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PUBLISHED PAPERS
Upright, Optimally-inclined and Tracking Grid-connected PV Systems Performance in Malta
C. Yousif, J.M. Franco Esteban*, D. Salvador Lopez* and L. Santana Zurita*
*E.T.S. Ingenieros Industriales, Universidad De Valladolid, Paseo del Cauce, s/n, 47005 Valladolid, Spain
The paper presents an analysis of the performance of various solar PV systems installed in Malta with a peak installed capacity ranging between 0.36 and 3 kW. These include the first vertically-installed PV system that aimed at providing information on future PV façade installations, an optimally oriented PV system operating in an industrial area, a stationary PV system that had some compromise with its orientation due to aesthetical reasons and a rooftop single-axis tracking system.
Analysis of data was carried out according to the guidelines set by the Joint Research Centre of Ispra Establishment. The average Performance Ratios varied from 0.46 for the upright PV system to 0.67 for the optimally-oriented system. Comparison of the optimally-oriented system was made with other similar systems published in IEA PVPS Task 2 Performance Database Programme. It was shown that the “optimum” PV system operated well, producing an average final yield of 3.97 kWh/kWp/day over three and a half years, which sets a new record for scientifically monitored PV systems in Malta. The previous value was set at 3.33 kWh/kWp/day, as the minimum guaranteed output based on five years of monitoring. The PV system that had a compromise, was installed facing the south-west, due to aesthetical reasons. This system produced a final yield of 3.37 kWh/kWp/day and had a Performance Ratio of 0.61. The single-axis tracking system had a fairly constant performance ratio for most months but peaking in spring at about 0.8.
Several studies were also made on the grid parameters of voltage and frequency. All systems operated at a grid voltage well within the standard of 230 volts a.c., except for the “optimum” system operating in the industrial area, which was gradually subjected to higher voltages reaching up to 250 V over the years. On the other hand, the grid frequency deviations were within safe limits.
REFER TO: Proceedings of the 22nd European Photovoltaic Solar Energy Conference & Exhibition, Milan, Italy, 3-7 September, 2007 (under print).
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High Concentration PV System
Oscar Pereles, Andreas Bett, Eckhard Lupfert, Andreas Haberle, Jesus Fernandez, Gerhard Srobl, David Faiman, Robert Soler, Charles Yousif
The aim of this project has been to develop, set up and test a new high concentration – 1000x or more – PV system with a large-area III-V-receiver. This has been achieved by integrating two technology fields: The high concentration of the sunlight has been obtained by using technologies experienced in solar thermal systems like parabolic dishes or tower systems. The high concentration photovoltaic receiver is based on the III-V solar cell technology. To deal with the high concentration, Monolithic Integrated Modules (MIM) have been developed and assembled to Compact Concentrator Modules (CCM). The CCM prototypes have been tested in a solar furnace (PSA) and in a parabolic dish (BGU). The results of the project will be presented in this paper.
REFER TO: Proceedings of the 4th International Conference on Solar Concentrators for the Generation of Electricity or Hydrogen, 12-16 March 2007, San Lorenzo del Escorial, Spain, under print.
Barriers and Incentives for the Widespread Application of Renewable Energy in Malta
R.N. Farrugia, M. Fsadni, E.A. Mallia and C. Yousif
Renewable energies have not been utilised to any significant extent in Malta. Domestic solar water heating seems to have a market penetration of about 8% of all households. Rooftop solar photovoltaic systems run into tens with an overall installed capacity of about 20 kWp. There are no medium or large wind turbines and plans to convert waste to energy are still being considered. The lack of an energy policy has been one of the hurdles, in spite of increasing interest. This paper also identifies other barriers that go beyond economics and politics, such as the general perception of different renewable energy applications. Evaluation of the present incentives is made and a number of measures that may enhance the diffusion of renewables are also proposed.
REFER TO: Proceedings of the World Renewable Energy Congress – IX, 19th-25th August 2006, Florence, Italy, under print.
