What's Better: VFAC-to-AC & Back or DC-to-AC & Back?

[crshears]

Hello all,

For the background to this post, see the thread entitled "Can 3 GT's Supply 1 HRSG?" in the Heat Transfer & Thermodynamics forum.

What I want to know is:

For an electrical machine rated at ~3 MW / 3MVA operating within a maximum-to-minimum speed ratio of 5:1 that could function as either a motor or generator depending on configuration, would it be better to

a] use a variable frequency AC machine with a range of from, say, 15 to 75 Hz paired with a bidirectional VFAC-to-AC convertor, or

b] a DC machine of some type with a bidirectional DC-to-AC inverter/rectifier?

Does the technology to construct a cost-effective, workable machine for Option B even exist?

If so, what config would the DC machine take? Which of the two options would be more efficient? Cost-effective? Controllable? Cause no to minimal scatter harmonics to show up in the balance of the vessel's AC electrical system?

Since the machine in question would be connected to a prime mover [steam engine] whose efficiency would be greatly optimized if its rotational speed could be controlled over a wide range, the need for either a VFAC or DC machine appears to be a sine qua non.

I envision the use of gearing/shafting of some suitable ratio [and capable of transmitting torque in either direction] between the machine and the engine so as to keep the size of the electrical machine from becoming unwieldy.

CR

 

[jraef]

What is your goal here? Your prodigious use of acronyms is making this hard to read, but it appears you are wanting to know if there is an existing technology to have a bi-directional power machine in the 3MW range? If so, the answer is absolutely, it is done all the time. But the DC option is problematic in that you cannot make a Brushless DC machine that big, so you will have a lot more maintenance compared to an all AC solution, But a "regenerative" AC drive is readily available for such a task. There are numerous technologies available, but deciding on what is best would involve a very detailed discussion of all of your performance criteria.

"Will work for (the memory of) salami"

 

[crshears]

Hi jraef, pleased to virtually meet you.

Sorry if my use of acronyms is creating problems; I'll try to avoid them.

It sounds like all alternating current would be the only sensible way to go; fair enough.

I have more than one 'goal' in view; here's the first one.

Since the plan under consideration in the original post is to supply one quadruple expansion steam engine with steam using the exhaust heat from either one, two or three combustion turbines paired with individual heat recovery steam generators, the amount of steam supplied to the engine would be [roughly] either 1/3, 2/3 or 3/3 of full engine nameplate rating. [The vessel's propellor would of course be decoupled.]

Since the steam consumption of the original engine was based on it developing its rated cruising output at more-or-less fixed expansion ratios, and since the new scenario envisions operating the engine at what would be two possible off-original-design-case modes, it would be sub-optimal to adjust the horsepower developed by the steam engine using solely the cut-off method. Attempting this would cause the lions' share of the power developed at part loads to be produced by the high-pressure cylinder, which would cause undesirable torque pulsations and uneven power output with a periodicity of a few to several cycles, depending on the gear ratio selected in the end design.

I therefore propose holding the individual expansion ratios to nominally optimal values by adjusting the individual cylinder cut-offs via their valve linkages. Once these optimal cut-off points have been attained, the volume of steam used by the engine would be roughly proportional to its rotational speed. By adjusting the engine's speed using the simple expedient of extracting more electrical power from the unit than is contained in the steam supplied, the steam consumption of the engine could be readly adjusted to match the available supply.

A second goal addresses the 'regenerative' portion of your response...

The likelihood that the old girl will ever again steam under her own power is slim to none...but I for one am not going to guarantee that it never happens by failing to incorporate the possibility into any design prepared. Should this concept ever be actualized, the proportions of the various components would be such that with the propellor coupled to the engine/electrical machine combination, one combustion turbine operating between 80 and 100% of full load could supply roughly two-thirds of the original design propulsion thrust via the path of its generator through the AC-to-AC convertor to what would become a propulsion motor, with the remaining third of the thrust supplied by the steam engine.

Thanks for listening.

