Susterel energy harvesting Roadmap for societal applications

RESEARCH REPORT VTT-R-05917-12

Susterel energy harvesting Roadmap for societal applications Authors: Marja-Leena Pykälä, Kari Sipilä, Ulla-Maija Mroueh, Margareta

Wahlström, Henrik Huovila, Tommi Tynell*, Jyrki Tervo

*) Aalto University

Confidentiality: Public


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Preface

The project SUSTEREL wasas a joint project financed by Tekes – the Finnish Funding Agency for Technology and Innovation, business partners Helsingin kaupunki/Helsinki City, Finnfoam Oy, Picosun Oy, Easy Led Oy, and research partners Aalto University and VTT. The steering group consisted originally of the following persons:

Mika Nummenpalo, Easy Led Oy Jarkko Piirto, Tekes

Yrjö Länkelin, Helsinki City Asso Erävuoma, Finnfoam Oy

Pekka Soininen, Picosun Oy Maarit Karppinen, Aalto University

Hisao Yamauchi, ” Tommi Tynell, ”

Erja Turunen, VTT Aino Helle, VTT

Mona Arnold, VTT Jyrki Tervo, VTT

We wish to address the warmest acknowledgements to the steering group for fruitful discussions and ideas for new materials and applications.

Espoo 21.12.2012

Authors


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Contents

Preface ........................................................................................................................ 3

1 Introduction ............................................................................................................ 6

2 Potential energy sources........................................................................................ 7

3 State of art of energy harvesting ............................................................................ 9

3.1 Technologies .................................................................................................. 9

3.1.1 Electrodynamics ................................................................................ 10

3.1.2 Vibration and kinetic energy - piezoelectric ...................................... 10

3.1.3 Thermoelectric ................................................................................... 10

3.1.4 Photovoltaic ....................................................................................... 10

3.1.5 Micro hydro, tidal and wave energy ................................................... 11

3.1.6 Heat from sewage and other warm water sources ........................... 11

3.1.7 Small scale wind power ..................................................................... 11

3.2 Enabling technologies .................................................................................. 11

3.2.1 Railways ............................................................................................ 11

3.2.2 New battery options and alternatives to batteries .............................. 12

3.2.3 Harvesting tolerant electronics and direct use of power .................... 12

3.2.4 Light harvesting for small devices ..................................................... 12

3.3 Sources and applications ............................................................................. 13

3.3.1 General.............................................................................................. 13

3.3.2 Commercial thermoelectric solutions ................................................. 14

3.3.3 Success in buildings and vehicles ..................................................... 16

4 Drivers and challenges of energy harvesting ....................................................... 21

4.1 Main drivers .................................................................................................. 21

4.1.1 Reliable Devices to Power Wireless Sensor Networks ...................... 21

4.2 Challenges ................................................................................................... 23

4.2.1 Market Drivers and Challenges ......................................................... 23

5 Potential new and developing applications .......................................................... 25

5.1 Sources of energy to harvest ........................................................................ 25

5.2 Thermal power potential in Finland .............................................................. 27


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5.3 Development of technologies ....................................................................... 35

5.4 Development of materials ............................................................................. 36

5.4.1 Thermoelectric Materials ................................................................... 36

5.4.2 Applications ....................................................................................... 38

5.4.3 Sustainability Analysis ....................................................................... 39

5.5 Competing technologies, their development and future potential ................. 41

5.6 Market forecast 2010-2020, 2030 of energy harvesting devices .................. 42

5.7 Cost efficiency .............................................................................................. 45

6 Environmental aspects of energy harvesting ....................................................... 47

7 Technology roadmap ........................................................................................... 48

8 Conclusions ......................................................................................................... 49

References ................................................................................................................ 50


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1 Introduction

Energy harvesting is the use of ambient energy to generate electricity. It provides potentially low cost, maintenance-free, long life equipment, by reducing the need for batteries (EH1 2011) or power chords. Energy harvesting (EH) is also known as power harvesting or energy scavenging, EH is considered to give benefits related in environmental friendlyness, safety, security, convenience and affordability. EH can be used for brand enhancing. Technically it can be used to make new things possible depending on engineering visionary.

So far, the main commercial successes include such things as photovoltaics on space vehicles, road furniture and consumer goods, electrodynamics in bicycle dynamos and wristwatches and piezoelectrics in light switches, indeed many forms of EH in building controls. Control devices with no battery or wiring have already been realised in many solutions. However, energy harvesting is now being made affordable and feasible for several new, large applications, (EH2 2011). They include:

1. 90% of envisaged uses of Wireless Sensor Networks (WSN) are impractical without energy harvesting. These mesh networks are rarely feasible (without energy harvesting) because, in the biggest projects envisaged, such as those where nodes are embedded in buildings and machines for life or on billions of trees, the batteries would be inaccessible or prohibitively expensive to access.

2. Getting almost free power for electronics and lighting to Africa where batteries are not affordable: indeed, they are rarely even obtainable.

3. Bionics and sensors are needed in the human body that stay there for the life of the patient. These are in the focus of a huge new research effort.

4. Mobile phones and laptop computers have batteries that frequently go down. Indeed, the power situation gets worse as more functionality is added, this inconvenience involves two billion people.

In all these applications there is now a delightful conjunction of progress by which new forms of lighting and electronics need far less electricity and new forms of energy harvesting are better able to provide it.


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2 Potential energy sources

Large scale energy production can be divided into three groups

1. Renewable : wind, large scale solar, hydroelectric, biomass, geothermal, tidal and waste energy sources

2. Fossil: coal, oil, methane, natural gas 3. Nuclear: fission, fusion

These large energy sources have been studied in numerous reports and articles elsewhere. Fuel cells and micro combustion engines are not included because they use fuel from external sources. As the energy harvesting is often understood as the use of ambient energy to provide electricity for small and/or mobile equipment, this report concentrates on the small-scale production like electrodynamics, microelectromechanical (MEMS), piezoelectric, thermoelectric, photovoltaic, micro hydro, tidal, biogas from waste, small scale wind power, vibration and kinetic energy.

Figure 1 shows how the output power of the harvesting devices is increasing and the demand for the electronic devices is decreasing. The button batteries up to AA or AAA size batteries can be replaced by energy harvesting systems.

Figure 1 Power demand of electronic devices vs. energy harvesting possibilities (EH2 2011): arrow upwards: Energy harvesting becomes more capable arrow downwards: Electronic devices become less power hungry.


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Figure 2 Comparison of the power density ranges of different energy technologies(EH2 2011)


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3 State of art of energy harvesting

Primary energy harvesting needs and efforts in Global efforts on energy harvesting development differ from country to country. In the report (EH1 2011) 160 case studies of energy harvesting in ongoing use in 31 countries is given. That is enough to give some idea of the leading countries, technologies and applications. The emphasis of energy harvesting in North America is on the applications for military, aerospace and healthcare. In Europe the industrial and healthcare devices are emphasised and in East Asia the consumer goods are the most important group of EH devices. The application sectors can be divided as shown in Figure 3. A combined approach is a use of a primary battery with energy harvesting device. That means that the battery can be much smaller and last longer. Is this a possible interim approach with Wireless Sensor Networks WSN as we await smaller, more efficient, more affordable multiple energy harvesting for these?

Figure 3 Number of EH cases in different application sectors (EH1 2011).

3.1 Technologies Although the price of specific technology erodes, our average unit value holds up because of change in mix. All the figures given below relate to the energy harvesting device such as a solar cell and not the associated energy storage, wiring or electronics.

Energy harvesting technologies are explained and exemplified in the reports (EH1 2011 and Frost & Sullivan 2007).


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3.1.1 Electrodynamics

The energy source of an electrodynamic device can be movement of an oscillating weight in such applications like a watch. Its power density varies from 1 to 200 mW/cm3. An advantage is the high power output and a disadvantage is the moving parts.

3.1.2 Vibration and kinetic energy - piezoelectric

Vibration and kinetic energy can be harvested using piezoelectric crystals, and piezoelectric composite fibres are considered to be the most promising energy harvesting devices for low-power applications. Power density of a piezoelectric device depends on the source: from machinery vibration 0.5 mW/cm3 and from shoe-insert moving 0.33 mW/cm3. Good features are simplicity and long life. Challenges are the availability of vibration or movement. One disadvantage is moving parts giving chance for fatigue failures.

