Phase-Change Thermal Storage Materials

Energy consumption for buildings can be divided into four general categories: electricity for devices and appliances, hot water, space heating, and space cooling. Of these, only electricity needs to be provided all the time. The applications that require heat do not really need to be filled immediately, since some fluctuations can be permitted. Thus these applications are potentially a well of deferrable demand that can be used to compensate for the intermittent nature of renewable power sources.

A very large proportion of the energy budget for a home goes into space heating & cooling and hot water. According to EREN's Buildings Energy Data Book, 55.2 % of residential energy consumption goes into the big three. Given that residential is about 20 % of the energy pie, that suggests thermal storage could transform about 10 % of our total energy requirements (or ~ 15 % of electricity production) into deferrable demand. That's a big hunk, and would provide a ton of breathing room to renewable power. Commercial and industrial uses of thermal storage are likely to come before residential, and they would provide additional capacity to thermal storage.

Of course, we as humans don't really like our nice cozy interior environment to have boomeranging temperatures controlled at the whim of the power utility. A potential solution is to introduce some thermal storage on-site which can act as a reservoir of heating or cooling. I have previously written that the benefits of thermal storage are underwhelming next to increased insulation, and that remains largely true. However, newish thermal storage mediums are looking more impressive. Furthermore, any dwelling needs some level of air exchange to flush odors and CO₂ and thermal storage can be retrofitted without completely gutting the interior of a house or apartment block.

Outside of people living in off-grid housing, there currently isn't any real incentive to install such equipment. However, if we look forward into the future of electricity production, the difficulties solar and wind face with intermittency feature large. The key prerequisite to making thermal storage workable is a regulatory structure that pays a premium to electricity consumers who are capable of deferring their demand to some later time (say a range of 1-4 hours) as a service to the electrical utility.

For any thermal storage medium, one wants a material with a high heat capacity so that the energy density is high. In addition, one generally wants a material that has high thermal conductivity, so that the power (Watts/second) that can be applied or extracted is high. Last but most important, the material has to be inexpensive.

In order to develop a material with an extremely high heat capacity, it is often useful to find one that has a phase change (i.e. solid to liquid) around the desired operating temperature. The transition used is always the solid to liquid phase because gases just don't have the desired density.

For example, the amount of energy required to freeze water is really quite amazingly high. If we were to build a water tank for cooling applications and ran it from 1 — 16 °C, we would have a energy density of 4.184 kJ K^-1 kg^-1 · 15 K = 62.8 kJ/kg. By way of comparison, the heat of fusion for water is 333.6 kJ/kg, or the equivalent of heating water by almost 80 °C. If we freeze that water, and operate from -1 — 14 °C, the stored heat energy density rises to 396.3 kJ/kg, an improvement of 530 % in spite of the fact that ΔT remains identical.

Figure 1: Enthalpy of Water from -25 °C to 125 °C.

By operating across a phase change, one needs less thermal storage medium and a smaller tank which is an economic advantage. It also allows one to store more heat across a given temperature gradient, which provides a boost to the efficiency of the heat engine supplying heating or cooling.

We can classify phase-change materials into three general categories depending on their application:

1. (0 — 15 °C) Space cooling and refrigeration.
   
2. (40 — 65 °C) Space heating and hot water.
   
3. (> 300 °C) Thermal storage for electrical power plants (i.e. concentrating solar thermal).

Both the space cooling and heating categories are essentially fulfilling the same function: storing energy at the residential or commercial level. Thermal storage for power plants is a slightly different issue. Briefly, if you overlaid a graph of electricity demand and solar radiation, you would notice a phase delay of about two hours from peak sunlight to peak demand. Thus, to make a solar-thermal power plant capable of 'peaking' (i.e. providing the expensive electrical power capacity above base-load) you need a little bit of storage, just to cover 1—4 hours. For this, molten salts provide the best mechanism proposed to date.

