concentrated solar power plants

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25 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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What is
concentrated solar power plants
?

When it comes to solar power, many people would have the picture of big photovoltaic panels
on someone’s roof popping out in their head. What is less obvious on the stage and more like a
hidden dragon (some calls it “sleeping solar giant”) is the technolog
y called concentrated solar
thermal power

harnessing the sun for heat at high temperature, which today generates the
same amount of the electricity sent to the grid as photovoltaic systems worldwide (about 500
GWh per year).
1

In places like Spain and the w
est of United States, there are already large
power plants in place, feeding households and industries in a steady flow.

With its potentially
large capacity, cheap energy storage system, reliable dispatchability and market price, National
Renewable Energy
Lab (NREL) has claimed it “can be a major contributor to our nation's future
need for new, clean sources of energy”.
2

I
n this page:




Concentrated Solar Power



Power Conversion



Other Applications



Different System Designs



Solar Thermal Power Plants In The World



Land Rush In The Southwest



Cost Reduction Of Csp Plants



Some Challenges And Environmental Concerns

Concentrated
Solar Power

(CSP) is a major utility
-
scale application of solar thermal
energy. Instead of being used directly to heat up houses or swimming pools, sunlight is focused
by mirrors or lenses to reach a high temperature (at least 570
°
F/300
°C

to be effective and
economi
cally applicable) to either generate steam to propel a turbine to produce an electric
current or convert heat to electricity directly using a
Stirling engine
. The former is the same
concept as in a conventional power plant, but rather than burn fossil fuel
s it collects the sun’s
radiation and sends off no pollution or greenhouse gases.

Power Conversion

Different temperature levels mean different conversion methods;
generally, the higher the temperature the more efficient the conversion. Different materials

and
technologies add up to different cost. Most commonly adopted steam turbines (i.e
Rankine
cycle
) have an efficiency of up to 41.7% while a combined cycle of Rankine and
Brayton

(has
gas turbine using pressured air) can achieve a reasonable target of 50

%
at a turbine inlet
temper
a
ture of 1200

C. Moreover, a
binary cycle

using
alkali
-
metal (i.e. most experiments use
potassium) as a second working fluid in the topping cycle

has demonstrated an efficiency of
57%.
3

Another method proposed for Solar Tower uses a liquid
-
fluoride
-
salt coolant system to
achieve to 1100 °C and operates through a multi
-
stage turbine system to obtain an efficiency of
60 percent.
4

Its high working temperature requires the plants to be built

in locations with
direct normal insolation

(DNI) above
1800KWh/ (m
2
day) (circa 5KWh/ (m
2
day))

to be
economical.

5

That is generally within the SunBelt

between the 35
th

northern and 35
th

southern latitudes. But via an efficient electric transmission system
, it theoretically has the
capacity to meet the world with its electricity demand.
7


Other Applications

CSP could also be integrated into other industries to provide power.
Desalination

using waste heat from power generation pumps out freshwater to the desert
regions where the mirrors are most ideally suited. The cold water can also be used to provide
air
conditioning
.
Solar electricity could also be used in the
production of hydrogen
, a
n increasingly
important clean fuel.
Solar furnace

made of parabolic dish or heliostat mirror can process
fullerenes and large carbon molecules with major potential commercial applications in
semiconductors and superconductors.

Different System Designs

The
re are currently three major types of CSP systems with
respect to how the sunlight is concentrated and different conversion processes
. They are
Parabolic Dishes
,

Solar Towers

and
Parabolic Trough Power Plants

(PTPP)
.

A list of operational solar thermal power plants in the world
6


(credit:wiki for a more complete list of plants under construction,
announced

in the U.S. and
elsewhere

click on here
http://en.wikipedia.org/wiki/List_of_solar_thermal_power_stations
)

Land
Rush in the Southwest

Until earlier this year, U.S.
Bureau of Land Management

had already received 125 applications
for solar energy development on federa
l land totaling around 4000km
2

(1544 mile
2
) or enough
land for 70GW.
7
,
8

While according to a 2003 NREL report on Southwest Solar Energy Potential,
it estimates an total area of 53,727 mile
2

of
land that has no primary use today, excluding land
with slope >

1%, <5 contiguous km2, and sensitive lands. Assuming 5 acres per MW, this size of
land have the potential of
6,877,055 MW from solar power.
9

