Investigation of the effect on the efficiency of phase change material placed in solar collector tank

Investigation of the effect on the efficiency of phase change material placed in solar collector tank

Thermal Science and Engineering Progress 5 (2018) 25–31 Contents lists available at ScienceDirect Thermal Science and Engineering Progress journal h...

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Thermal Science and Engineering Progress 5 (2018) 25–31

Contents lists available at ScienceDirect

Thermal Science and Engineering Progress journal homepage: www.elsevier.com/locate/tsep

Investigation of the effect on the efficiency of phase change material placed in solar collector tank

MARK



Sinem Kılıçkapa, , Emin Elb, Cengiz Yıldızc a b c

Bingol University, Faculty of Engineering and Architecture, Mechanical Engineering Department, 12000 Bingöl, Turkey Bitlis Eren University, Teknik Bilimler MYO, 13000 Bitlis, Turkey Firat University, Faculty of Engineering, Mechanical Engineering Department, 23200 Elazığ, Turkey

A R T I C L E I N F O

A B S T R A C T

Keywords: Solar power Latent heat storage Phase change material Efficiency

In parallel with the increase in population, natural energy resources on earth started to run out. And this brought along tendency to new energy resources. Therefore, studies on new and renewable energy resources became intensified and especially researches about solar power as the most important renewable energy resource gained intensity. In this study, tank with latent heat storage in combination with hot water collector was designed and tests were held under Elazığ climatic conditions. Thermal efficiencies obtained from collector system were compared to the efficiencies of a standard insulated tank. In accordance with this purpose the results of the tests, held on certain days in July-November, were commented. The highest thermal efficiency value in the study was obtained as 58% in July around hour 13:30 from the tank in which phase change material (PCM) was used.

1. Introduction In parallel with increasing population, countries need more energy. Within this scope use of renewable energy resources becomes mandatory. Being easily accessible, clean and cheap, renewable energy resources bring advantage when used. Also, it is important to design the systems in a way that they use the energy with maximum efficiency. It is determined that Turkey’s average annual solar radiation period is 2640 h (Daily total 7.2 h), average total radiation power is 1311 kWh/m2-year (Daily total 3.6 kWh/m2). It can be utilized in Turkey technically during ten months of a year and economically on 63% of the total country surface area and 17% all over the year. Turkey’s most solar energy receiving area is Southeastern Anatolia Region, it is followed by Mediterranean and Eastern Anatolia Region. Average annual radiation power is 1365 kWh/m2 in Eastern Anatolia Region including the Elazığ city where the tests were held and average annual solar radiation period is 2664 h [4]. Within this scope energy storage is used as a method for using energy efficiently. For this purpose, especially solar energy can be stored as sensible heat, latent heat and their combinations. Latent heat storage methods can be used in line with the requirements for heating and cooling purposes. Smaller tank volume is needed compared to sensible heat. Heat storage capacity is higher. Phase change materials (PCM) used for latent heat storage are more advantageous compared to other methods because they provide fluid



storage at constant temperature range. Shukla et al. [13], examined solar water heaters with phase change materials PCM) in heat energy storage. They classified their studies also according to collector type besides natural and latent heat storage. They compared solar water heaters with PCM and without PCM, they concluded that collectors with PCM had better thermal performance [13]. Koca et al. [8], performed energy and exergy analysis of a latent heat storage system with PCM on a flat plate solar collector. They used CaCl2·6H2O (Calcium chloride hexahydrate) as PCM. For system efficiency in changing periods, using first law equation of thermodynamics for energy analysis and second law equation of thermodynamics for exergy analysis they obtained the efficiencies of energy and exergy respectively as 45% and 2.2%. Finally, they stated that the exergy efficiency of the latent heat storage system with PCM was too low [8]. Da Cunha and Eames [5], examined storage of solar energy for applications at low and intermediate temperatures using PCM. They found out that phase transition temperatures were between 0 and 250 °C. When organic compounds and salt hydrates were under 100 °C, other mixtures varied between 100 and 250 °C. They remarked that mixture of sodium and potassium as PCM with melting temperature at around 170 °C was preferred more because of its cheaper price and moderate latent heat storage capacity [5]. Sharma et al. [12], performed examinations about thermal energy storage and its applications with phase change materials. They remarked that latent heat storage using PCM was the most effective way

Corresponding author. E-mail addresses: [email protected] (S. Kılıçkap), [email protected] (E. El), [email protected]firat.edu.tr (C. Yıldız).

http://dx.doi.org/10.1016/j.tsep.2017.10.016 Received 6 July 2017; Received in revised form 2 October 2017 2451-9049/ © 2017 Published by Elsevier Ltd.

