Energy Management System

[Peter Nachtwey]

Quickly make trial temperature changes.

Try and iterative technique where you make trial temperature changes.
A function that computes the undesirable index is still required. Instend of minimizing using advanced techniques one just evaluates U in a seres of nest loops making trial temperature changes.

Start each iteration with U set to a big number.
Now evalute U with all the combinations of temperatures of each zone using the range of Temp(z)-1 to Temp(z)+1. The number of calcuations can be large if there are a lot of zones.

However, in the case of controlling temperature when one has minutes to make many guesses ( trial temperature changes ) and check to see if a new minimum is found, this may a better way. One could continually make trial changes to the temperatures in each zone. If the new undesirable index is smaller than the last then the trial temperature for that zone that is new temperature set point. This should be fast enough to adjust for changing of weighting between zones or changes in the ambient heat. With ten zones that could be done in a MicroLogix with floating point and the computer function block. With 10 zones the U function would have to be calcuated 3^10 or 59000 times for each iteration. At one millisecond per U function calcuation that would take about a mimute per update.

At least this trial and error techinique could be understood by most. I still wouldn't use Fuzzy Logic. One must have an a function that must be evaulated and minimized. In this minimization is done by case by trial and error in stead of using calculus and arrays.

 

[rsdoran]

Peter I dont know if ya noticed or not but he is using thermostats...ie on/off. No temp values/analog is used. How will you know about the temp changes, the likelihood of the zones ever obtaining thermostat setting in hot months is unlikely if not given the freedom to run till that point.

You guys are the best but sometimes I think y'all enjoy making this stuff difficult.

To me the demands and restrictions make it a lose lose situation, so do something even if wrong.

My thoughts now are go with your first thoughts..use timers and a sequence, maybe allowing the priority zones a little more run time. Dont run when there is no demand, bypass those that dont need. Sequence what has demand. You will see that doing this they will more than likely (in hot months) cycle according to your sequence. Monitor it for 30 days etc and make modifications as you see fit.

I know HVAC, chillers, boilers etc. There is no accurate way to maintain a set temperature range using timers and thermostats in a zoned system with restrictions on the energy usage, unless you are very lucky. Might as well play ***** with a blindfold. The best you can do is work with it and make the key areas (where that may be only you can know) seem ok.

Get them to install lots of fans so there is more air flow.

Wish you lots of luck.

BTW I may not be good with PLC's but do know a little about this stuff. It can get complicated depending on conditions, like Terry said. The ambient external temperature ( the outside temp, like in Southern New Mexico in August) can have a more severe affect than Seattle, WA in August. There can be many more factors, is this in a plant? How high is the ceiling? What is the footage compared to the BTU/tonnage of the air conditioners? Are ovens used? Heat sealers? The list may go on and on.

The limitations imposed on you are mind boggling to me ( I have so little imagination). The concept of having X number of compressors but only allowed to use 50% at any time but its required to maintain a preset temperature ( level of comfort) is prepostereous.

Now that I think on it, forget about all the above. Make your boss and the real boss comfortable...dont sweat the rest. That is, dont allow yourself to sweat if ya get my drift.

 

[Allen Nelson]

Sorry about my previous posts. New shows more programming experience than most newbies we get here. And the last time I did this, I got analog inputs - so I assumed that this problem was the same (Why? - it was Friday night, ok?)

Don't know if this will help, but here's an idea. All things being equal (which they won't be - occupancy, humidity, thermostat settings will all change the theory, but assuming), then each zone will run for a certain time between the time the thermostat says "Please" to when it say's "Thank You". That time will vary per zone, but each zone should have it's own time constant. When demand exceeds capacity, make every zone suffer equally. Add up the time contants, multiply it by the MaxLoad divided by the sum of the demanded loads, and run each in turn for that smaller time.

Or maybe just do as Ron says - don't sweat it, as long as the guy who can fire you doesn't sweat.

Good luck.

 

[Orn Jonsson]

Energy Management

Your main concern here would be to minimize current draw on startup.
Companies are billed for electricity usage based on PEAK usage for any particular month.

This means that starting two 50Amp compressors at the same time is not a good idea since there will be an inrush of current on start up that may far exceed normal load ratings.

Keep this in mind when you sequence the starting pattern. Another thing to check is compressor capacity (nameplate rating). Find out if any of these 11 or so compressors are over-rated/under-rated for their load. Then you should be able to find out average run-time for each unit - do weighted avearage calculations and nail down your peak running load (not starting load). This will tell you what your Max load will be, just remember to sequence the motor starters so a whole bunch won't come on at the same time.

my two cents......

OJ
(Iceman)

 

[New2PLCs]

Orn,

So you're telling me the compressors draw more when they start up. That completely makes sense, but I hadn't thought about it. I was sort of just given constant values for what I guess was the maximum amperage the compressor could draw. I guess, due to many factors, this could vary from the "running" load of the compressor substantially. Do you think it will differ enough (in an "average" situation) for me to want to take it into account?