Yet this large
-
scale acquisition of
land has brought concerns about the desert environment and fragile ecosystems
there.
10

Cost reduction of CSP plants

During the 1980’s, the early parabolic trough power plants in Europe generated electricity at a
cost equivalent to 70
-
140 U.S. cents per KWh. It quickly went down to 30 cents when the SEGS 1
came into place in the U.S. Now it has reached a range of 8
-
16 ce
nts. These cost reductions
primarily come from larger plants being built, increased collector production volumes, building
projects in solar power park developments, and savings through competitive bidding. A general
rule is that the larger the size of the

plant the lower the per kW capital cost of power plants.
Today in Southern California for example, peak power costs
anywhere between US cent
10
-
18/kWh, almost no difference with CSP.
11
,45


These fast cost reduction is also a result of CSP’s fundamentally s
imple technology. It is the
same principle as you burn a piece of paper using a magnifying glass. With CSP, you just need to
have a good many of them and a traditional thermal power plants. There is no complex material
selection as in PV production, no hol
es to drill as geothermal has to.
30


A cost reduction study of PTPP and Solar Tower credit: NREL
12


Some
Challenges And Environmental Concerns

A big challenge for CSP to power greater area is transmission as the highest resource potential
does not match wi
th populous regions. High capacity power lines are needed for CSP’s
long
-
term development.
Competition with agricultural, industrial and residential use of water
would also be a spiny issue, water being sucked up from Colorado River. Some scientists have
b
rought up the concerns over the fragile ecosystem in the desert area. The only emission from
solar thermal power plants running on steam turbines, water vapor, clean as it is, yet
contributes to global warming. Some underlying safety concerns include the i
ncidental leakage
and explosion of some toxic oil heat transfer fluid.

After years of worldwide campaigns on global climate change, we finally do not have to
dedicate much energy in arguing for it. Now is the time for us to take our steps to actually shift
of our energy use.
Taking advantage of the non
-
sensitive deserts, no p
ollution and the lowest
carbon emission among other renewable energy technologies,
6
and with
the sun pouring more
than 7 KWh/m
2

day of its energy onto the golden landscape of the southwest,
43
concentrated
solar power has been quietly chasing around the su
n for some 20 years, just like the sunflowers.
CSP will and should exert a bigger play in the grand picture of America’s future renewable
energy mix with duly confidence.


Parabolic
Dish

systems use satellite
-
like mirror dish(es) to focus the light onto a single
central receiver in front of the mirror. They so
far ha
ve

the highest heat
-
electricity conversion
efficiencies among all CSP designs (up to 30
%
).
The size of the concentrator is determined by
its engine. A
dish/Stirling system’s concentrator
with a nominal maximum
direct normal
solar insolation

of 1000 W/m2 and

a 25
-
kW
capacity has a diameter of approximately 10
meters. It could also run on a single Brayton
cycle, where air, helium or other gas is
compressed, heated and expanded into a
turbine. Parabolic dish could be applied
individually in remote locations, or

grouped
together for small
-
grid (village power, 10 KW) or
end
-
of
-
line utility (100 MW) applications.

The electricity has to be used immediately
or
transmitted to the gird
as the system has no storage device.
1
,
13

Intermittent cloud cover can
cause
weakening

of highly concentrated receiver source flux. Sensible
energy

storage

in
single
-
phase materials was proposed to allow a
cylindrical absorber element not only absorb
the energy but also store it in its mass, thus reducing the amplitude of cloud cover
transi
ents.
14
Although

this design only allows short period energy storage, potential longer time

storage technology would make parabolic dish more
appealing
.

Dish/engine system schematic.

The
dish that
follows the sun on two axes focuses

the sunlight
onto one single point on a receiver posed right in
front of the mirror.



Stirling Energy System Inc.

s 300 M

first commercial one in California


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Cos
t
s

and Rates
O
ne dish costs
around

$250,000

averagely, depending on the
capacity of it.

O
nce production rates rise,
they could cost less
than $150,000
.

Southern Califor
nia Edison Electric Company
can
not give away the actual price per kWh, but they say it is well below the 11.33 cents seen
currently
.