The Renewable Energy Potential of Malta
R.N. Farrugia, M. Fsadni, E.A. Mallia and C. Yousif
Since its accession as member of the EU, Malta's interest in renewable energy applications has increased. This paper provides a realistic assessment of the technical potential of the different renewable energy sources in Malta based on a decade of monitoring and analysis of systems. Three different sources of renewable energy are directly available, namely solar and wind energies and energy from waste. The overall potential has been found to be 24% of the total fossil fuel-generated electricity of 2003, with 9.1% coming from rooftop solar photovoltaic systems, 5.4% from onshore wind farms, 3.4% from offshore wind farms and 5.6% being the contribution of energy from waste. Moreover, domestic solar water heating may offset an additional 4.8%. This assessment should be useful in the formulation of a renewable energy policy.
REFER TO: Proceedings of the World Renewable Energy Congress – IX, 19th-25th August 2006, Florence, Italy, under print.
Contribution of Solar Applications Towards Achieving a Renewable Energy Target for Malta Charles Yousif
Based on applied research and demonstration activities carried out during the past decade at the Institute for Energy Technology, solar photovoltaic applications offer a good prospective towards achieving a significant percentage of Malta’s electricity needs. This paper summarises the most important conclusions reached, describes the current installed systems in Malta, sets the achievable targets and discusses the challenges that face the widespread applications of this technology. Moreover, consideration is given to the widespread applications of solar water heating systems and their contribution towards lowering electricity consumption.
REFER TO: Proceedings of "Renewable Energies in Malta and Beyond" Seminar, 19th September 2005, Salina, Malta, published by the Institute for Energy Technology, University of Malta, pp. 17-21.
Investigation of Domestic Solar Water Heating Installations in Malta
S.P. Borg, *C. Yousif and *R.N. Farrugia
Malta enjoys favourable weather conditions that encourage the use of solar water heating systems, but their market penetration has so far remained low. In an effort to identify the reasons behind this lack of impetus, the Institute for Energy Technology has devised a number of study programmes. This paper deals with one of those studies, namely the technical aspects of installed domestic solar heating systems. Results showed that there are a number of issues that are contributing towards lower solar water heating systems performance. Most of these shortcomings can be easily overcome, but it is clear that there is a need for proper training programmes for installers as well as better diffusion of information on the proper utilisation of this technology.
REFER TO: Proceedings of "Renewable Energies in Malta and Beyond" Seminar, 19th September 2005, Salina, Malta, published by the Institute for Energy Technology, University of Malta, pp.30-38.
Energy Efficient Housing: A First Case Study for Malta
Buhagiar Vincent and Yousif, Charles
Due to higher demand for housing in Malta, local single-family dwellings, typically terraced houses, are being demolished to make way for maisonettes and multiple storey apartments. This increases the energy demand per square metre that would lead to a greater load on the infrastructure. This paper describes the first example of building an energy efficient housing block that integrates energy efficient measures and renewable energy systems, as a sample of Maltese housing stock working towards a sustainable housing vision.
REFER TO: Proceedings of the International Conference SB04MED Sustainable Construction: Action for Sustainability in the Mediterannean, 9-11 June 2005, Athens, Greece, published on a CD.
Analysis of Photovoltaic Projects in a Mediterranean Island
F.A. Ribeiro* and C. Iskander Yousif
* Loughborough University, UK
The paper presents an analysis of the four grid-connected systems installed in Malta and monitored by the Institute for Energy Technology of the University of Malta. The systems have varying power ratings, orientation and tilts. Three systems had stationary solar modules, while the fourth one used an active tracking device. The purpose of this study was to evaluate the performance of different systems under the local weather conditions of a typical Mediterranean country. Comparison to other systems in EU countries was made and answers to frequently asked questions were found. The mean monthly Performance Ratio of the systems ranged between 0.51 and 0.80, while the final yield was reported to be between 2.76kWh/kWp/day and 3.61kWh/kWp/day. The final efficiency was also found to be between 5.1% and 7.1%. This compared favourably with results reported by European researchers such as in the Thermie Project. The inverters proved to be safe and did respond as expected during times of grid disturbance. In view of Malta’s new membership in the European Union, this study will be an essential tool to support future widespread applications of solar electric systems in Malta.
REFER TO: Proceedings of the 19th European Photovoltaic Solar Energy Conference & Exhibition, Paris, France, 7-11 June 2004, pp. 2842-2845.
