Characteristics and application of road absorbing solar energy

Abstract

If the heat of road surface can be stored in summer, the road surface temperature will be decreased to prevent permanent deformation of pavement. Besides, if the heat stored is released, it can supply heat for buildings or raise the road surface temperature for snow melting in winter. A road-solar energy system was built in this study, and the heat transfer mechanism and effect of the system were analyzed according to the monitored solar radiant heat, the solar energy absorbed by road and the heat stored by soil. The results showed that the road surface temperature was mainly affected by solar radiation, but the effect is hysteretic in nature. The temperature of the solar road surface was 3°C–6°C lower than that of the ordinary road surface. The temperature of the solar road along the vertical direction was 2°C–5°C lower than that of the ordinary road. The temperature difference increased as the distance to the heat transfer tubes decreased. The average solar collector efficiency of the system was 14.4%, and the average solar absorptivity of road surface was 36%.

Keywords

solar energy / road-solar energy system / road surface temperature / solar absorptivity of road surface / solar collector efficiency of system

Introduction

At present, energy has become an issue of great concern in the world. The efficient use of renewable energy is the key to solving the energy problem and to promoting the sustainable development of the society. Solar energy is inexhaustible, and the photo-thermal conversion and the photo-electricity conversion of it have been widely used in living and industrial and agricultural production. The photo-thermal conversion is generally accepted by the community owing to its high efficiency and low investment, but its wide-scale use is limited due to the small space available. The roads are spacious both in urban and rural areas, and the temperature of road surface can be very high in summer, provided they are not obstructed seriously. If the heat can be carried away and stored in summer, it can be used to reduce the temperature of road surface to prevent permanent deformation of pavement, to provide heat for building heating, and to elevate the temperature of pavement for snow and ice melting in winter [1]. This way of utilizing solar energy of road not only mitigates the urban heat island effect [2], but also absorb the cooling capacity from road and store it for building cool supply and road temperature reduction in summer. It was shown that asphalt pavement could provide an energy of 90-150 kW·h/m² per year; the asphalt pavement of 30 m² could provide an energy of 2700-4500 kW·h for building heat supply. However, only 20%-30% of the heat from road surface was used to prevent road icing, and the rest (70%-80%) was used in heat supply [4,5]. A Zonneweg system was built in a business park at Arnhem in Netherlands to cool road in summer, melt snow and ice on the roads in winter, and provide heat and cool supply for a building in the park [6]. Water pipes were laid under the asphalt pavement in Scotland to store solar thermal to heat domestic water and to clear snow and ice on the roads in winter [7].

In China, domestic research on heat storage of roads is still in its infancy. By using simulation software, Wang and Li studied the solar-soil thermal storage and snow-melting system, analyzed the hourly subgrade gain heat, the hourly average surface temperature and the hourly exit temperature in summer [8]. They also initially established a heat transfer model of road snow-melting system and conducted a steady-state numerical simulation of heat transfer to analyze the effect of different buried depths and heating temperature on properties of road snow-melting based on the hourly meteorological data of a typical year and complex boundary conditions [9]. Liu analyzed the effect of decreasing environment temperature on snow melting road surface and the change law of stress according to the theoretical calculations. He recommended that the heat transfer tube depth should be more than 4.6 cm, the tube spacing should be more than 7.6 cm, and the best pacing should be more than 18.5 cm [10]. Han experimentally confirmed that the absorption rate of asphalt pavement was 0.87 for a radiant heat of 0.3-3 μm [11].

This paper is dedicated to the building of a road-solar energy system. By real-time monitoring of solar radiation, solar energy of road gain, heat stored by soil, etc, the heat transfer mechanism and effect of road energy can be analyzed to provide data support for intensive research and applications of road energy.

Heat transfer mechanism of road-solar energy system

The road-solar energy system, as shown in Fig. 1, consists of some U-shaped PE pipes with a vertical length of 120 m and an outer diameter of 32 mm, two stop valves, a filter, two flexible connections, a water pump, a check valve, a water separator and a water collector. In summer, roads absorb solar radiation heat and emit long-wave radiation to the sky, while the heat convection between the pavement and environment air is going on. The net heat absorbed by the road surface is transferred downward. Some heat is stored by the asphalt concrete for temperature rise while the rest is taken away by the fluid in the heat transfer tubes to be stored in soil, so that the soil temperature increases. In winter, in order to melt the snow and ice, the heat pump extracts the heat in the soil, and then transfers the heat to the road surface through the heat transfer tube.

