underfloor electric heating run by solar PV+ battery system

[scottishjohn]

been talking to a company in scotland who say next year they will be offering 48v underfloor heating to be run by pv+battery system
anyone seen one of these --could be very new .the house will be at least up to passiv standards

[Mike90250]

And very expensive . And very poorly planned. How do you heat on a cloudy day ? With your generator. Why not a Trombe wall/floor and direct heat with solar

The chance of that working well, is close to zero, unless you install an acre of PV to heat the floor. My aunt had concrete slab with electric heat wires in it, was wonderful heat, but really expensive with her electric bill, in the 60's - 80's

https://en.wikipedia.org/wiki/Trombe_wall

[J.P.M.]

been talking to a company in scotland who say next year they will be offering 48v underfloor heating to be run by pv+battery system
anyone seen one of these --could be very new .the house will be at least up to passiv standards

John: (Or "Jimmie") With only a little tongue in cheek (and a lot of Scottish friends in Glasgow), you're a Scot, therefore you're smarter and know better. There are better and more cost effective ways to heat a dwelling.

[scottishjohn]

YEs this is why Iask --if normal heating element is 200 w per m at 240v then to get same heat you would need to run a load of 5amps at 48v to get 180 w --I certainly won,t be rushing into it --,my preferred is solar thermal and BIG heat store --like1000 gallonsgrp tank --Ialready have one (chemical pressure vessel ) surrounded by at least 1ft of concrete and then phenolic insulation at 5 minimum"--thats where my head is going is leaning at this time --close to passiv house will not need a lot anyway--but always looking for info "everyday is a schoolday "

[J.P.M.]

YEs this is why Iask --if normal heating element is 200 w per m at 240v then to get same heat you would need to run a load of 5amps at 48v to get 180 w --I certainly won,t be rushing into it --,my preferred is solar thermal and BIG heat store --like1000 gallonsgrp tank --Ialready have one (chemical pressure vessel ) surrounded by at least 1ft of concrete and then phenolic insulation at 5 minimum"--thats where my head is going is leaning at this time --close to passiv house will not need a lot anyway--but always looking for info "everyday is a schoolday "

Believe it or not, but depending on the heat load, 1,000 gal. is not that unusual or necessarily large. Hell, I had ~ 500 gal. storage in a sunspace.

One way to look at storage size requirements in a relative sense is in units of hours that the store will provide useful heat. First, especially in your climate, insulate and tighten the home as much as practical and to the limits that a clean, healthy indoor environment will allow. That will lower the heat load and the heating bills. Then, if a retrofit, add thermal mass as much as practical. The goal in doing both is to get a parameter called the building time constant as low as possible. That will make any attempts at passive solar heat supplement (if that's the plan) easier and "more" practical, mostly by helping to avoid building overheating. Or looked at another way, with lots of daily and regular sun, you won't need much thermal mass to get you through, say, 24 hrs. or so. On the other end, if you get sun, say, 1 day/week, you better have a lot of equator facing windows, and/or a big collector array, and a lot of thermal storage, that sort of thing. That's why it's always more cost effective in a cold, cloudy climate to spend much more time/effort on lowering the load via lifestyle changes and conservation efforts than expecting solar to offset much of it a heat load.

Thermal mass divided by a low heat loss will, in the simple first analysis with the building treated as a lumped mass system, give a longer time until things cool down (and heat up). The more mass, or the lower the loss, the more time between heat input that can be attained. It's the quotient that matters. Between increasing thermal mass and lowering heat loss, work on the cheapest one firs then the other until the LCOE of the next unit of energy saved just equals the LCOE of the conventional heat input.

However, you'll get to that with your self education. My respectful guess is you'll find conservation will be easier and much more cost effective that solar. Passive retrofits can get tricky, with one of the big (and to my experience mostly unknown to the novice) challenges, among several, being even heat distribution. Large(r) storage adds its own considerations, which are many and also unknown to the neophyte.

Go slow and deliberate, and look before you leap.

