Storage arrangement for thermal energy

Heat of liquefaction and solidification is transmitted between a heat storing material undergoing the phase transition and a heating medium in a heat exchanger by means of a partly gaseous, partly liquid fluid sealed in a heat pipe a portion of which is received in the container for the aforementioned material, and which is equipped with fins in direct contact with the heat storing material while a second portion of the heat pipe is in thermal contact with the heating medium in the heat exchanger.

This invention relates to thermal storage arrangements, and particularly to 
an arrangement in which thermal energy is stored as heat of liquefaction 
or heat of solidification in a material undergoing a phase transition 
between the liquid and solid states when at a certain temperature. 
It is known to store thermal energy in the form of sensible heat or of 
latent heat. Storage arrangements relying on sensible heat may provide 
direct contact between a heating and cooling fluid and a solid in which 
thermal energy is to be stored and thus require relatively simple 
apparatus. It is usually necessary to separate the energy storing material 
and the heating and cooling fluid by a thermally conductive wall if the 
energy storing material undergoes a substantially isothermal phase 
transition when receiving or releasing thermal energy. The resulting more 
complex structure is acceptable where the advantages of approximately 
isothermal operation and smaller space requirements are important. Heat 
storage arrangements relying on storage of latent heat have normally been 
preferred where low bulk of the arrangement is important, as in space 
vehicles, but also in many terrestrial applications. 
The benefits derived from storing latent heat are commensurate to a large 
extent with the efficiency of the indirect heat transfer between the heat 
storing material and the heating and cooling fluid which supplies the heat 
to be stored and removes the stored heat in a heating and/or refrigerating 
system. It has now been found that a sealed tube enclosing a fluid which 
is partly liquid and partly gaseous at the transition temperature of the 
heat storing material most efficiently transfers thermal energy from and 
to the heating and cooling fluid. 
In its more specific aspects, this invention thus provides a thermal energy 
storage arrangement in which a container bounds a chamber. A heat storing 
material in the chamber undergoes a phase transition between the liquid 
and solid states at a predetermined temperature uniquely defined by the 
nature of the material. The arrangement further includes a heat exchanger 
having an intake port and a discharge port and defining a through passage 
which connects the ports for flow of a heating and cooling medium between 
the ports, the medium being a gas or a liquid. An elongated sealed tube 
has a first longitudinal portion in the chamber and a second longitudinal 
portion outside the chamber in thermal contact with the medium flowing 
through the passage of the heat exchanger. Fins of thermally conductive 
material transversely project from the first tube portion into direct 
contact with the heat storing material. The tube encloses a fluid which is 
partly liquid and partly gaseous at the phase transition temperature of 
the heat storing material.

Referring now to the drawing in detail, and initially to FIG. 1, there is 
seen a thermal storage device 1 including a prismatic casing 2. The 
chamber in the casing is almost completely filled with a heat storing 
material 4, typically a paraffin wax mixture melting at about 50.degree. 
C. A heat exchanger 6 has an upright wall in common with the casing 2 and 
another, parallel, upright wall in common with an additional heat 
exchanger 8. An intake port 10 and a discharge port 12 are connected by 
the hollow interior of the heat exchanger 6 for flow of a heating and/or 
cooling medium, and the interior of the heat exchanger 8 provides an 
analogous passage between intake and discharge ports 14,16. 
A sealed, cylindrical heat pipe 18 extends horizontally through the chamber 
in the casing 2 and the passages of the heat exchangers 6,8. The interior 
of the pipe 18 is divided into a circumferential zone 22 and a central 
zone 20 by a tube 24 of porous synthetic fabric having capillary pores, 
and is filled with one of the chloro-fluoro-alkanes, particularly 
halogenated methanes and ethanes, which are conventionally employed as 
heat transfer fluids in refrigeration systems. The pressure of the 
refrigerant in the heat pipe 18 is selected to keep the fluid partly 
liquid and partly gaseous at the transition temperature of the paraffin 
wax 4. The several longitudinal portions of the pipe 18 are supported in 
the outer walls and partitions of the apparatus. 
The pipe 18 consists of corrosion resistant stainless steel. Aluminum fins 
26 are closely and uniformly spaced along the portion of the pipe 18 in 
the chamber of the casing 1. They are rectangular and project at right 
angles from the pipe 18 to which they are welded for good thermal contact. 
Their circumferential edges bound gaps 28 with the four inner wall faces 
30 of the casing 1 so as to facilitate charging of the casing chamber with 
the molten heat storing material. The width of each gap 28 is but a small 
fraction of the distance between the gap and the pipe 18 which passes 
approximately centrally through the chamber in the casing and the 
similarly shaped passages in the heat exchangers 6,8. The gaps 28 provide 
some thermal insulation between the fins 26 and the casing walls. Little 
heat is lost or picked up by the ends of the pipe 18 which project 
slightly into the ambient atmosphere from the casing 2 and the heat 
exchanger 8. Additional aluminum fins 32 project from the portions of the 
pipe 18 in the passages of the heat exchangers 6,8 into the water or other 
heating and cooling medium that may flow through the passages. 
