Clathrate Hydrates for Thermal Energy Storage in Buildings: Overview of Proper Hydrate-forming Compounds

Increasing energy costs are at the origin of the great progress in the field of phase change materials (PCMs). The present work aims at studying the application of clathrate hydrates as PCMs in buildings. Clathrate hydrates are crystalline structures in which guest molecules are enclosed in the crystal lattice of water molecules. Clathrate hydrates can form also at ambient pressure and present a high latent heat, and for this reason, they are good candidates for being used as PCMs. The parameter that makes a PCM suitable to be used in buildings is, first of all, a melting temperature at about 25 °C. The paper provides an overview of groups of clathrate hydrates, whose physical and chemical characteristics could meet the requirements needed for their application in buildings. Simulations with a dynamic building simulation tool are carried out to evaluate the performance of clathrate hydrates in enhancing thermal comfort through the moderation of summer temperature swings and, therefore, in reducing energy consumption. Simulations suggest that clathrate hydrates have a potential in terms of improvement of indoor thermal comfort and a reduction of energy consumption for cooling. Cooling effects of 0.5 °C and reduced overheating hours of up to 1.1% are predicted.


Introduction
The building sector is the dominant energy consumer around the world with a total of 30% of the share of the overall energy consumption [1].
Moreover, in recent years, the energy demands for buildings have increased very rapidly due to an increase in the growth rate of the population, an evolving indoor comfort expectation and technological development worldwide.Increasingly, advanced technologies and methodologies have been investigated and developed to cope with the increasing demand of energy and resources [2][3][4].
Thermal energy storage is an efficient method to improve the energy efficiency of buildings and obtain a significant economic benefit [5].
Thermal energy storage can be accomplished either by using sensible heat storage or latent heat storage [6].Sensible heat storage has been used for centuries by builders to store/release thermal energy passively, but a much larger volume of material is required to store the same amount of energy in comparison to latent heat storage.
Latent heat storage is a very efficient way of storing thermal energy: unlike sensible heat storage, latent heat storage is characterized by a higher storage density and by a smaller temperature difference between storing and releasing heat [7].Latent heat storage is achieved using the so-called phase change materials (PCMs).
The principle of using the phase change materials is simple: as the heat is supplied, the material changes its phase from solid to liquid and vice versa at a constant temperature, until it completely converts into a solid.Similarly, when heat is released, the material changes phase from liquid to solid, again at a constant temperature, until it solidifies completely [8].
The latent heat storage by using PCMs in buildings is an attractive way to compensate for the small storage capacity of most existing modern buildings.Using PCM material in buildings can decrease the temperature fluctuation, particularly due to incident solar radiation loads, as proven in several numerical studies [9].
In the last 40 years, several materials have been studied, in particular hydrated salts, paraffin waxes, fatty acids and eutectics of organic and non-organic compounds [10][11][12][13][14].In addition to the mentioned materials, clathrate hydrates show an interesting behavior as a PCM.
Clathrates are crystalline structures in which molecules of one type are enclosed in the crystal lattice of another.Clathrate hydrates have been studied as a technology for gas storage and separation [15,16], but thanks to formation occurring also at ambient pressure and a high latent heat, they are also good candidates for being used as a PCM [17].
Generally, PCMs should meet several requirements, divided into three main groups:  Physical requirements (high density, high latent heat of fusion for unit mass, high specific heat);  Chemical requirements (chemical stability, chemical non decomposition, non-corrosiveness with respect to the construction materials, non-toxicity);  Economic requirements (wide availability of material, low material costs).
The objective of the present work is to provide an overview of groups of clathrate hydrates, whose physical and chemical characteristics meet the requirements needed for their application in buildings.
After a brief description of thermal storage applications for which clathrate hydrates are already used, data on the physical and chemical characteristics of clathrate hydrates for building applications are provided.
Simulations are carried out to evaluate the performance of clathrate hydrates in enhancing thermal comfort through the reduction of summer temperature swings and, therefore, in reducing energy consumption.The effect of clathrate hydrate PCM wallboard on indoor temperatures is examined for a building located in Rome using a dynamic building simulation tool and is compared to commercial wax PCM wallboards.

