Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
.
The bulk storage of heat or coolness at certain
temperature levels has many applica-tions such as solar heating
of buildings at 85 to 120~F, solar Rankine engines or absorption
refrigeration machines a-t 200 to 250`~F, off-peak hour opera-tion
of air conditioners at 30 to 600F, off-peak hour operation of
r-efrigeration plants at -20 to ~20~F, etc. The materials used,
except in the case of water at 320F, must be carefully mixed
in certain proportions with special equipment and techniques,
and must be kept away from materials that will corrode. Such
materials are bulky and heavy to transpor-t, and must be used
in contact with large area hea-t exchange devices because of the
poor thermal conductivity of these heat storage materials.
In order to minimize volume, weight, and cost~
heat of fusion materials with change of phase between solid and
liquid have been proposed, tested and tried experimentally
because 7,000 to 12,000 BTU's per cubic foot can be stored within
the above narrow temperature ranges, whereas if only a liquid
phase is used, such as water, capacity is limited to 2,000 to
3,000 BTU's per cu. ft. or so. These heat of fusion materials,
usually inorganic salt hydrates, must have provisions to prevent
stra-tification and subcooling.
Most prior designs have used air~ as the heat
transfer medium. Such prior ar-t designs have been very bulky
due to -the required volume of the air ducts and also have required
multiple encapsulation because of the requisite multiple air
passages of comparatively large cross sectional area. In certain
instances the prior art has attempted to u-tilize liquid as the
heat transfer medium; however such prior art arrangements have
been largely limited to the freezing of water in metallic tanks,
plates or tubes. However, such metallic devices suffer from
corrosion and cause galvanic ac-tion which can cause rapid deter-
ioration of various component parts. Moreover they are expensive
and in~lexible, subject to damage due to expansion of the phase
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~-hange material, and are heavy and difficult to transport.
O-~ner suggestions have involved multiple encapsulation with its
consequent high cost.
SUMMARY OF l'HE INVENTION
There are eight primary problems that have here-
tofore prevented the use of hea-t of fusion materials, or so-
called phase change materials, from being used in the storing
of thermal energy in a practical manner. They are cost of
equipment, poor thermal conductivity of phase change materials
(PCM's), corrosion, volume change during fusion, evaporation
` of water from sale hydrates, subcooling and stratification of
such materials, and cost of shipping. The way that the present
invention solves these eight problems is enumerated below.
1. Cost of equipment: The fi.rst advantage of the
present invention is that it enables the use of plas-tic heat
-transfer tubing whose relatively low thermal conduc-tivity can
be compensated for by greatly increasing the heat transfer
surfaces in accordance with the method and system of this
invention -thereby providing a large saving in cos-t. ~ne or ;
only a few plastic tanks are used instead of multiple encap-
sulation, thus also lowering cost.
~ 2. Poor thermal Conductivity: The limi-tation in
; heat transfer rates, moreover, is not in the liquid conduit
material but in the body of -the phase change material. Thus
the large amount of plastic heat transfer area is matched with
characteristics of the PCM by a multiplicity of small plastic
liquid transporting tubes distributed uniformly throughout the
entire mass of the PCM. The heat flow path at any point is
thus made very short.
3. Corrosion: Corrosion is of particular importance
because inorganic salt hydrates provide the necessary medium
for a battery if two dissimilar metals are present in any form
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w-;thin the salt. Severe corrosion of the metals can result
quickly. Plastics alone, including tubing, headers and fitting,
or with but one non-corrodible metal such as stainless steel
anywhere within the salt can be satisfactory.
4. Volume Change: The problem of volume change
during fusion is grea-tly lessoned by having a flexible plastic
material for both the outer container for the PCM ând for
the heat transfer surfaces all throughout the PCM. They will
take up any thermal expansion forces both on a large scale and
also locally in connection with a particular tube.
However, an element of this invention is that the
plastic tubes within the PCM are arranged so that the average
temperature between the liquid in any point in any tube throughout
the PCM and that in the adjoining -tube is approximately the same.
This is accomplished by means of mul-tiple parallel circuits with
U-bends at -the end of each circuit and every alterna-te tube
connected to a supply header and -the adjoining -tubes to a re-turn
header. See patent to C.D. MacCracken and Helmu-t Schmidt,
#3,751,935 dated August 14, 1973 for a method of crea-ting an
ice slab of uniform temperature for ice skating rinks which has
since become the leading way to build an ice rink in the U.S.A.
referred to commercially as the Icemat* ri~k.
