SOLAR TOWERS
SOLAR TOWERS
GUIDED BY PRESENTED BYDr VASUDEV M SREERAM NANDAKUMARPROFESSOR
14TH13FNITK MTECH THERMAL 2ND SEM
CONTENTSConcept of solar towerComponents of solar towerDesign
parametersEconomic analysisChallenges in indiaConclusion
WHY SOLAR ENERGY Solar energy is an important part of life and
has been since the beginning of time. Increasingly, man is learning
how to harness this important resource and use it to replace
traditional energy sources.Solar Energy Is Important as Clean
EnergySolar Energy Is VersatileFossil fuels, like gas and oil, are
notrenewable energy. They create massive pollution in the
environment.These fuels are expensive to retrieve from the earth
and they are expensive to use.Solar energy is a clear solution to
the problems faced by the use of fossil fuels. Energy reaching the
earth is incredible. By one calculation, 30 days of sunshine
striking the Earth have the energy equivalent of the total of all
the planets fossil fuels, both used and unused. [1]
CONCEPT OF SOLAR TOWERThesolar power tower, also known as
'central tower' power plants or 'heliostat power plants or power
towers, is a type ofsolar furnace using a tower to receive the
focused sunlight.It uses an array of flat, movable mirrors (called
heliostats) to focus the sun's rays upon a collector tower
(Reciever). [2]Heat Transfer Fluid (HTF) gains heat and transfers
this thermal energy to the power block to generate electric power.
[3]
The solar tower power plant mainly consists of two relatively
independent systems: solar collector and steam power generation
system.The solar collector system includes an array of heliostats
and a solar tower receiver, aiming to convert the solar radiation
into the high temperature thermal energy.The steam power system
comprises steam generator in the tower and power conversion heat
engine, whose purpose is to convert the high temperature thermal
energy into power output. [4]
CONVERSION CYCLESThere are commonly two conversion cycles that
are employed with a solar tower.[5]Molten salt cyclesDirect steam
cyclesMolten salt cycles feature a primary loop where cold molten
salt at 290C is pumped from a tank up to an external tube receiver
atop the central tower, heated up to 565 C and stored in a second
ground-based tank
Fig: molten salt based plant
The hot salt is then circulated through a steam generator to run
a conventional steam cycle, and goes back to the cold storage
tank.Given the amount of available molten salt, the storage
capacity may reach 15 hours and this way allow 24 hours of
operation on a summer day.Molten salt was chosen as the preferred
heat transfer fluid because it has good heat-transfer properties,
provides an efficient and low-cost thermal storage medium and
produces steam temperatures that are compatible with high tower-
efficiency Rankine-cycle turbines. [6]Molten nitrate salts are the
commonly used salts for thermal storage and power generation
application [7]
Direct steam cycles use no intermediate loop between the
receiver and the steam turbine: the feed water is directly pumped
and evaporated in a cavity tube receiver.The generated steam is
kept at saturation (e.g. 40 [bar], 250 [C] and is accumulated back
to the ground in a short-time buffer tank (45 minutes at half the
nominal power).It drives then the turbine, is condensed at about
0.06 [bar], 50 [C] and is pumped up in the tower again.The main
development opportunity for direct steam cycles is the generation
of superheated steam to allow the use of larger industrial turbines
and exceed 40% of cycle nominal efficiency. [5]
Fig: Steam based plant
NITRATE SALTSTEAMRECEIVER565 C550 C
PEAK FLUX ON RECEIVER1000 kW/m2
300 kW/m2
HOT STORAGE565 CDEPENDS ON TECHNOLOGYCOLD STORAGE290 C
DEPENDS ON TECHNOLOGYCONDENSER40 C HEAT REJECTION
40 C HEAT REJECTIONCOMPARISON BETWEEN MOLTEN SALT AND WATER
BASED PLANTS [7]SOLAR UPDRAFT POWER TOWER [8]Solar updraft tower
power plant (SUTPP, also called as solar chimney power plant is a
kind of device that produces buoyancy to drive air to ascend for
electricity generation.A conventional SUTPP consists of a circular
solar collector constructed on horizontal ground, a vertical solid
SUT situated at the centre of the collector, and turbine generators
installed at the collector outlet or at the SUT inlet.
