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Thermal energy recovery of air conditioning system––heat recovery system calculation and phase change materials development
Applied Thermal Engineering 24 (2004) 2511–2526 www.elsevier.com/locate/apthermeng
Thermal energy recovery of air conditioning system––heat recovery system calculation and phase change materials development Zhaolin Gu *, Hongjuan Liu, Yun Li School of Environmental and Chemical Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, PR China Received 15 January 2004; accepted 24 March 2004 Available online 6 May 2004
Abstract Latent heat thermal energy storage systems can be used to recover the rejected heat from air conditioning systems, which can be used to generate low-temperature hot water. It decreases not only the consumption of primary energy for heating domestic hot water but also the calefaction to the surroundings due to the rejection of heat from air conditioning systems. A recovery system using phase change materials (PCMs) to store the rejected (sensible and condensation) heat from air conditioning system has been developed and studied, making up the shortage of other sensible heat storage system. Also, PCMs compliant for heat recovery of air conditioning system should be developed. Technical grade paraffin wax has been discussed in this paper in order to develop a paraffin wax based PCM for the recovery of rejected heat from air conditioning systems. The thermal properties of technical grade paraffin wax and the mixtures of paraffin wax with lauric acid and with liquid paraffin (paraffin oil) are investigated and discussed, including volume expansion during the phase change process, the freezing point and the heat of fusion. 2004 Elsevier Ltd. All rights reserved. Keywords: Heat recovery; PCM; Phase change material; Air conditioning system; Domestic hot water
1. Introduction As the demand for air-conditioning increased greatly during the last decade, large demands of electric power and uncertain availability of fossil fuel have led to a surge of interest in the efficient energy application in air conditioning system. The rejected (sensible and condensation) heat from *
Corresponding author. Tel./fax: +86-2982-664-928. E-mail address: [email protected] (Z. Gu).
1359-4311/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.03.017
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Nomenclature cpr specific heat capacity of refrigerant under constant pressure (kJ/kg K) h specific enthalpy (kJ/kg) Dh heat of fusion (kJ/kg) latent heat of refrigerant during condensation (kJ/kg) Dhrc I the heating electric current (A) IEER1 IEER at the refrigeration mode, EER IEER2 IEER at the refrigeration and heat recovery mode m mass of experimental sample (kg) mass flow rate of refrigerant (kg/s) mr refrigeration capacity of air conditioning system (kW) Qe heat recovery capacity (kW) Qh sensible and condensation heat of air conditioning system (kW) Qk T absolute temperature (K) U electric voltage of heater (V) V volume (ml) DV % volume expansion during the phase change compressor power input (kW) We indicative power of compressor (kW) Wico power loss of compressor (kW) Wloss theoretic power of compressor (kW) W0 s time (s) ss;start melting start point ss;stop melting stop point Subscripts l liquid m melting r refrigerant s sensible, solid w whole Abbreviations COP coefficient of performance IEER integrated energy efficiency ratio LP liquid paraffin LA lauric acid PCM phase change material PW paraffin wax
air conditioning systems is a readily available energy source that can be used to produce lowtemperature hot water for washing and bathing [1–4].
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In summer, most hotels and restaurants provide both cooling and heating services for customers. Traditionally there are two systems, that is, hot water supplying system and air conditioning system, providing heat and cool load respectively. These two systems are independent. The main purpose of central water chiller plants of building air-conditioning systems is air conditioning. The heat from air conditioning systems is rejected into the air directly. On the other hand, great heat is needed to heat domestic hot water for washing and bathing, which is usually supplied by a boiler burning oil, gas or coal. Generally, the temperature of water in water pipe is between 15 and 20 C. The outlet temperature of cooling water for air conditioning system is between 35 and 40 C, which almost meets the temperature demand of washing or bathing water in summer. If the heat from air conditioning systems can be used to heat domestic hot water, it decreases not only the consumption of primary energy for heating the domestic hot water but also the calefaction to the surroundings due to the rejection of heat from air conditioning systems. There are some bottleneck problems to be solved in recovering the rejected heat from air conditioning system to generate domestic hot water. These problems are as follows: • The staggering and uncontinuity of domestic hot water demands. The problem would be solved if the rejected heat of continuous working air conditioning system were stored in an accumulator. Proper phase change materials should be developed. • The reasonable matching of the running condition of compressing condensing unit, and the optimizing of running mode to achieve the optimal economical performance. • Auxiliary heating to ensure the temperature and mass flow rate of hot water supplied. • Thermodynamic properties of the phase change materials (PCMs) compliant for recovering the rejected heat from air conditioning system. Early discussed heat recovery systems used water as the stored media or directly heat water to users [2]. However, the energy source and the demands of a house, in general, do not match each other at any time. Thermal energy storage provides a reservoir of energy to adjust this mismatch and to meet the energy needs at all times [5]. It is used as a bridge to cross the gap between the energy source, the air-conditioning system, and the users. Thermal energy storage is essential in the recovery of the rejected heat from air conditioning system. Sensible heating of water is the most widely used for energy storage. Although water is inexpensive and has good heat transfer characteristics, it requires a large volume because of its low energy storage capacity. Another drawback of water storage is the wide temperature range over which the stored energy is delivered. Meanwhile phase change materials (PCMs) are theoretically able to change phase at constant temperature and therefore store large quantities of energy [5]. They are of some superiority over sensible heat storage materials, such as very high heat of fusion relative to specific heat thus requiring smaller container volumes, a relatively stable and narrow temperature range of supplied water. The storage of thermal energy as latent heat represents a good attractive option to sensible energy storage. A large number of organic and inorganic substances are known to melt with a high heat capacity of fusion in any required temperature range. However, for their employment as heat storage materials in latent thermal energy storage (LTES) system, these PCMs must exhibit certain desirable properties, such as economic considerations of cost and large-scale availability.
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The main criteria that govern the selection of phase change heat storage materials are summarized as follows [6]: • Possess a melting point in the desired operating temperature range. • Possess high latent heat of fusion per unit mass, so that a less amount of material stores a given amount of energy. • High specific heat to provide additional significant sensible heat storage effects. • High thermal conductivity, so that the temperature gradients required for charging and discharging the storage material are small. • Small volume changes during phase transition, so that a simple container and heat exchanger geometry can be used. • Exhibit little or no super-cooling during freezing. • Chemical stability. • No chemical decomposition, so that a high LTES system lift is assured. • Non-corrosiveness to construction materials. • Non-poisonous, non-flammable and non-explosive. • Available in large quantities. • Inexpensive. Apparently, no single material can fully satisfy the long list of criteria. The organic substances serve as the important heat storage materials due to the several desirable properties they possess in comparison with inorganic compounds. Some of the advantages include their ability to melt congruently, their self-nucleating properties, and their compatibility with conventional materials of construction. A significant number of authors have based their work on organic materials such as alkanes, waxes or paraffin. Stritih [7] presented a solar wall, which absorbs solar energy into black paraffin wax, which is an example of phase change material. Inaba [8] studied a shape-stabilized paraffin as a new type of latent heat storage material, which keeps the same shape in a solid stage when the paraffin melts. Bo [9] described the thermal properties of tetradecane and hexadecane binary mixture and demonstrated their potential for use as PCMs for cool storage. Bo [10] discussed the technical grade paraffin waxes as phase change material (PCM) for cool storage and the cool storage system capital cost investment, and determined the potential for using cool storage systems. A review had been carried out of the history of thermal energy storage with solid–liquid phase change in 2003 [11]. It focused on the materials, heat transfer analysis and applications of thermal energy storage with phase change materials and listed over 150 materials used in research as PCMs, and about 45 commercially available PCMs. Although the information of phase change materials is quantitatively enormous, it is difficult to find a proper phase change material compliant for heat recovery of air conditioning systems. Paraffin wax calls growing attention to people because of its high heat of fusion, little or no super cooling during freezing, chemically stable, no chemical decomposition, non-corrosiveness, nonpoisonous, and it is available in large quantities [12]. It is a good media for energy storage. Presently, paraffin wax as phase change material mainly used in solar energy storage (including green house and building heating), building energy storage and district cooling systems, etc [7,9,10].
