Solar Asphalt Collector Investigation

Laboratory Investigation into the Collection Efficiency of Various Pipe Materials and Depths in Solar Asphalt Collectors

Bachelor of Engineering (Honours)

Lecturer:

Dr Michael McLoughlin

Students:

Luke Molloy

Civil Engineering

Jason Corbett Frank Kenna Module:

1

Highway Design

Abstract

This paper outlines an experiment to investigate the response to solar irradiation of square slabs of asphalt concrete. The energy source was provided for by halogen bulbs to simulate solar irradiation in the laboratory. The effects of the light energy on the asphalt slabs was monitored by the insertion of 8mm copper tubing and 16mm plastic tubing at 3 different depths from the surface within the asphalt layer in which water was circulated at a constant rate of 1.5 l/min. The thermal energy transmitted to the slabs was collected by the circulating water.

The temperature differential was monitored by placing thermocouples on the inlet and outlet locations. Thermocouples were also inserted into the asphalt slab at 20mm incremental depths from, and including, the surface to monitor the response of the asphalt to the energy source and the circulating water. The results show that the asphalt pavement can be cooled down by the solar collector and thus reducing the heat island effect. This was most prominent in the 8mm copper pipe at a depth of 20mm from the surface, which also showed an energy absorption rate of 537W/m2 while being exposed to approximately 900W/m2 of light energy.

1

2

Introduction

Physical issues associated with asphalt pavements subjected to solar radiation, which can result in high surface temperatures, include structural damage due to hardening as a result of thermal cycles and the environmental phenomena known as the heat island effect (Bobes-Jesus et al. 2013). Rapid changes in temperature induce thermal stresses within the pavement causing changes to the visco-elastic properties and gradual deterioration of the pavement (Merbouh 2012) while it has been well documented that overheating asphalt bitumen can lead to oxidation and stiffening (Dessouky et al. 2011). The rate of oxidation is dependent on temperature, thickness of the bitumen layer and time of exposure.

The results of oxidation produce more complex molecules in the bitumen which make the pavement harder and less flexible and are the main cause of ageing (Read et al. 2003).

Asphalt solar collectors (ASC) were firstly developed for an under-road heating system, to prevent pavement icing and to melt snow on bridge surfaces in Japan.

The first system of automatic collection and storage of energy and its use to melt snow, called the Gaia System, is currently still in service (Shaopeng et al. 2011).

There are many benefits from taking advantage of this energy source which has the potential to be in abundance especially in urban areas where parking lots and pavements are a considerable percentage of total urban area. Nicolas and Eleftherios (2010) have, in a Swedish context and climate conditions, investigated the application of an ASC for heat capture and a ground source heat pump with borehole storage. The intention is to use that heat for domestic hot water and heating demands in F책gelsten, a newly planned residential building area. The performance is being compared to the common use case of district heating and standard ground source heat pump systems in Sweden.

Sustainable energy sources can be exploited from the installation of ASC as in 2006 when the Highways Agency in UK had commissioned a scoping study for the Transport Research Laboratory (TRL) to explore available methods and assess the possibility that renewable energy generation could be exploited within the 2

highway network. The findings of this study resulted in trials on an access road near Toddington UK, where the use of an inter-seasonal heat transfer system incorporating ASC and shallow insulated heat stores in the ground were conducted. Two solar heat collecting arrays each 5m by 30m installed at a depth of 120mm were used to collect the heat and the identical arrays, but at a depth of 875mm, were used for the heat store. The results of this experiment showed that 6.5MWh of heat energy were exchanged from the ASC to the ground storage array during the good summer of 2006. This translates to approximately 43kWh/m2. Air temperatures peaked at 34oC and temperatures of 50oC and 38oC were recorded at the road surface and in the underground storage array respectively (TRL 2007). This type of experiment includes environmental factors such as variations in sunlight intensity over time and ambient air temperatures which are affected by wind direction and velocity. It has also been argued by Gao et al. (2010) that the average heat collecting capacity computed was approximately 150 to 250 W/m2 in the sunny days where the selective test time was from 9:30 AM to 4:30 PM in August 2008, in Changchun in the northeast of China.

Laboratory experiments will not provide as accurate results if environmental conditions are not replicated. Laboratory investigations can be carried out to investigate the thermal response of asphalt pavements to solar radiation.

