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Posted by feklar on September 11, 2003, 4:30 pm
 
Thread transfer from sci.energy.

Let's say my basic design for a solar greenhouse uses double
insulation (two panes with an air space between them), and uses a
large black tinted glass water tank for the heat storage mass.  Lets
say the greenhouse is 44 feet long by 14 feet wide by 12 feet high.
Lets say that the best height (and therefore best water storage
capacity) for the tank and greenhouse have yet to be resolved via
experiment.  For this example, let's use a height of 12 feet.

The external measurements of the tank would be 9 feet high, 12 feet
wide, and 42 feet long.  The internal measurements of the tank will be
6 feet high, 9 feet wide, and 39 feet long.  Or in other words, one
and a half feet thick glass walls.

For the time being, just ignore what currently passes for economics
for this, and assume that the cost of the glass will be very, very
low, inconsequential, irrelevant, only an example being used for the
purpose of illustrating a concept.  Ignore the cost issue and consider
for the tm ebeing that I somehow invented a machine that can create
the required glass for the tanks and greenhouses out of thin air, for
free. I didn't but just assume that for now and ignore the glass cost
issues.

Also, for the time being, just assume that the glass has the required
strength to stand up to water and steam pressures.  Assume that a
method has been found to produce ultra cheap ultra strong glass for
little or nothing, just for the time being.

The tank will have a black tinted glass pipe running through its
length, at or near the bottom of height and in the middle of the
center of width.  The pipe will be 20 inches in diameter and have an
internal open diameter of 6 inches (the pipe is 7 inches thick).  The
pipe will be solidly welded to the tank, so there is no danger of
water leaking out between the pipe and the end walls of the tanks.

So that would mean the tank capacity would be 9 x 6 x 39 = 2106 cubic.
feet, minus around 76 cubic feet or so for the space the pipe takes
up.

Let's say 2000 cubic feet.  Anyone know what that would be in gallons?
About 3000, somewhere in there?

Inside the pipe, electric heating elements will be located.

The idea of the pipe is to eliminate the need for a trough reflector
on the north side of the greenhouse.

Instead of using a trough reflector to gain the extra energy to get
the water boiling, the heating elements will do the job.  So, in the
morning, electricity from the power grid has to be used to kickstart
the heating elements, running them long enough to get the water
boiling inside the tube.  After the water gets boiling, it is assumed
that the solar energy alone, powering the steam turbine output, will
provide the excess wattage needed to run the heating elements.

A pulse system would be used to feed doses of water into the pipes
from the tanks as required to keep up the steam pressure.

With the surface area, and using a solar constant of 760 watts per
square meter loosely converted into square yards, the total wattage
from the sun incident upon the tank should be...

The external measurements of the tank would be 12 feet wide and 42
feet long.   So converted to yards, 4 by 14, a total of 56 square
yards.  Multiplied by a solar constant of 760 watts, a total of 42560
watts.

So roughly 40 kilowatts, more or less given that square yards do not
equal square meters.

So, what would the ideal figure be to divert back into the heating
elements?  For efficiency calculations, consider that any heat
produced by the heating elements that is not translated into steam
will be radiated into the water tank, and will not be lost.  Therefore
the larger the tank, the higher the efficiency should be.

What would the heat storage capacity of this size tank be?  How long
after the sun goes down until the break even point is reached, and the
turbine output is only enough to power the heating elements or less,
using that large a tank?

Here's one that's probably harder:  What is the ratio or the formula
that would determine the best number of watts to use for the heating
elements, given the storage capacity of the tank, that would allow the
longest period of positive energy production after the sun went down?
Or is there another concern that outranks that one?

If I had twenty such tanks, and all their pipes were connected, what
size turbine would be the best choice?  What are the considerations?

Where can I find details on the water recycling systems for steam
turbines, for the best way of returning the water to the tanks?
-----------------
end repost
----------------

On Tue, 2 Sep 2003 18:46:47 GMT, "Fred B. McGalliard"


Perhaps you could elaborate, I'm not usre what you mean.  I thought
that was more or less the intended result, although steam pressure
would be lower at startup, peak at or before noon, then slowly fall
off during the day and fall off faster after the sun went down to the
break even point.


The idea is to attempt to eliminate the mirror to increase land use
efficiency.  The idea is to stack as many solar greenhouses/tanks
together as possible, with little if any space between them.  

