Thermal Solar Panel V LPG Boiler To Heat Water Costings

[s2sap]

Thanks all so the calculator shows that we could generate 3640 kwh p/a and in the peak 2 months potentially generate 532 kwh per month. If we take this monthly figure of 532 divide it by 30 days and say that at best we could expect to produce 17 kwh on a given day - would that 17 kwh heat a 250l water cylinder?  If I understand @A_L quote below he states it would need 15.95kwh so 17kwh would do it. Am I interpreting this correctly? I am trying to see if we could use nearly all of the power generated by the PV even at its peak generating times and I think the answer is yes assuming we start each day at mid summer with no hot water in a cylinder?

 

  On 01/09/2017 at 09:41, A_L said:

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

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[Nickfromwales]

Don't forget you'll have white goods etc to consume any excess whilst the immersion kicks in and out.

Pv isn't just for hot water, and during the summer / peak times of generation you will export and get paid for each unit you 'sell'. 

[Jeremy Harris]

  On 03/09/2017 at 11:19, s2sap said:

Thanks all so the calculator shows that we could generate 3640 kwh p/a and in the peak 2 months potentially generate 532 kwh per month. If we take this monthly figure of 532 divide it by 30 days and say that at best we could expect to produce 17 kwh on a given day - would that 17 kwh heat a 250l water cylinder?  If I understand @A_L quote below he states it would need 15.95kwh so 17kwh would do it. Am I interpreting this correctly? I am trying to see if we could use nearly all of the power generated by the PV even at its peak generating times and I think the answer is yes assuming we start each day at mid summer with no hot water in a cylinder?

 

 

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To heat 250 litres of water from 8 deg C (typical cold main temperature) to 60 deg C (typical DHW temperature - could be higher or lower by 5 deg or so) then this is the calculation:

 

First you need to know the heat capacity of water, which is about 4181 J.K.l, or around 1.161389 Wh.K.l

 

Next calculate the temperature change which in this case is 60 - 8 = 52 K

 

Now multiply the volume, 250 l, by the temperature change, 52 K and the heat capacity of water:

 

250 x 52 x 1.161389 = 15,098 Wh, or around 15 kWh.

 

Bear in mind that this is the worst case, most of the time the tank will never get to the cold water temperature, and so will be heating from a higher starting temperature, as it's unlikely that it will be completely drained of DHW very often.

Edited September 3, 2017 by JSHarris

[s2sap]

OK then so if the maximum I will generate is about 17kwh per day in peak season and it would take 15kwh to replensish my 250L cylinder if empty I will probably be using most of my generated power heating the water and powering fridge freezers and other household items. Obviously I am happier with the generated power heating my water and powering appliances rather than exporting it at at 4p unit

 

Based on this assumption I am not going to have much power left over to heat a buffer tank for UFH if it is third on the list for receiving generated power after household requirements and DHW requirements am I ?

[Nickfromwales]

If you have an ASHP it'll also become white goods and power diverted into that will offset your space heating a little too. 

There is only so much you can squeeze out of pv, but it's so much better than ST imo. 

[s2sap]

Going with LPG boiler - no ASHP

[Stones]

  On 03/09/2017 at 15:21, s2sap said:

OK then so if the maximum I will generate is about 17kwh per day in peak season and it would take 15kwh to replensish my 250L cylinder if empty I will probably be using most of my generated power heating the water and powering fridge freezers and other household items. Obviously I am happier with the generated power heating my water and powering appliances rather than exporting it at at 4p unit

 

Based on this assumption I am not going to have much power left over to heat a buffer tank for UFH if it is third on the list for receiving generated power after household requirements and DHW requirements am I ?

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@s2sap You'll generate a lot more than 17kWh per day in peak season.  You could be looking at around 30 kWh on a good day.  You can work out fairly accurately what you will generate on a monthly or yearly basis, but day to day is weather dependent.

 

Over the course of the year, without an ASHP or other electricity hungry device, you'll probably use in the region of 20 to 25% of you generate, with an ASHP perhaps up to 40% (figure based on my own experience and those of friends who have PV), which leaves a lot 'spare' that will otherwise get exported.  If diverting to DHW, some days PV will provide all your DHW, others not.  Some days you may well use all you generate via  PV, others you simply won't be able to unless you invest in battery storage, or have an EV.

