Impact of cool roof on energy consumption for a railway station

Abstract

The roof reflectance surface and skylight shade reflectance are two significant factors affecting air-conditioning energy consumption and indoor lighting in a single-storey large volume building, such as a railway station. Based on qualitative analysis on the effect of cool roof on building performance, a model of a large tall railway station built in the software EnergyPlus was verified by field data. The model was applied to analyse the effect of cool roof reflectance and skylight shade reflectance on the installed capacity of air-conditioning system, lighting energy consumption and indoor thermal comfort under different climates. The results show that the high reflectance surface of opaque cool roof is beneficial by reducing the energy consumption for indoor cooling and the installed capacity of air-conditioning system and improve the indoor thermal comfort in summertime. However, the opposite is true in wintertime, and this could consequently lead to more annual energy consumption. The high reflectance of skylights could reduce the energy consumption for cooling in summer and limit the energy consumption for heating and lighting in winter, thus reducing the annual energy consumption by a large amount. This study indicates that the local climates and the inherent characteristics of roof structure should be considered in the practical use of cool roof technology.

Keywords

Cool roof Railway station Roof reflectance Skylight shade reflectance Energy consumption

Introduction

With the rapid development of urbanization in various parts of the world, there has been an increase in the volume of residential buildings and public buildings being constructed to improve people’s life standard. However, buildings can consume large amount of energy,¹ for example, in China, energy consumption in buildings accounts for about 25% of total national energy consumption.^2,3 For public buildings, the corresponding energy conservation potentials are 67.4% and 75.9% according to energy efficiency standards in Hot Summer and Cold Winter respective regions in China.⁴ Energy efficiency and acceptable indoor environmental quality are important consideration for buildings in China. There is now a Chinese standard (GB/T 50668-2011)⁵ for assessing indoor environmental quality of energy efficient buildings.⁶ Indoor environment quality should be simultaneously determined by assessing the thermal, acoustic, luminous environments and air quality.⁷ As a basic factor considered for building design,⁸ comfort is an important affecting factor in public buildings construction, such as for a railway station. To ensure a comfortable and convenient indoor environment, a huge amount of energy consumption is needed, especially in poorly designed buildings.⁹ Air-conditioning and artificial lighting would consume much energy in railway station because they provide comfortable indoor thermal environment and luminous environment, which are the basic requirements for passengers and workers.¹⁰

In addition to indoor thermal comfort and illumination, air-conditioning load and lighting energy consumption are affected considerably by the heat gained from the sunlight through roofs in a single-storey building.^11–14 For a large volume construction like railway stations, the temperature difference between the roof surface and outside-to-inside surface would usually increase quickly, which would lead to a rise in heat transfer through the roof due to the cold load increase in summertime and the heating load decreases in wintertime. The overheated roofs also heat ambient air and intensify the urban heat island.^15,16 With more sunlight through the roof, less artificial lighting is needed.¹⁷ Moreover, local weather and climates are another two important factors affecting the influence of cool roofs on air-conditioning energy consumption.¹⁰ Under cold climates, cool roofs show good performances in both of summer and winter; however, green roofs are not advantageous for well-insulated buildings in warm climates.¹⁸ The optical and thermal performance of the roofs should be fully taken into account to keep the building in a low cooling load,^19,20 a better luminous environment²¹ and a comfortable indoor environment.²²

