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Phase - Change Storage Systems

Rainer Tamme DLR - German Aerospace Center Institute of Technical Thermodynamics

· Introduction · Previous activities and experiences · Obstacles and solutions · EG / PCM Approach

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

1

Phase - Change Storage Systems

Definition

enthalpy

Thermal energy storage during phase change, preferably by melting process High storage capacity at small temperature variations

temperature

Latent heat storage

Example

Most activities directed to low temperature applications cold (ice) storage space heating heat protection temperature stabilizing

Quellenangabe

Ice Whisky

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

2

Phase - Change Storage Systems

PCM Storage for Parabolic Trough Plants

Heat Transfer Fluid - HTF

Temperature range in °C Operation pressure in bar

Synthetic Oil

250 - 400 10 - 20

Water / Steam

250 - 450 50 - 150

HTF Oil is cooled ~ 100 K HTF Steam is cooled ~ 150 K Single PCM cannot meet this requirements

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

3

Phase - Change Storage Systems

Selected Phase Change Materials - PCM

PCM NaNO3 NaNO2 NaOH KNO3 KOH NaOH/ Na2CO3 (7,2%) NaCl(26,8%)/NaOH NaCl/KCL(32,4%)/LiCl(32,8%) NaCl(5,7%)/ NaNO3(85,5%)/Na2SO4 NaCl/ NaNO3(5.0%) NaCl(5,0%)/ NaNO3 NaCl(42,5%)/KCl(20,5)/MgCl2 KNO3(10%)/NaNO3 KNO3/KCl(4,5%)

Quellenangabe

T Melting°C 310 282 318 337 360 283 370 346 287 284 282 385-393 290 320 342

HU [kJ/kg] 174 212 158 (316) 116 167 340 370 281 176 171 212 410 170 150 140

4

KNO3/KBr(4,7%)/KCl(7,3%)

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

Phase - Change Storage Systems

LUZ proposal for Solar Plant with Cascaded PCM Storage (1990)

HTF temperature end of charging

HTF temperature end of discharging

0

related distance from cold end

1

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

5

Phase - Change Storage Systems

DLR - ZSW proposal for Solar Plant with Hybrid - PCM/Sensible/PCM - Storage (1993)

Thermal Oil Cycle Thermal Storage System

Power Block

G

Quellenangabe

Solar Field

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

6

Phase - Change Storage Systems

NaNO3 (Tmelt=310°C) NaOH,NaCl (Tmelt=370°C) 10 m

Temperature distribution

200 MWht 3 h charging/1 h discharge Synthetic Oil as HTF Sensible Storage Concrete HTF

10 m

concrete as sensible storage medium

390

end charging TOil-hybrid TConcrete-hybrid

370

TConcrete-sensible

Temperature [°C]

350

TConcrete-sensible TOil-sensible

TOil-sensible

Hybrid Storage PCM/Concrete/PCM HTF

330

TConcrete-hybrid

310

TOil-hybrid

end discharging

290 0

Quellenangabe

50

100 Length [m]

150

200

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

7

Phase - Change Storage Systems

Resulting Material Consumption for Both Systems

200 MWht - 3 h charging/1 h discharge - Synthetic Oil as HTF

Sensible Concrete

Num ber of parallel tubes Storage volum e Storage m aterial m ass Tube m ass HTF m ass 24370 5574 m ³ 13544 to 1314 to 358 to

Hybrid NaNO 3 -Concrete-NaOH/NaCl

16260 Sensible 3347 8133 to PCM 372 745 to Total 3719 8878 877 239

?

what is the performance ? what are the cost ?

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

8

Phase - Change Storage Systems

PCM Storage Modules tested at DLR and ZSW 1)

Test Module

double tube internal tube diameter: 25mm / 18mm / 12mm double tube internal tube diameter: 25mm double tube multiple tube array 7 tubes; 18mm diameter buckled plate array 5 plates 505 x 1800 mm

1)

Quellenangabe

PCM Material

Melting Salt Mass Temperature [°C ] [ kg ]

305 265 - 293 142 305 305 60 60 60 540 850

NaNO 3 90% NaNO 3/10% KNO 3 40% NaNO 2 NaNO 3 NaNO 3

internal tube diameter: 25mm 7% NaNO 3/53%NaNO 3

Center for Solar Energy and Hydrogen Research, Stuttgart

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

Institut für Technische Thermodynamik

9

Phase - Change Storage Systems

Examples of test Modules at ZSW Lab 1990 - 1995

Double Tube Segment Segment of Buckled Plate Tubular System

Institut für Technische Thermodynamik

Quellenangabe

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

10

Phase - Change Storage Systems

Heat flux densities during charging of 90% NaNO3/10% KNO3 from 250 to 350 °C

In vertical tubes heat transfer during melting is supported by natural convection of the liquid phase

Quellenangabe

Double Tube; 25 mm tube diameter; HTF velocity 0.6 to 2.0 m/s

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

11

Phase - Change Storage Systems

Heat flux densities during discharging of 90% NaNO3/10% KNO3 from 350 to 250 °C

Heat transfer during discharging is limited by the formation of the solid phase with low thermal conductivity

Quellenangabe

Double Tube; 25 mm tube diameter; HTF velocity 0.6 to 2.0 m/s

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

12

Phase - Change Storage Systems

Lessons Learned

Straight forward design will not lead to an efficient and economic storage technology PCM has not is an intrinsic value Storage concept and material is caused only by economic aspects which means specific cost, reliability and scalable design PCM storage can only be applied commercially if heat transfer limitations can be solved

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

13

Phase - Change Storage Systems

Approaches to overcome the heat transfer limitations of PCM´s directed to:

Direct contact heat exchange - DCHX

No solid heat exchanger wall Composite with high matrix Small PCM dimensions high specific surface

Micro encapsulation of the PCM

Protection of heat exchanger surface against formation of solid PCM Influencing the melting/solidification process of the PCM

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

14

Phase - Change Storage Systems

Direct contact heat exchange - DCHX

1980-1990 DLR development of Oil /Salt-Hydrate system

Schematic of DCHX PCM storage; License to A. Schneider, Lahr

Extension to high temperature ? Will lead to pressurized storage vessels for oil HTF trough plants

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

15

Phase - Change Storage Systems

Encapsulation Approach

Conventional Approach Micro Encapsulation

Expanded Graphite - PCM composite material

Macro Encapsulation

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

16

Phase - Change Storage Systems

Formation of Expanded Graphite Matrix

2-dimensional graphite network determines thermal conductivity of the any composite material

Quellenangabe

Source: SGL Carbon AG

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

17

Phase - Change Storage Systems

Production of Expanded Graphite matrix

Source: SGL Carbon AG

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

18

Phase - Change Storage Systems

EG /PCM Processing

Quellenangabe

Source: SGL Carbon AG

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

19

Phase - Change Storage Systems

Anisotropic Thermal Conductivity of EG

EG / Paraffin Composite 15 % EG 80% Paraffin = 20 W/mK (ab-direction)

Source: SGL Carbon AG

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

20

Phase - Change Storage Systems Conclusions

PCM storage has achieved no or only minor technically application due non sufficient heat transfer PCM encapsulation - micro or macro - offers attractive solution to solve heat transfer limitations Expanded Graphite / PCM composite materials realized with Paraffin for low temperature applications Transfer to high temperature with salt PCM needs further development

Quellenangabe

Institut für Technische Thermodynamik

Workshop on Thermal Storage for Trough Power Systems February 20-21, 2003, Golden CO, USA

21

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