Senate Finance Committee Chairman Russell B. Long (D-La.) yesterday warmly embraced a two-year-old Nelson Rockefeller plan for a $100 billion energy development fund as a vehicle to increase energy production.

A major criticism of President Carter's energy proposals, has been that they concentrate on conservation and do too little to encourage more production of oil, natural gas or alternative fuels. Long has been saying for months that more should be done to spur production.

But wherever he or other oil-gas industry champions propose returning part of a suggested Carter oil tax to the industry to explore for more oil, it's denounced as a rip-off of the public - and it was defeated on these grounds in the House.

Rockefeller, while Vice President, headed a commission which proposed creation of an independent, government owned body that could spend up to $100 billion over 10 years to help private industry with big energy projects too expensive or risky for companies to tackle alone. This could include extracting oil from shale rock, converting coal to gas, and perhaps exploring for more oil and gas in hard to reach places.

Rockefeller's proposal didn't make it the first time around. It was shot down in the House as a handout to business. He offered it again yesterday to the Senate Finance Committee, and it got a friendly reception.

Rockefeller said the $100 billion could come from the oil tax which Carter proposed to discourage energy consumption by taxing the price of domestic crude up to world levels. It would bring in about $14 billion a year when fully effective in 1980. Rockefeller said he believes the American people would more readily accept the tax if it was to be used to help produce more energy. Carter has proposed rebating it to the public.

Long said the proposal has "tremendous amount of merit. I'm going to see that something along this line is done. All we are talking about is making money available to persuade business to take risks."

The Senate yesterday also neared passage of the second of five energy bills into which it has divided Carter's proposals. It was a catch-all conservation bill to which the senators attached a ban on sale of gas-guzzling cars after 1979. But the Senate refused to add other mandatory energy-saving provisions.

Sen. Lowell P. Weicker (R-Conn.) urged the Senate to add at least one mandatory provision in this puffball bill." But he lost on a vote of 83 to 7 when he tried to require that every automobile be kept off the road one day a week. His proposal that gasoline stations be closed on Sundays was also rejected, by a voice vote.

The bill requires local utilities to help customers insulate their homes, orders energy standards set for major home appliances and authorizes van-pooling programs to carry paying federal workers to and from work.

Added by the Senate to Carter's proposal were loans to install solar beating equipment, mandatory energy standards for industrial engines, pumps and other equipment, and a doubling of the fines on auto companies that fail to meet the fleet-average standards of 18 miles per gallon with 1978 models.

The Senate Energy Committee, which tied 9 to 9 Monday on whether to continue price controls on natural gas, yesterday sent Carter's bill continuing controls to the Senate floor without recommendation for action next week.

Carter would continue controls at a higher level than at present. Opponents contend only deregulation would produce more gas and avoid shortages. Backers of instant deregulation of new gas reportedly can count 43 to 50 votes in the 100-member Senate.

If that loses, Sens. J. Bennett Johnston (D-La.) and Pete Domenici (R-N.M.) have prepared a compromise for phased deregulation. Their plan would permit the price of new gas to rise from the present $1.46 per thousand cubic feet over four years to $2.41. After that, the price could not rise more than 7 1/2 per cent in each six-month period. Opponents have said deregulation could send gas prices up to $5.

Meanwhile, the President signed an order yesterday that puts the new Department of Energy into operation, effective Oct. 1 and told a news conference this fulfills his campaign pledge to begin reorganizing the federal government.

Carter also submitted to Congress the nominations of 10 top-ranking officials for the department, whose first annual budget will total almost $10.4 billion and which will include almost 20,000 employees.

Energy Secretary James R. Schlesigner presented to Carter a model of the new Energy Department seal - blue, green, and gold with a white eagle's head - and said it "indicates we will strike an appropriate balance between energy and the environment and we will keep the sky blue and the grass green."

Carter said he was optimistic that Congress will pass a comprehensive energy act before it adjourns from the present session. He noted that gasoline consumption reached a new high this summer and the oil imports have continued to increase despite the many public warnings of potential energy shortages.

Carter's nominations for the new department were headed, as widely expected by John F. O'Leary, now administrator of the Federal Energy Administration, who was proposed as deputy secretary.