Rethinking Low Grade Waste Heat Recovery for Thermal Desalination
Converting flue gas waste heat directly into water vapour without the penalties of conventional systems.
A Vapour Desal Technologies InnovationThe Challenge: Why Most Plants Don't Recover Flue Gas Heat
A major barrier to recovering flue gas heat in power and process plants is the additional electrical load required — particularly increased fan power consumption.
| Factor | Impact |
|---|---|
| Low flue gas temperature | 100–180°C — too low for economical steam production |
| Very low energy density | Makes pressurized steam generation unviable |
| Additional fan power | Increased ID/booster fan load to overcome pressure drop |
| Large circulation systems | Hot water loops require significant pumping power |
Limitations of Conventional Hot Water Generator Systems
- Bulky equipment — Large hot water circulation loops and separate flash chambers
- Hydraulically inefficient — High pumping power for large circulation rates
- Parasitic power loads — Additional electrical consumption on the host facility
- Complex interfaces — Multiple equipment items between heat source and MED
Our Solution: Vacuum Vapour Generator
The VVG directly converts low-grade waste heat into low-pressure water vapour under vacuum, eliminating intermediate loops.
Conventional Approach
VVG Approach
How It Works
Heat Source
Low-grade waste heat such as flue gas downstream of an ID fan (100–180°C), process exhaust, or other sensible heat sources.
Heat Transfer
Heat is transferred across a compact heat exchanger.
Vacuum Evaporation
Water is maintained under vacuum, allowing evaporation at significantly lower saturation temperatures (as low as 40–60°C).
Direct Vapour Output
The generated low-pressure water vapour is routed directly to a downstream MED unit.
VVG vs Hot Water Generator: A Comparison
All values expressed as multiples relative to a conventional hot water generator system.
| Parameter | Hot Water Generator | VVG | Improvement |
|---|---|---|---|
| Heat recovery approach | Sensible heat → hot water → flash vapour | Direct vapour generation under vacuum | Simplified |
| Number of equipment items | Multiple (HWG + pumps + flash chamber) | Single integrated device | Fewer components |
| Process-side flow rate | 1.0× (baseline) | ~0.03× | 97% reduction |
| LMTD (temperature driving force) | 1.0× (baseline) | ~1.9× | 87% higher |
| Heat exchanger surface area | 1.0× (baseline) | ~0.56× | 44% smaller |
| Equipment weight & footprint | 1.0× (baseline) | ~0.56× | 44% lighter |
| Component | Hot Water Generator | VVG System | Savings |
|---|---|---|---|
| Hot water circulation pump | 1.0× (baseline) | ~0.03× | 97% eliminated |
| Booster fan | 1.0× (baseline) | ~0.57× | 43% reduction |
| Overall auxiliary power | 1.0× (baseline) | ~0.52× | 48% less |
Key Benefits Summary
Energy & Power
- 40–60% reduction in parasitic power loads
- 90–95% reduction in process-side pumping
- 40–50% lower fan power impact
Equipment & Installation
- 40–50% smaller heat exchanger surface
- 40–50% lighter equipment footprint
- Single integrated device replaces HWG + flash chamber
Operations & Maintenance
- Elimination of hot water loops
- High retrofit suitability
- Simpler interface with MED — direct vapour delivery
Why This Matters
The VVG addresses the fundamental barrier that has prevented widespread adoption of flue gas heat recovery.
| Metric | Result |
|---|---|
| Total power savings | ~48% of auxiliary consumption |
| Heat recovery efficiency | Maintained — same thermal energy recovered |
| Retrofit suitability | High — can replace existing HWG |
| Annual cost savings | Significant — pays back VVG investment |
Sustainability & Circular Use of Heat
The VVG supports water-energy nexus and sustainability goals by:
- Enabling productive reuse of low-grade thermal energy
- Reducing auxiliary electrical consumption
- Improving water production efficiency in water-stressed regions
- Converting unavoidable heat losses into useful resources
Application Envelope
The VVG can be deployed wherever low-pressure water vapour is required and low-grade waste heat is available.
| Application | Heat Source |
|---|---|
| Flue-gas-based thermal desalination | Power plant stack gas (100–180°C) |
| Industrial evaporation systems | Process exhaust from cement, steel, glass plants |
| Process concentration | Waste heat streams in chemical, food processing |
| Engine heat recovery | Exhaust gas from cogeneration plants |
| Refinery waste heat utilisation | Various low-grade streams in petrochemical facilities |