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An Overlooked Danger of Solar Thermal Plants: Fire and Explosion
After doing some reading about solar energy I came across some information on a danger that had, quite frankly, not occurred to me before. It seems that many forms of solar thermal energy production carry a very high risk of severe fire and even explosion.
Solar thermal power plants use large mirrors to concentrate the suns light on collectors. In most systems this works through trough mirrors which focus the light onto long pipes. In others, an array of mirrors focuses the energy on a central receiver, which contains a fluid that is heated by the light. The fluid is normally pumped continuously through a system of collection tubes in order to transfer as much heat as possible.
Leaks have always plagued solar thermal power systems. The tubes must be thin to maximize thermal transfer and to keep costs down, and the scale of the systems is necessarily very large. This, combined with the stress of daily heating and cooling has lead to what might be called a “plumbers nightmare” and explains why so much labor is required to keep solar thermal plants up and running.
The engineering challenge of providing a leak-resistant way of transferring the fluid through miles, under such harsh conditions has resulted in a number of approaches, including the use of ball joints and flexible hoses. The sheer size of utility-scale installations has also been a problem, since the connections and tubing must be made as economically as possible.
Hot fluid leaks not only reduce system performance by changing the fluid pressure, but they increase the risk of insulation fires or auto-ignition. With risk of fire, high operation costs, and the necessity for large numbers of replacement parts, some pursued other options while improvements to flex hoses were developed.Ball joint assemblies, which have graphite seals and both rotation and angular deflection capabilities, were chosen by those with flex hose issues. Many suppliers and spare parts were available, and seal maintenance after thousands of hours of use resulted in lower maintenance costs.
The greater danger comes from the heat transfer fluid itself, which is generally oil-based. They may be synthetic or mineral oil based. The fluids themselves are usually designed to have a relatively high flash point and be less prone to combustion than other oil, but they are still flammable, especially when they are heated to high temperatures. To maximize the efficiency of solar thermal power plants, the highest temperatures are desired for the transfer fluid, although this increases the danger of fire. Since the fluid is often pumped at high rates, even a small leak can result in high pressure, high temperature fluid being sprayed.
A greater danger comes from the potential for the fluid to be over-heated within the system. Either errors in routing the hot fluid or blockages in the system could potentially cause the thermal transfer fluid to absorb too much heat and be pushed beyond the boiling point, potentially to the point of explosion.
YES, this has happened before. In 1982, Solar One, a concentrated solar tower was built with ten megawatts of capacity. In August of 1986, a problem with the facility’s heat transfer system resulted in 240,000 gallons of heat transfer oil erupting into flames. The explosion destroyed much of the plant.
A combined solar and gas fired power plant, SEGS I experienced similar fires at least twice. In 1990, it experienced an explosion resulting in a huge volume of superheated oil, which continued to burn for many hours before being brought under control.
Blasts Rip Desert Solar Power Plant
BARSTOW — A series of explosions and fire shut down electricity generation at the world’s largest solar power plant near here Wednesday.
Thick plumes of black smoke spiraled into the clear desert air when one of four natural gas-fired heaters used to back up the solar heating system exploded.
A short time later, a second natural gas heater caught fire and exploded as the first of 75 firefighters and 25 pieces of equipment were arriving at the site, about 140 miles northeast of Los Angeles.
“We had a series of explosions, more than two,” said Capt. Sharon Sellers of the San Bernardino County Fire Department. “Our first units got on-scene at 9:16 a.m. and a second explosion occurred at that point, then a series of them during the entire incident,” Sellers said.
“There was a mushroom cloud. The heat was real intense and there were explosions,” said an inmate from the Boron Federal Prison Camp who was pressed into service to help fight the fire. He would not identify himself.
Sellers said two workers at the plant suffered minor breathing problems and were treated at Barstow Community Hospital.
Operated by LUZ International Ltd. of Los Angeles, the $280-million Harper Lake solar plant began generating electricity on Dec. 28 and produces 80 megawatts, enough power to serve 115,000 people. The company operates eight such plants in the California desert. Combined, they generate 274 megawatts, which is sold to Southern California Edison Co. An Edison spokesman said there was no interruption of electric service to its customers.
“We had two oil heaters on line and were bringing up the third and fourth oil heaters when this explosion occurred,” LUZ International spokeswoman Kathleen Flanagan said in Los Angeles.
While no flames were visible 1 1/2 hours after the fire began shortly before 9 a.m., San Bernardino County firefighters had difficulty reaching the blaze deep within the generating equipment.
“There is fire up there somewhere still heating that oil,” Sellers said.
The blaze was contained, but continued to burn late Wednesday.
The 1990 fire was not directly the result of the solar power systems on site but of a gas-fired heater at the plant. None the less, it goes to show how difficult it can be to fight a fire involving such large volumes of heat transfer oil.
A second fire occurred in 1999 at SEGS I. As a result of “human error” the temperature of the oil exceeded the safe limit, resulting in a massive explosion that shook the area for miles. Some 900,000 gallons of oil erupted in flames.
Similar fires, involving heat transfer fluid have occurred in other industrial applications.
The danger of fire or explosion may also be increased by certain design features of some solar power stations. A number of fires have occurred when leaking thermal transfer fluid came into contact with insulting materials, used in some of the pipes at solar thermal plants. This has lead Dow Chemical, a manufacturer of thermal transfer fluid to issue warnings of the possibility of spontaneous combustion of hot fluids, should they come into contact with insulating materials.
Some newer solar thermal plants have proposed to use large tanks of molten salt as a means of providing extended energy storage. This has resulted in additional concerns over the possibility of reactions occurring between the salt and heat transfer fluid. Because the salt will solidify if cooled, and because of its corrosive nature, oil-based heat transfer fluid is still required for the collectors. DOE Safety tests found that a break in the barrier between the two fluids could be dangerous.
Therminol is a synthetic oil; Caloria is a mineral oil. Since the intention of the thermocline is to provide thermal storage for the Solar Trough power plants, we needed to investigate the reactivity between the oils used in the SEGS plants and the molten salt. It is possible that the oils could come into contact with the molten salt at the operating temperature of 400”C due to a failure in the oil-to-salt heat exchanger. The subsequent reaction could cause a dangerous result such as a fire or explosion.
It should, of course, be noted that no energy source is without risk, and there are certainly other industrial facilities that can also explode or produce massive oil fires. However, the danger has been largely ignored with solar thermal power plants. Citing such plants near population areas, without appropriate precautions, could be quite dangerous. Additionally, the sheer volume of oil and the nature of the fires that can occur could easily overwhelm most fire departments. Solar thermal plants generally have not been required to maintain the assets necessary to fight such a fire, should one occur.
2013-04-08 17:45:58
Source: http://depletedcranium.com/an-overlooked-danger-of-solar-thermal-plants-fire-and-explosion/
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