Power Plant: Reserves, Efficiencies and Alternatives

In this article we will discuss about:- 1. Size of Power Plant 2. Various Types of Reserves in Power System 3. Efficiencies 4. Economic Comparison of Alternatives.

Size of Power Plant:

The size of plant depends on the purpose (private industry supply, regional supply or grid supply), for which the plant is to be set up. If it is to be set up for a private industry; the size would be based on power requirements of different sections of the industry and the likely increase in power demand in near future. If it is to be set up as an emergency or standby plant, the size would be governed by the load that must be supplied by the plant in the event of failure of grid supply.

The size of the plant for supplying power to a given area or region will depend on the present requirements of the area and expected increase in power demand in next 5- 10 years. The size of the plant for supplying power to a power grid is based on the additional power requirement of the area being supplied by the grid and the likely increase in power demand in next 10-20 years or so.

The size of a proposed hydroelectric power plant for grid supply is, in addition to power demand, also governed by the rate of flow of water and height of fall (or water head). The size of steam plant to be added to the grid is, in addition to power demand, also governed by the fact whether the plant will be used as a base load plant or as a peak load plant and the extent to which it will be required to contribute to increase the firm capacity of the hydro plants.

Large sized plants have many advantages (mostly economic). Some of the costs are hardly affected by the plant size. The cost of office space, shops, docks and landscaping can be spread over more capacity. Coal handling equipment, cooling facilities and other appurtenances can also be operated at lesser cost per kWh generated in case of larger plants. Broadly speaking, a large sized plant will produce electricity at a lower cost.

However, the size of the plant is limited by some factors. Thermal power plants require lot of space for coal storage, ash disposal, cooling towers and large quantities of water. The reliability of the system is also dependent on the size of the plant. The maximum size of the plants and capability of system interconnection are also related. The environmental pollution problems are also more severe in case of larger plants.

The advantages of large power plants seem to outweigh the disadvantages and so large size plants are favoured. As a matter of fact, the increase in the power demand has forced the utilities to go in for larger size power plants.

In India six sites (Singrauli, Korba, Ramagundem, Farraka, Rihand and Vindhyachal) have been recommended for super thermal power stations of 2,000 MW each while at the time of independence, the maximum plant size was about 50 MW.

In USA, 5,000 MW size plants are being set up at present and in the coming decade the maximum plant size may be as high as 10,000 MW.

Various Types of Reserves in Power System:

Every system must have a certain amount of reserve generating capacity to provide for overhaul of generating equipment, forced outage of equipment etc.

Various types of reserves are given below:

Operating Reserve:

It refers to capacity in service in excess of peak load. It is provided for regulation within the hour to cover minute to minute variations, load forecasting errors, loss of equipment and maintenance of equipment.

The operating reserve is made up of the spinning reserve as well as the non-spinning or supplemental reserve.

The spinning reserve is the extra generating capacity that is available by increasing the power output of generators that are already connected to the power system. For most generators, this increase in power output is achieved by increasing the torque applied to the turbine motor.

The non-spinning or supplemental reserve is the extra generating capacity that is not currently connected to the system but can be brought on line after a short delay. In isolated power systems, this typically equates to power available from fast-start generators. However, in interconnected power systems, this may include the power available on short notice by importing from other systems or retracing power that is currently being exported to other systems.

Efficiencies of Various Power Plants:

Hydro turbines, the oldest and most commonly used renewable energy source, have the highest efficient of all power conversion processes. The potential head of water is available right next to the turbine, so there are no energy conversion losses, only the mechanical and copper losses in the turbine and generator and the tail end loss. The efficiency is in the range of 85 to 90%.

Steam (coals fired) power plants accounts for almost 41 % of the world’s electricity generation. Coal fired power plants operate on the modified rankine thermodynamic cycle. The efficiency is dictated by the parameters of this thermodynamic cycle. The overall coal plant efficiency ranges from 32 to 42%. This is mainly dictated by the superheat and reheats steam temperatures and superheat pressures.

