Thermal energy storage may refer to a number of
technologies that
store energy in a
thermal reservoir for later reuse. They can be employed to balance
energy demand between day time and night time. The thermal
reservoir may be maintained at a temperature above (hotter) or
below (colder) than that of the ambient environment.
The principal application today is the production of
ice, chilled water, or
eutectic
solution at night, which is then used to cool environments during
the day.
Thermal energy storage technologies store
heat,
usually from active
solar
collectors, in an
insulated
repository for later use in space heating, domestic or process hot
water, or to generate electricity. Most practical active solar
heating systems have storage for a few hours to a day's worth of
heat collected. There are also a small but growing number of
seasonal thermal stores, used
to store summer heat for space heating during winter.
Molten salt has been proposed as a means
to retain a high temperature thermal store for later use in
electricity generation.
Economics
High peak summertime loads drive the capital expenditures of the
electricity generation
industry. The industry meets these peak loads with low-efficiency
peaking power plants, usually
gas turbines, which have lower
capital costs but higher fuel costs. A
kilowatt-hour of electricity consumed at night
can be produced at much lower marginal cost. Utilities have begun
to pass these lower costs to consumers, in the form of Time of Use
(TOU) rates, or Real Time Pricing (RTP) Rates. Thermal energy is
cheaper than any other energy source.
Water based technology
Thermal energy storage is made practical by the large
heat of fusion of water. One
metric ton of water, just one cubic meter, can
store 334 M
J (317 k
BTUs,
93kWh or 26.4 ton-hours). In fact, ice was originally transported
from mountains to cities for use as a coolant, and the original
definition of a "ton" of cooling capacity (heat flow) was the heat
to melt one ton of ice every 24 hours. This is the heat flow one
would expect in a house in Boston in the summer. This definition
has since been replaced by less archaic units: one ton
HVAC capacity = 12,000 BTU/hour. Either way, an
agreeably small storage facility can hold enough ice to cool a
large building for a day or a week, whether that ice is produced by
anhydrous ammonia chillers or
hauled in by horse-drawn carts.
Air conditioning
The most widely used form of this technology is in large building
or campus-wide
air conditioning or
chilled water systems. Air conditioning systems, especially in
commercial buildings, are the most significant contributors to the
peak electrical loads seen on hot summer days. In this application
a relatively standard chiller is run at night to produce a pile of
ice. Water is circulated through the pile during the day to produce
chilled water that would normally be the daytime output of the
chillers.
A partial storage system minimizes capital investment by running
the chillers 24 hours a day. At night they produce ice for storage,
and during the day they chill water for the air conditioning
system, their production augmented by water circulating through the
melting ice. Such a system usually runs in ice-making mode for 16
to 18 hours a day, and in ice-melting mode for 6 hours a day.
Capital expenditures are minimized because the chillers can be just
40 to 50% of the size needed for a conventional design. Ice storage
sufficient for storing half a day's rejected heat will do.
A full storage system minimizes the cost of energy to run the
system by shutting off the chillers entirely during peak load
hours. Such a system requires chillers somewhat larger than a
partial storage system, and a larger ice storage system, so that
the capital cost is higher. Ice storage systems are inexpensive
enough that full storage systems are often competitive with
conventional air conditioning designs.
The efficiency of air conditioning chillers is measured by their
coefficient of
performance (COP). In theory, thermal storage systems could
make chillers more efficient because heat is discharged into colder
nighttime air rather than warmer daytime air. In practice, this
advantage is overcome by the heat losses while making and melting
the ice.
There are still some advantages to society from air conditioning
thermal storage. The
fuel used at night to
produce electricity is a domestic resource in most countries, so
that less imported fuel is used. This process also has been shown
in studies to significantly reduce the emissions associated with
producing the power for air conditioners, since inefficient
"peaker" plants are replaced by low emission base load facilities
in the evening. The plants that produce this power are often more
efficient than the
gas turbines that
provide peaking power during the day. And because the
load factor on the plants is higher, fewer
plants are needed to service the load.
A new twist on this technology uses ice as a condensing medium for
refrigerant. In this case, regular
refrigerant is pumped to coils where it is used. Instead of needing
a
compressor to convert it back in to a
liquid however, the low temperature of the ice is used to chill the
refrigerant back in to a liquid. This type of system allows
existing refrigerant based HVAC equipment to be converted to
Thermal Energy Storage systems, something that could not previously
be easily done with chill water technology. In addition, unlike
water-cooled chill water systems that do not experience a
tremendous difference in efficiency from day to night, this new
class of equipment typically displaces daytime operation of air
cooled condensing units. In areas where there is a significant
difference between peak daytime temperatures and off peak
temperatures, this type of unit is typically more energy efficient
than the equipment it is replacing.
Combustion gas turbine air inlet cooling
Thermal energy storage is also used for combustion
gas turbine air inlet cooling. Instead of
shifting electrical demand to the night, this technique shifts
generation capacity to the day. To generate the ice at night, the
turbine is often mechanically connected to the compressor of a
large chiller. During peak daytime loads, water is circulated
between the ice pile and a
heat
exchanger in front of the turbine air intake, cooling the
intake air to near freezing temperatures. Because the air is
colder, the turbine can compress more air with a given amount of
compressor power. Typically, both the generated electrical power
and turbine efficiency rise when the inlet cooling system is
activated.
This system is similar to the
compressed air energy storage
system.
Molten salt technology
Molten salt can be employed as a heat store to retain heat
collected by a
solar tower or
solar trough so that it can be used to
generate electricity in bad weather or at night.
It was demonstrated in
the Solar
Two
project from 1995-1999. The system is
predicted to have an annual efficiency of 99%, although it is not
clear if this is the
second law
efficiency.The molten salt is a mixture of 60 percent
sodium nitrate and 40 percent
potassium nitrate, commonly called
saltpetre. It is non-flammable and
nontoxic, and has already been used in the chemical and metals
industries as a heat-transport fluid, so experience with such
systems exists in non-solar applications.
The salt melts at . It is kept liquid at in an insulated "cold"
storage tank. The liquid salt is pumped through panels in a solar
collector where the focused sun heats it to . It is then sent to a
hot storage tank. This is so well insulated that the thermal energy
can be usefully stored for up to a week.
When electricity is needed, the hot salt is pumped to a
conventional steam-generator to produce
superheated steam for a turbine/generator
as used in any conventional coal, oil or nuclear power plant. A
100-megawatt turbine would need tanks of about tall and in diameter
to drive it for four hours by this design.
Several parabolic trough power plants under development in Spain
and
solar power tower developer
SolarReserve plan to use this thermal
energy storage concept.
See also
References
- Seasonal Thermal Energy Storage in Thermal
Banks.
- Advantages of Using Molten Salt Tom Mancini,
Sandia National Laboratories, Albuquerque, NM . Accessed December
2007
- Molten salt energy storage system - A feasibility
study Jones, B. G.; Roy, R. P.; Bohl, R. W. (1977) -
Smithsonian/NASA ADS Physics Abstract Service. Abstract accessed
December 2007
- Parabolic Trough Thermal Energy Storage
Technology Parabolic Trough Solar Power Network. April 04,
2007. Accessed December 2007
- "Prepared for the Thermal Energy Storage Systems Collaborative
of the California Energy Commission" and "Source Energy and
Environmental Impacts of Thermal Energy Storage." Tabors Caramanis
& Assoc.
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