Evacuated Tube System (Pentagon)

For six months in 1999, Capital Sun Group staff operated a 25-kW dish-Stirling demonstration for Science Applications International Corporation (SAIC) located at the Department of Defense Headquarters Pentagon building in Arlington, Virginia. We introduced senior Pentagon energy management staff we had worked with at that time to the concept of using evacuated tube solar collectors to heat service water. In 2002, the Pentagon issued a request for proposals for such a system, and Capital Sun responded together with ThermoTechnologies, Inc., the U.S. distributor of Thermomax evacuated tube technology.

In late 2002 and early 2003, Capital Sun and ThermoTechnologies surveyed the Pentagon water heating system in Wedge 1 of the Pentagon and suggested a novel instantaneous heating concept. Following approval of the final piping design and array locations in April 2003, Capital Sun completely installed 1080 solar collector tubes in June. The solar arrays were placed in six rooftop locations to make a 75-kilowatt-thermal system that offset natural gas in the central boiler plant.

The solar system design we originated for this project took advantage of the physical and performance attributes unique to evacuated tube collectors. To achieve the instantaneous heating concept, Capital Sun fabricated a piping manifold at the "top" of six building service hot water re-circulation loops. This "bridge" directs water to the roof when the solar array is hotter than the building water temperature and then the heated water returns to the existing indoor piping just at the point it starts to flow to the conventional boiler or hot water storage tank.

The following are key design attributes of the system installed:

• Use of existing water heating system infrastructure;


• Quick real-time response due to low solar piping mass and instantaneous heating design;


• Fewer square feet of collector area per therm delivered due to higher efficiency vacuum collectors;


• Array held on roof with ballast system, avoiding any building penetrations for support that would compromise roof integrity;


• Only penetrations to and from the roof for solar water supply and return pipes were through existing openings;


• Each of six array locations ties piping directly to hot water re-circulation loop in the shortest possible distance, minimizing heat losses;


• Multiple collector arrays connected to separate hot water risers, expanding potential for solar savings;


• Tube collectors and mounting hardware all boxed and easily transported to roof for assembly; and


• Freeze protection assured by pulsed flow.

The Pentagon Thermomax solar system is the largest thermal system of its type in North America. Overcoming the physical challenges of working in a 60-year old historic building was tougher than complying with substantial security requirements; but Capital Sun created an excellent working relationship with Pentagon staff and other contractors to smooth the installation process and hold down costs. Thermo-Technologies will be monitoring system performance for two years until July 2005. The Pentagon facilities personnel will also be monitoring the solar system with their own parallel instrumentation. Further information can be obtained from the Pentagon Energy Management Office.

Capital Sun Group completed a similar system at the Mid-Atlantic Social Security Administration Service Center in Philadelphia in January 2004 that employed 360 Thermomax tubes in two 180 tube arrays. The same instantaneous solar heating design was successfully employed. Again, Capital Sun partnered with ThermoTechnologies, Inc., under a GSA contract.