Energy Storage Part2

Lecture 4

Energy Storage for Power Systems

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Thermal Energy Storage (TES)

Introduction

Methods of Thermal Energy Storage

Sensible Heat Storage

Phase Change Energy Storage

TES for solar power plants

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Introduction

Developing efficient and inexpensive energy storage devices is as

important as developing new sources of energy.

The thermal energy storage (TES) can be defined as:

the temporary storage of thermal energy at high or low

temperatures

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Introduction

Characteristics of TES

Improves performance of energy systems by

smoothing supply and increasing reliability

The need for the storage of solar energy can not be

avoided as it is periodic source of energy

Have energy losses with time (not like Hydrogen )

Denser materials with high thermal capacity is

desirable for its small volume

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The first-law efficiency of thermal energy storage

systems can be defined as the ratio of the energy

extracted from the storage to the energy stored into it

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Maximum temperature during

discharge

Minimum temperature during

discharge

Maximum temperature during

the charge

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Methods of Thermal Energy Storage1. Sensible heat storage

Heating a liquid or a solid without changing the

phase

2. Latent heat storage

Heating a material which undergoes a phase

change (Usually melting)

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Sensible Heat storage Liquids for heat storage

Water (bellow 100C), heat transferoils, certain inorganic molten salts.

Solids for heat storage

Rocks, pebbles, refractory where

materials is collected in form of

porous media in a backed bed and

heat is stored by the flow of gas or

liquid in the voids

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Liquid storage media Water is stored in tanks made of steel, concrete or fiberglass.

These tanks is to be isolated with materials like glass wool,

mineral wool or polyurethane.

Heat transfer oils like Dowtherm and Therminol are used instorage of intermediate temperature ranges of 100 to 300C.

Heat transfer oils have disadvantages of:

Degradation () with time

possibility of ignition above their flash point.

High cost.

Inorganic salts have been considered for high temperatures(300C and above).

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Liquid storage media Advantages of water use in sensible heat storage.

1. Water is inexpensive, easy to handle, non-toxic, non-combustibleand 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 heatcarrier in the collector.

4. Natural convection flows can be utilized when pumping energy isscarce.

5. Simultaneous charging and discharging of the storage tank ispossible.

6. Adjustment and control of a water system is variable and flexible.

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Liquid storage media Disadvantages of water use in sensible heat storage.

Water might freeze or boil

Water is highly corrosive

Working temperatures are limited to less than 100C and

often have to be far below this boiling temperature.

Water is difficult to stratify.

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Solid storage media Energy can be stored in rocks or

pebbles packed in insulated

vessels.

This type of storage is used veryoften for temperatures up

to100C in conjunction with solarair heaters.

Rock or pebble-bed storages canalso be used for much higher

temperatures up to 1000C.

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Solid storage media The difficulties and limitations relative to liquids can be

avoided by using solid materials for storing thermal energy as

sensible heat.

larger amounts of solids are needed than using water, due tothe fact that solids, in general, exhibit a lower storing capacity

than water.

The cost of the storage media per unit energy stored is,however, still acceptable for rocks.

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Solid storage media Advantages of rocks in heat storage

Rocks are not toxic and non-flammable

Rocks are inexpensive

Rocks act both as heat transfer surface and storage

medium

The heat transfer between air and a rock bed is good, due

to the very large heat transfer area, and the effective heat

conductance of the rock pile is low, due to the small area of

contact between the rocks. Then the heat losses from the

pile are low.

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Solid storage media Magnesium oxide (magnesia), aluminum oxide (alumina) and

silicone oxide are refractory ( ) materials, and theyare also suitable for high-temperature sensible heat storage.

Bricks made of magnesia have been used in many countriesfor many years for storing heat.

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Latent heat storage In latent heat storage the principle is that when heat is

applied to the material it changes its phase from solid to

liquid by storing the heat as latent heat of fusion or from liquid

to vapor as latent heat of vaporization.

