SOLAR THERMAL ROOFING
FOR YEAR ROUND AND SEASONAL HEATING AT THE PENTAGON
John P. Archibald
American Solar, Inc.
8703 Chippendale Court
Annandale, VA 22003-3807
ABSTRACT
The Pentagon Reservation contains several buildings that
support the operation of the main building and other agencies in
the Washington, DC area. One of the buildings operates a
loading dock for the receipt of materials. A nearby building
supports an emergency generator to provide backup power in the
event of a power outage.
The paper will discuss the design and construction of two solar
thermal tile retrofits at the Pentagon in Arlington, VA. The
discussion covers air handling systems, thermal storage
installation, hybrid heat recovery and the design and
construction of the solar thermal tile retrofit for heating an
emergency generator
1. HEATING NEEDS DOMINATE BUILDING AND
INDUSTRIAL ENERGY USE
Approximately 60% of all residential, commercial, and
industrial energy use in the US is for heating buildings and
industrial processes. See Fig. 1. This onsite use of energy is
most often provided by conventional heating technology such as
burning natural gas, fuel oil, or propane, or by electric
resistance or heat pump heating equipment. The largest single
energy use is for space heating, followed by water heating, then
process heating.
1.1 Space Heating
Building space heating for occupant comfort is essentially an air
heating load, where the goal is to provide warm air next to the
inhabitants. Solar air heating systems hold promise to satisfy the
demand without conversion losses often associated with more
conventional heating sources. Often these conversion losses
involve the use of different heating media, such as water heating
in boilers, as an intermediate process in hydronic based radiator
or radiant floor heating systems.
Where water based systems are used for space heating, the most
visible conversion of energy is at the boiler or water heater. At
this equipment, the fuel is often visibly burned and in many
cases gas or electric meters may measure “energy” consumption.
However, “energy consumption” measured at the burner is often
twice the amount of energy required simply to heat air in the
occupied spaces. Exhaust losses up the stack, line losses from
the heated water pipes running through unconditioned spaces,
duct losses from air escaping in the middle of the run, excessive
heating of building envelopes from radiators or radiant floors,
excessive stratification of heat between ceiling and floor in
occupied spaces, and boiler or heater radiation losses at low
firing rates can reduce heating system efficiency within the
building to less than 50% when measuring the actual energy
consumed.
If electric heating energy is considered, a low 30%-40% power
plant conversion efficiency must also be taken into account. If
electric resistance heat is used, the true energy delivery for space
heating may be on the order of 20% to 25% of the energy burned
at the power plant. Space heating by heat pumps extracts heat
from “sun warmed local air” to double or triple the productivity
achieved by electric resistance heat. Even so, heat pumps only
achieve about a 60% conversion efficiency compared to the
energy consumed at the power plant. Poor off peak performance
drops overall heat pump efficiency down to below 50% in many
moderately cold climates.
In comparison, solar heating technologies, both air and water
heating, deliver between 10 and 30 times as much heat energy as
they consume to run fans or pumps. If air heating is required,
solar air heating technologies are often the most productive and
least expensive energy conversion technologies compared to the
energy consumed in the process.
1.2 Water heating
If water heating is all that is required, usually solar water heating
technologies are the most productive energy conversion method.
Usually, the conversion from solar heated water-to-air or air-towater adds cost and reduces productivity. Conversion, via
thermal storage, to coordinate supply with demand throughout
the day, will also reduce efficiency and increase cost. Despite
the technical capability of a particular solar technology at
meeting an apparent demand, such as solar water heating for
hydronic space heating, often a simpler solution, direct air
heating, is a more productive and less costly approach. However,
when solar air heating is installed for winter space heating, a
small additional expense can fund the installation of solar air to
water heating components for year round water heating.
1.3 Industrial Heating
For industrial energy use, process heating and facility space
heating consume nearly twice as much energy as all other
industrial uses combined. The loads include an enormous variety
of processes from; drying of lumber, textiles, and paper, to
dehydrating air for pharmaceutical and candy production, to
boiling water for canning foods and sterilizing production
facilities. In many industrial applications, intermediate heating
media are used to achieve a desired heating effect, e.g., water is
boiled to steam to heat air to dry products.
1.4 Diesel Generator Standby Heating
One particular industrial heating application is the standby
heating of emergency diesel generators. Diesel engines rely on
the compression of air in the cylinders to reach the ignition
temperature of the injected diesel fuel. In warm climates, the
engines and the outside air will always be warm enough to allow
the compression to achieve fuel ignition. However, in most US
climate zones, cold outside air exists for enough hours of the
year that ignition can not be guaranteed. In the colder climates,
both the engine and the combustion air may need to be heated to
guarantee year round cold starts.
