Ice/water instead of economizer, to make use off-peak electricity

[laudrup]

Hi,

I am working with a large factory that has an ammonia refrigeration plant. The plant has multiple screw compressors and an economizer. The refrigerant plant cools an ethanol-glycol-water solution to -7 deg C (19 deg F) into large holding tanks. The -7 deg C ethanol-glycol-water solution is then recirculated around the factory to cool jacketed product tanks to set temperatures.

There is interest in shifting more refrigeration plant operation to off-peak periods when electricity is cheaper. Phase change thermal storage is an option being considered. To save money on freezing-point suppressants, the factory is interested in the option of a simple water-ice storage system that could utilise an existing large tank at the factory that is available for much of the year. This ice-water couldn't be used directly for cooling because it is at 0 deg C (32 deg F) and the coolant is at -7 deg C.

To make use of the stored 0 deg C ice-water, there was interest in whether the ice-water could be heat-exchanged with the ammonia from the condensers, and the economizer bypassed.

Does anyone know of this having been done before?
Do you think it is possible?
Are there any potential negative operational/efficiency outcomes?

Thanks for your help. Any thoughts/advice is much appreciated.

[cadwaladr]

Glycol? May be the answer

[laudrup]

Thanks cadwaladr, but could it be made to work in the other way?

It is a very big ammonia system (Many MW of cooling) that already has a large glycol system and they are trying to avoid the cost/permanency of more glycol, and to take advantage of latent storage of cooling using ice without expensive encapsulated phase change materials.

[cadwaladr]

It's not my forte,but storing glycol
In an insulated tank may make more sense (

[Grizzly]

Hi laudrup.

To make use of the stored 0 deg C ice-water, there was interest in whether the ice-water could be heat-exchanged with the ammonia from the condensers, and the economizer bypassed.

I am only an Industrial/ commercial Engineer and not a design Engineer.
But the above makes absolute sense to me.

You could also look at your Process Water/Glycol usage?
Where if say, you cool down to -7.c but at some stage in the process you sterilise to say 90c or 123c for pasteurisation. Then cooled glycol is then turned back on to take the heat out of the product plant pre production.

Using the already chilled water to cool down after sterilisation before reintroducing the glycol through the jackets. Would save large amounts of Energy on the Glycol plant.
Any form of pre-cooling using said water if done properly would as I guess you have already worked out. Would be a huge benefit.
Please be aware though reducing an Ammonia Condenser pressure / Temp.

Is a fine art and taking to much heat out can be counter productive.
Off peak running is always a good idea (Load shedding) and cooling the tank down off peak is a good idea.
Have you considered have a 2 stage stat and running the Glycol at 2 diff set points. The lowest S.P. during the night when the tariffs are lowest.

Quite often a production set point is set lower than actually needed, especially if the glycol is then being used to counteract the Therms as I said.

Good luck Grizzly

[cadwaladr]

Tesla has an idea of storing power in a bank not too sure how it works but it's being trialed, good I idea

[laudrup]

Thanks Grizzly and cadwaladr. The factory has done most of the standard sorts of things already. I think they are already cooling the glycol down a bit more at night, but they also get cheap off-peak electricity over the weekends and would like to take advantage of this as well and make ice then and use it later. There is also a limited amount of cooling that you can store in the glycol because it is just sensible heat, while if could freeze water you get the latent heat storage and could store lots of cold over the weekend. Has anyone else had experience using ice water instead of an economizer? Thanks again.

[RANGER1]

Laudrup,
Not sure on your plant, Brewery, milk,as all have different processes.
If you can make ice in a purposefully designed system & have a use for it during the day, then it may be an advantage.
Have seen ice banks used in chicken processing, soft drink plant, milk industry, not a brewery though.

If you have mains water to be chilled for makeup to your process it would help.
Can you describe economiser, as usually a good way of making plant more efficient all the time, as long as screw at close to, or full load, otherwise not designed correctly.
Economiser to me is subcooling high pressure liquid line using economiser connection on compressor.
In industrial ammonia more used on lower temperature applications to subcool high pressure liquid line.
Because power prices increasing economiser more attractive on higher temperature applications.
Sometimes efficiencies can be lost with heat exchangers, pumps & initial costs to set it up correctly.
Can you fill us in more detail.

[laudrup]

Thanks Ranger1,

The ammonia goes from multiple parallel evaporative condensers to the economizer. The economizer is one that subcools the high pressure liquid ammonia line. This then goes to the economizer connection on multiple screw compressors in parallel that run as required.

Apologies that this is a bit light on detail. I will only get an opportunity to visit the site again in a few weeks, but I am trying to understand what could be possible and what the issues may be if they were to sub-cool the ammonia using the ice-water instead of the economizer.

I think that an economizer increases the COP of the cycle by ~5%. However, if they were able to get the same sub-cooling effect using the ice-water instead of having to divert some of the ammonia to the economizer, maybe they could get more cooling at the evaporator and might end up with a higher COP.

I am not necessarily convinced on this, anyway any guidance on what the implications could be is much appreciated. I will attempt some calculations as well but just trying to get a feel for the idea from people that might have done similar things before.

Thanks again for your help,
Laudrup

[RANGER1]

laudrup,
Knowing nothing about this plant I would leave economiser as it is.

For example Mycom 250VLD without economiser -10 sst 35 sct 1611.5 kw absorbed power 399.4 kw
with economiser 1735.8 kw absorbed power 403.2 kw
This equals 100 kw for next to nothing in power for 1 compressor

Cold water could be used for

make up water to processes, instead of ambient water if required
condenser water make up
cooling liquid line from condenser before economiser.

All things would have to be investigated, as some things sound good in theory, but are not worth investment.
Segei I am sure will respond when he spots this post.

Does plant have VSD control on all condenser fans?
Is there enough condensing capacity in summer?
do any compressors have VSD?
Do any compressors run unloaded?
Can suction pressure be raised at any point?
How is suction pressure controlled pressure or temperature?

[Brian_UK]

If you want some really heavy reading on ice banks then try this one... https://bit.ly/2k5DPc5

[laudrup]

Many thanks Brian_UK and Ranger1 for your responses. Appreciate the example info on compressor/economizer and advice on some of the factors to consider.

[Segei]

I agree with Ranger1. Sub-cooling liquid ammonia by cold water will give very limited reduction of the demand. Significant energy savings and reduction of demand can be achieved by optimizing/floating condensing pressure. For example. Lowering condensing pressure by 10-15 psig will save around 5% of compressor energy use and demand will be reduced by 5% as well. It will be good to have more info about operating set points and strategies of this plant.