According to the law of conservation of energy, the heat balance equation can be expressed as

(1)q=q_{\mathrm{sorlar}}-q_L-q_r=q_x+q_c,

where q is the net heat absorbed by road, W/m²; q_sorlar is the solar radiation heat absorbed by road surface, W/m²; q_L is the long-wave radiation of road surface, W/m²; q_r is the quantity of heat convection between road surface and air, W/m²; q_x is the heat absorption per unit area of road-solar energy system, W/m²; and q_c is the heat storage capacity of road materials, W/m².

The solar radiation heat absorbed by road can be calculated by

(2)q_{\mathrm{solar}}=\alpha I_T,

where α is the solar radiation absorption coefficient, which is set to 0.87 in this work according to Ref. [11]; and I_T is the total intensity of solar radiation on a horizontal plane, W/m².

The radiation heat, which is spread to the sky in the form of long-wave radiation by road surface, can be calculated by Eq. (3) [12]

(3)q_L=h_r(t_L-t_s),

where t_L is the temperature of road surface, °C; t_s is the temperature of the sky, °C, which can be calculated by Bliss empirical equation (Eq. (4)) [8]; h_r is the linear radiation parameter, which can be calculated by Eq. (5).

(4)t_s=\mathrm{0.0552}{(t_L+\mathrm{273.15})}^{\mathrm{1.5}}-\mathrm{273.15},

(5)h_r=\mathrm{4}ϵ\sigma {\left({\displaystyle \frac{t_L+t_s}{\mathrm{2}}}+\mathrm{273.15}\right)}^{\mathrm{3}},

where ϵ is the surface emissivity of road surface, ϵ = 0.86-0.90 for generic road surface; ϵ = 0.82-0.83 for road surface with a smooth surface, ϵ is set to 0.87 in this work; and σ is the blackbody radiation constant, σ = 5.67×10^-8 W/(m²·K⁴).

The heat convection between road surface and air can be calculated by Newton’s law of cooling:

(6)q_r=h(t_L-t_a),

where t_a is the air temperature, °C; h is the heat transfer coefficient of heat convection surface, W/(m²·K), which can be calculated by Eq. (7) [13,14]:

(7)h=\mathrm{698.24}\left[\mathrm{0.00144}{\left({\displaystyle \frac{t_L+t_a}{\mathrm{2}}}+\mathrm{273.15}\right)}^{\mathrm{0.3}}{\nu }^{\mathrm{0.7}}+\mathrm{0.00097}{(t_L-t_a)}^{\mathrm{0.3}}\right],

where v is the wind velocity of road surface. In this work, v is set to 3.5 m/s.

Research shows that the road surface temperature is mainly affected by solar radiation. Although the wind velocity has some effects on h, the effect is little. Only when the solar radiation is small or zero (at night), can wind velocity have a great impact on the temperature of road surface[15,16].

According to Eqs. (1)-(7), Eq. (8) can be obtained:

(8)q=\alpha I_T-\mathrm{4}ϵ\sigma {\left({\displaystyle \frac{t_L+\mathrm{0.0552}{(t_L+\mathrm{273.15})}^{\mathrm{1.5}}+\mathrm{273.15}}{\mathrm{2}}}\right)}^{\mathrm{3}}[t_L-\mathrm{0.0552}{(t_L+\mathrm{273.15})}^{\mathrm{1.5}}-\mathrm{273.15}]-\mathrm{698.24}\left[\mathrm{0.00144}{\left({\displaystyle \frac{t_L+t_a}{\mathrm{2}}}+\mathrm{273.15}\right)}^{\mathrm{0.3}}{v}^{\mathrm{0.7}}+\mathrm{0.00097}{(t_L-t_a)}^{\mathrm{0.3}}\right](t_L-t_a).

The heat gain per unit area of the road-solar energy system can be calculated by Eq. (9).

