November 1989 

Phase-Change Thermal 

Energy Storage 

Final Subcontract Report 

On the Symposium Held 

October 19-20,1988 

Helendale, California 

Luz International Ltd. 

Solar Energy Research Institute 

CBY Associates, Inc. 

Prepared under Subcontract No. HH-9-18108-1 
SERIISTR-250-3516 

UC Category: 202 

DE89009426 

Phase-Change Thermal 

Energy Storage 

Final Subcontract Report 

On the Symposium Held 

October 19-20,1988 

Helendale, California 

Luz International Ltd. 

Solar Energy Research lnstitute 

CBY Associates, Inc. 

November 1989 

SERl Technical Monitors: 

Russell Hewett and Bimleshwar Gupta 

Prepared under Subcontract No. HH8-18108-1 

Solar Energy Research lnstitute 

A Division of Midwest Research Institute 

161 7 Cole Boulevard 

Golden, Colorado 80401 -3393 

Prepared for the 

U.S. Department of Energy 

Contract No. DE-AC02-83CH10093 
NOTICE 

This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any 

agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, cornpleteness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 

Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. 

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can be found in the current issue of the following publications which are generally available in most libraries: Energy Research Abstracts (ERA); Government Reports Announcements and Index (GRA and I); Scientific and Technical Abstract Reports (STAR); and publication NTIS-PR-360 available from NTlS 

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PREFACE 

The research and development described in this document was conducted 

within the U .S. Department of Energy's Solar Thermal Technology 

Program. The goal of this program is to advance the engineering and 

scientific understanding of solar thermal technology and to establish the 

technology base from which private industry can develop solar thermal 

power production options for introduction into the competitive energy 

market. 

Solar thermal technology concentrates the solar flux using tracking mirrors 

or lenses onto a receiver where the solar energy is absorbed as heat and 

converted into electricity or incorporated into products as process heat. 

The two primary solar thermal technologies, central receivers and distributed receivers, employ various point and line-focus optics to concentrate 

sunlight. Current central receiver systems use fields of heliostats (twoaxes tracking mirrors) to focus the sun's radiant energy onto a single, 

tower-mounted receiver. Point focus concentrators up to 17 meters in 

diameter track the sun in two axes and use parabolic dish mirrors or 

Fresnel lenses to focus radiant energy onto a receiver. Troughs and bowls 

are line-focus tracking reflectors that concentrate sunlight onto receiver 

tubes along their focal lines. Concentrating collector modules can be used 

alone or in a multimodule system. The concentrated radiant energy 

absorbed by the solar thermal receiver is transported to the conversion 

process by a circulating working fluid. Receiver temperatures range from 

100°C in low-temperature troughs to over 1500°C in dish and central 

receiver systems. 

The Solar Thermal Technology Program is directing efforts to advance and 

improve each system concept through solar thermal materials, components, 

and subsystems research and development and by testing and evaluation. 

These efforts are carried out with the technical direction of DOE and its 

network of field laboratories that works with private industry. Together 

they have established a comprehensive, goal-directed program to improve 

performance and provide technically proven options for eventual incorporation into the Nation's energy supply. 

To successfully contribute to an adequate energy supply at reasonable cost, 

solar thermal energy must be economically competitive with a variety of 

other energy sources. The Solar Thermal Technology Program has developed components and system-level performance targets as quantitative 

program goals. These targets are used in planning research and development activities, measuring progress, assessing alternative technology 

options, and developing optimal components. These targets will be pursued 

vigorously to ensure a successful program. 

Thermal energy storage offers important operating advantages to solar 

electric power plants. The work described in this report was conducted 

under a subcontract to assess storage options. Although the primary focus 

of the work was on phase-change storage materials, other reasonable 

alternatives were considered. 

iii 
The symposium was jointly organized by Luz International Ltd. and SERI 

with the valuable assistance and planning capabilities of CBY Associates. 

Moreover, the assistance of other organizations that provided time and the 

travel expenses of the participants is both acknowledged and appreciated. 

These organizations were Oak Ridge National Laboratories, Pacific 

Northwest Laboratories, the University of Houston, Platforma Solar 

de AlmerialDFVLR, Argonne National Laboratories, and Luz International 

Ltd. 

Approved for 

SOLAR ENERGY RESEARCH INSTITUTE 

\<&- 

Russell Hewett 

Technical Monitor 

/4& 

B. P. Gupta, Leader 

Solar ~hermal Program 
CONTENTS 

INTRODUCTION ............................................... 1 

BACKGROUND INFORMATION ................................... 2 

Background Paper ........................................... 2 

Abstracts of Presentations ..................................... 2 

SUMMARY OF DISCUSSIONS ..................................... 6 

Cost Goals for SEGS Plants ................................... 6 

Phase Change Storage ........................................ 7 

Sensible Heat Storage ....................................... 9 

Chemical Storage ........................................... 10 

Ranking of Technologies and Approaches ......................... 10 

Phase Change Storage Approaches .......................... 10 

Alternative Thermal Energy Storage Options .................. 11 

Required Analysis and Cost Studies .............................. 12 

Research Needs ............................................ 13 

Program Issues ............................................. 13 

CONCLUSIONS ................................................. 14 

RECOMMENDATIONS ........................................... 14 

APPENDICES 

Background Paper 

Visual Aids from Presentations 

List of Participants 
INTRODUCTION 

Thermal energy storage (TES) offers important operating advantages to solar electric power 

plants. Luz International Ltd. has successfully commercialized solar electric plants at the 

30-80 MWe level as evidenced by the operation of SEGS I-V (&lar Electric Generating 

Systems) in the Mohave desert region of California, and the ongoing construction and 

design activities of SEGS VI-VIII. By the end of 1988, 194 MWe will be on-line. 

Heretofore, the use of thermal energy storage for these plants at the current operating 

temperature of the solar field, 735°F (397*C), has not been possible because cost-effective, 

reliable TES systems have not been available. 

In seeking a solution to this need, Luz developed two conceptual designs of TES systems 

utilizing phase change materials in the temperature range of interest for a SEGS plant. 

[These conceptual designs are described in the pre-assessment background paper by Luz 

included in this report.] Further, Luz proposed that the Department of Energy and Luz 

join in a cost-shared program to develop such a TES system. As a first step, it was decided 

that experts in phase change storage technology or related approaches should be assembled 

to provide advice and input into the feasibility, content and structure of a program designed , to provide cost-effective energy storage, with emphasis on phase change materials, for 

SEGS-type facilities. The Symposium described in this report is the result. 

While phase change storage was the primary focus of the Symposium, it was recognized that 

reasonable alternatives to this approach would be appropriate for discussion. Consequently, 

the experts assembled for the meeting represented a fairly broad spectrum of knowledge 

and expertise in thermal energy storage issues. The Symposium was organized to evoke 

discussion and seek tentative recommendations on key economic and technical issues 

rela,ted to the topic. More specifically, the primary subtopics of the Symposium were: 

The value of thermal energy storage for large intermediate temperature solar 

electric plants, 

The materials, engineering and economic issues relevant to phase change TES 

systems, 

Candidate TES systems for this application other than phase change materials, 

including the materials, engineering and economic issues for each, 

Expected key research and development needs for favored candidate TES 

systems, and the programmatic avenues to further development of the selected 

technologies. 
BACKGROUND INFORMATION 

Background Paper 

A paper was prepared by Luz Industries-Israel to provide both a background on the 

operation of the SEGS plants and the analysis of the thermal energy storage options 

already performed by Luz. The full paper is included in this report (Appendix A) and an 

abstract appears below. 

Thermal Energy Storage for 

Medium Temperature Solar Electric 

Power Plants Using PCM's: 

A Preliminary Assessment 

Luz Industries Israel 

The basic characteristics of five operating Luz solar 

power plants and two plants under construction are 

described and their performance is discussed. The 

first plant, which operated at a temperature below 

315°C (600°F) had short-term thermal energy 

storage in oil, but the high operating temperatures 

of subsequent plants have made oil storage 

economically infeasible. Although the current plants 

operate economically using auxiliary natural gas 

firing without storage, there are technical and 

economic benefits to including either long-term or 

short-term storage capacity in plants built in the 

future. A quantitative estimate of the value of longterm storage is about $14/kWht while the short-term 

storage, because of operational benefits, has an 

estimated value of $25/kWht. Two concepts 

employing the liquid-solid heat of fusion of salts are 

presented as candidates for achieving the desired 

cost goal of less than $25/kWht. 

Abstracts of Presentations 

One concept employs a cascade of PCM material 

between the solar loop and the steam loop with a 

single heat exchanger for charging and discharging 

each stage of the storage to the heat transfer fluid. 

Steam is generated and superheated in a separate 

heat exchanger. The second concept also involves 

multiple PCM materials, but the steam generator 

and superheater are integrated in the storage. The 

selection criteria for the PCM's are discussed and a 

preliminary identification of candidate salts is 

presented. The technical and economic considerations related to the selection and design of a 

thermal energy storage subsystem for the Luz power 

plants are presented in sufficient detail to serve as 

a basis for discussion in the Symposium. 

Several of the participating experts were invited to present brief descriptions of their work 

in solar energy storage or other research that was considered to be directly applicable to 

the goals of the symposium. Brief abstracts of those presentations are contained in this 

section. The visual aids that were used in the symposium presentations are contained in 

Appendix B. 
An Overview of ORNL TES Work 

and its Applicability 

to LUZ Solar Electric Needs 

M. Olszewski 

J. J. Tomlinson 

Thermal energy storage is being developed in three 

programs being conducted at Oak Ridge National 

Laboratory (ORNL). The DOE TES program has 

developed technology in the area of shell-and-tube 

and encapsulated PCM concepts. In addition, formstable composites and chemical systems (using 

complex compounds) are being examined. Other 

storage activities are in support of SDI and NASA 

needs. Encapsulated LiH is being developed for use 

in SDI burst power applications. Metallic PCM's are 

being used in support of advanced power supplies 

for the Space Station. In addition, a 3-D timedependent thermal performance code has been 

developed to predict the thermal behavior of the 

PCM in earth (1-g) and space (0-g) environment. 

Based on the existing TES work at ORNL several 

concepts were proposed for the LUZ SEGS 

application. They included: evaporating water and 

storing the vapor in zeolite, a slurry heat transport 

and storage system using microencapsulated PCM, 

a conventional shell-and-tube concept, open 

composites in a packed bed, sensible heat storage 

using a one-tank thermocline system, and a chemical 

storage concept using hydrated complex compounds 

or hydroxide/oxide reactions. A preliminary 

economic analysis of these concepts indicates that 

the chemical and sensible systems offer promise to 

meet the LUZ goal of $25/kWh,. 

A Review of Solar-Related 

Thermal Energy Storage Research 

Mark S. Bohn 

Several solar projects have been funded by the 

Department of Energy in which energy storage for 

solar thermal applications were investigated. For the 

most part, the projects have involved sensible heat 

storage (single-tank thermocline, two-tank molten 

salt) applied to parabolic trough or dish systems in 

the temperature range 200 to 600°C at pilot- or fullscale sizes. Operational experiences and performance of these systems was, in general, good although the storage systems are not necessarily 

applicable to solar thermal plants which utilize costly 

heat transfer fluids. One system study compared 

three storage methods applicable to the temperature 

range of interest: trickle-charge thermocline, twotank nitrate salt, and direct-contact phase change. 

Trickle-charge thermocline was the most costeffective for storage durations of less than 7 hours. 

The direct-contact phase change system was 

primarily intended to eliminate the heat transfer 

penalty generally associated with the solid phase of 

the phase change material and proved to be the 

most cost-effective of the three systems for storage 

durations greater than 7 hours. 

Latent Heat Storage for Solar Thermal Power 

N. Shamsundar 

Existing tools for design and performance recovery stage, for shell-and-tube heat exchangers. 

calculations for passive latent heat storage heat Additional development work is necessary to address 

exchangers are summarized. The tools are adequate the storage stage, viscous heat transfer fluids, finned 

to treat the more critical stage, namely, the heat tubes and system optimization. 
DFVLR/CIEMAT Study on Thermal Energy Storage 

for Commercial Applications 

Michael Geyer 

Within the Spanish-German cooperative program on 

solar thermal research at the Platforma Solar, a 

feasibility study of developing thermal energy storage 

for commercial applications (TESCA 200) was 

initiated and partly funded by DFVLR and 

CIEMAT, sharing costs with the companies INITEC, 

Siempelkamp, GERTEC and Flachglas and Luz. 

The study-started in June 1988- has a time span 

of one year. Technical and economic feasibility of 

three concepts is being analyzed: 

- TES in solid media 

- TES in molten salt sensible heat media 

- TES in cascaded phase change media 

Preliminary results of the frrst 3-month study period 

are: 

According to Siempelkamp's initial design 

and cost calculations, only concrete has the potential 

of meeting the $25/kWh, cost goal. Although piping 

costs are twice as expensive for the concrete as for 

the salt, media costs are much cheaper in the 

concrete case resulting in $27.50/kWht for the 

concrete system and $60/kWht for the solid salt. 

Basic cost reduction potential lies in increasing the 

temperature difference and reducing the discharge 

rate. Major problems to be solved include thermal 

expansion and thermal cycling. 

The storage costs of the two-tank molten 

salt system, including the tank and Hitec salt (with 

a salt price of $0.78/kg), amounts to $22/kWht to 

which $25/kWht have to be added for the 200 MWt 

heat exchanger. A 10 - 15% cost reduction is 

possible if the charging is extended to 3 hours and 

discharge is performed directly with steam. Specific 

costs decrease with higher capacities and higher 

temperature differences. With HITEC, such a 

system can be built without further research and 

development. 

The results for the PCM concepts still 

contain some uncertainties and are only preliminary. 

According to INITEC, between 26000 and 46000 m2 

of heat exchanger area are required for the cascaded 

PCM system depending on the selection of the melting points. This is 30% more than the 20000 m2 

required for the 200 MW active oil/salt heat 

exchanger, the assumed heat transfer coefficient of 

approximately 400 w/m2 K needs further experimental verification. Due to the heat exchanger costs, 

the total system cost rises to $SO/kWh,. 

Encapsulated PCM Concepts for Heat Storage 

Robert k Rapp 

The encapsulation of PCM eutectics in a strong, 

corrosion-resistant coating with a sufficient central 

void to accommodate the volume increase upon 

melting, offers the opportunity for direct contact 

with the heat transfer fluid (HTF) and savings in the 

fabrication of the heat exchanger. Metallic eutectics 

based on the Al-Si, Al-Si-Mg, etc. are attractive at 

temperatures greater than 560°C, but offer no 

promise for storage below 400°C as required for the 

Luz application. 

In general, if encapsulation of a PCM is possible, 

then a hypereutectic composition rich in the 

component of highest latent heat of fusion can be 

used to increase the heat storage capacity and widen 

its range of application to between the liquidus and 

the eutectic temperatures. 

If an encapsulated PCM eutectic or hypereutectic 

composition is arranged in a fured bed, then the 

cocurrent flow of the HTF through the bed (for the 

charging and utilization cycle) can yield a nearly 

constant outlet temperature if the bed exit is 

maintained at the eutectic temperature. 

