ASHP (Valliant?) + Sunamp + PV w/ battery queries.

[I wish I was a Trex]

Hi all, thanks for letting me join. 
 

We have a renovation scheduled to commence in April and I’ve been struggling to find a company that are able to take on a combined ASHP & PV project. In addition most of the standalone contractors seem to only be interested if I want the specific package they sell rather than a bespoke system. 
 

Our property is a 1930s solid brick trad semi with hip roof. We are extending to the rear with a new kitchen extension and renovating at the same time. 
 

The proposals include: 

All windows / doors replaced with modern argon filled A+ rated ~1.0u windows. 
100mm external insulation (looking like probably rock wool based on the contractors I have spoken to). 
100mm underfloor insulation. 
UFH full ground floor in screed. 
New extension to modern regs w/150mm warm roof. 
 

I want to replace the gas boiler with an integrated ASHP and Heat battery system that will work with the PV. 
 

I have been looking at heat batteries primarily due to the space I have in the house that I can use as a Plant Room and I don’t really like the idea of going back to a hot water tank, I really want better on demand. 
 

The ASHP external unit will be about 13m from the internal units. 
 

I have been looking at pairing a Valliant Arotherm with a Thermino 210 hpPV-VT (originally as this was the only matched option but I see that they now support Daikin and Samsung also so I guess this increases the options).
 

The HLC suggests a 9kw ASHP should suffice. 

I have managed to find a company (only in Glasgow 🙄no-one locally wants to do anything other than put 12 panels on a normal roof with an on roof system) who has worked to come up with a 15 panel roof integrated system with three arrays (two in hip roof one on garage roof) for a total of 6.3kW with a 8kw growatt battery. 
 

Is an integrated PV/Battery + ASHP/Sunamp feasible without designing / programming a bespoke system? 
 

My research suggests that a Eddi controller should do most of what I want, which is ensuring the PV tops up the battery/heat battery before exporting to the grid and supporting all of the switching itself. 
 

Some guidance on: 

 

Is a Sunamp worth it or are my fears over a DHW cyclinder unfounded (it would be a mixergy unit). Any real world experiences appreciate. 
 

ASHP (if Sunamp route) Valliant vs Samsung vs Daikin? 
 

Controllers? 
 

Assistance appreciated as I have been looking at the for about 6months and don’t seem to have gotten far and the point I need to place orders is, well now basically. 
 

Thanks in advance. 
 

Note: I’m a geotechnical engineer so most my built environment knowledge stop at the bottom of the grass but I’m keen to learn, just understand I’m not a M&E engineer. 
 

 

[JohnMo]

Think you are trying too hard. Break it down into bite size chunks.

 

Do you have G99 approval for 6+kW of PV?

 

Any electrical driven equipment will take up excess PV electric, so you don't need anything fancy or complicated. 

 

Basically during the heating season PV generates next to nothing.

 

A DHW cylinder with a 3m coil is all you need, half the cost or less than heat a bank, simple control, simple PV diverter.

 

Concentrate your efforts getting the heating system to run as low a temperature you can, that way you get best CoP.

 

[ReedRichards]

If you have radiators it's simplest if you can run them at the same low water temperature as your UFH.  This means they either need to have a really large surface area or instead you use UFH on both/all floors of your house.

 

I don't understand why you think a heat battery gives you "better on demand" hot water than a hot water tank.  Can you explain your "fears"?  

 

  

[I wish I was a Trex]

Thanks for the input, the major issue I have with the DHW cylinder is the space requirements as I’m a bit pushed. It would probably have to go in the new utility which might make it a bit cramped in there. 
 

Tank wise I was looking at a slim line mixergy but even that is 480mm x 1500mm. the equivalent Sunamp is half the size and would fit under the stairs or under a unit in the utility. However the mixergy I believe will work well with an Eddi controller for PV switching. 
 

