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