Waste heat is the new fuel: Inside the shipboard race to make fresh water cheaper

Marine vessels typically rely on onboard evaporators—thermal desalination systems that boil and condense seawater—to produce potable water. The three primary technologies at sea are multi‑stage flash (MSF), multi‑effect distillation (MED), and vapor compression (VC, also called MVC) (ResearchGate). With fuel costs high, “energy efficiency is the main influencing factor” in choosing a watermaker (Riviera).

Demand is real: large cruise ships may require on the order of 1,000–2,000 m³/day of freshwater—for example, two 825 m³/d evaporator trains (Riviera). Designing systems to use waste heat from the main engine is therefore standard practice (Wärtsilä), and many modern units can even start using only engine cooling water (Riviera) (Alfa Laval).

Multi‑stage flash design and operation

MSF (multi‑stage flash) heats seawater in a brine heater—often to ≈80 °C using engine jacket water or low‑pressure steam—then routes it through a series of flash chambers at successively lower pressures; the sudden pressure drop causes “explosive boiling (flashing)” of a fraction of the water (Wärtsilä). The pure vapor condenses on tubes or plates carrying incoming feed, recovering latent heat to preheat that feed (Wärtsilä).

A modern marine MSF might contain 4–6 stages. For instance, a five‑stage unit rated 300 t/day requires about 915 kW of drive power (≈191 kWh per tonne of water) (Riviera). Suppliers highlight MSF’s ruggedness—“sturdy construction” and the “lowest sensitivity to malfunction”—and product quality: residual salinity can be under 5 ppm (Wärtsilä) (Riviera).

The trade‑off is energy. In large‑scale terms MSF typically needs on the order of 10–16 kWh of heat per cubic meter of freshwater (kWh(th)/m³ denotes thermal energy) (MDPI), plus comparable electrical power for pumps. Wärtsilä cites plants producing up to 1,500 t/day (Wärtsilä)—but at the cost of substantial steam/jacket heat and electricity (915 kW for 115 t/day in the model data, or 191 kWh/t in the 300 t/d example) (Wärtsilä) (Riviera).

Multi‑effect distillation heat reuse

MED (multi‑effect distillation) evaporates seawater in a first “effect,” then condenses that vapor in a second effect to boil more water—reusing latent heat across multiple effects. Because each stage recycles heat, MED requires less heat input than MSF: about 5.5–9 kWh(th) per m³ (MDPI).

Modern plate‑based MED designs drive down electric power too. Alfa Laval’s MEP (multi‑effect plate) evaporator “employs combined plate‑type evaporator/condenser effects,” maximizing low‑grade heat use; the four‑effect MEP‑4‑750 model consumes less than 2.5 kWh of electricity per m³ of fresh water (Alfa Laval). In one cruise‑ship comparison, an 825 m³/d six‑effect plate MED drew only 91 kW of electrical power, versus 200 kW on a sister ship’s older system; the switch saved over 200 kW of shipboard power (Riviera).

Plate‑MED units are compact; Alfa Laval notes a “space‑saving design” with higher thermal efficiency per unit volume/weight than other evaporators (Alfa Laval). In practice, MED converters using waste heat tend to have a high gained output ratio (GOR, a measure of thermal performance) often >10 and very low net electrical load; they typically recycle 4–6 effects, capturing >95% of available heat (MDPI) (Alfa Laval).

Vapor‑compression footprint and power

VC (vapor compression, or MVC) uses a mechanical compressor to raise vapor temperature so it can heat incoming feed; shipboard units such as Alfa Laval ORCA evaporate seawater under vacuum and compress the vapor, running on electricity alone (Alfa Laval). Efficiency numbers are comparable to MED: typical specific energy consumption (SEC) is ~7–15 kWh of electricity per m³ of product (MDPI).

For reference, simpler single‑stage MVC units have reported much higher consumption—up to ~23–42 kWh/m³ (MDPI). One manufacturer claims minimal power consumption and “ultrapure water” (Alfa Laval). In practice, VC desalters achieve coefficients of performance (COPs) of ~5–6, meaning each kWh drives about 5–6 kWh of phase‑change heat. They are valued for reliability and low footprint but strictly require sufficient electrical power. For medium‑duty ships, MVC can be attractive if waste heat is scarce; large ships (and offshore rigs) often complement thermal systems with MVC for peak loads.

