BIOMASS
SUPPORTED SOLAR THERMAL
HYBRID POWER PLANT
INTRODUCTION
Biomass Supported Solar Thermal Hybrid Power Plant discuss about the use
of non- renewable energy cause environmental pollution and also they gets
depleted. Pollution free, freely available renewable sources are best alternative. Solar
power plants are widely used but
its main problem is variability. Biomass plants are also used but its fuel
availability in large amount is difficult. To overcome the disadvantages of
both power plants they are combined. By shared use of some of the equipments
effective cost is less than simple
addition. Effective operating hours and therefore overall
energy generation is higher. Hence
this can be considered
as technology for future.
Need of solar thermal power plant is increasing due to increased fuel
cost and environmental pollution. Working similar to conventional thermal power
plant. Steam is generated through solar thermal energy instead of fossil fuel.
BIOMASS
Biomass is a renewable energy resource derived from the carbonaceous
waste of various human and natural activities. It is derived from numerous
sources, including the by- products from the timber industry, agricultural crops, raw material from the forest,
major parts of household
waste and wood.
It is a biological
material derived from living, or recently living organisms. It most
often refers to plants or plant-based materials which are specifically called lignocellulosic
biomass.
As an energy source, biomass can either be used directly via combustion to
produce heat, or indirectly after converting it to various forms of biofuel.
Conversion of biomass to biofuel can be achieved by different methods
which are broadly classified into: thermal, chemical, and biochemical methods.
Wood remains the largest biomass energy source to date; examples include
forest residues (such as dead trees, branches and tree stumps), yard
clippings, wood chips and even municipal
solid waste.
numerous types of plants, including miscanthus,
switchgrass, hemp, corn, willow, sorghum, sugarcane, bamboo, and a variety of
tree species, ranging from eucalyptus to oil palm (palm oil).
Biomass can be converted to other usable forms of energy like methane gas
or transportation fuels like ethanol
and biodiesel. Rotting
garbage, and agricultural and human waste, all release methane gas also called
"landfill gas"
or "biogas". Crops, such as corn and sugar cane, can be fermented to
produce the transportation fuel, ethanol. Biodiesel, another transportation fuel, can be produced from left-over food products like vegetable oils and animal fats.
The biomass used
for electricity generation varies by region. Forest by-products,
such as wood residues,
are common in the United States. Agricultural
waste is common
in Mauritius (sugar cane residue)
and Southeast Asia (rice husks).
Animal husbandry residues, such as poultry litter, are
common in the UK.
The main contribution of waste energy
are municipal solid waste, manufacturing waste and landfill gas.
If the total annual
primary production of biomass is
just over 100
billion tonnes /year, and the
energy reserve per metric tonne of biomass is between about 1.5 – 3Kilowatt hours then biomass
could perhaps provide
only one tenth
of the approximate annual
150 Terra watt/hours required for the current world energy consumption.
SOLAR ENERGY
Solar energy is radiant light
and heat from the sun harnessed using a
range of ever- evolving technologies such as solar heating, solar photovoltaics,
solar thermal
energy, solar architecture and artificial photosynthesis.
It is an important source of renewable energy and its
technologies are broadly characterized as either passive solar or active solar depending on
the way they capture and distribute solar energy or convert it into solar power. Active solar
techniques include the use of photovoltaic systems,
concentrated
solar power and solar water heating
to harness the energy.
The development of affordable, inexhaustible and clean solar energy
technologies will have huge longer-term benefits. It will increase
countries’ energy security through reliance
on
an indigenous, inexhaustible and mostly import-independent
resource, enhance sustainability,
reduce pollution, lower the costs of mitigating global warming, and keep fossil fuel prices lower
than otherwise. These advantages are global.
The Earth
receives 174 petawatts (PW)
of incoming solar
radiation at the
upper atmosphere. Approximately 30% is reflected
back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum
of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with
a small part in the near-ultraviolet.
