Energy Reclamation Project

[MacMcMacmac]

Good Day Folks. I am trying to get some buy in for an energy reclamation project at our gas turbine engine test facility. We have a 2500hp centrifugal blower that provides air for the test cell. It also drives an 1800hp turbo expander. By mixing hot and cold streams, we can get the air temperature required for the engine under test.

The cell is evacuated by a compressor/exhauster that creates the necessary altitude conditions.

Due to the very dry air requirements to operate the turbo expander down to -100f without creating ice, we draw all of the air for the blower through two desiccant wheel dehydration units in series.

These do a very good job, except under the absolute highest flow conditions at the highest summer humidity and temperature.

What has bothered me about this system for the 10 years I've been working here, is the fact that a very large amount of air from the blower is constantly being discharged to atmosphere. It seems like a huge waste of very expensive air. The blower is almost always producing excess flow for the demands of the test. I do not know why it isn't being throttled via IGVs to match flow to demand, but I've met the engineering crew who designed the complete system and they are no fools, so I can assume there was a very good reason why it operates the way it does. Still, the blow-off air has been filtered, chilled, dried, filtered again, chilled again and dried again, so it is pretty high quality by the time it goes to process. Add in the cost of compression, and the fact that there are two, 1 Mbtu direct fired heaters constantly regenerating the desiccant wheel, and it soon adds up to a lot of money being blown away to no good effect.

I was wondering how I could quantify the cost of the air being blown to atmosphere. I assume there is a simple pressure/velocity/pipe size calculation to measure the mass of air being discharged from the silencer on the roof. I also have the inlet and discharge air temperature and pressure of the blower. Is there a way to calculate the energy required to raise the temperature of the mass of air? This would give us the compressor energy being squandered, but this does not take into account the amount of energy expended drying the air in the first place. If there is an easier method, I'm all ears.

My proposal is to direct the excess air from the blower back to the inlet of the first dehydration unit to dilute the incoming ambient air and lower the load on the desiccant wheels. There is an initial cost to chilling the hot air back to an acceptable approach temperature for the first chiller coil so we do not trip the refrigeration system, but I think the payback for the installation of a water to air heat exchanger would be fairly quick due to the reduced gas requirements for the two regen heaters. At the very least, it would help us reach some unattainable test points during the hottest and wettest parts of the summer. However, until I have some numbers, this is just a guess.

Any suggestions are greatly appreciated.

 

[MintJulep]

I'm making a guess about the ownership of this test facility.

Start by defining "Who's money are you saving?"

This facility is available for rent, right? So it's likely that the cost to operate it is built into the fees charged to rent the facility for a test.

i.e. the facility owner doesn't care because it doesn't affect their wallet.

If that is the case, then you need to sell this on the combination of:

"If we use less energy then we can lower our rate and perhaps gain some business by being less expensive than our competitors." If you have competitors.

"We can be greener."

If you can sell the business case in concept, then all you need to do is write the energy balance for each piece of equipment and crunch some numbers.

 

[EnergyProfessional]

MintJulep forget the 3rd, most important, reason to do that. You can keep the rental fees the same, and pocket the savings. That will pay for whatever measures you need to implement to save energy. And do the green propaganda in addition.

 

[MedicineEng]

EnergyProfessional:
Exactly my thoughts.

By the description this looks like a very specific testing facility, so I wouldn't expect that there are competitors under every rock fighting for business, so project's selling point is: "pocket the savings".
Unless it is a public institution, and then you might spin more the green credentials and eventually, saving taxpayer's money (a doubtful driving force in most public services, though) .


Mac:
Regarding the energy spent drying the initial fresh air intake, I would look into the psychrometric chart and plot initial and final air intake air and humidity, from there you can calculate enthalpy, water volume removed, etc. Then check desiccant dryer's manufacturer technical literature to see how much energy does it consume to remove the calculated amount of water.

 

[MacMcMacmac]

The money saved will be going directly to our customer's bottom line in the form of reduced utility charges each month. We have a recurring long term contract with them, so it benefits both of us to keep costs as moderate as possible, plus it will further their stated goal of reducing their environmental impact. The prospect of not missing test points during a campaign due to the dehydrators no longer being overwhelmed at high flow and humidity levels will also be a bonus.

 

[lilliput1]

Are you sure the heat from the exhaust air in not recycled by it passing through the approximate half of the desiccant enthalpy wheels while outdoor air for the intake air blower goes through the other half of the wheels thereby absorbing the heat from the exhaust air as the wheels rotate? There has to be exhaust air to simulate the lower pressure at high altitudes. Using the above heat recovery device minimize energy loss of exhausting air.