Wind Characteristics on the Caribbean island of Puerto Rico
K. Altaii* and R.N. Farrugia
Wind data was measured at a number of sites on the Caribbean island of Puerto Rico over a 24 calendar-month time frame. The wind data gathered at four sites is envisaged to shed new light on the wind characteristics of this tropical island with an emphasis on the climate's sustainability for wind energy technology applications. Characteristics such as the diurnal, monthly and annual wind speed are subjectively investigated to determine the sites' potential for further studies in the wind measurement field. Reasonable wind conditions for wind energy conversion system installation seem to exist in and around Aguadilla and Ponce.
*College of Integrated Science & Technology, James Madison University, Harrisonburg VA22807, USA
REFER TO: Renewable Energy, Vol. 28, No. 11, pp. 1701-1710, Sep. 2003.
A Multi-disciplinary Approach to University-level Renewable Energy Education
Robert N. Farrugia, Jonathan J. Miles*
Wind energy generation is currently one of the leading renewable energy technologies, making it a subject worth addressing in any engineering, energy or environment-oriented course. The general approach in secondary, post-secondary or tertiary education is to focus on the technology itself, its potential integration with the traditional modes of generation, or its impacts on the environment, depending upon the depth and breadth of the course being mentored. This paper focuses on what is arguably a unique and new way of exposing students and faculty alike to the benefits and possible impacts of wind generation subject to unique conditions on the Mediterranean island of Malta. Whilst the existence of a significant wind resource on the island has been established, this characteristic is in itself just part of a more holistic methodology required to determine the feasibility of wind generation in this island nation. Over the past six years, a number of intensive four-credit ‘Study Abroad’ programmes presented case studies evaluating the site-specific potential for wind generation subject to factors such as visual impact, noise and avian considerations, grid connection, and grid interfacing issues amongst others.
*College of Integrated Science & Technology, James Madison University, Harrisonburg VA22807, USA
REFER TO: ISES Solar World Congress 2003, Göteborg, Sweden, 14-19 June 2003
A Hybrid Wind/Solar Photovoltaic System for Malta: A Case Study for Supplying Power to a Reverse Osmosis Desalination Plant
Robert N. Farrugia & Charles E. Iskander Yousif
The paper describes the solar and wind potential at Marfa near a reverse osmosis (RO) desalination plant. An analysis is made to evaluate the possibility of installing a hybrid wind/photovoltaic power plant to offset a large part of the demand of this energy intensive process. The results have shown a marked level of compatibility between the monthly solar and wind trends although the diurnal trends call for a more careful analysis of any hybrid installation, since both renewable energy sources peak during the daylight hours. Both technologies have the potential of independently supplying the required power for water desalination with solar photovoltaics exhibiting better tracking of the RO electric consumption profile. However, wind energy systems showed the capability of producing excess power in the winter months that would help to offset any critical demands on the grid during the colder season.
REFER TO: Proceedings of the Renewable Energy Systems for Islands, Tourism and Desalination Conference, 26-28 May, 2003, Crete, Greece, pp. 269-276.
Wind Power in Virginia - An Innovative Strategy to Barrier Mitigation
Jonathan J. Miles*, Matthew G. Heck* and Robert N. Farrugia
A viable, and potentially valuable, wind resource exists in the Commonwealth of Virginia. However, this clean energy source is to date only partially characterized and barely exploited. In response to several initiatives put forth by the U.S. Department of Energy (DOE) and supported by the National Renewable Energy Laboratories and Virginia’s Department of Mines, Minerals, and Energy, most notably the Wind Powering America initiative, the Virginia Wind Energy Collaborative (VWEC) was formed in 2002. This organization links several academic, state, and private institutions seeking to attain a common goal – first the expansion of wind energy to become a key component in the Commonwealth’s energy supply portfolio, and second the development of a robust wind energy industry in Virginia. The collaborative has thus far identified, and is active in developing and implementing, a collection of technical resources, progressive policies, and legislative actions to support this goal.
REFER TO: American Wind Energy Association (AWEA), Windpower 2003 Conference and Exhibition, Austin, Texas, U.S.A., May 18 - 21, 2003.
*College of Integrated Science & Technology, James Madison University, Harrisonburg VA22807, USA.
Energy Technology Options for a Better Environment
R.N. Farrugia, M. Fsadni and C. Iskander Yousif
The Institute for Energy Technology's mission is to analyse the use of energy, to determine feasible measures to conserve energy, and to investigate and apply renewable energy technologies. A number of applied research projects are focusing on select priority areas such as energy management, energy use in buildings, solar and wind energy. It also participates in education programmes aimed at generating popular awareness in this field. The Institute has a number of ongoing collaborations with universities and other resarch centres, industry and international bodies.
