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Chemical Engineering and Processing 47 (2008) 484–489

Design of a thermochemical process for deep freezingusing solar low-grade heat

Le Pierres Nolwenn ∗, Stitou Driss, Mazet NathaliePROMES (Laboratory PROcesses, Materials and Solar Energy), CNRS Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France

Available online 12 January 2007

Abstract

A deep-freezing process has been designed and experimented to cool a cold box down to about −30 ◦C using only low-grade heat produced bysimple flat plate solar collectors operating at 70 ◦C. The original process involves two cascaded thermochemical systems using BaCl2 salt reactingwith ammonia. It works discontinuously, with one day phase of regeneration at high pressure and one night phase of cold production at low pressure.A global dynamic model allows the simulation of the different system components functioning depending on the hourly weather conditions. It takesinto account the transient periods and shows the temperature changes of the components, the chemical reactions in the system and its performances.This system will cover the cooling needs of a 560 L cold box at −20 ◦C during the 3 sunniest months of the year and provide more than 60% of the totalyearly cooling needs of this box for the weather conditions of Perpignan (South of France). The prototype is expected to show a system coefficient ofperformance (COP) of about 0.07 over the 10 sunniest months of the year, and a net solar COP of 0.05, taking into account the collectors efficiencies.© 2007 Elsevier B.V. All rights reserved.

Resume

Un systeme de production de froid tres basse temperature (de l’ordre de −30 ◦C) a ete concu. II utilise uniquement de la chaleur a bassetemperature (de l’ordre de 70 ◦C) issue de capteurs solaires plans. Le procede original defini dans ce but met en oeuvre 2 dipoles thermochimiquesfonctionnant en parallele et produisant du froid en cascade (Figs. 1 et 2). Le sel reactif selectionne est le BaCl2, reagissant avec 1’ammoniac. Leprocede fonctionne de facon discontinue, avec une phase diurne de regeneration (haute pression) et une phase nocturne de production de froid(basse pression). Au cours de la phase de nuit, la chaleur produite par la reaction chimique au sein du reacteur R1 est absorbee par 1’evaporationde 1’ammoniac au sein de l’evaporateur E2, et ce reacteur est descendu a environ 5 ◦C. Ainsi, l’evaporation de l’ammoniac au sein de E1 a tresbasse temperature est possible. Aucun transfert de masse n’a lieu entre les deux systemes, le transfert est seulement thermique.

Une modelisation dynamique mettant en œuvre l’ensemble des differents composants du systeme permet de simuler son evolution sur unejournee puis sur l’ensemble de l’annee en prenant en compte les phases transitoires du fonctionnement. La simulation du procede a ete confronteeaux resultats experimentaux obtenus grace a un prototype (Fig. 3 et dimensions Table 1) et une bonne concordance a ete obtenue. A partir de cettesimulation validee, le fonctionnement thermique du procede peut etre etudie (exemple a partir des conditions meteorologiques mesurees le 10 juin2005, Fig. 4). La simulation debute au lever du soleil et presente la phase de chauffage solaire des reacteurs jusqu’a la temperature d’equilibred’environ 50 ◦C (heures 0 a 5) et de decomposition (heures 5 a 13) suivies de celles de refroidissement nocturne des reacteurs et de production de froidpar l’evaporateur 1 (heures 13 a 24). Les puissances mises en œuvre sont presentees Fig. 5. La chaleur recue par les capteurs solaires est seulementen partie utilisee par la reaction chimique. La difference est due en partie au rendement des capteurs solaires et en partie a la necessite de chauffer lesmasses thermiques des reacteurs. Parallelement, les puissances de condensation sont environ deux fois plus faibles que les puissances de reaction,en raison de la difference d’enthalpie des deux processus. L’evaporation et donc la production de froid a lieu en continu au cours de la phase de nuit.

De plus, la simulation du systeme sur l’ensemble de l’annee a permis d’evaluer ses performances. Le froid produit pour des journees moyennesde chaque mois est compare aux besoins de froid d’une chambre froide de 560 L perdant 60 W en continu (Fig. 6). Le COP systeme de ce procedeoriginal de production de froid a −30 ◦C est egalement presente Fig. 6 en fonction du mois. Sa valeur moyenne atteint 0.07 sur les 10 mois lesplus ensoleilles et celle du COP solaire correspondant, 0.05, ce qui est comparable aux performances des procedes de production de froid solaire

◦

existants, mais pour des temperatures de l’ordre de −10 C.© 2007 Elsevier B.V. All rights reserved.