More Designs

The Stirling engine produces electricity using
the heat gathered by the receiver directly.

Click here for an animat
ion on how it works:

http://www.keveney.com/Vstirling.html







Infinia’s Modular Solar Thermal Dish



$50 million investment



20
-
30% cheaper energy production than PV
cells



334 dishes
per 1MW of power



designed to be assembled with mass
produced parts that an auto parts supplier
could manufacture



each dish costs approximately $20,000



The History Of Solar Dishes




-
Solar dishes have been in use since ancient Mesopotamian times



-
Polished gold dishes were used to concentrate the sun and light altar fires



-
In the 17
th

century glass lenses were used to smelt iron, copper, and mercury



-
In the 18
th

century, concentrated solar power was used to heat ovens and furnaces



-
Supposedly the G
reek scientist Archimedes used reflective bronze shields to focus sunlight
at wooden Roman ships to set fire to them

Dish/engine system with
stretched
-
membrane mirrors
: this design
allows wind to pass through to minimize the
destructive force of wind. Picture from
Sunlab, Department of Energy




Solar Tower,

sometimes called Central Receiver System, has rings of small individual
flat mirrors (heliostats) surrounding a central power tower (up to 100
-
200 m), on top of which
sits a receiver that gathers the reflected radiation. The receiver contains a kind of fl
uid medium,
be it water, air, mineral oil, liquid metal, molten salt or diluted salt. The heated fluid goes to a
hot fluid storage tank (where excessive heat is stored) and then to a steam generator to
engender electricity. The medium is then reused, retur
ning to a cold fluid storage tank and
being pumped up to the tower again.
Solar

tower can reach the highest temperature of all
concentrator designs. The scheme of a
solar tower

plant is shown in Figure 3.


Scheme of Solar Two, a molten
-
salt power tower
system
15

Solar tower possesses a higher efficiency than parabolic trough power plants (approximately 20%
vs. 15%) resulting from its higher concentrating ratio and higher temperature. Therefore they are
expected to be more cost efficient than parabolic trou
gh power plants when producing at a
large scale (100
-
200 MW) in a longer run. Pilot projects, Solar One (later converted into Solar
Two) in the Mojave deserts in the U.S have demonstrated well
-
maintained functionality. They
use molten/diluted salt which co
uld maintain the heat energy for several days. A big challenge
for solar tower now is the high cost of the overall construction and operation, with the heliostat
and the rest of the system each accounting for half of the total cost.
16
,
17

Several more solar
tower plants are scheduled for installation in the Mojave Desert, California

America

s pilot solar tower project that has been proven to operate functually
--
Solar Two
,

In
Daggett, CA, 10 MWe, HTF/Storage Molten Nitrate Salt, 30 Acres

in size


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A fully operation
PS10 solar tower plant near Seville
,

Spain

that
can generate 10MW of electricity.

Expansion

into 20 MW will be completed January 2009, enough to power 11,000
homes.(guardian.co.uk)


Parabolic Trough Power Plant

(PTPP) consists of a solar field filled with
hundreds or thousands of
solar collector assemblies

(SCA). Each SCA is an independently
tracking parabolic trough solar collector consis
ting of four major subsystems
:



parabolic reflectors (mirrors)



receiver

tube



metal support structure



tracking system that includes the drive, sensors, and controls.

A
lso
in this page:



PTPPs in the U.S.



PTPP around the world



Land Rush in
the

Southwest

In parabolic trough collector, long, U
-
curved mirrors focus the rays of the sun into an absorber
pipe. The mirrors track the sun on one linear axis from north to south during the day. The pipe is
seated above the mirror in the center along the focal line a
nd has a heat
-
absorbent medium
(mineral oil, synthetic oil, molten salt etc.) running in it. The sun’s energy heats up the oil, which
carries the energy to the water in a boiler heat exchanger, reaching a temperature of about
400°C.
The heat is transferred

into the water, producing steam to drive turbine. A study
supported by Japanese government found an annually
-
averaged collector efficiency using
supercritical CO
2

as the working fluid, higher than water/vapor.
18


Schematic of a PTPP with a thermal storage

system

The
Shape
a
nd Material

of the parabolic troughs differ from different designs as
well. The collector is generally composed of one bent glass mirror, with either silver or
aluminum coated on the backside of the glass. The glass is about four
-
millimeter thick and low
in iron, maxi
mizing the reflectance of incoming sunlight (about 93.5% with silver coating
protected by multilayer paint). Although National Renewable Energy Lab (NREL) uses silver for
its collector and it has a higher reflectance, aluminum is also adopted by others for

its cheaper
cost and stronger resistance to erodent environment.
12

Most current solar thermal power plants uses a parabolic trough design called
Luz system

