The background of thermal storage development at the Midtemperature Solar Systems Test Facility is reviewed. The problems which were encountered are discussed and a course of action for resolving the problems is outlined. Scaling effects of going from laboratory models to full-size applications were determined and applied to thermal storage needs in near-term solar projects

Harrison, T. D.

Randall, R. A.

English

NASA Technical Reports Server

THERMAL STORAGE EXPERIENCE AT THE MSSTF AND PLANS FOR THE FUTURE* 
Thomas D. Harr ison and Robert A. Randall  
Sandia Labora tor ies  
Albuquerque, New Mexico 8718 5 
SUMMARY 
The purpose of  t h i s  p re sen ta t ion  is 1) to review t h e  background of t he r -  
mal s to rage  development a t  t h e  Midtemperature Solar  Systems Tes t  F a c i l i t y  
(MSSTF) a t  Sandia Labora tor ies ,  2) to d e f i n e  the  problems which have been 
encountered, 3)  to o u t l i n e  a course of  a c t i o n  f o r  reso lv ing  these  problems, 
4) t o  determine s c a l i n g  e f f e c t s  of going from l abora to ry  models to f u l l - s i z e  
a p p l i c a t i o n s ,  and 5) to apply  the  l e s sons  learned  to thermal s t o r a g e  needs i n  
near -term s o l a r  p r o j e c t s  . 
WLTI PLE TANK THERMAL STORAGE 
The Mul t ip le  Tank Thermal Storage subsystem shown i n  Figure 1 was de- 
signed a t  Sandia t o  provide thermal s t o r a g e  f o r  the t e s t i n g  ope ra t ions  and to 
eva lua t e  t h e  mul t ip l e  tank concept i n  an ope ra t ing  environment. A three-tank 
des ign  was chosen because the  ope ra t ing  s t r a t e g i e s  and c o n t r o l  problems a r e  
r ep re sen ta t ive  of  those  encountered i n  t h e  f i e l d  with a l a r g e r  system. 
The b a s i c  des ign  requirement f o r  the  mul t ip l e  tank subsystem is to s t o r e  
860kWh ( 2 . 9 3 5 ~ 1 0 ~  Btu) thermal energy between the  temperature l i m i t s  of  241" 
t o  30g°C (467O t o  588°F). The h e a t  t r a n s f e r  and s t o r a g e  medium is Therminol 
66@- A volume o f  22.62 m 3  (6 000 g a l l o n s )  o f  Therminol 66@ w i l l  s a t i s f y  t h i s  
requirement. The subsystem was designed so t h a t  any two of the three  
i d e n t i c a l  tanks could hold a volume o f  23.24 m3 (6 140 ga l lons )  . Allowing 
for  10% u l l age ,  t he  volume of  each tank is 12.78 m3 (3 377 g a l l o n s ) .  
The subsystem w a s  requi red  to d e l i v e r  thermal  energy a t  a r a t e  o f  283 kW 
(0.966 Btu/h) and to r ece ive  thermal  energy a t  a r a t e  of 502 kW ( 1 . 7 1 3 ~ 1 0 ~  
Btu/h). To f a c i l i t a t e  pumping of  t h e  s to rage  f l u i d ,  a g a s  p re s su re  of 110 
kPa (16 p s i )  is maintained i n s i d e  the  tanks. Because of t h i s  pressure  t h e  
tanks  were designed to meet ASME pres su re  v e s s e l  s tandards.  
* Sandia Labora to r i e s  is a Department o f  Energy (DOE) f a c i l i t y .  This  work was 
supported by t h e  Divis ion o f  So la r  Technology, USDOE, .under Cont rac t  
DE-AC04-76 DP00789. 
https://ntrs.nasa.gov/search.jsp?R=19800014311 2020-03-21T18:14:43+00:00Z
m e  s p e c i f i c  dimensions and shape of the  t anks ' and  t h e  choice of  type 
and th i ckness  of tank i n s u l a t i o n  were determined wi th  t h e  a i d  of a computer 
program developed a t  Sandia. This  program predic ted  h e a t  l o s s  through the  
tank s u r f a c e s  f o r  t h e  d a i l y  ope ra t ing  cyc le  of  t y p i c a l  win ter  o r  summer days, 
g iven  tank shape and i n s u l a t i o n  types  and th icknesses .  Af te r  t he  tank shape 
