J. Inst. Eng. India Ser. C (April–June 2012) 93(2):171–176 DOI 10.1007/s40032-012-0014-4
REVIEW PAPER
Thermal Energy Storage using PCM for Solar Domestic Hot Water Systems: A Review S. A. Khot • N. K. Sane • B. S. Gawali
Received: 13 July 2011 / Accepted: 8 April 2013 / Published online: 30 May 2012 Ó The Institution of Engineers (India) 2012
Abstract Thermal energy storage using phase chase materials (PCM) has received considerable attention in the past two decades for time dependent energy source such as solar energy. From several experimental and theoretical analyses that have been made to assess the performance of thermal energy storage systems, it has been demonstrated that PCM-based systems are reliable and viable options. This paper covers such information on PCMs and PCMbased systems developed for the application of solar domestic hot water system. In addition, economic analysis of thermal storage system using PCM in comparison with conventional storage system helps to validate its commercial possibility. From the economic analysis, it is found that, PCM based solar domestic hot water system (SWHS) provides 23 % more cumulative and life cycle savings than conventional SWHS and will continue to perform efficiently even after 15 years due to application of nonmetallic tank. Payback period of PCM-based system is also less compared to conventional system. In conclusion, PCM based solar water heating systems can meet the requirements of Indian climatic situation in a cost effective and reliable manner. Keywords Phase change material (PCM) Thermal energy storage (TES) Latent heat thermal energy storage (LHTES) Solar domestic hot water system (SDHWS) S. A. Khot (&), Member Latthe Education Society’s Polytechnic, P-41, M. I. D. C. Kupwad, Sangli 416 436, India e-mail:
[email protected] N. K. Sane, Fellow B. S. Gawali, Non-member Mechanical Engineering, Walchand College of Engineering, Vishram Bhag, Sangli, India
Introduction Thermal energy storage (TES) is becoming an increasing concern in modern technology. The fundamental idea of a TES system is to support the energy management by storing thermal energy at periods when it is abundantly available and using it when and where it is required. TES has number of applications such as space and water heating, waste heat utilization, cooling and air-conditioning and so on. Fortunately, India is blessed with abundant solar radiation available almost throughout the year and over throughout its domain. One of the areas where solar energy is used extensively at present is water heating. Solar water heaters are getting popularity, since they are relatively inexpensive and simple to fabricate and maintain. They are viable supplement or alternative to electric or gas geysers. A solar water heater of 100 L per day capacity can prevent over 30 tons of carbon dioxide emissions during 20 years of its life span [1]. However, due to its intermittent and unpredictable nature; efficient, economical and reliable solar thermal energy storage devices and methods will have to be developed. Among the different possibilities to store energy, systems using PCMs are more fashionable for its consistency in latent heat storage. Nevertheless, the research work on the PCMs for thermal energy storage is still in its developing stage. Thermal energy storage using PCM for solar domestic hot water system can be alternative to the present day solar water heating systems. These systems have potential of conserving energy of the order 300 kwh/m2 per annum than the present system. Hence using smaller size solar collector can produce same output or ISI solar collector gives improvement in the efficiency at the additional cost
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of PCM. These PCM based solar system controls the temperature rise due to latent heat storage which will help in selecting non metallic materials. This can reduce the cost of the system. Increased life of the system and low maintenance are its other attractive features. The PCM based system may overcome the problems of scaling, corrosion, rusting and leakage. This paper covers literature review and the economic analysis of PCM based solar water heating system.
J. Inst. Eng. India Ser. C (April–June 2012) 93(2):171–176
Tg Tp
Glass Cover Blackence Tank Surface Water PCM in Capsules Insulation
Literature Review
Fig. 1 Built-in storage type water heater designed by Prakesh et al.
