A Techno-economic comparison of micro-cogeneration systems based on polymer electrolyte membrane fuel cell for residential applications
Introduction
Over the last years, micro-CHP PEM fuel cell (PEMFC) based systems overtook all other micro-CHP technology sales in terms of units, becoming a leading technology, also thanks to the high volumes deployed in Japan [1]. As of 2016, over 45,000 system units were sold worldwide for a total power of 200 MW, exceeding the sum of all the other fuel cell technologies [2].
The growing market and development of PEMFC in small scale power production systems relies on their high net electric efficiency, very low pollutant emissions, fast start-up and modularity making them a promising option when applied to distributed cogeneration in the urban areas [3]. The micro-CHP production, decentralized nearby the final user, can contribute to the reduction of greenhouse gas emissions thanks to the simultaneous exploitation of heat and electricity which results in remarkable primary energy savings [4]. Natural gas (NG) is usually considered as reference fuel thanks to its wide diffusion in industrialized countries. Therefore, significant progress on PEMFC based micro-CHP systems relies on the development of an efficient and compact fuel processor capable of converting the light hydrocarbons mixture into pure hydrogen or into a syngas rich in hydrogen with a low content of carbon monoxide CO (below 10 ppm), so suitable for PEM fuel cell application [5].
Conventional LT-PEMFC-based micro-CHP systems have typically an electric efficiency in the range of 30–33% and an overall efficiency in the range of 80–90% [6]. Moreover, both the modelling [7] and the lab-scale prototype tests [8] reveal that the system efficiency at partial load is higher than at design condition, thanks to the reduction of fuel cell losses: this is a remarkable feature for residential applications, generally characterized by fluctuating electrical and thermal loads [9].
In Europe, domestic gas and electricity suppliers, but also boiler manufacturers, are making significant efforts on the development and testing of micro-CHP systems based on PEMFC as well as on Solid Oxide Fuel Cells [10], aiming to achieve reliability and capital cost targets as good as conventional or other low carbon emission technologies [11]. Support schemes, field testing and project demonstrations, as Ene-Farm [12], Ene Field [13] and Callux [14] projects, have led to substantial progress in commercialisation, significant increases in production volumes and consequently considerable cost reductions [15]. Recent studies on experience curve development showed that the investment cost of conventional PEMFC based micro-CHP system could be reduced to around 2300–3800 €/kW for 1–2 kWel system or around 2900 €/kW for a complete larger unit, including also the auxiliary boiler and a hot water tank for thermal storage [16]. One of the main opportunities for micro-CHP cost reductions is system simplification [17], obtained reducing the fuel processor complexity and the BoP components (heat exchangers, connections between valves, sensors, manifolds, etc.) while improving their functional integration. Two new approaches, that are still not at commercial level, could be investigated for this purpose: one takes advantage of adopting an high temperature PEM fuel cell stack, allowing the use of lower purity reformed hydrogen as fuel [18]; another one regards the integration in the reformer reactor of hydrogen perm-selective membranes, thanks to their unique feature of separating pure hydrogen, avoiding the necessity of a multiple 4-stage fuel processor [19]. The first option could simplify the balance of plant with performance comparable to the conventional LT-PEM FC system [20]. The net electrical and total efficiency are in the range of 26–28% and 80–85% respectively [21]. Moreover the availability of high temperature heat makes this technology an attractive solution for the integration in a tri-generation system [22]. While a higher electric efficiency (over 40%) together with the reduction of the fuel processor complexity can be obtained with the second approach, relying on a membrane reactor [23]. The reactor generates pure hydrogen increasing the PEM FC operating voltage, thus the efficiency of the stack [24]; moreover the system takes advantage of a more efficient heat integration [7].
