Mixing solar thermal with solar PV

rugby_bull

Hi,
Apologies if what follows has been answered elsewhere on the forum or makes me sound like a self-confessed solar novice (which I am!).  We have been interested in installing solar panels for a while and have finally got round to giving it some serious consideration through the grants available for solar thermal through the Green Homes Scheme.  The company who've quoted for solar thermal also offer PV and so I'm wondering whether to get both installed at the same time.  I've seen mention of immersion diverters elsewhere on the forum with mixed views on these (particularly when batteries are also in use).  I would welcome any advice on whether we should install both solar thermal and PV panels or just stick to PV panels.  We have a 4 bed detached house in the Midlands with a S facing roof capable of taking 10 PV panels and an East facing roof which we've been told will take at least 2 solar thermal panels.  There are no shading issues that I'm aware of but yet to get a roof survey.  Thanks for your help with this.

frozen_wastes

I once considered solar thermal.  They are after all more efficient with the sunshine.  However they are capital intensive, and surplus solar thermal is completely curtailed.  Furthermore if you’re on mains gas, the savings on your gas bill are nowhere near as great as savings on an electric bill.

PV is less efficient, but far more versatile, with surplus power able to be exported.

I went for a hot water diverter.  Worthwhile if you’re on Propane or Oil, but not for mains gas users.  Every kWh that goes to solar hot water is a kWh that isn’t being exported at 5.5p/kWh in the case of OctopusEnergy customers.  When mains gas costs less than that, you’ll suffer an opportunity cost.

If you go for solar thermal,  the opportunity cost is sacrificing space on the roof that would otherwise go to a PV panel.

rugby_bull

frozen_wastes said:

I once considered solar thermal.  They are after all more efficient with the sunshine.  However they are capital intensive, and surplus solar thermal is completely curtailed.  Furthermore if you’re on mains gas, the savings on your gas bill are nowhere near as great as savings on an electric bill.

PV is less efficient, but far more versatile, with surplus power able to be exported.

I went for a hot water diverter.  Worthwhile if you’re on Propane or Oil, but not for mains gas users.  Every kWh that goes to solar hot water is a kWh that isn’t being exported at 5.5p/kWh in the case of OctopusEnergy customers.  When mains gas costs less than that, you’ll suffer an opportunity cost.

If you go for solar thermal,  the opportunity cost is sacrificing space on the roof that would otherwise go to a PV panel.

Thanks for your advice here.  It is much appreciated.  I should have mentioned that the Green Homes grant will mean that we contribute £1700 of the £5100 cost of a solar thermal system consisting of two Worcester solar thermal panels plus 250ltr twin coil Worcester cylinder.  
The company who have provided the quote think that we can fit 10 300W PV panels on our south facing roof, possibly 10 375W PV panels subject to a roof survey.  I'm assuming that we're better to push for something closer to a 3750W system than 3KW given the size of the property?  We are currently a household of 4 adults (as the kids don't look like moving out any time soon!).

Hexane

rugby_bull said:

frozen_wastes said:

I once considered solar thermal.  They are after all more efficient with the sunshine.  However they are capital intensive, and surplus solar thermal is completely curtailed.  Furthermore if you’re on mains gas, the savings on your gas bill are nowhere near as great as savings on an electric bill.

PV is less efficient, but far more versatile, with surplus power able to be exported.

I went for a hot water diverter.  Worthwhile if you’re on Propane or Oil, but not for mains gas users.  Every kWh that goes to solar hot water is a kWh that isn’t being exported at 5.5p/kWh in the case of OctopusEnergy customers.  When mains gas costs less than that, you’ll suffer an opportunity cost.

If you go for solar thermal,  the opportunity cost is sacrificing space on the roof that would otherwise go to a PV panel.

Thanks for your advice here.  It is much appreciated.  I should have mentioned that the Green Homes grant will mean that we contribute £1700 of the £5100 cost of a solar thermal system consisting of two Worcester solar thermal panels plus 250ltr twin coil Worcester cylinder.  

If they're not pulling the wool over your eyes in some way then this sounds like a very good deal. Solar thermal installations can be expensive. So this is not a case of an installer quoting an unrealistic price because the subsidy will make it affordable, as sometimes happens. Instead the price sounds realistic to begin with and the grant makes it apparently a very good deal. That's if it includes everything - a solar thermal install needs a pump and plenty of plumbing work of course.

With four adults in the house you do have a demand for the hot water it would generate, but that's partly balanced by earlier comments on how displacing gas heating of domestic hot water (or anything else) does not offer as large a saving as displacing electricity demand. On the flip side, the higher efficiency of solar thermal means that the proposed installation on your east roof would only displace a relatively small number of photovoltaic (PV) panels. And of course contributing to hot water heating in the morning (east facing) might be ideal for morning showers etc.

You will need to make sure that whatever heating controls you have or end up with, allow you to turn off the gas hot water heating while leaving the gas radiator heating on. Otherwise the gas-fired water heating can end up heating up the hot water at cost when it could've been heated up for free.

I've never used a thermal hot water solar system (had a salesman's survey and spiel and quote for one a long time ago) but you've probably seen the comments on this forum by someone who said that solar PV systems just stop generating power when they have a fault, but solar thermal systems set about destroying themselves when they have a fault.

rugby_bull said:

The company who have provided the quote think that we can fit 10 300W PV panels on our south facing roof, possibly 10 375W PV panels subject to a roof survey.  I'm assuming that we're better to push for something closer to a 3750W system than 3KW given the size of the property?  We are currently a household of 4 adults (as the kids don't look like moving out any time soon!).

With relatively limited roof space you should of course aim for the highest output panels that come at a reasonable price. 300W panels are old hat, my install was about two years ago and the installer provided 315W panels instead of 300W at no extra cost. There's one poster on this forum who installed 400W panels about a year ago, those are probably still a premium product though. You should probably expect to get 350W panels which if south-facing will provide useful total power generation. With only ten panels you will also be fine with a 3.68kW (or similar) inverter thus avoiding the need for a special application to your DNO to export more.

rugby_bull

Thanks also for your advice on this.  Much appreciated.  Out of interest, what is the limit in terms of the inverter capacity above which you need to make a special application to your DNO?

Hexane

My understanding is that the limit for installation without DNO approval is 16A if you're on a single phase supply;
And that 16A at a nominal 230V = 3.68kW which used to be misquoted as 4kW.
My DNO approved my larger system without charging anything, but many installers can't be bothered with the extra paperwork and back and forth for that. (The first two companies that gave me quotes told me that it was "impossible" to have more than a 4kW inverter.)