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In the framework of the European Committee for Standardization, CEN (Comité Européenne de Normalisation), the operation of a new technical committee dealing with solar thermal collectors and systems has been initiated. Specifically, CEN/TC 312, "Thermal solar systems and components," was created in 1994, following a request of the European Solar Thermal Industry Federation (ESTIF) to the CEN Central Secretariat. The scope of CEN/TC

312 is the preparation of European standards to cover terminology, general requirements, characteristics, and test methods of thermal solar systems and components.

The primary aim of the European standards is to facilitate the exchange of goods and services through the elimination of technical barriers to trade. The use of standards by industry and social and economic partners is always voluntary. However, European standards are sometimes related to European legislation (directives). Furthermore, conformity to such standards may be a presumption for solar projects to get a subsidy from national renewable energy systems supporting programs (Kotsaki, 2001).

For the elaboration of European technical standards, corresponding national documents as well as international standards (ISO) have been taken into consideration. It should be noted that, compared to the existing standards, the European norms under consideration are performing a step forward, since they incorporate new features, such as quality and reliability requirements.

In April 2001, CEN published eight standards related to solar collectors and systems testing. With the publication of these European standards, all national standards related to the same topic were (or have to be) withdrawn by the nations of the European Community. Most of these standards were revised in 2006. A complete list of these standards is as follows:

• EN 12975-1:2006. Thermal solar systems and components, Solar collectors, Part 1: General requirements. This European standard specifies requirements on durability (including mechanical strength), reliability, and safety for liquid-heating solar collectors. It also includes provisions for evaluation of conformity to these requirements. CEN publication date: March 29, 2006.

• EN 12975-2:2006. Thermal solar systems and components, Solar collectors, Part 2: Test methods. This European standard establishes test methods for validating the durability and reliability requirements for liquid-heating collectors as specified in EN 12975-1. This standard also includes three test methods for the thermal performance characterization for liquid-heating collectors. CEN publication date: March 29, 2006.

• EN 12976-1:2006. Thermal solar systems and components, Factory-made systems, Part 1: General requirements. This European standard specifies requirements on durability, reliability, and safety for factory-made solar systems. This standard also includes provisions for evaluation of conformity to these requirements. CEN publication date: January 25, 2006.

• EN 12976-2:2006. Thermal solar systems and components, Factory-made systems, Part 2: Test methods. This European standard specifies test methods for validating the requirements for factory-made solar systems as specified in EN 12976-1. The standard also includes two test methods for the thermal performance characterization by means of whole-system testing. CEN publication date: January 25, 2006.

• ENV 12977-1:2001. Thermal solar systems and components, Custom-built systems, Part 1: General requirements. This European pre-standard specifies requirements on durability, reliability, and safety of small and large custom-built solar heating systems with liquid heat transfer medium for residential buildings and similar applications. The standard also contains requirements on the design process of large custom-built systems. CEN publication date: April 25, 2001.

• ENV 12977-2:2001. Thermal solar systems and components, Custom-built systems, Part 2: Test methods. This European prestandard applies to small and large custom-built solar heating systems with liquid heat transfer medium for residential buildings and similar applications and specifies test methods for verification of the requirements specified in ENV 12977-1. The standard also includes a method for thermal performance characterization and system performance prediction of small custom-built systems by means of component testing and system simulation. CEN publication date: April 25, 2001.

• ENV 12977-3:2001. Thermal solar systems and components, Custom-built systems, Part 3: Performance characterization of stores for solar heating systems. This European pre-standard specifies test methods for the performance characterization of stores intended for use in small custom-built systems as specified in ENV 12977-1. CEN publication date: April 25, 2001.

• EN ISO 9488:1999. Solar energy, Vocabulary (ISO 9488:1999). This European-International standard defines basic terms relating to solar energy and has been elaborated in common with ISO. CEN publication date: October 1, 1999.

The elaboration of these standards has been achieved through a wide European collaboration of all interested parties, such as manufacturers, researchers, testing institutes, and standardization bodies. Furthermore, these standards will promote a fair competition among producers of solar energy equipment on the market, since low-quality/low-price products will be easier to be identified by customers, based on uniform test reports comparable throughout Europe.

The increased public awareness of the environmental aspects is reinforced by these standards, which help ensure the quality level for the consumer and provide more confidence in the new solar heating technology and products available.

4.8.1 Solar Keymark

The Solar Keymark certification scheme was initiated by the European Solar Thermal Industry Federation (ESTIF) to avoid internal European trade barriers due to different requirements in national subsidy schemes and regulations.

Before the European standards and the Solar Keymark were established, solar thermal products had to be tested and certified according to different national standards and requirements. The Solar Keymark idea is that only one test and one certificate are necessary to fulfill all requirements in all EU member states.

The Solar Keymark certification scheme was introduced to harmonize national requirements for solar thermal products in Europe. The objective is that, once tested and certified, the product should have access to all national markets.

This goal has now been achieved, except for some minor supplementary requirements in a few member states.

The CEN Solar Keymark certification scheme has been available for solar thermal products in Europe since 2003. The Solar Keymark states conformity with the European standards for solar thermal products. The CEN keymark is the pan-European voluntary third-party certification mark, demonstrating to users and consumers that a product conforms to the relevant European standard (Nielsen, 2007).

The Solar Keymark is the keymark certification scheme applied specifically for solar thermal collectors and systems, stating conformity with the following European standards:

• EN12975. Thermal solar systems and components, Solar collectors.

