Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
SYSTEM AND PROCESS FOR
USING SOLAR RADIATION TO PRODUCE ELECTRICITY
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No.
61/104,885 filed October 13, 2008.
BACKGROUND OF THE INVENTION
This invention generally relates to the equipment and process used to
capture solar radiation at a variable rate, store the radiation as heat and
then
transfer the heat at a constant rate and temperature. More particularly, this
invention is concerned with a solar power plant and a thermal storage
component
usable in the power plant.
Examples of patents and published patent applications that disclose
capturing solar energy and using it to produce electricity include US
4,286,141
and WO 2008/108870.
SUMMARY
Embodiments of the present invention provide the ability to capture solar
radiation as it becomes available, convert the radiation to heat and to
deliver the
heat at a constant rate. The ability to deliver heat at a constant rate
enables the
reliable production of electricity. In one embodiment, this invention is a
system
for using solar radiation to produce electricity. The system includes: (1) a
solar
collector that receives solar radiation at a variable rate, when measured as
joules
per hour, and uses the radiation to heat a thermal transfer fluid; (2) a
thermal
storage component connected to the collector; and (3) an electrical generating
1
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
station connected to the thermal storage component wherein the thermal
transfer
fluid transfers a constant amount of heat, measured as joules per hour, at a
constant temperature to the generating station. The thermal storage component
includes at least a first thermal storage zone and a second thermal storage
zone.
The zones include ceramic packing elements. The first zone's heat transfer
coefficient per unit volume exceeds the second zone's heat transfer
coefficient per
unit volume by at least 10 percent.
In another embodiment this invention is a system for using solar radiation
to produce electricity. The system includes: (1) a solar collector that
receives
solar radiation at a variable rate, when measured as joules per hour, and uses
the
radiation to heat a thermal transfer fluid; (2) a thermal storage component
connected to the collector; and (3) an electrical generating station connected
to the
thermal storage component wherein the thermal transfer fluid transfers a
constant
amount of heat, measured as joules per hour, at a constant temperature to the
generating station. The thermal storage component includes at least a first
thermal storage zone and a second thermal storage zone. The zones include
ceramic packing elements. The individual thermal conductivity of the first
zone's
packing elements exceeds the individual thermal conductivity of the second
zone's packing elements by at least 10 percent.
Another embodiment also relates to an apparatus for receiving solar
radiation at a variable rate and dispensing heat at a constant rate. The
apparatus
includes a first thermal storage zone and a second thermal storage zone. The
zones include ceramic packing elements. The first zone's heat transfer
coefficient
per unit volume exceeds the second zone's heat transfer coefficient per unit
volume by at least 10 percent.
In another embodiment, the invention relates to an apparatus for receiving
solar radiation at a variable rate and dispensing heat at a constant rate. The
apparatus includes a first thermal storage zone and a second thermal storage
zone.
The zones include ceramic packing elements. The individual thermal
2
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
conductivity of the first zone's packing elements exceeds the individual
thermal
conductivity of the second zone's packing elements by at least 10 percent.
Yet another embodiment relates to a process for the capture of solar
radiation to produce electricity. The process may include the following steps.
Receiving thermal radiation at a variable rate and converting the radiation to
heat.
Within a twenty-four hour period, storing at least twenty-five percent of the
heat
in a thermal storage component that includes at least a first storage zone and
a
second storage zone. Transferring the heat at a constant rate, measured as
joules
per hour, and a constant temperature to an electrical generating station.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic drawing of a first embodiment of a system for using
solar radiation to produce electricity;
Fig. 2 is a schematic drawing of a second embodiment of a system for
using solar radiation to produce electricity;
Fig. 3 is a perspective view of an embodiment of a packing element; and
Fig. 4 is a process flow chart.