Also if I understand correctly the Sunamp requires a high temp ASHP so  can run at a higher heating temp as well and the radiators upstairs would not need to be up spec’d as much? UFH upstairs wouldn’t be feasible at present. I’m hoping the insulation new windows and heat from the UFH on GF will make up for any radiator deficiency. 
 

RE: DHW demand I saw a comparison of flow rate of water a X temp vs Y rate for the Sunamp and another DHW tank and the Sunamp seemed better. Maybe I’m just being paranoid about cold showers…

 

The major problem is that as I can’t find one company to rule them all and install a full system, I’m trying to get a PV and Battery system from one installer and the ASHP/DHW tank and UFH from another. My concern is how they all integrate/interact. PV company is applying for G99. 
 

Not being an electrician/heating engine I’m concerned that it won’t be ‘plug and play’ enough that between the two of them and the builder it’s going to be a nightmare…

Not that it’s not logistically challenging already, the only place I can put the ASHP external unit is either on the patio or behind the garage, as due to shared access I can’t put it adjacent to the gable end, so it’ll be 12-15m way from either the utility/under stairs for the DHW. The PV inverter and batteries will be going in the garage. Im just trying to understand as well what supporting electrical equipment I’ll need to allow it all to communicate/divert power? 

 

 

 

[ReedRichards]

10 hours ago, I wish I was a Trex said:

Tank wise I was looking at a slim line mixergy but even that is 480mm x 1500mm. the equivalent Sunamp is half the size and would fit under the stairs or under a unit in the utility. However the mixergy I believe will work well with an Eddi controller for PV switching. 

I can't see any reason why a Sunamp wouldn't work equally well.  Sunamps are possibly a bit more temperamental in their operation than a tank of water; just an impression I have formed without ever having actually used one.  E.g. 

 

 

10 hours ago, I wish I was a Trex said:

Also if I understand correctly the Sunamp requires a high temp ASHP so  can run at a higher heating temp as well and the radiators upstairs would not need to be up spec’d as much?

I think it may be possible to get a Sunamp with a different lower-temperature phase change material that is better suited to a heat pump.  If you operate your ASHP at higher temperatures it just increases your running costs so it's really not a good option.   If you have to run your radiators at higher temperatures then you lose all the economic benefits of having UFH for as long as they are on, possibly for all the time if your heat pump is not smart enough.

 

10 hours ago, I wish I was a Trex said:

RE: DHW demand I saw a comparison of flow rate of water a X temp vs Y rate for the Sunamp and another DHW tank and the Sunamp seemed better. Maybe I’m just being paranoid about cold showers…

 

 Yes, that's just paranoia.  The comparison must have been between the Sunamp and a tank of water used as a thermal store.  Even then I don't see why it would be true [Edit Maybe I can.  A hot water thermal store cools as it supplies heat, a PCM used to store the same amount of heat supplies most of it at the phase change temperature].  But I [really] don't see that you can go wrong with a hot water cylinder that supplies hot water directly provided you can find the space to fit one.  You'll want to store your hot water at 50 C or less so you need a larger volume of storage than if you were using a higher storage temperature.  Or you could even combine a Sunamp with a smaller volume storage cylinder using the Sunamp to preheat the cold water feed to the cylinder.    

Edited March 3, 2023 by ReedRichards

[JohnMo]

As I said a ove you are over thinking it.  PV does not add would not integrated in to the ASAP.  If you have PV or battery power available the ASHP will naturally use that power instead of the grid.

 

Mixergy why not just a normal cylinder with a heat pump coil?

 

I believe the low temp heat pump suitable sunamp is no longer available.

 

Keep things simple, no need for integrating nothing really needs to talk a or ask questions of the other equipment.  Cylinder, on/off thermostat, call for heat from heat pump, 3 way valve opens heat ramps up temp, cyl thermostat closed, 3 way valve to and heat pump to heating duty. All simple bog standard stuff.

 

PV charges batteries or powers home when sun out.  If ASHP is running it will use that power.

