Thermal Masses for heating efficiency

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Thermal Masses for heating efficiency

Postby zetreque on February 16th, 2015, 6:07 pm 

When thinking about better ways to heat homes, I often find myself thinking about thermal masses, particularly water.

Hydronic heating has been on the rise over the past few years. These systems heat water that flows in plastic piping installed through the floor of your home.

I toured a geodesic dome a year ago in which the owners grow their garden year around despite the harsh winter seasons. This is another technology that is being developed in recent years. Inside of their dome they have a large tank of water which acts as a thermal heat mass throughout the winter.

I also learned about a home near me where the owner applied this concept by having a swimming pool inside of his house which apparently made it comfortable and very cheap to heat during the winter. He had designed south facing windows, and a garden that he basically lived in.

I don't think it's practical for homes to have swimming pools inside however. Pools often need water treatment, maintenance, and cause humidity problems to structures (possibly mold).

This leads me to try to think of an idea for sealed tanks of water inside homes. How much more efficient do you think we could heat our homes if we had a large enclosed tank of water in our homes? Instead of hydronic systems running water through tiny pipes scattered around the house, perhaps just heating a giant water tank in the center of your house? It could even double as the hot water tank for your domestic use. In fancy homes, you could install windows and possibly cool lighting of aesthetic fish tank affects in various shaped tanks.

This is also applying the concept of most "green" homes being built subsurface (sunken into the ground) to make use of a steady temperature under the ground.

Can anyone come up with some pros, cons, or improvements for this specific idea for boosting heating efficiency?

Keywords: heat capacity
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 16th, 2015, 6:24 pm 

How about having a large black tank filled with water outside in direct sunlight during the day while simultaneously having a large tank of water inside. At the end of the day and into the night that water that heated outside is then pumped and exchanged with the water from the tank inside?

Maybe as soon as shade falls on the exterior tank, it could be designed so a photocell triggers an insulation cover to slide over the tank?
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 16th, 2015, 7:19 pm 

I have given this a lot of thought and have settled on mirrors combined with thermal mass.

Underground has the same problem as the swimming pool as lack of sunlight leads to mold (for example my many basements I have owned).

My idea is to avoid the summer heat problem with south facing windows by putting my thermal mass on the north side. South facing mirrors could then focus sunlight on a smaller thermal mass. Less glass and no need for protection from the summer sun are the main advantages. The thermal mass of large tanks of water with black rear surfaces appeals to me because as you pointed out you could circulate the water on demand under various parts of the houses floor.

I never moved to actual design because funds are not available and besides my wife will never leave this dilapidated hundred and twenty year old pile of thermal mass bricks I live in.
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Re: Thermal Masses for heating efficiency

Postby Dave_C on February 16th, 2015, 10:26 pm 

A quick calculation of the amount of heat available from a tank of water:
10,000 gallon tank
Initial temperature: 100 F
Final temperature: 70 F
Heat released: 10,000 Watts for 3 days

That's a tank the size of a smallish swimming pool but large nevertheless. If it were a cylindrical tank, it would be 6 foot in diameter and 47 feet long.

But the amount of heat available from this tank won't get you very far through a winter. Your home may require more or less than 10,000 watts to heat it, but 3 days won't help regardless.

I saw a system once similar to what you're referring to but I think those systems must only use the heat from the water in the evening and then try to put that heat back into the water during the day. For example, if you had a house with a heat pump and needed to run that heat pump at night, I could see some benefit to heating the home plus a water tank during the day and then using the water tank as a thermal mass at night. You could either use the water directly or use the water as a heat source for the heat pump. The reason that might be of benefit is because night time temperatures can easily average 20 to 30 F lower than daytime temperatures making a heat pump relatively inefficient at night. But if you could warm a thermal mass during the day when you had that extra 20 to 30 degrees, you could then use that stored energy at night and the entire system would be more efficient.
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 16th, 2015, 11:01 pm 

Thanks for the responses.

