Review on Micro/nano Energy Generation (mechanical)

Renewable energy generation nowadays is crucial to preserve the coal reserves of our planet as well as reduce the carbon levels and other pollutants in the atmosphere. Micro/Nano energy generation means that the generation of energy is on a small scale. It was found in other researches that energy can be harvested from vibrations from cars, humans, etc and micro/nano energy generation is feasible. However, the current technology nowadays is still not able to increase the efficiency to a feasible level. Despite the low efficiency, it is still desirable to harness the clean energy.

Introduction

People have always relied on power since the Industrial Revolution in the period 1760 to the 1800s. As the demand for commodities increased during that period, there was a need to shift from hand production methods to machines. Machines were the solution for the increasing demands but needed a source of power to function properly. Early sources of power in England before Industrial Revolution were firewood, draught animals and human labor as seen in Figure 1.1.

The use of coal as a power source started becoming popular during the Industrial Period. It replaced wood as it yields more useful energy than firewood does as seen in figure 1.2. As a result of this massive shift in methods, products were manufactured very fast and the economies of every country rose exponentially. Many manufacturing plants were built to compensate the increase of demands. Hence, there is an increase of use of coal throughout the world. This resulted to a massive increase of pollutants in our atmosphere as seen in figure 1.3.

Coal is considered to be a nonrenewable resource. It takes millions of years to produce them naturally. Coal contains high level of carbon, nitrogen and other gases and when burned, these gases are released to the atmosphere where they cause the greenhouse effect. Greenhouse effect is the phenomenon where the sun’s warmth is trapped in the earth’s lower atmosphere due to the greater transparency of the atmosphere to visible radiation than infrared radiation emitted from the earth’s surface. This results to temperature increase and the gases remain on the atmosphere. These are very detrimental to Earth, as well as to us since it can cause extreme and abnormal phenomena. These effects have been increasingly noticeable nowadays such as smog in certain cities, stronger typhoons and increase of sea level due to global warming. Many researchers have taken initiative to search for alternatives to fossil fuels. There have been many researches that are rising today regarding new methods of energy generation. Many examples are cleaner and renewable energy such as solar, wind, and tidal. Current disadvantages of these methods are low efficiency and high cost. Researchers have been finding improvements to increase the efficiency so that the methods will be feasible in supporting the power demand of the world. Figure 1.4 shows the trend of world energy consumption from the year 1990 to the year 2040.

This study was conducted by the International Energy Outlook 2016. They predict that there will be a 48% increase in energy consumption between the years 2012 and 2040. It shows that 3 there will be an increase usage of renewable energy in the coming years. This may be linked to better access to high technology in the future. Since renewable energy usage is rising, it is natural for non-renewable sources of power to either fall, rise slowly or remain steady. Although, the renewables are seen to grow faster than the non-renewables, it is still stated that fossil fuels will still supply for almost ¾ of the energy consumption at year 2040.

Energy Harvesting Device

An energy harvesting device is usually comprised of four different components. The system is seen in figure 1.5. Environmental energy is readily available everywhere such as solar and heat energy from the sun, kinetic energy from the wind, vibrational energy from the cars passing, etc. An energy harvesting device converts these energies to useful energy. From figure 1.5, environmental energy is captured by an energy capture device and is presented to the transducer. The transducer converts the environmental energy which has no use to us to useful energy. The transducer outputs an electrical current. The resulting current may be unpredictable and may also require conditioning before being stored in a battery or used by an electric load. Such is the case in a solar panel setup, where the DC power produced by the solar panel is passed through an inverter to be converted to AC so that it can be used by our appliances. The role of the power conditioning unit is not only to condition the electric signal from the transducer but also to optimize the power flow from the transducer.

Current Status and Trends

Solar or PV cells are the most mature and long established of the types of small-scale energy harvesting applications. Example of small-scale solar cell application is a PV-powered pocket calculator. A common PV-powered pocket calculator is the Casio 991-ES calculator. The battery in the calculator is charged when exposed to the sun or any artificial light bright enough to produce power. Solar cells have been available now for over 30 years. Further development of the PV cells in the last few decades have focused on improving efficiency and cost. Thermoelectric devices for power generation have also been established but less extensively commercialized than PV cells however in recent years major research efforts have been done to increase the conversion efficiency by using sustainable materials such as nanostructured materials. Motion-based harvesting has been also developed for the past 10-15 years. Devices based on fluid flow such as microturbines have also been developed. A further harvesting method extracts power from the radiation made by humans. Example of these radiations are radio signals from cellphones, WiFi, and others. Harvesters were intended to be a substitute to batteries. Their intended purpose was to be perpetual, maintenance-free power by having long lifetimes and high reliability. One disadvantage of the micro harvesters are that they need to be designed according to their application, hence they do not achieve the versatility of batteries.