[Nickfromwales]

57 minutes ago, ReedRichards said:

I think it may be possible to get a Sunamp with a different lower-temperature phase change material that is better suited to a heat pump.  If you operate your ASHP at higher temperatures it just increases your running costs so it's really not a good option.   If you have to run your radiators at higher temperatures then you lose all the economic benefits of having UFH for as long as they are on, possibly for all the time if your heat pump is not smart enough.

 

NOPE! Off the 'net;

 

The new Thermino range contains a new and improved Plentigrade P58 phase change material (PCM) 

 

They did a PCM34 and then a PCM43 ( I was part of an 11 strong group to spearhead the design and implementation of this unit ) but they must not have pursued this as above. The lower temp units would have been way more reliable tbh, as the failure of the rest of these ( 58's ) has all been attributed to the immersion overheating ( and destroying ) the PCM.

 

To 'melt' the PCM58 you actually need to heat the SA with much hotter water or it takes an inordinately long time to phase-change. I routinely set wet heat to go into these at 65^oC which seemed to work quite well. The units with immersions were ( are? ) still prone to issues with spewing the overheated PCM out through the PRV. I've got a dedicated photo album of these gone 'pop', some where it was so violent that the nuts sheared off the lids as they bent upwards.

 

By comparison, I have been to 2 failed UVC's in my 30 years on the tools, and both were either installer error or where the client had neglected to service / maintain the installation, so basically a near zero fail-rate for UVC's.

 

I'm not a fan of the Mixergy tanks, sorry, just way too much faff / external components / flow switches / pumps and the associated heat losses from all the stuff mounted externally. A simple UVC wins the day, just get a low-slung secondary immersion installed and then you can use 100% of the low / sporadic / proportional energy from PV diversion control. 

 

 

[ReedRichards]

1 hour ago, Nickfromwales said:

They did a PCM34 and then a PCM43 ( I was part of an 11 strong group to spearhead the design and implementation of this unit ) but they must not have pursued this as above. The lower temp units would have been way more reliable tbh, as the failure of the rest of these ( 58's ) has all been attributed to the immersion overheating ( and destroying ) the PCM.

 

 

So I wasn't completely wrong; it's just that Sunamp (apparently) did not pursue the development of the 43^oC PCM unit that would have been much more suitable for use with a heat pump.   

[Nickfromwales]

2 hours ago, ReedRichards said:

 

So I wasn't completely wrong; it's just that Sunamp (apparently) did not pursue the development of the 43^oC PCM unit that would have been much more suitable for use with a heat pump.   

It never went public, therefore you made an assumption   

[ReedRichards]

I don't find Sunamp to be very forthcoming with a lot of the technical details of their product.  One is forced to make assumptions.  😞

[Nickfromwales]

14 hours ago, ReedRichards said:

I don't find Sunamp to be very forthcoming with a lot of the technical details of their product.  One is forced to make assumptions.  😞

You got that bit right. There's a reason they don't engage with the public any more   Some people buy a dog so it can do the barking, and sometimes, people who don't want to enter the ring anymore, buy one to do the fighting..

[markocosic]

Overthinking it.

 

Buy a heat pump and matching cylinder package from a heating installer.

 

Buy PV and (perhaps) an electrical battery from a PV installer.

 

They don't need to talk to each other.

 

Diverting PV via immersion at 100% efficiency is just plain stupid compared with using PV at 300% efficiency to heat water via the heat pump. Time the heat pump to make hot water when the sun is usually shining brightest and be done with.

[Nickfromwales]

15 minutes ago, markocosic said:

Diverting PV via immersion at 100% efficiency is just plain stupid compared with using PV at 300% efficiency to heat water via the heat pump. Time the heat pump to make hot water when the sun is usually shining brightest and be done with.

Good luck monitoring and facilitating that!!! By the time the suns out and the excess is generated, and the HP switches on, and then gets up to premium temp, and is then up and running long enough to be heating DHW effectively, the sun and any "excess" ( aka the requisite surplus thereafter base loads are satisfied ) will have disappeared, and the whole lot will have switched off again.

