Small-scale Combined Heat And Power Research Articles

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.

Bi-objective optimization of multiple agro-industrial wastes supply to a cogeneration system promoting local circular bioeconomy

This research analyses and proposes an optimization model for the supply of biomass to a combined heat and power (CHP) system with a supply of different biomasses at a local level under Mediterranean conditions. The research aims to quantitatively assess whether it is economical and environmentally beneficial to transport various types of biomasses to the CHP plant, instead of landfilling, determining the biomass required according to the availability of power generation for each biomass of agricultural and agro-industrial origin. To do this, a representative case study has been developed in the Maule Region, Chile to supply power and heat to public, private, and residential buildings. The main biomasses analyzed are olive pomace, fruit pits and vineyard pruning. The results demonstrate that the supply of residual biomass to the CHP plant avoids the emission of CO2e generated by final disposal. Regarding the minimization of CO2e emissions, pruning residues are identified as the first supply option, due to their high heating value. Regarding cost minimization, olive pomace is identified as the first option, followed by fruit pits and pruning material. Furthermore, transport is not a major contributing cost or environmental factor when biomass sources are close to the CHP system, up to a maximum supply radius of 30 km. Finally, despite seasonality of agricultural biomass supply under Mediterranean conditions, it is feasible to adequately supply a small-scale CHP plant. However, this increases the storage costs involved. Other lignocellulosic biomasses could be used to optimize costs and environmental benefits.

Test and optimization of a control algorithm for demand-oriented operation of CHP units using hardware-in-the-loop

This paper covers test and verification of a forecast-based Monte Carlo algorithm for an optimized, demand-oriented operation of combined heat and power (CHP) units using the hardware-in-the-loop approach. For this purpose, the optimization algorithm was implemented at a test bench at Reutlingen University for controlling a CHP unit in combination with a thermal energy storage, both in real hardware. In detail, the hardware-in-the-loop tests are intended to reveal the effects of demand forecasting accuracy, the impact of thermal energy storage capacity and the influence of load profiles on demand-oriented operation of CHP units. In addition, the paper focuses on the evaluation of the content of energy in the thermal energy storage under practical conditions. It is shown that a 5-layer model allows to determine the energy stored quite accurately, which is verified by experimental results. The hardware-in-the-loop tests disclose that demand forecasting accuracies, especially electricity demand forecasting, as well as load profiles strongly impact the potential for CHP electricity utilization on-site in demand-oriented mode. Moreover, it is shown that a larger effective capacity of the thermal energy storage positively affects demand-oriented operation. In the hardware-in-the-loop tests, the fraction of electricity generated by the CHP unit utilized on-site could thus be increased by a maximum of 27% compared to heat-led operation, which is still the most common modus operandi of small-scale CHP plants. Hence, the hardware-in-the-loop tests were adequate to prove the significant impact of the proposed algorithm for optimization of demand-oriented operation of CHP units.

Techno-Economic Analysis of Biofuel, Solar and Wind Multi-Source Small-Scale CHP Systems

The aim of the paper is the techno-economic analysis of innovative integrated combined heat and power (CHP) systems for the exploitation of different renewable sources in the residential sector. To this purpose, a biofuel-driven organic Rankine cycle (ORC) is combined with a wind turbine, a photovoltaic system and an auxiliary boiler. The subsystems work in parallel to satisfy the electric and heat demand of final users: a block of 40 dwellings in a smart community. A 12.6 kWel ORC is selected according to a thermal-driven strategy, while wind and solar subsystems are introduced to increase the global system efficiency and the electric self-consumption. The ORC can be switched-off or operated at partial load when solar and/or wind sources are significant. A multi-variable optimization has been carried out to find the proper size of the wind turbine and photovoltaic subsystems and to define the suitable operating strategy. To this purpose, several production wind turbines (1.0–60.0 kWel) and photovoltaic units (0.3–63.0 kWel) have been considered with the aim of finding the optimal trade-off between the maximum electric self-consumption and the minimum payback period and electric surplus. The multi-objective optimization suggests the integration of 12.6 kWel ORC with 10 kWel wind turbine and 6.3 kWel photovoltaic subsystem. The investigation demonstrates that the proposed multi-source integrated system offers a viable solution for smart-communities and distributed energy production with a significant improvement in the global system efficiency (+7.5%) and self-consumption (+15.0%) compared to the sole ORC apparatus.

