Concentrated Solar Vehicle (CSV) Technology

CSV needs these to work--

    Concentrated Solar Vehicle Components

Heat engine: a small closed cycle steam engine or Brayton cycle engine drives the vehicle just as an internal combustion engine or electric motor would. It runs the wheels and all the normal vehicle systems.

Solar Thermal Container: a fairly large insulated compartment, in the center of the chassis under the body holds the thermal storage medium. This is insulated and not pressurized. Heat exchange tubes draw heat and power the engine.

Heat Transfer to the CSV: A dual hose from the solar thermal system connects to the vehicle, transferring hot medium in and cooled medium out, just as fast as filling a gas tank and lots faster than charging batteries.

Cooling system: Radiators cool the condensed steam and return it to a small water tank. There is no air intake, no combustion, no exhaust.

Controls: The accelerator pedal controls the amount of heat going to the engine. A reverse neutral and park selector do the familiar things.

    Solar Thermal Energy System -- This is all explained in the section after the demo vehicle.

Concentrated solar field

Thermal energy collection and transfer system

Thermal energy storage

Here is only one example of a demonstration CSV:

Modified Vega demonstrates feasibility of automotive solar power Design News, Sept. 7, 1981. Daniel V. Edson, Associate Editor, Design News

On-board "thermal battery" can be charged by parabolic reflector to provide boiler heat for steam-driven experimental car. Widespread application of propulsion concept could result in sizable energy savings.

With a hiss and a burst of steam, the solar-powered test car jumped to life. I drove a short distance, eased to a stop, checked the steam-pressure and thermal-battery-temperature gages, shifted into reverse and returned. The car had moved smoothly with surprising power, and solely by heat energy stored on board.

An intriguing contrivance--a 1977 Vega wagon, stripped of its engine, transmission and drive shaft, and refitted with a 1915 Stanley engine, a monotube flash boiler and a 700 lb thermal battery--the experimental car effectively demonstrated the feasibility of using solar energy for vehicular propulsion.

The thermal battery, a unique heat reservoir and the key element in the design, contains a stable, high-temperature, heat-storage material that can be charged (heated) by a high-temperature solar concentrator, such as a parabolic reflector (for convenience a small oil burner charges the thermal battery during testing). Water pumped through the monotube boiler, which is submerged in the heat-storage material, produces steam to run the engine.

Solar-power test car built by American Solar Energy Corp., Arlington, VA, represents significant advance in applying solar energy to vehicular propulsion. Equipped with steam engine, flash boiler and "thermal battery" that can be charged by parabolic reflector, proof-of-concept vehicle theoretically can travel 35 miles at 55 mph on one, three-hour charge from the sun.

Designed and fabricated by Dr. Robert C. McElroy, president of the American Solar Energy Corp., Arlington, VA, the research vehicle has a theoretical range of about 35 miles at 55 mph on a single thermal-battery charge (roughly three hours with a 16-foot parabolic reflector in nominal sunlight). A production solar-powered car, built specifically for solar propulsion, would probably stretch that range to 80 miles or so, easily more than most Americans drive in a single day.

The solar-powered test car represents a basic, straightforward propulsion concept, one with the potential to reduce petroleum fuel consumption considerably.

400lb of BTU'S

Capturing the energy collected by a high-heat solar concentrator, such as a parabolic reflector, for use in a mobile application, is made possible by the heat-storage material in McElroy's patented thermal battery. The stable, tar-like, refinery byproduct, trademarked Thermal Liquid, can be heated to upwards of 1000F without igniting, degrading or creating high pressure.

Characterized by high density and high specific heat, Thermal Liquid absorbs and stores a large number of Btu's in a relatively small volume, minimizing the storage container surface area, and thus minimizing area to be covered by insulation against heat loss, and increasing portability. The Thermal Liquid composition in the test Vega has an auto ignition point of 620F (in oxygen), but chemists have advised McElroy that certain modifiers will raise the auto ignition point to 1000F or more. The material freezes at about 200F.