Heat storage through phase change has the advantage ofcompactness, since the latent heat of fusion of most

materials is very much larger than their specific heat.

For example, the ratio of latent heat to specific heat of wateris 80, which means that the energy required to melt one

kilogram of ice is 80 times more than that required to raise

the temperature of one kilogram of water one degree Celsius.

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Latent heat storage

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Latent heat storageAny latent heat thermal energy storage system should have at

least the following three components:

A suitable phase change material (PCM) in the desired

temperature range,

A containment for the storage substance,

A suitable heat carrying fluid for transferring the heat

effectively from the heat source to the heat storage

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Latent heat storageDue to its high cost, latent heat storage is more likely to find

application when:

1. High energy density or high volumetric energy capacity is desired,

2. The load is such that energy is required at a constant temperature

or within a small range of temperatures, or

3. The storage size is small. Smaller storage has higher surface area

to volume ratio and therefore cost of packing is high. Compactness

is then very important in order to limit the containment costs.

Similarly, heat losses are also more or less proportional to the

surface area. Compactness is also an important factor to limit the

heat losses in storages of small capacities.

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TES for solar power plantsImportance?

an output management tool to elongate operation aftersunset, to shift energy sales from off-peak hours to high

revenue peak demand hours, and to contribute to

guaranteed output;

an internal plant buffer, smoothing out insolation changesfor steadying cycle operation, and for operational

requirements such as covering steam production,

component pre-heating and freeze protection.

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TES for solar power plants

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ChargingDischarging

Sun rise Sun set


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the number of thermal kilowatt-hours per cubic meter (kWht/m3 ) is used here, since container volume and pumping power are the basic cost factors


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In a single medium storage system,the HTF is at the same time the storagemedium

If the liquid has low thermalconductivity and permits good thermalstratification, such as water andthermal oil, the one-tank thermoclineconcept requires the least tank volumesince the hot and cold medium arecontained in a single vessel.


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When thermal conductivity ishigher, as in molten salts orsodium, a rapid balancing of thetemperatures in the hot and coldregions takes place, makingseparate hot and cold tanksnecessary. Since in that case twiceas much tank volume as fluidcontent is required


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Dual medium concepts employ a storage medium that isdifferent from the HTF because the storage medium -usually solid - is cheaper than the transfer fluid

The transfer medium exchanges its heat in direct orindirect contact with the storage medium

Have the disadvantage of a drop in temperature betweencharging and discharging due to the intermediate heatexchange


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nts Thermal Storage for Oil-Cooled Solar Plants

Thermal, synthetic and silicone oils, with operatingtemperatures from 300C to over 400C, are of particularinterest as heat transfer media for thermal SPP. Unlikewater/steam, oils do not require high-pressure piping, norhave they freezing problems as with sodium or molten salt.

The 5 MWht capacity one-tank thermocline storagesystem, operated with 114 m3 of thermal oil attemperatures between 225 and 295C, was successfullytested at the IEA-SSPS project in Almeria, Spain, anddemonstrated a 92% roundtrip efficiency and excellentthermocline stratification.


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two tank oil storage system at SEGS I, Daggett/CA, operating between 241C and 307C (courtesy of LUZ IntI. Ltd. Los Angeles/CA, USA)


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Solar Plants

Molten salts are favoured central receiver coolantsbecause of their high volume heat capacity, low vaporpressure, good heat transfer and low cost, which makesthem economical enough to be used as a large bulkstorage medium while their thermodynamic propertiespermit compact and efficient receivers.

The storage tanks must be hydraulically separated fromthe receiver loop by an intermediate heat exchanger,


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Solar Plants

cylindrical hot tank concept for molten nitrate salt storage with corrugated liner (courtesy of SERl, Boulder/CO, USA) .


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Solar Plants

conical hot tank concept for molten nitrate salt storage with flat stainless steel liner (courtesy of Solar Energy Research Institute (SERI), Boulder/CO, USA), (courtesy

Energy Storage Part2

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