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Nomenclature Ac Cp ρ GT Qu Qr Ta Tci Tco Tt

To Ti ΔT ṁ V̇

surface area of solar collector [m2] specific heat [kJ/kgK] specific gravity of water [kg/m3] global solar radiation [W/m2] rate of useful energy gained [W] radiation energy [W] ambient temperature [°C] collector inlet temperature [°C] collector outlet temperature [°C] TANK water temperature [°C]

ɳi Wr Wt Wte x1, x2…

Outlet fluid temperature of solar collector [K] inlet fluid temperature of solar collector [K] temperature difference [K] mass flow rate [kg/h] volume flow rate [m3/h] instant collector efficiency average error that can be made in radiation measurement [W/m2] total error that can be made in temperature measurement [°C] Total error value error value

the small scale Trombe composite solar wall. They placed the phase change material into the wall in the form of a brick- shaped. While this material can store more heat than the same volume of concrete, but under dynamic conditions have seen a very different thermal behavior. They focused on the delay between the absorption of solar radiation and the energy supplied to the room. In their experiments, they determined the energy performance of the wall from heat flux measurements and enthalpy balances [17]. Veeraragavan et al. [14], in their work, they examined the solar thermophotovoltaic (SISTPV) system at night. We have seen that for any given PCM length in the results we have obtained, a combination of small taper ratio and large inlet hole-to-absorber area ratio are essential to increase the operation time and average power produced during the night time. They concluded that there was a balance between running time and the average power produced during the night time. They have stated that this solar thermophotovoltaic SISTPV) system can be a versatile solution that can be applied to work in different climatic conditions, even in space applications [14]. Veeraragavan and Shum [15], they have modeled heat losses from a phase change material PCM) storage tank for solar thermophotovoltaic SISTPV) systems. The heat losses from the side surfaces of the PCM tank are modeled using Newton's law of cooling. From the results they obtained, they determined that low thermal efficiencies would occur at high heat losses. As can be expected when thermal insulation is used on the lateral surfaces, they have found that approximately 40% thermal efficiency can be realized with low heat losses. It is expected that these thermophotovoltaic SISTPV) systems can be designed at steady-state when the PCM can be fully molten in order to maximize the thermal energy storage through latent heat. The analytical model they developed is said to be used to predict the design conditions in which the PCM tank is completely melted [15]. In this study, in terms of using solar energy more efficiently, tank with latent heat storage in combination with hot water collector was designed under Elazığ climatic conditions and the efficiency of the system was investigated experimentally.

