Critical review of energy storage systems Critical review of energy storage systems This review article critically highlights the latest trends in energy storage applications, both cradle and grave. Several energy storage applications along with their possible future prospects have also been discussed in this article. Comparison between these energy storage mediums, as well as their limitations were also thoroughly discussed. Suggestions and solutions in mitigating some of these challenges in order to improve the overall performance of these energy systems have also been analysed in this investigation. In spite of the accelerated growth in home energy storage system, there is still a grave need for further investigations, in order to reduce their costs. Further research activities will reduce the cost of some of these novel technologies, thereby accelerating their commercialization as well as making them better competitors against traditional energy storage mediums. Energy systems are dynamic and transitional because of alternative energy resources, technological innovations, demand, costs, and environmental consequences. The fossil fuels are the sources of traditional energy generation but has been gradually transitioned to the current innovative technologies with an emphasis on renewable resources like solar, and wind. Despite consistent increases in energy prices, the customers' demands are escalating rapidly due to an increase in populations, economic development, per capita consumption, supply at remote places, and in static forms for machines and portable devices. The energy storage may allow flexible generation and delivery of stable electricity for meeting demands of customers. The requirements for energy storage will become triple of the present values by 2030 for which very special devices and systems are required. The objective of the current review research is to compare and evaluate the devices and battery energy storage system presently in use and anticipated for the future. The economic and environmental issues as well as challenges and limitations have been elaborated through deep and strong consultation of literature, previous research, reports and journal. The technologies like flow batteries, super capacitors, SMES (Superconducting magnetic energy storage), FES (Flywheel Energy Storage), PHS (Pumped hydro storage), TES (Thermal Energy Storage), CAES (Compressed Air Energy Storage), and HES (Hybrid energy storage) have been discussed. This article may contribute to guide the decision-makers and the practitioners if they want to select the most recent and innovative devices and systems of energy storage for their grids and other associated uses like machines and portable devices. The characteristics, advantages, limitations, costs, and environmental considerations have been compared with the help of tables and demonstrations to ease their final decision and managing the emerging issues. Thus, the outcomes of this review study may prove highly useful for various stakeholders of the energy sector. The need for energy emerged as soon as human beings learned to cook food, although people were unknowingly benefitting from solar energy to protect their bodies from coldness and drying clothes in the sun etc. The first planned utilization of energy was from wood and fire. However, increasing awareness of nature for taking advantage of energy, various sources of energy were identified and put to versatile uses. People also acquainted to change forms of energy and storing it for the times when sources were not available, for example, solar energy at night, though the ways of conserving energy were very basic like storing wood under shelter and other safe places. However, increased populations and energy usage versatility added other sources like coal, steam, water, wind, and petroleum. The invention of electricity changed the whole scenario of energy. The olden sources of energy were replaced partially by the production and consumption of electricity. Some modern sources of energy like nuclear and renewable resources have been identified in the twentieth century. Presently, an energy mix is prevailing and being used in different parts of the globe. The demands for energy are increasing rapidly due to an increase in populations, economic development in developing countries, enhancement in per capita consumption, change in lifestyle, and supply at more remote places as stored energy. The world's primary energy consumption was 149,634 and 157,064 Terawatt-hours (TWh) in 2015 and 2018 respectively (Ritchie and Roser, 2019). According to their estimate, the regional consumptions were 69,615, 32,936, 23,859, 10,822, 10,494, 8164, and 5367 TWh for Asia Pacific, North America, Europe, CIS, Middle East, South and Central America, and Africa respectively. Thus, the biggest consumers of energy were Asia Pacific and North America while Africa used the least quantum of energy in 2018. The Gulf Cooperation Council (GCC) countries are although low populated, but are high consumer of energy, even in comparison to some of the developed countries (Al-Badi