Jump to content

ASHP v Immersion with PV


Marvin

465 views

 Share

Results for us as they come forward...(and not what I expected)

 

Our Cool Energy inverTech Air Source Heat Pump CE-iVT9 4.3kW-9.5kW has been on standby only, for the last few weeks, and I have measured the power consumption. It appears to use about 0.1 kWh an hour. That's about 2.25kWh a day in 24 hours.  

 

We use a Solic 200 to direct our excess electricity produced by the PV to the hot water immersion.

 

Whilst the Solic way of heat the hot water uses more energy, because it only runs using power we would otherwise give to the grid, its better for the bills to turn the ASHP off all together rather than use it to heat the hot water tank.

 

You would think that with a high COP it would be better to use the ASHP but with the other things on in the home and the car being charged, you can never be sure your not buying all the power.

 

Secondly I think the ASHP runs at  a minimum of about 1.5kW when heating the water. The Solic can use any spare power from the PV: 10 to 3000 Watts.

 

We could turn the ASHP off altogether and on only when we need hot water and I have decided this is a bit fiddly so won't do it.

 

Another benefit is the fact that the hot water tank is set to 70C ( the ASHP will only do up to 60 and that is at its least efficient) and this lasts us at least 2 or 3 days before needing to be heated up again! ( We do have a super insulated tank) Days when it could be cloudy and we would have to pay for the power. ( We will turn it up higher in the winter and use it as a thermal store for night time warmth...)

 

1830761436_TankStat.thumb.jpg.0b84874ebe9748abe7a987b6006abb93.jpg

 

And finally I would rather wear out the £50 immersion than the £3180+VAT ASHP!

 

Good luck with your project.

 

M

Edited by Marvin
Steamy tea clarification request

  • Like 2
 Share

28 Comments


Recommended Comments



Why do you try and force the ASHP to heat the cylinder so hot?  We have ours set to 48 degrees which the ASHP does comfortably, and that still leaves plenty of capacity for surplus solar PV to heat it further.

 

The ASHP is timed to only start heating the DHW at 11AM by which time there should be a reasonable chance of using a lot of solar PV power to contribute to it.

Link to comment
Just now, ProDave said:

Why do you try and force the ASHP to heat the cylinder so hot?  We have ours set to 48 degrees which the ASHP does comfortably, and that still leaves plenty of capacity for surplus solar PV to heat it further.

 

The ASHP is timed to only start heating the DHW at 11AM by which time there should be a reasonable chance of using a lot of solar PV power to contribute to it.

Hi @ProDave

We do not heat the water above 45C when using the ASHP (However I would like to point out that the tank temperature is very different depending where the probe is. At the moment the high probe says 71C and the ASHP probe says 48.9C

1667647306_ASHPprobetemp.thumb.jpg.b4573d47a88bf1a31d87d84bcfcb3794.jpg

 

Perhaps I written it wrong... let's try 4 scenarios:

 

Use ASHP on all sunny days

ASHP heats hot water to 48C for 3 days: Lets say 6kWh.

ASHP on standby for 3 days: about 6.75kWh 

Total power about 12.75kWh but at least 2.75kWh purchased for the hours from 9pm until 7am

 

Immersion heating system on all sunny days:

Set to heat to 70C: Uses about 9kWh of FREE power (well 10 pence per kWh from the PV)

 

Use ASHP on one sunny day and 2 cloudy days:

ASHP heats hot water to 48C for 3 days: Lets say 6kWh.

ASHP on standby for 3 days: about 6.75kWh 

Total power about 12.75kWh but at least 2.75kWh purchased for the hours from 9pm until 7am and 3kWh for the  cloudy 2 days when not enough PV.

 

Immersion heating system on one sunny day and two cloudy days:

Set to heat to 70C:  Fills up on sunny day and trickles bits in on 2 cloudy days. Uses about 9kWh of FREE power (well 10 pence per kWh from the PV)

 

One way we buy electricity, the other we don't, or at the very most it cost us about 10p per kWh for the system.

 

On cloudy days if we only have 0.5kWh spare being generated when the ASHP comes on it will grab 1kWh from the mains. Secondly the PV power not being used varies from moment to moment but the ASHP cannot currently accommodate this change but something like the Solic 200 can. Low and slow is the immersion way or high and fast, but either way only with excess energy.

 

And finally I would rather wear out the £50 immersion than the £3180+VAT ASHP!

 

( Goodness I am having to proof read all of this to try and avoid Steamytea's interventions)😂

 

Hope this helps to explain

 

 

 

 

.

 

  • Like 1
Link to comment

I have the ASHP set to heat water to 45c between 6am and 8am.... This is on our low rate tarrif of 10p. Plus the PV is generating something. This is purely to ensure there's always water for morning showers.

