On policy instruments for support of micro combined heat and power

The performance of residential micro combined heat and power (micro-CHP)—a technology to provide heat and some electricity to individual dwellings—is generally dependent on the magnitude of household thermal energy demand. Dwellings with larger and more consistent thermal consumption perform well economically and achieve greater greenhouse gas emissions savings. Consequently, the performance of micro-CHP is dependent on the level of thermal insulation in a dwelling. Therefore, emerging policy approaches regarding energy use in the residential sector, which generally support both energy efficiency measures such as thermal insulation and adoption of micro-CHP, may inadvertently incentivise micro-CHP installation where CO₂ reductions are meagre or not cost-effective. This article examines this issue in terms of the changes in economic and environmental performance that occur for three micro-CHP technologies under changing patterns of residential thermal insulation in the United Kingdom. The results of this analysis are used to comment on the structure of policy instruments that support micro-CHP. It is found that simultaneous support for energy efficiency measures and micro-CHP can be justified, but care must be taken to ensure that the heat-to-power ratio and capacity of the micro-CHP system are appropriate for the expected thermal demand of the target dwelling.     

Application of an improved operational strategy on a PBI fuel cell-based residential system for Danish single-family households

A proposed residential energy system based on the PBI (Polybenzimidazole) fuel cell technology is analyzed in terms of operational performance. Conventional operational strategies, such as heat-led and electricity-led, are applied to the simulated system to investigate their performance characteristics. Based on these findings, an improved operational strategy is formulated and applied in an attempt to minimize the shortcomings of conventional strategies. System parameters, such as electrical and thermal efficiencies, heat dumping, and import/export of electricity, are analyzed. The applied load profile is based on average data for a single-family household in Denmark and includes consumption data for electricity and heat demands. The study analyzes the potential of the proposed system on market penetration in the area of residential heat-and-power generation and whether this deployment can be justified as compared to other micro-CHP system technologies. The most important findings of this research study indicate that in comparison to non-fuel cell-based micro-CHP systems, such as Stirling Engine-based systems, the proposed system has significantly higher efficiencies. Moreover, the lower heat-to-power ratios allow the system to avoid high thermal surpluses throughout the whole annual operational profile.     

Cost-effective operating strategy for residential micro-combined heat and power

It is commonly assumed that dispatch of micro-combined heat and power (micro-CHP) should be heat driven, where the unit turns on when a heat load is present, and turns off or modulates when there is little or no heat demand. However, this heat led operating strategy—typical of large-scale CHP applications—may not be economically justified as scale decreases. This article investigates cost-effective operating strategies for three micro-CHP technologies; Stirling engine, gas engine, and solid oxide fuel cell (SOFC), under reasonable estimates of energy prices. The cost of meeting a typical UK residential energy demand is calculated for hypothetical heat led and electricity led operating strategies, and compared with that of an optimal strategy. Using central estimates of price parameters, and with some thermal energy storage present in the system, it is shown that the least cost operating strategy for the three technologies is to follow heat and electricity load during winter months, rather than using either heat demand or electricity demand as the only dispatch signal. Least cost operating strategy varies between technologies in summer months. In terms of environmental outcomes, the least cost operating strategy does not always result in the lowest carbon dioxide emissions. The results obtained are sensitive to electricity buy-back rate.     

The capacity credit of micro-combined heat and power

This article is concerned with development of a methodology to determine the capacity credit of micro-combined heat and power (micro-CHP), and application of the method for the UK. Capacity credit is an important parameter in electricity system planning because it measures the amount of conventional generation that would be displaced by an alternative technology. Firstly, a mathematical formulation is presented. Capacity credit is then calculated for three types of micro-CHP units—Stirling engine, internal combustion engine, and fuel cell systems—operating under various control strategies. It is found that low heat-to-power ratio fuel cell technologies achieve the highest capacity credit of approximately 85% for a 1.1 GW penetration when a heat-led control strategy is applied. Higher heat-to-power ratio Stirling engine technology achieves approximately 33% capacity credit for heat-led operation. Low heat-to-power ratio technologies achieve higher capacity credit because they are able to continue operating even when heat demand is relatively low. Capacity credit diminishes as penetration of the technology increases. Overall, the high capacity credit of micro-CHP contributes to the viewpoint that the technology can help meet a number of economic and environmental energy policy aims.     

