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The low temperature thermodynamic conversion of the solar energy -The history in brief and some small size ORC examples

Authors:
The low temperature thermodynamic
conversion of the solar energy – The history in
brief and some small size ORC examples
– an update –
Costante M. Invernizzi1
1Dipartimento di Ingegneria Meccanica e Industriale,
Universit`a di Brescia
5th February 2021
1
Contents
1 The thermodynamic conversion of the solar energy 3
2 The first historical low-temperature thermodynamic engines 9
3 The thermodynamic solar conversion at low temperatures in
the first half of the twentieth century in Italy 11
4 Some modern low temperature solar thermodynamic engines 16
2
1 The thermodynamic conversion of the solar
energy
The solar radiation The Sun is a sphere of very hot gases (its inner tem-
perature is estimated to be 10-40 millions of degrees) at high density (the
nucleus density of the Sun is estimated to be 100 times greater than the
water density). The energy from the fusion reactions in its “core” is then
radiated in the space as electromagnetic energy.
The energy from the Sun available outside our earth’s atmosphere, on a
surface perpendicular to the radiation, is 1367 W m2; our atmosphere then
attenuates the energy content of the radiation and appreciably modifies the
solar spectrum due to: (i) the absorption of energy by the ozone, nitrogen,
water vapour and carbon dioxide molecules, (ii) the diffusion (the deviation of
the radiation in all directions) when the radiation interacts with the molecules
of the atmosphere and with the dust in the air.
Due to the interactions between the solar radiation, the atmosphere and
the ground, a generally inclined and oriented flat surface on the earth’s soil
receive energy as
direct radiation the amount of the solar radiation that does not interact
with the atmosphere;
diffuse radiation the amount of the solar radiation that, as a consequence
of its interaction with the air, is accidentally diffused (scatterd) in all
directions;
reflected radiation the amount of the solar radiation reflected from the
ground and from the surfaces surrounding the receiver.
The sum of the direct, of the reflected and of the diffused components of
the solar radiation contributes to the available ˇ
total solar radiation at ground
level. In Figure 1 are the component fractions of the direct radiation in two
Italian places, at different latitude, for the different months of a typical year.
The thermodynamic conversion of the solar energy By thermodynamic
conversion of the solar energy we mean:
1. the collection of the solar radiation in the form of heat by surfaces with
a high absorption coefficient at the highest permitted temperature;
2. the transfer of the collected heat to a low temperature heat sink by
means of a thermodynamic engine with mechanical energy production.
3
Figure 1: The ratio between the amount of the direct radiation and the total
one in two Italian cities. Series 1 – Brescia, Series 2 – Ragusa. The considered
mean monthly radiation values are for a horizontal flat plane, [1, Appendice
D].
4
The solar collector By “surfaces with a high absorption coefficient” we
mean a solar collector. To obtain high temperature levels, concentration
collectors (named “concentrators”) are required, with a linear or punctual
concentration.
Increasing the concentration ratio, or as the ratio between the focal (ab-
sorbing) area and the open reflecting area reduces, the temperature of the
available heat increases. For example, a collation temperature of 100 C to
150 C is usual for a flat solar collector, 300 C to 500C for a parabolic col-
lector, and a temperature of 800 C is typical for a dish collector (with a
punctual concentration).
Since they do not require any particular movement, flat solar collectors
are preferable where simplicity and reliability are required and to reduce
maintenance to a minimum. Furthermore, they perform better on average
than sophisticated concentration systems in regions where scattered radiation
represents a significant fraction of the available solar energy.
A collector is always made up of a receiver (which absorbs the radi-
ation converting it into heat, consisting of the absorber, the necessary covers
and the thermal insulation) and, if the collector is also a concentrator, by
aconcentrator, an optical system that directs the incident radiation to-
wards the receiver. The concentration ratio is the ratio between the area
of the opening, through which the radiation reaches the concentrator, and
the area of the receiver. The temperature at which the receiver operates usu-
ally increases with the concentration ratio: in the case of flat collectors the
concentration ratio is almost unitary and the average temperature reached
usually is a hundred degrees higher than that of the environment.
The flat type collectors have the lower concentration ratios (at most about
few units) and the receiver absorbs also a significant amount of the diffuse
radiation. The concentrators with a small receiver area (compared to the area
of the concentrator) use only the direct component of the solar radiation and
require sun tracking mechanisms. Table 1 classifies by typology the different
solar collectors.
