The activities feasibility of high temperature underground thermal storage of energy was investigated. Results indicate that salt cavern storage of hot oil is both technically and economically feasible as a method of storing huge quantities of heat at relatively low cost. One particular system identified utilizes a gravel filled cavern leached within a salt dome. Thermal losses are shown to be less than one percent of cyclically transferred heat. A system like this having a 40 MW sub t transfer rate capability and over eight hours of storage capacity is shown to cost about $13.50 per KWh sub t

Collins, R. E.

English

NASA Technical Reports Server

HIGH TEMPERATURE UNDERGROUND THERMAL ENERGY STORAGE 
SYSTEM FOR SOLAR ENERGY 
R. Eugene C o l l i n s  
U n i v e r s i t y  o f  Texas a t  A u s t i n  
PROJECT OUTLINE 
P r o j e c t  T i t 1  e: H igh  Temperature Underground Thermal Storage o f  Sol  a r  
Energy 
P r i n c i p a l  I n v e s t i g a t o r :  R. E. C o l l i n s  
Organ iza t ion :  Energy Foundat ion o f  Texas 
U n i v e r s i t y  o f  Houston 
Houston, TX 77004 
Telephone: (713) 749-3887 
P r o j e c t  Goals: The o b j e c t i v e  o f  t h i s  p r o j e c t  i s  t o  e s t a b l i s h  t h e  
f eas i  b i  1  i t y  o f  h i g h  temperature underground thermal  
s to rage  o f  energy and a r r i v e  a t  a p r a c t i c a l  system design. 
P r o j e c t  Status:  Resu l t s  t o  date i n d i c a t e  t h a t  s a l t  cavern s torage o f  h o t  
o i l  i s  bo th  t e c h n i c a l l y  and economica l ly  f e a s i b l e  as a  
method of s t o r i n g  huge q u a n t i t i e s  o f  heat a t  r e l a t i v e l y  
low cos t .  One p a r t i c u l a r  system i d e n t i f i e d  i n  t h i s  s tudy  
u t i l i z e s  a  g rave l  f i l l e d  cavern leached w i t h i n  a  s a l t  
dome. Thermal losses  are shown t o  be l e s s  than 
one-percent o f  c y c l  i c l y - t r a n s f  e r red  heat. A  system 1  i k e  
t h i s  hav ing  a  40 MWt t r a n s f e r  r a t e  c a p a b i l i t y  and over 
e i g h t  hours of s torage c a p a c i t y  i s  shown t o  cos t  about 
$13.50 per  KWht. 
Con t rac t  Number: XP-9-8319-1 
Con t rac t  Per iod:  August 1, 1979 - J u l y  31, 1980 
Funding Level  : $110,500 
Funding Source: So la r  Energy .Research I n s t i t u t e  
https://ntrs.nasa.gov/search.jsp?R=19800014315 2020-03-21T18:14:50+00:00Z
High Temperature Underground Thermal Storage of Solar Energy 
Principle Investigator: R. .Eugene Col 1 ins, Professor of Petroleum Engineering 
University of Texas a t  A u s t i n ,  A u s t i n ,  Texas 78712 
Sub-Sub Contract to  the Umiversity of Texas at .Austin,  through the Energy 
Foundation of Texas from the Solar Energy Research Ins t i tu te ,  
Contract # XP-9-8319-1. 