The Operating Strategies of Small-Scale Combined Heat and Power Plants in Liberalized Power Markets

Distributed generation is a good option for future energy systems with respect to sustainable development. In this context, the small-scale combined heat and power (CHP) plants are seen as an efficient way to reduce greenhouse gas emissions due to lower fuel consumption compared to the separate generation of the heat and electricity. The objective of this paper is to establish operating strategies of the small-scale CHP plants to reduce operational cost and increase revenue in liberalized electricity markets. It analyzes a cogeneration plant with organic Rankine cycle and biomass fuel under the conditions of the Romanian electricity market and the green certificates support scheme for electricity generated in high efficiency cogeneration and from renewable sources. The main finding is that choosing an appropriate mode of operation and using correlated prices of heat and electricity can increase the trading profitability of a CHP plant in liberalized power markets. This can be done by an analysis of the particularities and the specific operating conditions of the CHP plant. The results show that the operating strategies of the CHP plant can yield substantial net revenues from electricity and heat sales. The CHP plant can be economically operated to a useful heat load of more than 40% when operating strategies are applied.

Statistical method for the determination of the minichannel heat exchanger’s thermal characteristics

The paper presents a new statistical method for the determination of the experimental thermal characteristics of mini- and microchannel heat exchangers. These exchangers are suitable for small scale ORC (Organic Rankine Cycle) installations, CHP (Combined Heat and Power) generation or other applications like electronic equipment. It is not possible in practice to measure the temperature inside mini- and microchannels and only statistical methods may be used to determine the exchanger’s characteristics. There are many such methods which are the modifications of the Wilson plot method. However, the presented method has some advantages. Both fluid flows are treated symmetrically and flow rates and temperature values may be changed simultaneously. In many other methods it is necessary to stabilize one fluid flow while changing the second one. The correlations for Nusselt number may easily be chosen without changing the method itself. The linear coefficients are calculated analytically, but in the case of additional nonlinear coefficients, the numerical method may easily be used. The presented statistical method was verified based on the experimental data of the prototype minichannel heat exchanger.

Experimental and economic study of small-scale CHP installation equipped with downdraft gasifier and internal combustion engine

This paper concerns the experimental and numerical analysis of a combined heat and power (CHP) installation equipped with a biomass downdraft gasifier, gas purification system and gas piston engine. The numerical studies were aimed at identifying waste heat in the CHP installation for further utilisation. In turn, a new innovative method of heat recovery inside the gasifier was proposed and subsequently applied in the experimental CHP installation. The newly developed facility is able to produce up to 75kWe of electrical power. An extensive parametric study was performed under the steady and unsteady facility operation and was supplemented by economic analysis of the process. The main objective of the study was to examine the possibility of sewage sludge gasification and the impact of produced syngas quality on gas engine performance. During the investigation a number of different types of biomass were investigated including wood pellets, sewage sludge and their blends. The results showed a stable operation of the CHP facility in terms of produced syngas load, calorific value and content – even when dried sewage sludge was used alone. Results indicate that a 40/60% blend of wood pellets with sewage sludge is recommended, which allowed the lower heating value (LHV) of 4.45MJ/Nm3. Additionally, it was confirmed that the fixed bed gasifier is able to self-adjust to temporary changes in gasifier agent load.The economic analysis was performed taking into account policies and regulations in the Polish energy market sector. The study showed that it is more profitable to use the generated electricity and heat for its self-consumption rather than selling it on the market. Even with the supporting policies the payback time is not less than about eight years. Taking into account calorific value and limited local biomass availability, it seems justified that the described CHP installation is suitable for decentralised power generation such as small farms or horticultural businesses, where the produced electricity and heat can be sufficiently utilised. This technology also contributes to reduction of CO2 emissions into the atmosphere, which is one of the most problematic issues for energy sector today.

Externally Fired Micro-Gas Turbine and Organic Rankine Cycle Bottoming Cycle: Optimal Biomass/Natural Gas Combined Heat and Power Generation Configuration for Residential Energy Demand

