These technologies are classified on the basis of their final product:
thermal or electric energy.
As in the case of energy sources and conversion processes, summary tables
are provided in this section as well.
A standard outline is not presented here, since the specific examples are
considered to be sufficiently clear.
These systems convert the chemical energy contained in fuels into thermal
energy (in the form of hot fluids: steam, water, oil or air).
In general, these systems are composed of:
The components listed in a) may be absent (e.g., wood-fired boiler with intermittent loading).
Classification is based on the physical state of the fuels used (gas, liquid or solid).
Burners
Burners mix comburent air with fuel to facilitate the process of oxidation.
Solids are generally placed in contact with a flow of air, liquids are
atomized in the comburent, and gases are simply mixed. Burners may contain
a feed system (e.g., a gas blower), whose size and cost can be significant
(especially in the case of solid fuels).
The comburent air can be sucked in by the boiler's natural draught
(atmospheric burners), or it can be forced in by a blower (blown air
burners).
In the case of solid fuels, a distinction is made between burners designed
for coarse fuels and those that use powdery fuels (or fine-grained fuels,
such as powdered coal or biomass, which can be transported in suspension by
the air).
Combustors are a particular kind of burner. In this case, the fuel is
gasified (with an air flow rate that is 30% that of normal), and the gas is
immediately burned (with the addition of the remaining quantity of
comburent air).
When the pieces of fuel are large (prisms whose longest side is over 50–60
mm), burners are replaced by a feed mechanism.
The proper regulation of the burners (or of the feed system) is extremely
important in terms of process efficiency.
The following forms of regulation should be mentioned:
Boiler Shell
This is where combustion and thermal exchange take place between hot gases
and heat-carrying fluids (water, steam, diathermic oil or air; type and
operating pressure should always be specified). The fluid may circulate
inside the tubes (tube boilers), outside them (fire-tube boilers), or in
proper air spaces.
Boilers are further distinguished on the basis of how the comburent air is
transported (depression boilers or presurized boilers; see also burners).
In the latter case, the flue gases can be forced to lick more complicated
(and hence more efficient) finning.
Condensation boilers are characterized by a large exchange surface, which
makes it possible to bring the temperature of the flue gases down to
55–60°C (the maximum temperature of the circulating fluid is thus limited
to these values). These boilers are only used for pure gases, since the
condensates of some fuels (e.g., biological gas, oil and coal) are
corrosive or create operating problems (soot or tar deposits, etc.).
The following equation provides the system efficiency μ:
μ=thermal power produced (Pp)/thermal combustion power (Pc)
Pc is the product of the fuel load Q and its heat value HV.
It is always important to specify whether this is gross or low HV
(generally the latter). Defined in this way, the efficiency is
instantaneous and refers to very precise operating conditions (value of Q,
temperature of heat-carrier, etc.).
The efficiency may also be calculated as the ratio between energies during
relatively long periods of time (one hour, one day, an entire season,
etc.). In this case, the value obtained is the average efficiency during
the time period under consideration.
The following factors have an effect on the efficiency:
incorrect system dimensioning or design (whether too large or too small).
Efficiency can be measured directly or indirectly. In the former case, the power supplied to the heat carrier Pp and the combustion power Pc are measured. Specifically:
| Pp=hsGδT; | Pc=Q*HV; |
where, in addition to the terms defined above, hs is the specific heat of
the fluid under consideration, G is the fluid flow rate and δT is the
temperature increase in the heat carrier.
Power measurement can pose some practical problems because of the
inevitable operating instability. Consequently, energy totals over
sufficiently long periods of time, t, are preferred.
It should be noted that the direct method requires installation of numerous
instruments (flow rate meters, etc.).
The indirect method, on the other hand, is based on measurement of the
temperature Tf and the concentration of O2 and CO2 in the flue gases (in
some cases, CO is also calculated). This information makes it possible to
determine whether combustion is taking place in an optimal fashion, and if
the boiler is operating properly. Indeed, when the concentrations of O2 and
CO2 are normal, E is correct. When Tf is lower than 250–300°C, the thermal
exchange is satisfactory.
It is also possible to estimate the value of μ using graphs and empirical
formulas. The method is fast and economical (it requires an outlet in the
flue and inexpensive instruments). The result provided, however, is not an
absolute value, but merely an indication of proper system functioning.
Values of μ (with respect to GHV) are always under one (this is also true with respect to LHV, except, sometimes, in the case of condensation boilers).
References: [9], [14], [15], [42].
| Thermal Energy Production | ||
| Final Technology - Summary Table: | BOILERS | |
| a) | operative flexibility: | high |
| b) | operation: simple with boilers that produce hot water; less simple with boilers that produce steam (the consumed water has to be treated). | |
| c) | most obvious limitations:1) liquid and gaseous fossil fuel and gasification and biological gas boilers: none in particular; 2) solidfuel boilers: i) cost of automatic feed systems; ii) low mass of unit volume of wood residues (large storage volumes); iii) particles in the flue gases (maintenance, safety and pollution problems). | |
| d) | auxiliary machinery needed: machinery to chip the wood. | |
| e) | recommended versions and their technological level: | |
| 1) | Liquid and gaseous fuel boilers: When gasification and biological gas are used, always be ready to feed the boiler with other kinds of fuel that are readily available; | |
| 2) | Solid fuel boilers: Automatic feed systems are hard to justify with thermal power of less than 150 kW. Therefore, it is best to use “gasifier” boilers that do not need to be reloaded for 4–5 hours; | |
| 3) | For all types: Models with very simple means of checking the combustion process can only be produced by workshops with a complete series of machine tools, heavy plate bending and welding capabilities and the possibility of obtaining cast pieces. The production of high-efficiency boilers (with adequate combustion control) is only feasible in high level factories located in rich countries. | |
| f) | energy transformation yields: | |
| - gaseous and liquid fuels: | > 70%; | |
| - solid fuels: | > 45% (generally no more than 70%). | |
| g) | can this technology be considered self-sufficient?(*) | |
| - used together with gasifier | YES | |
| - used together with digester | NO | |
| - solid fuels in general | YES | |
There are two different kinds of solar collectors: flat-plate
(operating temperature: ≃30–80°C) and focusing (≃80–300°C).