and AlMubarak, 2019). The consumption of electricity in the GCC region has grown from just 51 TWh in 1990 to almost 536 TWh in 2015 whereas the per capita use has been recorded as one of the highest rates. It is estimated that the GCC countries will be consuming 1094 TWh by 2025 (Almulla, 2014). Such a pattern is mostly due to rapid economic development and significant change in the lifestyle. The household energy storage system necessarily require smooth, balanced, reliable and quality supply (maintaining constant voltage and frequency) to the customers without any breaks and potential damage to electrical appliances. The strong variations always exist in demand of electricity at different times. Hence, there could be certain times when the energy production will be more than demand and vice versa. Just to quote an instance, the peak demand of GCC countries in summer is twice the off-peak summertime requirement due to the running of air conditioners and is thrice of winter peak times (Al-Badi and AlMubarak, 2019). For balancing and matching the demand and supply, the storage of energy is a necessity. The present trends indicate that the need for energy storage will increase with high production and demand, necessitating the energy storage for many days or weeks or even months in the future. According to estimates, requirements for storing energy will become triple of the present values by 2030 while the stationary energy could dominate in quantities of electricity supply through grids (IRENA, 2017). The energy storage techniques and devices have been changed and modernized simultaneously along with increasing production and demand. The devices conventionally were magnets, batteries, dry cells, and capacitors. However, besides changes in the olden devices, some recent energy storage technologies and systems like flow batteries, super capacitors, Flywheel Energy Storage (FES), Superconducting magnetic energy storage (SMES), Pumped hydro storage (PHS), Compressed Air Energy Storage (CAES), Thermal Energy Storage (TES), and Hybrid electrical energy storage (HES) were developed for sustainable and renewable usage (Frackowiak and Béguin, 2001, Doetsch, 2014; Haisheng et al. 2009; Luo et al., 2015; Silva & Hendrick. 2016; Stanley, 2012; UCS, 2006). However, energy storage mechanisms also face many challenges as well (Mohd et al., 2008) because none is complete in all respects due to one or more limitations like storage capacity and form, string time, special structural or implementation requirements, energy releasing efficiency, and operation time (Yae, et al., 2016). In addition, there are cost, and environmental aspects like CO emissions (IEA, 2019) associated with the energy storage technologies, which must be identified and considered when planning and deciding the selection of technologies for installation in the grid systems of an area. The aspects identified above need to be elaborated through a systematic study from the literature so that valuable research work of earlier authors is gathered, understood well, and arranged in a good array to clarify the study areas, which can contribute to support and ease the decision makers and practitioners for selection of best energy storage devices and mechanisms for their particular grid systems. Considering the high importance and problems of electric energy storage, some aspects of this subject are being discussed and highlighted with support from the literature review. 2. A dynamism in the forms and sources of energy The types and uses of energy had been dynamically changing in history because Beltran (2018) regarded energy as a living, evolving, and residential energy storage system, which remained an integral part of civilizations and their development. The sun was the only source of heat and light while wood, straw and dried dung were also burnt. The horses and other animals, wind, and water were used for transportation, working in the fields, grinding grains, pumping water, and driving the simple machines in very earlier times. Later, the power of steam was harnessed which dated back to ancient Alexandria. The steam engines remained in use till the 17th and 18th centuries. Simultaneously, coal was also used for heating and production of steam from water. By the late 1800s, petroleum was introduced as a fuel and is still in wider use. Thomas Alva Edison installed the first electric light plant in the city of New York in 1880. (UCS, 2006). The invention of electricity revolutionized energy usage and consequently, industrial revolutions happened on the globe. Currently, electricity is the dominating form of energy all over the world. The introduction of nuclear energy started in the 1950s and was increasing rapidly, but the Chernobyl accident in Russia (1986) and some later incidents in India and other countries discouraged its spreading due to safety concerns and social pressure (King, 2019). The modern biofuels, wind, and solar are finding their way again while geothermal and marine technologies are new additions in the field of energy. Advances in technology, alternative energy sources, costs