Electrical Energy Storage Systems and Batteries in Historic Buildings

The way that we generate, distribute and consume energy is changing with the need to decarbonise and meet the demand from the electrification of heat, vehicles and transport systems.

As part of this transition to greener energy, we have seen electrical power generation spread out from small numbers of large power stations to smaller localised installations. With this, and the increase in energy costs, there has been a growth in end-users and homeowners wanting to generate their own energy.

However, before deciding to generate your own energy, it is always advisable to reduce your demand first. Our energy efficiency web pages provide guidance on how to economise on energy usage.

Why store energy from a photovoltaic (PV) or other energy generation installation?

Energy generated from an installation such as PV can be fed back to the grid, which is done via an 'export meter' with the agreement of the Distribution Network Operator (DNO). An export meter records how much energy is exported to the grid and a fee is paid for the exported electricity.

The Smart Export Guarantee ensures small-scale generators are paid for the renewable electricity they export to the grid. However, the fee paid for exported electricity is at a far lower rate than what you would pay for importing electricity. This is why it is sometimes desirable to store all the energy locally for use when it is required, such as at night time.

In order to assess whether storage would be worth considering, you need to know when you use electricity. If it is mainly during the day when you are generating energy then it may not be worthwhile. You would need to know your annual electricity use, the generation capacity of your energy generation installation and the cost of the batteries and storage system (including maintenance) so you can compare how much you would gain from feeding back to the grid.

Storage can also provide the PV installation owner with greater resilience to be able to operate during dark hours or cloudy days when there is not enough sunshine to generate full power, as well as when there are power outages. Storing the energy generated on-site to use later requires an 'electrical energy storage system' (EESS) that consists of distribution and control equipment, and batteries.

Battery location and environmental considerations

Before preparing to install any form of battery system in a historic building, care must be taken to design a system that does not compromise the operational ability or maintenance of the equipment, or introduce a risk to the users or the building itself. The design and installation of building services systems in historic buildings need to be considered carefully as the historic interest of a building can be undermined by successive installations. Our web page on installing new services provides further advice.

Batteries need to be installed indoors in a cool, well-ventilated space, protected from direct sunlight and within 6-9 metres of PV arrays. The further the distance, the more electrical losses will be incurred.

Batteries should be preferably kept around 15^oC with 50% humidity. They can tolerate a wider temperature range for short periods, but this will affect their efficacy. Temperatures below 0^oC will reduce both efficiency and usable capacity, especially of lead-acid, providing 70-80% of the rated capacity.

The warmer the temperature, the shorter the battery's life and the greater the losses of stored energy. Elevated temperatures especially stress lead-acid and nickel-based batteries. Even when storing batteries under ideal conditions, all battery types will gradually discharge over time.

Charged lead-acid batteries can be stored for up to 2 years and nickel-based ones for up to 3-5 years even at zero voltage. Lithium-ion batteries must be stored in a charged state, ideally 40%.

As batteries and their associated equipment are electrically live and contain dangerous chemicals, they should only be worked on with the necessary protective gear.

It is not appropriate to locate any of the battery system in an escape route. It is essential for emergency evacuation that all fire exits are kept clear to ensure the safety of people using the building.

With domestic PV arrays, it can be tempting to install a battery system in the loft space or attic. This is not recommended as these sorts of spaces tend to get very warm in the summer months. The weight of the battery system and accessibility for maintenance also needs to be considered.

For example, a 3.3kWh energy bank can weigh 96kg (over 211 lbs) and a 4.2kWh unit can weigh up to 135kg (over 297 lbs). These weights will vary from manufacturer to manufacturer but are a considerable point load, especially when installed in a heritage building.

Likewise, installing this equipment under the floor of an occupied room or beneath an escape staircase or walkway will not provide an appropriate environment. It will cause issues with gaining access to the equipment and the safety of the occupants of the space should a wet-cell battery system be used.

It is not safe to install batteries near kitchens or any other source of ignition such as a boiler room.

Flooding

With weight in mind, another location that might be considered is a basement. It provides a cool environment and is out of direct sunlight, but care must be taken to ensure that there is no history of flooding for this building and that adequate ventilation can be provided.

Flooding is one of the most significant risks to historic buildings resulting from climate change. Many historic structures are in areas where there is a significant chance of river, coastal, surface water, groundwater or sewer flooding. When considering where batteries and associated electrical distribution equipment could be located, it is important to check the risk of flooding for that location. Our guidance on flooding provides further advice on protecting historic buildings.

Ventilation

Most batteries used in domestic installation tend to be lithium-ion, where ventilation is not an issue if they remain in a cool environment. However, ventilation is important when using lead-acid batteries because, when they charge, they give off hydrogen and oxygen gases that are a flammable, corrosive, unhealthy, and explosive mixture. Without proper ventilation, this gas can build up and contaminate the air or cause a potentially dangerous explosion.

Guidance on battery ventilation and how to use rechargeable batteries can be found on the Health and Safety Executive website.

