As time goes by, the demand for energy rises while finite resources gradually diminish. The concept of going ‘green’ or using infinite resources has become more and more common in the marketplace. With this in mind, the abundance and reliability of solar energy makes for an attractive alternative. This is because solar power is different. This statement, of course, begs the question of HOW solar power differs.
Common traditional power plants still utilizes finite fuel. Steam power plants, for example, use the fuel as an energy source to boil water which, in turn, allows the the steam to turn the turbine and drive the generator to produce electricity. Concentrated solar power systems, however, use heat energy from the sun as a heat source – which is renewable. This system works by using utilizing mirrors or mirror-like materials to concentrate energy from the sun and then takes that energy to produce steam. The system can also store the energy that is absorbed during the day, to be used at night when the sun is not present. There are a few different types of concentrated solar power systems which one can choose from.
Parabolic Trough: This type of solar power uses a curved mirror to focus the sun’s energy to a receiver tube with high temperature heat transfer fluid which absorbs the sun’s energy and passes it through a heat exchanger to heat water which produces steam.
Compact Linear Fresnel Reflector: The working principle of this solar power type is rather similar to parabolic trough, though instead of using a curved mirror, this application utilizes flat mirrors which are more economical. These mirrors act as reflectors to focus the solar energy into the tubes to generate high-pressure steam.
Power Tower: The power tower uses heliostats to track the sun movement and focus the solar energy to a receiver in the middle which is installed into an elevated tower. This application has been found to have better efficiencies compared to other types of solar power and can run on a higher temperature. The use of molten salt as a transfer fluid for the power tower applications is relatively common and helps improve efficiency.
Dish-Engine: This type of solar power utilizes mirrors that are designed to be distributed over a dish surface to concentrate solar power to a receiver in the middle. The application runs on a very high temperature and uses transfer fluid with a very high boiling point to power a high requirement engine.
Newer applications tend to lead to the installation and use of power tower design, since this design allows technology storage implementation which can be seen as a reliable option for the future of concentrated solar power application, not to mention the economic benefit it has compared to other technology storage implementation.
Global warming is a very popular topic at the present time. With the upwards trend of clean technology and the realization that strict climate policy should be implemented, demand of renewable energy has sky-rocketed while conservative plant popularity continues to fall. Additionally, the number of coal power plants have significantly dropped since its peak era, as they are now known as the largest pollutant contribution, producing nitrogen, sulfur oxide and carbon dioxides.
Renewable energy comes from many sources: hydropower, wind power, geothermal energy, bioenergy and many more. The ability to replenish and have no limit on usage and application makes renewable energy implementation attractive. To make this even better, it also produces low emission. Theoretically, with the usage of renewable energy, human-kind should be able to meet their energy needs with minimal environmental damage. With growth rates ranging from 10% to 60% annually, renewable energy is getting cheaper through the technological improvements as well as market competition. In the end, the main goal is to maximize profit while minimizing our carbon footprint. Since the technology is relatively new, capital costs are still considerably higher compared to more traditional (–and naturally harmful) implementations. This begs the question of exactly how we maximize the economic potential of a renewable energy power generation plant.
Living up to the full potential of any power generation plant starts with the design process. Solar power plants are one environmentally friendly option. During the design process, designers should take into consideration the type and quality of the solar panels as it is important to see the economic-efficiency tradeoff before jumping into an investment. Looking into the power conversion is also one of the most important steps one should take into consideration since it would be worthless to produce more energy than what is able to be transferred and put to use and low energy generation would mean less gross income.
Geothermal power plants are another option. Many studies have shown that boundary conditions on each component play a big role in determining the plant’s capacity and efficiency. High efficiency is definitely desired to optimize the potential of a power plant and minimized the energy loss. That being said, it is important to take into account the economic sacrifice. Regardless of how good the technology is, if it doesn’t make any profit, it would not make sense for one to invest in such technology. Low capital cost but high operating expenses would hurt the economic feasibility in the long run, whereas high capital cost and low operating expense could still be risky since that would mean a higher lump sum of investment upfront which may or may not breakeven or be profitable depending on the fluctuation of energy market.
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Though fossil fueled power plants aren’t as commonly used anymore, coal fired power generation is still a major source of global electricity, making up about 25% of the market in total. Compared to other options in fossil fuel power generation, coal is found to be the most economical choice as well as a reliable option. Making demands that are heavily reliant on other fuels, such as oil-fired for example, slowly levers to coal power generation. The global reserve of coal can be found in abundance when compared to other energy sources (such as oil for example) as there is about 3 times more of it. Also, IGCC comes with an economic benefit as the price of coal has remained relatively constant, which results in a higher degree of confidence when relying on coal as an energy source in the future.
How Does an IGCC Work?