In the solar collector, solar radiation passes through the
transparent roof and is received by the absorber, i.e., the ground
or an additional absorber laid on the ground, and thus the indoor
air is heated.Some heat is stored in the absorber when solar
radiation is strong during day time on sunny days. The heat is
released from the absorber when solar radiation is weak during
night time or on cloudy days. The density difference between the
warm air inside the SUT and the ambient air creates buoyancy that
acts as the driving force and is also called pressure potential.
The buoyancy drives the air to flow in the collector toward the SUT
base and rise in the SUT. Finally, the air current drives the
turbines powering generators to generate electricity. COMPONENTS OF
SOLAR TOWER [7]HELIOSTATS The heliostats field consists of a large
number of individual heliostats (from several hundreds to
thousands). Heliostats are mirrors that are managed by a dual axis
optical solar tracking system.The analog solar tracking circuit
controls two mechanical actuators that move a mirror plane on two
axes. The mirror plane will reflect the sunlight to a stationary
target during the day and then return to a preset morning position
after sunset.
Heliostat consists of:a sunlight reflector,a tracking unit with
the drive motor,the foundationthe electronic control systemDue to
economic considerations, large heliostats with areas from 100 to
200 m are applied in the current projects.
FACETED GLASS/METAL HELIOSTATS MEMBRANE TYPE HELIOSTATS
TOWER AND RECIEVER [7]The only receiver of central solar tower
power plant is located on the top of the tower. As support of the
receiver the tower is commonly with a height of 80 to 100m and made
of concrete or steel lattices.A higher tower is preferable for
bigger and denser heliostats field but it should to avoid the
shades or objects that block the sun.At the same time, the
technical factors, e.g. tracking precision and the economic
factors, e.g. tower costs should also be considered to determine
the height of the tower.The Receiver of solar tower power plant
transforms the solar energy collected by heliostats into the
thermal energy of working fluid.Water/steam receivers are the most
used receiver in solar tower power plants, e.g. in the early power
plant 10MW Solar One in the USA and in the world largest solar
tower PS20 in Spain.Meanwhile the molten salt receiver and open
volumetric air receiver are applied in some demonstrate
plants.MOLTEN SALT RECEIVERThe closed tube receiver system is
currently the favourite molten salt receiver system. In the closed
tube receivers the molten salt is pumped through the black coloured
receiver tubes and heated there.
Molten salt consists of sodium or potassium nitrate (NaNO3,
KNO3). In contrast to air, the molten salt has a much higher heat
capacity and can be directly used as heat storage medium. [7]
WATER/STEAM RECEIVER [7] The structure of the water/steam
receiver is essentially consistent with the previously described
molten salt tube receiver. Instead of the molten salt water is
evaporated in the receiver tube and possibly overheated so that the
steam turbine system is directly supplied with the saturated
steam.Most of them were provided by the two-phase flows
(water/steam) and the related difficulties in heat transfer and
material fatigue.In recent years the Spanish company Abengoa has
developed the technical mature saturated steam receiver and it is
applied in the solar tower PS10.A storage tank of saturated steam
was integrated into the system in order to ensure the continuous
operation during the time with insufficient or without solar
radiation.POWER CONVERSION SYSTEMS [7]
Rankine Cycle systemIntegrated Solar Combined Cycle System
(ISCCS) and other hybrid systemsStirling motor.The Rankine Cycle is
a mature solar only technology that provides a high solar
contribution. Meanwhile the ISCCS with a gas-fired hybrid facility
offers a low cost alternative for the solar powered electricity
generation. The Stirling motor is only implemented in the solar
dish-Stirling system.THERMAL STORAGE DEVICES [7]To store heat
energy in ST system, a choice can be made from several different
systems: solid salt, two-tank molten salt, thermocline, solid
materials (concrete), pressurized saturated water, etc. Currently
the most proven thermal storage technology is two-tank molten salt
