Posted by Gene on May 3, 2006, 4:58 am
Does anyone have any reference material or expertise in underground
thermal storage in a deep shaft similar to a water well (except that it
is sealed from outside water intrusion)? The heated water from solar
collectors would be circulated through the shaft in a closed system
heating the surrounding bed rock creating a large stable thermal
Posted by Adrian Brentnall on May 3, 2006, 6:20 am
The only references I've heard to something like this use the energy
already present in deep earth shafts to generate electricity (Google
for <hot rocks geothermal energy>).
All depends on the scale of the solar collectors - but I can't see
that you'd get enough energy to signifcantly raise the temperature of
your bedrock - which might well have its own natural water flowing,
and transporting your solar heat away...?
If you're not going very deep with your shafts then you might as well
use a heat pump system and extract the low-drade heat that's already
there, rather than try to store your solar heat down there....
I may have misunderstood what you're suggesting....
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Posted by nicksanspam on May 3, 2006, 10:43 am
That long skinny shape sounds wasteful for heat storage. How about this?
The Lyckebo [Sweden] system is a cavern of 100,000 m^3 capacity, cut out of
bedrock using standard mining methods, of cylindrical shape, with a central
column of rock left to support the overhead rock. The cavern is about 30 m
high and its top is about 30 m below ground level. It is water filled, and
inlet and outlet pipes can be moved up and down to inject and remove water
from controlled levels. The water is highly stratified with top to bottom
temperatures of about 80 to 30 C. Figure 8.7.2 shows temperature profiles
in the store at various dates in the second year of operation... No thermal
insulation is used, and there is a degree of coupling with surrounding rock
which adds some effective capacity to the system. Losses occur to a semi-
infinite solid and can be estimated by standard methods. Observed losses
from this system are higher than those calculated; this is attributed to
small but significant thermal circulation of water through the tunnel used
in cavern construction and back through fissures in the rock. It takes
several years of cycling through the annual weather variations for a storage
system of this size to reach a "steady periodic" operation. In the second
year of its operation, while it was still in a "warm-up" stage, 74% of the
energy added to the store was recovered.
from p 404 of section 8.7, "seasonal storage," of _Solar Engineering
of Thermal Processes_, by John A Duffie and William A Beckman, 2nd
edition, 1991, Wiley-Interscience ISBN 0-471-51056-4