August 13th, 2020
Category SOLAR ENERGY 3
Before there were computers we let nature be the computer. This was performed by using dimensionless numbers and developing correlations between them, particularly when designing complicated flow or heat transfer processes. The essence of the procedure was to take the differential equation applicable to the process at hand and make it dimensionless. The variables were all made dimensionless with process parameters such that they were all of order one in dimensionless form. How this allows nature to be the computer will become evident below.
Consider the velocity gradient dvz/dr for pressure driven flow of water in a pipe, see Fig. C.1. The appropriate velocity component of interest is that in the z-direction, vz for cylindrical coordinates. The radial position is given by r...Read More
Fig. B.1 Various properties of liquid water including: viscosity (д, •), density (p, o), heat capacity (Cp, ■), thermal conductivity (k, □), a group of parameters used in the calculation of the Rayleigh number (Ra = [gxp2Cp/kp]ATL3 = CRaATL3, see the text for the definition of the parameters used to determine CRa, *) and the Prandtl number (Pr, д). The dimensions for each parameter are given in the equation and their magnitude on the graph can be ascertained by putting 0 °C in the equation. All the equations in the graph have the temperature T in °C. Data are from J. R. Welty et al., ‘Fundamentals of momentum, heat and mass transfer,’ Third Edition, John Wiley & Sons (1984).
Fig. B...Read More
Table A.1 Numerical prefixes
*The prefix Quadrillion is used in the USA energy industry to represent 1015, such as Quadrillion BTUs or Quads.
Table A.2 Energy and power conversion factors
When a cover was placed on a flat plate solar energy collector it was found that the device became more thermally efficient despite the fact that the cover admitted less insolation. The gauge of the insolation reaching and absorbed by the absorber plate was || or ||, which is a complicated calculation requiring detailed knowledge of the cover and absorber material properties. In spite this, consideration of the product was given in Chapter 8; now the analysis becomes more challenging with a curved surface such as the envelope. In addition, the reflection optics, that is the reflection of insolation from the parabolic mirror to the absorber pipe, must also be considered to find an effective jar|...Read More
The basic parabolic trough STEGE process has a large area of parabolic reflectors that concentrate insolation to absorber pipes through which
a heat transfer fluid (typically) flows to receive the energy. The pipes have a selective surface and are surrounded by a clear glass concentric tube to minimize heat transfer to the surroundings, similar to the cover used in a flat plate solar energy collector.
The heat transfer fluid can be a synthetic oil, like Therminol VP-1, whose properties are given in Appendix B, that can withstand the high temperatures required to establish an efficient process. Yet, the development of new heat transfer fluids is an active area of research and in some cases molten salts are being considered...Read More
In order to use solar energy to produce steam and hence power a Rankine cycle to generate electricity, one must concentrate the solar radiation. The concentrated insolation is absorbed by a pipe that has the heat transfer fluid flowing through it. It was demonstrated in the previous chapter that a flat plate solar energy collector can heat water fairly efficiently to order 50 °C, however, much higher temperatures, approaching 400 °C, are required for electricity generation. Here the use of parabolic reflectors is considered for electricity production.
Previously we found that concentrating the sunlight can produce a large temperature rise over a short length of pipe in a solar concentrator, in Example 3.6...Read More
The Rankine cycle is the cycle of choice to produce electricity and is shown in Fig. 9.1 in a pressure (P)-enthalpy (H) diagram. Water is the usual working fluid in the cycle. Although there are more details to the cycle than shown, such as a reheat component, this will be ignored here so the reader may understand the basics of the cycle and the concept behind its design. It differs from the basic Carnot cycle in important aspects which will be discussed within the text below.
The four components to the cycle are the pump, boiler, turbine and condenser. We will begin with the pump and work around the cycle explaining how the steps between each component occur and the reason why they are designed that way...Read More
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 ...Read More