Experimental and theoretical assessment of an active PCM storage concept

Transcription

1 Experimental and theoretical assessment of an active PCM storage concept W.D. Steinmann, H. Pointner German Aerospace Center (DLR) Institute of Engineering Thermodynamics

2 Development of Latent Heat Storage Storage concepts Nitrate salts as phase change material (PCM) liquid PCM Steam solid PCM Heat transfer coefficient is dominated by the thermal conductivity of the solid PCM (<1.0 W/mK) Low thermal conductivity is bottleneck for PCM NaNO 3 at room temperature

3 Heat transfer concept for Latent heat energy storage State of the art: Embedded Finned tube heat exchanger Simplified PCM-storage concept Phase Change Material (PCM) Tube Fins Heat carrier: water/steam Radial finned tubes

4 Heat transfer concept for Latent heat energy storage State of the art: Finned tube heat exchanger embedded into PCM volume Pilot-scale latent heat storage unit: NaNO 3 as the PCM, Embedded parallel tube heat exchanger with radial aluminum Fins T melt 305 C Latent heat 175 kj/kg 1.4m x 1m x 6m PCM Mass: 14 tons Capacity: ca. 700 kwh axial finned tubes Increase of capacity requires increased heat exchanger Power not constant for constant steam pressure Heat exchanger not accessible

5 Active PCM concept solid liquid Transport of storage material (solid + liquid phase) Mechanical separation of heat transfer surface and storage material Constant power possible Capacity and power independent

6 PCMflux Concept Basic principle Container liquid PCM solid PCM fin Steam intermediate fluid layer tube Separation of PCM from heat transfer surface by an intermediate fluid layer Conductive heat transport through intermediate fluid layer Transport of PCM in thin walled containers

7 PCMflux Concept Basic principle feed rate central heat exchanger feed rate distributed heat exchanger Typical feedrate: mm/h

8 PCMflux Concept Basic geometry options horizontal vertical tubular

9 Theoretical analysis Comparison of PCMflux to state of the art Wärmestromdichte in W/m2 Heat flux [W/m²] PCMflux stationary Zeit in s PCMflux provides constant power PCMflux requires less tubes for the same average power Time [s]

10 Theoretical analysis Dependence of power on feed rate transferred thermal power too slow operating range too fast w optimal feed rate

11 Thermal resistance of intermediate fluid layer Lab scale experiment TT PCM PCM H FF TT 1 TT 1 TT 1 TT 2 TT 2 TT 2 Ceramic spacers W b H b ΔTT Q Heat Exchanger Pipe Fluid

12 Thermal resistance of intermediate fluid layer Experimental results: Hitec as intermediate fluid Thermal resistance [W/K] Thermal resistance RR ttt = ΔTT FF QQ Direct contact: resistance without fluid = 10 resistance with fluid Thickness of layer [mm]

13 Lab-scale experiment with moving PCM Trough shaped horizontal fin with integrated tube liquid PCM Container fin Steam solid PCM intermediate fluid layer tube

14 Lab-scale experiment with moving PCM PCM Wire IR-camera Container Containers for PCM Guide System Fluid Fin Heat Exchanger Pipe Transport Mechanism Insulation

15 Lab-scale experiment with moving PCM Principle of measurement Isolierung Sight glass for IR-camera tube with fin solid Processed image liquid

16 Lab-scale experiment with moving PCM Position stationary phase front dependent on feed rate (discharging) Feed rate = 0.7 w optimal Feed rate = w optimal Feed rate = 1.3 w optimal solid liquid

17 Demonstration of 10 kw test rig Design of test rig IR-camera 500 mm intermediate fluid pan for intermediate fluid fin tube

18 Construction of 10 kw test storage 30 parallel tubes Eutectic NaNO 3 /KNO 3 as PCM Distance for movement 0.5 m 400 kg PCM

19 Conclusions and Outlook PCMflux offers varius advantages: - constant power - reduced heat exchanger (accessible) -no direct contact between PCM and pressure tubes - only slow movements over limited distances required - flexibility in power by adjustment of feed rate Status: - Fundamentals for design of PCM have been elaborated by experiments and numerical analysis - Lab-scale proof of concept - Construction of 10 kw test rig has been almost completed

20 Outlook Extension to PCM also in solid state Integration of heat transfer structures to increase the thickness of PCM layers Utilization of different PCMs The nextpcm project is funded by the German Federal Ministry for Economic Affairs and Energy