Engines with working fluids other than steam, for example fluids used in the refrigeration industry and organic fluids in general, can easily be used to generate mechanical energy from heat sources at medium-low temperatures (from 100 to 400 °C). These engines are highly suitable for the production and distribution of mechanical or electrical energy and for exploiting any energy derived from cogeneration. Nowadays, organic fluid engines (known by the acronym ORC – Organic Rankine Cycles) are a firm reality and, at least in their more traditional versions, have acquired optimal levels of reliability and economic competitiveness. In Europe, there are around 180 engines in operation using the Rankine cycle and organic fluid, while, around the world, the power installed in plants has grown in the last twenty years from just a few MWe to 1600 MWe. Around 60% of the ORC units installed today are used in the thermodynamic conversion of biomass. Generally speaking, about 40% of the remainder are equally shared between geothermal applications (the so-called “binary cycles”) and heat recuperation, from internal combustion engines and industrial processes. Units for the thermodynamic conversion of solar radiation, though, still represent just a very small proportion. These notes represent the summary of a report prepared for the meeting at the Central State Archive (Piazzale degli Archivi, 27 – Rome) on 8 July 2013, organised by the “Group for the History of Solar Energy” (GSES, http://www.gses.it ). The GSES, founded by experts and researchers from various disciplines, is a voluntary cultural organisation which intends to spread awareness about solar energy. Its aims are to promote study of the history of solar energy use (in both direct and indirect forms) and, at the same time, promote greater awareness of this renewable natural resource. As far as heat engines with Rankine cycles and organic working fluids are concerned, greater technical detail can be found in the book: Costante M. Invernizzi Closed Power Cycles. Thermodynamic Fundamentals and Applications, Lecture Notes in Energy 11, Springer - Verlag, London, 2013.

ORGANIC FLUID ENGINES AND THE THERMODYNAMIC CONVERSION OF SOLAR ENERGY

Costante Mario Invernizzi

Dipartimento di Ingegneria Meccanica e Industriale (DIMI)

Università degli Studi di Brescia Via Branze, 38 – 25123 Brescia

http://www.costanteinvernizzi.it - Tel. +39 339 1351915 – costante.invernizzi@unibs.it 1. Introduction The systems for converting solar radiation into mechanical energy could naturally operate in areas that are remote and often inhospitable, involving great difficulty and technological challenges. The “dynamic” systems for thermodynamic conversion of solar radiation into mechanical energy were mostly proposed in the past for purposes of irrigation and, in general, the interest, research and study in the sector of small solar dynamic plants (a few dozen kW) has been growing over the last few decades. By thermodynamic conversion of solar energy, we mean: (i) collecting solar radiation from a surface with a high absorption coefficient, under the form of heat, generally at the highest temperature possible and (ii) the subsequent transfer of this heat to a thermodynamic cycle engine that produces mechanical energy. The earliest thermodynamic solar engines with a Rankine cycle used steam. For example, A. Mouchot’s engine which, built in 1866 (but with various versions made between 1872 and 1875), used a conical reflector to evaporate the water. Between 1868 and 1870, John Ericsson used parabolic collectors with steam-powered heat engines. In 1876 W. Adams constructed a 1.8 kW engine with a large solar collector, using numerous small flat mirrors, approximating the portion of a spherical surface. Other prototypes, also using steam engines, were made up until 1878. 2. The organic fluid Rankine engine The organic fluid Rankine engine is a heat engine that transforms heat energy into mechanical energy. This transformation occurs, in accordance with thermodynamic principles, by passing a working fluid (not the traditional water vapour) through a thermodynamic cycle, consisting of a series of transformations: evaporation at a relatively high pressure, expansion of the steam within a machine (typically a turbine) that generates mechanical work, condensation at the minimum pressure (reached at the expander outlet), and re-compression by means of a pump in order to restart the cycle. The heat for evaporating the working fluid must originate from outside and, in the case of heat energy of solar origin, this is usually provided by an oil, a fluid suitable for the heat

exchange, which, when crossing the solar field, is heated, prior to cooling in the engine evaporator. The condenser in the ORC unit may use either the surrounding air or water. In the all-solar case, there is usually a system of heat accumulation, too. The work fluids mainly used these days are refrigeration fluids, hydrocarbons, per-fluorocarbons, and silicon-based fluids (the siloxanes). 3. The first versions and the modern units The first experimenter to use a “low-boiling” fluid to make a thermodynamic solar cycle was a certain Schultz, in 1881, who employed sulphur dioxide (boiling temperature of -10 °C). In fact, the sulphur dioxide, in a closed circuit, caused the water evaporation which was itself the true engine fluid. Sulphur dioxide, ethers and ammonia were all adopted by Henry E. Willsie and John Boyle, between 1902 and 1908, whilst the last versions of their engine had a hot water heat accumulator, which guaranteed the system’s operation even in the absence of sunlight. Both flat and concentrated collectors have been employed, according to the power of the engine. In general, the flat collectors cost less than those with optical concentration and allowed thermodynamic cycles to be made with working fluids at relatively high pressures, even when temperatures were modest (even below 100 °C). Between 1960 and 1962, Lucien Bronicki and Harry Zvi Tabor built several solar engines with monochloride-benzene at maximum temperatures of 140-150 °C and power supplies from 2 to 20 kW. Numerous irrigation units, of varied power, consisting of an ORC engine alimented by solar radiation, connected to a pump for raising the water, were created during the 1970s-1980s, including an engine of 3 kW made in 1983 at the CNEN centre of Trisaia. The working fluids adopted over the years include acetone, ammonia, n-butane, toluene, various types of Freon (11, 12, 22, 113 e 114), and tetrachloride-etilene. We should also mention the solar plant of Borj Cedria (1982) in Tunisia (operating at low temperature and designed and built by Italians), of 12 kWe, as well as an engine of around 40 kW (1980), also entirely designed and built by Italian companies and Italian university institutes, operating at high temperature (300-340 °C), constructed in Australia, with a per-fluorocarbon as working fluid. 3. Engines built in Italy and the Italian school in the field of ORC engines Italy has a long tradition in the developing of heat engines that use organic fluid Rankine cycles. Between 1923 and 1931, Tito Romagnoli made an engine of about 1.5 kWe, using hot water at 55 °C and chloro-ethane as working fluid. He probably used simple flat collectors, with no concentration.

