Solar tubes Vs PV. ( Not again...)

bunderoon · 09-06-2019 5:28pm #1

Sorry if this has been flogged to death and the following spiel. But a bit of advice would be greatly appreciated.

Situation:
Family of 4. Wife and two daughters. Daughters are <10 years old.
Renovated south west facing bungalow in 2013 and got grant condensing boiler + heating controls. Also 18Kw Boiler stove. 300ltr Joule tripe coil stainless steel cylinder with the intention of getting tubes. 3Kw immersion, positioned 2/3s down the tank.
From Feb to Sept, the immersion provides hot water if the heating isn't on. If the heating is on, we divert to cylinder.
Three zones - bedrooms, living area and cylinder.
Myra pump shower from cylinder. Short runs for heating and hot water. So very efficient.
Backup electric shower which is rarely used.
LEDs throughout house.
Electric bill about 1100e a year. Switch providers each year.
Oil, about 500 ltr a year.

Back in 2013, was quoted 1770 for 40tube Firebird with full kit. 40 as we weren't south facing. Don't recall the labour cost quoted. I reckon 30 would suffice anyway since the house is unoccupied from 9-5.
All white goods used in evening time.

Wondering if any point going PV or just get 30 tubes and be done with it. As going big on PV with eddi, battery etc means a payback would be many many years, if ever.
I could go very small in PV (DIY) and see how it fares out.

I'd say I already know the answer.

n97 mini · 09-06-2019 6:31pm

What does you oil cost per kw/h? It might be cheaper to heat your hot water with oil instead of the immersion. We have mains gas, and a fully zoned system, so we can heat the cylinder by itself. It's substantially cheaper than daytime electricity, and slightly cheaper than night rate.

When working out payback times, compare to your cheapest option (night rate or oil). I have a feeling if it's just hot water you're after then PV is not the way to go.

bunderoon · 09-06-2019 8:05pm

n97 mini wrote: »

What does you oil cost per kw/h? It might be cheaper to heat your hot water with oil instead of the immersion. We have mains gas, and a fully zoned system, so we can heat the cylinder by itself. It's substantially cheaper than daytime electricity, and slightly cheaper than night rate.

When working out payback times, compare to your cheapest option (night rate or oil). I have a feeling if it's just hot water you're after then PV is not the way to go.

Oil cost per Kw/h:
Something I've always wondered about but don't know.
I've a water zone as well and hitting the boost button for an hour will more or less heat ~250ltr to 70c.
It may use 2.5ltr in that first hour. Oil is .76c per ltr at the mo.
.76c. * 2.5. = 1.90e

The immersion would heat the same after two hours.
3kw * 2hrs = 6Kw/h. Which would cost me ~15c * 6 =90c.
If I be generous and say it takes 3hrs to heat via immersion, then that's about 1.35e.

So I don't know. I would have thought electricity would be cheaper to hear the cyclinder unless I am heating some rads so while the water is hot in the boiler I can heat the cylinder as well for not much more

unkel · 10-06-2019 12:10am

Your figures are off. You must have measured incorrectly. Day rate electricity costs 18c/kWh, using an efficient condensing gas or oil boiler costs about 5c/kWh, an older inefficient boiler maybe about 8c/kWh

You need the same number of kWh (or joule to be precise) to heat your water, it doesn't matter what the heat source is

bunderoon · 10-06-2019 12:26am

unkel wrote: »

Your figures are off. You must have measured incorrectly. Day rate electricity costs 18c/kWh, using an efficient condensing gas or oil boiler costs about 5c/kWh, an older inefficient boiler maybe about 8c/kWh

You need the same number of kWh (or joule to be precise) to heat your water, it doesn't matter what the heat source is

I'm with energia. It's 15c per unit at present.
For boiler usage, I know how long it takes to heat nearly the full tank.
The only thing I dont know for sure is the rate of kerosene usage of the boiler for the hour.

I can only go with this ltr estimate. I'd love to perform a test at home with a 5ltr drum, but would be too much Hassel.

http://thehelpfulengineer.com/index.php/2011/02/how-much-does-an-oil-boiler-cost-per-hour/

Where do you get your figures from? From experience?

unkel · 10-06-2019 1:13am

bunderoon wrote: »

Where do you get your figures from? From experience?

No. Every fuel has its own energy density. If you have gas, you can see the conversion of m3 of gas into kWh on your gas provider's bill (or google it). And your bill's unit price is so many cents per kWh. I think mine is about 4.5c/kWh. Price of oil per kWh is similar (slightly higher)

lgk · 10-06-2019 1:29pm

bunderoon wrote: »

Where do you get your figures from?