Reflective surfaces are surfaces that can deliver high solar reflectance and high thermal emittance. The most well-known type of reflective surface is the cool roof. With high solar reflectance, high infrared radiation and low radiation absorbance, cool roofs reflect more sunlight than traditional roofs, reducing the amount of heat that is transmitted into the building,^23–25 which is useful to reduce the lighting energy and the cooling load, and to improve the comfort in non-air-conditioning buildings, and to mitigate the urban heat island.^26–28 Compared to green roofs and other optimization method for improving the performance of buildings, cool roofs are an economical technology.²⁹ Many research about cool roofs are in progress with reference to the indoor thermal and luminous environment. Compared to the cement concrete surface, the highly reflective bare soil, grey, white and green paint surfaces would provide a small sensible heat flux during the day time because of the low net radiation.³⁰ A light-coloured roof shows a 30% lower total (air temperature difference and solar driven) heat gain than a dark-coloured one³¹ and a 14–26% reduction in cooling energy consumption.²³ When cool roofs are applied, the expected depression rate of the average urban ambient temperature could vary between 0.1 and 0.33 K per 0.1 increase of the roofs albedo with a mean value close to 0.2 K,²⁹ which is beneficial to present relatively high heat island mitigation potential. The influences of membrane soiling and cleaning on solar absorbance of cool roofs are considerable because it can result in different heat gain through roofs and various indoor comfort levels.³² The increasing roof solar reflectance could reduce cooling load by 18–93% and the peak cooling demand of air-conditioned buildings by 11–27% and also improves the indoor thermal comfort conditions through reducing the discomfort hours by 9–100% in various climate conditions.³³ Combined with natural ventilation, a low cost cool roof can improve indoor thermal comfort by 16%.³⁴ The utilization of a cool roof can contribute to a 2.8 K reduction in the indoor air temperature and a 40% reduction in the annual cooling load in a non-cooled school building in Athens, Greece.³⁵

The review confirms the superiority of cool roofs for improving indoor environment and thus would reduce the cooling energy in summertime. Most of the literatures focus on a certain building without considering different climates and roof constructions; however, quantitative research on the performance of the cool roof technology and its effect on building energy consumption are still inadequate. In this paper, a model of a railway station with skylights has been built and verified by field data. A building energy simulation programme EnergyPlus³⁶ was used to analyse the effects of two kinds of cool roof albedos, under typical climate conditions of different cities in China, on air-conditioning capacity, lighting energy consumption and indoor thermal comfort.

Research objects

For a building with skylight, roof can consist of a non-transparent roof and skylight. The solar radiation of non-transparent roof is mainly affected by the reflectance of the roof outer surface, which we call roof reflectance. As for the skylight, solar radiation transmitting into building would be mainly through the glass. The amount of incident light could affect the building’s air-conditioning and lighting energy consumption. The heat transfer quantity and luminous flux are mainly affected by the solar reflectance of the outer surface of shading facilities, which we call shade reflectance.

According to the plan of Changsha South Station, this station building consists of two parts. One part is a check-in hall on the ground floor and a waiting hall on the upper floor. Another part is the corridors and the office area.

When measuring the dimension of the station building with a curved roof, we gauged the height above the floor to the ceiling (roof) with lighting fittings using laser measuring instrument and recorded the distance between the measurement points. The dimensions as obtained are listed in Table 1.

Table 1.

The dimensions of various parts of Changsha South Railway Station.

Zone	Dimensions
Check-in hall	Elevation: 0 m; rectangle: 40 m × 185 m
Waiting hall	Elevation: 9.5 m; rectangle: 195 m × 185 m
Skylight	Rectangle: 40 m × 130 m × 0.5 m

The modelling of the railway station is shown in Figure 1. Figure 1(a) and (b) illustrates that the building envelope is divided into transparent and non-transparent part. The outermost building envelope absorbs heat through radiation and convection. Then, the heat absorbed can be transferred into indoor environment across building envelope through heat conduction. In addition, extra heat can be transmitted across non-transparent building envelope via heat transmission and incident light; also, by using sunshade device that can reflect parts of solar radiation, heat absorbed by the building envelope would be reduced and transmitted through non-transparent part. However, sunshade can also reduce incident light, which may increase lighting energy consumption as shown in Figure 1(c).

Research methods

Effect of roof reflectance on building’s thermal performance

Under steady-state heat transfer theory, the indoor heat gain through the non-transparent roof can be described by equation (1)³⁷

q = U [T_{o} - T_{i} + (1 - ρ) (I_{b} \cdot \cos θ \cdotf + I_{s} \cdot \frac{1 + \cos ϕ}{2} + I_{g} \cdot \frac{1 - \cos ϕ}{2}) / h_{o}]

(1)

where q is the heat transferred from the roof to indoors (

W / m^{2}

); U is the overall heat transfer coefficient of the roof (

W / (m^{2} \cdot K)

); T_o is the outdoor air dry bulb temperature (℃); T_i is the indoor air dry bulb temperature (℃); ρ is the roof albedo; I_b is the direct solar radiation intensity (