Like I think you are saying Dave_C it appears to increase efficiency best in areas where there are large swings between night time and day time temperatures, and increases even further when pumping the water between either the ground or a solar operation like Wolfhnd is talking about.

The other thought is using it to keep constant temperature. A major inefficiency of heating and cooling systems is the start-up of appliances. Having a steady energy source heating your thermal mass might increase efficiency because it reduces cycle time.

Wolfhnd, you are welcome to design a system for me :D
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Re: Thermal Masses for heating efficiency

Postby Darby on February 16th, 2015, 11:32 pm 

I have to replace my 70 year old boiler this coming spring, and one of options I've been considering is a micro-CHP fuel cell system that directly converts natural gas into electricity and heat without having to combust it.

Example system: http://www.cfcl.com.au/BlueGen/

They're still quite expensive yet, and havent yet been optimized for small scale residential applications, so I may have to wait another 5-10 years for the tech to evolve further and before the price drops within reach, but it would be very cool to be able to live off the grid in the heart of suburbia, and sell power to the power company.
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 16th, 2015, 11:37 pm 

That would be awesome Darby. Combined Heat and Power (CHP) increases efficiency a lot. I too can't wait for when the price of those systems drops.
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Re: Thermal Masses for heating efficiency

Postby Darby on February 16th, 2015, 11:47 pm 

Right now the tech is starting to penetrate places like hospitals, car washes, supermarkets, offices and apartment buildings. One of the limiting factor with a micro-CHP that's slowing the penetration into the residential market (aside from availabiity and people rated to install and maintain them) is lifespan of the fuel cell matrix and how to max efficiency by fully utilizing all the waste heat. A 100 liter domestic hot water tank only moves the efficiency from from 60 to 85ish ... if money was no object I'd put in a swimming pool and get a Tesla electric car ... then I could run the system flat out, 24/7, sell all unused power to the power company, and dump all the excess heat (well in excess of domestic hot water needs) into the pool, and/or coils for a heated driveway, and the efficiency would be in the low 90's. That would be cool, but it's well beyond my current fiscal reach.

Better still would be a technological advance that allowed the system to directly convert some of the excess waste heat directly into more electricity ...

In any case, I think comparable systems like these are all still pushing like $ 50k+, which is 3-5x higher than the point where it will really begin to take off.
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 16th, 2015, 11:59 pm 

Yes,

As a hobby I did a whole study on how to implement a CHP system at a resort I worked for. I sized the system for the heat load of the pools and building (opposite of what you typically do), and had tons of excess energy. I talked with the local power company about the idea trying to convince them to support it because it would give them a new powerplant in the area stabilizing the grid (we get several power outages a year here). I also live in an area where it would be impossible to zone a power plant due to environmental regulations, but because of the cleanness of the system, they could do it.

One of the systems I used in my analysis output
1400kw
and
3.7 million btu/h at 120F
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Re: Thermal Masses for heating efficiency

Postby Darby on February 17th, 2015, 12:37 am 

Only 1400 kwh ? That a typo ? I'd guess at least 14000 kwh size, and for something like a resort I'd assume you'd probably need several running in tandem, depending on the size of the resort.

I think I figured my annual electric needs (as a residential homeowner) as roughly 7000 kwh/yr, and since the bluegen I linked generates 13000 that's 6000 kwh of excess capacity (if run flat out, 24/7) available for resale.
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 17th, 2015, 12:52 am 

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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 17th, 2015, 2:14 am 



If I did the math right that is enough for maybe 50 average homes?
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 17th, 2015, 2:16 am 



Sounds about right.
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 17th, 2015, 2:20 am 

Here is the cooling thermal mass idea lol

http://www2.buildinggreen.com/blogs/mak ... -buildings
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 17th, 2015, 2:23 am 

We could just get together with our neighbors and go nuclear.

http://www.nuscalepower.com/

50 megawatts is a lot of neighbors though :-)
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 17th, 2015, 2:25 am 

Dave is such a kill joy with all his practical comments JK Dave
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 17th, 2015, 3:08 am 

Cheap and easy to build but it could kill you or set you house on fire or so the disclaimer implies.

