Energy Efficiency
Building Design Guidelines
for Botswana
Developing Energy Efficiency and Energy Conservation in the Building Sector, Botswana
Project Funded by Danida
Department of Energy Ministry of Minerals, Energy and Water Resources
ENERGY EFFICIENCY
BUILDING DESIGN GUIDELINES
FOR BOTSWANA
September 2007
Developing Energy Efficiency &
Department of Energy
Energy Conservation
Ministry of Minerals, Energy
in the Building Sector, Botswana
and Water Resources
Author:
Andreas Groth Acknowledgments:
Jacob Knight of Arup Botswana wrote most of Section 8, Mechanical Systems, and made helpful comments on the other sections. Jesper Vauvert of Danish Energy Management A/S was the team leader for the project. He guided the preparation of the Guidelines throughout and reviewed the document.
A Task Force representing interested stakeholders reviewed the various drafts of the Guidelines as it developed and helped to guide the process. The following were members of this Task Force:
o Mr. J. McCrory (Architects Association of Botswana) o Mr. A. Ntlhaile (Botswana Bureau of Standards)
o Mr. M. Tafila (Association of Citizen Development Consultants) o Mr. T. Morewagae (Association of Consulting Engineers, Botswana) o Mr. N.Ofetotse (Botswana Housing Corporation)
o Mr. E. Mazhani (Botswana Institute of Development Professions) o Mr. H.T. Tumisang (Botswana Technology Centre)
o Mr. H.B. Brown (Department of Building and Engineering Services) o Mr. B. Kgaimena (Department of Energy)
o Mr. G. Kumar (Department of Energy) o Mr. A. Groth (Department of Energy) o Mr. J. Vauvert (Department of Energy) o Mr. A. Sebinyane (Department of Housing)
o Dr. Sajja (Department of Local Government and Development) o Mr. R.F. Rankhuna (Department of Town and Regional Planning) o Mr. F. Masuku (Gaborone City Council)
The project team for the project: Developing Energy Efficiency & Energy Conservation in the Building Sector, Botswana, and the staff of the Department of Energy, Ministry of Minerals, Energy and Water Resources, Government of Botswana all gave their full support and encouragement in the preparation of this document. Danida funded the work (contract no.: 104 Botswana. 1.MFS.15).
Layout:
The Guidelines has been formatted in landscape orientation in order to make it easy to read on screen as a pdf file. The font size and scale for images have been chosen to allow it to be read at a scale that shows one page at a time.
In print format the Guidelines is intended to be printed on both sides and bound on the left side of the odd pages. Comments and recommendations:
Comments and recommendations for revisions should be sent to:
Ministry of Minerals, Energy and Water Resources, Department of Energy, Private Bag 00378, Gaborone, Botswana Tel: +267 3914221, Fax: +267 3914201, email: [email protected], website:www.energyaffairs.bw
or the author:
Andreas Groth, Motheo (Pty) Ltd., P.O. Box 2224, Gaborone, Botswana, Tel: +267 3923462, Fax: +267 3923632, email: [email protected]
Published by Department of Energy
© Department of Energy, Danish Energy Management A/S, and Motheo Pty. Ltd. All rights reserved, 2007
REVISION TABLE
Revison No. Date issued: Sections Revised: Comments:0 July 2007 All Original document, based on revisions to Draft No. 3 as presented at a Workshop in Gaborone, Botswana on 7 March 2007.
1 September 2007
1, 2, 3, 4, 7, 8, 9, 10, 13 Amendments based on comments by Jacob Knight. Additional properties of materials and elements.
SECTION 1
INTRODUCTION
ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA
ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA
Sections:
1.
Introduction.
2.
Design
Brief.
3.
Climate.
4.
Indoor
Environment.
5.
Design
and
construction process.
6.
Planning.
7.
Building
envelope.
8.
Mechanical
Systems.
9.
Lighting - artificial and day lighting.
10.
Operation & Maintenance and Building Management Systems.
11.
Simulation.
12.
Life-Cycle
Cost
Analysis.
CONTENTS
1. INTRODUCTION 4 1.1. Background 4 1.2. Overview 4 1.2.1. Overall aim. 4 1.2.2. Classes of building. 51.2.3. Codes and Regulations. 5
1.3. Structure of the Guidelines 5
1.3.1. The Design Brief. 5
1.3.2. Technical Sections. 5
1. INTRODUCTION
1.1. Background
The project ‘Developing Energy Efficiency and Energy Conservation in the Building Sector, Botswana’ was established in the Ministry of Minerals, Energy and Water Resources in 2005 to address the Government policy as stated in NDP 9:
“… Improving energy efficiency and conservation is cost effective, offers a chance to defer new investment and helps reduce energy related pollution. During NDP 9,
Government will continue to support and encourage improved energy efficiency and conservation in all sectors of the economy. Planned measures to achieve the policy objectives are:
• Carrying out information and educational campaigns. • Conducting energy audits of energy intensive industries
and Government institutions
• Promoting energy efficient design and operation of
buildings.
• Developing and implementing a national energy
management plan.”
One activity of the project was to develop guidelines for the design of energy efficient buildings. This was done through a process of consultation with interested parties through a Task Force that has been established for this purpose. It is expected that this document will need to be regularly revised over the coming years to keep it up to date with
developments in the knowledge base and the regulatory environment of the building sector. The Guidelines and any subsequent revisions will be available as ‘pdf’ files on the website of the Department of Energy and the project website at http://www.eecob.com/.
1.2. Overview
1.2.1. Overall aim.
The Guidelines is intended to be a resource that will help in achieving the overall aim to improve energy efficiency and energy conservation in the building sector.
To achieve this, energy efficiency should be considered from the beginning of the lifecycle of a building. This is typically the stage when the initial Design Brief is prepared For this reason the Design Brief has been chosen as the core document around which these Guidelines are structured.
Energy efficiency needs to be considered at every stage of the lifecycle of a building. An optimum level of energy efficiency can be achieved when all aspects of the building design, construction and operation are integrated with each other in a coordinated manner to take full advantage of the opportunities that such synergies offer.
The Guidelines can assist in this by providing relevant information and guidance on key issues related to the various stages in the life of a building from inception, procurement, design, construction, commissioning,
This will hopefully facilitate timely incorporation and consideration of those aspects early in the design process. 1.2.2. Classes of building.
Requirements and opportunities for energy efficiency differ in certain ways for different types of buildings. The first edition of the Guidelines is specifically directed at the following broad classes of building:
o Office buildings.
o Public facilities, such as Police Stations. o Health facilities, e.g. hospitals and clinics. o Schools.
o Residential houses. 1.2.3. Codes and Regulations.
