Policymakers often portray energy sources as nearly at war with one another in an epic struggle of green versus brown, with the winner ultimately powering the world. But in today's real world of grid operation, renewable and fossil fuel resources are treated more as allies, as pieces of a puzzle, which when fit together properly, keep on the lights with minimum impact on prices and the environment.

Sometimes this means placing power plants strategically on an electric grid. Fossil fuel plants and wind farms, for example, might be sited near one another to ensure reliability. Or, increasingly, grid planners look to the cutting edge approach of building hybrid power plants, one facility that combines two technologies, such as coal and solar, gas and solar, wind and solar, or more conventional biofuel and coal. Such pairings allow the virtues of one fuel type to compensate for the failings of another.

"We're in a fairly early stage in the evolution of the hybrid combination projects. There are still problems to work out, but there is motivation to do this," said Roger Rosendahl, a partner at DLA Piper, which specialises in global project finance.

Indeed, the march toward hybridisation has led the Electric Power Research Institute to launch two demonstration studies in the US on solar energy and fossil fuel combinations, aimed at providing information for utilities that want to build similar projects. The demonstrations are underway in New Mexico at the 245 MW Escalante coal-fired plant and in Nevada at the 1102 MW Chuck Lenzie gas-fired plant. These hybrid systems inject steam heated by solar energy into the steam cycle of the fossil plants. The fossil fuels provide steam when the sun isn't shining. When it is shining, the solar power offers a lower-cost, low-emissions energy source.

The idea of using two fuels in the same plant is not new; for decades the power industry has used dual-fuel gas- and oil-fired plants. When natural gas is in short supply, plant operators switch to oil. Similarly, the pulp and paper industry has for years operated on-site generators that can use coal or woody byproducts as fuel.

"It is less of a technology play and more of a maturation of the industry. People are figuring out how to do these things using more or less conventional technologies," says Maurice Gunderson, senior partner at CMEA Capital. "The reason for doing the flexible-fuel power plants is different from the reason for doing the gas-supplemented solar. The gas-supplemented solar is for firming up the supply when the cloud comes over. And the flexible fuel is primarily driven by the need to generate power given the fuel you have available that day."

So the newer hybrid plants tend to hold answers to larger grid problems, particularly how to maintain reliability while introducing renewable energy into a system that Gunderson says in the US is more like a "pile of spaghetti" than carefully drawn lines. "The hybrid energy projects of today are really addressing a very particular problem: intermittency," adds David Huard, chairman of Manatt, Phelps & Phillips' Energy, Environment and Natural Resources practice. "Most renewable forms of energy, not all, have a period of time in which they have less reliability or production of power, especially if they are in a remote location," he says.

Connecting an intermittent source of power into transmission line is "very inefficient and expensive", he says, which lends weight to the idea of using a hybrid approach in grid planning. But hybrid systems have their own problems, particularly in defining how much of a hybrid's power can be used to meet state renewable portfolio standards in the US. "What do you call this? Is this renewable? If it's renewable for 16 hours a day but the dirtiest stuff on earth for 8 hours, do you average?" says Huard.

While problems of definition have yet to be resolved, hybrid systems are cropping up worldwide using various technologies.

Italy's New Molten Salt Hybrid

In Italy, Enel made history in July by unveiling the Archimede power plant in Sicily, which the company said is the first combined-cycle gas/solar thermal electric plant to use molten salts for heat transfer.

Like several other concentrating solar power plants, Archimede uses heated molten salts to store energy, so that at night or in overcast conditions it can still produce electricity. What's unique about the project is that the molten salts are not only used to store energy, but also to capture the sun's heat. Archimede contains 30,000 m² of parabolic mirrors that heat the transfer fluid pipes. By using the molten salts to capture heat, the plant can operate at higher temperatures (up to 550°C) than if it used oil for energy capture. Enel estimates that the 5 MW solar portion of the plant saves about 2100 tonnes of oil equivalent annually.

