Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
The present invention relates to a low temperature thermal
energy storage composition which utilizes the latent heat of phase change
to store heat.
Low temperature thermal storage materials are well known in
the prior art. Rocks, water and other fluids are often used, however the
excessive bulk and weight of the material needed to store a sufficient
amount of heat deters one from their use. The use o~ thermochemical heat
storage, wherei`n the latent heat of a phase change is utilized, permits one
to achieve compactness of the heat storage materi`al.
Materials suitable for heat of phase change storage have a
number of desirable properties, among which are a phase change in a practical
temperature range (usually about 90 - 200F), a high energy density (that
is, a high latent heat of phase change per unit volume), and low cost.
One such heat of phase change material is the decahydrate of
sodium sulphate, also known as Glauber's Salt or Mirabilite, occurring
` naturally or produced synthetically. It has the chemical formula Na2S04-10H20.
Glauber's Salt is particularly attractive because it is readily available,
inexpensive, and non-toxic, and the storage space required is small when
compared to non-latent hèat type storage materials. For instance, as re-
lated by Mr. F. Lindner in a paper given at the Energy and Politics Forum
of the Government of Baden-Wuerttembe~rg at the University of Stuttgard,
- May, 1977, to attain an equivalent heat storage capac;ty, a quantity of
rocks 34 times heavier and 27 times larger, or a quantity of water 6.5 times
- 25 heavier and 11.5 ti`mes larger than Glauber's Salt would be needed.
Glauber's Salt is known to melt in the crystal-bound water at
a moderate temperature of 90.8F, storing approximately 108 BTU/lb. as
latent heat of phase change. Recrystallization of the melt as it is
cooled releases the majority of this stored energy as recoverable heat.
The use of Glauber's Salt as a heat storage materi`al is re-
ported in U.S. Patents, 2,677,664 and 3,986,969, issued to Telkes.
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At least two maJor problems exist in any attempt to utilize the
salt hydrate for heat storage.
Firstly, upon cooling a melt of Glauber's Salt the mixture tends
to exhibit supercooling and thus the latent heat of recrystallization is not
fully recoverable. Telkes, in U.S. Patent 2,677,667 found that the problem
of supercooling could be overcome with the add;tion of a nucleating agent.
Particularly, sodium tetraborate decahydrate (Na2B407 10H20) has been proven
to be effective.
Secondly, &lauber's Salt, on melting, exhibits incongruent
melting; that is, two new phases are formed. One such phase is a metastable
supersaturated aqueous solut;on of sod;um sulphate, the water of solution
being ~holely derived from the water of hydration of Glauber's Salt repre-
senting 56% of the original mass. The other phase is solid anhydrous sodiumsulphate, representing approxi~ately 18% of the original mass of the unmelted
Glauber's Salt; this latter phase, having a density of approximately tw;ce
that of the solution phase, settles to form a layer on the bottom of the
container. On cooling, the sodium sulphate dissolved in the solution phase
begins to rehydrate with the water of solution to form Glauber's Salt crystals
which, having a higher density than the surrounding solution, settle on top
of the layer of anhydrous sod;um sulphate, thereby preventing a large fraction
of this material from rehydrating with the water of solution upon further
cooling. This large fraction is thus removed from further use for heat
storage, reducing the heat storage capacity of the system.
One solution to the problem of segregation resulting from in-
congruent melting has been to apply mechanical mixing to the meltedsolution andsettled or segregated layer of anhydrous sodium sulphate. As
developed by Herrick and reported in Business Week, January 16, 1978, the
unmelted Glauber's Salt is filled and sealed into a cylindrical container.
After melting and dur1ng cooling and further cycling, the cylinder is
continuously rotated slowly with its axis in the horizontal plane causing
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the segregated layer of anhydrous sodium sulphate to be lifted and then
overturned through the bulk of the solution, wherehy substantial rehydration
may be encouraged. However, this method suffers from the disadvantage of
requiring extra input of mechanical energy derived from an external power
source and maintenance of a rotating drive and suspension system.
Another approach reported by D. D. Edie and S. S. Melsheimer in
- "Sharing the Sun", Volume 8, 1976, Pages 262 to 272 considers providing
agitation and turbulence of the anhydrous sodium sulphate phase by circu-
lating an immiscible fluid of lower density than the salt solution from the
bottom to the top of the container. The bubbling action of the immiscible
fluid flowing up through the bulk serves to stir up the anhydrous layer,
thus exposing it to rehydration during cooling. In this approach, an
additional energy expense in the form of fluid pumping is required to
accomplish the objective of rehydrating the segregated anhydrous salt.
- 15 A better approach to this problem appears to be the provision
of a type of lattice network or dispersant to keep the anhydrous salt
suspended or trapped wi'thin the bulk of the solution. Telkes, in U.S.
