CA1104334A - Thermal energy storage composition - Google Patents
Thermal energy storage compositionInfo
- Publication number
- CA1104334A CA1104334A CA331,558A CA331558A CA1104334A CA 1104334 A CA1104334 A CA 1104334A CA 331558 A CA331558 A CA 331558A CA 1104334 A CA1104334 A CA 1104334A
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- CA
- Canada
- Prior art keywords
- thermal energy
- peat moss
- set forth
- energy storage
- storage composition
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
- F28D20/00—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00
- F28D20/02—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00 using latent heat
- F28D20/023—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00 using latent heat the latent heat storage material being enclosed in granular particles or dispersed in a porous, fibrous or cellular structure
-
- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
- C09K5/02—Materials undergoing a change of physical state when used
- C09K5/06—Materials undergoing a change of physical state when used the change of state being from liquid to solid or vice versa
- C09K5/063—Materials absorbing or liberating heat during crystallisation; Heat storage materials
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/14—Thermal energy storage
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E70/00—Other energy conversion or management systems reducing GHG emissions
- Y02E70/30—Systems combining energy storage with energy generation of non-fossil origin
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Combustion & Propulsion (AREA)
- Materials Engineering (AREA)
- Organic Chemistry (AREA)
- Dispersion Chemistry (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Central Heating Systems (AREA)
Abstract
"THERMAL ENERGY STORAGE COMPOSITION"
ABSTRACT OF THE DISCLOSURE
Peat moss is used in a thermal energy storage composition to provide a network in which to trap an incongruently melting salt hydrate capable of storing thermal energy as latent heat of phase change. The peat moss network is effective in preventing the segregation of a de-hydrated form of the salt between heating and cooling cycles. In a pre-ferred embodiment the salt hydrate is the decahydrate of sodium sulphate.
A nucleating agent such as sodium tetraborate decahydrate is included to prevent supercooling in the composition, and promote crystallization of the decahydrate of sodium sulphate.
ABSTRACT OF THE DISCLOSURE
Peat moss is used in a thermal energy storage composition to provide a network in which to trap an incongruently melting salt hydrate capable of storing thermal energy as latent heat of phase change. The peat moss network is effective in preventing the segregation of a de-hydrated form of the salt between heating and cooling cycles. In a pre-ferred embodiment the salt hydrate is the decahydrate of sodium sulphate.
A nucleating agent such as sodium tetraborate decahydrate is included to prevent supercooling in the composition, and promote crystallization of the decahydrate of sodium sulphate.
Description
lla4334 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.
3~
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 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.
llUat~3~
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.
~ 4 ~3 ~
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 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.
llU~33a~
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.
11S~4334 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.
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.
3~
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 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.
llUat~3~
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.
~ 4 ~3 ~
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 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.
llU~33a~
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.
11S~4334 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.
Claims (16)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A thermal energy storage composition which utilizes latent heat of phase change to store thermal energy comprising:
an incongruently 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.
an incongruently 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.
2. The thermal energy storage composition as set forth in claim 1 wherein the salt hydrate is the decahydrate of sodium sulphate.
3. The thermal energy storage composition as set forth in claim 2 wherein the peat moss is mascerated sphagnum peat moss.
4. The thermal energy storage composition as set forth in claim 3 wherein the nucleating agent is sodium tetraborate decahydrate, contained in the composition in the amount of approximately 3% by weight.
5. The thermal energy storage composition as set forth in claim 4 wherein at least 7% sphagnum peat moss on a dry weight basis of peat is included.
6. The thermal energy storage composition as set forth in claim 3 wherein at least 7% sphagnum peat moss on a dry weight basis of peat is included.
7. The thermal energy storage composition as set forth in claim 5 wherein the moisture content of the sphagnum peat moss is in the range of approximately 10 to 30% by weight.
8. The thermal energy storage composition as set forth in claim 6 wherein the moisture content of the sphagnum peat moss is in the range of approximately 10 to 30% by weight.