Resorting to the non imaging optics, a technique mainly developed
by Ronald Winston of UCMERCED (Schools of Engineering and Natural
Science, University of California, Merced) between 1966 and 1978, is it pos-
sible to design simple solar collectors working well even with a minimal or no
movement. All the solar radiation that falls within an “acceptance angle” is
reflected on the receiver, which can so absorb the scattered component also.
The non imaging optics, unlike the “imagining” ones, has not the aim
to create an “image” as precise as possible of the source, but optimises the
optical system in order to maximise (given an “angle of acceptance”) the
radiative exchange and to collect to the receiver the maximum amount of
5
Table 1: Rapporti di concentrazione indicativi, tipo di movimentazione e
tipiche applicazioni di collettori solari, [5].
CaApplicazione
1 to 4 collettore fisso riscaldamento, raffrescamento, foto-
voltaico (50 C to 250 C)
4 to 150 inseguimento su un
asse o movimento sta-
gionale
generazione di energia elettrica,
riscaldamento, raffrescamento, foto-
voltaico a bassa concentrazione
(100 C to 500 C)
500 to 10 000 inseguimento su due
assi (disco e torre)
generazione di energia elettrica
(500 C to 800 C)
20 000 to 100000 inseguimento su due
assi
fornaci solari (3000 C to 4000 C)
aRapporto di Concentrazione
radiation, [4].
Typical solar collectors designed by the non imaging optics are the com-
pound parabolic concentrators with tubular receivers. Like the one, for ex-
ample, in Figure 2.
A solar collector, of any type, provides a thermal energy (heat) lower
in quantity than that received (due to energy losses of various kinds) and
its efficiency (ratio between the amount of energy made available and the
amount of energy received in the form of solar radiation) is always less than
one and generally increases with the concentration ratio.
Therefore, in general, in the case of the thermodynamic conversion of solar
radiation, the choice of the collector is closely related to the thermodynamic
engine that is decided to use.
With the technological evolution the range of temperatures that can be
obtained, even with flat collectors, is wider than that at the time of the
first pioneers of thermodynamic conversion. Usually, once a type of collector
has been chosen, the thermodynamic and technical optimisation (also taking
into account the overall costs) then leads to the identification of an optimal
maximum operating temperature.
The thermodynamic engine Basically, a thermodynamic engine is a ther-
modynamic cycle which produces mechanical work by transferring the heat
from the solar collector (available at high temperature) to a thermodynamic
“reservoir” at a lower temperature (the “cold well”): for example, the envir-
6
Figure 2: Esempio di collettori composti parabolici (CPC, Compound Para-
bolic Collector) e loro sezione trasversale, [3].
onment, which generally is at a temperature lower than that at which the
collector operates.
The thermodynamic cycle is realised by making a suitable working fluid go
through a series of transformations which, in accordance with the principles of
thermodynamics, have as a global useful effect the production of mechanical
work (which can then be converted into electrical energy). Depending on
the temperature reached in the solar collector, the working fluid is either a
liquid that, at high pressure, evaporates, expands in a machine, condenses at
the minimum pressure and is then compressed again, or a gas, which never
changes phase. Usually, in engines that use solar concentrators with point
concentration the working fluid is a gas.
It is the high pressure steam, or the gas at high pressure and at high
temperature, which, expanding in a suitably designed machine, produces
mechanical energy. The ratio between the mechanical energy produced and
the thermal energy (the heat) absorbed by the engine and coming from the
solar collector represents the efficiency of the engine (always lower than the
unit).
The collation temperature and the performances of the engine
The thermodynamic efficiency of a heat engine strictly depends on the dif-
ference between the minimum temperature at which it operates (correlated
with that of the environment or in any case with that of the available cold
7
Figure 3: Il rendimento massimo di un motore termodinamico al variare
della temperatura massima quando la temperatura minima vale 30 C (linea
continua). Per confronto, i valori reali di rendimento di alcuni motori solari
a bassa temperatura realizzati in Italia fra il 1977 e il 1980.
reservoir) and the maximum temperature, correlated to that at which the
solar radiation is made available in the form of heat in the solar collector.