was e s t a b l i s h e d ,  t h e  c r i t e r i a  f o r  s e l e c t i o n  of i n s u l a t i o n  th i ckness  and type  
was minimum cost. The t o t a l  es t imated cost, using c u r r e n t  p r i c e s ,  was com- 
puted f o r  va r ious  th icknesses  and types  o f  i n s u l a t i o n .  Added to t h i s  was t h e  
cost of  e x t r a  c o l l e c t o r  a r e a  to compensate f o r  hea t  loss. The most econom- 
i c a l  choice f o r  m a t e r i a l  is in te rmedia te  s e r v i c e  f i b e r g l a s s .  The optimum 
th i ckness  is 0.4 metre (15.6 inches) .  Since t h e  cost inc reases  only  slowly 
a s  t h i ckness  i nc reases ,  t h e  t h i ckness  f i n a l l y  chosen f o r  t h e  tank i n s u l a t i o n  
was 0.533 metre (21 inches)  . 
Each tank con ta ins  ins t rumenta t ion  fo r  sensing both temperature and 
l i q u i d  l e v e l .  A t  t h e  bottom o f  each tank, a 45-cm (18-inch) diameter  "well" 
is provided so t h a t  nea r ly  a l l  the  l i q u i d  i n  each tank can be drained.  A l l  
l i n e s  f o r  f i l l i n g  and dra in ing  t h e  tank e n t e r  a t  t h e  wel l s .  
The s i g n i f i c a n t  problems encountered were 1) t h e  need fo r  a complex 
c o n t r o l  system, 2) an energy l o s s  o f  about  50% i n  excess  of des ign  c a l -  
c u l a t i o n s ,  and 3) accumulation of  law temperature f l u i d  i n  t he  bottom o f  t he  
tank. 
The c o n t r o l  complexity a r i s e s  from the  500 o r  more p o s s i b l e  combinations 
of t h e  many parameters  involved i n  t h e  ope ra t ion  of a three-tank system. The 
reasons  fo r  the  high thermal  l o s s e s  a r e  st i l l  under review, bu t  apparent ly  
i nc lude  1) l o s s  o f  hot  i n e r t  gas  from u l l age ,  2)  improper i n s t a l l a t i o n  and 
subsequent degradat ion of  i n s u l a t i o n ,  and 3)  convect ion,  o r  thermal siphon- 
ing ,  of  hea t  t r a n s f e r  f l u i d  along h o r i z o n t a l  p ipes  lead ing  t o  c losed  valves.  
Af te r  a per iod o f  s to rage ,  t hese  l o s s e s  r e s u l t  i n  t he  accumulation i n  t he  
bottom of t h e  tank o f  up t o  1.9 m3 (500 ga l lons )  of  f l u i d  which has cooled 
below the  usable  temperature.  This  c o o l  f l u i d  is the  f i r s t  t o  be de l ive red  
from t h e  tank ard  s p e c i a l  ope ra t ing  s t r a t e g i e s  a r e  requi red  t o  d e a l  wi th  it. 
THERMOCLINE THERMAL STORAGE 
The f i r s t  thermal  s to rage  subsystem i n s t a l l e d  i n  t h e  MSSTF was t h e  the r -  
mocline subsystem shown i n  Figure 2. This  system was used to eva lua t e  t he  
s to rage  of  thermal  energy i n  water a t  temperatures  up t o  232°C (450°F),  or  i n  
Therminol 66@ a t  temperatures  up t o  320°C (608°F). The volume of the  tank 
between t h e  d i f f u s e r s  is 5 m3 (1 563 g a l l o n s ) .  The t h e o r e t i c a l  energy 
s to rage  capac i ty  is 292 kWh ( l o 6  k J )  f o r  Therminol 66@ between the  tempera- 
t u r e s  243' t o  311°C (470" t o  592OF). The subsystem is composed of f i v e  
elements:  
1. A low-carbon-steel p r e s su re  ves se l ,  f a b r i c a t e d  to ASME Pressure  
V e s s e l  Codes, w i t h  2.5-cm (1-inch) t h i ck  w a l l s ,  
2. Vacuum f o i l  i n s u l a t i o n  around the  v e r t i c a l  w a l l s  o f  t h e  tank,  
3. D i f f u s e r s  a t  t h e  t o p  and bottom o f  t h e  tank to minimize flow d i s t u r -  
bances when f l u i d  is pumped i n  or  drawn o u t  o f  t h e  tank,  
