Question

design and perform your experimentation and evaluate the energy storage performance of these sensible heat thermal energy storage system natural number friction factor are to be found using friction and Reynolds number and what fraction attempt and develop more functional relationship for natural number and fractional factorial using first order

Answer 1

Chapter 1

Introduction

1.1 Background

Due to increase in energy consumption, a great deal of fossil fuels is being used. This is a consequence of the present environmental problems, such as global warming, acid rain, etc. In order to decrease these problems, the use of renewable energy sources is being promoted. But the most freely available renewable energy source, called as the mother of all energies, the solar energy, present the drawback that there is a mismatch between the energy demand and supply. To cover this mismatch, the use of phase change thermal energy storage systems is required. Thermal energy storage proves to be an attractive and economical alternative for small, medium and large-scale use. Energy is accumulated in a storage medium, and the storage mechanism could be classified as sensible heat, latent heat, or chemical storage. Latent heat storage is new area of study and it received much attention during the energy crisis of late 1970’s and early 1980’s where it was extensively researched for use in solar heating systems. Although research into latent heat storage for solar heating systems continues, it is increasingly being considered for waste heat recovery, load levelling for power generation, building energy conservation and air conditioning applications.

The interest in using renewable energy for producing electricity has risen with the potential threat of global warming due to greenhouse gas emissions and rising fuel prices. Solar and wind power are such technologies that are considered an important part of the future electricity supply. However for these technologies, there is often a mismatch between energy demand and energy production. Developing efficient technologies to store the harvested wind or solar power for use during high load is therefore considered an important part of making efficient systems with renewable energy sources [1].

The high-energy storage density of phase change materials (PCMs) and the small temperature swing of PCMs make them especially interesting to be used in several different fields, such as warming and cooling of buildings. However, due to the special characteristics of PCMs, case specific studies need to be conducted for every possible use of PCMs to verify that PCM implementation in a certain system is economically justifiable [2].

The researcher who wishes to do such an implementation in a system needs to look into the characteristics of the PCM, such as what melting point and temperature range is required, what container is compatible with the PCM and what would be a good heat exchanger for supplying efficient enough heat transfer so that energy can be extracted and inserted in the required time of the system [1].

This work focuses the heat transfer enhancement in Latent heat thermal energy storage in domestic applications.   

1.2 Introduction to Energy Storage Technology

The continuous increase in the level of greenhouse gas emissions and limited resources of Fossil fuels related to the climb in fuel prices are the main driving forces behind efforts to more effectively utilize various sources of renewable energy. One of the options is to develop energy storage devices, which are as important as developing new sources of energy. The storage of energy in suitable forms, which can conventionally be converted into the required form, is a present day challenge to the technologists. Energy storage not only reduces the mismatch between supply and demand but also improves the performance, efficiency and reliability of energy systems and plays an important role in conserving the energy. It leads to saving of premium fuels and makes the system more cost effective by reducing the wastage of energy and capital cost. For example, storage would improve the performance of a power generation plant by load levelling and higher efficiency would lead to better efficiency of energy conversion and less generation cost [3].     

1.3 List of Energy Storage Technologies

Mechanical storage systems

1) Pumped hydro storage

2) Compressed air energy storage

3) Flywheel energy storage    

Electrochemical storage systems

1) Secondary batteries   

2) Flow batteries   

Chemical energy storage    

1) Hydrogen

2) Synthetic natural gas   

Electrical storage systems

1) Double-layer capacitors   

2) Superconducting magnetic energy storage    

Thermal storage systems

1) Sensible heat storage

2) Latent heat storage

1.4 Thermal Energy Storage

Thermal energy storage through phase change material (PCM) is capable of storing and releasing large amounts of energy. The system depends on the shift in phase of the material for holding and releasing the energy. For instance, processes such as melting, solidifying or evaporation require energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa. Therefore, PCMs readily and predictably change their phase with a certain input of energy and release this energy at a later time. PCM depends on latent heat storage. Compared to the storage of sensible heat, there is no temperature change in the storage. In a sense every material is a phase change material, because at certain combinations of pressure and temperature every material can change aggregate state (solid, liquid, gaseous). In a change of aggregate state, a large amount of energy, the so-called latent heat can be stored or released at an almost constant temperature. Thus a small difference in temperature can be used for storing energy and releasing the stored energy Thermal energy storage can be stored as a change in internal energy of a material as sensible heat, latent heat and thermo chemical or combination of these. An overview of major techniques of storage of thermal energy is shown in Fig. 1 [4]