Calculations were made for the introduction of 

encapsulated Al-Si-Mg ternary eutectic spheres into 

the hot salt storage tank of a solar central receiver 

facility. Per unit volume, the PCM latent heat offers 

at least a 35% increase in storage capacity compared 

to the sensible heat change for the HTF (fused salt). 

With a minor change in the operational procedures 

the increased heat storage could be used to extend 

the duration of power output by the plant. 
An Overview of ANL 

Advanced Energy Transmission Fluids Work 

Argonne National Laboratory (ANL), under 

sponsorship of the U.S. Department of Energy, 

Office of Buildings and Community Systems, has 

been conducting a comprehensive, long-range 

program to develop high-performance advanced 

energy transmission fluids. 

ANL has identified two concepts that hold 

considerable promise for improving the performance 

of thermal system working fluids. These two 

concepts are (1) utilization ofvery low concentration 

of non-Newtonian additives to the carrier liquid to 

reduce flow frictional losses, and (2) utilization of a 

pumpable phase change slurry, comprising particles 

of material with a high heat of fusion conveyed by 

a liquid, to enhance-both bulk convective energy 

transport and heat transfer coefficients at the heat 

exchanger surfaces. The two concepts can be used 

separately or combined to achieve maximum 

benefits. - When the concepts are combined, the 

energy transmission fluid is composed of the phase 

change particles, friction-reducing additive, and 

carrier liquid. 

The current study focuses on the development of 

phase change slurries as advanced energy 

transmission fluids operating in the temperature 

range of 32 - 290°F. The objectives are (1) to 

establish proof-of-concept of enhanced heat transfer 

in a pure carrier liquid; (2) to investigate the effect 

of particle volumetric loading, size, and flow rate on 

the slurry pressure drop and heat transfer behavior 

with and without friction-reducing additives; and 

(3)to generate pressure drop and heat transfer data 

needed for the development and design of improved 

thermal systems. Two types of phase change 

materials were used in the experiments: ice slush for 

cooling, and cross-linked, high density polyethylene 

(X-HDPE) particles with diameters of 1/8 and 1/20" 

(3.2 and 1.3 mm) for heating. The friction-reducing 

additive used in the preliminary tests was Separan 

AP-273 at 65 wppm. 

The frictional loss associated with phase change-type 

slurry flows was found to be significantly less than 

that typically associated with heavy particle slurries 

(i.e., pp> >pl). Under some conditions, X-HDPE 

slurries with 1/20" diameter particles at loadings up 

to 30% and ice slush slurries exhibited significant 

friction reduction compared to pure water. No 

satisfactory correlations have been identified to 

accurately predict the slurry pressure drop and 

viscosity measured in the laboratory. The slurry 

tests also showed that the flow behavior is strongly 

influenced by the particle size, which is either 

neglected or considered to be a weak function in 

existing correlations. 

The heat transfer measurements for X-HDPE 

slurries with 1/8" diameter particles under nonmelting conditions showed a modest enhancement 

of heat transfer over that of water, but the data with 

1/20" diameter particles showed a modest 

degradation. The results also show a significant 

effect of particle size on slurry heat transfer, which 

requires further study. 

The results for combined-concept fluids have 

demonstrated that a friction-reducing additive can 

substantially reduce pressure drop for a slurry of 

water and X-HDPE particles. This effect should 

also exist for other types of particulate slurries. The 

use of phase change slurries holds promise for 

improving the thermal-hydraulic performance of 

SEGS including facilitating incorporation of thermal 

storage into the systems. In future work, slurry 

pressure drop and heat transfer with different PCM 

materials, particle sizes and carrier liquids should be 

examined and experiments performed under SEGS 

operating conditions to explore suitability for SEGS. 
SUMMARY OF DISCUSSIONS 

Cost Goals for SEGS Plants 

The Luz background paper on thermal energy storage for medium temperature solar 

electric plants (Appendix A) reviews the rationale, utilization and acceptable costs of 

thermal storage systems in SEGS applications. The paper served as background for the 

discussions at the symposium. Two types of storage systems were considered in an attempt 

to set realistic cost goals based on SEGS economics: 

Large storage systems [2500 MWh,] for use in base-load applications of solar power 

plants, and 

Small storage systems [200 MWh,] for improving solar plant system performance in 

applications by: 

Dampening solar field fluctuations 

Shifting electrical production to later times 

Utilizing peak excess solar field output 

Improving field and heat exchanger response 

Providing quicker morning startups, and freeze 

protection when needed. 

The Luz analysis was carried out using an hour-by-hour plant performance model, with the 

objective of comparing the incremental benefits (revenues) of a plant with storage to a 

base-case plant without storage. An important assumption in the analysis is that the 

average price of electricity was 6.5 cents/kWh,. While this is an optimistic assumption for 

base-load plants given the low avoided cost rates used by Southern California Edison (SCE) 

today it may be reasonable under future pricing scenarios. However, the 6.5 cents/kWh, 

price level is not at all unrealistic for the electricity revenue during peak periods under 

current pricing arrangements in the SEGS plants (which are, in essence, peaking plants). 

The analysis for the small storage systems was carried out for (a) consideration of the 

electrical production shifting only, and (b) the added value of the other benefits listed 

above. The results of the analysis, discussed more fully in the background paper, show the 

following acceptable costs for thermal energy storage: 

Storage Size Cost Goal 

MWh, $/kwh, 

Base-load plants: 2500 25 

Peaking plants: 

Saving peak excess 200 14 

AH benefits 200 25 
Phase Change Storage 

A number of technical issues were identified by those attending the symposium which must 

be addressed before commercial implementation of phase change storage systems can be 

achieved. 

The arrangement or configuration of the PCM's with respect to the HTF is an obvious 

concern and perhaps the first issue which must be resolved. It was noted that the Luz 

proposal of a cascaded system of PCM storage modules in series when compared to a 

single medium PCM system, makes more effective use of the latent heat capacity of the 

PCM in each module and minimizes the pinch-point problem (discussed in more detail 

below). A second storage system, applicable where steam was generated directly in the 

collector, also was presented by Luz (see Appendix A). Phase change storage is particularly 

applicable for the case of direct steam generation, which illustrates the importance of the 

system configuration and the method by which the storage system is integrated with the rest 

of the plant. 

As a result of the symposium discussions, three generic configurations for PCM systems 

were identified: 

- shell-and-tube heat exchangers (in which the PCM would be contained in the tubes 

and the HTF would be circulated on the shell side or vice versa), 

- a packed bed configuration of encapsulated PCM spheres contained in a pressure 

vessel, and 

- a direct contact configuration (in which heat transfer between the PCM and the HTF 

is brought about by bringing the two into intimate contact). 

From the standpoint of cost and required phase change temperature, it appeared that salts 

were the most practical candidates for the PCM in this application. A number of salts 

and salt eutectics are available at reasonable cost and with ranges of phase change 

temperatures satisfactory for the application to SEGS plants. 

Unfortunately, phase change salts have less favorable heat transfer characteristics than 

media such as sensible heat liquids. In either a shell-and-tube or packed bed configuration, 

heat transfer characteristics are very important, with the shell-and-tube arrangement clearly 

being more sensitive to these considerations. Heat transfer limitations result in more costly 

storage modules because their surface area must be increased to provide the desired charge 

and discharge rates. 

The first heat transfer limitation is primarily a result of the relatively poor thermal 

conductance across regions of solid PCM compared to convective heat transfer in the HTF. 

As the solid layer of PCM builds up during the discharge cycle, increased heat transfer 

surface area is required to accomplish the heat transfer in a specified discharge time. Thus, 
the system would need to be designed with the maximum heat transfer surface anticipated 

and this would likely have a negative impact on the cost effectiveness of the storage unit. 

A second heat transfer limitation is the pinch-point problem which would apply equally to 

either packed-bed or shell-and-tube. Pinch-point refers to the relatively small temperature 

differences between the PCM and the charging or discharging HTF which occur at the 

points in the heat exchanger where the PCM is just dropping below or rising above the 

phase change temperature. At these points, with relatively small temperature differences 

available for driving the heat transfer, large heat transfer surface area is needed. (The 

pinch-point effect noted here is analogous to that occurring in standard boilers where the 

feedwater enters below the saturation temperature.) Again, a potential exists for increasing 

the cost of the storage module due to a requirement for large heat transfer surface area. 

However, the pinch-point characteristic is either eliminated or greatly reduced for the 

configuration with direct steam generation. Likewise the pinch-point problem would be 

minimized if the cascade arrangement were replaced by a hypereutectic multicomponent 

salt for which the liquidus temperature equaled the maximum HTF temperature and the 

eutectic temperature equaled the turbine inlet temperature. Such operation would require 

concurrent flow of the HTF during the storage and utilization cycles. 

For the packed bed arrangement of PCM's, encapsulation and pressurized design are the 

critical issues. Encapsulation refers to provision of a protective outer layer for the PCM 

which isolates the PCM from the HTF and reduces their relative incompatibilities. 

Encapsulation for the packed bed arrangement plays the role of the tubes in the shell-andtube arrangement, but with the very important advantage of providing a high surface area 

for heat transfer and reducing the resistance for heat conduction within the encapsulated 

sphere. Although encapsulation has been investigated for metal alloy PCM's by ORNL and 

Lanxide/DuPont, only recently has consideration been given to encapsulation for salt-based 

PCM's. Commercial interest in this area may result in a commercially available salt-based 

PCM in the future. However, because encapsulation is in an early stage of development, 

the cost of such a process is not clear at this time. Therefore, perceived benefits of PCM 

systems due to the relatively low cost of the salts may be lost after encapsulation is 

considered, though clearly this is a promising area for research. 

A major issue related to encapsulation is the design of the storage vessel. Since the vessel 

must contain the working fluid, potentially at a pressure level of 10-15 bars, a pressure 

vessel will almost certainly be required, adding to the cost of the system. One advantage 

of the shell-and-tube configuration over the packed bed configuration is that if the HTF is 

contained in the tubes (the most likely arrangement), containment of the HTF vapor 

pressure is simpler and less expensive. 

Rapp suggested consideration of the containment of a hypereutectic salt in pipes of either 

steel or glass to minimize costs. These pipes would be positioned vertically and surrounded 

by the pressurized HTC which would flow from bottom to top for both cycles. However, 

glass tubes could suffer creep deformation. 

Microencapsulation of PCM was also discussed as a means for creating a HTF slurry in 

which the storage media would be directly incorporated in the collector loop. By 
introducing these small PCM particles in the collector loop, the effective specific heat of 

the HTF could be increased sufficiently to serve for short-term buffer storage. For longer 

term storage, the particles might be separated from the HTF and stored in a vessel until 

needed for discharging at which point they would be reintroduced into the HTF stream. 

The major unknowns are the potential for erosion of pumps, valves and other collector loop 

components, difficulties of encapsulation (as discussed in the previous paragraph) amplified 

by the problems associated with the small scale, and the complexities of the separation and 

storage phases. 

From a technical viewpoint, packed bed configuration has the potential to outperform the 

shell-and-tube configuration because the packed bed provides such large surface areas at 

potentially lower cost, subject to the cost of a pressurized containment vessel. Detailed 

system feasibility studies are required to size the system and to properly treat issues such 

as pressurization and encapsulation of the PCM. 

Sensible Heat Storage 

Several concepts were discussed in which heat storage is accomplished by a sensible 

temperature change in a single phase (without phase change). These systems include a 

single tank thermocline (in which hot and cold HTF is stored in the same stratified tank), 

a single tank trickle-charge thermocline (in which a the tank contains a large volume 

fraction of low-cost sensible heat storage media and the HTF is trickled over the media), 

two-tank molten salt system (in which heat is transferred from the HTF to a salt loop and 

two tanks are used to separate hot and cold salt), and dual media systems (in which the 

HTF is pumped through pipes imbedded in a massive sensible heat storage medium such 

as cast salt plates or concrete plates). 

Generally, these systems were viewed as being more nearly ready for implementation than 

the phase change systems, though not necessarily more attractive, and a comparison on the 

basis of a complete system study would reveal the best sensible TES candidates. However, 

certain technical issues must be considered. For the dual-media systems, sufficient heat 

transfer surface area may be a limitation due to the relatively poor conductive heat transfer 

through the solid heat storage media. In a thermocline system, storage of large quantities 

of HTF probably is not economical and reduction of this inventory of HTF in the trickle 

charge system may be attractive. The two-tank system requires heat exchangers, and the 

cost of these components may offset the cost advantage of using a low-cost heat storage 

medium in place of the HTF. Especially in the dual media systems, careful design is 

needed to assure that the HTF outlet temperature during discharge is sufficiently high. 

On the basis of preliminary work performed to date in the German-Spanish cooperative 

agreement, it appears that among the dual media concrete plate, dual media salt plate, and 

the two-tank salt systems, the dual media iron plate system is best positioned to meet the 

storage system cost goal of $25/kWh,. The remaining sensible heat storage options should 

be retained in this ongoing study so that a better comparison of all options can be made. 
Chemical Storage 

Storage of energy via reversible chemical reactions was discussed briefly. It appears that 

at least one class of reactions may hold promise for the SEGS and has received some 

attention in the past for transport of energy. The hydroxide/oxide reaction between CaO 

and H,O was mentioned as a possibility and, based on a preliminary cost assessment, such 

a system could have a lower installed capital cost than the sensible storage options. 

Although generally viewed as higher risk and possibly longer range than some of the other 

storage options, chemical storage appears to offer sufficient potential to warrant 

consideration for future developments. 

Ranking of Technologies and Approaches 

After the presentations and discussions of the critical issues associated with the phase 

change thermal energy storage, as well as some consideration of selected thermal storage 

alternatives, the experts at the symposium were polled to get opinions on the most 

promising approaches and alternatives. The basis for judging the alternatives was the ability 

to meet the cost goal of $25/kWh, within five to seven years. The rankings were not 

quantified but were based on interpretation of the individual statements of the experts when 

asked to evaluate the probability of each of the options meeting the cost goals within the 

time stated and given a reasonable budget for development. These opinions are based on 

expertise in different, sometimes narrow, fields and are derived from general beliefs rather 

than from the results of a thorough comparative analysis. 

Phase Change Storage Approaches 

The experts were asked to evaluate the probability of success of the following options: 

Continuous PCM media with embedded heat exchanger, . Continuous PCM media with direct contact heat exchange, 

Dispersed PCM media in some form of packed bed. 

Most participants either explicitly or implicitly expressed the view that a dispersed 

encapsulated PCM in a fixed packed bed is the most likely to meet near-term cost goals. 

There was, however, considerable conservatism regarding the probability of near-term 

technical success. It was generally agreed that simple encapsulation techniques could be 

developed using suitable mixtures of salts and glass or iron containers, but there was 

considerable skepticism about the ability of these simple approaches to meet the cost and 

performance goals. Spherical capsules suitable for packed beds were considered more 

difficult to develop, but more likely to be cost competitive. The major barrier to cost 

effectiveness of the packed bed approach, assuming a successful coating, was the possible 

cost of the pressure vessel to contain the packed bed and heat transfer fluid at pressures 

of 15 bars or higher. Specific design feasibility studies are needed to provide a solid basis 

for comparison. 
Although there were some questions regarding the available cost estimates of tube in shell 

PCM storage designs, the general consensus was that shell-and-tube systems were almost 

certainly too expensive because of the large area of tubes and fins required to compensate 

for the poor thermal conductivity of the salts. Further and more detailed analytical results 

are needed to quantify the heat transfer area question. 

