Patent Publication Number: US-2005126172-A1

Title: Thermal storage unit and methods for using the same to heat a fluid

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part of U.S. patent application Ser. No. 10/738,825, filed Dec. 16, 2003. 
    
    
     BACKGROUND OF THE INVENTION  
      This invention relates to thermal storage units (TSUs). More particularly, this invention relates to TSUs that provide sensible heat thermal energy storage and delivery in a way that increases efficiency and reduces costs compared to known TSUs.  
      TSUs are well known and are often used in power delivery systems, such as compressed air storage (CAS) systems and thermal and compressed air storage (TACAS) systems. Such systems, often used to provide an available source of electrical power, often use compressed air to drive a turbine which powers an electrical generator.  
      In TACAS systems, it is desirable to heat the compressed air prior to reaching the inlet port of the turbine. It is known that heated air, as opposed to ambient or cool air, enables the turbine to operate more efficiently. Therefore, a mechanism or system is needed to heat the air before providing it to the turbine. One approach is to use a suitable type of fuel-combustion system. Another approach is to use a TSU. While fuel-combustion systems usually emit polluting gases, TSUs may be preferable over fuel-combustion systems at least because they are not associated with such harmful emissions.  
      Although TSUs may offer advantages over fuel-combustion systems, existing TSUs have several shortcomings, as discussed below.  
      One known configuration of a TSU is shown in  FIG. 1 . TSU  10  of  FIG. 1  includes heated parallel plates  12  contained within housing  14  to create channels through which compressed air may flow. The heat transfer area and the gap between plates  12  may be adjusted for optimum heat transfer conditions. Such a TSU, however, is not optimally suited for high pressure operation as these plates do not provide optimum pressure containment for the compressed air, and instead result in leakage flow between plates  12  and housing  14 .  
      Another known TSU uses tube flow through elongated cavities embedded in a solid medium. As shown in  FIG. 2 , compressed air travels through through-holes  22 , which are bored out of bar  24 . Although tube flow, as provided by TSU  20  of  FIG. 2 , may provide more desirable pressure containment compared to channel flow TSU  10  of  FIG. 1 , it involves high fabrication costs. This is because it is usually costly to drill a plurality of small-diameter holes that extend throughout the entire length of a solid medium.  
      Therefore, it can be seen that the TSUs shown in  FIGS. 1 and 2  fail to provide means for effectively containing and delivering hot and compressed air in a manner that is cost beneficial.  
      In view of the foregoing, it is an object of this invention to provide a low-cost TSU that provides efficient heat storage, heat delivery and pressure containment.  
     SUMMARY OF THE INVENTION  
      This and other objects of the present invention are accomplished in accordance with the principles of the present invention by providing a TSU having at least one flow channel disposed annularly about an axis that is substantially parallel to the TSU&#39;s longitudinal axis. The annular channel may be contained between an inner member and an outer member, both of which may include thermal mass or thermal storage material having desirable energy or heat storage properties and may be fabricated using standard mill products. The annular channel may be coupled to a port or pipe on each end of the channel for either providing fluid thereto or projecting fluid therefrom. In one embodiment of the present invention, the TSU may include a single annular flow channel disposed about the TSU&#39;s longitudinal axis. In another embodiment of the present invention, the TSU may include multiple parallel annular flow channels, each being contained between the outer member and a different inner member. In yet another embodiment of the present invention, the TSU may include multiple parallel annular flow channels that may be coupled to each other via transverse channels such that various fluid routing arrangements and piping connections are made possible within the TSU. The TSU&#39;s size and shape may be optimized for manufacturing, packaging, transporting and storing.  
      The inner and outer members of the TSU may be heated to effectively heat a fluid flowing through the annular channel. Efficient heat transfer is realized with the annular channel because the ring-like channel maximizes the surface area of fluid contact with the inner and outer members. The inner members may be centered, offset or tilted in order to achieve different heating profiles. Moreover, insulation may be added to the TSU to limit heat loss from the TSU.  
      In addition to providing energy storage and efficient heat transfer, the outer member provides structural support for the TSU, thereby enabling it to contain pressurized fluids. For example, the TSU may be used in a TACAS system whereby compressed air may be sensibly heated in the TSU. The heated and compressed air may then drive a turbine which powers an electrical generator to provide an electrical output. Features that enable single-ended or same-side piping and mounting to supporting structures may be added to the TSU to limit the effects of thermal growth and provide alternative methods to couple the TSU to the TACAS system. The TACAS system may also include an inert gas purging system that minimizes fouling in the TSU channel(s). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features of the present invention, its nature and various advantages will be more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:  
       FIG. 1  is a top perspective view of a known thermal storage unit;  
       FIG. 2  is a top perspective view of another known thermal storage unit;  
       FIG. 3  is a partial sectional view of a thermal storage unit in accordance with the principles of the present invention;  
       FIG. 4  is a cross-sectional view of the thermal storage unit of  FIG. 3 , taken generally from line  4 - 4  of  FIG. 3 ;  
       FIG. 5  is a cross-sectional view of the thermal storage unit of  FIG. 3 , taken generally from line  5 - 5  of  FIG. 3 ;  
       FIG. 6  is a partial perspective view of another thermal storage unit in accordance with the principles of the present invention;  
       FIG. 7  is a cross-sectional view of the thermal storage unit of  FIG. 6 , taken generally from line  7 - 7  of  FIG. 6 ;  
       FIG. 8  is a partial schematic diagram of a thermal and compressed air storage system employing a thermal storage unit in accordance with the principles of the present invention;  
       FIG. 9  is a top perspective view of yet another thermal storage unit in accordance with the principles of the present invention;  
       FIG. 10  is another top perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 11  is a diagram indicating a flow arrangement in accordance with the principles of the present invention;  
       FIG. 12  is a diagram indicating another flow arrangement in accordance with the principles of the present invention;  
       FIG. 13  is a diagram indicating yet another flow arrangement in accordance with the principles of the present invention;  
       FIG. 14  is a partial sectional view of a portion of the thermal storage unit of  FIG. 9 , taken generally from line  14 - 14  of  FIG. 9 ;  
       FIG. 15  is a cross-sectional view of the portion of the thermal storage unit of  FIG. 14 , taken generally from line  15 - 15  of  FIG. 14 ;  
