6.heat Storage
6.heat Storage
6.heat Storage
Mass
Dr. Adam M Sebbit
1
Introduction
• Energy storage (ES) is important to the success of any
intermittent energy source in meeting demand.
• For example, the need for storage for solar energy
• Energy storage been developed to a point where it
can have a significant impact on modern technology.
• ES systems can contribute significantly to meeting
society’s needs for more efficient, environmentally
benign :
– energy use in building heating and cooling,
– and utility applications.
2
Introduction
The use of ES systems often results in such significant
benefits as:
• reduced energy costs.
• reduced energy consumption.
• improved indoor air quality.
• reduced initial and maintenance costs.
• reduced equipment size
• more efficient and effective utilization of equipment
• reduced pollutant emissions
3
Introduction
• Mechanical and hydraulic ES systems usually store
energy by converting electricity into energy of
compression, elevation, or rotation.
• There is a growing interest in storing low-
temperature heat in chemical form (reversible
chemical reactions)
• Electrochemical ES systems have better turnaround
efficiencies but very high prices.
• Thermal energy storage systems are varied, and
include designed containers, underground aquifers
etc. 4
Energy Storage Methods
• Electric utilities energy storage using a scheme called
pumped storage
• Heat storage ( water usage for at least one day)
• Advances in storage in benefit wind and solar energy
technologies.
• Thermal energy may be stored by elevating or lowering the
temperature of a substance (i.e., altering its sensible heat), by
changing the phase of a substance (i.e., altering its latent
heat) or through a combination of the two.
5
Thermal Energy
• E = mC(T2 − T 1) = ρVC(T2 − T 1)
• TES is useful for addressing the mismatch between the
supply and demand of energy.
• There are mainly two types of TES systems, sensible
(e.g., water and rock) and latent (e.g., water/ice and
salt hydrates
• The cooling loads, which coincide somewhat with
maximum levels of solar radiation but lag by a time
period, are often present after sunset.
• TES can provide an important mechanism to offset this
mismatch between times of energy availability and
demand.
6
Thermal Mass Storage
The ability of building materials to store heat
(thermal storage capacity). The basic characteristic of
materials with thermal mass is their ability to absorb
heat, store it, and at a later time release it.
• Phase Change Materials (PCMs) When a material
melts or vaporizes, it absorbs heat; when it changes
to a solid (crystallizes) or to a liquid (condenses), it
releases this heat. This phase change is used for
storing heat in PCMs.
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Thermal Mass Storage
• Typical PCMs are :
– water/ice,
– salt hydrates,
– and certain polymers.
• Since energy densities for latent TES exceed
those for sensible TES, smaller and lighter
storage devices and lower storage losses
normally they have a better performance.
8
Thermal Energy
• The solar energy is usually needed most when solar
availability is lowest, namely, in winter for heating
• TES complicates solar energy systems in two main ways.
– TES subsystem must be large enough to permit the system to operate
over periods of inadequate sunshine.
– A backup energy supply is an alternative but it adds to capital cost
and provides a unit that remains idle during summer.
• Solar energy can and probably needs to be integrated into
systems that also use conventional energy sources.
• In stand-alone solar energy systems may be desired in future
9
Energy storage Materails
• The most important criteria to be met by the storage material
for a latent TES in which the material undergoes a solid–liquid
or a solid–solid phase transition are as follows:
– high transition enthalpy per unit mass;
– ability to fully reverse the transition;
– adequate transition temperature;
– chemical stability and compatibility with the container (if
present);
– limited volume change with the transition;
– nontoxicity;
– low cost, in relation to the foreseen application.
10
Sample TES cycle characteristics
11
Thermal Energy
• Concentrating solar systems must cope with the intermittent
nature of direct sunlight on a cloudy day.
• The absorbers and boilers must be designed with care to
avoid burn-out problems when the sun suddenly returns with
full brilliance.
• Non-concentrating systems face the fundamental problem of
trying to provide sufficiently high efficiency at medium
temperatures to yield energy output at a reasonable cost.
Thus, TES costs must be reasonable
12
Thermal Energy
• Most solar energy systems use diurnal storage,
where energy is stored for at most a day or two.
Diurnal storage has advantages:
– capital investments for storage and energy loss are
usually low
– devices are smaller and can easily be
manufactured offsite
– sizing of daily storage for an application is not as
critical as sizing for larger annual storages
13
Thermal Energy
• The five main types of such passive systems
are:
– direct heat gain,
– heat collection and storage,
– sun space,
– roof-top heat storage,
– thermosyphon.
14
Building Applications of TES and Solar Energy
15
Building Applications of TES and
Solar Energy
Lightweight-building construction practices a
lightweight latent TES system which is easily installed in
a building could be beneficial.
