The basic characteristics of each source, methods for evaluation of its
energy potential and the extent of its availability are discussed below.
A summary table containing all the characteristics required for a clear
understanding of the most important aspects connected with the source's
use in milk processing is also presented.
The following is an outline of the table, including definitions of the
headings used:
| Summary Source Table: | identification |
operative flexibility: flexibility means the possibility of using the source as desired. For example, biomass is a very flexible source, since it can be obtained when needed, is easy to accumulate and can generate as much energy as required. On the other hand, solar energy is highly inflexible.
availability: information is provided about areas in which the source under examination may be considered relatively abundant.
essential data for preliminary evaluation: that is, the minimum information to be provided to an expert for the design of energy conversion plants.
typical energy intensity values: data that would enable anyone to make preliminary evaluations.
share (%) recoverable for the production of energy in the form of:
| 1 - water at 40–70°C | 2 - steam(*) | 3 - electricity | ||
Positions 1, 2 and 3 give the percentage of energy recoverable using common energy conversion technologies (on the source under examination) for the production of hot water, steam and electricity. The values are averages and refer to actual situations.
figure calculated from the characteristics of the source, needed to meet
the energy requirements of a center with a daily milk intake of 1000 l/day
(**)
(unit of measure: ):
| Simplif. Proc. Plants | Complete Proc. Plants | |||||
| Thermal req. | Electric req. | Thermal req. | Electric req. | |||
This section gives an indication of the degree to which the source may be utilized to meet the processing and treatment requirements of 1000 1/day of milk. Values relating to thermal energy tend to vary widely as a function of the type of process; minimum values refer to plant washing alone (necessary if refrigeration is the only process involved), while maximum values refer to pasteurization of all the milk. In practice, minimum and maximum values reflect the two extremes (lowest and highest, respectively) of the energy requirements of 1000 1 of milk, shown in Figures 1 and 3. Naturally, a more precise value can be calculated using these figures and simple ratios. The same is true of electric energy (Figures 2 and 4).
remarks: miscellaneous information.
(*) low-pressure, or in some cases water heated to 80–90°C
(**) with different intake levels, use proportional values
Solar energy is the perturbation emitted by the sun following the thermonuclear reactions that occur in its interior (the resultant power having an effect on the earth is estimated at 1.8×1017 W). There are two interpretations of this phenomenon, which are called the corpuscular and undulatory theories. The former involves a flow of particles (photons), while the latter is based on electromagnetic radiation. These interpretations explain the photoelectric effect and optic phenomena, respectively.
Outside the earth's atmosphere, radiation is practically constant over time, but on the earth it is highly variable. The reason for this is:
These factors affect the thickness, s, of the air which the radiation passes through. This in turn determines the amount of energy lost, which is mainly due to air molecules and the presence of steam and atmospheric dust. In other words, the larger the value of s, the more the radiation is weakened.
The following distinctions are generally made: direct radiation (coming
from the sun without any deflection) and diffuse radiation (subjected to
deflection and scattering). The former casts distinct shadows, while the
latter uniformly illuminates an object. The sum of these two types of
radiation is the total radiation.
On clear days, scattered radiation represents 20–22% of the total, while it
represents 100% on cloudy days.
Insolation (or sunshine or duration of the sun) represents the number of
hours per day, month or year in which there is direct radiation.
The term solar radiation can represent both the thermal power supplied by the sun and the energy made available to one m2 of surface area during relatively long periods of time (day, month or year). Consequently, it is important to clearly specify the unit of measure adopted.
Total radiation is measured by pyranometers. These instruments link
radiated energy to the difference in temperature between a black surface
and a light colored surface, both of which are exposed to radiation (or the
difference between a black surface and the environment).
Scattered radiation can be evaluated by screening the pyranometer's sensor
(so that it is only illuminated by light from the sky and the reflection
from the surrounding objects), while direct radiation can be measured through
difference (in this case, two instruments are needed).
Sensors based on the photoelectric effect or the sensitivity of some solar
energy resistors are also available.
Outside the earth's atmosphere, the power of radiation is 1.35 kW/m2 (solar
constant).
Seasonal variations are limited.
On the earth's surface, its power varies from 0 to 1 kW/m2.