Though Thermal Liquid performs satisfactorily and is readily available and reasonably priced, McElroy continues to assess other possible storage media, including synthetic lubricants, to improve heat-storage efficiency. Phase-change salts offer some of the properties desired, but are considered too corrosive.

Dr. Robert C. McElroy, solar car designer, points out steam release valve at rear of insulated, foil wrapped thermal battery. Ceramic-fiber insulation surrounds tank, holding ambient heat loss to 60F per 24 hours, from a 600F charge. Temperature gauge, visible at top of thermal battery, reads 475F. (Design News photo)

Sun-charged battery

Major components in the solar-power test vehicle include the thermal battery (with monotube flash boiler), feed water pump and engine. The Thermal Liquid and the monotube boiler are contained in a rigid, 275 lb. 1/4 inch steel tank positioned behind the front seats in the area normally occupied by the back seats and luggage. (The area available for the tank dictated the size of the storage battery--and likewise the estimated range.) To install the tank,

McElroy ripped out the floor and the front and back seats and replaced the floor with aluminum (diamond tread), and the front seats with light bucket seats.

The tank is insulated by Cer-Wool, a high R insulation holds ambient battery losses to about 60F per day from 600F plus.

A parabolic reflector would charge the thermal battery in a truly solar-powered configuration, but the reflector is considered a proven component and has not been used in testing. The reflector would be stationary, perhaps mounted beside the driveway or on top of the garage (or used in conjunction with residential heating/cooling equipment), with a quick disconnect optical universal connecting the reflector to the thermal battery. Solar energy reflected by the reflector face to a secondary receiver above the reflector would be transferred through the optic universal to a target plate in the thermal battery. The 400 lbs of Thermal Liquid in the thermal battery contains about 200,000 usable Btu, figuring a 375F to 620F working-temperature range. A 16-foot parabolic reflector, with 80% efficiency, could net a conservative 60,000 Btu/hour, considering system losses, and could recharge the battery in a little better than three hours. (Geographical location, of course, has an impact on charging time and the size of the parabolic reflector.)

The back-up heat source, which has been charging the thermal battery exclusively during tests, consists of a simple, high-efficiency oil burner, installed in a tube leading into the thermal battery. Roughly equivalent to a 16-foot parabolic reflector, the burner recoups one day's ambient heat loss in about 1 hour.

An expansion area at the top of the thermal battery accommodates the expansion and contraction of the Thermal Liquid as it is heated and cooled.

Mylar coated fiberglass parabolic reflector with interlocking sections combines strength and light weight. In nominal sunlight, 16-foot reflector could recharge thermal battery in solar-powered car in roughly three hours.

To keep the storage material from combining with oxygen, which would promote degradation, the expansion area is charged with a couple of pounds of nitrogen.

The flash boiler, submerged in the Thermal Liquid in the thermal battery, connects to the feedwater line at the top of the tank. (McElroy installed a removable panel in the roof to expose the connection). To keep steam in the boiler from backing up into the feedwater line, causing a vapor lock, the feedwater line connects first to an aluminum fitting--a thermal barrier--then to a steam check valve before meeting the stainless steel boiler.

The heat energy stored in the thermal battery has been satisfactory for the steam quantity and pressure required, but the monotube boiler is unable to keep up under high demand conditions. As a result, McElroy will replace the existing boiler by a boiler with a one-gallon reservoir inside the thermal battery, an engineering change that should alleviate the low-steam problem.

$50 per hp.

McElroy contacted a number of steam car owners before finding a steam enthusiast in Pennsylvania willing to part with an applicable engine, a 20 HP, 2-cylinder Stanley engine. Ironically, McElroy paid $1000 for the steam engine, manufactured in about 1915, but recovered only $125 when he sold the engine from the four-year-old Vega.

Steam engines, which are noted for simplicity and low parts count (21 moving parts in this engine), provide high torque at low speeds and do not require unique thermal battery provides 200,000 usable BTUs for solar-power test car. Containing stable, high-temperature, high-density, high-specific-heat "Thermal Liquid'" battery can be charged by stationary parabolic reflector through an optic universal.

gearing to accommodate low-speed operation. Because steam is injected into one end of each cylinder each stroke (exhausted at the opposite end), with intake and exhaust valves reversed for the return stroke, this two-cylinder steam engine has four power strokes per revolution and turns 773 revolutions per mile, in effect 3092 power strokes per mile.

