Cost of cooling – a case study with broccoli

Under the assumptions used in this study, vacuum cooling proved the most energy efficient and cheapest cooling method for broccoli, followed by forced air-cooling then hydrocooling. Room cooling was slow and inefficient, with top icing by far the most expensive method of cooling broccoli. Choosing the best method of cooling will depend on equipment efficiency, equipment costs and product volumes.

As cooling is ‘value adding with electricity’, it is important to use that energy efficiently.

The following case study for broccoli estimates the energy required to cool 1 tonne of broccoli from 20°C to 4°C (ΔT = 16) using the different methods available.

The rate of cooling will vary between the leaves, florets and stalk, and be affected by the surface area of the broccoli relative to its volume. These calculations assume an average thickness of 1.73cm, a surface area of 1.515cm2/g and a density of 0.385g/cm3 (values calculated from measurements of six heads).

Various cooling methods could be used, each of which has advantages and disadvantages.

The energy removal that is required to reduce the temperature of broccoli to the target is referred to as ‘sensible heat’.

This is calculated as: mass x change in temperature x heat capacity

The time taken to get broccoli to the target temperature is a key factor in the energy used, as losses occur during this cooling period.

In theory, 64MJ/tonne is needed to extract the sensible heat from broccoli, reducing temperature from 20°C to 4°C. Actual energy requirements will be higher than this figure.

Room cooling

Broccoli placed in a cool room will cool relatively slowly. Based on experimental data, broccoli inside a harvest bin can take three days to reach 4°C with passive room cooling alone. Investigations in 2014 indicated that most cool store floors are not insulated, although this could markedly increase energy efficiency. Energy efficiency within the cool store is estimated at 17%, mainly due to losses through the floor. Total energy needed is estimated at 233MJ/tonne, giving this method an overall efficiency of 27%.

Forced air cooling

Forced air systems can reduce the time required to cool broccoli from three days to 6−10 hours or even less. However, forced air systems do suffer the same losses as occur during room cooling, through heat conduction with structures as well as external heat sources such as fans and forklift motors. Total losses will depend on construction, throughput and operating temperatures. Forced air systems reduce the energy required for cooling in air from 233MJ/tonne to 90MJ/tonne, resulting in an overall efficiency of 71%.

Hydrocooling

Hydrocooling systems for broccoli typically involve chilled water used as a drenching shower over harvested bins of broccoli. The water extracts heat from the broccoli, so its temperature increases as it flows through the product as well as through the connecting pipes and reservoirs. The energy costs of pumping water are also significant.

Around 30 minutes are typically needed to cool broccoli, although this will depend on the rate at which water flows across the broccoli surface. As the heat capacity of water is three orders of magnitude greater than for the same volume of air, a much smaller amount of water is needed to cool the broccoli.

Although hydrocooling has the advantages of being fast and keeping broccoli hydrated, the inevitable losses in the system are significant. Its energy efficiency is therefore relatively low at 47%, with 136MJ/tonne energy required.

Vacuum cooling – broccoli

Vacuum cooling cools broccoli through the latent heat of vaporisation — that is, the heat absorbed by liquid water when it turns into vapour. To cool one tonne of broccoli from 20°C to 4°C, 26kg of liquid water needs to turn into vapour, assuming other materials do not add heat to the system. In this case there is no need to cool pipes, reservoirs or any equipment other than the bin the broccoli is contained in. Vacuum cooling is also rapid, typically taking 30−50 minutes for a cycle.

Vacuum cooling is the most efficient method of cooling broccoli from 20°C to 4°C, with an estimated energy consumption of 78MJ/tonne and an overall efficiency of 82%.

Top icing

In California, broccoli is often packed in the field, then cooled using an ice injection system. The semi-liquid slurry is pumped into the boxes under pressure using a jacket system. The slurry expands inside, filling the spaces between the broccoli heads. This is the primary method of pre-cooling the broccoli, which is then placed into refrigerated storage.

The process of icing broccoli in California. The ice slurry is pumped through the boxes, filling the air spaces inside. The melting ice pre-cools the broccoli before cold storage.

In Australia ice is not normally used to cool broccoli. Rather, it is used to maintain temperature during storage, transport and distribution. However, the costs associated with ice are included here for comparison.

Ice contact with broccoli is generally poor, leading to uneven cooling. It is also inefficient, as a large amount of energy is needed to remove the sensible heat from water in order to make ice. It is estimated that 1kg of ice is required to chill 3kg of broccoli from 20°C to 4°C, and that this would take approximately one day for a full bin. This would mean 927MJ energy is required to cool one tonne of broccoli, giving this method an overall efficiency of 7%.

Storage costs

While some growers now pack broccoli directly into cartons in the field, the more common practice is to harvest into bins, cool and then pack at a later date. It is then stored for a period before dispatch.

Broccoli may be packed into lined cartons, plastic crates or flow wrap films. Historically, broccoli was packed into polystyrene cartons and top-iced before transport, and many growers still pack broccoli this way. During packing broccoli may warm slightly, so a period of storage before transport can help to bring the temperature back down close to 0°C.

The total energy cost for room cooling broccoli packed at 4°C to 0°C then storing for two weeks is estimated at approximately 800MJ, or approximately $40/tonne.
If that same broccoli was packed in ice, over 30,000MJ energy would be required over the two-week period, costing a total of around $1,505/tonne.

In reality, a combination of the two is generally used. However, it is worth noting that ice is a highly inefficient method of cooling, and that large amounts are inevitably wasted.

Estimated energy required and total cost to reduce the temperature of one tonne of broccoli from 20°C to 4°C using different cooling methods. Electricity cost is estimated at $0.07/MW.

* Assumes that off peak electricity is used 50% of the time, and that the cost of off peak electricity is 40% that of normal daily rates.

Summary and conclusions

For broccoli, hydro-vacuum cooling appears to be the most energy efficient method by a considerable margin.

However, each business case is different. Previous studies have found large variations in energy efficiency between different systems, even ones using the same method of cooling. This can be related to the capacity of the system; an inefficient vacuum cooler run while only partly full can have lower energy efficiency than an efficiently operated hydrocooler. For example, a 2015 study of cooling systems used on potato farms found the cool room systems on farms were all operating well below the energy efficiencies that are possible with modern cooling equipment.

Moreover, estimates of cost do not include the capital cost of the equipment. They also do not consider the effects on product quality and, therefore, potential sale price. A vacuum cooler represents a significant investment. However, the fast cooling times achieved may preserve quality and extend storage life. This is particularly important for products such as baby spinach or lettuce, but may also be critical for broccoli during warm harvest conditions.