In the former models, solar radiation heats a surface (the
absorber), which contains a fluid (normally water or air). Other
collector components include a transparent cover, thermal
insulation and a container.
The absorber must be highly absorbent (capable of transforming
radiation into thermal energy) and should emit (or loose) little
heat through radiation.
The transparent cover should produce an intense greenhouse
effect, while the insulation should not absorb water and should
be able to operate under temperatures ≃100–140°C.
Focusing collectors can be fixed or mobile; they use optic systems to concentrate solar radiation on their small absorbers. The most complete versions contain an optic system (reflecting surfaces or lenses), a receiver (absorber), a tracking system (to determine the position of the sun and follow it) and a support structure. The ratio of the receiver's front surface to the optic system's surface is called the concentration ratio.
The main difference between flat-plate and focusing collectors is that the former use total radiation, while the latter employ only the direct component. Indeed, focusing collectors convert a lower degree of potential energy into thermal energy and require clear skies.
The efficiency (μ) is the ratio of the collector's thermal power to the power of the solar radiation incident on the receiving surface (in the case of flat-plate collectors, this coincides with the dimensions of the collector itself; in the case of focusing collectors, the front surface of the optic system has to be considered). It may be observed that:
μ = C1 - C2 * (Ti - Tamb)/I
where:
Consequently, μ increases as Ti approaches Tamb.
The fact that availability of the solar source and the user's requirements are almost never in tune with each other necessitates the use of some form of storage (generally tanks of water at a temperature Ts).
The operative rationale is the following: when the collector is
capable of supplying water with Tu>Ts, the solar plant starts to
work, and the temperature Ts increases over time. When Tu>Ts, the
plant stops working. This simple form of control can be
activated by a differential thermostat.
Each time a user takes H2O out of the tank, cold H2O is added
(and Ts decreases).
Consequently, the storage tank's specific volume Vs (1 per m2 of
collector) influences the temperature range Ti and hence μ.
In order not to have a negative effect on the latter value, Vs is
selected so that Ti (and consequentely Ts) remains below the
desired temperature.
When energy production must be guaranteed, the solar plant is
always combined with a system whose production is continuous
(e.g., fuel oil or biomass boiler).
The two plants are connected after the storage tank so that Ts
may be kept as low as possible.
Finally, it should be remembered that the insertion of exchangers
into the circuit will result in an increase in Ti and thus a
decrease in μ.
References: [17], [40], [44], [54].
| Thermal Energy Production | ||
| Final Technology - Summary Table: | SOLAR COLLECTORS | |
| a) | operative flexibility: | limited |
| b) | operation: simple with flat-plate collectors (hot water production); less simple with focusing collectors (steam production) | |
| c) | most obvious limitations: i) processing plants would have to be redesigned (e.g., to use water heated to 60°C in cheese-making), and the processes themselves would probably have to be modified to make the best use of the solar source; ii) the source is not dependable; iii) flat-plate collectors can only be relied on to supply hot water for washing and energy for processes that do not require high temperatures; iv) focusing collectors require clear skies and a certain level of plant complexity; they are not usually recommended. | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| 1) | Flat-plate collectors: produced with components manufactured locally and capable of being incorporated directly into processing center roofs (as an integral part). The traditional 1–2 m2 models are not recommended. | |
| 2) | Focusing collectors: only parabolic-cylinder or fixed (e.g., Winston) versions. It should be kept in mind that maintaining operating temperatures of even 100°C, with reasonable yields, can be difficult. | |
| 3) | For all types: production of flat-plate collectors can only be handled by workshops that are able to work on thin sheet metal, weld and put together simple hydraulic plants. This type of workshop could also produce focusing collectors (simple parabolic-cylinders with absorbers that are not insulated). The following are also required for the support structure and moving parts of focusing collectors: carpentry, electric motors, reducers, gears and low level electronics. | |
| f) | energy transformation efficiencies: | |
| - flat-plate collectors that operate at temperatures < 70°C: 40–60%; | ||
| - focusing collectors that operate at temperatures of ≃150°C for 4–6 h/day: 10–20%. | ||
| g) | can this technology be considered self-sufficient? (*) | |
| - All types of plants: | NO | |
| Solar plants always have to be combined with other energy systems (generally boilers). | ||
These machines draw heat from one environment at a temperature of T1 and
transfer it to another environment at a temperature of T2 (T1<T2).
Compression heat pumps make use of a fluid (generally freon) that boils at
a low temperature (-20/-40°C) and circulates in a closed circuit made up
of:
The fluid is liquid at the condenser outlet (at a pressure p1 and a
temperature T1, generally > 30–40°C).
As it passes through the expansion valve (which is a capillary with Φ ~1–2
mm), it reaches p2 (< p1 because of the narrow neck) and T2 (< T1 because
of the evaporation of 10–20% of the fluid).
In the evaporator, the fluid boils at a low temperature, drawing heating
from the outside and turning it into steam.
The steam is sucked in by the compressor, which brings it up to p3=p1 and
T3 > T1. The fluid then cools down in the condenser and returns to the
liquid state. At that point the cycle starts again. Temperature and
pressure levels depend on the refrigerating fluid.
These machines are equipped with numerous auxiliary devices designed to
optimize the cycle and guarantee safe operation.
