Peak Demand Management: Why Doesn't It Work?
Peak Demand Management
Just about every specification has a default clause to provide Peak Demand Management, but it rarely gets properly implemented. Possibly, because people don't fully understand the significant value it provides or because poor design ends up with it disabled.
WHY ISN'T PEAK DEMAND MANAGEMENT IMPLEMENTED?
In my opinion, Peak Demand Management is not implemented for two reasons:
- Peak Demand Management is often incorrectly associated to only managing the electrical load on the generators during a power failure and not as an energy efficiency strategy.
- Its perceived as too disruptive and impractical. Traditionally, the strategy switches off some mechanical equipment as we approach peak demand, for example, disable the 3rd chiller or switch off AHU's 6, 7 & 8.
HOW SHOULD PEAK DEMAND MANAGEMENT BE DESIGNED?
My general philosophy for energy efficiency is to make minor control strategy changes to the terminal units (FCU's, VAV's, PAC's etc). You would hardly notice these improvements, but multiply each tiny improvement by 500 VAV's, and it starts adding up. But more importantly, improvements at the end of the mechanical circuit feed back up the line and naturally result in improvements at the main AHU's , which in turn result in the chillers and boilers running more efficiently.
The same applies here, rather that switching off our primary equipment and causing major issues, lets start our strategy at the terminal units.
As the outside air temperature increases, for example, from 32°C towards 40°C, proportionally raise the entire buildings cooling temperature set point from 23°C to 24°C. This will result in the terminal units backing off, the AHU supply and return fans slowing down and reducing the demand on the chillers.
Another great function of this strategy is that if you have critical areas like executive boardrooms, operating theatres or communication rooms, you can have a button on that FCU/AHU graphic to disable the global temperature reset. This provides flexibility, and a tune-able control strategy, compared to simply switching off equipment or limiting the capacity of the chillers.
WHY DOESN'T THIS SIMPLE STRATEGY WORK?
If you implement the above strategy to control Peak Demand Management, it will get disabled within a year, because of temperature complaints.
The issue is that most terminal unit heating and cooling temperature control loops share a common set point: 22.5°C. If you raise the cooling set point from 23°C to 24°C (via the common set point), then you are also raising the heating set point, from 22°C to 23°C. Believe it or not, on a very hot day, when you are in full cooling, there will be less occupied internal areas that will over cool, and raising the heating set points will bring on heating. Building owners don't like it when you start burning gas on a 35°C day.
Typically, the suggested workaround is then not to raise the common set point but to widen the heating and cooling dead bands. The issue now is that as you widen the dead band by a further 1°C, the cooling set point increases from 23°C to 24°C, but the heating set point decreases from 22°C to 21°C. This solves the issue of not bringing on heating, however, in low occupancy internal areas or small meeting rooms, the temperature could then drop below 21°C.
WHAT IS THE SOLUTION?
Quite simply, we need separate heating and cooling set points. My experience is that people will tolerate higher space temperature on very hot days, but they will not tolerate a space temperature below 21°C, no matter how cold it is outside. With separate heating and cooling set points we can fix the heating set point at 22°C and only raise the cooling set point from 23°C to 24°C.
WHY DON'T WE HAVE THIS ALREADY? IT'S NOT ROCKET SCIENCE
The concept is quite simple, but the issue is that changing a terminal unit temperature control loop from a single common temperature set point to separate heating and cooling temperature set points, requires a download of the field controller. Now, if you have 20 controllers per floor and 30 floors, it's going to take a considerable amount of time to make this change, which will be considered as additional costs to the service contract.
Check out this blog, where I give you a few ideas of how to free up service time and get mini projects implemented for free.
THE ADVANTAGES OF PEAK DEMAND MANAGEMENT
- As the outside air temperature increases towards 40°C, we can proportionally raise the entire buildings temperature set point (start with only 1°C). This will save energy, reduce utility bills and improve your NABERS rating.
- It helps keep power usage below the peak demand threshold, which keeps our kWh cost rate down.
- It can assist in generator electrical load control, as the generators are approaching maximum power during a power failure.
- Reducing your electrical power demand on the grid, when the electricity supply authority instructs your building or the whole city to start load shedding. This is going to happen, it's just a matter of when.
- We should consider something similar for Peak Demand Management of gas usage. I don't think it is a big issue right now, but the cost of gas could overtake electricity in the future?
REDUCED COST PER KWH RATE
In Australia, the c/kWh rate is determined by the buildings peak electrical demand. If you can keep your peak demand down, then you can apply to have your kWh rate reduced. There are probably situations where buildings have reduced their peak demand by normal energy efficiency improvements, but are still paying the same kWh rate because they haven't applied for it to be reevaluated by the supplier. There is a potential utility bill cost reduction and all it takes is a little administrative time.