Passive solar heating

Traditionally, passive solar heating is not beneficial for commercial buildings because they have abundant internal heat gains from large numbers of people, lights, and equipment, and, except for morning warmup, require cooling much of the year. However, this convention breaks down for daylit buildings in which the electric lights no longer provide significant heat gains because they are often turned off during the day. Zion, BigHorn, Oberlin, TTF, and Cambria are heating-dominated buildings. Passive solar heating through indirect gain Trombe walls provide a significant portion of the heating at Zion, and passive solar direct gains contribute to the heating at TTF.

Many passive solar features were integrated into the TTF final design. The design team took advantage of Colorado’s sunny climate by carefully selecting, orienting, and placing windows and clerestories. The passive solar and daylighting design of the building incorporates 88% (1,134-ft² [105.4-m²]) of its total window area as a single row of view glass (492 ft² [45.7 m²]) and two rows of clerestories (642 ft² [59.6 m²]) along the southern facade. An additional 8% of the total view glass area is on the east (56 ft² [5.2 m²]) and west (56 ft² [5.2 m²]) facades; the remaining 1% is positioned on the north wall (38 ft² [3.5 m²]).

NREL engineered the building to provide passive solar gain during the winter months and minimize this gain during the summer months. The selected glass type allows solar energy to enter the building for passive heating energy. South-facing clerestory windows have a high solar heat gain coefficient (SHGC) of 0.68 (shading coefficient of 0.76); all others have a lower SHGC of 0.45 (shading coefficient of 0.51).

The ground-level windows were designed for viewing and are larger than needed from an energy perspective. To avoid overheating and glare from direct gain through these windows during the winter, the view windows have a lower shading coefficient than the clerestory windows. All windows were engineered with a low-e coating to increase thermal resistance. Engineered overhangs were designed to block direct solar radiation during the warm summer months when sun angles are high. Although not a prominent design feature, thermal mass in the floor and north wall helps to minimize temperature swings.

Opaque envelope components for the TTF were selected to be highly insulating and to expose thermal mass to the interior.

Direct gains are used at the TTF to provide a portion of the heating needs. The building typically requires heating only during the early morning hours to warm up from nightly temperature setback. Although the temperature set point is set back to 64°F (19°C) every night, the temperature in the TTF rarely drifts that low. Heating loads on a cloudy day and a sunny winter day were examined to understand the direct solar gain benefit (see Figure 3-22). For both days, the set point was 64°F (19°C) from 5:00 p.m. to 12:00 a.m.; the remainder of the day was set to 70°F (21°C). After the morning warmup, passive solar heating and internal gains met most of the building’s heating requirements.

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1 4 7 10 13 16 19 22 1 4 7 10 13 16 19 22

Hour of Day

Heating (kBtu/hr)

0 15 30 45 60 75

Inside Temperature (°F) .

Heating Tin

Cloudy Day Sunny Day

Figure 3-22 TTF heating loads on a cloudy day compared with heating loads on a sunny day

3.3.3.1 Passive solar heating with Trombe walls

The only building in the case studies to use a Trombe wall was the Zion Visitor Center. The Trombe wall was integrated into the envelope to provide a passive radiant heating source. Design details for this Trombe wall are shown in Figure 3-23. The 6-ft (1.8-m) high Trombe wall (740-ft² [68.7-m²] total area) is located on the entire length of south-facing walls. The wall makes up 44% of the south-facing wall area. The Trombe wall is constructed of 8-in (20-cm) grout-filled concrete masonry units (CMUs) with an R-value of 2.5 hr·ft²·°F/Btu (0.4 K·m²/W). The other walls are 6-in (15-cm) framed walls with an R-value of R-16 hr·ft²·°F/Btu (2.8 K·m²/W). The Trombe wall has a single layer of high transmittance patterned glass installed on a thermally broken storefront system.

Overhang Engineered for Summer Shading

Daylighting and View Windows

Shotcrete Wall Finish (Inside)

Selective Surface

Figure 3-23 Cross-section details of Zion Trombe wall 3.3.3.2 Thermal and construction details of Trombe walls are important

A major benefit of the Trombe wall is its ability to provide a uniform radiant heating source. This

improves thermal comfort during the heating season. Also, as the heat transfer is governed by the thermal difference and the thermal resistance, the thermal resistance on the interior of the wall must be minimized.