The HP and ancillary equipment will take a battering with this setup. Excess to immersion is the better choice AFAIC, and zero moving parts, zero LAG, and nigh-on 100% proportional ( so anything after 100W gets used by the immersion ) whereas those pockets of excess would NOT provoke the ASHP to fire at all. Add to this the extended longevity of the heatpump from NOT doing such a crazy duty cycle, christ knows how many times a day, and it's just not a great idea IMHO.

 

Nope.👎

[ReedRichards]

1 hour ago, markocosic said:

Diverting PV via immersion at 100% efficiency is just plain stupid compared with using PV at 300% efficiency to heat water via the heat pump. 

 

I disagree too.  My immersion draws 3 kW and my solar PVs are well capable of providing that (except in winter).  My heat pump draws up to 6 kW when heating the hot water and that would require some import.  Since I am paid for deemed rather than actual export then I can either heat my water for free using the immersion heater or at some cost using the heat pump.

[markocosic]

Not that Nick.

 

You time DHW production for a set time (e.g. midday to 3pm) when sun is most likely.

 

Sometimes it runs all on PV. Sometimes is runs on PV and import. Sometimes it runs in import only.

 

With zero money for exported electricity If the ratio of PV is over 30% you come out as a net winner vs burning the PV directly.

 

With some money for export then you perhaps only need to use PV 20% of the time for this to be more cost effective.

 

PV diversion is dumb.

 

If really fussed you install a battery and allow this to time shift the sun to when the heat pump in scheduled.

 

But run the numbers. It's unlikely to pay for itself if you have the alternative option of going nuts overprovisoning pv instead.

 

 

 

[markocosic]

1 hour ago, ReedRichards said:

 

I disagree too.  My immersion draws 3 kW and my solar PVs are well capable of providing that (except in winter).  My heat pump draws up to 6 kW when heating the hot water and that would require some import.  Since I am paid for deemed rather than actual export then I can either heat my water for free using the immersion heater or at some cost using the heat pump.

 

Being paid FITs is not an option for new installs.

 

I'd you have FITs and are paid deemed the agree PV divert is economically sound if morally bankrupt.

 

For new installs not the case.

 

Better heat pumps can be configured to draw less when heating DHW. Something like a 7 kW Arotherm Plus draws less than 3.6 kW in any scenario (16A breaker supply) and can be set to heat dhw at 50¥ compressor frequency not 100%.

[sharpener]

1 hour ago, markocosic said:

Not that Nick.

 

You time DHW production for a set time (e.g. midday to 3pm) when sun is most likely.

 

Sometimes it runs all on PV. Sometimes is runs on PV and import. Sometimes it runs in import only.

 

With zero money for exported electricity If the ratio of PV is over 30% you come out as a net winner vs burning the PV directly.

 

With some money for export then you perhaps only need to use PV 20% of the time for this to be more cost effective.

 

PV diversion is dumb.

 

If really fussed you install a battery and allow this to time shift the sun to when the heat pump in scheduled.

 

But run the numbers. It's unlikely to pay for itself if you have the alternative option of going nuts overprovisoning pv instead.

 

 

 

 

Been following this with interest as this is what I am planning/hoping to do. I would certainly not want the HP to cycle off every time the sun goes behind a cloud. I think the battery might be useful for filling in these dips in the PV output rather than time shifting per se. But not so easy to control because (for me at least) the HP will be outside the battery/grid control loop to avoid draining the battery into the HP at other times. Victron control is a bit clunky for this, to prevent discharge you have to set charge periods with an unrealistic target SoC (other solutions are also possible).

 

If and only if there is any export left after all that is it worth using it for resistive heating.

[ReedRichards]

3 hours ago, markocosic said:

I'd you have FITs and are paid deemed the agree PV divert is economically sound if morally bankrupt.

 

I'm paid on the assumption that I export half of what I generate.  I don't know how much I actually do export but it's still quite a fair percentage of my summer generation.  I don't see that this is "morally bankrupt"; what's the morally profitable course of action in my circumstances?  

