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					Chapter 5: Solar passive

Ecoserveis

Chapter 5 Passive solar
Introduction Passive solar is an expression applied to the interaction between solar radiation and buildings that requires no active component. When a fluid transfers and distributes the solar heat through the building then the term active solar is used instead (see Chapter 6: solar water heating); both active and passive solar use the infra red portion of the sun’s rays. Before the introduction of boilers and heat distribution systems like radiators or warm air flow, the primary form of controlling the climate inside a building was by means of passive solar architecture. The basic natural processes used in passive solar architecture are the thermal energy flows associated with radiation, conduction, and natural convection. When sunlight shines on a building, the building materials can reflect, transmit, or absorb the solar radiation. Additionally, the heat produced by the sun causes air movement that can be predicted. These basic responses to solar heat have lead to design elements, material choices and placements that can provide heating and cooling effects in a home. These can often be seen in older buildings and particularly in southern Europe where for example, windows have external shutters which can be used to limit the solar gain during the summer and retain heat in the winter. Passive architecture has the great advantage in that it requires no external energy source and therefore has neither a running cost nor does it contribute to environmental pollution. Such features can enhance the visual appearance of a building and will help to preserve its fabric. Whilst it is best considered when designing a new building, many of the techniques can be retrofitted to existing buildings. The potential of any building will depend upon the age, orientation and type.

5.1. Heat characteristics
All bodies radiate heat, the amount radiated depends on the nature of their surface and temperature. The greater the surface or the temperature, the greater the heat radiated. At very high temperatures, the radiation becomes visible as light from filament light bulbs or the sun. Heat flows from a hotter to a cooler body by radiation, conduction or convection. Sunlight heats buildings by radiation; the air does this by convection whilst heat is transferred through the walls by conduction (see Chapter 4). The amount of heat absorbed or reflected by a body depends on the intensity of the radiation and on the colour of that body. Black objects are the best absorbers of heat whilst white objects are the best reflectors. The concept of black and white refers to perfect or theoretical colours; real colours are never perfect so objects would neither absorb nor reflect all radiation. All bodies conduct heat from the hottest parts to the coldest parts, but at very different speeds depending upon the difference in temperature and the ability of their constituent materials to conduct heat; the bigger the temperature difference and the greater the conductivity, the greater the heat flow. These characteristics are very important for the thermal comfort at home. At ambient temperature, say 20ºC, a body with low conductivity will seem warm to us, such as wool or cork, but bodies with high conductivity will seem cool to us, such as metals. The solar greenhouse effect which is responsible for global warming arises from similar principles. The earth’s surface is able to absorb some of the light output of the sun which, when reradiated as heat is absorbed in the lower part of the atmosphere by greenhouse gases such as carbon dioxide. Thermal inertia is the resistance of a body to a change in temperature when the ambient temperature changes: the greater the mass of a body the greater its thermal inertia. This characteristic is important for the thermal comfort in the home. Low inertia buildings are quickly heated by the sun and so quickly cool at night. High inertia buildings keep a more constant temperature as the building acts as a thermal store storing energy in its walls during the day and then giving out this stored heat once the sun goes down and the air cools during the night.