The interior of the Zion Trombe wall includes in the design cast-in place concrete projections. These projections could be used as shelves to display product. The hope was to minimize the occupant’s desire to place bookcases or other obstructions in front of the wall. This has worked fairly well; however, the occupants have had a strong urge to place objects in front of the wall.

To reduce the thermal resistance, we verified construction of the CMU wall to ensure the concrete block cores were completely filled. This appears to have been effective. A problem area has been the shotcrete used as a finish material on the inside of the wall. Although a massive building material, there were air gaps between the material and the CMUs on the wall. These problems were detected using thermography as cooler temperatures on the wall surface. Placement of the footing insulation was also verified during the construction process to ensure proper installation. The location of this insulation is critical, as three-dimensional heat transfer to the ground can diminish Trombe wall performance. By thermally decoupling the footings from the ground with insulation, unnecessary heat loss is avoided and more heat from the Trombe wall is supplied to the building. This was effective as shown by minimal stratification of temperatures through the Trombe wall.

3.3.3.3 Trombe walls can meet a significant portion of the heat load

The Zion Trombe wall daily performance during the 2001–2002 heating season is shown in Figure 3-24.

The electric radiant heating system used 22,680 kWh during the course of one year and the Trombe wall contributed 20% of the total heating to the building. The Trombe wall imposed a heating load on the building for only two of the 151 days of the 2001–2002 heating season. For the other 149 heating days, the wall was net positive. The peak heat flux through the wall was 11.2 W/ft² (89 W/m²), or 8.3 kW over the entire Trombe wall area. The average efficiency of the wall over the 2001–2002 heating season (defined as the heat delivered to the building from the Trombe wall divided by the total solar radiation incident on the exterior of the wall) was 13%.

8-in. Grout-Filled CMU Wall

2-in. Air Gap

5/32”

Glazing Cast-in-Place Concrete Wall

Projections

1-1/2-in. Rigid Footing Insulation

During the first three months of the 2002–2003 heating season, the total electrical heating energy used was 5,389 kWh, while the Trombe wall provided approximately 41% of the energy or 3,800 kWh. This percentage was greater than the 2001–2002 heating season of only 20% because of improved controls of the electrical heating system and differences in weather.

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11/01/2001 11/08/2001 11/15/2001 11/22/2001 11/29/2001 12/06/2001 12/13/2001 12/20/2001 12/27/2001 01/03/2002 01/10/2002 01/17/2002 01/24/2002 01/31/2002 02/07/2002 02/14/2002 02/21/2002 02/28/2002 03/07/2002 03/14/2002 03/21/2002 03/28/2002

Daily Heating Energy Supplied to Visitor Center (kWh/day)

Heat Added by the Trombe Wall Heating System Use 2001-2002 Heating Season:

»5,470 kWh added by Trombe Wall

»22,680 kWh added by Trombe Wall

»20% of heating from Trombe Wall

»8.3 kW Peak heating from Trombe Wall

»17.2 kW Peak heating from Heating System

Figure 3-24 Zion Visitor Center Trombe wall and heating system performance, 2001–2002 heating season

3.3.3.4 Consider net annual performance when designing Trombe walls

A potential design issue to consider with the Trombe wall is overheating in the summer and swing seasons. The overhangs are designed to shade the Trombe walls during the cooling season. Even though the Trombe walls are shaded in the summer, the walls impose an additional cooling load on the buildings.

This is because early morning and late afternoon radiation is not shaded and diffuse and reflected radiation is not negligible. Additionally, the insulation values of these walls are low. For Zion, the additional cooling loads do not impact performance, as the passive direct evaporative cooling system provides an abundance of cooling. The annual net effect of the wall has to be considered in its design, as the additional cooling loads can affect the cooling system performance. For buildings with conventional air-conditioning systems, careful Trombe wall design is essential so summer gains are minimized. If buildings are flushed with evaporative cooling, the summertime cooling loads generated by the Trombe walls do not pose a problem.