Oaa en aug10


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Oaa en aug10

  1. 1. Solar Energy for Buildings Introduction: Solar Design Issues By Keith Robertson and Andreas Athienitis 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. 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: s Free after recovering upfront capital costs. Payback time can be relatively short. s Available everywhere and inexhaustible. s Clean, reducing demand for fossil fuels and hydroelectricity, and their environmental drawbacks. s Can be building-integrated, which can reduce energy distribution needs. 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).
  2. 2. Solar Energ y for Buildings 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 per cent 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 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. An active solar system uses mechanical equipment to collect, store and distribute the sun's heat. Active systems consist of solar collectors, a storage medium and a distribution system. Active solar systems are commonly used for: s Water heating; s Space conditioning; s Producing electricity; s Process heat; and s Solar mechanical energy. Hybrid power systems combine two or more energy systems or fuels that, when integrated, overcome limitations of the other, such as photovoltaic panels to supplement gridsupplied or diesel-generated electricity. Hybrid systems are the most common, except for the direct gain system, which is passive. S o la r E n e r g y o n a V e r t ic a l P l a n e 7 .0 0 6 .0 0 kWh/m2/day 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. Ha lif a x 5 .0 0 To r o n to 4 .0 0 Ed m o n t o n 3 .0 0 Yell ow kn i f e 2 .0 0 V anc ouv er 1 .0 0 0 .0 0 Jan Fe b Ma r A pr Ma y Ju n Ju l A ug Sep Oc t No v Dec Source: RETScreen1 Figure 1 – kWh/m /day on a ver tical surface, for selected Canadian cities 1 2 RETScreen is free energy assessment software that assesses renewable energy options against a base building model. Software modules are available at http://www.retscreen.net/ang/menu.php 2 Canada Mortgage and Housing Corporation
  3. 3. Solar Energ y for Buildings Glossary S o l a r En e r g y o n a H o r iz o n t a l S u r f a c e Absorptance—The ratio of absorbed to incident radiation. 7 .0 0 Energy rating (ER)—A rating system that compares window products for their heating season efficiency under average winter conditions. Halifax 5 .0 0 kWh/m 2/day 6 .0 0 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 buidling. Montréal Toronto 4 .0 0 Winnipeg 3 .0 0 Edmonton 2 .0 0 Yellowknife 1 .0 0 Vancouver 0 .0 0 Jan Feb Mar A pr May Ju n Ju l A ug Sep Oct No v De c Evacuated tube collectors—Solar collectors that use individual, sealed vacuum tubes surrounding a metal absorber plate. Figure 2 – kWh/m2/day on a south-facing horizontal surface, for five Canadian cities Source: RETScreen Flat-plate collectors—The most common type of solar collector. Can be glazed or unglazed. 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. 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. Solar balcony—An enclosed balcony that acts as a solar collector. Solar constant—1,350 W/m —The average amount of solar energy reaching the earth’s upper atmosphere. 2 Low-emissivity (low-e)—Coatings applied to window glass to reduce inside heat loss without reducing outside solar gain. 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. Passive solar—A solar heating or cooling system that operates by using gravity, heat flow or evaporation to collect and transfer solar energy. 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. Photovoltaic (PV) system—System that onverts sunlight into electricity. Can be autonomous or used with another energy source. (Can be connected to the main power grid, for example). Solar south—180 degrees from true or grid (not magnetic) north. Solarwall®—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 lowconductivity material. Canada Mortgage and Housing Corporation 3
  4. 4. Solar Energ y for Buildings Building design issues Careful solar design can: s Maximize possible solar transmission and absorption in winter to minimize or reduce to zero the heating energy consumption, while preventing overheating. s Use received solar gains for instantaneous heating load and store the remainder in embodied thermal mass or specially built storage devices. s Reduce heat losses using insulation and windows with high solar heat gain factors. s Employ shading control devices or strategically planted deciduous trees to exclude summer solar gains that create additional cooling load. s What is design integration? The most important factor for a successful solar building is “integration.” This concept includes not only the integration of design professionals at the project’s start, but also the integration of those who are responsible for the systems operation. This potential for synergy is usually overlooked because architects and engineers traditionally do not explore the concepts together closely enough to truly integrate systems, and they infrequently discuss new concepts with property managers, except when auditing a building failure. Employ natural ventilation to transfer heat from hot zones to cool zones in winter and for natural cooling in the summer; use ground-source cooling and heating to transfer heat to and from the underground, which is more or less at a constant temperature, and utilize evaporative cooling. Integrate building envelope devices such as windows, which include photovoltaic panels as shading devices, or roofs with photovoltaic shingles; their dual role in producing electricity and excluding thermal gain increases their cost-effectiveness. s Use solar radiation for daylighting,2 which requires effective distribution into rooms or onto work planes, while avoiding glare. s Integrate passive solar systems with active heating–cooling/air-conditioning systems in both design and operation. s not use the building enclosure as part of an integrated energy system in which the components fit together well. Collaboration between architects and engineers is increasing, but the traditional working relationships between architects, engineers, property managers and other professionals do not foster an integrated design approach.3 A preferable approach is to consider the building and its HVAC system as one energy system and to design them together, taking into account possible synergies such as electricity generation, thermal storage and control strategies. Direct Gain Solar Facade Collector-storage Wall Figure 3 – Two major options for thermal mass placement in passive solar design: direct gain and Trombe wall, or collector-storage wall The architect may design the building envelope to passive solar design principles while the engineer designs HVAC to extreme design conditions, ignoring the benefits of solar gains and natural cooling. The result is an oversized system that does 2 See Daylighting Guide for Buildings at: http://www.cmhc.ca/en/inpr/bude/himu/coedar/coedar_001.cfm 3 See Integrated Design Process Guide at: http://www.cmhc.ca/en/inpr/bude/himu/coedar/coedar_002.cfm 4 Canada Mortgage and Housing Corporation Passive solar heating systems (thermal) are separated into two broad categories, direct gain and indirect gain (see Figure 3). An indirect passive system insulated from the heated space is an isolated system.
  5. 5. Solar Energ y for Buildings Depending on climate and building function, certain heating/cooling systems are more compatible with passive systems. For example, the thermal mass in a floor may store passive solar gains and act as a floor-heating system. This is a control challenge that must be carefully planned if it is to achieve acceptable thermal comfort for the occupants. The key aspects of passive solar design are interlinked, dependent design parameters: s s s Fenestration area, orientation and type; s Thermal massing and envelope caracteristics; s Amount of insulation; s Shading devices—type, location and area; s Effective thermal storage insulated from the exterior environment, as well as amount and type; latent such as phase-change materials. Location and orientation of a building; s sensible—such as concrete in the building envelope with exterior insulation, or The ultimate objective of design integration is to minimize energy costs while maintaining interior comfort. A larger thermal mass within a building can delay its response to heat sources such as solar gains—the thermal lag effect. This thermal lag can avoid comfort problems if taken into account in selecting the thermal mass, choosing appropriate control strategies and sizing the heating–cooling system. Source: CMHC, at http://www.cmhc-schl.gc.ca/en/imquaf/himu/buin_018.cfm Figure 4 – Sixteen of the 42 units in this apar tment building in Amstelveen, the Netherlands, take advantage of solar energy from the atrium as an air pre-heating system. Solar domestic hot water panels provide about half the building’s domestic hot water energy. Canada Mortgage and Housing Corporation 5
  6. 6. Solar Energ y for Buildings Design procedure Building orientation The initial design steps in solar design are to: Orientation is crucial since it can provide free savings from the concept stage. There is a difference between true north and magnetic north. The deviation between magnetic north and true north—magnetic declination—varies between east and west coasts. In Nova Scotia, the compass points west of true north; in B.C., east of true north. 1. Set performance targets for energy sources and uses. 2. Minimize heating and cooling loads through orientation, massing, envelope and landscape design. 3. Maximize solar and other renewable energy to meet the building load, then to design efficient HVAC systems that are integrated with the building envelope performance characteristics. 4. Use simple energy simulation tools and detailed simulations in evaluating options at the early design stages and later to assess alternatives. Generally, deviations up to ±30º from due south reduce solar gains by up to about 12% and are thus acceptable in solar building design, providing significant freedom in choice of form. 6 The maximum difference (as a percentage) between south-facing and 30ºE (or W) orientations occurs when the sun is lowest and the days shortest (Dec. 21). When solar facades or roofs generate photovoltaic electricity that is sold to the grid at timeof-day rates, these rates may change the optimal orientation if their peak value is not at noon. Further information about magnetic deviation and a calculation routine is available at http://www.geolab.nrcan.gc.ca/geomag/ma gdec_e.shtml Canada Mortgage and Housing Corporation Generally, buildings with long axes running east and west have greater solar-heating potential if their window characteristics are chosen accodingly. For MURBs with a typical double-loaded corridor, this means half the units face south and half face north. A partial solution could be a south-facing central atrium or solar heater that pre-heats and delivers air for the north-facing units. Buildings with east-and west-facing orientations have greater potential for overheating in the non-heating season and get little solar gain in winter. In figure 5 the Foyer hongrois in Montréal angles the windows to the south creating a sawtooth plan, to avoid east-and west-facing windows.