KITH handbook for schools v2

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Chapter 5: Solar passive

Ecoserveis

The solar radiation reaches the surfaces at different angles, depending on the orientation of the surface and the position of the sun in the sky. The incidence angle is very important because it determines how much energy of the solar radiation can be captured or reflected by the surface. Maximum values are obtained when the radiation is perpendicular (90º) to the surfaces. When the radiation is parallel (incidence angle 0º) to the surface, no radiation is neither captured nor reflected by the surface. Bodies conserve their thermal energy unless they interchange it with other bodies or they transform it to other type of energy such as light or electricity. Related topics: Heat losses, Photovoltaic, Solar energy Figure 5.1: Double wall with cavity for high thermal inertia Activity 5.1: Heat absorption and reflection Activity 5.1: Heat absorption and reflection Colour plays a basic role in the absorption of the radiant solar heat as it does for the solar light. In this activity we will experience the effect of colour. A sunny day is needed for this activity. Tasks 1 Prepare three equal flat thin sheets of metal (aluminium, stainless steel) about 12x20 cm. 2 Keep one piece metallic. Paint one piece black and the other piece white. When dry, put them in direct sunlight on a wooden surface. 3 After one minute measure the temperature in front and behind each plate. If a thermistor is not available, use your hand to sense the temperature. Be very careful as the surfaces may be hot. Repeat after five minutes. 4 Discuss with your group which are the most suitable colours to reflect and absorb heat and complete the work sheet.
Notes for teachers: Background: Colour affects the heating and cooling of our homes, of our body and our cars. Choosing appropriate colours reduces the need for energy for heating and cooling. If the sun is not shining then a conventional filament lamp will have the same effect. Such lamps convert about 10% of electricity into light and the remainder into radiant heat. Prevent any contact with the hot surface of the light bulb. Aim: To understand the effect of colour under the solar radiation. Material: three equal flat thin sheets of metal (aluminium, stainless steel) about 12x20 cm. Key words: heat, colour, comfort. Skills: sensing different temperatures with the hand. National curriculum subjects: Age Range: 10-12 Key stage 2-3 Worksheet 5.1 Range of temperature 1 min 5 mins metallic plate white plate black plate Best to use at home To get heat from the sun To avoid heat from the sun

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Chapter 5: Solar passive

Ecoserveis

Activity 5.2: Thermal inertia Activity 5.2: Thermal inertia Thermal inertia is the resistance of a body to a change in temperature when the ambient temperature changes. It is the reason that different objects and materials have different temperatures at the same time and location. Thermal inertia smoothes the extreme ambient temperatures and can be used to improve the thermal comfort in our homes and cities. For this activity research with your group the monthly average and extreme air and water temperatures for a coastal city; air and water representing bodies with low and high thermal inertia. Tasks 1 Obtain data for the monthly average temperatures of both air and water for a coastal city, for a complete year; also note the monthly extreme temperatures for both 2 Record this information on the worksheet. Plot these temperatures. 3 Comment on the extreme values; when they occur, the difference between maximum and minimum values for the ambient and water temperatures. Explain the reasons.
Notes for teachers: Background: The temperature of the sea and of large lakes has a great influence on the ambient temperature of the surrounding cities due to the much greater thermal inertia of the water compared with the air. Extreme temperatures are also important in terms of maintaining a suitable inside temperature. Aim: To look for data, to learn about ambient temperatures of the air and the sea, to understand thermal inertia. Material: data on monthly average temperatures of a coastal city and its sea or lake, paper and pencil and/or a computer. Key words: heat, comfort, ambient temperatures, thermal inertia. Skills: getting local data, plotting monthly data, analysing graphs. National curriculum subjects: Age Range: 10-12. Key stage 2-3 Worksheet 5.2 Monthly temperature (ºC) May Jun Jul Aug Avg. Year

Jan Air mean extreme high low Water mean extreme high low

Feb

Mar

Apr

Sep

Oct

Nov

Dec

Plot both city and sea or lake monthly temperatures against each months. Draw horizontal lines for the yearly averages of the city and water temperatures. Comment on extreme values, differences between extreme values of the city and the water, months of maximum and minimum values for the city and the water, difference of the mean yearly values. Explain the reasons for these differences.