Literature Review

Many researchers carried out extensive research on various method of thermal energy storage and used these methods to analyzed problem in various applications. These researchers frame work in the analysis of heat transfer problem, development of various heat exchangers etc.

This literature survey gives past works on development of theoretical, numerical and experimental investigation of transient heat transfer phenomenon during melting and solidification in a shell-and-tube latent thermal energy storage unit. Research works which have been reported to realize thermal energy storage during last three decades can be classified into three major groups; theoretical, numerical and experimental studies. Among which majority of the research on shell and tube type storage unit has been carried out on design aspects and analysis of heat transfer phenomenon. Many software code used for formulation of mathematical (theoretical) and numerical model and their solution    studies have also been reported in literature on simulation of heat transfer problem in shell and tube type   heat exchanger. Subsequent section of this chapter presents a comprehensive report of the past research on various aspects of shell and tube type latent heat thermal energy storage unit.

2.1 Introduction to Heat Transfer in a Latent Heat Energy Storage System

Better understanding of the heat transfer characteristics of a PCM inside a LHESS will lead to more efficient energy storage as a result of better LHESS designs. Heat transfer accompanied by a change of phase can be classified into three modes: conduction-controlled phase change, convection-controlled phase change, and conduction and convection-controlled phase change. Conduction has been shown to be the main heat transfer mode during solidification and convection the main mode during melting (Agyenim et al. 9; Jegadheeswaran et al.10). As well, conduction is the main mode of heat transfer early in the melting process, and is replaced by convection only once gravity effects become significant (Khodadadi et al. 11).

2.2 Phase Change Material Properties and Selection

PCMs are the backbone of LHESS design, and can be broken into organic and inorganic PCM types. Organic materials, including fatty acids and paraffin’s, are the most commonly studied materials for PCM energy storage. Organic materials are known for congruent melting, or melting and freezing repeatedly without phase segregation or degradation of their thermal and material properties. They also self-nucleate, meaning they crystallize with little or no sub-cooling and are generally not corrosive (Sharma et al. 1). Sub-cooling occurs when the liquid PCM reaches a temperature below the solidification temperature before crystallizing. Paraffin wax PCMs were found to have good thermal stability after repeated cycling using a DSC, showing little to no degradation of the latent heat and phase transition temperature ranges (Zalba et al. 12). Dodecanoic acid (also called lauric acid) has been shown to have a melting temperature range that is suitable for a LHESS used for SDHW energy storage (40 to 50 ºC for a SDHW system) (Murray et al. 13). It also has stable thermal properties, is safe for use with domestic water, and is readily available and relatively inexpensive (Desgrosseilliers et al. 14).

Inorganic PCMs, which include salt hydrates, have a volumetric thermal storage density higher than most organic compounds due to higher latent heat and density. As well, salt hydrates go through relatively small volume changes during melting (Kenisarin et al. 15). However, most salt hydrates have poor nucleating properties which results in sub-cooling of the liquid PCM prior to freezing (Abhat et al. 16). In some cases, a small amount of sub-cooling may not be an issue, however, large amounts of sub-cooling which are seen in some inorganic PCMs are a problem for some systems (Farid et al. 17). Also, thermal cycling studies using a DSC showed that inorganic PCMs have a high level of deviation of thermal properties from their quoted and experimental properties (Shukla et al. 18).

