Denis A. Hayes, an outspoken advocate of solar energy and chairman of the Solar Lobby, yesterday was named director of the Department of Energy's Solar Energy Research Institute (SERI) in Golden, Colo.

In effect, the appointment makes Hayes the solar energy czar of the United States, since the $90 million being spent in the United States on solar energy research this year will be managed by or spent directly by the Solar Energy Research Institute.

Opened in 1977 and operated by the Midwest Research Institute of Kansas City, SERI has the status of a national laboratory, and is DOE's lead institution for solar research, development and demonstration.

"This is a happy occasion for the department and a happy occasion for solar energy," Energy Secretary James R. Schlesinger Jr. said in announcing the appointment. "If this country is to ride out the energy shortage it will be indispensable to develop solar energy. . . ."

Hayes also is a senior researcher of the Worldwatch Institute and chairman of the Center for Renewable Resources. Chairman of Sun Day, 1978, Hayes has written at least three Worldwatch papers on solar energy and a book, "Rays of Hope," advocating use of the sun for energy.

"It's a good time for me to leave the three institutions I've been associated with," Hayes said yesterday."The Solar Lobby is thriving, the Worldwatch institute and the Center for Renewable Resources have never been stronger financially than they are right now."

Hayes will take over next week as director of SERI, succeeding Dr. Paul Rappaport, who had been troubled by poor health and who will stay on as senior scientist. Hayes will inherit an organization whose staff now numbers 600 and is about to increase to 800.

While a surprise, the appointment of Hayes appeared to have broad political support. Labour and industry both praised him. In attendance at the Department of Energy news conference announcing the appointment were Sen. Gary W. Hart (D-Colo.) and Rep. Timothy E. Wirth, (D-Colo.), who represents the district where the Solar Energy Research Institute is located.

"Denis Hayes is a brilliant choice," Hart said. "He is the voice of hope at a time when the country needs hope in the field of energy."

"I think this is the most exciting job in the world," Hayes said of his new appointment. "And nobody will work harder at it than I will."

Besides the laboratory at Golden, the Solar Energy Research Institute operates three regional laboratories doing solar research. One is in Massachusetts, a second in Maryland and a third in Tennessee.

Hayes said he looks forward to the day that the sun can supply 20 percent of the nation's energy needs, a goal President Carter has set.

"I 'm not satisfied with the level of spending in the U.S. on solar research, but we have to realized that the Solar Energy Research Institute is spending more money than the rest of the world on solar research," Hayes said. "What I will be doing as its director will be to make the case for solar energy as strong as possible."