Concentrated Solar Power in Spain: thermal energy storage systems
Selvan Bellan Centre for transdisciplinary research
1 Outline
´ Evolution of Concentrating Solar Power technology ´ CSP in Spain ´ Thermal energy storage systems ´ Recent developments in latent thermal energy storage systems ´ Numerical Modeling of thermal energy storage system ´ Summary
2 Renewable Energy: Solution for global problems
Population Fossil increase Fuels Global warming Renewable Energy Energy Water CO2 demand shortage growth Solar Energy CSP PV Climate Crisis change Cost Increase
Disaster at nuclear power plant Concentrated solar Power
3 Installed solar thermal power plants since the 1980s
´ Early 1900s, interest in solar power was lost due to advances in internal combustion engines and availability of low cost fossil fuel ´ The first commercial plants had operated in California (USA) over the period of 1984–1991
Source: International Energy Agency(IEA) and www.cspworld.com. 4 As of March 2015….
´ CSP market has a total capacity of 5840 MWe worldwide ´ 4800 MWe is operational and 1040 MWe is under construction. ´ Spain had a total operational capacity of 2405 MW and 100 MW is under construction ´ USA having a total capacity of 1795 MW.
Solar thermal power plants in the planning
´ More than 10.135 GW; announced mainly by the USA and Spain ´ The Palen project includes two 250 MW adjacent power plants similar to Ivanpah technology is expected to be operational by the end of 2016 ´ Likewise, BrightSource is developing another two 500 MW projects named Rio Mesa and Hidden Hills. These two projects are still in the certification process. ´ Saudi Arabia has recently announced; a target of 25 GW in over the next 20 years. ´ Interest has grown in the Sun Belt countries such as Algeria, Morocco, India, Chile, South Africa, Australia, China and a few Middle East countries.
Source: Renewable and Sustainable Energy Reviews 23 (2013) 12–39. 5 6 Primary energy resources in Spain
Hydro 2.8% Renewabl e, 12 Wind 2.8% Oil, 44.5 Gas, 22.1 Biomass and Biogas 4% Nuclear, Coal, 9.9 11.5 Biofuels 1.2%
Solar Renewables 1.3% Renewable Gas Coal Nuclear Oil
Source: MITyC,Ministry of Industry,Tourism and Trade, Espana 2012 7 Electricity generation in Spain Biomass and Biogas 1.8 Renewables Solar Thermoelectri 1.3 Solar PV 2.9 Wind 18.1 Hydro 7.7 22% 32% 0 5 10 15 20 Nuclear Gas Energy production from renewable sources (ktoe) Coal Renewables
27% 19%
Source: MITyC,Ministry of Industry, Tourism and Trade, Espana 2012 8 Solar energy in Spain
´ Reports indicate that 71% of the feasible territory in Spain receives an annual Direct Normal Irradiance (DNI)1 between 1730 and 2310 kWh/m2 ´ Annual average global irradiation of 1640 kWh/m2 ´ Abundant solar resources makes Spain as one of the main solar energy markets in the world
Source: Sol. Energy 81 (2007) 1295e1305. 9 Factors boosting CSP technology
´ Numerous supports in various forms of incentives ´ Incentives in the form of feed-in-tariff, tax relief, capital cost grants encouraging electricity export rates for CSP-plants. ´ Support from National and international organizations (Banks, Agencies) ´ Pilot and demonstration level projects PS10, PS20 and SOLAR TRES have provided valuable information for the development of the CSP technology. ´ Up to 2030, the market potential is estimated at least at 7 GW in the EU-MENA. This offers the
opportunity to CO2 reduction of up to 12 million tons per year. ´ According to ECOSTAR report, about 50% of the intended reductions in costs of CSP-plants will be from technology developments, and the other half from scale up and volume production ´ Solar thermal power plants will be capable of delivering efficiently more than 3% of the EU’s electricity by 2020, and at least 10% by 2030
10 The cumulative capacity of CSP
Source: Renewable and Sustainable Energy Reviews 50 (2015) 1052–1068 11 Solar Thermal Projects ´ Andasol-1(AS-1) ´ Helios I(Helios I) ´ Puerto Errado 2 Thermosolar Power Plant(PE2) ´ Andasol-2(AS-2) ´ Helios II(Helios II) ´ Andasol-3(AS-3) ´ Solaben 1 ´ Ibersol Ciudad Real ´ Arcosol 50(Valle 1) (Puertollano) ´ Solaben 2 ´ Arenales ´ La AfricanaLa DehesaLa ´ Solaben 3 ´ Aste 1A FloridaLa Risca(Alvarado I) ´ Solaben 6 ´ Aste 1B ´ Lebrija 1(LE-1) ´ Solacor 1 ´ Astexol II ´ Majadas IManchasol-1(MS-1) ´ Solacor 2 ´ Borges Termosolar ´ Manchasol-2(MS-2) ´ Casablanca ´ Solnova 1 ´ Morón ´ Enerstar(Villena) ´ Solnova 3 ´ Olivenza 1 ´ Extresol-1(EX-1) ´ Solnova 4 ´ ´ OrellanaPalma del Río I Extresol-2(EX-2) ´ Termesol 50(Valle 2) ´ Extresol-3(EX-3) ´ Palma del Río II ´ Termosol 1 ´ Gemasolar Thermosolar ´ Planta Solar 10(PS10) Plant(Gemasolar) ´ Termosol 2 ´ Planta Solar 20(PS20) ´ Guzmán ´ Helioenergy 1 ´ Puerto Errado 1 Thermosolar Power Plant(PE1) ´ Helioenergy 2