The engine hangs under the car, between the rear axle and the rear bumper. (Integrating the steam engine to the Vega required as much dexterity with a torch as familiarity with automotive design.) Because the engine is built on four lag screws that pass through the axle housing, McElroy had to cut and weld the housing extensively. In addition, he was forced to grind out the housing to accommodate an 8-inch industrial flat-cut gear that had been hollowed to fit over the stock Vega carrier. A sheet metal cover (not installed during my test drive), holds oil for splash lubrication.

A small, 1/4-ton truck frame was substituted for the stock rear end to handle the weight of the Stanley and the thermal battery. The springs in the rear, however, are not adequate and McElroy has wedged a block of wood between the floor and springs to keep the suspension from bottoming until heavier springs can be installed. (Even with the added load in the rear, the Vega weighs approximately 400 lb less than delivery weight.)

To provide lubrication for the cylinders, McElroy chose a Detroit lubricator--an antique, but still rugged, reliable and easily metered.

Similar to lubricators found in later Stanleys, the pump squirts 600 weight cylinder oil into the engine each revolution (squirt being a stretch of the imagination). The pump runs so slowly that it took 3000 oscillations to move oil from the pump, mounted on the floor between the seats and the thermal battery, to the engine when the lubricator was installed.

The engine turns over at 100 psi, runs quite adequately at 200 psi, and is red lined at 600 psi; for my test drive we charged it to about 375 psi. A floor mounted lever controls direction--forward or reverse--and variable valve timing (the length of time steam is injected each stroke, an efficiency boosting economizer).

Priming and condensing

Once the Stanley is running, a power takeoff from the engine drives a water pump (an 800 psi high-pressure car-wash water pump in the Vega), assuring an adequate supply of feedwater. Getting started, however, requires a primer pump to jack up the steam pressure.

McElroy installed two primer pumps, in the test car, one hand-operated, the other driven electrically. The electrical pump, jerry-rigged to a 3/8 inch Black and Decker drill in a fixed position under the hood, has not performed as intended due to an inverter problem. At present, McElroy uses the drill only for static tests, since he must plug it into a wall socket. The hand-operated primer pump, infinitely more interesting, is an authentic Stanley brass pump, designed a good 75 years ago. Mounted on the floor between the seats, about where a gearshift might be positioned, the pump can achieve 1000 psi or better. At present, exhaust steam flows through a feed water preheater, an oil trap, then is exhausted.

Though a condenser is important--and though McElroy has experimented with a couple of designs--it is not a major thrust in this demonstration because a production solar powered car would not use water as a working fluid, eliminating the problem of separating heavy lubricant from steam.

Solar powered car in production

McElroy does not intend to develop this particular design much further. Beyond installing the new boiler with the one-gallon reservoir and making some full range road tests, the car will see few other refinements.

The next step involves applying the lessons of this proof-of-concept demonstration to a true solar powered prototype, built from the ground up. In a production solar-power-car-design, for example, the thermal battery might contain a heat storage material with higher temperature capability, higher specific heat and higher density, to increase the number of usable Btu's stored in a given volume.

Also, the thermal battery in a production car would be reconfigured for safety. McElroy suggests putting the well-insulated thermal battery inside the rectangle formed by the automobile frame, secured with shock absorber mounts. In a collision, the mounts and insulation would damp the tank movement, while the frame would physically protect it.

Attaching the passenger compartment to the battery, McElroy adds, could afford the passenger compartment the same shock absorption and cushioning given the thermal battery.

The engine in a modern solar power vehicle using McElroy's thermal battery would not be steam powered at all, but would use a low boiling point medium, such as Freon. In addition, the engine would have smaller bores, longer strokes and probably more than two cylinders, and would be designed for more complete expansion of the working fluid (or optimized variable valve timing) aiming for phase change at the end of the stroke. Engine efficiency in a production model might still be comparatively low, but it remains that the engine will operate only on demand (i.e., not when the car is stopped in stop-and-go traffic, or at traffic lights), and it will operate on an energy source that is renewable and, in a sense, free.