Piezoelectrics are being studied for use in self powered sensors and controls in the human body, in aircraft, trains and so on. The challenge is to control the energy (amplitude and band width of vibration frequency) going into the piezoelectric harvester. Too high mechanical energy input can lead to damage. Other opportunities such as thermoelectrics, magnetostriction and RF can be alternatives to piezoelectrics. Vibration can be harvested also by electrostatic or electromagnetic conversion. Kinetic energy can be converted to electrical energy using a spring-loaded mechanism.

3.1.3 Thermoelectricity

Thomas John Seebeck discovered the Seebeck effect in 1821. At that time there was no rush to commercialize thermoelectric generators. ean Charles Athanase Peltier discovered the Peltier effect eleven years later in 1834. For over a century, these effects remained little more than laboratory curiosities. The primary product based on the Seebeck effect became the thermocouple for temperature measurements. It was not until the 1950’s that significant resources were put into thermoelectric research that led to actual power generation products (Stabler 2006). Thermoelectric devices utilize temperature differences or thermal gradients for generating electricity. Power density of the thermoelectrics (TE) is quite high when the source is an engine with temperature differences up to 120 K: power density from a heat flux varies from 1-3 W/cm3 to 20 W/cm3. No moving parts exist. It is not possible to optimise power and voltage at the same time. Fabrication costs are high and the heat difference is not available at all times. Thus there is a need for energy storage to stabilize the power.

3.1.4 Photovoltaic

Photovoltaic cells area usually made of silicon-based material, although other materials are being studied (and used) as well. Wireless sensor systems can use the solar energy harvesting devices directly or for charging their batteries during daytime.


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3.1.5 Micro hydro, tidal and wave energy

Energy harvesting in general has been done for a long time, the earliest being structures such as water-wheels. Tidal and wave energy solutions may be large scale applications, but in this report the low-power applications have the main weight.

3.1.6 Heat from sewage and other warm water sources

Sewage water is utilised for heating in district heating systems especially in Nordic countries. Waste water temperature is over 10 °C after a water treatment establishment, so it is useable for heating by heat pump. The COP (coefficient of the performance) of the heat pump is high over the year. Condensate water from power plant or other sources give same heating possibility for using heat pump. Ground water and water in see, lakes and rivers are also potential sources for heat pump. Ground water in Finland e.g. is about 5 - 6 °C over the year, but the temperature of the other sources varies during the year following the annual seasons.

All the forms of heat pump applications also use electricity about one quarter of heat output, so the most desirable applications are warm water sources for utilising without the heat pump.

3.1.7 Small scale wind power

Nowadays wind power plants and wind farms present large scale energy production connected to the distribution or transmission network. Small scale wind power can still be installed to the regions not possible to be connected to the common network.

3.2 Enabling technologies

3.2.1 Railways

Energy harvesting options and potential benefits in the railway environment includes solar energy for outdoor deployment, vibration energy: from train for embedded applications or from passengers walking inside main train stations, huge electromagnetic fields close to catenaries (1500V DC or 25000 V AC) or dense WiFi traffic in train stations and a temperature gap for embedded applications Objectives include increasing Wireless Sensor Networks, WSN battery life and autonomy of the system. SNCF wants energy for all sensors, processing and communication parts to provide from external sources, with no need for maintenance.

Using WSN preferably with energy harvesting there are various specific environments and elements to monitor. Embedded systems can be used in freight, urban, regional and high speed trains. The possibilities lie in vibrations or collisions and in motor coach or locomotive as very high electro-magnetic fields.

Rail infrastructure including tracks, catenaries, signaling, civil engineering structures are all-metal.


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3.2.2 New battery options and alternatives to batteries

The batteries are a key enabling technology for energy harvesting devices, based on the huge number of times the batteries can be recharged, their form factor etc. They can enable wireless sensors to last for many decades. The batteries proved capable of accepting DC and AC signals, and delivering a high peak power for radio transmission on sensors. Virtually all types of energy harvesters can be used, such as photovoltaics to piezoelectrics to RF energy emitted from a cell phone. They provide enough energy storage required for wireless sensor network (WSN) nodes. Several new techniques have been developed like lithium laminar batteries, thin, lithium based rechargeable batteries, transparent printed organic batteries and bio batteries. The use of supercapacitors or Electrochemical Double Layer Capacitors (EDLC) – to balance power delivery and store power is receiving increased attention. Like traditional electrolytic capacitors, supercapacitors rely on ionic effects to provide exceptionally large capacitance in a small volume and thin film versions are possible. However, leakage currents are relatively high, so, the charge can be stored for weeks not years. The electrolyte also ages. Mini fuel cells are being developed by many organisations and those that are fuelled from their environment would come within the definition of energy harvesting. Unfortunately most of the devices being developed use methanol which is certainly not available naturally nearby.

3.2.3 Harvesting tolerant electronics and direct use of power

Central to the progress of energy harvesting is the development of electronic circuits that use less power and therefore place more modest demands on the harvesting technology. As there are more and more options for energy harvesting technology, there is a burden on the designers of circuits to make them cope efficiently with mismatched power. For example, Wireless Sensor Networks are currently a very hot topic because, if only they can be made long lived and affordable, there are huge markets from monitoring forest fires to vital signs in the human body. Yet a typical WSN node, that has harvesting, gathers unpredictable amounts of energy at unpredictable times.

The electronics designers are therefore looking closely at having circuits that tolerate varying voltages and currents and even frequencies of input and even send a size and length of message that takes account of state of charge of the battery or capacitor, where used, and nature of power input at that instant. After all, the whole idea of wireless sensor networks is that they are mesh networks where compressed signals pass fitfully depending on whether another node is in range and other factors.

Although the prototype captures and stores light energy, the scientists say that energy-harvesting radios could be powered by a number of different ways, including electrochemical, mechanical or thermal energy.

3.2.4 Light harvesting for small devices

Report (EH2 2011) presents a variety of electronic devices powered by electric harvesting methods. Infinite Power solutions produce thin, lithium based rechargeable batteries (Figure 4).


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Figure 4 Application to energy harvesting(EH2 2011)

Infinite Power Solutions has even embedded its batteries into printed circuit boards (PCBs), designed to last for a life and it removes the area of the battery as a discrete component, enabling smaller package units. Power density of a photovoltaic device varies from 1 to 150 mW/cm3 at sunshine at noon. Good features of this solution are that no moving parts exist and the power output is high compared to µW-range. Thin flexible versions become available as well as infrared versions. Availability of light or IR is challenging. So far, the majority of work on energy harvesting has been concerned with photovoltaics. Most of this has benefitted from huge investment in photovoltaics as renewable energy of which energy harvesting to power for small devices, is a minor part. There is now a change of emphasis however, with certain types of photovoltaic technology such as Dye Sensitised Solar Cells (DSSC) and organic photovoltaics substantially developed for energy harvesting. Even the highly sophisticated GaAs/Ge multilayer cells now standard in space are being developed for terrestrial harvesting (EH2 2011).

3.3 Sources and applications

3.3.1 General

Thermal energy sources can be utilised in various applications. The applications vary from grid connected thermal generators to separately operated sensors, control and display units. The applications for thermal generators are quite similar to photovoltaic (PV) systems a) grid connected and b) stand alone solutions. The stand-alone devices quite possibly need a battery backup and possibly also reserve power. Small scale applications are portable radios in areas difficult to reach, remote control devices, radio links and miniature lamps. The power demand for the previous examples is less than 10 W. One possible application is battery charging units in buildings or in vehicles. Depending on the battery size the power demand varies from watts to kilowatts (Lund 1999-2006) Constant temperature difference 200 K or more will allow the use of thermal generators of kW-range. Direct DC applications can be used for separately operated heating, cooling, ventilation or pumping. DC can be used for charging batteries e.g. in vehicles. The voltage level needs to be controlled.


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If the electric energy is converted into AC the possible applications are much wider, from separate devices to grid connection. In grid connection both voltage and frequency shall be controlled as well as the requirements for the connection.

Low energy sources with small areas or low temperature differences can be found for instance indoor/outdoor temperature in buildings or waste water treatment plants. The applications in these cases may be sensors, small displays, control units, charging units for small size batteries and objects like that. The display units may show temperature or time. In industry the display may indicate also pressure, mass flow, number of pieces, mass or identification number. In public transportation the information display could be used for showing the next stop, advertisement or other information purposes.