Figure 3 shows the (ideal) maximum efficiency values as the maximum tem-
perature varies, assuming a minimum available temperature equal to 30 C,
and the real net efficiencies of some low-temperature thermodynamic engines
made in Italy between 1977 and 1980.
8
2 The first historical low-temperature ther-
modynamic engines
Systems with point-focusing solar collectors allow relatively high temperat-
ures and potentially high efficiency engines can be obtained, but, using only
the direct component of the radiation, they must be mobile and must be
able to track the sun. Furthermore, they are ineffective when the diffuse
component of the radiation is high.
The use of flat collectors, with modest collation temperatures, can there-
fore be more practical, even if the efficiency of the thermodynamic engine
results necessarily penalised (see Figure 3).
Louis Abel Charles Tellier (1828–1913) Charles Teller, a French en-
gineer, born in Amiens (in the Somme), was the first who used a series of
flat collectors to make a solar pump of his own invention.
In 1856 he began his research on liquefaction and in 1865 he built a mech-
anical compression machine for refrigeration. In 1879 the ship Frigorifique
took a cargo of meat to Buenos Aires from Rouen in good condition after
105 days of navigation using a refrigerator engine designed by Tellier. The
fluids used by Tellier in his studies were ammonia, sulphur dioxide and car-
bon dioxide. He also developed the method of cascade refrigeration, to reach
very low temperatures with several refrigeration cycles each operating with
different fluids. He died in Paris in absolute poverty.
He wrote several books, among which, in 1860, La conquˆete pacifique de
l’Afrique Occidental par le soleil, in which he also describes a solar pump
of his own invention and built by him near Paris, using flat collectors and
ammonia as motive fluid, [8, p. 120-122], [9], [10].
Henry E. Willsie e John Boyle Starting from 1892, H. E. Willsie and
John Boyle, two American engineers, starting from Tellier’s patents, built a
series of low-temperature engines and in 1908 built a solar power plant in
Needles (California, Mojave Desert). The solar field, 100 m2with flat collect-
ors, was divided into two sections: the first one with single cover collectors
which heated water up to 66C; the second one, with collectors with a double
cover, which heated the water for further 17 C. In the collectors there was
no direct evaporation of the water, avoiding thus the circulation of a high
pressure fluid, but the hot water transferred (by means of a heat exchanger)
the absorbed heat to the working fluid in the engine (sulphur dioxide). The
system was also equipped with a hot water heat accumulator that guaran-
teed the system to function for 24 hours without sun. The maximum design
9
power was 15hp.
The company did not have a great commercial success but Willsie and
Boyle demonstrated that mechanical energy can actually be produced by
solar radiation even without concentrators and were the first to propose and
use a thermal storage, [8, p. 122-127].
Frank Shuman (1862–1918) Frank Schuman, an American engineer,
after carefully studying the pre-existing solar engines, in 1907 realised a solar
engine with flat collectors and diethyl ether as working fluid as a working
fluid. In 1910 he participated in the founding of the Sun Power Company
and by adding basic reflectors to the flat collectors previously used he then
developed a low temperature water vapour engine (with an evaporation tem-
perature lower than 100 celsius). The engine, after a new design of the
collectors, transformed from flat into linear parabolic collectors, was trans-
ferred in 1912 to Africa (in Meadi, a small agricultural community on the
Nile, Egypt), where, in 1913, it operated with a maximum power of 55hp,
sufficient to pump 22.7 m3min1of water, both while requiring more than
18.6 m2hp1with an estimated colectors’ efficiency of 40 percent, [8, p. 139-
141], [11].
10
Figure 4: Un estratto del brevetto del 1930 di Tito Romagnoli, [12].
3 The thermodynamic solar conversion at low
temperatures in the first half of the twen-
tieth century in Italy
Tito Romagnoli (...–1967) Between 1923 and 1930 Tito Romagnoli man-
ufactured a series of solar engines. His latest patent from 1930, see Figure
4, describes a solar engine that
converts solar heat into motive force by an intermediate me-
dium which is alternately in liquid state and in vapor state and
whose pressure is exhausted in an engine.
In the first engines of 1923, the working fluid used was sulfur dioxide,
which at that time was widely available as it was used in refrigeration sys-
tems; in 1930 Romagnoli built a methyl chloride engine, [13, p. 34]. With hot
water at a temperature of 55 celsius and with cold water at a temperature
of 15 celsius, the engine, in the latest versions, had approximately a power
of 2 hp, [13, p. 35].
