Solar Thermal Storage System

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Solar Thermal Storage System
One big shining point of Parabolic Trough Power Plant (PTPP), the so-called
dispatchability, is its potential to provide power 24 hours a day, by storing the heat
energy in a thermal storage unit for later use during peak hours, in the evening or on a
cloudy day. It enhances the annual capacity of a plant by 50 % over one without a
thermal energy storage system (TES). Within current technology, heat is much cheaper
to store than electricity. Nearly all current existing solar thermal plants that have backup systems are supported by fossil fuels, but a TES completely hoisted by the power the
plant generates itself is within reach. Several storage mechanisms have been put in
place while other proposals are still in lab-scale. Progress is being achieved by
improvements on old systems and alternative designs. ,

Two-tank direct storage system

Two-tank indirect storage system

Single-Tank Thermocline

Phase-Change Materials

Two-tank direct storage system
The early two-tank direct system was used in the first Luz mirror plant, the “Solar
Energy Generating System I (SEGS I)”in California. It has two tanks, one of low and one
of high temperature. Only one heat transfer fluid (HTF), in this case mineral oil (Caloria),
circulates from the low-temperature tank through the solar collectors picking up the
heat. Part of the heat goes to generate the steam to run the turbine and the excessive
heat goes back to the high-temperature tank for storage. After passing through a heat
exchanger, the cooled fluid flows back to the low-temperature tank to be reused. The
Solar Two power tower in California also uses this system, only with molten salt as the

Two-tank sensible heat storage

But as later SEGSs moved to synthetic oil (a eutectic mixture of biphenyl-diphenyl
oxide) to achieve a higher operating temperature and hence a higher efficiency, the
two-tank direct was no longer suitable. The old mineral oil has a high vapor pressure so
it cannot be used in the large unpressurized storage tank system as the one adopted
for SEGS I. Pressurized storage tanks are very expensive. In addition, the HTF in
some places is too expensive or not suitable to also serve as a storage fluid. It takes
the freezing point and local temperature (day and night) into consideration in
terms of choosing the transfer medium.

Two-tank indirect storage system
The subsequently developed two-tank indirect storage system has not only a HTF but
also a storage fluid (ST) and an extra heat exchanger. The storage fluid coming out of
the low-temperature tank absorbs the heat energy of the high-temperature HTF in the
extra heat exchanger. The now high-temperature ST flows back to a high-temperature
storage tank and the now low-temperature HTF moves on to the solar collector to start
the power cycle again. Despite the extra cost resulting from a second heat
exchangerand smaller temperaturedifference between the two tanks, the two-tank
indirect system with molten salt as the ST is still dominant in most of the PTPPs around
the world. The technology originated from the experiment of Solar Two power tower in
California. Two PTPP in plan, the 50MW AndaSol project in Granada, Spain and the
280MW Solana, in Gila Bend, Arizona, will both adopt the molten salt thermal storage
system.1,2, Andasol, for example, aims at a capacity of 1,010 MWh, equivalent to 7.5
hours of full load operation.

Two-tank indirect thermal energy storage system
for Andasol 1 and 2. The storage tank is 10m in
height and 37m in diameter. The storage fluid is a
mixture of 60% NaNO3 and 40% KNO3 Credit:
For high temperature thermal storage, above 400°C, organic HTFs tend to thermally
decompose, while molten-salt or liquid metal is still generally stable. It is also nonflammable and nontoxic and has been used in other industries . But problem with
molten salt is its relatively high freezing temperature 120 to 220°C (250-430°F). Special
operating maintenance needs to be done to make sure it doesn’t freeze during cold
night, especially in deserts.

Single-Tank Thermocline
To further reduce the cost of the storage fluid and the storage tanks, researchers moved
forward to a single tank called thermocline. Energy is stored in a tank made of solid
storage medium--commonly concrete or silica sand—instead of a storage fluid. Hightemperature fluid flows into in the tank from the top, all the way down through to the
bottom and cools. It creates two different temperature regions from high to low,
between which there is a space called temperature gradient or thermocline. When the

stored-up thermal energy is needed, the flow reverses taking up the heat on its way up.
Buoyancy effects make sure that hot, less dense materials stay on top of cool, dense
materials at the bottom, creating thermal stratification of the fluid.
Sandia National Laboratories in New Mexico has tested a 2.5 MWhr, backed-bed
thermocline storage system with binary molten-salt fluid, and quartzite rock and sand
for the filler material. The cost for a TSE system is reduced substantially by replacing
most of the storage fluid and cheap filling material for the tank.

