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Waste heat recovery system

A waste heat recovery system (WHRS) on a ship is an arrangement of heat exchangers, turbines, working-fluid circuits, and electrical or mechanical machinery that captures thermal energy rejected by the main propulsion plant and converts it into useful steam, electricity, or refrigeration. In a modern slow-speed two-stroke marine diesel engine, roughly 50% of the fuel energy emerges at the propeller shaft; the remaining energy leaves the engine as high-temperature exhaust gas (approximately 25% of fuel input), charge air cooler heat (approximately 15%), jacket cooling water heat (approximately 5%), and radiation or miscellaneous losses (approximately 5%). Without a recovery system that heat is discarded overboard. A well-engineered WHRS on a large containership or bulk carrier can recover energy equivalent to 5 to 15% of main-engine shaft power, directly reducing the specific fuel oil consumption (SFOC) attributed to the voyage and improving the ship’s Carbon Intensity Indicator rating and Energy Efficiency Design Index figure. ShipCalculators.com provides dedicated tools for WHR design and assessment; the waste-heat recovery calculator and the WHR exhaust economiser steam generation calculator are the primary entry points. The ShipCalculators.com calculator catalogue lists the full suite of related tools.

Contents

Background and energy balance

The first systematic attempts to recover exhaust heat from marine engines date to the introduction of exhaust gas boilers (also called exhaust gas economisers, or EGEs) on steamships in the late nineteenth century, where steam from the uptake section of a funnel casing was used to reduce auxiliary boiler firing. The transition to diesel propulsion from the 1910s onward shifted the character of waste heat: a diesel exhaust is cooler than a steam-plant furnace gas but the engine also rejects heat through cylinder jackets, charge air coolers, and lubricating oil coolers, each at a different temperature level and mass flow rate.

The quantitative basis for understanding WHRS design on modern ships is the energy balance of the main engine. For a well-tuned slow-speed two-stroke engine at full load, approximately 48 to 52% of the lower heating value (LHV) of the fuel appears as brake (shaft) power. Of the remaining 48 to 52%:

  • Exhaust gas carries approximately 22 to 28%, depending on engine load, turbocharger efficiency, and scavenging factor. The gas temperature at the turbocharger inlet is typically 250 to 350°C; after the turbocharger turbine the temperature falls to 220 to 280°C at the EGE inlet.
  • The charge air cooler rejects approximately 12 to 17% of the fuel energy as the compressed charge air is cooled from approximately 170°C at the high-pressure stage compressor outlet down to approximately 45°C before entering the cylinders.
  • Jacket cooling water carries approximately four to six per cent of the fuel energy, at temperatures between 70 and 90°C.
  • Lubricating oil coolers reject approximately two to four per cent, at oil temperatures of 45 to 55°C.
  • Radiation and miscellaneous losses account for the remaining two to five per cent.

The temperature level of each stream determines the thermodynamic quality of the recoverable heat. The exhaust gas, at 220 to 280°C at the EGE inlet, is the most valuable stream because it can generate steam above 100°C and drive steam or organic Rankine cycle turbines. The charge air cooler operates at temperatures up to 170°C at the first-stage cooler but the bulk of its heat is rejected at relatively low temperatures. Jacket water at 70 to 90°C is useful for low-pressure steam, feedwater preheating, or low-temperature ORC working fluids but cannot drive a conventional steam turbine efficiently. Lube oil heat at 45 to 55°C is generally only suitable for heavy fuel oil preheating or, with a suitable working fluid, very low-temperature power cycles.

Detailed thermodynamic modelling of the exhaust stream uses the WHR exhaust economiser steam generation calculator, which calculates steam production rate from exhaust mass flow, inlet temperature, and pinch-point temperature difference. The Rankine cycle efficiency calculator provides the theoretical thermal efficiency of the conversion stage. The engine thermal efficiency formula page presents the heat balance equations from which recovery fractions are derived. Steam saturation conditions at various pressures are available on the steam saturation temperature and pressure formula page, and latent heat values for steam at given conditions are on the steam latent heat formula page.

Engine load and exhaust temperature variation

The energy balance described above applies at full load (100% MCR). As engine load falls, the energy fractions shift significantly. Exhaust gas temperature at the EGE inlet on a modern slow-speed engine drops from approximately 260 to 280°C at 100% MCR to approximately 180 to 210°C at 50% MCR, and exhaust mass flow decreases roughly in proportion to fuel consumption. The combined effect is that the thermal power available to the EGE at 50% MCR is approximately 35 to 45% of the full-load value - a steeper reduction than the engine output itself because both temperature and mass flow decrease simultaneously.

This load sensitivity is the central constraint on WHR system sizing. A WHR system designed for maximum output at 100% MCR will be underloaded during slow steaming and may not generate sufficient steam to meet heating demands without auxiliary boiler support. A system designed to break even at 70% MCR (a more typical operational load for many ocean trades) will leave potential savings unrealised at full load. The balance is struck by vessel type, operating profile, and the relative economic weight given to capital cost versus operating cost recovery.

The charge air cooler heat also changes with load: at part load, the high-pressure turbocharger compressor outlet temperature is lower and the charge air mass flow is reduced, so the high-temperature fraction of charge air heat is less available at low loads. In two-stage turbocharged engines, the inter-stage intercooler and the final-stage aftercooler both deliver heat at temperature levels that vary with boost pressure and ambient conditions.

The engine thermal efficiency calculator provides the brake thermal efficiency (ηBTE) from which the heat rejection fractions can be estimated: if ηBTE is known, the sum of all waste heat fractions equals 1 − ηBTE. The engine CO2 per kWh calculator links SFOC to CO2 intensity, the metric through which WHR gains flow into EEDI and CII assessments.

