Answers Exam Theory Practice PDF

Power system Which are the global sources for electric power production? Rank them with the largest first. 1. Coal 2. Gas 3. Hydro 4. Nuclear 5. Wind 6. Oil 7. Solar 8. Other renewables How much has the average sea level changed the last 100 years and how much has t...

Power system Which are the global sources for electric power production? Rank them with the largest first. 1. Coal 2. Gas 3. Hydro 4. Nuclear 5. Wind 6. Oil 7. Solar 8. Other renewables How much has the average sea level changed the last 100 years and how much has the temperature changed the last 100 years ? Over the past 100 years, global temperatures have risen about 1 degree C (1.8 degrees F), with sea level response to that warming totaling about 160 to 210 mm (with about half of that amount occurring since 1993), or about 6 to 8 inches. List at least four challenges for a 100% renewable power system Increased consumption, electrification of transports and industry Power from wind and solar, intermittent production, harder to match demand and supply Power from wind and solar, power quality Power from wind and solar, power system stability Power from wind and solar, peak load Grid, more connections to other countries are needed Grid, matching production with consumption geographically Grid, reactive power Grid, use existing grid more efficiently Decentralized power system List and explain one way to better use the existing grid Maximize the hosting capacity of the distribution system/Using Active management strategies The local grids are passive with no active monitoring. Therefore grid owners calculate worst case scenarios when wind turbines should be connected. Since both load and production varies a lot over time, the probability for the worst case scenario to occur is very low. This means that unnecessary restrictions on connection capacity are used. To increase and maximize the hosting capacity active management strategies can be used: Reactive power compensation: Adding reactive power to the grid will reduce the risk of voltage rise leading to overvoltage => more wind power can be installed. Variable speed wind turbines can supply the needed reactive power to the grid by themselves and for other turbines reactive power can be added by using a static VAR compensator (SVC) or static synchronous compensator (STATCOM).  Coordinated on-load tap changer voltage control (C-OLTC): Tap changers in transformers are used to control the outgoing voltage. In C-OLTC voltage control, the on-load tap changer (OLTC) is controlled to keep the voltage on various critical points in the system within the acceptable voltage limits. This works well with wind and solar. Wind energy curtailment: Curtail the amount of power that the wind farm can produce to ensure that no overvoltage or overloading occur. —-----------------------------------------------------OR-------------------------------------------------------------- Active Management Strategies (AMS) are: Reactive power compensation: With the use of capacitors we can regulate the reactive power and stabilize the voltage. Coordinated on-load tap changer voltage control (C-OLTC): enable voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption. Wind energy curtailment: refers to the situation where the output of wind plants is reduced to a level below its maximum generation capacity. This happens when there is too much supply but little demand, which can cause overvoltage or overloading. If these are used much more wind power can be installed (up to 3% more).The wind housing capacity can be increased up to twice the capacity that could be installed based on worst case analysis. Dynamic line rating Hosting capacity can be limited by the temperature in the power cables. To keep the cable above the limit from the ground the power from connected wind farms can be curtailed. Dynamic line rating can increase the hosting capacity. Goes from a passive system to an active. Monitors the temperature in the cable, if it exceeds a certain value the wind farm is curtailed. So DLR is like the real-time flexibility your factory has depending on the environment. DLR: How much power a single line can carry based on real-time conditions. Dynamic transformer rating Dynamic transformer rating (DTR) looks at load and temperature variations to increase the rating of the transformer while maintaining safe operation. The result will be a smaller transformer with DTR: Lower cost Increased system and component reliability Decreased price of wind electricity How the whole power grid (network of lines) can be dynamically optimized for better performance. Describe the difference between centralized and decentralized power production and name benefits for both systems A centralized power system consists of a few large power stations and energy goes in one direction: from production to consumers [1p]. A decentralized power