DETAILED DESCRIPTION
The need to extract heat from a fluid has been recognized for many
decades. Procedures and equipment needed to accomplish the same have been
developed. In industrial processes, such as regenerative thermal oxidizers,
which
are used to improve the thermal and therefore economic efficiency of many
processes that generate heat which would be wasted if not recovered, the
ability to
extract heat from a first fluid and transfer the heat to a second fluid has
been well
developed as demonstrated in the patent literature including US 6,669,562 and
US
7,354,879. Regenerative thermal oxidizers are typically designed to receive a
heated flowing fluid such as a hot waste gaseous effluent from an industrial
3
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
process. Due to the controls exerted over the process that generates the
effluent,
the temperature of the effluent may be maintained within a relatively constant
temperature range. The thermal oxidizer receives the effluent and absorbs the
heat. The heat is absorbed by directing the hot gas to flow over, around and
through the ceramic media that are contained within the oxidizer. The media,
which may be referred to herein as packing elements or packing media, are
designed to have sufficient mass and surface area to quickly absorb a large
percentage of the heat in the fluid. The amount of time needed for the
absorption
of heat to occur depends upon the type of media used and various other
parameters such as the desired thermal efficiency of the process. A bed of
media
absorbs heat from a hot fluid and then desorbs heat to a fluid that is cooler
than
the media. The absorption and desorption of heat is then repeated. The time
from
the beginning of a first absorption cycle to the beginning of the next
absorption
cycle may be referred to as the duty cycle. Duty cycles for many regenerative
thermal oxidizers may last from a few seconds in one process to several
minutes
in a different process. Longer duty cycle times may not be feasible because
the
process that generates the effluent may be a continuously running process and
the
flow of effluent into the regenerative thermal oxidizer cannot be interrupted
for
extended periods of time without disrupting the process that generates the
effluent.
In contrast to regenerative thermal oxidizers which have duty cycles that
typically last less than thirty minutes, power plants that utilize solar
radiation
stored as heat in a thermal storage component have one duty cycle per day and
the
duty cycle lasts twenty-four hours. The inventors of this application
recognized
that this dramatic difference in the length and frequency of the duty cycles
of solar
power plants compared to conventional regenerative thermal oxidizers requires
the creation of unique systems for using solar radiation to produce
electricity and
thermal storage components that are specifically tailored to the duty cycle of
solar
power plants.
4
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
Solar power plants may be designed to include a solar collector and
thermal storage component that are capable of capturing and storing enough
solar
radiation as heat to provide heat at a constant rate to the electrical
generating
station for sustained periods of time up to and including twenty-four
consecutive
hours. The generating station may contain an expandable fluid, such as water,
that
reversibly converts from a liquid to a gas in response to increases in the
temperature of the expandable fluid caused by the transfer of heat to the
expandable fluid. The expansion of the generating station's expandable fluid
drives the generator which produces electricity. The heat supplied to the
generating station needs to be provided at a constant rate, measured as joules
per
hour, and at a constant temperature in order to power the turbine. As used
herein,
the rate at which heat is supplied to a generating station is defined to be at
a
"constant rate" for a defined period of time if the rate at which heat is
supplied in
every hour during the defined period of time does not vary more than five
percent
above or below the average amount of heat supplied per hour during the same
period. Similarly, the temperature at which heat is supplied to a generating
station
is defined to be "constant" for a defined period of time if the temperature at
which
heat is supplied in every hour during the defined period of time does not vary
more than five percent above or below the average temperature during the same
period. For example, within a ten hour period, if the average amount of heat
supplied to the generating station by the thermal transfer fluid is 10,000
joules per
hour at a temperature of 300 C, then the amount of heat supplied in any single
hour during the same ten hours cannot be less than 9,500 joules nor greater
than
10,500 joules and the temperature cannot drop below 270 C nor exceed 330 C. If
the heat is not supplied to the generating station at a constant rate and
temperature, the generator may slow down or speed up an unacceptable amount
thereby changing the electrical characteristics of the electricity generated
by the
generating station.
In some applications, a supplemental heat source may be used to precisely
increase the temperature of the thermal transfer fluid to a desired final
temperature
5
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
prior to the thermal transfer fluid flowing into the generating station. The
supplemental heat source could be, for example, a natural gas fired burner
assembly. Instead of, or in addition to, a supplemental heat source, the
thermal
transfer fluid may be directed into a large thermal equilibration tank prior
to the
thermal transfer fluid flowing into the generating station. The function of
the
thermal equilibration tank would be to moderate swings in the temperature of
the
fluid by allowing the temperature of the fluid to equilibrate before exiting
the
tank.
