Introduction
1.1 General
Energy is one of the most discussed topic in the world today, which plays a very important role in the development of the country .Whatever the activities that the country does depend upon the availability of the energy and it measures the developmental role in the country economy.
Today whatever the energy available is of the fossils fuels thus the attention is going towards the renewable energy sources and the fossil fuels is also causing lot of environmental disorders like global warming which is very acute and discussed problem in the world today.
There are lot of ways by which we can get the renewable energy like sun, tidal, biomass and sunlight which we call as solar energy and these all are renewable in nature. We know that the solar energy is easily available and environment friendly but we have to deal with an aspect that solar energy is intermittent in nature. And due to intermittent in nature we need to store the available energy when the energy is available during sunshine and use it when the energy is not available during non sunshine hours or low sunshine hours.
An energy storage unit is required to attach with solar collectors to store the energy and better utilization is done. There are 2 types of energy sources i.e renewable and non renewable , Non- Renewable is oil, gas , coal and nuclear fuels while the renewable is sun, wind, ,tidal, etc. The non renewable energy sources will exhaust up in due course of time and thus there is need to use the renewable source of energy as it is abundant in nature.
A energy storage not only improves the efficiency of the energy conversion and utilization but also it helps in bridging the supply and demand of energy with the variation of time, quantity, location and the demand for electricity also varies with time, therefore the energy should be stored during the off peak.
Energy Storage System can play an important role in meeting society’s needs for more efficient and environment friendly energy use in building heating and cooling, aerospace power, and utility applications. Dincer and Rosen [2] point out many significant benefits of energy storage system which are given as follows:
1. Decrease in energy cost.
2. Decrease in energy consumption.
3. Improvement in indoor air quality.
4. Gives flexibility in operation.
5. Better utilization of equipment.
6. Helps in Conservation of fossil fuels (by facilitating more efficient energy use and fuel replacement).
7. Decrease pollutant emissions (e.g., CO2 and chlorofluorocarbons (CFCs)).
1.2 Applications of Energy.
Demand of energy is in the various sectors like public, industrial, residential and other utility sectors.and demand fluctuates on a daily, weekly, monthly, seasonally and these energies are met by various energy storage devices. High demands are generally met by gas turbines, big generators which are cost effective .This high demand periods are the most difficult and costly times for energy storage devices The other applications for the energy storage are as follows: .
1. Industry: The heat coming out of the waste can be used for other purposes
2. Solar energy systems: Solar energy can also be stored for the cloudy days .
3. Utility: Gas and oil can be saved by storing the solar energy and used for
electricity generation.
4. Wind and run-of-river hydro: Wind energy can be used to function round the clock by charging the system during low demand hours and use it in the high demand hours, this increases the capacity factor which decreases the cost.
1.3 Techniques of energy storage
Generally, energy storage technology can be divided into two main categories which are thermal energy storage and electrical energy storage, both of which are also the main energy consumption types in our daily life. Dincer [3] classifies energy storage method in four categories which is shown in Figure 1.1
Figure 1.1: Classification of energy storage methods.
1.3.1 Mechanical Energy Storage
This type of storage is used where the kinetic and potential energy is stored.The mechanical storage system further classified as are water storage, air storage and flywheels.
1. Water Storage through pump.
It is a simple method of energy storage. At night, when energy demand is low, pumps circulate the water upward from the river to a reservoir. During the day, when energy demand is high, the reservoir releases water and the flowing water turns the turbine to generate electricity.
2. Compressed-air storage
In this system, the air is compressed and stored in the underground reservoirs, like gas fields, oil fields or manmade caverns.
3. Flywheels
They generally store the excessive energy of the engine and gives back when the energy is needed at the peak load condition..
1.3.2 Chemical energy storage
This energy is stored in the battery where the chemical energy is converted into electrical energy. There are lot of chemical reactions happens in the battery, which produces electrical energy. There are two types of chemical storage i.e electrochemical batteries and organic molecular storage systems, there are lot of devices which convert chemical energy in to electrical energy, examples are.
Batteries, fuel cells,.
Electochemical batteries store the energy in chemical form and upon the reactions this energy is converted in to electrical energy. e.g lead sulphuric acid battery., fuel cells are also very economical.
1.3.3 Magnetic storage
Here the energy is stored in the large electromagnet and at the absolute low temperatures they have no electrical resistance thus they can carry large amount of current which can be useful for many purposes. And they are called super conducting materials. And the losses are very less only energy is lost in converting from A.C to D.C and other energy is lost in maintaining low temperature.
1.3.4 Thermal energy storage
The thermal energy storage is classified as:
1) Sensible heat storage
2) Latent heat storage
3) Bond energy
The underlying principle behind the sensible heat storage is that temperature change of the system is with the highest possible capacity.