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This paper provides a heat recovery system using phase change materials (PCMs) to recover the rejected (sensible and condensation) heat of air conditioning system and producing low temperature hot water. The thermal properties of paraffin wax were studied. Paraffin wax based PCMs compliant for the thermal energy recovery of air conditioning system was developed.
2. Air conditioning system with heat recovery 2.1. System introduction Air conditioning system with heat recovery is a system using PCMs to store the rejected (sensible and condensation) heat of air conditioning and supplying low-temperature domestic water demanded for washing and bathing. The rejected heat of air conditioning system consists of sensible heat and latent heat, in which sensible heat accounts for about 15–20% of the total exhausting heat [13]. The exhausting temperature of compressor is relatively high, which is over 65 C when using Freon as refrigerant. Therefore, it can be recovered using an accumulator and gets heat of higher temperature. Thermal energy recovery system is consisted of a conventional air conditioning system, two heat recovery accumulators and an auxiliary electric water heater. The schematic diagram is shown in Fig. 1. The latent heat of exhaust is stored by accumulator 2, in which the PCM is paraffin wax with lower melting temperature, and the sensible heat of exhaust is stored by accumulator 1, in which paraffin wax of higher melting temperature is used as the PCM. In fact, condensation of the refrigerant will occur in accumulator 1 at some operating conditions, and the outlet temperature of refrigerant from accumulator1 could be higher than condensing temperature at some operating conditions. In accumulators 1 and 2, heat is transferred from hot refrigerant to PCMs. When domestic hot water is needed, water from water tank or water pipe flows into accumulators 2 and 1 successively, absorbing heat from the hot PCMs. The temperature of water outlet accumulator 2 could reach to about 35 C, and then was heated to about 40–45 C in accumulator 1. Electric water heater is put into operation whenever the heat recovered by the accumulators is insufficient to satisfy hot water heating requirement in a building or the outlet water temperature of accumulator1 cannot meet the temperature requirement of guests. Accumulators 1 and 2 are in series with cooling tower, which can successfully remove the remainder heat that could not be stored by the heat accumulators. The superheating refrigerant gas from the outlet of the compressor flows into a triple valve, which is used to control whether the refrigerant flows through the heat recovery accumulators, that is, whether the rejected heat is stored. The temperatures of PCMs in the accumulators vary with the change of condensing temperature. At the beginning, the control scheme turned-off the cooling tower circuit, heat is transferred from hot refrigerant to the PCMs in the accumulators, and the condensing pressure is rising with the temperature rise of phase change materials. When the condensing pressure reached to the rated value, the cooling tower circuit is turned on to remove the remainder heat of refrigerant and then decreased condensing pressure of the system. At this time, the accumulators still can store the sensible heat of superheating refrigerant gas and improve the temperature of phase change materials. When the temperature sensor of phase
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Z. Gu et al. / Applied Thermal Engineering 24 (2004) 2511–2526 9 Hot water users 10 4
11
12
3
13
5
2
6
1
7
8
Fig. 1. Schematics of the air conditioning system with thermal energy recovery devices. 1. Compressor, 2. Three-way valve, 3. Higher temperature accumulator (accumulator 1), 4. Lower temperature accumulator (accumulator 2), 5. Cooling tower, 6. Liquid storage tower, 7. Valve, 8. Evaporator, 9. Tap water tank, 10. Water pump, 11. Tap water valve, 12. Auxiliary electrical wayer heater, 13. Temperature sensor.
change materials detected that the temperature has reached to the setting value, the by-pass valve shut-off, and the system is operated in a normal refrigeration mode. The refrigerant leaving the accumulator 2 can be superheated vapor, saturated vapor and liquid or cooling liquid, depending on the thermal energy storage capacity of phase change materials. If the thermal storage capacity of PCM is adequate for storing all the sensible and latent heat of refrigerant, the refrigerant leaving the accumulator 2 can be saturated liquid or cooling liquid, which condition usually occurs at the start stage. If the PCMs could store the sensible heat and part of the latent heat, the refrigerant leaving the accumulator 2 can be saturated vapor and liquid. Otherwise, if the phase change material could store only part of the sensible heat, the refrigerant leaving the accumulator 2 can be superheating gas, which usually happens when the storage is nearly finished. The heat is not absorbed by PCMs in the accumulators and thus rejected to cooling water of the cooling tower. 2.2. Calculation model of thermal energy recovery system Generally, the temperature of the compressor discharging gas is over 65 C when using Freon as the refrigerant at the refrigeration cycle. The sensible heat and latent heat can be recovered by the accumulators to supply domestic hot water over 45 C for washing and bathing. Fig. 2 shows
Z. Gu et al. / Applied Thermal Engineering 24 (2004) 2511–2526 Qh
T Qc
7
2 3
4
8
5
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We
1
6 Qe
S
Fig. 2. T –S diagram of air conditioning system with heat recovery.
the Ts diagram of air conditioning system with heat recovery. To simplify the computation, the following assumptions were made in the model development. • Refrigerant pressure drops in all exchangers (accumulators, cooling tower and evaporator) are negligible. • The process taken place in the thermostatic expansion valve is isenthalpic. • There are no heat losses to the ambient from the system components. • The cycle was simplified without superheating and sub-cooling, that is, the degree of superheat of the refrigerant at the evaporator outlet and sub-cooling of the refrigerant at the cooling tower outlet are zero. The amount of sensible and condensation (latent) heat of air conditioning system can be calculated by Qk ¼ mr cpr ðT2 T4 Þ þ mr Dhrc ¼ mr ðh2 h5 Þ
ð1Þ
The refrigeration capacity of air conditioning system is Qe ¼ mr ðh1 h6 Þ
ð2Þ
The indicative power of compressor is given by Wico ¼ mr ðh3 h1 Þ The compressor power input can be calculated using Eq. (3) We ¼ mr ðh2 h1 Þ
ð3Þ
Generally, coefficient of performance (COP) is used to evaluate the property of refrigeration system. The integrated energy efficiency ratio (IEER) is established to estimate the energy saving capability of air conditioning systems with heat recovery, which includes the effective refrigeration capacity Qe and the effective heat recovery capacity Qh . The IEER can be defined as: P Q Qe þ Qh IEER ¼ P ¼ ð4Þ W W0 þ Wloss in which, W0 is the theoretic power of compressor and Wloss is the power loss of compressor.
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The IEER is different to heat recovery systems at different running mode. The IEER at the refrigeration mode, IEER1, and the IEER at the refrigeration and heat recovery mode, IEER2, can be calculated by Eqs. (5) and (6) respectively. IEER1 ¼ EER ¼ IEER2 ¼
Qe h1 h6 ¼ We h2 h1
ð5Þ
Qe þ Qh ðh1 h6 Þ þ ðh2 h8 Þ ¼ h2 h1 We
ð6Þ
If only the sensible heat air conditioning system is recovered, h8 ¼ h4 , IEER2 can be expressed as, IEER2 ¼ IEERs ¼
ðh1 h6 Þ þ ðh2 h4 Þ h2 h1
ð7Þ
If all the sensible and condensation heat of air conditioning system is recovered, h8 ¼ h5 , IEER2 can be expressed as, IEER2 ¼ IEERw ¼
ðh1 h6 Þ þ ðh2 h5 Þ h2 h1
ð8Þ
2.3. Data input of calculation The refrigerant used in the refrigeration cycle is R22. The isentropic efficiency of the cycle is 0.75. The nominal cooling capacity of the system is 3000 kcal/h at 54.4/7.2 C operating condition for our bench-scale test system. EER (IEER1), IEERs , and IEERw were calculated respectively. 2.4. Results and discussion System calculation was performed at different condensing temperatures i.e. 40, 45, 50, 55 and 60 C. During the calculation, the evaporating temperature was kept at 7.2 C. Fig. 3 shows that 5.0 4.5
System Energy/kW
4.0 3.5 3.0
Refrigeration Capacity Heat Recovery Capacity Compressor Power Input Sensible Heat of Heat Recovery
2.5 2.0 1.5 1.0 0.5 40
45
50
55
60
Condensing Temperature/ °C
Fig. 3. System energies at different condensing temperatures.
