Fluidized-Bed Heat Transfer Modeling for the Development of Particle/Supercritical-CO2 Heat Exchanger

Concentrating solar power (CSP) technology is moving toward high-temperature and high-performance design. One technology approach is to explore high-temperature heat-transfer fluids and storage, integrated with a high-efficiency power cycle such as the supercritical carbon dioxide (s-CO2) Brayton power cycle. The s-CO2 Brayton power system has great potential to enable the future CSP system to achieve high solar-to-electricity conversion efficiency and to reduce the cost of power generation. Solid particles have been proposed as a possible high-temperature heat-transfer medium that is inexpensive and stable at high temperatures above 1,000°C. The particle/heat exchanger provides a connection between the particles and s-CO2 fluid in the emerging s-CO2 power cycles in order to meet CSP power-cycle performance targets of 50% thermal-to-electric efficiency, and dry cooling at an ambient temperature of 40°C. The development goals for a particle/s-CO2 heat exchanger are to heat s-CO2 to ≥720°C and to use direct thermal storage with low-cost, stable solid particles. This paper presents heat-transfer modeling to inform the particle/s-CO2 heat-exchanger design and assess design tradeoffs. The heat-transfer process was modeled based on a particle/s-CO2 counterflow configuration. Empirical heat-transfer correlations for the fluidized bed and s-CO2 were used in calculating the heat-transfer area and optimizing the tube layout. A 2-D computational fluid-dynamics simulation was applied for particle distribution and fluidization characterization. The operating conditions were studied from the heat-transfer analysis, and cost was estimated from the sizing of the heat exchanger. The paper shows the path in achieving the cost and performance objectives for a heat-exchanger design.

Heat Flux Distribution Over a Solar Central Receiver Using an Aiming Strategy Based on a Conventional Closed Control Loop

Solar thermal towers are a maturing technology that have the potential to supply a significant part of energy requirements of the future. One of the issues that needs careful attention is the heat flux distribution over the central receiver’s surface. It is imperative to maintain receiver’s thermal stresses below the material limits. Therefore, an adequate aiming strategy for each mirror is crucial. Due to the large number of mirrors present in a solar field, most aiming strategies work using a data base that establishes an aiming point for each mirror depending on the relative position of the sun and heat flux models. This paper proposes a multiple-input multiple-output (MIMO) closed control loop based on a methodology that allows using conventional control strategies such as those based on Proportional Integral Derivative (PID) controllers. Results indicate that even this basic control loop can successfully distribute heat flux on the solar receiver.

Peak-Shaving Ratio Analysis of the Natural Gas Combined Heat and Power Plant With Distributed Peak-Shaving Heat Pumps

Distributed peak-shaving heat pump technology is to use a heat pump to adjust the heat on the secondary network in a substation, with features of low initial investment, flexible adjustment, and high operating cost. The paper takes an example for the system that uses two 9F class gas turbines (back pressure steam) as the basic heat source and a distributed heat pump in the substation as the peak-shaving heat source. The peak-shaving ratio is defined as the ratio of the designed peak-shaving heat load and the designed total heat load. The economic annual cost is taken as a goal, and the optimal peak-shaving ratio of the system is investigated. The influence of natural gas price, electricity price, and transportation distance are also analyzed. It can provide the reference for the optimized design and operation of the system.

Volume Element Model for Modeling, Simulation, and Optimization of Parabolic Trough Solar Collectors

In this paper we present a dynamic three-dimensional volume element model (VEM) of a parabolic trough solar collector (PTC) comprising an outer glass cover, annular space, absorber tube, and heat transfer fluid. The spatial domain in the VEM is discretized with lumped control volumes (i.e., volume elements) in cylindrical coordinates according to the predefined collector geometry; therefore, the spatial dependency of the model is taken into account without the need to solve partial differential equations. The proposed model combines principles of thermodynamics and heat transfer, along with empirical heat transfer correlations, to simplify the modeling and expedite the computations. The resulting system of ordinary differential equations is integrated in time, yielding temperature fields which can be visualized and assessed with scientific visualization tools. In addition to the mathematical formulation, we present the model validation using the experimental data provided in the literature, and conduct two simple case studies to investigate the collector performance as a function of annulus pressure for different gases as well as its dynamic behavior throughout a sunny day. The proposed model also exhibits computational advantages over conventional PTC models-the model has been written in Fortran with parallel computing capabilities. In summary, we elaborate the unique features of the proposed model coupled with enhanced computational characteristics, and demonstrate its suitability for future simulation and optimization of parabolic trough solar collectors.

