The engineers simulate real-world conditions in laboratory testing, but they do not often get the chance to follow the pumps into the field to track performance data. The intent was to improve the energy efficiency of the system without modifying the heating and cooling loads. In the lab, manufacturers can test the efficiency of a pump, but in the real world there are often unexpected variables and more complex behavior within the system that affect efficiency.
Columbia University’s Michael Waite, a postdoctoral research scientist in professor Vijay Modi’s Sustainable Engineering Lab, predicated a research thesis on a new mathematical model to assess a building’s energy profile that was derived from the research data. “There is a clear research gap for the evaluation of hydronic systems for large buildings,” Waite said. “Their complex systems demand a different mathematical model and approach to predict energy use. You also need to get into the building to see how (it has) responded to the energy conservation measures you have made.”
Prior to the retrofit, nearly 30 percent of the building’s common system utility costs were for pumping electricity, due primarily to the pumps being oversized for the demands of the system and the constant speed operation at partial loads. Oversized pumps were found to cause unnecessarily high pressure differentials and flow rates in the system.
Oversizing pumps is a common industry practice. There are a number of reasons why, including adding safety margins beyond those factored into the design by pump manufacturers and accounting for marginal over performance at system peak loads. However, this common practice comes at a cost—namely increases in the system’s operation, maintenance and capital costs over the system’s life cycle.
When considering both heating and cooling, the retrofit resulted in a computed annual pumping electricity usage of 316 megawatt hour (Mwh), a 41 percent improvement in pumping energy requirements and an estimated 12 percent reduction in the building’s central operations’ energy bills.
Commercial buildings account for 36 percent of all U.S. electricity consumption and cost more than $190 billion in energy every year, according to the U.S. Department of Energy. With more than 80 percent of the existing commercial and institutional buildings in the U.S. expected to operate beyond 2030, as reported by the Colorado-based Rocky Mountain Institute, demand for HVAC system retrofits will be great.
The existing environment presents an opportunity for saving energy, creating better value for building owners and promoting sustainability.
In addition, the populous states of New York and California have in place some of the country’s most stringent environmental policies—much stricter than existing federal rules—dictating that efficiency will continue to be a major consideration in selecting equipment for retrofit projects, regardless of potential federal policy changes.
The VFD Story
Centrifugal pumps installed in HVAC systems typically operate in variable load applications that see a fluctuation of flow requirements based on the heating or cooling load of a building at any given time. The original pumps specified for Astor Place were running at constant speed along with being oversized for the true operational demands of the building.
VFDs were the perfect solution to address the pump oversizing. Even at peak cooling load, electricity reduction was more than 50 percent compared to constant-speed pumps, according to the test data. VFDs do bring the most benefit in terms of energy consumption. The pumps consumed just enough energy to provide proper service for that part of the cooling loop.
In addition to the VFD testing, the research team set up four retrofit combinations to collect additional data to assess the energy savings contribution of the various retrofit measures installed at the same time. The following scenarios were analyzed:
- VFD and pressure independent control valves (PICVs) in operation (final post-retrofit condition)
- VFD in operation; original air handling unit (AHU) valves
- VFD bypassed; PICVs in operation
- VFD bypassed; original AHU valves (replacement pumps and primary-secondary loop modification only)
Sensors were installed at the end of each branch to provide vital performance data. All data was logged at one-minute intervals and accessed through a newly installed building management system (BMS).