Batteries - Time-shifting domestic PV POWER


The ever-increasing political pressures on the global power generation industry to meet demanding climate and energy targets is driving the increased use of renewable energy sources such as wind and solar power. As a result, electricity generation is becoming more decentralised and more intermittent. This calls for new types of power grids with both the flexibility and intelligence to receive generation of all qualities and quantities from diverse sources, and the capability of managing them to deliver reliable consumer supplies, explains Michael Lippert, Saft ESS (Energy Storage Systems) Division

Much of the debate on the nature of these new smarter grids has been focused on issues such as clean power generation, smart metering and information. Now though, there is a growing appreciation that smarter grids will almost inevitably feature some form of energy storage to provide the vital continuity and quality of supply needed to ensure electricity is available wherever and whenever demand - rather than supply - dictates.

A specific area where energy storage is set to make an early impact on smarter grids is in helping to boost self-consumption in grid-connected solar PV (photovoltaic) installations.

Boosting self-consumption for on-grid PV installations
By 2020, PV is expected to account for up to 12% of all generation in Europe, with a total installed capacity of some 390GW, with two-thirds of this being decentralised (source EPIA: ‘paradigm shift scenario'). PV installations with a permanent connection to the electricity grid are categorised as ‘on-grid' applications. This is currently the most popular type of PV system for homes and businesses in the developed world, comprising more than 90% of all PV installations.

A typical domestic PV installation in Europe, such as those now especially popular in Germany and Spain, is sized to deliver around 3,000kWh/year. With the average yearly energy consumption in those two countries running at 3,500kWh it is clear an energy conscious household with an efficient PV system could be capable of meeting all its energy needs itself. However, the current practice is to inject all of the PV energy produced by domestic schemes into the local electricity network, to be sold to the local utility. The household still imports all the electricity it needs from the network.

In the near future, it is expected we will see a significant change in this operating model as households aim to become energy autonomous. This means they will both produce and consume their own electricity, using a local energy storage system to store any excess PV energy until it is needed. In essence, the PV energy produced will need to be ‘time-shifted' from the day-time, peaking at noon, to make it available on demand in the evening.

The introduction of energy storage will both maximise local consumption and enhance the efficiency of the PV system. Only surplus energy would be fed back into the grid, and it is even possible the owner of the PV system might be remunerated at a higher tariff during peak demand periods. The indications are future legislation in Europe will favour this type of ‘self-consumption', especially as the clear indication of the change in energy value and availability throughout the day will encourage households to adopt a much more energy conscious attitude.

Security of supply and deferment of grid upgrades
In addition to helping the shift towards self-consumption, energy storage can also increase security of supply while making individual consumers less dependent on the grid. This will help to stimulate the development of energy self-sufficient houses and buildings and contribute to the continuous growth of PV as part of the global energy mix.

For utilities, the main benefit of on-grid energy storage is it will reduce the peak load on their grid while at the same time making PV a source of predictable, dispatchable power they can call on when needed. There is also the potential to defer costly grid upgrades needed to meet increasing demands for power.

The anticipated implementation of smart metering and real time pricing will enhance the use of demand side management techniques and serve as a major tool to help balance load versus demand in future distribution networks. With such market mechanisms in place, end users can play an active role in optimising energy consumption whilst maximizing the ROI (return on investment) of their PV system. Energy storage enables them to do this without any reduction in their home comforts.

On-grid energy storage - the operational model
A typical residential PV system with a panel size of 3kW produces a daily average of 8.5kWh throughout the year in Northern Europe, ranging from 3kWh in winter to a peak of 12kWh in summer. About 4.5kWh of the PV energy will be used directly (self-consumed), as soon as it is produced. There is therefore an average excess of 4kWh - with a seasonal range of 1kWh to 6kWh - that can then be stored until needed. So an energy storage system will need to ‘time-shift' between 1 and 6kWh per day - averaging 4kWh.

Li-ion battery technology
In grid-connected energy storage applications, the newest practical battery technology, lithium-ion (Li-ion), offers the potential for significant improvements in terms of performance and service life over conventional storage batteries, and it is also zero-maintenance. However, although Li-ion batteries are very well established in consumer applications, the more rigorous demands of PV applications means ordinary consumer cells are not suitable. Instead, a new generation of Li-ion battery systems designed specifically for industrial applications is under development, with the first systems already on field test.

