First and Second Law Analysis of a Flat Plate Collector Working With Nanofluids

The utilization of solar energy in thermal energy systems was and always be one of the most effective alternative to conventional energy resources. Energy efficiency is widely used as one of the most important parameters in order to evaluate and compare thermal systems including solar collectors. Nevertheless, the first law of thermodynamics is not solely capable of describing the quantitative and qualitative performance of such systems and thus exergy efficiency is used so as to introduce the systems’ quality. In this work, the performance of a flat plate solar collector using water based nanofluids of different nanoparticle types as a working fluid is analyzed theoretically under the climatic conditions in Greece based on the First and Second Law of Thermodynamics. A mathematical model is built and the model equations are solved iteratively in a MATLAB code. The energy and exergy efficiencies as well as the collector losses coefficient for various parameters such as the inlet temperature, the particles concentration and type are determined. Moreover, a dynamic model is built so as to determine the performance of a flat plate collector working with nanofluids and the useful energy that can be stored in a water tank. The exergy destruction and exergy leakage are determined for a typical day in summer during which high temperatures and solar intensity values are common for the Greek climate.

Performance Comparison and Parametric Analysis of sCO2 Power Cycles Configurations

A thermodynamic analysis and optimization of four supercritical CO2 Brayton cycles were conducted in this study in order to improve calculation accuracy; the feasibility of the cycles; and compare the cycles’ design points. In particular, the overall thermal efficiency and the power output are the main targets in the optimization study. With respect to improving the accuracy of the analytical model, a computationally efficient technique using constant conductance (UA) to represent heat exchanger performances is executed. Four Brayton cycles involved in this compression analysis, simple recaptured, recompression, pre-compression, and split expansion. The four cycle configurations were thermodynamically modeled and optimized based on a genetic algorithm (GA) using an Engineering Equation Solver (EES) software. Results show that at any operating condition under 600 °C inlet turbine temperature, the recompression sCO2 Brayton cycle achieves the highest thermal efficiency. Also, the findings show that the simple recuperated cycle has the highest specific power output in spite of its simplicity.

Ground Level Integrated Diverse Energy Storage (GLIDES) Cost Analysis

With the increasing penetration of renewable energy, the need for advanced flexible/scalable energy storage technologies with high round-trip efficiency (RTE) and high energy density has become critical. In this paper, a techno-economic model of a novel energy storage technology developed by the Oak Ridge National Laboratory (ORNL) is presented and used to estimate the technology’s capital cost. Ground-Level Integrated Diverse Energy Storage (GLIDES) is an energy storage technology with high efficiency which can store energy via input of electricity and heat and supply dispatchable electricity. GLIDES stores energy by compressing and expanding a gas using a liquid piston. GLIDES performance has been extensively studied analytically and experimentally. This study aims to develop a comprehensive combined performance and cost modeling environment. With the desired system storage capacity kilowattage, storage time (hours), and an initial RTE guess as inputs, the model optimizes the selection of system components to minimize the capital cost. The techno-economic model described in this paper can provide preliminary cost estimates and corresponding performance for various system sizes and storage times.

Design and Optimization of a Tidal Turbine and Farm

Tidal ocean power is a dependable and dense form of renewable energy which is a relatively underdeveloped field. This study optimizes a tidal turbine with respect to performance and economics, and then optimizes a farm to be economically feasible. It was determined that the southeastern portion of the Gulf Stream, Florida current, would be used for the tidal turbine system as it has some of the world’s fastest velocities and is relatively close to shore. The vertical axis designs were ruled out from extended research on turbine design for their lower efficiency in general. Only horizontal axis designs were tested in simulated environments. Using SolidWorks Flow Simulation and SolidWorks Simulation, turbine models were optimized and selected as having the potential for the greatest energy extraction. Static and fatigue analyses were conducted on the optimized models in order to prevent premature failure. Cost analysis was also performed on the turbine models and the model that had the lowest initial cost as well as the highest power generation was chosen for farm development. The optimized design produced reasonable amount of power considering varying velocities throughout the day having a diameter of about 30 m. Through fatigue analysis the optimized design also showed long enough lifetime so that a good return on investment can be acquired. The single optimized turbine was then placed in a farm, and the farm’s shape and arrangement were tested and optimized so that the best arrangement and distances between units could be found. It was found that a farm 1.25 kilometers by 20 kilometers consisting of 800 turbines would be optimal. The farm would produce an average of 249.33 megawatts for a profit of $294.88 million dollars annually. The farm would pay for itself in 7.12 years and have an expected life span of 26.1 years which was obtained through fatigue analysis.