Clathrate Hydrates as Phase Change Materials
An attractive application of clathrate hydrates is thermal storage as a PCM.In general, the use of PCMs depends on their melting temperature: (i) T < 15 °C, they are used for storing coolness in air conditioning applications; (ii) T > 90 °C, they are used for absorption refrigeration; and (iii) 15 °C < T < 90 °C, they are used in solar heating and in heat load leveling applications.Among these uses, residential building applications have been receiving growing attention in the last few years in order to obtain PCMs with the required melting point and thermal properties.Exhaustive surveys on various existing types of PCM can be found in the literature [5,6,[11][12][13][14][18][19][20][21][22][23][24][25][26][27], and data are reported in Table 1. Figure 1 shows the thermal properties of clathrate hydrates with respect to the other existing PCMs.Clathrate hydrates as PCMs have been mostly studied for cold storage in air conditioning systems [28][29][30].Since this type of novel cold storage medium was first proposed, research activities have obtained outstanding achievements [31][32][33][34][35][36].
Most refrigerant hydrates can be formed under low pressure (below 1 MPa) with a suitable phase change temperature for air-conditioning and large fusion heat (270-430 kJ/kg).Refrigerant hydrates have a larger fusion heat than eutectic salts, paraffin waxes and fatty acids.
Mixtures of solid CO 2 hydrate crystal and liquid aqueous solution are produced by non-mechanical generation processes, with a higher heat of fusion than that of ice.The production conditions can change melting the point of CO 2 hydrate slurries and their content of solid crystals, so that they can release plentiful latent heat in delayed time [39].
TBAB hydrates are the most studied as phase change materials.They have a melting temperature of 12 °C, good fluidity and a large cold storage density, which is about 2-4 times of that of chilled water.Researchers have conducted many investigations on the crystallization characteristics of TBAB hydrate crystals and their practical applications.The thermal and thermodynamic properties of TBAB semiclathrate hydrates, including their phase diagram [40,48,49], have been measured, and some data are presented in Table 2.
Real-sized prototypes have been implemented and studied [50].A compression cycle refrigeration unit (with a condenser installed outside the building) allowed, via a scraped surface heat exchanger (cooled down by the evaporator), the creation of TBAB hydrates slurries, which were stored in a tank during the night [36].
Since 2005, the JFE Engineering Corporation (Japan) has shown the feasibility of the large-scale utilization of clathrate hydrate slurry for air conditioning [43].The experiments demonstrated the feasibility of an air-conditioning system using TBAB hydrate slurry as a secondary refrigerant, with some technological problems to be solved, but different solutions have been proposed and tested.

Clathrate Hydrates of Organic Compounds
Another potential application of clathrate hydrates as PCMs is in buildings to deliver possible energy savings and peak load time shift.During the last few decades, clathrate hydrates of organic compounds have been characterized in terms of molecular structure, enthalpy of formation and melting temperatures.Among several hydrate-forming molecules, there are some that have the required thermodynamic features: (i) a fairly high melting point, which can also be near or equal to room temperature; (ii) the formation by cooling their aqueous solutions at atmospheric pressure; and (iii) a very wide concentration range from which the hydrate is separated out [44].
Studies on clathrate hydrates of quaternary ammonium halides, quaternary ammonium fluorides, bolaform salts, tetrabutylammonium carboxylates and tetrabutylammonium alkane-sulfonates are available in the literature.
Clathrate hydrates of tetraisopentylammonium alkanoates, [(i-C 5 H 11 ) 4 N]C n H 2n+1 COO (n = 0-9), instead present fairly high melting points between 10 and 28 °C [56], but to the extent of our knowledge, there are no data about their latent heat.