system has many small(er) production units distributed over a larger area and energy goes in both directions, both to and from consumers (prosumers) [1p]. A benefit with a centralized power system is that the power production is much easier to predict. [0.5p] A benefit with decentralization is that losses in the grid can be reduced. [0.5p] List three different types of energy storages Mechanical storage: Pumped hydro Compressed air Fly wheel Thermal storage: Water-based Salt-based Phase shifting material Electrical storage: Supercapacitor Superconducting magnetic energy storage Electrochemical storage: Lithium-Ion battery Redox flow battery Chemical storage: Hydrogen Synthetic natural gas Electro fuel (methane, ethanol etc) What is peak load and why will it be a challenge in a future power system? Peak load is the hour of the year with the highest electricity consumption. (In Sweden this occurs sometime between 16th of November to 15th of March.) The peak load will increase with much variable power in the system, but since it will only be needed a short number of hours per year, the power plant that will produce power during the peak load will be very expensive to run. Will there be someone who wants to operate it? List 4 different ways to incentivize/steer customers in demand side management Spot price Peak demand reduction Local flexibility markets Ancillary services to TSO Describe two ways to incentivize electricity customers to become more flexible. Different economic incentives could be used, such as peak power tariffs, real time electricity pricing or direct load control. Peak power tariffs will promote the customers to not use several high power demanding equipment simultaneously by charging for the actual electricity peaks. Real time pricing will let the end user pay the hourly electricity price and thereby an economic incentive to use electricity when it is cheap (e.g. when there is a surplus or high share of production with low marginal cost, such as solar and wind). With direct load control, the customers let an external party, e.g. the DSO, control part of their electricity consumption, e.g. EV charger or heat pump, and are compensated by a lower electricity price, loads in the future power system. Another alternative could be to use environmental incentives, e.g. inform about the importance of being flexible and show the possible CO2 reduction their flexibility leads to. 0.5 p per way (peak power tariffs and real time pricing) + description —----------------------------------------------------OR—------------------------------------------------------------ Demand management Real time pricing: Electricity price based on spot market Time of use: Different price during the day Critical peak pricing: Highest price during congested hours Direct load control: Lower electricity/grid fee if DSO can control some appliances Interruptible/curtailable service: Lower electricity/grid fee by restricting some loads Another way of demand side management is using V2X with EV. V2G (vehicle to grid): provide services to the grid V2H: increase resilience, create a microgrid V2V: exchange between vehicles V2L: use battery for devices on the “field” When is it better to use a supercapacitor as energy storage compared to pumped hydro. Describe why? Super capacitors are good to store smaller amounts of energy for high power applications, they can also withstand many rapid charge/discharge cycles.Even if they could store the energy without high static losses they are not commonly used for long term storage due to the low energy density, instead they are rather used in applications for e.g. power quality improvements. Pumped hydro has longer response time and can only be installed in a limited number of places, due to geographical reasons. What is dynamic line rating and how is it used in connection to renewable power? Dynamic line rating (DLR) is active monitoring of the cables in the grid. When looking at the load and temperature variation of the cable DLR can determine how much production that can be allowed from a wind farm and curtail the farm if necessary. This way more wind power can be installed in the existing grid. What is the typical frequency control sequence when a frequency disturbance event occurs? Frequency containment process to quickly limit the frequency drop and stabilize the frequency [1p], then frequency restoration process to bring the frequency back to 50 Hz How would you design the Swedish power system for 2050 with an electricity demand of 300 TWh? Which energy sources and how big should each be? Motivate your choices. To satisfy the 300 TWh demand, we see solar and wind as the main power producers with hydropower being used as back up power. During winter there needs to be an option to compensate for the lack of solar power. Because of this we have chosen to keep 30 TWh of CHP giving us the same ratio between solar