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Heat pumps are pinpointed as a key technology in decarbonizing buildings' heat supply, which is currently dominated by fossil-fuel based systems. To minimize the environmental impact of heat pumps over their lifetime, selecting natural working fluids (refrigerants) is the only viable long-term choice. Independent of the refrigerant, the efficiency of a heat pump is closely linked to temperature of the applied heat source, and to this end, wastewater is a promising candidate that is abundant in all urban environments. This experimental study investigates a novel two-stage, ejector-supported, brazed plate evaporator for its use in R744 (CO2) heat pumps with greywater as a heat source and compares it to more conventional evaporators. The work shows that splitting the evaporation process in two stages with an ejector in between allows reducing the evaporation pressure while the compressor suction pressure can be increased, in some cases significantly. Even if several possibilities for further optimization of the novel two-stage evaporator were identified, the results show heat pump efficiency improvements above 10 % compared to an equivalent heat pump with a well-performing one-stage evaporator. Heat pump systems aiming at utilizing maximum temperature difference for the greywater should use a high pressure lift ejector. Two-stage evaporator for R744 heat pumps using greywater as heat source

District heating has an important role in the shift to carbon-neutral energy systems through enabling the use of heat sources that would otherwise be wasted to cover buildings’ heating demands. The availability of many renewable and surplus heat sources is however in opposite phase with the heating demand, creating a demand for seasonal thermal energy storage. This study performs a techno-economic assessment of the heat supply system of a residential area in Norway, where seasonal storage storing excess heat from a waste incineration plant is being planned. A heat supply solution combining seasonal storage and low-temperature district heating was compared with two more conventional alternatives: high-temperature district heating and direct electric heating. The study shows that the seasonal storage is not cost optimal under the conditions assumed, in particular regarding the electricity market; however, the total costs were only 3% higher compared to electric heating. Seasonal storage additionally allows to reduce the use of peak heating units in the district heating system in the winter, thus reducing the costs and emissions related to heat production, and district heating alone has a significant impact in alleviating the pressure on the power grid. The peak power demand was reduced by 28% when investing to low- or high-temperature district heating, and seasonal storage was shown to enable up to 31% reduction in the peak heating demand. Moreover, it was shown that higher electricity prices in the winter and reduced grid capacity increase the economic viability of the solution and could make it competitive. © 2022 The Authors Assessing the potential of seasonal thermal storage for local energy systems: Case study for a neighborhood in Norway

Due to increased share of fluctuating renewable energy sources in future decarbonized, electricity-driven energy systems, participating in the electricity markets yields the potential for industry to reduce its energy costs and emissions. A key enabling technology is thermal energy storage combined with power-to-heat technologies, allowing the industries to shift their energy demands to periods with low electricity prices. This paper presents an optimization-based method which helps to select and dimension the cost-optimal thermal energy storage technology for a given industrial steam process. The storage technologies considered in this work are latent heat thermal energy storage, Ruths steam storage, molten salt storage and sensible concrete storage. Due to their individual advantages and disadvantages, the applicability of these storage technologies strongly depends on the process requirements. The proposed method is based on mathematical programming and simplified transient simulations and is demonstrated using different scenarios for energy prices, i.e., various types of renewable energy generation, and varying heat demand, e.g., due to batch operation or non-continuous production Keywords: thermal energy storage; optimization; steam; power-to-heat; renewable energy

District heating enables an economical use of energy sources that would otherwise be wasted to cover the heating demands of buildings in urban areas. For efficient utilization of local waste heat and renewable heat sources, low distribution temperatures are of crucial importance. This study evaluates a local heating network being planned for a new building area in Trondheim, Norway, with waste heat available from a nearby ice skating rink. Two alternative supply temperature levels have been evaluated with dynamic simulations: low temperature (40 °C), with direct utilization of waste heat and decentralized domestic hot water (DHW) production using heat pumps; and medium temperature (70 °C), applying a centralized heat pump to lift the temperature of the waste heat. The local network will be connected to the primary district heating network to cover the remaining heat demand. The simulation results show that with a medium temperature supply, the peak power demand is up to three times higher than with a low temperature supply. This results from the fact that the centralized heat pump lifts the temperature for the entire network, including space and DHW heating demands. With a low temperature supply, heat pumps are applied only for DHW production, which enables a low and even electricity demand. On the other hand, with a low temperature supply, the district heating demand is high in the wintertime, in particular if the waste heat temperature is low. The choice of a suitable supply temperature level for a local heating network is hence strongly dependent on the temperature of the available waste heat, but also on the costs and emissions related to the production of district heating and electricity in the different seasons c 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