KITH handbook for schools v2

Sept 06

Chapter 5: Solar passive

Ecoserveis

5.2

Solar shading

Heating by solar radiation is ideal during the winter, but not during the summer when it can cause overheating inside a building. Many cultures have learnt how to avoid this undesirable heating by shading the sunny side of the building during the summer. Suitable shading can provide good indoor climate control thereby avoiding air conditioning during the summer whilst helping with heating during the winter. To design good shading, it is necessary to know the solar radiation reaching the building throughout the day during the different seasons. Shading can be accomplished by many different means, depending on the location, the type and geometry of the building and the preferences of the designer. The basic principle is to place the shading so as to reduce the solar radiation during summer and to facilitate the solar gain during the winter. The following are the most common options.  deciduous trees – leaves provide shade during the summer but fall down in the autumn  shutters which are preferably mounted outside the window; these can have the sunlight falling on the window during the summer during the night in winter  blinds – comprise slats which can be inclined to control light (and heat): may be mounted either horizontally (Venetian blinds) or vertically  external horizontal surface – mounted above the window to cut off direct solar rays when the sun is high in the sky (summer, middle of the day); however, when the sun is low in the sky (winter and early morning and late evening during the summer) the rays can fall upon the window and enter the room  awning – an external blind that can be extended or retracted depending upon the strength of the sunlight during the summer  solar panels, flat or tubular, can be used to shade facades or terraces Examples of solar shading are shown in Figure 5.2.

KITH handbook for schools v2

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Chapter 5: Solar passive

Ecoserveis

Figure 5.2: Examples of solar shading

KITH handbook for schools v2

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Chapter 5: Solar passive

Ecoserveis

5.3

Solar heating

The basic characteristics of heat can be utilised to provide solar heating during to winter. The simplest method is by absorption of solar rays by an external, south-facing wall which enables heat to be conducted through to the inner wall of the dwelling. To be most effective, walls should not be shaded by trees or lie in the shadow of adjacent buildings for any length of time. Transmission of light through windows allows infra-red rays to heat the air in the room by convection. If the external window pane is coated with a suitable reflective layer on the inside, then the infra-red rays are reflected back into the room so retaining the heat. The larger the thermal inertia of a building, the more heat can be stored during the daytime thus reducing the need for heating during the night. Trombe walls Trombe is the name of the French engineer that first popularised this type of construction in the 1960’s. A Trombe wall is a black or dark wall that uses the greenhouse effect with a glass placed a few centimetres in front of it to form an air space. The outside of the wall gets heated by the sun and this in turn heats the air in front of it. Openings in the top and bottom of the thermal storage wall allow a convective heat transfer from the heated air cavity to the room inside. At sunset, the openings are closed to avoid a reverse movement of the air that would cool the building. With proper design of the wall (colour, holes, material, thickness) it will remain warm for some time after sunset providing comfort inside the building. Trombe wall Ventilated Trombe wall

Figure 5.3: Trombe wall principle

Figure 5.4: Houses with Trombe walls at Odeillo in France

KITH handbook for schools v2

Sept 06

Chapter 5: Solar passive

Ecoserveis

Activity 5.3: Trombe box Activity 5.3: Trombe box The idea is to build a ‘Trombe’ box and use it to illustrate the basis principles of absorption and reflection of heat. The box will then be used to demonstrate how a Trombe wall can be used to heat a building. Tasks 1. using the diagram provided build the box 2. insert white then black card 3. raise and lower the lamp to show how surfaces can cut of the heat source section through the side

view from the top lid removed

glass

coloured card

Side view of trombe box with light source

Notes for teachers This activity builds on of the some of the passive solar principles described in the main text. Aim: to demonstrate the principle of a trombe wall materials: key words: heat, radiation, reflection, absorption skills: subjects: design and technology, science key stages: 3 Age: 12-14

KITH handbook for schools v2

Sept 06

Chapter 5: Solar passive

Ecoserveis

5.4

Vented windows

Vented windows combine the characteristics of a window and a Trombe wall. As illustrated in Figure 5.5, a Venetian blind is contained within two glass panes with three openings A, B and C; two to the inside and one to the outside. The leaves of the blind are coated black on one side and white on the other.

wall

winter room space inner glass pane

A

B

summer venetian blind outer glass pane

C

wall

Figure 5.5: Vented window

Figure 5.6: A vented window To be effective, vented windows should be south facing. If the black surface of the leaves faces the sun, the air between the panes is heated. In winter with opening A open and opening B closed, the hot air escapes through opening A pulling the cooler air out of the room through opening C to be heated . In summer opening A is closed and hot air escapes through opening B drawing air out of out of the room through opening C. This natural ventilation can be aided during the summer by opening windows or balconies on the cooler side of the building.