When water is frozen to ice, which is one of the many
PCM's u-tilized in this invention, -the heat -transfer liquid enters
-the supply header and small tubes -typically at about 240F and
leaves the small tubes and return header at about 32F. With
a small plastic tube a-t 24F adjoining one at 320F the average
temperature is 280F and ice will form at a rate caused by that
average temperature. Halfway to the U-bends in each parallel
circuit the temperature in the supply tube will be 26F and the
adjoining return tube? 30~F, giving the same average temperature
of 28F. At the U-bends, where the supply and return small
plastic tubes are joined, the temperature will be 28 F in both.
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Therefore ice advantageously builds uniformly on allt~es entirely throughout the whole tank of water. The water
level rises in the tank because of the increased specific volume
of the ice formed but there is no sideward expansion forces as
-the ice joins from one spiral layer to the other because the
extra water volume has been sqeezed upwards previously. The rise
in water level provides a measure of the ex-tent of the fusion
process. The extra water on top is -the las-t to freeze.
Similarly in other PCM's the volume change is accom-
odated without thermal forces. Generally, it is a fact that
PCM's with a melting point above 32F shrink when they solidify
and for 320F and below they expand. For the PCM's that shrink
when they solidify, the tank is filled with liquid phase PCM
above the level of the tubes by the amount of the volume change -
shrinkage.
5. Evaporation: Evaporation of wa-ter from salt hydrates
changing their composi-tion and -thermal performance -take place
even through scaled plastic walls because of the property of
plastics called "water vapor transmission". This means that salt
hydrates sealed into multiple small plastic containers will
eventually change in performance with no way -to repair this
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except replacement. In the present invention only one or at
most a few, relatively large con-tainers are used to hold the
salt hydrates wi-th removable covers so that water may be poured
in to refill the lost water evaporated up to a mark showing
the proper level.
6. Subcooling: A major problem of salt hydrates is
subcooling, dropping below the freezing point without crystal-
lizing or ireezing taking place. This occurs because all the
crystals are melted when the salt hydrate is heated above the
melting(f~eezing) point and these crystals are not presen-t to
seed or nucleate upon recooling. Additives have been discovered
for many of the sale hydrates to promote nucleation (see patents
to M. Telkes numbers 2,677,66~ and 2~936,7L~1). Ano-ther very
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simple method is practical in -~hç case o~ the-presen~-
invention where the salt hydrates wi-th mel-ting points
above room temperature are held in a few large insulated
tanks. A very small projection from the tank outside of
the insulation keeps the salt hydrates in this projection,
or finger, from melting when heat is applied. Thus the
frozen crystals are present at all times and will nucleate
crystallization when the salt hydrate is cooled below its
freezing point. For example, the velocity of crystall-
ization of sodium thiosulfate pentahydrate is about one
inch per minute, so a 4 ft.diameter tank nucleates through-
out in an hour or less when cooling is provided by the heat
transfer liquid.
7. Stratificationo Another problem of salt hydrates
which this invention overcomes is the stra-tifica-tion of s~lid
c/rystals which, being heavier, in the case of mos-t salt
hydrates, sink to the bottom. They often nucleate into
different hydrate molecule combinations as they fall through
warmer areas. Because of incongruent phase change in these
different hydrate molecules the overall composition is
changed and consequéntly the performance. Also a permanent
precipitate forms at -the bottom. One solution to -this has'
been to limit -the vertical dimension to an inch or so. In
the present invention a straw~like mat of rubberized hair
or other inert low density matting is used as a spacer
between the -tubes. This effectively fills all the space
in the tank with such small openings like a filter that
there is no room for crystals to fall through. In addition
since the dual tubing averages the temperature uniformly
throughout, the crystal gr~owth will be also uniform through-
out and there will be no temperature differences to cause
large crystal build-ups in one area over another.
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~ 8. Shipping Cos-t: An average solar heated house is
required to store one million BTU's, and at 100 BTU's per
pound, five tons of PCM is needed and must be shipped.
The present invention provides for the PCM to be shipped in
heated tank trucks and pumped through a hose in liquid form
into the lightweight tanks and heat exchange tubing units
which have been previously installed and -tested. It is
well known that use of tank trucks is a much more economical
method of transporting and delivering :Large volumes of liquid
to many delivery points than by sealed containers.
