Heat recovery and aquifer heat/cold storage

[RicHoe]

Hello,

as a student, I am doing research on energy efficiency improvement for supermarkets in the Netherlands. We are trying to find out what the energy saving potential is of a combined system of heat recovery from refrigeration/freezing and aquifer heat storage. With such a system it must be possible to meet the total heat demand of a supermarket by storing the 'waste' heat into the ground.
We have several problems with calculating the system:
- what will be the COP improvement if the condensor is cooled by groundwater (T=11 ^oC)?
- how do we calculate this?

It would be very nice if someone has some information on this.

[NoNickName]

It depends on the refrigerant.
Normally, you would not be allowed to condense at a temperature lower than 25 or 30°. So it doesn't make any difference whether you are using acquifer or other waste waters.

[Andy]

It depends on the refrigerant.
Normally, you would not be allowed to condense at a temperature lower than 25 or 30°. So it doesn't make any difference whether you are using acquifer or other waste waters.

Low condensing pressures can be designed around, with bigger expansion valve sizes for lower pressure drops, or my favorite, electronic expansion valves.
On larger flooded systems head pressure is less of a problem, with low condensing pressures being the norm.

Kind Regards Andy

[Peter_1]

We have a pack which runs in winter conditions with a DP of +/- 3 bar.
And with normal Danfoss TEV's which were selected for a DP of 4 bar.

The COP improvement is huge, compared to what you mostly see in sueprmarkets.. a stable HP whatever teh outside temperature is.

You can use Coolpack for this. Free downloadable.

[US Iceman]

Normally, you would not be allowed to condense at a temperature lower than 25 or 30°.

Why so?

If the refrigerant flow control valves (TXV or EEV's) can accommodate the required flow rate at the available differential pressure the system should not be affected. In this I agree with Andy.

Now on the other hand, if the minimum condensing temperature is fixed due to heat recovery requirements the higher temp.'s might be acceptable or required.

...it must be possible to meet the total heat demand of a supermarket by storing the 'waste' heat into the ground.

This would not be much different than using bore holes for ground source heat pumps. Except... in an aquifer the ground water flow may remove some of your heat.

[Peter_1]

Richoe,

There are systems in The Netherlands where they used this technique already.
I think there was once an article in Koude en Klimaat.
Do you read sometimes Cool& Comfort, the Belgium HVAC/R magazine?

[NoNickName]

Envelopes of compressor do not allow such low Tc even with appropriately sized components.

[US Iceman]

Envelopes of compressor do not allow such low Tc even with appropriately sized components.

I have seen similar items for other compressors also.

My question to the compressor manufacturers is why can't they go lower?

Unless there is a specific design limitation of the compressor, the performance envelopes are just an arbitrary limitation that may have nothing to do with the actual compressor limits.

[Peter_1]

5.5 bar DP is within the limits of Bitzer which gives COP of +/- 4.2.
Some scrolls of Copeland only needs 15°C condensing; (-5°C evaporating) and only 10°C at -10°C evaporating (= +/- 3.75 DP)

[Andy]

Envelopes of compressor do not allow such low Tc even with appropriately sized components.

Reciprocating compressor performance improves at lower condensing pressures.
It's not that they can't operate at these conditions, just that they are not tested at these conditions.
Bitzer screws will run at very low heads
Copeland scrolls will not
Bitzer Octogons will
What about the new small rotary compressors that are coming onto the market
Kind Regards Andy

[Peter_1]

Copeland scrolls can at -10°C (14°F) go as low as 7°C (45°F) condensing temperature.
Selected via 'Select 6' of Copeland.

Same story for minimum alowable speed. Why may you not go to 10 Hz with for example a Bitzer if oil differential pressure is still 4 bar (60psi)?
Bitzer gave me once an answer on this (boundary layer lubrification or something like that)

[NoNickName]

Minimum condensing pressure is required, because at very low Tc the performance of the compressor increases with a lower refrigerant flow, and as such lower motor cooling.
Anyway, the point is that the working condition must be checked against the compressor envelope, which varies across manufacturers and models.

[Peter_1]

Minimum condensing pressure is required, because at very low Tc the performance of the compressor increases with a lower refrigerant flow, and as such lower motor cooling.
.

Yes and no.. with decreasing condensing pressure, also the needed power of the motor decreases, so there's much less heat generated by the windings.
Second, with a smaller compression ratio, the body is less heated so that more heat can be dissipated via the body.
You still stay below max current and there's allways the Kriwan in many compressors.
But I never have measured it and these are just some thoughts.