[scottishjohn]

thank you for the reply .
this time it will be on a new build and the idea is to get it as well insulated as possible to start with .
taking what you say then maybe it would be better to build from block/concrete and insulate to death around them so the walls as well as floor is thermal mass,which neither sips or ICF will really do .It sort of goes against most normal modern building types .working out where the balance is will be hard --but nothing started yet ,so time for much deliberation
could be a sips with internal block walls all round perimeter ,that would be different attached to insulated base slab making a proper thermal bridge between them .LOL

[J.P.M.]

thank you for the reply .
this time it will be on a new build and the idea is to get it as well insulated as possible to start with .
taking what you say then maybe it would be better to build from block/concrete and insulate to death around them so the walls as well as floor is thermal mass,which neither sips or ICF will really do .It sort of goes against most normal modern building types .working out where the balance is will be hard --but nothing started yet ,so time for much deliberation
could be a sips with internal block walls all round perimeter ,that would be different attached to insulated base slab making a proper thermal bridge between them .LOL

I designed (with an architect) what was to be my solar Magnum Opus in the desert that was to be a thermally massive home insulated on the "outside". It would have had a thermal time constant of about 10+ days, and be equipped with all legally required/building code HVAC (and mostly A/C) that would have been needed or used sparingly, if at all. I got the whole design through what's called preliminary plan check/review and approval to the point just shy of building permits as a hold point in the schedule. I then found out what I'd suspected: That even with a focused goal of using standard building materials and techniques (but in a few non standard ways perhaps), I wasn't going to get what I wanted, pretty much regardless of what I was willing to spend. Saying that, I appreciate a contractor's view: (S)he has a lot to lose in time, money, reputation and aggravation by trying new stuff in the middle of the desert that some retired P.E. (s)he doesn't know has been having wet dreams about for 30+ years. It also doesn't make my contractor selection any easier or their decision to take a chance that I also come off as a bit of an engineering prick - at least at first. So be it.

I offer that as thought to your comment of going against normal practice. Thinking from a builder's perspective, and with no rancor, a builder can probably make a lot more money for less hassle/risk by building stick/row production homes than off the wall stuff (s)he may be unfamiliar with. Nothing personal. Not charity or experiment. Just business. I'd probably do the same.

FWIW, I decided against ICF type stuff. Maybe things have improved, but 10-12 years ago, I wasn't comfortable with what I saw of ICF's that had been used up to that point. BTW, if I were you, and I used block, I'd consider filling the cores w/concrete. Also, my design called for insulating around the slab down 48" or to the footings. That acted to add some of the earth under the slab as an adjunct to the rest of the thermal mass in the exterior walls/slab/interior walls. I would have insulated under the slab, but soil conditions at the site would not permit it and neither would the AHJ. Long story, lots of details left out.

[scottishjohn]

a 10 day thermal constant would do just fine ,as weather here is very changable --average winter temp for heat calcs is +3c ,very few times when we get freezing conditions for more than a few days
,same goes for hot weather--this year has been longest hottest I have seen in 35 years here.so maybe big solar thermal array and my concrete encased+phenolic foam insulated thermal store along with the mega thermal mass of the house has a chance of working
-typical cost of heat pump system with hot water etc is around 8000pounds
30tube kingspan panels 03.02sqm cost 1100 pounds ,so i could have 150-180 tubes for same or cheaper than heat pump.
do you have any experience of the more economically priced panels form the far east
kingspan give 20 year warranty ,if fitted by approved engineer ,or 5 years if not.
maybe add a smaller buffer tank to run solar direct in worst conditions with auto switch between big and small tank depending on solar output/load

[J.P.M.]

a 10 day thermal constant would do just fine ,as weather here is very changable --average winter temp for heat calcs is +3c ,very few times when we get freezing conditions for more than a few days
,same goes for hot weather--this year has been longest hottest I have seen in 35 years here.so maybe big solar thermal array and my concrete encased+phenolic foam insulated thermal store along with the mega thermal mass of the house has a chance of working
-typical cost of heat pump system with hot water etc is around 8000pounds
30tube kingspan panels 03.02sqm cost 1100 pounds ,so i could have 150-180 tubes for same or cheaper than heat pump.
do you have any experience of the more economically priced panels form the far east
kingspan give 20 year warranty ,if fitted by approved engineer ,or 5 years if not.
maybe add a smaller buffer tank to run solar direct in worst conditions with auto switch between big and small tank depending on solar output/load