In the case of absorption heat pumps, the refrigerating fluid (e.g., NH3)
is sucked in by a fluid capable of absorbing it (a solvent; e.g., H2O).
The fluid is extracted from the latter substance by heating the mixture.
The advantage in this case is the use of thermal energy (85–200°C) rather
than mechanical energy. The most interesting aspect of the machine is the
fact that it does not have the compressor.
Generally, if the machine's primary function is to generate thermal energy
(by drawing it from any source), it is called a heat pump. If, on the
other hand, the goal is to draw thermal energy (dissipating it into the
environment), it is called a refrigerating machine.
These two operations may also coexist (e.g., a system that cools milk while
heating sanitary H2O).
Indeed, differences in construction are limited, and the two machines are
considered to be basically identical.
Models
Compression machines are classified on the basis of: the path followed by
the heat, the type of compressor and its operation.
Generally, the term “x-y” heat pump is used to described machines that draw
thermal energy from source x and transfer it to y.
Examples include air-air, air-water, water-air and water-water heat pumps.
The best sources are those that maintain a constant and sufficiently high
temperature throughout the entire period of their use.
When adjustments can be made to an x-y machine to turn it into a y-x
machine (that is, the evaporator will function as a condenser and vice
versa), it is reversible. Reversibility can be achieved with simple
hydraulic circuits, especially in the case of water-water models.
Performance
Performance is defined by:
Specifically:
COP = thermal power supplied (Qu)/mechanical power absorbed (L).
When nothing is lost:
COP = Qu/L = 1+ε.
The machine's performance depends on the temperatures of the hot (Tc) and
cold (Tf) source. The lower the value of δT=Tc-Tf (although δT>δTmin≃5-10°C),
the better the performance.
In addition, performance always has to be analyzed in relation to the value
of δT.
References: [5].
| Thermal Energy Production Final Technology - Summary Table: | HEAT PUMPS | |
| a) | operative flexibility: | high |
| b) | operation: simple when the cold source is the milk (or even whey) to be cooled. | |
| c) | most obvious limitations: i) the maximum temperatures that can be reached are ≃60°C, which is approximately the same as the limits posed by flat-plate solar collectors (see related table); ii) recovery is applicable only to electric machines; iii) refrigerating fluids (type R500) that limit the machine's refrigerating capacity (with respect to the traditional type, (R22) must be used to reach high temperatures at the condenser. | |
| d) | auxiliary machinery needed: | none. |
| e) | recommended models and their technological level: | |
| Machines whose evaporator and condenser are connected to refrigerating tanks (as in normal plants) and thermal storage tanks, respectively. For the sake of safety, the normal heat elimination procedure employing fans should be maintained. Production of heat pumps requires advanced technology. If compressors and regulating components are not supplied in-house, assembling workshops equipped with all machine tools are adequate. | ||
| f) | energy transformation efficiencies: | |
| A quantity of energy is recovered that is 30–50% greater than the heat drawn from the fluid to be cooled. | ||
| g) | can this technology be considered self-sufficient? (*) | |
| - All types of plant: | YES | |
| as far as the production of hot water (≃50°C) is concerned. | ||
The mechanical energy generated with wind and water power may be
transformed directly (by friction) or indirectly (through the use of
electric generators and resistors) into thermal energy.
The latter possibility is used most often (and is recommendable); in this
case, the frequency of the electricity produced does not have to be
checked.
| Thermal Energy Production | |
| Final Technology - Summary Table: | see WIND TURBINES |
| and WATER WHEELS AND TURBINES for electric energy production |
The purpose of this equipment is to exchange thermal energy between two
fluids (e.g., air and water) while keeping them physically separated.
Heat exchangers are normally made of metal (Al, Cu, etc.) because of its
excellent conductivity (when limited thickness is a requirement, however,
poor conductors, such as plastic, are also acceptable).
Heat exchangers are classified according to efficiency (εs), which is
defined by the ratio Q/Q∞, where Q is the thermal energy actually exchanged
and Q∞ is the thermal energy exchanged with a surface → ∞ under the same
conditions.
The simplest exchanger is composed of two coaxial pipes in which flow two
different fluids.
When the fluids circulate in the same direction, the exchanger is in
equiflow; otherwise, it is in counterflow.
When δT1 and δT2 indicate the differences in temperature at the two ends, then:
Q = kSδTml
where Q is the thermal power exchanged, k is the overall heat transfer coefficient, and δTml is defined by:
δTml = (δT1 - δT2)/ln(δT1/δT2)
It should be noted that:
k=1/(1/αa+1/αb+s/Γ),
where αa and αb are the convective thermal resistivities of the fluids (which depend on the type of fluid, its speed and on the type of wall), and s and Γ are the thickness and the thermal conductivity of the material.
Crossed currents exist when the two fluids move at right angles to each other. We can assume that:
δTml ≃ (Ti1-Ti2+Tu1-Tu2)/2
where Ti1, Ti2, Tu2 are the inlet and outlet temperatures. The subscript 1 always indicates the hotter fluid. More complicated situations should be compared with equicurrent and countercurrent exchangers for approximate evaluations. Whenever possible, ask the manufacturer for information about the exchanger's features.
The influence of any encrustations, oxidation or other deposits on the surface have to be evaluated in order to take the time effect into account.
