Power surge: the rise of photovoltaics

Efficient and reliable battery-based energy storage technologies are enhancing the viability and increasing the uptake of photovoltaic systems around the globe.

Michael Lippert, Saft Industrial Battery Group, France

The need for efficient and reliable battery-based energy storage systems is already well established in a wide range of off-grid photovoltaic (PV) applications à‚— from remote signalling and telecommunications systems through to hybrid renewable systems and spinning reserve support for large power grids. However, energy storage is also a key enabling technology for decentralized PV installations, where it can add significant value by making grid-connected PV more attractive to both customers and utilities.

PV installations with a permanent connection to the electricity grid are categorised as on-grid applications. This is the most popular type of PV system for homes and businesses in the developed world. PV can be installed on a roof or integrated into the roofs and facades of houses, offices and public buildings. An inverter converts the DC power to AC power for running normal electrical equipment.

The first systems of a new generation of Li-ion battery systems specifically designed for industrial applications are currently on field trial
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Private houses are a major growth area for roof systems as well as for building integrated PV (BIPV). A typical 5 kWp panel (about 50 m2) in southern Germany delivers 4000 to 5000 kWh per year à‚— sufficient to supply nearly all the annual electricity needs of an energy conscious household.

The current practice is to inject all the PV energy produced into the local electricity network and sell it to the utility. Electricity is then imported from the network to satisfy the household’s demand. However, in future, households are expected to become energy autonomous, producing and consuming their own electricity. Energy storage solutions will be used to store excess PV energy until it is needed. Effectively this ‘time-shifts’ PV energy produced during the day, peaking at noon, to make it available on demand. This both maximises local consumption and enhances the efficiency of the PV system. Only surplus energy would be fed back into the grid, for which the owner of the PV system might be remunerated à‚— possibly at a higher tariff during peak demand periods. The indications are that future legislation in Europe will favour this type of self-consumption.

Energy storage can also increase security of supply, while making individual consumers less dependant on the grid. It will help boost 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 that it will reduce the peak load on their grid and at the same time make PV a source of predictable, dispatchable power that they can call on when needed. Reduced grid losses (the energy lost transporting power from a centralized generator to the point of use) will also result in energy savings. Savings due to reduced consumption in PV-powered households are anticipated to be 10à‚—20 per cent.

One major barrier to the electrification of the developing world is the high cost of installing electric grids in countries with a low and scattered population. The reduced cost of PV systems, driven by the on-grid market, is making PV an increasingly attractive option, especially compared with diesel generators.

Typical off-grid PV applications include repeater stations for mobile phone networks, and electrification for remote areas or rural electrification in developing countries. Rural electrification means either small-scale home systems covering basic needs in a single household or larger PV mini-grids serving a small community.

If off-grid, standalone PV systems are to become fully established as a viable and sustainable alternative to grid electrification schemes à‚— cost-effective and reliable energy storage is needed to ensure that electricity is available overnight and in periods of no or low sunshine.

Energy storage technology

Suitable battery-based energy storage technologies for PV installations already exist and are well proven, although there is potential for further optimization. System integration is the key factor. Battery systems need to be both grid compatible and ‘smart’, that is, capable of integrating with PV panels, charge regulation and inverters. They must be reliable (require minimal maintenance) and able to operate with a partial charge over long periods. They must also have a high charge efficiency, good cycling capability, low weight and favourable life cycle cost.

Off-grid installations in outdoor, often remote, sites should be able to operate in extreme temperatures and offer good capacity availability at low temperatures. They must also be physically robust.

The two main battery technologies currently used in PV applications, lead-acid and nickel-cadmium (Ni-Cd), have been in use for well over 100 years, so failure modes and reliability issues are well known.

The newest practical battery technology, lithium-ion (Li-ion), potentially offers significant improvements in terms of performance and service life, and requires no maintenance. However, although Li-ion batteries are well established in consumer applications, the more rigorous demands of PV applications mean that 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 trial.

Single battery cells have relatively small voltages and capacities. This means that PV energy storage batteries are built up from multiple units, and it is therefore possible to construct them in a wide range of sizes, from small (10s watts) to very large (MW).

Lead-acid and Ni-Cd batteries are simple to install. In general, they only require bolted connections, there is no need for special installations or infrastructure and they require normal safety precautions. They are easy to service à‚— service requirements are well documented and based on many years of practical experience. Furthermore, they have a tolerant charge and discharge profile that is well suited to the vagaries of renewable energy systems. However, the more sophisticated nature of Li-ion batteries calls for an integrated system approach

Overall, the key to selecting the ideal PV battery is establishing the best match between technology and the application and requirement pattern. The Ni-Cd battery provides a highly robust, easy to install system in a remote, hostile environment, for example, while size and weight optimized nickel-metal hydride (Ni-MH) batteries are suitable where transport considerations are important. Li-ion offers unrivalled calendar and cycle life combined with a high energy capability à‚— high depth of discharge (DOD).

Whatever type of battery is selected, it must be optimized and adapted for PV applications. It is impossible to take a standard consumer cell packaged into a 12 V unit and expect to achieve a reliable, long-life system.

Storage models

Over 24 hours, a typical European household will consume 10 kWh of electricity à‚— 70 per cent during the day and 30 per cent at night. A small residential PV system will have a panel size of 3 kWp in northern Europe. In southern Europe, this will be 2 kWp. This will produce some 3 to 12 kWh per day, 3000 kWh a year.

Normally, about 50 per cent of the PV energy will be used directly, as soon as it is produced. But around 50 per cent must be stored until needed. Therefore an energy storage system will need to ‘shift’ between 4 and 6 kWh per day. Depending on the battery technology and operational pattern, this will call for a battery of between 5 and 10 kWh.

