thermal energy storage 03

TES CHALLENGES FOR SOLAR

APPLICATIONS

DR M. NAZIFIFARD

nazifi@kashanu.ac.ir

TES CHALLENGES FOR SOLAR

APPLICATIONS

TES subsystem must be large enough to permit

the system to operate over periods of inadequate

sunshine.

The second major complication imposed by TES

is that the primary collecting system must be

sufficiently large to build the supply of stored

energy during periods of adequate insolation.

Thus, additional collecting area (and its

additional capital cost) is needed.

TES TYPES AND SOLAR ENERGY SYSTEMS

In solar energy applications, TES can provide savings in systems involving either simultaneous heating and cooling, or heating and cooling at different times of the year.

Most solar energy systems use diurnal storage, where energy is stored for at most a day or two. Diurnal storage offers a number of advantages, such as

capital investments for storage and energy loss are usually low;

devices are smaller and can easily be manufactured offsite; and

sizing of daily storage for an application is not as critical as sizing for larger annual storages.

A variety of active and passive systems for

storage have been developed for the effective

utilization of solar energy. Passive systems,

which do not need pumps are often suitable for

small scale domestic applications, and are widely

used throughout Europe and the United States.

The five main types of such passive systems are:

direct heat gain,

heat collection and storage,

sun space,

roof-top heat storage,

thermosyphon.

BUILDING APPLICATIONS OF TES AND

SOLAR ENERGY

The ability to store thermal energy is important

for effective use of solar energy in buildings.

Today, much interest is focused on passive

systems for space heating and active systems for

water heating.

For building heating, conventional passive TES

materials include water, rocks, masonry, and

concrete.

To perform well, these storage materials must be

massive because their allowable temperature

swings are limited by comfort conditions that

must be maintained inside the building.

DUAL MEDIUM STORAGE SYSTEMS

PHASE CHANGING MATERIAL

IN SOLAR THERMAL ENERGY

STORAGE

CONTENTS

Introduction

Benefits and Drawbacks of PCM

PCM Options

Encapsulation

Increasing Thermal Conductivity

Conclusion

INTRODUCTION

Most systems have a disconnect

between supply and demand

Intermittent solar energy supply

can be maximized with a heat

storage system

Thermal energy can be

stored both as sensible

and latent heat

Continued efforts to find a phase changing material is currently underway

BENEFITS AND DRAWBACKS OF PCM

Benefits:

Higher storage density than sensible heat

Smaller volume

Smaller temperature change between storing and releasing energy

Drawbacks:

High cost

Corrosiveness

Density change

Low thermal conductivity

Phase separation

Incongruent melting

PCM OPTIONS

PCM OPTIONS Inorganic

Glauber’s salt, calcium chloride hexahydrate, sodium thiosulfate penthydrate, sodium carbonate decahydrate

Benefits:

Low cost and readily available

High volumetric storage density

Relatively high thermal conductivity

Drawbacks:

Corrosive

Decomposition

Incongruent melting

Supercooling

PCM OPTIONS Organic

Paraffin waxes and fatty acids

Benefits:

Melts congruently

Chemically and physically stable

High heat of fusion

Drawbacks:

More expensive and flammable

Low thermal conductivity in solid state

Lower heat storage capacity per volume

PCM OPTIONS

ENCAPSULATION Prevents reactivity towards environment

Compatible with stainless steel, polypropylene, and

polyolefin

Controls volume as phases change

Prevents large drops in heat transfer rates

INCREASING THERMAL CONDUCTIVITY Metallic fillers

Metal matrix structures

Finned tubes

Aluminum filling with VSP 25 and VSP 50

PCM-Graphite Matrix Finned Tubes

The VSP25 filling provided the highest thermal

conductivity of 1W/(mK), which is about six

times that of pure paraffin

Total solidification time of PCM is shorter

with fins and lessing rings, but the total

quantity of stored heat is slightly smaller

CONCLUSION

Thermal energy storage is imperative to make

solar energy more reliable and competitive

Further research in phase changing material can

improve the efficiency of energy storage

Design of the system is also important in

optimizing energy storage

OTHER APPLICATIONS

Cooling of heat and electrical engines

Cooling: use of off-peak rates

Cooling: food, wine, milk products (absorbing peaks in demand), greenhouses

Heating and hot water: using off-peak rates

Medical applications: transportation of blood, operating tables, hot–cold therapies

Passive storage in bio-climatic building/architecture (HDPE, paraffin)

Safety: temperature level maintenance in rooms with computers or electrical/electronic appliances

Smoothing exothermic temperature peaks in chemical reactions

Solar power plants

Spacecraft thermal systems

Thermal comfort in vehicles

Thermal protection of electronic devices (integrated in the appliance)

Thermal protection of food: transport, hotel trade, ice-cream, etc.

Thermal storage of solar energy