Thermal Solar Panel V LPG Boiler To Heat Water Costings

[s2sap]

Guys you have all been very helpful previously and wondered if anyone that is mathematically inclined could look at the doc attached ( boiler spec )  and tell me how much it would cost using the 30KW boiler to heat a 250L water cylinder using the cost per KW/hour detailed in the link below - BUT the unit cost in the link below for LPG is now 48 pence per unit not 36.75 as detailed in the link

 

http://www.nottenergy.com/energy_cost_comparison/

 

I am trying to work out the cost saving of installing solar thermal which would heat a 250L tank to a usable level nearly every day April - Oct

 

Is there enough information in these 2 documents to do this calc?

 

Thanks for any help

 

[Vijay]

Pretty sure I've read plenty of times to not bother with solar thermal and instead go for solar PV which can obviously supply you electricity which you can use anywhere, including an immersion heater to do your hot water

[ProDave]

If you think solar thermal will provide all your hot water for 6 months of the year you will be bitterly disappointed.  At best it can help to provide some.  you still need another means of heating it.

[SteamyTea]

1 lt of water takes 4.2 kJ of energy to increase by 1°C.

1 kJ = 0.0003 kWh (near enough).

 

So to raise 250 lt of water from 10°C to 65°C will require:

 

250 [lt] x (65 - 10) [°C] x 0.0003 [kWh]

 

4.125 kWh

 

The boiler has an efficiency of 90% (near enough)

 

4.125 kWh x (1/0.90) = 4.58 kWh (LPG needed)

 

The cost of LPG 48p/lt

There is 7.1 kWh of energy per litre of LPG, which works out at 6.7p/kWh (cheaper than E7)

 

So cost is

4.6 [kWh] x 6.7 [p] = 31p (near enough).

 

I often make mistakes in my arithmetic (statistics is so much better), but someone will be along to correct me I am sure.

 

This is only a very small part of the calculation.

You then have to take into account the real life performance of the boiler i.e. it is starting from cold and may only be running at peak performance for a short while.

Then there are the thermal losses from the 250 lt cylinder, over a day these may easily be 50% of the stored energy.

Then there is the maintenance costs of a gas boiler, and for that matter a solar thermal system.

And this is before looking at the volatile price of LPG and any fixed costs for the storage tank.

 

The main question you need to ask yourself is what you are trying to achieve?

Edited August 31, 2017 by SteamyTea

[Temp]

Looks good to me I made it 30.5p by using a slightly different method and numbers. 

 

3 hours ago, s2sap said:

nearly every day April - Oct

 

7 months at 30 days/month is 210 days so total saving is about £64.

 

 

 

 

Edited August 31, 2017 by Temp

[PeterW]

A lot of this misses the key point about not how you store the heated water but when you use it. 

 

Traditionally most households use hot water in the early morning and late evening. You would need to size a tank that could provide for all of this supply at a reasonable temperature with overnight losses, and then have sufficient solar thermal to reheat the tank in the daytime. You would get negligible heating before the early morning usage so you would be reliant on gas for that. 

 

I'd hazard a guess that's around a 4-500 litre tank however the restriction will be if your solar thermal can recharge it in time. 

 

 

[Nickfromwales]

@Trw144 has a ST system installed so maybe can provide some facts / feedback . 

[dpmiller]

I've a small ST system currently- my own copy of a Solartwin- and while it will heat the top of the tank enough for the odd handwash  from perhaps April-Oct yes, there's no way it's going to heat the whole tank in other than high summer.

 

My guess is that if you size the system for enough early and late in the year you're going to be dumping boiling water in the summer.

It would be nice to do something with that extra, and to have good efficiency under cloud. I'm going PV in our build.

[SteamyTea]

Really depends on how much DHW you actually use.

And for bathing, anything below about 30°C is useless.

[Nickfromwales]

I'd have ST if it was free, and only use it for DHW preheat. Pv is a no-brainer. If MCS and fits aren't attractive you can still DIY with kits quite cheap now. 

[SteamyTea]

I would have PV if it was free

 

You can check PVGIS for estimated yield from a system http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#

That will give you an idea of what can be generated, but yet again, is only part of the story as it relies on mean kWhs and not instantaneous power (watts).

Actually, just had a look at PVGIS and it does now do estimated mean power.

[Jeremy Harris]

There's a pretty good (but a bit old now) comparison calculator between solar thermal and water heated by solar PV on Ed Davies website, here: https://edavies.me.uk/2012/11/pv-dhw/

 

I looked at fitting a mix of solar thermal and photovoltaic panels initially, but a few calculations showed that the true efficiency of solar thermal was well below that of photovoltaics when used for heating water all year around, even though the peak efficiency of solar thermal is better.

 

The main problem is that solar thermal can only deliver sensible heat when the panels are hotter than the water to be heated.  This means they start delivering power later in the day, and stop delivering power earlier in the day, than photovoltaic panels.  It also means that there are many more days in the year when solar thermal panels deliver no useful power at all.

 

The reason is to do with the temperature differential required to get a solar thermal panel to work.  The panel always has to be a few degrees hotter than the tank, in order to drive heat from the panel to the hot water.  With photovoltaic panels, this is irrelevant.  Even if the panels are only generating a few watts of electricity, all of this can be delivered to the hot water tank, as long as the thermostat in the tank isn't satisfied and the electricity isn't being used elswhere.

 

The icing on the case is the cost saving, both the very much lower capital and installation cost of PV and the virtually zero maintenance cost.  Solar thermal needs maintenance checks every year, plus a high temperature antifreeze change about every five years, and altogether this pushes the cost per watt hour of useful energy delivered up to way above that a photovoltaic system.  Add in the small FIT benefit, and photovoltaic panels are a complete no-brainer.