, Objectives: T h i s  report documents the second year of a feas ib i l i ty  
study of underground storage of solar  energy as sensible heat. T h i s  e f fo r t  
addresses storage temperatures high enough t o  u t i l i z e  conventional steam- 
e lec t r i c  power generation on the recovery cycle. The method of storage now 
under evaluation u t i l i zes  cavern storage of heat t ransfer  o i l  a t  temperatures 
u p  to  650°F in leached caverns w i t h i n  s a l t  domes. A study of aquifer storage 
of hot water a t  these temperatures was discontinued when i t  became apparent 
that  such storage would encounter major problems from mineral ( s i l  ica)  solution 
and requirements for  down-hole pumps for  the recovery cycle. Research and 
development ef for ts  have been focused on the following technical problems; 
a )  Thermal losses 
b) Cavern. stabi 1 i ty 
c)  cavern construction 
d) We1 1 designs 
e)  heat exchanger interfacing 
f )  economics for  cavern storage systems; 
Conclusions. Studies indicate that  s a l t  cavern storage of hot o i l  will 
be both technically and economically practical as a method of solar  energy storage 
for e lec t r i c  power generation. The best .system identified in th i s  study i s  a 
gravel f i l l ed  cavern using a t  l eas t  one input and one output well, operated 
in a thermocline mode with injection and retrieval on a diurnal cycle. Thermal 
losses should be less  than one percent of cyclicly-transferred heat. The 
gravel f i l l i ng  would ac t  as a heat storage medium and as a s t ab i l i ze r  against 
cavern deformation due t o  plast ic  flow of s a l t .  (See Figure 1. ). 
During the second contract year i t  has been shown that  such a system 
can be bui l t  using existing technology and available materials. In part icular ,  
the design and operation of such a system for  interfacing t o  a 10 MWe central 
receiver 1 i ke the one being bui l t  a t  Barstow, California has been evaluated, 
A cavern storage system could be bu i l t  for  about $4 million having a 10 We 
t ransfer ra te  capability and 8' hours of storage capacity. Storage would be 
a t  about ti50°F and cost about $13.50.per kwht. This compares favorably w i t h  
DOE objectives, but larger cavern storage syitems would be more cost effective 
w i t h  costs estimated a t  a low $7.50 per k w h t  Thus ,  cavern storage would be 
preferred to  above-ground storage where i t  i s  geologically feasible. A review 
and summary of the various studies carried out during the past year isolate 
and explain primary conclusions. 
GRAYEL PACK + OIL 
Figure 1. Schematic of gravel f i l l ed  s a l t  cavern for thermocline 
storage of heat using heat transfer o i l .  
Thermal Losses from Cavern Storage Systems. In the f i r s t  year of the 
five-year study, we have reported results  on thermal losses, calculated 
w i t h  a computer simulator assuming a cavern with perfect mixing (homogeneous 
temperature inside),  for  cyclic operating conditions. Those studies showed 
that  for daily cycles of a hot o i l  storage cavern--8 hours injection and 16 
hours retrieval--thermal losses would decline rapidly from a moderately high 
value to less than one percent of the transferred energy by the end of one 
year of operation. This model has also been used to study effects of non-cyclic 
operation that would result with shut-down of the solar collector i n  cloudy 
weather. These studies indicated that losses would not be prohibitive for shut- 
down periods up to several days i f  the cavern were sufficiently large. 
The simulator under study i s  an oversimplified model, for i t  assumes 
perfect mixing of oil  within a ~pherical cavern. The model a1 so describes a 
storage system consisting of two caverns, a hot cavern and a cold cavern with 
a nitrogen gas cap in each cavern. The nitrogen gas cap i s  compressed during 
fluid injeftion and the expansion of the gas produces back-flow in the retrieval 
cycle, thus negating the requirement of a downhole pump. However this system 
requires caverns a t  significant depth to sustain required internal pressures 
and poses problems of mechanical s tabi l i ty .  
I t  now appears that the preferred cavern storage system will be a single 
gravel f i l  led cavern with two wells operated in a thermocline mode using oil  and 
rocks essentially l ike the above ground tanks; one well connects to the top of 
the cavern (the hot we1 1) and one to the lower end of the cavern (the cold well ). 
(See Figure 1. ) . 
Gravel f i  11 ing in a thermal storage cavern serves three purposes; 
1 )  the gravel i s  a storage medium for sensible heat and reduces the required 
oil  volume, 2) the gravel restr ic ts  thermal convection and stabilizes the 
thermocline, and 3)  the gravel provides mechanical support and rigldity to 
the cavern to prevent cavern deformation due to creep of the s a l t ,  or plastic 
flow, provided that internal fluid pressure i s  less than external geostatic 
pressure. 
