Setting up of "hybrid" energy centers, an initiative mainly about hybrid technology combining two renewable resources abundant in a particular area, is gaining momentum. In the context of CSP, there are projects, in which solar thermal technology is being combined with the existing combined-cycle generation unit.

For instance, California utility PG&E Corp earlier this year signed a deal with a renewable energy unit of Portuguese conglomerate Martifer for 106.8 megawatts (MW) of solar thermal-biofuel hybrid power. Two projects, which will provide enough power for nearly 75,000 homes in northern and central California, are to combine solar thermal technology with steam turbines powered by gas produced by local agricultural waste and livestock manure. This hybrid technology combines two renewable resources abundant in California -- solar energy and biofuel.

According to PG&E, the incorporation of biofuel increases the overall production of renewable power by allowing for around-the-clock production of clean energy, even at night or when sunlight is not at its strongest. Each hybrid project will require 250,000 tons of biofuel annually, to be supplied from a combination of locally-produced agricultural wastes, green wastes and livestock manure.

In July this year, Florida Power & Light Company had received approval from the Florida Public Service Commission (PSC) to begin construction of three solar energy centers. One of the projects is The Martin Next Generation Solar Energy Center.

Planned for construction at FPL's existing Martin Plant site, the Martin project will provide up to 75 megawatts of solar thermal capacity in an innovative "hybrid" design that will connect to an existing combined-cycle power plant. According to the company, it is the world's first project to integrate solar thermal steam generation into a combined-cycle steam turbine. When the power of the sun is being harnessed to produce electricity from steam, less natural gas is required. The Martin facility is expected to be on-line at the end of 2009 and completed by 2010.

According to the experts, hybrid systems can be designed to maximise the use of renewables, resulting in a system with lower emissions than traditional fossil-fueled technologies. Plus, hybrid systems can be designed to achieve desired attributes at the lowest acceptable cost. For instance, it is said there are opportunities to integrate wind and solar with other resources to design a more efficient bulk Transmission System.

Still, despite projects announced earlier this year, a section of the industry also acknowledges that there might be not enough clarity and it seems like the emphasis is on how to utilise existing combined-cycle plants more efficiently. For its part, CSP is designed to provide the benefits of relatively firm power during the peak times and "shoulders" of the peak on a stand-alone basis.

In the run-up to the second CSP Summit US, which recently took place in San Francisco, CSPToday.com spoke to several speakers regarding the same concept. Here is what they had to say:

Hal LaFlash, Director of Emerging Clean Technology Policy, Pacific Gas and Electric Company: Creating hybrid energy center when coupled with other renewable energy resources is a great way to create around-the-clock renewable energy generation that rivals the reliability provided by more conventional power resources.

We recently signed a power purchase agreement with Martifer Renewables Electricity for 106 MW of power from a similar set-up. Martifer is combining solar thermal technology with biofuel for the ability to provide extended renewable energy production. The project, which will be located in California's Central Valley near Coalinga, will source the biofuel from a combination of locally-produced agricultural wastes, green wastes and livestock manure.

Therefore, this project really exemplifies the potential within California to utilise diverse renewable energy resources to meet our climate goals.

(At the time of the announcement, it was reported - Advocates say the technology is a viable replacement for fossil-fueled power plants due to their large scale, but detractors argue the power produced is too inconsistent because it requires the sun to be shining. Martifer's hybrid technology attempts to solve that problem by using biomass gas to allow the solar thermal plants to keep running even when the sun is not shining.)

José C Martin, CEO, Sener Engineering and Systems, Inc.: Certainly, larger photovoltaic facilities will tend to be more economic than the smaller ones.  But, as it happens with the solar field in a CST plant, the PV plant is basically modular, so that no large economy of scale can be expected. 

With respect to the combination of a CST field with an existing steam turbine system (either CCTG or Rankine cycle), different solutions can be applied and the analysis of the potential alternatives is very complex. The solar heat can be applied in very different modes to an existing plant and a deep analysis of the off-design performance of the different equipment is required to find put the best solution, that will not be the same one for every plant. 

Optimising the size of the field (and thermal storage) for such an application may be even more complex than for a green-field one. And the economy of this kind of application is not clear - you save part of the development effort and the investment in the power island, but by using an existing power block, not optimised for the solar application and most probably with lower efficiency in the use of the solar heat.

Terry Murphy, president and CEO, SolarReserve: Integrated Solar Combined Cycle (ISCC) plants are being considered where natural gas is relatively inexpensive; however, it is a concept that will have only modest impact on reducing carbon emissions or providing renewable/sustainable energy.

The solar component at the identified projects is typically less than 25 percent of the overall fossil fuel plant capacity and requires natural gas to supplement solar thermal energy if attempting any storage or encountering intermittency due to weather.  The SolarReserve system from UTC/Rocketdyne, with integrated energy storage can provide all of the dispatchability attributes and continuous power characteristic that a gas turbine cycle can provide and with zero carbon footprint.