A higher initial investment in hybrid plants is balanced by reduced fuel consumption

Credit: Abengoa

Partnerships can be far stronger and achieve much more than individual efforts. So it is with hybrid concentrating solar-thermal plant. Together, these thermal and solar technologies have much to offer, both to each other and to a stable, secure and cleaner energy system, as David Appleyard discovers.

Attractive though they are on environmental grounds, renewable energy technologies face two principal challenges: the inherent variability of resources such as sun and wind, and their current tenuous economic viability.

An elegant solution to this two-fold challenge is hybrid concentrating solar power (CSP) and thermal power plant technology. A thermal energy partnership, such a development captures the environmental benefits of solar power, but offers the advantage of shared costs on major plant equipment. Items such as the steam turbine and generator, a good slice of the controls and instrumentation, switchyard, water treatment system and so on are all jointly financed. This significantly lowers the lifetime cost of energy (LCOE) of the solar component. There are operational advantages, too.

Stephan Ludewig, Package Frame Owner Steam Turbines SST-500/600/800 at Siemens, explains: “This configuration is effective in two ways. It not only minimizes the investment associated with the solar field by sharing components with the combined cycle, it also reduces the CO2 emissions associated with a conventional plant.

“The integration maximizes operation efficiency even though solar energy intensity varies according to the weather and time of day. The ISCC [integrated solar combined cycle] enables plant operators to continuously operate the steam turbine while the gas turbine is shut off during daytime and to cover peak times with the solar field,” he continues.

Abengoa spokesperson Luis Rejano Flores emphasizes that solar-thermal hybrids also take advantage of the features of a conventional fossil-fuelled power plant. In the case of a combined-cycle gas turbine thermal unit, this includes rapid startup capability, fuel flexibility, high thermal efficiency, reliability, availability and operating flexibility, such as 24-hour baseload or daily start-stop cycles.

Marco Simiano, technical director of Alstom’s Renewable Steam Plant R&D team, picks up on this key point: “From a grid operator point of view, a CSP-fossil hybrid can be considered fully dispatchable. Depending on the type of integration and dispatch possibility, the solar portion can also be used as a booster during daytime, to some extent replacing supplementary firing in combined-cycle power plant.”

While concentrated solar plants can be integrated with both conventional combined-cycle and other thermal steam-cycle plant configurations, Abengoa further argues that solar thermal systems can also be used to extend the lifespan of existing thermal facilities – for example, where regulatory changes require thermal plant to reduce emissions or face closure. The heat provided by the solar element adapts perfectly not only to new, but also to existing cycles, the company says.

In terms of cost, inevitably the initial investment in a hybrid facility is higher than in a standard thermal facility – not least because significantly more land is required to accommodate the solar field.

Nonetheless, the reduction on fuel consumption does deliver savings on operational costs. Simiano even adds: “Because of the lower lifetime costs of energy, regulators could achieve savings in their subsidy schemes to incentivize the penetration of CSP.”

Sizing up steam

CSP technology involves using mirrored surfaces to concentrate sunlight onto a receiver. Various mirror arrangements are possible, including parabolic trough, linear Fresnel and heliostat central receiver types. The receiver typically contains a heat transfer fluid – typically oil but now more frequently molten salt – which is heated to perhaps 380oC or more in the case of salt and is in turn used to generate steam in a boiler.

Rejano Flores says: “The main concept is similar for all the technologies: the solar field provides thermal energy – usually steam – that is injected into the other thermal cycle. The operating conditions, the fuel and equipment involved, and the size of the solar field are the key design parameters.

“The main modifications required to the conventional cycle affect the heat recovery steam generator (HRSG). It needs a customized design to accommodate the steam coming from the solar steam generation system. Additionally, the steam cycle design is modified to handle an optimum integration of systems.”

Scaling the conventional and solar systems appropriately is a key consideration, as Simiano explains: “Technical drawbacks may be limitations on the full utilization of the fossil portion of the hybrid plant, when the solar power is available, resulting in worsened economics.”

He continues: “An important trade-off to consider is that, if the steam turbine is sized to accept the full amount of solar steam while the fossil part is also on full load, the system pressure will become very low when the solar steam is not available, and the efficiency in fossil-only operation will be penalized.

“On the other hand, if the steam turbine swallowing capacity is not specifically optimized, when the solar portion is on full load, the fossil part of the plant will have to go in a very low part load.

“This is especially true for CSP add-on solutions to existing fossil plants, where especially the design of the steam turbine was not foreseen to accept additional steam coming from the CSP plant. However, these drawbacks can be eliminated in new-built hybrid plants.”

