Overview

In this lesson, we overview various Concentrating Solar Power (CSP) technologies, with emphasis on those that have been proved effective supplying utility scale power. The most well-known ones are parabolic trough systems, parabolic dishes, and 3D central receiver systems, such as solar power tower. The main purpose of a CSP technology is to convert solar radiation into thermal energy, which is in turn used to drive a heat engine. CSP plants have been commercialized and operate in a number of countries as part of power infrastructure. According to Renewable Energy World [1], the current global power generated by CSP exceeds 1095 MW. It is definitely a growing industry which works towards cost-effective solutions with renewable power, and may possibly become a major player in electricity generation in the U.S. over the following decades.

Here, we also review different types of power conversion cycles that are used for conversion of solar thermal energy to mechanical energy of the turbine. Such cycles are also called heat engines. They are crucial elements of any thermal plants, because in most cases mechanical energy or electrical energy are more practical than thermal energy. In this lesson, you will explore the principles and differences between several power conversion cycles and their varieties. One key question to answer will be which type of cycle would be suitable for a particular type of solar thermal plant and which parameters should be considered in that decision.

Learning Outcomes

By the end of this lesson, you should be able to:

1. Describe the principle of operation of the most important CSP technologies;
2. Apply performance metrics to characterize the utility scale CSP systems;
3. Understand the purpose and design of power conversion cycles;
4. Justify the choice of power conversion cycle based on solar system parameters and goals.

7.1 Introducing Concentrating Solar Power

The term Concentrating Solar Power (CSP) covers a range of technologies that utilize optical devices, such as mirrors and lenses, to concentrate the beam solar radiation and to provide for higher efficiency of its conversion into other forms of energy. Typically, in many sources, CSP systems are associated with the solar thermal power; although, in a general sense, CSP can work with both thermal solar power and photovoltaic applications. Conversion of the concentrated sunlight can follow three routes: (1) conversion to heat, (2) conversion to electricity, and (3) conversion to fuels. The large utility scale plants primarily use the concentration of thermal energy, which is used to operate a steam turbine generator and produce electric power on site. In this case, the solar heat is used as any other source of heat (such as coal combustion, etc.) to generate steam.

The video below (11 min.) introduces the main types of the CSP technology. For that conversion cycle to run smoothly, day by day generating power, a few key technologies are linked together. These include light concentration, thermal transfer fluids, steam powered turbines, and sometimes thermal energy storage. The video also discusses the system efficiency at every step of conversion.  

Video: CSP Video Tutorial Unit 1-01 – Overview of CSP Technologies (11:21)

CSP systems can only benefit from the direct beam radiation and therefore are best suited to the regions with a high percentage of clear sky days. The locations that have significant cloudiness, smog, or dust are not favorable.

By concentrator configuration, the commercial CSP systems are represented by:

• parabolic trough systems
• central receiver towers
• parabolic dish systems
• linear Fresnel concentrators

These are the main technologies that you will read about in this lesson.

The CSP technology is one of the competitive options in energy industry for combustion-free electricity generation. Because the fuel cost is zero, the cost of the CSP technology is mainly associated with the significant initial capital investment. However, cost reduction trend for CSP is confidently predicted in the near future. The main avenues for cost efficiency are linked to:

• technical progress in CSP technology, research and development efforts, and lesson learned from operating plants; and
• scaling up to larger plant size, which allows for the use of more cost effective turbines for power conversion.

The CSP technology has been commercialized and has experienced rapid growth since 2005, in part stimulated by the international concern for increased fossil fuel combustion and climate change. Worldwide, the CSP expansion was led by Spain, which has most CSP plants currently installed. Over the past decade, the CSP sector experienced a significant slowdown due to the surge of photovoltaic systems in the renewable energy markets. 

By technology type, CSP the current CSP market is led by the parabolic trough plants (over 75%). In spite of much slower growth than PV, solar thermal plants are nevertheless expected to be significant players in future energy economy. The main barrier to the CSP market growth remains high costs of electricity (average 0.20 USD/kWh compared to 0.05-0.10 USD/kWh for PV).  

CSP market shares (based on data from fortunebusinessinsights.com) 

Mark Fedkin

Further, in this lesson, you will learn about different configurations of CSP plants, which differ in design of optical systems (reflectors and light collectors), position of the receiver, and heat transfer networks. These various technologies have their pros and cons and are applied based on the target application, location, and other factors.

