Lesson 8 will cover a few important issues related to planning the large-scale CSP facilities. As we have already understood, performance of solar systems is very dependent on the locale, weather, and other physical conditions. Therefore, selection of a good site for a CSP project is a strategic step to take in order to ensure that the technology performs to its potential. So, criteria to consider in site selection are addressed in this lesson. Next, we will take a look at some factors in the social and economic spheres that pertinent to success of the solar projects. Finally, this lesson also touches upon the life cycle assessment of the CSP technology - LCA study examines a variety of metrics, which help to constrain the project with available resources and produced environmental impacts. In this lesson, you will be assigned to learn from a couple of published papers which present different methods of project assessment. Also, a lesson activity done in SAM software will give you some practical exercise with system parameters.
By the end of this lesson, you should be able to:
Report: Stoddard, L., Owens, B., Morse, F., and Kearney, D., New Mexico Concentrating Solar Plant Feasibility Study [1], Report to New Mexico Energy, Minerals and Natural Resources Department, 2005.
Journal article: Lechon, Y., de la Rua, C., and Saez, R., Life Cycle Environmental Impacts of Electricity Production by Solarthermal Power Plants in Spain [2], J. Solar Energy Eng., 130, (2008).
Journal article: Caldes, N., Varela, M., Santamaria, M., and Saez, R., Economic impact of solar thermal electricity deployment in Spain [3], Energy Policy 37, 1628-1636 (2009).
Solar energy systems rely on the natural solar resource, which is unevenly distributed over the planet. Clearly, some locations may be considered very favorable for harvesting sunlight, while others - much less favorable for a number of reasons. Choice of the CSP development site has therefore a strategic importance for the long-term feasibility of the project. Meteorological availability of sunlight is the first, but not the only, limitation to be considered when planning a CSP site development. One should also look at the physical geography of a potential site, available land area, available infrastructure, energy market, political and social situation. All these factors, being location-specific, can have a critical influence on the success of the CSP development in the area. In essence, the project development approach to CSP is no different from other project developments, such as those involving conventional fossil fuel power plants, photovoltaic solar, wind, etc. However, CSP has its own specific features that require special knowledge when site selection and feasibility study are performed.
Any site targeted for CSP should be carefully examined with respect to the key criteria. The most important of them are listed below:
The simultaneous analysis of the multiple factors make the site assessment a complex iterative process. The main successive phases of this process are illustrated below (Lovegrove and Stein, 2012):
Each subsequent stage in the above scheme requires more specific information and additional expertise. This process of site characterization and selection is to some extent illustrated in the report by Stoddard et al. (2005) - the study that considered various alternatives for CSP plant development in New Mexico:
Report: Stoddard, L., Owens, B., Morse, F., and Kearney, D., New Mexico Concentrating Solar Plant Feasibility Study [1], Report to New Mexico Energy, Minerals and Natural Resources Department, 2005.
Please read Section 3.0 of the document. Especially pay attention to Table 3-1, which provides a comparative matrix for nine different sites with potential for CSP development.
The water use strategy at the CSP facility is one of the keystones of the project, as it would influence many other factors and choices and may become a go/no-go tipping point for the project. This is because water shortage is being identified as a severe environmental problem in many regions, and, therefore, is subject to strict environmental regulations. So, let us take a closer look at the water requirements in CSP systems and associated technological options.
The biggest consumption point of water in CSP is cooling for the steam turbine condenser. Cooling systems can utilize both salt and fresh water, which can be taken from both surface and subsurface reservoirs. Agreement on water withdrawal should be reached prior to the project start, and that should be part of the project feasibility analysis. There are wet-cooling and dry-cooling systems. The wet cooling usually implies using a cooling tower, and that is the most efficient technology for cooling as long as water is affordable and continuously available for plant operation.
