Thermal energy storage unit

A thermal storage unit comprising a bed of particulate solid material, a liquid situated in heat-exchanging relation with said bed and cooperating with said bed to define a liquid-solid system containing a thermocline, and means for introducing liquid into and extracting liquid from said liquid-solid system.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to thermal energy storage systems and is 
particularly directed to a liquid-solid thermal energy storage system 
having a thermocline. 
2. Description of the Prior Art 
As the world's population becomes aware of the decline of the world's oil 
reserves, there is developing increasing interest in alternative energy 
systems. Thus, considerable study is being directed to solar energy 
systems. However, due to dirunal temperature changes, cloudiness and 
various other matters, the problem of thermal storage has been one of the 
major problems. In this connection, the late Dr. Farrington Daniels, noted 
physical chemist and past president of the Solar Energy Society, presents 
an excellent morphological survey of thermal energy storage concepts in 
his book, "Direct Use of the Sun's Energy, " Yale University, 1964. Dr. 
Daniels divides thermal energy storage concepts into three basic 
categories: 
1. Sensible Heat -- Storage by heat capacity 
2. Physcial Changes -- Particularly heats of fission and/or vaporization 
3. Reversible Chemical Reaction 
When economic analysis is made of these three categories, it becomes 
apparent that the two major considerations are: the inventory cost of the 
heat storage medium and container and, secondly, the engineering 
complexity of inputting and extracting the heat. Most analyses tend to 
favor sensible heat storage as the most economically and operationally 
attractive system. For temperatures up to about 200.degree. F., water is 
by far the best medium. To quote Dr. Daniels, "Water has about the highest 
heat capacity per kilogram per liter or per dollar of any ordinary 
material." The next lowest price heat storage medium is gravel or crushed 
rock, which is available at a cost of a few dollars per ton and is 
suitable for storage of heat at temperatures up to at least 1500.degree. 
F., the upper limit being determined by the fluid which flows through the 
rock bed to input or extract heat. Rock has been used as a heat storage 
medium for many years in "pebble bed" heaters, in which a gas (usually 
air) flows through the bed to input or extract heat. A major limitation of 
such heaters is that they do not have a thermocline, and the temperature 
of the exiting hot gas during heat extraction decreases rapidly from the 
storage temperature. It is known that water (and other liquids) may 
produce a "thermocline", that is, the hot and cold water may be made to 
separate into layers having a fairly distinct boundary, which rises or 
lowers within the container as water is added or withdrawn, so that the 
temperature of the water being drawn off can be substantially constant 
until the thermocline is reached and, at that point, will drop sharply to 
the temperature of the unheated water. This phenomenon is familiar in 
domestic hot water heaters. In contrast, where rock is the thermal storage 
medium in "pebble bed heaters", the temperature tends to increase 
gradually when heat is input, and to decrease gradually, as heat is 
withdrawn. 
It will be apparent that the thermocline principle is advantageous, but 
has, heretofore, been limited to relatively low temperature, all liquid 
thermal storage systems, whereas rock has been capable of storing heat at 
considerably higher temperatures but has been subject to supply 
degredation in temperature. 
BRIEF SUMMARY AND OBJECTS OF INVENTION 
These disadvantages of the prior art are overcome with the present 
invention and a novel thermal storage system is proposed which permits the 
thermocline principle to be employed with rock thermal storage systems 
and, consequently at considerably higher temperatures than have been 
economically feasible with the prior art systems. 
The advantages of the present invention are preferably attained by 
controlling such factors as bed material and particle size, fluid 
velocity, void fraction and method of fluid distribution so as to produce 
a thermocline in the bed of crushed rock or the like and, thus, to obtain 
the thermocline principle with storage temperatures up to at least 
1500.degree. F. 
Accordingly, it is an object of the present invention to provide an 
improved thermal storage system. 
Another object of the present invention is to provide a sensible heat 
storage system incorporating the thermocline principle and capable of 
storing heat up to at least 1500.degree. F. 
A further object of the present invention is to provide a sensible heat 
storage system incorporating the thermocline principle in a particulate 
bed system. 
A specific object of the present invention is to provide a sensible heat 
storage system incorporating the thermocline principle in a particulate 
bed system and capable of storing heat up to at least 1500.degree. F. by 
controlling the bed material and particle size, fluid velocity, void 
fraction and method of fluid distribution. 
These and other objects and features of the present invention will be 
apparent from the following detailed description, taken wth reference to 
the accompanying drawing.

DETAILED DESCRIPTION OF INVENTION 
In that form of the present invention chosen for purposes of illustration, 
FIG. 1 shows a thermal storage system, indicated generally at 2, 
comprising a fluid-tight container 4 filled with particulate solid 
material, indicated generally at 6, and having a liquid, indicated 
generally at 8, filling the interstices between adjacent particles of the 
particulate solid material 6. To input heat to the container 4 for 
storage, hot fluid from a suitable heat source, not shown, such as a 
furnace, nuclear reactor, solar furnace or the like, is supplied by pipes 
10 and 12 to a suitable input heat exchanger 14, as indicated by arrows 
16. Meanwhile, suitable means such as pipes 18, 20 and 22 and pump 24 
serve to draw the liquid 8 from the bottom of container 4 through pipe 18, 
pass the liquid 8 through input heat exchanger 16 to be heated by the hot 
fluid from the heat source, and supply the heated liquid 8 through pipe 22 
to the top of container 4, as indicated by arrow 26. To extract heat from 
container 4, suitable means, such as pipes 28, 30 and 32 and pump 34 serve 
to draw the heated liquid 8 from the top of container 4, as indicated by 
arrow 36, supply the heated liquid 8 to suitable output heat exchanger 38 
and return the liquid 8 to the bottom of the container 4. In addition, 
means such as pipes 40 and 42 serve to pass a suitable fluid through the 
output heat exchanger 38, to be heated by the liquid 8, and deliver the 
heated fluid to suitable heat utilization means, not shown, such as a 
turbine-generator set or a space heating system. 
