Industrial absorption refrigeration plants with storage facilities

Industrial absorption refrigeration plants with storage facilities G. M. H o l l d o r f f

Installations frigorifiques absorption industrielles avec

dispositifs de stockage

S//e chauffage, ou /es besoins, sont intermittents, /e syst#me frigor/fique b absorption incorpore un stockage de frigorigbne /iquide et d'une solution faib/e, ce qui procure des avantages thermodynamiques et de fonctionnement.

On peut uti/iser /a cha/eur perdue pour refroidir en se servant soit du syst~me frigorifique ~ absorption soit du systbme ~ compression m6canique entra?nb par cycle Rankine,

On exp/ique /e principe de fonctionnement d'une te//e installation frigorifique ~ absorption avec /e syst6me binaire NH3--H20 et on/e compare aux mbthodes usue//es.

Nomenclature Refrigeration capacity QH Heat energy Heat ratio AVK Storage volumen for refrigerant AVRL Storage volumen for strong solution AVA, Storage volumen for weak solution Specific evaporator enthalpy difference qo f Specific solution flow Oo

Waste heat can be used for refrigeration purposes by means of either the absorption refrigeration system or the mechanical compression systems driven by a Rankine cycle. If the heating, or the duty, is intermittent the absorption refrigeration system can

R e f r i g e r a t i o n systems using w a s t e heat Waste heat can be used for refrigeration purposes by three different methods

CK Ca Cr mK mRL mAL PK PRL PAL

Concentration of refrigerant Concentration of weak solution Concentration of strong solution Mass flow of refrigerant Mass flow of strong solution Mass flow of weak solution Specific density of refrigerant Specific density of strong solution Specific density of weak solution

incorporate the storage of liquid refrigerant and weak solution with thermodynamic and operational advantages. The working principle of such an absorption refrigeration plant with the binary system NH3--H20 are explained and compared with conventional methods.

absorber, solution pump, solution heat exchanger and desorber. For evaporation temperatures below O°C the binary system, ammonia/water, has to be used.

The author is chief engineer at BQRSlG GmbH, Berlin. Germany. Paper received February 1980

Mechanical refrigeration system driven by a Rankine ' cycle (Fig. 2). This system is not new: every refrigeration compressor driven by a steam turbine , applies this principle. Only recently working mediums other than steam - - preferably organic fluids - - have been considered for the recovery o f low temperature waste heat.

Volume 3 Number 5 September 1980

0i 40-7007/80/050273-06S02.00 © 1980 IPC Business Press Ltd and IIR

Absorption refrigeration Fig. 1 shows how the mechanical compressor of a refrigeration circuit is replaced by a "thermal compressor' consisting of

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I 2 3 4

A common working fluid is used for both circuits with a combined condenser (Fig. 3) which simplifies the system and offers operational advantages.

5

5 6 7 8 9

Use of i n t e r m i t t e n t heat e n e r g y Waste heat utilisation with any of these systems is difficult if.the heat energy is available only intermittently and does not coincide with the refrigeration demand. This occurs rather frequently in the chemical and food industries, whenever batch processes are used. Figs 4a-d illustrate typical examples plotting energy supply and demand versus time.

® --

~

Fig. 2 Refrigeration system driven by Rankine cycle (different working fluids) Fig. 2 Syst#me frigorifique entraTn# par cyc/e Rankine (diff#rents fluides actifs)

Fig. 4b is typical of the energy situation in breweries. The waste heat available in the water vapours from the kettle vent is out of phase with the chilling required by the wart. The phase shift i s caused by the fact that the same contents of the brew kettle are boiled first and chilled afterwards. Such cases however are not limited to breweries, in the chemical industry there are many similar processes. The problem of non-coincidence between heat energy and refrigeration demand can be managed either by adapting the momentary heat energy supply to the required refrigeration capacity or vice versa; both ways require storage systems. Low temperature waste heat cannot be stored economically, therefore it is necessary to store 'refrigeration capacity'.

Compressor Condenser Exponsion valve Evoporotor (chiller) Expander Heat exchonger Condenser Pump Evoporotor (boiler)

~oN

I 2 3 4 5 6 7 8

Compressor Expander Condenser Expansion valve Pump Evaporator (chiller) Heat exchanger Evaporator (boiler)

0oo Fig. 3 Refrigeration system driven by Rankine cycle (common working fluid) Fig. 3 $yst#rne frigorifique entrain# par cycle Rankine (fluide actif courant)

C o n v e n t i o n a l s t o r a g e systems The most common methods for the storage of refrigeration are: storage of ice; and storage of subcooted brine. Both have some undesirable features. Ice storage is restricted to operational temperatures above O°C. Considerable space is required for ice accumulating evaporators.