Most of the large power plants operate at steam pressures of 170 bars and 570°C superheat, and 570°C reheat temperatures. The efficiencies of these plants range from 35 to 38%. Super critical power plants operating at 220 bars and 600/600°C can achieve efficiencies of 42%. Ultra super critical pressure power plants at 300 bars and 600/600°C can achieve efficiencies in the range of 45% to 48%.

The efficiency of nuclear power plants is little different. On the steam turbine side they use the ranking thermodynamic cycle with steam temperatures at saturated conditions. This provides lower thermal cycle efficiency than the high temperature coal fired power plants. Thermal cycle efficiencies are in the range of 38%.

Since the energy release rate in nuclear fission is extremely high, the energy transferred to steam is a very small percentage—only around 0.7%. This makes the overall plant efficiency only around 0.27%. But one does not consider the fuel efficiency in nuclear power plants; fuel availability and radiation losses take center stage.

Diesel engines, large capacity industrial engines, deliver efficiencies in the range of 35-42%.

Natural gas fired (including CNG fired) power plants account for almost 20% of the world’s electricity generation. These power plants use gas turbines or gas turbine base combined cycles. Gas turbines in the simple cycle mode, only gas turbines running, have an efficiency of 32-38%. The most important parameter that dictates the efficiency is the maximum gas temperature possible.

The latest gas turbine with technological advances in materials and aerodynamics has efficiencies up to 38%. In the combined cycle mode, the new “H class” gas turbines with a triple pressure HRSG and steam turbine can run at 60% efficiency at ISO conditions. This is by far the highest efficiency in the thermal power field.

Non-Conventional Energy Sources:

Wind turbines have an overall conversion efficiency of 30-45%.

Solar thermal systems can achieve efficiency up to 20%. The moving path of the sun and the weather conditions drastically alter the incident solar radiation. The efficiency on an annual basis, around 12%, is considerably less than on daily basis.

Geothermal systems, on the other hand, also use the Rankine cycle with steam temperatures at saturation point. Since there is no other conversion loss, this plant can achieve efficiencies around 35%.

The power industry is trying to increase the conversion efficiency of power plants to maximise electricity generation and reduce environmental impact.

Economic Comparison of Alternatives:

Generally, limited alternatives are available in generation system planning. However, some alternatives concerning the number and size of units, auxiliaries and steam pressure and temperature (in case of steam plants) are usually available, especially for captive plants, and are to be compared for maximum benefits.

The methods available for economic comparison of alternatives of power generation are detailed below:

1. Annual Cost Method:

The generation cost can broadly be divided into fixed cost and operating or running cost. In annual cost method, all costs are converted into equivalent annual figures. The annual costs of the different alternatives are computed by using the same fixed charge rate and the same period of analysis. The alternative with minimum annual cost is considered the best. Annual cost is affected by both fixed and operating cost.

2. Present Worth Method:

Here the present worth of all alternatives is determined. The present worth is the sum which if invested at the start of the project would yield an amount equal to the annual cost each year. The present worth is determined using same period of analysis and same time base for each alternative irrespective of the fact whether they have a common life or not or whether they have been started in the same year or not. The alternative with the lowest present worth will represent overall expenditure and is the most suitable alternative.

3. Capitalized Cost:

This method assumes that the project once established will continue forever, otherwise it is similar to present worth method. The capitalized cost is the amount on which the annual interest will cover the total annual costs for ever. The alternative with the lowest capitalized cost will be the optimum alternative.

4. Rate of Return:

In this method, all items of annual cost except interest and depreciation are calculated as in method of annual cost. The difference between cost of purchasing power from utility and the annual cost (or the difference between the annual costs of different alternatives, excluding interest and depreciation in case power from utility is not available) represents a saving which is available for annual interest on interest and depreciation. If the difference between annual costs is A and investment is P, then rate of return r is determined from the following relation- A/P = r/ [1 – (1 + r)^-n] ,where n is the life of plant in years.

All alternatives in which value of r comes out to be less than the minimum rate specified are rejected. All the other remaining alternatives are arranged in increasing order of annual cost. The rate of return on increment investment i.e., r is determined by knowing the difference ΔA between annual costs of different alternatives and difference ΔP between investments in successive alternatives from the relation [ΔA/ΔP] = r/ [1- (1 + r)^-n].

Alternative is chosen by comparing this value of r.