It's that time of the year again in the hum of cooling units running tirelessly around the winelands drilling its way into every winemaker, assistant and cellar workers brain as we kick off the 2025 harvest. Along with water usage, electricity forms the backbone of costs associated with wine making in the cellar (excluding consumables used to make the wine). The impact on cost can be quite staggering and the optimised management of cooling crucial.
After a brainstorming session with Pieter Vergeer (a highly skilled senior engineer specialising in these processes at Stamicarbon Advance) over some good wine, I thought it would be valuable to outline how the cooling system operates in the cellar. It is imperative to understand the system so we can identify exactly what improvements can be made.
Cooling must be thought of in three layers and areas.
First, it is essential to consider the temperature of the fermentation tank. This temperature is typically determined by the desired quality and style of wine being produced and is the primary reason for implementing must cooling in a cellar. From a purely chemical standpoint, most of the total potential energy stored in the grapes is dictated by their sugar content, and this energy is released through the exothermic reaction of fermentation. Controlling this process allows us to influence the final product.
Secondly, consider the ambient temperature of the cellar. The closer the cellar's temperature is to the desired fermentation temperature in the tank, the easier and less energy-intensive it is to maintain the tanks at that desired temperature. This is because the primary variable we are dealing with is the temperature differential between the current temperature and the target temperature. The main objective is to minimize this differential to conserve the minimum amount of energy required to achieve the desired temperature.
Lastly, consider the outside temperature, which is especially important during the harvest season in South Africa's Western Cape, where it can get extremely hot. The intense heat from the sun can significantly disrupt the carefully controlled environment inside the fermentation tank. Therefore, it is crucial to insulate both the cellar and the tanks from those 35°C Stellenbosch days. Energy should be efficiently allocated to cooling the exothermic fermentation process rather than cooling the cellar itself.
On a biochemical advancement level, we are seeing the emergence of yeasts that can still produce quality white wines at a higher temperature than the traditional 12-13°C. With some strains claiming to be able to ferment delicate wines even at 16°C.These developments will also help us reduce the temperature difference for optimal operation.
Ultimately, we must work within the limitations of the environment to maximise efficiency. A well-insulated cellar can greatly reduce the pressure on the cooling system. One major source of heat entering the cellar is the grapes themselves, as they bring in high outside temperatures. We are aware of the negative effects that high temperatures can have on the quality of the final product, not only by altering the chemical composition due to the breakdown of heat-sensitive volatile components but also by causing premature fermentation outside the controlled environment of the cellar.
Additionally, high temperatures have a significant impact on energy usage. It is estimated that approximately 1 kWh of energy is needed to cool a ton of grapes by 2.5°C. On a hot day, reducing the temperature of grapes from 30°C to 15°C would require around 6 kWh of energy per ton of grapes entering the cellar.
As a little experiment, let’s do some quick napkin math. If we are considering the fermentation of wine in a 10,000-liter tank over a period of 21 days.
When fermenting at 13°C for 21 days compared to fermenting at 16°C for 16 days, the energy consumption can be contrasted as follows for a 10,000-liter tank:
- Fermentation at 13°C for 21 days: Consumes 524 kWh
- Fermentation at 16°C for 16 days: Consumes 392 kWh
The energy required for cooling is calculated at 33 kilowatt-hours (kWh) for each degree Celsius difference between the cellar's ambient temperature and the optimal wine fermentation temperature.
The energy consumption of the mash cooler is quantified at 2 kWh per ton for every 5°C it must cool down the grapes.
Additionally, our cost of electricity is approximately R0.30 per kWh (2024).
Considering the following scenarios for Sauvignon Blanc fermentation:
Minimum cooling requirement:
- Fermentation temperature: 16°C for 16 days
- Grapes temperature upon arrival: 20°C
- Cellar ambient temperature: 20°C
- Energy consumption: 419 kWh
- Cost: R125.70
Maximum cooling requirement:
- Fermentation temperature: 13°C for 21 days
- Grapes temperature upon arrival: 30°C
- Cellar ambient temperature: 25°C
- Energy consumption: 771 kWh
- Cost: R231.50
Please note that these figures only account for savings on direct cooling costs and do not encompass additional expenses such as water usage for running the cooling towers or the operational efficiency of the cooling units themselves.
It is important to remember that, similar to high school physics calculations, these estimates do not account for inefficiencies and energy losses that can vary from cellar to cellar. Instead, they should be viewed as potential percentage savings. When considered in this manner, the energy savings on this calculation could amount to approximately 45.7% of your cooling energy bill during harvest time.
Although the savings might be small on an individual tank scale, it demonstrates the significant impact that proper planning and optimization can have when scaled up to an entire cellar, without the need for additional expensive infrastructure.