Energy Efficiency Ratio
Z. Gu et al. / Applied Thermal Engineering 24 (2004) 2511–2526 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
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EER IEERw IEERs
40
45
50
55
60
Condensing Temperature tk/°C
Fig. 4. Energy efficiency ratios of the system at different condensing temperatures.
the amount of compressor power input is increased with the condensing temperature increase. Whereas the refrigeration capacity and heat recovery capacity is decreased with the increasing of condensing temperature. The percentage of sensible heat to the total rejected heat is increasing when condensing temperature increases. Fig. 4 illustrates that the energy efficiency of the system varies with changes of condensing temperature. In calculating IEERw and IEERs , the useful heating capacity (thermal energy recovered) for service low-temperature hot water is included as part of the total system output. IEERw includes all the rejected (sensible and latent) heat of air conditioning systems, while IEERs only includes the sensible part of it. The curves show that a lower condensing temperature results in a higher EER, IEERw and IEERs . Particularly, when the condensing temperature is 45 C, the EER, IEERw and IEERs is 4.5, 10.0 and 5.2 respectively. When the condensing temperature is 55 C, the EER, IEERw and IEERs is 3.3, 7.6 and 4.1 respectively. It is observed that the IEER of the system is improved effectively when all the sensible and latent heat of air conditioning systems are recovered.
3. PCMs development and experiments 3.1. Materials Chemically speaking, paraffin waxes consist primarily of straight-chain hydrocarbons with only a small amount of branching, such as 2-methyl groups, near the end of the chain. Paraffin contain in them one major component called alkanes, characterized by Cn H2n þ 2 , the n-alkane (CH3 – (CH2 )n –CH3 ) content in paraffin waxes usually exceeds 75% and may reach 100% [6]. Pure paraffins contain only alkanes in them, for example the well-known paraffin octadecane, C18 H38 . The melting point of the alkanes increases with the increasing number of carbon atoms; alkanes containing 14–40 C-atoms possess melting points between 6 and 80 C and are generally
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termed as paraffins. Commercial waxes, on the other hand, may have a range of about 8–15 carbon-number [6]. Paraffin wax qualifies as heat-of-fusion storage material due to their availability in a large temperature range and their reasonable high heat of fusion. Furthermore, they are known to freeze without super-cooling. Considering the cost, however, only technical grade paraffin wax may be used as PCM in latent heat storage. The various paraffin waxes, byproducts of petroleum refining, are available in a selection of melting point ranges, so that a good match can be made between melting range and system operating temperature. Paraffin can be classified as food paraffin, full-refining paraffin, semi-refining paraffin, crude paraffin and black paraffin etc. Each class can be divided into 52#, 54#, 56#, 58# and 60# paraffin on the basis of melting point as well. For example, 52# paraffin wax means that its melting range is from 50 to 52 C. Paraffin waxes melt without segregation of components and display no super-cooling, so that thickening agents and nucleating agents are not generally needed. These substances are non-toxic, non-corrosive, and chemically inert and have no unpleasant odor. They present little fire hazard, and they have been used for cool storage and in solar heat storage [7–9]. 52# paraffin wax can be used to recover the sensible part of the rejected heat from air conditioning system. If PCM is used to recover latent heat of refrigerant, it should have a melting temperature of approximately 45 C. Paraffin waxes with a phase-transition temperature near 50 C are available. Commonly, solvent can decrease the melting temperature of solute. Similarly, the additive can reduce the melting temperature of paraffin wax, so it would meet the demands for heat thermal energy storage of condensation. The additive also should meet the foregoing demands listed. Considered the safety, some food additives such as food essence (a kind of additive to make food smells good) or lauric acid, and paraffin of lower melting temperature such as liquid paraffin (paraffin oil) were selected. Our former experiments showed that food essence was not completely solved with paraffin wax, so it is excluded. Table 1 shows some properties of lauric acid and liquid paraffin. 3.2. Determination of phase change temperature Technical grade paraffin wax and nine mixtures with liquid paraffin or lauric acid (10, 20, 30, 40, 50 weight proportion of liquid paraffin with paraffin wax and 2.44, 4.76, 6.98, 9.90, 11.11 weight proportion of lauric acid with paraffin wax) were tested. Each mixture was stirred well in the test tube with a stirrer and heated over the melting temperature. The test tube was fixed on a hob and placed in the room temperature. The thermometer with a 1.0 C resolution and a range of Table 1 Some properties of lauric acid and liquid paraffin Material
Melting temperature
Physical and chemical properties
Cost
Lauric acid
44 C
$2.29/100 g
Liquid paraffin
–
White or fluorescent crystalline solid or powder, bright, specific odor, hardly soluble in water, solute in ethanol, chloroform and ethyl ether. Paraffin oil. Liquid under normal temperature
$1.81/500 ml
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100 C was inserted into the test tube. The temperature reading of the mixture was observed every 30 s. The temperature decreased continuously until it reached a constant value––the freezing point of the mixture. Only after the entire mixture was frozen the temperature start to decrease further. The test tube with the frozen contents was then immersed in a beaker of water (at temperature about 58 C). The temperature of the mixture increased slowly until it reached the melting point of the mixture. The temperature then increased faster after the entire mixture melted. 3.3. Volume change during phase change process A beaker (600 ml) and graduated cylinder (100 ml ± 1 ml) were used to measure the volume expansion during the phase change. A liquid sample (about 500 ml) was placed in the beaker and the volume (V1 ) was observed, the beaker was then placed in the room. After the sample was frozen completely, the paraffin wax separated from the wall of the beaker. Add water into the beaker gradually, at the same time, use a needle pressing the paraffin wax into the water. When the surface reached to a certain mark of the beaker (V2 ), the adding was stopped. Then the water in the beaker was transferred into the graduated cylinder and the volume was read (V3 ). The volume of solid sample was then determined. And the volume expansion during the phase change is given by DV % ¼ ðV2 V3 V1 Þ=V1 100%
ð9Þ
3.4. Thermal performance The heat of fusion is the needed heat for unit mass material to change its phase from solid to liquid. The temperature during melting is a range rather than a single point if the measured material is a mixture. The heat transfer of the material is very complex during melting process. There were solid and liquid phase at the same time, the temperature distribution in solid phase and liquid phase are different, and the phase change interface is changing continuously, and the changing rate is also different, which makes the heat transferred to the material must be divided into three parts. The major part of the heat is used to complete the phase changing, one part is used to improve the temperature of solid phase and the other part is used to improve the temperature of liquid part. The instantaneous heat balance equation is given by, dQ ¼ ms cps
oTs oTl dm þ ml cpl þ Dh ds os os
ð10Þ
Integral Eq. (10) from heating start to heating stop, we get Q ¼ mcp ðTm T s;start Þ þ mcpl ðT l;stop Tm Þ þ mDh
ð11Þ
where, Dh is the heat of fusion of the sample. From Eq. (11), the heat of fusion Dh can be obtained. If the melting start point ss;start and melting stop point ss;stop can be accurately determined, the heat of fusion Dh can be calculated by the following equation: Dh ¼
Qm IU ðss;stop ss;start Þ ¼ m m
ð12Þ
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where m is the mass of experimental sample, I is the heating electric current, and U is the electric voltage of heater, which is collected by a computer. Time s is measured by a second chronograph. The paraffin wax–lauric acid binary systems and paraffin wax–liquid paraffin were prepared in different weight proportion and their temperature distribution curves during melting were recorded using quasi-static calorimetric method. The heats of fusion of these mixtures were determined using equation (12).