Depending on the specific application and the nature of the heating and 
cooling medium, other heat storing materials and other heat transfer 
fluids will redily be chosen for charging the casing 2 and the heat pipe 
18. Parraffin waxes having 14 to 30 carbon atoms and mixtures of the same 
having melting points between about 6.degree. and 66.degree. C have latent 
heats of liquefaction of 55 - 60 Cal./kg and are eminently suitable in 
heat storing arrangements of this invention for domestic heating plants. 
Hydrated salts which may replace the paraffin wax specifically described 
above include Na.sub.2 HPO.sub.4.12H.sub.2 O and LiNO.sub.3.3H.sub.2 O, 
but others are known to have suitable heats of liquefaction and 
solidification and have been used for this reason in freezing mixtures. 
Various grades of polyethylene are useful at temperatures too high for the 
paraffin waxes, and metals such as gallium and potassium may be employed 
at even higher operating temperatures. Additional suitable materials will 
readily suggest themselves as other conditions may require, and materials 
of construction will be selected accordingly. 
The afore-mentioned halogenated alkanes cover a wide range of boiling 
points at ambient pressure, and their boiling temperatures can be modified 
by maintaining suitable pressures within the pipe 18, but they may be 
replaced by other fluids which are partly liquid and partly gaseous at the 
discharge temperature of the heat storing material. Water, ammonia, 
acetone, and other low-boiling organic solvents are effective at or near 
ambient temperatures, but potassium or sodium may also be used for 
operation at temperatures as high as 1400.degree. C for which apparatus 
analogous to that of FIG. 1 may be constructed. 
Heat transfer along the pipe 18 is rapid. Liquid fluid evaporates in 
portions of the pipe from which heat is to be removed, and the vapor 
condenses in other, cooler pipe portions to transfer heat of liquefaction 
to the ambient heat storing material or to a heating and cooling medium. 
The fabric 24 enlarges the effective surface area of the liquid fluid 
portion and thereby enhances the rate of evaporation and condensation. The 
capillary passages also facilitate distribution of the liquid along the 
pipe 18 which is otherwise caused by gravity. 
A typical application of the heat storage arrangement described above will 
be described hereinafter with reference to FIGS. 6 and 7. 
For other applications, two heat exchangers 6,8 may not be necessary, and 
the modified apparatus illustrated in FIG. 2 differs from that described 
with reference to FIG. 1 mainly by having a single heat exchanger 6' not 
significantly different from the heat exchanger 6. 
In the further modified thermal storage arrangement shown in FIG. 3, a 
single heat exchanger mainly consists of a coil 34 of copper tubing wound 
helically about the portion of the heat pipe 18 outside the container 2. A 
layer of polystyrene foam 36 covers the coil and associated portions of 
the heat pipe 10. 
The storage arrangements shown in FIGS. 1 to 3 readily lend themselves to 
the construction of modular assemblies in which the individual units are 
stacked. Thus, a device of the type shown in FIG. 1 may be interposed 
between identical higher and lower units, the ports 12,16 of the 
intermediate unit being connected to the ports 10,14 of the higher unit, 
and the lower unit being connected to the intermediate unit in an 
analogous manner. Storage arrangements varying greatly in their capacity 
may be built from a single type of basic units. 
Larger storage capacity may also be achieved in the manner shown in FIGS. 
4a,4b. Sixteen heat pipes 18', closely similar to the afore-described pipe 
18, are arranged equidistant in four rows and four columns in a large, 
common casing 2' of square cross section and in a flangedly attached 
single heat exchanger 6' of identical cross section. The portions of the 
pipes 18' in the chamber of the casing 2' carry square fins 26', and the 
fins welded to each pipe 18' bound narrow gaps with the fins associated 
with adjacent pipes. The separate fins facilitate installation, but do not 
provide optimum heat transfer between the fluid in the several pipes 18' 
and the surrounding heat storing material 4. Fins are normally also 
provided on the several pipes 18' in the heat exchanger 6', but have been 
omitted from FIG. 4a for the sake of clearer pictorial representation. 
The multiple pipe arrangement illustrated in FIGS. 5a, 5b has a casing 2" 
which is cylindrical and flanged to a correspondingly shaped heat 
exchanger 6". One heat pipe 18' passes coaxially through the casing 2" and 
the heat exchanger 6", and six additional pipes 18' are equiangularly 
spaced about the coaxial pipe. Circular fins 26" are almost equal in 
diameter to the chamber in the casing 2" so as to leave only narrow gaps 
28 between their outer circumferences and the inner cylindrical wall 30' 
of the casing 2". Each fin 26" is welded to each heat pipe 18' for good 
thermal contact. 
It will be appreciated that each of the storage arrangements shown in FIGS. 
3, 4a, 5a may be equipped with a second heat exchanger for use in the 
domestic heating and hot water systems illustrated in FIGS. 6 and 7. 