12 Solar Thermal Projects
Source: Sol. Energy 81 (2007) 1295-1305. 13 Comparison of the four CSP Technologies
CSP Technology Typical Plant peak Relative rise of Outlook for capacity (MW) efficiency (%) efficiency after improvements improvements (%) Parabolic 10–300 14–20 20 Limited trough (commercially proven) SPT- Central 10–200 23–35 40–65 Very significant receiver (commercial) Linear Fresnel 10–200 18 25 Significant (pilot project)
Dish Stirling 0.01–0.025 30 25 Via mass (demonstration production stage)
Source; Solar Energy 2011;85:2443–60. 14 Comparison of PTC and SPT
´ The capacity factor is the ratio of the actual output over a year and its potential output if the plant had been operated at full nameplate capacity ´ A lower cost in SPT technology is mainly due to a lower thermal energy storage costs ´ SPT plants, the whole piping system is concentrated in the central area of the plant; reduces energy losses
Source: Renewable and Sustainable Energy Reviews 22 (2013) 466–481 15 SPT- Central receiver system
´ Cost reductions associated with technology innovations of the heliostat, the receiver and the power block ´ Provides cheaper electricity than trough and dish systems ´ Provides better performance than trough system ´ Higher temperatures (up to 1000 C) and thus higher efficiency of the power conversion ´ Easily integrated with fossil plants for hybrid operation in a wide variety of options ´ It has the potential for generating electricity with high annual capacity factors (from 0.40 to 0.80 ) through the use of thermal storage ´ It has great potential for costs reduction and efficiency improvements (40–65%)
16 Central receiver solar thermal plants Demonstration solar power towers
Project Capacity, HTF year MW PSA SSPS-CRS 0.5 Liquid 1981 sodium
PSA CESA-1 1 Steam 1983 v Performance of the tower power TSA Air 1 1993 v Feasibility and the economical potential Pressurized Solgate 0.3 2002 v Components air v Hybrid concepts Eureka 2 Superheated 2009 steam v Heat transfer fluids and v Storage system
17 Central receiver solar thermal plants Commercial solar power towers Project Capacity Solar field Storage Heat transfer Receiver Type year MW area capacity fluid & Tout h m2 Planta solar 10 11.0 75,000 1 water Cavity 2005 250-300 C Planta solar 20 20.0 150,000 1 water Cavity 2006 250-350 Gemasolar 19.9 304,750 15 Molten salt 565 C 2011
18 Recent R&D activities in central receiver technology Cost reduction ´ Scaling up and mass production can contribute to about 50% in LEC reduction ´ The other half in LEC reduction is the result of R&D efforts ´ ECOSTAR study pointed out that the lowest LEC for large scale CSP-plants would be for solar tower concept with pressurized air and molten salt ´ R&D efforts have been growing sharply in many countries; performance improvements of the three major components can achieve very significant costs reduction
CTAER (Advanced Technology Center for Renewable Energy).
´ The Variable geometry central receiver solar test facility has been launched in Almeria; ´These helio-mobiles are placed over a mobile platform. ´The receiver is housed in a rotating platform
19 Thermal energy storage system
The importance of energy storage:
´ Facilitating the integration of renewable energy. ´ Mitigating the mismatch between energy supply and energy demand (dispatchability). ´ Shifting the generation period from peak hours of solar insolation to peak hours of power demand ´ It makes concentrating solar power (CSP) dispatchable and unique among all other renewable energy
Research efforts
´ The European DISTOR project: latent heat storage systems ´ The SunShot Initiative: Levelized cost of CSP-generated electricity to less than USD$0.06/kW h by 2020 with the cost of thermal storage less than USD$15/kWh and the exergetic efficiency greater than 95% . ´ The Australian Solar Thermal Research Initiative (ASTRI) ; the goal is to lower the cost of solar thermal power to AUD$0.12/kW h by 2020.
