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re: seek passive solar design faq/guide
9 may 1996
william r stewart   wrote:
>nick pine wrote:
>> william r stewart   wrote:
>> >... if you are considering passive solar as your main heat source.
>> >the passive solar industries council has a complete book and software
>> >program for this engineering problem.
>> >
>> >     passive solar industries council
>> >     1511 k st., #600
>> >     washington, dc 20005
>> >     (202) 628-7400
>> 
>> yeah, the brick people :-) give them a call if you want to fill up your
>> sunspace with thermal mass and cripple the performance, while raising
>> the price dramatically :-)
>
>thermal storage does not 'cripple' the performance of a sunspace,

i disagree. this basic high school physics is now well-understood, being
over 300 years old, invented by newton and others. in rhetoric, an assertion
demands no more than a counterassertion. i've gone beyond that already.
you have my numbers. where are your numbers, will?

>it simply evens out the temperature swings.

that it does, but that's not all it does. it also stores lots of solar heat
during the day, most of which radiates back out thru the sunspace glazing
at night, since that is a poor insulator. again, this is simple physics.

>brick is only one of several materials that can be utilized,
>including even water.

less delightful for the brick people, no doubt :-)

>> or if you want your "main heat source" to
>> provide less than half the heat for your house.
>
>you would have to provide the figures to support your assertion,

i'm confused here, will. or perhaps you are. i said that there are a number
of houses in the us that are 100% solar heated, with no backup heating
systems at all, some of which have long track records, in cloudier, colder
places than philadelphia. i also said that if you carefully follow the
orthodox psic "passive solar design strategies: guidelines for home
builders," you will end up with a house in the philadelphia area that is
no more than 41% solar heated (see the 13th line on the right hand side
of the table on page 31 of those guidelines.) this psic target seems
surprisingly low, given all these solar houses with no other form of heat.

>...i have seen passive solar homes where solar provided in excess of
>85% of the heating requirements.

so have i. but they were not built using those rotten psic guidelines.

>[199th repost of solar closet deleted]

perhaps you should read it and understand it once, will, instead of just
deleting it over and over... :-)

>> two views on sunspace design:
>> 
>>    it is hard to think of any other system that supplies so much heat
>>    (to an existing house) at such low cost...
>> 
>>    one could shorten the warm-up time of the enclosure and increase
>>    the amount of heat delivered to the rooms by making the enclosure
>>    virtually massless--by greatly reducing its dynamic thermal capacity.
>
>so that energy isn't stored for the evening and night hours?

correct. no heat lost via the sunspace at night, because the heat is
stored elsewhere. this is bill shurcliff, phd, physics, talking...

>>    this can be done by spreading a 2-inch-thick layer of lightweight
>>    insulation on the floor and north wall of the enclosure and then
>>    installing a thin black sheet over the insulation. then, practically
>>    no heat is delivered to the massive components of floor or wall;
>>    practically all of the heat is promptly transferred to the air.
>>    and since the thermal capacity of the 100 or 200 lb. of air in
>>    the room is equal to that of one fourth as great a mass of water
>>    (about 25 to 50 lb. of water), the air will heat up very rapidly.
>>    i estimate that its temperature will rise about 40 f. degrees in about
>>    two minutes, after the sun comes out from behind a heavy cloud cover.
>>    at the end of the day, little heat will be "left on base" in the
>>    collector floor or north wall and, accordingly, the enclosure will
>>    cool off very rapidly.
>
>i fail to see the advantage of such a system;
>what do you do for heat at night?

a solar closet, an attic warmstore, a rock bin, massy house walls,
an indoor pool, concrete furniture, a 5 year supply of diet coke, etc.

>>    a sunspace has extensive south-facing glass, so sufficient thermal mass
>>    is important. without it, the sunspace is liable to be uncomfortably hot
>>    during the day, and too cold for plants or people at night.
>
>just the opposite of what you state above.

right. i'm quoting the psic brick people here, not bill shurcliff, phd,
physics, harvard prof and author of a dozen or so well-respected books
on solar heating. bill shurcliff does not sell bricks :-)

>>    however, the temperature in the sunspace can vary more than in the
>>    house itself, so about three square feet of four inch thick thermal
>>    mass for each square foot of sunspace glazing should be adequate...
>
>how did you arrive at that size?

i didn't. the brick and concrete people did. i found this pearl of wisdom
on page 27 of my philadelphia psic guidelines.

