The facilities team at the Princess Margaret Cancer Research Tower have completed a project to upgrade the domestic water booster pumps at the site. Before I get into all the fun details, I’ll provide a quick breakdown of the savings numbers:
- Peak power savings: 24.6kW
- Annual power consumption savings: 188,432 kWh
- Annual GHG emissions reduction: 7.6 tons CO2 equivalent
- Annual cost savings: $23,365
- Payback: 3.7 years (including incentive)
What is a Booster Pump?
Water pressure from the city is typically sufficient to supply water to low rise buildings, but for taller buildings booster pumps are needed to enable domestic water flow to upper levels. For fun, let’s calculate the height that could be reached by city pressure of 275kPa (40psi)!
For a column of liquid, pressure (P) equals height (h) times density (rho) times acceleration due to gravity (g). Here’s some formulas and numbers:
Generally, a storey is 4.3m so water pressure of 275kPa at ground level can just reach 6.5 storeys. Anything above that, such as PMCRT, needs a pressure boost. This can be achieved by adding pumps to the incoming water supply. Of course, pumps require energy to run and different configurations can use much more energy than others. Here is another example of a booster pump replacement project by my colleague Songyang.
Previous Booster Pumps at PMCRT
Here are some beautiful photos of the old booster pump system at PMCRT. It consisted of three pumps (40HP, 40HP, 20HP) that could turn on and off based on water demand. The ability to stage the three pumps on and off suggests some potential for energy savings, but it turns out that the system was quite oversized for the actual demands of the site. Typically only one 40HP pump would run continuously, with one other pump being turned on during peak demand times.
The electricity consumption of the system was metered for four days and it was found that the system used an average of 30kW, which correlates to one 40HP pump running continuously. This measurement was done in November, so consumption could be higher in the summer due to cooling tower water consumption at roof level.
Note: power meter readings are negative due to orientation of the current transducers
New Booster Pump System
The new system, pictured below, looks somewhat similar to the old system, but it does have two major energy advantages. First, the system has been properly sized – the new system still contains three pumps, but they are 15HP each so that they can actually accommodate lower load operation more efficiently. The second major improvement is that the system includes variable frequency drives. With the old system, the pump ran full blast regardless of whether the pressure was higher than the setpoint. With the new system, the pumps ramp up and down to meet the pressure needs. Due to an exponential relationship between pump speed and power consumption, 80% pump speed correlates to 50% power consumption. At the time I took the photos, the new system was running at only 3.6kW, about 90% less than the old one. In addition to the energy benefits of the system, the facilities team will also benefit from the following:
- more stable operation – pumps ramp up and down continuously instead of banging on and off
- better information – BAS connection allows the building operators to see the status of the system, including power consumption in real time, allowing for proactive maintenance rather than reactive
- equipment renewal, reducing risk
Congratulations to the Black and McDonald facilities operations team on a successful project!