The following analysis of a practical case is designed to clarify the
concepts discussed in previous sections and provide a better example of the
objectives of this study.
The information contained in the following pages should be viewed as an illustration of a hypothetical analyst's possible conclusions, if that analyst were provided with the information contained in the various summary tables.
The following conditions are assumed to hold:
Analyst's identity: employee of the Ministry of Agriculture without any
special training in the field of energy and plant design.
Country under consideration: Tropical.
Pedologic and climatic characteristics: dry climate, average annual wind speed: 4 m/s; no specific data on solar radiation; availability of wellwater.
Other information: electric energy from the grid is not available; biomass (wood with a moisture content of 25% w.b.) is available at a cost of U.S. $0.02/kg; Diesel fuel at a cost of $0.60/kg (including transport) may also be available.
Processing center: currently does not exist. Expected quantities of milk to be processed: 1500 1/day; final products: pasteurized milk in plastic containers (1000 1/day); yogurt (500 1/day). Milk will be collected daily in metal cans. The products will be sold in a nearby city (30 km) by another organization.
Personnel: are not available, and it is not possible (for economic reasons) to have full-time engineers supervising the processing center's plants. There is an abundant supply of unskilled labor.
Purpose of the analysis: identification of several technical solutions for the supply of energy to the processing center on the basis of the information contained in this study.
Energy consumption is the first aspect that has to be evaluated. Indeed, this value makes it possible to analyze various technical possibilities with more confidence.
Given the context outlined above, simplified milk processing plants will no doubt be chosen for the processing center.
With reference to the equations contained in section 2.2.3:
|Et = 25mc + 180(mp + mt)|
|Ee = 145mc + 90(mp + mt)||[MJ]|
where: Et and Ee are the electric and thermal energy requirements; mc, mp
and mt are the quantities (t) of milk that are cooled, pasteurized and
processed (into milk or yogurt).
In the case under examination, 1000 1 of milk are pasteurized and 500 1 are processed into yogurt.
Et = 25*0 + 180(1.0 + 0.5) = 270 MJ/day
Ee = 145*0 + 90(1.0 + 0.5) = 135 MJ/day
By referring to graphs 3 and 4 (section 2), it may be observed that the
thermal energy requirements are equal to the maximum values and the
electric energy requirements are halfway between the maximum and minimum
This observation is important for analysis based on the summary tables contained in the text.
For the sake of information, it should be remembered that the chemical energy contained in 1 kg of Diesel fuel is 42 MJ (80% of which is transformable into thermal energy and 15–20% into electric energy with the use of normal energy conversion technologies).
The complete absence of a grid for the supply of energy means that selfsufficient solutions should be considered. Therefore, the profitability of employment of traditional sources (e.g., Diesel fuel for the generation of electric and thermal energy) and renewable sources has to be evaluated. Since the site under examination does not have any special resources (streams, geothermal sources, etc.), the only possible sources are the sun, wind and biomass (dry).
The summary table concerning solar radiation states that from 4 to 30 m2 of receiving surface are needed to satisfy thermal requirements and from 60 to 90 m2 are needed for electric requirements in the case of simplified plants and a daily intake of 1000 l. This means that the following surface areas are needed in the case under examination:
|For thermal requirements||→ 30 m2 * 1.5 ≃ 45 m2|
|For electric requirements||→ 75 m2 * 1.5 ≃ 110 m2|
1.5 is the multiplication factor used to take into account the larger quantity of milk considered here (1500 l/1000 1 = 1.5); 30 is the maximum value between 4 and 30 (as noted above, the thermal requirements equal the maximum values shown in Figure 3); and 75 is the intermediate value between 60 and 90 (the electric requirements fall halfway between the maximum and minimum values shown in Figure 4).
The summary table on wind energy (section 4.3) states that the employment of wind energy is only useful in areas in which annual wind speed is at least 5 m/s. In the case discussed here, wind energy appears to be excluded.