Determination of Static and Dynamic Injection Characteristics of a Common-Rail Direct Injection Diesel Engine Fuelled by CME-Diesel Blends

Much interest is given to the research in biodiesel these days. It is renewable and has similar properties to conventional diesel. Biodiesel is also generally seen to produce less emissions, hence it is seen as an attractive and a greener alternative source of energy. Biodiesels are also referred to as Fatty Acid Methyl Esters (FAME). They are obtained from the transesterification of oils from organic products such as animal fat or vegetable oil. Common biodiesel feedstocks are soybean (USA), rapeseed (Europe), palm, and coconut (Asia). The Philippines, being one of the largest producers of coconut in the world, should have a substantial interest in this. Biodiesel in the Philippines is obtained from coconut oil and is commonly called Coconut Methyl Ester (CME). There is a number of research works available that study the effects of biodiesel when used to run diesel engines, although there is notably less studies on CME and particularly Philippine-CME available. This work aims to show the fuel injection timing and duration of a Common Rail Direct Injection (CRDI) engine run by CME-diesel with neat diesel as baseline. There are two sets of injection parameters that describe the injection behaviour of an engine. The static injection parameters refer to the electronic commands given out by the Electronic Control Unit (ECU) while the dynamic injection parameters refer to the actual physical injection happening in the fuel injector nozzle. Knowledge of these information may help explain possible differences in performance and/or emissions observed in biodiesel-fed engines. The static injection commands were obtained by tapping into the solenoid signal wire from the ECU. The dynamic injection parameters were estimated from line pressure signals in the fuel injection line. All the tests were done on the AVL Eddy Current Engine Dynamometer in the University of the Philippines Vehicle Research and Testing Laboratory. Baseline data were recorded from 100% neat diesel, then volumetric blends B10 (10% CME biodiesel and 90% neat diesel) and B20 (20% CME biodiesel and 80% neat diesel) were mixed for the tests. The CRDI engine was ran at full load, sweeping the operating range at 400 RPM increments from 800 to 4000. The results showed no significant difference in the static injection parameters of the CME-diesel blend-fed engines as compared to being ran with neat diesel. As for the dynamic injection parameters, there were some significant differences observed in the higher engine speeds starting at 2800 RPM. The observed changes were attributed to the differences in the physiochemical properties of CME biodiesel as compared to neat diesel.

Heat Transfer Models of Moving Packed-Bed Particle-to-SCO2 Heat Exchangers

Particle-based concentrating solar power (CSP) plants have been proposed to increase operating temperature for integration with higher efficiency power cycles using supercritical carbon dioxide (sCO2). The majority of research to date has focused on the development of high-efficiency and high-temperature particle solar thermal receivers. However, system realization will require the design of a particle/sCO2 heat exchanger as well for delivering thermal energy to the power-cycle working fluid. Recent work has identified moving packed-bed heat exchangers as low-cost alternatives to fluidized-bed heat exchangers, which require additional pumps to fluidize the particles and recuperators to capture the lost heat. However, the reduced heat transfer between the particles and the walls of moving packed-bed heat exchangers, compared to fluidized beds, causes concern with adequately sizing components to meet the thermal duty. Models of moving packed-bed heat exchangers are not currently capable of exploring the design trade-offs in particle size, operating temperature, and residence time. The present work provides a predictive numerical model based on literature correlations capable of designing moving packed-bed heat exchangers as well as investigating the effects of particle size, operating temperature, and particle velocity (residence time). Furthermore, the development of a reliable design tool for moving packed-bed heat exchangers must be validated by predicting experimental results in the operating regime of interest. An experimental system is designed to provide the data necessary for model validation and/or to identify where deficiencies or new constitutive relations are needed.

Numerical Analysis of Centrifugal Pumps Running in Turbine Mode Under Dynamic Operating Conditions

The use of pumps as turbines (PAT) has gained importance in the recent years as a possible alternative to specifically developed turbine for mini/micro hydropower plants. The use of production pump for hydropower generation reduces the capital cost of the plant but the energy conversion efficiency can be remarkably lower. The paper analyses the performance of a production centrifugal pump running both in direct and reverse mode. The analysis calculates theoretically the behavior of the PAT under the best efficiency point and extends the investigation to other operating points using both a combined theoretical approach and CFD simulation under dynamic conditions. The effects of possible modifications to the initial design of the pump are investigated when running in turbine mode and their influence on the standard pump operation is also determined. Numerical simulation demonstrates that the impeller trimming leads to improvement in the PAT efficiency in some operating conditions. Conversely, the rotational speeds close to the values typical for the electric generator reduce the PAT performance. Finally, the modification of the impeller geometry at the turbine inlet increases the PAT efficiency but lowers the performance of the machine when running in pump mode.