20 Thermal energy storage
Sensible Latent Thermo- heat heat Chemical
Solid- vNominal temperature Solid liquid vSpecific enthalpy drop vOperational strategy Liquid Solid-gas vIntegration into the power plant
21 Sensible heat storage Commercially deployed storage material Material Melting point ( C) Max. Operating Temp. ( Cost, (USD $/kg) C) Solar Salt 220 585 0.49 (NaNO3–KNO3(60–40)
Hitec 142 450-538 0.93 (NaNO3–KNO3–NaNO2 (7–53– 40)
Hitec XL 120 480–505 1.43 (NaNO3–KNO3–Ca(NO3)2 (7– 45–48)
Therminol - 400 3.96
Feasibility, cost and performance of a parabolic trough plant with 6 h of storage; relative to an oil plant with Therminol ´ The molten salt plant can reduce the storage cost by up to 43.2% ´ Solar field cost by up to 14.8% and LCOE by 9.8–14.5% ´ Higher solar field outlet temperature, which will enable a higher Rankine power block efficiency and a lower cost energy storage system
22 CSP capacity with/without storage system
Source: National Renewable Energy Laboratory (NREL) 23 Annual solar-to-electricity efficiency
EASAC policy report. European Academies Science Advisoty Council; 16 November 2011. 24 Two-tank sensible storage system
´ Most commonly used storage technique in utility-scale CSP plants. ´ Some parabolic trough and most of the tower plants, which employ molten salt as the HTF, use the direct storage approach • Direct storage system: Same HTF • Indirect storage system: Storage-molten salt; HTF-Oil (Andasol 1 Plant) ´ Direct storage removes the need for a heat exchanger and hence reduces the cost and increases the overall efficiency Single tank thermocline storage
´ Eliminates one tank, enables a potential cost reduction of 35% compared to the two-tank storage ´ This concept has been patented and the first plant employing this technique will be implemented in Spain ´ Research has been conducted to use low-cost filler materials
25 Single tank thermocline storage Schematic of the packed bed storage Thermocline/packed-bed heat storage: ØHeat storage in a static packing of solid pebbles (packed bed) ØInterstitial fluid flow through the solid bed: ØFiller (solid) heat storage material: pebbles ØHeat transfer fluid (liquid or gas): Air ØSingle tank concept. ØStorage based on temperature stratification. ØTo maintain an appropriate temperature stratification profile: ØCharge: Hot fluid supply from the top of the tank ØDischarge: Cold fluid supply from the bottom of the tank
26 Latent heat storage system ´ Latent TES has attracted considerable attention for CSP applications ´ The storage capacity is governed by both the specific heat and the phase change enthalpy ´ Inorganic salts/salt eutectics and metals/metal alloys are the potential PCMs. ´ Salts have been the most studied PCMs to reduce the cost of thermal storage. ´ Investigations are limited to numerical modelling due to the difficulties and high cost associated with high temperature experiments Thermo-chemical storage ´ Offers high energy density and negligible heat loss ´ Potentially offers a long-term storage option with relatively small storage volume. ´ Thermochemical storage is still at a very early stage of development
Source: Renew Sustain Energy Rev 2014;32:591–610. 27 Recent R&D activities on Latent thermal energy storage
´ Exhibits desirable characters for CSP applications due to its high energy density and the isothermal behaviour. ´ Due to the high temperature requirements for CSP systems, inorganic salts/salt eutectics and metals/metal alloys are potential PCMs . Salts have been the most studied PCMs to reduce the cost of thermal storage. ´ However, salts have low thermal conductivity, which limits the heat transfer between the HTF and the PCM, particularly during discharging. ´ For charging and discharging processes, a small temperature difference is desirable to minimize the exergy losses and to improve the efficiency of the power cycle. ´ Reducing the thermal resistance between the PCM and the HTF is a critical issue; • Packed bed systems • Compositing high conductive materials • Heat pipes • Cascaded PCMs
28 Heat pipes Encapsulated PCMs
Cascaded PCMs
Cascaded PCMS
Source: Renewable and Sustainable Energy Reviews 53 (2016) 1411–1432 29 Heat storage capacity and cost of various PCMs
Renewable and Sustainable Energy Reviews 22 (2013) 466–481 30
2124 M. Liu et al. / Renewable and Sustainable Energy Reviews 16 (2012) 2118–2132
Table 4
Metals and metal alloys with potential use as PCM.
