Energo-Economic Evaluation of Macroencapsulated PCM and NePCM Integrated Concrete Roof for Passive Building Design

Abstract

Global energy consumption, specifically in buildings, has increased significantly over the past three decades, largely due to economic and population growth. Improving the energy efficiency of buildings is critical to reducing global energy consumption, mitigating the effects of global warming, promoting environmental sustainability, and strengthening energy security. In this context, phase change materials (PCMs) are highly effective in storing thermal energy as latent heat. PCMs are widely used in various industries and buildings because of their ability to store latent heat during solid-liquid phase transition at higher temperatures and release it during liquid solid phase transition at lower temperatures. However, the inherent low thermal conductivity of PCMs, particularly organic PCMs, limits their ability to respond quickly to thermal fluctuations during charge and discharge cycles. Incorporating nanoparticles, metal foam, and fins into PCMs, called nano-enhanced phase change material (NePCM), has improved their thermal conductivity. Roofs, being exposed to direct sunlight, experience significant thermal energy transfer into a building's interior. Employing PCMs has been identified as a promising solution for reducing heat transfer from roofs to interiors. This has significant practical implications for maintaining thermal comfort and optimizing energy savings. However, experimental studies on the thermal evaluation of PCM-integrated roofs, specifically those constructed with reinforced cement concrete (RCC), are very limited. In this study, a comprehensive experimental-based energo-economic assessment of RCC biaxial voided roof macroencapsulated with PCM and NePCM is performed. Further, thermal storage performance measures such as temperature profiles, heat flux, thermal loads, temperature time lag, and temperature decrement factor are evaluated. Financial feasibility indicators, such as electricity cost savings, payback period, and CO2 emissions savings for various fuels, such as lignite, coal, diesel, natural gas, and naphtha, are also calculated. Temperature fluctuations influence the working of PCM in the surrounding environment. Hence, selecting a suitable PCM is critical to ensure the daily charging and discharging of PCMs. This study evaluates a novel selection of the best PCM implemented in a building envelope, utilizing Multi-Criteria Decision-Making (MCDM) techniques while considering local climatic conditions. The AHP weighs, and TOPSIS and VIKOR decision-making methods choose the most preferred material. By integrating MCDM techniques with climate-specific considerations, the OM35/37 is identified to offer the best overall performance and suitability for the specific environmental context of the building location. Following PCM selection, thermophysical characterization using various techniques is performed. Due to the lack of established characterization standards, thermal characterization of PCMs and PCMs enhanced by carbon-based nanoparticles, including multi walled carbon nanotubes (MWCNT) and graphene nanoplatelets (GNP) are carried out. In addition to augmenting thermal conductivity, nanoparticle incorporation altered the phase change process, resulting in a narrower temperature range and reducing supercooling while preserving high latent heat capacity. The PCM-integrated roof reduced interior temperature by up to 7.2 °C during sunny hours, heat transfer to the interior by up to 60.6%, and thermal load by up to 54%. In addition, considering heating and cooling, it offers an average daily saving of 0.06 USD/kWh/m2 with a payback period of about 5.7 years. Further, it gives CO2 emissions savings of up to 13.7, 12.3, and 4.3 kgCO2/kWh for lignite-, coal-, and natural gas-fired power plants. In addition, its mean time lag is 4.2 hr., with a decrement factor of 0.75 compared to 3.9 hr. and 0.85 for the normal RCC unit. Similar trends were observed with the OM37 integrated roof. Furthermore, 2% and 4% GNP–NePCMs integrated roof reduced indoor temperatures by an average of 8.0, 9.3, and 9.7 °C during sunny hours, saving 89%, 42%, and 48% more electricity for space cooling than for heating, and have simple payback periods of 5.7, 5.8, and 7.3 years. The average heat flux in OM35, 2%, and 4% GNP–NePCMs is 63%, 79%, and 82% lower than the conventional slab. Further, 2% and 4% GNP–NePCMs save 67% and 65% more CO2 emissions and 40% and 46% less mean time lag than OM35. From the study, it can be concluded that the proposed PCM-integrated roof is promising and commercially viable.