Posted by: @ian33a

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Hi everyone,

 

we're in the very early stages of getting a heat pump and are exploring ways to load shift.

 

Our existing set up is a 4KWp solar array (circa 2013) on a generation FIT. The inverter is G98, so 3.68KWp. It is a DC/AC unit with no batteries.

We also have an AC coupled battery, set as G100. It has a 3KWp AC/DC inverter and 15KWh of batteries behind it.

 

If we install a heat pump, it will have to be substantial - probably 14 -18 KW , as our house is large and has a significant heat loss. The age of the house and the character makes massive improvements in insulation impractical. We have what we have.

 

Load shifting is essential, but a 3KWp inverter, to my mind, is not enough to drive a heat pump as well as a reasonable house load.

 

So, we are considering adding additional batteries, all behind another inverter - effectively, increasing the peak power capability of the stored power and thus giving the heat pump and house load as much headroom as possible.

 

Our FIT "partner" Scottish Power, has said that any changes that we make to the existing FIT hardware must not allow us to export any energy that we do not create on site. Fair enough.  Everything that goes through the generation meter must be self generated. I can't see a way that we can change the DC/AC inverter to a hybrid, add batteries to it and keep within the rules that they have stipulated. Perhaps you can?

So, my thought was that we add an additional hybrid inverter with batteries behind it and, if funds permit, some additional solar panels as well. This becomes a second solar array (we have space) as well as an energy store that we can fill either by solar energy or by purchasing energy when prices are cheap or both. This gives us a second repository for storing energy as well as a second inverter to provide a larger instantaneous power output to drive a heat pump and house load. We also have an opportunity to export excess charge too, in theory.

I'm wondering if we could set this second array up as a SEG based system (assuming G99 approval) but a problem comes in measuring exported energy. The original PV system may be exporting (and we are being paid for deemed export) as well as the planned second system which may be exporting under SEG. We are already being paid deemed export on the existing system.

Am I correct in thinking that I could surrender the deemed export section of the existing array (which is generation FIT) and, instead, seek to be paid for all actual export from the existing system and a new system under a SEG associated with the new array - all measured with the smart meter that we have at our property? 

What I don't want to do is lose the generation based FIT payments as they are quite decent , especially as we are paid for pure generation and not just for actual export.

 

If this isn't possible, is there another way to make this all work nicely together and still be financially decent?

thanks

 

@david999 The intention is to load shift as much as possible in winter (with an appropriate tariff) and to generate as much as possible in summer (with a different tariff) - what is generated and exported in summer being used to partially offset the high cost of running the system in winter. We have two wood burners (and our own wood) and these can be used to partially provide some of the heat needed in winter.

Posted by: @david999

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big house, poorly insulated V heat pump sounds like big bills does it not. 

On the right tariff, there is no reason a well installed heat pump cannot be significantly cheaper than gas (or oil) to run. The bills would be big no matter what the source of heating as they are a function of the heat loss regardless of the source of that heat.

Posted by: @ian33a

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The intention is to load shift as much as possible in winter (with an appropriate tariff) and to generate as much as possible in summer (with a different tariff) - what is generated and exported in summer being used to partially offset the high cost of running the system in winter. We have two wood burners (and our own wood) and these can be used to partially provide some of the heat needed in winter.

We do exactly that. We have an old stone cottage with ASHP, solar and battery, and we are net zero both in terms of our generation/consumption and electricity cost (inc. standing charges). For us, Octopus Cosy works perfectly with our Tesla PW3 battery. We can recharge 3 times daily during the cheap slots and the battery capacity is perfectly sized to see us through the 6h window until the next cheap rate slot meaning we are only ever importing at the Cosy cheap rate last winter.

The evening slot (4pm-10pm) is the hardest to cover as there is no solar to help out so we are totally on battery. On the very coldest of days we do need to prioritise heating by avoiding using the electric oven, so no 2h roast dinners at 6pm but that happens rarely (luckily we eat a lot of Asian food which we can cook on the gas hob) and we can always turn the flow temp down a degree or two on the heating to help see us through (or import electricity at the end of the period at day rate).

