Portugal's largest industrial company, Energias de Portugal, has announced that it has agreed to purchase Horizon Wind Energy for $2.9 billion. The big winner on this deal is Goldman Sachs (NYSE:GS), which acquired Horizon Wind in 2005.

In fact, it might interest you to know that Horizon Wind is just one of the many alternative energy companies in which Goldman is an early investor. The company has also invested in Iogen, a private company developing cellulosic ethanol; two solar businesses -- SunEdison and First Solar (NASDAQ:FSLR); and Nordex, which is a manufacturer of wind turbines.

As you may recall, Goldman also had a large supporting role in the private equity bid for TXU (NYSE:TXU), the Texas energy company, and most likely played an integral role in helping broker the deal with environmental groups to limit the number of new coal plants that TXU would build.

I am not recommending investing in Goldman Sachs on the basis of its investments in alternative energy alone. But I would encourage investors to begin paying more attention to these investments, because I expect that deals similar to today's acquisition of Horizon Wind Energy will begin adding more profits to Goldman's bottom line in the not-too-distant future.

For instance, just last week, venture capital superstar Vinod Khosla suggested that cellulosic ethanol will be cost-competitive with ethanol by 2009. Cellulosic ethanol is chemically identical to ethanol, but instead of corn it uses feed stocks, such as grasses, which promise to be easier to convert and thus require significantly less energy to produce. Khosla's prediction tells me that Goldman's investment in Iogen could represent its next "clean tech" payday. Whether Iogen will go public soon or be acquired by a larger firm such as Archer Daniels Midland (NYSE:ADM), which is seeking to strengthen its position in the area of cellulosic ethanol, I can't say. But I do know that it won't matter to Goldman because it will likely profit either way.

For investors interested in clean tech, I would further suggest that Goldman Sachs, while by absolutely no means a "clean tech" pure play, represents a unique opportunity to diversify one's portfolio while still offering some exposure to the alternative energy market.

Last year, the company earned almost $20 per share -- a figure that was up 76% from 2005. The company clearly knows how to make money, and now it is increasingly clear that it also understands that there is a lot of green to be made in being green.

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Fool contributor Jack Uldrich admits to being green about all of the aspects of Goldman Sachs' business, which is why he is not yet an investor in the company. The Fool has a strict disclosure policy.