mCHP Unit Research Articles

Dynamic study of hydrogen optimization in the hybrid boiler-fuel cells MCHP unit for eco-friendly house

This paper investigates an innovative study on the performance of a hybrid condensing boiler-fuel cell micro-cogeneration system for single residential applications. It combines experimental and dynamic numerical investigations including renewable energy for green hydrogen production and solid storage. Basing on the hourly measurements for a typical winter day, a dynamic model was developed and validated against different home heating need. Highlighting the influence home energy needs on fuel cell operating mode, it shows that single homes with low energy demand led to reduce the fuel cell's yearly operation below 50 %. In low-energy homes, a reduction in fuel cell size led to an increase in yearly operation to 60-70 % and achieved a 55 % electrical coverage rate. The integration of metal hydride hydrogen solid storage, powered by renewable sources, is emphasized. This setup increases heat demand, influencing fuel cell operation. In winter season, fuel cell operated for an average of 611 h, compared to 61 h in summer. System efficiency averaged 91 % annually, peaked at 95 % in summer, and sustained 93 % when both FC and condensing boiler were switched on.

Performance of Residential Micro-CHP Operating in a District Energy Network

To resolve the problem of the imbalance between micro-Combined Heat and Power (m-CHP) supply and demand, a district energy network has been established based on residential m-CHP units powered by natural gas-fueled internal combustion engine. The electric and thermal loads of a Harbin residential building in a typical winter heating week are calculated. The performance of the m-CHP systems operating in two operation strategies, i.e., Following the Electric Load (FEL) and Following the Thermal Load (FTL), are analyzed and evaluated from both energy and economic aspects. The result shows that the energy cost of the m-CHP is reduced by 10% when operating in FTL mode, and the income from selling excess energy is 2.7 times more than that in FEL mode, indicating lower operation costs in FEL mode. Moreover, compared with the traditional separate production of electricity and heat, the net cost reduction rate of the system in FEL and FTL mode is 13.68% and 38.83%, respectively. Micro-CHP operating in a district energy network is more profitable than traditional separate electricity and heat production.

Experimental Characterization of Transitory Functioning Regimes of a Biomass Stirling Micro-CHP

Micro-cogeneration (micro-combined heat and power) is a technology that simultaneously produces decentralized thermal and electrical energy with a power of less than 50 kWel. This technology consists of using the waste heat generated by a thermodynamic process to meet the heating and hot water demands of buildings. The use of biomass as a fuel offers important advantages: use of a renewable energy, carbon neutrality, availability, and low cost. Furthermore, the analysis and optimization of hybrid energy systems, which include existing micro-cogeneration systems powered by renewable energy, is a scientific challenge needing experimental characterization of such micro-cogeneration systems. In this context, a biomass Stirling micro-CHP unit (μCHP), was tested to characterize its energy performance. A dynamic model based on these experimental investigations was developed to evaluate its thermal power output and energy efficiencies. The dependence of the nominal load on the water flow rate of the consumer and the inlet temperature of the fluid heated by the cogeneration system was studied. Results showed that the flow rate of the heat transfer fluid rejecting heat from the μCHP unit influences the temperature of the heat transfer fluid exiting the μCHP to supply domestic hot water to the user, which, if too high, will prompt the self-guarding mechanism of the machine.

Modelling of an innovative and autonomous micro-grid based on a biomass - solar PV hybrid power system

