A Natural-Gas-Fired Thermoelectric Power Generation System
This paper presents a combustion-driven thermoelectric power generation system that uses PbSnTe-based thermoelectric modules.The modules were integrated into a gas-?red furnace with a special burner design.The ther-moelectric integrated system could be applied for self-powered appliances or micro-cogeneration.A mathematical model for the integrated energy system was established that considered irreversibilities in the thermal-to-electric energy conversion process.The electric power output and electrical ef?ciency of the system were simulated using the established model.A prototype system was developed and its performance was investigated at various operating conditions.Applicability of thermoelectric devices to self-powered heating systems was demonstrated.The thermoelectric integrated combustion system could provide the consumer with heating system reliability and a reduction in electric power consumption.The integrated system could also offer other advantages including simplicity,low noise,clean operation,and low maintenance.
Key words:Thermoelectric power generation,heat source,combustion,
modeling
INTRODUCTION
Thermoelectric power generation is a promising method for direct thermal-to-electric energy con-version.Thermoelectric generators have no moving parts,and are compact,quiet,highly reliable,and environmentally friendly.Thermoelectric devices can be integrated into combustion equipment and applied to self-powered heating appliances,auxil-iary power units for delocalized energy production and micro-cogeneration.1–5In these systems,fuel-?red heating equipment incorporates a power gen-erator to convert a portion of heat to electricity which drives the electrical components.Excess power can be used to charge batteries or be fed into the household grid.With this concept,the value provided to the consumer would be a combination of heating system reliability and a reduction in electric power consumption.
The thermal ef?ciency of the systems would be the same with or without thermoelectric genera-tion.In fact,electricity is generated at essentially
100%ef?ciency in combustion-heated thermoelec-tric systems since the heat dissipated from the power generation unit is used for space and water heating.
In the present study,we developed,constructed,and tested a thermoelectric integrated combustion system,with focus on system modeling,system development,integration,and optimization.A mathematical model for the combustion-driven power system was established.The in?uence of various parameters on power output and ef?ciency was examined.
MODELING
Model Description
The gas-?red thermoelectric energy generation system is illustrated in Fig.1.The burner consumes natural gas at a rate of Q fuel .The electric power P TE is generated by the two thermoelectric modules.The electrical ef?ciency of the system is de?ned as:
g ?
P TE Q fuel ?Q hl tQ hu Q fuel P TE
Q hl tQ hu
?g b g TE ;(1)
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where g b is the ef?ciency of the combustion heat source,Q hl and Q hu ,respectively,are the heat inputs to the lower and upper modules,and g TE is the thermal ef?ciency of the thermoelectric devices.If it is assumed that the burner surface and combustion products have the same temperature,T h ,and the overall heat transfer coef?cient includ-ing convection and radiation is K hl ,then the heat transfer from the heat source to the lower module may be calculated from the following expression:
Q hl ?K hl eT h àT 1l T;
(2)
where T 1l is the hot-side temperature of the lower module.Equation 2may also be expressed as:
Q hl ?g c Q fuel tQ air àK fl A E eT h àT 0T;
(3)
where g c is the combustion ef?ciency,Q air is the heat transferred to the combustion air in the recu-perator,K ?is the heat ?ux from the burner exhaust outlet,A E is the area of the burner exhaust outlet,and T 0is the ambient temperature.Equation 3can be written as:
Q hl ?K fl A E eT s àT h T;
(4)
with T s being the temperature of the heat source at
Q hl =0.T s is given by
T s ?
g c Q fuel tQ air
K fl A E
tT 0:
(5)
Heat transfer from the ?ue gases to the upper module may be calculated from the following expression:
Q hu ?K hu eT u àT 1u T;(6)
where K hu is the heat transfer coef?cient,T u is the ?ue gas temperature,and T 1u is the hot-side tem-perature of the upper module.Equation 6may also be written as:
Q hu ?K fu A E eT h àT u T;
(7)
with K fu being the heat loss ?ux from the exhaust ?ue gases outlet.
