SOLAR HEATING:
POLICY, TECHNOLOGY, AND CONSTRUCTION FOR SUCCESSFUL PROJECTS
John P. Archibald
American Solar, Inc.
8703 Chippendale Court
Annandale, VA 22003-3807
ABSTRACT
On a national level, lighting, cooling, electronics and motor
loads make up one third of all the energy used in buildings
and industrial facilities. The remaining two thirds of the
energy load is for heating needs. This simple fact draws to
mind a series of questions: Where should energy managers
to put the greatest resources to improve energy efficiency,
heating or non-heating loads. If heating is the largest load,
why not apply cost effective solar heating to the loads? How
can facility energy managers use reliable solar heating
energy to cut energy use in their electric, gas, fuel oil and
propane loads. Can solar heat provide other benefits such as
improved indoor air quality or reliability?
There are several techniques to apply solar energy to new
and existing buildings that can:
 Cut energy use
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




Improve reliability
Improve profit margins
Provide quick paybacks
Improve indoor air quality
Provide additional building space
Provide air conditioning load reduction
Provide environmental benefits to the company
Improve regional and national energy independence
The paper will discuss how solar heating techniques have
evolved since the 1970s to become reliable, attractive, and
high performing systems. Specific examples of the solar
thermal tile systems used for building and facility heating
will be provided, including heating and air conditioning
buildings at the Pentagon. Lessons learned in developing
successful solar heating projects will be provided.
Quads of Energy Use
Low Temperature Heati ng Do
Resi denti al , Commerci al , and Ind
20
Industrial End Uses
15
Heating End Uses
10
Residential End Uses
5
0
Commercial End Uses
Clothes
Total
Space Indus.
Various small
use
RefrigLighting
All
Boiler
Drying
Process
all eration/
heat
TV, ventilation,
other
fuels
heat
motors,office
equip
Process
uses
Air
W ater - use not
end
cooking
Cooling
specified
Condiheat
uses
tioning
Figure 1 Heating energy use dominates
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 Figure 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.
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 this energy demand without the
conversion losses often associated with more conventional
heating sources. See Figure 2. Often these conversion losses
from conventional heating technologies 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. Fuel is burned, to heat water,
to heat air next to the inhabitants.
Energy Delivered / Energy Consumed
Sola r Heating System Energy De livery
3 to 4 times more energy delivered than f rom
highest ef f iciency f ossil/electric systems
4
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. When heat
pumps are used for space heating, they extract 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 during weather below 60 degrees F 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. Even
when energy losses from the power plant and building
distribution systems are considered, the performance of
solar heating systems is between 3 and 12 times higher than
conventional sources. 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.
hi gh e ffi ci ency
3
norm a l effi ci ency
2
1
0
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 vs. energy delivered.
Electric Oil & Gas Electric
Heat Furnace Resistance
&
Pump
Water
Heat
Heater
Energy
Solar
Heat
Data S ources: Int'l
Agency, PNNL, NREL
Figure 2 Solar heating vs. conventional heating efficiency
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
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-to-water adds cost and reduces
productivity. Conversion, via thermal storage, to match the
timing of energy supply with energy 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. Usually
water heating systems are only sized to meet the daily water
heating load, which is much smaller than the space heating
load. Adding capacity to the solar water heating system can
provide some space heating capability, but it is generally
more expensive than adding a solar air heating system.
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.
Diesel Generator Standby Heating
One particular industrial heating application is the standby
heating of emergency diesel generators. Diesel engines rely
on the heat of 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 usually 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 these 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, but high cost 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 degrees 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 for battery
heating. Where such heaters are installed in outdoor
enclosures, annual heating loads can easily 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. --- Over the life of the generator, the heating
energy costs may add 50% to 60% to the installed cost of
the system.
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.
SOLAR HEATING PROJECTS AT THE PENTAGON
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 following sections discuss two successful projects
installed to provide solar air heating of the space and
equipment.
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.
Figure 3 Solar Thermal Tile System
To improve operating temperatures, overhead, gas fired unit
heaters are installed. Operators complain that the exhaust
from the overhead 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 Figures 3 and 4).
The solar thermal tile system is an evolutionary solar
heating system that improves on the air heating systems
developed in the 1970s. Unlike the old style flat plate
collectors, the solar tile system combines solar heating and
roofing technologies to create an aesthetically pleasing, cost
effective heating and roofing system. The use of solar air
heating, instead of the more well known solar water heating
systems, fits well with a number of heating and cooling
needs including; space heating, heat pump heating, boiler
air heating, and desiccant air conditioning.
