A two grade phase change material (PCM) based on CaCl2-water system has been investigated
in a PCM stuffed copper tube-water heat exchanger. The heat output of the two grades
has been measured in exhausting cycle. The heat storage system is to be applied to store solar
energy and the stored heat is used to preheat the water input of domestic hot water supply system

Bajnóczy, Gábor

Lovász, Anett

English

Periodica Polytechnica (Budapest University of Technology and Economics)

PERIODICA POLYTECHNICA SER. CHEM. ENG. VOL. 50, NO. 1, PP. 45–53 (2006)DOMESTIC HOT WATER PRE-HEATER UTITLIZING SOLARENERGYAnett LOVÁSZ and Gábor BAJNÓCZYDepartment of Chemical TechnologyBudapest University of Technology and EconomicsH–1111, Budapest, Budafoki u. 8.e-mail:gbajnoczy@mail.bme.huReceived: Sept. 27, 2005AbstractA two grade phase change material (PCM) based on CaCl2-water system has been investigated in aPCM stuffed copper tube-water heat exchanger. The heat output of the two grades has been measuredin exhausting cycle. The heat storage system is to be applied to store solar energy and the stored heatis used to preheat the water input of domestic hot water supply system.Keywords: phase change material, solar collector, heat exchanger, calcium chloride.1. IntroductionAs a consequence of Hungary’s joining to the European Union we must assignan increasing part to renewable energy sources in our energy consumption. Fromthe nature of the country biomass utilization, geothermic energy consumption cancause breakthrough, beside these we can not give up the consumption of otherenergy sources such as wind and solar energy.Relating solar energy often emerges an estimated value: the amount of theincoming energy is approximately 15000 times greater than the annual energy needof the Earth. Although these numbers are quite misleading, this quantity is convertedinto given purposes (30% reflects, 47% heats, 23% hydro geological cycle, less than1% of the energy is used by photosynthesis, which is only a rounding off). Theother limit of solar energy utilization is the relatively small surface energy density.The energy arriving from the Sun reaches upper fields of the Earth’s at-mosphere with approximately constant radiation power. Nowdays the acceptedvalue of solar power is approximately 1353W/m2 (solar constant).Table 1 gives information about the Hungarian Solar Energy Radiation [1].From the facts of Table 1 it can be seen that there is a great difference betweenthe specific value of radiation in wintertime and in summertime. What is thisradiation energy sufficient for? If we concern the heat consumption, the necessaryheat energy is 250 kWh/(m2·6month) in respect of living space and heating season.Which is almost 1kWh/(m2 day). It means a flat should have been equipped with asame sized collector. But it can assure the needful heat if the sun is shining.46 A. LOVÁSZ and G. BAJNÓCZYTable 1. Average solar energy radiation in Hungary [1]Month January JulyIncoming radiation 1.5 kWh/(m2day) 5.0 kWh/(m2day)Utilizable radiation 1.1 kWh/(m2day) 2.8 kWh/(m2day)It is much better if we talk about domestic hot water supply. Take a householdwith four people as a basis: the energy needs of hot water production is 14kWh/dayfor 200 dm3 hot water of 60˚C. The desired energy can be collected easily duringthe summer, with correspondingly sized collector and it is not a problem duringwintertime too.2. Producing Hot-water with Solar Collector SystemsThe temporal difference of energy source and energy needs made necessary thedevelopment of storage systems. The abundant amount of solar energy, which iscollected during the summer, could be stored in such a big storage unit, engineeringand economic requirement of which are disproportional. Systems which equalizethe temporal difference in one or two days, can be economic. In summer theenergy of the heat transfer fluid arriving from the solar collector at relatively