Charge air cooler heat recovery

The high-temperature section of the charge air cooler - the first-stage aftercooler in a two-stage turbocharged engine, where charge air enters at approximately 160 to 180°C - can in principle supply a WHR heat exchanger before the conventional seawater cooler. On some engine designs, this high-temperature charge air heat is tapped off to preheat fuel oil or feedwater, reducing the steam demand from the EGE. The benefit is modest compared with exhaust gas heat recovery but contributes to the overall system coefficient.

The low-temperature section of the charge air cooler, where charge air exits at approximately 45 to 55°C, produces heat at too low a grade for practical steam generation but is occasionally used to warm accommodation ventilation air in cold climates, reducing the HVAC steam demand. The HVAC sensible heat ratio calculator is relevant when assessing the interaction between charge air cooler heat and accommodation conditioning loads.

Exhaust gas economiser

The exhaust gas economiser (EGE) is the central component of almost every shipboard WHRS. It is a tube-and-shell or finned-tube heat exchanger installed in the engine uptake downstream of the main turbocharger, where it absorbs heat from the exhaust gas and transfers it to circulating water or a steam-water mixture. The exhaust gas boiler composite system calculator and its companion formula page model dual-pressure and oil-fired combined arrangements.

Steam generation and system pressure

On large slow-speed two-stroke engines, the EGE is typically a forced-circulation or natural-circulation steam generator operating at a gauge pressure of one to three bar, producing saturated or slightly wet steam at 120 to 134°C. This steam serves auxiliary heating loads: fuel oil heating to viscosity targets for purification and injection, cargo oil tank heating on tankers and bulk carriers, accommodation heating, fresh water generation, and HVAC supply air preheating. The steam generated at this low pressure level is not normally of sufficient temperature or quality to drive a steam turbine efficiently.

On vessels with dual-pressure WHR systems or composite exhaust-gas boilers, a second, higher-pressure section generates medium-pressure (MP) steam at 7 to 10 bar for power generation or feed to a steam turbine, while the LP section serves heating loads. The total thermal energy recoverable from the exhaust of a large slow-speed engine in the EGE alone is 15 to 25% of the exhaust enthalpy, or roughly four to six per cent of the fuel energy.

At full engine load, an EGE on a 14 MW main engine can produce three to five tonnes per hour of LP steam. When the engine load falls during slow steaming or manoeuvring, exhaust temperature drops significantly and steam production can fall below the minimum heating demand, requiring supplementary firing from an auxiliary oil-fired boiler. The system oil-fired boiler (Alfa Laval Aalborg) calculator sizes this supplementary boiler; its formula page at /docs/formulas/system-oil-fired-boiler-alfa-laval-aalborg provides the sizing methodology. The boiler efficiency direct method calculator provides an operational check on EGE thermal efficiency; the corresponding formula page explains the direct method calculation.

The boiler equivalent evaporation formula page converts actual steam output at a given pressure to the equivalent evaporation referenced to the standard condition of 100°C feedwater and 100°C steam, enabling comparison between different systems and operating conditions. Steam system safety considerations are supported by the boiler safety valve (ASME) calculator and the boiler safety valve formula page.

Fouling and cold corrosion

Two practical problems dominate EGE maintenance: soot fouling and cold corrosion. Both arise directly from the characteristics of the exhaust gas produced when burning heavy fuel oil (HFO).

Soot fouling occurs because HFO combustion produces particulate matter consisting of unburned carbon, metallic ash from vanadium, sodium, and iron in the fuel, and condensed heavy hydrocarbons. These particles deposit on the heat transfer surfaces of the EGE fins or tubes. At low engine loads, combustion is less complete, particulate concentration rises, and fouling rates increase. A significantly fouled EGE may lose 20 to 40% of its designed heat transfer capacity. The exhaust boiler fouling check calculator quantifies fouling-related heat transfer degradation from operating temperature differences; the methodology is detailed on the exhaust boiler fouling formula page. Soot blowing - steam or air jets periodically directed at the tube bundle - is the standard countermeasure and is typically automated on timed or manual-initiation cycles. Water washing at cold starts (with the engine shut down) is used for heavier deposits but requires careful drainage to prevent water ingress to the uptake.

Cold corrosion of the low-temperature sections of the EGE occurs when gas temperature falls below the sulphuric acid dew point, typically 130 to 160°C for HFO combustion products with 2 to 3.5% sulphur content. Below this temperature, sulphur trioxide (SO3) in the exhaust reacts with water vapour to form sulphuric acid (H2SO4), which condenses on heat transfer surfaces and corrodes carbon steel or low-alloy tube material aggressively. The problem intensifies in the LP section of dual-pressure EGEs and when operating in ECAs burning very low sulphur fuel oil (VLSFO, 0.5% S) at low load. Lower sulphur fuel generally reduces the acid dew point but does not eliminate it. Resistant materials (stainless steel, duplex alloys, or Cor-Ten cladding) or bypass dampers that limit gas flow through the low-temperature section at part load are the standard mitigation measures.

The dew point depression that accompanies a switch from HFO (3.5% S) to VLSFO (0.5% S) reduces the acid dew point by approximately 10 to 20°C, which can allow the EGE to extract more enthalpy from the exhaust by cooling the gas further before the dew point is reached. This is one secondary benefit of the IMO 2020 sulphur cap for WHR operators. However, switching fuel types also changes the particulate composition and can alter soot-blowing requirements.