Impacts of temporal precision in optimisation modelling of micro-Combined Heat and Power

When modelling the environmental and economic aspects of meeting a given heat and power demand with a combination of combined heat and power (CHP) and grid power, it is common to use a coarse temporal precision such as 1-h demand blocks in heat and power demand data. This may be appropriate for larger applications where demand is reasonably smooth, but becomes questionable for applications where demand exhibits substantial volatility such as for a single residential dwelling—an important potential market for the commercialisation of small-scale fuel cells and other micro-CHP. Choice of temporal precision is also influenced by the relative ease in obtaining coarse data, their compatibility with available energy price data, and avoidance of computational overheads when data sets expand. The thesis of this paper is that use of such coarse temporal precision leads to averaging effects that result in misleading environmental and economic outcomes for cost-optimal micro-CHP systems. Much finer temporal precision is required to capture adequately the specific characteristics of residential energy demand and the technical qualities of solid oxide fuel cell and stirling engine micro-CHP systems. This thesis is generally supported by the results of analysis, which shows that in some cases optimal design generation capacity of the CHP system is reduced by more than half between analyses using 1-h precision and 5-min precision energy demand data. When optimal dispatch of given generator and boiler capacities is considered, the quantities of energy delivered by the various components of the energy provision system (i.e. generation from CHP, heat from CHP, heat from an additional boiler, electricity from grid) varied by up to 40% between precisions analysed. Total CO₂ emissions reduction is overestimated by up to 40% by the analyses completed using coarse demand data for a given micro-CHP generator capacity. The economic difference is also significant at up to 8% of lifetime costs for a given micro-CHP generator capacity.     

The financial viability of an SOFC cogeneration system in single-family dwellings

In the near future, fuel cell-based residential micro-CHP systems will compete with traditional methods of energy supply. A micro-CHP system may be considered viable if its incremental capital cost compared to its competitors equals to cumulated savings during a given period of time. A simplified model is developed in this study to estimate the operation of a residential solid oxide fuel cell (SOFC) system. A comparative assessment of the SOFC system vis-à-vis heating systems based on gas, oil and electricity is conducted using the simplified model for a single-family house located in Ottawa and Vancouver. The energy consumption of the house is estimated using the HOT2000 building simulation program. A financial analysis is carried out to evaluate the sensitivity of the maximum allowable capital cost with respect to system sizing, acceptable payback period, energy price and the electricity buyback strategy of an energy utility. Based on the financial analysis, small (1–2 kW_e) SOFC systems seem to be feasible in the considered case. The present study shows also that an SOFC system is especially an alternative to heating systems based on oil and electrical furnaces.     

Techno-economic modelling of a solid oxide fuel cell stack for micro combined heat and power

Solid oxide fuel cell combined heat and power (CHP) is a promising technology to serve electricity and heat demands. In order to analyse the potential of the technology, a detailed techno-economic energy-cost minimisation model of a micro-CHP system is developed drawing on steady-state and dynamic SOFC stack models and power converter design. This model is applied it to identify minimum costs and optimum stack capacities under various current density change constraints. Firstly, a characterisation of the system electrical efficiency is developed through the combination of stack efficiency profiles and power converter efficiency profiles. Optimisation model constraints are then developed, including a limitation in the change of current density (A cm⁻²) per minute in the stack. The optimisation model is then presented and further expanded to account for the inability of a stack to respond instantaneously to load changes, resulting in a penalty function being applied to the objective function proportional to the size of load changes being serviced by the stack. Finally, the optimisation model is applied to examine the relative importance, in terms of minimum cost and optimum stack maximum electrical power output capacity, of the limitation on rate of current density change for a UK residential micro-CHP application. It is found that constraints on the rate of change in current density are not an important design parameter from an economic perspective.     

