But both rewarded those who snapped up their shares when they listed last year as part of a recent mini-boom in alternative energy stocks.

D1 aims to use Jatropha seeds to make biodiesel, an oil that can be added to conventional diesel and even replace it if cars are modified. Listed on London's Alternative Investment Market (Aim) in October at 160p ($3.04, E2.3), it was up at 266p on Wednesday, having risen 20% that day alone. ITM's membrane could cut the cost of fuel cells, essentially hydrogen fuelled batteries, enough to bring them into the mainstream. Its shares, launched in June at 50p, were up at 103p, also on Aim, where new energy stocks have been floating with startling frequency.

This month alone, Questair, whose technology helps purify hydrogen fuel, Voller Energy, which dreams up applications for fuel cells, and Renewable Energy Holdings, which invests in renewables projects, made their debut.

Stephen Tredget at Collins Stewart, the broker for Biofuels Corp, which listed in June, said: "There's a surprising appetite. You wouldn't have been able to float Biofuels a few years ago. Oil prices are high, which makes alternatives more competitive. The environmental driver is also strong. They're getting traction not because the world's gone green, but because they offer legitimate solutions to very real problems." 

The problem for the investor is that the stocks are anything but uniform. There is also the chance that today's positive sentiment could reverse. A boom in 2000, helped along by power blackouts in California, was followed by a crash in 2001. Charlie Thomas, the fund manager for Jupiter's Ecology fund is sanguine: "You've got to be selective in this space. 2003 was grim. There's a lot of hope value associated with these companies and valuations can get very stretched." 

The sector includes companies offering standard services tailored for the Kyoto regime, those investing in "traditional" renewables such as wind and waves, those looking at super-efficient cars, firms developing biofuels, and companies whose future depends on unproven technologies, around fuel cells or other small electricity generation units. 

ITM Power and Ceres Power are the key newly listed stocks in the last category, perhaps the highest risk. Another, Intelligent Energy may list this year. Ceres Power, floated out from Imperial College, is in some ways the safest, says Thomas, with its product further closer to development. 

ITM Power's membrane technology, he says, is potentially more exciting - it could make fuel cells cheap enough for the mass-market. But it is not scheduled to complete its first demonstration until June. Any negative results or delays could see the stock plummet. Turbogenset, the UK firm which benefited from the 2000 rally in fuel cell stocks saw its share price fall from over 500p in January 2001 to 11p in March 2003. Less risky technology bets are Romag and Porvair. The former has an established glass business bringing in reliable cash-flow and a joint venture with BP Solar to embed solar panels in glass. If solar power takes off, so should Romag. Shares were already up at 73.5p last week from 43p in December. 

Porvair is involved in membranes for fuel cells, but has steady cash flow from its chemicals business.

Renewable Energy Holdings, run by John Baker, former chairman of UK generator National Power, aims to secure a similar hedge by using reliable cashflow from wind developments and small hydro-electric projects to develop the more frontier wave power business.

The one listed company developing wave power alone is US-based Ocean Power Technologies. The company's shares have boomed from 80p to 112p already this year, but have yet to regain the 125p at flotation in October 2003. The company has installed electricity generating buoys for the US marines off Hawaii and has agreements to build more buoys in the US. But the test will be whether its joint venture with Spanish power firm Iberdrola to build a 1.25MW wave power station off Spain comes to fruition. On the fringes of the renewables technologies side are companies like Voller Energy and Solar Integrated Technology, which look for profitable ways to use the technologies of others.

Fuel cells are unlikely to replace the internal combustion engine, their most appealing application, for at least a decade. But hybrid vehicles, which achieve high fuel efficiency by combining a standard engine with a battery powered one, are already commercial.

Thomas says: "The hybrid play is becoming more and more interesting. It's an intermediate technology, there's no need for new infrastructure." For those who want to take advantage of this, Azure Dynamics is the best play, a hybrid which listed on Aim last August at 29p and is already at 42p.

Biofuels are the other obvious way to cut emissions. They are theoretically carbon neutral as plants absorb as much carbon dioxide growing as the fuel releases being burnt. France plans for all diesel in petrol stations to contain no less than 2% of biofuels by this year and intends to increase the percentage to 5.75% by 2010. The UK Treasury is considering a similar move.

Alongside D1, there is Biofuels, which is building Europe's biggest biodiesel factory on Teesside, and aims to sell 250,000 metric tons of biodiesel a year to petrol retailers once it begins production.

The record for biofuels is not encouraging. Past attempts to fuel power stations from plants have failed. The UK's Arbre scheme, which aimed to fuel a power station from willow coppice went bankrupt in 2002 after generating for only eight days. And only last month Holsworthy biogas, the UK's first plant to generate power from animal dung went into bankruptcy.

But the interest in biodiesel as a transport fuel looks likely to bring the sector into the mainstream. Indeed, the biggest risk, Thomas says, is that too many farmers and refiners set up biofuels businesses, leading to an excess of supply.

With Kyoto coming into force last week, it has become more certain governments will act to cut carbon emissions. And this means a continuing driver for new energy firms. But with many of the technologies still experimental, the sector remains speculative.

Ted Scott at F&C Stewardship fund says: "As with any sector, whenever you get too many companies being floated at once, and you get a lot of dross alongside the good ones."