Small-scale Combined Heat Research Articles

A High-Speed, High-Temperature, Micro-Cantilever Steam Turbine for Hot Syngas Compression in Small-Scale Combined Heat and Power

Abstract Coupling a biomass gasifier with a solid oxide fuel cell system through a high-temperature syngas compressor holds great promise to achieve low-emission, small-scale combined heat and power, since it reduces the number of heat exchangers and increases the system efficiency. However, due to the demanding operating conditions (high temperatures, toxic and explosive gases), electrical motors are not suitable to drive the syngas compressor. Therefore, a high-speed, small-scale cantilever steam turbine that can valorize the system?s waste heat to power the compression is designed and developed. An iterative process involving preliminary design, meanline analysis, commercial tools, and in-house codes is used for the design. The design is then numerically analyzed using computational fluid dynamics. The 2.8 kW cantilever steam turbine with a tip diameter of 21 mm runs up to 210 krpm at temperatures of 525°C while being supported on dynamic steam-lubricated bearings. A low-reaction, full-admission design has been chosen to lower the steam consumption, the axial forces, and the turbine backface leakage. The turbine rotor is made of Ti6Al4V and coated for structural integrity and to withstand high temperatures. Despite the small scale of this design, the results obtained from the established correlations based on large-scale turbines yield a remarkable concordance with the results from the numerical analysis, in particular for the isentropic expansion efficiency prediction.

A High-Temperature, High-Speed, Oil-Free Syngas Compressor for Small-Scale Combined Heat and Power

Abstract For low-emission, small-scale combined heat and power generation, integrating a biomass gasifier with a downstream solid oxide fuel cell system is very promising due to their similar operating conditions in terms of temperatures and pressures. This match avoids intermediate high-temperature heat exchangers and improves system efficiency. However, to couple both systems, a high-temperature and oil-free compressor is required to compress and push the low-density, high-temperature bio-syngas from the gasifier to the solid oxide fuel cell stack. The design and development of this high-temperature, high-speed, and gas-bearing supported compressor is presented in this work. An iterative process involving preliminary design, meanline analysis using commercial tools and in-house models is used for the design, which is then numerically analyzed using computational fluid dynamics. The goal is to achieve a design with a wide operating range and high robustness that withstands extreme working conditions. The 727 W machine is designed to run up to 210 krpm to compress 18.23kg/h of syngas at 350°C and 0.81bar. The centrifugal compressor has a tip diameter of 38 mm and consists of 9 backswept main and splitter blades. The impeller is made of Ti6Al4V and coated to prevent hydrogen embrittlement from the hot and highly reactive bio-syngas. The results obtained from the established models suggest a good concordance with the results from numerical analyses, despite the high temperatures and small scale of this design.

Assessing the emissions reduction potential and economic feasibility of small-scale (

Small-scale combined heat and power (CHP) for residential applications has been demonstrated in different regions throughout the world. CHP can enable resilience in energy supply and serve as a transitional technology as the grid progresses to higher fractions of low carbon electricity production. However, there are very limited feasibility assessments in the context of US climate zones, policies, and economic factors. To address this gap, we assess how the economic feasibility and emissions impact of a small-scale, natural gas CHP system compares considering the local electrical grid in seven climate zones and six subgrid regions across the United States. This is accomplished using an in-house Python program alongside EnergyPlus that is used to size the CHP and/or thermal storage systems according to representative electrical and thermal demand profiles for multifamily residential buildings, optimizes CHP and thermal storage energy dispatch throughout the year to meet demands, assesses the system’s economic feasibility, and calculates the carbon emission reduction or increase relative to the local electrical grid. For the baseline cases considered, carbon emissions can be reduced by up to 12% for regions with high required thermal load and/or electricity derived from high carbon emitting sources. In many scenarios, carbon emissions increased. Furthermore, the economic feasibility of the systems, even considering recent incentives, remains a significant barrier. The resulting Python program is open-source and designed for use by both researchers and building owners to conduct their own small-scale CHP feasibility assessments.