Direct contact heat exchange was considered the most promising approach to reduce the 

cost of the system, but most of the participants were dubious about the possibility of finding 

a compatible salt and heat transfer fluid that would also meet the system requirement. 

Alternative Thermal Energy Storage Options 

Several alternatives to the phase change storage concept were considered: 

e Chemical storage based on reactions such as hydrated complex 

calcium oxide - calcium hydroxide, 

Slurries of particles in the heat transfer fluid-either PCM or 

capacity materials, . Sensible heat storage in a single tank (thermocline), two-tank 

system. 

The majority of participants expressed the opinion that one or more of the 

compounds and 

other high heat 

system, or solid 

sensible storage 

options-could be successfully developed to meet the cost goals using current engineering 

knowledge. The development would require thorough and careful engineering development 

and small scale testing, but should not require new research. The choice between the three 

sensible heat storage options was not clear. Preliminary results from the German-Spanish 

study indicate that concrete castings with embedded steel tubes are already in the costeffective range. The massive structures would still require careful design and installation 

to deal with problems of thermal expansion and leakage. Single tank thermocline molten 

salt storage has not been shown to meet the cost goal, but there was some confidence 

among the participants that it could be accomplished. Some of the problems that have 

received attention for higher temperature levels, such as high radiant heat transfer through 

the optically transparent molten salts, appear to be less troublesome in the temperature 

range of interest for parabolic trough solar plants. Two-tank active storage using molten 

Hitec-type salts also was considered likely to be cost competitive since up to 420°C 

inexpensive steels can be used for piping and tanks. 

At least half the participants felt that chemical storage might have the potential to be the 

most cost-effective of the options considered. The promise of chemical reactions involving 

metal oxides and metal hydroxides would be further enhanced if the solar collector field 

working fluid were steam rather than oil. Clearly, additional research is required to identify 

the best reactions, establish chemical compatibilities, prove the process, and integrate the 

process into the complete system. Currently, work is in progress on hydrated and 

ammoniated complex compounds for high -and low - temperature applications. Success in 
this work gives confidence that the chemical energy storage concepts using complex 

compounds or oxide/hydroxide reactions could be developed within a few years. 

Engineering and development of a commercial system might be accomplished within five 

to seven years. 

Many of the participants found the concept of adding energy storage to the circulating 

working fluid, i.e., slurries, to be quite interesting and potentially effective. Considerable 

research is required to develop particles with the necessary thermochemical, thermophysical, 

fluid dynamic, and mechanical properties. Most of the participants considered slurry-based 

storage systems a higher risk development than chemical storage. 

Required Analysis and Cost Studies 

Although the symposium discussions produced opinions regarding the most promising nearand long-term options for thermal energy storage in SEGS plants, it was generally believed 

that more definitive feasibility studies are required to quantify the issues discussed in the 

symposium, probably at a relatively modest effort in analysis and cost estimation. Some of 

the work required already is planned as a part of the German-Spanish project and the 

results should be available in six to eight months. Other preliminary analyses are within 

the capabilities of the participants or their associates and might be performed on an 

informal ad hoc basis. The table below shows the tasks to be performed by storage option 

and possible organizations to accomplish the work. 

Storage Option Required Analysis or Study Means 

Sensible Storage Complete and verify results from German/Spanish study of concrete DFVLR 

and molten salt sensible storage. 

Extend study of trickle charge sensible storage to the temperature SERI 

range of interest for SEGS plants. 

Phase Change Determine the least lower bound on cost of phase change storage ORNL/DFVLR 

using spherically encapsulated PCM's in a fured packed bed by 

optimizing the size of spheres and ignoring the cost of encapsulation. 

Carry out more detailed analysis to better quantify the heat transfer LUZ/DWLR/UH 

area requirement. The model developed by the University of Houston 

could greatly simplify this analysis. 

Determine the least lower bound on cost of direct heat exchange ORNL/DFVLR 

phase storage by calculating the volume required for a salt with 

infinite thermal conductivity. 

Determine the maximum benefits available from adding salt PCM's to ANL 

heat transfer fluids in slurries. 

Chemical Identify one or more promising reactions and perform preliminary ORNL/SERI/DFVLR 

cost analyses for SEGS plants. 
Research Needs 

One of the original objectives of the symposium was to identify and define research and 

development tasks that would be required in a program to develop successful phase change 

storage components for SEGS power plants. Given the previous work carried out in the 

technical areas under discussion, the consensus of the participating experts was that the 

most probable routes to cost effective storage in the near-term require engineering 

development rather than research. However, recommendations on detailed development 

tasks could best be made after further exploratory analyses and prototype testing are 

completed. 

It was clear from the discussions that much could be learned from careful and complete 

evaluations of the most promising options. With respect to PCM's, such evaluations are 

made more difficult by the lack of crucial thermophysical property data. Consequently, 

determination of such data will be a requirement early in the development phase. 

The encapsulation of PCM's and the development of chemical storage are areas that will 

require further research. Since additional analysis is required to determine if it is necessary 

or desirable to initiate development of these options, the definition of research tasks was 

deferred until that determination has been made. 

Program Issues 

Some thermal energy storage research is currently funded by at least two offices of the U.S. 

Department of Energy and the German DFVLR. At present, none of the work supported 

by the DOE is specifically targeted for solar power plants operating at temperatures in the 

range of the SEGS plants. The work supported by the Solar Thermal Technologies Division 

of the Office of Solar Heat Technologies is tailored for solar power plants operating at 

considerably higher temperatures characteristic of central receiver or parabolic dish solar 

collectors. The Thermal Storage Office of the Office of Residential and Industrial 

Conservation supports storage research for a broad range of temperatures, but does not 

focus on solar applications. At present the Thermal Storage Office is not supporting any 

research that is directly applicable to the SEGS plant applications. The German and 

Spanish governments are currently funding a study that is directly related to the storage of 

heat in a temperature range suitable for parabolic trough plants. 

Commercial SEGS plants make substantial contributions to the displacement of fossil fuels 

by clean, renewable sources of energy. The recognition that these plants could be even 

more effective and more broadly applicable if cost-effective thermal storage were available 

presents a strong argument in favor of increasing resources devoted to thermal energy 

storage research and devoting a significant part of that research to tasks that could benefit 

the SEGS applications. 
CONCLUSIONS 

The conclusions reached by the participants in the Symposium can be described in terms 

that apply to the present status of thermal energy storage for plants of the SEGS type, and 

those that apply to the potential for future development. These conclusions are based on 

opinions, and suffer from lack of detailed system-scale comparisons to provide clarity to the 

technical and cost issues. 

For the present and the immediate future, phase change energy storage systems probably 

will not meet the cost goals for the SEGS plants. Sensible heat storage, however, using 

either solid media or molten salts does appear to offer a near-term possibility that may be 

achieved through careful engineering and development. 

The long-term potential for phase change storage systems using molten salt depends on 

providing the necessary heat transfer surface at low cost as well as upon solving various 

technological problems of materials compatibility and stability. Two questions related to 

heat exchange need to be answered: 1) do phase change salts offer enough benefits to offset 

any increase in heat exchanger cost, and 2) are there technically feasible and economically 

attractive options for improving heat exchanger effectiveness in salt based systems? The 

first question can be answered by a relatively straightforward engineering cost analysis of 

idealistic (most optimistic) systems. The second question requires the identification and 

evaluation of both conventional and innovative system configurations in a careful 

engineering feasibility study. 

Two potentially effective, but riskier, alternatives to the present storage options emerged 

which require further research efforts. Encapsulation, or microencapsulation, of phase 

change salts might provide a solution for the problems resulting from the poor thermal 

conductivity of salts by increasing the possible surface-to-volume ratio of the PCM. 

However, the cost issues of pressurized vessels and the encapsulation process itself were 

identified as critical, as well as the potential for reliability problems. Chemical energy 

storage using hydroxide/oxide reactions seems attractive in the longer term, and may offer 

low costs if suitable reactions can be found and if the process can be efficiently integrated 

into the power plant. 

RECOMMENDATIONS 

The symposium participants recommended that near-term detailed comparative system 

studies be conducted to evaluate the most-promising options, following the conclusions and 

discussions noted in this report. These studies should define the technical and cost issues, 

and establish a time-schedule for applications by 1992 or beyond. The most promising 

alternatives should be explored in addition to phase change materials. 

These evaluations will enable Luz to make well-informed decisions about designing and 

building thermal energy storage for its current series of SEGS plants and will provide 
guidance for Luz, DOE, and others about the most promising avenues for future research 

to facilitate the development of thermal storage for the next generation of SEGS plants. 

The results of these evaluations will provide the technical and economic bases for further 

efforts, including the design and installation of prototype field systems and detailed 

engineering design studies with supporting R&D activities. 
APPENDIX A 

Background Paper 
THERMAL ENERGY STORAGE 

for 

MEDIUM TEMPERATURE SOLAR ELECTRIC 

POWER PLANTS USING PCM's: 

A Preliminary Assessment 

1 October 1988 

Luz Industries Israel 

A. SYSTEM CONSIDERATIONS 

1. Solar Electric Generating System (SEGS) Power Plants 

The basic concept of the SEGS plants is to supply thermal energy via the solar field to 

produce steam to drive a Rankine cycle steam turbine, which in turn drives an electric 

generator to produce power. A Rankine cycle, which is a particular type of thermodynamic 

power cycle, is used in all conventional coal, oil-fired or gas-fired steam plants. Because 

the Rankine cycle conversion efficiency increases significantly with an increase in the 

temperature and pressure of the steam supplied to the steam turbine, it is advantageous to 

supply steam to the power cycle at the highest pressure and temperature possible given the 

energy source, piping systems, and other plant equipment and support systems. 

Luz International Limited has been developing line-focus parabolic-mirror technology for 

solar industrial process heat systems and commercial solar electric power plants since 1979. 

During the past four years, Luz has built five solar power plants with a combined generating 

capacity of 134 MWe and will bring two more 30 MWe plants on line in 1988. The seven 

power plants will produce 194 MWe of solar generating capacity, making Luz the single 

largest solar power generator in the world. The first five plants have demonstrated that 

power can be produced via the Luz technology on a commercial scale. 

Since the inception of SEGS I, advancements in the mechanical structure of the Luz solar 

collector technology have permitted a steady increase in the outlet temperature of the solar 

field, from 585°F in the LS-1 design to 660°F in the LS-2 design used in SEGS 111-V. 

Further advances, notably the introduction of a sputtered cermet selective coating on the 

heat collection element (HCE), have allowed solar field outlet temperatures of about 735°F 

in SEGS vI/W and future plants. 

Five plants were in operation in the Mojave desert of Southern California during 1988 - the 

first plant 13.8 MWe net capacity and the succeeding four plants 30 MWe net. All facilities 

are privately owned and sell the electricity produced to the Southern California Edison 

utility under individual power purchase agreements. Two additional 30 MWe plants are 

under construction for placement in service in late 1988. 

Workshop Background Page 1 
To optimize plant revenues, it is very important that maximum electrical output is delivered 

during the utility on-peak hours when electricity revenues are highest. This is partially 

accomplished with the aid of a natural gas boiler which can either supplement the solar 

field or operate independently. The energy supplied by natural gas is limited to 25% of the 

total effective annual plant energy input by regulations of the U.S. Federal Energy 

Regulatory Commission. 

The basic characteristics of the five operating plants (SEGS I-V), the two plants in 

construction (SEGS VI-VII), and the next plant (SEGS VIII) are given in Table 1. The 

first two plants are located at Daggett, California, about 110 miles northeast of Los Angeles. 

The next five plants are located at Kramer Junction, California, about 40 miles west of the 

Daggett site. SEGS VIII will be located at Harper Lake, between the previous sites and 

about 30 miles west of Daggett. Luz International Limited, a U.S. corporation with 

subsidiaries in Los Angeles and Jerusalem, has led the development of the SEGS plants 

since its inception in 1979. The ensuing period has seen continuous advancements in both 

the plant system design and the solar collector technology. 

SEGS I through SEGS V are presently in daily operation and producing electricity with 

excellent reliability. SEGS VI and VII will be operational in December, 1988. The design 

levels of annual electrical output are given in Table 1. Plant performance projections are 

derived from an hour-by-hour performance model that was developed by Luz and has been 

in use since SEGS 111. The model utilizes published insolation data and takes into account 

all of the significant factors influencing the solar field and turbine performance. 

Table 1. Basic Characteristics of SEGS I-VII 

First Full Turbine Cycle 

Operating Turbine Solar Field Efficiency Annua 1 

Plant Year Status Capacity Temp Size Solar Nat Gas Output 

(MWe net) (OF') (m2) (MWh net) 

I 1985 

I1 1986 

111 1987 

IV 1987 

V 1988 

VI 1989 

VII 1989 

VIII 1990 

Operational 

Operational 

Operational 

Operational 

Operational 

Construction 

Construction 

Design 

* includes natural gas superheating 

Workshop Background Page 2 
SEGS 111-V, the most recently operating plants, have achieved excellent performance during 

1987 and 1988 to date. These plants have benefited from the lessons learned in SEGS I 

and 11 in both the solar field and power block systems. After the initial two months of solar 

field checkout and acceptance procedures, the availability of the solar fields (fraction of the 

solar collector assemblies able to track the sun) has been consistently above 90-95% for 

SEGS I11 and IV, with an even better start in SEGS V. For reference, the mature solar 

field availability (percent capable of tracking) assumed in the performance projections is 

97%. 

As a result of the rapid progress in start-up, the electrical output of SEGS 111 and SEGS 

IV has been excellent with similar performance levels of both plants. The 1988 

performance of SEGS 111-V compared to warranty projections is given in Figure 1 for the 

period January through August. SEGS V shows excellent results for its start-up year 

performance in 1988. The 1988 performances of SEGS 111-IV are perhaps most significant, 

however, in that these plants are operating at expected design levels in their first normal 

operating year. These results demonstrate the success of the plant design and operation, 

as well as the validity of the performance modeling used to project plant outputs. 

A process schematic of the latest system design is shown in Figure 2. In this system, 

thermal energy collected by the solar field is added to the Heat Transfer Fluid loop which 

circulates through the solar heat exchangers to generate steam. Thermal storage is 

contemplated for the HTF side of the system, though it is conceivable that thermal storage 

be utilized on the steam side. In fact, a development objective for future SEGS plants is 

to eliminate the intermediate HTF loop and generate steam directly in the solar field. 

Workshop Background Page 3 


2. Thermal Energy Storage 

Solar thermal applications are particularly enhanced by the capability to store energy. 

