micro-CHP System Research Articles

Hybrid solar-biomass residential micro – Combined Heat and Power systems driven by the Partially Evaporating Organic Rankine Cycle – A case study in Southern Europe

In this study, a novel hybrid solar-biomass residential micro – Combined Heat and Power system driven by the Partially Evaporating Organic Rankine Cycle is proposed and its operation is simulated, by applying an in-house numerical model. The simulation tool is applied for a test case in Athens, Greece, where the micro-CHP system operates to cover the annual energy demand of a multi-family building. The proposed cogeneration unit covers 97.10 % and 25.85 % of the modeled building’s annual Space Heating and electricity demand, respectively, by using renewable energy sources. The annual average total energy and exergy efficiencies are equal to 53.33 %, and 10.35 %, respectively, while the power cycle can achieve thermal efficiencies as high as 9.73 %. Competitive Levelized Cost of Electricity (0.072–0.133 €/kWhel), and Levelized Cost of Heat (0.036–0.067 €/kWhth) values are estimated, with a PayBack Period between 5.17 and 9.64 years. The environmental impact of the proposed hybrid micro-CHP is anticipated to be significant, rising to 46.25 kWh of Primary Energy Savings, and 31.22 kg CO2-eq. reduction in carbon emissions per m2 of residence annually.

Rule-based energy management strategies for PEMFC-based micro-CHP systems: A comparative analysis

Commercial PEMFC-based micro-CHP systems are operated by rule-based energy management strategies. Each of these strategies constitutes a different way to meet the household energy demand (following the heat demand, following the electricity demand, the maximum of the two, etc.). Previous studies demonstrate that which of them is the best —i.e. the one that manages to meet the demand at the lowest operating cost— depends on the particular scenario in which the micro-CHP system works (gas and electricity prices, annual energy demands, ability to export electricity to the grid, etc.). This paper aims to explore this dependence relationship and to deepen our understanding of it. To this end, a parametric analysis is conducted and the performances achieved by four rule-based operating strategies are compared. The parameters whose influence is studied, and through which the scenario is jointly characterized, are: (1) energy prices (electricity and natural gas), (2) feed-in tariff, (3) stack degradation, (4) climate and (5) heat to power ratio of the demand. The results show this dependence relationship in a clear and more comprehensive way, and offer a better understanding of its nature. From this improved understanding it can be inferred, among other things, that adapting the strategy to the scenario can generate annual savings of up to 14.5 percentage points. Moreover, this enhanced characterization of that dependence relationship can be useful for the design of a new operating strategy, a strategy that, without falling into the complexity that an optimal energy management approach (based on linear programming) involves, manages to exploit the savings potential of micro-CHP systems, thus facilitating their future mass commercialization.

Residential Micro-CHP system with integrated phase change material thermal energy storage

This paper presents an analysis and experimental investigation of the effect of integrating Phase Change Material (PCM) within a heat exchanger within a Micro Combined Heat and Power (Micro-CHP) system intended for residential applications. A commonly used Micro-CHP layout is replicated in a test rig to characterise the performance of two alternative heat exchanger arrangements. The first is a conventional arrangement where the exhaust gas produced by the prime mover is directed to an air-to-water heat exchanger to heat water and store it in a tank. In the second arrangement, PCM material is encapsulated within specially designed compartments in a heat exchanger to store part of the exhaust thermal energy from the prime mover while heating the water in the storage tank. The time needed to heat the water and discharge heat is compared for both cases, as well as the amount of thermal energy stored during the same operating period. The study also explored the potential to improve the designed unit by using different PCM materials. The test results show that adding 4.7 L capacity paraffin compartments within the heat exchanger extended the discharge time of the hot water by over 400 %, which reflects a marked improvement in the heat storage capacity of the system. This would result in a significant increase in the viable operating period of a CHP system. The one-dimensional analysis revealed that replacing the paraffin with ClimSel C58 PCM can reduce the charging time of the water tank by around 54 % improve the heat storage capacity by factor of 2.35 comparing to Paraffin. The proposed heat exchanger with encapsulated PCM has a potential in other applications such as storage of excess renewable energy or integration with heat pumps to improve matching of supply and demand and thus flexibility of an energy system with integrated intermittent renewables.