Small-scale combined heat and power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly true when biomass fuels are used. In most cases, the use of both heat and electricity to serve on-site energy demand is a key issue to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and is influenced by climatic conditions, in particular in the case of residential and tertiary end users. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small-scale micro-gas turbine for heat and power cogeneration, a bottoming organic Rankine cycle (ORC) system can be coupled to the cycle, however, this option reduces the temperature and the amount of cogenerated heat available to the thermal load. In this perspective, the paper presents the results of a thermo-economic analysis of small-scale CHP plants composed of a micro-gas turbine (MGT) and a bottoming ORC, serving a typical residential energy demand. For the topping cycle, three different configurations are examined: (1) a simple recuperative micro-gas turbine fueled by natural gas (NG); (2) a dual fuel externally fired gas turbine (EFGT) cycle, fueled by biomass and natural gas (50% share of energy input) (DF); and (3) an externally fired gas turbine (EFGT) with direct combustion of biomass (B). The bottoming ORC is a simple saturated cycle with regeneration and no superheating. The ORC cycle and the fluid selection are optimized on the basis of the available exhaust gas temperature at the turbine exit. The research assesses the influence of the thermal energy demand typology (residential demand with cold, mild, and hot climate conditions) and CHP plant operational strategies (baseload versus heat-driven versus electricity-driven operation mode) on the global energy efficiency and profitability of the following three configurations: (A) MGT with cogeneration; (B) MGT+ ORC without cogeneration; and (C) MGT+ORC with cogeneration. In all cases, a back-up boiler is assumed to match the heat demand of the load (fed by natural gas or biomass). The research explores the profitability of bottoming ORC in view of the following trade-offs: (i) lower energy conversion efficiency and higher investment cost of biomass input with respect to natural gas; (ii) higher efficiency but higher costs and reduced heat available for cogeneration with the bottoming ORC; and (iii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid.

Small-scale biomass gasification CHP systems: Comparative performance assessment and monitoring experiences in South Tyrol (Italy)

During the last years, there was an increasing interest in small-scale biomass gasification system for combined heat and power (CHP) generation in South-Tyrol (Italy). The large number of plants based on various technologies and installed in a rather small area allowed carrying out a comparative study on a representative set of small-scale gasification-based commercial CHP unit.Each of the selected plants was monitored on-site for a time span of at least 5–6 h, measuring and quantifying mass and energy fluxes. The feedstock and the solid residue (char) have been characterized in terms of elemental composition, ash content and calorific value. The producer gas composition was measured, and tars collected in accordance with international sampling standard methodologies and analyzed. Moreover, environmental aspects have been taken into account, such as phytotoxicity levels in the char – the measurement of which has been performed by means of germination tests.This investigation resulted in a unique overview of the actual state-of-the-art of small-scale biomass gasification technology in Italy in terms of energy efficiency of the plants, effectiveness of the adopted solutions and characteristics of the products and by-products.

양방향 스털링엔진/발전기의 효율 특성 연구

This paper describes generating efficiency characteristics of the double acting Stirling engine/generator for domestic small-scale CHP (Combined Heat and Power) system. In small distributed generation applications, Stirling engine has competition from fuel cell, microturbine and etc. In order to be economical in the applications, a long life with minimum maintenance is generally required. Free piston Stirling engine (FPSE) has no crank and rotating parts to generate lateral forces and require lubrication. Double acting Stirling engine/generator has one displacer and two power piston which are supported by flexure springs. Two power pistons oscillate with symmetric displacement and are connected with moving magnet type linear generators for power generation from PV work. In experiments, 1 kW class double acting free piston Stirling engine/generator is fabricated and tested. Heat is supplied to hot end of engine by the combustion of natural gas and converted to electric power by linear generators which are assembled with power pistons. The electric parameters such as voltage, current and phase are measured with for variable flow rate of fuel gas. Especially, generating efficiency of FPSE is measured with three different measurement methods. Generating efficiency of the double acting Stirling engine/alternator is about 24%.

A Comparative Study of Open and Closed Heat-Engines for Small-Scale CHP Applications

In this paper the authors compare and contrast open and closed-cycle heat engines. First of all, by way of example and to aid discussion, the performance of proprietary externally heated closed-cycle Stirling engines is compared with that of internally heated open Otto cycle engines. Both types of engine have disadvantages and merits and this suggested that in order to accommodate the best of both engine types an externally-heated open-cycle engine might offer a more satisfactory solution for small-scale combined heat and power (CHP) systems. To investigate this possibility further the paper goes on to compare the performance of externally-heated and recuperated Joule hot-air cycle engines with that of an externally-heated closed Stirling cycle engines. The results show that an externally heated recuperated open Joule cycle engine can exceed that of a closed cycle Stirling engine operating between the same heat source and sink temperatures when a variable temperature heat source is used.

