One of Washington's first experiments in using solar energy as a fuel source for residential buildings begins this month with the completion of a 188-unit Fort Lincoln apartment building. The structure has a flat plate collector on the roof to gather and store solar energy to heat the water provided to residents, who will begin moving in next week.

The solar system is backed up by conventional hot water heating for use in the even of a stretch of cloudy days or problems with the solar equipment.

The apartments, at 3001 Bladensburg Road NE between South Dakota and Eastern Avenues, were built by Forrest City Dillon, Inc., Cleveland-based general contractors, for the Fort-Lincoln New Town Corp. They are being rented to participants in Washington's Section 8 leased housing rent supplement program for the elderly.

Savings in utility bills are difficult to project because of the newness of the solar energy system and the uncertainty of future costs of conventional fuels. Ralph Taylor, executive vice president of the Fort Lincoln corporation, believes, however, that fuel savings will amount to $6,000 to $7,000 annually in fuel bills.

The cost of outfitting a building to use solar energy can be sustantial. Equipment necessary for transforming the sun's rays into usable fuel is still expensive because technology is still developing and there is no mass production.

Fort Lincoln received a $129,000 grant from the Department of Housing and Urban Development to cover the initial cost of installing its flat plate collector and storage system and of making necessary changes in building design.

The principle behind the flat plate collector is "the coming thing," a "very simple" idea, said Kenneth M. Yarus, vice president of architecture and engineering at Forrest City Dillon. "It's like leaving your garden hose in the sun in the summer. When the water is turned on it comes out hot from sitting in the sun he added.

The Fort Lincoln collector consists of sheets of glass measuring three by six feet with small tubing running under them and exposed to the sun. A fluid, in this case an anti-freeze solution to avoid winter freezing, is run through the closed tubing to collect the heat, which is then transferred to the water supply pumped throughout the building. The system is most efficient at a little less than 90 per cent of its full 32,000-gallon capacity, Yarus said.

Yarus said that although the decision to use solar energy came sometime after the initial planning for the building, it required only minor changes in the construction plans.

David Engel, a marketing specialist in HUD's Division of Energy Building Technology and Standards Research, which has been making the energy demonstration grants, said that in addition to the D.C. project, seven homes in Columbia, Md., are being built with solar energy equipment financed by the department.