3.3.2 Commercial thermoelectric solutions

In the Diesel Engine-Efficiency and Emissions Research (DEER) Conference (DEER 2006) has been presented several solutions suitable for cooling or electric energy generating in vehicles. A thermoelectric waste heat recovery system has been modelled and key subsystem designs established with a path outlined to meet the program goal of 10 % fuel efficiency improvement The thermoelectric generator module, TGM has been designed incorporating thermal isolation and high density design principles. A secondary loop has been implemented to improve system efficiency and reduce material usage. Figure 5 shows possible locations for cooling or thermoelectric units. An article (Riffat 2003) presents commercial available thermoelectric cooling products or assemblies: like consumer products; recreational vehicle refrigerators; mobile home refrigerators; car refrigerators; portable picnic coolers; wine coolers; beer keg coolers; water coolers; motorcycle helmet refrigerators; insulin coolers (portable); residential water coolers/purifiers; beverage can cooler; industrial-temperature control NEMA enclosures; harsh environment protection for critical components; PC computer microprocessors; microprocessor and PCs in numerical control and robotics; stabilizing ink temperature in printers and copiers.Thermoelectric technology has been used in wide areas recently. The thermoelectric devices can act as coolers, power generators, or thermal energy sensors and are used in many the fields of society or as commercial products. The applications of small capacity thermoelectric coolers are widespread. But the applications of the large capacity thermoelectric coolers and power generators are limited by their low efficiency (Riffat 2003).

Rowe presents also an Automobile Thermoelectric Generator (ATG) which nowadays has an electrical output of 200 W (Rowe 2009).


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Figure 5. Primary Heat Exchanger of BMW 530i Exhaust System. Locations for coolers or thermoelectric generators (DEER 2006)

Micropelt manufactures devices for condition monitoring, smart sensors and measuring systems, mobile devices and tracking/tagging. All these devices are powered by thermo electrics. A low power range is suitable for wireless systems, which can be powered from waste heat. Figure 6 shows a solution for wireless sensor system. This commercial device operates at temperature difference from 10 K to 50 K producing power and it gives a possibility to understand thermal harvesting (Micropelt 2009).

Figure 6. TE-power NODE manufactured by Micropelt GmbH (Micropelt 2009).

SCTB NORD has a long experience of operation in the thermoelectric module market. At present, Peltier coolers are widely used by many hi-tech areas, such as telecommunications, space and medicine, having applications in advanced laser, optical and radio electronic systems. Consumer product applications are: portable refrigerators, compact air conditioners, air dryers and drinking water, beverage and wine coolers (SCTB 2010).

SCTB’s applications of generating thermoelectric modules: • direct conversion of heat to electricity (space equipment and self-contained

apparatus using released heat as an independent power supply) • pipeline galvanic protection, self-contained and emergency power supply systems

etc. • self-contained power supply of electronic unit for water boilers

• conversion of heat of natural heat sources (geothermal waters etc.) to electricity


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Table 1 shows an example of a thermoelectric generator of the TMG-450-0.8-1.0 (SCTB 2010) shows the dimensions of the corresponding element.

Table 1 Thermoelectric generator, TMG-450-0.8-1.0 (SCTB 2010)

MODULE TYPE Uoc, Rin, Rload, Wload, Uload, m, Hot ColdV Ohm Ohm W V W/°C side side

TMG-450-0.8-1.0 234 20.0 20.0 6.8 11.7ä 0.93 54.4 x 54.4 54.4 x 57.0 3.4

Height

Module operating properties are given at hot side temperature 175 °C and cold side temperature 50°C.

in which:

Uoc – open circuit voltage, (V) Rin – module internal resistance at 110 °C, (Ohm)

Rload – matched load resistance, (Ohm) Uload – output voltage, corresponded to matched load, (V)

m – module thermal conductance at 110 °C, (W/°C)

3.3.3 Success in buildings and vehicles

Report (EH1 2011) presents a variety of potential or already implemented applications using energy harvesting. Some of the cases that appear useful in Finnish society are considered here.

Canada institute de Cardiologie controls This example is an EnOcean Alliance project on a new temperature control for the institute of cardiologie control in Canada, 2009.

The targets were fast installation, enabling room temperature control and no opening of the walls.

Solution: Installation of wireless, battery-less temperature sensors with the possibility of monitoring and central control

Benefits obtained were less installation costs, easy and fast installation, reducing energy consumption.

Germany Premino 11 office building controls Another EnOcean Alliance project is called office building Premino 11.

Figure 7 New Office Building (EH1 2011)


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The targets were to build automation with WAGO I/O, DALI, radiator and floor control using 55 reed-contacts, 352 light switches, 321 sun-protection switches, 303 temperature sensors and system controllers in ceiling and floor, see Figure 7.

The benefits of the project were flexible room structure with less installation effort and low maintenance cost and interoperable products.

Germany OS3 office building controls Figure 8 shows a multi-building complex with 350 offices to be changed to wireless, battery-less solution. Installation was realized with programmable controllers and a web-based EC-Net system. Benefits obtained were easy and fast installation, reduced installation costs and an intelligent office

Figure 8 OS3 Intelligent office building(EH1 2011)

Netherlands DZ railway station Royal Boon Edam Group Holding has developed an energy generating revolving door for the Driebergen-Zeist railway station in The Netherlands, which not only saves energy but also generates energy with every person passing through the door. The station has a daily capacity of 8500 commuters and a calculation for this particular situation that indicated an energy saving of around 4600 kWh per year, a considerable saving compared to a conventional sliding entrance. Boon Edam has become the first manufacturer in the world to develop an energy generating manual revolving door. The revolving door is equipped with a generator that is driven by the human energy applied to the door whilst the generator controls the rotating speed of the door for safety. The ceiling of the revolving door is made of safety glass and gives a clear view of the technology. A set of super capacitors stores the generated energy and provides a consistent supply for the low energy LED lights in the ceiling. The control will switch to the alternative mains supply of the building when necessary and this ensures that the door is illuminated at all times, even when the passenger flow is minimal. The total amount of energy that is generated by the revolving door is accumulated and shown on a large display inside the building.


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Harnessing the energy of pushing through a revolving door is a simple idea but considering the number of revolving doors in busy city centres, this idea has great potential. The revolving doors in large office buildings and busy transport stations are always in use at any given moment during the day and by capturing that kinetic energy free electricity is supplied to the installation site, see Figure 9.

Figure 9 DZ railway station (EH1 2011)

Japan Toyota Prius In 2010, Toyota's newest hybrid Prius with regenerative braking and solar roof panel is a smash hit (EH1 2011)

The photovoltaic solar cells in the optional roof run ventilation fans that pull hot air out of the car's interior while it's parked, reducing the load on the air conditioning when the driver returns. That cuts fuel consumption and improves gas mileage. In some areas of the US, the 2010 Toyota Prius was essentially sold out at the beginning of the year.

Sweden Volvo hybrid bus The Volvo hybrid bus made in Sweden has regenerative braking, see Figure 10. It has higher purchase price than its conventional equivalent but it costs less to run and it is quieter.

Figure 10 Hybridbus (EH1 2011)


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Israel Power Paving Figure 11 shows Israeli company Innowattech has deployed three types of piezoelectric paving that will, it is claimed, generate huge amounts of power where there are people, trains or aircraft.

Figure 11 Power paving (EH1 2011)

Japan Toppan Forms The pressure is on for merchandising materials to be reusable and reprogrammable. There is also a need for printed electronics for brand enhancement and merchandising to avoid disposable batteries because of environmental concerns and the possibility of children choking on them. For example, the world uses 30 billion coin cells yearly with many malign consequences.

Organic thin film photovoltaic technology has been integrated with Add-Vision’s polymer organic light emitting display (P-OLED) and Toppan Forms’ Audio PaperTM technology to create a new breed of Point of Purchase POP display applications. Flashing lights and sound powered by ambient indoor light will be placed on store shelves and displays starting this month to test how they contribute to the promotion of products. Like the photovoltaics, Add- Vision’s P-OLED also allows seamless integration due to its thinness, flexibility, and lightweight features, Figure 12.


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Figure 12 Flat sheet type of charger that is flexible (EH1 2011)


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4 Drivers and challenges of energy harvesting

4.1 Main drivers

4.1.1 Reliable Devices to Power Wireless Sensor Networks

Wireless sensor networks employ batteries to power the devices in the network. Batteries are often not reliable for powering wireless devices. it is very time consuming to replace batteries and the replacement cost is also high. Hence, there was a look out for alternative technologies with high reliability and long life for powering network devices. This led to the development of energy harvesting technology. Energy harvesting devices have high reliability and do not require any replacement.

Multiple Energy Sources (Frost & Sullivan 2007): Ambient energy is utilized for energy harvesting application. Some of the available sources are vibrations, light, heat and human motion. Various ambient energy sources can be effectively utilized to harness electrical power. The utility of the source depends on the application domain. Self-powered systems can also be built using energy harvesting technology.

