CFD Evaluates Passive Cooling in Heritage Homes

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Dec 19 2024

A recent article published in Energy and Buildings investigated the efficiency of multiple natural ventilation methods in cooling a historic residential structure in San Antonio, Texas, a hot and humid climate area of the United States of America (USA), via computational fluid dynamics (CFD) simulations. Additionally, the potential of these methods to provide a thermally comfortable indoor environment during the spring and summer was analyzed.

Background

Indoor environmental quality in heritage buildings has recently gained significant attention. This is crucial while retrofitting historic structures to modern thermal comfort and energy consumption requirements. Simultaneously, preserving the inherent historical value of heritage buildings can be challenging.

Therefore, passive retrofit strategies are preferred over active systems to improve the energy efficiency of heritage buildings. Moreover, these structures often contribute to indoor environmental stability through their construction materials and methods. Their building characteristics, including the size, shape, envelope, and openings, govern their hygrothermal performance.

Global historic preservation guidelines stress considering alternative cooling techniques, such as natural ventilation, to ensure indoor thermal comfort without compromising valuable heritage materials, features, and values. Natural ventilation can help save energy in hot climates by reducing the need for mechanical systems. Moreover, it is a promising passive retrofit strategy for historic buildings.

Methods

This study explored the relation between natural ventilation and outdoor and indoor environmental conditions in residential heritage buildings for passive cooling under the adverse circumstances of ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) climate zone 2A (hot-humid).

Onsite data and ASHRAE standards were employed to create and validate CFD and energy models for six different natural ventilation methods. The simulations were performed on an early-listed 1900s residential building in San Antonio. This building, in a hot and humid climate, represents a common historic residential building typology in the USA.

Feasibility, cooling potential, and cultural preservation aspects were considered under six scenarios through the spring and summer seasons. The scenarios were: no natural ventilation, ventilation with openings at full capacity, ventilation with openings at half capacity, cross ventilation through opposite windows, stack ventilation using temperature differences to drive airflow, and night flush ventilation.

Furthermore, this study examined the balance between occupant thermal comfort and historic preservation requirements via mixed-mode ventilation and passive cooling instead of exclusive mechanical cooling and ventilation.

Results and Discussion

According to the analysis results, all the considered scenarios could contribute to energy savings in both seasons, especially in spring. Cross ventilation emerged as the most efficient strategy, as opening windows on opposite sides of the building improved airflow and regulated indoor temperature. Additionally, the size of the openings influenced thermal comfort.

To enhance the energy performance of heritage buildings while preserving their cultural and physical values, the inherent energy-efficient features were evaluated, along with the benefits of integrating them into the energy retrofit plan. Features, like existing shading devices (roof overhangs, porch, and awnings), large windows, high ceilings, and several envelope materials, such as brick, wood, and limestone, were identified and assessed for their impact on environmental conditions, cooling loads, and occupant thermal comfort.

The analysis revealed that existing shading devices could effectively lower indoor temperatures and cooling loads while barely increasing relative humidity. High ceilings also generally lower indoor temperatures, but their influence depends on outdoor temperatures. Additionally, replacing wood with high thermal mass materials, like brick or limestone, lowered temperature and enhanced thermal comfort. Alternatively, large windows led to higher temperatures but improved air circulation, reducing relative humidity.

Conclusion

Overall, the researchers demonstrated that heritage preservation and thermal comfort targets can be achieved simultaneously. Notably, cross-ventilation emerged as a promising strategy for energy saving and thermal comfort. However, opening windows is not always feasible or adequate to achieve a tolerable, livable environment.

Heritage buildings have distinct characteristics and specific requirements that frequently limit acceptable technological and architectural interventions. Nevertheless, this comprehensive analysis demonstrates that balancing the enhanced energy efficiency of historic buildings and preserving existing physical characteristics, especially in regions with hot and humid conditions, is possible.

Therefore, passive retrofit strategies should be prioritized, and alternative cooling methods, such as natural ventilation, should be explored. Moreover, these results can be replicated in multiple heritage buildings in similar climate regions worldwide.

Journal Reference

Iskandar, L., Bay-Sahin, E., Martinez-Molina, A., & Beeson, S. (2024). Evaluation of passive cooling through natural ventilation strategies in historic residential buildings using CFD simulations. Energy and Buildings, 114005. DOI: 10.1016/j.enbuild.2024.114005, https://www.sciencedirect.com/science/article/abs/pii/S037877882400121X?via=ihub

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