H Heat energy K Refrigeration demand Z Time

:~

,a Brine systems allow lower operatinq temperatures, but larger storage volumes are required. Storage vessels must be thermally insulated.

ondenser

I

I

~ Compressor ===b(~ ~)

Expansion valve

Solution heat exchanger

Expansion

~

Expansion~ )Solution valve ~pump

OI

I

~Evaporator

~Oo

valve

I

~ ~(2A

Evaporator V Oo

Fig. 1 Working principle of mechanical and absorption refrigeration Fig. I Principe de fonctionnement du refroidissement m#canique et du refroidissement ~ absorption

274

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Z

z

/

F"~

z

t

Z ~-- Average refrigeration \ capacity

c

Z

b

Oeso e,

)''1

/ ~:

z

Z

d

z

Fig. 4 Characteristics of heat supply and refrigeration demand: a - intermittent heat energy with almost continuous refrigeration demand; b - intermittent heat energy with mainly intermittent refrigeration demand, heat and refrigeration are out of phase; c - continuous heat energy with discontinuous refrigeration demand (peak loads); and d - totally independent characteristic of available heat energy and required refrigeration capacity Fig. 4 Caract#ristiques de I'apport de cha/eur et de/a demande de froid: a - energie thermique intermittente avec demande de froid presque continue," b - energie thermique intermittente, avec demande de froid surtout intermittente, /a cha/eur et le refroidissement ne sont pas en phase, c - apport d'#nergie thermique continu avec demande de froid discontinue (charges de pointe), d - caract&ristique tota/ement ind#pendante de /'#nergie thermique disponib/e et de/a puissance frigorifique exig~e

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No direct product cooling is possible. Large heat transfer surfaces have to be provided to allow the process material to make contact with the stored cold. Corrosion of equipment is often possible due to chemical or electrolyte action. There is also a higher energy demand because: lower evaporation temperatures are required by indirect cooling; additional lowering of the evaporation temperature at the end of the ice accumulation or to subcool the brine, in order to reduce the brine volume; and electrical energy for ice water pumps or brine pumps. Storage systems with eutectic brines offer advantages but cannot avoid all the undesirable features.

heat exchanger and then expanded, but to the storage vessel (PSS) and not to the absorber. During phase 2 weak solution from vessel (PSS) is fed to the absorber and absorbs instantly all the vapour evaporated from the liquid refrigerant, which was expanded from the vessel (RS) to the evaporator. The enriched solution is collected in the vessel (RSS); no solution flow occurs to and from the desorber. In practice there will normally be some base refrigeration load even during the heating periods. This means that the weak solution has to be distributed by the control loop (PC). A partial flow to the absorber, automatically adjusted to the required refrigeration load, and the excess weak solution accumulated in the vessel PSS.

Storage facilities for absorption refrigeration

With such a system all problems indicated in Figs 4a to d can be solved, provided that the following two conditions are met.

Uniquely with absorption refrigeration systems another very simple and effective way is possible. Refrigeration capacity may be stored by accumulating liquid refrigerant and weak solution. Fig. 5 shows the working principle of an absorption refrigeration plant with integrated storage facilities.

The first condition is that during the total working period the sum of the available heat energy has to be sufficient for the sum of the total required refrigeration capacity:

In a conventional absorption refrigeration plant all the refrigerant vapour, generated in the desorber (D) and purified to an ammonia concentration Ck t>0.998 in the rectifying column (R). is liquefied in the condenser (C) and expanded into the evaporator (E). In the absorber (A) the weak solution from the desorber (D), and precooled in the solution heat exchanger (T), absorbs the vapour from the evaporator (E). The solution pump (P) transfers the strong solution from the low pressure to the high pressure side. The subcooler (K) precools the liquid refrigerant by heat exchange with vapour from the evaporator and improves the energy balance. In order to adapt the absorption refrigerator for intermittent heat supply it is only necessary to enlarge the receivers for refrigerant RS and strong solution RSS and to provide a storage vessel for the weak solution PSS with an additional control loop. 'The best way to understand the working principle of the storage system is to regard the extreme cases: phase 1 - - transferring heat energy without refrigeration demand; and phase 2 --producing refrigeration capacity without simultaneous heat input. During phase 1 strong solution from the storage vessel (RSS) is pumped to the desorber for vapour generation. The liquefied refrigerant vapour is accumulated in the storage vessel (RS). The remaining weak solution is precooled in the solution

