IMECE2010-38682 Karthik by Caleb DeValve

Novel Thermal Energy Storage System for Concentrated Solar Power Karthik Nithyanandam and Ranga Pitchumani Advanced Materials and Technologies Laboratory Department of Mechanical Engineering Virginia Tech Blacksburg, Virginia 24061-0238 pitchu@vt.edu • http://www.me.vt.edu/amtl • (540) 231-1776

Paper No. IMECE2010-38682 presented at the ASME IMECE 2010 • November 15, 2010 • Vancouver, BC, Canada The work is being supported by a grant from the U.S. Department of Energy under Award Number DE-FG36-08GO18146

Introduction  Concentrated Solar Power (CSP) is a leading source of renewable energy for future power generation.  Due to intermittent solar availability, CSP plants require thermal energy storage.  Development of novel thermal energy storage approaches is necessary to decrease in the capital cost and increase the efficiency of a CSP plant.  Latent Heat Thermal Energy Storage (LTES) provides for isothermal operation and large energy storage for a given volume: compact storage.  A fundamental challenge with LTES is the low thermal conductivity of the phase change materials (PCM) used in LTES which reduces the heat transfer.

CONDENSOR

TURBINE PUMP

Heat Exchanger

PUMP

PCM-BASED TES

DISCHARGING CHARGING Advanced Materials and Technologies Laboratory

Approach and Objectives  To reduce the thermal resistance within the LTES by embedding heat pipes (HP).  Heat Pipe (HP) Thermal Cycle: 1) Working fluid evaporates absorbing thermal energy in the evaporator. 2) Vapor migrates along vapor core to lower temperature end 3) Vapor condenses back to fluid and is absorbed by the wick, releasing thermal energy. 4) Working fluid flows back to higher temperature end due to capillary GRAVITY pressure.

 Objectives  Conduct a numerical analysis on charging (PCM melting) and discharging (PCM solidification) of different configurations of LTES with embedded heat pipes.  Systematically demonstrate performance enhancements in LTES and obtain the optimum orientation of heat pipes and design configuration of the system which increased the melting rate of PCM. Advanced Materials and Technologies Laboratory

Domain Description  LTES/HP Configurations PCM

HTF

Heat Pipe

RHP

Rt HTF/PCM

LM HTF/PCM

Module 1:PCM surrounds the HTF tube

Module 2:HTF surrounds the PCM tube

 Heat Pipe Configurations 120o

120o

2 HHP

2 VHP

3 HP

120o

3 HP

4 HP

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Model Formulation Governing Equations

Boundary Conditions  k HTF THTF   k PCM TPCM THTF  TPCM

TPCM  0

THP  0

 .( V )  0 Module 1

THP  THTF  k HP THP   hA(THP  THTF )

Continuity

k PCM TPCM k HP  TPCM

THP  THP

 k HTF THTF   k PCM TPCM THTF  TPCM

THP  0

THP 

Module 2

THP  TPCM k THP  PCM TPCM k HP

hA (THP  THTF ) k HP

THP  THTF

Momentum  Dv   p  .  S Dt where  for HTF S  g   S  g (T  Tref )  A( )V for PCM

Energy HTF and Walls

DT  .(k eff T ) Dt PCM DH   .(kT ) Dt

c p

 The inlet and outlet flow were defined as periodic conditions while the front and back plane of PCM were implemented as symmetry boundary conditions. Heat Pipe Network Model For any thermal element ‘Ei’, the E6 conservation of energy can be Wall written as follows to solve for the Axial Adiabatic E5 Conduction temperature profile within the heat Wick pipes dT Radial ALM c p i  Qin  Qout Conduction dt E4 E3 E1 E2 Ti ,1  Ti Ti  Ti , 2 Q  , Q  Wick Wick Wall Wall out in R R Evaporator