11
Luigi D’Amelio (1893-1967) In 1935, Luigi D’Amelio, professor of “Thermal
and hydraulic machines” in what was then the “Regio Istituto Superiore
d’Ingegneria” in Naples, proposed and described in detail a solar engine with
ethyl chloride as a working fluid to be used for irrigation purposes in Libya
(Northern Tripolitania) as a substitute for internal combustion engines, [13].
Starting from an analysis of the Romagnoli engine, D’Amelio observed that
the low performance of the small power volumetric expanders, compromising
its overall performance, made the engine unattractive and useless:
so far the machine proposed for small powers has always based
on the volumetric (reciprocating) expander machines ... now, a
volumetric machine of a few horsepower is a formidable steam
eater ... as anyone who has experienced such small engines has
actually seen. The reasons are known: from a practical point of
view, the first is the low volumetric efficiency which is naturally
worsened as the expansion pressure ratio increases and as the
angular velocity decreases, [13, p. 37]
He therefore proposes the use of turbines instead of volumetric expanders.
In particular, in the engine he designed, a single-stage action turbine (see
Figure 5) with working fluid of suitable molar mass and molecular com-
plexity.
D’Amelio, after appropriate evaluations, concluding that
in the practical realisation of a thermal engine the choice of
the fluid operating in the cycle is not completely indifferent, [13,
p. 50]
excludes, for the particular application considered, among the substances
then widely used, water, ammonia (for thermodynamic reasons) and sulfur
dioxide (for chemical and safety reasons), concentrating on ethyl chloride
(chloro-ethane).
Daniele Gasperini (1895-1960) e Ferruccio Grassi (1897-1980), [6]
At the first Solar Energy Fair held in the United States in Phoenix, Arizona
in 1955 there was, among other numerous exhibited apparatuses, a “SOMOR
solar pump” (see Figure 6 and Figure 7).
Mario Dornig, then professor of “Fluid Machines” at the Politecnico di
Milano, wrote in his account of the conference in Phoenix, in 1956, that the
SOMOR solar-pump was the only one that
worked regularly and automatically for the entire duration of
the exhibition, drawing the general attention of the large audience
of visitors
12
Figure 5: Schema del motore progettato da Luigi D’Amelio nel 1935, [13].
1 caldaia, 2 turbina, 3 condensatore, 4 preriscaldatore, 5 pompa di circol-
azione dell’acqua nei radiatori, 6 guardia idraulica all’uscita dell’albero, 7
tenuta ad anelli di carbone e valvola di chiusura nei lunghi periodi di ri-
poso, 8 condotta dell’acqua pompata dal pozzo, 9 uscita dell’acqua per ir-
rigazione, 10 condotta dell’acqua ai radiatori, 11 condotta dell’acqua dai
radiatori, 12 scarico del vapore. Dati di progetto: temperatura di evap-
orazione 40 C, temperatura di condensazione 23 C, temperatura dell’acqua
calda all’ingresso dell’evaporatore 45 celsius, temperatura dell’acqua calda
all’uscita dell’evaporatore 42 C, potenza all’albero della turbina 5.7 hp. La
turbina, monostadio, parzializzata a 5000 rpm.
13
Figure 6: Una vista frontale della pompa solare SOMOR, [7].
The manufacturer of the solar pump was Daniele Gasperini, from Rovereto,
who conceived, as early as the 1930s, numerous solar motors and, from 1949,
after meeting with Ferruccio Grassi, founded SOMOR (SOciet`a MOtori Re-
cupero del calore solare e del calore perduto) to market its engines.
Different models of the SOMOR pump were exhibited in 1953 at the
VI Quinquennial Fair in Lecco, then in the Fourth International Technical
Exhibition in Turin, in 1954 and so on in other important fairs.
In the 1960s the company produced different types of pumps with different
flow rate and head, from 900 l h1to 60 000 l h1, with motor powers from
0.1 hp to 3.5 hp. Numerous models, about thirty, were installed in different
locations, in Italy and abroad. SOMOR was liquidated in 1963.
