By Dick Storm
This is the second of eight newsletters on energy and thermal power generation. The first one covered energy in America, this one will discuss the most common heat engines that we depend on each day.
During my career in power generation I have been enamored with “Heat Engines” and what they do for mankind to make our lives better. Most Americans take energy for granted, thinking nothing about flipping a light switch, adjusting the home thermostat, putting a key in the car ignition, stepping onto a jet airliner, or placing some food in the refrigerator. Yet without these engines our economic prosperity, manufacturing, transportation, clean water and sewage networks come to a complete stop. So let’s take a few minutes to explore these prime energy movers.
First, let’s review the basics of energy. In America, we typically use the term “British Thermal Unit” (BTU) to express the heat content of a fuel. For example, here are the approximate heat contents of several fuels:
- Diesel Oil: 19,500 BTU’s/pound (143,715 BTU’s/gallon and density of 7.37 #/gallon)
- Gasoline: 19,600 BTU’s/pound (120,344 BTU’s/gallon and density of 6.14 #/gallon)
- PA Bituminous Coal: 12,000 BTU’s (if western Lignite between 3,800-9,000 BTU’s/pound)
- Wood Chips: Approx. 6,000 BTU’s/pound (wet wood = 20% moisture)
All of these fuels are used to power “Thermal Power Plants” using a process of converting chemical potential heat energy into shaft horsepower, referred to as a “Prime Mover.” Heat Engines are used for propulsion of ships, airplanes, automobiles, trucks and of course, electric generators for electric power production.
Each BTU at 100% conversion efficiency is equivalent to mechanical energy of 778 Foot pounds of energy. This is important to remember. One BTU is equal to 778 foot pounds of work.
Useful mechanical energy is expressed as horsepower. One horsepower is equivalent to lifting 33,000 foot-pounds in one minute. One horsepower is equivalent to lifting 550 foot-pounds in one second. The use of a pulley system and a horse is shown in Figure 1. This is where the term horsepower originated. The amount of work that one horse could produce such as for plowing a field or lifting through a pulley system.
The efficiency of the conversion of heat energy to shaft horsepower is based on the amount of heat used to produce the mechanical or electrical output based on the energy input. Remembering that one BTU is equivalent to 778 foot-pounds of energy at 100% conversion efficiency.
The course “Introduction to Large Thermal Power Generation Plants” explains how “Heat-Engines” convert the chemical energy of fuels into useful work. Some heat engines are more efficient than others. Table 1 below shows the approximate efficiencies of common heat engines in use today.
Approximate thermal efficiencies of common heat engines at the most efficient operating point:
- Gasoline engine (Reciprocating Internal Combustion) 30%
- Diesel Engine (Reciprocating Internal Combustion) 38%
- Typical 1980’s design sub-critical Coal Power Plant 37%
- Diesel engine with Turbocharging, radiator and exhaust heat recovery 40%
- State of the Art Supercritical Coal Power Plant 40%
- Advanced Gas Turbine Engine 46%
- Gas Turbine, Steam Turbine Combined Cycle Power Plant 64%
- Steam Turbine Generator with Exhaust Steam utilized for process and heating. Also known as CHP (Combined Heat and Power) 70%+
A simple steam turbine heat engine is illustrated in Figure 2 below. This shows a simplified Rankine Cycle of expanding steam through a steam turbine to turn the shaft of a generator.
A more complicated and efficient application of the fundamentals of power generation includes a large 89% efficient boiler, multi-stage steam turbine, feed-water heaters, cooling towers and associated auxiliary equipment. A typical 1980’s vintage 2400 psi/1,000-degree F. superheat with 1,000-degree F. Reheat Utility scale coal power plant would be about 35% efficient. This is illustrated in figure 3:
The coal power plant example above is by design capable of about 38% efficiency at the best steam turbine valve point and operational load. Given its varying system demand and the fact that a plant does not operate at the “Best” single load point, means that the actual efficiency will be slightly less. In the example highlighted in Figure 3 the 500 MW coal plant’s overall efficiency is about 35%. This is typical efficiency rating for large coal plants that utilize the Rankine Steam Cycle.
