Siemens, which is now building the largest fully solar-powered steam turbine generator to date for BrightSource’s Ivanpah solar power plant, provides an insight into how turbines facilitate delivery of clean solar power.

Storing surplus solar thermal energy in large storage tanks and using it to extend the running hours of the steam turbine during the non-solar periods is one of the main advantages associated with the parabolic trough power plants. At the same time, in order to justify the investment cost for a CSP plant, which doesn’t run for 24 hours per day, high demands for efficiency and increasing economic returns are imposed on the steam turbine used in the process.

The industry has already witnessed the emergence of turbines, which use the reheat concept to improve the cycle efficiency and thus reduce plant investment costs.

The turbines are also designed to handle rapid start and stop times, which are necessary to allow night-time shutdown of the plant. Other than reducing the cost for generation of the required electrical power output, the surplus heat can be put into thermal storage to extend the production time for the plant. The reheat solution improves efficiency and reduces problems with erosion/ corrosion and moisture in the LP turbine.

A company like Siemens has worked on a turbine technology, which can fit with various CSP concepts - parabolic trough collectors, linear Fresnel collector plants with flat mirrors and central tower plants with slightly curved mirrors or heliostats. Its SST-700 DRH (dual-casing reheat) steam turbine is optimised for solar steam cycles and capable of generating up to 175 MW in CSP applications.

In terms of progress made by the company, Siemens, which was awarded a contract for Nevada Solar One in August 2005 for direct reheat SST-PAC-700RH steam-turbine generator set, is now building the largest fully solar-powered steam turbine generator to date for BrightSource’s Ivanpah solar power plant.

Daily cycling capacity

As far as daily cycling capacity of turbines is concerned, when focusing on annual power production, the short start-up times the turbine can provide are of great benefit to the CSP plant owner. Daily cycling and temperature variations require special attention, according to Siemens.

Turbines such as Siemens’ SST-700 DRH, with its low-mass rotors and casings, is termed as an ideal one for daily cycling and has a low minimum load, enabling maximum running hours per day for plants without heat storage. The cycle has also been optimised for stand-still at night and rapid restart in the mornings.

Providing an insight into the same, Markus Tacke, CEO of the Business Unit Industrial Applications, Steam Turbines of Siemens Sector Energy, says the casing and the rotor are the most important parts for rapid restarts and daily cycling.

The special casing design protects the steam turbine from cooling down too much at night and also significantly shortens the warm-up phase on start-up.

“The SST-700 DRH uses high quality materials specially chosen for long and trouble free operation in a solar plant, bearing in mind the potential wear and tear of the special daily cycle conditions with its problems with erosion/corrosion and moisture. The low-mass rotors support short startup times,” said Tacke.

In case of CSP plants, the pressurised steam powers the steam turbine, with excess passing back to the boiler to reheat the steam before it passes the low pressure turbine. The reheat enhances overall power plant efficiency.

Enhancing overall power plant efficiency

In the past, with reference to Nevada Solar One (in this CSP plant, developed by Acciona Solar Power, parabolic troughs collect heat from the sun and transfer renewable power during times of greatest need), it has been shared that steam is generated via the heat exchangers at 700°F/1,250 psia. After passing through the HP turbine it is taken back into the steam generator for reheating and bringing LP steam up to 700°F again. This will further reduce the oil temperature back to the solar collectors and improve the overall cycle efficiency of the plant. The reheated steam is now admitted into the LP turbine to further generate power; it then enters a vacuum condenser where remaining steam is condensed to be pumped back into the steam generator again. This careful use of water is vitally important considering the desert location and value of water sources. It has also been a consideration for the plant’s cooling towers.

Supplementing the heat cycle is a small, natural-gas-fired auxiliary boiler, which will be used at times when the sun’s radiation is not available, or during cleaning of the solar mirrors. The cycling capability of the plant is significant when dealing with likely variables in steam temperature and pressure.

The steam turbine is coupled to a generator, which produces electricity at 13.8 kilovolt (kV), 60 Hz. Power is then stepped up by a transformer, to be distributed to the grid at 230 kV from the huge switchyard.

In order to make the best use of large changes in volumetric flow from inlet to outlet, the SST-700 turbine steam expansion is divided into two different modules: one high-pressure turbine (HP) and one low-pressure turbine (LP).

On the utility of a turbine having two turbine sections — a high-pressure section and a low-pressure section, Tacke says a HP and a LP section is needed to operate the turbine in reheat configuration.

In a reheat cycle, heat is added to the steam cycle at a higher average temperature than to a non-reheat cycle. This improves the cycle efficiency (for instance, typically 1 -1.5 % higher output) and allows the steam turbine to provide a greater power output for the same heat input.

There will also be less steam going to the condenser and therefore lower cooling water losses.

The steam is reheated after it has expanded through the HP turbine. This reduces the moisture content in the LP turbine by 5-10 % compared with a non-reheat cycle, increasing efficiency and minimizing erosion caused by water droplets. All these factors increase the steam cycle efficiency.

“In terms of power distribution, for a SST-700 e.g. the high pressure sections contributes around 30 percent of the net output, the low pressure section 70 percent,” said Tacke.

Tackling lengthy production process

Siemens, which has already secured orders for more than 45 specially adapted steam turbines, covering the power output range from 1.5 MW up to 123 MW, still considers the duration of delivery time as a major challenge.

In December last year, when BrightSource Energy signed a contract with Siemens to purchase the steam turbine generator for BrightSource’s first 100MW plant at its Ivanpah Solar Power Complex in California’s Mojave Desert, the companies shared that due to a lengthy production process, turbine generators must be ordered approximately three years in advance of the planned delivery date.

For its part, Siemens is working hard to shorten the delivery time.

“In our steam turbine plant in Görlitz, we will open a new turbine assembly hall in May. This will give us the opportunity to react quicker on the demands of our customers,” said Tacke.

“Anyhow, it’s still recommendable to order a CSP steam turbine as early as possible. In most cases, this can be handled by the customer, because planning, engineering and erection of a CSP plant takes a few years, and the steam turbine is needed at the last stages of erection. A start-up in an early planning phase can also book a slot reservation for the manufacturing of their steam turbine in Siemens first, and place the final order at a later time.”

Expanding market

Solar power is an important part of Siemens environmental portfolio as it accounted for company revenues totaling EUR19 billion in 2008.

Till September last year, Siemens had secured orders for 45 steam turbines for solar thermal power plants:

CSP trough technology: 40 steam turbines for CSP trough technology; CSP solar tower technology: 3 steam turbines for solar tower; ISCCS trough technology: 2 steam turbines for ISCCS power plants.

The company expects a rapid growth of the overall solar thermal market, especially for parabolic trough technology as this technology is the most established technology currently.

“We also believe that the tower technology can be a major contributor in the future. In addition the big potential of CSP is the possible integration into combined cycle power plants and as well as use in desalination applications,” said Tacke.

Overall, the company estimates that by 2015, the market for solar thermal power plants will increase annually by more than 30 percent to over €5 billion euro.  

Related links: Reports on thermal storage on CSPToday.com