D.6.5 Passive and Active Solar Systems Passive and active solar systems provide sustainable methods of heating, cooling, and powering the buildings at Fort Carson. Based on a careful analysis of the loads and the energy needs across the seasons, these systems can be used to supplant, or at least augment, off-site energy sources.
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wall cross section with daylighting aperture |
A Trombe wall, or thermal storage wall, is a mass wall, with an airspace and an exterior glazing surface. The wall gathers solar heat through the glazing and a black or selective coating absorbs the heat. The heat moves slowly through the wall to help heat the interior several hours later. In Fort Carson, the Trombe wall should be adequately shaded from summer sun. Trombe walls are appropriate for providing supplemental heat and for building spaces with low levels of thermal control such as warehouses, loading docks, or storage areas.
D.6.5.3 Passive Cooling
The first step in passive cooling is to minimize the cooling load by providing effective external window shading and not oversizing the windows. Glazing selection is also important in reducing the solar loads on the building. In addition, turn off or dim electric lighting systems to take full advantage of the daylighting entering the building while at the same time reducing cooling loads. Finally, minimizing plug and equipment loads will also help the cooling loads (see Appendix D, section 7). Movable awnings, roll-down shades, or shutters can also shade building surfaces.
Landscaping can help reduce cooling loads. However, the building design should not rely on landscaping for shade because it takes years for new landscaping to mature, and then mature landscaping may die or be damaged. Low-fire-hazard landscape materials that shade building surfaces can help reduce the solar impact on the building envelope. Plantings may also be beneficial in blocking winter wind or channeling summer breezes. Section 10 contains more information on climate-sensitive landscaping.
D.6.5.4 Natural Ventilation
Fort Carson has many days when favorable outside temperatures can help condition the building. Take advantage of these conditions and use natural ventilation to reduce mechanical cooling loads. Natural ventilation systems work well whenever a traditional economizer cycle is a good design decision.
The building architecture will impact the success of a natural ventilation design. Operable windows located high in spaces can release hot air. Operable clerestory windows can easily provide all the ventilation requirements of a space, especially in high-bay areas. Tall ceilings produce a “stack effect” by inducing air movement as the warm air is drawn out through the high windows. Carefully coordinate the automatic control of these high, operable windows with the mechanical system design. Turn off the mechanical system when windows are open.
Under certain conditions, natural ventilation can be augmented with air movement from ceiling fans or outdoor breezes.
D.6.5.5 Evaporative Cooling
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cool moist air generated by air flowing through a wet medium at the top of the cooltower "drops" through large openings at the tower base to cool the space. |
Evaporative cooling is an adiabatic process in which warm dry air takes on moisture, lowering its temperature in the process (direct evaporative cooling). Indirect evaporative cooling can lower the air temperature without adding moisture to the building air by using a heat exchanger between the evaporatively cooled air (which is then purged to the outside) and the building supply air. A combined indirect/direct evaporative cooler extends the design conditions under which evaporative cooling can sufficiently meet space conditioning requirements.
Most evaporative cooling systems are “active” in the sense that blowers are used (see Appendix D, section 7). A passive alternative is the “cooltower” approach. This strategy involves integrating with the building architecture a tower with wetted surfaces exposed to the air. As the air hits the wet pads, it takes on moisture and cools. Because this moist air is denser than the surrounding air, it falls down the tower and into the building and generates a self-perpetuating air current, a form of natural ventilation.
Check the water supply properties before using evaporative cooling.
D.6.5.6 Passive Solar Heating Strategies
The climate at Fort Carson provides good opportunities for passive solar heating. The basic passive solar heating systems are direct gain systems (sunlight entering a window), indirect gain systems (sunspaces or atria), and thermal storage walls (Trombe walls). In warehouses or storage areas with only periodic occupancy, passive solar heating may be sufficient for all the space heating needs.
Glare can often be problematic in direct passive solar heating designs. Areas such as break rooms, hallways, and entries can tolerate direct solar gains for supple-mental heating because glare is not a big issue in these spaces.