No, but I'd not use evac. tube collectors for space heating applications (~ 60-70 C. or so). They're more suited for the higher temp. requirements of process heat applications (> 100C. or so). Match the source to the task and don't increase entropy any more than necessary - that's when cost inefficiencies begin. You'll get more useful energy for space heating per unit area of collector and money spent with a well made flat plate collector than an evac. tube unit, and pay less per unit of delivered energy while doing it. There is a difference between quality of heat (temperature) and quantity of heat (units of energy). Paying for more quality than needed for the task is a waste of energy, entropy and, in the end, money. Disabuse yourself of the idea that high temps. are always needed and available without consequences and you'll wind up with more serviceable equipment that costs less to acquire and maintain.

[scottishjohn]

I can understand why you only use flat panels with the amount and strength of sun you have in the desert ,but scotland is not like that and flat panels do not work best in cloudy conditions
and if you are only trying to get some hot water with relatively small buffer tank then they will work fine and will not stagnate as quick as evac tubes.the spec that most solar supplies go for
in uk is 1 x30tube panel or 2 x flat plate panels for hot water on 250litre tank ,and that will only give hot water nothing else and maybe not full year .2 x 20 tube evac panels ran all myhot water
and some of the underfloor ,but did stagnate too much in summer .I had them at an angle same as lat 54-but would this time set to winter sun angle 28? ,pretty close to vertical to help and maximise winter output

I keep coming back to the only major problem is volume of storage and getting best thermal stratification in the tank,so collectors can run effectively from daylight to dusk
,which means tank as tall as possible.
A bath like tank ,which is easy to make for DIY is pretty useless unless you use all the heat that day or next .
might even consider alpine type tank where they make it on gable end so is full height of building.still in panning mode and house thermal load requirement will make it all clearer
a video for those new to these things
uk.video.search.yahoo.com/search/video;_ylt=AwrIdiAwa7hbVlEAmEIM34lQ;_ylu=X3oDMTByZ mVxM3N0BGNvbG8DaXIyBHBvcwMxBHZ0aWQDBHNlYwNzYw--?p=solvismax&fr=mcafee&guccounter=1#id=11&vid=8ee9 84e806113f283c35bfcc622a2297&action=view

[J.P.M.]

I can understand why you only use flat panels with the amount and strength of sun you have in the desert ,but scotland is not like that and flat panels do not work best in cloudy conditions
and if you are only trying to get some hot water with relatively small buffer tank then they will work fine and will not stagnate as quick as evac tubes.the spec that most solar supplies go for
in uk is 1 x30tube panel or 2 x flat plate panels for hot water on 250litre tank ,and that will only give hot water nothing else and maybe not full year .2 x 20 tube evac panels ran all myhot water
and some of the underfloor ,but did stagnate too much in summer .I had them at an angle same as lat 54-but would this time set to winter sun angle 28? ,pretty close to vertical to help and maximise winter output

I keep coming back to the only major problem is volume of storage and getting best thermal stratification in the tank,so collectors can run effectively from daylight to dusk
,which means tank as tall as possible.
A bath like tank ,which is easy to make for DIY is pretty useless unless you use all the heat that day or next .
might even consider alpine type tank where they make it on gable end so is full height of building.still in panning mode and house thermal load requirement will make it all clearer
a video for those new to these things
uk.video.search.yahoo.com/search/video;_ylt=AwrIdiAwa7hbVlEAmEIM34lQ;_ylu=X3oDMTByZ mVxM3N0BGNvbG8DaXIyBHBvcwMxBHZ0aWQDBHNlYwNzYw--?p=solvismax&fr=mcafee&amp ;amp;amp ;amp;guc counter= 1#id=11& amp;vid=8ee984e806113f283c35bfcc622a2297&a mp;a mp;a mp;a ctio n=view

"Moran Taing".

Thinking/reading through about this thread and you others, I'm not sure I understand what your goals/priorities are, or if you want to confine this and/or other threads to your original question which Mike pretty much answered in a way I'd concur with - that is, from the info provided in your question, that outfit's offering sounds pretty much like B.S..