  7. 7. Solar Energ y for Buildings Although differences in assumptions and input data make comparisons difficult, a study of a Toronto building produced different results. RETScreen’s passive solar energy module was used for the Toronto building. The RetScreen model of a 110 m2 (1,184 sq. ft.), south-facing suite in Toronto with 7.2 m2 (75 sq. ft.) of windows (similar to the suites in Halifax) gave the following results. (Increases in cooling load were not calculated, as this was assumed to be an unconditioned building.): s s Figure 5 – Foyer hongrois in Montréal. South angled windows on a building with a long nor th-south axis. Sunshades shadow these windows in the summer time. Building Conditions NRC’s EE4 software4 was used to model the energy use of a Halifax MURB, and showed modest energy reductions from orientation, window performance and window size. The advantage of energy reductions due to orientation is that they are free, and the savings continue for the life-time of the building. Note also that these energy simulation results are specific to a particular location. The MURB had the following characteristics: s High insulation levels. Simulation Results s Using a higher Solar Heat Gain Coefficient (SHGC) glazing reduced the total annual heating cost by three to four per cent. Window-to-wall ratio: 19 per cent on primary facades. s Double-glazed, low-e vinyl windows. Increasing the window area on the south-facing suites reduced annual heating cost by less than one per cent. s Increasing the interior mass reduced annual heating cost by about two per cent. Four-storey, double-loaded corridor, wood-frame. s Orienting the building along the long east–west axis instead of north–south axis reduced annual heating cost by about one per cent. s s s 4 Increasing the glass Solar Heat Gain Coefficient (SHGC) from .45 to .65 saved 1,100 to 1,200 kWh annually. Doubling window area and increasing the SHGC gave a slight annual energy loss in a low-mass (wood-frame) building and a slight saving in a highmass (concrete-frame) building. s Increasing the glass R-value and maintaining a high SHGC saved about 900 kWh annually. s The best results came from increasing the R-value, increasing the mass, increasing the window area, and maintaining a high SHGC. These results are expected from basic solar design principles. Increasing the resistance of windows to thermal loss (low-e glazing) while admitting high solar gains reduces heating energy consumption if the building is well insulated and there is enough thermal mass to store the solar gains and prevent overheating. Obviously, the thermal performance of windows cannot be separated from solar gains, which relate to form, orientation and solar transmittance. Optimizing requires rigorous energy modelling and project-specific analysis. EE4 is the software developed for NRCan’s Commercial Building Incentive Program to check for compliance to its program requirements. Canada Mortgage and Housing Corporation 7
  8. 8. Solar Energ y for Buildings More details on the design of windows and glazing selection are presented in Selection and Commissioning of Window Installations5 approximately equal to the amplitude of the cyclic heat flow into the mass divided by its surface temperature amplitude or swing.) The analytical tool selected depends on the detail required. For basic energy flows, an analysis based on solar heat gain coefficients and thermal conductance provides an approximate estimate of the net energy transfer across the building envelope. The calculations can be performed in MathCAD, Matlab or a spreadsheet-based program such as RETScreen. A good design strategy for building orientation is to “tune” windows to admit or exclude solar energy based on their orientation. Generally, south-facing windows should admit winter solar gain and east- and west-facing windows should exclude low-angle solar gain. Window design strategies are discussed in more detail later. To determine room-temperature swings and associated thermal mass response, more detailed simulation tools are needed. However, even for the calculation of temperature swings and the effectiveness of thermal mass, simplified models exist which are based on thermal admittance calculations.6 “Thermal admittance” is essentially a dynamic U-value and is typically calculated for a daily cycle. (It is Another approach is control of solar gains with motorized blinds, which are widely used in airports, atriums and some commercial buildings in Europe. Along with other control technologies, such as electrochromic coatings, motorized blinds may soon become cost-effective. If active solar control is taken into account in sizing cooling systems, there may be significant savings from reduced energy consumption and reduced equipment sizing. Obstructions to sunlight Obstructions can have a significant effect on solar potential. For low- to mid-rise buildings, obstructions are usually buildings, terrain or trees. For larger buildings, obstructions are usually other large buildings. Obstructions can be identified on the sun path chart in figure 8. East and west obstructions can reduce solar gain in the summer and admit energy in the winter, when the sun rises in the southeast and sets in the southwest. 5 See http://www.cmhc.ca/en/inpr/bude/himu/coedav/upload/Article_Design_Aug31.pdf 6 Athienitis A.K. and Santamouris M., 2002. Thermal analysis and design of passive solar buildings, James and James, London U.K.. 8 Canada Mortgage and Housing Corporation
  9. 9. Solar Energ y for Buildings Direct-gain passive solar techniques Internal thermal mass reduces temperature swings within a space. In a properly designed passive solar system, thermal mass absorbs solar energy during the day, preventing the building from overheating, and releases the energy at night. Thermal mass is most effective when it can gain energy directly from the sun. An ideal thermal mass for passive solar heating has high heat capacity, moderate conductance, moderate density and high emissivity. Additional cost is negligible if the material is also structural or decorative. Concrete and masonry are good thermal mass materials. (Plaster, drywall, and tile are also useful in this respect, but calculations are needed to determine if they have sufficient mass, as was done in the Halifax study.) Passive solar design in single-family residences shows that operational energy can be reduced by 30 to 50 per cent through window sizing and thermal mass storage. A recent study of MURBs in Sweden reported that operational energy use in a heavy structure is only slightly lower than in a similar, lightweight structure.7 The additional energy used to build the heavy structure outweighed its operational advantage in a lifecycle analysis of costs. 7 Outdoor temperature Air temperature Pure passive solar design uses the sun’s energy directly, without mechanical intervention. In its simplest form, the sun shining through a window directly heats the space. Thermal mass within the building can absorb some of the heat and release it at night. Light timber-framed building Heavy building with external insulation Heavy building set into and partially covered with earth Time of day Figure 6 – Effect of internal mass on internal temperature swings It can be calculated as a percentage of the total area of the south-facing exterior wall—of limited use because it is not affected by what goes on beyond the wall—or as a percentage of heated floor area—which accounts for the volume of the building. Mass is known to be able to reduce peak cooling load when night temperatures are cooler than day temperatures. Exterior and interior masses cool down at night and reduce peak cooling demand while also delaying the time of the peak solar gain during the day. However, the effectiveness of thermal mass is proportional to the allowable room temperature variation over a day. A typical passive solar-heated building may have south-facing glazing equal to 10 to 15 per cent of the heated floor area. As the area of south glass increases, the amount of mass inside must also increase. The Advanced Buildings Technologies and Practices website, at http://www.advancedbuildings.org, recommends a window-to-exterior wall area ratio (WWR) of 25 to 35 per cent, similar to a typical MURB. Windows Window orientation, layout and performance are important in passive solar design. The goal is to provide an appropriate amount of window area in the right place. Where there is no fenestration, a conventional insulated wall is a solar barrier, transmitting little energy to the inside. Window sizing There are two ways to quantify a building's south-facing glass. WWR may increase with proper control of solar gains (for example, with motorized shading) and transfer of excess energy to north-facing zones. This could possibly approach 50 per cent when a large atrium is included with adequate thermal storage Stahl, Fredrik, The effect of thermal mass on the energy during the life cycle of a building, presented at Building Physics 2—6th Nordic Symposium Canada Mortgage and Housing Corporation 9
