Scholarly article on topic 'Optimization of Heat Exchange in a Heat Accumulator with Latent Heat Storage'

Optimization of Heat Exchange in a Heat Accumulator with Latent Heat Storage Academic research paper on "Materials engineering"

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Procedia Technology
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Keywords
{"solar energy storage" / "phase change material" / "thermal accumulator" / "heat transfer."}

Abstract of research paper on Materials engineering, author of scientific article — Luca Constantin, Daniel Dragomir-Stanciu, Ionut Vasile Crismaru

Abstract The paper analyzes the possibility to optimize the construction of a heat exchanger, which has the role of thermal accumulator in a solar energy storage system for residential use. The storage of solar energy is made in a phase change material (PCM), namely wax paraffin. Starting from a model of thermal accumulator tested experimentally, aiming at improving the heat transfer between wax paraffin and water takes the heat stored. For the new construction solutions proposed heat transfer modeling was performed, using Fluent module from ANSYS 14.5.

Academic research paper on topic "Optimization of Heat Exchange in a Heat Accumulator with Latent Heat Storage"

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Technology

Procedia Technology 19 (2015) 737 - 741 ^^^^^^^^^^^^^^

8th International Conferencelnterdisciplinarity in Engineering, INTER-ENG 2014,9-10 October

2014, Tirgu-Mures, Romania

Optimization of heat exchange in a heat accumulator with latent

heat storage

Luca Constantina, Daniel Dragomir-Stanciua*, Ionut Vasile Crismaraa

Technical University "Gheorghe Asachi" ofIa§i, Bid. D. Mangeron nr.61, 700050, Ia§i, Romania

Abstract

The paper analyzes the possibility to optimize the construction of a heat exchanger, which has the role of thermal accumulator in a solar energy storage system for residential use. The storage of solar energy is made in a phase change material (PCM), namely wax paraffin. Starting from a model of thermal accumulator tested experimentally, aiming at improving the heat transfer between wax paraffin and water takes the heat stored. For the new construction solutions proposed heat transfer modeling was performed, using Fluent module from ANSYS 14.5.

© 2015 TheAuthors.PublishedbyElsevierLtd.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 "Petru Maior" University of Tirgu Mures, Faculty of Engineering

Keywords:solar energy storage; phase change material; thermal accumulator; heat transfer.

1. Introduction

The considered solar energy system uses photovoltaic panels to produce electricity to residential users. In the periods in which the energy produced exceeds consumer demands, excess electricity is used for electric heating of a thermal accumulator. Heat is stored in a phase change material (PCM), namely paraffin wax. The advantage of using of PCM is that by using the latent heat of phase change can store of large amount of heat in a small volume [1].PCM absorbs and release heat at a nearly constant temperature. They store 5-14 times more heat per unit volume than sensible storage materials such as water or rock [2].

* Corresponding author. Tel.: +40 749 768485. E-mail address:ddragomir03@yahoo.com

2212-0173 © 2015 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 "Petru Maior" University of Tirgu Mures, Faculty of Engineering doi: 10. 1016/j .protcy.2015.02. 104

Paraffin wax is the mainly used commercial organic heat storage PCM. It consists of mainly straight chain hydrocarbons that have melting temperatures from 23 to 67 °C [3].

Inside thermal accumulator is mounted a coil through which water circulates. The water takes the heat stored in paraffin wax. Hot water thus produced is used for residential needs.

Such a thermal accumulator, capable of producing hot water, was tested experimentally [5].

Based on these results, the paper analyzes the possibilities to optimize the heat accumulator construction, in order to increase its efficiency and improve the heat exchange between paraffin wax and water circulating through the inner coils.

2. The first model of the heat accumulator

The first model of the heat accumulator is presented in Fig.l.a. May be observed the two electrical resistances located in the horizontal plane and the copper coil for water circulation located in the vertical plane. For this model was performed experimental tests and CFD analysis (Fig.lb).The results were similar [2].

Fig. 1. (a) The first model of the heat accumulator; (b) The temperature distribution in paraffin wax and flowing water.

