PASSIVE COOLING WITH RADIATION REFLECTION SYSTEM
By: Sayyid Habibullah
The World Green Building Council (WGBC) states the definition of a Green Building as follows: “Green building refers to a structure and using process that is environmentally responsible and resource-efficient throughout building’s life-cycle: from sitting to design, construction-operation, maintenance, renovation, and demolition. This requires close cooperation of the design team, the architects, the engineers, and the client at all project stages.”
According to a research conducted by U.S. Department of Energy (DOE) regarding the composition of energy use in residential buildings or Residential Buildings based on its activity sector, the building operational sector namely HVAC sector or Heating, Ventilation, and Air-Conditioning is the sector that most contributes to energy use in residential buildings in United States in 2006. Details on the distribution (in percentage) of energy use contributions in the activity sector are as follows: (1) Heating, Ventilation, and Air-Conditioning up to 39%, (2) Lighting up to 12%, (3) Heating Water up to 12%, (4) Electronics and Computers up to 9%, (5) Refrigerators up to 7%, (6) Cooking Needs up to 5%, (7) Other needs up to 10%.
Figure 1. Use of Energy in the United States in residential buildings. (Source: DOE, 2008 Buildings Energy Data Book, Section 2.1.5, 2008.)
- Passive Cooling
One of the main problems faced in designing a comfortable residential building is how to create a design that can create an atmosphere that suits the comfort needs of building occupants. Thermal comfort is one of the main aspects in creating a comfortable atmosphere. But this aspect is not something that is easy to achieve without having to pay expensive or excessive energy usage. Especially in tropical climates and adjacent to the equator where nature forces the sun to expose radiant heat throughout the day and year, cooling the room as an effort to create a comfortable space is a complex problem. With the sun exposes the earth with heat radiation continuously, the room temperature can easily reach level beyond the human comfort zone.
In 1991, Dr. Wolfgang Feist, an Austrian physicist pioneered the concept of innovative thermal comfort in a more pragmatic way by building the world’s first passive house. This innovation brings new concepts in the world of architecture by introducing ways to shape thermal comfort in a more natural way. This concept does not use mechanical coolers so that it eliminates the costs and energies for excessive cooling. Passive house produces very low energy building. The design concept of passive house that we took and then developed were radiation reflection systems, ventilation systems, and radiative cooling systems.
- Radiation Reflection System
One of the best ways to solve a problem is to prevent the occurrence of the problem from the start. So that in creating good thermal comfort, one of the efficient strategies is to prevent excessive heat entering the building.
- External Walls
The external wall is a vertical plane which also protects the building from the sun’s heat radiation. The external wall is not a source of solar heat radiation which absorbs a greater portion of radiation than the roof, however the external wall remains a factor contributing to heat radiation at the overall displacement thermal value, especially in many-storeys building. The most influential part of wall that contribute to entering the heat radiation into the building is a wall that faces the west and east wind directions so that in particular the wall of that sections require specific treatment.
The main source of heat accumulation in the room is solar radiation absorbed by roofs, walls and windows.  The value of the accumulation of thermal transfer caused by solar radiation into the building can be calculated as the Overall Thermal Transfer Value (OTTV) which is a number that shows the heat gain due to solar radiation passing through the square area of the building envelope.
External Wall OTTV Formula:
OTTVw = AW x UW α x TDEqw
OTTV = the overall thermal transfer value on the outer wall which has a certain direction or orientation.
AW = wall area
UW = Thermal wall transmission
α = Absorption of solar radiation
TDEq = The temperature difference is equivalent between the outer and inner sides
(source: Hong Kong Institute of Architects: “Calculation and Application of OTTV and U-Value”, 2012.)
From the formula above, it can be concluded that the value of absorption of solar radiation is a variable that is directly proportional to OTTV or the Value of Comprehensive Thermal Transfer. So, if we want a little thermal displacement by getting a low absorption value of solar radiatio, it will directly reduce the value of the overall thermal transfer. The value of absorption of solar radiation is an ability to absorb solar radiation contained intrinsically in different materials or building materials with different values. So choosing building materials that have a low absorption value of solar radiation is one of the design steps that can reduce the value of overall thermal transfer in buildings.
In addition, to reviewing building materials, one simple design approach that can reduce the value of absorption of solar radiation so as to reduce the value of overall thermal transfer is to use bright colors on the surfaces that are directly exposed to sunlight heat radiation. According to research conducted by the U.S. Department of Energy National Laboratories, the dark areas of the building’s exterior absorb 70% to 90% of the radiative energy from the sun concerning the outer surfaces of buildings.  From Table 1., we can compare the absorption value of solar radiation from different paint colors, so that we can not only choose color for aesthetic factors, but also as an objective consideration for the strategy to design a thermally comfortable building .
Source: SNI 6389: 2011, about building envelope energy conservation in buildings, National Standardization Agency, Jakarta.
Based on the table above, the color on the wall of a building that can maximally minimize the absorption of solar radiation is a white varnish color. This will further reduce the overall thermal transfer value.
Solar radiation that enters the building through the roof is the largest source of heat for building from the sun. This is because the roof is a wide area that is perpendicular to direct sunlight. Research results from the U.S. Department of Energy National Laboratories, stated that about one-third of unwanted building heat accumulated inside buildings entered through the roof.  So with the large portion of solar heat radiation penetrating across the roof, the roof design is one of strategic approaches that can have an impact on overall value of thermal transfer.
The OTTV value calculation on the roof is similar to the OTTV calculation on the wall, so that the roof can be considered as part of a wall facing vertically. Since, the roof directly exposed to the sun, it will reduce the heat that enters the building, if you choose a brightly colored roof. The roof of the building should be coated with white varnish paint that has a very low absorption of solar radiation.
National Renewable Energy Laboratory (NREL), 1994: “Cooling your home naturally”, DOE National Laboratory U.S., 1994