(182g) Energy Upgrade of a Building with TiO2 Coated Roof Tiles Using the Energy Plus Simulation Program

The huge energy demands buildings require for cooling have led the search for innovative solutions to ensure energy performance in the summer months. One of the most promising solutions is that of “cool roofs”. These have high solar reflective properties and absorb less solar energy. Materials with high reflectiveness (NIR) are in great demand in applications such as energy saving roofs, walls and pavements When an object absorbs solar radiation, energy from the sun is converted into heat energy, while approximately one third of the heat absorbed by one building originates from its roof. A study conducted by Selvaraj et al. [1] found that by increasing the heat reflection from the roof of their house the owners saved an average of 23% on house cooling costs. Another study by Akbari et al. [2] presented the results of applying a cool roof on a retail shop in Montreal, Canada, and found an annual saving of 60$ per 100m² roof. A simulation study by Konopacki et al. [3] concluded that having a cool roof can reduce annual energy costs in most regions of the world that require air conditioning in the summer season. In this study, the cooling loads required for the summer months by a nursery school located in Thessaloniki, Greece, were calculated for a roof of natural red clay tiles coated with TiO₂ and PEG. Simulation of the energy behavior of the building was performed using EnergyPlusâ„¢ software.

Methodology

Synthesis of TiO₂/PEG nanoparticles was conducted by adding 2.5 g of titanium dioxide into 100 mL of ionized water. The suspension was stirred and sonicated for 10 minutes at each phase of the development procedure. After this time, 180 μL of polyethylene glycol was added dropwise into the mixture and stirring was started for a total time of forty minutes with a sonication bath (20 minutes) at the half-way time point.

Natural and TiO₂ coated red tiles were characterized through solar reflectance and thermal emission measurements. Spectral reflectance measurements were carried out using the Jasco UV/VIS/NIR spectrophotometer, model V670, in the wavelength range of 250-2500 nm. Thermal emissivity measurements were carried out using a Devices and Services emissometer instrument, model AE1.

The EnergyPlus program was used for the energy simulations. Together with Open Studio, this program forms part of the usual energy modeling and simulation study of any building. Moreover, the Sketch Up program was used to construct the spaces of the energy model. This is a design program that allows the three-dimensional design of any object. The geometry of the building input into the program was based on the original design plan of the school. The three-dimensional thermal model was generated according to the geometry of the building. Each room/space was separated and building openings were included in the model. Additionally, the external conditions of each surface were defined. Following this, construction sets that correspond to the structure of each surface were created and the use of each space was defined. To estimate its energy efficiency, the building was divided into thermal zones, i.e., areas with similar uses and/or the same electromechanical systems. The complex design of the building and its variety of area usage led to the creation of six different thermal zones. These zones account for the air-conditioned spaces within the building. The roof is not air-conditioned but does form part of the building and does play a major role in its energy performance. Therefore, it was defined as an additional thermal zone. The building comprises one floor with an area of 1313 m² and a roof with an area of 1153 m². The roof is formed of the following construction elements: roof tile, roof membrane, roof insulation, roof membrane and wooden frame. Based on the above, the coated roof tile was evaluated as to how it influences indoor operative temperature, which impacts indoor thermal comfort perception.

The characteristics of the natural red clay tile used by the program to calculate thermal loads were: 0.4 W/m*K (conductivity), 2000 kg/m³ (density), 800 J/kg*k (specific heat), 0.42 thermal absorptance, 0.61 solar absorptance and 0.77 visible absorptance, while the characteristics of the roof tile coated with TiO₂ (anatase) and PEG were: 0.4 W/m*K (conductivity), 2000 kg/m³ (density), 800 J/kg*k (specific heat), 0.2 thermal absorptance, 0.19 solar absorptance and 0.15 visible absorptance. In this way, each change in the model would generate a corresponding change in its energy consumption and the real percentage of saved energy could be determined for each scenario. Thermal analysis of the coated roof tile was carried out in two seasons (summer and winter) to account for the climate of the particular region of the school’s location.

