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12 march 2010

Experience on a V161 Stirling motor as power unit of an EuroDish solar generator in Italy

Since 2002 an EuroDish dish – Stirling generator, equipped with a V161 solar version Stirling motor, has been installed and operated at ERSE (formerly CESI RICERCA) in Milan. Experience was aimed to test efficiency, reliability of the generator and its compatibility with the low tension grid. During more than 1700 hours of operation a number of detailed information regarding the V161 behaviour, performance, technical characteristics and maintenance demand have been achieved.

EuroDish is one of the few dish - Stirling generators fabricated and proved in Europe. Together with the following EnviroDish system represents the state of the art of the dish – Stirling technology in Europe. EuroDish is a solar thermal generator with a nominal electrical power of 10 kWe at 1000 W/m² of Direct Normal Insolation. The power is delivered at 400 V, three phases, 50 Hz by means of an asynchronous generator running at 1500 rpm suited for grid connection.
The EuroDish is comprised of a reflecting parabola which concentrates the direct normal solar radiation on the receiver of a V161 Stirling motor placed on the focus. By means of the heat supplied to the receiver at temperatures in the range of 800-650 °C, the working gas (helium o hydrogen) drives the pistons of the motor, producing mechanical work, which is converted in electricity by means of the generator. A circulation of water cools the gas at one side of the motor closing the thermodynamic cycle. A two axis tracking system, software driven, permits to maintain the dish pointed on sun during the day. Eurodish has been designed to operate autonomously without direct surveillance, following the sun path day by day with automatic switch off at sunset and restarts at down.

During usual operation the dish is pointed to the sun in such a way that a flux of concentrated radiation, equivalent to more than 2000 sun, strikes the flat tubular receiver of the Stirling motor. The receiver absorbs the radiation and is warmed over 800 °C. The motor is stably connected to generator that, being phased with the grid frequency, keeps the speed motor around 1500 rpm. The regulation of the power is obtained controlling the quantity of gas inside the motor: the more the thermal input rises the more the pressure of the gas increases. The increase of the power implies an increase of the phase angle in such a way that the speed passes from 1500 rpm at 25 bar up to 1530 rpm at 150 bar; the net overall efficiency approach 17% at the highest insolation using helium as working gas. A sophisticate software controls all the functions of the dish, including the motor regulation and protection.

During more than 1700 hours of operation EuroDish showed a good overall reliability. The behaviour of the motor was very good; only minor troubles emerged at the beginning due to  the water cooling circuit components. Some minor defects of the motor resulted during the experience mainly related to imperfect execution of mechanical details or due to a non perfect assembling, some of them were also originated by inappropriate procedures during the operation.

Extraordinary maintenance of the motor was needed, as expected, only for the replacement of the seals that separate the volume of the cylinder from the outside. Experience at ERSE Milan showed an average lifetime of the seals  around 600 hours, a good result, but still not sufficient to address the market. Improving the operation life of the seals remains the crucial problem to be solved to achieve the reliability usually required in the field of production of electricity. Reaching the goal of a maintenance in a year and starting the mass production may allow the dish - Stirling V161 finally to become competitive with the other solar energy - electricity technologies.

The experience of Eurodish at ERSE in Milan has been financed by the Research Fund for the Italian Electrical System. A special word of thanks to Davide Bombelli for his constant support  and dedication.

For more information

Vittorio Brignoli
ERSE Enea – Ricerca sul Sistema Elettrico
via Rubattino 54, 
20134 Milano (Italy)  

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