Sensible heat storage unit

A sensibe heat storage unit is provided which has a "step function" thermal gradient, and is constructed so as to have alternated regions of different thermal conductivity along the flow path through the unit such that, in one embodiment, there are spaced elements within the storage unit having transverse conductivities higher than the conductivity of the material in the spaces therebetween. This permits discharge of the storage unit at a uniform temperature until the storage unit is emptied.

FIELD OF INVENTION 
This invention relates to energy conversion systems and more particularly 
either to an improved sensible heat storage unit which can be discharged 
at a uniform temperature. 
BACKGROUND OF THE INVENTION 
While the subject invention relates to an improved sensible heat storage 
unit, it has particular utility in solar energy conversion systems, and 
especially those which use Brayton cycle engines. It is therefore useful 
to describe the solar energy conversion system for which the subject unit 
was designed. Although designed for a special type of solar energy 
conversion system, the subject storage unit may be utilized in any 
application where constant discharge temperatures are required. 
With respect to solar energy conversion systems, while "latent" heat 
switchable storage units have been utilized in steam cycle solar energy 
conversion systems as illustrated in U.S. Pat. No. 2,933,855 issued to E. 
K. Benedek et al., Apr. 26, 1960, the benefits of a "sensible" heat 
storage system have heretofore not been utilized with a Brayton cycle 
system involving a gas cycle turbine. 
One of the major problems with solar energy conversion systems utilizing 
steam is, in general, the extremely corrosive nature of superheated steam 
and the upper temperature limit associated with the tubing or plumbing 
used. In general, it is possible to heat up solar receivers to 
temperatures in excess of 2500.degree. F. in situations utilizing a 
central receiver positioned at the focus of a mirror field which redirects 
sunlight onto the solar energy receiver. Thus, the capability of central 
receiver type installations far exceeds the restraints on superheated 
steam systems which, in general, must operate below 100.degree. F. 
It will be appreciated that even the highest quality steels have limited 
strength at temperatures over 1650.degree. F. and, therefore, new types of 
solar receivers and storage equipment are necessary if solar energy is to 
be efficiently converted into electrical energy. It will be appreciated 
that the higher the temperature of the working fluid or gas, the more 
efficient will be the conversion process. 
In the present illustration an air or Brayton cycle system is used instead 
of a steam cycle. Brayton cycle engines have the advantages of proven 
outstanding realibility and efficiencies 10-20% higher than the steam 
cycle engines. As will be seen, they integrate well with low cost sensible 
heat storage units, and become optimum at very low pressure ratios, which 
allows even higher reliabilities and high component efficiencies. 
The solar energy conversion system described can withstand the high 
temperatures associated with central receiver type installations in which 
the receiver may be of the type that utilizes a ceramic honeycomb heat 
exchanger and in which sensible heat storage units of refractory materials 
are used so as to withstand the high temperatures. 
In one embodiment, an efficient "split cycle" solar energy system includes 
a solar energy loop isolated from a Brayton cycle engine (turbine) coupled 
to an electric generator. In the solar energy receiver loop a switchable 
sensible heat storage unit is charged by the solar receiver. The charged 
storage unit is then switched into the Brayton cycle turbine loop where it 
serves as the prime energy source of the engine. Finally, after it has 
been discharged, the unit may be utilized as a high temperature, high 
efficiency recuperator to recover waste heat from Brayton cycle turbine 
exhaust. The switchable sensible heat storage unit system is alternatively 
referred to as an "energy shift register" system. 
The use of the sensible heat storage unit as a recuperator permits the 
Brayton cycle engine to be run at extremely low pressure ratios and a 
thermal/electric conversion efficiency in excess of 60%. This can result 
in a solar/electric conversion efficiency in excess of 40%, as contrasted 
with steam cycle solar energy conversion efficients of less than 20%. 
