It's the middle of earnings season for clean tech companies, so the numbers are coming fast and furious. Whether it's solar power, alternative fuels, or efficient turbines, everyone is trying to make a case for your clean tech investing dollars.

Here is what happened this week.

Alternative fuels move forward, sort of
Rentech (AMEX: RTK) has been billing itself as a synthetic fuels company, but the biggest problem is that most of its revenue has come from nitrogen fertilizer products. After the company announced it had filed a registration statement for a public offering of Rentech Nitrogen Partners, the business' purpose may be clearer, but it doesn't leave Rentech with much in the way of revenue. The plan is for the new public company to be a master limited partnership, which pays out dividends and will fund Rentech's alternative energy business. For now, the businesses are one and fertilizer dominates earnings.

During the third fiscal quarter, Rentech reported revenue of $74.4 million, an increase of 47.3% from a year ago. Net income improved to $0.04 per share from a loss of $0.01 in the prior year. It's hard to see exactly what Rentech will be left with, so we'll keep an eye on further information coming out as the IPO process continues.

Shale drives turbine demand
Another company trying to change the way we think about energy is Capstone Turbine (Nasdaq: CPST), which makes microturbines for on-site power generation.

During the company's recently ended fiscal first quarter, revenue jumped 51% to $24.3 million as the company nearly doubled shipments. Gross margin was a measly 2% of revenue, and the company still has a loss of $2.9 million, but with sales ramping up there is a flicker of hope.

The growth of shale plays in the U.S. has given Capstone a growing demand base, which has given the company $71 million in new orders so far this year.

Another solar letdown
The quarter has been rough on solar stocks. First Solar (Nasdaq: FSLR) was hammered after reporting disappointing earnings and outlook. JA Solar (Nasdaq: JASO) has already told us it would have negative margins, and now SunPower (Nasdaq: SPWRA) made its weak quarter official.

Revenue grew an impressive 54% from a year ago to $592.3 million, but SunPower swung to a $147.9 million GAAP loss. On a non-GAAP basis, gross margins were 12.5% and loss per share was $0.19, in line with what the company projected in late July.

The biggest problem for SunPower right now is that price reductions have accelerated faster than cost reductions could keep up. The company is projecting $1.48 per watt for manufacturing costs to end 2011, which should be very competitive with Chinese manufacturers. But management mentions a "step reduction beginning Q411" and that has yet to help financial results.

I still like SunPower's long-term position as the efficiency leader, but it's going to be a bumpy road for a year or so.

Turning solar power into usable electricity
For every solar plant, there needs to be inverters to turn DC power into AC power we use every day. So when solar manufacturers ran into a rough patch, it's logical that inverter manufacturers would be hit hard as well. Two of the biggest inverter suppliers to solar plants, Satcon Technology (Nasdaq: SATC) and Power- One (Nasdaq: PWER), have seen their stock struggle this year, but financials look to be on the upswing.

At Satcon, revenue jumped 64.7% in the second quarter to $45.5 million despite tough conditions in Europe. The company's operating loss grew almost fourfold to $20.1 million, but that was mainly because of increased focus on product development.

Today the company also announced it had been chosen as the inverter supplier for a 10 MW utility scale solar plant in New Jersey. It's not a huge project, but it's certainly giving the stock a boost today.

Growing but not making money
The common theme among these companies is that their alternative businesses are growing but not terribly profitable right now. That will turn around eventually, but not every technology will be a big winner.

Which stock do you think is most promising? Leave your thoughts in our comments section and add your favorite stock to My Watchlist to keep tabs on the company.