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Implementation of a TPV integrated boiler for micro-CHP in residential buildings
Applied Energy 134 (2014) 143–149
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Implementation of a TPV integrated boiler for micro-CHP in residential buildings K. Qiu ⇑, A.C.S. Hayden CanmetENERGY-Ottawa, Natural Resources Canada, 1 Haanel Drive Ottawa, Ontario K1A 1M1, Canada
h i g h l i g h t s A TPV integrated boiler for micro-CHP application is designed, tested and demonstrated. Thermal radiation was emitted by a porous emitter in the TPV unit. The electric output of four TPV cell modules connected in series is measured and characterized under various conditions. 246.4 Electricity is generated at the emitter temperature of 1265 °C. This study shows that TPV generation in boilers/furnaces is feasible for micro-CHP application in residential buildings.
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Article history: Received 10 January 2014 Received in revised form 14 July 2014 Accepted 4 August 2014 Available online 23 August 2014 Keywords: TPV Boiler Power generation Micro-CHP
a b s t r a c t There is a growing interest in direct thermal-to-electric energy conversion using solid state devices such as thermophotovoltaic (TPV) generators. TPV devices convert thermal radiation from heat sources into electricity without involving any moving parts. TPV opens up possibility for efficient and stand-alone power generation in boilers and furnaces. In this paper, a TPV integrated boiler was designed, built and investigated for micro combined heat and power (micro-CHP) application in residential buildings. A full size gas fired residential boiler was used as a precursor for integration with TPV devices. Experiments were conducted with the prototype TPV boiler so as to evaluate various issues related to this new technology. The electric output of TPV modules installed in the boiler was characterized under different operating conditions. The TPV cell modules generated 246.4 W at an emitter temperature of 1265 °C, which would be enough to power the electrical components of the whole heating system. Moreover, such a TPV integrated boiler could be employed to form a micro-CHP system in residential homes, providing an effective means for primary energy savings, on-site power and energy security. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
1. Introduction Thermophotovoltaic (TPV) generators convert radiant energy from heat sources such as combustion into electricity (Fig. 1). This energy conversion is achieved by means of photovoltaic (PV) cells. Low bandgap PV cells are preferred for TPV generators since the photons emitted from heat sources at temperatures of practical interest are distributed at lower energies in comparison to solar radiation. Low bandgap PV cells are referred to as TPV cells. The advantages of TPV systems include: (a) a high fuel utilization efficiency is achievable due to that the fact that the heat dissipated can be recovered on-site for space conditioning and water heating needs, (b) no moving parts are involved in the power generation,
⇑ Corresponding author. Tel.: +1 613 996 9516; fax: +1 613 947 0291. E-mail address: [email protected] (K. Qiu). http://dx.doi.org/10.1016/j.apenergy.2014.08.016 0306-2619/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
and (c) this is a relatively low maintenance technology. Based the above features, the TPV generator is well suited for micro combined heat and power (micro-CHP) applications in residential buildings. A combustion driven TPV generator usually consists of a heat source, a thermal emitter, a spectral control filter and TPV cell modules. The thermal emitter converts the combustion heat into radiation energy (photons) at appropriate wavelengths. The emitter could be a solid surface heated by flame impingememt, or a porous medium burner with combustion taking placing inside the porous medium. The TPV cells play a pivotal role in the heat energy to electricity energy conversion process. Cells made of low bandgap semiconductor materials, such as GaSb, are favored because these cells can convert more infrared radiation (low energy photons) from the emitter to electricity. To maximize thermal to electric energy conversion efficiency, a spectral control filter should be used. The filter is capable of passing the desired portion of broadband thermal radiation and reflects the non-convertible
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Emitter
PV cells
Heat sources
(a) Hydrocarbon combustion (b) Concentrated solar radiation (c) Waste heat (d) Nuclear sources
Infrared radiation
Filter
Infrared radiation
(a) Silicon carbide (b) Tungsten (c) Ceramics (d) Metal alloy
(a) Si (b) GaSb (c) InGaAs (d) Ge (e) InGaAsSb
(a) Selective emitters (b) Quartz shield (c) Dielectric films (d) Back-surface reflector
Fig. 1. Illustration of TPV energy conversion.
