stand alone pv systems

[scottishjohn]

I looked at this 20 years ago  and what put me off in the end was the fact they could not say for certain if one hole or how  many holes would be needed to get the heat output  ,until first one was drilled 

certainly where i am it would be into solid rock with 10m for sure  so not a lot of casing needed .

I agree about trenchs -most are not deep enough and to me is only suitable for  wet ground and at least 1.5m depth -which I do not have

and digging in boggy ground that deep will cause problems in keeping trench from collapsing at that depth  

 freezing the ground shows they are not deep  or long enough  to recover from extraction of heat 

[scottishjohn]

21 hours ago, SteamyTea said:

Concrete

SHC 0.8 kJ.kg^-1.K^-1

Density 2700 kg.m^-3

Heat Capacity 2.16 MJ.m^-3

 

Liquid Water

SHC 4.18 kJ.kg^-1.K^-1

Density 1000 kg.m^-3

Heat Capacity 4.18 MJ.m^-3

 

when I was looking at solar thermal I did a test at home in the oven 

2 containers one with water one with water and granite sand 

and 80c temp in oven 

 the test was to see if I could use a large tank filled with water and rock to store a lot more heat -than water alone in same size tank-the down side was the limited heat input you could get from solar thermal -and then it also gives the heat out slower than just water -great if i had little geothermal vent to tap into  or in arizona   .LOL

yes water heated up more and much quicker over the test period , a lot more 

but when i turned oven and then logged the temp drop the granite sand stayed well above ambient for along time compared to the water ,at least twice as long 

which is why I say concrete etc will act as a thermal store and level out temp changes in the building ,presuming its inside the insulated envelope ,which will make it also good in hot summers It also is why pass house designs often have an atrium with glass facing onto a solid wall to control big temp swings  and I have seen some designs where they have holes 

this to me is why ICF buildings work  as they slow the flow of heat in both directions 

all non poly systems have the insulation on the outside portion of the ICF wall  for that reason  and even poly types have the most of the insulation on the outside as well.

 

Edited December 26, 2021 by scottishjohn

[joe90]

47 minutes ago, scottishjohn said:

when I was looking at solar thermal I did a test at home in the oven 

2 containers one with water one with water and granite sand 

and 80c temp in oven 

 the test was to see if I could use a large tank filled with water and rock to store a lot more heat -than water alone in same size tank-the down side was the limited heat input you could get from solar thermal -and then it also gives the heat out slower than just water -great if i had little geothermal vent to tap into  or in arizona   .LOL

yes water heated up more and much quicker over the test period , a lot more 

but when i turned oven and then logged the temp drop the granite sand stayed well above ambient for along time compared to the water ,at least twice as long 

which is why I say concrete etc will act as a thermal store and level out temp changes in the building ,presuming its inside the insulated envelope ,which will make it also good in hot summers It also is why pass house designs often have an atrium with glass facing onto a solid wall to control big temp swings  and I have seen some designs where they have holes 

this to me is why ICF buildings work  as they slow the flow of heat in both directions 

all non poly systems have the insulation on the outside portion of the ICF wall  for that reason  and even poly types have the most of the insulation on the outside as well.

 

Very Interesting, unlike @SteamyTea I don’t understand all the science etc but practical experiments I do. I built my house on common sense (and a bit of copying others here ?)

[ProDave]

If you have not looked already, take a look at the website of the guy that lives off grid at the north end or Raasay which includes a lot about his electricity system.

 

https://lifeattheendoftheroad.wordpress.com/

[SteamyTea]

3 hours ago, scottishjohn said:

I did a test at home in the oven 

How did you account for evaporation losses if the water?

 

One problem with storing energy in water is that the min and max temperatures are limited. Rocks stay solid over a much greater temperature range. Why storage heaters work. Harder to shift rocks about though, water is easy to pipe and pump.

 

The only good experiments are carefully designed ones to test just one aspect of the material properties, and are repeatable by just about anyone with access to the same equipment.

[Iceverge]

On 25/12/2021 at 08:16, scottishjohn said:

with current and projected price rises  I,m wondering if its getting close to be viable for stand alone pv system   and batteries and a stand by gen for worst times

rather than 30k for a supply ?

and use it for ufh +hot water and no ASHP?

 

 

 

This sounds similar to your predicament. 

 

 

Off grid is completely feasible provided you are happy to burn something in the winter and compromise your on grid living style. 

Your energy demand, how far North you are and the amount of cash you plan to spend will determine the extent of that compromise. 

 

Edited December 26, 2021 by Iceverge

[epsilonGreedy]

8 hours ago, SteamyTea said:

The only good experiments are carefully designed ones to test just one aspect of the material properties, and are repeatable by just about anyone with access to the same equipment.

 

 

Oh come on @SteamyTea time to get off your scientific High Horse and acknowledge that on Christmas Day 2021 there was a revelation by a Wise Man begotten from a land towards the Pole Star. Behold the new truth, the Old Testament according to the Lord Kelvin has been proven wrong in an oven, the heathen belief in Specific Heat Capacity was misguided for 200 years.

 

Time to load up your mule with frankincense, gold and myrrh and pay homage in the realm of Queen Nicola.

[Onoff]

12 hours ago, scottishjohn said:

when I was looking at solar thermal I did a test at home

 

9 hours ago, SteamyTea said:

The only good experiments are carefully designed ones

 

When I was playing with my solar thermal heater from soda cans I burnt my hand on the output. That works I thought.

 

? see ? do here. 

[joe90]

Whilst I like practical experimentation @SteamyTea has a point 

11 hours ago, SteamyTea said:

How did you account for evaporation losses if the water?

 

[Marvin]

1 hour ago, Onoff said:

 

 

When I was playing with my solar thermal heater from soda cans I burnt my hand on the output. That works I thought.

 

? see ? do here. 

Not fok that's hot then?

 

[SteamyTea]

8 hours ago, Onoff said:

When I was playing with my solar thermal heater from soda cans I burnt my hand on the output. That works I thought.

Temperature is not energy.

[scottishjohn]

19 hours ago, SteamyTea said:

How did you account for evaporation losses if the water?

 

One problem with storing energy in water is that the min and max temperatures are limited. Rocks stay solid over a much greater temperature range. Harder to shift rocks about though, water is easy to pipe and pump.

 

The only good experiments are carefully designed ones to test just one aspect of the material properties, and are repeatable by just about anyone with access to the same equipment.

that is why I only went to80c -to restrict the evaporation of the water  from both trays  and as both had water  the level in the trays at end of test were very much same as at beginning of test

  quoting you (thats Why storage heaters work.)

 which is why i know stone concrete are good heat store and will smooth out temp change 

[scottishjohn]

 where I got the idea  was from  an article

If I can find the article again 

i wil post a link

 It was an office complex in uk 

 with a large car park  -pipes buried  not that deep below tarmac --a vast area which the water mix was pumped in closed loop around  to  what could looked at  as  large well insulated sub basement filled with granite sand and small stones -and a closed loop taking heat from this to the heating system  of the building 

all summer it heats up and is then used in winter 

 obviously when ground temp is lower than the heat sink - it does not circulate .

 have you ever touched tarmac when the sun is shining  

 

 

 

 

 

[scottishjohn]

1 hour ago, SteamyTea said:

Temperature is not energy.

but you need energy to to raise or lower  the temp of anything -

 and stone whilst taking a lot longer to heat up also takes longer to loose it once warmed up to target value

and whilst most will say there is no insulation value in stone

 it does have slow transfer properties 

does  it not therefore have a long decrimental delay  in heat transfer ?

 the magicword for those not subscribing to the heat sink camp

 the reason in hot countries they build stone/clay houses with thick walls to control the diurnal temp swings in some countries

Edited December 27, 2021 by scottishjohn

[SteamyTea]

40 minutes ago, scottishjohn said:

have you ever touched tarmac when the sun is shining  

Yes, but that is more to do with the energy input and the absorption.  If you pump 800 W.m^-2 into something that absorbs 95% of all the energy hitting it, it is going to get hot.

41 minutes ago, scottishjohn said:

the reason in hot countries they build stone/clay houses with thick walls to control the diurnal temp swings in some countries

Very different climates, why most of the studies are done in cloudless places.  Go to undeveloped equatorial regions i.e. rain forests and the building style is very different.

43 minutes ago, scottishjohn said:

and stone whilst taking a lot longer to heat up also takes longer to loose it once warmed up to target value

and whilst most will say there is no insulation value in stone

 it does have slow transfer properties 

It is a balance, and non linear, between the heat capacity, shape, thermal conductivity, area exposed to energy inputs and outputs, it is not as simple as just mass.

This is why there are no units for 'thermal mass'. 

Granite has a thermal conductivity of between 1.7 and 4 W.m^-1.K^-1 so yes, it does slow thermal transfer.  But then Oak has a conductivity of 0.16 W.m^-1.K^-1.  So it slows at least 10 times more. So even if, for the same raise in temperature stone stored the same amount of energy per unit mas or volume, which it does not, it also looses to the semi infinite heat sink that is the environment at a faster rate, leaving very little energy left to be transferred to the interior.

 

Quite simply, if just adding mass to a building stabilised the temperature, we would all have been doing it for millennia.   Part of the reasons that in the UK we think that adding mass helps is that we have an odd climate for our geographic location, stick the UK 1000km East and we would not be building in the same method.  Find a low lying island 1000 km West and thing would be different again.

There are also historic reasons that the UK likes 'brick', some go back 1800 years, other just 80.

 

[joe90]

But on a practical level we know stone cottages with thick walls stay cool in hot weather and are more stable temp wise all year. It’s difficult to build a house from water ?. Despite there being no value for mass my “heavy” new build is very stable temp wise and uses very little to heat it. My temporary caravan (lightweight) got very cold and very hot like the temp outside.

[SteamyTea]

37 minutes ago, joe90 said:

But on a practical level we know stone cottages with thick walls stay cool in hot weather and are more stable temp wise all year.

Well that is wat all this debating is about.

Built two equally well insulated cartoon house out of stone and timber, ventilation rates identical, and input the same energy to warm them, and you will not see a difference.

This is what Physics tells us.

Now if you build a stone wall, of normal dimensions i.e. 0.12 m thick, in the stone house and the timber house, you will see no difference.

But if you built a timber wall in the timber house, you may see a slight variation, probably too small to notice.

 

This is an interesting topic, skewed greatly by the local build types, but the Physics are the same.

If I get time later (feeding my Mother at the moment and then exercises/looking up tax information) I shall sketch up some scenarios to highlight the problems in calculation all this (there are a couple of different ways to calculate it all, the energy model and the power model).

No promises though.

Edited December 27, 2021 by SteamyTea

[scottishjohn]

7 hours ago, SteamyTea said:

Built two equally well insulated cartoon house out of stone and timber, ventilation rates identical, and input the same energy to warm them, and you will not see a difference.

you will how fast the temp changes if all heating ceases the insulated one with a stone wall inside the envelope will stay warmer longer as it releases the stored energy /heat slower and it will be slower to heat up for same reason

the modern passion for mega size windows + no curtains only make temp swings more violent between day and night and need for air con more probable -- that uses energy 

 

[scottishjohn]

7 hours ago, SteamyTea said:

Now if you build a stone wall, of normal dimensions i.e. 0.12 m thick, in the stone house and the timber house, you will see no difference.

my outer walls are 0.7m thick before i build the new house inside it 

[SteamyTea]

1 hour ago, scottishjohn said:

will stay warmer longer as it releases the stored energy /heat slower and it will be slower to heat up for same reason

This is the whole point.

The time it takes to cool is governed by the thermal conductivity, not the heat capacity.

1 hour ago, scottishjohn said:

my outer walls are 0.7m thick

When in a steady state, the temperature of the mid point of the wall will be the mean temperature of the wall i.e. 10°C for an internal temperature of 20°C and an external temperature of 0°C.

This has the effect of halving the stored energy in the wall.  Because of the high thermal conductivity of stone, as the energy escapes to the environment, that 10°C point moves inwards, reducing the stored energy even more as heat only moved from hotter to colder. It effectively reduces the thickness of the wall, so what energy there is stored, is released to the room faster.  It follows the Cooling Law, which is an exponential decay that is proportional to temperature differences.  It is this proportionality that often causes confusion.  It is not just the temperature difference between the inside and outside face of the wall, it is also to do with the temperature gradient inside the wall.

 

If you really want to stabilise temperature, get an oversize MVHR unit and scavenge the energy out of the air, easier and more controllable.

[epsilonGreedy]

Is there a single reference residential home in the UK that proves the viability of heating an offgrid home through the winter using a thermal store?

