Categorizing Hybrid Renewable Energy Systems
By John Whitney, AIA
What circumstances make hybrid power systems reasonable, economical, and/or highly desirable?
This is a work in progress, but I would like to achieve more clarity in my thinking about hybrid renewable energy systems. Here’s what I’ve got so far (I’m looking for feedback):
By their very nature, hybrid energy systems are developed to solve a problem with a stand-alone generation system. The problem can be situational (remote location makes access to conventional fuels and power grid difficult or impossible) or it can be a system-specific shortcoming (i.e., the intermittent nature of some renewable generation technologies). Additional stimulus for their use may include environmental (desire to lessen impact of conventional generation plant), or economical (use of multiple energy sources to drive a shared generation technology and transmission access) motives.
My working definition of hybrid energy systems:
Hybrid power systems combine two or more energy conversion mechanisms, or two or more fuels for the same mechanism, that when integrated, overcome limitations inherent in either. Hybrid systems provide a high level of energy security and reliability through the integrated mix of complementary generation methods, and often will incorporate a storage system (battery, fuel cell) or fossil-fueled power generation to ensure consistent supply.
Hybrid Energy Systems | Situational Issues
Remote locations and islands have been the obvious and traditional use for hybrid power generation. Located beyond the reach of the conventional power grid and requiring special shipment of fuel to power generation plants, they pose a problem that can, in many places, be solved with renewable hybrids. The cost of extending the conventional electrical grid to these locations is typically cost-prohibitive if not impossible.
In these locations hybrid renewable energy systems, while costly, are typically more economical (and reliable) than conventional fossil-fuel power generation. Examples will frequently include a mix locally available renewable intermittent generation (wind or solar), reliable baseload generation (diesel or biodiesel generation), and storage (batteries, fuel cells, pumped hydro).
Hybrid Energy Systems | System-Specific Shortcomings
By their nature, some renewable energy technologies (wind, solar, and run-of-river hydro) provide intermittent generation. This lack of consistent, reliable power as required by demand can stresses utilities which will then often place limits on how much intermittent power their grid can absorb.
This issue can be somewhat offset if, prior to feeding power into the utility grid, the renewable energy generation plant can combine complementary generation technologies to provide more reliable and consistent service. Examples can include paired renewable technologies (CSP + biomass, PV + wind, or wind + biodiesel), renewable generation plus storage (wind + hydrogen generation + fuel cell, wind + pumped hydro, wind/PV + batteries), or renewable plus conventional generation (CSP + combined-cycle gas). In each of these hybrid systems, technologies are integrated and the power that is fed into the utility grid is significantly more (if not completely) consistent and predictable.
Hybrid Energy Systems | Environmental Amelioration
We have no magic wand and the transition to a renewable energy future will be years in the making. However, during our long interregnum, there are strategies that can be implemented to lessen the environmental impact of conventional generation plants. Among these efforts is the use of renewable energy “fuel” to assist/ augment in the generation of power at a fossil-fuel power plant. Examples may include dual-fuel combustion (coal + biomass) and integrated solar combined-cycle (CSP + natural gas). In these cases, the use of renewable generation, when available, allows for the ramping back of fossil-fuel consumption to operate the power plant.
Hybrid Energy Systems | Economic Model Enhancement
Because many renewable generation technologies are intermittent in nature, generation equipment will be shut down when the renewable resource (sun, wind, hydro) is not available. Simply put, significant cost savings can be realized if generation equipment (turbines and generators) can be shared by multiple complementary technologies, keeping the equipment in continuous use. Multi-fuel steam generation (CSP + coal, CSP + natural gas, CSP + biomass, biomass + coal) is the most typical example.
Also playing into this financial decision-making matrix is the fact that renewable energy generation plants are often located far from demand centers, requiring construction of significant power transmission infrastructure. It should make economic sense to combine generation technologies to share the cost of infrastructure development (PV + wind, wind + hydro, geothermal + PV, etc.).
Economies in generation costs can also be offset with synergistic efficiencies. Examples include combined heat and power (CHP) systems where waste heat from generation is used productively, combined solar PV + CSP/ thermosolar installations, and biomass + ethanol production plants (where by-products of ethanol production are used in biomass power generation).
About the Author
John Whitney AIA is a registered architect with over 25 years of experience in all aspects of project development, design, management, and construction. As Co-Founder and President of Taylor-Whitney Architects, his significant focus was on higher education clients and sustainable energy. As the Founder and Principal of the Clean Energy Action Project, he now provides consulting services that include master planning, feasibility studies, and research in the realm of cleantech and renewable energy. John is currently coordinating a solar PV database research project for AASHE (Association for the Advancement of Sustainability in Higher Education).