and CHP as the solar 2040 scenario. Hydropower is a reliable source of energy and hence it will provide that amount of power production well into the future. Hydropower cannot be increased beyond the 69 TWh mark in Sweden due to environmental concerns. Wind is cheap and sustainable and works generally year round, although it seems slightly better during winter when consumption is at its highest. Therefore we want to expand on wind power. We foresee wind power being the main source of energy providing up to 40% of the consumption requirement. We have considered nuclear energy production as a base power to help supply the critical infrastructure with reliable power. Since the infrastructure already exists we would keep some nuclear power until we are able to fulfill the consumption needs using only renewable sources. We are relying heavily on solar and wind. This may lead to an increased need to import energy from other countries. However we may also increase exports during peak sun/wind hours. If other countries also increase their reliance on wind/solar, chances are that while we have low production, other countries will have high production, giving them a need for export when we have a need for import. This scenario of import and export brings into focus the need for a thoroughly interconnected grids between multiple countries, enabling efficient transfer of power between the various regional grids. We believe that along with the changing power supply grid, there will be a drastic change in the way the appliances consume electricity. By 2050 we foresee that a smart grid ecosystem will be established which will help to enable efficient transfer of electricity. This smart grid will be characterized by the use of smart meters, smart appliances, V2G charging infrastructure and other technologies. These technologies will work together to maintain the delicate balance between production and consumption, ensuring a safe and secure supply of electricity Based on current energy consumption in Sweden, which sectors are using most fossil fuels? Name two ways to reduce the fossil fuel consumption in this sector. The transportation sector consumes most fossil fuel [1p]. Two ways of reducing the usage is to electrify the transport sector, use hydrogen vehicle [0.5p] or bio based fuel Explain qualitatively the basic optimization framework for market clearing of the Nordic day-ahead electricity market (including the objective function and main constraints). Explain when the market prices can be the same and different between the market areas Let's say that between areas A and A' there's a transmission line with 100 TWh capacity If area A has an energy surplus of 200TWh than area A', the transmission line in the middle can be - overloaded with 200TWh (not good for the cable) and the prices will be kept equal - utilised for 100TWh (good for cable) but there will be different prices (low in A and high in A') Explain which power system issues that the adequacy and reliability address respectively. Explain at which time scale of the power system phenomena that the ancillary service aims to handle Adequacy (the state of being sufficient for the purpose concerned): generation capacity adequacy & grid capacity adequacy – 0.5 p Reliability: balance + various power system stability -0.5 p Time scale: within an hour or 15 min. – 1 p Consider a simple two-area market for a given hour as shown in Figure 1. Each circle represents one generator (supplier) of 100 MW available capacity and each building represents one load (customer) of 100 MW. The bid price (€/MWh) for each generator is shown in the figure. Assume that the market is the single auction market with only supply-side bids. Determine the market price for each area and generation schedules (ON/OFF) of generators to satisfy the load demand for the cases: a) Unconstrained market settlement (without considering the transmission capacity limit). Motivate your answer. The market will be settled as one single market without the transmission constraint. The supply curve is constructed according to the bid prices in the increasing order. The total demand is 600 MW. The market price is cleared where the curve meets the total demand of 600 MW. The market price will be the last accepted bid price, which is 14 €/MWh, for both areas, with generators with bid prices 10, 11, 12, 13 in Area-1 and 13, 14 in Area-2 ON.  b) Constrained market settlement (considering the transmission capacity limit). Motivate your answer Constrained market settlement considering transmission capacity limit: The solution in i) will lead to the power transmission of 200 MW from Area-1 to Area-2 which is greater than the transmission capacity of 100 MW. In this case, the market will be cleared in each area separately, considering the maximum export power of 100 MW from Area-1 to Area-2. The market price will be determined using the bid