The energy produced by radiation is influenced by the latitude, the local
climatic conditions and the position of the receiving surface (orientation
and slope with respect to a horizontal plane).
Average daily radiation on a horizontal plane is usually available for each
month (see summary table).
It is always important to determine whether these values have been measured
or calculated (by simulation models).
The validity of the latter data is closely connected with the programmer's
skill (since the must also be aware of the site's climatic characteristics),
and these data are acceptable for comparative evaluations only.
References: [9], [13], [17], [21], [29], [37], [40], [43], [44] [49], [50].
| Summary Source Table: | SOLAR RADIATION |
| a) operative flexibility: | very poor |
| b) availability: generally good at sites whose latitude is between 35°N and 35°S | |
| c) essential data for preliminary evaluation: average monthly radiation values on a horizontal surface | |
| d) typical energy intensity values: | |
| • maximum power: | 1 kW/m2 |
| • maximum daily radiation: | 25–30 MJ/m2 |
| • average annual radiation in the Tropics: | 17–22 MJ/(m2.day) |
e) share (%) recoverable for the production of energy in the form of:
| 1 - water at 40–70 fC 40–60 | 2 - steam (*) 15–20 | 3 - electricity 5–10 |
f) figure calculated from the characteristics of the source, needed to
meet the energy requirements of a center with a daily milk intake of
1000 1/day (**)
(unit of measure: m2 of receiving surface):
| Simplif. Proc. Plants | Complete Proc. Plants | |||||
| Thermal req. 4.30 | Electric req. 60–90 | Thermal req. 4–100 | Electric req. 80–160 | |||
g) remarks:
Reference is made to average Tropical conditions (results are also valid
for summers in many other locations).
(*) low-pressure, or in some cases water heated to 80–90°C
(**) with different intake levels, use proportional values
Solar energy absorbed by the earth produces the upward motion and expansion
of air which create areas of high and low pressure.
The latter contain air currents (winds) whose direction is influenced by
the earth's rotation and the force of gravity.
The kinetic energy in these currents is called wind energy.
The horizontal component of wind speed is generally larger than the
vertical component. There is a negative and positive correlation,
respectively, between the two components and the ambient temperature.
The vertical component always generates weather disturbances (gusts or
blasts of wind, etc.).
The horizontal component, V, equals zero at ground level (z=0) and varies
according to an undefined law until it reaches the top of any obstacles it
encounters (trees, buildings, hills, etc.). It then follows a curve
represented by the expression:
V = V′* 1n[(z-d)/z′],
where:
V′ is the speed at z→ ∞;
d is a value which is slightly lower than the height of the obstacles;
z′ is a coefficient that depends on the irregularity of the contour of the
land.
Standard meteorological data refer to the wind speed measured at a height
of 10 m (Vs).
The speed V can be evaluated at height z by using the equation:
V=Vs*(z/10)n
where n = 0.143 for open spaces. This equation is used to obtain an initial orientation (n varies during the course of the day and according to the season) and when z < 50 m.
In addition, the behavior of the wind speed over time (on an annual basis)
is an essential element.
Experience shows that the distribution of the probability φv that a given V
will occur can be estimated by using Weibull's function:
and that the probability φv of there being a V > V′ is equal to:
where:
k may vary from 1.6 to 3;
c ≃ 2Vm / √ ∏
Vm is the average speed at the site.
When k=2 (an acceptable value in the majority of cases), Weibull's function is the same as Rayleigh's function:
In addition, the maximum value of φv is reached when V=0, 8*Vm. Therefore, knowing Vm is sufficient for obtaining preliminary values. The directional distribution of the wind is also important.
Wind energy is calculated on the basis of knowledge of the wind speed, V.
Anemometers placed at a height of 10 m are normally used to obtain this
value. These instruments provide a signal (usually an impulse) which is
proportional to the number of rotations.
Readings should be taken continuously so that fluctuations in wind speed
and direction (which have a significant impact on, processing efficiency and
the machine's endurance strength) can be evaluated.
The power Pv of wind with a speed V is equal to:
| Pv = AμV2/2 = 1.24*AV2/2 = 0.62*AV2 | [W] |
where:
μ is the mass of a unit volume of air;
A is the area of the section under consideration (perpendicular to the wind's flux lines).
