How to use cold air mass flow to support supermarket cooling?

[tjrk]

TL;DR Summary

Cold air mass flow to support supermarkt cooling

An air mass flow of approx. 120 kg/h is available, which can or should be heated up from 0 °C to up to 150 °C. The cooling circuit based on propane R290 is to support the cooling of a supermarket. I have the load curve of the power consumption of the supermarket refrigeration to cool food at a temperature of 4 °C and the air mass flow of the cold air.

How can I conclude from the cold air mass flow to the saved electricity consumption for cooling and is this system reasonable?

Background of this idea is a techno-economic analysis of an energy storage based on compressed air. The usage of cold air could enhance the overall efficiency, the only question is how much energy this could save for cooling the supermarket.

 

[Chestermiller]

Please provide a schematic diagram of what you envision the system to be.

 

[tjrk]

Please provide a schematic diagram of what you envision the system to be.

A schematic overview of the system is attached to my post.

 

[Chestermiller]

A schematic overview of the system is attached to my post.

Sorry. I really don't follow what you are trying to do.

 

[tjrk]

Sorry. I really don't follow what you are trying to do.

Please excuse my inaccuracy.

The subject of the study is an existing supermarket with a cooling system (load: 12-20 kW) based on R290 and a solar roof.
An A-CAES (adiabatic compressed air energy storage) is being developed to temporarily store the surplus PV electricity so that it can also be used at night. When the A-CAES is discharged, the compressed air is expanded and the air cools down. This cold air has a mass flow rate of 120 kg/h at a temperature of 0 °C.

This is to be the starting point for developing a refrigeration machine. My focus is on the economic consideration, which is why the amount of energy saved by the original cooling system is relevant for me.

Currently, I have the load profile of the cooling system from the supermarket and from the PV generation. I hope this clarifies my question.

 

[berkeman]

@tjrk -- Welcome to PF.

Is this by chance a school project? What is your educational background so far?

 

[Andrew Mason]

TL;DR Summary: Cold air mass flow to support supermarkt cooling

An air mass flow of approx. 120 kg/h is available, which can or should be heated up from 0 °C to up to 150 °C. The cooling circuit based on propane R290 is to support the cooling of a supermarket. I have the load curve of the power consumption of the supermarket refrigeration to cool food at a temperature of 4 °C and the air mass flow of the cold air.

How can I conclude from the cold air mass flow to the saved electricity consumption for cooling and is this system reasonable?

It is not clear what you are asking. If you are trying to use waste energy from a cooling system, you need to find some use for heat flow from the condenser.

Modern supermarkets may have a central refrigeration system that will serve all the refrigerators and freezers. Instead of letting that energy dissipate to the outside, it can be used to heat warehouse space or heat water. This is not a way to store energy, however.

You can't retain the compressed refrigerant to store energy. The output of the compressor has to be cooled in the condenser and then throttled and expanded and cycled through the refrigerators and freezers back to the compressor.

AM

 

[russ_watters]

The subject of the study is an existing supermarket with a cooling system (load: 12-20 kW) based on R290 and a solar roof.
An A-CAES (adiabatic compressed air energy storage) is being developed to temporarily store the surplus PV electricity so that it can also be used at night. When the A-CAES is discharged, the compressed air is expanded and the air cools down. This cold air has a mass flow rate of 120 kg/h at a temperature of 0 °C.

This is to be the starting point for developing a refrigeration machine. My focus is on the economic consideration, which is why the amount of energy saved by the original cooling system is relevant for me.

Currently, I have the load profile of the cooling system from the supermarket and from the PV generation.I hope this clarifies my question.

Some.... In the diagram you appear to be blowing the cold air across the condenser of an air conditioning unit. Why? If the air is at 0C already, the compressed air system is already "a refrigeration machine". Why can't you use the cold air directly? Just blow it into an air conditioning duct, to mix with the return air to the system?

Back of the envelope, 120 kg/hr at 0C is roughly 1.2 kW of cooling, which for a typical air conditioner costs about 260 W of electrical input. That's about what you could save by injecting the cold air into the return air duct of an AC unit.

 

[Andrew Mason]

I have looked at this again. If the idea is to use the excess output of a solar panel array to compress air adiabatically in a tank, let the tank cool down and then release the air to cool something, you are wasting most of the energy.

First of all, the Joule-Thompson co-efficient (the rate of change of temperature to change in pressure with no change in enthalpy) for air is effectively zero. While the compressed air does cool when expanding into surroundings at lower pressure (because the expansion into the atmosphere does work), the cooling is not comparable to the cooling of a hydro-fluorocarbon refrigerant (in which the greater cooling in rapid expansion is due to the conversion of molecular kinetic energy to molecular potential energy - the latter not contributing to temperature). To maximize cooling when a fluid expands you need a substance with a low negative Joule-Thompson coefficient.

Second, if you are using air as a coolant, in order to make the air as cool as possible you would need to start with a compressed gas at ambient temperature. But this means that much of the energy stored in the hot air in the tank after adiabatic compression is lost. You need to insulate the tank to retain all the stored energy.

The best way to maximize cooling from the stored hot compressed air would be to use it to run the compressor in the refrigeration system. For example, the hot air under high pressure could be used to drive a reciprocating piston or turbine fan that would then drive the compressor in the refrigeration system.