THE DRAWINGS
The invention may be better understood by reference
to the drawings.
Fig. 1 is an elevational view of the -thermal storage
device showing a cylindrical, open--top -tank with spiral
-tubing and spacer mats in cross-section.
Fig. 2 is an elevational view of the flexible tubing
grid and spacer matting being rolled up into a heat exchanger
assembly.
Fig. 3 is sectional plan view of the spiral tubing
g*id and spacer matting rolled up and installed in the
cylindrical tank.
Fig. 4 is an elevational sectional view similar -to
Fig. 1 of the thermal storage apparatus showing a rectangular
-tank with -tubing mats tensioned around spacer bars and running
up and down vertically.
Fig. 5 shows a schematic arrangement of a thermal
storage apparatus connected to pump, heat input and output
devices and piping.
- Fig. 6 shows an enlarged partial sectional view of
a portion of the apparatus of Fig. 1 showing tubing, spacer
material and phase change material in a partially frozen
condi-tion O
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DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS
Refering now -to the drawings in greater detail,
Figure 1 illustrates a phase change thermal storage unit 2
which stores and releases a large amoun-t of heat and cool-
ness per unit volume owing to heat capacity of the phase
change materials. There are many materials and additives
which when phase change takes place release a large amount
of heat when it freezes and absorbs large amounts of heat
when it melts. Examples of PCM's are indicated further
below.
Thermal storage device 2 consists of a semi-flexible
walled open-top tank or container 4 which preferably is made
of thermoplastic material to provide flexibility and resis-
tance to corrosion, an important factor. Most phase change
ma-terials (PCM's) are corrosive to metals.
Prior to filling the tank wi-th PCM24 a preformed roll
of flexible tubing mat 8 and rubberized hair 22 is placed
in the tank. This roll of mat 8 and rubberized hair 22
fills the space in the tank so that no region within the
entire tank is more than a short distance away from the mat
tubing which carries the heat transfer fluid 26 for hea-ting
(mel-ting) and/or cooling (freezing) of -the PCM. Heat transfer
fluid 26 may be water, or if used below 32F (o C), an anti-
freeze solution must be utilized such as e-thylene glycol
with water. The flexible -tubing mat 8 is prefabricated in
the factory using extrudel twin tubings of small diameter
1/4" approx.) usually made of synthetic plastic material
which are kept clo'sely spaced and parallel to one another
by means of a spacer s-trip assembly which consists of a
rigid plastic strip 12 and a flexible plastic strip 14
attached together in such a way -that they form tight pockets
for -twin tubing 10. A more popular methocl of pre-forming
this grid of mat 8 is heat sealing. For uses involving
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temperatures too high for common plastics, synthetic rubber
or elastomeric compounds may be used such as EPDM (ethylene
propylene diene terpolymer). A number of -twin tubings 10
(number of tubings is dependent on the width of the mat grid,
a popular width is 4ft. nominal which requires 32 twin
-tubings spaced 1 1/2" center to center) are placed at a
given spacing and parallel to one another. A rigid vinyl
strip 12 is placed under the tubings 10 and a flexible
vinyl strip 14 is placed over the tubings 10 in such a way
that it is located right over the rigid strip 12. These
two strips 12 and 14 are then heat sealed together between
the dual tubes so that they form loops around individual
tubings. These mats 8 may be fabricated to any desired I
length. At one end of the mat 8 two headers are installed;
one is supply header 16 and the other return header 18. On
-the o-ther end of the mat 8 the 'U' bends 20 are ins-talled.
Referring -to Fig. 2 the flexible tubing mat 8 and
rubberized hair matting 22 are rolled together by laying
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out on a long table a longer length of the horse hair
matting 22 on top of the flat extended flexible tubing grid.
Then a roll is formed starting from the 'U' bend end of the
mat keeping the starting circle as small as possible (about
2'-l~'' diameter). When one completes the rolling process,
a roll is formed which has alternate layers o:F flexible
-tubing 8 and a spacer medium in -the form of a flexible
fibrous low density material having relatively large spaces
be-tween fibers, for example, a rubberized hair 22 is used
in pole vault and high jumping landing pi-ts. The supply
header 16 and return header 18 are on the outside of the roll,
but as can be seen in Figure 3 an extension of the layer
of spacer matting separates the tubing and tank wall.