In Colorado, Xcel Energy is now testing a demonstration hybrid solar-coal plant. The Colorado Integrated Solar Project adds parabolic-trough technology, developed by Abengoa Solar, to Unit 2 of the coal-fired Cameo Generating Station. Eight rows of solar troughs decrease the 49 MW plant's use of coal and lower its carbon dioxide emissions. The $4.5 million pilot project will attempt to discern if the approach is commercially viable and efficient.

"If this project produces the successful results we expect, this type of solar thermal integration will help move the use of solar energy one step closer to being a potential technology for improving the environmental performance of coal-fired power plants for Xcel Energy and for utilities around the country," said Kent Larson, Xcel vice president and chief energy supply officer.

While the Archimede plant is cutting edge, other hybrid technologies have been around for years, such as coal-fired/biofuel hybrid plants, which offer one of the simplest ways to utilise biomass. No specialised technology is necessary, nor is much modification made to existing coal plants. Biomass is simply burned in the furnace of a conventional coal plant.

Such plants are more prevalent in Europe than in the US. Europe has roughly 100 coal firing/biomass units, many operating commercially, according to the European Biomass Industry Association. The International Energy Agency reports that the US has about 40 coal-firing/biomass plants and Australia has 10.

Biomass/coal combinations, however, sometimes come under criticism from environmental groups. They admit that hybrids emit less carbon dioxide than plants solely fuelled by coal, but argue that biofuels could extend the life of less-efficient coal plants that would otherwise be retired because of high emissions.

Moreover, biomass from wood is under increased scrutiny in the US. The state of Massachusetts may prohibit utilities from using wood-fuelled plants to meet its renewable portfolio standard, except in combined heat and power plants. The state's Department of Energy Resources is contemplating the rule change based on a report from the Manomet Centre for Conservation Sciences that says biomass may not be carbon neutral, depending on how trees are harvested, and could increase greenhouse gases more than coal-fired generation over the long term.

Pairing Green With Green

Meanwhile, new and more innovative combinations of fuel and technology are being matched into hybrid operations.

California company Skyline Solar has developed an all-solar hybrid that the company says uses the best aspects of tracked photovoltaics and concentrating solar power to lower energy costs through higher energy yield per peak watt installed. "The product we are commercialising is a hybrid with a rack PV system using conventional flat plate silicon and parabolic trough solar thermal," says Bob MacDonald, Skyline Solar's co-founder and chief technology officer. "We are using a lot of the attractive attributes of CSP, but we are using silicon based flat panels that focus sunlight."

Skyline calls its technology High Gain Solar (HGS). Manufactured with the same materials as traditional solar panels, but smaller in size, the systems include a backing plate, silicon cells, encapsulant, and junction box. What's different is that the HGS system includes a metal heat sink that allows passive convection cooling.

The application works best in projects up to 20 MW, which makes HGS projects ideal for municipalities and manufacturers, says MacDonald. In addition, because the components are PV rather than CSP, they are "human scale" versus the "Jurassic scale" of a typical CSP plant, he says. As a result, Skyline project components are easier to manufacture and transport than CSP, the company claims.

In Turkey, Solimpeks Solar Energy has also merged solar electric and thermal energy capture into one technology, called the Volther hybrid solar collector, designed for buildings. Water circulates near the PV panels to absorb their heat. The hot water can then be used in the building. By producing both electricity and heat from the same panel, such hybrid systems offer a quicker return on investment, according to Solimpeks. The water absorption also acts to cool the PV panel, increasing its conversion efficiency and longevity.

A hybrid solar PV/solar thermal installation in Instanbul (Credit: Solimpeks)

California company PVT Solar offers a variation on this theme with its Echo solar system, which combines PV and thermal solar for homes. Rather than capturing the heat by running water through a copper tube, which could damage the roof if it leaks, Echo uses a computerized fan to draw hot air from under the solar panel at a rate of about 14 m³/m. The air is used to heat the home or its hot water.

Gordon Handelsman, PVT Solar president, says Echo is highly efficient because it uses the same panel to produce both electricity and heat. While a solar panel typically converts 15% of sunlight to energy, with the rest lost as heat, the Echo system hit 50%, he says.