>and what material would you use for the thermal mass, as energy 
>capacities vary widely?

i would use sealed containers of water myself, but i would not put them in
a sunspace. i'd keep them somewhere inside the house, ideally above room 
temperature inside a solar closet, where they wouldn't lose all of their
heat overnight or during a week without sun to the outside world thru the
glazing, which is a good heat conductor. i'd let the sunspace itself get
icy cold very quickly at night, so it loses little heat.

>>    the sunspace floor is a good location for thermal mass. the mass floors
>>    should be dark in color. 
>
>like brick?  :-)

sure, if you sell bricks :-)

>>    no more than 15-25% of the floor slab should be
>>    covered with rugs or plants... another good location for thermal mass
>>    is the common wall (the wall separating the sunspace from the rest of
>>    the house)... water in various types of containers is another form of
>>    energy storage often used in sunspaces.
>
>yes, a water wall is an effective thermal storage device.

it is indeed, if it has insulation between itself and the outside world.
  
>> so, which is the most energy-efficient sunspace in a partly cloudy climate
>> like philadelphia? 

>> shurcliff's plastic film sunspace, wearing the green uniform in this
>> contest, might cost about $1/ft^2, and on an average december day at 36 f,
>> it would receive about 1000 btu/ft^2 of sun, like the psic sunspace. let's
>> assume that both sunspaces have a perfectly insulated wall between them and
>> the house, to avoid the thermal disaster of a poorly insulated trombe wall
>> in a partly cloudy climate, and let's assume there is no air infiltration
>> from the outside in either case.
>
>two major assumptions that are unacceptable in a real world situation,
>especially the lack of air infiltration.

ok, put in an imperfectly insulated wall, say r20, and some air infiltration,
eg 2 air changes per hour. the results hardly change at all. trust me, i know
what i'm doing. i won't bore you with those details.

>that would negate the benefits of an air storage attempt in a sunspace.

nobody's trying to store heat in air... (?)

>> >some of the variables involved in such a design include;
>> 
>> >what is the heat loss rate of your structure?
>> 
>> yes, that's a good thing to know... "ohm's law for heatflow"... note glass
>> is a very poor insulator... a 30 x 30' x 2 story house with r20 walls and
>> ceiling might have a thermal conductance of 2000 ft^2/r20 = 100 btu/hr-f.
>> make 10% of the wall area windows by adding 200 ft^2 of r2 glass and this
>> doubles to 200 btu/hr per degree f--unless the glass is in a thermally
>> isolated sunspace, in which case the thermal conductance and heat loss
>> of the house go down, not up...
>
>try using r4 windows with window quilts for even more night insolation.

r4 is poor, compared to a wall. and try using the word "insolation" for sun,
and "insulation" for heatflow. and recall that people don't use manual movable
insulation for long. they get tired of operating it. nobody seems to have
come up with good, simple, cheap, automatically-movable window insulation,
after all these years. for one thing, it's not easy to seal the edges. 

>and the sunspace you refer to with mylar windows will have less than an
>r1 rating, so energy retention in the sunspace, including air infiltration,
>will be negligent.

for starters, i guess you mean "negligible" instead of "negligent" (as in
"the great american colonial composer william billings was said to be
'a man of uncommon negligence,' since he spent a lot of time in the gutters
of portsmouth.") more substantively, air infiltration should be minimal in
a sunspace made with a very large piece of plastic film, and i assume that
by "energy retention" you mean something having to do with solar collection
efficiency, not heat storage... i can't find very complete information about
the thermal resistance of mylar (polyester) film in my greenhouse engineering
book, perhaps because it has not been used for a long time in greenhouses,
but it does say that mylar has an ir transmittance of 30%, vs 50% at the
same temp for r0.8 polyethyene film, so it must be at least r0.8. so instead
of a loss of 6 hr (80-36)1 ft^2/r1 = 264 btu/ft^2 for a 74% solar collection
efficiency of a single-layer glass sunspace in the phildaelphia area, we
might have a loss of 330 btu/ft^2/day and a solar collection efficiency of
67%, so we would need a few more square feet of sunspace glazing. no big deal,
at 10 cents/ft^2 (?)

>> >what is the solar insolation in your area and when does it occur?
>> 
>> also good to know, eg the amount of sun that falls on a south wall on a
>> december day, as well as the average temperature in december. if your
>> house stores heat for several days, these averages are good enough for
>> design. 
>
>this is a little over-general, as passive solar mistakes
>have borne out in the past.

i disagree. i've posted some detailed computer simulations that prove this.
perhaps you would like me to email you a 262,980 line solar simulation for
such a house using hourly nrel data for the last 30 years.
 
