Solar Thermal Electric Power Plants

Types of Solar Thermal Electric Power

One of the most promising renewable energy sources of electricity for the future is solar thermal electric power plants. Solar thermal electric power plants generally use concentrated sunlight obtained through various mirror configurations to focus the sun’s energy to produce high-temperature heat. The heat energy is then transferred to a fluid or gas, which is used in a typical power plant cycle to convert the heat energy to mechanical energy and then electricity. The two major parts of a solar thermal electric power plant are the component that collects the solar energy and converts it to heat and the component that then converts the heat energy into electricity. One of the major benefits of solar thermal energy is that it involves a thermal intermediary, so it fossil fuels can easily be used integrated into the system as an alternative source of fuel if the sun is not providing enough energy unlike photovoltaic solar panels. In some cases the heat produce by the sun’s energy can go into thermal storage for periods of low to no sunlight, further reducing the average cost of the electricity produced. The three most commonly used solar thermal electric power plant designs are the parabolic trough design, the power tower design, and the parabolic dish/engine system.

Parabolic Trough

Overview

A parabolic trough is a type of solar thermal energy. Parabolic trough power plants use concentrated sunlight, in place of fossil fuels. They provide the thermal energy required to drive conventional power plants. These plants use a large field of parabolic trough collectors which track the sun during the day and concentrate the solar radiation on a receiver tube located at the focus of the parabolic shaped mirrors. A heat transfer fluid passes through the receiver and is heated to temperatures required to generate steam and drive a conventional Rankine cycle steam power plant.

Construction and Design

A parabolic trough is constructed as a long parabolic mirror that is usually coated silver or polished aluminum. It has a Dewar tube down its length at the focal point of the mirror. Sunlight is reflected off of the mirrors and is concentrated on the Dewar tube. The figure below is a drawing of how the sunlight hits and is reflected off of the parabolic trough.

Figure 1: Schematic of Parabolic Reflector [4]

The intensity of the sun can be multiplied by a concentration ratio of 30-80. To achieve such concentration, a trough tracks the sun in one axis continually throughout the day. To maximize the sunlight incident on the absorber, the reflectance of the parabolic reflector must be as high as possible. This is why aluminum or silver reflectors are used. Silver has the higher reflectance, but is harder to protect against the corrosive effects of the outdoor environment. It is also important to keep the reflectors clean since dirt will degrade the reflectance of light from the parabola.

The receiver of a trough concentrator (Dewar tube) is typically a metal absorber surrounded by a glass tube. The absorber is coated with a selective surface that has a high absorptance for incoming light in the visible range, and a low emittance in the infrared wavelength. The glass insulates the pipe from the effects of the wind and greatly reduces convective and conductive heat loss. Glass is also a radiation barrier to infrared light so it reduces heat loss due to radiation. Heat transfer fluid (usually oil) runs through the tube to absorb the concentrated sunlight. The heat transfer fluid is then used to heat steam in a standard turbine generator.

Efficiency

The peak optical efficiency of a parabolic trough is in the range of 70-80%. Since thermal losses from the receiver are relatively small and increase only moderately as operating temperatures increase, at peak conditions, a trough can be expected to deliver 60+% of the radiation incident on the collector even when taking into account heat losses in the solar field piping.

Renewable Energy

Current commercial plants utilizing parabolic troughs are hybrids; fossil fuels are used during night hours, but the amount of fossil fuel used is limited to a maximum 27% of electricity production, allowing the plant to qualify as a renewable energy source. Because they are hybrids and include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of space ranges enormously.

As this renewable source of energy is inconstant by nature, methods for energy storage have been studied, for instance the single-tank (thermocline) storage technology for large-scale solar thermal power plants. The thermocline tank approach uses a mixture of silica sand and quartzite rock to displace a significant portion of the volume in the tank. Then it is filled with the heat transfer fluid, a molten nitrate salt.