In the usual case of circular sections with a diametre D:
| Pv = 0.487*V2D2 | [W] |
The average power Pm may be calculated by considering the average speed Vm during the time period under examination. In fact, it can be shown that:
| Pm ≃ μA(Vm)3 = 1.24*A(Vm)3 | [W] |
Going back to the distribution of Weibull's and Rayleigh's functions, the
product P *Φv represents the distribution of the various wind powers as a
function of V.
The maximum value of Pv*φv is obtained when V=1.6*Vm.
Evaluation of the duration of periods in which V ≃ 0 is also very useful. In actuality, when V m>12 m/s, some Pv is always present. When 5<Vm<8 m/s, periods of Pv≃0 are brief; when Vm<5 m/s, the duration of periods in which pv≃0 may be unacceptable.
It is estimated that approximately 1% of the solar energy that reaches the earth is transformed into the kinetic energy of wind (1.2*1015 W). Wind energy is generally available along the coast.
References: [9], [20], [27], [36], [50].
| Summary Source Table: | WIND |
operative flexibility: average only in “windy” places
areas of acceptable use: anywhere, as long as the average wind speed is equal to or greater than 5 m/s
essential data for preliminary evaluation: average annual speed; information about the duration of periods in which there is no wind
typical energy intensity values:
annual energy available at an average speed of 5 m/s:
490 MJ/m2 of surface perpendicular to the wind flux
share (%) recoverable for the production of energy in the form of:
| 1 - water at 40–70°C 10–15 | 2 - steam (*) 10–15 | 3 - electricity 10–15 |
figure calculated from the characteristics of the source, needed to
meet the energy requirements of a center with a daily milk intake of
1000 1/day (**)
(unit of measure: m2 of surface receiving the wind at 5 m/s):
| Simplif. Proc. Plants | Complete Proc. Plants | |||||
| Thermal req. 60–450 | Electric req. 250–450 | Thermal req. 60–1500 | Electric req. 250–700 | |||
remarks:
The contributions made by wind energy are highly dependent on the
average speed at the site.
(*) low-pressure, or in some cases water heated to 80–90°C
(**) with different intake levels, use proportional values
The term hydro energy is used to indicate the potential energy that a stream loses (turning into kinetic energy) as a result of a head or sloping course.
The following aspects are important for the utilization of this source:
the usable head.
The flow rate of natural streams generally depends on:
extent of rain and snow.
If the stream is only fed by rain, the maximum flow rate occurs during rainy seasons (normally the spring and autumn); if, on the other hand, the stream is fed by glaciers or snow fields, the maximum flow rate occurs at the start of the summer, and the minimum flow rate is observed in the winter.
The volume of water that crosses a given section of the stream during a
certain period of time (a month or a year) is called the discharge flow.
Information on the discharge flow is generally available from national
agencies; when data is lacking, however, direct measurements must be
taken.
Official or measured data should make it possible to construct an average
duration curve for the flow rate, which is essential for evaluation of the
usefulness of energy transformation of this source.
The gross or geodetic head (Ho) represents the difference between the height of the water surface above and below the head. The latter value is usually considered to be small (low head) when 2<Ho<12 m; it is considered average when 12<Ho<100 m and large when Ho>100 m.
The power P generated by a water flow rate Q [m3/s] with a head of Ho [m] is equal to:
| P=μQgHo=1000*9.81*Q*Ho=9810*Q*Ho | [W] |
where μ is the mass of a2 unit volume of water [kg/m3] and g is the acceleration of gravity [m/s2].
In the absence of official data, Q may be calculated in the following manner:
for average Q's, by measuring the speed Vs of a floating object, two-thirds of which are submerged (e.g., a half-full bottle), in a part of the stream (e.g., 50 m long) which is sufficiently straight and has a constant cross-section. Assume that the stream's average speed Vm is around 0.8*Vs ( the operation should be repeated several times in order to obtain a reliable figure). Then it is necessary to calculate the area A of that part of the bed by measuring the depth Zn at various points which are Yn from one bank:
A ≃ Y1Z*12+(Y2-Y1)*(Z1+Z2)/2+(Y3-Y2)*(Z2+Z3)/2+….