radiation back to the emitter to increase the overall system efficiency. A number of research groups have been developing cogeneration systems based on TPV devices [1–4]. Schubnell et al. [1] have analyzed the influence of various parameters including cell bandgap and flue gas temperature on the electricity conversion efficiency of a TPV central heating unit, however, they did not provide their experimental results. Horne et al. [2] presented a liquid fuel-fired TPV system for portable power applications where a resonant metal mesh infrared band pass filter was used, yet a TPV system with such a filter would be difficult to implement in practical applications. The investigators at JX Crystals Inc. reported on the development of a prototypical propane-fueled TPV stove, generating up to 100 W of electric power [3]. In a study by Nelson [4], a self-powered, gas-fired, warm air furnace was evaluated as a candidate for the autonomous generation of electric power. In addition to the above work, TPV systems were shown to have the potential for application in high temperature industries [5,6]. There have been other investigations that have examined the combination of TPV with other power generation approaches such as thermoelectrics [7] and low temperature Rankine cycles [8,9]. The reviews of TPV research [10–12] have shown that TPV systems have many advantages, including no moving parts, high reliability and the capability of converting radiation from a variety of heat sources directly into electricity. The same reviews have also shown that TPV systems have certain limitations, for example, the electrical efficiency of combustion-driven TPV systems developed to-date has been relatively low [3,12–14]. The major energy losses in combustion-driven TPV systems arise from the limited conversion of fuel to radiation energy and low fraction of in-band radiation. It has been suggested that heat recovery or recuperation from the flue gas exhaust could be an effective means of increasing TPV system efficiency [15–20]. TPV devices can suitably be integrated with heating equipment to form micro-CHP systems for use in residential buildings. Residential combined space/water heating equipment, such as fuel fired boilers, use both fuel for heat production and electric power to drive its electrical components. If a boiler utilises a TPV generator to convert part of the combustion heat energy to electricity that then operates its electrical components including a combustion fan/blower, pumps, valves, and controls, it becomes a so-called self-powered heating system. Any surplus electricity generated can be provided to other electrical loads within the home, realizing
the CHP concept in a residential setting. This provides a possible means for power supply, energy security for the home, and overall primary energy savings as well. Although a number of TPV systems were investigated [3,7,13–28], no work on practical or full-size TPV systems has been reported in the literature and little attention has been paid to an integrated system in the earlier investigations. In this paper, a TPV integrated boiler was designed, built and investigated for micro-CHP application in residential buildings. A full size gas fired residential boiler was used as a precursor to integrate with TPV. Recently-developed GaSb TPV cell modules were employed in the direct thermal-to-electric energy conversion system. Experiments were conducted with the prototype TPV boiler in order to address certain issues related to this technology, which must be resolved, before TPV can be applied in practice. While the development of a self-powered boiler was considered as a first target, upgrading the equipment to a micro-CHP system was also addressed. 2. TPV integrated boiler for micro-CHP in residential buildings 2.1. Concept description Fig. 2 shows the schematic diagram of the TPV power generation process in a gas-fired boiler. Natural gas is converted by a radiant burner into thermal radiation and the sensible heat of flue
DHW heat exchanger
Hydronic space heating
Diverter valve
Flue exhaust
Domestic hot water Electrical power output
Flue-water heat exchanger
Cold water Pump
Power supply Cooling system
Control system Electricity TPV converter
Heat
Photons Optical filters In-band photons
Gas burner
Emitter
Out-of-band photons
Blower Air Gas
Heat
Fig. 2. Schematic diagram of the TPV power generation process in gas-fired residential boiler.