(9)q_x={\displaystyle \frac{m\rho c_p(t_{\mathrm{1}}-t_{\mathrm{2}})}{\mathrm{3600}F}},

where m is the water flow in the heat transfer tubes, m³/h; ρ is the density of water, kg/m³; c_p is the specific heat capacity of water, kJ/(kg·K), c_p = 4.187 kJ/(kg·K); t₁ and t₂ is the inlet and outlet water temperature of heat transfer tubes, respectively, °C; and F is the road area, m².

The heat storage capacity per unit area of road materials can be calculated by Eq. (10)

(10)q_c={\displaystyle \sum c_{pi}}{\rho }_i{\delta }_i{\displaystyle \frac{d{t}_i}{d\tau }},

where τ is time, s; c_pi is the specific heat capacity of the ith layer material of roads, J/(kg·°C); ρ_i is the density of the ith layer material of roads, kg/m³; δ_i is the thickness of the ith layer material of roads, m; and t_i is the temperature of the ith layer material of roads, °C.

Experiment of road-solar energy system

The experimental site was selected in an open space of urban area in Tianjin, China, as shown in Fig. 2. The road materials are listed in Table 1. An area of 10 m × 8 m of the road was the road for solar energy utilization, under which heat transfer tubes were laid. Besides, an area of 2 m × 8 m of the road was ordinary road without heat transfer tubes for reference. According to Ref. [10], the heat transfer tubes were laid 6-8 cm under the road surface, with a tube space of 20 cm. The tube is made of PE-RT, with an inner diameter of 15 mm and an outer diameter of 20 mm. Temperature sensors were respectively planted in different locations of the pavement: 9 measuring points were set on each cross section of the test pavement (6 measuring points were set within the limits of the solar road while 3 measuring points were set within the limits of the ordinary road), whereas 7 measuring points were set in corresponding locations on vertical section. The whole layout is demonstrated in Fig. 3.

The temperature sensor is Pt1000, whose accuracy is±0.2°C. The data acquisition system is presented in Fig. 4. The analogy signals of the temperature sensor are converted to digital signals by the temperature collecting module, which are then transmitted through the RS485 interface to the screen and stored in the computer.

The outdoor temperature sensor and solar radiometer, as shown in Fig. 5, were installed at the test site. The monitoring range of the outdoor temperature sensor is -30°C to 70°C with a monitoring precision of±0.5°C; The monitoring range of the solar radiometer is 0-2000 W/m² with a monitoring precision of±2%. The signals collected by the outdoor temperature sensor and the solar radiometer were converted to digital signals by the A/D conversion module and stored in the computer.

According to the hydraulic calculation, the pump head is 2.9 m, water flow is 0.5 L/s, and the volume of open expansion tank is 0.015 m³.

In summer, the water which is heated by the heat transfer tubes flows into the vertical double U-shaped PE pipes. The heat carried by the warm water is transmitted to the underground soil via the PE pipes. The transferred heat is stored in the underground soil, and the water continues to flow back into the heat transfer tubes buried underneath the road to absorb heat by the circulating pump cooling water, forming a closed thermal cycling circuit.

Experimental data and analysis

The debugging and operation of the system began on July 21, 2012. The temperature sensors at the measuring points recorded the data from the start, while the outdoor temperature sensor recorded the data from July 29, 2012, and the solar radiometer recorded the data from Sept. 3, 2012.

Temperature of road surface

Temperature of ordinary road surface

The data were collected every 10 min during the test. The comparison of the variation of the ordinary road surface temperature and the outdoor temperature is illustrated in Fig. 6. It can be observed that the difference between the road surface temperature and the outdoor temperature fluctuates between -4°C and 4°C in rainy days (Sept. 6, Sept. 7, Sept. 11, and Sept. 22); The temperature of the ordinary road surface is 10°C higher than the outdoor temperature in sunny days, and the maximum temperature difference amounted to 21°C, appearing at 13:00, Sept. 13, when the solar radiation is 698 W/m². The variation of road surface temperature is similar to that of solar radiation intensity, but is hysteretic in nature. The maximum of solar radiation intensity appears at 12:00 to 13:00, while the peak temperature of the road surface usually appears 30 to 60 min later than that of solar radiation intensity. The solar radiation intensity is proportional to the road surface temperature. The road surface temperature, the solar radiation intensity and the outdoor temperature of a given day are tabulated in Table 2. It can be seen from Table 2 that the outdoor temperature varies between 24.7°C and 30.1°C, with a fluctuation of 5.4°C; at 12:00-17:00, the outdoor temperature is comparatively stable, while the road surface temperature varies greatly, with a fluctuation of 17.2°C.