prices (i.e., supply curves) for each area separately. Area-1 will have market price of 12 €/MWh (generators with bid prices 10, 11, 12 ON) to supply for 200 MW demand in Area-1 and 100 MW export to Area-2, while Area-2 will have the market price of 15 €/MWh (generators with bid prices 13, 14, 15 ON and 100 MW import) to supply for 400 MW demand in Area-2. This market solution will ensure that the power transmission from Area-1 to Area-2 is 100 MW. Hydro Power Which are the main components in a hydro power station? Dam Penstock (gate or intake structure that controls water flow, or an enclosed pipe that delivers water to hydro turbines) Turbine Generator Explain how a pumped hydro storage system works and name 2 benefits with it With a pumped hydro storage facility you have two water reservoirs at different elevations. When there is a low demand for electricity and electricity is cheap, you pump water to the higher reservoir. When there is higher demand of electricity you release water from the higher reservoir and the water flows through the hydro power station and generates electricity Benefits: You can store power Relative high efficiency 70-85% Possible to build on any scale Discharge times ranges from several hours to a few days List at least 3 advantages and disadvantages with hydropower. Disadvantages: The construction of dams has a large environmental impact: it can alter aquatic habitat, flood previously dry areas, may require the expropriation of land, leading to the relocation of local communities, changes in water quality, etc. Cost of building a hydropower plant is very high Not just any piece of land is suitable for building a dam, which limits the availability of space for renewable energy generation. Hydropower production will depend on the amount of rainfall in the area: in periods of drought, electricity production can drop considerably Advantages: It is flexible energy. This means that, thanks to the fact that the amount of water that passes from its natural or artificial source to the turbines can be controlled, It is clean energy. The production of electricity through the use of water does not produce waste. It is a safe energy. Due to the high safety standards of dams and reservoirs, accidents due to water leaks are nowadays very unlikely. Pairs well with other renewables Is inexpensive in the long run Explain how a Pelton turbine works. Nozzles direct forceful, high-speed streams of water [0.5p] against a series of spoon-shaped buckets [0.5p], which are mounted around the outer rim of a drive wheel (also called a runner). When the jet hits the buckets in the Pelton wheel it induces an impulsive force. [0.5p] This force makes the turbine rotate. Pelton turbines transform kinetic energy in the jet to rotational energy. The rotating shaft runs a generator and produces electricity. [0.5p] Control the turbine by regulating the water flow rate in the nozzle with a spear head. [0.5p] This gives the turbine a constant RPM. Which are the main ecological impacts from a hydro power station? Loss of forests, wildlife habitat, species Degradation of upstream catchment areas due to inundation of reservoir area Rotting vegetation also emits greenhouse gases  Loss of aquatic biodiversity, fisheries, other downstream services Cumulative impacts on water quality, natural flooding Disrupt transfer of energy, sediment, nutrients Sedimentation reduces reservoir life, erodes turbines Which are the main types of hydro power turbines and at which heights of water are they typically used? Wave Energy Describe shortly three different wave energy converters. The tapered channel: Waves make water flow into the onshore water basin through the tapered channel. The water in the reservoir then flows through the turbine out to sea again, generating power Overtopping: Overtopping devices use the wave’s ability to reach a higher level than the average sea level. Waves will pass into the converter and into a reservoir that is above sea level. The water in the reservoir is let out through the bottom of the converter, passing through a turbine Oscillating water column: Oscillating water columns consist of a semi-submerged chamber that is open to the sea from below. Above the water in the chamber is a trapped air pocket. When waves reach the chamber, they increase the water height  inside the chamber and force the air up through a small column and through a turbine placed there. When the waves retract, the water level in the chamber decreases and air flows back through the turbine. Point absorber: Point absorbers utilize the vertical motion of the wave and generate power as it moves up and down. They can be either fixed on the seabed or floating in the water. This motion generated by the waves is converted to electricity. Attenuators: Attenuators are floating structures that capture energy from waves passing them. The sections of the attenuator