W / m^{2}

); I_s is the sky diffused radiation intensity (

W / m^{2}

); I_g is the ground reflective-diffused radiation intensity (

W / m^{2}

); f is the direct sunlight coefficient, which can be calculated from the building’s shadow; θ is solar azimuth (degree); ϕ is the solar elevation angle (degree); h_o is the overall heat transfer coefficient of the outer surface of the roof (

W / (m^{2} \cdot K)

From equation (1), it can be seen that the heat transfer process of the roofs has the following characteristics. First, the heat transfer process is bidirectional; in daytime T_o > T, I_b, I_s and I_g > 0, q > 0, heat transfers from outdoors to indoors; at night I_b, I_s and I_g are 0, and because of the heat dissipation from occupants and equipment, therefore T_o < T, q < 0 and then heat transfers from indoors to outdoors. Second, the heat transfer process can also be affected by the comprehensive heat transfer coefficient of the roof (U). So, the cool roof is a complete structure. We can never analyse its effect on buildings without its structural characteristics.

Effect of shade reflectance on building’s thermal performance

At a certain moment, two parts of solar radiation (direct and diffuse radiation) are incident on the skylight. The heat gain resulting from the transmission can be calculated by equations (2) and (3)³⁷

Q_{τ}^{dir} = f \cdot I_{dir, norm} \cos φ T_{sys}^{dir} (φ, φ_{s})

(2)

and Q_{τ}^{dif} = T_{sys}^{dir, dif} [(1 - \frac{| \cos γ |}{2}) I_{s} + \frac{| \cos γ |}{2} I_{g}] + T_{sys}^{gnd, dif} [\frac{| \cos γ |}{2} I_{s} + (1 - \frac{| \cos γ |}{2}) I_{g}]

(3)

where

Q_{τ}^{dir}

is the direct radiation heat (

W / m^{2}

Q_{τ}^{dif}

is the diffuse radiation heat (

W / m^{2}

I_{dir, norm}

is the direct solar radiation of skylight normal surface (

W / m^{2}

T_{sys}^{dir} (φ, φ_{s})

is the function for the shading system of direct solar radiation transmission coefficient,

T_{sys}^{dir, dif}

is the sky diffused radiation transmission coefficient of shading systems,

T_{sys}^{gnd, dif}

is the ground reflected radiation transmission coefficient of shading systems, φ is the solar radiation incidence angle (degree),

φ_{s}

is the direct solar radiation profile angle (degree), and γ is the angle between the skylight plane and the horizontal plane (degree).

T_{sys}^{dir} (φ, φ_{s})

T_{sys}^{dir, dif}

and

T_{sys}^{gnd, dif}

would decrease as the sunshade reflection coefficient increases and, therefore, the solar radiation transmitted into the interior can be effectively reduced by raising the shade albedo.

At a certain moment, equations (4) and (5) would be used to calculate the architectural lighting energy changes as the reference point illuminance value of the work surface³⁷

L_{i} = \sum_{\binom{window}{elements}} L_{w} d Ω \cos γ

(4)

and

L_{w} = \frac{1}{π} \int_{θ_{min}}^{θ_{max}} \int_{π / 2 - φ_{w}}^{π / 2} L (θ, φ) T (β) \cos β \cos φ d θ d φ

(5)

where L_i is the luminance of an indoor point (

lux

), L_w is the emission luminance of a window (

cd / m^{2}

d Ω

is the spatial angle of the point and the windows (spherical degree), γ is the clip angle between connection line of reference point and the windows and normal direction of working plane (degree),

L (θ, φ)

is the reflected luminescence brightness function of the sky or the ground (

cd / m^{2}

T (β)

is the skylight shading system transmission coefficient function, θ is the sky or ground elements azimuth (degree), φ is the elevation angle of the elements of the sky or the ground (degree), and β is the angle of solar incident (degree).

T (β)

is the temperature decrease due to shade albedo increase, meaning that lighting energy consumption would increase as the incident luminous flux is reduced. From the above theoretical analysis, the impact of a cool roof on a building can be the result of many factors and a complex process; therefore, we choose computer simulation as the primary means to analyse this process comprehensively.