Steam dumping and condensate management

Shipboard steam systems require careful management of the balance between steam generation and steam demand. When the EGE is generating steam at full load and the heating demand is low (for example, a tanker in ballast with no cargo heating requirement in warm tropical waters), the excess steam must be managed. Options include dump condensers (steam directed to a seawater-cooled condenser and condensate returned to the feed system), bypass dampers that divert part of the exhaust flow around the EGE, and in systems with a steam turbine generator, increasing electrical output to absorb excess steam.

Condensate management is critical for EGE reliability. Return condensate from steam heating services contains corrosion products, hardness salts, and dissolved oxygen if the condensate system is not properly maintained. The boiler blowdown conductivity check calculator and the boiler water hardness calculator support water quality monitoring to prevent scale formation in the EGE and steam turbine damage from carry-over of dissolved solids. Feed pump reliability is essential because the EGE can be damaged by loss of water level; the boiler feed pump (multi-stage centrifugal) calculator and its formula page size the primary and standby feed pumps with adequate suction head and delivery pressure margins.

Steam traps throughout the heating circuit require periodic checks to confirm correct operation. A failed-open trap vents live steam to drain, creating a direct and continuous fuel waste. The steam trap thermostatic or float formula page covers sizing and the steam trap fail check formula page provides the diagnostic method.

Power turbine and turbo-compound systems

Power turbine after main turbocharger

A power turbine (also called a compound turbine or free power turbine) is positioned in the exhaust stream after the main turbocharger. In a standard engine arrangement, the turbocharger turbine extracts sufficient energy from the exhaust to drive the compressor; the exhaust gas still leaves the turbocharger at 220 to 280°C and at a gauge pressure of 0.05 to 0.15 bar above ambient. This residual pressure and enthalpy is available to a power turbine.

The power turbine drives either a reduction gearbox connected to the propeller shaft (shaft power recovery) or a dedicated generator. On large slow-speed two-stroke engines, the power turbine output at full engine load is typically four to seven per cent of the main engine brake power, or 500 kW to 3,000 kW on engines in the 10 to 50 MW class. This is sufficient to supply a significant fraction of shipboard electrical demand from exhaust energy that would otherwise be rejected. The turbocharger axial/radial exhaust system formula page covers the thermodynamic analysis of exhaust gas extraction.

MAN Energy Solutions’ Turbo Compound System (TCS) is the best-known commercially deployed power turbine arrangement. The TCS power turbine is connected via a step-up gearbox to a synchronous generator. The system is matched to the specific engine and vessel operating profile; at typical sea-going loads (70 to 85% MCR), the power turbine operates within its design flow range and can contribute four to seven per cent of main engine power. MAN ES quotes a fuel saving of approximately 3 to 7% on fuel consumption attributed to the main engine at full load conditions. The turbocharger surge margin calculator is relevant to power turbine integration because the additional back pressure introduced by the power turbine must remain within the turbocharger compressor surge margin; the analytical method is given on the turbocharger surge margin formula page.

Grid synchronisation and dynamic load management

The output of a power turbine generator varies with engine load and exhaust mass flow. When the main engine load changes - for example during acceleration, deceleration, or heavy weather - the power turbine output changes in step, creating a transient imbalance between generator output and shipboard electrical demand. Managing this imbalance requires either a controlled load-shedding system, a frequency converter between the power turbine generator and the switchboard, or integration with a battery energy storage system (BESS) that absorbs transient surplus energy and supplies deficit power. The shaft generator output calculator and the shaft generator credit calculator quantify the electrical output and EEDI credit from shaft-connected generation.

Load dumping - the rapid diversion of power turbine electrical output to resistive dump loads (steam heaters, dummy loads) - is used when the ship’s demand suddenly falls below generation capacity. Proper sizing of dump loads and control logic is part of the power turbine commissioning scope.

Steam Rankine cycle power generation

Single-pressure steam turbine systems

Where a power turbine uses the exhaust gas pressure difference directly, a steam turbine system instead uses the EGE to raise steam, which then expands through a turbine to generate shaft or electrical power. The Rankine cycle converts the EGE’s thermal output from a low-grade heat stream into mechanical work with a thermal efficiency that depends on the turbine inlet pressure and temperature and on condenser temperature. The steam Rankine cycle formula page presents the cycle equations and the method for calculating net electrical output, condenser duty, and cycle thermal efficiency.

On a typical shipboard steam Rankine WHR system, the EGE raises steam at 7 to 15 bar gauge. The steam drives a small single- or multi-stage impulse or reaction turbine (output 500 kW to 4,000 kW on large vessels), which is coupled to a generator. The steam exhausts to a condenser, where it is condensed by seawater cooling. Condensate is pumped back to the EGE feed system. The overall Rankine cycle efficiency from exhaust enthalpy to shaft output is typically five to nine per cent. The steam Rankine cycle calculator computes the ideal cycle efficiency from the turbine inlet enthalpy, turbine exit enthalpy, condensate enthalpy, and pump work.

Combined EGE plus steam turbine systems on large containerships and tankers typically generate 5 to 10% of main engine power as electricity at full load, sufficient to supply all auxiliary electrical demand and eliminate the need to run auxiliary diesel generator sets during sea passages. The shaft generator credit calculator translates this electrical output into the EEDI credit defined by IMO guidelines.

Dual-pressure and combined power turbine and steam turbine systems

Some large containerships and very large crude carriers (VLCCs) install combined systems that integrate a power turbine and a steam turbine in sequence. The power turbine extracts work from the exhaust gas pressure after the main turbocharger; the exhaust gas then enters the EGE, which raises both MP steam (7 to 10 bar) for the steam turbine and LP steam (one to three bar) for heating services. The combined output of the power turbine and steam turbine represents 8 to 15% of main engine power.

