Thermal energy storage composition

Abstract

The present invention provides a thermal energy storage composition, the composition comprising a phase change material of Formula I Formula I wherein each of R1 and R2 is an alkyl group; and wherein the phase change material has a melting temperature of greater than or equal to 10 ºC and less than or equal to 15 °C.

Description

[0001] THERMAL ENERGY STORAGE COMPOSITION

[0002] Field of Invention

[0003] The present invention provides a thermal energy storage composition, a thermal energy storage unit comprising the thermal energy storage composition and an air-conditioning system comprising the thermal energy storage unit.

[0004] Background

[0005] The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0006] Over the last few years, use of air conditioning has become more widespread due to an improvement in the quality of life for many people. As a consequence of this, the demand for energy has significantly increased. One of the possibilities for improving the cooling efficiency of air conditioning is the use of phase change materials (PCMs). Phase change materials can absorb, store and release large amounts of thermal energy and so these materials are a promising storage medium for air conditioning systems.

[0007] Phase change materials should be considered in relation to the air conditioning system that they are to be used with in order for the entire system to work properly and efficiently. This is because the properties of each phase change material is different (e.g., phase change transition temperature, latent heat of fusion, specific heat and thermal conductivity.

[0008] Therefore, there exists a need for alternative and / or improved phase change materials for use in an air-conditioning system.

[0009] Summary of Invention

[0010] It has surprisingly been found that ethyl myristate and close analogues thereof are particularly useful as a phase change material in an air-conditioning system device. Aspects and embodiments of the invention will now be described by reference to the following numbered clauses.

[0011] 1. A thermal energy storage composition, the composition comprising a phase change material of Formula I

[0012] Formula I wherein each of Ri and R2is an alkyl group; and wherein the phase change material has a melting temperature of greater than or equal to 10 °C and less than or equal to 15 °C.

[0013] 2. The composition according to Clause 1 , wherein one or both of the following apply:

[0014] Ri is a Ci to C3 alkyl group; and

[0015] R2is a linear or branched Ci2to C14 alkyl group.

[0016] 3. The composition according to any one of the proceeding clauses, wherein the phase change material is ethyl myristate.

[0017] 4. The composition according to any one of the proceeding clauses, wherein the composition comprises an antioxidant.

[0018] 5. The composition according to Clause 4, wherein the antioxidant is

[0019] 6. The composition according to Clause 4, wherein the antioxidant is present in the composition in an amount of from 0.05 to 0.2 wt%, such as about 0.1 wt% of the total weight of the composition.

[0020] 7. A thermal energy storage unit comprising a thermal energy storage composition according to any one of Clauses 1 to 6.

[0021] 8. The thermal energy storage unit according to Clause 7, wherein the thermal energy storage unit is configured to be retrofitted to an air-conditioning system. 9. An air conditioning system comprising a thermal energy storage unit according to Clause 7 or Clause 8, wherein the thermal energy storage unit is configured to: enhance cooling provided by the air conditioning system by the phase change material transitioning from a solid phase to a liquid phase when the air conditioning system is in use for cooling; and when the air conditioning system is not in use for cooling, the phase change material returns to a solid state through application of a suitable means or apparatus within the air conditioning system to achieve this.

[0022] Drawings

[0023] FIG. 1 is the DSC curve of ethyl myristate.

[0024] FIG. 2 includes DSC curves of comparative examples i.e., (a) isobutyl stearate, (b) decyl tetradecanol and (c) 1 -decanol.

[0025] FIG. 3 depicts the experiment set-up for the charge-discharge test according to the present disclosure.

[0026] FIG. 4 includes the temperature profiles of 1 kg ethyl myristate in accordance to the chargedischarge test at different temperature ranges during (a) freezing (4-20°C, 6-20°C, 8-20°C and 10-20°C) and (b) melting (4-20°C, 6-20°C and 8-20°C).

[0027] FIG. 5 includes the temperature profile of 1 kg ethyl myristate in accordance to the charge / discharge test for (a) 1 cycle and (b) 10 cycles.

[0028] FIG. 6 includes the results of the charge / discharge test of ethyl myristate during (a) freezing and (b) melting at a temperature range of 6-20°C.

[0029] Description

[0030] It has been surprisingly found that the thermal energy storage composition according to the current invention is compatible with the operating conditions of a heat pump. Specifically, the phase change material present in the composition has a melting temperature of greater than or equal to 10 °C and less than or equal to 15 °C which is ideal in keeping the efficiency of the heat pump high. In addition, the phase change material is able to absorb at least 150 J / g during its phase transition, is able to be produced using low cost, widely available and sustainable raw materials, safe for humans and for the environment (no toxic, flash point >100 °C). Further desired properties are low subcooling (<5 C°), compatibility with the thermal energy storage unit materials (steel / copper / aluminum) at the working temperatures (10-30 °C).