This roll is then installed in t he tank 4. Two ports
are provided in the tank cover 6 for inlet connection
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assembly 27 and outlet connection assembly 28 which are
connected to -the supply header 16 and return header 18
and -to tubing nipples 17 by known plumbing methods. For
large tanks more than one tubing mat 8 may be placed end
to end in Figure 2 and rolled up in one roll so as to limit
the pressure drop of fluid 26 in the -tubing.
The device 2 is filled with a suitable PCM 24
in accordance with the temperature requiremen-ts of -the
application. The tank 4 is kept covered using a clamped
plywood or plastic cover 6, fill plug 25, and gaske-t 9 held
by clamps 37 as a prevention against dust accumulation,
evaporation, leakage in shipment, and con-tamination of the
PCM 24 and also to keep the lightweight buoyant heat
exchanger from raising up ou-t of the often heavier density
PCM when in the liquid s-tate. Insula-tion 32 is provided
all around the tank 4 with insula-ted base 34. The tank 4 is
also provided wi-th a nuclea-ting element in the form of a
tubular conduit which protrudes ou-tside the insulation 32
and is exposed to ambient temperature. The purpose of
nuclea-ting device 30 is to retain some frozen crystals of
the PCM 24 while all the PCM 24 inside the tank 4 is in
molten state. These trapped crystals in the nucleating
device 30 are very helpful in initiating the crystallization
of the PCM for avoiding subcooling. It is a -tendency of
the liquified PCM when there are no crys-tals present within
the material that it becomes subcooled which is undesirable
~'l because it delays the phase change and reduces its effect-
iveness.
Fig. 3 shows a sectional plan view of Fig. 1 to
show how the spacer matting is located with respect to the
tubing grid. Depending upon the heat transfer cycle time
for charging and discharging the PCM storage material, a
- ~thinner or thicker matting may be selec-ted with consequent
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change in length of tubing grid and matting. Details of
this are discussed later. From the description of ~ig. 1
the detail elements may be understood.
Fig. 4 illustrates another -thermal storage
device 2. The device includes a rectangular tank 5 which
is preferably made of a thermoplastic material which there-by
provides its quali-ty of beingsemi-~le~:ible and resistant
to corrosion to various chemicals used as PCM's. However,
the tank may be metallic~ particularly if the PCM is water,
and a plastic liner or coating 3 is used over the metal.
The rectangular design of the tank 5 facilitates the use
of flexible tubing mat 8 without -the use of spacer material
throughout the length of the mat 8. The mat 8 is installed
differently by festooning the mat up and down around the
spacer rods 40 which are lnstalled in two rows, one near the
top of the tank 5 and the other near the bottom. In each
row spacer and support rods 40 are equally spaced for example
at approximately 1 1~2" to 6" center-to-center distance and
are arranged parallel to one another. Supply header 16
and return header 18 are located on the flange into the
upper portion of the tank 5 where they are secured in place
by header holding clips 38 which are shown a-ttached to the
flange wall of the tank 5. The other end of -the ma-t 8 which
has 'U' bends 20 is located on -the flange of the upper por-tion
of the tank on the opposite side from the headers. The 'U'
bends 20 are secured in place on the flanges by using an
anchor strip 36 which has the same number of hooked fingers
as the ? U' bends. After assembly the tank 5 is filled with
.
the PCM 24. The tank 5 has a cover 7 to prevent evaporation
and contamination by falling foreign matter but need not be
the clamped cover of Fig 1 because the tubing assembly is
anchored by spacer rods 40. The tank is located on an
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insulated surface 34 and is well insula-ted all around with
insulation 32. A nucleating device 30 extends through the
insulation 32 outside the tank. As described under Figure
1, this nucleating device 30 retains some crystals while
all the other PCM 24 is in molten state inside -the tank 5.
Theæ trapped crystals in the nucleating device 30 help start
the freezing cycle without undergoing subcooling.