References: [34].
| Thermal Energy Production | ||
| Final Technology - Summary Table: for thermal recovery and to reduce refrigeration requirements | HEAT EXCHANGERS | |
| a) | operative flexibility: | high |
| b) | operation: | simple |
| c) | most obvious limitations: i) milk precooling requires well water at temperatures under 15°C and in quantities equal to or greater than those of the milk; ii) the temperature of the heated water is at least 3–5°C lower than that of the cooled fluid (i.e., 25–28°C). The production of water for washing is the only feasible final use. | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| Countercurrent exchangers. Exchangers not produced by sheet metal forming (e.g., plate exchangers) require workshops that are capable of welding stainless steel pieces (in addition to working on sheet metal and pipes). | ||
| f) | energy transformation efficiencies: | |
| By cooling 1 1 of milk from 30 to 15°C, 1.1–1.2 1 of water can be heated to 25°C. With whey at 40°C, the same quantity of water can be heated to 35–37°C. | ||
| g) | can this technology be considered self-sufficient? (*) | |
| - All types of plants: | NO | |
| The result is the preheating of water for washing, which saves a small amount of energy. | ||
| Thermal Energy Production | ||
| Final Technology - Summary Table: | HEAT EXCHANGERS for geothermal media | |
| a) | operative flexibility: | high |
| b) | operation: simple when exchanger is made with the right material and used with fluids that are not overly saline. | |
| c) | most obvious limitations: | |
| 1 | For exchangers alone: i) chemical composition of the geothermal fluids; ii) temperatures reached by the fluids themselves (when < 80°C, which is very common, the problems encountered are similar to those observed with flat-plate solar collectors; see related table). | |
| 2 | For geothermal plants: iii) two wells are usually necessary; iv) cost of the wells; v) use of high quality pumps, when required; vi) thermal storage tanks are needed when low-capacity sources (or small plants) are used. | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| 1 | For exchangers alone: countercurrent exchangers. Exchangers produced without sheet metal forming (e.g., plate exchangers) require workshops that are capable of welding stainless steel pieces (in addition to working on sheet metal and pipes). | |
| 2 | For geothermal plants: the level of plant design depends on the temperature T. T<80°C does not present any particular problems (the same components and contractors involved in drilling drinking-water wells are also acceptable in this case); T>80°C requires a certain degree of specialization in terms of components (e.g., water-cooled pumps) and contractors. | |
| f) | energy transformation efficiencies: | |
| See table on geothermal sources. | ||
| g) | can this technology be considered self-sufficient? (*) | |
| - All types of plants: NO | ||
| The result is the preheating of water for washing, which saves a small amount of energy. | ||
This is the classic solution for the production of electricity in isolated areas. Diesel engines are usually used because of their low level of consumption.
Generally, with machines whose capacity is:
Specific consumption is higher in small machines, since smaller engines and
generators tend to be less efficient (mainly for economic reasons).
This aspect is more evident in generators (see Appendix 3 for general
characteristics) than in engines.
Endothermal engines use part of the energy freed by fuel combustion; these
include Diesel and Otto engines.
The consumption levels of the latter type of engine are generally 20–25%
higher than those of Diesel engines.
It should be noted that specific consumption increases for all engines as
the load decreases. For more complete information about performance, the
following values have to be known:
Parameters a) and c) make it possible to identify the machine's basic features and determine whether it is suitable for use. Map c) makes it possible to evaluate the behaviour of the specific consumption curve as a function of the load and the related efficiencies.
The following fuels can be used:
As mentioned above, renewable liquid fuels are not treated as potential
energy sources since their small-scale production for self-consumption
(i.e., as part of the processing center's activities) is not recommended,
at least initially.
It is felt that the problems connected with their production should be
handled on the macroeconomic level (and thus are beyond the scope of this
report).
However, Appendix 4 contains information on the general characteristics of
these fuels.
The summary tables presented below consider the practical performance of
engine-generator sets in relation to type of feeding. It is assumed that
no information is necessary with regard to feeding with traditional fuels.
It should be emphasized that Otto engines can be gas-, ethanol- and
methanol-fed.
Diesel engines can be fed with vegetable oils, or with any of the renewable
sources listed above with the aid of the “dual-fuel” system.
References: [1], [2], [19], [32], [38], [41], [48], [55], [56].
| Electric Energy Production Final Technology - Summary Table: | GENERATOR SETS FED BY GASIFICATION GAS | ||
| a) | operative flexibility: | high | |
| b) | operation: fairly simple when the gasification gas (see related intermediate technology table) is cooled (so that all condensables are eliminated) and filtered (to eliminate all particles). But operation is more difficult than in the case of similar engines fed with standard quality fossil fuels. The lubricating oil has to be changed more frequently, and various components have to be checked more often. | ||
| c) | most obvious limitations: (with respect to standard models) | ||
| 1) | Otto engines: i) the feed system has to be modified substantially and the ignition system has to be adjusted; ii) the use of fossil fuels is no longer possible; iii) although high energy transformation efficiencies can be obtained, the maximum power that can be developed is approximately 50% that generated by fossil fuels (thus, bigger engines are needed); | ||
| 2) | Dual-fuel Diesel engines (where the gas is mixed with comburent air): i) the feed system has to be modified substantially (in terms of both air intake and injection equipment); ii) a fairly sophisticated governing system is required; iii) at maximum load, approximately 15% of the energy requirement has to be supplied by Diesel fuel; with variable loads (which is almost always the case), this percentage can reach 50%; | ||
| 3) | For all types of engines: i) the cleanness of the components has to be checked continually (especially the valves); ii) the noise level should be considered (this problem also exists in standard models). | ||
| d) | auxiliary machinery needed: | none | |
| e) | recommended models and their technological level: | ||
| 1000–1500 rpm engines with water cooling to make use of thermal recovery (see cogeneration table); electronic ignition (Otto engine). Self-exciting generators without brushes. | |||
| The technological level required for construction of these engines is well known. Simple transformation (for the use of renewable fuels) does not require special equipment, but a good understanding of this field is still necessary. | |||
| f) | efficiencies (%) (*): | ||
| element | 30% of load | max. load | average real values |
| otto engine | 10–15 | 20–25 | 15–20 |
| diesel engine | 18–22 | 30–35 | 25–30 |
| 5 kW generator | - | - | 70–75 |
| 20 kW generator | - | - | 80–85 |
| 100 kW generator | - | - | >90 |
| 5 kW otto set | 7–12 | 10–15 | ≃10 |
| 5 kW diesel set | 12–16 | 20–25 | ≃18 |
| 20 kW otto set | 10–14 | 17–22 | ≃14 |
| 20 kW diesel set | 15–19 | 24–30 | ≃22 |
| 100 kW otto set | 10–15 | 20–25 | ≃18 |
| 100 kW diesel set | 18–22 | 27–33 | ≃24 |
| g) | can this technology be considered self-sufficient? (**) | ||
| YES | |||
| Since the gasifier can provide a constant supply of gas. | |||
(*) Calculated in relation to the fuel. Multiply these values
by ≃0.7 to take the gasification process into consideration as well
(in which case, efficiency will refer to the chemical energy of the
biomass).