Temperatures of solar road surface

The comparison of the variation of the solar road surface temperature and the outdoor temperature is shown in Fig. 7. The variation of solar road surface temperature was similar to that of solar radiation intensity, but is hysteretic in nature. The solar road surface temperature is 3°C-6°C lower than the ordinary road surface temperature, and 6°C higher than the outdoor temperature, with a maximum temperature difference of 19°C.

Figure 7 shows that the temperature of road surface is mainly influenced by the solar radiation instead of by the outdoor temperature. Based on the data, the fitting curve of the solar road surface temperature and solar radiation intensity in sunny days was established, as shown in Eq. (11). In Figs. 6 and 7, relevancy is represented by R, which takes R² = 0.984 in this paper.

(11)t_L=-\mathrm{0.9192}{A}^{\mathrm{2}}+\mathrm{24.544}A+\mathrm{158.64},

where

A=\mathrm{12.4229}\pm \sqrt{-\mathrm{0.03484}{I}_T+\mathrm{30.68}}

in which+ means that the temperature is taken before 12:30, and - means that the temperature is taken after 12:30. The comparison of the fitted temperature and the measured temperature of solar road surface on a sunny day is depicted in Fig. 8. It is seen from Fig. 8 that the maximum difference between the calculated temperature and the measured temperature is 2.4°C, relative errors are within the limits of±6.5%, and the average relative error is 1.0%.

Variations of road temperature in vertical direction

Ordinary road

The 7 temperature measuring points were arranged in such a way that the first was laid on the road surface, the second was laid just below the road surface, the third, fourth, sixth and the last were laid 2, 4, 6, 8 and 10 cm, respectively, under the road surface. The average temperature of the ordinary road along the vertical direction of a typical day is shown in Fig. 9.

It is seen from Fig. 9 that the solar radiation intensity increases first and then decreases. The outdoor temperature varies slowly, but the fluctuation of the outdoor temperature is greater than that of the solar radiation intensity within a short time. The maximum of the solar radiation intensity appears at 12:10, and declines sharply after 15:00.

The minimum of road surface temperature appears at 8:30. It begins to rise with the solar radiant heat increasing, and reaches its maximum at 13:10, which is 60 min later than that of solar radiation intensity. As the depth of the measuring point increases, the temperature decreases, and the time of peak temperature postpones. The maximum of the deepest point (seventh point) appears 2 h later than that of road surface temperature. The temperature difference between layers increases with the solar radiation intensity increasing. The temperature of the lowest layer was 2°C-14°C lower than that of the second layer, and the maximum temperature difference appears at 12:40. The temperature of road surface declines sharply after 15:20. After 17:00, the temperature of road surface is lower than that of other layers below road surface. The fluctuation range of temperatures along the vertical direction gradually decreases. Apparently, the solar radiation has a direct impact on the road temperature.

Solar road

To compare the solar road temperature with the ordinary road temperature in the vertical direction more intuitively, the solar road temperature of the same day is analyzed, and the results are shown in Fig. 10. Because the water in the proposed system takes away some heat from the road surface, the solar road temperature of each layer is lower than the ordinary road temperature, the average difference being 3°C-5°C. However, with the solar radiation intensity increasing, the water in the system is unable to carry away the heat of the road surface in time, the temperature of the road surface gradually increases, the maximum reaching 48.5°C, 3°C lower than the ordinary road surface temperature. The variation of the solar road temperature along the vertical direction is similar to that of the ordinary road, but the solar road temperature lower than the ordinary road temperature.