Figure 5 shows a schematic diagram of a thermal
s-torage system with a tank 50 containing a PCM for storing
thermal energy by the latent heat of fusion, a pump 52 for
pumping heat transfer liquid 26 through multiple small tubes
8 and 10 as shown in Figure 1, and a variety o~ heat inpu-t
devices on the left side and heat output devices on the right
side connected with piping 53 and valves 55, some of which
are not shown because it is obvious to selec-t various piping
circuits by valving. Fig. 5 depicts an illus-trative system
only,
For example starting at the top left and proceeding
down are shown examples of heat input equipment solar col-
lector 54, air coil 56, heat pump 58, electric resistance
liquid heater 60, fossil fuel boiler 62, ice skating rink
grid 64, and cold storage room 66.
Starting at top righ-t and proceeding down are
shown examples of heat output equipment including agri-
cultural or industrial process heater 70, water heater
72, heating coil in air duct 74, heat pump 76, radiant base-
board heater 78, radiant floor heater 80, Rankine engine 82,
absorption air conditioner 84, and air cool or cooling tower
86.
The various heat input devices shown 54-66 may
all be used -to melt a PCM in tank 50 selected for -the
appropriate temperature level of -that hea-t input device.
For example solar collector 54 may heat fluid-. 26 to 130F
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as it is being pumped t-hrough collector 54 on its way to tank
50. Pipe insultation 57 prevents substantial change in
temperature of fluid 26. The reader will understand that
insulation 57 should be distributed throughou-t the system
but is omitted to simplify the drawing,. In tank 50 so~id
PCM sodium thiosulfate pentahydrate, which melts at 118~F,
will begin to melt by taking heat from fluid 26, dropping
its temperature from 1300F or slightly less to perhaps 1250F
as it leaves tank 50 and is pumped back to collector 54.
After several hours, the length of time depending on the
total area and spacing of small tubes 8 and 10, the PCM ~ ,~
will be totally melted except for what is in nucleating
device tube 30 in Figure 1.
The hea-t stored in the above example in -tank 50
may be pumped via fluid 26 -to various heat outle-t devices
on the right side when desired by suitable valve operation.
For example heating coil in air duct 74 may be selected
and heating provided -to a structure, not shown, in the usual
manner. Or water heater 72 may be heated by fluid 26.
Similarly agricul-tural process 70, such as grain drying,
may be performed or room radiant heating 80.
If a higher temperature PCM were selected, such
as trisodium phosphate dodecahydrate at 150F, or magnesium ,'
chloride hexahydra-te at 243~F, solar collec-tor 54 could be
utilized advantageously to supply heat to heat output
devices such as baseboard radiation 78, Rankine engine 82,
and absorp-tion air conditioner 84. Heat pump 76 could be
best utilized with a PCM ,at 32 F or 55 F supplying heat
to the evaporator.
In similar manner the other heat inputs may be
advantageously connected through tank 50 to many of the
heat outlet units. One example of each will be men-tioned.
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Air coil 56 can be used to melt ice in -tank 50
created by operation of heat pump 76.
Heat pump 58 can be used during night off-peak
hours to melt a 1180F PCM tank 50 which will in turn provide
heat during the day to air coil in duct 74.
Electric heater 60 can supply heat to melt PC~
at 150F in tank 50 during off-peak hours to be used during
the day in baseboard radiator 78.
Fossil fuel boiler 62 7 undersized for direc-t
heating application in a church, can store heat ahead of
time in tank 50 and release it into the church on Sunday
morning through radiant heater 80.
Ice rink 6L~ can be kept frozen during peak daytiMe
hours by coolness stored a-t 12F in a PCM such as 22% ethylene
glycol and water the previous night by operation of heat
pump 76.
Cold storage room 66 may si~milarly be kept cold
by storing coolness from the heat pump during off-peak
hours.
Cooling tower 86 can be operated at night to
freeze a 55 F~PCM and supply air conditioning through air
coil 56 in the day-time.
There are many other combinations for which
thermal storage may be used. It will be understood that
there may be multiple heat inlets and multiple outlets which
may be interconnected in various cross-combinations. Fig.
5 illustratively shows ~only some of the possible heat input
and heat output equipment that might be advantageously used.
Figure 6 is an enlarged cross-sectional view of
a section of the thermal storage device 2. The section
shows two layers of mat 8 spaced apart approxima-tely 1" by
rubberized hair 22 which has an open wiry appearance.
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Rubberized hair has very large air spaces within the matted
structure and actually leaves most of the space for PCM
24 while also keeping the layers of mat 8 spaced properly.
