Scholarly article on topic 'Energy Accumulation Using Encapsulated Phase Change Materials with Recycled Material Components'

Energy Accumulation Using Encapsulated Phase Change Materials with Recycled Material Components Academic research paper on "Materials engineering"

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Energy Procedia
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{"solar energy" / PCM / sphere / "paraffin wax" / "thermal accumulation" / graphite}

Abstract of research paper on Materials engineering, author of scientific article — Ance Ansone, Mikelis Dzikevics, Aivars Zandeckis

Abstract Phase change materials (PCM) are often used for solar energy accumulation. PCMs have large latent heat capacity, which is why they are suitable for energy storage. Solar radiation is cyclic, so it is important for storage materials to have quick response time, when the temperature of the environment is changing. An advantage of PCMs is that they allow to accumulate not only sensible, but also latent energy during phase change, however PCM also has a problem - low thermal conductivity, which needs to be improved. To improve energy accumulation, PCM can be mixed with materials that have better thermal conductivity, for example metals or graphite, but these composite materials can be expensive. To reduce costs of encapsulated PCM spheres, recycled materials can be used. There are several publications on energy related calculations, but none of them involve energy accumulation using encapsulated PCM together with recycled material components. In this research experiments are carried out with recycled graphite powder and steel spirals from a local industrial enterprise which has these materials as waste, to see how recycled components affect energy accumulation for encapsulated PCM.

Academic research paper on topic "Energy Accumulation Using Encapsulated Phase Change Materials with Recycled Material Components"

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Energy Procedía 95 (2016) 153 - 158

International Scientific Conference "Environmental and Climate Technologies", CONECT 2015,

14-16 October 2015, Riga, Latvia

Energy accumulation using encapsulated phase change materials with recycled material components

Ance Ansone*, Mikelis Dzikevics, Aivars Zandeckis

Institute of Energy Systems and Environment, Riga Technical University, Azenes iela 12/1, Riga, LV-1048, Latvia

Abstract

Phase change materials (PCM) are often used for solar energy accumulation. PCMs have large latent heat capacity, which is why they are suitable for energy storage. Solar radiation is cyclic, so it is important for storage materials to have quick response time, when the temperature of the environment is changing. An advantage of PCMs is that they allow to accumulate not only sensible, but also latent energy during phase change, however PCM also has a problem - low thermal conductivity, which needs to be improved. To improve energy accumulation, PCM can be mixed with materials that have better thermal conductivity, for example metals or graphite, but these composite materials can be expensive. To reduce costs of encapsulated PCM spheres, recycled materials can be used.

There are several publications on energy related calculations, but none of them involve energy accumulation using encapsulated PCM together with recycled material components. In this research experiments are carried out with recycled graphite powder and steel spirals from a local industrial enterprise which has these materials as waste, to see how recycled components affect energy accumulation for encapsulated PCM.

©2016 The Authors.PublishedbyElsevierLtd. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. Keywords: solar energy; PCM; sphere; paraffin wax; thermal accumulation; graphite

* Corresponding author. Tel.: +371 67089943 E-mail address: ance.ansone@rtu.lv

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. doi:10.1016/j.egypro.2016.09.037

1. Introduction

World energy demand is incessant and from the attention to climate change prevention in the last decade not only in Europe, but all over the world, we can conclude that renewable energy sources have one of the highest development potentials [1]. Solar energy systems is among these renewables, which has optimization and energy efficiency enhancement potential [2, 3]. Sharma et al. [4] say that solar energy is dominating among other renewable energy sources and it plays a significant role in world energy politics.

One widely known solar accumulation method is use of encapsulated phase change materials (PCM) [5]. PCMs are used because of their wide work temperature range and they can accumulate not only physical energy, but also latent energy which is also known as "hidden" energy. Latent energy is energy which is used to change phase of the material, for example, from liquid to solid. A common PCM is paraffin wax, because it is relatively cheap, easily accessible, not toxic and can have several work cycles, but its biggest drawback is low thermal conductivity X=0,2 [5]. Thermal conductivity can be enhanced by adding composite materials with higher thermal conductivity, for example graphite powder or metallic details [3, 4]. There are several publications where metal or graphite powder are added to PCM with the aim to enhance thermal conductivity of the material, but these composite materials can be expensive [8]. That is why it is important to find a good alternative, to reduce costs of encapsulated PCM with good thermal conductivity. In a world where resources are limited, it is crucial to expand the life cycle of everything we have. In experiments with spherically encapsulated PCM, graphite powder and metallic parts are taken as waste materials from a local enterprise which specializes in graphite brush production. Since this enterprise has no use for these materials, it is interesting to experiment with these materials in solar energy accumulation field and have its lifecycle expanded, making the end product (encapsulated PCM with better thermal conductivity) more sustainable.