A TRACKING SOLAR CONCENTRATOR

http://www.ida.net/users/tetonsl/solar/page_iii.htm
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Re: Thermal Masses for heating efficiency

Postby Darby on February 17th, 2015, 7:46 am 

Ah, I was using kilowatt hours, and you were using kilowatts.

That beast generates in 5 hours as much power as I use in an entire year. ;)

re: 7000 kwh/1400 kw = 5 hrs of output
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Re: Thermal Masses for heating efficiency

Postby Dave_C on February 17th, 2015, 10:37 am 


I like the idea of using the phase change of water. That's a great way to store thermal energy, in this case for air conditioning. But you could do the same at higher temperatures for heating. Unfortunately, I don't think there are any fluids that change phase (solid / liquid) that are nearly as economical as water, but perhaps something like a custom tailered wax.
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Re: Thermal Masses for heating efficiency

Postby Dave_C on February 17th, 2015, 10:46 am 

Darby » February 16th, 2015, 10:32 pm wrote:I have to replace my 70 year old boiler this coming spring, and one of options I've been considering is a micro-CHP fuel cell system that directly converts natural gas into electricity and heat without having to combust it.

Example system: http://www.cfcl.com.au/BlueGen/

They're still quite expensive yet, and havent yet been optimized for small scale residential applications, so I may have to wait another 5-10 years for the tech to evolve further and before the price drops within reach, but it would be very cool to be able to live off the grid in the heart of suburbia, and sell power to the power company.

Hi Darby, I work in the industrial gas industry working primarily to supply hydrogen to fuel cell applications such as the material handling market (ie: forktrucks / forklifts) and the emerging automotive market. I wonder if providing hydrogen to fuel cell powered homes, similar to the oil or propane business, would be economical. It gets rid of a lot of capital and problems with operating and maintaining your own natural gas reformation unit, but you still get all the benefits of power with excess heat.
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Re: Thermal Masses for heating efficiency

Postby Dave_C on February 17th, 2015, 10:56 am 

Idea: Install a 25 gallon water tank in your car (about 206 pounds of water) and heat it using the waste heat from your car. If a car had let's say 10 hp of waste heat for 1 hour (typical drive day?) you could recover roughly 7500 watts. Not a tremendous amount but pretty easy to capture. Water temperature would rise from 70 F to about 195 F.
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Re: Thermal Masses for heating efficiency

Postby Darby on February 17th, 2015, 11:44 am 

Hi Darby, I work in the industrial gas industry working primarily to supply hydrogen to fuel cell applications such as the material handling market (ie: forktrucks / forklifts) and the emerging automotive market. I wonder if providing hydrogen to fuel cell powered homes, similar to the oil or propane business, would be economical. It gets rid of a lot of capital and problems with operating and maintaining your own natural gas reformation unit, but you still get all the benefits of power with excess heat.


Hi Dave,

I don't really know enough about the industrial gas industry to have an informed opinion, so I'm pretty much limited to conceptualization and conjecture as I write this.

I would imagine that the business options for delivered hydrogen would be very different for that of fuel-cell based vehicles than it would for fuel-cell powered buildings. The former, by definition, carry a limited & refillable fuel supply with them, and thus it seems feasible deliver and sell same so that such vehicles can carry and utilize it. The latter however involves larger non-moving structures whose far greater needs are better met by direct connection via gas lines to local utilities.

An off-the grid house however, would seem to have a limited enough rate of annual consumption that deliveries could be a viable option. However, the problem I see is one of internal compatibility. If a rural home or small building has already been rigged for propane storage because there are no gas lines nearby (as would be more typical in a suburban home or an urban setting), and if they were considering a CHP system, it seems to me like it would make little sense to add a separate system for a second type of gas when they could just the propane they already have. I think that market would likely be an uphill climb.