system.The storage medium used in this system is a mixture of 60%
sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3), which has
been proved as a favourable combination. On sunny days, the heat
energy is transported by synthetic oil from the solar field to the
oil-to-salt heat exchanger and then this heats the salt in the cold
tank to 384C, which will then be stored in the hot tank. In the
evening or on the cloudy days, the salt mixture is pumped to the
exchanger and heats the oil to provide thermal energy for
electricity generation.SOLAR TOWERS AND THEIR SPECIFICATIONS
[3]
PS10 AND PS20 SPAINGEMASOLAR SPAIN IVANPAH TOWER USAACME SOLAR
TOWER INDIA[9]
Technology:Power
towerStatus:OperationalCountry:IndiaCity:BikanerRegion:RajasthanLat/Long
Location:2811 2.0 North, 7314 26.0 EastLand Area:12 acresStart
Production:April 2011Heliostat Solar-Field Aperture Area:16,222
mNumber of Heliostats:14,280Heliostat Aperture Area:1.136 mTower
Height:46 mHeat-Transfer Fluid Type:Water/SteamReceiver Inlet
Temp:218CReceiver Outlet Temp:440CTurbine Capacity (Gross):2.5
MWTurbine Capacity (Net):2.5 MW
DESIGN PARAMETERSOverall efficiency of conversion of solar to
electric energy [3]
Equivalent capacity [3]
The mirror area and land area per MWe of rated capacity varies
due to the number of thermal storage hours employed. Hence there is
a necessity to normalise the mirror and land area taking in to
consideration the number of hours of thermal storage.Assume a plant
with no thermal storage operates for 9 hours a day. Then a plant
with x hours of thermal storage have an increased capacity by a
fraction of (9+x)/9.
where x is the number of hours of thermal storage.
MIRROR AND LAND AREA COMPARED WITH EQUIVALENT CAPACITY [3]
PACKING DENSITY [3]The packing density of a plant is defined as
the ratio of mirror area to land area.It gives an indication of the
extent of utilisation of land.
The actual receiver area is higher than that of the calculated
area inorder to accomadate spillages,varying flux
distribution,thermal losses etc.TOWER HEIGHTIt has been seen that
there is no direct correlation between tower height and equivalent
capacity.
The ratio of farthest distance of heliostat to the tower height
for most plants fall in between 5.7 and 5.8
ECONOMIC ANALYSISINVESTMENT COST [7] Site development &
Infrastructure Heliostat field Receiver Tower & Piping Power
Block/Balance of Plant(BOP) Thermal Storage Indirect costsSite
development & Infrastructure [7]The activity typically involves
site selection & planning as well as land-disturbing tasks such
as clearing, excavating and grading.These costs also include the
land cost and construction costs for buildings (power house,
storage depot and administration building) and roads inside the
power plant and those connecting to the main roads.
HELIOSTAT COSTThe investment cost for the heliostat is composed
of the costs for heliostats (including drive and foundation), the
wiring, process control and assembly.
RECEIVER COST
TOWER AND PIPING COSTThe investment cost of tower is related to
its height and the figure given by Fichtner is calculated using the
following formula:
The investment cost of the tower is influenced by the price of
the construction materials, and therefore cost will be different
every year. The piping efficiency will increase due to larger
piping and shorter lengths per kWe in the large scaled project,
ultimately resulting in lower costs.
POWER BLOCK AND BALANCE OF POINT COSTThe power block and balance
of the plant costs include: the steam turbine and generator, steam
turbine and generator auxiliaries, steam generator, feed water and
condensate systems, condenser and cooling tower system, water
treatment system, fire protection, piping, compressed air systems,
closed cooling water system, instrumentation, electrical equipment,
etc.
THERMAL STORAGE COSTThe main components for the current two-tank
thermal storage system are the hot storage tank, cold storage tank
and piping. The advanced thermal storage concept, for instance the
direct thermocline molten-slat storage system can reduce the
thermal storage cost significantly.
TOTAL INVESTMENT COST
According to this table the total investment cost for solar
tower plant is currently from 4500 to 16900$/kW(Rs.28000 to
10lakh/kW). These costs are much higher than for the parabolic
trough power plant of 3000 to 6700$/Kw.
