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Thermal Energy of Asphalt Pavements1

Lajos Kisgyörgy2, Balázs Plesz

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ABSTRACT

The surface of roads and parking places could be regarded as a low-cost solar collector

system. Using road pavements as alternative energy sources has several advantages.

Renewable energy could be harvested, and the effective life span of the pavement structure

could be extended by cooling down the hot pavement. By the reduction of the heat radiating

from the road surfaces, the air temperature of densely built urban areas could be decreased as

well during the hot summer days.

The utilization of road pavements as alternative energy sources is a new and interesting

topic. Currently some introductory implementation happened, mainly to cool down the

pavement and to produce hot water for the premises beside the road. In some cases the energy

gained in summer was used to prevent icing on the road in winter.

The following questions should be answered to create an optimal technology:

• Which is the most effective way to harvest the heat energy of pavements?

• How the roads and parking places could be built to maximize their heat absorbing

abilities?

• What are the operational and maintenance issues?

• What road pavement maintenance technologies could be applied?

We have analyzed the following three basic approaches in detail:

• application of circulated heat transporting liquid

• integrated thermoelectric block traversing the courses of the pavement

• cascading thermoelectric elements between the pavement courses

Among them the system of circulated heat transporting liquids proved to be

implementable in practice. At the creation of the technology besides the potential amount of

the harvested energy the implementability and the long-term operability were also significant

factors.

The result of the research is a technology which makes it possible to harvest the heat

energy of road pavements economically and to increase the life-span of the pavement

structure.

Keywords: asphalt, thermoelectric generator,

1 Kisgyörgy, L., Plesz, B. (2014) Thermal energy of Asphalt Pavements; Magyar Építőipar (ISSN: 0025-0074)

64: (1) pp. 3-7. 2 Dept. of Highway and Railway Engineering, Budapest University of Technology and Economics, Hungary

3 Dept. of Electron Devices, Budapest University of Technology and Economics, Hungary

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INTRODUCTION

One of the possible uses of renewable energy is the application of solar thermal

collectors where the harvested energy could be used for heating. In summer the temperature

of the surface of asphalt pavements can rise to 55 – 65°C which can cause plastic deformation

in the pavement. Harvesting the thermal energy of the pavement has several advantages: a

renewable, green energy is gained and the plastic resistance of the asphalt pavements could be

increased.

One of the generally mentioned disadvantages of the solar collector is that the collector

surfaces are visible and can cause aesthetical problems. From this aspect the application of

asphalt pavement as solar collector has pronounced advantages. With its large surface it far

substitutes the more efficient tubular solar panels, the created tube system can remain in a

“hidden” form. The asphalt’s other advantage is in the longer “operating time” due to its heat

reservoir ability, as we can obtain heat even after the sun goes down. System operation is

favourable from the aspect of asphalt pavement too, because the heat - detracted during the

summer time warm-up – decreases the possibility of plastic deformation, in the winter time

we can rewind the ‘detracted” heat, which decreases the cryogen stress of the pavement, such

as the possibility of cracks in the paving, furthermore it can make the melting by salting

unnecessary.

In view of the above mentioned it’s not accidental that several Universities in the USA

and the dutch company named Ooms in Europe deals with the utilization of heat energy

nascent in the asphalt. In spite of it the utilization of surface pavements as energy source is a

brand new task, only initiative steps occurred at international level, they primarily brought the

summer cooling into focus. In the extant cases the energy nascent in the asphalt was

principally used for water heating. The primary purpose was the reduction of the surface

pavement’s temperature, or rather the wintry, de-icing utilization of the energy exploited in

the summer time. Only one research dealt with the technical application of thermoelectric

generators, here the goal was also the cooling of surface pavement [1-3].

INSPECTIONS OF ASPHALT MIXTURES

The purpose of our examination is the determination of different content asphalt

mixtures’ thermal conductivities and the analysis of the content’s effect. We executed the

measurements, determining the asphalt’s thermal conductivity in all cases at three different

temperatures. According to the measurements the thermal conductivities determined at

different temperatures fall upon one direct, this facilitated the construction of calculation

model dramatically.

