Energy saving with VFD motor control

The building I administer is quite ancient, with a central heating / cooling system.
*** note: the motors are rated 3.7kW, but were consuming around 4.5kW ***

Heating:
A thermostatically controlled On/Off keroseneboiler heats water in the tank. Pump#1 on a3.7kW 3phase induction motor pumps that hot water through the whole building.

Cooling:
A thermostatically controlled On/Off chiller cools the water.Same loop as hot, butpump#2 on a 3.7kW motor pumps the cold water. Pump[size=small]#3pumps the hot[/size]waste water to the cooling tower, where a fan (1.5kW motor) cools the warm water down.

Old photos, but looks like this. The 3x 3.7kW pumps:



The cooling tower with the 1.5kW fan:





The original architect/engineer apparently designedthe system with the capacity tohandle the worst casescenario, addeda super wide margin, and then doubled that again, just in case. Thus the motors aretotally oversized and was wasting a _lot_ of energy circulating water like crazy. Solution: VFD (Variable Frequency Drive) with some temperature sensors to reduce the motor speed to just what is needed.

The sensors only cost a dollar or so:



And I've simply attached them to the pipes with some thermal grease. Added some insulation on top, so that I'm measuring the pipe(water) temperature, and not the air temperature.



Again, old photo. My first plan was to use thermostat modules (3 in thetop left corner, https://www.aliexpress.com/item/32845952005.html) to switch between low rpm/ high rpm. And that certainly worked. Later I figured out how to use the PID controller on theVFDs (3 big units in the bottom right) to dynamically adjust the speed, which works much better and saves more energy.
https://en.wikipedia.org/wiki/PID_controller - used in cruise control, robotics, etc to dynamically adjust the motor output to counter changes in the environment.






How does it work now?
Heating:
The boiler thermostat keeps the water between60C~65Cdegrees. I've attached the temperature sensor on the IN port of the boiler, thus measuring the luke warm water returning from the building. The VFD tries to keep that temperature at 45C. If it drops below 45C, it speeds up the pump to get more hot water to the building. If it goes above 45C, it slows down the pump.
On warmer days, pump#1 runs at a leisurely 300W (the minimum allowed).Even in thecoldest of days at just around1.3kW.

Cooling:
Very similar, except the chilling unitkeeps the waterat 5C~10C. Pump#3 (waste heat water) runs at between 300W~2.1kW, depending on whether the chiller is in standby or actively working. The cooling tower fan runs at between 200W~1.2kW, depending also on the outside temperature and humidity.


The savings are considerable, especially in the shoulder seasons when the pumps are semi-idling most of the time. Electricity costs went down very noticeably,recouping the cost of the VFDs in just a few months.
And the users in the building are none the wiser - all the cooling/heating works just as well as before.

Very Cool!

A few more details:

Induction 3phase motors do not like running at very low speeds (they overheat), so they are kept spinning at a leisurely rate, even when not really required. Plus there has to be a certain minimum flow to see if more heat/cold is needed in the building.

I've also installed some smaller A/C units that workindependently of the main unit. They are programmed to turn on several hours before the main unit, thus pre-cooling / pre-warming the building.
[size=small][size=small]Our demand charge[/size][/size][size=small][size=small]is determined[/size][/size][size=small][size=small]by the[/size][/size][size=small][size=small]peak kWh (30 min brackets[/size][/size][size=small][size=small])[/size][/size][size=small][size=small]of the last 12 months. Generally, that peak was[/size][/size][size=small][size=small]in the morning of[/size][/size][size=small][size=small]a[/size][/size][size=small][size=small]summer day right after a long weekend, as the building has soaked upthe[/size][/size][size=small][size=small]heat and the[/size][/size][size=small][size=small]cooling[/size][/size][size=small][size=small]system would run[/size][/size][size=small][size=small]full power for more than 30 mins, in addition to everyone turning on their PCs, etc. With the pre-cooling andsavings on the circulation pump and fan motor, I've managed to cut down on the demand charge significantly.[/size][/size]

I believe the VFDs are Mitsubishi FR-D720

Next step would be to set your forward temperature depending on outside temperature.

I've also installed thermostats in each room, so that there is no need to regulate the hot/cold water temperature.

And circulator fans to shove cold air near the floor up to the ceiling. Before, some spots had 16C cold floor and 30C hot ceiling right above it.

Both have lead to reduced fuel&electricity use.

The purpose of controlling/reducing forward temperature, is to mitigate heat losses in the pipes. Not to control room temperature. A lot of thermal energy is wasted pumping 65 C water through pipes if 40 C could do the job. Since you said its an old building, I assumed pipe insulation could be better. Also depending on the lengths of the whole piping. But you can easily safe kilowatts by reducing the temperature.

For some boilers though you need to make sure they stay on temperature or you will get condensing issues. But that can be prevented with a mixing valve.

Right... but that would require a separate controller for both the boiler and VFD. And some rooms on the shade side are unexplainably chilly even when ambient temps are up... might need a lot of fine tuning.
It's an old building, but they didn't skimp on pipe sizing or insulation. Besides, all pipes run through the core of the building, so no heat is wasted - just ends up warming the walls and floors.