Recent Developments in the Energy Harvesting Systems from Road Infrastructures

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Review Recent Developments in the Energy Harvesting Systems from Road Infrastructures

Niloufar Zabihi * and Mohamed Saaﬁ Department of Engineering, Lancaster University, Gillow Avenue, Lancaster LA1 4YW, UK; m.saaﬁ@lancaster.ac.uk * Correspondence: [email protected]

Received: 17 June 2020; Accepted: 15 August 2020; Published: 20 August 2020

Abstract: The rapid increase in energy demand has resulted in more dependence on fossil fuels, which leads to higher CO2 emissions every year. To overcome this problem, shifting from fossil fuel-based energy resources to renewable and sustainable ones is essential. One of the new research areas developed in this context is the harvesting of energy from urban infrastructures and, in particular, roads. A large amount of energy in the form of heat or kinetic energy is wasted annually on roads. Recovering these local forms of energy as electricity would improve the energy efficiency of cities. In this review paper, recent developments in the field of energy recovery from roads using solar panels, piezoelectric, thermoelectric and electromagnetic harvesters are discussed along with their efficiency, cost and field implementation. Moreover, new advancements in developing compatible energy storage systems are also discussed and summarised. Based on the review, although all of these systems have the potential of recovering at least a part of the wasted energy, only one of them (the electromagnetic converters) is capable of generating a considerable energy level. In addition, based on the evaluation of the maturity of the technologies, and their cost analyses, more studies are required in order to fill the gap between the current state of the technologies and their full operational form.

Keywords: energy; harvesting; power; piezoelectric; thermoelectric; photovoltaic; electromagnetic; storage

1. Introduction Most of our current energy resources are based on fossil fuels. Due to the increasing trend in energy demand, there is an essential need for renewable and more sustainable sources of energy. One of the most significant energy-consuming sectors is transportation. In 2017, the transportation industry in the UK consumed energy equivalent to 56.5 million tons of oil, of which more than 97% was resourced from the oil industry [1]. While this massive amount of energy is consumed, a considerable part of it is also wasted in different forms such as vibration and heat [2,3]. Zhao et al. investigated the potential energy from trucks for a road with 600 V/h of traffic in one lane and estimated that almost 150 kWh energy is produced by passing vehicles in 1 km of the road [4]. This substantial energy is used to deform, vibrate and warm up the surface of the road and can be a good source for converting and harvesting it. Implementing energy harvesting technologies can turn these infrastructures into a source of energy. A few methods are known to have the potential of harvesting energy from roads, which can be divided into two groups based on their resources, namely solar energy heat or solar radiation and kinetic energy (mechanical vibration), which includes electromagnetic and piezoelectric generating systems [5,6]. It is worth mentioning that most of these methods are categorised as micro energy harvesters, which generate energy at a small scale (a joule or less) [5].

Sustainability 2020, 12, 6738; doi:10.3390/su12176738 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 6738 2 of 27

SustainabilityThe focus 2020 of, 12 this, x FOR paper PEER is REVIEW on the most significant energy-harvesting technologies. Considering2 of 27 that harvesting energy is about capturing, converting and storing it, these aspects are reviewed, with The focus of this paper is on the most significant energy-harvesting technologies. Considering a focus on only field and experimental studies. Electromagnetic, piezoelectric and thermoelectric that harvesting energy is about capturing, converting and storing it, these aspects are reviewed, with systemsa focus and on only solar field panel and implementation experimental studies. are discussed Electromagnetic, as the energy-convertingpiezoelectric and thermoelectric units. Briefly, insystems electromagnetic and solar converters, panel implementation the mechanical are discusse displacementd as the of energy-converting the road pavements units. is usedBriefly, to spinin anelectromagnetic electromagnetic converters, engine. Piezoelectric the mechanical materials displaceme can convertnt of the mechanical road pavements vibrations is used to to an spin electrical an potential,electromagnetic thermoelectric engine. materials Piezoelectric give materials out electrical can convert voltage mechanical once there isvibrations a temperature to an gradientelectrical in theirpotential, bodies, thermoelectric and solar panels materials are well-known give out electric for convertingal voltage solaronce radiationthere is a temperature to electrical energy.gradient All in of thesetheir concepts bodies, haveand solar real-life panels applications are well-known as well. for The converting “Magic solar Carpet” radiation by MIT to Mediaelectrical Laboratory energy. All and NASA’sof these deep concepts space have voyager real-life are applications well-known as practical well. The examples “Magic Carpet” of piezoelectric by MIT Media and thermoelectric Laboratory harvesters,and NASA’s respectively deep space [7,8]. voyager Electromagnetic are well-known generators practical also have examples several applications,of piezoelectric such and as in windthermoelectric turbines [9 ]. harvesters, respectively [7,8]. Electromagnetic generators also have several applications,This paper such is a as review in wind of theturbines formerly [9]. conducted research works in this field. In the following section,This the paper concepts is a review of diff erentof the energyformerly conversion conducted research systems works are explained in this field. briefly. In the Afterwards,following ansection, overview the of concepts the recent of different studies of energy piezoelectric conversi harvesterson systems with are explained different geometriesbriefly. Afterwards, are reviewed, an whichoverview is followed of the byrecent a summary studies of of piezoelectric the new works harvesters and developments with different in geometries thermoelectric are reviewed, harvesters, andwhich electromagnetic is followed by systems a summary are subsequently of the new works presented. and developments Finally, recent in advancesthermoelectric in energy harvesters, storage and electromagnetic systems are subsequently presented. Finally, recent advances in energy storage methods are outlined, and the last section summarises the main points of this paper and presents methods are outlined, and the last section summarises the main points of this paper and presents suggestions for future studies in the field. suggestions for future studies in the field. 2. Road Energy Harvesting Systems: Concepts and Experimental Studies 2. Road Energy Harvesting Systems: Concepts and Experimental Studies The available energy sources in roads are vibrations (mechanical) caused by passing vehicles, The available energy sources in roads are vibrations (mechanical) caused by passing vehicles, solar radiation on the large surface of the road and the temperature gradient (thermal energy) between solar radiation on the large surface of the road and the temperature gradient (thermal energy) the hot surface and colder bottom layers, induced by passing vehicles and solar radiation on the between the hot surface and colder bottom layers, induced by passing vehicles and solar radiation surfaceon the [ 5surface]. Therefore, [5]. Therefore, piezoelectric, piezoelectric, electromagnetic, electromagnetic, thermoelectric thermoelectric and and solar solar panel panel systems systems are suitableare suitable for converting for converting these formsthese forms of energy of energy into electrical into electrical potential. potential. Figure Figure1 is a diagram 1 is a diagram showing theshowing possible the energy possible conversion energy conversion in roads. in roads.

FigureFigure 1. 1.Energy Energy harvestingharvesting potential from from pavement pavement roads. roads.

2.1.2.1. Piezoelectric Piezoelectric Harvesters Harvesters

2.1.1.2.1.1. Piezoelectricity Piezoelectricity PiezoelectricityPiezoelectricity is is the the fundamental fundamental featurefeature of piezoelectric materials. materials. There There are are two two types types of of piezoelectricity.piezoelectricity. Direct Direct piezoelectricity piezoelectricity is the is conversionthe conversion of mechanical of mechanical vibrations vibrations to electrical to electrical potential, potential, and inverse piezoelectricity is the conversion of electrical potential to mechanical Sustainability 2020, 12, 6738 3 of 27

Sustainability 2020, 12, x FOR PEER REVIEW 3 of 27 and inverse piezoelectricity is the conversion of electrical potential to mechanical vibrations. One of the famousvibrations. and One everyday of the functionsfamous and of piezoelectriceveryday functions materials of piezoelectric is in quartz watches. materials Piezoelectric is in quartz materialswatches. canPiezoelectric be either materials natural or can synthesised, be either natural and once or theysynthesised, are subjected and once to stress, they electricalare subjected charge to occursstress, throughelectrical themcharge (piezoelectricity) occurs through [them10]. To (piezoelectric describe theity) state [10]. of To a piezoelectric describe the material,state of a e.g., piezoelectric measure thematerial, voltage e.g., output measure under the vibrationvoltage output or the under strain vibration under applied or the voltage,strain under the constitutional applied voltage, law the of piezoelectricconstitutional materials, law of piezoelectric according tomaterials, Equation according (1), is followed. to Equation (1), is followed.

" #ε " E t #"σ # ε = S d σ D= E (1)(1) D ddεεσ E In Equation (1), ε is the strain vector (m/m), D is the electric displacement vector (C/m2), referring to theIn displacement Equation (1), ofε is the the dipoles strain vectorcaused (m by/m), an Delectric is the electricfield, and displacement SE denotes vectorthe compliance (C/m2), referring matrix toconstants the displacement in an electric of the field dipoles (m2/N). caused Piezoelectric by an electric strain field, matrix and is S Ed denotes(m/V), and the complianceεσ is the matrix matrix of constantselectric permittivity in an electric constants field (m(F/m)2/N). at Piezoelectricconstant stress, strain which matrix indicates is d (mthe/ V),polarizability and εσ is theof a matrixdielectric of electricmaterial. permittivity Stress vector constants (N/m2) (F is/m) σ, atand constant E stands stress, for whichthe vector indicates of the the applied polarizability electric of field a dielectric (V/m). material.Figure 2 Stressillustrates vector the (N concept/m2) is σof, anddirect E stands piezoe forlectric the vectorconversion of the within applied the electric context field of (V road/m). Figureapplication.2 illustrates the concept of direct piezoelectric conversion within the context of road application.

Figure 2. Direct piezoelectric harves harvester—applicationter—application in a road.

The essentialessential properties properties of theseof these materials materials are the ar piezoelectrice the piezoelectric charge constant charge (d)constant and permittivity (d) and (permittivityε). The former (ε). indicatesThe former the indicates created polarisationthe created polarisa under unittion mechanical under unit stress,mechanical and the stress, latter and is the polarizabilitylatter is the polarizability of charges underof charges stress under perunit stress electrical per unit field. electrical Figure field. of meritFigure (FOM) of merit is (FOM) a quantity is a definedquantity todefined evaluate to andevaluate optimise and optimise the performance the performance of these materials.of these material A numbers. A ofnumber figures of of figures merit existof merit for exist piezoelectric for piezoelectric materials, materials, depending depending on their on application their application and loading and loading conditions. conditions. For energy For harvestingenergy harvesting purposes, purposes, the electromechanical the electromechanical coupling factorcoupling is one factor of the is one assessment of the assessment factors utilised factors for theutilised materials for the under materials equal under mechanical equal mechanical energy: energy:

2 FOM=2 k²d =E = Stored Electrical Energy (2) FOM = k = = (2) ε Input Mechanical Energy Another figure of merit is defined for the materials under equal mechanical stress: Another figure of merit is defined for the materials under equal mechanical stress: FOM = (3) d2 FOM = (3) where d is the piezoelectric charge constant, E is theε Young’s modulus and ε is the dielectric permittivity. whereHigh d is theperforming piezoelectric synthetic charge piezoe constant,lectric E ismaterials the Young’s for energy modulus harvesting and ε is the functions dielectric are permittivity. PZT (lead zirconateHigh titanate), performing PZN-PT synthetic (lead piezoelectric zinc niobate-lead materials titanate) for energy and PMN-PT harvesting (lead functions magnesium are PZT niobate- (lead zirconatelead titanate.). titanate), Examples PZN-PT of(lead typical zinc piezoelectric niobate-lead ha titanate)rvesters and are PMN-PT cantilever (lead beams, magnesium disks, cylinders niobate-lead and titanate.).stacks [11–17]. Examples of typical piezoelectric harvesters are cantilever beams, disks, cylinders and stacksIn [ 11addition–17]. to the materials’ properties, environmental properties, such as loading conditions and temperature,In addition to affect the materials’ the output properties, of piezoelectri environmentalc harvesters. properties, Their suchoutput as loadingbecomes conditions higher if andthe temperature,applied stresses affect are the parallel output ofto piezoelectricthe polarisation harvesters. direction. Their Loading output becomesfrequency higher also ifsignificantly the applied stressesinfluences are the parallel output, to theand, polarisation regarding the direction. temperat Loadingure, the frequency Curie point also is significantlythe threshold influences temperature the beyond which the material loses its piezoelectricity [12,18–20].

