thermal storage system

8/12/2019 thermal storage system

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THERMALENERGYSTORAGEWITH

PHASECHANGEMATERIAL


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INTRODUCTION

Scientists all over the world are in search of new and renewable energy sources to reduce

the CO2 emissions from the combustion of fossil fuels, particularly in areas where low

temperature applications are involved.

Solar energy has an enormous potential for the heating and cooling of buildings,

producing hot water for domestic and industrial purposes, cooking, warming greenhouses

for agricultural crops, etc.

Intermittent energy source, unpredictable, and available only during the day.


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The total energy we receive each year from the sun is around 35,000 times

the total energy used by man. However, about 1/3 of this energy is either

absorbed by the outer atmosphere or reflected back into the space.

Solar thermal energy can be stored as:

1.Sensible heat (water and rock),

2.Latent heat (water/ice and salt hydrates), and

3.Heat of reaction, or combination of these.

One major drawback of sensible heat storage is the large volume required,

especially when the allowable temperature swing is small.

Latent heat storage is more attractive than sensible heat storage because ofits high storage density with smaller temperature swing


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Latent heat storage has main advantage:

(i) it is possible to store large amounts of heat with only small temperature changes

and therefore to have a high storage density

Their use often results in such significant benefits as:

Reduce energy costs,

Reduces energy consumption,

Increased flexibility of operation, Reduces initial and maintenance costs,

Reduces equipment size,

More efficient and effective utilization of equipment,

Conservation of fossil fuels (by facilitating more efficient energy use and/or fuel

substitution), and

Reduces pollutant emissions (e.g. CO2 and CFCs).


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TES deals with the storing of energy by heating, cooling, melting,

solidifying or vaporizing a material, the thermal energy becoming available

when the process is reversed.

Thermal energy storage (TES) systems have an enormous potential to

make the use of thermal energy equipment more effective and for

faci l itating large-scale energy substitutions from an economic perspective.


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PHASECHANGEMATERIAL

A phase change mater ial (PCM ) is a substance with a high heat of

fusion which , melting and solidifying at a certain temperature, is

capable of storing and releasing large amounts of energy.

Heat is absorbed or released when the material changes from solid

to liquid and vice versa; thus, PCMs are classified as latent heat

storage (LHS) units.


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PCMs are classified into three types


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ORGANICPCMS

Paraffin (CnH2n+2) and Fatty acids CH3(CH2)2nCOOH).

Advantages:

1. Availability in a large temperature range

2. Freeze without much super cooling

3. Ability to melt congruently

4. Self nucleating properties

5. Compatibility with conventional material of construction

6. No segregation

7. Chemically stable

8. High heat of fusion

9. Safe and non-reactive

10. Recyclable


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Disadvantages:

1. Low thermal conductivity in their solid state. High heat transfer rates are duringthe freezing cycle

2. Volumetric latent heat storage capacity is low

3. Flammable. This can be easily alleviated by a proper container

4. Due to cost consideration only technical grade paraffins may be used which are

essentially paraffin mixture and are completely refined of oil


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INORGANIC

Salt hydrates (MnH2O)

Advantages:

1.High volumetric latent heat storage capacity

2. Low cost and easy availability

3. Sharp melting point

4. High thermal conductivity

5. High heat of fusion

6. Non-flammable


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Disadvantages:

1. Change of volume is very high

2. Super cooling is major problem in solid-liquid transition

3. Nucleating agents are needed.

Advantages:

1. Eutectics have sharp melting point similar to pure substance

2. Volumetric storage density is slightly above organic compounds

Disadvantages:

1. Only limited data is available on thermo-physical properties as the use of

these materials are very new to thermal storage application


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CHEMICALPROPERTIES

(i) Chemical stability.

(ii) Complete reversible freeze / melt cycle.

(iii) No degradation after a large number of freeze/ melt cycles.

(iv) Non-corrosiveness to the construction materials.

(v) Non-toxic, non-flammable, and non-explosive materials for safety.


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Exper imental setup

The set up consists of a solar collector and a thermal energy storage unit, a

water tank and a pump as shown in Fig. The collector is well insulated and

collects the solar energy and transfer heat energy to water. This hot water

passes to the thermal energy storage unit where hot water passes though a

coil and heat transfers from copper coil to the storage media HTF loses heat

energy and is stored in the storage unit during charging (day time). During

night time or early mornings cold water is passed through storage unit to

recover the stored energy.


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Mathematical Modeling

A Mathematical model has been developed for the storage unit using water as

sensible heat storage material is shown fig. In storage unit, storage material fills the

annular shell space with mass m and heat capacity Cp around the coil, while the heat

transfer fluid flows within the coil and exchanges heat with the Water


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Energy Equation for Water as sensible heat storage material

Consider storage tank is filled with water initially. The water in the storage tank gets heated

up by circulating HTF which circulates continuously between solar collector, storage unit

and water tank. HTF collects heat from solar collector and dissipates it to storage unit bywhich the temperature of water in the storage unit raises and energy will be stored in the

form of sensible heat. The energy balance equation for liquid storage tank charging and

discharging simultaneously is given by

[(VCp)l + (VCp)t] dTl/dt = qu - qload - (UA)t (Tl -Ta)


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PRO- E DESIGN OF STORAGE TANK:


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THANK YOU

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thermal storage system