Unlike traditional electric power plant fuel sources, such as petroleum, natural gas, and 

coal, solar "fuel" can not be directly stored, and solar energy supply is independent of 

electric demand. Thermal energy storage primarily enhances the solar electric power 

system by adding the capability to store energy during peak insolation periods for later use 

during afternoon and evening hours, though there are a number of secondary benefits as 

discussed below. In general, the economics, operational effectiveness and operating strategy 

of the SEGS plants can all benefit significantly from the use of thermal energy storage. 

a. TES - General 

A schematic representation of a solar power plant with a thermal energy storage system is 

shown in Figure 3. Different system designs are discussed in section 2 of this assessment. 

The power plant energy storage system can be operated in four modes. 

Storage charging only. This mode applies when there is solar field output, 

but the solar output can be more effectively used at a different time of the 

day. In this case, the HTF flows from the solar field through the thermal 

storage heat exchangers and back to the pumping station. 

Steam generation and storage charging. In this case, thermal energy that is 

not used to generate steam is stored. The HTF flows from the solar field 

through both sets of heat exchangers in parallel and then back to the pumping 

station. 

. Steam generation from storage discharge only. This situation applies when 

there is no solar field output and steam is required. The HTF runs in a loop 

between the two sets of heat exchangers -- that is, from the storage heat 

exchangers to the preheater, steam generator, superheater, and reheater, and 

then back to the storage heat exchangers. 

Steam generation from solar field and storage discharge. This applies when 

there is solar field output, but it is desirable to supplement that output (e.g., 

to reach peak output) with energy from storage. The HTF flows from the 

solar field and thermal storage heat exchangers (the solar field and heat 

exchangers act in parallel) to the process steam heat exchangers. From there, 

HTF returns to both the pumping and the storage heat exchangers. 

b. TES - Phase Change Materials 

A number of thermal energy storage schemes have been proposed and investigated to date. 

These schemes differ in energy storage density, operational characteristics during storage 

and retrieval, and the state of development of the storage technology. Comparison of these 

schemes indicate that thermal energy storage based on latent heat of phase change 

materials (PCM's) provides the highest energy storage densities. PCM storage based on 

reversible solid-liquid transition requires relatively small volumes, with the additional 

advantage that energy is stored and retrieved at a constant operating temperature. 

Workshop Background Page 6 
It is envisaged that the PCM heat exchanger will consist of a large number of small 

diameter tubes immersed in an insulated PCM holding tank. The tubes would incorporate 

an enhanced external heat transfer surface using spines or fins. Further PCM conductivity 

enhancement may be obtained with the addition of aluminum or stainless steel honeycomb 

or wool. Individual tube length and the total number of tubes will be specified according 

to the design rate of heat transfer and the required total energy storage capacity. Tube 

geometry may be either strait or coiled, depending on the heat transfer fluid type and rate 

of flow. Modular construction would be used where possible to simplify inspection, 

maintenance, and repair. 

c. TES - SEGS Example 

The specific requirements for a thermal energy storage system will, naturally, be plantspecific. The general requirements for a thermal energy storage system compatible with an 

actual 80 MWe SEGS plant -and compatible with the SEGS plants that are planned for 

the next several years - are as follows: 

Nominal Operating Characteristics of the Solar Field 

- Solar field nominal output temperature 390 "C 

- Solar field nominal input temperature 293 "C 

- Full load operation 240 MWt 

- Flow at full load 3.63~10~ kg/hr 

. Storage Charging 

- Storage capacity 200 MWh 

- HTF inlet temperature 390 "C 

- Maximum HTF outlet temperature 315 OC 

- Maximum flow through stirage 

a) Storage mode (80% full flow) 3x10~ kg/hr 

b) Hybrid mode (25% over flow)0.9lxl0~ kg/hr 

Storage Discharge 

- Minimum HTF outlet temperature 350 "C - Maximum load (80% turbine load) 198 MWt 

- HTF inlet temperature 265 OC 

- Maximum flow 3x10~ kg/hr 

- Period between charge and discharge 12 hr 

As specified, such a system would provide approximately one hour of thermal storage-a 

very small system. The use of thermal energy storage with the current configuration of the 

SEGS plants is still developmental, though use of storage has been quite successful in 

SEGS I. Near-term SEGS plants, therefore, are designed to be economic without energy 

storage. 

Workshop Background Page 7 
d. TES -Economics 

The addition of thermal energy storage to a solar power plant provides significant operating 

advantages. Perhaps the most significant, albeit long-range, advantage of thermal energy 

storage with solar power plants is the potential to meet the crucial need for solar-only 

systems for base load electrical generation without fossil fuels. 

A base load solar/storage plant would require 10 to 12 hours of thermal storage capacity. 

The economics of incorporating a thermal energy storage system into a solar power plant, 

naturally depends on the specific contract, system and component design, and field 

condition. 

A comparative economic analysis was performed between two 80 MWe SEGS plants-a 

base load solar/storage plant and a SEGS plant without storage. Using a cost of $135 per 

square meter of installed solar field (a value consistent with actual Luz LS-3 solar collector 

experience) and $25 per kwh, of storage (the current target), a large-size thermal energy 

storage system is justified at approximately 6.5 cents per kwh,. Other important differences 

between the two plants include: 

Solar Solar with 

Onlv S torane Difference 

Solar field size (m2) 464,000 1,000,000 536,000 

Annual Output (MWh,) 245,000 455,000 210,000 

Annual Output (MWh,) 565,000 1,165,000 600,000 

Storage (MWh,) 

(Hours) 

Solar field multiplier 2.5 

Base load electric payments of 6.5 cents per kWh, are much greater than realized today. 

However, an average price of 6.5 cents per kwh,-that is, the average price of off-, mid, 

and on-peak -is realistic. For example, the mid-peak period of Southern California Edison 

ends at llpm during the summer and 9pm during the winter. The average rate during this 

period is approximately 6 cents per kwh,. According to the Pacific Gas and Electric 

(PG&E) energy storage study, at 6 cents per kWh, a 6-hour storage system is optimal, 

based on PG&E's system demand characteristics. 

Using the prior SEGS plant example, except reducing the storage capacity to 1,500 MWh, 

(approximately 6 hours), 6.5 cents per kWh, can be achieved with a solar field of 800,000 

square meters. Such a system is well sized to generate electricity during high-value onpeak and mid-peak evening hours. 

Utility time-of-use peak period definitions vary considerably between utilities, and are 

subject to change depending on changing utility needs. Whereas most U.S. Southwest 

utilities have summer peak periods in the afternoon from noon until 5 or 6 pm, at least 

Workshop Background Page 8 
one utility (in Las Vegas) has its peak in the late afternoon and early evening. Energy 

shifting is clearly desirable in this case: this is but one example of unique conditions that 

exist in specific utilities where a thermal storage option would allow better matching of a 

solar electric plant to the electricity demand curve. 

Several external factors further improve the economics of large-size solar/storage systems. 

For example, U.S. Federal Energy Regulatory Commission (FERC) policy limits the 

quantity of back-up natural gas to approximately 25 percent of a SEGS plant's electric 

output. Because of this restriction, the plant can not operate during approximately half of 

the winter mid-peak period. Energy storage could be used to fill this gap. 

In addition, regulations tightening emissions standards may further limit the allowable 

quantity of natural gas and fossil fuels burned for power production back-up, thereby 

improving the relative economics of energy storage. 

As previously stated, energy storage is still developmental, and, therefore, near-term SEGS 

plants are designed to be economic without storage. A comparative economic analysis was 

performed between an 80 MWe SEGS plant and one with a very small storage system-200 

MWh,, corresponding to approximately 45 minutes. Such a system would be used to smooth 

fluctuations during intermittent cloud cover and to extend operating time. 

A large-size storage system is justified if built for approximately $14 per kwh, -much less 

than the current target of $25 per kwh,. A small-size system, however, has other daily 

operating benefits which increases its value to $25 per kwh,. These operating advantages 

include: 

. Dampens solar field fluctuations. The storage system can provide a thermal 

buffer to dampen solar field output fluctuations due to intermittent clouds, 

thus providing for smoother turbine and power system operation. 

. Matches supply and load characteristics. Heat energy can be shifted to more 

closely match electric load characteristics. For example, energy absorbed 

during the day can be stored and then used later for peak demands in the 

evening. 

. Absorbs excess solar field output. The size of the solar field is generally 

optimized for total annual production without storage. The result of such a 

design is that on peak summer days the solar field can actually produce 

thermal energy in excess of the power block design capacity. This excess 

energy can be stored and used to generate steam in the evening, thus 

extending the power block operating hours. This energy would otherwise be 

rejected by the system. 

. Responds quickly. Because the storage operates at the same temperature 

and can use the same heat exchangers as the solar field to generate steam, 

it can be brought on-line much more quickly than auxiliary boilers. Fossilfuel boilers operate with many more cycling restrictions due to the need to 

balance flame and metal temperatures. 

Workshop Background Page 9 
Provides options for quicker morning startup. The power block is designed 

to work under constant temperature conditions. It, therefore, takes time each 

morning for the HTF to reach working conditions and run the turbine. 

Anti-freeze protection. Storage energy could be used as HTF anti-freeze 

protection. 

B. PROPOSED CONCEPT 

The Luz storage concept uses a series of separately contained phase-change materials 

(PCM's), each having a different liquid-solid phase-change temperature and each storage 

bath having its own individual heat exchanger. The cascading of several PCM's achieves 

the same energy transfer at flow rates lower than a single PCM system; or, greater energy 

transfer at lower flow rates. Three general configurations are possible. 

HTF/HTF-Hot HTF is pumped through the heat exchange tanks in one 

direction to melt the salts, and "charge" the storage, while cold HTF will be 

pumped through the same heat exchangers in the opposite direction to freeze 

the salts and "discharge" the storage. 

Figure 3 is a schematic representation of such a system with five heat 

exchange tanks. Each tank is filled with a PCM of different T, (temperature 

of phase change) and arranged from top to bottom, in a sequence of 

diminishing T,'s. 

. HTFIsteam-Figure 4 presents the concept of charging the storage with HTF 

and discharging with waterlsteam. The thermal storage system acts as the 

steam-generating equipment, thus eliminating the need for two sets of heat 

exchangers (steam-generating heat exchangers and thermal-storage heat 

exchangers). The basic system operation would, however, be essentially the 

same as an HTF/HTF system. The T, sequence is this system is 

Tpc22Tpc3 ' Tpcl* 

. Steamlsteam-Steam is used to charge and to discharge the PCM storage, 

thus eliminating the need for HTF and heat exchangers. Because of the 

shape of the waterlsteam phase diagram, only a cascading PCM storage 

system would likely be economic with this system. 

Workshop Background Page 10 


C. APPLICABLE PCM's 

1. PCM's - Selectioncriteria 

The selection of applicable salts for a molten salt storage system was based on three groups 

of criteria - thermodynamic properties, safety, and economics. 

Thermodynamic properties. The initial screening of salts included the 

evaluation of phase change temperature and heat of fusion. Salts that melt 

outside of the appropriate temperature range or with an energy density too 

low were excluded. Also reviewed in this step was the phase diagram, volume 

change characteristics and thermal conductivity of each salt mixture. 

. Safety. Materials were examined for stability, volatility, corrosiveness, and 

toxicity. Of course, most industrial chemicals have some associated hazard, 

and some of the materials which passed the screen have potential problems 

with regard to corrosion, fire and explosion hazards. Most of the selected 

candidates, however, are well-known chemicals which can be safely handled 

using existing engineering practices. 

Economics. In addition to the cost per unit weight of salt, many other 

factors effect the system economics of using a particular salt. Some of these 

other economic factors include heat of fusion and storage volume, 

corrosiveness and containment costs, heat transfer costs, and overall system 

costs. 

2, PCM's - Candidates 

Based on these criteria a preliminary list of PCM candidates for a five cascade molten salt 

storage system has been prepared. These candidates, with their important properties are 

listed in Table 2. 

Workshop Background Page 13 
Table 2. Candidate PCM9s 

Temperature Composition 

(mole%) 

367 NaOH, NaCl(20) 

KOH 

Tf Ht.FusionRelative 

(C) (kJ/kg) Cost 

............................................................................ 

308 NaC1, NaOH(93.7) 314 ? ? 

NaN0, 310 174 1.35 

NaF, NaN0, (96.5) 304 ? ? 

LiOH, KOH(60) 314 341 3.57 

NaOH , NaCl(7.8) , Na2C03 (6.4) 282 316 2.90 

1 Based on NaCl at 0.05 $/kg and NaOH and 0.35 $/kg, the resulting 

storage capacity cost is 2.7 $/kwh. This is given only as an 

indication for comparison. 

2 NaOH is reported to have an additional solid state transition. The 

higher heat of fusion is the sum of both transitions. The cost is 

based on this combined value. 

D. TECHNICAL ISSUES 

One important objective of the TES Symposium is to discuss the technical problems and 

operational experience with phase-change materials, with emphasis on heat exchanger 

configuration, heat transfer phenomena, long-term chemical stability, material compatibility 

and safety. The experts participating in the meeting will address these issues from the 

perspective of their own knowledge and experience. From these discussions, the 

development steps necessary to evaluate and test the most promising candidates will be 

tentatively developed. 

Workshop Background Page 14 
APPENDIX B 

Visual Aids 
INTRODUCTORY REMARKS BY 

THE CHAIRMAN 

Phase Change Thermal Energy Storage Symposium 

October 19-20 

Inn at Silver Lakes 

Helendale, California 

Charles A. Bankston 

CBY Associates, Inc. 

Washington, DC 

October 19, 1988 
ISSUES FOR DISCUSSION 

ECONOMICS 

ENGINEERING ISSUES 

RESEARCH NEEDS 

RESEARCH TASK DEFINITIONS 

PROGRAM ISSUES 
ECONOMICS 

1. DEFINITION OF TERMS 

2. VALUE OF STORAGE 

a) Operational Benefits 

b) Rate Shifting 

c) Base Load Plants 

3. COST OF STORAGE 

a) Storage Media 

b) Containment and Insulation 

c) Heat Exchangers 

d) Transport Media 

4. OPTIMIZATION 

a) Least Capital Cost 

b) Greatest ROR 

c) Least Cost Power 
ENGINEERING ISSUES 

1. THERMODYNAMIC CONSIDERATIONS 

a) Latent or Latent/Sensible Storage & Fluids 

b) Single or Multiple Stage 

2. STORAGE MEDIA 

a) Heat Capacity 

b) Media Form (bulk or discrete particles) 

c) Surface to Volume Ratio 

d) Insulation Optimization (Cyclic) 

e) Material Compatibility 

f) Safety, Maintenance, and Life 

3. HEAT TRANSFER 

a) Delta T and Surface Area 

b) Heat Exchange Fluids 

c) Safety, Maintenance, and Life 
ENGINEERING ISSUES (cont.) 