District heating (DH) will play an important role in the future fossil-free energy systems by enabling increased utilization of waste heat and renewable heat sources to cover buildings' heat demand. A prerequisite for this is a reduction in the distribution temperature and shift towards decentralized heat production. In this study, dynamic modeling has been applied to study the technical, energetic and environmental impacts of including prosumers – customers who both consume and produce heat – in a local low-temperature DH grid. Four different scenarios were studied for a planned building area in Trondheim, Norway: high- and low-temperature scenarios with the entire heat demand being covered by a heat central, and two low-temperature scenarios including heat supply from prosumers. A data center and two food retail stores were considered as the prosumers, each with different location and individual characteristics for the heat supply, allowing to study their impact on the water flow in different parts of the grid. The results show that utilizing local surplus heat is a significant measure to reduce the heat demand and the environmental impact of the DH grid. Decentralized heat supply additionally contributes to reduced heat losses, due to overall lower distances to transport the heat. © 2018 Elsevier Ltd. All rights reserved.

Thermal energy storage (TES) is a key technology for enabling increased utilization of industrial waste heat in district heating. The ability of TES to equalize offsets in demand and supply depends strongly on the sizing, control and integration in a heating plant. We consider the problem of sizing TES in heating plants utilizing a varying waste-heat source. To this end, we propose a combined dynamic simulation and model predictive control approach that accounts for the dynamics and optimal control of the heating plant with TES. A case study has been carried out on a district-heating plant located in Norway, with 90% of its annual heat production being heat recovered from the off-gas from a ferrosilicon plant. We evaluate the effective peak-heating reduction with different TES sizes and the energy-to-heat-flow-ratio for the TES discharging periods as performance metrics. For the case study, our results suggest that a modest TES tank volume of 1500 m3 is sufficient to achieve a half-year peak-heating reduction of 12% and comparable performance with larger volumes. The proposed methodology constitutes a numerically tractable means of incorporating the impact of model predictive control on the sizing of TES for heating plants with time-varying waste-heat supply and demand. Keywords Thermal energy storageWaste-heat utilizationDistrict heatingModel predictive control

Industrial plants organized in clusters may improve their economics and energy efficiency by exchanging and utilizing surplus heat. However, integrating inherently dynamic processes and highly time-varying surplus-heat supplies and demands is challenging. To this end, a structured optimization and control framework may significantly improve inter-plant surplus-heat valorization. We present a Modelica-based systems model and optimal-control scheme for surplus-heat exchange in industrial clusters. An industry-cluster operator is assumed to coordinate and control the surplus-heat exchange infrastructure and responsible for handling the surplus heat and satisfy the sink plants&rsquo, heat demands. As a case study, we use an industry cluster consisting of two plants with surplus heat available and two plants with heat demand. The total surplus heat and heat demand are equal, but the availability and demand are highly asynchronous. By optimally utilizing demand predictions and a thermal energy storage (TES) unit, the operator is able to supply more than 98% of the deficit heat as surplus heat from the plants in the industry cluster, while only 77% in a corresponding case without TES. We argue that the proposed framework and case study illustrates a direction for increasing inter-plant surplus-heat utilization in industry clusters with reduced use of peak heating, often associated with high costs or emissions.

District heating is an under-researched part of the energy system, notwithstanding its enormous potential to contribute to Greenhouse Gas emission reductions. Low-temperature district heating is a key technology for energy-efficient urban heat supply as it supports an efficient utilization of low-grade waste-heat and renewable heat sources. The low operating temperature for such grids facilitates the integration of seasonal thermal energy storage, enabling a high degree of operational flexibility in the utilization of both uncontrollable and controllable heat sources. Yet, an inherent challenge of optimizing the operation of low-temperature district heating networks and its flexibility is the underlying uncertainty in heat demand. We develop a new stochastic model to minimize the total operational cost of district heating networks with local waste heat utilization, seasonal storage and uncertain demand. We consider in particular how demand side management and seasonal storage can improve the operational flexibility and thereby reduce costs. We analyze different set-ups of a local low-temperature district heating network under development in a new residential area in Trondheim, Norway. We find up to 37% reductions in carbon dioxide emissions, 29% generation reduction in peak hours, and 10% lower operational costs. These large values highlight the significance of flexibility options in low-temperature district heating networks for cost-effective, large-scale deployment.