At present the Codes and Regulations relating to buildings in Botswana make little or no reference to energy
efficiency.
In the absence of a specific Botswana code for energy efficiency in buildings, building developers may wish to use the Guidelines as a tool to achieve energy efficiency in new buildings. This may be done by encouraging
consultants to work in accordance with the
recommendations of the Guidelines throughout the design and construction process.
It is the intention that the information and
recommendations contained in the Guidelines will be helpful in the development of an Energy Efficiency Code for buildings if and when this happens.
1.3. Structure of the Guidelines
1.3.1. The Design Brief.
When the need for a building has been established, it is good practice to prepare a Design Brief for the building. This should define all the requirements of the building, including the overall objectives that the building is intended to meet, the specific spaces that it will provide, their
characteristics and relationships to each other, how the building will respond to its environment, constraints imposed by the site, the budget, the programme, and many other issues relating to the project.
A well-prepared Design Brief should guide the project throughout the design and construction process. The client and the design team can use the Design Brief as a tool for monitoring the development of the project, to ensure that the original objectives and requirements are being
achieved.
These Guidelines have been structured around the Design Brief. The core document is Section 2, Design Brief. This sets out a suggested format for the Design Brief, and gives guidance for the preparation of each section of this
suggesting how it can assist to enhance energy efficiency. 1.3.2. Technical Sections.
The core document has deliberately been kept short and simple, so that it can be useful to a wide variety of people. The more detailed and technical content relating to specific aspects of building design are included in Technical
Sections that are referred to in the relevant parts of Section 2, Design Brief.
The Technical Sections themselves also refer to other reference material including Standards, Codes of Practice, books, papers, websites, etc. where relevant information may be found.
Section 13, Appendices provides data on the thermal properties of materials and construction details, and other relevant information.
Technical Sections:
3.
Climate.
4. Indoor
Environment.
5. Design
and
construction process.
6. Planning.
7. Building
envelope.
8. Mechanical
Systems.
9.
Lighting - artificial and daylighting.
10.
Operation and Maintenance & Building
Management Systems.
11. Simulation.
12. Life-Cycle Cost Analysis.
13. Appendices.
1.4. Who is the Guidelines intended for?
It is hoped that the Guidelines will be of interest to all people involved in the process of procurement, design and operation of buildings. This includes the following groups of people:
Owners and developers. o Building owners. o Developers.
o Company employees with responsibility for property development.
o Government employees with responsibility for property development.
Planners and design consultants. o Town Planners. o Landscape architects. o Architects. o Civil Engineers. o Structural Engineers. o Electrical Engineers. o Mechanical Engineers. o Quantity Surveyors.
People responsible for operation and maintenance of buildings.
o Facility Managers. o Property Managers. o Owners.
However it is specifically intended to be used by those involved in preparing and implementing the Design Brief. This includes the ‘client’ and the consultant team
responsible for the design, construction and commissioning of the building.
SECTION 2
DESIGN BRIEF
ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA
ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA
Sections:
1.
Introduction.
2.
Design
Brief.
3.
Climate.
4.
Indoor
Environment.
5.
Design
and
construction process.
6.
Planning.
7.
Building
envelope.
8.
Mechanical
Systems.
9.
Lighting - artificial and day lighting.
10.
Operation & Maintenance and Building Management Systems.
11.
Simulation.
12.
Life-Cycle
Cost
Analysis.
CONTENTS
2. DESIGN BRIEF. 5
2.1. Project Objectives. 6
2.2. Project Requirements. 7
2.2.1. Schedule of accommodation. 7
2.2.2. Indoor environment specifications. 7
2.2.3. Lighting requirements. 8
2.2.4. Aesthetic considerations. 9
2.3. Opportunities and Constraints. 10
2.3.1. Siting. 10
2.3.2. Climate. 10
2.3.3. Budget. 11
2.3.4. Time. 11
2.4. Performance Targets. 12
2.4.1. Financial performance targets. 12
2.4.2. Energy performance targets. 12
2.5. Environmental Rating Schemes 13
2.6. Design Approach. 14
2.6.1. Procurement Strategy. 14
2.6.2. Integrated design approach. 14
2.6.3. Planning and landscape. 15
2.6.4. Envelope and structural design. 16
2.6.5. Lighting and electrical design. 19
2.7. Operation and maintenance. 20
2.8. Resource Material 21
2.8.1. Books and reports. 21
2. DESIGN
BRIEF.
The Design Brief for a building project is essentially a Terms of Reference for the project consultants, setting out the client’s objectives, requirements, constraints, targets and the design approach to be implemented in addressing these.
Typically the amount of thought and effort that is applied to preparing the Design Brief varies enormously from one project to another, as does the amount of attention that is later paid to the document during the implementation of the project.
A well-prepared, accurate and comprehensive Design Brief can make an important difference to the quality of the final building, and can also be a focus for ensuring that issues are raised and resolved before they become problems. A well-prepared Design Brief can be used throughout the project as a reference to ensure that the original objectives are achieved. It should be revised as necessary to reflect any changes that are agreed with the client.
The more competitive the procurement process for consultants is, the more the pressure on them to reduce costs, and hence the need to verify performance against an agreed scope of work. A well-prepared, detailed design brief is a valuable component of the contract between a client and the consultants.
In the following sections, some key elements of a design brief are considered with an emphasis on energy efficiency considerations.
A typical structure for a Design Brief is shown in the table below.
2.1. Project
Objectives.
A description of the background to the project will be followed by general statements of project objectives to indicate what is required from the project, and what is important to the client.
This will include the ‘direct’ objectives that have motivated the client to initiate the project, e.g. the need for additional office space, a building that is needed to implement a business plan, or a new art department for a school. It can also include indirect or secondary objectives that relate to the client’s overall philosophy, or mission statement. These could include an emphasis on
environmental sustainability, a desire to promote the local economy, or the wish to communicate a particular
corporate identity.
A general statement could be included here relating to energy efficiency, such as:
The development shall be designed to achieve an appropriate level of energy efficiency, taking into account life cycle costs and having due
consideration for the likely increase in energy costs relative to other costs over the design life of the building.
DESIGN BRIEF - STRUCTURE
Project Objectives. Project Requirements.
• Schedule of accommodation. • Indoor Environment requirements. • Aesthetic considerations.
Opportunities and Constraints. • Siting. • Climate. • Financial. • Time. Performance Targets. • Financial • Energy
Design and Construction Approach. • Procurement strategy.
• Integrated design approach • Planning and landscape.