Shaopeng et al. (2011) used 20mm diameter copper piping in 300mm square asphalt slabs at a depth of 75mm from the surface where a laboratory irritation simulation test was performed to heat up the asphalt slabs. The effects of flow rate, time of start of collection and the initial temperature distribution of the slabs on the process of heat collection were all factors in the solar collector’s performance. The results show that the asphalt pavement can be cooled down by the solar collector and thus is good for reducing the effect of heat-island in a city, but the temperature gradients between the slabs’ surface and the pipe were noticeable.

There are many factors influencing the effectiveness of thermal transfer from slab to pipe and in turn to the circulating fluid within the pipe network. Test 3

results obtained by Mallick et al. (2008) from small-scale samples showed that the use of aggregates with high conductivity can improve the efficiency of heat capture and using quartzite as an aggregate significantly increases heat capacity.

Adding filler material to the asphalt mix in order to improve the efficiency of the collector has proved effective and is a key issues relating to performance optimisation (Wu et al. 2009).

Pipe spacing and fluid flow rates also affect the solar collection efficiency.

Shaopeng et al. (2011) show that circulating water temperature in the pipe network varies as a function of the applied flow rate, while there proved to be only minimal variation in surface temperatures (1.87oC), the maximum extracted heat energy increases as flow rate increases with a maximum value of 400W/m2 being extracted in that system.

The increase of flow rate results in increased fluid velocity and thus improves the heat transfer coefficient of fluid in the pipes (Shaopeng et al. 2011).

Furthermore, the pipe configuration, especially the effective total length in a certain area is also important for the amount of heat that can be extracted which is determined by pipe configuration in the slab (Wu et al. 2009).

The process of optimising the effectiveness of solar collection may lead to secondary effects such as pavement durability. It is found that locating the heat exchanger tubes at shallow depths can extract more energy but results in higher stresses in the asphalt, and thus reduced durability of the pavement (Bijsterveld et al. 2001). Asphalt pavement durability is considerably increased with the addition of reinforcement in the pavement layer. Reinforcement refers to the ability of an interlayer to better distribute the applied load over a larger area and to compensate for the lack of tensile strength within structural materials. Elseifi and Al-Qadi (2005) showed that placing steel reinforcement at the bottom of the hot mix asphalt layers, the range of improvement for the pavement structure was between 15 and 257% in the transverse direction, and between 12 and 261% in the longitudinal direction compared to the pavement without any reinforcement.

4

The purpose of this research is to investigate the performance of two different types of materials to be used for the pipes. The first pipe is high-density polyethylene (HDPE), which has a diameter of 16mm and a thermal conductivity of 0.45W/m.K. The second pipe is copper with a diameter of 8mm and a thermal conductivity of 401W/m.K. The performance of each pipe will be compared in terms of extracted heat energy as was carried out by Shaopeng et al. (2011). The pipes will be placed at three different depths and the performance of each pipe at the various depths will be investigated in terms of their ability to transfer the heat from the asphalt to the fluid flowing through the pipes.

The use of steel reinforcing mesh could also provide a means of conducting heat from areas surrounding the collector pipes in the asphalt layers, provided there is contact between the reinforcing mesh and the pipes themselves. To investigate this possibility, steel mesh will be placed into two of the test samples and the results obtained will be compared with each other and with the samples without the mesh.

3 3.1

Experimental Test configuration

The experimental asphalt solar collection systems (ASC) were constructed with imbedded tubing arranged in a serpentine configuration within 0.5 x 0.5 x 0.11m asphalt slabs as can be seen in Figure 1, Figure 2 and Figure 5. Two types of imbedded tubing were incorporated in the experimental system, namely an 8mm diameter copper tubing configuration at 58mm centres and a 16mm diameter plastic tubing configuration at 175mm centres. Three depths within the asphalt slabs were selected for the placement of the tubing configurations at 20, 40 and 60mm below the slab surface, with two ASCs incorporating zinc coated mild steel mesh sheets. The asphalt slab mix consisted of 200pen bitumen; 6mm singled sized limestone aggregate and 6mm limestone dust aggregate with an average density of the asphalt slab being 2692kg/m3. The main test apparatus incorporated the ASC, inflow manifold with flow meters, an acquisition unit 5

connected to thermocouples and 400W tungsten lamp rig. Water is fed to the system from the mains supply, whereby the flow rate to the ASC was controlled by flow meters on the inflow manifold with the water being discharged directly from the system instead of recirculation.