Based on the evaluation of test results we can say that mainly the bitumen content and

the voids content determine the thermal conductivity factors (Figure 1)

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Figure 1: Relation between thermal conductivity and voids content

PAVEMENT OPTIMAL IN THE ASPECT OF ENERGY EXPLOITATION

The thermal conductivity measurements executed with the different asphalt mixtures

closed with the adequate result from the aspect of surface pavement construction. It’s

essentially definable, that although there are variations according to the content – stone

material type and quantity - , their effect is smaller correlating to the voids content. This can

be perceived to be lucky, because from the aspect of pavement the advantageous case is if the

asphalt layer’s voids content decreases towards the pavings’ surface.

In case of mixes the asphalt concrete wearing course is 2,5 – 4,5 vol%, while in case of

SMA mixture the applicable voids content is between 2,0 – 4,5 vol%. Considering the asphalt

mixtures’ other properties the voids content around 4 vol% is the generally used value. In case

of binding courses the specification orders a higher gap between 3,0-5,5 vol%.

Taking the performed tests and the above regulations into consideration we have to fix

the following statements regarding the applicable asphalt mix types and content properties:

• All versions of AC 11 and AC 16 can be built as wearing course.

• From the SMA mixtures only the SMA 11 can be applied because of the installation

thickness.

Voids content of wearing courses has to be chosen in an interval between 3,5-4 vol%.

Lower voids contents are dangerous mainly due to the asphalt mixtures’ plastic deformation

habit. Considering that the asphalt layer – due to the heat-removal – is continuously under

“cooling”, application of 2,5-3 vol% seems to be possible, which is more favourable from the

aspect of heat conduction. The application possibility of versions with low voids content was

not testable between laboratory conditions, so that can be/must be examined by large model

test.

It is worth to note that from the aspect of voids content the application of wearing

course with grain size dmax = 8 mm would be the better solution. This possibility can be

examined during the installation experiments, considering that the pipe allocation requires at

least 50 mm thick installation, which is not possible in case of wearing course with grain size

dmax = 8 mm according to the standards.

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Mastic asphalt mixtures with voids content 0 vol% do not fall out from the utilization

circle totally. Its primary reason is the application possibility not in case of surface

pavements, which we deal with in case of other utilization areas.

From the aspect of heat conduction certain dolomite types are favourable, but their

absorption capability and so the recoverable heat flow is smaller due to their light colour.

Taking the absorption and heat conduction properties into consideration the application of

andesite ballast stones and andesite- or dolomite crushed sand is the effective in case of

wearing course-mixtures. As the relative small difference nascent in the heat conduction can

be eliminated already with 0,5 vol% voids content decrease, so the application of basalt

ballast stone can be a solution too.

Binding course of pavements planned for heat extraction can be any of the asphalt

mixtures from the specification. In case of binding course mixes, considering the importance

of their mechanical properties, utilization of 4,5-5,5 vol% voids content is suggested under the

wearing course applied for heat extraction. This provides the lower heat-removal of the lower

layers, maximizing by this the energy recoverable from the wearing course.

In the content of binding course the basalt and andesite sand or their ballast stones can

be applied. Dolomite ballast stones can be used too, but in their case voids content falling into

the upper interval is suggested.

TRANSFORMATION OF THE EXTRACTED HEAT ENERGY WITH

THERMOELECTRIC GENERATORS

Utilization possibilities of the extracted heat energy

In our Research we dealt with the transformation potential of asphalt pavement’s heat

energy to electric power, applying thermoelectric generators for the task. In Hungary the heat

energy arriving to the roads is annual 1200 kWh/m2

an average on horizontal planar surface,

which means an annual 300 TWh if we consider the 240 km2 national public roads can be

found in the country. Put in context the average applied electric energy is an annual 50 TWh

in Hungary. According to these data it’s worth to examine the energetic utilization of lost

heat, even if the available small temperature-differences make only a moderate transformation

efficiency possible.