[Jeremy Harris]

  On 03/09/2017 at 15:21, s2sap said:

OK then so if the maximum I will generate is about 17kwh per day in peak season and it would take 15kwh to replensish my 250L cylinder if empty I will probably be using most of my generated power heating the water and powering fridge freezers and other household items. Obviously I am happier with the generated power heating my water and powering appliances rather than exporting it at at 4p unit

 

Based on this assumption I am not going to have much power left over to heat a buffer tank for UFH if it is third on the list for receiving generated power after household requirements and DHW requirements am I ?

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Most of the time the DHW won't need anything like 15 kWh.  You can work out how much by using the rough rule of thumb that a decent shower will use around 3 kWh at most, more probably around 2.5 kWh.  A bath would be around the same.

 

Baths and showers are the two biggest users of DHW, so work out how many of those you will have a day, then add on around another couple of kWh fo washing up, hand washing etc (again, probably a worst case) and that should give you a rough idea of the true DHW energy requirement.

[A_L]

  On 03/09/2017 at 15:21, s2sap said:

OK then so if the maximum I will generate is about 17kwh per day in peak season and it would take 15kwh to replensish my 250L cylinder if empty I will probably be using most of my generated power heating the water and powering fridge freezers and other household items. Obviously I am happier with the generated power heating my water and powering appliances rather than exporting it at at 4p unit

 

Based on this assumption I am not going to have much power left over to heat a buffer tank for UFH if it is third on the list for receiving generated power after household requirements and DHW requirements am I ?

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The average you will generate is 17kWh per day, on a perfect day in U.K. a 4kWp PV system can generate in excess of 28kWh

 

You will, unless you pay for an export meter, be on a 50% 'deemed export' and receive 4p for half your generation regardless, even if you do not export any at all

 

During the heating season after appliance and DHW heating the amount left over will be insignificant

[SteamyTea]

As an example, during the summer, I use 3 to 4 kWh/day (excluding washing days, which adds another 1.5 kWh).

Now I have relatively high base load as there is only me in the house.

 

You could try and work out your DHW usage, it is not hard, just tedious to do.

[Jeremy Harris]

  On 03/09/2017 at 16:29, SteamyTea said:

As an example, during the summer, I use 3 to 4 kWh/day (excluding washing days, which adds another 1.5 kWh).

Now I have relatively high base load as there is only me in the house.

 

You could try and work out your DHW usage, it is not hard, just tedious to do.

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I worked out ours.  Our DHW usage follows a pretty set pattern, two showers, both in the morning, about an hour apart usually.  The first uses around 100 litres at 38 deg C, the second uses around 70 litres at 38 deg C.  Our incoming mains water is about 8 deg C, so in total the showers use around (100 + 70) x (38 - 8) x 1.161389 = 5.92 kWh. 

 

Next came hand washing and hand washing up.  Both the basin and sink taps deliver around 6 litres/minute, and the hot water is usually around 45 deg C.  The total time per day they are used is around 15 minutes, so that gives: 15 x 6 x (45 - 8 ) x 1.161389 = 3.867 kWh.

 

Our total DHW usage is therefore around 5.93 + 3.867 = 9.797 kWh/day.  That's on the high side, because our showers are quite long, and at the moment the shower flow rate is quite high.  I may well put back the shower flow rate restrictor, and that would reduce the shower DHW use by around 20%.

Edited September 3, 2017 by JSHarris

[s2sap]

  On 03/09/2017 at 15:37, A_L said:

You will, unless you pay for an export meter, be on a 50% 'deemed export' and receive 4p for half your generation regardless, even if you do not export any at all

 

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Yes I'd forgot about that thanks

[s2sap]

So if I go with a 4kw PV system that will probably not have any surplus to power an UFH buffer tank, what should I do with regards the heating system that will be an LPG boiler and UFH upstairs and downstairs. Whats the thinking on buffer tank or not for the heating ( as it probably wont have any input from PV ).

 

[Jeremy Harris]

I found that a small buffer tank was pretty much essential with our low heating requirement (UFH) and small ASHP.  Without the buffer the ASHP would short cycle, with it the ASHP runs continuously at it's lowest power for an hour or so ever other day in winter, maybe an hour or so every day in very cold weather.  At it's lowest modulation level the ASHP draws around 600 W from the supply, and delivers around 1.8 kW to the house.

