Their Powers Combined: NREL Postdocs Study Turbulent Winds To Boost Concentrating Solar Efficiency
Most people have seen solar photovoltaic panels mounted on rooftops or lined up across sunny fields, but fewer people are as familiar with concentrating solar power (CSP). This form of solar power uses mirrors to concentrate sunlight onto a receiver filled with fluid. The sunlight heats the fluid, producing energy, which can be used to generate electricity. CSP technology can efficiently and inexpensively store some of that thermal energy, too, which means it can provide power even when the sun is not shining.
It makes sense to build CSP facilities in areas with abundant solar resources, like the American Southwest. However, these environments can also face turbulent, high-speed winds that put a lot of stress on the CSP facility’s mirrors and support structures—which can decrease efficiency and increase design and maintenance costs.
That is where three National Renewable Energy Laboratory (NREL) postdoctoral researchers (or postdocs, for short) come in. As part of a two-year project supporting NREL’s CSP program, these postdocs are working hard to understand the impact of turbulent wind conditions on parabolic troughs—a type of CSP collector that uses a tube positioned along a curved, mirrored structure to collect sunlight.
Working alongside established NREL researchers, the postdocs collect, assess, and model field measurements of wind conditions at the Nevada Solar One facility. Their findings will help the CSP community predict the impact of these conditions on CSP structures and performance and develop solutions for more resilient CSP facilities.
We sat down with these three postdocs to discuss the paths that led them to their current roles, find out what lessons they have learned from their time at NREL, learn about their passions beyond the lab, and get a glimpse of their visions for a greener future.
Ulrike Egerer
You might say Ulrike Egerer’s head has always been in the clouds, but this NREL postdoc’s FOCUS is grounded in protecting the planet we call home.
When she was younger and still living in her native country of Germany, Egerer flew sailplanes in her free time and dreamed of becoming a professional pilot. As an adult, she earned her Diplom—a combination of a bachelor’s and a master’s degree—in aerospace engineering at the Dresden University of Technology. Diplom in hand, Egerer began her career as a mechanical engineer in the private sector. Then, she had her first child and went on maternity leave.
“That time off with my baby allowed me to ask myself what I wanted to do with my life,” Egerer said. “I started reading about renewable energy as a solution to climate change, and I decided I want to work in one of those fields—climate science or renewable energy.”
That decision led Egerer to start a Ph.D. in atmospheric science at Leipzig University, which took her on a research expedition to the Arctic. There, she led a campaign collecting wind turbulence data to feed climate change models. Her first postdoc role was at the University of Colorado Boulder (also called CU Boulder). This experience prepared her for the next stage of her professional journey, as a postdoc at the Department of Energy’s NREL.
To support the CSP project, Egerer analyzes wind measurements at the Nevada Solar One plant.
“I study how the incoming wind changes as it passes over the solar collector rows and how the different wind conditions affect the collector structures at different locations,” she said.
Nine months into her postdoc, Egerer has found that she loves the supportive, collegial environment at NREL, as well as the mission to create a healthier, greener future for our planet.
“I think because we are all passionate about fighting climate change, everyone I work with is willing to help,” Egerer said. “Not only that—I get to bike to work two to three days a week. Quite a few of my colleagues do too. When the weather isn’t good for biking, we often carpool.”
Outside of NREL, Egerer still finds ways to spend time in the clouds. In addition to the usual outdoorsy Colorado hobbies, like hiking and rock climbing, Egerer and her husband recently started paragliding.
“Paragliding is great because it combines flying—my first passion—with hiking up a mountain and just being outdoors,” Egerer said. “It’s also easier to manage with kids. I have three now, and we try to take them outside as much as possible.”
Egerer has enjoyed working on the CSP project and wants to stay at NREL for a few more years. Then, she will decide where and how to further stretch her wings.
“I’m not exactly sure where my career will take me,” she said, “but I know that the experience I’m getting at NREL will help me no matter where I go next.”
Stephanie Redfern
While working on a Ph.D. in atmospheric science at CU Boulder, Stephanie Redfern became familiar with NREL through her advisor, Julie Lundquist, who holds a joint appointment with CU Boulder and NREL. When Redfern saw an open postdoc position that called for someone with experience in wind resource assessment, she knew she had to apply. Redfern landed the job and now uses her wind modeling skills to support the CSP program as a postdoc in power engineering.
“I take the observed wind data that Ulrike collects, and I compare it against our weather model’s output to see how well the model is measuring and forecasting the winds within and around the solar plant,” Redfern said. “The hope is to develop a high-resolution wind resource data set akin to NREL’s Wind Integration National Dataset Toolkit for our study regions.”
