Distributed Cogeneration Systems Research Articles

The Optimization of Steam Generation in a Biomass-Fired Micro-Cogeneration Prototype Operating on a Modified Rankine Cycle

According to the United Nations, one of the sustainable development goals is to ensure access to affordable, reliable, sustainable, and modern energy for all. Among other options, these goals can be achieved by developing and introducing micro-scale combined heat and power systems powered by renewable energy sources, including solar and biomass energy. Considering renewable energy-powered cogeneration technologies, the most promising are steam/vapor turbines, Stirling engines, and thermoelectric generators. This paper focuses on the selected operational aspects and retrofitting optimization of the prototypical micro-cogeneration system powered by a biomass-fired batch boiler and operating according to the modified Rankine cycle. The existing installation was tested, and the amount of energy transferred from the oil to the condensate and steam and the efficiency of the evaporator and the superheater were determined. A retrofitting optimization aimed at maximizing the piston engine’s power output was conducted based on the results. In particular, it was shown that the system’s power output might be as high as 9 kWe. Moreover, the analyzed system featured a high energy utilization factor of 97.9% at optimal operating conditions. In general, it was shown that the micro-scale steam Rankine system may successfully serve as an alternative technology for micro- and distributed cogeneration systems. As a technology supplied with renewable biomass energy and operating on a cheap and environmentally friendly working medium (water), it fits very well into the idea of sustainable energy system development.

Techno-economic feasibility study of a solar-powered distributed cogeneration system producing power and distillate water: Sensitivity and exergy analysis

To satisfy the increasing demand for energy and potable water, large-scale cogeneration is widely integrated. However, in remote locations, the lack of power system infrastructure limits the integration of large-scale systems. Consequently, a large portion of inhabitants has no access to electricity and the pressure on groundwater resources increases drastically. To address this power and water scarcity, a distributed cogeneration system consisting of a solar-powered micro Gas Turbine and desalination system is considered. Since the integration of solar energy in distributed cogeneration systems is uncertain, we performed a feasibility study. This paper covers the modelling, sensitivity and exergy analysis and 3 desalination systems designs, each making a trade-off between smaller plant size and higher performance. Introducing solar energy in the micro gas turbine results in an increase in electrical efficiency by 3.2% absolute. The proposed designs achieve a levelized cost of water between 1.78$/(m3/d) and 1.92$/(m3/d), which is comparable with conventional solar-powered desalina-tion plants. Therefore, these designs demonstrate the feasibility of integrating solar energy in distributed cogeneration systems and provide a promising solution towards cost-efficient, renewable-based power and water cogeneration in remote locations. The future work will enhance the economic analysis by including an intermittent solar energy supply.

Analysis on Cooperation between Central Power Utility and Distributed Cogeneration Systems Based on a Mixed-Integer Linear Model

Analysis on Cooperation between Central Power Utility and Distributed Cogeneration Systems Based on a Mixed-Integer Linear Model

A distributed cogeneration system with a two-stage solar-driven biomass gasifier for heating, power and hydrogen in Northern China

In this work, a cogeneration system with a two-stage solar-driven biomass gasifier is proposed to achieve the multiple outputs, i.e., generating power and heating during the heating period and producing hydrogen during the non-heating period in Northern China. The thermodynamic performances of the system under the design and off-design conditions are investigated. The solar-biomass gasification is divided into two stages, i.e., pyrolysis and gasification in sequence. The mid-temperature solar energy (around 673 K) from parabolic trough collectors is utilized to drive biomass pyrolysis, and the high-temperature solar energy (around 1150 K) from heliostats with point-focus solar collection technology is used to drive the gasification. In the heating period, the gasified syngas as a solar fuel is directly fed into a gas turbine for generating electricity, then a heat recovery steam generator (HRSG) is deployed to produce steams for heating. An energy storage module (i.e. gas tank) is introduced to offset heating at night. The primary energy efficiency of the system is 56.1%, while the rate of stored energy to the syngas based chemical energy is up to 35.66%. The energy storage module weakens the influence of time-varying solar irradiations on system performances, and the steady and efficient utilization of solar energy are achieved. In the non-heating period, the gasified syngas can be converted into hydrogen with the water-gas shift reaction. The above results indicate that the proposed system provides a promising complementary utilization approach of multiple energies (solar energy and biomass) in Northern China.

Complex thermal energy conversion systems for efficient use of locally available biomass

This paper is focused on a theoretical study in search for new technological solutions in the field of electricity generation from biomass in small-scale distributed cogeneration systems. The purpose of this work is to draw readers' attention to possibilities of design complex multi-component hybrid and combined technological structures of energy conversion plants for effective use of locally available biomass resources. As an example, there is presented analysis of cogeneration system that consists of micro-turbine, high temperature fuel cell, inverted Bryton cycle module and biomass gasification island. The project assumes supporting use of natural gas and cooperation of the plant with a low-temperature district heating network. Thermodynamic parameters, energy conversion effectiveness and economic performance are examined. Results show relatively high energy conversion performance and on the other hand weak financial indices of investment projects at the current level of energy prices. It is however possible under certain conditions to define an optimistic business model that leads to a feasible project.

