Phase Change Material (PCM) Performance
|✅ Paper Type: Free Essay||✅ Subject: Engineering|
|✅ Wordcount: 3519 words||✅ Published: 18th Oct 2021|
1.0 Literature Review
This literature review will investigate the techniques being used for analysis of PCM performance, with emphasis on investigating if any properties are thought to have notable influence on successful PCM integration. Further exploration will look into the limitations of PCMs and the techniques to mitigate them by enhancing properties.
1.1 A Review of PCMs in the Built Environment
PCMs are materials with a high latent thermal energy storage capacity. Large enough quantities of energy from the surroundings can be absorbed or released by the material during phase change to allow useful heating or cooling in buildings. The appropriate use of PCMs can reduce the peak heating and cooling loads. Minimising temperature fluctuation gives PCMs the ability to keep indoor temperature within the comfort range of occupants without or with reduced energy consumption through HVAC (Souayfane et al., 2016). Souayfane et al. (2016) found many studies investigating the use of PCMs and giving evidence that they had the capability to significantly improve the energy performance of buildings when used correctly.
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The first documented use of a PCM was in the passive solar heating system for a residential building in 1948 by Dr Maria Telkes (Kośny, 2015). In the seventy years since, understanding of these materials has grown gradually as their potential has been investigated further. Despite the notable potential for energy savings and continual development of the technology, the widespread implementation of PCMs has not been achieved. Kośny (2015) found that the slow introduction of PCMs into the built environment is predominantly due to a lack of market acceptance which can be traced to a high cost-to-benefit ratio which discourages consumers from considering PCMs as a viable design choice. Recent awareness of the issues posed by climate change have highlighted the need for development of these technologies so that they can be more widely implemented (Rastogi et al., 2015). This realisation has brought on a period of detailed analysis and investigation relating to PCM chemistry (Silakhori et al., 2014) (Bo et al., 1999), performance (Krupa et al., 2014) and methods of application such as microencapsulation (Krupa et al., 2014).
1.2 Analysis of PCM Performance
1.2.1 Methods of Incorporation
Research conducted into a ranking system for PCMs used in concrete emphasises the method of incorporation into the built environment as a factor to consider for performance analysis (Tang et al., 2018). The appropriateness of a materials incorporation to its environment is relatable to how effective it will be at regulating temperature. Techniques discussed include direct incorporation such as powdered PCM or small solid particles and indirect incorporation such as microencapsulation and microencapsulation. Tang et al. (2018) identify the ease with which direct incorporation can be applied to various mixtures as an advantage, however drawbacks including leakage and incompatibility with construction materials reduce the feasibility of these PCMs for most applications. The encapsulation method is used to mitigate the negative effects of direct incorporation; material leakage, phase separation, chemical and physical stability issues (Huang et al., 2019). Microencapsulated PCMs are some of the most popular options for use in latent thermal energy storage systems. Microencapsulation helps alleviate issues with low thermal conductivity by reducing the surface area-to-volume ratio in comparison to a macroencapsulated PCM. There is limited literature focusing on the use of macroencapsulation in the built environment. The little literature that does exist on macroencapsulation cites such advantages as less chance of damage during handling and the allowing a larger volume of PCM into the building envelope (Rathore, Shukla, 2019). These are factors that require consideration when it comes to deciding suitability of PCM for a given application. However, they do not hold any influence over the thermal performance of the PCM and advantages such as larger volume are misleading as more volume is not necessarily associated with more thermal storage. A more appropriate measure would be surface area-to-volume ratio which does directly correlate with efficiency of chemical reactions – and one in which microencapsulation performs noticeably better.
1.2.2 Properties Suggested to Influence PCM Performance
Rastogi et al. (2015) identifies the significance of phase change temperature as the most important factor in PCM performance and therefore deciding appropriate PCM. The study suggests mass density, latent heat of fusion, thermal conductivity and specific heat capacity at constant pressure as further properties which affect the performance of a PCM. Tang et al. (2018) studied the influence of many technical and environmental considerations, particularly for use in concrete. This research suggests that despite the main purpose of PCM use to reduce energy consumption in an effort to combat climate change, environmental sustainability of the material should be low importance when considering performance, as the energy savings made by any material worth using as a PCM should offset the negative environmental impacts of using the material, otherwise it is by default not worth using as an energy saving material. According to Tang et al. (2018) the most desirable properties for PCM selection are low costs and strong thermal performance; high latent heat of fusion, high thermal conductivity and appropriate phase change temperature.