Energy benchmarks and RO comparison

For large ships using waste heat, the three thermal technologies diverge in both heat and electrical demand. MSF requires roughly 10–16 kWh(th)/m³ of heat (MDPI) plus ~0.19 kWh/L of electricity (Riviera). MED needs about 5–9 kWh(th)/m³ (MDPI) and far less pump power; plate‑MEDs use ≤2.5 kWh/m³ electric (Alfa Laval). VC operates without external heat but uses ~7–15 kWh/m³ electric (MDPI).

By comparison, modern reverse osmosis (RO) consumes only ~3 kWh/m³, explaining the shift toward RO in many applications (Riviera). In procurement, some operators weigh RO packages like ship‑grade seawater RO alongside evaporators; for context, see sea‑water RO and broader membrane systems.

Typical figures cited in the field:

• MSF: Up to ~1500 t/day capacity (Wärtsilä); thermal demand ~10–16 kWh(th)/m³ (MDPI); electrical ~0.19 kWh/L (e.g., 191 kWh/t) (Riviera). GOR ~8–12.
• MED (plate): Typically 3–6 effects; thermal ~5–9 kWh(th)/m³ (MDPI); electrical ≤2.5 kWh/m³ (Alfa Laval). High GOR (>10).
• VC (MVC): Smaller units (20–120 m³/d); electric‑only ~7–15 kWh/m³ (MDPI). (COP ~5–6.)

Evaporators still produce purer “technical” water and are unaffected by feed fouling.

Efficiency levers: waste heat and controls

All three technologies benefit from heat recovery and modern controls. Engine heat recovery is routine: MSF units can be started and run solely on engine cooling water (no boiler steam) to use “surplus heat” (Riviera). MED and single‑effect systems are engineered to operate on whatever hot water or exhaust heat is available (Alfa Laval). Pre‑heating seawater with outgoing brine or lube‑oil heat can raise efficiency further.

Electric power optimization matters: variable‑frequency drives and high‑efficiency motors are standard on pumps. Wärtsilä specifies frequency‑controlled pumps to “save energy” (Wärtsilä), and one source notes an Aqua Blue (MED) design uses roughly half the seawater flow of peers, cutting pump load. Designs today can operate down to very low engine loads without adjustment; an Alfa Laval unit runs stably at 10% main‑engine power with only ~130 kWh/m³ of thermal input (Riviera). Careful control of vacuum and brine circulation also reduces irreversibilities.

Process enhancements include increasing the number of effects or adding recompression units to raise thermal performance ratio. Multi‑effect designs may recycle vapor from the coldest effect back to the hottest (thermal vapor compression). New plate architectures reduce fouling and scale—by separating heat‑transfer surfaces—allowing consistently high performance (Wärtsilä). Optimized MED plants can reach the theoretical limit of their waste heat. For MSF, controlling brine concentration (e.g., via added stages) can reduce blowdown and feed needs (Wärtsilä).

Procurement choices and system integration

These improvements mean contemporary MED and MSF units often match or even exceed the electrical efficiency of RO systems (on a total‑energy basis). Suppliers underscore the point: Alfa Laval says its MED “offers the lowest possible consumption of electric energy” (Alfa Laval), and Wärtsilä notes optimized MSF power draw now rivals RO with pretreatment (Riviera).

In practice, operators choose between MSF, MED, and MVC by weighing required capacity, available waste heat, and space. MED plate systems lead in heat utilization and low power use (Alfa Laval) (Alfa Laval), MSF units excel at heavy‑duty reliability (Wärtsilä), and MVC provides compact, robust distillation when electricity is available (Alfa Laval). Where RO is selected instead of thermal desalination, shipyards often consider ship‑class seawater systems such as sea‑water RO; some buyers also evaluate broader membrane systems portfolios in parallel.

Source notes and references

Benchmarks and case details cited above draw on industry data and research: Riviera; Wärtsilä; peer‑reviewed analyses on energy consumption (MDPI); and manufacturer specifications for plate‑MED and MVC (Alfa Laval) (Alfa Laval), along with a review of freshwater generator technologies (ResearchGate). Specific numbers are from manufacturer data and case studies (Riviera) (Alfa Laval) and technical notes on fouling/scale and blowdown optimization (Wärtsilä) (Wärtsilä).