4. Experiment results and discussion 4.1. Freezing temperature of paraffin wax–liquid paraffin (PW–LP) mixtures Paraffin wax and paraffin wax–liquid paraffin binary systems in different weight proportions were prepared and their freezing properties were measured in order to determine the mixture ratio and the freezing temperature of the PCM. The freezing curves of these mixtures are shown in Fig. 5. It shows that the freezing temperature of paraffin wax is about 50.8 C, which is a bit higher for the recovery of condensation heat from air conditioning system. The freezing temperature of PCM decreased as the weight portion of LP increased. The desired value is approached when the weight proportion of liquid paraffin in the mixture is 40.0% as weight. The freezing temperature of paraffin wax with 40.0% of LP is about 44.7 C. The variation of freezing temperature of the PCMs with the weight proportion of liquid paraffin was shown in Fig. 6. The temperature decrease was nearly linear with the weight proportion of liquid paraffin. 4.2. Freezing temperature of paraffin wax–lauric acid (PW–LA) mixtures The paraffin wax–lauric acid (PW–LA) binary systems were prepared in different weight proportions and their thermal properties were measured in order to determine the mixture ratio and 60 LP LP LP LP LP
55
t/˚C
50
_10% _20% _30% _40% _50%
45
40
35
30 0
5
10
15
20
25
30
35
40
45
time/min Fig. 5. Freezing curves of PCM with different weight proportion of liquid paraffin.
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Freezing temperature/°C
52
50
48
46
44
42 0
10
20
30
40
50
Weight proportion of liquid paraffin in PCMs
Fig. 6. Variation of freezing temperature of PCMs with different weight of liquid paraffin.
65 PW / LA (40/1) PW / LA (40/2) PW / LA (40/3) PW / LA (40/4) PW / LA (40/5)
60
Temperature°C
55 50 45 40 35 30 25 0
5
10
15
20
25
30
35
40
45
50
55
60
65
Time/min
Fig. 7. Freezing curves of PCM with different weight proportion of lauric acid.
the freezing temperature of the PCM. The freezing curves of these mixtures are shown in Fig. 7. The freezing temperature of PCM decreased as the weight portion of LA increased. The desired value is approached when the weight proportion of LA in the mixture is 11.11% (weight ratio of paraffin wax over lauric acid is 40:5). 4.3. Melting temperature of PCMs The melting curves of technical grade paraffin wax, PW/LP (60:40), and PW/LA (40:5) were measured using the thermometer while the phenomena of melting were observed. The melting
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55 40% Liquid Paraffin
Temperature°C
50
Paraffin Wax
45
40 11.11wt% Lauric Acid 35
30
25 0
2
4
6
8
10
12
14
16
18
20
time/min
Fig. 8. Temperature distribution during melting of PW, PW–LP (60:40) and PW–LA (40:5).
ranges of these mixtures were obtained from temperature curves shown in Fig. 8. The materials melt over a range of temperature, rather than at a single temperature. This is because these materials are mixtures rather than pure substances. The melting range of paraffin wax is 44 C– 50.5 C, which is a bit higher when it is used as a PCM to recover the condensation heat of air conditioning system directly. The melting range of paraffin wax with 11.11 wt.% LA is from 38.6 C to 45.3 C, while the melting range of paraffin wax with 40 wt.% LP is from 37.8 C to 45.1 C. Therefore, these two materials already meet the temperature demands of PCMs for heat recovery of air conditioning system. 4.4. Volume contraction during the phase change The volume expansion of paraffin wax during the phase change was measured. The results indicate that the phase change of paraffin wax from liquid to solid state is accompanied by a volume contraction. The volume contraction is less than 12%. Such a volume contraction of paraffin wax during freezing should be considered in a heat thermal energy storage system. 4.5. Thermal performance The PW–LP and PW–LA binary systems were prepared in different weight proportions and their temperature distributions during melting process were recorded using quasi-static calorimetric method. The heat of fusion of these mixtures was determined using Eq. (12). The calculated data are given in Tables 2 and 3. In PW–LP binary system, the desired value is approached when the weight ratio of LP in the mixture is 40.0% as weight. The heat of fusion of the mixture is about 92 kJ/kg with a freezing temperature of 44.42 C. The freezing point of paraffin wax with 14.89% LA is 45.46 with a heat of fusion of 99.9 kJ/kg. It provides some useful information for the design
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Table 2 Thermal properties of PW–LP mixtures at different combination ratios Liquid paraffin (wt.%)
Heat of fusion (kJ/kg)
Freezing point (C)
0 10 20 30 40 50
165.9 142.1 126.8 103.5 91.7 73.6
50.77 49.05 47.79 46.27 44.42 43.38
Table 3 Thermal properties of PW–LA mixtures at different combination ratios Lauric acid (wt.%)
Heat of fusion (kJ/kg)
Freezing point (C)
0 2.44 6.98 11.11 14.89
165.9 139.2 123.0 104.3 99.9
50.77 49.87 48.40 46.84 45.46
of accumulators for heat recovery in air conditioning system. Compare with single substance, the heat of fusion of PCMs is decreased with the addition of additives. Although the heat of fusions for paraffin wax with 40 wt.% of liquid paraffin or paraffin wax with 11.11 wt.% of lauric acid is not very high, it has some superior to the sensible water storage, such as a relatively stable and narrow temperature range of supplied water, etc. Moreover, the melted PCM also can absorb some sensible heat to increase the storage capacity.
5. Conclusions A heat recovery system using PCMs to store the rejected heat (sensible and latent) from air conditioning system has been developed and studied, making up the shortage of other sensible heat storage system. It decreases not only the consumption of primary energy for heating domestic hot water but also the calefaction to the surroundings due to the rejection of heat from air conditioning systems. Through thermodynamic calculation, the integrative energy efficiency ratio (IEER) of the system was calculated and analyzed by changing the condensing temperature of the system. It is observed that the IEER of the system is improved effectively when all the rejected (sensible and latent) heat from air conditioning systems is recovered. The phase change temperatures of technical grade paraffin wax and the mixtures with liquid paraffin and lauric acid were obtained by thermometer measurement. The experimental results indicate that technical grade paraffin wax with the additive, LP (liquid paraffin) or LA (lauric acid), qualify as PCM for heat recovery of air conditioning system. Paraffin wax with 40% LP or 14.89% LA satisfies the temperature demand of heat recovery of air conditioning system. Furthermore, they are able to melt congruently and, due to their self-nucleating properties, they freeze
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without super-cooling. They also show a good stability over twenty heating and cooling cycles. The volume contraction of the phase change process from liquid to solid is below 12%, although the volume contraction of these mixtures should not be neglected in heat thermal energy storage system. The measurement of heat of fusion for PCMs provides some useful information for the design of accumulators for heat recovery in air conditioning system. The technical grade paraffin wax and the mixtures with LP and LA are proper PCMs for heat thermal energy storage. However, because of the high cost of lauric acid (>97.5%, $2.29/100g in Xi’an, 2003), liquid paraffin (paraffin oil) is preferred to lower the melting point of paraffin wax.
Acknowledgements This work was supported by Shaanxi Science & Industrial Development Projects, Grant No. 2002 K07/G19.