The total solar energy absorbed by the Earth's
atmosphere, oceans and land masses is approximately 3,850,000 exajoules per year.
APPLICATIONS OF SOLAR TECHNOLOGY
Solar energy refers primarily to the use of solar radiation for
practical ends. However, all renewable energies, other than geothermal and tidal, derive
their energy from the sun. Solar
technologies are broadly characterized as either passive or active depending on
the way they capture, convert and distribute
sunlight.
v Active solar techniques use photovoltaic
panels, pumps, and fans to convert sunlight into useful outputs.
v Passive solar techniques include
selecting materials with favorable thermal
properties, designing spaces that naturally circulate air, and
referencing the position of a building to the Sun.
Active solar technologies increase the supply of
energy and are considered supply
side technologies, while passive solar technologies reduce
the need for alternate resources
and are generally considered demand side technologies.
SOLAR THERMAL
ENERGY
Solar thermal energy is a form of energy and a technology for
harnessing solar energy to generate thermal energy or electrical energy for use in industry, and in the residential
and commercial sectors.
Solar thermal
collectors are classified as low, medium, or high temperature collectors.
Ø Medium-temperature collectors
are also usually flat plates but are used for heating water or air for
residential and commercial use.
Ø High-temperature collectors
concentrate sunlight using mirrors
or lenses
and are generally used for fulfilling heat requirements up to 300 deg C / 20
bar pressure in industries, and for electric power production.
The existing instability of fossil fuel cost and the
impact of environmental pollution from fossil fuel, solar thermal power is
being used to generate electrical energy as a great potential of renewable
energy technology. The Solar Thermal Power Plant (STTP) works like as conventional thermal power plant,
but instead of fossil fuel it uses solar thermal
energy as a heat for the generating steam.
SOLAR THERMAL
COLLECTORS
There are different methodologies available. Some types of methodologies
used for STPP such as parabolic trough Collector system, parabolic dish system,
and central receiver system.
TROUGH COLLECTOR SYSTEM
Parabolic Trough collector
In the above
diagram we can see that how a parabolic collector focuses sunlight in to focal
point. It is the most cost effective and reliable among the three.
It is straight in one dimension and curved as a parabola in the other two,
lined with a polished metal mirror.
The energy of sunlight
which enters the mirror parallel to its plane of symmetry is focused along the focal line,
where Heat transfer fluid (usually oil) runs through the tube to absorb the
concentrated sunlight.
Heat transfer fluid
(usually thermal oil)
runs through the tube to absorb
the concentrated sunlight. This increases the temperature of the fluid to some
400 °C. The heat transfer fluid is then used to heat steam in a standard
turbine generator. The process is economical and, for heating the pipe, thermal
efficiency ranges from 60-80%. The overall efficiency from collector to grid, i.e. (Electrical Output Power)/ (Total
Impinging Solar Power) is about 15%, similar to PV
(Photovoltaic Cells).
PARABOLIC DISH
SYSTEM
With a parabolic dish collector, one or more parabolic dishes
concentrate solar energy at a single focal point, similar to the way a reflecting
telescope focuses starlight, or a dish antenna focuses radio
waves. This geometry may be used in solar furnaces and solar
power plants.
Parabolic dish Collector
CENTRAL RECEIVER SYSTEM
In a central receiver system (also known as a power
tower), thousands of two-axis tracking mirrors (heliostats) track the sun and reflect
its light towards
a central point at the top
of a tower. The reflected suns energy is used to heat up a liquid inside of the
tower which in turn is used to run a steam engine. A form of molten salt is
normally used due to its high specific heat capacity. Currently, the largest
power tower plan exists in Spain, producing 20 MW (PS20). Several other projects are under development in the US, the largest
being the 150
MW Rice Solar
Energy Project proposed in
the Mojave Desert.
