Keeping It To Yourself!

8th May 2013
ES Admin

As photovoltaic arrays become more common, consumers need to make informed decisions about the energy storage systems they use, so how should a consumer choose? Dr Alexander Hirnet, General Manager, Energy Storage Systems, VARTA Micro Storage GmbH, explores this issue further in this article from ES Design magazine.

For owners of photovoltaic (PV) panel arrays, the economics of their investment are changing: on the one hand, governments around Europe are reducing the feed-in tariffs payable on residents’ surplus solar energy. On the other, the price of energy sourced from the grid is rising rapidly, as utilities seek to recoup from consumers the cost of investment in new energy supplies and infrastructure.

This explains the rapidly rising interest in residential Battery Energy Storage Systems (BESS), which allow energy captured in daylight hours to be used when it is dark. A BESS offers consumers the chance to increase their energy independence, often generating up to 70% of their own energy requirement, and therefore to reduce their usage of expensive grid-sourced energy.

But the purchase and installation of a BESS is not to be undertaken lightly. The quantity of energy harvested from residential PV arrays can be substantial, and requires a correspondingly large array of batteries for energy storage. Since a BESS forms an essential part of the user’s autonomous power supply in tandem with the PV panels, and should last as long as the panels themselves, the quality and durability of the chosen batteries assumes especial importance.

But consumers are not used to making investments in battery systems; in most consumer products, the choice of battery can appear to be trivial. So what will consumers need to consider for the first time as they weigh up their choice of BESS for their PV array?

Rechargeable batteries as an energy storage technology are used every day by consumers: most commonly in mobile phones and cars, but also in hundreds of other products such as electric toothbrushes, laptop computers and cordless home telephones.

In these devices — even in vehicles — the battery is a small and cheap enough item to be regarded as disposable. Consumers might not study the technical data about the lead acid battery in their car or the lithium-ion battery in their mobile phone, but they generally expect them to fail after a certain number of years of use or charge/discharge cycles; and the cost of replacing the failed battery is small and insignificant compared to the cost of the host device.

In a residential BESS for a PV array, however, the calculation is quite different, and the battery must last as long as the array itself — typically 20 years.

In most installations, the home owner will require sufficient energy capacity to provide for a large proportion of daily energy consumption. A typical European household will use between 5kWh and 20kWh of electricity per day. In the residential sector, most energy is consumed in the early morning and evening, when the building is occupied. But PV energy is generated during the day, when in many cases the occupants are absent, and only a small proportion of the energy generated may be used instantly by appliances and devices in the home. For this reason, many BESS installations will need to be made large enough to store most of the energy harvested from the sun.

Typical installations will therefore require a capacity of between 5kWh and 10kWh. To give some perspective, the chassis to accommodate such a battery system will be nearly as large as a residential hot water tank. There are two fundamental features in the design of a BESS which dramatically affect how long it lasts, and how stable its operation is over its lifetime; cell chemistry and circuit design.

Cell Chemistry

Battery packs or modules are made up of multiple individual cells, connected electrically so that they can be charged and discharged as a single large unit. (A module in a BESS is typically around the size of a standard car battery).

Each cell contains a chemical compound which is able to store electrical energy when charged. A number of different cell chemistries, each with its own set of characteristics, is in use in consumer and industrial batteries today. The most common in consumer batteries are nickel metal hydride (NiMH) and lithium-ion, which is used in mobile phone and laptop computer batteries. Car batteries are traditionally lead-acid types.

BESS available today generally feature one of three chemistries.

Lead-acid (the chemistry used in car batteries) has the principal advantage of being cheap; its energy density, however, is relatively low. Lithium-ion can provide around four times greater capacity in a given volume than lead-acid. But beyond this, there is one main reason that lead-acid is less suitable for a BESS; its lifetime is too short. Just like a car battery, after 3-5 years of use, the battery’s energy storage capacity will rapidly decline, leading soon thereafter to complete failure.

