Once upon a time, not too long ago, renewable-energy projects sized in kilowatts (kW) were considered large. The biggest solar array in 1963 had a mere 242 watts of capacity, and was installed on a lighthouse in Japan. Wind power, which took off more quickly, reached its first 100-kilowatt system in 1931, in Yalta, then part of the Soviet Union. And at that time, wave and tidal power plants were still twinkles in researchers’ eyes.

But renewable-energy projects have grown inexorably larger, from kW-size systems to megawatt-size systems and now to gigawatt-size systems. “It’s just the natural progression of what’s happened to renewable energy,” said Clean Edge principal Ron Pernick, a firm that picked “megaprojects” as one of its top five trends for 2010. Starting with 150-kW wind turbines at Altamont Pass in California, one of the first U.S. wind projects, turbines have grown to 3.5 MW and even 5 MW today, he said, and solar projects have expanded from off-grid homes to commercial and industrial buildings and now to utility-scale solar farms.

The gigantic renewable systems in the works today match – and in some cases even exceed – the size and scope of some conventional fossil-fuel power plants. If they materialize, these projects will represent a major turning point, as renewable energy becomes just, well…energy.

The Reason for Growth

Why do renewable energy projects seem to get bigger and bigger? The most obvious answer is cost, said Marianne Boust, a senior analyst at IHS Emerging Energy Research (EER). “When you start a project, you have to do a lot of studies [and paperwork], so companies are looking at larger projects to amortize those upfront costs,” she said. In addition, the high cost of building transmission to ferry power from a wind farm to the grid, for example, works out more cheaply, compared to revenues, if the project is bigger, she added.

Another big reason for the growth has been government policies. As federal, state and local governments set ambitious renewable-energy goals and create incentives to encourage them, larger utility-scale projects are cropping up, Pernick said. China is a critical player, aggressively developing very large projects – many in the GW size – projected to go online in the next few years, he added. And in the United States, utilities are getting involved in creating huge projects to meet state renewable portfolio standards while innovative financing models also are helping push larger projects.

Of course, not every proposed project will get completed — far from it. Transmission has been a major obstacle for many big projects, with one of the most public transmission-related failures being T. Boone Picken’s plans to build the largest U.S. wind farm in Texas earlier this year. Financing has been another daunting challenge in the recession, and large projects have large upfront costs, even if the cost per kW-hour ends up being cheaper. New technologies tend to have a tougher time than more-proven technologies in today’s risk-averse climate.

“There are clearly financial challenges — we’re not out of the woods — but we’re certainly seeing some projects get done and we expect to see quite a few more happen in the next few years,” Pernick said. “The fact that there are some wind farms and solar PV installations approaching the gigawatt scale shows that it’s not just pie in the sky; it will be possible to get there.”

The Biggest of the Big

We’ve sifted through announcements, spoken with experts and rounded up the hugest of the huge: the largest dream projects that developers have proposed in five different categories: solar, wind, geothermal, wave and tidal. While we’ve focused on projects with evidence of at least some chance of success, it’s unlikely that all of these biggest planned projects will succeed. Their very massiveness makes them challenging, but their vision and audacity also makes them inspiring — and potentially important milestones to launch renewables into their next phase. And in cases where they fail big, they will also illustrate — and underline — the remaining challenges in a way that smaller projects couldn’t. Take a look at five visions of what the future could look like for the renewable energy industry.

SOLAR: Desertec, North Africa and the Middle East — 100 GW

At a scale that spans three continents, Desertec may well be the most ambitious renewable-energy project ever proposed. The concept seems simple enough: huge concentrating solar-thermal projects in the Sahara Desert and other sunny areas in Africa and the Middle East will supply the whole region, including Europe, Africa and the Middle East, with electricity via high-voltage direct-current transmission lines. Wind farms on the coast of Europe and Africa, as well as geothermal, photovoltaic, hydropower and biomass projects, mainly in Europe, will help balance out the grid’s power supply, but the concentrating solar-thermal projects will make up the largest piece — 100 GW, or the equivalent of 100 nuclear power stations.