3
Compound Melting Heat of fusion Density (kg/m ) Specific heat Thermal conductivity References Note
1 1
temperature (◦C) (kJ kg− ) (kJ kg− K) (W/m K)
Solid Liquid Solid Liquid Solid Liquid
Pb 328 23 [43]
Al 660 397 [43]
Cu 1083 193.4 8930 [76]
8800 350 [40]
Mg–Zn (46.3/53.7 wt%) 340 185 4600 [42]
Mg–Zn (48/52 wt%) 340 180 [41]
Zn–Al (96/4 wt%) 381 138 6630 [42]
Al–Mg–Zn (59/33/6 wt%) 443 310 2380 1.63 1.46 [41]
Al–Mg–Zn (60/34/6 wt%) 450.3 329.1 [45] 1000 thermal
cycles
Mg–Cu–Zn (60/25/15 wt%) 452 254 2800 [41]
Mg–Cu–Ca (52/25/23 wt%) 453 184 2000 [41]
Mg–Al (34.65/65.35 wt%) 497 285 2155 [42]
Al–Cu–Mg (60.8/33.2/6 wt%) 506 365 3050 [42]
Al–Si–Cu–Mg (64.6/5.2/28/2.2 wt%) 507 374 4400 [42]
Al–Cu–Mg–Zn (54/22/18/6 wt%) 520 305 3140 1.51 1.13 [41]
Al–Si–Cu (68.5/5/26.5 wt%) 525 364 2938 [42]
Al–Cu–Sb (64.3/34/1.7 wt%) 545 331 4000 [42]
Al–Cu (66.92/33.08 wt%) 548 372 3600 [42]
Al–Si–Mg (83.14/11.7/5.16 wt%) 555 485 2500 [42]
Al–Si (87.76/12.24 wt%) 557 498 2540 [42]
Al–Si–Cu (46.3/4.6/49.1 wt%) 571 406 5560 [42]
Al–Si–Cu (65/5/30 wt%) 571 422 2730 1.30 1.20 [41]
Al–Si–Sb (86.4/9.6/4.2 wt%) 575 471 2700 [42]
Al–Si (12/86 wt%) 576 560 2700 1.038 1.741 160 [44]
Al–Si (20/80 wt%) 585 460 [44]
Zn–Cu–Mg (49/45/6 wt%) 703 176 8670 0.42 [41]
Cu–P (91/9 wt%) 715 134 5600 [41]
Cu–Zn–P (69/17/14 wt%) 720 368 7000 [41]
Cu–Zn–Si (74/19/7 wt%) 765 125 7170 [41]
Cu–Si–Mg (56/27/17 wt%) 770 420 4150 0.75 [41]
Mg–Ca (84/16 wt%) 790 272 1380 [41]
Mg–Si–Zn (47/38/15 wt%) 800 314 [41]
Cu–Si (80/20 wt%) 803 197 6600 0.50 [41]
Cu–P–Si (83/10/7 wt%) 840 92 6880 [41]
Si–Mg–Ca (49/30/21 wt%) 865 305 2250 [41]
Si–Mg (56/44 wt%) 946 757 1900 0.79 [41]
Renewable and Sustainable Energy Reviews 16 (2012) 2118–2132 interface through the growing solid layer to the heat exchanger 31
surface. Hence, the heat transfer coefficient is dominated by the
thermal conductivity of the solid PCM. However, most PCMs usu-
1
ally provide low thermal conductivity around 0.5 W m− K), which
results in poor heat transfer between the HTF and the storage mate-
rial. Therefore, the design of a cost effective phase change thermal
storage system requires the development of proper thermal per-
formance enhancement technique. Based on the literature on high
temperature phase change storage systems, the following tech-
niques (as presented in Fig. 6) have been employed to enhance the
thermal performance of the storage system: increasing the thermal
conductivity of the PCM by compositing high conductive materi-
als, extending heat transfer surfaces by fins and capsules, using
intermediate heat transfer medium or heat pipes and employing
multiple PCMs.
Fig. 5. PCM test module during assembly [39].