Conventional lithium-ion cell chemistry is favoured in small consumer devices, such as mobile phones, in which both size and weight are kept to an absolute minimum. Lithium-ion offers the highest energy density of all mainstream cell chemistries. When used in today’s consumer devices, lithium-ion batteries are protected by complex electronics systems which are intended to prevent ‘thermal run-away’ — dangerous overheating which can cause the battery to combust. While electronic protection keeps the risk of fire in consumer devices very low, the potential for damage and injury if a large lithium-ion system such as a BESS were to catch fire can be very high, and this counts against its use in residential PV systems.

Lithium-iron-phosphate chemistry offers energy density almost as high as that of a consumer lithium-ion battery; a large PV installation can store a vast amount of electrical energy in a lithium-iron-phosphate battery. But unlike lithium-ion, lithium-iron-phosphate is not prone to thermal run-away; it is extremely safe. It also offers a significantly longer operating lifetime than lead-acid systems. In addition, when comparing BESS systems it is easy to be misled by comparisons of the nominal capacity of the systems, expressed as a price ratio in €/kWh. It is important to check the actual effective capacity (different from the nominal capacity), which ranges from just 40-60% for lead-acid systems up to 80-90% for lithium systems, including lithium-iron-phosphate.

Circuit Design

A battery can ‘fail’ in two ways. The most obvious way is a complete or — to use the technical term — ‘catastrophic’ failure. When this happens, the user is completely unable to draw any power from the battery (to ‘discharge’ it), or to replenish its store of energy (to charge it). The battery is dead.

But a battery can also fail by operating at a fraction of its nominal capacity. Let us take the example of a consumer who buys a laptop computer with a rated battery run-time of three hours from fully charged. Having charged the battery fully, the user then takes the computer onboard an aeroplane, expecting to use it for the whole of a three-hour flight. The battery runs out of charge after one hour. From the user’s point of view, the battery has ‘failed’ even though it was able to operate for a limited time, and was not completely dead.

Either type of failure — catastrophic or material functional impairment — is unacceptable in a BESS. And this is where the battery’s circuit design is of crucial importance. The circuit must operate so that:

-the failure of a single cell does not disable an entire module;

-the failure of a module does not disable the entire BESS;

-the capacity of each module can be monitored, so that a module with severely reduced capacity can be identified, and;

-a failing module can be removed and replaced with a new module in order to maintain energy storage capacity at the specified level. The system should be capable of accommodating a mix of original and replacement modules.

It is clear, then, that different BESS products can vary significantly in terms of performance and lifetime. While BESS manufacturers typically claim their products offer a 20-year lifetime, the purchaser should not rely solely on the manufacturer’s own rating.

As this article shows, both the wrong choice of cell chemistry, and an ineffective circuit design, can dramatically impair the BESS’s operation, and lead to premature catastrophic failure, a reduction in energy storage capacity of more than 20% before 20 years has elapsed, and heightened risk of fire caused by thermal run-away.

The new BESS from VARTA, Germany’s biggest and oldest manufacturer of industrial and consumer batteries and a pioneer of advanced battery technology, incorporates all the most important features described above in order to offer a reliable and high-performance battery system for PV installations, combined with long expected system lifetime of >20 years.

Engion home by VARTA Micro Storage is a fully integrated system, comprising a 4kW inverter, an electronic Energy Management System (EMS) and a modular battery which offers capacity ranging from 3.7kWh to 13.8kWh. The EMS automatically switches from charge to discharge mode and from PV supply to battery supply or grid supply as appropriate.

Adding modules to boost capacity (up to the 13.8kWh maximum) is simple, and can be done by the user even years after the purchase of the original system. New, improved modules will work alongside existing modules.

Each Engion BESS module, which is made from lithium-iron-phosphate cells, is rated for 6,000 charge/discharge cycles; this provides for more than a 20 year lifetime for typical users. After a module has exceeded its rated lifetime, it can easily be removed and replaced with a new module.

The features and advanced design of the Engion BESS from VARTA Storage therefore give users confidence that their investment in energy storage may last as long as their PV panels, and that they can reliably power their home more independently of the grid over a potential 20 year lifespan and even longer.

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