HTF and HTM in CSP plants:
definition and examples.
Dr. Anna chiara Tizzoni
Rome, 15 September 2014
HTM and HSM
• In parabolic trough solar plants (PTSP), a sun-orientable
parabolic mirror is employed to concentrate solar energy on a
fixed receiver pipeline containing a heat absorbing material
(heat transfer fluid, HTF).
• The thermal fluid transfers the energy to a heat storage tank,
containing a heat storage material (HSM), which in turn will
feed a steam generator to produce electric energy by, for
instance, a Rankine cycle.
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HTF and HTM
Tube
receivers
Mixtures of molten salts
(nitrates) have been
extensively employed in
various CSP plant
typologies.
In general, these
materials can operate
in a temperature range
between 270 and 550600 °C
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HTF and HTM
HTM and HSM
• Where the HTF and the HSM are the same material, there is
no need of an intermediate heat exchanger (HX) between
them. This configuration is employed by the ENEA developed
plants and facilities, in particular, it is present in the
Archimedes solar power plant located in the south eastern
part of Sicily .
• As expected, the thermal exchange and storage materials are
fundamental components of a PTSP plant, affecting both the
efficiency and the costs of the generated electric power.
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HTF and HTM
Diathermic oils
• To date, most of the operating PTSPs employ diathermic oils
as HTF, that are composed by a mixture of organic (generally
aromatic) compounds, mostly diphenyl and diphenyl oxide.
• Nitrate alkaline mixtures are generally used as HTMs. The
heat transfer between the two fluids is performed by an
intermediate HX.
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HTF and HTM
Diathermic oils - advantages
• The main advantage in the employment of diathermic oils, is
their low freezing point (-18÷12 °C), which avoids the HTF
solidification in the plant receiver tube and pipelines;
• In this way there is not the necessity for a heating system to
maintain the plant lines at a temperature higher than the one
in the external ambient.
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HTF and HTM
Diathermic oils - disadvantages
• On the other hand, these materials are expensive, toxic for
humans and very toxic for the environment, especially for the
aquatic species;
• They present a relatively low thermal stability, they can be
employed up to about 250 °C at atmospheric pressure, and
under pressure from nitrogen or inert gases up to around 440
°C.
• Above this temperature they undergo an irreversible
degradation and are also very flammable materials.
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HTF and HTM
State of art
• Molten salts mixtures are known to exhibit satisfactory
thermal and physical features, both for heat exchange and
storage, in the temperature range concerned, together with
low corrosion properties and a relatively low cost .
Advantages of molten salts (nitrates/nitrites) :
• safe
• non-toxic
• available at low cost
• stable at relatively high temperatures
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HTF and HTM
Thermochemical properties
of molten salts
Solar Salt (NaNO3-KNO3 60-40 % w/w corresponding to 64/36
mol/mol) is currently the most employed material both as HTF
and HSM.
 quite inexpensive
 not flammable
 presents a relatively high thermal stability point (≈ 600 °C)
 low viscosity
 high heat capacity
 the only drawback is a relatively high freezing point (238 °C) 9
HTF and HTM
“Solar salt” properties
Physical Properties :
Phase Diagram NaNO3-KNO3
400
Freezing Temperature: 238°C;
Melting point
350
Freezing point
T (°C)
300
Melting Temperature: 221°C;
250
200
Heat of Fusion: 161 kJ/kg;
150
Molar Fraction NaNO3
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
100
Volume Change on Fusion: 4,6%.
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HTF and HTM
State of art – Solar Salt
• It is clear that “solar salt” also presents several drawbacks.
• A primary disadvantage is a relatively high freezing point
ranging from about 100°C to 230°C, depending on the
mixture's composition, as opposed to about 13°C for organic
fluids.
• As such, considerable attention must be paid to avoid salt
freezing in the CSP plant, which can seriously affect the
power plant's operating conditions, by plugging valves and
pipes, and reducing heat transfer surface.
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HTF and HTM
Looking for new ones - reasons
• It is possible to roughly estimate that the heat storage
represents, in a typical CSP plant, from 10 to 20% of the total
plant expenditure.
• Considering that the cost of “low freezing” molten salts is
about four times the cost of the solar salt mixture, it is possible
to realistically estimate that, in case of “low freezing” molten
salt mixtures used in place of the “solar salt mixture”, the
expenditure related to heat storage will rise up to the 20-30%
of the total plant cost.