hightemperature (70◦C-90◦C) can be stored in insulated hot-water container.Except in summer, specially in winter, the temperature of the heat transferfluid coming from the collector is relatively low (35-60◦C). In this period of timewhich means nine months in a year, one way of storage is to use solid-liquid phasechange materials. In comparatively small volume the phase change materials havegreat storage capacity in small temperature interval. Storage systems using thisheat accumulator materials can store the energy from the solar collector at lowertemperature level in winter. The stored energy can be used for pre-heating thecold incoming water, so in the households the unit which is actually producinghot-water (gas or electric boiler) is supported by the unit storing solar energy. Sothe traditional system integrated with solar energy storage might moderate the costof the energy consumption.3. Phase Change Materials as Heat AccumulatosIn practice several phase change materials (PCM) are known, such as: paraffins,fatty-acids, organic and inorganic salt hydrates, organic and inorganic eutectic com-pounds [2, 3]. Theoretically isotherm heat storage, in accordance with phase changetemperature can be achieved by these materials.DOMESTIC HOT WATER PRE-HEATER UTITLIZING SOLAR ENERGY 47Conditions of using phase change materials are: relatively high latent heat,high heat conductivity (more than 0.5 W/m◦ C), material’s melting temperatureshould be in the functional interval (15◦C < T < 90◦C) if it stores solar energy,congruent melting, minimal supercooling, chemical stability, economic efficiencyand aspects of environmental protection [4, 5, 6].In the course of the experiments a special phase change material was examinedthat stored heat in two different temperature grade [7]. The grade which stores heaton lower temperature is isotherm (29 ◦C). The other links to higher temperaturegrade (29 – 42 ◦C), that can be considered a storage system with high apparentspecific heat. The reason for the high apparent specific heat is the continuouscrystallization of the PCM during the temperature drop and energy production inthe interval mentioned above.4. Heat Storage by two Grade Phase Change MaterialThe two grade phase change material, examined in a pre-heater that can be linkedto the domestic hot-water system, is a solution containing 211.4 mol of CaCl2/1000mol H2O. The way of heat storage process is shown on the phase diagram of CaCl2- H2O (Fig. 1).20253035404550180 200 220 240[mol CaCl2/1000 mol H2O]T  [oC]T+LCaCl2˙6H2O (H)CaCl2˙4H2O (T)P1P2H+TH+LL(211,4mol ;43,3oC)(200mol ;29,9oC)L: liquid phaseFig. 1. Phase diagram section of the CaCl2-water system48 A. LOVÁSZ and G. BAJNÓCZYThe two grades can be connected to two peritectic points P 1 (211.4 molCaCl2/1000 mol H2O ; 43.3◦C) and P 2(200 mol CaCl2/1000 mol H2O ; 29.9 ◦C).If the phase change material is heated above the appropriate temperature (P 1),the crystals melt. During the cooling the melt preserves the peritectic composition,until it reaches point P 1. From this point the crystallization of CaCl2 4H2O startswhich causes the release of crystallization heat and goes on until the temperaturereaches the temperature of point P 2. During the phase change the temperaturefollows the liquidus line as a function of the amount of solidified CaCl2 4H2Oand gradually decreases to 29.9oC. Continuing the heat extraction in the presenceof seeding crystals (SrCl26H2O and Sr(OH)2 8H2O) the rest of melt solidifies asCaCl2 6H2O.The temperature remains constant during this process.The heating part of the cycle involves the semicongruent melting of CaCl26H2O and the peritectic melting of CaCl2 4H2O. The peritectic melting is accompa-nied by the segregation of solid CaCl2 2H2O. This solid material should be dissolvedin the melted phase otherwise loss of heat storage capacity happens.5. The Heat Exchanger Charged with Two Grade Phase Change MaterialWe