Volume 3 Num6ro 5 Septembre 1980

7"QH = ~'Qo/[ The second condition requires adequate dimensions of the storage vessels. Due to the high specific evaporation heat of ammonia the necessary storage volume of liquid ammonia is relatively small, 5.0 to 6.0 m 3 MWh -1 storage refrigeration capacity according to the following formula: AV,=

Oo/qopK

It is possible to accumulate the liquid ammonia on the low pressure side instead on the high pressure side. This results in a slightly reduced storage volume caused by the higher specific density. On the other hand this vessel has to be insulated and even then radiation losses cannot be avoided. This arrangement is only recommended for special applications. The solution vessel volumes are larger and within a wider range. The reason is that the specific solution flow:

f =mRL/rnK= ( CK--Ca)/( Cr--Ca) varies with the process conditions and depends mainly on the evaporation temperature and the cooling water inlet temperature. Normal values are between 5 and i 2 kg kg -1. The storage volumes are, for the strong, solution: AVRL= (Qof)/(qoPRL) and for the weak solution: AVAL= [Oo(f--

1)]/(qoPAL) 275

In practice storage volumes of between 12 and 30 m3 MWh -1 stored refrigeration capacity can be expected for each of the solution receivers.

Features of the integrated storage system The advantages of this method are as follows. There are no temperature level restrictions, therefore the evaporator temperature can be between - 6 0 ° C and + 10°C for the ammonia/water system. All the accumulating fluids have approximately ambient temperature, no insulation is required and no refrigeration is lost. Storage time is virtually unlimited. No thermodynamic degradation occurs during the storage period, the refrigeration machine operates always with the optimum heat ratio. Direct product cooling without additional heat transfer surfaces and circulation pumps requiring energy. Minimum mechanical equipment particularly in companson with Rankine systems. Only one low cost working medium. Furthermore, all the economical and operational advantages typical for the absorption refrigeration system such as high reliability; low costs for spare parts, repair and maintenance; excellent part oad behaviour; flexible operation with changing working conditions; no oil problems; low noise level; and outdoor installation, still apply.

Further applications Apart from the operation with intermittent heat energy there are other interesting applications for an absorption refrigerator with integrated storage facilities. In some processes there are periodically short but high peak loads, sometimes many times of the average capacity (Fig. 4c). Conventionally one has to provide a refrigeration plant with compressor and al other components designed for the maximum peak load. The expenses are high and most of the time the compressor is running under partial load with corresponding low efficiencies. An absorption refrigeration plant with storage system can operate continuously under average load accumulating liquid refrigerant and weak solution. In this case only the absorber and the evaporator have to be sized for the peak capacity and the other components for the average load only. The savings can be remarkable particularly in processes with a high peak ratio.

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Sometimes the refrigeration is required only in one or two shifts (8 or 16 h d - l ) , butthere is waste heat or other low price heat energy available either during 24 h d -1 or during the night time only. Such operatiQn requires &similar system as the pre~/ious case; the results are substantial savings in energy and investment cost. In both the previous cases the required refrigeration load can be higher than the nominal load or the theoretical load distributed over 24 h. A modified Storage system is also advantagous, if there are loads much smaller than the minimum economical partial load. Such an application was realized in one of Europe's largest ice cream factories. During the production time, five days a week, the refrigeration capacity is defined by all the freezers and coolers in operation. But during the weekend only 1 to 2% of this load is required for the cold store. With decreasing production on Friday afternoon enough liquid ammonia and weak solution can be accumulated to run a 'weekend circuit'; the main solution circuit including the direct-fired desorber remains shut down during this time. So the plant is operated during nights and weekends without any attention and the considerably improved partial load efficiency reduces the operational cost as well. The integrated storage system can be applied to heat pumps based on the absorption principle for all the above mentioned modes of operation as intermittent heat energy, peak load and minimum load.