(**) NO/YES: frequent replacement of the source by another, more readily available source is/is not suggested. Here, reference is to the gasifier-engine-generator.
| Electric Energy Production Final Technology - Summary Table: | GENERATOR SETS FED BY BIOGAS | |
| a) | operative flexibility: | average |
| b) | operation:fairly simple when the engine does not contain copper parts (e.g., all the bearings must be made of white metal), and the biological gas (see related intermediate technology table) has been sufficiently cooled (so that all the vapour is eliminated). But operation is more difficult than in the case of similar engines fed with standard quality fossil fuels. The lubricating oil has to be changed more frequently. | |
| c) | most obvious limitations (with respect to standard models): | |
| 1) | Otto engines: i) the feed system has to be modified and the ignition system has to be adjusted; ii) although high energy transformation efficiencies can be obtained, the maximum power that can be developed is approximately 60–80% that generated by fossil fuels (thus, bigger engines are needed); | |
| 2) | Dual-fuel Diesel engines (where the gas is mixed with comburent air): i) the feed system has to be modified substantially (in terms of both air intake and injection equipment); ii) a fairly sophisticated governing system is required; iii) at maximum load, approximately 15% of the energy requirement has to be supplied by Diesel fuel; with variable loads (which is almost always the case), this percentage can reach 50%; | |
| 3) | For all types of engines: i) the cleanness of the components has to be checked continually (especially the valves); ii) the noise level should be considered (this problem also exists in standard models). | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| 1000–1500 rpm engines with water cooling to make use of thermal recovery (see cogeneration table); electronic ignition (Otto engine). Self-exciting generators without brushes. | ||
| The technological level required for construction of these engines is well known. Simple transformation (for the use of renewable fuels) does not require special equipment, but a good understanding of this field is still necessary. The transformation procedure for biological gas is simpler than that for gasification gas. | ||
| f) | efficiencies (%) (*): | |
| See values for gasification gas. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| NO | ||
| Since digesters (especially smaller models) may vary or stop gas production for numerous reasons. Normalization or restarting can take days (3–10). However, biological gas-fed engines can also operate with other fuels. | ||
(*) Calculated in relation to the fuel.
(**) NO/YES: frequent replacement of the source by another,
more readily available source is/is not suggested.
| Electric Energy Production Final Technology - Summary Table: | GENERATOR SETS FED BY ALCOHOL | |
| a) | operative flexibility: | high |
| b) | operation: similar to fossil fuel-fed models. | |
| c) | most obvious limitations: (with respect to standard models) | |
| 1) | Otto engines operating on gasoline-alcohol mixtures (generally 5–25% ethanol or methanol): i) possible problems connected with the mixture's instability; | |
| 2) | Otto engines operating on alcohol alone: i) the engines have to be modified substantially; ii) the use of fossil fuels is no longer possible; | |
| 3) | Dual-fuel Diesel engines (alcohol is added to the comburent air): i) the feed system has to be modified substantially (in terms of both air intake and injection equipment); ii) a fairly sophisticated governing system is required; iii) at maximum load, approximately 15% of the energy requirement has to be supplied by Diesel fuel; with variable loads (which is almost always the case), this percentage can reach 30–50%. | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| 1000–1500 rpm engines with water cooling to make use of thermal recovery (see cogeneration table); electronic ignition (Otto). Selfexciting generators without brushes. | ||
| The technological level required for construction of these engines is well known. Transformation for the use of mixtures (Otto engines) does not require special equipment or experience. However, this is not the case with transformation for the use of alcohol alone. | ||
| f) | energy efficiencies (%) (*) | |
| See values for gasification gas. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| YES | ||
| As long as the fuel is constantly supplied. | ||
(*) Calculated in relation to the fuel. See Appendix 4 for information
concerning the fuel production process.
(**) NO/YES: frequent replacement of the source by another, more readily
available source is/is not suggested.
| Electric Energy Production Final Technology - Summary Table: | GENERATOR SETS FED BY VEGETABLE OILS | |
| a) | operative flexibility: | high |
| b) | operation: fairly simple when the oil has been carefully filtered and degummed, but operation is more difficult than in the case of similar, standard quality, Diesel-oil-fed engines. The various components have to be checked more frequently. | |
| c) | most obvious limitations: (with respect to standard models) | |
| 1) | Feeding with Diesel-vegetable oil mixture vv (generally 20% vegetable oil): none; | |
| 2) | Feeding with vegetable oil alone: i) precombustion Diesel engines have to be used (although these engines are being phased out in agriculture); ii) possible paraffin formation at temperatures as low as OfC; iii) the components' condition must be checked constantly; iv) the noise level should be considered (this problem also exists in standard models). | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| 1000–1500 rpm/l engines with water cooling to make use of thermal recovery (see cogeneration table) and good fuel filtering. Selfexciting generators without brushes. | ||
| The technological level required for construction of these engines is well known. Simple transformation (for the use of vegetable oils) does not require special equipment, but a good understanding of this field is still necessary. | ||
| f) | efficiency (%) (*): | |
| element | 30% of load | max.load | average real values |
| diesel engine | 18–22 | 30–35 | 25–28 |
| 5 kW generator | - | - | 70–75 |
| 20 kW generator | - | - | 80–85 |
| 100 kW generator | - | - | >90 |
| 5 kW set | 12–16 | 20–25 | ≃18 |
| 20 kW set | 15–19 | 24–30 | ≃22 |
| 100 kW set | 18–22 | 27–33 | ≃24 |
| g) can this technology be considered self-sufficient? (**) |
| YES |
| As long as the fuel is continuously supplied. |
(*) Calculated in relation to the fuel. See Appendix 4 for information
concerning the fuel production process.
