Master's Thesis Katharina Brinkmann
Integration of operational characteristics into the design process of air-to-water heat pumps
Deficits in design, installation and operation of heat pump heating systems lead to lower efficiencies compared to normative estimated ones. Higher system efficiencies can be achieved by integrating operational characteristics into the previously static design process of heat pump heating systems.
Analyzing operational characteristics of the system, we develop a signal flow diagram according to DIN IEC 60050. With respect to the typical building stock, we design a monoenergetic heating system with an air-to-water heat pump according to the current VDI guideline 4645.
On/off controlled heat pump is controlled in accordance with current installation practice.
Therefore, the outside-temperature-controlled heating curve specifies the buffer storage set temperature of the system.
The hysteresis controlling heat pump and electrical auxiliary heating as well as the specified setpoint temperatures are decisive factors for the system efficiency identified by the signal flow diagram. In addition, an influence of the rule-based implementation of minimum lockout time and minimum runtime is expected.
The calculation of the seasonal coefficient of performance according to VDI 4650 is not sufficient to determine the influence of the identified variables on the efficiency and to confirm the hypotheses. Hence, annual simulations with different operating parameters are executed to determine overall efficiency.
The annual efficiency of the system is defined as the quotient of provided heat and electrical energy consumed by the aggregates considering the starting losses.
The annual system efficiency is mainly determined by the efficiency of the idealized system and the starting losses. The heating curve has the biggest influence on the efficiency of the idealized system. The average temperature is descisive over the operation of the heat pump. Lowering the heating curve at this operating point from 50 °C to 30 °C results in a 32.2 % higher overall efficiency. Furthermore, start losses due to starting processes per year are dependent on the hysteresis of the heat pump controller. The dependence of the number of starts on the hysteresis of the heat pump controller is 50 times higher than the dependence on the other analyzed parameters.
Consequently, a dependency of the system efficiency on the operational parameters is shown. Therefore, an integration of the operational characteristics into the design process of the heat pump system is necessary.
Furthermore, in order to estimate the influence of the start losses on the overall efficiency, it is necessary to precisely determine the losses of all system components. Thereby, both partial load of the heat pump and start losses must be taken into account.