This section reviews research work reported by several investigators related to SWHS, PCM and its use in solar domestic hot water systems. PCMs are latent heat storage materials. As the source temperature rises, the chemical bonds within the PCM break up as the material changes phase from solid to liquid. The use of latent heat storage has two vital advantages. First, it is possible to store large amounts of heat with small temperature changes and therefore to have a high storage density. Secondly, because of the finite time required for phase change, it is possible to have smooth temperature variations [2]. A large number of organic, inorganic and eutectic materials have been reported, based on melting temperature and latent heat of fusion. As no single material satisfies all the ideal required properties, one has to have a compromise using the available materials with an adequate system design. Ogueke et al. [3] presented a review on solar water heating systems for domestic and industrial applications. They are grouped into two categories as passive and active solar water heating systems. The active systems generally have higher efficiencies than those of the passive systems. The best efficiencies of passive solar hot water systems (PHWS) are in the range of 30–50 %. The integrated collector storages (ICS) are of the order of 30 % while those with thermosiphon systems are of the order of 50 %. Solar hot water systems (SHWS) have very high potentials to significantly contribute to hot water requirement. Kenisarin et al. [4] concentrates on thermal properties of PCMs, methods of heat transfer enhancement and uses in solar water heating systems. An effort was made by Sharma et al. [5] discussed a heat exchanger with ways to enhance the heat transfer which will also help to provide a variety of designs to store the heat using PCMs for different applications. Zalba et al. [2] carried out a review on TES using solid– liquid phase change with reference to potential materials together with their thermophysical properties, heat transfer and applications. Commercial PCMs have also been listed. Different methods of thermal properties determination and
problems in long term stability of the materials and their encapsulation are discussed. Prakesh et al. [6] analyzed a built in storage type water heater containing a layer of PCM filled at the bottom (Fig. 1). During the sunshine hours, the water gets heated up which in turn transfers heat to the PCM below it. The PCM collects energy in the form of latent heat and melts. During off sunshine hours, the hot water is withdrawn and is replaced by cold water, which gains energy from the PCM. The energy is released by the PCM on changing its phase from liquid to solid. But its use is very limited because of the poor heat transfer between PCM and water. Tiwari et al. [7] presented an analysis of PCM storage for water heater by incorporating the effect of water flow through a parallel plate placed at the solid–liquid interface. In order to reduce the night heat losses from the exposed surface, movable insulation has been made. It contains movable insulation so that night losses from exposed surface are minimized. Hence hot water can be obtained throughout the day and night, and the fluctuations in water temperature decrease with an increase in the melted region of the PCM water heater. Kurklu et al. [8] have developed a new type of waterPCM solar collector and investigated its short-term thermal performance (Fig. 2). Water-PCM solar collector is much more advantageous than traditional hot water collectors. Because total system weight and cost of water-PCM solar collector is low as compared to traditional hot water collectors. Nallusamy et al. [9] have made an experimental investigation on a combined sensible and latent heat storage unit integrated with solar water heating system (Fig. 3). It consists of an insulated cylindrical TES tank, which contains PCM encapsulated spherical capsules, solar flat plate collector, flow meter, and circulating pumps. The paraffin is used as PCM with a melting temperature of 60 ± 1 °C and latent heat of fusion of 213 kJ/kg. It is concluded from experimental results that the combined sensible and latent storage concepts reduces the size of the storage tank quantitatively compared to conventional
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Fig. 4 The PCM combined cylindrical heat storage tank designed by Canbazog˘lu Fig. 2 Water-PCM solar collector designed by Kurklu et al.
Fig. 3 Experimental set-up designed by Nallusamy et al.
storage system and the combined storage system employing batch wise discharging of hot water from the TES tank is best suited for application where the requirement is intermittent. Canbazog˘lu et al. [10] have presented the results of investigation of solar energy storage performance using sodium thiosulfate pentahydrate in a conventional solar water-heating system. Figure 4 shows a detailed crosssectional view of a heat storage tank combined with a PCM. The total mass of PCM used in the heat storage tank was approximately 180 kg, using the density of the solid state of the PCM of 1,666 kg/m3. The storage time of hot water, the mass of hot water produced to use, and the total heat accumulated in the heat storage tank that contains some hydrated salts are approximately 2–3 times greater than that of conventional solar energy systems with a heat storage tank that does not include a PCM. The additional cost required for a PCM be compensated by smaller size of storage tank that of conventional solar energy storage systems. Therefore, it is obvious that the use of PCMs in the system may not cause an important increase in cost.
