Post on 31-Jan-2018
Producing Power from exhaust heat more efficiently
with innovative organic rankine cycle solutions
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introduction to orc
“ in both industrial and power generation process, the efficiency range of energy conversion is usually between 20% and 50% with a great amount of heat at a variety of temperatures, lost through exhaust gases or cooling stream. in parallel, the increasing electricity price combined with the introduction of environmental regulation leads to a higher cost for electricity production and purchase.
1 orc flow diagram
“ under this scenario of increasing electricity price and the tightening of environmental regulations the demand for waste heat recovery is continuously growing in popularity.
This paper focuses on a method to recover part of the waste heat
from industrial processes and engines and to produce electricity
using low-medium enthalpy sources through an Organic Rankine
Cycle with the innovative Radial Outflow Turbine (ROT) developed
by Exergy.
Organic Rankine Cycles are Rankine cycles operating with
organic fluids selected on thermodynamic criteria and
techno-economic optimization.
An intermediate circuit extracts the heat from the thermal source
to evaporate the working fluid in a heat exchanger. The organic
fluid is expanded in a turbine to produce electric energy.
At the turbine outlet, the fluid is liquefied releasing heat to a cold
medium (air or water) and then is pumped at the maximum cycle
pressure.
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“ considering the small number of components, the system can be installed rapidly and with a small footprint.
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Where there is a high temperature of the working fluid at the
turbine outlet, the use of a recuperator to pre-heat the liquid can
dramatically increase the efficiency of the overall system; this
solution is usually implemented for higher temperature sources.
The fluid selection mainly depends on the application and the key
characteristics of the energy source. The use of organic fluids,
such as refrigerants or hydrocarbons, enables efficient and cost-
effective solutions with advantages like:
vv Retrograde saturation curve that brings to super-heated steam
at turbine outlet.
vv Possibility to choose fluids in a wide range of critical
temperatures and pressures to optimize the heat exchange
process with the hot and cold source.
vv Possibility to choose in a wide range of fluids that can be non-
flammable or non-toxic for particular hazard restrictions.
The main restrictions related to the working fluids are:
vv Environmental limits like Global Warning Potential or Ozone
Depletion Potential
vv The explosive or high flammable nature of hydrocarbons.
vv The high cost for some organic fluids.
“ the turbine and cycle know-how combined with the fluid selection allows to perform an optimized design of the plant archiving high values of efficiency.
Based on the Exergy experience, a list of the typical fluids adopted
in our applications is reported in Table 1.
The main components of an ORC system are:
vv The turbine: it’s the key component of the entire plant. It
expands the working fluid producing mechanical energy that is
converted in electricity by a generator directly coupled with the
turbine shaft.
vv The heat exchangers: the working fluid flows through the heat
exchangers, extracting the heat from the intermediate thermal
fluid. Shell and tube heat exchangers are usually applied but they
can vary geometry and configuration depending on the energy
source and the total thermal input.
vv The feed pump brings the organic fluid from the condensation
pressure to the maximum pressure of the cycle. The pump can
be horizontal or vertical driven by an electric motor at variable
rotating speed.
vv The condenser: with the direct air to fluid heat exchanger, the
organic fluid is cooled and liquefied before entering the pump.
The use of air eliminates the requirement for water to treatment
and make up. It is possible to use also a water cooled condenser.
Considering the small number of components, the system can
be installed rapidly and with a small footprint.
To recover the heat from the source, a Waste Heat Recovery
Sytem must be provided. The use of an intermediate loop with a
boiler has three main advantages:
vv It is possible to recover exhaust gases with high concentration
of particulate or corrosive substances without decreasing the heat
exchanger efficiency of the ORC system. Only the boiler must be
built with adequate material to prevent corrosion and deposition.
vv It is possible to adjust the heat input without affecting the
industrial process or the engines only bypassing part of the
thermal fluid flowing in the loop.
vv When the ORC must stop for the periodical maintenance,
the industrial process or the engines can still run without no
interferences.
The solution here described is suitable to recover heat from a
variety of industrial process or engines like:
vv Cement Factories
vv Steel Mills
vv Glass Mills
vv Furnaces
vv Internal combustion engines
vv Gas turbines
vv Compressor stations.
main comPonents aPPlications limits of aPPlicability Price notes
hydrocarbons heat recovery flamable nature low unfit for small units because of the small volumetric flows
refrigerants heat recovery high cost low
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“ the application of the orc to heat recovery from industrial process ranges from temperature above 100°c and sizes up to 68mw approximately.
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steam vs. organic fluid
When the enthalpy level of the heat source (or the temperature)
is too low or the size of the application is too small to warrant the
complexity of a steam system, the Organic Rankine Cycle is the
most efficient and appropriate.
In fact, when compared to steam, organic fluids are characterized
by lower evaporation/condensation temperatures, at the same
pressure. This makes possible the design of turbines with higher
efficiency than steam turbines, at the same boundary conditions.