If you are not familiar with those, watch the following video (3:16) to see how they work:
However, cooling towers use up to 85-90% of all process water. When water is in short supply, dry-cooling systems can be an alternative. Dry-cooling systems use ~10 times less water than wet cooling systems do; even truck-based supply may suffice. However, from an economic point of view, dry cooling systems are less beneficial. In dry cooling systems, air is used as heat transfer medium, and air has much lower heat transfer coefficient than water. Furthermore, the cooling effect of evaporation, which is the core mechanism of cooling in cooling towers, is not available. This results in lower efficiency of the water-steam cycle. Another drawback of the dry-cooling systems is additional power consumption by fans blowing air for cooling. For the above reasons, the wet-cooled projects have an economic advantage over dry-cooled projects. The decision involving the trade-off of water versus energy is to be made individually in each particular case based on available resources. One of the compromise options for water use is hybrid cooling tower, which combines dry cooling and wet cooling. In this technology, water is sprayed on the condenser allowing for evaporative action, but the water consumption is significantly lower compared to conventional wet cooling method.
According to water use estimation by Andrew Eilbert (Worldwatch Institute), on the average, CSP plants use only 120 Gal of water per megawatt-hour of energy. For comparison, this number is lined up against typical values for other types of power plants in Table 8.1. [Eilbert, 2010]. Visit the WorldWatch website [6] for specific data on water use by different CSP plants in California.
Type of power plant | Average lifecycle water use |
---|---|
CSP (with dry or wet cooling) | ~120 gal/MWh |
Powder River Basin coal power plant | 523 - 1,084 gal/MWh |
Conventional natural gas combined cycle power plant with wet cooling | 152 - 525 gal/MWh |
Conventional nuclear power plant with wet cooling | 475 - 900 gal/MWh |
Other water uses in CSP plants include:
The water involved in water-steam cycle needs to be relatively pure and often needs to be de-mineralized. Requirements for water purity is specified by the turbine manufacturers. These requirements impose an additional limitation on the water sources. If raw water contains significant amounts of ions and other chemicals, a special water treatment plant may need to be added to the facility. Much of this water is recycled, and its total volume is not substantial.
Please answer the following self-check questions before proceeding to the next section.
What are the main types of site-related costs to consider in site selection?
CSP deployment has a number of positive collateral impacts on environment and social welfare, and those impacts are important to consider in the project feasibility analysis. Sometimes, when project evaluation is solely based on the energy prices, renewable energy technologies may not look competitive enough at the modern energy market. However, including externalities, such as greenhouse gas emission reduction effect, improved diversity, security of the energy supply, employment, etc., into the evaluation process can help to justify the value of a renewable energy project more fairly.
One of the prominent impacts of CSP in the socio-economic area is stimulation of the economy and creation of new jobs at the local level. This is largely due to relatively "low-tech" profile of this technology: the main components are mirrors, steel, concrete, and labor. These local impacts are realized through the increase in demand for goods and services and creation of jobs. These impacts can be classified as direct, indirect, and induced.
The direct effects imply the increased demand for good and services that are required to construct, operate, and maintain the CSP facility. The indirect effects involve the one the new investment has on new sales and material flows among other productive sectors of the economy. The induced effects are related to expansion of private expenditure (for example, from workers employed) in goods and services, such as food, health care, transportation, etc. [Lovegrove, Stein, 2012]. For proper accounting of all levels of external benefits, there should be a way to quantify them and to assign them a monetary value.
One of the analytical methods to quantify the externalities of energy projects is based on Input-Output (I-O) analysis. The I-O analysis is an economic theory that was developed by Wassily Leontieff, a Russian economist who received a Nobel Prize for it. This model describes the inter-industry relationships within an economy, connecting the outputs from one part of the economy to the inputs to another part of the economy. The data are typically expressed as monetary values and are organized as a matrix, with column entries representing inputs to an industrial sector, and rows representing outputs from that sector. [Input-Output Model from Wikipedia.org [8] ]. Essentially, this approach recognizes that spent investment becomes income to other industries, and thus stimulates their development.
