ice-e info pack 9 heat reclaim-recovery

8/13/2019 ICE-E Info Pack 9 Heat Reclaim-recovery

1/4

Heat recovery and reclaim

All the heat extractedfrom the cols store is

lost to the ambient ifnot recovered: It canbe a very competitiveheat source.

In cold stores a refrigeration system is lifting

the heat load from the room temperature to a

temperature high enough to reject it to

ambient and hereby losing it. Also

compressed air systems produce waste heat

but the mail focus of this Info Pack in on therefrigeration system.

In theory all the heat can be

recovered but unfortunately some

constraints exists the temperature levels being

the largest.

Amount of heat

The heat rejected from the refrigeration

system is the sum of the heat load (the cooling

load) and the power supplied to therefrigeration compressor as illustrated in the

figure.

The efficiency of the heat recovery can be

expressed as

Refrigeration

system

Heat load

on room

Rejected

heat

Electrical

power

COPHP relate the COP of refrigeration (COPR)

as

If the heat is recovered without changing the

running condition (the condensing pressure) of

the refrigeration system one could argue that

the heat is for free as the electrical power

supplied to the compressor with or without the

heat recovery.

Heat sources

As described in the Info Pack Compressors

the most used compressor types in cold storesare reciprocating and oil injected screw

compressors.

On oil injected screw compressors quite a

considerable part of the heat (up to 15%) is

rejected through the oil cooler at quite high

temperature (up to 90C) whereas the other

compressor type only the heat rejected in the

condenser is in question.

The heat recovery is normally done by installing

an extra heat exchanger in series and upstreamthe condenser / oil cooler. This makes it

possible to operate the refrigeration system

when no heat recovery is done.

Constraints from themedia to be heated

Just looking at the heat available for recovery is

unfortunately not enough. The major constraints

are

1. Inlet temperature of media to beheated

The only reason forrecover heat is toreduce the cost ofheating based onanother source

ICE-EINFORMATIONPACK


2/4

2. Flow of media to be heated

3. Required outlet temperature of

media to be heated

4. Simultaneity between waste heat

production and requirement for

heating

Of 1: This is the lowest possible temperature

for the condensation temperature if all heat is

to be recovered (not taking into account

possible sub cooling of the refrigerant).

Of 2 and 3: This is to some extend given by

the amount of heat required or the outlet

temperature of the media to be heated.

Of 4: A storage tank is often needed in orderto level out the difference in the time of

production and the consumption of the waste

heat.

Temperature levels

As mentioned the heat recovery can be

considered as for free if the running condition

of the refrigeration system is not changed.

When considering haet recovery the systems

have to be divided into

Traditional refrigeration cycles

Transcritical CO2cycles

Traditional refrigeration cycles

Considering the traditional refrigeration

cycles (R404A, R717 etc.) the condensing

temperature has a direct impact on the

running cost and therefor minimized and this

normally limits the temperature of the

recovered heat too much. The temperature

can depending on the refrigerant be raised a

little by installing an extra heat exchanger

before the condenser (a desuperheater) in

which the quite warm discharge gas is

cooled before entering the condenser.

In figure 1 the heat recovery is shown for a

R717 system operating at 75C condensing

temperature cooled by a media being

heated from 40 to 80C. The heat

production is 100kW. In this example the

condensing temperature is raised compared

to normal refrigeration system.

The process is limited at the lowest possible

temperature difference which is found at the

pinch point. In figure 1 this is found at the

point where the condensation starts (at

80kW).

Figure 2 shows the same process but now

the water is heated to 95C. This process is

not possible as the refrigerant (red line)

always has to have a higher temperature

than the media to be heated. In this case

the condensing pressure has to be raised to

approx. 85C to have the pinch point at

80kW above the green line.

Transcritical CO2cycle

The CO2cycle has some advantages when

it comes to heat recovery and as the

process is not that well known there is avery short introduction in the following:

As CO2has a so-called critical point at 31C

the heat rejection will often be a supercritical

process: Above the critical point it is not

possible to condense the refrigerant. The

CO2 is to be considered as a more and

more heavy gas as the heat rejection is

progressing. This also means that the

temperature is falling during the heat

rejection: It has a glide.