The city of Thessaloniki is located in northern Greece within climate zone C. The school is heated from 15^th October until 30^th April each year, and is cooled from 1^st June to 31^st August each year. The total number of heating degree days differing from the optimal 20°C is estimated as 2184 and the total number cooling degree days for the limit of 26°C is estimated as 135. Hourly meteorological data site were collected from the database Europe WMO Region 6 (Thessaloniki 166220 (IWEC) and used by the EnergyPlus engine.

Results

Measurements of total solar reflectance (TSR) and UV, VIS, NIR were carried out on the natural clay tile, the clay tile coated with PEG, the clay tile coated with TiO₂ anatase, and the clay tile coated with TiO₂ anatase and PEG. The results showed that the reflectance spectrum is significantly improved in the UV, VIS and NIR regions until approx. 1900 nm and that the optical absorption of TiO₂ anatase significantly increases at 400nm and above. Regarding the tile coated with the TiO₂ anatase and PEG mixture, the TSR increased by 51% compared to the TSR of the natural red clay roof tile. Values of natural tiles and tiles coated with PEG alone were at the same level, indicating that PEG does not contribute to reflectance. Thermal emission measurements showed that tiles coated with TiO₂ (anatase) and PEG have slightly higher emission values than the natural clay tiles. Results of the simulations with Energy Plus to calculate the cooling and heating loads required per month when the roof is covered with natural clay tiles or clay tiles coated with TiO₂ and PEG, showed that the cooling loads required to maintain the temperature inside the building below 26^ΟC in the summer, reduced significantly when the proposed coated roof tiles were applied. In July and August when the hottest environmental temperatures are recorded the cooling loads were reduced by 24% and 23%, respectively. The cooling loads were zero in the winter months. Regarding the thermal loads, there is an average increase of around 7% in the winter. The number of hours in summer with temperatures higher than 26°C was calculated within each of the building thermal zones when, a) applying natural red clay roof tiles, and b) applying roof tiles coated with TiO₂. It was found that when the roof tile coated with TiO₂ was applied, the percentage of hours with T>26°C inside the building reduced by 20-44% depending on the thermal zone.

Conclusions

The results of this study showed that there is a significant difference in cooling load consumptions when applying natural red clay roof tiles and clay roof tiles coated with TiO₂ & PEG. The total cooling load required to maintain temperatures below 26^oC in the summer is 63.450 kWh when roof tiles coated with ΤiO₂ & PEG are applied and 82.828 kWh when uncoated natural red clay roof tiles are applied. On the contrary, the thermal load required in winter to maintain a temperature >20^ΟC is 42.308 kWh when roof tiles coated with ΤiO₂ & PEG are applied and 39.203 kWh when uncoated natural red clay roof tiles are applied. Therefore, energy consumption is reduced by 13.3%. Given that the price of 1 kWh in Greece for the first half of 2022 was set at 0.2305 euro/kWh (higher than the average EU price of 0.2525 euro/kWh) [4] the percentage reduction of energy required for cooling brought about by the use of cool roofs can be economically significant for both private households and the public sector, not only in Greece but also in other countries that experience high summer temperatures.

Funding sources

This research study was funded by the Operational Programme Competitiveness, Entrepreneurship and Innovation 2014-2020 (EPAnEK) of the Ministry of Economy and Development, aiming to develop a new bioclimatic product which will have photoreflective and photocatalytic properties for use in cool roofs.

References

1. Meenakshi, P. and M. Selvaraj, Bismuth titanate as an infrared reflective pigment for cool roof coating. Solar Energy Materials and Solar Cells, 2018. 174: p. 530-537.
2. Hosseini, M. and H. Akbari, Effect of cool roofs on commercial buildings energy use in cold climates. Energy and Buildings, 2016. 114: p. 143-155.
3. Konopacki, S., Cooling energy savings potential of light-colored roofs for residential and commercial buildings in 11 US metropolitan areas. 1997.
4. Eurostat. Electricity price statistics. 2023 7 March 2023, at 16:19; Available from: https://ec.europa.eu/eurostat/statistics-explained/index.php?title=Elect....