When the bed is irregular, the number of reading points should be increased. Therefore, Q≃Vm*A;
for average and high Q's, through the use of a grate and a speedometer, or a standard size weir. In the latter case, Q is a function of the height of the stream through the section under examination (see specific manuals on the subject).
All these methods provide an instantaneous value for the flow rate. The analysis should be continued for at least one year to determine the minimum and maximum values of Q. Calculation of Ho, on the other hand, is easier, and that value may be obtained by using normal topographic methods.
Calculation (and related energy evaluation) of heads that generate less power is practically impossible. This estimate is, especially complex since only heads that are an acceptable distance from potential users should be considered.
References: [3], [9], [15], [23], [24], [30].
| Summary Source Table: | HYDRO |
| a) operative flexibility: | generally very high |
| b) areas of acceptable use: anywhere that a head with the required characteristics is available near a processing center (preferable distance < 500 m) | |
| c) essential data for preliminary evaluation: changes in the water's flow rate over the course of the year | |
d) typical energy intensity values:
| - power generated by 1 l/s that falls from 1 m: | 9.8 W |
| - energy generated annually by 1 l/s that falls from 1 m: | 309 MJ |
e) share (%) recoverable for the production of energy in the form of:
| 1 - water at 40–70°C 70–80 | 2 - steam (*) 70–80 | 3 - electricity 70–80 |
f) figure calculated from the characteristics of the source, needed to meet the energy requirements of a center with a daily milk intake of 1000 l/day (**) (unit of measure: product flow rate*head in m3/S):
| Simplif. Proc. Plants | Complete Proc. Plants | |||||
| Thermal req. 0.3-2.5 | Electric req. 1.2-2.0 | Thermal req. 0.3-8.0 | Electric req. 1.3-3-1 | |||
g) remarks:
It is assumed that the necessary energy can be supplied in three hours
of generator functioning.
(*) low-pressure, or in some cases water heated to 80–90fC
(**) with different intake levels, use proportional values
In the innermost parts of the earth, temperatures of 4,000°C are reached
and maintained by nuclear reactions (radioactive decay of uranium, thorium,
potassium, etc.). This produces a gradient of less than 30°C/km (e.g., at
a depth of 35 km, it is normal to observe a temperature equal to 500°C) and
a thermal energy flux, calculated on the earth's surface, of 0.06 W/m2.
Therefore, the average energy intensity of this source is negligible.
At some sites, however, 10–20 W/m2 are reached at a depth of 5 km for
sufficiently long periods of time (over 20 years). In these cases, 100
MW/km2 are available in the form of a 50–70°C aquifer or, more rarely, a
steam bed with a temperature ≥ 150°C.
The term geothermal energy is generally used to indicate the thermal energy
available at a depth of less than 6 km.
From an energy standpoint, a temperature gradients G≥80°C/km (along the
borders of tectonic plates, for example) is considered to be high. If
40<G<80°C/km, the temperature is average (the origin is due to
irregularities in the earth's crust), and if G<40°C/km, the temperature is
normal.
Fluids (liquids and steam) coming from geothermal basins almost always have a complex composition. This is because they contain a high number of minerals (alkali, sulfates, bicarbonates, etc.) and/or dissolved gas (CO2, H2S, CH4, H2, NH3, Ar, Rn, etc.), which always pose problems for the plant (corrosion, encrustation, etc.).
At least two kinds of analyses are required:
chemical-physical, of the geothermal fluid for design of the user plant.
Evaluation of the source's thermal power P (i.e., of the energy producible) is carried out on the basis of:
the maximum temperature head δT obtainable.
Indeed:
P=csQδT
where cs is the fluid's specific heat (when liquid, consider the cs of the water).
The value δT depends on the temperature Ts of the source and the
temperature Tf to which the geothermal fluid is cooled (δT=Ts-Tf).
Ts is a characteristic (not modifiable) of the site; Tf, on the other
hand, is a function of the type of plant and user considered.
If 25<Tf<35, only space heating is possible. If 80<Tf<120, the generation
of thermal energy and refrigeration is also possible. Finally, if Tf>150,
electric energy can be generated.