 

I have a 12 kW heat pump that draws up to 6 kW when heating the hot water, that's half power.  That would seem much the same as a 7 kW heat pump heating the water on half power, 3.6 kW except that it will take twice as long.

[markocosic]

Not morally bankrupt:

 

Export the PV to reduce the amount of gas burned in power stations to generate electricity.

 

Import a smaller amount of electricity to generate hot water efficiently via the heat pump. 

 

Even burning gas in a combi results in less overall gas use/emissions than using pv to a cylinder instead of using that pv to reduce gas use at generation plant elsewhere.

 

 

Depends what your objectives are.

 

If you're the slightest bit thinking of the greater good then you wouldn't use pv diversion whilst there is still fossil generation on the grid. 

 

For others it's more important that their electricity is used for their water irrespective of economics; or that you maximise the economic outcome rather than the energy/carbon outcome.

 

Personally I'm not a fan of what Putin or Xi represent; therefore minimising the gas use is the only "morally profitable" option in this day and age.

 

 

Heat pumping:

 

3.6 kW is the max that unit could possibly run at given the 16A breaker. That isn't half power.

 

In practice for this application you'd use the eco mode (50% reduction in compressor frequency) to minimise the draw / extend the runtime charging hot water.

 

Per the charts below that's say 5.2 kW at 7C external and 55C flowtemp with COP 2.9; so an electrical draw of 1.8 kW. 

 

 

Most of that would come from PV if you have a sizeable array. Any time that there's enough sun to PV up a cylinder by immersion there'll also be enough sun to PV up the cylinder by heat pump. And when there's not enough sun to pv up a cylinder the heat pump will use 3x less import.

 

At the last part of heating it'd be 4.9kW at cop 2.4 so 2.2 kW input. And at the first part 6.9 kW at a cop of 4.8 so 1.4 kW input.

 

Look at actual production graphs. Draw lines at 1.4 to 2.2 kW. You'll be above that level for significant amounts of time on a 4 kW array; even moreso on an 8kW array.

 

Use it via the heat pump into the cylinder instead of funding Putin by diverting PV and causing more energy to be used overall.

 

Run some numbers on pv generation plots and compare letting a heat pump do it's thing all year Vs using pv diversion and being paid for the export. 

 

(Ignore some stupid legacy got very lucky arrangement where you're still paid for export even if you self consume - that's a very nice lotto win for an early player but it's not relevant to this NEW installation that the chap is asking for advice on)

[Nickfromwales]

10 hours ago, markocosic said:

With zero money for exported electricity If the ratio of PV is over 30% you come out as a net winner vs burning the PV directly.

 

With some money for export then you perhaps only need to use PV 20% of the time for this to be more cost effective.

 

PV diversion is dumb.

 

If really fussed you install a battery and allow this to time shift the sun to when the heat pump in scheduled.

 

But run the numbers. It's unlikely to pay for itself if you have the alternative option of going nuts overprovisoning pv instead.

 

Still a nope, sorry.

10 hours ago, markocosic said:

 

PV diversion is dumb.

Defo not the case at all.

A good PV diversion controller will dump 100w and upwards into the immersion, so not even 1% indiscriminate. Every single drop of excess is grabbed and pointed at the dump load, so, dumb, it is NOT.

 

10 hours ago, markocosic said:

Better heat pumps can be configured to draw less when heating DHW. Something like a 7 kW Arotherm Plus draws less than 3.6 kW in any scenario (16A breaker supply) and can be set to heat dhw at 50¥ compressor frequency not 100%.

Which means for the same size cylinder they will need to run for longer, requiring much longer / sustained periods of 'excess'. A kW is a kW, regardless of how you get there.

 

5 hours ago, markocosic said:

If you're the slightest bit thinking of the greater good then you wouldn't use pv diversion whilst there is still fossil generation on the grid. 

It may be past my bedtime, but, "WHAT!?".  