During a twenty-four hour period, the heat supplied to the generating
station may be (a) completely supplied directly from the solar collector; (b)
a
combination of heat supplied directly from the solar collector and heat from
the
thermal storage component; (c) completely supplied by heat from the thermal
storage component; or (d) a combination of heat supplied by a supplemental
heat
source and heat directly from the solar collector and/or the thermal storage
unit.
In a typical twenty-four hour period, which is defined herein as beginning one
hour after sunrise, heat supplied to the generating station may be provided by
a
combination of heat flowing directly from the solar collector and heat which
had
been captured the previous day and is now flowing from the thermal storage
compartment. The period of time during which heat is provided by both the
solar
collector and the thermal storage component may be referred to herein as the
morning transitional period. The morning transitional period begins when heat
provided directly from the solar collector begins to supplement heat provided
by
the thermal storage compartment and ends when the quantity of heat provided
directly from the solar collector exceeds the amount of heat needed to power
the
generating station. The excessive heat is then diverted to the thermal storage
component where it may be stored in thermal storage media. The amount of heat
captured by the solar collector between mid-morning and mid-afternoon must be
sufficient to provide all of the heat needed by the generating station during
that
period of time while also capturing and storing excess heat in the thermal
storage
component. Between approximately mid-afternoon and sunset, the heat supplied
6
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
directly from the solar collector to the generating station may need to be
supplemented by heat supplied from the thermal storage component. During this
transitional period, the amount of heat from the thermal storage component may
need to be gradually increased until all of the heat provided to the
generating
station flows directly from the thermal storage component. Beginning at
approximately sunset, all of the heat supplied to the generating station must
be
supplied by the thermal storage component because the solar collector is no
longer
capturing solar radiation from the sun. From sunrise to approximately mid-
morning, heat supplied directly from the solar collector begins to increase
and
heat supplied from the thermal storage component may be decreased.
To supply heat to an electrical generating station as described above, the
solar power plant's thermal storage component may need to absorb and retain
large quantities of thermal radiation at a rapid rate for several hours
(between
mid-morning and mid-afternoon), and then release the heat at a variable rate
for a
few hours (the afternoon transitional period), a constant rate for several
hours
after sunset, and a variable rate for another few hours (the morning
transitional
period). This unusual thermal regime may be difficult to achieve using (a)
conventional thermal storage media such as rocks or a bed of ceramic media
designed for use in regenerative thermal devices; and (b) a single thermal
storage
zone such as a vessel filled with homogenous thermal storage media. The
inventors of this application have recognized that the demands of a solar
power
plant's thermal regime can be met by providing a thermal storage component
that
has at least a first thermal storage zone and a second thermal storage zone
provided the thermal storage zones differ in at least one thermal
characteristic that
allows the rate and/or quantity of heat provided by each zone to be controlled
so
that the combination of heat from the first storage zone, the second storage
zone,
the solar collector and the supplemental heat source can be combined to supply
the thermal regime described above. A system for capturing and using solar
radiation to produce electricity is disclosed in Fig. 1 and described below.
7
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
Shown in Fig. 1 is a general schematic of a system that captures and uses
solar radiation to produce electricity. The system may include a concentration
solar collector 20, thermal storage component 22 and generating station 23.
Optionally, supplemental heat source 25 and/or thermal equilibration tank 27
may
be inserted between thermal storage component 22 and generating station 23.
Thermal transfer fluid, which flows throughout the system, may move from the
solar collector 20 to one or more of the thermal storage zones, through the
electrical generating station and then back to collector 20. Alternately, the
transfer
fluid may flow directly from mechanism 20 to generating station 23 and then
back
to mechanism 20. By-pass line 29 may be used to route the heat transfer fluid
around the supplemental heat source and thermal equilibration tank. In one
embodiment, a solar collector includes an elongated concave shaped trough
lined
with a reflective material that focuses the solar radiation onto tubing that
contains
thermal transfer fluid and is located at the convergence of the reflected
thermal
radiation. Absorption of the solar radiation by the fluid in the tubing
increases the
energy content, measured as joules per kilogram, of the fluid. The rate at
which
the solar radiation is absorbed by the fluid may be measured as joules per
hour.
