The widespread use of combined heat and power (CHP) distributed generation (DG) for buildings could significantly increase energy efficiency and reduce greenhouse gas and air pollution emissions. By displacing both electricity from conventional centralized power plants and heat from decentralized boilers, CHP DG could reduce primary feedstock fuel consumption in the U.S. by approximately 20%, or 6,000 terawatt hours. However, optimally integrating CHP DG within buildings is challenging. This work aims to elucidate optimal system sizing and design of micro-CHP fuel cell systems (FCSs) integrated with commercial buildings.
This modeling effort compares and contrasts the performance of high temperature polymer electrolyte membrane (PEM) fuel cell systems (HTPEM FCSs) and solid oxide fuel cell (SOFC) systems for commercial buildings. A parallel research effort is independently analyzing measured data from HTPEM FCSs installed in commercial buildings. Measured data from that effort is integrated into this modeling work. In certain regions, there has been a research and development and commercialization trend moving from using low temperature PEM FCSs (e.g. with a stack temperature of around 80°C) to using HTPEM FCSs (e.g. with a stack temperature of around 160°C) and to using SOFC systems (e.g. with a stack temperature of around 700°C) for CHP building applications, given the higher temperature of the available waste heat from these systems.
In this work FCS performance data is coupled with building energy system models from the U.S. Department of Energy (DOE) using EnergyPlus™ whole-building energy simulation software. Using these baseline reference commercial building model data, parameters are examined including heat demand for space heating and for domestic hot water heating over time, temperatures and water flow rates associated with this heat demand, and building electrical demand over time, to evaluate FCS integration within the building. Examining the data obtained through the simulation exercise in this work, it is found that in a large office building, with heat demand temperatures in the range of 82°C for space heating and 60°C for hot water heating, an HTPEM FCS with an exhaust temperature of 47°C can potentially access, at a maximum, 19% of the total building heating demand. By contrast, in a small office building, with heat demand temperatures in the range of 23°C (supply air temperature) for space heating and 60°C for hot water heating, it is found that this HTPEM FCS can potentially access, at a maximum, 90% of the total building heating demand.
Examining the temporal characteristics of the building heat demand to determine FCS sizing, it is found that a maximum of 50% of the time, the heat demand can be served with an HTPEM FCS with a thermal capacity of 8 kilowatts (kW) (0.05 kW for small office) and an electrical capacity of approximately 4.5 kilowatts-electric (kWe) (0.45 kWe for small office). A maximum of 80% of the time, the heat demand can be served with an HTPEM FCS with a thermal capacity of 85 kW (0.16 kW for small office) and an electrical capacity of approximately 73 kWe (0.14 kWe for small office).
The simulation results further indicate that an SOFC has advantages over an HTPEM FCS that originate from its higher exhaust temperature (between 25°C and 315°C), which allows it to meet a greater percentage of the building heating demand (up to 100%). This enables an SOFC to serve a larger percentage of the building stock and a wider variety of building heating systems. Furthermore, if the CHP FCSs are grid independent (i.e., it is not possible to supply electrical power back to the grid), then the heat-to-power ratio of an FCS can be an important parameter. In such a scenario, the heat-to-power ratio of an SOFC (approximately 0.33) is closer to the heat-to-power ratio of a building (approximately 0.081, averaged over an entire year). In a stand-alone configuration, when the CHP DG has a heat-to-power ratio that more closely matches that of the buildings, the utilization of the DG system is likely to be higher and its economics and environmental impacts more favorable.