Thermoeconomic modeling of micro-CHP (micro-cooling,heating, and power) for small commercial applications

The increasing demand for electrical power as well as energy for heating and cooling of residences and small commercial buildings is a growing worldwide concern. Micro-cooling, heating, and power (micro-CHP), typically designated as less than 30 kW electric, is decentralized electricity generation coupled with thermally activated components for residential and small commercial applications. The number of combinations of components and parameters in a micro-CHP system is too many to be designed through experimental work alone. Therefore, theoretical models for different micro-CHP components and complete micro-CHP systems are needed to facilitate the design of these systems and to study their performance. This paper presents a model for micro-CHP systems for residential and small commercial applications. Some of the results that can be obtained using the developed model include the cost per month of operation of using micro-CHP versus conventional technologies, the amount of fuel per month required to run micro-CHP systems, the overall efficiency of micro-CHP systems, etc. A case study is used to demonstrate differences in the system performances of micro-CHP systems driven by a natural gas internal combustion engine and a diesel engine. Some of the results show that both systems have similar performance and that system total efficiencies in cooler months of up to 80% could be obtained. Also, modeling results show that there is a limit in fuel price that economically prevents the use of CHP systems, which is $11 MBTU⁻¹ for this specific case. Copyright © 2008 John Wiley & Sons, Ltd.     

Fuel cell micro-CHP techno-economics: Part 2 – Model application to consider the economic and environmental impact of stack degradation

This article presents a literature review regarding the mechanisms of fuel cell degradation, accompanied by the reported range of observed degradation rates in experimental, demonstration and early commercial systems. It then synthesises and exploits this information to investigate the influence of degradation on the economic and environmental credentials of fuel cell micro-combined heat and power (micro-CHP) for the UK residential sector. The investigation applies a techno-economic model developed in the companion article designed to demarcate the key characteristics of commercially successful systems. Two distinct modes of degradation are examined; one proportional to power density in the stack, and the other proportional to thermal-cycling rate of the stack. It is found that limiting the power-density related degradation rate is very important from economic and environmental viewpoints, but thermal-cycling related degradation is less important when thermal energy storage is available because cycling can be avoided. Furthermore it is noted that techno-economic studies that ignore degradation can overestimate the marginal value of a micro-CHP system with respect to the conventional alternative by up to 45% and the CO₂ emissions reduction potential by up to 57%, for performance degradation rates of 2% per MW_eh output. This conclusion is noteworthy because most techno-economic analyses of fuel cells ignore degradation, potentially providing misleading results. Finally it is concluded that existing commercial degradation targets, such as the SECA targets, are appropriate for achieving marketable systems.     

Energy harvesting, reuse and upgrade to reduce primary energy usage in the USA

Two-thirds of input energy for electricity generation in the USA is lost as heat during conversion processes. Additionally, 12.5% of primary fuel and 20.3% of electricity are employed for space heating, water heating, and refrigeration where low-grade heat could suffice. The potential for harnessing waste heat from power generation and thermal processes to perform such tasks is assessed. By matching power plant outlet streams with applications at corresponding temperature ranges, sufficient waste heat is identified to satisfy all USA space and water heating needs. Sufficient high temperature exhaust from power plants is identified to satisfy 27% of residential air conditioning with thermally activated refrigeration, or all industrial refrigeration and process heating from 100 to 150 °C. Engine coolant and exhaust is sufficient to satisfy all air conditioning and 68% of electrical demands in vehicles. Overall, this study demonstrates the potential to reduce USA primary energy demand by 12% and CO₂ emissions by 13% through waste heat recovery. A detailed analysis of thermal energy demand in pulp and paper manufacturing is conducted to demonstrate the methodology for improving the fidelity of this approach. These results can inform infrastructure and development to capture heat that would be lost today, substantially reducing USA energy intensity.     

Performance evaluation of different configurations of biogas-fuelled SOFC micro-CHP systems for residential applications

Three configurations of solid oxide fuel cell (SOFC) micro-combined heat and power (micro-CHP) systems are studied with a particular emphasis on the application for single-family detached dwellings. Biogas is considered to be the primary fuel for the systems studied. In each system, a different method is used for processing the biogas fuel to prevent carbon deposition over the anode of the cells used in the SOFC stack. The anode exit gas recirculation, steam reforming, and partial oxidation are the methods employed in systems I–III, respectively. The results predicted through computer simulation of these systems confirm that the net AC electrical efficiency of around 42.4%, 41.7% and 33.9% are attainable for systems I–III, respectively. Depending on the size, location and building type and design, all the systems studied are suitable to provide the domestic hot water and electric power demands for residential dwellings. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required for the SOFC stack to generate around 1 kW net AC electric power, the thermal-to-electric ratio (TER), the net AC electrical and CHP efficiencies, the biogas fuel consumption, and the excess air required for controlling the SOFC stack temperature is also studied through a detailed sensitivity analysis. The results point out that the cell design voltage is higher than the cell voltage at which the minimum number of cells is obtained for the SOFC stack.     