To understand how this method can be applied to CSP projects, we refer to the following publication, which analyzes the socio-economic impacts of parabolic-trough and solar-tower plants in Spain. The authors come up with impressive numbers for increase in demand and employment impacts, demonstrating the remarkable potential of CSP for benefiting local economy.
Journal article: Caldes, N., Varela, M., Santamaria, M., and Saez, R., Economic impact of solar thermal electricity deployment in Spain, Energy Policy 37, 1628-1636 (2009).
This article is available online through the PSU library system. See copy of the article in Canvas Module 8.
This article nicely demonstrates the quantified benefits from solar thermal energy projects, which are rarely taken into account in feasibility analysis. The type of analysis presented there is a convenient tool to determine whether the government subsidies provided to defray the cost of the renewable technology are justified in terms of social welfare.
Some main points in the above-mentioned study are included in the self-check questions below.
As a technology based on the renewable solar resource, the CSP has a great potential to reduce greenhouse gas (GHG) emissions. Emissions typically associated with conventional electricity production result from coal or natural gas burning and have been a severe factor in global pollution in climate change. Hence, installation of CSP offsets generation from fossil fuel plants. It does not mean, however, that CSP technology is emission free: while operational emissions are negligible compared to fossil-fuel power plants, lifecycle emissions may be still significant. To estimate the GHG emission level and the magnitude of the emission reduction benefit, one can employ Life Cycle Assessment (LCA) - a comprehensive methodology dealing with inventory of all processes and materials involved in a technology.
Life Cycle Assessment (LCA) is a “cradle-to-grave” approach for assessing products, processes, industrial systems, and the like. “Cradle-to-grave” begins with the gathering of raw materials from the earth to create the product and ends at the point when all materials are returned to the earth. LCA evaluates all stages of a product's life from the perspective that they are interdependent, meaning that one operation leads to the next. LCA enables the estimation of the cumulative environmental impacts resulting from all stages in the product life cycle, and, as a result, allows selecting a path or process that is more environmentally benign.
LCA helps decision-makers select the product, process, or technology that results in the least impact to the environment. This information can be used with other factors, such as cost and performance data, to find optimal solutions. LCA identifies the transfer of environmental impacts from one media to another (for instance: a new process may lower air emissions, but creates more wastewater, etc.) and between different lifecycle stages. The diagram below illustrates the main lifecycle stages to be considered in LCA:
The diagram above lists the main stages of product lifecycle: (1) raw material acquisition, (2) manufacturing / construction, (3) operation or use, and (4) recycling or waste disposal (decommission stage). Each of these stages has inputs of materials and energy and outputs of atmospheric emissions, waterborne wastes and solid wastes. Each stage creates the main useful input to the next stage, and usually the operational stage of the lifecycle is where the main product of the technology is produced. Any co-products (desirable or undesirable) are also identified and taken into account in the analysis. The LCA based on this scheme is a complex process (even for small systems), which requires large amount of data and interdisciplinary expertise for proper assessment. A typical LCA project plan includes the following main steps:
Life Cycle Assessment: Principles and Practice [9], EPA/600/R-06/060, 2006.
When the LCA analysis is applied to a CSP plant, a number of impact metrics need to be identified. One of them, as mentioned above, is greenhouse gas emissions, but there are also other impacts that involve environmental harm. Other typically assessed impacts are acidification and eutrophication potential. Acidification is referred to as increase in acidity of natural waters and soils. It may cause loss of aquatic life, forests, and other plants in the area and increase ecotoxicity. Eutrophication is related to nutrient enriching in aquatic and terrestrial environments. It can cause harm to ecological system through a chain of feedbacks, change in dissolved oxygen contents.
With respect to above metrics, CSP offers significant benefits, according to various studies reviewed in Lovegrove and Stein (2012). Values of GHG emissions by CSP plant (in case of solar only operation - no hybrid systems with natural gas backup) are estimated in the range from 11 to 90 g CO2 equivalent per kWh of electric energy generated. For central tower plants, the emissions are on the lower side, and for the parabolic trough plants, the emissions are on the higher side of that range; but, in either case, these numbers are well below the values typical for other electricity generation facilities (Table 8.2).