But putting together such a vast project, really made up of dozens (or even hundreds) of separate projects connected by the all-encompassing intercontinental transmission lines, is anything but simple. The challenges can hardly be overstated. Creating such a large grid — agreeing on electricity standards across, not only cities and provinces, but different countries and even continents — and hashing out how to share the costs and benefits of building, maintaining and managing it is a gargantuan task rife with political landmines. Issues of national energy security are involved. The logistics of building so much solar power, of getting the materials, the people and the planning in place, is nearly unfathomable. And then there’s the cost: an estimated $555 billion.

The project might sound like nothing but a hazy dream, unlikely to materialize, except for the fact that a consortium of a dozen big companies, including Siemens, Munich Re, E.ON, RWE and Deutsche Bank, last year signed an agreement to try to raise the money. That’s not money in the bank, however. Aside from an expected 1 billion euros from the European Union, it will take plenty of government and private funding to make the project happen, and the donation buttons on the website make it clear the Desertec foundation is collecting wherever it can. The consortium doesn’t even plan to complete the plan to raise the money until 2012. And even with the funding, the project is expected to take decades, with the goal of completion by 2050.

WIND: Dogger Bank, UK — 9 GW

Moving wind-power projects offshore opens up vast amounts of space and also the potential to take advantage of steadier, faster-moving wind. The largest such project in the pipeline today is the Dogger Bank development, which is part of the United Kingdom’s third round of offshore wind licensing, according to EER. The project, with a whopping target installation capacity of 9 GW — and the potential for some 13 GW — blows away the current largest wind farm, a 782-megawatt onshore farm in Roscoe, Texas, that was completed in October 2009.

Forewind, a consortium of major energy companies including Scottish and Southern Energy, RWE Innogy’s RWE npower Renewables subsidiary, Statoil and Statkraft, won the license to develop the Dogger Bank zone in January. The site is 3343 square miles large, 77 to 150 miles from shore, with depths of between 59 and 206 feet, and its unparalleled size, distance and depth create a number of logistical challenges in constructing the project and connecting it to the grid. Make no mistake, this project is years away from completion. Forewind hasn’t had set a target opening date, but has said it plans to make initial investment decisions about the project in 2014.

TIDAL: Incheon, South Korea — 1.32 GW

Completed in 1966, the first tidal power plant in the world, France’s 240-MW Rance plant, remains the largest today. Now South Korea is planning a project more than five times as large in the Incheon Bay. GS Engineering and Construction Corp. (GS E&C), a publicly traded company based in nearby Seoul, said in January that it plans to begin building the Incheon tidal plant in the second half of next year, if regulators approve the project. Korea Hydro & Nuclear Power Co. will run the plant, expected to cost $3.4 billion and start operations in 2017. The project involves a barrage, or an ocean dam, which traps water in a basin and uses turbines to make electricity from the water-level difference created by the tides.

As countries aim to get more electricity from renewable sources, it’s possible that another project, the U.K.’s proposed Severn Barrage, could surpass the South Korean plant. The project, which could install up to 10 miles of dams and sluice gates across the Severn Estuary, has been bandied about for nearly 30 years and a timeline remains uncertain. The government is considering five different ideas for the barrage, ranging from 1.05 to 8.6 GW in capacity, as well as three alternate concepts. Tidal power interests utilities because it is reliable – the tides occur twice a day without fail — and although it is not considered constant, or baseload, power.

But both the Incheon and Severn projects face significant environmental concerns and opposition. Like river dams, these tidal barrages have caused some unintended environmental and ecological consequences, making them unpopular with environmentalists. For one thing, they reduce the flow of water from the tides and the exchange of water from the basins, which impacts the water, the surrounding wetlands and the wildlife that lives in them.

Other types of tidal-power technologies — including underwater turbines that operate like wind turbines, using the current itself to generate electricity without needing a barrage — could help avoid those issues, but such technologies are still under development. Three 200-MW projects are racing for the title of the largest tidal-current project. Utility SSE Renewables in March won a bid to develop two such projects in the UK’s Pentland Firth off Scotland’s northern coast: one on what’s called the Westray South site and the other, in partnership with Irish tidal-energy company OpenHydro, on the Cantick Head site. SSE’s already raised some £3.8 billion in bonds, loans and stock offering proceeds to support all its renewable projects. Meanwhile, Crest Energy has applied for approval to install up to 200 submerged turbines near the entrance of the Kaipara Harbour, on the northwestern site of New Zealand’s North Island.

