Ammonia vs CO2 vs Glycol: Choosing a Refrigeration System for Your Cold Storage Facility

Ammonia vs CO2 vs Glycol: Choosing a Refrigeration System for Your Cold Storage Facility

Lead paragraph:

Choosing the right refrigeration system is the most consequential decision after temperature spec on a cold storage project. The system you specify affects upfront capital cost, operating cost, regulatory burden, footprint, serviceability, and product safety risk for the entire 30+ year life of the facility. The wrong choice can add millions in operating cost over the building's life or lock you into specialty service contracts that limit operational flexibility.

This guide compares the five refrigeration system architectures used in modern cold storage construction — ammonia, CO2, glycol secondary loop, synthetic refrigerant, and cascade — across the variables that matter for buyer decisions.

The Five Refrigeration System Options

There is no universal best system. The right choice depends on facility size, operating temperature, regulatory environment, operations team capability, and long-term cost priorities. Below is the working framework for matching system to application.

Ammonia (NH3) — The Industrial Standard

Ammonia has been the dominant industrial refrigerant for over a century. It remains the most cost-efficient and energy-efficient option for large cold storage facilities. Industrial ammonia systems handle the thermodynamic heavy lifting of large-scale frozen storage, food processing, and multi-zone distribution facilities better than any alternative.

Why ammonia wins on operating cost: Ammonia has the highest coefficient of performance (COP) of any commercial refrigerant. It moves more heat per unit of compressor work, which means lower energy consumption per cubic foot of cold space. Over a 30-year facility life, this efficiency advantage can add up to millions of dollars compared to alternatives.

Why ammonia is more expensive upfront: Ammonia systems require Process Safety Management (PSM) compliance under EPA regulations when refrigerant charge exceeds 10,000 pounds. PSM adds documentation, training, written procedures, mechanical integrity programs, and audit requirements. Mechanical room construction is more rigorous — leak detection, ventilation, isolation valves. Ammonia-rated piping and components cost more than standard refrigeration components.

Why operations matters: Ammonia is toxic at concentrations above 25 ppm. A leak isn't just an equipment failure — it's a safety incident that requires evacuation procedures and emergency response protocols. Operating an ammonia facility requires trained refrigeration operators, not just facility maintenance staff. Many large cold storage operators have in-house refrigeration teams. Smaller operators contract this out.

When ammonia is the right choice:

• Facility size 100,000+ SF with high refrigeration load
• Long-term operations (30+ year hold strategy)
• Operations team with refrigeration expertise (or willingness to contract specialists)
• Facility located outside dense residential areas
• Long-term operating cost is the priority

When ammonia isn't the right choice:

• Facility under 30,000 SF where PSM costs exceed energy savings
• Dense urban locations where ammonia release would be high-risk
• Operations team without refrigeration capability
• Short-term hold strategy where operating cost amortization is limited

CO2 Transcritical — The Growing Alternative

CO2 transcritical refrigeration has gained significant market share in cold storage construction over the past decade, particularly in supermarket cold rooms, mid-scale distribution centers, and facilities subject to ammonia restrictions.

The advantages: CO2 is non-toxic, non-flammable, naturally occurring, and has zero ozone depletion potential. It's not subject to phase-down regulations like synthetic refrigerants, so investment in CO2 infrastructure today won't become regulatory legacy in 10 years. Operating costs are competitive with ammonia in moderate climates.

The challenges: CO2 systems operate at very high pressures — sometimes 10 times higher than ammonia or HFO systems. This requires specialized piping, components, and pressure relief systems. Service technicians need specialized training. Equipment availability is more limited than ammonia or synthetic alternatives. In hot climates, CO2 transcritical efficiency drops significantly because CO2 transitions through its critical point at ambient temperatures around 87°F. This means Houston or Phoenix cold storage facilities running CO2 transcritical may need parallel compression or other efficiency strategies that add capital cost.

When CO2 is the right choice:

• Mid-scale facility (30,000 to 150,000 SF)
• Moderate climate (Pacific Northwest, Northeast, Mountain West, Midwest)
• Long-term refrigerant strategy with regulatory durability priority
• Facility in dense urban or suburban location where ammonia is restricted
• Owner with sustainability or ESG reporting requirements

When CO2 isn't the right choice:

• Hot climates without parallel compression strategies (Texas, Florida, Arizona, Southern California)
• Very large facilities (200,000+ SF) where ammonia operating cost advantages compound
• Facilities requiring sub-zero temperatures below -40°F (cascade is typically better)
• Small facilities where simpler synthetic systems are operationally easier

Glycol Secondary Loop — The Multi-Zone Solution

Glycol secondary loop systems use ammonia (or occasionally CO2) as the primary refrigerant in a contained mechanical room, with glycol as the secondary fluid distributing cold throughout the building. The glycol loop circulates to evaporators in each cold zone, eliminating refrigerant from the production spaces.