       FIG. 16  is a partial sectional view of a portion of an alternative embodiment of the thermal storage unit of  FIG. 9 , taken generally from line  14 - 14  of  FIG. 9 ;  
       FIG. 17  is a cross-sectional view of the portion of the thermal storage unit of  FIG. 16 , taken generally from line  17 - 17  of  FIG. 16 ;  
       FIG. 18  is a partial sectional view of a portion of another alternative embodiment of the thermal storage unit of  FIG. 9 , taken generally from line  14 - 14  of  FIG. 9 ;  
       FIG. 19  is a cross-sectional view of the portion of the thermal storage unit of  FIG. 18 , taken generally from line  19 - 19  of  FIG. 18 ;  
       FIG. 20  is a cross-sectional view of the portion of the thermal storage unit of  FIG. 18 , taken generally from line  20 - 20  of  FIG. 18 ;  
       FIG. 21  is a cross-sectional view of the portion of the thermal storage unit of  FIG. 18 , taken generally from line  21 - 21  of  FIG. 18 ;  
       FIG. 22  is a cross-sectional view of the thermal storage unit of  FIG. 9 , taken generally from line  234 - 234  of  FIG. 9 ;  
       FIG. 23  is a cross-sectional view of an alternative embodiment of the thermal storage unit of  FIG. 9 , taken generally from line  234 - 234  of  FIG. 9 ;  
       FIG. 24  is a cross-sectional view of another alternative embodiment of the thermal storage unit of  FIG. 9 , taken generally from line  234 - 234  of  FIG. 9 ;  
       FIG. 25  is another cross-sectional view of the portion of the thermal storage unit of  FIG. 14 , taken generally from line  15 - 15  of  FIG. 14 ;  
       FIG. 26  is a partial sectional view of a portion of the thermal storage unit of  FIG. 9 , taken generally from line  26 - 26  of  FIG. 9 ;  
       FIG. 27  is a partial perspective view of a portion of another embodiment of the thermal storage unit of  FIG. 9 ;  
       FIG. 28  is a partial sectional view of a portion of the thermal storage unit of  FIG. 9 , taken generally from line  28 - 28  of  FIG. 9 ;  
       FIG. 29  is a perspective view of the thermal storage unit of  FIG. 3  or  FIG. 6  in accordance with the principles of the present invention;  
       FIG. 30  is a perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 31  is another perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 32  is yet another perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 33  is yet another perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 34  is yet another perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 35  is yet another perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 36  is yet another perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention;  
       FIG. 37  is yet another perspective view of the thermal storage unit of  FIG. 9  in accordance with the principles of the present invention; and  
       FIG. 38  is another partial schematic diagram of a thermal and compressed air storage system employing a thermal storage unit in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 3  depicts an embodiment of thermal storage unit (TSU)  100 , in accordance with the principles of the present invention. TSU  100  may be cylindrical in shape and may have longitudinal axis  150 . Persons skilled in the art will appreciate that the general shape of TSU  100  is not limited to cylinders and may be constructed to fulfill any design criteria.  
      TSU  100  may include three main compartments, namely, middle portion  110  and end portions  120 . Middle portion  110  may be defined as the portion of TSU  100  that is between lines  101 , whereas end portions  120  may be defined as the portions of TSU  100  that extend beyond lines  101  to both ends of TSU  100 . When fluid is applied to TSU  100 , it is directed into one of end portions  120 , flows through middle portion  110 , and is directed out of the other end portion  120 . Fluid may be matter in the liquid, gas or plasma phase.  
      When fluid is routed through middle portion  110 , it flows in a ring-like channel, which is referred to as annular flow channel  115 . Annular channel  115  may extend generally along middle portion  110 , between outer member  114  and inner member  112 . Annular channel  115  may extend along the length of middle portion  110 , in a direction that is substantially parallel to longitudinal axis  150 .  
       FIG. 4  shows a cross-sectional view taken along line  4 - 4  of  FIG. 3 . Annular channel  115  may have an inner diameter and an outer diameter. Inner diameter  116  and outer diameter  118  of  FIG. 4  define the cross-sectional area of annular channel  115 . The portion of inner member  112  contained in middle portion  110  may have a cylindrical outer surface, thereby providing a basis for inner diameter  116  of annular channel  115  (i.e. the diameter of inner member  112 ). Similarly, the inner surface of outer member  114 , which may be cylindrically shaped and which is contained in middle portion  110 , provides a basis for outer diameter  118  of annular channel  115  (i.e. the diameter of outer member  114 ). The length of a mean diameter (depicted by dotted line  117 ) of annular channel  115  may be calculated as the mean value of the length of inner and outer diameters  116  and  118 .  
      Referring back to  FIG. 3 , because inner member  112  extends partially into end portions  120 , and because outer member  114  extends through the entire length of TSU  100 , annular channel  115  may also partially extend into end portions  120 . Starting approximately at each end of middle portion  110 , mean diameter  117  of annular channel  115  may taper into end portion  120 , in a direction parallel to longitudinal axis  150 .  
      End portions  120 , which may be identical, may each include a hollow or tubular enclosure, namely, port  125 , within a portion of outer member  114  that extends into each of the end portions. Port  125  may be coupled to the portion of annular channel  115  that extends into the end portion for either providing fluid thereto or projecting fluid therefrom. In this arrangement, annular channel  115  may decrease in mean diameter from a point within TSU  100  (e.g., a point proximal to line  101 ) to the point on the end portion where port  125  couples to annular channel  115 . This arrangement enables fluid delivery to and from the TSU. Port  125  may be also seen in  FIG. 5 , which shows a cross-sectional view taken along line  5 - 5  of  FIG. 3 . Port  125  may be a tubular aperture (e.g., an inlet or outlet) for facilitating the delivery or projection of fluid to or from TSU  100 .  
      In a preferred embodiment of the present invention, inner member  112  may be constructed from solid material(s) that have adequate thermal conductivity and other desirable thermal properties such as high volumetric heat capacity. Outer member  114  may be constructed from the same material(s) as inner member  112 . Therefore, both inner and outer members  112  and  114  may provide thermal mass for energy storage. Alternatively, outer member  114  may be constructed from material(s) capable of withstanding high pressure, in addition to possessing desirable thermal properties. Such materials may include iron, steel, aluminum, any alloys thereof or any other suitable material(s).  
      According to the principles of the present invention, TSU  100  may be heated to a desired temperature by heating inner and outer members  112  and  114 . Fluid may then be heated by routing it through TSU  100  such that it enters one of ports  125  at one end, flows through annular channel  115 , and exits through port  125  at the opposite end.  