The method of energy-transformation

The thermo-electrical modules can be applied for the electrically forced delivery of

heat, or rather for electric energy production for the debit of heat flowing over them. Thermal

convection thanks to the Peltier-effect, while the electric power output is based on the

Seebeck-effect: the temperature-difference between the two points forms into electric power

with the aid of a semiconductor device applied as a thermo-electric generator (Figure 2). The

previous effect is mainly used in cooling/heating applications. In these applications

temperature-difference gets on between the two sides of cooling modules, through electric

power input. However there is a possibility for the modules’ converse utilization too and so

electric energy can be produced. This can be achieved by the application of temperature-

difference between the two sides of the module. Although the so-produced energy is slim and

the efficiency is low, useful energy can be won in such cases, when redundant heat source is

available.

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Figure 2: Thermoelectric generator’s basic structure [4]

Thermo-electric generators are used since many years for spaceships, furthermore oil-

and gas industrial telemonitoring stations’ energy supply. In the previous years this

technology became available for the public at large too, and the revolution of thermo-electric

energy began. Electric energy can be won out almost from any kind of lost heat by thermo-

electric generators, even from the heat of the body.

Construction of the system

In case of road pavements liquid circulated in single- or two-circuit system is applicable. In

the wearing- or binding course of the road there is a heat exchanger tube coil, in the ground

there is a heat sonde, in the summer time the previous one means the thermoelectric

generator’s warm heat tank, the latter one means its cold heat tank. Circulating pump(s),

expansion tank(s) and the devices suitable for energy extraction (for heating possibly)

(Seebeck- battery, thermocouple, heat engine) are allocated concentratedly, in the mechanical

area based beside the road or into the soil (Figure 3). At the extant roads only the wearing

course has to be taken up and relayed after laying the tubes. At new roads soil sampler can be

placed under the road, at old roads it can be placed next to the road. In wintry operation, for

keeping the roadway frostless it can be enough to overcirculate the thermal convection agent

through the heat sonde based in the soil, as the temperature in a few meter depth is

sufficiently above the freezing-point. Installation of the mechanical area is suggested under

the ground because it ensures higher protection in front of the environmental and human

effects too. System has to be built up so that fittings sensitive possibly for all temperature

(pump, expansion tank, check valve, emergency valve, etc.) have to be placed in a branch

with temperature approaching the temperature of soil sampler after the heat exchanger (Figure

4).

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Figure 3: Single-circuit circulated system’s principle scheme

Figure 4: Mechanical area’s principle scheme

The single-circuit system’s mechanical area consists of the followings:

The pump circulating thermal convection agent is allocated on the pipe incoming from

the soil sampler, so it suffers less from the daily and annual heat variation.

One thermocouple placed between the liquid arms overflowing in the two directions

makes possible the transformation of temperature-difference to electric energy by the

Seebeck-effect. During the winter time, if we want to use the heat - took up through

the soil sampler with the aid of the system – for the road’s malleabilization this device

is totally disconnectible or applicable for assistant heating.

The electronical block provides the control of the pump, the communication and

telemonitoring, furthermore the transmission of the produced electric energy.

System applying the circulated liquid has to be filled up with antifreezing heat conveyor

liquid because of the continuous annual consumption, like in case of solar collectors. It’s

important, that for this purpose only non-toxic antifreezing agent can be applied, because if

the heat exchanger holes out, then the antifreezing agent can get into the natural water system.

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In the collector systems we generally use propylene glycol-water mixture, which is non-toxic

and furthermore totally biodegradable, does not load the environment either.

As the TEG devices available in trade are typically a few mm tips, it makes their terrain

maintenance easy if we arrange them into modules. The concentration of the plan’s

mechanical system accounts for this, as the heat efficiency accumulating there is able to drive

a lot of specimens from such TEG devices. Figure 5 shows the construction of TEG-unit. The

module consists of one pipe with thin wall and good heat conductivity (Figure 5, B), and

TEG-s placed on its periphery (Figure 5, T). This is surrounded by an outward pipe or tank

possessing with liquid circulation, which provides the constant temperature cold junction

(Figure Figure , K). In the inner pipe we circulate the warm water extracted from the

pavement, while cold water (extracted from the soil) gets into the outward pipe.

