since the high volumes, the load on the network is reduced by storing.
© Janina Moshövel, ISEA

Storages in grid-connected PV systems

Photovoltaik storages can specifically relieve the grids and reduce the net expansion to a necessary measure. Which use PV storages have, depends on the operating method of the storage and the grid. First results gives the analysis "PV Benefit".

Project status lifetime of different storage technologies
Typical system size Energy [MWh] 0-25 kWh
Typical system size Output [MW] 0-30 kW
Volumetric energy density [Wh/l] 50-350 Wh/l
Gravimetric energy density [Wh/kg] 40-200 Wh/kg
Volumetric power density [W/l] 20-200 W/l
Gravimetric power density [Wh/kg] 40-400 Wh/kg
Efficiency AC/AC 85-98 %
Storage loss [1/d] 0-0,4 % / d
Cycle durability (80 % discharge level) 200-10.000 full cycle eqivalent
Service life oft he system (1 cycle/day) 5-20 years
Typical discharge time 1 h – 1 day
Response time when preparing the energy 20 ms
Typical period between storage and withdrawal 1/day
Example application areas Decentralised storing of PV electricity, feed-in management, Peak shaving, optimisation of own consumption
Project duration December 2012 until May 2015

The projects aim is the quantitative presentation of the benefit of storages in PV systems in consideration of

  • storage technology, dimensioning and modes of operation,
  • effects on distribution and transmission networks,
  • business and economic effects, and
  • ecological and social issues.

The identification of modes of operation of storages with positive effects on the entire power supply system, the associated general conditions and their analysis concerning the acceptance among potential operators serves to provide recommendations for system-relevant embedding of storages in PV systems and to derive the necessary conditions and potential funding mechanisms. The focus is on battery storage systems in private households with an on-grid PV system.

  • Overloaded parts of the grid: spatiotemporal © IFHT RWTH Aachen
  • While improper, only own consumption optimised operation strategy of PV Storage systems there is the risk that the storage is already fully charged before lunchtime and therefore the PV systems feed to the grid with full power and hence stress the grid additionally. © IÖW
  • Qualitative comparison of the grid workload during  hight load;  Change due to different  opareting strategies respectively further utilities (storages). © PV-Nutzen
  • The figure shows how the battery could be used in the perfect case. Since the high volumes, the load on the network is reduced by storing. © Janina Moshövel, ISEA

Independent investigation

In the project PV Benefit all major aspects of the use of battery storages in PV systems are examined and quantified in an independent study. Previous studies have generally examined only partial aspects or they have been conducted by affected interest groups. In addition, this project brings various partners together who are able to analyze and assess all technical, economic and ecological aspects of this project at the highest academic level and based on years of experience. Due to the rapidly increasing interest of the industry and various interest groups, independent expertise is urgently needed. This is a prerequisite, in conjunction with the wide dissemination of the results, that the findings will ultimately trigger promising innovation processes and readiness to act among many stakeholders from industry to politics.

Workshop shows first results

The project PV-Benefit is in its first operational year in which an initial analysis of the combination of storages and PV systems will be conducted. The results of the initial analysis will be presented and discussed during a validation workshop in Q4 (presumably in November 2013). This ensures high transparency of the project and validity of the assumptions made.

Analysis analyzes During the subsequent 1.5 years of the projects duration, an evaluation of the expected storage lifetime will be worked out. Special attention will be given to the technology-specific dependence of the lifetime with regard to the modes of operation. In addition, the dimensioning of the storage system will be optimized with regard to private benefits to the operator of the system, as well as to the overall social benefit concerning costs and grid stability. To examine the effects on the distribution network, the battery model will be interlinked to a grid simulation model so that feedback mechanisms can be examined. This is followed by an analysis of the overall effects of the use of storages on the German power supply system. The project will be completed with a consideration of the overall ecological effects. In addition to the economic benefit of PV storages the acceptance and the willingness to invest of potential users will be analyzed. In due consideration of these aspects, recommendations for different stakeholders will be developed. Towards the end of the project, there will be another workshop to present the results of the analysis.