of storing thermal energy and PCMs had high energy storage density. In their study, they examined PCM usage at heating and cooling applications in buildings for last 10 years and they summarized their analyses on thermal energy storage system [12]. Mehling et al. [10], examined hot water heat storage with PCM (phase change material) module. They explained that the system provided high thermal storage density when PCM was used at the upper side of the water tank and heat loss through the upper side of the tank was prevented. In their studies, they made the numerical simulation of the system using finite difference method and explained the test results. Thus, it was stated that PCM kept the water hot longer by 50–200% and increased average energy density by 20–45% [10]. Mazman et al. [9], examined PCM usage in solar hot water systems. They stated that PCM modules placed on water tank were high storage density systems and heat loss was reduced because of PCM’s latent heat. In their studies, they used PCM graphite composed of paraffin and stearic acid PS), paraffin and palmitic acid PP), stearic and myristic acid SM) with optimized thermal properties and 80:20 percentage rates. In their tests, they explained that average tank water temperature dropped under PCM melting point in 6–12 h and when reheating PCM raised the temperature of 14–36 L, of water in the upper side of the tank by 3–4 °C. Also, they concluded that paraffin and stearic acid PS) were the best PCM as they increased the efficiency by 74% [9]. Papadimitratos et al. [11], they worked on phase change materials PCMs) in solar collectors for solar water heaters SWH). In this method, the heat pipe is immersed inside the phase change material that is stored for an extended period of time. Thus, they provide hot water at times when the solar intensity is insufficient. In the solar collector, were utilized two distinct phase change materials PCM), namely Tritriacontane and Erythritol, with melting temperatures of 72 °C and 118 °C respectively. From the results of this study, they have obtained efficiency improvement 26% for normal operation and 66% for stagnation mode, compared with standard solar energy water heaters with that lack phase change material [11]. Fazilati and Alemrajabi [6], examined the effects of PCM usage in a solar water heater. They used paraffin as PCM. They compared energy and exergy efficiencies of the water heater with and without using PCM. They remarked that PCM usage enhanced energy and exergy efficiencies respectively up to 39% and 16%. Also, they found out that from the PCM using solar water heater hot water was provided for 25% longer time [6]. Cardenas and Leon [3], researched on thermal energy storage and observed PCMs commonly used in latent heat storage systems. They stated that thermal energy storage via latent heat was superior than the other thermal storage methods. They explained that salt components used as PCM were generally comprised of chloride, fluoride and nitrate. Also, they saw that in literature there were great differences between the data of PCM melting temperature, thermal conductivity and density and they concluded that this was rooted from lack of international PCM test methods [3]. Zalewski et al. [17], they have studied the phase change material on

2. Material and method In the study, heat storage via latent heat method in a flat plate solar collector-natural circulation tank system was observed. The flat plate solar collectors consist of, glass cover, absorber plate, flow tubes, heat insulation and the collector frame (Fig. 1). Open loop systems with natural circulation are the systems that fluid heated in collector circulates by itself because of reduced density of the heated fluid (Fig. 2). Cold water accumulated in the lower level of the tank passes to the collector and becoming lighter by heating it rises to the top level of the tank. This event continues all day long and the water in the tank is heated. The water in the collector of the natural circulation open loop system is same as the water used and these systems are of high efficiency. The pump is not used in our system because the use of the pump causes more cost increase in all systems. This has provided us 26

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Table 1 Technical specifications of Calcium chloride hexahydrate [2]. Melting point Density Latent heat stored Supercooling temperature Heat storage capacity No. of heat cycles Inflammability and toxicity Cpk (specific heat) Cps (specific heat)

28–35 °C 1500 kg/m3 188.406 kJ/kg 1 °C (with nucleant addition) 31.84 * 104 kJ/m3 Acc. to the literature, 3500–5000 None 1.42 kJ/kg °C 2.09 kJ/kg °C

Fig. 1. Sections of a collector [1].

to create our system economically with less cost. In such kind of systems, heat storage is important in terms of energy efficiency and sustainability. Heat energy is stored as sensible heat with the change in material’s internal energy, latent heat, heat of reaction and combination of these. Phase change materials are used for latent heat storage. For this purpose, materials, which show melting, evaporation and other phase changes at certain temperatures are utilized [16]. Also, the cost of an energy storage system with phase change is very important. A hard to find material with high cost can abolish the applicability of this system. With phase change storage materials meeting these properties, working at lower temperature range, storing in lower volume and storing more heat due to high heat capacity, can be realized [16]. In this study, calcium chloride hexahydrate (CaCl·6H2O) is used as phase change material because of its low phase change temperature. Calcium chloride hexahydrate gives better result than other phase change materials in terms of economy, thermal and chemical stability. Also, it is nonhazardous on human health, economic and easy to provide. This material is supplied anhydrous and by adding (51% by weight CaCl and 49% H2O) a bit more water it is solved. The most conspicuous properties of salt hydrates are they have high fusion latent heat with only a small change in volume and pretty high thermal conductivity (compared to organic materials). In Table 1, some of the technical specifications of calcium chloride hexahydrate are shown.

Fig. 3. Testing apparatus.

Also, to prevent nucleation, potassium nitrate (KNO3) of 2% by weight was added in the PCM. Collector efficiency is known as the ratio of solar energy collected in the heat-conveying liquid to the solar energy received by the collector. When the difference between the collector inlet water temperature and the ambient temperature rises, the efficiency tends to drop so instead of a general efficiency it will be better to speak of an instantaneous collector efficiency. The instantaneous collector efficiency of the collectors used in this study is defined as the ratio of the solar energy obtained from solar radiation received on the absorber plate to useful energy transferred to the fluid and, Fig. 2. Open loop system with natural circulation.