Use computer simulations to evaluate the effect of more glazing on the annual energy loads before increasing the amount of glazing on the building for more winter solar gains. Some spaces having high internal loads may not need additional heating, even in the winter. In these spaces, size the glazing to only provide the desired amount of daylight. Overhangs must be properly sized to avoid overheating of the space during the summer. If incorporating passive solar heating strategies, then select glazings having a high SHGC to maximize the passive solar potential.
D.6.5.7 Active Solar Heating Systems
Typical applications of active solar heating include domestic hot water heating, space heating (air or water), and ventilation air preheating. Of these, ventilation air preheat tends to be the most economical.
First, determine that the building use is appropriate for an active solar application. Determine the feasibility of using active solar systems in conjunction with the over-all mechanical system design (see Appendix D, section 7).
Solar hot water system collectors can be either mounted directly on the building or rack-mounted near the building. Installation is usually simpler and the collectors are more attractive if they are integrated into a roof surface or installed flush with it. Even if solar collectors are not part of the initial design, it may make sense to design roof surfaces with future solar installations in mind.
A general rule of thumb is that the vertical tilt angle of the south-facing collector should equal the latitude angle (38.8°N for Fort Carson) for year-round use such as domestic hot water heating. A solar space heating system would benefit from a steeper tilt angle (about 50°) to maximize solar gain in the winter, when the sun is lower in the sky. The collectors should have an unobstructed view of the sun path from at least 9:00 a.m. to 3:00 p.m. throughout the year. Beware of light reflecting off the glass-covered collectors as it can create uncomfortable glare in nearby buildings.
D.6.5.8 Transpired Solar Collector
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A transpired collector is a simple, efficient method of heating ventilation air. The system consists of a dark, south-facing, perforated, metal surface that also acts as the building’s exterior protective skin. The sun heats the dark perforated wall. A centrally located fan at the top of the wall draws air through the perforations. The air is heated as it travels up behind the heated wall. The fan then distributes the warm air into the interior space.
Transpired solar collectors are particularly effective in areas requiring large amounts of heated ventilation air during the daytime hours, such as warehouses or garages. On the other hand, spaces with large heated ventilation air requirements on a 24-hours-per-day basis may be better served by a system that transfers heat between the exhaust and ventilation air (see Appendix D, section 7).
D.6.5.9 Solar-Electric Systems
Solar electric systems (also known as PV systems) use the direct conversion of sunlight to direct current (DC) electricity. The four major types of PV cells in order of highest to lowest performance and cost are single crystal, polycrystalline, thin film, and amorphous silicon cells. The cells have a long life and are almost maintenance free. The cells are assembled into modules and the modules are connected into arrays. The type of cell connection determines the voltage and current of the array.
The power generated by a PV array is instantaneous direct current while the sun shines. Most systems include alternating current (AC) inverters and some include batteries for storage.
PV systems may be stand-alone or utility-grid-integrated. A stand-alone system is applicable to remote locations that are at least one-quarter mile from utility connections. For stand-alone systems, a fuel generator and/or batteries may be used to provide electricity during periods of insufficient solar radiation. A grid-integrated system supplements utility power. In buildings having an uninterruptible power supply (UPS) system, the PV system can charge the UPS battery bank and supply supplemental power to the building.
Like active solar collectors, PV arrays may be mounted directly on the building or on nearby racks. Building-integrated PV modules are available as roofing tiles, shingles, standing seam metal roofing, spandrel panels, or as partially transparent shading elements. The building site might incorporate PV arrays as shading devices for parking areas, pedestrian walkways, or outdoor gathering spaces.
PV systems are still quite expensive when grid power is available, but improvements in efficiencies, manufacturing, and storage systems promise to reduce the total system cost of future PV installations. It may be desirable to plan building surfaces with proper solar access and wiring access points for a future PV system. PV systems sometimes make economic sense when very high-quality, uninterruptible power is needed.