Monte Carlo studies of the Ising square lattice with competing interactions

We use improved Monte-Carlo algorithms to study the antiferromagnetic 2D-Ising model with competing interactions $J_1$ on nearest neighbour and $J_2$ on next-nearest neighbour bonds. The finite-temperature phase diagram is divided by a critical point at $J_2 = J_1/2$ where the groundstate is highly degenerate. To analyse the phase boundaries we look at the specific heat and the energy distribution for various ratios of $J_2/J_1$. We find a first order transition for small $J_2 > J_1/2$ and the transition temperature suppressed to $T_C=0$ at the critical point.Comment: 4 pages, 4 figures, accepted for publication in the proceedings of the conference on Highly Frustrated Magnets 2008 in Braunschwei

Quantum disordered ground state for hard-core bosons on the frustrated square lattice

We investigate the phase diagram of hard-core bosons on a square lattice with competing interactions. The hard-core bosons can be represented also by spin-1/2 operators and the model can therefore be mapped onto an anisotropic $J_1$-$J_2$-Heisenberg model. We find the N\'eel state and a collinear antiferromagnetic state as classical ordered phases to be suppressed for small ferromagnetic exchange terms $J_{1,2}^{x,y}$ and a ferromagnetic phase which orders in the x-y-plane for large $J_{1,2}^{x,y}$. For an intermediate regime the emergence of new quantum states like valence bond crystals or super-solids is predicted for similar models. We do not observe any signal for long-range order in terms of conventional order or dimer correlations in our model and find an exponential decay in the spin correlations. Hence, all evidence is pointing towards a quantum disordered ground state for a small region in the phase diagram.Comment: 8 pages, 9 figures; version 3 includes erratum Phys. Rev. B 84, 219902(E) (2011): correction of formulas in Sec. III and replacement of "DPT transition line" in Fig. 1, (no other changes

Mindergie: A pervasive learning game for pro-environmental behaviour at the workplace

This chapter reports about a pervasive learning game to increase the environmental awareness and pro-environmental behaviour at the workplace. Based on a discussion of the theoretical background and related work we introduce the game design and game elements. Results of a formative evaluation study are presented and discussed. Results show that incentive mechanisms are less important than challenging game components that involve employees in proposing solutions for energy conservation at the workplace. Conclusions are drawn for future games and energy conservation activities at the workplace.This project has been partially funded by a SURFnet innovation grant for sustainable ICT solutions and partially by the Welten Institute – Research Centre for Learning, Teaching and Technology of the Open University of the Netherlands

Ongoing Commissioning of a High Efficiency Supermarket with a Ground Coupled Carbon Dioxide Refrigeration Plant

A significant reduction in the energy consumption and greenhouse gas emissions of supermarkets can be reached by the combination of several innovative components and the continuous optimization of their operation. A German food retail chain developed a new supermarket concept combining several innovative solutions for the refrigeration, lighting and heating/ventilation with the goal to reduce the energy consumption by about 30% compared to a standard subsidiary. A highly insulated building envelope, the use of daylight and covered refrigeration units contribute jointly to reach the goals. The key component of the concept is a carbon dioxide refrigeration plant with waste heat recovery. To reduce the efficiency losses in supercritical operation, carbon dioxide is cooled through a borehole heat exchanger using the ground as a heat sink. In the paper the design concept, the results of simulation studies and of the first monitoring year are presented and discussed

Prädiktive Regelung thermoaktiver Bauteilsysteme unter Berücksichtigung der hydraulischen Topologie

Thermoaktive Bauteilsysteme (TABS) ermöglichen die Einbindung von Umweltwärmequellen und - senken für die Gebäudeheizung und -kühlung, wodurch der Endenergiebedarf gesenkt werden kann. Aufgrund der hohen thermischen Trägheit von TABS ist die Regelung zur Erzielung eines hohen thermischen Komforts erschwert. Die durchgeführten Forschungsarbeiten fokussieren die Entwicklung prädiktiver Regelungen für TABS. Unter Berücksichtigung von drei hydraulischen Topologien zur Anbindung der TABS an die zentrale Wärme- und Kälteerzeugung werden 8 prädiktive Regelungsvarianten untersucht. Auf Basis eines thermo-hydraulischen Simulationsmodells eines Bürogebäudes in Dymola/Modelica werden dynamische Optimierungen der Regelungsvarianten durchgeführt. In einer vergleichenden Analyse der Regelungsvarianten werden der thermische Raumkomfort und der thermische Energieaufwand ausgewertet

Model-based optimization of control strategies for lowexergy space heating systems using an environmental heat source

This paper focuses on the optimization of control strategies with regard to the end energy consumption and the thermal comfort of low-exergy systems for space heating in non-residential buildings. The survey building uses a small-scale heat pump system (2.8 kWtherm) with a floor heating system. Firstly, the thermal as well as the hydraulic system was modeled in Modelica. The control parameters of the applied conventional control strategy were investigated concerning their impact on the end energy consumption and the thermal comfort. For the most sensitive control parameters a model-based optimization was performed to obtain the parameter set that achieves an optimum for the objective criteria