(**) NO/YES: frequent replacement of the source by another, more
readily available source is/is not suggested.
The most interesting models to consider are steam and Stirling engines. The basic feature of these engines is external combustion. Therefore, they can hypothetically use any type of fuel whatsoever (including solids).
Steam engines are already in use in some developing countries. When less power is needed, they provide some advantages over steam turbines, which are not recommended for the applications discussed here (because of the limited power required). Steam engines naturally have to be combined with steam boilers (adapted to operate at 10 bar and ≃350°C).
Stirling engines are operated by the expansion and contraction of a gas (usually air or helium) through a hot and cold source, both of which are located outside the machine. Engines with small capacities (4–5 kW) are currently available, but they are expensive and difficult to find (for commercial reasons).
References: [4], [5], [9], [26], [51].
| Electric Energy Production Final Technology - Summary Table: | STEAM SETS FED BY VARIOUS FUELS | |
| a) | operative flexibility: | high |
| b) | operation: fairly simple. Considerable amount of labor required to load the fuel into non-automatic systems (10–12 hours/day for continuous function systems). | |
| c) | most obvious limitations: none that are particularly serious. However, it should be noted that: i) water consumption is high (water must be filtered and chemically treated); ii) the simplest models have a limited number of revolutions (200–300 rpm), which necessitates the use of multipliers for connection with generators. | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| Existing models are available in small numbers, and consequently it is difficult to recommend a specific version. The technological level required for construction of the simplest models (manual fuel loading and release of exhaust steam into atmosphere) is not high, and it is feasible for manufacturers located in underdeveloped countries. It is similar to the level required for production of simple, single-cylinder Otto engines. | ||
| f) | efficiencies (*): | |
| - simple models: 5%; | ||
| - improved models: 5–10%. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| YES | ||
| As long as the required fuel is available (e.g., wood). | ||
(*) Calculated as the ratio of the electric energy produced to the energy
of the fuel employed (usually solid).
(**) NO/YES: frequent replacement of the source by another, more readily
available source is/is not suggested. Here, reference is made to a
steam boiler-engine-generator set.
| Electric Energy Production Final Technology - Summary Table: | STIRLING SETS FED BY VARIOUS FUELS | ||
| a) | operative flexibility: | high | |
| b) | operation: fairly simple, except in the case of fuel loading. | ||
| c) | most obvious limitations: none, except the fact that commercial models are hard to find. | ||
| d) | auxiliary machinery needed: | none | |
| e) | recommended models and their technological level: | ||
| Existing models are available in small numbers, and consequently it is difficult to recommend a specific version. | |||
| The technological level required for construction of modern Stirling engines suitable for operation at a fixed site is high. In fact, research into these engines is currently being conducted in developed countries only. | |||
| f) | efficiencies (*): | ||
| - mechanical efficiençies with modern versions operating at full load: | |||
| i) | using helium as working gas: | 20–25%; | |
| ii) | using air as working gas: | ≃6%; | |
| - corresponding overall efficiencies (electric energy production) at full load: | |||
| i) | with small capacities (< 5 kW): | 5–15%; | |
| ii) | with highly efficient generators and engines: | 18–23%. | |
| g) | can this technology be considered self-sufficient? (**) | ||
| YES | |||
| As long as the fuel is continuously supplied. | |||
(*) Calculated in relation to the fuel, and considering a heat generatorengine-generator set.
(**) NO/YES: frequent replacement of the source by another, more readily available source is/is not suggested.
These machines are divided into two groups: water wheels and water turbines. The former are used for the production of mechanical energy (low power with a small number of revolutions), while the latter are normally combined with generators for the production of electric energy.
Water Wheels
Wheels that convert the potential energy of a head are equipped with boxes
(overshot wheels), while those that convert the kinetic energy of streams
have paddles (undershot wheels).
The power P obtainable with overshot wheels is calculated as follows:
| p=10*QHμ | [kW] |
where Q is the flow rate [m3/s], H is the available head, and μ is the
wheel's overall efficiency (0.5–0.7).
For example, if Q=20 l/s and the wheel's diameter and width are 3 and 0.17
m, respectively, then p= 0.4 kW (at 9.5 rpm).
For undershot wheels:
| p=0.25*AV3μ | [kW] |
where A is the submerged section measured perpendicularly to the flow [m2],
V is the speed of the current [m/s], and μ is the overall efficiency (0.5–
0.7).
Given the low number of revolutions (6–20 rpm), wheels are not highly
recommended for the production of electric energy (a velocity ratio of
≃1/100 is required).
Water Turbines
Water turbines are basically composed of a nozzle (or stationary guide
vanes) and a runner (or rotor, or propeller).
The purpose of the nozzle (or stationary guide vanes) is to direct the
water to the runner and transform (completely or partially) its pressure
energy into kinetic energy. The runner is composed of vanes that convert
the energy of the water Ea into mechanical energy Em (rotation around a
fixed axis; machine efficiency: μ=Em/Ea).