Central receiver system
WORKING PRINCIPLE OF SOLAR
THERMAL POWER PLANT
Solar energy consists of thermal radiation emitted by the sun. The
radiation of the sun which reaches
the Earth’s atmosphere is called solar irradiance. Solar technologies can use the un-scattered solar irradiance called direct irradiance or
“beam” irradiance and the remaining and
scattered irradiance diffuse
irradiance. Both irradiances together are in sum the global solar irradiation. This high irradiance is
used in concentrating solar power plants for electricity generation by optical
concentration of solar energy to obtain high-temperature fluids or materials to
drive heat engines and electrical generators.
The production of electricity from solar radiation is a direct process.
Solar energy is not very dense, it is necessary to concentrate it to produce
exploitable temperatures usable for the production of electricity. The
radiation may concentrate on a point or on a line, where thermal energy
is transferred to the heat transfer fluid.
The intensity of concentration is defined
by the concentration factor, the more this one is higher, the more reached
temperature will be important.
The existing instability of fossil fuel cost and the impact of
environmental pollution from fossil fuel, solar thermal power is being used to
generate electrical energy as a great potential of renewable energy
technology. The STPP works like as conventional thermal power plant,
but instead of fossil fuel it uses solar thermal energy as a heat for the
generating steam. There are different
methodologies available. Some types of methodologies used for STPP such
as parabolic trough Collector system, parabolic dish system, and central
receiver system.
Among solar thermal power plants which are operating on medium
temperature and using the line focusing parabolic trough collector technology
at a temperature of about 4000C have established to be the most cost effective and reliable. The parabolic trough
collectors have been used to
heat the absorber tubes which are contained a synthetic oil or water.
The synthetic oil is used for producing super-heated high pressure steam
which drives the turbine to generate electrical energy. The parabolic dish
focus systems reflect light in to a receiver
at dish’s focus and the maximum temperature is generated is 8000 - 10000 C.
The parabolic
dish system tracks the sun by rotating about two axes and the sun’s rays are brought to a point focus. The diagram of this typical
solar thermal plant is shown in Fig. 4.
Diagram of Parabolic Dish STPP
1---- PD’S 5----
Steam Generator
2---- Receiver 6---- Turbine
3----
Heat
Exchanger 7---- Alternator
4, 4`---- Heat Exchanger 8----
Condenser
A working fluid (oil/toluene/helium) flowing through a receiver at the
focus is heated and this heat is used for producing
steam that drives
a prime mover to generate
electricity. This parabolic
dish system is also used to drive stirling engines to generate electrical
energy. The parabolic dish solar thermal power plant contains
parabolic dish, receiver,
heat exchanger, heat storage systems, steam generator,
steam turbine, alternator, and condenser.
In central
receiver STPP, solar radiation reflected from arrays of large mirrors which are
called heliostats is concentrated on a receiver situated at the top of a
supporting tower. A
molten salt is used as working fluid and medium of
thermal storage system. The sun rays are reflected using several hundreds
of heliostat tracking
mirrors and the maximum temperature is generated in the central receiver is 5000-20000 C. A working
fluid (molten salt, water, air) flowing though receiver absorbs the
concentrated radiation is used for producing steam and this steam drives the
steam turbine to generate electrical energy.
The characteristics of solar intensity, energy stored in the hot tank,
and electric power output as functions of time of day are shown in Fig. 5. The
STPP starts collecting thermal energy after sunrise and stores it in the hot
tank throughout the day.
Characteristics of STPP with molten salt central
When the field energy production is insufficient compared to the
operating schedule, the storage energy is used balance out the missing from the
field. Due to the storage system, power output from the alternator remains
constant through fluctuations in solar intensity and until all of the energy
stored in the hot tank is depleted. Since the storage capacity is limited, the
STPP cannot supply electrical energy continuously. After sunset or in the case
of insufficient solar radiation, the storage energy will be used in electricity
generation until the depletion of storage energy. So energy storage plays an
important role for the successful operation of STPP, and working fluids are
reliable media to be the key of the cost effective energy storage system.