1.4 Sensible Heat Storage
In the case of sensible heat storage systems, energy is stored or extracted by heating or cooling a liquid or a solid, which does not change its phase during this process. A variety of substances have been used in such systems. These include liquids like water,heat transfer oils and certain inorganic molten salts, and solid like rocks, pebbles and refractory. In the case of solids, the material is invariably in porous form and heat is stored or extracted by the flow of a gas or a liquid through the pores or voids. The choice of the substance used depends largely on the temperature level of the application, water being used for temperature below 100°C and refractory bricks being used for temperatures around 1000°C. Sensible heat storage systems are simpler in design than latent heat or bond storage systems. However they suffer from the disadvantage of being bigger in size. For this reason, an important criterion in selecting a material for sensible heat storage is its (ρCp) value. A second disadvantage associated with sensible heat systems is that they cannot store or deliver energy at a constant temperature.
1.4.1 Liquid media storage
With its highest specific heat water is the most commonly used medium in a sensible heat storage system. Most solar water heating and space-heating systems use hot water storage tanks located either inside or outside the buildings or underground. The sizes of the tanks used vary from a few hundred liters to a few thousand cubic meters. An approximate thumb rule followed for fixing the size is to use about 75 to 100 liters of storage per square meter of collector area. Water storage tanks are made from a variety of materials like steel, concrete and fiberglass. The tanks are suitably insulated with glass wool, mineral wool or polyurethane. The thickness of insulation used is large and ranges from 10 to 20 cm. because of this, the cost of the insulation represents a significant part of the total cost and mean to reduce this cost have to be explored. Shelton has shown that in an underground tank, the insulating value of the earth surrounding the tank may be adequate and this could provide the bulk of the insulation thickness required. However, it may take as much as one year for the earth around a large storage tank to reach a steady state by heating and drying, and a considerable amount of energy may be required for this purpose. If the water is at atmospheric pressure, the temperature is limited to 100°C. It is possible to store water at temperature a little above 100°C by using pressurized tanks. This has been done in a few instances. In order to reduce the costs, an alternative way, which is being examined for large-scale storage, is to use naturally occurring confined underground aquifers which already contain water . The advantages and disadvantages of such storage can be is as follows:
Advantages:
1.Water is inexpensive, easy to handle, non-toxic, non-combustible and widely available.
2. Water has a comparatively high specific heat and high density
3. Heat exchangers may be avoided if water is used as the heat carrier in the
collector.
4. Natural convection flows can be utilized when pumping energy is scarce.
5. Simultaneous charging and discharging of the storage tank is possible.
6. Adjustment and control of a water system is variable and flexible.
Disadvantages:
1. Water might freeze or boil
2. Water is highly corrosive
3. Working temperatures are limited to less than 100°C and often have to be far
below this boiling temperature.
1.4.1.1 Well-mixed liquid storage
This is the most widely used method of sensible heat storage. In this type of storage systems the temperature in the storage tank is assumed to be uniform.
1.4.1.2 Stratified liquid storage
Figure 1.3: Well-mixed liquid storage tank.
In real situations, the liquid temperature in the storage will not be uniform, especially in the vertical direction. The warmer, lighter water is stored on the top of the colder, heavier liquid which results in thermal stratification. The incoming hot liquid will seek its own density level provided it enters the tank with low velocity, without agitating the liquid in the storage. As shown in Figure 1.4 hot water from solar collector at 60°C slowly enters the water storage tank at a level where water temperature is 50°C and it rises upward due to its higher buoyancy until it reaches the level where water temperature is 60°C. In practice, perfect stratification is not possible since the water entering the tank will also cause a certain amount of mixing.
Figure 1.4: Stratification
1.4.2 Solid media storage
Solids are also very common medium to store sensible heat. Varieties of solid substances are used to store sensible heat. These include rocks, pebbles, concrete, cast iron and bricks. The energy can be stored at low as well as high temperatures, since these solid materials do not freeze or boil.
1.4.3 Dual media storage
In this type of system solid and liquid both can be used as packing element . The water tank is surrounded by the rocks or pebbles and there is away where the cool air is passed from one side and heated air is passed from the other end, and with this the energy is stored in the tank by using these two medium .Figure 1.5 shows a hybrid storage system with water tank surrounded by rock bed has been used for solar space heating.
Figure 1.5: Solar storage tank using both water and stone as storage.
1.5 Latent Heat Storage
This is a type of system where the heat is stored when the materials melts and the heat is extracted when it freezes, example is wax. Latent heat storage is particularly attractive technique. Since it provides a high energy storage density and has the capacity to store heat as latent heat of fusion at a constant temperature corresponding to the phase transition temperature of the phase change materials (PCMs). For water, energy required to melt 1 kg of ice is 80 times more than to raise the temperature of 1 kg of water by 1oC. It means that a much smaller weight and volume of material is needed to store a certain amount of energy.
