220 Performance of Solar Collectors ISO 9806-1:1994 and ASHRAE Standard 93:2003 give information on testing solar energy collectors using single-phase fluids and no significant internal storage. The data can be used to predict the collector performance in any location and under any weather conditions where load, weather, and insolation are known. Solar collectors can be tested by two basic methods: under steady-state conditions or using a dynamic test procedure. The former method is widely used and the test procedures are well documented in the aforementioned standards for glazed collectors and in ISO 9806-3:1995 (ISO, 1995b) for unglazed collectors. For steady-state testing, the environmental conditions and collector operation must be constant during the testing period. For clear, dry locations, the required steady environmental conditions are easily satisfied and the testing period requires only a few days. In many locations of the world, however, steady conditions may be difficult to achieve and testing may be possible only in certain periods of the year, mainly during summertime, and even then, extended testing periods may be needed. For this reason, transient or dynamic test methods have been developed. Transient testing involves the monitoring of collector performance for a range or radiation and incident angle conditions. Subsequently, a time-dependent mathematical model is used to identify from the transient data the collector performance parameters. An advantage of the transient method is that it can be used to determine a wider range of collector performance parameters than the steadystate method. The dynamic test method is adopted by EN 12975-1 standard. The European standards are generally based on the ISO ones but are stricter. These are briefly introduced in Section 4.8. To perform the required tests accurately and consistently, a test ring is required. Two such rings can be used: closed and open loop collector test rings, as shown in Figures 4.1 and 4.2, respectively. For the tests, the following parameters need to be measured: 1. Global solar irradiance at the collector plane, Gt. 2. Diffuse solar irradiance at the collector aperture. 3. Air speed above the collector aperture. 4. Ambient air temperature, Ta. 5. Fluid temperature at the collector inlet, Ti. 6. Fluid temperature at the collector outlet, To. 7. Fluid flow rate, m. In addition, the gross collector aperture area, Aa, is required to be measured with certain accuracy. The collector efficiency, based on the gross collector aperture area, is given by η
p (To Ti ) mc Aa Gt
(4.1)
In this chapter, the steady-state test method is thoroughly described. The dynamic method is presented later in the chapter.