PROPOSED
STPP FOR STABLE AND CONTINUOUS
OPERATION
BIOMASS SUPPORTED SOLAR THERMAL HYBRID POWER PLANT FOR CONTINUOUS ELECTRICITY GENERATION
STPP can operate when the sun is shining. But for any type of power
plant, it is desirable to be operated continuously. For STPP, solar thermal
storage system can contribute to operate continuously in generating electrical energy.
In the case of insufficient solar irradiation and after day time, a large amount of budget will be required for the building
of large storage system. Under the circumstances a special arrangement can
contribute to run the STPP continuously.
The biomass supported STHPP can contribute to generate electrical energy
continuously after sunset or in the case of disappearance of solar radiation.
The diagram of proposed hybrid plant is shown in Fig. 6. In the case of
insufficient solar irradiation, biomass plant contributes to operate STHPP for
continuous operation.
Solar thermal power plant with an auxiliary boiler
1---- Receiver 6---- Turbine
2----
Auxiliary
boiler 7---- Alternator
3----
Heat
Exchanger 8----
Condenser
4, 4`---- Heat Storage 9---- Deaerator
5----
Steam
Generator 10----
Steam Storage
The fuel will be provided from the biomass plant to heat water of
auxiliary boiler, producing steam which will drive steam turbine to generate
electricity. The auxiliary boiler ensures to maintain
rated parameters of steam of the STHPP.
The characteristics of STHPP are shown in Fig. 7.
energy
can be used for a short period and for a long period biomass fuel can be used
for the continuous operation of the hybrid plant.
CONTRIBUTION OF THERMAL ENERGY STORAGE SYSTEM
One of the major problems associated with the utilization of solar energy
is its variability. For this reason, most applications require
some type of energy storage
system. The purpose of such a system is to store thermal energy
in well insulated container with glass wool,
mineral wool or polyurethane when it is in excess of the requirement of an
application and to make it available for extraction when the supply of solar
energy is absent or inadequate.
At present, insulation with ceramic fiber mattress protected by aluminum
sheeting is being used to increase solar thermal storage efficiency. Moreover,
stainless steel liner corrugated both transversely and longitudinally, the
shell consisting of carbon steel sheeting, a covering isolating bricks are
being used in building of large storage system.
A potentially important
advantage of STPP systems is that thermal
energy is, relatively easier and less-costly to store
compared to electrical or other forms of energy. Cost-effective storage will
enable a high penetration of solar thermal energy into markets.
THERMAL STORAGE
There are three
basic approaches to storing thermal energy:
·
Heating a liquid or solid which does not melt or otherwise change
state during heating. (This is called “sensible-heat” storage, and the amount of energy stored
is proportional to the
system’s temperature.)
·
Heating
a material which melts, vaporizes, or undergoes some other change of state at a
constant temperature. (This is called “latent-heat” storage.)
·
Using
heat to produce a chemical reaction ‘which will then release this heat when the
reaction is reversed.
Sensible heat storage is most commonly used in current parabolic trough
and central receiver STHPP systems where hot heat transfer fluids such as
water, oils or molten salts are stored in tanks or underground caverns.
The thermal storage system of STHPP consists of two distinct tanks. The
first tank contains the working fluids leaving the solar reflector field during
normal working condition
of STHPP, where the second tank contains the cooled
working fluids leaving from the steam generator after performing their
job. The capacity
of both tanks must be sized so as to hold the entire quantity of working fluids
present in STHPP, in case of unavoidable disturbance of one tank it must be fed the contents to the other tank. So, the
specification of both tanks must be identical to work alternate. Working fluids
with high temperature are collected from receiver and then passed through the
heat exchanger to the thermal storage container. Under running condition of
STHPP, thermal energy is passed to storage if the temperature of heat source
higher than heat loads and thermal energy from the storage will be passed to
heat loads if the temperature of heat source lowers than heat loads. The
present capacity factor for biomass plant is approximately 90% and efficiency
32-34% and with solar thermal storage system, capacity factor is 65% and
efficiency 30%.