Thermoacoustic micro-CHP system for low-grade thermal energy utilization in residential buildings

Effectively utilizing low-grade thermal energy is a promising approach to mitigating greenhouse gas emissions while reducing the burden on centralized power grids. Current thermoacoustic heat pumps and power generators face challenges such as high onset temperature differentials and low performance. This paper addresses these challenges by introducing a gas-liquid resonator into a thermoacoustic combined heat and power systems to recover low-grade thermal energy in residential buildings. Through Sage modeling and calculations, the internal characteristics of the proposed system and its output performance under different operating conditions are explored. At a heating temperature of 350 °C, the system can generate 6.4 kW of output thermal power, 0.9 kW of electricity, and overall exergy efficiency is 79.3 %. Combining neural network models with case studies conducted in Spain and Finland, the system can annually save 5.6 MWh and 20.7 MWh in fuel energy, reduce emissions of 1374 kg and 5180 kg of carbon dioxide, and save a total cost of €611 and €2324, respectively. Furthermore, comparisons with other emerging micro-CHP systems highlight the efficiency of the proposed system. These results indicate the high potential of thermoacoustic combined heat and power systems in recovering low-grade thermal energy and achieving energy savings and emission reductions.

Green hydrogen energy source for a residential fuel cell micro-combined heat and power

In this paper, fuel cell micro combined heat and power with green hydrogen production is investigated for a residential house. Solar and wind energy were investigated for green hydrogen production. The performance of the micro-CHP system was evaluated using yearly energy coverage rate as an index, defined by the energy production and need ratio. A dynamic mathematical model of PEM fuel cell combined with the condensing boiler, supplied by local production of green hydrogen was developed. It was validated based on the yearly measurements in the FC micro-CHP unit installed in a French residential house. The key factors in the micro-CHP operating strategy were the fuel cell heat generation, stop phases, and regeneration which control water temperature, fuel cell life cycle, and annual performance. For conventional residential house in Normandy, the fuel cell operates for a short period during a summer day and the stored hot water was sufficient for heat demand. For winter typical day, the fuel cell operates continuously with maximum heat production. Electricity demand is satisfied and the condensing boiler is used to fulfill heat demand. Operating under heat led strategy, the yearly heat coverage rate is of 100% and the electricity coverage rate is usually upper 100% with 20 to 60% of stored electricity. Impact of the heating degree day and solar irradiance were analyzed using climatic zone factor as an index. Micro-CHP minimum hydrogen self-sufficiency of 57% is obtained in the coldest region where the climatic zone factor is 71°Cm2/W. Micro-CHP 100% hydrogen autonomy is achieved in regions where the climatic zone factor is lower than 39°Cm2/W.

Realistic operation and strategic energy, economic and environmental analyses of a hybrid renewable energy based - micro combined heat and power

The expansion of emerging energy sources that are sustainable and cleaner has opened the chance for CHP systems to be integrated for higher efficiency and performance. Prior to the operation in a real system, the integrated micro-CHP system needs a comprehensive performance analysis. Combining with a realistic energy management, the strategic operation must be dealing with the complexity of the system components at the optimal energy serving cost and keep it environmentally friendly. This study develops and analyses the m-CHP system that integrates renewable energy sources coupled with a realistic load-based management. The components control and system management are developed through a validation of the actual data for each component and integrating them in a simulation environment using LABVIEW to ensure the system works as intended. The comprehensive assessments are conducted in terms of energy, economic and environmental aspects to evaluate the feasibility of implementing the system in the long-term practices. In addition, a critical comparison of the proposed method with the existing projects locally in Malaysia and globally is presented to illustrate the potential of the m-CHP for public and private buildings.During 25 years of the operation, the proposed m-CHP system achieved a primary energy savings ratio of 44.7 %, with a net present value of USD 17,790.42. For electrical and thermal loads, the proposed m-CHP could reduce carbon dioxide emissions by 53.1 % and 73.5 %, respectively.

