The solar heating, air conditioning, power, and lighting system installed at the guard station of the Pentagon Heating and Refrigeration Plant is one of the most advanced solar energy systems in the United States. Despite its modest appearance, the system incorporates 12 advanced features that demonstrate new and improved ways of using solar energy to reduce fossil fuel and electrical energy use.

It demonstrates these new and advanced technologies in an unobtrusive way within the conventional construction of a small 400-square-foot building.

The guard station solar project incorporates the following 12 technologies:

1. Solar thermal tile air heating roof system.

2. Reflective roofing laminates and selective surface absorbers to boost solar roof air temperature.

3. High temperature, multi-stage solar roof with peak operating temperatures above 212 degrees F.

4. Photovoltaics (PV) beneath solar thermal tiles for electricity generation and heat production.

5. PV panels separate from the solar roof for grid independent power generation and operation.

6. Grid connected, off-peak, supplemental battery charging controlled by PV-sensing relays.

7. PV-powered cooling fans for PV temperature control, switch gear cooling, and solar roof heat recovery.

8. Desiccant dehumidification of outside air using solar "waste heat" in the summer.

9. Solar heat driven desiccant-evaporative cooling of outside air.

10. Solar pre-heating and pre-cooling of a heat pump to boost heat pump performance and cut electrical energy use.

11. Rainwater recovery from the solar roof to supply the indirect evaporative cooling stages.

12. Automatic winter tank drain-down to prevent freezing.

Many of these features have never before been demonstrated, such as the solar air heating tiles with PV absorbers below for simultaneous electricity and heat production in one weather-tight roof. The desiccant evaporative cooling system is also a unique development, since it relies on solar air heating to drive a desiccant air conditioning system. The high temperature summer airflow from the solar roof is an ideal energy source for the desiccant regeneration, which is accomplished with hot air. In the wintertime, the solar roof supplies heating energy to the guard station. The electric power produced drives the heating and cooling system fans and pumps throughout the year and provides security lighting at night.

Solar Heat And Electricity

The new use of photovoltaic materials in a solar heating system is made possible by the use of "air" as the heat transfer agent under the solar thermal tiles. Sunlight passing through the tiles hits the PV materials, which simultaneously generate heat and electricity. The electricity from the PV runs the heat recovery and cooling fans that collect solar heated air from the PV surfaces below the tiles. The electricity from the PV also energizes controls in the lighting and battery charging circuits. Placing the PV system below the tiles keeps the PV warm, which improves the electric generating capacity of the amorphous PV panels. The fans also keep the PV cool enough (below 180 degrees F) during peak summer conditions, to protect the panels from thermal damage. The PV deployed below the roof surface represents just 3 percent of the total roof collector area. The PV panel surface area contributes 68 watts of electric power and 345 watts of thermal air heating to the roof's peak summer heating capacity of 11,700 watts thermal (40,000 Btuh).

Solar Air Conditioning

The new solar-desiccant-evaporative air conditioning system reduces summer humidity levels of outside air and cools the air before supplying it to the guard station. The desiccant drying stage removes the humidity from the air. The dry air allows ultra-efficient evaporative cooling to take place even in humid climates from the mid-Atlantic to the Gulf coast. Because indirect evaporative cooling is used, no humidity is added to the air headed to the guard station. This aspect of the system demonstrates how outside air can be pre-conditioned before entering an existing building HVAC system, using "excess" solar heat in the summer. This is particularly important for buildings like laboratories or industrial facilities with l00 percent outside airflow and high energy use and cost in dehumidifying and cooling the air.

The solar-desiccant-evaporative system has reduced dew point temperatures by as much as 16 degrees and reduced dry bulb temperatures by 10 degrees F during a mid-day test in July. When minor adjustments are made to the water flow and airflow between stages, a 20+ degree drop in dry bulb temperature is expected. At peak performance, the existing system has demonstrated 3.6 units of cooling/dehumidification output for every 1 unit of electrical input and all of the electrical input from the utility grid is at night, during "off-peak" hours.

Solar-Assisted Heat Pump

Another advanced feature of the system is a modern update of an older solar heating technology that was conceived during the 1970s, but never commercialized. At that time, "solar-assisted heat pumps" were recognized as beneficial for cutting energy use by heat pumps. In the 1970s, heat pump technology was at an early development stage and showed marginal efficiency improvement from solar pre-heating. However, modern heat pumps have overcome those inefficiencies and can substantially reduce energy use with solar air pre-heating.

During cold weather, heat pump energy use can be cut by 35 percent or more with the addition of solar air pre-heating systems. In many cases, solar heated air from the roof or walls can be readily directed to the nearby rooftop or ground mounted heat pumps. Similarly, cooler air supplied to the heat pump in summer will cut electricity use by the heat pump in delivering air conditioning. The Pentagon system was designed to demonstrate how solar air pre-heating and pre-cooling of heat pumps can cut high electricity use in the winter and summer.

Direct Current Power

All electrical equipment in the system operates off of direct current (DC) power that is delivered at 24 volts DC to a battery bank within the building. The use of DC power instead of AC power saves energy in three ways. First, it eliminates conversion losses from converting DC to AC power in an inverter. Second, the external rotor DC motors in the fans use about one-third of the power of comparable AC motors moving the same amount of air. The third reason DC power saves energy is related to the use of peak demand reduction during the summer cooling season.

The PV system is sized to supply all the power needed during the winter months. During the peak air conditioning season in the summer, the solar-desiccant-evaporative system will often consume more power than the PV panels can generate. The batteries provide the necessary capacity to operate the solar-desiccant-evaporative system throughout the day. When the sun sets, the PV system activates a "110 Volt AC to 24 Volt DC" battery charger that brings the batteries up to full charge during the nighttime hours. This hybrid battery charging approach makes the maximum use of the PV output during peak electric demand and shifts the grid connected battery charging to an "off-peak" period when electrical demand on the utility grid is lower.

Rainwater Recovery

The Pentagon system collects rainwater for the evaporative cooling stages because the guard station's remote location has no ready source of water. Rainwater recovery was actually the lowest cost option, since installation of a "city" water system would have required hand digging 200 feet of trench over other utility lines buried under asphalt. However, the rainwater system offers other benefits, such as reduced storm water runoff from the roof, and reduced consumption of "city" water. The PV system provides automatic pumping for the evaporative cooling stages and drain down of the storage tank for winter freeze protection.

Lessons Learned

One of the lessons learned from this project is that the multi-staged roof and external PV panels will not be required in future versions. Only a single tile roof surface, with PV panels integrated below the solar thermal tiles, is required to provide the necessary high temperature heated air and electricity for the heating, air conditioning, lighting, and power. Water heating and thermal storage for night time and cloudy day use can be easily accommodated with an air-to-water heat exchanger.

The project was initiated in 2003 and the system began automatic operation and testing in the summer of 2004. Dr. Get Moy, director of Installations Requirements and Management for the Office of the Under Secretary of Defense (Installations and Environment), said, "I am excited that the Pentagon has demonstrated the successful application of these advanced energy technologies, where they will be visible to energy users across the Department of Defense and the federal government."

Reprinted from FEMP Focus, Winter/Spring 2005, a publication of the office of Federal Energy Management Programs, U.S. Department of Energy. For additional information contact Terri Robertson, Pentagon energy manager, at 703-695-8004 or John Archibald of American Solar Inc. at 703-346-6053 or download the Summary Report at

Publication date: 05/16/2005