• Envelope and structural design. • Lighting and electrical design. • HVAC design.
Similar statements may be included for other
environmental considerations, such as water management, waste management, etc.
2.2. Project
Requirements.
This section will contain the specifications for the building and other developments. The actual structure and content will vary depending on the type of development that is required.
2.2.1. Schedule of accommodation.
The schedule of accommodation will indicate the main types of space that are required, how large they need to be, and any particular requirements related to the use of each space.
It is also helpful define as far as possible the way in which different spaces should relate to each other.
2.2.2. Indoor environment specifications.
The primary requirement is likely to relate to the comfort of the building’s occupants. Specifications for comfort are considered in more detail in Section 4, Indoor
Environment. They should include consideration of the types of activity for which the building is intended. Comfort conditions are affected by the overall approach to air conditioning. Different specifications may apply to buildings that are mechanically air conditioned than to those that are naturally ventilated. Initially both
specifications could be included, so that the decision on air conditioning approach may be delayed.
With regard to energy efficiency, it is important that the specifications are appropriate to the actual needs of the building. An unduly restrictive specification may result in higher capital and recurrent cost as well as increased energy consumption.
The specification may also indicate the period of time for which the specifications could be exceeded. If the indoor temperature exceeded the limit for say one week of the
TYPICAL INDOOR ENVIRONMENT SPECIFICATIONS
Fresh air to achieve required air quality: o Air volume 8 – 12
litres/second/person o Air changes Min. 0.5ACH
Temperature:
Air conditioned buildings
o Summer 23-27°C o Winter 20°C min
Naturally ventilated / evaporatively cooled buildings
o Summer 22-29 °C o Winter 19-26 °C Relative Humidity:
year, this may not cause great problems, but could result in a substantially smaller capacity HVAC system, with savings in energy consumption as well as capital and recurrent cost
Requirements for air quality should also be considered, as these will affect the need for ventilation. Again,
unnecessarily demanding specifications will lead to increased cost and energy consumption.
2.2.3. Lighting requirements.
Lighting requirements should be specified in the Design Brief, as well as some indications of the approach to be taken in the design of lighting.
Lighting levels required in different areas or rooms should relate to the intended use of these spaces. Section 9, Lighting – artificial and daylighting gives indications of typical specifications for light level for different activities, as well as references to various standards and codes that provide more detailed information.
TYPICAL INDOOR LIGHTING REQUIREMENTS
Public spaces – no visual tasks 50 lux Background lighting, offices 150 lux Task lighting, office work 300 lux Task lighting, detailed work 750 lux Task lighting, very fine work 3,000 lux
2.2.4. Aesthetic considerations.
One of the greatest challenges in improving energy
efficiency in public and commercial buildings is to develop an architecture that is both aesthetically satisfying, and meets the technical requirements determined by the local climate and available material options.
It is important to set clear objectives regarding how the buildings should look, and to understand the implications on energy performance, initial cost and life cycle cost. If it is regarded as an important objective for the building that it makes a particular architectural statement, then the cost, energy and other implications should be clearly stated and agreed to.
2.3. Opportunities and Constraints.
2.3.1. Siting.
If the client already has a site for the project, then an
assessment should be made of opportunities and constraints of the site that are relevant to the project. These are
discussed in more detail in Section 6, Planning.
Energy considerations will include the orientation of the site in relation to the sunpath and typical wind directions, shading features such as trees, hills, other buildings, and other factors affecting the local climate such as vegetation, ponds / rivers, wind breaks, etc.
2.3.2. Climate.
The energy performance of a building is largely determined by how well the design is adapted to the local climate. It is therefore important that the design team has a clear understanding of the local climate with its daily and seasonal variations.
During the course of a year the climate changes with the seasons, and there are also variations in climate from one year to the next. It is therefore necessary to define the climate for a typical year for use in building design.
Fig. 2.1 DB Temperature in Gaborone, by month.
Fig. 2.2 Relative Humidity in Gaborone, by month.
RH DATA MONTHLY GABORONE 2000-2002
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
MONTH
RH %
MIN MAX AVG TEMP DATA MONTHLY GABORONE 2000-2002
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
MONTH DE G C MIN MAX AVG MAXDIFF
An assessment of the variation in climate for different locations in Botswana suggests that for purposes of building design at least two climatic zones should be considered. The northern zone includes Chobe District and Ngamiland District. The southern zone includes all the remainder of the country.
Generally the winters in the northern zone are sufficiently mild that there is little or no requirement for heating in buildings. In the southern zone heating in the winter is generally required, and may require more energy than summer cooling, depending on the building design and the amount of heat generated by activities in the building. Maun has been taken as a typical location in the northern zone, and Gaborone as a typical location in the southern zone. For each of these locations, the most relevant climate parameters have been determined for each hour of a typical year.
Any particular features of the local climate should be noted, such as dominant wind direction, shading effect of any tall trees, hills or buildings, etc.
Further details are included in Section 3. Climate. 2.3.3. Budget.
Opportunities and constraints regarding the financing of the project should be considered at this stage. The chapter on
Project Cost and Energy Efficiency in Section 5, Design and Construction Process is relevant here.
Possible trade-offs between initial cost and life-cycle costs may affect the way the project is financed. If access to capital finance is restricted this may reduce the scope for investment choices that will reduce life cycle cost. Section 12, Life-Cycle Cost Analysis gives a background to the methods that can be used to analyse various options and determine the most cost effective solution based on assumptions regarding future energy costs, maintenance interventions and other relevant parameters
It may be cost effective to invest in additional work and cost in the design stage in order to optimise the energy performance of the building. The anticipated costs and benefits should be carefully considered.
2.3.4. Time.
The client’s particular requirements regarding the project programme should be defined. This may then be
subdivided into pre-contract and post-contract programmes, to determine the amount of time available for the design process.
Detailed analysis of different approaches with regard to energy efficiency takes time to carry out. The costs, both in consultant fees and project timing need to be considered and evaluated in relation to the opportunity to achieve a more cost effective and better quality project.
2.4. Performance
Targets.
2.4.1. Financial performance targets.
A building represents a substantial, long-term investment by the owner. In many cases an important objective in making this investment is to obtain a financial return, either in the form of rental income, or saving of rental expense that would otherwise be incurred in the case of owner-occupied buildings.
It is therefore helpful to establish financial performance targets for the building against which actual performance can later be assessed. This will also inform decisions made during the design phase of the project, and guide the Quantity Surveyor in making recommendations regarding the cost of different components.
The financial performance targets should be broken down into capital costs, recurrent costs and recurrent income. These figures may if appropriate be developed into a life-cycle performance model to show the long term return on investment and predicted cash flows for the project. 2.4.2. Energy performance targets.