2.1.2. Experimental Studies on Road Piezoelectric Energy Harvesters Sustainability 2020, 12, 6738 4 of 27 output, and, regarding the temperature, the Curie point is the threshold temperature beyond which the material loses its piezoelectricity [12,18–20].

Sustainability 2020, 12, x FOR PEER REVIEW 4 of 27 2.1.2. Experimental Studies on Road Piezoelectric Energy Harvesters EnergyEnergy generation generation from from piezoelectric piezoelectric materials materials has has been been studied studied for for a a long long time, time, almost almost since since theirtheir discovery discovery in in the the late late 19th 19th century. century. However, However, on onee of of the the recently recently focused focused fields ﬁelds is is the the applicability applicability ofof this this technology technology on on roads roads in in order order to to harvest harvest ener energy.gy. In In this this regard, regard, roads’ roads’ loading loading conditions conditions are are thethe controlling controlling condition condition to to test test the the harvester, harvester, i.e., i.e., the the amount amount of of load load should should be be limited limited to to vehicles’ vehicles’ load,load, the the frequency frequency is isrelated related to tothe the average average speed speed of the of the vehicles, vehicles, and and the damping the damping properties properties of the of pavementthe pavement material material should should be considered. be considered. The magn The magnitudeitude of the of load the is load usually is usually between between 25 KPa 25 and KPa 0.7and MPa 0.7 in MPa different in diﬀerent references references (depending (depending on the on pa thessing passing vehicle), vehicle), and the and frequency the frequency is 30 Hz is 30 or Hz less or [21–23].less [21 –Figure23]. Figure 3 illustrates3 illustrates the shapes the shapes of fo ofur fourpiezoelectric piezoelectric transducers transducers used used in roads. in roads.

(a) (b) (c)

FigureFigure 3. 3. SideSide view ofof differentdifferent shapes shapes of of road road piezoelectric piezoelectric transducers: transducers: (a) cylinder, (a) cylinder, (b) stack, (b) stack, (c) bridge. (c) bridge. Each of these transducers has a different level of mechanical strength based on its geometry and materials.Each of For these practical transducers purposes, has thesea different transducers level of are mechanical placed in strength designated based boxes onor its other geometry protective and materials.structures. For Cymbal, practical cantilever purposes, and these hybrid transducers systems are are placed the other in designated forms of piezoelectric boxes or other transducers. protective structures. Cymbal, cantilever and hybrid systems are the other forms of piezoelectric transducers. Bridge and Cymbal Piezoelectric Road Harvesters BridgeBridge and Cymbal piezoelectric Piezoelectric harvesters Road have Harvesters been the focus of several research works. As shown in FigureBridge3, in thesepiezoelectric modules, harvesters a rectangular have piezoelectric been the focus element of several is placed research between works. two protective As shown caps in Figurein the top3, in and these bottom. modules, a rectangular piezoelectric element is placed between two protective caps in theZhao top and et al. bottom. investigated three different bridge transducers through simulation and experiments withZhao trapezoidal et al. investigated and arc-shaped three different caps. It bridge was found transducers that arch through bridge simulation transducers and are experiments capable of withproducing trapezoidal potential and of arc-shaped almost 285 Voltscaps. underIt was 0.7 found MPa pressurethat arch and bridge 10 HZ transducers frequency. are It is capable also shown of producingthat the modulus potential of of elasticity almost 285 and Volts the under thickness 0.7 MPa of the pressure cap material and 10 isHZ significantly frequency. It eff isective also shown on the thatoutput. the modulus At the same of elasticity modulus and of elasticity,the thickness increasing of the cap the material thickness isof significantly the cap from effective 0.3 to 0.5on mmthe output.decreases At thethe voltagesame modulus by almost of 9%. elasticity, A higher increasing modulus the of elasticitythickness results of the in cap a steady from 0.3 decrease to 0.5 inmm the decreasesoutput voltage. the voltage Regarding by almost the 9%. compressive A higher modulus strength, of the elasticity trapezoidal results bridge in a steady performed decrease better in thanthe outputarc-shaped voltage. caps Regarding and could the withstand compressive up to 1streng MPath, pressure the trapezoidal [23]. bridge performed better than arc-shapedIn another caps study,and could bridge withstand transducers up to were 1 MPa fabricated pressure by attaching[23]. horizontally polled piezoelectric rodsIn together, another using study, a silver bridge paste totransducers form a bridge were transducer. fabricated Eventually, by attaching one harvester horizontally unit containing polled piezoelectric64 modules ofrods transducers together, using was tested.a silver Experimental paste to form a results bridge showed transducer. that, Eventually, under a load one of harvester 0.7 MP at unit5 Hz, containing 26 to 31 mWatt 64 modules power of was transducers harvested [was3]. tested. Experimental results showed that, under a load ofCymbal 0.7 MP transducersat 5 Hz, 26 to are 31 mWatt similar power to a cymbal was harvested in shape, [3]. holding a piezoelectric layer in the middle.Cymbal Palosaari transducers et al. designed are similar and to studieda cymbal the in potentialshape, holding of cymbal a piezoelectric piezoelectric layer harvesters in the middle. under Palosaarilow-frequency et al. compressivedesigned and forces. studied The the results potentia indicatedl of cymbal that, under piezoelectric a frequency harvesters of 1.19 HZ, under the cymbal low- frequencywith a 250 compressiveµm cap had forces. theoptimal The results performance, indicated that, with under an output a frequency voltage of of 1.19 almost HZ, the 47 Vcymbal under with24.8 a N 250 (approximately μm cap had the 25.8 optimal KPa) [ 24performance,]. Another recent with an study output on cymbalvoltage harvestersof almost 47 investigated V under 24.8 their N (approximatelyperformance under 25.8 frequencies KPa) [24]. lower Another than 30recent HZ. Anstudy open on circuit cymbal (OC) harvesters power output investigated of almost 0.2 their mW performancewas obtained under from frequencies the harvester lower under than 500 30 N HZ. and An 20 open HZ. Moreover,circuit (OC) in power OC conditions, output of thealmost voltage 0.2 mW was obtained from the harvester under 500 N and 20 HZ. Moreover, in OC conditions, the voltage output would not change remarkably by frequencies of more than 20 HZ (almost 52 V) [22,25]. Figure 4 shows one example of embedding a bridge energy harvester in the road. Sustainability 2020, 12, 6738 5 of 27 output would not change remarkably by frequencies of more than 20 HZ (almost 52 V) [22,25]. Figure4 shows one example of embedding a bridge energy harvester in the road. Sustainability 2020, 12, x FOR PEER REVIEW 5 of 27

(a) (b)

(c)

Figure 4. (a) Bridge transducer module embedded in the road.road. ( (b)) A harvester module of cymbal transducer. ReprintedReprinted with with permission permission from from [22]. ([22].c) The (c module) The module under test. under Reprinted test. Reprinted with permission with frompermission [22]. from [22].

Disk and Cylinder Piezoelectric Road Harvesters Roshani etet al., al., in in 2016, 2016, designed designed and testedand tested piezoelectric piezoelectric disk harvesters disk harvesters and found and that found increasing that theincreasing frequency the andfrequency applied and load applied has a positiveload has eaﬀ positiveect on the effect harvester’s on the harvester’s power output. power A durabilityoutput. A testdurability on the test harvesters on the harvesters showed a constantshowed a output constant in 8output h, and in the 8 feasibilityh, and the studyfeasibility of the study harvesters of the indicatedharvesters that indicated interstate that highways interstate with highways high tra withﬃc load high are traffic suitable load alternatives are suitable for alternatives installing these for harvestersinstalling these [26]. harvesters Figure5 shows [26]. the Figure disk 5 harvester shows the embedded disk harvester in the asphaltembedded sample in the being asphalt tested. sample beingIn tested. another study, Xiong et al. fabricated harvester modules consisting of piezoelectric disks. It was foundInthat another the power study, outputXiong et is highlyal. fabricated related harveste to the totalr modules axle loading consisting of the of vehicles. piezoelectric The averagedisks. It powerwas found output that was the almostpower output 3 mW, whichis highly is enoughrelated to to the power total small axle loading electronic of devicesthe vehicles. embedded The average in the roadpower [27 output]. was almost 3 mW, which is enough to power small electronic devices embedded in the road In[27]. a comparative study, several shapes of piezoelectric transducers were investigated theoretically by ZhaoIn et al.a Itcomparative was found thatstudy, piles several and multi-layered shapes ofpiezoelectric piezoelectric transducers transducers have were higher investigated conversion capability.theoretically Furthermore, by Zhao et al. in It a was theoretical found that optimisation piles and multi-layered study, it was suggestedpiezoelectric that transducers a number have of 8 2 tohigher 16 cylindrical conversion piezoelectric capability. Furthermore, piles in each in 0.04 a theo mreticalof pavement optimisation are requiredstudy, it was in order suggested to have that a considerablea number of 8 energy to 16 cylindrical outcome [4 piezoelectric,28]. piles in each 0.04 m2 of pavement are required in order to have a considerable energy outcome [4,28]. Sustainability 2020, 12, 6738 6 of 27 Sustainability 2020, 12, x FOR PEER REVIEW 6 of 27