PCM selection for LHESS depends on the desired application of the system. Some criteria for choosing a PCM are as follows (Abhat et al. 16; Agyenim et al. 9; Cabeza et al. 19):

• Melting point in the desired temperature range for the application to assure storage and release of heat at a useful temperature;
• High latent heat of fusion to achieve high storage density;
• High specific heat so that sensible heat storage effects may play a role;
• High thermal conductivity;
• Small volume changes during phase transition;
• Exhibit little or no sub-cooling during solidification;
• Possess chemical stability, no chemical decomposition and no destructive corrosion of materials used in the LHESS;
• Contain no poisonous, flammable or explosive elements;
• Reasonable price and easily accessible.

Based on the above criteria, a PCM can be selected that meets all or most of the requirements for a LHESS. However, the selection of a PCM for a SDHW system should be done carefully in order to produce hot water in an acceptable range of temperatures, and to minimize safety concerns (such as PCM leaking into the building water supply) in the event of an accident (El-qarnia et al. 20). Out of the two types discussed, organic PCMs were shown to be more advantageous for use in a LHESS because they are less corrosive, have less sub-cooling, and have less deviation of thermal properties during melting and freezing cycles than inorganic PCMs (Sari et al. 21).

2.3 Experimental Studies Section

2.3 presents’ experimental setups commonly used by other researchers, along with the results and conclusions made concerning LHESS geometry, operating parameters, and heat transfer enhancement designs.

2.3.1 Experimental Setups

One of the most common experimental setups consists of a LHESS with a HTF pipe passing through the centre, with hot and cold constant temperature water baths (with temperature controllers to limit fluctuations) from which the HTF is circulated to charge/discharge the system. Thermocouples are commonly placed throughout the PCM and on the storage container, while PCs and data acquisition (DAQ) systems of various kinds are used to record and save data (Ettouney et al. 22; Jian-You, et al. 23). Centrifugal pumps are used to circulate the HTF through the PCM, and flow meters report flow rates to the DAQ (Akgun et al. 24). Flow controls, such as throttling valves, are used to keep the HTF flow rate constant. Heat exchangers or radiators are used in some experimental setups to discharge the energy in the HTF that has not been collected by the PCM (Kaygusuz et al. 25). Figure 2.1 shows a common setup for experimental studies of PCMs.

Fig. 2.1 Typical experimental setup [9]

All of the previously discussed experiments are designed to only study consecutive or separate charging/discharging since there is commonly only a single pass of the HTF pipe through the PCM, as shown in Fig. 2.1. However, (Liu et al. 26) performed simultaneous charging/discharging experiments on a heat pipe heat exchanger, not to be confused with a LHESS, with paraffin wax PCM storage. During simultaneous charging/discharging, heat transfer from the hot water directly to the cold water took up a major portion of the total heat recovered by the cold water, and only a small amount of heat went to, or was taken from, the paraffin wax. This is due to the small thermal resistance from the hot water to the cold water through the heat pipe (which is in direct contact with both the hot and cold water) compared with the thermal resistance from hot to cold fluid through the paraffin wax. In this heat pipe setup, the hot and cold water are separated only by a thin wall. In a LHESS the hot and cold HTFs are separated by the high thermal resistance PCM, therefore the same results are not expected. In order to have more control over the amount of heat transferred from the hot/cold source to the PCM, an increase in the heat transfer surface area between the PCM and the heat pipe wall was recommended (Liu et al.26).