The SEAI publish fuel cost comparison data.

unkel · 10-06-2019 8:47pm

Those are standard rates though. If you switch every year, you pay a lot less. Nobody needs to pay 7c for gas and 24c for electricity

But yeah going by that table it costs almost 4 times as much to heat your water with electricity than it costs to heat it with gas

bunderoon · 10-06-2019 9:58pm

It's difficult for me to be sure as I said before, I don't know how much ltrs of kerosene the boiler used for 1hr, but going by the link above, I would hazard a guess at 2-2.5 ltrs as its heating 300ltrs.
To keep it simple as I've done it above. If it takes an hour to heat 250 ltrs to 70c and say 3 hours to heat the same to the same via immersion (3Kw element) then I don't see how it's 4 times more expensive to heat with electricity than with kero.

Maybe I am missing something huge here and I simply don't see it. Especially the official figures.

bunderoon · 10-06-2019 10:01pm

I would think my annual 500 ltrs would run out much sooner and my annual electric speed wouldnt be reduced by the equivalent amount.

n97 mini · 10-06-2019 10:03pm

Something to consider is turning down the thermostat. You don't need to heat to 70 degrees every day, 42 is enough. We heat our 100L tank to 42, which is enough for 3 gravity showers. If you've a shower pump, consider turning that down too. (Every couple of weeks the tank is heated to approx 65 for Legionnaire's, though I'm not sure even that is necessary)

bunderoon · 10-06-2019 10:12pm

Thanks for that.
No, it wouldn't be a daily thing. Every second day the the least. If I heat to 70, then it's about 30c on the second day even after two showers the previous day. So I would only be heating back up to do is for ~2 days. Stainless steel cylinder so legionnaire s is always in the back of my mind.

bunderoon · 10-06-2019 10:20pm

Not to get side tracked (though I nvery grateful for the info), I guess it's a no brainer to just get the 30 tubes. After grant, net to be would be much. and as Unkel said in another thread, depending on the tubes I get, may not need to get the system serviced or glycol changed every three years as some makes do have vents which don't allow the fluid to over heat as much and degrade as quick.

rolion · 11-06-2019 9:39am

And again,no one mentioned about the gas boiler modulation.
Those boilers have a minmum running KW consumption,a minimum modulation that overall gets the cost of the 4.5c/KWh way over above that figure.
My boiler is a 30KW Viessman.Maxim 30Kw output and a minimum of ... 4.5c !? Not a chance ...

But lets play with the figures and compare gas with electricity.
Let me ask you something : why there are not available instant gas consumption meters,like are with the electricity.
Leaving aside the danger of a gas vs a danger of a cable...

Read more H E R E ....and... H E R E

Quote: "Our Linea One 38kW combi boiler boasts a class-leading 10:1 modulation ratio, enabling it to modulate its output to 10 per cent of its maximum – 38kW down to 3.8kW. " SO ... 4.5c/kw X 3.8Kw = 17.1cents without any taxes adn other levyes.

Q E D

Take care.

antoinolachtnai · 11-06-2019 9:54am

rolion wrote: »

And again,no one mentioned about the gas boiler modulation.
Those boilers have a minmum running KW consumption,a minimum modulation that overall gets the cost of the 4.5c/KWh way over above that figure.
My boiler is a 30KW Viessman.Maxim 30Kw output and a minimum of ... 4.5c !? Not a chance ...

But lets play with the figures and compare gas with electricity.
Let me ask you something : why there are not available instant gas consumption meters,like are with the electricity.
Leaving aside the danger of a gas vs a danger of a cable...

Read more H E R E ....and... H E R E

Quote: "Our Linea One 38kW combi boiler boasts a class-leading 10:1 modulation ratio, enabling it to modulate its output to 10 per cent of its maximum – 38kW down to 3.8kW. " SO ... 4.5c/kw X 3.8Kw = 17.1cents without any taxes adn other levyes.

Q E D

Take care.

Electricity is priced per kWh not per kW.

Your calculation is only correct if you run the boiler for a full hour. You don’t have to run it for a full hour (although there are benefits to running for longer, but less frequently).

rolion · 11-06-2019 10:14am

antoinolachtnai wrote: »

Electricity is priced per kWh not per kW.

Your calculation is only correct if you run the boiler for a full hour. You don’t have to run it for a full hour (although there are benefits to running for longer, but less frequently).

