virtual laboratory for combined solar energy system

VIRTUAL LABORATORY FOR COMBINED SOLAR ENERGY SYSTEM

NAGY Istvan

Virtual Laboratory for Combined Solar Energy System Janos Hamar*, Rafael K. Jardan*, Istvan Nagy*, Hiroyuki Ohsaki** *

Dept. of Automation and Applied Informatics, Budapest University of Technology and Economics Goldmann Gyorgy ter 3, 1111 Budapest, Hungary, European Union Phone: +36-1-463-1165, Fax: +36-1-463-3163 E-mails: [email protected], [email protected], [email protected], ** The University of Tokyo, Graduate School of Frontier Sciences, Dept. of Advanced Energy 5-1-5 Kashiwanoha, Kashiwa 277-8561, Japan E-mail: [email protected]

Acknowledgements The authors wish to thank the Hungarian Research Fund (OTKA T046240, T049640, F049152), the Control Research Group of the Hungarian Academy of Sciences (HAS), the support stemming from the cooperation between HAS and the Japanese Society for the Promotion of Science (JSPS) and for the support of the Innovation Fund of Research and Technology in the frame of the scientific and technology cooperation between the Hungarian and Japanese Government.

Keywords Renewable energy systems, Photovoltaic, Virtual instrument, Software

Abstract The basic objective of the current paper is to present a remote laboratory for a combined solar-energy system. The system generates electrical energy as well as heat energy. The remote laboratory facilitates the Internet-based access to the electrical and thermal parameters during the operation of the system. Even external intervention to operation is possible for the remote clients. The virtual laboratory can be considered as an extension of an earlier started international collaboration, called Yoto project, where contributors from Europe, Japan and Australia are trying to increase the academic and industrial education by multimedia-rich e-learning tools and virtual laboratory. It can have an important role in the distance education too.

Introduction European Union is the global leader in renewable energy technologies, with an annual turnover of approx. €10 billion, employing over 200,000 people and representing one of the EU’s most dynamic industry sectors [1]. The renewable energy industry is strategic for a united European Energy and Climate Policy aimed at reversing climate change, increasing energy security, job opportunities and clean energy in the EU and the world [2]. International Energy Agency (IEA) studies have projected cost reductions to 2010 and 2025 in utilization of renewable energy sources. IEA projects that solar PV electricity costs will decline to 6-30 cents per kWh (from 18-50 cents today) [3]. Directives of the European Commission have established aggressive policy targets for shares of primary energy (12%), electricity (21%), and transport fuels (5.75%) from renewables by 2010, as well as a solar hot water target (100 million m2 of collector area). All EU countries also have individual targets for share of electricity, ranging from 3.6% to 78%, that together should achieve the 21% EU target. During 2005/2006, the European Commission prepared a “green paper,” “roadmap,” and “biomass action

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plan” on strategies to achieve these targets and go beyond them, proposing targets of 20% primary energy and 10% transport energy by 2020. For low temperature heat production, the current share of renewables in Europe is 6-7%, about 90% of that from biomass. Some of the scenarios put the share at 14-20 % by 2030 [3]. The remote laboratory being presented in this paper has been built upon a recent research project, aimed at efficient and combined utilization of solar energy, contributing to the EU strategic plans on renewables. The main objective of the laboratory is to show up-to-date research results for university students and practicing engineers and popularize solar-energy utilization.