The system uses a high pressure gasifier to turn coal and other carbon based fuels such as high-sulfur coal, heavy petroleum residues and biomass into pressurized clean coal synthesis gas (also known as syngas). The solid coal is gas-fired to produce syngas by gasifying coal in a closed pressurized reactor with a shortage of oxygen to ensure that coal is broken down by the heat and pressure. Before going out of the system, the syngas runs through a pre-combustion separation process to remove impurities, starting with water-gas-shift reaction to increase concentration of hydrogen and efficiency during combustion process, to a physical separation process (through variable methods). After that, a fairly pure syngas is used as a fuel in a combustion turbine that produces electricity. Waste heat contained in a gas turbine’s exhaust is used to produce steam from feed water that further turns a steam turbine to generate additional electricity.
What are the Advantages of IGCC?
IGCC is currently found to be the cleanest of coal technology with lower emission (especially for carbon dioxide by 10%) and is about 30-40 percent more efficient. Using syngas in gas turbines results in a higher output that is less dependent on temperature when compared with natural gas. Additionally, looking into the economic benefit of this technology, IGCC produces couple by-products, from chemicals to materials for industrial use that could be sold for side economic benefits.
In recent days, many people find themselves spending time and resources on uncovering the best solution to optimize the power generation cycle. Until recently, 80% of power plants worldwide (whether fossil fuel, nuclear, or clean technology) used steam as its main working fluid and while it is still the most common option, today’s power plants are finding another fluid to use.
Although supercritical CO2 study began in the 1940’s, it was disregarded as an alternative fluid option because it was expensive to explore and steam was still perfectly reliable at the time. Nowadays due to increasing quantity and quality demand in power, researchers are looking into the possibility of replacing steam with supercritical carbon dioxide. The discover of this property, increases the incentive of exploring the technology further. This year, the US Department of Energy is awarding up to $80 million towards projects to build and operate a supercritical CO2 plant.
Getting back to the basics, it is important to establish what supercritical CO2 is. SCO2 is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure. When carbon dioxide is heated above its critical temperature and compressed above its critical pressure, the fluid inherits both liquid and gaseous phase properties. SCO2 has many unique properties that allow the fluid to dissolve materials like a liquid but at the same time flow like a gas. It also carries the advantage of being non-toxic, non-flammable and environmentally friendly.
Supercritical CO2 is believed to improve the efficiency of thermal power plants that utilize coal, natural gas, solar, geothermal or nuclear energy. At its supercritical state, carbon dioxide is able to generate a higher amount of electricity from the same fuel compared to a steam power plant. Accordingly , it will drop down carbon dioxide & greenhouse gas emissions as well as operating cost. The use of carbon dioxide as a working fluid also allows for the usage of smaller and more economically feasible machines. Supercritical carbon dioxide is twice as dense as steam, thus easier to compress. With this in mind, smaller components can be used, for example, to decrease the turbine size compared to a steam generating power cycle, resulting in lower costs. Although an economically feasible SCO2 plant has yet to exist due to the early stage of technology and the still high research and development costs, we may be able to expect one in the near future as it is beneficial both economically as well as environmentally compared to a traditional steam power cycle.
Implementation feasibility of power plant design relies heavily on the economic benefits. More often than not, newer technology cannot be implemented due to high cost of electric generation which would not be acceptable in the market since energy is a price sensitive commodity. Sometimes while deciding on a design to choose, we are given a choice between a high initial equipment cost and efficiency versus a lower capital cost with lower efficiency. The designer must be able to choose which design would fit best with their needs and goals.
While running a power generation plant, there are three types of cost that need to be taken into consideration: capital cost, operational cost and financing cost. With point one and two to being of higher priority.
Capital cost generally covers the cost of land, construction, equipment and so on. In other words, capital cost includes all costs in the initial phase of building the plant itself. Capital costs varies from time to time, and from one location to another. Largely, it is a function of labor costs, material costs and regulatory cost –which all is dependent on investment time and the availability of resources as well as the administrative regulation that governs the area. For example, building a power plant in an engineering hot spot like Texas would be much easier then it would be in a residential area such as near a neighborhood in California due to environmental laws as well as construction regulations. Consequently, the time needed to build a plant of the same size in both cases could be significantly different, thus making a noticeable gap in the capital cost. In common practices, capital costs are not necessarily paid in advance as cash, rather sometimes in debt and equity. This fact brings us to financing cost, which would be the cost of paying off the capital expenditure for a period of time.
In practice power plants take into consideration three main things while calculating for operating cost: fuel, labor and maintenance. With that being said, there are many other aspects to consider that could vary based on each individual designs. Operational cost usually varies with the capacity of the plant or with plant operations. In most cases, fuel cost dominates the marginal cost of a conservative power plant, say fossil-fuel, whereas newer technology such as biomass or geothermal, the cost of fuel is generally “free” though higher capital cost. The trade-off between operating and capital cost investment should be taken into consideration while designing a power plant.
