Buffer Tank Sizing

Hi all!
I am trying to size a buffer vessel...(thermal storage tank). It on-ditioning of a Commercial Building, using a water cooled chilled water system. I intend to have the buffer vessel so that i can discharge it during the time period of 6 pm to 10 pm...(4 horus)...so that i can reduce the peak electrical consumption of the building.
Can anybody help me with where to find details for the sizing. any formulae...or anything..book, website!
The total design load of the building is 600 tonnes, we can safely assume that the average load is around 80% of that.the operating temperatures are between 7 C and 12 C, and no glycol is expected to be used.
Also how do i design the pressure vessel, what all pressures should i consider?
Regards
Deepak Bhat.

You definitely have to store the energy in ice, otherwise the storage tank will become larger then the building.

I once went to France to a company called Cristopia a technical meeting where they made some promotion for these systems

Have once a look at http://www.cristopia.com/

Furthermore, there is Baltimore Air Coil who makes ice-banks.

Packo, a large stailness steel manufacturer in my street makes also ice-banks.

Do once a Google on ice-banks.

In the ASHRAE books can information be found regarding ice-banks.

I saw energy calculation programs - posted once here some links - where you could enter ice-banks.

The average load is 600 *.8 or 480 ton and this for 4 hours so 2.000 ton or +/-600 kW.

So you can calculate now theoretically the ice volume you need

http://www.ibkcompac.nl/inhoud/toepa...e/overige.html and click on the pdf link Ijsbuffer-installatie.
It's in Dutch be you have an idea of the size, +/- the systems you need.

So, this can be start.

I agree with Peter. Ice makes a much better thermal storage media. You get more energy storage potential in every cubic meter of volume.

In the thermal storage sector we normally use the term ton-hours (here in the US). As Peter described, 480 Tons X hours hours is approximately 2000 Ton-hours of cooling (1920 to have the exact number).

You have to include the pumping energy as a heat gain, since this can increase the heat load on the storage media. These heat loads also include possible heat gain through the tank.

The actual storage volume will be higher than the calculated volume due to the additional heat loads that will be found.

Storage tanks are a little tricky to design.

For water storage you have to consider the stratification of the cold water and warmer water in the tank. This level of stratification is called the thermocline layer. Cold water on the bottom, warmer water on top. The thermocline layer is were the difference in water density starts to appear (a simple explanation).

Diffusers and specific piping installations are required in the tank to prevent disrupting this thermocline layer, otherwise you have problems with mixed water temperatures, instead of two distinct layers; cold and warm. Some tank designs use a bladder to separate the warm and cold water in the tank.

Ice storage has it's own problems. Ice banks must use an air agitation system (usually an air blower) to promote even melting of the ice on the tubes. Other problems include uneven melting, partial melting, and others.

For another source for ice builders look at:
http://www.evapco.com/

(the information is under the products menu for thermal ice storage)

If the ice storage is for ice harvesters (a fancy term for ice released from the ice forming surface), the ice forms a pile in the storage tank in the water. The ice can build up in what is called an angle of repose.

This is like pouring sand out of a bucket. The pile of sand forms at a certain angle as the sand accumulates. This angle requires a design consideration during the selection of the ice/water storage tank.

For information on ice harvestors and ice slurries review this link:

http://www.muel.com/products/thermalstorage/

ASHRAE is a good source for design information.

Hope this is useful for you.

Actually I prefere chilled water storage tanks instead of ice storage tanks because it is easier to be controlled.

The sizing procedure is as in the attached file that I used to give it to my customers.



I hope this will help

regards

A. Fraij

But then you will need them much larger then ice banks.


Chemi:)

Fraij,
The system you describes in your Word file may not be seen as a buffer tank to store huge amounts of energy.
It's only fitted between the system to prevent short cycling of the compressor.
You see or I have to say, you saw it in chillers where there was only 1 or 2 compressors which couldn't work in part-load.
The chillers they make now these days have multiple compressors (scrolls) and some are even VFD ontrolled. So the available capacity can perfecly be adapted to the needed capacity. So you don't need the buffers anymore.

Given that a DT of 5 K is acceptable in your system, you need a tank of +/- 10 ft x 10 ft x 300 ft, so where will you gonna put these? On the roof?

And, if your system was designed with a leaving water temperature of 6°C (so evepaorating at 0°C), you will need to start with a water temperature of +/- 0°C (to store some energy. If you start at 6°C, then as soon the pumps starts, the stored energy doesn't match the needed energy anymore.
But making water of 0°C will say that you have to lower your evaporating temperature to -6°C, so the COP of your machines drops seriously.
So what are the savings then?