References [1] Z. Sun, S. Li, Analysis of energy reclamation of air-conditioning system, Energy Conservation Technology 17 (6) (1999) 26–28. [2] G. Rong, Mode of reusing Chiller’s heat energy in summer building, Heat Energy Ventilated Air Conditioner 28 (2) (1998) 27–29 (in Chinese). [3] X. Wu, B. Xia, L. Lu, Y. Zhang, Development and application of heat recovery unit, Refrigeration and AirConditioning 1 (6) (2001) 29–32 (in Chinese). [4] X. Yang, Analysis of thermal usage from condenser heat in middle and high standard hotels in summer, Journal of Sichuan University of Science and Technology 21 (2) (2002) 57–59 (in Chinese). [5] A. Sari, A. Kaygusuz, Thermal energy storage system using stearic acid as a phase change material, Solar Energy 71 (6) (2001) 365–376. [6] A. Abhat, Low temperature latent heat thermal energy storage: heat storage materials, Solar Energy 30 (4) (1983) 313–332. [7] U. Stritih, P. Novak, Solar heat storage wall for building ventilation WREC (1996) 268–271. [8] H. Inaba, P. Tu, Evaluation of thermophysical characteristics on shape-stabilized paraffin as a solid–liquid phase change material, Heat and Mass Transfer 32 (1997) 307–312. [9] H. Bo, E. Mari Gustafsson, F. Setterwall, Tetradecane and hexadecane binary mixtures as phase change materials (PCMs) for cool storage in district cooling systems, Energy 124 (1999) 1015–1028. [10] B. He, F. Setterwall, Technical grade paraffin waxes as phase change materials for cool thermal storage and cool storage systems capital cost estimation, Energy Conversion and Management 43 (2002) 1709–1723. [11] B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23 (2003) 251–283. [12] J. Wang, Review of latent thermal energy storage (a) characteristics of PCMS and performance optimization of storage system, New Energy 22 (2000) 31–35. [13] J. Zhang, Energy saving technology of refrigeration devices, Mechanical Industry Press, Beijing, 1999.
Thermal energy recovery of air conditioning system––heat recovery system calculation and phase change materials development Zhaolin Gu *, Hongjuan Liu, Yun Li School of Environmental and Chemical Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, PR China Received 15 January 2004; accepted 24 March 2004 Available online 6 May 2004
Abstract Latent heat thermal energy storage systems can be used to recover the rejected heat from air conditioning systems, which can be used to generate low-temperature hot water. It decreases not only the consumption of primary energy for heating domestic hot water but also the calefaction to the surroundings due to the rejection of heat from air conditioning systems. A recovery system using phase change materials (PCMs) to store the rejected (sensible and condensation) heat from air conditioning system has been developed and studied, making up the shortage of other sensible heat storage system. Also, PCMs compliant for heat recovery of air conditioning system should be developed. Technical grade paraffin wax has been discussed in this paper in order to develop a paraffin wax based PCM for the recovery of rejected heat from air conditioning systems. The thermal properties of technical grade paraffin wax and the mixtures of paraffin wax with lauric acid and with liquid paraffin (paraffin oil) are investigated and discussed, including volume expansion during the phase change process, the freezing point and the heat of fusion. 2004 Elsevier Ltd. All rights reserved. Keywords: Heat recovery; PCM; Phase change material; Air conditioning system; Domestic hot water
1. Introduction As the demand for air-conditioning increased greatly during the last decade, large demands of electric power and uncertain availability of fossil fuel have led to a surge of interest in the efficient energy application in air conditioning system. The rejected (sensible and condensation) heat from *
Corresponding author. Tel./fax: +86-2982-664-928. E-mail address: [email protected] (Z. Gu).
1359-4311/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.03.017
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Nomenclature cpr specific heat capacity of refrigerant under constant pressure (kJ/kg K) h specific enthalpy (kJ/kg) Dh heat of fusion (kJ/kg) latent heat of refrigerant during condensation (kJ/kg) Dhrc I the heating electric current (A) IEER1 IEER at the refrigeration mode, EER IEER2 IEER at the refrigeration and heat recovery mode m mass of experimental sample (kg) mass flow rate of refrigerant (kg/s) mr refrigeration capacity of air conditioning system (kW) Qe heat recovery capacity (kW) Qh sensible and condensation heat of air conditioning system (kW) Qk T absolute temperature (K) U electric voltage of heater (V) V volume (ml) DV % volume expansion during the phase change compressor power input (kW) We indicative power of compressor (kW) Wico power loss of compressor (kW) Wloss theoretic power of compressor (kW) W0 s time (s) ss;start melting start point ss;stop melting stop point Subscripts l liquid m melting r refrigerant s sensible, solid w whole Abbreviations COP coefficient of performance IEER integrated energy efficiency ratio LP liquid paraffin LA lauric acid PCM phase change material PW paraffin wax
air conditioning systems is a readily available energy source that can be used to produce lowtemperature hot water for washing and bathing [1–4].
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In summer, most hotels and restaurants provide both cooling and heating services for customers. Traditionally there are two systems, that is, hot water supplying system and air conditioning system, providing heat and cool load respectively. These two systems are independent. The main purpose of central water chiller plants of building air-conditioning systems is air conditioning. The heat from air conditioning systems is rejected into the air directly. On the other hand, great heat is needed to heat domestic hot water for washing and bathing, which is usually supplied by a boiler burning oil, gas or coal. Generally, the temperature of water in water pipe is between 15 and 20 C. The outlet temperature of cooling water for air conditioning system is between 35 and 40 C, which almost meets the temperature demand of washing or bathing water in summer. If the heat from air conditioning systems can be used to heat domestic hot water, it decreases not only the consumption of primary energy for heating the domestic hot water but also the calefaction to the surroundings due to the rejection of heat from air conditioning systems. There are some bottleneck problems to be solved in recovering the rejected heat from air conditioning system to generate domestic hot water. These problems are as follows: • The staggering and uncontinuity of domestic hot water demands. The problem would be solved if the rejected heat of continuous working air conditioning system were stored in an accumulator. Proper phase change materials should be developed. • The reasonable matching of the running condition of compressing condensing unit, and the optimizing of running mode to achieve the optimal economical performance. • Auxiliary heating to ensure the temperature and mass flow rate of hot water supplied. • Thermodynamic properties of the phase change materials (PCMs) compliant for recovering the rejected heat from air conditioning system. Early discussed heat recovery systems used water as the stored media or directly heat water to users [2]. However, the energy source and the demands of a house, in general, do not match each other at any time. Thermal energy storage provides a reservoir of energy to adjust this mismatch and to meet the energy needs at all times [5]. It is used as a bridge to cross the gap between the energy source, the air-conditioning system, and the users. Thermal energy storage is essential in the recovery of the rejected heat from air conditioning system. Sensible heating of water is the most widely used for energy storage. Although water is inexpensive and has good heat transfer characteristics, it requires a large volume because of its low energy storage capacity. Another drawback of water storage is the wide temperature range over which the stored energy is delivered. Meanwhile phase change materials (PCMs) are theoretically able to change phase at constant temperature and therefore store large quantities of energy [5]. They are of some superiority over sensible heat storage materials, such as very high heat of fusion relative to specific heat thus requiring smaller container volumes, a relatively stable and narrow temperature range of supplied water. The storage of thermal energy as latent heat represents a good attractive option to sensible energy storage. A large number of organic and inorganic substances are known to melt with a high heat capacity of fusion in any required temperature range. However, for their employment as heat storage materials in latent thermal energy storage (LTES) system, these PCMs must exhibit certain desirable properties, such as economic considerations of cost and large-scale availability.
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The main criteria that govern the selection of phase change heat storage materials are summarized as follows [6]: • Possess a melting point in the desired operating temperature range. • Possess high latent heat of fusion per unit mass, so that a less amount of material stores a given amount of energy. • High specific heat to provide additional significant sensible heat storage effects. • High thermal conductivity, so that the temperature gradients required for charging and discharging the storage material are small. • Small volume changes during phase transition, so that a simple container and heat exchanger geometry can be used. • Exhibit little or no super-cooling during freezing. • Chemical stability. • No chemical decomposition, so that a high LTES system lift is assured. • Non-corrosiveness to construction materials. • Non-poisonous, non-flammable and non-explosive. • Available in large quantities. • Inexpensive. Apparently, no single material can fully satisfy the long list of criteria. The organic substances serve as the important heat storage materials due to the several desirable properties they possess in comparison with inorganic compounds. Some of the advantages include their ability to melt congruently, their self-nucleating properties, and their compatibility with conventional materials of construction. A significant number of authors have based their work on organic materials such as alkanes, waxes or paraffin. Stritih [7] presented a solar wall, which absorbs solar energy into black paraffin wax, which is an example of phase change material. Inaba [8] studied a shape-stabilized paraffin as a new type of latent heat storage material, which keeps the same shape in a solid stage when the paraffin melts. Bo [9] described the thermal properties of tetradecane and hexadecane binary mixture and demonstrated their potential for use as PCMs for cool storage. Bo [10] discussed the technical grade paraffin waxes as phase change material (PCM) for cool storage and the cool storage system capital cost investment, and determined the potential for using cool storage systems. A review had been carried out of the history of thermal energy storage with solid–liquid phase change in 2003 [11]. It focused on the materials, heat transfer analysis and applications of thermal energy storage with phase change materials and listed over 150 materials used in research as PCMs, and about 45 commercially available PCMs. Although the information of phase change materials is quantitatively enormous, it is difficult to find a proper phase change material compliant for heat recovery of air conditioning systems. Paraffin wax calls growing attention to people because of its high heat of fusion, little or no super cooling during freezing, chemically stable, no chemical decomposition, non-corrosiveness, nonpoisonous, and it is available in large quantities [12]. It is a good media for energy storage. Presently, paraffin wax as phase change material mainly used in solar energy storage (including green house and building heating), building energy storage and district cooling systems, etc [7,9,10].
