- Accuracy of savings estimates.
- Better savings opportunities.
Accuracy of Savings Estimates
A favourite energy conservation measure (ECM) of energy engineers is slowing down fans and pumps. There is something called the Cube Law that states the energy requirements of a fan or pump is proportional to its speed cubed. In other words, if you reduce a fan speed by 10%, you only need (0.90)^3 of the energy or 73% of the energy and you save 27% of the energy. If you reduce a fan speed by 20%, you achieve almost a 50% energy savings. You can see why variable speed (frequency) drives (VFDs) are so popular.
But wait. This only works when the system remains unchanged. Most systems such as variable air volume systems, hydronic heating loops, and domestic water booster systems do not remain static. Reducing the fan or pump speed will cause the system to react by opening dampers or valves. The system must be taken into account when calculating savings. Simply using the Cube Law will greatly overestimate the savings.
Another favourite ECM is installing condensing boilers. A typical non-condensing boiler is rated at around 82%, but may be lower for an atmospheric boiler. A condensing boiler, however, is between 92% and 95% efficient…a huge improvement.
But again, if we don’t look at the whole system and how it is controlled, you won’t achieve the calculated savings. Many buildings are designed to have 180F heating water on the coldest day, with the temperature “reset” or adjusted based on outdoor air temperature. But at 180F, a condensing and a non-condensing boiler have almost identical efficiencies. You only start gaining efficiencies from condensing when the return water temperature is below 140F. Depending on the reset curve, a condensing boiler would only start condensing when the outdoor air temperature is above freezing.
Better Savings Opportunities
Don’t despair, achieving higher savings just takes a little digging. You have to understand the entire system, not just the equipment.
If you put a VFD on a fan, it doesn’t make sense to simply control the fan speed based on duct pressure (typical). Instead, monitor the dampers (VAV boxes) and reduce the fan speed until some of the boxes are wide open. In doing so, you’ll be further reducing fan speed, duct pressure, and associated losses. You will also be reducing duct leakage.
If you install condensing boilers, make sure that the return water loop is as low as possible. If control valves are all less than 80% open, reduce the supply water temperature until valves are 100% open. The boilers will then be able to operate in condensing mode for much more of the year.