General Principles of Energy Management
Abstract
Energy is ubiquitous because it permeates every aspect of human endeavor. At first glance, the diversity of energy end-uses is so great as to defy any attempted classification or synthesis. However, certain general principles exist and these can be employed to use energy resources more efficiently, to extend and prolong existing supplies, and to bring the benefits of low-cost energy to a greater share of the world’s population. This chapter discusses the basic approaches to energy management and describes 16 general principles that provide a starting point for initiating energy management efforts in any activity.
Keywords
Approaches; general principles; energy quality; efficiency; fuel switching; cascading energy; energy containment; material economy
Introduction
Life was a reward for successful energy management by prehistoric man. Limited by what could be hunted, dug from the ground, or carried on the back or as body fat, our ancestors had little latitude in dealing with the vicissitudes of nature. Predators, droughts, disease, and natural disasters took their toll, and the bands’ elders were lucky to reach 40 years of age when times were good. For primitive peoples—even for those living today—energy management requires balancing the roughly 2000 to 3000 kcal per day (approximately 3.0 to 4.6 GJ per year or 2.9 to 4.3 MBtu/year) work and metabolic energy expenditure with an equivalent food intake. Peak demand also was, and still is, important; even with adequate energy supply, an excessive power requirement (such as subzero weather) could lead to failure of the human system. Thus, the concept of energy management is not new to human affairs, but has been an essential aspect of human survival for centuries.
The industrial revolution changed this situation by allowing humans to draw upon a greater diversity of nature’s stored sources of energy—first firewood, then fossil fuels, and eventually nuclear energy. Humans have also learned to use energy from the sun, wind, and water much more effectively than our ancestors did, and we have developed increasingly innovative ways to capture energy from geothermal resources, agricultural crops, and biological waste. While these developments have been significant only in the last few hundred years—really, in the last century—of man’s million year-plus history, they represent a revolutionary accomplishment of enormous significance to human and other life forms.
Today, most citizens of our energy-intensive cities manage or mismanage the equivalent of 100 times as much energy as our early ancestors—much more than is needed for bare survival. In the face of diminishing natural resources, a worldwide increase in demand for energy, and the threat of global climate change, efficient and judicious use by all people will be needed if costs are to remain reasonable and environmental impacts manageable. Energy management can contribute to this goal in the home, on the farm, and in factories and cities.
Approaches
At the most elementary level, energy management may be thought of as task energy use, that is, the provision of as much energy as is needed, when it is needed, where it is needed, and with the quality required. Since there is often limited flexibility in the timing and locational aspects of task energy use, the primary areas of focus for energy management are to maximize utilization of energy quality and minimize utilization of energy quantity.
Appropriate “cascading” of energy use from high quality to progressively lower quality forms ensures maximum utilization of energy quality for each task at hand. Therefore, implicit in energy cascading is meeting energy requirements with “waste” heat, or recovered energy dissipated from another process whenever lower quality energy is acceptable for the task. This concept is in itself a fundamental aspect of energy management, since “not every Joule (or Btu) is created equally.” Why not? Because the energy content of a swimming pool of lukewarm water is roughly the same as a liter of gasoline. However, while the energy content is the same, the useful “work” that can be accomplished by a liter of gasoline is much greater than the useful “work” that can be done by a swimming pool of lukewarm water! By the same token, when people use high grade fuels such as oil or natural gas to generate hot water or low-temperature process steam, in a strict thermodynamic sense they are misusing the potential value of the fuel, even though this practice is quite commonplace. Extending this concept to the use of electric resistance elements for water heating shows the potential for even greater misuse of high quality energy, since only about 1 unit of electricity is generated for every 3 units of fuel combusted in a conventional coal-fired power plant. An example of the best use would be to employ a topping cycle such as a gas turbine to extract work initially and then use the low-quality “waste” heat for process steam production or water heating.
There are three basic approaches for minimizing the quantity of energy used:
• Reduce use by downsizing or eliminating an end-use (or shifting usage to a different time period, which effectively redefines the “when it is needed” portion of task energy use) due to a self-imposed change or because of regulatory or economic pressures. In this approach, unless the end-user has been operating systems superfluously, the user generally makes a sacrifice to reduce energy quantity. That is, they no longer realize the same level of “functionality” they previously obtained from their end-use systems. Except in circumstances where the financial benefits far outweigh the sacrifices [e.g., participation in a utility’s demand response program may be a very attractive option with minimal detriment to end-use functionality (see Chapter 5)] or in cases of needless energy use (e.g., process equipment in use when no longer needed, unused space with 24-h per day illumination, or unoccupied rooms being heated and cooled), this approach should usually be viewed as a last resort.
