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Engineering questions? HVAC issues? This is the place to dwell on technicalities.

Postby flaja » Sat Feb 25, 2006 9:47 am

American Ingenuity’s website claims that thermal mass does not lead to any savings in energy costs because you have to expend energy to either heat up or cool off thermal mass before you can heat or cool the air. If you want to warm a room you have to use energy to heat the thermal mass, and if you want to cool a room you have to use energy to counter any heat the thermal mass has stored. And you also have an uncomfortable space for much of the time because of the long heating-cooling cycle caused by the thermal mass.

I know from personal experience that thermal mass in Florida is not a good thing. I grew up in a house that had a wooden floor set on concrete block pillars. The house could get very cold and very hot, but it never took the ac or heater very long to make the house comfortable. But, I now live in a house that is on a concrete slab foundation. Since it takes so long for the slab to absorb or release heat I often have to have the ac running in October when outside temperatures are comfortable and I also have to have the heater on in May even though the outside temperatures have been reaching 90 degrees by the end of April.
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Postby gearweasle » Sat Feb 25, 2006 10:40 pm

flaja wrote:I know from personal experience that thermal mass in Florida is not a good thing. I grew up in a house that had a wooden floor set on concrete block pillars. The house could get very cold and very hot, but it never took the ac or heater very long to make the house comfortable. But, I now live in a house that is on a concrete slab foundation. Since it takes so long for the slab to absorb or release heat I often have to have the ac running in October when outside temperatures are comfortable and I also have to have the heater on in May even though the outside temperatures have been reaching 90 degrees by the end of April.


I'm not so sure I'm with you on this one, flaja. I think you're describing an air
infiltration/heavy humidity situation, and the concrete is just wicking out
the heat and having to be cooled off again -thermal weight or no, it's just
not insolated from the heat.

Two examples in my mind. They recommend in Minnesota, for enegy
conservation, to put used milk jugs of water in your refridgerator, instead
of having an empty refridgerator, to make it more efficient. That way the
compressor doesn't kick so often, trying to keep the fridge at the proper
temperature. That's thermal mass.

Also, I'm sure down in Florida you guys have those big 30gal ice chests.
If you have two chests that are the same, and start them out at
50 degrees; and then add 1) a 50 degree ham sandwich to one chest, and
2)6 24-pack cases of 50 degree Coke to the second; then the chest of the
ham sandwich will warm up to room temperature real fast as soon as you
set them both out. The Cokes will lag far behind. They're the concrete.


Where you are quite right, is the humidity problem, coupled with the need
for a constant, though slow, fresh air exchange for the house. All that air
will need to be conditioned. Constantly, I guess.

--gearweasle

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Postby flaja » Sun Feb 26, 2006 8:32 am

gearweasle wrote:Two examples in my mind. They recommend in Minnesota, for enegy conservation, to put used milk jugs of water in your refridgerator, instead of having an empty refridgerator, to make it more efficient. That way the compressor doesn't kick so often, trying to keep the fridge at the proper
temperature. That's thermal mass.


Did the compressor not have to run longer than it would have in the first place in order to cool the thermal mass?
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Postby Insight Driver » Sun Feb 26, 2006 2:38 pm

Consider just a box made of plywood. Next to it put an identical box, but coat the outside with 4" of polyurethane foam. Run an airconditioner in each box. Which one will use the most electricity? Clearly the one in the box without insulation. This is, in a highly-simplified form, the principal in a monolithic dome. It is a highly-insulated structure. Once the interior space is properly conditioned it takes much less energy to maintain that temperature because it is highly insulated from the outside temperature.

It does take time to stabilize a high-mass insulated structure, of any form, whether it be poured in place walls, airform monolithic dome or tilt-up prefabricated walls. The point is it takes less long-term energy to maintain the interior temperature than it would in an un-insulated box.

As far as a slab is concerned, it also must be properly insulated or it is just a large heatsink tied to the earth temperature. While it is common in northern climates to insulate the slab, I don't think it is common in Florida, so you end up with a slab that constantly sinks or sources heat, depending on whether it is warmer or cooler than the air inside the box you are trying to condition.

The subject of heat flow is a science and it may not be intuitive. The monolithic dome structure is a highly insulated mass. In any environment it takes less energy to maintain a constant indoor temperature in such a structure versus any structure with mass or not that is not insulated as well from the outside environment.