[s2sap]

  On 03/09/2017 at 17:38, JSHarris said:

I found that a small buffer tank was pretty much essential with our low heating requirement (UFH) and small ASHP.  Without the buffer the ASHP would short cycle, with it the ASHP runs continuously at it's lowest power for an hour or so ever other day in winter, maybe an hour or so every day in very cold weather.  At it's lowest modulation level the ASHP draws around 600 W from the supply, and delivers around 1.8 kW to the house.

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But how would my setup compare to yours ie LPG 

[SteamyTea]

Is there any reason why you cant put a small immersion heater in a buffer tank?

[s2sap]

  On 03/09/2017 at 17:56, SteamyTea said:

Is there any reason why you cant put a small immersion heater in a buffer tank?

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There isn't - but through this thread I have concluded that I will prob not have any spare energy from the PV to heat an immersion in an UFH buffer so I would be heating any buffer with LPG rather than electric - and now trying to ascertain should I have a buffer for UFH or not if it is being heated by LPG boiler

[SteamyTea]

Would the capital costs and service of an LPG system not be greater than the imported power of an electrical system?

[Jeremy Harris]

  On 03/09/2017 at 17:43, s2sap said:

But how would my setup compare to yours ie LPG 

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We don't have E7, so every kWh of output from the ASHP costs between 3.5p and 5p, depending on COP.  LPG costs around 5p to 6p per kWh, I believe, so the ASHP is a bit cheaper to run, at least for low temperature heating.

[Nickfromwales]

  On 03/09/2017 at 18:31, s2sap said:

 

There isn't - but through this thread I have concluded that I will prob not have any spare energy from the PV to heat an immersion in an UFH buffer so I would be heating any buffer with LPG rather than electric - and now trying to ascertain should I have a buffer for UFH or not if it is being heated by LPG boiler

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You need to calculate your delta flow temp and then work out your ventilation and fabric heat losses. From there you will know your 'idle' kw requirement ( when the house is at ambient and just being maintained as so ) and then see if that's = to or lesser than the lowest output of your chosen boiler. 

The likelihood is it will be less than the boiler can efficiently produce which means it'll modulate the output down and live under the condensing threshold aka at a quite reduced efficiency. 

To combat that and get maximum efficiency you ideally want the boiler to come on at a higher temp and pulse heat into a thermal battery ( buffer ) to store the produced heat energy ready for the UFH to consume it as required. Running the UFH directly from the boiler would see it short-cycling a lot and that's not a good design. 

For my curiosity, why LPG when you could have an ASHP ?

[s2sap]

  On 03/09/2017 at 18:43, Nickfromwales said:

 

For my curiosity, why LPG when you could have an ASHP ?

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I have done this decision to death and resurrected it many times There are several reasons why we are going LPG mainly initial capital outlay and potential length of stay in new property but not that reason alone - and we now need to make the LPG system as efficient as possible. We may change to ASHP in the future but as things stand right now its going to be LPG boiler so I need to figure out the right setup 

 

  On 03/09/2017 at 18:43, Nickfromwales said:

You need to calculate your delta flow temp and then work out your ventilation and fabric heat losses. From there you will know your 'idle' kw requirement

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How do I do this - will a SAP calc be sufficient - if so I can upload one - if not where do I start?

[Nickfromwales]

Iirc @JSHarris posted a spreadsheet for this. He'll reply shortly. 

Im a little less refined and use 'plumbers eye' for most of my specifying. I'm never far out TBH, but usually over rather than under. 

"You can turn a big boiler down but you can't turn a small boiler up" etc. ?

[Jeremy Harris]

  On 03/09/2017 at 19:01, Nickfromwales said:

Iirc @JSHarris posted a spreadsheet for this. He'll reply shortly. 

 

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Here it is: http://www.mayfly.eu/wp-content/uploads/2017/01/Fabric-and-ventilation-heat-loss-calculator-Master.xls

[Nickfromwales]

Thanks. 

[s2sap]

  On 03/09/2017 at 20:17, JSHarris said:

 

Here it is: http://www.mayfly.eu/wp-content/uploads/2017/01/Fabric-and-ventilation-heat-loss-calculator-Master.xls

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Thank you for posting the link. However it really is beyond my interpretation. My eyes glazed over and I nearly fell backwards off the chair