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