Mechanical Engineering: Radiant Ceiling Panels Analysis

The main heating and cooling system for the ELEMENT is a variation on a “cooled ceiling” system which is most commonly used as a cooling technique for tall buildings. The system used in the ELEMENT design can theoretically be referred to as a radiant ceiling system. This incorporates ceiling panels which comprised of small tubes with refrigerant running through them. The system uses a vapor compression cycle and is almost identical to a heat pump and air conditioner unit with the exception of fans. In a typical heat pump and air conditioner system fans are used to blow air over these coils, which in turn, heats or cools the air. In the system used by the ELEMENT, the fans are removed and the air movement across the coils will occur through natural convection. In that sense, the system is similar to a traditional radiator system. Another slight difference between the standard vapor compression system and the ELEMENT heating and cooling system is that a traditional system will use a refrigerant such as R-12, R-134a or R-410a whereas the ELEMENT’s system uses CO₂ as its refrigerant.

The Typical Vapor Compression Cycle

The major issue with this type of cooled ceiling technique is the possibility of condensation on the piping of the ceiling panels. To avoid an indoor rain storm, the humidity levels of the air must be closely monitored and controlled with a humidifier / dehumidifying system. In order to adhere to the strict guidelines of the Solar Decathlon competition, this monitoring must take place anyway, so this issue really becomes less of a problem. The monitoring of the heating and cooling system will also benefit several other areas of indoor comfort. For example, the ELEMENT will not have any operable windows (firstly because of an increased cost and secondly because of the poor air tightness which would result) therefore, in order to maintain a healthy Indoor Air Quality (IAQ) a Heat Recovery Ventilator (HRV) or a Energy Recovery Ventilator (ERV) will be used to supply fresh air to the house (the principle difference between the two being the HRV only transfers sensible heat while the ERV transfers latent heat as well). The main reasoning behind the decision to avoid operable windows was decreased air tightness that would result from having operable windows.

Construction Progress: Week of May 20

Now that the school semester has ended, volunteer workers have been in short supply. This will mean a slower construction pace until students return for the summer semester. This week the majority of the work will be focusing on preparing the building for window and drywall installation. For the most part, this means finishing attaching the exterior sheathing around the rest of the modules. Before this takes place sheets of expanded polystyrene (commonly referred to as XPS or XEPS) are being placed above the sprayed-in polyurethane where the roof parapet meets the roof and walls. This insulation will help limit air infiltration and heat transfer. Without it, air could more easily flow above the sprayed-in insulation (in front of the roof parapet) and down behind it into the house.

The Kitchen Module with XPS Being Installed In Front of the Roof Parapet

Also, before the rest of the exterior sheathing is installed, gaps in the wall insulation will need to be filled. These gaps have mostly occurred around areas where thicker conduit is running through the exterior wall. For more information on the sprayed-in insulation and the insulation installation, see “Construction Techniques: Spray Foam Insulation.”

Construction Progress: Week of May 13

The mounting track and conduit of the vapor panels have been installed and polished off this week. The electricians took longer than expected this week which pushed the drywall installation and window installation back another half week or so. The window on the north side of the Bedroom Module (that would be the window in the bathroom) was installed as a test. Tyvek was installed on the exterior face the wall before the window was installed. Tyvek is a permeable membrane which limits air infiltration while allowing moisture to escape to the outside if it gets to the inside of the wall. When the rest of the windows and doors are installed, this same process will be used.

The North Wall of the Bedroom Module with Permeable Barrier and Window Installed

By June 15 all exterior finishes should be on and interior nearing completion. At this point the mechanical engineering team will be able to install the heating, ventilation and air conditioning system (also referred to as the HVAC system). Once installed, the HVAC system will be tested to insure the optimum indoor air quality (IAQ), an adequate number of air changes per hour (this plays a large part in the providing an optimum IAQ), and one of the most important tests will be to insure the most efficient performance of the vapor compression panels. These tests will make the ELEMENT a pleasant, healthy and energy efficient house in which to live.