  10. 10. Solar Energ y for Buildings capacity. Utilization of double facades with blinds in the cavity, or exterior controlled shading reduces cooling loads during summer. (Figure 4 – Urban Villa, Amstelveen)8 Glazing This section describes some the most important parameters of window and glazing design. Solar heat gain coefficient (SHGC) The Solar Heat Gain Coefficient (SHGC) is a useful measure of a window's ability to admit solar energy. SHGC is the amount of solar gain a window allows, divided by the amount of solar energy available at its outside surface, a number between zero (solid wall) and one (open window). SHGC can be measured for the window unit, including the frame, or the glazed area. The higher the SHGC, the better the window will perform as a solar collector. If overheating is a concern, low-SHGC windows exclude solar energy to reduce cooling loads. A single pane of clear glass facing the sun will admit most of the visible solar radiation, some of the infrared and very little ultraviolet and have the highest heat loss from inside to outside. Ways to modify windows to enhance their performance include: s Adding a second or third layer of glass, which can dramatically lower the U-value (increase the R-value), while maintaining a large SHGC. Additional layers of glass also permit thin, lowemissivity (low-e) coatings to be applied onto a protected glass surface. Low-e coatings still allow solar gain (short wavelength radiation) and they Figure 7 – Double-glazed, low-e window help retain heat by reducing longwave (infrared) radiation losses. This is very helpful from a passive solar heating point of view. s There are reflective coatings that block unwanted solar gain (reduce the SHGC) to reduce the cooling load. There are many types of spectrally selective glazings that block out selective wavelengths that can change the SHGC and levels of visible light transmittance. s Evacuating the space between the panes, using an inert gas such as argon or krypton, or transparent insulation, can reduce heat loss by conduction and convection. Because gas-fills perform well and are low cost, they should be used whenever a low-e coating is used in a glazing unit. High-performance windows may make it possible to move heating outlets further from windows to eliminate ducting or piping. 8 A recent glazing development is switchable glazing. These can vary their optical or solar properties according to light (photochromic), heat (thermochromic) or electric current (electrochromic). Initial computer simulations show that electro chromic glazing has the most promise for improving comfort. These are prototype systems. They will likely be able to reduce cooling loads and glare and improve visual comfort if high solar transmittance is not needed. Switchable glazings may have poorer optical properties and not be suitable in residential buildings. Visible light transmittance Visible light transmittance (VT) measures the visible spectrum admitted by a window. Typical daylight strategies require windows with a high VT. A low SGHC is also desirable where heat gain is a concern. Reflective glass is not recommended for daylighting. Table 1 shows typical values for light transmittance and SHGC of common glazing systems. See Innovative Buildings Case Studies — “Atrium, Solar shading and ventilation for residents’ confort”, Amstelveen : http://www.cmhc.ca/en/inpr/bude/himu/inbu_001.cfm#CP_JUMP_58686 10 Canada Mortgage and Housing Corporation
  11. 11. Solar Energ y for Buildings Table 1 – Visible Light Transmission–solar heat gain coefficient (per cent) Glazing system (6 mm glass) Clear Blue-green Grey Reflective Single 89–81 75–62 43–56 20–29 Double 78–70 67–50 40–44 18–21 Double, hard low-e, argon 73–65 62–45 37–39 17–20 Double, soft low-e, argon 70–37 59–29 35–24 16–15 Triple, hard low-e, argon 64–56 55–38 32–36 15–17 Triple, soft low-e, argon 55–31 52–29 30–26 14–13 Source: ASHRAE Fundamentals 1997,Table 11, page 29 Frames Frames are often the weakest thermal part of a window. Although frames (sash and mullion assemblies) are only 10 to 25 per cent of window area in commercial buildings, they can account for up to half the window heat loss and be the prime site for condensation.9 Thermal performance of frames is improved either by using a low-conductivity thermal break in metal frames or a frame of a lowconductivity material, such as wood, vinyl or fibreglass. Low-conductivity window frames reduce energy consumption in all types of buildings. For MURBs the designer should note that Canadian fire codes state that the area of windows with combustible framing materials must be less than 40 per cent of the building wall area and that non-combustible materials must separate windows.10 Spacers Spacers separate panes of glass in a sealed window to prevent the transfer of air and moisture in and out of the glass cavity. 9 Warm-edge spacers use low-conductivity materials, rather than aluminum, and are important in reducing heat loss through the window. By reducing the likelihood of condensation on the glass surfaces, they can also influence daylighting performance. The low cost and good performance of warm-edge spacers make them suitable for all window systems and should be considered mandatory whenever low-e coatings and inert gas fills are used.11 Window orientation The greatest amount of solar energy is generated at noon on any given day in the year. The greatest amount of energy received through a window is when the sun is perpendicular to the window and 30 to 35 degrees above the horizon. South, east and west windows receive about the same annual maximum of solar radiation. The time and date that the maximum energy is received depends on the building’s latitude and wall orientation. The earth rotates 15 degrees an hour; when a window is oriented 30 degrees east of south, the maximum heat gain will be about two hours before solar noon. East and west facades receive maximum solar gain in the summer; a south-facing surface receives its annual maximum in the late fall or winter. Figure 8 shows a sun path chart for latitude 44ºN. The sun’s path varies by a project’s latitude. The X-axis gives the direction of the sun; the Y-axis the sun’s angle above the horizon. The curved lines show the arc of the sun across the sky on the 21st day of each month. The dashed lines show the time of day. An accurate location of the sun can be determined by plotting the time of day and month. Obstructions are also plotted to show when a building will be shaded. Sun charts for any latitude can be generated through a University of Oregon online program at http://solardat.uoregon.edu/SunChartProgr am.html Figure 9 shows the intensity of solar energy striking a vertical surface facing the sun. The maximum energy entering a window occurs when the sun is 30 to 35 degrees above the horizon and directly in front of the window. Website: Advanced Buildings: Technologies and Practices http://www.advancedbuildings.org/_frames/fr_t_building_low_conduct_window.htm 10 Website: Advanced Buildings: Technologies and Practices http://www.advancedbuildings.org/_frames/fr_t_building_low_conduct_window.htm 11 Website: Advanced Buildings: Technologies and Practices http://www.advancedbuildings.org/_frames/fr_t_building_warm_edge_windows.htm Canada Mortgage and Housing Corporation 11
  12. 12. Solar Energ y for Buildings Superimposing Figure 9—Solar energy intensity – over the sun path chart shows the effect of window orientation on solar gain Figure 10 aligns the solar intensity chart to south on the sun path chart. This shows that the maximum solar gain occurs at noon in October and February. To indicate the solar gain on a west window, align the solar intensity chart with west on the Sun path chart, as shown in Figure 11. This clearly shows how window orientation affects the time of day and the time of year of maximum solar gain. North-facing windows provide consistent indirect light with minimal heat gains, but can also create heat loss and comfort problems during the heating season. South-facing windows provide strong direct and indirect sunlight that varies during the day. Controlling heat gain can be a problem during the cooling season. Shading is easily done with horizontal shading devices in these windows. East- and west-facing windows can create more problems with glare and heat gain and are more difficult to shade because the sun is closer to the horizon. In Canada’s North, the sun is at a low angle in the sky during winter, when sunlight is most needed to contribute to heating. This is when south-facing clerestory windows have an advantage over horizontal roof glazing. However, the sun also creates glare. Overhangs over south windows may need to be large to prevent this. Also, when the sun is low, buildings and trees can create shade, which is desirable in some seasons. Note that south-facing surfaces receive more energy in the winter and less in the summer than east- and west-facing surfaces. A strategy to control overheating Adapted from Edward Magria “The Passive Solar Energy Book” Figure 8 – Sun path char t Figure 10 – Energy striking a south window for latitude 44ºN Figure 9 – Solar energy intensity Figure 11 – Energy striking a west window for latitude 44ºN 12 Canada Mortgage and Housing Corporation