– The challenge is the effective and economic containment
of a (Phase change materials) PCM in its liquid phase. ( not
economical now)
– A more interesting approach is a wallboard containing a
PCM. With the wallboard providing PCM containment as
well as serving an architectural function, the economics
are improved. Further, the large heat-transfer area of the
wallboard supports large PCM
– heat fluxes driven by small temperature 16
differences.
Building Applications of TES and Solar Energy/
Integrated design/ A solar rock-bed TES system
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Sensible TES
• In sensible TES, energy is stored by changing
the temperature of a storage medium such as
water, air, oil, rock beds, bricks, sand, or soil.
The amount of energy input to TES by a
sensible heat device is proportional to
– the difference between the storage final
– initial temperatures,
– the mass of the storage medium, and its heat
capacity.
18
Available media for sensible and
latent TES systems
19
Thermal capacities at 20◦C of some
common TES materials
20
Concrete Material TES
• Concrete is sometimes chosen because of its low
cost, availability throughout the world, and easy
processing. Inexpensive aggregates to the concrete
are widely available. Concrete has the following
characteristics as a storage medium:
• high specific heat
• good mechanical properties
• a thermal expansion coefficient near that of steel
• high mechanical resistance to cyclic thermal loading.
21
Latent TES
• Effective utilization of time-dependent energy resources
requires appropriate TES methods to reduce the time and rate
mismatch between energy supply and demand.
• The latent heat change is usually much higher than the
sensible heat change for a given medium, which is related to
its specific heat.
• Most practical systems using phase-change energy storage
involve solutions of salts in water.
• A heat storage substance that undergoes a phase transition
within the desired operating temperature range, and wherein
the bulk of the heat added is stored as latent heat and a
containment for the storage substance.
22
Latent TES
• The three most favoured storage media for solar
energy systems, water, rock, and Glauber’s salt,
• The storage cost for water is $54 and for Glauber’s
salt it is $146. However, only 0.18m3 of Glauber’s salt
is required, which is one-quarter of the necessary
0.72m3 of water.
• Rock storage for the same collector would cost $217
at $8/t for the required 2.46m3.
23
Latent TES
• Latent TES is a promising storage technique, it
provides a high energy storage density, second only
to chemical energy storage, and can store and
release heat at a constant temperature
corresponding to the phase transition temperature
of the heat-storage medium.
• An important material category capable of storing
energy through phase change is paraffin waxes.
• These have the advantage of very high stability over
repeated cycles of latent TES operation without
degradation. 24
Operational aspects of latent TES
• The most important criteria to be met by the storage
material for a latent TES undergoes a solid–liquid or a
solid–solid phase transition are as follows:
– high transition enthalpy per unit mass;
– ability to fully reverse the transition;
– adequate transition temperature;
– chemical stability and compatibility with the container (if
present);
– limited volume change with the transition;
– nontoxicity;
– low cost, in relation to the foreseen application.
25
Some of the Thermodynamic criteria for PCMs
27
Solar thermal storage systems
28
Solar thermal systems/climate
HOT HUMID TEMPERATE COLD HOT ARID
Use of high mass Well Winter heating Both winter
construction is designed houses predominates in heating and
generally not should require no these climates summer cooling are
recommended in supplementary although some very important in
hot humid climates heating or cooling. summer cooling is these climates.
due to their limited The usually necessary. High
diurnal range. predominant High mass mass construction
Passive cooling in requirement for construction combined with
this climate is cooling in these combined with sound passive
generally more climates is often sound passive solar heating and cooling
effective in low suited to design and high principles is
mass buildings lightweight, low level insulation is the most effective
mass construction an ideal solution. and economical
Good solar access means of
is required maintaining
in winter to heat thermal comfort.
the thermal mass.
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THERMAL MASS LOCATION – INSIDE
INSULATED BUILDING ENVELOPE
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THERMAL MASS LOCATION – INSIDE
INSULATED SUN-FACING FACADE
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Effect of floor coverings on energy
consumption (GJ)
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Thermal mass Material Characteristics
35
Heat storage by interior building
elements
• The estimated amount of heat for heating up the
room, heat Q flowing into the component the heat
transfer coefficient hi and
• the temperature difference between the component
surface Ts,1 and the room air Ti ;
• The Heat Q= hiA (Ti-Ts)
36
Heat storage by interior building
elements
• After the time-step the new temperature of the
component Ts,2 results from the stored amount of
heat Qs = Q
37
Heat storage by interior building elements
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Heat storage by interior building elements
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Heat storage by interior building elements
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Heat storage by interior building elements
42
Required Luminance Vs Power
• Approximate values for nominal flux of light, and
specific connected loads of energy saving
lighting concepts
43