REFER TO: Proceedings of the “Chamber of Engineers Conference on the Environment May 2003 (under print).
Comparison Study Between the Performance of Tracking and Stationary Solar Photovoltaic Systems in Malta
C. Iskander Yousif
The paper compares the performance of a 1.8 kWp stationary solar photovoltaic grid-tied system and a 360 Wp single-axis tracking system, for the period January to December 2001. Results show that the tracking system had succeeded in maintaining a relatively stable performance ratio, while the stationary system suffered from lower values, throughout summer. Wind gusts of up to 14 m/s were logged during this period but the tracking system, weighed down by concrete blocks, has remained in position on the roof. The tracking mechanism design proved to be rigid and simple to operate however a higher power-rated double-faced pilot solar photovoltaic module had to be used to drive the tracking motor. Analysis of data was carried out according to the guidelines set by the Joint Research Centre - Ispra Establishment.
REFER TO: Proceedings of the "PV in Europe from PV Technology to Energy Solutions" Conference & Exhibition, Rome, Italy, 7th-11th October 2002, pp. 1065-1068.
"Sikka l-Bajda": A Case Study for Offshore Wind Power
R. N. Farrugia, J. J. Miles*, et al
*College of Integrated Science & Technology, James Madison University, Harrisonburg VA22807, USA
This paper presents select details from a case study investigating the potential for an offshore wind turbine installation on Sikka l-Bajda, a reef approximately 1.5 kilometres off Malta’s NE coastline. The project considered prime factors associated with current offshore wind generation technology. These included an assessment of the offshore wind resource, turbine and array options, as well as estimates of turbine performance. In addition, an exercise identifying the effects of wind farm development using a multi-disciplinary holistic approach was undertaken. Parameters such as aesthetics, safety, grid-connection and other social and environmental factors were investigated. Finally, financial considerations including cost distribution and economic implications of present-day offshore wind technology are presented.
REFER TO: Proceedings of the “Enemalta's 25th Anniversary Conference on Energy and the Environment”, Mediterranean Conference Centre, 18th-19th October, 2002, Valletta, Malta
Recent Developments of Applying Solar Photovoltaic Technologies in Malta
C. Iskander Yousif
The paper presents an overview of some solar photovoltaic grid-tied installations in Malta, and gives a description of their purpose and date of commencement, besides other data. A presentation of the Maltese past and present situations and the future prospects of solar photovoltaics is given. A brief comparison between the performance of existing grid-tied PV systems is made to demonstrate the good potential of generating electricity from the sun, thus making photovoltaics a future contributor to the energy mix in Malta. Finally, some proposals are presented, which could be used by national legislative and statistical offices, in order to foster the wide-spread application of solar photovoltaics in a professional and orderly manner.
REFER TO: Proceedings of the “Enemalta 25th Anniversary Conference on Energy and the Environment”, Mediterranean Conference Centre, 18th-19th October, 2002, Valletta, Malta
Wind as a Sustainable Generation Option
Ing. Robert N. Farrugia
In 1994, the Institute for Energy Technology of the University of Malta embarked upon a wind resource assessment with the primary aim of quantifying the local wind potential. This first phase of the research programme used a multi-disciplinary approach, which coupled field studies to numerical modelling procedures, in an attempt to determine wind characteristics for particular locations around the Maltese Archipelago. The Institute’s research is now focusing on multiple-level wind monitoring at two specific sites. The wind data taken at these localities is very relevant as it includes wind measurements taken at, or very close to a realistic wind turbine’s hub height. This paper will present appropriate results from both phases of the research program and strive to address issues and concerns associated with medium to large-scale wind energy conversion system installations as perceived in the local context.
REFER TO: Proceedings of the “Enemalta's 25th Anniversary Conference on Energy and the Environment”, Mediterranean Conference Centre, 18th-19th October, 2002, Valletta, Malta
Comparison Study Between the Performance of Tracking and Stationary Solar Photovoltaic Systems in Malta
C. Iskander Yousif
The paper compares the performance of a 1.8 kWp stationary solar photovoltaic grid-tied system and a 360 Wp single-axis tracking system, for the period January to December 2001. Results show that the tracking system had succeeded in maintaining a relatively stable performance ratio, while the stationary system suffered from lower values, throughout summer. Wind gusts of up to 14 m/s were logged during this period but the tracking system, weighed down by concrete blocks, has remained in position on the roof. The tracking mechanism design proved to be rigid and simple to operate however a higher power-rated double-faced pilot solar photovoltaic module had to be used to drive the tracking motor. Analysis of data was carried out according to the guidelines set by the Joint Research Centre - Ispra Establishment.