Elephant Grass (Pennisetum purpureum): A Bioenergy Resource Overview

Elephant grass (EG), or Pennisetum purpureum, is gaining attention as a robust renewable biomass source for energy production amidst growing global energy demands and the push for alternatives to fossil fuels. This review paper explores the status of EG as a sustainable bioenergy resource, integrating various studies to present a comprehensive analysis of its potential in renewable energy markets. Methods employed include assessing the efficiency and yield of biomass conversion methods such as pretreatment for bioethanol production, biomethane yields, direct combustion, and pyrolysis. The analysis also encompasses a technoeconomic evaluation of the economic viability and scalability of using EG for energy production, along with an examination of its environmental impacts, focusing on its water and carbon footprint. Results demonstrate that EG has considerable potential for sustainable energy practices due to its high biomass production and ecological benefits such as carbon sequestration. Despite challenges in cost competitiveness with traditional energy sources, specific applications like small-scale combined heat and power (CHP) systems and charcoal production show economic promise. Conclusively, EG presents a viable option for biomass energy, potentially playing a pivotal role in the biomass sector as the energy landscape shifts towards more sustainable solutions; although, technological and economic barriers need further addressing.

An integrated agroforestry-bioenergy system for enhanced energy and food security in rural sub-Saharan Africa

Most people in rural sub-Saharan Africa lack access to electricity and rely on traditional, inefficient, and polluting cooking solutions that have adverse impacts on both human health and the environment. Here, we propose a novel integrated agroforestry-bioenergy system that combines sustainable biomass production in sequential agroforestry systems with biomass-based cleaner cooking solutions and rural electricity production in small-scale combined heat and power plants and estimate the biophysical system outcomes. Despite conservative assumptions, we demonstrate that on-farm biomass production can cover the household’s fuelwood demand for cooking and still generate a surplus of woody biomass for electricity production via gasification. Agroforestry and biochar soil amendments should increase agricultural productivity and food security. In addition to enhanced energy security, the proposed system should also contribute to improving cooking conditions and health, enhancing soil fertility and food security, climate change mitigation, gender equality, and rural poverty reduction.

Organic Rankine Cycle (ORC) Systems: A fundamental Overview of Small-scale Applications Fuelled by Low-grade Heat Sources

Environmental issues shift energy production from conventional methods to new and more efficient alternatives. One of these alternatives is the use of organic Rankine cycles (ORC) in low-grade heat sources to generate both heat and power at small scales. Among different technologies available for this purpose, ORC-based systems seem to be the most suitable and promising option due to their simplicity and versatility. Thus, such systems have been investigated intensively. However, current studies often focus on only one aspect of these systems due to the massive research scale in this field. Therefore, this study aims to provide a fundamental and holistic overview to evaluate ORC-based low-heat sourced and small-scale applications from multiple perspectives. As a result, the basic operating principles and application areas of ORCs, selection and design criteria of their working fluids and all other system components, methods of improving their performance, and other thermodynamic cycles that can be ORC alternatives are examined in detail. The results of this study show that ORC applications can enable small-scale combined heat and power generation, while geothermal and solar energy sources have the potential to scale the size of such applications up to kW capacities. The results also showed that dry & isentropic fluids and vane & scroll expanders are the most suitable refrigerant and expander types, respectively, for small-scale ORC applications. Furthermore, the implications of all findings are critically discussed.

Operational Strategies of Two-Spool Micro Gas Turbine With Alternative Fuels: A Performance Assessment

Abstract Micro gas turbines (mGT) have not yet succeeded in conquering the small-scale combined heat and power (CHP) market. One reason is that their electrical efficiency is not high enough to maintain a cost-effective operation. A two-shaft intercooled mGT has the potential to meet the current market demand. This technology maintains a high electrical efficiency even at part-load and coupled with its fuel-flexible combustion chamber, it is an ideal candidate for CHP concepts in a renewable future. In this paper, performance analysis on two-spool mGT is carried out with various fuel blends. Attention is given to the low-pressure and high-pressure compressors and the variation of surge margin by adding hydrogen and syngas. Two control strategies for the mGT are adopted. In the first scenario, the two shafts have equal rotational speeds while in the second, the speeds are controlled independently. As the engine is operated at equal speeds, the maximum performance with 100 vol. % of syngas is observed at 85% of the nominal load while 100 vol. % of hydrogen shows maximum efficiency at a load of 63.7%. At electric power lower than 60% and for high amounts of syngas in natural gas, the low-pressure compressor (LPC) operates closely to surge line. In the second scenario, the efficiency increases as the load decreases and the LPC runs in an efficient and safe operating region. Additionally, the amount of nitrogen in syngas affects the part-load performance of the two-spool mGT.