At the determination of the TEG-unit length we consider that tube-length, which results

10 °C temperature-difference at the output besides a given rate of flow, becasue according to

the characteristic TEG does not produce electric energy under this temperature practically.

Further parameters - have to be sized from energetic aspect - are the length of tube coil

belonging to one TEG unit and the rate of flow of the liquid inside. More tube coils can be

bonded to one TEG module parallely. We supposed steady-state during modelling and we

took some environmental parameters average in Hungary into consideration. So we perceived

the soil temperature in large depth (where we can find the heat sonde) to 12 oC, the cooling

temperature of the roadway to 20 oC, the input power density of sunlight to 600 W/m

2.

The outlet liquid’s temperature does not depend on the time spent in the tube coil

linearly, but it shows a relation decaying exponentially. It’s not worth to apply too short tube

coil, as this time the distinct temperature would decrease dramatically (large curve gradation

under 40 °C), which is unfavourable regarding the characteristics and the non-linear

temperature-dependence of TEG jointed in the system. Too long tube coil is not favourable

either, because above 50 °C the curve starts to flatten out, so the enlargement of tube-length is

not in commensurate with the achieved temperature-growth. Accordingly an outlet water

temperature between 40 °C – 50 °C is suggested.

Figure 5: Construction of TEG unit.

Modules T TEG can be found along the cylinder placed between the walls F with good

heat conductivity of diameter d inner B and outer K volume. In the inner volume we circulate

the warm agent extracted from the asphalt, while the cold agent (from the soil) gets into the

outward one.

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For the TEG-units with length choosen at sizing we executed the simulations for

different rate of flow. The optimal tube coil-length depends on the speed of circulation too.

Calculations demonstrate that application of low rate of flow is practical due to the road

pavement’s relatively small thermal efficiency-density, so we can achieve a larger electric

efficiency and better harnessing of TEG devices.

Recoverable amount of energy

On behalf of the conditions of practical realization and the system’s larger efficiency

it’s worth to optimise the system to an inlet temperature 40 °C and a rate of flow 0,1 m/s.

Figure 6. sums up the electric outputs cumulated along the length of TEG unit beside these

parameters.

The amount of energy recoverable from the road surface depends on the quantity of

solar radiation reaching the road, so the geographical situation of the road affects the

recoverable amount of energy significantly.

Figure 6. Cumulated electric outputs.

INSTALLATION ISSUES

Tube coil raw material

Tubing of the roads brings up several questions. In order to implement an operation-safe

system with a long life cycle, careful and accurate planning and execution are required. Tubes

must be made of cheap and recyclable materials and they must endure mechanical stress and

temperature extremities. The pavement’s operation temperature varies between -20 0C and 80

0C and the temperature during construction can be as high as 150-180

0C.

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The pipe system intended to be built under the pavement is to ensure both low and high

temperatures. Provided that the temperature will not exceed 95 0C during construction and

operation PVC (plastic) tubes can also be applied. Above this temperature other materials are

to be chosen. Copper tubes used by solar systems can be a proper alternative but applying

them might easily exceed the budget. Steel tubes must be made of plated steel due to its

corrosion-proof property. Aluminium is not suitable as it is a highly deformable metal. On top

of these, some complex types of plastic can provide further options.

Heat extraction out of the asphalt is basically the reversed process of the wall heating

and furthermore in the winter “melting” operation heating is practically going on. In

accordance PEX or PE-xc reticulated PE-tubes can be best applied. Due to high temperature

installation of diecast asphalt types, it is recommended to use copper tubes but special plastic

types can also be applied.

