Towering achievement

In April Abengoa Solar successfully completed a three-day production and operational testing period of its 20 MW PS20 plant located near Seville. This marked the start of commercial operation of a key project in Spain, and a milestone in the development of solar power tower technology. The plant (Figures 1-4) is the largest such solar tower in the world. Its creators are hoping eventually to see most of the 650 000 population of Seville supplied with electricity produced solely from sunlight.

The first commercial plant of this type, the 11 MW, 115m (377 ft) high PS10, was installed by Abengoa at the same site and has been operating successfully since 2007. PS20 is Abengoa’s second commercial power tower system, and it has plans for a third that will utilise high temperature technology recently developed at a small demonstration plant on the same site.

Both plants are located at Abengoa’s Solúcar Platform, at Sanlúcar la Mayor. PS20, which in principle and general layout is a scaled up copy of PS10 (Figure 5), consists of a solar field made up of 1255 mirrored heliostats, each with a surface area of 120 m2. The heliostats reflect solar radiation to a central receiver located at the top of a 162 m (530 ft) high tower. The radiation is transferred to a superheated fluid that is used to raise steam for a steam turbine-generator also located at the top of the tower.

The start-up of PS20 comes at a time of growing interest in concentrating solar power (CSP) technologies such as solar power towers, despite the fact that producing it is three times more expensive than getting it from conventional sources. Last year Abengoa announced plans to construct two 50 MW CSP plants in Andalucia, Spain. The two plants – Helioenergy 1 and 2 – will use parabolic trough technology and will be constructed near the city of Ecija at a cost of

r500 million. The firm is also developing a 280 MW parabolic trough plant in Tuba, Arizona, USA (Figure 6). Trough systems are considered to be the most mature of the CSP technologies.

PS20 represents an advance on the PS10 design. The fact that it surpassed its predicted power output during the three-day testing period validates its design, says Abengoa.

‘Generating more power during production testing than the design output is indeed a significant milestone,’ said Santiago Seage, CEO of Abengoa Solar. ‘The technological breakthroughs we have achieved, coupled with our cumulative expertise, have enabled us to take a qualitative leap forward in our power tower technology.’ PS20 features a number of significant technological improvements over the PS10 design. These enhancements, developed by Abengoa Solar, include a higher-efficiency receiver, various improvements in the control and operational systems, and a better thermal energy storage system. And with a capacity of 20 MW the new PS20 solar power plant will on its own avoid the emission of the approximately

12 000 tonnes per year of CO2 that a conventional power plant would have produced.

The Solúcar site

Ultimately the Solúcar platform is to have a total capacity of around 300 MW from a variety of solar sources – 50 MW from tower technology, 250 MW from heliostat troughs, 1.2 MW produced by photovoltaics, and 80 kW from a system known as dish Stirling technology which uses parabolic reflectors to direct heat energy at the hot end of a Stirling engine. The site, which is scheduled to be fully operational by 2013, occupies 800 hectares and is expected to create 300 permanent jobs for a total investment of r1.2 billion. It will also prevent the emission of 185 000 tonnes of CO2 per year, amounting to 4 million tonnes of CO2 during its lifetime.

Currently, the platform has three plants operating of which two, Sevilla PV and PS10, are supplying grid electricity via the Sanlúcar la Mayor substation. PS20 will soon be ready to supply the grid. Two other plants are currently under construction, Solnova 1 and Solnova 3, both heliostat trough types.

Sevilla PV is apparently the largest low-concentration (1.5x and 2.2x) PV plant in the world. It is composed of 154 twin-axis tracking units. Each unit is 100 m2 (1076 ft2) in area. The plant, which employs Abengoa's proprietary ‘Sevilla PV’ thin film technology, delivers 2.4 GWh of electricity to the grid annually under the Special Regime production tariff.

In June Abengoa Solar’s first high-temperature power tower, Eureka, was unveiled at Solúcar by Martín Soler Márquez of the Andalusian regional government. The tower is intended to test, on an experimental basis, a new type of receiver that will achieve the higher temperatures needed for higher-efficiency thermodynamic power cycles. This experimental plant is relatively small at 16000 square ft in area, with 35 heliostats and a 164 foot tower which houses the experimental superheating receiver. The power output is approximately 2 MW. The plant includes a thermal energy storage system supplying power to the grid for short periods when there is no insolation. According to Rafael Osuna of Abengoa Solar New Technologies, ‘This marks the beginning of the next experimental phase for this high-potential solar power tower technology which could lead to an important step forward in our goals of generating clean electricity at competitive prices. Our significant investment in research and development has made this groundbreaking concentrating solar power technology a reality.’

