FAQs

Q: How a trough system works?

Q: Why use a parabolic trough instead of a flat plates or evacuated tubes?

Q: How can I use a IPH trough system to reduce my energy bills?

Q: Can I produce electricity?

Q: How a trough system works?

A: Below is an explanation of the concept of parabolic trough solar collectors, factors that influence their design and the practicality and usefulness of the technology to meet a wide range of energy requirements.

Concept

It is a principle of geometry that a parabolic reflector pointed at the sun will reflect parallel rays of light to a focal point of the parabola. A parabolic trough is a one-dimensional parabola that focuses solar energy onto a line. Physically, this line is a pipe with flowing liquid inside that absorbs the heat transmitted through the pipe wall and delivers it to the thermal load.

A trough captures sunlight over a large aperture area and concentrates this energy onto a much smaller receiver area, concentrating the intensity of the sun. The concentration ratios, which are the amount of solar energy on a receiver with a reflector divided by the amount that would normally be on the receiver, range between 30 and 80. It is the process of concentration that allows troughs to deliver high temperature thermal energy. In order to achieve the desired concentration, a trough tracks the sun in one axis continually throughout the day. The required tracking accuracy is within a fraction of a degree.

Systems

Almost all of the IPH systems work on a “closed loop.” As the schematic below shows, this means that the fluid being heated in the concentrator never leaves the system. In order to use the energy absorbed by the system, the heat is transferred from the working fluid to another medium. One of the greatest advantages of solar thermal technologies is the possibility of simple storage. Insulated tanks containing the heated working fluid can hold energy for use at a later time.

For a more detailed explanation of how a trough system works, please see the FAQ “How does your product work?” To see some of the ways solar troughs have been used for industrial processes, see our “Project Library.”

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Q: Why use a trough instead of a flat plates or evacuated tubes to deliver heat

A: Flat plate collectors or evacuated tubes are designed for residential water and space heating applications, but they are not well suited for large-scale commercial and industrial applications.The reasons why troughs are more appropriate are:

Operating Temperature and Efficiency

Flat plate collectors have a large absorber surface from which to lose heat and hence their performance rapidly degrades as operating temperatures rise.The practical limit for the efficient delivery of energy is in the range of 140 – 160 F (60 – 70 C).Evacuated tubes are generally non-concentrating or only slightly so, and similarly have a large area for heat loss.A vacuum is designed to limit conductive and convective heat loss so they have a slightly higher practical limit for thermal energy delivery compared to flat plates.However, on loss of vacuum, they perform much worse than a flat plate.

A parabolic trough only uses the direct component of solar radiation, whereas non-concentrating collectors can absorb the direct and diffuse component of solar radiation.However, because troughs track, in good solar areas, even taking out the diffuse component, they intercept more solar energy.

Abengoa troughs concentrate solar energy about 40 times onto a small area of absorber tube.Thus, there is only a small area for heat loss.This allows troughs to operate with good efficiency even when delivering energy at temperatures up to 260 C.At temperatures around 100 C, which is about the upper limit for evacuated tubes and essentially beyond the capabilities of flat plates, troughs will be operating at efficiencies better than 60% in converting solar energy into useful heat.
Energy delivery

Because troughs intercept more energy in good solar areas, and because they operate at a higher efficiency (except for the lowest of temperatures, such as heating a swimming pool), they deliver much more energy than flat plates or evacuated tubes.They can also deliver energy over a longer time period.For instance, in the summer the sun will rise and set to the north (in the northern hemisphere).Flat plates and evacuated tubes are mounted tilted to the south.Hence, they will not start collecting energy until late in the morning when the sun illuminates their surface.In contrast, a trough tracks to face the sun early and late in the day and therefore delivers energy for a much longer period during the day.This can be important when trying to meet loads not in the middle daylight hours, such as cooling loads that peak in late afternoon.

