A numerical assessment of a latent heat storage system for district heating substations

Highlights

• A physical model to analyze a latent heat storage system for an urban district heating system

• Apparent heat capacity approach to simulate phase transition in PCM

• Dynamic performance of latent storage tank during discharge process

Abstract

This work deals with the development and sizing of a latent heat storage system for an urban district heating system (UDHS). The investigated system is a cylindrical tank containing spherical phase change material (PCM) capsules. A physical model based on the apparent specific capacity method was adopted and validated to describe the phase transition within the capsules. The effects of various designs and operating factors on storage performance are detailed and addressed, including the thermal energy provided by the system during the discharge period, the melting temperature, tank volume, and water return temperature. The results show that the energy efficiency of such a device can be enhanced only through a judicious choice of the mentioned parameters. For example, it was found that increasing the UDHN thermal power demand influences the system's performance and leads to reduce the discharging duration. Based on the presented analyses, it can be concluded that the proposed numerical tool can be used by thermal designers of urban heating networks to size, optimise, and predict the actual dynamic behaviour of the storage system under real exploitation conditions.

Introduction

Comparatively, among the European countries, France relatively has very high electricity consumption for heating purposes in buildings and for other domestic uses [1], [2]. Therefore, France's electricity system is thermally sensitive. This high thermo-sensitivity is expected to be made more prominent by the gradual phasing out of the gas and oil boilers and the rise of electric mobility. Due to this thermo-sensitivity, France is scheduled to close its last coal-fired power plants' which could create heating energy shortage that could pose a risk to the winter supply, even if the French electricity system seems to have sufficient margins [3], [4]. The situation could likely create additional pressure for the electricity network, to cope with peak energy consumption periods especially in the morning, night or the winter period. District heating is a promising solution to this problem [5], [6], [7]. Today, the adoption of district heating has an actual carbon footprint and cost advantages, when considered in full-cycle terms., It has been noted that district heating has initial capital investment but energy and running cost savings during operation and few components recycling at a later stage of use makes it economically feasible [1], [2], [3]. District heating has several other advantages which includes (i) the opportunity to explore many heat networks, abundant local inputs, such as municipal waste, sewage sludge, local biomass or waste heat which are not susceptible to any other valorisation than energy, especially in areas not served by natural gas networks; (ii) security of supply, in particular regarding the inputs mentioned above; and (iii) local employment of labour [4], [5]. Additionally, district heating has the ability to integrate solar energy into its network [6]. The large-scale integration of renewable energies in district heating systems requires an extension of the number of energy generators and/or the adoption of energy storage systems. The first approach requires additional capital costs to accommodate significant penetration of renewable energy via the so-called net metering alternative [7], [8], [9], and, the second approach (see Fig. 1) operates as a form of load sharing by ensuring a reliable supply and a peak smoothing strategy [10], [11], [12].

At present, three possible strategies can be envisaged for thermal energy storage [13], [14]:

• Traditional thermal storage via sensible heat [15], [16];

• Latent thermal storage via phase change materials (PCM) [17], [18];

• Thermochemical storage via a reversible chemical reaction [19], [20].

Currently, thermal latent heat storage (LHS) is widely used because it can store a large amount of energy in a small volume and heat storage process can be performed at a very small temperature range with little volume [21]. Various PCMs have been studied and are commercially produced for different applications so far [22], [23]. The principal drawback of latent storage systems is the poor thermal conductivity of current phase change materials, which considerably reduces both the charging and discharging time and the efficiency of the storage units [24], [25]. Since large-scale latent storage systems are constantly under debate, enhancing the thermal performance of the PCM used is critical to integrating them into district heating systems. Thermal performance of PCM can be improved by a variety of methods including the use of extended surface fins, nano-particles, etc. [26], [27], [28].

Substation PCM heat storage devices and their effect on water temperature input and output have been intensively examined by numerous academics over the past few years [29], [30]. In an urban district heating system (UDHS), the latent heat storage device can also be positioned near to the production plant or substation without taking up much area [30]. The dynamic behaviour of a latent storage system in an UDHS has rarely been studied. Recently, Bentivoglio et al. [31] conducted the only previous experimental effort on this topic, which experimentally conceptualized and analyzed the dynamic behaviour of a latent heat storage device in an UDHS. These authors [31] adopted the tubes-and-shell latent storage device in their study.

The PCM fills the space between the tubes and the heat transfer fluid (HTF) flowing into the tubes. Recently, a numerical work was published by Lamrani and Kousksou [32], in which authors have modelled the latent storage system developed by Bentivoglio et al. [31]. Instead of being set up in a shell-and-tube arrangement connected to a DHS, in this paper, the proposed system uses PCM contained in spherical capsules. For industrial applications, spherical capsules are often used [32]. This configuration can enhance the heat transfer between the PCM and the HTF. Numerous researches have been conducted on the numerical modeling of the phase change process in a tank filled with PCMs in spherical shells [33], [34], [35]. In order to evaluate the thermal performance of this system, various physical models [36], [37], [38], [39], [40] have been developed. The packed bed structure is viewed as acting as a porous medium in all of these models [41], [42].

The thermophysical properties of PCMs during phase transition, such as heat capacity and enthalpy as a function of temperature, are required for precise mathematical modeling of heat transfers in the thermal storage system.

In this paper, the apparent heat capacity method [43], [44], [45] is adopted to analyze the energy performance of a latent storage device integrated into UDHS. The novelty of this research lies in first presenting a new numerical tool to forecast the practical running of a large-scale PCM storage during the discharging process for the district heating system. Secondly, it aimed at providing a detailed analysis of coupled PCM storage tank to a real UDHS in Grenoble city (France). The proposed tool will be advantageous for fast simulating and sizing of this coupled thermal system. In fact, this system must be sized to offer steady thermal power for a set length of time while also delivering hot water at temperatures of 60 °C. Thanks to this numerical tool, the influence of some specific parameters like the PCM type, the PCM position in the storage system, the targeted power, and the district heating network (DHN) return temperature on the discharge time under constant power can be analyzed.