move relative to each other and this motion is converted to electricity. Describe shortly three different tidal/ocean currents converters. Tidal barrage A tidal barrage is a dam-like structure with turbines placed at the bottom of the reservoir. The dam creates a barrier between the sea and the tidal basin taking advantage of the change in the tide levels to produce power. Gates are used, opening and closing regularly to control the flow of water into the basin, when the tide is at its highest the gates close, allowing the tide to go back down but still keeping water in the basin generating a head difference. In some cases, pumping may be performed, using electricity from the grid, to further raise the water levels in the basin, creating a larger difference between the sea and the basin water levels (head of water) and thus resulting in the increase of power generation. When there is enough head of water, the water is released through a turbine pass, thus generating electricity. Tidal lagoon Tidal lagoons work in a similar way to tidal barrages by capturing a large volume of water behind a man-made structure which is then released to drive turbines and generate electricity. Unlike a barrage, where the structure spans an entire river estuary in a straight line, a tidal lagoon encloses an area of coastline with a high tidal range behind a breakwater. Tidal stream Tidal stream turbines are similar to wind turbines since they are turbines that convert kinetic energy to electric energy, but instead of wind it is water that flows through the turbine. The stream is generated by low and tides in the water. No barrage or dam is needed. Wave energy converters (WEC) come in many types and shapes. Please classify WEC into three categories, either from a wave energy capture approach or from a wave direction approach. From a wave energy capture approach: (i) Wave activated bodies; (ii) oscillating water column; (iii) overtopping devices OR From a wave direction approach: (i) point absorber; (ii) attenuators; (iii) terminators. Describe, by an explained formula, the transported time average wave power per unit of wave length. The transported time-average wave power per unit of wave length for a wave can be described using the formula: Where: P is the wave power per unit of wave length (W/m), This represents the amount of energy transported by the wave over a certain length, and it's related to both the potential energy stored in the height of the wave and the kinetic energy due to the wave motion. ρ is the density of water (approximately 1000 kg/m3), g is the acceleration due to gravity (approximately 9.81 m/s) A is the wave amplitude (half the wave height), reflecting the fact that the wave power is proportional to the square of the wave height. Taller waves carry significantly more energy. c is the phase velocity (the speed at which the wave crests move). How is the power calculated from a wave energy converter? Nuclear Energy Describe the most probable fusion reaction and explain where the elements for this reaction are harvested. deuterium + tritium => helium + neutron + energy [1p] Deuterium comes from sea water. [0.5p] Tritium is radioactive and has to be produced from Lithium. Lithium is available in earth-crust and sea-water Name the two main types of Light Water Reactors. Boiling water reactor (BWR): Cooled by water at medium pressure (70 bar). Pressurized water reactor (PWR): cooled by water at high pressure (155 bar) List at least two of the advantages and two of the disadvantages of fusion Advantages Potential to become a large-scale, safe and environmentally friendly energy option. Fusion fuels are virtually inexhaustible and available everywhere. Waste from fusion will not be a long-term burden on future generations. No byproducts that can be used in nuclear weapons. Fusion power stations can be made inherently safe. Energy production requires active control Only 1 gram fuel in the reactor No risk for meltdown Disadvantages  Nuclei must be extremely close together to fuse Nuclei electrically charged -repel each other! Needs high particle speeds Reactions are independent, hard to get going Expensive Nuclear power – describe the main advantages and drawbacks with Nuclear Power of today. Advantages: Low Greenhouse Gas Emissions: emit very low levels of carbon dioxide during operation High Energy Density: a small amount of nuclear fuel (such as uranium) can produce a large amount of electricity. This makes it highly efficient in terms of fuel usage. Reliable and Continuous Energy: Nuclear power plants provide a stable and continuous power supply Reduced Land Use: Compared to renewable sources like solar and wind Drawbacks: Nuclear Accidents Radioactive Waste: High-level waste requires secure, long-term storage (thousands of years), and many countries still lack permanent solutions. High Initial Costs: Nuclear Proliferation: The spread