There are several overlapping properties that Tang et al. (2018) and Rastogi et al. (2015) highlight as high priority for efficient performance, suggesting that these are specifically important by all measures and would need to be considered in any analysis performed in this paper. It can also be inferred that there is a lack of agreement about what properties beyond this core few are also important. Further properties which have been mentioned but not investigated in depth include: supercooling, cyclic stability, toxicity, small volume changes on phase change, corrosivity and flammability. This leaves opportunity for this paper to investigate how they can be incorporated into any multiple attribute decision making process in a way that precisely reflects their importance to the built environment.
1.2.3 Existing Performance Ranking Methods
The existing literature on PCM performance analysis by Rastogi et al. (2015) and Tang et al. (2018) use 35 and 20 PCM respectively for their analysis. The later began analysis on 24 individual products but 4 were removed due to their poor cyclic stability. The focus of the study performed by Rastogi et al. (2015) was primarily on PCMs which have been developed by Rubitherm, PCMProducts, etc. specifically for PCM purposes. The research undertaken by Tang et al. (2018) was more focused on the identification of materials which show promising PCM characteristics and could be feasibly developed into a successful PCM product. Although the model developed was not validated by any means and thus the reliability is questionable. Regardless, the shared focus of both is the identification of a material which provides the basis for a highly efficient thermal energy storage system, a focus which will be shared with this paper. However, this research will use a broader range of materials, both existing and potential products, to be able to make effective comparisons with more conclusive results. This will allow clearer identification of trends in PCM behaviour and highlight how companies are currently enhancing the properties of materials to make superior products.
No literature exists on the analysis of performance using predicted energy savings as a key measure. Many reviews discuss the importance of energy savings, but little effort is made to quantify the value. This study will begin to investigate how energy savings can be quantified reliably, allowing energy saving predictions to analyse performance and if it could ultimately be used to guide selection for consumers and designers.
1.3 Limitations of PCMs and Enhancement
1.3.1 Chemical Limitations and Solutions
Average installation cost of a PCM into the building envelope is around £5,000 with an annual average energy saving of roughly £200. This means the average required service life of a PCM which is financially viable is 25 years (Bland et al., 2017). 25 years of use equates to over 9000 freeze/melt cycles. Bland et al. (2017) identify one of the major issues with current PCMs as their lack of chemical stability over many cycles. Over the course of 25 years the material could become susceptible to natural degradation which renders the PCM useless. This is the same problem that plagued the first documented use of a PCM in 1948, which saw the salt used decompose within a few weeks.
While the technology has seen substantial improvement, it is still not viable in many applications, so developments need to be made. One way to tackle the issue of chemical degradation is through maintenance and replacement. The use of PCMs in other industries has successfully achieved this however the maintenance of PCMs to extend lifetimes in the building envelope is restricted by incorporation methods which don’t allow direct access to the PCM. Essentially, once the PCM stops being effective there is almost no way to replace it without destructive techniques which are costly and inconvenient for consumers.
Supercooling is a type of chemical failure that the PCM can experience. Supercooling is the state where a liquid solidifies below its normal freezing point (Al-Shannaq et al., 2015). This study emphasises the effect of supercooling as limiting PCM’s widespread introduction. Understanding the crystallisation process of PCMs is crucial to understand the phenomena of supercooling and therefore act against it. The influence of supercooling on PCMs varies greatly; inorganic PCMs are highly susceptible compared to organic materials which are far less vulnerable. However, studies have shown that when a PCM is microencapsulated, even an organic material, it can become extremely vulnerable to supercooling (Farid et al., 2004) which makes this a crucial area for consideration.
Technology and techniques exist to mitigate the effects of supercooling. The size of a microencapsulated PCM will affect the crystallisation temperature of the PCM. A simple solution to the problem is to manufacture capsules which are large enough to offset the ability of a PCM to supercool. Nucleating agents are another solution which has been widely implemented to combat the effects of supercooling (Al-Shannaq et al., 2015). Many studies have investigated the effectiveness of different chemicals as nucleating agents. Fan et al. (2004) and Zhang et al. (2005) evaluated the effectiveness of paraffin and 1-octadecanol respectively as nucleating agents and found certain quantities of each as a proportion of the weight of the microcapsule can act as an effective agent against supercooling.
