Project Brightfield has a nice ring to it. Chevron, the California-based oil giant, is turning the site of an old oil refinery into an eight-acre field of solar panels, showcasing seven new technologies from an array of cutting-edge companies. It seems to fit the company's current online slogan: "Finding newer, cleaner ways to power the world".

But there is a problem for Chevron, which has over a thousand Texaco filling stations in Britain. It plans to use the solar energy to help power pumps and pipelines at what will remain one of the oldest, dirtiest and most greenhouse-unfriendly oil fields on the planet – the Kern River heavy oil facility near Bakersfield.

The company is proud enough of the solar panels to have a promotional video on Operation Brightfield. Chevron's local vice president, Bruce Johnson, calls the solar facility "a clear example of Chevron's efforts to find ways to integrate innovative technologies into our business."

But the Rainforest Action Network, a California-based NGO, put out a natty little video of its own charging the company with "greenwash" in the California sun.

Chevron is the biggest greenhouse-gas emitter in California, according to RAN. And its global green reputation could do with some refurbishing. The company is still living down the environmental damage caused by past involvement of Texaco, a company it bought in 2001, while grabbing oil from the rainforests of Ecuador.

And it faces new criticism for its prominent role in developing tar sands in Canada. This latter is a big problem, as the California governor, Arnold Schwarzenegger, seeks to cut the state's carbon dioxide emissions.

RAN says last year Chevron hit a "new all-time low in renewable energy investments", with just 1.96 per cent of its capital and exploratory budget going green.

So the plaudits Chevron has won for its Brightfield test rigs, along with a planned solar project in New Mexico, are green gold dust.

But its dirty old ways still look like the main game at Chevron. You can see its real business down the road from the shiny new solar panels, at the Kern River heavy oil facility. The field is more than a century old and contains some 10,000 "nodding donkey" rigs pumping away. The field is largely exhausted, with production declining every year, but Chevron is reluctant to call a halt to its ancient money-spinner.

But bringing the oil to the surface is increasing difficult, and energy-intensive. The thick tar-like dregs of the oilfield won't flow on their own. They have to be heated first. So Chevron burns natural gas to make steam, which it pumps underground to raise temperatures and get the gunge moving. They call it "steam flooding". One reporter invited to Kern River by the American Petroleum Institute describes the scene on The Oil Drum.

Chevron is a specialist in extracting heavy oil round the world. In Venezuela and Indonesia, for instance. But bringing the stuff to the surface has a very large carbon footprint, according to Tony Kovscek of Stanford University's Energy Resources Engineering department, who has studied Kern River.

He estimates (pdf) that the carbon footprint of producing heavy oil at Kern River is around 50kg of carbon dioxide for every barrel of oil.

That is only half the footprint of tar sands in Alberta, he says, "but the carbon footprint of conventional oil is a great deal smaller."

The company spokesman Alex Yelland said the 750-kW solar facility, which has an expected lifetime of 25 years, is intended "to evaluate competing next generation solar technologies". He denied any attempt at greenwash. "That the oil field nearby produces heavy oil was not relevant to the siting of the solar test."

Kovscek says, "some of the largest point sources of carbon dioxide in California are from these types of oil field operations." Solar panels powering the pipeline pumps won't change that.

But, if Chevron wants to carry on pumping heavy oil from Kern River, there would be a way for the company to make a serious difference, he says. It could harness the power of the sun big time to make the steam.

A lot of entrepreneurs in California want to develop what they call "concentrated solar thermal power". Rather than covering the desert in photovoltaic panels, they want to install mirrors to concentrate the sun's rays and boil water to make steam. Their main idea is to use the steam to run turbines. But why not, says Kovscek, use it directly to free up the heavy oil?

"Relatively conservative designs could reduce the heavy-oil carbon footprint by at least 30%," he told the Guardian. "More aggressive designs could achieve even greater reductions." Yelland said that the company plans a "solar-to-steam" demonstration facility to replace some of its natural gas needs at another oil field in California.

Now that really will "integrate innovative technologies" into Chevron's business. It would put Project Brightside in the shade. Until then, Chevron seems to be using a few solar panels to greenwash a thoroughly filthy oilfield.