Ease of design

It is much easier to design large parabolic trough fields than flat plate or evacuated tube systems.This is because a single flow path of a trough field removes heat from a large area of collectors.In contrast, the area of flow paths for non-tracking collectors is very small.Thus, you have many, many parallel flow paths.In addition, within each flow path there are tens of individual flow paths or risers in parallel.It is a major challenge to maintain equal flows throughout flat plate and evacuated tube collector fields.Usually, some type of balancing valve is employed or reverse return piping is employed.All of these solutions increase cost and complexity, while reducing thermal output. Nevertheless, they are necessary since the penalty for failure is large.Collectors that do not receive adequate flow will operate with poor efficiency.

Reduced complexity, cost of balance of plant and running costs

Parabolic trough collector efficiency is much less affected by operating temperature than non-tracking collectors.Hence, Abengoa Solar parabolic troughs typically operate at differential temperatures across the solar field that are a number of times greater than the differentials across non-tracking flat plates or evacuated tubes.This means that flow rates for non-tracking collectors are correspondingly larger and piping and pumps are larger and more expensive.These differences result in increased heat loss, slower startup up and increased electricity consumption.Other components in a trough system, such as heat exchangers are also less expensive because of the great temperature driving force and energy storage tanks can be smaller since they can be efficiently heated to higher temperatures.

Installed cost

Parabolic trough collectors use much less materials than non-tracking collectors.Added to lower balance of plant costs and the use of steel for piping compared to copper in flat plates and evacuated tubes, the installed cost of parabolic trough collector fields is much less than these less satisfactory alternatives.

Ease of maintenance

The fact that a trough moves is considered a major disadvantage, in terms of complication, compared to non-tracking collectors.While tracking is an added complexity, drive and control systems are so reliable and require so little maintenance as to outweigh the disadvantages of not tracking.For instance, if needed for maintenance, a trough can be de-focused. The collector and the heat transfer fluid cools down so that systems can be worked on.However, it is not possible to “turnoff” a non-tracking system and they remain hot in the presence of sunlight.In fact any reduction in flow can cause major problems.Temperatures in flat plates under no-flow or stagnation conditions rise to over 300 F.This is far beyond the operating temperature and can degrade the collectors and the collector fluid.System pressures rise, temperatures approach the melting point of solder and leaks can develop.Evacuated tubes stagnate at very high temperatures to the point where the vacuum can be compromised and they are rendered useless for energy collection.

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Q: How can I use a IPH trough system to reduce my energy bills?

A: Troughs can be regarded simply as a source of heat with delivery temperatures up to 500 F
(260 C).So that anything that you can do with heat from a conventional energy source, such as natural gas or fuel use, you can do with solar energy.The difference is that the interface between the solar system and the application must be carefully designed to account for the fluctuating nature of solar energy.Examples of solar thermal applications include:

Hot Water Heating for large-scale domestic uses at military barracks, hospitals and prisons.
Hot water heating for industrial applications such as cooking, pasteurization of milk and beverages, vegetable frying, heating of metal processing tanks, heating of reaction vessels or distillation columns at chemical, biofuels or pharmaceutical operations, and sterilization and cleanup at food processing plants.
Steam generation wherever steam is used as the heat transfer medium in the food, chemicals, petroleum refining and other industries
Space Heating and Cooling (using an absorption chiller) for any large occupied space such as retail establishments, college campuses, large office and manufacturing buildings.
Hot air heating for drying operations in the mining and chemical industries.
Enhanced Oil Recovery whereby hot water or steam is injected into petroleum reservoirs to recover previously abandoned oil reserves or as the primary method of recovery from heavy oil formations.
Desalinization using thermal energy as an alternative to the use of electricity for reverse osmosis.
Preheat energy for high temperature processes, such as boiler feedwater preheating in power plants or air preheat in combustion or processing operations.
Inlet air cooling of gas turbines used for electricity production to eliminate the decline in power output at high ambient temperatures.

See the project library for examples of trough applications.

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Q: Can I produce electricity?

A: It is technically feasible to use Abengoa Solar IPH troughs to produce electricity on a small scale using an organic Rankine cycle (ORC) engine.However, such engines are typically expensive and much less efficient than utility-scale steam turbines.Therefore, it is usually not economically attractive to generate electricity in this way.For information on large-scale electricity generation view the Concentrated Solar Power (CSP) pages of the Abengoa Solar web site.

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