I just now we ran for several weeks a pack with Bitzers at a minimum frequency of 10 Hz and with a DP of a little bit more then 3 bar.
We monitored/logged closely LP and OP those weeks and you couldn't see almost no difference in OP when speed decreased (perhaps 0.2 bar)

[US Iceman]

...because at very low Tc the performance of the compressor increases with a lower refrigerant flow,...

The mass flow does not change due to a decrease in Tc. The mass flow is fixed at a constant speed and suction pressure (fixed density & volume).

The compressor capacity does increase due to the lower liquid enthalpy, and the motor amps (& motor cooling)will decrease due to the lower pressure ratio from the reduced Tc.

[NoNickName]

The mass flow does not change due to a decrease in Tc. The mass flow is fixed at a constant speed and suction pressure (fixed density & volume).

The compressor capacity does increase due to the lower liquid enthalpy, and the motor amps (& motor cooling)will decrease due to the lower pressure ratio from the reduced Tc.

Error in translation. I meant ...because at very low Tc the performance of the compressor increases compared to the refrigerant flow,...
But no, the motor amps will not decrease, because as the Tc lowers, Te lowers as well.

[Peter_1]

Error in translation. I meant ...because at very low Tc the performance of the compressor increases compared to the refrigerant flow,...
But no, the motor amps will not decrease, because as the Tc lowers, Te lowers as well.

Do once a calculation with Select 6 of Copeland or Bitzer software, you will see that for a given capacity with decreasing TC it decreases a lot.

[Josip]

Hi,

Quote:

...because at very low Tc the performance of the compressor increases with a lower refrigerant flow,...

The mass flow does not change due to a decrease in Tc. The mass flow is fixed at a constant speed and suction pressure (fixed density & volume).

The compressor capacity does increase due to the lower liquid enthalpy, and the motor amps (& motor cooling)will decrease due to the lower pressure ratio from the reduced Tc.

Quote:

The mass flow does not change due to a decrease in Tc. The mass flow is fixed at a constant speed and suction pressure (fixed density & volume).

The compressor capacity does increase due to the lower liquid enthalpy, and the motor amps (& motor cooling)will decrease due to the lower pressure ratio from the reduced Tc.

Error in translation. I meant ...because at very low Tc the performance of the compressor increases compared to the refrigerant flow,...
But no, the motor amps will not decrease, because as the Tc lowers, Te lowers as well.

Sorry, maybe I am the only one (not understanding your points) please, can you write down what is constant if decrease Tc (condensing temperature) only.

Best regards,

Josip

[Dan]

As far as low compression ratios go, I think you can enter the realm where reed valve flutter can occur. This was painfully demonstrated with Carlyle and Copeland compressors in two-stage applications. I believe this is why an envelope is drawn that limits the condensing temperature to 70 Deg F or whatever it is these days.

I honestly don't know why a rotating compressor would have such a limitation.

[NoNickName]

Do once a calculation with Select 6 of Copeland or Bitzer software, you will see that for a given capacity with decreasing TC it decreases a lot.

We have already discussed this.
You can't simulate this. Because you don't know how more kW will result in the evaporator. They will result in a lower Te, and as such a higher current absorption.
This is also a side effect of a higher subcooling.

[RicHoe]

Thanks for your information, this is really an interesting forum.
We've just made an easy calculating using Coolpack and EKS for an ADAP-Kool system which has, according to EKS, an average COP of 4,1. By fixing the condensing T to 20 C, the COP would become 4,8 (Coolpack). We assumed that it must be possible to set the condensor temp. to 20 C + 4 K of subcooling by cooling it with 11 C ground water, but couldn't find figures for this.

[Dan]

Copeland indicates that the minimum condensing temperature is 70 deg F in their performance data, but you might look at their application engineering bulletin No. 1334. Much lower condensing temperatures are discussed in this bulletin as well as heat recovery and unloading considerations. Not sure the link will work if you haven't registered, but here it is.

https://opi.emersonclimate.com/CPID/...EB/ae-1334.pdf

[US Iceman]

Dan,

Thanks for that link. I have been looking for something like this for a long time.

[Dan]

Iceman, it was actually a discussion we shared regarding the 70 deg F low limit that encouraged me to hunt this bulletin down. I wish Copeland's software would let us extrapolate the compressor performance curves to points that are "outside the envelope."

[US Iceman]

Dan, I have been having a running battle with Copeland this week trying to find information. I asked some questions about compressor limits and they told me to read the bulletins. That's it.

About as helpful as a dog would be to a cat.

I did quickly look at the bulletin you found. It seems they are now saying it is OK to run at 40F (4.4C) for the condensing temperature for certain refrigerants.

I'm glad they didn't know about this 20 years ago.