>> >what are your backup systems (eg, masonry fireplace, ground-source
>> >heat-pump, etc)?
>> 
>> ideally none. this is how some people define a "solar house," ie one with
>> no other form of heat... simple, no? such a house can be easily designed
>> with some high school physics and algebra, as licensed professional engineer
>> norman saunders has been doing in cold, cloudy new england since 1944.
>
>again, the specifics of the weather play an important role, because
>if you have 6 days of cloudy or rainy weather in january, then you will
>freeze without supplementary heating of some form.

i don't think you will freeze, but you might have to put on a sweater every
35 years or so, the way norman saunders, pe, calculates these events, on a
statistical basis, like 100 year floods... of course one can never build a
"100% solar house," as you say, because no matter how much thermal storage
a house has, mother nature will eventually supply a string of cold cloudy
days that exhaust it, and the house temp may dip to 67 or 66 f (horrors!)
despite our best laid plans. my best laid plans might include a simple solar
house simulation based on hourly local nrel/noaa data for the last 30 years.
it seems to me that most people would be happy over the next 30 years if
their house would not have needed any backup heat for the last 30 years...  

>> here's the scoop. the way to do this is simple. start by finding 3 numbers:
>> 
>> 1. find the heat loss for your house, eg 200 btu/hr per degree f.
>
>a fairly well-insulated house.

true. and why not. as steve baer says, the greatest discovery of solar
investigators has been that if you use lots and lots of insulation, a
house will need very little heat, from the sun or any other source. however,
insulation does cost money, and it doesn't make hot water. perhaps future
houses will have more glazing and thermal mass and less insulation. perhaps
not. perhaps they will have backup heating systems, perhaps not. this is
a matter of economics, inter alia. but, there will always be purists, sailors
who like to sail boats without motors, and heroic natural homeowners...

>> 2. find the average temperature in december where you live, eg 36 f. 
>> 3. find the average amount of sun that falls on a south wall in december
>>    where you live, eg 1000 btu/ft^2/day, using nrel's numbers for
>>    philadelphia, assuming a little more ground reflection.
>
>from http://solstice.crest.org/cgi-bin/solrad , we get 2.9 kwh/m2/day
>for a vertical wall surface in philedelphia in december.

fine.

>out of "principles of solar engineering", kreith/kreider, we find for 40o
>north latitude; dec 21, we find 702 btuh/ft2 for an ideal day, with no
>overcast or other insolation impediment.

that sounds more like an average day, not an ideal day, especially since
you mention the latitude. your 2.9 kwh/m^2/day nrel number (919 btu/ft^2/day)
with a built-in 0.2 ground reflectivity is probably more accurate for an
average day... either one can be improved with something more reflective
lying on the ground in front, eg snow or ice with a 0.6 reflectivity. .   

>> size a low-thermal-mass sunspace to provide 100% of the heat for the house
>> on an average december day, with some sun. there are several steps here: 

>> 4. find how much heat your house needs on an average december day. if it
>>    needs, say, 200 btu/hr/degree f, using "ohm's law for heatflow," on
>>    an average 36 f day, it will need 24 hours (70-36) 200 = 163k btu/day
>>    to stay at 70 f inside.
>
>> 5. find how much net heat a square foot of low-thermal-mass sunspace can
>>    gather on an average day where you live. suppose the sunspace takes
>>    in 1000 btu/ft^2/day with r1 single glazing. then if we let the sunspace
>>    temperature rise to, say, 80 f during an average 6 hour december day,
>>    so it can provide warm air to heat the 70 f house, the loss will be about
>>    6 hours (80-36)1 ft^2/r1 = 264 btu, for a net gain of 736 btu/ft^2/day.
>
>what is the surface area of the sunspace?

i had in mind a fairly shallow sunspace, perhaps some south "solar siding."

>it wouldn't be identical to the floor square footage,

true. it would be quite a bit larger. making a 16' tall x 12' long lean-to
sunspace 8' deep instead of 8" deep adds another 128 ft^2 or so of endwall
losses, so if the original sunspace were designed to collect (1000-264)x16x12'
=141k btu/day, the 8' deep one would want to collect enough sun with the
south facing glazing area a so that (1000-264)a=128*264+141k btu/day, making
a about 238 ft^2, ie we might use a 16' x 16' sunspace.

