re: concentrator questions & clarifications
27 apr 1997
the redoubtable gene a. townsend wrote:
>a few reflections on this thread.
numbered, even :-)
>1. anyone that says it's easy to get 30-40% efficiency from a briggs
>and stratton converted to steam has never tried it.
"easy" is relative...
that might be 30-40% efficiency for the steam engine alone, not the solar
collection (which might be very efficient with one of pe norman saunders'
(us pat no. 4,129,120, expired 12/12/95) steam generator receivers.)
i imagine rudy has not tried this yet, but other people have, over hundreds
of years, and rudy is a competent hands-on aerospace engineer who says he's
built a number of ae systems, including a dozen hydro systems, the largest
some 400 kw, and he's doing a good job of making his own off-grid power for
his commercial indoor hydroponic plant growing factory, running propane and
diesel 50 kw generators now, with plans to set up some large windmills to
replace them, using some outdoor concentrating cassegrainian dome and
quonset-shaped greenhouse structures. the carnot efficiency seems to be here
with this steam system. why can't we convert a substantial part of it to
>these effeciencies are attainable in complex commercial steam plants only
>when things like reheat, regeneration, recuperation, and other
>complicated processes are added.
i imagine a system with only condensation (an automobile radiator? a 34' tall
pipe, used as a "barometric condenser"?) might work well. what's the ratio of
carnot/overall efficiency for a small well-insulated 100-200 psi steam system?
>2. small, simple steam systems, where the only parts are boiler,
>engine, and condenser, operate in the 1 to 5% efficiency range.
starting with what carnot efficiency? does this overall 1-5% include steam
generation efficiency, eg boiler stack losses? paraphrasing norman's patent:
...sufficient water is provided in the tank so the top level is above
the heat absorbing grid, and the level is kept below the outlet so that
only steam exits... given enough heat energy from the sun coming through
the transparent bottom, a strong vertical thermal gradient is created
between the grid and the bottom which is accompanied by a strong density
gradient in the water due to the temperature difference [like an upside-
down solar salt pond, a boiler with insulation on top and water acting
as "transparent insulation" underneath:-)]
for example, with a spacing of 45 mm between the bottom and grid, a thermal
gradient of between 70 and 80 c will provide a stabilizing force of about
10 newtons per square meter holding the cold water to the bottom. so long as
the vertical temperature gradient remains substantially constant, the water
flow into the tank will remain laminar, so the water creeps across the
entire bottom and only starts to rise as it is warmed and displaced by new
inlet water under it. a 0.5 x 3 m tank with a 45 mm spacing between the
hottom and heat absorbing grid and thermal gradient of about 80 c would take
about 5 minutes for cold incoming water to reach the far side of the tank,
and about 4 hours for water to reach the grid and evaporate into steam.
[a shallower tank would have less volume and boil more quickly and safely,
with somewhat higher thermal losses.]
on a bright sunny day, sufficient reflectors are provided so that for a
spacing of about 30 mm between bottom and grid, a concentration of about
12 kw/m^2 [12 suns] is provided (for a spacing of 45 mm, about 8 kw/m^2
should be provided and for a spacing of about 60 mm, a concentration of
about 6 kw/m^2 [6 suns, as howard reichmuth, pe independently suggests] in
order to provide about an 80 c thermal gradient so as to generate steam.
the manner of operation is easily explained based on the following facts.
first, it is known that the specific thermal conductivity of water is about
0.6 watt per (m^2 x c/m); that water is essentially transparent to sun
radiation and essentially opaque to its own heat radiation [ie it blocks
its own black body radiation, david! :-)] and that for most temperatures
water expands when heated, ie at a rate of about 400 ppm/c between about
30 and 50 c.