Thermoeconomic Analysis of a Solar MVC Desalination System

Globally, about 10% of the world population does not have access to enough fresh water. In many hot-and-dry coastal regions and islands, the desalination of seawater might be the only practical option to have a fresh water supply. Therefore, low-cost desalination system is critical for freshwater demands. To address this issue, a desalination system consisting of solar photovoltaic (PV) and mechanical vapor compression subsystem is proposed in this study. The entire desalination system was modeled and designed to produce 10,000 m3 of fresh water per day at the coast of San Francisco, California. Key components such as water vapor compressor, solar PV panel, and three-stream heat recovery unit were designed, and their performances were analyzed. The effects of design variables and operating conditions on the system performance were investigated through a parametric study. Finally, an economic analysis was conducted and compared with current desalination technologies. The analysis results show that the specific power consumption of desalination system can reach 14.4 kWh/m3 when the evaporation temperature is 70°C. It is found that the evaporating temperature has a great influence on the heat pump system efficiency and evaporator design. The levelized cost of the proposed system is $0.76 per m3 of fresh water which is lower than current grid-powered vapor compression desalination system and other thermal desalination systems. The proposed solar PV driven desalination improves thermoeconomics of desalination system by applying low-lift operating condition to the vapor compression cycle so that it can contribute to solving the fresh water supply challenges.

Non-Isothermal Phase Change Behaviors of Binary Mixtures of D-Dulcitol and Pentaerythritol As Novel Heat Storage Materials

As a promising Phase Change Material (PCM) candidate for low-to-medium temperature (100–250 °C) latent heat storage, sugar alcohols undergo serious supercooling during cool-down for crystallization. Technical efforts need to be dedicated to suppression or control of the supercooling of sugar alcohols. In this work, the supercooling of D-dulcitol, with a melting point of around 186 °C, was attempted to be reduced by mixing with a solid-solid PCM Pentaerythritol (PE) as the nucleation agent, which has a solid-solid phase transition temperature (∼186 °C) similar to the melting point of d-dulcitol. Such novel binary mixtures were prepared by dispersing PE powders at various mass fractions into d-dulcitol melt. The non-isothermal phase-change-related properties, with emphasis on the crystallization properties, were tested on a heat-flux differential scanning calorimeter at a constant heating/cooling rate of 5 °C/min. The preliminary results showed that both the crystallization point and latent heat of crystallization strongly depend on the mass fraction of PE, and both decrease in magnitude with the increasing in mass fraction of PE. The degree of supercooling of the binary mixtures also depend on the mass fraction of PE, and a reduction of up to 10 °C was obtained at 50 wt.% PE, as a result of the decrease in the melting points of the binary mixtures.

Using Forecasted Daily Maximum Temperatures to Control a Chiller Thermal Storage System

Residential buildings in Canada and the United States are responsible for approximately 20% of secondary energy consumption. Over the past 25 years, air conditioning has seen the single largest increase of any residential end use. This load currently places a significant peak load on the electrical grid during later afternoon periods during the cooling season. One method to reduce or eliminate this peak load being placed in the grid is the use of a chiller coupled with a thermal storage system. The chiller operates during off-peak periods, predominately over-night to charge the thermal storage tank, and the stored cooling potential is realized to meet the cooling loads during peak periods. In previous studies, the use of a chiller has seen a reduction in annual operating costs, however a significant increase in energy occurs as a result of decreased performance of the chiller. To improve system performance, a new control scheme was developed, which uses the forecasted daily high for the next day to predict the cooling load for the day during peak periods for the day. The predicted cooling load is then used as the set-point for the cold thermal storage tank, allowing the peak cooling load to be met using stored cooling potential. This control scheme was implemented into a modelled house located in each of the 7 major ASHRAE zones, with a storage tank with a previously found optimal tank volume. Across each of the locations, a reduction in annual utility costs and overall energy required to meet the building loads observed, with the total cost savings between 0.3% and 1.5% and total electricity required to meet the cooling demand decreasing by as much as 10.2%.

