Radiant Success: Designing For Energy-Efficient Comfort In California
It’s not often you get to work on a project with an enthusiastic, knowledgeable client, a renowned architect, and a very resourceful contractor. IBE Consulting Engineers was retained by Claremont McKenna College (CMC), located in Claremont, CA, to design an energy-efficient building, with a mix of classrooms and office space, in which occupant comfort and energy conservation would be a priority. The goal was to provide comfort levels at 10% percentage of person dissatisfied (PPD) or less for each space and at the same time consume the least amount of energy against both California’s Title 24 requirements and ASHRAE 90.1-2007 for LEED® points.
The building has a gross area of 182,000 sq ft, and contains a basement with four levels above grade. The spaces are distributed in the following manner:
•Server room, classrooms, a parking garage, and mechanical and electrical rooms in the basement
•The first and second floors contain a mix of classroom studios as well as office and support facilities
•The third and fourth floor contains the main administrative offices, faculty offices, and ancillary support spaces
ENGINEERING THE ARCHITECTURE
The place to start in creating comfortable spaces is with the architectural design and not the conditioning systems. IBE spent considerable time working with the architects, analyzing different glazing alternatives, and investigating the inside surface temperature for the glass as this drives the mean radiant temperature (MRT) in the occupied spaces. A dynamic comfort simulator was used that could analyze space conditions for a single day, month, or year. Having a better understanding of the building shade characteristics and thermal conditions, the team improved overall thermal comfort while reducing energy consumption by implementing some or all of the investigated strategies.
Claremont McKenna College is located at 34.1 degrees latitude. Using a software program, a sun path diagram was created to show the total solar radiation on south and west facing surfaces of a 90-degree structure. The sun path diagram reveals the maximum solar radiation potential for September and July are 144 Btuh/sq ft, and 168 Btuh/sq ft respectively. The design peak days selected for the analysis were July 30th for the western facing windows and September 24th for the southern facing windows.
On the fourth floor of the southern façade of the college, there are 1.5-ft fins protruding from both sides of the windows. There is also a 1.5-ft overhang above the windows.
The material characteristics of the fins are very important. The material should have a high reflective factor to reflect solar radiation from being absorbed into the shade. In Claremont, CA, the peak solar intensity is 168 Btuh/sq ft. By allowing only minimal radiation to hit the windows, the solar gain to the space is reduced significantly. At the same time, the solar radiation penetrating the fins must be utilized to enhance the natural daylighting of the spaces.
The inside surface of the fins must also be carefully selected. If the surface has a higher reflectance than any radiation reflected from the glass after being allowed to hit the glass, it could be reflected back into the building from the shade. If the inside surface of the fins is not reflective, the solar radiation reflected from the glass will be absorbed by the fins.
The glazed surfaces of the college were carefully selected as the glass had to perform to reduce solar loads, yet permit natural daylight to enter the spaces. During the winter, the glazing must have a low U-value to reduce heat losses. A low U-value is most often obtained by having a coating on either the second or third surface of the double-glazed construction. The ideal glazing is one with a balance between a high visible light transmittance and low shading coefficient. This is often a difficult compromise to maintain a clear appearance yet achieve the required shading performance.
The glazing type used in the analysis for the college was an insulating glass with a low shading coefficient of 0.32 and high visible transmittance of 62%, a winter nighttime U value of 0.29 Btuh/sq ft and a summer U value of 0.25 Btuh/sq ft.
The choice of an appropriate conditioning system was based upon the required comfort compliance requirements. But the different characteristics of classrooms and offices would lead to two different conditioning systems.
Based upon previous design for academic buildings, such as Cooper Union, we had some excellent operational feedback that would help us select a system for CMC. Each classroom was designed for 30 students, with and without computers. Experience in designing academic buildings over the years requires a flexible solution, taking into consideration the amount of students attending classes and at what time of day will the classes be held. The basis of the design is a variable volume ventilation air supply, and we chose to provide 20 cfm of outside air for each person present. Providing a 20-cfm ventilation rate qualifies for the LEED point for extra ventilation. The cooling provided by supplying 20 cfm/student and with a maximum of 30 students in the room is nearly sufficient to maintain a space temperature of 74°F.