FigureFigure 5. Piezoelectric 5. Piezoelectric disk harvester disk harvester embodied embodied in the inasphalt the asphalt sample. sample. Reprinted Reprinted with permission with permission from [26,29] from [26,29] Stack Piezoelectric and Other Systems of Road Harvesters Stack Piezoelectric and Other Systems of Road Harvesters ThereThere areare alsoalso severalseveral studiesstudies onon stackstack piezoelectricpiezoelectric harvesters harvesters [ 30[30,31].,31]. InIn 2014,2014, JiangJiang etet al.al. studied thethe potentialspotentials of suchof such harvesters harvesters through through simulation simu andlation experiments. and experiments. Thirty-six layersThirty-six of piezoelectric layers of transducerspiezoelectric were transducers connected were as one connected module and as testedone module under three and di fftestederent compressiveunder three loadsdifferent and fourcompressive different loads frequencies. and four The different highest frequencies. power output The of highest 0.2 W was power obtained output from of 0.2 1360 W N, was under obtained 6HZ frequencyfrom 1360 and N, under almost 6HZ 300 K frequencyΩ resistive and load. almost The same 300 K loadingΩ resistive scenario load. leads The to same an output loading of 0.05scenario mW inleads OC to conditions an output [32 of]. 0.05 mW in OC conditions [32]. StackStack piezoelectricpiezoelectric harvestersharvesters werewere thethe focusfocus ofof anotheranother studystudy donedone byby KhaliliKhalili etet al.al. whichwhich simulatedsimulated aa piezoelectricpiezoelectric harvester,harvester, andand thethe powerpower outputoutput fromfrom thethe stackstack harvesterharvester matchedmatched thethe experimentalexperimental data.data. The The harvester harvester was was investigated under dynamic loads from 1.1 to 11 KNKN andand frequenciesfrequencies rangingranging fromfrom 2.52.5 toto 6262 Hz.Hz. Based on the characterisation results,results, itit waswas estimatedestimated thatthat thethe powerpower outputoutput peak peak of of 1.5 1.5 Watts Watts could could be be obtained obtained at 11at kN11 kN load, load, 62 HZ62 HZ and and 500 K500Ω resistiveKΩ resistive load load [25]. [25]. In another study, various piezoelectric stack harvesters were built and compared for their design processIn andanother performance. study, various Two different piezoele fabricationctric stack methodsharvesters were were employed built and to compared build the stackfor their harvesters. design Bothprocess harvesters and performance. were tested Two under different 0.7 MPa andfabricat 10 HZion cyclicmethods load. were It was employed observed to that build the harvesterthe stack builtharvesters. by the multilayerBoth harvesters adhesive were process tested hadunder a higher 0.7 MPa power and output 10 HZ levelcyclic of load. 32.4 mW,It was with observed an optimal that resistancethe harvester load built of 30 by kΩ the. In multilayer comparison, adhesive the harvester process made had through a higher the power process output of monolithic level of 32.4 co-firing mW, hadwith a an lower optimal power resistance output, ofload 9.6 of mW, 30 k withΩ. In an comparison, optimal resistance the harvester load of made 20 kΩ through. The power the process output ofof themonolithic system inco-firing the OC had condition a lower was power attuned output, to 3.30 of 9.6 mW mW, [33 ].with an optimal resistance load of 20 kΩ. The powerIn another output study, of the Guo system et al. in in 2018 the focusedOC condition on the was shapes attuned of the to harvesters 3.30 mW and [33]. the filling materials (insulator)In another in the study, pavement. Guo Theet al. cross-section in 2018 focused of the roadon the was shapes divided of intothe threeharvesters layers: and two the layers filling of conductivematerials (insulator) asphalt and, in the in between,pavement. a piezoelectricThe cross-sect layer,ion of which the road was was analysed divided with into di threefferent layers: shapes two of piezoelectriclayers of conductive curved asphalt roofs, balls and, and in between, cylinders. a piezoelectric The stiffness layer, of the which fillermaterial was analysed in the with piezoelectric different layershapes was of seenpiezoelectric to be a criticalcurved factorroofs, inballs the and output. cylinders. In the The fillers stiffness with lowof the sti fillerffness, material piezoelectric in the ballspiezoelectric had higher layer voltage was outputseen to thanbe a the critical others, factor while, in bythe increasing output. In the the sti fffillersness, with the voltage low stiffness, output decreasedpiezoelectric remarkably, balls had byhigher almost voltage 100%. output Given than that thethe piezoelectricothers, while, layers by increasing were modelled the stiffness, as a layer the filledvoltage with output discrete decreased piezoelectric remarkably, elements, by thealmost output 100%. was Given significantly that the di ffpiezoelectricerent dependent layers on were the stressmodelled distribution as a layer [2 ].filled with discrete piezoelectric elements, the output was significantly different dependentHybrid on energy the stress harvesters distribution containing [2]. piezoelectric harvesters were also the focus of a study conductedHybrid by energy Fan et al.,harvesters coupling contai electromagneticning piezoelectric and piezoelectric harvesters harvester were also units the together focus toof generatea study powerconducted at low by frequencies. Fan et al., Thecoupling results electromagnetic showed that, under and piezoelectric hand-shaking harvester vibrations, units enough together power to wasgenerate generated power to at illuminate low frequencies. light emitting The results diodes showed (LEDs). that, However, under hand-sha a largerking portion vibrations, of this enough power waspower generated was generated by the electromagnetic to illuminate light harvester emitting [34 ].di Figureodes (LEDs).6 shows However, a module ofa larger a stack portion piezoelectric of this roadpower harvester. was generated by the electromagnetic harvester [34]. Figure 6 shows a module of a stack piezoelectric road harvester. Sustainability 2020, 12, 6738 7 of 27

Sustainability 2020, 12, x FOR PEER REVIEW 7 of 27

(a) (b)

FigureFigure 6.6. (a) Stack piezoelectric harvester module. module. ( (bb)) The The module module being being tested tested in in a amechanical mechanical testing testing andand simulationsimulation servo-hydraulicservo-hydraulic system (MTS) ma machine.chine. Reprinted Reprinted with with permission permission from from [33] [33.].

PiezoelectricPiezoelectric CementitiousCementitious CompositesComposites InIn additionaddition to to the the design design and and fabrication fabrication of the of conventionalthe conventional piezoelectric piezoelectric harvester harvester units, anotherunits, areaanother of interest area of is interest developing is developing cementitious cementitious piezoelectric piezoelectric harvesters, harvesters, ideally to serve ideally both to asserve construction both as materialconstruction and energymaterial harvester and energy simultaneously. harvester simult Theseaneously. composites These arecomposites made by are mixing made piezoelectric by mixing ceramicpiezoelectric particles ceramic (mostly particles PZT) (mostly as additives PZT) as to addi cement,tives polarisingto cement, polarising the mixes the or both.mixes Soor both. far, these So studiesfar, these have studies been have conducted been conducted at the laboratory at the labora level,tory and level, no field and tests no field have tests been have reported. been reported. InIn oneone study,study, piezoelectricpiezoelectric cementcement composites were were made made by by mixing mixing PZT PZT particles particles in in cement cement by by 3030 toto 90%,90%, resultingresulting inin a significantsignificant increase in in diel dielectricectric and and piezoelectric piezoelectric properties properties compared compared to to the the plainplain cementcement mixes.mixes. The features remarkably improved improved by by increasing increasing the the volume volume of of PZT. PZT. Relative Relative dielectricdielectric constantconstant andand piezoelectricpiezoelectric constant ( εr andand d d3333) )increased increased from from 79 79 and and 0 0in in the the plain plain mix mix to to almostalmost 300300 andand 4545 pCpC/N/N inin the sample with 90% PZ PZTT addition, addition, respectively respectively [35]. [35]. The The same same researcher researcher investigatedinvestigated thethe eeffectffect ofof PZTPZT particleparticle size distribution on on the the resulting resulting composite composite by by mixing mixing three three didifferentfferent PZTsPZTs withwith didifferentfferent averageaverage particleparticle sizes.sizes. Larger Larger PZT PZT particles particles were were found found to to be be more more eeffectiveffective inin thethe overalloverall improvementimprovement of the piezoele piezoelectricctric properties properties of of the the composites, composites, which which was was due due toto thethe smallersmaller contactingcontacting surfacesurface areaarea between cement and PZT PZT particles particles [36]. [36]. Similarly,Similarly, Sladek Sladek et et al. al. developed developed a a numerical numerical method method to to predict predict the the piezoelectricity piezoelectricity of cement-PZTof cement- compositesPZT composites through through the finite the finite element element method. method. The predictabilityThe predictability of the of method the method was comparedwas compared to the previousto the previous experiments, experiments, showing showing good agreement good agr betweeneement between the results the and results also aand remarkable also a remarkable increase in increase in d33 by adding more PZT particles [37]. d33 by adding more PZT particles [37]. OneOne ofof the the challenges challenges associated associated with with PZT-cement PZT-cement composites composites is their is complicatedtheir complicated poling poling process, whichprocess, needs which high-temperature needs high-temperature treatment. treatment. This challenge This challenge has beenaddressed has been addressed in research in by researchGong et by al. , whoGong mixed et al., carbon who mixed nanotubes carbon (CNTs) nanotubes in PZT-cement (CNTs) in composites PZT-cement to enhance composites the conductivityto enhance the and simplifyconductivity the polarisation and simplify process. the polarisation According process. to the results, According the additionto the results, of CNTs the byaddition up to 1.3%of CNTs enabled by polarisationup to 1.3% enabled at room polarisation temperature. at Improvementroom temperat inure. piezoelectricity Improvement propertiesin piezoelectricity happened properties with an optimumhappened volume with an percentage optimum volume of 0.3% percentage [38]. of 0.3% [38]. OneOne ofof thethe crucialcrucial limitationslimitations of piezoelectric harvesters is is the the low low power power outcome. outcome. It Itcan can be be summarised that piezoelectric harvesters, independent of their configuration and material properties, summarised that piezoelectric harvesters, independent of their configuration and material properties, still have an outcome of almost 200 microwatts to 30 milliwatts [6,20,27,39,40]. Figure 7 illustrates the still have an outcome of almost 200 microwatts to 30 milliwatts [6,20,27,39,40]. Figure7 illustrates the internal structure of a cementitious piezoelectric composite. internal structure of a cementitious piezoelectric composite. Sustainability 2020, 12, 6738 8 of 27 SustainabilitySustainability 2020 2020, ,12 12, ,x x FOR FOR PEERPEER REVIEWREVIEW 8 8of of 27 27

((aa)) ((bb))

FigureFigure 7.7. ( ((aa))) SEM SEMSEM image imageimage of ofof a a cement cement + PZTPZT particles particles composite.composite. composite. ReprintedReprinted Reprinted with with with permission permission permission from from from [36]. [36]. [36 ]. (b(b) ) Internal Internal structurestructure structure ofofof aaa piezoelectricpiezoelectricpiezoelectric cementcement composite.composite. Reprinted Reprinted with withwith permission permissionpermission from fromfrom [37] [[37]37].. .

It is noteworthy that the experimental studies’ power outputs of the road piezoelectric harvesters ItIt isis noteworthynoteworthy thatthat thethe experimentalexperimental studies’ies’ powerpower outputsoutputs ofof thethe roadroad piezoelectricpiezoelectric are all within the range of milliwatts. To provide an overview, Figure8 shows the power or voltage harvestersharvesters are are allall withinwithin thethe rangerange of milliwatts. To provide anan overview,overview, Figure Figure 8 8 shows shows the the power power outputor voltage of some output of theof some highlighted of the highlighted studies. studies. or voltage output of some of the highlighted studies.