In the conventional Brayton cycle, large pressure losses occur in the heat 
addition cycle because heat is being added to a high velocity fluid 
stream. Also, penalizing temperature and pressure losses occur in the 
large recuperator needed to make low pressure ratio engines operate at 
high thermal efficiencies. In some types of ceramic wheel heat exchangers, 
there is significant leakage to further penalize performance. 
The "energy shift register" system utilizing sensible heat storage improves 
the efficiency of Brayton engines by minimizing these losses. Heat 
addition occurs efficiently and slowly without significant pressure loss 
in a large insulated tank filled in one embodiment with alternated 
materials of different thermal conductivity which produce low conductivity 
in the flow direction. In one embodiment the storage unit is formed by 
spaced ceramic matrices or perforated ceramic elements. As the air passes 
through the matrices at velocities of 1 m/sec or less, a sharp thermocline 
(called herein a "step function" thermal gradient) develops; i.e., in a 
narrow region of the tank a major temperature gradient develops, and 
travels at approximately 1/1000 of the air velocity. As will be seen, this 
permits discharge of the tank at a uniform temperature. When the step 
function thermal gradient reaches either the top or bottom of the tank, 
the tank is considered full and must be switched out of one position of 
the Brayton cycle into another. Hence, the name "energy shift register." 
As can be seen, the sensible heat storage is utilized to isolate the 
receiver loop from the engine or electric power generating loop. Thus, the 
sensible heat storage unit provides a large buffer for the turbine and 
allows a high degree of flexibility in plant operation by allowing 
different rates of thermal energy collection and consumption. 
In short, the isolation between the receiving loop and the engine loop 
buffers the engine against changes in solar flux due to the passing of 
clouds over the sun, etc., or from any receiver-related condition. Thus, 
the engine loop can be made and designed to run at maximum efficiency 
regardless of the operating conditions in the receiver loop. 
Moreover, because of the isolation between the receiving loop and the 
engine loop in the subject invention, the solar receiver loop may operate 
at a different pressure than the engine loop, since the storage unit to be 
described can be discharged at any desired pressure. Separating the 
receiver from the pressurized engine loop permits the use of an 
"open-ended" ambient pressure solar receiver in which a "window" need not 
be used. The "open-ended" receiver typically operates at ambient pressure 
to reduce sealing requirements and for safety and low cost. This receiver 
also uses air which is a non-polluting working fluid. Moreover, when 
working at atmospheric pressure, the heat exchanger in the receiver may be 
assembled loosely to its housing to allow room for thermally induced 
motions. 
While a split cycle solar energy conversion system with sensible heat 
storage has been described in which a Brayton cycle engine is utilized, it 
will be seen that the subject system involves improvements in the Brayton 
cycle system itself. The improvements to the Brayton cycle system include 
the use of a sensible heat storage unit both for recuperation and as a 
prime energy source. 
As a prime energy source, operating the storage unit at low pressure makes 
it possible to run the Brayton cycle engine at highly efficient low 
pressure ratios. Moreover, energy for the Brayton cycle engine may be 
provided not only from the sun, but also from extremely "dirty" fuels. 
This is because deposits from the fuels are not picked up by the low 
velocity gaseous working fluid and do not reach the Brayton turbine blades 
to corrode them. 
When the sensible heat storage unit is used as a recuperator, because of 
its extremely high effectiveness, the entire efficiency of the Brayton 
cycle system is significantly increased. 
SENSIBLE HEAT STORAGE 
It should be noted that the storage unit envisioned for use herein is a 
"sensible" heat storage unit as distinguished from a "latent" heat storage 
unit. The distinction between sensible heat storage and latent heat 
storage is that sensible heat is energy stored in the heat capacity of the 
materials in the storage unit so that every time a BTU of sensible heat is 
stored, the temperature of the material goes up proportionately. Thus, 
with every BTU added, the temperature of the material goes up, whereas in 
latent heat storage, there is a phase change in the material such that for 
every BTU added there is not necessarily any temperature change, but 
rather part of the material changes state, e.g., goes from liquid to gas 
or solid to liquid. In latent heat storage there is no change in 
temperature until all the material has experienced a phase change. It 
should be noted that the above-mentioned Benedek et al steam cycle plant 
utilizes latent heat storage. The major problem with latent heat storage 
is the corrosive nature of the phase-change materials used. In the Benedek 
et al. patent sodium salt (NaNO.sub.3) which is exceedingly corrosive is 
used as the phase-change material. Also the temperature is fixed for a 
given phase change material, which limits the temperature change over 
which latent heat storage units may operate. 