Sizing and siting parabolic reflectors is another application detail, but not insurmountable. Available sun in a particular locale, of course, will determine size of parabolic reflector and length of charge.

To accommodate commuter cars, cars driven to the train station, cars not at home during the day (where a reflector most commonly would be), employers and municipalities might erect parabolic reflectors in parking lots, charging stations paid for by the hour. Going a step further, collapsible reflectors could be designed--reflectors folded on the top of a car that the driver could pop open to charge the thermal battery while the car is parked.

McElroy recognizes the innate inconvenience of being tethered to a parabolic reflector, and even now is working on a solar car design with self-contained charging components. He envisions concentrating lenses built into the body of the car, lenses that would provide energy for a flash boiler or charge the thermal battery, depending on the need, whenever the car is in the sun, driven or parked. Ultimately, he projects, an integrated solar powered car could be driven nearly nonstop on a sunny day without using backup energy sources.

A concept stalled?

Plugging into a backyard parabolic reflector may not be every driver's idea of convenience, but given the outlook for fuel availability and cost, that prospect will be less odious as time goes on. McElroy estimates that a four-seat, compact solar car could be manufactured in production for close to the cost of a conventional compact car, less the parabolic reflector. He also estimates that such a car could be on the road in the next decade--if research continues.

The climate for funding for this type of venture has grown foul in recent years, and shows no sign of improving. Both government and industry in this country, for a multitude of reasons, have cut spending on numerous promising energy-saving ideas, flywheel propulsion and continuously variable transmissions being two excellent examples. Whether the concept demonstrated by McElroy is investigated further to determine its true potential, or whether the red, white and blue Vega with the NOGAS license plate remains parked in a driveway in Arlington, VA, remains to be seen.  

[Dr. McElroy's contact information ]

Solar Thermal Energy from wikipedia.

Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy. The CSV application for private vehicles and residences requires development and demonstration.

High-temperature collectors concentrate sunlight using mirrors or lenses and are generally used for fulfilling heat requirements up to 300 deg C / 20 bar pressure in industries, and for electric power production. Two categories include Concentrated Solar Thermal (CST) for fulfilling heat requirements in industries, and Concentrated Solar Power (CSP) when the heat collected is used for electric power generation. CST and CSP are not interchangeable in terms of application. MicroCST is proposed for CSV for private vehicles and residences, primarily autonomous systems off the grid.



Augustin Mouchot demonstrated a solar collector driving refrigeration equipment making ice cream at the 1878 Universal Exhibition in Paris. The first installation of solar thermal energy equipment occurred in the Sahara approximately in 1910 by Frank Shuman when a steam engine was run on steam produced by sunlight. Because liquid fuel engines were developed and found more convenient, the Sahara project was abandoned, only to be revisited several decades later.[1]

Vehicle applications have an historical precedent in the work of John Ericsson in solar power operation of heat engines.

Ericsson's solar engine

Stirling/Ericsson engines give higher efficiency than the kinds we use in trucks and cars today. But they're naturally heavier and slower. Ericsson tried to power a ship with one and it was far too big and heavy. Later he saw that his engine would be effective when heated with a parabolic solar heater. He did not have an interest in storage or road vehicles.  John Leinhart discusses this.

Many fireless steam locomotives are in use in switching and tug applications, and these are still commercially offered. Usually these are heated by burning fuel and even electrically, but there is no reason solar heat could not be used. These would be commercial CSVs with relatively little development.  

High-temperature collectors

Main article: Concentrated solar power

Part of the 354 MW SEGS solar complex in northern San Bernardino County, California.

The solar furnace at Odeillo in the French Pyrenees-Orientales can reach temperatures up to 3,500C .

The efficiency of heat engines increases with the temperature of the heat source. To achieve this in solar thermal energy plants, solar radiation is concentrated by mirrors or lenses to obtain higher temperatures--a technique called Concentrated Solar Power (CSP). The practical effect of high efficiencies is to reduce the plant's collector size and total land use per unit power generated, reducing the environmental impacts of a power plant as well as its expense.