  13. 13. Solar Energ y for Buildings is to maximize window area on the south and use less on the east and west. For mainly cloudy regions, where overheating is less of a problem, interior spaces benefit from larger windows (including the north facade) that allow more light into a building. There can be a trade-off between allowing more daylight and increasing heat loss. In mainly clear regions, glare and heat gain are more problematic. In direct sunlight, smaller windows can provide adequate daylight. Direct sunlight can also be reflected or diffused, or both, with window shading. Window performance and tuning Window orientation, size, layout and performance are important in passive solar design. Proper glazing and frame selection can enhance daylighting and energy performance. General rules for tuning window orientation include: s Determine the window size, height and glazing treatments separately for each facade. Overhang s Maximize southern exposure. Shading s Optimize northern exposure. s Minimize western exposure when the sun is lowest and most likely to cause glare and overheating. Windows themselves can be oriented differently from the plane of the wall in a “sawtooth” arrangement. Shading may be exterior, interior, fixed, motorized or between an exterior glazing and an interior facade in double-facade systems. Figure 12 shows some examples of shading systems. A good shading system permits lower levels of artificial illumination, because the eye can accommodate itself without strain to function within a wide illumination range. Larger window areas increase the risk of glare, overheating in summer and heat loss in winter. For areas with direct sun, shading needs to reduce transmittance to 10 per cent or less to prevent glare. Exterior shading devices are the most effective at controlling solar gain. Interior window shading allows much of the solar energy into the building and allows more heat, sometimes an unwanted partner of daylight, to enter the building. Lightcoloured interior shading will reflect some of this energy back through the window. However, a minimum of about 20–30 per cent of incident solar radiation will come indoors as transmitted or be absorbed and re-emitted as heat when interior blinds are used. Exterior blinds collect dust and may be difficult to maintain and clean. One solution is to place reflective blinds between the two glazings and possibly to have airflow within the cavity—a double-facade. Glare from windows can occur when the incoming light is too bright compared with the general brightness of the interior. Punched windows can create strong contrasts from the interior between windows and walls. Horizontal strip windows provide more even daylight distribution and, often, better views. This article discusses other interior design guidelines later. Louvred Overhang Lightshelf Vertical Fins Figure 12 – Common types of exterior shading Canada Mortgage and Housing Corporation 13
  14. 14. Solar Energ y for Buildings South-facing windows are the easiest to shade. Horizontal shading devices, which block summer sun and admit winter sun, are the most effective. East- and westfacing windows are best shaded with vertical devices, but these are usually harder to incorporate into a building and not limit views from the window. On lower buildings, well-placed deciduous trees on the east and west reduce summer overheating and allow desirable winter solar gains. Some practioners are testing vines hung on metal lattices to reduce overheating. Interior shading is most effective at controlling glare and can be controlled to suit the occupants. Energy Rating—ER Energy Rating (ER) is a rating system developed by the Canadian Standards Association and the window industry. It compares window products for their heating season efficiency under average winter conditions. The ER is the value of energy gained or lost in watts per square metre (W/m2). RSI value is a misleading measure of energy efficiency because it often only accounts for the heat loss through the centre of the glass. The ER considers all the energy flows through the window, the total glass R-value, the frame R-value, air infiltration and average solar gain. The solar gain is an average of the four orientations. Figure 13 – Double facade in a residential building, Klosterenga, Oslo, Norway 14 Canada Mortgage and Housing Corporation Because the ER relies on an average solar gain, it cannot be used to compare actual performance for a specific location orientation and window size. Further calculations can determine the Energy Rating Specific (ERS). This determines a specific ER value for a window based on the climate of a particular location, the window-to-floor area ratio and the window orientation on the building. Both the ER and ERS are part of CSAA440.2 Energy Performance of Windows and Other Fenestration Systems standard. Figure 14 – Glazed solar pacade from the outside of Klosterenga
  15. 15. Solar Energ y for Buildings Solar cooling Traditionally, passive solar cooling is associated with much warmer climates than Canada’s. In Canada, the most effective method is to exclude solar gain through fenestration design, window glazing selection and shading devices. Another common strategy is to use the mass of the building, which cools down at night to mitigate overheating by absorbing solar energy during the day. Harnessing the stack effect, that is the upward movement of warmer, more buoyant air, is possible if a building is designed to capture solar heat and exhaust it at roof level. This warm air can be released to the outside, drawing cooler ground-level air into and up through the building. An atrium can act as a solar chimney with motorized windows to harness the stack effect and help the natural ventilation process. Using thermal mass in an atrium helps prolong the chimney effect well into the night to draw cool air into the building. In Europe, cool night air is passed (using fans) through hollow core floors to store coolness. During the day, room air is recirculated through the cool floor to provide free cooling. Absorption cooling involves hightemperature solar collectors connected to an absorption chiller operating at around 100°C (212°F). The device uses a solar collector to evaporate a pressurized refrigerant from an absorbent–refrigerant mixture. Absorption coolers require little electricity to pump the refrigerant Solar balconies compared to that of a compressor in a conventional electric air conditioner or refrigerator. This system is not yet efficient enough for conventional buildings and requires a large, upfront investment. Glazed, stacked balconies can also work as passive collectors. They passively re-radiate heat or actively ventilate to the rest of the unit or to the outside. Desiccant cooling uses a desiccant, a chemical drying agent, in contact with the air to be cooled. The air becomes so dry that moisture can be injected into it without affecting comfort. The moisture droplets evaporate and cool the air. The drying agent is regenerated by hot air that is heated through solar air collectors or a coil connected to liquid-based collectors.12 An effective method is to inset the balcony into the building envelope. This simplifies the building envelope and eliminates the need to separately support or cantilever the balcony. It also reduces the amount of thermal bridging across the envelope, but may require additional shading devices if the room is to be occupied regularly or if temperature fluctuations are not desirable. Of course, the balcony becomes less effective as a solar collector as it is oriented away from south. An enclosed balcony partially or entirely projecting from the exterior allows solar gains in units without direct southern exposure. The Rankine-cycle cooling process is a vapour compression cycle similar to that of a conventional air conditioner. Solar collectors heat the working fluid, which has a very low vaporization point, which then drives a Rankine-cycle heat engine. This technology is mainly experimental and not used often because it needs a large system size to do any meaningful amount of cooling.13 In the CMHC study of renewable energy at the building envelope, energy modelling of a six-storey MURB in Halifax predicted that solar balconies would reduce energy consumption by about four per cent. Overheating Overheating tends to occur more from unshaded west-facing windows and, to a lesser extent, east windows. Late summer is often the most crucial time of year. Design strategies include minimizing the amount of east- and west-facing glass, selecting glazings with a low SHGC to exclude heat and provide shading. Thermal mass inside the building can also have the effect of reducing the peak-cooling load in some climates. A Dutch study14 looked at solar balconies in renovating post-war, multi-family residential buildings with aged and failing envelopes. The study showed that the new solar elements were a cost-effective way to upgrade while reducing energy consumption by about 35kWh per square metre. Optimizing thermal, glazing and ventilation parameters and using simple venting and solar shading enhanced occupant comfort. 12 Natural Resources Canada website: http://www.canren.gc.ca/tech_appl/index.asp?CaID=5&PgID=164#desiccant 13 U.S. Department of Energy website: http://www.eren.doe.gov/consumerinfo/refbriefs/ac2.html 14 Advanced glazed balconies: Integration of solar energy in building renovation, W/E consultants, The Netherlands, EuroSun'96 Canada Mortgage and Housing Corporation 15