An important element in the financial performance of a building is energy cost. This requires estimates of the energy consumption for different purposes, as well as estimates of the price of different energy supplies. The major energy source for the types of building under
consideration in these Guidelines is electricity. The cost of electricity is subject to change based on the changing
conditions of supply and demand, as well as the policies of the authorities that set the price.
In many countries codes and standards for energy performance of buildings have been introduced. In some cases these are voluntary and give guidance to investors regarding what can be achieved. They can be used as specifications that the design team is expected to achieve, with or without financial incentives (see Section 5, Design and Construction Process )
Information regarding the actual energy performance of different types of building in Botswana is becoming available through the work of the EECOB project in the Department of Energy, both through audits of existing buildings and simulations of typical ‘generic’ building types.
The following figures for specific energy consumption (energy consumption per unit area) may be used as targets in the interim until actual energy performance standards become available.
Specific annual energy consumption [kWhr/m2.yr]
Building type Total Lighting HVAC Office
Equipment
Other
Office 150 34.5 63 43.5 10.5
School 40 14.8 3.6 2.8 18.8
Residential (high cost, air conditioned) 89 21.4 23.1 0 44.5
2.5. Environmental Rating Schemes
In many countries, environmental ratings are being adopted by private clients and governments as a way of
demonstrating that they are environmentally responsible. Well known rating systems include BREEAM (Building Research Establishment Environmental Assessment Method) and LEED (Leadership in Energy and
Environmental Design). A client may make it a part of their brief that their building should achieve a BREEAM
"excellent" rating, or a company may make it part of its sustainability policy that any new buildings which it procures will be constructed to achieve a BREEAM "very good" rating. For example, in 2003, the UK government made it a condition that all government departments when undertaking new or refurbishment projects carry out an environmental assessment, and that all new build projects must achieve a BREEAM "excellent" and refurbishment projects a "very good" rating.
These ratings consider a wide range of factors and compare them against a local benchmark of "typical" construction practice. They can therefore be adapted to any country, although there are initial costs involved in establishing suitable local benchmarks, particularly if there are no regulatory requirements as in Botswana.
The BREEAM rating assesses the following: o management: overall management policy,
commissioning, site management and procedural issues such as controlling noise, dust and waste materials on the construction site.
o energy use: operational energy and carbon dioxide (CO2 ) issues such as the predicted energy use of air
conditioning systems.
o health and well-being: indoor and external issues affecting health and well-being such as provision of
adequate daylight, artificial lighting and good air quality.
o pollution: air and water pollution issues, such as use of refrigerants, pollution from coal fires etc.
o transport: transport-related CO2 and
location-related factors, such as availability of public transport links to the building and whether occupants are encouraged to use alternative forms of transport to private cars.
o land use: greenfield and brownfield sites, whether the project is a refurbishment or built on a site already developed, or whether it is built on virgin ground.
o ecology: ecological value conservation and
enhancement of the site, including landscaping etc. o materials: environmental implication of building
materials, including life-cycle impacts. o water: consumption and water efficiency.
Credits are available under each of these headings, and the total number of credits obtained determines the final score achieved (from Pass to Excellent).
2.6. Design
Approach.
2.6.1. Procurement Strategy.
There are a number of different procurement strategies that can be used for the appointment of the professional team and the contractors for a building project.
These have implications for the energy performance of the building, which are discussed in more detail in Section 5, Design and Construction Process.
The most appropriate approach for a particular project should be determined based on the priorities and resources of the owner.
2.6.2. Integrated design approach.
A simple building such as a low cost residential house can be fully designed by a competent, experienced designer such that all aspects of the building work well together in a coordinated, sensible way. The designer takes into
consideration decisions that relate to one aspect of the building when making decisions on other aspects. Since the same person is making all the design decisions, she or he can easily consider the implications of a decision about say the location of windows on the planning of the rooms and the switching of lights.
Larger, more complex buildings require a team of
specialised designers, each working on different aspects of the overall design. Often they work for different firms located in different places, with limited communication. They will be coordinated by a team leader, often the architect or project manager, who is responsible for
ensuring that the different elements of the building work in relation to each other.
Typically energy efficiency has not been a key
requirement of ensuring that the different design aspects work together to achieve optimum energy efficiency has tended to be overlooked.
By deliberately adopting an integrated approach to energy efficient design, the design team can be encouraged to take advantage of opportunities to achieve improved energy performance.
This is discussed in more detail in Section 5. Design and Construction Process.
The integration of the different design aspects almost always requires that changes in approach be made in each aspect to accommodate the others. Such changes should be
made as early in the design of the building as possible, since the time and work required in making changes increases rapidly as the design becomes more detailed. It is helpful therefore to have a systematic approach to the coordination of these approaches, and the Design Brief is a good opportunity for providing this. It is suggested that some initial indications are included in the Design Brief at the project inception stage, and that the consultants amend these as the design develops.
2.6.3. Planning and landscape.
The planning of the building on the site provides many opportunities for improving energy performance. The overall approach to energy performance should be considered, e.g. whether the building will require
mechanical systems to control the indoor climate, or whether passive heating and cooling approaches will be used. In practice a combination of these may often be appropriate. Different solutions may be needed for different types of building. For buildings such as residential houses and classrooms it should be possible to achieve the required comfort levels with little or no mechanical equipment. In office buildings comfort conditions can be achieved with passive methods for much of the year, but some form of mechanical cooling may be required to deal with summer conditions.
The overall shape of the building is important to achieving energy efficiency. It has been found that the walls perform an important role in removing heat from a building, suggesting that a high surface area to volume ratio is useful. This also allows for maximum use of daylight, reducing the energy needed for lighting, and indirectly helping to keep the building cool as well, since artificial lighting also generates heat.
Plants can be used very effectively to amend the local climate on site, e.g. using trees for shade and wind breaks, ground cover to reduce reflected heat, climbing plants on frames to provide shade and evaporative cooling, etc. Planning and landscaping are discussed further in Section 6, Planning.
2.6.4. Envelope and structural design.
The building envelope consists of all the different elements that make up the fabric of the building, such as the floor, walls, windows and roof.
Most of the design decisions relating to the building envelope are the responsibility of the architect and structural engineer. They have a large impact on the thermal performance of the building, and it is therefore essential that the performance of the envelope is coordinated with the design of the HVAC system.
This is the area that offers most opportunities for improved building performance through an integrated design
approach.