• Increase efficiency with better operation and maintenance procedures, more efficient equipment, advanced controls, improved processes, or different material inputs. The aim of this approach is to achieve the same level (or, ideally, a much improved level) of functionality from the end-use systems while minimizing energy quantity and lowering lifecycle costs. Increasing efficiency is a primary focus of this book.
• Substitute energy form with another in less demand or with a more appropriately matched energy quality for the task in question (this is also referred to as “fuel switching”). Preferably, the replacement energy form would serve both purposes: reduce demand for the constrained or expensive energy form and provide the suitable quality of energy. However, in some cases, the most cost-effective energy substitute may not reduce overall energy quantity, but it would still reduce the required quantity of the original energy source.
General Principles
Although there is a very great diversity in energy end-use technology, there are certain basic approaches or general principles that apply to a wide range of applications. Identification of fundamental principles for energy management is an attractive concept because it suggests an initial approach to the problem. The principles alone will not improve energy use efficiency, but they can provide a rational basis for developing more specific technological responses.
In Table 3.1 we summarize some general principles that are applicable to a wide variety of situations. The table also provides an approximate, highly qualitative assessment of relative costs, implementation time, complexity, and benefit based on our experience. The following discussion helps clarify how these principles could be applied to a typical energy user.
Table 3.1
General principles of energy management
Review Historical Data
The first principle is to review historical energy use. It helps establish typical seasonal, monthly, and even daily energy use patterns and facilitates identification of anomalies such as unexpected spikes or dips in usage, energy use during non-business periods, or even gradual energy increases over time that may signal degradation of equipment. Sometimes seasonal variations or scheduling discontinuities are present but unrecognized; the review process brings them to light and may suggest ways of combining operations, reducing demand charges, or otherwise affecting savings. For example, a plant may experience a surge of manufacturing during a certain season, yet maintain space conditioning all year-round. Often the question “why do we do this?” and the answer “that’s the way we’ve always done it” flag an area for immediate savings. Chapter 4 discusses historical review in greater detail.
Energy Audits
Historical energy use data are never sufficient, however, since they provide the total picture but not the details. It may be necessary to collect other types of data to better understand the factors driving energy use which might include weather, production, or occupancy. In addition, energy audits are the means for investigating energy use by specific processes and machines, and provide insight into inefficient operations. Chapter 6 provides a comprehensive discussion of building and site energy audits.
Operation and Maintenance
Improving operation and maintenance in the plant will generally save energy. Well-lubricated equipment has reduced frictional losses. Cleaned light fixtures transmit more light. Changing filters reduces pressure drop. Repairing steam leaks prevents waste of high quality energy. Operation and maintenance practices are applicable to all types of end uses. Chapters 8, 9, 11, and 12 describe common measures for HVAC, lighting, process, and building envelope systems.
Analysis
Analysis goes hand-in-hand with the energy audit to determine how efficient equipment is, to establish what happens if a parameter changes (reduce flow by 50%), or to simulate operation (computer models of building or process energy use). Chapter 7 presents general techniques for energy analysis.
Economic Evaluation
Economic evaluation is an essential tool of energy management. New equipment, processes, or options must be studied to determine costs and returns. The analysis must include operating costs, investment tax credits, taxes, depreciation, and the cost of capital for a realistic picture, particularly when considering escalation of energy prices. Chapter 13 discusses various methods of cost effectiveness analysis.
More Efficient Equipment
More efficient equipment can often be substituted to fulfill the same function (e.g., LED or high output T5 fluorescent lamps rather than T12 or even T8 fluorescent lamps for area lighting, or premium efficiency motors instead of standard or high efficiency motors). Most types of industrial, commercial, and residential equipment are now rated or labeled in terms of their efficiency; there are wide variations among different manufacturers depending on size, quality, capacity, and initial cost, but there are many online resources for comparing the different technologies. Chapters 8, 9, and 11 provide several examples of high efficiency HVAC, lighting, and process equipment.
More Efficient Processes
More efficient processes can often be substituted without detrimental effect and often yield improved product quality. A classic example is a continuous steel rolling mill, which uses a continuous process to produce steel products, avoiding energy loss involved in cooling and reheating in batch production. Another example is powder metallurgy rather than machining to reduce process energy; still another is a dry papermaking process which reduces energy expended to remove water from the finished product. Inert atmosphere ovens can reduce energy used for drying solvent-based paints, compared to ultraviolet bake ovens. Membrane separation in food processing can result in better tasting products than heat treatment technologies.1 Drying with microwave or radio-frequency radiation increase drying rates and minimizes surface drying and cracking relative to conventional drying processes. For example, an energy management team conducted a study to find a replacement for gas-fired drying oven used in the processing of agricultural feed additives. The study tested a microwave oven, electric resistance heating, and a solar oven. The relative drying time using these three technologies was in the approximate ratio of 1:10:100. Not only was the microwave process the fastest, but it reduced waste heat losses and improved product quality. See Chapter 11 for additional examples of efficient processes.