When all the energy at the runner's inlet is kinetic, the machine is called
an impulse turbine; when the energy is mixed (i.e., in the form of
pressure and velocity), it is called a reaction turbine. The latter are
also equipped with a diffuser, which connects the rotor's outlet to the
tailrace. Its purpose is to suck in the water (this is important for low
heads). Each kind of turbine is suitable for different values of available
head.
Propeller or Kaplan Turbines (reaction turbines)
These turbines operate with low heads (2–20 m) and high flow rates. The runner is composed of a bulb-shaped hub and 4–6 adjustable vanes, which guarantees high efficiencies μ (defined as the ratio of the mechanical energy produced to the total energy of the flowing water) even with variable flow rates (0.8<μ<0.9).
Francis Turbine (reaction turbine)
Suitable for average heads (15–150 m); composed of a rotor (rotation speed: 250–1000 rpm) with stationary guide vanes. (0.8<μ<0.9).
Pelton Turbine (impulse turbine)
Requires large heads (> 100 m) and the runner (rotation speed: 500–1000 rpm) is composed of a disk around which a set of vanes (in the shape of a double spoon) are placed. Injectors (1–6) direct an equal number of jets towards the paddles, thereby generating torque (0.8<μ<0.9).
Other kinds of turbines also exist, including the Banki turbine which is suitable for heads between 1 and 200 m; in this case, the water passes through the rotor.
To calculate the amount of power that can be produced, it is necessary to evaluate:
μt=μμcμg
where μ is the water machine's efficiency (defined above), μc is the water pipe's efficiency (if this exists; ≃ 0.93–0.98), and μg the electric generator's efficiency (0.9–0.98 with average and high capacities).
When the plant has been well constructed, μt=0.65–0.88.
References: [3], [23], [24], [30].
| Electric Energy Production Final Technology - Summary Table: | WATER WHEELS | |
| a) | operative flexibility: | high |
| b) | operation: fairly simple. | |
| c) | most obvious limitations: i) requires the presence of a stream with the right characteristics near the processing center; ii) the number of revolutions is low: therefore, overgears are needed for connection with generators; iii) when thermal energy is desired, heat storage is required to limit the amount of installed power. | |
| d) | auxiliary machinery needed: none in particular, except that related to water intake and conveyance. | |
| e) | recommended models and their technological level: | |
| Versions are based on the type of stream under consideration. Naturally, the most efficient paddle design should be selected. | ||
| The technological level required for construction of water wheels is not high. Workshops must be capable of working with and welding sheet metal, or working with wood. | ||
| f) | efficiencies (*): | |
| - electricity generation (regulated electricity): 20–25%; | ||
| - electricity generation (unregulated electricity for heat production): 30–35%. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| YES | ||
| As long as the flow rate is constant. | ||
(*) Calculated in relation to the energy of the stream.
(**) NO/YES: frequent replacement of the source by another,
more readily available source is/is not suggested.
| Electric Energy Production Final Technology - Summary Table: | WATER TURBINES | |
| a) | operative flexibility: | high |
| b) | operation: fairly simple. | |
| c) | most obvious limitations: i) requires the presence of a stream with the right characteristics near the processing center; ii) operations connected with water conveyance are generally laborious and sometimes complex; iii) when thermal energy is desired, heat storage is required to limit the amount of installed power. | |
| d) | auxiliary machinery needed: none in particular, except that related to water intake and conveyance. | |
| e) | recommended models and their technological level: | |
| Versions are based on available head. The Michell-Banki model is recommended for use in poor countries because of its operative flexibility. | ||
| The technological level required for construction of Pelton, Francis and Kaplan turbines is fairly high when satisfactory efficiencies are desired. Workshops must be able to carry out all mechanical and foundry operations. However, Michell-Banki turbines can be produced in simpler workshops (capable of working with and welding sheet metal and doing basic work on metal parts), but adequate technical back-up is required. | ||
| f) | efficiencies (*): | |
| - electricity generation (regulated electricity): 60–90%; | ||
| - electricity generation (unregulated electricity for heat production): 70–90%. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| YES | ||
| As long as the flow rate is constant. | ||
(*) Calculated in relation to the energy of the head.
(**) NO/YES: frequent replacement of the source by another, more readily
available source is/is not suggested.
When certain materials are reached by solar radiation, they generate an electromotive force (photovoltaic effect). Examples include silicon crystal wafers (thickness: 0.2–0.4 mm) cut from bars and contaminated by impurities in order to turn the two sides into positive and negative semiconductors, respectively.
When solar radiation is present, electric energy is supplied by connecting the bottom surface (metalized) to the top (to which a metal grate is applied).
Other materials can be used in addition to silicon. Examples include indium phosphorus (InP), gallium arsenide (AsGa) and cadmium sulfide combined with copper sulfide (CdS-Cu2S). Amorphous silicon can be very useful (because of its low cost), but its duration is limited.
The cells are characterized by peak power (Pp), which is the electric power supplied with 1000 W/m2 of radiation.
A complete photovoltaic plant is composed of:
References: [9], [21], [43].
| Electric Energy Production Final Technology - Summary Table: | PV SYSTEMS | |
| a) | operative flexibility: | low |
| b) | operation: fairly simple. | |
| c) | most obvious limitations: i) solar energy varies over time, and hence photovoltaic collectors have to be connected with other systems to ensure continuous service; ii) complete plants (i.e., with storage and inverter for the production of alternate current) are required for application to processing plants; to simplify the system, existing milk processing plants would have to be modified (changeover to direct current motors, etc.); iii) high construction costs. | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| The characteristics of existing milk processing plants require systems composed of: collectors, regulating equipment, electric storage and inverter. | ||
| The technological level required for their construction is very high, and only developed countries are currently in a position to produce these systems. | ||
| f) | efficiencies (*): | |
| - complete systems: 5–10%. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| NO | ||
| Since a solar source cannot be relied on to power a continuous process, unless the system is highly oversized. | ||
(*) Calculated as a ratio of electric energy produced (A.C.) to
incident solar energy.