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This paper provides a heat recovery system using phase change materials (PCMs) to recover the rejected (sensible and condensation) heat of air conditioning system and producing low temperature hot water. The thermal properties of paraffin wax were studied. Paraffin wax based PCMs compliant for the thermal energy recovery of air conditioning system was developed.
2. Air conditioning system with heat recovery 2.1. System introduction Air conditioning system with heat recovery is a system using PCMs to store the rejected (sensible and condensation) heat of air conditioning and supplying low-temperature domestic water demanded for washing and bathing. The rejected heat of air conditioning system consists of sensible heat and latent heat, in which sensible heat accounts for about 15–20% of the total exhausting heat [13]. The exhausting temperature of compressor is relatively high, which is over 65 C when using Freon as refrigerant. Therefore, it can be recovered using an accumulator and gets heat of higher temperature. Thermal energy recovery system is consisted of a conventional air conditioning system, two heat recovery accumulators and an auxiliary electric water heater. The schematic diagram is shown in Fig. 1. The latent heat of exhaust is stored by accumulator 2, in which the PCM is paraffin wax with lower melting temperature, and the sensible heat of exhaust is stored by accumulator 1, in which paraffin wax of higher melting temperature is used as the PCM. In fact, condensation of the refrigerant will occur in accumulator 1 at some operating conditions, and the outlet temperature of refrigerant from accumulator1 could be higher than condensing temperature at some operating conditions. In accumulators 1 and 2, heat is transferred from hot refrigerant to PCMs. When domestic hot water is needed, water from water tank or water pipe flows into accumulators 2 and 1 successively, absorbing heat from the hot PCMs. The temperature of water outlet accumulator 2 could reach to about 35 C, and then was heated to about 40–45 C in accumulator 1. Electric water heater is put into operation whenever the heat recovered by the accumulators is insufficient to satisfy hot water heating requirement in a building or the outlet water temperature of accumulator1 cannot meet the temperature requirement of guests. Accumulators 1 and 2 are in series with cooling tower, which can successfully remove the remainder heat that could not be stored by the heat accumulators. The superheating refrigerant gas from the outlet of the compressor flows into a triple valve, which is used to control whether the refrigerant flows through the heat recovery accumulators, that is, whether the rejected heat is stored. The temperatures of PCMs in the accumulators vary with the change of condensing temperature. At the beginning, the control scheme turned-off the cooling tower circuit, heat is transferred from hot refrigerant to the PCMs in the accumulators, and the condensing pressure is rising with the temperature rise of phase change materials. When the condensing pressure reached to the rated value, the cooling tower circuit is turned on to remove the remainder heat of refrigerant and then decreased condensing pressure of the system. At this time, the accumulators still can store the sensible heat of superheating refrigerant gas and improve the temperature of phase change materials. When the temperature sensor of phase
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Z. Gu et al. / Applied Thermal Engineering 24 (2004) 2511–2526 9 Hot water users 10 4
11
12
3
13
5
2
6
1
7
8
Fig. 1. Schematics of the air conditioning system with thermal energy recovery devices. 1. Compressor, 2. Three-way valve, 3. Higher temperature accumulator (accumulator 1), 4. Lower temperature accumulator (accumulator 2), 5. Cooling tower, 6. Liquid storage tower, 7. Valve, 8. Evaporator, 9. Tap water tank, 10. Water pump, 11. Tap water valve, 12. Auxiliary electrical wayer heater, 13. Temperature sensor.
change materials detected that the temperature has reached to the setting value, the by-pass valve shut-off, and the system is operated in a normal refrigeration mode. The refrigerant leaving the accumulator 2 can be superheated vapor, saturated vapor and liquid or cooling liquid, depending on the thermal energy storage capacity of phase change materials. If the thermal storage capacity of PCM is adequate for storing all the sensible and latent heat of refrigerant, the refrigerant leaving the accumulator 2 can be saturated liquid or cooling liquid, which condition usually occurs at the start stage. If the PCMs could store the sensible heat and part of the latent heat, the refrigerant leaving the accumulator 2 can be saturated vapor and liquid. Otherwise, if the phase change material could store only part of the sensible heat, the refrigerant leaving the accumulator 2 can be superheating gas, which usually happens when the storage is nearly finished. The heat is not absorbed by PCMs in the accumulators and thus rejected to cooling water of the cooling tower. 2.2. Calculation model of thermal energy recovery system Generally, the temperature of the compressor discharging gas is over 65 C when using Freon as the refrigerant at the refrigeration cycle. The sensible heat and latent heat can be recovered by the accumulators to supply domestic hot water over 45 C for washing and bathing. Fig. 2 shows
Z. Gu et al. / Applied Thermal Engineering 24 (2004) 2511–2526 Qh
T Qc
7
2 3
4
8
5
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We
1
6 Qe
S
Fig. 2. T –S diagram of air conditioning system with heat recovery.
the Ts diagram of air conditioning system with heat recovery. To simplify the computation, the following assumptions were made in the model development. • Refrigerant pressure drops in all exchangers (accumulators, cooling tower and evaporator) are negligible. • The process taken place in the thermostatic expansion valve is isenthalpic. • There are no heat losses to the ambient from the system components. • The cycle was simplified without superheating and sub-cooling, that is, the degree of superheat of the refrigerant at the evaporator outlet and sub-cooling of the refrigerant at the cooling tower outlet are zero. The amount of sensible and condensation (latent) heat of air conditioning system can be calculated by Qk ¼ mr cpr ðT2 T4 Þ þ mr Dhrc ¼ mr ðh2 h5 Þ
ð1Þ
The refrigeration capacity of air conditioning system is Qe ¼ mr ðh1 h6 Þ
ð2Þ
The indicative power of compressor is given by Wico ¼ mr ðh3 h1 Þ The compressor power input can be calculated using Eq. (3) We ¼ mr ðh2 h1 Þ
ð3Þ
Generally, coefficient of performance (COP) is used to evaluate the property of refrigeration system. The integrated energy efficiency ratio (IEER) is established to estimate the energy saving capability of air conditioning systems with heat recovery, which includes the effective refrigeration capacity Qe and the effective heat recovery capacity Qh . The IEER can be defined as: P Q Qe þ Qh IEER ¼ P ¼ ð4Þ W W0 þ Wloss in which, W0 is the theoretic power of compressor and Wloss is the power loss of compressor.
