Innovative thermal energy storage technology for enhanced energy storage
By Dr. Jundika Candra Kurnia
In recent decades, demand for energy has exponentially increased due to significant growth of the global population and surging industrial activities to maintain sustainable socio-economic development.
In the current energy landscape where fossil fuels remain the primary source of energy in many countries, the increasing demand for energy is accelerating the depletion of available fossil fuel sources and further contributes to greenhouse gas emissions and global warming. Coupled with limited resources, non-uniform distribution, non-optimal energy conversion and mismatch between energy supply and demand, the risk of serious energy crises has become more pronounced.
To minimise the adverse effects of energy crises, world leaders have been working together to reduce the strong reliance on fossil-based energy and significantly reduce greenhouse gas emissions through various policies, regulations and incentives. Consequently, several initiatives have been proposed: (1) exploring various alternative energy resources to complement or even replace primary fossil-based energy, (2) increasing the performance and efficiency of current energy systems, and (3) developing energy storage to eliminate the mismatch between energy supply and demand and relieve stress on energy availability.
The first initiative has been the major topic in most countries in recent times. Malaysia in its energy transition plan has set a target of 31 per cent renewable energy (RE) in its installed capacity in 2025 and 40 per cent in 2035. Presently, the installed capacity in Malaysia is 7,995 MW. By 2035, the RE installed capacity is projected to double to 18,000 MW.
The second initiative has been the primary focus in the energy and thermal fields with various enhancement strategies having been proposed and evaluated.
The last initiative, meanwhile, has been gaining traction of late with the boom in the exploration of renewable energy, which has typically intermittent availability, and the growing mismatch between energy supply and demand.
Not surprisingly, therefore, various energy storages have been identified, proposed and evaluated. They include thermal energy storage, electrical energy storage, mechanical energy storage, chemical energy storage and biological energy storage. These energy storages are becoming very much a part of our modern life but have gone largely unnoticed much of the time.
Presently, energy conversation and utilisation are realised mostly through thermal systems such as heating and air conditioning systems, powerplants, heat exchangers, internal combustion engines and portable power generators. As a consequence, thermal energy has become one of the most widely adopted and utilised forms of energy storage.
Thermal energy storage has become an inherent part of most thermal systems, particularly for large applications where fluctuation on the demand is large. There are three main thermal systems based on the mechanisms used to store the energy – sensible TES (STES), latent TES (LTES) and thermochemical energy storage (TCES). The first system is the most mature and technologically simplest storage while the last is the most recent and relatively more complicated than the others.
The sensible TES can be commonly found in typical thermal applications such as district cooling or heating plants, air conditioning systems of buildings, or water and/or air heater plants. There are various materials that can be implemented in STES depending upon its operating conditions, including fluid (water, oil, and brine water), solid (sand, rock, gravel, brick, and concrete), molten salt, and liquid metal. Except for the last two materials, energy storage materials for STES are relatively low-cost and widely available.
The inherent drawback with STES is its energy storage mechanism, where energy is stored by increasing the temperature of the storage medium. Therefore, it suffers inevitable heat loss due to temperature gradient with the surroundings. Moreover, the storage medium typically has low thermal capacity, limiting its storage capacity.
To achieve higher storage capacity, STES needs to have larger volume and/or higher storage temperature. Either one incurs significant cost and larger potential of heat loss to the surroundings. Moreover, larger storage leads to higher potential for non-uniform temperature within the storage resulting in non-constant outlet temperature profile.
To minimise this problem, some studies have proposed incorporation of phase change materials (PCM), mainly in the form of encapsulated PCM, to the sensible TES to augment the overall storage performance. Owing to their relatively high latent heat of fusion, PCM can keep large amounts of thermal energy at fairly constant temperature. The concern with this strategy, however, is the inherent low conductivity of PCM which hinders heat storage and extraction.
Our research group at Curtin Malaysia, working with our counterparts at McGill University in Canada and Shandong University of Science and Technology, China, is exploring the possibility of using PCM with low thermal conductivity properties as the insulation for the TES. In traditional sensible TES, an insulation layer made of cement, clay or fibreglass is commonly used to prevent heat energy from escaping into the environment.
By incorporating a PCM layer on the outer side of STES, the low thermal conductivity of PCM can function as an effective wall insulation by slowing down the heat transfer into the environment while providing additional energy storage capacity. Results have shown that better insulation, indicated by higher retained temperature within the storage, can be achieved by Hybrid TES with PCM wall layer as compared to the typical sensible TES without it. In addition, TES heat storing capacity is increased by introducing the PCM wall.
Among the investigated parameters, HTF inlet temperature has the most significant positive effect on the performance of Hybrid TES. Our study reinforces the potential of PCM wall incorporation into TES as wall insulation and additional thermal energy storage medium. More studies will be conducted to evaluate other key factors affecting the performance of this hybrid thermal energy storage and obtain optimum design and operating parameters.
Dr. Jundika is an associate professor in the Mechanical Engineering Department, Faculty of Engineering and Science, Curtin University Malaysia. He was previously a senior lecturer in the Mechanical Engineering Department and Head of Green Hydrogen Technology Centre (GHTeC) and Solar Thermal Advanced Research Centre (STARC) of Universiti Teknologi PETRONAS. Prior to these academic appointments, he was a post-doctoral researcher at Masdar Institute of Science and Technology (now Khalifa University), Abu Dhabi, United Arab Emirates, and the Mineral Metal Material Technology Centre, National University of Singapore.
He obtained his Bachelor Engineering (BEng) degree from Universitas Gadjah Mada, Yogyakarta, Indonesia and Doctor of Philosophy (PhD) in Mechanical Engineering from National University of Singapore. He is registered as a Chartered Engineer with the UK Engineering Council and a Professional Engineer with the Institution of Engineers Indonesia.
Dr. Jundika has to 15 years of research and development experience in the field of transport phenomena and energy systems in addition to more than 15 years of academic experience. He has published extensively in international journals and conference papers with a high number of h-index citations, and has secured a number of competitive national and international research grants. He is also actively involved in the academic community as advisory board member, visiting professor, invited speaker, journal guest editor, journal reviewer, conference committee member and research grant reviewer.
Dr. Jundika can be reached by email to email@example.com or by phone at +60 85 630100 ext. 2517.