Figure 1- ASC with plastic embedded pipes

Figure 3 – Manifold and pipe connections

Figure 2 – ASC with copper embedded tubing

Figure 4 – ASC being operated with halogen light rig in place

6

Figure 5 – ASC layout

3.2

Test method

The solar collector surface was subjected to radiation heat from a rig of halogen lamps, as seen in Figure 4, in an arrangement similar to Wu et al. (2009) which was first proposed by Mrawira and Luca (2006) and gives an approximate exposure of 900W/m2, determined by the relative positioning of the lamps. The total test period for each ASC continued for 4 hours and comprised of 3 phases, a 1 hour phase with the slabs exposed to the radiated heat source without water flow, a 2 hour phase with radiated heat and a constant water flow rate of 1.5 l/s and finally a 1 hour phase with the flow rate maintained without the presence of the radiated heat source. Temperature readings were taken at three minute intervals by thermocouples at the outlet and inlet of the ASC and at 20mm increments from the surface of the asphalt at a location adjacent the imbedded tubing of the systems.

7

4

Results and discussion

4.1

Temperature profile

Figure 6 shows the temperature differential at different depths within the asphalt for the various depths of the 16mm diameter plastic pipe.

Temperature distribution 30

35

40

Temp OC 45

50

55

60

Thermocouple Depth (mm)

0 20 40 60

60mm cover with mesh 60mm cover witout mesh

80

40mm cover without mesh 20mm cover without mesh

100

16 mm Diameter Plastic Pipe 120

Figure 6 – Temperature distribution at 180 min

The tests were performed in a laboratory environment where the effects of wind velocity and heat transfer downward were not considered variables. Ambient air temperature was recorded at a constant 20oC. The pipe placed at 20mm below the surface showed the lowest surface reading as expected peaking at a temperature of approximately 51oC, in comparison to the pipe at its lowest position of 60mm depth from the surface in which the surface temperature peaked at 550C. The flow rate was maintained at a steady 1.5l/min throughout the duration of the test.

The rate of decent in temperature from the surface to the respective pipe locations can be seen from the graph. This rate is reduced in the locations below each pipe giving the indication that less heat energy is passing the pipe than is

8

being absorbed by the pipe in each case, thus being absorbed by the flowing water in the pipe.

The plastic 16mm diameter pipe at a depth of 20mm proved to be the most effective location for the test conditions for the reduction in asphalt temperature at the surface of the slab. The pipe placed at 40mm did reduce the core temperature of the asphalt at that location however the overall energy gain was not produced as can be seen in Table 3.

Figure 7 shows the temperature differential at different depths within the asphalt for the various depths of the 8mm diameter copper pipe.

Temperature distribution Temp oC

10

20

30

40

50

60

0

Thermocouple Depth (mm)

20 40

60

60mm cover with mesh 60mm cover without mesh

80

40mm cover without mesh 20mm cover without mesh

100

8 mm Copper Pipe 120

Figure 7 – Temperature distribution at 180 min for ASC with embedded copper pipes

The test was carried out under the same conditions as the plastic piping.

Significant variations were found especially in the 20mm depth. The maximum temperature recorded was 36oC on the surface at the 20mm depth after 180 minutes of irradiation. Both 60mm depth pipes showed similar surface temperatures of just over 50oC. The available energy absorbed into the slab was 9

most efficiently captivated by the copper piping in general, with the 20mm pipe depth being the most efficient as outlined in Table 2.

4.2

Energy transfer efficiency

The temperature difference between the incoming and outgoing water, ΔT, is a key element when establishing the efficiency of the system for extracting heat energy. The temperature difference between inlet and outlet from the heat carrier directly reflects the absorbing heat ability (Gao et al. 2010), while Shaopeng et al. (2011) stated that ΔT acts as an indicator of the efficiency of the system for extracting heat energy from pavements.

To investigate the effectiveness of the asphalt solar system as an energy collector, the change in temperature between the inlet and outlet for each sample was used. An average value of ΔT was calculated by taking the ΔT values for each sample box between the times of 72 and 180 minutes of irradiation.

