Experimental investigation of a rotating drum heat exchanger for latent heat storage

Highlights

• Experimental study of a rotating drum heat exchanger for latent heat storage.

• Investigation of heat transfer, layer thicknesses and required mechanical energy.

• Constant and fully controllable heat transfer during the discharge process.

• Complete separation of power and capacity achievable.

• Measured heat transfer of up to 6.8 kW⋅m⁻² based on the total drum shell surface.

Abstract

With latent heat thermal energy storages, thermal energy can be stored at a constant temperature level with high storage density using the enthalpy of the solid-liquid phase change of a material. During the discharge process of a latent heat thermal energy storage, phase change material (PCM) solidifies at the heat transfer surface and increases the thermal resistance. This decreases the transferred thermal power with time and the state of charge. The rotating drum heat exchanger, experimentally investigated in detail for the first time in this paper, overcomes this limitation by removing the PCM layer. While a heat transfer fluid passes through the inside of the rotating drum, which is partially immersed in liquid PCM, the PCM solidifies at the outer side. The solidified layer is removed at each rotation by a fixed scraper. Thus, the layer thickness and the thermal power are kept constant over time. The solidified PCM can be stored separately from the liquid phase, resulting in a complete independence of thermal power and storage capacity. A commissioned experimental test rig using a low temperature PCM is used for the investigation of the heat transfer potential, the layer thicknesses, the mechanical energy needed for the removal of the solidified layer and as a proof of concept. The experimental data show a consistent heat transfer which is increasing for higher rotational speeds. With the presented test rig, the heat transfer density is up to 6.8 kW⋅m⁻² based on the total drums shell surface of the rotating drum at a temperature difference of 5 K between the melting point of the PCM and the temperature of the heat transfer. Adhering liquid PCM increases the total heat transfer by up to 60% as the liquid PCM solidifies also after the surface left the liquid PCM. While the measured solidified layer decreases to below 0.05 mm with higher rotational speeds, the adhering layer is slightly increasing. The results show the high potential of the rotating drum heat exchanger concept for the generation of steam out of a high temperature PCM.

Introduction

For the transition of the energy system from limitedly available and carbon dioxide emitting fossil fuel consumption towards the usage of fluctuating renewable energy, the demand of energy needs to be decoupled from its production. This can be achieved by using suitable energy storage systems. Renewable energy from solar and wind will be available primarily in form of electricity. However, the pure demand of thermal energy in industrial applications for process heat in the temperature range of 100–400 °C in the EU28 member states amounts to around 2200 PJ per year, which is around 5% of the total final energy consumption throughout all sectors [1]. Storing the energy in form of the required thermal energy offers advantages regarding the limited availability of critical resources for electrochemical energy storages and cost savings. Apart from that, there is the possibility to convert electricity from stored thermal energy by using a suitable thermodynamic cycle [2].

Latent heat thermal energy storages (LHTES) utilize the phase change enthalpy, usually between liquid and solid state, of the storage material (phase change material, PCM). Their essential characteristic is the isothermal temperature level during the melting (charging) and solidification (discharging) process. In applications that include isothermal processes, the required temperature difference for the heat transfer can be kept constant or minimized for an increased exergetic efficiency of the storage system. This includes the supply of process steam with specific requirements for industrial applications as well as the supply of organic- or water steam for electricity production. The exergetic efficiency is further maximized by an isothermal charging process e.g. by a condensing fluid (Fig. 1). By overheating the liquid PCM above its melting point or subcooling the solid PCM below its solidification temperature, the sensible heat of the PCM can be used for energy storage as well. This increases the storage density and decreses the specific costs of the storage material simultaneously. For the generation of steam in a temperature range between 150 °C and 350 °C, nitrate salts are well suited. Especially sodium nitrate is cost attractive and has an abundant availabillity [3]. However, these materials have a limited thermal conductivity. During the discharge process, liquid PCM is solidifing at the cooled heat exchanger surface and a solidified PCM layer is growing. The phase change enthalpy released during the phase change is emitted at the boundary between solid and liquid PCM as shown in Fig. 2. Thus, the thermal energy has to be transferred through the growing solidified layer. This causes a decline of the heat transfer capability during the discharge process with time and state of discharge. During the charging process of a LHTES the solidified PCM layer is melted at the heat exchanger surface. The heat transfer from the liquid PCM to the heat exchanger surface is increased by natural convection [4], [5] and can be further increased by forced convection. Therefore, most activities to improve LHTES are focusing on the discharge process as is the case in this work.