REFER TO: Proceedings of the "PV in Europe from PV Technology to Energy Solutions" Conference & Exhibition, Rome, Italy, 7th-11th October 2002, in print.
The Wind Shear Exponent in a Mediterranean Island Climate
R.N. Farrugia
The difficulties in estimating the long term mean wind speed and subsequently wind turbine energy output derive from the fact that more often than not, available data is taken at a level other than machine hub height. The 1/7th power law has been recognised as a handy tool to carry out vertical wind speed extrapolation to the desired hub height. It is also understood that using an exponent of 1/7th could lead to underestimation of the actual long-term mean wind speed aloft. This paper strives to evaluate the Power Law with respect to wind data taken on a 25m mast on the central Mediterranean island of Malta. Whilst deriving a site-specific factor affiliated to a typical terrain type, it also strives to determine characteristic variations of the power law exponent over appropriate sampling intervals.
REFER TO: Renewable Energy, Vol. 28, No. 4, pp. 647-653, Apr. 2003
A Five-Year Report on a Solar Photovoltaic Grid-Tied system Operating under a Typical Mediterranean Climate
C. Iskander Yousif & E. Scerri
The paper presents the performance results of the first solar photovoltaic grid-tied solar system, installed in Malta. This 1.8 kWp system has supplied the Institute with more than 60% of its electricity needs, i.e. 10,000 kWh. Within this period of study, four solar modules were replaced while the inverter experienced two faults. Analysis of data was carried out according to the guidelines set by the Joint Research Centre – Ispra Establishment.
REFER TO: Proceedings of the 17th European Photovoltaic Solar Energy Conference & Exhibition, Munich, Germany, 26th – 30th October 2001 (under printing).
REFER TO: Proceedings of the 17th European Photovoltaic Solar Energy Conference & Exhibition, Munich, Germany, 22nd-26th October 2001, pp. 720-723.
Integrating Renewable Energy Studies in Under- Graduate Curriculum: The IET/ISAT Experience
C. Iskander, E. Scerri, J. Miles*
During the past 3 years, collaboration between the Institute for Energy Technology (IET) at the University of Malta and the Program of Integrated Science and Technology (ISAT) at James Madison University, Virginia, U.S.A., has led to the formulation of a programme of studies captured within the course entitled “International Energy Studies”. This is a 3-credit offering that permits students from ISAT to travel abroad and to study and work on specific renewable energy projects and related topics such as economic feasibility and environmental issues.
In many parts of the world, including the Mediterranean region, the field of renewable energy, in most of its aspects, is limited to post-graduate studies, and is rarely taught in undergraduate courses. The end result is that very few people know about the potential of utilising these energies in their everyday life, in particular solar and wind energy.
This paper describes the unique experience gained from teaming up between IET and ISAT. Renewable energy technologies are taught to undergraduates, teasing and challenging them to devise sustainable forms of energy in various test cases. The paper will focus on one section of the programme, which is solar photovoltaics. A programme of studies will be presented and a number of practical suggestions will be given, to integrate photovoltaics in the curriculum of universities and other institutions of higher education.
* College of Integrated Science and Technology, James Madison University MSC 4102, Harrisonburg, VA 22807, U.S.A.
REFER TO: Proceedings of the World Renewable Energy Congress – VI, 1st to 7th July, 2000, Brighton, United Kingdom, pp. 2454-2457.
Sustainability in the Energy Sector in Malta
M. Fsadni, E. A. Mallia, S. Nasser and A.A.M. Sayigh
Primary energy consumption in Malta has stopped rising, largely due to a reduced consumption of water. The electricity demand in the domestic and commercial sectors continues to increase, mostly for air conditioning. The energy intensity has fallen but remains higher than other Mediterranean countries due to the energy requirement for water production and to an inflated road transport sector. The environmental impact of electricity generation and road traffic is high. A considerable potential exists for energy conservation in electricity generation and in buildings. The potential for renewable energy - solar thermal, photovoltaics and wind remain largely untapped. There is a need for an energy policy that promotes energy efficiency and conservation as well as the use of renewable sources at all levels.