Figure 2: Arrays of parabolic troughs [2]

Figure 3: Sketch of a Parabolic Trough Collector [5]

Solar Power Tower

How it works

The solar power tower is a system that uses many solar reflectors called heliostats to reflect the energy of the sun to a central tower. The energy of the sun works to heat a fluid like water, air, liquid metals, or molten salts which circulates through the tower. This hot fluid can be used to turn a turbine for power generation. In order to increase the capacity factor (the percent of a day the tower can operate) the hot fluid stores its energy in a hot salt tank. The hot salt tank is a large tank containing a molten salt. Molten salts are used due to their ability to store thermal energy and because they are liquid at standard atmospheric pressure and temperature. The heat in the hot salt tank is used as a heat source for use in power generation through a turbine. The stored heat allows the power tower to create electricity up to 80% of a day (19.2 hours).

Figure 4: Graph of all 3 forms of energy used or stored over the course of a day [6]

Heliostats

The mirrors which reflect the sun’s energy are made of 2 sheets of glass. The glass is the same quality as is used in most windows. In between the sheets of glass is a thin film of silver. Only about one ounce of silver is used in each heliostat. Each heliostat is mounted on top of a post and rotated and flipped up and down by a small motor. A computer controls each motor to ensure that each heliostat reflects the sun’s energy to the correct spot on the tower.

Figure 5: Schematic displaying the size and orientation of a heliostat [7]

Technical Data

Technical data is listed for the National Solar Thermal Test Facility Power Tower (larger systems exist).

Tower Height: usually 200 ft tall
Heat Energy Focused on the Tower: 5 MW
Electrical Output: 1.5 MW
Peak Flux: 260 W/cm²
Number of Heliostats: 222
Area of Land: 8 Acres-Heliostats, 1 Acre Other Components
Cost: < $21 Million
Peak Temperature: 4,000 ºF
Operating Fluid Temperature: 1,000-1,500 ºF

Figure 6: Picture of the National Solar Test Facility [6]

Other Uses

In addition to power generation, the solar power tower can be used for research applications. The thermal performance of components and materials can be measured by placing them in the tower. Tests on how aerodynamic heating effects radar can be conducted. Nuclear thermal flash can be simulated. Furthermore, the light from the power tower can be used for astronomical observations and satellite calibration

Concerns

Power towers are usually placed in very sunny places so they can generate the most possible power each day. There is little environmental impact from power towers; they only require the use of fossil fuels as a back-up for peaks in power demand and night time power supply. The 9 Acre footprint is less than many other electric power plants. The heliostats are too high for animals to be harmed by them; however, birds may be harmed if they fly between the tower and the heliostats. The heliostats are fully automated. However, accumulated dust can substantially decrease their ability to reflect the sun. Rain is actually beneficial in that it cleans the dust off of the heliostats. Hail can break the glass on the heliostats; however, it would have to be over 1 inch in diameter.

Parabolic Dish

Parabolic Dish Array/Concentrator

The third and final type of solar thermal power system is the parabolic dish/engine design. Parabolic dish/engine systems utilize an array of parabolic dish-shaped mirrors to reflect and concentrate the incoming solar insolation directly hitting the array of dishes back onto a single receiver located at the focal point of the dish as shown in Figure 7. The solar energy taken over a large array of parabolic mirrors is concentrated into a large amount of thermal energy focused on a very small area, the receiver. Motors are used to enable the entire parabolic mirror array to be able to continuously track the sun in two axes so that the incoming solar radiation is always hitting the mirrors at the optimal angle to reflect and concentrate the most sunlight to the receiver.

Figure 7: Array of Parabolic Mirrors and Receiver [11].

The parabolic mirrors used in the main array are made of glass with a reflective surface of aluminum or silver deposited on top of it. The main advantages of using glass mirrors are that they are relatively inexpensive, can be fairly easily cleaned, and reflect approximately 92 % of the sunlight that hits them. The mirrors are made parabolic in shape because the ideal shape for concentrating sunlight is the paraboloid of revolution.