An Evaluation of Financial Incentive Policies for Solar Photovoltaic Systems in the U.S.

This paper analyzes some of the existing incentives for solar photovoltaic (PV) energy generation in the U.S. to investigate how effectively those existing incentive policies can promote PV adaptions in the U.S. market. Two common building types (i.e., hospitals and large hotels) located in five different U.S. states, each having their own incentives, are selected and analyzed for the PV incentive policies. The payback period of the PV system is chosen as an indicator to analyze and critique the effectiveness of each incentive by comparing the payback periods before and after taking the incentive into consideration. In this way, the existing incentive policies implemented by utility companies in each state are analyzed and critiqued. Finally, a parametric analysis is conducted to determine the influence of the variation in key parameters, such as PV system capacity and PV capital cost, on the performance of PV system. The results show how the existing incentives can be effectively used to promote the PV systems in the U.S. and how variations of the parameters can impact the payback period of the PV systems. Through the evaluation of the existing incentive policies and the parametric study, this paper demonstrates that the type and level of incentives should be carefully determined in policy-making processes to effectively promote the PV systems.

Optimization of a Small Size CHP System by Means of a Fully Transient Numerical Approach

The paper investigates the performance of a combined heat and power system by means of a fully dynamic numerical approach. An ad-hoc library for the simulation of energy conversion systems is developed under the OpenModelica open source platform; the library includes the main components that usually equip a Combined Heat and Power (CHP) system and they can be connected as they are logically connected in the real plant. Each component is modelled by means of equations and correlations that calculate their performance on a time dependent basis. Therefore, many configurations can be evaluated not only in terms of cumulative annual results or average performance, but the instantaneous behavior can be investigated. The numerical library is constructed using the lumped and distributed parameter approach and it is validated by comparing the numerical results with the measured values over a one-year time period. The prediction capabilities of the proposed numerical approach are evaluated by simulating a case study of a health spa. This case study is selected since it is characterized by significant requirements of both thermal and electric energy. The comparison demonstrated that the calculated results are in good agreement with the measurements in terms of both annual values and distribution over the reference period. Furthermore, an optimization algorithm is adopted and linked to the developed library in order to estimate the best size of different components of the CHP system according to a number of constraints. This feature is particularly important when addressing the energy efficiency of a complete system that is depending on a number of interdependent variables. Therefore, the case study is investigated by accounting also for additional technologies that can be further enhance the performance of the system both in terms of energy consumption and economic investment. In particular, the numerical model is used to optimized the CHP energy efficiency by estimating the best trade-off between the reduction of the energy purchased and the overall cost of the system. The application of PV panels and electric energy accumulators is also investigated and the simulation demonstrates that the size of the cogeneration unit equal to 48 kW, the number of PV panels of 299 and the battery capacity of 45 kWh provide the lowest amount of energy purchased, while the best return of investment is obtained by the CHP unit of 40 kW along with 109 PV panels and a battery of 40 kWh.

Flexible Natural Gas/Intermittent Renewable Hybrid Power Plants

Increasing grid penetration of intermittent renewable power from wind and solar is creating challenges for the power industry. There are times when generation from these intermittent sources needs to be constrained due to power transmission capacity limits, and times when fossil fuel power plant are required to rapidly compensate for large power fluctuations, for example clouds pass over a solar field or the wind stops blowing. There have been many proposals, and some actual projects, to store surplus power from intermittent renewable power in some form or other for later use: Batteries, Compressed Air Energy Storage (CAES), Liquid Air Energy Storage (LAES), heat storage and Hydrogen being the main alternatives considered. These technologies will allow power generation during low periods of wind and solar power, using separate discrete power generation plant with specifically designed generator sets. But these systems are time-limited so at some point, if intermittent renewable power generation does not return to its previous high levels, fossil fuel power generation, usually from a large centralized power plant, will be required to ensure security of supplies. The overall complexity of such a solution to ensure secure power supplies leads to high capital costs, power transmission issues and potentially increased carbon emissions to atmosphere from the need to keep fossil fuel plant operating at low loads to ensure rapid response. One possible solution is to combine intermittent renewables and energy storage technologies with fast responding, flexible natural gas-fired gas turbines to create a reliable, secure, low carbon, decentralized power plant. This paper considers some hybrid power plant designs that could combine storage technologies and gas turbines in a single location to maximize clean energy production and reduce CO2 emissions while still providing secure supplies, but with the flexibility that today’s grid operators require.