Measurements on real heat generator plants show that efficiency parameters determined by calculation sometimes deviate considerably from the values recorded by measurement technology. This applies in particular to heat pumps and micro combined heat and power plants. The annual performance factor is decisive for their economic success. For heat pumps there are normative procedures such as VDI 4650, with which the annual performance factor can be determined. Despite relatively simple test measurements, a new method should deliver significantly more realistic energy key figures.
The simplified measurement technique was developed in a three-year research project. It can be used to determine realistic annual performance figures. In contrast to previous static methods, the researchers at the three universities TU Dresden, RWTH Aachen and the University of Stuttgart also wanted the new method to take dynamic operating conditions into account, such as start-up and shutdown processes and thermal storage losses. They have a noticeable influence on the energy system performance.
Hardware-in-the-Loop combines measurement and simulation
The analyses were based on the "Hardware-in-the-Loop" (HiL) method. Hardware and software components are connected to form a system. This method combines the advantages of simulation and measurement: The measurement of real system components captures the specific behavior of aggregates and control technology available on the market. The hardware, in turn, is embedded in a simulated environment with which any boundary conditions such as usage profiles or weather situations can be taken into account, which allows test measurements in short, typical measurement cycles.
For example, the flow temperature and the pressure difference in the heating network are transferred to the simulation. This in turn supplies the mass flow and the return temperature of the heating network, which result from the simulated system dynamics. The demand for hot drinking water is stored in the simulation's tap profile and transferred to the hydraulic module. Suitable interfaces and balance limits must be defined for the loop, i.e. for a coherent combination of simulation and hardware. In addition, the technical components of the test stand must be able to adjust the variables obtained from the simulation sufficiently quickly.
Practical results and possible applications
In order to be able to record the real plant behaviour over a whole year in the shortest possible measuring cycles, representative scenarios are required: With a few so-called type days, the thermal conditions are to be mapped for an entire year. Using the k-medoids clustering method, 4 type days were defined on the basis of the weather data of a test reference year, which together are representative of the real thermal conditions and thus enable a short, realistic test procedure.
The results of the new test method show that fundamental trends and effects can be confirmed in direct comparison to the field measurements. The process is surprisingly practical and realistic. In addition to the previous normative procedures, there is now another test procedure for realistic evaluation of heat pumps and CHP systems.
The flexibility of the process resulting from the methodical approach allows it to be used or further developed for various tasks. It can also be used as an analysis tool for the review or further development of normative procedures. It is also interesting for manufacturers of system and control components because they can test and further develop their hardware in the laboratory under realistic conditions.
The researchers want to further develop the test procedure in a follow-up project. It is currently based on a single-family house as a model building and on the test reference year for Potsdam. It could be transferred to other building types and adapted to other climatic regions. Other system types, such as hybrid systems, and more complex control modules could also be mapped in the process.