  16. 16. Solar Energ y for Buildings Cour tyards, atriums and common spaces A south-facing atrium can collect pre-heating air to be circulated throughout the building. This requires airtight construction and a high level of insulation. Overheating in the atrium can be avoided with properly sized and located motorized shades and a passive ventilation system. Architects must recognize the fire safety issues of atriums and provide protection for their occupants. This is addressed in a separate article on the CMHC website Fire Safety in High-rise Apartment Buildings.15 The difficulty in dealing with smoke control and using an atrium to pre-heat building air becomes a challenge. In high-rise and mid-rise apartments, it may be easier to consider common spaces, such as entry and elevator lobbies and stairwells, as solar space. This makes orientation of individual units more flexible and may allow greater variations in temperature swings. An efficient flat-plate solar hot water heater can collect approximately 2GJ of energy per m2 (550 kWh/m2) of collector area per year in most of southern Canada. Other systems available include thermosiphon systems, common in southern Europe, that eliminate the need for pumps. project consists of 100 well-insulated units; each with 140 m2 (1,506 sq. ft.) heated area, and assessed configurations of collector area (900 m2 to 1,500 m2) and insulated underground water storage 1,600 m3 to 6,300 m3 (56,503 cu. ft. to 222,482 cu. ft.).16 Several projects in Europe are working with prototypes of seasonal storage, the “Holy Grail” of the solar world. These projects use large solar arrays to collect heat in the summer and store it in large, well-insulated underground water tanks. The heat is extracted from the water during the heating season. To illustrate the scope of such systems, they use approximately 10 m2 to 20 m2 (107 sq. ft. to 215 sq. ft.) of collector and 20 m3 to 40 m3 (706 cu. ft. to 1,412 cu. ft.) of storage for every flat or house. Performance projections indicate that they would provide from 30 to 60 per cent of a buildings’ energy. Planning for a 100-unit solar demonstration housing project in Bavaria assessed systems capable of providing 60 to 90 per cent of heating using seasonal solar heat storage. The In Hamburg, 24 single-family, detached houses used 3,000 m2 of collector with 4,500 m3 (158,916 cu. ft.) insulated underground water storage. A sister project in Friedrichschafen used 5,600 m2 (60,277 sq. ft.) of collector with 12,000 m3 (423,776 cu. ft.) of storage for 570 flats in eight buildings. Both projects anticipate solar energy will cover 50 per cent of heating and hot water needs.17 Solar water heating Solar domestic hot water heating systems vary in complexity, efficiency and cost. Modern solar water heaters are relatively easy to maintain and can pay for themselves in energy savings well within their lifetimes. In MURBs, they may pre-heat water for the boiler in hot water heating systems. This works best in large projects that have significant system heat losses (when the return water is cooled sufficiently that solar can reheat it). For boilers heating water for space heating and hot water, solar panels may allow the boiler to be shut down in the summer and provide hot water from solar energy alone. In much of Canada we have clear cold winters and under these conditions a substantial amount of solar energy is available when needed, so short-term (1-2 days) storage is more cost effective. An equivalent climate in Canada for these European examples would be the lower mainland of British Columbia. Glazing Box Tube Outlet Inlet Inlet Header Absorber Plate Note: for further information on collector design and performance, see manufacturers’ specifications Insulation Bottom Plate Figure 15 – Glazed flat-plate collector 15 http://www.cmhc-schl.gc.ca/en/imquaf/himu/upload/Fire-Safety-in-High-Rise-Apartment-Buildings.pdf 16 D. Lindenberger et al., Optimization of solar district heating systems: seasonal storage, heat pumps and cogeneration, May 1999 17 B. Mahler et al. Central solar heat plants with seasonal storage in Hamburg and Friedrichschafen 16 The design shown is an example of a typical liquid-cooled collector. Air-cooled collector design will vary accordingly Canada Mortgage and Housing Corporation
  17. 17. Solar Energ y for Buildings Unglazed flat-plate collectors are the most common North American collectors, as measured by area installed per year. They are used most for warming water up to 30°C (86°F) for outdoor and indoor swimming pools. They are inexpensive, simple systems that can provide all the heating needs for residential outdoor swimming pools, eliminating both fossil fuel consumption and the capital costs of conventional heating equipment. They are simple to install and generally have a three- to sixyear-year payback.18 In Canada, their use is limited to non-heating seasons. impractical on high-rise buildings. To avoid heat loss during transit, a glycol collector with a well-insulated circuit may be used close to the pool. Southern or overhead glazing can also provide direct solar energy and cut conventional lighting costs. Solar energy can supply between 30 and 100 per cent of the required heat, depending on variables, including location, collector angle and orientation, desired pool temperature, size of pool and use of a pool cover. Evacuated tube collectors are individually sealed vacuum tubes surrounding a metal absorber plate. The vacuum minimizes conductive heat loss, like a thermal jug. These collectors are commonly used in very cold climates. Evacuated tube collectors are able to provide higher water temperatures, but are also more expensive, with longer payback periods. RETScreen calculations show that an evacuated tube collector can deliver about 1.2 kWh/m2/day in winter and up to 2.9 kWh/m2/day in June. Simple RETScreen calculations show that unglazed collectors deliver about 2.0 to 2.4 kWh/m2/day during summer. Outdoor pools are usually seasonal and in warmer months a solar blanket can be used, or solar collectors and pumps can heat the pool directly. When indoor pools are at or below grade, rooftop collectors are Natural Resources Canada Figure 16 – Unglazed flat-plate collector 18 Sheltair Group et al, Healthy High-Rise: A guide to innovation in the design and construction of high-rise residential buildings, (Canada: CMHC, 1996) p. 49 Canada Mortgage and Housing Corporation 17
  18. 18. Solar Energ y for Buildings Active solar space heating Evacuated tube SDHW—Solar domestic hot water systems Solar Domestic Hot Water (SDHW) systems supplement traditional hot water heating. The most common system uses glazed, flat-plate collectors in a closed glycol loop. A heat exchanger transfers the energy from the glycol to one or more solar storage tanks. These are usually connected in series to the hot water system. The traditional water heater comes on to keep the water at the required temperature if the solar heat is not enough. Glazing Inlet Outlet Figure 17a – Evacuated tube collectors Source: Natural Resources Canada1 Cross section of evacuated tube Outer Glass Tube Inner Glass Tube Fluid Tube Copper Sheet Evacuated Space There are seasonal variations in the energy they collect, depending on location, collector efficiency, collector angle and orientation, ranging from about 0.6 to 1.0 kWh/m2/day in winter up to about 2.4 kWh/m2/day in summer. It is easy to get 50 per cent of hot water energy from the sun. A reasonable target for fossil fuel displacement is 30 to 40 per cent. This allows the panels to operate at a more efficient temperature. These systems are easily integrated into current hot water systems and have a payback in the range of 10 years. In Canada, this varies tremendously, depending on funding incentives and fuel cost. 18 Figure 17b – Evacuated tube collectors Source: Natural Resources Canada Figure 17c – Rooftop evacuated tube collector Source: Architectural Graphic Standards Canada Mortgage and Housing Corporation
  19. 19. Solar Energ y for Buildings Anti-Freeze Solution Solar Heated Water Hot Water to House Solar Collectors Pump Controller Pump Cold Water in Gas or Electric Water Heater Heat Exchanger Solar Storage Tank Figure 18 – Solar domestic hot water system Source: www.AdvanceBuilding.org Table 2 – Cost and benefits of solar collectors Collector Capital cost $/m2 Energy delivered annually kWh/m2 Not for freezing temperatures. 150–350 210–250 (summer only) Economical. Needs glycol protection from freezing. 450–750 500–600 Provides hotter water. Expensive; needs glycol protection from freezing. 1,100–1,500 800–840 Typical uses Advantages Unglazed Swimming pools Economical, efficient at low temperature differentials. Glazed DHW pre-heat Evacuated tube DHW pre-heat Disadvantages Canada Mortgage and Housing Corporation 19
  20. 20. Solar Energ y for Buildings Solar air heating Table 3 – Common elements in solar air-heating systems The following summary is based on Solar Air Systems: A Design Handbook, edited by S. Robert Hastings and Ove Mørck. The authors looked at European and North American applications. Cost analyses are in Canadian dollars, unless otherwise noted. Collector systems s Flat-plate collector s Window air collector s Six principal solar air-heating systems are summarized below. All systems consist of the following common elements in one form or another: collector, distribution system (ducting), storage unit and control system. s A total system can consist of any combination of the four different components. s The applications analyzed in the study were for industry, dwellings (apartments, row and single-family houses), offices, schools, sports halls and swimming pools. The factors affecting system performance are type and mass of building, insulation level and climate. s s Design procedure The Solar Air Systems design handbook recommends the following design steps. More technical details can be found in the guide. Storage systems Perforated unglazed collector (Solarwall®) Double facades and double-shell collector s Hypocaust (ceiling or floor slab) s Murocaust (wall) s Rock beds s Water s s Define necessary basic data about building and climate. Determine if it is possible to obtain enough collector area. 20 Continuous performance s Temperature control s Solar cell control s Timer control s Usually through ducting. Spatial collector (atriums, sunrooms, greenhouses) Determine ventilation rate through the solar air collector. Determine if there are restrictions on inlet temperature from ventilation system. s Investigate if it is appropriate to include storage in system. Define the required control strategy. s Choose a solar collector. s Investigate if the system may serve other purposes. Canada Mortgage and Housing Corporation s Distribution Phase-change material s s Control systems s Determine the collector area. s Size the ducting. s Choose a fan. s Choose diffusers. Using an integrated design approach will enable the building design team to better consider any possible alternative purposes for the various systems, which could help reduce the payback time or provide other benefits to the occupants.