Patent 3,986,969, has taught suspending the salt hydrate in a thixotropic
gel, as provided by an aqueous solution of attapulgite clay.
The present applicant has investigated this clay-salt mixture
and has found, after subjecting it to a number of heat-cool cyclesg that the
thixotropic gel can ~reak down, allowing a portion of the anhydrous salt
to settle out of solution.
SUMMARY OF THE INVENTION
The inventor has discovered that peat moss provides an
excellent lattice network in which to trap or suspend an incongruently
melting salt hydrate. Thus, in accordance with the present invention, a
thermal energy storage composition is provided which comprises an in-
congruently melting salt hydrate, capable of storing thermal energy as
latent heat of phase change, and a nucleating agent, both being trapped in
a network of peat moss. The composition has been shown to be effective.
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with Glauber's Salt.
While not wishing to be bound b~ this explanation, it appears
that the ability of peat moss to form a network ;n which to effectively
trap anhydrous sodium sulphate is due to the reabsorptive properties of
peat moss. Peat moss occurs in nature containing up to 96% water. A
large portion of this water can be removed and reabsorbed through a number
of cycles without destroyi~ng the bulk appearance of the peat. It is
believed that the peat moss, in the composition of the present invention,
absorbs the solution created by melting the Glauber's Salt. Both the
solution and the anhydrous salt are trapped ;n the network of peat fibres,
holding both components in close proximity for rehydration of the salt as
the composition is cooled.
To form this novel heat storage composition, the peat moss i5 dried,
preferably to a moisture content of 10 to 3a%, and preferably mascerated
to reduce the fiber size of the peat,preferably in the range of 1 - 3 mm.
The peat is then mixed with an incongruently melting salt hydrate,such
' as Glauber's Salt,and a nucleating agent,such as sodium tetraborate
decahydrate. This mixture is then heated to at least the phase change
temperature of the incongruently melting salt hydrate and preferably 5
to 10F higher, to form a melt, whereby the anhydrous salt, the nucleating
' agent and the solution formed on phase change are held in the peat network.
On cooling, the anhydrous salt rehydrates, and the salt hydrate and nucleat-
ing agent remain trapped in the network of peat moss.
Preferably, the peat moss usèd in the heat storage composition
is sphagnum peat, included on a dry weight basis of peat in at least 7%.
The nucleating agent is preferably sodium tetraborate decahydrate
included in the composition in an amount of about 3~ by weight.
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DESCRIPTION OF THE DRAWING
Figure l is a schematic of a solar energy storage unit
utilizing the thermal energy storage composition of the present invention.
_S RIPTION OF THE PREFERRED EMBODIMENT
The present invention seeks to provide a solution to the problem
of segregation arising from incongruent melting of salt hydrates being
used in thermal energy storage. The most preferred salt hydrate is the
decahydrate of sodium sulphate, however other incongruently melting salt
hydrates having favorable heat storage properties could be applicable.
Exemplary of other heat of phase change compounds which may be suitable are:
sodium thiosulphate pentahydrate, sodium carbonate decahydrate, and various
eutectic salt mixtures incorporating sodium sulphate decahydrate.
The thermal energy storage composition includes the incongruently
melting salt hydrate and a nucleating agent dispersed in a network of
peat moss.
Three types of peat moss are known and are biologically
classified as sphagnum, reed-sedge and humus peat moss. The present
invention has been demonstrated with sphagnum peat moss, which, on the basis
of low cost, ready availability, excellent absorptive properties and high
bulk/weight ratio, appears to be the most preferred type of peat moss to
employ. The other peat moss types do however have good absorptive pro-
perties and varying bulk/weight ratios. Thus a ready supply of these peats
may render them suitable for the purposes of the present invention.
Naturally occurring sphagnum peat moss can contain as much as
96% by weight water. Much of this water is removed in commercial
dra;ning and drying processes to reduce the moisture level to approxi-
mately 35 to 50% by weight. Although peat moss having such high moisture
contents can be used in the composition, the excess water necessarily dilutes
the salt hydrate, and thereby reduces the latent heat storage capacity
or energy density of the system. Generally, the lower the water content
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included in excess of the crystal-bound water in the salt hydrate, the
greater the energy density of the system.
Thus in a preferred composition, the sphagnum peat moss is
dried to as low as 10% by weight water by heating the peat in an oven at
a temperature less than about 150F. Higher drying temperatures and lower
moisture contents should be avoided since they tend to destroy the re-
absorptive properties of the peat.
Additionally, the sphagnum peat moss should be mascerated or
ground to reduce the fibre size. This has been found to improve the
ability of the peat moss network to effectively trap the anhydrous salt.
