David Gelbaum, the super-secretive investor behind the Quercus Trust, just took a day job.

He will serve as CEO of Entech Solar, one of the Trust's investments. He takes over for Frank Smith, who became Entech's CEO in March 2008.

The product of a merger between struggling solar installer WorldWater and Solar Technologies, Entech has devised a solar unit that contains silicon photovoltaic cells, which can convert sunlight into electricity, as well as a fluid-filled pipe that collects heat from the sun. The collected heat is then used to warm water and displace the need to run a natural gas-burning water heater.

The devices will be set up in arrays ranging from 100 kilowatts to 5 to 10 megawatts in size and sold largely to industrial customers and corporate campuses. The units, which measure three feet wide, 12 feet long and two feet deep, can also be made fairly inexpensively.

The company is part of a small group of firms that are focusing on the development of multifunctional devices. Others include skylights into their products to also take advantage of solar lighting. Although the devices are appealing, sales have yet to take off.

The Quercus Trust has been struggling of late. Last year, Gelbaum had to restrict chairitable giving because of a downturn in the Trust's energy investments. The Trust is also sprawling. At last count, it had 48 companies in its portfolio, an unusually high number for any sort of VC firm, particularly one that seems to be managed by a handful of people.

The most interesting aspect of the Entech deal will be to see how active Gelbaum becomes in day-to-day management. He shies away from publicity and public speaking. He rarely gives interviews. Will he break that tradition to help sell commercial customers and utilities on Entech? He may have to. And if he does, expect him to get swamped with business plan proposals. Researchers and small companies regularly try to figure out ways to get in touch with the Quercus Trust.

Quercus means 'oak' in Latin.