Evaluating the Effective Solar Absorptance of Dilute Particle Configurations

Ray-tracing and heat-transfer simulations of discrete particles in a representative elementary volume were performed to determine the effective particle-cloud absorptance and temperature profiles as a function of intrinsic particle absorptance values (0 – 1) for dilute solids volume fractions (1 – 3%) representative of falling particle receivers used in concentrating solar power applications. Results showed that the average particle-cloud absorptance is increased above intrinsic particle absorptance values as a result of reflections and subsequent reabsorption (light trapping). The relative increase in effective particle-cloud absorptance was greater for lower values of intrinsic particle absorptance and could be as high as a factor of two. Higher values of intrinsic particle absorptance led to higher simulated steady-state particle temperatures. Significant temperature gradients within the particle cloud and within the particles themselves were also observed in the simulations. Findings indicate that dilute particle-cloud configurations within falling particle receivers can significantly enhance the apparent effective absorptance of the particle curtain, and materials with higher values of intrinsic particle absorptance will yield greater radiative absorptance and temperatures.

Update on NREL Outdoor Exposure Campaign of Solar Mirrors

Beginning in 2018, the National Renewable Energy Laboratory restarted exposure campaigns on new and archived samples as part of a multiyear project, some with outdoor exposure for more than a decade. By resuming exposure and collecting and analyzing data on thousands of samples going back decades, several goals can be advanced that can be difficult to determine within the timeline of most projects: 1) correlating an accelerated exposure campaign to outdoor aging, specifically with xenon arc lamp exposure chambers; 2) drawing conclusions between specific corrosion mechanisms and weather patterns; and 3) finding novel relationships between mirror composition and performance. In addition to building and mining a database, we will experiment with new characterization techniques, primarily focused on macroscopic and microscopic imaging. In introducing these techniques more broadly, it may be possible to reveal a more direct line between optical performance and exposure campaigns by better understanding the degradation mechanisms occurring.

System Design of a 2.0 MWth Sodium/Molten Salt Pilot System

Nitrate molten salt concentrating solar power (CSP) systems are currently deployed globally and are considered state-of the art heat transfer fluids (HTFs) for present day high-temperature operation. Although slightly higher limits may be possible with molten salt, to fully realize SunShot efficiency goals of $15/kWhth HTFs and an LCOE of 6¢/kWh, HTF technologies working at higher temperatures (e.g., 650 °C to 750 °C) will require an alternative to molten salts, such as with alkali metal systems. This investigation explores the development of a 2.0 MWth sodium receiver system that employs a sodium receiver as the HTF, as well as with a ternary chloride (20%NaCl/40%MgCl/40%KCl by mol wt.%) salt as a thermal energy storage (TES) medium to facilitate a 6-hr. storage duration. A sodium-to-salt heat exchanger model as well as a salt-to-sCO2 primary heat exchanger model are employed and evaluated in this investigation. A thermodynamic system design model was developed using Engineering Equation Solver (EES) where state properties were calculated at inlets and outlets along both hot and cold legs of the pilot-scale plant. This investigation assesses receiver performance as well as system efficiency studies for the pump and system operational ranges. Results found that high efficiency sodium receivers were found to have higher heat transfer coefficients and required far less spreading of incident flux. The system performance model results suggest that for a pump speed of 2400 RPM, respective hot and cold pump TDH values were determined to be 260.1–307 ft. and 260.1–307 ft for pump flow rates of 90–120 GPM.

Heat Pump-Organic Rankine Cycle Hybrid Systems for Co/Tri-Generation Applications: A State-of-the-Art Overview

