2nd Lecture

Heat Transfer and Energy Balance Notes

Introduction to Heat Transfer and Climate Change

Main Topic: Heat transfer and energy balance, crucial for understanding and mitigating the impacts of human activities on the built environment.
- Importance of climate change and its profound impact on future scenarios, necessitating adaptive and resilient building designs.
- Focus on the reuse of materials in construction as a key strategy to reduce embodied energy, minimize waste, and promote a circular economy in the building sector.

Reusing Materials

Considerations for demolishing existing buildings versus renovating or deconstructing them to build new ones.
- The significant possibility and economic benefit of recovering components from demolished buildings, reducing landfill waste and demand for virgin materials.
- In New Zealand, concerted efforts are being made by government bodies and industry to maximize material recovery from demolitions through specific policies, incentives, and infrastructure for material sorting and processing.

Climate Change Impacts

Risks Related to Climate Change:
- Flooding: Increased frequency and intensity of extreme rainfall events and rising sea levels pose significant risks to coastal and low-lying areas.
- Heat waves: Prolonged periods of unusually hot weather leading to increased energy demand for cooling, health risks, and impacting material durability.
- Drought: Extended periods of water scarcity affecting water supply for construction and building operations, and potentially impacting vegetation around buildings.
Example: Auckland floods caused by Cyclone Gabriel (February-March 2023) highlighted the vulnerability of urban infrastructure and housing to extreme weather events, leading to widespread damage, displacements, and economic losses.
- Widespread and varied impacts of climate change experienced globally, from melting glaciers and wildfires to changes in agricultural productivity and biodiversity.

Climate Data Resources

NIWA (National Institute of Water and Atmospheric Research) provides crucial climate information and projections for New Zealand:
- Detailed data on average rainfall, temperature, wind patterns, solar radiation, humidity, and extreme weather event frequencies, all of which are relevant to informing resilient building design and site selection.
Sea Level Rise:
- A direct consequence of global warming and the thermal expansion of ocean water combined with the melting of glaciers and ice sheets.
- Projections indicate significant increases in sea levels, leading to permanent inundation of coastal areas, increased coastal erosion, and saltwater intrusion into freshwater systems.

Analyzing Site Conditions

Use GIS maps (Geographic Information Systems) to accurately identify and visualize potential flood-prone areas, landslide risks, and other environmental hazards affecting site selection for building projects. These maps can overlay topographic data with flood hazard zones.
Areas in Auckland (e.g., coastal zones, floodplains), Dunedin (coastal and low-lying areas), Christchurch (swampland, liquefaction zones), and Wellington (seismic activity, coastal exposure) are at heightened risk of flooding and other climate impacts.
Considerations for existing homes include assessing their current vulnerability and implementing risk management strategies by local councils, which may include zoning regulations, property buy-outs, or mandated flood protection measures.

Building Design Considerations for Climate Resilience

Strategies to Design Durable and Resilient Buildings:

Design Principles:
- Utilizing durable materials and robust construction systems that can withstand projected climate impacts, such as increased moisture, UV exposure, and extreme temperatures.
- Thoroughly assessing materials like timber versus engineered timber for specific applications, considering their moisture resistance, strength, and embodied energy.
- Incorporating climate-resilient design practices:
- Elevating buildings in flood-prone areas through techniques like pile foundations, raised concrete slabs, or stilts, to keep critical building elements above predicted flood levels.
- Designing for optimal natural ventilation, cross-ventilation, and stack effect to reduce cooling loads and improve indoor air quality.
- Implementing effective solar shading strategies (e.g., overhangs, louvers, fins, vegetation) to manage solar heat gain and prevent overheating.
- Integrating rainwater harvesting systems and permeable paving to manage stormwater on-site.
Incremental Housing:
- Designing expandable structures that can accommodate future changes in family size, functional requirements, or climate conditions (e.g., additional room, repurposing spaces).
- Incorporating flexibility in spaces for changing needs or activities (e.g., classrooms that can be easily reconfigured or subdivided, or residential units that can be adapted for multi-generational living).