4.1. Enhancement using high conductive materials
further testing in an electric heater. Sun et al. [44] tested the thermal
The heat transfer within a PCM storage system can be enhanced
reliability of metal alloy Al–Mg–Zn with 59.36% Al, 34.02% Mg and
by composing high thermal conducting material (sensible heat
6.62% Zn (wt%). DSC results showed that the melting temperature
phase) into the PCM (latent heat phase). In the PCM/ceramic com-
and the latent heat of fusion of the alloy decreased by 3.06–5.3 K
pound, the molten PCM is retained and immobilized within the
and 10.98% respectively after 1000 thermal cycles.
micro-porosity defined by the ceramic network by capillary forces
and surface tension, which offers the potential of using direct
4. Methods of performance enhancement contact heat exchange [45,46]. Petri et al. [45] tested a packed-
bed laboratory scale storage unit containing 1.22 kg composite
During the discharging process, the energy released by solid- Na2CO3–BaCO3 (melting point of 700 ◦C)/MgO. The composite has
ification of the PCM must be transported from the solid–liquid been pressed into cylindrical pellets with a diameter of 2 cm High Temperature Processes Unit R&D – Research topics One observation….
TODAY • Solar fuels and Conservative first-generation schemes chemistry • Brayton cycle Solid particles • Air heating receivers
Source: IMDEA Source: Energía NEXT GENERATION Ceramic Efficiency (high-temperature/high-flux) receivers • Brayton cycle High P • Air heating Dispatchability (storage/hybrid) / Solar fuels High T Ceramic Modularity (small size) receivers Low P • Air heating Environmental impact (water) conceptsAdvanced High T Solarized Stirling engines • Disco Stirling
• Brayton cycle • Air Pre-heating Volumetric air receivers (metallic)
Molten salts receivers • Air heating Sodium • Rankine cycle • Steam heating Receivers Current Water/Steam • Rankine cycle
Present concepts Present receivers • Steam heating Oil receivers • Steam heating 500 ºC 1000 ºC 1500 ºC Temperature
32 High Temperature Processes Unit R&D – Research topics
Modularity Efficiency Dispatchability Integration & Environmental impact New CSP concepts (High T / High Flux) (Energy storage & Solar chemistry) (Small size) (New receivers & thermal fluids) (water)
4 / 20
33 High Temperature Processes Unit Scientific results – Achievements Efficiency Dispatchability Integration Modularity (High T / High Flux) (Energy storage & Environmental impact (Small size) (Receivers / thermal fluids) & Solar chemistry) (water)
Vertical Heliostat Field
CFD analysis of PCM pellets HT electrolysers Design, contruction and melting optical characterization of 7 Integration of supercritical kW Solar Simulator cycles. Dual receivers. Re-heating schemes
150 m2 heliostat solar field
Kinetic analysis of MnxOy reduction under high radiation CFD analysis, design and fluxes construction of 2 kW solar RE integration in buildings Multitower analysis reactors
34 High Temperature Processes Unit Collaborations / Networking Universities R&D centres
Companies
European Powder & Process Technology 8 / 20
35 High Temperature Processes Unit
Principal researcher Senior researcher People
Manuel ROMERO José GONZALEZ AGUILAR Optical engineering & characterization CFD/Heat transfer Power conversion and performance Concentrators/Receivers Absorbers/Storage/Heat transfer fluids analysis Postdoc researcher Postdoc Researcher Postdoc Researcher
Jian LI Selvan BELLAN James SPELLING
Receiver engineering & Reactor engineering & Integration & Integration & High flux/High temperature Solar energy Solar chemistry Power conversion characterization storage & Materials Heat transfer fluids
Fabrisio GOMEZ Elisa ALONSO Lucía ARRIBAS Sandra ALVAREZ Alessandro GALLO Javier SANZ “ Doctorands
36 Innovative Latent Thermal Energy Storage System for Concentrating Solar Power Plants
Numerical modeling of latent TES system
37 Fabrication of PCM Pellets Ø 20-30 mm diameter hemispheres
Ø CARVER automatic hydraulic press( under 7000 pound pressure)
Ø Average weight of the pellet: 7.5-9 gm.
CARVER automatic press Hemispheres pellet Two hemisphere pellets one on another
38 Encapsulation of PCM pellets
Ø Precoating – A layer of polymer is coated around the salt pellet Ø Pressed by using automatic press Ø Coating is metallized using electroless and electroplating chemistry Ø After passing 1000 cycles, the cycling duration will be increased to 3 hours
Encapsulated Salt Capsule after 1000+ Thermal Pellet Cycles
39 Ø Different polymer thickness capsules were investigated Ø Polymer and metallization methods were used for encapsulation of salt Ø Electroless/Electroplating methods were pursued for metal (Nickel) deposition
Polymer Coated Salt Metallization of Salt Pellets Pellets
40 Thermal cycling
Ø Coating material: polymer and metal Ø Cycle: 1 hour at 370 o C then 310 o C for 30 minutes Ø Polymer coated pellets passed 101+ cycles (continuing).