Modeling, Control and Optimization of a Small Scale CHP System in Island operating Mode based on Fuzzy logic controller

This paper presents the modeling, control scheme and optimization of a small scale combined heat and power (SSCHP) in island operating mode. A thermo dynamic model is carried out for SSCHP engine using Carnot machine method, while an optimization technique is formulated to identify CHP engine parameters. In addition, a dynamic model is built based on a system identification toolbox for SSCHP engine to study the dynamic behavior of the SSCHP during operating mode. In island mode, the Fuzzy logic controller (FLC) is used to regulate the inputs of SSCHP, and to match electric load demand. The complete system was represented and simulated in Matlab/Simulink.

Investigations into the performance and emissions of a small-scale CHP system using producer gas obtained from gasification of forest residues

The cost of wood, which form traditional feedstocks for biomass gasification, has been sharply increasing in recent years. Hence, low cost alternative feedstocks, such as forest residues, need to be explored to retain the economical edge. Forest residues (FR) are the materials left behind on the forest floor after forest harvesting and management activities. The present study explores the effect of forest residues as feedstock in a downdraft gasifier coupled with a small-scale combined heat and power (CHP) system, consisting of a diesel engine-alternator setup. The findings reveal that the FR mixtures change the flow behaviors of both feedstock and air in the reactor. While the average composition of the producer gas is similar to that of wood chips, a stratified bed with uneven flow leads to an unpredictable nature in the equivalence ratio which is a process controlling parameter. This is also reflected in gas parameters such as the heating value and cold gas efficiency, which in turn leads to unpredictability in the overall performance of the CHP system, evident in its electrical and thermal efficiencies, and exhaust gas components like CO, CO2, and NOx. Diesel substitution rates (DSR) of up to 59% and 46% for partial and full electrical loads respectively were realized. While emissions of CO and CO2 increased with increasing DSR, supplementing diesel with producer gas has benefits in terms of the pollutants NOx and PM, which were found to be generally lower for the FR mixtures. In the case of NOx, reductions of over 65% was achieved.

Techno-economic investigation of integrated biodiesel internal combustion engines and transcritical organic Rankine cycles for small-scale combined heat and power generation

The work presents an integrated system for combined heat and power generation based on a topping biodiesel internal combustion engine (ICE) and a bottoming transcritical organic Rankine cycle (TORC). A 0D/1D mathematical model is developed to evaluate the performance of the innovative biodiesel system in design and off-design conditions for a wide range of rated power and organic fluids. The analysis highlights that the TORC integration improves the total full load electric power by more than 12%, and the engine size significantly influences the integrated system performance. The total electric efficiency increases from 35.0% to 47.9%, whereas the thermal efficiency decreases from 27.2% to 19.8% when the ICE’s nominal power moves from 10 kWel to 500 kWel.Afterward, a multi-objective investigation is carried out to define the most suitable power of the proposed biodiesel system and its capability to satisfy the energy request of specific users. A thermal self-consumption equal to 100%, an electric self-consumption larger than 94%, and a payback time of 5.2 years are found. Lastly, a sensitivity analysis demonstrates the economic feasibility of the proposed biodiesel integrated system for a wide range of prices of energy vectors and system components.