In 1956 a pump was transferred to the “Stanford Research Institute”
(Menlo Park, California) for analysis and functional testing. The engine
studied, with direct evaporation in the solar collectors, used sulfur dioxide
(SO2) as working fluid, and a volumetric expander at 80rpm to 90 rpm. The
engine was found to have, under the test conditions, an overall efficiency of
about one percent, [7].
14
Figure 7: Una vista posteriore della pompa solare SOMOR, [7].
15
4 Some modern low temperature solar ther-
modynamic engines
Studies and manufacturing of low-temperature solar engines were never aban-
doned and significant applications were also made in the second half of the
twentieth century. Some of them – of small size – are briefly described below.
First, however, it is also necessary to mention those systems that go by the
name of “solar ponds”, [2, Chapter 18].
The solar ponds The solar pond is basically a large flat solar collector
consisting of a body of water with controlled and variable salinity according
to the layers. The surface layer (with low salinity) acts as a transparent cover
to radiation and as a thermal insulator too, the deeper layer (with a high
concentration of salt) acts as a radiation absorber and thermal accumulation,
the intermediate layer acts as an additional insulator and it is characterised
by variable salinity and temperature, from the relatively high values typical
of the deepest layer to the characteristic values of the superficial one.
The first documented experiences with sun-heated lakes date back to
1900, when in Transylvania, near Szov´ata, in a lake (the Bear Lake) at the
depth of 1.3 m, at the end of the summer, were reached 70 C. The bottom
salinity was about 26% in sodium chloride.
Studies and researches for the thermodynamic conversion of the heat
accumulated in solar ponds were carried out in Israel in the 1950s and 1970s
in the areas around the Dead Sea with the aim of producing useful electrical
energy. The basic scheme adopted today in systems of this type is in Figure
8and an example of a modern system is in Figure 9. A first small 6 kW
turbine was coupled in 1978 to a 1500 m2pond to demonstrate the feasibility
of the concept. In 1979 a greater model of 150 kW was put into operation in
Ein Boqeq (Israel), [15].
· · ·
As we said, there has always been an interest in having small solar units
(with electrical or mechanical power of 3kW to 10 kW for isolated areas,
where other sources of energy are not available, [14]. However, it is essential
that such systems have constructive and operational simplicity, ensure high
reliability and have low costs.
· · ·
16
Figure 8: Schema di principio per un impianto con stagno solare per la
produzione di energia elettrica, [16].
Figure 9: L’impianto di Bet Ha’arava (Mar Morto, Israele). Un sistema con
stagni solari da 5 MWe, [16]
17
Figure 10: The 600 W solar ORC pump built in Mali (1966), [18]
The pioneering work of D’Amelio inspired the design and the development
of new turbine engines with organic working fluids, the so called Organic
Rankine Cycles (ORC).
The design of small solar turbines in the size range from 2 kW to 10 kW,
was presented and discussed in 1961 in [17]. The selected working fluid was
monochlorobenzene and, in the evaporation temperature range from 140 C
to 200 C (assuming a condensation temperature of 30 C), the estimated
overall cycle efficiencies resulted of the order of 15-20%, according also to
the size power.
A600 W solar thermal pump
In Figure 10 there is a picture of a small power solar thermal pump
developed in 1966 and designed for applications in remote areas of Israel and
Africa, [18]. The plant in the Figure is that one installed in Mali (Africa).
The small ORC unit (with a nominal power of about 600 W) used monochloro-
benzene as working fluid. Some characteristics of this unit are in Table 2.
A3.0 kW solar engine for pumping water
Two similar solar engines [19], [20] and [21] were built for pumping water
between 1977 and 1978 in Italy.
In the spring of 1978 a solar pumping system was installed on the roof
of a building of the Ansaldo factory (Genoa) with the aim of studying its
performance (see Figures 11).
18
Table 2: Some data for a small solar pumping station. From [18].
Turbo-generator
Nominal output 600 W
Boiler temperature 90 C to 125 C
Maximum power output 700 W
Average output 17 kW h d1
Collectors
Total net collector area 43 m2
Total mirror area 16 m2
Maximum heat output 12 kW
Average useful heat output 35 kW h d1
Pump
Maximum flow at 40m 3000 l h1
Average quantity of water pumped per day 1100 l d1
The maximum evaporation temperature (obtainable with flat collectors)
of the working fluid was about 80 C to 120 C. The pumped water also
provided for the cooling of the engine, as it was essential for the modest
evaporation temperatures reached to had a cold well at the lowest possible
temperature. The design engine conditions were: evaporating temperature
75 C, condensing temperature 30 C, mechanical power 4.0 kW, a turbine
expander.