7.2. Parabolic Trough CSP Technology

As was noted earlier in this course, parabolic trough technology is the most widespread among utility-scale solar thermal plants (Figure 7.1). The potential of this type of solar concentration is very high and can provide output fluid temperatures in the range 400-500°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 7.1. 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 [6]

We covered the basics of light concentration by parabolic reflectors in Lesson 2. Now, we go on to look at all different aspects of the parabolic trough technology, including materials, operation parameters, system design, field layout, energy storage associated with this kind of plant. Please refer to the following reading material:

Based on the above reading, please go through the following self-check questions.

7.3. Central Tower CSP Technology

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 (Figure 7.2). 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 7.2. 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 [7]

Please proceed further to the following chapter reading, which covers some basic configurations and design of the central receiver CSP technology, approaches used to convert and store energy, specifications of the heliostats and receivers utilized in some known facilities of this type.

It is clear that central receiver CSP 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 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 parabolic concentrators. Therefore, to reach the 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.

7.4. Parabolic Dish CSP Technology

Parabolic dish geometry concentrates light in a single focal point, i.e., all sun rays that are parallel to the axis of the parabola are directed towards the central receiver. This allows this type of collector to achieve the highest concentration ratios among all other type of solar collectors. The dish concentrator must be oriented towards the sun. Usually, losses in this technology are associated with the imperfections of dish alignment and non-ideality of reflection. The engine that converts the concentrated solar energy into electricity is placed at the focal point. This technology can be used for both large-scale power plants (with many dishes grouped in arrays) and autonomous small-scale power generation systems that would provide power to off-grid remote facilities. Example of such a system is shown in Figure 7.3.

Figure 7.3. EuroDish stirling parabolic dish solar collector is an example of de-centralized solar technology. Some spec: diameter 8.5 m, aperture 56.7 m², average concentration factor 2500.

Credit: Schlaich Bergermann via Wikimedia Commons [8]

Go to the following chapter reading to learn the fundamentals of the parabolic dish CSP technology:

As we can see from this reading, the parabolic dish is a very efficient and flexible technology, which comes in various designs and is suitable for various applications. There are many cases (some described in the history section) of applying this technology in remote locations to provide self-sustained power for water heating, water pumping, and alternative power. One of the features of the dish Stirling systems is their low "inertia" - they start producing power very quickly as soon as direct beam radiation hits the reflector, but, at the same time, they are very sensitive to variations in solar intensity and results in sharp variation in power output and frequent interruptions if the meteorological conditions are not perfect. Because of the high intermittence, energy storage applications which would buffer the power output, would be desirable, but at the moment are not well developed.

7.5. Thermal - electric power conversion

To make usable energy from solar heat collection in CSP plants, thermodynamic power conversion cycles (heat engines) are used. The main idea is quite simple. The heat transfer fluid, which is directly heated in the solar receivers, delivers heat to the boiler, which generates steam. Further steam is used in the heat engine to generate mechanical work to run electric generator. This scheme is very much the same as at conventional fossil fuel power plants, except for the heat being created not through combustion but through concentration of solar radiation.

General concept of solar thermal conversion system 

Credit: Mark Fedkin (modified after Duffie and Beckman, 2013)

In general, the thermodynamic power cycles can be categorized into gas cycles and vapor cycles. In gas cycles, the working fluid is only present in the gas phase throughout the entire cycle. In vapor cycles, the working fluid can be transformed from the vapor phase to the liquid phase in different parts of the cycle. Rankine cycle, commonly used in stationary steam power plants, is an example of a vapor cycle with water as a working fluid.

In the course of a thermodynamic power cycle, the working fluid goes through a series of temperature and pressure (T,P) parameters. Changes in temperature and pressure result from heating, condensing, pressurizing, and expanding the fluid medium. Expansion creates the physical force to perform mechanical work, which is the main purpose of the system. The efficiency of the cycle is typically higher when the difference between the lowest and highest temperatures is maximized. For example, Carnot efficiency is:

$$\eta =\frac{T_{\max }-T_{\min }}{T_{\max }}$$

The maximum temperature is set by the heating source - for example, solar concentrator. The minimum temperature is set by the ambient conditions or cooling system - for example, air, river. The Carnot efficiency of 50% is considered good for a real system.

