Advances in Passive Cooling

Mat Santamouris

• Publisher: Routledge
• Year: 2012
• ISBN:
  9781136547331
• Edition: 1
• Topic: Architecture
• Subtopic: Architecture General

Following a rapid increase in the use of air conditioning in buildings of all types, the energy demand for powering such devices has become a significant cause for concern. Passive cooling is increasingly being thought of as the best alternative to air conditioning.

This book offers the latest knowledge and techniques on passive cooling, enabling building professionals to understand the state of the art and employ relevant new strategies. With separate chapters on comfort, urban microclimate, solar control, ventilation, ground cooling and evaporative and radiative cooling, this authoritative text will also be invaluable for architects, engineers and students working on building physics and low-energy design.

Advances in Passive Cooling is part of the BEST series, edited by Mat Santamouris. The aim of the series is to present the most current, high quality theoretical and application oriented material in the field of solar energy and energy efficient buildings. Leading international experts cover the strategies and technologies that form the basis of high-performance, sustainable buildings, crucial to enhancing our built and urban environment.

1

Progress on Passive Cooling:
Adaptive Thermal Comfort and
Passive Architecture

Fergus Nicol and Susan Roaf

INTRODUCTION

This chapter seeks to show how the decision to air condition a building does not only have a fundamental influence on the comfort of the building’s occupants, but also profoundly influences its sustainability and even that of the other buildings within the community, city and culture of which it is a part. The chapter also shows that the philosophy of comfort embodied in international standards promotes mechanically cooled environments, as do the assumptions that the standards incorporate and the solutions they impose on building designers. Existing international standards are derived from laboratory experiments and tend towards static solutions to preferred indoor conditions. This approach is contrasted below with the essentially dynamic ‘adaptive’ approach to designing for comfort, based on observations of human behaviour in real buildings, in a wide range of climates, cultures and economic environments. This approach is shown to be more compatible with reducing energy use in buildings and, in particular, with passive approaches to the design of buildings. The chapter ends by suggesting that there is now an environmental imperative to develop a generation of ‘modern vernacular’ buildings that combine the lessons of traditional buildings with the benefits of appropriate modern technology to provide truly sustainable buildings for the 21st century.

Our thermal experience

In our daily lives most people will have a pattern of ‘normal’ thermal experience and behaviour that reflects their own personal circumstances and the culture and climate in which they live. Examples are given by Roaf (1988) and by Nicol (1974).

In her thesis on the wind catchers of Yazd in Iran, Roaf (1988) describes the daily thermal routine of the local adapted population occupying traditional mud brick houses, with their wind catchers set in the one- to three-storied buildings over deep cellars:

In contrast to the Western approach to comfort and design in which the individual chooses the climate for a room, the Yazdi living in a traditional house selects a room for its climate. Such choice and movement around a house during a day constitutes a behavioural adjustment that has been an essential adaptive strategy evolved by the people of such hot desert regions, enabling them to inhabit a seemingly hostile environment with some degree of comfort. In the heat of the Yazdi summer, starting out from sleeping on the roof, they will migrate to the courtyard, which provides shade and relative cool in the morning and, thence, to the cellar to rest through the hottest hours of the day. Towards evening they will come out into the relative heat of the courtyard, which may initially be cooled a little by water thrown on to the hot surfaces, and will then grow cooler as night draws near. In late autumn, a different migration occurs, horizontally from the shaded north-facing summer wing, to the south-facing winter rooms of the courtyard, deliberately warmed by the sun. The consequence of this daily movement is that by recording climate in one or two spaces, one does not cover the diurnal range in climate experienced by the occupants of the houses. In Yazd, it has been necessary to follow the occupants around the house, climatically, in order to record and, in turn, understand, the nature of the ‘occupied’ summer climate in the houses of Yazd.

In a different context Nicol (1974) quotes a description by M. R. Sharma of the daily routine in laboratories and offices in the Central Building Research Institute in Roorkee, India:

The room is full of warm air in the mornings. The windows are opened and the fans run at full speed to churn cool air into the room. Within half or three quarters of an hour the air is cool enough for work to begin. Conditions remain comfortable with fans running throughout the forenoon.

