Washington (CNN) -- The Department of Defense has put a lot of money and effort into finding alternative fuels to replace petroleum-based fuels it uses now, but a new study concludes the military will not benefit from alternative energy research.

No organization in the world uses more oil-based fuel than the U.S. Department of Defense, about 337,000 barrels of fuel a day as of 2008. In recent years it has been looking for ways to replace petroleum with fuel made from algae, coal, garbage, even a plant similar to the mustard plant.

The Air Force and Navy have been testing various aircraft and ships, and have found they can operate on a 50/50 mixture of traditional fuel and alternative fuel. The Air Force hopes to use 50% alternative fuel in all its domestic flights by 2016. And the Secretary of the Navy, Ray Mabus also has set a goal to use alternative energy sources to provide half of the energy for all the Navy's warships, planes, vehicles and shore installations by 2020.

The DoD "has spent hundreds of millions of dollars" on these testing and research programs, but for all the cost and time, there is little promised benefit over using fossil fuels, according to the congressionally-mandated study by the Rand Corp.

"There is no direct benefit to the Department of Defense or the services from using alternative fuels rather than petroleum-derived fuels," the study said. But the authors conceded that the United States as a whole will benefit, somewhat like the American economy profited from the early space missions that gave the country everything from smaller computers to better baby food.

Much of the DoD alternative fuel research has focused on turning vegetable matter, like algae, soybeans or camelina seeds into fuel. The fuel will burn, but the study said that doesn't make it useful.

"Too much emphasis is focused on seed-derived oils that displace food production, have very limited production potential and may cause greenhouse gas emissions well above those of conventional petroleum fuels," said James Bartis, lead author of the study and a senior policy researcher at Rand, a nonprofit research organization.

As an example, Bartis said to make just 200,000 barrels of fuel from seeds or beans, which is one percent of the fuel the United States burns in one day, would require 10% of all the cultivated farm land in the United States. Bartis said that would mean farmers might have to stop growing food and start growing crops for fuel and that would likely lead to higher food prices, but not necessarily cheaper or cleaner fuel for the military.

"I disagree vehemently with that report," Mabus said Tuesday at a speech in Washington, after the Rand study was released.

Tom Hicks, deputy assistant secretary of the Navy for energy, told CNN he has a number of specific problems with the Rand study.

Hicks said the study doesn't take into account that camelina, which is basically a weed similar to the mustard plant, can be grown while fields are not being used for food crops," Hicks said.

"It can be used in rotation with wheat, and other grains and provide nutrients back into the soil."

While the study said alternative fuels cost more than ordinary oil, Hicks points out that the process is right now a research-and-development effort, that as the military starts using more alternative fuels, more farmers and companies will start producing them and market forces will cause prices to drop. Already the cost of the fuels the Navy has been using in its tests have dropped 50% in each of the past two years.

The study by Rand also questions whether the alternative fuels will be environmentally friendly.

Hicks said, before the military starts using the fuels on a regular basis they have to meet ecological standards. "We are held by law ... any barrel of oil that we replace, petroleum, has to have a life-cycle greenhouse gas emissions equal to or less than that."

But, in the Navy's eyes, the most important reason to use alternative energy is national security.

"Having more homegrown sources of fuel, more homegrown sources of energy," Hicks said "I think every barrel of oil that we can take out of countries that don't care for us very much -- that don't like our policies or politics very much -- and replace those with fuels that are produced in this country, with American labor, on American land,I think that goes right to our national security."

Mabus said Tuesday that conservation and fuel efficiency also are important because they reduce the risks to U.S. troops

"Every few days we have to refuel our ships, whether at sea or in port. And it's during refueling that our ships are most vulnerable. We saw this on the Cole 10 years ago when it pulled into port in Aden, Yemen. It was attacked. It was in Aden to get refueled," he said, referring to a terrorist attack in October 2000 on the U.S. Navy destroyer Cole that killed 17 sailors.

Mabus also said that Marines die in Afghanistan protecting convoys that bring the huge amounts of fuel from Pakistan or other countries to bases in the war zone. If the United States can reduce fuel use in the war zone, there will be fewer convoys for Marines to guard.

The DoD's reaction to the study was lukewarm. What Rand found will not stop the military from looking towards fuels that don't come from petroleum

"The DoD is committed to aggressively cutting energy risks by identifying improved efforts to manage energy, to include alternative fuels" according to a DoD spokesperson.

In fact, even while the study was being done, the DoD created a new office of assistant secretary of defense of operational energy. That office will review the study "to improve the Department's strategic approach to mitigating energy challenges."