Sensible heat storage has been utilized in the steel industry for over 150 
years through the use of what are known as blast furnace stoves. A blast 
furnace stove is a heat exchange device used since the early 1800's in the 
glass and steel industries. In general, it consists of an insulated 
pressure shell containing an internal air duct and a large array of 
refractory bricks called "checkers." The checkers are arranged in stacks, 
often 30 meters high, forming a large number of individual air passages 
called flues, through which the air can flow. Heat is alternately stored 
in the checkers or removed from them during opposing portions of the 
process cycle. In the steel industry, these stoves are used to supply vast 
quantities of hot air into the blast furnaces which are charged with iron 
ore, coke, and limestone. The hot efflux of the furnace is piped into 
another "cold" stove where heat is extracted for use during the next blast 
period. 
In a typical installation, a furnace will have three or four stoves 
manifolded together with automatic valves, with multiple stoves "on 
blast." The outputs of these stoves are controlled by heat sensors and 
valving so that the combined output temperature from the manifolded stoves 
remains constant. It should be noted that the output temperature of these 
stoves varies with time during the discharge cycle and, therefore, it is 
necessary to add and subtract heat as necessary depending on the sensed 
temperature of the air delivered to the blast furnace. Through an 
arrangement called staggered parallel operation the stoves are valved from 
one position to another to maintain constant output temperatures during 
the continuous operation of the furnace. 
The ideal operating temperature for current blast furnace stoves is 
2000.degree. F. which has now been found to be ideal for efficient gas 
turbine operation. It has also been found that typical stoves deliver air 
at the same pressure as that required if the stove were to exhaust into a 
turbine designed for recuperated operation. Additionally, since blast 
furnace stoves are designed to handle large flow rates with low internal 
pressure losses, high Brayton cycle efficiency and thus, high 
thermal/electric conversion efficiencies can be maintained. 
In summary, it has been found that the adaptation of blast furnace stoves 
to Brayton cycle power plants is unusually efficacious because of the 
ability to store and release huge quantities of heat at high temperatures; 
because of the ability to deliver large air flows; because of the ability 
to operate at the desired pressure levels; and because of the existence of 
automatic valving techniques to rapidly connect and reconnect stoves. 
STEP FUNCTION THERMAL GRADIENT STORAGE 
However, one problem with the use of the traditional blast furnace stove is 
that the outlet temperature drops 400.degree.-500.degree. F. during 
discharge, an undesirable condition for operating Brayton cycle machinery 
because it means either a loss in efficiency or that makeup heat must be 
provided. 
As will be seen hereinafter, it is not desirable to manifold and control 
numerous individual stoves, it is desirable to provide a novel sensible 
heat storage unit in which the above temperature drops are not 
experienced. This unit is called a step function thermal gradient storage 
unit. The term "step function" refers to a sharp temperature discontinuity 
between a "spent" region of the unit and a "charged" region of the unit. 
In other words, the temperature discontinuity is confined to a narrow 
region of the unit, typically a region 1/10 the length of the unit. 
Because of the "step function" operation, this sensible heat recovery unit 
is characterized by a constant output temperature at temperatures in 
excess of 2000.degree. F. and makes possible efficient engine design. 
For purposes of this portion of the invention, step function thermal 
gradient storage units are characterized in that they have an overall 
anisotropy in that there is a low thermal conductivity in the flow 
direction as opposed to the lateral direction. In one embodiment, this is 
accomplished by spacing isotropic ceramic matrices along the flow path 
within the storage unit. In general, this results in a structure in which 
the spaced apart matrices or elements have a higher intrinsic as well as 
overall transverse conductivity than the conductivity of the material in 
between the elements, while the net longitudinal conductivity is lower 
than the matrix or the interstitial material due to the alternating 
"series" arrangement. This results in a battery-like operation of the 
storage unit, such that until the storage unit is completely discharged, 
the output temperature of the fluid from the storage unit stays constant. 