Energy codes and standards for buildings typically specify the performance requirements for the building envelope in terms of an ‘overall thermal transfer coefficient’ (OTTC), which gives an indication of the amount of heat that will flow between the building and the environment. In some cases the standard defines the requirements for the thermal properties of different building elements.
Typical values for the total thermal resistance of the walls and roof proposed for the South African Standard SANS 283 and 204, The Energy Efficiency Standards are given in Table 2.2.
Building
Element Total ‘R’ value [m
2.K/W] Total ‘U’ value [W/m2.K] Typical construction to achieve this value:
Wall Min. 1.4 Max. 0.71 Sand-cement brick cavity wall with 25mm insulation in cavity plastered both sides. Roof and
ceiling Min. 2.7 Max 0.37 Galvanised roof sheets, 100mm insulation and 6mm ceiling.
Table 2.2. Thermal properties of building envelope elements (draft SANS 204) Source: TIASA.
Frequently energy codes offer an alternative method of demonstrating compliance based on a computer simulation of the proposed building using approved methods to verify whether it achieves the required minimum standard of performance.
Details regarding computer simulation of building energy performance are provided in Section 11, Simulation. Windows and other glazing elements are frequently responsible for more heat gain and loss than any other building element. Assuming that the roof is insulated to the level recommended in Table 2.2, the greatest source of solar heat gain in most buildings will be glazing. Glazing however also provides the opportunity to admit natural daylight into the building, reducing the energy consumption for artificial lighting. It is therefore important to achieve an optimum balance whereby the opportunity for effective daylight is achieved with minimal unwanted solar heat gain.
2.6.4.1. Summary of simulation results.
Simulations of three building types; Classroom, Residential and Office have been carried out for the Gaborone climate to quantify the effect on energy consumption of various alternative envelope and operational parameters.
Some of the key results are summarised below. In each case references to changes in energy cost refer to total heating and cooling energy, not total building energy. Full tables of results are included in the EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’.
Orientation:
Orientation in the N-S direction resulted in a 6% increase in energy consumption over an E-W orientation for the
classroom type of building. For the office it was only 0.8% and for the residential house it was 1.8%. This suggests that orientation is less significant than expected. However local effects within the building and impact on quality of
daylight are also important considerations that are strongly related to orientation. It is therefore recommended that an
E-W orientation be used whenever possible, particularly where daylight is an important consideration as in offices and classrooms.
Roof:
In the classroom building a white roof reduced energy by 45% compared to a galvanised roof with no ceiling insulation in both cases.
In the residential building, with ceiling insulation the white metal roof is comparable in performance to a concrete tiled roof also with insulation.
In the three storey office building, a white metal roof reduced energy consumption by 5% compared to a green coloured metal roof.
The addition of 100mm insulation on the ceiling reduced energy consumption by 43% in the classroom (galvanised roof), 2.7% in the residential house (tiled roof), and had no effect in the office building (green metal roof).
Wall.
An insulated cavity wall in place of a standard 220mm solid wall increased energy consumption by 8% for the classroom and by 5% for the office. However it reduced energy consumption by 27% in the residential building. This energy saving was due to reduced heating cost. The insulated cavity wall was almost three times as effective as an uninsulated cavity wall, so the small extra cost of providing insulation in the cavity is well rewarded.
A 500mm wide mass wall with insulation on the inside gave similar results, with energy cost increased by 10% for the classroom and 7% for the office. In the residential house the energy saving increased to 30% compared with the insulated cavity wall.
The simulation showed that the walls provide some cooling during the day when they absorb radiant heat from the ceiling. A width of 220mm seems to be about optimum; 115mm walls are worse, as are wider walls.
The simulation confirmed that different solutions are appropriate for different types of building. Solid 220mm walls are best for classrooms and offices that are primarily occupied during the day, and insulated cavity walls or mass walls are effective for residential houses that are occupied more during the night.
Floor.
The ground floor is also an important cooling element in all buildings in summer, and also in winter for office buildings that require cooling all year. For the classroom, providing floor insulation resulted in a 23% increase in annual energy cost. In residential buildings there is some unwanted heat loss to the ground floor in winter, but this is marginal compared to the benefit in summer.
Windows.
Heat flow through the windows from direct and indirect solar radiation is in many cases the largest source of heat gain to the building. The easiest way to reduce this is to reduce the size of windows to the minimum required to
provide daylight and views. It was found that a glazing ratio of 20% (window to wall area) provided more than enough daylight in the classroom.
Ventilation.
The use of ventilation to control indoor temperature was found to be highly effective in the office building, resulting in a 28% energy saving. It was less effective in the
residential house (11% saving) and in the classroom (2% saving). This should be considered as an option for office buildings, and would need to be included in the HVAC design approach, as it may require increased duct sizes, larger fans, and different control systems. It appears that the substantial savings that can be achieved would justify this extra expense.
Further suggestions for appropriate design approaches for different building envelope elements are described in Section 7, Building Envelope.
2.6.5. Lighting and electrical design.
Optimal use of daylight can result in reduced energy consumption, and also has other benefits. Studies have shown that people perform better under daylight than artificial light. Views of the world outside the building are also important for the well-being of the occupants, and have been found to improve performance and productivity. There are a number of opportunities to improve the
effectiveness of daylight without excess heat gain, including use of light shelves, light tubes and skylights.
The design of artificial lighting should aim to provide an adequate level of background illumination for general purposes, with higher levels of task lighting in the specific areas where more light is needed. This results in energy savings and also allows for more flexibility should the use of spaces change in future.
Control of lighting should be designed to ensure that lights are only on when and where they are needed.
This is discussed in more detail in Section 9, Lighting – artificial and daylighting.
2.6.6. HVAC design.
The mechanical systems or HVAC (heating, ventilation and air conditioning) are designed to amend the indoor climate of a building to achieve the requirements of the particular application in buildings for which this cannot be achieved using natural ventilation alone.
The first decision that is required is therefore whether such systems are required or not. This depends on how stringent the indoor environment requirements are, the internal loads from occupants and equipment, the local climatic
conditions and the design of the building envelope.
If an HVAC system is required, the design approach should be coordinated with the envelope design to ensure that the building requirements are achieved with an optimal energy performance.
It has been found that HVAC systems designed using hourly computer simulation are more accurately matched to the needs of a particular building in relation to the local climate than those designed using steady state methods. It is recommended that for all projects that will have a centralised HVAC system installed, computer simulation be used to determine the system capacity that is required. Use of ventilation to control indoor temperatures offers substantial energy savings, particularly in buildings with high heat gains from occupants, equipment and lighting. It is recommended that centralised HVAC systems be
designed to use ventilation for this purpose as well as providing adequate indoor air quality.