Construction Progress: Week of May 6

Electricians came in this week and began running wires. Conduit for the vapor compression panels are also being prepared this week. The conduit was put in place in the exterior walls before the insulation was sprayed in (this is the blue conduit you can see from the exterior). It is surprising to see that the conduit was run through the walls as apposed to above the panels in a drop ceiling. The reasoning behind this placement is uncertain. It would seem more logical to run the conduit on the interior of the building to ensure the maximum amount of insulation and the minimum amount of air transfer between the interior and exterior. Aesthetically, running the conduit in the wall assembly may be more pleasing; however, one also must consider the adverse affect this decision will have on the performance of the building’s envelope. For more information about the insulation used in the building envelope see “Construction Techniques: Spray Foam Insulation.”

Construction Progress: Week of Apr 29

This week the exterior sheathing is being applied. The exterior sheathing will help the modules better protect against shearing forces applied at the roof level during transportation and after the modules have been installed. This sheathing also helps better protect the insulation for the harsh exterior. Also this week, workers will be retro-fitting the insulation so that next week the modules are ready for the electricians to come and run the circuits to power the modules.

Roof Drain Installation Transferring Rain Water from Module to Module

Exterior Sheathing Nearing Completion on the Kitchen Module

Construction Progress: Week of Apr 22

Insulation spraying was completed this week, inside and out. Since in typical construction the majority of heat and energy loss is through the roof, the insulation in the roof was sprayed on both the exterior and interior. This detail allowed for much thinner roof members while nearly doubling the amount of insulation. For more information about the installation process of the insulation used see “Construction Techniques: Spray Foam Insulation.”

The Bedroom Module showing the Interior Roof Insulation between Roof Joists

ELEMENT Design Development: Solar Panel

The solar panels being placed on the roof are being supplied by BP Solar. The panels are 170 Watt Photovoltaic Module units (Model BP 7170). The efficiency of these panels is rated at 13.5%. This number may seem small; however, the most current photovoltaic cell converts sunlight into DC power at an efficiency rate of at most 15%. For more detailed information about BP Solar’s products or services visit their residential solar applications website at:

http://www.bp.com/subsection.do?categoryId=3050503&contentId=3060168

After the module shift took place, we were able to more adequately maximize the roof area available for solar panels. As seen below, the roof of each module has a solar array consisting of a 12 photovoltaic module units in a 3 x 4 grid. Then attached to the façade of each module there are an additional 4 photovoltaic module for a total of 48 photovoltaic module units to power the house and the electric car.

South Elevation showing the Solar Array

Performance and transportation where the two main drivers behind the design of the solar arrays. The solution to both these issues came when we decided to hinge each group of panels. The 12 units on the roof fold down on top of the roof for ease of transportation while the 4 units on the south façade fold inward against the façade during transport. This hinging will help protect the panels during the trip to Washington DC and it will also help us maximize the performance of the photovoltaic cells. By placing the panels perpendicular to the rays of the sun the cells while be able to convert more of the light into power. The hinged panels attached to the south façade will also help shade the windows on the south façade to prevent unwanted solar heat gain during the summer months.

Conceptual Design of Hinged Solar Arrays

ELEMENT Design Development: Panoramic Window

The panoramic window was designed based on the eye height of an average person. This feature was designed purely for its aesthetic appeal without taking into consideration the efficiency of the window and wall assembly. This became a very big problem in the design development and also in the construction of the house. However, it was decided that the panoramic window was such a nice feature it would be left in the final design even though it would have a very negative effect on the overall performance of the house. The panoramic window also raised several structural issues. The framing for the panoramic window was further complicated when a second panoramic window was added directly below the higher one. The second window was designed based on the eye height of the average person while seated.