The flexible tubing mat 8 is factory fabricated using
small diameter -twin tubings 10 which are extended out of
thermoplastic material suitable for a wide range of tem-
perature and is corrosion resistant.
Area 24A of PCM denote the frozen crystals around
the tubes during a discharging cycle when the PCM is giving
up heat. Areas 24B between the tubes show the melted
unfrozen part. During a charging cycle this would be
reversed. Heat domain divider line 42 denotes the location
equidistant between the tubes where heat flow divides
between the domain of each tube. It should also be no-ted
that whichever of -the -tubes 10 of each pair is connected
to inlet header 16 will have more frozen PCM surrounding
it during a discharge cycle because it is colder and will
have more melted PCM surrounding it during a charging,
,
heating cycle. Fig. 6 is shown near the halfway point
(close to the U-bends) so little difference in temperature
is noted, and thus the frozen PCM 24A will be fairly
symmetrical.
It is understood -that the charging or freezing
~eriod involves solid PCM being around the tubes and melted
PCM being out halfway between the tubes, while the melting
period involves melted PCM around the tubes and frozen PCM
a~ the halfway point. Since liquid can transfer heat by
conduction and convection, that is, moving around within its
melted space, while solid PCM can only transfer by conduction,
the freezing up period will take longer.
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EXAMPLE ~ 2~ ~
1. An electric utility company introduces an
off-peak rate of $.060 per KWH for 10. A.M. -to 10 P.M.
and $.015 from 10 P.M. to 10 A.M. A small office building
uses 20 tons of chilled water air conditioning at 44 F
from 9 A.M. to 6 P.M. during the summer season. The
following will show the storage equipment as shown in the
present invention which is required and how much operating
cost is saved.
Ice at 32F provides 44 F water with a 12
differential. The amount of ice required is compu-ted as
follows:
(a) 12 hours x 20 tons x 12,000 BTU's per hour
per ton 144 BTU's per pound of ice 56 pounds ice per (
cubic foot requires 357 cubic feet of ice in -the PCM tanks
50 or 5OA. A 6 ft. diame-ter plas-tic -tank five feet high
is a practical maximum size and this holds 120 cubic feet
up to a 4 1/4 ft. level. Therefore three 6 ft. diameter
tanks are required.
The spacing of the plastic tubes within the ice
must be such that all the ice be melted in 9 hours and all
the water refrozen in 12 hours because there is a time
period described above when the rates are low. The spacing
determines the total hea-t transfer area and thus the length
of the spiral tublng mat to be installed in the tan~.
I have found that 14 BTU/hr/sq/ft./ F can be
transferred from an ice slah on both sides of -the mat up
to 1" thick.
Assuming an average temperature differential of
40F when freezing the ice, it would mean the chilled anti-
freeze solution would enter at about 24F and leave at 32~F
with an average of 28~F. A refrigerant suction temperature
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- of about 180F produces this desirable temperature pattern.
The calculation, then, is as follows:
(b) 20 tons x 12,0~0 BTU/hr/ton
14 BTU/hr/ft i F x 4 F = 4,286 sq.f-t.
and with mats 4 ft. wide, this meant 4286/4 - 1072 running
ft. of mat
located in three 6 ft. diameter tah~s. Such -tanks have a
combined cross-sectional area of 85 sq. ft. The spirally
coiled mats take up substantially the whole space in the
cylindrical tanks. Thus each coil path has a width of
85/1072 = .079 ft. or .95 in. Therefore 1072 running feet
in the three tanks provide 357 running feet per tank.
I have found out that pressure drop consider-
ations as a practical matter limit mats to about 90 ft. in
length. Thus you install more -than one mat in each tank 3
4 mats per tank each 89 ft. long, or 12 89 ft. mats in all.
The spacer material would be ~95" less -the width of the mat
which is .31", thus about 5/8". An alternative is to use
6 mats 60 ft. long which would raise the cost of more headers
and U-bends but would reduce pressure drop, allow for more
low, and provide faster response.
The present opera-ting cost of -the offlce building
air conditioning equipment would be about
(c) $.06/KWH x 12,000 BTU/hr/-ton
746 BTU/KWH x 3.0 C.O.P. $-32/ton-hr
Assuming that this small office building is
operating 25 weeks, 5 days per week at 50% of full load
9 hours per day. the calculation is as follows:
(d) 25 x 5 x 9 x .5 x 20 = 11,250 ton-hrs., or
$3,600 cost.