(LS
-
1,
2 and 3) collectors. Made from galvani
zed steel
to support its torque
-
tube structure, Luz
collector represents the standard design.
Solargenix Energy and NREL collaborated to
have developed a new collector structure that
uses extruded aluminum. Solargenix SGX
-
1
collector thus weights less than

steel design and
is easier to assemble and be aligned.
13,
19


A simpler design called
compact linear fresnel reflector (CLFR)
solar collector reduces the cost
significantly. It uses simple flat (or slightly curved) mirrors, an optical system originally
developed by French
engineer Augustin
-
Jean Fresnel
. It
weighs 3 kg/m
2
, only one third of parabolic trough
mirror.
20

It has a much lower concentrating
temperature, at
285
°C

(545
°F)

21
,
22
,
23

Ausra Inc.’s Fresnel
Principle technology, originally developed by founder
David Mills at Sydney University, currently can operate in
a $10
-
cent
-
per
-
KW range, about the same as the cur
rent
market price in terms of grid base load in the U.S.
24

In
October 2008,
Ausra just launched a 5
-
MW solar thermal plant in
Bakersfield
, California, with a
177
-
MW plant in planning.

The
Absorber Pipe
, also
called heat collection element
(HCE), is made up of a
Ausra

s 5
-
MW plant in Calf.
S
ource:
Ausra.com

Heat collection element (HCE)
used in Luz system
(Source: Flabeg Sola
r International)

the end of a Luz
-
2 collector

credit:

Henry Price

several
-
meter
-
long metal tube and mostly a glass envelope covering it. In between these two
usually resides either air or a vacuum to reduce convective heat losses and allow for
thermal
expansion
. A glass
-
to
-
metal seal is crucial in reducing heat losses as

well. The metal tube is
coated with a selective material (chrome black, cermet etc.) that has high solar radiation
absorbance (filters out infrared rays) and low thermal remittance (attracts more visible light).
The HCE is the core part that enables PTPP
to acquire high efficiency

(with only a 10% heat
losses).

25
,
26
.

Other supporting structures

of an SCA include pylons, drive, controls, collector
interconnect. Pylons are the foundations that hoist the mirrors; drive enables the collector to
track the sun. T
he local controller for each SCA, connected to a central computer, keeps track of
the drive and also watches out for any abnormal conditions. Collector interconnect are the
insulated hoses that link up the whole power cycle.
27
.

U.S. Parabolic Trough Power
Plant
s


11
Parabolic Trough Power Plants have been operating in the southwestern U.S. (9 of them in
California) since 1980s, producing roughly 420 megawatts of annual net output. The recently
completed
Nevada Solar One

PTPP has a capacity of 64 MW.

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Florida Power & Light is investing a 300 MW CSP plant, bigger than any existing ones.
28

It will
adopt Aus
ra Inc.'s compact linear fresnel reflector solar collector and steam generation system.
Spain has a layout of 1000 MW capacity for solar thermal power plants, the first 200 MW already
in place.
29

Despite the just launched Kimberlina concentrating solar ther
mal power plant in
Bakersfield, Calif.by Ausra Inc., Governor Schwarzenegger mandated a Solar Task Force of
implement 3,000 MW of new solar power by 2015. New Mexico has even outlined a CSP specific
task force.
30


(credit
: NREL for specific information of each of the plant click on
http://www.nrel.gov/csp/troughnet/power_plant_data.html

)

Solar Chimney Tower Plant



Prototype of the solar tower prototype plant at
Manzanares,

Spain
(
Schlaich, J. et al

31
)

S
chematic presentation of a solar
chimney
tower
1


“Hot air rises.” This is the most basic fact
employed in the design of
the gigantic
solar chimney
tower plant.