From the foregoing description, it will be seen that the liquid 8 in 
container 4 flows downward during thermal input and flows upward during 
thermal output. This arrangement serves to maintain the liquid 8 hotter 
near the top of container 4 than near the bottom of container 4. 
In a typical example, the container 4 would be a cyindrical tank, 64 feet 
in diameter and 57 feet high, containing 11,000 tons of crushed granite 
rock and coarse sand plus 310,000 gallons of a commercial heat transfer 
fluid, such as that sold by Exxon Corporation under the tradename "Caloria 
HT43". This container would have a thermal storage capacity of about 200 
thermal megawatt-hours operating over a temperature range between 
218.degree. C. and 302.degree. C. 
Assuming that the rock and liquid in the container 4 are initially at an 
equilibrium temperature of 302.degree. C., a thermocline will be 
established at the bottom of the container 4, as indicated by vertical 
line 46 in FIG. 2. In order to extract heat from the thermal storage unit 
2, fluid is extracted from container 4 via outlet pipe 28 at a temperature 
of 302.degree. C., gives up some of its heat in the heat exchanger 38 and 
is returned to the bottom of container 4 via inlet pipe 32 at a 
temperature of about 218.degree. C. As this occurs, the thermocline will 
move upward in the container 4, as indicated at line 48. Above the 
thermocline 48, the temperature of the fluid will be 302.degree. C., while 
below the thermocline 48 the temperature of the fluid will be 218.degree. 
C. As more heat is extracted from the thermal storage unit 2, the 
thermocline will continue to move upward within the container 4, as 
indicated by lines 50 and 52. Eventually, the thermocline will approach 
the top of the container 4, as indicated by line 54. When this occurs, the 
temperature of the fluid drawn through oulet pipe 28 will fall rapidly to 
the lower temperature of 218.degree. C. Preferably, however, the heat 
extraction will be discontinued prior to reaching this level and heated 
fluid at 302.degree. C. will be supplied to the container 4 via input pipe 
22 to drive the thermocline downward to recharge the thermal storage unit 
2. Thereafter, the heat extraction operation can be renewed. 
FIG. 3 is a curve showing the temperature of the fluid extracted from the 
container 4 via outlet pipe 28 as a function of time. As described above, 
it has been found that the temperature of the exit fluid remains 
essentially constant at about the top operating temperature, as indicated 
by line 56, until most of the energy in the thermal storage unit 2 has 
been extracted. Then, the exit fluid temperature begins to drop rapidly, 
as indicated by line 58. Preferably, a cut-off point, such as point 60, 
will be established at which the heat extraction operation will be 
discontinued. If the cut-off point is set at 293.7.degree. C. (that is, 
8.3.degree. C. below the upper limit of 302.degree. C.), some of the 
stored thermal energy will remain in the container 4, as indicated by the 
shaded areas in FIGS. 2 and 3. This energy can be recovered, but only at 
continually decreasing temperatures, as indicated by curve 58 of FIG. 3. 
However, dividing the volume of the shaded area of FIG. 2 into the total 
area of FIG. 2 yields a value which may be termed the "extraction 
efficiency" of the thermal storage unit 2. Using the figures given above, 
the extraction efficiency of the typical example is 95%. 
There are many design and operating parameters which are necessary or 
desirable in order to have a successful efficient and economical thermal 
storage system of the type described above. For example, the system must 
be operated in an orientation such that the relatively cold fluid enters 
or leaves adjacent the bottom of the container 4, while relatively hot 
fluid enters or leaves adjacent the top of the container 4. Moreover, the 
fluid distribution systems at the top and bottom of container 4 should be 
designed to minimize turbulence. 
In order to assure that a thermocline will be established within the tank 
4, there are several parameters which must be observed. Among these 
parameters, the ratio of the void volume to the total volume, (this ratio 
is referred to as the "void fraction") should be less than about 0.4. 
Similarly, where two sizes of solids are used, the ratio of the average 
diameter of the large size solids to the average diameter of the small 
size solids should be not less than about eight. In addition, the 
superficial velocity should be in the range of about 4 to 20 feet per 
hour. The superficial velocity is the fluid volumetric flow rate divided 
by the tank cross-sectional area, (.pi.D.sup.2 /4). Finally, the ratio of 
the height to diameter of container 4 should be in the range of about 0.2 
to 1.5. 
Obviously, numerous variations and modifications may be made without 
departing from the present invention. Accordingly, it should be clearly 
understood that the forms of the present invention described above and 
shown in the accompanying drawing is illustrative only and is not intended 
to limit the scope of the present invention.