The following paper aims to explore a heat pump’s (HP) as well as an organic Rankine cycle’s (ORC) novel combination for the development of both an efficient and low-emissions heating and cooling systems. This latest review examines both benefits and possibilities of a combined HP-ORC system. Previously, studies have explored several different combinations, such as directly-coupled and reversible combination units as well as parallel configurations units in addition to indirectly-coupled ones. Following defining aforementioned configurations, a discussion on their performance is carried out in detail. Considerations for the optimisation of the architecture, overall of such hybrid systems via utilising the same sources while also discussing heat source, sink selection and operating temperatures as well as thermal energy storage, expander/compressor units, control strategies in addition to working fluids’ selection and managing seasonal temperatures that are increasingly variable, have been identified. Additionally, the experimental studies that have been performed reveal increasingly practical obstacles as well as other areas that require more research while serving to shed light on experimental techniques, which can be applicable to this research’s area. Based upon research, it has been revealed that regional conditions including temperatures and annual weather as well as the cost of energy produce a colossal effect on such systems’ economic feasibility framework as well as partially dictating the overall system configuration’s selection. Additionally, the review disclosed how important the following elements are: 1) a greater temperature differential amid the source of heat and heat sink; 2) proper source of heat and sink selection; 3) working fluid selection; and 4) thermal storage for the maintenance of the difference. Comparatively, from the research works from the past, additional optimisation based on individual component level as well as through control strategies of either an advanced or predictive method, these produce a smaller effect and are worth performing an evaluation on economically due to them not being feasible for the current system. Lastly, based on investigated research, there are certain areas for which recommendation have been provided with regard to future research and this includes a technology configurations’ comparison for understanding different regions’ optimal system, a sensitivity analysis for understanding key system elements for both optimisation as well as design, both an investigation as well as testing carried out for available units and applicable systems that are presently available, and identifying novel use cases.

Performance Investigation of Solar Organic Rankine Cycle Systems With and Without Regeneration and With Zeotropic Working Fluid Mixtures for Use in Micro-Cogeneration

Globally there are several viable sources of renewable, low-temperature heat (below 130°C) particularly solar energy, geothermal energy, and energy generated from industrial wastes. Increased exploitation of these low-temperature options has the definite potential of reducing fossil fuel consumption with its attendant very harmful greenhouse gas emissions. Researchers have universally identified the organic Rankine cycle (ORC) as a practicable and promising system to generate electrical power from renewable sources based on its beneficial use of volatile organic fluids as working fluids (WFs). In recent times, researchers have also shown a preference for/an inclination towards deployment of zeotropic mixtures as ORC WFs because of their capacity to improve thermodynamic performance of ORC systems, a feat enabled by better matches of the temperature profiles of the WF and the heat source/sink. This paper demonstrates both the technical feasibility and the notable advantages of using zeotropic mixtures as WFs through a simulation study of an ORC system. The study examines the thermodynamic performance of ORC systems using zeotropic WF mixtures to generate electricity driven by low-temperature solar heat source for building applications. A thermodynamic model is developed with an ORC system both with and excluding a regenerator. Five zeotropic mixtures with varying compositions of R245fa/propane, R245fa/hexane, R245fa/heptane, pentane/hexane and isopentane/hexane are evaluated and compared to identify the best combinations of WF mixtures that can yield high efficiency in their system cycles. The study also investigates the effects of the volumetric flow ratio, and evaporation and condensation temperature glides on the ORC’s thermodynamic performance. Following a detailed analysis of each mixture, R245fa/propane is selected for parametric study to examine the effects of operating parameters on the system’s efficiency and sustainability index. For zeotropic mixtures, results showed that there is an optimal composition range within which binary mixtures are inclined to perform more efficiently than the component pure fluids. In addition, a significant increase in cycle efficiency can be achieved with a regenerative ORC, with cycle efficiency ranging between 3.1–9.8% and 8.6–17.4% for ORC both without and with regeneration, respectively. Results also showed that exploiting zeotropic mixtures could enlarge the limitation experienced in selecting WFs for low-temperature solar organic Rankine cycles.

Analysis on Energy Consumption of Urban Central Heating Network in North China

Urban central heating network (UCHN) is an important infrastructure in northern China. However, as the technology and management of heating operators are still at a low level, a lot of energy is wasted in heating network. Based on the actual operation data of nearly 100 heating stations in northern China in 2018–2019 heating seasons, this study comprehensively analyzes the problems existing in the operation of UCHN in China from the perspective of time and space, investigating the correlation between the hourly energy consumption of a single thermal station and the hourly outdoor temperature in the time dimension and examining the uniformity of heat supply of different thermal stations in the space dimension. This paper focuses on the analysis of the heat consumption per unit area (HCA), temperature of primary side return water (TPSRW), heat consumption per unit area per unit temperature difference (HCATD) and its standard deviation (SHCATD) of each heating station in the early, middle and end period of heating season. The results show that the TPSRW among different thermal stations varies greatly and distributes unevenly, suggesting that the problem of thermal imbalance is serious. The SHCATD in these two periods is dramatically higher than that in the middle period, indicating that the control systems of current heating systems fail to compensate for the weather. Two measures are proposed to solve the problems of the operation of UCHN.