The example of the insulated chests is germain. An empty chest will not stay cold inside for as long a time as a chest full of ice because it takes a lot more energy leaking into the chest to bring the ice up to the outside temperature. The insulation is key. The mass inside the monolithic dome soaks up the daily outside fluctuations in temperature that leak through the insulation. This means the air conditioner for the space inside does not have to cycle on and off as much as it would if it were just an insulated box without mass inside. Basically all you have to do now, once the mass of a monolithic dome has been stabilized is to cool what heat the leaks through the insulation, but not having it change rapidly due to outside air temperature and insolation changes it just reacts to the slow change caused by the mass that is able to act as a sponge that holds heat energy.

As a layman I do not have good examples to give. I do have, however, some textbooks that quantify the energy transfer in measured units.

I suggest you start here:

http://www.monolithic.com/thedome/climate/index.html

From here you can find information that answers your questions better than I am able to.
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Postby gearweasle » Sun Feb 26, 2006 6:10 pm

flaja wrote:
gearweasle wrote:Two examples in my mind. They recommend in Minnesota, for enegy conservation, to put used milk jugs of water in your refridgerator, instead of having an empty refridgerator, to make it more efficient. That way the compressor doesn't kick so often, trying to keep the fridge at the proper
temperature. That's thermal mass.


Did the compressor not have to run longer than it would have in the first place in order to cool the thermal mass?


Yes. You have to cool all that mass initially. Then just keep the temp from
leaking out thru conduction, convection, or radiation of heat.

You're explaining a good strategy, but it depends on what your situation is.
We use both strategies, here in Minnesota for winter; one for the car, one
for the monolithic house.

The car is used in spurts with long periods of being unused, and exposed
to the temps (cold in winter, hot in summer). Having a light-weight double
wall with no insulation to speak of in your car, allows the heater/AC to
quickly effect the surge of new temperature. The light-weight wall
then starts re-radiating the new temp back into the passenger
compartment, letting the user get quick results. Thermal mass is bad for
quick change. It wants to stay cold or hot or what ever it is. And if you
use it in spurts, you waste your effort to get the thermal mass up to
temperature and stay there.

In the house, however, you want to use it for long periods of time, even
when you're not there, to keep things at room temperature. You don't want
change. Therefore, you do use insulation and you use thermal mass to
keep things that way, for the long periods of time, like a half year.

Also, high humidity weather really accelerates the temperature change in
a house. Up here, we have both hot-dry spells of weather, and hot-humid
spells of weather. In a somewhat tight, but not well insulated house -if
you keep the doors and windows closed-, the house will take 3-4 days to
heat up by itself during a hot-dry spell. A fast day or less if it's hot-humid.

I can't imagine endless days of summer in high humidity anymore. And
that concrete slab you lived on was just wicking the heat of the ground
into your house. You needed to insulate the slab on the bottom as
well as walls and roof.

--geaweasle

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The hyper-efficient, safe-school-building

Postby hitssquad » Sun Feb 26, 2006 7:15 pm

flaja wrote:I now live in a house that is on a concrete slab foundation. Since it takes so long for the slab to absorb or release heat I often have to have the ac running in October when outside temperatures are comfortable

There is something missing from your anecdote, and that is: what heat source made the slab uncomfortably hot and how would you have dealt with that heat source if the slab had not been soaking up the heat.

If the answer is approximately, "What made the slab uncomfortably hot was the sun streaming in through the windows during the day, I time period when I wasn't home and therefore didn't have a concern for the house being too hot since I would have been easily -- without the slab -- able to cool the house off at night by simply opening the windows," perhaps the design of your southern-exposure window visor could stand improvement in such a way, as Cloud Hidden (Jim Kaslik) has suggested elsewhere on these forums, that the visor blocks the high sun in the summer but allows the low winter sun to enter and hit the slab.

As for your school, I would design it without any windows at all. There would be less distractions that way for the students, and in case of an atmospheric overpressure event emanating from outside the students would not be impaled by millions of high-velocity glass shards. Without windows, the school would be cheaper and faster to build, and cheaper to HVAC (with also a smaller up-front investment for HVAC hardware). The school buildings would be their own disaster-resistant safe-rooms. (But you would need to make sure you had at least minimal back-up power for lighting in case of a power failure, for the security of the students.)
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Postby flaja » Sun Feb 26, 2006 9:52 pm

Insight Driver wrote:Consider just a box made of plywood. Next to it put an identical box, but coat the outside with 4" of polyurethane foam. Run an airconditioner in each box. Which one will use the most electricity? Clearly the one in the box without insulation. This is, in a highly-simplified form, the principal in a monolithic dome. It is a highly-insulated structure. Once the interior space is properly conditioned it takes much less energy to maintain that temperature because it is highly insulated from the outside temperature.