Modeling and off-design performance of a 1 kWe HT-PEMFC (high temperature-proton exchange membrane fuel cell)-based residential micro-CHP (combined-heat-and-power) system for Danish single-family households

A novel proposal for the modeling and operation of a micro-CHP (combined-heat-and-power) residential system based on HT-PEMFC (High Temperature-Proton Exchange Membrane Fuel Cell) technology is described and analyzed to investigate its commercialization prospects. An HT-PEMFC operates at elevated temperatures, as compared to Nafion-based PEMFCs and therefore can be a significant candidate for cogeneration residential systems. The proposed system can provide electric power, hot water, and space heating for a typical Danish single-family household. A complete fuel processing subsystem, with all necessary BOP (balance-of-plant) components, is modeled and coupled to the fuel cell stack subsystem. The micro-CHP system is simulated in LabVIEW™ environment to provide the ability of Data Acquisition of actual components and thereby more realistic design in the future. A part-load study has been conducted to indicate performance characteristics at off-design conditions. The system is sized to provide realistic dimensioning of the actual system.     

Fuel cell micro-CHP techno-economics: Part 1 – model concept and formulation

This article presents the concept and mathematical treatment for a techno-economic modelling framework designed to enable exploration of fuel cell micro combined heat and power (micro-CHP) system design and control. The aim is to provide a tool that can help to focus research and development attention on the system characteristics critical for commercial success of these technologies, present cost targets for developers, and to ensure policy makers provide appropriate instruments to support commercialisation. The model is distinctive in that it applies mixed integer unit commitment formulation to link design and control decisions for micro-CHP, and explicitly characterises stack degradation in a techno-economic framework. It is structured to provide depiction of the fuel cell stack and balance-of-plant, supplementary thermal-only system (e.g. tail gas burner), thermal energy storage, and electrical power storage. Technically, the fuel cell stack is characterised by steady-state thermal and electrical efficiencies for full and part-load operation, its nameplate capacity, minimum operating set-point, and stack degradation via performance loss rate proportional to power density and thermal cycling rate. The dynamics of operation are emulated via ramp limits, minimum up-time and minimum down-time constraints, and start-up and shutdown costs and energy consumptions. The primary performance evaluation metric adopted is the maximum additional capital cost a rational investor would pay for the fuel cell micro-CHP system over and above what they would pay for a competing conventional heating system. The companion article (Part 2) applies the developed model to consider the impact of stack degradation on economic and environmental performance.     

Controlling micro-CHP systems to modulate electrical load profiles

As micro-CHP systems move towards mass deployment an increasing emphasis will be placed on their effect on time-varying demands for network electricity. A 50 dwelling data set of heat and power demands was employed to investigate the implementation of various penetrations of μCHP system on the resultant electrical load profile using two control methodologies: heat-led and a proposed method for modulating the aggregate electrical load. The first caused the daily load factor of the net load profile to decrease from 42.5% to 28.6% on a January day and the after diversity maximum demand to decrease from 2.0 to 1.2 kW. The second caused the daily load factor to increase from 42.5% to 48.6% and the after diversity maximum demand to decrease from 2.0 to 0.9 kW. The extent to which these improvements in load factor can be achieved was investigated in detail and maximum resultant load factor values were identified for a day in January, April and July. Further improvements in the modulating capability of this control approach may be realised if prime movers capable of rapid start-up, shut-down and cycling can be developed. The control of micro-CHP systems in this manner offers a mechanism for managing the load at distribution transformers.     