      Inner member  112  and/or outer member  114 , may be heated through radiation by means of an external or internal heater. For example, a ceramic fiber heater that annularly surrounds—without coming into contact with—TSU  100  may heat both inner and outer members  112  and  114  through radiation when actuated. Alternatively, one or more heating rods may be placed into one or more cavities extending through at least a portion of or the entire length of TSU  100 . When such heating rods are actuated, they radiate heat energy to heat both inner and outer members  112  and  114 .  
      Due to the thermal conductivity of the inner and outer members  112  and  114 , heat energy is effectively conducted through these members. Moreover, because annular channel  115  maximizes the surface area of fluid contact with the thermal mass in inner and outer members  112  and  114 , the fluid flowing in the channel may be sensibly heated through convection from inner member  112  and/or outer member  114  to the fluid. Accordingly, heating either member or both enables the efficient heating of the fluid flowing through the channel. Thus, when fluid having a predetermined temperature (e.g., ambient temperature) is supplied to TSU  100 , its temperature rises as it flows through annular channel  115  formed between inner and outer members  112  and  114 .  
      Persons skilled in the art will appreciate that electronic circuitry (not shown) may be used to monitor the temperature of TSU  100  and control the mechanism (e.g., the external ceramic heater or internal heating rods) used to heat TSU  100 . A more detailed discussion of such electronics is provided below in connection with  FIG. 8 .  
      An example of a fluid that may be routed through TSU  100  is compressed air. Compressed air may be heated using TSU  100 , as discussed above. Moreover, TSU  100  provides structural integrity against pressure exerted from the compressed air flowing in the channel. This is due to the fact that outer member  114 , which contains material capable of withstanding high pressure, cylindrically surrounds the annular channel, thereby containing the pressure exerted by the air on the outer member. Therefore, not only is TSU  100  adequate for providing heat storage, TSU  100  is conducive to high pressure operation, unlike the parallel-plate channel flow TSU  10  of  FIG. 1 .  
      Moreover, unlike drilling multiple small-diameter holes that extend through the entire length of a bar in order to implement tube flow as shown in connection with TSU  20  of  FIG. 2 , fabricating TSU  100  may be significantly easier and less costly. This is because TSU  100  may be fabricated using conventional mill products having cylindrical shapes such as pipes, tubes and round bars. For example, inner member  112  may be a round bar that is machined to achieve the desired diameter and profile.  
       FIG. 6  depicts an alternative embodiment of thermal storage unit (TSU)  200  that utilizes multiple annular flow channels, in accordance with the principles of the present invention. TSU  200  may be cylindrical in shape and may have longitudinal axis  250 . Persons skilled in the art will appreciate that the general shape of TSU  200  is not limited to cylinders and may be constructed to fulfill any design criteria.  
      Like TSU  100  of  FIG. 3 , TSU  200  may include three main compartments, namely, middle portion  210  and end portions  220 . End portions  220 , which may be identical, may each include a hollow or tubular enclosure, namely, port  225 , for either providing fluid to middle portion  210  or projecting fluid therefrom. When fluid is routed through middle portion  210 , it flows through multiple annular flow channels  215 . Annular flow channels  215  may be parallel to one another and may extend generally along middle portion  210 .  
      Each one of annular channels  215  may be disposed annularly about an axis that is substantially parallel to longitudinal axis  250 , such as axis  251 . Each annular channel  215  may be formed by drilling or casting a relatively large-diameter hole in a round bar, which may be referred to as outer member  214 , and inserting a smaller round bar, which may be referred to as inner member  212 , such that each inner member  212  extends at least along the length of middle portion  210 . Because the holes in outer member  214  are relatively large, at least compared to the holes bored through TSU  20  of  FIG. 2 , TSU  200  can be fabricated relatively easily using conventional mill products. Not only does TSU  200  benefit from ease of manufacturing, it also provides efficient energy storage, heat transfer and pressure containment consistent with that discussed above in connection with TSU  100  of  FIG. 1 .  
       FIG. 7  shows a cross-sectional view taken along line  7 - 7  of  FIG. 6 . Each one of annular channels  215  may be formed between the inner cylindrical surface of a hole in outer member  214  and the outer cylindrical surface of one of inner members  212 . Each inner cylindrical surface in outer member  214  provides a basis for outer diameter  218  in one of the annular channels, while each outer cylindrical surface of inner members  212  provides a basis for inner diameter  216  in the same annular flow channel. The length of a mean diameter (depicted by dotted line  217 ) of each annular channel  215  may be calculated as the mean value of the length of inner and outer diameters  216  and  218  for the annular channel.  
      In a preferred embodiment of the present invention, each mean diameter of annular channels  215  may be substantially equal in length. Moreover, inner and outer members  212  and  214  may be constructed from the same material as members  112  and  114  of TSU  100  of  FIG. 3 , and may be heated using the same means described for heating TSU  100 . Fluid may therefore be heated by routing it through heated TSU  200  such that it enters one of ports  225  at one end, flows through annular channel  215 , and exits through port  225  at the opposite end.  
      The present invention may be used in many applications.  FIG. 8  illustrates one such application. More specifically,  FIG. 8  shows a thermal and compressed air storage (TACAS) system  600  for providing output power utilizing TSU  100  of  FIG. 3 , described above. For example,  FIG. 8  may represent a backup energy system that provides backup power to a load in the event of a disturbance in the supply of power from another power source (e.g., utility power failure.) Naturally, TSU  200  of  FIG. 6  may be used instead of TSU  100  in TACAS system  600 .  
      The following discussion of TACAS system  600  is not intended to be a thorough explanation of the components of a TACAS, but rather an illustration of how TSU  100  or  200  can enhance the performance of a TACAS system. For a detailed description of a TACAS system, see commonly-assigned, co-pending U.S. patent application Ser. No. 10/361,728, filed Feb. 5, 2003, which is hereby incorporated by reference herein in its entirety.  
      As shown in  FIG. 8 , TACAS system  600  includes storage or pressure tank  623 , valve  632 , TSU  100 , electrical input  610 , turbine  641 , generator  642  and electrical output  650 . When electric power is needed from system  600 , compressed air from pressure tank  623  may be routed through valve  632  to TSU  100 . TSU  100  may heat the compressed air before it is provided to turbine  641 .  
      The hot air emerging from TSU  100  may flow against the turbine rotor (not shown) of turbine  641  and drive turbine  641 , which may be any suitable type of turbine system (e.g., a radial-flow turbine). In turn, turbine  641  may drive electrical generator  642 , which produces electric power and provides it to electrical output  650 .  
      Also shown in  FIG. 8  is turbine exhaust  643  (e.g., the exhaust gases emerging from turbine  641 ). Turbine exhaust  643  may be vented through an exhaust pipe (not shown), or simply released to recombine with atmospheric air.  
