October 26, 2020
Renewable sources of energy from natural resources such as solar and wind are indispensable in the fight against climate change. However, the problem associated with renewable energy is its intermittent supply. E.g., the sun doesn’t shine at night and the wind doesn’t always blow. This natural fluctuations in renewable power sources do not coincide with the worlds’ electricity demand, resulting in a mismatch between demand and supply. Therefore, to effectively use renewable energy, it is imperative to have a cost-effective and large-scale energy storage methods that can absorb surplus energy during the off-peak hours and supply the stored energy during peak hours.
There are several means to store energy, and the phase change materials (PCMs) are one of the most appropriate materials store thermal energy from renewable energy resources effectively. PCMs are materials that have an intrinsic ability to absorb and release heat during phase transition cycles. Latent heat storage using PCMs finds application in various field including building energy storage systems, waste heat recovery systems, thermo-regulating fibers, smart textile materials, thermal management of the batteries, temperature management of the microelectronics, photovoltaic thermal (PV/T) applications, space and terrestrial thermal energy storage applications, and in the temperature management of greenhouses1. Several PCM materials categorized as organic, inorganic, or eutectic mixture are available for energy storage and the selection of the PCMs for the real-world application depends on its material properties. The key material properties considered are the melting point, latent heat, thermal conductivity, toxicity, flammability, cost, and availability. Among these criteria, accurate prediction of thermal conductivity is crucial in determining how readily the PCM can charge and discharge the stored thermal energy and plays an important role in the technology’s operational performance of the using PCMs.
Most phase change materials have low thermal conductivities, especially organic compounds, and thermal conductivity enhancement is one of the most important design criteria for PCM storage application. The most applied heat transfer enhancement methods use fins, insertion, or dispersion of high thermal conductivity materials, multitube, and micro or macro encapsulation2.
A single solar water heater can reduce approximately 50 tons of CO2 emissions in 20-years. Integrating a solar thermal heating system with solid-liquid PCMs based TES technology can absorb more heat than a conventional solar water heater. The PCM based solar water heater can not only enhance efficiency but also avoids fluctuations in the temperature of the stored water. In the beginning, the water heaters were supported by filling the bottom of the heaters with PCMs. However, the quantity of the available energy in the storage system was limited by the PCM’s low thermal conductivity. Several researchers currently focus on increasing the thermal conductivity of PCMs for efficiently storing thermal energy for water heating. A layout of a typical solar water heater is shown in Fig.1.
Figure 1: Layout of a solar water heater using PCMs based TES technology 3
Concentrating solar power (CSP) is another proven technology that utilizes PCMs for thermal energy storage. A dozen large-scale power plant installations using PCMs, coupled with a CSP tower plant, are currently operating in the USA and Spain. A schematic of a PCM integrated CSP is provided in Fig 2. The PCMs that have been investigated for the CSP systems include organic compounds (sugar alcohols (< 200 oC), molten salts (>300 oC), and metallic alloys (>500 oC). In a typical PCM integrated CSP, the PCMs are heated and stored in an insulating container during the off-peak hours. When the stored energy is required, the PCM is pumped into a steam generator that boils water, spins a turbine, and generates electricity. The cooled PCM is pumped back into the storage tank to be heated and reused.
In all these applications the rate of charging and discharging of the PCMs plays an important role in the commercial application’s feasibility, which in turn depends on the thermal conductivity of PCMs.
Figure 2: Layout of CSP tower with PCM integrated TES 4