90 90 80 80 70 70 Power (mW) 60 Power (mW) 60 50 Voltage (V) 50 Voltage (V) 40 40 30 30 20 20 10 Power (mW)-(V) Voltage 10 Power (mW)-(V) Voltage 0 0 5 103015300.3660 5 103015300.3660Frequency (Hz) Frequency (Hz) Figure 8. Maximum power or voltage output of experimentally studied road piezoelectric energy Figureharvesters 8. Maximum [3,21,22,41–44] power . or voltage output of experimentallyexperimentally studied road piezoelectric energy harvesters [[3,21,22,41–44]3,21,22,41–44].. 2.2. Thermoelectric Harvesters 2.2. Thermoelectric Harvesters 2.2.1. Thermoelectricity 2.2.1. Thermoelectricity Thermoelectricity is the fundamental property of thermoelectric materials, which convert the Thermoelectricity is the fundamental property of thermoelectric materials, which convert the temperatureThermoelectricity difference is to the electrical fundamental voltage. property The Seebeck of thermoelectric effect, named aftermaterials, the German which scientist,convert theis temperature difference to electrical voltage. The Seebeck effect, named after the German scientist, temperatureassociated with difference thermoelectric to electrical materials voltage. and The indica Seebecktes the effect, generation named of after the electrical the German voltage scientist, across is is associated with thermoelectric materials and indicates the generation of the electrical voltage across associatedthe temperature with thermoelectric gradient between materials the sides and indicaof a thtesermoelectric the generation material. of the In electrical these materials, voltage across heat thecarriers temperature are charged gradient particles, betweenbetween which thethe can sidessides be electrons ofof aa thermoelectricthermoelectric in n-type or material.material. holes in In p-type these thermoelectrics. materials, heat carriersFigure 9are shows charged a power particles, generating which p-n can thermoelectr be electrons ic in setup. n-type Thermoelectric or holes in p-typegenerators thermoelectrics. have a use Figurein temperature9 9 shows shows a acontrollers. power power generating generating p-n p-n thermoelectric thermoelectric setup. setup. Thermoelectric Thermoelectric generators generators have have a use a use in temperaturein temperatureSubsequently, controllers. controllers. thermoelectric harvesters depend on the temperature difference between their two Subsequently,sides, making the thermoelectric electrical charges harvesters (heat depend carriers) on displace the temperature from one side difference di fftoerence another between and create their twoelectrical sides, potential making the(Seebeck electrical effect). charges This (heat phenom carriers)carrieenon,rs) togetherdisplace withfrom the one Peltier side toeffect another and andThomson create electricaleffect, are potential “thermoelectric (Seebeck effects” effect). effect). [45–47]. This phenom phenomenon, enon, together with the Peltier effect effect and Thomson eeffect,ffect, are “thermoelectric eeffects”ffects” [[45–47].45–47]. Sustainability 2020, 12, 6738 9 of 27 Sustainability 2020, 12, x FOR PEER REVIEW 9 of 27

(a) (b)

FigureFigure 9.9. (a(a)) Single Single p-n p-n power power harvesting harvesting junction. junction. (b) Arrangement (b) Arrangement of p-n junctions of p-n in junctions a thermoelectric in a thermoelectricgenerator. generator.

TheThe Seebeck Seebeck coefficient coeﬃcient is is the the fundamental fundamental proper propertyty of the thermoelectric materials, which isis the theratio ratio of changeof change in thein the generated generated voltage voltage by temperature.by temperature.

∆V∆V SS== (4)(4) ∆T∆T where ΔV is the change in the electrical potential and ΔT is the change in temperature. The where ∆V is the change in the electrical potential and ∆T is the change in temperature. The performance performance of thermoelectrics depends on their Seebeck coefficient, electrical conductivity and of thermoelectrics depends on their Seebeck coefficient, electrical conductivity and thermal conductivity, thermal conductivity, as well as their environmental conditions, i.e., temperature difference between as well as their environmental conditions, i.e., temperature difference between the sides. Similar to the the sides. Similar to the piezoelectric materials, a dimensionless figure of merit is also defined for piezoelectric materials, a dimensionless figure of merit is also defined for thermoelectric conductors, thermoelectric conductors, which is calculated according to the following formula: which is calculated according to the following formula: SσT ZT = 2 (5) SKσT ZT = (5) K Another assessment variable for thermoelectricity is the power factor: Another assessment variable for thermoelectricity is the power factor: PF = Sσ (6) In Equations (5) and (6), ZT is the dimensionlessPF = S2σ figure of merit, PF is power factor, S is the(6) Seebeck coefficient of the matter (V/K), σ is the electrical conductivity (1/Ω·m), T is the temperature differenceIn Equations (K), and (5) K andis the (6), thermal ZT is the conductivity dimensionless (W/m·K). figure ofUnlike merit, the PF power is power factor, factor, which S is the is Seebeckrelated coefficient of the matter (V/K), σ is the electrical conductivity (1/Ω m), T is the temperature difference only to the electronic features, ZT also depends on the thermal conductivity· and the temperature (K), and K is the thermal conductivity (W/m K). Unlike the power factor, which is related only to the difference. · electronicMost of features, the thermoelectric ZT also depends materials, on the such thermal as bismuth conductivity telluride and and the lead temperature selenide, diarefference. synthetic. However,Most there of the are thermoelectric also claims of materials, naturally suchoccurring as bismuth ones, such telluride as zinc and tetrahedrite lead selenide, [13,45,46,48,49]. are synthetic. FigureHowever, 10a thereshows are the also change claims in the of naturally famous thermoel occurringectric ones, materials’ such as zincfigure tetrahedrite of merit by [13 temperature.,45,46,48,49]. Figure 10a shows the change in the famous thermoelectric materials’ figure of merit by temperature. According to Figure 10, most thermoelectric materials have a high figure of merit at temperatures much higher than room temperature (almost 300–320 K) and the only ones performing in room temperature are CsBi4Te6, Bi2Te3 and Yb0.15Co4Sb12. High electrical conductivity, which is critical for charge transfer within the material, and low thermal conductivity for maintaining the temperature difference over a more extended period are the other two crucial and favourable properties of thermoelectric materials. Sustainability 2020, 12, 6738 10 of 27 Sustainability 2020, 12, x FOR PEER REVIEW 10 of 27

(a)

(b) (c)

FigureFigure 10. 10.( a()a) Famous Famous thermoelectric thermoelectric materials’materials’ figure ﬁgure of of merit. merit. Reprinted Reprinted with with permission permission from from [46]. [46 ]. (b()b SEM) SEM image image of Bi2Te3. of Bi2Te3. Reprinted Reprinted with permissionwith permission from [ 50from]. (c )[50]. SEM ( imagec) SEM of CsBi4Te6.image of ReproducedCsBi4Te6. withReproduced permission with from permission [51]. from [51].

2.2.2. ExperimentalAccording to StudiesFigure on10, Road most Thermoelectric thermoelectric Energy materials Harvesters have a high figure of merit at temperatures much higher than room temperature (almost 300–320 K) and the only ones performing Usingin room Ceramic temperature Thermoelectric are CsBi4T Generatore6, Bi2Te3 (TEG) and Yb0.15Co4Sb12. RoadHigh pavements electrical haveconductivity, large surfaces, which which is critical are exposed for charge to sunlight transferduring withinthe the day. material, The combination and low ofthermal sunlight conductivity radiation and for maintaining friction from the passing temperature vehicles difference can create over a a hot more surface, extended especially period are in warm the seasons.other two However, crucial and the temperaturefavourable properties decreases of gradually thermoelectric according materials. to the depth of the road, creating a temperature gradient. In some areas, a gradient of 35 degrees has been measured along 23 cm of road 2.2.2. Experimental Studies on Road Thermoelectric Energy Harvesters proﬁle depth, such as that detected on 5 consecutive days in San Antonio, Texas [52]. In addition to the temperature gradient provided by the pavement, in some works, a pipeline system or storage has Using Ceramic Thermoelectric Generator (TEG) been constructed in the pavement in order to provide and maintain the gradient and transfer it to the thermoelectricRoad pavements modules [have6,53– large56]. surfaces, which are exposed to sunlight during the day. The combinationThe converter of sunlight unit of theseradiation systems and is friction a ceramic from TEG. passing Figure vehicles11 shows can an examplecreate a ofhot a TEG.surface, They areespecially comprised in warm of p-type seasons. and However, n-type thermoelectric the temperature junctions decreases connected gradually in according parallel thermally to the depth and of in seriesthe road, electrically creating (multiple a temperature junctions gradient. connected, In some similar areas,to a gradient Figure9). of 35 degrees has been measured alongSustainability 23 cm of 2020 road, 12, xprofile FOR PEER depth, REVIEW such as that detected on 5 consecutive days in San Antonio,11 of Texas 27 [52]. In addition to the temperature gradient provided by the pavement, in some works, a pipeline system or storage has been constructed in the pavement in order to provide and maintain the gradient and transfer it to the thermoelectric modules [6,53–56]. The converter unit of these systems is a ceramic TEG. Figure 11 shows an example of a TEG. They are comprised of p-type and n-type thermoelectric junctions connected in parallel thermally and in series electrically (multiple junctions connected, similar to Figure 9).

(a) (b)

FigureFigure 11. (11.a) Thermoelectric(a) Thermoelectric generator generator (TEG).(TEG). ( (bb)) Internal Internal arrangement arrangement of a of TEG. a TEG. Reprinted Reprinted with with permissionpermission from from [13 ].[13].