These two thermal ‘diaries’ share one characteristic: in each case the building occupants are using their building to make themselves comfortable – in the case of Yazd, by internal vertical and horizontal migration using the different temperatures in different parts of the building; in Roorkee, by using the ceiling fans to offset the increasing temperature as the day wears on. This interaction between the buildings and their occupants is crucial to the approach to thermal comfort that is presented in this chapter. It arises from observations of people in their normal environment and separates what might be called the ‘traditional’ approach to thermal comfort (concerned mainly with physics and physiology) and the ‘adaptive’ approach that is being increasingly used to inform building design.

What we try to show in this chapter is that the ‘traditional’ approach to thermal comfort arises from the needs of the heating and cooling industry predicated on their need to provide a predefined set of indoor conditions that are calculated to be optimal, with a minimum of active participation by occupants. Most passive buildings, on the other hand, require their occupants to take an active role in controlling the indoor environment. This makes the adaptive approach, developed from observations of user behaviour, better suited to the needs of designers who use passive cooling and heating in their buildings. The chapter introduces the thinking behind the adaptive model of comfort and shows how the choice of comfort approach has deep and far-reaching implications for the design of buildings and cities in the coming decades in which all buildings will increasingly need to rely on passive methods for their thermal performance.

THE HEAT BALANCE APPROACH TO DEFINING COMFORT IN BUILDINGS

Underlying assumptions and methodology

Thermal comfort is famously defined as ‘that state of mind which expresses satisfaction with the thermal environment’ (ASHRAE Standard 55, 2004). Despite the fact that this is a psychological definition (a state of mind), it has usually been modelled in terms of physiology and physics. The aim of those investigating comfort has been to define the thermal environment that a heating, ventilating and air-conditioning (HVAC) system has to provide in order to ‘ensure’ a comfortable environment. Much of the research on which such standards are based has been done in climate chambers – thermally controlled laboratories. Subjects’ reactions are typically monitored over a period of three hours in any one set of thermal conditions. The final response of subjects is measured using the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) scale shown in Table 1.1. In order to generalize the results, the responses have been related to a heat balance model – the assumption being that a pre-condition of thermal neutrality (a 0 response on the ASHRAE scale) will be a (steady state) balance between metabolic heat production and overall heat losses to the environment.

Table 1.1 American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and Bedford scales of user response

ASHRAE descriptor	Numerical equivalent	Bedford descriptor
Hot	3	Much too hot
Warm	2	Too hot
Slightly warm	1	Comfortably warm
Neutral	0	Comfortable
Slightly cool	–1	Comfortably cool
Cool	–2	Too cool
Cold	–3	Much too cool

Fanger’s predicted mean vote (PMV)

The best known of such heat balance models is the predicted mean vote (PMV) of Fanger (1970), based on experiments in US and Danish universities during the 1960s. Fanger proposed values of skin temperature and sweat secretion for thermal comfort. He obtained data from climate chamber experiments, in which sweat rate and skin temperature were measured for people who were comfortable at various metabolic rates. Optimal sets of environmental conditions for thermal comfort were deduced from the metabolic rate and the clothing insulation.

Fanger extended his work by proposing a method by which the mean thermal sensation of a group of people (on the ASHRAE scale in Table 1.1) could be predicted. His assumption for this was that the sensation experienced was a function of the physiological strain imposed by the environment. This he defined as ‘the difference between the internal heat production and the heat loss to the actual environment for a man kept at the comfort values for skin temperature and sweat production at the actual activity level’ (Fanger, 1970). He calculated this thermal load for people involved in climate chamber experiments and used it to predict their comfort vote.

The final equations for optimal thermal comfort and for PMV are presented in textbooks (e.g. McIntyre, 1980; Parsons, 2002). Fanger presented the results in the form of diagrams and tables from which optimal comfort conditions can be read, and CEN-ISO 7730 (ISO, 1994) and ASHRAE 55 (2004) include a computer program that can be used to calculate PMV for a group of people in a particular environment given a knowledge of their mean metabolic rate and clothing insulation.

