The DC (Direct Characterization) test method offers a second way of characterizing solar combisystems, especially factory-made systems. DC tests are carried out at an indoor test facility. Solar input is generated by means of a solar simulator and the actual collector or by a heater simulating the solar collector. In the latter case, collector parameters determined according to EN 12975—2 should be available. The space heating load is emulated following specified climate conditions and is based on a specific low-temperature heat distribution system. The core phase of the test method consists of six days, with two days simulating winter, two days summer and two days spring/autumn. Both the climate and the domestic hot water draw-offs are arranged so that evaluation of the measuring data to derive annual system performance is relatively simple. No numerical system model is needed for this.
The major performance indicator of the solar combisystem given by the DC test is the final energy used by the auxiliary heater. This implies that the solar combisystem is always tested in combination with the auxiliary heater. This feature is considered to be favourable, because many problems in system operation appear to be due to improper control strategy for the coupling of the solar and auxiliary parts of the system. Hence, the test method forces manufacturers to think about the integral system design. In special cases, a well defined laboratory auxiliary heater can be used. However, even in these cases, the performance indicator is presented for only the specific combination of the solar combisystem and its auxiliary heater.
The applicability range of the DC test method includes solar combisystems with a collector area smaller than 15-20 m2 and a heat store volume of 1500-2000 litres. In these cases, the prediction error in the annual system performance derived by the method remains smaller than 5%. Determination of the annual final energy used is only possible for annual conditions that correspond more or less to the test conditions. This is one of the reasons why the system performance has to be derived for the combination of one out of three climate zones, one out of three space heating loads and one domestic hot water demand, following the references in Chapters 2 and 3.This approach reduces the threshold for export, as there is no need for translation into specific national conditions with respect to climate and load.
The DC test originated in Sweden (Bales, 2002), where test sequences based on the average climate throughout the year were processed by a numerical model to reveal the annual system performance. This so-called AC/DC method was simplified into the DC test procedure.
The concept of the CCT (Concise Cycle Test) method is similar to that of the DC test method in that both are indoor test methods. With a core phase of 12 days, the CCT method uses a longer test cycle. The building is simulated on-line and the system with its controller(s) decides how heat is supplied to the building by flow temperature and flow rate, whereas the DC method predefines the space heating load according to a load file. A significant advantage of the floating load of the CCT is that all of the system functions may be assessed. The disadvantage is that there is no uniform or predictable energy use for space heating, which complicates characterization of the system’s energy-related performance. Unlike the DC method, the CCT method can in principle be used to characterize solar combisystems where the system intentionally uses the building’s thermal mass to optimize its heat storage strategy, e. g. when there is a heavy heating floor. Extreme examples of systems using the floor for heat storage and distribution are the French direct solar floor systems (see Sections 4.4.3 and 4.4.5). A test facility is shown in Figure 10.4.
At the time of publication, both test procedures still need validation and more practical experience. CEN/TC 312 is aware of the developments in this respect.
More information on the DC and the CCT methods can be found in Technical Reports of Task 26 of the IEA Solar Heating and Cooling Programme (Visser, 2002; Naron and Visser, 2002;Vogelsanger 2002).
Figure 10.4. Installation space of the solar combisystem test facility at SPF in Rapperswil, Switzerland