Compact Devices Apart from performance factors that are required for specific applications, the need was to develop small, light-weight, portable power sources (devices) for wireless sensor networks and other sectors where batteries are found to be unreliable sources of power. Energy harvesting modules are lightweight portable devices. Control Electronics

Advanced control electronics complement the growth of energy harvesting technology. Power electronic circuits can be used for power management along with the transduction unit. This improves on the performance and efficiency of the energy harvesting device. The efficiency of the energy harvesting device can be improved by using microcontrollers, digital signal processors, neural networks, fuzzy logic controllers or software tools. All these capabilities can be seen in the long run.

Manufacturing Process The flexibility of energy harvesting technology with MEMS manufacturing process contributes to the adoption rate of this technology. Energy harvesting devices are available as modules with all the sensing and control units. Advanced manufacturing process provides reliable and scalable modules. Storage Devices

The harvested energy can be either directly used to power devices or can be stored using supercapacitors. Sometimes, they can be used to recharge batteries also. The availability of advanced storage devices that are compatible with the transduction and control unit supports the growth of energy harvesting technology compared to battery powered devices. Rechargeable battery technology is not near to achieving this in affordable form so the long life means no battery and perhaps an energy storage capacitor for now (EH1 2011).


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Ideally, energy harvesting units should last at least twenty years in most applications, see Table 2.

Table 2. Examples of the primary motivation to use energy harvesting by type of device (EH1 2011).

Device Primary reason for energy harvesting

Mobile phones, e-books, Laptops

Convenience – no drained batteries, never need to find a charging point or carry a heavy charger

Wireless Sensor Networks

Mobile and inaccessible nodes become feasible in huge deployments such as monitoring trees in forest fires and sensors embedded in buildings and engines. Support costs greatly reduce

Military equipment Operational availability – security of use.

Medical implants Safety Operational availability – security of use. Intrusive procedures reduced.

Healthcare disposables Cost, convenience, reliability, better user interface eg. spoken, prompting, scrolling instructions

Consumer goods and Packaging

Cost, new merchandising features, better user interface eg. moving color images become safe and viable

Another application scenario is given by (Frost & Sullivan 2007). Any kind of low power battery operated device can be powered by energy harvesting. Some of the applications of energy harvesting devices are given below:

Communication Networks: Sensor networks, traffic control and radio frequency identification (RFID) applications

Building technologies: temperature control and structural health monitoring (SHM) systems

Medical: implant devices and portable health monitoring devices

Defense: Chemical & biological sensors and intrusion detection sensors

Consumer devices: Low power devices such a mobile phones, personal digital assistants (PDA’s), portable audio video players and other handy devices

Industrial / Commercial: Machine-to-machine communication devices, equipment monitoring, automation and process control applications.

Automotive / Aerospace: Safety and monitoring systems and radio frequency (RF) communication

Both references list quite similar applications. In this context the military, medical and aerospace applications will not be referred further.


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4.2 Challenges Constant Availability of Ambient Energy

Ambient energy sources such as vibrations, light, do not provide sufficient power to energy harvesting devices at all times. In such cases, it is difficult to generate power when there is no input source. However, energy can be stored and can be utilized during such critical situations.

Output Power and Efficiency The output power of an energy harvesting device is only in the range of a few milliwatts or microwatts. Vibration-based devices are claimed to provide higher energy density (J/cm3 ) compared to photovoltaic energy harvesters. High output power influences the deployment of energy harvesting devices in various fields. The device efficiency depends on associated control circuitry. The transduction, conversion, and regulation circuits dissipate power and this results in low efficiency. Energy harvesting devices should be capable of converting the available input to effective output with high efficiency to power the wireless networks. Research and Development Activities

Research activities are taking place in the universities, but there are very few manufacturing companies in this domain who are developing commercial products and prototypes. More number of manufacturers across the globe, are required to take energy harvesting technology to the mass production stage. They can collaborate with university researchers through technology transfer programs and develop innovative and effective energy harvesting technologies that can boost its market growth.

4.2.1 Market Drivers and Challenges

DRIVERS Proliferation of Wireless Technologies The growing need for wireless technologies increases the demand for reliable and efficient power sources. Highly reliable power devices fulfills the demands of sensors, remote networks, micro machines, and many applications. Market growth of wireless networking led to the development of energy harvesting technology (Frost & Sullivan 2007).

Cost Energy harvesting devices eliminate the cost of replacing batteries. This reduces the maintenance cost also (Frost & Sullivan 2007). Funding Activities

Energy harvesting technology is also backed by some amount of funding from government agencies and venture capitalists. The funding attracts manufacturers and researchers to develop advanced and innovative energy harvesting technologies for various applications, thereby contributing to the growth of the technology (Frost & Sullivan 2007).


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CHALLENGES High Volume Production Considering piezo-based energy harvesting technology, commercial products are available. Prototypes are under development for other types for energy harvesting technologies such as thermoelectric- and electromagnetic-based scavenging devices. It will take at least five to ten years to mass produce these energy harvesting modules (Frost & Sullivan 2007).

Rapid Technology Developments Effective power management systems and ultra low power battery technologies lower the need for energy harvesting technology. R&D activities are heading in this direction to deploy ultra low power batteries for wireless networks. In such a scenario, energy harvesting devices can be utilized to recharge the batteries. In this spectrum, Ubiwave of Belgium, Europe develops ultra low-power and highly reliable wireless mesh networking technologies. Ubiwave’s wireless sensor products are based on their patented technology. The low power technology increases the life time of the batteries (about six to seven years). Ubiwave also provides battery-free wireless mesh networks. Solar panels are utilized for powering the networks. Though, low power networking technologies are developed, energy harvesters are still capable of replacing batteries (Frost & Sullivan 2007).

Replacing Batteries Though energy harvesting devices offer benefits over batteries, it is highly difficult to replace batteries. This is due to the matured (high) market potential of battery technology. However, few manufacturers are developing energy harvesting technologies that can completely replace batteries. Ubiwave of Belgium is one such example. Yet another example is MicroStrain Inc., of Williston, USA, that develops piezoelectric based energy harvesters for aerospace applications that can replace batteries. Recently, the technology was successfully demonstrated in Bell model 412 (M412) helicopter (Frost & Sullivan 2007). Device Manufacturers

There are few device manufacturers across the globe, who are developing products in this domain; though activities are active in universities. There is a need to bridge the gap in order to produce commercial energy harvesting products. Creating awareness on energy harvesting technology will probably attract many manufacturers in this domain. Also, government intervention in promoting the technology through energy harvesting programs can possibly drag the attention of technology developers, system integrators and original equipment manufacturers (OEMs) in increasing the market potential. Universities can collaborate with manufactures through technology transfer programs in developing innovative and improved technologies for commercial applications (Frost & Sullivan 2007).


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5 Potential new and developing applications

5.1 Sources of energy to harvest A variety of different sources exist for harvesting energy, such as solar power, thermal energy, wind energy, salinity gradients, human movement and kinetic energy. There is rarely a good choice of options for the types of energy that can be harvested in a specific application. Electromagnetic radiation, visible light, sometimes radiated heat can be harvested for small devices. Inside machines, people and so on, movement and thermal gradients are usually the most accessible sources of energy (EV2 2011). By gross sales value and numbers used, the favourite technologies are: photovoltaic, electrodynamic, piezoelectric and thermoelectric solution. Figure 13 shows the predicted share of the different energy harvesting technologies in 2020.

Figure 13. Consumer market value by technology 2020 (EH2 2011).

Table 3 gives the relative importance/unimportance of those four favourite technologies and the most promising solution of energy harvesting is loading the button batteries and rechargeable AA and AAA size batteries.


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Table 3. Primary reason for relative importance or unimportance of energy harvesting technologies (EH1 2011)

Technology in order of successful adoption in ongoing programs

Reason for importance

Photovoltaic Affordable, can be very efficient, thin and new versions are flexible. No moving parts. Usually non polluting

Electrodynamic

Affordable, efficient and can take the greatest variety of forms e.g. dynamos, regenerative braking using motors in reverse, vibration and footfall harvesting, foot treadle, rip cord, hand cranking

Reason for relative unimportance so far

Piezoelectric Affordable and efficient but reliability and life problems in some applications, limited acoustic frequency response and tunability in vibration harvesting

Thermoelectric Expensive and inefficient but no moving parts and applicability where there is no light provides niche applications


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5.2 Thermal power potential in Finland Thermal energy sources in Finnish municipal facility and in industry for thermal energy harvestin are listed in the Table 4

Table 4. Thermal electrical sources.