Why glycol secondary makes sense: The architectural separation between primary refrigerant (in the mechanical room) and the production spaces solves several problems simultaneously. Ammonia stays contained in a properly engineered mechanical room with leak detection and ventilation. The cold spaces have only glycol piping — non-toxic, food-grade, no PSM trigger. Multi-zone facilities benefit because one ammonia plant can serve many independent temperature zones through different glycol loops at different setpoints.

The cost tradeoff: Glycol secondary adds significant capital cost. The glycol pumps, heat exchangers, and additional piping infrastructure run 8 to 15 percent more than direct expansion ammonia in the same building. Operating efficiency is also slightly lower because of the additional thermodynamic step (ammonia cooling glycol cooling air).

When glycol secondary is the right choice:

• Multi-zone facilities with 3+ independent temperature zones
• Pharmaceutical cold storage requiring zero refrigerant exposure to product spaces
• Food processing facilities with washdown environments where refrigerant safety is critical
• Facilities in regulatory environments that limit refrigerant in production spaces
• Multi-tenant facilities where individual tenants want independent temperature control

When glycol secondary isn't the right choice:

• Single-temperature facility where simpler direct expansion is cost-efficient
• Cost-sensitive projects where the 8 to 15 percent capital premium is prohibitive
• Very small facilities where the additional complexity isn't justified

Synthetic Refrigerants (HFO/HFC) — The Simpler Option

Synthetic refrigerants like R-448A, R-449A, R-407F, and emerging HFO blends are the simplest cold storage refrigeration option to install and service. They're widely available, service technicians are trained on them everywhere, and they don't trigger PSM requirements at typical facility scales.

The simplicity advantages:

• Service available from any commercial refrigeration company
• No PSM documentation requirements
• No specialized training for facility staff
• Equipment widely available with shorter lead times
• Lower upfront capital cost than ammonia or CO2

The phase-down problem: HFC refrigerants are subject to phase-down under the AIM Act in the US and similar regulations globally. R-404A is already significantly restricted. R-448A and R-449A are transitional refrigerants — better than R-404A but still subject to future phase-down. HFO blends are improving but pricing volatility on these refrigerants over the past five years has been significant. Operating cost is also higher than ammonia by roughly 20 to 30 percent due to lower thermodynamic efficiency.

When synthetic refrigerants are the right choice:

• Smaller facilities (under 30,000 SF) where ammonia PSM overhead is disproportionate
• Retrofit projects where existing infrastructure is HFC and replacement is constrained
• Operations teams without refrigeration expertise
• Short-term hold strategies (5 to 10 years) where operating cost amortization is limited
• Specialty applications with lower refrigeration loads

When synthetic refrigerants aren't the right choice:

• Long-term facility ownership where 30-year operating cost matters
• Large facilities where ammonia efficiency advantages compound
• Facilities subject to ESG or sustainability reporting requirements
• Markets with refrigerant phase-down regulatory pressure

Cascade and Hybrid Systems — The Sub-Zero Solution

Cascade refrigeration uses two refrigerant cycles operating in series — one cycle handles the high-temperature side (typically ammonia or CO2), the other cycle handles the low-temperature side (typically CO2 or specialty refrigerants). The two cycles meet at a heat exchanger where the high-side cools the low-side.

Why cascade is required for sub-zero: Single-cycle refrigeration has thermodynamic limits. Going from ambient to -40°F or colder in a single cycle is technically possible but extremely inefficient and creates equipment durability issues. Cascade systems split the work — the high-side cycle handles the larger temperature drop from ambient to roughly -10°F, then the low-side cycle handles -10°F to -40°F or colder.

The complexity tradeoff: Two complete refrigeration systems mean roughly 40 to 60 percent higher capital cost than equivalent single-cycle systems. Two compressor systems, two refrigerant charges, two control systems, two service contracts. Operating cost is competitive with single-cycle alternatives because the cascade efficiency at sub-zero temperatures is dramatically better.

When cascade is the right choice:

• Blast freezer applications below -20°F
• Pharmaceutical ultra-low temperature suites (-70°C and below)
• IQF (individual quick freezing) protein and seafood processing
• Facilities with multiple temperature programs requiring deep freeze capability

When cascade isn't the right choice:

• Facilities operating above -10°F where single-cycle is more cost-effective
• Cost-sensitive projects where the capital premium is prohibitive
• Operations teams without specialized refrigeration capability

System Sizing Benchmarks

Refrigeration system size is measured in BTU per hour or in tons of refrigeration (1 ton = 12,000 BTU/hr). Starting-point benchmarks for cold storage facilities:

These are starting-point estimates. Real engineering scope requires accounting for dock door cycles, pallet positions, product warmth at receiving, ambient design conditions, and facility usage patterns.

Under-sizing is the #1 cause of cold storage facility failures. A facility refrigeration system sized at 80 BTU/h per SF for a frozen storage that actually demands 120 BTU/h per SF will run continuously, never recover from door-open events, develop ice buildup, and progressively fail.

