High temperature thermal
energy storage and heat
transfer using eutectic
metals in concentrating
solar power
Johannes P. Kotzé
Stellenbosch University – STERG
Energy Postgraduate Conference 2013
Thermal energy storage – State of the art
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•
•
•
Two tank molten salt
565°C
Subcritical steam
Power block efficiency:
–
36-40%
• Excellent from a thermodynamic
point of view but limited thermal
efficiency
Cost breakdown of a CSP plant – US-DOE
Cost breakdown of LCOE (All costs)
Heliostat cost
22.1 %
Indirect costs
20.8 %
Operations and maintenance
12.1 %
Power plant cost
12.1 %
Receiver cost
10.1 %
Tax
8.1 %
Storage cost
7.4 %
Balance of plant cost
4.0 %
Site cost
2.0 %
Tower cost
1.3 %
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•
•
DOE – CSP calculation at
15 US$ cents/kWh
41% of the costs are due
to indirect costs and is site
specific
Still, heliostats relates to
37.5% of the total
hardware cost
Increased
thermal
efficiency (less
heliostats)
Heliostat cost
reduction
Most direct path for cost reduction
Most direct
path to
cost
reduction
• Consider a 10% increase in thermal efficiency from a usual subcritical
steam power block to a ultra supercritical steam power block:
– 26.3% savings in thermal input from the heliostats.
– 26.3% less heliostats
– And a 5.8% reduction in LCOE, and a 9.8% reduction in plant cost based on
heliostats alone.
• Requires higher temperature thermal storage temperatures
Technology
Source temperature
Thermal efficiency
Subcritical steam
540°C
~36%-40%
Ultra supercritical steam
600°C
~48%-52%
CO2 Brayton cycles (S-CO2)
550°C
~45%
Metallic PCM’s
800.00
Si(56)–44Mg
LFC
CPC/CTC
700.00
PTC
Selected alloy for research is AlSi12:
PTC with steam
600.00
PDR/HFC
Latent heat of fusion (J/g)
87.76Al–12.24Si
500.00
88% Aluminium, 12% Silicon (by mass)
Melting point :
±577 °C
Heat of fusion :
490 to 546 J/g
Density@ 577 °C :
± 2650 kg/m^3
Thermal conductivity @ 577 °C :
± 190 W/m.K
Low cost casting alloy known as AA4047 or LM6
[4]
83.14Al–11.7Si–5.16Mg
86.4Al–9.4Si–4.2Sb
Cu(56)–27Si–17Mg
46.3Al–4.6Si–49.1Cu
400.00
64.1Al–5.2Si–28Cu–2.2Mg
60.8Al–33.2Cu–6.0Mg
Al(59)–35Mg–6Zn
300.00
Al(65)–30Cu–5Si
Cu(69)–17Zn–14P
66.92Al–33.08Cu
68.5Al–5.0Si–26.5Cu
Mg(47)–38Si–15Zn
64.3–34.0Cu–1.7Sb
Al(54)–22Cu–18Mg–6Zn
34.65Mg–65.35Al
46.3Mg–53.7Zn
96Zn–4Al
Zn(49)–45Cu–6Mg
Mg(55)–28–17Zn
[5]
Mg2Cu
Mg(52)–25Cu–23Ca
200.00
Si(49)–30Mg–21Ca
Mg(84)–16Ca
Zn2Mg
Mg(60)–25Cu–15Zn
[5]
Cu (91)–9P
Cu(80)–20Si
Cu(74)–19Zn–7Si
Cu(83)–10P–7Si
100.00
0.00
0.00
100.00
200.00
300.00
400.00
500.00
600.00
700.00
800.00
900.00
1000.00
Melting temperature (°C)
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Higher storage temperatures possible
High thermal conductivity – Less complicated heat exchanger design
High density storage
Metallic heat transfer fluids - NaK
Heat transfer fluid
Liquid sodium
Molten nitrate salt
Steam vapour
Air
NaK eutectic system
NaK receiver capable of 3MW/m2
Water
Sodium (atm)
NaK46 (10bar)
NaK78 (atm)
Thermal oil
Hitec Solar Salt
Hitec
Hitec XL
-50
0
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100
Operational Temperature - C
Peak flux
(MW/m2)
1.5
0.8
0.4
0.22
• Sodium – Potassium alloy (NaK)
• Eutectic composition:
• 77.8% potassium
• Melts at -12.8°C (no trace
heating)
• Traditionally used in experimental
nuclear reactors for safety reasons
• Highly reactive with water
• NaK46 is better suited for solar
applications
• 46% potassium
• 20°C melting point
• Higher specific heat capacity
• Compact and economic receiver design
for even higher efficiency.
Concept evaluation
Steam/
Water
out
NaK in:
780°C
NaK to storage
AlSi12 PCM
Re-heater
Housing
Steam/water
pipes
Steam drum
AlSi12 PCM
NaK Pipes
G
Superheater
HP-Turbine IP-Turbine
LP-Turbine
Boiler
Collector field
NaK to Recievers
NaK out:
577°C
Steam/
Water
in
Cross-section
• Combined storage and steam generator
concept
• Design tool: Flownex SE
• 100MW electrical output
• Subcritical steam cycle
• Aim is to investigate design methodology
and process control parameters
FW Pump
O-FWH
FW Pump
Condenser
Heat transfer analysis: Stefan problem
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Two key components to analysis:
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Heat transfer to the heat exchange pipes
Tracking and predicting the solidification front in the melt
This is a classical Stefan problem
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–
Solved using a enthalpy tracking method
Heat of fusion is measured using DSC
Experimental validation
One way valve
One way valve
Header tank
(Oil)
TC
Ball Valve
Vapour
condenser
P
Gate valve
Heaters
Primary oil
coolant loop
Test section
cooling loop
Ball Valve
Test section
Cooling tower
Water
cooling loop
F
Flow meter
TC
V-1
F
Ball Valve
Water pump
TC
Flow meter
TC
P
Solenoid valve
Dump tank
Conclusions
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Higher storage temperatures relate to lower LCOE
Metallic phase change materials can offer storage temperatures that can be
used with high efficiency power cycles
Metallic heat transfer fluids, like NaK can offer high temperature heat transfer
solutions, will not solidify at night, and results in high performance receiver
designs.
A concept was developed and evaluated. The design tools was evaluated
using a experiment.
The experiment serves as a prototype, and is a proof of concept .