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Table 2 Technical specifications of the solar collector [7]. Dimensions Weight Gross Area Test Pressure Working Pressure Carrier wing Collection tube manifold Boru Plate coating Up cover Frame and Glass Back sheet Insulation

ηi =

930 mm × 1930 mm × 87.5 mm 31 kg 1.82 m2 9 bar 6 bar ETIAL 60 Extruded integral wing 16 mm, 12 tubes ETIAL 60 Extruded 30 mm

3. Experimental study The experimental study was realized on the terrace of Firat University Engineering Faculty Mechanical Engineering Department Heat Technics Laboratory in the city Elazığ. The tests were performed in July–November 2013. In this study, the aim was to observe the effect of using phase change material for latent heat storage in solar energy systems in terms of efficiency of the system. For this purpose, as shown in Fig. 3, flat plate solar collectors were designed in combination with tanks having PCM. In testing apparatus, conventional insulated tank and tank with PCM are both used and measurements are received at the same time. For the tests, standard solar energy hot water collector with aluminium oval tubes, flat plate and 930x1930x87.5 mm dimensions was used. The collector mounting racks were adjusted to the slope angle of 38° as the latitude of the city Elazığ. Two pieces of flat solar collectors (see Table 2 for the specifications) were used at the same time. In studies in the literature conducted on this area; it has been identified that a cylindrical standard fiberglass insulation tank having a diameter of 50 cm, a length of 100 cm and a thickness of 5 cm is used. In addition, it has been determined that there is a interval of 15 cm between the hot water tank and the tank in general. In our work, we have manufactured solar heating system considering these measurements. The interval of 15 cm outside the hot water reservoir in the tank was filled with PCM to store the latent heat. To measure the temperature of the PCM, a plastic tube was placed in the PCM and the tube was drilled at regular intervals. It was determined that the PCM temperatures showed variations that could generally be taken into consideration at intervals of 10 cm, and the tube were opened holes at intervals of 10 cm. Thermocouples were placed in of these opening holes to measure the temperatures and measurements were taken. The interior of a standard tank with PCM is shown in Fig. 4. To measure radiation values pyranometres of the brands Kipp and Zonen (CC12) were used. The unit of the values measured by the pyranometer was W/m2. Pyranometre was adjusted at collector slope angle of 38° to measure solar radiation. The measurements were taken in 30 min time zones. Elimko E-680 series multi channel digital thermometer was used for temperature measurement. The device is 32 inputs, corresponding to IEC 668 norms, an industrial device on which universal inputs and outputs can easily be programmed. Iron constantan thermocouples were used for temperature measurement. Temperature

Fig. 4. Inside of the tank with PCM.

Useful energy transferred to the fluid: (1)

Solar energy received on the absorber plate:

Qr = A cGT

(2)

Instantaneous collector efficiency:

ηi =

ṁ Cp ΔT Qu = Qr A cCT

(4)

A cGT

Are defined so.

Black flat paint, Selective coating 4 mm glass Aluminium Aluminium 50 mm Fiberglass sheet

Qu = ṁ Cp Δ T= ρV̇Cp (Tco−Tci)

ρV̇Cp (Tco−Tci)

(3)

Fig. 5. The change of radiation and temperature values in time (17 July 2013).

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Fig. 6. The change of radiation and temperature values in time (01 November 2013).

Fig. 7. The change of radiation and temperature values in time for standard tank with PCM.

Fig. 8. The change of radiation and temperature values in time for tank with PCM.

radiation values were measured [7]. It has been determined that there are net changes in 30-min time intervals in experiments conducted in accordance with studies in the literature. For this reason the measurements were continuously taken every 30 min in the study. The change of the values, measured within the day, according to solar radiation in time and thermal efficiency values are placed on the graphics below (Figs. 5–9). As an example of the taken measurements and calculated efficiency values, 17.07.2013 and 01.11.2013 dated result graphics are shown below.

and radiation values were taken in 30 minutes’ time zones.