This is because there is a sharp "step function" differential in 
temperature within the storage unit as energy is withdrawn from or added 
to the unit. The sharp temperature change takes place in a narrow region 
of the unit and travels from the inlet end of the storage unit towards the 
outlet end during the discharge operation. Prior to the step function 
thermal gradient reaching the output port of the storage unit, energy is 
taken out of the storage unit at essentially a constant temperature. In 
essence, therefore, the storage unit can be conceived of as a battery 
whose output does not vary during the discharge cycle until the battery is 
completely discharged. 
It is, therefore, an object of this invention to provide a step function 
thermal gradient storage unit for utilization in an energy conversion 
system. 
It is another object of this invention to provide a sensible heat storage 
unit with preferred anisotropic properties. 
These and other objects will be better understood in view of the following 
detailed description when viewed in light of the accompanying drawings in 
which:

DETAILED DESCRIPTION 
Referring now to FIG. 1, in one embodiment, the illustrated system includes 
a charging loop generally designated by reference character 10 and a power 
loop generally designated by reference character 12, separated by dotted 
line 14. An open-ended solar receiver 16, in one embodiment, is located in 
the charging loop and is mounted on a tower 18 which is at the focus of a 
mirror field generally indicated at 20. Mirror field 20 redirects solar 
rays 22 through the open-end 24 of the solar receiver and onto a heat 
exchanger 26 located within the receiver. The heat exchanger may be a 
ceramic honeycomb matrix made of silicon carbide. In one embodiment, this 
open-ended receiver operates at atmospheric pressure so that its output 
over line 28 carries air at 2000.degree. F. and 1 atm. to a sensible heat 
storage unit 30 at position 30a. The input to the sensible heat storage 
unit at 30a is designated 32, and its output 34. In one embodiment, prior 
to being completely filled, the sensible heat storage unit is initialized 
at 1200.degree. F. Output 34 is connected to a suitable fan or blowing 
system 36 which exhausts to the front end of the solar receiver as shown 
by return line 38. 
In operation, air is sucked from return line 38 and is heated as it passes 
through heat exchanger 26 which is heated by the focused solar radiation 
(called "insolation") from the mirror field. With proper pressure 
adjustments, the pressure drop across the open end 24 of the receiver can 
be reduced to zero thereby eliminating the need for a window. The hot air 
downstream of the heat exchanger is coupled to the inlet of the sensible 
heat storage which initially is at 1200.degree. F. 
The sensible heat storage unit at 30a is charged to capacity by the 
incoming hot air such that at some time after the charging has begun, the 
entire sensible heat storage unit is at 2000.degree. F. The flow through 
the sensible heat storage unit is adjusted by a mass flow regulator 42 
which adjusts the mass flow of blowing system 36 in accordance with the 
sensed temperature so as to maintain the receiver outlet air temperature 
at 2000.degree. F. 
When the sensible heat storage unit at 30a is charged to capacity, it is 
shifted by conventional valving techniques from its position shown to the 
left of dotted line 14 to the position 30b to the right of dotted line 14 
as illustrated by arrow 44. In the position illustrated by 30b, the 
sensible heat storage unit acts as a prime source of energy for the power 
loop. 