In order to be competitive with other micro-CHP technologies, these systems benefits should correspond with economic benefits reducing the energy bill of a real residential application. A micro-CHP system is usually integrated in a distributed generation scenario as supplier of electricity and heat to one or more residential users, replacing partially or completely the electric grid and the domestic boilers. Latest literature on model-based framework investigated the optimal operating strategy modes of conventional PEMFC-based system [26] and their annual primary energy consumptions [27] and corresponding economic savings [28]. Results showed a reduction of total primary energy use compared to the centralized power scenario (without micro-CHP) producing savings in the operational cost [29] where specific values depend on the micro-CHP size, operational mode [30], different residential loads [31] and the interconnection with other prime mover and thermal [32] or electric storage [33].
This work, developed within Italian project MICROGEN 30 (‘Industry 2015′ program), aims at investigating the benefits of applying to residential users a 10 kWel PEMFC based system with an innovative membrane reformer. The first novel contribution of the analysis is the assessment of the three different PEMFC based micro-CHP solution with the same assumptions and target, carried out in order to compare their performance both at rated power and partial load. The second novel aspect is related to the approach used in the development of a preliminary economic analysis. It is proposed an annual simulation analysis which takes into account different country-specific scenarios in terms of heat and electric load and NG/electricity prices and investigates, through an in-house heuristic model which optimizes the unit operating strategy: (i) the benefits of a distributed small power production with respect to centralized power generation and conventional boilers, in terms of economic saving and fuel consumption; and (ii) the maximum bearable capital cost of the overall system based on PEM fuel cell. Few works studied the complex interaction of the micro-CHP with the residential user in its entire working range with a meaningful objective function. This analysis covers the whole problem, from the micro-CHP design to the short and long term optimization of the micro-CHP system, taking into account also possible government subsides, start-up costs, different scenarios in different countries and detailed hourly energy consumptions profiles.
Section snippets
Micro-CHP PEMFC based system definition
Micro-cogeneration solutions based on PEM fuel cells and natural gas as feedstock are usually based on the concept shown in Fig. 1. The conventional fuel processing chain for producing the required hydrogen consists of: (a) the reformer which can be a steam reformer (SR) or an autothermal reformer (ATR) operating at temperatures above 800 °C, (b) two water gas shift reactors (WGSR), operating at different temperatures (200–350 °C) to promote carbon monoxide conversion to hydrogen, and (c) a
Conventional SR-LT layout
In a conventional fuel processor, shown in Fig. 2, the preheated reactants (stream number {1}), natural gas and steam, are fed to the SR where endothermic reactions are favoured by high temperatures (700–900 °C) and the heat required to sustain the process is supplied by combusting the exhaust fuel from FC anode outlet {7} and additional NG {11}. The heat exchange within the SR reactor is implemented in the model and calculated as counter-current. After cooling, the syngas is sent to water gas
Economic and energy balance methodology
Although results of the thermodynamic analysis both at rated and partial load already evidence a clear advantage of the MR-LT solution in terms of electrical efficiency, an economic analysis is appropriate for understanding the market potentialities of the three configurations when facing a combined heat and power application. Micro-CHP systems are usually integrated in a distributed generation scenario as supplier of electricity and heat to one or more residential users, replacing partially or
Results
Results of the heuristic model are the annual economic savings in the system energy bill (i.e. the total positive cash flow), which are not dependent from the plant capital costs. Fig. 9(left) clearly shows a general increase of the savings when going from few to many dwellings, with few partial exceptions.
Increasing the number of dwellings yields different positive effects:
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the electric and thermal demand is high enough to result in a continuous operation of the micro-CHP system, thus cutting
Conclusion
This work summarized the potentiality of micro-cogeneration proton exchange membrane fuel cell systems to supply two or more residential users in a distributed generation scenario replacing the common electricity and thermal production technology. In particular three solutions were investigated and compared: conventional fuel processor for hydrogen production with a low temperature PEM fuel cell stack, a simpler conventional fuel processor coupled with high temperature PEM fuel cell and an
Acknowledgment
This work was partially funded within the Microgen30 project (EE01_00013) funded by Italian Ministry of Economic Development with the program Industria 2015.
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