Solar Thermal or Solar PV?

Roadtoad

Has the dust settled on an obvious choice between these two?

I've a nice SE and SW facing back roof in Kildare, and have just installed a new HW cylinder with a low 'solar' coil, but otherwise I haven't invested yet one way or the other.

The four kids are going (baby in 5th Year), so its two people using hot water 5 days a week, with perhaps extras at the weekends, unless we have them returning!

Long term, I've enough useless grass to fit tracking PV units, if the figures went really favourable. This isn't the subject of today's question.

I'm techi enough to understand the jargon, and may be brave enough to try a self install. House insulation and draught proofing are ongoing tasks.

(I see some older threads, but figure it might be worth a fresh look if the numbers have changed, e.g. energy yield, panel cost).


Thanks for listening!

rolion

Solar tubes, 10 tubes for every 100L of the cylinder.

I have both...each for its best designed usage.
Even is there is not enough "light" to excite PV panels,always will be "rays" to trigger movement of heating agent in to your solar tubes.
Unless you want to get big array and a diverter that does maxim 3kw heating power versus a nice heat transfer coil.

So,basically not too much changed and constructive arguments are alive.

unkel

I wouldn't bother with solar thermal for just two people, unless you can get a really cheap install (and benefit from the subsidy). I've a 40 tube Kingspan solar thermal myself and a 360l cylinder, but we're a family of 5 with 4 females, and we use lots of water

In your situation I would concentrate on solar PV, go big and take maximum benefit of the new subsidy (€3,800)

Things have changed completely because of this new subsidy. Without it I would not have made above recommendation.

rolion

You can see my systems live, here:

Solar Tubes

PV Panels

Based on day,time and season i hope it will give an indication of how efficient the systems,each individual,are for its designed purpose.

Have fun

unkel

Output on line is cool rolion. So what you got an ABB 5kW inverter (non-hybrid)? What model? With how many separate strings, or just one as your panels are all same direction (south) facing?

rolion

unkel wrote: »

I wouldn't bother with solar thermal for just two people, unless you can get a really cheap install (and benefit from the subsidy). I've a 40 tube Kingspan solar thermal myself and a 360l cylinder, but we're a family of 5 with 4 females, and we use lots of water

In your situation I would concentrate on solar PV, go big and take maximum benefit of the new subsidy (€3,800)

Things have changed completely because of this new subsidy. Without it I would not have made above recommendation.

Thats correct.

If you can afford to go with bigger PV array versus space,then is much better.
Costwise better RoI due to PV grant and can be used to "divert" to electricity as well when hot water,well,is hot at 70 degrees.

If that is the case,i will search for a new cylinder that has the dual immersion connectors adn make sure you have the 3KW longest loop availbale for that cylinder.
I'll get a diverter with two outputs,select first top and then bottom of cylinder. Get top hot for first fast water / needs and then warm-up the bottom at relax or ramaining time / energy.

If current cylinder is ok,then go for a smaller one,maybe 180 or 210 vertical thin type and use it with a diverter output only for pre-heating,feeding hot water to cold input of the big cylinder.


I advise minimum 5Kw array,even more if you can afford.
Not the standard 2KW installed around most of the homes.

I still say dual systems are better but your choice...you get grants for both !
Have fun...

wexfordman2

rolion wrote: »

You can see my systems live, here:

Solar Tubes

PV Panels

Based on day,time and season i hope it will give an indication of how efficient the systems,each individual,are for its designed purpose.

Have fun

What size is your over system again rolion?

randombar

rolion wrote: »

You can see my systems live, here:

Solar Tubes

PV Panels

Based on day,time and season i hope it will give an indication of how efficient the systems,each individual,are for its designed purpose.

Have fun

How are you measuring the thermal data?

SemperFidelis

The issue as I see it is that for PV you'll need planning for anything over 12m2 (more than 7 panels), the thermal will be under that so should be exempt.

niallers1

SemperFidelis wrote: »

The issue as I see it is that for PV you'll need planning for anything over 12m2 (more than 7 panels), the thermal will be under that so should be exempt.

The 12m2 came about due to the maximum SEAI grant for Solar thermal when the solar thermal grant was first announced. Planning took direction from SEAI not the other way around.

Now the Maximum Solar PV grant is for approx: 22m2 of solar PV. (1.6*14 300w)
So planning needs to catch up and allow an increase from 12m2 to at least 22m2 .

I have been told by somebody in SEAI that this is being discussed at the moment.

There will be political pressure to get more renewables in the mix so it makes sense that the array size allowed by planning will increase.

1874

niallers1 wrote: »

The 12m2 came about due to the maximum SEAI grant for Solar thermal when the solar thermal grant was first announced. Planning took direction from SEAI not the other way around.

Now the Maximum Solar PV grant is for approx: 22m2 of solar PV. (1.6*14 300w)
So planning needs to catch up and allow an increase from 12m2 to at least 22m2 .

I have been told by somebody in SEAI that this is being discussed at the moment.

There will be political pressure to get more renewables in the mix so it makes sense that the array size allowed by planning will increase.

That isnt the case now though, planning is required over 12m^2 and its still a maximum of 50% of available roof area

an_fathach

I think the main advantage of solar thermal is that the energy is easy to store, and the waste energy (hot water cooling in your cylinder) provides home heating.

niallers1

an_fathach wrote: »

I think the main advantage of solar thermal is that the energy is easy to store, and the waste energy (hot water cooling in your cylinder) provides home heating.