Keywords: Solar thermochemical process; Deep freezing; Low-grade heat; Experimentation

∗ Corresponding author. Tel.: +33 4 79 44 45 58; fax: +33 4 79 68 80 49.E-mail address: [email protected] (N. Le Pierres).

0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.cep.2007.01.011

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. Introduction

The increasing use of air conditioning in developed areasnd the growth of refrigeration and deep-freezing needs ineveloping countries are the cause of peaks in electricity con-umption during summer months. To cover these cooling needs,ew technologies with low environmental impacts (greenhouseas emission and ozone depletion) are thus needed. The devel-pment of solar cold production sorption systems is then aromising solution, as sorption systems only use heat sourcesnd environmentally friendly refrigerants (no CFC or HCFC).

Today, sorption systems are used to produce cold for aironditioning and refrigeration purpose, at cooling temperaturesround respectively, 10 and 0 ◦C [1,2]. These processes need heatources with temperatures higher than 80 and 100 ◦C, respec-ively. Complex and costly solar collectors are necessary toroduce heat at these temperature levels. The implementation ofhese technologies in housing is thus slow and difficult. More-ver, no deep-freezing system has been developed using sorptionechnologies. The aim of this project was to design a thermo-hemical process producing cold at deep-freezing temperatureslower than −18 ◦C), and using only low-grade solar heat fromimple flat plate solar collectors (below 70 ◦C). This systemhould be as cheap as possible and should work without anyoving parts.

. Description of the process

.1. Thermochemical processes

The thermochemical process used is based on a reversibleeaction between a solid reactant S and a refrigerant gas G:

1 + γG2↔1S2 + γ�Hr (1)

n direction 1, called synthesis, gas G is fixed on solid S1. Thishemical reaction is exothermic and releases �Hr. In direction, called decomposition, S2 releases the gas G. This endothermic

eaction needs the heat quantity �Hr.

The simplest thermochemical cold production system asso-iates a solid/gas reaction with a liquid/gas phase change. Thisonnection between a reactor and an evaporator/condenser is

dcpc

Fig. 1. The cascaded cycle during the day (decomposition) and night (synth

and Processing 47 (2008) 484–489 485

n example of a thermochemical dipole [3]. These two pro-esses are monovariant and their equilibrium conditions followhe Clausius–Clapeyron equation:

n(p) = −�Hi

RT+ �Si

R(2)

hus, the thermodynamic equilibrium conditions of the reactorsre determined by only one parameter (p or T). The degree ofonversion of the reaction, X, determines the quantity of solidhich has reacted with the gas.For these thermochemical processes, the solids reactants

chloride) are inserted in consolidated blocs of natural expandedraphite (GNE). This implementation was developed to enhanceeat transfer in the reactors [4].

.2. Solar process

The cold production system associates two thermochemi-al dipoles, which are thermally connected. The primary dipolewhich produces cold in the cold box) is cooled during the nighthase by evaporation of ammonia in the secondary evaporatorFig. 1). This thermally cascaded process was obtained throughn original exergetic analysis of ideal processes [5].

The two systems (primary (1) and secondary (2)) work in par-llel during the daily and night periods. They contain the sameeactants BaCl2 and ammonia. During the day phase, the twoeactors R1 and R2 are heated by solar radiation and decom-osition occurs. Ammonia desorbed by the salt is condensed inondensers C1 and C2, respectively, both at ambient tempera-ure. Liquid ammonia is accumulated in tanks 1 and 2 (Fig. 2).

During the night period, both reactors are first cooled byatural convection with outside air and radiation toward theocturnal sky and their pressure decreases. When pressure invaporator E2 becomes lower than pressure in tank 2, ammoniahich was stored in tank 2 flows to E2 through the check-valves.2 is then in synthesis conditions. Ammonia evaporates in E2,hich is located inside R1. This evaporation absorbs heat from1 through the evaporator walls. The temperature of R1 then

ecreases to about 5 ◦C and R1 is then brought to synthesisonditions. Ammonia can thus evaporate in E1 at very low tem-eratures, down to −33 ◦C. Evaporation of ammonia in E2 isontinuous during the night phase. This evaporation maintains

esis and cold production) phases in the Clausius–Clapeyron diagram.