The turbine itself is characterized by an efficiency higher than 80%.
The application of the ORC to heat recovery from industrial
process ranges from temperature above 100°C and sizes up to
68MW approximately as show in Figure 2.
Compared to the traditional steam technology, the ORC has many
advantages including:
vv Fully-automated operation and easy maintenance, resulting
in minimal impact on the existing production facility in terms of
dedicated personnel or O&M costs.
vv Flexibility of operation and superior off-design performance
allowing electricity production at a reduced heat inputs.
vv High reliability and simple maintenance means low cost and
high operating hours during the year
vv Flexibility to choose the most appropriate working fluid
depending on the boundary conditions, environmental
requirements and features of the heat source (i.e. heat source
release curve, flammable or non-flammable); the solution is
completely customizable on client requests.
vv No need of make-up water for the cycles operation.
In addition to the advantages already mentioned, differently
from the steam, the shape of the organic fluids saturation curve
(complex moleculas) ensure that no liquid drops can inpact on
the turbine’s blades (since the expansion transformation occurs in
the vapor phase only), resulting in longer life for the turbine and
superior fluidodynamic efficiency.
2 orc aPPlicability limits comPared to steam cycles
3 comParison between orc and steam cycle on t-s diagram
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“ exergy’s orc solutions are unique in the marketplace as they utilize the proprietary technology, the radial outflow turbine (rot), which allows power plants to reach higher efficiency and flexibility when compared to competitors using axial turbines.
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exergy: about us
Exergy is the worldwide developer, engineer and producer of
Organic Rankine Cycle (ORC) systems with the innovative and
pioneering Radial Outflow Turbine.
EXERGY’s ORC solutions are unique in the marketplace as they
utilize the proprietary technology, the Radial Outflow Turbine
(ROT), which allows power plants to reach higher efficiency and
flexibility when compared to competitors using axial turbines.
EXERGY undertakes the entire product development and
manufacturing process of the ORC turbine and plant internally.
The process includes:
vv R&D and product development both for the turbine and for the
cycle components (special design of heat exchangers and package
arrangements)
vv Material testing and technology validation - pre and post
construction testing is standard practice
vv Engineering and project management - customized service for
all clients ensures tailor-made solutions every time
vv Design, manufacturing and assembly of the turbine and
packaging, with strong quality control procedures
vv After sales service and maintenance to ensure reliability and
ongoing assistance for the life of the product.
EXERGY has a global outreach and concentrates on the markets
that show the greatest development opportunities, such as
Turkey, North and South America, and Asia, particularly Indonesia.
In the next few years, Africa is set to become one of the most
interesting regions for energy exploitation and expansion.
4 exergy facilities in olgiate olona
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the radial outflow turbine
(rot)Designed and patented by Exergy, the ROT is an innovative and
pioneering technological breakthrough and it is the first turbine
of its kind to be utilized in an ORC system. The ROT, different
from the axial and radial inflow configurations, is capable of
converting the energy contained in the fluid into mechanical
power with higher efficiency than competing technology on
the market (over 80%).
In ROT the fluid enters axially and is deviated by 90 degrees with
a nose cone. The fluid expands radially through a series of stages
arranged on a single disk. At the end the fluid is discharged in a
radial diffuser to recover the kinetic energy and then is conveyed
to the recuperator or the condenser.
“ the new frontier of exergy r&d department is the addition of a final axial stage at the turbine to ensure a higher enthalpy jump with same dimensions and weight.
The ROT has several advantages:
vv Inherent higher efficiency than an axial turbine, therefore
allowing up to 20% more power. This high increment in
performance is related to:
1. Up to 6 stages on a single disk reducing the size and length of
the turbine compared to anaxial turbine
2. Less tip leakage and disk friction loses
3. Minimum 3D effect thanks to the low blade height and low
blade height variation
vv No gearbox thanks to the low rotational speed (3000-1500
rpm) that allows a direct coupling with a generator with 2 or 4
poles and more reliability for the overall system .
5 the radial outflow turbine
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Figure 6 helps to better understand the mechanical differences
between the ROT and the axial turbine. The stages are arranged
radially in the ROT, the distance between the center of gravity of
the machine and the bearings is minimized (see distance B). This
leads to the possibility to place up to 6 stages on a single disk,
while the actual limit for the overhanging axial turbine is 3 stages.
This allows to exploit higher enthalpy drops while reducing the
vibrations and enhancing the life of the bearings (the maintenance
is twice as cheaper as competing technologies).
