Measurements on real heat generator plants show that efficiency parameters as determined by calculation sometimes deviate considerably from the values registered by measurement technology. This is especially true for heat pumps and micro CHP plants. The annual performance factor is decisive in terms of their economic success. There are normative procedures such as VDI 4650 for heat pumps that can be used to determine the annual performance factor. Despite relatively simple test measurements, a new method ought to deliver significantly more realistic energy characteristic values.

The new metrological method was developed in a three-year research project. It can be applied to determine realistic annual performance factors. In contrast to previous static methods, the researchers at the three universities TU Dresden, RWTH Aachen University and Stuttgart University also wanted the new test method to take dynamic operating conditions into account, such as start-up and shutdown processes as well as thermal storage losses. They have a noticeable impact on the system’s energy efficiency.

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 joined to form one system. This method combines the advantages of simulation and measurement: The measurement of real system components captures the specific behaviour of units and control technology available on the market. The hardware, in turn, is embedded in a simulated environment with which any constraints such as usage profiles or weather situations can be taken into account— this allows for test measurements in short, typical measurement cycles.

For example, the supply temperature and the pressure difference in the heating network are transferred to the simulation. The simulation, in turn, provides the mass flow rate and the return temperature of the heating network that result from the simulated system dynamics. The demand for domestic hot water is stored in the simulation's tap-use profile and is transferred to the hydraulic module. Suitable interfaces and balance boundaries must be defined for the loop, i.e. for a coherent linkage of simulation and hardware. In addition, the technical components of the test stand must be able to set the parameters obtained from the simulation with a sufficient speed.

Representative scenarios are required in order to be able to record the actual system behaviour over a whole year in the shortest possible measurement cycles: With a few so-called type days, the thermal conditions for an entire year are to be represented. Using k-medoids clustering, 4 type days were defined on the basis of the weather data of one test reference year, which taken together are representative of the real thermal conditions, thereby enabling a short, realistic test method.

Practical results and possible applications

The results of the new test method show that fundamental trends and effects can be confirmed in direct comparison to field measurements. The method is surprisingly practical and realistic. In addition to current normative procedures, there is now another test method for the realistic evaluation of heat pumps and CHP systems.

The flexibility of the method resulting from the methodical approach allows it to be used or further developed for other tasks. It can also be used as an analysis tool for the review or further development of normative methods. It is also of interest to manufacturers of plant and control components because they can test and further develop their hardware in the laboratory under realistic conditions.

Outlook

The researchers intend to develop the test method further in a follow-up project. It is currently based on a single-family home as a model building and on the test reference year for Potsdam. It could be transferred to other building types and be adapted to other climatic regions. Other system types, such as hybrid systems and more complex control modules could also be represented in the process.

This news item has already been published on the website of the ENERGIEWENDEBAUEN research initiative.

(jl)