You can store +/- 80 times more energy in ice than in water, so why not use ice?

I'm loud thinking now perhaps a silly idea, but suppose you make an ice buffer during the night (in Belgium is the kWh price during the night hust half the price as you pay during the day, 0,16 €/kWh)
You use this stored energy during the day to condens very low or to subcool??

Some process applications can have high instantaneous loads with precise temperature control requirements. In these applications a buffer tank is required to store the cooling capacity, rather than by providing a chiller with a larger capacity.

This does not occur in an office building, so the tank would be used for cool energy storage to offset energy costs.

Chilled water storage tanks are very, very big. However, the application of chilled water storage is easier for some people to accommodate, since the chiller is usually already in place.

Ice storage systems typically need refrigeration engineers to design them, since you cannot pick the system from a catalog.

The ice harvesters or slurry ice appears to be the best method in my opinion. Although the ice builders have been around longer and more people are familiar with them.

A good ammonia refrigeration system and an ice storage system. A good way to go!;)

Hi guys
hey peter i think u are right about the fact ice is a better thermal storage medium than water. Besides your point of ststing the ice can be mande in the night is cent percent correct!....I am based in india...bombay...here there is a chance that time of day tarrif may be introduce beacuse of which using the chillers in the night time, making a buffer capacity enough to take care of the laod during the peak electricity load hours like in the evening tmie is what i am looking for.
Besides, in india cristopia here is working in collabration with a firm called KHEMS engg. who do these kinds stuff.....i have worked with them, just that i thought i must get as much knowledge as i can!
I have conducted energy audits for a few hotels here and when i draw up their consumption chart(tonnage VS time of day) there seems to be a gr8 potential to reduce the peak load of the building( for HT consumers it is important that the peak load reduces!) and also save up on the energy cost be effective use of the buffer tank.
Well it was gr8 to read the replies...thought i read it quite late...thanks for all the help!
Regards
Deepak Bhat

We had once the idea - but never tried it - to make some sort of a copy of the Cristopia system by filling PET Cola Bottles with a water/glycol mix - we also used a water/salt mix and found this the best mix - and store these bottles in an isolated tank.
We did dome test with these bottles in a freezer to see how it should react when you solidify the bottles.
That's all we did but with the oil prices going up.....perhaps we should start again some thinking around this idea.

There is for me a big disadvantage with this system, and I will try to explain why. We discussed this while we sat together with a representative of Cristopia and our client.
A Cristopia system is a large isolated reservoir filled with plastic balls. In these balls is a mix with an eutectic point +/- 6 to 10 K lower than then the needed supply water.
So, our client was thinking about peak shaving and the Cristopia system could be a solution for all the meat cutting rooms which has to be held at 10°C (supply water of +/- 2°C)
Suppose a common waterchiller application; if you need a supply water of 2°C, the corresponding evaporating temperature will be -5°C (+/-), so you can use almost fresh water.

But with the Cristopia system, the same solution (water/glycol mix) that circulates to the coils is also used to solidify the balls.
During the night, you have to solidify the mix in the balls (-6°C eutectic point) so you need to circulate the liquid at a temperature of -12°C. So a lot of glycol has to be used in the whole system, penalising the overall performance of the chiller (adding glycol makes the unit less powerfull, the COP drops)
This lower VCOP due to the glycol is allways valid, during day and night.
But to achieve a temperature of -12°C for the mix, you need to evaporate at -18°C. So, in a conventional set-up, we can evaporate at -5°C where we have to do this at -18°C during the night.
Only this is counts for a COP drop of +/- 30%, so much of the savings of the cheaper electricity is lost due to this lower COP.

I read an article saying that Ice Storage would reduce the efficiency of your chiller. Is that true?

I prefer binary ice (slurry), SST can be some what higher than a convensional ice builder, but I also think it is just as important to allow the SCT to drop in the colder hours (at night), this of course can bring its own problems.
But all these systems are out of the normal range of the high efficient centrifs(not feild of speciallty) i would look at a high temp phase change material (melting point between 6 & 8C) by managing load profile, the higher temp circulated water will meet actual demand (no need to upgrade the refrigeration equipment)

Hi

FYI, In the M.E. these central cooling plants use very large buffer tanks for storing chilled water at less han 4.5 deg C charged during the night to reduce peak power during the day. On a 25000 Rt plant the storage tank is over 3 storeys high. Dont have storage volume.

No glycol used, only water.

Alan

Mr.Bhat can you give your contact no...I am also in same field of HVAC

Regards,
Atul Kulkarni
9322277717