C[O.sub.2] ice rink.
The efficiency of transcritical C[O.sub.2] compressors varies tremendously. The compressor efficiency is most impacted by the discharge pressure and the outlet temperature of the gas cooler. Based on best case winter conditions with no heat reclaim, a C[O.sub.2] transcritical compressor requires 1.3 hp/ton, which is less efficient than an ammonia compressor in an identical application.
To recover heat for a hot water application, the energy consumption increases 55%. Moreover, the amount of useful heat to be recovered from a transcritical C[O.sub.2] system is surprisingly low. Even if the heat recovery exchangers were 100% efficient with a 0[degrees]F approach temperature, less than 70% of the waste heat is usable to heat the building. Of this waste heat, only 20% is available for hot water heating.
If the refrigeration system for the C[O.sub.2] rink does in fact consume 25% less energy that a refrigeration system of a "comparable" ammonia system, the difference must be entirely due to external factors such as dehumidification, insulation, solar loads, and control systems.
Dave Malinauskas, Member
The Author Responds
My answer is simple. We have to see the ice rink refrigeration system as a complete system, not look only at the compressors.
The efficiency of the compressors working with C[O.sub.2] varies a lot with ambient temperature (sub or transcritical cycle), but we can easily prove that the compressors have an annual efficiency equal to or slightly better than ammonia. The main reason is that a C[O.sub.2] direct expansion system works with a higher evaporating temperature than any brine system.
The part-load efficiency: a C[O.sub.2] direct expansion system has a multi-compressor design (piston compressors) and no unloader is used. The control computer activates the right number of compressors needed to match the load, and the smaller compressor is 15 hp. The result is a constant efficiency all of the time. Ammonia-brine systems use large capacity compressors (mainly screw) with sliding valves that affect a lot the efficiency at part load. A rink with 80 hours of occupancy per week will be at part load 53% of the time.
Pump power: A C[O.sub.2] direct expansion system will use a 3 kW liquid C[O.sub.2] pump to feed the concrete slab, with VFD to modulate the flow according to the load. An ammonia-brine system will have 20 to 25 kW brine pumps, and many times this pump is not equipped with VFD and runs all the time.
Hot water pump: A C[O.sub.2] direct expansion system will use a 1/6 hp water pump with variable speed to maintain a constant water outlet temperature of 175[degrees]F and will provide all hot water needs for the facility. An ammonia-brine system will have a 5 hp water pump with continuous operation, and this is not enough to provide the entire hot water needs for the facility.
The C[O.sub.2] direct expansion system will beat any conventional brine system in our climate (Canada and northern USA).
Luc Simard, Ing., Associate Member ASHRAE, Les Coteaux, QC, Canada