  21. 21. Solar Energ y for Buildings Preheated Ventilation Air Solar Collector System 1 Conserval Figure 19 – Ouellette Manor, Windsor, uses Solarwall ® to pre-heat corridor ventilation air System 1: Solar heating of ventilation air, such as Solarwall ® This system provides the simplest, and usually least costly way to bring solarheated fresh ventilation air into a building. It uses mainly off-the-shelf components in its design. Its major disadvantage is that it will reduce cost-effectiveness of the building’s ventilation heat recovery unit. An example of this type of system developed in Canada is Solarwall®, in which a southfacing wall is clad with dark metal panels, typically steel or aluminum, perforated with Figure 20 – System 1, solar air pre-heat system concept small holes. A gap is left between the cladding and the wall so that outside air passes through the holes in the collector panel. Air is aspirated into the airspace between the collector and the wall, is heated, and rises as a result of the stack effect and the lower pressure zone above, which is created by fans moving air through the system to the interior. This pre-heated ventilation air is then incorporated into the building's normal distribution system. A recirculation damper controls the mix of air from the collectors and from inside the building to maintain a constant air temperature for distribution. Using the sun to pre-heat air for ventilation in this way is a fairly new Table 4 – Solar heating of ventilation air Benefits Limitations Less cost to heat ventilation air Requires large, south-facing wall area Recaptures heat loss through wall Reduces opportunity for south facing glazing May replace conventional cladding (new construction) Conceals old cladding (retrofit) Reduces the cost-effectiveness of ventilation heat recovery (because owner pays less to heat incoming air) technology. In the last 10 years, about 35,000 m2 (376,737 sq. ft.) of Solarwall® collector systems have been installed in buildings, including low-rise and high-rise residential. Pre-heated ventilation air systems can be integrated into new construction or as a retrofit (see figure 19). In the early 1990s, Ouellette Manor, a 24-storey, 400-apartment seniors residence in Windsor, reclad part of its complex with Solarwall®. The new Solarwall® had an incremental cost of about $30,000 and the energy savings provided a simple payback of about six years. There is more information about Ouellette on the CMHC website at http://www.cmhc-schl.gc.ca/en/imquaf/ himu/buin_006.cfm Solarwall® is ideally suited for applications that require large quantities of ventilation air during the day and has proven effective at pre-heating ventilation air in MURBs. In new and retrofit situations, it has the benefit of offsetting the cost of traditional cladding materials. As a result, it can have very quick, and sometimes immediate, payback. Doesn't replace normal heating system Canada Mortgage and Housing Corporation 21
  22. 22. Solar Energ y for Buildings System 2: Open collection loop with radiant discharge storage In this system, air circulates, either naturally or mechanically, through the collector, distribution system, room space and back to the collector. It can be built with or without storage, and may require a separate ventilation system. Open Loop Air Circulation System 3: Double envelope (facade) systems In a double-envelope or double-facade solar air system, solar heated air is circulated through cavities in the building envelope, surrounding the building with a layer of solar-heated air. This creates a buffer space that reduces the building’s heating and cooling load. Inner comfort is improved because inner surfaces of the external walls are warmer. The outer envelope can be made of opaque materials (traditional cladding materials with an air space) or glass. The Klosterenga project in Oslo, Norway uses the space between double layers of south-facing windows to preheat the air. The figures in Table 5 are for glass-enclosed systems. Questions of cleaning and maintenance for this type of system must be addressed. Solar Collector System 2 Figure 21 – System 2, without storage Solar Collector Cavity Wall This system is versatile and integrates into most existing heating systems, but is usually much more expensive than other systems. In North America, costs are reported to be four to five times that of traditional, low-cost cladding systems,19 but the effective cost may drop if the double facade reduces energy consumption. Radiant Heat Solar Air Surrounds Envelope System 3a Figure 22 – System 3a, double-envelope system with storage 19 Meyer Boake, Terry et al. Canadian Architect, August 2003, p. 38 22 Canada Mortgage and Housing Corporation
  23. 23. Solar Energ y for Buildings There are numerous concepts for double facade. The following example demonstrates the heating effect of an airheating solar collector with a motorized blind as the surface absorbing the solar radiation. Major design parameters are the spacing between the two skins of the facade, the air velocity and the properties of the blind, which is controlled by the building automation system, with manual override and automatic refresh every hour or so. The blind, even when closed, must allow enough daylight into the space. This requires a 20 per cent transmittance depending on window area. The glazing must be clear. The airflow-window type of double facade was considered for the Seville adaptive reuse project in Montréal.20 Each floor may be separate (with box window types) with individual inlets and outlets or connected to form one large “chimney.” Figure 23 shows double glazing for the outer skin with low-emissivity coating facing the skin cavity to reduce heat losses in winter. However, this coating, which increases the outlet temperature by a couple of degrees, may possibly be excluded as it can deteriorate in this case. The inner glazing may be operable. The inlets and outlets of the airflow window need to be carefully designed. 20 Open Loop Air Circulation Solar Facade System 3b Figure 23 – System 3b, double-facade design option “Seville Theatre Redevelopment Project Integrated Design Process,” CMHC Technical Series (63175) Research Highlight 03-102 Canada Mortgage and Housing Corporation 23
  24. 24. Solar Energ y for Buildings The following results show an example of the air temperature rise of the solar collector due to varying the distance between the two “facades” or skins. v=air velocity: 0.1–0.2 m/sec w=width of the space=3.6 m, Outdoor air temperature of -5ºC L= the distance between the two skins; Outer glazing, clear double; inner glazing single low-emissivity coating on inner side of outer glazing (double) blind solar absorbance: 60 per cent, transmittance 20 per cent. Height of the space=4 m Note that the larger the gap width between the skins, the smaller the air velocity needed to achieve the required fresh-air flow rates. Figure 24 – Fresh-air pre-heating in double facade (Klosterenga, Oslo, Norway) 1. For L=20 cm: for v=0.1 m/sec, the collector air temperature will rise to about 15ºC (rise of 20ºC) when the blind is closed with incident solar radiation of 600 watts/m2. Solar Collector 2. For L=30 cm: for v=0.2 m/sec (L=30 cm), the collector air temperature will rise to about 5ºC (rise of 10ºC) when the blind is closed with incident solar radiation of 600 watts/m2. System 4: Closed-collection loop with radiant discharge storage In this system, an air collector is connected to the building’s integrated heat storage. The air is circulated in a closed loop, normally with the aid of fan-forced convection, through the collector to the storage and back to the collector. The room-facing surface of the storage discharges heat by radiation and convection to the room space. The collector system can be used as part of the building envelope, with lower extra costs. 24 Solar Collector Radiant Heat Closed Loop Charge Mass System 4 Figure 25 – System 4, with storage Canada Mortgage and Housing Corporation
  25. 25. Solar Energ y for Buildings System 5: Closed-collection loop with open-discharge loop This system provides comfort, even in rooms with high internal and solar gains and small losses, because it allows controlled discharge of stored solar energy to the heated room. This increases the solar system’s efficiency and reduces the risk of overheating. It can use existing building components and can be combined easily with existing HVAC systems. It is more expensive than other systems. Solar Collector Solar Collector Radiant Heat System 6: Closed-collection loop with heat exchange to water The closed-loop solar-air system has advantages over liquid systems, as there is no risk of leaking, boiling or freezing. It might also be chosen for its economy or for architectural reasons. Solar-air heated water can provide space heating, domestic hot water heating or be used for industrial applications. Apart from the collector, the system consists of standard HVAC components. This system can be used for heating hot water during the summer. It requires that the air temperature in the system be hotter than ventilation pre-heat systems. It is usually bulkier than liquid systems. System design Open Loop Discharge Mass System 5 Figure 26 – System 5, with storage Air to Water Heat Exchanger Solar Preheated Water Solar Collector For more technical details, see pp 103-104 of Solar Air Systems: A Design Handbook s Step 1-Profile the loads s Step 2-Select collector type s Step 3-Decide on air mass flows s Step 4-Specify the heat exchanger s Step 5-Size the storage and determine heat loss System 6 Figure 27 – System 6, with hot-water storage Canada Mortgage and Housing Corporation 25
