Saving energy through responsive HVAC
A study of 29 commercial buildings has shown that energy usage can be cut and occupant comfort improved by making the internal environment more in tune with outside conditions.
The Investa Property Group’s Investa Sustainability Institute has conducted a two-year trial encompassing 29 commercial buildings that may finally bring an end to office air-conditioning wars. The results of the trial show that making the internal environment of a facility more in tune with outside conditions, through the use of more dynamic temperature setpoints, can reduce air-conditioning energy usage and, thus, costs. In addition, this improves occupant comfort as well.
The trial was conducted by Craig Roussac (director, Investa Sustainability Institute), Jesse Steinfeld (data analysis, Investa Sustainability Institute) and Richard de Dear (associate professor, Faculty of Architecture, Design and Planning at the University of Sydney). It culminated in a paper, entitled ‘A preliminary evaluation of two strategies for raising indoor air temperature setpoints in office buildings’, which was presented at the Australian New Zealand Architectural Science Association Conference in Sydney in November 2011.
EXPERIMENTAL SETPOINT STRATEGIES
A facility’s internal environment is often kept stagnant day after day regardless of the weather outside. Investa notes that air-conditioning in a typical Australian office building is generally set to target 22.5 degrees Celsius in summer, yet this temperature setpoint is well below the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ (ASHRAE’s) comfort standard, which recommends a range from 23 to 26 degrees Celsius for lightly active occupants wearing typical summertime office attire. The company adds that previous research it conducted exhibited that roughly one-third of summertime air-conditioning complaints are because it’s too cold inside a building.
During the 2006/07 summer, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) carried out a Smart Thermostat trial in one of Investa’s Melbourne buildings. The building’s internal temperature setpoint was raised by one degree Celsius. Heating, ventilation and air-conditioning (HVAC) savings of five to 10 percent were realised with every one degree Celsius the setpoint was raised.
Investa built on this discovery during the 2009/10 summer, when it extended the one-degree Celsius setpoint raise across 33 buildings. This resulted in HVAC savings of approximately six percent from the sample of analysed buildings and a reduction in cooling complaints by three percent compared to 2008/09.
The company then developed and selected new strategies for the following summer that it considered most likely to reduce HVAC energy demand and enhance occupant comfort in each building. After conducting a detailed analysis of building energy, weather and air-conditioning complaint data from the previous year, it pinpointed a number of strategies, including:
- floating setpoints – varying internal temperature setpoints throughout the day in response to the external temperature
- time of day based setpoints – a step-up in the temperature setpoint at a certain time during the day, and
- a simple setpoint raise.
In addition to the three strategies, Investa looked at implementing complementary strategies, such as air-conditioning optimum start and modifying plant operation times in some buildings.
Investa implemented its floating setpoints strategy in 11 buildings. The company notes that during the previous summer, some buildings automatically switched on their heating during cool summer mornings to try to meet the higher summer temperature setpoint.
It states that varying the setpoint in response to external temperature avoids the possibility of the building switching its heating on during cooler summer mornings. In addition, more actively responding to changing external conditions better aligns the expectations of occupants as they enter the building – posing the question: who wants to enter a 22.5 degrees Celsius building when its 40 degrees Celsius outside?
In one building, a high proportion of ‘too hot’ complaints were measured in the morning during the previous summer. Investa’s solution was to provide cooler internal conditions during the morning to improve comfort and then to step the setpoint up in the afternoon to save energy when minimal complaints were measured.
SIMPLE SETPOINT RAISE
A simple setpoint raise provides internal conditions that are more in line with ASHRAE’s temperature recommendations for summer, suggesting that comfort will be improved, according to Investa. If the simple setpoint raise was successful previously, or if the buildings couldn’t implement another strategy, a simple setpoint strategy was employed. This strategy was implemented in 12 buildings.
STRATEGIES PROVE SUCCESSFUL
Simple weather adjusted energy models for each building were used to work out how successful the various strategies were. Overall, in the summer of 2010/11, HVAC savings of 4.1 percent were measured, compared to 2009/10 and eight percent, compared to 2008/09 based on an analysis of a sample of 15 buildings.
Air-conditioning complaint data collected from a tenant help desk revealed that air-conditioning complaints fell by 13 percent in 2010/11, compared to 2009/10, indicating that occupants inside were generally more comfortable.
The trial shows that implementing more dynamic setpoints in existing buildings is a good early step to more responsive building management. Investa is continuing to use trial and error to optimise these strategies. It states that the strategies in place are an encouraging early step towards an ultimate goal of creating adaptive buildings that can respond to the weather for the benefit of the energy bill and the people inside, and encourages other buildings to get involved with these simple trials.