Unmascerated peat moss comprising the naturally occurring long fibres of
peat is not as effective in trapping the anhydrous salt. Thus a finer
network is desired. Conversely, reducing the fibre size below about 1 mm.
and drying below 10% moisture, reduces the wetability of the peat, which
makes subsequent blending difficult. A fibre size in the range of 1 - 3
mm. has been found effective.
A nucleating agent should be included in the heat storage
composition to overcome the problem of supercooling and promote nucleation
of the decahydrate of sodium sulphate. As provided by the teachings of the
Telkes patents, an effective nucleating agent is sodium tetraborate deca-
hydrate.
In accordance with the above teachings a suitable and most
preferred thermal energy storage composition includes in approximate
weight percentages:
90% Glauber's Salt;
7% mascerated sphagnum peat (on dry weight basis) having a
moisture content of about 10%; and
3% sodium tetraborate decahydrate.
This heat storage composition has thus far been subiected to
more than lOQ heat-cool cycles with no visible signs of either water or
anhydrous salt separation.
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The present applicant has found that 7% sphagnum peat moss,
calculated on a dry weight basis of peat, is approximately the minimum
amount of peat moss which can be included which will effectively keep the
anhydrous salt trapped. For practical purposes, the amount of Glauber's
Salt included should be maximized to achieve a high heat storage capacity
in the composition. In the above described composition, the heat storage
capacity is approximately 95 BTU/lb. of composition.
The abovedescribed composition has been disclosed with the
decahydrate of sodium sulphate, which is readily available in many lo-
cations in this hydrated form. While it is preferred to utilize thisform of the salt, it will be realized that in many locations the deca-
hydrate is not available. In such cases it may be possible to rehydrate
the anhydrous form of sodium sulphate; however it is difficult to achieve
the 56% water, 44% sodium sulphate ratio naturally present in Glauber's Salt.
Additionally, compositions prepared from the anhydrous salt are already
segregated, lowering the heat storage capacity of the system.
To form a suitable heat storage composition, the mascerated
sphagnum peat moss may first be dry mixed with Glauber's Salt and the
nucleating agent. The mixture is then heated with mixing to at least the
temperature of phase change, or somewhat higher, in this case to about 95F.
This temperature is maintained to form a melt of the mixture wherein the
solution and anhydrous sodium sulphate phases thus formed are blended
substantially uniformly through the network formed by the peat moss. On
cooling the composition, the anhydrous salt in close proximity to the water
of hydration can rehydrate, releasing the latent heat of phase change.
To utilize the abovedescribed thermal energy storage composition
in a heating embodiment it is generally combined with a source of thermal
energy, the availability of which does not correspond with demand, and a
heat transfer medium capable of transferring thermal energy between the
source, the storage composition and a space to be heated.
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In the embodiment shown in Figure 1, the compos;tion ;s
included in a simplified solar heat;ng system. It should be understood
that the composition could be us:ed with a number of thermal energy sources.
For instance, electrical energy could be stored at off-peak demand hours
for load levell;ng of energy demands on util;t;es.
The solar heat;`ng sys:tem includes a solar collector 1 wh;.ch
is effective in absorbing heat from the solar rays. Conventional air or
water pan collectors or more efficient vacuum tube collectors are well
known in this art. A heat trans.fer medium 2, ;:n th;s case a;r, ;s c;rcu-
lated by way of hlower 3 over or through. the solar collectors and intoa heat storage unit 4 contai:ning the heat storage composition.
The heat storage compos;tion ;.s prefera~bly conta;ned and sealed
;n shallow conta;ners 5, commonly referred to as trays. In th;s way the sur-
face area/volume rat;o, wh;'ch ;:s a limi:ting factor of heat exchange, is
maximized.
Once the temperature of the s~pacè to be heated has fallen below
comfortable li;mits, as determi:ned by thermostat control, a;r is c;rculated
over the heat composit~on and through the heated space 6. 3ypass ducts 7,
may be appropr;ately employed either when solar energy ;s not available or
space heating i:s not reqùired.
Whi.le the thermal energy storage composi:tion has been disclosed
in a heat;:ng embodiment, it should be understood the composit;on is also
effect;.ve for cooling purposes. In th.is case, the latent h.eat of crystal-
lization i:s used to remove thermal energy from the space to 6e cooled via
the heat transfer medi.um. The expressi.on "utili:zing the latent heat of
phase change to store thermal energy" i.s meant to include both heating
and cooling purposes.
While the present invention has been disclosed in connect;on
with preferred embodi:ments thereof, ;t s.hould be understood that there may
he other emhodiments which fall within the spir;t and scope of the invention
as defined in the following cla;ms.