Based on the phase change process, PCMs can be classified into solid-liquid, liquid-gas and solid-solid PCM. Among these three types of PCMs, solid-solid PCM is rarely suitable for the thermal storage in buildings. Liquid-gas PCM experiences a very significant volume change due to the difference of molecular intervals between the gas and liquid. Thus, in general only solid-liquid PCM is suitable for the normal applications.
1.7 Comparison of energy storage technologies
There are different parameters on which the energy storage techniques are compared like efficiency, use and other specifications, each energy storage techniques have there own advantages and disadvantages but it is being observed that sun energy storage gives highest efficiency thus there is highest possibility that in future sun energy application will be seen highest. In all of the energy storage methods, the sensible heat storage is the highest efficiency and this technology is found to be of higher use.
1.8 Packed bed
A packed bed consists of a packing elements which are packed tightly or loosely according to the need , and during the charging mode the hot air is being circulated from top to bottom of the bed while the direction is reversed on discharging mode.
is transferred during the charging phase as shown in Figure 1.6
In a packed bed the density of air is very low therefore it is necessary to transfer the heat to a higher denser medium and thus different packed material are used in the bed which stores the energy.
Figure 1.6 shows the schematic of a solar air heating system with a packed bed storage unit and attached energy source.
Figure 1.6: Schematic of a packed bed energy storage system
In a packed bed the density of air is very low therefore it is necessary to transfer the heat to a higher denser medium and thus different packed material are used in the bed which stores the energy.
Above Figure shows the schematic of a solar air heating system with a packed bed storage unit and attached energy source.
Coutier and Farber [6] suggested that the packed bed generally represents the most suitable energy storage unit for air based solar system. Packed bed is the porous system composed of uniform or non-uniform, regular or irregular solid elements which packed randomly or regularly within the space confined by the wall of packed bed.
The rate of heat transfer depend upon lot of thermal and physical properties of the bed and the kind of geometries of the elements used and the porous nature of bed. The packing is usually random, where elements of apparently the same size and shape are packed in an arbitrary manner into the container.
1. There are several variables that determine the performance of a packed bed thermal energy storage unit. These variables can be divided into following three groups: These are different shapes and sizes of bed elements and bed length and other geometric parameters of the bed..
2. Those describing the characteristics of the flowing fluid as it pass through the bed like fluid properties and the mass velocity.
3. The other parameters are initial temperature of bed, inlet and outlet temperature of air and heat transfer co-efficient
1.8.1 Advantages of packed bed energy storage system
The advantages of packed bed are:
a) High degree of stratification temperature in the bed has high heat transfer.
b) There is temperature difference between the materials due to which we get high efficiency.
c) Increase in heat transfer coefficient between medium and storage material.
d) There is no need of insulation in the system as the elements have high thermal conductivity.
e) Increase in storage temperatures are possible.
f) There is no need of heat exchangers..
f) Freezing and corrosion problems are not present.
g) Storage material is inexpensive, easily available, easy to handle, non-toxic and non- combustible.
1.8.2 Disadvantages of packed bed energy storage system
Each energy storage system also has some limitations. Packed bed energy storage
systems have some limitations, which are listed below:
a) The size is so large that it is difficult to handle.
b) We need to have a large pumping power for the air to flow in the bed and pressure drop in the bed is also high due to that.
c) Discharging and charging at the same time is not possible.
1.9 Objectives of the study
1. To investigate the effect of system parameters (void fraction and aspect ratio of packing elements) and operating parameter (Reynolds number) on heat transfer
and pressure drop characteristics of 6 grooved packed bed heat storage system. Aspect ratio of element (le/de) has been used to define the shape of the material
elements.
.
2. To develop the correlations for Nusselt number and friction factor as a function of the system and operating parameters.
3. Validation of present correlation by comparing experimental data of Nusselt number and friction factor and that predicted by the correlation for Nusselt number and Friction factor respectively. Validation of correlations also checked
by comparing the values predicted by the present correlations and those predicted by the previous correlations reported in the literature.
1.10 Chapter outline
Chapter 1
Introduces various types of energy storage systems as sensible heat storage and latent heat storage system. It also introduces about the packed beds solar energy storage system and its advantages/disadvantages. Objectives of the thesis have been explained.
Chapter 2
Describes the published research work on packed bed energy storage system with emphasis on material properties, effect of system and operating parameters on heat transfer and pressure drop in packed bed.
Chapter 3
Explains about experimental set-up and packing arrangement. Experiment procedure and data reduction also discussed in this chapter.
Chapter 4
Describes the effect of system and operating parameters on the heat transfer and pressure drop in packed bed energy storage system.
Chapter 5
Explains the development of correlations for Nusselt number and friction factor for large sized cylindrical elements. Validation of correlation also discussed in this chapter.
Chapter 6
Concludes the major observations of the present experimental investigation.