Clathrate Hydrates for Building Applications: Thermal Comfort Evaluation
The effect of quaternary ammonium halide clathrate hydrates (n-C 4 H 9 ) 4 NF•30H 2 O incorporated into the wallboard of the building envelope was examined using the PCM Express Building Simulation tool, for a building located in Rome, Italy.
PCM Express, developed by Valentin Energy Software, in collaboration with the Fraunhofer Institute for Solar Energy (ISE), is a simulation software for evaluating the effect of phase change materials in building.The effectiveness of the PCM boards was evaluated in terms of indoor thermal comfort, indoor temperature leveling and the increase in free cooling.
PCM Express was used to carry out simulations for three types of PCM: paraffin waxes Micronal ® with operating temperature of 23 °C and 26 °C (Case 1 and Case 2 in Table 4) and the selected clathrate hydrate with an operating temperature of 25 °C (Case 3 in Table 4).
Simulations were carried out considering a one-room structure with a length of 5 m, a width of 5 m and a height of 3 m.The building model had one south-facing window of 2 m 2 with double thermal-insulating glass.The stratigraphy of the envelope is shown in Table 4.The thermo-physical properties of the clathrate hydrate used in the simulation are shown in Table 5. Micronal ® PCMs are microencapsulated.Clathrate hydrates are assumed to be stored in a wallboard of the same thickness.PCM wallboards were compared to the base case, i.e., a structure with ordinary gypsum wallboards.A heating system output of 50 W/m 2 is assumed during a heating period from October 15 to March 31, with a heating threshold temperature of 20 °C.A heat recovery and ventilation system operates with an efficiency of 85%, typical of passive house constructions [59].

Results and Discussion
Table 6 summarizes the results obtained for the three PCM used in a building located in Rome, Italy.
Results show that in all cases, PCMs improve the indoor comfort in terms of a reduction of hours in which the indoor temperature exceeds the comfort range.This means that, with respect to the base case, all PCMs provide free cooling.It is noteworthy that clathrate hydrates theoretically provide freer cooling that wax PCMs. Figure 2 shows that the effect of all PCMs is to increase the number of times in which temperatures are in the range from 20 °C to 26 °C.The frequency of the room's temperature being inside the comfort range is higher in the case of clathrate hydrates (Case 3) than in the case of waxes (Case 1 and Case 2).On the other hand, clathrate hydrates shift room temperatures towards 26 °C and 27 °C, but the effect is negligible (0.1%).  Figure 3 shows temperature profiles during the days with the best PCM effect.The largest free cooling is obtained in Case 1 and Case 2 with a reduction of 1 °C in room temperature during the daytime, while clathrate hydrates in Case 3 allow one to obtain a reduction of 0.5 °C.Anyway, the damping effect of clathrate hydrates is comparable to that of commercial PCM.

Conclusions
Clathrate hydrates have been suggested as a potential group of PCMs.The performed overview on the thermo-physical characteristics of different clathrate hydrates allows one to assess which of them are suitable for PCM applications in buildings.
The effect of quaternary ammonium halide clathrate hydrates incorporated into the wallboard of the building envelope was examined using the PCM Express Building Simulation program, for a building located in Rome, Italy.
Simulations suggest that clathrate hydrates have a potential in terms of the improvement of indoor thermal comfort and a reduction of energy consumption for cooling.Cooling effects of 0.5 °C and reduced overheating hours of up to 1.1% are predicted.Therefore, there is an argument for the experimental investigation of clathrate hydrates in building applications.
Anyway, further experimental work is necessary to better comprehend the kinetic and chemical properties of clathrate hydrates and to assess if they meet the needed chemical and economical requirements.
Kinetic properties, such as a high nucleation rate and little or no supercooling of the liquid phase and a high rate of crystallization, and chemical properties, such as completely reversible melt/crystallize cycles, long-term chemical stability and no degradation after a large number of melt/crystallize cycles, non-corrosiveness and compatibility with construction materials, non-toxic, non-flammable and nonexplosive, as well as economic properties, such as abundance, availability and cost effectiveness, should be investigated.Properties, such as low embodied energy, separation facility from the other materials and recycling potential, low environmental impact and non-polluting, are environmentally requested.Studies on methods of micro-or macro-encapsulation should be also carried out.
The effectiveness of the wallboard is climate dependent, and care must be taken to choose the most appropriate phase change temperature [59].
Since melting temperature and melting enthalpy depend on molecular effects and properties, such as different chain lengths, ions, crystal structure and the position of the chain of anions, future activities should focus on modifying molecular structures and synthesizing original molecules starting from the selected clathrate-forming molecules.The objective is to obtain hydrate-forming compounds with proper phase change temperatures and a high formation enthalpy.

Figure 3 .
Figure 3.The day with the greatest PCM effect (PCM internal temperature in blue, conventional system internal temperature in red).

Table 6 .
Results: the number of hours in which indoor temperature exceeds the comfort range (>26 °C).