Possibilities for the reduction of the storage volumes Of course the required storage volumes influence the initial cost of the plant, that is why one must consider how to reduce these volumes. Both solution storage vessels RSS and PSS can be made smaller, if during the heating period the

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J

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Phase I: end of cooling period, beginning of heating

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Fig. 5 Absorption refrigeration system with integrated storage facilities Fig. 5 Syst~me frigorifique ~ absorption avec dispositifs de stockage incorpor#s

International Journal of Refrigeration

Table 1. A comparison o f the ways of converting waste heat into refrigeration Tableau 1. Comparalson des modes de transformation de la cha/eur en froid Mechanical comoression system

Absorption refrigeratfon system

Temperature of heating medium, °C

130

150

"30

~50

Evaporation temperature, °C

-22.5

-22.5

-15

- 15

15

lOto -17,5

Brine temoerature, °C

-lOto

1 50

-17.5

Refrigeration capacity, MW

1.08

1.17

1.92

1.85

1.85

Energy for pumps, kW

50

60

20

19

19

Required storage vowme, m 3

31 5

320

230

130

80"

Wim combined solution storage vessel

Table 2. A refrigeration plant with peak load operation Tableau 2. Une installation frigorffique avec fonctionnement en conditions de po~nte Mechaqical compression system

Absorption refrigeration system

Case (a)

Case (b)

Energy input for peak load Nmax

1.72 MW

35.7 GJ

Average energy input ~J

1.20 MW

10.7 GJ n -~

10.7 GJ h -1

RI/N . . . . %

70

30

30

Equipment cost, %

100

128

30

Case (c) 35.7 GJ h - '

n -1

concentration difference can be increased above the normal values.

To / from

condenser

R

Another possibility is the combination of both solution receivers in one common vessel. In a system according to Fig. 5, depending on the storage conditions, the vessels RSS and PSS can be either full, empty or have any level in between. The volume of both vessels together must be at least twice of the required storage volume.

Separation line between rich and poor solution,

--~

Beginning of cooling period,

L.-L e_

)z Beginning of heating period, end of cooling

~1 ~

In a combined vessel according to Fig. 6 the liquid level is nearly constant and only the separation layer between strong and weak solution fluctuates. Weak and strong solutions may be separated by a floating ,device, but experiments have proven,- that both . fluids having the same pressure and approximately the same temperature do not mix in spite o f different concentrations (patents pending).

olution storage vessel

From evaporotor

Pe

---~

PhaseI ] Phase Tr j~ operation

Fig. 6 Absorption refrigeration system with combined solution storage vessel Fig. 6 Syst#me frigorffique ~ absorption avec r#servoir de stockage combin# de solutions

E v a l u a t i o n s and conclusions Table 1 gives a survey and comparison of different ways to convert intermittent waste heat into refrigeration. For the example case the following assumptions are made. The available heat energy amounts to 40 GJ for 1 h with the following 2 h without heating:total cycle 3 h. The heating medium is a condensing fluid.

Volume 3 Number 5 September 1980

The generation of refrigeration capacity is continuous and used to chill ~ product from - 5 ° C to -10°C. Cooling water inlet temperature is 27°C. Mechanical compression system with Rankine drive and brine storage, using R l 1 4 as working medium, ammonia as refrigerant and ethyleneglycol as brine.

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Absorption refrigeration with the binary system ammonia/water. The indicated energy demand for the solution pump, working fluid pump and brine pump is the average rate during the whole cycle. For another example case - - a refrigeration plant with peak load operation - - s o m e significant data are compiled in table 2. Compared are: mechanical refrigeration with centrifugal compressor, driven by electric motor (a); absorption refrigeration, heated with process heat 1 35°C (b); absorption refrigeration as previous case but with integrated storage system (c). The following operational conditions are assumed: constant base load, 1.0 MW; additional peak load

every 8 h for 1 h, 4.0 MW; maximum load, 5.0 MW; evaporation temperature, - 2 0 ° C ; cooling water inlet temperature, 25°C. Absorption refrigeration systems with integrated storage facilities can economically and reliably recover waste heat otherwise not yet utilized hitherto. The range of applications is remarkably wide and promises to make absorption refrigeration even more attractive in the future.

References 1 Holldorff, G. Revisions up absorption refrigeration efficiency Hydrocarbon Processing (J uly 1979)

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Manufacturing Engineering Proceedings of the 2nd Joint Polytechnics Symposium, t 4-'~3June 1979, Lanchester Polytechnic, UK The aim of this symposium was to bring together industrialists, researchers and students in a common forum where their problems and achievements could be examined and discussed. Topics covered include: Machine tools; Bearings and tribology; Computers/Microprocessors; Forming and fabrication; Electrochemical machining; Material removal and processes,

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July 1979/cloth/260 pages/O 86103 013 3 .£15.00net [S39.00]

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