Applying what he calls the "principle of the baked potato," a George Washington University professor of engineering has invented and patented a method of storing solar energy that preserves the warmth of the summer sum so it can be used to heat homes in the winter.

From April through November, Shao W. Yuan collects the sun's heat in a panel of water-filled solar collectors located outside an area research facility. The collectors are connected to an underground storage system of coiled plastic piping which in turn heats the surrounding ground. Throughout the warmer months the sun-heated water is pumped through the underground coils and back again through the solar collectors.

"I collect heat in the summer when the summer sun is stronger and the days are longer," says Yuan.

By the first of November, he adds, about the time when most Washington area homeowners' furnances are running, the temperature of the ground surrounding the coils will have risen to 170 to 180 degrees, in effect creating an underground reservoir of solar heat. (The normal winter minimum underground temperature is 55 to 65 degrees.) It will remain hot enough throughout the winter, Yuan says, to heat the water in the coiled plastic piping and provide sufficient heat and hot water until spring.

"The ground will hold heat longer than water will," says Yuan. "It's something like the difference between a cup of coffee and a baked potato.

When you go out to dinner, order a coffee and a baked potato with your meal and then set them aside when the waiter brings the food. At the end of the meal, the baked potato will still be hot, but the coffee will be lukewarm. The ground under the house is like the baked potato, storing the solar heat all winter."

Basically, the problem with conventional solar heating is that most systems use storage tanks to store the solar heated water, and they will provide only 30 to 50 percent of the total annual requirement for space and water heating for homes. Like the after-dinner cup of coffee, the water in the tanks cools quickly.

In a prolonged period of good weather with lots of sunshine, most solar heating systems will build up a sufficient level of heat to supply adequate heat and hot water for most buildings. But during a long stretch of cloudy, snowy or rainy weather the systems lose heat after a few days. Thus, most solar-heated buildings need supplemental or backup systems.

A native of Shanghai, Yuan grew up in Peking. He came to the United States in 1934 to study engineering at Stanford University and California Institute for Technology. After the Japanese attack on Pearl Harbor he couldn't get back to China, so he remained in the United States.

From 1941 to 1945, he held a number of research jobs with the aircraft industry and later served as a consultant to industrial firms and government organizations, including the Oak Ridge National Laboratory.

He has been on the faculties on the University of Texas and the Polytechnic institute of Brooklyn. For the last 10 years, Yuan has worked at George Washington University, where he was chairman of the Department of Civil, Mechanical and Environmental Engineering until last summer. He and his wife live in Falls Church.

Ironically, it was scientific principles he observed during his boyhood in China, not his years in academia, that supplied the inspiration for his plan to store solar heat underground.

"In the old days you cut ice from the river in the winter and stored it in the cellar, and it would stay cold all summer," he recalled. "When you have a watermelon, you want it to be cold, so you sink it in a well overnight and when you take it out in the morning it is ice-cold.

"The earth," he observes, "is one of the best insulators. It is also one of the cheapest."

For about five years, Yuan 65, has been working on a method for underground storage of solar heat, for which he was granted a patent-number 4,138,995-in February. $"The system provides for long-duratoion earth storage of the heat energy, which can later be used for both space and hot water heating in homes, multiple unit housing, commercial buildings, public buildings etc . . . ," says the patent.

Yuan estimates that after two more years of testing, his system could be installed for about $18,000 to heat a house with 1,500 square feet of living space.

Over a 20-year period, he predicts, the annual cost of heating such a building with the solar collector and earth storage system would be one-third that of electric heating and two-thirds that of conventional solar heating.

His predictions are based on a yearly inflation rate for other fuels of 12 percent, a $2,000 tax credit for the installation of the solar underground system and a 20-year amortization at 9 percent interest.

Yuan's system is being tested on a one-tenth scale model on the grounds of Atlantic Research Corporation off I-395 South in Fairfax County. There, a solar collector panel is already gathering the heat of the springtime sun for storage underground against the cold of next winter.

In an instrument room inside the test house, meters monitor the rate of water flow while tapes record temperatures of water in the pipes leading to and from the soil. Pressure dials measure the water pressure in the glass tubular collector where water is being heated almost to the boiling point by the sun's rays. CAPTION:

Picture, Shao W. Yuan's System uses water-filled solar collectors. By Joel Richardson-The Washington Post; Illustration, The ground stores and insulates solar heat. By Richard Furno-The Washington Post