9. A process for forming the thermal energy storage composition of claim 1 comprising:
mixing peat moss with an incongruently melting salt hydrate, capable of storing thermal energy as latent heat of phase change, and a nucleating agent; and heating the mixture to at least the phase change temperature of the salt hydrate to form a melt which when cooled results in the salt hydrate and nucleating agent being trapped in a network of peat moss.
mixing peat moss with an incongruently melting salt hydrate, capable of storing thermal energy as latent heat of phase change, and a nucleating agent; and heating the mixture to at least the phase change temperature of the salt hydrate to form a melt which when cooled results in the salt hydrate and nucleating agent being trapped in a network of peat moss.
10. The process as set forth in claim 9 wherein the salt hydrate is the decahydrate of sodium sulphate.
11. The process as set forth in claim 10 wherein the peat moss is mascerated sphagnum peat moss.
12. The process as set forth in claim 11 wherein the nucleating agent is sodium tetraborate decahydrate, contained in the composition in the amount of approximately 3% by weight.
13. The process as set forth in claim 12 wherein at least 7%
sphagnum peat moss on a dry weight basis of peat is included.
sphagnum peat moss on a dry weight basis of peat is included.
14. The process as set forth in claim 11 wherein at least 7%
sphagnum peat moss on a dry weight basis of peat is included.
sphagnum peat moss on a dry weight basis of peat is included.
15. The process as set forth in claim 13 wherein the moisture content of the sphagnum peat moss is in the range of approximately 10 to 30% by weight.
16. The process as set forth in claim 14 wherein the moisture content of the sphagnum peat moss is in the range of approximately 10 to 30% by weight.
Priority Applications (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA331,558A CA1104334A (en) | 1979-07-06 | 1979-07-06 | Thermal energy storage composition |
| DE19803025401 DE3025401A1 (en) | 1979-07-06 | 1980-07-04 | Heat storage medium using Glauber's salt - suspended in matrix of peat moss together with hydrated borax used as nucleant, suitable for solar heat collectors |
| JP9075580A JPS5641289A (en) | 1979-07-06 | 1980-07-04 | Regenerating composition and manufacture thereof |
| IT8009476A IT8009476A0 (en) | 1979-07-06 | 1980-07-04 | COMPOSITION BY ACCUMULATION OF THERMAL ENERGY, PROCEDURE FOR ITS FORMATION |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA331,558A CA1104334A (en) | 1979-07-06 | 1979-07-06 | Thermal energy storage composition |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1104334A true CA1104334A (en) | 1981-07-07 |
Family
ID=4114659
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA331,558A Expired CA1104334A (en) | 1979-07-06 | 1979-07-06 | Thermal energy storage composition |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS5641289A (en) |
| CA (1) | CA1104334A (en) |
| DE (1) | DE3025401A1 (en) |
| IT (1) | IT8009476A0 (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5987455U (en) * | 1982-11-30 | 1984-06-13 | 日本キム株式会社 | bag-shaped container |
| FR2554790B1 (en) * | 1983-11-15 | 1986-11-21 | Vittel Eaux Min | BAG IN FLEXIBLE SYNTHETIC MATERIAL COMPRISING A REMOVABLE RIGID ELEMENT AS A CLOSING ELEMENT |
| JPS61137932A (en) * | 1984-12-10 | 1986-06-25 | 松下電器産業株式会社 | Drain apparatus of bathtub |
| US4702853A (en) * | 1986-10-06 | 1987-10-27 | The United States Of America As Represented By The Department Of Energy | Phase change thermal energy storage material |
-
1979
- 1979-07-06 CA CA331,558A patent/CA1104334A/en not_active Expired
-
1980
- 1980-07-04 JP JP9075580A patent/JPS5641289A/en active Granted
- 1980-07-04 DE DE19803025401 patent/DE3025401A1/en not_active Ceased
- 1980-07-04 IT IT8009476A patent/IT8009476A0/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5641289A (en) | 1981-04-17 |
| DE3025401A1 (en) | 1981-01-08 |
| IT8009476A0 (en) | 1980-07-04 |
| JPS5721542B2 (en) | 1982-05-08 |
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