KITH handbook for schools v2

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Chapter 5: Solar passive

Ecoserveis

5.5

Natural ventilation
Natural ventilation is an effective way of removing hot air from the interior of buildings using methods such as natural breezes, air temperature differences or the effect of chimneys. In some cases hot dry air can be being cooled and humidified by small scattered fountains (Figure 5.7). An important feature of traditional architecture is to use natural ventilation; that is allowing air to flow from a north to south facing window or east to west during the summer. This air flow, which can be assisted by a fan, enables the dwelling to be cooled by the transfer of air from the cooler to the hotter side of a building. Potential depends upon dwelling orientation and position and size of windows.

Figure 5.7: Hot dry air cooled and humidified by fountains Opening windows on opposite sides of a room will induce natural ventilation. However some buildings may have glass walls that cannot be opened or windows that have to be kept closed due to the traffic noise and pollution. One solution is to use the vented windows as described above. They may remain “closed” but allowing some ventilation, sun protection and visibility. Trombe walls are very effective in south facing buildings and can have attractive designs. They are most common in southern Europe.

5.6 Heat storage
For long term storage such as summer to winter, the ground, water or air can store large amounts of heat. This storage is possible because of their low thermal transfer properties (poor conductivity) and large mass (thermal inertia). They remain cooler than the average summer ambient temperatures and warmer than the average winter ambient temperatures. Use can be made of this property to warm homes in winter and cool them in summer. The heat pump system is a proven technology that can provide this service. For short term storage such as day/night, the thermal mass of the building can be used to store energy passively. Typically the outer fabric will be heated during the day with heat being conducted through the walls to warm the inside of the house. At night the process will be reversed so insulation of the outer walls will reduce the heat loss. The amount of heat stored will depend upon the material(s) with which the outer walls are made. Bricks and cement based blocks will have a high mass and so high storage capacity whereas wood has a low mass and so a correspondingly lower storage capacity.

5.7

Potential in the school and home

Given this basic knowledge, it is now possible to consider how these techniques can be applied. Start by taking temperature measurements in and around the classroom, and then observe the construction and orientation of the building to assess the potential for passive solar techniques. Repeat this procedure for your home and discuss your findings with other members of your group.

5.8

Advice

Your parents, or better still, your grandparents, will be able to explain how they managed without air conditioning or heating. If you look at older buildings in your town you should be able to recognise some of the passive solar techniques described earlier. Further sources of advice could include builders and architects or your local library.

KITH handbook for schools v2

Sept 06

Chapter 5: Solar passive

Ecoserveis

Activity 5.4: Passive solar potential in your school Activity 5.4: Passive solar potential in your school This activity investigates the temperature differences in various areas of the classroom; where it is too hot or too cold and where the temperature is comfortable. Then, after considering any measures that could be used inside, go outside to consider what external passive solar measures could be used to keep your classroom at a comfortable temperature. Tasks 1. measure temperatures in different parts of the classroom 2. as well as considering the inside of your classroom, look at the outside of the building to decide what passive solar options might be suitable e.g.  shutters  window openings/blinds  trees for shading 3. what would be needed to install these?
Notes for teachers This activity to investigate the suitability of passive solar measures to the student’s local environment. Aim: To identify any passive solar measures that would make the classroom more comfortable. materials: pen and paper key words: heat, radiation, reflection, absorption skills: observations, measurements subjects: science, geography key stages: 2-3 Age: 9-12