 

Rest off the time the Solic diverter heats the tank to 56c. I think the ASHP has been on only once or twice in the last week or so after I set this up- that was a day after lots of washing machine /dryer action and extra showers.

 

 

  • Like 1
Link to comment
6 hours ago, Marvin said:

Goodness I am having to proof read all of this to try and avoid Steamytea's interventions

And it looks alright.

 

Alt + 248 gives you a °

Edited by SteamyTea
  • Like 1
Link to comment

It's a right (expletive deleted)up isn't it - the UK electricity market that it.

 

Why they won't do "half hourly net metering" (bill you based on the net import/export over each half hourly period) is beyond me.

 

Avoids all the faffing with "diverting the pv" or investing in batteries just so that you can run small loads intermittently whilst the PV is running at less than this etc at peak but more than this on average.

 

It should not be a good thing to encourage use of PV instead of the heat pump but...that's what they're doing with the pricing system!

 

Standby on that heat pump is disappointing too. Not quite mitsi but far from ideal. Is there any "force stop" mode that can be set on a manual timer?

Link to comment

Hi @markocosic

 

Now we have invested in PV were not interested in giving it away.  The aim with PV should be to reduce the amount of power you need to buy.  Selling at 8p per kWh and buying it back at 40p per kWh is of no interest to me because in the winter we will need to buy a lot of power which would cost more with such a deal. ( summer sale v winter buy back). We also charge our EV from the PV.

 

You touched on another point: we are also setting up an off grid battery supply of power which will be isolated from the mains, but charged from excess PV. Whilst we cannot store lots of power to last the winter (not without £80k haha!) the aim in to try and cover the background usage over night which would save about £550 a year at 28p a kWh.

 

The on off thing with the ASHP is tricky but can be done: a timer, switching a minimum 20amp relay ( I would use about a 30amp) on and off would do the trick. However I would only be turning it off in the summer months, so I will need to calculate the cost saving benefit, as at the moment I just flick the fuse switch.

 

Good luck

M

Link to comment
19 minutes ago, markocosic said:

Why they won't do "half hourly net metering" (bill you based on the net import/export over each half hourly period) is beyond me.

Pre FiT days, the PV industry was asking for Net Metering.

But in the best Labour Government fashion, we got a highly controlled and stupidly priced system.

 

I think the real problem is that the DNOs modeling assumed much lower domestic uptake (systems were £20k for 4 kWp back then) and did not ask for better control/finance of the local grid.

  • Like 1
Link to comment
1 hour ago, Marvin said:

Hi @markocosic

 

Now we have invested in PV were not interested in giving it away.  The aim with PV should be to reduce the amount of power you need to buy.  Selling at 8p per kWh and buying it back at 40p per kWh is of no interest to me because in the winter we will need to buy a lot of power which would cost more with such a deal. ( summer sale v winter buy back). We also charge our EV from the PV.

 

You touched on another point: we are also setting up an off grid battery supply of power which will be isolated from the mains, but charged from excess PV. Whilst we cannot store lots of power to last the winter (not without £80k haha!) the aim in to try and cover the background usage over night which would save about £550 a year at 28p a kWh.

 

The on off thing with the ASHP is tricky but can be done: a timer, switching a minimum 20amp relay ( I would use about a 30amp) on and off would do the trick. However I would only be turning it off in the summer months, so I will need to calculate the cost saving benefit, as at the moment I just flick the fuse switch.

 

Good luck

M


 

what about a wifi spur?

https://www.timeguard.com/products/time/immersion-and-general-purpose-timeswitches/wi-fi-controlled-fused-spur
 

 

Link to comment

Or just get a 240v relay, timer with override switch used to switch the relay on, relay takes care of the amps.

Link to comment
TonyT

Posted (edited)

Use it to switch a contactor 

Edited by TonyT
  • Thanks 1
Link to comment
1 hour ago, TonyT said:

Use it to switch a contactor 

Yes. Silly me! 

Link to comment

It's not a criticism of what you're doing @Marvin - that is logical given the rules of the game - it's a criticism of the rules!

 

 

We would like people to "use electricity when it is available" so to speak.

 

We already have this tool called the electricity grid. Within reason we can each draw 23 kW peak (230 VAC / 100A) whilst using an average of 350W over the course of a year (3,000 kWh). That's very useful. Thanks to this grid that spreads out the peaks in demand we don't need as much infrastructure.

 

 

What can we do with PV?

 

 

The "smart" thing to do would be to use the grid.

 

Did you consume some electricity? Did you generate some electricity? Ok. Then your net consumption / generation was X. That's what you pay for (or get paid for).

 

You can't average this out over a year or even a day because the electricity that you used (in the  nighttime, in winter) might be worth a lot more than the electricity that you generated (in the daytime, in summer)

 

 

The UK has an "import tariff and an export tariff" today.

 

You pay (a lot) for anything imported. You get paid (almost nothing) for anything exported.

