In the world of clean technology, numerous subsectors make up the overall arena. In the fast-growing world we live in, next generation transportation, natural gas and liquid natural gas (LNG), smart grid and next-generation lighting are all spaces where growth is expanding exponentially and offering investors a new frontier not only for technology, but for portfolio profits.

The analysts at Cowen have initiated coverage of this exciting area with top stocks to buy in all four of the subsectors of clean technology. For investors with an aggressive growth portfolio, and a long time horizon, these could be some of the best stocks to add for the next few years and perhaps way beyond.

Acuity Brands Inc. (NYSE: AYI) leads of the list and is a top stock to buy in the next-generation lighting space. The company is the leading provider of North American lighting fixtures (22 brands, 1.7 million SKUs). The Cowen analysts acknowledge that, trading at 24.6 times fiscal year 2015 earnings estimates, the stock is not cheap on a relative basis. However, they think that non-residential sales trends, combined with LED margin improvement can continue to drive the share price higher. The Cowen price target is $142. The Thomson/First Call target is $137.30. Shares closed trading Thursday at $123.04.

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Maxwell Technologies Inc. (NASDAQ: MXWL) is a company that makes the Cowen Outperform list in the next-generation transportation arena. The company is a leading supplier of innovative ultra-capacitor solutions targeted at automotive, wind, heavy-duty transportation, rail, consumer electronics, industrial motive power and utility metering applications. Maxwell has solid opportunities in China with sales for regenerative braking systems for municipal buses. Cowen places the initial price target at $12, while the consensus target is much higher at $16. The shares ended Thursday at $9.33.

Pacific Ethanol Inc. (NASDAQ: PEIX) is a top biofuel name rated Outperform at Cowen. The company is the leader in California’s busy ethanol market. Margins have been much better in 2014 after a difficult 2013 with low corn prices relative to ethanol pricing. The Cowen team expects California ethanol prices to rise with increased rail costs from the Midwest to the state. The Cowen opening price target is $29, and the consensus target is $30. The stock closed on Thursday at $21.31.

Polypore International Inc. (NYSE: PPO) is another top stock in the next-generation transportation sub-sector at Cowen, and it is listed as the top pick in the clean tech coverage. This is a stock that is very levered to the fast-growing electric car arena as it produces specialty chemical separators used in the battery industry. After two disappointing years in 2012 and 2013, the Cowen teal feels that expectations have been reset and the company is poised to take advantage of long-term deals with Samsung and Panasonic. The Cowen price target is $53, and the consensus target is $42.30. Shares closed at $43.58.

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Silver Spring Networks Inc. (NYSE: SSNI) rounds out the top five names in the clean tech field to buy at Cowen. The company is a smart grid firm and is one of the leading providers of IP-based secure networking platforms that also provides suite of solutions to utilities. While Silver Springs has had a tough couple of quarters on regulatory delays with a key customer, and the culmination of multiyear deployments at PG&E, BG&E and Florida Power & Light, the Cowen analysts are very positive on the company’s industry-leading 30% share of utility networking equipment. The Cowen price target is $14, and the consensus is at $14.44. The stock closed Thursday at $12.23.

The question with clean technology is not if, but when do all of these top sub-sectors and applications become ubiquitous. In a world choked with ever-growing populations, especially in urban areas, and increasing environmental hazards, the sector could very well prove to be a leader for years and perhaps decades to come.

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