I'm wondering if vacuum distillation might be the best route to
efficiency for this. It is many times more efficient than just using
heat. Overall, the apparatus seems more economical than what is needed
to re-capture all waste heat.
Bob Stuart
On 07/08/2010 11:17 AM, David G. LeVine wrote:
On 08/01/2010 01:15 AM, Ken Gotberg wrote:
<snip>
Water takes on the order of 2.2 Mega Joules/liter to boil at 100
degrees Celsius and more with the salt present. This mixture can take
up to 130 0C to boil and the options available to me as I
see them are: 1. rocket stove, 2. gasification stove, 3. charcoal
stove, 4. solar thermal concentrator, etc. The 130 0C is
no problem for the stoves, but I'm curious about what the maximum
efficiency of any of these is. It seems to me that the rocket or
gasification would have about the same efficiency, but perhaps I'm
wrong. A charcoal stove may be useful in some situations, but
probably not in most. And I suppose reverses osmosis and flash
boiling etc. are also possible, but seem pretty sophisticated for
local people?
Ken,
Let's ask some questions:
1. Is this for use in the Philippines? I will assume so.
2. Aren't the Philippines relatively high humidity and warm?
3. How many MJ/l would it take to condense the water from the air?
My gut feel is that you will need to reject 2.26 MJ/L and cool it
from 30° C to 20° C for it to be useful, which means 420 KJ/L or
2.7 MJ/L. To boil water will take 2.26 MJ/L and to to raise it to
130° C from 30° C will take 4.2 MJ/L. So heating it seems to need
6.5 MJ/L, then returning it to 20° C will take rejection of of
about 7 MJ/L.
4. Might it be worth looking at another technology to pull the water
from the air? We are talking putting in about 2 KWH/L and
rejecting a bit more heat for 1 liter of water, 4 liters (not an
unreasonable amount for a 70 KG man who works 1 hour per day)
means needing 8 KWH/day per person who only works 1 hour per day.
Working 8 hours/day in a warm climate means probably 16-32
KWH/day/person.
5. We receive an average of 164 W/M^2 per day from the sun, this
implies about 100 meters^2/person just for water at 100%
efficiency, and solar collection is well under 100% efficiency.
Did I miss something here? Biomass for a family of four would to be
400 m^2 per day to supply water if 100% of the sunlight is harvested,
and plants use a narrow band of energy, so collection is less than
100% efficient.
It sounds like a poor choice, there may be a better way to spend the
energy.
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