Nickel plated capsules Pellets for 370 o C
41 Encapsulation process
42 Numerical studies
Thermo physical properties of NaNO3
Property NaNO3 ������� (��/�3) solid phase 2118 mushy zone 250122.6 − 428� MODEL VALIDATION
Liquid phase � = �� /�(� − �� ) + 1 3 • PCM: n-octadecane �� (��/� ) 1904 ������� ��������� (��/� �) 0.0119 − 1.53�10−5� • Melting temperature : 28°C • Inner radius: 50.83 mm ������ ℎ��� �� ������ (�/��) 182000 • Shell thickness: 1.5 mm ������� ����������� (℃) 306.8 • Initial and boundary conditions �������� ℎ��� (�/���) 444.53 − 2.18� • Initial temperature: 27°C �ℎ����� ��������� �����. (�−1) 6.6�10−4 • Wall temperature : 40°C �ℎ����� ������������ (�/��) 0.306 + 4.47�10−5�
43 Numerical studies on latent TES system
Ø Preliminary study to design and construct TES prototype Cyclic charging and discharging
Charging PCM: NaNO3 ü Effect of fluid flow rate HTF: Therminol 66 ü Influence of capsule size Tank Height : 1.5 m ü Influence of HTF Temperature (Stefan number) Tank radius: 0.352 m ü Effect of storage material Storage Tank (L/D) ratio: 2.13 ü Influence of insulation layer thickness Tank Capsule radius: 0.01 m ü Cyclic charging and discharging Fluid flow rate: 1 m3/h
DT (Tf - Tm) : 50 K
Energy capacity: 50 kWh Discharging
44 Thermal energy storage system with encapsulated PCM
Inner layer: Polymer Outer layer: Nickel
Schematic of (a) thermal storage tank and (b) spherical capsule Objectives Ø Numerical models: • 2-D continuous solid phase • Effective packed bed model Ø To study the behavior and performance of a thermal energy storage system for high temperature applications • Spherical capsules • PCM: Sodium nitrate • HTF: Therminol 66 Ø To predict the temperature distribution, fluid flow, melting, solidification and thermocline behavior of the system
45 Continuous solid phase model
Modelling approach Computational domain Assumptions: • PCM capsules behave as continuous medium • Flow inside the tank is laminar and incompressible • Radiation heat transfer between the capsules is negligible. • Capsule distribution inside the tank is defined by
0.87 �−� � � = ���� 1 + − 1 ��� −5 ���� � • Axial velocity along the radial direction is obtained by
2 �� 1−� � � 1−� � � � � �� = −� � � − � � � 2 + ��� � �� ε3 �2 � � 3 � � �� ��
• Temperature distribution throughout the tank is obtained by
�� �� �2� �2� 1 �� � �� � + �� � � = � � + � � + � + ℎ � � − � � � �� � � �� �� ��2 �� ��2 � �� � � � �
�� �2� �2� 1 �� 1 − � �� � = � � + � � + � − ℎ � (� − � ) � � �� � ��2 � ��2 � �� � � � � • Model is validated using C. Arkar and S. Medved, Thermochemica Acta 438(2005) 192-21
46 Model validation
Experimental (Arkar and Medved, 2005) and numerical prediction of temporal variation of temperature at16th row (left) and 35th row (right) during discharge and charge process
Storage tank : Diameter =0.34 m HTF = Air Height =1.52 m capsule diameter = 50 mm PCM = RT20 paraffin average porosity = 0.388.
C. Arkar and S. Medved, Thermochemica Acta 438(2005) 192-21
47 Parametric analysis
Capsule HTF flow rate Tank (L/D) Insulation layer Study ∆T = T -T (K) Ste radius (m) f m (m3/h) ratio thickness (m) Case 01 0.010 50 0.46948 1 2.13 - Influence of Case 02 0.015 50 0.46948 1 2.13 - capsule size Case 03 0.020 50 0.46948 1 2.13 - Case 04 0.025 50 0.46948 1 2.13 - Case 05 0.010 5 0.04694 1 2.13 - Effect of HTF Case 06 0.010 10 0.09389 1 2.13 - temperature Case 07 0.010 20 0.18779 1 2.13 - Case 08 0.010 50 0.46948 1 1.50 - Influence of L/D ratio of tank Case 09 0.010 50 0.46948 1 2.50 -
Case 10 0.010 50 0.46948 2 2.13 - Effect of HTF Case 11 0.010 50 0.46948 3 2.13 - flow rate Case 12 0.010 50 0.46948 4 2.13 - Effect of Case 13 0.010 50 0.46948 1 2.13 0.02 insulation layer Case 14 0.010 50 0.46948 1 2.13 0.03 thickness Case 15 0.010 50 0.46948 1 2.13 0.05 48 Influence of capsule size
Storage tank : Radius =0.352 m Height =1.5 m Storage capacity = 50 kWh PCM = NaNo3 HTF = Therminol 66 Average porosity = 0.388.
