Evaluation of the thermal-energy performance of naturally “cool” gravel covering for cool roof and cool pavement application: a case study

Anna Laura Pisello1, CIRIAF – Interuniversity Research Centre, University of Perugia (Italy) Veronica Lucia Castaldo, CIRIAF – Interuniversity Research Centre, University of Perugia

(Italy) Gloria Pignatta, CIRIAF – Interuniversity Research Centre, University of Perugia (Italy) Franco Cotana, CIRIAF – Interuniversity Research Centre, University of Perugia (Italy)

ABSTRACT

In the last few years, researchers and designers showed an increased interest in the development of innovative technologies for the reduction of buildings’ energy requirement. Efforts have been made in order to develop solutions for roof coverings and urban paving with high performance in terms of thermal emittance and solar reflectivity, with the purpose being (i) the enhancement of the “cool roof behavior” at the single-building scale, and (ii) the mitigation of local climate conditions i.e. urban heat island at inter-building level. In this paper, the evaluation of the optic-thermal behavior of a gravel with natural intrinsic cool properties for roof and paving application is carried out, through in-lab and in-field experimental analysis. Additionally, its cool roof effect is investigated through dynamic simulation in a properly selected case study building. Specific evaluation about the cooling potential is carried out with varying grain size of the same local gravel, and with varying stone typology, in summer and winter conditions. The case study is a university building situated in central Italy. Six scenarios were simulated to compare the cooling potential of the natural, low impact gravel with the thermal-energy performance of the traditional bitumen membrane currently installed over the roof. The results show differences up to 24% in terms of solar reflectance with varying the only grain size, and a 30pt. difference of SRI. Additionally, the gravel with the smallest grain size is shown to be the most performing both in terms of summer energy saving for air conditioning and indoor thermal comfort conditions.

Introduction

Over the course of the years, many strategies and techniques have been developed by scientists and researchers with the purpose of enhancing buildings’ energy efficiency and reducing energy requirements (Sozer 2010), in the perspective of a sustainable development of construction technologies. This is due to the high impact of the construction sector in terms of greenhouse gases emissions and energy consumption (Lombard, Ortiz, and Pout 2008). In particular, several innovative cool covering materials with high performance in terms of solar-reflectance and thermal-emittance have been proposed (Levinson, Akbari, and Berdhal 2010a), given the huge impact on the local climate of the urban environment i.e. urban heat island (Santamouris 2014). Therefore, the suitability of materials used in outdoor urban spaces and buildings’ surfaces in order to contribute to lower ambient temperatures and fight heat island effect has been assessed (Kolokotsa, Santamouris, and Zerefos 2013). Moreover, the mitigation

1 Corresponding Author: pisello@crbnet.it, +39 075 5853796.

potential of “cool roof” solution characterized by high solar reflectance and high thermal emissivity has been demonstrated in different climatological contexts (Kolokotsa et al. 2012). In this scenario, Ferrari et al. (2013) developed innovative white-engobe ceramic tiles with high solar reflectance able to limit the summer overheating of buildings by integrating into the traditional existing tiles the suitable raw materials and pigment. The spectral response data for development of cool colored tile coverings have been studied by Libbra et al. (2011) by testing, measuring and comparing the solar reflectance of few covering solutions with different colors. Additionally, “cool” tiles similar to the traditional ones but with a 15% increased reflectance have been developed by Pisello et al. (2013), in order to be applied in historic buildings typically subjected to several architectural constraints. Moreover, the cool roof effect in terms of thermal comfort conditions and energy saving was investigated through dynamic simulation in several climate conditions around the world (Sinnefa, Akbari, and Santamouris 2007). Many other studies suggested the use of specific membranes and coatings likely to be applied over flat roofs in order to decrease the energy demand for heating and cooling (Santamouris, Synnefa, and Karlessi 2011). By taking into account all the contributions about the measurement techniques of the optic- radiative properties i.e. albedo and solar reflectance (Levinson, Akbari, and Berdhal 2010b), this paper builds upon previous study about the thermal-energy performance of local gravels (Pisello et al. 2014), and concerns the evaluation of the thermal-energy behavior of an innovative high-performance gravel for possible building roofs and urban paving applications. Both in-lab and in-field experimental campaigns were carried out in order to measure the optic-thermal properties i.e. solar reflectance and thermal-emittance of the selected material. In particular, four different grain size of the same local gravel were evaluated and compared to a more common and mixed granulometry. These local gravels were therefore compared to the thermal-energy performance of a traditional bitumen membrane, through dynamic energy simulation with refer to a specific case study building. The final purpose was to determine the cooling potential of the different evaluated gravels with varying weather boundary conditions, in terms of both energy saving and indoor thermal comfort.