Condenser

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Numerical Implementation  The model was implemented in a finite-volume based CFD solver, FLUENT, in which the thermal network model for the heat pipes was implemented as a user defined Parameter MODULE 1 function (udf). MFR (kg/s) 2.89  The PCM (KNO3) was initially in the solid state for T (K) 664 charging and in the liquid state for discharging with the char,in Tdisc,in (K) 568 initial temperature being its melting temperature.  The HTF (Therminol) mass flow rate per unit volume of the PCM is kept constant for both the configurations.  The charging and discharging of the PCM was carried out for 12 hours each which was assumed to be the average length of day and night.  Sodium-Stainless steel heat pipe is used. Effectiveness LTES System Initial Energy Q temperature stored in   t,HP Qt of PCM  Specified material PCM properties of HTF, PCM and Heat pipes Initial temperature  Specified dimensions of HTF

MODULE 2 0.917 664 568

Qt,HP – Energy stored/discharged from LTES embedded with heat pipes. Melt fraction of the PCM  Qt – Energy stored/discharged from LTES with no heat pipes.

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Tube Only

Module 1

 PCM melting up to 4.16 h is symmetric and purely conduction dominated.  After ~6.94 h, natural convective circulation forms due to temperature difference between the hot wall and the cold solid PCM.  Convective circulation results in relatively faster melting of the PCM at the top of the tube compared to the bottom.

Module 2

 The convection cells formed in the case of Module 2 are seen to be stronger due to the smaller amount of PCM confined in the tube.  A symmetry about the vertical diametral axis is seen for Module 1, whereas in Module 2, the HTF flow (outside the tube) from left to right causes an asymmetry.

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Module 1 Charging: 2 HHP and 2 VHP

 Initially (< 4.16h) melting is conduction  Initially (< 4.16h) melting is conduction dominated, but convection cells are formed dominated but convection cells form around along the HHP’s condenser section. the HPs; with time, the convection leads to faster melting near the top HP.  As time progresses, convection cells extend  At 12 h, the PCM adjacent to the top HP is over the tube after about 6.94 h completely melted but not at the bottom:  25.1% of PCM volume is molten at 12 h final molten PCM volume fraction is 21.1% which is 1.46 times greater than compared to  Because of natural convection, the bottom a tube without any heat pipe. HP is less effective in the enhancement.

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Module 1: Energy Stored and Effectiveness

 The sharp increase in the effectiveness curve at a time of approximately 4.2 hours for both horizontal and vertical heat pipes is due to the start of the convection effects.  Module 1’s effectiveness with 2 HHP was 1.46 at the end of 12 hours, whereas Module 1 with 2 VHP was effective to only 1.2 (21% difference).  The energy stored with time for a LTES embedded with 2 horizontal stainless steel (HSS) rods shows that the HPs provide for better thermal enhancement.

No HP 2 HHP 2 VHP 2 HSS

0 (b)

2 HHP 2 VHP

1.6

Effectiveness, 

 The charging (melting) rate is the highest for Module 1 embedded with 2 HHP.

Energy stored in PCM, Q [MJ]

1.4

1.2

1.0 0

6 Time, t [h]

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Module 2 Charging: 2 HHP and 2 VHP

 The convection cells along the tube and heat pipes coalesce into one zone resulting in an efficient melting in the upper part of the PCM while the PCM at the bottom part of the tube remains mostly in the solid state.  71.8% of the PCM volume is melted at the end of 12 h, which is 1.42 times greater than that of Module 2 without any heat pipes.

 The melt front in the bottom half of the PCM grows faster; due to thermal stratification in the top part of the PCM, the circulation zone extends to the middle of the PCM only.

 The final melt fraction is higher than the corresponding case in Module 1.

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Module 2: Energy Stored and Effectiveness

 The effectiveness of the heat pipes follows the same pattern as observed in Module 1, with a convection aided jump around 3 h.  The effectiveness value for LTES embedded with horizontal heat pipes was 1.42 at the end of the charging cycle.

 The difference in the energy stored between HHP and VHP is 9%, compared to the 21% for Module 1.

