Solar security: grid connection and batteries give the best of both worlds
Posted by Nick Roach on 20 January 2012 at 9:49 am
We’re pioneer microgenerators who installed PV in 2006 before the feed-in tariff or the spurious ‘when do I get my money back’ argument appeared on the scene.
We live in a rural area with no mains gas, with overhead power cables flapping around in ever more frequent storms; we are solely reliant on electricity for power – and to power oil-fired central heating.
To ‘secure’ hot water, we installed solar thermal in 2009 with a solar-powered pump, i.e., independent from mains power.
As all solar PV owners know, they are as useful as a chocolate fireguard when there’s a power cut because the system shuts down (if it didn’t exported electricity could cause serious injury to people working down the line). So when a YouGen newsletter from Cathy Debenham arrived heralding a battery back-up system, this appeared to offer a solution.
After several false starts, we managed to locate an installation company, which was as committed to the concept as we were; there are a few outstanding issues, but the system is up and running.
The system Cathy highlighted came from SMA, whose units are very widely used in homes throughout Germany & Scandinavia.
The system provides battery power to the house in times of power failure, or may be manually switched on to use battery power when required.
It comprises two separate, integrated boxes of tricks: unit one isolates the home from the grid whenever there is a power cut or supply is momentarily disrupted, and engages battery power. The second unit regulates power flow into the home, and any solar PV power that is being generated at the time continues to be the first choice, but is supplemented by battery power instead of grid power. The changeover takes milliseconds, ensuring computers, TVs etc don’t go into death mode.
Unit one senses when grid power is restored, the process is reversed, restoring grid power once batteries are isolated safely.
In daylight, the excess electricity generated by the solar PV is used to recharge the batteries, rather than being exported to the grid; at night, mains electricity tops up the batteries ready for the next grid failure.
For our home, the back-up system provides battery power for around four days, but we shepherd our power use, which others may not.
As with all installations, there are compromises to be recognised and accepted, eg:1. both units remain on standby (around 40w between them) in readiness for grid failure and battery power flow,
2. space is required for unit installation & battery storage.
The greatest compromise is cost – the final bills haven’t arrived yet, but I am reckoning on around £5,500 + 5% VAT. To our way of thinking, this represents savings exchanged for 20+ years of energy security in a rural area with a dubious national grid, which fails around four times a year.
Failures which are increasing due to the lack of investment in rural areas, which isn’t going to get better any time soon, and the increasingly worrying phenomenon of metal theft – several sub-stations in our region (SE) have been plundered for scrap metal in recent months; they are easy targets.
In an increasingly uncertain world, we have taken the initiative to secure the power supply for our home and home-based business. We’re happy to provide more detailed information to anyone interested in this type of system, and equally happy for people to view the installation.Our suppliers: PV battery back-up: RS Eco Company Ltd. Solar thermal (independent of mains): Solartwin
…and for the record, we have no financial interest in either of the two companies mentioned above; we merely think they are a very useful additions to solar PV.
If you have a question about anything in the above blog, please ask it in the comments section below.
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