Charging mode Initial temperature :Tliq - ∆T (529.95 K for case 1, Tliq=579.95 K, ∆T= 50 K). HTF Temperature at inlet : Tliq + ∆T Discharging mode Initial temperature :Tsol +∆T HTF Temperature at inlet : Tsol + ∆T
49 Influence of HTF Temperature (Stefan number)
Ø Molten fraction of the bed
↓∆T about 90% (from 50 K to 5 K)
↑MVF about 77% (from 1.65 to 7.00 h)
Ø Stored energy of the bed
↑∆T about 90% (from 5 K to 50 K)
↑Energy storage capacity, about 45%
50 Influence of tank (L/D) ratio Ratio: 1.5 Ratio: 2.13 Ratio: 2.5
Case 8 Case 1 Case 9 Temperature distribution of the tank at 50 min. during charging
Effect of fluid flow rate
51 Influence of insulation layer thickness
Melt fraction of the tank and the Instantaneous heat transfer rate through the wall during charging and discharging
wall = hw (Ta Tw )
Ta = 300K
52 Temperature distribution at the axial symmetry
Ø The maximum and minimum cut-off temperatures, Tc and Td, are assumed to be 301.8 and 311.8ºC respectively.
Ø Eutl is defined as the ratio between the useful energy that can be recovered in a cycle to the total energy storage capacity of the tank.
Initial and boundary conditions Cycle 1 Charging mode Discharging mode Initial temperature: 256.8 ºC final state of the charge process HTF Temperature at inlet: 356.8 ºC 256.8̊ C HTF is introduced at top boundary at bottom boundary
Cycle 2 Charging mode Discharging mode Initial temperature: final state of the cycle 1 final state of the charge process discharge process HTF Temperature at inlet: 356.8 ºC 256.8̊ C Cycle 3 Charging mode Discharging mode Initial temperature: final state of the cycle 2 final state of the charge process discharge process HTF Temperature at inlet: 356.8 ºC 256.8̊ C
53 Melt fraction of the storage tank as a function of time during cyclic charge and discharge process
54 EFFECTIVEEffectivePACKEDPackedBEDBed(EPB)ModelMODEL
Assumptions • The flow is laminar and incompressible • The radiation heat transfer between the capsules is negligible. • The contact between the capsules is point-to-point. • The void space between the capsules is assumed; the porosity of the 2-D model is 2 VHTF n (d / 2) avg 1 VTANK D H Governing equations • Heat transfer and fluid flow in the void region of the system are: f . ρ u 0 t f
u T 2 (u. )u P u u u f t f 3 T C f C u. T . T p f t p f f f f • The energy equation in the PCM region is: T C s . T p s t s s • Natural convection effect present in the liquid phase: eff C Ra m l
55 MODEL VALIDATION
v Comparison of the numerical results and the reported experimental data • Cylindrical storage tank • Diameter: 0.34 m • Height:1.52 m • HTF:Air • PCM: RT20 paraffin • Capsule diameter: 50 mm
56 Cylindrical storage tank THERMO-FLUID FLOW Height :1.5 m INFLUENCE OF CAPSULE SIZE… Radius :0.352 m Porosity : 0.388 Radius of capsule: The temperature, melt Ø 10mm fraction and velocity Ø 15mm distribution of the storage Ø 20 mm tank at 1800 s during charge mode Charge process Initial state of the system: fully discharged state Flow rate = 1 m3/h. At t =0, Tinit= 529.95 K. v This model not only At t > 0, Tf= 629.95 K provides the accurate results than continuous Discharge process solid phase model, but it Initial state of the system: enables to obtain more fully charged state details of the fluid flow Flow rate = 1 m3/h. and heat transfer At t =0, Tinit= 629.95 K. characteristics of the At t > 0, Tf= 529.95 K system.
57 …INFLUENCE OF CAPSULE SIZE v The temperature distribution and melt fraction of the packed bed during charge process for various capsule sizes:
Ø In each plot, the left side represents the temperature and the right side represents the melt fraction.