This is precisely my point. A layer of concrete would provide much more thermal mass, i.e., heat storing ability, than plywood would. But, the main benefit to a monolithic concrete dome is its insulation, not its thermal mass.

As far as a slab is concerned, it also must be properly insulated or it is just a large heatsink tied to the earth temperature. While it is common in northern climates to insulate the slab, I don't think it is common in Florida, so you end up with a slab that constantly sinks or sources heat, depending on whether it is warmer or cooler than the air inside the box you are trying to condition.


I can’t say if the slab for my house is insulated or not since it was an existing house when I moved in.

The example of the insulated chests is germain. An empty chest will not stay cold inside for as long a time as a chest full of ice because it takes a lot more energy leaking into the chest to bring the ice up to the outside temperature.


Civil preparedness advice is that you should keep your refrigerator and freezer as full as possible so a short-term power failure won’t harm the food. But, it is better to use food as the thermal mass since you have to cool the food anyway.

The insulation is key. The mass inside the monolithic dome soaks up the daily outside fluctuations in temperature that leak through the insulation. This means the air conditioner for the space inside does not have to cycle on and off as much as it would if it were just an insulated box without mass inside.


But, once the concrete mass has absorbed heat that has gotten through from the outside, where does it go? Does it pass back through the insulation to the outside, or does it simply radiate into your living space?

Basically all you have to do now, once the mass of a monolithic dome has been stabilized is to cool what heat the leaks through the insulation, but not having it change rapidly due to outside air temperature and insolation changes it just reacts to the slow change caused by the mass that is able to act as a sponge that holds heat energy.


And in Florida you would still have to run the ac constantly because at some point the thermal mass is going to be saturated with heat and it will send heat into the space you are trying to cool.
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Re: The hyper-efficient, safe-school-building

Postby flaja » Sun Feb 26, 2006 10:09 pm

hitssquad wrote:
flaja wrote:I now live in a house that is on a concrete slab foundation. Since it takes so long for the slab to absorb or release heat I often have to have the ac running in October when outside temperatures are comfortable

There is something missing from your anecdote, and that is: what heat source made the slab uncomfortably hot and how would you have dealt with that heat source if the slab had not been soaking up the heat.


In my part of Florida it is very common to have winter days that are windy, rainy with the day’s high never making out of the 50s. On such days I usually have to have the heat on all day. But, the next day could easily be clear, sunny and with a temperature in the 80s. On that day my house is uncomfortably warm because of all the heat that the slab absorbed. I had to spend the previous day using the heater to warm the slab before the air got comfortable and then when the temperature makes it into the 80s outside the next day, I have to run the ac to make the house comfortable again.

If the answer is approximately, "What made the slab uncomfortably hot was the sun streaming in through the windows during the day,


My lot used to have several pine, cedar and oak trees, which I removed because of the pollen and hurricane risk. The yard had heavy shade.

I time period when I wasn't home and therefore didn't have a concern for the house being too hot since I would have been easily -- without the slab -- able to cool the house off at night by simply opening the windows," perhaps the design of your southern-exposure window visor could stand improvement in such a way,


My house is a ranch style that has a wide overhang over the southern windows, so they do not get much direct sunlight. And the few northern windows I have are still heavily shaded in the summer.

As for your school, I would design it without any windows at all. There would be less distractions that way for the students, and in case of an atmospheric overpressure event emanating from outside the students would not be impaled by millions of high-velocity glass shards.


I have thought of this, but since the school will cater to academically gifted students, distractions by windows may not be a problem. However, I used to teach at a school that catered to students with learning disabilities such as ADD (adults didn’t discipline). The school had no windows at all and a lot of times I got too much of a boxed in feeling.

BTW: The windows at every public school I ever attended in Florida had plexiglass rather than glass since public schools here are used as hurricane shelters.
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Postby Insight Driver » Sun Feb 26, 2006 11:00 pm

The insulation minimizes the heat transfer into the interior space. That said, the interior space of an insulated box with no mass will ramp up and down with the outside temperature changes. An air conditioner will run more during the day when it's hot and humid and less during the slightly cooler night. What the mass does for you is it averages out the heat gain so that the heat of the day is not radiated into the space through the leakage of the insulation. Your air conditioner then runs at a nearly constant rate. It is the properly-sized air conditioner in a highly insulated space that is more efficient if it runs nearly constantly versus starting and stopping frequently in the hottest parts of the day.

The thermal mass sits, at an average temperature of the interior space you are cooling. The heat of the noon sun in the summertime beating down on a monolithic dome cannot cause the air temperature in the interior to rise because it has to heat up the concrete first to get through to the interior. Meanwhile the air conidtioner is running steady-state taking the seasonal average difference between the interior and exterior, not the daily highs and lows.