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Table 1Technical specifications of the main components of the prototype

Reactive medium (BaCl2 + GNE) mass system 1 15.3 kgArea of solar collector 1 2 m2

Reactive medium (BaCl2 + GNE) mass system 2 30.6 kgArea of solar collector 1 3.8 m2

Volume of tank 1 15 LVolume of tank 2 20 LArea of evaporator 1 5 m2

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ig. 2. Schematic representation of the cascaded cold production freezing pro-ess.

he temperature of R1 low enough to ensure an efficient synthe-is reaction in R1 and thus an efficient evaporation of ammonian E1.

A cold storage is included in the cold box to manage theay/night discontinuous cold production of the process and thenefficient operating conditions during bad weather. For the

eteorological conditions of Perpignan (Mediterranean climate)nd a process working during the 6 sunniest months of the year,old storage for 3 days is necessary. This cold storage is pro-ided by a phase change material (PCM) (salt solution [6]). Tonsure a cold box temperature always lower than −18 ◦C, thehase change temperature is around −20 ◦C.

This process does not involve any rotating part. The onlyoving components are four check valves in the secondary sys-

em, which regulate the flow of ammonia within the system [7].he opening and closing of these check-valves is controlled by

he pressure difference between both sides of the valves. Theystem operation is thus totally autonomous (no electricity oruman intervention for its functioning), robust and silent.

. Process simulation

A dynamic simulation of the process has been developed [7].imulations describe the process evolutions, depending on theeather conditions and on the sizes of the different components.hey take into account the transient periods of the process oper-

tion, the influence of the inert material masses and the kineticsf the chemical reactions, according to kinetic equations derivedor this type of solid–gas reactions [8]. For the solid–gas mono-ariant reversible reactions, the kinetics depend linearly on the

aps

Fig. 3. Pictures of the solar deep freezing prot

rea of condenser 1 9 m2

rea of condenser 2 22 m2

ocal equilibrium drop, pop − peq and this is presented in (3) forhe decomposition and in (4) for synthesis reactions.

dX

dt= kcinX

pop − peq

pop(3)

dX

dt= kcin(1 − X)

pop − peq

pop(4)

hese simulations were used to size a prototype, which was builtn Perpignan (Fig. 3) and has been running since May 2005. Itsimensions are presented in Table 1. Simulation results haveeen compared to experimental ones and the model parametersave been adjusted [9]. The efficient solar flat plate collectorsave an optical efficiency of 0.85 and a heat losses coefficient ofW/(m2 K). The kinetic coefficient kcin of the reaction betweenaCl2 and NH3 is of 10−4 s−1. The cold box of 560 L loses.4 W/K. When the cold box at −20 ◦C is located in a room at0 ◦C, the cold power needed is thus 60 W.

.1. Temperature evolution

An example of simulation results is presented in Fig. 4. It isased on measured hourly weather conditions (solar radiation,mbient temperature and wind speed) on 10 June 2005. Sim-lation begins at sunrise, at 6:00 a.m. These simulations wereerformed assuming an empty cold box (without PCM or icenside).

The evolution can be separated into two phases: between 0nd 15 h, the day phase (sunrise to sunset) during which decom-osition occurs, and between 15 and 24 h the night phase withynthesis in the reactors. During the first hours (between 0 and

otype. (A) Front side and (B) back side.

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ig. 4. Temperatures and degrees of conversion of the system for the weatheronditions of the 10 June 2005 in Perpignan (South of France).

h), the degrees of conversion of the reactors X1 and X2 remainpproximately constant while the temperatures of the reactorsncrease. Once these temperatures reach about 55 ◦C, reactionsegin. There is a plateau in the temperature evolution, as theeat absorbed by the solar collectors is mainly used by thendothermic decomposition process. After about 9 h, the reac-ion kinetics in R1 decreases (following equation (3)) and theegree of conversion X1 reaches zero, the decomposition pro-ess in this reactor is thus finished. As solar radiation goes on,he temperature of R1 further increases, up to about 110 ◦C. Aseaction is not completed in R2, the temperature in this reactor isaintained approximately at the equilibrium value. After about

1 h, the solar radiation decreases and becomes too low to coun-eract the heat losses of the collector to the surrounding air. Thus,he temperatures of the reactors decrease progressively. Thisooling changes the pressure conditions in the different partsf system 2 and the ammonia contained in tank 2 flows to E2.hus, the wall temperature of evaporator 2 decreases strongly at4 h.