Redfern’s work at NREL is the culmination of an impressive academic career that started with a bachelor’s degree in mechanical engineering and a master’s degree in global policy studies, both from the University of Texas at Austin, followed by a master’s degree in civil and environmental engineering from Stanford University, then added on that Ph.D. from CU Boulder.
“My interest in renewable energy really came into FOCUS when I developed a better understanding of climate change through an undergraduate engineering communication class,” Redfern said. “When I went back to school a couple of years later to study policy, my FOCUS was on climate change and learning how renewable energy could help.”
Similar to her postdoc colleagues, Redfern has found a culture of support at NREL that feels unique.
“Everyone at NREL is friendly, relaxed and open, so it’s easy to talk to people about anything from technical issues to just personal life,” she said. “I don’t think I have worked in a place before that had this openness and community feel.”
When she is not comparing wind data to wind model outputs, Redfern likes to read, run, play sand volleyball, and hike. She also used to play ultimate Frisbee.
Now living back in her hometown of Austin with her partner and family, Redfern is preparing to add to her already impressive degree collection—and to execute an ambitious career pivot. This summer, she will conclude her two-and-a-half-year chapter at NREL to start medical school at Pennsylvania State University, where she will study to become a physician.
“Climate change and renewable energy are still two things I care deeply about,” Redfern said, “but when I saw the impact the coronavirus pandemic had, I thought, ‘This is where I really want to make a difference.’”
Still, Redfern will take many valuable lessons from her time at NREL into the next chapter of her career.
“As a postdoc, I learned how to manage my time well, speak to a wide variety of people about different topics, tailor the conversation to fit their backgrounds, and how to do research,” she said. “Those skills will help me during my time in medical school and my future career as a physician.”
Principle of concentrating solar power
The principle of concentrating solar power is to collect sunlight to the solar collector device through the reflector, use the solar energy to heat the heat transfer medium (liquid or gas) in the collector device, and then add water to form steam to drive or directly drive the generator to generate electricity. The principle of concentrating solar power and thermal power generation is basically the same, and the back-end technical equipment is exactly the same. The biggest difference is that the heat source used for power generation is different. Concentrating solar power uses solar energy to collect heat, and thermal power generation uses coal, natural gas, etc. to obtain heat.
Concentrating solar power vs pv power difference comparison
The working principle is different
PV power converts photons into electrons, then into chemical batteries, and then from direct current to alternating current to power the house and grid. A lot of energy is lost in each link of the process, and this is only on a small scale of home energy storage. If it reaches a utility scale, it is conceivable that more energy is lost. Concentrating solar power generates electricity through the conversion process of light energy-thermal energy-mechanical energy-electrical energy.
Concentrating solar power uses reflectors, condensers and other concentrators to gather the collected solar radiation heat energy to the heat collection device, and heat the heat transfer medium such as heat transfer oil or molten salt in the heating device. Then, the water is heated to high temperature and high pressure steam through the heat exchange device, and the steam drives the steam turbine to drive the generator to generate electricity.
The heat source used in the front-end of concentrating solar power is the same as that of photovoltaic, and the principle of back-end technical equipment and thermal power generation is basically the same. The advantages of both are combined to achieve the purpose of clean and stable power supply.
Different application scenarios
In addition to centralized power generation, photovoltaics can also be distributed, such as solar facilities built on the roof of the home, which convert sunlight into electrons, and distribute electricity for home use or distribution to the public grid. Concentrating solar power can only be centralized at present, especially suitable for construction in areas with vast terrain and long sunshine time.
Concentrating Solar Collectors
Different ways of storing energy
PV power is greatly affected by the weather and the power supply is unstable. The traditional way of storing solar energy is to use lithium batteries. Regular energy storage lithium battery companies have high quality requirements for batteries. Lithium batteries are relatively expensive and may have safety issues. Concentrating solar power can store the excess heat energy in the energy storage container and release it at night or on rainy days, ensuring continuous and stable power generation for 24 hours. In contrast, the energy storage cost is low and it is clean and environmentally friendly.
The composition of concentrating solar power plant
Large concentrating solar power plant can be divided into four parts: heat collection system, heat transfer system, heat storage system, power generation system.
Heat collection system: The heat collection system is responsible for absorbing solar radiation energy, heating the heat transfer medium, and providing energy for subsequent power generation. It is the core component of the concentrating solar power system. The heat collection system includes two core components: a concentrating device and a receiver.