Several thermodynamic cycles that may be considered in connection to solar thermal applications are:

• Rankine Cycle
• Brayton Cycle
• Stirling Cycle
• Kalina Cycle

One of the criteria to combine these cycles with the solar thermal plants is the compatibility of temperature. The concentrating technologies must be efficient enough to generate high temperatures for efficient power conversion in the thermodynamic cycle. So, depending on the technology and type of solar collectors, one or another cycle may be chosen.

Another important criterion to consider is the choice of the working fluid. For higher temperatures (600 ^oC), steam is the best choice. For lower temperature conversions (100-400 ^oC), organic fluids are more suitable (Batton, 2000). The desirable properties of the working fluid are:

• low cost
• non-corrosive nature
• thermal stability
• high cycle and turbine efficiency

Possible choices for the working fluids include:

• water
• refrigerants
• organics
• ammonia
• toluene (under development)
• mixtures of above
• fully fluorinated benzene ring fluids

Currently, Rankine cycles are most promising for handling the collector temperatures.

7.6. Rankine cycle

We are going to overview the principle of thermodynamic cycle operation using Rankine cycle example, since most of solar power cycles currently operating are Rankine cycles.

The Rankine cycle system consists of a pump, boiler, turbine, and condenser. The pump delivers liquid water to the boiler. The boiler heated by the solar heat converts water to superheated steam. This steam is used to run the turbine which powers the generator. Steam leaves the turbine and becomes cooled to liquid state in the condenser. Then the liquid is pressurized by the pump and goes back to the boiler. And the cycle continues. Schematically, this process is represented in Figure 10.1.

Figure 10.1. Rankine cycle layout.

Credit: Andrew.Ainsworth via Wikimedia Commons [9]

In an ideal Rankine cycle, all the units operate with the steady-state flow, and the kinetic and potential energy of the fluid are considered to be negligible compared to the temperature and pressure effects.

The work terms for each component of the cycle can be expressed as follows.

The work done by the pump to compress water (W_pump) can be represented as the change in enthalpy (H) of the water (fluid) before entering the pump and after leaving the pump:

$$W_{\text{pump}}=H_2-H_1$$

In this case, we assume there is no heat exchange with the surroundings, so the energy is not lost (which is an ideal scenario). The process, which is not accompanied by any heat exchange with the environment, is termed "adiabatic." So, this step 1-2 is adiabatic compression.

The next process 2-3 takes place in the boiler. The energy balance in the boiler can be expressed as the change in enthalpy of the fluid from the "before" state (compressed liquid) to "after" state (superheated steam):

$$Q_{\text{in}}=H_3-H_2$$

Where Q_in is the heat used by the boiler. This heat is supplied to the boiler from the solar concentrator. There is no pressure change in the boiler, only heat transfer to the fluid; therefore, no mechanical work is done here.

The next process 3-4 is expansion of the steam in the turbine. The work done through that process is the useful work, which is the main purpose of the cycle:

$$W_{\text{turbine}}=H_3-H_4$$

Here, we again assume that there is no heat exchange with the surrounding, so all the fluid energy change is converted to work. Note that the enthalpy change is written as "before" minus "after" because the energy of the superheated compressed steam is higher than the expanded steam after it exits the turbine. So, this expression gives us the positive work value.

Lastly, the process 4-1 is steam cooling and condensation. The energy balance on the condenser will be:

$$Q_{\text{out}}=H_4-H_1$$

At this stage, the extra heat is withdrawn from the system, and water returns to liquid state. This is important for the Rankine cycle from technological standpoint, since pumps employed in the system require liquid medium to work efficiently and may have problems with water-vapor mixtures.

The energy balance for the whole cycle is then can be expressed via the following equation:

$$(Q_{\text{in}}-Q_{\text{out}})-(W_{\text{turbine}}-W_{\text{pump}})=0$$

The net work done by the system is W_turbine-W_pump. Therefore, the thermal efficiency of this cycle can be presented as follows:

$$\eta =\frac{W_{\text{net}}}{Q_{\text{in}}}=1-\frac{Q_{\text{out}}}{Q_{\text{in}}}$$

The basic Rankine cycle is presented in terms of temperature and entropy change in Figure 10.2. The ideal state of this cycle is reflected in the vertical lines 1-2 and 3-4, when the fluid compressed and expanded. Those processes are shown to proceed isentropically, i.e., without entropy change. That rarely happens in real life. Some fluid friction losses and dissipation of some heat to the surrounding usually makes this system deviate from the ideality (as for example, shown by the dashed lines).

