air-liquid solar water heaters SOLAR.KnowledgePublications.com

air-liquid solar water heaters
16 oct 1996
daniel@laser.net wrote:

>>the trick is, as =
>>
>>mr. pine has figured out, to isolate the sunspace =
>>
>>from the living area. =
>
>good idea :-)

and put our bare collectors in the sunspace, eg big fins, with an ordinary
water heater upstairs that seldom turns on, heated by natural warm water
convection, with no collector glazing nor insulation nor pumps nor controllers
nor antifreeze nor heat exchangers if possible, just a well-managed sunspace
that never freezes.

or perhaps we can hold up 2 plastic 55 gallon drums full of water under the
peak of a roof, with a redundant u shaped rope sling around the rafters, and
some glazing to the south, and insulation overhead, and some foil underneath, 
using a few stud cavities for the supply airpath, to make a gravity-feed
solar water heater like this:
                                
                      ........                        .               ---
                      drums                     . drum drum .
polyethylene       .g b  b                 .    .collar beam.    .---
film sunspace --> . g foil                 .    . solar air .    .    11'
                 .  g     polycarbonate    .    .   heater  .    . 8'
   pond         .   g <-- single glazing   .    .           .    .
            .....................      ...................................
                                           |          12'        |

during the day, the air from the drum space would slide down the inside of
some south wall stud cavities, between the outside wall and some reflective
insulation, then south through a hole at the bottom, then up through a 6"
air gap between the dark north wall of the sunspace and a single layer of
polycarbonate glazing, and back into the drumspace. there might be a diagonal
layer of oxidized stucco mesh in the air heater, sloped from north to south.

something like this would be nice for a rustic cabin, as a part of solar
heating our 10'x12' shed. the sunspace frame has curved 1x3 spaced beams
on 4' centers (i just snapped 4 1x3s in bending them to less than an 8'
radius) and the 4x8x8' solar closet might be in the sunspace itself, which
leaves a 4x8' patch of sunspace wall. it would be nice to have a warmwater 
hose in the sunspace, for showers. at some point, the "solar closet" might
become an epdm-lined strawbale hot tub, as well, with a rigid automatically
movable reflective and insulating cover.

the water supply might be a 16x24' pond to the south, as well as the 8x12'
sunspace and 10x12' roof and 12x16 adjacent greenhouse area, about 800 ft^2
of rain catchment area, which should provide 64 gallons of water a day, where
i live, for an average water consumption of 8 gallons per hour over an 8 hour
day. enough water for 16 showers with 4 minutes of water usage at 2 gpm.

heating the water from 65 f to 105 f takes about (105-65)64x8 = 20k btu/day,
which could come from the 4x8' air heater, and raising it from 32 f to 65 f
takes about (65-32)64x8 = 16k btu/day. could this be done with a 4" x 10'
tempering pipe hanging inside the cabin, near the ceiling? it would hold 
about 56 pounds of water, with a surface area of about 10.5 ft^2, and the
main thermal resistance might be a (us) r2/3 still air film on the outside,
which would make rc = 0.67/10.5x56 = 3.5 hours. hmmm. we need something
closer to a half hour to keep up with the average water usage.

add some sort of fins and a small fan and a 6" concentric pipe to double the
first pipe area and reduce the r-value to about 0.2, by moving air past the
pipe at 8 mph? with a half-hour time constant, 7 gallons of water in a 70 f
room would warm up to 70-(70-32)exp(-1/0.5) = 65 f in an hour. or should we
put the pipe outside, near the peak of the warmer sunspace, with some
insulation above it? 

or hang a dark drum in the sunspace, and let it lose heat all night? a 55
gallon drum has about 25 ft^2 of area, so rc = 2/3/25x55x8 = 12 hours. suppose
we surround it with insulation on the back, and r1 glazing on the front. most
of the heat loss will be through the glazing, and on an average day, with some
sun, if the drum starts out at 32 f, and the sunspace is 80 f for 6 hours,
the water temp at the end of the day would be 80-(80-32)exp(-6/12) = 51 f.
adding 12ft^2x0.9x0.9x1000 btu/ft^2/dayx1.3reflective gain = 13k btu of direct
sun to this might raise the water temp another 13k/450 lb = 28 f to 79 f.
looks like the right ball park.

jade mountain's $59 4 gpm 12v 7a #wp100 amazon submersible pump might run
for 2 minutes every hour, with a float switch or 2 in one of the drums.

nick