In many situations it may be possible to achieve the required comfort conditions using evaporative coolers in place of air conditioning systems, with far lower recurrent cost and energy consumption.
Further information is included in Section 8, Mechanical Systems.
2.7. Operation and maintenance.
The decisions that are made during the design phase of a building have implications for how it will be operated and maintained.
The overall approach to operation and maintenance should be specified in the Design Brief, so that this can guide the decisions taken in the design process.
In particular, the human resource requirements for the operation and maintenance of the building should be considered.
The Design Brief should specify the requirement for the design team to prepare a draft Operations and Maintenance Manual as one of their tasks. This should be developed as an ongoing process during the design, to ensure that the O&M implications are given consideration
The draft O&M manual will then be revised and finalised during and following the commissioning of the building. Possibly the greatest opportunity for reducing energy consumption in buildings, and certainly the cheapest and quickest to implement is to encourage occupants to turn off lights and other equipment when these are not needed. The simulation of the office building indicated that 39% of total energy use could be saved through such behaviour change. Operation and Maintenance considerations are discussed in more detail in Section 10, Operation & Maintenance and Building Management Systems that also includes a suggested format for an O&M Manual.
2.8. Resource
Material
2.8.1. Books and reports.
TIASA, The Thermal Insulation Guide for Energy Efficiency in Buildings. Thermal Insulation Association of Southern Africa. January 2006.
EECOB Report: ‘Energy Efficiency and Energy Conservation in the Building Sector, Botswana, Report on Baseline Energy Surveys’, Department of Energy, Government of Botswana, July 2005.
Bauer, C. and Groth, A. EECOB Report: ‘Parametric simulation of the energy performance of three generic building types in Gaborone, Botswana’. Department of Energy, Government of Botswana, January 2007.
2.8.2. Web resources
BREEAM Building Research Establishment Environmental Assessment Method
http://www.breeam.org
LEED Leadership in Energy and Environmental Design.
SECTION 3
CLIMATE
ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA
Revision 1
September 2007
HOURLY AVERAGE TEMPS GABORONE 2000-2002
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 00:0 0 01:0002:0003:0 0 04:0005:0006:0 0 07:0008:0009:0 0 10:0011:0012:0013:0014:0015:0016:0017:0 0 18:0019:0020:0 0 21:0022:0023:0 0 TIME DEG C JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
HOURLY AVERAGE RH GABORONE 2000-2002
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 00:0 0 01:0 0 02:0 0 03:0 0 04:00 05:00 06:00 07:00 08:00 09:0 0 10:0 0 11:0 0 12:0 0 13:0 0 14:0 0 15:0 0 16:0 0 17:0 0 18:0 0 19:0 0 20:0 0 21:00 22:00 23:00 TIME RH % JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
ENERGY EFFICIENCY BUILDING DESIGN GUIDELINES FOR BOTSWANA
Sections:
1.
Introduction.
2.
Design
Brief.
3.
Climate.
4.
Indoor
Environment.
5.
Design
and
construction process.
6.
Planning.
7.
Building
envelope.
8.
Mechanical
Systems.
9.
Lighting - artificial and day lighting.
10.
Operation & Maintenance and Building Management Systems.
11.
Simulation.
12.
Life-Cycle
Cost
Analysis.
CONTENTS
3. CLIMATE 5 3.1. Overview 5 3.1.1. Climate of Botswana. 5 3.1.2. Elements of climate. 5 3.1.3. Climatic zones. 5 3.1.4. Climate patterns. 53.1.5. Climate and simulation. 5
3.2. Climate of Botswana 6
3.2.1. Classification. 6
3.2.2. Cycles of climate and global warming. 8
3.3. Elements of Climate 9
3.3. Elements of Climate 10
3.3.1. Meteorological data 10
3.3.2. Temperature 10
3.3.3. Design Day Conditions 12
3.3.4. Humidity 13
3.3.5. Radiation 13
3.3.6. Wind 14
3.4. Climatic Zones. 15
3.5. Climate Patterns. 18
3.6. Resource Material 19
3.6.1. Books and papers 19
3. CLIMATE
3.1. Overview
This Section addresses the subject of climate and its impact on building energy performance in Botswana. The topics that will be covered are briefly outlined below.
3.1.1. Climate of Botswana.
The section begins with an overview of the climate of Botswana in a global context. The classification of the climate is considered, and various cycles in the climate are identified.
3.1.2. Elements of climate.
This section includes a general discussion of the principal elements of climate and how they relate to building energy performance. The ways in which data are collected and made available are also considered.
3.1.3. Climatic zones.
The variation in climate with location is considered, with particular reference to the implications for building energy performance. It is recommended that the country be divided into two climatic zones for the purposes of these
Guidelines.
3.1.4. Climate patterns.
In addition to the geographical variations in climate, there are also patterns of climate within one locality. In
considering the impact of climate on building energy performance, it is important to consider the different
patterns that occur, and differentiate these from the average characteristics, which may never be experienced.
3.1.5. Climate and simulation.
Building energy performance may be predicted using software that simulates the interaction of the building with the climate.
Typical meteorological year data has been prepared for Gaborone and Maun, which have been taken as typical of the Northern and Southern climate zones.
This data is available in a format that may be used for computer simulation of building energy performance.
3.2. Climate of Botswana
3.2.1. Classification.
In the Köppen Climate Classification System, theclimate of most of Botswana falls in the classification ‘BSh: semi-arid steppe, hot’. The exception is the extreme north of the country, which is classified, as ‘Aw: tropical wet-dry (low sun dry) – savanna’. Approximately two thirds of the area of the country is within the tropics. The Tropic of
Capricorn crosses the Jwaneng - Ghanzi road just north of Kang, runs through the middle of Khutse game reserve, and crosses the Gaborone - Francistown road just north of Dibete.
Generally Botswana experiences a very high proportion of clear, sunny days, with little cloud cover or rain.
The summers are warm to hot in the day and cool at night, particularly in the southwest of the country. Rainfall generally occurs in the time between October and April, which coincides with the summer months.
Winters are warm in the day and cool at night, with minimum temperatures lower in the south, and increasing as one moves further north.
Summer maximum daytime temperatures are closely related to rainfall, rising rapidly in times of drought. In years of reasonable rainfall, the highest average maximum temperature often occurs in October, before the rain begins, after which temperatures drop due to increased cloud cover
and evaporative cooling from the moisture in the soil. In years of drought, and in regions that receive less rain the maximum temperatures continue to rise until January or February.
