Conversion of solar to mechanical and electrical energy has been the objective of experiments for over a century.

— Duffie and Beckman

Solar radiation is a great source of heat for generating steam. Steam is a highly useful working fluid that has been used to drive mechanical systems for centuries. Heat is heat, no matter what its origin. As such, as solar thermal technologies continue to be developed and decrease in cost, more and more thermal systems that are currently driven by steam that is generated from the burning of fossil fuels can have their fuel source replaced by the sun.

Learning Objectives

• Identify the steps of the processes required to convert solar energy to mechanical and electrical energy.
• Compare the thermal behavior of solar thermal power systems.
• Summarize the technical details of the solar thermal power systems existing and emerging over the globe.

What is due for Lesson 10?

This lesson will take us one week to complete. Specific directions for different assignments are given in the table below and within this lesson pages.

Questions?

If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Steam has been used for centuries to perform mechanical work. Steam locomotive engines are probably one of the most popular machines known for converting steam to mechanical work. Any modern steam turbine does a similar conversion at higher energy conversion efficiency. Many steam turbines are used because of their high efficiency at turning steam energy into kinetic rotational energy. This rotational energy can be further used to drive an electricity generator or any other process that requires mechanical energy to operate. Historically, the steam required for such processes was derived from burning fossil fuels such as coal or natural gas, while solar thermal energy was used experimentally for over a century. Steam that is generated by renewable methods (such as solar radiation) is identical to steam generated by burning a fuel to heat water, and the principles of conversion of solar heat to mechanical and electrical energy are fundamentally similar to those used in combustion systems. Concentrating solar thermal technologies are best suited to achieve high temperatures under higher pressures, simultaneously meeting the demands of large-scale turbines that require a significant amount of high-quality steam. The general strategy of energy conversion using solar thermal energy is presented on the diagram below.

Figure 10.1: Schematic of a generic solar thermal power system

Credit: Mark Fedkin

The solar energy obtained and converted to heat by the collector system is transferred by the thermal fluid to the storage and further to a boiler, where steam is generated. Further steam is supplied to a turbine in the heat engine, where it is converted to mechanical energy, while some heat is rejected. If the electric power is desirable output, the mechanical energy is supplied to a generator, where it is converted to electricity. At each conversion step, we can expect some losses due to non-100% efficiency. One of the challenges here is that the efficiency of the solar collectors decreases with increasing operating temperature, while the efficiency of the heat engine increases at a higher temperature (Duffie and Beckman, 2013). Therefore optimization is needed to select system operation conditions. Typically the temperatures delivered by the flat-plate collectors are too low for heat engines to be efficient; thus concentrating collectors (e.g. parabolic systems) or evacuated tubular collectors are more preferable choices.

The main configurations of solar thermal power systems include: 

• Parabolic troughs
• Parabolic dish
• Central receiver systems (power tower)
• Solar updraft tower

You can re-visit those technologies on the Energy Information Association website [2]

The overall efficiency of the power conversion system is composed of the efficiency of the solar collectors (with parabolic troughs, max ~75%), the efficiency of the heat engine (~35%). Minus field losses, the typical average overall efficiency of solar trough thermal plants is around 15-20%.

The following pages of this lesson refer you to various types and system designs.

Parabolic trough technology is the most widespread among utility-scale solar thermal plants. The potential of this type of concentrating collectors is very high and can provide output fluid temperatures in the range up to 500°C. Parabolic trough is the linear-focus collector, which consists of a cylindrically curved parabolic mirror, which reflects the sunlight onto a tubular receiver positioned in the focus line of the parabola. The tubular receiver contains the fluid that absorbs heat and transfers it via circulation to the boiler or another device to produce steam.

Figure 10.2: Kuraymat parabolic trough solar plant, Egypt. The plant has the total solar aperture area of 130,800 m² and expected electricity generation of 34,000 MWh/year. It has been operating since 2011.

Credit: Green Prophet via Flickr [3]

Rows of parabolic mirrors are mounted in parallel on either a north-south axis (typical) or an east-west axis (there are pros and cons to each orientation based on location and energy production requirements) and move to track the sun across the sky. The tubes are very carefully designed to absorb solar radiation and transfer the heat to the heat exchange fluid passing through the tube. Fluid is pumped through the absorber tubes that are connected in series and parallel. Some systems employ an insulated storage tank to enable power generation when the solar resource is either intermittent (due to something like cloud cover) or unavailable (typically during the early evening hours). The heat transfer fluid is then passed through the storage tank, if it exists, and then pumped to heat exchangers to transfer the heat to water (except in the case of direct steam generation where water is already the heat transfer fluid and a heat exchanger is not needed) to generate steam for expansion in a steam turbine to generate electricity.

Solar Energy Generating Systems (SEGS) is the name of the world’s largest parabolic trough solar thermal electricity generation system, developed by Luz in southern California, USA. SEGS is the second largest solar thermal power plant in the world at 354 MW (surpassed by the 377MW Ivanpah Solar Power Tower system discussed in the next section). The three largest plants in the world currently range in size from 250 MW to 354 MW and are all located in the US. The next twelve largest plants in the world range in size from 100 MW to 200 MW and are all located in Spain.

Unlike linear concentrating systems (troughs), which reflect light onto a focal line, the central receiver systems send concentrated light onto a remote central receiver. A typical example of such a system is a solar power tower system, which consists of multiple tracking mirrors (heliostats) positioned in the field around a main external receiver installed on a tower. Such systems are capable of reaching of much higher levels of concentration than linear systems. Concentrated radiation is further used as heat to produce steam and convert it to electricity (like in a regular power plant), or the generated thermal energy can be stored in a molten salt storage.