Small scale biomass CHP: Techno-economic performance of steam vs gas turbines with bottoming ORC

Small scale Combined Heat and Power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly valid when biomass fuels are used. In most cases, the use of both heat and electricity to serve on site energy demand is a key to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and it is influenced by climatic conditions. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small scale biomass CHP plants, a bottoming ORC system can be coupled to the cycle, however this decreases the temperature and quantity of cogenerated heat available to the load. In this perspective, the paper proposes a thermo-economic analysis of small scale CHP plants based on steam turbine (ST) or externally fired micro gas turbine (EFGT) coupled to different typologies of bottoming Organic Rankine Cycles (ORC). The research assesses the influence of the thermal energy demand and CHP plant operational strategies on the global energy efficiency and profitability of the proposed cogeneration options, taking into account the part load efficiency and the heat to electricity ratio flexibility that could be achieved through a switch on-off of the bottoming ORC. The thermodynamic cycles and their part load efficiency are modeled by Gate-Cycle (Brayton cycles) and Cycle-Tempo (Rankine cycles). The research explores the profitability of bottoming ORC in view of the higher efficiency and electricity generation revenues but higher costs and reduced heat available for cogeneration in the case of bottoming ORC. The results indicate the optimal CHP technology and configuration for each energy demand segment and the relative key technical and economic factors in the Italian legislative framework.

Thermo-Economic Assessment of a Olive Pomace Gasifier for Cogeneration Applications

Abstract A thermo-economic analysis of a combined heat and power (CHP) plant fed by syngas produced through the gasification of dry olive pomace is presented. The plant is composed by a 800 kWt downdraft gasifier, a gas clean-up system, a 200 kWe microturbine (MGT) and a heat recovery system to cogenerate hot water. Surplus heat is used to dry olive pomace from 50% to 17% wb moisture content. The plant is modeled in ASPEN Plus. Real data from experimental tests are used to calibrate the gasifier model, while the technical specification and performance of the CHP plant are collected from commercial plants in operation and data from manufacturers. Mass and energy balances are reported throughout the paper. The thermodynamic simulation of the biomass gasifier coupled to the MGT, the thermal and electrical conversion efficiency and temperature of cogenerated heat available are also presented. A thermo-economic assessment is then proposed, to investigate the economic profitability of this small scale CHP plant in the Italian energy policy scenario and considering the subsidies available for renewable electricity in the form of feed-in tariffs. For this purpose, the case study of base load CHP plant operation and heat supplied to different typologies of energy end user is assumed. The results allow quantifying the most influencing economic and technical factors that affect the performance and profitability of such investment and the bottlenecks that should be faced to facilitate a broader implementation of such CHP schemes for on site generation.

Experimental characterization of an ORC (organic Rankine cycle) for power and CHP (combined heat and power) applications from low grade heat sources

An ORC (organic Rankine cycle) module, designed and built for a specific CHP (combined heat and power)) application, is tested in this paper. The aim of the work is to characterize the system performance in the operating range allowed by the ORC. For this purpose, a test procedure has been conducted in a test bench. The heat source has been simulated through a natural gas boiler and a thermal oil heat transfer loop to control the temperature in the low grade range of 90 °C–150 °C. The heat sink has been developed using a dry cooler to control the hot water temperature in the range of 30 °C, corresponding to a power application, to 80 °C, of a small-scale CHP application that provides hot water at 90 °C. Thereby, the results show that the thermal power captured by the ORC, electricity and useful heat produced, increase with the rise of the thermal oil temperature and larger pressure ratios. Moreover, the expander electrical isentropic effectiveness is maximized about 70% for a pressure ratio suitable for a CHP system. The cycle efficiency slightly continues increasing for higher pressure ratios, up to a net electrical efficiency of about 8%.

Comparative Assessment of Combined-Heat-and-Power Performance of Small-Scale Aero-Derivative Gas Turbine Cycles

This paper considers comparative assessment of combined-heat-and-power (CHP) performance of three small-scale aero-derivative industrial gas turbine cycles in the petrochemical industry. The bulk of supposedly waste exhaust heat associated with gas turbine operation has necessitated the need for CHP application for greater fuel efficiency. This would render gas turbine cycles environ-mentally-friendly, and more economical. However, choosing a particular engine cycle option for small-scale CHP requires information about performances of CHP engine cycle options. The investigation encompasses comparative assessment of simple cycle (SC), recuperated (RC), and intercooled-recuperated (ICR) small-scale aero-derivative industrial gas turbines combined-heat-and-power (SS-ADIGT-CHP). Small-scale ADIGT engines of 1.567 MW derived from helicopter gas turbines are herein analysed in combined-heat-and-power (CHP) application. It was found that in this category of ADIGT engines, better CHP efficiency is exhibited by RC and ICR cycles than SC engine. The CHP efficiencies of RC, ICR, and SC small-scale ADIGT-CHP cycles were found to be 71%, 60%, and 56% respectively. Also, RC engine produces the highest heat recovery steam generator (HRSG) duty. The HRSG duties were found to be 3171.3 kW for RC, 2621.6 kW for ICR, and 3063.1 kW for SC. These outcomes would actually meet the objective of aiding informed preliminary choice of small-scale ADIGT engine cycle options for CHP application.