examined the thermal properties of the heat storage tubes containing the twograde phase change material, in a heat exchanger special for this experiment. Waterwas the circulating agent among tubes, heat transfer was stagnant inside the tubes.The PCM was a solution of 211.4 mol CaCl2 /1000mol H2O. It was situated in coppertubes (diameter 28 mm), which were placed horizontally in the heat exchanger.Table 2 shows the parameters of the heat exchanger.Table 2. Main parameters of the storage system containing two grade phase change mate-rial.Outer dimensions: 450×450×515mmWall : 2 mm steel plateInsulation: 15 mm plastic foamInner volume: 88.2 dm3Volume of heat transfer fluidinside the exchanger: 58 dm3Volume of the PCM: 12.6 dm3Number of the heat storage tubes: 18Surface of the heat storage tubes: 3.28 m2Diameter of the tubes: 28 mmThe hot water coming from the solar collector was simulated by a flow through21 kW electric boiler, which can be adjusted between 40◦C-60◦C transformationrange. Fig. 2 shows the diagramatic make-up of the experimental equipment. TheDOMESTIC HOT WATER PRE-HEATER UTITLIZING SOLAR ENERGY 49Fig. 2. The heat exchanger during the cooling of the heat accumulator tubesheat exchanger did not contain the appropriate flow fences, so we avoided thechannelling by using alternating hot and cold water input. The hot water havinglower density streamed to the highest point of the heat exchanger, the cold waterhaving higher density streamed to the bottom. The volume flow of the heat transferfluid was 3.5 dm3/minute. The temperature of the effluent and influent heat transferfluid was recorded by thermometers connected to computer.The tests were broadened to the corrosion property of the PCM situated in thecopper tubes. We have assigned the potential difference between the copper tubeand the solder in the case of air connected, melted PCM. And we have measured thecorrodibility of the copper tubes in the case of dissolved oxygen and oxygen-freeheat storage material.6. ResultsThe accumulated heat by the two grade phase change material can be calculated bythe given relation:Q =M ·Tinitial∫Tf 1cpldT +Tf 1∫Tf 2cp,virt.︸ ︷︷ ︸I.grade+MCaCl2·6H2O · 1Hf + M ·Tf 2∫Tf inalcps,averagedT︸ ︷︷ ︸I I.grade50 A. LOVÁSZ and G. BAJNÓCZY102030405060700 10 20 30 40 50 60 70 80 90 100 110 120 130 140time [min]temperature [oC]Fig. 3. Cooling curve of the two grade PCM measured by 30◦C and 15◦C water coolantcoolant 115,794,8100,086,776,5 73,8 72,323,113,2 10,9020406080100120140100 90 80 70 60 50 40 30 20 10accumulated heat ratio %capacity [W/m2]I. gradeII. gradeFig. 4. Heat power of accumulator tubes based on cooling curve measured by 30◦ C and15◦C water coolantDOMESTIC HOT WATER PRE-HEATER UTITLIZING SOLAR ENERGY 51whereQ storage capacity [kJ]Tf 2 the start of crystallization of CaCl2 6H2O (29.9 ◦C)M mass of the phase change material [kg] in the second gradeMCaCl2.6H2O the mass of CaCl2 6H2OTf inal temperature of the PCM at the end of heat recovery solidifiedfrom 29.9 ◦C [kg]Tf 1 the start of crystallization of CaCl2 4H2O in the grade I.(43.3◦C) in the range of Tinit ial − 43.3◦Ccpl specific heat of fluid phase PCM 2.09 kJ/kg ◦C,Tinit ial the temperature of the PCM at the beginning of heat recovery11.5 kJ/kg ◦C (the cooling of liquid phase is accompaniedcp,virt virtual specific heat in the interval of Tf 1 - Tf 2 Tinit ial > Tf 1by partial crystallization)1Hf phase change enthalpy of CaCl2 6H2O =188 kJ/kgcpsaverage specific heat of the solidified PCM 0.8 kJ/kg ◦CTable 3 shows the ratio of the withdrawable and theoretically accumulatedamount of heat.Table 3. Theoretical and available storage capacity of the two grade phase change materialGrade Temperature Theoretical Available storage Available storageinterval of storage capacity capacity capacityaccumulation [kJ/dm3] [kJ/dm3] [%]I. 60 ◦C – 30 ◦C 175 156 89II. 30 ◦C – 12 ◦C 199 196 98I + II 60 ◦C – 12 ◦C 374 352 94Because of the