Materials for Energy Efficiency and Thermal Comfort in Buildings

Matthew R Hall

• Publisher: Woodhead Publishing
• Year: 2010
• eBook ISBN: 9781845699277
• Topic: Architecture
• Subtopic: Sustainability in Architecture

Almost half of the total energy produced in the developed world is inefficiently used to heat, cool, ventilate and control humidity in buildings, to meet the increasingly high thermal comfort levels demanded by occupants. The utilisation of advanced materials and passive technologies in buildings would substantially reduce the energy demand and improve the environmental impact and carbon footprint of building stock worldwide.Materials for energy efficiency and thermal comfort in buildings critically reviews the advanced building materials applicable for improving the built environment. Part one reviews both fundamental building physics and occupant comfort in buildings, from heat and mass transport, hygrothermal behaviour, and ventilation, on to thermal comfort and health and safety requirements.Part two details the development of advanced materials and sustainable technologies for application in buildings, beginning with a review of lifecycle assessment and environmental profiling of materials. The section moves on to review thermal insulation materials, materials for heat and moisture control, and heat energy storage and passive cooling technologies. Part two concludes with coverage of modern methods of construction, roofing design and technology, and benchmarking of façades for optimised building thermal performance.Finally, Part three reviews the application of advanced materials, design and technologies in a range of existing and new building types, including domestic, commercial and high-performance buildings, and buildings in hot and tropical climates.This book is of particular use to, mechanical, electrical and HVAC engineers, architects and low-energy building practitioners worldwide, as well as to academics and researchers in the fields of building physics, civil and building engineering, and materials science.- Explores improving energy efficiency and thermal comfort through material selection and sustainable technologies- Documents the development of advanced materials and sustainable technologies for applications in building design and construction- Examines fundamental building physics and occupant comfort in buildings featuring heat and mass transport, hygrothermal behaviour and ventilation

Part I

Fundamental issues and building physics: understanding energy efficiency and thermal comfort in the built environment

1

Heat and mass transport processes in building materials

M.R. Hall, University of Nottingham, UK

D. Allinson, Loughborough University, UK

Abstract:

This chapter introduces heat and mass transport in terms of the fundamentals and their application in the field of materials and building physics. It is the scientific topic that underpins all aspects of energy efficiency and thermal comfort in terms of the materials that make up our buildings and occupied spaces. An overview of thermodynamics and the conservation laws are provided to serve as a refresher for some readers and as a basic introduction for others. The chapter then deals with heat transfer by providing explanations of the fundamental science and then applying this to topics that are relevant to material properties and their application in buildings. The introduction of mass to these materials (e.g. water) adjusts the thermal properties, which in turn can alter the driving potentials for mass transport, which affects the thermal properties, etc., hence the true situation in materials is fully transient and highly time dependent. It is essential to consider this for accurate analysis and understanding of fabric behaviour, or of the indoor environment behaviour in response to the fabric it is made of. It is also an essential approach for studying phenomena such as surface and interstitial condensation, mould growth, as well as implications of changes to fabric (e.g. retrofit upgrades) and for thermal comfort. Therefore the next section in the chapter introduces mass transport where the approach is to, again, provide explanations of the fundamental science and then apply this to topics that are relevant to material properties and their application in buildings. Clearly mass transport is a subject in its own right, as is heat transfer. However, the chapter concludes by making the important point that in reality the two occur simultaneously and are inter-dependent, which leads on to the subject of hygrothermal behaviour.

Key words

heat and mass transfer

porous materials

thermal properties

hygric properties

1.1 Introduction

This chapter aims to explain and discuss the fundamental laws and processes that apply to the transport of heat energy and mass within the context of the fabric of buildings, i.e. the materials from which they are made. The chapter does not attempt to replace the multitude of excellent key texts already available on the general subject of heat and mass transfer and instead points to references and sources of further information where the reader may wish to deepen their knowledge. Its objective is to serve as a learning tool for keen students as well as a ‘one stop’ revision/reference tool for experienced researchers.

1.1.1 Laws of thermodynamics

As far as we understand, all matter possesses a quantity of mass and a quantity of energy. These two values are inextricably linked and we can assume that their total quantities never change – referred to as the laws of conservation of mass and conservation of energy, respectively. The reasons for this are explained in the following section. The further implications are that energy and mass are constantly moving from one place to another. Energy is formally described as the capacity of a system to do work and can occur either as potential energy (i.e. that stored in a body such as nuclear, electrical, etc.) or kinetic energy (i.e. energy of motion). The internal energy, U, of a body is the sum of potential and kinetic energies between component atoms and molecules, the total quantity of which is measured in Joules, and manifesting itself as the physical property of temperature. One can express this as a thermodynamic temperature, T in Kelvin (K) or as a Celsius temperature, θ in degrees Celsius (°C). Note that a temperature of zero on the Kelvin scale (− 273.15 °C) is called absolute zero because in theory it is the coldest possible temperature when internal kinetic energy is zero. When the temperature of matter in one region is higher than that in another region, transport of energy will attempt to occur from the hotter body to the cooler body until temperature equilibrium is restored. When energy is in transport from a higher temperature body to a lower one, it is described as heat energy. Mass is formally described in terms of its inertia (i.e. its resistance to acceleration), although it can also be measured in terms of the gravitational force it exerts on other objects, or (in practice) as the force by which it is gravitationally attracted to the Earth’s mass, i.e. its weight. Mass transfer is when transport of matter occurs from a higher concentration of mass in one region to a lower concentration in another. One can immediately appreciate how and why heat and mass transport occurs and the importance that this has within the context of this book.

A popular phrase that is used to describe the first law of thermodynamics is that ‘heat is work and work is heat’. Work, W is the fundamental physical property in thermodynamics and simply describes motion against an opposing force, i.e. 1 Joule (J) is equal to the energy needed for 1 Newton of force to push an object over a distance of 1 metre. The energy of a system can be changed either by the system doing work, doing work on the system, or by transferring energy to or from another system in the form of heat. A ‘system’ in this sense can include a control volume of a material, e.g. a collection of molecules. An ‘open system’ is one where both matter and energy are free to enter or leave. In a ‘closed system’ only energy is able to enter or leave, whilst in an ‘isolated system’ neither mass nor energy can enter or leave. In a non-isolated system, the internal energy (U), can be changed by the transport of mass, transport of heat energy (Q; unit = Watts), or by the system doing work. An adiabatic system is one where, theoretically, there is no transfer of heat. The first law of thermodynamics can now be written such that for an adiabatic system Q = 0 and so ΔU = W, whereas in a non-adiabatic system ΔU = Q + W, i.e. the conservation of energy law applies. We can expand this to include the concept of enthalpy, H, which describes the thermodynamic potential of a system, summing internal energy and the product of pressure and volume so that H = U + PV.

The second law of thermodynamics can be explained by the assertion that heat energy cannot of itself pass from one body to a hotter body. This suggests that the natural process of heat transfer is irreversible since heat cannot pass between two systems in thermal equilibrium with one another, nor can it pass from a cooler ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Energy
• Preface
• This publication was sponsored by The Austin Company of UK Limited
• Part I: Fundamental issues and building physics: understanding energy efficiency and thermal comfort in the built environment
• Part II: Materials and sustainable technologies: improving energy efficiency and thermal comfort in the built environment
• Part III: Application of advanced building materials and design: improving energy efficiency and thermal comfort in the built environment
• Index