Electricity costing

The electric furnace is an alternative to the reverberatory furnace in environmentally sensitive areas where electricity costs are not too high. The electric furnace is versatile, produces small volumes of effluent gases, and the SO2 concentration can be easily controlled. Operating costs, however, are high. 

Means Electrical Cost Data, R. S. Means Co., Kingston, Mass., 1992. 

Several industries are highly dependent on cheap electric power. These include the aluminum industry, the Portland cement industry, electrochemical industries such as plating and chlorine production, the glass industry, and the pulp and paper industry. Other industries such as the petrochemical industry, which is highly competitive, depend on low priced power. About two-thirds of the cost of producing ammonia is electrical cost. 

Factored electrical cost as a percentage of total installed plant cost for specific types of plant... 

Electrical costs involve four main components (1) power wiring, (2) lighting, (3) transformation and service, and (4) instrument and control wiring. A breakdown of these component costs as a percentage of total elec trical cost is given in Table 9-60. 

TABLE 9-60 Component Electrical Costs as Percentage of Total Electrical Cost... 

We have an aeration basin that currently operates at 3.2 mg/Liter DO. Compare this operation where the DO concentration is 1.3 mg./Liter. The temperature of the basin is 18.0° C and 200 kW of aeration power is used. The average electricity cost is 8 cents per kWhr. Determine (a) the current average electricity consumption for aeration (b) the daily electricity costs for the operation (c) what you could save on a daily basis and per year by lowering the DO concentration (d) determine the yearly savings on a percentage basis. 

The cost of N2, like that of O2, is particularly dependent on electricity costs, though plant maintenance and transport costs also obtrude. Typical prices in 1992 for No in the USA were about 32 per tonne for bulk liquid (exclusive of transportation and handling charges). Costs for small-scale users of N2 from gas cylinders are proportionately much higher. 

The efficient heat pump reduces energy use by 1,676 kWh per year on average. Is the efficient model heat pump a good investment Suppose the incremental cost of the efficient unit, as compared with the less efficient unit, is 1,000, and electricity cost 10 cents per kWh. With this price of electricity, the efficient heat pump reduces electricity costs by 167.60 per year. Taking a simplified approach for purposes of illustration and assuming that each unit lasts indefinitely and has no repair, maintenance, or replacement costs, and ignoring possible tax effects, the internal rate of return may be calculated as 1,000= 167.60/r, which is 16.76 percent per year. If the household can borrow money at, say, 10 percent per year and earn 16.76 percent, the investment makes economic sense. If we assume a 10 percent discount rate, the present value of the investment is 1,676, which exceeds the initial investment cost. The net present value is 676, which indicates that the investment is feasible. 

Even when the time comes to make a purchasing decision, an energy-efficient motor purchase is not a certainty. Sometimes an energy-efficient motor will be the economically efficient choice at other times, not. The capital investment decision is based on the cost in relation to performance, efficiency and reliability. Moreover, the decision depends on the application and the amount of time the motor is in operation. It can be the major component of a product (drill or mixer), or a minor component (computer disk drive) it can be the major component cost of a product (fan), or it can be a minor component cost (stereo tape deck) it can run almost constantly (fan, pump, and machinery), or only a few minutes a day (vacuums and power tools). For example, contractors purchase circular saws almost solely based on performance and reliability. Time is money, and since the saw is operating only a few minutes a day and the contractor is often not responsible for the electricity costs to run the motor, energy efficiency is not a consideration performance and reliability are what matter most. On the other hand, an industrial user, who runs huge electric motors twenty-four hours a day to work pumps, machinery, and ventilation equipment, is very concerned tvitli energy efficiency as well as performance and reliability. 

The only significant electrical cost involved in ventilation systems is operation of fans. Other electrical equipment such as dampers, compressors, etc. generally mn for such short periods that costs are negligible. 

Electrical costs for fans can be estimated from the following formula ... 

Example 6.7 In Example 6.1, the required plant capacity is 218 kW and the running time is 2000 h/year at an electricity cost of 5 p/ (kW h) and a motor efficiency of 75%. In order to achieve the condensing temperature of 85°F (29.4°C) the condenser would cost 7250, while a smaller condenser for a temperature of 100°F (37.8°C) would cost 4600. (Prices of evaporative condensers at April 1987.) Estimate the break-even time if the larger condenser is fitted. 

This is a rough calculation, based on direct capital cost and not on interest rates, and needs to be analysed in terms of the general plant economics. It should also be borne in mind that this is based on present-day electricity costs, and a greater saving will be made as fuel costs rise. 

In all forms of condenser pressure control, the minimum maintained pressure should he the lowest which will give satisfactory operation, in the interests of running economy. An indication of the relative electricity costs for a 350 kW air-conditioning plant is given in Table 9.1. 

Condensing temperature rc) Coefficient of performance Weekly electricity costs ( , 5 p/unit)... 

Energy recovery turbine systems are available for very high pressure RO (seawater) plants, whereby brine reject water under high pressure can be used to spin an energy recovery turbine, reducing the RW hp requirements (and hence electricity costs) by 30 to 32%. 

It will have occurred to the reader that aU of these energy forms need to be produced from electricity. Magnetic and light energy can be partly exceptions to this rale. In assessing the overall economics of a process, electricity cost and the efficiency of converting electricity to the desired energy form will play a vital role. 

Example 13.4 A centrifugal pump is required to deliver 100 m3 of water per hour with an increase in pressure of 5 bar. If the driver for the pump is to be an electric motor with an efficiency of 90%, electricity costs 0.06 KW h-1, operating for 8300 hours per year, estimate the annual cost of power. 

Despite the technical success of electrochemical H2 purification, it has not found commercial use. The reason is economic at a total voltage of 200 mV and an electricity cost of 0.05/kWh, the electric cost alone amounts to 2.36 per million BTU, an unacceptable price for commercial-grade H2. The real value in this process is the high purity of the product, and if a need for this purity arises the process will be attractive. At present, electrolytically-produced by-product H2 is often discarded. 

At present the operating practice is to set the fresh brine flow according to load alone and not take into account electricity cost. This clearly represents an opportunity to use the model to optimise the brine flows. 

Prices for electricity fluctuate depending on the time of day. England has electricity costs of 22 cents per kilowatt hour and is further north than most inhabited parts of Canada and receives limited solar radiation over much of the year. This makes conventional solar power not very competitive at grid delivered costs. However, kilowatt hour photovoltaic costs have been in an exponential decline for decades, with a 20-fold decrease from 1975 to 2001. 

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