Between 1918-1922, Prof. Mario Dorning and Prof. Luigi d’Amelio were among the first to propose using particular working fluids in the creation of low-powered solar engines. In 1935, Prof. d’Amelio carried out detailed research into the design of a turbine solar engine with ethylene chloride, at low temperature, with the aim of building solar pumps for use in Libya. A prototype was made on the island of Ischia in 1940 (using a geothermal heat source). A pump driven by a solar engine with sulphur dioxide was built by Daniele Gasperini who, in collaboration with Ferruccio Grassi, sought to market the SOMOR motor pump, named after the company founded specifically for the purpose, presenting it at the first fair on solar energy held at Phoenix (Arizona) in 1955. The first patent for Gasperini’s engine (a patent bearing the names of both Andri and Gasperini) is dated 1936 and concerns an “engine operating by heat energy, mainly from the sun’s rays, but which may also operate with poor fuels and agricultural waste”. The first engine was exhibited at Turin in 1935 and in July of 1936 at the Fair of Tripoli. At least fifteen pumps were sold and tested before the end of the 1960s, in Italy, the United States, in several African countries and in Costa Rica. However, the real “Italian school” of organic fluid Rankine engines, which would significantly and systematically study and develop numerous engines, appeared in the late 1960s at the Politecnico of Milan. Its founder was Prof. Gianfranco Angelino (1938-2010), together with Prof. Mario Gaia and Prof. Ennio Macchi, who started as his pupils, before becoming his colleagues and friends. Between 1967 and 1984, this group at the Politecnico designed and helped build 14 organic fluid engines with power supplies from 3 to 500 kW, for solar applications, for exploiting geothermal heat sources and for recycling the heat from industrial processes. 3. Conclusions Thanks to its versatility and its good thermodynamic qualities even at low temperatures, the organic fluid engine, based on the Rankine cycle, make it particularly suitable now (as in the past) to the rational exploitation of solar energy via thermodynamic conversion. It is particularly appropriate for small-medium power ranges and for generating electricity destined for distribution. Its capacity for converting heat from whatever source into mechanical energy makes it useful, too, in exploitation of other energy sources, from renewable sources (not just solar radiation, but also biomass and geothermal energy) and for recycling waste heat. In the specific sector of low temperatures, with its inevitably modest conversion efficiency (albeit associated with an optimal thermodynamic quality), the ORC now has stiff competition from traditional photovoltaic systems (with their comparable efficiency and similar costs). The concentration and the high temperatures, though, would give reasonably high and more interesting conversion efficiencies. Naturally, the ORC engine can also operate in cogeneration configurations.

Maintaining the good reflecting characteristics of the optical concentrators, maybe associated with sun following, still remains a challenge. The engine reliability is already optimal, even though further research and development is advisable in the sector of working fluids (costs and availability, environmental and safety aspects) and in developing heat exchangers and expanders, especially for low-power engines.

Fig. 1 –Simplified layout for a modern Rankine engine with organic fluid. The working fluid, in liquid

phase, (i) is pre-heated, evaporated, and, if necessary, super-heated, in the heat exchanger “A” (transformation 3-4), (ii) expands in the turbine (transformation 4-5), (iii) cools, in the steam phase

(transformation 5-6), in the recuperator, heating the liquid at high pressure (transformation 2-3), (iv) condenses (transformation 6-1) and, once again in liquid phase, starts the cycle again.

This diagram is an adapted version of a figure in Costante M. Invernizzi Closed Power Cycles. Thermodynamic Fundamentals and Applications, Lecture Notes in Energy 11, Springer - Verlag, London,

2013.

Fig. 2 – Detail of the organic fluid engine installed at Varna (Val d’Isarco, Bolzano). The power station, for

a biomass district heat-distribution, which began operating in 2008, uses a biomass boiler of 6500 kW alimented by untreated wood (chips, bark and sawdust) deriving from the saw-mill or from forestry activities in the area. The engine is around 1000 electric kW and 3800 thermal kW. By courtesy of

Turboden. Copyright © Turboden S.r.l. All rights reserved.