CR

 

[waross]

Two problems (well maybe three).
1> Charge more for visitors to see a working steam engine. (Some will pay but many won't. Beware that the lower visitor count times the increased unit revenue does not invoke the law of diminishing returns.)
2> Generate and sell power commercially.
It is a wonderful thing when synergy kicks in and a common solution is mutually beneficial. Such may not be the case here.
Now to make the problem more interesting, we want to preserve the ability of a really old ship with unusable boilers to steam again.
Not going to happen. The cost of inspections and repairs to return such a vessel to service may result in a pay back time that is not only in centuries, but recedes further with every year of service.

But to be positive, there may be an economical way to both generate commercial power and utilize the old recip engine.
As I see it, the big thing is to have the original engine in operation driven by steam.
What is your house load? Use a fairly small generator to supply all or part of your house load. I'm thinking 500 KVA to 1500 KVA (NOT MVA)
About 8 years ago, I saw a DC 3, refurbished and equipped with on-the-ground A/C. This was to service the "Nostalgia factor." The venture was a fairly quick financial failure.
You may find that your costs are a few orders of magnitude higher and the "Nostalgia factor" significantly less.
Build a conventional combined cycle power plant to service your commercial interests.
Bleed off a small amount of steam to turn the old recip' and generate a modest amount of electricity for in house use. Better yet use an electric boiler for the recip' and keep the oily condensate away from the main system.
You are talking about a 3 MW or 4 MW motor/generator. The cost of a drive to run this at varying speeds for propulshion will probably be a deal breaker.
Consider dropping the steam pressure to the recip. A steam to steam exchanger may also be a way to supply steam to the recip. At low pressure and low load, the recip' may last a long time.


The mind boggles at the thought of using such a large generator on the old recip' and then trying to motor it. Yes it can be done but the cost will be high.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Thanks, waross; your brutally, perhaps even painfully honest appraisal of this concept is the first more or less comprehensive assessment of this idea that I have received, and is just the sort of thing I need.

In turn I'll state my fervent hope that not everyone agrees with you! )

Thanks again.

CR

 

[waross]

Thanks for your understanding.
Another thought that may be doable. Can you refurbish one of the original auxiliaries to generate power? Again I would look into a heat exchanger type steam generator to isolate the oily recip' steam from the turbine steam.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Hi again, bill.

You asked: 'Can you refurbish one of the original auxiliaries to generate power?'

The short answer would be no.

A longer answer is that, for various reasons, it's highly unikely; in a way, the idea of generating power is only a means to an end, namely to have the historic and well-preserved engine actually operating on steam such that visitors can 'go back in time 100 years; experience the sights, sounds, and smells of a Titanic-era steam engine as if it's driving a ship at cruising speed - - all without leaving the safety of the harbour' [my fanciful descriptive language].

Additionally, the planned overall end state is for the old girl to be [a] a floating museum that is a sort of crown jewel in the middle of a large residential / resort / shopping esplanade / marina-type 'destination development;' the chance to put any type of separate power generating facility anywhere near close enough by to supply extraction or bleed steam is therefore a non-starter, hence the only option is to incorporate whatever is built within the vessel's hull...which should by no means be an impossibility, since there are empty cargo holds that could be turned into machinery rooms; careful design and heavy acoustic insulation would however have to be employed to ensure that no local disamenity is caused [you may be able to tell that I've thought about this a lot].

If re-purposing the main engine into some manner of prime mover that can earn its keep is not feasible, the best that can be accomplished [and it's a very distant second best] is to have the engine rotating on what I call 'kinetic display' at 1-2 rpm so the relative motions of the moving parts can be seen.

Carl

 

[ScottyUK]

Slightly off-thread (maybe). Steam turbines for shipboard lighting were in use when Victoria was on the throne and we Brits were still running the show. Only a few years later they were running on the exhaust steam from triple expansion engines, with the turbine taking LP exhaust steam down to condenser vacuum. Guess who has been reading about the work of Charles Parsons while he was on vacation.