Energy storage development for solar thermal processes

Solar Energy Materials

Solar Energy Materials 24 (1991) 386-396 North-Holland

Energy storage d velopment for solar thermal processes Rainer T a m m e a,., Ulrich Taut b, Christian Streuber c and Horst Kalfa c a DLR - German Aerospace Research Establishment, Institut fi~r Technische Thermodynamik, Pfaffenwaldring 38-40, D-7000 Stuttgart 80, Germany h University of Stuttgart, Germany c Didier AG, Wiesbaden, German),

Modified concrete for existing solar farm plants and refractory ceramics for proposed air co~led solar tower plants have the best prospects for near-term realization of large scale solar thermal energy storage systems. In the near future, significant improvements and cost reduction are feasible by utilization of phase change materials. Composite salt/ceramic hybrid materials for high temperature applicatiom are under development in a joint research program. Manufacturing and testing in commercial scaie has already been started.

1. Introduction For the two major solar plant technologies - solar farm and solar tower different energy storage and conversion possibilities exist with respect to the time schedule for their realization (table 1). For the already existing solar farm systems operated with oil as the heat transfer fluid (HTF) and for the proposed air-cooled solar tower PHOEBUS sensible thermal energy storage (TES) systems, working as a buffer storage, are available. In addition, for the molten salt cooled solar tower, also a sensible storage system, the two-tank molten salt system, is available. For

Table 1 Energy storage technologies for solar power plants Near term - buffer storage

Solar farm (oil) Solar tower (air) Mid term - diurnal storage'.

Solar farm (oil/steam) Solar tower (air) Long term

Energy storage and transport

Sensibel TES

Modified concrete Regenerator ceramics PCM concepts

Cascaded salts Composite hybrid materials Solar chemical com'ersion

Chemical storage Solar chemicals Solar fuels

0165-~,533/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

R. Tamme et al. / Energy storage dez,elopmen;for solar thermal processes

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the next plant generation of the solar farm concept - producing directly steam and of the tower concept - operating at elevated temp,..ratures t,sing ceramic absorber materials - phase change materials (PCM) storage concepts are under development. The final goal is long-term storage and transport of solar energy and the way to realize that, may be by means of chemical storage using a reversible chemical reaction operated in a closed loop, may be by production of high-value chemicals or by producing transportable fuels. This paper deals with the first two items, the near- and mid-term oriented aspects of solar energy storage techniques.

2. TES for solar farm plants For near-term realization of a commercial buffer TES system for solar farm plants a feasibility study was performed in 1988 through 1989 by Flachglas Solartechnik, Luz Industries, Siempelkamp, C I E M A T and DLR [1]. The assumption was made that the design of the investigated TES system has to be adapted to the operation conditions of the reference plant SEGS VI. This means, an optimization of the complete plant design including the thermal storage was not aspired. The objective of the study was to identify a buffer stn~orageof 1 h discharge and 200 MWh storage capacity that could be h i l t within few years. Therefore, only existing techniqurs, available und useful for scale up, were considered. Advanced storage concepts, proposed or currently under investigation in the labs, were excluded. Remaining under the cost goal of 50 D M / k W h storage capacity, Luz would include such TES system in the SE(3S plants, to be built in the early 90's. The essential results are summarized in table 2. Included are the numbers for the reference storage system, an active two-tank sensible storage with synthetic oil

Table 2 Summary of TES f¢.asibility-slud~results TES concept

Storage material

Reference: active, sensible two tank storage

synth, hydrocarbon Monsanto VP-I

Storage volume (m3) 6500

Cost goal Sensible storage

(DM/kWhth) 120

50 7000 5200 5400

60 90 85

Sensible + PCM

Concrete Solid salt.. NaCI Liquid nitrates 3 salts KNO3, NaNO3, NaNO2 Concrete+ 3 sails

4100 6600

270 (180) ~

LUZ proposal cascaded PCM

5 salts

(2600) ~

Two tank, sensible Cascaded PCM

"~ Estimated.

(50) ~'~

388

R. Tamme et al. / Energy storage development for solar thermal processes

currently used as the HTF in the parabolic trough absorbers. Such storage system would consist of about 6500 m 3 oil leading to specific costs of i20 DM/kWh. The presented results of the analyzed systems indicate, that the cost goal could not be achieved. Best restiits were obtained for a concrete sensible TES system with 60 DM/kWh. This concept has some potential for further improvements by optimization of the design and by modification of the concrete material. Therefore it is expected to meet the cost criteria in the future. Luz made a proposal of a cascaded PCM storage, and estimated specific costs of about 50 D M / k W h assuming ideal use of the latent heat, But, for the analyzed system, based on proven dcsign and PCM materials, the specific costs were significant higher than expected. On the other hand, for the PCM concept, there exists a significant potential of reducing the costs by using a better design and materials with higher amount of latent heat than the alkali metal nitrates. Based on the study, further investigations have been started. In order to demonstrate feasibility of the concrete storage concept, and in order to validate the calculation model, two test modules of the concrete material, used in the study, were built by Siempelkamp and are currently investigated in a small test loop at the Plataforma Solar de Almeria, Spain. In addition, also the PCM storage development was continued with emphasis on material selection and design improvement.