The equations governing the heat input and heat rejection rates for the two modules are obtained by considering the energy supply or removal to over-come the Peltier effect,the heat conduction,and the Joule heating 6:
Q hl ?a n I l T 1l tK n eT 1l àT 2l TàI 2
l R n
2;(8)
Q hu ?a n I u T 1u tK n eT 1u àT 2u TàI 2
u
R n
2
;(9)
Q cl ?a n I l T 2l tK n eT 1l àT 2l TtI 2
l R n
2
;(10)
Q cu ?a n I u T 2u tK n eT 1u àT 2u TtI 2
u
R n
;(11)
where K n is the thermal conductance of the ther-moelectric device consisting of n couples,a n is the apparent Seebeck coef?cient,R n is the electrical resistance of the thermoelectric device,and I l and I u are the electric currents generated by the lower module and upper module,respectively.The heat removal rates from the two modules to the cooling medium may be expressed as
Q cl ?K c eT 2l àT cl T;(12)Q cu ?K c eT 2u àT cu T;
(13)
where K c is the heat transfer coef?cient between the cold side and the cooling medium.The electric power produced by the two modules is obtained from the energy balance:
P TEl ?Q hl àQ cl ?a n I l eT 1l àT 2l TàI 2l R n ;
(14)P TEu ?Q hu àQ cu ?a n I u eT 1u àT 2u TàI 2
u R n ;
(15)
or
P TE ?P TEl tP TEu ?R L eI l tI u T2;
(16)
where R L is the load resistance.
For known or given thermal conductance,Seebeck coef?cient and electrical resistance of a thermoelectric module,heat transfer coef?cients,and fuel combustion properties,we can solve the above equations for power output and electrical
Q T T T N
N
1u
T Q T w w w
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ef?ciency at various operating conditions.First,we obtain the temperatures of the hot and cold junc-tions for the thermoelectric converters,and the heat input and rejection rates.Then,the power output and electrical ef?ciency are calculated.
Modeling Results
Figure2shows the variation of electric power output with the burner(heat source)operating temperature for the two thermoelectric modules. The electric power outputs of both modules increase markedly with the burner operating temperature. Figure3illustrates the electrical ef?ciencies of the power system as a function of the burner oper-ating temperature with and without heat recuper-ation.As expected,heat recuperation improves the electrical ef?ciency of the combustion-driven power system.There is seemingly an optimum heat source temperature.The reason is that,for given heat transfer conditions,the electrical ef?ciency of the thermoelectric device increases with heat source operating temperature,but the heat source ef?-ciency decreases with the temperature.These com-bined effects lead to an optimum heat source temperature.
EXPERIMENTAL
Two thermoelectric modules were integrated into a gas-?red furnace.Figure4shows the modules, one consisting of325thermoelectric couples.The thermoelectric elements of the couples are made from PbSnTe doped to have either p-or n-type properties(Fig.5).The burner is made of a high-temperature alloy and has a perforated structure. Natural gas burns in the burner to heat the inner surface wall(hot junction)of the thermoelectric module.The outside surface(cold junction)of the module is maintained at a low temperature by cooling water that circulates through a jacket.The two modules were arranged in tandem in the
Fig.4.Thermoelectric modules. w
.
z
h
u
l
o
n
g
system.The lower one has a ?at heat transfer sur-face at the hot side while the upper one contains a number of heat-conducting ?ns to enhance heat transfer (Fig.4b ).Various measuring and control devices,including temperature sensors,?ow meters,voltage/current analyzers,and a data acquisition system,were installed.The personal computer (PC)-based data acquisition unit collected signals from temperature sensors and gas ?ow me-ters and the outputs of the modules.The power outputs of the modules were obtained from the load voltage and the electric current or from the load voltage and the load resistance.Figure 6shows the experimental setup.