The initial indications are Li-ion technology will offer both very high efficiency, of around 95%, combined with a long calendar and cycle life - 20 years at 60 percent DOD (depth of discharge)/day.

The compact, sealed for life design of Li-ion batteries also offers considerable advantages. Considering a minimum capacity of 5kWh, then using Li-ion batteries it would be possible for a compact domestic battery to only take up 50 litres or so of space - similar to the footprint of a fridge-freezer.

Guadeloupe grid-connected energy storage project
A current project on the Caribbean island of Guadeloupe is testing the viability of using Li-ion batteries in conjunction with PV systems. 15 PV systems have been deployed over 10 sites, each consisting of an array of 2kW PV panels and a 210/280 V, 10kWh Saft Li-ion battery system that provides buffer storage for the grid-connected PV units.

During peak periods, the PV systems provide a controlled injection of 4kWh daily to the grid, upon utility demand - one hour in the morning and three hours in the afternoon, simulating the substitution of fuel powered generators.

Results from the two-year test period have shown the average daily cycle for the batteries is 45% DOD. This corresponds to about 50% of the generated PV energy stored at a battery efficiency of 97%. The expected payback time on the investment is between six to 10 years, depending on the prevailing cost of peak power.

US DOE SEGIS and SMUD projects
A Saft Li-ion battery system, sized at around 10kWh, will provide energy storage for one of the ‘Solar Energy Grid Integration Systems' (SEGIS) projects funded by the US Department of Energy (DOE). The objective of the SEGIS program is to develop high performance products that will allow PV to become a more integral part of household and commercial smart energy systems.

Similarly, a Saft Li-ion battery will supply renewable energy storage for the Sacramento Municipal Utility District's (SMUD) PV storage pilot programme at Anatolia, Ill, a high penetration PV community within SMUD's service territory. The two-year pilot project is being funded by the DOE to examine the value of distributed PV coupled with energy storage in 15 homes and three sites on SMUD's distribution system within the community.

Efficient energy storage will enable solar power to be time-shifted to support SMUD's ‘super-peak' from 4pm to 7pm, particularly when PV output drops off after 5pm

Sol-ion, Europe's largest PV energy storage development project.
In the EU-backed Sol-ion project, Saft has joined forces with industrial partners Voltwerk and Tenesol, as well as with French and German research institutions. The aim is to create an integrated energy conversion and storage kit, capable of production on an industrial scale, for decentralised on-grid, residential PV systems.

The development phase of the project, which commenced in August 2008, has been completed recently, and it is now moving into its test and evaluation phase. This involves the deployment of 75 Sol-ion energy kits for field trials across France and Germany.

The Sol-ion trials will see Li-ion (lithium-ion) batteries used in PV systems on the largest scale ever tested in Europe. The trials will be used to assess the performance of the technology, its economic viability, the added value of energy storage in an on-grid system and the benefits to stakeholders. The project will also investigate the impact of energy storage on demand side management issues such as peak shaving effects and the potential for integration within future smart grid concepts.

The Sol-ion kit has been developed to accommodate PV energy production of 5kWp (peak) with a battery rated from 5 to 15kWh and a nominal voltage of 170V to 350V. Li-ion is the only technology that meets the project's need for 20-year battery life in demanding environmental conditions.

The energy conversion and system management systems are designed to handle four system functions: multidirectional energy flows; self-consumption; grid support; back-up. They are also intended to handle requirements for demand side management such as control over storage and loads using smart metering, and integration within future smart grids that will need to handle demand response and dynamic pricing.

The Sol-ion battery is based on Saft's high energy Li-ion modules, with a nominal voltage of 48V and 2.2kWh capacity. These compact, maintenance-free modules feature an advanced and robust industrial design, and they can easily be connected in series or parallel to create the desired voltage and capacity for each installation.

- Energy storage is a vital element in smarter grids
- Distributed on-grid PV systems with battery energy storage can effectively ‘time-shift' production, making electrical power available when it is needed.
- Decentralized storage provides value to all stakeholders
- Li-ion is a promising energy storage technology and industrialized systems are being developed and trialled