Many test procedures have been proposed by various organizations to determine the thermal performance of solar water heaters. Testing of the complete system may serve a number of purposes. The main one is the prediction of the system’s long-term thermal performance. System testing may also be used as a diagnostic tool to identify failure and causes of failure in system performance. Other purposes include the determination of the change in performance as a result of operation under different weather conditions or with a different load profile.
The International Organization for Standardization (ISO) publishes a series of standards, ranging from simple measurement and data correlation methods to complex parameter identification ones. ISO 9459 was developed by the Technical Committee, ISO/TC 180—Solar Energy, to help facilitate the international comparison of solar domestic water-heating systems. Because a generalized performance model, which is applicable to all systems, has not yet been developed, it has not been possible to obtain an international consensus for one test method and one standard set of test conditions. Therefore, each method can be applied on its own.
A total of five parts comprise ISO 9459 on solar domestic water-heater performance testing as described below with their current status:
ISO 9459-1:1993. Solar heating, Domestic water-heating systems. Part 1. Performance rating procedure using indoor test methods. → Active.
ISO 9459-2:1995. Solar heating, Domestic water-heating systems. Part 2. Outdoor test methods for system performance characterization and yearly performance prediction of solar-only systems. → Active (referenced in EN 12976).
ISO 9459-3:1997. Solar heating, Domestic water-heating systems. Part 3. Performance test for solar plus supplementary systems. → Withdrawn in 2005.
ISO/DIS 9459-4. Solar heating, Domestic water-heating systems. Part 4. System performance characterization by means of component tests and computer simulation. → Under development (ISO/FDIS 9459-4 published in October 2012 [FDIS = Final Draft International Standard]).
ISO 9459-5:2007. Solar heating, Domestic water-heating systems. Part 5. System performance characterization by means of whole-system tests and computer simulation. → Active (referenced in EN 12976).
Therefore, ISO 9459 has at the time being three active parts within three broad categories.
Rating test
• ISO 9459-1 describes test procedures for characterizing the performance of solar domestic water-heating systems operated without auxiliary boosting and for predicting annual performance in any given climatic and operating conditions. It is suitable for testing all types of systems, including forced circulation, thermosiphon, and Freon-charged collector systems. The results allow systems to be compared under identical solar, ambient, and load conditions.
• The test methods in this standard define procedures for the indoor testing of solar water heaters with a solar simulator. The characteristics of the solar simulator are defined in ISO 9845-1:1992 (see Chapter 4, Section 4.1.3). The entire test sequence usually takes 3–5 days and the result is the daily solar contribution for one set of conditions. An indoor test procedure in which the solar simulator is replaced by a controlled heat source, used to simulate the solar energy gain, is also described. This test has not been widely adopted.
Black box correlation procedures
• ISO 9459-2 is applicable to solar-only systems and solar preheat systems. The performance test for solar-only systems is a “black box” procedure, which produces a family of “input–output” characteristics for a system. The test results may be used directly with daily mean values of local solar irradiation, ambient air temperature, and cold-water temperature data to predict annual system performance.
• The results of tests performed in accordance with ISO 9459-2 permit performance predictions for a range of system loads and operating conditions, but only for an evening draw-off.
• ISO 9459-2 is one of the methods used in EN 12976 for performance testing.
Testing and computer simulation
• ISO/DIS 9459-4 draft standard (published so far only as ISO/FDIS-final draft international standard) suggests a procedure for characterizing annual system performance and uses measured component characteristics in the computer simulation program TRNSYS (described in Chapter 11, Section 11.5.1). The procedures for characterizing the performance of system components other than collectors are also presented in this future part of ISO 9459. Procedures specified in ISO 9806-1 (see Chapter 4) are used to determine collector performance, whereas other tests are specified for characterizing the storage tank, heat exchangers (if used), and control system.
• ISO 9459-5 presents a procedure for the dynamic testing of complete systems to determine system parameters for use in a computer model. This model may be used with hourly values of local solar irradiation, ambient air temperature, and cold-water temperature data to predict an annual system performance.
• ISO 9459-5 specifies a method for outdoor laboratory testing of solar domestic hot water systems. The method may also be applied for in-situ tests and indoor tests by specifying appropriate draw-off profiles and irradiance profiles for indoor measurements. The system performance is characterized by means of whole-system tests using a “black box” approach, i.e., no measurements on the system components or inside the system are necessary. Detailed instructions are given on the measurement procedure, processing and analysis of the measurement data, and presentation of the test report.
• ISO 9459-5 is one of the methods used in EN 12976 for performance testing.
• The results of tests performed in accordance with ISO/DIS 9459-4 or ISO 9459-5 are directly comparable. These procedures permit performance predictions for a range of system loads and operating conditions. The disadvantage of these procedures is that a detailed computer simulation model of the system is required.
The procedures defined in ISO 9459-2, ISO/DIS 9459-4, and ISO 9459-5 for predicting annual performance allow the output of a system to be determined for a range of climatic conditions, whereas the results of tests performed in accordance with ISO 9459-1 provide a rating for a standard day.
One of the most used system test methods is standard ISO 9459-2. This is because it requires the least investment in equipment and operator skills. In this standard, the system is preconditioned at the start of each test day and charged to the required temperature, Tc, then it is left to operate with no loads applied and the only measurements required are those of solar radiation and ambient temperature. Energy monitoring is required at the end of the day, during the single draw-off, and this can be achieved with either a simple manual temperature and volume measurements or a data acquisition system. The daily energy gain is determined for a range of clear and cloudy days with irradiation between 8 and 25 MJ/m2 day, with approximately the same (Ta − Tc) value for each day. The correlation parameter (Ta − Tc) is varied, however, by testing for a range of initial tank temperatures, Tc, for each day. The useful delivered energy at the end of the day, Qu, is correlated to the test results by:
(5.79)
α1, α2, and α3 = correlation coefficients.
The effects of thermal stratification and mixing in the storage tank are evaluated by a load calculation procedure using the temperature profiles measured during draw-off at the end of the day.
The long-term performance of the system is determined by a calculation procedure that accounts for the climatic conditions, energy carry-over from day to day, and the load volume. Additionally, a 1 h time step procedure is specified for the determination of the nighttime heat loss and the energy carry-over from day to day.
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