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3.1 Introduction (see Appendix 1 for an explanation of the terminology used below)

This report considers only those energy requirements connected with milk processing. In fact, the following forms of energy consumption are not considered:

  1. indirect;
  2. related to the production of milk on the farm and its transport to processing centers.

It is not a simple matter to provide information which is valid for all situations. Indeed, practice has shown that energy requirements and the types of energy used are highly dependent on the technological level of the equipment used at the processing center. For example, in the case of home cheese production, the milk container is heated directly over the flame (using wood).

In this context, it may be observed that:

  1. the energy used is almost exclusively thermal;
  2. the system's efficiency is limited (the ratio of energy requirements to energy consumption, in fact, is on the order of 10–15%);
  3. electricity is used little or not at all (the milk is manually stirred and poured from one container to another).

On the other hand, the following situation characterizes a modern cheese factory:

  1. thermal energy consumption is generally low, since high efficiency generators and heat recovery are used (unless particular, energy-intensive techniques are employed, such as the production of powdered milk);
  2. electric energy consumption is significant as a result of milk chilling (using refrigeration equipment) and the widespread use of electric machinery (pumps, stirrers, etc.).

In any case, it was impossible to consider anything other than modern plants (though of varying complexity and completeness) in this report. The reasons for this are obvious:

  1. only modern plants are capable of guaranteeing quality, hygienic final products and long-term uniformity in milk and cheese production;
  2. milk refrigeration should be introduced in all rural settings, and this can only be made possible by the availability of plants which incorporate advanced technology. Under these conditions, both the necessary basis for a fresh milk market (highly requested in urban centers in poorer countries) and the opportunity for rural populations to finally leave behind the stage of familial self-sufficiency become possible;
  3. the employment of rationally designed (i.e., modular) plants by individual countries may make it possible to plan the location of standardized processing centers in order to increase the probability of their successful introduction. In addition, standardization may lead to the development of local industrial activity based on the manufacture (at least partial) of the necessary plants;
  4. all energy plants designed for the use of renewable sources have reached a certain technological level. This is true of even the simplest models.

In other words, the kind of engineering that goes into energy plants (e.g., solar collectors manufactured with locally produced components and used to heat water to 50°C) is decidedly more complicated than that required by simple milk-cheese plants (e.g., containers heated directly over wood fires).
Consequently, the possibility of using modern milk processing and transformation plants is implicit in any analysis of the possibility of producing energy. Indeed, it is logical to imagine a system in which energy and processing aspects are technologically compatible.

It should also be noted that the optimal use of energy sources would naturally lead to the modification of commercially available plants. For example, photovoltaic collectors would be better employed if the final users were direct-current electric motors (in order to do away with inverters and increase the plant's utilization factor). Another example concerns the use of 70–75°C water (instead of traditional steam) to heat milk, which would make it possible to employ relatively simple solar collectors.
It is clear, then, that the possible application of renewable sources is closely connected with the type and level of technology employed in the plants used to transform the milk.
Here, only brief mention is made of possible modifications of processing plants, since it is felt that the actual introduction of new technologies requires that their innovative impact be limited and spread out over time.

Finally, it should be observed that the use of modern technologies does not imply that poorer countries must be forced to import technologies; instead, this should be seen as an incentive to engage in semi-industrial activity (naturally in relation to available local labor), which could make it possible to create or consolidate commercial activities in rural areas.

3.2 Energy Requirements

In evaluating energy requirements, direct reference has been made to a study carried out by the FAO [49].
The results are provided below.
Here, reference is made to thermal and electric requirements (and not consumption). The former concern water-milk or steam-milk heat exchangers, while the latter relate to the electric motors found in normal milk-cheese plants (e.g., pumps, stirrers, refrigeration plant compressors and various servomechanisms).

Requirements, and not consumption, are examined for the following reasons:

  1. these plants should be viewed as equipment which has already been perfected and for which substantial modifications are not suggested (at least in principle and during the initial phase, as noted above). Attempting to modify the traditional set-up of these plants would, in fact, mean increasing their cost, since non-standard modifications would be required;
  2. the energy consumption of electric motors (e.g., cooling plant compressors) and heat exchangers (e.g., to heat water and milk) has to coincide with the output of electric or thermal generators. Therefore, this output represents the share of energy that has to be provided.