 

This is dumb. It is encouraging people to waste electricity.

 

 

Say there is 2.5 kWh of PV available.

 

- (1) One could use 1 kWh of electricity to put 2.5 kWh into the hot water tank (run the heat pump) AND sending the other 1.5 kWh out to the grid so that we burn 4.5 kWh less gas to generate that 1.5 kWh. 

 

- (2) One could also export the entire 2.5 kWh of electricity (so that we burn 7.5 kWh less gas to generate that 2.5 kWh) and burn 1.25 kWh of gas in an 80% efficient combi boiler to make the hot water. That would...actually be better for the world as it stands today.

 

- (3) Or one could spend money on a diverter to throw all 2.5 kWh using an immersion into the hot water tank, leaving the grid to burn 4.5 kWh of gas to supply 1.5 kW of electricity elsewhere. This is the worst possible outcome for the world...but the one that the rules of the game make you play.

 

 

One could also spend money on a battery to store that excess then return 80-90% of it back to yourself later. This is much less bad for the world than (3) but it's still not as good as (2) or even (1).

 

 

Net metering can also be dumb.

 

Lithuania has net metering. You pay (a lot) for anything imported. You get paid (a lot) for anything exported. They do this on an annual basis.

 

This encourages people to generate as much as possible in the summer (when it is easiest / cheapest to generate with PV) and does nothing to discourage consumption in spring / autumn (when the electricity is most expensive to generate from gas).

 

It isn't as ridiculous as ti could be because electricity actually gets a bit cheaper in winter...because they are using the heat from the power plant to heat the cities / need the heat from the power plant to heat the cities. But it's not ideal either.

 

 

Perfection?

 

1) Let suppliers charge whatever they like for import and export but force them to do "net metering" on a half hourly basis. Yes, you can put the kettle on at 3 kW for the minute or two it takes to make tea, even if your PV is only generating 1 kW for the other 28-29 minutes in that half hour, and you would be charged £0 for net import and paid £X for the net export. 

 

This way you don't need fancy diverters tracking minuscule buckets as @Radian is trying to do and you don't need heat pumps that can turn down to virtually nothing. You just need a basic forecast of what the PV will generate in the next half hour and a way to tell your discretionary appliances to "please, if you could, randomly use 0.5 kWh in the next 30 minutes to do your thing" so that the net import/export is near zero. (that's a much easier technical problem to solve)

 

It maximises value from the grid that we already have. It avoids sending money to China or Elon for batteries that are totally pointless except for playing an accounting game at the expense of those who can't afford batteries.

 

What this wouldn't do is encourage you to export the (high value) electricity and burn (low value) gas to make your hot water if this is the best thing for the world right now. That's because of the difference between import and export.

 

 

 

2) Force the suppliers to charge the same for import kWh as they pay for export kWh...and let them choose the timescale down to some regulatory minimum (e.g. half hourly). Those that have the technology will probably choose the shortest timescales to avoid needing to supply electricity at night at the same price as they are having to pay for it during the daytime. Those that don't...won't be competitive for anybody with self generation. 

 

This would give a clearer signal about when it is better to export the (high value) electricity and burn (low value) gas to make your hot water if this is the best thing for the world right now. 

 

 

1&2 are not mutually exclusive. You could do both.

  • Like 1
Link to comment

@markocosic

Interesting.

Electrical generation price should be set on CO2 emissions, not resources used. A CCGT has an efficiency of around 0.6.

Also, if we did have net metering, why would the delivery companies want to pay more than the half hourly wholesale rate? Or the even cheaper long term wholesale rate, which carries severe financial penalties for non delivery.

Should domestic supplies have the same rate of income without the risks, or should they accept a lower rate?

Until the last year, the mean half hourly price of electricity was around £50/MWh. Pretty close to what people are getting paid now.

Link to comment

Efficiency:

 

 

Gas?

 

If it's all from the north sea 1 kWh = 1 kWh. 

If we're importing LNG then say 1 kWh actually = 1.1 kWh (due to cost of getting it here) and it's the LNG imports that would reduce if you use less.

 

Generation?

 

Yes CCGT operating under perfect baseload conditions is 60% efficient.

But in practice it's the peaker plants / part loaded plants that would be turned down if you were to use less. They're not as efficient. Say 40%.

 

Distribution?

 

This isn't without loss. On average it might be around 8% but this is grossly distorted by lots of large loads that are served relatively efficiently at high voltages.

 

The losses en route to LV domestic customers will be higher. Say 20%.

 

So 1 kWh used = 1.25 kWh supplied (80% efficient) = 3.125 kWh burned (40% efficient) = 3.4 kWh imported (allowing for LNG faffery?)

 

It's not as great as you might first think. 