[0031] Thus, in a first aspect of the invention, there is provided a thermal energy storage composition, the composition comprising a phase change material of Formula I

[0032] Formula I wherein each of Ri and R2is an alkyl group; and wherein the phase change material has a melting temperature of greater than or equal to 10 °C and less than or equal to 15 °C.

[0033] In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components / features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of’ or “consists essentially of’). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of’ or the phrase “consists essentially of’ or synonyms thereof and vice versa.

[0034] The phrase, “consists essentially of’ and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greaterthan 99% pure, such as greaterthan 99.9% pure, such as greaterthan 99.99% pure, such as greater than 99.999% pure, such as 100% pure.

[0035] Unless otherwise stated, the term “alkyl” refers to an unbranched or branched, acyclic and saturated hydrocarbyl radical. In some embodiments, Ri may be a Ci to C3alkyl group; R2may be a linear or branched Ci2to C14 alkyl group such that the melting temperature of the phase change material is greater than or equal to 10 °C and less than or equal to 15 °C. In more particular embodiments, the phase change material may be ethyl myristate. The phase change material according to the current invention has a melting temperature of greater than or equal to 10 °C and less than or equal to 15 °C. Advantageously, this is compatible with the operating conditions of a heat pump to keep the efficiency of the heat pump high. As a comparative example, while ice has been considered as a PCM material, the melting temperature of water (0°C) is too low for a heat pump as the heat pump efficiency is lower when the cold side temperature is lowered.

[0036] In some embodiments, the thermal energy storage composition may comprise an antioxidant. Advantageously, the antioxidant increases the oxidation stability / lifetime of the thermal energy storage composition. The antioxidant may be any suitable antioxidant that does not interfere with the phase change material. In some embodiments, the antioxidant may be:

[0037] The antioxidant may be present in the composition in any suitable concentration that does not negatively affect the heat storage capacity and the charging and discharging performance of the thermal energy storage composition. In some embodiments, the antioxidant may be present in the composition in an amount of from 0.05 to 0.2 wt% of the total weight of the composition. In more particular embodiments, the antioxidant may be present in the composition in an amount at about 0.1 wt% of the total weight of the composition.

[0038] When used herein, the term "about", in the context of concentrations of components of the formulations, may mean a variation of ± 5% of the stated value, more typically + / - 4% of the stated value, more typically ± 3% of the stated value, more typically, + / - 2% of the stated value, even more typically ± 1 % of the stated value, and even more typically + / - 0.5% of the stated value.

[0039] As noted above, the thermal energy storage composition according to the current invention is compatible with the operating conditions of a heat pump, improving the overall efficiency of the heat pump. Thus, in a second aspect of the invention, there is provided a thermal energy storage unit comprising a thermal storage composition as defined herein.

[0040] In a third aspect of the invention, there is provided an air-conditioning system comprising a thermal energy storage unit as defined herein, wherein the thermal energy storage unit is configured to enhance cooling by way of a phase transition of the PCM from solid to liquid when the air conditioning system is in use for cooling; and it returns to a solid state when the air conditioning system is not in use for cooling (e.g. through the application of a suitable means or apparatus built into the air conditioning system to achieve this).

[0041] In a general embodiment, the PCM composition changes from liquid to solid (and vice versa) when the air conditioning system is operating. In a charging phase, this can occur through an exchange of thermal energy between the PCM and an intermediate fluid, such as, but not limited to, water or directly between the PCM and refrigerant, depending on the air conditioning system and the integration of the thermal energy storage unit into the system of air conditioning. In a cooling phase, the solid PCM composition can absorb thermal energy from a carrier fluid, which can be for example, but not limited to, air or, preferably, water, so as to transition back to a liquid state. As will be appreciated, both the charge phase and cooling phases may operate in tandem with each other, though they may operate better separately.

[0042] Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples below.

[0043] Examples

[0044] All reagents were purchased from commercial sources and used without further purification.

[0045] Example 1 : Characterization Test

[0046] Various characterization tests including viscosity, thermal conductivity, flash point and density were performed on ethyl myristate in accordance to ASTM standards. Differential scanning calorimetry (DSC) was also performed on ethyl myristate and other comparative examples. Table 1 summarises properties of ethyl myristate.

[0047] FIG. 1 is the DSC curve of ethyl myristate. FIG. 2 includes DSC curves of the comparative examples i.e., (a) isobutyl stearate, (b) decyl tetradecanol and (c) 1 -decanol.

[0048] Table 1 : Properties of ethyl myristate

[0049]

[0050] Results and Discussion

[0051] Ethyl myristate is identified as an effective PCM candidate based on its melting temperature and thermal energy storage ability. Most importantly, ethyl myristate also has low viscosity, which facilitates the convection of the PCM.