Micro Combined Heat and Power (micro-CHP) Systems for Household Applications: Techno-economic and Risk Assessment of The Main Prime Movers

ABSTRACT To limit global warming and meet international environmental targets, more environmentally benign technologies must be used in the residential market. Combined Heat and Power (CHP) at the micro-scale (<50 kWe) is seen as one of the best solutions that offers simultaneous generation of both electricity and heat with high overall efficiencies using environmentally friendly fuels (e.g., biofuel, hydrogen, syngas). In this study, four major micro-CHP prime movers (i.e. Micro-gas turbine, Gas engine, Stirling engine, and Fuel cell) have been evaluated in terms of their performance and the currently available technologies and products. The most suitable options for household applications were identified using Political, Economic, Social, Technological, Legal, and Environmental (PESTLE) risk analysis and a Multi-Criteria Decision Analysis (MCDA). Fuel cells display the best environmentally friendly properties despite their price and high start-up time. Gas engines (Reciprocating engines) have the most developed technology with the fastest start-up time and high efficiencies but have vibration and noise issues. Micro-gas turbine micro-CHPs are emerging into the market with relatively cheaper prices, lower maintenance, and good start-up time. Stirling engines are fuel flexible with reasonable prices and available products. Micro-CHP systems, particularly when running on biofuels and hydrogen, provide excellent energy solutions for the future net zero buildings.

Optimizing the use of forestry biomass producer gas in dual fuel engines: A novel emissions reduction strategy for a micro-CHP system

Low energy density fuels, like producer gas from biomass gasification, can be profitably used in compression ignition (CI) engines by means of a proper combustion strategy, such as the dual-fuel mode. Experimental investigations have proven the environmental benefits of the use of producer gas in terms of green-house gas emissions but critical issues related to their use in CI engines have been also highlighted. In particular, under specific operating conditions, the performance and the emissions of the engine might be derated reducing the attractiveness of the technology. In this work a new data-driven emission optimization strategy for a micro-cogeneration system based on an open-top biomass gasification unit coupled with a CI engine running in dual-fuel mode (producer gas/diesel) is presented and discussed. The main purpose of the work is to provide an appropriate tool to address one of the typical problems of dual-fuel systems, namely the nitrogen oxides (NOx) and carbon monoxide (CO) emission trade-off, by calculating the optimal diesel substitution rate (DSR) at a certain required power load. The methodology baseline is the experimental characterization and parametric modeling of the system in terms of NOx, CO, and electrical efficiency (ηe). Then a trade-off objective function (TOF) is defined. The novelties of the proposed approach stands in the building of an ad-hoc designed TOF so as to describe the problem with a single-objective optimization, and thus avoiding the computational and time complexity of a multi-objective optimization. Moreover, a second peculiar aspect is the introduction in the TOF of an internally dependent coefficient (namely the load factor Kl) able to take into account the effect of load and DSR on CO emissions: the adding of such a physical-related feature represents an effective enrichment of the minimization strategy, especially considering the nature of the optimization, which is purely data-driven. Finally, sequential quadratic programming (SQP) minimization of the TOF is carried out by means of the available MATLAB SQP libraries, and two optimization strategies – namely, CO-driven and efficiency-driven - are discussed. Results show (i) promising potentialities in identifying the best effective utilization range of dual-fuel combustion; (ii) high flexibility in the use of the optimization algorithm thanks to the definition of the TOF, which can be easily adapted to different objectives (e.g., NOx mitigation, maintenance of high electrical efficiencies, CO emission bounding) while maintaining external constraints; (iii) high robustness even in the extreme cases in which the imposition of a particular constraint (e.g., high electrical efficiency) is not compatible with the capabilities of the system.

Development of a Dual Fuel ICE-Based Micro-CHP System and Experimental Evaluation of Its Performance at Light Loads Using Natural Gas as Primary Fuel

