AN EASY SOFTWARE TO ESTIMATE THE PRODUCTION OF THE EURODISH SOLAR GENERATOR IN ITALY

Transcription

1 AN EASY SOFTWARE TO ESTIMATE THE PRODUCTION OF THE EURODISH SOLAR GENERATOR IN ITALY Dr. Vittorio Brignoli 1, Davide Bombelli 2 1 ERSE SpA (formerly CESI Ricerca SpA) via Rubattino, MILANO ITALY tel: , fax brignoli.vittorio@erse-web.it 2 ERSE via Rubattino, MILANO ITALY Abstract Information gained during a number of years of experimental operation of an EuroDish solar generator have been utilised to implement a software capable to model its detailed behaviour and estimate the production of electricity under real conditions. The software, named ERSE-DISH, is an easy tool developed in Visual Basic, capable to calculate and show the production of electricity starting from data of DNI - Direct Normal Insolation - and other main meteo parameters. A number of functions, tuneable by the operator, permit to take into account the intimate characteristics of the dish, its main performance curve, its parasitics and behaviour during the non operation due to random troubles generated from the grid. A special function pre-processes the meteo data to transform typical hourly means meteo data files into 5 minutes means data files to make feasible the simulation of the short transients that affect the performance of the machine. Extended checks with the experimental data gained in Milan in a wide range of weather conditions permitted to refine and qualify the model. Application of the tool to some reference locations in South Italy completed the test of the ERSE-DISH and gave a first estimation of the production of electricity obtainable by means the available dish Stirling technology in Italy. Comparisons between helium and hydrogen versions of the Stirling motor have also been performed. Keywords: dish Stirling Italy model Eurodish 1. The EuroDish generator 1.1. Introduction EuroDish is a dish Stirling solar thermal generator with a nominal electrical power of 10 kwe at 1000 W/m² of Direct Normal Insolation (Fig. 1.). 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 normai solar radiation on the receiver of the Stirling motor placed on the focus. By means of the heat supplied to the receiver at temperatures in the range of C, a gas (helium) drives a closed Stirling thermodynamic cycle inside the motor, producing mechanical work, which is converted in electricity by means of the asynchronous generator. 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 restart at down. Dish Stirling technology, whose EuroDish is a positive example, made important progresses during the last decade. Even though there is still room for improvement of reliability and efficiency and in the overall design, this technology is today quite close to the market standards. Independently of the specific model, EuroDish or others, a limited production of hundreds of dishes per year would imply a dramatic reduction of installation cost and, as a consequence, a direct competition with other small generators in sunny isolated areas and in small islands.

2 Fig. 1. EuroDish in operation at ERSE Milan 1.2. Performances and behaviour of the EuroDish During a number of years of test, EuroDish showed a good overall reliability and, despite of the whether condition of north Italy, was operated for 1800 hours only during the working days. The behaviour of the motor was excellent although minor troubles emerged from some other conventional components of the system. Ordinary maintenance was quite reduced, extraordinary maintenance of the motor was performed periodically. Experience gained at ERSE permitted to analyse deeply the behaviour of the EuroDish and to evaluate its impact on power generation. Operation criteria implemented in the control software, together with the intrinsic efficiency and of course the meteo parameters, have a crucial role in determining the daily production. According to the expectations, due to the semi continental climate and the latitude of 45,5 N, the average productivity resulted lower than the maximum potential of the generator; nevertheless it was possible to estimate the performance of the system, even at the highest intensities of radiation, and how the logics of the control software and the intrinsic characteristics of the generator can reduce it. In general, dish Stirling production is reduced mainly by the fact that they have to track the sun path, need start up and switch off procedures and that the efficiency of the Stirling unit depends from air temperature and wind velocity. From this point of view the relationships between power production, behaviour of the generator and efficiency curve found for the EuroDish are valid, in principle, for any other dish Stirling system. Obviously the relationships have to be analysed in details and accounted differently for each generator. The behaviour of the generator is originated by a number of internal and external inputs that drives it trough a series of logic status, each of them corresponding to a different physical state. Logic status are defined by the values of a number of internal parameters and the functioning of the components of the system; some status have also a logic name that permits to the operator to know at a first look what is the status of the generator at any time. Examples of these main status are stow position, tracking in standby, tracking on target. In the status of stow position the dish is switched off with the parabola downward to protect the mirrors, during the status of tracking standby the dish tracks the sun path at a certain angular distance, in such a way that it can rapidly pass into the tracking on target status which corresponds to the condition of real power geeration. During the night the dish enters into the status of idle where all the subsystems are switched off. During the experimental operation, the dish performances have been investigated to identify three different classes of relationship: the trends of the efficiency, the trends of the parasitic consumes, the dynamic behaviour of the dish. The main experimental relationship can be presented as a linear formula as follow: GrosPower = m(t out ) DNI - q (T out ) (1) where GrosPower is the gross electricity produced from the alternator, DNI is the Direct Normal Isolation

