Matthias_VetterMatthias Vetter, Ph.D., Head, Electrical Energy Storage, Fraunhofer Institute for Solar Energy Systems ISE is a presenter during the Emerging Energy Storage Applications conference at the 34th Annual International Battery Seminar & Exhibit in Fort Lauderdale.

Grid-Connected PV Systems, Off-Grid Applications,
Stationary Battery Storage and Optimization Potential

1) Can you describe your experiences with stationary battery systems, through prior projects and current ones at the Fraunhofer Institute?

At Fraunhofer ISE we have been involved for decades in projects in the field of stationary battery storage. All started with battery systems for PV off-grid applications, from small solar home systems to single-house AC power supplies up to hybrid PV mini-grids in the MW range, in which batteries allow high solar fractions. In those days, mainly lead-acid batteries had been used and batteries for storing solar energy were a pure off-grid topic, particularly in rural electrification projects. Therefore we have been active in international projects since the beginning of this activity.

Today battery storage also plays an important role in grid-connected PV systems. So we can see that the two worlds – off-grid PV and on-grid PV – are growing together from a technical point of view: both need storage, and in both a reasonable and optimized operation can only be secured by an energy management system. So in the meantime we are also active within projects of grid-connected battery storage applications. Results from our monitoring projects show that lithium-ion batteries are very promising in stationary storage applications, but there is still a huge optimization potential on the system level, which has to be addressed.

2) How has energy storage in power systems with high fractions of renewables evolved during the last decade? What are some of the significant achievements in your field?

With all our experiences we gathered with lead-acid batteries in off-grid PV applications, we can say the battery storage was almost the Achilles verse in such systems. In contrast to this, lithium-ion batteries show very beneficial characteristics for PV applications, e.g., for partial state-of-charge operations. Furthermore, electromobility pushes this technology strongly and worldwide. Therefore huge cost reductions can be estimated for the next years. On the other hand we do not have long-term experience with this technology, either in the mobility sector or in the stationary sector. Long-term experience only exists with lithium-ion batteries in consumer products. So to summarize, there exist huge chances and the potential of this technology is amazing, but there are also some risks and challenges. To name some of them: safety, reliability, lifetime, efficiency and effectiveness. Amongst others we address these topics within our quality assurance services.

3) What challenges have persisted for integrating battery storage in grid-connected photovoltaic systems or PV diesel mini-grids to enable high solar fractions?

In the meantime we have installed over 40,000 residential PV battery systems in Germany with the purpose of increasing self-consumption and to reduce the purchased electricity and therefore the electricity bill. But there is still optimization potential, e.g., in terms of efficiency over a wide range of the operating range and also in terms of the reaction time in case of fast changing power generation or consumption. Furthermore safety is a very important issue, and this is not only true for new systems but even more for aged systems, in which for example the inner resistances of the lithium-ion cells increase and as a result also the heat production. Therefore we are investigating together with partners several home storage systems within a huge project over several years.

In commercial-scale and utility-scale lithium-ion storage systems we see the necessity to increase performance and efficiency. Topics are inverters with high efficiencies over a wide range of the operating range but also the thermal management of the battery modules and the entire system. Model-based development of highly optimized cooling systems as well as model predictive control strategies are enablers to reach also high efficiencies on the system level. Actually for example we monitored a district battery storage for one year and the annual average efficiency reached only 81% (AC-AC round trip). If this value is compared to efficiencies on the cell basis – 98% are possible – it becomes obvious, that there is still a lot of room for improvement.

In hybrid PV diesel mini-grids battery storage allow the maximization of solar shares and the diesel consumption can be reduced to a minimum. Besides lithium-ion technologies lead-acid batteries still play an important role. Furthermore we see a potential for redox-flow batteries in case huge capacities are needed to buffer a couple of days with limited PV power. Unfortunately non-technical risks are in such projects higher – uncertainty of future user structure and uncertainty in political decisions, e.g., in terms of providing concessions – and therefore more conservative investments are made. This fact results in smaller sizes of the PV generator and the battery capacity and finally in smaller solar shares.

4) What future developments do you foresee over the next five years?

I think in the coming five years lithium-ion batteries will reach mass market volumes in all PV applications from the residential sector and district storage applications to the commercial-scale and utility-scale sector. All of them are grid connected and are fulfilling various purposes. In the off-grid and mini-grid sector it might take a little bit more time due to the non-technical uncertainties and the comparable high investments for lithium-ion technologies, e.g., in comparison to lead-acid batteries. Hybrid storage solutions, a combination of both technologies could be a solution. At our institute we developed such a system, in which only 10% of the capacity are lithium-ion batteries and they are used to avoid the worst operating conditions for the lead-acid batteries. This enables higher lifetimes of the lead-acid battery and helps to reduce the specific storage cost. Furthermore redox-flow batteries will have the chance to enter this market with significant shares as they offer the huge advantage to dimension power and capacity independently from each other. Well, we will see…

Speaker Information:

Dr. Vetter is an Electrical Engineer with 18 years of experience at Fraunhofer Institute for Solar Energy Systems ISE. His Ph.D. thesis was in the field of modeling and development of control strategies for fuel cell systems at University of Karlsruhe. Until 2005, he was project manager in the field of modeling, simulation and development of control strategies for distributed power generation systems at Fraunhofer Institute for Solar Energy Systems ISE. Until 2010, he was head of the group for “off-grid power supply” at Fraunhofer Institute for Solar Energy Systems ISE. Since 2011, Dr. Vetter has been head of department in “electrical energy storage” at Fraunhofer Institute for Solar Energy Systems ISE. His work topics among others: autonomous systems and mini-grids, decentralized grid connected PV battery systems, development of battery systems for stationary and automotive applications. He also works on development and optimization of battery management systems, energy management systems as well as supervisory control strategies. Dr. Vetter is the contact person for battery systems at Fraunhofer Battery Alliance.

Presentation: Wednesday, March 22 during the Emerging Energy Storage Applications conference: