good video on net zero energy home

[scottishjohn]

a good video   quite long 

 

but worth a watch   could even have made me think more serious about PV

moderators --move if you like to more appropiate forum

[joe90]

?They mention thermal mass !!!!!! (Don’t tell Jeremy?).

[Jeremy Harris]

It's so common I'm beginning to wonder why SI haven't yet invented a new unit for it.  I've given up pointing out that mass has sod all to do with the thermal time constant of the building, and that it's the combination of heat capacity and thermal conductivity (both of which have units and can be measured and so quantified) of the structure that are the dominant factors.

 

Easy enough to build a net zero energy/carbon home, ours is actually net negative energy/carbon, in that it both generates more energy than it uses and is, in effect, a CO₂ sink (the EPC reports the CO₂ "emissions" as being -0.9 tonnes per year).  Given that the structure of our house is an off-the-shelf build system, then there's no real  reason for more houses not to be built to a similar standard. 

[scottishjohn]

I don,t think its quite as black and white regarding thermal mass as some would like to believe 

If it was of no value --how you going to get solar gain -if no thermal mass to heat up --wheres the heat going --air is a very poor conductor and storage medium of heat 

why have a thick slab for your UFH -if thermal mass was not in play 

something has to store the energy and the name thermal mass is as good as any 

[ToughButterCup]

John, have you reviewed the wealth of information on BH about thermal mass?     It took me at least a year to get it.....

 

Here's a shortened reading list.

[joe90]

It’s not that it does not exist, people just use the wrong name/terminology .

[Jeremy Harris]

There's only one question that needs to be asked and answered about this, and that is, if "thermal mass" is real, then what are the units used to measure it? 

 

What would be a "good" value for "thermal mass", and what would be a poor value?

 

Heat capacity can be measured, and has defined units, see here: https://en.wikipedia.org/wiki/Heat_capacity

 

Thermal conductivity can also be measured, and has defined units, see here: https://en.wikipedia.org/wiki/Thermal_conductivity

 

Decrement delay and the decrement factor are a useful measures of the interaction of heat capacity and thermal conductivity, in the context of the materials from which a house is constructed, and they can also be measured and defined: http://www.greenspec.co.uk/building-design/decrement-delay/

 

The thermal time constant of the inside of a building can also be measured, and has units: https://en.wikipedia.org/wiki/Time_constant#Thermal_time_constant

 

I've yet to discover any units that can be used to describe "thermal mass", and frankly if it cannot be measured, then it isn't a real physical property, just a loose and ill-defined term to try to describe the complex interaction between heat capacity and thermal conductivity in the context of house building and the way that might relate to its thermal time constant. 

 

There is a Wikipedia entry (under review) https://en.wikipedia.org/wiki/Thermal_mass that attempts to describe "thermal mass", but it is flawed is several ways, not least the fact that it tries to tie the heat capacity of a structure to its mass, which is a poor way of trying to describe something, as mass is generally a poor indicator of the heat capacity of any material. 

 

For example, plaster has a heat capacity of around 1.09 J.g^-1.K^-1, yet concrete only has a heat capacity of 0.88 J.g^-1.K^-1, so a given mass of concrete will store less heat energy at a given temperature than plaster.  Going even further, water has a heat capacity of around 4.18 J.g^-1.K^-1,so will store about 4.75 times as much heat energy as concrete for a given mass.  Clearly, trying to use mass as a measure of the ability of a structure to store heat is not really valid, as there are very large variations in the heat capacity of a given mass, depending on the material that that mass consists of.

[Ed Davies]

3 hours ago, JSHarris said:

Easy enough to build a net zero energy/carbon home, ours is actually net negative energy/carbon, in that it both generates more energy than it uses and is, in effect, a CO₂ sink (the EPC reports the CO₂ "emissions" as being -0.9 tonnes per year).

 

It's entirely possible for a house which is net positive energy/carbon to cause less emissions than a house which is net zero or negative.

[ProDave]

Our house has no obvious large mass structure, but it certainly holds it's heat for a long time, and likewise takes a long tome to heat up.  Probably the largest source of heat capacity is the plasterboard walls and ceilings, and the biscuit mix screed for the under floor heating.

 

It is a very comfortable house to live in. you don't have to worry about when the heating is on or off, as the temperature will not drop noticeably for a long time when it goes off.  Likewise the slow warm up from the low level UFH means it does not overshoot when the thermostats click off.

[Jeremy Harris]

2 minutes ago, Ed Davies said:

 

It's entirely possible for a house which is net positive energy/carbon to cause less emissions than a house which is net zero or negative.

 

Not sure about this.  By definition, a house that has a negative CO₂ emission rate can't be worse for emissions than one with a positive emission rate, can it?

[scottishjohn]

I see exactly what you are getting at and do not disagree in principle 

but the term thermal mass is usually used in the same way as "insulation" is 

now just saying insulation

does not give you a value --but we all know that a house needs to built with insulation and it generally needs to have a thermal mass to store  the  energy 

so when people say they want a good thermal mass its same as saying i want the house well  insulated

I ask  this question

what would you call your 300mm slab floor if not  a thermal mass,as it stores energy 

,using the commonly understood   definition that that a thermal mass stores energy

at what rate it gains  or holds or looses it is another defining attribute  all together

 

 

[Tennentslager]

I’m going to buy some popcorn ???

[Jeremy Harris]

25 minutes ago, scottishjohn said:

I see exactly what you are getting at and do not disagree in principle 

but the term thermal mass is usually used in the same way as "insulation" is 

now just saying insulation

does not give you a value --but we all know that a house needs to built with insulation and it generally needs to have a thermal mass to store  the  energy 

so when people say they want a good thermal mass its same as saying i want the house well  insulated

I ask  this question

what would you call your 300mm slab floor if not  a thermal mass,as it stores energy 

,using the commonly understood   definition that that a thermal mass stores energy

at what rate it gains  or holds or looses it is another defining attribute  all together

 

 

 

But insulation has units that define its performance and it can easily be measured, whereas  "thermal mass" has no units and cannot be measured.

 

I can go into a builders merchant and buy insulation that will have well-defined performance.  It will be defined in terms of it's thermal conductivity, λ, which is given in units of W/m.K in its specification.

 

Also, we don't have a 300m  thick slab in our house, it's 100mm thick.

[Ed Davies]

1 hour ago, JSHarris said:

By definition, a house that has a negative CO₂ emission rate can't be worse for emissions than one with a positive emission rate, can it?

 

It's a matter of definitions, really. Unless a house is actually sequestering carbon (making coal in the cellar or something) it can't really have a negative CO₂ emission rate, all it can do is produce energy which results in other people emitting less carbon. In the (not so) long run [¹] we need to get pretty close to zero emissions in total. When that happens any energy exports from the house will no longer be negative emissions as they'll only be displacing zero-carbon energy production at another place or time. That might well be a good thing to do but it'll not be negative CO₂ emissions.

 

Suppose you have a house which doesn't export any energy at all but imports a small amount of electricity, say 1000 kWh evenly spread throughout the year so causing (going by my recent SSE bill which indicates above-average emissions per kWh) 300 kg of CO₂ emissions. If all the 25 million houses in the UK did that total household emissions would be 7.5 Mt. The house next door imports 3000 kWh during the winter but exports 4000 kWh in the summer so it's “net negative”. Still, if all the houses did that there'd be total emissions of 22.5 Mt during the winter (or a huge investment needed in technology (electricity -> gas, perhaps) to save the spare energy from the summer for use the next winter which will result not insignificant emissions in itself). So, yes, this “negative emissions” scenario does result in greater emissions.

 

[¹] A lot less than the lifetime of houses we're building now.

[joe90]

I tend to refer to our house as “heavy” instead of high thermal mass after reading here about the truth about thermal mass, perhaps thermal capacity is a better term?

[Jeremy Harris]

13 minutes ago, Ed Davies said:

 

It's a matter of definitions, really. Unless a house is actually sequestering carbon (making coal in the cellar or something) it can't really have a negative CO₂ emission rate, all it can do is produce energy which results in other people emitting less carbon. In the (not so) long run [¹] we need to get pretty close to zero emissions in total. When that happens any energy exports from the house will no longer be negative emissions as they'll only be displacing zero-carbon energy production at another place or time. That might well be a good thing to do but it'll not be negative CO₂ emissions.

 

Suppose you have a house which doesn't export any energy at all but imports a small amount of electricity, say 1000 kWh evenly spread throughout the year so causing (going by my recent SSE bill which indicates above-average emissions per kWh) 300 kg of CO₂ emissions. If all the 25 million houses in the UK did that total household emissions would be 7.5 Mt. The house next door imports 3000 kWh during the winter but exports 4000 kWh in the summer so it's “net negative”. Still, if all the houses did that there'd be total emissions of 22.5 Mt during the winter (or a huge investment needed in technology (electricity -> gas, perhaps) to save the spare energy from the summer for use the next winter which will result not insignificant emissions in itself). So, yes, this “negative emissions” scenario does result in greater emissions.

