1.1 Importance of Building Energy Efficiency
- Buildings are significant users of energy and building energy efficiency is a high priority in many countries.
- Efficient use of energy is important since global energy resources is finite and power generation using fossil fuels (such as coal and oil) has adverse environmental effects.
- The potential for energy savings in the building sector is large.
- Energy efficient building design is location-dependent. The local climate must be considered when selecting appropriate design strategies.
- A cooling-dominated climate is assumed here. However, some of the general principles are also applicable to other climate types.
2. BASIC PRINCIPLES
2.1 Climate and Site
- Climate has a major effect on building performance and energy consumption. Energy-conscious design requires an understanding of the climate.
- Buildings will respond to the natural climatic environment in two ways:
- Thermal response of the building structure (heat transfer and thermal storage).
- Response of the building systems (such as HVAC and lighting systems).
- To gain the maximum benefits from the local climate, building design must "fit" its particular climate.
- When faced with unfavourable climatic conditions, optimal siting and site design may solve all or part of the problems. Site elements to be considered include:
- Topography - slopes, valleys, hills and their surface conditions.
- Vegetation - plant types, mass, texture.
- Built forms - surrounding buildings and structures.
- Water - cooling effects, ground water, acquifiers.
- The six important aspects of architectural planning which will affect thermal and energy performance of buildings are:
- Site selection
- Mutual relationship
- Architectural and landscape designs should be closely integrated. If possible, should provide wind breaks in cold winter and access to cooling breezes in summer.
2.2 Building Envelope
- Elements of the building envelope (= "protective skin"):
- Walls (exterior)
- Underground slab and foundation
- Three factors determining the heat flow across the building envelope:
- Temperature differential
- Area of the building exposed
- Heat transmission value of the exposed area
- The use of suitable thermal mass and thermal insulation is important for controlling the heat flow. Remember, the envelope components will respond "dynamically" to changing ambient conditions.
- Some people also consider the "embodied energy" (include energy for producing and transporting) of building materials when making the selection.
2.3 Building Systems
- Heating, ventilation and air-conditioning (HVAC) systems are installed to provide for occupant comfort, health and safety. They are usually the key energy users and their design is affected by architecture features and occupant needs.
- While being energy efficient, HVAC systems should have a degree of flexibility to allow for future extensions and change.
- To achieve optimum energy efficiency, designers should evaluate:
- Thermal comfort criteria
- Load calculation methods
- System characteristics
- Equipment and plant operation (part-load)
- Lighting systems is another key energy user and additional cooling energy will be required to remove the heat generated by luminaires.
- Energy efficient lighting should ensure that:
- Illumination is not excessive.
- Switching is provided to turn off unnecessary light.
- Illumination is provided in an efficient manner.
- General design strategies for lighting design:
- Combination of general and task lighting.
- Electric lighting integrated with daylight.
- The use of energy efficient lamps and luminaires.
- Use light-coloured room surfaces.
- Other building services systems consuming energy include:
- Electrical installations
- Lifts and escalators
- Water supply systems
- Town gas supply system
3.1 Passive Cooling and Sun Control
- Passive systems - internal conditions are modified as a result of the behaviour of the building form and fabric.
- General strategies for passive heating and cooling:
- Cold winters - maximise solar gain and reduce heat loss.
- Hot summers - minimise solar gain and maximise heat removal.
- Correct orientation and use of windows.
- Appropriate amounts of thermal mass and insulation.
- Provision for ventilation (natural).
- Strategies for shading and sun control:
- External projection (overhangs and side fins).
- External systems integral with the window frame or attached to the building face, such as lourves and screens.
- Specially treated window glass, such as heat absorbing and reflecting glass.
- Internal treatments either opaque or semi-opaque, such as curtains and blinds.
- For hot and humid climate like Hong Kong, extensive shading without affecting ventilation is usually required all year round. Shading of the east and west facades is more important.
- Daylight can be used to augment or replace electric lighting. Efficient daylighting design should consider:
- Sky conditions
- Site environment
- Building space and form
- Glazing systems
- Artificial lighting systems
- Air-conditioning systems
- The complex interaction between daylight, electric lights and HVAC should be studied carefully in order to achieve a desirable solution.