4. COLLECTOR FIELD 

a) Heat Transfer Fluid 

b) Operating Temperature 

5. CONTROL SYSTEMS AND STRATEGIES 

a) Short Term/Long Term 

b) High Loss/Low Loss Storage 


RESEARCH NEEDS 

1. ANALYTICAL TASKS 

a) Heat Exchange Design/Optimization 

b) Control Strategies 

c) Economic Optimization 

2. LABORATORY RESEARCH 

a) Materials Characteristics 

b) Heat Transfer Studies 

3. PLANT SCALE EXPERIMENTS 

a) Heat Exchanger Performance 

b) Control Systems and Strategies 

c) Safety, Operations, Maintenance, and Life 
RESEARCH TASK DEFINITIONS 

1. NEED 

2. GOAL 

3. DESCRIPTION 

4. PROMISING APPROACHES 

5. MAJOR OBSTACLES 

6. PROBABLE TIME AND COST 
PROGRAM ISSUES 

1. Who (has the expertise to do the research)? 

2. When (can the results be expected)? 

3. How (can the research results be integrated)? 

4. Why (should DOE participate in research)? 
A Review of Solar-Related 

Thermal Energy Storage Research 

Symposium on Phase-Change 

Thermal Energy Storage 

October 19-20, 1988 

Mark S. Bohn 
Contents 

Background on Phase Change Storage Research 

Coolidge Irrigation Project 

Shenandoah Total Energy Project 

* CRTF Molten Salt Two-Tank Test 

Comparison of three storage concepts 
Phase Change Storage Research 

In the temperature range of interest only 

1 system study and 1 experimental study 

appear applicable 

Probable reasons for lack of interest 

- heat transfer across solidified pcm 

generally poor 

- expansion/contraction of pcm 

= containment is difficult 

= most sensible methods appear simpler 

Potential for phase change has been recognized 

but not exploited in DOE solar-related storage 

research 
Coolidge Irrigation Project 

1979-1982 

* 23,000 ft* parabolic trough system 

Caloria heat transfer fluid 

392OF 5OO0F 

Toluene organic Rankine cycle 

30,000 gallon thermocline storage tank - 5 hours 

- efficiency measured 

- thermocline stability determined 

- no problems specifically mentioned 

- no economic analysis provided 

Thermocline probably not applicable due to 

- high cost of Syltherm in Luz system 

Technical feasibility demonstrated in event a 

lower-cost h ig h-temperature HTF is developed 

SAND83-7125 

Collector Field Size: 

Flu id: 

Temperature: 

48 Acurex collector groups with 

N-S axis orientation - 23 040 ft* 

Caloria" HT-43 

Inlet, 392OF; outlet, 550°F 

Thermal Storage Type: Stratified liquid (thermocline) 

Tank size: 50,000 gal - 13.67 ft dia by 494 

length (30,000-gal usable storage) 

Storage temperature: 392O to 550°F 

Storage medium: CaloriaB HT-43 

Insulation: 12-in-thick fiberglass 

Cooling System Type: Vapor condenser 

Water (make-up): 10 gallmin 

Condensing temperature: 105OF 

Power Generation Type: Organic Rankine cycle 

Working fluid: Toluene 

Gross efficiency: 20 010 
Shenandoah Solar Total Energy Project 

1982-1984 

47,000 ft2 parabolic dishes 

Syltherm 800 heat transfer fluid 

5OO0F * 75OoF 

Steam Rankine cycle 

11,000 gallon thermocline storage tank - 1 hour 

- heat loss measured 

= thermocline stability determined 

- storage capacity measured 

- need for more storage was identified for 

improved load matching 

SAN D87-7108 

Molten Salt Storage Tests at CRTF 

Two tank storage system 

Nitrate salt 

Hot tank- 1050°F (565OC), 7 MWh,, 

Cold tank - 550°F (288OC) 

Test objectives 

- determine if thermal performance degraded since 

installation in 1982 

- validate computer model 

- estimate efficiency of 1200 MWh, system 

- compare performance with 2 European systems 
Molten Salt Storage Tests at CRTF 

1987 (continued) 

Results 

- hot tank losses same as in 1982 

- cold tank losses were less due to wet insulation in 1982 

- computer model gives good prediction of tank performance 

- efficiency of 1200 MWh, system is 98%' 

- thermal performance of test system was similar to the 

European systems 

- demonstrates technical feasibility 

Comparative Ran king of Thermal Storage Systems 

Comparison of 3 storage systems for Shenandoah-type plant 

- Sylthermltaconite trickle charge 

- Hitec 2-tank 

- direct contact phase change 

Results 

- Syltherm trickle charge is best for < 7 hours storage 

- phase change is best for > 7 hours 

Caveats/issues 

- compatibility between Syltherm and taconite 

- phase change system has not been proven experimentally 

SERIITR-631-1283, by Stearns-Roger Services, I nc. 

SERIISTR-230-2065 describes direct-contact p hase-change 

research by Grumman Aerospace Corp. 




Conclusions 

Considerable solar-related experience with various storage 

systems in temperature range of interest 

Thermocline systems 

- adequate field experience, but requires large quantities 

of HTF 

- probably not cost effective for Syltherm-based systems 

Trickle charge 

- possibly most cost effective for short duration 

storage ( < 6 hours) 

- no technology development required 

Two-tank, salt 

- good field experience 

- most cost effective solution over narrow range of 

duration (8-12 hours) 
Conclusions 

(continued) 

Phase change 

- many possible configurations 

- no field experience 

- development required 

- good potential for cost effective long-term storage 

(> 15 hours) 
AN OVERVIEW OF ORNL TES WORK AND ITS 

APPLICABILITY TO LUZ SOLAR 

ELECTRIC NEEDS 

M. Olszewski 

J. J. Tomlinson 

Oak Ridge National Laboratory 

Prepared for 

EXPERT'S MEETING 

October 19 - 20, 1988 
APPLICABLE TES TECHNOLOGY HAS BEEN 

DEVELOPED THROUGH A NUMBER OF 

PROGRAMS AT ORNL 

DOE Thermal Energy Storage Program 

- building applications with applicable temperature 

range of -20C to 200C 

- industrial applications covering temperature range 

of 100C to 1 100C 

Burst Power Reject Heat Management for SDI 

- low (30C) and high (400C to 800C) needs 

Dynamic Solar Power System TES for NASA Space 

Station 

- development of performance model 

- advanced metal PCM development 
WIDE VARIETY OF TECHNOLOGIES ARE BEING 

(OR HAVE BEEN) DEVELOPED IN 

THE DOE PROGRAM 

Salt PCM Systems with Shell and Tube Heat 

Exchangers 

Thermocline Sensible Heat Storage 

0 Slurry Heat Transport and Storage Systems 

Form-Stable Composite Media 

Encapsulated Metal Alloy Storage Media 

Chemical Reaction Systems Using Complex 

Compounds 
MANY SHELL AND TUBE CONCEPTS 

HAVE BEEN STUDIED 

The primary issue with shell and tube designs is that 

reduction in conductance due to increasing solid salt 

layer leads to large surface area requirements 

o Techniques studied include 

- scrapers to keep solid from adhering to heat 

exchange surface 

- finned surfaces that extend beyond the solid salt 

layer 

- reticulated metal foams to provide continuous 

conduction path to the liquid 

- vibrating heat transfer surface to prevent 

adhesion of solidifying salt 

In general results were not greatly positive 

- mechanical means for removing solid salt from 

HX surface did not work well 

- finned surfaces enhance conductivity by about 

25% 

- reticulated metal foams had higher conductivity 

enhancements (about 35 to 45%) if good contact 

was made with heat transfer tubes 

a Even with conductivity enhancement shell and tube 

concepts require rather large surface areas 
THERMOCLINE SYSTEM STUDIES BY DOE HAVE 

PROVIDED TECHNICAL BASIS FOR 

SENSIBLE HEAT STORAGE DESIGN 

Thermocline TES design benefits include 

- single tank resulting in low system 

reduced heat losses 

- high volumetric efficiency 

cost and 

- textbook knowledge developed for diffuser design 

- no need for membrane 

Richardson 

stratification 

number identified as nondimensional 

parameter 

For stratification, Ri > -114 

Sharp thermoclines achieved in chilled water systems; 

better performance expected with SEGS TES 

Rationale: poi, = 20 pH, 
PUMPED PCM SLURRY INITIALLY STUDIED 

BY DOE MAY BE AN OPTION 

a Application in collector loop 

Potential benefits include 

- increased heat transport through latent heat 

- smaller pipes and pumps 

- greater thermodynamic availability especially in 

small AT systems such as SEGS 

Attractive features include 

- extensive SEGS collector loop volume serves two 

roles: heat transport and TES 

- slurry solids displace expensive heat transfer oil 

o Problems 

- PCM slurry material not available 

- slurry management and control 
OPEN COMPOSITES HAVE BEEN DEVELOPED FOR 

INDUSTRIAL REJECT HEAT RECOVERY APPLICATIONS 

Concept uses a PCM immobilized within a ceramic 

matrix 

Composition using Na,C03-BaCOJMgO was focus of 

development work 

Cold-pressing and extrusion processes developed that 

produced pellets with density of 92% T.D. 

Pellets typically 50 to 60% PCM by weight (heat of 

fusion of Na,CO,-BaCO, is 172 KJIKG) 

Performance test of 160 thermal cycles (>3500 hours) 

showed good media stability with only about 1% 

weight loss and no significant cracking 

At lower temperatures concept has not been as 

successful 

- nitrates and chlorides 

- density and PCM content have been lower than 

that achieved with carbonates 

- weight losses have been high 
Thermodynamic and practical advantages of packed bed 

of PCM pellets long-recognized by DOE and others as 

useful in low AT TES systems 

Central issue: development of PCM coating that is 

- economic 

- probably seamless 

- chemically stable 

- strong enough 

DOE-sponsored work (through OSU) focused on 

development of integral shell 

- by controlled quenching of Si-AI hypereutectic to 

form Si shell (Si-AI eutectic m.p. = 577C) 

- by aluminothermic reaction coating (alloy reacts 

with SiO, to form AI,O,) 

- by packed bed cementation siliconizing 

- results only partially successful 

- further work at ORNL on Si-A1 hypereutectic 

showed that Si shell not stable above 600C 

0 Joint venture between Lanxide and DuPont formed to 

continue coating development and applications work 

a PCM coating processes now being developed at 

lower temperatures by others. 
Thermochemical reactions currently being studied by DOE 

for chill storage applications may be useful in SEGS TES 

a Reaction: 

salts(s) + ligand (g) + complex compound(s) + A H 

- chill storage ligand is ammonia 

- chill storage salt: metal halides 

For SEGS , metal oxide - hydroxide reactions such as 

CaO(s) + H,O(g) 3 Ca(OH),(s) + ~h 

appear promising. 

System based on transfer of ligand (H,O) between 

two containers 

\ ' SATURATED YO \ 

HYDROXIDE \ 

- TEMPERATURE 
SDI ACTIVITIES HAVE FOCUSED ON 

BURST POWER APPLICATIONS 

Large amounts of reject heat generated for only a 

small portion of the orbit 

High-temperature need for prime power system uses 

Li H 

- between 700K and 11 00K storage density is >4 

M J/KG 

- encapsulate LiH spheres with metal shell 

- shells of 304L SS (7-mil) and MOLY (5-mil) have 

been used 

Lower temperature (300K) needs have been met 

using complex compounds 

- use ammines or hydrates 

- storage unit acts as heat pump and storage 

system 

- can get lifts exceeding 100K in a single stage 

unit 
ASSESSMENT ACTIVITIES PARALLEL 

THE HARDWARE DEVELOPMENT 

Examined thermal performance of five candidate 

storage unit designs 

- shell and tube PCM tube side 

- shell and tube PCM shell side 

- concentric annular regions 

- parallel slabs 

- packed bed 

. Assessment based on specific mass (WKG) basis 

Thermal performance results indicated that mass of 

encapsulating material and thermal conductivity of 

PCM were critical parameters 



DEVELOPMENT OF ADVANCED TES MEDIA FOR SPACE 

APPLICATIONS FOCUSES ON METALLIC PCM 

Fluoride salts (used in baseline system concepts) 

have problems associated with 

- low conductivity 

- restricted melt temperatures 

- large thermal expansion (20% to 30%) 

Advanced concepts will use metallic PCM's to 

overcome problems of salt systems 

- higher conductivity 

- alloy compositions available with melt 

temperatures spanning the operating temperature 

range of interest (1 100 to 1500K) 

- thermal expansion problem greatly mitigated 

- higher latent heat 

Silicon-based compositions under study include 

- germanium - silicon solutions (1 21 1 to 1687K) 

- eutectics with "iron" group 

Ni-Si eutectic (1 239K) 

- eutectics with alkaline earth group 

Be-Si eutectic (1363K) 

Key technology issue is encapsulation of PCM 
THERE ARE A RELATIVELY FEW MEDIA 

THAT DESERVE ATTENTION 

Within the 31 8C to 370C range there are about 500 

potential PCM's 

o Screening was done using the following criteria 

- cost 

- property data availability 

- toxicity 

9 Welookedat 

- salts 

- metallic compounds 

- brazes and solders 

I organics 

Potential candidates are in following families 

- hydroxides 

- chlorides 

- fluorides 

- nitrates 
BASED ON PREVIOUS WORK IT APPEARS THAT 

SEVERAL CONCEPTS ARE CANDIDATES 

FOR THE LUZ SOLAR APPLICATION 

Evaporating water and storing the vapor in zeolite 

Slurry heat transport system using microencapsulated 

PCM 

Shell and tube system 

Open composite media in packed bed 

Sensible heat storage using thermocline tank 

9 Chemical means using hydrated complex compound 

or hyd roxide/oxide reaction 


IN SUMMARY 

Many of the technologies developed by the DOE/ESD 

TES program are applicable to the LUZ application 

PCM is limited to a few salts 

Technologies potentially attractive include sensible, 

phase change, and chemical concepts 

- one-tank thermocline 

- PCM slurry 

- open composite 

- reversible reactionlhydrated complex compound 

o More detailed studies will be required before 

conclusion can be reached 

LUZ application could be addressed with some 

restructuring of current DOEIESD TES program 
DFVLR/CIEMAT 

Study on Thermal 

Energy Storage for 

Commercial Applications 

Presentation for 

Phase Change Thermal Energy Storage Symposium 

October 19-20 

Inn at Silver Lakes 

Helendale, California 

Dr. Michael Geyer 

Platforma Solar de Almeria 

Aptdo. 7 

Tabernas (Almeria) 04200 

Spain 

October 19, 1988 
SOLID SALT STORAGE 

Preliminary Cost Analysis 

Pipes: 760 000 rn 

304 m per stack 

Salt: 8300 m3 

1000 plates (8,3t) 

including melting 

and forms 

and assembly 

l nsulation Concrete ( 5600 m2 ) 

and steel cladding ( 17 0001 m2 ) 

and rockwool insulation ! 19 550 m2) 

TOTAL DM 22 270 000 

Rate 1.85 
CONCRETE STORAGE 

PRELIMINARY COST ANALYSIS 

Pi pes 'I 6500~0 diameter 22 mrn outside 

20 mm inside 

welded, mounted into the form work 

including 10710 tube turns 

i ncl udi ng 2 orbital we1 di ng machines 

and 1 pipe bending machine 

i ncl udi ng 6 di stri butors 

Concrete 10000 m3 and 330 t reinforcement steel 

mounted i ncl udi ng i nsul ati on 

concrete DM Z ?46 o@O 

Outside insul ation : 200 mm rock wool plate cladded 

7 ,pee 2 

Oi 1 520 m3 Monsanto 

Total 

Exchange rate: 

1 US $ equal to DM l,85 

Total 





















s 

Advanced Energy Transmission 

Fluids for Thermal Systems 

K. E. Kasza, U. S. ~.ho, and K. V. Liu 

Experimental ~hermaj Hydraulics Section 

Materials and Components Technology Division 

Argonne National Laboratory 
Scope 

Develop phenomenological understanding and 

engineering information for the use of phase change slurries (PCSs) and friction-reducing 

additives (FRAs) in thermal systems to improve thermal-hydraulic performance 

Justification 

Significant potential exists for improving a thermal 

system's overall performance through 

higher end-use temperatures reduced flow, allowing smaller pipes and 

,pumps lmproved system thermal storage 

reduced system weight increased heat transfer, allowing reduced heat 

exchanger size 

Lack of data on heat transfer, fluid mechanics, and 

system performance hinders implementation of 

the PCS and FRA concepts 
Advanced fluids concepts 

identified at ANL 

Low concentrations (20-200 wppm) of nonNewtonian additives (high:molecular-weight linear 

polymers, surfactants, synthetic fibers) to reduce 

frictional flow losses 

Phase-change (melting-freezing) particulate 

slurries (cross-linked high-density polyethylene, ice slurries, waxes) to enhance bulk convective 

energy transport and heat transfer coefficients 

Combining above concepts to achieve maximum 

benefits 
Technical Issues 

Can robust FRAs and PCSs be identified which 

are resistant to flow shear and compatible with 

temperatures of various thermal systems? 

