Posted by Curbie on March 24, 2009, 10:09 pm
Morris,
Two things jump through my foggy recollection of Striling cycles and
solar concentration, here is what is confusing me:
1) Gas law is Gas law. P = (nRT) ÷ V Whether your moving a solid (a
piston) or liquid (water) you still have to heat the volume of gas to
the desired pressure, for each cycle, to the work.
2) Volume of heat from a parabola is a function of the area of the
collector and temperature of heat is a function of the "Concentration
Ratio" (geometric or ideal) of the collector aperture area / receiver
aperture area. I don't see how you create the volume of heat or
temperatures cited to heat the gas to pressurize enough cycles to
create 1hp (or anything like it)?
More math cheese:
8' x 4' = 32 Ft^2
250 BTU per Ft^2 (on a reasonably clear day)
32 (Ft^2) x 250 (BTU) = 8000 BTU per hour
Plug the 8000 BTU per hour into gas law and chop the pressure into any
bore, stroke, and frequency per hour combination you like and I can't
find the Hp.
I have a spread sheet on gas law into Hp if or interested, maybe it's
all goofed up and you point out my mistake, won't be the first time.
Good Luck.
Curbie
wrote:
>Curbie wrote:
>> I suspect you're just goofing on me but on the off chance:
>>
>> Looks like beta or gamma type Stirling engine with one or more liquid
>> pistons inspired by Home Power #76, April / May 2000 by Jay Wilson. It
>> probably competes "tooth and nail" with a hamster wheel in terms of
>> hp.
>I'm working on a pair of fluidynes (Stirling cycle with only a fluid and
>a gas as moving parts): a low-temperature/low-efficiency version to be
>used as a solar-powered pump; and a high-temperature/high-efficiency
>version for other purposes.
>> Stirling's where another one those AE cures I chased, the numbers
>> worked (simple gas law) until you got into design materials - pretty
>> exotic/expensive stuff.
>The low-temperature version is what was shown in the videos at the link.
> Those (student) builders have only been working on that engine since
>the beginning of the month and the performance isn't exactly "gee-whiz"
>yet, but I expect they'll end up with a 1.5 - 2 hp pump if they keep at
>it. Even if it produces only 1 hp, it'll do hamsters proud. :)
>Other than the blue dye (whatever it is), I don't think there's anything
>exotic/expensive there.
>So far, there isn't anything more expensive than a sheet of
>polycarbonate mirror and finned copper tubing in the high-temperature
>version - but that version isn't yet complete so I can't make any claims
>for it. As you said, at this stage the numbers /do/ look pretty good,
>with a 70+% thermal -> mechanical energy conversion efficiency.
>You can see sketches of both types at
> http://www.iedu.com/DeSoto/Projects/Stirling/Fluidyne.html
>although the most recent plan calls for a reflector that's 4' long and
>8' wide (instead of vice versa), to produce a hot head temperature in
>the 1450F ballpark. It should provide some "interesting" problems. :)
>> The one that set the record for converting solar thermal into
>> electricity is somewhere on a sunny mountain top, has a round parabola
>> of 35' (roughly 1000 Ft.^2) that generates 275,000 BTU per hour and
>> they only run it on crystal clear days because the ring-less pistons
>> have such a short operational life.
>Sounds terribly spendy! The beauty of fluidynes is that there isn't any
>significant wear. I'm not much so much concerned with efficiency as with
>ability to get a job done reliably with a reasonable price tag.
>>
>> The only commercial endeavor (a good indicator of viability) I've seen
>> is the Whisper-Gen, 800 watts per hour @ $5K+ with a something like a
>> 20,000 hour operational life.
>>
>> The hamsters don't look so bad.
>Thanks - you're making one of my points for me. When what's needed is
>mechanical energy (rather than electric energy), sometimes there're
>other ways to get and apply it. I'm perfectly happy to leave electrical
>power generation to others...
>...although if you /do/ figure out a really efficient way to produce
>electrical energy from one of these things, I'd really like to hear
>about it!
Posted by Morris Dovey on March 24, 2009, 11:22 pm
Curbie wrote:
> Morris,
>
> Two things jump through my foggy recollection of Striling cycles and
> solar concentration, here is what is confusing me:
> 1) Gas law is Gas law. P = (nRT) ÷ V Whether your moving a solid (a
> piston) or liquid (water) you still have to heat the volume of gas to
> the desired pressure, for each cycle, to the work.
