Energy generation: using various power sources

Despite (or as a result of) the economic downturn, the use of renewable energy options have grown over the past several years, supported by federal and state programs including federal tax credits, state renewable portfolio standards, and a federal renewable fuels standard.

12/28/2017


This article has been peer-reviewed.Learning objectives:

  • Discuss the renewable energy options available to engineers when designing power systems.
  • Review various alternative power sources, such as combined heat and power (CHP).
  • Learn about resilient power options.

Failure of power generation systems serving hospitals, airports and transportation systems, water- and waste-treatment plants, police stations, and public safety food distribution could result in supply shortages, considerable disturbance to public order, and a significant economic impact both regionally and nationally. The need for reliable power at every building is essential, but in recent years, several blackouts in the U.S. have highlighted the need for resilient power systems at critical facilities.

Figure 1: This multifuel combined heat and power (CHP) plant, located in the U.K., is based on grate boiler technology with two boiler streams and two steam turbines. Courtesy: ArupBecause maintaining these facilities is crucial to societies, it is imperative to improve the resilience of critical infrastructure to failures and attacks. With increasing urgency, we must ask ourselves whether renewable energy options can serve as suitable backup options. It is widely understood that the renewable energy options currently available cannot be stand-alone, reliable power sources for these facilities. However, renewable energy sources can become part of an effective solution when integrated with conventional energy options (see Figure 1).

The frequency and causes of power outages, as well as the availability of renewably energy, vary with geographic location. Figure 2 shows the main causes of power outages that occurred in 2016. Note that extreme weather has the largest impact. These outages not only create regional issues, but also compound and can become national catastrophes if not mitigated effectively. Figure 3 shows power outages by region throughout the country for 2016.

Figure 2: This graph shows a variety of causes for power disturbances or outages including weather, birds/animals, equipment failure, and human error. Courtesy: Eaton

There are two main types of resilience: engineering and ecological. Engineering resilience makes the ecological systems close to a stable steady state and time, which is required to return to the same situation. There are so many definitions for resilience, but the important factors are robustness, redundancy, resourcefulness, and rapidity.

  • Robustness is the ability to withstand a shock without significant degradation or loss of performance.
  • Redundancy is the diversity and capability of satisfying functional requirements if significant degradation or loss of functionality occurs.
  • Resourcefulness is the ability to mobilize when threatened as well as to prioritize problems to initiate solutions.
  • Rapidity is the ability to contain losses and recover in a timely manner without disruptions.

Figure 3: This graph provides an analysis of the power outages by state, region, and year. Courtesy: Eaton

Defining dispatchable and non-dispatchable technologies

Owners of critical infrastructures need to reconsider their power and backup power systems to be more resilient with the constant updates in technology. Rather than relying on more diesel generators or stand-alone nondispatchable options, which in most cases are very expensive, integrating nondispatchable designs with dispatchable systems is a good practice. Part of this is understanding the definitions of both dispatchable and nondispatchable technologies. A dispatchable source of electricity is an electrical power system, such as a power plant, that can be turned on or off and can adjust its power-output supply based on demand. Most conventional power sources, such as coal or natural gas power plants, are dispatchable systems. In contrast, many renewable energy sources are nondispatchable. Renewable sources, such as wind and solar power, generate electricity based on variable sources, which affects the flow of output energy.

Figure 4: A proprietary gasifier system converts woodchips into a wood gas suitable for fueling a CHP plant’s spark ignition engine. Courtesy: ArupOne technology that combines dispatchable and nondispatchable systems can be seen by combined heat and power (CHP) plants, which use biomass and fuel cells as a nondispatchable energy source. Combining such systems can provide ongoing benefits, such as continuous power, energy-cost savings, and emergency power coverage of critical loads when the utility grid is down. This type of integration can be designed and evaluated as a resilient power system for a specific single facility, or designed for larger-scale systems. These technologies even can be used in smaller scales for schools or universities, where the buildings are normally used as shelters in the event of power outages due to natural disasters, terrorist attacks, or any number of events. Bio-fueled CHP can be a solution for resilient energy generation for a single facility. This system effectively uses the waste streams as raw materials. For example, a proprietary gasifier system converts woodchips into a wood gas suitable for fueling the CHP’s spark ignition engine (see Figures 4 and 5).

This 50-MW biomass-fueled power plant is located in Ireland. The project was awarded and permitted in July 2011 as Best Available Techniques (BAT) and the most efficient plant of its kind in the world. Courtesy: Arup/photo by RKD ArchitectsWhen designing to have resilient power for critical facilities using nondispatchable sources, it is important to understand the available sources, tools, and calculations that are open to the public. While many factors must be considered when designing a combined resilient system with a nondispatchable source, the geographical location of the facilities determine one or two nondispatchable energy sources that are more efficient and cost-effective than others. Therefore, it is important to note that one of the key factors for the relationship between the annual production (kWh) of the system and the annual cost is the geographical location. The annual cost can be defined as the sum of the present-value initial cost of construction, annual operating-cost expenses, the cost of the fuel source, the discount rate (driven by government tax credits and incentives), and the capacity factor of the system. The capacity factor of a system, such as a power plant, is the ratio of its actual output over a period of time to its potential output if it were possible for it to operate at full nameplate capacity continuously over the same period of time.

Experience curves that describe how unit costs decline with cumulative production have always challenged technology-related decisions. Although stand-alone nondispatchable technologies have a higher initial capital cost, increasing effectiveness of the present technologies is allowing for further acceptance of these solutions. Therefore, the ability to incorporate nondispatchable technologies with current dispatchable facilities is becoming more prevalent. The importance when comparing the system relevance is geographical region and initial cost.


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