There are a number of general categories of materials for phase-change thermal applications: organic materials which are typically oils, water and hydrated salt solutions, and salts. Organic compounds and saturated salts are used for low temperature (< namespaceuri="urn:schemas-microsoft-com:office:smarttags" name="stockticker">

Material	Melting Point (°C)	Sensible Heat (kJ kg-1K-1)	Latent Heat of Fusion (kJ/kg)	Thermal Conductivity (W m-1K-1)
Space Cooling Materials
Water - H2O	0	4.2	334	2.18 (ice)
Paraffin C14	4.5	-	165	-
Polyglycol E400	8	-	99.6	0.187
ZnCl2·3 H2O	10	-	253	-
Space Heating Materials
Paraffin C­22-C45	58-60		189	0.21
Na(CH3COO)·3 H2O	58	-	264	-
NaOH	64	--	227.6	-
Electrical-quality Heat Storage Materials
31.9 % ZnCl2 + 68.1 % KCl	235	-	198	0.8
NaNO3	310 (d 380)	1.82	172	0.5
KNO3	330 (d 340)	1.22	266	0.5
38.5 % MgCl + 61.5 % NaCl	435	-	328	-
NaCl	800		463-492	5

One thing that really stands out in the literature on phase-change materials is how poorly characterized so many materials are. A great number of salts (high temperature) or hydrated salts (lower temperatures) form eutectics with other salts, allowing hybridization of thermal properties. Eutectic means two materials form a crystal alloy at a given concentration of each material. Hence the number of potential permutations is enormous. The field of organic materials is similarly enormous.

In the case of the salts, the phase change materials are highly corrosive, so it would be poor design practice to use them as the working fluid. Rather, one uses a common well-established working fluid (such as water). On the other hand, hot water is pretty corrosive as well, while oils generally are not.

Now, if we go back to the original criteria for thermal storage, recall we want both high heat capacity for energy, but also high thermal conductivity to provide power. If we compare the thermal conductivity of copper (400 W m^-1K^-1) to that of phase-change materials, we see that the thermal storage materials are not very conductive of heat.

The obvious solution is to build some sort of composite material where you have a high thermal conductivity lattice paired with a phase change material for heat storage. The simplest example would be a water tank equipped with aluminium fins. For molten salts, this becomes more challenging as the material has to be refractory (i.e. does not react with the molten salt). The ideal choice is typically carbon, which pairs strong covalent bonds with exceptional thermal conductivity. Graphite has the highest thermal conductivity (around 1950 W m^-1K^-1) of any material around (exception: superfluid helium) but only along the plane of the sheets.

In 2000, Fukai et al. proposed using a structure of carbon fibre inside a tank of paraffin as a phase-change composite [2]. They found that by including a volume fraction of 2.4 % carbon fibre they could improve the thermal conductivity 24-fold to 6.25 W m^-1K^-1. However, carbon fibre is relatively expensive.

A cheaper alternative would be to used expanded graphite as the lattice material instead. Think perlite/ vermiculite, but composed of carbon; it is similar to the anode of a battery. A recent study explored the potential for using expanded graphite for use with molten salts for high temperature solar-thermal applications [3]. This is the first study to examine carbon paired with molten salts to my knowledge. The approach of expanded graphite requires considerably more graphite by weight (20 % for most of the results) which in turn will reduce the energy storage density. The results demonstrate that NaNO₃ and KNO₃ phase-change materials in a matrix of expanded graphite had a thermal conductivity of around 4 W m^-1K^-1, or roughly a 8x increase. The authors state that this is still below their desired figure at a given graphite concentration.

In conclusion, the most heartening aspect of phase-change thermal materials is the shear variety of options available. The development of composite phase-change materials is interesting but evidently proceeding slowly. The carbon fibre approach seems to offer superior performance for a given concentration almost certainly because it provides a continuous conduction pathway for heat along the length of a fibre. The expanded graphite is by nature, a more chaotic material so there will be many small zones where heat is forced to travel across the less conductive phase-change material. For the spacing heating and cooling applications, feasibility is largely a function of regulatory structure. It's only worth doing on a large scale, so the political will would have to be present to move forward.

References
[1] Belén Zalba et al., Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23(3): 251-283.

[2] J. Fukai et al., Thermal conductivity enhancement of energy storage media using carbon fibers, Energy Conversion and Management 41(14): 1543-1556.

[3] S. Pincemina et al., Highly conductive composites made of phase change materials and graphite for thermal storage, Solar Energy Materials and Solar Cells 92(6): 603-613.

Update: Density numbers for the materials listed in Table 1. All numbers in kg/m^3.
Water: 998 (@ 20 ^C) / 917 (@ 0 ^C)
Parrafin C14: n.a.
Polyglycol E400: 1125 @ 20 ^C
ZnCl2*water: n.a.
Parrafin C22-C45: 0.795 @ 70 ^C
Na(CH3COO)*water: 1450
NaOH: 1690
31.9 % ZnCl₂ + 68.1 % KCl: 2480
NaNO3: 2260
KNO3: 2110
38.5 % MgCl + 61.5 % NaCl: 2160
NaCl: 2160

There are many more in the references.