Encyclopedia of Global Resources part 41 doc

Solar Heat Storage When converted into heat, solar energy can be stored in the form of sensible heat and latent heat.. Sensible heat is stored in a material by raising its temperature..

Trang 1

Advantages of pumped hydro units include simple

operation, high reliability, low maintenance, long life,

quick start from a standstill, and economic

genera-tion of peaking electrical energy Several such systems

are in operation in the United States

Power-generat-ing capacities of these systems vary between 5

mega-watts and 2,000 megamega-watts or higher The overall

effi-ciencies of these power plants vary between 65 and 90

percent (these figures include the efficiencies of

pumps, hydraulic turbines, and generators, and losses

from the upper reservoir) In spite of the technical

and economic viability of pumped hydro, the

require-ment of a specific type of topography and some

envi-ronmental concerns limit its application To

over-come these problems, underground pumped hydro

storage can be used In this case a large cavern or an

aquifer can be used as the lower reservoir

Solar Heat Storage

When converted into heat, solar energy can be stored

in the form of sensible heat and latent heat Sensible

heat is stored in a material by raising its temperature

The amount of sensible heat stored in a material is

equal to the product of the mass, specific heat, and

the temperature rise of the material The most

com-mon sensible heat storage materials include water,

propylene glycol, rocks, and molten salts Water has

the highest specific heat value The higher the

tem-perature rise, the greater the amount of heat stored

However, the highest temperature is limited by the

properties of the material

Thermal energy can also be stored as latent heat in

a material when it changes phase, as from solid to

liq-uid or liqliq-uid to vapor Some materials also change

phase from solid to vapor directly or from one solid

phase to another The amount of latent heat stored in

a material is equal to the product of the mass of the

material and its latent heat Because materials change

phase at a constant temperature, latent heat is stored

and retrieved at a fixed temperature known as the

transition temperature Some common phase change

materials (PCMs) used for heat storage are paraffin

waxes, Glauber’s salt (sodium sulfate decahydrate),

calcium chloride hexahydrate, sodium acetate

tri-hydrate, and cross-linked high-density polyethylene

Solar heat storage has major applications in space

heating, crop drying, cooking, electric power

genera-tion, and industrial process heat Heat storage in

water is the most economical and well-developed

tech-nology Hot water is stored in tanks made of glass- or

stone-lined steel, fiberglass, reinforced polymer (plas-tic), concrete with plastic liner, and wood The storage tanks may be located above or below ground In North America and China, aquifers have been used for long-term storage of hot water Molten nitrate salt (50 percent sodium nitrate, 50 percent potassium ni-trate), also known as Draw salt, which has a melting point of 222° Celsius, has been used as a storage mate-rial for a solar thermal power system in an experiment

in Albuquerque, New Mexico This was the first com-mercial demonstration of generating power from storage Solar Two, a 10-megawatt solar thermal power demonstration project in Barstow, California, also was designed to use this molten salt to store solar energy It led to the development of Solar Tres Power Tower near Seville, Spain

PCMs encapsulated in tubes, trays, rods, panels, balls, canisters, and tiles have been used for solar space-heating applications The most common PCMs used are hydrated salts of sodium sulfate, sodium thiosulfate, sodium acetate, barium hydroxide, mag-nesium chloride, and magmag-nesium nitrate For build-ing space-heatbuild-ing applications, PCM can be encapsu-lated in the building components themselves They can be incorporated in the ceiling, wall, or floor of the building For example, paraffin wax mixtures have been used for heat storage in wallboards

D Yogi Goswami and Chand K Jotshi

Định dạng
Số trang	10
Dung lượng	86,66 KB