One of the disadvantages of modern lightweight construction is its lack of thermal mass, which means this type of building can overheat in the summer and can’t retain heat in the winter. Often, heating and cooling systems are installed to maintain temperatures within the comfort zone. However, it is also possible to replicate the effect of thermal mass of the building using phase change materials (PCM).
Thermal energy storage through PCM is capable of storing and releasing large amounts of energy. The system depends on the shift in phase of the material for holding and releasing the energy. For instance, processes such as melting, solidifying or evaporation require energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa. Therefore, PCMs readily and predictably change their phase with a certain input of energy and release this energy at a later time.
As Figure 1 illustrates, PCM depends on latent heat storage. Compared to the storage of sensible heat, there is no temperature change in the storage. In a sense every material is a phase change material, because at certain combinations of pressure and temperature every material can change its aggregate state (solid, liquid, gaseous). In a change of aggregate state, a large amount of energy, the so-called latent heat can be stored or released at an almost constant temperature. Thus a small difference in temperature can be used for storing energy and releasing the stored energy.
Today, classic cooling systems are used for room cooling, and these ensure that the rooms are cooled to a comfortable temperature in all environment climates. These systems are effective, however they would be a lot more energy-efficient if they used the natural temperature differences between day and night for cooling purposes. The use of the building mass as a storage medium is well known. This concept of night-time cooling has already been successfully implemented in many construction projects.
The objective of PCM is illustrated in Figure 2. As can be seen, PCM limits excessive temperatures by storing the excess heat during the day, and releasing it during the night. This mimcs the effect of thermal mass, which also stores heat during the day and releases it during the night. Phase-change materials (PCMs) allow large amounts of energy to be stored in relatively small volumes, resulting in some of the lowest storage media costs of any storage concepts.
As mentioned, essentially all materials are phase change materials. However, the characteristics required for effective and predictable thermal energy storage excludes a large number of materials. In Figure 3, several PCMs are illustrated with their corresponding temperature ranges and enthalpy energy storage characteristics. The Figure also illustrates the temperature band which represents a typical temperature comfort zone in households. It shows that parrafin and salt hydrates are useful PCMs for households. Salts and sugar alcohols are used for higher temperature ranges. An example of a high temperature energy storage use is in a Concentrated Solar Power (CSP) plant, which uses salt to store energy for later use. This alleviates part of the intermittency problem of solar power.
The application of PCMs in buildings can have two different goals (Pasupathy, Velraj & Seeniraj, 2008). First, the PCMs can be used to utilize natural heat and cold sources. For instance, solar energy for heating during the evening/night or the use of night cold for cooling during the day. Second, PCMs can use manmade heat or cold sources.
In addition, different ways of using PCMs are available. In buildings, these again fall into two groups. PCMs can be located in building components such as walls or ceilings, or can be arranged in separate heat or cold stores. Using PCMs within buildings components are generally passive systems. The heat or cold is stored automatically and released when indoor or outdoor temperatures rise or fall beyond the phase change point of the material. Using PCMs in seperate heat or cold stores are usually based on active systems. The stored heat or cold is in containment seperated from the building itself and heat or cold transfer is not automatic, but used on demand.
Types of PCMs
Multiple types of PCMs exist. The main categorization of PCMs is the differentiation between inorganic PCMs and organic PCMS. The commonly used phase change materials for technical applications are: paraffins (organic), salt hydrates (inorganic) and fatty acids (organic) (IEA, 2005). Additionally,ice storage can be used for cooling applications. The differentiation between organic and inorganic is especially important for building based PCM use. Many other differentiations and catgeorizations exist, which are illustrated in Figure 4.
Early efforts in the development of latent heat storage materials used inorganic PCMs. Inorganic PCMs are salt hydrates. The advantages of these materials are: high latent heat values, non-flammable, low-cost and readily available. However, the disadvantages of inorganic PCMs has led to the investigation of organic PCMs. Some of these disadvantages are corrosiveness, instability, improper re-solidification, and a tendency to supercool.
The high storage density of salt hydrate materials is difficult to maintain and usually decreases with cycling. This is because most hydrated salts melt congruently with the formation of the lower hydrated salt, making the process irreversible and leading to the continuous decline in their storage efficiency.
Segregation can be prevented changing the properties of the salt hydrate with the addition of another material that can hinder the heavier phases to sink to the bottom . This can be achieved either with gelling or with thickening materials. Gelling means adding a cross-linked material (e.g. polymer) to the salt to create a three dimensional network that holds the salt hydrate together. Thickening means the addition of a material to the salt hydrate that increases the viscosity and hereby holds the salt hydrate together .
Subcooling is another serious problem associated with all hydrated salts. It appears when a salt hydrate starts to solidify at a temperature below its congelating temperature (Fig. 9). Several approximations have been studied to solve this problem. One is the use of hydrated salts in direct contact heat transfer between an immiscible heat transfer fluid and the hydrated salt solution . Another solution is the use of nucleators [6,125].