Park et al., in 2016, focused on the low energy output and the key factors affecting the output of thermoelectric harvesters on roads. A harvester module was developed using a TEG sandwiched between two conductive plates, which were connected to hot and cold sources separately. The performance of the designed harvester could be improved by matching their thermal and electrical impedances and reducing the heat loss between the heat exchange and the environment [57]. Concrete structures in the road were the focus of another study by Lee et al. Laboratory experiments were conducted using a halogen lamp on a concrete block, simulating sunlight radiation on a concrete road barrier. The effect of the surface material was studied by measuring the voltage on two different surfaces of wood and aluminium (1000 times more thermally conductive than wood), placed on the concrete block. It was found that the output voltage was 1.5 times higher on the aluminium surface than the wooden one [47]. Tahami et al. designed and tested a novel thermoelectric harvester, embedded in the pavement and thermally attached to the surface. The hot side of the ceramic thermoelectric module was connected with an L-shaped piece to the surface to transfer the heat, and the cold side was connected to a heat sink made of aluminium, holding a phase change material. Maximum power of 34 mW was harvested at 32 K temperature difference [52]. Hasebe et al. developed a pavement cooling system with a thermoelectric generator, using an integrated pipeline system in the pavement as the heat collector and river water transfer as the cooler side, both connected to the layer of ceramic thermoelectric harvesters. Results of simulation and experiments showed that the highest power was harvested at 30 Ω and there was a temperature difference of 40.5 K between the hot and cold sides [53]. In another study, a thermoelectric module was connected to the hot surface by a Z-shaped piece, and its cold side was connected to a heat sink filled with water. Two different configurations of modules were built, a two-TEG and a four-TEG prototype (each with different sizes). It was observed that the two-TEG prototype (larger total surface area) and the four-TEG (smaller surface area) gave 8 mW and 11 mW on average over the testing period. Field test results showed that the power output of the harvesters peaked in May, although the maximum temperature gradient occurred in June and July [55]. Thermoelectric generators were also employed to develop a harvesting system by Jiang et al. The system was comprised of a water tank as the cooling side and attached to one of the TEGs. The hot side of the TEG was attached to the hot pavement surface via an aluminium vapour chamber. The water tank was placed under a shading box. Small scale experiments showed that the generated voltages in indoor and outdoor tests were 0.73 V and 0.41 V, respectively; however, on larger scales, this output was reduced [54,58]. Figure 12 shows some applications of TEGs on road pavements. Sustainability 2020, 12, 6738 11 of 27

Park et al., in 2016, focused on the low energy output and the key factors affecting the output of thermoelectric harvesters on roads. A harvester module was developed using a TEG sandwiched between two conductive plates, which were connected to hot and cold sources separately. The performance of the designed harvester could be improved by matching their thermal and electrical impedances and reducing the heat loss between the heat exchange and the environment [57]. Concrete structures in the road were the focus of another study by Lee et al. Laboratory experiments were conducted using a halogen lamp on a concrete block, simulating sunlight radiation on a concrete road barrier. The effect of the surface material was studied by measuring the voltage on two different surfaces of wood and aluminium (1000 times more thermally conductive than wood), placed on the concrete block. It was found that the output voltage was 1.5 times higher on the aluminium surface than the wooden one [47]. Tahami et al. designed and tested a novel thermoelectric harvester, embedded in the pavement and thermally attached to the surface. The hot side of the ceramic thermoelectric module was connected with an L-shaped piece to the surface to transfer the heat, and the cold side was connected to a heat sink made of aluminium, holding a phase change material. Maximum power of 34 mW was harvested at 32 K temperature difference [52]. Hasebe et al. developed a pavement cooling system with a thermoelectric generator, using an integrated pipeline system in the pavement as the heat collector and river water transfer as the cooler side, both connected to the layer of ceramic thermoelectric harvesters. Results of simulation and experiments showed that the highest power was harvested at 30 Ω and there was a temperature difference of 40.5 K between the hot and cold sides [53]. In another study, a thermoelectric module was connected to the hot surface by a Z-shaped piece, and its cold side was connected to a heat sink filled with water. Two different configurations of modules were built, a two-TEG and a four-TEG prototype (each with different sizes). It was observed that the two-TEG prototype (larger total surface area) and the four-TEG (smaller surface area) gave 8 mW and 11 mW on average over the testing period. Field test results showed that the power output of the harvesters peaked in May, although the maximum temperature gradient occurred in June and July [55]. Thermoelectric generators were also employed to develop a harvesting system by Jiang et al. The system was comprised of a water tank as the cooling side and attached to one of the TEGs. The hot side of the TEG was attached to the hot pavement surface via an aluminium vapour chamber. The water tank was placed under a shading box. Small scale experiments showed that the generated voltages in indoor and outdoor tests were 0.73 V and 0.41 V, respectively; however, on larger scales, this output was reducedSustainability [54 2020,58, 12]., x Figure FOR PEER 12 REVIEWshows some applications of TEGs on road pavements. 12 of 27

(a) (b)

FigureFigure 12. ( 12.a) Thermoelectric(a) Thermoelectric pavement pavement harvesting harvesting using using heat heat chamber. chamber. Reprinted Reprinted with with permission permission from [54]. (b) Internal arrangement of a TEG. Reprinted with permission from [52]. from [54]. (b) Internal arrangement of a TEG. Reprinted with permission from [52].

ThermoelectricThermoelectric Cementitious Cementitious Mixes Mixes In addition to assembling the thermoelectric harvesters by TEGs, another recently developed In addition to assembling the thermoelectric harvesters by TEGs, another recently developed approach utilises the thermoelectric properties of the cementitious materials and develops approach utilises the thermoelectric properties of the cementitious materials and develops cementitious cementitious thermoelectric mixes, capable of generating power as well as being a construction material. Similar to piezoelectric mixes, the focus of these works is on building cementitious composite mixtures with known thermoelectric materials, for example, ordinary Portland cement and metallic oxides or carbon fibres and evaluating their thermoelectric properties and potentials. Several of these additives have been studied for their thermoelectric properties [49,59,60]. Enhancement of the thermoelectric properties of cement composites with ZnO and Fe2O3 nanoparticles was studied by Tao et al. in 2018 [61]. Cement composite paste samples were made by adding the nanoparticles in various percentages, characterising and testing them for their thermal and electrical conductivity and Seebeck coefficients. It was observed that by adding 1 to 5% of these nano-powders, the Seebeck coefficient and electrical properties of the mixes improved significantly. A similar observation was made by Ghahari [62]. In an earlier study, cement composites with carbon nanotubes (CNTs) and fibres were also studied by Zuo et al. In addition to the thermoelectric properties and characterisation, they also studied the mechanical features. It was realised that the addition of a small percentage of carbon nanotubes improves the mechanical properties and changes the conventional cement paste into a P- type thermoelectric mix. By having a composite with 0.5% of CNTs, the Seebeck coefficient reached a maximum of 23.5 μVK−1 [63]. A similar observation was made by Tzounis et al., who studied cement composites with both p- and n-doped carbon nanotubes. The moisture content of the samples also affected their properties [64]. Composites of carbon fibre and carbon fibre powder in cement have also been studied in some investigations. It was found that the charge density and mobility is affected by the amorphous structure of the composites, which is influenced by the percentage of carbon fibre addition. Addition of 1% carbon fibres to the cement paste mix was found to maximise the figure of merit of the samples (equal to 1.334 × 10−7) [65,66]. Expanded graphite cement composites (EGCC) were studied Wei et al., building three cementitious composites with different amounts of EG (expanded graphite). The tested composites were all n-type, with a maximum figure of merit of 6.82 × 10−4 and power factor of 6.38 μWm−1K−2 obtained by mixing 15% of expanded graphite. In addition to the thermoelectric properties, high compressive strength (more than 100 MPa) was also achieved from the mixes, making them a promising composite for energy harvesting as well as a structural material [67,68]. Figure 13 shows the test setup of some thermoelectric cementitious materials. Sustainability 2020, 12, 6738 12 of 27 thermoelectric mixes, capable of generating power as well as being a construction material. Similar to piezoelectric mixes, the focus of these works is on building cementitious composite mixtures with known thermoelectric materials, for example, ordinary Portland cement and metallic oxides or carbon fibres and evaluating their thermoelectric properties and potentials. Several of these additives have been studied for their thermoelectric properties [49,59,60]. Enhancement of the thermoelectric properties of cement composites with ZnO and Fe2O3 nanoparticles was studied by Tao et al. in 2018 [61]. Cement composite paste samples were made by adding the nanoparticles in various percentages, characterising and testing them for their thermal and electrical conductivity and Seebeck coefficients. It was observed that by adding 1 to 5% of these nano-powders, the Seebeck coefficient and electrical properties of the mixes improved significantly. A similar observation was made by Ghahari [62]. In an earlier study, cement composites with carbon nanotubes (CNTs) and fibres were also studied by Zuo et al. In addition to the thermoelectric properties and characterisation, they also studied the mechanical features. It was realised that the addition of a small percentage of carbon nanotubes improves the mechanical properties and changes the conventional cement paste into a P-type thermoelectric mix. By having a composite with 0.5% of CNTs, the Seebeck coefficient reached a 1 maximum of 23.5 µVK− [63]. A similar observation was made by Tzounis et al., who studied cement composites with both p- and n-doped carbon nanotubes. The moisture content of the samples also affected their properties [64]. Composites of carbon fibre and carbon fibre powder in cement have also been studied in some investigations. It was found that the charge density and mobility is affected by the amorphous structure of the composites, which is influenced by the percentage of carbon fibre addition. Addition of 1% carbon fibres to the cement paste mix was found to maximise the figure of merit of the samples (equal to 1.334 10 7)[65,66]. × − Expanded graphite cement composites (EGCC) were studied Wei et al., building three cementitious composites with different amounts of EG (expanded graphite). The tested composites were all n-type, with a maximum figure of merit of 6.82 10 4 and power factor of 6.38 µWm 1K 2 obtained by mixing × − − − 15% of expanded graphite. In addition to the thermoelectric properties, high compressive strength (more than 100 MPa) was also achieved from the mixes, making them a promising composite for energy harvesting as well as a structural material [67,68]. Figure 13 shows the test setup of some Sustainabilitythermoelectric 2020, 12 cementitious, x FOR PEER REVIEW materials. 13 of 27

(a) (b)

FigureFigure 13. 13. AnAn example example of ofcementitious cementitious thermoelectric thermoelectric composite composite (a) samples (a) samples and andtesting testing setup; setup; (b) testing(b) testing procedure. procedure. Reprinted Reprinted with with permission permission from from [64]. [64].

Figure 14 shows the maximum power output of road thermoelectric harvesters, or the measured figures of merit of cementitious thermoelectric samples, in one diagram.

100 Power (mW)

80 ZT (µvk^-1)

Power (mW)- (µvk^-1) ZT 20

0 4.5 4.6 65 25 16 Module's area (cm^2) Figure 14. Maximum power output or figure of merit of experimentally studied thermoelectric energy harvesters [52,53,55,63,65,66].

2.3. Solar Panel Road Energy Harvester Solar panels are well-known for converting solar radiation into electrical energy. They are compiled of several solar cells connected to each other in order to increase the electrical energy output of the whole panel. Solar cells are made of silicon-based crystals or, in more expensive types, made of scarcer materials such as gallium arsenide [69]. Solar panels are widely used, especially in sun- catching environments and on residential buildings’ rooftops to generate off-grid electricity. The efficiency of these panels depends on a number of environmental factors, such as the weather conditions, as well as production factors, such as the type of solar cells and the surface area of the panel. Nowadays, they have versatile applications and even can be found on buildings’ rooftops for domestic energy generation. In addition to energy harvesting, one of the advantages of using solar panels on roads is the reduction of the urban heat island effect. Implementing solar panels on roads or on road facilities has been investigated in some projects. In SolaRoad, solar panels were implemented on a biking lane pavement. In laboratory results, the Sustainability 2020, 12, x FOR PEER REVIEW 13 of 27

(a) (b)

SustainabilityFigure2020 13. ,An12, 6738example of cementitious thermoelectric composite (a) samples and testing setup; (b13) of 27 testing procedure. Reprinted with permission from [64].

Figure 14 shows the maximum power output of roadroad thermoelectric harvesters, or the measured ﬁguresfigures ofof meritmerit ofof cementitiouscementitious thermoelectricthermoelectric samples,samples, inin oneone diagram.diagram.