6 axial turbine vs. rot
vv Less limitation on cycle pressure and on blade manufacturing:
technological and manufacturing limits related to the minimum
blade height of the first turbine stage and the maximum blade
height of the last stage can be overcame with the ROT, therefore
Exergy ROT allows for designing cycles with lower condensation
pressures and higher evaporating pressures, extending the range
of application.
vv Large increase in volumetric flow achieved without the need
for extreme changes in the blade height, the stages are arranged
radially instead of axially and therefore the accommodation of the
volumetric flow is reached also by the increasing of the fluid flow
area.
vv Simpler construction technology: compared to the twisted
blade of the axial turbine, blades are straight since the velocity of
the fluid across the stage do not experiment a great modification
in values and direction.
vv Less vibrations, therefore longer life for the bearings.
vv Easy and low cost maintenance: it is possible to easily remove
the mechanical group (a module containing the bearings, oil
lubrication system and seals) of the turbine without the need of
draining all the organic fluid away from the cycle, by simply sliding
back and locking the rotor disk towards the cage. This dramatically
reduces the downtime of the plant, since the mechanical group
can be removed and the bearings can be substituted in 6 hours
compared to one week downtime of axial turbine and competing
technologies.
“ orc systems are suitable to recover heat from cement factories, steel mills, glass mills, furnaces, internal combustion engines, gas turbines, compressor stations.
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The ORC units produce enough energy to cover the electric
consumption of the filtering system exploiting a reduction of
costs on the overall industrial process.
Considering a total revenues calculated on 8000 hours of plant
operation and a price of the electricity for industrial customer
in Italy equal to 0.22 €/kWh, the payback time of the solution
installed could be calculated in first approximation as:
where Reveneus = Pnet * Electricity price * hours of operation.
The payback time for a plant is equal 3.5 years.
The plant now produces electricity with zero extra emissions
saving 4,692 tons of CO2 and 1,499 tons of oil equivalent per
year.
The high efficiency of the Exergy product reduces the plant’s
energy consumption and carbon footprint contributing positively
to the environment.
case study 1
heat recovery from steel mill
In traditional fume treatment from arc furnaces, in order to
protect the downstream filtering system, the off gases are cooled
down by water-cooled panels. The traditional systems employ
water to absorb heat from the fumes, which is then cooled by air
coolers. The solution proposes an ORC system that cool down the
water extracting heat and producing electricity.
The main advantages of this system are:
vv Simple completely automatic machine
vv No influence on the steel making process
vv Minimal O&M costs
vv Choice of the right organic fluid to get the best heat recovery
and conversion efficiency
vv Suitable for frequent start&stop cycle
vv Good off-design performances.
Plant at glance
date of imPlementation sePtember 2014
aPPlication heat recovery (steel mill)
model used ePsd 110
flow of Pressurized water 250 t/h
temPerature of Pressurized water 200 °c
electrical generation net 860 kwe
water or cooling agent cooling water from cooling towers
generator voltage lv
installation outdoor
PBT = C A P E X
Revenues - Opex
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case study 2
heat recovery from internal combustion engine
This ORC solution recovers the waste heat from internal
combustion engine exhaust gases (twelve engines burning
natural gas). Here, an intermediate oil circuit cools down
the engine exhaust gases from 489°C to 140°C recovering
approximately 10 MWth of low temperature heat.
The ORC module produce 2.15 MWel of electricity. An air cooled
condenser cools down the organic fluid. This means no make up
water or water treatment system for the condenser.
With the same algorithm as before, the calculated payback time
for this solution results 3 years.
Plant at glance
date of imPlementation november 2015
aPPlication heat recovery (internal combustion engines)
model used ePs 240
flow of exhaust gases 95 t/h
temPerature of exhaust gases 489 °c
electrical generation net 2150 kwe
cooling system air cooled condenser
generator voltage mv
installation outdoor
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Combining the ORC with the innovative Radial Outflow
Turbine by Exergy, the solutions proposed reach high values
of efficiency for low-medium enthalpy sources.
All the solutions are designed to best fit the client requirements
and recover the maximum amount of heat. The simplicity of
the plants and the high reliability brings to low operating and
maintenance costs decreasing the payback time of the plant.
The plant produces electric energy with zero emissions
converting waste heat in a valuable resource lowering the
cost of the industrial process.
There are many world wide applications where ORCs would
benefit all stakeholders and Exergy are particularly focused on:
vv Waste heat streams within refineries
vv Waste heat from industrial applications
vv Engines run on natural gas or diesel, particularly those in
remote locations.
Exergy supports green technologies, whatever their form,
however it is clear that if more effort were made to maximize
the efficiency of existing installations, be them processes
or generation, by the use of heat recovery, fewer new
power stations would be needed. This would reduce capital
expenditures, carbon output, land usage and fossil fuel
consumption – a solution with true worldwide benefit.
conclusions
“ exergy supports green technologies with the aim to reduce capital expenditures, carbon output, land usage and fossil fuel consumption – delivering solutions with true worldwide benefit.
HEAD OFFICE
Via degli Agresti, 6
40123 Bologna (BO) ITALY
OPERATIVE HEADQUARTERS
Via Santa Rita, 14
21057 Olgiate Olona (VA) ITALY
Tel +39 0331 18 17 711
Fax +39 0331 18 17 731
www.ExERgy-ORC.COm
InFO@ExERgy.IT