Installation of heat conductive tube coil

The tube coil is recommended to be built in the pavement with a 15-20 cm spacing and

connection to the collective tubes is recommended at every 10-15 metres. This is benefitial

from the maintenance prospective because in case of any reparation, only this length (or twice

this length in worst case) of the road will be concerned. In order to achieve the best possible

sustainability, 50-60 cm tubeless sections are to be installed. This way any necessary

wrecking or milling can be managed without damaging the neighbouring tubes.

The installation depth highly depends on the temperature and the tension generated in

the pavement. The tube coil – if its material can endure the load of the pavement – can be

directly fixed to the surface of the binding course. If this is not manageable then we have to

use the plastic frames generally applied for underfloor heating. In this case the plastic frame

absorbs the load from the paver (paver) and the vehicles transporting asphalt.

The input and output junctions of the tube coil can be connected with the collective-

distributive channels in the roadside bench. These channels transport the heated fluids to the

heat exchanger.

In the process of building the wearing course, the paver and the trucks transporting the

asphalt are to move on the tube coil which is fixed to the binding course. The tube coil must

bear the load of these. The paver’s load can be significantly reduced if we use a chain-track

one instead of one with rubber tyres. This is not too much of a compromise because the

pavers in our country are 50-60% equipped with chain-tracks.

In case the wheel-load of the trucks transporting the asphalt is huge and there is a risk of

deformation on the tube coil then the previously mentioned plastic frames are to be applied.

After the asphalt is spreaded, it is compacted with rollers, their load is distributed in the

asphalt layer already, so the tube coil suffers no major load.

Installation temperature of the asphalt mix

The hot asphalt mix is installed at different temperatures according to the type of the

bitumens applied. In case of normal bitumens this temperature is 150-165 0C, when applying

modified bitumens it is 170-190 0C. During the process of spreading – in the zone of the coils

– in depths close to the binding course it reduces to around 80 0C in 10-15 minutes due to the

heat-absorbing effect of the lower asphalt layer. So the applied coils must endure this

temperature. This aspect affects the selection of the coil type.

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In the recent years the association of road constructors has made developments to

reduce the energy consumption. Nowadays WMA (warm mix asphalt) types are also used in

our country although not in big volumes so far. The description in Hungarian “moderately hot

asphalt” reflects the difference from traditional ones. These asphalt types can be processed,

mixed and inserted at 30 0C lower temperature compared to the previously mentioned

spreading temperatures. Based on all other features and properties being the same as those of

the normal asphalt coatings, the usage of WMA asphalts does not create problems, however

thanks to the lower temperature the tube coil application will be safer.

CONCLUSION

It is important to know that nowadays green energy is less economical so far. The

previously presented system has a relatively long return on investment, compared to solar

cells. However we have to pay attention on renewable energy as it reflects the future trends

and it effectively reduces the size of our ecological footstep.

The system we have hereby examined is suitable to utilize the temperature of the road.

The return on investment of this system highly depends on the solar effect on the road surface

and also on the costs of certain system elements difficult to estimate so far. According to the

hypothetic calculations the research is definitely worth to continue and the construction of a

test road is ultimately recommended.

By constructing and continuously surveying a test road section we can set up the

fundamentals of accurate economic calculations. Based on these possible implementations it

can be decided.

REFERENCES

[1] Alternative Energy Hits the Road - Research at WPI Explores Turning Highways and

Parking Lots into Solar Collectors. [Online]

http://www.wpi.edu/news/20089/asphaltnews.html.

[2] Innovation in The Production And Commercial Use of Energy Extracted From Asphalt

Pavements. Sullivan, C., at al., 6th Annual International Conference on Sustainable

Aggregates, Asphalt Technology and Pavement Engineering : Liverpool John Moores

University, United Kingdom, 2007.

[3] de Bondt, A. H. és Jansen, R. Generation and Saving of Energy via Asphalt Pavement

Surfaces. The Netherlands, Fachbeitrag in OIB : Ooms Nederland Holding bv., 2006.

[4] www.tegpower.com [Online]