Micro-Combined heat and power (m-CHP) systems fuelled by renewables, such as residual biomasses, are today of great interest to produce energy in an efficient and green way. The aim of the present work is to develop a numerical model of a hybrid energy system including a biomass powered m-CHP unit based on gasification and coupled with solar photovoltaic (PV) modules and to an electrical storage device, able to provide energy to remote areas. Main advantages of the considered configuration are its ultra-low environmental impact and just its autonomous operation by local resources. The developed unsteady multiphysical model accounts for the main plant components, namely the syngas powered engine, the PV panels and the battery, as well for a dynamic use of the produced energy. Main features of the adopted schematization of the actual system rely on the proper description of syngas combustion, that takes into account the extreme difficulty of working with a non-conventional gas, and on a PID control that is considered for energy flow management to meet the electrical demand curve. The PV panel current is calculated through an ad-hoc function that reads, at any given time, actual solar irradiance data in the city of Naples, Italy, the battery voltage and the main parameters of the PV module. The battery, with a voltage of 48V and a capacity of 100kWh, is modelled as an equivalent circuit with its proper State of Charge (SoC) versus Open Circuit Voltage (OCV) curve. The voltage response to a current at a particular SoC and temperature are calculated. This dynamic modelling permits the optimal control of the whole system to meet the electrical and thermal user’s demand and also to better design any possible change in the storage or PV size to meet specific real uses of the produced energy.

Economic benefits of Smart Microgrids with penetration of DER and mCHP units for non-interconnected islands

The coordinated control of Distributed Energy Resources (DER) allows the full exploitation of DER advantages. Smart Microgrids (SM) seem to be the best solution for the management of modern Low Voltage (LV) grids with DER especially for Non-Interconnected Greek Islands. The main purpose of this paper is to investigate the economic benefits that can be obtained from the coordinated control of DER and micro-Combined Heat and Power (mCHP) Units in SM operation. For each examined scenario, various market prices expressed by the System Marginal Prices (SMP) and DER-mCHP penetrations are considered for annual operation. All different cases are compared between them to determine which one is the most advantageous in terms of operational cost. A typical LV SM, standard DER units and typical market values have been considered to provide initial results. In addition, a sensitivity analysis is carried out in order to determine the effect of installed DER power on grid operation, such as in operating cost, power losses, power injected from the upstream distribution network, local DER production, etc. The presented results show clear the economic benefits in SM operation.

Modelling of the Biomass mCHP Unit for Power Peak Shaving in the Local Electrical Grid

In the article, the method and algorithm for a control strategy of the operation of a micro combined heat and power (mCHP) unit and for reducing the power consumption peaks (peak shaving) are proposed and analyzed. Two scenarios of the mCHP’s operation, namely with and without the control strategy, are discussed. For calculation purposes, a boiler fired with wood pellets coupled with a Stirling engine, manufactured by ÖkoFEN, was used. These results were used to analyze two scenarios of the control strategy. In this study, the operation of mCHP was simulated using the energyPRO software. The application of this control strategy to dispersed mCHP systems allows for a very effective “peak shaving” in the local power grid. The results of calculation using the new algorithm show that the electricity generated by the mCHP system covers the total demand for power during the morning peak and reduces the evening peak by up to 71%. The application of this method also allows for a better reduction of the load of conventional grids, substations, and other equipment.

Comprehensive preliminary experimental performance investigation of a micro-CHP system by using an equipped test room

By using an equipped test room, the performance of a gas engine based micro-CHP (m-CHP) system with the nominal capacity of 49 kW (14 and 35 kW electricity and heat, respectively) is investigated experimentally. Both common operation strategies of m-CHP units, i.e. following electrical and thermal loads are studied. Experiments are done for electrical loads of 7, 8.75, 10.5, 12.25 and 14 kW as well as water outlet temperature of 55, 60, 65, 70 and 75 °C. Subsequently, based on the reported data comprehensive sensitivity analyses to find performance map and beneficial points to retrofit performance of the system are done. The impact of variation of heat and electricity demands on different parameters like caloric input value of fuel, generated heat and electricity, the temperature of exhaust gases and overall efficiency of the system are discussed during the sensitivity analyses. The results show that the possible minimum water outlet temperature is the best magnitude for it. Moreover, using the m-CHP unit in moderate to full electrical loads does not change the values of parameters like overall efficiency. Therefore, the full electrical loading operation is suggested. In this condition, the extra electricity can be either sold or given to the hot water provision system, which does not only have efficiency but also technical and economic benefits.