Again, I dont know the industry. Perhaps someone else will chime in who is better informed.
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 20th, 2015, 7:19 pm 

Here is an idea I came up with.


Image
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 20th, 2015, 8:06 pm 

That's pretty cool, your greenhouse protects the side of the house and possibly adds light inside. It would only work for specific locations though. The problem with green building is every home has to be built specifically to the location. There is no one size fits all unfortunately.
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 20th, 2015, 9:01 pm 

The problem with thermal mass homes is aesthetics, light and air. If it were not for these factors a cave would work just fine.
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 20th, 2015, 9:08 pm 

Here is something we looked at in class today (R values for insulation).

Insulation though is a different topic than thermal mass. They are related though maybe when you have a wall facing the sun maybe though.

http://www.coloradoenergy.org/procorner/stuff/r-values.htm
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Re: Thermal Masses for heating efficiency

Postby Dave_C on February 20th, 2015, 10:15 pm 

zetreque » February 20th, 2015, 8:08 pm wrote:Here is something we looked at in class today (R values for insulation).

Insulation though is a different topic than thermal mass. They are related though maybe when you have a wall facing the sun maybe though.

http://www.coloradoenergy.org/procorner/stuff/r-values.htm

Insulation often works by trapping air and preventing gross movement. Take a look at the insulations on that page and see which ones are trapping air. If you can stop the air from moving, you can stop it from transporting heat from the hot side to the cold side (ie: stopping convective heat transfer). You'll find typical insulations have a thermal conductivity that's close to the thermal conductivity of air. That's why so many of those insulating materials in the first column of the second table have roughly the same value. The solid part of the insulation, be it glass fiber, open cell foam or whatever has only a small impact on thermal conductivity of typical housing insulating materials.

If you use the insulation in a different gasseous environment therefore, such as helium or hydrogen, the thermal conductivity of those materials changes dramatically because the thermal conductivity of the gas changes.

Of course, the best way to insulate something is to remove the air altogether and create a vacuum. Very hard to do with a flat panel though because of stresses on the materials needed to confine the vacuum. Because of the cost of something like that, you won't see vacuum panels used in homes, though I've always thought it would be a neat way to improve a home's insulation.
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Re: Thermal Masses for heating efficiency

Postby zetreque on November 1st, 2015, 4:33 pm 

Thinking about this topic again and thinking about the rate of heat loss from the water tank into the room air.

Say you have an enclosed room heavily insulated to the outside.
Inside of that room is a large tank of water. This tank of water is perhaps hanging or supported by supports so that it does not have that much direct contact with the floor, ceiling or surface of the walls to loose heat through conduction.

The larger the volume of water, the more heat is stored of course. Also if you were able to insulate a certain percentage of that tank surface so that you have a part of that tank surface that more easily transfers heat to that room you can figure out how to design it appropriately as to not overheat your room.

It is a confusing thought thinking about how you can have a really hot tank of water sitting there, but your room air will not be as hot. This is due to the thermal conductivity of the air right? I think that is something I need to really understand when designing my system.

Air is a poor conductor of heat so I am curious about how long it would take a room to heat up given a certain temperature and insulation factor of your water tank. Any thoughts?
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on November 2nd, 2015, 11:44 pm 

Image

Windows are not very efficent insulators so reflectors give you the option to keep them small. Reflectors also solve the what to do in summer problem better than blinds as the window can still be an aesthetic element on the north side of the building. A closed tank does not appeal to me as it would be difficult to clean and any dark surface area will have minimal contact with the water. You could use a couple of solar powered water pumps because you want the water fall to just be a thin sheet of slowly moving water.
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Re: Thermal Masses for heating efficiency

Postby zetreque on November 3rd, 2015, 12:18 am 

open system will have evaporation and humidity problems though.
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