Sunny future for parabolics in Granada and Nevada

There are two main solar technology subgroups: photovoltaic and solar thermal systems. In contrast to photovoltaic plants, solar thermal generation is not based on the PV effect, but rather the direct generation of electricity from the heat produced by sunlight.

Solar thermal generators can be subdivided into three main concentrating solar power (CSP) systems: parabolic troughs, central towers, and parabolic dishes (such as the Stirling dish). Amongst these, parabolic trough power plant (PTPP) technology is the best established for large scale development. Currently CSP outputs range from 1 MW (parabolic dishes), to 10 MW (central towers), up to around 100 MW (PTPPs).

The core of a PTPP is the solar field. This consists of parallel arrays of trough shaped collectors, more than 400 m in length, which track the sun’s path from east to west. These troughs are composed of parabolically shaped reflectors made of highly transparent silver covered glass that concentrate solar radiation 80-fold. The resulting

radiation is focused onto centrally located glass receivers through which a thermally stable synthetic oil circulates in steel covered tubes. The oil, which reaches temperatures of around 400ºC, is pumped through a series of heat exchangers to produce superheated steam to drive a turbine.

PTPPs are claimed to be unique among solar technologies in being able to incorporate energy efficient and cost effective storage technology. By using thermal storage systems using media such as molten salts, stored solar heat can be extracted after sunset or during cloudy periods, to maintain turbine operation.

The five most promising regions in which solar power is seen as a viable substitute for fossil fuels are Spain, the Middle East, North Africa, the southern states of the USA, and Australia. These areas combined have some 1000 MW of solar thermal projects planned to be in place by 2025.

Europe’s first PTPP

Spain was the first European country to introduce a ‘feed-in tariff’ funding system for solar thermal power, in 2002. This set a price of h0.12/kWh of solar power output for plants between 100 kW and 50 MW capacity. It was later decided that this was not a sufficient financial incentive, and consequently in 2004 the payment was increased to h0.18/kWh. The law, which guarantees energy supply payments for solar thermal energy for twenty five years, has paved the way for a planned 200 MW of solar thermal plants to be in operation by 2010.

One such development is AndaSol 1, a project being developed by Solar Millennium AG. The plant will be operated by AndaSol 1 SA (owned by ACS/Cobra Group) and the Solar Millennium group. In June 2006, construction of the h300m plant began near Guadix in Andalusia, in the province of Granada. The project, which is the first of its kind in Europe, received a h5m grant from the European Commission’s ‘Fifth Framework Programme’, as well as financial assistance from the German federal ministry for the environment. It is hoped that the 50 MW plant will be commissioned during 2008, whereupon it is expected to supply 179 GWh per year, at a cost of h0.15/kWh.

New collector design

During the 1990s a European development consortium, supported by the European commission, designed two advanced collector generations called EuroTrough I and II (ET). Based on two ET pilot installations in Almeira, Spain, a German development consortium composed of Flagsol, Solar Millennium, Schlaich, Bergermann and Partner, and the German Aerospace centre (DLR) focused on improving the collector design for next generation PTPPs. The improved collector design, called SKal-ET (SKaled-up EuroTrough), has been used successfully for more than 7000 hours of operation in an 800 m demonstration collector loop, at the solar thermal SEGs V plant in Kramer Junction, California.

Adoption of the torsion resistant design has afforded a cost reduction of roughly one third for collector components by the development of a larger collector unit length of 150 m, compared to 100 m in past designs. In addition, the SKal-ET design has a reduced weight per square metre of reflector surface and improved optical accuracy under heavy wind loads, resulting in a 10% performance gain compared to the previous collector generation.

Additionally, Flagsol has developed a highly precise collector control unit, and the DLR, the accompanying measurement technology to help optimise the capture of radiation.

The 2 km2 AndaSol 1 plant will have 624 collectors of the SKal-ET design, each with 366 mirrors. These 200 000 mirrors will have a combined area of 510 120 m2, reported to be the largest array in the world. In addition, 14 m by 36 m thermal storage tanks, requiring 28 000 tons of a sodium and potassium nitrate mix, are to be installed. When fully loaded, these will support steam turbine operation for approximately 7.5 hours.

Two further plants are planned, replications of the AndaSol 1 project. It is envisaged that AndaSol 2, which is to be identical to AndaSol 1 in design, will begin construction immediately following financial closure, which is anticipated to occur soon. The combined investment for the first two projects is expected to amount to h600m.

Solar Millennium is also presently working on EPC contracts and tender invitation documents for the financing of the AndaSol 3 project.

The plant’s grid connection has already been approved. Construction is forecast to begin, at the earliest, towards the end of 2007.