Power plant Steam power plant

Condenser Exhaust gas

Motors Fuel cell: exhaust gas Diesel motor: exhaust gas, cooling water Jet turbine; airplane Car motors; exhaust gas, cooling water Industry Metal industry Pulp & paper industry Chemical industry

Saw mils Laundry Bakery

Building Radiator DHW, domestic hot water Roof Cooling machine condenser Indoor to outdoor temperature in winter Indoor to outdoor temperature in summer Solar Concentrated collector Flat plate collector Waste water Waste water treatment

Highest temperature differences are obtainable from power plants, motors and industry. The main points are how thermal energy can be collected from the sources for utilising in buildings, local heating network, etc. Solar or electricity based cooling machines produce condensed heat for utilising in house heating and local heating network.

A method was developed to evaluate the best solutions and practises to curry out the harvesting of natural energy sources. The potential of power output and annual energy is evaluated for most of the heat sources named in the table 4. One way to improve the sustainability of our electricity production is through the scavenging of waste heat with thermoelectric generators, i.e. thermoelectric materials. Home heating, automotive exhaust and industrial processes all generate waste heat that could be converted to electricity by using thermoelectric. The exploitation of the thermoelectric phenomena will increase by using nanotechnology. Thermoelectric generators are silent, reliable and scalable, making them ideal for small, distributed power generation.

Heat sources for applications could be listed:

Buildings and households: cooling machines, stoves, hot tap water, ventilation


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Electric motors Lights Heavy vehicles like busses, trucks and working machines Industrial processes and WWTP processes

It is foreseen that thermoelectric elements will most likely appear first in applications like sensors, control units, information signs or displays and pay terminals. These societal applications are numerous. The essential factor for a heat source is the temperature on both sides of the element (temperature difference), heat flow through the element (heat conductivity) and area of the element as well as the material of thermo-electrical element and heat transfer coefficient from heating side to that element. Some of the heat sources limit utilisation of electricity only in the same application as the heat source, e.g airplane. The material of thermo-electric elements are chosen based on temperature, where they work. The physical properties of semiconductor materials and the ZT –value are defined by working temperature.

Potential of the thermo-electric power is about 21 MW and annual energy about 71 GWh in Finland. (0,08 % of Finnish electricity consumption nowadays). The potentials of these sources (Table 4) are presented in Table 5. Table 5 shows that highest thermal resources for societal applications could be in steam power plants and in industry as well as exhaust gas of diesel motors of heavy vehicles. In vehicles the stand-alone systems are possible at both AC and DC. One solution is charging the battery of the vehicle; this requires a control system which can operate in parallel with the vehicles charging system. The investment cost of those examples are not evaluated in the table 5. If we would take the cost based potential evaluation, most of those cases would not be very realistic. But if we look at the price development of solar cells during the few last years, the future of thermo-electric elements might be much more positive.


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Table 5. Power potential of thermal electrical sources.

Heat sourceTemp. max

Temp. min

Sebeck factor

Thermalcondactivity Number of

elementseff. Cross-section

Service area

Heat tr.conf.

Temp. diff.

Thermal power

Power potential

Energy potential

Number of heat source

Total energy

potential NoteTh [°C] Tc [°C] zT 10-3 V/K mW/cm,K ps. m2 m2 °C W W kWh/a MWh/a

1 Power plant Steam power plant 150 5 0,63 0,007384 12,293 5 000 000 20,00 26,91 0,02 2,9 35 651 0,049 1 750,8 10 505 100 1 050

2 Condenser 50 5 0,64 0,007224 10,370 5 000 000 20,00 26,91 0,10 4,5 46 667 0,018 853,9 5 123 100 512

3 Motors Fuel cell: exhaust gas 400 5 0,38 0,004413 39,893 50 000 0,20 0,27 0,01 4,0 1 576 0,064 101,1 607 1 000 607

4 Diesel motor: exhaust gas 300 5 0,52 0,007007 17,053 50 000 0,20 0,27 0,01 3,0 503 0,070 35,2 211 2 000 422

5 Jet turbine; airplane 300 -50 0,58 0,006905 15,047 50 000 0,20 0,27 0,01 3,5 527 0,095 49,9 249 5 000 1 246 height 10000

m

6 Car motors 200 5 0,61 0,007363 13,630 10 000 0,04 0,05 0,01 2,0 53 0,059 3,2 1 1 000 000 948 20 000km/ 70 km/h

7 Industry Metal industry 200 20 0,60 0,007454 14,080 5 000 000 20,00 26,91 0,02 3,6 50 687 0,053 2 702,2 19 726 20 395

8 Pulp&paper industry 100 5 0,64 0,007332 11,207 5 000 000 20,00 26,91 0,02 1,9 21 293 0,035 753,6 6 051 20 121

9 Chemical industry 50 5 0,64 0,007224 10,370 5 000 000 20,00 26,91 0,10 4,5 46 667 0,018 853,9 6 233 20 125

10 Laundry 50 5 0,64 0,007224 10,370 500 000 2,00 2,69 0,10 4,5 4 667 0,018 85,4 256 200 5111 House Radiator 70 20 0,64 0,007463 10,930 3 000 0,01 0,02 0,10 5,0 33 0,019 0,6 3 6 000 000 16 58512 DHW 55 5 0,64 0,007237 10,443 1 000 0,00 0,01 0,10 5,0 10 0,020 0,2 0 1 000 000 42113 Roof 50 18 0,64 0,007401 10,564 500 000 2,00 2,69 0,10 3,2 3 380 0,013 43,2 158 150 000 23 678 in summer

14 Cooling mach. condenser 40 5 0,64 0,007196 10,233 2 000 0,01 0,01 0,10 3,5 14 0,014 0,2 2 500 000 902

15 Indoor temp. 22 -5 0,63 0,006988 9,902 50 000 0,20 0,27 0,10 2,7 267 0,012 3,1 11 1 000 000 11 348 winter16 Indoor temp. 24 12 0,64 0,007254 10,118 50 000 0,20 0,27 0,10 1,2 121 0,005 0,6 3 1 000 000 3 133 summer

17 Solar Concentrated collector 100 12 0,64 0,007412 11,344 300 000 1,20 1,61 0,10 8,8 5 990 0,032 194,4 709 10 000 7 094

18 Flat plate collector 50 12 0,64 0,007320 10,473 300 000 1,20 1,61 0,10 3,8 2 424 0,015 37,1 135 10 000 1 355

19 Community Waste water 20 5 0,64 0,007136 9,989 100 000 000 400,00 538,24 0,10 1,5 318900 0,006 2 043,1 17 897 40 7160,9 1,2 21 10 678 500 71

Mm2 Mm2 MW pcs. GWh/a


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The temperature difference, potential of application power capacity and annual energy are illustrated in Figures 14, 15 and 16.

-1

0

1

2

3

4

5

6

7

8

9

10

-100

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Used

tem

p. d

iff. (

°C)

Tem

pera

ture

(°C)

Heat sources

Temperature for thermoelectricity

Temp. max Temp. min Used temp.diff.

1. Steam power plant2. Condenser3. Fuel cell: exhaust gas4. Diesel motor: exhaust gas5. Jet turbine; airplane6. Car motor7. Metal industry8. Pulp&paper industry9. Chemical industry

10. Laundry11. Radiator in house12. DHW13. Roof14. Cooling machine condenser15. Indoor temperature (winter)16. Indoor temperature (summer)17. Concentrated collectorl18. Flat plate collector19. Waste water

Figure 14. Temperatures in potential heat source applications and effective temperature difference.

Maximum temperature is a source temperature for thermo-electric element and minimum the cold temperature between which the power generator is operated. Temperature difference is the effective utilised T of the temperature difference, which generates electricity. The highest power potential is estimated to metal industry, steam power plant and waste water (Fig.15). However, because of the short-time utilising the amount of energy is not high. In waste water the utilisation time is long enough, but temperature difference makes the amont of energy low. The most energetic sources (Fig.16) are evaluated to be the radiators in buildings and the indoor-outdoor temperature difference in winter because of the amount of buildings and radiators.


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0

500

1 000

1 500

2 000

2 500

3 000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Powe

r (W

)/uni

t

Heat sources

Thermoelectric power

1. Steam power plant2. Condenser3. Fuel cell: exhaust gas4. Diesel motor: exhaust gas5. Jet turbine; airplane6. Car motor7. Metal industry8. Pulp&paper industry9. Chemical industry10. Laundry11. Radiator in house12. DHW13. Roof14. Cooling machine condenser15. Indoor temperature (winter)16. Indoor temperature (summer)17. Concentrated collector18. Flat plate collector19. Waste water

Figure 15. Evaluated power potential of heat sources.