Geothermal energy is rare at high temperatures and relatively widespread at
average and low temperatures.
The majority of uses involves the extraction of hot water, its cooling by
an exchanger and its replacement in the aquifer.
The exchanger and the pump (often submergible) operating on the source
must be suitable for the temperature and the chemical composition of the
fluid.
Replacement of the fluid is required to prevent lowering of the aquifer and to solve the problem of disposal of the refluent (which is usually considered to be a pollutant).
Wells for extraction and replacement must be sufficiently far away from
each other and correctly laid out; naturally, replacement water must not
cool extraction water.
The economic impact of these wells (including any geological studies)
should always be carefully analyzed and confirmed.
It should also be remembered that wells tend to age; that is, they loose
their initial characteristics (in terms of flow rate).
References: [34], [50].
| Summary Source Table: | GEOTHERMAL ENERGY | |
| a) | operative flexibility: | high where available |
| b) | areas of acceptable use: anywhere geothermal fluids are available with a temperature of approximately 80°C | |
| c) | essential data for preliminary evaluation: analysis and temperature of geothermal fluid; available flow rates. | |
| d) | typical energy intensity values: | |
- power generated by 1 l/s extracted at 80°C: | 170 W [1] | |
| e) | share (%) recoverable for the production of energy in the form of: | |
| 1 - water at 40–70 fC 70–90 | 2 - steam (*) 70–90 [2] | 3 - electricity 5–10 [3] |
| f) | figure calculated from the characteristics of the source, needed to meet the energy requirements of a center with a daily milk intake of 1000 l/day (**) (unit of measure: liquid flow rate in l/s): |
| Simplif. Proc. Plants | Complete Proc. Plants | |||||
| Thermal req. 0.04–0.3[4] | Electric req. 0.7–1.0 [3] | Thermal req. 0.04–0.9 [4] | Electric req. 0.7–1.7 [3] | |||
| g) | remarks: | |
| It is assumed that the necessary energy can be supplied in three hours. | ||
| [1] | For calculation of d), it is assumed that the fluid will be cooled to 40°C. | |
| [2]–[3] | Possible when the fluid's temperature is on the order of 150°C. | |
| [4] | Hot fluid at ≃100°C. | |
(*) low-pressure, or in some cases water heated to 80–90°C
(**) with different intake levels, use proportional values
The term biomass is used to describe organic substances that are directly
(vegetable) or indirectly (animal) derived from photosynthetic activity.
The two types of biomass generally considered for energy purposes are
vegetable substances and animal waste.
In order to characterize the various materials from an energy standpoint,
certain physical (moisture content, heat value of the dry substance, mass
of a unit volume) and chemical properties (composition of the dry
substance, C/N ratio, total solid content) must be known.
Physical Characteristics
The moisture content U (here evaluated on wet basis) gives an indication of
the difficulty involved in conservation of the substance as it is and its
suitability for fermentation processes.
In the case of vegetable substances, 10<U<90%; in animal wastes, 60<U<99%.
The apparent mass of a unit volume ma is connected with the state of
fragmentation, collection and packing methods and the product's energy
intensity.
The following are some indicative values (kg/m3):
loose vegetable substances, 40<ma<80 (straw, pruning residues); baled
vegetable substances, 150<ma<250 (straw); densified vegetable substances,
600<ma<800; manure with a considerable amount of straw and just removed
from the stable, 180<ma<250; ripe manure, 550<ma<800; waste in general,
900<ma<1050.
The heat value is the quantity of thermal energy produced by the complete
combustion of 1 kg of liquid or solid fuel (1 Sm3 in the case of gases;
“S” indicates that reference is being made to a temperature of 0°C and to
atmospheric pressure).
The gross heat value (GHV) includes the latent heat of the steam that is
formed during the process by combination of the hydrogen and oxygen
contained in the fuel and in the air. The latent heat is not considered in
the low heat value (LHV). GHV and LHV are determined by the “calorimetric
bomb” method, and they always refer to 1 kg of anhydrous substance. LHV is
generally 90–95% of GHV.