建筑形式对太阳能热利用的影响研究

以上海地区的住宅建筑为研究对象,通过模拟分析的方法,采用DeST软件计算确定建筑逐时的采暖、空调能耗,研究分析窗墙比对建筑全年采暖能耗、全年空调能耗以及全年采暖、空调总能耗的影响规律,研究分析太阳辐射热增加所导致采暖能耗的降低幅度与外围护结构保温性能两者之间的定量关系。计算结果表示在夏季外窗遮阳和夜间通风的条件下,加大南向窗墙比可增强太阳能的热利用效率,降低建筑全年的采暖、空调总能耗;而外围护结构保温性能的增强则可降低室内向室外散热的程度,相应提高对冬季太阳辐射的热利用程度,从而达到降低采暖能耗的目的。     

1kW家用SOFC-CHP系统建模及性能分析

构建一个以天然气为燃料的SOFC-CHP系统,推导SOFC传热传质及电化学方程,建立各个组件的数学模型,编写计算程序,对发电功率为1kW的家用SOFC-CHP系统在设计工况下进行性能模拟并探讨不同系统参数对性能的影响。计算结果表明:在设计工况下,系统热电联供效率远高于电池单独发电的效率;此外,随着燃料入口流量的增大,系统发电功率存在一个最大值,燃料利用率与发电效率不断减小,系统热电联供效率不断增大,SOFC的温度梯度分布则越来越平缓;同时发现降低过量空气系数可以提高该CHP系统的性能。     

Thermoeconomic assessment of a multi-engine,multi-heat-pump CCHP (combined cooling,heating and power generation) system – A case study

Design and operation of complex systems for combined cooling, heating and power generation (CCHP) are always a matter of matching performance and demand characteristics of a thermal system set to supply electrical, cooling and heating loads, according to specific usage demands. Equipment selection and operation require the characterization of power, heating and cooling load demands, and their time variation during years, seasons, months and even hours or minutes. The paper aims at utilizing a general model for complex CCHP systems. The proposed model is based on the general theory of exergy cost and structural coefficients of internal links. A general model is presented, and a simple hypothetical cogeneration case is studied. The system operates with two heat engines, with waste heat recovery driving a chiller, in order to meet electrical power and refrigeration loads.     

Optimization modeling method for coal-to-electricity heating load considering differential decisions

Heating by electricity rather than coal is considered one effective way to reduce environmental problems. Thus, the electric heating load is growing rapidly, which may cause undesired problems in distribution grids because of the randomness and dispersed integration of the load. However, the electric heating load may also function as an energy storage system with optimal operational control. Therefore, the optimal modeling of electric heating load characteristics, considering its randomness, is important for grid planning and construction. In this study, the heating loads of distributed residential users in a certain area are modeled based on the Fanger thermal comfort equation and the predicted mean vote thermal comfort index calculation method. Different temperatures are considered while modeling the users’ heating loads. The heat load demand curve is estimated according to the time-varying equation of interior temperature. A multi-objective optimization model for the electric heating load with heat energy storage is then studied considering the demand response (DR), which optimizes economy and the comfort index. A fuzzy decision method is proposed, considering the factors influencing DR behavior. Finally, the validity of the proposed model is verified by simulations. The results show that the proposed model performs better than the traditional method.     

Domestic micro-generation: Economic,regulatory and policy issues for the UK

Micro-generation in individual homes has been the subject of increasing policy and industry attention in recent years. Whilst it has been estimated that micro-generation could meet 30–40% of UK electricity demand by 2050, deployment to date has been slow. In its Micro-generation Strategy the UK government has started to outline how deployment could be increased. Various technical, economic, behavioural and institutional changes are needed to establish a UK market for micro-generation. This article discusses how different deployment models for domestic micro-generation might attract investments in these technologies. It considers not only investments by individual households but also by energy companies. Starting from an economic analysis of payback times for three different technologies (micro-CHP, micro-wind and solar PV) it identifies policy and regulatory recommendations. It argues for technology-specific support policies in the short term. It also suggests that a ‘level playing field’ for micro-generation technologies as a result of fiscal and market reforms could considerably increase the attractiveness of micro-generation technologies.     

Heat distribution and the future competitiveness of district heating

The competitiveness of present and future district heating systems can be at risk when residential and service sector heat demands are expected to decrease in the future. In this study, the future competitiveness of district heating has been examined by an in depth analysis of the distribution capital cost at various city characteristics, city sizes, and heat demands. Hereby, this study explores an important market condition often neglected or badly recognised in traditional comparisons between centralised and decentralised heat supply.