Lets say using the size I described in the OP, 12 by 42 feet.  A nine
by nine array of greenhouses of this size, with the center position
occupied by the turbine and generator, and conencted to the eight
surrounding center greenhouses to make one large center greenhouse 9
times the size of the others.  So, a total incident solar energy of
42560 watts per greenhouse, or a total of 3404800 watts for the entire
array.  At a 20 percent efficiency, that would fork out an average of
680960 watts during the day.  I suspect the efficiency will be twice
that, but I don't have all the facts yet so I can only go with what is
safe to say at the present time.  Its too bad I'm not still in Denver,
I'd go to SERI's library in Golden and get what I need.  But the point
is, the land footprint would be well-used with no empty space.  In
this case, 108 feet by 378 feet.

A reflector might increase efficiency by 5 percent of so, but an
entire greenhouse can fit in that same space the refector requires,
and double the total output from that land area instead of boosting it
merely five percent.   (requires, if it is to avoid casting shadow on
the next greenhouse to the north)

If you use a mirror, the mirror casts shade upon the greenhouse to the
north.  You take an efficiency hit using my approach vs. using a
reflector, but you free up twice as much land area.  Per square yard
of land area, packed together this way without reflectors there is a
total efficiency improvement, because you have twice as many
greenhouses covering a given area as you would have if you used
reflectors.

Also, more importantly, there is a longer period of startup in the
morning, and using reflectors you have to shut down before the sun
even goes down, where my design will operate for a fair amont of time
after the sun sets.

I should have been clearer.  I am not talking about a backyard
installation, I am talking about massive terraforming for large scale
commercial power production.

You can't ask for more than a reflector when it comes to proving that
power can be generated solely from the sun, but in large scale
practice and application its better to tap the power grid or stored
reserves to jump start in the morning like I said, and do away with
the reflectors.  For a small scale application. land waste is
acceptable, but in a very large scale implementation, the most needs
to be gotten from every square inch of land.  Consider the price of
land.


Like I said.  It is a very important point to differentiate between
the end result you thought I was aiming for and the one I am actually
aiming for.  Let me know if I haven't explained my intent enough or if
you don't follow the overall land use efficiency angle.

Consider that in 50 year's time, I want to pack greenhouses in like
sardines in some given area in New Mexico, leaving little if any space
between them..

I also realized there is another very important cogent point that
needs to be made regarding my design, namely the steam return from the
turbine.  If the turbine returns low temperature steam, that steam
would be routed back to the greenhouses and released inside them,
between the tank and the greenhouse.  There it would give up its heat
energy to the tank and condense and colelct on the floor, and the
resulting water would then be pumped back into the tanks.  Obviously,
the steam inside the space between the greenhouse and the tank will
not interfere with the transmutation of radiant solar heat to
conductive heat inside the tank, since the steam reacts very little to
incident radiant energy, transmitting more than 99.9 percent of it.

That's a pretty important consideration, I'm surprised I didn't think
to put it in the original post.


Don't get me wrong now.  I'm not trying argue that you are mistaken.
You are entirely correct in what you say, if the intended application
is the one you thought it was.


Is this preferable and more efficient that hydrolysis for nighttime
operation? For very large scale power generation?   And in answering,
please ignore the economics, obviously with what they have to work
with now, electrolysis is a lot more expensive.  Just take my word for
now that I can change that.

Original post:
On Tue, 2 Sep 2003 18:46:47 GMT, "Fred B. McGalliard"



Posted by Joe Fischer on September 11, 2003, 8:53 pm
 
: Please reply in alt.solar.thermal
:
: Thread transfer from sci.energy.
:
: Let's say my basic design for a solar greenhouse uses double
: insulation (two panes with an air space between them), and uses a
: large black tinted glass water tank for the heat storage mass.  Lets
: say the greenhouse is 44 feet long by 14 feet wide by 12 feet high.
: Lets say that the best height (and therefore best water storage
: capacity) for the tank and greenhouse have yet to be resolved via
: experiment.  For this example, let's use a height of 12 feet.

       Your assumptions are all wrong, sunlight should
not be let in a glass tank, algae grows.
       There are about 7 gallons per cubic foot.

       The big thing is that without concentrating,
the sun will not boil water, the container loses
heat too fast due to conduction, convection, and
radiation.

       Which is why PV is building a market, it
is the least complicated way to make electricity
from the sun.

