HTF SYSTEM

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1. MAIN PUMPING SYSTEM.
2. ULLAGE SYSTEM.
   
3. ANTI-FREEZING SYSTEM.
4. NITROGEN SYSTEM.
5. EXPANSION TANKS.
6. SALTS STORAGE SYSTEM.
7. AUXILIARY BOILER.

The first question is why an organic syntetic fluid is chosen to circulate by the solar field. The main reason is because in the range of working temperature there is no phase change. If it was water, the phase change provoque that it would have lot of pressure at the working temperature (293-393ºC) so more resistant pipes and powerful pumps would be required.

 Table 1. Characteristic parameters of the HTF system.

The collector field consists of a large number solar collector which absorb the solar radiation, conected by pipes.  This pipes are made of carbon steel, and are insulated with rock wool. IN the power island, parts of them are heat traced-pipes. The main features that can describe the solar field piping are:

1. Pipes are subjected to strong temperature variations. (Expansions and thermal strains).
2. There are two parallel pipes flowing through the hot and cold fluids.
3. Each knot has a connection to the cold fluid pipe (inlet) and other one to the hot one (outlet).
4. U bend tubes are needed to absorb expansions caused by temperature changes, more or less one every 70 meters.
5. Expansion joints are needed at certain points to absorb expansions.
6. Pipes can not be traced-pipes. It makes the system more expensive because it consists of too many miles.
   
7. To prevent freezing, it is necessary to make thermal oil flow continuously, avoiding the fluid to be retained.
8. Joints can not be made by pipe flanges. They are welded to avoid leaks. It means that maintenance is required.
   

Figure 2. Collectors field.

HTF system consist of the following subsystems:

1. MAIN PUMPING SYSTEM.

The pumping system consists of several large pumps in charge of making the thermal oil flows throughout the plant. The power of these pumps is very high, between  0,7 - 2MW each. Normally, there are between 2 and 6 pumps in a 50 MW power plants, in a n+1 configuration (n of them working in parallel and the other is spare one). The discharge pressure is usually 30 bar. 

It is possible to find two types of main pump:

- Centrifugal pump type API 610 OH2. This pumps are usually used a cantilevered impeller, one side double closure, horizontal suction and vertical discharge.

- Centrifugal pump type API 610 BB2, with double support impeller, both sides double closure, vertical suction and vertical discharge.

2. ULLAGE SYSTEM.

Waste disposal system is responsible for cleaning thermal oil from oxidation and cracking of the contaminated products. If waste materials would not be removed, it would suppose filters obstruction, pumps and valves deterioration, exchange capacity reduction and a drop in the inflammation point.

Waste disposal system is based on extracting 1-2% of the total of the oil flow volume. This oil is evaporated reducing the pressure from 16 to 4 bar, separating the components with a high boiling point. Then, thermal oil is cooled to make it liquid again. In this way, it is achieved the separation of the products with lower liquefaction temperature.

Figure 4. Waste disposal system.

3. ANTI-FREEZING SYSTEM.

Its function is to provide heat to the HTF system to avoid reaching its freezing temperature. This system also can be used to add heat to the system, though it is not specifically to prevent freezing. It can be used to produce HTF extra warming to generate more energy than the one which belongs to the collected radiation. Also, this system can heat oil without radiation, to generate energy from fossil fuel, natural gas.

To prevent improper use of natural gas, in Spain the maximum quantity of Natural Gas that can be consumed in a solar thermal power plantis limited between 12-15%.

Some of the pipes are traced to prevent oils viscosity and their solidification. Trace can be made by electric resistances or by steam heated jacketed piping. But, electric tracing is the most used because of the following features:

1)    It is easy to control the product temperature.

2)    All heating circuits are central control systems.

3)    Continuous monitoring control and very low operating expenses.

4)    There are not parts subjected to friction

5)    Heating wires are easily placed.

4. NITROGEN SYSTEM.

High pressure nitrogen tanks are used to prevent oil degradation by oxidation and cracking inside them. Their pressure is higher than the steam one, about 11 bar, relative pressure.

Rendered inert with nitrogen tanks are:

1)    All the waste disposal system tanks, except the flash evaporation one.

2)    The expansion tank.

3)    The overflow tank.

5. EXPANSION TANKS.

 It is responsible for absorbing the differences in volume when the fluid is heated. It is placed in the higher point of the power plant, according hith international standard. Vent valves should be watched because water steam comes out from them. Expansion tank should be high pressure nitrogen. It is associated with the overflow tank and the recirculation pump from overflow tanks.

6. SALTS STORAGE SYSTEM.

Molten salt can be employed as a heat store to retain heat collected so that it can be used to generate electricity in bad weather or at night. Molten salts are used because to store the same quantity of energy with thermal oil we should use bigger tanks.

There are two tanks: the cold storage tank, normally at 280ºC, and a hot storage, normally at 380 ºC. The molten salt is a mixture of 60% sodium nitrate and 40% potassium nitrate, commonly called saltpeter. No phase change exists in it at the working temperatures. Molten salt has high heat transfer coefficient, between 0,6-1,2 MW/m2, and high thermal storage capacity. Its melting point falls between 220-250 ºC, so electric tracing is needed.

       Characteristics of molten salts storage in a 50 MW power plant.

1. It is designed to store 1010MWh.
2. 28800 tons of molten salts are needed.
3. Thermal load is carried out in 7,7 hours with a heat exchange between heat thermal fluid-salts of 131MW.
4. Storage discharge to empty the hot tank is done in 8,5 hours with a heat exchange of 119 MW.
   
5. A flow of 935 kg/s is pumped from the cold tank to the hot one. Hot storage tank has a flow rate discharged of 874kg/s flow. In this process HTF temperature changes from 287ºC to 379ºC.
6. Exchangers, valves and pipes have electric heat tracing to prevent salts freezing.
7.  Tanks are provided of electric resistances in the central area and on the ground.
8. If there is a long stop, there is in the cold tank a salts recirculation system to prevent their stratification.
9. Tanks are rendered inert with nitrogen to prevent oxygen contact with the HTF in case of leak.
10. Drainage deposit collets drainages from pipes and exchangers and returns them to the cold tank
    

Figure 6. Salts storage scheme.

6.2    System elements.

1) Cold salts storage.

- Cold salts storage tank.

- Electric heaters submerged in the tank.

- Cold salts storage pumps with electric motors and speed variators.

2)Heat exchangers for molten salts. Exchangers train is provided in serie in effect from cold to hot tank, heating in this way the salts and the HTF that flows trough the tubes, while molten salts are circulating trough the shell. Pipes have electric heat tracing to prevent salts freezing.

Figure 7. Heat exchangers.

3) Hot salts storage.

- Electric heaters submerged in the tank.

- Hot salts storage pumps with electric motors and speed variators.

4) Drainage system.

- Drain container is used to empty heat exchangers and pipes. It is about 30 m³, located 2 meters below ground level. Its function is to collect pipes and heat exchangers drainages. It is provided with electric tracing to prevent salt solidification.

- Drainage pump used to return the salts to the cold salts tank.

- It is provided with leaks detection and a HTF condenser system. It detects, removes HTF from salts circuit and identifies the exact leak location.

7. AUXILIARY BOILER.

It is responsible for keeping oil temperature in the correct values for the system continue operating even when, for any reason, sunlight collectors do not provide enough energy. Usually feed on natural gas.

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