PCMs’ performance and the benefit enhancement

At present, non-renewable energy sources such as oil are being consumed in large quantities and, along with this energy consumption, large quantities of carbon dioxide (greenhouse gases) are being produced, contributing to increased global warming. The energy used to control the interior temperature of a building area cannot be disregarded since the building industry, as a significant energy consumer and CO2 emitter, has a significant influence on the reduction of greenhouse gas emissions. Due to their enormous thermal energy storage capacity, phase change materials may be employed in building envelopes, where they can help conserve energy and somewhat lower carbon emissions. In this paper, the thermal performance of phase change materials that can be applied to the building sector is summarised and compared, while various ways of improving the ratio of thermal efficiency to cost are proposed based on their practical use, from material enhancement, production method enhancement, and the choice of use and location for efficiency enhancement.


Introduction
The construction industry, one of the sectors with a large proportion of energy usage and carbon emissions, has a significant influence on attaining global decarbonization objectives in light of the growing shortage of energy resources and the urgent climate change situation.2009, the building sector accounted for 23% of global carbon dioxide emissions (directly or indirectly), and by 2030, building energy consumption will account for 30% to 40%, while carbon emissions will reach around 30% [1].The manufacturing of building materials, temperature regulation energy consumption, such as heating and cooling, etc., are the key components of energy consumption in buildings, with heating and cooling systems accounting for 20% of the overall energy consumption in the construction industry.High energy consumption is accompanied by high carbon emissions, so the control of energy consumption for temperature regulation in the building space will have a significant positive impact on achieving energy savings, reducing its environmental impact and achieving the goal of "Carbon neutrality" [2].

Building envelope
Traditionally, the temperature regulation of building spaces has been dominated by electricity, gas and wood.All three of them can be used to maintain space heating, while the use of electricity is the main way of cooling equipment, and these heating and cooling systems are accompanied by energy consumption.The smaller the regulated temperature range, the less energy will be consumed, so it is proposed that passive regulation of the building envelope, complemented by active heating and cooling systems, will reduce energy consumption.Phase change materials (PCMs) have thermal qualities that allow them to store and release thermal energy during melting and freezing (changing from one phase to another).This material transforms from a solid to a liquid by absorbing an equal amount of energy from its surroundings; when it freezes, it releases a large amount of energy in the form of latent heat of melting or crystallization energy.By controlling the amount of energy absorbed and released, the phase change material's environment can be kept at a specific temperature.When integrated into building envelopes and building services, phase change materials can increase the energy storage capacity of the envelope and reduce the emissions of carbon dioxide and energy use used to regulate the temperature of the building space [3].The building envelope, the external shield that wraps around the building, uses elements such as roofs, floors and wall panels to separate the internal space from the outside.Phase change materials can improve the performance of a building by enhancing the thermal mass of the building structure, thereby regulating thermal loads and helping to manage human comfort.

The way phase change materials are actually used
The main methods currently available for applying PCM to the actual building envelope are immersion, direct incorporation, encapsulation, shape stabilized PCM materials and composites.The immersion method involves using a porous structural material and immersing it in a liquid phase change material so that it penetrates inside the structural material due to capillary action, but when the immersion method is applied to concrete, it is prone to contact with the reinforcement causing corrosion of the reinforcement and affecting structural safety and service life.However, the biggest drawback of this approach is that PCM is mostly used in the construction sector as an organic material, which is prone to leakage when in the melting stage, and organic materials are flammable, which increases the risk of fire.Encapsulation, as the name suggests, uses a shell to encase the phase change material to avoid the problems caused by leakage.However, the choice of the shell has a major impact on the heat transfer efficiency of the PCM, so the encapsulation method aims to expand the region for heat transfer while ensuring thermal conductivity [4].At present, encapsulation has become the mainstream method of using PCMs due to the influence of the building structure and thermal efficiency, but due to its production technology and raw material costs, the total cost is high and the ratio of thermal benefits to costs obtained is not satisfactory.Therefore, consideration will be given to improving the benefit-tocost ratio in terms of the material itself, production methods and use [5].