Methodology

The research work consisted of the following steps:

- In-lab optic-energy experimental campaign; - In-field experimental 7-months monitoring campaign (i.e. September 2013-March 2014); - Selection of a case study building and elaboration of the energy model in Energy Plus

environment; - Calibration of the energy model through the continuously indoor-outdoor monitored data; - Dynamic energy simulation of the building’s thermal-energy performance and analysis of the

results. In particular, the thermal-energy properties of four prototypes (i.e. “1”, “2”, “3”, “4”)

belonging to the same local gravel but characterized by different grain size (Table 1), were evaluated and compared to the more common and generic gravel i.e. “5” characterized by the same grain size of gravel type “1”, but different stone typology. The five natural gravels were therefore compared to the more traditional bitumen roofing membrane i.e. “0”, which is currently positioned on the roof of the case study university building selected for the analysis.

Table 1: Description of the evaluated covering gravels.

Gravel type

Granulometry name Maximum Grain size [mm]

“1” Shingles 8-22.4 “2” Chippings 4-12.5 “3” Pebbles 2-5.6 “4” Sand 0-4 “5” Rubble 4-12.5, as “2”

In-Lab and In-Field Experimental Campaign

The in-lab gravel’s characterization in terms of solar reflectance was performed by means of the Shimatzu Solid Spec 3700 UV-VIS-NIR spectrophotometer with integrating sphere according to the ASTM E1918 Standard test method (2006). The gravels‘ thermal-emittance was measured with the use of a portable emissometer, according to the ASTM C1371 Standard Test Method (2010), by using selfmade 10×10cm prototype samples. The in-field experimental campaign for the measurement of the albedo was carried out by means of albedometer, according to ASTM E1918 (2006). In particular, an on-going monitoring since September 2013 was performed, by measuring the albedo of each gravel every day from 9:00 a.m. to 6:00 p.m. The experimental field-test (3m×3m each) was situated over the roof of the case study university building located in Central Italy. It consisted of 5 fields, one for each tested gravel typology, and included the four grain sizes belonging to the same gravel, and the more common gravel i.e. “5”. Specific evaluation about the cooling potential was carried out with varying period of the year and available global solar radiation conditions during the course of the monitored days. All the field measurements were carried out in parallel with a weather continuous monitoring, through a complete weather station located over the same roof, useful for the calibration process and for the albedo campaign thermal data analysis (Pisello et al. 2014).

Description of the case study

The case study is a university campus situated in Perugia, Italy, (Latitude: 43°07’04’’ N; Longitude:12°.21’03’’E) composed of several buildings hosting professors’ offices, laboratories, conference rooms and lectures rooms (Figure 1a). The structure consists of reinforced precast concrete columns and beams. The opaque envelope consists of external brickwork (0.10 m), rock wool insulation panel (0.10 m), air gap (0.10 m) and internal gypsum plasterboard (0.020 m), with a global thermal transmittance of 0.34 W/m2K. The internal partitions are made of two layers of gypsum plasterboard (0.025 m) with internal air gap (0.10 m). The roof contains an internal layer of plasterboard (0.015 m), a concrete slab (0.20 m), XPS extruded polystyrene insulation (0.10 m) and an external bituminous membrane (0.004 m), and it presents a global thermal transmittance of 0.23 W/m2K. The internal ceiling consists of ceramic tile pavement (0.01 m), aerated concrete slab (0.25 m), an air gap (0.30 m), and mineral fiber (0.01 m). The windows have double clear glass panes (4 mm–3 mm with 10 mm air) with internally positioned venetian blinds and an aluminum frame with a thermal break. In particular, the control area selected for the dynamic simulation of the energy performance includes the main body of the campus (Figure 1b) which hosts both professors offices (conditioned areas) and study halls (free-floating conditions), without considering the underground floors.

