Energy efficiency can't be read from a single number

A roaster can use gas to supply heat and electricity for the fan, controls, cooling, or auxiliary equipment. So total electricity or gas consumption alone doesn't say whether the machine is operating efficiently or poorly. To compare, you need to know how much product that energy served and under what operating conditions.

Bottazzi and colleagues built a roasting model accounting for system components and the ability to predict fuel consumption alongside operating variables. This provides a basis for viewing efficiency as a systems problem rather than just reading a fuel meter.

Gas and electricity answer different questions

Gas usually relates directly to the heat source in fuel-burning machines, while electricity can power fans, motors, controls, and cooling systems. If these two groups are added into one number without converting to the same energy basis, the result can be misleading.

Even once converted to the same unit, you still need to know which portion of energy actually goes into heating and which is lost or used for auxiliary equipment. A system with more fans or different heat recovery can have a very different consumption structure from another.

The important denominator: energy per unit of product mass

A more meaningful comparison converts energy consumption to the same mass of green coffee or roasted coffee, depending on the analysis goal. If one machine uses more energy but processes a significantly larger mass, total consumption alone isn't enough to conclude the machine is less efficient.

Besides energy per mass, you should record batch time, charge weight, post-roast weight, and endpoint conditions. When these variables travel together, an analyst can see how much product that energy actually produced.

High power doesn't mean low efficiency

Power is the rate of energy supply, while efficiency relates to the level of useful energy obtained relative to energy input. A high-power machine may need high power in the early stage to compensate for heat and stabilize the system, then reduce afterward.

Bottazzi and colleagues showed the model can be used to compare control strategies and the effect of operating conditions on the roasting process. This shows looking only at instantaneous power misses the progression of the whole batch.

Heat loss and heat recovery change the comparison

Two machines using the same amount of gas may transfer heat to the bean differently due to different roasting-chamber structure, airflow, and heat-recovery ability. Similarly, electricity used for fans can change with the airflow the machine needs.

So if comparing two differently configured machines, the scope needs to be stated clearly: comparing input energy, energy per kilogram of product, or heat-conversion efficiency. Don't use one indicator for every purpose.

What data should be recorded for each batch

A basic dataset can include: gas consumed, electricity consumed, charge weight, post-roast weight, batch time, ambient temperature if meaningful, operating mode, and points where power changed. If the system has separate meters for auxiliary equipment, the data should be kept separate rather than summed from the start.

Data needs the same measurement time window. If gas is recorded for the whole day but electricity is only recorded for one batch, the two numbers can't be directly used to calculate that batch's efficiency.

Comparing machines requires equivalent conditions

A 5 kg roaster and a 30 kg roaster shouldn't be assessed by the total energy of one batch. Charge weight, roast level, time, cooling method, and operating conditions need to be considered before comparing.

Hernández and colleagues showed the roasting process depends simultaneously on temperature, heat transfer, moisture, and the bean's progression. So an energy indicator can't be separated from the quality of the roast batch it produces.

Efficiency only carries meaning alongside product results

The goal of energy optimization isn't reducing every consumption number at all costs. If reducing energy extends roasting time or the product fails to meet requirements, a lower energy indicator isn't necessarily a better result.

A more practical approach is tracking energy per product mass, batch time, mass loss, and relevant quality indicators together. When data is recorded with a consistent method across many batches, a business can identify trends and find points that need improvement.

That said, the per-kilogram indicator still needs to go together with quality conditions. A batch with low consumption but a roast level that doesn't meet requirements can't be considered optimal. Energy analysis only carries meaning when placed within the same goals for output, quality, and operating time.

If the goal is long-term tracking, an indicator like kilowatt-hours per kilogram of roasted coffee, or equivalent energy per kilogram of product, can be more useful than total daily electricity. This standardization helps reduce the effect of varying batch counts and allows viewing trends by output.

Energy data needs to be tied to the same product unit

For gas, the unit and meter-reading method need to be clearly stated; for electricity, you need to know whether the reading covers the whole batch or just part of the equipment. If the system uses several energy sources, the method for summing and converting should be defined before starting the comparison. Standardizing the measurement matters because a discrepancy in recording scope can be larger than the actual difference between two batches.

References

• Bottazzi et al. (2012). A numerical approach for the analysis of the coffee roasting process. Journal of Food Engineering, 112, 243–252. https://doi.org/10.1016/j.jfoodeng.2012.04.009
• Hernández et al. (2007). Analysis of the heat and mass transfer during coffee batch roasting. Journal of Food Engineering, 78, 1141–1148. https://doi.org/10.1016/j.jfoodeng.2005.12.041
• Susilawati et al. (2023). The Analysis of Heat Transfer on Hot Air Coffee Roasters at the Appropriate Technology Research Centre. VANOS Journal of Mechanical Engineering Education.