Vikram et al. [11] have experimentally investigated the thermal behavior and feasibility of a cylindrically encapsulated PCM. A storage tank containing latent heat storage material is used to analyze the performance of latent heat thermal energy storage system. Experiments have been carried out at a constant flow rate of heat transfer fluid for which the thermal characteristics of the LHTES system and efficiency of the system is calculated. It is concluded that LHTES systems are a commercially viable option for solar heat energy storage. Hinti et al. [12] empirically investigated the performance of water-PCM storage for use with conventional solar water heating systems. Paraffin wax contained in small cylindrical aluminum containers is used as the PCM. The storage performance was investigated when connected to flat plate collectors with conventional natural circulation. The use of paraffin wax as PCM is simple and inexpensive thermal energy storage. The suitability of the melting temperature of paraffin wax enables the storage of excess energy available in daytime hours as latent heat, and then the release of this stored heat to maintain the water temperature in an acceptable range for most domestic applications.
Economic Analysis: A Case Study of PCM Based SDWHS The economic analysis of a system has to be done in order to compare cumulative savings, life cycle savings and payback period of PCM based SWHS with conventional system. The value of the system utilizing solar energy, directly or indirectly, must ultimately be judged on the basis of its economy. Energy system using conventional energy sources is characterized by a relatively low initial cost and a relatively high annual cost. As is evident by now, a solar system is characterized by a relatively high initial cost and
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J. Inst. Eng. India Ser. C (April–June 2012) 93(2):171–176
a low annual cost. On the other hand, PCM based solar water heating system further improves performance of conventional SWHS system and simultaneously increases cumulative savings and reduces payback period. Thus an economic analysis of an energy system has to consider these aspects. In this section, brief information about methods used for making economic evaluations are given with reference to PCM based solar water heating applications. A stand- alone 100 L per day solar water heating system with and without PCM installation is considered with following assumptions. 1.
2.
3.
4.
The solar system requires a total investment C of which a fraction fl is taken as a loan and interest rate on the loan is dl and that is to be paid back in equal annual installments over a period of nl years. Let the annual energy load to be met by the solar system be E. This would result in an annual savings of E units of conventional energy. Assume that the cost of this energy is cf per unit of energy and that it increases at the rate of if every year. The solar system will normally require some annual expenditure by ways of maintenance, electrical energy for running subsidiary equipments and local taxes. We assume that the extra cost associated with these minor items of expenditure is M in the first year of operation and that it increases at the rate of im ever year. The rates im and if are related to inflation. Finally, it is assumed that if the system used for commercial/industrial applications, tax deductions are allowed both on the interest component of the annual loan repayment installment as well as on depreciation of the system. The depreciation to be at a uniform rate rd per year. The income tax rate is rr. Otherwise, subsidy per collector specified by MNRE is taken for calculation for first year only.
The cumulative savings over a period of n years for the system data given in the statement of the problem is obtained by summing the present worth of the annual savings, subtracting the initial down payment on the solar system and adding the cost of an equivalent conventional system. Data used for calculation of cumulative savings, life cycle savings and payback period is given in Table 1. Arrangement of spherical capsules inside the thermal storage tank of 100 LPD capacities is shown in Fig. 5a. Efficiency of the Flat Plate Collector (FPC) and Evacuated Tube Collector (ETC) as a function of operating temperatures are shown in Fig. 5b. Efficiency of the FPC at 60 °C is represented in the graph as 56 % [13]. From the economic analysis, it is found that, PCM based SWHS gives 23 % more cumulative and life cycle savings for the period of up to 15 years predicted as life of conventional SWHS. But, PCM based SWHS systems will
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Table 1 Data of conventional and PCM-based SWHS for economic analysis Sr no. Particular 1
CSWHS PCMSWHS
Thermal storage tank capacity, LPD 2
100
100
2
Surface area of solar collector, m
2
2.3
3
Operating temperature, °C
70
60
4
Solar collector thermal efficiency, g
50 %
56 %
5
Energy absorbed in the collector, kJ
18,000
23,184
6
Number of PCM balls
00
239
7 8
Mass of PCM, kg Quantity of water, ltrs
00 100
43.98 47
9
Sensible heat stored by PCM, kJ
00
4617.48
10
Latent heat stored by PCM, kJ
00
10882.07
11
Sensible heat stored by water, kJ
18,000
8306.16
12
Total heat stored, kJ
18,000
23785.71
13
Initial cost of solar system,
20,000
23,000
14
Government subsidy @ 33%,
6,600
7,590
15
Net cost of system less subsidy,
13,400
15,410
16
Initial down payment,
2,680
3,082
17
Loan amount,
10,720
12,328
18
Rate of interest
5%
5%
19
Repayment of loan, no of years
5
5
20
Annual expenses for first 5 years pa
–
21
Annual expenses for after 5 years pa 400
1,300
22
Rate at which maintenance cost increases annually
5%
5%
900
23
Cost of conventional fuel saving,
9,600
12,684
24
Rate at which energy cost increases annually
5%
5%
25
Cost of conventional equipment,
5,500
5,500
continue to perform efficiently even after 15 years because of use of non-metallic tank. Payback period of PCM-based system is also less with and without discounting compared to conventional system.