4. Evaluation of test results In this study, tests were performed by connecting a standard tank and a tank with PCM to natural convection flat plate solar collectors. The tests were performed in July–November. In natural convection collector tests; tank water temperature (Tt), collector inlet temperature (Tci) and outlet temperature (Tco), ambient temperature (Ta) and solar 29

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Fig. 9. The change of instantaneous collector efficiency.

determined for natural convection system under the same working conditions. The evaluation of the test results is summarized below:

As shown in Fig. 5, the test results for insulated standard tank without are as follows:

• It can be seen that (Fig. 5) the water inlet and outlet temperatures of



• In tests performed in July; the water temperature of the standard

the collector measured in 17 July and the water temperatures of the tank without PCM are rising continuously until the hour 15.30 and then decreasing. The maximum water temperature of the tank is 87 °C. It is seen that the calculated instantaneous collector efficiency changes according to the hours of the day and radiation and at 13.30 the maximum collector efficiency is 56% as shown in Fig. 9. It can be seen that (Fig. 6) the water inlet and outlet temperatures of the collector measured in 01.11.2013 and the water temperatures of the tank without PCM are rising continuously until the hour 13.30 and then decreasing. The maximum water temperature of the tank is 70 °C. It is seen that the instantaneous collector efficiency changes according to the hours of the day and radiation and at 12.30 the maximum collector efficiency is 51% as shown in Fig. 9.

• • • • •

For the system with tank having PCM, tests were performed like the solar collector system with standard tank without PCM. In tests for standard tank with PCM, it was seen that the temperature of the collector output water was changing with solar radiation. The test results of the tank with PCM are as follows:

• •

• It can be seen that (Fig. 7) the water inlet and outlet temperatures of



the collector measured in 17.07.2013 and the water temperatures of the tank are rising continuously until the hour 15.30 and then decreasing. The maximum water temperature of the tank is 89 °C. It is seen that the instantaneous collector efficiency changes according to the hours of the day and radiation and at 13.30 the maximum collector efficiency is 58% as shown in Fig. 9. It can be seen that (Fig. 8) the water inlet and outlet temperatures of the collector measured in 01.11.2013 and the water temperatures of the tank are rising continuously until the hour 13.30 and then decreasing. The maximum water temperature of the tank is 73 °C. It is seen that the instantaneous collector efficiency changes according to the hours of the day and radiation and at 13.00 the maximum collector efficiency is 54% as shown in Fig. 9.

tank with PCM was obtained as 89 °C, while maximum water temperature at standard storage tests carried out was 87 °C at 15.30. In similar tests performed in November, the maximum tank water temperatures were obtained at 13.30 under the same conditions. In the standard tank was 70 °C, in the tank with PCM was 73 °C. In tests performed for the day selected out of the tests done in July, maximum collector efficiency of the flat plate solar collector was; 56% for the standard insulated tank and 58% for the tank with PCM. In tests performed in November maximum collector efficiency was; 51% for the standard tank and 54% for the tank with PCM. As it is seen from the results of the tests performed, the highest collector efficiency was obtained from the tank with PCM. The fact that the water temperature of the tank with PCM and efficiency is high, is caused by the decrease of thermal losses by showing the heat storage and insulation properties of PCM. Also, the system with PCM transferred the heat stored during day time to water in the tank at night time and provided water at proper temperature for 1–1.5 h more. Experimental errors are usually caused by carelessness, inexperience, incorrect selection of measuring instruments, and reading of values. The total error value can be calculated by the following equation by taking into account constant errors, manufacturing mistakes and random errors to determine the total error in any parameter.

Wte = [(x1)2 + (x2)2 +…+(x ∞)2]1/2 Total error (Wt) that can be made in the temperature measurement = ± 0.173 °C Average error (Wr) that can be made in the radiation measurement = ± 0.1 W/m2 According to these results, in solar collector systems the use of hot water in tank for a longer time can be possible by using latent heat storage systems, thus the efficiencies of the systems can be increased and more economic operation of the system can be possible.

5. Conclusions Appendix A. Supplementary data

In this study; tank with PCM in combination with hot water collector was designed and the collector efficiency of the system was determined experimentally. In the tests, temperature and efficiency values of standard tank with flat plate solar collector and tank with PCM were

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tsep.2017.10.016. 30

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