> 2) Volume of heat from a parabola is a function of the area of the
> collector and temperature of heat is a function of the "Concentration
> Ratio" (geometric or ideal) of the collector aperture area / receiver
> aperture area. I don't see how you create the volume of heat or
> temperatures cited to heat the gas to pressurize enough cycles to
> create 1hp (or anything like it)?
>
> More math cheese:
> 8' x 4' = 32 Ft^2
> 250 BTU per Ft^2 (on a reasonably clear day)
> 32 (Ft^2) x 250 (BTU) = 8000 BTU per hour
>
> Plug the 8000 BTU per hour into gas law and chop the pressure into any
> bore, stroke, and frequency per hour combination you like and I can't
> find the Hp.
>
> I have a spread sheet on gas law into Hp if or interested, maybe it's
> all goofed up and you point out my mistake, won't be the first time.
Here's another way to come at the problem (different units): figure
1kW/m^2 for total input. The trough uses a 4x8 sheet of mirror, but the
trough width is noticably less due to curving the mirror sheet.
Still, we end up with more than 2 m^2 of capture area which should
provide 2kW of input power. We're going to have losses at the focal tube
and losses due to Carnot cycle efficiency limitations. In the best of
all possible worlds [ :) ] the efficiency losses will be close to 26%
(at 70F/1450F head temperatures).
Those 26% efficiency losses drop our 2kW to 1.48kW, and I'm hoping
(because I don't yet have enough experience to predict) that the losses
at the focal tube leave enough to get at least 746W out as mechanical
energy.
Not factored into any of this is heat re-cycled by the fluidyne's
regenerator, possible water jacket cooling of the cold head to improve
Carnot efficiency, or collection efficiency boost resulting from tricky
(artsy-fartsy) sun side insulation and/or reflector shielding of the
collection tube.
In summary - I think there /should/ be enough juice to squeeze out
somewhere /around/ a horsepower - less if I do a shoddy job - and maybe
more if I can both design and build well - which is certainly still in
question, since I'm not an ME or physicist, and I don't have any
thermodynamics or heat transfer background. :P
I have a Stirling cycle web page (let me know if you spot errors) at
http://www.iedu.com/DeSoto/Projects/Stirling/StirlingCycle.html
that tries to describe the Stirling cycle in both prose and formulas. I
used those formulas (formulae?) to build a software simulator - which
still doesn't account for everything it should, but does seem to agree
more or less with what I've been able to measure.
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
Posted by Curbie on March 25, 2009, 12:00 am
Morris,
I'll take a hard look at all in post a little later tonight and from
this your perspective and see if if I can't find your focus. NCIS now.
Curbie
wrote:
>Curbie wrote:
>> Morris,
>>
>> Two things jump through my foggy recollection of Striling cycles and
>> solar concentration, here is what is confusing me:
>> 1) Gas law is Gas law. P = (nRT) ÷ V Whether your moving a solid (a
>> piston) or liquid (water) you still have to heat the volume of gas to
>> the desired pressure, for each cycle, to the work.
>> 2) Volume of heat from a parabola is a function of the area of the
>> collector and temperature of heat is a function of the "Concentration
>> Ratio" (geometric or ideal) of the collector aperture area / receiver
>> aperture area. I don't see how you create the volume of heat or
>> temperatures cited to heat the gas to pressurize enough cycles to
>> create 1hp (or anything like it)?
>>
>> More math cheese:
>> 8' x 4' = 32 Ft^2
>> 250 BTU per Ft^2 (on a reasonably clear day)
>> 32 (Ft^2) x 250 (BTU) = 8000 BTU per hour
>>
>> Plug the 8000 BTU per hour into gas law and chop the pressure into any
>> bore, stroke, and frequency per hour combination you like and I can't
>> find the Hp.
>>
>> I have a spread sheet on gas law into Hp if or interested, maybe it's
>> all goofed up and you point out my mistake, won't be the first time.
>Here's another way to come at the problem (different units): figure
>1kW/m^2 for total input. The trough uses a 4x8 sheet of mirror, but the
>trough width is noticably less due to curving the mirror sheet.
>Still, we end up with more than 2 m^2 of capture area which should
>provide 2kW of input power. We're going to have losses at the focal tube
>and losses due to Carnot cycle efficiency limitations. In the best of
>all possible worlds [ :) ] the efficiency losses will be close to 26%
>(at 70F/1450F head temperatures).