This study presents the implementation of a micro-generation system and its operation procedure, based on a dual fuel diesel engine using natural gas as the primary fuel and conventional diesel as the pilot fuel. On the other hand, the evaluation and validation results by experimental testing of a model according to International Energy Agency—IEA—Annex 42, applied to dual fuel diesel micro-cogeneration system, are also presented. The control procedure for experimental operation depends of both inputs’ electric power generation demand and desired substitution level due a given natural gas availability. The heat recovery system of the micro-generation system uses a gas–liquid compact heat exchanger that was selected and implemented, where wasted heat from exhaust gases was transferred to liquid water as a cool fluid. Effective operation engine performance was determined by measurement of masses’ flow rate such as inlet air, diesel and natural gas, and also operation parameters such as electric power generation and recovered thermal power were measured. Electric power was generated by using an electric generator and then dissipated as heat by using an electric resistors bank with a dissipation capacity of 18kW. Natural gas fuel was supplied and measured by using a sonic nozzle flowmeter; in addition, natural gas composition was close to 84.7% CH4, 0.74% CO2 and 1.58% N2, with the rest of them as higher hydrocarbons. The highest overall efficiency (electric efficiency plus heat recovery efficiency) was 52.3% at 14.4 kW of electric power generation and 0% of substitution level. Several substitution levels were tested at each engine electric power generation, obtaining the maximum substitution level of 61% at 17.7 kW of electric power generation. Finally, model prediction results were closed to experimental results, both stationary and transient. The maximum error presented was close to 20% associated to thermal efficiency. However, errors for all other variables were lower than 10% for most of micro-cogeneration system operation points.

Employing Taguchi method to optimize the performance of a microscale combined heat and power system with Stirling engine and thermophotovoltaic array

This study proposes a biosyngas-fueled power system, which is a fusion between a micro-thermophotovoltaic (micro-TPV) system and a Stirling engine. The combustor in the micro-TPV is a coaxial tube made of platinum. Biosyngas was simulated using different compositions of H2 and CO mixture. The H2/CO/air mixture was delivered to the inner channel of the micro-TPV combustor, while the CH4/air mixture was delivered to the outer channel. This study realized a micro-CHP system with a combustion-driven TPV cell array, and a Stirling engine-driven power generation system. This micro-CHP system harvests energy generated through thermal radiation from the reactor's surface as well as thermal energy from hot flue gas. The results indicate that the overall efficiency of the biosyngas-fueled micro-CHP system was strongly dependent on fuel composition, fuel/air ratio, and flowrate mixture. Thus, Taguchi method was employed to find optimal operative conditions; the highest efficiency was achieved under a biosyngas composition of 80% CO and 20% H2, with a flow velocity of 6 ms−1, and an equivalence ratio of 1.2, and its corresponding overall efficiency reached 43%, incorporating 0.84 W electricity output by TPV cell array, 3.25 W electricity output by Striling engine-driven power generation system, and 325.5 W of water energy gained.

Energy and exergy analyses of a coal-fired micro-CHP system coupled engine as a domestic solution

In this study, a coal-fired stoker boiler and a beta-type rhombic-drive Stirling engine were manufactured and tested. The performance of the coal boiler with a Stirling engine has been investigated experimentally for the heating system of a three-storey house. The aims of this study are to investigate the Stirling engine's electricity generation potential and thermal energy production. Also, energy and exergy analyses of the micro-CHP system were performed to evaluate thermodynamic performance. Measurements were taken at 10-min intervals for three months. Thus, the temperature values, thermal power, thermal performance, efficiency, energy rates, exergy rates, and electrical parameters of the micro-CHP system can be monitored. The heating demand of the building was covered totally by the micro-CHP system, and the thermal power of the stoker boiler was reduced by approximately 1.29 kW due to the integration of the Stirling engine. The daily total of 2.6 kWh of electricity was generated with the Stirling engine and stored in the battery, and the rest of the electrical energy demand was taken from the grid. If the heating system is supported by PV, it can be operated off-grid. This study will provide insight into the usage of micro-CHP systems in residential applications.

Techno-economic analysis of a combined heat and power system integrating hybrid photovoltaic-thermal collectors, a Stirling engine and energy storage

