1. E2C 2013 October 27-30, 2013 Budapest, Hungary Experimental Study of Phase Change Materials for Thermal Storage in the Temperature Range of 300-400 o C R. Adinberg, D. Zvegilsky, M. Epstein

2. Discussion Topics  Thermal Energy Storage (TES) for Concentrating Solar Power (CSP) plants  Selection of Phase Change Materials (PCM) for Solar TES  Thermal analysis of PCM and Main results  Reflux Heat Transfer Storage (RHTS) experimental system  TES prototype design  Conclusions 2

3. Q1Q1 Power Block Q L CSP System Q 0 ~ Q3Q3 Q 2 QFQF Electricity Grid FOSSIL FUEL Power Control System: Q 0 …....Q L (HTF flow), E S QRQR SOLAR POWER Solar-Fossil Hybrid Power System Thermal Storage Operation  Buffering during transient weather conditions  Dispatchability / time-shifting into peak/night hours  Achieve full load operation of the steam turbine cycle  Increase of annual capacity factor 20 to 40/50% 3

4. State of the art Thermal Storage Technology 2-tank Molten Salt System Tank Size 14 m height 38 m diameter Amount28,500 tons Capacity1010 MWh Molten Salt (K-Na)NO 3 Temperature 250-400 o C Andasol, Spain Solar Power Plant 50 MWe Temperature 300-400 o C (Parabolic Trough CSP) MethodEffectPhaseMaterials Sensible Heat CP ΔTCP ΔT Liquid Thermal oil Molten salt Pressured water Latent Heat Δ H T @ T melt Solid/LiquidPCM: Molten Salts Metal Alloys Thermo- Chemical Δ H R (Endo-Exo thermal reaction) Solid Gas CaO/H 2 O NH 3 → N 2 + H 2 4

5. Reflux Heat Transfer Storage -RHTS Design Specifics Isothermal Process:T steam  PCM m.p. Overall Thermal Conductivity:PCM-to-Steam  Thermosyphon Energy density by volume: 2-5 times of 2-tank MS system Pressure-tight tank:5-10 bar Challenge : PCM-HTF Compatibility 1PCM Selected Materials: Zn Alloys, Molten Salts 2 HTF Diphyl Diphenyl Oxide + Biphenyl (Dowtherm A; Therminol VP- 1) 3 Temp. < 400 o C Key Features PCM-HTF Direct Contact HTF Vapor Natural Convection & Liquid Reflux Heat Exchanger External to PCM To Steam Generator PCM HTF Boiling 5

6. PCM-HTF Thermo-Chemically Stable Systems HTFPCMMelting Temp. Density (solid) Latent Heat Max Test Temperature Diphyl Zn70Sn30 370 o C 7. g/ cm 3 107 J/g 400 o C Zn95Al53847.101400 KNO 3 3342.110360 (K/NA/Li) 2 CO 3 392/72.5280400 Thermal Analysis Applied TGA/DTADSCFTIR Q600 TA InstrumentsDSC 4000 PerkinElmerTensor 27 Bruker Optics Calorimetric results for the 3C-Diphyl chemical system including a few sequential heating-cooling cycles between 350 to 420 o C Time 6

7. RHTS Lab-scale Experimental System Thermocouples K, T Sample Pan Sample Vessel Stirrer Water-filled Condenser (4 L) Insulation HTF: Diphyl 0.25 L 3C-PCM 0.9 kg Metering Valve Pressure Sensor HTF Flow Electric Furnace 1 2 Cross section of the sample pan containing 3C PCM after the long-term test Multiple Cycles 7

8. RHTS Prototype Design Calculated parameters of a thermal storage tank for a 50MWe solar power plant PCM KNO 3 3C Amount PCM, ton 17,000 14,000 Amount HTF, ton ca. 2,000ca. 1,000 Height, m 20 Diameter, m 3224 Thermal capacity, MW th h 5001000 Thermal Storage for 7 operational hours (solar capacity factor 0.4) Number of tanks21 8

9. Experimental Study of Phase Change Materials for Thermal Storage in the Temperature Range of 300-400oC SUMMARY  RHTS is a synergistic process that integrates the PCM solid–liquid and HTF liquid–vapor reversible phase change effects for respectively storing and transferring heat in the thermal storage system.  The experimental results of this work show that the (K/Na/Li) 2 CO3 - Diphyl storage medium is thermally and chemically stable and suitable for RHTS up to 400 o C.  The experimental system has been demonstrated quite effective in heat transfer for improving the performance of thermal storage systems.  A feasibility study is currently being done on integration of 3C-Diphyl RHTS module into a 50 MW solar thermal electric power plant.