Passive solar design features

Passive solar home in Minden, NV. Source: Donald Aitken via NREL

Shading for view window and LightLouver for daylight window at NREL's Research Support Facility. Source: NREL

Interior of NREL's Research Support Facility. Credit: Dennis Schroeder / NREL

The concept of passive solar design is to minimize heating and cooling loads through architectural design features, and reducing (or perhaps even eliminating) the need for mechanical heating and cooling. Passive solar design ideally finds a balance between 1) beneficial solar heat gain to offset heating energy, 2) unwanted solar heat gain that causes an increase in cooling energy, and 3) useful daylight that reduces the need for electric lighting. Due to these complex interactions, BEM is an essential tool for evaluating and optimizing passive solar design concepts.

Impact of Passive Solar Design

A primary goal of most passive solar design is to affect both the magnitude and timing of space thermal loads. A common application is a design with windows that admit solar heat gain during cool days into spaces that contain adequate thermal mass to absorb a portion of that heat gain. With the right balance, the need for space cooling will be avoided during the daytime, and evening heating demand will be reduced by the heat released from the thermal mass.

Through its influence on thermal loads, passive solar design can have a number of impacts:

• Space air temperature and thermal comfort: increases the number of hours that thermal comfort can be maintained without active heating or cooling.
• Resilience and passive survivability: helps keep buildings suitable for occupancy when heating or cooling is not available.
• Energy consumption for heating and cooling: lowers heating and cooling loads and associated energy consumption, utility cost, and carbon emissions; may provide beneficial load shifting, changing time-of-day of energy consumption.
• HVAC peak loads and system sizing: reduces the peak heating and cooling loads, allowing for smaller system sizes to meet the space conditioning requirements of the building.
• Electric lighting energy: provides beneficial daylight and associated electric lighting energy savings and reduced cooling load, with attention to passive design for daylight distribution and glare control.

Embodied carbon will also be affected by passive solar design decisions, as is true of most building fabric and system decisions.

Passive Solar Design Alternatives

The passive solar designer has a number of strategies to choose from that affect solar heat gain and space thermal loads.

Building massing and space programming may be the most impactful design choice. The potential impact of passive solar design depends on the fraction of floor area comprising perimeter space and the type of occupancy in those perimeter spaces. See Compare Massing Options.

Orientation of facades is another fundamental factor in passive solar design. For example, surfaces facing the equator (south-facing in the northern hemisphere) can receive beneficial solar heat gain in winter and can be shaded in summer. See Analyze the Building's Orientation.

Fenestration design is key in finding the right balance between beneficial and unwanted solar heat gain. Factors include window area, window orientation, exterior shading and interior shading and glazing type. Glazing is available with a wide range of performance, especially in terms of solar heat gain coefficient, and BEM can be used to evaluate the tradeoff between windows that admit solar heat and those that reject solar heat. Glazing type can also be varied by orientation. See Fenestration and daylighting options - conceptual design.

Thermal mass plays a role in most passive solar designs and can be useful in a few different ways:

1. A heavy mass surface, such as a concrete floor, can be located near a window to receive direct solar radiation, storing heat and releasing it in the space over time.
2. Thermal mass surfaces surrounding a space or objects within a space will absorb heat in a space and delay a rise in air temperature.
3. Thermal mass within exterior envelope constructions will delay and dampen heat transfer

See: Constructions and thermal mass options - conceptual design.

Surface thermal properties, the solar reflectance and thermal emittance, for both exterior and interior surfaces affect performance. Exterior surfaces with high solar reflectance and high thermal emittance reduce heat gain and are a time-tested strategy to keep buildings cool in warm climates. Low solar reflectance (high solar absortance) is sometimes used in passive solar design for interior surfaces exposed to direct sun when the goal is to capture as much heat as possible.

Landscaping and outdoor surfaces can be employed to control solar heat gain. Deciduous trees provide shade in the summer and admit sun in the winter. The solar reflectance of ground cover or paving near buildings can also have an impact on building thermal loads.

Natural ventilation is often a good complement to passive solar design, which can reduce cooling loads and increase the number of hours when passive cooling via operable windows or other openings can provide thermal comfort. In some cases, with the appropriate climate, passive solar design can eliminate the need for active cooling. Natural ventilation can also complement passive solar design by providing passive ventilation for indoor air quality and reducing the need for active mechanical ventilation.

Other strategies can be a good fit for passive solar design:

• Dynamic glazing (also known as switchable glazing or smart glass) can be controlled to admit solar gain when the heat is needed and can reduce solar heat gain when it is unwanted.  
• Super-insulating windows use multiple layers with low-e coatings to achieve a high R-value.
• Trombe walls are traditional passive solar design elements that place a dark mass wall behind a layer of glass so that it collects and stores heat during the day and employs passive air circulation to deliver heat to an adjacent space in the evening.

Guidance on Modeling Approach

For an early-design study of passive solar design options, both simple box models and shoebox models may be useful. The shoebox model of a single space may be quicker to develop and modify when comparing alternative strategies, while a simple box model is useful for quantifying and analyzing whole-building impacts.

There are two basic approaches to take in a BEM study of passive solar design alternatives, and one or the other or both may be appropriate depending on project goals.

• With HVAC: estimate energy consumption, along with other outcomes such as peak heating and cooling loads. In this case, the BEM software will calculate the amount of heat that needs to be removed from or added to each space to keep the air temperature at a given setpoint and then estimate energy consumption by the HVAC system.
• Without HVAC: evaluate space conditions that occur for a space without active heating and cooling to understand the impact on thermal comfort. The software calculates the zone air temperature that occurs with no heat added or removed by an HVAC system.

In either approach, here are some important considerations:

• Use the material layer approach to defining constructions in order to represent the impact of thermal mass in the building envelope.
• Check the interior thermal mass inputs, which are often default inputs in BEM software, to make sure they are reasonable for your project. Read the software documentation to understand how thermal mass is modeled in your simulation software.
• For passive solar strategies that use direct solar gain on interior surfaces for heat storage, such as a dark floor near a window, review documentation for your simulation tool regarding how direct solar heat gain through windows is distributed to interior surfaces. The modeling approach varies among simulation tools, and some tools may offer more than one modeling option.
• Use realistic assumptions for interior heat gains

There are some additional considerations for the “without HVAC” analysis approach:

• Pay attention to model inputs for infiltration and natural ventilation because they can have a big impact on space temperature results. Understand how natural ventilation control, i.e. logic for opening and closing of windows, is implemented in your simulation program.
• Review hourly zone temperature outputs.
• Consider running simulations with future year weather data or extreme weather years.

Guidance on Presenting Results

The following are examples of visual presentation of results that can be used to illustrate the impact of passive solar design strategies.

Example heat-map chart showing simulated indoor air temperature in a naturally ventilated building. The same type of chart can be used to illustrate other results, such as heat index for indoor spaces, which is a function of both air drybulb temperature and relative humidity

Example heat map of indoor temperature and histogram illustrating thermal autonomy, which is the fraction of time a building can passively maintain comfort conditions without active systems. Source: https://coolshadow.com/wp-content/uploads/2023/04/LU_Thermal-Autonomy-as-Metric-and-Design-Process.pdf

Resources

Passive Solar Technology Basics.

Passive Solar Home Design.