This paper presents a comprehensive analysis of the energetic, economic and environmental performance of a micro-combined heat and power (CHP) system that comprises 29.5 m2 of hybrid photovoltaic-thermal (PVT) collectors, a 1-kWe Stirling engine (SE) and energy storage. First, a model for the solar micro-CHP system, which includes a validated transient model for the SE micro-CHP unit, is developed. Parametric analyses are performed throughout a year to evaluate the effects of key component sizes and operating parameters, including collector flow rate, storage tank size, SE micro-CHP flow rate, and battery capacity, on the energetic, economic and environmental performance of the proposed system using real hourly weather data, and thermal and electrical energy demand profiles of a detached house located in London (UK). The optimum component sizes and operating parameters are determined accordingly. The daily and monthly operating characteristics of the system are evaluated, and its annual performance is compared to those of a reference system (gas boiler plus grid electricity), as well as of other alternative solar-CHP systems including a PVT-assisted heat pump system and a standalone PVT system. The results indicate that the installation of such a system can achieve an annual electricity self-sufficiency of 87% and an annual thermal energy demand coverage of 99%, along with annual primary energy savings and carbon emission reduction rate of 35% and 37% relative to the reference system. Over 30 years of operation, the net present value (NPV) of the proposed system is £1990 and the discounted payback period is 28 years. The economics of the proposed system is very sensitive to utility prices, especially the electricity purchase price. Relative to the alternative solar systems, the proposed system offers greater environmental benefits but has a longer payback period. This implies that although the energy saving and emission reduction potential of the proposed system is significant, the initial/capital investment, especially of the SE CHP unit and the PVT collector array, are currently high, so efforts should focus on the cost reduction of these technologies.

Experimental Investigation of a Concentrating Bifacial Photovoltaic/Thermal Heat Pump System with a Triangular Trough

The heat absorbed by the heat transfer fluid for cooling a concentrated photovoltaic thermal (CPVT) solar collector can be used for purposes such as residential heating and cooking. Because of the combined production of heat and power, these systems are proposed for individual or commercial use in rural areas. In this study, a hybrid system was proposed to increase the electrical efficiency of the system. Experiments were conducted in winter conditions. Two operational modes were compared, namely a CPVT system with HP (HP-CPVT) and without HP (CPVT). The evaporator of the heat pump was settled inside the triangular trough receiver. The effects of cooling the PV system with a heat pump in the bifacial CPVT system on the electrical and thermal energy efficiencies were investigated. The electricity and thermal energy efficiencies of the CPVT system were calculated as 12.54% and 38.37% in the HP-CPVT system, respectively, and 10.05% and 81.97% in the CPVT system, respectively. The electrical exergy efficiencies of the CPVT system with and without HP were 14.65% and 10.73%, respectively. The thermal exergy efficiencies of the CPVT system with and without HP were 82.47% and 85.63%, respectively. The thermal heat obtained from the HP-CPVT system can be used for heating needs. Thus, the bifacial HP-CPVT system was an example of the micro-CHP system.

Performance Testing and Case Studies of Rural Household Biogas Micro-CHP Systems

Against the backdrop of clean rural heating and dual carbon targets, this paper investigates and develops a household combined heat and power (CHP) unit. The performance of the unit under natural gas operating conditions was tested and evaluated in the laboratory. The test results show that at rated operating conditions the unit has an electrical efficiency of 29% and an overall efficiency of 96%. The micro-CHP was then tested in a civil building in a demonstration site in Xuzhou, Jiangsu province, for trial operation under biogas working conditions. The test results show that when the unit in biogas working condition is heating the room, the unit has a rated working condition power generation of 3.2 KW and still has an average electrical efficiency of about 26.8% and a thermal efficiency of 65.6%, which remains at a high level. And the room temperature data shows that the heat supply meets the heating demand. Finally, this paper compares the performance with micro-CHP units available in the European and American markets. The results show that the performance of this unit is superior to many micro-CHP units available overseas. This paper demonstrates through test results that rural micro-CHP system can meet the daily energy needs of farmers, improve energy use efficiency and reduce carbon emissions.

Parameter uncertainty modeling for multiobjective robust control design. Application to a temperature control system in a proton exchange membrane fuel cell

Advanced control systems are tuned using dynamic models and optimization techniques. This approach frequently involves satisfying multiple conflicting objectives. Tuning robust controllers requires considering a framework that represents the system uncertainties, and its definition is not a trivial task. When dealing with a nonlinear model with many parameters, a high-quality representation requires a massive sampling of variations. In many cases, this represents an inaccessible computational cost for the optimization process.This work presents a new methodology for parameter uncertainty modeling that is oriented to tuning robust controllers based on multiobjective optimization techniques. The uncertainty modeling formulated represents a feasible computational cost and leads to robust solutions without attributing excessive conservatism. The novelty of this process consists in using the multiobjective space to define a set of scenarios with highly representative properties of the global uncertainty framework that formulate the control problem under a predefined minimization strategy.To demonstrate the effectiveness of this methodology, we present a temperature control design in a micro-CHP system under worst-case minimization. Based on the results, particular interest is given to verifying the appropriate formulation of the uncertainty modeling, which represents a 92.8% reduction of the computational cost involved in solving the robust optimization problem under a global uncertainty framework.