2.3.2 Storage Geometry

Vertical cylindrical containers are the most common geometry for LHESS. A figure showing the various types of PCM container geometries is given by Agyenim et al. [9] Vertical concentric pipe-in-pipe configurations with the HTF circulated inside the internal pipe through the PCM, which is in the annular space outside the metal HTF pipe, is a common configuration for consecutive charging/discharging experiments using the experimental setup discussed in Section 2.3.1. Horizontal cylindrical LHESS have been used as well. In one experiment a 1200 mm long, 375 mm diameter copper cylinder was used to hold RT-58 PCM, and a 65 mm diameter copper HTF pipe of the same length, with longitudinal fins for heat transfer, was placed centrally through the PCM. Although both vertical and horizontal cylindrical containers are common, there has been no direct comparison between the two. A less common cylindrical container design is an inclined outer shell with the PCM in the annular space, which is used for heat transfer enhancement by natural convection. Results show, using paraffin wax PCM, an approximate 30 % decrease in the total melting time using this configuration over a vertical outer shell. Multi-tube arrays involve a cylindrical PCM container with two or more HTF tubes through the container. The multi-tubes result in significantly more melting than a single pipe-in-pipe geometry and experimental measurements have demonstrated earlier onset of natural convection resulting in multiple convective cells, which enhances melting. Multi-tube arrays were shown to enhance heat transfer for a horizontally oriented cylindrical container with CaCI2∙6H2O as the PCM (Sari et al. 25). Vertical multi-tube arrays were shown to be beneficial as well, due to the increased heat transfer surface area from the multiple HTF tubes, which results in higher heat fluxes. Higher heat fluxes in the vertically oriented container allow for increased natural convection heat transfer (Hamada et al. 27). Although there are clear advantages of this geometry, due to its more complicated nature, heat transfer and phase change behaviour is more difficult to study experimentally.

Rectangular flat plate containers were chosen for PCM storage by a research group because melting/solidification is symmetric about a plane at the centre of the plate, and the surface area to volume ratio for heat transfer is largest compared with other geometries studied (Zalba et al. 5). Experimental results for a stainless steel container, 100 mm wide, 100 mm thick, and 20 mm high, have shown that a rectangular container only requires half of the melting time of a cylindrical container with the same volume and heat transfer area (Zivkovic et al. 28).

Spherical containers are not as commonly used for LHESS, however research has been done on micro-encapsulated PCMs inside other LHESS geometries. Encapsulated PCMs, which have a barrier protecting the PCM from harmful interaction with the environment, provide sufficient surface area for heat transfer, and provide structural stability. Microencapsulation is not commonly used because it is more expensive and the matrix reduces the heat transfer through the PCM because it limits natural convection (Regin et al. 29).

Researchers have also studied experimental systems in which PCMs are built directly into solar collectors (Alva et al. 30; Mettawee et al. 31). In these cases the PCM is in direct contact with the energy source (i.e. the solar collector), which has been shown to increase the energy storage rate. It was found using this type of setup that having a smaller solar collector surface area for heat collection and larger PCM surface area for heat transfer results in lower outlet HTF temperatures and more energy stored in the PCM, but an increased cost of the LHESS due to a larger amount of PCM required (Koca et al. 32). The possible use of this design is limited to special applications, such as space and soil heating of greenhouses located in arid zone areas, during winter (Rabin et al. 33).

2.3.3 System Operating Parameters

There are a number of operating parameters that can be adjusted to achieve optimum operation and storage capacity of a LHESS. These parameters include the HTF temperature at the inlet of the LHESS, and the flow rate and direction of the HTF (Castell et al. 34). Of the parameters studied experimentally, an increase in the HTF inlet temperature has been shown to lead to a decrease in the melting time (Akgun et al. 24) and an increase in the amount of energy that can be stored (Agyenim et al. 9). In one experiment in which dodecanoic acid filled the annular space between the PCM container and the HTF pipe, the average heat transfer rates were found to increase when the inlet HTF temperature was increased during melting, and the average heat transfer rates decreased when the inlet HTF temperature was decreased during solidification (Sarı & Kaygusuz, 2002). Also, at lower flow rates during charging the PCM temperatures took longer to increase, resulting in lower energy storage rates (Jian-You et al 23; Mettawee et al. 31).     

It was shown for a vertical pipe-in-pipe LHESS configuration that natural convection effects played a major role during melting when the HTF entered the container from the bottom of the LHESS, but were not significant when the HTF entered from the top of the container (Ettouney et al. 22). Also, in a 1200 mm long and 90 mm diameter vertically oriented pipe-in-pipe LHESS configuration, with CaCl2 as the PCM, using air as the HTF was shown to increase reliability of results by allowing for longer cycle times (Bajnoczy et al. 35).