When solar radiation is provided by allowing the sun to shine on the
reflective
material, the mechanism inherently receives the solar radiation at a variable
rate
throughout a 24 hour period. As used herein, the rate at which solar radiation
is
received is defined to be at a "variable rate" if the rate at which radiation
is
received in any hour during a twenty-four hour period of time varies more than
fifty percent above or below the average amount of radiation received per hour
in
the same twenty-four hour period. In a solar application, the rate at which
solar
radiation is received may vary from zero joules per hour during portions of
the
night to a maximum rate, which may be kilojoules per hour, during the middle
of
the day.
As shown in Fig. 1, collector 20 is connected to thermal storage
component 22 which may include all of the items shown within dotted line 24.
The thermal storage component includes at least first thermal storage zone 26,
8
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
second thermal storage zone 28 and may include additional thermal storage
zones
such as third thermal storage zone 30. The thermal storage zones may contain
packing elements made from materials, such as ceramics, which are capable of
absorbing and desorbing heat. The packing elements may be individual elements
that are randomly loaded into a thermal storage zone and are commonly known as
randomly oriented packing elements. An example of a suitable randomly oriented
packing element is shown in Fig. 3 and described below. Alternately, packing
elements known as monoliths may be loaded into a thermal storage zone by
manually placing each element beside another element so that the passageways
in
one element align with the passageways in the adjoining element. The first
thermal storage zone may differ from the second thermal storage zone in at
least
one of the following thermal or physical characteristics. The first
characteristic is
heat transfer coefficient per unit volume of the thermal storage zone. When
exposed to the same operating conditions, such as the incoming heat transfer
fluids having the same temperatures and flow rates, the first zone's heat
transfer
coefficient exceeds the second zone's heat transfer coefficient by at least 10
percent. For example, if the second zone's heat transfer coefficient is 40
W/m2K,
then the first zone's heat transfer coefficient must be at least 44 W/m2K. The
second characteristic is the geometric surface area per unit volume of the
thermal
storage zone. This characteristic is determined by measuring: the geometric
surface area of an individual packing element; the number of elements within a
fixed volume, such as one cubic meter; and then multiplying the number of
elements by the geometric surface area per element. The units of measurement
used may be expressed as square meters per cubic meter. The first zone's
geometric surface area per unit volume exceeds the second zone's geometric
area
per unit volume by at least 10 percent. The third characteristic is the
thermal
storage zone's specific heat capacity. The first zone's specific heat capacity
exceeds the second zone's specific heat capacity by at least 10 percent. The
fourth characteristic pertaining to the storage zones is mass per unit volume.
The
9
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
first thermal storage zone's mass per unit volume exceeds the second thermal
storage zone's mass per unit volume by at lest 10 percent.
Instead of differences between thermal or physical characteristics of the
storage zones, the packing elements in the zones may differ in one or more of
the
following thermal or physical characteristics. A packing element
characteristic is
the individual mass of a zone's packing elements. The individual mass of the
first
zone's packing elements exceeds the individual mass of the second zone's
packing elements by at least 10 percent. Another packing element
characteristic
is the individual thermal conductivity of a zone's packing elements. The
individual thermal conductivity of the first zone's packing elements exceeds
the
individual thermal conductivity of the second zone's packing elements by at
least
10 percent. Yet another packing element characteristic is the individual heat
capacity of a zone's packing elements. The individual heat capacity of the
first
zone's packing elements exceeds the individual heat capacity of the second
zone's
packing elements by at least 10 percent.
A thermal storage component that has two or more thermal storage zones
which differ in one of more of these thermal or physical characteristics can
be
used to supply all or some of the heat to the electrical generating station.