Practical Design Considerations

Accessibility:
- The imperative need for carefully designed ramps, lifts, or other accessible pathways in building designs to comply with universal access standards, especially when elevating ground floors for flood resilience.
- Addressing challenges in creating raised ground floors while ensuring seamless and dignified accessibility for individuals with disabilities, often requiring integrated design solutions.
Material Recovery and Recycling:
- Traditional demolition practices often prioritize speed over material recovery, resulting in vast amounts of usable materials ending up in landfills, necessitating better planning through pre-demolition audits and deconstruction methodologies.
- Important questions raised about the necessity of all new constructions versus the more sustainable option of redeveloping, retrofitting, or adapting existing structures to extend their lifespan and reduce embodied carbon.

Circular Design Principles

Material Choices for Sustainable Building:
- Biogenic materials: Prioritizing the use of natural, renewable materials like untreated timber, bamboo, hempcrete, and straw bales, which sequester carbon and have lower embodied energy compared to conventional materials.
- Implementing efficient material usage techniques, such as prefabrication, modular construction, and design for disassembly, to minimize construction waste and promote the reusability of components at the end of a building's life.
Second-hand materials:
- Utilizing examples include reclaimed bricks for new facades, recycled asphalt for road bases, re-purposed concrete aggregates, recycled glass in new products, recycled plastics in insulation or structural elements, and salvaged timber for both structural and aesthetic applications.
- Developing and understanding robust methodologies for assessing, processing, and integrating various second-hand materials into new construction, ensuring quality, safety, and compliance with building codes.

Heat Transfer Concepts

Types of Heat Transfer:

Conduction:
- Example: Feeling the heat transfer from a hot cup of coffee directly to your hand when holding it.
- Heat transfer through direct contact between materials, occurring at a molecular level as vibrating molecules transfer kinetic energy to adjacent molecules.
- Solids are generally better conductors than liquids, and liquids are better than gases, due to the closer packing and stronger intermolecular forces. Metals are particularly good conductors due to the presence of free electrons that can rapidly transfer energy.
Convection:
- Heat transfer through the bulk movement of fluids (liquids and gases).
- Example: Heating a room through convection heaters, where air is warmed, becomes less dense, rises, and is replaced by cooler, denser air, creating a circulation current (natural convection).
- Forced convection involves external means like fans or pumps to facilitate fluid movement.
Radiation:
- Heat transfer through electromagnetic waves, which do not require a medium for transmission (e.g., the sun’s rays heating the Earth).
- The interaction between different materials is based on their surface characteristics, specifically their emissivity (ability to emit radiation) and absorptivity (ability to absorb radiation).
- The greenhouse effect is explained through radiation dynamics: short-wave solar radiation passes through the atmosphere and is absorbed by the Earth's surface, which then re-emits longer-wave infrared radiation. Certain atmospheric gases absorb this outgoing infrared radiation, trapping heat and warming the planet.

Interaction of Heat Transfer Mechanisms

Heat transfer in buildings is a complex interplay involving all three mechanisms simultaneously, depending on system design, material properties, and environmental conditions. For example, heat moves through a wall by conduction through solid layers, convection and radiation across air gaps, and radiation from the outer surface to the environment.

Energy Balance in Buildings

Winter Energy Balance:

Heat Losses vs. Gains: During winter, the primary goal is to minimize heat loss and maximize heat gain to reduce heating demand.
- Transmission Losses ( $Q_{trans}$ ): Heat loss occurring through the building envelope elements (walls, roofs, floors, windows, and doors), calculated as:
 $Q{trans} = U \cdot A \cdot (T{in} - T_{out})$
where $U$ is the U-value, $A$ is the surface area, $T{in}$ is indoor temperature, and $T{out}$ is outdoor temperature.
- Ventilation Losses ( $Q_{vent}$ ): Heat loss through controlled air exchange (mechanical ventilation) and uncontrolled air leakage/infiltration, calculated as:
 $Q{vent} = \rho \cdot cp \cdot V{air} \cdot (T{in} - T_{out})$
where $\rho$ is air density, $cp$ is specific heat capacity of air, and $V{air}$ is the volume flow rate of air.
- Gains from Appliances ( $Q_{int}$ ): Internal heat gains generated by electrical devices, lighting, and occupants within the building.
- Solar Gains ( $Q_{solar}$ ): Heat gained from solar radiation passing through windows and opaque building elements, significantly influenced by window orientation, size, and glazing properties.
- Total heating demand is mathematically expressed as:
 $H{demand} = (Q{trans} + Q{vent}) - (Q{int} + Q_{solar})$
or, more simply:
```
 $\text{Heating Demand} = \text{Total Losses} - \text{Total Gains}$ 
```