Figure 1. (a) Engineering faculty in Perugia and (b) case study building (in red).

Elaboration- Calibration of the Energy Model and Dynamic Simulation

After the analysis of the characteristics of the case study building, the energy model was elaborated by means of the Energy Plus simulation tool (Crawley et al. 1993). Building features are described within the dynamic simulation environment by considering real indoor thermal zones and occupancy. Therefore, the daily occupancy level was elaborated according to the real observed use of the offices and of the study halls. According to the real buildings’ system setup, the energy systems operate every day from Monday to Friday, from 8:00 a.m. to 6:00 p.m. Additionally, the cooling system operates in the period June 1st- September 30th. The heating system operates in the period October 15th- April 15th. Setup air temperature is 24°C in summer and 21°C in winter. Therefore, the validation of the energy model through the experimentally measured data was carried out. The outdoor data were collected by an exhaustive weather station situated over the university roof, able to measure several climate parameters i.e. air temperature and velocity, wind direction, global and solar radiation and humidity. Additionally, the indoor air temperature was measured by two dedicated temperature sensors positioned in the study area located at the first floor of the case study building. In particular, experimentally measured data were used to populate the weather file of the specific location, which was used for calibrating the model during the monitored period i.e. November 5th- December 9th, 2014. The dynamic simulation of the validated model was therefore carried out with the reference TMY weather file (Oko and Ogoloma 2011) of Perugia, Italy. The calibration of the model (Clarke, Strachan, and Pernot 1993) consisted of an iterative process of simulation runs with the purpose to reduce discrepancies between simulated and actual building thermal-energy behavior. The iterative process was performed by calculating two main parameters at each runtime- the Mean Bias Error (MBE) and the Root Mean Square Error (RMSE) (Soebarto 1997; Pernetti, Prada, and Biggio 2013) in order to make the model representative of the real behavior of the case study building. This process was repeated until the two calibration indexes i.e. MBE (1) and RMSE (2) reached sufficiently low values, when the calibration is considered appropriate according to ASHRAE Guidelines 14 (2002).

The calibration process included a set of 30 dynamic simulations of the building. After the first simulation over the monitored period, the energy model reported a MBE and RMSE of 2.8 and 3.5, respectively. Therefore, the buildings’ infiltrations/ natural ventilation rates, the internal gains and HVAC operating schedules were identified as the main parameters influencing the difference between predicted and simulated data, and were iteratively modified in order to re-tune the energy model. A second model was then elaborated by iteratively reducing the buildings’ infiltration level during the day, and enlarging the occupancy level, in order to reduce

the thermal excursion between day and night. Additionally, other models were elaborated by modifying the setup, the COP and the operational schedule of the heating system with reference to the real occupancy schedule of the building. At the end of the procedure, the last model presented a MBE and RMSE of 0.02 and 0.3, respectively, calculated with a 10-minute time step. This was within the ranges according to Ashrae Guidelines 14 (2002) i.e. MBE=0±10% and RMSE=0±30%, and therefore the calibration process was considered to be exhaustively concluded, as the gap between measured and simulated values was properly reduced. The dynamic simulation of the validated model was therefore carried out, in order to assess the thermal-energy performance of the case study building in Energy Plus environment. Six different scenarios were elaborated, with the purpose to simulate (i) the current thermal-energy behavior of the real building, characterized by the presence of the bitumen roofing membrane i.e. “0”, and (ii) the building’s energy performance with the application of the each of the evaluated gravels i.e. “1”,“2”, “3”, “4” and “5”. For each scenario, the dynamic energy simulation was performed by considering the real thermal-physical properties of the gravels in terms of density, solar absorbance, solar reflectance and thermal-emittance according to the in-lab measurements.

