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An overview of the developed activity in respect with the efficiency increasing of PV pumping systems highlights a temporal evolution structured on two levels: components and subsystems. Components Efficiency Level A huge variety of pumps have been used. Related with technical problems, some of them have practically disappeared (e.g. shaft driven pumps with surface mounted motors). Others reached only a laboratory or test stage (e.g. different types of displacement pumps) and are suitable for special applications (e.g. high pumping head). The most frequently used type, that has been imposed as the most suitable, is centrifugal submersible pump with asynchronous AC motor[1]. The inverters' efficiency, usually around 95%, doesn' t have major implications into the system performances. It remains the problem of the wave form distortion related with the harmonics. PV array usage is limited only to crystalline silicon cells because of the drastically increasing of the price in thin- films or other high efficiency cells and the unsolved degradation in a:Si cells. Practically all PV generators are designed based- on fixed structure concept. Although tracking systems increase overall systems efficiency they are not used due to their costs, high maintenance needs and friendly environment requirements[1]. Subsystems Efficiency Level Since, the individual components' optimisation seems to reach its limit and becomes evident that further steps could be done only with costs' sacrifices or the activity focused on the yield increasing of the subsystems. Several techniques are recognised as being appropriate to obtain important practical results: · Mechanical coupling of the pumps[2]; · Tandem connection of the pumps[3]; · Short- term energy storage[5]; · Efficient water distribution systems[7]. Currently, this second level reached its maturity and further achievements are difficult to be preview. This fact generates the need of optimisation concept reshapement on a third level, the overhall system one. The PV pumping system has to be seen as a dynamic entity integrated in its environment in a proactive way. The two characters linked under the framework of this new way of thinking (integration into the environment and dynamicity of functioning) could be reflected, from practical point of view, in two directions of PV pumping systems optimisation: preliminary design phase and "intelligent" management. Preliminary Design Phase The New University of Lisbon, Faculty of Science and Technology has two PV pumping facilities (15,4 kW and 7,8 kW) developed under the framework of several EU research projects. The lessons learned from both laborious activities of designing, purchasing, installing and careful surveillance of their operation highlight the importance of the preliminary design phase for an optimal PV pumping system achievement and its proper operation. One of the most important elements of the environment that must be careful analysed in the phase of the system design is the water source used. We consider that the study of following items gives relevant design inputs: · usage of an existing or a new well; · surface or deep well; · well location; · well efficiency; · quality of the water; · geometrical characteristics of the well; · avoiding possible well collapse phenomena. On the other side it also must be analysed all the aspects concerning water demand[7]. The usually peak water demand used for system designing leads to an over dimensioning of the systems with direct impacts on the costs. Because of the enormous number of particular situations, we could only recommend the usage of a water demand average based on a complex integration of the following factors: · peak water demand; · daily, monthly and annual distribution curve of water demand; · meteorological data and their periodical distribution; · water storage facilities. We find that a lot of problems in the reliability of the system come from an equipment selection inadequate with the environment conditions. The water level sensors are among the parts that normally are purchased from shelf. Most of them are not suitable for all kind of water chemical ranks and stuck by corrosion. This phenomenon was present also in parts of the submersible pumps that are not made in stainless steel. An other apparently minor aspect that could generate important damages to the system is connected with the wave form of the inverter. If the harmonics distortion of the inverter frequency is reflected only in small power losses, the voltage distortion could have important impacts in electrical coil insulation. The varnishes withstand a large range of voltages but tend to degrade with time in presence of high voltage harmonics typically above of 700V. The problem can be accelerated if by some reason the motor heats. The pipe system, many times ignored, is also very important for a good design PV pumping system. A summary of our considerations regarding it is presented in the Table 1.
"Intelligent" management Often, the analysis of a PV pumping system efficiency during a period of operation in real conditions points out an efficiency lower than the predicted one. This happens even to well-designed systems. Some times this difference is considerable. This fact is related with the following: · randomness of the external conditions; · largesse of the boundary conditions range in system operation; · luck of energy management inside the system; · knowledge level of the operators. Our optimisation concept considers that a good solution for such problems solving is the development of a monitoring and control system based on Artificial Intelligence tools, namely expert- systems[8]. In order to improve the efficiency of one of our PV pumping facilities, we already started to design such an "intelligent manager". Its mainly structure is presented in Fig. 1.
Fig. 1 "Intelligent" Monitoring and Control System Through its implementation we intend to achieve: · decision making support for users/ operators; · autonomous control for remote and/ or unsupervised PV pumping systems; · training tool for users/ operators. Based- on a new concept of PV pumping systems optimisations, our team find that interesting results could be achieved considering two directions: a preliminary design phase and an "intelligent" management engine. Due to the deep connection of the first one with the particularly conditions of each system development, we could only propose a general action framework that will be adapted by each designing team. The second direction is opening a new field of computer applications into the PV field. From that beginning point, it rests a lot of work until the achievement of a real intelligent expert- system. The most difficult part of it is the transposition of the relevant specialists' experiences and knowledge into the symbolic representation of the expert- system knowledge database and inference engine. 1. A.Hanel, P.Helm, Ph. Malbrance, J.M. Servant, Recent Development in PV Pumping Applications and Research in the European Community, Proceedings of 12-th European Photovoltaic Solar Energy Conference, 476, (1994). 2. J. Heegaard, Water Supply by Means of Solar Energy, European Directory of Renewable Energy, 121, (1993). 3. R. Schroer, Photovoltaic Tandem Pumping Systems, European Directory of Renewable Energy, 161, (1994). 4. M.N. Eskander, A.M. Zaki, Maximum Efficiency- Photovoltaic- Induction Motor Pump System, , Proceedings of 11-th European Photovoltaic Solar Energy Conference, 1399, (1992). 5. M. Landau, J. Sachau, A. Raatz, PV Pumping System for Intermittent Operation, Proceedings of 11-th European Photovoltaic Solar Energy Conference, 1391, (1992). 6. V. Gerhold, W. Vaassen, PV Supplied Multi- Pumping Systems: Improuvement of Daily Efficiency by Energy Management Systems, Proceedings of 12-th European Photovoltaic Solar Energy Conference, 1965, (1994). 7. A.S. Bahaj, A.S. Mohammed, Sizing of a PV Pumping System and its Storage Capacity to Meet Crop Water Requirments in Remote Areas, Proceedings of 12-th European Photovoltaic Solar Energy Conference, 1969, (1994). 8. C. Caracaleanu, L. Guimarães, A. Ribas, C. Olteanu, Development of a Real PV Expert-System, Proceedings of 13-th European Photovoltaic Solar Energy Conference, 1034, (1995). |