Nine work packages

The project partners are the “Institut für Stromrichtertechnik und elektrische Antriebe“ (ISEA), the “Institut für Hochspannungstechnik“ (IFHT) and the “Institut für ökologische Wirtschaftsforschung“ (IÖW). They provide their expertise in storage technology, network simulation, environmental and sustainability research for the dimensioning of PV storage systems, technical assessment, and general ecological considerations. ISEA provides recommendations for the dimensioning based on the technical properties of various battery technologies and evaluates the costs taking into account among others the effects of aging. IFHT studies the effect of storages on distribution networks and identifies grid expansions that potentially could be saved by the use of storages. The technical impact (including power plants and grid traffic loads) of different modes of operation are analyzed for superimposed network levels. IÖW examines the acceptance and willingness to invest of potential system operators, and the environmental impacts of the identified storage dimensioning and modes of operation.

Work Package 1: WP 1 will mainly provide an overview of existing work and tools of project partners and serves to define the work which is potentially needed to guarantee the projects feasibility. Additionally, it is also necessary to identify those points at which the various tools interact.

Work Package 2:
The study focusses on private households and their respective load profiles. At first reference cases (representative sample configurations) and basic parameters, which serve as the basis for the simulations and calculations, are defined. To estimate future development, future scenarios regarding the development and use of PV systems, storage systems and grids are projected, also taking into account the 2010 pilot study of the BMU.

Work Package 3:
Based on available experimental laboratory results about the lifetime of different storage technologies, existing models predicting the lifetime of storages will be combined with up-to-date parameters. These lifetime models will be integrated into an overall system model to facilitate time series simulations. Existing modeling approaches can be used to model the battery lifetime.

Work Package 4:
The aim of this work package is to determine the optimal dimension of the storage system in order to optimise the economic efficiency for the user. This takes into account, on the one hand, own consumption, and on the other hand, avoiding shutdowns of PV systems during power overloads in low voltage networks. To do so, the benefit of different dimensioned battery systems are quantified for different sample grid structures and different rates of penetration of PV systems, different radiation conditions, and different operation strategies (eg. taking into account a radiation forecast). The influence of including control power or system services on the optimal storage dimension will also be considered.

Work Package 5: 
Due to storing, the low voltage grid segment can be relieved during times of high solar radiation by absorbing the energy partially. It will be examined how much the absorption capacity of grid segments of different structures can be improved depending on the number and dimension of installed PV systems, as well as storage systems. This results in a benefit for the PV systems operators through avoiding future limitation of the systems and potentially the prevention of further low voltage network expansion.

Work Package 6:
The results of WP 3 and WP 5 form the basis for the modeling in WP 6 which examines the impact of the application of decentralized storing on the German power supply system. The analysis includes changes in the scheduling of power plants, as well as the technical impact on a simplified high voltage grid.

Work Package 7:
Based on the previous work packages, modes of operation of decentralized storages with positive effects on the entire power supply system will be derived. It is still largely unclear to what extent the associated conditions (eg. smart grids, smart metering, remote access to storages by network operators) and the potentially implied behavior meet with system operators acceptance. Therefore, the study shall determine the benefit of storages to system operators and empirically investigate their willingness to invest and their acceptance of the relevant conditions.

Work Package 8:
Taking into account the ecological perspective is extremely important for sustainable business. The effect of the whole system, which consists of a PV system and a storage at the household level, are composed of several aspects. To begin with the individual products respectively their production, their use and their disposal exert ecological effects. Furthermore the overall effect depends on the development of the total German production of electricity. Decisive questions are to what extent and which plants are able to shut down their production of electricity as soon as decentralized storage for PV electricity is available to a significant extent. For further investigation, a product-related life cycle assessment will be performed. Based on this, as well as on the results of the previous WP, the ecological effects from changes in the deployment of power plants will be examined on LCA base.

Work Package 9:
Finally the results of all previous WP will be merged, assessed and discussed. As a result, there is a detailed and well-structured report on the economic, technical and ecological benefit of storages in PV systems. The opportunities and risks of different modes of operation of storages are presented and potential conditions, ensuring a beneficial management of storages for the entire power supply system, are discussed. Including questions of acceptance allows deriving specific recommendations for target groups. Finally, recommendations will be developed and the results will be prepared specifically for different target groups. These recommendations will be distributed electronically and free of charge to those interested.

Supported by: The Federal Government on the basis of a decision by the German Bundestag


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  • Prof. Dr. Sauer
    Institut für Stromrichtertechnik und elektrische Antriebe (ISEA), RWTH Aachen
Other Addresses


Research funding

The information system EnArgus provides information on research funding, including on this project (German only).