In the illustrated case, the sensible heat storage unit at position 30b, is 
discharged at 60 psia and at a temperature of 2000.degree. F. over line 50 
to the turbine section 52 of a Brayton cycle engine. The engine includes a 
compressor 54 which compresses ambient air at 60.degree. F. and 15 psia to 
raise the temperature of the incoming air to 300.degree. F. and to raise 
the pressure to 60 psia. The output of the compressor at 56 is connected 
to a conventional recuperator 58. At this point, the 300.degree. F. air 
from the compressor section captures waste heat from the output of turbine 
section 52 via line 60 which is coupled to recuperator 58. This line 
carries air at 15 psia at 1300.degree. F. In the process, the temperature 
of the air from the compression section goes from 300.degree. F. to 
1200.degree. F. and is delivered over line 62 to the input end of the 
sensible heat storage unit at position 30b. When the unit at 30b is 
discharged from 2000.degree. F. to 1200.degree. F., it is switched back to 
position 30a for recharging. 
It should be noted that the output 64 of the recuperator corresponding to 
input line 60 exhausts air to the atmosphere at approximately 400.degree. 
F. 
As shown, the mechanical turbine output is illustrated by dotted line 66 
and is coupled to a conventional electric generator 68. 
The advantage provided by the system of FIG. 1 is the ability to separate 
the solar collection cycle from the turbine cycle of using highly 
efficient sensible heat storage. Operationally, this offers the user the 
advantage of scheduling power outputs to meet demand without direct 
dependence upon instantaneous availability of focused sunlight. As 
mentioned hereinbefore, sensible heat storage in the form of checker 
stoves may, if properly manifolded, be utilized. Single sensible heat 
storage units may also be used, especially if modified for step function 
thermal gradient operation. 
In the illustrated system, all energy passes through a storage unit. This 
provides a large buffer for the turbine as the only storage losses are 
those associated with insulation losses through the storage unit wall and 
losses involved in pressure/switching of the sensible heat storage units. 
These sensible heat storage units also provide a high degree of 
flexibility in the plant operation by allowing different rates of thermal 
energy collection and consumption. 
By the ability to isolate the charging loop from the power loop, it is now 
possible to utilize a solar receiver which differs from conventional 
configurations in that it does not utilize a standard high pressure 
tube/boiler technique. The open-ended solar receiver captures and 
transfers heat at pressures near atmospheric, utilizing a material and a 
configuration that is a highly efficient absorber of solar radiation. The 
aforementioned honeycomb heat exchanger operates with a very low pressure 
drop (less than 1 inch H.sub.2 O) thus reducing the air sealing 
requirements and permitting the honeycomb components of the heat exchanger 
to be assembled loosely to allow room for thermally-induced motions. Since 
the entire receiver operates at near atmospheric pressure, safety hazards 
and the cost of the pressure vessel are minimized. 
In passing, the open cycle Brayton concept has several important 
advantages. The basic gas turbine cycle is simply implemented compared to 
steam cycles. The reduced maintenance associated with gas turbines is 
enhanced by the reduction of the corrosion problems due to fuel combustion 
products. In addition, the energy efficient, open-cycle air system permits 
direct rejection of residual (waste) heat to the atmosphere, eliminating 
the need for large quantities of cooling water. 
Moreover, the engine can be mounted at the concentrator focus and closely 
coupled to the solar receiver. As will be seen, when the recuperator is in 
the form of a sensible heat storage unit, it is lightweight and can also 
be mounted with the receiver/engine unit, providing an integrated thermal 
conversion system with short piping and low thermal losses. 
In summary, because of the use of the split cycle, an open-ended receiver 
can be used which uses air as the working fluid. Moreover, heat collection 
and power generation is accomplished in separate, independent heat 
transfer loops. Additionally, since the power generation system works from 
storage, it is independent of short term fluctuations and solar radiation. 
Finally, the use of sensible heat storage units for storage significantly 
increases system efficiency because it actually fulfills a 
recuperator/heat exchanger function as well as storage/delay. 
In the system illustrated, either one or two storage units are used which 
are sequentially valved into one of the two positions shown. In another 
type system, a third and even a fourth unit may be used for additional 
storage and recuperation. 