The solar heated air from the loading dock system is
delivered to the work area at the center of each loading dock
door. 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.
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 solar tile system and waste heat from the
industrial process 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.
The system has been designed with a larger tank capacity
than required for thermal storage from the solar roof. This
larger capacity has incorporated to accommodate a second
available source of heat from an industrial process within
the building. The solar heating system will heat 130 to 170
gallons. The full 530 gallon capacity will be used when the
heat recovery system is installed. 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.
Figure 4 Solar Tile System Installed
The solar thermal roof is installed on a section of south
facing roof adjacent to the loading dock. The roof consists
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. The air handler inside the
loading dock includes three equally sized in-line fans and 4
backdraft dampers to control air flow. 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 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 early-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.
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 a
single fan in an enclosure on the north side of the building.
The fan is rated at 150 cfm at .75 inch water static pressure
for the supply and 79 cfm at .5 inch water for the return.
Supply fan running power use is about 53 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 roof. The generator enclosure provides
very little containment of heated air due to the ventilation
openings on all doors and the elevation above the
foundation slab of the entire generator skid and enclosure on
shock dampers.
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
Figure 5 Emergency Generator Solar Heating Schematic
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.
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.
INITIAL OPERATIONS
The Emergency Generator System began operation in
February of 2003. The Loading Dock system began
operation in April of 2003. Both systems are working within
specs to deliver solar heated air to the loads.
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 turned on under automatic
control on the first partly sunny day following the storm.
the solar air duct below the generator and jacket water and
ambient temperatures.
On the industrial building, the snow also accumulated on
the solar tile system as well as 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, in accordance with
the design, 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. There was no impact on the systems
performance when operation began in April.
During the beginning of this period, the ambient
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 supply temperatures from the solar tile
system.
MONITORING PERFORMANCE
The return water temperature to the electric cartridge heater
Figure 6 Solar heat vs. generator heater current
remained between 92 and 80 degrees F with occasional
excursions up to 109 F during the engine tests. The return
water 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
temperatures caused an increase in heater average power,
The Emergency Generator system was monitored from
January through April. See Figure 6. A current meter was
placed on the 115Volt AC line at the circuit breaker feeding
the combined electric cartridge heater and the battery
charger. Temperature data loggers were installed to monitor
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 typically 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 40+ degrees F
above the ambient temperature at the generator
corresponding to an approximate 150 CFM of air flow.
Maximum temperatures during early spring testing were
around 120 F.
These temperatures were achieved before any
improvements were made to fan air flow and with no
insulation of the fans or supply and return ducts in the fan
enclosures. Both adjustments are planned before final
turnover of the system. The modifications are expected to
increase temperatures to about 50 to 60 F above ambient,
leading to peak summer temperatures in the preferred range
of 140-160F.
An additional modification of the duct work will be tested
to determine the effect of converting the return air path to a
second supply air path. Since the generator enclosure
provides little containment of the warm solar heated air at
the top of the enclosure, the return air temperatures are close
to ambient and provide little temperature boost when
delivered back to the solar roof. An alternative approach is
to reverse the flow in the return air duct and use a single
supply fan for both ducts. This will reduce the air flow rate
over the collector and boost delivered air temperatures for
the same air volume flow. Expected air temperatures with
the revised duct configuration are in the range of 65 to 75 F
above ambient. Solar Thermal Tile Systems can generate
heated air at temperatures as high as 100 degrees F above
ambient temperatures and in most climates the maximum air
temperature can reach 200 degrees F on a hot summer day.
RECENT DEVELOPMENTS
The Pentagon Energy Office has recently contracted for the
installation of a solar thermal tile system that will use solar
heated air to heat and cool a guard station. This will be the
first deployment of a new solar air conditioning system
which uses solar heated air and desiccant and evaporative
cooling to deliver cool dry air from a solar roof.
CONCLUSIONS
Diesel electric heater energy use is proven to vary with
outside ambient temperature.
Solar air heating of the enclosures can raise the temperature
directly adjacent to the diesel engine and within the
enclosure and cut diesel electric heater use.
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 and is providing space heating for a loading dock.
Solar thermal storage via air to water heat exchange and
water storage have been installed and demonstrated.
A hybrid thermal storage system using heat recovery from
industrial process has been designed into the installed
system.
Solar thermal tile system operation immediately after
significant snowstorms has been demonstrated.
REFERENCES
1.
American Solar, Inc. 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.