Apparent Power-Based Anti-Islanding Protection for Distributed Cogeneration Systems

In this paper, the performance of a passive anti-islanding method is experimentally tested for three phase $(3\phi)$ cogeneration systems. The tested method is based on determining the wavelet packet transform (WPT) high-frequency subbands present in the $d$ – $q$ -axis components of instantaneous $3\phi$ apparent powers $(s_{d} \mbox{and} s_{q})$ , when evaluated at the point of common coupling (PCC). This passive anti-islanding method is founded based on the nature of instantaneous $3\phi$ apparent powers that have components continuously exchanged between both sides of the PCC. An islanding condition can be considered as a transient disturbance that creates nonperiodic and nonstationary high frequency components in $s_{d}$ and $s_{q}$ . These frequency components can be parameterized by WPT high-frequency subbands, which can provide accurate detection of the islanding condition. The $d$ – $q$ WPT-based anti-islanding method is tested for a $3\phi$ cogeneration system under various loading and power delivery conditions. Performance results reveal accurate, fast, and reliable detection and response to the islanding condition.

Effect of control strategy on performance and emissions of natural gas engine for cogeneration system

Because of the global increase in the consumption of low-calorie natural gas, and increasing awareness of the mid-to long-term problem with the decrease in the calorific value of natural gas, actions are needed to minimize the problems with gas appliances. Most natural gas engines for small distributed cogeneration systems operate with a lean fuel/air mixture and do not employ an excess air ratio control and optimized spark advance timing. The engine performance can be influenced by calorific value changes in the natural gas without the excess air ratio control.In the present study, the performance and emission characteristics of a natural gas engine were experimentally investigated under different HHV (higher heating value) of natural gas fuel. Nitrogen was introduced to simulate a decrease in the HHV, and the injection duration of the injector, spark advance timing, and throttle valve opening were maintained to realize the non-control of the excess air ratio at the baseline condition, where the HHV was 10,300 kcal/Nm3. To recover the level of torque with the HHV baseline, the combustion and emission characteristics for each control strategy were analyzed, and the feasibility of lowering the HHV of the natural gas fuel in the field was also assessed.

Optimization of a Distributed Cogeneration System with solar district heating

The aim of the paper is to identify the optimal energy production system and its optimal operation strategy required to satisfy the energy demand of a set of users in an industrial area. A distributed energy supply system is made up of a district heating network, a solar thermal plant with long term heat storage, a set of Combined Heat and Power units and conventional components also, such as boilers and compression chillers. In this way the required heat can be produced by solar thermal modules, by natural gas cogenerators, or by conventional boilers. The decision variable set of the optimization procedure includes the sizes of various components, the solar field extension and the thermal energy recovered in the heat storage, while additional binary decision variables describe the existence/absence of each considered component and its on/off operation status.The optimization algorithm is based on a Mixed Integer Linear Programming (MILP) model that minimizes the total annual cost for owning, maintaining and operating the whole energy supply system. It allows to calculate both the economic and the environmental benefits of the solar thermal plant, cooperating with the cogeneration units, as well as the share of the thermal demand covered by renewable energy, in the optimal solutions.The results obtained analyzing different system configurations show that the minimum value of the average useful heat costs is achieved when cogenerators, district heating network, solar field and heat storage are all included in the energy supply system and optimized consistently. Thus, the integrated solution turns out to be the best from both the economic and environmental points of view.

Application of the multi-objective optimization and risk analysis for the sizing of a residential small-scale CCHP system

Multi-objective optimization for sizing of a small-scale combined cooling, heating, and power generation (CCHP) system is performed. In multi-objective optimizing the s CCHP system, the three objective functions including the exergetic efficiency, total levelized cost rate of the system product and the cost rate of environmental are optimized, simultaneously. The environmental impact and thermoeconomic objective are minimized while the exegetic objective is maximized. A comprehensive emission assessment framework suitable for addressing of distributed cogeneration systems is formulated according to an electrical output-based emission factor approach and the environmental impact objective function are defined and expressed in cost terms. The economic analysis is conducted in accordance with the total revenue requirement (TRR) method. The genetic algorithm is applied to find the set of Pareto optimal solutions with respect to the aforementioned objective functions. In the present work, reliability and availability are introduced in the developed models of the system, so that redundancy is embedded in the optimal solution. In this regard, risk analysis is used as a decision-making tool for the selection of the final optimal solution from the obtained Pareto optimal frontier. The sensitivity of the optimal solution respect to variations of input parameters is analyzed.