How much do FRAs reduce heat transfer? 

How much do PCSs increase pressure drop? 

Are FRAs effective when combined with a PCS? 

What are the mechanisms and parameters 

dictating FRA and PCS performance? 

How do advanced fluids interact with thermal 

system components (pumps, heat exchangers, 

piping, valves, etc.)? 
Benefits 

Flow rate reduction due to PCS latent heat 

- Heating 0.6 Qo 

- Cooling 0.2 Qo 

System pressure drop and pumping power 

reductions due to FRA and PCS 

- Heating O.lAPo (Wo) 

- Cooling O.OIAPo (Wo) 

Pipe size reductions due to combined use of both 

concepts 

- Heating 0.5D0 - Cooling 0.3Do 

Energy storage tank volume reductions due to 

PCS 

- Heating 0.4 Vo 

- Cooling 0.1 Vo 

Friction-Reducing Additives 

Additives Tested: Dilute aqueous solutions of highmolecular weight linear polymers 

Polyox 

Separan 

Guar gum 

Aqueous suspensions of large- aspect-ratio synthetic fibers 

Ion Fiberglass 

lyester Iron Whiskers 

Conditions: Temperatures of 45200°F 

Prototypic turbulent piping flow 

shear 

Fresh solutions in various 

viscometers and long-term recirculatory flow 

Parameters Pressure drop Viscosity 

Measured: Heat transfer Degradation rate 

Outcome: Separan and nylon fibers 

identified as candidates for further 

testing 



Phase Change 

Slurries tested: 

Conditions: 

Parameters 

Measured: 

Outcome: 

Slurries 

Cross-linked high-density 

polyethylene (X-HDPE) 

Ice crystals 

Temperatures of 32-285°F 

Turbulent flow 

Loadings of 0-45 vol.% 

Particle sizes of 1/8 and 1/20 in. 

Pressure drop 

Temperature profiles 

Heat transfer 

Highly loaded slurries under 

certain conditions can be 

pumped with only marginal 

pumping power increases and 

exhibit increased heat transfer 

relative to pure water 

Ice slurry and X-HDPE have 

similar effects on pressure drop 

Pressure drop and heat transfer 

are strong functions of particle 

diameter 

Existing correlations are 

inadequate for desian 


Combined-Concept Fluids 

Materials Tested: X-HDPE + water + Polyox 

Conditions: Temperature of 75°F 

Turbulent flow 

Loadings of 0-35 vol.% 

X-HDPE, 200 wppm Polyox 

Particle sizes of 1/8", 1 /20" 

Parameters Pressure drop 

Measured: Temperature profiles 

Outcome: A friction-reducing additive can 

significantly reduce the pressure 

t@ 

drop for a slurry with < 30% 

particle loading 


Conclusions 

Testing of Advanced Energy Transmission Fluids 

has established their feasibility 

Improved thermal-hydraulic performance and 

component cost reductions can be realized 

Further testing is needed to evaluate performance 

in prototypic systems and identify additives for 

various operating conditions 

An engineering thermal-hydraulic data base must 

be generated when enhancement mechanisms 

are better understood 
Modeling of Fixed Bed Heat Storage 

Units Utilizing Phase Change Materials 

V. ANANTHANARAYANAN, Y. SAHAI, C. E. MOBLEY, and R. A. RAPP 

A computer model has been developed for the cafculation of the heat exchanged and temperature 

profiles in a packed bed containing a phase change material. The packed bed is intended as a heat 

storage unit in which an inert fluid flowing through the bed exchanges heat witb an encapsulated 

spherical shot of the phase change (melting and freezing) material. Examples of predicted bed 

temperature profiles during heat storage and utilization cycles are given. For Al-12 wt pct Si and 

Ai-30 wt pct Si shot, a sequence of storage and utilizat~on cycles with cyclic cocurrenr fluid flow was 

found to utilize the high latent heat of fusion of the shot efficiently and permit the ut~lization of the 

bed as a near isothermal (577 "C) heat recuperator. 

I. INTRODUCTION 

ECONOMIC heat storage systems are used for heat recovery and for load leveling under circumstances where the 

demand for and availability of energy do not coincide chronologically. Consequently, load leveling in a power plant 

with the utilization of stored heat for power generation during peak demand hours, utilization of alternative sources of 

energy (e.g., solar), and waste heat recovery are areas with 

potential for cost reduction using efficient heat storage. 

Heat storage systems that utilize only the sensible heat 

of a storage material (no phase change) may be one of 

two types. The first one is a system in which the energy 

transporting fluid itself serves as the storage medium. The 

other system has an enthalpy transporting fluid which exchanges heat with a separate storage medium. Domestic hot 

water heating systems using water as both the transporting 

and heat storing fluid fall into the first category. An example 

of the second type is a system utilizing the sensible heat of 

solids such as a firebrick checkerwork. A storage medium 

utilizing sensible heat should have high values for its heat 

capacity per unit volume and for thermal diffusivity; it 

should be chemically and geometrically stable under conditions of reversible heating and cooling. It should be noncombustible and nontoxic, have a low vapor pressure, and 

be inexpensive. ' 

Storage systems that utilize the latent heat of transformation are currently receiving considerable attention. 

'The storage media used by these systems, called Phase 

Change Materials (PCMs), offer advantages such as a small 

temperature difference between the storage and retrieval 

cycles, small unit sizes, and low weight per unit storage 

capacity. Selection of the storage material depends largely 

on the mean storage temperature. Some desirable characteristics of a PCM for use as a heat storage medium are a high 

latent heat of transformation (usually fusion), a narrow 

freezing range or a congruent melting point, nontoxicity, 

long term chemical stability, inertness with respect to a suitable container material, low cost, and ready availability. ' 

Inorganic salts such as halides, nitrates, sulfates, carbonates, etc, in multicomponent eutectic systems have been 

V. ANANTHANARAYANAN, Graduate Student, Y. SAHAI, Associate Professor, C. E. MOBLEY, and R. A. RAPP, Professors, are with the 

Department of Metallurgical Engineering, The Ohio State University, 

Cniumbus, OH 43210. 

Manuscript submitted January 8, 1986. 

studied for a Iong time as PCM alternatives to sensible heat 

storage in water, fused salts. or bricks. Tye, Bowine, and 

Desjar1is"pecified PCM salts with temperature ranges to 

match principal heat generating sources: LiN03 at 527 K 

(254 "C) for a pressurized water reactor, 63LiQI-I-37LiCI at 

533 K (260 "C) for a boiling water reactor, LiOH at 743 K 

(470 "C) for a fossil-fueled supercritical steam reactor, and 

Na2B,O7 at 1015 K (742 "C) for a high-temperature gascooled graphite moderated reactor. Thus lithium salts were 

the basis for no st workable PCbf materials in that study. 

Birchenall and ~iechman' and Farkas and Birchenal14 

have surveyed the potential of metallic eutectics and congruently melting intermetallic compounds for heat storage. 

They showed that the heat of fusion of a heat storage alloy 

can be maximized by the use of a large number of components of high beats of fusion in nearly equai proportions in 

the alloy. They found that alloys rich in the relatively plentiful element Si offer high heat storage densities. 

Research has been in progress at The Ohio State University toward the development and fabrication of spherical 

self-encapsulated phase change material shot, which can be 

brought into direct contact with an energy transporting fluid 

in a ceramic lined column, obviating the need for an expensive and corrosion-resistant heat exchanger. The heat 

storage shot would have a relatively high melting, corrosion 

resistant outer shell that would enclose and contain a lower 

melting heat storing eutectic core. 

The fabrication of self-encapsulated PCM shot as described above is being considered through (i) controlled 

cooling of a hypereut&tic Al-Si shot in-the temperature 

range between the liquidus and the eutectic, (ii) chemical 

oxidationlanodizing, and (iii) pack cementation siliqonizing. The shot as fabricated would have a shrinkage cavity 

at its center that would accommodate the expansion upon 

melting of the shot core during temperature cycling. 

The purpose of this work was to develop a mathematical 

model that would describe the thermal response of packed 

beds which utilize encapsulated eutectic or hypereutectic 

Ai-Si PCM shot as a heat storage medium. Models available 

in the literature on heat exchange in packed beds adequately 

cover sensible heat storage without a phase change. These 

models were extended to include the latent heat of fusion for 

the PCM shot in addition to differing values for the specific 

heat above and below the transformation temperature. 

Figure 1 shows the ALSi equilibrium phase diagram.5 

Figure 2 shows a schematic representation of the shot and 

RIETALLURGICAL TRANSACTIONS 13 VOLUME 18B. JUNE 1987-339 
Atomic Percentage Silicon 11. BACKGROUND 

Weight Percentage Silicon 

Fig. 1 - Aluminum-silicon phase diagram. 

/Silicon Shell of 

Thickness as per 

Hot Fluid During Storage Cycle or Cold Fluid During Utilization Cycle 

Ceramic Lined 

Column Containing 

At - Si Shot as Above 

Out let Fluid a\ Temperature 

Nearly Equal to the Eutectic 

Temperature Dur inq Both Storage 

and Utilization Cycles 

Fig. 2-Schematic figure sliowing Al-Si shot with shell and a ccramic 

lined column containing the shot. 

the mode in which the shot may be used in a ceramic-lined 

column based on this study. 

Such a bed could be operated above and below the transformation temperature as a near-isothermal recuperator to 

utilize the high latent heat of fusion of the PCM shot. The 

model described here was developed to devise modes of 

operating the bed in a most efficient manner. 

Two models are conventionally used to describe the transient response of a packed bed heat storage unit. One is the 

single-phase conductivity model which is based on the assumption that the temperatures of the fluid and solid are 

equal at any given axial location. 

The energy balance equation for the single phase model is 

The use of dimensionless parameters describing the response of the packed bed results in simplified differential 

equations and general soIutions valid for many different 

combinations of independent variables involved. 

~iaz' provided solutions of the conductivity model, plotting dimensionless temperature as a function of dimensionless distance and time. defined as below: 

dimensionless distance 

dimensionless temperature 

dimensionless time 

The nondimensional energy equation became 

The alternative Schumann model for the solidifluid energy balance assumes infinite thermal conductivity in the 

transverse direction, no axial heat conduction. perfectly insulated container walls, temperature-independent physical 

and thermal properties, constant fluid flow velocity along 

the length of the bed, and negligible radiation effects. The 

basic differential equation for the energy balance for the 

fluid is: 

The equivalent equation for the storage medium is 

Riaz compared the results of the two-phase Schumann 

model with those obtained using the conductivity model and 

indicated that the two models agreed within errors of less 

than 10 pct for 7 less than 10. 

~effersod presented a detailed evaluation and comparison 

of the various methods f~r the determination of the transient 

response of packed beds. He found that Biot number, thermal capacity ratio, and Peclet number as defined below were 

significant in determining the conditions under which the 

simplified two-phase model could be modified to account 

for intraparticle conduction and axial fluid dispersion. The 

dispersion effect is the mixing action within the fluid that 

results from the eddy currents created as the fluid flows 

through a complex set of passages. 

340--VOLUME 18B, JUNE 1987 hIETALLURGICAL TR.4NSACTION.S 0 
Biot number 

Heat capacity ratio 

Thermal capacity ratio 

P = VJ(VH + 1) F 101 

Peclet number 

The Biot number was found to provide the sole criterion for estimating the relative importance of intraparticie 

conduction. For a storage medium with a high thermal 

conductivity, the Biot number is smaIl and the storage medium will have a uniform temperature within each particle. 

At large values of VH, Jefferson lumped together axial dispersion and intraparticle effects to obtain an effective convective film heat transfer coefficient he to be used with the 

Schurnann model. 

Schmidt and Willmottl evaluated lz, using the above expression in typical beds of steel shot with air as the fluid and 

showed that the difference between h and h, is small. 

None of these previously available solutions accommodates 3 phase change in the shot. To generate temperature 

profiles for a bed of PCM shot shown in Figure 2, these 

models must be extended to include in the calculations an 

expression for the latent heat of transformation. The purpose 

of this work was to develop such a model. Since temperature 

profiles in a bed of AI-Si shot of high thermal conductivity 

heated or cooled by air are proposed to be generated, two 

simplifications become clear. The high thermal conductivity 

of the shot makes the Biot number small and the assumption 

of uniform temperature in the transverse direction becomes 

reasonable. Secondly, for fluids such as air with high values 

of V,, the use of a modified heat transfer coefficient in a 

Schumann-type model, as proposed by Jefferson, becomes 

applicable. However, as the change in the heat transfer 

coefficient due to such modification has been shown to be 

small,' a Schumann model with incorporation of terms for 

the phase change in the energy equation appears to be satisfactory. Regarding other simplifying assumptions that may 

be made, the assumption of negligible axial conduction appears to be reasonable as the shot are only in point contact. 

Since it is proposed to use the bed of AI-Si shot around the 

eutectic temperature of 577 "C, the operating temperatures 

are not high enough to make radiation effects appreciable. 

However, the model developed for the Al-Si shot should 

agree with the Schumann model below the transformation 

temperature. For short dimensionless times less than ten, the 

Schurnann model agrees with curves published by ~iaz,' 

which can be used to check the validity of the model. 