Results and discussion

The establishment and verification of calculation model

In this study, the computer programme EnergyPlus, developed by the US federal agency and Lawrence Berkeley National Laboratory based on development experience with two existing programmes: DOE-2 and BLAST, was used as a simulation tool to model and solve the problem. The advantages of the software calculation accuracy have been validated by many research.³⁸ The initial condition parameters of the model were set according to a simulation model of Changsha South Railway Station and related literature as shown in Table 2.³⁹

Table 2.

The initial parameters used for modelling.

Parameters	Details
Check-in hall	Storey height: 0 m, rectangle: 150 m × 40 m × 30 m
Elevated waiting room	Storey height: 10 m, rectangle: 150 m × 160 m × 20 m
Heat transfer coefficient of envelope	0.65 W/(m²K)
Skylight	Rectangle: 50 m × 100 m × 0.5 m, solar heat gain coefficient: 0.8,light transmittance: 0.3
Indoor lighting density	10 W/m²
Illumination level	150 lux
Lighting control mode	10% continuous dimming control
Skylight control mode	Open in the day and close at night in summertime; turned off during the day and on at night in wintertime
Air-conditioning system	Variable air volume system, running time: 6:00–24:00

In order to verify the validity of the model, we used the field meteorological data of Changsha on 7, 8, 10 and 11 September 2013 as input term to start simulation. Then we verified the result by 48 groups of hourly (8:00–19:00) indoor dry bulb temperature values and work surface (0.85 m) illuminance values which were measured in the elevated waiting room of Changsha South Station on 7, 8, 10 and 11 September 2013. The results are shown in Figure 2.

As illustrated in Figure 2, the model calculations are in perfect agreement with the measured data. The absolute error between the calculated and measured dry bulb temperature is less than 2.2 ℃, and the relative error is less than 9.2%. For indoor illuminance values, although the calculated and measured values of points 11, 23, 35, 40 and 47 varied greatly, the absolute errors of the rest of the data are less than 260 lux and the relative errors are less than 9.9%. Therefore, the research about impacts of the cool roof on the building performance based on the above model is rational.

Based on the comprehensive analyses of the impacts of the cool roof technology on building energy consumption and indoor comfort, we studied the effects of the cool roof on installed capacity of the railway station, air-conditioning and energy consumption for lighting and indoor thermal comfort on the conditions of

\bar{U}

(average heat transfer coefficient of building envelope), which are 2.43, 0.65, 0.35

W / (m^{2} K)

, respectively. The

ρ_{r}

(solar reflectance roof) and

ρ_{w}

(solar reflectance of skylight shade) values are: 0.0, 0.3, 0.6, 0.9. According to the Chinese building climate divisions and solar resources divisions in Chinese standard,^40,41 we chose the following combinable mode of six typical climate and solar energy resources as the natural meteorological parameters for the calculation. The differences in characteristics with using cool roof in the different climatic conditions in China were analysed, and these are shown in Table 3.

Table 3.

Comparison of using cool roof in the different climatic conditions in China.

Group	Climatic characteristics	Solar resource	Prototypical city
A	Hot summer cold winter	Quiet poor	Changsha
B	Cold	Quiet rich	Beijing
C	Hot summer warm winter	Quiet rich	Guangzhou
D	Cold	Rich	Lasha
E	Hot summer cold winter	Poor	Chengdu
F	Chilly	Quiet rich	Ha’erbin

Impact of cool roof on energy consumption of air conditioner

In order to explain the results of the impacts of cool roof on energy consumption of air conditioners, the concept of building heat capacity was considered. Building heat capacity refers to the ability of maintaining the original thermal state under some external heat disturbances of certain intensity, which is mainly determined by the local meteorological environment, thermal insulation performance of building envelope and air-conditioning load demand.⁴² In general, when the temperature difference between indoors and outdoors is big, the outdoor environment can quickly neutralize the building thermal disturbance. When the average heat transfer coefficient of the building envelope is high, the indoor heat can be quickly transferred to outdoors.⁴³ Moreover, when the air-conditioning load demand is great, the load caused by thermal disturbances can almost be neglected. A building under these three circumstances therefore has a high heat capacity (i.e. it has good thermal stability).

The impact of shade reflectance on energy consumption for cooling and heating of air conditioner can be classified into four groups, represented by the results of Group A, Group B, Group D and Group E, as shown in Figure 3.