No HP

(a)

2 HHP 3

2 VHP

0 (b)

2 HHP 2 VHP

Effectiveness, 

 Even though the melt fraction for Module 2 is much greater than that of Module 1, the energy stored in Module 2 is slightly less due to lesser melt volume inside the tube.

Energy Stored in PCM, Q [MJ]

1.4

1.2

1.0 0

6 Time, t [h]

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Module 1 and Module 2 Charging: 4 HP

ď ą Melting is more pronounced around the top and the horizontal heat pipes, with the bottom heat pipe contributing the least.

ď ą Thermal stratification at the top causes the circulation pattern to intensify as the charging progresses leading to nearly completing melting of the entire PCM volume

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Comparison Charging Module 1 Module 1 2 HHP 2 VHP

Module1 4 HP

Module 2 Module 2 Module 2 2 HHP 2 VHP 4HP

Energy stored, MJ

3.70

3.11

6.23

3.39

3.10

4.69

Energy density, MJ/m3 PCM

50.28

42.26

84.76

144.45

132.09

200.14

Energy density â&#x20AC;&#x201C; per heat pipe, MJ/m3 PCM

25.14

21.13

21.19

72.23

66.05

49.96

Discharging Module 1 Module 1 Module 1 Module 2 Module 2 Module 2 2 HHP 2 VHP 4 HP 2 HHP 2 VHP 4 HP Energy discharged, MJ

2.09

1.86

2.71

1.76

2.24

Energy density, MJ/m3 PCM

28.40

25.28

36.83

75.00

95.59

Energy density â&#x20AC;&#x201C; per heat pipe MJ/m3 PCM

14.2

12.64

9.21

37.50

23.90

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Conclusions  The paper presented a methodology to enhance performance of LTES by embedding heat pipes.  Module 2 resulted in higher melt fraction compared to Module 1 while the energy stored in the case of Module 1 was higher.  The results showed that the natural convection plays an important role in the performance of the LTES with embedded heat pipes.  Among the 2 HP configurations, 2HHP provided the maximum effectiveness for both Module 1 and Module 2.  Overall, Module 1 embedded with 4 HP provided the highest thermal enhancement in terms of energy stored and effectiveness.  The energy stored/discharged was greater in Module 1 embedded with 4 HP while the energy density was greater for Module 2 embedded with 4 HP.  The energy density per unit heat pipe was greater for Module 2 embedded with 2 VHP.

 Future studies will investigate static and dynamic performance of the LTES system to determine the optimum configuration of heat pipes so as to maximize the effectiveness of the system. Advanced Materials and Technologies Laboratory

3 HP Configuration

120o

120o 120o

Charging

120o

Discharging

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

System Parameters Geometrical parameters used in the computational modeling

Physical parameters of HTF , PCM and wall used in computational modeling PCM KNO3

Heat Pipe Outer radius, RHP [m]

0.018

Condenser section length, Lc [m]

0.14

Evaporator section length, Le [m]

0.1

Adiabatic section length, La [m]

0.06

Wall thickness, wHP [m]

0.001

Wick porosity, p

0.9

Effective wick thermal 45 conductivity, keff [W/m-K] Effective wick heat capacity, ρcpeff, 1.05106 [J/m3-K] Tube Module Length, LM, [m]

0.12

Wall thickness, wt [m]

0.003

Outer radius, Rt [m]

0.5

Density, ρ [kg/m3] Thermal Conductivity, k [W/m-K] Specific heat, cp [J/kg-K] Dynamic Viscosity, μ [Pa-s] Melting point, Tm [K] Thermal expansion coefficient, β [K-1] Latent heat of fusion, hsl [J/kg]

HTF Stainless Therminol Steel

2109

709

7900

0.5

0.078

20.1

953

2588

559.9

2.5910-3

0.15210-3

664

20010-6

95103

The dimensions of the LTES as presented in Appendix 1 are representative of those used for large scale energy storage. Sodium-Stainless steel heat pipe is used while the HTF and PCM materials are KNO3 and Therminol respectively.

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