The main phase change materials used in construction
PCMs can be controlled in temperature by the latent heat of phase change.When the temperature rises to a certain point (melting point), the material starts to absorb large amounts of heat from the surrounding area gaining energy capable of breaking chemical bonds and changing from a solid to a liquid state.As the temperature decreases, the substance releases energy (usually in the form of heat) into the surrounding area, re-forming the chemical bonds and returning to the solid state.Considering that the melting temperature of the phase change material is near the desired comfortable room temperature, a more stable and comfortable indoor climate temperature can be obtained.This is why the melting point of phase change materials used to regulate room temperatures is usually required to be within the temperature regulation range required for most buildings [6].Depending on their chemical components, PCMs can be categorized as either organic, inorganic, or eutectic (as shown in figure 1).
Due to their diverse molecular architectures, which are mostly formed of hydrocarbon molecules, organic PCM materials, which are polymers with long chain molecules primarily consisting of carbon and hydrogen, offer a wide variety of thermal characteristics.They typically exhibit high-order crystallinity when frozen and most undergo phase changes above 0°C.Most organic PCMs consist of kinds of paraffin (CH3 (CH 2 ) n CH 3 ), and fatty acids (CH 3 (CH 2 ) n COOH), which are the second largest group of organic PCMs, and are largely within the human comfort temperature range.In addition to this there are organic polymers such as polyethylene glycols and modified polyurethanes which also can be used as PCM materials due to their high latent heat.Inorganic PCM materials consist of two main categories of materials, namely salt hydrates and metal PCMs, which have a higher latent heat than organic PCMs.Eutectic PCMs, on the other hand, are phase change materials made up of aggregates of two or more types of PCM.They can be inorganic mixtures, mixtures of organic and inorganic as well as organic mixtures [7].In the construction sector, in addition to the human comfort temperature, there is also a joint check on its latent heat, thermal conductivity, the PCMs' effects on the building's construction, safety, economics, and other factors.Paraffin waxes undergo phase changes at temperatures ranging from 18°C to 35°C, while fatty acids, for example, between 20 and 60 degrees Celsius, experience phase transitions.Inorganic salt's phase changes from -100°C to 1000°C, while the temperature of eutectic phase change materials is determined by a combination of several materials and their ratios.All of these have phase change temperatures that are within the range of comfortable human temperatures, and they all have latent heats that are within the range of temperature regulation for buildings.Organic PCMs such as paraffin have low thermal conductivity and are flammable, whereas inorganic PCMs have strong thermal conductivity and latent heat, but they tend to leak and cause corrosion to the internal steel reinforcement of the structure.The raw material cost of organic PCM is higher, while the raw material cost of inorganic PCM (except metal PCM) is lower compared to organic [8].A comparison of the thermal properties, flammability and cost of several common phase change materials is shown in the table 1 below.

Technologies required for the application of phase change materials in construction and their costs
Buildings that employ PCMs as latent heat storage systems, it is important to consider not only their applicable materials, but also how to apply them in practical situations and how to use them to achieve effective heat transfer.In practical scenarios, in addition to the material properties mentioned above, how to ensure the stability of the PCMs in use and not to affect the surrounding structural materials, such as concrete, steel, etc., during the phase change process.This is why the concept of encapsulation was introduced.As most of the phase change materials used in construction pass through the liquid phase, encapsulation is very important to avoid leakage of the phase change material.Current encapsulation methods are broadly classified into micro-encapsulation and macro-encapsulation.Micro- encapsulation means that the material that is the main PCM is wrapped inside with another stable material, forming tiny capsules with a size of 1μm to 300μm.Due to their tiny size, they can be considered to be mixed and dispersed directly inside materials such as concrete and gypsum.Macroencapsulation, on the other hand, means that the PCMs are encapsulated in any type of container, such as spheres, panels etc. to be used as heat exchangers in building materials.Due to the increased surface area, microencapsulation improves the heat transfer between the and the surrounding environment, but usually at an increased cost.For example, the average cost of an inorganic salt hydrate PCM is US$ 1.98 to 3.96/kg, and the price of the product after microencapsulation reaches US$ 3.08 to 4.95/kg, with the encapsulation step accounting for 20% to 35% [9].

Material improvements-Hybrid porous stable support materials
Although organic PCMs generally have the problem of low thermal conductivity, some of them possess excellent properties such as phase change temperatures in the comfortable temperature range of the human body, sufficient latent heat, almost no supercooling, the stable state almost not easily subject to chemical reactions and non-corrosive.So for regulating the temperature of indoor spaces, some of them can be used as suitable materials, such as paraffin wax.However, the thermal conductivity of paraffin wax is very low, about 0.2W/(m℃), which makes the heat transfer efficiency greatly reduced.Some studies have proposed that composites can be made by doping with materials that improve thermal conductivity.The materials involved in the composite need to have good physical and chemical compatibility to facilitate the composite.In addition, the stability of the material under repeated temperature changes should be considered, including the stability of physical properties and chemical properties such as thermal conductivity, density, porosity, etc.For instance, expanded graphite, which is formed when graphite expands at high temperatures and has a loose, porous internal structure, has a substantial adsorption impact on molten phase transition compounds like paraffin and lowers their mobility.Therefore, graphite can be used as a support structure to enhance the thermal conductivity of paraffin by compressing it so that it fully absorbs paraffin wax [10].In addition to graphite, there are also, for example, ceramic materials and porous foams, which have high porosity and large specific surface area, high thermal cycle stability and corrosion resistance.