Notable examples include the waste heat recovery systems installed on CMA CGM ultra-large containerships. The CMA CGM Bougainville (23,112 TEU, delivered 2018) and Antoine de Saint-Exupéry (21,000 TEU, delivered 2018) are equipped with dual-pressure WHRS supplying electrical power from both a power turbine and a steam turbine, reducing auxiliary diesel generator consumption by several megawatts at sea.

Organic Rankine cycle systems

Principle and working fluid selection

The organic Rankine cycle (ORC) is a variant of the Rankine cycle that uses an organic working fluid instead of water. Organic fluids have lower boiling points and lower latent heats than water, making them better suited to heat sources at temperatures below approximately 250°C - including the jacket cooling water, charge air cooler circuits, and post-turbocharger exhaust gas on marine engines.

Working fluid selection is central to ORC design. The key parameters are: normal boiling point, critical temperature and pressure, latent heat of vaporisation, specific heat capacity, fluid stability at maximum cycle temperature, environmental properties (global warming potential, ozone depletion potential), safety classification (flammability and toxicity), and cost. Common working fluids used or proposed for marine ORC applications include:

  • R-245fa (1,1,1,3,3-pentafluoropropane): a hydrofluorocarbon with a boiling point of approximately 15°C at atmospheric pressure and a GWP100 of approximately 1,030. Widely used in commercial ORC units but subject to phase-down under the Kigali Amendment to the Montreal Protocol.
  • R-1233zd(E) (trans-1-chloro-3,3,3-trifluoropropene): a hydrofluoroolefin with a GWP100 below five, promoted as a lower-GWP replacement for R-245fa with similar thermodynamic properties.
  • Pentane (cyclopentane or n-pentane): hydrocarbon working fluids with very low GWP but classified as flammable. Shipboard use requires additional fire and explosion protection of the ORC machinery space.
  • Toluene and cyclohexane: higher-temperature organic fluids suited to exhaust gas temperatures above 200°C. Toluene is stable up to approximately 370°C. Both are flammable.

The cycle configuration for marine ORC typically consists of an evaporator (heated by exhaust gas or jacket water), an expander (scroll, piston, or radial turbine), a condenser (seawater-cooled), and a feed pump. Cycle efficiencies from jacket water heat at 80 to 90°C are approximately five to eight per cent; from post-turbocharger exhaust at 150 to 200°C, efficiencies of eight to 15% are achievable.

Commercial ORC systems for ships

Several manufacturers market modular ORC systems targeted at the marine sector.

Wärtsilä’s Aquarius system uses a proprietary working fluid and is designed to recover heat from jacket water and exhaust gas simultaneously, with electrical outputs from approximately 50 kW for small vessels up to several hundred kilowatts for medium-sized vessels. The modular design is intended to fit into existing machinery spaces without major structural modification.

MAN Energy Solutions (through its after-market service, PrimeServ) offers the Organic Efficiency Upgrade (OUE) ORC system as a retrofit for existing vessels. The system is matched to the specific MAN ES main engine installation and targets waste heat from the high-temperature jacket water and exhaust streams.

Climeon (Sweden) produces a modular heat-power module rated at approximately 150 kW per module. The units are designed for low-temperature heat sources in the range 70 to 120°C - particularly jacket water at 80 to 90°C on diesel engines and heat at similar temperature levels from gas turbine exhaust streams. Multiple modules can be installed in parallel to match available heat load. The system has been fitted on cruise ships and ferries in the Nordic region, including vessels operated by Stena Line and Wallenius Wilhelmsen.

Alfa Laval, which manufactures the Aalborg range of marine boilers and heat exchangers, has developed ORC and combined cycle heat recovery systems for the marine market, building on the company’s established position in EGE supply.

ORC systems for marine application must meet additional requirements not shared with land-based plants: vibration and shock resistance, operation at varying pitch and roll angles (affecting liquid levels in separators and condensers), salt-air corrosion protection, containment of refrigerant leaks in confined machinery spaces, and the ability to operate and shut down within the control infrastructure of the ship’s integrated automation system.

Kalina cycle systems

The Kalina cycle, developed by Alexander Kalina and introduced commercially in the 1980s and 1990s, uses a mixture of ammonia (NH3) and water as the working fluid in place of pure water (as in the Rankine cycle) or a single organic compound (as in the ORC). The key thermodynamic advantage of the ammonia-water mixture is that its boiling point varies with composition and with pressure; this variable boiling temperature allows the working fluid temperature profile in the evaporator to be better matched to the temperature profile of the heat source, reducing irreversibility in the heat exchange process. For a given heat source and sink, the Kalina cycle can in principle achieve 8 to 12% higher thermal efficiency than a basic Rankine cycle operating on steam.

For marine waste heat recovery, the Kalina cycle has been proposed and investigated for recovering heat from the exhaust gas and jacket water of slow-speed and medium-speed diesel engines. The practical barriers have limited commercial deployment in the marine sector: the ammonia-water system is more complex than a pure-fluid Rankine cycle, requires a separator and rectifier in addition to the standard evaporator-turbine-condenser-pump loop, and introduces the safety complications of an ammonia circuit on a vessel with accommodation spaces and limited ventilation capacity. Ammonia’s toxicity (IDLH 300 ppm), flammability at high concentrations, and corrosive effect on copper alloys all add to the design and regulatory burden.