Comparative assessment of the effectiveness of a free-piston Stirling engine-based micro-cogeneration unit and a heat pump

In this article, a comparison of the thermodynamic and economic effectiveness of two heating systems dedicated to residential applications is presented: a natural gas-fueled micro-cogeneration unit based on a free-piston Stirling engine that generates additional electric energy, and a heat pump system. The measurements of the heat pump system as well as those of the energy (electricity and heat) demand profiles in the analyzed heating season were conducted in a single-family house. The measurements of the μCHP unit were made using a laboratory stand prepared for simulating a variable heat demand. The efficiency of electric energy generation in the μCHP unit was in the range of 7.2%–12.7%, and the overall efficiency was in the range of 88.6%–92.4%. The economic evaluation of the μCHP unit revealed a slight loss compared to the operation of the heat pump throughout the heating season. Sensitivity analysis of the unit price of natural gas and electric energy was performed, and revealed that a reduction in the gas price or an increase in the purchase price of electricity at 2% would have a beneficial financial effect when operating a μCHP system as compared to a heat pump.

Energy management and primary energy optimization of a thermally interconnected semi-autonomous commercial district via optimized μ-CHP dispatch strategy

The paper presents the energy balance and Primary Energy Consumption of a pertinent innovative energy management concept for districts with buildings coupled by thermal and electric micro-grids. A group of centrally controlled μ-CHP units produces heat and power within the district bounds. Power imports/exports to the public grid and heating auxiliary boilers sustain the balance between production and demand. The energy conversion devices (gas boilers, ICE, Stirling, SOFC units and back-up boilers) are considered for two settings: a “Reference” (central power generation and separate gas boilers) and a “μ-CHP” (decentralized μ-CHP units) case. In the latter case the district acts as a small scale energy producer and can export to the market. A fictitious commercial district, located in the central/commercial part of Munich is simulated as an example. The Matlab based, in-house developed software DEPOSIT simulates the complete district and all energy flows. The importance of heat-led control, of primary energy factor following and heat demand forecasting is presented. A primary energy reduction of 12% is shown in the cogeneration case, despite the low electrical PEF for the electricity generation mix in Germany and the very few units installed.

A methodology for thermal analysis of complex integrated systems: Application to a micro-CHP plant

Thermal analysis and management studies are commonly conducted from early design stages of new products by numerical means. However, the application of three-dimensional tools for evaluating the thermal performance of multi-component systems, exhibiting high geometrical and phenomenological complexities may become unpractical in view of the long computational times. Therefore, a methodology for thermal analysis of complex thermal systems is developed to improve the overall numerical convergence rate. The methodology is based on an iterative two-way coupling procedure between two sets of simulations: simulation set where critical components are individual and fully simulated (component-level simulations); and simulation framework where the overall system is considered with hollow components (system-level simulation). A communication strategy between both simulation sets is established. The methodology can be readily applied for performing thermal management studies. A verification procedure for the suggested methodology is firstly carried out. Afterwards, the methodology is applied to investigate the thermal performance at the system-level of a fuel cell based μ-CHP unit, namely regarding the feasibility of components integration. The methodology application allowed the identification of critical hot-spots (with temperatures up to about 400°C) on the surface of the unit internal components, mainly due to an inadequate insulation thickness and shape.

Comparison of different operation modes of a single effect thermal desalination plant using waste heat from m-CHP units

This work is an in-depth analysis of the performance behavior of a small scale single effect thermal desalination plant. The prototype, which refers to the distributed micro cogeneration field, consists of a 1 kWe Stirling engine coupled with a single effect thermal desalination plant for the simultaneous production of electricity and >150 L/day of fresh water.The prototype was able to work in two different operation modes: batch operation and continuous operation. In the former, the evaporator tank is initially filled with the salt water up to its maximum and the evaporation process ends as the salt water level reaches the top of the coil. In the latter, concentrate is continuously extracted and salt water is fed inside the evaporator tank in order to maintain brine salinity almost constant.In a previous work authors investigated the plant performance in batch operation mode finding a good agreement with the predicted results. In this work, plant performance is evaluated in continuous operation and compared to the batch operation mode. In both cases, fresh water production reached a maximum of about 7 L/h in the best operating conditions. However, despite the higher complexity of the plant, performance is worse in the continuous operation mode due to the negative effect of the continuous seawater flow. Indeed, the lower the salt content of the treated water, the lower the fresh water production due to process limits which will be extensively discussed in the paper. More precisely, the plant's average productivities were about 1.3 L/kWh and 1.16 L/kWh of thermal energy input in batch and continuous operation modes respectively.In any case, the apparatus exhibited a very good response to varying thermal power input thus confirming the opportunity to feed the desalination plant also with different forms of waste heat.Hence the proposed solution, studied for a coupling with a 1 kWe Stirling engine, can be easily applied also to the other micro-CHP technologies.