Market diffusion strategies for the PEM fuel cell-based micro-CHP systems in the residential sector: scenario analysis

At present, one of the most prominent support mechanisms for sustainable energy is implementing Feed-in Tariffs. This study analyzes Feed-in Tariffs for distributed electricity generation in Iran and Feed-in Tariffs for electricity generated by fuel cells in other countries. Based on reviews of the regulations and the support plans for renewable energy development, CHP generators, and fuel cells, four scenarios were designed for pricing the electricity generated from the fuel cells and how to support its market development. Based on these scenarios, the Feed-in Tariffs of electricity from fuel cells or the expected amount of support for each fuel cell unit was calculated. In the case of using a production tax credit (PTC) model, assuming the total export of the generated electricity to the grid, the cost per kilowatt-hour of electricity varied by 9.89–60.78 cent/kWh based on the utilization of different PEM fuel cell products of different companies. Using Iran's small-scale generator support guideline, the electricity generation cost was calculated between 7.032 and 57.921 cents/kWh.

Advances on a free-piston Stirling engine-based micro-combined heat and power system

As a promising supplement to traditional central electric generation technologies, a distributed combined heat and power (CHP) system is conducive to renewable energy deployment and mitigation of carbon emissions. Among the candidate technologies, the free-piston Stirling engine (FPSE)-based CHP technology is competitive in micro- or small-scale applications. In this work, the performance of a FPSE-based micro-CHP system is improved through a combination of numerical optimization and experimental validation. First, by using a third-order model and thermoacoustic analysis, the impact of regenerator length, displacer moving mass, and force factor of the alternator on the free-piston Stirling generator (FPSG) inside acoustic field, supplied thermal power and exergy loss is studied numerically. The numerical results indicate a rational increase of the force factor, the regenerator length as well as the displacer moving mass is positive for improving the CHP performance. Based on the numerical optimization, the FPSG prototype is updated accordingly, and adequate measurements of the internal acoustic field of the FPSG have been deployed. Then, a series of experiments are conducted to evaluate the CHP performance of the refurbished FPSG prototype. The experimental results show that the highest CHP efficiency reaches 95.7%, the output electric power is 4.03 kW, and the corresponding thermal-to-electric efficiency is 32.2% when the average temperature of supply and return water is as high as 75 °C. Compared with the work before upgrade, the output electric power and CHP efficiency are improved significantly by 39% and 9.4%, respectively. An insight into the impact of the temperature difference between supply and return water indicates that it has a slight impact on electric power generation, especially under higher charge pressure. In contrast, thermal production exhibits an apparent negative correlation with the temperature difference.

4 E analysis of DG set based micro combined heat and power (CHP) system and its employment for shelter heating in high altitude areas

Modern spectrum of combat and warfare calls for increased flexibility in operations of military bases through sustainable and adaptable designs churned out of various technological advancements. Micro CHP system is a niche technology which can be employed to reduce usage of fossil fuel for space heating in military bases and forward areas. Micro CHP systems improve energy efficiency, reduce CO2 emissions, and help to reduce cost of fuel burnt for space heating thereby reducing burden on huge logistic supply chain employed for maintenance of forward logistic bases and isolated posts. Considering their huge cumulative contribution in energy conservation and environment protection such systems have immense potential to be used not only in defence but across the nation in energy sector to help us achieve, goal of “ATMA-NIRBHAR BHAARAT” simultaneously ensuring achievement of national Sustainable Development Goals (SDGs).This paper highlights results of theoretical and experimental analysis carried out on DG set based Micro-CHP system at College of Military Engineering, Pune. It covers Energy, Exergy, Environment and Economic analysis of the system when employed for space heating of military shelters. It includes discussion on design of radiant heating system for military shelters using waste heat energy produced from DG set based Micro-CHP system in high altitude areas. 4E analysis reveals that efficiency of 62.5 kVA DG set is 31 % which after retro-modification into Micro-CHP system increases to 45 %. Exergy efficiency of DG set is 29.9 % which after retro-modification into micro-CHP system increases to 38.9 %. Employment of DG set post retro-modification into micro-CHP system for space heating results in 24 % reduction in carbon dioxide emissions and will lead to savings to the tune of INR 5.6 lacs annually on account of cost of fuel consumed in Bukharies/Kero-heaters used in high altitude areas for heating of military shelters.

