By Christian Cianfrone, Cornell University, Ithaca, New York
View the pdf of this article
Air pathlines colored by the mean age of
air entering from the diffusers
The U.S Department of Energy
is hosting its second Biennial Solar Decathlon Competition in Washington, DC in October 2005. Eighteen schools will exhibit their fully-functional solar powered houses on the Washington mall. As the name suggests, the competition comprises ten different contests that focus on many facets of solar powered energy. Some contests, such as the Comfort Zone contest have very strict requirements. This contest requires that the house be between 72°F and 76°F. The house must also have excellent air quality and maintain a relative humidity between 40 and 55%.
Iso-surfaces showing only the regions above 76°F (green) and below
72°F (blue) during heating
A computer rendering of the solar house
The solar house is currently under construction
The only energy coming into the house is provided by photovoltaic panels, which are used to supply electricity not only to the HVAC system, but also to several appliances and lighting. Thus, energy efficiency for all systems plays a major role in the house design. For the Comfort Zone contest, the Cornell University Solar Decathlon (CUSD) Team used Fluent software to generate a unique design that would distinguish their house from the others in the competition.
Airpak was used to create the geometry and to generate a mesh for the house. The values for insulation and material properties were entered accurately according to the house specifications. An air-handler and three diffusers were used for circulation. The diffusers were the only geometry variable and their placement helped determine the temperature distribution and relative humidity throughout the house, as well as the overall air circulation. The diffuser macro in Airpak allowed the students to change the values for the throw and velocity of each diffuser, based on the data provided by the different manufacturers.
The most difficult obstacle to overcome was the limited number of places that the diffusers could be placed. Due to the structurally insulated panels (SIPs), there was no room to run ducts through the walls or roof. Hence, they had to be placed in the floors between the structural joists. The only exception was the diffuser in the bedroom that could be positioned in a utility wall, where other equipment was stored.
The computational model predicted that the variable speed air-handler could run at a low speed, even in the harshest external conditions. The results also showed that all of the livable space is within the desired temperature range during heating, except for the region immediately downstream of the air diffusers. To achieve the relative humidity requirements, it was determined that the air-handler needed to condition the air to deliver a relative humidity of 30. Finally, the mean age of air did not exceed 10 minutes throughout the entire house. This is well within ASHRAE standards that suggest 0.35 air changes per hour for residential ventilation.
Since the construction and testing of the house and equipment is being performed by students under stringent time constraints, it was very important to determine all of the details of the HVAC system before actually building and testing it. CFD provided a time- and cost-effective way of testing the system with respect to diffuser placements, energy consumption, and equipment requirements.
|
