|
|
re: contemplating installation of solar water heating panels (dhw)
16 jan 2001
john becich wrote:
>...as we live in so. california, almost all of our gas usage is for
>hot water. we have plenty of sunshine, although not a lot of place
>to put solar panels, except on the roof of the garage...
suppose you live in la, where 890 btu/ft^2 of sun per day falls on
a horizontal surface in january, and 1300 falls on a south wall and
i guess sqrt(890^2+1300^2) = 1575 btu falls on a surface tilted up
90-atan(890/1300) = 56 degrees to the south. the 24-hour average temp
is 56.8 f, with a 65.7 daily max, about 61 f during daylight hours,
and a 23 f record low. the annual average (wellwater?) temp is 63 f.
>as there is no freeze here, a direct system should suffice, without
>the need for a heat exchanger.
but maybe a pump and a hot water storage tank. it seems to me that
most solar hot water systems are undersized.
>i wonder how i might quantify the thermal output of a solar panel.
>i'd like to equate it to "therms," so i might get some idea of the
>cost benefit of this, before taking the plunge.
a square foot of solar heating panel with an r1 single cover with 90%
solar transmission and 130 f water inside might gather 0.9x1575 = 1418
btu of sun and lose about 6h(130f-61f)1ft^2/r1 = 413 on an average day,
for an average net gain of 1005, so you'd need about 100 ft^2 of panels
to make a therm of hot water per day, heating 100000/(8(130-63)) = 187
gallons of 63 f water to 130. you might use an unpressurized insulated
7' diameter x 5' tall 1500 gallon $400 ag poly tank to store hot water
for a cloudy week, along with a $75 12v shurflo rv pump with built-in
pressure switch for the tank output.
for the collectors, you might make something like this with a horizontal
water-filled epdm rubber bladder and nielsen's mylar, $0.09/ft^2 in 4'
wide rolls from http://www.snomo.com/mylar.html...
the most serious mistake was making the outer container of the receiver
of plywood. we thought that the plywood would be sufficiently insulated
from the copper panel which was the receiver proper, that it would not
get too hot. the copper panel was separated from the plywood by 4" of
fiberglass insulation. nevertheless, the plywood caught fire and the unit
was completely destroyed. we suppose this is a success, of sorts...
from "a solar collector with no convection losses," (a downward-facing
receiver over a 4:1 concentrating parabolic mirror) written by
h. hinterberger and j. o'meara of fermilab, ca 1976
nick
|
|
