Passive Cooling of Buildings

D. Asimakopoulos, M. Santamouris, M. Santamouris

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781134254897
• Edition: 1
• Topic: Arquitectura
• Subtopic: Arquitectura general

Energy use in buildings in the EU represents about 40% of the total annual energy consumption. With greater awareness of the need to reduce energy consumption comes a growth of interest in passive cooling, particularly as an alternative to air-conditioning. This book describes the fundamentals of passive cooling together with the principles and formulae necessary for its successful implementation. The material is comprised largely of information and results compiled under the SAVE European Research Programme.

1

Cooling in buildings

Ever since humans have moved into shelters, in search of a more stable environment, they have looked for ways to improve indoor conditions. Inevitably, however, the indoor environment is influenced by prevailing outdoor conditions, daily and seasonal changes in climate and varying occupant requirements due to the type and operation of the building. Depending on the location and season, emphasis is given to either cooling or heating of indoor spaces, in an attempt to counterbalance the unfavourable outdoor conditions and achieve indoor comfort by controlling the indoor temperature, humidity, light availability and air quality.

Historically, for practical reasons and owing to natural laws, humans have been most successful in controlling their environment in situations requiring heat. Maintaining a warm environment has always been considered necessary during the cold season. However, in modern society, maintaining a cool environment during the warm months has proven to be as important for the optimum utilization of human resources and productivity.

The greatest advance has been in the change from rather artful design practices of indoor spaces to the detailed analytical methods that are necessary in order to handle complex building structures. Modern buildings, taking full advantage of currently available state-of-the-art technology, provide an indoor environment with high living and working standards.

Air-conditioning (A/C) systems can be used for year-round environmental control, in terms of temperature, moisture content, and air quality. However, like all mechanical systems, A/C systems consume valuable electrical energy for their operation. The shortage of conventional energy sources and escalating energy costs have caused reexamination of the general design practices and applications of A/C systems and the development of new technologies and processes for achieving comfort conditions in buildings by natural means. Indoor thermal comfort implies that humans are satisfied with the prevailing air conditions in the space – neither too warm nor too cool.

The historical development of the various processes and systems for mechanical or passive cooling, along with current trends and practices in the field, are reviewed in the following sections.

HISTORICAL DEVELOPMENT

Cooling is the transfer of energy from the space or the air supplied to the space, in order to achieve a lower temperature and/or humidity level than those of the natural surroundings. The development of cooling processes has passed through several stages, starting from simple intuitive applications of natural cooling techniques, such as shading, evaporative cooling and air circulation for enhancing the comfort sensation, to mechanical cooling systems, known as air conditioners, based on mechanical refrigeration cycles.

In fact, it appears that there has been a return to the utilization of several well-known techniques and processes that were used successfully even in the early periods of civilization. The principles of passive cooling are the same, but they are now enhanced with the available technological know-how and they are optimized so that they can be successfully incorporated into the building design and operation, in a suitable form for providing the best results.

In the early stages of history earth shelters were used by humans as a readily and naturally available living space, which provided protection from high and low temperatures, as well as from other unfavourable weather conditions. Building architecture quickly developed as an art, as the needs and demands of humans were changing with time, along with the appropriate know-how and availability of tools.

Well before the development of mechanical systems, though, several techniques for providing cooling and comfortable indoor conditions were applied in building architecture. The use of these techniques was not based on the understanding of the physical processes involved, but rather on conceptual experience. The majority were simple applications, like air movement through open spaces, external and internal shading, appropriate arrangement of the immediate surrounding spaces (vegetation, open pools and ponds) and use of proper building materials (‘cold’ marble and light surface colours). In addition, human mobility in indoor spaces was used extensively in order to avoid spaces with uncomfortable conditions during the day, as a result of direct solar gains.

Building design incorporated various fundamental and simple, but effective, principles. The large openings of the buildings, allowed for ample cross air movement, which can have a significant cooling effect. Even if the outdoor air is not at the desired temperature, air movement creates a cooling sensation as it moves around the human body.

The building itself provided sufficient protection to the occupants by properly shading the living spaces from direct solar gains. The landscaping around the buildings was primarily designed for aesthetic reasons, but at the same time improved the microclimate around the building, by providing shading and evaporative cooling. Extensive use of vegetation around the buildings provided the necessary shading, while absorbing large amounts of incident solar radiation and maintaining lower air temperature, which is further reduced by evapotranspiration from the trees. Open pools, fountains, ponds and running water were quite popular in the historical development of architecture, especially in southern dry climates. The phase change during water evaporation can decrease the dry bulb temperature of the air, though at the same time, there is an increase of the water content of the air.

Light coloured outer surfaces have been used extensively in traditional Greek architecture. The picturesque white villages in the Greek islands provide more than an aesthetically pleasing feeling. White surfaces reflect much of the incident solar radiation, thus reducing the heat transferred into interior spaces.