Fire risk and mitigation

Although PVs or other electrical energy storage systems are no greater risk than other electrical equipment, it is still important to understand the risks and how to mitigate them.

Some types of battery such as lithium-ion can be subject to something called thermal runaway, which in extreme cases can lead to cell rupture, explosion and fire. However, battery packs have control circuitry that protects against such hazards.

Other battery types also have this potential and care must be taken to ensure that the manufacturer’s installation guidance is followed, and that control circuitry is not damaged or modified. Care must be taken to ensure that the batteries are not damaged, that they are connected correctly, and that they are only connected to approved charge/discharge units.

Where an electrical energy storage system has inverters or switchgear installed in a remote or rarely visited location, it is recommended that suitable fire detection equipment to British Standard BS 5839 – 6:2019 is installed.

The type of detector to use is likely to be a smoke, heat or multi-sensor detector.

Where the location is infrequently visited, it is recommended that the detector(s) are interlinked with others located in occupied parts of the building so that any audible or visual alarm is acted upon.

Our Fire Alarms for Historic Buildings web page provides general information when installing or upgrading a fire alarm system in a historic building.

Permissions and consents

If the electrical energy storage system and batteries cannot be located within the main building, and adaptation or another structure is needed, then it may be necessary to obtain planning permission. The Planning Portal provides useful guidance. If there are any doubts, it is important to liaise with your local planning authority.

You should also check whether consent is required if the building is listed or scheduled.

Electrical Energy Storage Systems (EESS)

The main components of an EESS are:

• Batteries
• The battery charge controller: The charge controller ensures that a consistent amount of electrical power is sent to the batteries so that they are not overcharged, and so that the backup batteries do not discharge back through the system at night. In many ways, this component is like a car battery charger.
• Power isolating switch: This is an electrical safety device that can manually disconnect itself from the modules in the solar photovoltaic system. In photovoltaic applications, direct current (DC) isolators are used to manually disconnect solar panels for maintenance, installation or repair.
• Power inverter: Since energy is stored in the form of direct current (DC) in batteries, and the building’s electrical distribution system is alternating current (AC), the EESS will require equipment to convert from one to the other and this is done by an inverter.
• Battery Management System: The management system allows the safe charge/discharge of the batteries and the supply of loads. Batteries are protected to avoid degradation by not permitting fast charge/discharge cycles.

In addition, when the source is low voltage direct current, as you would have with a photovoltaic array, then compatible power conversion equipment (PCE) will have to be fitted along with a direct current coupling device. All of this will require a suitable area within the building to be located.

Storage Systems can be either packaged or assembled on-site.

Pre-assembled or packaged EESS

This is essentially an off-the-shelf item, usually purpose-built for the specified rating of a PV installation. It can come as an all-in-one enclosure or as several parts, with separate power conversion equipment depending upon which arrangement works best with the available space.

With packaged EESS, the controller and battery are specified by the manufacturer based on a set of pre-determined input and output power conditions, which means you must ensure that your electrical load, generation output and grid supply profiles are matched to this packaged system. The system designer, or in the case of domestic installations the installing contractor, must ensure that the installation meets the requirements of the relevant legislation and follows the guidance in the IET Code of Practice for Electrical Energy Storage Systems 2nd edition (2021).

The main advantage of this system is that the manufacturer's spares equipment, and the instructions for the operation, maintenance and decommissioning of the installation, are all readily available.

Typical space requirements for a packaged EESS are:

• 3.3kWh unit - a space in excess of 70cm x20cm x 65cm
• 6kWh unit - a space in excess of 125cm x 95cm x 65cm

As dimensions vary from manufacturer to manufacturer, the above figures are just a guideline.

Each form of battery has its own characteristics and it is important to consider these when a particular type of packaged EESS is suggested. Battery manufacturers can provide this information as to their product’s specification and performance.

You will also need to allow space around the EESS to maintain the system.

Assembled on-site EESS

The main advantage of this arrangement is that the system designer is not limited to using the components of just one manufacturer. However, it is strongly dependent on the competence and experience of the designer. They must ensure that this discrete-component EESS matches the load, grid and generation profiles of the installed system.

They are also responsible for compiling all of the associated documentation and drawings, including the user’s instructions and the future availability of compatible spares.

Where space is at a premium or restricted in some other way, the assembled-on-site EESS may be the preferred option as it gives the maximum flexibility on layout and space required.

For domestic installations, a preassembled EESS will be preferable as a neat self-contained unit, usually only taking up a space anywhere from the size of a small computer to that of a washing machine.

Small systems can be wall mounted, while larger ones sit on the floor. Some companies offer 'stackable' batteries that can be used together.

Batteries

The best way to choose the optimum battery for your solar energy system is to ask the following questions when evaluating your options:

• How long will the battery last and how much power it can provide?
• What type of battery to use?
• How much space do you have?
• The costs against your budget

How long will a battery last and how much power can they provide? 

