Energy Harvesting from Pavements

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ment of the pavement by traffic and to solar energy converted directly to ... things about solar energy are that it is an assured source of energy for the foresee-.
 

               

Published in: Climate Change, Energy, Sustainability and Pavements, 2014 Editors: Gopalakrishnan, Kasthurirangan, Steyn, Wynand JvdM, Harvey, John (Eds.)

Energy Harvesting from Pavements  

  Andrew Dawson1, Rajib Mallick2, Alvaro García Hernandez 3, Pejman Keikhaei Dehdezi 4  

   

Abstract  

 

         

 

Against a background of the immense solar radiation incident with available pavement surfaces, the opportunity for energy to be harvested from pavements is investigated. While the emphasis is on the harvesting of solar-derived heat energy, some attention is also paid to the collection of energy derived from displacement of the pavement by traffic and to solar energy converted directly to electricity via photovoltaic systems embedded in pavements. Basic theory of heat collection is covered along with a discussion of the relevant thermal properties of pavement materials that affect heat transmission and storage in a pavement. Available technologies for pavement energy harvesting are reviewed and some of their advantages and limitations reviewed. The chapter continues with some descriptions of the ways in which the harvested energy can be stored and then used before ending with sections on evaporative cooling of pavements and system evaluation.

 

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A. Dawson, R. Mallick, A. García, P. Keikhaei Dehdezi

  1.1 Introduction  

 

With the increase in world population and industrialization, there has been a continuous increase in consumption of energy. Therefore, this important question is raised; will fossil energy resources (i.e. coal, oil, and gas) in the future account for the energy needed to survive and develop? Although opinions differ as to when fossil fuels will be depleted, there is no doubt that supplies are limited. Oil, one of the most consumable types of fossil fuels, is being consumed about one million times faster than it was made (Armstrong and Blundell, 2007). In addition, environmental pollution is a serious threat to vegetation, wild life, and human health. Generating energy from fossil fuels increases the level of carbon dioxide into the upper earth atmosphere and causes anthropogenic climate change; an acceleration of the ‘greenhouse effect’ (Armstrong and Blundell 2007). The depletion of oil reserves, the need to arrest global warming, climate change, or ozone layer depletion caused by the combustion of fossil fuels, all mandate new thinking from all those with concerns for the future. Hence, governments and industries everywhere are striving, more than ever, to capture, harvest and generate energy in every possible way by discovering new potential energy supplies and reservoirs, and developing innovative technologies to extract the energy available from them. In terms of harvesting renewable energy, there is none more researched than solar energy. The two most attractive things about solar energy are that it is an assured source of energy for the foreseeable future, and it is omnipresent on any exposed surface on the earth during daylight hours. To make the capture and harvesting of this energy feasible, the solar radiation needs to be of a minimum intensity for sufficient period of time during the year. While there are many areas of the world that are blessed with such sunshine, harvesting technologies need to be of sufficient surface area to capture a meaningful amount of solar radiation. The most commonly used technology – photovoltaic cells – includes cells that themselves have a significant environmental footprint (estimated energy payback times (energy generated/energy consumed to make and deliver) between 0.7 and 4 years and carbon footprints of between 15 and 38g CO2-equivalent per kWh generated) (de Wild-Scholten, 2003). For these reasons it would be attractive to find an existing common material, existing in all parts of the world, of significant surface area that would be exposed to sunlight all year round and that has the ability to “hold” the energy in the form of “heat” that can be extracted. Pavements - asphalt and concrete, are such materials. They cover millions of square kilometers all over the world and are exposed to the environment throughout the year. As an example, considering the total paved surfaces of 158,000 square kilometers in the US, then an average of 4.8 kilowatt-hours (kWh) of incident solar radiation per square meter per day means that here are 758 terawatt-hours (TWh) of solar energy per day that is incident on pavements. These pavement surfaces, because of their relatively high absorptivity (and hence low reflectivity) and low conductivity, absorb a significant amount of

 

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  this radiation (as much as 80-90% of the energy reaching the earth’s surface – see Figure 1.1) and then hold it as heat energy. Hot pavement surfaces, especially during warm weather, are common observations in most places of the world and are implicated in contributing to the Urban Heat Island (UHI) effect. Their surfaces emit that stored heat, particularly in evenings, leading to increased temperatures of adjacent buildings, use of more cooling energy, and hence depletion of fossil fuels, with consequent CO2 and particulate emissions (Wong & Chen 2009). Furthermore, hot pavements are more likely to experience structural and functional failures sooner, thus requiring more frequent maintenance. Moreover, rutting is a major temperature-related distress in asphalt pavements that occurs as a result of high temperature. Hence this heat absorption leads indirectly to greater consumption of natural resources and results in more harmful emissions that contribute towards climate change (Figure 1.2). Collecting heat from the pavement could reduce the UHI effect and rutting potential of the asphalt pavement (Mallick et al. 2009; Wu et al. 2011).  

