Heating and cooling buildings is a large part of global energy demand and a significant source of CO2 and greenhouse gas emissions, and in the coming decades the energy demand for heating and cooling - also known as thermal energy - is expected to grow considerably. Scientists and engineers have made many advances in lowering building energy demand by improving energy efficiency in building technologies and reducing energy loss through the building walls and windows.
Now, researchers are concerned that simply tackling the problem through energy efficient technology and design will reach its practical limits, and they are pioneering a new framework that determines the minimum thermal energy required to keep building occupants comfortable.
Researchers from Lawrence Berkeley National Laboratory (Berkeley Lab), the National Renewable Energy Laboratory and UC Berkeley recently published a study in the journal Joule that makes the case for calculating the theoretical minimum thermal load in order to dramatically lower the energy required for heating and cooling buildings.
For the last 50 years, scientists and engineers have focused on modeling building energy use based on whole-building energy demand, and this means that energy is used to heat or cool the entire/partial building space in order to meet occupant comfort needs. With this new framework, the calculation focuses on only providing the thermal energy needed to keep an occupant comfortable, which is defined as maintaining the human body at 98.6°F for a specific set of boundary conditions and thermal properties.
“Our work shows that current thermal loads in buildings are more than an order-of-magnitude higher than the theoretical minimum thermal load,” said Associate Laboratory Director of Berkeley Lab’s Energy Technologies Area Ravi Prasher, who is the corresponding author of the paper. “In fact, the theoretical minimum thermal load showed that in residential buildings the energy used for heating or cooling an entire building for occupant comfort could be between 19 to 40 times lower.”
The theoretical minimum thermal load does not calculate the amount of heating or cooling that would be needed to make an uncomfortable space comfortable, but rather it sets up a new baseline for occupant comfort with different building parameters. By calculating this baseline, the researchers are identifying the physical limit for reduced thermal energy use, that is, the point at which further reduction in thermal energy would cause occupant discomfort.
"In addition to calculating the theoretical minimum thermal load, we also explore options for approaching it,” said Berkeley Lab Research Scientist Prakash Rao, who is a co-author on the publication. “Using very simple approximate models, we find that reducing the space that is actively conditioned by 80% and creating moderate temperature setbacks for the balance of the building can deliver energy savings upwards of 60% for some climate regions."
Berkeley Lab Research Scientist Vi Rapp, who is also a co-author on the study, added, “Many technologies are already in development, such as zonal thermal controls in buildings, thermally adaptive clothing, and tunable thermal storage for walls of buildings.”