Organic PCMs have a number of characteristics which render them useful for latent heat storage in certain building elements. They are more chemically stable than inorganic substances, they melt congruently and supercooling does not pose as a significant problem. Moreover, they have been found to be compatible with and suitable for absorption into various building materials. Although the initial cost of organic PCMs is higher than that of the inorganic type, the installed cost is competitive.
However, these organic materials do have their quota of unsuitable properties. Of the most significant of these characteristics, they are flammable and they may generate harmful fumes on combustion. Other problems, which can arise in a minority of cases, are a reaction with the products of hydration in concrete, thermal oxidative ageing, odour and an appreciable volume change. Appropriate selection and modification have now eliminated many of these undesirable characteristics.
A comparison between organic and inorganic materials for heat storage is shown in Table 1.
- No corrosiveness
- Low or no undercooling
- Chemical and thermal stability
- Greater phase change enthalpy
- Lower phase change enthalpy
- Low thermal conductivity
- Phase separation
- Phase segragation, lack of thermal stability
Techniques for heat transfer between PCM and the fluid cycle
Heat transfer between the PCM and the fluid cycle is necessary to charge and discharge the PCM (IEA, 2005). Different techniques are available, including:
1) Direct contact between phase change material and heat transfer fluid: this needs materials that are chemically stable for long periods of direct contact and the solidification of PCM occur in small particles, securing sufficient heat transfer during subsequent melting.
2) Macroscopic-capsules: this is the most frequently used encapsulation method. The most common approach is to use a plastic module, which is chemically neutral with respect to both the phase change material and the heat transfer fluid. The modules typically have a diameter of some centimetres.
3) Micro-encapsulation: this is a relatively new technique in which the PCM is encapsulated in a small shell of polymer materials with a diameter of some micrometres (in the moment only for paraffins). A large heat-exchange surface results and the powder- like spheres can be integrated into many construction materials or used as aquaeous pumpable slurry. Plasters incorporating micro-encapsulated PCM are on the market since 2004. PC;-slurries are still under development.
Advantages and disadvantages of PCM use compared to conventional water storage
The three main advantages of PCM over conventional water storage techniques for thermal energy storage are (IEA, 2005):
1) Higher thermal energy storage capacity compared to the sensible energy storage in water. This leads to smaller required storages. Only a true advantage if only small useful temperature differences can be achieved.
2) Relatively constant temperature during charging and discharging.
3) Burner cycles for the back-up generation unit and therefore their CO and HC emissions can be reduced.
The four main disadvantages of PCM compared to conventional water storage techniques are (IEA, 2005):
1) Higher investment costs
2) Peak power during discharge is limited due to limited heat conduction in the solid state of PCM. This is the main limit determining the acceptable size for the storage modules.
3) Limited experience with long-term operation of many thousands of charge-discharge cycles.
4) Risks of loss of stability of the solution and deterioration of the encapsulation material.
While phase change materials in general are abundant, since all materials are essentially phase change materials, certain ideal characteristics can be identified for effective use of PCMs. These important characteristics are identified in Table 2.
|Thermal Properties||Chemical properties||Physical Properties||Economic Properties|
|Phase change temperature fitted to application||Stability||Low density variation||Cheap and Abundant|
|High change of enthalphy near temperature of use||No phase separation||High density|
|High thermal conductivity in both solid and liquid phases||Compatibility with container materials||Small or no subcooling|
|Non-toxic, non- flammable, non polluting|
While these characteristics outline the important aspects that need to be considered to determine the appropriate PCM, the principle of the technology itself can be applied in any structure. Climates that put high demands on cooling and heating are suitable for PCM. Large day-night differences are especially suitable for PCM, since the PCM would be able to smoothen and streamline the temperature differences throughout the day and therefore significantly reduce energy use for cooling and heating.
Figure 5 shows that storage applications in general will become more valuable and important when renewable energy penetration climbs. As can be seen in the figure, the Strategic Energy Analysis and Applications Center of the National Renewable Energy Laboratory of the U.S., currently deems energy storage a valuable technology but is not considered necessary with the current U.S. electricity grid. However, the future situation of a low-carbon economy will demand high levels of energy storage applications.
While quite a large share of energy storage applications has already reached a mature stage, PCM is still in a developmental phase. Consider Table 3, which shows several thermal energy storage technologies and their maturation stage regarding use in a Concentrated Solar power plant. PCM is placed in the developmental or demonstration phase. EPRI concludes that "storage systems involving PCMs are still in their infancy, and will require further study to determine the compatibility of these systems with CST plants using heat transfer fluids" (EPRI, 2009)
|Commercial||Pre-commercial prototype||Demonstration phase||Developmental stage|
|Steam accumulators||Two tank direct
Two tank indirect
|Two tank direct
Single tank thermocline
Phase change materials
|Phase change materials|
The use of PCM leads to a reduced need for peak generation capacity. Storage provides an alternative to the construction and operation of new generation and reserve capacity. For instance, the CPUC notes that for California peak demand is a major concern. Especially due to strongly growing populations in the hotter central and southern parts of California. The CPUC concludes that the value of the avoided cost of peak generation capacity will continue to increase as peak demand grows and as carbon emissions become more expensive (CPUC, 2010). In such a case, implementation of PCMs would lead to a reduced peak demand for air-conditioning and will alleviate the strain on peak generation capacity.