Comparison of solar road temperature and ordinary road temperature

The variation of the temperature between layers of the solar road and ordinary road is shown in Fig. 11. As can be seen that, because the heat transfer tubes are buried between the fifth layer and the sixth layer, the variation of the temperature of ΔT₅ and ΔT₆ are greater than that of others, which increases first and then decreases after 17:00, the maximum variation being 5.3°C; the peak temperature variation appears 4 h later than that of solar radiation. The heat absorbed by water is proportional to the variation of temperature. The nearer the layers are to the road surface, the smaller the variation of the temperature. ΔT₁ always remains approximately at 2°C, ΔT₂ increases first and then decreases; the variations of ΔT₃ and ΔT₄ are similar to those of ΔT₅ and ΔT₆, but those of ΔT₃ and ΔT₄ are smaller than those of ΔT₅ and ΔT₆. Because the seventh point is under the heat transfer tubes, the variation of ΔT₇ is similar to that of ΔT₅ and ΔT₆.

Endothermic character of road-solar energy system

In order to study the efficiency of the road-solar energy system, η₁ (solar absorptivity of road surface) and η₂ (solar collection efficiency of the system) were introduced. η₁ refers to the ratio of heat absorption per unit area of the road surface and the net heat absorbed by the road. η₂ refers to the ratio of heat absorption per unit area of the system and solar radiation intensity.

(12){\eta }_{\mathrm{1}}={\displaystyle \frac{q_x}{q}},

(13){\eta }_{\mathrm{2}}={\displaystyle \frac{q_x}{I_T}},

where q and q_x are calculated by Eqs. (8) and (9). Figure 12 shows the heat absorption per unit area of the system and the net heat absorbed by the road surface, while Fig. 13 shows η₁ and η₂.

It can be seen from Figs. 12 and 13 that the variation of the heat absorbed by the road surface is similar to that of the solar radiation intensity. It increases first and then decreases, one cycle per day. The variation of the long-wave radiant heat is in contrast to the solar radiation intensity. It decreases first and then increases. The heat absorbed by the road surface is the least, while the long-wave radiant heat is the largest. The peak of the heat absorbed by the road surface is 440 W/m², accounting for approximately 50% of the solar radiation intensity. This shows that the endothermic ability of the road surface is very big. The heat absorption peak per unit area of the system appears 30-60 min later than that of the heat absorbed by the road surface. The heat absorption peak of the system per unit area is 104 W/m². The solar collection efficiency of the system is 12%–25%, the average being 14.4%. The solar absorptivity of the road surface is 27%-52%, the average being 36%. The data above shows that the endothermic ability of the system is strong, and the water can take away 36% of the heat of the road surface.

Conclusions

The following conclusions can be reached after the analysis of the operating data of the road-solar energy system in summer:

1) The temperature of the road surface is mainly affected by solar radiation, but there is a certain lag in temperature variation. The peak of the road surface temperature usually appears 30-60 min later than that of the solar radiation intensity. The solar road surface temperature is 3°C-6°C lower than the ordinary road surface temperature.

2) In each layer, the temperature of the solar road is 2°C-5°C lower than that of the ordinary road. The closer the layer to the heat transfer tubes, the greater the variation of temperature; the nearer the distance between the layers and the road surface, the more inconspicuous the variation. ΔT₁ always remains approximately at 2°C.

3) When the solar radiation intensity is 836 W/m², the maximum heat absorbed by the road surface reaches 440 W/m², accounting for approximately 50% of the solar radiation intensity. The peak heat absorption per unit area of the system is 104 W/m², accounting for 27% of the effective heat gain, and 18% of the solar radiant intensity. The average heat collection efficiency of the system is 14.4%, while the average solar absorptivity of the road surface is 36%.

4) If 14.4% of the solar radiation heat in summer can be stored underground for winter use, the energy saving potential can be impressive.

5) An energy conversion device should be added while the system provides cold and heat source for buildings. In summer, the water in the pipelines absorbs the cooling capacity stored in the soil; the cooling capacity is converted to cool supply for buildings through the energy conversion device. The water transfers the heat to the road surface through the heat transfer tube; the heat capacity of buildings and road is taken away and stored in the soil. In winter, in contrast, the water absorbs the heat stored in the soil. A part of the heat is converted to heat supply for buildings through the energy conversion device, and the rest of the heat is transferred to the road surface to make the road surface temperature above the freezing point. The cool capacity of buildings and the road surface is taken away by the water and stored in the soil. According to the seasonal transform, the energy balance cycle will always continue.

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