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The IEER is different to heat recovery systems at different running mode. The IEER at the refrigeration mode, IEER1, and the IEER at the refrigeration and heat recovery mode, IEER2, can be calculated by Eqs. (5) and (6) respectively. IEER1 ¼ EER ¼ IEER2 ¼
Qe h1 h6 ¼ We h2 h1
ð5Þ
Qe þ Qh ðh1 h6 Þ þ ðh2 h8 Þ ¼ h2 h1 We
ð6Þ
If only the sensible heat air conditioning system is recovered, h8 ¼ h4 , IEER2 can be expressed as, IEER2 ¼ IEERs ¼
ðh1 h6 Þ þ ðh2 h4 Þ h2 h1
ð7Þ
If all the sensible and condensation heat of air conditioning system is recovered, h8 ¼ h5 , IEER2 can be expressed as, IEER2 ¼ IEERw ¼
ðh1 h6 Þ þ ðh2 h5 Þ h2 h1
ð8Þ
2.3. Data input of calculation The refrigerant used in the refrigeration cycle is R22. The isentropic efficiency of the cycle is 0.75. The nominal cooling capacity of the system is 3000 kcal/h at 54.4/7.2 C operating condition for our bench-scale test system. EER (IEER1), IEERs , and IEERw were calculated respectively. 2.4. Results and discussion System calculation was performed at different condensing temperatures i.e. 40, 45, 50, 55 and 60 C. During the calculation, the evaporating temperature was kept at 7.2 C. Fig. 3 shows that 5.0 4.5
System Energy/kW
4.0 3.5 3.0
Refrigeration Capacity Heat Recovery Capacity Compressor Power Input Sensible Heat of Heat Recovery
2.5 2.0 1.5 1.0 0.5 40
45
50
55
60
Condensing Temperature/ °C
Fig. 3. System energies at different condensing temperatures.
Energy Efficiency Ratio
Z. Gu et al. / Applied Thermal Engineering 24 (2004) 2511–2526 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
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EER IEERw IEERs
40
45
50
55
60
Condensing Temperature tk/°C
Fig. 4. Energy efficiency ratios of the system at different condensing temperatures.
the amount of compressor power input is increased with the condensing temperature increase. Whereas the refrigeration capacity and heat recovery capacity is decreased with the increasing of condensing temperature. The percentage of sensible heat to the total rejected heat is increasing when condensing temperature increases. Fig. 4 illustrates that the energy efficiency of the system varies with changes of condensing temperature. In calculating IEERw and IEERs , the useful heating capacity (thermal energy recovered) for service low-temperature hot water is included as part of the total system output. IEERw includes all the rejected (sensible and latent) heat of air conditioning systems, while IEERs only includes the sensible part of it. The curves show that a lower condensing temperature results in a higher EER, IEERw and IEERs . Particularly, when the condensing temperature is 45 C, the EER, IEERw and IEERs is 4.5, 10.0 and 5.2 respectively. When the condensing temperature is 55 C, the EER, IEERw and IEERs is 3.3, 7.6 and 4.1 respectively. It is observed that the IEER of the system is improved effectively when all the sensible and latent heat of air conditioning systems are recovered.
3. PCMs development and experiments 3.1. Materials Chemically speaking, paraffin waxes consist primarily of straight-chain hydrocarbons with only a small amount of branching, such as 2-methyl groups, near the end of the chain. Paraffin contain in them one major component called alkanes, characterized by Cn H2n þ 2 , the n-alkane (CH3 – (CH2 )n –CH3 ) content in paraffin waxes usually exceeds 75% and may reach 100% [6]. Pure paraffins contain only alkanes in them, for example the well-known paraffin octadecane, C18 H38 . The melting point of the alkanes increases with the increasing number of carbon atoms; alkanes containing 14–40 C-atoms possess melting points between 6 and 80 C and are generally
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termed as paraffins. Commercial waxes, on the other hand, may have a range of about 8–15 carbon-number [6]. Paraffin wax qualifies as heat-of-fusion storage material due to their availability in a large temperature range and their reasonable high heat of fusion. Furthermore, they are known to freeze without super-cooling. Considering the cost, however, only technical grade paraffin wax may be used as PCM in latent heat storage. The various paraffin waxes, byproducts of petroleum refining, are available in a selection of melting point ranges, so that a good match can be made between melting range and system operating temperature. Paraffin can be classified as food paraffin, full-refining paraffin, semi-refining paraffin, crude paraffin and black paraffin etc. Each class can be divided into 52#, 54#, 56#, 58# and 60# paraffin on the basis of melting point as well. For example, 52# paraffin wax means that its melting range is from 50 to 52 C. Paraffin waxes melt without segregation of components and display no super-cooling, so that thickening agents and nucleating agents are not generally needed. These substances are non-toxic, non-corrosive, and chemically inert and have no unpleasant odor. They present little fire hazard, and they have been used for cool storage and in solar heat storage [7–9]. 52# paraffin wax can be used to recover the sensible part of the rejected heat from air conditioning system. If PCM is used to recover latent heat of refrigerant, it should have a melting temperature of approximately 45 C. Paraffin waxes with a phase-transition temperature near 50 C are available. Commonly, solvent can decrease the melting temperature of solute. Similarly, the additive can reduce the melting temperature of paraffin wax, so it would meet the demands for heat thermal energy storage of condensation. The additive also should meet the foregoing demands listed. Considered the safety, some food additives such as food essence (a kind of additive to make food smells good) or lauric acid, and paraffin of lower melting temperature such as liquid paraffin (paraffin oil) were selected. Our former experiments showed that food essence was not completely solved with paraffin wax, so it is excluded. Table 1 shows some properties of lauric acid and liquid paraffin. 3.2. Determination of phase change temperature Technical grade paraffin wax and nine mixtures with liquid paraffin or lauric acid (10, 20, 30, 40, 50 weight proportion of liquid paraffin with paraffin wax and 2.44, 4.76, 6.98, 9.90, 11.11 weight proportion of lauric acid with paraffin wax) were tested. Each mixture was stirred well in the test tube with a stirrer and heated over the melting temperature. The test tube was fixed on a hob and placed in the room temperature. The thermometer with a 1.0 C resolution and a range of Table 1 Some properties of lauric acid and liquid paraffin Material
Melting temperature
Physical and chemical properties
Cost
Lauric acid
44 C
$2.29/100 g
Liquid paraffin
–
White or fluorescent crystalline solid or powder, bright, specific odor, hardly soluble in water, solute in ethanol, chloroform and ethyl ether. Paraffin oil. Liquid under normal temperature
$1.81/500 ml
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100 C was inserted into the test tube. The temperature reading of the mixture was observed every 30 s. The temperature decreased continuously until it reached a constant value––the freezing point of the mixture. Only after the entire mixture was frozen the temperature start to decrease further. The test tube with the frozen contents was then immersed in a beaker of water (at temperature about 58 C). The temperature of the mixture increased slowly until it reached the melting point of the mixture. The temperature then increased faster after the entire mixture melted. 3.3. Volume change during phase change process A beaker (600 ml) and graduated cylinder (100 ml ± 1 ml) were used to measure the volume expansion during the phase change. A liquid sample (about 500 ml) was placed in the beaker and the volume (V1 ) was observed, the beaker was then placed in the room. After the sample was frozen completely, the paraffin wax separated from the wall of the beaker. Add water into the beaker gradually, at the same time, use a needle pressing the paraffin wax into the water. When the surface reached to a certain mark of the beaker (V2 ), the adding was stopped. Then the water in the beaker was transferred into the graduated cylinder and the volume was read (V3 ). The volume of solid sample was then determined. And the volume expansion during the phase change is given by DV % ¼ ðV2 V3 V1 Þ=V1 100%
ð9Þ
3.4. Thermal performance The heat of fusion is the needed heat for unit mass material to change its phase from solid to liquid. The temperature during melting is a range rather than a single point if the measured material is a mixture. The heat transfer of the material is very complex during melting process. There were solid and liquid phase at the same time, the temperature distribution in solid phase and liquid phase are different, and the phase change interface is changing continuously, and the changing rate is also different, which makes the heat transferred to the material must be divided into three parts. The major part of the heat is used to complete the phase changing, one part is used to improve the temperature of solid phase and the other part is used to improve the temperature of liquid part. The instantaneous heat balance equation is given by, dQ ¼ ms cps
oTs oTl dm þ ml cpl þ Dh ds os os
ð10Þ
Integral Eq. (10) from heating start to heating stop, we get Q ¼ mcp ðTm T s;start Þ þ mcpl ðT l;stop Tm Þ þ mDh
ð11Þ
where, Dh is the heat of fusion of the sample. From Eq. (11), the heat of fusion Dh can be obtained. If the melting start point ss;start and melting stop point ss;stop can be accurately determined, the heat of fusion Dh can be calculated by the following equation: Dh ¼
Qm IU ðss;stop ss;start Þ ¼ m m
ð12Þ
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where m is the mass of experimental sample, I is the heating electric current, and U is the electric voltage of heater, which is collected by a computer. Time s is measured by a second chronograph. The paraffin wax–lauric acid binary systems and paraffin wax–liquid paraffin were prepared in different weight proportion and their temperature distribution curves during melting were recorded using quasi-static calorimetric method. The heats of fusion of these mixtures were determined using equation (12).