Research publications from institutions including NTNU (Norwegian University of Science and Technology) and technical universities in Korea and China have demonstrated cycle efficiencies from marine exhaust heat that compare favourably with ORC alternatives on a thermodynamic basis. Whether the efficiency gain justifies the system complexity remains a commercial judgement that depends on fuel price, voyage profile, and the available space and weight allowance on the particular vessel.

Low-temperature sources: jacket water and lube oil recovery

The jacket cooling water circuit on a slow-speed two-stroke engine is maintained at 75 to 90°C at the engine outlet. This stream carries approximately four to six per cent of the fuel energy - on a 14 MW engine, approximately 600 to 840 kW of thermal power. At this temperature level, direct recovery options include:

  • Freshwater generation: an evaporator/condenser unit using jacket water as the heat source can produce fresh water from seawater at low vacuum pressure (steam at approximately 70°C and 0.03 bar absolute), with typical production rates of 15 to 35 tonnes per day per unit depending on heat input and seawater temperature. This reduces the auxiliary boiler load.
  • Heavy fuel oil heating: HFO must be heated to approximately 100 to 135°C to achieve the low viscosity required for centrifugal purification and the even lower viscosity required for injection. Jacket water at 80 to 90°C can supply the preheat stage before the HFO heaters, reducing steam consumption. The fuel heater (steam heat exchanger) formula page covers the heater sizing method.
  • Low-temperature ORC: jacket water at 80 to 90°C is a suitable heat source for organic Rankine cycle systems using low-boiling-point working fluids.
  • Absorption refrigeration: jacket water at 80 to 90°C can drive a single-effect lithium bromide-water absorption chiller to supply cooling for air conditioning, provision stores, or cargo refrigeration, as discussed in the following section.

Lubricating oil heat at 45 to 55°C has low thermodynamic quality and in practice is rarely recovered for power generation. Its most common use is preheating heavy fuel oil or boosting the district heating circuit to a low-level accommodation heating load.

Absorption chiller systems

An absorption refrigeration machine (ARM) uses heat instead of mechanical work to drive its refrigeration cycle. The most common configurations for marine application are lithium bromide-water (LiBr-H2O) for air conditioning (refrigerant temperatures above 0°C) and ammonia-water (NH3-H2O) for lower-temperature cargo refrigeration.

In a single-effect LiBr-water absorption cycle, a generator heated by steam or hot water drives water vapour from a strong LiBr solution. The water vapour condenses in a condenser (seawater-cooled), expands to low pressure, and evaporates in the evaporator, providing cooling. The dilute LiBr solution is returned to the generator where it is reconcentrated. For a generator supplied with steam at approximately 80 to 90°C (available from a low-pressure EGE section or jacket water heat exchanger), the coefficient of performance (COP) is approximately 0.65 to 0.75 - that is, approximately 0.7 kW of cooling per 1 kW of heat input.

Double-effect absorption chillers, which require a higher-temperature driving heat source (steam at 120 to 150°C or exhaust gas directly), achieve COPs of approximately 1.1 to 1.4, roughly double the single-effect performance. The higher driving temperature means the EGE or LP steam generator must be designed with sufficient pressure and temperature capacity.

For large cruise ships and passenger vessels, absorption chillers driven by exhaust or steam waste heat can supply a substantial fraction of the vessel’s cooling demand (several megawatts of refrigeration), directly displacing electrical cooling demand from vapour-compression systems. The refrigeration COP (heating) calculator provides a COP reference calculation for comparison.

Thermoelectric generators

Thermoelectric generators (TEGs) exploit the Seebeck effect to convert a temperature gradient directly into electrical voltage without any moving parts. A TEG module consists of p-type and n-type semiconductor elements (typically bismuth telluride at low temperatures or lead telluride at higher temperatures) arranged between hot and cold faces. The conversion efficiency of current TEG materials at the temperature differences available in marine exhaust systems (200 to 300°C between exhaust gas and cooling water) is approximately two to five per cent, significantly below the eight to 15% achievable with Rankine cycle systems at similar source temperatures.

TEGs have attracted interest for marine applications because they are mechanically simple, silent, vibration-resistant, and require almost no maintenance over lifetimes of tens of thousands of hours. Several research programmes, including projects funded under European Commission Framework programmes, have investigated TEG arrays for marine exhaust recovery. Commercial deployments on ocean-going vessels remain limited to prototype and pilot installations as of 2025; the low efficiency and high cost per watt of thermoelectric materials have not yet produced a competitive economic case against steam turbine or ORC alternatives for large ships. TEGs may find niche roles on small vessels, inland waterway craft, or for powering sensor systems where the simplicity advantage outweighs the efficiency penalty.

EEDI, EEXI, and CII regulatory credits

MEPC.1/Circ.815 and the innovative technology category

IMO’s Energy Efficiency Design Index (EEDI) framework, established under MARPOL Annex VI Chapter 4, allows WHR systems to claim a reduction in attained EEDI through the innovative energy-efficient technology category. The relevant IMO guidance is MEPC.1/Circ.815 (2013), which defines the calculation procedure for Category B (wind and waste heat recovery) technologies, updated by MEPC.1/Circ.896.

Under this procedure, the shaft-power equivalent output of the WHR system (PWHR, in kW) is credited against the EEDI numerator. Because the EEDI is expressed as CO2 per tonne-mile and the WHR power output displaces auxiliary engine fuel consumption, the credit reduces the attained EEDI in proportion to the WHR electrical output relative to total main engine power. For a typical large containership where the WHR system generates 6% of main engine power, the attained EEDI improves by approximately five to six per cent depending on the specific EEDI formula terms. The EEDI innovative technology credit calculator implements the MEPC.1/Circ.815 calculation. The EEDI attained calculator computes the overall attained EEDI including the WHR credit, and the EEDI attained formula page presents the complete calculation methodology.