Material improvements-Composite eutectic materials
Inorganic PCMs have the benefits of high latent heat, low cost, abundant resources and wide temperature range, but also because of its wide range of phase change temperatures, it often requires two or more molten salts in a certain ratio to form a composite PCM with the right temperature.In addition, the inherent defects of inorganic PCMs, drastically restrict its utilization, such as supercooling and phase separation.For supercooling, the addition of nucleating agents with similar lattice parameters to water and salt can effectively reduce the problem of supercooling hydrated salts.For phase separation, as the number of thermal cycles increases, phase separation leads to a continuous decrease in the hydrated salt's ability to store heat, so the addition of thickening agents is considered to solve the phase separation, and polymer thickening agents are considered to be selected so that they can act as both a solution to phase separation and as highly adsorbent substances to prevent leakage of inorganic PCMs.In combination with the above properties of porous support materials, composite inorganic eutectic phase change materials and porous support materials can be mixed to make composite inorganic eutectic phase change materials with more stable performance, higher ambient temperature suitability and thermal properties [11].Fatty acids have a higher latent heat capacity and greater thermal stability than other organic PCMs, giving them additional benefits as phase change materials.but the phase change temperature of a single fatty acid is usually not consistent with the human comfort temperature, i.e. the environmental suitability for practical applications.It is possible to eutectic binary or poly fatty acids to give them a suitable temperature range with the advantages of high latent heat, low subcooling and good stability [12].

Material improvements-Highly conductive nanoparticles optimize the thermal conductivity of PCMs
Dispersing nanoparticles in PCMs can improve their thermal conductivity and the potential to maintain thermal efficiency.The inherent thermal conductivity of the nanoparticles and the compatibility of the phase change material with the nanoparticles combine to influence the total electrical conductivity of the composite.Nanoparticles can exhibit favorable thermophysical properties when diffused into PCM.Nanoparticles can be utilized as nucleating agents and fillers to improve phase change materials' effective thermal conductivity and thermal storage potential.This will increase the materials' long-term stability, thermal storage capacity, and thermal conductivity.Nanomaterials such as metal oxide nanoparticles such as TiO 2 , Al 2 O 3 , ZnO and SiO 2 , in addition to carbon-embedded nanostructures and graphite nanoparticles can also be added to phase change materials [13].

Economic analysis of PCMs selection
The choice of phase change materials can be determined in a number of ways for different temperatures and different environments around the building.When PCMs are used in the building envelope, the result of their interaction with insulated wall panels, roofs etc. may directly determine their energy efficiency.Phase change materials regulate the temperature of the building environment in a passive form, performing cooling and heating.The selection of PCM should be made in relation to its phase change temperature, thermophysical properties and latent heat to ensure that it will work effectively in the building envelope.But which part of the envelope, such as roofs and walls, to use PCM for requires a combination of different climates and temperatures, including solar absorbance and emissivity, for example, the different temperature regulation efficiencies that arise if PCM is integrated into walls and roofs.Another example is whether the PCM is placed on an external surface to ensure its activation directly, or whether it is placed on an internal surface to passively receive the transformation of energy from the external temperature.
The choice of phase change material is not a blind pursuit of its high latent heat.In areas where the need for thermoregulation is not so great, the material of choice can be chosen purposefully, taking into account local market prices, to be within the appropriate temperature range with the lowest cost and highest benefit.The cost-effectiveness of the material can then be assessed based on the building's thermal mass resulting from the envelope position chosen by the PCM above.The total life cycle cost of the envelope area can be obtained from the the energy cost in the current and the PCM cost of energy consumption over a 30-year life cycle, and ultimately the payback period after the application of PCM to determine the cost-effectiveness of the integration of PCMs and to complete the economic analysis [14].

Conclusion
As one of today's rapidly developing renewable and environmentally friendly materials, the ability of phase change materials to be used on a large scale and to accelerate the progress of the 'zero carbon' era depends more on how well they work in practice and how sustainable they are.Within the constraints of encapsulated production conditions and other sources of raw materials, the total cost of the material itself will not be low due to the nature of the production process.But the balance between benefits and costs is not just about costs, but also about benefits.If costs can be balanced with benefits, even if costs cannot be reduced significantly, the benefit-to-cost ratio will increase accordingly and the market will be more successful.If the cost is low and there is a large market demand for the material, then more people may emerge to research and promote the material, further reducing the cost; but if the cost is high and the market demand is low, research into the material may lag and the cost may rise.If the benefits offered by the material itself can be enhanced while at the same time controlling costs, then there will be more scope for the use of PCM.Today, however, the control of the cost of PCMs and their use in practical building construction is somewhat flawed.The immaturity of the technology makes the cost of PCM still high, and its unstable nature makes it difficult to control the impact on structural materials, especially in large buildings with long life cycles.The rapid growth of the global economy is

Figure 1 .
Figure 1.Phase change materials classifications[6].Paraffin waxes undergo phase changes at temperatures ranging from 18°C to 35°C, while fatty acids, for example, between 20 and 60 degrees Celsius, experience phase transitions.Inorganic salt's phase changes from -100°C to 1000°C, while the temperature of eutectic phase change materials is determined by a combination of several materials and their ratios.All of these have phase change temperatures that are within the range of comfortable human temperatures, and they all have latent heats that are within the range of temperature regulation for buildings.Organic PCMs such as paraffin have low thermal conductivity and are flammable, whereas inorganic PCMs have strong thermal conductivity and latent heat, but they tend to leak and cause corrosion to the internal steel reinforcement of the structure.The raw material cost of organic PCM is higher, while the raw material cost of inorganic PCM (except metal PCM) is lower compared to organic[8].A comparison of the thermal properties, flammability and cost of several common phase change materials is shown in the table 1 below.Table1.Several typical PCMs comparison.