 

 

Suppliers:

 

Suppliers need to "settle the bill for what they have purchased"

 

This is settled on a half hourly basis. If in that settlement window you import 3 kWh and export 2 kWh then the suppliers will only need to buy 1 kWh.

 

To charge you for 3 kWh, then pay you a pittance for 1 kWh, is a joke.

 

 

It is a legitimate argument, if you use 0 kWh and export 2 kWh, to say that they should pay you wholesale rate for that 2 kWh. 

 

They absolutely shouldn't be double dipping by charging retail rates for the gross import though. Only for the net import. That absolutely encourages socially / environmentally stupid behaviour.

 

 

 

But I'd counter that argument by saving that actually, most of the cost of each kWh (in a normal time) is recovering fixed overheads (fixed cost of dealing with a consumer, fixed cost of distribution network) not the cost of the kWh.

 

And the way that you deal with this incentive to (mis)use your own rather than export the electricity is to shift those fixed costs onto the standing charge (look, it's £500 per connection per year, ok, because that's what it actually costs to have the cable there and ready to use) then the import rate per kWh could be much closer to the export rate per kWh.

 

In Sweden the distribution and generation were separated on your bill. Distribution cost more than generation by some margin for low (domestic) consumers. Heat pumps? Please. Funny that.

  • Like 1
Link to comment

Not sure North sea gas is 1kWh = 1kWh.  Firstly offshore they use gas compression to push pressure up to get it onshore, then when onshore it is further cleaned, re-compressed to get into the distribution network, on it way to England could be re-compressed a couple of times.  1kWh is 1kWh +++.

 

Also LNG, uses huge amounts of energy, plenty of gas compression and refrigeration to get it from its gaseous state to the liquid state.

 

If you heat your water with gas, using PV via an immersion directly displaces gas use from the grid.

  • Like 1
Link to comment

For sure 1 kWh (extracted) is not the same as 1 kWh (delivered) for north sea gas. Highlighting that it's even worse for LNG was the purpose of that multiplier.

 

Using 1 kWh of PV generation to immersion heat hot water, instead of using 1.25 kWh of gas to heat hot water, will increase overall natural gas use and carbon emissions. 

 

That's because to avoid burning 1.25 kWh of gas yourself, you just took 1 kWh of electricity away from the grid, and more than 1.25 kWh of gas will need to be burned to generate that 1 kWh of electricity. The only time this stops being true is when there are so many renewables that we are burning zero gas on the grid. We are not there yet.

 

Diverting PV is economically the right thing to be doing but it is also the environmentally wrong thing to be doing.

 

  • Like 1
Link to comment
3 hours ago, markocosic said:

They absolutely shouldn't be double dipping by charging retail rates for the gross import though. Only for the net import. That absolutely encourages socially / environmentally stupid behaviour.

 

Amen to that. Why do we never seem to see any media coverage of yet another sickening extortion perpetrated by the energy sector? I would have thought it was quite a juicy story.

  • Like 1
Link to comment

Like many on here I installed the PV, no grants, no payment for export.  In fact I want to install loads more PV, but to that I either go fast track G99 and have to limit export to 16amp.  

 

The system is rubbish, the generation infrastructure doesn't really want our PV.

Link to comment
47 minutes ago, JohnMo said:

The system is rubbish, the generation infrastructure doesn't really want our PV.

And no one wants to pay to upgrade it either.

  • Like 1
Link to comment
11 minutes ago, JohnMo said:

A new leader of country will fix it - not!

Yes, been top of the policies they are promising, the only ones above it are everything else.

Rishi has already said 'no onshore turbines' or in other words, no change in policy.

At least Boris promised us that all houses will be powered by windpower by 2030, and he was an honest man.

  • Haha 1
Link to comment
1 hour ago, JohnMo said:

In fact I want to install loads more PV, but to that I either go fast track G99 and have to limit export to 16amp.  

Time for a bit of 'direct action' perhaps? For self builders that can self install PV for no more than the cost of the equipment, I'd say there's a definite case to be made for adding redundant capacity to extend the generation seasons into winter. What you do with it in the summer, once all the batteries are full, HW max'd out is... just switch it off. If the politicians can't make reasonable or sensible policies that address our energy needs then perhaps those that can should just get on with it.

  • Like 1
Link to comment

Or 

 

- always get 3ph on self builds and install 11 kW worth of PV / 11  kW worth of EV charge

 

- install a load more than 3.7 kW and export limit; then reprogramme to fill export the minute their back is turned

 

Part of the reason that they double dip is metering capabilities. Precious few can even get basic billing right; let alone the half hourly settlement that would be needed not to double dip using the current metering tech. (the meters report gross in / gross out and it's up to the reader to work out the net import/export...and guess this in the event they meter falls to communicate for any period) 

 

Hence the suggestion that unit rate should be identical both ways. Force the swines to fix their technical capabilities else find themselves unable to price differentially.

  • Like 1
Link to comment

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...