Solar system

Will Darby, solar PV, battery storage, metering, Carlo Gavazzi UK
Will Darby

Imagine being able to harness the sun’s energy during the day for use during the night. With a battery storage system this is exactly what can happen. During the day a roof mounted photovoltaic system can gather electrical energy from the sun then, at night the occupants can draw on the electrical energy stored in the battery.

Battery storage applications for solar PV come in all shapes and sizes, from domestic to industrial in scale. The London Borough of Hounslow is an early adopter of battery storage. It has employed the technology at Western International Market in a scheme which incorporates more than 6,000 solar panels on the rooftop of the wholesale market for fresh produce and flowers. The array is claimed to be the largest set up by a local authority. The £2 million scheme is also the first by a council to adopt battery storage.

Battery storage

The 1.73 MW array of 6,069 panels and four 60kW lithium batteries now generates half the site’s required electricity and Hounslow council says the solar system will contribute 2% of its carbon reduction target, cutting emissions by more than 780 tonnes a year. It will also save £148,000 in energy costs which, along with £100,000 in generation tariff payments and £7,000 in export tariffs, means that the council expects to be £255,000 better off in the first year of operation. Not only can it expect to see immediate savings on its electricity bills, but it is anticipating a return on investment in about five years.

According to a report on renewable energy investment from independent energy firm Smartest Energy, some 2.3GW of battery storage, via 150 projects, is now in planning. The solar battery market is rapidly growing as more manufacturers start looking at ways to innovate and drive down costs.

Metering is an essential part of any battery storage installation and Carlo Gavazzi is receiving a significant upsurge in interest. Solar PV panels generate direct current (DC) and an inverter is needed to convert this DC to alternating current (AC). A generation meter is required to record the kWh generated and used in the home or business or is exported to the grid.

Converting current

The batteries in a solar PV charge using direct current (DC) from an external source (the solar PV system) and discharge DC when energy is required. DC is converted to AC by an inverter. The meter measures export/import so the system automatically charges/discharges the battery in order to increase self-sufficiency.

There are two main ways of linking battery storage into a solar PV system and useful guidance is available from the Building Research Establishment (BRE) and others:

Will Darby, solar PV, battery storage, metering, Carlo Gavazzi UK
The future of energy generation - battery storage is key

• DC coupled: batteries are installed on the same side of the inverter as the solar PV panels; they charge from the panels, and their DC is only converted to AC when it’s used

• AC coupled: the batteries are installed on the grid-side, where the solar PV’s DC has already been converted to AC. A separate inverter converts the AC back to DC for storing in the battery. When the battery discharges, the same separate inverter converts the DC back to AC.

Part of the future

An AC-coupled system is more appropriate when adding battery storage to an existing solar PV system and in such retrofit applications the installer will need to verify that the new equipment being installed is compatible with the existing equipment, says BRE.

Many DC-coupled systems will not operate in a power-cut and they may affect FIT income. In DC-coupled battery storage systems, the solar electricity that goes to charge the batteries isn’t registered by the generation meter at the time, but only when it is subsequently discharged by the batteries or when the batteries are full. Since all batteries lose energy in the charge-discharge cycle, some of the original solar PV output will be lost in the process. This will impact on FIT payments. However, the savings available through buying less electricity will still be significantly greater than the loss of export tariff, claims the BRE.

From grid-connected energy storage such as E.ON’s £4 million facility, located at Willenhall substation near Wolverhampton, through to the likes of Tesla and Nissan’s products for the domestic sector, this is a market set to soar. Energy managers are advised to make sure they’re a part of it.

Will Darby is managing director of Carlo Gavazzi UK

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