Real Time Micro Gas Turbines Performance Assessment Tool: Comprehensive Transient Behavior Prediction With Computationally Effective Techniques

Abstract Conventional centralized power generation is increasingly transforming into a more distributed structure. The periodic power production that is created by the renewable production unit generates the need for small-scale heat and power units. One of the promising technologies which can assist flexible power grid is micro gas turbines (mGTs). Such engines are competent candidates for small-scale combined heat and power (CHP). mGTs, as compensators for demand fluctuations, are required to work on transient and part-load conditions, creating new research challenges. A complete characterization of their dynamic behavior through a real-time simulation tool is necessary to establish effective control systems. Moreover, the energy transition requires the conversion of conventional mGTs to more sophisticated high-efficient cycles with the addition of extra components (saturation unit, aftercooler, etc.). Consequently, a modular and computationally fast real-time tool offers an asset in the development of future cycles based on the mGT concept. This paper presents the development of a numerical in-house tool implemented in Python programing language for the performance prediction of the mGT. The fundamental target of our work is to achieve high fidelity of the simulated dynamic responses. The key benefit of this tool is the low complexity component modules. The model is validated with experimental results from the VUB T100 test rig. The code reproduces the experimental data well during steady-state and transient operations as the key cycle parameters present a deviation from the measurements within the range of 1.5%.

Optimization of configuration for home micro-grid cogeneration system based on Wind-PV/T-PEMFC

Residential energy consumption has increased rapidly with the improvement of living standards of Chinese households, in particular, heating demand in North China has a direct impact on the haze. The integration of renewable energy with the combined heat and power (CHP) technology provides an effective method to solve the household energy consumption and environmental impacts, but the construction, capacity, and cost of the system seriously affect the economy of the system, environmental protection, and operational reliability. Therefore, a family-oriented small-scale combined heat and power system was designed based on wind, photovoltaic, fuel cell, and hydrogen storage. The scheduling strategy of heat determined by power, meeting the requirements of all electrical loads and partial heat loads, and the minimum investment for the whole life cycle of 20 years was used as the objective function to design the system. Combined with the actual environmental conditions and thermoelectric load, an adaptive particle swarm optimization algorithm was improved for solving optimal allocation. The system was analyzed through thermal and power conditions, economy, the reliability of meeting the load, and other evaluation criteria in the optimal system configuration, the prediction, and evaluation of the prospects and development of the system provide basis and reference for the family cogeneration micro-grid applications.

Thermo-economic investigation of solar-biomass hybrid cogeneration systems based on small-scale transcritical organic Rankine cycles

• An innovative small-scale solar/biomass hybrid system is investigated. • A novel transcritical organic Rankine cycle is studied at full and partial loads. • The comparison with single-source systems shows the advantages of the hybrid solution. • Biomass and solar source integration assures cost decrease and clean production. • Biomass saving (–32.8%) and better solar exploitation (+8.8%) are reached. The present study aims at investigating the energy, exergy, and economic performance of an innovative multi-source renewable hybrid energy system for small-scale combined heat and power (CHP) applications. The system is based on a transcritical organic Rankine cycle (ORC) able to exploit solar and biomass sources, through a concentrated solar power unit and a biomass boiler. A thermo-economic model has been developed and a parametric analysis has been implemented to define the proper configuration of the hybrid system balancing energy and economic purposes. The investigation, performed for a residential complex in Southern Italy, highlights that the suitable hybridisation of biomass and solar energy increases the useful production and system flexibility compared to the single-source configurations. The proposed small-scale system guarantees a 48.9% decrease in the biomass consumption compared to the corresponding biomass-only apparatus, increases the exploitation (+8.8% of electric production) of low solar radiations that are not adequate to feed a full-solar ORC, also overcoming the stochastic and intermittent characteristics of the solar source. The comparison with the conventional scenario, where the electric grid and natural gas boilers satisfy the energy demands, separately, demonstrates that the suggested solar/biomass hybrid ORC system guarantees a 24.2% primary energy saving, a 53.5% decrease in the unit electric and thermal production costs, and a 57.7% drop in greenhouse gas emissions, with a payback period equal to 7.5 years.