KITH handbook for schools v2

Sept 06

Chapter 5: Solar passive

Ecoserveis

Activity 5.5: Passive solar potential in your home Activity 5.5; Passive solar potential in your home For this activity, you will consider the conditions in your home and how they change during the year. Ask the other members of your family for their ideas. Tasks 1. Write down the location of the hottest area in your home in the summer. What time of day is it hottest and how do you explain this? 2. Write down the location of the coldest place in your home in winter. What time of day is it coldest and how do you explain this? 3. Write down any measures that you think would improve the solar passive gains in your home during the winter. 4. Write down any measures that you think would reduce overheating in your home during the summer. 5. Ask your Grandparents, or any older family friends, how they made use of the sun in their homes when they were young. Describe any unusual or interesting responses. Are there any measures that they used that could be used in your home today? 6. Consider where you would find further information on passive solar measures that could improve the comfort of your home.
Notes for teachers For this activity the students are asked to apply what they have learnt about passive solar measures to improve the comfort of their own homes. Aim: To identify any passive solar technologies that could be applied to the student’s home. materials: pen and paper key words: temperature, cooling, heating, comfort skills: observation, subjects: science, geography key stages: 2-3 Age: 10-12

KITH handbook for schools v2

Sept 06

Chapter 5: Solar passive

Ecoserveis

Activity 5.6: Previous knowledge. Activity 5.6: Previous knowledge People who lived sixty years ago or more usually have knowledge of how to get thermal comfort at home without using fuels or minimising their use. We can learn from them by asking how they managed when they were young. Tasks 1. Identify a family member or family friend that would be willing to talk to you about living conditions fifty or sixty years ago when they were young. 2. Prepare the set of questions you want to ask. 3. Get in contact and explain what you want to ask. 4. Ask advice on how you may approach the questions. 5. Take notes during the visit, or record it if you are allowed to do so. 6. Fill in Worksheets 5.6 at home.
Notes for teachers: Background: Aim: to learn how to interview an older person to get answers to a set of prepared questions, to learn how to prepare visits, to identify the questions necessary to learn about traditional comfort in the home. Material: paper and pencil, recorder (optional). Key words: thermal comfort, traditional way of leaving, previous knowledge. Skills: grasping the key questions for thermal comfort at home, conducting interviews, getting on with older people. National curriculum subjects: Age Range: . Key stage ?? Worksheet 5.6a Ambient thermal comfort problems What were the problems? How they were managed In winter In summer Worksheet 5.6b How was ambient thermal comfort achieved by applying solar passive concepts:
Windows Curtains, shadows shutters or Ventilation Floor carpets Arrangement of furniture Other

In winter In summer

KITH handbook for schools v2

Sept 06

Chapter 5: Solar passive

Ecoserveis

Activity 5.7: Getting advice. Activity 5.7: Getting advice. It is difficult to get advice on improving the thermal comfort of the home and on other energy matters. Nevertheless, there are several sources of information available that we may not have thought about. Tasks 1 Consider where you would go for advice on how to improve the thermal comfort at your home. 2 Fill in worksheet 5.6 showing the sources of information and advice you would use (Yes/No) and which you prefer to use (Pr.).
Notes for teachers: Background: Proper advice on improving the thermal comfort of the home can make sound economic sense if implemented. This activity offers the opportunity to identify the student’s preferences when looking for information and advice. Aim: This simple activity has two purposes: 1) to illustrate the multiple potential advice sources to students, and 2) to inform teachers regarding the preferential sources of information for their students. Material: internet, phone book. Key words: energy advice, information providers. Skills: looking around for information, asking the right questions. National curriculum subjects: Age Range: . Key stage ?? Worksheet 5.7 Pr consumer association energy advice centres energy day/week local energy exhibition/fair energy seminar/course friends installers internet magazines manufacturers neighbours NGOs Y N parents telephone advice centres plumbers public library relatives school library school peer group school teachers science/technical museum shops TV programmes utility companies Pr Y N

Any other sources of advice you would like to use:

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