In summer we are net exporters, so switch to a tariff like Flux or IOF which maximises our export over summer and builds credit for winter. IOF was especially good last summer as we averaged ~26p per kWh for our export and then paid an average of 14.8p in the winter to import. This leverage meant that for every 1kWh we exported in summer we could by back in 1.75kWh of electricity (or 7kW of heat at a SCOP of 4) whoch really makes the system work for you.

Hopefully others can advise on a suitable way forward with your solar, but I hope the above serves as an indication as to what is possible when you get an ASHP, solar and batteries working well together.

We have an 1860s grade II listed victorian pile with solid stone walls that we heat with air/air heatpumps. We do have a FIT for the original 3kW solar but also 4kW of non grid tie solar and hopefully soon (waiting LBC) another 11kW of grid tied. It's costing us a lot less than the gas did.

You can combine FIT and non FIT systems. You can also move your FIT to a different FIT provider if the one you currently have is being an idiot. Our is for historic reasons with Ecotricity (before Dale turned weird) and they were very helpful on this, if infuritatingly pedantic about diagrams and the like. You probably do need to find an installer who knows the (somewhat complex) rules of the game and what you can and cannot do but the only real big ones are that you don't put anything the wrong side of the generation meter and you don't try and claim export payments under SEG and keep your deemed export too.

We have an 8kW inverter that runs most stuff, but some of the heatpumps (it's air/air so it's not one single monster) are run off non grid tied batteries and 4kW of non grid tie solar (all self installed). In summer our aircon is basically free, in winter the additional non grid tie batteries charge from the grid overnight but also give us some generation - and often on the coldest days as they are the clear ones. Currently we have a bit under 50kWh of batteries but it's a very large building. If we were doing it today we'd have used a much bigger inverter for the main system and a pile of big Fogstar batteries but at the time the initial 20kWh and inverter cost more than 64kWh of Fogstars and a 60A capable Sunsynk inverter would do today!

The big problem is I think going to be your existing inverter setup. If it's 3kW export that's fine, but if it's also 3kW total output then yes that'll not cover most high heat loss buildings especially if you insist on heating the whole building evenly. With an 8kW inverter we had to be careful in winter and time shift stuff as well as juggle loads a bit when that and 20kWh of batteries was all we had. It worked on Cosy with time shifting. Going to an EV tariff involved adding the non grid tie stuff. Batteries are at least much cheaper now (or will be once Iran has finished with Trump - right now there's a serious demand and supply mismatch that shipping has to catch up with) so it's not unreasonable to put 32kWh of batteries into a house (bit over 3 grand for the battery itself) but that's only going to help if you've got as way to get the power back out and into the heatpumps.

The other option is to simply add some air/air. The air/air grant is looking pretty pointless and without it air/air is still much more affordable and you can add the air/air whilst keeping your gas and then just use the gas to top up when needed, perhaps expanding the air/air and battery over time. You'll get a COP of 4 with any vaguely reasonable installation and you get a lot of flexibility depending what your battery charge level is, what the prices today are and whether reform has jacked electric up and gas down or labour the reverse a few years from now.

Thank you, everyone, for your replies.

Our FIT provider came back in the past couple of days and said that we can, in principle, replace the 3KW solar inverter DC/AC with a hybrid inverter and add batteries, "like for like" provided the system only records nett generation. I take this to mean that we should replace the generation meter with a bidirectional meter and send the nett energy recorded (ie. Outgoing energy from the hybrid inverter to the grid - incoming energy to hybrid from the grid }

This, in principle, is probably better than we have as it would allow us to stack a load of batteries behind the hybrid inverter to soak up PV and cheap electricity and it doesn't require a G99.

However, it limits us to 6KW of instantaneous power from our batteries and likely means that we have to take some power from the grid. It also means that if we install a large array of batteries, we can only charge them at 3KWp - which is pretty useless.

I will send a further email to our potential installers suggesting that we explore a standalone PV array , hybrid inverter, and batteries and G99 - that way we can get traction from SEG while having a high capacity power store with a decent power delivery.