Within a
single system for using solar radiation to produce electricity, the rates and
quantities of heat obtained from two or more thermal storage zones that have
different thermal and/or physical characteristics may be coordinated to supply
heat at a constant rate and temperature to the generating station. For
example, a
system may be designed to have a first thermal storage zone and a second
thermal
storage unit wherein the first storage zone has a relatively small specific
heat
capacity compared to the specific heat capacity of the second thermal storage
zone. The first storage zone may be quickly heated to its maximum specific
heat
capacity by the solar collector while the second zone may require a much
longer
time to reach its maximum specific heat capacity. The first zone may then be
used to provide a rapid infusion of heat to the thermal transfer fluid if the
amount
of solar radiation transferred directly from the solar collector should
suddenly
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
decline due to clouds temporarily blocking direct sunshine from reaching the
collector. In this example, the first storage zone could be used as a quick
charge/discharge zone while the second zone, which would take longer to heat
to
maximum specific heat capacity, would be used to provide a constant rate of
heat
over a much longer period of time.
Some of the thermal storage component's physical parameters that may be
controlled in order to maintain the differences in the thermal storage zones'
thermal characteristics described above will now be addressed by comparing
thermal storage zones 26 and 28. As shown in Fig. 1, the length of the second
thermal storage zone is at least ten percent greater than the length of the
first
thermal storage zone. If the first and second zones are pipes that have the
same
inside diameter and are filled with the same randomly oriented ceramic media,
then the specific heat capacity of the second zone will be at least 10 percent
greater than the specific heat capacity of the first zone. Zones having
different
specific heats of capacity could also be accomplished by using different
thermal
storage media in zone 26 versus in zone 28. For example, if the geometric
surface
area per unit volume of the randomly oriented packing media in zone 28 was 20
m2/m3 and the geometric surface area per unit volume of the randomly oriented
packing media in zone 26 was 15 m2/m3, the geometric surface area per unit
volume of the randomly oriented packing media of the two zones would differ by
more than ten percent of the geometric surface area per unit volume of the
randomly oriented packing media of zone 26. Yet another way to achieve the
same objective is to construct the thermal storage component so that zone 28
includes a pipe which has a constant inside diameter that is at least 10
percent
greater than the diameter of the pipe in zone 26. If the lengths of the pipes
are the
same, the heat storage media in each pipe are the same and the diameters of
the
pipes differ by at least 10 percent, then the specific heat capacities of the
zones
must differ by at least 10 percent. Yet another way to achieve the same
objective
is to construct the thermal storage components so that the interior volume of
zone
28 is equal to the interior volume of zone 26 and the individual heat capacity
of
11
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
the thermal storage media in zone 28 exceeds the individual heat capacity of
the
thermal storage media in zone 26 by at least 10 percent.
While the basic concept of using a thermal storage component was
explained with reference to only two thermal storage zones, the same
principles
apply to a thermal storage component that includes three or more thermal
storage
zones. The existence of a third zone would provide even greater flexibility
and
control over the amount of heat provided by the thermal storage component. In
one embodiment of a three zone thermal storage component, the mass per unit
volume of thermal storage zone 30 could be made at least ten percent larger
than
the mass per unit volume of zone 28 which could be made at least ten percent
larger than the mass per unit volume of zone 26.
If desired, the thermal storage component could include more than three
zones. Each zone may be connected by a network of valves (not shown) and
pipes which function to deliver heat transfer fluid from the solar collector
to the
zones and may be referred to herein as fluid distribution system 34. Each of
the
zones may also be connected by a network of valves (not shown) and pipes which
function to collect and deliver the heat transfer fluid from the zones to the
generating station and may be referred to herein as fluid collection system
36.
Delivery pipe 21 allows the heat transfer fluid to flow directly from solar
collector
20 to the generating station. Return pipe 32 allows the heat transfer fluid
that
exits the generating station to return to the solar collector.
With reference to Fig. 1, thermal storage zones 26, 28 and 30 could be
three pipes, three vessels or a combination of pipes and vessels. If a pipe is
used
as a thermal storage zone, the pipe may have an inlet, an outlet and ceramic
heat
transfer media disposed within the pipe. The media could be randomly oriented
or secured within the pipe. An example of media secured to the pipe is one or
more ceramic tubes that have been inserted into and fit closely within the
pipe so
that the tube's smooth outer circumferential surface abuts the interior
surface of
the pipe and the tube's interior surface is grooved and contoured to
facilitate a
circular, turbulent flow of transfer fluid through the pipe. Fluid
distribution
12
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
system 34 may be connected to the pipe at the inlet and fluid collection
system 36
may be connected to the pipe at the outlet. The pipe may have a length to
width
ratio of at least 5:1 and a constant inside diameter between the inlet and
outlet.