Energy Containment
Energy containment seeks to confine energy, reduce losses, and recover energy. Examples include repair of steam and compressed air leaks, better insulation on boilers or piping, air sealing of building envelopes, and installation of heat exchangers or power recovery devices. For example, the flue gases from boilers and furnaces and other systems that depend on combustion provide excellent opportunities for heat recovery. Depending on flue gas temperatures, the exhaust heat can be used to raise steam or to preheat the air to the boiler. Figure 3.1 shows an example of such a system, where an ammonia reformer heater is designed to conserve fuel by using a steam generator and air preheater to recover heat from the stack gas.2
There is overlap between this energy management principle and the operation and maintenance principle discussed above (Principle 3) and the energy cascading principle discussed below (Principle 13).
Material Economy
Material economy implies recovery of scrap, reduction of waste, and “design for salvage.” The powder metallurgy example cited above also illustrates this principle. Product design that permits salvage or recovery of reusable parts, motors, and components is another example. Structures, in fact, can be designed for reuse and relocation.
Substitute Materials
Substitute materials can sometimes be used to advantage. For example, in low-temperature applications, low melting point alloys can substitute high-temperature materials. A material that is easier to machine, or that involves less energy to manufacture, can replace an energy-intensive material. Water-based paints can be used without baking in certain applications. An emerging technology for primary aluminum production that uses wetted cathodes and inert anodes instead of carbon anodes promises to reduce energy use and lower greenhouse gas emission while increasing productivity and lowering costs over the conventional Hall-Héroult process; these savings are due largely to the fact that current carbon anodes are consumed during the process whereas the inert anodes do not corrode or release carbon dioxide emissions.3
Material Quality Selection
Material quality selection is extremely important, since unnecessary quality almost always means higher costs and often means greater energy use. For example, is distilled water needed, or is deionized sufficient? Purity of chemicals and process streams has an important impact on energy expense; trace impurities may not be important for many applications.
Aggregation of Energy Uses
Aggregation of energy uses permits greater efficiency to be achieved in certain situations. For example, in a manufacturing plant it is possible to physically locate certain process steps in adjacent areas to minimize the energy use for transportation of materials. Proper time sequencing of operations can also reduce energy use, for example, by using temperatures generated by one step of the process to provide preheating needed by another step.
Cascade of Energy Uses
Heat recovery is an example of cascading energy use, whereby high temperature heat is used for one purpose and the waste heat from that process applied to another process step, and so on. There are many sources of waste heat in commercial and industrial facilities. Figure 3.1 showed an example of recovering heat from a gas-fired reformer furnace. Energy in the form of heat is also available at a variety of noncombustion sources such as electric motors, crushing and grinding operations, air compressors, and air thickening and drying processes. These units require cooling in order to maintain proper operation. The heat from these systems can be collected and transferred to some appropriate use such as space heating. An example of this type of heat recovery is shown in Figure 3.2.4 All the energy supplied to the motor in electrical form is ultimately transformed into heat and nearly all of it is available to heat buildings or for domestic water or mine air heating.
As the temperature of waste heat decreases, the opportunities for applying it to other processes diminish; however, in some cases industrial heat pumps may be a viable and efficient option for accepting the low temperature waste heat and delivering it at a higher temperature for applications requiring higher quality energy.5
Energy Conversion and Energy Storage
Careful consideration of the energy source and form can lead to improved efficiency, environmental benefits and costs savings. Consider whether an alternative energy source, different energy conversion process, or energy storage is applicable. For example, onsite solar photovoltaic panels for electricity production are becoming more cost effective as the technology matures. In addition, solar thermal technology is an effective means for water heating. Also, thermal energy storage using ice banks or eutectic salts is a useful means to shift cooling loads to off-peak periods and battery technology is rapidly advancing, which will make onsite storage of electric energy increasingly viable in the future.
Conclusions
The three basic approaches and 16 general principles for energy management we present here provide a starting point for initiating energy management efforts in any activity. Applying these concepts can take place at several levels. Modification and retrofit can be applied to existing equipment and facilities and involve either operating budget or capital dollars, depending on project size and complexity. Many utility companies offer incentives to lower the costs of energy efficiency improvements. In new facilities, plant designers have numerous opportunities to improve process efficiency, often without increasing capital costs, simply by planning that takes into account the anticipated costs and availability of energy resources. In building construction, builders can specify the most efficient equipment and materials economically justified. When energy costs are negligible, the initial cost of the project is often the important consideration. Now, with increasing operating costs to be expected, a higher initial cost may be justified if it saves money over the project’s lifetime.