Results and Discussion

In-Lab Measurements

The laboratory values of solar reflectance and thermal-emittance of the evaluated gravels are reported in Table 2, together with the SRI calculated values.

Table 2. Solar reflectance, thermal emittance, and SRI of the gravels (hc=5 Wm-2K-1).

Gravel type Solar reflectance (300-2500 nm) [%] Thermal emittance [%] SRI [%] “0” 13 82 14 “1” 38 93 42 “2” 50 94 58 “3” 45 90 50 “4” 62 88 72 “5” 27 93 28 A general increasing trend of the solar reflectance with decreasing grain size was

detected for all the evaluated prototype gravels. Therefore, the gravel type “4”, which is characterized by the smallest grain size, presents the highest solar reflectivity i.e. 62%. Additionally, a consistent behavior in the whole solar spectrum has been registered. The results confirm the good natural properties of the local gravel with respect to the generic stone i.e. “5”, which presents the lowest values of solar reflectance according to the spectrophotometer i.e. 27%. As concerning the thermal emittance, the values measured by the portable emissometer are all around 0.9, for both the prototype samples belonging to the same natural gravel. Therefore, from the laboratory analyses it is possible to assess that the evaluated local gravel presents good properties in terms of solar reflectance and thermal emittance. In particular, the gravel type “4” represents the most effective natural “cool” material with respect to all the other grain sizes, to the mixed one (i.e. “5”), and to the more traditional bitumen membrane i.e. “0”, given that the

SRI increases of 44pt. from gravel type “5” to “4”, and of 70 with respect to a bitumen membrane i.e. “0”, by considering a convection coefficient of 5 Wm-2K-1.

Diurnal and Seasonal albedo variability

The in-field measured albedos with reference to the gravel with the highest solar reflectance i.e. “4” are reported in Figure 2a, with varying weather conditions. No significant changes of the albedo trends have been detected during the monitored days. In particular, a variability < 20% was registered in two typical days of September-October. More constant trends are detected in the mid-day measured albedos i.e.12:00 a.m.-2:00 p.m. time interval. Figure 2b shows the mutual mid-day measured albedos of the evaluated gravels during the monitored period. The monthly average mid-day albedo measured between 12.00 p.m.-2:00 p.m. varies between a minimum of 28.9% (gravel type “5”) and a maximum of 46.7% (gravel type “4”). A large variability in the albedos of the five different gravels is detected, with non-consistent measurements’ results within the monitored months due to the variable weather conditions. In particular, more linear and predictable measurements are found in association with sunny clear days and months i.e. March and September.

Figure 2.(a) In-field albedos (9:00 a.m.- 6:00 p.m.) of gravel “4”, and (b) monthly average albedo (12:00-14:00) with varying gravel field.

Analysis of gravel roof thermal behavior

The simulation of six different scenarios of the case study building was carried out, in order to evaluate the energy performance of the five evaluated gravels when compared to the more common and traditional bitumen membrane i.e. “0”. In particular, the dynamic simulation was performed by taking into account the thermal emittance and solar reflectance values corresponding to the in-lab experimentally measured data by means of portable emissometer and spectrophotometer, respectively. First, a comparison between the outdoor air temperature and the gravels’ surface temperature was performed for two significant monitored days i.e. September, 25th and October 23th (Figure 3a, b), in order to compare the effect of the solar radiation on the different evaluated roof coverings. In particular, the temperature difference between the surface temperature and air temperature generated by each evaluated gravel (“1” to “5”) was compared with the temperature difference generated by the more traditional bitumen covering membrane i.e. “0”. The analysis shows that during September 9th the gravel type “4” generates a maximum temperature difference between the surface temperature and the outdoor air temperature of 4°C, while the traditional bitumen membrane i.e. “0” generates a much higher temperature difference i.e. 28°C. Additionally, in October 23rd, the maximum difference between the air temperature