100 Power (mW)

80 ZT (µvk^-1)

Power (mW)- (µvk^-1) ZT 20

0 4.5 4.6 65 25 16 Module's area (cm^2)

Figure 14. MaximumMaximum power output or figure ﬁgure of merit merit of of experimentally experimentally studied thermoelectric energy harvesters [[52,53,55,63,65,66].52,53,55,63,65,66].

2.3. Solar Panel Road Energy Harvester Solar panels areare well-knownwell-known forfor convertingconverting solarsolar radiationradiation intointo electricalelectrical energy.energy. They are compiled ofof several solar cells connected to each other in order to increase the electrical energy output of thethe wholewhole panel.panel. SolarSolar cellscells areare made made of of silicon-based silicon-based crystals crystals or, or, in in more more expensive expensive types, types, made made of scarcerof scarcer materials materials such such as gallium as gallium arsenide arsenide [69]. Solar[69]. panelsSolar panels are widely are widely used, especially used, especially in sun-catching in sun- environmentscatching environments and on residential and on residential buildings’ rooftopsbuildings’ to rooftops generate to off generate-grid electricity. off-grid The electricity. efficiency The of theseefficiency panels of dependsthese panels on a numberdepends of on environmental a number of factors, environmental such as the factors, weather such conditions, as the weather as well asconditions, production as well factors, as production such as the factors, type of such solar as cells the and type the of surfacesolar cells area and of the panel.surface Nowadays,area of the theypanel. have Nowadays, versatile they applications have versatile and even applications can be found and even on buildings’ can be found rooftops on buildings’ for domestic rooftops energy for generation.domestic energy In addition generation. to energy In addition harvesting, to energy one of harvesting, the advantages one of the using ad solarvantages panels of using on roads solar is thepanels reduction on roads of theis the urban reduction heat island of the e urbanffect. heat island effect. Implementing solar panels on roads or on road fa facilitiescilities has been investigated in in some some projects. projects. In SolaRoad, solar solar panels panels were were implemented implemented on on a abi bikingking lane lane pavement. pavement. In laboratory In laboratory results, results, the the output energy was almost 3 MW h over the first six months and the estimated energy density was 50–70 kWh per m2 per year [70]. Similarly, in other projects developed by Soluxio, flexible solar panels were formed in order to cover the surface of the lampposts in roads. The 4 to 12 m long cylindrical panels are installed around the lamppost, providing enough off-grid power to illuminate them. Each panel also consists of a microcontroller unit, to maximise the power, and lithium battery storage [71]. However, it is noteworthy that these systems are associated with many challenges, such as protecting their fragile components, their dependence on the weather conditions and the partial shadow on them caused by the surroundings or by passing vehicles and, therefore, they are not easily compatible with the road conditions [72,73]. Figure 15 shows some examples of the mentioned solar panel applications on roads or on road facilities. Sustainability 2020, 12, x FOR PEER REVIEW 14 of 27

output energy was almost 3 MW h over the first six months and the estimated energy density was 50–70 kWh per m² per year [70]. Similarly, in other projects developed by Soluxio, flexible solar panels were formed in order to cover the surface of the lampposts in roads. The 4 to 12 m long cylindrical panels are installed around the lamppost, providing enough off-grid power to illuminate them. Each panel also consists of a microcontroller unit, to maximise the power, and lithium battery storage [71]. However, it is noteworthy that these systems are associated with many challenges, such as protecting their fragile components, their dependence on the weather conditions and the partial shadow on them caused by the surroundings or by passing vehicles and, therefore, they are not easily compatible with the road Sustainabilityconditions2020 [72,73]., 12, 6738 Figure 15 shows some examples of the mentioned solar panel applications14 on of 27 roads or on road facilities.

(a)

FigureFigure 15. 15(.a ()a Solar) Solar pavements pavements onon asphaltasphalt samples.samples. Re Reprintedprinted with with permission permission from from [74]. [74 (].b) ( bFlexSol) FlexSol lamppost.lamppost. Reprinted Reprinted with with permissionpermission from [71]. [71]. ( (cc)) Layers Layers of of solar solar roadway roadway panels. panels. Reprinted Reprinted with with permissionpermission from from [75 [75]]. .

2.4.2.4. Electromagnetic Electromagnetic Energy Energy Harvesters Harvesters

2.4.1.2.4.1. Electromagnetism Electromagnetism ElectromagneticElectromagnetic generators generators areare thethe corecore of the electromechanical conversion conversion systems. systems. They They are are usedused in in several several systems systems such such as as windwind andand water turbines. They They consist consist of of a acoil coil rotor rotor and and a permanent a permanent magnet.magnet. Once Once thethe coilcoil rotorrotor hashas startedstarted rotation, it it creates creates a atime time changing changing magnetic magnetic field, field, which which accompaniesaccompanies an an electric electric field, field, based based onon Faraday’sFaraday’s law [76,77]. [76,77]. The The rate rate of of change change in inthe the magnetic magnetic field field hashas a directa direct relationship relationship with with the the generatedgenerated electricelectric field: field: ∇×= −∂B (7) E = (7) ∇ × − ∂t where ∇ is the Del operator, E is the electric field, B is the magnetic field and t is the time. where In isaddition the Del to operator, the generators,E is the electromechanical electric field, B is theharvesters magnetic also field need and an tapparatus is the time. to capture ∇ theIn mechanical addition toenergy the generators, and convert electromechanical it into a rotational harvestersmovement. also For needexample, an apparatus wind turbines to capture consist the mechanical energy and convert it into a rotational movement. For example, wind turbines consist of large turbines rotating with the wind flow. In road energy harvesters, conversion of the road surface displacements is often done by devices such as hydraulic cylinders and rack and pinions [78].

2.4.2. Experimental Studies on Road Electromagnetic Energy Harvesters Harvesting energy from roads using electromagnetism is a relatively new approach in this ﬁeld and is categorised as a mechanical system, converting the roads’ kinetic energy into electrical energy using electromagnetic generators. Depending on their energy transfer mechanism, these harvesters can be divided into rack and pinion, hydraulic and roller. Experimental and theoretical studies have been conducted on developing and testing each of these harvesting systems, some of which are discussed brieﬂy in the following section. Sustainability 2020, 12, x FOR PEER REVIEW 15 of 27

of large turbines rotating with the wind flow. In road energy harvesters, conversion of the road surface displacements is often done by devices such as hydraulic cylinders and rack and pinions [78].

2.4.2. Experimental Studies on Road Electromagnetic Energy Harvesters Harvesting energy from roads using electromagnetism is a relatively new approach in this field and is categorised as a mechanical system, converting the roads’ kinetic energy into electrical energy using electromagnetic generators. Depending on their energy transfer mechanism, these harvesters can be divided into rack and pinion, hydraulic and roller. Experimental and theoretical studies have been conducted on developing and testing each of these harvesting systems, some of which are Sustainabilitydiscussed 2020briefly, 12, in 6738 the following section. 15 of 27

Rack and Pinion Electromechanical Energy Harvester Rack and Pinion Electromechanical Energy Harvester In these harvesters, the vertical movement of the road surface is converted into a rotational movementIn these through harvesters, a rack the and vertical pinion movementsystem connect of theed roadto the surface surface. is These converted harvesters into a are rotational usually movementdeveloped throughwithin a aspeed rack andbump pinion structure. system connected to the surface. These harvesters are usually developedIn a recent within study, a speed Gholikhani bump structure. et al. developed a speed bump system with a moving surface, whichIn arotates recent a study, pinion Gholikhani by the co etnnected al. developed rack. The a speed pinion bump is systemthen connected with a moving to a clutch surface, to which have rotatesunidirectional a pinion rotations. by the connected The preliminary rack. The results pinion of is thenthe plain connected system to resulted a clutch toin havea maximum unidirectional power rotations.output of The1.96 preliminary mW; however, results their of the following plain system prototypes resulted included in a maximum a gearbox, power which output increased of 1.96 mW; the however,number of their rotations following and hence prototypes the power included output a gearbox, up to a whichmaximum increased of almost the number16 W [79–81]. of rotations In another and hencestudy, thethey power evaluated output the up critical to a maximum factors affectin of almostg the 16 output W [79– 81of]. their In another harvester study, and they identified evaluated the thesupports’ critical spring factors stiffness, affecting loading the output and of unloading their harvester and the and magnitude identified of the the supports’ load [77]. spring stiffness, loadingIn a and prior unloading study, Duarte and the et magnitude al. developed of the a simi loadlar [77 system]. to be implemented in the regions of speedIn reduction a prior study, (similar Duarte to speed et al. bumps) developed [82]. a similarInstead systemof having to bevertical implemented rack, their in theharvester regions was of speedcomprised reduction of a double (similar crank to speed system bumps) hinged [82 on]. Insteadthe surface of having plate. Vertical vertical displacement rack, their harvester of the plate was comprisedmade the ofcranks a double move crank horizontally, system hinged spinning on the the surface pinions plate. connected Vertical displacementto the electromechanical of the plate madeconverters. the cranks Their move developed horizontally, harvester spinning was implemen the pinionsted connected in both pedestrian to the electromechanical and road pavements converters. [82– Their85]. Figure developed 16 shows harvester two different was implemented mechanisms in both of mechanical pedestrian androad road energy pavements harvesters [82– based85]. Figure on rack 16 showsand pinion. two di fferent mechanisms of mechanical road energy harvesters based on rack and pinion.

(a) (b)

FigureFigure 16.16. (a) Horizontal rack and pinion pinion road energy harvester. Reprinte Reprintedd with permission from [82] [82].. ((bb)) VerticalVertical rackrack andand pinionpinion roadroad energyenergy harvester.harvester. ReprintedReprinted withwith permissionpermission fromfrom TaylorTaylor && FrancisFrancis Ltd.,Ltd., http:http://www.tandfonline.com//www.tandfonline.com[77 [77]]. .

HydraulicHydraulic ElectromechanicalElectromechanical EnergyEnergy HarvesterHarvester UnlikeUnlike thethe rackrack andand pinionpinion system,system, inin thesethese harvesters,harvesters, thethe loadload fromfrom thethe passingpassing vehiclesvehicles isis transferredtransferred throughthrough aa hydraulichydraulic mediummedium toto movemove aa pistonpiston andand spinspin thethe motormotor andand thethe connectedconnected generatorgenerator toto generategenerate power.power. SpeedSpeed bumpsbumps areare alsoalso consideredconsidered aa suitablesuitable hosthost forfor thesethese harvesters.harvesters. Ting et al. conducted one of the most prominent experimental studies of these systems. In their research, they developed a harvester specifically for downhill deceleration purposes. It is composed of several pistons with a stroke of 3 cm connected to a hydraulic potential energy collector and a generator. The collector’s role is to save energy in case the applied load is not enough to produce energy efficiently. The results show an average power output of almost 600 W and working efficiency of almost 41% for the system [86]. A similar hydraulic system was developed and analysed, theoretically giving an output of efficiency of almost 68% [87]. In 2017, a speed bump hybrid harvester was developed, combining a hydraulic collector and a crank and pinion system. Their theoretical analysis showed a similar power output over the first 24 h [88]. A few other studies have employed the concept of the hydraulic harvester to develop an energy harvesting system using shock absorbers and vehicle suspensions. Li et al. designed a damper energy harvester using a hydraulic motor and a generator to produce electrical power. A maximum output of Sustainability 2020, 12, x FOR PEER REVIEW 16 of 27

Ting et al. conducted one of the most prominent experimental studies of these systems. In their research, they developed a harvester specifically for downhill deceleration purposes. It is composed of several pistons with a stroke of 3 cm connected to a hydraulic potential energy collector and a generator. The collector’s role is to save energy in case the applied load is not enough to produce energy efficiently. The results show an average power output of almost 600 W and working efficiency of almost 41% for the system [86]. A similar hydraulic system was developed and analysed, theoretically giving an output of efficiency of almost 68% [87]. In 2017, a speed bump hybrid harvester was developed, combining a hydraulic collector and a crank and pinion system. Their theoretical analysis showed a similar power output over the first 24 h [88].