Modeling the dynamic operation of a small fin plate heat exchanger – parametric analysis

Abstract Given its high efficiency, low emissions and multiple fuelling options, the solid oxide fuel cells (SOFC) offer a promising alternative for stationary power generators, especially while engaged in micro-combined heat and power (μ-CHP) units. Despite the fact that the fuel cells are a key component in such power systems, other auxiliaries of the system can play a critical role and therefore require a significant attention. Since SOFC uses a ceramic material as an electrolyte, the high operating temperature (typically of the order of 700–900 °C) is required to achieve sufficient performance. For that reason both the fuel and the oxidant have to be preheated before entering the SOFC stack. Hot gases exiting the fuel cell stack transport substantial amount of energy which has to be partly recovered for preheating streams entering the stack and for heating purposes. Effective thermal integration of the μ-CHP can be achieved only when proper technical measures are used. The ability of efficiently preheating the streams of oxidant and fuel relies on heat exchangers which are present in all possible configurations of power system with solid oxide fuel cells. In this work a compact, fin plate heat exchanger operating in the high temperature regime was under consideration. Dynamic model was proposed for investigation of its performance under the transitional states of the fuel cell system. Heat exchanger was simulated using commercial modeling software. The model includes key geometrical and functional parameters. The working conditions of the power unit with SOFC vary due to the several factors, such as load changes, heating and cooling procedures of the stack and others. These issues affect parameters of the incoming streams to the heat exchanger. The mathematical model of the heat exchanger is based on a set of equations which are simultaneously solved in the iterative process. It enables to define conditions in the outlets of both the hot and the cold sides. Additionally, model can be used for simulating the stand-alone heat exchanger or for investigations of a semiadiabatic unit located in the hotbox of the μ-CHP unit.

Design and test of a single effect thermal desalination plant using waste heat from m-CHP units

This work refers to an innovative integrated system for the simultaneous production of fresh water and electricity. In particular, a 1 kWe Stirling engine coupled with a thermal desalination plant has been considered for the purpose. The prototype, which refers to the distributed micro cogeneration field, has the final aim of building and testing a single effect distillation plant with a fresh water production of about 150 L/d.Firstly, thermodynamic theories and numerical analysis have been carried out to define the final prototype configuration. Then, an experimental test phase has been carried out to evaluate the actual plant performance.The experimental analysis has been in good agreement with the predicted results. In particular, at nominal operating conditions (@50 °C) the maximum heat transfer rate was higher than the evaporator heat exchanger designed condition (5.5 kWt). Despite the non-ideal plant thermal insulation, fresh water production reached a maximum of about 7 L/h at best operating conditions, proving a good process efficiency. According to the behavior predicted by the model, fresh water production is strongly dependent on the temperature difference between the heating fluid and the salt water in the evaporator tank while it is weakly influenced by the salt content of the treated water. Moreover, the apparatus exhibited a very good response to varying thermal power input thus confirming the opportunity to feed the desalination plant also with different forms of waste heat. More precisely, the plant average efficiency was about 1.3 L/kWh of energy input with minimum and maximum values equal to 1.16 and 1.42 L/kWh.Definitely the proposed solution, studied for a coupling with a 1 kWe Stirling engine, can be easily applied also to the other micro-CHP technologies.

