|
|
re: storage for heat pump
20 mar 2001
pete prossen wrote:
>...due to the expense of the ground-source collection loop i've been
>wondering about alternatives. here's one of them:
>bury a 1500-gallon concrete septic tank and fill it with... about
>12,500 pounds of water that will stablize at 61 to 65 degrees (here
>in san diego county) through an earth-contact area about 190 square
>feet excluding the lid.
that's a lot of water, but heat flow seems more important than heat storage.
do you really need heat in san diego? nrel says it's 57.4 f in january, with
an average daily max of 65.9, and 1420 btu/ft^2 of sun falls on a south wall,
and a 1-axis ew horizontal tracker can collect 1205 btu of that.
what kind of soil do you have, and what kind of groundwater movement?
the 1993 ashrae hof lists "apparent thermal conductivity values" for soils
for use when no site-specific data are available, with conservatively low
values of 5.4 btu-in/h-ft^2-f (1/5.4 = r0.185 per inch or r2.22 per foot)
for sands, 11.4 for silts, 7.8 for clays and 6.6 for loams, "for use in
soil heat exchanger or earth-contact cooling applications."
we might think of the tank as an equivalent circuit with a few decreasing
series resistors between the water and deep earth temp. the first would be
the tank itself, say r0.8/190ft^2 = 0.0042 for 4" of concrete. now move
a foot out into the soil from the tank surface. if the tank is 5'x5'x10',
a 1' thick earth shell below and around it would be 6'x7'x12' od, with an
od surface of 312 ft^2, excluding the top, and a thermal resistance of
r2.22/312ft^2 = 0.0071 in sandy soil. another move, and a 7'x9'x14' shell
adds 0.0050, then 8'x11'x16' and 0.0037, 9'x13'x18' and 0.0028, and
10'x15'x20' and 0.0022, totaling 0.025. the next shell only adds 0.0018,
about 7% of the total, so we might stop there, with 0.0268.
so if the deep soil is say, 64 f, with 40 f tank water, (64-40)/0.0268
= 895 btu/h flows into the water.
>that's very close to the total contact area of 1,000 feet of the 3/4"
>polyhose normally used in ground-source collection fields. this length of
>hose can provide about 14,000 btu/hr to the heat pump...
try this with the hose. say r0.1/(1000'xpix3/4/12) = 0.00051 for the wall,
plus r0.185/(1000xpix2.75/12) = 0.00026 for the first cylinder, 1" thick,
plus r0.185/(1000xpix4.75/12) = 0.00015 for the second inch, plus 0.0001
for the third, then 0.000081, then 0.000066, then 0.000055, then 0.000048,
then 0.000042, then 0.000038 for an 18.75" cylinder, less than 3% of the
0.00135 total. the heat flow would be (64-40)/0.00135 = 17.8k btu/h.
>...enough heat to make up the geo-deficit could be provided by above-
>ground solar collectors.
you might put the tank above ground and heat it to 180 f with a parabolic
reflector and just use the heat pump for air conditioning. heat pumps are
noisy and expensive and energy-inefficient compared to solar heating. with
enough old tires at -$1.25 each, it would pay to build one of these, even
if you never used it.
>another alternative would be an insulated tank to store solar-heated water
>at much higher temperatures.
good idea.
>the water would have to be tempered down to
>about 70 degrees for presentation to the heat pump.
gee, you really like heat pumps :-)
nick
|
|