SustainabilityA few2020 other, 12, 6738studies have employed the concept of the hydraulic harvester to develop an energy16 of 27 harvesting system using shock absorbers and vehicle suspensions. Li et al. designed a damper energy harvester using a hydraulic motor and a generator to produce electrical power. A maximum output 435of W435 was W was obtained obtained under under 3 cm 3 excitation cm excitation and 0.8 and Hz 0.8 vibration Hz vibration [89]. Another [89]. Another prototype prototype with a with similar a conceptsimilar resultedconcept inresulted an average in an poweraverage output power of output 34–100 of W 34–100 under variousW under testing various factors testing [90 factors]. Examples [90]. ofExamples mechanisms of mechanisms of hydraulic of road hydraulic energy road harvesters energy harvesters are shown are in Figure shown 17 in. Figure 17.

(a)

(b)

FigureFigure 17. 17.( a(a)) Hydraulic Hydraulic harvester harvester speedspeed bump.bump. ReprintedReprinted with permission from [91] [91].. ( (bb)) Hydraulic Hydraulic pistonpiston plate plate harvester. harvester. Reprinted Reprinted with with permissionpermission fromfrom [[86]86]..

RollerRoller Electromechanical Electromechanical Energy Energy Harvester Harvester TheseThese harvesters harvesters were were designeddesigned asas speedspeed bumpsbumps placed across the roads, and and once once a a car car passes passes overover them, them, due due to to friction, friction, thethe rollerroller rotatesrotates andand spins a generator to to produce produce electrical electrical power power [92]. [92]. Sarma et al. designed a set of these harvesters and obtained 1.67 watts from only one motorbike vehicle passing [93]. An illustration of a roller electromagnetic road harvester is shown in Figure 18. Sustainability 2020, 12, x FOR PEER REVIEW 17 of 27 Sustainability 2020, 12, x FOR PEER REVIEW 17 of 27

SustainabilitySarma et al.2020 designed, 12, 6738 a set of these harvesters and obtained 1.67 watts from only one motorbike17 of 27 vehicle passing [93]. An illustration ofof aa rollerroller electromagneticelectromagnetic roadroad harvesterharvester isis shownshown inin FigureFigure 18.18.

(a)

(b)

Figure 18.18. ((aa)) RollerRollerRoller harvesterharvesterharvester speedspeedspeed bumpbumpbump design.design.design. ( (b)) RollerRoller harvesterharvester speedspeed bumpbump laboratorylaboratory testtest setup.setup. Reprinted with permission from [[93]93]... The power outputs of various mechanical road energy harvesters from diﬀerent experimental The power outputs of various mechanical road energy harvesters from different experimental studies have been plotted in Figure 19. studies have been plotted in Figure 19.

700 600 600 Power (W) 500

400

300 200 Power output (W) Power output 200 Power output (W) Power output 100

0 Rack and pinion 1 Rack and pinion 2 Hydraulic Roller Rack and pinion 1 Rack and pinion 2 Hydraulic Roller

Figure 19. Power output of mechanical harvesters [77,82,86,92,93] FigureFigure 19.19. PowerPower outputoutput ofof mechanicalmechanical harvestersharvesters [[77,82,86,92,93]77,82,86,92,93].

3. Experimental Studies on Energy Storage Infrastructures Energy storagestorage isis anan essentialessential phasephase ofof harvestingharvestingting energyenergyenergy afterafterafter thethethe conversionconversionconversion phases.phases.phases. SomeSome suggestions ofof energy energy storage storage for for road road energy energy harvesting harvesting systems systems include include super capacitors, super capacitors, big batteries big andbatteries hydraulic and energyhydraulic storage. energy In the storage. latter case, In the the energylattertter case,case, is stored thethe inenergyenergy the form isis ofstoredstored mechanical inin thethe hydraulic formform ofof energy,mechanical which hydraulic will be energy, converted which to electrical will be converte energyd once to electrical a threshold energy is passed. once a threshold These facilities is passed. are oftenThese additional facilities are components often addition of energyal components harvesting of systems, energy taking harvesting place systems, eithersystems, within takingtaking the placeplace mechanical eithereither modulewithin the or asmechanical a separate module module or by as the a sideseparate of the mo road.dule Figure by the 20 side shows of the examples road. Figure of these 20 facilities shows fromexamples various of these references. facilities from various references. Sustainability 2020, 12, x FOR PEER REVIEW 18 of 27 Moreover, a new approach addresses a crucial point about the existing energy storage infrastructure, which is to have components which are more mechanically compatible with the rest

Sustainabilityof the structure.2020, 12 In, 6738 this regard, a newly developed approach is to build energy storage composites18 of 27 based on building materials and composites.

(b)

(a) (c)

FigureFigure 20. 20.Examples Examples ofof energyenergy storagestorage devices. ( a) A A hydraulic hydraulic tank tank as as storage storage device. device. Reprinted Reprinted with with permissionpermission from from [ 86[86].]. ((bb)) TheThe energyenergy storagestorage device within a a me mechanicalchanical harvester harvester module. module. Reprinted Reprinted withwith permission permission fromfrom [[94].94]. ((cc)) AA samplesample energyenergy storagestorage componentcomponent made made of of geopolymer geopolymer (building (building material).material). ReprintedReprinted withwith permissionpermission from [[95]95]..

Moreover,In a recent a study, new approach Saafi et al. addresses developed a crucial and te pointsted a about potassium the existing geopolymer energy cement storage paste infrastructure, capable whichof serving is to haveas a capacitor components and whichsensor are(Figure more 20). mechanically The power density compatible of the with paste the was rest measured of the structure. to be Inalmost this regard, 0.33 kW/m a newly2, discharging developed in approach around 2 ish. toBased build on energy density storage functional composites theory (DFT) based simulations on building materialsand experiments, and composites. the performance of this paste as a capacitor depends on the diffusion of cations of K and,In atherefore, recent study, when Saafi the eteffective al. developed thickness and of testedthe samples a potassium increases, geopolymer there are cementmore K+ paste ions capable in the ofmatrix, serving making as a capacitor the sample and work sensor better (Figure [95]. 20). The power density of the paste was measured to be almostMoreover, 0.33 kW/m Zhang2, discharging et al. developed in around a structural 2 h. Based su onper density capacitor functional based on theory cement (DFT) and simulationsgraphene. andSuper experiments, capacitors thewere performance made by making of this paste thin assheets a capacitor of cement depends paste onwith the water diffusion to cement of cations ratios of K and,starting therefore, from 0.3 when up theto 0.5. effective Once thicknesscured, they of were the samples sandwiched increases, between there the are sheets more of K+ conductiveions in the matrix,graphene making electrodes. the sample The results work bettershowed [95 that]. the samples’ specific capacitance peaks at a water to cementMoreover, ratio equal Zhang to etalmost al. developed 0.4 and decreases a structural after super that capacitorand also as based the hydration on cement proceeds. and graphene. The Supermaximum capacitors capacity were (almost made by19.5 making F/g) was thin seen sheets for ofthe cement samples paste with with a 0.4 water water to to cement cement ratios ratio starting at the fromage of 0.3 three up todays 0.5. [96]. Once cured, they were sandwiched between the sheets of conductive graphene electrodes.Meng Theet al. results conducted showed one that of the earlier samples’ works specific in this capacitance field. They peaks developed at a water a cement to cement matrix- ratio equalbased to battery, almost 0.4using and manganese decreases afterdioxide that particles and also for as thethe hydration cathode, zinc proceeds. for the The anode maximum electrode capacity and (almostcarbon 19.5black F/ g)dispersion was seen in for both the as samples the conductive with a 0.4 additive. water to The cement electrolyte ratio at of the the age battery of three is the days pore [96 ]. solutionMeng in et the al. cement conducted paste, one placed of the earlierbetween works the el inectrodes. this field. The They experimental developed aresults cement of matrix-basedthis battery battery,showed using a 0.72 manganese V open-circuit dioxide voltage, particles a maximum for the cathode, current zincof 120 for μ theA and anode a highest electrode power and output carbon density of 1.4 μW/cm2 black dispersion in both [97]. as the conductive additive. The electrolyte of the battery is the pore solution Moreover, Adam et al. developed a thin-film capacitor ideal for light structures, with a specific in the cement paste, placed between the electrodes. The experimental results of this battery showed capacitance of 35.98 μF/g, maintaining 1500 cycles of charging and discharging. In another study, a a 0.72 V open-circuit voltage, a maximum current of 120 µA and a highest power output density of multifunctional energy storage laminated composite was built, giving a maximum specific capacity 1.4 µW/cm2 [97]. of almost 7.4 F/g [98–100]. Moreover, Adam et al. developed a thin-film capacitor ideal for light structures, with a specific capacitance of 35.98 µF/g, maintaining 1500 cycles of charging and discharging. In another study, Sustainability 2020, 12, 6738 19 of 27 a multifunctional energy storage laminated composite was built, giving a maximum specific capacity Sustainability 2020, 12, x FOR PEER REVIEW 19 of 27 of almost 7.4 F/g [98–100]. CompressedCompressed air air energy energy storage storage (CAES) (CAES) is another is another newly newly developed developed plan for storingplan for and storing transferring and energytransferring on large energy scales. on This large system scales. is usually This system adopted is for usually wind turbineadopted harvesters. for wind However, turbine harvesters.considering theHowever, potentially considering large amounts the potentia of energylly large generated amounts by of electromagnetic energy generated road by energyelectromagnetic harvesters, road this storageenergy planharvesters, can also this be storage a suitable plan match can also for thesebe a su harvesters.itable match Figure for these 21 shows harvesters. a schematic Figure diagram21 shows of thisa schematic system and diagram an illustration of this system of the and storage an illustration plan. of the storage plan.

(a)

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FigureFigure 21.21. ((aa)) CompressedCompressed air storage system main main parts. parts. ( (bb)) Illustration Illustration of of an an adiabatic adiabatic air air storage storage system.system. ReprintedReprinted with permission from [101] [101]..