>Those 26% efficiency losses drop our 2kW to 1.48kW, and I'm hoping
>(because I don't yet have enough experience to predict) that the losses
>at the focal tube leave enough to get at least 746W out as mechanical
>energy.
>Not factored into any of this is heat re-cycled by the fluidyne's
>regenerator, possible water jacket cooling of the cold head to improve
>Carnot efficiency, or collection efficiency boost resulting from tricky
>(artsy-fartsy) sun side insulation and/or reflector shielding of the
>collection tube.
>In summary - I think there /should/ be enough juice to squeeze out
>somewhere /around/ a horsepower - less if I do a shoddy job - and maybe
>more if I can both design and build well - which is certainly still in
>question, since I'm not an ME or physicist, and I don't have any
>thermodynamics or heat transfer background. :P
>I have a Stirling cycle web page (let me know if you spot errors) at
> http://www.iedu.com/DeSoto/Projects/Stirling/StirlingCycle.html
>that tries to describe the Stirling cycle in both prose and formulas. I
>used those formulas (formulae?) to build a software simulator - which
>still doesn't account for everything it should, but does seem to agree
>more or less with what I've been able to measure.
Posted by Curbie on March 25, 2009, 6:27 am
Ok,
Your Stirling cycle web page looks fine which confuses me all the
more. Where did you consider gas law in your post? I don't know spit
about the metric system except that two liters is about the size of a
coke bottle, they didn't teach it when I went to school in the
stone-age.
After reading, and re-reading your post I'm still at a loss over the
same two questions I'll re-word slightly:
1) Whether your moving a solid or liquid you still have to produce
enough VOLUME of heat to heat the of gas to the desired pressure, for
EACH cycle, to do the designed work.
2) VOLUME of heat from a parabola is a function of the area of the
collector and TEMPERATURE of heat is a function of the "Concentration
Ratio".
I got into my solar concentrator spread-sheet and set-up a 2m^2
concentrator and adjusted the receiver area for a temperature of
1440F, I got a geometric "Concentration Ratio" of 127 to 1 and
requiring a receiver AREA of 6.5 Inches^2 or 403mm^2. Since you are
talking about a parabolic tough 4x8 feet, I think your talking about a
48" long receiver that has a total receiver AREA of 6.5 Inches^2?
I based my spread sheet on
http://etd.lib.fsu.edu/theses_1/available/etd-01052007-181917/unrestricted/Newton_Manuscript.pdf
I think submitted by an engineering student as his master's thesis. I
bring this up only because I know you've done software simulation and
you might be looking to cross-check for results. I like this Newton
manuscript because he proved all his calculations with answers which
is great for the metrically-challenged (me).
http://eosweb.larc.nasa.gov/sse/RETScreen/ is NASA site where you give
it your latitude and longitude and they give you a pile of data
including "Daily solar radiation".
You have heat volume and peak temperature calculations which are over
my head, but I'm easily confused.
Good luck.
Curbie
wrote:
>Curbie wrote:
>> Morris,
>>
>> Two things jump through my foggy recollection of Striling cycles and
>> solar concentration, here is what is confusing me:
>> 1) Gas law is Gas law. P = (nRT) ÷ V Whether your moving a solid (a
>> piston) or liquid (water) you still have to heat the volume of gas to
>> the desired pressure, for each cycle, to the work.
>> 2) Volume of heat from a parabola is a function of the area of the
>> collector and temperature of heat is a function of the "Concentration
>> Ratio" (geometric or ideal) of the collector aperture area / receiver
>> aperture area. I don't see how you create the volume of heat or
>> temperatures cited to heat the gas to pressurize enough cycles to
>> create 1hp (or anything like it)?
>>
>> More math cheese:
>> 8' x 4' = 32 Ft^2
>> 250 BTU per Ft^2 (on a reasonably clear day)
>> 32 (Ft^2) x 250 (BTU) = 8000 BTU per hour
>>
>> Plug the 8000 BTU per hour into gas law and chop the pressure into any
>> bore, stroke, and frequency per hour combination you like and I can't
>> find the Hp.
>>
>> I have a spread sheet on gas law into Hp if or interested, maybe it's
>> all goofed up and you point out my mistake, won't be the first time.
>Here's another way to come at the problem (different units): figure
>1kW/m^2 for total input. The trough uses a 4x8 sheet of mirror, but the
>trough width is noticably less due to curving the mirror sheet.