The spread
-
out solar collectors
receive the sunlight and act like a greenhouse
together with the ground. Air in the “greenhouse” is heated and pushed toward the turbines at
the bottom of the chimney at speeds of up to 70km/h (43.5 mi/h)
. The
buoyancy effect

created by the pressure difference from the air under the collectors and ambient

(surrounding/outside) air produces a driving force to make sure the air
moves fast
.

The size of the collector and area and the height of the chimney decide the capacity of t
he
electricity production. The larger the collecting area, the more air flow and heat it traps; the
higher the height of the chimney, the greater the pressure difference. This is called the
stack
effect

in physics.

Heat Ca
n Be Stored by
t
he Ground
.

The gro
und beneath the collector roof
absorbs the heat and re
-
radiates it during the night, therefore able to provide energy 24 hours a
day. Other uses for the space in between the roof and ground have been proposed, such as
dehydration of fruits or vegetables.


Principle of thermal energy storage with water
-
filled black tubes for additional thermal storage capacity.
This works better than soil alone as water as water’s heat capacity is five times larger than that of soil.
Also
heat transfer between water tubes
and water is much higher than that between ground surface
and the soil layers underneath.

(
Schlaich, J. et al

32
)


The
First Prototype Plant

was established in Manzanares, Spain in 1981, jointly
invested by German government and a Spanish Utility .
1
33

The

chimney is 194.8

m
eter
(639.1 ft)
in height and 10 meter(32.8 ft) in diameter ;

collector
zone(greenhouse)

of 244m(800.5ft) in
diameter. It pr
oduced an upwind velocity of 15

m/s(33.5mi/h)
, reaching a total output of 50 KW
.

It was set up mainly for experim
ental use to test different materials and other parameters. One
sections of the collector zone is actually used as a greenhouse to grow plants
.

Here is video clips
of the plant:

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A Future Plant
In 2002,
an
Australian
company EnviroMission acquired the permission
from the
government
to build

a 1000

m high by 7

km diameter
solar chimney

plant.
A

power
output of 200

MW
34

is expected. T
he greenhouse will use
heat enhancing properties materials
including glass, polycarbonate and polymer while the chimney will just be forged with
reinforced concrete.

It will
prevent over 900,000 tons of greenhouse
gases otherwise to be
created by fossil fuel plants.


In
Terms Of Conversion Efficiency,

the Australian SCPP project estimated that
they can utilize

about 0.5 percent,

or 5

W
/m² of 1

kW
/
m², of
the solar radiation the sun pours
onto the whole collecting area. It is a rather low conversion rate considering
the 15%
-
30% of
other concentrated solar power technologies (PTPP and Parabolic dish
respectively). But the
reliability of these calculations remains to be fu
r
ther investigated because of insufficient testing
data.

Click here for an animation of what it looks like!





http://www.enviromission.com.au/SolarTower%20Animation%20Metric.wmv

T
he
Bul
k
o
f
t
he Cost
o
f a

SCPP

falls on the initial construction of the plants
. It
involves relatively less sophisticated technologies and therefore very ideal for less developed
countries with optimal solar insolation and large area of unused inferior flat land.

Countries like
Botswana and Namibia have been looking into the possibility of investing such a plant.
Carbon
credits will also help reduce the overall leveled cost of the plant.
35

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 back
-
up systems are supported by fossil fuels, bu
t 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 de
signs.
36
,
37



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 thr
ough 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 HTF.
1,3



Two
-
tank sensible heat storage
38

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 oi
l 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 term
s of choosing the transfer medium.
1,2

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
-
temperatu
re HTF in the extra heat
exchanger. The now high
-
temperature ST flows
back to a high
-
temperature storage tank and the
Two
-
tank indirect thermal energy storage
system for Andasol 1 and 2.
T
he storage
tank is 10m in height and 37m in diameter.
The storage fluid is a mixture of 60%NaNO
3

40% of KNO
3

Credit: Flagsol

now low
-
temperature HTF moves on to the solar collector to start the power cycle again.
Despite the extra cost resulting from a second hea
t exchanger and smaller temperature
difference 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, t
he
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,
39

Andasol, for example, aims at a capacity of 1,010
MWh, equivalent to 7.5 hou
rs of full load operation.