Multi-Fidelity Combustor Design and Experimental Test for a Micro Gas Turbine System

A multi-fidelity micro combustor design approach is developed for a small-scale combined heat and power CHP system. The approach is characterised by the coupling of the developed preliminary design model using the combined method of 3D high-fidelity modelling and experimental testing. The integrated multi-physics schemes and their underlying interactions are initially provided. During the preliminary design phase, the rapid design exploration is achieved by the coupled reduced-order models, where the details of the combustion chamber layout, flow distributions, and burner geometry are defined as well as basic combustor performance. The high-fidelity modelling approach is then followed to provide insights into detailed flow and emission physics, which explores the effect of design parameters and optimises the design. The combustor is then fabricated and assembled in the MGT test bench. The experimental test is performed and indicates that the designed combustor is successfully implemented in the MGT system. The multi-physics models are then verified and validated against the test data. The details of refinement on lower-order models are given based on the insights acquired by high-fidelity methods. The shortage of conventional fossil fuels and the continued demand for energy supplies have led to the development of a micro-turbine system running renewable fuels. Numerical analysis is then carried out to assess the potential operation of biogas in terms of emission and performance. It produces less NOx emission but presents a flame stabilisation design challenge at lower methane content. The details of the strategy to address the flame stabilisation are also provided.

Thermoeconomic and environmental evaluation of a combined heat, power, and distilled water system of a small residential building with water demand strategy

The most important challenges facing humans are global warming and lack to freshwater. Large-scale desalination has always been of interest, but producing freshwater using heat recovery in small-scale combined heat and power systems has not yet been sufficiently discussed. The small scale desalination is highly required, because it can use small scale recoverable waste heat from the micro/small scale power generators. Decentralized small scale desalination systems are more feasible and reliable than centralized big desalination systems for the time of crises such earthquake, war and terrorist attacks, and water loss reduction due to water transmission pipelines. In this research, a cycle is designed for simultaneous production of power, heat and freshwater in small scale for residential buildings. The power generator is a micro gas turbine by Capstone Company. The cycle feasibility is studied by energy, exergy, economic, and environmental criteria, and the design of the desalination is presented in details. The sizing of the system’s components is done according to the following water demand strategy. Evaluations show that the proposed system is feasible from thermodynamics, economic, and environmental viewpoints. The simulation is done by code generation in MATLAB software. The footprint area to install the small scale multi-stage flash desalination is 0.25 m2. The net present value is 25 million United States dollar with payback period of less than 2 years. It also reduces carbon dioxide about 39 tons/year.

Modeling and Economic Operation of Energy Hub Considering Energy Market Price and Demand

This paper discusses the economic operation strategy of the energy hub, which is being established in South Korea. The energy hub has five energy conversion devices: a turbo expander generator, a normal fuel cell, a fuel cell with a hydrogen outlet, a small-scale combined heat and power device, and a photovoltaic device. We are developing the most economically beneficial operation strategy for the operators who own the hub, without making any systematic improvements to the energy market. First, sixteen conversion efficiency matrices can be achieved by turning each device (except the PV) on or off. Next, even the same energy must be divided into different energy flows according to price. The energy flow is controlled to obtain the maximum profit, considering the internal load of the energy hub and the price fluctuations of the energy market. Using our operating strategy, the return on investment period is approximately 9.9 years, which is three years shorter than that without the operating strategy.

Paths to market for stationary solid oxide fuel cells: Expert elicitation and a cost of electricity model

Solid oxide fuel cells (SOFCs) can efficiently generate continuous electricity for buildings but face technical and economic challenges to achieving mass market acceptance. We conducted a workshop comprising 23 experts to assess near-term markets, system cost and production scale, and system and stack degradation rates. Experts identified commercial- and residential-scale systems as the most favorable entry-level markets in the U.S. and world, respectively. The 2020, 2035, and 2050 median 250 kW electric-only system costs assessed by experts were $2,400/kW, $1,909/kW, and $841/kW (2018 USD) at production volumes of 210, 4000, and 10,000 systems per year, respectively. For small-scale combined heat and power applications, median 1 kW system cost assessments were $11,000/kW, $7,750/kW, and $5,750/kW at 50,000, 80,000 and 175,000 systems per year, respectively. Experts suggested voltage degradation could achieve 0.2%/1000 hrs in 2035–2050 and identified chromium poisoning, microstructural cathode degradation, and Ni agglomeration and coarsening as the most substantial barriers to mitigating degradation. To assess lifecycle performance, we incorporated experts' assessments into a cost of electricity (COE) model. The 250 kW system COE competed with anticipated internal combustion engine, microturbine, and U.S. grid COEs in 2035–2050. The 1 kW system COEs were three times anticipated grid prices in 2035–2050; however, 1 kW systems may be better suited to countries with higher spark spreads. Our results could inform business decisions, funding prioritizations, and technology roadmap development.