>i would estimate you would have roughly 3 times the surface area to floor
>space ratio, at a minimum. 

let's check that: (16x16'+128')/16x8' = 3. exactly :-) 

>that makes an enormous difference in your calculations.

no, this is just one of those little refinements i mentioned before. using
a deeper sunspace than solar siding requires a little more glazed area.

>and don't forget air infiltration.

ok, i won't forget that. should we forget internal heat generation?

>> there are a few more little details to check, but this isn't rocket science,
>> or even college physics. 
>
>you need to construct one, collect data, and provide empirical results.

like the one with the electronic data logger inside that has been up on
the roof of the local college since november 4? no, i don't need to do that.

>> ...you might use a trombe wall, invented by felix trombe in 1964 (and
>> patented by edward morse of salem, ma, in 1881) or a picture window in the
>> living room, with a masonry floor in front of that... a "direct loss" house,
>> like the one architect george f. keck called a "solar house" in 1934 :-)
>
>you are making specious claims.

i disagree. trombe walls are even poorer performers than sunspaces full of
bricks, since they have little thermal resistance. at least you can put an
insulated wall between your living space and your brick sunspace, so most of
your backup heat stays in the house during a cloudy week. you have my numbers.
which number is incorrect? where are your numbers? most people don't even
need numbers to see this.

>trombe wall effectiveness has been proven with empirical data. 

they work pretty well in the southwest, in places with less than 5 cloudy
days per year, but even there, people call these "direct loss houses" :-)
in cloudier climates, they are thermal disasters.

>>and the architect said, "i agree with you completely, but if you do that,
>>you will violate the integrity of the traditional trombe wall, which has
>>a magical, wonderful way of *flywheeling*, and transporting the heat
>>through the wall, so it is available at the other side *precisely* when
>>it is needed, the next morning!" and he went on and on about this conceptual
>>delight, this conceit, completely ignoring numerical performance... :-)
>
>perhaps you should have listened to his argument for a change.  

i did listen... 

>> i'm amazed that so many people, even in these newgroups, are still so
>> interested in trombe walls, or their passive solar equivalents, like
>> direct gain houses or high-thermal mass sunspaces. 
>
>what you have described is a direct gain air collector with no thermal mass
>for storage.

perhaps you didn't have time to read this carefully.

>>a lot of people are apparently still willing to settle for high-cost,
>>low-performance passive solar house heating techniques, that get them a
>>30% yearly savings in backup space heating costs over a 20 year payback
>>period, vs. warmstores, solar closets, sunspaces and transparent siding,
>>which really can save close to 100% of the space heating energy needed for
>>a house and provide close to 100% of the hot water needed for a house...

>you make these claims yet provide no empirical data to back them up.

i have some data. norman sauders has lots of data, and many people with
firmer grasps on simple high school physics can understand how well this
works without building a thing.

>>>how do you plan to provide for effective daylighting while preventing glare?
>> 
>>how about skylights with reflective south-facing sunscoops over the top?
>
>what are the r-values?  how many lumens do they provide?

perhaps you have heard people say "asking questions is the mark of a fool"? :-)

>what do you do when you have day lighting requirement in the basement
>and 1st floor of a two-story building?

i don't know. what do you do? windows, transparent floors, light pipes,
a walkout basement?

>> >i'm presently looking for windows that block uv radiation...
>> >but are not low-e (because of the amount of infrared radiation they block).
>> 
>> you seem to be seriously confused, will. for solar heating, blocking ir is
>> good, as is passing visible light... 
>
>i don't want to block incoming infrared, which the low-e glazings do.

it is nice to pass solar ir, from 0.78 - 3 microns. it is not nice to pass
80 f ir re-radiation at 3 microns and over... one way to do this is to use
plain old glass in a low-thermal-mass sunspace. another is to use inexpensive
single layer polycarbonate glazing. polyethylene film is not good at blocking
either kind of ir.

>> >uv will fade furniture 
>> something fades furniture next to south-facing windows... is it the visible
>> light or the heat or the tiny amoutn of uv that gets through the glass?
>> at any rate, an intelligently-designed solar home (not yours, apparently)
>> will not have a lot of glass opening into the living space.
 
>you imply that i am not intelligent...

let's just say that you don't seem to understand how to design an inexpensive,
energy-efficient solar house, and you have been bamboozled by psic. that's ok.
ignorance is no crime, save when combined with arrogance, stupidity, rudeness,
direspect, stubborness, short-sightedness and superficial orthodox thinking.
 
perhaps you should take up religion, or law.

still trying to help...

cheers,

nick




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