Literature review

A study conducted by Zhou, M., et. al shows a review on heat and mechanical energy harvesting from human. The study states that heat and mechanical energy are available to be harvested from human daily activities. Specifically, in mechanical energy harvesting, power can be taken from human motion. The human motion manifests itself in many different ways: dislocating body center/parts. Performing basic body functions like respiration and blood circulation, redirecting leg motion with the help of foot strike, etc. It is stated that the energy available from dislocation of body center/ parts come in two forms: the motion of body center and the motion of different joints. The other two harness energy obviously due to the cyclic nature of those actions. The study also stated that mechanical energy available from human motion can be generally divided into two groups: kinetic energy due to rigid body motion and elastic energy due to elastic deformation. There are different possible mechanical energy harvesting devices that are seen in the study. They are electrostatic, electromagnetic, piezoelectric and triboelectric. The corresponding advantages and disadvantages are also clearly stated in the study. The study also stated that a combination of mechanical and thermal harvesters or hybrid are a good candidate for harvesting energy from humans. It is usually achieved in two ways: the integration of different energy harvesting devices and the coupling of multiple energy conversion mechanisms. This study explained that the conversion of heat/mechanical energy to electricity using human motion is a highly nonlinear process. These methods are confronted with several challenges such as complexity and difficulty in fabrication. Since it is a relatively new research, there is still very little developments on these methods hence would result the study being infeasible.

A study by Selvan, K., et. al shows a review of methodological performance of micro-scale energy harvesting devices in the last decade. It is stated in the study that micro-scale energy production is very crucial and useful for powering devices without the need for large power storage elements especially in remote areas. There are different micro-scale harvesters known in our current technology. These are thermoelectric, thermo-photovoltaic, microbial fuel cells and piezoelectric means. The micro-scale harvesters, on paper are very desirable in terms of clean power generation, however they also face several problems and limitations such as being only optimistic. Usually, these micro-power harvesting devices are augmented to produce higher voltages and good system efficiencies. This leads to negative feedbacks and greatly affects the performance. Piezoelectric devices can produce high voltages but only small electrical current flow because of its high impedance. They are prone to cracking or breaking hence have only limited amount of applied strains and low operating frequency.

A study by Abdelkareem, M., et.al shows a detailed review on vibration energy harvesting in automotive suspension system. The study shows the energy harvesting potentiality for different kinds of vehicles as well as the improvement of fuel efficiency when using regenerative energy shock absorbers. According to the study conducted, mechanical energy can be transformed to electrical energy through devices which uses the concept of electromagnetic or piezoelectric means. The study states that the power harvested due to vibrations produced from road irregularities could reach up to 350 W for a medium sized sedan car which has four energy-harvesting dampers). It is also stated that for heavy-duty and off-road vehicles, the power harvested can reach over 1kW. The energy harvesters can offer up to 3% fuel savings or an estimated 0.3-0.5L of fuel per 100 km.

A study conducted by Arafa, M., et.al shows the power harvested from gas pipeline vibrations. The experimental setup used consists of an air blower and brass pipe held in position by two flexible rubber bands. The blower vibration is then isolated from the pipe by inserting a flexible rubber coupling between the pipe and blower. This ensures that the power gathered is solely from the vibrations produced by the passing gas. The pipe vibration was measured using an accelerometer. It was found that the maximum power delivered was 0.4 µW for base excitations of about 0.02g.

A study by Ye, G. shows a study of energy harvesting from water distribution systems. A model in the study shows a piston harvesting power due to pressure fluctuations within the water flow. It was shown that when the system works in the 0.1 Hz Sine data, the resonant system produces more power of 15.65 mW than in the nonresonant system where it produces less power of 10.83 mW. Both systems dynamic displacements (peak-to-peak) are 0.1m. However, when the system works in the real data, both power outputs drop. This is because the real data is hard to model. It was found that the resonant system produces 8.11 mW but its dynamic displacement is much greater than 0.1 m. On the other hand, in the nonresonant system, the power produced is still less than the resonant at 3.43 mW however its displacement is closer to 0.1 m. The study assumes that all energy consumed in the damper has been transferred to electrical output power. The study suggests that an electromagnetic generator should be used than a piezoelectric generator since the frequency of the vibration generated by the pressure fluctuation which is less than 1 Hz might be too low to drive a piezoelectric generator hence it may not be able to read the data. Although the piston movement is very small because of low frequency pressure fluctuations, a gear box can easily convert the slow motion to a high-speed rotation which will be suitable for a commercial electromagnetic generator.