The perfect mixing model has therefore been p u t  aside and atteetion 
devoted to the formulation of computer simulators for the study of fluid mover 
ment, heat transfer, and thermal losses in a gravel f i l led  cavern operated as 
a thermocline with o i l .  preliminary results from these models show that the 
order of magnitude of the thermal loss 'I'n tR$s systm i s  no t  rndtcally dffferent 
from that in the homogeneous cavern. 
Two new simulators are being evaluated; the f i r s t  i s  designed 
to describe in detail the mass and heat flow within the cavern as well as the 
heat flow within the sa l t ,  while the second simulator i s  designed for accurate 
description of the heat flow across the cavern boundaries and wlthin the s a l t  
while the mass and heat flow w i t h i n  the cavern a re ' t r ea ted  by more approximate 
means. T h i s  program is operational and i s  being used t o  generate systematic 
data on cavern operations. 
The model has already been used t o  show tha t  a thermocline cavern storage 
system' fo r  a small scale system (10 MUe) w i t h  eight hours storage would lose 
about 2.6% of the useful stored heat during one daily cycle a f t e r  about three 
months of continuous operation. The loss ra te  a t  the end of one year of con- 
tinuous operation i s  approximately 1.5%. For systems of larger s ize  (100 MWe or 
more) the long term loss ra te  would be 1%, or less ,  of the cyclicly transferred 
heat . 
Cavern Stabi l i ty  Studies. The major problem anticipated for  the cavern 
storage system was cavern deformation due to creep of the s a l t  a t  high tempera- 
tures,  or  plast ic  flow. Published experimental data for  a wide range of s t ra ins ,  
rates of s t ra in  and temperatures have been curve-fitted t o  provide an adequate 
description of the viscoelast ic/plast ic  properties of ha1 i te .  This rheol ogical 
model for  hal i te  i s  given as a general equation of s t ra in  ra te  as a function of 
s t ra in ,  differential  s t ress  and temperature for  u n i  -axial conditions. For 
particular constant s t ress  and temperature values i t  can be integrated numeri- 
cal ly t o  give the to ta l  s t ra in  as a function of time. For a temperature of 
59g°F and an expected differential  pressure of 300 psi ,  the integration yields 
a total  s t ra in  on an element of hal i te  of 1.8% in 20 years or 2.2% i n  50 years. 
The same element of hal i te ,  when subjected to  a differential  pressure of 600 psi 
a t  the same temperature would suffer  a total  deformation of 8.4% i n  20 years 
or 14% i n  50 years. Note that  these resul ts  do not take into account the r ig idi ty  
of the gravel pack which would functton t o  reduce these d e f g r m ~ t i ~ n s ,  
Gravel f i l l i n g  will be a greater deterrent to  cavern deformation than 
i n i t i a l l y  anticipated. I t  is well known that  a container f i l l e d  with r igid 
granules i n  firm contact cannot undergo a shear deformation without expansion of 
the container volume. Thus ,  since the pressure of the overburden must be over- 
come, i n  order for  the cavern t o  expand, the gravel should give great r ig idi ty  
t o  the cavern. A simulator i s  currently being developed t o  determine the defor- 
mation and ra tes  of deformation experienced by a spherical cavity f i l l e d  w i t h  
saturated gravel, the boundary of which i s  subjected t o  hydrostatic loading. 
T h i s  simulator i s  now in  the debugging stage. A survey of the l i t e ra tu re  is 
a1 so underway t o  find sufficient  data to  determine the el astSc/creep constants 
of saturated sands t o  be used as input t o  the simulator. 
