• Energy Research

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.

The application of the Organic Rankine Cycle to high temperature heat sources is investigated on the case study of waste heat recovery from a selected biogas plant. Two different modes of operation are distinguished: pure electric power and combined heat and power generation. The siloxanes hexamethyldisiloxane (MM) and octamethyltrisiloxane (MDM) are chosen as working fluids. Moreover, the effect of using mixtures of these components is analysed. Regarding pure electricity generation, process simulations using the simulation tool Aspen Plus show an increase in second law efficiency of 1.3% in case of 97/03 wt % MM/MDM-mixture, whereas for the combined heat and power mode a 60/40 wt % MM/MDM-mixture yields the highest efficiency with an increase of nearly 3% compared to most efficient pure fluid. Next to thermodynamic analysis, measurements of heat transfer coefficients of these siloxanes as well as their mixtures are conducted and Kandlikar’s correlation is chosen to describe the results. Based on that, heat exchanger areas for preheater and evaporator are calculated in order to check whether the poorer heat transfer characteristics of mixtures devalue their efficiency benefit due to increased heat transfer areas. Results show higher heat transfer areas of 0.9% and 14%, respectively, compared to MM.

Abstract In this study, a life cycle assessment (LCA) for geothermal power generation by binary power plants is carried out. The selected case scenarios are based on representative geothermal conditions in Germany. For this purpose, subcritical one-stage and two-stage Organic Rankine Cycle (ORC) power systems as well as supercritical cycles are considered. The LCA evaluates potential power plant concepts under consideration of working fluid losses and the associated environmental impact. Due to the restrictive regulations by the European Union for the use of fluorinated refrigerants, a special focus is laid on the evaluation of so-called low-GWP working fluids in ORC systems. In particular, the substitution of R245fa and R134a by working fluids like R1233zd and R1234yf or natural hydrocarbons is examined by a second law analysis. In addition, the environmental impact of the considered power plant concepts is calculated. The results show that the investigated low-GWP fluids lead to equivalent second law efficiency and significant lower environmental impact in comparison to common fluorinated working fluids. In case of a low-temperature heat source, the second law efficiency decreases by 2% and the global warming impact of the ORC is reduced by 78% by using R1233zd as a working fluid instead of R245fa. For the supercritical cycle with R1234yf an efficiency increase of 37% and also a significant decrease of the CO 2 -equivalent is obtained. For geothermal conditions with higher temperatures of the geothermal fluid and a limitation of the reinjection temperature, like in the Upper Rhine Rift Valley, the considered optimization approaches lead to an efficiency increase of up to 7%. In this context, the concept of a two-stage ORC is favorable. Compared to a subcritical one-stage system with R245fa as a working fluid, the two-stage ORC with R1233zd leads to 2% higher exergetic efficiency and a reduction of the global warming impact from 78 gCO 2 /kWh e to 13 gCO 2 /kWh e .

Abstract The organic Rankine cycle (ORC) and the Kalina cycle (KC) are well established thermodynamic concepts for decentralized power generation based on waste heat at low and medium temperature level. In a previous exergetic analysis, it has been shown that second law efficiency of KC can be increased by applying alcohol/alcohol mixtures as working fluid instead of ammonia/water. The aim of this work is to provide a detailed evaluation of operational parameters of a novel ethanol/hexanol mixture as a working fluid for the KC. Therefore, process simulations in ASPEN PLUS V8.0 are conducted. As a benchmark the KC with standard working fluid ammonia/water and the ORC are examined. Next to thermodynamic aspects, a techno-economic evaluation of the KC and the ORC is conducted. For 200 °C, 300 °C and 400 °C heat source temperature the pressure, power output, heat exchange capacity and the size parameter are analyzed. Compared to ammonia/water alcohol/alcohol mixtures offer an up to 1.5 times higher power output, an up to 66.6 % lower pressure and heat exchange capacity, but lead to 5.6 times higher size parameters. Compared to the KC, the subcritical ORC leads to an up to 3.4 % lower power output. The heat exchange capacity is at least 33.3 % and the size parameter up to 6.3 times lower. For the considered concepts, ammonia/water leads to the lowest specific cost with 619.4 €/kW. However, the cost estimation for the KC is related to several uncertainties. Therefore, the pure fluid ORC should be preferred in terms of techno-economic considerations.To sum up, the results show that the pure fluid ORC should be preferred in terms of techno-economic considerations.

Abstract This article investigates the potential of economic model predictive control of complex residential energy systems with electric coupling to the public grid. The examined system includes a battery energy storage system, photovoltaic power generation, an air-to-water heat pump, thermal energy storage and a building model. The said power generation provides energy for electric loads as well as domestic hot water and space heating. Model predictive control algorithms manage the energy system by nonlinear global optimization. Within this optimization, a time-varying state space model, which is derived from the energy system simulation model, reflects the system dynamics. Owing to the resulting high complexity, two algorithms for distributed model predictive control are developed. In addition, the developed approaches are compared to a common reference control concept as well as centralized model predictive control. For the comparison of annual operational costs, current German energy prices and subsidies are implemented into the economic calculation. Results show an improved performance of the developed approaches with 11.6% cost reduction in comparison to the reference. This is achieved through an increase of the heat pump seasonal performance factor by 3.4%, reduced curtailment of electrical photovoltaic energy to 21.5% of the reference value and prevention of auxiliary heater operation. Furthermore, increased photovoltaic self-consumption by the heat pump results in a slight reduction of battery storage operation. In conjunction with monetary and energetic advantages, model predictive control increases the comfort in regards to the violation of minimum limits for the comfort criteria.

Abstract This study aims to investigate techno-economic benefits of biomass retrofit for practical concentrated solar power (CSP) organic Rankine cycle (ORC) power plants. In this regard, technical parameters of an existing CSP-ORC plant currently running in Ottana (Italy) have been adopted for analyses. The plant consists of linear Fresnel collectors solar field, coupled with a two-tank oil-based thermal energy storage (TES) device, feeding a 630 kWe ORC plant. Fixed and modular biomass hybridization approaches were proposed for retrofit. In the fixed approach, biomass furnace constantly supplies thermal energy to ORC such that a pre-determined minimum power production is guaranteed, with or without solar thermal energy availability. In the modular approach, biomass furnace is regulated to supply balance thermal energy needed to continuously operate the ORC at its full capacity. Furthermore, beyond the biomass retrofit case study, a newly integrated design case study was introduced, with modified solar field and TES capacity. By using the meteorological conditions of Ottana, results of techno-economic performance for the retrofit case study showed that annual net electrical efficiency increases by 4–5 percent points, relative to the solar-only ORC plant. Marginal levelized cost of electricity (LCOE) of between 103 €/MWh and 109 €/MWh were obtained. Also, marginal net present value (NPV) ranges from 1.83 M€ to 3.22 M€. For the newly integrated design case study, results showed that design case with modular hybridization approach represents the highest annual net electrical efficiency, at 19.8%. For this approach, economic results showed that LCOE ranges between 130 €/MWh and 146 €/MWh. Beyond current state-of-the-art, the results obtained in this study show that biomass retrofit for existing CSP-ORC plants can lead to higher full-load operation hours and more prospective plant economics. Moreover, the implemented modular approach enables scheduled power profile to be followed, thereby enhancing plant dispatchability.

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).