Joe Fischer  

--
3

Posted by feklar on September 11, 2003, 10:10 pm
 On 11 Sep 2003 16:53:07 -0400, Joe Fischer


I said a black tinted tank, but I should have said black tinted and
black coated tank.  Using purified water and/or algicide will prevent
or reduce that.  Any return to the tank will be through a distiller,
so there won't be a whole lot of nutrients available for any algae to
feed on.


Thanks.


I'm not sure you read the entire post.  The design uses the tank as a
water preheater, then uses electric heating elements to add the extra
(approx. based on season and latitude) 2 to 42 extra degrees of heat.
Only a small (compared to the tank capacity) amount of water is
boiling inside the steam pipe that contains the heating elements.  It
is my the intent to boil all the water in the tank, only to get it
fairly close to boiling, then repetitively feed charges of that
preheated water into the steam pipe.

On the hottest days of the year down in Texas or Mexico, the tank
water might boil for an hour or two a day, but on the average the tank
temperature should be around 190 or 200 degrees or so (a little more
than the standard 180 to 190 value, because of the presence of the
heating elements adding their unused waste heat output to the tank,
and possibly because of a very small amount of steam entering the tank
each time a water charge is taken from the tank.

The heating elements serve the same function as a reflector would.

There is an efficiency loss doing it this way versus using a
reflector, but doing it this way doubles the number of greenhouses per
given square area or square mile, because the greenhouses can be
packed side by side with no space between them.  If you use reflectors
then you have to alternate rows of greenhouses, reflectors,
greenhouses, reflectors, etc. from south to north.  This setup uses
greenhouse, greenhouse, greenhouse.

So which is a more efficient land use, 1 greenhouse running at 25 or
30 percent efficiency and one refector, or 2 greenhouses running at 20
percent efficiency with a greater insolation area (averaging around at
least 150 percent greater except at noon when the insolation is the
same) and therefore greater total output per comparative efficiency
unit, if both setups require the same land footprint?  If you see what
I mean...


Posted by feklar on September 11, 2003, 10:20 pm
 On Thu, 11 Sep 2003 22:10:50 GMT, feklar@rock.com (feklar) wrote:


Only a small (compared to the tank capacity) amount of water is
boiling inside the steam pipe that contains the heating elements.  It
is not my intent to boil all the water in the tank, only to get it
fairly close to boiling, then repetitively feed charges of that
preheated water into the steam pipe.


Posted by Joe Fischer on September 11, 2003, 10:44 pm
 
On Thu, 11 Sep 2003 22:10:50 GMT, feklar@rock.com (feklar) wrote:

            I read most of it, possibly the only thing I am not sure
of what you plan to use the steam for.
            Apparently you are not aware of the latent heat of
vaporization, which is what make PV more reasonable in
relation steam.
            It isn't just "2 to 42 degrees" that have to be added
to make steam, let's define BTU British Thermal Units
which may be easier for me.

            A BTU is the a amount of heat required to raise the
temperature of one pound of water one degree Fahrenheit,
not counting losses.

           And the latent heat of vaporization of water is in the
neighborhood of 900 BTU per pound.

           I like the way you are thinking, and I hope you do
continue to think constructively and with optimism.
           But to make a pound of water into steam it takes
that 900 BTU per pound of water plus the 2 to 42 BTU
to get up to 212 degrees.  

            I think it takes a little more than 3 BTU per hour
to equal one watt.


          Sure, but probably would use more power than you would get.


          But the idea is to get power output, not pack greenhouses
close.


         I see what you mean, but Texas has enough land area to fit
the entire population of the Earth and still only have 20 people
per acre.

        Your idea is good and I am surprised that power companies
have not built some type of solar plants all across the sunbelt,
even with little or no storage they could feed the peak power
users in the middle of the morning and the late afternoon all
across the US.

         Also, steam power efficiency goes up with working
temperature, at 100 PSI (about 300 degrees) the efficiency
is about 10 percent, and at 2000 degrees it goes up to about
30 percent.    
        New York is in a big problem partly because of old
low temperature power plants, I suppose when planning
was done they were building nuke plants right and left,
then the government regulations and labor costs went
way up and made nukes a poor investment.
      
       But alternative energy is needed, so please continue
to think and dream, and learn as much as possible about
the physics.
       The good thing about concentrators is that the heat
can be used for other things, whether it is from PV or
steam boilers.

Joe Fischer
  


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