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In order to have a better understanding of energy efficient design, it is essential to have a clearer picture of the relevant problems and definitions. There are two very important distinctions to be made between energy efficiency and energy conservation and between energy and power efficiency. Energy conservation is frequently confused with energy efficiency. Energy efficiency may be achieved by keeping the net energy output (total work) of a system constant while reducing the energy input. Energy conservation, on the other hand, refers to lowering the total energy input without necessarily keeping the energy output the same.
Almost 56% of the total primary energy used (about 100 Quads in 2010) in USA is wasted. 40% of the total energy is used to generate electricity with an overall efficiency of about 32% (or 1/3rd). The transportation sector uses about 27% (1/4th) with an overall efficiency of 25%. The industrial/commercial sector uses 47% with an overall efficiency of 55%. Energy Efficiency has been a topic of great interest in the Petroleum and Chemical Industry (PCI) since 7-8% of all electricity generated in the US is used by the PCI. This paper discusses recent advancements and best practices in energy efficient design techniques and the impact of new legislation on energy efficiency. It also addresses economic considerations and provides some application guidelines.
The word “efficiency” as commonly used with reference to electric devices like motors and transformers typically refers to the power rather than the energy used. Energy consumed by a device is the total amount of power integrated over time. This can be easily understood from the following illustrations. Figure 1 depicts a typical efficiency curve of a motor or transformer. The efficiency number is defined at discrete points like half-load, full-load or 20% over-load, etc. Figure 2 shows load cycle for the same device over several daily load cycles. The energy consumption is given by the area underneath the curve, the integral of the power over the time it is supplied. The input energy will depend on the variable (power) efficiency values during the load cycle. Differing load cycles each day will cause the device to consume daily different amounts of energy. As a simple example, a motor 100HP (74.6kW full-load output) motor may have an efficiency of 94% at full-load and 93% at half-load. If the machine is delivering full-load for 24hrs./day, the machine energy efficiency over that period of time is 94%. However, if the same machine is running at half-load for 12 hours per day and full-load for the remaining 12 hours, the resulting energy efficiency is now only 93.5%. If the same motor is running at no-load for the entire day while doing no actual work, the resulting energy efficiency is now 0%.
|Energy Efficient Design: Tools, Techniques and Applications to the Petroleum and Chemical Industry|