Since the refrigeration suction temperature will
be lower because of the freezing of the ice, about 18 F vs.
3~ F for chilled water, the C.O.P. (coefficient of perform-
; ance) of the chiller heat pump can be assumed to be about
2.5 instead of 3Ø The cost is calculated as follows:
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(e) $.015 x 12,000 = $.0g6/-ton-hr
and for 11,250 ton-hrs, the cost would be $1,080. A
savings of $2,520 per year, or a savings of 70%, compared
to operating without storage in the small office building.
II. Example II using sodium thiosulfate
pentahydrate, a phase change material, PCM-118, which
melts at 118 F, or 115 F in this case because of certain
impurities, is to store heat for one cloudy day and -two
nights during 30~F average temperature weather in a house
that takes 50,000 BTU's/hr at O F. Since the design base
is 65F, 35/65 x 50,000, or 26,923 BTU's per hour for 40
hours, or 1,076,920 BTUts must be able -to be s-tored.
With 92 BTUs per pound la-tent heat and 18 BTUs
per pound sensible heat between 100 F, 1,076,920 BTUs divided
by 110 BrrUs per pound shows -that 9,790 pounds of salt are
requlred .
Since salts are more conveniently loaded at the
factory into tank or tanks 50, the weight of loaded con-
tainers is a factor, and a practical limit of about 1,000
pounds for shipping and moving into a house basement is
assumed. PCM-118, with a specific gravity of 1.6, will
store over 10q,000 BTUs and weigh about 1,000 pounds in a
plastic container 2 feet in diame-ter and 4 feet high.
Ten such tanks 50 are needed for this example,
; providing about 1,100,000 BTUs. Heat transfer liquid from
the solar collectors at about 1300F enters the tank tubing
leaving at 116F for an average of 1259F,= 8 F above the
~usion point at llS~F. Opposite from the other example,
charging the tank 50 involves melting liquid around the
tubes first which speeds heat transfer by conventional
motion of the liquid.
Assuming two sunny days to charge the tanks with
heat~ assuming 900 BTUs per square foot per day from the
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': ' '
collectors~ and assuming 25,000 BTUs per hour are needed
to hea-t the house during the 8 hour sunny days, -then
1,276~000 BTUs would be needed along with 1,476,000/900
x 2 = 820 sq. ft. of collector. The charging time is 16
hours (the two 8 hour solar days) and the discharging
(freezing) time is 40 hours.
Since PCM-115 has a thermal conductivity about
1/3 less than ice, an overall coefficient of abou-t 5 BTU/
hr/ft / F from each side is reasonable, or 10 overall
when the salt is solid. The partially liquid phase should
be higher but can be assumed to be the same. The calcu-
lation for the mat area would be
Lo7TU0-- BTUs -- - - = 840 sq. Et.
hr ft2 F x 8 F x 16 hrs
and with 4 feet high mats, it would mean 2L0 running feet
divided into 10 -tanks, or 21 fee-t length per ma-t. Since
the ten tanks are each 2.0' diameter, their area is 3.14
sq. ft., and 3.14 sq, f-t./21 ft. = .150 ft. or 1.8 in. is
the width of each coil path of the spiral. Sub-tracting
the mat thickness of 0.3 in., the spacer material is 1.5"
thick.
It is to be noted that since the salt shrinks
as i-t freezes, the mol-ten salt should more than cover the
tubes and the frozen salt will be totally wi-thin the height
oE -the tubes in tank 50.
The headers 16 are made of ABS or CPVC plastic
pipe with ABS or CPVC nipples 17 solvent ceménted to the
headers for low cost, adequate heat resistance and elimin-
ation of corrosion. The mat tubing is a medium or high
density polyethylene with butyl rubber additive for flexi-
bili-ty to aid in making tight sealing joints. Stainless
steel U-bends and s-tainless steel tubing clamps are -the
only metal in contact with the salt to avoid ga~Lvanic ac-tion.
-- 19 --
.
i2~
The examples given are illustrative of various
applications that may be made of phase change material
thermal energy storage according to my invention. These
examples are not to be thoughtof as limiting as to any
particular use, dimension, or material. It is intended
that various modifications which might readily suggest
themselves to those skilled in the art be covered by the
scope of the following claims.
. . .
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