The c r i t i c a l  factor for  the stabil izing role of the grayel on the cavern 
appears t o  be maintenance of f lu id  pressure w i t h i n  the pore space less  t h a n  
the confining geostatic pressure of the s a l t .  An analyttcal solution fo r  the 
pressure distr ibution w i t h i n  the cavern has been obtained, valid w i t h i n  some 
simp1 i fying assumptions, which shows that  f lu id  pressure increases required 
t o  maintain f lu id  flow will generally be on the order of a few hundred psi. 
Such pressures could be tolerated i n  caverns a t  reasonable depths, Further 
study of these flow effects  i s  underway, using both analytical and numerical 
methods. 
Thermal Storage Fluids. The optimum f luid  t o  be used in the cavern 
storage system would minimize replacement costs for  a given power from the 
system. Replacement costs are directly proportional to  f lu id  loss ra te  due 
to  thermal degradation. Published experimental data for  f lu id  loss ra tes  of 
some commercially avai 1 able heat t ransfer  f lu ids  have been examined from this 
viewpoint. Calovia HT43, SUN 21 and Therminol 66 were considered. Of these 
i t  appears that  Caloria HT 43 would be most suitable w i t h  a loss ra te  of about 
6.25% per year a t  600PF. However, more information is  requlred on effects  of 
contact with metals, a i r ,  water and s a l t  on ra te  of degradation. 
Well Designs and Cavern Construction. A comprehensive review of the 
1 i terature indicated that  existing equipment, materials, and procedures for  oi l  
wells can be adopted fo r  construction of wells for  the cavern storage system. 
The win body of the second-year report on this project provides a detailed 
description of the process of d r i l l ing ,  completing the wells, leaching the 
caverns, and placing gravel i n  the caverns. 
The major consideration i n  the well designs i s  the requirement of adequate 
cement bond i n  the ins ta l la t ion of the well casing pipe t o  constraln the pipe 
against thermal expansion. The thermal expansion of the injection tubing can 
be accomnodated by thermal expansion joints .  A r ig id ,  porous s i l i c a  foam 
coating on the exterior of this tubing can provide good thermal insulation. 
Other major considerations i n  the design of cavern systems are brine 
disposal while leaching and the technique of gravel placement. 
Detailed casing designs have .been formulated fo r  wells of three f l ~ w  
rates,  1500 gpm, 2000 gpm, and 2500 gpm, and two depths, 3000 f t  and 5000 f t .  
Detailed cost estimates account for  materi.al s ,  instal  la t ion,  and supervision, 
Costs have also been estimated for cavern leaching and gravel f t l l i ng .  
Heat Exchangers and Power Generation. Since a specific solar  col lector 
has not yet  been identif ied t o  interface w i t h  a cavern heat storage system, 
the 10 We system being bu i l t  a t  Barstow, California, was used fo r  evaluation 
purposes. Using data provided i n  the McDonnell Douglas report on this system, 
design studies were carried out to  determine the type of modifications of the 
heat exchangers and power generation equipment that  might be required i n  order 
t o  interface the solar  collector w i t h  a cavern storage system. Though some 
modifications would be necessary, none are considered t o  be of major s$gnifl- 
cance. 
Larger systems--100 MWe t o  1000 Me-- would require a much more complex 
interfacing of the solar  collector,  cavern storage, and steam-electric turbines, 
One proposed design for such larger systems i s  a ~ross~compound system using 
steam-electric conversion a t  two different  temperature levels ,  one di rect  
from the collector and one di rect  from the cavern, w i t h  cross coupling, T h i s  
would be desirable since output from the solar tower would be a t  900-1000°F 
while cavern storage i s  limited t o  about 650QF by the oil  storage f luid.  
Detailed cost estimates were prepared for  the 10 MWe system and some 
preliminary estimates of costs for  larger systems were made; these are  shown 
below. 
Economics. The to ta l  cost ,  C ,  of a cavern storage system i s  given by 
where 
Cc = u n i t  volume cost of cavern and contents (gravel, oi 1 ) 
V = cavern volume 
Cw = costs of hot and cold wells 
CD = cost of brine disposal well 
Ca = cost of above ground equipment. 
