Abstract:
Solar Energy for Buildings presents basic information on solar building design, which includes passive solar heating, ventilation air heating, solar domestic water heating and shading. The article suggests ways to incorporate solar design into multi-unit residential buildings, and provides calculations and examples to show how early design decisions can increase the useable solar energy.
This Introduction to Solar Design Issues, presents basic notions of solar design and describes different passive, active and hybrid systems and the solar aspects of design elements, which include window design, cooling and control, and water heating.
Upon reading this article, the reader will understand:
1. The benefits of solar energy in building design.
2. The difference between passive, active and hybrid solar technologies.
3. The design opportunities available for multi-unit residential
buildings (MURB).
THE PRINCIPLES OF SOLAR DESIGN
Benefits of solar energy
For both new and retrofit projects,solar energy can substantially enhance building design.
Solar energy offers these advantages
over conventional energy:
- Free after recovering upfront capital costs. Payback time can be relatively short.
- Available everywhere and inexhaustible.
- Clean, reducing demand for fossil fuels and hydroelectricity, and their environmental drawbacks.
- Can be building-integrated, which can reduce energy distribution needs.
The amount of energy that reaches earth’s upper atmosphere is about 1,350 W/m2— the solar constant. The atmosphere reflects, scatters and absorbs some of the energy. In Canada, peak solar intensity varies from about 900 W/m2 to 1,050 W/m2, depending on sky conditions. Peak solar intensity is at solar noon, when the sun is due south.
Energy from the sun reaches earth as direct, reflected and diffuse radiation.
Direct radiation is highest on a surface perpendicular to the sun’s rays (angle of incidence equal to 0 degrees) and provides the most usable heat. Diffuse radiation is energy from the sun that is scattered within the atmosphere by
clouds, dust or pollution and becomes non-directional. On a cloudy day, 100 percent of the energy may be diffuse radiation; on a sunny day, less than 20 per cent may be diffuse.
The amount of the sun’s energy reaching the surface of the earth also depends on cloud cover, air pollution, location and the time of year. Figure 1 shows the solar energy available in five Canadian cities at different times of the year.
The amount of solar energy reaching a tilted collector significantly changes the result. Figure 2 shows the amount of solar energy received by a horizontal collector, such as window, for a passive solar design. Note that even Yellowknife receives a significant amount during part of the heating season.
Passive, active and hybrid solar energy
Solar buildings work on three principles:
collection, storage and distribution of the sun’s energy.
A passive solar building makes the greatest use possible of solar gains to reduce energy use for heating and, possibly, cooling. By using natural energy flows through air and materials—radiation, conduction, absorptance and natural convection.
A passive building emphasizes passive energy flows in heating and cooling. It can optimize solar heat gain in direct heat gain systems, in which windows are the collectors and interior materials are the heat storage media.
The principle can also be applied to water or air solar heaters that use natural convection to thermosiphon for heat
storage without pumps or fans.
Solar Energy
Figure 1 – kWh/m2/day on a vertical surface, for selected Canadian cities
Glossary
- Absorptance : The ratio of absorbed to incident radiation.
- Active solar : A solar heating or cooling system that operates by mechanical means such as motors, pumps or valves to sort and distribute the sun’s heat to a building.
- Energy rating (ER) : A rating system that compares window products for their heating season efficiency under average winter conditions.
- Evacuated tube collectors : Solar collectors that use individual, sealed vacuum tubes surrounding a metal absorber plate.
- Flat-plate collectors : The most common type of solar collector. Can be glazed or unglazed.
- Hybrid power systems : Combines active and passive solar power systems or involves more than one fuel type for the same device.
- Latent Heat : Also called heat of transformation. Heat energy absorbed or released by a material that is changing state, such as ice to water or water to steam, at constant temperature and pressure.
- Low-emissivity (low-e): Coatings applied to window glass to reduce inside heat loss without reducing outside solar gain.
- Passive solar : A solar heating or cooling system that operates by using gravity, heat flow or evaporation to collect and transfer solar energy.
- Photovoltaic (PV) system : System that converts sunlight into electricity. Can be autonomous or used with another energy source. (Can be connected to the main power grid, for example).
- R-value (imperial), RSI-value (metric) : A measure of resistance to heat flow through a material or assembly a numerical inverse of the U-value.
- Solar balcony : An enclosed balcony that acts as a solar collector.
- Solar constant 1,350 W/m2 : The average amount of solar energy reaching the earth’s upper atmosphere.
- Solar Domestic Hot Water (SDHW) : A supplement to traditional domestic hot water heating. The most common system uses glazed, flat-plate collectors in a closed glycol loop.
- Solar Heat Gain Coefficient (SHGC) : Equal to the amount of solar gain through a window, divided by the total amount of solar energy incident to its outside surface.
- Solar south : 180 degrees from true or grid (not magnetic) north.
- Solar wall : A proprietary system that uses perforated metal panels to pre-heat ventilation air.
- Switchable glazing : Glazing materials that can vary their optical or solar properties according to light (photochromic), heat (thermochromic) or electric current (electrochromic).
- Thermosiphon solar collector : A system in which the circulation of hot water in the loop is based only on buoyancy.
- U-value : A measure of heat flow through a material or assembly. Measured in Watts/m2/°C.
- Warm-edge spacers : Separate a window’s glazing layers with thermal break or a low conductivity material.
Solar Energy
Figure 2 – kWh/m2/day on a south-facing horizontal surface, for five Canadian cities