Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage

Abstract

This paper addresses a numerical and experimental investigation of a thermal energy storage unit involving phase change process dominated by heat conduction. The thermal energy storage unit involves a triplex concentric tube with phase change material (PCM) filling in the middle channel, with hot heat transfer fluid (HHTF) flowing outer channel during charging process and cold heat transfer fluid (CHTF) flowing inner channel during discharging process. A simple numerical method according to conversation of energy, called temperature & thermal resistance iteration method has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of heat transfer fluid (HTF, including HHTF and CHTF) on the thermal energy storage has been studied. Comparison between the numerical predictions and the experimental data shows good agreement. Graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axial position, time-wise variation of energy stored/released by the system were presented and discussed.

Introduction

Energy resources on the earth will be exhausted one day if they are used in an unchecked way. This will cause serious problems and even crisis and jeopardize the survival of mankind. In China, with the rapid development of economy and the much improvement of people’s living standard, the problem of energy shortage becomes serious. Especially after early morning until midnight, this problem becomes more serious for more power consumption of industrial, commercial and residential activities during that period of time.

There are two ways to relieve this serious situation to some extent. One is to shift electricity usage from peak periods to off peak periods. The other is to recover the energy which is used to be ejected directly to the environment as wasted heat such as the ejected heat from the condenser of air conditioner, or to collect and utilize the diurnally fluctuating thermal energy such as the solar energy. In any case, an energy storage system should be necessary.

Due to worldwide energy shortage, the development of efficient and cost effective thermal storage system has received considerable attention due to the impending shortage and increasing cost of energy resources (Hamada, 2003). A thermal storage system is expected to provide to timely and complete energy management by excessive thermal energy at energy-rich periods to utilize it at energy-poor periods. The main task of the energy storage is then to overcome the discrepancies between energy supply and energy demand. There are three main methods of thermal energy storing: sensible, latent and thermal–chemical heat storage. A latent thermal energy storage system, with solid–liquid phase change, has received a considerable attention due to its advantages, such as storing a large amount of energy in a small volume i.e., high storage density and heat charging/discharging at a nearly constant temperature, which result in a greater flexibility and more compactness of the PCM storage system in choosing a location for the storage system (Setterwall, 1996).

Experiments have been conducted to study the thermal performance of PCM storage systems and numerous publications have been found (Banaszek et al., 2000, Al Hallaj and Selman, 2000, Neeper, 2000, Py et al., 2001, Liu and Chung, 2001). According to these publications, paraffin waxes have been extensively used as PCM for many applications. This is because not only paraffin waxes are nonpoisonous, non-corrosive, chemically stable, relatively high latent heat capacity, but also they have a negligible degree of subcooling during nucleation, no phase separation and only a small volume change in the phase change process.

In this paper, a triplex concentric tube with HHTF flowing outer channel, CHTF flowing inner channel and PCM filling in the middle channel was designed for energy recovery of wasted heat in industry, ejected heat from the condenser of air conditioner or solar energy.

Report about heat transfer of triplex concentric tube with phase change material for thermal energy storage has not appeared so far. Only phase change heat transfer of dual concentric tube or shell-and-tube for thermal energy storage can be found for reference. Heat transfer in these types of thermal energy storage system represents a transient conjugate phase change-forced convection problem. Since phase change heat transfer is non-linear due to the moving solid–liquid interface, analytical solutions are only known for a few so-called moving boundary problems with simple geometry and simple boundary conditions. Therefore, a numerical approach has to be used to achieve a sufficiently accurate solution for heat transfer in the latent thermal energy storage unit. The most common methods used for solving phase change problems are the enthalpy methods and temperature-based equivalent heat capacity methods. These methods have been used in mathematical approaches by many authors (Zhang and Faghri, 1996, Ismail and Abugderah, 2000, Zivkovic and Fujii, 2001, Sari and Kaygusuz, 2002, Kayansayan and Ali Acar, 2006).

Although so many researchers focus on the problem of phase change of PCM as mentioned above, their models are still too complex and need huge workload of calculation. In this paper, a simple numerical method, called temperature & thermal resistance iteration method, has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of HTF on the thermal energy storage has been studied. From the point of view of application, the variations of heat transfer rate, fluid temperature, the front movement during solidification and melting with time are important quantities to study the characteristics of latent heat thermal storage. So graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axis position, time-wise variation of energy stored/released by the system were presented and discussed.