3. TES for solar power plants

For the solar tower technology, fig. 1 may introduce the basic concept of connecting TES and fossil back-up firing. Such combination allows prolongation of operation times and guarantees a defined output. For electricity production, it allows shifting of energy output from low price periods to peak periods, and for production of fuels and chemicals, continuous operation of suitable endothermal chemical processes is also feasible [2,3]. For gas-cooled solar tower plants, the proven technology of the existing industrial high-temperature TES systems, commonly known as regenerators, recuperators and cowpers, can be considered as the straightforward solution. But between industrial and solar applications there is a significant difference regarding the utilization factor of the storage system. While industrial TES systems are operated continuously with typically 24 charge/discharge cycles per day, for a solar TES system only one complete charge/discharge cycle per day is feasible. This is the reason for the low value of about 25 DM/kWh for the specific costs of industrial high-temperature TES systems in comparison to the 190 D M / k W h for the proposed PHOEBUS storage system. The goal is to reduce the costs and, basically, there are two ways to do this: 1. variation of the storage design including the size and shape of the storage materials and 2. utilization of storage materials with higher storage capacities than the available refractory materials but with comparable materia| costs.

R. Tamme et al. / Energy storage del'elopment for solar thermal processes

389

4. TES system improvements Examples of sensible ceramic materials developed for solar applications are shown in fig. 2. Fig. 2a represents a stacked arrangement of checker bricks, but with a higher specific heating surface than the available materials. Exactly this material was used for the calculations of the PHOEBUS storage design. Fig. 2b shows MgO honeycomb ceramics. At present, they are not a commercial product. but they indicate the direction, where we like to go for further improvements. An additional potential in reducing the size and the costs of the TES system can be derived from higher storage charging temperatures. The influence of increasing the inlet temperature is calculated for the above mentioned PHOEBUS plant [4]. Fig. 3 shows the flow sheet for the reported design. Important is the fact that during the three hours of discharging the storage outlet temperature decreases from 700 to 640 o C, causing a reduction of the steam generator output to an Thermal Storage

Radiation

to Receiver Solar

Burner

i : I)

i !

~_~Burner Fossil

[.......] Turbine ! ~]=~Generator I

I

!----~-'-'-i... ~" Products

....F-Feed Thermal

Radiation

to Receiver [__r~> Solar Burner

~

Storage

[~~~'Burner

.".......l Turbine ~ [~]=~Generator i

Fossil

F-"I... b

........

Fig. 1. Hybrid solar plant concept. (a) Solar operation and TES charging. (b) Hybrid mode, discharging and fossil burner operation.

TES

390

R. Tamme et al. / Energy storage de~,elopment for solar thermal processes

Fig. 2. (a)' Stacked arrangement of checker brick~. (b) Stacked arrangement of honeycomb ceramics,

intermediate load level. An increase of the receiver output temperature to 900 ~ C, using a ceramic receiver, leads to higher outlet temperature for the discharge mode. That modification was calculated for two cases. The first case - modification A - represents the case with the same temperature difference of 60 degree at the storage outlet, as we already noticed for the basic design (fig. 4). A mixture of hot air of the storage outlet with cold air of the steam generator outlet constantly provides the steam generator with the necessary mass flow of 151 k g / s air of 700 ° C. Modification B (fig. 5) presents discharge operation from 900 to 700 ° C.

Table 3 Summary of modified operation conditions

Receiver outlet temp. Storage discharging from to Storage material mass Discharge time Storage capacity Steam generator inlet from to

Base case

Modification

Units

a

b

700 700 640 7400 3 250 700 640

900 900 840 5000 3 250 700 700

900 900 700 4000 3 250 700 700

°C °C oC t h MWh °C °C

R. Tamme et al. / Energy storage development for solar thermal processes

391

RECEIVER

700"C

151 k g / s

700°C --> 151 k g / s

L TN 700*CI

640"C

1

IOUT 7 0 0 " C TO 640*C

I

STEAM STORAGE GENERATO]

i

BASE BASE

CASE:

I

1850C

CASE

BASE CASE AND . . . . >| < . . . . M O D I F . A A N D B

T E M P E R A T U R E F R O M R E C E I V E R 700 *C TEMP. FROM STORAGE WHEN DISCHARGING

700

-->

640

*C

Fig. 3. PHOEBUS TES subsystem, base case.

