Energy Harvesting Transducers

EE174 – SJSUTan Nguyen

Solar cell 

A solar cell is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect.The solar cells that you see on calculators and satellites are also called photovoltaic (PV) cells, which as the name implies (photo meaning "light" and voltaic meaning "electricity"), convert sunlight directly into electricity.

Solar Cells: Converting Photons to Electrons

Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors

are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current -- that is, electricity. This electricity can then be used to power a load, such as a light or a tool.

• A module is a group of cells connected electrically and packaged into a frame known as a solar panel.

• Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module.

• Multiple modules can be wired together to form an array.

• Solar panels or arry can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.

Solar Cells: Converting Photons to Electrons

A solar system consists of: Solar Panels, a Charger Controller, a Power Inverter, andBatteries.• Solar Panels: supply the

electricity and charge the batteries. A very small system could get away with a couple 240 watt panels but figure at least 4 to 8 for a small to medium system.

• Charge Controller: is needed to prevent overcharging or draining too much of the batteries. Proper charging will prevent damage and increase the life and performance of the batteries.

• Power Inverter: is the heart of the system. It makes 120 volts AC from the 12 volts DC stored in the batteries. It can also charge the batteries if connected to a generator or the AC line.

Typical Solar Power Setup

• Batteries: store the electrical power in the form of a chemical reaction. Without storage you would only have power when the sun was shining or the generator was running.

Types of Solar PanelsMonocrystalline solar panels : The most efficient (15 – 20%) and expensive solar panels are made with Monocrystalline cells. These solar cells use very pure silicon and involve a complicated crystal growth process. Long silicon rods are produced which are cut into slices of .2 to .4 mm thick discs or wafers which are then processed into individual cells that are wired together in the solar panel.Polycrystalline solar panels : Often called Multi-crystalline, solar panels made with Polycrystalline cells are a little less expensive & slightly less efficient than Monocrystalline cells because the cells are not grown in single crystals but in a large block of many crystals. This is what gives them that striking shattered glass appearance. Like Monocrystalline cells, they are also then sliced into wafers to produce 12 the individual cells that make up the solar panel.Amorphous solar panels : These are not really crystals, but a thin layer of silicon deposited on a base material such as metal or glass to create the solar panel. These Amorphous solar panels are much cheaper, but their energy efficiency is also much less so more square footage is required to produce the same amount of power as the Monocrystalline or Polycrystalline type of solar panel. Amorphous solar panels can even be made into long sheets of roofing material to cover large areas of a south facing roof surface.

How long do solar panels last?

Solar photovoltaics slowly lose their generating capacity. Although some solar panels are still working satisfactorily 40 years after installation, the conventional view is that most will dip below 80% of their rated capacity within about 20 years. This will vary slightly between manufacturers and between different types of silicon.

A single solar cell produces only about 1/2 of a volt. However, a typical 12 volt panel about 25 inches by 54 inches will contain 36 cells wired in series to produce about 17 volts peak output. If the solar panel can be configured for 24 volt output, there will be 72 cells so the two 12 volt groups of 36 each can be wired in series, usually with a jumper, allowing the solar panel to output 24 volts. When under load (charging batteries for example), this voltage drops to 12 to 14 volts (for a 12 volt configuration) resulting in 75 to 100 watts for a panel of this size.Multiple solar panels can be wired in parallel to increase current capacity (more power) and wired in series to increase voltage for 24, 48, or even higher voltage systems. The advantage of using a higher voltage output at the solar panels is that smaller wire sizes can be used to transfer the electric power from the solar panel array to the charge controller & batteries. Since copper has gone up considerably in the last few years, purchasing large copper wiring and cables is quite expensive. (that's why pennies are made of mostly zinc today).

EXAMPLES

• Piezoelectric transducers are a type of electroacoustic transducer that convert the electrical charges produced by some forms of solid materials into energy. The word "piezoelectric" literally means electricity caused by pressure.

• With piezoelectric devices, energy from vibration can supply low-power applications, such as wireless sensors that are difficult to reach and maintain, for equipment or structural monitoring.

Piezoelectric Transducers

Piezoelectric wafers are fabricated from a ceramic such as lead zirconate titanate (PZT) or fluoropolymer film such as polyvinylidene fluoride (PVDF). When subjected to mechanical forces that cause them to flex, these devices produce an ACvoltage proportional to the amplitude of the flexing movement. For use as vibration transducers, these materials are typically mounted in cantilevered structures, mounted to a fixed platform at one end and affixed with a tuning mass at the other.

How Piezoelectric Transducer Works?

Piezoelectric Resonance Frequency

• By changing the loading mass on a piezoelectric transducer, engineers can adjust its resonant frequency to match that of the attached object. Manufacturers typically provide load-dependent resonant frequency curves that document this effect. The datasheet for the Measurement Specialties LDT Series devices, for example, documents the changes in resonant frequency from 180 Hz with no loading to 90, 60, and finally 40 Hz as the loading mass is increased in increments of approximately 0.26 g.• Piezoelectric transducers can provide a significant energy source when combined with suitable power management circuits in micro-harvesting designs. By matching the transducer's vibration frequency to its attached object—and matching the power-management circuit load to the transducer's—engineers can extract maximum power from sources such as motors, structures, and the like.

Piezoelectric Resonance Frequency

EH Summary

EH Summary

EH Summary

When is energy harvesting appropriate?

Energy harvesting – a great solution Energy harvesting is useful when: • There is a match between the available energy and the energy needed • Energy harvesting provides a benefit that is not achievable using batteries or grid electricity

Energy harvesting – not a great solution • Free energy often comes at a cost, and thinking through the complete system is key to determining whether energy harvesting is going to solve your power needs.

Green energy A myth that keeps cropping up is that energy harvesting is a small scale demonstration of how we can collect freely available energy in the environment, and that in the future these same technologies will be scaled up and used to power our homes and businesses. This is false. With the exception of photovoltaic cells for collecting sunlight, the technologies are completely different.

References:

http://en.wikipedia.org/wiki/Solar_cell

http://science.howstuffworks.com/environmental/energy/solar-cell.htm

https://www.google.com/search?q=solar+cell&espv=2&biw=1280&bih=699&tbm=isch&tbo=u&source=univ&sa=X&ei=l15FVLqFJLGGigKN0IHIAg&ved=0CD4Q7Ak

http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/

http://www.offgridwinchester.com/Solar%20Power-101.pdf

http://www.apec-conf.org/wp-content/uploads/2013/09/is2.1.1.b.pdf

http://www.psma.com/sites/default/files/uploads/tech-forums-energy-harvesting/presentations/2012-apec-112-multiple-harvesting-transducer-optimization-continuous-power-needs_0.pdf