Assuming you want to continue some dialog re: solar thermal: I believe I understand what you are saying and your skepticism of my ability to understand you cloudy situation. Perhaps a sentence or two of background will add some light. For the first half century or maybe a bit less of my life I lived in a climate that was probably cloudier and for sure colder than Scotland for about 6+ months of every year. I became an engineer in the mid/late '70's because of solar, after getting into solar as a possible way to stay warm in the winter without going broke doing it, and found out how badly I could get screwed by shysters and con men, but mostly my own ignorance. I make different mistakes now. FWIW, the cons are still running and still with us as your recounting of the pitch for underfloor heating with PV may demonstrate.

Back on your situation, my training and experience over the years such as it may be during and since that time has left me with the opinion that, because the solar availability under what you (and I) may consider "cloudy" conditions (or what you'll come to understand from Duffie & Beckman is a monthly "clearness index" of ~ 0.2 - 0.3 or so in winter) any cost effective use of solar energy over that period becomes a real challenge, and if that solar climate is a common occurrence for several months of the year, all the harder. For such times, and in such climates, cost effective active solar space heating, and to about the same degree solar domestic water heating is, for the most part, an exercise in futility bordering on wishful thinking.

On active solar assisted domestic water heating, like most other solar applications, its efficacy and cost effectiveness are largely dependent on the availability of the solar resource. Evacuated tube collectors will indeed produce hotter water than flat plate collectors. But, hotter water does not necessarily mean more collected energy that's deliverable to a load, nor does it say anything about cost effectiveness relative to other, conventional ways to supply heat to a load ( fossil fuel, electric resistance heating, maybe heat pump). That's the difference between quality (temperature) and quantity (heat delivered). Raising 1 lbm of H2O 100 deg. F. requires the same energy input as raising 100 lbm of H2O by 1 deg. F. The first has more energy quality (lower entropy) and the result is that the hotter water more useful (has more available energy and more versitilty for different tasks). The second, raising 100 lbm by 1 deg. F. is easier to do (and so will cost less), and for DHW purposes will, in the end accomplish the same task - raising the temp. of the water in the tank. Those 2 conditions also have some applicability to your storage size situation.

In an average climate, whatever "average" may mean in such a context, over the course of a year, an evac. tube unit will usually collect slightly more useful heat than a flat plate collector, and will produce hotter fluids. That hotter fluid situation may seem nice to a user, but the evac. tube unit's advantage of hotter fluids can turn into a liability when it's producing, or capable of producing, lower entropy fluids than required for the task (capable of producing 150 C. water - but who needs that capability for DHW ?) That costs money. Analogy: Do I want a 750 kW roadster as a grocery hauler when a 50 kW EV will perform the task as well ? Either will get the job done. Which do you think will be more cost effective ? You understand Thermodynamics. Lower entropy costs money. Lowering entropy more than necessary for a task wastes money. Match the energy production to the task and it'll be easier to make it cost effective.

In a "cloudy" climate such as you have, or at cloudy times of say, an hour or so, an evac. tube unit will probably collect more useful energy than a flat plate collector. BUT, the rub with that is that there is a lot less energy to collect in the first place. A rough example: Under clear sky conditions, for an hour, say, 1,000 units of energy may be incoming to a collector per unit area of collector. With an evac. tube unit, maybe 750 of those units might be collected and stored in a tank over that hour per unit area of collector. Also, say 550-600 units might be collected with a decent flat plate under the same conditions for the same unit area. That's advantage to the evac. tube unit, with most of that advantage being due to the likely much lower heat loss coef. of the evac. tube unit, but with that advantage being somewhat offset or mitigated by the usually lower optical efficiency of evac. tube units vs. flat plate units that reduces the amount of energy intercepted by the evac. tube unit.