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
non
-
flammable
and nontoxic and has been used in other
industries
40
. 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
.
41



Single
-
Tank Thermocli
ne

To further reduce the cost of the stora
ge 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. H
igh
-
temperature 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 materia
ls at the bottom
, creating thermal stratification of
the fluid.


Sandia
National Laborator
ies 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
.
2,3


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 system.
42

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 th
e 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
Figure 11
The German Aerospace Center
constructed a facility at the University of Stuttgart
for testing a concrete, thermal energy storage
system.

handle and has higher strength, it faces problems such as maintaining good contact between
the concrete a
nd 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 feasibility.

Despite its encouraging prospect, however, PCMs is challenged by the complexity of the system
itself, unstable life
span 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.
1,2,

43
,
44

Proposal of a cascaded latent heat storage

tank with 5 PCMs according to Dinter et al. (1991)


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.
45

"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 conjun
ction with Semicon West.
46

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.
47
An increasing number of major venture capital
also flows into researche
s that focus on more cost efficient solar thermal storage technologies.

Solar radiation

is a general term for the electromagnetic radiation emitted by the sun.
Solar thermal energy
captures

the
radiation and converts it into heat
to produce energy
.
Concen
trated Solar Power
utilizes the high temperature heat to generate electricity.

Photovoltaic systems
in contrast convert the radiation into electricity directly.
The sun’s waves
hit a photovoltaic cell and excite the electrons within layers of the cell. The excited electrons
jump back and forth, creating electricity. This electricity is captured by wires running through
the PV cells and sends the electricity into y
our home.

Unlimited Solar Resources



In one hour, enough sunlight
(
1000 Wh per m² = 1 kWh/m²)

falls on the earth to power the
world for an entire year



If 1% of the Sahara Desert were covered in solar thermal systems, enough energy would be
produced to power the entire world



Solar radiation

along with secondary solar resour
ces such as
wind

and
wave power
,
hydroelectricity

and
biomass

account for
99.97%

of the available
renewable energy

on
Earth.



53,727 mile
2

of
land in the American southeast that has no primary use today

has the
potential of
6,877,055 MW from solar power.
, (excluding land with slope > 1%, <
5
contiguous km2, and sensitive lands)
and a
ssuming 5 acres

per MW
.
48

Types of Solar Radiation



Diffuse Solar Radiation is the sunlight that is absorbed, scattered and reflected by all kinds
of particles in the air
(such as water vapor and clouds)
.




Direct So
lar Radiation is the
solar radiation that reaches the Earth's surface without being
diffused.



Direct
-
Normal Radiation

refers to the portion of sunlight that comes directly from the Sun
and strikes a surface at a 90
-
degree angle.



The sum of the diffuse and direct solar radiation is called global solar radiation.

Measurement




Insolation

is a measure of
solar radiation

energy received on a given surface area in a given
time. It is commonly expressed as average
irradiance

in watts per square meter (W/m²) or
kilowatt
-
hours per square meter per day (kW
∙h/(m²∙day)) (or hours/day). Direct estimates
of solar energy may also be expressed as watts per square meter (W/m
2
).



In
photovoltaics

it is commonly measured as kWh/kWp•y (kilow
att hours per year per
kilowatt peak rating).



Radiation data for solar water heating and space heating systems are usually represented
in British thermal units per square foot (Btu/ft
2
).


Some
History of the
Concentration Use of Solar Energy

Reclaim
the Su
n!

Ancient Greeks and Romans saw
great benefit in what we now refer
to as passive solar design

the use
of architecture to make use of the
sun’s capacity to light and heat
indoor spaces. Romans advanced
the art by covering south facing
building openings
with glass or mica
to hold in the heat of the winter sun.
Through calculated use of the sun’s energy, Greeks and Romans offset the need to burn wood
that was often in short supply.