Summer Energy Balance:

Similar assessments are made for cooling, focusing on minimizing internal and solar heat gain to prevent overheating. Strategies include controlling solar radiation, promoting natural ventilation, and using high-performance glazing.
- Cooling strategies heavily rely on proper building orientation, effective solar shading, and efficient natural ventilation to expel heat and draw in cooler air.

Thermal Performance and Properties of Building Materials

Thermal Conductivity ( $\lambda$ ):
- Measured in \text{W/(m\cdot K)} (Watts per meter Kelvin), indicating a material’s intrinsic ability to conduct heat. Materials with high thermal conductivity (e.g., metals) transfer heat easily, while those with low thermal conductivity are good insulators.
- Low thermal conductivity is highly desirable for insulating materials to minimize heat transfer across building elements.
Thermal Resistance (R-Values):
- Represents a material layer’s or building assembly’s resistance to heat flow. It is crucial for assessing insulation performance.
- An R-value is derived from the ratio of a material's thickness ( $d$ ) to its thermal conductivity ( $\lambda$ ), with units typically $\text{m}^2 \cdot\text{K/W}$ :
  $R = \frac{d}{\lambda}$
- Higher R-values indicate better insulating properties.

U-Values:

The overall heat transfer coefficient ( $U$ ) reflects the rate of heat transfer through an entire building element (like a wall or window) by combining conduction, convection, and radiation effects. It is the inverse of the total thermal resistance ( $R_{total}$ ) of the assembly:
$U = \frac{1}{R_{total}}$
Measured in \text{W/(m}^2\text{\cdot K)} , a lower U-value indicates better thermal performance and less heat loss or gain.

Considerations for Windows and Glazing

Windows typically represent the weakest point for thermal performance in buildings due0 to their relatively high U-values compared to opaque walls.
Use of Low E Glass: A low emissivity coating applied to one or more glass surfaces that selectively reduces heat transfer by reflecting long-wave infrared radiation while allowing visible light to pass, significantly improving thermal performance.
Double and triple glazing effectiveness is achieved by trapping a layer of air or inert gas (like argon or krypton) between two or three panes of glass, which acts as an insulating barrier, reducing all three forms of heat transfer (conduction, convection, and radiation).

Building Form and Performance

Compact Buildings: Reduce heat loss due to lower surface-to-volume ratios.
- The surface-to-volume (S/V) ratio of a building is a critical indicator for its thermal performance. A lower S/V ratio (i.e., a more compact building form) means there is less external surface area exposed to the environment relative to the building's enclosed volume. This characteristic is highly desirable for energy efficiency because it directly minimizes heat loss in winter (fewer pathways for heat to escape) and minimizes heat gain in summer (less solar radiation absorption and heat transfer from the exterior). Consistency in achieving and maintaining a low surface-to-volume ratio throughout the design of a building ensures a more predictable and stable thermal performance. This consistency contributes significantly to reducing the overall energy demand for both heating and cooling, making the building more energy-efficient and inherently resilient to external climatic variations.
- Importance of studying compactness during design phase for energy efficiency.
- Consideration of orientations, porosity, and volumes in designs.

Conclusion

Reiterate the critical importance of adopting sustainable building practices that holistically consider climate impacts, resource scarcity, and energy demand throughout a building's lifecycle.
The necessity for high-performance materials and integrated designs that accurately account for fundamental heat transfer principles and rigorous energy balance calculations is paramount for creating resilient, comfortable, and energy-efficient buildings that contribute positively to environmental stewardship.