  26. 26. 26 Canada Mortgage and Housing Corporation 6 5 4 3 2 1 System Window collectors may overheat adjacent rooms; rocked storage bulky Furniture can't be placed against walls Increased installation costs because of facade shell Bulkier than liquid systems Risk of freezing in heat exchanger Overall efficiency reduced due to temperature drop over heat exchanger Existing building components used Can combine with heat and ventilation system No problem with freezing, boiling and leaking in collector Standard ventilation equipment can be used Can be used to heat hot water in summer lowers extra costs for the solar system Using collector as part of building envelope retrofits Relatively more expensive May require separate ventilation Simple, can be used with or without storage High degree of integration possible, even in Decreased performance of heat recording unit Collector materials must be non-toxic Disadvantages Simple, inexpensive Low temperature air usable All collectors usable System uses standard components Advantages Table 5 – Comparison of six solar-air heating systems $250–$650/m2 $90–$475/m2 $200/m2 Solarwall® $194/m2 Cost 175–375 kWh/m2 80–240 kWh/m2 (Heating season) Payback time depends on integration Payback 25 years 600–800 kWh/m2 System performance 300–400 kWh/m2 (sunny–cold) 120–130 kWh/m2 (cloudy–temperate) 30–150 kWh/m2 (sunny–cold) 10–100 kWh/m2 (cloudy–temperate) 100–425 kWh/m2 (sunny–cold) 50–200 kWh/m2 (cloudy–temperate) 150–400 kWh/m2 (sunny–cold) 100–225 kWh/m2 (cloudy–temperate) 80–200 kWh/m2 (sunny–cold) 40–75 kWh/m2 (cloudy–temperate) 110–550 kWh/m2 (sunny–cold) 90–300 kWh/m2 (cloudy–temperate) Saved energy Solar Energ y for Buildings
  27. 27. Solar Energ y for Buildings Photovoltaic (PV) systems The photovoltaic effect converts solar energy directly into electricity. When sunlight strikes a photovoltaic cell, electrons in a semiconductor material are freed from their atomic orbits and flow in a single direction. This creates direct current electricity, which can be used immediately, converted to alternating current or stored in a battery. Whenever sunlight arrives at its surface, the cell generates electricity. PV cells normally have a lifespan of at least 20-25 years; however, they usually last longer if frequent overheating—temperatures in excess of 70ºC (158ºF) is prevented. PV systems can be used as a building's sole electricity supply or with other sources, such as a generator or a grid connection. Autonomous PV systems include an array of PV cells and a power conditioner that connects to the building's electrical loads. To have electricity when there is no sun, this system must have storage batteries. Battery storage must be sized to the anticipated load and solar access. A weakness of the system is that the supply of solar energy may be intermittent. Hybrid PV systems have at least one additional electricity source, such as a fuelfired generator or a wind turbine. These systems can still be off the utility grid and can minimize or eliminate the problem of intermittent solar energy. The cost of PV technology is now much more expensive than traditional electricity and has a very long payback period. In 2000, Natural Resources Canada assessed the breakeven point for PV products in Canada using market data from the past 25 years. Based on annual growth rates of 20 per cent (growth has been closer to 30 per cent for the past six years), the break-even point for competing with bulk electricity generation was calculated to be between 2020 and 2030.21 This was based on lowest production cost but does not consider technological advancements or the advantage of reduced greenhouse gas production. Figure 28 – PV as window shading elements (overhangs) at Queen’s University, Kingston Source Kawneer While autonomous systems can be immediately cost-effective in remote locations, they are not likely to be costbeneficial for MURBs. Grid-connected PV systems cancel out the need for onsite generators and batteries and eliminate the problem of intermittent solar energy. In many jurisdictions it is possible to supply excess solar-generated electricity to the grid and receive credit from the power company. Building-Integrated PV systems (BIPVs) A more recent trend is the development of Building—Integrated Photovoltaics (BIPVs). The PV cells are incorporated into a building element. Currently there is much development in PV roofing, PV shading elements (see figure 28) and PV cladding or semi-transparent curtain wall components (see figure 29). With PV cladding it is best to have a vented cavity behind the panels so as to operate at lower temperatures. By following such a construction approach one may also develop an effective rainscreen system which hinders rain penetration. PV roofing is installed much the same way as 21 Ayoub, J., Dignard-Bailey, L. and Filion, A., Photovoltaics for Buildings: Opportunities for Canada: A Discussion Paper, Report # CEDRL-2000-72 (TR), CANMET Energy Diversification Research Laboratory, Natural Resources Canada, Varennes, Que., November 2000. Canada Mortgage and Housing Corporation 27
  28. 28. Solar Energ y for Buildings conventional roofing and is available in shingles, tiles and metal standing-seam roofing (see Figure 30). PV shading can be effective as a window shading element, entrance canopy or walkway shading. PV panels can be opaque, used where no light transmission is needed, or semi-transparent for areas where light is wanted, such as atriums or skylights, but some shading is needed to reduce cooling loads. Figure 29 – PV integrated in curtain wall elements at the Mataró Librar y, Mataró, Catalonia, Spain. (The facade is also used for fresh air pre-heating). Sol Source Engineering Figure 30 – BIPV metal-standing seam roof, Toronto 28 Canada Mortgage and Housing Corporation
  29. 29. Solar Energ y for Buildings Until the use of BIPVs becomes more widespread, there are barriers to overcome. Canadian utilities are often not familiar with small, decentralized energy production. Consequently, utility interconnection for BIPVs is a major barrier to their use. Another barrier is the absence of technical standards and installation codes. Non-technical barriers include the lack of experience among builders and electrical inspectors; lack of financing for systems with large capital costs; additional permit, insurance and inspection fees for net-metering systems; unawareness of potential and long-term benefits to system integrators.22 Photovoltaic hybrid heating system (PV-thermal system) A typical crystalline silicon PV panel has an efficiency of 10–15 per cent. PV solar panels produce more than four times as much heat as electricity. This heat is normally lost to the environment. A PV cell can have a stagnation temperature of 50°C (122ºF) above the ambient if the heat is not removed. The cooler the PV cells, the higher the efficiency. A solar air collector has a typical thermal efficiency of 40–70 per cent. Drawing outside air in across the back of panels pre-heats the HVAC supply air and also increases the PV efficiency by keeping them cooler. Combining these two systems produces both heat and electricity. (This is equivalent to a co-generation power plant.) Test results show that using PV panels to generate both electricity and useful heat substantially improves the overall efficiency (electrical plus thermal). The payback for a 22 Simplified Analysis Incident solar radiation= reflected+ electricity+ heat transfer to air+ heat lost to exterior 1,000 W= 100+ 100+ 400+ 400 grid-connected PV electrical system was decades. By 2004, the cost of panels had fallen to $4.5 per kW-peak (one kW-peak is electricity generated with 1,000 W/m2 incident solar radiation). Annual electrical production is generally in the 70-200 kWh/m2 range, depending on climate. of thermal power. This gives the following energy balance on the PV panels: Consider a simplified analysis of a PVthermal system—a PV panel with airflow behind it (see figure 10, page 12). Assume that 1,000 W/m2 of solar radiation is incident on the solar panel, which converts 10 per cent to electricity to produce 100 watts of electricity for one square metre of panel (a panel costs about $450 at mid-2004 prices). About 5–10 per cent of incident solar radiation is reflected but the rest becomes heat. By bringing in fresh air through an inlet at the bottom and passing it behind the panels the air is heated the same way as in a Solarwall® system. The faster the airflow, the more heat is transferred to the flowing air and less is lost to the outside air. Optimal cavity width and air velocity are selected by taking into account fan energy, required outlet temperature and fresh air requirements. The PV can extend over multiple stories, with multiple inlets. In any case, if the inside heat transfer coefficient hi is equal to the exterior film coefficient ho (about 12 W/m2 for still air), then the flowing air may capture about 400 watts/m2 This shows that four times more thermal energy is generated than electricity. The electrical efficiency is 10 per cent, while the thermal efficiency is 40 per cent. This gives an overall efficiency of 50 per cent. If thermal energy is worth half as much as electricity, then this system generates about three times the revenue of a simple PV system on the facade. This simplified analysis shows why PV–thermal applications are the key to early, cost-effective use of PV. Integration into MURBs For MURBs, facades have the highest potential for cost-effective BIPVs. In facades, they can easily generate thermal energy. Semi-transparent panels can also provide daylighting. There are two main options for using the hot air. Like the Solarwall® system, the PV system can be applied in vertical strips with a fan drawing the air into the HVAC system. An alternative is installation into box-type, airflow windows. If they project from the facade, they need a separate support structure, adding to installation costs. The applications may range from small overhangs to large continuous facade areas. Figure 30 shows a double facade with PV o overhangs, in Freiburg, latitude 48 N. Ayoub, J., Dignard-Bailey, L. and Filion, A., IBID. Canada Mortgage and Housing Corporation 29