To overcome the discharge limitation, extensive reaserch has been conducted on so called passive LHTES. Among others, the mainly used basic principles are to increase the effective heat transfer surface or to increase the effective thermal conductivity of the PCM. The effective heat transfer can be increased by integrating a heat transfer structure in the PCM [6], [7], [8], [9] or a macro-encapsulation of the PCM [10]. The effective thermal conductivity can be increased by modifying the PCMs thermal properties e.g. by incorporating of nanoparticles [11], [12], micro-encapsulation [13], [14] or compression of the PCM to a composite [15]. However, the thermal power and thermal capacity of conventional passive LHTES with fixed heat exchangers and non-moving PCM is defined by its design at a certain ratio. Thus, if the capacity is enlarged, the heat transfer structure has to be incresed as well. This limits possible cost savings due to scaling effects especially for large long-time storages. Another possibility to overcome the limited heat transfer potential of available low cost PCMs is the use of active LHTES. Here, an active motion transports the PCM. This results in a constant heat transfer as the solidified layer thickness is kept constant in average. Beside the rotating drum firstly introduced in [16] and discussed in detail in this paper, further concepts of active LHTES have been published. Among others, these are mainly:

• The Screw-Heat-Exchanger concept, in which two parallel screws, known from the plastic extrusion, are transporting the PCM and serve as heat exchanger [17].

• In the PCM-Flux concept, PCM is moved in a container linearly over a locally fixed heat exchanger [18].

• In [19], a plate heat exchanger is presented, whose heat transfer surface is periodically scraped off by a linearly moving scraper.

• A vertically rotating, scraped rotating drum with PCM around the drum in a closed enclosure is presented in [20].

All of these concepts have been demonstrated experimentally on an experimental scale. However, all of them have in common, that there is a layer thickness of solid PCM given at the heat exchanger surface, which dominates the heat transfer. Further concepts of scraped surface heat exchangers are mainly optimized for the process of freezing or for the increase of the convective heat transfer without solidification [21], [22], [23], [24]. Within Fig. 3, the thermal resistances of a typical heat transfer problem of latent heat storages are given. The solidified PCM layer clearly dominates the heat transfer resistance if it is thicker than 0.1 m. Only with a further reduction of the solidified PCM layer thickness the thermal resistances of the steel wall and the evaporating water become dominant.

The novel concept of the rotating drum heat exchanger is designed from scratch to minimize the layer thickness of the solid PCM and thus maximize the surface specific heat transfer of the heat exchanger. Furthermore, a separation of the solidified PCM from the liquid PCM is a key element for the separation of power and capacity. The basic concept of the rotating drum heat exchanger is shown in Fig. 4. A rotating drum is partially immersed into liquid PCM. An evaporating fluid extracts thermal energy while passing the inside surface of the heat transfer wall. Simultaneously, liquid PCM releases its phase change enthalpy during solidification at the outer surface of the wall. To maintain a minimum thickness of the solidified PCM layer, a fixed scraper removes the layer with each rotation. The scraped PCM can be stored separately from the liquid PCM. This leads to a complete separation of the thermal power that can be transferred and the thermal energy that can be stored with the thermal energy storage system. The separation leads to cost savings for medium and long-term storages with high storage capacities, since the heat exchanger as the main part of the storage system can be chosen independently in its size. The transferred thermal energy can be controlled by the rotational speed. This leads to a flexible system for the provision of changing steam requirements. The overall specific thermal energy density of the storage material can be increased by extending the temperature range used above and below its phase change temperature. Thus, the energy density is composed of the latent heat of the phase change and the sensible heat stored in both the liquid and the solid phase of the storage material. The maximum temperature of the liquid PCM is limited by the decomposing temperature of the PCM or the material specific maximum operating temperature of the rotating drum and its components.

Within this paper, the concept of the rotating drum heat exchanger for latent heat storage is examined experimentally in detail for the first time. Therefore, an experimental test rig using a low-temperature PCM is commissioned. The use of a low-temperature PCM allows a more detailed investigation of the various effects while the basic physical properties of the heat transfer remain unchanged compared to a high temperature system. The methodology of the experimental investigation is described in the following section. The main objective of the research is a proof of concept and to identify the technologies potential regarding the heat transfer. Furthermore, the influence of different effects such as adhesion and different temperature differences are examined.