Decision Framework — How to Choose

Run through these questions in order:

1. What's your operating temperature?

• Above 32°F: any system works; choose on cost and operations
• 0°F to 32°F: ammonia, CO2, or glycol secondary
• −10°F to 0°F: ammonia, glycol secondary, or cascade
• Below −20°F: cascade is typically required

2. What's your facility size?

• Under 30,000 SF: synthetic or CO2 typically wins
• 30,000 to 100,000 SF: CO2, glycol secondary, or ammonia all viable
• 100,000+ SF: ammonia operating cost advantages compound

3. How many independent temperature zones?

• Single zone: direct expansion ammonia, CO2, or synthetic
• 2 zones: direct expansion with separate evaporators
• 3+ zones: glycol secondary loop almost always wins

4. What's your operations team capability?

• In-house refrigeration team: any system viable
• Standard facility maintenance only: synthetic or fully-managed CO2
• Mixed capability: glycol secondary isolates complexity to mechanical room

5. What's your hold strategy?

• Build to sell within 5 years: capital cost is paramount, synthetic or simple CO2
• Long-term hold (10+ years): operating cost compounds, ammonia or CO2 transcritical
• Build for legacy (30+ years): ammonia for industrial scale, glycol secondary for multi-zone

6. What are your regulatory and ESG requirements?

• Standard commercial: any system
• ESG / sustainability reporting: avoid HFC, prefer ammonia or CO2
• Pharmaceutical GMP: glycol secondary for refrigerant isolation
• High seismic or wind zones: equipment selection affects structural design

Don't Choose the Refrigeration System Alone

The refrigeration system is one of the most consequential decisions on a cold storage project, and the decision is best made by a team that includes the cold storage GC, the refrigeration engineer, the facility operations leadership, and the owner's financial planner. The capital and operating cost tradeoffs need to be modeled together over the building's expected hold period.

We work with refrigeration partners like FrigoSys, Mecalux, and others to engineer the right system for each project. The system we recommend is the one that wins on total cost of ownership for your specific operating profile — not the system we install most often.

[Talk to us about refrigeration system selection →]

Frequently Asked Questions

Is ammonia or CO2 better for cold storage?

Ammonia wins on operating cost and efficiency for large facilities (100,000+ SF) with experienced operations teams. CO2 transcritical wins for mid-scale facilities (30,000 to 150,000 SF) in moderate climates, dense urban locations where ammonia is restricted, and facilities with ESG or sustainability priorities. There is no universal answer — the right choice depends on facility size, climate, operations capability, and long-term cost priorities.

What is a glycol secondary loop?

A glycol secondary loop uses ammonia (or CO2) as the primary refrigerant in a contained mechanical room, with food-grade glycol as the secondary fluid circulating to evaporators throughout the building. This isolates ammonia to the mechanical room, eliminates refrigerant exposure in production spaces, and enables one refrigeration plant to serve multiple independent temperature zones. Common in multi-zone facilities, pharma cold storage, and food processing.

Are ammonia refrigeration systems safe?

Ammonia is toxic at concentrations above 25 ppm and requires Process Safety Management (PSM) compliance when refrigerant charge exceeds 10,000 pounds. Properly designed ammonia systems with leak detection, mechanical room ventilation, and trained operators have an excellent safety record across thousands of US cold storage facilities. The safety risk is in poorly maintained systems or facilities without trained operations staff.

Why is cascade refrigeration used for blast freezers?

Single-cycle refrigeration has thermodynamic limits at sub-zero temperatures. Reaching -40°F or colder in a single cycle is technically possible but extremely inefficient and creates equipment durability issues. Cascade systems split the work between two refrigeration cycles operating in series — one handles ambient to roughly -10°F, the other handles -10°F to -40°F or colder. This dramatically improves efficiency and equipment longevity at deep freeze temperatures.

What's the cheapest cold storage refrigeration option?

Synthetic refrigerants (HFO/HFC) have the lowest upfront capital cost and don't trigger PSM compliance requirements. However, they have the highest operating cost — roughly 20 to 30 percent higher than ammonia per unit of cooling delivered. They're also subject to phase-down regulations under the AIM Act, creating long-term legacy risk. For small facilities (under 30,000 SF) with short hold strategies, synthetic is typically the best choice. For larger or longer-held facilities, ammonia or CO2 wins on total cost of ownership.

Internal links to add

• /refrigeration-facility-construction (main refrigeration service page)
• /frozen-storage-construction (when discussing frozen applications)
• /industries/pharma-biotech-cold-storage (when discussing pharma/glycol secondary)
• /industries/food-beverage-cold-storage (when discussing food processing)
• /resources/cold-storage-construction-cost-per-square-foot (Article 1)
• /resources/how-long-cold-storage-construction-takes (Article 2)
• Cost Guide download CTA mid-article

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• Hero: industrial ammonia compressor in mechanical room
• Mid: CO2 transcritical equipment
• Mid: glycol pumps and secondary loop piping
• Mid: cascade refrigeration mechanical room
• Final: refrigeration controls / monitoring station