  30. 30. Solar Energ y for Buildings Table 6 – Description of collector types Collector type Advantages Disadvantages Capital cost $/kW Efficiency Single crystal High efficiency High cost, fragile, uniform look 5,000–10,000 11–15% Polycrystalline High efficiency High cost, fragile, non-uniform look 5,000–10,000 10–14% Thin-film amorphous Flexible, can be applied to different types of surfaces Low efficiency, degrades Spheral solar (crystalline family) Low cost, flexible, can be applied to different types of surfaces. Low efficiency 5–8% 4,500 9–10% Table 7 – BIPV manufacturers BIPV product Manufacturer–country Sloped roof Atlantis Solar Systeme AG, Switzerland Ecofys,The Netherlands BMC Solar Industrie GmbH, Germany BP Solar, United Kingdom Canon Inc., Japan Lafarge Brass GmbH, Germany MSK Corp., Japan United Solar Corp, U.S.A. Facades Atlantis Solar Systeme AG, Switzerland Pilkington Solar Inter., Germany Isophoton Inc., Spain Saint-Gobain Glass Solar, Germany Sanyo Solar Engineering Ltd., Japan Schuco Int. KG, United Kingdom Shading Ecofys, Netherlands Colt Solar Technology AG, Switzerland Kawneer, U.S.A. Flat roof Powerguard, U.S.A. Figure 31 – Double facade with PV overhangs in Freiburg, Germany 30 Canada Mortgage and Housing Corporation
  31. 31. Solar Energ y for Buildings Summary Passive solar is best for buildings that have low internal heat gains and in which direct solar gain is directed to absorbent thermal mass. The housing market today may object to hard floor surfaces out of concern for comfort and impact noise, but increased drywall thickness and concrete ceilings may compensate for the lack of hard flooring. Mass is most effective if it receives direct solar gains, i.e. usually on the floor. However, if this is not possible, a concrete ceiling will absorb much of the energy from air heated by the floor; this air will rise through buoyancy. Generally, about 5-10 cm of concrete—or equivalent—on the floor provides adequate mass. s s The cost of passive solar is minimal, but must be planned during the initial design stages. Orientation in most MURBs provides challenges. An effective strategy is to tune the selection of glazing based on the orientation of each facade. s Solar domestic hot water can have a reasonable payback time and is relatively easy to install in new buildings and retrofits. s Solar water heating is less likely to be effective for space heating, except in very large heating systems. s Solar water heating for swimming pools is very effective for seasonally used pools, with short payback times. s Solar air-heating systems for preheating ventilation air can have very short—even immediate—payback. Their drawbacks are the need for prime southern exposure and their industrial aesthetic. Such considerations need to be part of the architectural design of MURB installations. s Presently, photovoltaics are an expensive way to provide electricity and are more cost-effective combined with heat recovery as well. However, the cost of building-integrated PV (BIPV) systems is coming down as competition and market share increase. Tools and resources Canada Mor tgage and Housing Corporation www.cmhc.ca key word: Innovative Buildings NRCan and CMHC’s Advanced Buildings Technologies program is being phased out in 2007. However the EE4 software and CBIP goals are part of the LEED prerequisites. Renewable Energy Deployment Initiative (REDI) REDI provides an incentive of 25 per cent of the installed cost of renewable energy systems for space and water heating and cooling. Eligible systems include: s active solar hot water systems s active solar air heating systems s highly efficient, low-emitting biomass combustion systems. www.advancedbuildings.org PV systems are not eligible but receive accelerated depreciation under Class 43 of the Income Tax Act. C2000 Commercial Building Program Natural Resources Canada RETScreen This is a Natural Resources Canada demonstration program for design assistance in energy-efficient commercial buildings. The goal is 50 per cent less energy consumption than a building constructed to the Model National Energy Code of Canada for Buildings (MNECB). RETScreen is free energy-assessment software that assesses renewable energy options against a base model. RETScreen has specific modules for passive solar, solarair heating and solar domestic hot water heat. Output includes financial analysis, payback periods and energy displaced. Modules are available at http://www.retscreen.net/ang/menu.php Commercial Building Incentive Program (CBIP) CBIP is probably the largest federal government initiative to reduce energy use in commercial buildings. Building owners are given a financial incentive of three times the annual energy savings if the predicted building energy use is 25 per cent below that required by the MNECB. Computer simulations are done with NRCan's EE4 software. This program has no provision for analyzing PV systems. Energy from PV is a credit towards meeting the energy target and contributes to the eligibility of the incentive. The Canada Mortgage and Housing Corporation 31
  32. 32. 32 Canada Mortgage and Housing Corporation Atrium pre-heats ventilation air, solar DHW, heat recovery ventilation, SOcLAR shading of some units Thermally isolated balconies; meeting room–greenhouse; external shading Passive solar heating; large thermal mass; 21 m2 solar panels for DHW 336 m2 Solarwall® pre-heats ventilation air 228 m2 solar DHW pre-heat system 47.5 m2 flat-plate solar collectors heat DHW with surplus going to radiant slab; 2 PV panels for pumps; no fossil fuels consumed on site Combined heat and power units (CHP)—wastewood combustion delivers electricity and space heat via centrally located domestic hot water cylinders; PV array to recharge cars Greenhouse in common area; space heating from rooftop solar collectors; solar balconies 218 m2 rooftop solar collectors for radiant slab and DHW pre-heating; passive solar double glazed south facade to pre-heat ventilation air; summer shading 4-storey apartments, new construction 10 units, 4-storey MURB, 1997 new construction 400 units, 24 storeys 232 units, 10 storeys 2-storey inn (9 suites plus common areas) Carbon-neutral residential development Social housing project, 10,000 residents; “green retrofit” of 1970 s building 35-unit 6-storey green apartment demonstration Conservation Co-op, Ottawa Uster Condominiums, Switzerland Ouellette Manor, Windsor Quinpool Towers, Halifax Chantrelle Inn, North River, N.S. BedZED, U.K. Gardsten, Gothenberg, Sweden Klosterenga, Norway Technologies used, solar features 42-unit MURB new construction Building description Amstelveen, Amsterdam,The Netherlands Project–location Case studies — visit CMHC website for other examples 11,589 euros/m2 $36,700 ($772/m2) after incentive $93,000 total system ($408/m2) before incentive $90/m2 incremental cost 200 USD/sq. ft. construction cost Incremental cost $733 per balcony System cost $645/ m2 $152, 386 Cdn per unit Project cost Part of IEA solar program International Energy Agency 60% decrease in total unit energy demand; 90% decrease in total heat demand 46 GJ (12,800 kWh) produced each year—50% of DHW and 25% of space heat requirements; savings of $2,660/year—10.5-year payback (with incentive) 630 GJ/yr solar delivered; 6-year payback (with incentives) Collector delivers 584 kWh/ m2; 6-year payback; 2,000–4,000 Cdn savings in energy/yr Total energy consumption 112 kWh/ m2-35% of typical MURB 5,320 GJ/ m2 ; energy savings small but other benefits Energy use: 13 kWh/m2 for space heating, 15 kWh/ m2 for DHW, Comments Solar Energ y for Buildings
  33. 33. Solar Energ y for Buildings References Tap the Sun: Passive Solar Techniques and Home Designs (Canada: CMHC, 1998) Canada Mortgage and Housing Corporation (CMHC) Sheltair Group, Healthy High-Rise: A Guide to Innovation in the Design and Construction of High-Rise Residential Buildings, (Canada: CMHC, 1996) Ayoub, J., Dignard-Bailey, L. and Filion, A., Photovoltaics for Buildings: Opportunities for Canada: A Discussion Paper, Report # CEDRL-2000-72 (TR), CANMET Energy Diversification Research Laboratory, Natural Resources Canada, Varennes, Que., November 2000, pp. 56 (plus appendices). Solar Air System: A Design Handbook Editors S. Robert Hastings, Ove Morck, James & James 2000 Edward Mazria, The Passive Solar Energy Book, Innovative Building Case Studies, CMHC website. Canada Mortgage and Housing Corporation 33
  34. 34. Solar Energ y for Buildings Questions 1. Name three benefits of using solar energy. 2. Why is passive solar a good choice for new construction MURBs? 3. What four window technologies can improve a window’s performance as a solar collector? 34 4. What is the difference between active and passive solar energy systems? 5. What are the main elements of a hybrid grid-connected photovoltaic system ? 6. What are the main elements of a typical solar domestic hot water system? Canada Mortgage and Housing Corporation 7. Name three types of active solar collectors. 8. What are three pros and three cons of a Solarwall® type of ventilation air preheat system? 9. Describe two ways of integrating PV in MURBs and improving their cost-effectiveness