The capacity rating measured in kilowatt-hours (kWh) tells you how much energy the battery can store. It does not tell you what the battery can provide at any given moment. For this, you need to know the battery power rating.

The power rating of a battery is the amount of electricity that it can deliver at one time, measured in kilowatts (kW). A 1kWh battery could in theory run 20 LED lamps for about 9 hours, or a modern refrigerator for 6 hours.

Many solar battery systems have a fixed storage capacity starting at around 2kWh to 14kWh. However, there are arrangements designed to be multiple battery systems, which means they can be added to if you require extra capacity.

Battery types

Some of the types that are available include:

• Lithium-ion (Li-ion) such as lithium-titanate or lithium-cobalt: Lithium-ion batteries are used in most energy storage technologies. Lithium-ion batteries are lighter and more compact than other types of batteries, and they have a higher depth of discharge and a longer life span. However, lithium-ion batteries are more expensive than their lead-acid counterparts, so in an installation where large amounts of storage are required, lead acid is considered.
• Lead-acid: Lead-acid batteries are a reliable technology that has been used in energy systems for a long time. However, they have a relatively short life and a lower depth of discharge than other types of batteries. They are one of the least expensive options currently on the market. For building owners who want to go off the grid and need to install lots of energy storage, lead acid can be a good option. However, they are the most hazardous type of battery.
• Lithium-iron-phosphate (LiFePO4): These batteries have a much better discharge rate than lithium-ion and can handle higher temperatures. This means they are better suited to systems that need to run for long lengths of time without recharging. The downside is that they do not have as good an energy density as lithium-ion. They tend to be used in applications where safety is a factor, as lithium-iron-phosphate is non-toxic and easily disposed of by manufacturers.
• Saltwater: This is a new type of energy storage battery. Unlike others, saltwater batteries do not contain heavy metals, relying instead on saltwater electrolytes. While batteries that use heavy metals need to be carefully disposed of, a saltwater battery can be easily recycled. However, as a new technology, it is relatively untested.

Battery disposal

As most batteries are toxic to the environment, their disposal must conform to UK law. The Waste batteries: treat, recycle and export guidance explains the rules.

Grid connection

When you are considering installing a storage system, you may want it to connect to the grid so you can feed back energy to the grid for a fee. To connect to the grid, you will need approval from your local Distribution Network Operator (DNO). To find out who your DNO is, check with the Energy Networks Association (ENA). You may also have to inform your local council.

To connect the system to the grid, you will need to apply the Energy Networks Association (ENA) Engineering recommendations G98 or G99 depending on the size of the installation. The ENA recommendations set the connection and commissioning requirements for connecting an electricity generating system to the UK distribution network. The ENA have guidance documents and forms available online in their resource library.

Warranty and manufacture

Typically, a manufacturer will offer a 10-year limited product warranty (materials and labour) with the following:

• A 25-year limited power warranty (typically 10 years at 90% power output and 25 years at 80% power output)
• Workmanship and materials warranty of one or two years
• Batteries (non-grid systems/hybrids) warranties roughly 5-15 years
• Inverter(s) warranty of between 5-10 years

However, these are average figures and they will vary from manufacturer to manufacturer.

You will also need to check what warranties your installer offers. The Microgeneration Certification Scheme provides a list of certified contractors.

PV batteries vary in cost depending on their capacity and energy rating. Domestic PV battery systems start from about £400 per kWh upwards to around £800 per kWh, depending on the battery’s life cycle, storage capacity, usable capacity, the chemical materials used, and how it is made.

The cost of some domestic battery systems is decreasing due to the increase in electric cars, and the subsequent involvement of some car manufacturers recycling used car batteries into home systems. These recycled batteries can make storage more affordable.

Maintenance, inspection and testing

As with any electrical equipment, PV batteries must be inspected and tested to the requirements of British Standard BS 7671; as specified by the EESS manufacturer or the manufacturer of the components; and as required by the DNO under the Engineering Recommendations G98 or G99 as relevant.

The inspection should be performed before the energising of the system and the testing performed as per the prescribed sequence. This can be performed in the same manner as that applicable to any other alternating current (AC) circuit, and as outlined in British Standard BS 7671 and supporting document Guidance Note 3 Inspection and Testing.

The procedure for commissioning an EESS will depend upon the type of system and the manufacturer’s requirements.

Lithium-ion batteries, the most common type of solar battery installed in homes and businesses, require very little maintenance but are not maintenance-free. Other types will require a trained technician to perform an annual check-up. This will include checking the voltage of each battery and cleaning the terminals.

Wet-cell batteries such as lead-acid require the most specific maintenance, including checking water levels every two to four weeks and making sure that only distilled water is used. As this type of battery is dangerous, suitable protective equipment must be employed when carrying out this work.

Your regular maintenance schedule should include keeping the energy storage system and its batteries clear of dust and debris. The wiring between the inverters and the PV panels should also be regularly inspected.

Learn more

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