Thats the right question... how long does it take for a boiler to heat the water of a certain volume and required temperature VERSUS same elements of the electrical immersion !? We've been here before Antoin, with calculus of joules and kilowatts / kilowatts hour and litres and so on...
We are disputing here the plus or minus of the gas boiler's very performant heat exchange vs immersion inneficient stick and "personal fight" of the PV diverter.
I guess,at the end of the day and i stand to be corrected,home owner / user will take advantge of whatever is available and connected. Difficulty is when you have multiple choices and needs to chose.

I want to heat the water with my diverter and avoid the gas as is costing me nothing today BUT i had to pay €450 for the diverter 3 years ago ...how do you make the maths on that ?! Financial value versus personal feelings ?
For the past few days,gas boiler was off,water done by solar tubes and PV diverter.Was that free ? Yes,but NO,cause i paid years ago so ... how do i make the maths with the cents per KW / KWh !? Again,is paid for 3 years ago,do i sit in the bath and think or ROI and so on...no flipping well,just let th etap more hot water as is Sun that gives me constant flow of hot water. Is hard to express it .

Corect also... leaving aside the efficiency of the boiler vs running time vs power on/off cycles. Mine is powered on all the time when cold times and i demand heat or water from the EvoHome controller.

Thanks.

rolion · 11-06-2019 10:27am

bunderoon wrote: »

Not to get side tracked (though I nvery grateful for the info), I guess it's a no brainer to just get the 30 tubes. After grant, net to be would be much. and as Unkel said in another thread, depending on the tubes I get, may not need to get the system serviced or glycol changed every three years as some makes do have vents which don't allow the fluid to over heat as much and degrade as quick.

30 tubes of PVs will do nicely 300L of cylinder.
Can i recommend to chose 40 tubes,20 and 20 for a 300litres, to increase that efficiency with a small kick over the cold days , which are more than the good few weeks over the summer. And make sure get a variable speed pump.

Regading maintenance,again,highly advisable to do it every couple of years irespective what the supplier says. In event of troubles,spilling or malfunction / underperforming,you are the one suffering the consequences. Even if the tubes are replaced under warranty T&Cs, you will need to clean the attic or repaint those walls... They will be covered by the small print in the website or documentation.

You can check the meters in relation to pressure and check temperatures on the gauges and for noise from the pump and from pipes.
The circulating agent will have to be tested every few years and eventually replaced. You may need to engage a professional and test the liquid's chemical characteristics.Not a big job but needs to be done properly as the whole system needs to be properly vented and sealed.

Cannot say anything about those automated venting solution that unkel says is workign for him,mine is the old Joules manufacturer style tubes ,with a valve set a a certain temperature / pressure and just dumps the very hot water outside.
Thanks

antoinolachtnai · 11-06-2019 10:56am

The issue at the end of the day is that you can heat a lot of water for 300 euros a year using either off peak electricity or oil. It will take years to get your investment back at that rate, especially if the unit requires regular maintenance.

If I were going to do a green project I would do PV with a battery because it doesn’t need regular maintenance.

rolion · 11-06-2019 11:42am

antoinolachtnai wrote: »

The issue at the end of the day is that you can heat a lot of water for 300 euros a year using either off peak electricity or oil. It will take years to get your investment back at that rate, especially if the unit requires regular maintenance.

If I were going to do a green project I would do PV with a battery because it doesn’t need regular maintenance.

Agree with you again,in relation to maintenance only !
BUT,law of physics below you cannot beat a good solar coil :

HERE

You will need a good insulated cylinder /puffer with maybe 2-3 immersion elements at 3Kw power each.
Fitted top and below of the cylinder and run some priority mode,top first heat to 50" and then bottom .
Then, a minimum 5KW installed power of PV panels...dedicated to hot water.

As well, been there already and easy to say but hard to manage...no one technology can beat the dynamics of a solar tubes in relation to efficiency in a good day or a bad day on this type of weather to heat water.

Good luck and please share the plans with us...

JOULE%20STAINLEES%20STEEL%20CLYLINDER-600x600.JPG

JOULE%20STAINLEES%20STEEL%20CLYLINDER-600x600.JPG

antoinolachtnai · 11-06-2019 4:17pm

Or you could just heat the water when the wind is blowing at night.

The problem is that it is hard to make the cost of any of these systems work financially in a domestic setting. Not many families use more than 3000kWh on heating water.

rolion · 11-06-2019 9:02pm

antoinolachtnai wrote: »

If I were going to do a green project I would do PV with a battery because it doesn’t need regular maintenance.

I am going to share a project,long term one,a little secret of mine.

