BTU's. I looked online for a way to do this but only came up short (that's what she said, again and again). I want to try to figure out many BTU's my solar collector is putting out. I assume I'd need to know the temperature of the air (got that) and the amount of air coming out. So, I guess I have two questions. How do I figure out the volume of air coming out to go along with the temp to figure out BTU, then what do I do with those numbers?
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Jan. 22, 2012 7:13 p.m. DrBoost SuperDork
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Jan. 22, 2012 7:19 p.m. ransom Dork
Hrm.
Sounds like you need a MAF sensor for your solar collector?
I would think you'd also need to know the temp of the air going in. Though I guess all you need for that is a thermometer in the room...
Aren't BTUs a measure of thermal energy?
The upshot its that you'd need to know how much air you warmed and by how much you warmed it to know how much energy was added.
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Jan. 22, 2012 8:12 p.m. FlightService Dork
I hope your solar collector isn't putting out any BTU's
Now smartassed-ness is over, ransom is right. Get your delta and your volumes.
BTU/hr. = CFM * delta-T * 4.5
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Jan. 22, 2012 10:23 p.m. DrBoost SuperDork
Ah, yes you guys are right. I need to know both temps. I got about 60 degree temp increase. So FlightService. If I understand your formula correctly, my BTU/hr equals the CFM (flow, correct?) times temp difference, multiplied by 4.5?
Since I'm relying on convection (no fan specs to use) how do I figure out CFM? I'm dying to know how many BTU's this heater is, providing.
Oh, please explain the smartassed-ness. I just want to understand the terminology I should be using and why. -
Jan. 22, 2012 11:34 p.m. ransom Dork
In reply to DrBoost:
I may be just about to make myself look dumb, but I assume he's pointing out that your collector should be, based on its name, taking in BTUs.
I need sleep too badly to follow through on this, but I wonder if you can figure out an estimate of CFM based on the temperatures at the top and bottom of the collector, the temperatures at the top and bottom of the room out in the middle, and the cross-sectional area of the collector (or the tightest restriction in the airflow path).
Seems like it would be pretty rough, but the temperatures would give you the ability to calculate relative density, and the pressure should be more or less constant for all four points. What I don't know how to find is the convective effect.
Perhaps this calculator can help, though it would appear to take the "outside" (collector?) and "inside" (living room) temperatures as single values. I'm not sure what to call the gradient of temperatures you have going on in the collector. Do you use the max value? Average value? I would guess the former, but I'm really not sure.
I do, however, suspect that by now you really wish someone who actually knew what they were talking about would chime in
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Jan. 23, 2012 7:23 a.m. DeadSkunk Dork
Start with one of these, www.pro-measure.com/SearchResults.asp?Cat=48
You need to measure the velocity of the air exiting your collector. Take several readings across the opening. Normally flow gradients look parabolic. You'll get an average velocity and then you can calculate the volume using the flow rate and opening area. Plug that CFM number into FlightService's formula.
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Jan. 23, 2012 12:18 p.m. Schmidlap HalfDork
Instead of worrying about the speed of the air flowing through the collector just put a thermometer inside your collector and seal off any openings. Estimate the mass of the air inside from the dimensions and measure the change in temperature of the air over time. Throw those numbers into a standard heat transfer equation (I'll try to find the right one) and you'll have a rough estimate which is probably a lot more accurate than if you were trying to estimate CFM.
On edit: Off the top of my head, BTU/hr = (change in energy)/(change in time) and (change in energy) = (change in temperature) * (specific heat of material) * (mass of material) so you'd need to find specific heat of air, and the mass of the air in your collector. Work through the equations making sure you've got the right units and you'll get an estimate of the BTU your collector is collecting from the sun. I would recommend doing this after the temperature of the collector has stabilized so that you don't have to worry too much about how much heat it is absorbing from the sun (it will be approximately constant).
Does my initial equation make sense? It's been a long time since I've done any kind of temperature calculations.