The energy content (EC) is also evaluated in the case of products
containing moisture (e.g., wood). EC takes into account the energy
absorbed by evaporation of the water incorporated into the material's
structure:
Chemical Characteristics
The composition of the dry substance makes it possible to evaluate the C/N
ratio, which, together with the moisture content U, is determinant for
selection of the right energy conversion process.
If C/N>30 and U<30, the product may be used as solid fuel suitable for
thermochemical transformations (e.g., sufficiently dry vegetable
substances).
If C/N<30 and U>30, the product is acceptable for biochemical
transformations (e.g., wet vegetable substances and animal waste).
In the case of animal waste, the content of the following substances is
important: total solids (TS), volatile solids (VS), N (organic and mineral
content), P and K.
The following aspects must be evaluated for a calculation of the potential energy supplied by the biomass:
the costs of collection and possible transport to a user point:
For a), reference may be made to average subproduct/product ratios in the
case of secondary agricultural products. The procedure is approximate and
should be employed with caution, especially in the case of wood residues.
Quantities of animal waste are evaluated with reference to the species, the
type of breeding and the average weight of the animals.
For b), the type of energy process in which the product will be utilized
must be clearly defined.
When thermochemical processes are involved, the dry substance's LHV and the
moisture content U at the time of use are always required for a calculation
of the energy content. This value is needed to determine the amount of
energy that is actually available.
In the case of anaerobic processes, the content of volatile substances (VS)
must be known. One kg of VS can theoretically be transformed into ≃ 0.8
Sm3 of gas. In actuality, this value is limited to 0.1–0.4 Sm3/kg of VS
(depending on the type of process involved).
Evaluation of the costs of collection and transport c) makes it possible to obtain an initial parameter for analysis of the feasibility of the product's use as energy.
The availability of biomass varies widely as a function of the location under consideration. It is virtually impossible to provide general indications.
References: [9], [11], [12], [14], [25], [35], [50].
| Summary Source Table: | DRY BIOMASS [C/N>30] |
| a) operative flexibility: | high |
| b) areas suitable for its use: anywhere dry wood-pulp products and residues are available | |
| c) essential data for preliminary evaluation: type of biomass, its moisture contect and available quantities | |
| d) typical energy intensity values: | |
- material with 10–20% moisture: 13–16 MJ/kg | |
| e) share (%) recoverable for the production of energy in the form of: | |
| 1 - water at 40–70 fC 40–60 | 2 - steam (*) 40–60 | 3 - electricity 5–25 | ||
| f) figure calculated from the characteristics of the source, needed to meet the energy requirements of a center with a daily milk intake of 1000 l/day (**) (unit of measure: kg/day of wood residue): | ||||
| Simplif. Proc. Plants | Complete Proc. Plants | |||||
| Thermal req. 4–30 | Electric req. 100–150 | Thermal req. 4–100 | Electric req. 110–250 | |||
| g) remarks: | ||||||
(*) low-pressure, or in some cases water, heated to 80–90°C
(**) with different intake levels, use proportional values
| - Summary Source Table: | WET BIOMASS [animal waste] |
| - a) operative flexibility: | poor |
| b) areas suitable for its use: anywhere there are animal breeding farms near a processing center | |
| c) essential data for preliminary evaluation: quantity of waste; volatile solid content; chemical analysis | |
| d) typical energy intensity values: | |
- 100 kg of live weight provide 25–35 MJ/day | |
| e) share (%) recoverable for the production of energy in the form of: | |
| 1 - water at 40–70 fC 70–80 | 2 - steam (*) 70–80 | 3 - electricity 5–20 | ||
| f) figure calculated from the characteristics of the source, needed to meet the energy requirements of a center with a daily milk intake of 1000 l/day (**) (unit of measure: live animal weight in t): | ||||
| Simplif. Proc. Plants | Complete Proc. Plants | |||||
| Thermal req. 0.12–0.80 | Electric req. 3–5 | Thermal req. 0.12–3 | Electric req. 4–8 | |||
| g) remarks: | ||||||
| A chemical analysis of the waste is always necessary to confirm the possibility of energy production (in the form of biological gas) | ||||||
(*) low-pressure, or in some cases water heated to 80–90°C
(**) with different intake levels, use proportional values