Ø The heat transfer between the PCM capsules and the fluid is rapid except the phase change region (mushy zone) due to the latent heat transfer and thermal resistance caused by the molten/solidified layer.
Ø The heat transfer rate at near wall region is high since the velocity of the HTF is high in near wall region than the rest of the region.
Ø It can be seen that the heat transfer rate of the bed is high for the small size capsules than the large size capsules.
58 Temporal variation of the melt fraction of the bed …INFLUENCE OF CAPSULE SIZE
Ø Complete melting/solidification↑=Capsule size↑. Complete melting (melt Ø Complete solidification time is longer than the melting time: natural convection fraction=1) and solidification (melt fraction=0) time of the Ø Natural convection effect: not significant influence on small size capsules storage tank
Ø Natural convection effect in the molten PCM↑= capsule size↑
Ø Difference between the complete melting and solidification time ↑= capsule size↑
59 Temporal variation of temperature distribution of the tank COMPARISON OF CSP AND EPB MODELS
Cylindrical storage tank • Height :1.5 m • Radius :0.352 m • Radius of capsules: 20 mm • Porosity : 0.388 Thermal storage capacity: 50 kWh
Charging process • Initial state of the system: fully discharged state • Flow rate = 1 m3/h. • At t =0, Tinit= 529.95 K. • At t > 0, Tf= 629.95 K
Ø The heat transfer between the PCM capsules and fluid is rapid except around the melting point (mushy zone)
Ø During melt process, the heat transfer between the encapsulated capsules and the HTF is decreased
(a) 900 s (b) 1800 s (c) 3600 s (d) 5400 s 60 Temperature distributions of the PCM and HTF at various axial positions …COMPARISON OF CSP AND EPB MODELS
Difference between these two models is found in the HTF and PCM temperature profiles, due to; • CSP model: overall heat transfer coefficient • EPB model: effective thermal conductivity
61 Melt fraction of the storage system predicted by CSP and EPB models …COMPARISON OF CSP AND EPB MODELS
Ø Melting/solidification time of the tank: CSP model is about 5% higher than EPB model.
Ø By giving the appropriate coefficients in correlations both models can be used Temperature, melt fraction, velocity and pressure distribution of the tank Ø EPB model: • Fluid flow through the void • Thermal gradient inside the capsules
Ø Difficult to implement large scale systems: • Complex to create computational geometry • Computational time is much higher than the CSP model.
62 63 Effective Packed Bed Model Ø Temporal variation of thermo-fluid flow, and temperature distributions of the points P1 and P2 of the capsules at 2nd and 8th row of the tank during (a) charge and (b) discharge modes.
Continues Solid Phase Model
64 Summary
Sensible heat storage system
´ Developing new molten salts with lower freezing temperatures and higher decomposition temperatures ´ Developing ionic liquids for next generation HTFs ´ Utilizing nanobased technology to improve the specific heat capacity and thermal conductivity and ´ Developing low-cost solid storage media and evaluating them through compatibility testing with molten salts.
Latent Heat storage system ´ Maximize the extraction rate of the stored heat from the storage system by reducing the thermal resistance between the PCM and the HTF through encapsulation ´ Utilizing heat pipes ´ Cascaded PCM systems indicate that they can offer a high energy and exergy efficiency storage solution compared to a system containing only one PCM ´ Future research should focus on PCM characterization to facilitate accurate design of the latent TES system. ´ Pilot and demonstration scale tests are needed to verify the deployment of latent TES in CSP applications.
65 Summary The important results obtained from the numerical study are summarized as follows: ´ The complete melting time is shorter compared to the solidification time due to the high heat transfer coefficient during melting. ´ The natural convection effect in the molten PCM is gradually increasing when increasing the capsule size, consequently, the difference between the complete melting and solidification time of the storage tank is gradually increasing with increasing the capsule size. ´ The charging and discharging rate are significantly higher for small size capsules compared to large size. Small size capsules yield higher total and latent energy storage, and the thermocline region is gradually increasing when increasing the capsule size. ´ It is found that an increase in the Stefan number is accompanied by a decrease in the thermocline region which finally increases the effective discharge time and the total utilization. ´ Significant difference is not found in the complete melting/solidification time of the system when the tank length to diameter ratio varied between 1.5 and 2.5 for the given conditions ´ The thermocline region increases with the increase in fluid flow rate, and consequently decreases the effective discharge time and the total utilization. The heat loss through the wall is increases with a decrease in the insulation layer thickness. ´ Transient two-dimensional continuous solid phase and effective packed bed models are developed. The difference between these models is compared in terms of temperature distribution and melt fraction.
66 Thank you
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