These times were chosen because a steady state temperature was observed in between these times at depths equal to the location of the pipes for each test sample as observed in Figure 8 and Figure 9.

10

Temperature at depth of pipe (Degrees Celcius)

Location of Steady State Temperature for Copper Pipes 40 35 30 25 20 15 Copper with mesh at 60mm depth

10

Copper at 60mm depth

5

Copper at 20mm depth Copper at 40mm depth

0 0

50

100 72

150 Time (Minutes)

200

250

300

180

Figure 8 – Steady state water temperature (copper pipe)

Temperature at depth of pipe (degrees Celcius)

Location of Steady State Temperature for Plastic Pipes 60

50 40 30 Plastic at 40mm depth

20

Plastic at 20mm depth Plastic with mesh at 60mm depth

10

Plastic at 60mm depth

0 0

50

72

100

180 200 150 Time (Minutes)

250

300

Figure 9 – Steady state water temperature (plastic pipe)

11

This method of observing the steady state temperature differential between the inlet water temperature and the outlet water temperature is used, as an average maximum achievable temperature differential (Shaopeng et al. 2011).

The ΔT varies for each of the samples tested which was expected as the depths of the embedded pipes varied as did the material of the pipe itself. The results of the ΔT are outlined in Table 1. The average ΔT for the various depths was greater in the samples that used the copper pipes compared to the plastic pipes by approximately 50%. For each material the ΔT was greatest where the pipe was embedded at 20mm from the top surface of the slab, with ΔT for the copper being 1.28 oC and 0.87oC for the plastic. The addition of the mesh in the slab with copper had an insignificant effect on the ΔT but when used with the plastic pipe it had a significant effect, in this case the addition to the mesh resulted in a reduction of 57% of the ΔT. Temperature variations at 60mm depth 50

Temperture (0C)

40 30 20

Plastic tubing 60mm cover with steel mesh Plastic tubing 60mm cover Copper tubing 60mm cover with steel mesh Copper tubing 60mm cover

10 0 0

30

60

90

120

150

180

210

240

Time (mins)

Figure 10 – Temperature variations at 60mm depths over the 4 hour test period

The temperature variation profile at the 60mm depth, which includes the steel mesh, increase and dissipate at approximately the same rate for both the copper and plastic tubing configurations when compared to the depths without mesh.

However, the temperature is as much as 4oC lower between equivalent systems.

This would suggest that the mesh is inhibiting the potential ∆T of the ASC at this 12

depth by conducting heat away from the imbedded tubing. This is also reflected by the maximum ∆T being recorded in the systems without mesh at this depth.

It can be seen from Table 1 that the close configuration spacing of the copper system, coupled with the materials higher conductivity returns the temperature profile to a steady state much more efficiently than the plastic system.

Table 1- Temperature Differential Temperature Differential between Inlet and Outlet During Steady State Temperature Plastic Copper Pipe 60mm 60mm 40mm 20mm 60mm 60m 40mm 20mm Mesh Mesh Δt (Max) 0.68 0.29 0.34 0.87 1.01 1.02 0.94 1.28 o C The quantity of heat energy which can be extracted by the circulating water was found using the following expression by Shaopeng et al. (2011) as: qout = Cp x V x ΔT/Aslab

(1)

Where: qout = Heat energy absorbed, W/m2 Cp = Specific heat capacity of the water, kJ/kg.K V = Flow rate, l/s ΔT = Maximum change in temperature, oC or Kelvin Aslab = Experimental slab area, m2

The following data was used for the energy value calculation: Cp, specific heat capacity of water = 4.2 kJ/kg.K V, water low rate, 25 l/second A, area of surface = 0.25 m2 qin, irradiation rate = 900 W/m2

13

The maximum value of energy extraction of 536.9 W/m2 is representative of idealised laboratory conditions of the copper pipe at 20mm depth from the irradiated surface exposed to 900 W/m2. This is higher than the average heat collecting capacity of between 150 and 250 W/m2 outlined by Shaopeng et al.

(2011).

The above energy calculations were based on the solar collectors reaching a steady state condition which were experienced between 72 and 180 minutes of irradiation.