System structure and operation The exploitation of solar energy can be achieved by photovoltaic (PV) cells to produce electric power directly and by solar thermal (T) panels to generate heat power. The heat power can be applied directly e.g. for utility hot water service, the solar thermal panels can be economic as their construction is simple and their efficiency is quite high. However the cost of the PV cells is relatively high and the efficiency is moderate, resulting in long payback time. Even if the utilization of the solar energy is necessary only to generate electric power, it is worth considering the option of using thermal panels in combination with some heat / electric power conversion technology. The solution can be viable and economical, even if the overall efficiency of the system is moderate. The power generated by utilizing the solar energy absorbed by a given area of solar panel can be increased if the two technologies, photovoltaic and thermal cells, are combined so that the resulting unit will be capable of co-generation of heat and electric power. The two technologies offers together distinct advantages of cost savings at production, installation and by the reduced demand in roof or land area. The main disadvantage of the solution is that the efficiencies of both the PV and the thermal parts will be reduced as compared to the individual application. The research work aimed at improving the performance of the combined panels resulted in a special construction with considerable increase in the overall efficiency and reduction of the production cost. A simplified diagram about the basic layout is seen in Fig. 1. GS PL / WM Si

C

TI

Fig. 1 Construction of the combined PV / T panel The thermal part of the construction is built by applying transparent plastic layers (PL) with cellular inner walls where working medium (WM) is circulated. Under the plastic layers the Silicon (Si) photovoltaic cell is fixed and hermetically insulated. The solar ray penetrates through the plastic layers and working medium, generating electric power in the PV cell. The heat power is absorbed partly in the WM and Si layer. Thermal insulation (TI) is placed beneath the Si layer. The construction is held by a case (C) and covered by a glass layer (GS) having a special filter coating in order to reduce the power loss caused by reflection. A system proposed for converting the energy, obtained from solar radiation, into electric energy is shown in Fig. 2. The combined Photovoltaic / Thermal panels (PV / T) are connected to a heat / electric energy conversion system that consists of a heat exchanger and a turbine-generator-converter unit connected to the utility mains (parallel mode of operation - PM) or a group of loads (stand-alone mode of operation – SAM). In parallel mode, by adding a static switch (SW), UPS operation can also be realized by using energy storage devices: storage tank in the thermal side or batteries in the dc link of the AC/AC converter.

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Mains Pump

Working SW

PV/T panel

+

TurbineGeneratorConverter

Heat exchanger

DC

Charge controller

AC

Battery Load

Microcontroller Supervisory System

Fig. 2 Combined PV / T system The task of the charge controller is to limit the battery voltage to a maximum level in case of surplus charge and disconnect the load to prevent deep discharge in case of deficiency in charge. The heat conversion and turbine-generator section of the system can be seen in Fig. 3. The circuit of the primary working fluid is circulated through the PV/T panel, storage tank (ST) and heat exchanger (HE). The secondary circuit includes the heat exchanger, the turbine (T) and the condenser. Additional heat energy can be supplied by the help of an auxiliary steam generator using an independent energy source, e.g. natural gas. HE Heat Exchanger

Storage Tank ST

Solar Thermal Collector

Generator

Pump

PV/T

Turbine

G To AC/AC Converter

ET

Expansion Tank

Aux. Steam Generator

Condenser ASG

Heat input

Fig. 3 Heat Conversion and Turbine-Generator It is noted, that a similar configuration can be used in another application, when our objective is the utilization of waste energy. In this case the system is used to replace a throttling valve thus utilizing energy that can be extracted during pressure reduction. The working medium (steam or gas) is fed to a special high speed turbine through a control valve and the torque of the turbine is transferred to the generator. Since only the turbine, generator and condenser play a role, of course Fig. 3modifies.