The EEDI fVSE (virtual specific energy consumption) calculator and its formula page also interact with WHR when the system contributes to shaft power output rather than only electrical power: the shaft power delivery from a power turbine directly reduces the required main engine fuel input at the reference speed, affecting both numerator and denominator of the EEDI fraction.

EEXI credit

The Energy Efficiency Existing Ship Index (EEXI), introduced by IMO MEPC.75 (2021) and applicable from 1 January 2023, applies the same thermodynamic principles as EEDI but to existing ships. A vessel that retrofits a WHR system of sufficient output can claim an EEXI reduction by the same method as the EEDI credit. The EEXI attained calculator and the EEXI required calculator allow assessment of whether a WHR retrofit brings an existing vessel into compliance. For tankers and bulk carriers that face EEXI deficiencies, a WHR retrofit may in some cases provide sufficient improvement to avoid engine power limitation (EPL). A detailed account of the EEXI framework is in the what is EEXI article.

CII rating improvement

The Carbon Intensity Indicator (CII) is an operational metric, calculated annually from actual fuel consumption and distance sailed. WHR systems improve the CII rating through two mechanisms. First, by generating electricity from waste heat, WHR reduces or eliminates auxiliary diesel generator operation, reducing the fuel consumption attributed to auxiliary engines in the CII numerator. Second, if the WHR system delivers power to the propeller shaft (via a power turbine or gear connection), it reduces main engine fuel consumption at a given vessel speed.

For a bulk carrier operating at 70% load factor and 12 knots, a WHR system generating 1,500 kW of electrical output (displacing auxiliary generator consumption) can improve the annual CII figure by approximately two to five per cent depending on the vessel’s CO2 reference line. The CII attained calculator and the CII rating calculator quantify the improvement. The CII attained formula page explains the CII calculation structure. The CII quick check via SFOC calculator provides a rapid estimate of how a reduction in effective SFOC (from WHR credit) shifts the CII score.

The interaction between slow steaming and WHR is important. Slow steaming reduces main engine exhaust temperature and mass flow, which reduces EGE steam production and power turbine output. At 50 to 60% MCR, a power turbine may produce only two to three per cent of main engine power rather than the four to seven per cent at full load. This means that WHR’s CII benefit is relatively smaller precisely when slow steaming provides the largest CII gain. Combined systems integrating WHR with battery storage can smooth this interaction by storing excess WHR energy at higher loads for use during the transit between load points. The voyage slow steaming savings calculator and its formula page quantify the fuel benefit of speed reduction, enabling it to be compared with WHRS savings on the same voyage.

Economic assessment

Capital cost and payback period

The economic case for WHR on ocean-going vessels depends on fuel price, vessel utilisation, load factor, and the cost of capital. For a large containership or VLCC with a main engine in the 20 to 50 MW range, a complete dual-pressure WHR system (EGE plus power turbine plus steam turbine plus electrical switchboard integration) has a typical installed cost in the range of US$3 million to US$8 million, depending on system complexity, classification society certification scope, and whether the system is factory-fitted or retrofitted at a shipyard. A standalone EGE with LP steam generation only costs US$500,000 to US$1.5 million installed.

A system generating 3,000 kW of electrical output at 70% annual load factor displaces approximately 3,000 × 0.7 × 8,760 ÷ 1,000 = 18,400 MWh per year of auxiliary generator energy. At an SFOC of 200 g/kWh for the displaced auxiliary engines and a fuel price of US$600 per tonne, this represents annual fuel savings of approximately US$2.2 million, implying a simple payback period of one and a half to four years at 2024 fuel price levels. At lower fuel prices (US$350 per tonne, as prevailed in 2015 to 2016), the payback extends to three to six years.

For smaller vessels with main engines below 10 MW, the economics are less attractive because fixed system costs (heat exchangers, steam turbine or ORC expander, electrical switchboard, engineering and commissioning) scale less than linearly with power, and because the annual operating hours may be lower.

Under carbon pricing frameworks such as the EU Emissions Trading System for shipping, the CO2 reduction from WHRS also attracts financial value: each tonne of CO2 avoided has a value equal to the prevailing EUA price, which ranged from EUR 50 to EUR 100 per tonne CO2 during 2023 to 2024. For a vessel saving 4,500 tonnes of CO2 per year through WHRS, this represents EUR 225,000 to EUR 450,000 in avoided EUA purchases annually, improving the investment case substantially.

The waste-heat recovery calculator and the voyage slow steaming savings calculator together enable assessment of combined WHR and slow steaming fuel-saving potential on a specific voyage or route. For multi-year fuel cost projections, the lifecycle fuel total cost of ownership calculator provides discounted cash flow comparison across fuel scenarios.

Bulk carrier and tanker specific considerations

Bulk carriers and tankers present some of the most economically attractive applications for WHR retrofit because they tend to operate at near-constant speed and engine load on long ocean voyages, maximising the utilisation of the WHR system. A capesize bulk carrier operating at 85% MCR for 70% of the year (a typical utilisation pattern for the spot market) offers approximately 6,000 running hours per year at close to full WHR capacity.

