Heat Recovery System – Understanding what’s happening in it?
To start with, it must be emphasized that every ship Owner or Manager aims at cutting down the operating costs, improving efficiency and looking at avenues to implement in every possible way. And recent strict amendments on IMO’s emission norms made it even more challenging for the Engine Designers.
Here we are about to discuss one such latest system which falls under the topic of “Heat Recovery”.
When it comes to heat recovery systems used on board, here are few which we already know:
- Exhaust Gas Economizer,
- Fresh Water Generators,
- Turbochargers and some more systems.
From all the above, we can arrive at a conclusion that the heat energy available with the exhaust gas is huge and it is still left untapped. One such system which recently came into existence is the Exhaust Gas Heat Recovery System. Here is a brief description of such system and its importance.
Components involved in system:
1. Main Engine,
2. Exhaust gas Boiler with Low pressure, High Pressure and Super heater section,
3. Low Pressure Drum,
4. High Pressure Drum,
5. Turbo generator with LP and HP stage with Gas turbines coupled,
6. Shaft Generator with PTI and PTO mode.
The system Circuit – Explained:
The exhaust gas from the main engine is pumped by the pistons into the exhaust manifold, which forms a part of Constant Pressure turbocharging system. Depending upon the size of the engine, it may be possible to have 2 or 3 turbochargers and even more. These Primary turbochargers help in pumping fresh air for scavenging through the under piston space.
The exhaust gas, after performing some useful work in rotating the turbine of the turbocharger, find its way into the Economizer, which houses the Low Pressure (LP), High Pressure (Hp), and Super heater Sections and also has an exclusive steam service outlet for engine room heating purpose.
TCS – PTG ?
What is TCS – PTG? Sounds weird? Every Mariner has to get acquainted to such acronyms as these are the future of marine propulsion systems. It stands for Turbo Compound System – Power Turbine & Generator. This system tries to utilize and extract the exhaust gas energy to its extreme extent.
The Power Turbine Generator system as the name implies, has a turbine driven by exhaust gas from the main engine, which is coupled to a generator on the other end of the shaft. Thus it is called as Power Turbine Generator. It is said that up to 13% of exhaust gas is diverted from the main turbochargers to this PTG system and this generates sufficient power which can be utilized for sea-going needs of a ship. This means, with the help of shaft generator and PTG, no auxiliary diesel engines are required to run when the vessel is at sea.
When at sea, the volume and pressure of exhaust gas is sufficient to spin the turbine, which is on the PTG which is coupled via reduction gear to a generator. This power along with shaft generator can power the entire ship when at sea.
One more aspect is apart from exhaust gas spinning the turbine, of the PTG system, the High pressure steam and Low pressure steam from the Economizer is fed to a steam turbine which is again coupled via reduction gear to the same generator. This enables sufficient speed of turbine which can take up part sea load and the remaining is taken up by the shaft generator.
Image Credits: MAN Diesel & Turbo SE
So what happens in a Port?
As usual, our auxiliary diesels take over to power up for cargo and manoeuvring operations. Typical Capabilities of Turbo Generators are from 2 MW and that of the shaft generators are from 400 to 600 kW. It is usual to have ships of auxiliary diesel engines of 1200 kW each, four in number for port/manouevring/and emergency power needs with emergency generator acting as 3rd stand-by. Thus one can claim that the power redundancy is thrice when compared to other ships. Of course due to modern methods of slow steaming, it is posing a challenge to whether slow steam with spending fuel on auxiliary diesel engines or to propel fast and claim the advantage of this heat recovery system.
When Main Engine is running, the exhaust is led to economizer which in turn generates steam and fed to PTG as said before, in case at higher RPM the excess exhaust gas which is monitored by scavenge pressure(higher Scav Pr means Higher Ex Gas) the valve to steam turbine is opened with monitoring scav prerruse. This automatically ensures that the scavenge pressure does not drop below the makers limit.
Practical Experience and Exposure:
When the Main Engine is running, Shaft generator is also running and power is produced in the region of approximately 900 to 1000 Kw, However the normal sea load is just 700 kW.
So what will you do with the excess power of 300kW?
The Shaft Generator has two modes.
PTI – Power Take In
PTO – Powe Take Out.
Shaft Generator running as motor takes “In” power and thus called as Power Take-In Mode. When in normal condition and the power generation is within limits, this is called as Power Take Out mode as power is taken from the shaft generator and supplied to the Bus Bar.
Why should it run as motor?
When it is running in motor mode it drives ME thereby reducing SFOC. Ultimately this aims at improving Economy and saving fuel.