3 and the parameter m and q depend on the temperature T out of the cooling water at the output from the motor, which in turn depends on the temperature of the air T air. and experimentally was found: T out = T air + DNI x ( x T air ) (2) The dependence of the parameters m and q from the T out was derived crossing a large number of data; a valuable reduction of the gross power is due to the increase of the temperature of the cooling water of the motor. As the cooling water passes trough the water gas heat exchanger, its temperature has a direct effect on the efficiency of the thermodynamic cycle. The decrease of efficiency is around 1 kwe at 1000 W/m² passing from 45 C to 65 C. Alternatively the effect of the air temperature can be seen on the net efficiency of the system with a reduction from 11% to 7% at 400 W/m² and from 17 to 16% at 900 W/m² as shown in the Fig.2. where the net efficiency is plotted for temperature T out in the range 45 C - 65 C. Regarding the parasitics consumes, it has to be noted that the EuroDish generator mounts a number of motors and devices whose operation can vary depending on the status of the dish, on the stability of the weather conditions and on the settings introduced in the control software Tout < 45 C 10 9 Tout C Tout C 5 4 Tout >65 C DNI (W/m²) Fig.2. Trends of the net efficiency as a function of the DNI and the cooling water temperature. During the operation, the consumes vary continuously according to the status of the generator. Experience has shown that daily consumes, including the night ones, can range among 5 15% of the gross daily production depending on the degree of cloudiness of the day: the more you have variable weather conditions the more you have parasitics consumes because of the increase of the switch on - off cycles. Parasitics can be divided in two categories, the ones related to the Stirling motor and the other ones related to the dish. While the first ones are strictly connected to the generation, p.e. during switching on or switch off or during the regular operation, the latter are related to the dish motion, p.e in case of normal operation or during the fast movements needed to pass from one angular position to another. Regarding the behaviour, it can be noted that the logic status can be divided in 2 categories: stationary and temporary. The duration of the first ones is not predetermined and depends on external inputs such as the variation of the intensity of the radiation, the time of the day or a command from the PC keyboard, the duration of the second ones is predetermined from the logic of the system as they are typically transient status during which the dish is moved from a position to another. For example, the status tracking on target can last for many hours from dawn to sunset during a sunny stable day, the transient status standby tracking

4 - Tracking on target lasts about a minute during which the dish passes from a predetermined point away from the sun path to the sun position. This fast movement implies a certain amount of consumes of energy from the servo motors that drives the parabola, roughly 700 We for 1 minute. 2. Model of the EuroDish generator 2.1. Model description The EuroDish generator has been tested in Milan essentially for logistic reasons. Like the other solar thermal power systems, its application should be advisable below 40 N of latitude where the cloudiness is reduced and the annual solar energy due to the DNI is more then 1800 kwh/m².y; this latitude corresponds in Italy roughly at the position of the city of Naples. Due to its intrinsic characteristics dish Stirling generators could found application in Italy even at highest latitudes, typically at locations along the coasts that benefit of the Mediterranean climate, in central Italy. To evaluate the real potential of the EuroDish generator in different locations around Italy has been decided to realise an easy model capable to estimate the production of the dish based on its technical characteristics and capable to use the available meteo data. Solar radiation data are usually available as hourly averages. The Eurodish and in general dish Stirling systems, are usually very reactive to any variation of the DNI or the other meteo data. These variations can imply also an increase of the non generation time and consequently of the parasitics consumes; for these reasons a delta t of an hour due to the availability of the solar radiation data doesn t permit to describe with sufficient precision the trend of generation during the day. For these reasons it was conceived a model that accepts in input the hourly averages data and recalculates the solar radiation DNI and the other data meteo at steps of 5 minutes using a spline curve. In this way you obtain a file that should approximate a more realistic daily trend of the solar radiation (Fig. 4.). The behaviour of the EuroDish is evaluated on the base of this new data file. At each step ( 5 minutes) the model decides what is the status that the dish would assume, and calculates the gross production as a function of DNI, air temperature, the parasitics related to the generation, and, in case the dish had change the status respect the previous step, calculates the parasitics involved in the change. To try to describe as better as possible the behaviour of the generator, the status have been simplified and represented in the model appropriately. A list of these status are listed below (Tab.5.). Most of the transient status last less then 5 minutes and consume a fixed amount of energy; in case the transient status corresponds to a passage from non production to production, the model takes care of the fixed consume and subtracts its to the production. Fig. 3. ERSE DISH model video interface

