Finally Back! Solar Heat

[Derek]

It's been a while since I've worked on any solar or free energy projects, as well as a long time since I've been here on the forums.  I hope to get back into frequenting them again now.  So what have I been doing lately?  Well working on a very effecient method of solar heating.

I'm still in the beginning stages of this heater, but need to get to work since its starting to get cold!  So far what I've got is this:  I have an 8" by 8" square area, with only 1 layer of 1/8" glass thats putting out some serious heat.  With natural convection only, its capable of taking 65 degree input temps to outputting over 155 degrees.  So a boost of of 90 degrees in a VERY small amount of space.  I imagine that an increase in size of this type of unit will make a big difference.  I'm using a few constants in my tests such as 8x8 front area, and only single glass.  So making it bigger and going dual paned glass will probably help.  Plus I plan to add reflective outer panels similar to solar oven reflectors to both the top and bottom of the unit, but not on the sides, so that way it gets light from sunrise to sunset.

I've still got quite a few tests to do, but eventually plan on scaling it up, and adding at least a 100 cubic foot per minute fan to it to supply heat to my living room at least.  As far as BTU's, that will very much depend on the end project.  But we'll see, it may be possible to supply nearly all of the heat with some fairly good sized units.  Maybe an array size of 6 foot by 6 foot tall and wide could provide enough heat by my initial estimates.  But we shall see!

[GaryGary]

Hi Derek,

Here is one way to think about how much collector you need to make a dent in the fuel bill:

For most areas on a sunny winter day, each square foot of collector will receive about 1600 BTU for the full day.  Most collectors that are descently designed will end up with efficiencies somewhere between 40% and 60% under winter conditions (say 50% as an average).  So, each square foot of collector on a sunny winter day deliver around (50%)(1600 BTU/day) = 800 BTU per day to your house.

To have enough collector area to save you the equivalent of a gallon of propane burned in an 80% efficient furnace, you need:

   (92000 BTU/gal)(0./(800 BTU/sqft -day) = 90 sqft of collector

My 2 cents is that people often don't put enough emphasis on making good sized collectors.

But, I think that experimenting around with small collectors to get the design down is a great idea.

Gary

[wdyasq]

I remember seeing "collector should be 20% of heated floor area" as a practical number and, after a lot of actual calculation of btu/sf, "R" factors, surface areas and various other heat gain/loss things, decided they were about right. There is only so much energy available from the sun. Keeping the "deltas" low and the glazing material isolated from extreem heat is important. Having as few 'transfers' of heat is important also.

Collelction of low grade energy sources makes conservation, storage and use of the energy even more critical. Many times, it enough effort is placed in conservation, renewable energy becomes very low in priorities as the bills become very managable.

For some, myself included, the desire is to eliminate the necessity of relying on outside resources and make as low impact on the environment as possible, not as is practicle. For most, the desire is to lower energy costs. Some want a "grid-tie" and net meetering to only have a zero electrical bill. For another group, the electrical grid is not available due to isolation. These are goals that require different approaches as well as different choices in tools and equipment.

Ron

[Derek]

Well, by my measurements, I'm actually getting over 200BTU's per hour with this small design.  Maybe I have something completely wrong in my entire formula, but I dont think so.  I've researched that quite a bit now.  The 90 degree increase is where it really juices up the BTU's on this thing.  And this is only an 8x8 one.  So I would say I'm getting closer to 75% effeciency with this.  I know everyone's initial reactions will disagree, but I've got a pretty good design here!  Send me your email and I will send you this formula sheet and you can check it out.  

[wdyasq]

Well, ~1000 watts/meter squared is the figure I have found for solar energy falling on the earth, perfect conditions. You are claiming 200 btu/hr out of .0412 m squared. Conversion, watts to BTU; 1W = 3.412 BTU. So by calculations, you are getting 4854 BTU/m or 1423 W/m.

Congratulations, you have just exceeded theoretical by a good margin including glazing losses.