 

So, solar PV, bought and fitted, it's producing excess energy beyond your consumption, and you'd not use it, WHY? Reducing countrywide consumption by each adopter of PV utilising as close to 100% of it as they can will drive down fossil fuel consumption at grid generation locations.

 

I'm lost here, sorry.

[markocosic]

25 minutes ago, Nickfromwales said:

So, solar PV, bought and fitted, it's producing excess energy beyond your consumption, and you'd not use it, WHY? Reducing countrywide consumption by each adopter of PV utilising as close to 100% of it as they can will drive down fossil fuel consumption at grid generation locations.

 

I'm lost here, sorry.

 

Look at the big picture instead of looking within your own four walls.

 

 

Mr & Mrs Loadsamoney have a gas boiler and PV

 

Mr & Mrs Conscientious have a heat pump and PV

 

Miss Miggins needs electricity to something non-discretionary (e.g. the fridge)

 

 

Mr Sun sells sunshine to PV generators for £0.0/kWh

 

Mr Putin sells gas for £0.10/kWh; so it's £0.125/kWh for heat from a boiler or £0.25/kWh for electricity

 

(boiler 80% efficient; power generation and distribution 40% efficient)

 

 

 

Option 1:

 

Mr & Mrs Loadsamoney send 1 kWh of PV into their hot water tank

 

Miss Miggins buys 1 kWh of electricity from the grid which gives £0.25 to Mr Putin for gas

 

 

Option 2:

 

Mr & Mrs Loadsamoney buy 1 kWh of hot water by giving £0.125 to Mr Putin for gas and sending 1 kWh of electricity out to the grid

 

Miss Miggins buys 1 kWh of electricity from the grid. Mr Putin gets nothing.

 

 

Option 2 halves overall gas use and payment of tribute to Putin vs Option 1.

 

This is why many folks would consider you to be a selfish penny pinching piece of work if you're diverting PV to generate hot water.

 

 

Option 3:

 

Mr & Mrs Conscientious buy 1 kWh of hot water using 0.5 kWh of PV in the heat pump and send 0.5 kWh to the grid

 

Miss Miggins buys 1 kWh of electricity from the grid which gives £0.125 to Mr Putin for gas

 

 

Option 3 also halves overall gas use vs Option 1.

 

The COP of a heat pump in summer is over 3 though.

 

 

So Option 4:

 

Mr & Mrs Conscientious buy 1 kWh of hot water using 0.3 kWh of PV and send 0.7 kWh to the grid

 

Miss Miggins buys 1 kWh of electricity from the grid which gives £0.075 to Mr Putin

 

This is why you should be using PV in heat pumps.

 

 

There is a separate debate to be had about how broken our electricity markets are. The suppliers should not be charging you for electricity imported (with a half hour period) that you subsequently export (within that same half hour period). It really doesn't matter if a little electricity shuffles back and fort in the wires as the clouds pass or kettles boil. We should have what they call "net half hourly" metering such that you only pay for what you have, net, used at the end of each half hour.

 

But that debate on the commercial terms is different to the debate about what the morally responsible thing to be doing is. The morally responsible thing to be doing is minimising overall fossil gas consumption. Reduce fossil gas consumption for electricity by generating PV. Then reduce fossil gas consumption for heating y using electricity to move heat more efficiently than you can burn gas to generate heat.

 

 

Does this make the absurdity of PV diversion clearer?

 

It absolutely categorically does not achieve the reduction in fossil consumption that you thought it did.

 

It's for selfish people who miserably count every last penny. It's not for people who care about resource efficiency.

 

But you'd be forgiven for thinking that it is "eco" given how it is greenwashed.

 

 

 

BTW the reason that its useful to run for longer at a lower power level is clouds etc.