The insulation reduces the heat load, the mass averages the heat load.

Please read the links provided to learn more about heat transfer in a dome and the two batteries in a monolithic dome.
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Postby Insight Driver » Sun Feb 26, 2006 11:36 pm

I want to add some more considerations. A stick-built house has a much greater degree of air infiltration than a monolithic dome. Reading about energy star techniques for a standard house, one thing recommended is to seal leaks to minimize air infiltration. Obviously if the outside hot air is leaking into the house, no amount of insulation in walls will reduce the amount of air conditioning required to cool the space.

Heat is transferred by conduction, convection and radation. Radiant heat is the visible sun beating down on a structure. If you have windows facing south you get a lot of radiated heat coming in through those windows. Conducted heat is transfer through materials. A slab on grade, un-inulsated, will sit at the ground temperature of the site. Ground temperature varies depending on type of soil, percentage of moisture, ground cover and seasonal factors. If a seasonal ground surface temperature in Florida, for example, in the summer is 75 degrees then the slab will tend to sit at 75 degrees.

convection is the heat that is emitted from a surface. Convection always goes from hotter to cooler until the difference in temperature is equalized. A loaf of bread taken out of an oven will make your face feel warm from the convection of heat in the bread.

Passive solar design takes seasonal changes in heating and cooling into account. Part of passive solar design is to moderate daily changes based on daily sun cycles. Obviously, in Florida, in the summer, radiant heat is a dominant factor. Shading is effective. Light colored surfaces absorb heat at different rates based on their reflective characteristics and their color. A flat black surface absorbs radiant heat readily. In Florida you want a light-colored reflective surface on a dome to help reflect radiant heat. In Florida you don't want south facing windows to allow the radiant heat of the sun in. North-facing windows are ideal in Florida because only diffuse light enters the dome, not the direct radiant heat of the sun. Shading of south-facing windows is important.

The greater the rate of change in temperature inside a dome, the greater energy required to maintain that rate. The mass inside the dome reduces the rate of convection into the air space of the interior. An air conditioner does not have to run at full blast as it would if you left it off for a week and the interior temperature climbed up into the 80,s. Not only would the air conditioner have to remove the heat that is in the air volume, but it has to constantly cool the concrete that has also risen up to approximately that 80 degree point. The benefit of mass is that it reduces the rate of temperature change. Only conducted heat gets through the insulation and the concrete. Because they are both poor conductors, what happens is the daily fluctuations in radiant heat from the sun get averaged out in the concrete such that this effect happens: if a dome were stabilized at 70 degrees and the air conditioning shut off it would take days before the interior temperature would rise three degrees in spite of the fact that the outside temperature has been in the 100's in the daytime and mid 80's at night. It is this thermal inertia that makes air conditioning load less in a monolithic dome.

If one were to get into the mathematics of it, the calculations would take into account the libido of the surface of the dome (degree of reflectivity). The insulation thickness then would determine how much heat is conducted into the concrete versus how much is re-radiated into the atmosphere through the dome coating at night when convection would cause heat to transfer back out because the surface of the dome would be cooler now. The insulation would limit how much daily energy can leak into the concrete. The mass of the concrete then can be taken and the rate of temperature change inside the dome can be calculated. What you aim for in passive solar design is that the rate of change inside is very slow, in the matter of days, where outside the rate of change is within a 24 hour cycle and can vary plus and minus 60 degrees (taking into account both radiant heat of direct sunlight and contributions from conducted heat and convected heat.

Think of the heat shield tiles on the shuttle. Take the conduction of the tiles, calculate the load of the aluminum sub-structure of the shuttle. Size the air conditioning inside such that the heat of re-entry will not raise the interior temperatures of the shuttle to an unsafe level for human life. Because the tiles are such great insulators the air conditioner does not have a huge heat load to shed. The same effect is what you get in the insulation of the monolithic dome. The intense sunlight cannot get through the insulation easily. The concrete mass averages out the rate of change of the interior temperature. It does not take a huge air conditioner to maintain a comfortable indoor temperature.

In a monlithic dome the air infiltration is minimized. The leakage path is through penetrations through the dome only. Radiant heat is deflected, both by the libido of the surface of the dome and also the shape of the dome (a flat surface perpendicular to the sun would have more radiant heat absorbed where much of a domed surface would reflect the radiant heat due to it's angle away from perpenducular). Finally very poor conduction of heat occurrs through the insulation. Convective heat is averaged in the concrete mass. The air conditioner, once the air and concrete are stabilized has a fairly constant heat load to deal with. That constant heat load is roughly 15 percent of the heat load you have on a box structure, typical.