Technology | GHG emissions g CO2 equiv/kWh |
---|---|
CSP (solar thermal) | 11-90 |
Solar photovoltaic (PV) | 57-109 |
Nuclear power | 14 |
Heavy oil condensing | 726 |
Light oil gas turbine | 436 |
Coal/lignite | 690-820 |
Natural gas combined cycle | 391 |
The CSP emissions have been shown to increase by 650 g CO2 equiv./kWh if a fossil-fuel back-up is used and by 60 g CO2 equiv./kWh if heat storage is used. The different studies indicate that CSP's GHG emissions are mainly associated with steel and concrete used in solar field and tower, and salts used in storage systems. The use of synthetic salts instead of naturally mined salts for storage system results in an increase in GHG emissions by ~13 g CO2 equiv/kWh. Also, non-renewable electricity and materials may be used at the manufacturing stage of the project and in transportation to the site. It is known that the use of dry-cooling system instead of wet-cooling system increases GHG emissions by ~2 g CO2 equiv/kWh.
Acidification impact of CSP reported in the range 70-100 mg SO2 equiv/kWh. In case of hybrid plant (solar + fossil fuel backup), this number is substantially higher - 590-612 mg SO2 equiv/kWh. In case of eutrophication, the impact range is 6-10 mg PO4/kWh for solar only mode and ~50 PO4/kWh for hybrid mode.
It is important to note that these impacts take place during the operational phase only because the plant consumes power from the grid rather than power being produced on site.
A published study by Lechon and co-authors (2008), referenced below, describes the LCA analysis for CSP systems in more detail. The first two sections of the article define the method and scope of LCA, so you can just briefly look through those. It is more important to look at the data tables that contain specifications of the assessed facilities and various metrics. Usually, LCA studies collect considerable amounts of data, and our goal here is to learn to read and to interpret that information.
Journal article: Lechon, Y., de la Rua, C., and Saez, R., Life Cycle Environmental Impacts of Electricity Production by Solar thermal Power Plants in Spain, J. Solar Energy Eng., 130, (2008).
This article is available through the PSU library system. See a copy of this article in Module 8 in Canvas.
As you can understand from the above article, the energy consumption at different stages of the system lifecycle is still the main source of the environmental impact. The numbers characterizing two different CSP facilities in terms of energy consumed and energy generated are listed in Table 3, which is the basis for finding the lifecycle emissions. Also, pay attention to Table 6, where the emissions are itemized by system component. This itemization can be helpful in finding engineering solutions to further decrease the environmental impact of the technology. Note that the plants assessed in this study are hybrid (solar thermal + natural gas 15%), and this is the reason for higher overall GHG emissions calculated for this case compared to solar-only values given in Table 8.2 above.
What sources of GHG emissions can you assume at the construction phase of the CSP plant?
The biggest challenge in prediction of the CSP performance coming from a projected solar thermal plant is the unsteady nature of the solar resource. Models involve weather and insolation data which bring in uncertainties in the final power output. So, it is not sufficient to calculate the annual energy yield simply by expected load hours, as it is usually done for conventional fossil fuel power stations.
The energy flow within a CSP plant follows a complex chain of transformations, each provided by a certain technology block. This step-wise energy flow can be represented by the following diagram:
Each step in the power conversion chain is performed by technological units, each characterized by a number of parameters, which can be optimized depending on the scale of the facility, external conditions, available resources, and other preferences. Simultaneous optimization of those parameters and computation of the power output can be done by System Advisor Model (SAM) software, distributed by the National Renewable Energy Laboratory (NREL). SAM currently is one of the widely used and most developed tools for solar plant modeling, which incorporates both energetic and economic parameters. SAM has a user-friendly graphic interface, which allows the operator to see diverse technical performance information in table and graphs.
In SAM, the setting for each of the above mentioned technological units can be predefined and controlled separately, which creates opportunities for simulating different solar systems at different locations.
