Interview by Rikki Stancich

This week, Torresol Energy announced that its Valle 1 and Valle 2 plants - two identical 50 MW parabolic trough plants, located in Cadiz, in the South of Spain, have commenced commissioning.

The plants are equipped with thermal storage, which will allow them to continue to produce power for up to 7.5 hours in the absence of solar radiation. The Valle 2 plant will road test Sener’s single tank storage system, as well as its latest trough collector, the SENERtrough 2.

Construction on Valle 1 and Valle 2 began in December 2009, and was completed in December 2011. Valle 1 & Valle 2 will produce 160 GWh of power per year, respectively. The twin projects were financed through seven Spanish commercial banks in 2009 for a total financing of EUR 540 million.

Torresol Energy, the joint venture between Abu Dhabi-based renewable energy company Masdar, and Spanish engineering and construction firm, SENER, has now brought a total of three CSP plants online, including the innovative Gemasolar plant that started commercial operations in May last year. The company is due to bring a fourth CSP plant, Shams1, online later in 2012.

CSP Today: Sener is testing a prototype single tank thermocline storage system in the Valle 2 plant. What is the anticipated advantage of single vs. two-tank storage?

Ignacio Larrea: The single-tank thermal storage system simplifies the design of the molten salts storage system, achieving a 25% reduction in the storage costs. This system would be an alternative to the current molten salt storage system that SENER installs in the majority of its CSP projects consisting of two tanks: one to store cold salts, and the other to store hot salts.

The dual-tank system functions correctly and has been proved in commercial operation in various plants, demonstrating a high level of operational reliability. Even so, it is necessary to keep moving forward in the search for solutions that allow the cost of power generation to be reduced.

With this in mind, based on the experience acquired in the design, construction and operation of the dual-tank system, SENER is working on a single-tank storage system with an insulation barrier. We have designed a reduced-scale prototype to be tested in the Valle 2 commercial plant, which SENER is constructing for Torresol Energy in Cadiz (Spain).

This innovative system uses just one tank to contain salts in both their cold and hot states, separated by an insulation barrier that naturally moves up and down according to the quantity of salts in each state.

In the single-tank system, the single tank has a capacity similar to that of one of the dual-system tanks, and it is permanently full. In this system, all of the pumps have short shafts. These pumps are more common in the marketplace and, therefore, they are also less expensive. 

With the new system comes not only the expectation of a significant reduction in storage costs, that can achieve the 25%, but also of an increase in production, as thermal losses are reduced by having just one tank instead of two. Once this prototype is validated, SENER expects to start applying it on a commercial scale.

CSP Today: Sener is also testing a new collector system in the Valle 2 plant, the SNT2. How does this newer model improve on the SNT1?

Ignacio Larrea: The SENERtrough 2 system increases the size of the collectors, allowing a 20% reduction of the number of loops required in a parabolic trough plant while maintaining the same efficiency. Therefore, it cuts down the cost of the solar field in an 8%.

CSP Today:  The construction of Valle 1 and 2 is unique in that they were built in parallel. What were the key advantages of this approach, and will Sener repeat this approach for future Spanish projects?

Ignacio Larrea: The aim of this sequential construction was to improve, most of all, the schedule of the works, reducing the construction period. Construction on Valle 1 and Valle 2 officially began in December 2009 and was completed in December 2011, when the plants were ready for commercial operation.

But it was the first time that SENER carried out a sequential construction of two solar power plants, and it was a challenge for our project management team. Regarding your question about repeating this sequential construction in future projects, it will depend on the characteristics of the project.

CSP Today: What challenges did these combined projects present and how were these overcome?

Ignacio Larrea: SENER led the EPC contract for the two projects that were carried out in consortium with Cobra. SENER was in charge of both consortia, as Project Manager, and provided 100% of the technology and engineering for both plants.

Being a sequential construction, it implied an additional effort in the logistics: processing various types of supplies (purchasing, manufacturing, inspection, authorization of shipments to the site, transportation, receiving, and on-site storage), overseeing multiple contractors, quality control of the execution and, above all, planning and supervising construction as it progresses.

It bears mentioning that most of the launch for the plants was performed using power supplied by generators, since no external power connection was available until mid-September 2011. Afterwards the plants were connected to the 400 kV Spanish Electrical Network. Not being able to use power from the network for the internal launch activities slowed down the final phase of the projects.

CSP Today: Initial construction was delayed three months due to poor weather conditions, yet the project was brought back on track. How did you achieve this comeback?

Ignacio Larrea: As I said before, SENER has done an outstanding effort in the construction works of these projects. The construction schedule has been extraordinary, with a rate of assembly in the solar field that reached a peak of 120 collectors per day in the assembly line. Besides, we have also implemented different ‘speed-up plans’ in several construction Works.

CSP Today: In terms of assembly and installation of the solar field, were any new processes introduced to enable such a swift installation?

Ignacio Larrea: The main difference from previous plants was the use of two assembly lines simultaneously.

CSP Today: How could the assembly and installation process be further improved?

Ignacio Larrea: It is still possible to slightly increase the rate of production of collectors with greater synchronization of time in each jig mounting.

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