>Still, we end up with more than 2 m^2 of capture area which should
>provide 2kW of input power. We're going to have losses at the focal tube
>and losses due to Carnot cycle efficiency limitations. In the best of
>all possible worlds [ :) ] the efficiency losses will be close to 26%
>(at 70F/1450F head temperatures).
>Those 26% efficiency losses drop our 2kW to 1.48kW, and I'm hoping
>(because I don't yet have enough experience to predict) that the losses
>at the focal tube leave enough to get at least 746W out as mechanical
>energy.
>Not factored into any of this is heat re-cycled by the fluidyne's
>regenerator, possible water jacket cooling of the cold head to improve
>Carnot efficiency, or collection efficiency boost resulting from tricky
>(artsy-fartsy) sun side insulation and/or reflector shielding of the
>collection tube.
>In summary - I think there /should/ be enough juice to squeeze out
>somewhere /around/ a horsepower - less if I do a shoddy job - and maybe
>more if I can both design and build well - which is certainly still in
>question, since I'm not an ME or physicist, and I don't have any
>thermodynamics or heat transfer background. :P
>I have a Stirling cycle web page (let me know if you spot errors) at
> http://www.iedu.com/DeSoto/Projects/Stirling/StirlingCycle.html
>that tries to describe the Stirling cycle in both prose and formulas. I
>used those formulas (formulae?) to build a software simulator - which
>still doesn't account for everything it should, but does seem to agree
>more or less with what I've been able to measure.
Posted by Morris Dovey on March 25, 2009, 11:31 am
Curbie wrote:
> Ok,
>
> Your Stirling cycle web page looks fine which confuses me all the
> more. Where did you consider gas law in your post? I don't know spit
> about the metric system except that two liters is about the size of a
> coke bottle, they didn't teach it when I went to school in the
> stone-age.
I have the same problem with metric - but this is the first problem of
its kind that I've tackled, so I converted familiar units to metric and
forged ahead in the hope that most of the physics/engineering experts
from whom I might need to beg help would be more comfortable with that
system.
The approach I took in my post was "follow the power". I can guesstimate
the input power, and can identify losses at the focal tube and losses
within the engine because of the Carnot cycle efficiency limitations.
I know that there will be other losses since the mirror won't be a
perfect reflector, the focal tube won't be a perfect absorber, air
viscosity increases with temperature to prevent perfect flow, etc ad
nausea. I doubt that I'll ever be able to isolate and individually
quantify all of the loss mechanisms. My choices were to either shrug and
continue or declare myself incompetent and quit. I shrugged.
> After reading, and re-reading your post I'm still at a loss over the
> same two questions I'll re-word slightly:
> 1) Whether your moving a solid or liquid you still have to produce
> enough VOLUME of heat to heat the of gas to the desired pressure, for
> EACH cycle, to do the designed work.
At this stage in /my/ development, there isn't a design output
requirement. I'm coming at this as a learning experience (which allows
me to re-invent wheels as necessary) to discover, given a particular
engine configuration, just how much work the thing can be tweaked to
perform...
I think [1] is true. From the energy perspective, if I increase the
energy level of the gas by 1kW, I should be able to see 1kW worth of
effect - and if I then reduce the energy level by 1kW, I should be able
to see 1kW worth of effect in the other direction. Carnot tells me that
this isn't quite so, and that my effects will be dependent on the
temperature extremes - and provides a formula for how much of my input
power can result in actual output power: (1 - Tc/Th, temps in K).
Since I have (a guesstimate of) the input power, and values for the
temperature extremes, I can approximate the maximum (ideal) output power
directly, without needing to involve pressure and volume variables.
What this tells me is that if I can manipulate the Th and Tc values so
as to produce a Carnot cycle efficiency of 74.6% (and if my engine were
"ideal"), an input of 1kW would result in a 746W (1 hp) output.
The big question is: "Just how far from 'ideal' will the engine be?" The
only way I know to get an answer is to build it and see how well it does. :)
> 2) VOLUME of heat from a parabola is a function of the area of the
> collector and TEMPERATURE of heat is a function of the "Concentration
> Ratio".
>
> I got into my solar concentrator spread-sheet and set-up a 2m^2
> concentrator and adjusted the receiver area for a temperature of
> 1440F, I got a geometric "Concentration Ratio" of 127 to 1 and
> requiring a receiver AREA of 6.5 Inches^2 or 403mm^2. Since you are
> talking about a parabolic tough 4x8 feet, I think your talking about a
> 48" long receiver that has a total receiver AREA of 6.5 Inches^2?
