Stop hyperventilating, say energy efficiency researchers

Customising ventilation in large office buildings based on the number of people in a room could reduce annual energy bills by 18%, with no loss of comfort, according to a report from the US Department of Energy’s Pacific Northwest National Laboratory (PNNL).

The report outlines an advanced building control currently in development that customises the level of ventilation by sensing the number of people in different areas or zones of a building and adjusting fan speed and air movement accordingly. PNNL carried out extensive simulations of the impact of the advanced building control, which are outlined in the report.

Current sensor-based ventilation systems operate ventilation at full blast, regardless of whether one or 100 people are in a room. However, PNNL researchers say, a room with just a few people doesn’t need as much ventilation as a crowded room.

“This is the reason you often feel cold when you’re in a big space like a conference room or cafeteria without a lot of people,” said engineer Guopeng Liu, the lead author of the report.

“Technology available today doesn’t detect how many people are in a room, and so air flow is at maximum capacity nearly constantly. That creates a big demand to reheat the air before it enters the rooms. It takes a lot of energy to keep you comfortable under those circumstances.”

While existing occupancy sensors have saved significant amounts of energy by turning off lights when rooms are vacant, the PNNL team estimates that more advanced versions which count the number of people in rooms will save up to 28 times as much energy when used for both lighting and ventilation.

A piece of equipment called a variable air volume terminal box determines how much air to move to different zones of a building, based on the number of people in a room. This equipment is likely to become available in the next few years. While currently very expensive, the technology is improving rapidly and the cost is expected to reduce, according to team leader Michael Brambley.

“We undertook this study to try to determine if this is a technology worth pursuing vigorously. The answer, clearly, is yes. Using the number of people in a room as a factor in determining the level of air flow offers great promise for saving energy and money,” said Brambley.

The study focused on a 12-storey large commercial office building with a total area of about 46,452 m². The simulation was programmed to heat the building if temperatures dropped below 21°C, and heat it at temperatures above 23°C. These figures were set back several degrees for weekends and evenings.

In 13 of the US’s 15 climate regions, the PNNL team estimates the advanced controls would save at least $40,000 annually for buildings of a similar size. In Baltimore and Fairbanks, savings could exceed $100,000 each year because of the greatly reduced need to heat new air being pumped in from outside. Even in the two cities where savings would be the least, El Paso and Miami, estimated savings still come to $33,400 and $23,500 respectively.

A prime target for these advanced controls is retrofitting existing buildings, Liu and Brambley say. Since heating and cooling usually draw much more energy than lighting, retrofitting HVAC systems offers a greater opportunity for savings. The PNNL team found that advanced controls for ventilation offer eight times as much savings as controls for lighting. When the HVAC system alone is considered, the advanced controls cut energy usage by nearly 40%.

The study was supported by the Department of Energy’s Office of Energy Efficiency and Renewable Energy. The report is available here.