However, we were looking for comfort compliance, so a radiant ceiling was introduced mainly for heating during the brief and relatively mild winters in California. The choice of a radiant ceiling was based upon the system being able to control radiant temperatures in the space, especially for the first lesson of the day, and with only a minimum of students present. The radiant ceiling would provide heat to the space and control space radiant temperatures, and the ventilation air would be supplied in amounts determined by individual space CO2 sensors.
Another spinoff from this methodology is the reduction in fan power for the ASHRAE 90.1 energy performance. Once the choice for a radiant ceiling was made, investigations then took place to look at the utilization of cooling from the radiant ceiling. It was basically the same scenario as heating: if the class is partially occupied, the ventilation air would be reduced and the cooling and radiant temperature control would be performed by the radiant ceiling.
The results show that comfort conditions comply with ASHRAE Standard 55 when a radiant ceiling is introduced as part of the conditioning system for the classrooms.
We decided to use active beams to condition the offices and administrative spaces at CMC. The choice was based upon our quest for occupant comfort and individual control in each space. Constant volume primary air is supplied to each beam and the sensible cooling from the primary supply air is only about 15% to 20% of the space sensible cooling load. The larger portion of the cooling load is provided by the control of cooled water flowing through the beam. By putting the control emphasis on the waterside control of the system, the response time is improved and this increases the efficiency of the system.
A central cooling and heating plant was provided to serve this building. The central plant is located at the basement level to the north of the building.
The chiller plant consists of two 160-ton frictionless chillers. Each chiller has a variable-speed primary pump. The chillers also have the capability of having their speed varied to improve efficiency. Condenser water for the chillers is cooled by a single cooling tower having variable-speed fans. The condenser water loop is constant volume.
There are two variable volume chilled water loops:
•There is a 42° loop that transports water to the AHUs, computer room air conditioning (CRAC) units and fancoils in the IDF rooms.
•The second loop has a variable supply temperature from 55° to 58° for the active beams and the radiant ceiling panels.
Two boilers each with a 2 MBtuh capacity provide water at a constant volume to a common header.
There are two variable volume heating hot water loops:
• There is an 180° loop that transports
water to the AHUs.
•The second loop has a variable supply temperature for the active beams and the radiant ceiling panels.
An energy model was constructed to explore the building’s performance against the California Energy Code (Title 24). This code provides a measuring stick based upon the size and use of a building.
•The reference baseline building shell is comprised of metal frame wall with R-13 batt insulation, insulated glazing with a T-24 maximum shading coefficient, and roofing with a R-19 insulation.
•Lighting systems were specified to meet Title 24 allowances of 1.2 watts/sq ft.
•The reference baseline mechanical system was an overhead VAV system and a central heating and cooling plant as allowed by Title 24 standards.
Figure 8 shows the EnergyPro output for the energy analysis. The reference standard design is a building of the same size and usage built in accordance with the prescriptive requirements of Title 24. By taking the performance approach, we do not need to follow the prescriptive requirements as long as our proposed building outperforms the standard building.
Based on the preliminary model, the proposed building is performing 32.3% better than the standard model, although the value of 37.9% better than Title 24 is used for savings by design, as this excludes process loads.
The building includes the following features to increase the performance of the building to exceed Title 24 minimum standards by 37.9%:
•High-performance lighting systems in classrooms, seminar rooms, meeting room and offices, with occupancy sensors and daylight harvesting sensors
• High-performance glazing
• High-efficiency frictionless chillers
•Wall insulation increased to R-19 and roof insulation increased to R-30
• Daylight harvesting sensors
For the LEED submittal, the percentage of energy savings was 63.5% and the cost savings were 46.7%, which was good for 10 LEED points. ES