Ageing studies of a PEM Fuel Cell stack developed for reformate fuel operation in μCHP units: Development of an accelerated degradation procedure

Proton Exchange Membrane Fuel Cells (PEMFC) durability of stationary systems should be improved for large scales development and commercialization. The aim of this work is to improve degradation understanding in order to propose operating strategies enhancing lifetime and to design specific accelerated tests experiments for micro-CHP application and reformate operation. This paper is focused on results obtained on performance and membrane electrodes assemblies (MEAs) degradation during ageing tests of PEM fuel cells stacks.Durability tests conducted on short stacks over hundreds of hours (up to 900 h) showed how the fuel composition, particularly carbon monoxide content, and the load cycles profiles are impacting on performance degradation rates. The analyses of each cell by electrochemical diagnostics show the modifications of MEAs properties caused by the degradation occurring in both cathode and anode catalyst layers. Results allowed proposing major accelerating features and subsequent relevant accelerated tests, enhancing particularly the ruthenium dissolution identified as the specific limiting mechanism.

Optimisation of the capacity and the dispatch of decentralised micro-CHP systems: A case study for the UK

Cogeneration of heat and electricity constitutes an important pillar in energy and climate policy. The optimal dimensioning and operation of residential micro-combined heat and power (mCHP) systems is an important area of ongoing research, especially because these systems are prohibitively expensive. This paper presents an optimisation model for the capacity and dispatch planning of residential mCHP systems, which is applied with field data from the UK. The method possesses several novelties, including the consideration of technical and economic constraints previously overlooked, such as economies of scale for investments. The application of the model to empirical data demonstrates that economic savings of up to 30% in total annual costs can be achieved through an optimal sizing and operation of a system consisting of a mCHP unit, a peak load boiler and a hot water storage tank. The CO2 emissions in this optimised case are approximately the same as in the case with a mCHP unit only, but substantially (around 22%) below those in a case with a separate generation of heat and electricity from the grid. The new method has been validated in several ways, including a comparison to investments based on constant specific investments, which demonstrate that the latter consistently underestimated the total annual cost by up to 17%. This figure would be higher for larger, or a higher range of electrical CHP capacities. The largest uncertainty relates to the assumed load profiles, from one year, which are taken as representative for the whole time horizon of several years. Further research should focus on more accurately representing discrete operating modi as well as applying the method to other building objects and CHP technologies.

Modeling Platform for a Micro-CHP System with SOFC Operating under Load Changes

Paper presents a novel approach to modeling of a micro-combined heat and power (μ-CHP) unit with solid oxide fuel cells (SOFC). The proposed numerical simulator can be applied both to the analysis of a system operation in the design point and in off-design. Main components of the power system have been represented by dedicated sub-models, incorporated in the numerical simulator of a complete μ-CHP unit. The proposed modeling platform offers the possibility of analyzing system with different solid oxide fuel cells, its operation at partial loads and with various fuels. Components of the system can be modified, technical specifications can be adjusted in order to allow simulation of other components. The main equations for electrical and overall efficiency calculations are given and discussed.

Economic, energy and GHG emissions performance evaluation of a WhisperGen Mk IV Stirling engine μ-CHP unit in a domestic dwelling

This paper presents an assessment of the energy, economic and greenhouse gas emissions performances of a WhisperGen Mk IV Stirling engine μ-CHP unit for use in a conventional house in the Republic of Ireland. The energy performance data used in this study was obtained from a field trial carried out in Belfast, Northern Ireland during the period June 2004–July 2005 by Northern Ireland Electricity and Phoenix Gas working in collaboration with Whispertech UK. A comparative performance analysis between the μ-CHP unit and a condensing gas boiler revealed that the μ-CHP unit resulted in an annual cost saving of €180 with an incremental simple payback period of 13.8years when compared to a condensing gas boiler. Electricity imported from the grid decreased by 20.8% while CO2 emissions decreased by 16.1%. The μ-CHP unit used 2889kWh of gas more than the condensing gas boiler.