Thermocline test at Sandia National Laboratories. Credit: Sandia National Laboratories
The research goals now directing current R&D in solar thermal storage encompass
finding heat-transfer fluid that can operate at higher temperature with low freezing
point, hence a higher overall heat transfer efficiency. Another goal is to develop a
storage fluid that has high heat capacity so that less amount of fluid is needed in the

Although these above-mentioned systems are very reliable technically, they still pose a
high overall cost. Other concepts for a cheaper cost are being explored and investigated
too. Some research is under way to find more efficient and less costly filler materials for
the one-tank system which possesses high potentiality for cost reduction.

Phase-Change Materials
Although using concrete as the filler materials is very cost efficient(it is much cheaper to
hold the same amount of energy than molten salt), easy to handle and has higher
strength, it faces problems such as maintaining good contact between the concrete and
pipelines and low efficiency of heat transfer from the concrete to the HTF.

Another rather promising solution is phase-change
materials (PCMs), use d in high temperature latent
heat thermal energy storage system (HTLTTES) for
direct steam generation (DSG).Its primary
advantage resides in its ability to hold up large
amounts of energy in relatively small volumes, at
one of lowest costs among other storage materials.
It utilized different PCM’s different latent heat of
fusion (melting), which should be matched to the
temperature of the incoming sensible HTF. The
PCMs are cascaded from low melting temperature
at the bottom of the tank to high temperature at
the top (maximum operating temperature around 390°C).

The HTF flows downward when charging (melting the PCMs) and upward when
discharging providing heat to generate steam (solidifying the PCMs). Current researches
propose nitrate/nitrite salts and eutectic mixtures of these salts, such as lithium nitrate
and potassium nitrate as the PCMs for HTLHTES, for their enthalpy and economic
Despite its encouraging prospect, however, PCMs is challenged by the complexity of the
system itself, unstable lifespan of the PCMs and low heat conductivity. Researchers are
looking for other material sources that possess more sufficient heat of fusion,
corrosiveness and high heat conductivity (at least 2 W/(m K)). Or it can also be
improved by developing proper heat transfer techniques to offset the low conductivity
of PCMs. ,

A cost comparison of the three storage concepts in different parts
including 2-tank direct liquid salt, thermocline (concrete, solid salt
and liquid salt), and PCM. The latter two are only in testing phrase.
"Thermal energy storage is the killer app of concentrating solar power technology," said
Andrew McMahan, vice president of SkyFuel, New Mexico, told a packed solar
technology conference last month held in conjunction with Semicon West. This month,
the U.S. Department of Energy (DOE) just announced a funding of $35 million to
facilitate developing lower-cost energy storage for CSP technology. An increasing
number of major venture capital also flows into researches that focus on more cost
efficient solar thermal storage technologies.

Nava P, Herrmann, U. Trough Thermal Storage Status Spring
2007 NREL/DLR Trough workshop -Denver Mar 2007
Leopold, G., Solar thermal technology heats up Electronic
Engineering Times August 2008 4 Pg 38
“ DOE to invest $35 million in concentrating solar plant
projects” National Renewable Energy Lab, Sep 19,

Michels, H., Pitz-Paal, R., Cascaded Latent Heat Storage For Parabolic
Trough Solar Power Plants Solar Energy 81 (2007) 829–837
Guo, C., Zhang, W. Numerical simulation and parametric study on
new type of high temperature latent heat thermal energy storage
system Energy Conversion and Management Volume 49, Issue
5, May 2008, Pg 919-927
Michels, H., Pitz-Paal, R., Cascaded Latent Heat Storage For Parabolic
Trough Solar Power Plants Solar Energy 81 (2007) 829–837
Guo, C., Zhang, W. Numerical simulation and parametric study on
new type of high temperature latent heat thermal energy storage
system Energy Conversion and Management Volume 49, Issue
5, May 2008, Pg 919-927
“Solar Storage And Research Development”, U.S Department of
Energy Efficiency and Renewable
Solar Power; Sunny Future For Parabolics In Granada And
Nevada Modern Power System February 14, 2007
“National solar thermal testing facilities” Sandia National
Taggard, S., Parabolic troughs: CSP’s quiet achieverRenewable
Energy Focus Volume 9, Issue 2, March-April 2008, Pages 46-48,
“Parabolic Trough Thermal Energy Storage Technology”NREL
“Thermal Storage” U.S Department of Energy Efficiency and

Stine, W.B., Harrigan, R.W. an online update version of the book
"Power From The

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