0

5 000

10 000

15 000

20 000

25 000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Ener

gy (M

Wh)

/a

Heat sources

Thermoelectric energy


Figure 16. Annual electricity output potential of heat sources.


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ZT characteristic for materials Z is the thermoelectric figure-of-merit of the materials.

ZT can be defined

(1)

where T is average temperature, electrical resistivity, thermal conductivity, S Seebeck factor, A is cross-area of the element, is electricity efficiency and can be defined

(2)

where Th is a hot temperature of the element and Tc a cold temperature. ZT is defined (Fig. 17) for two n-type semiconductor material Bi2Te3 working in the temperature range of 20 -167 °C and CoSb3 in the range of 167 – 700 °C.

Thermal conductivity and electrical resistivity as a function of temperature are presented for the same materials in figures 18 and 19. As seen in the figures the curves are irregular in temperature of 167 °C, because the material works in different range of temperature and properties are different.

zT for n-Type materials

y = -3E-06x2 + 0,0053x - 1,4654R2 = 0,9959

y = -1E-05x2 + 0,0063x - 0,3474R2 = 0,9863

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000

Temperature [K]

zT

'n-Bi2te3'

'n-CoSb3'

Poly. ('n-CoSb3')

Poly. ('n-Bi2te3')

440-973 K167-700 °C

293-440 K20-167 °C

Figure 17.ZT for the n-type semiconductor materials Bi2Te3 and CoSb3

TSzTATh

QQP 2,;

h

ch

TTTZ

TZTT

1

11


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Thermal condactivity for n-Type materials

y = 8E-05x2 - 0,1156x + 76,789R2 = 0,9943

y = 0,0002x2 - 0,0919x + 19,915R2 = 0,9959

0,0

5,0

10,0

15,0

20,0

25,0

30,0

35,0

40,0

45,0

250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000

Temperature [K]

(m

W/c

m,K

)

'n-Bi2te3'

'n-CoSb3'

Poly. ('n-CoSb3')

Poly. ('n-Bi2te3')440-973 K167-700 °C

293-440 K20-167 °C

Figure 18. Thermal conductivity of n-type semiconductor materials Bi2Te3 and CoSb3 as a function of temperature.

Electrical resistivity for n-Type materials

y = -5E-05x2 + 0,0414x - 5,3989R2 = 0,9998

y = -1E-06x2 + 0,0021x + 0,0981R2 = 0,9986

0,00

0,50

1,00

1,50

2,00

2,50

3,00

250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000

Temperature [K]

(1

0-3cm

)

'n-Bi2te3'

'n-CoSb3'

Poly. ('n-Bi2te3')

Poly. ('n-CoSb3')

440-973 K167-700 °C

293-440 K20-167 °C

Figure 19. Electric resistivity of the n-type materials Bi2Te3 and CoSb3 as a function of temperature.

There are also other semiconductor n- and p-type materials used in thermoelectric materials as shown in figure 20. Those two n–type material are just chosen as an example for calculation in the Table 5.


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Figure 20. Thermoelectric materials with highest figure of merit as a function of temperature.

ZT is a function of S(T), (T) and (T) [i.e ZT=F(S (T), (T), (T))], so the working temperature is an important parameter in working conditions. Good thermoelectric materials have ZT > 1. The impact of ZT on power and energy output is presented in figures 21 and 22 for ZT=1.0 and ZT = 2.0. What is the impact of ZT to power and energy output in the same examples as shown in figures 15 and 16? When ZT = 1.0 (+63 %), the power output increases about 40 % and utilised energy about 40 % and respectively when ZT = 2.0 (+226 %) power output and energy increases 116 % compared to the original cases in figure 15 and 16.


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0

500

1 000

1 500

2 000

2 500

3 000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Powe

r (W

)/uni

t

Heat sources

Thermoelectric power sensitivity

1. Steam power plant2. Condenser3. Fuel cell: exhaust gas4. Diesel motor: exhaust gas5. Jet turbine; airplane6. Car motor7. Metal industry8. Pulp&paper industry9. Chemical industry10. Laundry11. Radiator in house12. DHW13. Roof14. Cooling machine condenser15. Indoor temperature (winter)16. Indoor temperature(summer)17. Concentrated collector18. Flat plate collector19. Waste water

-- zT= 1.0 (+ 63 %) -- zT= 2.0 (+ 226 %)

Figure 21. Thermo-electric power potential, if all the thermoelements have ZT values of 1.0.

0

5 000

10 000

15 000

20 000

25 000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Ener

gy (M

Wh)

/a

Heat sources

Thermoelectric energy sensitivity


-- zT= 1.0 (+63 %) -- zT= 2.0 (+ 226 %)

Figure 22. Thermo-electric power potential, if all the thermoelements have ZT values of 2.0.

5.3 Development of technologies Photovoltaics and electrodynamics do not efficiently scale down to microscopic size. In future, there will therefore be a place for technologies that do scale down such as capacitive and piezoelectric harvesting, a particular focus of research being their use in micro-electromachnical systems MEMS e.g. for use in the human body and consumer goods including cars. There are also other possible technologies and new applications of energy harvesting. However, photovoltaics is in a period of fast evolution with many


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new technologies providing such things as conformability, rollability, transparency and ability to harvest UV, IR and light together and even stretchable and edible versions may appear one day. This underwrites the ongoing dominance of photovoltaics in applications and expenditure. However, photovoltaics and piezoelectrics seem likely to retain the greatest breadth of interest as EH media for the next five years. The reason for piezo attracting so much new development includes its high power density, its superior efficiency and its suitability for vibration and other motion – ambient power forms that are exceptionally ubiquitous.

A timeline for areas of EH primary deployment is given in

Table 6.

Table 6. Suggested timeline for widespread deployment of energy harvesting (EH2 2011)

5.4 Development of materials

5.4.1 Thermoelectric Materials

The variety of thermoelectric (TE) materials that can be used in energy harvesting is quite large, and the optimal material for a given application depends mainly on the temperature range that the material is to be used in. Although thermoelectric materials exhibit thermoelectric behavior in all temperatures, their figure-of-merit (ZT) is quite strongly influenced by temperature and typically the ZT value peaks in a certain temperature range. Therefore, when evaluating the feasibility of thermoelectric materials for different applications, the material performance in the required temperature range in addition to other factors like cost and availability should be taken into account.


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Nanostructuring has led to significant improvement of the properties of thermoelectric materials. The main strategy has been to decrease the thermal conductivity via phonon scattering. Research is required for understanding the interaction between thermal, electrical and entropy transports; controlling nanostructures that can be used in actual devices; and improvements in materials for soldering, ceramics, packaging, and so on. (Benesch, 2012)

The main focus of the research is still on tellurides due to their outstanding properties. Some of the most interesting alternatives to make cheap and less toxic TE materials include Mg2Si, CoSb3, ZnSb, ZnO and other oxides. All these materials have been known for a long time and more explorative work is required to find totally new materials. (Benesch, 2012) Materials manufacturing methods such as sol-gel or electrodeposition are considered to be scalable to industrial level, and new reliable and cheap techniques to sinter and compact the nanostructured material into a nanostructured bulk material are crucial. (Benesch, 2012). Conducting polymers are a class of materials that could be produced directly from the solution without sintering. Benesch (2012) suggests that another completely different strategy barely explored: to consider an electrochemical reaction for the thermoelectric power generation. The materials considered in this report are the ones that are most commonly used today and the ones that show the greatest promise for applications in the near future. Only bulk materials will be discussed, as nanostructured or otherwise exotic materials are still being researched on the laboratory scale and quite far from commercialization. Table 7 lists the TE materials to be discussed and some of their relevant properties.

Table 7: The properties of some promising thermoelectric bulk materials.

Temperature Range (°C)

Conductivity Type ZT Toxicity Price

Bi2Te3 0 - 300 n, p 0.5 – 1.0 High High MnSi1.73,

Mg2Si 200 - 700 n, p 0.5 - 0.7 Low Low

Zn4Sb3 200 - 500 p 0.5 – 1.2 Medium Low TAGS 200 - 600 p 0.5 – 1.2 Medium Very high PbTe 200 - 550 n, p 0.5 – 1.0 High High

Skutterudites 200 - 600 n, p 0.5 – 1.2 Low Low Oxides 400+ n, p 0.5 – 0.8 Low Low La3Te4 500+ n 0.5 – 1.1 High High

The temperature range listed for the materials in Table 7 is the range where their ZT values are at least 0.5, which is admittedly too low to compete with other methods of waste heat recovery, but still high enough to make them useful in applications where the advantages of thermoelectrics outweigh the disadvantages. Based on these temperature ranges, the materials in Table 1 can be roughly divided into low, intermediate and high temperature materials. Bi2Te3 is the only material in the low temperature category, oxides and La3Te4 go into the high temperature group, while all the other materials belong to the intermediate temperature group. The ZT values of the materials are not constant within the temperature range listed, so the variation of the ZT value is listed in the ZT column of Table 1. The ZT value is a good way to compare thermoelectric


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materials to each other, and it also gives an estimate of the efficiency of the material in different temperature gradients. Figure 23 shows the estimated efficiencies of heat recovery for various ZT values in a temperature gradient.