5 Fig. 4. Example of 5 minute average data spline curve of DNI recalculated from the hourly average data STATUS IDLE FINDING ZERO STAND BY TO TRACKING TRACKING COLD TRACKING HOT STOP RUN TRACKING NO RUN TRACKING TO STAND BY FROM TRACKING TO STAND BY TO STOW OPERATION Non operation status during the night or when the sky is heavy cloudy Initial rotation of the parabola for the position check Tracking of the sun path not aiming to the sun Shift from the STAND BY status to the TRACKING HOT/COLD status Operation with the motor not in regime temperature Operation with the motor at regime temperature Motor operated by the alternator to repump the gas into the bottle Motor stopped with the parabola tracking (cloudy) Fast movement from the status of STOW to the status of STAND BY Fast movement from the status of TRACKING to the status of STAND BY Fast movement from the status of STAND BY to the status of STOW Table. 5. List of logic status and the physical correspondence status or functioning of the generator At the generic step N the model considers the status of the generator at the step N-1 and decides what will be the status at the step N according to the DNI value and the other meteo conditions. If the status N implies the operation of the Stirling motor, it calculates the gross production at the step N using the formulas (1) and (2) and subtracts the parasitics related at the status N. In case at the step N the DNI is no more sufficient to drive the motor, the model changes the status, for example from TRACKING HOT to STOP RUN TRACKING and applies the related consumes. The software has been developed in Visual Basic language and conceived to permit the user to check easily

6 the flow of the runs (Fig.3.). The simulation is managed on monthly base but the operator can decide to slow down the process and verify the recalculation of the DNI daily trend and the behaviour of the dish step by step. Alternatively the simulation can flow automatically for the entire month. The program requires in input a monthly file of hourly averages data of DNI, air temperature and wind velocity and generate an output file with 5 minute averages data of GrosPower, Stirling parasitics and dish parasitics. To permit the operator to follow the process, the video interface shows the spline lines that interpolate the hourly averages of the original input data file and the other data calculated by the model, including the status of the dish at any moment. To verify the capability of the model to simulate correctly the performance of the EuroDish generator, the model has been tested on a number of different days on the real conditions in Milan. In general, the daily average difference between the calculated data of the net power and the same data experimental ranges from 1% in case of stable weather up to 3% in case of variable cloudiness. Comparisons have been based both on hourly average data and real data achieved at a rate of 1 minute from the DAS of the EuroDish facility. An example of the curves of GrosPower obtained experimentally and via simulation are compared in the Fig. 6. The curves have a different degree of smoothness because of the different dt rate. The simulation is based on the 5 minutes DNI data and includes also the period before the 8:30 in the morning while the dish is not in operation because of the presence of shadow created from some trees close to the facility. A part from the first hour, the integral of electricity calculated during the rest of the day is about 1% higher than integral of energy measured. Fig. 6. Comparison between simulated and experimental daily GrosPower of EuroDish 2.2. Model application ERSE-DISH model has been applied in some location in south Italy to estimate the potentiality of the EuroDish under the Mediterranean weather conditions. The simulations have been based on meteo data, including DNI, achieved in the 90 in some reference locations in south Italy. One of them is Gela, along the Mediterranean coast of Sicily. To have a good significance of the simulations a reduced TRY DNI experimental data file, based on 3 years period, was first realised, using the Global Horizontal Irradiance data as guide parameter. In the graph in Fig. 7. are plotted the trends of the production of an EuroDish solar generator in the site of GELA, calculated using the ERSE DISH model. It can be seen that the number of hours of operation is slightly inferior than the potential usable number per month. This result is due essentially to the wind effect, that in south Sicily can be valuable, and to the time that the dish looses because of its behaviour. The total amount of net energy produced annually is estimated at Gela as kwhe. This data corresponds to a