While air in the cylinders is what needs to be heated, a
convenient approach to providing this heated air is to use an
intermediate fluid, the cooling water that “jackets” the cylinders
of the engine (jacket water). The most common approach is to
install electric heaters in the jacket water circuit. These heaters
are typically electric resistance type cartridge heaters. For large
engines, (>1,500 KW) the fluid may be pumped through the
engine. For small and mid sized engines, the heater is usually set
up in a convection loop beside the engine. Heating of the jacket
water in the cartridge heater causes it to rise up in the cylindrical
cartridge and flow out through a pipe connected near the top of
the engine. Within the engine, this warm water flows through the
cylinder jackets to a pipe connected near the bottom of the
engine, and then back to the bottom of the cartridge heater.
Typically these heaters are set to maintain 130 degree F water
temperature leaving the heater. Water returning from the engine
is typically about 120 F. Even with the system properly
installed, the engine is not evenly heated. Typically, the engine
“block” will show increasing temperatures from bottom to top,
with the bottom of the engine at ambient temperatures as low as
O F in cold climates and the top of the engine at 80 to 90 F.
Similarly, the engine temperature decreases rapidly with
increasing horizontal separation from the heater supply and
return lines.
In general, the heaters seem to be sized at about 1 KW of heater
capacity for each 100 KW of engine capacity. The heaters may
only operate at their rated capacity during the coldest winter
conditions. However, the electricity use during warm summer
conditions can still be quite high, at several KW during 70
degree weather. Annual energy consumption for mid sized
generators will be about 17,000 kilowatt hours per year in
moderate climates. In some generator enclosures, additional
heaters are also installed for fuel tanks and fuel lines, for space
heating and battery heating. Where such heaters are installed in
outdoor enclosures, annual heating loads can exceed 25,000
kWhr/yr. The heating elements may cost only a few hundred
dollars to install, but cost thousands of dollars per year to run.
While jacket water heating is almost universal, air heating
can actually be less expensive and more productive. Air heating
provides:
 both a heated engine and heated combustion air,
 more even heating of the engine,
 greater thermal storage in the mass of the engine and
generator and enclosure to better respond to temperature
changes,
 heating of the fuel and battery systems, and

heating of the operating fluids such as oil in the sump
which improves lubrication during startup.
2. SOLAR SPACE HEATING OF A LOADING DOCK
One of the buildings at the Pentagon operates a loading dock for
the receipt of materials, primarily during early morning through
the afternoon. Trucks back to the two overhead roller doors to
discharge materials. To the sides of the roller doors are air intake
louvers which permit air flow through the loading dock as a
supply air source to the industrial process within the building.
Even with the loading dock doors closed, outside air can flow
through the loading dock via the louvers. Despite the use of
loading dock door seals, winter time operations are at close to
outdoor ambient temperatures. Operators also complain that the
exhaust from the overhead gas unit heaters is very localized at
the center of each roller door, and is too hot when working off
the back of a truck bed.
The Pentagon Energy Office has contracted for the installation
of solar thermal tile air heating system on the roof of the
building (See Figs. 1 and 2). The solar heated air from this
system will be delivered to the perimeters of the loading dock
doors. Excess heated air at the top of the loading dock will be
returned to the solar roof for re-heating and re-delivery to the
dock doors.
within the building. Hot air from the solar tile system heats the
water in a coil in the air handler. The hot water is circulated to
the storage tank during sunny weather. Hot water is drawn from
the tank to the coil in the air handler when solar hot air heating is
not available in the early morning or on cloudy days.
A separate water to water heat exchanger has been designed into
the system to circulate hot water to the tank from the industrial
process. This hybrid system will increase the availability of
stored heat in the tank even when solar heating is limited.
Fig. 1 Solar Thermal Tile System
To provide solar heat during the early morning hours, the solar
air heating system incorporates water thermal storage. The water
tank is heated via a hybrid system using solar heated air from the
Fig. 3 Air handler
The solar thermal roof is installed on a section of south facing
roof adjacent to the loading dock. The roof consist of 100 square
feet of Classic Slate solar tiles installed over corrugated metal
absorbers. Air moving under the tiles is drawn into the building
from a plenum at the east end of the roof section. Return air
from the building is ducted along the top of the roof to a plenum
at the west end. Supply and return air ducts penetrate the exterior
wall of the loading dock above the roller doors. An air handler,
inside the loading dock, incorporates 3 fans, water heating coils
and backdraft dampers to control the air flow.