and surface temperature is equal to 22°C and to 2°C for the bitumen membrane i.e. “0” and the gravel type “4”, respectively. As concerning the other gravels, the difference between the surface temperature and the outdoor air temperature always ranks between the values found for the bitumen i.e. “0” and gravel type “4”.

Figure 3. Surface temperature trends of each gravel for (a) September and (b) October.

By comparing the in-field results and these thermal analyses it is possible to assess that

the surface temperature of the gravels increases with the decrease of the albedo. For this reason, the surface temperature of the gravel are always lower than the bitumen membrane’s values. Therefore, the evaluated gravels could be considered as “cool” roofing material suitable for both cool roof and cool paving applications, given their promising potential in terms of solar reflectivity, albedo, and surface temperature.

Indoor thermal analysis with varying gravel roof typology

Here the indoor thermal conditions are studied in free-running conditions with varying gravel granulometry and typology for the simulated case study building. In particular, in a selected summer day (Figure 4a), an indoor operative temperature reduction of 2-3°C is detected between bitumen roofing membrane i.e. “0” and the best performing gravel granulometry i.e. sand (gravel type “4”). On the contrary, no significant differences in terms of indoor operative temperatures are found in winter conditions by comparing the five covering gravels (Figure 4b). Therefore, a slightly temperature difference of 1°C is registered between the application of gravel type “4” and the bitumen membrane i.e. “0” in the middle hours of the day.

Figure 4. (a) Summer day (September 29th) and (b) winter day (February20th) indoor operative temperature trend of the gravels and the bitumen membrane i.e. “0”.

The mutual trends of the operative temperatures of the six different scenarios over a year are plotted in Figure 5.

Figure 5. Mutual trends of the monthly average indoor operative temperatures in the six different scenarios.

From the analysis of the monthly average temperatures it is possible to determine the

seasonal benefits of the application of the gravel type “4” over the traditional bitumen membrane i.e. “0”. Therefore, the application of the gravel type “5” produces the lowest improvement with respect to the bitumen membrane. A significant reduction of the indoor operative temperature is found between May and September, with a strong enhancement of the indoor thermal comfort inside the building. On the contrary, the cooling effect due to presence of the natural gravel “4” on the roof of the building doesn’t have a significant impact if compared to a traditional bitumen membrane (“0”) during the winter-autumn period. Therefore, in summer conditions the application of the natural local gravel i.e. “4” presents promising cooling benefits on the indoor operative of the building, with a consequent increase of the indoor comfort level, while the winter effect is negligible.

Indoor energy analysis with varying gravel roof typology

The purpose of this analysis is to quantify the reduction of the energy requirements of a selected conditioned area of the case study building i.e. office area due to the application of the gravels over the more traditional bitumen membrane i.e. “0”, both in winter and summer conditions. In particular, Figure 6(a, b) shows the daily trend of the energy requirements for heating and cooling of the selected office area with reference to a summer and winter day, in the six scenarios assessed.

Figure 6. Profile of the energy requirements for cooling and heating in one (a) summer and (b) winter selected day, in the six evaluated scenarios.

Figure 6(a) shows that the energy requirement for cooling is strongly reduced in the scenario characterized by the use of the gravel type “4” rather than the traditional bitumen membrane i.e. “0”. In particular, the offset of the energy consumption for the office area’s conditioning during summer reaches up to a maximum of 4.4 kWh in the middle hours of the day. Therefore, the cooling effect of the gravel type “4” is non negligible during summer. As concerning the winter energy requirements, figure 6(b) shows that the application of the gravel type “4” over the bitumen membrane (“0”) generates an increase in the energy requirement for heating up to 1.2 kWh. Figure 7(a, b) shows the comparison between the monthly energy requirement for cooling and heating in the six different scenarios over a year of operation.