Bernd Beyer, head of sales for steam turbines in South West Europe for Siemens, observes that for newbuild, “the most-used technical solution for ISCC plants is to oversize the steam turbine of the combined-cycle plant. During solar operation, the gas turbine waste heat is used for reheating/superheating, while solar heat is used for steam generation to operate the turbine with an increased amount of steam. In this way, the capacity of the steam turbine can be increased by up to twice the amount of a conventional combined-cycle power plant. In hybrid cycle steam plants, the steam produced by the solar field is normally used to displace turbine extraction steam to the feed-water heaters.”

Indeed, Alstom has developed a design concept based on its GT24/26 gas turbines in which the steam turbine is sized to accept full solar steam while the GT is at about 80 per cent load, but still close to its optimum efficiency. In this way, the ISCC operates at maximum efficiency in full hybrid operation, while the efficiency penalty in fossil-only operation is kept to a minimum, the company says, noting that further adaptations involve oversizing of the reheater surfaces in the HRSG to accommodate increased steam flow.

Through sharing the cost of major plant components, hybrid plants can optimize LCOE

Credit: Siemens

New build vs retrofit/gas vs coal?

Optimized solar-thermal operational characteristics are plant-specific, but there are options available for retrofit. For example, Alstom has proposed a retrofit solar feed-water heating integration system based on central receiver tower technology.

“However,” says Simiano, “it is for new-build that the full potential of solar integration can be exploited.”

The company is installing its Solar Receiver Steam Generator (SRSG) direct steam central tower technology with surrounding heliostat field at the new Ashalim plant, under construction in Israel. “It produces high pressure, high temperature steam, which can be directly integrated with the high pressure steam of a gas turbine combined cycle,” says Simiano.

Alstom has also developed an integrated solar system for coal-fired power plants, which similarly offers to inject high pressure, high temperature steam directly into the steam turbine and give an option to operate the plant in fuel saving or boost mode.

Considering the varying types of thermal plant, Ludewig observes: “There are two main differences in the plant design. Whereas steam turbines for thermal power plants (i.e. coal, biomass, solar, etc) typically need many steam extractions to support the preheating system, the units for combined cycle power plants are usually designed without steam extractions.

“Similar integration concepts can be applied to biomass and geothermal plant, as long as the optimized integration design can keep to a minimum any disturbance to the other part of the plant,” Simiano concludes.

Market prospects

Certainly, CSP technology is rapidly maturing. There are some very substantial CSP installations already operating, such as the 392 MW Ivanpah plant in California, US, commissioned in 2013.

Manufacturers are optimistic given the potential market available. Abengoa suggests any countries with natural gas reserves and high solar resource would be potential markets – a profile which matches almost all MENA region countries, Mexico, the southwestern US, and parts of Asia.

Simiano also picks up on the MENA hotspot: “Hybrid systems have been mostly developed in North Africa (Egypt, Morocco, Algeria) and the US when associated with gas combined-cycle. The market is expected to mainly grow in the MENA region, although there are some discussions in other part of the world as well, such as India and Latin America.”

Beyer highlights the ISCC in Kuraymat, Egypt, where two gas turbines of about 40 MWe each and one roughly 70 MWe SST-900 steam turbine are in operation together with a parabolic trough-type solar field covering an area of about 650,000 m2. Located some 90 km south of Cairo, the solar field generates about 200 GWh of solar heat annually. Further enlargement of the solar field in future is possible, potentially raising plant capacity to around 150 MW.

Indeed, overall the future market for hybrids is expected to be dominated by newbuild.

Rejano Flores notes: “New-built solar-thermal hybrid plants should represent the main share of these markets. In the short term, greenfield plants are more likely to be installed in emerging countries where new power generation is needed.” He adds that the retrofit market “is limited, since it is usually complicated to find an operating facility with room next to it to build a solar field and, in any case, a good solar resource is key to get a profitable project”.

In terms of market development, Simiano calls for greater integration of the solar elements of hybrid systems at an early stage of a power project’s life: “For the case of ISCC, projects are sometimes developed as two separate plants, with solar being a mere add-on to a gas plant with limited steam contribution.

“Such an approach limits the potential of hybridization and generates sub-optimum solutions since we are missing the ‘integration’ and optimization dimension. It is clear that a plant developed as a real integrated solution since the inception of the project is expected to be more optimized and economical, and show the full value of hybridization.”

Abengoa also calls for change: “Create an adequate legislation which recognizes the solar generation within the whole hybrid plant production and allows the development of these plants. If this solar share is not recognized as renewable there will be small appetite to introduce changes in conventional facilities.”

Advancing CSP integration

CSP technology is naturally advancing along a number of fronts, in particular towards higher temperature and pressure steam conditions. Explains Ludewig: “When the CSP technology was market-ready, one saw much lower power outputs and steam conditions than today. This was mainly due to market regulations and supply limitations to the grids and did not depend on technical solutions.