Figure 3 demonstrates that the main factors affecting the energy consumption of air conditioner are climatic conditions and building envelope thermal insulation performance. The shade facilities, which are closed during the day in winter, would not affect the solar radiation incident. As a result, changes in shade reflectance have little effect on energy consumption for heating of air conditioner. There are four types of variations in energy consumption for cooling:

Represented by Groups A and C, energy consumption for cooling would be reduced slightly when shade reflectance ranged from 0 to 0.6 and reduced significantly in terminal with good thermal insulation when shade reflectance ranged from 0.6 to 0.9;

Represented by Group B and F, energy consumption for cooling would be reduced with shade reflectance ranging from 0 to 0.6 and reduced dramatically in terminal with the best thermal insulation when shade reflectance ranged from 0.6 to 0.9;

Represented by Group D, the energy consumption for cooling would be reduced rapidly with the increase of shade reflectance, while the thermal insulation of the building envelope has little effect on it;

Represented by Group E, energy consumption for cooling would be reduced when the shade reflectance ranged from 0 to 0.6 and reduced dramatically when shade reflectance ranged from 0.6 to 0.9.

A lower thermal insulation of building envelope would contribute to a higher heat capacity of terminal, causing thermal disturbance due to changes in shade reflectance to transmit heat rapidly to outdoors. This is the reason why energy consumption of most areas’ terminal cooling did not change significantly when shade reflectance was ranging from 0 to 0.6, but this declined dramatically with the shade reflectance was ranging from 0.6 to 0.9. In summertime, the big indoor–outdoor temperature difference would help to improve building heat capacity, so the building can still retain thermal stability even with high building envelope thermal insulation, such as results given by Group A, B and E. For areas with small summer cooling, energy demand like Group D, the changes in solar incident radiation would affect the cooling load a lot and energy consumption for cooling would yield a nearly linear respond as the shade reflectance was increased. From the above results, the high shade reflectance and thermal insulation of the building envelope can effectively reduce the air-conditioning energy consumption in summer.

The heat transfer process in the non-transparent structure of the roof is more complex than skylight heat transmission process. The impacts of roof reflectance on the terminal’s cooling and heating energy consumption would keep varying all the time and the results are shown in Figure 4.

As shown in Figure 4, the roof reflectance would increase as heating energy consumption of each city terminal gradually increases and the terminal with a poor thermal insulation would increase more. However, the energy consumption for cooling would vary as local climate conditions change. This confirms that the increase of roof reflectance would reduce the heat transfer from the roof surface to indoors, but the results show the effects of a variety of situations that was due to local climatic conditions, the envelope thermal insulation and the bidirectional of roofs heat transfer process.

Impact of cool roof on lighting energy consumption

By improving the shade reflectance would reduce the natural light flux into the interior, and therefore increases the lighting energy consumption. Figure 5 shows the effect of skylight and shade reflectance on lighting energy consumption.

As shown in Figure 5, factors affecting the energy consumption for lighting in railway stations are the annual sunshine hours, the light transmittance of the skylight glass and shading reflectance. First, the order of annual sunshine hours is Group D (3973 h) > Group B (3747 h) > Group F (3574 h) > Group A (3392 h) > Group E (3383 h) > Group C (3192 h). This is consistent with the distribution of the curve. However, the actual flux into the interior is small due to the high latitudes of the northern cities where the sun incidence angle is small, and that is why the annual lighting energy consumption of Group B and Group F is higher than that of Group A and Group E. Second, the high transmission coefficient of the skylight glass would contribute to more luminous flux entering indoors and thus would reduce the lighting energy consumption. Finally, the raised shade reflectance would reduce the luminous flux into the interior, thus causing an increase in lighting energy consumption. The impacts of lighting energy consumption trends on the transmission coefficient of the skylight glass are not significant, as the shade and the skylight are two relatively independent components and the impact of their internal reflection phenomenon on lighting energy consumption is very small, even negligible in engineering designing.

