Preview Script - Full Script available for subscribers!
In this lesson, we're going to cover geothermal chillers, often referred to as central geothermal.

They differ from our other heat pumps because they support any system type. As you can see in this diagram, we're going to be modeling VAV.

Instead of having heat pumps in each of the rooms or zones, there is a centrally located plant.

We're going to save this file as going to call it GeoChiller1.

Here, per our diagram, we can change this to a variable volume VAV reheat system.

When we change the system, we'll apply that and we want to set the the fan types because a VAV system is not very useful if we have a centrifugal constant volume fan as we have here.

Look at assign rooms to systems, all of the assignments should remain the same and the zoning should remain the same as well.

Here, when we make a zone on a VAV system, just as a reminder, this makes a single VAV box.

That takes us to plants. The best way to model a Geothermal Chiller, is seen here.

The default in this screen is bidirectional cascade setup.

There's much more to learn about bidirectional cascade, but that would fall into HVAC theory that we do not need to cover in this lesson.

..

We'll look at the cooling equipment and we can see under our equipment tag that we have Chiller 1 and Chiller 2, and they happen to be identical.

And they have identical thermal storage, identical cooling towers.

If we look at the controls and then the controls again, we can see that it defaults to the IGSHPA and 100% flow rate with the variable volume pump.

It does enter a pump head here where you would probably want to enter the horsepower or the kilowatt draw.

We can make a note that there is also a secondary distribution pump and you'll want to make sure that you have your pumps documented.

As we mentioned before, this secondary distribution pump is going to operate whether Chiller 1 is on, whether Chiller 1 and 2 are both running.

We do have a heat exchanger in the diagram, but we can assume that it's a perfect heat exchanger and that the approach is zero degrees, at least for this theoretical scenario.

So that's modeling a Geothermal Chiller.

However, the details that go into it are not very simple.

If we look at the default Geothermal Chiller heater, the options are entered at typical numbers.

This is where things get complicated. The unloading curves are very detailed for these Chiller heaters.

Thus, if you are making a custom one, this takes a lot of work to make an unloading curve like these, but it's necessary to get the accuracy of a Chiller heater.

Thus, if you do not have a bidirectional cascade Chiller heater system, you can still use this Geothermal Chiller heater, but we'll have to set this up slightly differently.

And you may want to change some of the operating points, such as the minimum operating temperature.

Let's close out of this and back into our scenario.

What we can do here is we can go to the first Chiller and we can make it 100% of the plant capacity.

We can make the heat recovery prorated based on that and that will be 80% of the plant capacity.

And then what we'll do is we'll just delete this Chiller too.

One thing you want to remember in the equipment tag, if you bring something in from the Chiller wizard and there's a percentage in the tag name,

..

Of course, the sequencing would be single since there's only a single Chiller.

Notice it defaults to reject the condenser heat to the ground loop, which is typical of any Geothermal system.

And then for the Geothermal Chiller, it will reject the heating plant too, which was created through the plant wizard.

We'll get to some of the details of modeling specific boiler plants in a moment.

If we have a single Chiller, we want to go to the controls button again.

..

Then ensure the flow scheme is no longer bidirectional cascade but fully mixed since we have a single piece of equipment as opposed to two Chillers operating in bidirectional cascade mode.

And here we have our other option for a Geothermal Chiller that's not modeled in the bidirectional cascade default.

..

And for the heating system, we can assign all of these to one heating plant.

However, in the diagram, we did not indicate that the reheat was hydronic, but we will assume hydronic reheat.

If the reheat coil was in fact electric, we would need to create another electric plant and then assign this coil separately.

You can see that we can split up the main heating / reheat coil from the preheat coil, which is the central coil.

If you ever have confusion on that, make sure you go to create systems and the schematic to see how the coils are labeled.

In reality, you may only have one boiler plant, and here we have this default boiler.

And then you go to set up a domestic hot water load.

I have some custom libraries that you may not have that you may have noticed.

And you want to assign this to the heating plant, which is the boiler.

So for instance, if you have hot water that is met by the boiler system, it seems natural to assign this here.

However, this is going to be met by the geothermal chiller.

If in fact your domestic hot water is going to be met by a boiler independently of the geothermal chiller,

..

And now we have two boilers in our model, even if in reality we only had one boiler.

The purpose here is so that boiler 2 operates as an actual boiler, unlinked to the geothermal chiller.

..

If it's going to run from a boiler only, we want to create that boiler in a separate plant and assign the domestic hot water load to this fictitious plant that runs at the same efficiency as our actual boiler plant.

..

And now we have a geothermal chiller with a backup boiler modeled and a boiler meeting the domestic hot water load.