Combined Cycle Power Plants are among the most common type of power generation cycle. Demand of CCP application has risen across board due to the rising energy demand (and consumption) as well as growing environmental awareness. Combined cycle is a matured energy that has been proven to generate much lower CO2 (and other environmental footprints) compared to a traditional fossil fuel steam or gas turbine power generation cycle Consequently, this application is often looked as a “better” substitute compared to other a fossil fuel technologies. That being said, CCP is still a temporary alternative to substitute SPP since although CCP generally is more environmentally friendly, CCP process still requires the combustion of fossil fuel (though at a significantly lower degree compared to SPP) for initial heat/energy source.
The application takes two kinds of thermodynamic cycle in assembly to work together from the same heat source. Fluid Air and fuel enters a gas turbine cycle (Joule or Brayton) to generate electricity, waste heat/energy from working fluid will then be extracted then go through a Heat Recovery Steam Generator and towards steam turbine cycle (Rankine) to generate extra electricity. The main advantage of this cycle combination is the improvement of overall net efficiency (around 50-60% higher compared to each cycle alone), thus, lower fuel expenses. With that being said, net efficiency of a CCP is often inflated especially on systems which use a low-temperature waste heat.
There are two configurations of a combined cycle power plant – single-shaft and multi-shaft. The first configuration has one gas turbine and one steam turbine coupled to one generator and one heat recovery steam generator. A multi-shaft has one large steam turbine, condenser and heat sink for up to three gas turbines — each gas turbine and each steam turbine also has its own generator. Each configuration comes with its own advantages and disadvantages, for example single shaft design has a slightly smaller initial cost and smaller footprint whereas multi-shaft is found to be more economical in the long run due to the number of gas turbine to operate in conjunctions. Though overall it’s hard to say which configuration is best to be applied, judgement should be based on needs and consideration of the designer since each wins and losses in different categories.
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Supercritical carbon dioxide cycles have slowly become more popular in the engineering market for electricity generation from various sources. SCO2 is found to be an ideal working fluid for generating power cycles due to its high efficiency –more than supercritical or superheated steam, which results in lower cost of electricity.
Supercritical carbon dioxide is a fluid state where carbon dioxide is operated above its critical point which causes the compound to behave as both a gas and a liquid simultaneously with the unique ability to flow as a gas though at the same time dissolve materials like a liquid. SCO2 changes density over small difference in temperature or pressure, though stay in the same phase; allowing large amount of energy to be extracted at higher temperatures.
This cycle works in a similar manner to other power generation cycles, and is potentially applicable to wide variety of power generation applications. Hypothetically speaking, any cycle that is running with steam as the working fluid should be able to be upgraded to SCO2 application. In an example for applications using fossil fuel as a main heat source, cycle could be designed as an indirectly-heated non-condensing closed-loop Brayton cycle or directly fired SCO2. In the first event, CO2 is heated non-directly through a heat exchanger. After that, the hot CO2 flow expands in the turbine where the mechanical energy is extracted and any remaining heat is extracted in the recuperator to preheat the CO2 going back to the inlet loop, resulting to high efficiency systems. Where for second arrangement, fossil fuel is directly combusted with oxygen, resulting to steam/CO2 mixture to drive the turbine and generate electricity. The remaining heat in the fluid mixture will be recuperated to preheat the CO2 that is used as the combustion diluent.
There are many benefits that come with SCO2 power conversion technology when compared to other power cycles such as higher efficiency (which correspondent to higher productivity with the same thermal input), environmentally friendly/low greenhouse gas emission, and lower capital cost from reduced size compared to a conventional steam cycle.
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Geothermal energy has become more and more popular globally due to its sustainability and economic stand point. Geothermal power plants run on a variety of temperatures and utilize hydrothermal resources (water/steam and heat) from below the earth surface to generate electricity for people’s daily consumption. Resources can come from dry steam or hot water wells.
There are three kinds of Geothermal cycle for power plants: binary cycle, dry steam and flash steam. Binary cycle power plants use the heat transfer from geothermal hot water to secondary fluids with a low boiling point at the lower end of standard geothermal temperature (225 to 360 F). This heat will cause the secondary fluid to bubble and turn into steam in the heat exchanger, which is then used to turn the turbine. Since water and secondary fluids are kept apart in the cycle, air emission is minimized.
Dry steam is the first geothermal power plant to ever exist from a natural rupture of steam, though considerably uncommon since it demands sustainable underground heat sources to work. The steam used as a working fluid will be piped directly from the underground geothermal reservoirs to turn a turbine and generate electricity.