Optimal Integration of Renewable Sources and Latent Heat Storages for Residential Application

Given the large amount of energy required in the building sector, an interesting opportunity to reach future sustainable energy systems is the path towards low energy buildings. This work proposes an approach for optimally integrating building-scale energy technologies (both traditional and renewable) to enhance the transformation of the existing buildings (often energetically inefficient) in low-carbon systems. The approach promotes a transition sustainable from both the economic and environmental perspectives. Both operation and design optimization are considered with the aim of suggesting the best set of capacity of the technologies to be installed taking into account the expected operations. The building-scale technologies are integrated with proper storage units: Li-ion batteries and thermal storage (latent heat, that requires low installation space). As a dispatchable renewable technology, a biogas small-scale combined heat and power unit is included in the system. Once the key role played by this component in meeting the loads is proved, an analysis of the impact of the cost of the primary energy carrier of this technology on the system design is carried out. Two optimization approaches have been adopted (both based on non-linear programming). Results show that operation costs can be reduced by up to 29%. The adoption of a combined approach that takes into account both operation and design optimization lead to a reduction in installation and operating costs by up to 27%. In the analyzed cases, the use of the combined optimization confirms that latent heat storage is more suitable to be installed than electric storage (about −4.5% cost).

Mining Smart Meter Data to Enhance Distribution Grid Observability for Behind-the-Meter Load Control: Significantly improving system situational awareness and providing valuable insights

Distributed Energy Resources (DERs) are playing an increasingly important role in power systems. In 2023, five categories of DERs-distributed solar, electric vehicles (EVs), energy storage, residential smart thermostats, and small-scale combined heat and power-are expected to contribute about 104 GW to the U.S. summer peak (see GTM, 2018). With the increasing integration of DERs in power distribution systems, distributed load control is imperative to smooth the fluctuations that they introduce. However, a main challenge is that distribution systems lack systematic situational awareness because of their limited sensors. Furthermore, most customer-level behind-the-meter (BTM) DERs, such as rooftop photovoltaics (PVs), are being integrated into distribution systems, which complicates the system monitoring and control. Enhanced electric grid monitoring is needed to promote renewable integration while ensuring reliability, but current approaches rely on expensive sensors.

Optimal Operation of Combined Heat and Power in Competitive Electricity Markets: a Case Study in IAUN

Energy efficiency management, in addition to the balance between energy supply and demand, requires thorough policy and elaborate decision-making by governments to adopt appropriate strategies to achieve the targeted goals. Combined heat and power systems featuring a decentralized and independent generation model of both electricity and heat, which procures efficient utilization of wasted energy, prevention of losses in distribution and transmission grid. Furthermore, the deployment of such a scheme results in reducing fuel consumption as well as the emission of environmental pollutants, increasing competitiveness, and liquidity in electricity markets. Optimal scheduling of small-scale combined heat and power systems in response to the consumption pattern and the electrical and heat demand of a typical customer as well as the type of interactions and interoperability with the aggregators and the market is an innovative paradigm to improve the efficiency of distributed generation resources. Due to the physical correlation between these two types of energy, the effective implementation of such schemes entails meticulous configuration and arrangement, precise modeling, and even rigorous simulation. Hence, to get a convincing and superb performance, particular attention must be paid to the model. This paper delves into the impacts of energy exchange between CHP and the upstream grid and the way to optimize it subject to satisfaction of the optimal capacity constraints. In this regard, the linear modeling method is employed and the optimization problem is solved by Grasshopper Optimization Algorithm (GOA) using MATLAB software. The Islamic Azad University of Najafabad (IAUN) complex is considered as the case study in this paper. The results imply that a significant part of the energy generated in the combined heat and power unit can be sold to the grid, this will play a vital role in reducing the cost of electricity supply.