Figure 23: The efficiency of a thermoelectric device based on its ZT value and magnitude of temperature gradient.

The efficiencies of current state-of-the-art thermoelectric devices are in the 5-10 % range, and it is clear from Figure 23 that the ZT values will need to be raised significantly for thermoelectrics to become competitive with traditional heat recovery methods. TE materials must be: efficient, stable, environmentally friendly, composed of elements abundant in nature, and synthesized with a scalable method. Also, low-cost manufacturing process of the TEGs must be addressed. Nowadays manufacturing constitutes 50% of the cost for a TEG. At the moment, such materials and manufacturing methods do not exist or they are not explored sufficiently and constitute the main bottle neck for using this technology. (Benesch, 2012)

5.4.2 Applications

Judging by efficiency alone, thermoelectric devices would seem to be at a great disadvantage compared to other methods of heat recovery, but thermoelectrics have the advantages of scalability and ruggedness (from having no moving parts), which make them promising for a variety of applications.

Automotive Applications Thermoelectrics can be used in automotive applications in two ways: either use the Peltier effect to support or even completely replace the typical heating and cooling devices inside the car, or use the Seebeck effect to utilize some of the waste heat produced by the engine to provide electricity for the onboard systems of the car. Both


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approaches can lead to improvements in fuel efficiency (DOE has a goal of 10 % fuel efficiency improvement via thermoelectrics), if the challenges of implementing them can be overcome.

Thermoelectrics are already used in some models for seat heating and cooling, but for replacing the entire air conditioning system with thermoelectric, the design of the car will need to be modified extensively in order to accommodate the extra wiring and other systems required for integrating Peltier elements all over the vehicle. However, if this can be done, the overall efficiency of the air conditioning system should improve, (Yang 2009).

Utilizing the waste heat of the engine with thermoelectrics is even more challenging because of the unique requirements placed on the materials by the constant vibrational stresses and thermal cycles present. The thermal cycles are especially problematic, because typically the ZT of TE materials peaks close to the temperature where they melt or otherwise decompose. Since the temperature of the engine exhaust is typically 500 – 600 °C, but can rise as high as 1000 °C from time to time, it is very challenging to design a material that performs optimally in normal operating conditions but can still withstand the occasional higher temperatures (Yang 2009).

Industrial Heat Sources Thermoelectric materials can also be used in industrial processes to convert some of the waste heat into usable electricity. Unfortunately, the advantages of TE materials (scalability, ruggedness) are not very important on an industrial scale, so thermoelectrics are quite inefficient compared to more traditional methods of utilizing the heat. Thermoelectrics could, however, be utilized in industrial processes to power wireless sensor networks that can be used to monitor some aspects of the process. TE materials have already seen some use in remote sensors that monitor gas or oil pipelines.

Electronics In the field of electronics, thermoelectric modules have been used on a limited scale for cooling or energy scavenging. Thermoelectric cooling has moved from military applications into computing, where Peltier modules can be used to cool chips in order to make them perform better. The cost of the cooling versus the benefits is so far too high to justify larger scale use in computer chips, so until the ZT value of the materials can be raised further, TE cooling in computer chips will be limited to high-end and specialist applications. Energy scavenging with TE materials in small electronics has been demonstrated in the thermoelectric watch by Seiko, which runs on the body heat of the person wearing the watch. Using the same method, other wearable electronics could be made that are powered solely by the thermal gradient between the body and the ambient temperature. As electronic devices become smaller and more efficient, this application of thermoelectric modules should become more and more promising.

5.4.3 Sustainability Analysis

The sustainability of using the materials outlined in previous sections for thermoelectric power generation has been evaluated based on the annual production rates, the known reserves and the price development of the constituent elements. The analysis is based on the Mineral Commodity Summaries 2011 report by the U.S. Geological Survey


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(Geological Survey 2011). Table 2 contains the world production rates, reserves and price of the elements under consideration.

Table 8: Annual production of the elements used in thermoelectric materials, in metric tons (Geological Survey 2011).

Element Annual world production

Reserves World Resources

Price (2010) $/kg 2009 2010

Antimony 155,000 135,000 1,800,000 ? 8.16 Bismuth 8,200 7,600 320,000 ? 18.12

Germanium 120 120 450 (in the U.S.) ? 940

Lead 3,860,000 4,100,000 80,000,000 >1.5 billion tons 2.07

Magnesium 608,000 760,000 sufficient virtually unlimited 5.73

Manganese 10,800,000 13,000,000 630,000,000 large but irregular 0.008

Rare Earths 133,000 130,000 110,000,000 ? 6.87 Silicon 6,310,000 6,900,000 sufficient abundant 3.09 Silver 21,800 22,200 510,000 ? 571

Tellurium >118 >125 22,000 ? 210 Zinc 11,200,000 12,000,000 250,000,000 1,900,000,000 2.20

The column “Reserves” in Table 8 reflects the current known deposits of the minerals that are commercially viable, while the column “World Resources” lists the estimated total amounts of the minerals (data on this was largely unavailable). Based on these figures, out of the minerals needed for the thermoelectric materials under consideration, only germanium is in danger of being depleted in the near future. However, because of the projected rise in their demand, the U.S. Department of Energy estimates that the status of tellurium and lanthanum supplies is going to be near critical in the short term (0-5 years) and for tellurium also in the medium term (5-15 years) (Department of Energy 2010).

Concerning the price of the elements, germanium and silver are significantly more expensive than the other raw materials, and tellurium is also quite expensive. It is very difficult to predict future trends in these prices, but comparing the fluctuations of the mineral prices in recent years offers some insight into the stability of the prices. Figure 24 shows the changes in mineral prices over 5 years compared to the level of 2006.


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0

0,5

1

1,5

2

2,5

3

3,5

4

2006 2007 2008 2009 2010

Sb

Bi

Ge

Pb

Mg

Mn

Rare Earths

Si

Ag

Te

Zn

Figure 24: Fluctuation of mineral prices compared to the level of 2006.

Looking at Figure 24, it is apparent that there is quite a lot of fluctuation in many of the mineral prices, but perhaps the greatest variation can be seen in the prices of manganese, bismuth, tellurium and magnesium. This is not a certain sign of future fluctuations, and at least in the case of manganese the mineral price is so low that large fluctuations can occur more easily. Nonetheless, it is good to keep in mind the volatility of the prices when assessing the potential of different thermoelectric materials.

5.5 Competing technologies, their development and future potential The competing technologies for energy harvesting devices are on one hand conventional power sources and direct wiring and on the other hand batteries. For example, most wireless sensors require a constant power source. The efficiency, output power, and lifetime of batteries have improved, but batteries cannot provide power infinitely.

Most power obtained from the mains is from nonrenewable sources, as only a few regions use entirely renewable sources such as solar and wind energy. The wiring also implies lack of mobility and the need to provide infrastructure to supply power, something which is hard and cost ineffective in remote locations. (Frost & Sullivan 2007). Figure 25 illustrates the efficiency (electrical power out/heat in) for several heat sources (geothermal, industrial waste, solar, nuclear and coal) in combination with several thermal-to-electric conversion technologies.The systems shown in Figure 25 represent an estimate of ‘best practice’, meaning these values are based on the actual performance of up-to-date systems. The existing, practical mechanical systems are far more efficient in primary energy production than thermoelectrics, and are more efficient than thermoelectrics are likely to become in the foreseeable future (Vining 2009).


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Figure 25 Assessing thermoelectrics. Efficiency of ‘best practice’ mechanical heat engines compared with an optimistic thermoelectric estimate (Vining 2009).

