/ Solar PV and heat pump

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Philip on 06 Jul 2014
Anyone chosen solar PV to complement a heat pump? If so, what size PV did you go for. We're looking into the maximum grid connection we can have without paying for upgraded cable, but assuming we could go to 8 (which is about the limit of capital I want to invest) how did you match PV size to heat pump. I've got 90m2 of roof on each of due E and due W, so I'll only get 75% of S performance.

I was thinking that trying to match the spring / autumn output not to exceed the running of the HP would mean being able to use generated energy in the day to match the heat pump. Anyone got a better idea?
Ali.B - on 06 Jul 2014
In reply to Philip:

Solar PV is normally fed into the grid.
HeatPumps normally are active 24/7, regardless of load.
Therefore, i cannot quite see what you are getting at when you say you wish to match PV to Heatpump size?

Ali.B - on 06 Jul 2014
In reply to Philip:

ps... i dont look into the forums that often. If you pm me, happy to discuss and offer further thoughts.
John2 - on 06 Jul 2014
In reply to Ali.B:

It's a little more complicated than that - solar PV is used to power any appliances that are currently operating in the house in which it is installed, then any surplus is exported to the grid. So the heat pump would be powered for free when the sun was shining, and would use paid for electricity the rest of the time (which would doubtless be the majority of the time).

I have solar PV in my house, a 4 kw installation pays me a little over 1000 a year (and also reduces my electricity bill when the sun is shining).
Philip on 06 Jul 2014
In reply to Ali.B:

Heat pump will have a near continuous 2-3 kW requirement. So during spring and autumn a PV giving 2-3kw during daylight would be consumed saving me the ~13p of importing. Any more would only be exported at ~4p. Less (which a 4kw array on E/W in autumn / spring would almost certainly be) will mean importing.

The reason the HP is significant is that no other space heating type (except maybe storage heaters) would give an opportunity to use most of the generated electricity. Using it rather than exporting it and importing at night means payback is much longer.
John2 - on 07 Jul 2014
In reply to Philip:

'Using it rather than exporting it and importing at night means payback is much longer'

Are you assuming that the electricity that you export to the grid is measured? That's not the case - a meter will not be installed to measure electricity exported, only one to measure electricity generated. The assumption is made that 50% of electricity generated is exported, so all installations receive the export tariff for 50% of their output.
jkarran - on 07 Jul 2014
In reply to Philip:

8 what?

Isn't the limit you can grid tie without additional negotiations (and potentially network upgrades) just shy of 4kW nominal (16A at line voltage)? You could fit more to run your heat pump if it needs it bearing in mind they'll be generating far below capacity when you actually need the heat and at peak times unless you're running aircon or running the pump in reverse to warm your heat source you'll have no use for the power you're generating, some of which will go to waste. The additional capacity won't be easy to get connected to the grid. Over all I doubt you'd cover your costs.

East and West facing grid connected solar installs should be encouraged to spread the availability of solar power across the day.

Optimising the heat pump's controls to ensure the pump isn't just shut down during the day due to solar gains and running at night would be essential to make best use of a domestic PV install to cut electricity bills. You could see your home temperature fluctuating a fair bit unless you invest in some significant storage or your home has the ability (thermal mass inside the insulated envelope) to soak up and release that heat.

None of that is said to put you off, I think PV and heat-pumps are both worthwhile technologies for the home but up front prices are significantly inflated by the government subsidy schemes.

jk
Philip on 07 Jul 2014
In reply to John2:

> Are you assuming that the electricity that you export to the grid is measured? That's not the case - a meter will not be installed to measure electricity exported, only one to measure electricity generated. The assumption is made that 50% of electricity generated is exported, so all installations receive the export tariff for 50% of their output.

Ah. Yes, I must admit I assumed they arranged the metering so you had total generated, exported and imported. I'll need to consider this a bit more.


In answer to JKarran. 8kW - I thought that would be obvious, sorry. Maximum size without consent from DNO is just under 4 (equiv to 16A). Above this requires them to do a survey. In our area they don't charge for this (in the SE and in Wales I've heard they do). If they decide that they can accept more than 4 then you can put more on. The firm I'm using are going to contact Western Power for the limit in that area.