STEP FUNCTION THERMAL GRADIENT STORAGE UNIT 
As will be seen, it is desirable for the storage unit to have what is known 
as a "step function thermal gradient" in which the temperature difference 
between two adjacent locations is very shape. This means that the 
transition between one temperature and another temperature within the 
storage unit occurs in a transition zone of less than 1/10 the total flow 
path length of the unit. This is accomplished in one embodiment by 
providing high density, high heat capacity, high conductivity matrices at 
spaced locations along the flow path such that, in general, the thermal 
conductivity in the direction of flow is mimimized. 
As outlined in an article by D. J. Close entitled, Rock Pile Thermal 
Storage for Comfort Air Conditioning, Instrumentation Engineering 
Australia (Mechanics & Chemical Engineering Transcripts), Vol. MC.1 (#1), 
at page 14, the works of Schumann (Heat Transfer: A Liquid Flowing Through 
a Porous Prism, J. Franklin Inst., Vol. 208, September, 1929, pp. 405-16) 
and Furnas (Heat Transfer from a Gas Stream to a Bed of Broken Solids, 
Amer. Inst. Chem. Engrs. Transcripts, Vol. 24, June, 1930, pp. 142-69) on 
packed beds indicate that there is an assumption that the bed has zero 
conductivity in the flow direction and infinite conductivity in planes 
normal to it. However, Close, later on in his article, indicates that 
these assumptions for the rock pile thermal storage are incorrect because 
so little is known about the actual operation of rock piles when used as 
thermal storage. On page 17 of the Close article Close says that certain 
factors suggest that the simple theory is inadequate and that it would be 
instructive to examine the validity of the main assumption of zero 
conductivity of the packing in the flow direction and infinite 
conductivity perpendicular to it. Thus, according to Close, the 
assumptions by Schumann and Furnas are all subject to scrutiny and 
extensive testing. 
On the other hand, a computer program and algorithm have now been developed 
which evaluate all of the types of energy transfer within the rock pile on 
a point-by-point basis. It has been found that, indeed, as Close 
suspected, there is not infinite conductivity in planes normal to the flow 
path, nor is there zero conductivity in the flow direction. 
In the subject invention these anisotropic characteristics can be made to 
occur within the sensible heat storage unit by specialized structure 
within the unit, and that having done so, the transition zone for the 
thermal gradient can be kept small, of the order of 1/10 the overall flow 
path length. This provides a step function thermal gradient which permits 
the storage unit to discharge at a substantially constant temperature 
until the gradient travels from the inlet end of the storage unit 
completely to the outlet end. 
In other words, there is a large thermal gradient between the "charged" 
portion of the storage unit and the "spent" or uncharged portion of the 
unit. Because the gradient travels from the inlet to the outlet end at a 
relatively low speed, and because the transition is kept to a small area, 
the discharge temperature of the unit is maintained substantially constant 
throughout the discharge cycle. This type of operation is not possible 
with uniform packed beds operating at the temperatures and pressures 
specified in the above-mentioned articles. What is therefore necessary is 
to modify the packed bed structure to give it an anisotropic property of 
low thermal conductivity in the flow direction and higher thermal 
conductivity lateral to the flow direction. 
This anisotropic property results in the step function thermal gradient and 
is made to occur in the subject invention by the alternation or lamination 
of materials of different thermal conductivity along the flow path in the 
storage unit, or by the use of anisotropic materials in the storage unit. 
In order to produce a step function thermal gradient, it is desirable to 
reduce transverse gradients while maximizing gradients in the flow 
direction, with the sharpeness of the thermocline being dependent upon low 
conductivity in the flow direction. Therefore, it is a characteristic of 
the subject storage unit that high conductivity in the flow direction is 
materially reduced, thereby to provide a step function thermal gradient. 
It is possible to produce this step function thermal gradient by using 
laminated structure made up of isotropic materials. Thus, it is a finding 
of this invention that isotropic elements may be used to achieve a step 
function thermal gradient of the type described, while helping uniformity 
of heat transfer in the transverse direction. 
It is also a finding of this invention that the step function thermal 
gradient can be achieved by providing spaced regions having a conductivity 
transverse to the flow path which is higher than the conductivity of the 
material along the flow path in the spaces inbetween these regions. 