The actual width will be close to 88" and the target area will be 3/8",
for a ratio closer to 88/0.375 or 234:1. 3/8" * 48 = 18 in^2. (I plan to
wear welding goggles.)
> I based my spread sheet on
>
http://etd.lib.fsu.edu/theses_1/available/etd-01052007-181917/unrestricted/Newton_Manuscript.pdf
> I think submitted by an engineering student as his master's thesis. I
> bring this up only because I know you've done software simulation and
> you might be looking to cross-check for results. I like this Newton
> manuscript because he proved all his calculations with answers which
> is great for the metrically-challenged (me).
One of the nice things about the simulator is that I can enter all my
information in familiar units and do a front-end conversion to metric,
then display results in both units.
I'm very conscious that the simulator (so far) only simulates an ideal
engine. The only thing I hoped for in writing it was a tool that allowed
a "sanity check" on actual results. As I learn more about the real-world
operation of these things, I may be able to make the simulator more
realistic.
> http://eosweb.larc.nasa.gov/sse/RETScreen/ is NASA site where you give
> it your latitude and longitude and they give you a pile of data
> including "Daily solar radiation".
I appreciate the links. I've saved 'em for when I've had more sleep.
> You have heat volume and peak temperature calculations which are over
> my head, but I'm easily confused.
I suspect that you aren't - and I think this stuff does take a bit of
time to internalize. All the calculations I have on the web page were
either lifted directly or derived from a handful of university web sites
and wikipedia articles - and I've had a good bit of help from some of
the folks who hang out on alt.solar.thermal.
If it weren't for the Internet I'd still be saying: "I wonder if..."
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
XML site map
>> I suspect you're just goofing on me but on the off chance:
>>
>> Looks like beta or gamma type Stirling engine with one or more liquid
>> pistons inspired by Home Power #76, April / May 2000 by Jay Wilson. It
>> probably competes "tooth and nail" with a hamster wheel in terms of
>> hp.
>I'm working on a pair of fluidynes (Stirling cycle with only a fluid and
>a gas as moving parts): a low-temperature/low-efficiency version to be
>used as a solar-powered pump; and a high-temperature/high-efficiency
>version for other purposes.
>> Stirling's where another one those AE cures I chased, the numbers
>> worked (simple gas law) until you got into design materials - pretty
>> exotic/expensive stuff.
>The low-temperature version is what was shown in the videos at the link.
> Those (student) builders have only been working on that engine since
>the beginning of the month and the performance isn't exactly "gee-whiz"
>yet, but I expect they'll end up with a 1.5 - 2 hp pump if they keep at
>it. Even if it produces only 1 hp, it'll do hamsters proud. :)
>Other than the blue dye (whatever it is), I don't think there's anything
>exotic/expensive there.
>So far, there isn't anything more expensive than a sheet of
>polycarbonate mirror and finned copper tubing in the high-temperature
>version - but that version isn't yet complete so I can't make any claims
>for it. As you said, at this stage the numbers /do/ look pretty good,
>with a 70+% thermal -> mechanical energy conversion efficiency.
>You can see sketches of both types at
> http://www.iedu.com/DeSoto/Projects/Stirling/Fluidyne.html
>although the most recent plan calls for a reflector that's 4' long and
>8' wide (instead of vice versa), to produce a hot head temperature in
>the 1450F ballpark. It should provide some "interesting" problems. :)
>> The one that set the record for converting solar thermal into
>> electricity is somewhere on a sunny mountain top, has a round parabola
>> of 35' (roughly 1000 Ft.^2) that generates 275,000 BTU per hour and
>> they only run it on crystal clear days because the ring-less pistons
>> have such a short operational life.
>Sounds terribly spendy! The beauty of fluidynes is that there isn't any
>significant wear. I'm not much so much concerned with efficiency as with
>ability to get a job done reliably with a reasonable price tag.
>>
>> The only commercial endeavor (a good indicator of viability) I've seen
>> is the Whisper-Gen, 800 watts per hour @ $5K+ with a something like a
>> 20,000 hour operational life.
>>
>> The hamsters don't look so bad.
>Thanks - you're making one of my points for me. When what's needed is
>mechanical energy (rather than electric energy), sometimes there're
>other ways to get and apply it. I'm perfectly happy to leave electrical
>power generation to others...
>...although if you /do/ figure out a really efficient way to produce
>electrical energy from one of these things, I'd really like to hear
>about it!