Optimal lay-out and operation of combined heat & power (CHP) distributed generation systems

Abstract The paper presents an optimization model of a distributed cogeneration system with a district heating network, applied to a real city centre situation. The distributed urban cogeneration system includes both a set of micro-gas turbines, located inside some public buildings, and a centralized cogeneration system based on a Internal Combustion Engine. The objective function adopted for the optimization is the Total Annual Cost for owning, maintaining and operating the whole system. To face the problem a Mixed Integer Linear Program (MILP) is defined and solved by a commercial software. Starting from the thermal and electrical demand of the buildings, the MILP model allows to define the possible installation of the centralized cogeneration ICE (Internal Combustion Engine) and the number of microturbines in the different buildings, the optimal lay-out of the district heating network and the optimal operation strategy for the whole system as well. In particular the energy performance and global CO2 emissions are evaluated.

Global and local emission impact assessment of distributed cogeneration systems with partial-load models

Small-scale distributed cogeneration technologies represent a key resource to increase generation efficiency and reduce greenhouse gas emissions with respect to conventional separate production means. However, the diffusion of distributed cogeneration within urban areas, where air quality standards are quite stringent, brings about environmental concerns on a local level. In addition, partial-load emission worsening is often overlooked, which could lead to biased evaluations of the energy system environmental performance. In this paper, a comprehensive emission assessment framework suitable for addressing distributed cogeneration systems is formulated. Local and global emission impact models are presented to identify upper and lower boundary values of the environmental pressure from pollutants that would be emitted from reference technologies, to be compared to the actual emissions from distributed cogeneration. This provides synthetic information on the relative environmental impact from small-scale CHP sources, useful for general indicative and non-site-specific studies. The emission models are formulated according to an electrical output-based emission factor approach, through which off-design operation and relevant performance are easily accounted for. In particular, in order to address the issues that could arise under off-design operation, an equivalent load model is incorporated within the proposed framework, by exploiting the duration curve of the cogenerator loading and the emissions associated to each loading level. In this way, it is possible to quantify the contribution to the emissions from cogeneration systems that might operate at partial loads for a significant portion of their operation time, as for instance in load-tracking applications. Suitability of the proposed methodology is discussed with respect to hazardous air pollutants such as NO x and CO, as well as to greenhouse gases such as CO 2. Two case study applications based on the emission data of real microturbines are illustrated in order to highlight the effectiveness of the proposed assessment techniques. The numerical results exemplify the emission impact of distributed cogeneration systems operating under general and realistic loading conditions with respect to average and state-of-the-art conventional technologies.

Optimal operational loads for distributed cogeneration power plants

AbstractThe purpose of this paper is to determine the operating mode of a cogeneration system that minimises its overall cost of operation. The optimisation procedure is achieved in two steps:• First, we consider a single unit and we develop a convex but non differentiable formulation for the operating cost. Furthermore an explicit expression for the optimal operational mode parametrized with heat and power consumer demand and with electricity market price, is obtained;• Then, we express for the distributed cogeneration system the overall operating cost. This objective function is still convex and non differentiable and the optimisation procedure requires subdifferential calculus, via the Moreau‐Rockafellar and the Duboviskii‐Milyutin theorems [1].The final result describing the best operating mode for the overall set of cogeneration units relies on the ordered sequence of marginal costs associated with the different operational modes for each unit. It indicates that the optimal operation of the distributed cogeneration system is achieved through an economic dispatch mode. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

From hospital to municipal cogeneration systems: an Italian case study

A mixed integer linear programming model combined with a more traditional design by scenarios is proposed to optimize facilities size and operation mode of a municipal energy system involving significant civil centres and a hospital. Moving from the need of a new heat and power station for the local hospital due to the construction of new pavilions, the opportunity of involving other centres in the neighbourhood in a distributed cogeneration system is analysed, increasing system complexity step by step. Smaller cogeneration units tailored to hospital needs are rewarding ventures with relatively low risks but, in a country whose traditional power generation systems heavily rely on fossil fuels and where energy policy and market conditions can make it profitable to sell surplus power, district heating systems foster the installation of larger cogenerators and lead thereby to higher profits and to better performance as for primary energy savings and greenhouse gases emission reduction. Copyright © 2006 John Wiley & Sons, Ltd.

703 マイクロガスタービン・コージェネレーションシステムの評価研究(マイクロガスタービンシステムの熱力学的特性)(OS.11 マイクロガスタービンと分散電源)

Distributed cogeneration systems, which supply both electric power and heat, offer many advantages over conventional central power stations for reducing the primary energy consumption. Micro gas turbines (μGT) have recently been developed as an electric power generation device for small-scale cogeneration systems. This paper reports the energy saving capability assessment based on the simulation of the annual fuel consumption in a micro gas turbine cogeneration system (μGT-CGS) including a boiler, a mechanical compression heat pump unit and an absorption chiller unit. Assuming the installation into three types of buildings (office, hotel and residence), the annual energy saving and the power supply fraction of the μGT-CGS are assessed. The results show that the advantage of the μGT-CGS significantly increases when the efficiency of μGT reaches 40%.