As described above, thermal transfer zones in thermal storage components
may use heat transfer media, such as ceramic media, to reversibly store heat.
Ceramic media used in heat transfer applications may be designed to
accommodate one or more of the following design parameters. First, the thermal
conductivity of the ceramic material. Second, the thermal capacity of the
ceramic
material. Third, the thermal efficiency of the media. Fourth, pressure drop
across
a bed of the media. Depending upon the heat transfer application, the media
may
need to be modified to increase one thermal characteristic, such as thermal
conductivity. Unfortunately, the modification made to increase the thermal
conductivity may inherently and undesirably decrease another one of the
media's
thermal characteristics such as the media's thermal capacity. In a specific
application, such as a regenerative thermal oxidizer, media may be designed to
minimize pressure drop due to the high fluid flow through the oxidizer. In
contrast, the rate at which the thermal transfer fluid moves through a typical
solar
collector may be much slower than the rate of fluid flow through a thermal
oxidizer. Consequently, heat transfer media that have been designed for use in
a
regenerative thermal oxidizer may not be appropriate for use in a system that
captures and converts thermal radiation to electrical energy.
Shown in Fig. 2 is a second embodiment of a system for using solar
radiation to produce electricity. Part numbers in Fig.s 1 and 2 identify the
same
parts unless otherwise noted. Relative to Fig. 1, Fig. 2 has been modified by
connecting fluid collection system 36 to return pipe 32 via extension 38. As
indicated by arrow 40, extension 38 allows thermal transfer fluid in one or
more
of the thermal storage zones to flow back to collector 20 for additional
heating by
solar radiation before flowing to the generating station. This setup is
particularly
useful if the quantity of heat in a thermal storage zone has dropped below an
13
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
acceptable level and the temperature of the thermal transfer fluid needs to be
increased without using supplemental heat source 25.
Referring now to Fig. 3, there is shown a perspective view of a first
embodiment 60 of a ceramic packing element, also referred to herein as ceramic
media and heat transfer media useful in a system of this invention. This
particular
embodiment includes peripheral wall 62, first end face 64 and second end face
66.
The packing element may be manufactured as described in US 6,699,562 which
generally discloses the use of any suitable ceramic materials such as natural
or
synthetic clays, zeolites, cordiertes, aluminas, zirconia, silica or mixtures
of these.
The formulation can be mixed with bonding agents, extrusion aids, pore
formers,
lubricants and the like.
Shown in Fig. 4 are process steps for converting thermal radiation to
electrical energy. Step 100 represents receiving thermal radiation at a
variable rate
and converting the radiation to heat. Step 102 represents, within a twenty-
four
hour period, storing at least twenty-five percent of the heat in a thermal
storage
component that includes at least a first storage zone and a second storage
zone. In
step 104, the heat is transferred at a constant rate, measured as joules per
hour,
and a constant temperature to an electrical generating station. Within a
twenty-
four hour period of time, the heat may be received for at least 2 hours and no
more
than 18 hours. In many applications, much more than twenty-five percent of the
heat received may be stored in the thermal storage component. For example,
forty
percent, fifty percent or as much as seventy-five percent of the radiation
received
may be stored as heat in the thermal storage component. Heat may be
transferred
to the generating station for much more than two hours, such as eight, ten or
twelve hours in a single day. The heat transferred from the thermal storage
component may flow solely from the first thermal storage zone to the
generating
station and then solely from the second thermal storage zone to the generating
station or the heat may be transferred simultaneously from both zones.
The above description is considered that of particular embodiments only.
Modifications of the invention will occur to those skilled in the art and to
those
14
CA 02740431 2011-04-12
WO 2010/045130 PCT/US2009/060305
who make or use the invention. Therefore, it is understood that the
embodiments
shown in the drawings and described above are merely for illustrative purposes
and are not intended to limit the scope of the invention, which is defined by
the
following claims as interpreted according to the principles of patent law,
including
the Doctrine of Equivalents.