of nuclear technology and materials increases the risk of nuclear weapons proliferation. Limited Fuel Supply: Although uranium is relatively abundant, the supply of economically extractable uranium is finite What is reprocessing of nuclear waste and what is the point of doing it? Reprocessing of nuclear waste is a process that extracts usable materials, such as uranium and plutonium, from spent nuclear fuel that has been used in a nuclear reactor. The primary goal is to recycle these materials for use in new nuclear fuel, thereby reducing the amount of high-level radioactive waste and optimizing the use of natural resources. The point of reprocessing is to reduce waste volume, extend fuel supply, reduce radiotoxicity, energy efficiency and reduce proliferation risks. The temperature in a fusion plasma needs to be very high (on the order of 100 million degrees), and sustained for a long time. Why? Nuclei (such as deuterium and tritium, common fusion fuels) are positively charged and naturally repel each other due to the Coulomb force (electrostatic repulsion). For fusion to occur, the nuclei need to get close enough for the strong nuclear force (which is attractive and acts over short distances) to overcome this repulsion. This requires extremely high kinetic energy, which can only be achieved at temperatures in the range of millions of degrees. To achieve a sustained and useful energy output, the plasma must be hot enough for the nuclei to frequently collide with enough energy for fusion to happen at a significant rate. For a reactor to produce continuous power, the fusion plasma must be sustained for a long time under high temperatures. If the plasma cools down too quickly, the reaction rate will drop Wind Power Where in Sweden is it optimal to place new wind turbines from a power system perspective? Motivate your answer. You should place new electricity production close to where the consumption is, i.e. close to where people live and industries that use a lot of electricity. By doing this the electricity doesn’t need to be transported long distances and you reduce the losses. What information do you need to be able to calculate the energy outcome from a wind turbine on a specific site? Average wind speed Wind speed distribution Height of wind measurement Turbine Specifications (rotor diameter, hub height) Air density Turbine power coefficient Operational time Losses Which two ways is there to control the power production of the wind turbine? Pitch or stall control. Explain why not all energy can be harvested from the wind in a wind turbine To transform the kinetic energy of the wind to electricity, it needs to slow down. All energy can not be transformed, since then the wind speed at the rotor would be 0 and the wind wouldn’t want to go through the rotor. Which two forces are acting on a wind turbine blade and where do they come from? Lift and drag forces, Lift is created by a higher pressure on the underside of the blade and a lower pressure on the top of the blade. The drag is the friction of the blade moving through the air. Describe shortly three electrical systems for variable speed operation in a wind turbine. Doubly-Fed Induction Generator (DFIG): Uses a partial power converter connected to the rotor, allowing variable speed by controlling rotor currents. It is efficient and widely used in modern turbines. Disadvantages: Limited speed variation. Limited power of the converter. Slip rings=maintenance Advantages: Good efficiency. Good control of P, Q Full-Converter System: The generator (typically a permanent magnet or synchronous generator) is fully decoupled from the grid through a power converter, enabling broad speed control and high efficiency across a range of wind conditions. Advantages: full control of P & Q since all power goes through the converter, the generator can be AG, SG, PM. Disadvantages: Higher losses. Synchronous Generator with Direct Drive: No gearbox is used, and the generator operates at lower speeds. Full power converters manage grid connection, ensuring efficient variable speed control. It's robust and has fewer mechanical components but requires larger generators. Induction generator for fixed speed operation: Robust generator. Low maintenance. Simple system. High mechanical forces. Dominating system from 1980- 1990. Not so common on large machines> 1.5 MW Name and describe/draw the three most common electrical systems in a wind turbine. Generator: It converts the mechanical energy from the spinning rotor into electrical energy. When the wind turns the blades, the rotor is connected to a shaft, which drives the generator Converter: The converter system helps control the electrical output of the wind turbine. Since wind speeds fluctuate, the power generated can be inconsistent, so the power electronics work to regulate and optimize this energy for smooth transmission. Transformer: The transformer