Building as a power plant: Modeling and selection of a combined heat and power system for an advanced commercial building

Abstract

In this Mechanical Engineering senior project, combined heat and power (CHP) systems were evaluated based on their effectiveness in supplying the energy requirements of a planned building on the Carnegie Mellon University campus. Initial feasibility studies found that three system types could potentially meet the energy needs of the building in a cost-effective manner: a diesel engine-generator system with heat recovery exchangers, a gas microturbine with an exhaust gas boiler, and a high temperature fuel cell with heat recovery exchangers. Using engineering equation solver (EES) software, the thermodynamics of each system was modeled to assess its useful thermal output for different system sizes. The thermodynamic analysis determined the necessary system size needed to meet the predicted maximum thermal load of the building. The required system sizes are (reported as maximum electrical power output): two 115 kW diesel engine-generator sets, a 250 kW microturbine, or two 250 kW high temperature fuel cells. After sizing the systems, a cash flow analysis model was constructed to evaluate each system under varying assumptions regarding utility rates, fuel costs, and renewable energy incentive programs. Systems utilizing commonly used fuels (natural gas and diesel fuel), as well as biofuels (biogas and biodiesel) were considered in the analysis. Potential systems were also compared to a traditional system comprised of a natural gas fueled HVAC system with building electrical energy needs being supplied by the local utility. The economic model incorporates realistic system and fuel pricing from quotes received from system manufacturers and distributors. The economic model uses operating parameters for each system, along with three different utility rate structures, and calculates the internal rate of return assuming a 20 year system life. Based on the results obtained from the economic analysis, a natural gas fueled microturbine and diesel fueled engine-generator set are best choices for the combined heat and power system. They offer comparable rates of return in each scenario, and are unaffected by the unpredictability of renewable energy buyback incentives. The microturbine offers more adaptability while the diesel engine-generator offers a lower capital investment. © American Society for Engineering Education, 2007.