The 2022 climate law has accelerated investments in clean-energy projects across the United States. It has also delivered financial windfalls for big banks, lawyers, insurance companies and start-up financial firms by creating an expansive new market in green tax credits.

The law, signed by President Biden, effectively created a financial trading marketplace that helps smaller companies gain access to funding, with Wall Street taking a cut. Analysts said it could soon facilitate as much as $80 billion a year in transactions that drive investments in technologies meant to reduce fossil fuel emissions and fight climate change.

The law created a wide range of tax incentives to encourage companies to produce and install solar, wind and other low-emission energy technologies. But the Democrats who drafted it knew those incentives, including tax credits, wouldn’t help companies that were too small — or not profitable enough — to owe enough in taxes to benefit.

So lawmakers have invented a workaround that has rarely been employed in federal tax policy: They have allowed the companies making clean-energy investments to sell their tax credits to companies that do have a big tax liability.

That market is already supporting large and small transactions. Clean-energy companies are receiving cash to invest in their projects, but they’re getting less than the value of the tax credits for which they qualify, after various financial partners take a slice of the deal.

Clean-energy and financial analysts and major players in the marketplace say big corporations with significant tax liability are currently paying between 75 and 95 cents on the dollar to reduce their federal tax bills. For example, a buyer in the middle of that range might spend $850,000 to purchase a credit that would knock $1 million off its federal taxes.

The cost of those tax credits depends on several factors, including risk and size. Larger projects command a higher percentage. The seller of a tax credit will see its value diluted further by fees for lawyers, banks and other financial intermediaries that help broker the sale. Buyers are also increasingly insisting that sellers buy insurance in case the project does not work out and fails to deliver its promised tax benefits to the buyer.

The prospect of a booming market and the chance to snag a piece of those transaction costs have raised excitement for the Inflation Reduction Act, or I.R.A., in finance circles. A new cottage industry of online start-up platforms that seeks to link buyers and sellers of the tax credits has quickly blossomed.

An annual renewable energy tax credit conference hosted by Novogradac, a financial firm, drew a record number of attendees to a hotel ballroom in Washington this month, with multiple panels devoted to the intricacies of the new marketplace. The entrepreneurs behind the online buyer-seller exchanges include a former Biden Treasury official and some people in the tech industry with no clean-energy or tax credit experience.

Tax professionals and clean-energy groups say the marketplace has widely expanded financing abilities for companies working on emissions-reducing technologies and added private-sector scrutiny to climate investments.

But those transactions are also enriching players in an industry that Mr. Biden has at times criticized, while allowing big companies to reduce their tax bills in a way that runs counter to his promise to make corporate America pay more.

“I wouldn’t call it irony. I would call it, sort of, this unexpected brilliance,” said Jessie Robbins, a principal of structured finance at the financial firm Generate Capital. “While it may be full of friction and transaction costs, it does bring sophisticated financial interests, investors” and corporations into the world of funding green energy, she said.

Biden administration officials say many clean-tech companies will save money by selling their tax credits to raise capital, instead of borrowing at high interest rates. “The alternative for many of these companies was to take a loan, and taking that loan was going to be far more costly” than using the credit marketplace, Wally Adeyemo, the deputy Treasury secretary, said in an interview.

Some backers of the climate law wanted an even more direct alternative for those companies: government checks equivalent to the tax benefits their projects would have qualified for if they had enough tax liability to make the credits usable. It was rejected by Senator Joe Manchin III of West Virginia, a moderate Democrat who was the swing vote on the law.

A modest federal marketplace of certain tax credits, like those for affordable housing, existed before the climate law passed. But acquiring those credits was complicated and indirect, so annual transactions were less than $20 billion — and large banks dominated the space. The climate law expanded the market and attracted new players by making it much easier for a company with tax liability to buy another company’s tax credit.

“There weren’t brokers in this space, you know, a year ago or 14 months ago before the I.R.A. came out,” said Amish Shah, a tax lawyer at Holland & Knight. “There are lots of brokers in this space now.” Mr. Shah said he expected his firm to be involved in $1 billion worth of tax credits this year.

“The discussion goes like this,” said Courtney Sandifer, a senior executive in the renewable energy tax credit monetization practice at the investment bank BDO. “‘Are you aware that you can buy tax credits at a discount, as a central feature of the I.R.A.? And how would that work for you? Like, is this something that you’d be interested in doing?’”

Financial advisers say they have had interest from corporate buyers as varied as retailers, oil and gas companies, and others that see an opportunity to reduce their tax bills while making good on public promises to help the environment.

Experts say large banks are still dominating the biggest transactions, where projects are larger and tax credits are more expensive to buy. For the rest of the market, entrepreneurs are working to create online exchanges, which effectively work as a Match.com for tax credits. Companies lay out the specification of their projects and tax credits, including whether they are likely to qualify for bonus tax breaks based on location, what wages they will pay and how much of their content is made in America. Buyers bid for credits.

In order to sell tax benefits under the law, companies have to register their credits with the Treasury Department, which created a pilot registry website for those projects this month. The online platforms to connect buyers and sellers of the credits are not regulated by the government.

Alfred Johnson, who previously worked as deputy chief of staff under Treasury Secretary Janet L. Yellen, co-founded Crux, one of the online exchanges, in January. The company has raised $8.85 million through two rounds of funding.

Mr. Johnson said his business helped replace the “low-margin” administrative work that happens to facilitate deals. Lawyers and advisers will still be brought in for the more complicated parts of the deal.

“It just requires more companies coming into the market and participating,” he said. “And if that doesn’t happen, the law will not work.”

Seth Feuerstein created Atheva, a transferable credit exchange, last year. He has no clean-tech experience, but he has brought in green-energy experts to help get the exchange started.

Atheva already has tens of millions of dollars in projects available for tax-credit buyers to peruse on the site, with hundreds of millions more in the pipeline, he said. On the site, buyers can browse credits by their estimated value and download documentation to help assess whether the projects will actually pay off. Mr. Feuerstein said that transparency helped to assure taxpayers that they were supporting valid clean-energy investments.

“It’s a new market,” Mr. Feuerstein said. “And it’s growing every day.”