If you are not yet familiar with how the SAM software works, please refer to the following video (35:08), which explains the basic things about the program interface.
As an activity in this lesson, you will use a few examples from SAM to explore the differences between different cases of CSP installations.
Different types of fluids are commonly used for storing thermal energy from concentrating solar power (CSP) facilities. CSP plants typically use two types of fluids: (1) heat-transfer fluid to transfer the thermal energy from the solar collectors through the pipes to the steam generator or storage, and (2) storage media fluid to store the thermal energy for a certain period of time before it is used on demand.
These are some available heat transfer and storage fluids currently used:
Water is the most available and cheapest fluid to use, but the problem with water is that it has very limited temperature range when it is liquid. Keeping water in liquid state above 100 oC requires high pressure, which adds significantly to system complexity and cost. Oils and other synthetic liquids are commonly used in CSP plants, as they have a much wider working temperature range. Molten salts are probably the most common storage medium (Wu et al., 2001), but are not the best heat transfer medium, because salt tends to solidify in tubes at lower temperatures, blocking transport. Additional heating will need to be provided in that case to start up the plant. There are developments for novel fluids that can be used for both heat transfer and storage at the same time (Moens and Blake, 2004). Some commonly used fluids and their working temperature ranges are shown in the diagram in Figure 9.1.
Molten salt storage is employed at many existing solar thermal plants, so we are going to look at it in some more detail. As was mentioned above, salt "freezing" (i.e., transforming from a molten state to solid state) can present some problems, because the salt is supposed to circulate through the tubes to deliver thermal energy to the steam generator or another application. So, to achieve the lowest possible "freezing" temperature, a eutectic mixture of salts is used.
A eutectic system is a homogeneous mixture of two or more components, which together have a lower melting point than each of them separately. The eutectic mixture melts as a whole only at a specific ratio of those two components in the mixture. A generic eutectic phase diagram is shown below.
Typical molten salt mixture used for energy storage is represented by the ternary eutectic
53 wt % KNO3 (potassium nitrate)
40 wt % NaNO2 (sodium nitrite)
7 wt % NaNO3 (sodium nitrate)
or binary eutectic
45.5 wt % KNO3 (potassium nitrate)
54.5 wt % NaNO2 (sodium nitrite)
The eutectic temperature for those compositions is in the range 142 to 145 oC.
The molten salt is used for high temperature energy storage applications (above 400 oC) because typical thermal fluids, such as synthetic oils, have temperature limitation and decompose beyond the maximum allowable temperature (410-430 oC).
This is how the molten salt storage is employed in a solar thermal plant. First, the solar energy is caught by the collectors and concentrated on the receiver tube filled with heat transfer fluid. The heat transfer fluid (with temperature of ~393 oC) is circulated in a closed loop to deliver heat to the steam generator, which produces superheated steam, and then the thermal fluid flows back to the solar collectors (with temperature of ~ 293 oC). Such a loop can only operate during sunshine hours. To extend the steam generation beyond sunshine hours, molten salt thermal energy storage is used. The thermal storage usually consists of two salt storage tanks. In this case, the closed loop with the heat transfer fluid is passed through one of the salt tanks, where salt is heated to the temperature of ~ 384oC. The tank is insulated, so salt can stay hot for a substantial period of time (estimated heat loss ~0.5 oC per day). The molten salt is stored in the tanks at ambient (atmospheric pressure). To discharge heat during night hours, the molten salt from the hot tank is pumped through the steam generator to produce steam, and then to the cold storage tank (at ~ 292 oC). In this configuration, some part of the heat transfer fluid loop is diverted to the heat exchanger between the cold and hot salt tanks. The cooled molten salt is then pumped through the heat exchangers and returns to the hot salt tank.
Solar tower systems can use molten salt as heat transfer fluid and heat storage medium without involving any additional thermal transfer fluid loops due to higher radiation concentration temperatures. In this case, molten salt is flowing through the tower-mounted molten salt receiver, where it is heated to 565 oC. Then the salt is supplied to the hot salt tank, from where it flows to the steam generator. This concept is illustrated on the eSolar website [11].