During the day phase, evaporator E2 is empty and its tem-erature follows exactly R1 temperature. The cold box and1 heat up slowly from −23 to 0 ◦C: this is the natural tem-erature evolution of the empty box when cold production istopped.

The night phase begins at sunset (15 h) with the opening of theide walls of the secondary solar collector in order to increaseatural convection and cooling of R2. Thus, there is a step inhe evolution of reactor R2 temperature at 15 h on Fig. 4. Thealt in R2 reacts with gaseous ammonia and the reaction heat isejected to the environment. Pressure then decreases in system 2nd the evaporation of ammonia in E2 increases. Consequently,he primary reactor R1 is cooled, R1 is brought to synthesisonditions and the reaction heat of the exothermic reaction in R1s transferred to E2. The temperature of R1 thus decreases belowhe outside temperature (18 h). The synthesis reaction continueslso in R1 and X1 increases steeply. The ammonia temperaturen E1 decreases to about −30 ◦C during the night phase (23 h).

he temperature of the air in the cold box is about 10 ◦C higher,ue to the low heat transfer coefficient with E1. However, theemperature of the air in the box decreases to −23 ◦C during thisight.

ifta

ig. 5. System power for the weather conditions of the 10 June 2005 in Perpig-an.

.2. Power study

Fig. 5 shows the simulation of the power involved in the sys-em on 10 June 2005, as in Fig. 4. The heat received by the solarollectors follows the radiative heat flux received by the Earthurface at this point. As solar collector 2 is twice as large asollector 1, heat absorbed is also twice as large. Only a part ofhe input heat is used for salt decomposition, as the collectorfficiency is lower than 1 and a part of this energy is neededo heat up the metal walls of the reactor. Moreover, as decom-osition in R1 is already complete after only 10 h, the wholeolar energy received by collector 1 after that decompositioneriod is lost and only used to heat up the collector parts overhe decomposition temperature. For R2, as the decompositions not complete before the end of the day, the reaction powerollows the evolution of the received solar power.

During the day phase, condensation occurs at outside tem-erature. Thus, condensing heat is rejected to the condensersurrounding air. This heat is about two times smaller than theeaction heat, according to the ratio of ammonia latent heat tonthalpy of ammonia/BaCl2 reaction.

During the following night phase, synthesis occurs in theeactors and evaporation produces cold in the evaporators. Theeaction and evaporation rates are almost constant, except a smalleak at the beginning of the night (15 h). This is due to the open-ng of the collectors side walls at sunset to increase convectionith outside air, and the enhancement of the flow of ammonia

rom tank 2 to evaporator 2 to cool R1. After that peak, theeaction rates decrease very slowly until the sunrise. During theight phase, this process can provide an average cooling powerf 110 W to the cold box during 10 h for that day of June.

. Performances

.1. Cooling needs coverage

The simulation results show the efficiency of the process dur-

ng the whole year. Simulations are obtained for ‘average days’or each month of the year. These average days weather condi-ions are calculated from the hourly weather data of years 1996nd 1997 in Perpignan. The cooling needs of the cold box and

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488 N. Le Pierres et al. / Chemical Engineering

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ig. 6. Cold production and needs of the cold box, system and net solar COPuring the different months in Perpignan for the system described in Table 1.

he cooling capacity of the process are compared in Fig. 6. Sim-lations show that the system could cover at least 100% of theooling needs of the cold box during the 3 sunniest months ofhe year (from May to July). The results obtained for the other

onths are also shown. During these months, the process pro-ides an average of 53% of the cooling needs. Globally, therocess should provide more than 65% of the total yearly cool-ng needs of this box for the weather conditions of Perpignan.n auxiliary cooling system would be needed to maintain the

old box temperature low enough for food storage.