of energy and pressures of social issues associated with energy production are the driving forces behind the above changes, but the static fact is the consistent increase of energy utilization during the global history (Ritchie and Roser, 2019). Ritchie and Roser (2018) reported that the total global energy consumption in 2018 was 160,228 TWh while different energy production sources contributing to this huge production are oil, coal, gas, hydropower, wind, solar, nuclear, and other renewals. According to them, the biggest sources are Oil, Coal, and Gas contributing energy (TWh) as 54,220 (33.84%), 43,869 (27.38%), and 38,489 (24.02%), respectively. Thus, these three major sources are meeting 85.24% of global energy requirements. The respective shares of Hydropower, Wind, and Solar were 6.89% (11,034 TWh), 2.09% (3342 TWh), and 0.96% (1539 TWh). The contribution from nuclear resource was 4.43% (7109 TWh) and Other Renewals 0.38% (626 TWh). Due to CO2 emissions during electricity generation from fossil fuels, demand is increasing to shift gradually to renewal sources, but it is not possible in the short-term because demand of electricity may go thrice by 2040. According to estimates of World Energy Council (2019), global emission of CO2 might stabilize by 2030 and reductions could be expected afterwards. These days an energy mix (electricity, the solar, wind, and nuclear) is being consumed in various countries of the world. However, all the other forms contributed only less than 1% of the total energy utilization (BP Statistical Review, 2019, Ritchie and Roser, 2019). 3. Global status of the consumption of energy The energy consumption has increased tremendously after the industrial revolutions due to an increase in population, invention of new techniques and machines, economic development, accessing remote and far flanged areas, and big changes in the lifestyle. According to estimates, energy use was doubling in each decade in earlier times (UCS, 2006). Simultaneously, a significant increase also took place in the production of energy, especially electricity. Among other drivers of increasing demand for energy are selling the electricity even below the actual cost in GCC and some other countries, wastage due to usage and building designs, and lower efficiency of generation and delivery equipment (Al-Badi and AlMubarak, 2019). Nevertheless, production could not match demands in so many developing countries. According to estimates, the world's primary energy consumption in 2015 remained as 146,000 terawatt-hours (TWh), 25 times higher than the year 1800 (Ritchie and Roser, 2018). As the data values are not mostly same when reported by different sources, in another report (BP Statistical Review, 2019), the global energy consumption was 136,129 TWh in 2008 and 161,250 TWh in 2018. There has been a 2.9% increase in consumption for the 10 years. World Energy Council (2019) while finding scenarios and exploring innovative pathways to 2040, contemplate that the globe will be entering in a new energy era promising enough, clean, and residential solar energy storage for all communities with increasing uses and users. About 10% increase is presumed in demand of energy by 2040. However, there will be more emphasis on renewal sources considering environmental protection, but fossil fuels (especially gas replacing major part of coal) will remain dominating although decreasing as source of electricity generation. The energy consumption is highly variable in different countries of the world, not necessarily proportional to the populations but also many other factors; economic development, lifestyle, and climate. The top ten high consuming countries in the descending order are China, USA, India, Russia, Japan, Canada, Germany, South Korea, and Brazil (Table 1). It is very clear that these ten countries swallow 66% of energy utilization of the world. Only China consumes 23.9% while USA takes 16.6%, thus these two countries share 40.5% of the word's energy consumption. If India and Russia are added too, the energy dissipation of the four biggest countries rise to 51.5%, which means that the whole of the rest world consumes even a bit lesser (0.5%) than 50% (Table 1). The per capita consumption of electricity is also highly variable in different countries. The values range from 52921.73 KWh (Iceland) to 8.32 KWh (Liberia). This rate for GCC countries ranged from 5340 KWh to 17,610 KWh in 2010, compared to 3378 KWh and 2728 KWh, the respective means for the Middle East and the globe (IRENA, 2012). The countries ranking in the top ten list during 2015 are Iceland (52921.73), Norway (25018.59), Kuwait (18818.11), Bahrain (18491.19), UAE (18213.33), British Virgin Island (18035.13), Qatar (15784.42), Canada (14501.59), Finland (14328.50), and Sweden (12589.75). At the bottom of the list are Burundi, Sierra Leone, Guinea-Bissau, Chad, and Liberia in the descending order. The per capita consumptions of Saudi Arabia and Oman were 10248 and 5987 KWh, respectively while the other four countries of GCC have already been included in the top ten list above (CIA, 2019).