The Hassi R’Mel integrated solar combined cycle plant in Algeria

Credit: Abengoa

“Whereas the first steam turbines delivered to CSP plants typically had lower power output and inlet steam temperature of about 380oC, we now see a strong tendency to higher steam temperature and power output. The limit of about 380oC is set by the thermal oil. Most thermal power plants erected so far are with the parabolic trough technology which uses thermal oil. However, more and more solar tower projects are developed which provide higher inlet steam conditions resulting in better cycle efficiencies.”

Rejano Flores also picks up on this transition: “The most developed technologies are the ones that hybridize parabolic trough and combined cycle systems (ISCC). The most recent development by Abengoa Solar, now available commercially, is the SolGasMix, a combination of a conventional gas combined cycle with a molten salt tower.

“The first parabolic trough plants that came into operation were designed with a 5 per cent solar share approximately, and current SolGasMix projects are able to get more than 30 per cent solar energy production, at a lower LCOE,” says Rejano Flores, adding, “Reductions in cost in the solar field are especially remarkable and, of course, directly applicable to hybrid plants.

“The most important challenges of this technology have to do with increasing the solar share included in the solar-thermal hybrid plants, [and] the addition of thermal storage to improve the dispatchability and lower the cost and [accelerate] the reduction of costs.”

Simiano has perhaps a more sober outlook: “We do not see major technology changes for hybridization on the horizon. Combined-cycle power plants are not going to supercritical steam conditions, but integration with an ultra-supercritical coal-fired steam plant would require the development of a new SRSG with matching steam conditions. However, this may be non-economical, and the option always remains to integrate the solar steam at the intermediate pressure steam turbine.”

Nonetheless, he notes: “As for cost, hybrid CSP/fossil plant will benefit of the general cost reductions that the CSP technology is achieving.”

Hybrids with storage

CSP-thermal-storage systems are expected to offer additional operational advantages.

Says Rejano Flores: “The most efficient thermal storage in STE technology nowadays is the use of molten salts in tanks at different temperatures. The energy can be recovered at any time to generate steam. This concept splits the solar energy collection and the electricity production.”

He continues: “It can be used to operate the plant up to 24 hours a day, modulating the load and avoiding shutdowns and efficiency decreases during nights or cloudy periods. Including some hours of thermal storage increases the capex of the project, but decreases greatly the cost of electricity produced, which means that including thermal storage decreases the LCOE of a hybrid plant.”

Rejano Flores explains that while the first solar trough systems had no storage capacity, this type of demand is critical in today’s energy market. “This is due to various facts. First, the increasing penetration of intermittent renewables in the grid make it more unstable and makes dispatchability a real necessity to keep the grid safe and electricity distribution secure. The more wind and PV built, the more necessary STE will be to balance the grid while increasing the share of renewables.”

Beyer highlights the advantages of the move towards molten salt thermal storage: “There is a trend to molten salt storage systems – used in both solar tower and parabolic trough technology – that makes the plants capable of running in baseload with considerably fewer stops, which is a tremendous benefit for the steam turbine.

“Generally, Siemens sees a trend towards larger storage capacities due to specific power purchase agreements, for example”.

Simiano, though, sounds a more pessimistic note when it comes to CSP-hybrid plants with storage capability. “The reason to install storage in a CSP plant is to increase the capacity factor, thereby increasing the utilization of the power plant and thus the overall economics,” he says.

“As a rule of thumb, half of the cost of a CSP plant comes from the power island, half from the solar equipment. In a hybrid plant, the power block is in shared use with the fossil portion and thus is already fully utilized. So, half of the incentive to install storage is not there anymore. Storage can still increase the utilization of the solar portion, make easier the control of the solar power and improve the plant reliability (buffer), so it finally depends on the specific project economics if storage makes sense or not for a hybrid plant.”

On storage technology choice, Simiano is emphatic though: “It will only be a molten salt central tower receiver with tank storage and steam generator. This technology is mature; any other would be an experiment.”

Storage technology using molten salts in hybrid plants is currently a commercial technology. Abengoa Solar, for example, offers such a system. “This design improves substantially the performance of the ISCC in terms of efficiency, solar contribution and dispatchability. This means the electricity generated has a higher percentage of solar originated and is cheaper than it would be without the storage,” says Rejano Flores.

“Obviously, any advance in terms of storage related to any technology under development will directly impact the hybrid plant.”

Lighting up the future

“Nowadays, developers are more confident about STE technologies because many power plants are successfully operating in many locations, including some hybrid ones,” says Rejano Flores, adding that where hybrid technology is appropriate, there is a market opportunity to develop the technology to get a ‘soft transition’ from fossil fuels to a solar energy source.

Certainly, the initial investment for integrated solar combined-cycle projects is higher than for standard configurations, and even moreso when considering thermal storage. But, proponents argue, the electricity produced from these hybrid plants is ultimately both cheaper and cleaner than either could produce alone.

David Appleyard is a freelance journalist focused on the energy and technology sectors.