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Abstract
Micro-cogeneration or micro-combined heat and power, which can meet both the electrical and thermal needs of the residential sector, is growing and can offer reduced energy usage and emission. Micro-combined heat and power differs from typical combined heat and power plants in size and in the way these systems are often controlled to follow the space heating load as opposed to the electrical load. Among the different technologies available, fuel cell-based micro-combined heat and power fundamentally differs from the combustion-based technologies and has the potential to achieve high electrical efficiency, low emissions, reduced noise, and reduced energy consumption for the residential sector. However, these benefits are not always realized by fuel cell micro-combined heat and power as they depend on factors such as start-up/shutdown time, turndown capability, and the application, among other things. While the opportunities are clear and well-documented in the literature, the means of assessing fuel cell micro-combined heat and power energy usage, efficiency, and emissions varies. Advanced models and methods, such as the quasi-two-dimensional or three-dimensional approach, have been developed for fuel cell system analysis, but simplified models are often used as part of the building integrated micro-combined heat and power assessment. Many of the building integrated micro-combined heat and power assessments only investigate the average annual energy usage and the associated greenhouse gas emissions of the micro-combined heat and power systems without fully accounting for the seasonal and day-to-day variations in the load and supply. Differences in the fuel cell models used for building integrated micro-combined heat and power assessment has led to conflicting results and conclusions about the potential for this technology in the residential sector. A literature review is conducted in this work to assess the opportunities and obstacles for fuel cell micro-combined heat and power in the residential sector and the methods of assessing these systems. The review reveals that certain fuel cell model types, such as the black box approach, tend to give inconsistent results when the assumptions do not align with the actual operation of the fuel cell micro-combined heat and power system. Recommendations are made for employing these models while assessing building integrated fuel cell micro-combined heat and power systems. A unified approach that integrates more sophisticated fuel cell micro-combined heat and power models into building integrated energy performance simulation platforms is recommended.
Nomenclature
| Abbreviations | ||
| AC | = | alternating current |
| BoP | = | balance of plant |
| DC | = | direct current |
| Mchp | = | micro-combined heat and power |
| TER | = | thermal electric ratio |
| Symbols | ||
| Ai | = | surface area of ith enclosure |
| = | body forces | |
| Di | = | diffusion coefficient for species i |
| E | = | reversible cell potential |
| e | = | specific energy |
| Erev0 | = | reversible cell potential at standard conditions |
| F | = | Faradays constant (96,485 C.mol−1) |
| = | heat flux | |
| Qstack | = | stack heat generation |
| Qinternal | = | heat recovered for internal preheat and reforming |
| i0 | = | exchanger current density |
| iL | = | limiting current density |
| LHV | = | lower heating value of the fuel |
| ṁfuel | = | mass flow rate of fuel |
| = | mass flux of species i | |
| N | = | number of moles |
| n | = | number of electrons per mole of fuel |
| P | = | pressure |
| PAC | = | AC power output from fuel cell system |
| Paux | = | power for auxiliary equipment |
| Pstack | = | electrical power output of fuel cell stack |
| Pth | = | thermal power output of the fuel cell mCHP |
| Q | = | building heating load |
| R | = | universal gas constant |
| Rc | = | cathode equivalent resistance |
| S | = | volumetric heat source term |
| T | = | temperature |
| t | = | time |
| TER | = | thermal electric ratio |
| Ufv | = | fuel utilization |
| Ui | = | overall heat transfer coefficient of ith enclosure |
| = | velocity vector | |
| V | = | volume |
| Vstack | = | stack voltage |
| Vth,stack | = | thermoneutral voltage of the stack |
| w | = | source/sink of mass |
| wi | = | source/sink of species i |
| Yi | = | mass fraction of species i |
| α | = | apparent charge transfer coefficient |
| ρ | = | density |
| ρj | = | material resistivity |
| ηact | = | activation polarization |
| ηconc | = | concentration polarization |
| ηohm | = | ohmic polarization |
| μ | = | dynamic viscosity |
| Φ | = | equivalence ratio |
| ϵstack | = | fuel cell stack electrical efficiency |
| ϵCHP | = | efficiency of the mCHP system at design loads |
| ϵDC/AC | = | efficiency of the DC/AC converter |
| Δg0 | = | change in Gibbs free energy between products and reactants at standard conditions |
| Δs | = | change in entropy between products and reactants |
| ΔT | = | temperature difference between indoor setpoint and design heating load temperature |
| λ | = | thermal conductivity |