      Not only is system  600  advantageous because it uses a relatively inexpensive and efficient TSU, it is also non-polluting. That is because, unlike conventional systems that use fuel-combustion systems to provide hot air to the turbine, it does not require a fuel supply to heat the air that is being supplied to turbine  641 . Instead, TSU  100  may be powered by electrical input  610 , which provides the energy needed to heat the compressed air, while providing effective pressure containment. For example, TSU  100  may include an external or internal radiant heater, as discussed above, which may be powered by electrical input  610 . System  600  therefore provides the benefits of heating compressed air from pressure tank  623  before it is supplied to turbine  641 , without producing the harmful emissions associated with combustion systems.  
      It will also be understood by persons skilled in the art that, alternatively, the thermal storage material of TSU  100  may be heated by any other suitable type of heating system. For example, a resistive heater may provide a heat source that is in physical contact with the thermal storage material of TSU  100  and may heat this material to a predetermined temperature. Alternatively, electrically conductive thermal storage materials, such as iron, may be heated inductively using induction heating circuitry that causes current to circulate through and heat the thermal storage material of TSU  100 . Thus, the invention is not limited to the specific heating manners discussed above.  
      TACAS system  600  may also include control circuitry  620  which may be coupled to both TSU  100  and electrical input  610 . Control circuitry  620  may include means for measuring the temperature of TSU  100 . Control circuitry  620  may also include electric circuitry for controlling the temperature of TSU  100 . Control circuitry  620  may control the temperature of TSU  100  by, for example, controlling the electric power provided to the heat source. This may be achieved by providing instructions to electrical input  610 , such as instructions to activate, deactivate, increase or decrease the output of electrical input  610 . Control circuitry  620 , along with electrical input  610 , may therefore be used to monitor and control the temperature of TSU  100 . As a result, the TSU  100  may be heated to and maintained at a desired temperature.  
      Moreover, valve  632  may be coupled to piping (not shown) that bypasses TSU  100  and feeds into turbine  641  along with the output from TSU  100 . By controlling the portion of the total compressed air flow through the TSU, the ratio of heated to non-heated air provided to turbine  641  may be modified, thereby providing another means for controlling the temperature of the air being supplied to the turbine.  
      Another advantage of utilizing TSU  100  is that larger pressure tanks are not required as is the case with compressed air storage systems that do not utilize thermal storage units or combustion systems.  
       FIGS. 9 and 10  depict an alternative embodiment of thermal storage unit (TSU)  300  that also utilizes multiple annular flow channels, in accordance with the principles of the present invention. TSU  300  may be rectangular in shape and may have longitudinal axis  350 . The rectangular shaped body of TSU  300  allows for optimized packaging, transporting and storing. Persons skilled in the art will appreciate that the general shape of TSU  300  is not limited to the particular shape depicted in  FIGS. 9 and 10  and may be constructed to fulfill any design criteria.  
      Like TSU  200  of  FIG. 6 , TSU  300  may include multiple annular flow channels  315  through which fluid may flow. Annular channels  315  may be parallel to one another. Each one of annular channels  315  may be disposed annularly about an axis that is substantially parallel to longitudinal axis  350 , such as axis  351 . Each one of annular channels  315  may extend generally along the length of TSU  300 , between outer member  314  and one inner member  312 .  
      TSU  300  may also include one or more transverse flow channels formed by transverse members  330 , through which fluid may flow. Two adjacent annular channels  315  may be coupled through transverse member  330  close to one end of TSU  300 . One of these two annular channels may be coupled to a different annular channel  315  that is adjacent to it through another transverse member  330  away from the first end and/or close to the other end of TSU  300 . Transverse members need not extend through the entire width of TSU  300 , so long as each transverse member couples two adjacent annular channels to each other such that they are in fluidic communication with one another.  
      Two transverse members  330  may be disposed close to a first end of TSU  300  wherein one transverse member  300  may couple a set of annular channels  315  that is different than the set of annular channels  315  coupled by the other member. At least one transverse member  330  may be disposed close to the other end of TSU  300  for coupling a set of annular channels  315  that is different than the sets coupled by the members disposed close to the first end. The sets of annular channels coupled by two members disposed close to the same end of TSU  300  preferably do not have an annular channel in common. Moreover, the sets of annular channels coupled by two members disposed close to opposite ends of TSU  300  preferably do have one annular channel in common. Furthermore, the two members disposed close to the same end preferably are parallel to each other and perpendicular to the at least one member disposed close to the other end.  
      The foregoing arrangement of annular channels  315  and transverse members  330  allows for much flexibility in routing of fluid through TSU  300 . For example, fluid may be routed through one or more annular channels simultaneously via different paths. Such an arrangement also enables fluid to be supplied to and retrieved from the same end of TSU  300 , provided the fluid flows through an even number of annular channels. Examples of different fluid flow arrangements are depicted in  FIGS. 11-13 .  
       FIG. 11  illustrates a two-pass/two-path flow arrangement that may be implemented through TSU  300  of  FIGS. 9 and 10 . In this arrangement fluid may be supplied to channel  315   a  and may then flow along two different paths: 1) through channel  315   a  then channel  315   b , and 2) through channel  315   d  then channel  315   c , before exiting through channel  315   c . Such a flow arrangement requires four transverse members  330  for coupling each channel to one that is adjacent to it.  
       FIG. 12  illustrates a four-pass/one-path flow arrangement that may be implemented through TSU  300  as well. In this arrangement fluid may be supplied to channel  315   a  and may then flow along a single path through channel  315   a , then channel  315   b , then channel  315   c , then channel  315   d , before exiting through channel  315   d . Unlike the flow arrangement discussed in connection with  FIG. 11 , such a flow arrangement requires only three transverse members  330 . Therefore, one of the transverse members  330 —namely, the member coupling channels  315   a  and  315   d  together—preferably is omitted when fabricating TSU  300  according to an alternative embodiment of the present invention.  
      Similarly, several transverse members may be omitted when fabricating TSU  300  according to another alternative embodiment of the present invention. Specifically, multiple TSUs  300  may be coupled together in order to provide even longer flow paths. For example,  FIG. 13  illustrates a four-pass/two-path flow arrangement where fluid may flow along two different paths, each of which involving at least four different annular channels through which fluid may flow. Alternatively, the flow arrangement of  FIG. 13  may be implemented through a single TSU having multiple sets of annular channels that are disposed next to each other and coupled through an appropriate number of transverse members. In this situation, a single transverse member may or may not couple multiple annular channels to one another.  