Bob
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Jan. 23, 2012 12:51 p.m. SVreX SuperDork
Your solar collector isn't putting out any BTU's, it's collecting them. The rate of collection will vary greatly based on weather, ambient temperature, flow rates, etc.
You then have a distribution system which is distributing what was collected. Your losses from both your collection system and your distribution system will effect usable energy.
What are you trying to compare to? Are you trying to estimate reduced load on your electric meter, or compare to the heat output of your boiler, or what?
I am assuming you have built an air/air solar collector. Is that correct? Or is it air/ water, or photovoltaic, etc?
Is the heat circulated by natural thermal currents, or are you using a mechanical device (fan, etc) to move the heat?
Hard to answer your question the way you asked it.
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Jan. 23, 2012 1:17 p.m. DrBoost SuperDork
SVreX wrote:
Hard to answer your question the way you asked it.
Yeah, not surprised since I don't really know what I'm doing. Here's what I want to figure out. How many BTU's am I putting into my room when this collector/heater is doing it's job. I have diesel fired heaters that are for vehicle applications and they are rated in BTU' and I'd just like to know how this compares, or how it compares to a space heater.
I appreciate the help guys. I plan on building some larger one's and the ability to estimate how much heat I'll have would be helpful in the future. -
Jan. 24, 2012 6:25 a.m. FlightService Dork
Sorry for the delay.
find something that will measure wind velocity at the exit. figure the cross sectional area of the location measured.
the down side to fan data is that it doesn't take into account losses across the path of the fluid you are moving.
If you aren't using a fan and just movement due air expansion and you know the air, delta, altitude you can probably assume expansion is the movement.
As SVreX said difficult to detemine.
pics?
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Jan. 24, 2012 7:23 a.m. foxtrapper SuperDork
Even with all the fancy detection equipment, you will never determine a simple BTU value because it's a solar heater and the thermal uptake of the unit varies tremendously depending on sun position and sky clarity. You can establish a table of variable BTU capacities based on those variables.
If you wish to do a down and dirty, very crude BTU determination, go back to what a BTU is. The amount of energy required to raise the temperature of 1 lb of water 1 degree (there's more to it, but that will suffice). Take a thermometer and 1 lb of water, place in the exhaust stream and record how long it takes to raise the temperature 1 degree. Play a guessing game about the percentage of the solar heater air you used in heating that water, and adjust accordingly. That will get you a very crude but reasonable BTU value.
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Jan. 24, 2012 9:38 a.m. DrBoost SuperDork
foxtrapper wrote:
Even with all the fancy detection equipment, you will never determine a simple BTU value because it's a solar heater and the thermal uptake of the unit varies tremendously depending on sun position and sky clarity. You can establish a table of variable BTU capacities based on those variables.
If you wish to do a down and dirty, very crude BTU determination, go back to what a BTU is. The amount of energy required to raise the temperature of 1 lb of water 1 degree (there's more to it, but that will suffice). Take a thermometer and 1 lb of water, place in the exhaust stream and record how long it takes to raise the temperature 1 degree. Play a guessing game about the percentage of the solar heater air you used in heating that water, and adjust accordingly. That will get you a very crude but reasonable BTU value.
Crude and reasonable, that's really all I'm looking for. I know there are variables at play. I'd just like to be able to quantify what I've built here.
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Jan. 24, 2012 9:39 a.m. DrBoost SuperDork
FlightService wrote:
pics?
Follow this link http://grassrootsmotorsports.com/forum/off-topic-discussion/it-works-it-works/4455.... In that link, there's a link to the original post.
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Jan. 25, 2012 8:45 p.m. FlightService Dork
Now I see, you are building a modified solar radiator. With that set up Convection is the primary unit and radiation is secondary (I didn't see an panel on the door so I am making the assumption that conduction is not used.)
Either way Fox trappers idea would be a good one. Use a gallon ziplock bag hold it flat. Or a metal cup. That way you can figure out the area measured and multiply it by the whole door.
You could then get the R value of the material used. Remember to seal it so you do not get loss of water mass.
Get the delta of the 1 lb of water and if you can't figure it out I am sure someone here can help you.
Keep us posted.
