Modeling & simulation of hybrid renewable energy-CHP systems for distributed generation applications

Abstract

Maturing distributed energy resources (DER) have prompted the exploration of alternative energy sources for commercial building applications due to their potential to supply on-site power and heat at lower costs and emissions rates compared to centralized generation. Deployment of renewable energy (RE) technologies and battery storage are of increasing interest as building owners seek to not only reduce utility costs but also address sustainability and resiliency requirements. While several software tools exist for evaluating the techno-economic potential of integrated renewable energy-combined heat and power (CHP) systems for distributed generation applications, many suffer from poor accuracy in capturing off-design (part-load and changes in ambient air temperature and pressure) performance characteristics. The goal of this work is to improve off-design simulation of various CHP technologies, including microturbines, stationary engines, and heat recovery equipment, thereby enabling a higher precision techno-economic evaluation of hybrid RE-CHP technologies in commercial building applications. One of the outcomes of this effort is to increase the capabilities of the Renewable Energy Integration \& Optimization (REopt) tool developed by the National Renewable Energy Laboratory (NREL). This tool seeks to evaluate and maximize the value proposition of applying integrated hybrid RE-CHP systems for specified commercial building applications and serves as a valuable technology screening tool for various industry users. In this thesis, detailed, performance models of microturbines and stationary engines are developed in the gPROMS modeling platform to improve off-design performance predictions in REopt. Off-design modeling of the prime movers includes part-load analysis, assessment of ambient sensitivities, and mapping heat recovery heat exchanger response to hot water or steam grade requirements; the latter two investigations appear to offer a new level of granularity that enables higher accuracy evaluations in REopt. Using model results, baseload operating tests illustrate hourly reactions of power, electric efficiency, and heat recovery that prime movers exhibit when subject to unique ambient temperatures and elevation. This analysis is extended to REopt simulations of hybrid PV-CHP systems subject to local building loads, economic drivers, and a life-cycle-cost minimizing objective function. Simulations reveal that sizing of microturbine-PV systems is highly sensitive to climate conditions; one case study of a particularly hot climate shows a 50 \% decrease in microturbine size and a 70 \% decrease in life-cycle-savings when compared to models using only design-point CHP performance. Likewise, in the same extreme climate, an engine experiencing power derate prompts a 45 \% increase in PV size to offset hourly variation in CHP power capacity. Contrarily, select cooler climates may prompt an increase in CHP sizing upwards of 10 \% due to more efficient operation while maintaining constant capacity. Ultimately, specific climate types heavily affect CHP performance while other types do so marginally. Because of this reality, the study endorses adoption of these methods such that REopt exhibits flexibility to adjust CHP off-design performance according to any site location.