[0052] Example 2: Oxidation Stability

[0053] Oxidation stability represents the ability of a material to resist the action of oxidizing agents and is an important property for a PCM, as it determines its durability over time. Oxidation stability can be improved by using suitable antioxidant additives, however, a high amount of antioxidant could negatively affect the heat storage capacity and / or affect the charging / discharging performance of the phase change material. Therefore, for this Example, only 0.1 % wt. of antioxidant (AOX) was used. The antioxidant used in this Example is

[0054] Oxidation stability test was performed on ethyl myristate in accordance with ASTM standards (ASTM D8206) using an oxidation stability tester (RapidOxy) under accelerated conditions (140°C, 7 kPa O2).

[0055] Results and Discussion

[0056] Table 2 shows the oxidation stability result of ethyl myristate with and without 0.1 % AOX. The product lifetime improved by 7 times from 125 min to 867 min. As such, the antioxidant helps to improve the oxidation stability of the phase change material. Table 2: Oxidation Stability of ethyl myristate with and without 0.1 % AOX

[0057] Example 3: Charge and Discharge Test

[0058] FIG. 3 depicts the experiment set-up for the charge-discharge test according to the present disclosure. A Syrris batch reactor was used to scale-up the testing to 1 kg. The reactor was jacketed and the set point min temperature was set between 4 and 10 °C to verify at which temperature ethyl myristate can undergo through a complete charge / discharge cycle (freezing / melting cycle). 10 consecutive runs were performed once the optimal set point min temperature was identified.

[0059] Results and Discussion

[0060] The charge-discharge test was performed at different temperature ranges to study the temperature profile of ethyl myristate. FIG. 4 includes the temperature profiles of 1 kg ethyl myristate during (a) charging / freezing (4-20°C, 6-20°C, 8-20°C and 10-20°C) and (b) discharging / melting (4-20°C, 6-20°C and 8-20°C). By setting the set point min temperature at 8 °C, it is clear that the freezing rate (charging rate) almost doubled. Furthermore, complete solidification was not achieved at 10°C. The results were very similar for 4°C and 6°C to 8 °C, so 6°C was determined to be the optimal set point min temperature.

[0061] Once the optimal set point min temperature was identified (6°C), the charge-discharge test was performed for 10 consecutive cycles (at 1 °C / min cooling rate and 6 / 20 °C as the set point temperatures). FIG. 5 includes the temperature profile of 1 kg ethyl myristate for (a) 1 cycle and (b) 10 cycles. It is therefore shown that ethyl myristate can complete 10 charge / discharge cycles at temperatures that simulate the operating conditions of a heat pump.

[0062] FIG. 6 and Table 3 show the results of the charge / discharge test of ethyl myristate (a) charging / freezing and (b) discharging / melting at a temperature range of 6-20°C. Table 3: Charqe / discharqe test results of ethyl myristate

[0063] Example 4: Safety Test Safety data sheet (SDS) was completed for ethyl myristate. No hazards were observed.

[0064] In addition, it is noted that ethyl myristate can be produced from 100% renewable raw materials. Specifically, it can be produced starting from economic and widely available sustainable raw materials (ethyl alcohol and myristic acid) through esterification. The phase change material also shows flash point >100 °C and full compatibility with metals within the temperature range 10-30 °C.

[0065] Ethyl myristate has been identified as a suitable phase change material in the application of air-conditioning systems. Specifically, ethyl myristate may be used in a thermal energy storage (TES) unit for an air-conditioning system. Further, differential scanning calorimetry on >98% purity grade ethyl myristate revealed melting temperature within the desired range with less than 5 °C of subcooling and enthalpy of melting >180 J / g.

Claims

Claims1. A thermal energy storage composition, the composition comprising a phase change material of Formula IFormula I wherein each of Ri and R2is an alkyl group; and wherein the phase change material has a melting temperature of greater than or equal to 10 °C and less than or equal to 15 °C.

2. The composition according to Claim 1 , wherein one or both of the following apply:Ri is a Ci to C3 alkyl group; andR2is a linear or branched Ci2to C14 alkyl group.

3. The composition according to Claim 1 , wherein the phase change material is ethyl myristate.

4. The composition according to Claim 1 , wherein the composition comprises an antioxidant.

5. The composition according to Claim 4, wherein the antioxidant is6. The composition according to Claim 4, wherein the antioxidant is present in the composition in an amount of from 0.05 to 0.2 wt%, such as about 0.1 wt% of the total weight of the composition.

7. A thermal energy storage unit comprising a thermal energy storage composition according to Claim 1 .

8. The thermal energy storage unit according to Claim 7, wherein the thermal energy storage unit is configured to be retrofitted to an air-conditioning system.

9. An air conditioning system comprising a thermal energy storage unit according to Claim 7, wherein the thermal energy storage unit is configured to: enhance cooling provided by the air conditioning system by the phase change material transitioning from a solid phase to a liquid phase when the air conditioning system is in use for cooling; and when the air conditioning system is not in use for cooling, the phase change material returns to a solid state through application of a suitable means or apparatus within the air conditioning system to achieve this.

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