      The foregoing flow arrangements are only illustrative of the principles of the present invention. Various modifications and fluid routing arrangements can be implemented by those skilled in the art without departing from the scope and spirit of the invention.  
      In a preferred embodiment of the present invention, inner and outer members  312  and  314  may be constructed from the same material as members  112 ,  212 ,  114  and  214  of TSUs  100  and  200  of  FIGS. 3 and 6 , and may be heated using the same means described for heating TSU  100 . Fluid may therefore be heated by routing it through heated TSU  300 .  
      Because TSU  300  utilizes annular flow channels, it provides efficient energy storage, heat transfer and pressure containment consistent with that discussed above in connection with TSU  100 . Unlike TSUs  100  and  200  of  FIGS. 3 and 6 , which allow for only single-pass flow arrangements, TSU  300  allows for various fluid routing arrangements. Having fluid undergo multiple passes and/or multiple paths increases the length of fluid flow and flow area in the TSU, thereby allowing for retrieving more thermal energy in and from the TSU. However, this may also lead to an increase in pressure drop across the TSU. Nevertheless, TSU  300  allows for much flexibility in choosing a flow arrangement that balances flow length and flow area with pressure drop in order to optimize thermal energy transfer and pressure conditions.  
      Moreover, similar to TSUs  100  and  200 , TSU  300  can be fabricated relatively easily without incurring severe costs, as compared to drilling multiple small-diameter holes that extend through the entire length of a bar in order to implement tube flow as shown in connection with TSU  20  of  FIG. 2 . This is because TSU  300  may be fabricated using conventional mill products having rectangular or cylindrical shapes such as pipes, tubes and round bars. TSU  300  may be fabricated by machining, drilling, extruding or casting a block of solid material or an integrated manifold to form outer member  314  having longitudinal holes that are parallel to axis  350 . The longitudinal holes preferably have inner cylindrical surfaces and need not extend through the entire length of the manifold. Transverse members  330  may be formed by machining, drilling or casting parallel transverse holes connecting two adjacent longitudinal holes. The transverse holes need not extend through the entire width of the manifold so long as each transverse member  330  couples two adjacent longitudinal holes. Inner members  312 , which may be round bars, rods or any elongated solid structures having outer cylindrical surfaces, may then be inserted in the longitudinal holes, thereby forming annular channels  315 . For a particular annular channel, the diameter of the outer cylindrical surface is smaller than that of the inner cylindrical surface. Inner member  312  need not extend through the entire length of the manifold. Transverse members  330  also form flow channels through which fluid may flow from one annular channel  315  to another. Appropriate welded or threaded plugs may be used to seal drilled entry points and ends, or other entry points and ends. One or more holes may be drilled in these plugs, thereby forming an inlet and/or an outlet for a particular annular channel, as shown in  FIG. 28 , or for a transverse flow channel. Piping connections, such as the ones shown in  FIGS. 30-34  may be coupled to TSU  300  at such inlets and outlets for either providing fluid thereto or projecting fluid therefrom.  
       FIG. 14  is a partial sectional view of a portion of TSU  300 , taken generally from line  14 - 14  of  FIG. 9 .  FIG. 15  is a cross-sectional view of the portion of  FIG. 14 , taken generally from line  15 - 15  of  FIG. 14 . As can be seen, inner member  312  is positioned such that its longitudinal axis  351  does not intersect outer member  314  and such that inner and outer members  312  and  314  are not in contact with one another. In this embodiment, longitudinal axis  351  may be substantially concentric to the axis of the longitudinal hole in which inner member  312  in inserted. It may be said that inner member  312  is centered according to this embodiment of the present invention.  
      Centering inner members  312 , as shown in  FIG. 22 , which is a cross-sectional view of TSU  300 , taken generally from line  234 - 234  of  FIG. 9 , produces a uniform fluid velocity profile around axis  351 . Heat transfer at the surface of centered inner members  312  and outer member  314  may result in heat flow along lines  2202  and  2204 , respectively in these members. This, in turn, results in circumferentially uniform heat transfer from TSU  300  to the fluid flowing through it, thereby maximizing total thermal energy extraction from the thermal mass in inner and/or outer members  312  and  314 . As a result, less material is required in the TSU. Therefore, providing uniform heat transfer in multiple-pass/multiple-channel units allows for reducing the size of the TSU.  
      In alternative embodiments of the present invention, inner member  312  may be positioned as shown in  FIGS. 16 and 18 .  
       FIG. 16  is a partial sectional view of a portion of an alternative embodiment of TSU  300 , taken generally from line  14 - 14  of  FIG. 9 .  FIG. 17  is a cross-sectional view of the portion of  FIG. 16 , taken generally from line  17 - 17  of  FIG. 16 . As can be seen, inner member  312  is displaced from the center and positioned such that its longitudinal axis  351  does not intersect outer member  314  and such that inner member  312  comes in contact with outer member  314 . In this embodiment, longitudinal axis  351  may be parallel to the axis of the longitudinal hole in which inner member  312  in inserted. It may be said that inner member  312  is offset according to this embodiment of the present invention.  
       FIG. 18  is a partial sectional view of a portion of another alternative embodiment of TSU  300 , taken generally from line  14 - 14  of  FIG. 9 .  FIG. 19  is a cross-sectional view of the portion of  FIG. 18 , taken generally from line  19 - 19  of  FIG. 18 , while  FIG. 20  is a cross-sectional view of the portion of  FIG. 18 , taken generally from line  20 - 20  of  FIG. 18 , and  FIG. 21  is a cross-sectional view of the portion of  FIG. 18 , taken generally from line  21 - 21  of  FIG. 18 . As can be seen, inner member  312  is displaced from the center and positioned such that its longitudinal axis  351  intersects outer member  314 . More specifically, inner member  312  may be positioned such that it is centered close to the middle portion of annular channel  315 , while inner member  312  may come in contact with outer member  314  at or close to the ends of annular channel  315 . In this embodiment, longitudinal axis  351  may also intersect the axis of the longitudinal hole in which inner member  312  in inserted. It may be said that inner member  312  is tilted according to this embodiment of the present invention.  