A typical 2 kW system produces more heat than a home can use, he says. "We don't run out of electricity; we run out of load." The company has field-tested the systems, beginning in 2004 with a US Department of Energy grant. By the first quarter of this year it had 50 systems on rooftops, and has been adding them at a rate of about 50 per month. Meritage Homes, a major American home builder, is incorporating the system in relatively modestly priced homes — under $180,000 — in the Phoenix, Arizona area. The Echo system also can be found in Utah and soon California, says Handelsman.

Meanwhile, Urban Green Energy has developed a wind/solar hybrid to light up streetlamps. The Sanya Wind Solar Hybrid Streetlamp combines a 600 W vertical-axis wind turbine with 80 W solar panels, and a battery bank that can store up to five days worth of energy. The lamps can also be hooked up to a grid, feeding off any excess energy produced and providing emergency backup in case of prolonged periods of low wind, although the company expects the units to generate energy 365 days a year.

Urban Green Energy, which had orders to ship the units to South Korea, Poland, the US, and the US Virgin Islands, says the lamps are customised to specific locations with the goal of making the streetlamps as maintenance-free as possible. Those using the units include a US Air Force base in Kansas, a girls' school in the Great Rift Valley in Kenya, and the Raising Malawi Academy for Girls, an orphan care initiative sponsored by performer Madonna.

Common Ground for Cows and Computers

An even more futuristic possibility for hybrid energy pairs energy-hungry data centres and grass-hungry cows. Hewlett-Packard researchers presented a paper in May at the ASME International Conference on Energy Sustainability in Phoenix, Arizona that found a medium-size dairy farm, one with about 10,000 dairy cows (that's about 200,000 tonnes of manure per year) could fulfil the energy needs of a 1 MW data centre. The system would even have enough power left over to support the needs of the farm.

The researchers propose a combined heat and power system that would provide power to the data centre, which would in turn feed its waste heat back into the CHP system. The heat from the data centre would speed up the anaerobic digestion of the manure to create methane gas for generating electricity for the data centre.

HP's model addresses the problem that farmers have in disposing of cow manure. It may even make the farmers some money: Researchers suggest farmers would break even within two years, and make roughly $2 million a year selling waste-derived power to data centres. The idea of co-locating data centres and farms makes sense, given that Google, Yahoo, Microsoft and others have begun to build on inexpensive rural land, the study said.

"The idea of using animal waste to generate energy has been around for centuries, with manure being used every day in remote villages to generate heat for cooking. The new idea that we are presenting in this research is to create a symbiotic relationship between farms and the IT ecosystem that can benefit the farm, the data centre and the environment," said Tom Christian, principal research scientist of HP's Sustainable IT Ecosystem Lab.

Hybrids as Transitional

One of the most talked about US hybrid projects is being built by renewable energy leader, Florida Power & Light. Called the Martin Next Generation Solar Energy Centre, the plant will use 75 MW parabolic troughs to supplement its natural gas. The project broke ground in 2008 and is scheduled to be finished in 2010.

The fossil/green hybrid may not be an end in itself, but could prove to be transitional, a vital step away from fossil fuel dependency to a renewable energy future, says QGEN, a Boston-based development company. CEO Wael Almazeedi, who also founded international energy development company BTU Power, plans to sell hybrid designs as intellectual property. Still somewhat in stealth mode, QGEN was close to finalising a hybrid industry consortium as REW went to press. The consortium's objective is to lead the way in hybrid design and development.

On its website, QGEN says we stand at "a watershed moment" in energy history. Transition to clean energy "will require more than disruptive new energy technologies; the energy industry is not known for embracing radical change. It will also require game-changing evolutionary technologies such as hybrid generation in which state-of-the-art renewable energy technologies are integrated into conventional fossil fuel plant designs. Hybrid generation will ensure that renewable energy technologies are commercially competitive by deploying them as a complement to existing fossil fuel plants, rather than an outright substitute."

At some point renewable energy may be the outright substitution for fossil fuels. But for now it appears plenty of room exists for fossil fuels and green supply to work in a kind of détente that brings new efficiency to the power grid.

Elisa Wood is US Correspondent for Renewable Energy World