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Laboratory Experiments via WEB The new, WEB-based remote laboratory prototype has especially been designed for educational purposes using Java programming language. It facilitates the continuous monitoring of the combined solar energy system for the remote clients and furthermore they can also intervene in its operation. The proposed architecture is shown in Fig. 4. The system described above is connected to the server via the system connector interface. It implements some relevant methods, like starting, stopping the system or adjusting its operation parameters according to the request, it directly received from the data processor. On the other hand it manages the regular data-sampling and collection of the previously selected operation parameters in the system, that is, it arranges for the measurements of voltages, currents, temperature, heat, etc. time by time with the given sampling rate. The remote clients are divided into two groups. The clients with standard access rights are identified in the figure as Regular Remote Clients, while the clients with administration-level access rights are marked as Privileged Remote Clients. The WEB Service component and the Administration Servlets provide the interface between the remote clients and the server. The basic operation, omitting the initialization process of the system is shortly as follows: The solar system can be started by any of the privileged clients from a WEB browser after appropriate authorization. As the client software sends the start request, it is forwarded to the data processor and the system connector starts the solar energy system. Furthermore the start event is also logged to the database. After starting the solar-system, the so-called system connector component begins the acquisition of the measurement data with the preadjusted sampling rate. Each time new data are acquired, they are forwarded to the data processor and finally stored in the database. Any of the external clients can send a request to the WEB service component to get the measured parameters. In this case the request goes further to the data processor and the last measurement results are returned from the database to the WEB service and finally to the remote client. The adjustment of the operation parameters is very similar. The client sends the request to the WEB Service with the name of the parameter and the desired new value. It is forwarded to the data processor, which on first hand sends an order to the system connector to adjust the required parameters to the new values and afterwards logs the adjustment details into the database.

Fig. 4 Basic architecture of the system In fact, details of each important system event are stored in the database. The database content can be accessed by any of the privileged remote clients and the stored data can be checked and exported for further utilization e.g. for visualization in spreadsheets, diagrams etc. Depending on the sampling rate

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of the data acquisition, this may result in huge amount of data. The max. quantity of data to be kept in the database should therefore be set in advance in the configuration settings. HTTP/SOAP protocol is used for the interprocess-communication between the server and client sides. SOAP is a simple XML-based protocol to let applications exchange information over HTTP. Applying SOAP has several relevant advantages especially considering the flexibility. Since the communication uses HTTP transport binding, the firewall problems that often come up with many other protocols (e.g. socket-based communication) can be avoided. Taking into consideration the XML-based SOAP standards the clients can be designed and implemented using one of the widely used programming languages without limitations, like, C++, Java, Macromedia Flash AS, also other ones tied to the .NET framework, etc., furthermore the clients can run on different platforms (Windows, Unix, Linux, Solaris etc.). On the other hand, its main drawback comes from the lengthy XML format; SOAP is considerably slower than competing middleware technologies. However the prototype-tests confirmed that the achieved communication speed is more than enough for this system. A test client operated in a simulation environment can be seen in Fig. 5 and Fig. 6. It was implemented in Macromedia Flash using ActionScript language.

Fig. 5 Remote client: Thermal part It shows the combined photovoltaic/solar thermal system. Two tabs are used on the upper part of the screen. One of them shows the schematics of the thermal part with the thermal parameters, like temperatures, flow rates and heat as shown in Fig. 5. The other tab presents the schematics of the electrical circuit, including the PV panel, solar controller, inverter and battery (Fig. 6). The parameter values are being continuously collected on the server and refreshed on the client. The displayed data were refreshed each 2 sec in the test system, but this rate can be further improved if necessary. The messages/warnings shown in the bottom of the client conduce the revealing of the communication problems if any. If the connection is lost due to any trouble in the communication network, the client keeps trying to reconnect. The number of successive connection-trials and lost of data packets can be traced in the bottom right corner. They are especially for debugging purposes.

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Fig. 6 Remote client: Electrical part

Fig. 7 System operation report The content of the database can be easily visualized in system operation reports (Fig. 7), that is, the stored parameter values can be displayed in tabular format for a given time period. The data obtained can also be imported into spreadsheet software like Excel for further edition and graphical visualization.

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Conclusions A concept for a remote laboratory of a combined solar system has been proposed. The system produces direct electrical energy by the photovoltaic panels as well as thermal energy by heating up the circulating medium. The system is designed to utilize solar energy for the production of electric energy based on the application of combined photovoltaic/solar thermal panels. The remote laboratory is built upon the outcome of a recent project, targeted the efficient utilization of the solar energy for residential/farm applications. It is devoted to the academic and industrial education and to manifest the advantages of the recently proposed, combined solar-energy system.

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