Botswana is completely landlocked, and is located in the centre of the southern African plateau. The country is approximately equidistant from the Atlantic Ocean coast, 1,000km to the west, and the Indian Ocean coast about 960km to the east (measured to the middle of the country). The country is relatively flat, at an average elevation of approximately 1000m above sea level. As a result moist air from the oceans seldom reaches Botswana without having first shed its moisture on the escarpments between. The distance from the ocean together with the relatively high altitude result in low, intermittent and unreliable rainfall. The rain that does occur is a result of localised regions of low pressure that draw in moist air from the coast.
Not only is the average rainfall in Botswana low, it is also very variable, both within a particular year, and from one year to the next. There is a trend for average rainfall to reduce and variability to increase from north to south, and from east to west.
3.2.2. Cycles of climate and global warming.
A number of different climatic cycles have been observed, including a short-term cycle of about 6-10 years during which a few years of good rain are followed by years of below average rain or drought. This takes place within the framework of a longer cycle spanning several centuries, and another even longer cycle of several thousands of years. Although the Kalahari has generally been a semi-arid area for millions of years, during that time there have been periods of sufficient rainfall to maintain large inland seas and perennial rivers that now remain as fossil river valleys. Over the past century the natural long-term climatic cycles of the earth have been subject to increasing influences from human activity, particularly the enormous increase in energy consumption from fossil fuels and resulting
emissions of carbon dioxide. This has resulted in increased concentrations of greenhouse gasses in the atmosphere. These act as a radiation filter surrounding the earth, which allows solar radiant heat to pass through, but reflects thermal radiant heat back to the earth, as does the glass in a greenhouse. The consensus view of the Intergovernmental Panel on Climate Change (IPCC), the world authority on global warming, is that this could result in an increase in average temperatures over southern Africa of between 2-5°C over the coming century. The following excerpt from an article by Mike Davis in The Science News suggests that this may be a highly optimistic view.
The actual rate of change of climate may not be accurately predictable, but there seems to be little doubt that increases
in temperature will be experienced throughout this century, together with increased energy cost, both in economic and environmental terms. This makes it even more urgent that buildings are designed and built to achieve human comfort with minimal energy consumption.
The Science News
Scientific discussions of environmental change and global warming have long been haunted by the specter of nonlinearity. Climate models, like econometric models, are easiest to build and understand when they are simple linear extrapolations of well-quantified past behavior: when causes maintain a consistent proportionality to their effects.
But all the major components of global climate - air, water, ice and vegetation - are actually nonlinear: at certain
thresholds they switch from one state of organization to another, with catastrophic consequences for species too finely-tuned to the old norms. Until the early 1990s, however, it was generally believed that these major climate transitions took centuries if not millennia to accomplish. Now, thanks to the decoding of subtle signatures in ice cores and sea-bottom sediments, we know that global temperature and ocean circulation can change abruptly - in a decade or even less. The paradigmatic example is the so-called 'Younger Dryas' event, 12,800 years ago, when an ice dam collapsed, releasing an immense volume of meltwater from the shrinking Laurentian ice-sheet into the Atlantic Ocean via the
instantly-created St. Lawrence River. The freshening of the North Atlantic suppressed the northward conveyance of warm water by the Gulf Current and plunged Europe back into a thousand-year ice age.
Abrupt switching mechanisms in the climate system, like relatively small changes in ocean salinity, are augmented by causal loops that act as amplifiers. Perhaps the most famous example is sea-ice albedo: the white, frozen Arctic Ocean reflects heat back into space, thus providing positive feedback to cooling trends; alternatively, shrinking sea-ice increases heat absorption and accelerates its own melting and planetary warming.
Thresholds, switches, amplifiers, chaos - contemporary geophysics assumes that earth history is inherently revolutionary. This is why many prominent researchers - especially those who study topics like ice sheet stability and North Atlantic circulation - have always had qualms with the consensus projections of the Intergovernmental Panel on Climate Change (IPCC), the world authority on global warming.
3.3. Elements of Climate
3.3.1. Meteorological data
In Botswana the responsibility for the collection,
processing, storage and dissemination of meteorological data rests with the Department of Meteorological Services (DMS) in the Ministry of Environment, Wildlife and Tourism. The DMS maintains synoptic weather stations at the following locations around Botswana:
o Francistown o Ghanzi o Jwaneng o Kasane o Letlhakane o Mahalapye o Maun o Pandamatenga o Selebi-Phikwe
o Sir Seretse Khama Airport o Shakawe
o Sua Pan o Tshabong o Tshane
A wide range of variables are measured, including the following:
o Dry Bulb Temperature o Humidity o Wind Speed o Wind Direction o Rainfall o Sunshine hours o Evaporation o Air pressure o Soil Temperature
In addition, rainfall and temperature are measured at a large number of other locations by volunteers who regularly submit their data to DMS.
3.3.2. Temperature
Air temperature (Dry Bulb temperature) is the characteristic of climate that most directly affects comfort. It determines the rate of heat transfer by conduction and convection. Assuming that there are no significant sources of radiant heat transfers, DB temperature is the main determinant of human comfort, and therefore the most significant variable to be specified when defining indoor climate requirements. Heating and cooling equipment is generally controlled by thermostats that are set to a particular target temperature or temperature range.
Dry bulb temperatures in Gaborone vary throughout the year, between an average daily maximum temperature of 32°C in October, and an average daily minimum
temperature of 4°C in July. [Bauer Consult].
The maximum daily temperature in summer typically occurs at about 3.00pm, and the minimum daily temperature in winter typically occurs at 7.30am.
20.0 25.0 30.0 35.0
Fig. 3.2 Temperatures in Gaborone, by month.
Fig. 3.4 Relative Humidity in Gaborone, by month.
Fig. 3.3 Temperatures in Gaborone, by hour.
Fig. 3.5 Relative Humidity in Gaborone, by hour.