2. Methods

2.1. Experimental materials and runs

In the experiment paraffin was used as a PCM and it was encapsulated in 4 spheres (diameter 5 cm) - paraffin mixed with graphite powder (graphite mass in sphere is5%± 1 %), paraffin mixed with steel spirals (mass 10%±1 %), paraffin mixed with burned steel spirals (mass 10 % ± 1 %) and clean paraffin. Some steel spirals were burned for one hour in 550 °C temperature. This was done to burn off all industrial oils and other materials that may influence thermal conductivity of the material. When spheres with different material combinations were made, spheres were put in a water tank, which was heated to 58 °C (melting temperature of paraffin 58 °C). The length of the experiment is 130 minutes, during this time all spheres are melted completely.

Data of the experimental material parameters are summarized in Table 1.

Table 1. Parameters ofmaterials used in experiment [9, 10].

cp cp liquid

Sphere No. Material solid phase, kJ/kg-K phase, kJ/kg-K Mass, kg Hf, kJ/kg T beginning, K T melting, K T max., K

n—1 Paraffin 2.48 2.76 0.0449 266 296 327 331

n—1 Graphite 0.71 - 0.0024 - 296 327 331

n=2 Paraffin 2.48 2.76 0.0429 266 296 327 331

n=2 Burned steel 0.6 - 0.0040 - 296 327 331

n=3 Paraffin 2.48 2.76 0.0395 266 296 327 331

n=3 Steel 0.6 - 0.0040 - 296 327 331

n=4 Paraffin 2.48 2.76 0.0498 266 296 327 331

Parameters for numerical calculations, such as specific heat capacity or heat of fusion are taken from scientific publications. Other parameters are measured by the authors. Each PCM sphere has its own number, where the sphere

which consists of the paraffin and graphite composite is marked n=l; sphere with burned steel spirals is marked n=2, paraffin sphere with steel spirals is marked n=3 and sphere which consists only of paraffin is marked n=4.

2.2. Numerical calculations

The maximum energy capacity in the experiment for each individual sphere can be calculated using Eq. 1.

Q~ =Qp+Qmp+a. (1)

Qtotai energy accumulated in the sphere during the time of the experiment, kJ; Qsp energy accumulated in solid phase, kJ;

Qmp energy accumulated during melting (also known as latent energy) phase, kJ; Qip energy accumulated during the liquid phase, kJ.

The maximum, accumulated energy for the sphere filled only with paraffin, is calculated using Eq. 2.

Qparajfn ~ mPCM CpPCMs (T2 ^ ^

трем paraffin mass in sphere, kg;

CppcMs specificheatcapacityforparaffininsolidphase, J/(kg-K); Ti temperature of paraffin at the beginning of the experiment, K;

T2 paraffin melting temperature, K.

If the sphere consists ofPCM and other composite material, then Eq. 3 is used.

Qsp ~ трем CppcMs (T2 T,) ^ mc Cpcs (T2 T1) ^ ^

mc composite mass, kg;

Cpcs specific heat capacity of composite material in solid phase, J/(kg K).

In this experiment latent energy calculations only apply to paraffin, because it is the only material which changes its phase in the temperature range of the experiment. Calculation of latent energy is similar to each sphere, the only parameter that changes is that of the mass of paraffin (see Eq. 4).

Qmp = mPcM ■ Hf (4)

Qmp accumulated latent energy in melting phase, kJ;

Hf PCM heat of fusion, kJ/kg.

Similar as in solid phase, calculations are made for theoretical maximal accumulated energy in liquid phase. Since in this experiment composite materials are not going through phase change, specific heat capacity of solid phase is used (see Eq. 5).

Qu,=mp

'■PCM CpPCMl T 3

TT+mc • Cpcs' TT

CppcMi

specific heat capacity ofliquid PCM, J/(kg-K); end temperature ofthe experiment, K; melting temperature of PCM, K.

To describe energy accumulation speed, a time unit is necessary. Based on measurements time ts is selected. In this time PCM with beginning temperature Tb reaches melting temperature Tm and tm is the time in which PCM accumulates latent energy and ti is time in which PCM reaches the experiment's water temperature or ending temperature Te. The scheme oftemperature reaching is shown in Fig. 1.