These windows were the cause of several framing redesigns for the North, East and West Walls of the Living Module. The North was the most complicated to design because this is where the two windows overlapped. Two different framing schemes where devised. First, a truss system was devised which would transfer the load from the roof and take it over or between the windows and move it to the corners of the module. The major benefit of this scheme was that a large header would be needed above the windows and more wall area could be filled with insulation. The second scheme followed typical framing techniques which involved using a very large header above each window in order to transfer the load from the roof around the windows and to the foundations. Inversely, this scheme would need much more wood and would leave less room for insulation. Thus, a great deal of energy loss would occur through this wall. The second scheme was chosen to be constructed despite the weaker energy performance. Either scheme would have resulted in a poor energy performance simply based on the design and placement of the windows. Therefore, the second scheme was constructed because it could more easily be constructed by the volunteer labor. To construct the truss system would have required a specific knowledge of truss construction and performance in order for it to be built and constructed properly.

Panoramic Window on North Façade (framing based on first scheme)

Panoramic Windows seen on North Façade (framing based on second scheme)

In the case of the panoramic windows, the aesthetic was designed without any consideration to either structural or energy performance needs. When this takes place it is rare that the desired aesthetic will be achieved. Clearly, the structural elements must be satisfied in order for the building to stand. In the case of the panoramic window, the initial design had two pieces of glass meeting at the corner so that you would have a clear unobstructed view to the exterior. However, in order for the living module to stand without using any overly complicated construction methods, the roof load most be transferred through the corner essentially eliminating the full panoramic view. Thankfully, this oversight in the design of the window will help the window perform a little better in terms of thermal transfer.

Panoramic Window from Living Room

Once the panoramic window was put into the design of the ELEMENT the thermal performance of the house decreased dramatically. Then, because the structurally capabilities of the window design were none existent, the concept of the panoramic window design was not able to be realized. This brings to question, which is more important, aesthetics or performance? In the case of the ELEMENT Panoramic Window, the full aesthetic design was not meet and the performance of the house substantially reduced. This brings to light the importance design. From the onset, designers must consider aesthetics, structure and thermal performance. Each one affects the other, if they are designed together, at the early stages of design, they can work together. However, if only one aspect is taken into account at the beginning of the design, the other elements of the design will be adversely affected.

ELEMENT Design Development: Battery Box

As seen in the Floor Plan seen in the “ELEMENT Design Development: Layout” the Battery Box was originally placed on the North Façade in front of the Bathroom. In one of the first design charrettes it was thought that the battery box could be made into a display. The battery box was then designed to attached to the North Façade and placed along the tour path so that before patrons entered the house the could see the “guts” of the building. The battery box was imagined as a mostly glass frame structure which housed the batteries and a computer monitoring system which could display the total output from the solar panels as well as the total energy consumption of the building. However, mechanical engineers where worried about the aesthetic appeal of the battery box saying that the batteries themselves where not much to look at and the computer system would be most likely not be able to be displayed at all. The Battery Box was further complicated after the mechanical engineers meet with the event organizers in Washington DC. It was recommended that the number of batteries be drastically reduced and detached from the structure entirely. Taking their recommendations into consideration, the battery box was detached from the house and moved into a separate structure to the east of the house.

This move caused several other small changes in the overall layout of the house. In particular, the inverter closet (once on display on the North Façade) was moved next to the bathroom on the East Façade. This move caused the size of the bathroom to shrink by roughly 2 ½ feet causing the bathroom to know longer be accessible (aka meeting ADA requirements). In terms of program requirements for the competition, this reduction is not a problem because the bathroom is not in the general tour path. However, the fact that the bathroom is no longer accessible may hurt the overall marketability of the house.

Final Floor Plan

Construction Progress: Week of Apr 15

The insulation spraying was continued over the next two weeks. Uncooperative weather delayed the spraying yet again. The spraying needs to take place outdoors do to the chemicals used during the spraying. The entry “Construction Techniques: Spray Foam Insulation” covers the spraying methods and chemical bases more thoroughly. When weather kept the ELEMENT indoors, the student workers finished work installing electrical outlets and running HVAC ducts and piping in the Mechanical Room. For more information about the Mechanical System see “Mechanical Systems Integration: Vapor Compression System.”

Modules are Rolled Outdoors on the Rail System which serves as its Foundation

Modules are Lifted off Rails (This Same System will be used to Install the ELEMENT during Competition)

Installation of the Forced Air Portion of the HVAC System

The Living and Bedroom Modules Outdoors awaiting Insulation Installation