Solar thermal energy generated electricity (STEGE) is produced similarly to a typical, coal-fired power station used today, with the only difference being the heat source. The power cycle for a contemporary power plant occurs by first pulverizing coal into a fine powder which is then ignited to generate heat within a furnace. Water is pumped through pipes within the furnace that are in thermal contact with the inferno and high pressure steam is produced. This is passed through a turbine that turns a generator to produce electricity. The exhaust from the turbine is low pressure steam that moves through a condenser in thermal contact with a heat sink to produce liquid water. The water is then pumped back through the boiler and the cycle is closed.
Of course a key factor in this technology is the heat source. Coal is used in approximately 50% of the power plants in the United States and 42% worldwide; other sources are oil, natural gas, nuclear energy and to a much lesser extent the Sun. All these energy sources perform the same function, boil water. A nuclear power plant does nothing else other than use a controlled thermonuclear reaction to boil water! This is an important point since nuclear power is frequently equated to a high technology energy source. Certainly, controlling the nuclear reaction and all the periphery instrumentation and equipment surrounding the nuclear reactor is fairly sophisticated, yet, what it does is merely boil water to turn a turbine; not high technology at all.1
A solar energy generated power plant will operate on the same principle as a coal-fired plant; why is high pressure steam necessary? It is required to turn a turbine, similarly to a person blowing on a pinwheel. The high pressure steam is forced over the blades of the turbine which is exhausted at a lower pressure and the lost energy is used to turn the shaft. The rotating shaft has permanent magnets on it, away from the blades of course, which are located within wound wire. The rotating magnetic field produces an electric field within the wires and so electricity is produced. This is a simple and cost effective way to produce electricity and the entire technology rests on producing a rotational motion. A hydroelectric plant and wind turbines do the same thing by using flowing water and the wind, respectively, to produce electricity. Indeed it has been proposed to put turbines on the seafloor and catch tidal currents to the same effect. Basically, linear motion is translated
Solar Energy, An Introduction, First Edition, Michael E. Mackay © Michael E. Mackay 2015. Published in 2015 by Oxford University Press.
to rotational and the fact that the magnetic and electric fields are perpendicular to each other is used to make electricity.
The efficiency of the coal-fired power plant is reasonably good since it translates about 40% of the coal energy content to electrical energy; the distribution of energy use and loss in a typical power plant is shown in Table 9.1. Surprisingly, most energy is lost to the cooling part (condenser) of the process. This is a necessary part of the operation, as explained in Chapter 3, a cycle does not work unless heat is rejected. Regardless, over 50% of the energy is lost in this component to the process which is immense in absolute magnitude. Consider the rest of the losses to understand how much energy is lost to the condenser. Even though they are small and of order 1%, if one were to consider that this is a 500 MW power plant, a 1% loss means that there is 5 MW of power which is not being used! The percentage is small, yet, the absolute number is staggering. The average residential household in the United States uses electricity at a rate of 1.3 kW on average and so a 1% loss is equivalent to 3800 households, quite a large number. Fifty times more energy is rejected to the environment via the condenser.
Although power systems engineering is not the focus of this chapter, due to the similarity of all these technologies; i. e. coal, nuclear, gas, oil and solar, in producing electricity, the basic Rankine cycle is discussed. This will give the reader a grounding in power cycles and then the way energy (heat) is produced in a STEGE plant can be fully appreciated, as discussed in subsequent sections. Dimensionless numbers and correlations between dimensionless numbers will be used to design the devices in this chapter, if the reader is not familiar with these conceptsn reading Appendix C may help in understanding what they are and why they are used.