Peak load

Petroleum. Thermal cracking (pyrolysis) of petroleum or fractions thereof was an important method for producing gas in the years following its use for increasing the heat content of water gas. Many water gas sets operations were converted into oil-gasification units (55). Some of these have been used for base-load city gas supply, but most find use for peak-load situations in the winter. 

Hydrogen-storage alloys (18,19) are commercially available from several companies in the United States, Japan, and Europe. A commercial use has been developed in rechargeable nickel—metal hydride batteries which are superior to nickel—cadmium batteries by virtue of improved capacity and elimination of the toxic metal cadmium (see BATTERIES, SECONDARYCELLS-ALKALINe). Other uses are expected to develop in nonpolluting internal combustion engines and fuel cells (qv), heat pumps and refrigerators, and electric utility peak-load shaving. 

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

If the cycle has a short-time rating, with a period of energization and one of rest, the motor will obviously cool during the de-energizing period, and depending upon the peak load and tlie rest periods, a comparatively lower output motor can perform duties at higher loads. The equivalent output for the load cycle of Figure 3.16 is... 

From the peak load P2 and from the available h.p, of the motor Feq. we can determine the energy to be shared by the flywheel, i.e. 

It is so much less than that there is no risk of fast fracture, even at peak load. 

Intermittent operation and peak-load operation lead to more problems and reduee the life of many of the hot parts. Peak-load units aeeount for only about 20% of the units above the 10 MW range. The smaller units (between 1 and 10 MW) are usually operated as standby power generators or for eompressor drives mostly on offshore platforms. Small-unit repair eosts average about 55,000- 100,000. These units are very rarely used for peaking serviees. 

Determination of peak loads and dimensioning of ElVAC system components... 

Proof of compliance with building regulations or codes expressed in terms of either peak load targets or energy targets... 

Hdchst-belastung,/. maximum load, peak load, -besetzungszahl, /. maximum number of electrons in a shell, -betrag, m. Tnarifniim... 

XNBR (wt%) PS (wt%) OPS (wt%) Impact energy (J) Peak load (N) Ductile ratio... 

In the single fiber pull out test (SFPO), a small portion of the fiber is embedded in the bulky matrix and the interfacial strength is calculated from the peak load when the fiber is pulled out of the composite. 

The late-1980s also saw the initiation of more widespread programs operated by utilities to promote energy-efficient appliances in the marketplace. Many of these focused on reducing peak loads by offering rebates for more efficient air conditioners and chillers in both residential and commercial applications. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Cooling loads are most significant on hot summer afternoons when electric utilities typically have annual peak loads. Lower solar heat gains lead to reduced peaks, which reduce peak demand charges and power plant capacity requircliiclits. 

For close examination of systems having varying latent heats and flow rates, it is wise to examine several points in the tower, even tray by tray in some cases, to be certain that the number of holes in that tray does not place its performance too close to the weep or flood conditions. Towers have been built and operated at rated peak loads with every tray having a significantly different number of holes. 

The advantage of modulating control is that it maintains a constant working water level and therefore the boiler is always in its best condition to supply steam for peak loading. These controls may also be fitted to boilers below 9000kg/hF and A100°C particularly if severe loads are present or when the working pressure is above 10 bar. 

Most boiler plants can be installed using one or more boilers, which can accommodate minimum to maximum loads. Occasionally, heavy peak loads occur for only relatively short periods, and here there may be an advantage on economic running grounds to install boilers whose firing rate will not meet these peaks. In these cases, there are two methods which may be used one is thermal storage and the other is with an accumulator. 

Due to interruptible gas tariffs, it is often necessary to adopt gas as the primary fuel and burn oil in periods of peak loads. This means that the economizer has to be arranged so that when oil firing, the flue gases are bypassed around the economizer. The bypass duct must also contain a damper to simulate the economizer gas resistance so that the burner back pressure remains the same for both fuels. Figure 25.6 shows a typical installation layout. 

Do not over tension belts as it shortens belt and bearing life. The ideal tension is the lowest at which the belt will not slip under peak load conditions. 

The effectiveness of the various protective measures adopted should be assessed by making shut-down tests and comparing the results with tests made during peak-load periods (see Fig. 10.36). 

Example 21.5 The coldroom in Example 21.3 has 12 lighting fittings labelled 280 W. The four evaporators each have three fan motors of 660 W gross per fan and 18 kW defrost heaters which operate alternately for 15 min twice a day. The fork-lift truck is rated 80 A at 24 V and will be in the store 20 min each hour during the 8-h working day. Two packers will be present for 10 min each hour. Estimate the average and peak loads (see Table 21.2). 

Examination of the items which comprise the total cooling load may throwup peak loads which can be reduced bylocalized treatment such as shading, modification of lighting, removal of machines, etc. A detailed analysis of this sort can result in substantial savings in plant size and future running costs. 

The usual method for smaller units is thyristor speed control of the compressor, and such compressors can be slowed to 40% of full speed or less. Some can also be run at higher speeds than normal, to deal with peak loads, but the COP will fall off at this condition. As many as eight fan-coil units can be connected to one condensing unit, and the installed number can be nominally greater than the compressor capacity, on the basis that not all will be cooling at the same time. 

Online boilers may be idle or may run on low- or high-fire and may operate under a variety of conditions, such as steady-state, peak-load, or lead and lag arrangements. 

Metal oxide and other deposits are laid down, as may occur in peak-loading power boilers. 

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