Cargo heating on tankers creates an additional synergy. A product tanker or crude oil tanker requires substantial steam for heating cargo tanks, steam heating coils in cargo pipes, and HFO heating. If a well-sized EGE can supply this steam demand from exhaust heat, it eliminates a significant portion of auxiliary boiler fuel consumption. The tanker cargo heating duty calculator and the cargo steam heating time calculator quantify this heating load. The cargo heater (steam coil) formula page covers the coil sizing method, and the cargo heating cost calculator translates it into fuel cost, enabling direct comparison with WHR installation cost.

The what is EEDI and what is EEXI articles provide the regulatory context within which these economic calculations must be placed for vessels built or converted since 2013.

Practical implementation and commissioning

System integration and space requirements

Integrating a WHR system into a ship’s engine room or uptake space requires coordination with the engine builder, classification society, marine automation vendor, and switchboard manufacturer. The EGE is installed in the exhaust uptake between the main turbocharger outlet and the funnel casing. Space constraints in this region are tight, particularly on vessels with compact funnel arrangements or on retrofits where structural members cannot be removed.

Power turbines or steam turbines are typically installed on a mezzanine deck adjacent to the exhaust uptake or on a separate level connected by exhaust ducting. The generators driven by these turbines must be aligned with the shipboard 60 Hz or 50 Hz electrical supply system and synchronised with the main switchboard. Grid synchronisation requires a static frequency converter in variable-speed systems (power turbines, ORC systems) or a speed governor maintaining constant speed in steam turbine systems.

Commissioning time for a new WHRS on a large vessel typically requires two to four weeks of sea trials, during which the system is brought up to operating conditions, control parameters are tuned, and performance is verified against the maker’s guarantees. Classification society surveyors attend the commissioning sea trial to verify that performance, safety systems (safety valve settings, high-temperature alarms, dump-load operation, automatic shutdown sequences), and documentation meet classification requirements. The steam relief valve (spring-loaded) formula page provides the capacity sizing method for relief valves protecting the WHR steam circuit.

Maintenance intervals and service philosophy

The EGE is a static heat exchanger with no rotating components; its maintenance is dominated by tube cleaning (soot blowing and periodic water washing), inspection for corrosion and leakage, and replacement of tubes or finned elements at four- to eight-year intervals. The classification society surveys at 2.5-year and five-year intervals inspect the EGE as part of the boiler and pressure vessel scope.

Steam turbines used in WHR service are typically small impulse turbines with maintenance intervals of 12,000 to 24,000 running hours for blade and nozzle inspection. Bearing clearance checks and governor calibration are performed at intermediate intervals of approximately 8,000 hours. Power turbines in the MAN TCS arrangement have comparable intervals.

ORC expanders (scroll or radial turbines) and their associated heat exchangers are generally designed for very long maintenance intervals - 40,000 to 60,000 hours between major overhauls for small scroll expanders - but require careful attention to working fluid purity (moisture and non-condensable gas content) and to the integrity of all seals to prevent fluid leakage into the machinery space.

Hybrid WHR and battery storage

Integration of WHR systems with shipboard battery energy storage has gained practical traction from the early 2020s. The combination addresses two limitations of standalone WHR: the variable output of the WHR system as engine load changes, and the inability of WHR generators to respond rapidly to electrical demand peaks (for example, bow thruster operation or large accommodation load switching).

In a hybrid arrangement, the WHR generator supplies the base electrical load while a battery bank absorbs any surplus (for example, when WHR output temporarily exceeds demand) and supplies deficits. The battery also buffers the WHR start-up and shut-down transients and can allow auxiliary diesel generators to be shut down more rapidly when the vessel departs port and the EGE reaches operating temperature. This integration reduces auxiliary diesel running hours and associated maintenance costs beyond what is achievable with WHR alone. The generator parallel operation calculator supports analysis of parallel WHR generator and battery inverter operation on the switchboard.

Interaction with exhaust aftertreatment systems

Modern large engines frequently combine WHR with exhaust aftertreatment devices occupying the same uptake. The most common co-installed systems are the exhaust gas cleaning system (wet scrubber) for sulphur oxide control and the selective catalytic reduction (SCR) catalyst for nitrogen oxide reduction.

SCR catalysts require a minimum gas temperature of 300 to 400°C to operate effectively. Positioning the SCR upstream of the EGE (high-temperature position) delivers adequate catalyst temperature at all loads above approximately 25% MCR, but the EGE then receives cooler exhaust. Positioning the SCR downstream of the EGE (low-temperature or slip-catalyst position) allows the EGE to receive maximum temperature, but the SCR catalyst sees reduced temperatures at part load and may require exhaust gas bypass or supplementary heating at low loads. The choice of SCR position is a design trade-off between NOx reduction reliability and WHRS heat recovery.

Wet scrubbers for HFO sulphur compliance remove SO2 from the exhaust but also cool and humidify the gas. If the scrubber is placed upstream of the EGE, the EGE receives cooler, wetter gas with altered dew-point characteristics. The interaction between scrubber placement and EGE performance is a design variable that affects both WHR output and acid dew-point corrosion risk.

Regulatory and classification requirements

Classification societies - including Lloyd’s Register, DNV, Bureau Veritas, American Bureau of Shipping (ABS), and ClassNK - require WHRS installations to comply with their boiler and pressure vessel rules, which reference international standards such as EN 12952 (water-tube boilers) or ASME Section I (power boilers). Steam pressure vessels on ships are classed items subject to survey at five-year intervals. The EGE drum is hydrostatically tested at 1.5 times design pressure during new-build survey.

Safety requirements of particular note include: relief valves sized to pass full boiler output at 10% above set pressure, safety interlocks to isolate the WHRS on detection of exhaust temperature anomalies or drum water level faults, and documented procedures for emergency shutdown of the STG on loss of steam supply.