Impact on energy requirements and emissions of heat pumps and micro-cogenerators participating in demand side management

The potential impacts of participating in demand side management (DSM) on the performance of air source heat pumps (ASHP) and micro-combined heat and power (mCHP) units are considered by this study. As significant consumers and generators of electricity at the distribution level, large numbers of heat pumps and micro-cogenerators would provide considerable scope for participation in DSM systems. However, it is possible that operating regimes which are optimised for grid considerations will not achieve the maximum performance that is possible from the units.Modelling has been conducted to investigate the significance of this effect, considering the case where local distribution constraints are the main driver for demand side interventions. A model of domestic electrical demand has been adapted to consider a neighbourhood of 128 dwellings in order to identify when interventions are necessary. This has been combined with dynamic models of two combustion engine micro-cogenerators, a solid oxide fuel cell micro-cogenerator and two ASHPs. A simple thermal model of each building is combined with a range of user preferences in order to determine the preferred operating profiles of the heating units.The DSM scheme analysed here is likely to have minimal impact on the emissions and energy requirements associated with each heating unit. Its effect is similar to that which occurs without DSM if the control system gain is relaxed such that equivalent thermal comfort is achieved. DSM can reduce the peak electrical demand of the neighbourhood. However, in the scenarios investigated, it is unlikely that the peaks can be reduced sufficiently such that they do not exceed the capacity of the local distribution transformer if ASHPs are used in all dwellings. By using a combination of mCHP units with ASHPs, it is possible to supply heating to all dwellings without exceeding this capacity. In this case, the use of DSM can increase the ratio of ASHPs used. In the context of a low carbon grid electricity supply, this will reduce the average carbon emissions associated with the neighbourhood.

Performance prediction of micro-CHP systems using simple virtual operating cycles

This paper presents a general methodology to roughly estimate in advance the actual performance of μ-CHP (micro Combined Heat and Power) systems in one year of operation, by means of limited information on the CHP prime mover efficiency and emission factors in selected set points and by means of a simplified prediction model of the operating cycle. The carried out analysis has been applied to several market-available and under development μ-CHP units of different technologies (Internal Combustion Engines, Micro Gas Turbines, Organic Rankine Cycles, Stirling, Thermo Photo Voltaic, Fuel Cell), operated under a hypothetical virtual operating cycle. The virtual cycle is obtained in this paper on the basis of the year thermal demand of a domestic user, assuming thermal load following of the CHP system. The methodology can be generalized to different applications and different management logics of the CHP system.

Load sharing with a local thermal network fed by a microcogenerator: Thermo-economic optimization by means of dynamic simulations

The cogeneration is the combined production of electric and/or mechanical and thermal energy starting by a single energy source; in particular in this paper the analysis will be focused on a cogeneration system with electric power lower than 15 kW (micro-cogeneration). The paper analyzes a system consisting of a natural gas-fired micro-cogeneration unit (MCHP), a heat storage and a peak boiler. The system provides thermal and electric energy to two end-users, the former is a tertiary building (office), where the generation system is located, and the latter is a residential building connected to the former through a district heating micro-grid. In order to analyze the influence of climatic conditions, two different geographical locations in Italy (Benevento and Milano) are considered, that are also characterized by different natural gas and electricity tariffs. Particular attention is paid to the choice of the users, in order to obtain more stable and continuous electric and thermal loads (load sharing approach) and to increase the operating hours per year of the MCHP unit.The operation of the MCHP is governed by a control system, aimed to optimize a thermo-economic objective function. The models representing the components, the thermo-economic objective function and the buildings have been implemented in a widely used commercial software for building simulations. The models are calibrated and validated through data obtained from experimental tests carried out in the laboratory of the University of Sannio (Benevento). The results of the simulations highlight the potential benefits of the thermal load sharing approach. In particular, this study shows that an MCHP unit connected by means of a thermal micro-grid to different users in “load sharing mode” can obtain a high number of operating hours as well as significant energy (Primary Energy Saving) and environmental (avoided CO2 equivalent emissions) benefits with respect to an appropriate reference system, even in Mediterranean areas, where the climatic conditions are not always suitable for cogeneration.