• EN12976. Thermal solar systems and components, Factory-made systems.

Solar Keymark is the key to the European market because:

• Products with the Solar Keymark have access to all national subsidy schemes in EU member states.

• In some member states (e.g., Germany), it is now obligatory that solar collectors show the Keymark label.

• People expect the Solar Keymark; most collectors sold now are Keymark certified.

The main elements of the party Keymark certification are:

• Type testing according to European standards (test samples to be sampled by an independent inspector).

• Initial inspection of factory production control (quality management system at ISO 9001 level).

• Surveillance: annual inspection of factory production control.

• Biannual "surveillance test": detailed inspection of products.

4.9 DATA ACQuisiTioN sYsTEMs

Today, most scientists and engineers use personal computers for data acquisition in laboratory research, test and measurement, and industrial automation. To perform the tests outlined in this chapter as well as whole-system tests, a computer data acquisition system (DAS) is required.

Many applications use plug-in boards to acquire data and transfer them directly to computer memory. Others use DAS hardware remote from the PC that is coupled via a parallel, serial, or USB port. Obtaining proper results from a PC-based DAS depends on each of the following system elements:

• The personal computer.

• Signal conditioning.

The personal computer is integrated into every aspect of data recording, including sophisticated graphics, acquisition, control, and analysis. Modems connected to the Internet or an internal network allow easy access to remote personal computer-based data recording systems from virtually any place. This is very suitable when performing an actual solar system monitoring.

Almost every type of transducer and sensor is available with the necessary interface to make it computer compatible. The transducer itself begins to lose its identity when integrated into a system that incorporates such features as linearization, offset correction, and self-calibration. This has eliminated the concern regarding the details of signal conditioning and amplification of basic transducer outputs.

Many industrial areas commonly employ signal transmitters for control or computer data handling systems to convert the signal output of the primary sensor into a compatible common signal span. The system required for performing the various tests described in this chapter, however, needs to be set up by taking the standard requirements about accuracy of the instruments employed.

The vast selection of available DAS hardware make the task of configuring a data acquisition system difficult. Memory size, recording speed, and signal processing capability are major considerations in determining the correct recording system. Thermal, mechanical, electromagnetic interference, portability, and meteorological factors also influence the selection.

A digital data acquisition system must contain an interface, which is a system involving one or several analog-to-digital converters and, in the case of multi-channel inputs, a multiplexer. In modern systems, the interface also provides excitation for transducers, calibration, and conversion of units. Many data acquisition systems are designed to acquire data rapidly and store large records of data for later recording and analysis. Once the input signals have been digitized, the digital data are essentially immune to noise and can be transmitted over great distances.

One of the most frequently used temperature transducers is the thermocouple. These are commonly used to monitor temperature with PC-based DAS. Thermocouples are very rugged and inexpensive and can operate over a wide temperature range. A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermoelectric voltage is known as the Seebeck voltage, named after Thomas Seebeck, who discovered it in 1821. The voltage is nonlinear with respect to temperature. However, for small changes in temperature, the voltage is approximately linear:

where

AV = change in voltage. S = Seebeck coefficient. AT = change in temperature.

The Seebeck coefficient (S) varies with changes in temperature, causing the output voltages of thermocouples to be nonlinear over their operating ranges. Several types of thermocouples are available; these thermocouples are designated by capital letters that indicate their composition. For example, a J-type thermocouple has one iron conductor and one constantan (a copper-nickel alloy) conductor.

Information from transducers is transferred to a computer-recorder from the interface as a pulse train. Digital data are transferred in either serial or parallel mode. Serial transmission means that the data are sent as a series of pulses, 1 bit at a time. Although slower than parallel systems, serial interfaces require only two wires, which lowers their cabling cost. The speed of serial transmissions is rated according to the baud rate. In parallel transmission, the entire data word is transmitted at one time. To do this, each bit of a data word has to have its own transmission line; other lines are needed for clocking and control. Parallel mode is used for short distances or when high data transmission rates are required. Serial mode must be used for long-distance communications where wiring costs are prohibitive.

The two most popular interface bus standards currently used for data transmission are the IEEE 488 and the RS232 serial interface. Because of the way the IEEE 488 bus system feeds data, its bus is limited to a cable length of 20 m and requires an interface connection on every meter for proper termination. The RS232 system feeds data serially down two wires, one bit at a time, so an RS232 line may be over 300 m long. For longer distances, it may feed a modem to send data over standard telephone lines. A local area network (LAN) may also be available for transmitting information; with appropriate interfacing, transducer data are available to any computer connected to the local network.

4.9.1 Portable Data Loggers

Portable data loggers generally store electrical signals (analog or digital) to internal memory storage. The signal from connected sensors is typically stored to memory at timed intervals, which range from MHz to hourly sampling. Many portable data loggers can perform linearization, scaling, or other signal conditioning and permit logged readings to be either instantaneous or averaged values. Most modern portable data loggers have built-in clocks that record the time and date, together with transducer signal information. Portable data loggers range from single-channel input to 256 or more channels. Some general-purpose devices accept a multitude of analog or digital inputs or both; others are more specialized to a specific measurement (e.g., a portable pyranometer with built-in data logging capability) or for a specific application (e.g., temperature, relative humidity, wind speed, and solar radiation measurement with data logging for solar system testing applications). Stored data are generally downloaded from portable data loggers using a serial or USB interface with a temporary direct connection to a personal computer. Remote data loggers may also download the data via modem through telephone lines.

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Solar Panel Basics

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