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Author Topic: Solar thermal compared to Solar PV  (Read 1163 times)
nickhlx
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« on: March 14, 2020, 05:30:55 PM »

I have a friend in Northern France who spends about 500 a month on his electricity bill. He runs a Bed + Breakfast most of the year with up to about 20 guests, all, no doubt, taking long showers (and probably twice a day !) and being cooked for

Looking at his situation the reason for this is everything he does is with electricity - all his water is heated by electricity as well as all his cooking and heating (!) of the house and apartment rooms.

Being "into solar PV" (and impressed with our system, which is only solar PV only and does heat our water between about March and October) he asked if "a few panels would help"... 

Obviously a few panels would help (a little) but I think the better solution would be to fit some solar thermal panels to heat the water and as many as space he has left for, to fit solar PV panels. 

Can anyone tel me what the comparable output between a sq mtr of solar thermal and of solar PV is in watts output is please ? 

Thanks,

Nick

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andrewellis
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« Reply #1 on: March 14, 2020, 05:51:32 PM »

https://www.navitron.org.uk/sfb-al-slimline-20-tube-58mm-solar-panel
This is about twice the size of a pv panel. Therefore Pv approx 700w.  Solar approx 1200. Solar thermal is better for the area but you can get higher temperatures with electric when it is cloudy.
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Countrypaul
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« Reply #2 on: March 14, 2020, 06:09:54 PM »

It using a lot of hot water (which seems to be the case) then heating it with an ASHP driven off a PV system would give him a much higher amount of heat than a resistance heater. An ASHP may not get it to the full temperature required so it might need topping up by the normal heating but that might be from 45 to 60C rather than 15 to 60C. The ASHP might be capable of heating to 60C  but a significant efficiency hit.

An advantage in sumer is that there would also be a cold air supply! I am not clear on whether you can combine split system so that you get AirCon and hot water from the same system (nor whether he wants it) but is another aspect to consider.

The ASHP system could be considered aong with Solar Thermal if beneficial.
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A.L.
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« Reply #3 on: March 14, 2020, 07:06:44 PM »

I would suggest that a solar thermal collector would collect 3x the energy of a PV panel of equal area (solar thermal measured by aperture area). PV is typically 17% efficient and solar thermal around 50% for typical temperature regime
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nowty
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« Reply #4 on: March 14, 2020, 07:56:53 PM »

From what I understand Solar Thermal is better per m2 but if you heat the water with a heatpump then Solar PV is just as good.
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brackwell
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« Reply #5 on: March 14, 2020, 09:03:36 PM »

If we assume that is 400/month on HW then that could be the order of 80kwh/day av. just on HW  Using a HP at COP 2.5 as long as it is used to preheat the cold water in the bottom of the tank in a similar method to solar thermal then we need 32kwh/day to fully fulfill the need.  This is the max daily that you could get from a 4kwp system in the height of summer (assuming south facing and no shading).

To deliver 80kwh/day of heat over the middle 10 hrs of the day requires a HP of 8kw output. Eminantly doable.

Of course thats on a good day but in the bad day, PV wise, he is still better off because of the effect of the HP.

Then of course you need a tank big enough to hold 80kwh + worth of hot water !   That could be quite a big tank if one expected to heat the water one day to do that evening and the next morning,  perhaps 1000L but then he might already have that size of tank.

So one could run the HP from say 7am to 5pm and select a topping up regime with the present system (would be good if this was a just in time method but is probably too difficult/expensive).

The area available for the solar is probably critical but as always the more the better.


Ken

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A.L.
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« Reply #6 on: March 14, 2020, 09:19:49 PM »

From what I understand Solar Thermal is better per m2 but if you heat the water with a heatpump then Solar PV is just as good.

Probably yes from a kWh point view but there is the capital costs and the availability of PV electricity when needed
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brackwell
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« Reply #7 on: March 14, 2020, 10:42:35 PM »

If using solar thermal then you would need a number of panels and these are not adaptable in overall size to fit the area available wheras PV made up of smaller panels can fit the available area more effectively.   Also one always have the use of the HP even if using mains leccy wheras thermal could offer nothing/very little some days.  Therefore i expect the extra capital cost of the PV/HP route will quickly pay for itself in this case.
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nickhlx
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« Reply #8 on: March 15, 2020, 12:09:54 AM »

Thanks for all the information...  I think funds are going to be limited and the more optimised solutions are not going to be affordable.  However, I think he could do a lot worse than fitting some solar thermal panels and plumbing those in, so I will get some prices for those over to him.

Thanks again,

Nick
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System:   7.5 kWp, 2 SamilPower 4500 Inverters, array bearing 145 / 43 deg slope.
               Two 200L DHW cylinders fed via Intelligent Immersion I3, using
               Willis (external) heaters, 2nd I3 diverter for space heating.
                Now looking to add wind turbines
billi
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« Reply #9 on: March 15, 2020, 09:51:40 AM »

PV and a heatpump !!  One should not forget that PV produces some watts on  not so good days , while solar thermal produce fu(k all, beneficial  to arrive in the tank .  On an overcast day the efficiency of a PV-heatpump idea wins  genuflect genuflect
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brackwell
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« Reply #10 on: March 15, 2020, 11:18:00 AM »

If money is tight just fit the heat pump and then the savings in two yr from that will pay for PV!
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Countrypaul
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« Reply #11 on: March 15, 2020, 12:21:19 PM »

I can see a 9KW ASHP for sale for 1500+VAT, so if that saved 66% of the energy needs on heating water the payback would be about 4 yrs, there may well be cheaper ones around. I am working on teh basis this is not essential (house occupants won't freeze if it stops working) and that if it did fail he could go back to the current heating method whilst it was fixed. Obviously a branded make would be more expensive and might be more reliable (though it does seem to be a little hit and miss).  If you got a COP of 3 from the ASHP (I assume North France is a little warmer than Northern England) then that would require only 3KW of electricity, so in future a PV system to drive that would be easily feasible. Not sure on the rules in France, but if grid tied then even during cloudy weather the ASHP could run (from the grid) and so would still save 2/3 of the electricity. In winter it may be a little less efficient but should still easily outperform direct electric heating.
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nickhlx
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« Reply #12 on: March 15, 2020, 01:01:40 PM »

Thanks for the additional comments - I will discuss with him...

Nick
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System:   7.5 kWp, 2 SamilPower 4500 Inverters, array bearing 145 / 43 deg slope.
               Two 200L DHW cylinders fed via Intelligent Immersion I3, using
               Willis (external) heaters, 2nd I3 diverter for space heating.
                Now looking to add wind turbines
brackwell
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« Reply #13 on: March 15, 2020, 02:16:35 PM »

Paul,
His leccy bill is 500/MONTH  Assuming HW is 400/month and the HP saves 66% then we have a saving of 3168/YEAR  thats well less than a yrs payback !!

Ken
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Countrypaul
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« Reply #14 on: March 15, 2020, 02:47:27 PM »

Paul,
His leccy bill is 500/MONTH  Assuming HW is 400/month and the HP saves 66% then we have a saving of 3168/YEAR  thats well less than a yrs payback !!

Ken

Whoops, you are indeed right Ken, problem with beng disturbed by young kids whilst trying to retain my train of thought - at least they are playing Minecraft now!

He could save the cost of the heatpump before PV output gets to the point of making a significant contribution.
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