Using Equation 1 the following maximum energy extraction was obtained:

Table 2 – Energy values from copper pipe Copper Pipe cover 60mm 60mm with Mesh 40mm 20mm

qout (W/m2) 423.0 428.3 394.5 536.9

Efficiency (%) 47.0 47.6 43.8 59.7

Table 3 – Energy values from plastic pipe Plastic pipe cover 60mm 60mm with Mesh 40mm 20mm

qout (W/m2) 284.7 120.6 141.6 364.2

Efficiency (%) 31.6 13.4 15.7 40.5

The efficiency is calculated based on: [qout/qin]*100

(2)

The efficiency reported here is based on a laboratory condition of irradiation at a constant value approximating 900W/m2 which is relative to typical summer day

14

in Ireland (MetÉireann 2013). Laboratory conditions do incorporate other typical climatic conditions such as wind direction, speed and ambient temperature.

The results indicate that the copper pipes perform better in terms of heat energy collection efficiency than the plastic pipes. As can be seen in Table 2 and Table 3 the maximum percentage energy efficiency achieved from the ASC with the copper pipes reached 59.7% while the corresponding value in the ASC with the plastic pipes reached only 40.5%. This suggests that the copper pipes give 150% of the energy efficiency of the plastic pipes. Given the thermal properties of the two materials this result is expected. For both materials, the energy efficiency was greatest for the pipes at 20mm depth. This suggests that the pipes of either material will potentially be more efficient if utilised in a system when they are close to the surface of the pavement but placing pipes close to the surface may have negative impacts such as loss of strength of the pavement and problems associated with the physical embedding of the pipes in the pavement without causing damage to the structure of the pipe itself.

With both the plastic and copper pipe the efficiency at a depth of 60mm was greater than at a depth of 40mm. These results were unexpected, considering that the pipes at 60mm were further away from the surface and therefore further away from the heat source. These results may be explained by the fact that 50mm of insulation was place at the bottom of the boxes. As the heat reached the bottom of the box it may have been deflected upwards and this may have led to an increased temperature at the lower depth. Another explanation is the larger volume of material above the pipe at a depth of 60 mm could have had more potential heat energy stored above it due to the larger mass of material under which it was embedded, which resulted in a more efficient transfer of heat through the system.

The results from Table 2 indicate that the addition of the mesh in the ASC that use the copper pipes has little effect on the energy efficiency of the system.

Without the addition of the mesh, at a depth of 60mm, an efficiency of 47.0% was achieved in the ASC. When this value is compared to 47.6% which was achieved for the ASC with the mesh at the same depth the difference between 15

both values is relatively insignificant although a slightly better performance was achieved as a result of adding the mesh. The effect of the adding the mesh was far greater in the ASC that used plastic pipes. The results from Table 2 show that the use of the mesh reduced the energy efficiency of the ASC from 31.6% to 13.4%. These results represent a disadvantage associated with the addition of mesh as the omission of the mesh has resulted in the ASC without the mesh having an efficiency of 235% of the ASC with the mesh. As can been seen in Figure 8, Figure 9 and Figure 10 no apparent advantage can be observed by the addition of the mesh after the heat source is removed. Initially it was considered that the addition of the mesh would lead to the ASC to retain the heat energy in the system for longer but no significant results were achieved which suggest that this is the case.

5

Conclusion

Asphalt solar collection systems were prepared and tested using two different pipe materials at various depths, with two having zinc coated, mild steel mesh placed under the pipes. Each collection system was exposed to 900W/m 2 of heat energy from a rig of halogen lamps and the difference between the incoming and outgoing temperature of the water along with the temperature at various depths throughout the asphalt slab was recorded. The results show that the closer to the surface the pipes are embedded in the asphalt slab, the greater the reduction in the surface temperature of the slab. A surface temperature difference of 4oC was recorded between the asphalt slab with the plastic pipes embedded at 20mm and the asphalt slab with the pipes embedded at 60mm.

The results of the ASC with the copper pipes show that the temperature difference between at the surface of the slab is 16oC cooler when the pipe is 20mm from surface as opposed to 60mm from the surface.

The results indicate that embedding pipes that are part of an ASC system can be effective in reducing the surface temperature of the asphalt. The results also indicate that copper pipes will perform better at reducing the surface 16

temperature of the asphalt than plastic pipes and the closer to the surface that the pipes are embedded, the greater the reduction of temperature of the surface of the asphalt.