EXPERIMENTAL RESULTS
Burner Performance
The results of the burner combustion performance are given in Table I .The premixed gas combustion occurs in the burner chamber,producing hot gas-eous combustion products.The burner surface at temperatures of 950°C to 1050°C emits thermal radiation that heats the inner wall of the lower module effectively.Heat recuperation via air pre-heating was observed to affect combustion perfor-mance.
Electric Power Output
Table II illustrates the power output character-istics of the two modules in the combustion-heated power system when not using the power condi-tioner.The power output was observed to increase dramatically with module inner wall temperature.The electric power outputs reached 565.8W and 486.4W for the lower and upper modules,respec-tively.The electric power output of the upper mod-ule is somewhat lower than that of the lower module due to the more effective radiative heat transfer between the burner and the lower module.
The two modules generated 1052.2W of electri-city,which is enough to power all the electrical components for a residential heating system.Excess electricity can charge batteries or be fed into the household grid.The advantages provided to the consumer are both increased on-site energy security and a reduction in electric power consumption.This would provide an effective means of reducing greenhouse-gas emissions.Note that in practice the hot-side temperature of the module should be below 600°C in order for the PbSnTe materials to operate long term.
CONCLUSIONS
A thermoelectric integrated combustion system has been developed for self-powered appliances or micro-cogeneration that is capable of providing increased system reliability and a reduction in electric power consumption.A mathematical model for the combustion-driven power system was established that considers various irreversibilities
P-type leg
N-type leg
T h
T c
Fig.6.Experimental setup.
Table I.Burner Combustion Performance
Heat
Recuperation Combustion Load a
(W/cm 2)
Burner Surface Temperature (°C)
Temperature of Gaseous Combustion Products (°C)
No 53.29611078Yes 46.89701086No 59.810251136Yes
51.7
1033
1130
a
Combustion load refers to the burner surface area.
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in the direct thermal-to-electric energy conversion process.Modeling results show the in?uence of operating conditions on electric power output and electrical ef?ciency.The model would be useful in system design optimization.A prototype system was constructed and tested.In the prototype system,PbSnTe-based thermoelectric modules were inte-grated into a natural-gas-?red furnace with a spe-cial burner design.With the power system,good applicability of thermoelectric power generation to fuel-?red heating equipment was demonstrated.The thermoelectric devices in the integrated system generate approximately 1kW of electricity.The thermoelectric integration concept could offer many advantages and the potential for applications in certain situations.
REFERENCES
1. D.Allen and J.Wonsowski,The Annual Winter Meeting of the
American Society of Heating,Refrigeration and Air-Conditioning Engineers ,18–21January 1998,San Francisco.
2. D.Allen and W.C.Mallon,The 18th International Conference
on Thermoelectrics ,29August–2September 1999,Baltimore.doi.10.1109/ICT.1999.843339.
3.J.C.Bass and J.W.Thelin,The 19th International Conference
on Thermoelectrics ,20–24August 2000,Cardiff.
4. D.M.Rowe,Renew.Energy 16,1251(1999).doi:10.1016/
S0960-1481(98)00512-6.
5.K.Qiu and A.C.S.Hayden,J.Power Sources 180,884(2008).
doi:10.1016/j.jpowsour.2008.02.073.
6. D.M.Rowe and C.M.Bhandari,Modern Thermoelectrics
(London:Holt Rinehart and Winston,1983).
Table II.Results of Power Output Characteristics
Heat Recuperation Burner
Operating
Temperature
a
(°C)TE Module Hot-Side Temperature (°C)Cold-Side Temperature (°C)Open-Circuit Voltage (V)Load Voltage (V)Load Current (A)Power Output (W)No 959Upper
module 5477444.123.818.0428Lower module 6017949.825.919.2497Yes 1020Upper module 5717545.724.018.5444Lower module 6158051.026.620.0532No 1053Upper module 5917647.024.118.8453Lower module 6268252.227.120.2547Yes
1082
Upper module 5987849.525.619.0486Lower module 6388252.727.620.5566
a
Burner operating temperature is assumed to be the average between the burner surface temperature and the temperature of gaseous combustion products.
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