Grid Tied PV but controlling export

[Iain]

After reading about the controller Alan has made,I started to think how useful it could be.

http://www.navitron.org.uk/forum/index.php/topic,9072.0.html

I have a PV system and hopefully I will be on the FIT scheme. As the payments are mainly based on Generation capacity with a small amount for export(3p) I started to wonder if there was a way of storing the exported capacity (or some of it) and use it in the evening,  so as not to import at 14p. (or to reduce the import in the evenings/night)

The only way I can see it working is:-

Have a controller with a similar principle to the one Alan designed and use the excess PV power (normally exported) to charge a battery bank . My evening use, I think is about approx 4kwh.
I would need a separate grid tie inverter that could be supplied from a battery bank controlled again (output controlled by a similar circuit to Alan's) so that it only supplied my household needs and not export. (just hold the system in balance) If the battery dropped below a preset capacity the inverter would just stop and the system would then import from the grid.

It is similar to the Sunny Back up system except everything stays grid tied and the battery inverter only supplies my needs and doesn't export, although still connected to the grid.

Is this type of system possible?
Is it possible to control the output of a battery fed grid tied inverter so that it supplies enough for my use but not export?(the reverse power monitor to control ?)

Just lots of thoughts , Would it work?
Or is it just not possible or practical?
Iain

[EccentricAnomaly]

This sort of scheme tends to be pretty marginal though I don't know what the effect of FITs would be.  Generally, electricity out of a wall socket is cheaper than electricity out of a battery even if the battery will be recharged for free (e.g., from PV) because of the depreciation of the battery combined with the inefficiencies of a charger → battery → inverter chain.

Perhaps it's better to make as much good use of the PV electricity as possible when it's available.

I've suggested this before but one possibility would be to have the thermostat on your freezer turned to a lower temperature when the sun shines to store energy as cold.  That way the freezer probably wouldn't need to run during the evening or night as it'll only warm slowly and might not reach the normal thermostat setting.  A refinement to this would be to have a phase change material (PCM) in the freezer which freezes at, say, -18°C.  If you arrange for the thermostat to be set to -20°C when there's PV available then the material will freeze.  At night the thermostat could be at -16°C so the PCM will melt, absorbing heat so extending the time until the compressor will need to run.

Saturated salt water might make a suitable PCM for this application.

I haven't come across anything suitable (melting point around 2 to 4°C) for analogous use in a fridge.  Hydrazine has a melting point of 1°C which is a tad low and also you probably don't want it anywhere nearby (unless you normally hazmat suits around the house).

[Alan]

Perfectly Doo able

Device to read house load and produce a

0 to 5 volt signal in proportion to house load.

Use 0 to 5 volt signal to vary the battery out put voltage using a I.G.B.T. fet

Use a decent wind grid inverter ( Aurora ) where you can program the curve from 50 volts to > 700 ish

Down side. Cost of batteries every few Months / Years. Dooooo

They don’t last long and they cost lots.

If I was not on the grid, batteries / energy storage would be a requirement.

Super capacitors are available for storage. Costs not dropping as yet.

Options

Use excess electrical solar above requirement for house load to heat water. Use water for heating / washing when required.

In my village there is a redundant water tower for sale. If I lived next door. Use excess solar above requirement for house load to Pump water from bore hole to tower.
Use water from tower through hydro turbine back to bore hole and grid connect in proportion to used load. ( I don’t live next door )

If you lived next door to a redundant tall chimney use the constant suction to drive a wind turbine and generate power.


Must get a life part two. Started Waffling.

Regards

Alan

[guydewdney]

Use a decent wind grid inverter ( Aurora ) where you can program the curve from 50 volts to > 700 ish

500 - not 700 - the magic smoke comes out at 600V

[Iain]

Hi
thanks for all the replies. I can see the technology is there but as you say, is it worth it. I suppose if SMA or someone came up with a grid tie inverter that could change from Solar - day to wind - night and allow this to happen, it could be viable. Just add batteries! . I was just impressed with Alan's controller and it seemed to fit with my idea. I suppose if I found a cheap inverter to play around with it might be worth an experiment, I have some batteries already being charged from a small set of panels. When I have a bit of free time possibly!!!
Thanks all
Iain

[tange179]

Just to chip in here....I believe Panasonic are working on a home battery store that will work in conjunction with solar pv. 

http://70.32.84.178/ecofriend/10428-panasonic-li-ion-battery-stores-surplus-renewable-energy-for-home-use

Although there is little on the web about how the system will work I can only assume that it's PC controlled and knows when to divert export into the battery for use when the sun is not shinning i.e. feeds back through your existing or its specially adapted inverter.

I cannot see these systems being cheap, probably in the thousands.

John.