Effect of Waste Vegetable Oil on Cooling Performance and Lifetime of Power Transformers

During the operation of a power transformer, a large amount of heat is generated due to the electrical and magnetic energy losses in its core and windings, causing a temperature rise in transformers. This generated heat is known as the main factor for aging the electrical insulating system of a transformer. In this research, we numerically studied the ability of a vegetable-based oil — as an alternative coolant for the petroleum-based oils — on the cooling performance of a power transformer. The studied oil was a biodiesel produced from waste cooking vegetable oils, having lower viscosity compared to traditional mineral oils. We also calculated the aging rate of the transformer in the presence of the biodiesel. The results indicated that compared to the mineral oil, the average hotspot temperature of the transformer is 3 degrees lower when the biodiesel was used. The life expectancy of the transformer with the vegetable-based oil was also significantly longer than the case with mineral oil. In conclusion, this study provided a sustainable way to use an eco-friendly material produced from a waste resource as an alternative insulating liquid for the cooling of power transformers.

Near Optimal Model Predictive Control of Thermal Energy Storage

Efficient plant operation can be achieved by properly loading and sequencing available chillers to charge a thermal energy storage (TES) reservoir. TES charging sequences are often determined by heuristic rules that typically aim to reduce utility costs under time of use rates. However, such rules of thumb are in most cases far from optimal even for this task. Rigorous optimization, on the other hand, is computationally expensive and can be unreliable as well if not carefully implemented. Model-predictive control (MPC) that is reliable, as well as effective, in TES application must be developed. The goal is to develop an algorithm that can reach ∼80% of achievable energy efficiency and peak shifting capacity with very high reliability. A novel algorithm is developed to reliably achieve near optimal control for charging cool storage in chiller plants. Algorithm provides a constant COP (or cost per ton-hour) for 24-hr dispatch plan at which plant operates during most favorable weather conditions. Preliminary evaluation of this novel algorithm has indicated up to 6% improvement in plant annual operating cost relative to the same plant operating without TES. TOU rate used in both cases charges 7.4cents/kWh during off peak hours and 9.8cents/kWh during peak hours (Peak hours are 10 am to 10 pm).

Unmanned Aerial Vehicle Path Generation for Image Collection to Assist Heliostat Field Optical Characterization

This paper focuses on applications of unmanned aerial vehicles (UAVs) for measuring optical error of heliostats in concentrating solar power (CSP) plants. In CSP, there is a need to measure solar-field optical errors, which is critical for future production improvement as well as for operations and maintenance of a heliostat field. This latter need is particularly challenging because of the large number of heliostats (over 10,000 for a utility-scale power plant) that individually track the sun in the field. To address this issue, a camera-equipped UAV, with an optimized drone flight path developed and uploaded to it, collects images of a precise reflection of the tower on each heliostat to evaluate optical error sources without interrupting plant operation. Generation of the drone path for capturing the reflected images is affected by a number technical and realistic constraints, which include the camera angle used to capture the image, the blocking of the camera view due to surrounding heliostats, the location of the camera in reference to the target heliostat, and the target heliostat position with reference to the tower. The effect of these constraints on calculating the camera position will be discussed in detail in this article. An effective drone-path algorithm is generated to fulfil the need of image collection under various constraints.

Comparison and Implementation of Thermally Massive Wall and Roof Models for Use in Simplified Building Energy Models

An increasing trend in building energy simulations is to use simplified models to reduce simulation time, evaluate different model configurations, and analyze for energy consumption across different constructions and weather climates. Simplified models tend to share some common benefits such as ease of calibration and reduced setup and operation time. All of which allows for shorter time and simpler program to evaluate different situations or systems. Some of these simplified models ignore thermal capacitance within walls and roofs; removing thermal capacitance can decrease simulation time but may alter loading due to ignoring the delay between when exterior surfaces receive loading and when the load is transferred to the interior. While this simplification is sometimes useful, it often overlooks the delay that occurs between the external wall heating and that heat being transferred to the interior. This paper will explore alternative methods for evaluating conduction loads in opaque surfaces for use in building energy models. Specifically, a differential equation conduction method with numerical integration, closed form solution, and forward difference calculation. These methods will be evaluated for how different conduction simulation techniques can be used in different situations to provide a potential increase in accuracy for simplified models while simultaneously reducing computational loads. Understanding the physics of dynamic envelope loading can change how much energy a building uses and when room conditioning needs to occur.