What does this mean in terms of capacity or storage material reduction? Table 3 summarizes the results for the previous discussed examples. The base case gives the numbers of the proposed PHOEBUS TES system. For a storage capacity of 250 MWh and 3 hours discharge time the necessary amount of storage material was calculated to be 7400 tons. For the modified design a reduction of the material mass can be observed to 5600 (modification A) and 4000 tons (modification B). Assuming that the costs for the ceramic receiver would be in the same order than the metallic receiver, the costs of~the TES system could be reduced to about 60% of original value.

5. Advanced storage materials

The second possibility of storage system improvements exists by utilization of materials with higher storage capacities. The use of phase change materials (PCM) provides a number of desirable features, e.g. high volumetric storage capacities and heat availability at rather constant temperatures. However, the growth of the solidified phase on the solid surface of conventional heat exchangers during discharge seriously affects the internal heat exchange between the PCM and the heat transfer fluid.

R. Tamme et aL / Energy storage decelopment for solar thermal processes

392

l l 700"C l 151 kg/s l l l l l STEAM

RECEIVER ..... > -----> -----> -----> ..... > -----> ----->

900"C

IN

107

900°C

I

OUT TO

kg/s

900°C 840°C

BYPASS 185"C | 44 kg/s

STORAGE

GENERATOR

l l l 1850C

MODIFICATION

MODIF.

A:

TEMPERATURE TEMP. FROM

A

. . . . >| < . . . .

FROM RECEIVER 9 0 0 *C STORAGE WHEN DISCHARGING

900

BASE CASE AND MODIF. A AND B

-->

840

*C

Fig. 4. PHOEBUS TESsubsystem, modification A.

The advanced "composite salt/ceramic TES media concept" has the potential of using latent heat via direct contact heat exchange and, therefore, the potential of significant cost improvement through reduction of storage materials and containment vessel size. This salt/ceramic approach may be explained as microencapsulation of a PCM within the submicron pores of a ceramic matrix. The liquid salt is retained within the solid ceramic network by surface tension and capillary forces. Heat storage occurs as latent heat of the PCM and as sensible heat of the basic ceramic material and the PCM. Therefore, the use of salt/ceramic materials represents not a pure latent heat but a latent/sensible hybrid storage concept. Salt/ceramic development at DLR was started in 1986 with laboratory scale material researck In 1988 Didier AG joined the project, responsible for manufacturing the hybrid material in technical scale and investigating low-cost processing procedures. The presently used preparation method of lab scale materials is cold pressing of the raw mixture. Essentially, it consists of the basic ceramic, the salt and further additives, mainly chemical binders and water. Usually, we prepare cylindrical pellets with diameters varying between few millimeters for thermoanalyticat investigations and 26 or 40 millimeters for testing thermal .zx:d mechanical stability. Main influence on the stability of the composite materials "s caused by the

R. Tamme et al. / Energy storage development for solar thermal processes

_________i!1---~ RECEIVER

9oo*c

I 1 7000C

l o ~ . . . . > 1~1 k q / ~ I z s z

IN 9 0 0 * C

393

kg/s

I

I

OUT'900*C TO 700*C

I

! I iBYPASS I

STORAGE

185"C 44

-o

STEAM

| GENERATOR

kg/sl

, 185"C

II

|

[ MODIFICATION MODIF.

B:

B .... >|< ....

TEMPERATURE F R O M R E C E I V E R 9 0 0 *C TEMP. FROM STORAGE WHEN DISCHARGING

900

BASE CASE AND MODIF. A AND B

-->

700

*C

Fig. 5. PHOEBUS TESsubsystem, modification B.

particle size of the main components, the particle size distribution of the ceramic, type and quantity of the binders, and by temper- and burning processes [5]. Detailed investigations have been conducted with the salt/ceramic systems Na-BaCO3/MgO and Na2SO4/SiO2 . They included essentially chemical, thermal and mechanical, and X-ray investigations and single pellet testz :,n laboratory

Table 4 Thermophysical properties of lab-scale hybrid materials

PCM content (wt%) Density ( g / c m 3)

Fusion temperature T r ( ° C) Specific heat (600 ° C) (J/gK) (800 ° C) (J/gK) (990 ° C) ( J / g K ) Latent heat ( J / g ) Storage capacity ( J / g ) (AT = 200 K) Compressive strength ( N / m m 2) (100 K > T F)

Na 2504/SiO 2

Na-BaCO 3 / M g O

50 2.0 880 1.15 1.17 1.23 80

45 2.7 700 1.19 1.35 82

338

336

> 0,2

0.11

R. Tamme et al. / Energy storage development for solar thermal processes

394 750.00

690.00

630.00

s7o.oo

o 510.00

i ..'"

