ENERGYSTORAGE
Considering a minimum capacity of 5 kWh, then using Li-ion batteries it would be possible for a compact domestic battery to only take up 50 litres or so of space – about the same footprint as a fridge-freezer.
do this without any reduction in their home comforts.
On-grid energy storage 28
A typical residential PV system with a panel size of 3 kW produces a daily average of 8.5 kWh throughout the year in Northern Europe, ranging from 3 kWh in winter to a peak of 12 kWh in summer. About 4.5 kWh of the PV energy will be used directly (self-consumed), as soon as it is produced. There is therefore an average excess of 4 kWh – with a seasonal range of 1 kWh to 6 kWh - that can then be stored until needed. So an energy storage system will need to ‘time-shift’ between 1 and 6 kWh per day – averaging 4 kWh.
Li-ion battery technology
In grid-connected applications, the newest practical battery technology, lithium-ion (Li-ion), offers the potential for significant improvements in terms of performance and service life, 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 that ordinary consumer cells are not suitable. Instead, Saft is developing a new generation of Li-ion battery systems designed specifically for industrial applications, with the first systems already on field test.
The initial indications are that Li-ion technology will offer both very high efficiency, of around 95 per cent, combined with a long calendar and cycle life – 20 years at 60 per cent DOD (depth of discharge)/day.
The compact, sealed for life design of Li-ion batteries also offers considerable advantages.
Deployment of grid-connected energy A current project taking place 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 2 kW PV panels and a 210/280 V, 10 kWh 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 4 kWh daily to the grid, upon utility demand, - 1 hour in the morning and 3 hours in the afternoon, simulating the substitution of fuel powered generators.
The first results during the summer of 2008 showed that the average daily cycle for the batteries is 45 per cent DOD. This corresponds to about 50 per cent of the generated PV energy stored at a battery efficiency of 97 percent. The expected pay back 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 10 kWh, 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 4 pm to 7 pm, particularly when PV output drops off after 5 p.m. Sol-ion, Europe’s largest PV energy storage development project. In the EU- backed Sol-ion project, Saft has joined forces with
www.solar-pv-management.com Issue X 2010
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