Passive Solar Design

New construction offers the greatest opportunity for incorporating passive solar design features, as demonstrated by the design of the Solar Energy Research Facility in Golden, Colorado. For retrofit projects, consider daylighting strategies, heat control techniques, and using passive solar heating strategies to allow modification of HVAC systems.

Passive solar systems make use of natural energy flows as the primary means of harvesting solar energy. Passive solar systems can provide space heating, cooling load avoidance, natural ventilation, water heating, and daylighting. This section focuses on passive solar heating, but the other strategies also need to be integrated and coordinated into a whole-building design. Passive solar design is an approach that integrates building components—exterior walls, windows, and building materials—to provide solar collection, heat storage, and heat distribution. Passive solar heating systems are typically categorized as sun-tempered, direct-gain, sunspaces, and thermal storage walls (Trombe walls).

New construction offers the greatest opportunity for incorporating passive solar design, but any renovation or addition to a building envelope also offers opportunities for integration of passive methods. It is important to include passive solar as early as possible in the site planning and design process, or when the addition or building is first conceived. Ideally, an energy budget is included in the building design specifications, and the RFPs require the design team to demonstrate their commitment to whole-building performance and their ability to respond to the energy targets. This commitment is emphasized during programming and throughout the design and construction process.

For retrofit projects, consider:

• Daylighting strategies, such as making atria out of courtyards or adding clerestories, along with modification of the electric lighting system to ensure energy savings

• Heat control techniques, such as adding exterior shades or overhangs

• Using passive solar heating strategies to allow modification of HVAC systems-perhaps down-sizing if the passive strategies reduce energy loads sufficiently.

When renovating older buildings, determine whether passive features that have been disabled can be revitalized.

Sun tempering is simply using windows with a size and orientation to admit a moderate amount of solar heat in winter without special measures for heat storage. Direct gain has more south-facing glass in occupied spaces and thermal mass to smooth out temperature fluctuations. A Trombe wall puts the thermal mass (e.g., tile floors) directly behind the glazing to reduce glare and overheating in the occupied space.

A sunspace keeps the glass and mass separate from the occupied space but allows for the transfer of useful heat into the building by convection or a common mass wall; temperatures in a sunspace are allowed to fluctuate around the comfort range.

Highlight passive solar as a project goal. A good general project goal is "to produce a beautiful, sustainable, cost-effective building that meets its needs, enhances productivity, and consumes as little nonrenewable energy as possible, through the use of passive solar design, energy efficiency, and the use of other renewable resources."

Establishing energy performance goals conveys the seriousness of energy consumption and the use of passive solar as a design issue. For small offices, warehouses, and other smaller projects—10,000 sq ft (930 m2) or less—facility managers or their contractors can develop energy budgets easily using software such as Energy-10. For larger multi-zone projects (for example, laboratories or high-rise office buildings), national average energy consumption data by building type can be cited as targets to be exceeded, or more complex analyses can be run by consultants.

The building design should allow passive solar strategies to be effective (for example, large multistory core zones are hard to reach with passive solar). Building requirements, such as privacy and security, may influence the type of passive solar heating system that can be used.

Thirty to fifty percent energy cost reductions below national averages are economically realistic in new office design if an optimum mix of energy conservation and passive solar design strategies is applied to the building design. Annual savings of $0.45 to $0.75 per sq. ft ($5 to $8/m2) is a reasonable estimate of achievable cost savings.

Passive solar design considers the synergy of different building components and systems.
For example:

• Can natural daylighting reduce the need for electric light?
• If less electric light generates less heat, will there be a lower cooling load?
• If the cooling load is lower, can the fans be smaller?
• Will natural ventilation allow fans and other cooling equipment to be turned off at times?

Passive solar design is often more challenging than designing a mechanical system to accomplish the same functions. Using the building components to regulate temperature takes a rigorous analytical approach to optimize performance while avoiding such problems as overheating and glare.

Generic design solutions or rules of thumb are of limited value. Rules of thumb may be useful in anticipating system size and type, but only early in the design process. Computer simulation provides much more accurate guidance because of the complexity of system combinations and interactions. Some of the variables involved include:

• Climate (sun, wind, air temperature, and humidity)
• Building orientation (glazing and room layout)
• Building use type (occupancy schedules and use profiles)
• Lighting and daylighting (electric and natural light sources)
• Building envelope (geometry, insulation, fenestration, air leakage, ventilation, shading, thermal mass, color)
• Internal heat gains (from lighting, office equipment, machinery, and people)
• HVAC (plant, systems, and controls)
• Energy costs (fuel source, demand charges, conversion efficiency).

An hourly simulation analysis combines all of these parameters to evaluate a single figure-of-merit, such as annual energy use or annual operating cost. The integrated interaction of many energy-efficient strategies is considered in passive solar design. These include: passive solar heating, glazing, thermal mass, insulation, shading, daylighting, energy-efficient lighting, lighting controls, air-leakage control, natural ventilation, and mechanical system options such as economizer cycle, exhaust air heat recovery, high-efficiency HVAC, HVAC controls, and evaporative cooling.

Cost and technical analyses are conducted at the same time in passive solar design to optimize investments for maximum energy cost savings. It is rarely feasible to meet 100% of the building load with passive solar, so an optimum design is based on minimizing life-cycle cost: the sum of solar system first-cost and life-cycle operating costs. It is difficult to separate the cost of many passive solar systems and components from other building costs because passive solar features serve other building functions-e.g., as windows and wall systems.