Now, considering the more common situation (for Scotland anyway), and for comparative purposes only, but maybe not terribly far off reality, under cloudy skies, there may be, say, only 200 units of incoming energy per unit area (not the 1,000 as in the sunny, prior example). Assume for the time being, although it's not strictly correct, that under these cloudy conditions, the efficiencies of the evac. tube unit stay the same as it's efficiency under sunny skies, but the efficiency of the flat plate drops to, say, 40%. So, the evac. tube unit collects 200*.75 = 150 units of energy while the flat plate collects 200*.40 = 80units of energy. WOW ! The evac. tube unit collected almost 2X as much as the flat plate !! But notice how much less was collected under cloudy skies for either collector, 750 vs. 150 for the evac. tube, and 575 vs. 80 for the flat plate. Point is, while the evac. tube unit will, in a relative sense, indeed collect substantially more energy than the flat plate under cloudy conditions, both outputs suffer under cloudy skies because the energy isn't there in the first place. The comparison between evac. tube and flat plate aside for a bit, whether or not the absolute performance of evac. tube units under more cloudy conditions is sufficient to make it cost effective relative to other (conventional) ways of meeting a load is an individual determination as is that for the flat plate case. But and FWIW only, my experience with the two collector types is that evac. tube units cost a fair amount more, are less reliable and require more maint. and babysitting. They do have an advantage in colder /freezing climates, but since they need glycol as a coolant (unless they're a heat pipe type), and since flat plates can, and have, for a long time operated with direct circulation of potable water as a coolant with either a drainback or freeze protection circulation system used to prevent freeze up, I'd suggest that evac. tube collectors are often more high tech than necessary and thus more money. Quite honestly, I think evac. tube units are overcomplicated with the resulting cost increases that make them less cost effective for the application of heating water for domestic use or space heating. Just opinion.

On your stratification concerns, things get complicated. Precis: System thermal efficiency is largely but not entirely determined by collector average temp. and the diff. between it and the ambient air temp., and also the collector heat loss coeff. High flowrates will mean lower average plate temps. That will lower the overall heat loss, which increases system efficiency, with one result being higher storage temps. Also, higher thermal efficiency will often be attained due to less thermal resistance between the fluid and the collector (but not always, depending of two dimensionless flow parameters, the Reynolds # and the Prandtl # or the Nusselt # if the flow is mostly laminar.) But, higher flowrates will also destroy tank stratification. Bottom line: The two effects - lower collector temps. and less tank stratification tend to cancel one another out, but overall, more heat will be collected with higher flowrates. An added bonus with higher flowrates being that higher flowrates also tend to keep the system "cleaner", that is, less clogged up, or a lower rate of fouling which tends to keep thermal efficiencies from declining as fast.

Summing up, if I was in Scotland and building a home with one goal of many being as close to zero energy use and as long term cost effective as possible, I'd build a highly/super insulated and sealed high mass dwelling insulated outside the thermal mass to the limits and as discussed in a prior post. I'd plan on some passive solar heating via fenestration and affix moveable insulation to all the windows. I'd consider a batch heater as a solar DHW boost and abandon the idea of meaningful solar assist in the winter. I'd place most of the windows on the south and east side of the dwelling, and fewer on the north and west side. I'd put some shading by planting trees, etc. to the west, probably at 270 deg. azimuth or perhaps a bit more for your latitude. I'd avoid elec. resistance heat except for small task related applications (under desk, elec. blankets, etc.) and be careful about heat pump economics vs. fossil fuel. If available, I'd prefer fossil fuel, but in a semi-moderate temp. climate like Scotland, I'd perhaps be persuaded to change my mind to heat pumps depending on power and equipment costs and equipment reliability. I'd be careful about indoor air quality, particularly in a tight dwelling, but knowing what I know, I'd avoid the air-air heat exchangers that seem to be available these days. I took up about half an engineering career designing industrial heat transfer equipment, mostly heat exchangers and power boilers of various types. The stuff on the market today for home use is utter garbage, a waste of money and in many cases unsafe. I'd live with the penalty of not intentionally heating incoming/makeup air until something comes along that's not a potential health hazard and a waste of money.

Sorry for the long post.

Fortan Leat.

J.P.M.