Fig. 2 Solar thermal tile system installed
solar tile system and waste heat from the industrial process
The air handler for the system includes three equally sized inline fans and 4 backdraft dampers. The fans are rated at 100
CFM at 1” static pressure. They are designed to move 1 to 2
CFM per square foot of solar tile roof. Each fan consumes only
about 80 watts of power. In comparison, the system delivers
about 2,900 watts (10,000 BTU/hr) of peak heating thermal
energy. The supply fan draws air from the solar roof, and
Fig. 4 Solar heating emergency generators
exhausts across the water heating coils. The return fan sends the
coil exhaust air back to the solar tile system for re-heating.
Water is constantly pumped through the coils to store solar heat
in two 265 gallon water tanks. In the early morning, the tanks
deliver heat to the coils. By mid-morning, the solar roof supply
fan is delivering heat to the coils for storage in the tanks.
The door fan draws air from the return plenum, downstream of
the water coils, to feed the warm air ducts around the loading
dock doors. When the supply and return fans are not running, the
door fan draws air from the room, across the hot water coils. If
the door fan calls for heat while the solar supply and return fan
are running, the door fan will draw air from both the solar
supply fan and the room, across the hot water coils. The use of
spring loaded backdraft dampers balances pressures across the
fans and coils ensures air flow from the intended source with
minimal controls.
The system has been designed with a larger tank capacity than
required for thermal storage from the solar roof. The design has
incorporated the larger capacity to accommodate a second
available source of heat from an industrial process within the
building. Issues of funding for the second heating source have
delayed the installation of the heat recovery system. In the
meantime, the system is operated at reduced volume to ensure
adequate temperatures are maintained in the thermal storage
tanks.
The water heating system is designed to take water from the
colder tank bottom and feed it to the pump and the coils for solar
heating of the water. However, when the loading dock door fan
is started, to deliver heat to the loading dock, the water flow to
the coils needs to be reversed to deliver warm water from the top
of the “hot” tank. This is accomplished by two solenoid actuated
valves and a piping loop to the top of the hot tank. The two
solenoid actuated valves are wired in series with the loading
dock door fan. Whenever the door fan is activated, the source of
water from the tanks is reversed and hot water flows to the coils.
3. SOLAR HEATING OF AN EMERGENCY GENERATOR
Reliable electrical power is essential to ensure safe operation of
the industrial equipment inside the building. As a result, an
emergency generator is installed in an outdoor enclosure
adjacent to a small storage building. The shed roof of the storage
building faces south at a slope of 4 in 12. The generator is
located about 4 feet from the north side of the building.
The Pentagon Energy Office contracted for the installation of an
emergency generator solar heating system. The system
incorporates 100 square feet of Classic Slate Solar Tiles installed
on 6 foot wide by 17 foot tall section of the shed roof. The solar
tile system is the weather tight roof of the building. Supply and
return ducts from the roof are enclosed in a vertical trunk on the
north side of the building down to a fan enclosure near the
generator.
Solar heated air is drawn from the solar roof down to the fan
enclosure on the north side of the building. The fan enclosure
consists of two fans rated at 150 cfm at .75 inch water static
pressure for the supply and 79 cfm at .5 inch water for the
return. Total combined wattage is about 200 watts. The solar
heated air is blown about 3 feet across to the generator
enclosure, in a trunk above ground. The solar heated air is
discharged directly under the diesel engine. Warm air from the
upper part of the generator enclosure is drawn back to the roof
by a return fan. The return fan takes this air to the bottom of the
roof and discharges it into a plenum on the lower half of the
west side. From the plenum, air moves across the bottom half of
the collector to a plenum on the east side, then up through the
plenum to the top half of the roof. The air then moves across the
top half of the roof before being drawn into the supply duct and
fan. This serpentine approach was required to ensure that the air
move across more than 10 feet of collector surface to achieve a
proper temperature rise.
4. STRUCTURAL INTEGRATION WITH THE BUILDINGS
Both solar roofs are attached to the main structural walls of the
building and rest on top of the existing standing seam metal
roofs. In the case of the storage building, the structural members
that tie the roof to the walls, actually penetrate the existing roof
to attach to the walls. The weather tight solar tile roof
completely covers the old existing roof from ridge to eaves so
the structural openings are of no consequence to the weather
integrity of the building.
Both of the solar roofs installed at the Pentagon, use the side of
the roof as supply and return plenums. The generator roof has
over 34 feet of side (17’ east and 17’ west). The industrial
building has only 10 feet of side (5’ east and 5’ west by 20’
long)
One disadvantage of the supply and return plenums at the side is
that the side edge leakage can have a greater impact on delivered
temperature than with supply suction at the center of the tile
field. However, very simple sealing of the tiles at the roof edge
greatly reduces the leakage. Sealing is accomplished by caulking
with a small volume of high temperature sealant in the triangular
openings below the tiles and above the drip edge.