4. Experiment results and discussion 4.1. Freezing temperature of paraffin wax–liquid paraffin (PW–LP) mixtures Paraffin wax and paraffin wax–liquid paraffin binary systems in different weight proportions were prepared and their freezing properties were measured in order to determine the mixture ratio and the freezing temperature of the PCM. The freezing curves of these mixtures are shown in Fig. 5. It shows that the freezing temperature of paraffin wax is about 50.8 C, which is a bit higher for the recovery of condensation heat from air conditioning system. The freezing temperature of PCM decreased as the weight portion of LP increased. The desired value is approached when the weight proportion of liquid paraffin in the mixture is 40.0% as weight. The freezing temperature of paraffin wax with 40.0% of LP is about 44.7 C. The variation of freezing temperature of the PCMs with the weight proportion of liquid paraffin was shown in Fig. 6. The temperature decrease was nearly linear with the weight proportion of liquid paraffin. 4.2. Freezing temperature of paraffin wax–lauric acid (PW–LA) mixtures The paraffin wax–lauric acid (PW–LA) binary systems were prepared in different weight proportions and their thermal properties were measured in order to determine the mixture ratio and 60 LP LP LP LP LP
55
t/˚C
50
_10% _20% _30% _40% _50%
45
40
35
30 0
5
10
15
20
25
30
35
40
45
time/min Fig. 5. Freezing curves of PCM with different weight proportion of liquid paraffin.
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Freezing temperature/°C
52
50
48
46
44
42 0
10
20
30
40
50
Weight proportion of liquid paraffin in PCMs
Fig. 6. Variation of freezing temperature of PCMs with different weight of liquid paraffin.
65 PW / LA (40/1) PW / LA (40/2) PW / LA (40/3) PW / LA (40/4) PW / LA (40/5)
60
Temperature°C
55 50 45 40 35 30 25 0
5
10
15
20
25
30
35
40
45
50
55
60
65
Time/min
Fig. 7. Freezing curves of PCM with different weight proportion of lauric acid.
the freezing temperature of the PCM. The freezing curves of these mixtures are shown in Fig. 7. The freezing temperature of PCM decreased as the weight portion of LA increased. The desired value is approached when the weight proportion of LA in the mixture is 11.11% (weight ratio of paraffin wax over lauric acid is 40:5). 4.3. Melting temperature of PCMs The melting curves of technical grade paraffin wax, PW/LP (60:40), and PW/LA (40:5) were measured using the thermometer while the phenomena of melting were observed. The melting
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55 40% Liquid Paraffin
Temperature°C
50
Paraffin Wax
45
40 11.11wt% Lauric Acid 35
30
25 0
2
4
6
8
10
12
14
16
18
20
time/min
Fig. 8. Temperature distribution during melting of PW, PW–LP (60:40) and PW–LA (40:5).
ranges of these mixtures were obtained from temperature curves shown in Fig. 8. The materials melt over a range of temperature, rather than at a single temperature. This is because these materials are mixtures rather than pure substances. The melting range of paraffin wax is 44 C– 50.5 C, which is a bit higher when it is used as a PCM to recover the condensation heat of air conditioning system directly. The melting range of paraffin wax with 11.11 wt.% LA is from 38.6 C to 45.3 C, while the melting range of paraffin wax with 40 wt.% LP is from 37.8 C to 45.1 C. Therefore, these two materials already meet the temperature demands of PCMs for heat recovery of air conditioning system. 4.4. Volume contraction during the phase change The volume expansion of paraffin wax during the phase change was measured. The results indicate that the phase change of paraffin wax from liquid to solid state is accompanied by a volume contraction. The volume contraction is less than 12%. Such a volume contraction of paraffin wax during freezing should be considered in a heat thermal energy storage system. 4.5. Thermal performance The PW–LP and PW–LA binary systems were prepared in different weight proportions and their temperature distributions during melting process were recorded using quasi-static calorimetric method. The heat of fusion of these mixtures was determined using Eq. (12). The calculated data are given in Tables 2 and 3. In PW–LP binary system, the desired value is approached when the weight ratio of LP in the mixture is 40.0% as weight. The heat of fusion of the mixture is about 92 kJ/kg with a freezing temperature of 44.42 C. The freezing point of paraffin wax with 14.89% LA is 45.46 with a heat of fusion of 99.9 kJ/kg. It provides some useful information for the design
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Table 2 Thermal properties of PW–LP mixtures at different combination ratios Liquid paraffin (wt.%)
Heat of fusion (kJ/kg)
Freezing point (C)
0 10 20 30 40 50
165.9 142.1 126.8 103.5 91.7 73.6
50.77 49.05 47.79 46.27 44.42 43.38
Table 3 Thermal properties of PW–LA mixtures at different combination ratios Lauric acid (wt.%)
Heat of fusion (kJ/kg)
Freezing point (C)
0 2.44 6.98 11.11 14.89
165.9 139.2 123.0 104.3 99.9
50.77 49.87 48.40 46.84 45.46
of accumulators for heat recovery in air conditioning system. Compare with single substance, the heat of fusion of PCMs is decreased with the addition of additives. Although the heat of fusions for paraffin wax with 40 wt.% of liquid paraffin or paraffin wax with 11.11 wt.% of lauric acid is not very high, it has some superior to the sensible water storage, such as a relatively stable and narrow temperature range of supplied water, etc. Moreover, the melted PCM also can absorb some sensible heat to increase the storage capacity.
5. Conclusions A heat recovery system using PCMs to store the rejected heat (sensible and latent) from air conditioning system has been developed and studied, making up the shortage of other sensible heat storage system. It decreases not only the consumption of primary energy for heating domestic hot water but also the calefaction to the surroundings due to the rejection of heat from air conditioning systems. Through thermodynamic calculation, the integrative energy efficiency ratio (IEER) of the system was calculated and analyzed by changing the condensing temperature of the system. It is observed that the IEER of the system is improved effectively when all the rejected (sensible and latent) heat from air conditioning systems is recovered. The phase change temperatures of technical grade paraffin wax and the mixtures with liquid paraffin and lauric acid were obtained by thermometer measurement. The experimental results indicate that technical grade paraffin wax with the additive, LP (liquid paraffin) or LA (lauric acid), qualify as PCM for heat recovery of air conditioning system. Paraffin wax with 40% LP or 14.89% LA satisfies the temperature demand of heat recovery of air conditioning system. Furthermore, they are able to melt congruently and, due to their self-nucleating properties, they freeze
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without super-cooling. They also show a good stability over twenty heating and cooling cycles. The volume contraction of the phase change process from liquid to solid is below 12%, although the volume contraction of these mixtures should not be neglected in heat thermal energy storage system. The measurement of heat of fusion for PCMs provides some useful information for the design of accumulators for heat recovery in air conditioning system. The technical grade paraffin wax and the mixtures with LP and LA are proper PCMs for heat thermal energy storage. However, because of the high cost of lauric acid (>97.5%, $2.29/100g in Xi’an, 2003), liquid paraffin (paraffin oil) is preferred to lower the melting point of paraffin wax.
Acknowledgements This work was supported by Shaanxi Science & Industrial Development Projects, Grant No. 2002 K07/G19.
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