111. MODEL DESCRIPTION 

The computer program (herein called the model) was 

developed for the estimation of temperature profiles of the 

solid and the fluid along the length of a packed bed of 

seff-encapsulated PCM shot as functions of distance along 

the bed and time during a series of heat storage and utilization cycles. The model ignores the thin encapsulation shell 

on the PCM and considers cnly the heat storing core of the 

shot in the calculations. In this two-phase model, an energy 

balance is established between the fluid and the storage 

material. The bed is divided into a series of small height 

segments within which both the solid and fluid temperatures 

are assumed to be uniform within each phase. The model 

assumes infinite thermal conductivity in the transverse 

direction, no axial heat conduction, a perfectly insulated 

container wall, negligible radiation effects, constant fluid 

flow velocity along the length of the bed, and no axial 

dispersion effects in the fluid. Temperature-dependent properties of the fluid are used in the caIculations. 

In a given height segment, heat exchanged between the 

shot and a small amount of fluid that enters the bed during 

an incremental period of X,/U, is considered. The heat 

transfer coefficient for heat exchange between the fluid and 

the height segment nearest to the inlet end is first calculated according to the following expressions proposed by 

Baumeister and en net:^ 

where a and z are given in Table I.~ 

The Ergun analogy factor J, represents an analogy 

between the friction factor and the hear transfer coefficient. 

In fact, Jh is proportional to the friction factor. 

h,, = 6h/d [151 

The heat exchanged from the fluid to the solid is then calculated as: 

where m and n are the indices for the mth time increment 

and the nth height segment of the bed, respectively. The 

temperature change in the fluid resulting from passage 

through the segment is given by: 

The corresponding temperature change in the shot is given 

by: 

ATb(m,n) = x,H(~. n)/m,CbUf [ 181 

The temperature of fluid entering the next height segment is 

given by: 

The temperature of the bed for the next quantity of fluid that 

enters the bed during the next period X,/Uf is given by: 

Table I. Constants Used in the Calculation of .lh9 

hfETALLURCICAL TRANSACTIONS B \'OI.LIIII 18B. JUNE 1987-341 
During a storage cycle, AT,(m, n) is negative and AT,(m, 12) 

is positive, with opposite signs during a utilization cycle. 

The flowing fluid with the revised temperature then exchanges heat with the next height segment, which is again 

calculated as per Eqs. [13] through [20]. 

When the temperature of the shot reaches the eutectic 

temperature in any segment, its temperature is not changed 

until the latent heat of fusion of the shot in the segment is 

absorbed by the shot. During this period, the percentage of 

shot melted in a given segment is calculated as the percentage of the heat of fusion absorbed by the segment. 

The model accounts for the time element by adding X,/Uf 

to the time elapsed for each incremental volume of fluid that 

passes through the bed. The output variables are nondimensionalized as in Eqs. 1213 through [B]. However, it should 

be noted that a change in the inlet fluid temperature relative 

to the PCM eutectic or transformation temperature would 

change the profiles. 

The instantaneous rate of heat storage is calculated as: 

where y is the nvmber of height segments into which the bed 

is divided and Q is the instantaneou~~rate of heat storage in 

the bed. During a utilization cycle, Q has ? negative value 

since heat is removed from the bed. The Q values for the 

entire duration of a storage or utilization cycle are summed 

up to calculate Q, the total heat stored in or removed from 

the bed during the cycle, as follows: 

where N is the number of fluid elements that pass through 

the bed during the cycle. The bed utilization factor is evaluated as: 

The above model was used to generate temperature profiles in a bed of ALSi shot as described in Figure 2. Inlet 

fluid temperatures of 750 "C during storage cycles and 

27 "C during utilization cycles were used in the calculations. 

The physical properties required for the calculations are 

presented in Tables II and 111. The profiles generated and the 

modes of heating and cooling were then examined to devise 

modes of operating the bed as a near isothermal source of 

heat during both storage and utilization cycles. 

IV. RESULTS AND DISCUSSION 

Program runs were first made with dimensionless parameters calculated using Eqs. [4], [S], and [6] for comparison of temperature profiles prior to the onset of phase 

Table 11. Physical Properties Used in Calculations 

A1-12 Si Shut A1-30 Si Shot Air 

Density 2.34 X 103 2.34 X lo3 1.281 

(Kg/m3) 

Heat capacity 8.87 X 10' 8.57 X 10' 

(Joules/Kg-C) for Th < 577 OC 

2.415 X lo3 

for Tb > 577 OC 

Heat of fusion 5.16 X lo5 4.13 X lo5 

(Joules/Kg) 

The other physical properties of air ihat were used arc given in Table 111. 

Table 111. Physical Properties of Air Used in Calculations 

Yf , Cf 

(x 10- 1 oise) (X lo3 ~oules/Kg-C) 

184 I. 005 

184 1.005 

227 1 .014 

265 1.030 

299 1.051 

33 1 1.075 

362 1.099 

39 1 1.122 

4 19 1.143 

445 1.161 

change in the PCM shot, i.e., considering only sensible heat 

exchange. Agreement within approximately 5 pct between 

the temperature profiles generated by the present model and 

those published by Riaz was observed as shown in Figure 3. 

The calculations of the present model had been carried out 

with a step height of 2.5 x meter. 

Figure 4 is a plot of dimensionless temperature vs dirnensionless distance for a bed of eutectic 1 x lo-' meter diameter Al-12 wt pct Si alIoy shot at different times during 

an initial heating cycle for a bed from a uniform initial temperature of 26.7 "C (80 OF). The heat exchange between the fluid and the shot is very fast as expected for a 

packed bed owing to the high heat transfer surface area to 

volume ratio. The temperature of the bed at and near the 

top (inlet) quickly reaches the bed eutectic tenlperaturz 

(750 "C or 1382 OF), while the bottom of the bed is still 

at lower temperatures. Hence. if the latent heat of the 

- Present Work - --- R~az's Work 

ti 1 'ItI1'I f"~~ll~~"i' 

0 0 2 17 4 0 6 0 8 I C 

Dimensionless Distance 

Fig. 3 -Comparison of temperature profiles generated by present model 

with those of ~iaz.' 

342-VOLUME 18B. JUNE 1987 
D.1 mensionless Distance 

Fig. 4-Profiles in a bed of eutectic Al-12Si alloy shot during the first 

heating cycle. Hot inlet fluid flows from left to right. 

shot is to be efficiently utilized by raising the outlet of the 

bed to the eutectic temperature, the bed inlet needs to 

approach the inlet fluid temperature. If conventional cyclic 

countercurrent cooling of the bed were done after the first 

storage cycle, the outlet would cool quickly and generate 

steep temperature gradients in the bed for the subsequent 

heating cycle, as seen in Figure 5. Hence, the efficient utilization of the latent heat of fusion would not be realized with 

the scheme of cyclic countercurrent heating and cooling. 

Therefore, the program was run to generate temperature 

profiles during a cyclic cocurrent cooling cycle for the bed, 

starting from the profile at the end of the initial heating cycle 

shown in Figure 4. The intention was to transfer heat from 

the hotter inlet of the bed to the cooler outlet of the bed and 

thereby raise the temperature of most of the bed to the 

eutectic temperature. Figure 6 is a plot of dimensionless 

temperature vs dimensionless distance for different dimensionless times during the first cocurrent cooling cycle following the storage cycle of Figure 4. At the end of this 

cycle, essentially all of the bed had been brought to the 

eutectic temperature, as desired. 

Figure 7 is a plot of temperature profiles in the bed at 

different dimensionless times during the subsequent heating 

cycle starting from the profile at the end of the cocurrent 

cooling cycle shown in Figure 6. Through step 5, a major 

portion of the bed is at the eutectic temperature and the heat 

input is utilized mainly to provide the latent heat of fusion 

for the shot, while the fluid outlet temperature remains at or 

near the eutectic temperature. 

Dimensionless Distance 

Fig. 5 -Profiles in a bed of eutectic AI- 12Si alloy shot during a subscqucnt 

countercurrent cooling cycle. Cool inlet fluid flows from right to left. 

Ill 111 I 1 ,:I 

OCblll 0 2 l1Il' 04 C 6 0 8 10 

Dimensionless Distance 

Fig. 6-Profiles in a bed of eutectic Al-l2Si alloy shot during a cocurrent 

cooling cycIe following the heating cycle of Fjg. 4. Cool inlet fluid flaws 

from left to right. 

Dimensionless Dlstonce 

Fig. 7- Profiles in a bed of eutectic AI-12Si alloy shot during the second 

heating cycle after the cocurrent coohg cycle of Fig. 6. Hot fluid flows 

from left to right. 

For all subsequent cyclic cocurrent cooling and heating 

cycles, about 75 pct of the bed re~nains at the eutectic temperature with the shot at each height exhibiting a differing 

melted fraction, while the bed inlet temperature cycles between the two temperature extremes, as shown in Figures 8 

and 9. 

Figures 10 and 11 present temperature profiles for the 

hypereutectic Al-30 wt pct Si alloy. This hypereutectic alloy is chosen as a composition for which the outer containment shell could be pure Si. The shell thickness would vary 

according to the lever rule from the phase diagram. By the 

use of fabrication procedures for the shot as suggested earlier and the selection of cycling temperatures, a typically 

minimum shell thickness of 1 x lop3 meter would be rnaintained. The melting of the hypereutectic alloy has been 

treated to provide an effective (average) specific heat above 

the eutectic temperature. This was done by taking into account the heat of fusion of silicon as it progressively melts 

above the eutectic temperature for AI-36 wt pct Si alloy. 

Upon comparing Figure 10 with Figure 4, the inlet of the 

bed of Al-30 wt pct Si shot is not heated up quite as much 

as the inlet of the bed with A142 wt pct Si shot, because the 

effective specific heat for the A1-30 wt pct Si alloy above 

the eutectic temperature is 2.415 X lo3 Joules/Kg-C as 

compared with 8.87 x 102 Joules/Kg-C for the A!-12 wt 

pct Si alloy. 

METALLURGICAL TRANSACTIONS 3 VOLUME 1SB. JUNE 1987-343 
Dimensionless Distance 

Fig. Y -Profiles in a bed of eutectic AI-12Si alloy shot during the second 

cocurrent cooling cycle following storage cycle of Fig. 7. Cool fluid flows 

from left to right. 

oL'~~~~''~'~' 0 2 0 4 'l'llli'ltl'll 0 6 0 8 10 

D~mensionless Distance 

Fig. 9-Profiles in a bed of eutectic Al-12Si alloy shot during the third 

heating cycle following utiIization cycle of Fig. 8. Hot fluid flows from left 

right. 

Dimensionless Distance 

Fig. 10- Profiles in a bed of hypereutectic AI-30Si alloy shot during the 

first heating cycle. Hot fluid flows from left to right. 

Figure 11 shows profiles for the A1-30 wt pct Si alloy 

after the third heating cycle combined with cocurrent cooling, similar to the curves shown in Figure 9 for the Al12 wt pct Si alloy. Figure 1 l further ill~.istsates that only 

the inlet of the bed cycles in temperature while the bed 

effectively provides an isothermal reservoir of heat at the 

eutectic temperature. Computer runs for the A1-30 wt pct 

alloy corresponding to successive cycles, such as in Fig- 

, I, ,I oL'~'~!~~~'!' 0 2 0 4 0 6 0 8 i 0 

Dimensionless D~stance 

Fig. 1 I -Profiles in a bed of hypereuicctic AI-30S1 alloy shot during the 

third heating cycle following thc satnc Iieat~ng and cooling cycles as in 

Figs. 4 through 9. (This figure correaporlds io Fig. 9 for the Al-I2 wt pct 

Si alloy.) 

ures 5 through 8 for the A1- 12 wt pct alloy, resulted in sirnilar- profiles. 

Figure 12 is a plot of the instantaneous rate of heat storage 

in the bed during successive heating and cocurrent cooling 

cycles in a bed of A1-12 wt pct Si shot. After a few initial 

cycles of heating rind coaling. ths amount of heat stored in 

the bed during each heating cycle (area under curve) and that 

removed from the bed during L'JC~ cooling cycle are nearly 

constant. The model can force it?;: areas under the curve to 

bc equal for each successive hcating and cooling cycle by 

continuing a given heating or cooling cycle until a specified 

value of BUF is reached. This would ensure that the total 

ainount of heat stored in the bed during a heating cycle 

exactly equals the heat removed during the succeeding cooling cycle. The model can also provide the relative periods 

of heating and cooling needed to maintain such equality for 

a given set of hot and cold fluid inlet temperatures. 

For an industrial system con~prised of two identical columns which are operated out-of-phase for constant time 

periods, the hot inlet process temperature would be fixed. 

The thermal balance could then be satisfied most readily by 

adjustment of the flow rate of the cool inlet stream orland 

adjustment (raising) of the temperature of the cool inlet 

stream l~ia heat gained from an auxiliary conventional heat 

exchanger. 

The use of a balanced pair of heat storage units, each 

utilizing a sequence of different PCM shot with different 

l'l:lll'~"l'L1'l:~l"l 1 0 

Time (m~n) 

Fig. I2-Plot of heat storage rates (Al-3OSi alloy). 

34.1- VOLUME 188. JUNE 1987 \IETALLURGICAL TRANSACTIONS B 
transformation temperatures, could yield a set of outlct fluids with different, but nearly constant, temperatures for 

power generation or other purposes. A schematic representation of such a system is shown in Figure 13. Indeed, other 

eutectic alloys high in Si have been proposed for heat storage at various temperatures.12 

V. CONCLUSIONS 

A computer model has been developed for the estimation 

of temperature profiles of PCM shot in a packed bed in 

contact with a hot or cold inert flowing fluid as functions of 

distance along the bed and time during successive heating 

and cocurrent cooling cycles. Successive cyclic cocurrent 

heating and cooling cycles were shown to maintain a major 

portion of the bed at or near the transformation temperature, 

so that heat gained by or removed from the bed contributed 

largely only to a change in the percentage of shot melted. 

Such cycles were also shown to provide a nearly constant 

outlet fluid temperature equal to the eutectic temperature. 

The model can specify relative periods of heating and cooling cycles needed to make the heat removed during a cooling cycle equal the heat stored during a heating cycle. This 

is done by specifying a desired bed utilization factor for each 

heating and cooling cycle that would enable the continucd 

operation of a major portion of the bed at or near the eutectic 

temperature after the initial few transient cycles. The model 

was found to compare favorably with results published in 

the literature for temperatures below the onset of melting in 

the PCM shot. A sequence of such heat storage beds inPower Genera tion - or Other Use or Other Use 

Cold 

Fluid / 

Outlet Fluid 1 01 -T2 

Other Valves ond Serles of Beds Contorning 

Shot of Successtveiy Lower 

Tronsformat~on Temperot ure 

i Low Tempercture i Low Temperolure 

Hgat Feat 

Fig. 13-,4 schematic representation of a balanced pair of sequential 

packed beds utilizing PCM shot. Indicated valve positions provide heat 

storage for be& on left and utilization on right. Roration of all valves 

90 deg clockwise provides opposite functions. 

volving different PCMs with differmt eutectic temperatures 

could provide isothermal streams of waste heat at several 

levels of temperature. 