The use of batteries is already well established in off-grid PV applications. However, further work is required to define the ideal specification for on-grid installations. It would be possible to produce a compact 5 kWh domestic battery, using Li-ion batteries, that would only take up 50 l of space.

The battery would be housed in a cabinet and located in a garage, basement or loft. It would probably be marketed by system integrators as part of a power system package, complete with the power conversion and power management system, and possibly the panels. The system integrator would handle installation and maintenance à‚— such a battery system is unlikely to be purchased for DIY installation.

In the long term, the cost of PV generation and energy storage à‚— in terms of the unit cost per installed kWh à‚— is expected to become competitive. It could even beat electricity retail prices during peak periods. However, for this to happen, high-volume production of dedicated storage solutions will be needed.

Current industrial off-grid systems power a variety of remote sites, in the telecom, marine, oil and gas sectors, for example. Batteries provide energy during periods without sunshine, mainly during the night. They are sized to power the equipment for several days, or sometimes several weeks in bad weather conditions. The typical daily DOD is about 10 per cent. The batteries can normally be fully recharged during the summer, but are often operating at a partial state of charge during the winter, when there is insufficient daylight for a full recharge.

In rural electrification applications a typical off-grid household in the developing world consumes 1 kWh of energy per day, which corresponds to a theoretical panel size of 100 Wp. This is expected to increase to 300 Wp in future.

Energy storage is required to ensure three days of autonomy, based on an average daily cycle of 20 per cent DOD. This calls for an energy storage system of 0.5 to 1 kWh per household. In reality, it is expected that a single PV system would provide energy to several households at a time, at community level, for example. The system size and energy storage required would then be greater.

Innovative energy storage projects


Off-grid projects


Terracon energy container

Terracon Energy is using an Ni-Cd battery system to provide the vital energy buffer for an innovative, self-contained, transportable ‘energy container’, designed to provide an almost instant source of up to 10 kW of renewable energy in remote locations.

The Terracon Energy Container integrates a complete hybrid renewable power system à‚— comprising wind generator, PV modules, diesel generator, batteries, and associated control and monitoring systems and inverters à‚— within a standard 6.1 m ISO shipping container.

On arrival at site, the container simply has to be craned into the required position. There is no need for a base plate and the container also acts as the foundation for the 15 m-high wind generator. This makes the power system fast to set up, and it can be ready to produce electricity in less than a day.

The battery system provides the vital balance between the energy generated and the energy consumed to ensure continuity of supply. It acts as a buffer to make up the difference when the available renewable energy is less than the demand. It also supplies all the energy needed when renewable energy is unavailable and stores excess energy when demand falls below the output from the wind generator and PV modules. If needed, the battery can be recharged using the diesel generator.

The Energy Container guarantees a peak output of 10 kW and a continuous output of 5 kW. Efficient use of renewable energy sources alone enables it to produce between 5000 and 15 000 kWh per year, and in combination with the diesel generator, it could supply up to 37 000 kWh.

The Terracon Energy Container, self-contained renewable power system
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Terracon anticipates that the Energy Container will be installed in a wide range of applications where the lack of a fixed electricity grid calls for an easy-to-transport, self-contained power system. The majority of installations will be in remote locations and subject to extreme environmental conditions, with limited access for maintenance. This calls for a highly efficient and reliable battery with low maintenance requirements.

Saft Sunica.plus Ni-Cd batteries were selected by Terracon for the Energy Container, as they are designed specifically for renewable energy applications, especially in remote installations. They offer exceptional performance, are highly efficient and require little maintenance (topping up intervals have been extended to up to four years, depending on the application). They are highly reliable, able to operate in extreme temperatures (à‚—50 à‚°C to +70 à‚°C) and at any state of charge, and have an extremely predictable 20-year service life.

The exact specification of the battery installed in each Energy Container will vary according to specific customer needs. Typically, it will be a nominal 48 V and 800 Ah capacity.


The Renewable Energy Mobile Utility System (REMUS) developed by Titan Energy Development incorporates wind, solar and battery power to supplement a diesel generator and provide utility-scale electrical services in situations where reliable fuel sources are unavailable or too costly. It is an important development in the emergency energy industry, as it offers a solution that is less dependent on traditional fuels than other systems, an important advantage in emergency and remote situations where fuel sources may be limited.

REMUS requires a small, lightweight battery capable of controlling numerous operations while maximizing fuel efficiency. Saft’s advanced Li-ion technology, which is part of a standard line of products for hybrid electric military vehicles, offers the ideal solution.

On-grid projects


Guadeloupe initiative

A current, two-year project on the Caribbean island of Guadeloupe is testing the viability of using Li-ion batteries in conjunction with PV systems. Some 14 PV systems have been deployed, each consisting of an array of solar panels and a 48 V, 11 kWh Saft Li-ion battery system providing buffer storage for grid-connected PV units.


Sol-ion is a Franco-German project dedicated to the development of a new concept in energy conversion and storage for grid-connected PV systems. Saft, Conergy and Tenesol are partners in the project.

The objective of the Sol-ion partnership is to develop an integrated energy kit able to be produced on an industrial scale for decentralised on-grid, residential PV systems. The project will introduce large Li-ion batteries into PV systems on a scale not yet seen in Europe. Li-ion technology is required to meet the need for 20 years of battery life under demanding cycling conditions. A total of 75 systems will be deployed in Germany and France in trials that will establish the performance of the system, its economic viability, the added value of energy storage in an on-grid PV system and the benefits for stakeholders.

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