small difference between the theoretical and the availablestorage capacity we could avoid the channelling, disabling heat recovery by theappropriate water input.We have measured the relation of pre-heated water temperature and timeduring the period of heat recovery in two different ways; the storage capacity ofthe given grade was obtained by applying water being pre-heated on two differenttemperatures (30◦C and 15◦C) (Fig. 3), otherwise we exhausted the heat capacityof the double grade by continuously applying water on 15◦C (Fig. 5).We have determined the heat power of the heat accumulator tubes from thecooling curve as a function of percental storage capacity ((Figs. 4, 6). Approxima-tion of the percental storage capacity from the cooling curve was assumed constantheat loss.The two grades can be well traced on Fig. 6. The crystallization of calcium-chloride-tetra-hydrate attended by a temperature drop and a power decrease canbe observed on the heat accumulator tubes. During the isotherm grade the output52 A. LOVÁSZ and G. BAJNÓCZYmeasured by 15 C water coolant 1020304050600 10 20 30 40 50 60 70 80time [min]temperature [oC]Fig. 5. Cooling curve of the two grade PCM measured by 15◦C water coolant050100150200250300350400100 90 80 70 60 50 40 30 20 10accumulated heat ratio %Capacity [W/m2]I. gradeII. gradethe Fig.6. The crystallization of calcium-chloride-tetra hydrate attended Fig. 6. Heat power of the accumulator tubes based on cooling curve measured by 15◦ Cwater coolantDOMESTIC HOT WATER PRE-HEATER UTITLIZING SOLAR ENERGY 53slowly decreases (during the crystallization of CaCl2 6H2O). The cause of slowerdecrease is the formation of insulating layer from crystals deposited on the insidewall of the heat accumulator tubes.Inorganic salt hydrates having peritectic melting character, as in CaCl2 – watersystem (along advantageous properties) show a decrease in heat storage capacityafter several storage cycles because of the segregation of phases during melting.The task of further experiments is to define this effect correctly, and to solve theproblem of containment.In accordance with the results of corrosion tests a number of decimal voltpotential difference can be measured in the melted PCM between the copper andsolder. The copper tube definitely indicates the signs of corrosion in the meltedheat storage material with the presence of dissolved oxygen. However in the closedtubes the corrosion stopped after the running out of dissolved oxygen.AcknowledgementThe authors have to thank Fiorentini Solar Kft. for assuring the experimental heat exchangerand OTKA for the financial aid, number T037496.References[1] GYURCSOVICS, L., Heat from the Sun Műszaki Könyvkiadó, Budapest, 1987.[2] FARID, M. M., A Review on Phase Change Energy Storage: Materials and Application EnergyConversion and Management 45, Iss. 9-10, 2004, pp. 1597–1615.[3] HISHAM, E.– HISHAM, EL DESOUKKY – EMAN,A., Heat Transfer Characteristics duringMelting and Solidification of Phase Change Energy Storage Process Industrial & EngineeringChemistry Research, 43 Iss. 17., 2004, pp. 5350–5357.[4] KAYGUSUZ, K.: Phase Change Energy Storage for Solar Heating Systems Energy Sources, 25,2003, pp. 791–807.[5] FAN, Y. F. – ZHANG, X. X. – WANG, X. C.– LI, J. – ZHU, Q. B., Super-cooling preventionof microencapsulated Phase Change Material Thermochimica Acta, 413 Iss. 1-2, 2004, pp. 1–6.[6] ZALBA, B.- MARIN, J. M.- CABEZA, L.F.- MEHLING, H. , Review on Thermal EnergyStorage with Phase Change Materials, Heat Transfer Analysis and Applications Applied ThermalEngineering, 23 Iss. 3, pp. 251–283, 2003.[7] BAJNÓCZY, G. – GAGYI PÁLFFY, E. – PRÉPOSTFFY, E., Periodica Polytechnica Chem. Eng,43 No.2. 1999, pp. 137–147.

DOMESTIC HOT WATER PRE-HEATER UTILIZING SOLAR ENERGY

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DOMESTIC HOT WATER PRE-HEATER UTILIZING SOLAR ENERGY

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