As the temperature increases, different forms of conversion become practical. Up to 600C, steam turbines, standard technology, have an efficiency up to 41%. Above 600C, gas turbines can be more efficient. Higher temperatures are problematic because different materials and techniques are needed. One proposal for very high temperatures is to use liquid fluoride salts operating between 700C to 800C, using multi-stage turbine systems to achieve 50% or more thermal efficiencies.[30] The higher operating temperatures permit the plant to use higher-temperature dry heat exchangers for its thermal exhaust, reducing the plant's water use--critical in the deserts where large solar plants are practical. High temperatures also make heat storage more efficient, because more watt-hours are stored per unit of fluid.

Commercial concentrating solar thermal power (CSP) plants were first developed in the 1980s. The world's largest solar thermal power plants are now the 370 MW Ivanpah Solar Power Facility, commissioned in 2014, and the 354 MW SEGS CSP installation, both located in the Mojave Desert of California, where several other solar projects have been realized as well. With the exception of the Shams solar power station, built in 2013 near Abu Dhabi, the United Arab Emirates, all other 100 MW or larger CSP plants are either located in the United States or in Spain.

The principal advantage of CSP is the ability to efficiently add thermal storage, allowing the dispatching of electricity on demand. With current technology, storage of heat is much cheaper and more efficient than storage of electricity. In this way, the CSP plant can produce electricity day and night. If the CSP site has predictable solar radiation, then the CSP plant becomes a reliable power plant.

With reliability, unused desert, no pollution, and no fuel costs, the obstacles for large deployment for CSP are cost, aesthetics, land use and similar factors for the necessary connecting high tension lines. Although only a small percentage of the desert is necessary to meet global electricity demand, still a large area must be covered with mirrors or lenses to obtain a significant amount of energy. An important way to decrease cost is the use of a simple design.

When considering land use impacts associated with the exploration and extraction through to transportation and conversion of fossil fuels, which are used for most of our electrical power, utility-scale solar power compares as one of the most land-efficient energy resources available:[32]

System designs

During the day the sun has different positions. For low concentration systems (and low temperatures) tracking can be avoided (or limited to a few positions per year) if nonimaging optics are used.[33][34] For higher concentrations, however, if the mirrors or lenses do not move, then the focus of the mirrors or lenses changes. A tracking system that follows the position of the sun is required. The tracking system increases the cost and complexity. With this in mind, different designs can be distinguished in how they concentrate the light and track the position of the sun.

Parabolic trough designs

Main article: Parabolic trough

Sketch of a parabolic trough design. A change of position of the sun parallel to the receiver does not require adjustment of the mirrors.

Parabolic trough power plants use a curved, mirrored trough which reflects the direct solar radiation onto a glass tube containing a fluid (also called a receiver, absorber or collector) running the length of the trough, positioned at the focal point of the reflectors. The trough is parabolic along one axis and linear in the orthogonal axis. For change of the daily position of the sun perpendicular to the receiver, the trough tilts east to west so that the direct radiation remains focused on the receiver. However, seasonal changes in the angle of sunlight parallel to the trough does not require adjustment of the mirrors, since the light is simply concentrated elsewhere on the receiver. Thus the trough design does not require tracking on a second axis. The receiver may be enclosed in a glass vacuum chamber. The vacuum significantly reduces convective heat loss.

A fluid (also called heat transfer fluid) passes through the receiver and becomes very hot. Common fluids are synthetic oil, molten salt and pressurized steam. The fluid containing the heat is transported to a heat engine where about a third of the heat is converted to electricity.

Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land. Since 1985 a solar thermal system using this principle has been in full operation in California in the United States. It is called the Solar Energy Generating Systems (SEGS) system.[35] Other CSP designs lack this kind of long experience and therefore it can currently be said that the parabolic trough design is the most thoroughly proven CSP technology.