The concentrating device is controlled by a central control system to track the position of the sun to collect and reflect (redirect) the maximum amount of sunlight, and concentrate the radiant energy on the receiver. The receiver uses the collected energy to heat the internal medium to achieve energy absorption, storage and transportation.
Heat transfer system: The heat transfer system is an intermediate link that transfers the heat energy collected by the heat collection system to the subsequent system using the heat transfer medium. The most mainstream working fluid at present is molten salt.
Compared with water and heat transfer oil used in the early days, molten salt can maintain a wider working temperature range in the molten state, allowing the system to absorb and store thermal energy under low pressure conditions. However, since the high temperature molten salt has certain corrosion on the inside of the pipeline and the heat storage tank, the material requirements are relatively high.
Heat storage system: Through the heat storage tank, the concentrating solar power system can centrally store the medium heated by the collector, and then pump it out to exchange heat with water to generate steam to drive the steam turbine to generate electricity. The cooled working fluid can then flow back to the collector system again for reheating.
Why Design Now?: Z-20 Concentrated Solar-Power System
The heat energy is stored in the heat storage tank, and it can continue to work for a period of time at night or in the case of insufficient light, thereby breaking the limitation of light time and realizing ultra-long power generation time. At the same time, the energy storage tank also has the ability to adjust the output power, and can adapt to the grid dispatching power generation according to the local electricity load. Compared with photovoltaic power station, concentrating solar power has its own heat storage system.
Power generation system: The power generation system of concentrating solar power is not much different from traditional power plants. It still obtains high-quality superheated steam by heating water, and drives various steam turbines to generate electricity. Since the heat transfer medium used in concentrating solar power is recycled and generates almost no emissions, the power generation process is undoubtedly more environmentally friendly.
Concentrating Solar Thermal Overview
Concentrating solar thermal (CST) technologies produce high-temperature, high-quality energy that can be used to drive a variety of engineering processes. CST is attractive because of free and abundant solar radiation, but significant engineering challenges need to be overcome.
▲ Figure 1. Reflectors move throughout the day to concentrate solar radiation onto a central receiver at the PS10 concentrating solar power (CSP) plant in Seville, Spain. Image courtesy Solúcar.
A CST system is composed of a field of distributed reflectors that move over the course of a day to concentrate sunlight onto a solar receiver (Figure 1). CST technologies have different geometries and arrangements, but the same principle of operation: A solar concentrator focuses solar radiation onto a solar-thermal receiver that absorbs the solar radiation, converting it to thermal energy.
CST is not a new idea. Stories of its application go as far back as antiquity. Folklore within the scientific community suggests that Archimedes led attempts to FOCUS solar radiation onto Roman ships using many reflective surfaces. The first modern commercial application of CST was the implementation of a trough system in Egypt constructed in 1913 to produce hot steam to drive a pump for irrigation of semi-arid farmland (1).
The Solar Energy Generating System (SEGS), commissioned between 1984 and 1990 in the U.S., was the first use of concentrating solar for power generation. The SEGS project consists of nine separate trough power plants that have a total 354 MWe nameplate power generation capacity (2). The project marked the beginning of a resurgence of interest in CST technology.
All of the components in a CST plant are designed to facilitate the absorption of well-focused sunlight by the solar receiver; thus, all the interactions that the solar radiation undergoes must be considered.
▲ Figure 2. Solar radiation from the sun passes through Earth’s atmosphere and is concentrated onto a solar receiver. The thermal energy produced can be used, stored as thermal energy, or converted to chemical energy and stored.
The entire atmosphere — from the top of the Earth’s atmosphere to the concentrator, and between the concentrator and receiver (Figure 2) — affects the quality of the solar radiation. The night and day cycle, clouds, and aerosols in the atmosphere cause intermittencies in the solar radiation reaching the receiver. Intermittencies must be accounted for, but energy storage technologies can mitigate their detrimental effects on CST performance.
Designing and characterizing solar concentrators for CST applications involves optimizing the concentration ratio of the radiation considering the collector cost and material limitations. A low concentration ratio implies low-temperature thermal energy and thus low exergy; a high concentration ratio implies large thermal losses from the solar receiver. In addition, heat losses at the solar receiver must be considered (as discussed in more detail later).
This general discussion of CST technologies details the different types of solar collectors and the various applications to which CST can be applied, including electricity generation (i.e, concentrating solar power [CSP]), desalination, advanced oil recovery, and solar thermochemistry incorporating the production of solar fuels.