*this page is contributed by Molly 11


Hampshire College and Ally 09


Hampshire College

MORE LINKS

Here are more links about concentrated solar power

t
echnologies

National Renewable Energy Lab in Golden, Colorado:

http://www.nrel.gov/csp/


U.S. Department of Energy, Energy Efficiency

and Renewable Energy

http://www1.eere.energy.gov/solar/csp.html


SolarPACES, an international cooperative organization, one of a number of collaborative
programs managed under the umbrella of the
International Energy Agency:

http://www.solarpaces.org/


Solar Energy Industrial Association, works to expand the use of solar energy and promote
research

http://www.seia.org/


The official website for the book “Profit from clean energy”

http://www.profitfromcleanenergy.com/index.asp


An encyclopedia of alternative energy and sustainable living

http://www.daviddarling.info/encyclopedia/AEalphindex/AE_categories.html


Solar Thermal Power Plants, Technology Fundamentals

http://www.volker
-
quaschning.de/articles/fundamentals2/index_e.html


Renewable Energy World, latest news on renewable energies where you can post your
resumes as well

http://
www.renewableenergyworld.com/rea/home













1

Pitz
-
paal, R. “How The Sun Gets Into The Power Plant”,
Renewable Energy: Sustainable Energy Concepts For The Future

Wengenmayr, R.; Buhrke, T. Eds. Wiley
-
VCH,2008 pp.26
-
33

2

“CST Research
-
Technology Basics”
National Renewable Energy Lab

http://www.nrel.gov/csp/technology_basics.html


3

Angelino,G., Invernizzi,

C.

Binary Conversion Cycles For Concentrating
Solar

Power Technology
Solar Energy

Volume 82,
Issue 7
, Jul 2008, Pages 637
-
647

4

C. H. Forsberg et al, High
-
Temperature Liquid
-
F
luoride
-
Salt Closed
-
Brayton
-
Cycle Solar Power Towers Journal of Solar
Energy Engineering
May 2007, Vol. 129
141
-
146







5

Müller
-
Steinhagen H, Trieb F. Concentrating Solar Power

A Review Of The Technology.
Ingenia

18, 2004

6

List of solar thermal power stations, http://en.wikipedia.org/wiki/List_of_solar_thermal_power_stations

7
Woody, T. The Southwest desert's real estate boom
CNN.com
July 11 2008
http://money.cnn.com/2008/07/07/technology/woody_solar.fortune/index.htm

8
Bureau of Land Management Initiates Environmental Analysis of Solar Energy Development
http://www.blm.gov/wo/st/en/info/newsroom/2008/may_08/NR_053008.html

9

Kennedy; C.E.
Advances in Concentrating Solar Power Collectors: Mirrors and Solar Selective Coatin
gs

National
Renewable Energy Laboratory
NREL/PR
-
550
-
43695, Oct 2007

10
Bowles J.,
Hearings to debate impact of solar farms on threatened species
The Press
-
Enterprise
http://www.pe.com/localnews/inland/stories/PE_News_Local_S_solar15.48dbdb9.html

11

Solarpaces.org http://www.solarpaces.org/CSP_Technology/docs/solar_trough.pdf

12

Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts,
Oct 2003
NREL/SR
-
550
-
34440

13

“Solar dish engine”
SolarPaces
http://www.solarpac
es.org/CSP_Technology/docs/solar_dish.pdf

14
Lund, K. O.A., Direct
-
Heating Energy
-
Storage Receiver for Dish
-
Stirling Solar Energy Systems,
J. Sol. Energy Eng.

Feb1996 Volume 118, Issue 1, 15

15

SolarPaces.org http://www.solarpaces.org/CSP_Technology/docs/so
lar_tower.pdf

16

“Learning About Renewable Energy: Concentrating Solar Power”
NREL
http://www.nrel.gov/learning/re_csp.html

17

Farret, F.A.; Simoes, M.G.

Integration of Alternative Sources of Energy
IEEE Press 2006 pp.112
-
127

18

Zhang, X.R., Yamaguchi, H. An
experimental study on evacuated tube solar collector using supercritical CO
2

Applied
Thermal Engineering
, 28 (2008) 1225

1233

19

Gee, R.C. and Hale, M.J.