Device configurations that enable a unidirectional propagation of carriers in a semiconductor are fundamental components for electronic and optoelectronic applications. To realize such devices, however, it is generally required to have complex processes to make p-n or Schottky junctions. Here we report on a unidirectional propagation effect due to a self-induced compositional variation in GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly reproducible rectifying behavior, where the rectifying direction is determined by the NW growth direction. Combining the results from confocal micro-Raman spectroscopy, electron microscopy, and electrical measurements, the origin of the rectifying behavior is found to be associated with a self-induced variation of the Sb and the carrier concentrations in the NW. To demonstrate the usefulness of these GaAsSb NWs for device applications, NW-based photodetectors and logic circuits have been made.

eTransport is a linear optimization tool for evaluating energy supply alternatives for building areas. This report describes an improved, more realistic district heating (DH) module that has been developed for eTransport. The new module includes several improvements as compared to the previous module: • Varying mass flow that depends on the heat load, as opposed to constant mass flow. • Pressure included as a variable, with certain limits for minimum pressure and minimum pressure drop at the loads. • Calculation of pumping power is included in the module, and pumping power due to pressure losses in pipes and at loads is included in the objective function • A more realistic calculation of heat losses included, and the heat losses are included in the heat load • The module allows supply flow to both directions in a pipe; a property which is relevant when more heat sources are present in a DH grid. This feature is however yet to be tested properly. The report presents the main equations required for mathematical description of a district heating system are presented, followed by the approach taken for linear representation of these equations, required for eTransport. The report includes a brief evaluation of the module using a simple test network, and discusses the simplifications and limitations of the present module, giving suggestions for further improvements.

Current district heating (DH) systems with high temperatures are facing many challenges that may decrease its competitiveness. Some of the challenges are decreased heat demands due to energy efficient buildings and high return temperatures that decrease possibilities for utilization of renewable heat sources. Low temperature DH (LTDH) systems have opportunities for utilization of waste heat and renewables and lower distribution losses. Therefore, the aims of the study were to analyze the challenges in the transition to LTDH and to estimate the increased competitiveness in low heat density areas. Since the heating density is an important factor for the DH competitiveness, the high and the low heat density area were analyzed. A building area consisting of the passive house and low energy buildings in Trondheim, Norway, was analyzed. The hourly DH network model was developed included both thermal and pressure losses. The results showed that the heat loss could be reduced by lowering the supply temperature from 80 °C to 55 °C. Analysis of the return temperature showed that LTDH could provide a lower return temperature than the existing DH system, regardless of the faults. Competitiveness of LTDH might be decreased for the heat densities lower than 1 MWh/m. © 2018 Elsevier Ltd Challenges and potentials for low-temperature district heating implementation in Norway

In Norway, a large portion of the building stock originates from the period from 1955 to 1990. Many of these buildings fail to comply with the current building regulations regarding the energy consumption. In this study, the possibility for upgrading a hypothetical apartment building with an oil-based heating system has been investigated employing simulations from the IDA Indoor Climate and Energy software. For the construction of the original building, customs and regulations from the period 1981-90 were employed, and the building envelope was upgraded to the requirements of the Norwegian research centre on Zero Emission Buildings. Two alternative heating systems have been investigated: solar thermal collectors (i) alone and (ii) as combined with borehole thermal storage and a ground-source heat pump. For each case, the energy consumption, thermal comfort and indoor climate were studied. The simulations predict a reduction in the total annual heat demand to one third of the original with the upgrading. For the alternative heating systems, with solar collectors alone the demand for additional electric heating was still considerable, however in the combined system it was negligible. Regarding thermal comfort, in the upgraded building longer periods with elevated temperatures were observed.

By applying a correlated micro-photoluminescence spectroscopy and transmission electron microscopy (TEM) approach, we have utilized molecular beam epitaxy grown self-catalysed GaAs nanowires (NWs) with an axial GaAsSb insert to determine the band offsets at the crystal phase heterojunction between zinc blende (ZB) and wurtzite (WZ) GaAs. Two distinct PL emission bands originating from the ZB GaAsSb insert were identified. The lower energy PL emission allowed an independent verification of the maximum Sb molar fraction to be ∼30%, in agreement with quantitative high-angle annular dark field scanning TEM performed on the same single NW. The higher energy PL emission revealed a low temperature ZB/WZ band offset of 120 meV at the interface between the two GaAs crystal phases occurring at the upper boundary of the insert. Separate conduction and valence band offsets develop at a higher temperature due to the different temperature dependence of the ZB and WZ GaAs band gaps, but both offset values show a relatively little variation in the range of 10–150 K. Published by AIP Publishing. Locked until 28.12.2018 due to copyright restrictions. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Applied Physics Letters and may be found at http://aip.scitation.org/doi/10.1063/1.4960193http://aip.scitation.org/doi/10.1063/1.4991884