      Positioning inner members  312 , as shown in  FIGS. 23 and 24 , produces non-uniform fluid velocity profiles around axis  351 . Heat transfer at the surface of displaced inner members  312  and outer member  314  may result in heat flow along lines  2302  and  2402  in the inner members shown in  FIGS. 23 and 24 , respectively. Similarly, heat transfer at the surface of outer member  314  may result in heat flow along lines  2304  and  2404  in the outer members shown in  FIGS. 23 and 24 , respectively. This, in turn, results in non-uniform heat transfer from TSU  300  to the fluid flowing through it.  FIG. 23  is a cross-sectional view of an alternative embodiment of TSU  300 , taken generally from line  234 - 234  of  FIG. 9 , whereby inner members  312  are offset towards axis  350 . Such an arrangement maximizes total thermal energy extraction from the extremities of the TSU.  FIG. 24  is a cross-sectional view of another alternative embodiment of TSU  300 , taken generally from line  234 - 234  of  FIG. 9 , whereby inner members  312  are offset away from axis  350 . Such an arrangement maximizes total thermal energy extraction from the center of the TSU.  
      Centering, offsetting or tilting inner member  312  as shown in  FIGS. 14-21  may be achieved by adding features such as the ones shown in  FIG. 25 , which is another cross-sectional view of the portion of  FIG. 14 , taken generally from line  15 - 15  of  FIG. 14 . Such features may be added to either the outer surface of inner member  312  or the inner surface of outer member  314  at desired location(s) along axis  351 , in order to control the radial position of inner member  312  with respect to outer member  314 . Such features may include inserting threaded studs  2502 , such as set screws, pressed pins  2504 , weld beads  2506  or any other appropriate fixture. Adding such features may help maintain the position of inner member  314  within outer member  314  and reduce variability among different TSUs.  
      Other features, such as the ones shown in  FIGS. 26-28 , may be added to prevent or reduce fluid flow blockage at the inlet or outlet of an annular channel or at the various transverse flow channels in TSU  300 . Reducing fluid flow blockage reduces flow and pressure variations that may result from inner members  312  potentially coming in direct contact with different portions of TSU  300 . This may be due to positioning of inner member  312  or due to movement of inner member  312  in a direction along or parallel to axis  351  prior to or during fluid flow.  
      The features shown in  FIGS. 26 and 27  may be provided to prevent fluid flow blockage resulting from inner member  312  coming in contact with outer member  314  close to one end of TSU  300 . In certain embodiments, as described above, annular channel  315  may be formed by drilling a hole through a manifold forming outer member  314  and inserting inner member  312  therein. As can be seen in  FIGS. 26 and 27 , the end of outer member  314  may include conical surface  370 , which may have a conical shape resulting from such drilling. The features shown in  FIG. 28  may be added to prevent fluid flow blockage resulting from inner member  312  coming in contact with drilled plug  2802  at another end of TSU  300 . This end of TSU  300  may correspond to the entry point of drilling of the manifold in order to form the hole through which inner member  312  may be inserted.  
       FIG. 26  is a partial sectional view of a portion of TSU  300 , taken generally from line  26 - 26  of  FIG. 9 .  FIG. 26  shows elements  2602 , which may include a bead or pin that is mounted, or a tab that is welded, to inner member  312  or outer member  314 .  FIG. 27  is a partial perspective view of a portion of another embodiment of TSU  300 .  FIG. 27  shows features which may be realized by cutting away or removing portions  2702  from inner member  312 . Portions  2702  may be circular segments defined by each of parallel chords  2704 . Accordingly, inner member  312  may locally rest on outer member  314  at surface  370 .  
       FIG. 28  is a partial sectional view of a portion of TSU  300 , taken generally from line  28 - 28  of  FIG. 9 .  FIG. 28  shows features which may include welding tab  2804  to inner member  312  or outer member  314 , or may include mounting pin  2806  on inner member  312  or on drilled plug  2802 , or adding any other appropriate fixture. Accordingly, adding features such as the ones depicted in  FIGS. 25, 26  and  28  may restrict movement of inner member  312  in a direction transverse to or parallel to longitudinal axis  351 .  
      Alternatively, features to prevent fluid flow blockage may include machining, forging or welding the ends of inner member  312 , or using any other appropriate method in order to maintain a gap between inner members  312  and outer member  314  or drilled plug  2802  discussed in connection with  FIGS. 26-28 . Preferably, none of the aforementioned features annularly surround inner member  312 . Instead, these features preferably create more gaps allowing fluid to flow more freely than it would had such features not been included.  
      As discussed above, TSU  300  allows for various fluid routing arrangements that enable retrieving fluid from the same end it is supplied. Allowing for various fluid routing arrangements also provides more flexibility in choosing optimum piping connections to TSU  300  in order to supply and retrieve fluid to and from the TSU. This aspect is further enhanced by the existence of transverse members  330 , which allow for making piping connections to the side of TSU  300 . Examples of such piping configurations are illustrated in  FIGS. 30-34 .  
       FIG. 29  illustrates pipes  2902  and  2904  being mounted to opposite ends of TSU  100  or TSU  200  of  FIGS. 3 and 6  for delivering and retrieving fluid to and from the TSU. Because each of TSU  100  and TSU  200  allows for a single-pass fluid flow arrangement, fluid may not be retrieved from the same end it is supplied. In contrast,  FIGS. 30-34  illustrate piping connections being mounted to the same end and/or to the side(s) of TSU  300 , due to the mutli-pass arrangements made possible by the way TSU  300  is designed.  
      For example,  FIG. 30  illustrates a piping arrangement whereby pipes  3002  and  3004  may be mounted to one end of TSU  300 , such that they are coupled to diagonally-spaced (non-adjacent) annular channels. This piping arrangement may be suitable for the two-pass/two-path fluid flow arrangement of  FIG. 11 . More specifically, inlet pipe  3002  of  FIG. 30  may be coupled to channel  315   a  of  FIG. 11 , for supplying fluid to TSU  300 , whereas outlet pipe  3004  of  FIG. 30  may be coupled to channel  315   c  of  FIG. 11 , for retrieving fluid from TSU  300 .  
      As another example,  FIG. 31  illustrates a piping arrangement whereby pipes  3102  and  3104  may be mounted to one end of TSU  300 , such that they are coupled to adjacent annular channels. This piping arrangement may be suitable for the four-pass/one-path fluid flow arrangement of  FIG. 12 . More specifically, inlet pipe  3102  of  FIG. 31  may be coupled to channel  315   a  of  FIG. 12 , for supplying fluid to TSU  300 , whereas outlet pipe  3104  of  FIG. 31  may be coupled to channel  315   d  of  FIG. 12 , for retrieving fluid from TSU  300 .  