[A_L]

11 hours ago, SteamyTea said:

1 lt of water takes 4.2 kJ of energy to increase by 1°C.

1 kJ = 0.0003 kWh (near enough).

 

So to raise 250 lt of water from 10°C to 65°C will require:

 

250 [lt] x (65 - 10) [°C] x 0.0003 [kWh]

4.125 kWh

I often make mistakes in my arithmetic (statistics is so much better), but someone will be along to correct me I am sure.

 

'Fraid so Steamy...........Basically you forgot to multiply by 4.2, also 0.00027 would be better

 

It takes 15.95kWh (not 4.125) to raise 250l of water by 55°C, 0.25*1.16*55 = 15.95

 

The boiler may be 90% efficient but I bet the efficiency of transferring the fuel energy to the tap is 70% or less so have to allow a factor here

 

14 hours ago, s2sap said:

 

I am trying to work out the cost saving of installing solar thermal which would heat a 250L tank to a usable level nearly every day April - Oct

 

To get anywhere near this I think you would need at least 7.5m² of ST and two days storage capacity

[SteamyTea]

3 hours ago, A_L said:

Fraid so Steamy...........Basically you forgot to multiply by 4.2,

Yes, it was late.

So 16 kWh times 6.7p/kWh is £1.1

3 hours ago, A_L said:

The boiler may be 90% efficient but I bet the efficiency of transferring the fuel energy to the tap is 70% or less so have to allow a factor here

I agree, why annoys me about quoting efficiency sometimes.  Once you add in all the losses the figures often look dreadful.

 

3 hours ago, A_L said:

also 0.00027 would be better

Just easier to remember 3 zeros after the decimal point and then a 3.

But when put into its mechanical equivalent it is only a tiny difference and probably lost in 'noise'.

[Temp]

Darn it.

[s2sap]

Thank you to everyone who has contributed to this question

 

In particular thank you to @SteamyTea for doing the calc twice for me  - I liked you first answer better but will now go in a different direction because of you revised answer.

 

If I go in the direction of PV instead of ST and I put up a 4KW PV array is this how it would work - I would plan to use the PV for these requirements in this order is that correct - 

1. domestic electricity requirements - if there is no requirement for this then
2. heating DHW - then
3. heating  buffer tank for UFH when both of above are not calling for input ( UFH buffer would be topped up by gas boiler as required )
4. export back to grid when no domestic requirements

 

 Is that how the system would work? 

Edited September 2, 2017 by s2sap

[Nickfromwales]

In a nutshell, yes. Would be dependant on the type of pv diverter if you can cascade from DHW to the buffer. 

[PeterW]

11 minutes ago, Nickfromwales said:

In a nutshell, yes. Would be dependant on the type of pv diverter if you can cascade from DHW to the buffer. 

 

Most of the decent ones do it automatically - immersun has a secondary load terminal.

[Nickfromwales]

What he said ?

[s2sap]

14 hours ago, s2sap said:

I would plan to use the PV for these requirements in this order is that correct - 

1. domestic electricity requirements - if there is no requirement for this then
2. heating DHW - then
3. heating  buffer tank for UFH when both of above are not calling for input ( UFH buffer would be topped up by gas boiler as required )
4. export back to grid when no domestic requirements

 

This topic may be more appropriate to the PV subforum but based on my above usage plan as I would nearly always be using whatever the PV produces ( as I would think that 4Kw would nearly always be used with a 250L water cylinder and UFH buffer )  can anyone give a very rough idea of annual output and therefore potential savings. The orientation is S SW and would be on a 20 degree roof and location is 55 degrees north. 

 

Is this information enough to give a rough idea?

[Nickfromwales]

3 minutes ago, s2sap said:

 

This topic may be more appropriate to the PV subforum but based on my above usage plan as I would nearly always be using whatever the PV produces ( as I would think that 4Kw would nearly always be used with a 250L water cylinder and UFH buffer )  can anyone give a very rough idea of annual output and therefore potential savings. The orientation is S SW and would be on a 20 degree roof and location is 55 degrees north. 

 

Is this information enough to give a rough idea?

One of many Online calculators

[A_L]

The latest version of perhaps the most respected simulator http://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#PVP

[Jeremy Harris]

25 minutes ago, s2sap said:

 

This topic may be more appropriate to the PV subforum but based on my above usage plan as I would nearly always be using whatever the PV produces ( as I would think that 4Kw would nearly always be used with a 250L water cylinder and UFH buffer )  can anyone give a very rough idea of annual output and therefore potential savings. The orientation is S SW and would be on a 20 degree roof and location is 55 degrees north. 

 

Is this information enough to give a rough idea?

 

As @Nickfromwales said, any one of the online calculators will give a reasonable estimate of the output.  PVGIS is my preferred one, as it seems to be within about 5% of our actual output, which seems pretty accurate.  Bear in mind that you will get loads more energy than you can use in summer and very little energy at all for maybe three or four months in winter - that's just the nature of the UK climate.

[Nickfromwales]

12 minutes ago, JSHarris said:

Bear in mind that you will get loads more energy than you can use in summer and very little energy at all for maybe three or four months in winter - that's just the nature of the UK climate.

But at least you can export that energy with Pv . 

[Jeremy Harris]

4 minutes ago, Nickfromwales said:

But at least you can export that energy with Pv . 

 

 

Indeed you can, and in winter even a very small bit of excess generation will all go to heating the water if it needs it, irrespective of the storage device temperature.