Philip on 07 Jul 2014
In reply to John2:

Thanks for putting me right on the metering. It seems that a lot of installations have an unofficial export meter, so you know what you're exporting but to make it official you have to have an MPAN associated with it and pay a yearly cost.

Based on some numbers I've run it probably won't be in my favour to do that. It seems the 8 months of the year Sept-April produce about 50% of the electricity that solar PV generates. During these I would be able to consume all electricity based on a HP running all day for heating. The other four months I'll be able to consume a reasonable amount just storing hot-water, so the yearly average will be over 50%.

After that I've run the payback time for 1 - 10 kW in 1kW units. After 4 kW install the payback time flattens out 8 years for top-end of on-site usage and is in the range 8.6-9.3 years for bottom-end on-site usage.

So I think the answer to my original question is to stick with 4kW as 6kW is only going to save me 1 month in payback and then provide 230/year extra after that.

The biggest error is probably not factoring in energy price rises. But it's put my mind at rest.
alexcollins123 - on 07 Jul 2014
In reply to Philip:

I actually did some research on this sort of thing as part of my degree.

The maximum you can use for feed-in is 4kW as others have said, which amounts to a 16A breaker being added to your consumer unit. Any more than this, I assume, would mean additional inverters and cabling.

We have a 4kW system installed which provides 3.85kW maximum on a sunny day with no cloud cover whatsoever and no shading (shading makes a massive difference to output). On a really good day we make, maybe 25kWh, on an average day in July about 18, and on a shitty day in December maybe 3.

Taking this into account, if we say a GSHP (or any type of medium-sized HP) uses about 2-3kW constantly, during 24h it will use 48-72kWh. This means even on the sunniest day (when your house is lovely and toasty from the sun anyway) you will be paying out for the majority of the energy used. Of course you get payments from the government too, but that probably wouldn't cover it.

On that freezing cold rainy day in december when your panels are covered by snow and not producing anything, the GSHP will be drawing its maxiumum power, and you will be paying out for all of that!

Ontop of all of this, a heat pump requires a highly efficient heating system to be used. Underfloor is perfect because it has a massive surface area. The GSHP will only provide a low flow temp of about 40-50 degrees, so the bigger the heating surface the better it will work. Radiators (unless the are huge) won't cut it and heat will just be wasted.

You also have to consider that it will never be HOT in the house, you won't be able to crank up the heat quickly, it will just remain warm all of the time. You can do day/night temperatures though to save it heating on full whack all night.

Using a heat recovery system may also be beneficial, although still, thats more power being drawn from your already underpowered PV system.

Oh and finally, all of this is worthless if you still have normal haolgen bulbs throughout, if your loft is only marginally insulated etc... etc...

When we had ours installed we doubled the insulation in the loft (which was already twice the minimum requirement), insulated all of the walls, upgraded the old windows and replaced the majority of the lighting with low-energy or LED lamps. Its saved a fortune in electricity, and now I don't actually think we pay for our power in the summer months!

I hope this helps, and please don't hesitate to get in touch if you want more information :D
John2 - on 07 Jul 2014
In reply to Philip:

My actual monthly figures for a SW-facing installation are:

Jul 668
Aug 504
Sept 349
Oct 236
Nov 159
Dec 104
Jan 97
Feb 206
Mar 369
Apr 488
May 540
Jun 631

As the years go by, the panels become slightly less efficient, but the tariff is index-linked, so I expect the financial payback to remain pretty constant.
Ali.B - on 07 Jul 2014
In reply to John2:

Yes, it definitely would appear from the particular applications being discussed, to be more complicated than simply wiring the PV into a single breaker onto the consumer unit, which is the most simple and least costly way of integrating PV.
Philip on 07 Jul 2014
In reply to John2:
Thanks for your figures. They are slightly worse than what I used, but only a couple of %. Not enough to change the outcome.

As for the decrease in performance with time. I wanted a <10 year payback because new inverter and replacement panels may well be necessary by then.

Improvement to insulation may also reduce demand with time. I have to put ASHP in now because there is no alternative central heating in this house. So although underfloor heating, with solid insulated walls and LED lighting would be ideal, the reality is 12k on ASHP or 8k on oil.
Post edited at 12:50

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