Latent thermal storage

Latent TES is based on the latent heat of phase change of a material between liquid to solid or liquid to gas at a constant temperature level. Latent TES is typically called phase change materials (PCM),

Figure 7: Temperature range and phase change enthalpy (latent heat of fusion) per unit volume for some common PCM groups (ZAE Bayern,2017).

and is characterized by the ability to store energy at a higher density. It is also beneficial for several applications to be able to extract heat at a constant temperature. The storage capacity of PCMs can be calculated by

Q=mL (2)

where Lis the specific latent heat of phase change of the material.

Typically the phase change between solid and liquid state is applied, due to the large change in volume between liquid and gas phases. Energy density is one of the main advantages of PCMs compared to sensible heat storage, but another important feature is the constant temperature of the heat charge and discharge. While sensible heat storage changes temperature during storage and release, the phase changes takes place at a constant temperature. A drawback of PCMs is the thermal conductivity, which normally is lower than for sensible heat storage media. This requires large heat exchanger surfaces in order to achieve fast charging and discharging rates, making the technology more complex and expensive.

Due to the high energy density of PCMs, studies have also been performed to investigate the possibility of transporting surplus heat from one location to another. This can give great advantages in areas where the surplus heat and the heat demand are not co-located, such as in Norway, where the industries are often not located in areas with dense populations. In one test project, trucks filled with molten PCM have been used to transport up to 2 MWh from a biogas facility to a district heating network approx. 6km away (Miró et al., 2016). A few studies, and some demonstration projects regarding such mobile TES have been performed, but few have shown promising results, especially regarding the total CO₂ emissions (Miró et al.(2016) and Kaizawa et al.(2008)).

Latent thermal storage is generally divided into four types:

• Organic

• Inorganic

• Metallic (sometimes categorized as a subgroup of inorganic PCMs)

• Eutectic

These groups of materials are discussed briefly below; a more detailed discussion can be found in separate reports ((Drexler-Schmid and Kauko,2017) and (Sevault et al.,2017)).

Organic PCMs

General: Organic PCMs, characterized by having carbon atoms in their structure, are among the most commonly applied PCMs (A.S. Fleischer, 2015;Pereira da Cunha and Eames,2016). Organic PCMs exhibit relatively high latent heats (in the range of 100-200 kJ/kg), are physically and chemically stable, affordable, readily available, and easy to work with. However, their thermal conductivity is typically low (in the order of 0.1-0.7 W/(m·K) (Pereira da Cunha and Eames,2016)), and this has to be taken into consideration in the heat exchanger design. Furthermore, organic PCMs have relatively low densities and do not exhibit a sharp phase transition at the melting temperature but rather an extended transition taking place around the melting temperature (A.S. Fleischer,2015).

Temperature: 0-170^◦C (Pereira da Cunha and Eames,2016)

Development & cost:Organic PCMs are widely studied for low-temperature applications: paraffins for thermal management in electronics, and fatty acids for applications related to thermal comfort, e.g., in residential buildings (A.S. Fleischer,2015). Their practical applications are apparently still few. Incorporation of polymers to heat exchangers for different applications in the industry has been studied byZauner et al. (2016); however, this application is still at a low TRL level. Prices of organic PCMs vary from around 45 NOK/kWh for fatty acids to 250 NOK/kWh and beyond for amides and other less common PCMs (Pereira da Cunha and Eames, 2016). Note that the price only includes the price of the material, excluding the costs of the heat exchanger system, storage tank and other infrastructure.

Inorganic PCMs

General:Common inorganic PCMs for higher temperature applications include salts and salt hydrates.

Salt hydrates are combinations of members of the inorganic salt families (oxides, carbonates, sulphates, nitrates and halides) with water molecules, following a specific ratio (A.S. Fleischer,2015). Salts and salt hydrates are the most commonly applied PCMs in the high-temperature range. Inorganic PCMs have sharp phase transitions at the melting temperature, latent heats comparable to those of organic PCMs, and higher thermal conductivities. Furthermore, they have higher densities, and exhibit smaller changes in volume during the phase change than organic PCMs. However, salts and salt hydrates tend to degrade over repeated thermal cycling, and are corrosive, hence not compatible with most materials. Moreover, they have a tendency for supercooling, meaning that the PCM might solidify at a temperature below the actual melting temperature. This problem may however be tackled with adding nucleating agents into the PCM.

Temperature: Salt and salt hydrates can be found with melting temperatures ranging from 10 to 900^◦C (A.S. Fleischer,2015).

Development & cost:Salts and salt hydrates are commonly suggested for TES in e.g. concentrated solar power applications Xu et al.(2015), and some pilot installations exist (see section4). Salts and salt hydrates are inexpensive; the prices range from 11 to 45 NOK/kWh for the salt hydrates listed by Pereira da Cunha and Eames (2016).

Metallic PCMs

General: Metallic PCMs are little studied but promising group of materials for high-temperature applications, such as surplus heat recovery from hot exhaust gases (Maruoka and Akiyama, 2006).

Metals and metal alloys are the only groups of PCMs that do not suffer from low thermal conductivities, and most of them are safe and easy to work with. Their main drawbacks are their low latent heat and high density, which results in a high mass for the thermal storage. In the low-temperature regime, the latent heats of gallium and cesium are one order of magnitude lower than the organic PCMs with similar melting temperature. In the high-temperature ranges, however, metallic PCMs exhibit similar latent heats as salts and similar melting temperatures.

Temperature: Most potential for high-temperature applications, from 400 to >1000^◦C (A.S. Fleis-cher,2015;Maruoka and Akiyama,2006).

Development & cost: It seems that the use of phase change metal alloys for high-temperature TES has been underestimated by researchers though they are superior to salts in many respects (Kenisarin, 2010). Some studies on using metallic PCMs for surplus heat recovery from hot exhaust gases are presented in section4. No price information was found for this group of PCMs.

Eutectic PCMs

General:Eutectics are generally defined as alloys or mixtures exhibiting a distinct melting point for the whole mixture, which is lower than that of any other alloy or mixture composed of the same constituents in different proportions (A.S. Fleischer,2015). Commonly applied eutectic PCMs are mixtures of salts and water, applied for operation temperatures close to/below 0^◦C (Oró et al., 2012). Furthermore, binary and ternary mixtures of inorganic salts have been widely studied for high-temperature appli-cations (Pereira da Cunha and Eames, 2016). Due to their high density and stability in the liquid state, they have been used widely in high-temperature sensible thermal storage systems (thermonuclear energy, concentrated solar thermal power).

Temperature:Mixtures of salts and water are applied for operation temperatures close to/below 0^◦C.

Eutectic mixtures of inorganic salts appear promising in the range from 130^◦C up to 1250^◦C (Pereira da Cunha and Eames,2016).

Development & cost: Application of aquatic salt solutions as PCMs for cold thermal storage are commercially available (Oró et al., 2012). The application of eutectic mixtures as PCMs for high-temperature applications is on the research stage. Prices range from 16 to 340 NOK/kWh (Pereira da Cunha and Eames,2016).