Shortly after completion of the exterior portions of the solar tile
systems, the Washington DC area experienced the 5 th largest
snowstorm ever recorded. Approximately 18” of snow fell over
a 2 day storm. Snow accumulated on both roofs. Within 36
hours, the snow slid off of the storage building solar roof. The
system operated on automatic control again on the first partly
sunny day following the storm.
On the industrial building, the snow also accumulated on the 50
foot slope of the standing seam metal roof above the solar tile
system. Over the next several days snow banked up above the
solar tile roof and very gradually slid up and over the solar tile
roof section. Melting snow continued to flow under the solar tile
system on the existing metal roof. After a week, there was no
damage to any of the structure, tiles, or trim. Final connection of
the mechanical systems in the building were still in progress
when all snow had melted.
6. MONITORING PERFORMANCE
The Emergency Generator system was monitored from 1/27/03
to 2/18/03. A current meter was placed on the 115Volt AC line
at the circuit breaker feeding the combined electric cartridge
heater and the battery charger. A temperature data logger was
installed to monitor the return water temperature to the cartridge
heater and the outside temperature. A current meter was also
installed to monitor the operating hours of the supply and return
fans.
In the case of the industrial building, the solar tile roof floats
over the existing standing seam roof, with no penetration of the
weather tight envelope. In this case, rainwater and snow melt
flow under the solar tile roof.
During this period, the weather in Washington remained
unseasonably cold and outdoor temperature varied between 50F
and 25F. The power draw on the heater cycled between full on
and full off. Peak current was 13.8 amps. Average hourly
current for the heater varied between 8.5 amps and 12.5 amps.
Only during the weekly 30 minute generator tests did the current
draw drop below this range as the engine achieved normal
operating temperatures. The electric current rises and falls in
opposition to the changes in the ambient temperature.
5. INITIAL OPERATIONS
Previous solar roofs have taken supply air from the center of the
tile field and introduced outside air at the sides to the tile field.
The return water temperature to the electric cartridge heater
remained between 92 and 80 degrees F with occasional
excursions up to 109 F during the engine tests. The return water
Temp top of radiator
120
100
80
60
40
20
Temp return to heater
Heater current
Ambient Temp
16
14
12
10
8
6
4
2
0
257
209
193
177
161
145
129
113
97
81
65
49
33
17
1
241
Daylight
0
225
Temp Deg F &
Lumens/sq ft
140
Current (amps @
115VAC)
Pentagon EDG monitoring
hours start 1/27/03 14:00
Fig. 5 Monitored emergency generator
temperature varied as the ambient temperature varied. In
contrast, the temperature of the water hose at the top of the
radiator varied in opposition to the ambient temperature
changes. This occurred because the colder ambient temperature
increased heater average power, which increased the temperature
of the water at the top of the jacket water circuit, which is the
top of the radiator.
In February, the solar tile system for the emergency generator
begins automatic operation of the fans, on a clear sunny day at
about 10 AM. It continues to run until about 6PM, providing 8
hours of collection with a differential thermostat setting of 16
degrees between the roof temperature and the sensor strapped on
the top of the radiator hose. Mid day temperatures of the air
discharged from the duct below the diesel engine were about 40
degrees F above the ambient temperature at the generator.
These temperatures were achieved before any edge sealing of
the solar tiles and with no insulation of the fans or supply and
return ducts in the fan enclosures. Both are planned after testing
of the roof discharge temperatures during warmer weather.
7.
CONCLUSIONS
Diesel electric heater energy use is proven to vary with
outside ambient temperature.
Solar air heating of the enclosures can raise the temperature
within the enclosure and directly adjacent to the diesel engine.
The solar thermal tile air heating systems are currently
delivering solar heated air to emergency generators.
The solar thermal tile system has been demonstrated to
provide a weather tight “solar roof” over a storage building and
provide solar heated air to an adjacent emergency generator.
Solar thermal roofing technology has been designed and
installed to provide space heating for a loading dock. Thermal
storage via air to water heat exchange and water storage have
been installed. A hybrid thermal storage system using heat
recovery from industrial process has been designed into the
installed system.
8.
REFERENCES
1.
American Solar analysis of US Department of Energy,
Energy Information Administration data from
Residential Energy Consumption Survey, Commercial
Building Energy Consumption Survey and
Manufacturing Consumption of Energy.
2.
Archibald, J.P., 1999, Building Integrated Solar
Thermal Roofing, Solar 99 Conference Proceedings of
the ASES Annual Conference
3.
Archibald, J.P., 2001, Design And Construction Of
Solar Thermal Tile Systems For Stand-By Heating Of
Emergency Diesel Generators, Forum 2001Proceedings
of the ASES Annual Conference
ACKNOWLEDGMENT
The author would like to acknowledge the support of the
Pentagon Energy Office and Defense Protective Service during
the construction and initial operation of these solar thermal tile
systems.