The ISM code requires that WHRS operating procedures be documented in the ship’s safety management system (SMS) and that crew be trained in fault identification and emergency shutdown. Planned maintenance system (PMS) entries should include soot-blowing intervals, water treatment checks, and turbine inspection schedules.

Port state control inspectors occasionally examine boiler water treatment records and pressure vessel survey certificates, and deficiencies in these areas can result in detentions. The MARPOL convention provides the parent regulatory framework under which the EEDI and EEXI requirements - and their WHR credits - sit.

Notable installations and vessel examples

Maersk’s Triple-E class containerships (18,000 TEU, 20 vessels, delivered 2013 to 2015) were among the first ultra-large containerships to incorporate an integrated WHR system as part of the vessel’s efficiency design package. The triple-E concept (Economy of scale, Energy efficiency, Environment) included a waste heat recovery power plant generating electricity from EGE steam and supplementing the main engine shaft output, with the WHR contribution sized to reduce auxiliary generator running time during ocean voyages significantly.

The CMA CGM Antoine de Saint-Exupéry (21,000 TEU, delivered 2018) and CMA CGM Bougainville (23,112 TEU, delivered 2018) are equipped with dual-pressure WHRS combining a power turbine and a steam turbine, with a combined electrical output of several megawatts at full main engine load, credited in their EEDI assessments under MEPC.1/Circ.815.

Stena Line ferries and Wallenius Wilhelmsen ro-ro vessels have been fitted with Climeon Ocean modular ORC units in commercial service, with verified electricity generation from jacket water heat on medium-speed four-stroke diesel engines. The modular units allow installation without drydocking in some cases, reducing retrofit cost.

Gas carriers with large slow-speed two-stroke engines on LNG trades have particular potential for EGE-based WHR because the vessels operate at near-constant speed and load over long ocean passages, maximising EGE utilisation hours. LNG as marine fuel produces exhaust gas with lower SO2 content and a lower acid dew point than HFO, allowing the EGE to cool the gas to lower temperatures and extract more energy. The FuelEU Maritime regulation’s GHG intensity trajectory to 2050 provides a long-term regulatory signal that should support continued WHRS investment on vessels burning any fuel type, since the intensity reductions required by 2035 and 2040 will likely require WHRS in combination with alternative fuels on many vessel types. The ShipCalculators.com calculator catalogue at ShipCalculators.com provides calculation tools covering all these interacting factors.

See also

References

  1. IMO. MEPC.1/Circ.815. Guidelines for calculation of reference lines for use with the Energy Efficiency Design Index (EEDI) - Category B innovative technologies. International Maritime Organization, London, 2013.
  2. IMO. MEPC.1/Circ.896. Amendments to MEPC.1/Circ.815 - Calculation of reference lines for use with EEDI. International Maritime Organization, London, 2021.
  3. MAN Energy Solutions. Waste Heat Recovery System for Reduction of Fuel Consumption, CO2 Emissions and EEDI. Technical Paper 5510-0136-01ppr. MAN Energy Solutions, Copenhagen, 2012.
  4. Larsen, U.; Nguyen, T.V.; Knudsen, T.; Haglind, F. “System analysis and optimisation of a Kalina split-cycle for waste heat recovery on large marine diesel engines”. Energy, 64 (2014): 484-494.
  5. Tchanche, B.F.; Lambrinos, G.; Frangoudakis, A.; Papadakis, G. “Low-grade heat conversion into power using organic Rankine cycles - a review of various applications”. Renewable and Sustainable Energy Reviews, 15(8) (2011): 3963-3979.
  6. Sprouse, C.; Depcik, C. “Review of organic Rankine cycles for internal combustion engine exhaust waste heat recovery”. Applied Thermal Engineering, 51(1-2) (2013): 711-722.
  7. Wärtsilä Corporation. Aquarius ORC - Waste Heat Recovery System. Product Brochure. Wärtsilä, Helsinki, 2019.
  8. Climeon AB. Climeon Ocean - Heat Power Module. Technical Specification. Climeon, Stockholm, 2022.
  9. Kalina, A.I. “Combined cycle and waste heat recovery power systems based on a novel thermodynamic energy cycle utilising low-temperature heat for power generation”. ASME Paper 83-JPGC-GT-3, 1983.
  10. Wang, T.; Zhang, Y.; Peng, Z.; Shu, G. “A review of researches on thermal exhaust heat recovery with Rankine cycle”. Renewable and Sustainable Energy Reviews, 15(6) (2011): 2862-2871.
  11. MAN Energy Solutions. MAN Turbo Compound System - Exploiting Exhaust Energy. Technical Documentation. MAN Energy Solutions, Copenhagen, 2018.
  12. IMO MEPC.203(62). Amendments to the Annex of the Protocol of 1997 (EEDI for new ships). 2011.

Further reading

  • Shu, G.; Liu, L.; Tian, H.; Wei, H.; Liang, X. “Analysis of regenerative organic Rankine cycles (ORCs) used in combined heat and power systems”. Energy Conversion and Management, 77 (2014): 816-827.
  • Theotokatos, G.; Livanos, G. “Techno-economic analysis of single pressure exhaust gas waste heat recovery systems in marine propulsion plants”. Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, 227(2) (2013): 83-97.
  • Suárez de la Fuente, S.; Roberge, D.; Greig, A.R. “Safety and CO2 analysis of using ammonia with organic Rankine cycles on ships”. Ocean Engineering, 151 (2018): 91-105.