The SEGS is a collection of nine plants with a total capacity of 354 MW and has been the world's largest solar power plant, both thermal and non-thermal, for many years. A newer plant is Nevada Solar One plant with a capacity of 64 MW. The 150 MW Andasol solar power stations are in Spain with each site having a capacity of 50 MW. Note however, that those plants have heat storage which requires a larger field of solar collectors relative to the size of the steam turbine-generator to store heat and send heat to the steam turbine at the same time. Heat storage enables better utilization of the steam turbine. With day and some nighttime operation of the steam-turbine Andasol 1 at 50 MW peak capacity produces more energy than Nevada Solar One at 64 MW peak capacity, due to the former plant's thermal energy storage system and larger solar field. The 280MW Solana Generating Station came online in Arizona in 2013 with 6 hours of power storage. Hassi R'Mel integrated solar combined cycle power station in Algeria and Martin Next Generation Solar Energy Center both use parabolic troughs in a combined cycle with natural gas.

Enclosed trough

Main article: Parabolic trough

Inside an enclosed trough system

The enclosed trough architecture encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system.[36]

Lightweight curved solar-reflecting mirrors are suspended within the glasshouse structure. A single-axis tracking system positions the mirrors to track the sun and focus its light onto a network of stationary steel pipes, also suspended from the glasshouse structure.[37] Steam is generated directly, using oil field-quality water, as water flows from the inlet throughout the length of the pipes, without heat exchangers or intermediate working fluids.

The steam produced is then fed directly to the field's existing steam distribution network, where the steam is continuously injected deep into the oil reservoir. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up as a result from exposure to humidity.[36] GlassPoint Solar, the company that created the Enclosed Trough design, states its technology can produce heat for EOR for about $5 per million British thermal units in sunny regions, compared to between $10 and $12 for other conventional solar thermal technologies.[38]

GlassPoint's enclosed trough system has been utilized at the Miraah facility in Oman, and a new project has recently been announced for the company to bring its enclosed trough technology to the South Belridge Oil Field, near Bakersfield, California.[39]


MicroCSP is used for community-sized power plants (1 MW to 50 MW), for industrial, agricultural and manufacturing 'process heat' applications, and when large amounts of hot water are needed, such as resort swimming pools, water parks, large laundry facilities, sterilization, distillation and other such uses. Individual solar storage systems for personal use are even smaller as the heat requirements for a residence and vehicles is much less than 1MW.

Enclosed parabolic trough

The enclosed parabolic trough solar thermal system encapsulates the components within an off-the-shelf greenhouse type of glasshouse. The glasshouse protects the components from the elements that can negatively impact system reliability and efficiency. This protection importantly includes nightly glass-roof washing with optimized water-efficient off-the-shelf automated washing systems.[36] Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure.[37] Water is pumped through the pipes and boiled to generate steam when intense sun radiation is applied. The steam is available for process heat. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors as a result from exposure to humidity.[36]

Heat collection and exchange

Heat in a solar thermal system is guided by five basic principles: heat gain; heat transfer; heat storage; heat transport; and heat insulation.[53] Here, heat is the measure of the amount of thermal energy an object contains and is determined by the temperature, mass and specific heat of the object. Solar thermal power plants use heat exchangers that are designed for constant working conditions, to provide heat exchange. Copper heat exchangers are important in solar thermal heating and cooling systems because of copper's high thermal conductivity, resistance to atmospheric and water corrosion, sealing and joining by soldering, and mechanical strength. Copper is used both in receivers and in primary circuits (pipes and heat exchangers for water tanks) of solar thermal water systems.[54]

Heat gain is the heat accumulated from the sun in the system. Solar thermal heat is trapped using the greenhouse effect; the greenhouse effect in this case is the ability of a reflective surface to transmit short wave radiation and reflect long wave radiation. Heat and infrared radiation (IR) are produced when short wave radiation light hits the absorber plate, which is then trapped inside the collector. Fluid, usually water, in the absorber tubes collect the trapped heat and transfer it to a heat storage vault.

Heat is transferred either by conduction or convection. When water is heated, kinetic energy is transferred by conduction to water molecules throughout the medium. These molecules spread their thermal energy by conduction and occupy more space than the cold slow moving molecules above them. The distribution of energy from the rising hot water to the sinking cold water contributes to the convection process. Heat is transferred from the absorber plates of the collector in the fluid by conduction. The collector fluid is circulated through the carrier pipes to the heat transfer vault. Inside the vault, heat is transferred throughout the medium through convection.