      An alternative piping arrangement to that of  FIG. 31  may also be suitable for a two-pass/two-path fluid flow arrangement similar to the one shown in  FIG. 11 . More specifically, inlet pipe  3102  of  FIG. 31  may be coupled to channel  315   a  of  FIG. 11 , for supplying fluid to TSU  300 , whereas outlet pipe  3104  of  FIG. 31  may be coupled to channel  315   b  of  FIG. 11 , for retrieving fluid from TSU  300 . By coupling outlet pipe  3104  to channel  315   b  instead of  315   c , the fluid flow arrangement of  FIG. 11  may be modified such that fluid flowing through channel  315   c  may subsequently flow through one transverse member towards channel  315   b , as opposed to having fluid from channel  315   b  be directed through that same transverse member towards channel  315   c.    
      As yet another example,  FIG. 32  illustrates a piping arrangement whereby pipe  3202  may be mounted to one end of TSU  300  and pipe  3204  may be mounted to one side of TSU  300 , such that they are coupled to diagonally-spaced annular channels. This piping arrangement may also be suitable for the two-pass/two-path fluid flow arrangement of  FIG. 11 . More specifically, inlet pipe  3202  of  FIG. 32  may be coupled to channel  315   a  of  FIG. 11 , for supplying fluid to TSU  300 , whereas outlet pipe  3204  of  FIG. 32  may be coupled to channel  315   c  of  FIG. 11  for retrieving fluid from TSU  300 . Alternatively, inlet pipe  3202  may be mounted to the other side of TSU  300  and coupled to channel  315   a . Also, outlet pipe  3204  may be mounted to the same end of TSU  300  and coupled to channel  315   c.    
      As yet another example,  FIG. 33  illustrates a piping arrangement whereby pipe  3302  may be mounted to one end of TSU  300  and pipe  3304  may be mounted to one side of TSU  300 , such that they are coupled to adjacent annular channels. This piping arrangement may also be suitable for the four-pass/one-path fluid flow arrangement of  FIG. 12 . More specifically, inlet pipe  3302  of  FIG. 33  may be coupled to channel  315   a  of  FIG. 12 , for supplying fluid to TSU  300 , whereas outlet pipe  3304  of  FIG. 33  may be coupled to channel  315   d  of  FIG. 12 , for retrieving fluid from TSU  300 . Alternatively, inlet pipe  3302  may be mounted to the other side of TSU  300  and coupled to channel  315   a . Also, outlet pipe  3304  may be mounted to the same end of TSU  300  and coupled to channel  315   d.    
      An alternative piping arrangement to that of  FIG. 33  may also be suitable for a two-pass/two-path fluid flow arrangement similar to the one shown in  FIG. 11 . More specifically, inlet pipe  3302  of  FIG. 33  may be coupled to channel  315   a  of  FIG. 11 , for supplying fluid to TSU  300 , whereas outlet pipe  3304  of  FIG. 33  may be coupled to channel  315   b  of  FIG. 11  through a transverse member  330 , for retrieving fluid from TSU  300 . By coupling outlet pipe  3304  to channel  315   b  instead of channel  315   c , the fluid flow arrangement of  FIG. 11  may be modified such that fluid flowing through channel  315   c  may subsequently flow through one transverse member towards channel  315   b , as opposed to having fluid from channel  315   b  be directed through that same transverse member towards channel  315   c.    
      As yet another example,  FIG. 34  illustrates a piping arrangement whereby pipe  3402  may be mounted to one side of TSU  300  and pipe  3404  may be mounted to the same side of TSU  300 , such that they are coupled to adjacent annular channels. This piping arrangement may also be suitable for a two-pass/two-path fluid flow arrangement similar to the one shown in  FIG. 11 . More specifically, inlet pipe  3402  of  FIG. 34  may be coupled to channel  315   a  of  FIG. 11 , for supplying fluid to TSU  300 , whereas outlet pipe  3404  of  FIG. 34  may be coupled to channel  315   b  of  FIG. 11 , for retrieving fluid from TSU  300 . By coupling outlet pipe  3404  to channel  315   b  instead of channel  315   c , the fluid flow arrangement of  FIG. 11  may be modified such that fluid flowing through channel  315   c  may subsequently flow through one transverse member towards channel  315   b , as opposed to having fluid from channel  315   b  be directed through that same transverse member towards channel  315   c.    
      As illustrated by some of the examples discussed in connection with  FIGS. 30-34 , any combination of end and side piping connections may be used depending on the desired fluid routing arrangement and the manner by which TSU  300  is to be mounted, installed or oriented in a thermal and compressed air storage system or other system. The foregoing piping arrangements are only illustrative of the principles of the present invention. Various modifications and piping arrangements can be implemented by those skilled in the art without departing from the scope and spirit of the invention.  
      Like TSUs  100  and  200 , TSU  300  may be heated to a desired temperature by heating inner and/or outer members  312  and  314  through radiation, by placing one or more heating rods into one or more cavities extending through at least a portion of or the entire length or width of TSU  300 , or by any method or apparatus disclosed in U.S. patent application Ser. No. ______, filed Sep. ______, 2004 (Attorney Docket No. AP-53), which is hereby incorporated by reference herein in its entirety, or by any other suitable method or apparatus.  
      Heating a TSU, especially for extended time periods at elevated temperatures, may result in dissipation of thermal energy through its walls in the form of heat. To counter such heat loss, insulating material may be coupled to the outer surface of the unit, as can be seen in  FIG. 37 .  
      For example, insulating material fabricated in single or multiple layers with stepped interfaces between adjacent faces may be used. Effective insulation material would not corrode or degrade significantly over time. Preferably, such material may be inert, such as ceramic microporous material. Moreover, such material may be in either pressed rigid board form or fabric-coated stitched form, having thermal conductivities ranging from 0.023 W/m-K to 0.050 W/m-K. The thickness of such high efficiency insulation material need not be considerable to attain a substantial reduction in heat loss. Therefore, not only does a TSU surrounded by or coated with such material benefit from a reduction in heat loss, which reduces the cost of maintaining a TSU, using such material does not result in a significant increase in the volume of the unit. This, in turn, does not substantially affect packaging, transporting and storing of the unit negatively.  
      Heating a TSU may also lead to thermal growth due to the expansion of the materials used to fabricate it. This may have harmful effects on the system in which the unit is used. For example, axial expansion of a unit having piping connections on opposite ends to a thermal storage system may strain or stress and therefore fracture such piping connections.  
