With the world turning to renewable energy as its resources are being depleted, researchers and scientists around the world are working to develop ways to utilize renewable energy in hopes to create a more sustainable way of life. However, irregularities in environmental conditions have made this task challenging. There is an emphasis on developing consistent energy sources that can provide reliable energy despite weather/environmental conditions. One way scientists are tackling this challenge is by employing thermal energy storage mechanisms.
Thermal energy storage mechanisms can account for shortcomings of current renewable energy systems, such as solar energy. With sunlight only powering solar panels for the limited hours of daylight, this system lacks consistency. The addition of a thermal energy storage mechanism can help to store a portion of the solar energy for use at times when there is no solar radiation. Hence, thermal energy storage is an incredibly important process that could become the basis of energy production/usage in the future.
Thermal energy storage refers to the ability of a material to retain heat for use at a later time. This time frame can range from seconds to years. Thermal energy storage can be broken down into 2 categories: latent and sensible. Latent thermal energy storage refers to the region of phase change in which the temperature of the material does not change. This is typically seen in a solid-liquid phase change, but can also occur during evaporation/re-humidifying. In rare cases, solid-solid phase changes can be exploited in crystalline structures. Sensible thermal energy storage describes the region in which an increase/decrease in temperature can be detected, or sensed (figure 1). An accessible example of sensible heat storage is placing hot rocks in a sauna to keep the temperature high.
Figure 1: Sensible vs latent thermal energy storage
Phase change materials (PCM) provide a promising environmentally friendly way to store thermal energy due to their large supplies of latent energy. However, pure PCMs tend to have a low thermal conductivity value, and therefore their heat transfer efficiency tends to be lower than desired. Thus, thermal conductivity is an integral property at play when discussing thermal energy storage. High thermal conductivity additives are often integrated into PCMs to maximize the efficiency of the heat transfer system.
Thermal conductivity is the rate at which heat can transfer through a given material. This property directly affects the heat exchange efficiency, and plays an important role in determining appropriate applications of a given material. Materials with a high thermal conductivity value have faster response times to a change in environmental temperature. A material with a high thermal conductivity value can transfer heat to its exterior, and therefore into the environment, at a faster speed than a material with a low thermal conductivity value. It is crucial to characterize a material’s thermal behavior to control and use the thermal energy storage material safely and effectively.
In a paper published in Solar Energy Materials and Solar Cells, Maryam Fashandi et al examine the impact of adding hexagonal boron nitride (hBN) and graphene nanoplatelets (GNP) to sodium acetate trihydrate (a PCM) in an attempt to improve phase change performance and thermal conductivity. hBN and GNP were chosen as additives due to their low density and high thermal conductivity. Fashandi et al were able to successfully increase the thermal conductivity of the PCM by increasing the wt. % of hBN and GNP (Figure 2). Thermal conductivity was measured by a C-Therm Technologies Ltd. MTPS sensor. This sensor is available on Trident, which can be used to test thermal conductivity and related thermal properties using the three most prominent methods in thermal analysis. The MTPS sensor is constructed with patented guard ring technology to ensure unidirectional heat flow, offering the most precise and reproducible measurements of any thermal conductivity instrument on the market (complies with ASTM D7984). As a result of the increased thermal conductivity, 1wt% hBN increased the PCMs heat transfer efficiency by 35%.
Figure 2: Thermal conductivity vs hBN and GNP content
In a study published in the Journal of Nanomaterials, Liye Zhang et al investigate the thermal conductivity of nanofluids containing silver nanowires (AgNWs) of different sizes (Figure 3). Size/shape of thermal additives is largely unexplored, with the default shape being spherical. In this study, spherical AgNWs are compared to those with a larger aspect ratio. The Hamilton-Crosser modification of Maxwell’s Law was used in this study to predict the thermal conductivity while considering the effect of the thermal additive size/shape:
(1)
In this equation, K represents thermal conductivity, with subscript c, a, and e representing the nanofluids containing AgNWs, discontinuous phase AgNWs, and continuous phase ethylene glycol, respectively. Va represents the volume fraction of discontinuous phase, and (n-1) represents the shape factor.
The nanowires were prepared in an oil bath with magnetic stirring. After thermal conductivity testing, it was confirmed that a higher aspect ratio of AgNWs led to a definitively higher thermal conductivity value (Figure 4). This can be attributed to the somewhat organized/linear alignment of the nanowires, which enhance the heat transfer efficiency.
Figure 3: SEM images of silver nanowires. (a) larger aspect ratio (K30 AgNWs); (b) smaller aspect ratio (K88 AgNWs)
Figure 4: K30 vs K88 AgNWs
It is not possible to understand the kinetics of a thermal energy storage material unreservedly without first understanding its thermal conductivity. When PCMs are impregnated with high thermal conductivity additives, heat can spread at a faster rate. This leads to faster responsiveness to changes in temperature and therefore increases the heat transfer efficiency.
In the first case study, thermal conductivity was the key property examined to improve the heat transfer efficiency of the PCM. With the addition of hBN and GNP, thermal conductivity was increased, resulting in a vast increase in heat transfer efficiency.
The second case study emphasizes the importance of thermal additive size/shape. To maximize the efficiency of the PCM for thermal energy storage and heat transfer efficiency, it is important to control the thermal additive composition as well as its aspect ratio.
C-Therm Technologies produces powerful thermal analysis instruments that can measure various thermal properties, such as thermal conductivity, in solids, liquids, powders, and pastes. Accurate measurements are available in seconds, with extremely high reproducibility. Trident is the most powerful thermal analysis instrument on the market, with the ability to test a substantial range of sample compositions. To learn more, contact us at info@ctherm.com
Written by Meaghan Fielding, C-Therm Laboratory Technologist.