4. Two "T" va lves  to a l l aw  i n j e c t i o n  of  f l u i d  a t  310°C (590°F) i n t o  t h e  
t o p  o f  t h e  tank whi le  s imultaneously withdrawing f l u i d  a t  243OC 
(470°F) from t h e  bottom o f  t h e  tank , and v i c e  ve r sa ,  and 
5. A temperature probe to, measure t h e  v e r t i c a l  temperature  p r o f i l e  o f  
t h e  f l u i d  w i t h i n  t h e  tank.  
The major d i sadvantages  i n  t h i s  thermocline s t o r a g e  subsystem a r e  
1) t h e  thermocl ine reg ion  i n i t i a l l y  occupies  20% of  f l u i d  volume, 2)  t h e  
thermocline e n l a r g e s  with t i m e ,  3) energy i n  t h e  thermocl ine reg ion  is 
u s u a l l y  no t  usab le  because o f  l o w  q u a l i t y ,  4) thermal  energy losses a r e  i n  
excess  o f  de s ign  c a l c u l a t i o n s .  The advantages a r e  1) c o n t r o l  is s i m p l i f i e d ,  
2) withdrawing ho t  f l u i d  from t h e  t o p  of  t h e  v e s s e l  g i v e s  h igh  assurance  t h a t  
it w i l l  be a t  t he  proper temperature ,  3) t h e  subsytem is adaptab le  to mult i -  
medium s to rage ,  and 4) t h e  subsystem is 20% less expensive than  a m u l t i p l e  
tank system o f  the  same thermal  s t o r a g e  capac i ty .  Because o f  t he se  
advantages,  Sandia is concen t r a t i ng  f u t u r e  e v a l u a t i o n  e f f o r t  on t h e  
thermocline concept.  
Other thermocl ine subsystems a r e  being eva lua ted  by Sandia. The sub- 
systems i n s t a l l e d  a t  t h e  Wi l l a rd ,  NM, and Coolidge, AZ,  I r r i g a t i o n  P r o j e c t s  
a r e  p a r t  of  l a r g e  s o l a r  systems and a r e  no t  instrumented fo r  thorough evalu- 
a t i o n  of  t h e  thermal  s to rage  subsystem. Never the less ,  some u s e f u l  informa- 
t im  is being obtained.  
FUTURE ACTIVITIES 
The problems def ined  i n  t h e  two e x i s t i n g  thermal  s t o r a g e  subsystems a t  
t he  MSSTF cannot be solved using t h e s e  f a c i l i t i e s ,  e i t h e r  because instrumen- 
t a t i o n  is lack ing  o r  because o f  t h e  d i f f i c u l t y  of  modifying e x i s t i n g  hard- 
ware. The thermal  s t o r a g e  subsystems a t  Wil lard and Coolidge a r e  instrumented 
f o r  a n a l y s i s  o f  g r o s s  o p e r a t i o n a l  e f f e c t s  and not  f o r  t h e  s tudy o f  macroscopic 
even t s  which occur i n  a thermocl ine s t o r a g e  f a c i l i t y .  For t h e s e  reasons  a 
new thermocl ine tank was designed. 
A new thermocline tank is p r e s e n t l y  being i n s t a l l e d  i n  p l a c e  of  t h e  o l d  
one a t  t h e  MSSTF (see F igu re  3 ) .  The o b j e c t i v e  is to produce u s e f u l  d e s i g n  
information f o r  t he  i n s t a l l a t i o n  and e f f i c i e n t  ope ra t i on  o f  thermocl ine stor- 
age subsystems f o r  use w i th  l ine- focus ing  s o l a r  collectors used i n  t o t a l  
energy and i n d u s t r i a l  process hea t  app l i ca t i ons .  It w i l l  be opera ted  a s  a 
single-media, d i r e c t  s t o r a g e  thermocl ine tank.  The c a p a c i t y  w i l l  be 4.5 m3 
(1 179 g a l l o n s ) .  The tank is made o f  4.76-mm (3/16-inch) l o w  carbon steel 
with  s t a i n l e s s  steel l e g s  to minimize conduction. Mul t ip l e  p o r t s  a t  t h e  t o p  
and bottom g i v e  access  f o r  plumbing and ins t rumenta t ion .  The e n t i r e  t o p  can  
be removed i f  requi red  for i n t e r n a l  modi f ica t ions  o r  changes i n  instrumen- 
t a t i o n .  A 45-cm (18-inch) diameter  hatch p rov ides  personnel  access .  The 