LEVELIZED ENERGY COST [7]Levelized Energy Cost (LEC) is defined
as the total cost of a system over its lifetime divided by the
expected energy output over its useful lifetime.LEC includes all
costs through the lifetime of a plant including the initial
investment, operations and maintenance, cost of fuel, and cost of
capital.It is a measurement of the cost of producing energy from a
technology and is an important parameter to gauge the commercial
viability of any electricity generation technology.The LEC is the
minimum price at which energy must be sold for an energy project to
break even.
crf : capital recovery factor Cinvest : total investment cost of
the plant CO&M : annual operating and maintenance costs Enet :
annual net electricity output
kd : real debt interest rate = 8%n : life time = 25 yearsA
capital recovery factor is the ratio of a constant annuity to the
present value of the total investment cost of the plant.The life
time is defined as the useful life of the major technology
components which are usually within a range of 20 to 30 years.
PAYBACK PERIOD [5][10]The payback period is the amount of years
needed to start being profitable, based on a given sale
price.Simple payback period N is given as N = Total cost/(Annuity
+O & M)N is the number of years, the annuity is the profit from
the sale of the electrical energy, O&M is the yearly operation
and maintenance costs and total cost is the total cost of the power
plant.NET PRESENT VALUENPV is the sum of the actualised values of
all expenses and incomes.NPV is preferred here to provide an
estimate of the total actualised amount earned until the end of the
defined plant lifetime.NPV = (P/A)*Annuity + (P/A)* O&M Total
cost
NPV in INR = 500 croresCHALLENGES IN INDIATECHNICAL As seen from
the r/h ratio the farthest heliostat can be as much a km from the
tower. When a sun ray is reflected from such a large distance ,it
is bound to undergo attenuation losses. These losses occur due to
the scattering and diffusion of reflected solar radiation by
aerosols,moisture content and dust in the atmosphere.Most areas in
India which have abundant solar irradiation (for example, Gujarat
and Rajasthan) are also areas which are prone to very high dust
factors.Dust in the atmosphere,not only causes attenuation losses
but also settles on the heliostats,thereby reducing their life and
efficiency.In these cases,maintenance of each heliostat (mirror
cleaning)is of prime importance and this is not an easy taskina
field with thousands of mirrors .OPPORTUNITIESMost parts of India
receive high solar resource, i.e., Direct Normal Irradiance(DNI)
almost through out the year, which has the potential to be
tapped.Thebest sites in India receive around 2100 kWh/m2/annum.
India receives around 300 sunny days in a year , thus enabling
harnessing of solar energy for the setting up of powerplants. It
can be seen from the map,that most parts of India enjoy a daily
solar radiationof 4.55 kWh/m2/day. Out of the3.28 million km2 of
land area that India spans,around18% receives solar radiation of
5.56 kWh/m2/day which is suitable for solar thermal power
generation.
LANDMicro-siting of wastelands has not been conducted for all
states in India. This needs to be performed in order to estimate
the potential or possibility of ST technology by assessing terrain,
soil, wind conditions etc.The wasteland locations have to be
coupled with matrices like sub-stations, water avail- ability and
road connectivity in order to identify suitable waste- lands for ST
installations. Approximately 0.4 million km2 of waste land is
available in India .Even if 1% of this waste land is utilised,the
potential for ST technology is around 105GWe. FINANCIALCosts of
waste land are relatively cheap in India and would be of the order
of about12% of the total capital costs . For zones which receive
high DNI in India,land banks have not yet been identified.
CONCLUSIONSolar tower technology has a vast potential for future
energy needs. If properly tapped we can eliminate the need of any
fossil fuels.However the technical expertise needed for this needs
more attention and researches have to be done to improve the
efficiency of each and every components so that the cost comes
down.India has a great solar resource to use ST technology. The
government has to take initiative to bring in more advanced
technologies and also have to introduce policies that can
financially assist ST projects.Area of research and can be the next
big thing in the
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in China,Institut fr Energiewirtschaft und Rationelle
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http://mcensustainableenergy.pbworks.com/
