There are a number of ways of reducing energy requirements of cool stores.  Perhaps the most important is regularly checking the operating temperature of the room and considering whether this is the most cost-effective temperature for storage. For example, although the optimum storage temperature for many (non-chilling sensitive) products is close to zero, if they are only being stored for a short time then higher temperatures may be just as good. Monitoring the temperature will also indicate how often the system is defrosting and whether the cooling system is working effectively given external and internal heat sources.

Other possibilities include;

• Pre-cooling product before it is placed in storage reduces heat load on the equipment and permits use of a smaller refrigeration plant. It can be more energy efficient to pre-cool with forced air in one room, then transfer product to a less regularly accessed second cool room for longer term storage.
• Defrost the system regularly – ice build up on the evaporator greatly reduces efficiency. However, defrost cycles which overheat the air inside the cool store are also not desirable.
• In some areas it may be possible to focus on cooling well insulated, fully packed stores at night using off-peak power. During the day they can be fully sealed.
• Reduce fresh air exchange as much as possible – keep the door shut as much as possible, ensure door seals work well and check how air circulates through the cooling system
• Use LEDs for lighting and turn lights off when not in use
• Reflective paints applied to the outside of cool stores can reduce external heat load
• Ensure cool room insulation is intact – damp insulation is no longer insulating

Using CO₂ as a refrigerant gas can reduce energy consumption and increase cool room efficiency.

Refrigeration plant is generally a mature technology. Several refrigerants are in common use including R22 (being phased out due to its ozone depleting potential), R134a and R404a.

Although the refrigerants R134a and R404a do not significantly affect ozone depletion they have high global warming potentials (GWPs) of 1,300 and 3,260 CO₂-e respectively. From 1st July 2012 synthetic refrigerant are taxed using the current carbon price and the above GWPs.

Perhaps surprisingly, CO₂ itself has negligible greenhouse gas impact compared to synthetic refrigerants. As it has excellent heat transfer properties, smaller evaporator and condenser systems can be used to provide similar cooling to synthetic refrigerants. The drawbacks are a currently lower coefficient of performance (COP), mainly due to the less developed compressor technology, as well as its higher pressures needed which increase system cost.

CO₂ has been used as a refrigerant for decades, but demand has only recently increased due to issues with synthetic refrigerants. It is increasingly used in Europe and there are some limited examples of its use in Australia, but apparently none in the vegetable industry.

Note that the CO₂ system does not release CO₂ into the cool room, it is contained within the sealed refrigeration system.

Comparison of refrigeration costs

Growers thinking of investing in new refrigeration plant will most likely compare the cost of continuing with their current system compared to the total cost of new equipment. The example shown below compares the net present value of three refrigeration plants; a paid off plant running on R134a, a new plant running on R134a and a new plant running on CO_2.

This analysis shows that the paid-off R134a plant is likely to cost in the order of $5,096/kW_e compared to $5,870/kW_e for a new R134a plant or $5,482/kW_e for a new CO₂ plant. However, if the CO_2. plant is able to achieve a lower capacity factor due to its better heat transfer, it may already be the most cost effective option. Although a new CO₂ plant is currently 10-25% more expensive than a new R134a plant, this difference is likely to disappear within the next 15 years.

Importantly, refrigerant replacement represents a small component of total cost in all cases, even if R404a (with its higher global warming potential) is used and leakage rates were up to 30%. Electricity accounts for more than 80% of operating costs. Plant costs are secondary, with refrigerant replacement, operating and maintenance together accounting for roughly 10% of net present value.

A lesser known feature of CO₂ based refrigeration is that it has a high temperature of heat rejection – roughly 120ºC. Recovering this waste heat may be useful, especially if large quantities of hot water are required to, for example, heat a greenhouse or manufacture a processed product. The financial viability of a CO₂ based refrigeration plant is significantly improved if such waste heat can be used on-site.

As CO₂ plant improves it will become a better option.  Modelling for 2030 indicates that by then this technology will outperform synthetic refrigerants. Again, this is not because of the carbon price, but rather because of savings in electricity costs.