REFER TO: Proceedings of the World Renewable Energy Congress – VI, 1st to 7th July, 2000, Brighton, United Kingdom, pp. 1755-1778.
Sowing the Seed for an Energy Conscious Society in Malta
C. Iskander, R.N. Farrugia, M. Fsadni
A one-day programme of studies aimed at school students between 13 and 16 years old has been devised and implemented for the last 3 years. The programme aims at increasing the awareness level of students and teachers, on the present energy situation in Malta and the detrimental effects that it is having on health, the environment and buildings. It also stimulates them to realise that their present social and living habits, whilst providing them with a temporarily better quality of life, frequently conflicts with the long-term sustainability of that quality. Remedies to these problems are discussed, which focus on energy conservation methods, energy efficiency implementations and use of renewable energy sources.
This is the first time that such a programme has been implemented in Malta. During the scholastic year October to May of every year, schools are invited to apply for a one-day presentation, held at the Institute’s premises. The programme includes presentations, hands-on experience on operating renewable energy projects and video shows. Discussions are held and questions are asked in the course of the day.
The short-term effects are already appearing by raising the interest of the younger generation in energy conservation and renewable energy applications. Some of the students who had visited our premises have opted to work on projects that deal with energy, the environment, buildings and renewable energy, for their project in “Systems of Knowledge”. This subject is compulsory for the final year of pre-university colleges, where a project involving “Technology and the Quality of Life”, has to be prepared and presented. It requires that every student presents a written report and builds a working model to demonstrate the concept of the project.
On a long term basis, it is hoped that such initiatives will help promote energy consciousness among the citizens who will be more responsive towards the protection of the environment and the implementation of renewable energy applications in their own lives. It is also envisaged that university students will opt for specialised studies in these fields. Only then, one can launch national programmes to help alleviate the energy dilemma in Malta, where consumption is directly proportional to the rate of increase of the gross national product.
REFER TO: Energy Globe 2000: The Award for Sustainable Energy, Austria, 9th to 12th March, 2000.
Offshore Wind Potential of a Central Mediterranean Archipelago
R.N. Farrugia, E. Scerri
An ongoing research program initiated by the Institute for Energy Technology of the University of Malta indicates that the prospects for land based wind energy conversion systems are encouraging. The next natural step is to extend this analysis and evaluate the potential for offshore wind generation. This sector is all the more important as being a small archipelago imposes additional constraints on land availability and utilization. Based upon results derived by the WAsP program and focusing on other multi-disciplinary aspects commonly associated with offshore wind generation, this paper will attempt to lay the ground for future research in the offshore wind assessment field.
REFER TO: Proceedings of the Offshore Wind Energy in Mediterranean and Other European Seas Seminar (OWEMES 2000), Syracuse, Sicily, April 2000.
A Wind Resource Assessment for the Maltese Islands
R.N. Farrugia, E. Scerri
This paper presents results deriving from an ongoing research program to evaluate the wind resource at different sites in the Maltese Islands. The aims of this study are being attained by a combination of short- and long-term wind measurement programs coupled to analysis of results generated by the WASP program. This methodology attempts to establish parameters appertaining to the local wind resource with the primary aim of determining the islands’ wind generating potential.
REFER TO: Proceedings of the 1999 European Union Wind Energy Conference and Exhibition, Nice, France, 1st to 4th March 1999.
Interfacing a Solar Photovoltaic System with the National Electricity Grid in Malta
C. Iskander & E. Scerri
A 1.8 kWp grid-connected solar photovoltaic system is being monitored at the Institute for Energy Technology, since June 1996. The average performance ratio for 22 months of operation was found to be 0.715, which translates to a final yield of 3.44 kWh/kWp/day. The average final efficiency was found to be 7.6%. These values exclude the month of October 1997, during which the system was not operating.
Problems encountered, relating to the interface between the electric utility grid and the PV system are discussed.
REFER TO: Proceedings of the Fifth World Renewable Energy Congress, Florence, Italy, 20th to 25th September, 1998. Renewable Energy, Vol. 1, pp. 577-580, Pergamon Press.
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