For systems of the s ize  envisioned i n  t h i s  study, one brine disposal well 
(CD = $620,000) would be adequate fo r  the cavern leaching operation. The u n i t  
3 
volume cost of the cavern and i ts  contents is approximately $2.85/ft , Thls 
i ncl udes costs of leaching, gravel placement, and oi 1 , The cavern volume i s  
dependent upon the desired storage capacity while Cw and Ca depend upon the 
desired transfer  rate.  
Cost estimates fo r  the components o f  a cavern system w i t h  a t r a n s f e r  
r a t e  o f  33 MWt and 8-hour storage period, given i n  vartous sect ions o f  the  
complete second year  repor t ,  a re  sumar jzed below: 
Tota l  storage system costs f o r  33 MWt 
t ransfer  r a t e  and 8-hour storage per iod. 
m i l l l o n  $ 
3 Cavern Contents (139,000 ft @ cc = $2.85) 0.396 
Cw (1  ho t  we1 1, 1 co ld  we1 1 ) 1.638 
CD ( 1  b r i n e  disposal  w e l l )  .620 
Ca (heat exchangers, pumps) - ,733 
TOTAL $3.387 
The t o t a l  cos t  o f  t h i s  system i s  approximately $3.4 m i l l i o n .  Th is  sum 
corresponds t o  t o t a l  storage system costs of $103/kWt and $13/kWht. These 
f i gu res  compare favorably  w i t h  Department o f  Energv cos t  goals f o r  near term 
sensible heat storage. Cost goals (Ct) can be compared t o  power r e l a t e d  costs 
(Cp) and capaci ty  r e l a t e d  costs (CS). Power r e l a t e d  cos t  (C ) depends upon P 
the c a p a b i l i t y  of the storage system t o  accept and d e l i v e r  thermal energy a t  
a given r a t e  (heat exchangers, wel ls,  pumps, etc.) .  Capacity r e l a t e d  cos t  
(Cs) i s  r e l a t e d  t o  the maximum amount o f  energy t h a t  can be contained w i t h i n  
storage ( o i l ,  gravel ,  cons t ruc t ion  costs of cavern capaci ty) .  These r e l a t i o n s  
are shown be1 ow: 
Storage System Costs 
Here: Ct($/kWt) = C + h0Cs P *DOE 6 hour costs converted t o  8 hour costs f o r  d i r e c t  compari- 
son w i t h  t h i s  study. 
Ct($/kWht) = (Cp + h*Cs)/h 
h 
hours o f  
storage 
6 
8 
8 
C~ 
power cos t  
$/kWt 
45 
4 5 
9 1 
t Cs Gapaci ty  
cos t  
$/kwht 1 
7.50 
7.50 
1.50 
.- 
$/kWt 
DOE Goals 90 
DOE Goals* 105 
This  study 103 
(small system) 
$/kwht 
15 
13.3 
13 
The underground storage value for the power cost i s  about twice the DOE goal, 
b u t  the capacity cost is  much less.  This difference exis ts  because the 
underground system has very low containment costs which reduce capactty 
related costs (CS), b u t  the power related costs ( C  ) are increased primarily P 
due to  the well costs. 
Cost figures quoted are  for  a rnInlmum underground system, costs for  
larger commercial scale systems would be less. Minor modificatfons In well 
design would conceivably allow doublfng the flow rates used In this study, 
With th i s  change, the cost of a large storage system (100 We or larger) 
would be approximately $60/kWt or $7.50 k w h t  Therefore, cavern storage 
appears to be an a t t ract ive  option for near term sensible heat storage for 
solar power systems of large s f  ze. Cavern storage may a1 so be economically 
favorable for  storage periods long enough (16 hours) to  provide baseline 
e lect r ic  power. 


High temperature underground thermal energy storage system for solar energy

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