i n s t a l l a t i o n  of  s t a i n l e s s  steel l e g s  and o f  t he  t h i n n e s t  w a l l s  p o s s i b l e  is a 
r e s u l t  o f  p a s t  exper ience  . 
Various d i f f u s e r  des igns  can be i n s t a l l e d  f o r  eva lua t ion .  The tank is 
instrumented wi th  350 temperature s enso r s .  Two v e r t i c a l  thermocouple probes 
w i l l  be  i n s t a l l e d  i n s i d e  the  tank to measure t h e  temperature a t  5-cm (2-inch) 
i n t e r v a l s .  Also, thermocouples w i l l  be a t t ached  to both t h e  i n s i d e  and out -  
s i d e  wa l l s  of  the  tank ,  and t h e r e  w i l l  b e  thermocouples between each l aye r  of  
i n s u l a t i o n .  A displacement-type gage w i l l  measure f l u i d  l e v e l .  Four t u r b i n e  
flowmeters w i l l  measure r a t e  of  flow of  hot  and co ld  f l u i d  i n t o  and o u t  of  
t h e  tank. The d a t a  from t h e  s e n s o r s  a r e  acqui red  and s t o r e d  by a minicom- 
pu te r .  Plumbing connect ions al low hot  f l u i d  t o  be rece ived  from e i t h e r  t he  
s o l a r  c o l l e c t o r s  o r  from t h e  Mul t ip l e  Tank Thermal s t o r a g e  subsystem. The 
tank l e g s  w i l l  r e s t  on a 5-cm (2-inch) pad of  load-bearing i n s u l a t i o n .  The 
tank w i l l  be i n su la t ed  wi th  38 c m  (15 inches)  o f  f i b e r g l a s s .  
Subscale  models of  d i f f u s e r  des igns  w i l l  be t e s t e d  i n  t he  l a b o r a t o r y  and 
then f ab r i ca t ed  f u l l  s c a l e  f o r  t e s t i n g  i n  t h e  system. The f i r s t  des ign  has  
been completed and the  d i f f u s e r  f abr  i ca t ed  . 
Subscale tests a r e  a l s o  being conducted to i n v e s t i g a t e  t h e  s t a b i l i t y  o f  
thermoclines.  Fu l l - s ca l e  t e s t i n g  of  t he  new tank w i l l  begin e a r l y  i n  1980. 
Tes t ing  w i l l  i nc lude  1) hea t  loss measurements under s t eady- s t a t e  flow condi- 
t i o n s  a t  temperatures  of  200°, 260°, and 315°C (400°, 500°, 600°F); 2)  s t a t i c  
hea t  l o s s  tests with i n i t i a l  temperature a t  t h e s e  same l e v e l s ;  and 3 )  tests 
of  thermocline s t a b i l i t y  and du ra t ion  under s t a t i c  cond i t i ons ,  a t  va r ious  
charge and d ischarge  r a t e s ,  and a t  var ious  temperatures .  A l l  t h e s e  tests 
w i l l  be  done both wi th  the  tank completely f i l l e d  using an a u x i l i a r y  tank t o  
accomodate expansion, and then  with u l l a g e  space i n  t h e  tank. 
The d i f f e r e n c e  i n  performance between l abo ra to ry  s c a l e  models and re -  
s u l t i n g  components of  t h e  new thermocline tank w i l l  p rovide  a measure o f  
s c a l i n g  e f f e c t s .  
One c u t p u t  o f  t h e  eva lua t ion  w i l l  be a des ign  handbook f o r  t h e  i n s t a l -  
l a t i o n  and e f f i c i e n t  ope ra t ion  o f  s t o r a g e  concepts  f o r  i n d u s t r i a l  p roces s  
h e a t  and o the r  s o l a r  app l i ca t ions .  
REFERENCE 
1. Sarr i son ,  T. D., - e t  a l .  : Solar  Energy Tes t  F a c i l i t y  Resu l t s ,  High 
Temperature Thermocline Storage Subsystem. SAND77-1528. Sandia 
~ a b o r a t o r  ies, Albuquerque, NM, A p r i l  1978. 
Figure 1. The Multitank Thermal Storage Subsystem 
70  um ( 2 . 7 5  in.) 
TOP DIFFUSER 
ANNULUS 
CONTAIN1 
VACUUM 
BOTTOM DIFFUSER 
TOP-VIEW 
25.4 m (1 .0  i n . )  
CROSS-SECTION 
Figure 2. Cross Section and Top View of 
High-Temperature .Storage Tank 



Thermal storage experience at the MSSTF and plans for the future

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