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GEOTHERMAL: Sarulla, Indonesia 330 MW

After a series of delays since the project was first announced in 2006, it looks like an $800 million geothermal project slated for North Sumatra is finally underway. The consortium of companies developing the project, headed by Indonesian oil and gas company Medco Energi Internasional, in April announced it has negotiated an agreement to sell the power to PT Perusahaan Listrik Negara, a state-owned power company, for a levelized price of 6.79 cents per kilowatt-hour.

The talks had held up the project, originally expected to be completed next year, for the last four years, and the price will still need to win regulatory approval before the project can go forward. But barring financial or regulatory difficulties, the consortium plans to finish the project in five years, with the first phase — representing about one-third of the capacity — beginning operations within three years. Aside from Medco, the consortium includes Nevada-based Ormat Technologies (NYSE: ORA), Japan-based Itochu Corp. and Japan-based Kyushu Electric Power Co.

Even when it’s finished, the 330-MW project — which EER called the largest planned project with advanced development — will seem small compared to the Geysers in California, which is the largest project ever completed. The capacity of the geothermal field at the Geysers started out at 1.5 GW when it was first developed in the 1970s, then peaked at 2 GW in 1987 before sinking back to about 700 MW today due to poor maintenance, according to EER.

Why is the largest planned geothermal project today smaller than the current largest project? The Geysers was originally developed as multiple projects— not a single project, although it’s now maintained by one company, Calpine, which is working to expand the production by up to 80 MW. The Geysers consists of 19 facilities spread over the geothermal field. The Sarulla project’s 330 MW make it the world’s largest single-contract geothermal project so far, which Ormat pointed out back in 2006. And it has a point.

But it’s also true that geothermal project sizes are indeed falling, at least in the United States, because some of the sites with the best resources— such as the Geysers — have already been tapped, EER says. Meanwhile, in places like New Zealand and Indonesia, which have better resources, project sizes will likely get larger. Indonesia is aggressively pursuing more geothermal, targeting a whopping 4 GW by 2014. Cost remains a challenge, as geothermal competes largely with cheap, but dirty, coal in the developing country. But geothermal remains highly alluring because it provides constant, not intermittent, power at a lower potential cost than solar or wind.

WAVE: Orkney, Scotland, 200 MW

Clean-energy developer SSE Renewables is in an enviable position – it’s tied with itself for the title of the largest wave project in development today. In March, the company won exclusive rights to lease two sites in the U.K.’s Pentland Firth, an area off the northwestern coast of Scotland’s Orkney, for wave projects. It’s building a 200-MW wave farm at the Costa Head site and is also partnering with Scottish ocean-energy company Aquamarine Power to develop the Brough Head site. SSE holds a 47 percent stake in Aquamarine.

At Brough Head, the companies plan to install clusters of Aquamarine’s Oyster (left) wave-power devices, which look like hinged rafts that flap as waves pass, driving hydraulic pistons that in turn spin a conventional hydro-electric turbine. The wave farm is expected to begin operations in 2013. Including this project, the partners have signed an agreement to develop up to 1 GW of such wave farms together in the U.K. and in Ireland by 2020. SSE Renewables hasn’t yet said whether its Costa Head project would also use Aquamarine’s Oysters, only saying it is working with partners and stakeholders to come up with a more specific plan for the site.

Wave power has long been an attractive concept because of the large area of the global covered by oceans, the constantly moving nature of waves— which could deliver dependable baseload power, instead of the intermittent power of solar and wind — and the potentially low cost. But so far, the technologies have faced challenges, including cost and reliability issues. For one thing, sea conditions (think constantly moving saltwater) can be very rough on equipment, and maintenance and repairs more expensive than if the same equipment was easily accessible on land. Still, as governments push for more renewable energy, several companies and utilities are working on demonstration projects around the world.

Freelancer Jennifer Kho has been covering green technology since 2004, when she was a reporter at Red Herring magazine. She has more than nine years of reporting experience, most recently serving as the editor of Greentech Media. Her stories have appeared in such publications as The Wall Street Journal, the Los Angeles Times, BusinessWeek.com, CNN.com, Earth2Tech, Cleantechnica, MIT’s Technology Review, and TheStreet.com.