7 number of 1056 full power equivalent hours. The annual number of operation hours should be around 2430 with an average power of 4,34 kwe. The average net efficiency results to be 12.5% while the parasitics consumes are roughly 7% of the gross energy produced. Its also interesting to see that the average efficiency of the generator is slightly higher in winter than in summer, despite of the fact that the intensity of DNI should be higher in summer. This behaviour is due to the increase of the air temperature in summer that can reduce the efficiency of the Stirling cycle. On the other hands during the test in Milan was clarified that the radiators are slightly undersized respect the potential amount of energy to discharge when the ambient temperature approaches 35 C. Fig. 7. Trend of production of an Eurodish system using helium as working gas located at Gela Sicily, according to the model ERSE-DISH The estimated production doesn t account of the possibility to have failures during the normal operation of the generator. It is known that dish Stirling systems can have unwanted stops for a number of reasons, internal and external. Regarding the external ones, the experience at ERSE in Milan showed that the grid can have a relevant role in these lacks of production: micro interruptions of power, spikes, fast variations outside the standards of the tension parameters can generate signals that force the electronics to move the system out of the sun in safety position. Fast reactions of electronics and software permits to save the generator but can result in a extra reduction of annual net production. If you assume that the generator, at the today stage of development, has a stop of one hour every week of regular operation, you have a reduction of production of 230 kwhe/y that corresponds roughly to 2 % of the estimated net annual production. This reduction is not relevant in terms of energy but can be valuable from the extra manpower requested as the stops imply often a survey of the system. Moreover, due to the today degree of development, the substitution of the seals of the motor has to be foreseen every hours, adding at least hours of lack of production per year. Other tests based on the data of other locations along the coast in south Italy showed a reduction of net annual production roughly proportional to the decrease of DNI, in such a way that the number of equivalent full power hours ranges from 1000 to 900, including a 250 real hours of missed production for failures and maintenance which corresponds roughly at 100 full power equivalent hours or 1000 kwhe net. To have a first evaluation of the impact of the substitution of helium with hydrogen as working gas, the formulation of the production was substituted inside the model with a curve that implies a maximum net efficiency of 24%. Tests performed with the hydrogen hypothesis showed that the energy produced could

8 be improved by 30 35% with a number of full power equivalent hours of per year, including failures and maintenance stops. In these cases the production should be comparable with an equivalent PV system with the same nominal pick capacity. 3. Conclusions. An EuroDish dish Stirling generator was installed and tested since 2002 at ERSE, formerly CESI Ricerca, in Milan. The experimental operation permitted to achieve a number of detailed information regarding the efficiency of the system in different weather conditions, the parasitics consumes and the behaviour of the generator due to the control software. All these information have been introduced into an easy software named ERSE-DISH capable to simulate the dish operation in real conditions and useful to estimate the production of the system in sunny locations in south Italy. The model has the possibility to recalculate the hourly average data of direct solar radiation, at a 5 minute average data in such a way to permit a refined description of the dish Stirling systems that have, in general, a fast response to any input. The simulation of the behaviour of the EuroDish implied the description of the internal logic status dynamics which in turn, is associated to the variability of production and parasitics consumes. In general was found that daily parasitics consumes can vary from 5 to 15 % of the gross power produced according to the average solar radiation intensity and stability of the weather condition. Ambient temperature and wind velocity can affect the production as well; the radiator system should be designed to maintain the cooling water below 55 C even when the ambient temperature approaches 45 C at the maximum power rate. Tests performed with the ERSE DISH model, based on experimental DNI meteo data of some location in south Italy, showed that net production could range between kwhe/y, including a reasonable number of failures and stop for maintenance, using helium as working gas for the Stirling motor. Alternatively, using hydrogen as working gas, the production could increase of % yearly resulting in full hours equivalent, maintenance included, which is a value comparable with the equivalent full hours of production of a PV plant of the same nominal pick power in the same locations. Last but not least the model can also be adapted to other dish Stirling systems, modifying the logics of the behaviour and intrinsic performances, to permit the comparisons from different machines, or estimate the improved evolution of the same system, in different location, using the typical available hourly average meteo data files. Acknowledgements This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between CESI RICERCA and the Ministry of the Economic Development General Directorate for Energy and Mining Resources stipulated in June 21, 2007 in compliance with the Decree n.73 of June 18, References [1] T. Kech, W. Schiel, EnviroDish and EuroDish System and Status, ISES Int. Solar Energy Congress Gotegorg June 2003 [2] V. Brignoli, A new dish Stirling solar generator in Italy, ISES Int. Solar Energy Congress Gotegorg June 2003 [3] V. Brignoli, One year of operation oa a Solo 161 Stirling solar unit in Italy 11 th Int. Stirling Conference Rome Nov 2003 [4] T. Mancini, M. Mehos, R. Diver - The development of Dish Stirling systems in the U.S., 11 th SolarPACES Symposium May 2002 [5] P. Heller, A. Baumuller, W. Schiel, EuroDish the next Milestone to decrease the costs of dish/stirling systems towards competitiveness SolarPACES Symposium, Sydney 2000