AM

 

[sophiecentaur]

The best way to maximize cooling from the stored hot compressed air would be to use it to run the compressor in the refrigeration system. For example, the hot air under high pressure could be used to drive a reciprocating piston or turbine fan that would then drive the compressor in the refrigeration system.

This would be a heat engine with very low thermodynamic efficiency.

Compressed air is a poor energy storage medium. Energy storage is a hot issue these days and there are very few systems that really work or are economic to run. How about an enormous flywheel? -it's been done before. This link makes good reading.

If you want to 'store cool' then make loads of ice in the daytime. Water based systems are cheap and low tech. They used to have cold stores in large country houses in historic times and chilled storage worked from one winter to the next.

I think the OP is attracted to a particular new system and wants to use it at all costs. Bottom line is the prices of the goods for sale. I would wait for others to put the first toes in the water.

 

[Andrew Mason]

This would be a heat engine with very low thermodynamic efficiency.

No. It would not be a heat engine at all. All work done by the expanding air would be adiabatic work.

 

[sophiecentaur]

All work done by the expanding air would be adiabatic work.

OK but how would that be achieved in practice?

 

[Andrew Mason]

OK but how would that be achieved in practice?

Just connect a hose from the air tank to a pneumatic motor set up to run a compressor for the cooling system. It is adiabatic work because there is no heat flow - no source of heat involved.

It would be more efficient to use the excess output from the solar array to charge a battery that would then be used to power an electric compressor motor. But if the OP wants to insist on using compressed air for cooling, this is much more efficient than just letting the compressed air flow over the parts to be cooled.

AM

 

[sophiecentaur]

Just connect a hose from the air tank to a pneumatic motor

Filling a diver's air bottle will heat it up. Is there any way that the big bottle elevated temperature could be maintained? Energy will be wasted - particularly because of the very high temperatures involved.
Choosing to do a job with one particular method is not usually a good thing because usually a random choice will not produce the best outcome. Batteries are a known quantity and that particular cycle is favoured for good reasons.

 

[russ_watters]

I feel like you guys are violently agreeing with each other except on one pedantic and irrelevant point.

 

[Andrew Mason]

I feel like you guys are violently agreeing with each other except on one pedantic and irrelevant point.

We agree that the use of compressed air proposed by the OP is a bad idea. Re: the point about the use of compressed air to run a pneumatic motor is not irrelevant. Pneumatic motors are actually quite efficient - not quite as efficient as electric motors but close - and much more efficient than any heat engine.

AM

 

[jrmichler]

Pneumatic motors are actually quite efficient - not quite as efficient as electric motors but close - and much more efficient than any heat engine.

No. Pneumatic motors are useful in certain cases, such as very hot environments. The efficiency is very low.

For example, a Gast 6AM-NRV-7A air motor requires 128 CFM of compressed air at 100 PSI to deliver 4.0 hp. An Ingersoll-Rand "Next Generation R-Series Oil-Flooded Rotary Screw Air Premium Efficiency" air compressor rated to deliver 123 CFM at 110 PSI has a 25 hp motor. Air compressors are designed to draw the full motor power when operating at design load, so that compressor is using the full 25 hp. The air motor efficiency is power out divided by power in: $Efficiency = 4/25 = 16$%.

Data sheet for the Gast motor: https://www.gastmfg.com/sites/default/files/2019-07/std133.pdf
Data for the I-R air compressor: https://www.ingersollrand.com/en-us/air-compressor/oil-flooded-ac/ng-r-series-15-22-kw-20-30-hp

 

[berkeman]

Filling a diver's air bottle will heat it up.

That's one of the reasons we often fill our scuba tanks in a water bath. What's the other reason?

 

[sophiecentaur]

What's the other reason?

Spotting bubbles due to a leak?

 

[Andrew Mason]

No. Pneumatic motors are useful in certain cases, such as very hot environments. The efficiency is very low.

For example, a Gast 6AM-NRV-7A air motor requires 128 CFM of compressed air at 100 PSI to deliver 4.0 hp. An Ingersoll-Rand "Next Generation R-Series Oil-Flooded Rotary Screw Air Premium Efficiency" air compressor rated to deliver 123 CFM at 110 PSI has a 25 hp motor. Air compressors are designed to draw the full motor power when operating at design load, so that compressor is using the full 25 hp. The air motor efficiency is power out divided by power in: $Efficiency = 4/25 = 16$%.

Data sheet for the Gast motor: https://www.gastmfg.com/sites/default/files/2019-07/std133.pdf
Data for the I-R air compressor: https://www.ingersollrand.com/en-us/air-compressor/oil-flooded-ac/ng-r-series-15-22-kw-20-30-hp

I was referring to the efficiency of a pneumatic motor to convert the energy in a compressed air source into useful mechanical work. Turbine fans working with compressed air can realize efficiencies of 65%. See: https://library.automationdirect.com/pneumatic-efficiency/

But you are right that, overall, using compressed air as an energy storage device is not efficient for energy storage, as pointed out earlier. There are huge losses in setting up the compressed the air source . But once you have the compressed air, converting it to mechanical work can be fairly efficient.

AM

 

[berkeman]

Spotting bubbles due to a leak?

Not exactly...

Spoiler

Unless you count an exploding scuba tank as a "big bubble".

 

[Vanadium 50]

Energy storage is a hot issue t

I saw what you did there.