Figure 3 - Daylighting design in an atrium
- Advanced window technologies have been developed to change/switch the optical properties of window glass so as to control the amount of daylight. There are also innovative daylighting technologies now being investigated:
- Light pipe systems
- Light shelves
- Mirror systems
- Prismatic glazing
- Holographic diffracting systems
3.3 HVAC Systems
- Energy efficiency of many HVAC sub-systems and equipment has been improved gradually over the years, such as in air systems, water systems, central cooling and heating plants.
- Energy efficient HVAC design now being used or studied include:
- Variable air volume (VAV) systems to reduce fan energy use.
- Outside air control by temperature/enthalpy level.
- Heat pump and heat recovery systems
- Building energy management and control systems.
- Natural ventilation and natural cooling strategies.
Figure 4- Waste heat recovery in a doule-bundle chiller plant
- Thermal storage systems (such as ice thermal storage) are also being studied to achieve energy cost saving. Although in principle they will not increase energy efficiency, they are useful for demand-side management.
3.4 Active Solar and Photovoltaics
- Solar thermal systems (active solar) provide useful heat at a low temperature. This technology is mature and can be applied to hot water, space heating, swimming pool heating and space absorption cooling.
- The system consists of solar collectors, a heat storage tank and water distribution mains. An integrated collector storage system has also been developed recently to eliminate the need for a separate storage tank.
Figure 5 - Schematic of a typical solar hot water system
- Photovoltaic (PV) systems convert sunlight into electricity using a semi-conductor device. The main advantages of PV systems include:
- Reasonable conversion efficiencies (6-18%).
- PV modules can be efficiently integrated in buildings, minimising visual intrusion.
- Their modularity and static character.
- High reliability and long lifetime.
- Low maintenance cost.
- In practice, PV technology can be used for central generation or building-integrated systems (BIPV). The systems can be of the standalone type, hybrid type or grid-connected type. Although the cost of PV is still high at present, it may become cost-effective in the hear future.
Figure 6 - Grid-connected solar photovoltaic system
4. Evaluation Methods
4.1 Bioclimatic Design
- The integration of design, climate and human comfort -- the bioclimatic approach to architectural regionalism -- was first proposed in mide-1950s by Victor and Aladar Olgyay.
- Their intention was to highlight the belief that architectural design should begin with understanding of the physiological needs of human comfort and take advantage of local climatic elements to optimise these requirements naturally and efficiently.
- Building design itself is conceived as a natural energy systems that restores environmental quality to its site.
- The aim is to creat a supportive and productive environment that ultimately can contribute to sustaining the regional and global environment.
4.2 Building Thermal and Energy Simulation
- Nowadays, building energy design often require the analytical power to study complicated design scenerio. Computer-based building energy simulation will provide this power and allow greater flexibility in design evaluation.
- The simulation method is based upon load and energy calculations in HVAC design. The purpose is to study and determine the energy characteristics of buildings and their building systems.
- The cost effectiveness of any energy conservation measures will be a compromise between initial, maintenance and energy costs. Simulation techniques can provide the tools for assessing different design options based on their energy performance and life cycle costs.
4.3 Building Energy Audits
- Building energy auditing can be defined as "measuring and recording actual energy consumption, at site, of a completed and occupied building (expressed in units of energy, not monetary value); fundamentally for the purposes of reducing and minimising energy usage".
- Energy audits identify areas where energy is being used efficiently or is being wasted, and spotlight areas with the largest potential for energy saving. They are useful for establishing consumption patterns, understanding how the building consumes energy, how the system elements interrelate and how the external environment affects the building.
- There are different approaches to conducting a full building energy audit, but the following stages are often adopted:
- Stage 1 - An audit of historical data
- Stage 2 - Survey
- Stage 3 - Detailed investigation and analysis
- A proper energy audit is useful for more than energy conservation goals. Energy audits can be employed to assist in areas such as:
- Establishment of data bank and consumption records.
- Estimating of energy costs.
- Determining of consumption patterns and utility rates.
- Establishment of an operational overview.
- Building energy design challenges building designers to think about climate, orientation, daylighting, and the qualities of environment as part of the initial design conception.
- It also requires the architectural and engineering disciplines to work as a team early in the design phase and to conceptualise the building as a system.
- Architects and engineers who incorporate energy design concepts and methods into their design projects can play a significant role in reducing energy consumption and achieving sustainable energy structure for our society.