CSP is an important application of CST technology. The term encompasses both CST plants that are used to generate electricity and concentrating photovoltaic (CPV) power plants. This article focuses on CST’s application for power generation and does not discuss CPV.
Solar radiation and the atmosphere
Solar radiation incident at the outer edge of the Earth’s atmosphere has an intensity of about 1,370 W/m 2 — the so-called solar constant. Because of the high temperature of the sun, the radiation has a very high exergy. As viewed from the Earth, the sun subtends a half-angle of about 4.65 mrad. Propagation through the atmosphere affects the intensity and the angular distribution of the incoming solar radiation. Aerosols scatter the incoming radiation, increasing the amount of circumsolar radiation, which affects the concentration and distribution of the image at the receiver of a CST collector. Clouds attenuate and scatter the radiation.
Thus, it is important to understand the time dependence and spatial distribution of clouds and aerosols. Meteorological models and experimentally derived data from ground stations and satellites provide an overview and inform predictions of time-dependent changes in the atmosphere, which help to define the expected functionality of CST technologies.
Radiation incident on the surface of the Earth is categorized as either diffuse or direct. In direct solar radiation, most of the angular distribution of the solar source is preserved, and this direct radiation can be concentrated and used in CST applications. Diffuse solar radiation cannot be concentrated because the angular distribution is randomized and optics cannot FOCUS it.
Therefore, CSP plants rely on direct solar radiation, the intensity of which is described by the direct normal irradiance (DNI), expressed in SI units as Watts per square meter. Another important metric is the global horizontal irradiance (GHI), which includes the diffuse component of the solar radiation and is measured at a plane parallel to the Earth’s surface.
▲ Figure 3. This map of the global direct normal irradiation shows that the southwest U.S., northern Chile, Peru, central Australia, Saharan Africa, and South Africa are optimal regions for concentrating solar thermal (CST) technologies. Source: DNI Solar Map © 2016 Solargis (23).
Figure 3 is global map of the direct normal irradiation, which is a time integral of DNI. High direct normal irradiation makes the southwest U.S., northern Chile, Peru, central Australia, Saharan Africa, and South Africa prime real estate for development of CST technologies. Spain is a favorable location for CST development in Europe.
Solar concentrators
▲ Figure 4. Four types of concentrators are typically used in high-temperature CST applications: (a) linear-Fresnel reflectors, (b) parabolic trough reflectors, (c) central receiver (or power tower) systems with heliostat reflectors, and (d) paraboloidal dish reflectors. Source: Adapted from (24).
Choosing a concentrator (i.e., reflector) type is one of the chief optimization challenges of the fledgling solar thermal industry. CST plants use four different types of concentrators: linear Fresnel reflector (LFR), parabolic trough reflector, central receiver system with heliostats, and paraboloidal dish reflector (Figure 4). Each reflector type is defined by its ability to intercept and guide solar radiation to a thermal receiver that is engineered specifically for the reflector type and application. LFR and parabolic trough collectors are classified as line concentrators and track the sun along one axis. Central receiver systems with heliostats and paraboloidal dish collectors are called point concentrators and track the sun in two directions.
Linear Fresnel reflectors (Figure 4a). A LFR is composed of strips of mirrors. Each strip rotates around the long axis to guide incident solar radiation to a stationary receiver located above the reflectors. The collectors are aligned north-south.
LFRs are low-cost, low-concentration line-FOCUS collectors. Benefits include small reflector size, low structural cost, a stationary receiver position, and noncylindrical receiver geometry that reduces convection losses. Receivers designed for use with LFRs often incorporate a secondary concentrator to.
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Solar Dish Collector
Another type of concentrating solar collector that optically reflects and focuses the suns incident solar energy onto a small receiving area using mirrors or lenses is called a Solar Dish Collector, or more technically, a point focusing collector.
By concentrating the sunlight to a single spot, the intensity of the receiving solar energy is magnified many times over with each mirror or lens acting as a single sun shining directly at the same focal point on the dish meaning that more overall power per square meter of dish is achieved.
The concentration factor, also known as the “number of suns”, of a solar dish collector can be greater than 1,000 suns reaching temperatures at the focal point of the receiver (called the “target”) approaching several hundred or even several thousand degrees Celsius depending upon the size of dish and its location.
Unlike the previous solar collector which was in the shape of a long trough, a parabolic solar dish collector is very similar in appearance to a large satellite TV or radar dish making it much smaller than a long trough collector. The curved parabolic shaped dish, which is generally referred to as a “solar concentrator” is the main solar component for this type of solar heating system.