In this case, the use of molten salt for both heat transfer and thermal energy storage minimizes the number of storage tanks and salt volumes needed.
The following video (~2 min) provides a simple illustration of the molten salt thermal energy storage concept.
Storing energy in fluids involves exchanging heat between different types of fluids in heat exchangers. For example, transfer of heat from a thermal fluid in solar-heated tubes to the molten salt reservoir requires a heat exchanger; further transfer of heat from the molten salt to water to produce steam would involve another heat exchanger. There is a thermal physics method to calculate the effectiveness of the heat transfer in different types of heat exchangers and to evaluate their performance. One example of such calculation is given in Section 3.17. of the book "Solar Engineering of Thermal Processes" by Duffie and Beckman (2013), referred to below.
Book chapter: Duffie, J.A. and Beckman, W.A., Solar Engineering of Thermal Processes. Section 3.17 Effectiveness-NTU calculations for heat exchangers. pp. 168-170.
This reading is available online through PSU Library system. It is an optional material to study if you are interested in more insight in how heat is transferred from one medium to another, and what kind of losses can be expected. More thorough consideration of thermal topics is included in EME 811, which is also part of RESS Solar Option program.
Strategic planning of a CSP facility involves a number of steps - site evaluation, socio-economic assessment, environmental assessment, and system modeling and optimization. All these steps must be taken before investment is made, and work has begun. Feasibility analysis requires several iterations and special expertise to justify the decision. This lesson introduced you to a few useful tools and methods to start with.
After you have covered the assigned readings for this lesson, please complete the following assignments:
Type | Description/Instructions | Deadline |
---|---|---|
Discussion Forum | Discussion Forum "Social benefits of solar thermal systems": Lesson 8 What, in your opinion, is the most important benefit of CSP technology for local economy and society in general?
|
Sunday night |
Activity | SAM exercise
Note: I hope you are familiar with the SAM software, since it was used in your EME 810 course. If you are missing that prerequisite, go to this EME 810 page [12]to learn how to install and run SAM. |
Wednesday night |
Lovegrove, K., Stein W., Concentrating Solar Power Technology, Woodhead Publishing, 2012.
NREL, 2013 / http://www.nrel.gov/docs/fy13osti/56290.pdf [7]
Eilbert, A., The Trade-Off Between Water and Energy: CSP Cooling Systems Dry Out in California, Revolt - Worldwatch Institute, 2010.
Caldes, N., Varela, M., Santamaria, M., and Saez, R., Economic impact of solar thermal electricity deployment in Spain, Energy Policy 37, 1628-1636 (2009).
Lechon, Y., de la Rua, C., and Saez, R., Life Cycle Environmental Impacts of Electricity Production by Solar thermal Power Plants in Spain, J. Solar Energy Eng., 130, (2008).
Links
[1] http://www.emnrd.state.nm.us/ECMD/Multimedia/documents/NMCSP-draft-final-rpt-02-05_000.pdf
[2] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Lesson08/Lechon%2C%20Rua%2C%20Saez.pdf
[3] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Lesson08/Caldes%2C%20Varela%2C%20Santamaria%2C%20Saez.pdf
[4] http://solargis.info
[5] https://www.youtube.com/watch?v=QfLCjj73MZA
[6] http://blogs.worldwatch.org/revolt/wp-content/uploads/2010/12/CSP.Chart_.jpg
[7] http://www.nrel.gov/docs/fy13osti/56290.pdf
[8] http://en.wikipedia.org/wiki/Input%E2%80%93output_model
[9] http://nepis.epa.gov/Exe/ZyPDF.cgi/P1000L86.PDF?Dockey=P1000L86.PDF
[10] https://www.youtube.com/watch?v=XkuXDoSsZQI&list=PLBC8A9F2F04C4AC23
[11] http://www.esolar.com/applications/ms-power/
[12] https://www.e-education.psu.edu/eme810/node/568