5.6 Market forecast 2010-2020, 2030 of energy harvesting devices The market forecasts concern autonomous devices and ones where the harvesting and the driven device are no more than one meter apart as with bicycle dynamos driving lighting. Microbial fuel cells, harvesting floor movement and, in vehicles, harnessing exhaust heat and shock absorbers or use of regenerative braking are omitted (EH2 2011). Figure 26 shows the consumer market total value by sectors. The value refers to the harvesting element, such as the photovoltaic unit, not the product using it, such as a satellite. The value of any interfacing electronics or storage is omitted. The adoption of energy harvesting in wireless sensor networks (WSN) has been slow despite the particularly acute need for it. This is because there are other delays with WSN such as protocols that permit affordable, deployable systems with large numbers of nodes and because the EH challenge here is a severe one in terms of price, size and performance. Figure 26 gives the trend of bicycle dynamos, watches, calculators, toys, wind-up lighting, laptops, mobile phones, and radios.


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Figure 26 Consumer market total value by sector (EH2 2011)

The presentation Thermoelectric Waste Heat Recovery (Rowe 2009) was concluded by the following points:

Huge amounts of waste heat, is discharged into the environment.

Vast quantities of untapped natural heat is available most of which is below 100 °C.

Thermoelectric generation (TEG) is an environmentally friendly technology able to convert low and high temperature waste heat into electricity.

Both high and low temperature recovery technology have been successfully demonstrated on a laboratory scale and in prototype commercial systems/vehicles

Wide scale application of this technology can only be achieved by substantial improvements in thermoelectric material performance.

Including thermoelectrics in EU research program calls has resulted in an upsurge of collaboration between European Universities and Industry and is already having an impact particularly in automobile exhaust heat recovery.

Figure 27 illustrates the market requirements for TEG systems until 2025.

US$ million


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Figure 27. Market requirements roadmap for TEG Systems (Rowe 2009)

The estimated market value is shown in Figure 28, and the market size for the Thermoelectric Power Generation Devices in Japan is depicted in Figure 29.

Figure 28. Market value estimate in 2021 by technology (IdTechEx, 2011)


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Figure 29. Market Size for the Thermoelectric Power Generation Devices in Japan - transition and forecast (Yano Research Institute, 2011)

5.7 Cost efficiency Payback from energy harvesting has been estimated in the report (EH1 2011). With consumer, marine, military, automotive and healthcare products, using energy harvesting, paybacks are not usually part of the sales proposition. That tends to focus on convenience, novelty, making new things possible and other factors. For example, someone buying a bicycle with a dynamo will not calculate how many batteries they save but they will value the convenience of no maintenance. The same is true of those buying an electrodynamic or photovoltaic wristwatch.If these paybacks were calculated, they would not be very compelling given the higher upfront cost. Price and payback is not a major issue here. Typical growth curves for cost and volume of the thermal electrical devices are shown in Figure 30. Applications such as integrated circuit spot cooling and systems for battery replacements can be relatively high cost because of the cost of the alternatives. For most of these applications, volume is likely to be limited unless there is an indication that lower cost, higher volume thermoelectric modules will be available in the foreseeable future. The vision of low cost, high volume applications such as automotive power generation or cooling and industrial or home power generation can be invaluable to the launch of even the high value commercial products (Stabler 2006).


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Figure 30. Products that will pay the high cost of initial applications are needed to allow time to develop the processes and build the high volume manufacturing facilities (Stabler 2006).


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6 Environmental aspects of energy harvesting

Primarily, the environmental argument for energy harvesting is not saving power stations and their attendant pollution directly. EH is powering small electronic devices not acting as a heavy power source for heating, motive power and so on. Information and communication technology ICT represent only two percent of the energy consumption in the world but they can lead to huge environmental savings if deployed more widely and appropriately to optimise heavy power creation and handling by utilities and others.

38% of energy is consumed in buildings but it would be much less if electronic controls were cheaper and easier to install. More affordable building controls of longer life are the focus of companies describing how they have installed 4200 wireless and battery-less light switches, occupancy sensors and daylight sensors in a new building construction in Madrid. These are powered by energy harvesters and embedded in the building. This saved 40 % of lighting energy costs by automatically controlling the lighting in the building, 20 miles in cables, 42,000 batteries (over 25 years) and most of the cost of retrofitting. Batteries usually contain harmful compounds, so the environmental benefits are wide ranging and substantial. EH is likely to replace many of the 30 billion button batteries sold yearly, many containing poisons. The primary motivations for use of energy harvesting are for instance convenience and reducing costs in battery changes and use of wireless sensors.


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7 Technology roadmap

The technology roadmap explains the existing and emerging energy harvesting technologies over the time. Currently, vibration-based (piezoelectric) energy harvesting technology is the most popular method of powering sensor networks. Products are being developed for commercialization and many new application segments are being explored using piezoelectric. However, high volume production of energy harvesting modules is expected in the near future. Long term development of electromagnetic and electrostatic vibration-based energy harvesting techniques are under investigation. Prototypes have been demonstrated for thermoelectric energy harvesting technique. Simultaneously, researchers are trying to investigate the feasibility of using electroactive polymers for power generation (Frost & Sullivan 2007).

A lot of work is going on for improving the thermal electric materials. So, materials are expected to be more efficient and cheaper in the future.

Figure 31. Technology roadmap for energyharvesting (Frost & Sullivan 2007).


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8 Conclusions

Energy harvesting with thermoelectric generators is expected to have an important position among future sustainable energy technologies. With thermoelectric generators waste heat can be converted into electricity. The existing sources of waste heat within societies were mapped. In these cases temperatures are typically lower than 300 oC and the heat sources are ubiquitous. The two main drawbacks of thermoelectrics are high cost and low efficiency, which limit practical utilization of thermoelectrics to low power applications such as wireless sensors and sensor networks. Such applications have not been available before, and therefore the novel low-power devices provide new opportunities for energy harvesting applications. TE materials must be efficient, stable, environmentally friendly, composed of elements abundant in nature, and synthesized with a scalable method. Enhanced research on thermoelectrics has been able to improve materials properties, but progress is considered to be relatively slow. Another important factor affecting cost is manufacturing. Nowadays manufacturing constitutes 50% of the cost for a TEG. At the moment low-cost materials and manufacturing method do not exist or they are not explored sufficiently and constitute the main bottle neck for using the technology. Recommendations given (Benesch, 2012) include further material science research on materials structure, electrical properties and performance, as well as processing and manufacturing to overcome obstacles related to price and production up-scaling. The work requires multi-disciplinary activity and co-operation. Utilization of advanced modelling should be investigated.


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References

Benesch, J.V. (editor), Report on Forward Looking Workshop on Materials for Emerging Energy Technologies. EUROPEAN COMMISSION, Directorate-General for Research and Innovation, Industrial Technologies Material Unit, EUR 25350 EN, 2012.

DEER 2006, Diesel Engine-Efficiency and Emissions Research (DEER) Conference, http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2006/session6/2006_deer_lagrandeur.pdf

Department of Energy 2010, Critical Materials Strategy, U.S. Department of Energy, December 2010.

EH1 2011: P. Harrop, R. Das, Energy Harvesting in Action 2011, IDTechEx, 2011, 231 p

EH2 2011: P. Harrop, R. Das, Energy Harvesting and Storage for Electronic Devices 2010-2020, IDTechEx 2011, 357 p

Frost & Sullivan 2007, Advances in Energy Harvesting Technologies, DOC2, Technical Insights, Frost & Sullivan, 2007, 93 p.

Geological Survey 2011, Mineral Commodity Summaries 2011, U.S. Department of the Interior, U.S. Geological Survey.

Riffat 2003, Xiaoli MA: Thermoelectrics: a review of present and potential applications, Applied thermal Engineering, Pergamon 2003, pp- 913-935

Micropelt 2009, Thermoelectric power generation, Nanopower Forum, San Jose, CA,

Mike Rowe 2009, An Overview of Thermoelectric Waste Heat Recovery Activities in Europe, Thermoelectrics Applications Workshop, September 29-October 2, 2009, 51 p.

SCTB NORD 2010, Specialized Constructive Technological Bureau NORD, Internet (8.12.2010) http://www.sctbnord.com/

Francis R. Stabler 2006, Commercialization of Thermoelectric Technology, Materials Research Society Symposium Proceedings Vol. 886, Materials Research Society, 10 p.

Vining 2008, ZT Services Auburn, AL, The Limited Role for Thermoelectrics in the Climate Crisis, Solutions Summit, New York City, May 1 2008, 42 p

Vining 2009, An inconvenient truth about thermoelectrics, Nature Materials Vol. 8, February 2009, www.nature.com/naturematerials , pp. 83-85

Yang 2009 and Stabler, F. R., Automotive Applications of Thermoelectric Materials, J. Electr. Mater. 38 (2009) 1245-1251.

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Susterel energy harvesting Roadmap for societal applications

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