Disc Turbine for Energy Harvesting

Energy harvesting is part of a current paradigm of distributed production and use of energy. However, we are aware that the size of turbomachinery used to produce the energy can be an advantage, in terms of higher efficiencies when using larger machines. This is one main reason why distributed production and use of energy has not known a very large popularization in the past. Still, with present day fluid dynamics tools and new production technologies it is now becoming possible to expand this concept. A less used turbomachinery is the disc or Tesla turbine or pump. This is based on a series of parallel discs that rotate under the action of a common shaft. These parallel discs impart a movement to the flow by means of viscous effects, due to viscous adherence of the fluid to the rotating discs. Disc turbines offer and inherent advantage to provide a capability of energy harvesting when fluids include living elements, or when the fluid is mostly comprised of slurries. In this paper we present an experimental and numerical analysis of the flow in this special kind of turbomachine. The experimental activities use a direct operating mode to assess the operation of the pump-as-turbine. An experimental set-up provides a set of results that allow to characterize the design conditions of the disc turbomachine under analysis. Further, a numerical simulation is performed for direct and inverse operation modes. After careful validation of the numerical model using the experimental results, the performance of the disc turbine is verified for a range of conditions. Among these are pure water and slurry fluids. The results allow us to provide the set of conditions in which the disc turbine can be used for energy harvesting.

Examining Ice Storage and Solar PV As a Potential Push Toward Sustainability for Qatar

Soaring electricity demand from space cooling and excellent solar photovoltaics (PV) resources are creating an opportunity for the financial viability of low-emission solutions in Qatar that can compete with existing approaches. This study examines the big picture viability of combining large utility-scale PV with decentralized building-scale ice storage for cooling in Qatar. Qatar is found to have consistently high repeatable solar radiation intensity that nearly matches space cooling requirement. A means to exploit the low installed costs of PV, combined with low cost and long lifetime of ice storage (as opposed to batteries) are examined to meet space cooling loads. Space cooling is responsible for about 65% of Qatar’s annual electric load (which averaged 4.68 GW in 2016). While multiple gas prices are considered, a scenario with the current gas price of $3.33/MMBTU, a PV system of 9.7 GW capacity and an aggregate ice-storage capacity of 4.5 GWh could reduce the gas-fired power generation in Qatar by nearly 39%. Here, gas-fired generation capacity to meet current load exists and hence is not costed.

Radiative Transport and Hydrodynamic Modeling of Microalgae Photosynthesis in Bio-Flow Reactors

A simplified two-phase flow PCH (physicochemical hydrodynamics) model is developed for modelling and simulation of microalgae growth in bio-flow reactor. The model considers carbon balance through coupled gas-phase and liquid-phase transport equations. The transport model accounts for interfacial transport of CO2 from gas bubble/slug to liquid, and microalgae photosynthesis reactions. A simplified photosynthesis reaction is adopted in the model, which assumes a pseudo-first order reaction for glucose pathway. The reaction rate is calculated assuming that it is proportional to the solar absorption rate by microalgae in the liquid. The reaction model also includes a simplified photoinhibition sub-model which assumes that the rate of photoinhibition is proportional to the square-root of solar irradiation reaching the algae cell. The Beer-Lambert law is used to calculate the radiative transfer of solar flux in seeded microalgae liquid flow. Analytical solution was obtained for single-channel bio-flow reactor. Decrease of the CO2 concentration in gas bubble/slug and in liquid flow is assumed to be the result of the microalgae growth in bio-flow reactor. Two efficiency parameters are defined: CO2 conversion efficiency and photosynthesis efficiency. The conversion efficiency is calculated based on the decrease of CO2 between the bio-flow reactor inlet and exit. The photosynthesis efficiency is based upon the heating value of microalgae yield versus solar irradiation. The rate of microalgae yield is calculated by multiplying the mass stoichiometric coefficient of photosynthesis reaction to CO2 consumption rate. Model analysis provided some insight of the microalgae formation in bio-flow reactor as interpreted from the PCH-coupled photosynthesis model that includes a dimensionless number as a potential scaling parameter for gas-phase only CO2 supply operation; photosynthesis efficiency increases with increasing CO2 molar concentration (i.e., number of moles per unit volume) at the reactor inlet for both gas-phase and liquid-phase only CO2 supply; an optimal irradiation flux for maximum photosynthesis efficiency — a factor to consider should artificial light source be used for harvesting algae.