Strengths of the heat balance model in defining standards for highly serviced buildings

Creating thermal comfort for man is a primary purpose of the heating and air-conditioning industry, and this has had a radical influence ... on the whole building industry ... thermal comfort is the ‘product’ which is produced and sold to the customer. (Fanger, 1970, pp14, 15)

Fanger’s approach is justified by his aim to define the ‘product’ (comfort) that the HVAC industry is ‘selling’ to the customer. The requirement is to define conditions for comfort in a building serviced by heating or air conditioning. From this flows the assumption that a stable and closely controlled indoor environment is required, as it is assumed that any deviation from optimal conditions will increase the risk that the subjects will become uncomfortable. This assumption is embodied in international and European standards, such as ISO 7730 (ISO, 1994) and CEN 15251 (CEN, 2007). International standards are now dividing buildings into A (best), B or C categories solely on how closely their indoor environment is controlled (±0.2PMV, 0.5PMV or 0.7PMV, respectively).

Methodological and philosophical weakness
of the heat balance approach in a real situation

One obvious strength of this approach that helps to explain why PMV is so widely used is that it provides a ‘number’ that engineers, facilities managers or building occupants can set the thermostat to, only changing it (if at all) at the transition between winter and summer. The difference between indoor and outdoor temperatures will consequently be wide when the weather is particularly cold or hot, causing more energy to be used to run the systems than would be the case if the indoor temperature was to a greater extent linked to the outdoor temperature. When the method was being developed it may have been necessary to manually set the thermostat of the system. Today the technology exists to enable the indoor temperatures to track outdoor weather conditions in such a way as to maintain indoor comfort while reducing energy use (Nicol and McCartney, 2001).

For environmental designers, the calculation of PMV poses other problems (Nicol et al, 1995):

• First, we must assume that conditions in the building approach those of the steady state in the climate chamber.
• We must also know the mean clothing insulation of the building occupants and their mean metabolic rate (and there is an additional problem for buildings where a number of activities are taking place in the same space). Both of these variables are difficult to measure and, in the absence of accurate knowledge, the tendency has been to make an assumption (e.g. a clothing insulation of 0.5 in summer and 1.0 clo in winter) (CIBSE, 2006).

A further fundamental question has also been raised by Humphreys and Nicol (1996) about the assumption behind the PMV equation itself. The PMV equation is based on the assumption that the thermal sensation of a person (away from neutral) is a function of the thermal load on the body, which is then expressed as a deviation from a state of thermal neutrality. This is different from the comfort criteria in the equation for comfort expressed in terms of heat balance, mean skin temperature and sweat secretion (Fanger, 1970). This leads to an internal contradiction when applied to people not in thermal neutrality who are wearing different levels of clothing so that for a single load, different levels of thermal sensation and physiological response (skin temperature and sweat rate) would occur. In addition, if there is a net thermal load, then theoretically the body will either warm up or cool down and this is incompatible with a model that assumes thermal balance. Experimental investigations by Parsons et al (1997) tended to confirm these doubts about PMV.

The heat balance method applied
to passively cooled buildings

Conditions in passively cooled buildings often cannot be controlled to the same extent as in buildings with mechanical air conditioning. Using natural phenomena such as the wind, the sun and the outdoor temperature, such buildings cannot be closely regulated to a single temperature in the same way as those with fully mechanical systems. If the ‘highest’ leve...

Table of contents

• Front Cover
• Half Title
• Title
• Copyrights
• Contents
• List of Figures, Tables and Boxes
• List of Contributors
• Preface: Why Passive Cooling? By Mat Santamouris
• List of Acronyms and Abbreviations
• 1 Progress on Passive Cooling: Adaptive Thermal Comfort and Passive Architecture
• 2 Opportunities for Saving Energy and Improving Air Quality in Urban Heat Islands 30
• 3 Solar Control
• 4 Ventilation for Cooling
• 5 Ground Cooling: Recent Progress
• 6 Evaporative Cooling
• 7 Radiative Cooling
• Index