Considering I just wrote an article titled "Why Now is the Time to Buy Coal," you'd probably think I'm one of the last people who'd delve into the subject of green alternatives. On the contrary, I enjoy researching companies that have sustainable and renewable business models -- I'm just hypercritical when it comes to new technologies because many are still years away from being viable -- and by "viable" I mean "profitable."

Since today is Earth Day, I thought it would be worthwhile to take a look at what I consider to be five sustainable green ideas for your portfolio. Remember, these five companies are all having a positive impact on the environment, but they're also sustainable on an operational basis. Perhaps that will explain why you won't find any solar or biofuel producers on my list -- because pricing and profitability in these sectors is more or less a pipe dream right now.

Without further ado, I give you five Earth-friendly ideas for your portfolio:

Zipcar (Nasdaq: ZIP)
When you think green, a car-service company is probably the last thing on your mind -- but Zipcar is more than just your average car-service provider. The company, and its fleet of approximately 9,000 vehicles, serves as a pay-per-hour car-rental service to the 673,000 members it had as of the end of 2011. And what the company does has multiple environmental and social benefits.

First, according to Zipcar, for every car rented, 15 new vehicles are taken off the roads, reducing carbon emissions and the headaches we get from being stuck in traffic. Second, while Zipcar's fleet is still operating predominantly on gas, it is beginning to see a transition toward hybrid and zero-emission electric vehicles. Last month, the company rolled out a line of electric vehicles in Chicago and already boasts an average in its San Francisco fleet of more than 100 miles per gallon. These zero-emission vehicles also provide a supplemental income boost for Zipcar, which can sell its ZEV credits for a profit. In 2011, the company netted a $3.4 million benefit from those credits.

As the infrastructure for EVs becomes more prominent, it's likely that Zipcar's purchases of EVs and hybrids will rise even further. The company is doing a great job of taking vehicles off our roads and is projected to grow at 50% annually over the next five years. It's a company I'm proud to maintain a CAPScall of outperform on.

Intel (Nasdaq: INTC)
This is where that "Wait, what?" look comes across your face. Would it surprise you if I said that Intel is the world's largest purchaser of renewable green-energy credits? These credits, known as RECs, act as "green currency" and signaled Intel's proactive commitment to purchase 1.3 billion kilowatt hours per year in renewable energy from sources that include wind, solar, hydroelectric, and biomass. Based on estimates from the Environmental Protection Agency, Intel's purchase is the equivalent of removing 185,000 vehicles from the roads each year.

Intel isn't just purchasing RECs, either -- it has been proactively working to make its factories energy-efficient for years. Within the past seven years, Intel has invested $20 million in approximately 250 energy projects that have saved 500 million kilowatt hours of energy -- enough to power 50,000 homes annually.

But as I said, I demand operational sustainability in addition to green sustainability, and Intel simply doesn't disappoint. Intel's dominance in the microprocessor market as well as its burgeoning growth in providing hardware to the rapidly growing cloud-computing sector earned it my selection as the right stock to power your IRA over the long term.

Westport Innovations (Nasdaq: WPRT)
If the first two names didn't get you scratching your head, then my choice of Westport certainly will, considering that I just made my case less than two weeks ago for why natural-gas vehicles are years away from being viable. But there's plenty of reason Westport doesn't fit the mold of your typical natural-gas-related stock.

For Westport, a company that converts gasoline-burning engines to run on natural gas, the bounty to be paid on such a conversion is amazingly high right now, and that translates into huge margins. A recent report on CNBC says the average light-truck conversion will run around $9,000, with larger-diesel engines running significantly more. But with the huge disparity in natural-gas prices and gasoline or diesel fuel (in some cases $2 a gallon), the costs from converting can be recouped possibly as quickly as one to two years for business operators. That doesn't even factor in the environmental benefits of burning natural gas as opposed to gasoline, reducing greenhouse gas emissions by 20% to 29%.

Westport is the one lone unprofitable company on this list of five green stocks for your portfolio, but with the company projected to grow by 38% over the next five years, and with high-profile partnerships in place with Ford, General Motors, and Cummins, profitability isn't very far off. Westport is truly a unique company for the green advocate.

Waste Management (NYSE: WM)
There's no need to scratch your head here, either. Rather than thinking about Waste Management as an operator of landfills, which are among the primary sources of methane in the world, think of Waste Management as leading the charge in turning waste into energy. Remember, Waste Management didn't create the trash, but it is revolutionizing how the trash gets dealt with.

Waste Management is using trash to make sustainable treasure in countless ways. First, the company has upgraded some of its landfills to catch the methane released by the trash and transform it into usable energy for households. Second, Waste Management has electronics-recycling agreements with some of the world's largest electronics producers, including Sony. Waste Management is also using its technology to convert trash directly into usable energy that's capable of powering more than 1 million households. It's the company's goal to double this figure to 2 million households by 2020 while, at the same time, improving the fuel efficiency of its fleet of trucks by 15%.

The company's highly sustainable business model (let's face it, there's always going to be trash) and huge recycling operations are what earned it a spot back in March as a great dividend stock you could buy right now. Consider this trash company a renewable-resource lover's treasure.

Hain Celestial (Nasdaq: HAIN)
We all need to eat, so what better way to round out this list of Earth-friendly companies than with an all-natural and organic food supplier, Hain Celestial?

Aside from the obvious benefits to your body of putting in all-natural products and leaving out man-made preservatives, Hain Celestial is translating its loves for the "all-natural" over to its operations. The company has been aggressively transforming its production facilities into green, sustainable operations that are minimizing energy usage and maximizing recycling potential. Another key component to Hain's green platform is using biodegradable packaging that's safe for the environment.

But when push comes to shove, Hain can also deliver for you in the earnings column. In the fourth quarter, Hain's sales climbed 32%, while income jumped 35%. Whereas input costs and slow growth are eating into its competitors' bottom lines, Hain's all-natural product line is catering to a growing health-conscious audience and boosting sales far faster than Wall Street had predicted.

Foolish roundup
Believe it or not, "green" and "profitable" can go hand in hand, as these five companies are making serious strides to make the Earth a better place for our future generations. What socially responsible companies do you have on your radar? Share them in the comments section below, and consider adding these five stocks to your free and personalized Watchlist.

If you'd like our analysts take on three companies that could help you retire rich, then today is your lucky day! Get access to our latest special report for free!