I have my Leaf 24KW,is 4 years old,120K KM and 11 bars on the battery.
I estimate that in 5 years time,it will be no more efficient as a car.
Im going to take it and use it as an engine in the house.
reDesigned all lighting for 12V LEDs.
Heating,using heat recovery and the heating pump. How water ,the solar tubes on the house and gas boiler whenever needed.
Most of appliances are working at 12V,the camping type such as heating water and few more others,even fridge.
For multimedia,use a 220v 2KW inverter.
CCTV,computers,radio,alarms,fans,all 12V healthy ok.

Cannot use it for heavy appliances such as cooking,gas_boiler and cleaning and ironing but i can live with a few 220V sockets in the house. Maybe change the sockets only for 220V UK type and all others just disconnect or use EU type sockets (to protect against accidental usage)

All above DIY with no problems... even the curent house can be done easily.

Happy dreaming.

unkel · 11-06-2019 10:07pm

rolion wrote: »

"Our Linea One 38kW combi boiler boasts a class-leading 10:1 modulation ratio, enabling it to modulate its output to 10 per cent of its maximum – 38kW down to 3.8kW. "

3.8kW is perfect for heating a bit of water. Isn't an immersion 3kW?

You have just proven that modulation is not an issue at all. Use an efficient boiler for half an hour and it will have heated up the water more than using an immersion for half an hour. At a fraction of the cost of day rate electricity.

n97 mini · 12-06-2019 12:05am

unkel wrote: »

3.8kW is perfect for heating a bit of water. Isn't an immersion 3kW?

You have just proven that modulation is not an issue at all. Use an efficient boiler for half an hour and it will have heated up the water more than using an immersion for half an hour. At a fraction of the cost of day rate electricity.

I don't get rolion's point about modulation. It's so that boilers will be more efficient, not less, especially at lower temperatures.

unkel · 12-06-2019 12:23am

My general impression is that rolion, as an environmental leader, does all the right things for the environment, despite the costs, and has done so for several years. Which is highly applaudable. If it wasn't for people like him, people like you and I on a much tighter budget and far more conscious of value for money and pay back times would not have been able to follow this soon after.

And because of people like you and I, and ever further dropping prices of hardware, the masses will hopefully adopt shortly after

josip · 12-06-2019 12:33am

If getting the tubes, make sure your system has a way of dumping the excess heat.
Ours didn't initially and until we got an aftermarket one installed, the service costs ate into any savings.
If going again, I think I'd go with PV for the lower maintenance.

unkel · 12-06-2019 12:40am

josip wrote: »

If getting the tubes, make sure your system has a way of dumping the excess heat.

You don't need that if you have a proper Kingspan Thermomax system. It does not go over 95C and vents at the tubes. So the interval between coolant changes is much longer than if you use a more basic system where temps can reach 200C and where indeed you need a thermal dump for when you are away for more than a few days in summer!

josip · 12-06-2019 9:20am

unkel wrote: »

You don't need that if you have a proper Kingspan Thermomax system. It does not go over 95C and vents at the tubes. So the interval between coolant changes is much longer than if you use a more basic system where temps can reach 200C and where indeed you need a thermal dump for when you are away for more than a few days in summer!

I'd consider venting at the tubes as another way of dumping the heat?

dathi · 12-06-2019 9:53am

unkel wrote: »

You don't need that if you have a proper Kingspan Thermomax system. It does not go over 95C and vents at the tubes. So the interval between coolant changes is much longer than if you use a more basic system where temps can reach 200C and where indeed you need a thermal dump for when you are away for more than a few days in summer!

i have cheep Chinese tubes with heat dump. the whole point of the heat dump is to keep the system under 100c , its not for when you go away for a few days in the summer.

bunderoon · 12-06-2019 10:18am

dathi wrote: »

i have cheep Chinese tubes with heat dump. the whole point of the heat dump is to keep the system under 100c , its not for when you go away for a few days in the summer.

Is the heat dump a radiator?
I already have a 'heat dump' in the form of a towel rad in the bathroom for when I have the stove on. I wonder if I could tap into that or if a separate dump is required.

josip · 12-06-2019 10:40am

bunderoon wrote: »

Is the heat dump a radiator?
I already have a 'heat dump' in the form of a towel rad in the bathroom for when I have the stove on. I wonder if I could tap into that or if a separate dump is required.

Yes, some people often use an old rad in the attic as a dump, although others use some or all of their normal house rads.
Venting tubes such as Unkels will be the best dumping solution imo, and wouldn't need any extra installation work.

bunderoon · 12-06-2019 1:23pm

Thanks for that.

I guess these would be the job:

https://www.kingspan.com/irl/en-ie/product-groups/renewable-technologies/solar-thermal-solutions/solar-evacuated-tube-collectors/thermomax-hp400-evacuated-tube-collectors

Solar tubes Vs PV. ( Not again...)

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