Effect of cool roof on the installed capacity of air conditioner

Air conditioner units can be classified as cooling units and heating units based on their different function. Figure 6 shows the changes in installed capacity of the railway station’s cooling units with different cool roof reflectance. First, the installed capacity of cooling units of southern cities is greater than that of northern cities, while the performance of the building envelope and cool roof could not affect the result. So, the main factor that determines installed capacity of cooling units is the natural climatic conditions. Second, the installed capacity in buildings with shade reflective cool roof matching medium insulation performance of the building envelope or roof reflective cool roofs matching high thermal insulation performance of the building envelope is generally low. In addition, when the shade reflectance is enhanced, the installed capacity would show a clear downward trend, and the trend is not affected by the thermal insulation of the building envelope. The effect of roof reflectance on installed capacity would change significantly as reflectance is increased when the thermal insulation is low. The roof reflectance would affect installed capacity slightly when thermal insulation is high. The reason is that the thermal insulation of the building envelope would not affect the solar radiation that penetrates into the room through shade reflective cool roof. So the shade reflective cool roof can function well under the case of different thermal insulation and would perform best in the case of medium thermal insulation. While the roof reflective cool roofs would transfer heat by conduction, the thermal insulation of the building envelope rather than the cool roof is the main factor. In fact, increasing the thermal insulation of the building envelope would exert a better effect on reducing installed capacity than retrofitting cool roof. Regardless of solar radiation, the thermal insulation of the building envelope instead of the cool roof albedo would determine the installed heating units’ capacity.

Figure 7 illustrates the trend of installed heating units’ capacity on conditions that the average heat transfer coefficient ( $\bar{U}$ ) of building envelope being 2.43, 0.65, 0.35. Figure 7 shows that the order of heating units installed capacity is: F > B > D > A > E > C (i.e. the installed capacity of northern cities is greater than that of southern cities). The performance of building envelope does not have an effect on this order. Therefore, the local natural climatic conditions are the primary factor of sizing installed heating units’ capacity. In addition, the installed heating units’ capacity would be reduced and the reduction is more in the northern areas than southern areas when increasing the thermal insulation. Therefore, increasing the thermal insulation of the building envelope is an effective way to reduce the heating load. The greater heating load means the more significant thermal insulation effect on the building envelope.

Impact of cool roof on the whole building energy consumption

The energy consumption for air-conditioning could count for about 40–60% of the annual energy consumption of a railway station and the lighting energy consumption could count for about 10%. According to the above study, impact of the cool roof on the energy consumption for air-conditioning and lighting of a railway station are outlined in Figure 8.

In Figure 8, as the shade reflectance is enhanced (although the thermal insulation of the building envelope is different) the total energy consumption of most railway stations would produce a downward trend, especially in the terminal with high shade reflectance, good thermal insulation and with a greater energy consumption for cooling. Therefore, the reduction in energy consumption for cooling would be much greater than the increase in the energy consumption for heating and lighting due to small shade reflectance. Therefore, a huge energy saving potential could be attained when the thermal insulation is sufficient. Sometimes, energy consumption for cooling could not be reduced when thermal insulation is poor but could be reduced under a higher shade reflectance. This means that the function of cool roof in terminal with large heat capacity could be restricted. As the roof reflectance is enhanced, the total energy consumption of the terminal would show, mostly, an upward trend, but this is not as obvious if the thermal insulation is increased. Therefore, the increase of the roof reflectance would lead to an increase in the total energy consumption in the building in most cases. However, when cool roofs technology is in application, a combination of the actual situation for the rational design would be required to enable further reduction in energy consumption.

Impact of cool roof on indoor comfort

The heat gain and infrared radiation from the inner side of the roof to indoors would be reduced due to an increase in the roof reflectance, therefore would improve the thermal environment for occupants to feel comfortable in summer and cold in winter. This paper uses the predicted mean vote–predicted percentage dissatisfied index⁴⁴ as the parameters for evaluating indoor comfort. There are four types of results through simulation, as shown in Figure 9; viz: (1) Group A; (2) Group B, E; (3) Group C; (4) Group D, F from 6:00 to 24:00 indoor environment uncomfortable hours during any typical year.