Thermal degradation assessment study of a direct vaporization ORC based micro-CHP system under close-to-real operating conditions

• Most of the thermal degradation assessment methods are far from real conditions. • The thermal degradation temperature is affected by the assessment method. • Static approaches can provide over-restrict thermal degradation temperatures. • Surface temperatures don’t need to be limited by values taken from static methods. • Degradation studies must quantify the effective time over the degradation limit. Organic Rankine cycles have the potential to be implemented at a micro-scale and to fulfil the technological gap that is preventing the retrofit of the wall-hang residential combi-boilers by cogeneration systems. Such ability requires the use of high-temperature combustion gases to vaporize the organic fluid and needs to deal with the risk of its thermal degradation. Limits to the fluid bulk temperatures are well known and easily controllable, nevertheless, the relevance of the thermal boundary layer and the temperature of the heat-transfer surfaces over the thermal degradation must be considered and analyzed in depth because of the significant temperature differences between the combustion gases and the organic fluid. With the objective of understanding the thermal degradation temperature limits presented in the literature, an extensive and updated review of the most relevant works was presented. Due to the dispersity of the information regarding the thermal degradation methods and procedures found, a new framework was developed to classify them according to their key essential features: the thermal stress, the degradation assessment and the determination of the limit temperature. The output of that classification is presented together with the degradation temperature value to expose their dependency. The review also allows identifying that the large majority of the thermal degradation temperature values were determined using operating conditions that are far from reality. Complementing the review analysis, this paper offers a first insight into the study of the effect of the thermal boundary layer on its chemical integrity. This is achieved by coupling a detailed comprehensive characterization of the system operating conditions, which comprise the determination of the heat-transfer surface temperatures in contact with the organic fluid, with the realization of dynamic thermal stress tests that include the experimental evaluation of the organic fluid thermal degradation. Operating conditions that lead to heat-transfer surface temperatures well above the threshold found in the literature are considered in this work. The absence of thermal degradation in this situation may indicate that i) the current threshold should not be used to impose upper bounds on the evaporator heat-transfer surface temperature and ii) the time (and not only the temperature) has a significant effect on the thermal degradation of the organic fluid in real operating systems.

Augmented ε-Constraint Algorithm Applied to Multi-objective Optimization Programs of Residential Micro-CHP Systems

Micro-combined heat and power systems (Micro-CHP) are expected to play a major role in reducing carbon dioxide emission, increasing the primary energy and economic saving in the future. In this paper, the optimal planning of a residential Micro-CHP system for a single-family house situated in Iran is investigated. In order to achieve this target, a new mixed-integer linear programming model is developed. Mathematical modeling and optimization were carried out for the Micro-CHP system using natural gas, at the residential building level and for three different operating strategies, cost-driven, primary energy driven, and carbon emission driven. Three competing objective functions are simultaneously minimized using an augmented ε-constraint optimization algorithm. This work has multiple novelties, containing the consideration of some economic and technical constraints previously neglected, such as the energy, economic, and environmental effects of replacing conventional energy systems with a residential Micro-CHP system and the effects of integrating an electrical heating element for storing thermal energy (electrical heating element can act as a power sink when required). A detailed case study on a residential building situated in Tabriz city, Iran, was carried out by applying the developed model and the optimum strategies and optimal sizes for Micro-CHP, axillary boiler, and other equipment are obtained. Results have shown that the optimal values of CSR, PESR, and ERR, in the augmented ε-constraint method, were 12.46%, 1.19%, and 88.38%, respectively. In this case, the obtained result for a nominal capacity of Micro-CHP and boiler was 3.6 kW and 1.05 kW, respectively. Finally, the payback period was obtained for 2 years. Furthermore, to understand the influence of key parameters on the planning of the Micro-CHP system, the sensitivity analysis has been performed on energy prices. Sensitivity analysis of electricity price indicated that, by increasing the electricity price, the overall annual cost-saving value clearly increases, while increasing gas prices significantly reduces the profitability index.