METHOD FOR REMOVING GAS FROM HIGH-TEMPERATURE HEAT-TRANSFER FLUIDS IN SOLAR THERMAL POWER PLANTS

Abstract

The invention provides a process for removal of gaseous decomposition products from high temperature heat transfer fluid HTF of an operational solar thermal power plant having an HTF circuit, in which a volume increase of the HTF in the HTF circuit which is caused by incident solar radiation in an HTF-traversed solar field and consequent heating by day takes place regularly in a day-night cycle and the additional volume formed by the volume increase is collected from the HTF circuit in an expansion vessel, a portion of the additional volume of the HTF is transferred into a drainage vessel operated at relatively low pressure in which gaseous decomposition products and low-boiling constituents escape from the HTF, wherein the low-boiling constituents are condensed, and during the volume contraction of the HTF occurring during the night-time cooling a portion of the additional volume of the HTF is recycled from the drainage vessel into the expansion vessel and from the expansion vessel into the HTF circuit, wherein the volumes in the expansion vessel and the drainage vessel becoming vacant as a result of the transferrals of the HTF are filled with inert gas.

Claims

1. A method for the removal of gaseous decomposition products from a high temperature heat transfer fluid (HTF) of an operational solar thermal power plant having an HTF circuit, in which a volume increase of the HTF in the HTF circuit which is caused by incident solar radiation in an HTF-traversed solar field and consequently heating by day takes place regularly in a day-night cycle, comprising: collecting from the HTF circuit the additional volume formed by the volume increase in an expansion vessel; transferring a portion of the additional volume of the HTF into a drainage vessel operated at relatively low pressure in which gaseous decomposition products and low-boiling constituents escape from the HTF, wherein the low-boiling constituents are condensed; and recycling a portion of the additional volume of the HTF, resulting from the volume contraction of the HTF occurring during the night time cooling from the drainage vessel into the expansion vessel and from the expansion vessel into the HTF circuit or directly from the drainage vessel into the HTF circuit, wherein the volumes in the expansion vessel and the drainage vessel becoming vacant as a result of the transferrals of the HTF are filled with an inert gas.

2. The method according to claim 1, wherein the HTF is silicone oil.

3. The method according to claim 1, wherein the condensation of the low-boiling constituents of the HTF is carried out as a multi-stage condensation.

4. The method of claim 1, wherein the expansion vessel has smaller dimensions than the drainage vessel.

5. The method of claim 1, wherein the inert gas is selected from the group consisting of He, Ar, Ne and N.sub.2.

6. The method of claim 1, wherein the HTF in the solar field in daytime operation has a temperature between about 150 C. to 475 C.

7. The method of claim 1, wherein the HTF circuit has a pressure of about 15 to 50 bar.

8. The method of claim 1, wherein the low-boiling constituents have a vapour pressure of at least 1 mbar at 20 C.

9. The method of claim 1, wherein heat is intermediately stored in a salt melt.

10. The method of claim 1, wherein the expansion vessel has a fill-level control means installed which actuates a liquid-side flash valve and decompresses into the drainage vessel the additional volume of the HTF transferred into the drainage vessel.

Description

EXAMPLES

[0072] A CSP power plant having salt storage tanks passes heat (W) to a conventional steam power plant with a nominal electrical output of 50 MW. The CSP power plant is operated with Helisol 5A (Wacker Chemie), a mixture of methylpolysiloxanes, as the HTF. During the day in the solar field (S) the HTF entry temperature is 300 C. and the HTF exit temperature is 425 C. Overnight the HTF on average cools to 200 C. Annual operating hours at nominal output are 3219 hours. The total mass of HTF in the CSP power plant is 1200 t. The daily heating time taken to reach operating temperatures in the solar field is 1 hour. The volume increase of the HTF during the daily heating time results in a mass displacement of 253 t. (21% of the total HTF mass) from the HTF circuit into the expansion vessel (E) and into the drainage tank (D). During night-time cooling the volume contraction in the HTF circuit results in a reverse mass displacement from the drainage tank ultimately into the HTF circuit. Formed daily in the CSP power plant under these conditions are 0.03 kg of H.sub.2, 2.7 kg of alkanes (methane, ethane) and 1.6 kg of methylsilanes (trimethylsilane and tetramethylsilane) which, for long-term steady-state operation, require removal.

Example 1

[0073] The pressure in the expansion vessel is set to 24.8 bar and the fill level is kept constant. The pressure in the drainage tank is set to 1.5 bar. In both containers ambient-temperature nitrogen is used for pressure control and simultaneous inertization. The expansion vessel and the drainage tank are ideally insulated (boundary condition).

[0074] During morning heating of the solar field the fill-level control means in the expansion vessel actuates a flash valve that conveys HTF from the expansion vessel into the drainage vessel and decompresses it to the pressure thereof. The evaporated low-boiling HTF constituents are recovered from the inert gas (N.sub.2) and the uncondensable decomposition products by a two-stage partial condensation.

[0075] The first partial condenser is a dry air cooler operated at a condensation temperature of 65 C. Only 0.7 t of gaseous and vapourous products exit the first partial condenser over the day and these are post-treated in the second partial condensation stage. The second partial condensation stage is a low-temperature condenser operated at a condensation temperature of 20 C. The off-gas from the 2.sup.nd partial condenser is only 0.56 t/d and consists of >98% nitrogen, 0.5% uncondensable gases (methane and ethane) and <1.5% low-boiling siloxanes and silanes. The HTF losses for replacement are altogether <11 kg/d.

[0076] As a result of the condensate reflux a temperature of 166 C. is established in the drainage tank. The nitrogen consumption in the drainage vessel for volume compensation during return of HTF from the drainage tank into the HTF circuit during night-time cooling is 550 kg/d.

[0077] Due to the fill-level control means the nitrogen consumption in the expansion vessel is negligibly small.

[0078] The following steady-state gas concentrations are established in the HTF: N.sub.2 421 ppm, H.sub.2 0.1 ppm, alkanes (methane, ethane) <10 ppm.

Example 2

[0079] The average temperature in the non-insulated expansion vessel and in the drainage tank is 200 C. After the two-stage partial condensation (condensation temperatures 65 C. and 20 C. as in example 1) there remain 0.57 t/d of off-gas consisting of 90% nitrogen, 0.5% uncondensable gases (methane and ethane) and <0.5% low-boiling siloxanes and silanes. The HTF losses for replacement are altogether <6 kg/d.