From Figure 9, the winter uncomfortable hours of terminals with good thermal insulation are low for areas with large winter heating energy demand like Group B. In areas with large cooling energy demand like Group C, poor thermal insulation of the building envelope would help to take full advantage of the warmer outdoor weather and high thermal insulation efficiently to prevent indoor heat from dissipating outward. Both of these scenarios can help to reduce the indoor cold uncomfortable hours. In addition, as the shade reflectance is enhanced, the cold uncomfortable hours would become slightly increased, which suggests that although shading facilities are turned off during the day, they could still block part of solar radiation (this part is small and the impact is not obvious) into the interior.

Hot uncomfortable hours would be reduced as the shade reflectance is enhanced, but thermal disturbance would be small and the increased value could vary from 0 h to 100 h due to the limited area of the skylight. For type 1 and 2, the better the thermal insulation of the building envelope is, the longer the hot uncomfortable hours would be. The outdoor temperature of Group B is lower than that of Group A in summertime, the building’s thermal stability of Group B is better, so the results of $\bar{U} = 0.65$ and $\bar{U} = 0.35$ are similar. For areas with large heating load like Group C, if thermal insulation of building envelope is low, the indoor heat can rapidly radiate to the outdoor atmosphere at night; if thermal insulation is high, heat of outdoor high temperature can be isolated by building envelope during the day. So the terminal’s summer comfort of Group C would be at the lowest with the middle average heat transfer coefficient. On the contrary, the high thermal insulation of the building envelope of Group F would make it difficult for indoor temperature to cool to the low outdoor ambient temperature. In the case of poor thermal insulation of the building envelope, the heat transfer from outside to inside cannot be suppressed if there was a rise in outdoor temperature or due to direct sunlight, so the terminal’s summer comfort of Group F would be at the best with the middle range average heat transfer coefficient.

As shown in Figure 10, the impact of roof reflectance on indoor uncomfortable hours is similar with that of shade reflectance, but differences can still exist. At first, there are increases in variation range and most are more than 300 h and are proportional to heating or cooling energy demand, such as the cold hours of terminal of Group D which could be increased by 769 h when $\bar{U} = 2.43$ and the hot hours of terminal of Group C which could be decreased by 430 h with the same. Second, the curve of uncomfortable hours is linked to the roof reflectance in terminals with different thermal insulation. For example, the change in cold hours of Group C with $\bar{U} = 0.65$ and hot hours of Group C with $\bar{U} = 0.35$ could be resulted due to a change in the diurnal heat transfer direction of the roof. Viewing the implementation in a whole year, the uncomfortable hours were mostly shown in an upward trend as enhancement was made to the shading and roof reflectance. But the increase rate could be affected by local meteorological environment, thermal insulation and heating and cooling energy consumption. For shade reflective cool roofs, the variation range is large when the local solar energy resources are abundant. The variation range would be small when the thermal insulation of the building envelope is modest.

Conclusion

This study developed a model to investigate the impacts of the cool roof on energy consumption of railway station by taking Changsha South Railway Station as a case study. Some important findings were obtained by the analysis. First, for skylight, because of the proper shading strategy, the increase in reflectance of the shading device would not only reduce the installed capacity of chiller and energy consumption for cooling, but also would improve indoor thermal comfort in summer. In addition, the enhancement in reflectance would only just increase the energy consumption for heating and lighting and would therefore worsen the indoor thermal comfort in winter to a limited extent, without raising the installed capacity of the heating unit. Thus, the enhancement of roof reflectance would markedly reduce the total energy consumption, as well as to maintain an acceptable thermal comfort for occupants. Second, for non-transparent cool roof, the enhancement of reflectance on the roof would reduce the installed capacity of chiller and improve thermal comfort in summer, but would increase the installed capacity of the heating unit and the energy consumption for heating and would worsen the thermal comfort for occupants in winter. Moreover, the energy consumption for cooling could not be reduced due to the complications of thermal insulation and heat transfer of the roof. Thus, the total energy consumption would be increased for most railway stations due to the increase in the roof reflectance.

In summary, the cool roof can play a significant role in the energy saving with appropriate design, and by taking the local climate and roof structural characteristics into account.

Footnotes

Authors’ contribution

All authors contributed equally to the preparation of this manuscript.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant No.21376274), and supported by The National Key Technology R&D Program of China (2015BAL04B02) and was also supported by the Fundamental Research Funds for the Central Universities of Central South University (2014zzts194, 2014zzts196).

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