Today's district heating (DH) networks in Norway are 2nd and 3rd generation systems, with supply temperatures ranging from 80 to 120 °C. In new developments, it is desirable to shift to 4th generation, low-temperature district heating (LTHD) in order to reduce the heat losses and enable better utilization of renewable and waste heat sources. A local LTDH grid for a new development planned in Trondheim, Norway, has been modelled in the dynamic simulation program Dymola in order to study the effect of lowered supply temperatures to heat losses and circulation pump energy use. Different scenarios with supply temperatures ranging from 55 to 95 °C, lowered return temperature as well as peak shaving were analyzed. Real DH use data for buildings in Trondheim were employed. The environmental impact in terms of the total produced CO2equivalent emissions was estimated for each scenario, assuming a heat production mix corresponding to that of the local DH provider. The results showed that by lowering the supply temperature to 55 °C, the heat losses could be reduced by one third. The total pump energy increased significantly with reduced supply temperature, however the pump energy was generally an order of magnitude lower than the heat losses. © 2017 Elsevier Ltd © 2017 Elsevier Ltd. All rights reserved. This is the authors' accepted and refereed manuscript to the article. Locked until 29 July 2019 due to copyright restrictions

Summary The surface characteristics of a large set of commercial lightweight coated paper grades are explored. The quantification of the 3D structure is revealed by atomic force microscopy, laser profilometry and X-ray microtomography. This comprehensive study demonstrates the suitability of different and modern methods for assessing critical coating layer properties, thus identifying the right tools for specific structural analyses. Based on the assessment of the top and bottom surfaces of 25 commercial lightweight coated samples, three main conclusions can be drawn: (1) the facet orientation polar angle is a function of roughness, (2) skewness did not describe the surface details affecting the gloss of the commercial lightweight coated samples assessed in this study and (3) surface roughness at wavelengths below approximately 1.0 μm does not affect the paper gloss significantly. This is important knowledge for the understanding of lightweight coated paper surface structure and its properties.

The Sb concentration in axial GaAs(1-x)Sb(x) inserts of otherwise pure GaAs nanowires has been investigated with quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The Sb concentration was quantified by comparing the experimental image intensities normalized to the incident beam intensity with intensities simulated with a frozen lattice multislice approach. Including static atomic displacements in the simulations was found to be crucial for correct compositional analysis of GaAs(1-x)Sb(x). HAADF intensities of individual nanowires were analysed both across the nanowires, exploiting their hexagonal cross-sectional shape, and along the evenly thick central part of the nanowires. From the cross-sectional intensity profiles, a decrease in the Sb concentration towards the nanowire outer surfaces was found. The longitudinal intensity profiles revealed a gradual build-up of Sb in the insert. The decrease of the Sb concentration towards the upper interface was either gradual or abrupt, depending on the growth routine chosen. The compositional analysis with quantitative HAADF-STEM was verified by energy dispersive X-ray spectroscopy.

The near-surface reduction of the Sb mole fraction during the growth of GaAsSb nanowires (NWs) and GaAs NWs with GaAsSb inserts has been studied using quantitative high-angle annular dark field scanning transmission electron microscopy (STEM). A model for diffusion of Sb in the hexagonal NWs was developed and employed in combination with the quantitative STEM analysis. GaAsSb NWs grown by Ga-assisted molecular beam epitaxy (MBE) and GaAs/GaAsSb NWs grown by Ga- and Au-assisted MBE were investigated. At the high temperatures employed in the NW growth, As-Sb exchange at and outward diffusion of Sb towards the surface take place, resulting in reduction of the Sb concentration at and near the surface in the GaAsSb NWs and the GaAsSb inserts. In GaAsSb NWs, an increasing near-surface depletion of Sb was observed towards the bottom of the NW due to longer exposure to the As beam flux. In GaAsSb inserts, an increasing change in the Sb concentration profile was observed with increasing post-insert axial GaAs grow...

We demonstrate a method for compositional mapping of AlxGa1–xAs heterostructures with high accuracy and unit cell spatial resolution using quantitative high angle annular dark field scanning transmission electron microscopy. The method is low dose relative to spectroscopic methods and insensitive to the effective source size and higher order lens aberrations. We apply the method to study the spatial variation in Al concentration in cross-sectioned GaAs/AlGaAs core-shell nanowires and quantify the concentration in the Al-rich radial band and the AlGaAs shell segments.

GaAs nanowires are a highly promising candidate for future optoelectronic applications. Here quantitative high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) has been used for studying the Sb content x in zinc blende (ZB) GaAs1−xSbx axial inserts in otherwise pure wurtzite (WZ) GaAs nanowires. Direct comparison of the normalized experimental HAADF-STEM image intensities with simulated intensities displayed a good match for the pure structure. It was discovered that the Sb content in a 20 nm long insert is not uniform but increases from 13 ± 3 at.% at the lower to 21 ± 3 at.% at the upper WZ-ZB interface. Non-compositional effects on HAADF intensity have been identified.