 

Lost in the debate here is the essential arithmetic namely, how many kW hours does it take to keep a property warm through the coldest 100 winter days, how kW hours can a thermal store hold in late October and how many will be lost from the store to outside thermal envelope.

[SteamyTea]

39 minutes ago, epsilonGreedy said:

Is there a single reference residential home in the UK that proves the viability of heating an offgrid home through the winter using a thermal store?

I think a few institutions, government agencies and architects have tried.

None have worked as designed as the parasitic losses and large temperature differences in winter put a stop to this.

But why try, get a HP and use energy that is already stored in the ground, water or air.

 

My house needs around 25 kWh.day^-1 in the depth of winter i.e. half of December, all off Jan and half of Feb.  So around 1500 kWh.

If I could store that in shortage heater bricks which have a SHC of around 2 kJ.kg^-1.K^-1 and charged them with solar thermal to 70°C, (actually does not matter how they are charged) but using a minimum draw off temperature of 40°C, I would need 90 tonnes of them.  Around 30 m³.

To reduce the losses to an average of 2 kWh.day^-1, and assuming a cube for the store (3.1 m each side), the surface area would be a shade under 58 m².  Using a thermal conductivity of 0.02 W.m^-1.K^-1 for insulation, the insulation would need to be 0.62 m thick, that would increase the sides of the cube to 4.34 m, and the total volume to 82 m³.  This would fill the ground floor of my house.

And it would still not be enough in reality.

So to store enough energy to run a house, you have to fill the house with storage.

 

 

Edited December 27, 2021 by SteamyTea

[scottishjohn]

it seems I have been misunderstood 

I never thought that having a concrete /granite wall inside the house would store enough energy to provide heating in the winter with no other input 

that would not be possible 

this started off with my suggestion that the second floor could be supported on block +beam or  hollow core planks  and that would give a thermal store to help keep house at a stable temp  

 yes I might have a feature wall of granite where we will have a large window downstairs to absorb some of the excess heat  from direct sunshine  during the day  and release it back in the evening .

house faces s/w 

 I already get a smaller version of that with present house which has tiled floor where we have a double sliding window 

1 hour ago, SteamyTea said:

This is the whole point.

The time it takes to cool is governed by the thermal conductivity, not the heat capacity.

When in a steady state, the temperature of the mid point of the wall will be the mean temperature of the wall i.e. 10°C for an internal temperature of 20°C and an external temperature of 0°C.

This has the effect of halving the stored energy in the wall.  Because of the high thermal conductivity of stone, as the energy escapes to the environment, that 10°C point moves inwards, reducing the stored energy even more as heat only moved from hotter to colder. It effectively reduces the thickness of the wall, so what energy there is stored, is released to the room faster.  It follows the Cooling Law, which is an exponential decay that is proportional to temperature differences.  It is this proportionality that often causes confusion.  It is not just the temperature difference between the inside and outside face of the wall, it is also to do with the temperature gradient inside the wall.

 

If you really want to stabilise temperature, get an oversize MVHR unit and scavenge the energy out of the air, easier and more controllable.

again i think i have not been clear or you have picked me up wrong

my 0.7m granite outer wall will have an air gap before the new house built inside which is where the insulation will be  as normal practise 

not expecting the outer wall to do much other than slow down  heat loss  or gain and far better than some thin exterior cladding or screed

 as for thickness of insulation  "kingspan tech man " told my anything more than 150mm of PIR  is basically a waste as extra value gained  is so small above that  it would never pay , 

this was when i was trying to configure  solar thermal a good alternative .as I already had a smaller system on my present house

which could only be done by having very large storage(15-20000litres) in this country ,works fine in the alps  with a fraction of that volume where they have bright sunny days all through the winter but not uk 

[joe90]

I agree with @scottishjohn that a “heavy” house will be more stable heat/temp wise. (I lived for a short while in a caravan ?) If mass is good enough for storage heaters then it must be good within the house. Yes insulation works on diminishing returns, the first inch does the most good and I have read (somewhere) that over 300mm in the walls gives little benefit cost wise. I plumped for 200mm as a compromise between cost vs wall thickness.

Edited December 27, 2021 by joe90

[SteamyTea]

Your best options are to reduce losses as much as possible, then find a way to self generate. Some PV and a couple of small wind turbines, around 4 kW each, would supply you with a fair amount of power.