(**) NO/YES: frequent replacement of the source by another, more readily
available source is/is not suggested.
Wind generators can be divided into two groups: those with horizontal axes
and those with vertical axes. The former (unlike the latter) do not have
moving parts that are faster than the wind, and they must rotate around a
vertical axis for the rotor to be in operating position.
Machines with 1–3 blades are generally used for electricity production.
The most complete versions include:
The supply of electric energy is dependent on the wind speed v. Once three typical values of v (v1, v2, v3, where v1 <v2 <v3) have been established, the machine will operate in the following manner:
| - when v<v1, | the machine will not start; |
| - when v1<v<v2, | variable power is supplied; |
| - when v2<v<v3, | constant power is supplied, |
| - when v>v3, | the machine stops working to prevent damage. |
| Generally: | v1≃ 5 m/s; v2≃ 12 m/s; and v3≃ 25–30 m/s. |
Between v1 and v2, electric energy is not regulated (variable frequency).
Theoretically, the transformation efficiency μ (defined as the ratio of energy produced to wind energy) can reach 59% (Betz's criterion), but it generally ranges from 10 to 40%.
In brief:
When the machine is connected to the national grid, speed can be controlled by a generator excited by the grid itself (hence, the machine is forced to rotate at a fixed number of revolutions).
References: [9], [20], [27], [36], [50].
| Electric (and Thermal) Energy Production Final Technology - Summary Table: | WIND TURBINES | |
| a) | operative flexibility: | low |
| b) | operation: fairly simple when the machine has been designed well | |
| c) | most obvious limitations: i) wind energy is extremely variable over time, and the production of regulated electricity is always a problem; the procedure can be simplified by equipping milk processing plants with direct current motors; ii) storage is always necessary in order to avoid oversized plants; iii) possible operation in parallel with other generators requires fairly complex energy plants. | |
| d) | auxiliary machinery needed: | none |
| e) | recommended models and their technological level: | |
| The need to store energy (see Appendix 5) means that direct current machines have to be chosen. | ||
| The technological level required for construction of a good wind system is high. A clear understanding of the basic problems encountered in this field is the most important requirement. | ||
| f) | efficiencies (*): | |
| Average operating efficiencies are closely connected with the average wind speed v. | ||
| In places where average annual v > 5 m/s: | ||
| - electricity generation (regulated electricity): 8–10%; | ||
| - electricity generation (unregulated electricity for heat production): ≃15%. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| NO | ||
| Since wind speed is extremely variable over time. | ||
(*) calculated as the ratio of electric energy to the wind's kinetic
energy. Set considered: wind generator-storage-inverter.
(**) NO/YES: frequent replacement of the source by another, more readily
available source is/is not suggested.
As we have seen, processing centers require electric and thermal energy. Consequently, a plant capable of the simultaneous generation of these two types of energy certainly merits discussion here.
Cogenerators are based on the recovery of waste heat from an engine (internal or external combustion) connected to a generator. For example, with generators based on Otto (fed with any kind of fuel) or Diesel engines, 10–30% of the fuel's energy is transformed into electric energy, and the remaining portion is dispersed as heat by the exhaust gas (30–35%) and engine and lubricating oil cooling (30–40%). Thus, it is possible to recover (using simple exchangers) thermal power that is 1.2–2.5 times greater than electric power (total efficiency: 75–95%).
Positive results can only be obtained with water-cooled engines. When all the exchangers (operating on the exhaust gas and engine, respectively) are in series, thermal energy at ≃80°C can be produced.
Cogeneration can also be applied to steam and Stirling engines.
Steam engines, unlike all other types of engines, can produce thermal energy independently from electric energy. Indeed, in addition to recovering steam from the engine's exhaust, desired quantities of this product can also be drawn directly from the boiler.
References: see section 6.
| Cogeneration Final Technology - Summary Table: (Internal and External Combustion Engines) | GENERATOR SETS | |
| a) | operative flexibility: | average |
| b) | operation (apart from the engines' requirements): no problem when the plants have been designed well. | |
| c) | most obvious limitations (apart from those connected with the engines): i) the thermal power supplied (except in the case of steam engines) is closely connected with the electric power by precise ratios that vary from 1.5 to 2.5; however, at processing centers, the ratios of thermal to electric requirements very from 0.3 to 2.7; therefore, cogeneration is not always suitable for the center's production requirements; ii) production of thermal and electric energy is simultaneous, but related requirements are not; thus, thermal storage is necessary; iii) steam (except in the case of engines fed with this medium) can only be produced with exhaust gas (which supplies slightly less than half the thermal energy recoverable). | |
| d) | auxiliary machinery needed (in addition to that required for the engines): | none |
| e) | recommended models and their technological level: | |
| Water-cooled engines are recommended because of the ease of thermal recovery. Steam engines (and their related boilers) are best suited for these purposes. | ||
| The technological level required for transformation of engines and cogenerators is quite high. | ||
| f) | energy efficiencies (*): | |
| - Otto, Diesel and Stirling engines: 60–80%. | ||
| g) | can this technology be considered self-sufficient? (**) | |
| YES | ||
| As long as the processing center has ratios of thermal to electric requirements that are similar to the cogenerator's production. | ||
(*) Calculated as the ratio of total energy produced (electric + thermal) to that of the fuel used.
(**) NO/YES: frequent replacement of the source by another, more readily
available source is/is not suggested.
