Development of high-temperature sensible thermal energy storage systems for advanced concentrating solar power generation
Date
2018
Authors
Mohan, Gowtham
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Abstract
Solar energy is increasingly considered as one of the most
favourable alternative sources of energy to conventional fossil
fuels to mitigate carbon emissions. Despite the advantages, the
intermittent availability of the solar resource causes a gap
between demand and supply, and solar power plants cannot meet
energy demands at night without energy storage. Concentrating
Solar Power (CSP) plants have the distinctive feature that
thermal energy storage (TES) is relatively easy to integrate to
support energy demand in the absence of solar radiation for
several hours. Thus, development of energy efficient and
cost-effective thermal storage systems is vital for the CSP
industry.
The selection of a particular thermal storage media requires
understanding of thermo-physical properties, operating
limitations, and chemical compatibility. This thesis focusses on
the development of novel high-temperature TES media for next
generation concentrating solar power plants. At present, the
Levelised Cost of Electricity (LCOE) for CSP plants is higher
than for conventional fossil fuel power plants. It is essential
that efficiency improvements and cost reductions are made, to
achieve cost-effective clean electricity through CSP. A key focus
for next-generation CSP plants in to enable high-efficiency power
cycles (such as the supercritical CO2 Brayton cycle, >700ºC),
requiring a higher maximum temperature compared to the
state-of-the-art. An increase in temperature in the power block
impacts design decisions for most components (including the solar
field, solar receiver design, and containment materials) and
choice of the heat transfer fluid and TES media. Nitrate salts
used in state-of-the-art CSP plants cannot be used above 600°C,
and possible alternative salts include carbonates, chlorides,
fluorides, and hydroxides. Of these, based on extensive critical
review of different alternatives, chloride salts are of
particular interest due to their natural abundance, and hence low
cost.
A novel ternary eutectic salt mixture for high-temperature
sensible thermal energy storage (HTSTES), composed of sodium
chloride, potassium chloride and magnesium chloride (NaKMg–Cl)
is developed and its thermo-physical properties are tested and
compared with other potential binary and ternary mixtures. Based
on the analysis, NaKMg–Cl has an acceptable melting point
(387°C), reasonable heat capacity (1.18 J g-1 K-1), high thermal
stability (>700°C) and low storage media cost (4.95 USD/kWh).
These combined traits of NaKMg–Cl make it a potential thermal
storage solution for HTSTES. One of the desirable characteristics
of HTSTES media is low melting point, to avoid freezing the pipes
and other parasitic losses. In further attempts to identify a
‘sweet spot’ between the melting temperature of the mixture
and storage media cost, multiple quaternary chloride mixtures
(NaKMgZn–Cl) are developed by adding different proportions of
ZnCl2 (10% to 90%) to the existing ternary NaKMg–Cl salt. The
results show a significant reduction in melting temperature is
possible, but with a steep increase in the storage media cost.
The last part of the thesis explores the system level
techno-economic analysis for three salt candidates (the ternary
NaKMg–Cl and two of the quaternary NaKMgZn–Cl salts)
identified for HTSTES application. LCOE calculations are made
using the default System Advisor Model (SAM) power tower model,
with cost and performance inputs based on 2020 SunShot CSP
targets. The exception is the thermal energy storage system, for
which costs are calculated separately and depend on the
thermo-physical properties and cost of the salt media. For the
salts identified, a trend of increasing LCOE with decreasing
melting point is identified. The standout is the ternary
NaKMg–Cl eutectic, which has the lowest LCOE of the salt
candidates. However, there may be certain applications where even
lower melting point is required, and in these cases, this work
serves as a guide to the likely LCOE penalty for selection of a
more expensive storage medium with more favourable melting point.
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Keywords
Concentrating Solar Power, Thermal storage, Molten Salts, Eutectic, LCOE
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Thesis (PhD)
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