• 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.

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.

The combination of a high-temperature packed bed thermal energy storage (TES) with an organic Rankine cycle (ORC) - named ORCTES - is analysed and optimised numerically for different boundary condi...

Classical numerical methods for solving solid–liquid phase change assume a constant density upon melting or solidification and are not efficient when applied to phase change with volume expansion or shrinkage. However, solid–liquid phase change is accompanied by a volume change and an appropriate numerical method must take this into account. Therefore, an efficient algorithm for solid–liquid phase change with a density change is presented. Its performance for a one-dimensional solidification problem and for the quasi two-dimensional melting of octadecane in a cubic cavity was tested. The new algorithm requires less than 1/9 of the iterations compared to the source based method in one dimension and less than 1/7 in two dimensions. Moreover, the new method is validated against PIV measurements from the literature. A conjugate heat transfer simulation, which includes parts of the experimental setup, shows that parasitic heat fluxes can significantly alter the shape of the phase front near the bottom wall.

Abstract Organic Rankine Cycle (ORC) is a promising technology for the utilization of low-grade waste heat. However, due to tailor-made power plants for different heat source temperatures, specific investment costs are still too high to be profitable. This study compares two different methods to configure a modular ORC in the temperature range of 373–463 K. The first method assumes a simple adaption of the mass flow rate within the ORC (mass flow method). In the second method, simultaneous adaption of mass flow rate and working pressure (combined method) take place. The common purpose of both methods is the optimization of the net power output for heat source temperatures lower and higher than the reference plant. Analyses are carried out for common fluorinated refrigerants (R227ea and R236ea) as well as for iso-alkanes (isobutane and isopentane). It is shown that within a wide range of temperatures the deviation in net power output between the simpler mass flow method and the more sophisticated combined method is below 10%. However, the deviation strongly depends on the location of the pinch point and on the choice of the working fluid. In general, it is shown that the mass flow method in combination with a working fluid, for which the pinch point is located at the beginning of the preheating, is thermodynamically favorable for plant manufacturers. Economic analyses subsequently compare both methods with respect to payback period and cash flow. Additional investment costs for the combined method are allowed to be up to 10% in order to reach higher profitability than units with mass flow method.

The design of phase-change material (PCM)-based thermal energy storage (TES) systems is challenging since a lot of PCMs have low thermal conductivities and a considerable volume change during phase-change. The low thermal conductivity restricts energy transport due to the increasing thermal resistance of the progressing phase boundary and hence large heat transfer areas or temperature differences are required to achieve sufficient storage power. An additional volume has to be considered in the storage system to compensate for volume change. Macro-encapsulation of the PCM is one method to overcome these drawbacks. When designed as stiff containers with an air cushion, the macro-capsules compensate for volume change of the PCM which facilitates the design of PCM storage systems. The capsule walls provide a large surface for heat transfer and the thermal resistance is reduced due to the limited thickness of the capsules. Although the principles and advantages of macro-encapsulation have been well known for many years, no detailed analysis of the whole encapsulation process has been published yet. Therefore, this research proposes a detailed development strategy for the whole encapsulation process. Various possibilities for corrosion protection, fill and seal strategies and capsule geometries are studied. The proposed workflow is applied for the encapsulation of the salt hydrate magnesiumchloride hexahydrate (MCHH, MgCl 2 · 6 H 2 O) within metal capsules but can also be assigned to other material combinations.

Abstract The simulation of melting phenomena in the frame work of continuum theory can be handled through many different approaches, among which the fixed-grid methods are the most common ones. Even though such methods have been in use for several decades, only scarce publications exist addressing their performance when solving moving boundary problems. Recently, five of the most used energy formulations with a strong coupling between temperature and enthalpy have been studied in an extensive benchmark both qualitatively and quantitatively. The present paper extends this work by including the role of the formulation of the convective term in the energy equation. Two cases are considered: a first one where all the enthalpy is convected, while in the second one it is the sensible enthalpy only. Similarly to what was done in the aforementioned study, this analysis also includes investigations concerning the modeling of the phase transition, through the associated temperature range for the mushy zone, and several numerical parameters, namely the time step, the mesh coarsening, the CFL condition and the tolerance for the energy equation. The numerical results are compared to a well known quasi-2D melting experiment from the literature in order to determine clearly wrong results. In addition, the liquid fraction and its variance are analyzed for every solver. Except for the solver using an apparent heat capacity method, all of them give reasonable results for a broad range of parameters. The sensible enthalpy convection formulation is in general more stable than the all enthalpy convection formulation. For the chosen parameters and the most stable solver - which is practically not affected by the tolerance, the CFL or the maximum time step - it appears that parameters having the strongest influence on the liquid fraction are the width of the mushy zone followed by the mesh and the convective formulation.

Abstract Galactitol, in terms of its phase change enthalpy and temperature, is a promising phase change material (PCM) for medium temperature (150–200 °C) latent heat storage of solar cookers. This study aimed at determining the effect of upper cycle temperature on thermal behavior of galactitol in bulk thermal cycling. Three bulk samples were repetitively melted and frozen with each sample having fixed upper cycle temperature different from the others. Temperature histories of the samples were recorded whereas phase change enthalpies and specific heat capacities were obtained by differential scanning calorimetry. Thermal diffusivities of fresh galactitol within a range of 20–240 °C were determined by a flash diffusivity instrument. The results show that the upper cycle temperature has a great influence on the attainable number of melting and freezing cycles, the degree of subcooling, the rate of change of degree of subcooling as well as the phase change enthalpy and temperature. The upper cycle temperatures above but close to the melting temperature are favorable. The lowest upper cycle temperature was around 200 °C and yielded about 90 thermal cycles feasible for solar cooking at temperatures greater than 150 °C. Therefore, galactitol as a PCM in thermal energy storage of solar cookers that are thermally cycled at least once a day, can afford a lifespan of less than 100 days, which is far lower than lifespans of the other parts of the cooker system. Galactitol was thus found to be unstable and with a too short lifespan for practical application as PCM for medium temperature thermal energy storage purposes.