The maximum temperature of paraffin wax was 148 °C. The maximum temperature of the outlet water is 88°C.

From the experiments it was observed that at the bottom part of the thermal accumulator the paraffin wax remains in a semisolid state, any heat would provide electrical resistances. The thickness of this semisolid layer is about 50 mm.

3. The optimization of the heat accumulator

To improve heat accumulator operation are proposed some constructive changes. Determining of the functional parameters, for the new constructive changes, was done by CFD analysis. The CFD analysis used three mathematical models to obtain the melting of the wax, the heat transfer inside the wax, through the copper wall of the water coil and inside the flowing water and the turbulence inside the fluids. The dominant heat transfer mechanisms are conduction and natural convection in liquid phase [4].

The thermal energy which melts the paraffin wax is introduced in to the model as a thermal boundary condition on the external surfaces of the electric resistances. The phase change of the wax and the heat transfer to the water were modeled using the Fluent module from Ansys 14.5. In modeling performed, the maximum temperature of paraffin wax was maintained 148 °C.

3.1. The first version

For this first version was followed the homogenization of the temperature inside the mass of paraffin wax. For this purpose two exterior pipes were installed on two opposite side surfaces of the heat accumulator (Fig. 2.a). These pipes set up a circulation of paraffin wax from the top to the bottom of accumulator, due to the density difference.

Another change from the initial model is that water enters in the accumulator at the top of the coil, which helps to maintain a higher temperature in the lower layer of paraffin wax. It was considered that water flows through one single coil, as in the first model (Fig.2.b). The electrical resistances were placed closer to the bottom of heat accumulator.

The amount of paraffin wax inside accumulator is in this case 0,017 m3. The heat exchange surface of the coil for water circulation is 0,054 m2.

The temperature distribution in the heat accumulator, represented in Fig.3, shows that the thickness of the semisolid layer in the bottom part of accumulator decreases, having 20 mm.

Fig. 2. (a) The heat accumulator with external pipes; (b) Coil and electrical resistances position.

Fig. 3. Temperature distribution in the heat accumulator

Convective heat transfer coefficient from paraffin wax to coil has values in the range 96,3 - 172,13 [W/m2K] and heat transfer coefficient from coil to water has values in the range 344,26 - 516,4 [W/m2K] .

3.2. The second version

In order to increase the heat flux over water it is proposed solution added two more coils for water circulation. The three coils a connected to a common pipe at the inlet and outlet (Fig.4). Increasing the number of water coils will lead to more uniform heat taking from the paraffin wax volume.

The volume of paraffin wax is 0,016 m3. The heat exchange surface of the coils for water circulation is 0,162 m2.

Fig. 4. Heat accumulator with three water coils

Fig. 5. Temperature distribution in the heat accumulator with three water coils -section through middle coil

Temperature distribution in the middle coil is presented in Fig.5. Temperature distribution through sections of lateral coils is similar. The water massflow through middle coil is greater than in lateral coils.

Convective heat transfer coefficient from paraffin wax to coil has values in the range 90,2-206,8 [W/m2K] ,. Convective heat transfer coefficient from coil to water has values in the range 341,37-874,60 [W/m2K] . The higher values corresponding to middle coil.

4. Conclusions

Proposed constructive changes contribute to increased heat accumulator performance. The thickness of semisolid

layer from the bottom part of heat accumulator dropped. For both constructive versions proposed, thermal

convection coefficients are higher than the initial version. Their growth is between 20-25%.

The maximum temperature of hot water leaving the accumulator is 96°C compared to 88°C for the initial model.

This temperature increase is due increased heat transfer between paraffin wax and water.

Modeling demonstrates that such heat accumulator, that stores solar energy, can be used to deliver hot water.

Photovoltaic panels and heat accumulator can be together a low power cogeneration system for residential sector.

References

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[10] Amrit OM, Gowtham M, Vinod R, Ramkumar G. Analysis of PCM Material in Thermal Energy Storage System, International Journal of environmental Science and Develpoment, vol.2, n.6, 2001: 437-441.