Numerical and experimental investigation of a PCM-based thermal storage unit for solar air systems

Highlights

• The latent heat thermal storage unit for air-based solar systems was investigated.

• The CSM panels with the phase change material Rubitherm RT42 were used.

• The investigations were done both experimentally and numerically.

• The experimental and numerical results are in a relatively good agreement.

• Results confirmed a potential of a phase change materials usage in thermal systems.

Abstract

A general problem of most solar thermal systems is the need for thermal storage in order to balance supply and demand of heat over a certain period of time. A possibility to employ latent heat of fusion in phase change materials (PCMs) for thermal energy storage in air-based solar thermal systems was investigated using laboratory experiments and numerical simulations. A heat storage unit containing 100 aluminium panels filled with a paraffin-based PCM was used in the investigations. The experiments were carried out in a lab environment with an electric air heater as a heat source. A numerical model of the unit was developed and implemented as a type in the TRNSYS 17 simulation tool. The results of the simulations with the developed model show a good agreement with experimental results. Subsequently, the model was used for a parametric study analysing the influence of certain parameters. The performed investigations showed a potential of the use of latent heat thermal storage in air-based thermal systems with a narrow temperature operation range.

Introduction

Solar air systems, though not as common as water-based solar systems, can be used in a number of applications. One of the most common applications is space heating or ventilation air heating [1]. Drying of various products is another area in which solar air systems can be used [2]. Solar heated air can also be used for regeneration of desiccant wheels in air-conditioning systems [3]. However, many of these applications require thermal energy storage in order to operate effectively [4]. Water can easily be used as a heat storage medium in water-based solar energy systems but it is usually less practical for air-based systems. Building structures can be employed as thermal storage mass in some solar applications utilizing air as a heat carrier but this approach is not applicable in all situations. The Tromb wall [5] is one of the examples of thermal storage integrated with building structures. Another possibility is the use of packed beds where solid materials (usually pebbles) are used for sensible heat storage [6]. Though rather simple to use, the packed beds containing sensible heat storage materials have certain disadvantages; they use a lot of space, they are difficult to clean and the thermal storage density is relatively small. With regard to the thermal storage density and operation temperature range, latent heat thermal storage, which makes use of phase change of a heat storage material, offers certain advantages over sensible heat storage for many technical applications [7].

The materials used for latent heat thermal storage are generally referred to as phase change materials (PCMs). The phase change of a material provides a rather high thermal storage capacity and also energy storage density in a relatively narrow temperature interval around the phase change temperature. Though both the solid–liquid and the liquid–gas phase changes can be employed for latent heat storage, it is the solid–liquid phase change that is used in almost all the latent heat storage systems [8], [9]. A large number of papers dealing with the phase change materials and their use in different technical applications, e.g. cold thermal energy storage [10], building integrated thermal storage [11], or air conditioning [12] have been published in recent years.

Various ways of integration of PCM-based heat storage in the air-based solar thermal systems have been reported. One of the simplest ways is the integration of the PCMs directly with the solar air collector [13]. Tyagi et al. [1] presented a review of a variety of solar air heating systems with and without thermal energy storage. Thermal energy storage in solar air systems is mostly intended to provide the heat storage capacity for hours or days. Since the volumetric thermal capacity of water is approximately 3500-times higher than that of air, it makes much more sense to use water-based solar systems when long-term (e.g. seasonal) thermal storage is needed and to employ water–air heat exchangers for air heating.

Several studies, both experimental and numerical, were carried out into latent heat thermal storage with air as a heat carrier. Hed and Bellander [14] reported mathematical modelling of a PCM-air heat exchanger for thermal storage in case of night cooling. The exchanger contained six horizontal layers of PCM in aluminium pouches with 8 mm air gaps between the layers. The length of the layers in the direction of airflow was 0.48 m. The air velocity of 4 m s⁻¹ was considered in all air gaps. The model was implemented as a single node finite difference model. The heat loss to the ambient environment was neglected. The authors used the model to investigate the influence of the surface roughness on heat transfer in the heat storage unit, but the surface roughness is not described in detail what makes the results quite difficult to interpret. The authors concluded that the rough surface can significantly intensify heat transfer between the fluid and the heat storage material, but this intensification is subsequently paid for by a higher fan power. The increase of the fan energy consumption is not quantified in the paper.