Use of Biomass
The summary table concerning dry biomass (section 4.6) suggests that consumption of this material (as in the case of solar radiation) is equal to:
|For thermal requirements||→||30 kg * 1.5 ≃||45||kg|
|For electric requirements||→||125 kg * 1.5 ≃||190||kg.|
The summary table regarding the production of thermal energy from a solar
source (section 6.2.2) states that the production of hot water is not a
significant technological problem as long as flat-plate collectors are used
(thus, milk processing plants that are capable of operating at low
temperatures have to be employed).
The most serious problem is the fact that the source is not self-sufficient and hence requires the installation, in parallel, of a thermal energy generator. In this particular case, the generator could be a biomass-fed boiler.
Generation of electric energy could be handled by photovoltaic plants (section 6.3.4), but this equipment is very expensive and hard to manufacture in underdeveloped countries. In addition, the application of this technology would require the redesign of current processing plants. It should be remembered, however, that the consumption of electricity for cooling (and hence the surface area of the required photovoltaic collectors) could be reduced considerably if the available well-water were cold enough (under 15–20 fC) and the heat exchangers were instantaneous (summary table in section 6.2.5).
As noted above, wind energy does not appear to be applicable here. However, this source could be considered for pumping water (using water pumps that operate satisfactorily with wind speeds between 3 and 4 m/s).
The required thermal energy could easily be produced (in the form of hot
water or steam) by the right kind of boilers (section 6.2.1). These
boilers could be manufactured on the semi-industrial level in numerous
countries with varying socioeconomic levels.
The production of electric energy could be handled by reciprocating steam engines (see related summary table, section 6.3.2), which could also produce the necessary thermal energy with the aid of steam-water exchangers.
In addition, these technologies can be considered self-sufficient.
Thermal energy requirements could be reduced through the adoption of
refrigerating machines for milk cooling with heat recovery (section 6.2.3).
Electric energy could also be produced by gasification of the biomass and
use of suitable electric generators (section 6.3.1).
The use of generators (with possible heat recovery) fed by alcohol or vegetable oils (sections 6.3.1 and 6.4) is a special case. These solutions are naturally only applicable when these fuels are available (at least on the regional level).
Given the discussion of sources and technologies in the previous section, the use of biomass seems to be the most rational choice for the following reasons:
Once the most suitable energy source has been preliminarily identified
(more detailed studies can be carried out later by energy experts), the
economic aspect has to be tackled.
The objective here is to compare the classic technological solution (a liquid or solid fuel-fed boiler and Diesel generator) with what appears to be the most promising solution from a practical standpoint, based on the analysis carried out in the previous sections (i.e., the steam generator with heat recovery). Naturally, two solutions that employ renewable sources can also be compared.
Even an approximate economic analysis requires a certain amount of data and, as we saw in section 7, some familiarity with the technologies and their general problems. Therefore, the analyst has to gather enough information to make it possible for him to evaluate the following parameters (which are defined more precisely in section 7.8):
For the sake of simplicity, in the case at hand electric energy production alone is analyzed, and it is assumed that requests for estimates (from specialized companies) and technical information (from specialists) would make it possible to fill in the following table:
|Parameter||Diesel Generator||Steam Generator|
|Energy to produce||(GJ/yr)||50||50|
|Fuel type||Diesel oil||wood|
The amount of energy to be produced has been evaluated on the basis of the daily electric requirement (section 8.2). The maintenance cost includes all routine expenses (including labor for manual fuel feeding).
At this point, the question becomes: is it profitable to invest U.S. $11,000 more and spend $900 a year more for maintenance in order to use a less expensive fuel?
With reference to the symbols used in section 7, and keeping in mind that in this case the goal is a reduction in the cost of the energy source, the following values can be calculated:
Annual fuel costs with the proposed solution: 50,000 [MJ/year]*$0.03/MJ≃$1500/year.
ST(0)=(900+1500)*10 = U.S. $24,000
It may be observed that the current energy cost ($0.14/MJ) is higher than
In conclusion, the steam engine appears to be suitable for the production of electric energy, and its economic performance exceeds expectations. Indeed, IRR is over 20%.
In order to complete the analysis, the problem of thermal energy production also has to be tackled (thermal energy can be recovered by the engine itself, as shown by more detailed studies).