Efektywność energetyczna i ekonomiczna perspektywicznych technologii wytwarzania energii elektrycznej

The paper presents the analysis of energy and economic effectiveness of electricity generation technologies of the future in: system power plants, large and medium scale combined heat and power (CHP) plants, and small scale power plants and CHP plants (distributed sources). For particular generation technologies were determined the quantities characterizing their energy effectiveness, unitary emission of CO2 (kg CO2/kWh) and unitary discounted electricity generation costs of 2013. DOI: 10.12736/issn.2300-3022.2014214

Small-scale combined heat and power as a balancing reserve for wind – The case of participation in the German secondary control reserve

Increasing amounts of intermittent renewable energy sources (RES) are being integrated into energy systems worldwide. Due to the nature of these sources, they are found to increase the importance of mechanisms for balancing the electricity system. Small-scale combined heat and power (CHP) plants based on gas have proven their ability to participate in the electricity system balancing, and can hence be used to facilitate an integration of intermittent RES into electricity systems. Within the EU electricity system, balancing reserves have to be procured on a market basis. This paper investigates the ability and challenges of a small-scale CHP plant based on natural gas to participate in the German balancing reserve for secondary control. It is found that CHP plants have to account for more potential losses than traditional power plants. However, it is also found that the effect of these losses can be reduced by increasing the flexibility of the CHP unit.

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.

Experimental Characterization of a T100 Micro Gas Turbine Converted to Full Humid Air Operation

Micro Gas Turbines (mGTs) were considered very promising for small-scale Combined Heat and Power (CHP) production. They however never fully penetrated the CHP market, resulting in a limited market share. Due to their low electric efficiency (30% versus 80% CHP efficiency), the operation of the mGT is strongly heat-driven. In applications with a non-continuous yearly heat demand, like domestic heating systems, the mGT would be shut down for long periods, resulting in a reduced amount of yearly running hours. These limited running hours make the mGT as small-scale CHP less profitable compared to other technologies, like reciprocating engines. Converting the mGT into a micro Humid Air Turbine (mHAT) offers a solution. The waste heat in the exhausted gases is recovered in the mHAT by evaporating auto-raised hot water behind the mGT compressor, resulting in a higher electric efficiency and a more flexible mGT operation. In this paper, we present the results of experiments on a modified Turbec T100 mGT. As a proof of concept, the mGT has been equipped with a spray saturation tower to humidify the compressed air. The goal of these experiments was to evaluate the beneficial effect of compressed air humidification on the mGT performance. Two stable runs at constant pressure ratio and rotation speed were recorded during water injection. Final electric efficiency results show a relative electric efficiency increase of 1.2% and 2.4%. These changes are however in the range of the accuracy of the measurements. Further experiments are therefore necessary to fully characterise the behaviour of the mHAT at part and nominal load. Fine-tuning of the water injection control strategy is also identified as a key parameter to reduce the auxiliary power losses.

Evaluating municipal energy efficiency in biorefinery integration

In this study biomass-based energy production was introduced to an urban city area of Helsinki, Finland. The study compared two cases in integration with a municipality: (1) biomass fuelled small-scale CHP (combined heat and power)-plant and (2) a biorefinery. The comparison was made according to primary energy consumption, primary energy factors, CO2 (carbon dioxide) emissions and the price of produced biowax. It was also studied how results are influenced by different assumptions. The results showed that the primary energy consumption and CO2 emissions were higher in the biorefinery case in absolute amounts as more products i.e. biowax was produced. The results indicated the primary energy factors were almost the same for both cases. Additionally, the primary energy use was very low for district heat and electricity produced in the biorefinery, when the primary energy use of the biorefinery was allocated only to the biowax. The sensitivity analysis of biowax pricing showed that a biorefinery is a competitive alternative for a CHP-plant if the prices of biomass and market electricity are low and the price of CO2 allowance is high. In terms of overall energy efficiency comparison, the comparison cannot be properly completed, because of the different end-products of the plants.