Receiver/Heat Engine

After the array of mirrors focuses the sunlight, the concentrated sunlight then heats up the working fluid to temperatures of around 750 OC within the receiver. The heated high temperature working fluid is then used in either a Stirling or Brayton heat engine cycle to produce mechanical power via rotational kinetic energy and then electricity for utility use with an electric generator. An example of a Brayton cycle used to produce electricity for a parabolic dish power plant is shown in Figure 8. In the cycle the concentrated sunlight focused on the solar fluid heats up the compressed working fluid of the cycle, air, replacing altogether or lowering the amount of fuel needed to heat up the air in the combustion chamber for power generation. As with all Brayton cycles, the hot compressed air is then expanded through a turbine to produce rotational kinetic energy, which is converted to electricity using the alternator. A recuperator is also utilized to capture waste heat from the turbine to preheat the compressed air and make the cycle more efficient.

Figure 8: Schematic of Parabolic Dish/Brayton Cycle Power Plant [9].

A Stirling cycle would generate mechanical power in a similar way by using the heat from the concentrated sunlight to move pistons to produce rotational kinetic energy like an internal combustion engine in an automobile. The rotation of the engine’s crankshaft could be used to drive an electrical generator and produce electricity. Currently, Stirling engines are more commonly used than Brayton cycles in dish/engine systems, but analysis of dish/Brayton applications done predicts potential possible thermal to electric efficiencies of over 30%.

Efficiencies and Benefits

When considering all three of the major solar thermal electric technologies, the parabolic dish/engine systems have the highest efficiency in the conversion of solar energy to electricity with an efficiency of 29.4% achieved. High optical efficiencies and low startup losses aid in making the dish/engine systems very efficient, which gives it the potential to eventually become one of the least expensive forms of renewable energy. Compared to solar power towers or parabolic trough power plants, parabolic dish/engine systems are typically designed for smaller, high value applications, such as for remote power needs, requiring only 5-25 kW of power generation provided by a single dish or a small farm of several parabolic dish systems connected together to provide power for a small grid of end-of-the-line utility applications. An example of a parabolic dish system provides power for a village consisting of four 25 kW dish/engine units is shown in Figure 9.

Figure 9: Schematic of a Dish/Engine system for 100 kW of power for a village [9].

A major benefit of parabolic dish/engine systems is that they have a hybrid capability in that the Brayton cycle can get its heat from either the concentration of solar energy from the parabolic dish or from a combustion chamber within the Brayton cycle burning a fossil fuel, such as natural gas. Depending on the availability of sunlight, the parabolic dish/engine system has a very high reliability as a power source, since it can produce power using two types of fuels almost directly eliminating the need for a second entire separate cycle for generating power.

Concerns

Unlike parabolic trough systems or power towers, parabolic dish arrangements are not able to produce large-scale power needs and are intended for small power generation in the range of kilowatts of power instead of the megawatts of power produced by the other two systems. Another major problem is the high technology development risk for dish/engine systems. A low cost concentrator system will have to be developed to concentrate sunlight with a high efficiency cheaply and at least one commercially viable engine must be developed by the dish/engine system with high efficiencies. Currently, parabolic dish/engine systems are not used to produce electricity commercially and are only used for small-scale remote projects, research, and demonstration of the technology. Parabolic dish/engine system also can have high operations and maintenance costs, since moving parts are needed for the two-axis tracking of the dish and in the Brayton cycle for producing the electricity, a turbine and compressor are needed.

Comparison between Different Types

Table 1 below shows a comparison between the three major types of solar thermal electric power plants. Both the power tower and parabolic trough systems are used for large grid-connected utility power project on the scale of tens to hundreds of megawatts, while the parabolic dish systems are used on a much smaller scale for small villages or remote applications on the scale of tens of kilowatts. Parabolic trough plants are by far the most used system commercially for power generation today and are the most technologically developed solar technology. However, they have limited thermal storage and the lowest efficiency of the three types of solar thermal plants. Power towers have fairly well developed technologically, low cost, efficient thermal storage, and mid-level efficiency. Parabolic dish/engine systems have the highest efficiencies, but existing systems are primarily only prototypes, they have a high technology development risk, and batteries are required for energy storage.

Table 1: Characteristics of solar thermal electric power systems. [10]