.2. Coefficient of performance

The system coefficient of performance (COP) is defined ashe ratio of the heat extracted by evaporator E1 in the cold boxo the heat absorbed by the reactors of both systems. It is calcu-ated for the same time sequence as presented above. Results arehown in Fig. 6. The average system COP over the 10 sunniestonths of the year given by the simulations is 0.071. The systemOP evolution is slightly different from the cold production. It

emains approximately constant from February to November,ith a small increase in March–May and September–October.Indeed, during January and December, the daily solar energy

nput received by the solar collectors is not sufficient to coveroth the heating of the collectors to over the decomposition tem-erature and the reaction heat. Most of the heat received is usedo heat up the walls and inert components of the reactor. Fromebruary to November, the most favourable months are not theunniest in Mediterranean conditions, as the outside tempera-ures are also the highest during these months. The increase inutside air temperatures decreases the system performances forwo reasons:

at night time, the high outside air temperature slows downthe cooling of the solar collectors and indeed of the reactors.Thus, both time available for the synthesis reactions and the

reaction rates are low and this process can be stopped beforecompletion. Moreover, the synthesis temperature for reactorR2 is high, thus increasing the boiling temperature of ammo-nia in evaporator E2, and the temperature of R1 during the

and Processing 47 (2008) 484–489

night. This increases the boiling temperature of ammonia inE1 and, consequently, the cold box temperature;during the day time, the high outside temperature increasesthe condensing temperature of ammonia in the air-cooledcondensers. Thus, to counteract this high temperature, theequilibrium temperature of the reactors must also be high. Asthe solar collectors efficiencies are low in these conditions andas a large amount of input radiation is used after decomposi-tion to warm up the metal parts of the process to temperaturepeaks of the reactors, the net solar COP is low.

The net solar COP of the solar system (Fig. 6) is defined ashe ratio of the heat extracted by evaporator E1 in the cold box tohe solar energy input received by the solar collectors. This COPakes into account the efficiency of the solar collectors. Its aver-ge over the 10 sunniest months of the year is 0.047. This is closeo the COP obtained by other solar cold production systems butor a higher evaporation temperature of about −10 ◦C [10–12].he average ratio of the system and the solar COP is 67% andpproximately constant for the different months. It results fromhe solar collectors efficiencies.

For this deep freezing thermochemical system, the idealOP [8] is 0.23. It is defined as the ratio of cold producedy evaporator E1 in the ideal thermodynamic cycle to theeat required for regeneration of the salt. The COP of 0.071btained by simulations of the experimental prototype takesnto account the dynamics of the process and both the sensibleeats of the salts, ammonia and of the inert components of therocess.

. Conclusion and outlooks

A new thermochemical process has been designed and stud-ed, to produce cold at low temperature (below −20 ◦C) usingow-grade heat from simple flat plate solar collectors. This pro-ess associates two cascaded cycles working in parallel withaCl2 and NH3. The numerical simulation of this process and

he construction of a prototype:

proved the feasibility of this new concept of solar freezing,to produce cold at a temperature as low as −30 ◦C using lowtemperature solar heat in the range of 60–80 ◦C;demonstrated its potential use in housing, by the simplicity ofits cheap and efficient flat plate collector technology and bythe acceptable size and weight of the system;showed the system performances during the year in a cho-sen location with a system COP of about 0.07 over the 10sunniest months of the year and a net solar COP of 0.05. Fur-ther work will be carried out by considering different weatherconditions and locations for implementation of this solar deepfreezing system. The potential use of other types of low-grade

for this process will also be considered. Another point understudy is the use of PCM in the cold box to ensure a lowtemperature over the whole day from a discontinuous coldproduction.

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N. Le Pierres et al. / Chemical Engin

cknowledgements

This work was supported by the French governmental Agencyor Energy Management and Environment (ADEME) and theENERGIE’ program of the CNRS through the project PRIFroid Solaire’ 6.1.

ppendix A. Nomenclature

condenserevaporatorgas

H enthalpy change (J mol−1)cin kinetic coefficient (s−1)

pressure (Pa)reactorideal gas constant (J mol−1 K−1)solid

S entropy change (J mol−1 K−1)temperature (K)degree of conversion

reek letterstoechiometric coefficient

ubscriptsmb ambientq equilibrium

reaction, evaporation or condensationp operatoring

reaction

[

and Processing 47 (2008) 484–489 489

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