Ron

[Derek]

Ok, here is what I've got as far as the formula goes for coming up with this:

I have a single opening on this unit, which is 3.14 square inches (.022 sq ft)

I've figured up that heat rises on its own at 88 ft per minute, which is 1.5 feet per second.  At least when we are talking a 90 degree difference.

The average air density would be .068, and the BTU/lb would work out to .267.

Now basically, for BTU's per minute you multiply the following:

sq ft duct area X ft/min air velocity X air density X temperature difference X BTU/lb

this means: .022 X 88 ft/min X .068 X 90 degrees X .267 BTU/lb = 3.14 BTU/min = 188 BTU/hour

If there is any problem with the formula here let me know, but I've checked several places and this is what I've seen is the way to figure this up.

[wdyasq]

I'm not following your method of 'work done' but may have time next week to try to get into your way of thinking. Air weighs 1.29 e-3 g/cm^3 at 0C. This means it takes takes a few cubic meters to make a pound. I don't know the specific heat of air but that varies with heat and pressure. It can easily vary 50% or more with temperature. Just a few 50% errors can wrought havoc with calculations.

Good luck with your project.

Ron

[Derek]

GaryGary had given me an equation, plus there is one listed here: http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

This is how I've figured up the BTU's of my solar heater.

[GaryGary]

Hi Derek,

If you are getting 188 BTU/hour from the 8X8 panel, that would be (144 in^2/64 in^2)(188 BTU/hr) = 423 BTU/sqft-hr -- as I think someone pointed out, this is more than the total solar input by quite a bit. So, unless you have an over 100% efficient collector, there must be an glitch in the calcs somewhere.

I'm guessing that the 88 ft/min might be the source of the problem.  The flow velocity through a collector depends on flow resistance, vertical height of the collector, how well the absorber absorbs, how well it transfers heat to the air, how much heat you lose out the glazing, ...  There are just too many variables to allow accurate calculation -- you need to measure it.

There are some cheap ways to do this here:

http://www.builditsolar.com/References/Measurements/measurments.htm#Airflow

If you can get the outlet airflow to go horizontal, the little $25 Dwyer gage works really well for thermosyphon collectors.  The Kestrel meter is also good, but costs more -- its the one I use most of the time -- but, it will not read realiably below about 80fpm.

The method I use to make a guess at collector efficiency is here:

http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

Its not by any means perfect, but it gives a pretty good idea how well a collector is doing.  The comments about temperature rise through the collector might also be worth looking at -- a large temperature rise usually means an inefficient collector, because it goes with large collector losses out the glazing.

This is small deal, but the specific heat of air is 0.24 (not 0.267).

But, the basic point I was trying to make is that even if your collector was 80 or 90% efficient (very, very, very  hard to achieve in winter conditions), you still need to have a fairly large amount of collector area to generate useful amounts of heat for space heating.  This is not so bad, since air collectors are cheap to build, and you can make them big without a lot of expense.  Here is one from Mother Earth News that was built of 10 cents a sqft!!

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Passive

(look for "Very Inexpensive Solar Shop Heater")

Gary

[Derek]

Hey Gary!  Sent you an email with my spreadsheet.  Check it out, see what you think, and let me know if I have anything messed up.  Now I remember getting some info about the speed of heat rising here: http://www.fieldlines.com/story/2006/6/16/171153/285

88 feet per minute out of the top of the unit would mean 1.5 feet per second.  I think thats about the speed at which heat would come out of the top of this thing.  I just dont have a real good way to measure that I suppose.  So 60ft/min would be 1 foot per second.  That would give me a total of 115 BTU/hr.  This is also after I changed it from .267 to .24 for specific heat of air.  That took it down from 188 to 169 BTU/hr.  So either way, still rather high.  Check out that email I sent and let me know what you think.  In either case, I may have a pretty good design I feel.  It certainly puts out good heat.  90 degree increase in 8 inches is pretty good right?