Concluding Remarks The review is based on the findings of the investigations on the SWHS, PCMs and PCM-based solar domestic hot water systems and its economic analysis. 1.
2.
SHWSs have very high potentials to contribute to hot water demand. However, present literature indicates that more research and development work are needed to further improve the existing level of efficiency for it to serve effectively as a viable alternative to the conventional means of hot water generation. Thermal energy storage using PCMs has received considerable attention in the past two decades.
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Fig. 5 a Schematic of thermal storage tank b Comparative performance of FPC and ETC
Fig. 6 Conventional and PCM-based SDHWS Systems a Cumulative savings b Life cycle savings
3.
4.
A large numbers of organic and inorganic materials have been identified as PCM based on their melting temperatures and latent heat of fusion. As no single material can have all the desired properties for an ideal storage media, one has to use the available materials and try to accommodate the poor physical property with proper design. Cost of available PCMs is comparatively high in the market. The utilization of these products at present prices is very limited and they are used only in demonstration projects or for specific application. There should be constant search of similar low cost materials and further investigation should be carried out.
5. 6.
7.
Since there is no standard test system, a standard test system based on the use of PCM should be developed. Low cost PCM material must be given serious attention. The stability of materials during thousands of thermal cycles has to be studied. Based on economic analysis of PCM-based SDHWS, cumulative and life cycle savings are more compared to conventional solar systems. Payback period is also more.
Finally, it can be concluded that thermal energy storage using PCM’s for solar domestic hot water system is a commercially viable option. The designs are still of preliminary in nature and no commercial design and system is
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available in the international market. It is necessary to coordinate joint efforts among researchers and manufactures of PCM products for its commercialization.
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J. Inst. Eng. India Ser. C (April–June 2012) 93(2):171–176 8. A. Kurklu, A. O’ Zmerzi, S. Bilgin, Thermal performance of a water-phase change material solar collector. Renew. Energy 26, 391–399 (2002) 9. N. Nallusamy, L.N. Rao, S. Sampath, R. Velraj, Effective utilization of solar energy for water heating applications using combined storage system’’. In proceedings of the International Conference on ‘‘New Millennium-Alternative Energy Solutions for Sustainable Development (Coimbatore, India, 2003) pp. 103–108 10. S. Canbazoglu, A. Sahinaslan, A. Ekmekyapar, Y.G. Aksy, F. Akarsu, Enhancement of thermal energy storage performance using sodium thiosulphate of a conventional solar water heating system. Energy Build. 37(3), 235–242 (2005) 11. D. Vikram, S. Kaushik, V. Prashanth, N. Nallusamy, An Improvement in the Solar Water Heating Systems using Phase Change Materials, 3rd edn, ed. by S.P. Sukhatme, J.K. Nayak. Proceedings of the International Conference on Renewable Energy for Developing Countries-2006. Solar energy: Principals of thermal collection and storage (Tata McGraw Hill companies, New Delhi, 2008) 12. A. Hinti, A. Ghandoor, Maaly, A.N.Z. Al-Khateeb, O. Al-Sheikh, Experimental investigation on the use of water–phase change material storage in conventional solar water heating systems, Gcreeder 2009, Amman-Jordan, March 31st–April 2nd (2009) 13. S.P. Sukhatme, Solar Energy—Principles of the Thermal Collection and Storage (Tata McGraw-Hill publishing company ltd, New Delhi, 2008)