Solar Dish Reflection
The solar dish is formed into a paraboloidal shape by stamping them out from thin sheet metal or thin aluminium coated mylar, and which themselves can be anywhere between a few feet to several metres in diameter. The parabolic dish collects the incoming solar energy directly from the sun and concentrates or focuses it on a small focal point area positioned in front of the dish.
The parabolic solar dish is covered with many small mirror reflectors all around its shape to help concentrate the thermal energy into a single focal point were the heat absorber is located producing more overall thermal energy per square meter of dish.
These highly polished mirrors can reflect more than 90% of the sunlight that hits them increasing the efficiency of the dish by more than 20% compared to the parabolic trough collector. Mirrors are generally used instead of a single highly polished dish because they are relatively inexpensive, can be easily cleaned and last a long time in an extreme outdoor environment, making them an excellent choice for the reflective surface of a solar dish collector. Also individual mirrors can be easily changed if damaged.
As well as the solar dish collector, some form of thermal receiver is required to convert the focused beam of intense solar energy into heat. The solar receiver can be as simple as a small evacuated tube or a more complex solar heat engine, such as a Stirling engine generator.
Due to the very high temperatures at the focal point, a thermal oil type fluid is generally used instead of water inside the receiver, which transfers the intense heat created by focusing the sunlight on the receiver. Like the trough collectors, solar dish collectors can be used singly or linked together for larger industrial type applications.
Solar Dish Collector type systems can also be part of another solar technology called a “solar dish-engine” system. The dish part of a solar dish engine system is very similar to the one described above, but may include many individual but smaller parabolic mirrors instead of one large single dish all angled and focused to the same focal point.
Solar Dish Collector Engine
As their name suggests, dish-engine solar collectors include a special type of solar engine built into the solar receiver. This so called heat engine, is driven by the solar thermal energy converting it into rotational mechanical output by the cyclic compression of the engine’s working gas, which is usually helium or hydrogen.
The mechanical power that is produced is then used to directly drive an electrical generator or alternator producing a significant amount of AC electrical power. These types of solar powered heat engines are commonly called a Stirling Engine after its inventor Robert Stirling in the year 1817.
Stirling engines belong to the group of closed-loop hot-gas machines that work on the basic principal that a gas will change its volume when subjected to a heat change producing an isothermal compression of the cold and isothermal expansion of the hot gas at a constant volume. This temperature change, and thus the continuous operation of the engine, is produced by moving the gas between two different chambers producing a constantly high and a constantly low temperature.
The efficiency and operation of the Stirling heat engine is determined by the operating temperature of the gas which is kept between 650 o C and 750 o C. To constantly keep the reflected solar radiation at the correct focal point and temperature during the whole of the day, a two-axis sun tracking system is used with the dish which continuously rotates the solar concentrator.
Like the other types of concentrating solar collector technologies, practical solar dish collectors or Stirling collectors are not suitable for domestic hot water systems due to their size, cost and very high operating temperatures. As with a trough reflector, or solar dish/Stirling systems. They are also modular in design allowing them to be connected together to form a collector field were they are connected in parallel rows ranging in size from a few kilowatts to tens of megawatts.
Solar Dish Collector As A Cooker
As well as using solar dish collectors to generate electricity at very high temperatures, the concentrating type parabolic solar dish can also be used to cook food. Something as simple as an old abandoned one metre diameter satellite dish covered in aluminium foil can be turned into a solar cooker with a black cast iron cooking pot located at its focal point. In fact a parabolic solar cooker can even be made using an umbrella and covering the inside with ordinary aluminium foil but its efficiency would be limited.
Solar cookers can be used in camping or remote areas to boil water, fry eggs or cook cakes and breads at temperatures well over 200 o C, which is hot enough for cooking most foods without the danger of a fire.
Solar cookers, and ovens are relatively inexpensive and easy to make for remote cooking applications, even backpack versions are sold for camping. However, solar cookers require frequent adjustment to stay focused on the sun as well as supervision for safe operation.
A word of warning about using solar cookers, the focal point and cooking pot gets very hot so it is advisable to wear good quality sunglasses and gloves as the focused solar energy glare can create nasty burns or could damage your eyes for good. So avoid standing on the sunny side of a solar dish collector.
In the next tutorial about Solar Heating we will look at another type of solar thermal collector which is designed to concentrate the received solar radiation even more into a single focal point using an array of adjustable mirrors around its base.
This type of solar thermal collector can reach temperatures well into the thousands of degrees and is generally known as a Solar Power Tower. Solar power towers FOCUS the sun’s energy onto a single point hot enough to melt salt.