The analysis carried out in this section considers two basic operations:

  1. milk collection (refrigeration and supplementary treatments);
  2. milk processing (milk packaging, production of cheese, yogurt, etc.).

The energy consumption of centers whose plants are at varying levels of completeness are analyzed for each individual operation. In terms of point c), these include:

c1) modern collecting centers for chilling only;

c2) modern collecting centers for chilling and container washing.

For point d), they are:

d1) plants with modern equipment (complete or simplified);

d2) centers using electric energy only.

3.2.1 Milk Collection

Modern collecting centers for chilling only (medium-sized and large scale)

Refrigeration of one ton of product generally requires 100–120 MJ of electric energy. The energy is needed to remove heat (using icebank or direct expansion refrigeration systems), stir the milk and pump water to wash containers or equipment (approximately 150 l/t of milk).

Modern collecting centers for chilling and container washing (small scale)

Farmers bring their milk to these centers in small containers (quantities normally range from 1–2 to 100 1); the milk arrives at a temperature which is slightly lower than that of the freshly obtained product. Electricity consumption connected with refrigeration (including stirring and pumping) is higher than that noted above (120–145 MJ/t) because of the plants' increased energy losses and low utilization factors. In addition, approximately 25 MJ of thermal energy per ton of milk should be added for container washing (water consumption may reach 300 l/t of milk in these types of plants).
By way of comparison, the consumption of electric energy is limited to approximately 65–85 MJ/t in large industrial plants (to which approximately 25–30 MJ/t of thermal energy must be added).

3.2.2 Milk Processing

Centers using modern equipment (complete or simplified)

Analysis has been restricted to small and medium-sized centers using electricity, low pressure steam boilers, simple filling and sealing machines and pasteurizers (usually batch type).

In the case of simple plants (with milk packaged in plastic containers), the following requirements have been estimated:

- pasteurization:180 MJ of thermal and 90 MJ of electric energy;
- cheese and yogurt production:180 MJ of thermal and 90 MJ of electric energy.

In the case of complete plants (the type that is generally used in Europe and produces bottled milk), the requirements are the following:

- pasteurization:600 MJ of thermal and 200 MJ of electric energy;
- cheese and yogurt production:450 MJ of thermal and 270 MJ of electric energy.

Centers using electric energy only (without steam)

These plants have been created solely for the packaging of non-pasteurized milk. Their electricity consumption is estimated to be on the order of 120 MJ/t of final product.

3.2.3 Observations

The electric energy consumption and thermal requirements mentioned above are summarized in Table 1, which also contains data on the production of condensed milk. By way of comparison and in order to supply information about other products, Table 2 also provides typical values for complete modern plants (data obtained from developed countries).
Based on the comments made earlier and with reference to the most important cheese products, only two kinds of plants have been considered in order to make summary evaluations possible:

  1. complete modern plants;
  2. simplified plants.

In both cases, milk cooling (plus equipment and container washing) and pasteurization are the operations carried out.

Using the following terms (quantities in t/day):

mc: chilled milk;
mp: milk packaged in small containers (i.e., pasteurized);
mt: milk processed into yogurt or cheese (not including whey processing);
me: condensed milk.

As well as (values in MJ/t of processed milk):

Ee: electric energy requirements;
Et: thermal energy requirements.

An initial approximation shows that for type a) plants:

Et = 25mc + 600 mp + 450mt + 1060 me 
Ee = 110mc + 200 mp + 270mt + 220me[MJ]
and for type b) plants: 
Et = 25mc + 180 (mp + mt) 
Ee = 145mc + 90(mp + mt)[MJ]

For example, using a simplified plant working on the following quantities:

The related thermal and electric energy requirements are:

Et = 25×0 + 180 (0.5+0.7) = 216 MJ/day
Ee = 145×0 + 90 (0.5+0.7) = 108 MJ/day

and using a complete plant:

Et = 25×0 + 600×0.5 + 450×0.7 + 1060×0 = 615 MJ/day
Ee = 110×0 + 200×0.5 + 270×0.7 + 220×0 = 189 MJ/day

It may be observed that consumption largely depends on the final destination of the milk that comes into the center. To illustrate this aspect, Figures 1, 2, 3 and 4 show the maximum and minimum requirements of centers with a daily intake that ranges from 500 to 2000 1 (excluding the production of condensed milk).
In the case of both simplified and complex plants, the maximum and minimum thermal and electric requirements are measured for simple cooling and complete pasteurization (or processing) of all the available milk. For example, with reference to the figures provided above and with 1000 1/day of milk (simplified plant), the electric and thermal requirements would range from 90–140 and 24–180 MJ/t of fresh milk, respectively. In the case of complete plants, these figures would be 25–600 and 100–240 MJ/t, respectively. The energy plants will be dimensioned later on the basis of the variations illustrated by the figures.