      One way to avoid such effects is to design the TSU such that piping connections to other portions of the system need not be made on opposite ends of the unit. As evidenced by the discussion relating to  FIGS. 30-34 , TSU  300  exhibits such characteristics. If the inlet and outlet pipes are coupled to the same end, the opposite end may remain unsecured, thereby allowing for unrestrained growth of the TSU that does not impact the inlet and outlet pipes nor does it impact the system in which it is used.  
      Moreover, features, such as the ones illustrated in  FIGS. 35-36  may be added to the TSU to help limit the impact of thermal expansion of the piping connections coupled to the TSU. Such features may include coupling threaded rods  3510  to TSU  300 , as shown in  FIG. 35 . A pair of threaded rods  3510  may be coupled to beam  3512 . Beam  3512  may include an “I” section, a “C” channel section, a rectangular section or an angle section for mounting TSU  300  to a support structure. The other pair of threaded rods  3510  may also be coupled to another beam for mounting in order to provide further support for TSU  300 . Alternatively, other features may be used, such as welding tabs  3610  to TSU  300 , as shown in  FIG. 36 . A pair of tabs  3610  may be coupled to bushings  3614  which are coupled to beam  3612 . Beam  3612  may include an “I” section, a “C” channel section, a rectangular section or an angle section for mounting TSU  300  to a support structure. The other pair of tabs  3610  may be coupled to bushings and another beam for mounting in order to provide further support for TSU  300 .  
      TSU  300  may be mounted to a support structure in order to minimize the impact of thermal growth of outlet pipe  3604  which may heat substantially. By mounting TSU  300  to a support structure close to the end of TSU  300  where the inlet and outlet pipes  3602  and  3604  are situated, the outlet pipe may grow in a direction away from the TSU and opposite to the direction of growth of the TSU, thereby allowing for unrestrained growth of the outlet pipe that does not impact TSU  300 . TSU  300  may be coupled to the support structure through any of fixtures depicted in  FIGS. 35 and 36  or through any other appropriate fixtures. The support structure may be coupled to TSU  300  and sized to meet load requirements at elevated temperatures. Such a support structure may be integrated into the frame of the system the TSU is used in or may be self-supporting.  
       FIG. 37  shows self-supporting structure  3700 . Using a self-supporting structure may facilitate fabricating the unit and treating it as a separate component for packaging purposes. A self-supporting design also provides: a structure to support the insulation  3708  coupled to TSU  300 , an envelope, such as metallic sheets  3710 , to protect insulation  3708  from incidental damage, and a mounting surface for any heating rods that may be placed into apertures  3720  for heating TSU  300 . Self-support structure  3700  may also include beams  3712  for optionally mounting TSU  300  to another supporting structure for further support. Shipping restraints (not shown) may be integrated in the base of the self-supporting design in order to limit the movement of the unit during pre-installation handling of the assembled unit and shipment of a completed system, such as a thermal and compressed air storage (TACAS) system, in which the unit may be used.  
       FIG. 38  shows TACAS system  3800  for providing output power utilizing TSU  300  of  FIGS. 9 and 10 , described above. TSU  300  may be mounted or installed in TACAS system  3800 . For example, TSU  300  may be coupled to a support structure that may be integrated into TACAS system  3800  or that may be self-supporting, as shown in  FIG. 37 . Similar to  FIG. 8 ,  FIG. 38  may represent a backup energy system that provides backup power to a load in the event of a disturbance in the supply of power from another power source.  
      Similar to TACAS system  600  of  FIG. 8 , TACAS system  3800  includes storage or pressure tank  623 , valve  632 , TSU  300 , electrical input  610 , turbine  641 , which is coupled to turbine exhaust  643 , generator  642  and electrical output  650 . When electric power is needed from system  3800 , compressed air from pressure tank  623  may be routed through valve  632  to TSU  300 . TSU  300  may heat the compressed air before providing it to turbine  641 . TSU  300  may be heated, and its temperature controlled, by any method discussed in connection with  FIG. 8 . When driven by the hot air supplied to it, turbine  641  may drive electrical generator  642 , which produces electric power and provides it to electrical output  650 .  
      TACAS system  3800  benefits from the advantages discussed in connection with TACAS system  600 . Moreover, because TSU  300  is coupled to other components of TACAS system  3800  such that piping connections to the other components are made on the same end of TSU  300 , the opposite end of TSU  300  may remain unsecured. As described aove, this limits or avoids the negative impact of thermal expansion on the piping connections and on TACAS system  3800 .  
      TACAS system  3800  also includes gas tank  3823  and valve  3832 . Gas tank  3823  may contain a pressurized inert purge gas. After electric power is needed from system  3800 , and compressed air from pressure tank  623  is routed to TSU  300 , heated and provided to turbine  641 , valve  3832  may be actuated such that the inert gas is routed from gas tank  3823  to TSU  300 . The inert gas may be used to purge and replace some or all of the air present in TSU  300 . Valve  3832  may be used to control the portion of inert gas supplied to TSU  300 , thereby modifying the ratio of inert gas to air in TSU  300 . Preferably, gas tank  3823  may contain an inert gas that is heavier than air, such as argon. TSU  300  may be oriented such that the piping connections to other components of TACAS system  3800  are above the body of TSU  300 , thereby allowing gravity to assist the purge gas to displace the air in TSU  300  and remain in TSU  300 .  
      The benefit of using an inert gas to purge the air in TSU  300  is the prevention of oxidation that could cause fouling of the flow channels in the TSU. Fouling refers to the build-up of an oxide residue layer on the internal surfaces of the flow channels, thereby partially or completely blocking the flow channels and degrading performance. High temperature operation increases the rate of oxidation in the flow channels. Accordingly, fouling may be prevented by lowering the operating temperature which would require a larger TSU for the system to operate efficiently. This, in turn, may lead to increased manufacturing, packaging, transporting and storing costs relating to the TSU. Alternatively, occasionally substituting an inert gas for air in a TSU reduces the rate of oxidation in the TSU&#39;s flow channels. Therefore, implementing a purge gas system in a thermal and compressed air system, or any other system that utilizes a TSU, may be a more desirable alternative.  
      The present invention was presented in the context of industrial backup utility power. The present invention may be alternatively implemented in a continuously operating electrical generation system such as a continuously operating TACAS system. Such a system may use a compressor that constantly provides compressed air to a TSU that may be continuously heated to ensure that the air is sensibly heated before being provided to a constantly operating turbine.  
      Alternatively, the present invention may be used in any application associated with generating power, such as in thermal and solar electric plants. Furthermore, the present invention may be used in any other application where thermal storage, fluid heating or heated fluid delivery may be desirable.  
      The above described embodiments of the present invention are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.