Figure 10.3: SOLUCAR PS10 (Planta Solar 10) solar power plant, Spain. Operational since 2007, PS10 produces 23,400 MWh/year. Aperture size is estimated at 74,880 m². The light is concentrated on the top of the 115 m high tower.

Credit: Afloresm via Wikimedia Commons [4]

Central receiver systems are typically large-scale plants that are usually built to power a steam cycle. The central position of the receiver offers a universal advantage to collect all energy at one location and save on transport networks. At the same time, the fixed position of such a central receiver results in a limitation of light collection: heliostats are always oriented at an angle to the direct beam, so the amount of energy collected is less compared to a parabolic concentrator. Therefore, to reach necessary efficiencies of light concentration, the size of the collecting field is increased, which brings into considerations such issues as land use, higher environmental impacts, and higher capital costs. Significant potential for developing large-scale central receiver solar plants is hence attributed to deserts and flat arid areas which have plenty of sunshine and lower land value with respect to other applications and industries.

The world’s current largest solar thermal power system is a power tower system named Ivanpah. Located in Southern California on the border with Nevada, Ivanpah has three main towers, nearly 2.5 million square meters of heliostats (mirrors), and can generate as much as 377MW of power under the right conditions [5]. Worldwide, solar power tower systems have been used for decades to generate steam for both electricity generation and various industrial processes, with large-scale implementations in the tens of MW often for research purposes. Ivanpah is the first and only solar power tower plant in operation in the world that is larger than 20MW. While there are plans and ongoing construction in many countries around the world to build plants that are similar in size and even five times as large as the Ivanpah system, our global experience with such large solar power tower systems is very limited.

In the Ivanpah plant, there are several buildings near the base of the tower that contain the components of a typical steam electricity generation station. Once high-quality steam is generated in the tower and pumped down to the generation station at the base of the tower, the remaining components of the electricity generation system are no different than conventional electricity generation components.

Globally, some countries have much more history and subsequent experience with solar power towers. Spain and the USA are the two leading nations, with many other countries operating small power tower systems or currently developing plans to construct large (>10 MW) solar power tower electricity generation stations. The USA currently houses the largest solar power tower plant in the world and has the history of Solar One and Solar Two, which are currently decommissioned, but were 10 MW in size. Spain houses three active solar power tower systems larger than 10 MW with plans to build three more of that magnitude.

Further please proceed further to the following sources to learn about basic configurations and design of the central receiver solar power technology and some specifications of the heliostats and receivers utilized in known facilities of this type.

Solar updraft towers for generating electric power were first conceived over a hundred years ago. Several prototypes have been developed over the decades, and some have been implemented and operated over the course of several years. These prototypes vary in size and scale, with the largest ones capable of producing tens of kilowatt of power with towers that are a couple of hundred meters tall (most notably,  the solar updraft tower in Manzanares, Spain [6] with a tower of 194 m tall and capable of producing 50 kW of electrical power).

Figure 10.4: Solar Chimney prototype at Manzanares, Spain. The tower is seen through the polyester roof.

Widakora via Wikimedia Commons

The principle of operation of solar updraft towers is based on the stack effect: difference in the density of air due to temperature and humidity differences can drive air movement. Solar energy is used to increase the air temperature at the bottom of the tall chimney (tower), creating a gradient in density, which creates upward air movement. This solar-induced "wind" is used to rotate wind turbines installed in the confined space in the chimney. The power output of a solar updraft tower depends on two design variables - solar collector area and height of the tower. The collector is represented by a greenhouse-like structure at the surrounding ground at the base of the tower, and the commercial size collectors are designed up to 4-5 square miles in area. A greater height of the tower creates greater pressure gradient due to stack effect and results in more usable wind power. The known system heights are in the range 100-200 m, while systems as tall as 1500 m are being proposed (Wikipedia- Solar Updraft Tower [7]).

There have been and continue to be many proposals for projects in countries around the world for solar updraft tower systems that are incredibly large compared to the scale that has been proven to date. The high capital cost remains the main barrier to commercialization and widespread use of this technology.

A lot of solar technologies appear to be very similar at face value. However, the history of various technologies as well as knowledge of how each system works help reveal why certain technologies are selected over other solutions. Parabolic trough was mentioned as most widespread solar thermal power technology, and there should be reasons.

For this lesson assignment:

1. Summarize in a table the main cons and pros of the four types of the solar power systems listed in this lesson - parabolic trough, central receiver, parabolic dish, and solar updraft tower (Take into account both technical and economic factors).
2. Provide a discussion as to why certain technologies in this list out-pace others. What are the primary barriers preventing any of these technology options from becoming more widely accepted? Do you have any reason to think that one of the current technologies will become the sole winner in solar thermal power market?

Some supplemental reading resources that can be helpful in this assignment are provided in Module 10 in Canvas.

Summary

Solar thermal power systems are a great way to convert solar radiation to electricity. Historically, there have been many complex issues that have been addressed one by one, making utility scale solar thermal power systems a reality today. Concentrating solar power systems are the main enabler of this reality. By concentrating solar radiation with parabolic troughs or heliostats, higher temperatures and, thus, higher quality steam can be achieved at competitive cost.

Reminder - Complete all of the Lesson 10 tasks!