Solargenix Energy Advanced Parabolic Trough Development
Solargenix Energy
Conference
Paper
NREL/CP
-
550
-
39206 No. 2005

20

Ford, G., CSP: Bright Future For Linear
Fresnel

Technology?
Renewable Energy Focus

Volume 9, Issue 5
,Sep
-
Oct 2008,
Pages 48
-
49, 51

21

“How Ausra’s technology works”, Ausra Inc
.,

http://ausra.com/technology/

22

García
-
Valladares, O.; Velázquez, N., Numerical
Simulation Of Parabolic Trough Solar Collector: Improvement Using
Counter Flow Concentric Circular Heat Exchangers

International Journal of Heat and Mass Transfer
2008.08.004

23

Inslee, Jay; Hendricks, Bracken,

Apollo's fire : igniting America's clean
-
energ
y economy

Island Press for economic and
social association, 2008 pp 84
-
87

24

“Corporate Overview” Ausra Inc,. http://ausra.com/about/

25

Farret, F.A.; Simoes, M.G.

Integration of Alternative Sources of Energy
IEEE Press 2006 pp.112
-
127

26

Wengenmayr, Roland;
Buhrke, Thomas
Renewable Energy: sustainable energy concepts for the future

Wiley
-
VCH,2008
pp.26
-
33

27

“CST
-
how it works”
SolarPACES
http://www.solarpaces.org/CSP_Technology/csp_technology.htm

28


300
-
MW Array and More Planned for Florida, California”
Engineering News

October 8, 2007 Pg. 14 Vol. 259 No. 13

29

“CSP project developments in Spain”
SolarPaces

http://www.solarpaces.org/News/P
rojects/Spain.htm

30

Jones, J. Concentrating Solar Thermal Power,
Renewable Energy World Magazine

Sep 2, 2008

31

Schlaich, J. et al Design of
Commercial Solar Updraft Tower Systems


Utilization of Solar Induced Convective Flows for Power Generation J. Solar Energy Engineering Feb 2005, Vol. 127

32

Schlaich, J. et al Design of Commercial Solar Updraft Tower Systems


Utilization of Solar Induced
Convective Flows for Power Generation J. Solar Energy Engineering Feb 2005, Vol. 127

33

Pasumarthi, N. and Sherif, S.A. Experimental and theoretical performance of a demostration
solar chimney

model

Part 1: mathematical model development,
J Energy Res

22

(1998), pp. 277

288.

34

http://www.enviromission.com.au/f
aqs/faqs.htm

35

Fluri, T.F. et al
Cost analysis of
solar chimney

power plants J.
Solar Energy

July 2008


36
“Parabolic Trough Thermal Energy Storage Technology”
NREL

http://www.nrel.gov/csp/troughnet/thermal_energy_storage.html#direct

37

“Thermal Storage”
U.S Department of Energy Efficiency and Renewable Energy






http://www1.eere.energy.gov/solar/thermal_storage.html

38

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

From The Sun
"
http://www.powerfromthesun.net/Chapter11/Chapter11.htm

39

Solar Power; Sunny Future For Parabolics In Granada And Nevada
Modern Power System
February 14, 2007

40

“National solar thermal testing facilities” Sandia National Laboratories
http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm

41

Taggard, S.,
Parabolic troughs: CSP’s quiet achiever

Renewable Energy Focus

Volume 9, Issue 2
, March
-
April

2008,
Pages 46
-
48, 50

42

“Solar Storage And Research Development”,
U.S Department of Energy Efficiency and Renewable Energy

http://www1.eere.energy.gov/solar/thermal_storage_rnd.html#storage_systems

43

Michels, H., Pitz
-
Paal
, R., Cascaded Latent Heat Storage For Parabolic Trough Solar Power Plants
Solar Energy

81 (2007)
829

837

44

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

45

Nava P, Herrmann, U.
Trough Thermal Storage Status Spring 2007
NREL/DLR

Trough workshop
-
Denver Mar 2007

46

Leopold, G., Solar thermal technology heats up
Electronic Engineering Times
August 2008 4 Pg 38

47

“ DOE to invest $35 million in concentrating solar plant projects”
National Renewable Energy Lab,

Sep 19, 2008
http://www.nrel.gov/csp/news/2008/634.html

48

Kennedy; C.E.
Advances in Concentrating Solar Power Collectors: Mirrors and Solar Selective Coatings

National
Renewable Energy Laboratory
NREL/PR
-
550
-
43695, Oct 2007