This material is an extract from “Solar Thermal Power Systems” Keith Lovegrove and Andreas Luzzi, Encyclopedia of Physical Science and Technology, 3rd Edition, Vol 15.

1 Introduction

Various devices for collecting solar radiation thermally have been devised. At the simplest level, a flat metal plate painted black and placed in the sun will heat up until it reaches a temperature where the heat that it looses to the air around it and also by radiating itself, exactly balances the amount of energy it receives from the sun. This “stagnation temperature” occurs at around 80oC for a simple flat plate. If water, for example, is passed through passages in the plate, then it will stabilize at a lower temperature and the water will extract some of the energy in being usefully heated up. This is the essence of solar thermal energy collection.

Greater levels of sophistication are aimed at reducing the amount of “thermal loss” from the collector surface at a given temperature. This allows energy to be collected more efficiently and at higher temperatures.

Starting with the flat plate, a cover layer of glass helps by cutting down the energy lost by the circulation of cold air. If metal tubes and glass cylinders are used instead of plates, then the space can be completely evacuated, so that air convection losses are completely eliminated. Clever use of coating materials to produce an optically selective surface that absorbs as much as possible of visible solar wavelengths whilst emitting as little as possible of thermal radiation from the plate helps to reduce radiation losses. Various combinations of these measures are used in the production of systems for the production of solar hot water that are used around the world for domestic and industrial applications. In principle, solar collectors of this nature could be used for electricity production. However the low temperatures achievable limit the conversion efficiencies possible to low levels.

Further increasing the temperature at which energy can usefully be recovered requires some method of optically concentrating the radiation so that the size of the absorbing surface and hence its thermal loss, is reduced. The conceptually simplest approach is to employ a series of flat mirrors (called heliostats) that are continuously adjusted to direct solar radiation onto the absorbing surface. This is illustrated conceptually in Figure 1. Large plants called “Central Receiver Systems” or “Power Towers” have been built based on this principle and are discussed in detail in section 2. The concept can also be adapted to linear absorbers and long strips of mirror to create a “Linear Fresnel” concentrator.

Alternatively, the mathematical properties of a parabola can be exploited. The equation for a parabola in the x y plane is;

y=x2/4f

Figure 1. The central receiver concept; a field of plane mirror heliostats all move independently to each keep a beam of solar radiation focussed on a single central receiver (Figure H. Kreetz).

Rays of light parallel to the y axis of a mirrored parabola will all be reflected and focused at the focal point at a distance f from the vertex.

As illustrated in Figure 2, this effect can be used in a linear arrangement, where a mirrored “trough” with a parabolic cross section will focus solar radiation onto a line focus when it is pointed directly at the sun. The largest Solar Thermal Power plants constructed to date employ this principle, they are reviewed in section 3. Alternatively a mirrored dish with a parabolic cross section (a paraboloid) will focus solar radiation to a point focus (see figure 3). Dish systems are reviewed in section 4. Both dishes and troughs require continuous adjustment of position (or at least frequent readjustment) to maintain the focus as the sun moves through the sky.

Similar focusing effects can obviously be achieved with lenses of various kinds, but this has not been employed on the scales needed for solar thermal power systems.

Another alternative which potentially avoids the need to track the sun is to employ “non imaging” concentration. As illustrated in Figure 4, this involves the construction of a mirrored “light funnel” of some kind. Such a device will be able to collect rays into its aperture over a range of incidence angles and cause them to exit via a smaller aperture via multiple reflections. Non imaging concentrators have not found application as the “primary” means of concentration for Solar Thermal Power systems but they are frequently applied as “secondary concentrators” at the focus of central receivers, dishes or troughs, where they serve to further reduce the size of the focal region.

Figure 4. (right). A non-imaging concentrator concentrates solar radiation without the need to track the sun.

The rays of light from the sun are not exactly parallel. This means that even a perfect optical system will produce an image of finite size, with an intensity distribution that is a maximum in the center and tapers off to zero at the edges. Imperfections in mirror shape and tracking accuracy have the effect of further spreading out the sun image.

Each of these approaches to concentration has a typical ratio of collected radiation intensity to incident solar radiation intensity, termed the “concentration ratio”. Table 1 summarizes the options discussed and lists typical concentration ratios, the resultant operating temperatures and the consequent thermodynamic limiting efficiency with which electricity could be produced. The limiting conversion efficiency arises from the second law of thermodynamics. The maximum efficiency for conversion of heat from a constant high temperature source given by;

Maximum conversion efficiency = 1- Tcold/Thot

This is the “Carnot limit”. A simple understanding of why there should be such a limit can be developed by realizing that empirically heat will not spontaneously flow from cold objects to hot ones. If all the heat flow from a hot object could be converted to electricity then this could in turn all be used to heat an even hotter object which we know to be impossible. In real Solar Thermal Power systems, conversion efficiencies around one third or less of the ideal maximum are typically achieved.

Clearly higher concentration ratios give higher efficiency, however they also lead to potentially higher complexity and cost. The ultimate challenge with Solar Thermal Power systems is to produce the desired output as economically as possible. This invariably means a trade off between system efficiency and capital investment drives the design process.

2 Central Receiver Systems

The central receiver concept was first proposed by scientists in the USSR in the mid 1950s. The first experiment was established in Sant' Ilario near Genova, Italy, in 1965 by Professor Giovanni Francia. He installed 120 round mirrors each the size of a 'tea-table', focusing on a small steam generator on top of a steel frame. The product was superheated steam (500 oC, 10 Mpa).
Central receivers have the advantage that all the energy conversion takes place at single fixed point. This avoids the need for energy transport networks and allows investment to improve the efficiency and sophistication of the energy conversion process to be made more cost effectively. Associated disadvantages are that they must be built as single large systems, without the modularity benefits of distributed systems like troughs or dishes. The fixed position of the receiver also means that heliostats do not point directly at the sun, so that the amount of collected solar radiation per unit area of mirror is less than with the other technologies.
Major investigations during the past twenty years have focused on four heat transfer fluid systems; water/steam, sodium, molten salt and air.An overview of the major demonstration grid connected power plants built to date is given in Table2.

3 Trough Systems

Solar thermal power in the form of mechanical energy for water pumping was established for the first time near Cairo in 1914 (~ 40 kW). It incorporated a water/steam operated parabolic trough array (5 units, 4m x 62m) and a low pressure condensing steam engine. Solar electric trough development was re-activated by the U.S. Department of Energy in the mid 1970's. The first experimental system started operation in 1979. At the same time a private Research and Development company from Jerusalem, Israel, (LUZ) decided to design and implement commercial scale parabolic trough 'Solar Electric Generating Systems' (SEGS). This decision was strongly motivated by favourable power purchase agreements and tax credits offered in the state of California.
The nine SEGS plants built by LUZ between 1984 and 1989, have a combined capacity of 354MWe. They are all continue to operate on a commercial basis and together they represent by far the majority of operating solar thermal power station capacity in the world.
During the early 1980s small demonstration trough based solar thermal power systems were constructed in the USA, Japan, Spain and Australia. Table 3 lists the details of these plants. Specifications for the 9 SEGS plants are given in Table 4.

4 Paraboloidal Dishes

The first