[scottishjohn]

My priorities are to make solar thermal work if at all possible on the new house where i do not have issues for having as large a storage as required to do the job.
with of course an eye on the initial cost total cost -
If i can get it to work ,then initial cost will soon be repaid by no fossil fuel bills for heating--I will use 6 years as my time period for repayment ,same as heat pump grants
the previous solar thermal system ,which suffered damage 10 years ago ,but carried on working at lesser rate ,was caused by incorrect specification by supplier of certain parts
like the plastic air separation device on top of panels ,which melted with the failure of a temp sensor ,and me assuming that the system was correct and not having redundancy
in the over heat cut off system or alarms ,use to be quite a joke ,when next door nieghbour asked what the steam was coming out of the soffit vent pipe when if lifted the pressure safety valve .LOL
this of course was my 300 litre tank getting to boiling on a good day ,as my tubes were a 135c regulated type --a mistake on my part --all part of the learning curve
as there was little sensible help 20+years ago in Uk
I continued using it with half the tubes damaged until last year when heat pump went in.
I guesstimate the effect on fuel cost after it was damaged went up by 25-35%,hard to be accurate even allowing for rise in fuel prices ,but somewhere around that .

[scottishjohn]

2 years ago I fitted air source heat pump+combined tank to replace a fairly new self condensing LPG boiler .rest of system the same
the result was a drop of total energy costs by 1100gbp
lpg bill was 1600gbp ,elec bill went up up 500gbp.
add to that the RHI grant of 1000gbp for 6 years and 70%of heat pump is paid by that -
so if not on mains gas in uk,which is half price of LPG its a no brainer if you can afford initial outlay
due to contractor not telling me the whole truth ,eg I could not specify when the heat pump would heat water ,so i could let solar do most
of it ,the solar became fairly usless
the anti legonella function meant it cycled hot water evey few hours to 60c -which meant solar could not run at its best
50c is plenty for hot water -or you are adding cold all the time ans it is a closed system with one way valves i see no legonella danger.
my original own designed thermal store had hot water coil fitted on outlet with a cold supply and thermo mixer
definitely the way to get most out of solar hot water ,as i only need 40c for under floor -it just runs longer

[scottishjohn]

I agree basically with your view on evac /plate panels ,buts that the thing i want to get every bit of energy to pump up my tank when it can .
I also found out very quickly that the expansion vessel supplied with kit was not big enough -so that was doubled in size--caused fluid loss ,which then needed
topping up with more antifreeze solution
when i first fitted my system it was in feb and a good clear day solar panel temps were 45c and it ran all underfloor at 26c --not enough i know but it does work
which is why I say the only problem is large enough storage so the panels never max out .
I had my solar controller --very crude in those days set at a 6 degree between tank and panel -it worked fine on good days I could see a 15 -20 degree difference
between panels and tank return by playing around with flow speed on pump
I had simple thermostats upper and lower to decide when boiler could come on or not -
I did work out a complicated system of control but that would have meant computer control and that was not easy to find then or sensibly priced
It just on a cloudy winters day the max temp might be only 40c -so i need volume and stratification to allow me to use that .
which means you do not want to stir up the contents of the tank if you can help it
simple physics really 1000gallons @40c has same energy as 500gallons at 80c--so my idea is to get main tank to 85-90c in summer with a another tank of smaller size
say 70gallons so when collector temp is lower I can heat that first or only in winter--basically a cascade system of tanks by temp control+pump
and that one would have a back up in line electric boiler just in case .
bear in mind UK gallons are bigger than US
so this where the tank surrounded in concrete and wrapped in insulation comes in to keep what i make for long periods
a normal person would just go air source heat pump --but by now you know I,m not normal

[scottishjohn]

car batteries --thats my field been motor engineer for 45 years --buy the most expensive one for same rated out put --very price sensitive --so you get what you pay for
then are you wanting it to sit doing nothing for long periods or will it be cycled often -that changes battery type to choose
how old is the car? --old cars had machine sensed alternators ,which just keep charging battery to full--which is why old cars you often got acid corrosion on terminals from the venting from over charging
modern cars use "battery "sensed altenators which basically will return battery charge state to what is was when car was started
so if car not used much the battery level will slowly drop due to on board alarms etc l --
so to get it full you need to remove it and manually charge it right up -same goes for the new battery --it will not be fully charged when you buy it
they never are --so charge up fully before fitting and then it will perform as designed and last much longer
be aware some new type of batteries fitted to newer cars cannot be charged with an old type charge it will damage them -
simple question --but not a simple answer --as usual with these things
some cars won,t even run when you fit new battery if you don,t tell the on board computer what the capacity of it is --
they look at resistance of battery and know its capacity it decides on charge rate --so it needs to know spec of battery to work correctly