The results indicate that the copper pipes are more effectual in terms of energy efficiency than the plastic pipes. The energy achieved from the from each asphalt slabs which had the copper pipes embedded in them performed significantly higher than those with the plastic pipes with a maximum energy collection efficiency of 59.7% and 40.5% being achieved respectively. The least efficient results for both the copper and plastic pipes were achieved at a depth of 40mm, however, the use of insulation under the asphalt slab may have given erroneous results of the pipes at a depth of 60mm. The inclusion of the zinc coated, mild steel mesh has resulted in no significant increase in the energy collection efficiency of the asphalt solar collection systems containing the copper pipes, but in fact has resulted in a decrease of 18.5% in the system containing plastic pipes.

The method of extracting heat energy from asphalt slabs prepared in the lab has been investigated in this present work. The results suggest the most efficient configuration for such system involves the integration of copper pipes close to the surface of the asphalt. The addition of the zinc coated, mild steel mesh may not increase the efficiency of system in terms of heat energy collection but its inclusion may have benefits in terms of the structural integrity of the asphalt.

17

6

References used

Bijsterveld, W.T.V., Houben, L.J.M., Scarpas, A. & Molenaar, A.A.A. (2001) Using Pavement as Solar Collector: Effect on Pvement Temperature and Structural Response. Journal of the Transportation Research Board, 1778 pp. 140-148.

Bobes-Jesus, V., Pascual-Muñoz, P., Castro-Fresno, D. & Rodriguez-Hernandez, J. (2013) Asphalt solar collectors: A literature review. Applied Energy, 102 (0) pp.

962-970.

Dessouky, S., Reyes, C., Ilias, M., Contreras, D. & Papagiannakis, A.T. (2011) Effect of pre-heating duration and temperature conditioning on the rheological properties of bitumen. Construction and Building Materials, 25 (6) pp. 27852792.

Elseifi, M.A. & Al-Qadi, I.L. (2005) Effectiveness of Steel Reinforcing Nettings in Combating Fatigue Cracking in New Flexible Pavement Systems. Journal of Transportation Engineering, 131 (1) pp. 37-45.

Gao, Q., Huang, Y., Li, M., Liu, Y. & Yan, Y.Y. (2010) Experimental study of slab solar collection on the hydronic system of road. Solar Energy, 84 (12) pp. 20962102.

Mallick, R.B., Chen, B.L., Bhowmick, S. & Hulen, M.S. (2008) Capturing Solar Energy from Asphalt Pavements. International Symposium on Asphalt Pavements, pp. 161-172.

Metéireann (2013, 2013-3-10) Met Éireann - The Irish Weather Service. [Online].

Available at: http://www.met.ie/climate-ireland/sunshine.asp [Accessed: 10/03/2013].

Merbouh, M.H. (2012) Effect of Thermal Cycling on the Creep-Recovery Behaviour of Road Bitumen. Energy Procedia, 18 (0) pp. 1106-1114.

Mrawira, D.M. & Luca, J. (2006) Effect of aggregate type, gradation, and compaction level on thermal properties of hot-mix asphalts. Canadian Journal of Civil Engineering, 33 (11) pp. 1410-1417.

Nicolas, S. & Eleftherios, Z. (2010) Asphalt Solar Collector and Borehole Storage.

Thesis (Masters). CHALMERS UNIVERSITY OF TECHNOLOGY, Sweeden.

Read, J., Whiteoak, D. & Shell, B. (2003) The Shell Bitumen Handbook. Thomas Telford Shaopeng, W., Mingyu, C. & Jizhe, Z. (2011) Laboratory investigation into thermal response of asphalt pavements as solar collector by application of small-scale slabs. Applied Thermal Engineering, 31 (10) pp. 1582-1587.

TRL (2007) Performance of an Interseasonal Heat Transfer Facility for Collection, Storage and Re-use of Solar Heat from the Road Surface. (PPR 3023/302_81): Transprt Research Laboratory.

Wu, S.P., Chen, M.Y., Wang, H. & Zhang, Y. (2009) Laboratory Study on Solar Collector of Thermal Conductive Asphalt Concrete. International Journal of Pavement Research and Technology, 2 (4) pp. 130-136.

18