Fig. 1. Temperature reaching in time ofPCM.

It is possible to conclude what the melting speed is based on experimental results, but it is not possible to describe the speed oftemperature increase, which also changes within one period.

Calculations ofrelative values are important, because they allow to objectively compare values using size rate.

The relative energy accumulation amount shows how much energy in percentage from total energy, is being accumulated in each phase. It is assumed that each PCM sphere accumulates 100% energy. To calculate the relative proportion ofeach phase from the total amount ofaccumulated energy, Eq. 6 is used.

■100%

where qn

relative accumulated energy in n-phase, %;

energy amount which is accumulated in n-phase (solid, melting, liquid), kJ.

And by using Eq. 7, it is possible to calculate relative energy accumulation speed. This speed shows what percentage from the total accumulated energy in each phase, material can accumulate in one hour. The length of latent phase and the amount of energy accumulated in each phase depends on the physical properties of material. If the thermal conductivity is higher, the length of latent phase is shorter. To compare different materials, the relative energy accumulation speed which shows what percentage from the total accumulated energy in each phase, for specific material can accumulate in one hour.

q =—■ 60 (7)

-l speedn

qspeedn relative energy accumulation speed in n-phase, from total accumulated energy, %/hour; tn lengthof phase,min.

3. Results and Discussion

Based on experimental data and numerical calculations maximal accumulated energy in each sphere is shown in Table 2.

Table 2. Accumulated energy.

Q solid phase, kJ Q latent phase, kJ Q liquid phase, kJ Q total, kJ

1 (with graphite) 3.51 11.95 0.50 15.96

2 (with burned steel) 3.37 11.40 0.48 15.25

3 (with steel) 3.11 10.52 0.45 14.08

4 (clean paraffin) 3.83 13.25 0.55 17.63

Results show that most of the energy is accumulated as latent energy when material is changing its phase. To calculate energy accumulation of latent phase, Eq. 4 was used. The smallest energy accumulation is happening when all the material has melted and paraffin is in a liquid state. In this case it is because of the fact, that temperature of water is only 4 degrees higher than melting temperature of paraffin. In this experiment, most energy was accumulated by the 4th sphere withjust paraffin (17.63 kJ) and 1st sphere with graphite (15.96 kJ).

Since theoretical maximal accumulated energy depends on the mass of the material and in this experiment each sphere has a different weight, for an objective comparison, the results were calculated to their respective, relative values. Relative accumulated energy in each sphere is shown in Table 3.

Table 3. Relative accumulated energy.

Q solid phase, % Q latent phase, % Q liquid phase, %

n—1 (graphite) 21.97 78.88 3.15

n=2 (burned steel) 22.09 74.74 3.17

n=3 (steel) 22.13 74.71 3.17

n=4 (paraffin) 21.72 75.16 3.12

Relative values results are pretty similar, and have difference only in hundredth part. Results show the tendency, that most of the energy is accumulated during the melting phase, while in the liquid phase, a relatively small part of energy is accumulated. In this case, the burning process of steel does not have any essential influence on accumulated energy amount. Further studies need to be carried out to evaluate other important PCM parameters, such as, high thermal conductivity, small volume changes during phase change, high specific heat capacity. Also recycled composite materials need to be studied and compared with new materials in terms ofusing them in encapsulated PCM.

4. Conclusion

Mathematical calculations were used to analyze spherically encapsulated PCM energy accumulation and influence of composite material on accumulated energy amount and speed in each phase. Recycled graphite powder and metallic material from a local enterprise were used in the experiments. The main problems with recycled

materials are that their thermodynamic parameters are unknown and that their composition is not homogeneous, for example material can be covered with oils from the production process. Most of the energy is accumulated in latent phase (74.71-78.88 %), while there is a tendency which emerges that a relatively insignificant energy amount is accumulated after all PCM has melted and composite material influence in this phase is small (clean paraffin accumulates 3.12 % of energy, while paraffin with steel 3.17 %). In general results show that the highest energy accumulation indicators are in spheres with added components. Best energy accumulation performance is in graphite sphere, which shows, that during the melting phase, graphite enhanced thermal conductivity of material and energy is accumulated faster which is a preferable feature.

Mathematical calculations provide results of tendencies how thermal conductivity can be enhanced with recycled materials, but further numerical and experimental analysis should be considered to achieve more detailed and specific results.

Acknowledgements

The work has been supported by the National Research Program "Energy efficient and low-carbon solutions for a secure, sustainable and climate variability reducing energy supply (LATENERGI)".

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