steps up the voltage of the electricity generated by the turbine so it can be transmitted efficiently over long distances. Describe at least 4 ways that wind power affects the environment. Humans: Sound/noise Lights Nature: Birds Bats Fish Visual: Landscape Influence on culture What kind of birds are at most risk of being killed by wind turbines and why? What can be done to mitigate the problem? Birds of prey and fowls are most vulnerable to wind turbines. Birds of prey are not used to having enemies in the sky and look for prey on the ground and have little attention to risks in the air. Fowls are disturbed by people moving through the forest. Make buffers around nests for birds of prey and around play areas for fowls. Alternative answer to the last part: Look at all things that impact the birds, not just wind power. Set population goals for the different species, based on biology. Focus on population and not on specific individuals. Describe how offshore wind power affects the environment and how problems can be mitigated. The fish is mostly disturbed during the building of the turbines. They are affected by piling [0.5p] and that the water gets muddy [0.5p]. The disturbance with piling can be minimized by: Choosing the right time Scare away Some positive effects with offshore wind power are that the foundations act like artificial reefs [1p] and since no fishing is allowed inside wind farms, they create safe playgrounds for fish. [1p] Name two reasons for introducing variable speed operation in wind turbines. Operation at optimal Cp Perfect control of the torque in the drive train. Low operation speed of the turbine at low wind speed will also reduce the noise from the turbine How close to each other is good to place the wind turbines in a wind farm? Short motivation is required. Wind turbines in a wind farm should generally be placed 7 to 10 rotor diameters apart in the direction of the prevailing wind and about 3 to 5 rotor diameters apart in the crosswind direction. This spacing minimizes the effects of wake turbulence, where the airflow behind one turbine slows down and becomes more turbulent, reducing the efficiency and performance of downstream turbines. Why is it important to be able to calculate the eigenfrequency of a wind turbine tower? Calculating the eigenfrequency of a wind turbine tower is essential for avoiding resonance, ensuring structural integrity, preventing fatigue, and improving the dynamic response of the tower to external loads. This contributes to the safety, reliability, and longevity of the wind turbine. Describe shortly a typical wind/diesel and a typical wind/solar system for stand-alone operation and what are the main benefits with these systems ? A wind/diesel system combines a wind turbine with a diesel generator to provide reliable electricity in remote or off-grid areas. The wind turbine generates electricity when wind conditions are favorable, while the diesel generator can take over during low wind periods or high demand. Benefits include reliability, reduced fuel consumption, and flexibility. Solar power Describe how one technology for Concentrated solar power works Parabolic trough collectors Parallel rows of mirrors/reflectors Curved in one dimension to focus the arrays on the absorber tube Can be more than 100 m long Stainless steel pipes, with special coating (absorber tubes), collects the heat. Uses synthetic oil as heat transfer fluid The reflectors and the pipes move together to follow the sun The synthetic oil in the pipes transfer the heat from the sun to a heat exchanger that boils water to steam that drives a steam turbine that generates the electricity. Parabolic troughs are the most mature CSP technology. Can be connected to storage. The efficiency from collector to grid is 15% Solar power towers Solar towers use hundreds or thousands of small reflectors (heliostats) Concentrate the sun’s arrays at the central receiver at the top of a central tower. Heliostats turn with the sun to maximize heat transfer. Molten salt as both heat transfer fluid and storage. A heat transfer fluid is heated at the central receiver that transfers the heat to a steam turbine – generating electricity. Direct steam generation (DSG) can be used in the receiver Solar parabolic dishes Parabolic dishes concentrate the sun rays at a focal point. The reflector follows the sun. Most dishes have their own generator. No heat transfer fluid is needed. Often a Stirling engine is used to generate power. Highest solar to electric efficiency, around 30%. Harder to connect with storage Linear Fresnel reflectors Uses rows of mirrors to reflect sunlight to an absorber tube. Mirrors can be flat or curved. The absorber tube is fixed here Uses direct steam generation. No heat transfer fluid is needed. Less efficient than parabolic troughs. More difficult to combine with storage. Very few Linear Fresnel reflectors are built.