A term used in the utility industry to express overall efficiency is “Heat-Rate.” That is the amount of BTU’s that must be applied to a boiler to create electric energy output. In this case the average Heat-Rate is 9,751 BTU’s input to equal 3,412.6 BTU’s equivalent in electrical energy. The efficiency is expressed as Efficiency= 3412.6/9751 x 100 which is 35%.
The most efficient large thermal power plants employ the Gas Turbine Combined-Cycle (GTCC) to generate electricity. As illustrated in Figure 4, the combined-cycle means that a gas turbine is used to turn a shaft-driven generator. The exhaust steam is sent through the turbine exhaust to heat a boiler which in turn generates steam that is sent to a steam turbine which provides the torque for a second electrical generator. This process is a referred to as a combined cycle since the “Brayton Cycle” of the Gas Turbine is combined with the “Rankine Cycle” of a steam turbine. The combination of the two results in an overall efficiency from heat to electricity generation of up to 64% design.
This figure shows the use of two generators, one driven by a gas turbine and one driven by a steam turbine. This is typical of the current highly efficient Gas Turbine Combined-Cycle plants. Combined-Cycle means combining the “Rankine Steam Cycle” with a “Brayton Gas Turbine Cycle”. This combination of cycles is what makes the overall high Thermal Efficiency possible.
Gas turbines are referred to as “Aero-Derivative” This is because the advanced technologies used for maximum efficiency and power output has utilized the development of the most advanced aircraft jet engines. Gas Turbines that are used for power generation derive their high efficiency from the aeronautical advances over the last 70 years.
Figure 6 illustrates the progress of “Heat-Engine” Efficiencies since the founding of Williamson College (1888) and the experiments in Central Station Electric Power generation by Thomas Edison:
The advances of GTCC utilizing the technologies of aircraft engine efficiencies and combining these with the best steam turbine design efficiencies has provided the capability for Thermal Power Plants to approach 65% Thermal Efficiency. This is a marvelous achievement. However, there is an even more efficient use of heat energy; Combined Heat & Power usually abbreviated “CHP.”
What is the absolute best & most efficient use of energy? It is CHP.
Even more efficient than GTCC plants in overall heat utilization are those driven by CHP. This is where the exhaust heat of the cycle is used for building heating or process. Three examples are paper mills, food production, and the Williamson Energy Island. At Williamson, we use Combined Heat and Power to generate electricity with steam turbines while using the resultant exhaust heat for building heating and kitchen cooking steam. The overall efficiency of CHP systems is over 70%. That is because the heating energy for the buildings or kitchen cooking steam would be required anyway. By first passing the steam through the steam turbines to generate electricity and then using the exhaust steam for heating, the electricity used on the Campus is generated with no additional heat energy beyond which is already required. In essence, the steam turbines function as steam pressure reduction devices with the electrical output being a by-product.
Below, and at the top of the page, are a couple pictures of the Williamson College of the Trades steam turbines which are utilized for CHP.
The continuing education course planned for July 2018 will address all of the forms of Thermal Power Generation and will include plant tours of power plants which utilize coal, oil and natural gas fuels. The upcoming summer 2018 course also tours the Williamson Energy Island which is a “Micro-Grid.” Our micro-grid includes steam turbines, diesel generators, a solar array and a reciprocating natural gas engine. Planned for the near future are another reciprocating gas engine with heat recovery, thermal storage and a (natural gas fueled) micro-turbine.
The Electric Utility Scale plants toured include:
- A 400 MW Peaking, Conventional Rankine Steam cycle, oil and natural gas fueled
- Two 500-600 MW Gas Turbine Combine Cycle Power Plants
- A 200MW Coal-Fueled Plant that provides Process Steam to a neighboring Chemical Processing Plant. An example of Utility Scale CHP.
This is a brief overview of heat engines and how they convert the chemical energy of fuels into useful heat energy to provide the motive force to turn electric generators. The three-day course in July will expand greatly on the short description provided in the foregoing.
Richard F. (Dick) Storm, PE, CEM
Williamson class of 6W2