 

[¹] A lot less than the lifetime of houses we're building now.

 

I agree wholeheartedly.  When giving a talk in our village hall about our house, I was asked what the " -0.9 tonnes of CO₂ emissions" bit meant on the EPC.  It's not real, of course, for the reasons you've given, in that it just means less CO₂ is emitted from other energy generation sources.  However, I did work out that it's roughly the same as having around 40 mature trees on our plot, in terms of CO₂ sequestration per year.  Perhaps a bit far-fetched but it serves to show how much impact even a small area of PV generation can have, relative to using the same area for growing things in order to reduce CO₂ emissions (shades of an old, and long-running, debate with a certain biomass fan on another forum!).

[Jeremy Harris]

3 minutes ago, joe90 said:

I tend to refer to our house as “heavy” instead of high thermal mass after reading here about the truth about thermal mass, perhaps thermal capacity is a better term?

 

It's really about having a high heat capacity internal structure, and a low thermal conductivity outer envelope, plus having an outer envelope that has a high enough decrement delay as to largely damp out the impact of diurnal external temperature variation.

[Big Neil]

'THermal Mass' as a phrase seems to me to represent most peoples understanding about energy conservation with a physical space (at least in the contexts i've seen it used). Maybe it's something which could be expressed as a figure which is arrived as by a combination of these things like u value - decriment delay - air-tightness and such like. If it doesn't really have number, why do we not give it one, or express it as a factor of these other measurements; 'My house has a thermal factor of 7 because it has a u factor or .19, has 1.2 ACH, but poor decriment delay.

[Jeremy Harris]

If someone wants to define some units to measure it and get the Système International (d'unités) to accept those units, so that they have international acceptance, like those for heat capacity, thermal conductivity, etc, then I can't see a problem, other than the fact that the term "mass" is misleading.   We already have all the units we need to define the comfort level of a house (and this is about perceived comfort, not energy conservation, no one considers "thermal mass" to be a measure of efficiency, AFAIK), perhaps the one most people might relate to is the thermal time constant, which is roughly how long it takes for the temperature inside the house to change when there is a change in outside temperature.

 

Many would accept that living in a caravan can be uncomfortable because it heats up quickly on hot days and cools down quickly on cold days, and we tend to perceive that temperature variation as being uncomfortable.  On the other hand, a stone building that stays at roughly the same temperature no matter what happens outside (within reason) is often seen as being more comfortable.  The stone building isn't more thermally efficient than the caravan, it may well be a lot less thermally efficient, but it has a high heat capacity and that tends to even out variations in temperature, even though it may need a greater heat input to stay comfortable.

[TerryE]

I take a slightly different view to Jeremy in that I am happy to use the phrase thermal mass as an informal description of a general property, though when it comes to actual modelling the thermal performance of a house, you'll actually use the more precisely defined coefficients of thermal capacity, specific heat, conductivity, etc.

 

We will never have a near energy neutral house, mainly because our planners discouraged / forbade our use of PV, and severely limited our window dimensions, so we have little scope for large solar gain in winter.  But then again I think that solar gain is very much a two edged sword, and overall I'm glad we don't have a large area of glass: what most seem to forget is that an energy efficient house is pretty much energy neutral for 3 season a year, and that solar gain during the seasons can be a real cause of severe overheating, unless you put a lot of effort into the design to avoid this.

 

 

 

Edited February 17, 2019 by TerryE

[Ed Davies]

Yes, very good video for Colorado. It''s worth remembering, though, that the whole of the contiguous US is south of the whole of Great Britain (by at least 60 km) and that Colorado's winter weather, though a lot colder and snowier than most of Britain's, has much more reliable sunshine so the emphasis on passive solar there makes a lot more sense than it might in the UK.

 

In the UK it's often said that south facing glazing is a net gain of energy in winter. It might well be, averaged over the whole winter, but I'm sceptical about it being so over the darkest fortnights (of which there are usually two or three in a winter) which brings us back to our net-zero discussion. A house with a large amount of south-facing glazing might well import less energy over the whole winter but it'll likely import more during the darkest bits so pushing the grid towards less efficient generation (assuming electrical heating of some sort). This might be fine if a lot of that power's coming from wind or a limited but dispatchable source like geothermal or large-scale hydro, but still a lot doesn't.

 

It seems to me that passive solar with a lot of “thermal mass” works well if you're dealing diurnal variations resulting from at least a bit of sunshine most days. If you have to deal with longer timescales of multiple days to weeks then the temperature swings involved mean that it's probably better not to try to live in your solar collector.