In a non-ideal cycle, fluid friction results in the lower pressure in the line. To compensate for this pressure drop, the water needs to be pumped to a higher pressure. Heat loss can happen when steam flows through the connecting pipes and the cycle components, which are not perfectly insulated. To maintain the same work output, more heat needs to be transferred to the steam in the boiler.

Figure 10.2. Temperature-entropy diagram of the ideal Rankine cycle.

Credit: Olivier Cleynen via Wikimedia Commons [10]

Varieties of the Rankine Cycle

There are several scenarios of employment of the Rankine steam cycle in power plants, including solar plants. Those scenarios intend to increase the overall efficiency of the system.

There are three ways to increase the efficiency of the basic Rankine cycle (Gramoll, 2015):

1. Decreasing condenser pressure. This results in lower heat rejection temperature of the fluid in the condenser (pushing point 4 on the diagram in Fig. 10.2 downward), thus allowing the system to produce greater net work.
2. Superheating steam to a higher temperature allows achieving higher temperature differential, thus increasing the amount of work done by the cycle.
3. Increasing the boiler pressure results in higher average steam temperature in the boiler. This effect allows additional work to be done in phase 2-3 (Fig. 10.3c). However, there is some loss of useful work in phase 3-4 because of necessity to re-heat the steam. Reheating is used to mitigate higher moisture content of the high-pressure steam.

The above-described efficiency modifications are illustrated in Figure 10.3.

Figure 10.3. Effects of different parameters on the work output of the Rankine cycle: (a) effect of decreasing condenser's pressure, (b) effect of superheating steam to a higher temperature, (c) effect of increasing boiler pressure. The shaded area shows the extra work performed by the system due to each parameter change. The red curve corresponds to the basic cycle, and green curve shows the adjusted cycle.

Credit: Kurt Gramoll [11]

Another variety of Rankine cycle is the Regenerative Rankine. The idea behind the regenerative cycle is to increase the temperature of the feed water that is supplied to the boiler. Why is it desirable? Higher water temperature would allow some energy savings for steam generation. So, some of the steam that exits the turbine is used for pre-heating the feed water for the boiler. This process is called regeneration. Heat transfer can be achieved using a heat exchanger (regenerator). There are two types of feed water heaters: open and closed.

The open feed water heater is essentially a mixing chamber, where the steam extracted from turbine is combined with water from the pump. The fluid that exits the mixing chamber is saturated water. The closed feed water heater is a heat exchanger, inside which the steam condenses on the outside of the tube carrying the feed water. As a result, the feed water temperature is increased. The pressures of the steam and the feed water do not need to be matched since the flows are not mixing. Without heat regeneration, the feed water temperature would be much lower and would require more energy from the heat source for the boiler. Regeneration helps raise the overall efficiency of the system.

Figure 10.4. T-S diagram of the regenerative Rankine cycle. In case of open feed water-heater, the phase 2-7 corresponds to the mixing, and 7-8 is the second compression of the fluid before the boiler.

Credit: Donebythesecondlaw via Wikimedia Commons [12]

Summary and Activities

In this lesson, we looked at the types, design, and components of the Concentrating Solar Power (CSP) systems - parabolic trough, solar power tower, and parabolic dish. All of these types of CSP systems essentially differ in the way how they collect and concentrate light, and how the thermal energy is transferred. There is not as much difference in the final application (with some exceptions of course), as the output energy is primarily used to produce steam and run a turbine for electricity generation. The second half of this lesson reviewed the power converion cycles that accomplish that task. Based on this lesson, you should have a clear idea on the principle of operation of each of those types of systems and should be able to explain and compare the key specifications of CSP plants. Certainly, planning of a large-scale CSP plant is a complex task, which includes not only the technical principles of energy concentration and conversion. The major decisions are also made regarding the climate setting, solar resource, policy, and cost. Those considerations involved in strategic planning of CSP projects will be addressed further in Lesson 8.

The thermodynamic power conversion cycles used to convert heat into mechanical energy (and eventually electric power) are not novel technologies. Most of those cycles were technologically developed through 20th century and were widely applied in traditional fossil fuel fired plants. However, integration of those systems in solar CSP power plants may require some adjustments to match both system parameters. Rankine cycle has been most applicable to solar power plants to date as it operates in the lowest temperature range and therefore requires the lower concentration ratios. Stirling and Brayton cycles require higher temperature range and are suitable for higher concentration ratios, like those in central receiver systems. This lesson gives you a few examples of practical implementation of power conversion cycles in existing solar facilities.

References