 

   

Figure 1.1 Albedo for a range of pavement types in Phoenix, AZ (Cambridge Systematics, 2005). Notes: Albedo is the proportion of incident energy reflected, thus 100 x (1-Albedo) gives the percentage absorbed; HMA = Hot mix asphalt; UTW = Ultra-thin whitetopping (concrete)

If this heat energy were to be extracted, two principle benefits would be obtained:

                                             

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A. Dawson, R. Mallick, A. García, P. Keikhaei Dehdezi

Figure 1.2 Effects of high temperature in pavements (James 2002, Akbari, 2005)  

   

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The energy could be used beneficially, allowing a reduction in fossil fuelderived energy The energy would be removed from the location where it is currently causing a problem (both to the pavement and to the UHI)

To give some context to the amount of solar radiation incidence on pavements, 758 TWh as mentioned above, consider that there are a little over 300 million households in the US using, on average, about 30 kWh of energy per day – a total of about 9 TWh. Thus it is apparent that the complete household consumption of energy in the US could be provided from pavements if only 1.2% of the solar energy incident on the pavement could be captured! Also consider: 1. the fact that there is no need to set up a collector system to capture this energy, (although there is a need for a system to “harvest” it), the system already exists and is functioning as part of the transportation network! Indeed, as Figure 1.1 shows, pavement surfaces are among materials with low albedo (i.e. they don’t reflect the energy back into the atmosphere well), so they are relatively efficient at energy collection without any special treatment. 2. that no additional material is needed for the solar “collector” and this means an avoidance of a significant amount of energy, money and time that are involved in the manufacturing process.

 

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  3. because the heat is retained inside the pavement, it could be used even at night, when the demand for energy could be significant and maybe even higher than that during the daylight hours.  

Due to the nature of solar energy, two components are required in order to have a functional solar energy system; a collector and a storage unit. These are required whether a PV system is considered or, as here, a pavement. In any solar energy system, the collector simply receives the radiation that falls on it and converts it to another form of energy, such as electricity in a PV system, or in the case of a pavement, converts it to heat. The beauty of a pavement solar energy system is that, properly considered, the pavement not only provides the collector but can also act as an energy storage unit due to its large thermal mass thereby largely overcoming the non-constant nature of the supply of solar energy. Of course, there are many barriers to practical harvesting, among which are the following: 

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consumers of the energy may not be close to the pavement giving energy conveyance problems, particularly in rural areas. A considerable proportion of the 158,000 km2 of pavement mentioned above will have this problem, constructing harvesting arrangements that don’t negatively impact the pavement from delivering its primary function, carrying traffic, will not be a trivial issue, maintenance of the harvesting arrangements and of the pavement must be achievable without disrupting the other, conventional pavement construction sequences, materials and plant may not be best suited to installing energy harvesting equipment, in the developed world most pavements are already constructed so harvesting systems would usually need to be retro-fitted and this will probably be difficult/expensive/disruptive, initially, the durability of pavements equipped with energy harvesting arrangements won’t be understood very well, causing planning difficulties for pavement managers, as with all solar systems, the energy abstractable is weather and time-ofday dependent so consumers will almost certainly require an alternative source as well, decreasing economic efficiency.

Nevertheless, in the light of the very significant opportunity, even at low efficiencies, as mentioned above, extraction of heat energy from pavements is very likely to be a worthwhile effort! It might be even more attractive in developing countries where pavement infrastructure is developing rapidly and where energy consumption rates are lower at the moment but increasing rapidly (so demand for new energy sources is extremely strong). Furthermore many of those developing countries are solar-gain-rich.

 

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A. Dawson, R. Mallick, A. García, P. Keikhaei Dehdezi

  A few companies have been installing systems for the last decade or so, but, overall, given the barriers listed above, we can anticipate slow take-up of pavement energy harvesting technology. Furthermore there will be need for significant research efforts to address these, and other, barriers. For this reason most experience to date is restricted to academic research, yet research and practical implementation is rapidly growing all over the world!  

   

1.2 Energy Incidence with Pavement Surface  

 

 

As can be seen from Figure 1.3, approximately, half of the world’s incoming solar energy is absorbed by the earth’s surface (Russell, 2007). Most obviously, the sun provides solar energy to our planet’s surface at a rate of about 100 000 TW; therefore, the energy from one hour of sunlight is approximately equivalent to all the energy mankind currently uses in a year (Armstrong and Blundell 2007). When this energy is incident with a pavement it, inevitably, heats the pavement. Most likely we have all experienced hot pavement surfaces in the summer months. But what are the factors that control how hot the pavement will get? As Figure 1.4 shows, the energy balance in pavements involves five factors: 1. Solar radiation (also known as irradiance) 2. Absorption and reflection 3. Conduction 4. Convection, and 5. Thermal radiation. The pavement absorbs (and partly reflects) the solar radiation. The absorbed heat is partly conducted down through the pavement’s layers and partly accumulated in the material. Denser pavements can accumulate more energy than porous pavement (as they almost always have a higher specific heat) and the speed at which an asphalt (or other) mixture loses heat depends on the thermal conductivity of the mixture (a function of the thermal conductivity of its individual compounds). The surface is cooled down by the effect of wind that blows air at lower temperature to the pavement surface, by the infrared radiation emitted to the space by the hot pavement and by the convective losses that happens when air in contact with the surface of the pavement heats and moves away from the surface (Figure 1.4). Porous pavements have greater surface areas in contact with air than dense pavements, and thus will experience higher convective losses. Furthermore, porous pavements may accumulate water moisture, which accelerates the heat loses as it evaporates (see Section 1.8). For this reason, porous pavements are recommended to reduce the widely researched Urban Heat Island (UHI) effect, through which the near surface air temperatures in urban areas remain at a higher level than that in rural areas during nighttime.

 

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                                                  Figure 1.3 Solar energy budget (NASA)

 

 

From this model of energy balance, it is obvious that the greater is the surface absorptivity (and hence the lower is the reflection), and lower is the thermal conductivity to underlying layers, then the greater is the amount of energy that is “captured” and “retained” by the pavement. In general, asphalt pavements, primarily because of their dark surface color, have lower reflectivity (also known as albedo) (