Building Energy Studies

 

Assessing Energy-Saving Potential of Different Roof Systems

Student: Stephen Ray, Ph.D. student in Mechanical Engineering

Previous studies suggest potential for energy savings through cool and green roofs, but do not always consider the many factors that affect potential savings or the relative advantages of different technologies. To further investigate these factors, a tool has been developed to allow architects the ability to quickly assess the energy-saving potential of different roof systems. A first principles heat transfer model has been developed for each of the roof technologies, with particular care for green roof heat and mass transfer. Experimental data from Japan and Florida validate the models by predicting roof surface temperature. This model is incorporated into the existing MIT Design Advisor to allow users the ability to include roofing systems in their energy analysis of a building.

 

 

Infrared Thermography for the Comparison of Building Energy Envelope Performance in Residential Homes

Student: Kaitlin Ryan Goldstein

There are 120 million residential homes in the United States that are responsible for 20% of the United States’ total carbon emission. As energy has been relatively cheap for the past 20 years, the efficiency of these homes has been an afterthought during the construction process. We are thus left with an existing building stock with the potential to improve their consumption through efficiency measures alone from 15-40%.

In order to make a difference in these homes we must first understand where the problems are, and, which homes are the greatest consumers. Unfortunately, the current means by which this is accomplished, home energy audits, are time-consuming, involve full participation and commitment form the home-owner, and require a large, as of yet, untrained labor force. While these audits are essential there needs to be an independent means by which to understand the homes’ energy consumption, and in particular, heat transfer ( the loss or gain of energy) through the building envelope. This includes all of the parts of the building that interact with the outdoors including its exterior walls, windows doors and roof.

To understand energy consumption in buildings and to compare one home to another in a fast, efficient manner, we are developing a suite of tools centered around infrared thermography. Our research looks to take these pictures which give a representation of surface temperatures on the building surface from a remote vehicle driving past the neighborhood homes. The next step, and the focus of this research is to utilize these maps of surface temperatures along with measurements of prevailing external conditions to understand how the homes are interacting with their surroundings. From this, we will be able to back out the insulation, or R-values of the homes. We also hope to characterize how heat moves through the windows and doors and how much air is escaping from inside the building through infiltration.

The end goals is to be able to compare one home to another in a neighborhood and provide recommendations as to which homes should be targeted for retrofit and the most effective means by which to improve their envelopes. In addition, we hope to develop a map of efficiency potential within a neighborhood and calculate the respective returns on investment for various suites of improvements. This method is not only fast and efficient in the identification process but it also allows for return examination through the same mechanism and the comparison of performance pre and post retrofit to determine the efficacy of improvements.

Efficient Cooling Technologies for the Built Environment

Principal Investigator: Les Norford Sponsor: Masdar Institute of Science and Technology

One promising method for reducing the energy consumption of cooling systems is to reduce the pressure rise across the refrigerant compressor, a systems approach known as low-lift cooling that was pioneered by Armstrong at the Masdar Institute of Science and Technology. This requires increasing evaporator temperatures and decreasing condenser temperatures. Radiant cooling systems promote the former, through use of water at higher temperatures than the output of all-air cooling systems. Thermal storage in building mass promotes night cooling, when outdoor conditions allow lower condensing temperatures. Efficient motors and variable-speed motor drives for the compressor and auxiliary fans and pumps provide efficient low-load operation. Finally, a dedicated outdoor air system meets latent cooling loads.

Operation of this system to maximize performance requires model-based predictive control, which in turn demands component models that accurately capture component operation under a very wide range of conditions. Research at MIT includes test-stand measurements of component performance, physics-based modeling with parameter identification from measurements, identification of optimal operating points for a given cooling load and indoor and outdoor conditions, and evaluation of the performance of conventional and low-lift systems in a full-scale test chamber.

MIT Design Advisor

Principal Investigator: Leon Glicksman Sponsor: Permasteelisa Group

The MIT Design Advisor is a multi-purpose simulation tool designed to evaluate the performance of advanced building facade systems. By defining a set of building parameters and operating conditions, a building designer can simulate in realtime the energy requirements (heating, cooling, and lighting) and comfort levels (daylight, temperature) of a proposed design. This simulation presents the user with a convenient method of examining facade performance.

Existing analysis tools are typically very complicated, difficult to learn, and require a fully developed building design, making them unsuitable for preliminary design analysis. Efforts to improve building efficiency are typically left for the later stage of the design process, after the critical design decisions have already been made. Because early stage design decisions can have a dramatic impact on building performance, we offer this tool as a fast, simple way for a non-technical user to evaluate preliminary designs.

Building codes help to ensure that buildings meet a minimum standard of energy efficiency. To assist building designers, we are implementing a tool to test a proposed building design against two building code standards: ASHRAE Standard 90.1-2001, and the UK Building Code Part L.

The Design Advisor allows a user to simulate a single side of a building facade or an entire four-sided building. The four-sided simulation assumes that the features on each side of the building are identical. In practice, this is often not the case, and so we are developing added functionality to simulate a building with four different sides.