RH DATA MONTHLY GABORONE 2000-2002
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
MONTH
RH %
MIN MAX AVG
HOURLY AVERAGE TEMPS GABORONE 2000-2002
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 00:0 0 01:0002:0003:0 0 04:0005:0006:0 0 07:0008:0009:0 0 10:0011:0012:0013:0014:0015:0016:0017:0 0 18:0019:0020:0 0 21:0022:0023:0 0 TIME DEG C JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC TEMP DATA MONTHLY GABORONE 2000-2002
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
MONTH DE G C MIN MAX AVG MAXDIFF
HOURLY AVERAGE RH GABORONE 2000-2002
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 00:00 01:00 02:00 03:0 0 04:00 05:00 06:0 0 07:00 08:00 09:0 0 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:0 0 18:00 19:00 20:0 0 21:00 22:00 23:00 TIME JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
dry bulb
°C wet bulb °C relative humidity ASHRAE design temperature Gaborone Airport (Jan) based on 0.4% chance of exceedance
(derived using IES software) 37.7 19.9 20%
CIBSE A guide (5th Ed) design temperature Maun (October) 39 22 24% CIBSE A guide (5th Ed) design temperatures Maun (Jan) 37 25 39% CIBSE A guide (5th Ed) design temperatures Ghanzi (Nov) 38 23 29% CIBSE A guide (5th Ed) design temperatures Ghanzi (Jan) 37 24 36% Standard design conditions in common usage (Gaborone) 38 25 36%
More extreme design conditions (Gaborone) 40 27 38%
Based on the Typical Meterological Year (TMY) for Gaborone and Maun generated by Meteonorm:
Heating Dry Bulb temperature (99% chance of no lower temperature, Gaborone) 2.5
Cooling Dry Bulb temperature (1% chance of higher temperature, Gaborone) 34.1 25.6 61% Heating Dry Bulb temperature (99% chance of no lower temperature, Maun) 6.3
Cooling Dry Bulb temperature (1% chance of higher temperature, Gaborone) 39.1 22.4 45%
Table 3.1 Design Day Conditions for Gaborone, Maun, and Ghanzi
3.3.3. Design Day Conditions
Although it is recommended that buildings are simulated using real weather data (see section @@) some buildings may continue to be designed using “design day” methods. Typical design temperatures for both cooling and heating design are provided in Table 3.1 above.
The choice of design day temperatures is something that the client must sign off, since it involves a choice about how often the building is likely to overheat, versus the risk of oversizing plant. Generally, for an energy efficient building
it is desirable to use lower design temperatures and allow the building to overheat occasionally.
One of the reasons that more extreme design conditions are used is to give a design margin and effectively to give the client future flexibility for increased heat loads or for variations/defects in the construction of the building post design stage. However, this should be avoided as it is likely to result in over sizing of plant.
3.3.4. Humidity
Humidity is a measure of the moisture content of the air. It is generally measured as relative humidity, which indicates the percentage saturation of the air.
Relative humidity (RH) is an important characteristic of climate with regard to building design for the following reasons:
o It is a determinant of the comfort zone temperatures. o It determines the effectiveness of evaporative
cooling.
Generally RH varies inversely with temperature through the day. It is higher in the summer months when rain occurs than in the dry months of winter.
For Gaborone the highest hourly average RH is 90% and occursin June. The lowest hourly average RH is 28% and occurs in September. [Bauer Consult]. Maximum RH typically occurs at 7.00am, while minimum RH typically occurs at 5.00pm.
3.3.5. Radiation
Radiation is a critically important characteristic of climate, both at a macro, outdoor level and in relation to indoor climate.
Heat transfer by radiation is proportional to the difference in temperature of the surfaces raised to the fourth power. It is therefore a minor component of total heat flow between surfaces where the temperature difference is small, and rapidly becomes the major component of heat flow when
temperature difference increases. It is also affected by other characteristics of the surfaces, including colour and texture, as well as the translucence of the intervening space.
During the day radiant heat transfer between a building and its surroundings is primarily in the form of solar heat gain, and includes direct, diffuse and reflected radiation.
During the night, radiant heat loss to the night sky occurs from any surface in view of the sky.
Total solar radiation received on a horizontal surface has been recorded at Sebele since 1977. For other locations it has been calculated from recorded measurements of bright sunshine duration using the Angstrom formula.
The annual average daily total radiation on a horizontal surface varies between 19.6 MJ/m2.day in Sebele, to 22.0 MJ/m2.day in Tsabong. [Bhalotra]
The monthly average daily total radiation on a horizontal surface for Gaborone varies from 14.6 MJ/m2.day in June, to 26.2 MJ/m2.day in December. [Bhalotra]
The indoor radiant environment is often underestimated as a factor in determining comfort. A space may feel
uncomfortably hot even when the air temperature is several degrees below the minimum comfort level, if there is a hot surface in view (such as the sun, seen through a window, or even a warm wall). Likewise, a space with an air
temperature higher than the maximum comfort level may feel cold if there is a view to a cold body such as the night sky. (See Section 4, Indoor Environment.)
3.3.6. Wind
Wind is significant in energy efficient building design as a driving force for ventilation, which is of benefit in the following ways:
o Natural ventilation to improve air quality.
o Natural ventilation to provide cooling air movement. o Wind driven evaporative cooling.
Wind driven infiltration is a problem in the following ways: o Heat loss through infiltration.
o Heat gain through infiltration.
o Excessive air speeds due to infiltration in high winds. o Entry of dust or other contaminants due to
infiltration.
Wind direction for most of Botswana is predominantly from the East, with a significant component from the south to southwest in the extreme southwest of the country. There are extensive periods of calm, e.g. 37.7% for Gaborone. It would be important to analyse wind data to determine whether there is a difference between the dominant wind direction for light winds and for strong winds.
3.3.7. Rainfall
Rainfall has limited direct effect on building energy performance, but is important since it is closely linked to other climate variables. For example, in a year of good rainfall, the hottest month of the year is frequently October, which in such years is generally still dry with little cloud cover. Rainfall during the months of December and January helps to reduce temperatures through evaporation and reduced sunshine hours. In years of drought, the reverse is the case, with temperatures in December and January exceeding those of October.
Rainfall must be taken into consideration in designing the landscape around a building. Plants that require much irrigation should be avoided, since water is a scarce resource in Botswana. Opportunities for using greywater should be considered in any building project. The website at www.oasisdesign.net has useful information on practical greywater design solutions.
3.4. Climatic
Zones.
Botswana extends from latitude 17°,50’ at Kasane in the north, to latitude 26, 59’ at Bokspits in the south. The western border with Namibia runs along longitude 20, 0’ E, while the confluence of the Limpopo and Shashe rivers in the east is located at longitude 29°, 30’E. The country spans approximately 1,100 km from north to south, and 965km from west to east.
The variations in climate across the country are such that they need to be taken into consideration in building design for comfort and energy efficiency.
Fig. 3.7 shows the monthly mean maximum and minimum temperatures for various locations around Botswana.
Fig. 3.7 Temperatures in different locations. (1961-1990) 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
F'town Max F'town Min Gab Max Gab Min Ghanzi Max Ghanzi Min Kasane Max Kasane Min M'h'pye Max M'h'pye Min Maun Max Maun Min Shakawe Max Shakawe Min Tsabong Max Tsabong Min Tshane Max Tshane Min