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HTF and HTM
Looking for new ones
• Also taking into account that the Rankine electric power
generating block is slightly affected by a decrease of the lower
operative point of the thermal fluids below 270 °C , the “solar
salt” formulation can be considered the only realistic choice for
heat storage in CSP applications;
• Given its low cost, relatively high thermal stability and very low
toxicity, it is also considered the most feasible material to be
used as HTF.
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HTF and HTM
Low freezing mixtures
• quaternary ones containing Na/K nitrates-nitrites,
• ternary ones with lithium nitrate,
could be advantageously employed as HTF
so as to
overcome the difficulties related to specific maintenance
operations, such as shutting down and starting up plants.
In the near future, the problems related to the employment of
more innovative low melting molten salts can be investigated,
and the improvement of their economical and chemicalphysical features is expected to be a very interesting and
challenging research field.
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HTF and HTM
New mixtures
• Besides diathermic oils and “solar salt”, other materials
present some application as HTF:
 A commercial ternary mixture, produced by Coastal Chemical
Co., L.L.C. and named Hitech® Heat Transfer Fluid, consists
of NaNO3-KNO3-NaNO2, with a weight percentage ratio of
7/53/40 (see also table 2). The liquidus point was lowered to
141°C, and this material found a relatively wide application,
but it suffered from a marked decrease in the material thermal
stability, which is limited to 430° in air.
 Inert gases, however at an experimental stage, have also
been used as solar heat thermal fluids.
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HTF and HTM
Advantages and disadvantages of the
alternative HTFs employed
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HTF and HTM
New mixtures – on going
• In recent times, also the addition of calcium nitrate Ca(NO3)2
and lithium nitrate LiNO3, together or separately, were also
studied .The obtained ternary mixtures presented a liquidus
point of 120 °C when lithium was added, and of 133 °C when
calcium was added.
• Regarding the thermal stability, if lithium nitrate is employed,
the ternary material is stable up to 550 °C, but this value is
limited below 500 °C if calcium nitrate is used, because at
higher temperatures a solid phase is observed.
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HTF and HTM
New mixtures – on going
• Furthermore, the addition of calcium nitrate leads to an
increase of the mixture's viscosity, especially at temperatures
near the freezing point .
• Quaternary mixtures containing both calcium and lithium
nitrates present very low melting points, but suffer the same
problems of the ternary formulations containing lithium and
calcium.
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HTF and HTM
New mixtures
Liquidus
T
(°C)
Max T
(°C)
notes
54
221
na
eutettico
60
40
238
550-600
“solar salt”
7
53
141
450-538
“HITECH®” salt
15
42
140
505
AdP
18
53
30
120
550-600
Archetype
18
40
22
<95
na
NaNO3
(%p)
KNO3
(%p)
46
Ca(NO3)
2
(%p)
LiNO3
(%p)
NaNO2
(%p)
40
42
21
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HTF and HTM
Characterization steps in our labs
STEPS to FOLLOW:
Samples preparation
 mixed inside a glass flask
 heated up at around 300 °C
 mixed by mechanical stirring
 rapidly cooled down and mechanically crushed
phase diagrams
specific heat
viscosity
density
Compared
to Solar salt
heat conductivity
thermal stability
XRD diffraction
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HTF and HTM
Characterization steps
Properties
Interest for HTF
Interest for HSM
phase diagrams
Determination of the lowest Tliq
Determination of the lowest Tliq
specific heat
Capacity of solar heat tranfer to the storage system
Capacity of heat storage
viscosity
Determination of the necessary pumps hydraulic head
It depends on the storage system. In“Archimede “configuration HTF and HSM
are the same fluid.
density
Related to heat capacity; capability of heat storage per volume
Related to heat capacity; capability of heat storage per volume
heat conductivity
Necessary parameter to determine the heat exchange surfaces
Necessary parameter to determine the heat exchange surfaces
thermal stability
Maximum operative T
Maximum operative T
XRD diffraction
Integration to investigate solid nitrates structures: predition for
phase diagrams
Integration to investigate solid nitrates structures: predition for phase diagrams
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HTF and HTM
Next future
In the near future, problems related to the
employment of more innovative low melting
molten salts can be investigated, and the
improvement of their economical and chemicalphysical features is expected to be a very
interesting and challenging research field.
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HTF and HTM