Figure 7. Profile of the energy requirements for (a) cooling and (b) heating in the six scenarios.

During summertime, especially between June and September, the application of the gravel type “4” over the traditional bitumen membrane (“0”) generates a reduction of the energy requirement for cooling up to 6.7 kWh/m2 (July). On the contrary, the analysis of the winter energy requirements shows a reduced energy consumption for heating i.e. 1.2 kWh/m2 (February) in the case of the application of the bitumen membrane (“0”) on the roof of the building, given the high solar reflectivity of the natural gravel type “4”. Therefore, the energy saving due to the application of the gravel type “4” in summertime is higher than the winter energy penalty due to the slight increase of the energy requirement for heating between January and March. The global energy requirement for heating and cooling of the conditioned office area are therefore plotted in Figure 8.

Figure 8. Total energy requirements for heating and cooling of the conditioned area in the six evaluated scenarios.

It is evident that gravel type “4” is best solution among all the evaluated gravels and the bitumen membrane i.e. “0”, as it is able to generate the lowest (i.e. 125.6 kWh/m2) total energy requirement for heating and cooling over a year of operation.

Conclusions

In this paper the experimental and numerical evaluation of a local, low-cost, and natural cool covering for roofs and pavements application was carried out. Four different grain sizes of the same reflective local gravel were evaluated and compared to a more common and mixed gravel type, finally with respect to the traditional bitumen membrane. Both in-lab and in-field analyses were performed, in order to characterize the thermal-optic properties of the gravels in terms of solar reflectivity, emissivity, and albedo with varying weather boundary conditions. Therefore, the dynamic simulation of a case study university building’s thermal-energy performance was carried out, with reference to (i) free-running conditions and (ii) HVAC operative conditions. The laboratory analyses showed important differences with varying grain size and type of stone. In particular, a variability of about 24% with varying grain size was detected in terms of solar reflectance, and a SRI difference of about 30 pt. with varying grain size. Additionally, the in-field analyses confirmed such differences in terms of field monitored albedo. In particular, albedo differences are less visible, given the huge environmental variability. The results demonstrated that the measured albedo of the local high reflective stone is higher with respect to the common gravel i.e. “5” and that the albedo values decrease with the grains’ size within the same gravel type. Additionally, the cooling effect due to increased solar reflectance/albedo is more evident in sunny clear days. Surface temperature decreases up to 5°C with varying grain size and stone type in winter conditions. More impressive results are expected for summer, since the in-field monitoring is on-going.

Therefore, gravel type “4”, which presents the smallest grain size, is shown to be the best performing gravel, and can be considered as a local, sustainable, effective cool coating for roofs and pavements in urban areas affected by urban heat island phenomenon. In fact, the thermal-energy dynamic simulation showed that the gravel type “4” is able to decrease the indoor operative temperature of about 3°C in free-floating conditions with refer to the non-conditioned case study area during a summer day, with respect to the traditional bitumen membrane (“0”). Additionally, the same gravel “4” is able to reduce the energy requirement for cooling of the selected conditioned office area up to 6.4 kWh/m2 during summer. Therefore, it is possible to assess that the summer energy saving due to the application of the gravel type “4” over the traditional bitumen membrane (“0”) is higher than the winter penalty.

Future Developments

Future research will investigate the effects of the application of the local gravels in real full scale buildings through dynamic validated simulation. Additionally, the analysis of directional properties of grains in outdoor pavement application will be carried out. Moreover, a time performance and humidity content analysis of such solution will be performed.

Acknowledgments

The authors’ acknowledgments are due to LUIGI METELLI S.p.A. for providing the gravel stone materials and to Dr. Emanuele Piccioni for assisting the experiment setup. The authors also acknowledge Francesca Lia, for her contribution.

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