Thermal and Electrical Energy Generation

1
Thermal and Electrical Energy Generation in Cold
Climates Using Low-Cost Wood Stoves This
presentation overviews the principles behind the
heating system I am working on, intended mostly
for cabin use but expandable to nearly any size
application. The wood stove principles we propose
to implement have been known for hundreds of
years, and many modern heating systems employ the
same hydronic principles. This stove is
primarily designed for small cabins (that heat primarily with wood and have no
electrical connection to a central power grid.
There are currently few, if any, wood stove
systems like this on the market, largely because
this market is small and dwindling. However, the
system we propose could be integrated into
a typical fossil fuel based hydronic system as a
backup, providing both heat and pump
power. Similarly this power generation technology
has been used for dozens of years, since the
early NASA satellites. These thermoelectric
generators are now available as off-the-shelf
products, and are composed of thermocouples
operating on the Seebeck effect. Their unit cost
compares reasonably well compared with mid-winter
costs of solar panels production, but it remains
to be seen whether a complete value-added package
will remain economically competitive.
2
Typical Wood Stoves
Wood stoves are typically operated in a slow burn
mode, with low air flows. This allows burns to
last all night, but also causes low efficiency
and high soot.
3
Hot fires, created by allowing more oxygen into
the fire box, extract more heat from the wood but
much of this heat is lost up the flue or is
dumped into the house, causing large temperature
swings.
4
Wood Stove Requirements What is needed is a means
to burn very hot fires, recirculate flue gasses
through the flame to ensure complete combustion,
and to extract as much heat as possible from the
flue without causing condensation or downdraft,
as well as not turning the house into a sauna.
5
The system we propose does all this, hopefully at
a cost that is not prohibitive.
6
Wood Stove Operation
7
Wood Stove Operation
8
Wood Stove Operation
9
Wood Stove Operation
The system needs to be operable without
electricity in case of pump failure, but without
allowing the liquid (likely a glycol mixture) to
boil in this case. Rather than employ a drain,
we will test the use of a second flue for the
heat extraction, controlled using a damper. This
system can also be used in a slow burn mode by
limiting incoming air flow.
Front view
10
Wood Stove Operation
The main flue will be equipped with a heat sink
and thermostatically controlled fan such that the
maximum heat possible can be extracted from it as
well, such as when trying to heat a frozen house.
Front view
11
Thermoelectric Generators (TEGs)

• Composed of thermocouples (72 in this case)
• Generates electricity when exposed to a
  temperature gradient
• Retail price 150/20W (TEG only)

12
Thermoelectric Generators
Individual units generate low voltages, but can
produce approximately 20W in normal operating
range (dT200C, max T 250C). Therefore several
units must likely be combined to both power pumps
and fans and still generate enough power for
external loads. A DC-DC charge controller is
needed to maintain 12V-13V (depending on
application).
13
Thermoelectric Generators
TEG Design Possibilities - Contact with hot
air - Contact with hot metal Note TEGs must
always have cool side to maintain a temperature
gradient.
14
Hot Gas Applications Remote locations often use
TEGs to generate power by burning a stored gas to
create the heat source needed. It is conceivable
that we can put the TEGs inside the stove (either
fire box or smoke chamber) such that they have a
direct connection with the hot gas. It is
unlikely that these locations will be chosen,
because it is difficult to control temperature
here and difficult to providing cooling.
15
Hot Metal Applications Stack Robber. This unit
employs two thermoelectric generators with a fan
for cooling. This is likely the simplest method
for energy extraction, and has the advantage of
being an easy retrofit for existing stoves. It
is also relatively easy to provide cooling and
maintain an appropriate temperature gradient. We
will focus on initial efforts and testing using
this method.
16
Hot Metal Applications Stove tops can serve as
the hot side of the thermoelectric generator.
The major design problems here are maintaining
the coupling as the metal expands and taking up
useful space on the stove.
Our wood stove has plenty of surface area for
TEGs, which can affixed tightly without reducing
surface area useful for other purposes (such as
cooking).
17
Hot Gas Applications Another design location is
affixed to the hot water tank. The advantage
here is the temperature difference is maintained
for a longer duration than in the wood stove.
The water or glycol temperature, however, is only
about 100C warmer than room air temperature,
decreasing power generation.
18
Hot Gas Applications However, by locating the
water tank appropriately, the TEG can use the
exterior air for passive cooling, easily
providing a 200C temperature difference in
Alaskan winters.
House Exterior
19
Hot Gas Applications Similar, by locating the
wood stoves incoming air connection
appropriately, this air can be used to cool the
TEG, eliminating the need for a cooling fan and
increasing electrical efficiency while the stove
is in use.
House Exterior
20
ThermoElectric Generators How much power is
useful? Off-the-grid homeowners are typically
employ the lowest power technology. A 1000 ft2
cabin can currently be very well lit using ten
1-watt LED lightbulbs. Therefore 15 watts of
power generation can provide all of lighting
power needs. In mid-winter, a 50 watt solar
panel may only be producing 1 or 2 watts at peak
insolation. Such panels cost 250 typically,
meaning that it would take 10 or more such panels
(2500, or 50/watt) to generate the equivalent
of the TEG system we propose (20/watt). Further
the solar panel equivalent would essentially be
generating unused power in summer when only two
or three panels are needed due to abundent
sunshine.
21
Outcomes
We expect to prototype and test two commercially
viable products - A high efficiency wood stove
(1500 retail) - An electrical power generation
device (20/watt) We do not anticipate selling
and marketing these ourselves and do not believe
there are patent possibilities. We do intend to
market and sell the plans to build these devices,
either privately or through organizations such as
the Cooperative Extension.
22
Thank you for your attention.