Using PCMs in an active system, where manmade heat or cold is stored, can also ensure more efficient use of renewable and other off-peak generation. A supply of energy when demand is low can be stored for later use when demand is high. For instance, the CPUC notes that wind in California tends to blow most strongly at night, which causes a mismatch between the supply and demand of energy. Storage applications, such as PCM, can allow excess wind and other off-peak energy to be stored and used during high demand times (CPUC, 2010). However, in the case of excess electricity, other storage applications are likely to be more effective. Especially in a case where excess heat, which will be used for electricity generation, needs to be stored, PCMs would be a more effective storage application compared to several other storage technologies.
The California Public Utilities Commission (CPUC) identifies a climate related advantage with the use of storage applications. CPUC argues that due to shifting on-peak energy use to off-peak periods storage applications can lower GHG and other emissions. Application of PCM leads to a smaller demand for energy on the hottest part of the day, since there is a reduced need for energy for air-conditioning. In addition, PCM lowers the energy needed for heating because the stored energy is used.
There are a number of factors that influence the cost of the PCM technology. Storage tends to be an application-specific resource and therefore the costs (and benefits) can vary greatly (CPUC, 2010). One of the complications in developing detailed cost estimates of PCM technologies and methods, and with storage applications in general, is that the costs of a given technology are greatly influenced by the particular application in which that technology is deployed. Thus, any generalized cost estimates are of questionable value.
However, some general issues can be mentioned for storage applications in general and for PCM applications in particular.
For storage applications in general the total installed cost varies on two dimensions : power (which is the amount of electricity, heat or cold which can be discharged at one time) and energy (the amount of hours that the application can discharge continuously). These two dimensions greatly influence system size and therefore installed cost. In addition, the system costs are influenced by the system efficiency and the frequency of use. System efficiency is determined by measuring the number of useable KwH or GJ that can be discharged compared to the amount charged. The frequency of use is how often and how deeply the system is discharged. All of these factors (size, efficiency, and frequency) mean that an EES technology’s cost cannot be meaningfully estimated independently of the way in which it is used (IEA, 2005).
Operating and maintenance costs (O&M) is the other main financial aspect to a storage system. O&M costs include the cost of buying the energy used to charge the system (when it is an active system; passive system used natural temperature fluctuations), fixed costs that do not depend on how much or often the system is used, and variable costs which is mostly replacement costs (IEA, 2005).
For the PCM storage technique in particular several issues can be mentioned. In a study on the effect of PCM when integrated with brick in Iran Haghshenaskashani & Pasdarshahri (2009) found that one of the important factors which has both a influence on economic factors as well as thermal efficiency is the quantity of PCM used.
Pasupathy, Velraj & Seeniraj (2008), Phase change material-based building architecture for thermal management in residential and commercial establishments. Renewable and Sustainable Energy Reviews Vol. 12, No. 1 January 2008 pp. 39 - 64. doi:10.1016/j.rser.2006.05.010
Trox, no date. Cooling naturally with phase change materials. Document can be found online at: www.troxtechnik.com
A. Abhat, Low temperature latent heat thermal energy storage: heat storage materials, Solar Energy 30 (1983) 313-332.
Haghshenaskashani, S., & Pasdarshahri, H., 2009. Simulation of Thermal Storage Phase Change Material in Buildings. World Academy of Science, Engineering and Technology 58 2009 pp. 111- 115
Demirbas, F., 2006. Thermal energy storage and phase change materials: an overview. Energy Sources Part B 1 85-95. Document can be found online at: doi:10.1080/009083190881481
IEA, 2005. Inventory of Phase Change Materials (PCM). A report of IEA Solar Heating and Cooling Programme – Task 32.”Advanced storage concepts for solar and low energy buildings”. Report C2 from Subtask C. International Energy Agency Solar Heating and Cooling Program. Document can be found online at: http://www.iea-shc.org/publications/category.aspx?CategoryID=64
CPUC, 2010. Electric Energy Storage: An Assessment of Potential Barriers and Opportunities. Policy and Planning Division Staff White Paper. California Public Utilities Commission. This document can be found online at: www.cpuc.ca.gov/PUC/energy/reports.htm
EPRI, 2009. Program on Technology Innovation: Evaluation of Concentrating Solar Thermal Energy Storage Systems. Electric power research institute. Document can be found online at: www.epri.com