The process is shown in Figure 3. Three

Figure 1 Temperature slope in heatrecovery condenserred line: refrigerant

blue line: water, heated 40 > 80C

Superheat

Subcooling

Condensing

Figure 3 Trans critical CO2refrigeration cyclesEvaporating temperature -5C, gas cooler outlet temperature 40CGas cooler pressure: Green: 90 bar a, red: 100 bar a and blue: 120 bar a(Source: Danfoss)

Figure 2 Identical to Figure 1

green line: water, heated 40 > 95C

ICE-E INFO PACK


3/4

ICE-E INFO PACK

identical cycles are shown, all rejecting heat

down to 40C. The three COPs shows that in

contrast to traditional cycles the COP can

raise at raising compressor discharge

pressure. The critical parameter for the cycle

is the temperature out of the gas cooler, if allheat is to be recovered the media to be

heated has to have a low inlet temperature.

As the super critical CO2 has a temperature

glide it better fits the slope of for instance

water as media to be heated. This is shown in

Figure 4 where water is heated from 15 to

75C. Even though water is quite cold at the

inlet it is not possible to utilize this to cool the

CO2 all the way down and the gas cooler is

rejecting the last part of the heat to ambient

(and losing it).

If a traditional refrigeration cycle was used for

making 75C water, the condensing

temperature had to be raised to 78C. The

CO2 cycle was not changed!

The energetic cost ofrecovering heat

As stated the recovered heat is from an

energetic point of view for free (excluding

extra pumps etc.) as long as the conditions for

the refrigeration system are not changed.

But often raising the condensing temperature

is needed which will cause the electrical

power consumption to go up: The recovered

heat is no longer for free.

Instead of using the COPHR as the measure

for the efficiency of the recovery the cost of

recovery can be compared to the extrapower

consumption due to the altered running

condition. This can be expressed as the

Efficiency of Heat Recovery, EHRdefined as

EHRis sometime called marginal COP or COP

of heat recovery.

EHR can be very high even when the no

change is done to the running condition.

Actual it can turn negative implying that

energy is produced! This is due to the fact that

in the case of 100% recovery the ambient

condition does not affect the running condition

any longer. In the case of on heat recovery

a further raise in ambient temperature could

have caused an increase in running

condition compared to the situation of 100%

recovery. This situation is possible ontranscritical CO2systems.

EHR can also be very low: In the case of

only a minor part of the heat is recovered

and the condensing pressure is raised also

the heat rejected to ambient is done at the

elevated pressure. In other words the COP

for the whole refrigeration capacity is

affected negatively in order to recover only a

little part of the heat.

Heat pumpsThe conditions shown in Figure 1 and 2 is

not possible to achieve in ordinary NH3

refrigeration systems as the condensing

pressure (> 40 bar a) will be too high for a

standard compressor. In this case a heat

pump compressor can be (or a complete

heat pump system) installed turning the

system into a kind of two stage system (see

the Info Pack on Refrigerant cycles).

Doing this will decouple the running of therefrigeration system and the heat recovery

and the before mentioned situation of very

low EHRwill be eliminated. Or more precise

it will be eliminated in some case:

When installing a heat pump the

refrigeration system (and cold store) has to

be thoroughly analyzed and optimized: The

installation of a heat pump can look very

promising when the condensing pressure is

high but it can turn out to be bad in the case

the condensing pressure is optimized and

lowered.

Strategy of heatrecovery

If the heat is recovered to a storage tank

which is often the case in facilities where the

recovered heat is used for cleaning

purposes two strategies exists:

1. Build up a stratified layer in the

tank having cold water in the

Figure 3 Heat recovery on transcritical CO2

refrigeration cycles.Water heated form 15 to 75C.Pinch point inside the HRHX (Heat RecoveryHeat eXchanger)CO2further cooled in an air cooled gas cooler

Figure 5 Heat recovery heat exchanger for oil

system on screw compressor


4/4

ICE-E INFO PACK

bottom part and recovering the heat

to the required temperature (e.g.

60C) in the top.

2. Charging the tank being fully mixed

The last strategy is the best from an energeticpoint of view: The EHR will be high in the

beginning and decline throughout the

charging the tank. In strategy 1 the EHR will

have the low value throughout the charging.

And the amount of energy in the tank will be

the nearly the same: Strategy 1 will charge a

little bit more resulting from the lower EHR. In

other words an electrical heating element in

the tank would have given the same.

Investment and paybackThe only reason for recover heat isto reduce the cost of heating basedon another source.

One has to keep this in mind when making

the cost benefit analysis of heat recovery. The

sensibility to future change in price of the

competing heat source, future change in the

operation of the cold store etc. must be taken

into account. Into this picture has to be taken

the possibility of selling the recovered heat.

ICE-E INFO PAC

ICE-E INFO PACK

ICE-E INFO PACK

ICE-E INFO PACK

For more information, please contact: Lars Reinholdt ([email protected])

Figure 5 Heat recovery heat exchanger fordesuperheating on screw compressor system

ice-e info pack 9 heat reclaim-recovery

Documents