A study by Edwards, R., et. al. states a review on micro-energy harvesting technologies. The paper discussed the different energy harvesting technologies in use today namely photovoltaic, thermoelectric, magnetic, piezoelectric and pyroelectric, electrostatic and capacitive, and RF technologies. In this review, the advantages and disadvantages of each methods are also discussed as well as their applications and their future developments.

Discussion

The preceding section listed recent researches towards micro/nano energy generation. Micro/nano energy harvesting means harvesting energy at small scale usually best described by microwatts, usually 10s-100s of microwatts, or sometimes nanowatts. They can also refer to the size of the harvesters. There are several ways in micro-energy harvesting. These are through photovoltaic, thermoelectric, magnetic, piezoelectric and pyroelectric, electrostatic and capacitive, and RF technologies. From the methods mentioned in the literature review, piezoelectric, electrostatic and capacitive, electromagnetic, and triboelectric can be classified as mechanical energy harvesting devices.

A. Piezoelectric Energy Harvesting

Piezoelectric and pyroelectric energy harvesting are very closely linked to each other as they both result in changing the conditions of a crystalline material. It can be clearly seen from figure 2.1 that in order for electricity to be produced using the piezoelectric effect, mechanical stress must be applied to a piezoelectric material. The voltage produced is dependent upon the intensity of the stress applied.

Advantages and Disadvantages

The main advantage of using piezoelectric energy harvester is that there are a vast number of sources of vibration that can be exploited. Some examples are from the vibrations caused by the passing cars on our roads, the vibration produced from the irregularity of the road, vibrations produced by high pressure fluids in pipes and from human motion. Another set of advantages of PE energy harvesters are:

• High Voltage Output
• Small mechanical damping
• No voltage source required
• No mechanical stoppers needed
• High energy density
• High capacitance.

The disadvantages however are:

• Need for piezoelectric materials
• Difficult integration
• Self-discharge at low frequency
• Poor coupling at the micro scale
• High impedance and low current.

B. Electrostatic and capacitive

Energy Harvesting Electrostatic energy harvesting uses movable plates of variable capacitors that are periodically charged and discharged. The structure of the system converts mechanical energy from the oscillation of the plates against the magnetic field into electrical energy. Capacitive energy harvesting consists of inserting a conductive shell between the ground and a conductor forming a capacitive voltage divider.

Advantages and Disadvantages

One of the main advantages of the capacitive energy harvester is that its implementation is independent from the load and is not affected by external weather conditions. Another set of advantages are:

• Easy fabrication
• Low operation frequency
• High electrical damping
• High voltage output
• High power density.

The disadvantages are:

• Dielectric breakdown
• External power source required
• Low capacitance
• High output impedance and low output current.

C. Electromagnetic Induction

Energy Harvesting EM induction is a technique in which a conductor, usually a coil, is used to break magnetic flux lines which induces an EMF and current onto the coil that generates electrical power.

Advantages and Disadvantages

The advantages are:

• No voltage source required
• No need for contacts
• Small mechanical damping
• Robust and durable operation
• High output current
• Low output impedance.

The disadvantages are:

• Complicated to be miniaturized
• Low efficiency
• Coil losses
• Low voltage output
• Difficult integration.

D. Triboelectric Energy Harvesting

Triboelectric energy harvesting relies on the electrification of a given material when it comes into frictional contact with a different material. A common example of triboelectricity is observed when you rub glass with fur or when you use plastic comb through your hair.

Advantages and Disadvantages

The advantages are:

• High power density
• Easy fabrication with nanoscale size
• Low operation frequency
• High conversion efficiency
• Device flexibility.

The disadvantages are:

• Low current at high voltage
• Mechanism not fully understood
• Durability not satisfactory
• Electrostatic charge accumulation
• Not easy to be integrated.

Conclusion

This study held a review of micro/nano energy generation. It stated the current status and trends of this technology. It was found that despite the potential of several energy harvesting devices such as the piezoelectric and triboelectric energy harvesting device, the efficiency and cost of fabrication was too high for it to be feasible in today’s standards. Hence, it is concluded that the energy should be used and harvested despite its low efficiency since it is free and clean energy. This could help minimize the pollutants in the atmosphere hence reducing the devastating effect of global warming.