From all the information on the monolithic dome web site I've gained this understanding, and from some passive solar design engineering textbooks I have.

Far from being of no benefit, the mass in the dome is of direct benefit in reducing the energy requirements to maintain a comfortable air temperature in the dome. Even in Florida.
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Postby Insight Driver » Sun Feb 26, 2006 11:51 pm

One more time I have another thought.

Consider the action of freezer. When the freezer is first turned on, it takes a hour or so to get the temperture down to it's setpoint. For arguements sake, let's say the air temperature was 70, the freezer setpoint was zero and it took an hour to decrease the freezer temperature to the setpoint. The rate of change would be 70 degrees per hour. The compressor would be running full blast for that hour, then would turn off and begin cycling. The amount of insulation will determine the rate of temperature change when the compressor is shut off.

If the freezer then is filled with mass, it will then cause the compressor to turn on and run as long as it takes to reduce the temperature of the mass to zero. Also, if you have a lot of mass in the freezer, it takes a longer time before the freezer temperature will rise until the compressor turns back on. The mass of food reduced the rate of temperature change. The leakage of the insulation determines how much energy is leaking into the freezer

The monolithic dome is in effect, a huge freezer compartment, highly insulated and filled with a mass that reduces the rate of temperature change.
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Black in the visible spectrum vs black in other bands

Postby hitssquad » Mon Feb 27, 2006 8:16 am

Insight Driver wrote:A flat black surface absorbs radiant heat readily.

There is minimal relationship between blackness and albedo in terms of radiant heat. This is because radiant heat is technically far-infrared only. What would be important is whether or not a surface is black in far-infrared illumination, and that is not determinable simply by viewing it in visible-spectrum lighting.

Since light-colored (in terms of visible illumination) things reflect visible light well, they receive less heat from visible light; but that is not the same thing as reflecting thermal radiation.
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Postby Insight Driver » Mon Feb 27, 2006 12:36 pm

Albedo is effective for radiant energy, such as that from the sun. As far as being an emitter of heat, I agree, it does not matter. As an emitter of heat that is convective the body color has minimal relationship.
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Postby Missouri Dome Builder » Mon Feb 27, 2006 7:17 pm

:lol: My dome is north of I 70 in north Mo. It was my plan to run a small airconditioner only at night when the cooler outside air makes the ac more efficient. It also pays to put compressor in the shade. But guess what?? With my thermal mass it has never reached a temperature where the ac was needed. The warmest it has been is 74 dgrees. At that temperature a ceiling fan is enough to keep people comfortable. Note, that even with the largest outside temperature change, the inside temperature will not change more than a degree in one day. Some domes will change more than this and the reason is because of the mass of the thermal battery. I have 6" thick walls.
I feel sure the same idea would work in Florida. Only run the ac at night and the mass will keep the house cool through the day.

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Postby flaja » Mon Feb 27, 2006 7:51 pm

Missouri Dome Builder wrote::lol: My dome is north of I 70 in north Mo. It was my plan to run a small airconditioner only at night when the cooler outside air makes the ac more efficient. It also pays to put compressor in the shade. But guess what?? With my thermal mass it has never reached a temperature where the ac was needed. The warmest it has been is 74 dgrees. At that temperature a ceiling fan is enough to keep people comfortable. Note, that even with the largest outside temperature change, the inside temperature will not change more than a degree in one day. Some domes will change more than this and the reason is because of the mass of the thermal battery. I have 6" thick walls.
I feel sure the same idea would work in Florida. Only run the ac at night and the mass will keep the house cool through the day.

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I doubt that any house in Florida (even a dome) would have a naturally occurring maximum indoor temperature of 74 degrees (or even 84 degrees) anytime from June through September. And I would venture that the reason your dome's inside temperature does not have drastic changes is that the outside temperature does not have drastic changes. During the winter my part of Florida can easily be 30 degrees at day break and then be 80 degrees by mid-afternoon. And in the summer our daily temperature range can be very narrow- upper 70s to mid-90s. And because of the humidity the air feels hotter than it really is because it cannot lose its heat. So without ac, your house would get hotter and hotter.

The owner of Emega Technologies, maker of soy-foam building blocks, has told me that a house built with these blocks in Akron, Ohio has only a 15 degree temperature difference from the coldest temperature in the winter to the warmest temperature in the summer. And, an Emega block building relies on insulation rather than thermal mass.
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