Grid-tie Inverter vs Feed-In Inverter

Hi all,

New to the forum so please excuse the ignorance.

I am looking to use a few second-life EV cells to make a home energy storage solution. I'm pretty comfortable with the more basic aspects of charging and battery management, but the inverter area I am finding a tad confusing.

I would like the system to have future flexibility in case the market here in the UK moves to one where energy export back to the grid is rewarded during times of high consumption. From what I can see, solar inverters have the potential to benefit from any 'Feed-in-tariff' as solar panels where marketed with the idea that any excess energy not consumed by the domestic circuit could be exported to the grid. Grid-tie inverters for use with battery systems don't seem to have this capability. It seems as though they are able to sync the 230v output with the grid to allow seamless transition from grid to battery power, or even battery-assisted power, but exporting power back into the grid wouldn't be possible as the output voltage just isn't high enough. Is this correct?

Does anyone know of an inverter where there is a potential for energy export, and where the export current/power can be set/adjusted? I assume that to export power back to the grid here in the UK the AC output would need to be in sync with grid power + a few volts to result in energy flow in the required direction.

Could a grid-tie inverter be modified to do such a task by identifying the control circuitry and using a step-up transformer with a feedback circuit to give you a marginally higher output voltage? I presume the PLL circuitry used in the majority of these devices are performed by the same texas instrument microcontrollers.

Thanks.

Welcome to ETO!

splking said:

Does anyone know of an inverter where there is a potential for energy export

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Any legal export back to the grid would require an inverter to be type-approved (and probably provided/installed) by the DNO, for safety reasons at least.

I think that's a Tesla powerbank style of product (there are other makes) you are describing and as far as I can see it's approved for use in the UK BUT anything connected to the grid has to be installed by a licensed installer MCS. The battery voltage has no bearing on grid connection, think of a UPS that might only use a 24 or 48V pack and still produce 240VAC at the output.

Hi, thanks for the replies.

Makes sense it would need to be certified. I guess that without the support for these elastic energy storage systems the products aren't going to be manufactured yet.

With V2G technology then, I assume those charger/units would need the circuits to be able to convert DC to AC and provide a sufficient over-voltage to result in energy flow to the grid.

fourtytwo said:

I think that's a Tesla powerbank style of product (there are other makes) you are describing and as far as I can see it's approved for use in the UK BUT anything connected to the grid has to be installed by a licensed installer MCS. The battery voltage has no bearing on grid connection, think of a UPS that might only use a 24 or 48V pack and still produce 240VAC at the output.

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I think we got confused, I wasn't referring to the battery voltage, rather the inverter output voltage and its bearing on being able to support a domestic circuit vs being able to feed energy back into the grid.

Something I thought of if you want to use your batteries is to use an inverter with a transfer switch so you can either power your house from the inverter or the grid. On the subject of grid-ties they are fundamentally different from inverters, grid ties source a current into the grid but rely upon the grid for voltage regulation whereas an inverter sources a voltage into the load providing voltage regulation itself. I have seen some DIY designs that combine the two features (in a switchable fashion) but not any commercial products (probably due to approval issues).

How would a household inverter system feeding the grid recognizes if the grid goes off ? By total collapse of inverter output voltage if the grid is not on ? Would it blow a inverter output fuse/trip some breaker or turn the inverter off or the transfer relay off ?

Externet said:

How would a household inverter system feeding the grid recognizes if the grid goes off ? By total collapse of inverter output voltage if the grid is not on ? Would it blow a inverter output fuse/trip some breaker or turn the inverter off or the transfer relay off ?

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An Inverter should never be connected to feed the grid, it will simply be destroyed in the event! That is why the required device is called a grid-tie.
Maybe you mean in the case of using a transfer switch, but the way such a switch should be used is to power the house EITHER from an inverter or the grid, in such a system the inverter is never connected to the grid.
It is also necessary to stop motors etc between powering from the inverter and grid as they will not be synchronized, bit like using a standby generator.

Externet said:

How would a household inverter system feeding the grid recognizes if the grid goes off ? By total collapse of inverter output voltage if the grid is not on ? Would it blow a inverter output fuse/trip some breaker or turn the inverter off or the transfer relay off ?

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They would simply shut down - it's an absolutely vital safety feature. Isn't a grid-tie just a type of inverter anyway?.

Nigel Goodwin said:

They would simply shut down - it's an absolutely vital safety feature. Isn't a grid-tie just a type of inverter anyway?.

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I am sorry for any confusion I was just trying to point out they are the same but different

A regular household inverter cannot feed the grid. Period. Grid tie inverters have circuits and logic to syncronize the AC cycle to that of the grid, and adjust the voltage high enough to make the power go "into" the grid. Grid tie inverters are pure sine wave as well, to match the frequency and shape of the grid AC waveform.
Most household inverters are "modified sine wave" which means they are really a square wave with many steps. Not true AC voltage. Even if you happen to have a pure sine wave home inverter, its frequency stability could be off by a few percent, and the AC phase would never match that of the power grid.

Most grid tie solar systems (I have one) have some form of power company contract. Where one is paid for power produced, there is a separate meter showing what is fed to the grid, and you are paid based on that meter. Your house consumption is based on the other meter, billed at a different rate.
If you have a "net use" contract, that means you have one meter that runs forward or backwards, depending on house demand and what you generate. So if you generate more than you use, the meter gives you a credit. If you use more than you generate, the meter accumulates charges. Net effect for "net use" is to try and balance overall generation to overall use in kWH for a specific period of time (usually a year). If you generate more than you use, you do not get any carry forward or payment for a "net use" contract.
My contract is to get paid for generation, and the contract is for 20 years. The payment is way more than I'm charged for household use, in part to defray the capital costs of installing the system (government program). After 20 years, it becomes "net use".
As a note, most grid tie solar contracts prohibit the attachment of battery storage to their systems. That is to prevent people using the lower household rate to charge the batteries and feed that back to the grid at the higher payment rate. For "net use" systems, I would see no reason to ban batteries, as efficiency losses would prevent any benefit of cycling the same grid power to charge batteries...