 

Look at the actual data from PV installations. It's much easier to meet 100% of a 1.5 kW load for 6 hours than 100% of a 3 kW load for 3 hours. (clouds rarely cover 100% of the sky)

 

You CAN actually steer heat pump power use with the appropriate controls; but only between ~50% and 100%; so in time there will be devices to offer this type of service; but in the meantime what you should if you really care is buy a 1-2 kWh battery; which charges in the morning before the heat pump hot water; infills the heat pump hot water when clouds pass; then charges again after it's finished ready for the evening.

 

Alternatively buy an imperial duck tonne of PV; such that you're invariably generating over the required amount; and spill / curtail the rest as appropriate. It lasts much longer than house batteries; is less inclined to catch fire or get nicked etc; and like insulation is one of things that you really can't have too much of.

 

(Generation is an investment that you buy to create. Batteries are just paying to trade.)

[Nickfromwales]

15 minutes ago, markocosic said:

 

Look at the big picture instead of looking within your own four walls.

 

 

Mr & Mrs Loadsamoney have a gas boiler and PV

 

Mr & Mrs Conscientious have a heat pump and PV

 

Miss Miggins needs electricity to something non-discretionary (e.g. the fridge)

 

 

Mr Sun sells sunshine to PV generators for £0.0/kWh

 

Mr Putin sells gas for £0.10/kWh; so it's £0.125/kWh for heat from a boiler or £0.25/kWh for electricity

 

(boiler 80% efficient; power generation and distribution 40% efficient)

 

 

 

Option 1:

 

Mr & Mrs Loadsamoney send 1 kWh of PV into their hot water tank

 

Miss Miggins buys 1 kWh of electricity from the grid which gives £0.25 to Mr Putin for gas

 

 

Option 2:

 

Mr & Mrs Loadsamoney buy 1 kWh of hot water by giving £0.125 to Mr Putin for gas and sending 1 kWh of electricity out to the grid

 

Miss Miggins buys 1 kWh of electricity from the grid. Mr Putin gets nothing.

 

 

Option 2 halves overall gas use and payment of tribute to Putin vs Option 1.

 

This is why many folks would consider you to be a selfish penny pinching piece of work if you're diverting PV to generate hot water.

 

 

Option 3:

 

Mr & Mrs Conscientious buy 1 kWh of hot water using 0.5 kWh of PV in the heat pump and send 0.5 kWh to the grid

 

Miss Miggins buys 1 kWh of electricity from the grid which gives £0.125 to Mr Putin for gas

 

 

Option 3 also halves overall gas use vs Option 1.

 

The COP of a heat pump in summer is over 3 though.

 

 

So Option 4:

 

Mr & Mrs Conscientious buy 1 kWh of hot water using 0.3 kWh of PV and send 0.7 kWh to the grid

 

Miss Miggins buys 1 kWh of electricity from the grid which gives £0.075 to Mr Putin

 

This is why you should be using PV in heat pumps.

 

 

There is a separate debate to be had about how broken our electricity markets are. The suppliers should not be charging you for electricity imported (with a half hour period) that you subsequently export (within that same half hour period). It really doesn't matter if a little electricity shuffles back and fort in the wires as the clouds pass or kettles boil. We should have what they call "net half hourly" metering such that you only pay for what you have, net, used at the end of each half hour.

 

But that debate on the commercial terms is different to the debate about what the morally responsible thing to be doing is. The morally responsible thing to be doing is minimising overall fossil gas consumption. Reduce fossil gas consumption for electricity by generating PV. Then reduce fossil gas consumption for heating y using electricity to move heat more efficiently than you can burn gas to generate heat.

 

 

Does this make the absurdity of PV diversion clearer?

 

It absolutely categorically does not achieve the reduction in fossil consumption that you thought it did.

 

It's for selfish people who miserably count every last penny. It's not for people who care about resource efficiency.

 

But you'd be forgiven for thinking that it is "eco" given how it is greenwashed.

 

 

 

BTW the reason that its useful to run for longer at a lower power level is clouds etc.

