• Energy Research

Abstract In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the outer surface of the tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the investigation of the replacement of R245fa by the low GWP fluid R1233zd(E) in geothermal applications. The nucleate boiling HTC on a horizontal tube and the electrical power of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained results, regarding power output as well as heat transfer characteristics, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and pressure, and the lower viscosity. The HTC of R245fa is exemplarily up to 43.2 % increased in comparison to R1233zd(E).

Abstract In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the outer surface of the tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the investigation of the replacement of R245fa by the low GWP fluid R1233zd(E) in geothermal applications. The nucleate boiling HTC on a horizontal tube and the electrical power of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained results, regarding power output as well as heat transfer characteristics, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and pressure, and the lower viscosity. The HTC of R245fa is exemplarily up to 43.2 % increased in comparison to R1233zd(E).

Abstract In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the external surfaces of tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate pool boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the experimental evaluation of nucleate pool boiling heat transfer correlations for R245fa and its possible replacement R1233zd(E) in ORC applications. The nucleate boiling HTC on a horizontal tube and the electrical power generation of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained measurement results, regarding heat transfer characteristics as well as power output, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and saturation pressure, and the lower viscosity. The HTC for R245fa is up to 43.2% higher in comparison to R1233zd(E). The experimental HTC are compared to selected nucleate pool boiling HTC correlations. The evaluation reveals that correlations according to Cooper and Gorenflo et al. show the slightest mean absolute deviations between 4.75% and 15.65% for both working fluids.

Abstract In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the external surfaces of tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate pool boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the experimental evaluation of nucleate pool boiling heat transfer correlations for R245fa and its possible replacement R1233zd(E) in ORC applications. The nucleate boiling HTC on a horizontal tube and the electrical power generation of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained measurement results, regarding heat transfer characteristics as well as power output, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and saturation pressure, and the lower viscosity. The HTC for R245fa is up to 43.2% higher in comparison to R1233zd(E). The experimental HTC are compared to selected nucleate pool boiling HTC correlations. The evaluation reveals that correlations according to Cooper and Gorenflo et al. show the slightest mean absolute deviations between 4.75% and 15.65% for both working fluids.

Abstract This study investigates the potential of model predictive heat pump control in detached houses in terms of electric energy consumption, thermal comfort and photovoltaic energy self-consumption. Two comparable test rigs with identical devices are set up. The test rigs include electrical air source heat pumps with variable compressor speed, thermal energy storages and heat dissipation by heat exchangers. The heat demand is controlled by valves, which are coupled to real-time simulation of building models in compliance with the principle of energy conservation. Measurements confirm test rig comparability. After introducing the model predictive control (MPC) concept, a successive series of six measurements of 120 h each within the heating season is presented. The model predictive heat pump controller is evaluated by comparison to a standard heat pump controller implemented into the reference test rig. Results show an average increase of the heat pump coefficient of performance of 22.2%, an average increase of 234.8% in terms of photovoltaic energy self-consumption as well as a resulting average heat pump operational cost reduction of 34.0% by application of MPC.

Abstract This study investigates the potential of model predictive heat pump control in detached houses in terms of electric energy consumption, thermal comfort and photovoltaic energy self-consumption. Two comparable test rigs with identical devices are set up. The test rigs include electrical air source heat pumps with variable compressor speed, thermal energy storages and heat dissipation by heat exchangers. The heat demand is controlled by valves, which are coupled to real-time simulation of building models in compliance with the principle of energy conservation. Measurements confirm test rig comparability. After introducing the model predictive control (MPC) concept, a successive series of six measurements of 120 h each within the heating season is presented. The model predictive heat pump controller is evaluated by comparison to a standard heat pump controller implemented into the reference test rig. Results show an average increase of the heat pump coefficient of performance of 22.2%, an average increase of 234.8% in terms of photovoltaic energy self-consumption as well as a resulting average heat pump operational cost reduction of 34.0% by application of MPC.

This article presents a 125-day experiment to investigate model predictive heat pump control. The experiment was performed in two parallel operated systems with identical components during the heating season. One of the systems was operated by a standard controller and thus represented a reference to evaluate the model predictive control. Both test rigs were heated by an air-source heat pump which is influenced by real weather conditions. A single-family house model depending on weather measurement data ensured a realistic heat consumption in the test rigs. The adapted model predictive control algorithm aimed to minimize the operational costs of the heat pump. The evaluation of the measurement results showed that the electrical energy demand of the heat pump can be reduced and the coefficient of performance can be increased by applying the model predictive controller. Furthermore, the self-consumption of photovoltaic electricity, which is calculated by means of a photovoltaic model and global radiation measurement data, was more than doubled. Consequently, the energy costs of heat pump operation were reduced by 9.0% in comparison to the reference and assuming German energy prices. The results were further compared to the scientific literature and short-term measurements were performed with the same experimental setup. The dependence of the measurement results on the weather conditions and the weather forecasting quality are shown. It was found that the duration of experiments should be as long as possible for a comprehensive evaluation of the model predictive control potential.

This article presents a 125-day experiment to investigate model predictive heat pump control. The experiment was performed in two parallel operated systems with identical components during the heating season. One of the systems was operated by a standard controller and thus represented a reference to evaluate the model predictive control. Both test rigs were heated by an air-source heat pump which is influenced by real weather conditions. A single-family house model depending on weather measurement data ensured a realistic heat consumption in the test rigs. The adapted model predictive control algorithm aimed to minimize the operational costs of the heat pump. The evaluation of the measurement results showed that the electrical energy demand of the heat pump can be reduced and the coefficient of performance can be increased by applying the model predictive controller. Furthermore, the self-consumption of photovoltaic electricity, which is calculated by means of a photovoltaic model and global radiation measurement data, was more than doubled. Consequently, the energy costs of heat pump operation were reduced by 9.0% in comparison to the reference and assuming German energy prices. The results were further compared to the scientific literature and short-term measurements were performed with the same experimental setup. The dependence of the measurement results on the weather conditions and the weather forecasting quality are shown. It was found that the duration of experiments should be as long as possible for a comprehensive evaluation of the model predictive control potential.