Table 1 - Specific energy requirement in milk collection and processing (from [49], modified)

Final productPlant typeEnergy requirement [MJ/t of milk]
collected milkA-100–130
milk in bottlesC600200
D (1)-120
Cheese, yogurt (2)C450270
Condensed milkC1060220

(1) unpasteurized
(2) without whey processing
A: collecting centres with chilling only
B: collecting centres with chilling and can washing
C: plants, large or small, with modern and compléte equipment
D: centres with electric power but without steam
E: centres with a varying degree of simple mechanization usingelectric power and steam (simplified plants)

Table 2 - Specific energy requirement in modern milk processing plants (from [49], modified)

Final productEnergy requirement [MJ/t of milk]
Milk in bottles:  
- pasteurized
- sterilized
Milk in one-way containers:  
- pasteurized
Skim milk powder and butter:2100325
Full cream milk powder:1900290
Ripened cheeses:  
- without whey processing
- with whey processing
Evaporated and condensed milk:1060220

Figure 1

Figure 1 - Thermal requirements versus daily quantity of milk processed in complete plants. The lower line represents the energy requirements of chilling alone (sanitary hot water for washing), while the upper one represents pasteurization, which requires more energy.

Figure 2

Figure 2 - Electric requirements versus daily quantity of milk processed in complete plants. The lower line represents the energy requirements of chilling alone, while the upper one represents the production of yogurt and cheese, which requires more energy.

Figure 3

Figure 3 - Thermal requirements versus daily quantity of milk processed in simplified plants. The lower line represents the energy requirements of chilling alone (sanitary hot water for washing), while the upper one represents the pasteurization or the production of yogurt and cheese, which require more energy.

Figure 4

Figure 4 - Electric requirements versus daily quantity of milk processed in simplified plants. The lower line represents the energy requirements of the pasteurization and of the production of yogurt and cheese, while the upper one represents the chilling, which require more energy.

3.3 Comments on Renewable Sources in Relation to Their Use in Milk Treatment and Processing Centers

Energy is used on three qualitative levels at milk treatment and processing centers.

Specifically, energy is used in the form of:

  1. hot water, heated to temperatures under 80°C (low temperature washing and processes);
  2. steam at various temperatures and pressures (the traditional energy medium for all treatments and processes). As mentioned above, steam may be replaced (with slight plant modifications) by water heated to 80–90°C;
  3. electricity (220 or 380 V).

It should be observed that the availability of a heat-carrying fluid and electricity is essential. Low-temperature water could be generated (at least partially) from process waste.
If our analysis is limited to actual practice, this type of situation would lead us to consider only a small number of sources and, most importantly, a limited number of energy conversion technologies.

In fact, for the production of low-temperature hot water, the following technologies may be considered:

For the production of steam (or average-temperature water at 80–90°C), these technologies may be evaluated:

Consideration of unusual technologies for the production of thermal energy (i.e., electric generators) is justified by the very real possibility of discovering significant water resources quite near treatment and processing centers. Naturally, this solution would only be useful (since it would simplify plant design considerably) in the case of reduced thermal power requirements or if special kinds of thermal storage were employed.
In addition, it is obvious that technologies that are suitable for average-temperature energy production can also be used for low-temperature production.

The following technologies may be considered for electricity production:

Naturally, it would be logical to concentrate on technologies that offer the possibility of meeting all the user's energy requirements with a single, easily obtainable source. An example of this type of technology is the steam engine.

Technologies which are considered to be experimental or impractical are not analyzed here for obvious reasons. For example, focusing collectors (for generating average-temperature thermal energy or even electricity) require significant maintenance, meticulous plant design and efficient heat storage and are not easy to find on the market. Their on-site construction (in significant quantities and with a high level of quality and completeness) may pose problems.
Similarly, steam or organic fluid turbines are not discussed.

Sections 5 and 6 contain information about the considerations that have led to selection of the technologies listed above.

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