The energy for PV is easy to store too. You can divert the excess into your hot water cylinder or into a battery.
Should be zero maintenance with PV too. Solar Thermal will have maintenance for sure.

unkel

niallers1 wrote: »

Solar Thermal will have maintenance for sure.

For sure, but not much. Depending on the system. In a Kingspan Solartherm system, the coolant doesn't get overly hot, so I expect to replace it once every 5-10 years. Same for the pump, it won't last forever but I'm expecting about 10 years out of it

Solar PV is not totally without maintenance either, the inverter has an expected life about 10 years and batteries dependent on what type you use and how you treat them. Could be a few years, could be 15 years

Calahonda52

an_fathach wrote: »

I think the main advantage of solar thermal is that the energy is easy to store, and the waste energy (hot water cooling in your cylinder) provides home heating.

Hot water is a low-grade form of energy, max 60 degrees so once you have washed yourself and the dog a few times, as you say the rest goes to waste, as doses any excess once ur cylinder is maxed out.

Elec is a much higher grade of energy...

unkel

Calahonda52 wrote: »

Hot water is a low-grade form of energy, max 60 degrees so once you have washed yourself and the dog a few times, as you say the rest goes to waste

How do you mean? We have a 360l hot water cylinder and if the whole cylinder is heated to 63C, it will last us just 2 days. There is little or no waste here apart from when we are away. We are a family of 5 that uses lots and lots of hot water. If you don't, then don't get solar thermal as the initial cost is high of the system is high and thus your payback will be long if you don't use much water.

Calahonda52 wrote: »

Elec is a much higher grade of energy...

LOL. If you don't use it immediately, it will go to waste. If you use your excess to heat water, same as above for solar thermal applies, except it is far less productive per m2 and it is far less efficient to go from heat to elec to heat compared to just heat

an_fathach

unkel wrote: »

How do you mean? We have a 360l hot water cylinder and if the whole cylinder is heated to 63C, it will last us just 2 days. There is little or no waste here apart from when we are away. We are a family of 5 that uses lots and lots of hot water. If you don't, then don't get solar thermal as the initial cost is high of the system is high and thus your payback will be long if you don't use much water.



LOL. If you don't use it immediately, it will go to waste. If you use your excess to heat water, same as above for solar thermal applies, except it is far less productive per m2 and it is far less efficient to go from heat to elec to heat compared to just heat

Unused hotwater also contributes to home heating so no waste as long it isnt summer, but Id love to see how significant this is.

I can understand the appeal of PV but I guess it is really case by case on which is best.

unkel

an_fathach wrote: »

Unused hotwater also contributes to home heating so no waste as long it isnt summer, but Id love to see how significant this is.

Insignificant. My own 360l hot water cylinder lis so well insulated it is supposed to lose just 1C per 24h

Unless of course the system was designed as a thermal heat store to help with space heating. But this is not the norm.

an_fathach wrote: »

I can understand the appeal of PV but I guess it is really case by case on which is best.

Indeed. Simple rule of thumb: if you are a very heavy user of hot water: go thermal (plus PV if you can). If you are an average to low user of hot water: don't go thermal, but do go PV. If you want to install an immersion diverter, get at least 3kwp, but the more the better. If you have a small PV setup like below 2.5kwp, do NOT install an immersion diverter as it will cost you money, not save you money. Not even over a very long period.

Gumbo

an_fathach wrote: »

I think the main advantage of solar thermal is that the energy is easy to store, and the waste energy (hot water cooling in your cylinder) provides home heating.

Your hot water cyclinder should provide no space heating at all. If it does, this is a sign that the cylinder is not adequately insulated.

The old fashioned hot press design was a by product of lack of insulation on your hot water storage vessel.

an_fathach

kceire wrote: »

Your hot water cyclinder should provide no space heating at all. If it does, this is a sign that the cylinder is not adequately insulated.

The old fashioned hot press design was a by product of lack of insulation on your hot water storage vessel.

No I don't agree. Mine is very new factory insulated 300 l Joule. That hot press room is nearly always very warm. I haven't done the maths on how much heat it loses per a day but one online source mentions approximately 8 Degrees per 24 Hrs for new cylinders *. I don't know if they include uninsulated pipe junctions which could be releasing the bulk of heat.

I understand it is a reflection of not using the hot water (I live alone and my house is cooler than some) and some households use it all but many wont and it contributes to heating.



* https://churchfieldhomeservices.ie/hot-water-cylinder-replacement/

unkel

kceire wrote: »

Your hot water cyclinder should provide no space heating at all. If it does, this is a sign that the cylinder is not adequately insulated.

The old fashioned hot press design was a by product of lack of insulation on your hot water storage vessel.

+1

Since we replaced the original 120l cylinder in our '00 house with a 360l very well insulated one in '16, our "hot press" is no longer any warmer than the surrounding area.

Mad Benny

Interesting discussion about factory insulated cylinders and hot presses. I'm getting this installed. I'm surprised with the low energy rating. It's part of an air to water heat pump system.

unkel

Mine is a Joule too.

dixiefly

I have a setup with an insulated cylinder and coil for solar as well as pumps and also piping to the roof.

Can people here recommend one or two companies that could review my setup and recommend / supply the best options?

Thanks