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Another limitation which occurs over the cyclic lifetime of the material’s use is phase separation. This occurs when materials inside the microcapsule are at different phases during the freeze/melt cycle. Multicomponent PCMs, mainly eutectics, are highly susceptible to this influence. The difference in properties, although often minor allow the natural separation of materials over extended periods of time (Bland et al., 2017). For example, a eutectic mixture with components of different densities is separated by gravity and causes the materials to act independently of each other.
Research has been conducted into the potential for thickening agents to reduce the phase separation of hydrated salt PCMs. Ryu et al. (1992) discovered that absorbent polymers can act as effective thickeners which prevent the segregation of components in PCM capsules. It was found that generally salt hydrates were stabilised by adding 3 to 5 wt% of thickening agent to the PCM. While promising, this study only considers the effects on salt hydrates which only make up some of the PCM products commercially available. Saeed et al. (2018) develops on the ideas presented by Ryu et al. (1992) to propose a thermally enhanced form stable eutectic PCM that utilises nano-graphene platelets to thicken the PCM mixture and prevent unwanted phase segregation.
1.3.2 Fire Safety Limitations and Solutions
Asimakopoulou et al. (2015) describes the materials used in the building envelope as significant influences on the building’s flammability. Gypsum board impregnated with 24% paraffin-based PCM has been found to reduce the fire safety behaviour of the board resulting in a failure to meet fire safety regulations (Banu et al., 1998). This effect is because the paraffin becomes a vapour when exposed to the temperatures felt in a fire which can be ignited, hence exacerbating the potential for fires to spread in a building. Not all PCMs are detrimental to the fire safety of a building, although for the ones that are this characteristic will completely discourage consumers from considering a PCM entirely, regardless of strong thermal capabilities.
Research has examined the possibility of using fire retardants to mitigate the negative effects of some PCMs on fire safety (Bland et al., 2017), like the way in which thickening agents or nucleating agents would be used for phase segregation or supercooling. Cabeza et al. (2011) discuss the ways in which fire retarding materials could be applied proofing as:
- treating the board surface with a non-flammable material which will provide a protective layer for the flammable plasterboard and will also act to prevent the wicking action of plasterboard covers
- Using specific PCMs which are capable of self-extinguishing – brominated hexadecane and octadecane are mentioned.
- Sequential treatment of plasterboard in PCM followed by fire retardant during manufacture. This will displace some of the PCM but will act to positively enforce the fire-resistant abilities of the plasterboard.
There is a lack of consideration on how flammability issues can be tackled in other building materials and how effective these proposed solutions would be at achieving what they claim. This research will aim to develop ideas on this area and propose more detailed solutions for a wider range of applications.
The most practical solutions to develop PCMs will be dependent on the PCM in question. Each has unique disadvantages and will therefore require careful consideration of the circumstances to effectively evaluate the most appropriate techniques which would enhance the properties of the PCM.
1.4 Conclusions and Implications
The assessment of a PCM’s performance can be associated primarily to several thermal properties of the material as is agreed across existing research. The core properties which are widely agreed to be of importance are phase change temperature, latent heat of fusion and thermal conductivity. What factors are important beyond these is still open to interpretation and therefore will be discussed in this study, with consideration of the physical, chemical, environmental and commercial aspects of materials. The main purpose of this dissertation will be to determine a definitive set of properties which can accurately model the performance of various PCMs against each other. This research will use the existing methods laid out by Rastogi et al. (2015) and Tang et al. (2018) as guides to help develop the model which can perform this task.
The model proposed should help to highlight areas of weakness in existing PCMs and allow investigation into the enhancement of certain properties to improve performance. The limitations and enhancements mentioned in this literature review were discussed because of the suitable literature which already exists on those topics. This research may identify further limitations which are worthy of evaluation and will be discussed as necessary.
PCMs are often overlooked by potential users due to negative cost implications and lack of understanding, and although there is still a large amount of development that needs to occur until PCMs become more widely implemented, the potential for these products as common building materials in the future is considerable. The existing materials and the literature to support them, while not perfect, prove that there is a possibility for ideal PCMs that can be used in the built environment albeit through some management of their properties.
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