 

Look at the actual data from PV installations. It's much easier to meet 100% of a 1.5 kW load for 6 hours than 100% of a 3 kW load for 3 hours. (clouds rarely cover 100% of the sky)

 

You CAN actually steer heat pump power use with the appropriate controls; but only between ~50% and 100%; so in time there will be devices to offer this type of service; but in the meantime what you should if you really care is buy a 1-2 kWh battery; which charges in the morning before the heat pump hot water; infills the heat pump hot water when clouds pass; then charges again after it's finished ready for the evening.

 

Alternatively buy an imperial duck tonne of PV; such that you're invariably generating over the required amount; and spill / curtail the rest as appropriate. It lasts much longer than house batteries; is less inclined to catch fire or get nicked etc; and like insulation is one of things that you really can't have too much of.

 

(Generation is an investment that you buy to create. Batteries are just paying to trade.)

That post deserves a reply where I haven't drunk 4 Brewdog Punk IPA's, so bear with.

 

It's not my fault, they made it very drinkable.

[markocosic]

The numbers are easier after a jar or two.

 

 

Want to save polar bears / eastern europeans?

 

 

Create something (electricity) then use it to replace the most gas-intensive thing currently out there.

 

At the moment this is electricity. Don't displace gas boiler use in your own home (80% efficient) by chucking it in a tank. Displace power station gas use in somebody else's home (40% efficient) by chucking it in the grid. It's a bigger win. Simple as that.

 

 

Create something (heat) more efficiently than you could before.

 

Don't burn gas at 80% to make heat. Burn gas at 40% then use the electricity to create heat at 300% for a relative efficiency of 120%.

 

 

Ideally both.

 

[ReedRichards]

5 hours ago, markocosic said:

Don't displace gas boiler use in your own home (80% efficient) by chucking it in a tank. Displace power station gas use in somebody else's home (40% efficient) by chucking it in the grid. It's a bigger win. Simple as that.

 

 

Is it really as simple as that on a local level?  From my house I can glimpse the tops of two wind turbines, part of a set of 10 (I think) nearby.  So when the wind blows,which is most of the time, then my locale as a whole (which is not densely populated) will be exporting electricity so the main issue will be the transmission losses before that electricity gets somewhere big enough to use it all.  If I export my solar PV electricity I'm just adding to the electricity that loses power in transit.  Wouldn't it be better using it locally, in my house?    

[Marvin]

21 hours ago, markocosic said:

Diverting PV via immersion at 100% efficiency is just plain stupid compared with using PV at 300% efficiency to heat water via the heat pump. Time the heat pump to make hot water when the sun is usually shining brightest and be done with.

Hi @markocosic

I beg to differ. What you say seems to be right when you do the maths, but in fact this is not what happens. 

 

To be clear our electricity diverter (which diverts energy to our immersion heater in our hot water tank) only diverts energy which is not being used and would otherwise be diverted to the main grid. As we do not sell our electricity we direct the energy to the immersion. (In the winter when the hot water tank is up to temperature we redirect to an electric storage heater)

 

This is especially useful during the winter when the ASHP is on.

 

It is normal for an ASHP to 'cycle'. This means it heats up and then stops heating every, what 20 mins or so.

 

When the ASHP is not running in our house we average about 600 Watts but this is not constant ( freezer, fridge on/off, kitchen equipment on/off, and so on.)  So in between all these on and off demands are moments when surplus energy goes to the hot water immersion.

Also it uses the ASHP less but the immersion more: ASHP £ 3,429. Immersion heater under £100. (or under £500 including diverter)

Also we set the immersion at 61 degrees centigrade just to make sure we avoid any legionella issues.

 

Also the higher temperature means that we can store more energy from a sunny day to the next not so sunny day as our tank will last the two of us for about 3 days: A day when all the electricity used to heat the water via our ASHP would come from the grid.

 

Since the end of October we have used the ASHP about 4 times to heat the water when there was not enough sun.

 

Oh, and we charge the car during the day when available as well. 

 

Hopefully anyone reading this who has PV and a hot water tank with an immersion will check out what I am saying.

 

It is 8.54am and right now the PV is heating our hot water.

 

Marvin