thus, water introduced to the tank will be more dense and will remain at
the bottom until warmed and replaced by fresh cooler water. the sun's
radiation passes freely through the water until it strikes the absorbing
grid. when there is no flow of water, the temperature of the water below
the grid continues to rise until there is substantial heat loss through
the transparent bottom [sounds like inherent safety to me :-)]
for example, if the grid temperature is 100 c and the bottom is 25 c,
the downward heatflow is then 0.6 w/m^2-c/m x (100-25)/0.045m = 1000 w/m^2.
of the 8000 w/m^2 of insolation, this leaves about 7000 w/m^2 to turn the
water into steam [an 88% "boiler efficiency".] it requires about 626 wh of
energy to convert a kilogram of water at its boiling point into steam. thus
approximately 11 kg of water will be boiled off each square meter each hour
and the mass of water in the tank will be flowing upward at the rate of
about 11 mm per hour. to heat 11 kg of water from 25 to 100 c in one hour
requires 1.162 wh/kg-c x 11 kg x (100-25) = 975 watt hours per hour.
thus, substantially all of the heat being conducted downward in the tank
will be used to heat the incoming water. in effect, what we have is a
stationary temperature gradient with heat moving downward and water moving
upward at a balanced rate which just maintains that gradient.
now, is that clever, or what? norman seems to have done the hardest part
of the thinking for this system, vs making a little steam engine...
>3. nobody really seems to know how much pollution is contained in
>flat panel pv panels, but we all know how much cost is there.
right. (flat panels will work with low-temp low-pressure low-boiling point
systems, but they have less carnot efficiency, they are more expensive, and
the working fluids tend to be toxic or dangerous.)
>4. pv concentrators combine the best features of both: the concentrator
>needed with the steam system, but lack the inefficient engine,
it's my impression that the inefficient engine is at least as efficient as
the inefficient pv (which also needs cooling...), and a heck of a lot cheaper.
>and the boiler danger.
i don't know much about boiler dangers and avoidance, but it seems to me that
something like this might be designed to be inherently safe, eg by making the
volume small, and it might have some modern redundant safety systems as well.
perhaps a lot of those old boiler explosions were caused by faulty old safety
systems, eg sticky (or tied down, during steamboat races) mechanical pressure
relief valves, with people watching pressure and temperature gauges instead of
electronic sensors and controls, and no redundancy, and massive boilers with
massive fires, vs relatively small thermal targets that can be quickly moved
out of a focus, with no exposed high temperatures or open flames.
>5. small steam systems without condenser have even lower efficiencies
>than the 1 to 5% figure. with condenser, there will be no useful
>waste heat output.
that seems like an oversimplification. suppose the "condenser" is a fan-coil
unit that supplies space or water heating for a house, e. g. the $139 all-
copper 2' x 2' shw 2347 duct heat exchanger made by magicaire, which transfers
45k btu/hour between 125 f water and 68 f air (800 btu/hr-f) with 1400 cfm
of airflow, eg from a window fan, with a 0.1" h20 pressure drop--5 hp at 30%
efficiency means condensing about 25k btu/hr... (present day economics say
house heating is a lot more important to most rational on-grid people than
trying to be "energy independent" by making their own electricity, while
heating the house with oil, etc.)
>6. steam systems require the water to be of a certain quality. this
>makes use for water recycling (often suggested) impractical.
or less practical, requiring a heat exchanger, and evaporation perhaps,
instead of boiling. suppose the condenser is 20 ft^2 of 40 btu/hr-f-ft^2
pipe running through a septic tank in the basement...
>9. a simple solution to the pollution "problem" is to move out of town.
that works for some people, for a while. some industries (eg inco nickel in
canada) "solve" their pollution problems by building taller smokestacks...
>10. people don't do this because they are too busy making large
>incomes so they can pay the high cost of living in large cities.
>11. unplug from the mess: move to the country, produce your own power
>because you want to, and don't worry about greenhouse gasses.
that might be a good interim step, perhaps with solar steam engines...
>12. perhaps the internet will allow the reruralization of america?