4.4. ComparisonComparison of Energy Conversion Systems, Their Their Applications Applications and and Challenges Challenges ConsideringConsidering the energy harvesting techniques discussed discussed in in the the previous previous sections, sections, one one of of the the notablenotable didifferencesfferences betweenbetween themthem is their power outp output.ut. Electromagnetic Electromagnetic harvesters harvesters (apart (apart from from their their medium)medium) usuallyusually have high power output, rangin rangingg between between almost almost 15 15 W W and and 600 600 W W in in different different references.references. ThisThis powerpower output is sufficient sufficient enough enough for for powering powering street street lighting, lighting, facilities facilities and and wireless wireless connections.connections. However,However, thethe application of this technology technology in in roadways roadways is is limited limited because because it it influences influences thethe tratrafficffic flow,flow,requires requires maintenancemaintenance andand isis relativelyrelatively new.new. PiezoelectricPiezoelectric energyenergy harvesters harvesters are are another another method method of harvestingof harvesting energy energy from from roads. roads. Their Their power outputpower isoutput significantly is significantly lower than lower electromagnetic than electromagnetic systems, and systems, their fabrication and their and fabrication maintenance and are relativelymaintenance expensive. are relatively However, expensive. they can However, directly they be placedcan directly in the be pavement placed in system the pavement (in a protective system case),(in a protective and, unlike case), some and, other unlike methods, some their other performance methods, their is not performance dependent onis not the weatherdependent conditions. on the Althoughweather conditions. their power Although output is their low, power they can output be utilised is low, in they remote can sensorsbe utilised and in systems remote whichsensors require and micro-levelsystems which power. require micro-level power. ThermoelectricThermoelectric and photovoltaic cell cell harvesters harvesters are are also also devices devices which which can can be be installed installed in in civil civil structuresstructures includingincluding roadroad pavementspavements for energy harv harvestingesting purposes. purposes. They They are are similar similar to to each each other other inin termsterms ofof beingbeing dependentdependent on the weather condit conditionsions and and the the climate climate of of the the region, region, which which limits limits theirtheir application.application. They also have low power outp outputut compared compared to to electromagnetic electromagnetic systems. systems. Both Both of of them are usually easy to install and integrate into the road structure and are cheaper than piezoelectric harvesters. However, similar to piezoelectric harvesters, the power output of these harvesters is also low, which makes them suitable for low power sensors and devices. Table 1 describes the positive and negative features of each studied technology. Sustainability 2020, 12, 6738 20 of 27 them are usually easy to install and integrate into the road structure and are cheaper than piezoelectric harvesters. However, similar to piezoelectric harvesters, the power output of these harvesters is also low, which makes them suitable for low power sensors and devices. Table1 describes the positive and negative features of each studied technology.

Table 1. Advantages and disadvantages of the major road energy harvesting technologies.

Technology Advantages Disadvantages

– Good mechanical properties – Very low power output Piezoelectric – Easy installation – Expensive fabrication and maintenance – Performance not dependent on weather

– Very low power output Thermoelectric – Easy installation – Expensive fabrication and maintenance – Dependent on the weather and temperature

– Dependent on the weather – Expensive fabrication and maintenance Photovoltaic – Average power output – Fragile and not mechanically compatible with roads – Relatively low power output

Electromagnetic – High power output – Influencing traffic flow – Relatively low maintenance cost

4.1. Cost-Beneﬁt and Technology Readiness Level Assessment (TRL) Besides the advantages and disadvantages of each technology, technology readiness levels (TRL) and cost-beneﬁt analysis are essential to classify and assess them. Originally developed and practiced by NASA for many years, TRL is a systematic measurement methodology used to appraise technologies’ maturity levels. It is categorised from 1, standing for having only the basic principles recognised and described, to 9, being fully developed, successfully tested and ready. The full description of each stage of TRL is shown in Table2[102,103].

Table 2. Description of each stage in TRL classiﬁcation, according to the European Commission [103].

TRL Level Description 1 Investigation of the fundamental principles of the technology 2 Technology’s concepts are formulated 3 The concept of the technology is experimentally proven 4 The technology is validated in the laboratory environment 5 The technology is approved in a suitable environment 6 The technology is exhibited in a suitable environment 7 The technology’s prototype is exhibited in an operational environment 8 The technology’s system is finalised and qualified 9 The qualified system of the technology is approved in the operational environment

Moreover, cost-benefit analysis which financially evaluates the technologies is another useful method to classify and compare them. However, it is worth mentioning that most of these mechanisms are still in their development process. Hence, their full and detailed financial appraisal, considering their long-term performance and their interactions with other pavement facilities, is still not available. Still, estimated cost analyses have been prepared, considering the costs of building the unit and its installation in the testing environment, and it is compared with the cost of energy provided by the conventional power plants, which is, in the US, for each Kwh, between $ 0.15 and $ 0.30, ignoring the environmental costs. Sustainability 2020, 12, 6738 21 of 27

Table3 shows the TRL and cost-beneﬁt (as described) analysis results of the developed road energy harvester units.

Table 3. Cost-beneﬁt and TRL analyses of diﬀerent mechanisms.

Mechanism Reference TRL Cost-Beneﬁt ($/KWh) [29] 4 ~187 Piezoelectric [17] 5 240 [104] 3 19.15–57.46 [55] 3 0.89 Thermoelectric [6] 3 2.31 [105] 4 19.8 Photovoltaic [70] 9 Not available Electromagnetic [77] 4 37.36

As reported in Table3, most of the energy harvesting technologies are still in the initial or median level of readiness, meaning that they are still in the development process. Most of them have not been installed or approved in their relevant environment (TRL < 5) and all of them generate more expensive power than fossil fuel power plants. It is worth mentioning that, although they produce more expensive power than conventional power plants, their power generation has less CO2 emission, which is a remarkable improvement considering the environmental costs of energy generation. Therefore, future studies are still needed to ﬁll this gap between the laboratory demonstration and their full operational mode in roads and eventually have more energy-eﬃcient cities.

4.2. Road–Harvester Interaction Challenges As mentioned previously, except for solar panels harvesters, none of the investigated technologies have been installed and performed on roads. However, in this section, considering the available harvesters from the literature, some future challenges have been recounted for each of them. In the piezoelectric harvester developed by Jasim et al. [3], the piezoelectric pieces have high mechanical properties (Young’s modulus = 63 GPa), which makes them a suitable option for bearing mechanical loads. However, these pieces are placed in a harvester module case, which is mostly a metallic box and eventually embedded in the depth of the road (almost 10 cm deep). While the elastic modulus of a mid-range aggregate asphalt pavement is 488.95 MPa [106], the elastic modulus of aluminium (an alternative for the material of the case) is 69 GPa. This significant incompatibility between these materials, especially between the asphalt and the metallic case, makes them perform differently under mechanical loads. Some additional cracking and degradation will occur on the surface and depth of the roads under operation. These materials also have different thermal properties, which can trigger thermal cracks on the pavements as well. Moreover, in the long term, the modules will also need maintenance. For example, fatigue damages due to the cyclic loads from vehicles is a common reason for the piezoelectric pieces breaking down and failing, in which case the piece should be replaced. In addition, the modules’ insulation against moisture penetration is a critical matter, which has to be checked and maintained. The thermoelectric road harvester which was developed by Datta et al. [55] has a Z-shaped piece made of copper which is embedded 2 cm in the road. Similar to the discussion of piezoelectric harvesters, the difference between the mechanical and thermal properties of the asphalt and the copper piece could trigger some cracks and rupture in the pavement in the long term. Since the copper piece is placed by the side of the road, the thermal cracks are more critical than the mechanical ones. Moreover, the TEG ceramics, which are also used in this study, are fragile pieces and once they break, they should be replaced. Therefore, maintenance of these systems, and specifically the TEG components, is crucial. In addition, this system works based on the temperature difference. While the heat is planned to be provided from the hot surface of the pavement, the cold sink is provided by a tank filled with water Sustainability 2020, 12, 6738 22 of 27 installed at almost 20 cm depth in the road. The water in the tank needs constant monitoring for its temperature and occasional refilling. Compared to piezoelectric harvesters, this type of harvester has better insulation. However, it still needs monitoring for potential moisture leakage. Solar pavements developed and implemented in the Solaroad project [70] are the most developed technology, with a TRL of 9. In this project, exclusively designed solar panels are placed on the surface of the biking lane. Following the performance of this project reveals that the cracks appearing on the surface of the road are critical and need maintenance. Furthermore, as the project was extended to develop a Solaroad bus lane, due to the irreparable damage to the panels, the project was stopped. Electromagnetic road energy harvesting systems are mainly developed as speed bumps, which are known facilities of roads, to increase safety. Therefore, they have an established form of interaction with the pavement, in terms of mechanical performance. As in the work by Todaria et al. [94], the conversion and storage components of the system are all within the speed bump structure placed under the cap, and if one of these parts required repair, the whole system would be affected. The performance of the suspension system (springs and dampers) is crucial as it is constantly under dynamic loads. They need regular monitoring, especially for fatigue. The whole speed bump harvester should be monitored for corrosion or other erosive phenomena. Moreover, the speed bump structure should also be well-insulated, especially against moisture and liquid seepage.

5. Conclusions and Future Perspective In this paper, the available technologies of energy harvesting from roads and their related infrastructures are summarised. Harvesting technologies of piezoelectric systems, thermoelectric systems, solar panels and electromagnetic modules have been explained, and the recent studies conducted on them have been discussed briefly. Moreover, to address the fundamental issue of energy resources which was initially mentioned, special attention was paid to the experimental and fieldwork outcomes of these works in terms of their efficiency and energy output. Accordingly, comparing the energy output of the systems, electromagnetic-based systems outperform the rest of the technologies significantly, by an average of least 15 W. Most of these electromagnetic harvesters were designed within the structure of road speed bumps, which are fundamental devices in roads to lower vehicle speeds and improve roads’ safety. Therefore, for future research, improving these systems in terms of their mechanical efficiency and their maintenance is recommended in order to develop more sustainable infrastructures. Another promising topic in this field is developing building materials which are capable of energy harvesting. In particular, cement-based piezoelectric and thermoelectric and energy storing composites are recently developed topics which have high potential for further improvements and future applications. Although their power output is relatively negligible compared to electromagnetic systems, their high compatibility with their host environment (the same material) is a significant advantage over all other systems. Finally, based on the TRL and cost-benefit analysis results, it is shown that most of these technologies are still in their laboratory demonstration stage, and their power generation is still more expensive compared to the economic cost of fossil fuel power generation. Future research is demanded in order to address these issues and develop and prepare these systems for actual real-field operations.

Author Contributions: Conceptualization, N.Z. and M.S.; investigation, N.Z.; writing—original draft preparation, N.Z.; writing—review and editing, N.Z. and M.S.; supervision, M.S.; project administration, M.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript. Funding: This research is funded by SAFERUP! from the European Union’s Horizon 2020 research and innovation program, under the Marie Skłodowska-Curie grant agreement no. 765057. Acknowledgments: This project is affiliated with the SAFERUP! network, aiming to research “Sustainable, Accessible, Safe, Resilient, and Smart Urban Pavements”. Conflicts of Interest: The authors declare no conflict of interest. Sustainability 2020, 12, 6738 23 of 27

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