It will be appreciated that the anisotropy in the above-mentioned 
embodiment is due to the laminated configuration of the storage unit, as 
opposed to any anisotropic property in the materials used. It will, 
however, be appreciated that anisotropic materials could be used in place 
of the laminated structure. These anisotropic materials exist and in 
general, are single crystal solids. However, single crystal solids are 
usually too expensive for use in the subject system. 
Referring to FIGS. 2A and 2B, there are shown two different types of 
sensible heat storage units in which a step function thermal gradient can 
be achieved. 
With respect to FIG. 2A, the unit may have a housing 70 having an inlet 72 
and an outlet 74 in which the flow direction and path is indicated by 
dotted arrow 76. This unit is packed with spaced apart isotropic 
structures 78 which may be rods or matrices of metal or materials such as 
mullite or cordierite. It will be appreciated that these structures are 
spaced apart along a flow path indicated by arrow 76. In between these 
structures are regions 80 which may be filled with material having a lower 
thermal conductivity in the flow direction, such as clay refractories 
which have a conductivity of 0.1 BTU/Hr./sq. ft. 
As shown in FIG. 2B, unit 70, may carry segments 82 which may be honeycomb 
discs or perforated blocks of mullite or cordierite to provide for the 
higher thermal conductivity in the transverse direction than the spaces 
therebetween. The low thermal conductivity areas 84 are merely provided by 
spaces occupied by air or other gases passing through the unit. A property 
of the above-mentioned materials is also that they retain heat. Thus, 
alternatively, what is provided are spaced segments of high heat retaining 
material. 
Alternatively, any higher thermal conductivity segment transverse to the 
flow path may be utilized, such as corrugated materials interspaced with 
flat materials of the same material structure (not shown). Transverse 
metal plates may be used now and then to get maximum net anisotropy if 
needed. 
The important aspect of the unit is that in order to achieve the step 
function thermal gradient, there are regions spaced apart and transverse 
to the flow path which have higher thermal conductivity than that of the 
spaces in between these regions. 
Referring to FIG. 3, a graph is shown of the step function thermal 
gradient. This step function thermal gradient is shown by solid line 90 
for the charging or discharging case. During charging, the step function 
thermal gradient moves from the left to the right, from the inlet end of 
the thermal storage unit to the outlet end as indicated by arrow 94. 
During discharge, the step function moves in the reverse direction as 
indicated by arrow 95. The transition region which carries the step 
function thermal gradient has a length in the flow direction indicated by 
x. As indicated hereinbefore, a step function thermal gradient is, in 
essence, defined by the fact that the transition region between one 
temperature and another in the storage unit occupies a distance along the 
flow path of less than some small fraction of the total flow path distance 
between the inlet to the storage unit and the outlet. Thus, in one 
embodiment in the charging cycle the temperature in the storage unit goes 
from 1300.degree. F. to 2000.degree. F. over a distance of x which is less 
than 1/10 the total flow path length through the unit. 
While fraction 1/10 is utilized, for explanation purposes, it should be 
appreciated that the step function may vary substantially. It is 
distinguished from a gradual thermocline in that there is a narrow 
transition region which is well defined within the storage unit where the 
thermal gradient occurs. Portions of the storage unit to either side of 
the transition region therefore exist at a substantially uniform 
temperature. 
It will be noted that due to the symmetry of the step function thermal 
gradient storage units, these may be charged and discharged in any 
direction. As such, they are said to be "bi-directional." 
Alternatively, the thickness and thermal conductivity of the materials 
utilized within the unit may be given asymmetric properties such that they 
are no longer bi-directional. In this case, it might be useful to make the 
transition portions at the outlet end of greater thickeness than those at 
the inlet end. 
Although preferred embodiments of the invention have been described in 
considerable detail for illustrative purposes, many modifications will 
occur to those skilled in the art. It is, therefore, desired that the 
protection afforded by Letters Patent be limited only by the true scope of 
the appended claims.