Thanks,

-Don

 

[New2PLCs]

Terry,

I'm waiting hopefully to see if you have further comments on this. In the meantime, here's what I've got in mind.

I'm thinking of setting up each of the compressors with a starting priority, 1-11, with 11 being the most important. I've got 1 timer per compressor. When a thermostat calls for cooling AND the compressor kicks on successfully, I'm going to use that timer to lock that compressor in for a predesignated minimum amount of time. As long as the thermostat is still calling, it gets to run for X minutes. Now, if the thermostat is calling for cooling but the compressor can't come on due to load restrictions, I'm going to use the timer to start counting 5-minute (arbitrary number) intervals. For every 5-minute interval, I'm going to take the compressor's starting priority, add 2 (arbitrary number), and store it as an overall priority for that compressor. When any compressor's overall priority gets high enough, it's allowed to shut off any compressors for which the overall priority (when it last kicked on) is lower, provided that the running compressors have been allowed to run for their minimum time. The overall priority for a particular compressor gets reset to the starting priority for that compressor once it has run.

Problematic case: All thermostats calling, a big compressor has gradually achieved highest overall priority because it hasn't been allowed on for a while. Now the big compressor has to wait for as many smaller compressors to finish their minimum-time run as needed to free up enough load space for it to run. In this case, you're potentially going to be wasting load space (but only if a thermostat for one of those compressors you're waiting on to time out stops calling--if not, you just let the compressor run until it's time for the big compressor to shut them all off together.)

While my head spins when I think of trying to implement this in ladder logic, it should be possible, and I think it is about the best one can do, given the nature of the problem. I think there are enough "fudge factors" to ensure an adequate amount of "tweakability":

1.) Starting priorities for each compressor
2.) Minimum time each compressor can run
(My tactic for handling the case where a large compressor gets locked out for a longer period than desired is to just increase it's minimum time--it will eventually come on; when it does, let it run longer.)
3.) Time interval length after which you increase overall priority when a unit has been calling but not allowed to run due to load restrictions
4.) Amount that you increase overall priority per interval for which a unit has been calling but not allowed to run

Any comments, further suggestions (especially on how I'd want to implement this with ladder logic!) are welcome and appreciated.

Thanks all,

-Don

 

[rsdoran]

Don
I am not as good as these guys (by no means) when writing code, but I am fair at dealing with things like this in the real world. I know you are familiar with programming but ladder is new to you, so you are looking at what you were told and trying to lay it all out on paper beforehand to get the thoughts straight. I know I didnt state too clearly before what I meant but will attempt it again.

You are new to ladder, so make it easy on you to start out, remember that you can alaways edit (and probably will anyway) the code after you get it running. To begin with write a code that just sequences the compressors allowing equal time to all and not allowing the load rating to be over 50%.

Here are my thoughts, you arent totally familiar with the system(s) or doing ladder with a PLC, so you may need a little assistance from the electrician or hvac guy (anyone that can monitor the load ratings and temperatures in the zones.)and help you get full details on HP and Amperage draw per compressor motor.

Not being familiar with ladder make the code at the beginning just a sequence with timers.This will give you time to monitor temperatures in the different zones and the average load conditions At first dont sweat running at say %60 to %80, unless there is physical reason not to. That way everyone should be comfortable in most areas, as I said before jokingly, it is a good idea to keep the bosses comfortable. You can then make adjustments to fine tune the system, may even see a more efficient way to write the code.

You may want to see if someone has a Non Contact Thermometer to use to monitor temperatures in different places and zones.

You havent stated how in depth you have studied the hvac systems or the zones. You make want to determine the BTU's of each unit and the approximate square footage of the zones they are cooling. May want to determine the correlation to space vs btu, that may or may not help determine cycling time.

Keep it simple at first and allow yourself time to expand on it over a few weeks. Its a fact, no matter what you do in the beginning the odds are you will spend time making changes in the next few weeks.

 

[Tom Jenkins]

"So you're telling me the compressors draw more when they start up. "

Yes, but.....

Many utilities charge larger customers for "Peak Demand". This is usually a charge of $X per kW for the largest 15 minute kW usage in a month during peak usage periods. That means that running a 5 hp motor for 15 minutes during peak periods can cost as much as running a 50 hp motor for hours during off peak times. Most energy management systems include demand limiting as one strategy, eliminating or "shedding" loads when demand gets above a pre-set threshold during peak times.

The influence of motor starting on demand charges is often over estimated. It is true that inrush current on a motor is several times the full load amps. However, starting across the line, which causes maximum inrush, only has a duration of a few seconds. When this is averaged across the 15 minute demand window the impact is negligible. It is running the motor for the 15 minutes that pushes up demand charges.

If you use "soft starts" the impact of inrush amperage is eliminated.