Heat storage enables solar thermal plants to produce electricity during hours without sunlight. Heat is transferred to a thermal storage medium in an insulated reservoir during hours with sunlight, and is withdrawn for power generation during hours lacking sunlight. Thermal storage mediums will be discussed in a heat storage section. Rate of heat transfer is related to the conductive and convection medium as well as the temperature differences. Bodies with large temperature differences transfer heat faster than bodies with lower temperature differences.

Heat transport refers to the activity in which heat from a solar collector is transported to the heat storage vault. Heat insulation is vital in both heat transport tubing as well as the storage vault. It prevents heat loss, which in turn relates to energy loss, or decrease in the efficiency of the system.

Heat storage for remote use

Main article: Thermal energy storage

Heat storage allows a solar thermal application to produce power at any time.

Heat is transferred to a thermal storage medium in an insulated reservoir during the day, and withdrawn whenever required. Thermal storage media include pressurized steam, concrete, a variety of phase change materials, and molten salts such as calcium, sodium and potassium nitrate.[55][56]

Steam accumulator

The PS10 solar power tower stores heat in tanks as pressurized steam at 50 bar and 285C. The steam condenses and flashes back to steam, when pressure is lowered. Storage is for one hour. It is suggested that longer storage is possible, but that has not been proven in an existing power plant.[57]  This is essentially the type of heat supply used in fireless locomotives.

Molten salt storage

See also: Thermal energy storage

The 150 MW Andasol solar power station is a commercial parabolic trough solar thermal power plant, located in Spain. The Andasol plant uses tanks of molten salt to store solar energy so that it can continue generating electricity even when the sun isn't shining.[58]

Molten salt is used to transport heat in solar power tower systems because it is liquid at atmospheric pressure, provides a low-cost medium to store thermal energy, its operating temperatures are compatible with today's steam turbines, and it is non-flammable and nontoxic. Molten salt is also used in the chemical and metals industries to transport heat.

The first commercial molten salt mixture was a common form of saltpeter, 60% sodium nitrate and 40% potassium nitrate. Saltpeter melts at 220C (430F) and is kept liquid at 290C (550F) in an insulated storage tank. Calcium nitrate can reduce the melting point to 131C, permitting more energy to be extracted before the salt freezes. There are now several technical calcium nitrate grades stable at more than 500C.

This solar power system can generate power in cloudy weather or at night using the heat in the tank of hot salt. The tanks are insulated, able to store heat for a week. Tanks that power a 100-megawatt turbine for four hours would be about 9 m (30 ft) tall and 24 m (80 ft) in diameter.

The Andasol power plant in Spain is the first commercial solar thermal power plant using molten salt for heat storage and nighttime generation. It came on line March 2009.[59] On July 4, 2011, a company in Spain celebrated an historic moment for the solar industry: Torresol's 19.9 MW concentrating solar power plant became the first ever to generate uninterrupted electricity for 24 hours straight, using a molten salt heat storage.[60]

In January 2019 Shouhang Energy Saving Dunhuang 100MW molten salt tower solar energy photothermal power station project was connected to grid and started operating. Its configuration includes an 11-hour molten salt heat storage system and can generate power consecutively for 24 hours.[61]

Phase-change materials for storage

Phase Change Material (PCMs) offer an alternative solution in energy storage.[62] Using a similar heat transfer infrastructure, PCMs have the potential of providing a more efficient means of storage. PCMs can be either organic or inorganic materials. Advantages of organic PCMs include no corrosives, low or no undercooling, and chemical and thermal stability. Disadvantages include low phase-change enthalpy, low thermal conductivity, and flammability. Inorganics are advantageous with greater phase-change enthalpy, but exhibit disadvantages with undercooling, corrosion, phase separation, and lack of thermal stability. The greater phase-change enthalpy in inorganic PCMs make hydrate salts a strong candidate in the solar energy storage field.[63]

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