The system shown in FIG. 6 has a primary hydraulic circuit 40 and a 
secondary hydraulic circuit 42 in which water is circulated as a heating 
and cooling medium by pumps 44,50 respectively, and which are connected by 
a tube-and-shell heat exchanger 48. In addition to the shell of the 
exchanger 48 and the pump 44, the primary circuit 40 includes a solar 
heating panel 46 and a shut-off valve 74. 
In the secondary hydraulic circuit 42, a two-way valve 52 having discharge 
ports 76,78 directs the output of the pump 50 either through the tubes of 
the heat exchanger 48 or through a by-pass 54 to another two-way valve 56 
whose discharge port 80 is connected to the intake port 10 of the storage 
device 1 more fully shown in FIG. 1. The discharge port 12 of the latter 
and a by-pass connected to the other discharge port 82 of the valve 56 are 
connected to a coil in an auxiliary heater 60. The output of the heater is 
returned to the pump 50 through a further two-way valve 64 and either 
through a thermostatically controlled radiator 62 or a by-pass 65. The 
single illustrated radiator 62 represents the several radiators employed 
for heating a house. 
The hot-water supply of the house, illustrated by a broken line 66, draws 
water from a well, a reservoir, or a public water system 68 under a 
pressure sufficient to drive the cold water through a two-way valve 70 to 
another coil in the auxiliary heater 60 either through a discharge port 84 
of the valve 70 and the heat exchanger 8 in the storage device 1, or 
through a by-pass connected to the other discharge port 86 of the valve 
70. From the auxiliary heater 60, hot water is supplied to the several 
faucets and other taps in the house, such as the illustrated shower 72. 
The several valves are provided with temperature responsive actuators which 
are also linked to the energizing circuits of the pumps 44,50 so that the 
system illustrated in FIG. 6 operates in the following automatic manner. 
When the sun supplies sufficient energy to the panel 46, the pumps 44,50 
are energized, the valve 74 is opened, toward the ports 76,80,84. Solar 
heat is transferred to the heat storing material in the storage device 1 
and supplied to the radiator 62 in circulating water. Depending on the 
thermostat setting for the radiator 62, the circulating water may be 
heated further by burning oil or gas in the auxiliary heater 60. 
When the temperature in the primary circuit 40 drops below a set minimum, 
the valve 74 is closed, and the pump 44 is shut off. If freezing of the 
water in the circuit 40 is to be guarded against, the circuit may be 
provided with a non-illustrated valve and pump which drain the circuit 40 
and fill it again with water as the temperature of the panel 46 rises and 
falls. 
When the temperature in the heat exchanger 48 drops to a value lower than 
the temperature of the water discharged from the radiator 62, the valve 52 
switches to the by-pass 54 to prevent heat loss toward the circuit 40. If 
the temperature of the water discharged from the storage device 1 drops 
below a set minimum, the valve 56 switches to the by-pass 58, and all 
heating energy thereafter is supplied by the auxiliary heater 60. The 
valve 70 in the hot-water system is operated in an analogous manner when 
the heat storage device 1 cannot maintain a desired water temperature at 
the discharge port of the heat exchanger 8. 
An alternate heating system is shown in FIG. 7. It has a single heating 
circuit equipped, in series, with a centrifugal circulating pump 50', a 
solar panel 46' and a by-pass 54' arranged in parallel and alternatively 
supplied with heating medium by the discharg4e ports 76',78' of a valve 
52', another valve 56' with discharge ports 80',82' leading through the 
heat exchanger 6 of the storage arrangement 1 or through a by-pass 58' to 
an auxiliary heater 60' and thence back to the pump 50' through a radiator 
62' or a by-pass 65' depending on the setting of a valve 64'. 
Hot water is furnished by a circuit 66' from a supply 68 to a shower 72 
through a valve 70' having discharge ports 84',86' connected to the heat 
exchanger 8 and a by-pass respectively, and through the auxiliary heater 
60'. The operation of the system illustrated in FIG. 7 is closely similar 
to that described with reference to FIG. 6, but the apparatus of FIG. 7 is 
designed for the use of a gaseous heat transfer medium, such as hot air, 
and it will be appreciated that the radiator 62' in this instance may be 
replaced by a system of registers conventional in itself. 
During adequate sunshine, the valves 52',56',70' open their discharge ports 
76',80',84', and the heated, gaseous medium yields some of its thermal 
energy to the storage device 1 and may be heated to a higher temperature 
by the heater 60'. When the gas leaving the collector panel 46' does not 
maintain a desired minimum temperature, the valve 52' is switched to the 
by-pass 54', and other valves operate in a manner obvious from the 
afore-described mode of operation of the system shown in FIG. 6. 
The devices for storing thermal energy illustrated in FIGS. 2 to 5b may be 
substituted in an obvious manner in the systems of FIGS. 6 and 7 if an 
independent hot-water supply is furnished, and other variations of these 
solar heating systems will readily suggest themselves to those skilled in 
the art. 
It should be understood, therefore, that the foregoing disclosure relates 
only to preferred embodiments of the invention, and that it is intended to 
cover all changes and modifications in the examples of the invention 
chosen for the purpose of the disclosure which do not constitute 
departures from the spirit and scope of the invention set forth in the 
appended claims.