Energy

Demand Response From an Underground Freezer: Turning the Compressors Off on Purpose

By the operator · October 27, 2026

Most of my job is keeping the compressors running. Once a year, I shut them all off on purpose.

I run an underground freezer, and we’re enrolled in a utility demand-response program. The deal, in plain terms: when the grid is stretched, the utility wants big users to drop load for a set window, and a freezer with a large ammonia plant is a big user. To prove we can do it, we run tests. Here’s what those tests look like from the engine room, and what I’d tell another operator before they sign up.

Why a freezer can do this at all

A freezer is a battery for cold. Once the product, the racks and the structure are frozen down, they don’t warm up the moment the compressors stop. Underground, that effect is bigger. The rock around the rooms sits at about 58 °F year-round, so the outside weather isn’t pushing heat in, and the frozen pillars hold cold like giant ice cubes. That stored cold is what lets you shed load for a window without the rooms running away from you.

It’s commonly claimed that underground freezers use 40 to 50% less energy than buildings on the surface. Whatever the real number is for a given site, the same stability that saves energy day to day is what makes shutting off for an hour realistic.

What we shut off, and what we kept on

This is the part I’d get clear before the first test.

The compressors come off. That’s where the load is, and it’s what the first test measured.
The fan coils in the freezer stay on. We chose to keep the evaporator fans running. That’s a real load of its own, and it means the drop you show isn’t as deep as “everything off.” But moving air keeps the rooms even while the plant is down, and I’d rather give up a little load than have warm spots in the racks.
Pumps are the next step. For the longer test, the plan added shutting off some pumps on top of the compressors. If you’re trying to hit a bigger drop, look at your pumps before you look at your fans.

Write this down as a load list before anyone from the program asks. Every motor you plan to stop, every one you plan to keep, and why. You’ll use it for every test after.

The first test: a few minutes, and a rough restart

Our first test was in February 2025, a few minutes long, first thing in the morning, just long enough for the load change to show up on the meter. Shutting down was the easy part.

The restart did not go smoothly. We learned two things that morning that weren’t on our start sheet:

The king valve has to be open before the screw compressor will run.
The boosters have to start unloaded. They were supposed to come up unloaded on their own, and they didn’t. We put the booster unloaders in hand and brought them up manually, and asked the controls contractor to fix it on their side.

We had everything back by about 9:15 that morning. That afternoon I started rewriting our startup procedure. The full restart order we use now is in Restarting a Two-Stage Ammonia Plant After a Shutdown.

That’s the real lesson of demand response for a refrigeration plant. The shutdown costs you nothing. The restart is where a test turns into a bad morning, and a short test is the cheapest way to find out your restart has holes.

The longer tests

After the first test, the next step was a full one-hour test with the deeper load list. We run that hour-long test every year. During the hour, the room temperatures didn’t noticeably move. That’s the stored cold doing its job.

One year, we had a power outage the night before the test, and the plant wasn’t fully running yet in the morning, so we asked to postpone. The program let us move it as long as it was done before the end of the month. Don’t stack a planned shutdown on top of an unplanned one. If the plant just came back from an outage, it isn’t ready to be a test subject.

Before you run your first test

Run a short test first. A few minutes is enough to see the meter move and find out whether your restart works.
Pick a cool, quiet time. Early morning, low dock traffic, no defrosts due. Every door opening during the window is heat the stored cold has to absorb.
Make a load list. What goes off, what stays on, and why. Decide about your evaporator fans before test day.
Have the restart sheet in hand, and the person who’ll run it. The restart is the risky half.
Don’t test right after an outage. Postpone. The program would rather move the date than have you fail it.
Log your room temperatures before, during and after. That log is your proof the product stayed safe, and it tells you how long a window your building can really ride through.

That last one is what our temperature log is for. If you want a starting point, get the free cold storage temperature log template through the signup box below.

Related: Why Underground Cold Storage Works, on what the rock does for a freezer, and The Checklist That Keeps Our Underground Freezer Running.

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