BIOMASS
FOR STPP
In biomass plant the main component is the boiler, which is where biomass
is burnt to generate superheated steam. Energy generated in the combustion
process is used to heat the feed water (economizer), generate steam
(evaporator) and superheat the steam to its final temperature and pressure (superheater).
Each biomass plant consists from the same principle and refers to
renewable energy coming from biological material such as plants, trees, manure,
bagasse, wastes, sewage gas, landfill gas etc. The effectiveness power generation requires
availability of biological materials and their transportation. It is important decision to
integrate the correct sources of energy for optimal power supply to the region.
Practically, the integration of alternative sources, such as biomass into
the other regional energy planning depends on the geographical characters of
the region, the transportation, and transmission and distribution lines. The
optimal sites and sizes of biomass plant will depend on availability of biomass
materials and their storage, and transportation of materials. The biomass fuel
is transported from the fuel storage to the boiler within the fuel system. The fuel is burned in the auxiliary
boiler and steam is generated. The combustion gases from the auxiliary boiler are
conveyed to the flue gas cleaning system and then cleaned gases are released to
the atmosphere. The steam flows to the steam turbine produces mechanical
energy. The shaft of the steam turbine
and the alternator are coupled, so the alternatADVANTAGES OF BIOMASS
SUPPORTED STHPP
In sunny day, solar thermal power plant generates reasonable amount of energy
by using the solar radiation. During sunny day there is no need of biomass .In
sunny day solar radiations are absorbed by the solar collectors and the
absorbed radiation is used to generate rated steam. This steam is used to run
the turbines. But when the parameters of steam cannot reach the rated value
in insufficient solar radiation, then biomass plant starts to operate. During this period the steam generated
from biomass is mixed with steam from the receiver so the mixed steam reaches to the rated value. After sunset STHPP
starts to use the storage
energy to generate steam and
in the case of deficiency of storage energy, the auxiliary boiler of biomass plant reheats the steam to
maintain the rated value of steam parameter. In the absent of solar radiation
or depletion of storage energy, the biomass plant starts to operate
independently to generate electricity stably and continuously.
The main advantage
is that auxiliary boiler uses biomass fuel, producing steam for
continuous and stable operation of STHPP and ensures 100% uses of renewable sources
to generate electricity. So, no consumption of fossil fuel is needed in
electricity generation that will
contribute to reduce
the environmental pollution
and increase the efficiency of the STHPP.
From the study, it can be estimated
capital cost of proposed biomass
hybrid power plant with thermosolar which is shown in
Table 1
ESTIMATED COST OF PROPOSED
BIOMASS SUPPORTED SOLAR THERMAL
HYBRID POWER PLANT
|
Sources
|
Estimated cost /kw (in USD )
|
Estimated combined cost/kw (in USD)
|
|
Solar Thermal Plant
|
3,149
|
5,018
|
|
Biomass Plant
|
1,869
|
Table 1 Estimated cost of
proposed biomass supported solar
thermal hybrid power plantor converts electrical energy from mechanical energy.
CONCLUSION
By using solar thermal energy it is possible to generate power in large scale in STHPP.
It is essential to generate
electrical power from 100% renewable sources, so that environmental
pollution can be reduced considerably and it can reduce use of fossil fuel.
Based on the above discussion, the following conclusions are made:
·
The proposed system
will ensure to use 100% renewable sources
to generate electricity and encourage using other
renewable sources for the different type of power plants.
·
The storage system of STHPP contributes to save
biomass fuel that will increase the overall efficiency of the plant.
·
The biomass supported solar thermal hybrid
power plant is the optimal
operating mode of electrical
energy generation.
·
The proposed biomass supported STHPP is one of
the most important steps of generating electrical energy for the stable and
continuous mode of operation.
·
The combined operation of biomass and solar
thermal plant could be one of the most important modes to use the solar thermal
and biomass power in the future.