Low gas Nevada project

Meanwhile, at a more advanced stage of construction is Solargenix Energy LLC’s (formerly Duke Solar) Nevada Solar One PTPP which is currently undergoing installation and commissioning. In 2003, the company signed a long-term power purchase agreement with Nevada Power Co and Sierra Pacific Power Co for the development of the 64 MW parabolic trough plant to be built in Boulder City near Las Vegas. The plant’s construction began in February 2006, and it is anticipated that the 1.5 km2 plant will be providing grid electricity in June 2007 from a solar field of over 350 000 m2. However, unlike the AndaSol plants, Solar One will not have molten salt heat storage facilities.

The backdrop to this deal is the state of Nevada’s 1997 ‘renewable portfolio standard’ which required that electric utilities, in this case Nevada Power Co and Sierra Pacific Power, source a gradually increasing proportion of power from renewable sources. As of 2005 the standard stipulated that by 2015 the two companies will be required to have reached 25% renewable energy sourcing, one quarter of which must come from solar energy systems.

The Nevada Solar One project is not the first of its kind. There are nine similar projects, with a combined capacity of over 350 MW, already running in the Mojave desert in California. Two of these plants have a unit capacity of 80 MW. Solar One will be the first parabolic trough plant built since the last of the Californian plants in 1991, but will differ from these older projects in a number of ways.

Notably, it will have only a 2% dependence on natural gas fired reheat backup (to minimise fluctuations in the temperature of the heat storage medium), in contrast to the Californian plants’ 25% dependence. Additionally, Solar One is to have a more efficient trough motoring system, as well as using lighter aluminium trough frames in place of galvanised steel. The German glass company, Schott, which is providing the receiver tubing for the AndaSol projects, was also the provider for the nine Californian plants. In 2004 the company

developed its own higher performance receiver (PTR 70), which will be used in Solar One.

It is vital that receivers are able to withstand temperatures of 400°C. For this a borosilicate glass was developed with the same thermal expansion coefficient as steel. This means that the vacuum packed steel receiver, which holds the heat transfer fluid, has the same thermal coefficient as the glass absorber tubes. This results in the two materials’ reacting in unison to the same temperature stresses, which puts less stress on glass-to-metal seals. This is a crucial improvement if one bears in mind the changeable nature of the desert climate – from cool nights to hot days.

The surface area within the receiver available for radiation capture has been increased to 96%, which is reported to improve overall efficiency by 2% over previous models. This was achieved by redesigning the receiver’s bellows and glass-to-metal seals to increase the tube’s exposed active area.

The PTR 70 design also employs a new antireflective glass coating that is designed to better withstand abrasion. Use of a different absorptive steel coating on the tubing inside the outer glass envelope is expected to improve radiation absorption rates to 95%.

Power unit

Both the AndaSol and Solar One plants are to use Siemens SST-700RH steam turbines, the design of which was considered most appropriate for such low temperature reheat applications. The turbine consists of one high pressure/high speed unit and a low pressure/low speed unit. The latter drives the 2-pole 3600rpm generator directly.

There are various reasons for this turbine’s suitability for combining with a PTPP. A ‘two body’ solution – that is, a turbine consisting of high and low speed modules – increases efficiency, allowing the turbine to run for longer, for instance during periods of zero or low sunlight. Reduced volumetric flow at a high pressure for a given temperature can be exploited in the high pressure/high speed unit. In a high speed turbine this allows for longer blades at a low hub diameter with reduced losses as a result. Additionally, since all shaft seals have a smaller diameter, steam leakage is reduced. The low pressure/low speed is designed to allow for proper turbine exhaust conditions (volumetric flow conditions) and in this case connects directly to the generator.

Since the inlet steam is at a high pressure and low temperature it expands quickly below the saturation line, the resulting high temperature moisture potentially causing erosion-corrosion problems on stator as well as rotor parts. But with the reheat concept employed the HP module exhaust steam is brought back into the steam generator, still under superheated conditions. It now reheats the steam to the same temperature as the inlet conditions for the HP module but at a lower pressure level. The ‘hot’ reheat steam then enters the LP turbine and follows a regular steam turbine expansion path, with minimal impact from moisture droplets.

The turbine’s steam path design offers much flexibility for feed water extraction. This is especially useful in areas such as Nevada and Granada with limited water availability for cooling purposes. For instance, this offers the option of extracting as much steam as possible to reduce steam flow to the condenser.

The HP and LP casings are designed to allow for rapid start-up and shut-downs, thereby maximising daily production, in addition to allowing the plant to conform to local regulations. For example, to qualify as a renewable power plant in Nevada, a plant can only generate a maximum of 2% of its power from non-renewable fuels. Therefore during periods without sunlight the plant can only run on heat remaining in the transfer oil. During extended periods of no sunlight the unit will shut down until sunlight returns and the transfer oil is sufficiently reheated. In the AndaSol plants the presence of thermal storage systems helps alleviate this.