NOTATION 

Area of cross-section of the bed (m2) 

Heat transfer surface area in each height 

segment (m2) 

Biot number (hd/kh) 

Bed utilization factor (fraction) 

Specific heat at constant pressure (Jouies/Kg-C) 

Specific heat at constant pressure of storage medium 

above the eutectic temperature (Joules/Kg-C) 

Diameter of shot in the bed (m) 

Diameter of the bed (mi 

Rate of heat exchange (Jouies/sec) 

Heat of fusion of the shot core material (Joules/Kgi 

Convective film heat transfer coefficient 

(Joules/rn2-C-sec) 

Effective film heat transfer coefficient 

(Joules/m2-C-sec) 

Volumetric heat transfer coefficient 

(~oules/m'-C-sec) 

Ergun analogy factor 

Thermal conductivity (Joules/(sec) (m) ("C)) 

Length of the bed (m) 

Mass rate of flow (m/sec) 

Weight of shots in a segment (Kg) 

Prandtl number (Cfpf/X-F) 

Reynolds number (pJdCTI /p,.) 

Represent the nth height segment and the :nth 

time element 

Peclet number (plCj Ll, )'ALE /kjA,, 

Heat stored in or removed from the bed in t storage 

or utilization cycle (Joules) 

Rate of heat storage or rernoval (Joules/se-:) 

Time (sec) 

Eutectic temperature (Ci 

Temperature (C) 

Temperature change (C) 

Superficial velocity of fluid (m/sec) 

Interstitial fluid velocitj. (m/sec) 

Heat capacity ratio [p,, Ci,( 1 - ~j/p, Cf] 

Volume of shot in a segment (m') 

Distance from heating fluid inlet (m) 

Length of a height segment (m) 

Number of height segments into which bed is divided 

Greek Letters 

Thermal capacity ratio ! lJH /I1, f 1 ) 

Dimensionless time as used in Riaz's work7 

irhp, C,/(p,C, U,YI 

Dimensionless time (th,,/p, C,) 

Porosity of the bed (fraction) 

Density (Kg/m3) 

Dimensionless distance as used in Riaz's work7 

(~kb / 4 P~ C!) 

Dimensionless distance (L - X)/L 

Dimensionless temperature 

[TI(], 1) - Tdm. n)]/!J,( 1, 1) - Tdl, 01 

METALLURGICAL TRANSACTIONS B VOLUhlE 18B. JUNE 1987 - 335 
f Fluid 

b Storage medium 

ACKNOWLEDGMENTS 

Financial support from the Department of Energy Subcontract No. 19X-43394C from Martin Marietta Energy 

Systems, Inc.. Oak Ridge, Tennessee 3783 1, is gratefully 

acknowledged. The authors appreciate technical discussions 

with h4. Olszewski, J. Martin, and L. S. Fan. 

REFERENCES 

1. F. W. Schmidt and A. J. Willmott: Thermal Et~cr-g Storage citrri 

Regeneration, Hemisphere Publishing Corporation. Washington. 

1981, chs. 1' and 12. 

2. R. P. Tye, 3.G. Bowine, and A. 0. Desjarlis: NASA Report No. 

NASA-CR-135098, Dynatech Corp.. August 1976, available from 

NTIS. 

3. C. E. Birchenall and A. F. Riechrnan: Mcrilll. Trans. A, 1980. 

vol. 11.4, pp. 1415-20. 

4. D. Farkas and C. E. Birchenall: ,VI~rall. Trons. A. 1985, vol. 16A, 

pp. 323-28. 

5. I>. T. Ha\vkins and R. Hultgrcn: i1S:t.I M~va1.s Handbook. 8th ed.. 

vol. 8. p. 263. 

6. T. W. E. Schurnann: J. FranX-li~l I~l~rilrrte, 1919. voi. 208. pp. 415-16. 

7. IM. Riaz: J. Near Trcln$er, Trcrtr.r. :?SME, 1977, x'ol. 99. pp. 189-92. 

8. C. P. Jefferson: A1Ch.E J., 1972. ~ol. 18, pp. 309-16. 

9. E. B. Baunleister and C. 0. Rennet: ,4IChE J., 1958. vol. 4. 

pp. 69-74. 

10. R. H. Perry and C. H. Chilton Chcmiciil EI~~~IICJY~S' Hu~~dhook, 

5th ecl., McGraw-Hi:l Book Company, Ncw York. NY. 1973. 

Section XVI. 

11. R. Hultgren, P. D. Desai, D. T. IIawkins, M. Gleiser, and 1). D. 

Wagnian: Selected Values of T!lemoh.tlamic Properties qf Bait1at-y 

Alloys. ASM, 1973. 

12. V. Ananthanarayanan, R. Gallo. V. Ravi, W. Thacker, I. Varol. 

C. Mobley, and R. Rapp: Annual Report of Subcontract No. 

19X-433946, prepared for Marlin Marietta Energy Systems. hc., 

January IS, 1985, p. 37. 

346--VOLUhIE 188. JUNE 198'7 3IETALLURGICAL TRANSACTIONS B 
N, SHAMSUNDAR 

ASSOC. PROF. IN MECH. ENGINEERING 

UNIVERSITY OF HOUSTQN 

HOUSTON, TEXAS 

Prepared for LUZ-DOE-SERI Workshap, 

LJi~.t~rviile~ California, October 1988. 
N. SHAMSUNDAR 

Puepared for LUZ-DOE-SERI Horkshop , 

Vt i 11, California, October 1988, 

ABSTRACT 

Existing tools for design and perfoumance 

calculations for passive latent heat storage heat 

exchangers are summar i zed. The to~ls are adequate 

to treat the more cri tical stage, namely, the heat 

recovery 5taje, for shellitube heat exchangers. 

Addi t i anal development wurk is necessary ta address 

the storage staye, viscous heat transfer fluids, 

finned tubes and system ap tirnization. 
Thermal Anal y c,i 5, Peuf ormance Calculat i ons, 

Thermal Design Consideration=. and Methods 

TYPES OF LATENT HEAT STORAGE UNITS 

Shell-and-tube : (a) single pas5 -- multiple pas5 

(b) finned -- unf inned 

(cj PCM in she1 1 -- PCM in tubes '- 

Other types -- plate-f in, divect-contact, etc. 

CIESI GP-j AND ANALYSI S Of PERFORMANCE 

Storage Unit in Discharge Mode - Storage Mode 

Flowrate and Inlet Temperature of Fluid Vary 

in Time in a Prescribed Manneu ,may haue Jumps 
1 Latent Heat S~oyage ir :GOT 

150 thermal : Temp, Di f f . needed 

for Conduct i an, Convection 

2. Freeringielting will probably NOT . 

be Axially Uniform 

3. Double arid Multiple Tube Passes of 

the Fluid-Fluid Type to be Avoided. 

Uniform phase growth not desiuable 

4. Fins can Provide Significant Irnprouernent 

in Perfcarrnance, close to the Point of 

Isothermal Operation 

6 

LS . I-->o 1 ume t Y i c Heat Cap ac i t y i s Mcr r e 

Important than Good Conductivi ty 

in FCM Selection 

6. Melting is not always dominated by 

Convect ion 
Effectiveness -- bITU char t 5 

Csrnpu ter pregram for calculating effect iueness. 

Handles freezing and melting on 

unf i nned tubes, t ime-dependen t HTF flow 

and inlet temperature 

Computer pr~grarn for finned tubes, wi th 

constant HTF flow an.--! T-inlet 

.Design pYoguarri CdeTaiIs next3 

FEM code for -~*irtgfe tube, used as diagrmstic 

tool f~r srmple models 

Simulation subroutine far heat uecovery 

Shrinkage cavity model (experimentally ueuified) 
Select PCM wi+h melting point 

slightly below stsring temperature. 

Decide upon maximum and minimum outlet 

temperatures during heat recovery. 

Collect thermal properties of fluid 

and PCM. 

Run design program {any PC, VAX, etc.) 

Examine output design parameters from 

prog~arn and redesign if necessary . 

Limi tations of Existing Design Software: 

Heat recoueuy only. 

Ns fins, 

Heat transfev coefficient uniform 

-- not valid for viscous HTF 

C~nstant toad 

The5-e limi tat ions DO MOT apply to rrtcrdels 

and analysis software. 
Include melting, finned tubes and 

time-dependent HTF flow in design tool 

Develop model for deudoping flaw for 

u5.e with tri~.cou~ HTFs 

Verify me1 t ing modal by FEM 

LJerify fin riiodel by FEH 

Merge storage 5imulati~rn 5ubroutine with 

solar system simulator, optimize system 

Include melting, finned tubes, holding 

stage into simulator 

C~st opt irnizat ion, structural/safety needs 






APPENDIX C 

List of Symposium Participants 
PHASE CHANGE THERMAL ENERGY STORAGE SYMPOSIUM 

Dr. Charles A. Bankston 

CBY Associates, Inc. 

5039 Cathedral Avenue, NW 

Washington, DC 20016 

(202) 363-6693 

Dr. Mark Bohn 

Solar Energy Research Institute 

1617 Cole Boulevard 

Golden, CO 80401 

(303) 231-1755 

Mr. Kevin Drost 

Batelle Pacific Northwest 

Laboratories 

Batelle Boulevard 

Wichland, WA 99352 

(509) 375-2017 

Mr. Yoel Gilon 

Luz Industries Israel 

c/o Luz International Ltd. 

924 Westwood Boulevard 

Los Angeles, CA 90024 

DialCom 05:L,I1015 

Dr. Michael Geyer 

Platforma Solar de Almeria 

APTDO. 7 

Tabernas (Alrneria) 04200 

Spain 

Mr. Bimleshwar P. Gupta 

Solar Energy Research Institute 

1617 Cole Boulevard 

Golden, CO 80401 

Mr. Russell Hewett 

Solar Energy Research Institute 

1617 Cole Boulevard 

Golden, CO 80401 

(303) 231-1000 

Mr. Kenneth E. Kasza 

Materials and Components 

Technology Division 

Argonne National Laboratory 

9700 South Cass Avenue 

Argonne, IL 60439 

(312) 972-5920 

October 19 - 20, 1988 

Helendale, California 

Dr. David M. Kearney 

Vice President 

Advanced Technology Development 

Luz International Ltd. 

924 Westwood Boulevard 

Los Angeles, CA 90024 

(619)481-8044 

Mr. Leon Mendelovici 

Luz Industries Israel 

c/o Luz International Ltd. 

924 Westwood Boulevard 

Los Angeles, CA 90024 

DialCom 05:LII015 

Dr. Mitchell Olszewski 

Oak Ridge National Laboratory 

P.O. Box Y 

Oak Ridge, TN 37831 

(615) 574-0768 

Professor Robert A. Rapp 

Department of Metallurgical 

Engineering 

141 A Fontana Laboratories 

116 West 19th Avenue 

Columbus, OH 43210- 1179 

(614) 292-6178 

Dr. M. Shamsundar 

Mechanical Engineering Department 

University of Houston 

4800 Calhoun 

Houston, TX 77004 

(713) 749-2347 

Dr. John Tomlinson 

Oak Ridge National Laboratory 

P.O. Box Y 

Oak Ridge, TN 37831 

(615) 574-0768 
LIMITED DISTRIBUTION LIST 

Battelle Pacific NW Laboratory 

P.O. Box 999 

Richland, WA 99352 

Dr. Ben Johnson 

Mr. Tom A. Williams 

Bechtel Corporation 

P.O. Box 3965 

San Francisco, CA 94 1 19 

Mr. Pascal de Laquil 

Mr. Tom Brumleve 

Consultant 

15 12 N. Gate Road 

Walnut Creek, CA 94598 

Department of EnergylHQ 

Forrestal Building 

1000 Independence Ave., SW 

Washington, DC 20585 

Mr. Cliff Carwile 

Dr. H. Coleman 

Mr. S. Gronich 

Mr. K. Klein 

Mr. M. Scheve 

Mr. Frank W ilkins 

Department of Energy/SAO 

16 17 Cole Blvd. 

Golden, CO 80401 

Dr. Paul K. Kearns 

Industrial Solar Technology 

5775 West 52nd Avenue 

Denver, CO 8021 2 

Mr. Randy Gee 

LaJet Energy Company 

P.O. Box 3599 

Abilene, TX 79604 

Mr. Monte McGlaun 

Pacific Gas and Electric Company 

3400 Crow Canyon Rd. 

San Ramon, CA 94583 

Mr. Gerry Braun 

Sandia National Laboratories 

Solar Energy Department 6220 

P.O. Box 5800 

Albuquerque, NM 87 185 

Dr. W. Marshall 

Science Applications 

International Corp. 

1 040 1 Rosselle Street 

San Diego, CA 92121 

Dr. Barry Butler 

Solar Energy Research Institute 

16 17 Cole Blvd. 

Golden, CO 80401 

Dr. L. M. Murphy 

Electric Power Research Institute 

P.0, Box 1041 2 

Palo Alto, CA 94303 

Mr. John Bigger 
Page I SI:RI/STR-250-3516 1 ~389009426 

4 Title and Subtitle 

Phase-Change Thermal Energy Storage: Final 

Subcontract Report on the Symposium Held 

October 19-20, 1988, Helendale, California 

5. Publication Date 

November 1989 

8. Performing Organization Rept. No. 

9. Performing Organization Name and Address 

Luz International Ltd. 

924 Westwood Blvd. 

10. ProjectlTasWWork Unit No. 

ST91 7001 

11. Contract (C) or Grant (G) No. 

Los Angeles, CA 92024 I (c) Hll-9-18108-1 

12. Sponsoring Organrzation Name and Address f 13. f ype of Report & Period Covered 

Solar Energy Research Institute 

1617 Cole Blvd. 

I Technical Report 

Golden, CO 80401-3393 I 14. I 

15. Supplementary Notes 

SERI Technical Monitors: Russell Hewite and Bimleshwar Gupta 

16 Abstract (Limit: 200 words) This document reports on the symposium held October 19-20, 198! 

at Helendale, Calif., on phase-change thermal energy storage. Thermal energy 

storage is important to solar electric power plants, but currently there is no 

cost-effective reliable storage system that will work at 735OF' (397*C), the currenl 

operating temperature of the Luz International Ltd. Solar Electric Generating Systems (SEGS) . As a possible solution, Luz developed two storage designs using 

phase-change materials and asked symposium participants to advise on the feasibility of using these materials in SEGS-type facilities. 

Symposium participants concluded that phase-change materials would probably not 

meet the cost goals in the immediate future. However, two alternative storage 

options emerged: encapsulation of phase-change salts and chemical energy storage 

using hydroxide/oxide reactions. Participants recommended that the most promising 

options be evaluated and that schedules for applications by 1992 or beyond be 

established. These evaluations will provide the technical and economic bases for 

further efforts, including designing and *installing prototype field systems and 

detailed engineering design studies with supporting R&D activities. 

Document Control ( 1. SEA1 Report No. 1 2. NTlS Accession No. 1 3. Recipient's Accession No. 

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1 

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17 Document Analysis 

a. oescrlptors Phase-change materials; sensible heat storage; solar thermal power 

plants; energy storage systems; energy storage. 

b. Identif~ers!Open-Ended Terms 

c. UC Categories 

2 02 

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8. Avarlab~lity Statement 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, Virginia 22 161 

19. No. of Pages 

20. Price 

A08