Also on the site, ‘Casaquemada’ is a 1.9 MW photovoltaic plant with twin-axis tracking technology that went into operation in September last year. The plant has the specific purpose of serving as a test field for high-concentration technology by Concentrix Solar that enables sunlight to be concentrated by a factor of about 500 onto a special module. This is Abengoa’s first commercial plant to integrate this new technology.

Solnova 1, 3 and 4, all at 50 MW, are parabolic trough collectors, each with 300 000 m2 of collector area that occupies 120 hectares (297 acres) of land area. These are the first three parabolic trough collector plants of a total of five identical units to be built. In 2007 construction began on Solnova 1 and 3, and during 2008 on Solnova 4.

There are two other units under development, a 20 MW tower technology plant with similar characteristics to PS20 and designated AZ20 (20 MW), and a CSP plant, Aznalcóllar TH, (80 kW) based on dish Stirling technology.

Tower technology

Power tower systems use a field of heliostats that track the sun on two axes and reflect solar radiation toward a central tower which houses a receiver in which the incident radiation is concentrated and transferred to a superheated fluid at up to 600°F. Steam is generated in secondary heat exchangers and used to operate a conventional power cycle, in this case a steam turbo-generator.

Each heliostat is composed of a flat reflective surface, a supporting structure and a solar tracking mechanism. Currently, the most commonly used reflective surfaces are glass mirrors. The main advantage of this technology is its potential to achieve higher efficiencies than competing CSP technologies.

Solar towers at Solúcar

The PS10 solar tower plant was built by Abengoa Solar after several years of research and development and began operation on 30 March, 2007. As shown by the schematic (Figure 5), the plant generates pressurised steam to run a conventional power cycle with an 11 MW capacity. Plants PS10, PS20 and the yet to be constructed AZ20 (20 MW) are of similar type.

The PS20 solar field of 1255 mirrored heliostats, designed by Abengoa and each with a surface area of 1291 square feet, works the same way, reflecting the solar radiation that falls on it to the receiver located at the top of the 530 foot high tower, also producing steam fed to a steam turbo-generator. This makes it is a double scale version of PS10 which consists of 624 heliostats each of 1300 ft2, making a total surface area of 18.5 acres (75 000 m2). Each heliostat has an independent solar tracking mechanism that directs solar radiation toward the receiver.

The heliostat fields do not completely surround the receiver towers but are grouped on the north side of each tower to optimise the solar radiation collected while minimising heat loss. The receiver is located in the upper section of the tower and is a ‘cavity’ design consisting of four vertical panels 5.5.m (18 ft) wide and 12 m (39 ft) high. The panels are arranged in a semi-cylindrical configuration and housed in a square opening 11 m on a side. Under normal conditions, the receiver is capable of operating at 92% thermal efficiency and delivering 55 MWt of saturated steam at 500ºF (250ºC) and 40 bar.

Uniquely, PS10 has two different solar functions. One field of heliostats (moveable mirrors) concentrates sunlight on a solar thermal receiver; another smaller field of heliostats, the Sevilla PV plant, a large, low concentration collector, concentrates sunlight on photo-voltaic panels on the other side of the same tower. The two generators share the 66 kV line running to the Sanlucar la Mayor substation.

PS10 has the capability to store 1 hour’s worth of pressurised steam for electricity generation. Additionally, under low radiation conditions it is capable of supplying 12-15% of its output capacity from natural gas. The total efficiency, from solar radiation to electricity, is approximately 17%. This is a fairly high number considering that the efficiency of the steam cycle is approximately 27%.

Operational challenges

One of the main difficulties in operation is controlling the heliostat field. Maintenance or control problems can lead to damaging, perhaps dangerous, outcomes. For example, if a group of heliostats was not properly focused, the receiver could be seriously damaged because of the hot points that may appear. And in order to function properly, heliostats must be cleaned regularly. Heliostat surfaces obscured by dirt can greatly reduce the efficiency of the entire system. Wind poses another difficult for heliostats. In winds greater than 22.5 mph (36 km/h) the heliostats are set vertically to avoid structural damage. If winds exceed 87 mph (140 km/h) it could result in the loss of structural integrity.