One of the most effective ways, however, of dealing with the problem of high temperatures during the day has also been the behavioural response of people. Use of different, cooler indoor spaces during the day or changes in building use altogether during summer, were very common behavioural actions. Gatherings in cooler open spaces, during the day or at the midday break, and outdoor sleeping during the night were also some simple ways of dealing with high temperatures and uncomfortable conditions.

Progress in science and technology has introduced tremendous changes in all fields, including the management of indoor conditions. In particular, advances in the fields of thermal sciences (thermodynamics, heat transfer, fluids) had as a result the design, development and production of mechanical systems capable of satisfying practically all needs in the field of cooling and refrigeration. The development of a practical cooling and refrigerating machine dates back to the middle of the nineteenth century, although there is evidence of the use of evaporative effects and ice for cooling in very early times [1]. Mechanical cooling for comfort got its start early in the twentieth century, but advances in this area have been rapid.

Mechanical cooling is achieved by several means [2], including:

• Vapour compression systems;

• Gas compression systems involving expansion of the compressed gas to produce work;

• Gas compression systems involving throttling or unrestrained expansion of the compressed gas; and

• thermoelectric systems.

Vapour compression is the most commonly used method in air-conditioning systems. The first equipment dates back to a British patent application by Jacob Perkins in 1835 [3]. The early systems were driven by steam engines and used ether as the working fluid. Compressors and overall equipment size became smaller as electric motors were substituted for the original steam engines and as vapour compression systems were applied to space air conditioning and commercial refrigeration.

Compression is accomplished mechanically or through absorption methods. The simple (theoretical) single-stage vapour compression refrigeration cycle is shown in Figure 1.1.

Cooling is accomplished by evaporation of the working fluid (a liquid refrigerant) under reduced pressure and temperature, in the evaporator, as a result of heat transfer from a high-temperature space. The refrigerant then enters the compressor as a slightly superheated vapour at a low pressure. It leaves the compressor and enters the condenser as vapour at some elevated pressure, where it is condensed as a result of heat rejection to ordinary cooling water or atmospheric air. The relatively high-pressure liquid is then throttled as it flows through the expansion valve. The thermodynamic cycle is completed as the remaining low-pressure liquid again enters the evaporator.

Figure 1.1 A simple vapour compression cycle

The most common refrigerants used in the 1920s and 1930s, were ammonia, carbon dioxide, methyl, ethyl, and methylene chloride, and isobutane, but they exhibited major disadvantages. Finally, the development of nontoxic, nonflammable working fluids, with acceptable operating temperatures and pressures and high efficiencies solved the problem for many decades. Eventually, CFC-12 was accepted as a standard working fluid for air-conditioning applications and refrigerators, HCFC-22 for residential air conditioning, CFC-11 for most commercial air-conditioning applications, and HCFC-502 for low temperature refrigeration.

Gas compression systems involving expansion of the compressed gas to produce work have found commercial applications in air refrigeration systems used for cooling aircraft spaces. The two gas compression systems are also used in the liquefaction of various gases.

Finally, there are some systems that produce cooling by thermoelectric means [4]. The thermoelectric device, such as a conventional thermocouple, utilizes two dissimilar materials. One of the junctions is located in the space under cooling and the other in ambient air, as shown in Figure 1.2. When a potential difference is applied, the temperature of the junction located in the cooled space decreases and the temperature of the other junction increases. Under steady-state operating conditions, heat is transferred from the cooled space to the cold junction. The other junction reaches a higher temperature than the ambient and, as a result, heat will be transferred from the junction to the surroundings.

Figure 1.2 A thermoelectric device

Since in most cases there is a need both to cool and to heat a space, depending on the season, it appears that a system which can be used for both cooling and heating would be most attractive. This can be achieved with a heat engine, which is also known as a heat pump. The principle of the heat pump was introduced in the late mid-nineteenth century by Lord Kelvin. The system, however, received widespread application in the USA after World War II [5].

Technology advances that have provided systems with a high efficiency and performance and lower initial cost are described later in this chapter. However, the apparent concern for an overall reduction of energy consumption, including the increased cost of energy, and inherent problems of mechanical compression systems that operate with refrigerants which have been linked to atmospheric pollution, have caused a re-examination of alternative technologies and systems. Many of the previously described techniques of pass...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1 Cooling in buildings
• 2 Passive cooling of buildings
• 3 The Mediterranean climate
• 4 Microclimate
• 5 Urban design
• 6 Thermal comfort
• 7 Cooling load of buildings
• 8 Heat attenuation
• 9 Natural ventilation
• 10 Solar control
• 11 Ground cooling
• 12 Evaporative cooling
• 13 Radiative cooling
• 14 Simplified methods for passive cooling applications
• Biographies of the authors
• Index