Modelling and experimental validation of a similar arrangement of a PCM-air heat exchanger for free cooling applications was presented by Lopez et al. [15]. The authors considered an exchanger consisting of parallel horizontal PCM slabs with air gaps between the slabs. The simulation model was created in MATLAB with the use of control volume method. The same air flow rates were considered in all air gaps and thus only a half of the slab thickness with a half of the air gap was modelled. The heat loss to the surroundings was neglected. The authors reported a good agreement between the results from the developed numerical model and the experimental data from the laboratory experiment. However, the phase change of the material (indicated by inflection points) is absent in the temperature curves during the heat storage and heat release periods. The temperature curves without inflection points are typical for sensible heat storage materials.

A quite extensive study of a PCM-air heat exchanger containing aluminium containers filled with a PCM (compact storage modules – CSM) was presented by Dolado et al. [16], [17], [18]. The authors studied a PCM-air exchanger in which the CSM panels were positioned vertically and the direction of air flow was also vertical with air supply at the top and air return at the bottom of the unit. The paraffin-based PCM RT27 was used in the investigations. The total weight of the PCM in the unit was 135 kg. Both empirical [16] and numerical [17] models of the investigated heat storage unit were developed. The numerical model of the PCM layer was developed with the use of finite difference method as a one-dimensional implicit formulation. An influence of thermophysical properties of the PCM as well as other parameters such as air flow rate, surface rugosity and thermal conductivity of the encapsulation material were numerically investigated with the developed model.

Halawa and Saman [19] reported the thermal performance analysis of a phase change thermal storage unit for space heating. The PCM used in the study was calcium chloride hexahydrate (referred to as PCM29 in the paper) with the melting temperature of 28 °C. The PCM-air heat exchanger consisted of parallel slabs in a rectangular duct with air passing between the slabs. The authors analysed the influence of various parameters such as the air flow rate, the PCM slab thickness or the air gap on the performance of the unit. The adiabatic walls were considered for the unit but such an assumption is reasonable for a parametric study in which the influence of various design and operation parameters is investigated. However, the melting temperature of 28 °C seems to be relatively low for space heating application. Though the parallel PCM slabs with the air channels between them is the most often considered configuration of an PCM-air heat exchanger, some other designs are also possible. Dubovsky et al. [20] presented an analytical model of a shell-and-tube PCM-air heat exchanger where a PCM was in the tubes and the air flowed inside the shell. According to Agyenim et al. [21], the shell-and-tube exchangers represent the most frequently studied case by the number of published papers, however, most of these cases are water-based systems.

As pointed out above, many of the models neglect the heat exchange between the thermal storage unit and the ambient environment [14], [19]. Such simplification can be justified for thermal storage in passive cooling applications where the temperature difference between the PCM and the ambient air is rather small (usually less than 10 K) and the heat storage cycles are relatively short. In case of thermal storage for space heating, the temperature difference between the PCM and the ambient air can exceed 30 K, resulting in a non-negligible heat loss over a certain period of time. Another assumption often used in modelling of the storage units with PCM slabs is the same air flow rate in all air channels. This flow pattern can be achieved in laboratory experiments but it is less likely in case of the thermal storage units in actual building energy systems.

The aim of the study presented in this paper was to develop and validate a simulation model for the heat storage units comprising CSM panels filled with PCMs. The simulation model allows to take into account the distribution of air flow rates in different air channels as well as the heat exchange with the ambient environment. The model was used for a parametric study analysing the influence of certain parameters.