PV, UPS & grid-tie

[dan_aka_jack]

Hi there,

I'd like a system whose main aim is to reduce my power draw from the grid.  But an important second aim of the system is to provide backup power for the house if the grid goes down.

The system would comprise of:

Solar PV (about 200Wp to start with...hopefully growing when funds allow)
A battery bank (again, starting quite small but growing when funds allow)
A grid-tie inverter

Are there any "single box solutions" which behave something like this:

condition: mains is OK but sun isn't shining
action: keep battery bank topped up

condition: mains is OK and sun is shining
action: keep battery bank topped up and push power from the PV array onto the grid

condition: mains has failed and sun isn't shining
action: I manually isolate my house from the grid.  The system then uses batteries to power inverter which powers my house

condition: mains has failed and sun is shining
action: I manually isolate my house from the grid. use batteries and PV to power inverter

Any suggestions as to how I can achieve this?  One system I considered would consist of two components: a grid-tie solar inverter and a UPS.  The problem with this system is that if the grid fails then the grid-tie solar inverter will shut off.  I could put the grid-tie solar inverter on the 240v output of the UPS but I expect that might fry the UPS.

So, I suppose my question boils down to: is it possible to get a grid-tie inverter which also be persuaded to run off-grid?

[dhaslam]

I am going to use UPS circuits.  One  for  some  lights  and the other for circulation pumps etc.   Although they only store  a few hundred watts each they should keep adequate lights etc  going and they can be supplemented by external batteries.  I already have several UPS used for computers as well but they don't stay going for long.    It isn't really practical to store electricity for high usage appliances.    I am planning a wind generator, grid linked once I can check the wind speed for a period and I expect it will disrupt power on ocassions, also  being a rural location line faults will take longer to repair.     There is a system available which uses batteries and is grid linked as well which also has the advantage of using more home produced energy but batteries can double the cost of a system both in capital cost and maintenance. 

[Alan]

Quote “  But an important second aim of the system is to provide backup power for the house if the grid goes down. “

How often do you expect the grid to fail. Here in a rural location I have gone for a 7.5 K.W. electric start diesel generator. Sits in garage and gets used in anger once / twice a year. When you look at the cost of a battery system with its expected life of ? years, is it worth the trouble / expense of installing.

If no grid supply is available then all options need to looked at.

I would suggest a grid connected inverter, your solar cell system of 200w should / help to cover the base load of your house. If you go larger in the future then consider selling excess to your electricity supplier.

Regards

Alan

[dan_aka_jack]

How often do you expect the grid to fail.

Good question!  I honestly can't remember the last time it failed.

However, I'm getting increasingly worried about the UK's energy security in particular so I'd like to start preparing before the other 60.8M inhabitants of this fair isle start to worry about power cuts.  I work from home as a freelance video editor so a domestic powercut would also kill my ability to earn a living.

[billi]

jack

your ideas are not far away from my setup 

i am not so sure with the battery idea ( its like adding a girlfriend to the household --- too much voltage then in the end  )

A battery bank (again, starting quite small but growing when funds allow)

adding a new battery to an old one can be a loss ( the new ones will adapt the bad habits of the old one ..i guess)

My system is based on an inverter/charger idea -------- so you connect the incoming grid to him (her  ) program her to say how much to use from the battery the rest is provided by the grid...

or/and tell her to keep the battery topped up ...

But if you go down that road , i would consider a system based on the night unit tarifs -- so charge the battery in the night to use the units during the day( save money for the investment and add on more PV panels  year by year -- as a christmas gift  )

These inverter/charger units are available in every size ( in the occasion of a powercut  that inverted power is all you have) but you can parallel upto five units to increase power (by time)

did anybody understand me ? i guess not


perhaps this helps

http://www.victronenergy.com/upload/documents/Achieving_the_impossible.pdf


billi

[RichardKB]

Good system there Billi bit does not supply power to the grid if you have excess power from some renewable source, from what I can see on their site.

But I assume you could use a grid tie inverter from a renewable source before this system this would satisfy all requirements.

Rich

[billi]

Rich

I think its not a big problem to feed the grid (if wanted)
There is an inbuilt relay that can be (for example) setup to start AC heatpump
 when battery is full and switch back to battery charging when battery is going down

I assume that relay  can do that as well to feed in the grid (but don't know what regulations and meters you need)

In my case having no grid here i use the diesel generator as "my Grid"
Until the Grid supplier here in ireland is not willing to give me a acceptable amount for the feeded in units expensively invested( photovoltaic) , i will use my units preferable to heat or to store


billi

[stephend]

The SMA sunny island inverters seem to offer similar functionality: http://download.sma.de/smaprosa/dateien/5613/SI332-SI4248%20PVN_1.PDF

Although it also looks as though you'd need to manually switch your PV panels between the 2 AC grids if you want to both feed power back to the grid and power your own equipment when the grid goes down.