Flash steam is the most common type of geothermal application, using a underground high-pressure hot water reservoir with a minimum temperature of 360 F, converting it to steam as it moves up to the surface from change in pressure. After steam gets separated from water, it drives the turbines to produce electricity. As the steam cools down and condenses to water, fluid then will be injected back to the reservoir to be reused.
Each one of these system designs comes with its own advantages and disadvantages. For example, binary cycle allows low temperature geothermal sources to be used thus can be used in more wide spread applications. This kind of cycle also does not release geothermal fluid into the system, thus the technology is more environmentally friendly. On the other hand flash steam power plant gives you the advantage of sustainability as well as cost effectiveness in the long run, though it’s rather geographically sensitive. Dry steam application is hard to implement due to the rather rare natural resource used to be able to implement such a cycle, though it generates less of a footprint and require simpler technology which results to lower initial cost. The better application is really dependent on the designer’s needs and goals.
Geothermal energy is categorized as a “green energy”, with low emission of approximately 5% of carbon dioxide, 1% H2S, 1% sulfur dioxide and less than 1% of the nitrous oxide of an equal sized fossil or coal power plant. Concentrations of each environmentally disruptive gases are controlled by temperature, composition of fluid, and geological setting. Although most of the geothermal emissions commonly come from existing geothermal resource gas, some percentage of the emission also comes from various processes of the energy conversion process. Non-condensable gases are also emitted as a part of high temperature process of geothermal energy conversion.
According to various studies, the type of geothermal power plant design would really impact the production rate of the mentioned gasses. The selection between open-loop and closed (binary)-loop system is essential while taking into consideration air emission. Geothermal plants to this date are commonly separated into three main cycle design: dry-steam, flash-steam or binary –the first two extensively generate more greenhouse gasses (GHGs) compared to the last. In a binary loop system, gases which are removed from the system will not be transferred to the open atmosphere, instead, after transferring the heat gasses will be run through back to the ground, and result in minimal air pollution. In contrary, open-loop system emits all of the emission gas contained such as hydrogen sulfide, carbon dioxide and many more. There are also different factors which cause the technology to emits gases that are naturally present in the fluid such as fluid chemistry/composition, fluid phase, and geological setting to temperature.
The main types of air emission or contamination within the application of geothermal energy are commonly found to be carbon dioxide and hydrogen sulfide. Hydrogen sulfide reacts to produce SO2 once touched with the atmosphere. SO2 is known for its hazardous nature to health and environment, causing acid rain and respiration problem. Even though the concentration of this gas emission is significantly smaller than a conventional fuel power plant, reduction of hydrogen sulfide emission is still desirable for any conditions. Types of condensers installed to the design determines the ratio between the condensable and non-condensable gas. Consequently, with the right selection of condenser as well as implementing other reduction plans such as installation of adsorption tower, etc, hydrogen sulfide emission could be minimized.
Over the past couple of years, energy storage technology has significantly evolved to meet engineering demand and political regulations. This wasn’t initially looked as a desirable investment due to the high production cost, however over time, exploration of such technology by bigger companies has driven down the manufacturing cost and generated more demand. With occurrences such as rapid capital raise of smaller start-up companies, to the acquisition of Solar City by Tesla, the market of energy storage is predicted to continue growing. The technology allows for collection of energy produced to be used at a later time. Energy storage systems have wide technology variation to manage power supply – from thermal, compressed air to everyday batteries.
Molten Salt Usage
The usage of molten salt in thermal energy storage applications has become more common. In commercial solar energy storage, molten salt (from potassium nitrate, lithium nitrate and more) is used in conjunction with concentrated solar energy for power generation. Molten salts are able to absorb and keep heat energy transferred from the fluid mediator, then to transfer it again when it’s needed. In the liquid state, molten salt has a similar state to water. It also has the capacity to retain temperatures of 1000 Fahrenheit. Though efficiency is known to be lower than other storage media such as batteries, (70% vs 90%), the main advantage of the usage of molten salt is lower costs which allows the technology to be implemented in a higher volume production.
How Molten Salt Energy Storage Works
Using solar energy as the main source of energy, heliostats (mirrors used to track sun/solar heat) are used to reflect the solar radiation into an energy receiver at the power plant. Molten salt then is used to collect this heat energy from the concentrated pool. The molten salt will later be stored. When power is needed, hot molten salt is transferred to a HX (or steam generator) to produce steam at a high pressure and temperature. The steam then will be used for electricity generation as the live steam in a conventional steam power plant. After exiting the generator, molten salt will then be transferred back to the thermal storage tank to again absorb energy.
The Benefits of Molten Salt Energy
There are three main benefits of molten salt energy storage – reliability, economic savings and environmental friendliness. While in a liquid state, molten salt improves long term reliability as well as reduces operation and maintenance cost. The capital cost of the material itself is also relatively cheap and easily accessible. Molten salt is also known as a non-toxic compound, thus completely green and comparable to fertilizer.