A8.0 kW medium concentration solar engine In 1979, as part of the
national “Programma Finalizzato Energetica del CNR” project, in Italy, a
prototype of a solar engine of about 8 kW was designed and built, [22].
The engine, based on a Rankine cycle, had an evaporation temperature
of about 200 C and used mono-chlorobenzene as working fluid. The main
data for the thermodynamic cycle and for the turbine are in Table 3.
In Figure 12 there is an image of the built two-stage turbine.
The Borj C´edria solar engine In 1984 in Borj Cedria (Tunisia) a solar
engine powered by solar heat from a field of flat collectors was built, [23].
Single-glazed flat collectors with a selectively coated copper absorbing plate
were used. The system was equipped with a hot water heat storage made
with a carbon steel vessel with glass wool insulation. The main data of the
plant in the design conditions are in Table 4.
19
Figure 11: Il motore solare da 3.0 kW per il pompaggio di acqua realizzato
nel 1977 a Milano, [19].
20
Table 3: I dati principali del ciclo e dell’espansore per il motore descritto in
[22].
Fluido di lavoro C6H5Cl mono-
clorobenzene
Massa molecolare 112
Temperatura/Pressione critica 359.3C/45.5 bar
Temperatura/Pressione di evaporazione 176.7C/3 bar
Temperatura/Pressione di condensazione 33 C/0.025 bar
Temperatura ingresso/uscita rigeneratore 80/50 C
Salto entalpico isoentropico della turbina 134.6 kJ kg1
Numero di stadi 2
Grado di ammissione del primo stadio 0.275
Rapporto di espansione del primo stadio 8.15
Grado di reazione del secondo stadio 0.4
Rapporto di espansione del secondo stadio 15
Diametro medio I stadio/II stadio 68/88.5 mm
Velocit`a di rotazione 24 000 rpm
Table 4: Dati principali dell’impianto di Borj C´edria nelle condizioni di
progettoa, [23].
Superficie totale dei collettori 750 m2
Temperature di entrata/uscita per i collettori 85.4/100 C
Temperature di entrata/uscita nel motore 98.5/86.5 C
Temperatura dell’acqua di condensazione 20 C
Portata di acqua al condensatore 6 kg s1
Rendimento medio giornaliero del capo solare 0.33
Energia elettrica netta giornaliera 80 kWh
Volume di accumulo termico (acqua calda) 45 m3
Consumi giornalieri degli ausiliari del campo solare 13.8 kWh
Consumi giornalieri degli ausiliari del motore 1.6 kWh
Potenza netta del motore 12 kW
ainsolazione giornaliera: 5 kWh/m2d, corrispondente ad un giorno sereno di
riferimento, prossimo all’equinozio.
21
Figure 12: La turbina del motore solare da 8 kW with mono-chlorobenzene
as working fluid described in [22].
22
The chosen working fluid for the engine was perchlorethylene (tetra-
chloro-ethylene, Cl2C
CCl2). A non-flammable fluid with a high critical
temperature (Tcr =346.85 C), critical pressure Pcr = 44.9 bar and a normal
point of boiling of 121.25 C. Its molar mass is 165.83, with a molecular
complexity parameter of σ= 4.0. Perchlorethylene, thanks to its excellent
solvent properties, is still widely used for dry cleaning in industry and for
cleaning and degreasing metals.
???
It is interesting to note that the first engines operating according to the
Rankine cycle and using organic working fluids (the ORC cycles) were devised
and built for solar pumps: an excellent application for the application of a
small size engine, and a good opportunity to test its technology feasibility.
Now, as it is well known, the ORC are spread and used in various in-
dustrial sectors and for the production of mechanical/electrical power from
renewable energy sources, with power sizes from a few hundred kW up to
about 20 MW.
23
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Incluye índice Incluye bibliografía El presente volumen trata de las aplicaciones de la energía solar a tecnologías de calentadores, equipos fotovoltaicos y agrícolas, con los apartados siguientes: hornos solares, desalinización solar, secadores de alimentos con energía solar, bombas de agua, invernaderos y celdas colectoras de energía solar.
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