~ 450.00

IF

aso.eo

330.00

270.00

r • 210,00 ~

0.00

,

----

atr

~

TES

.'

temperELture material

temp.

"

2.00

4.00

6.00

8.00

10.00

HEISHT

~2.00

14.00

16.00

1B.O0

20.00

H

Fig. 6. Time-dependent temperature profile for PHOEBUS (base case) sensible TES system.

furnaces under ambient atmosphere. For both systems, the results have demonstrated thermal and chemical stability of the composite materials, and proven that the presently realized compressive strength above the melting temperature of the PCM is sufficient to build up a stacked arrangement of about 3 m for the magnesia composite and 5 m for the silica composite. Their properties are summarized in table 4. With the sodium sulfate/silica composite material, Didier has already started manufacturing in technical scale. For designing a TES system consisting of composite materials, a specific simulation code considering the PCM effects has been developed. To demonstrate the influence of such salt/ceramic composites, we selected the PHOEBUS TES system design again. Fig. 6 presents the time-dependent temperature curves of a pure sensible storage material for charging and discharging. The dotted lines represent the air temperatures. The results from this calculations completely agree with Didier results, conducted with their sensible ceramic material storage design model. Therefore, we assume, that the results for the composite materials will be in the correct magnitude. Fig. 7 presents the results for the same charging/ discharging conditior~s, but assuming utilization of a composite salt/ceramic

R. Tamme et al. / Energy storage decelopment for solar thermal processes

395

750.00

690.00

630.00

570.00

o 510.00

W

45O. O0

ago.oo

330.00

air temlaerature

- - -

270.00

TES material teml3. 210.00

150.00 L _ . 0.00

2.00

4.00

6.00

8.00

10.00 HEZGHT

12.00

14.00

16.00

18.00

20.00

N

Fig. 7. Time-dependent temperature profile for PHOEBUS composite hybrid material TES system.

material with a melting temperature of 5 0 0 ° C . You can see the significant influence of the PCM melting and crystallization process on the temperature profile of the storage° On the other hand, from the results you can derive, that only in a certain part of the storage, latent heat is stored and released. That means, for applications with high t e m p e r a t u r e differences between inlet and outlet, we need several composite materials with different melting temperatures. The calculated improvement of the P H O E B U S TES system by using such latent h e a t / s e n s i b l e heat hybrid materials is presented in table 5. With only one hybrid material, the amount of materials can be reduced from 7400 tons to about 5900

Table 5 TES improvement by use of hybrid materials Sensible TES, ceramic Hybrid TES, 1 comp. Hybrid TES, 3 comp.

Mass (ton) 740t~ 5900 4700

Vhybr./V~¢,.~" 0.80 0.64

396

R. Tamme et al. / Energy storage decelopment for solar thermal processes

tons, and with three hybrid materials the material mass can be further reduced to about 4700 tons.

6. Conclusions - For the existing solar farm and the proposed solar tower plants, sensible TES systems are available, - significant improvements of TES systems are feasible by optimization of the design and by utilization of advanced TES materials, and - manufacturing and testing of composite materials has already been started.

References [1] F. Dinter, M. Geyer, R. Tamme (Ed.): Thermal En,'rgy Storage for Commercial P cations, Springer, Berlin (1990), ISBN 3-540-530054-1. [2] M. Geyer, H. Klaiss, R. Tamme, "Entwicklung zu h6heren Verfiigbarkeiten - Speicher, fossile Zusatzfeuerung und hybride Kraftwerke", VDI-Ber. 704 (1988) 73-89. [3] R. Tamme, M. Hermann, U. Grfizinger, A. Gliick, H. Kanwischer, "High-Temp:~'ature Heat Storage for Solar Thermal Electricity Generation and Solar Fuels and Chemicals", Proc. (1989) Congr. of ISES, Kobe, Japan, September 1988. [4l PHOEBUS, Phase lb - Feasibility Study (03. 1990). [5] R. Tamme, A. Gliick, U. Griizinger, H. Kaiiwischer, "HTWS - Verbundprojekt Hochtemperaturwiirmespeicher", BMFI" Statusbericht 1989 Thermische Energiespeicherung.