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gases. A TPV converter generates electricity to power electrical auxiliaries of the boiler, including an air blower, a fan, water pumps, an igniter, valves and a control unit and surplus electricity is supplied to other electrical loads or is exported to the grid. The exhaust flue gases flow through a heat exchanger and heat is thus recovered for home space and/or water heating needs. A microprocessor is used in the system control unit, which is capable of initiating the air blower, the pump, the igniter, and the gas valve from a battery. The battery supplies the required electric power for a short period during the start-up process until the TPV generator is functioning. The battery will also power the pump to run for a certain period to avoid overheating of the TPV converter during the system shut-down process. The control unit controls the battery charging circuit, flame sensing and safety shutoff. The working procedure of the control system is as follows: (1) When the control unit receives a signal for hot water or space heating needs from a thermostat, it will activate switches to turn on the combustion air blower and the water pump. (2) The igniter will start and the fuel solenoid valve will be switched on. (3) Upon successful burner ignition and when the electricity being generated by the TPV exceeds the power being consumed by the boiler auxiliaries, the power supply will be switched to the TPV generator. (4) The surplus power generated can recharge the battery or be conveyed to other electric loads, and/or exported to the grid. (5) If the control unit detects any demand to shut-off the boiler, e.g. from the thermostat or safety controls, the fuel solenoid valve will be first closed, the power switched immediately from the TPV generator to the battery, and then the blower will be turned off. The water pump will continue running for enough time to prevent the TPV modules from being overheated.
(a) Burner assembly Exhaust gases
Heat Shield Emitter Combustion chamber Filter
Water jacket
TPV cells
2.2. Development of TPV integrated boiler design
Combustible
A full size TPV integrated boiler was designed and developed during the course of this work. A commercially available gas fired residential boiler was selected as a precursor to integrate a TPV generator. The boiler selected was a wall-mounted combi-boiler that serves as both a home space heater and domestic hot water provider. The power consumption of the boiler’s electrical auxiliaries was 150 W. The highest electrical consumption in the boiler was from the pump that circulates the water and the fan that evacuates the exhaust gases. The balance of the power consumed was due to the electronics that control the boiler system. A TPV burner assembly, as shown in Fig. 3, was devised for the integrated boiler. The TPV burner assembly consists of an air/gas premixing tube, a natural gas/air mixture plenum, a combustion chamber, porous thermal emitters, optical filters, TPV cell modules and a cell cooling system. The natural gas and air are well mixed prior to entering the combustion chamber and the gas mixture burns within the combustion chamber. The high temperature combustion products flow through the porous emitters, which are heated and produce thermal radiation. The optical filters reflects the non-convertible radiation, helping to keep the TPV cells cooler. The TPV cells then produce electricity from the infra-red radiation incident upon them. The electrical efficiency of a combustion driven TPV system is defined as the ratio of electric output power, Pel, to fuel chemical energy input, Qchem:
gTPV
Pel ¼ Q chem
ð1Þ
or
gTPV ¼
pel Acell Q chem
ð2Þ
(b) Cross section of the TPV burner Fig. 3. Design of the TPV burner assembly.
where pel is the cell power density and Acell is the cell area. Since the TPV cell modules were arranged very close to the emitter, the view factor can be assumed to be one. In this case, the system electrical efficiency may also be calculated from the efficiencies of thermal emitter, spectral control and TPV cells:
gTPV ¼ grad gspe gcell
ð3Þ
where grad is the radiant efficiency, gspe is the spectral efficiency and gcell is the cell efficiency. The radiant efficiency is defined by [13]:
grad ¼
Q rad Q rad ¼ Q chem mfuel DHL
ð4Þ
where Qrad is the total radiant power, mfuel is the fuel mass flow rate and DHL is the natural gas lower heating value. The thermal efficiency of the system is defined by:
gth ¼
Q heat Q chem
ð5Þ
where Qheat is the thermal output of the system. During TPV operation, unconverted heat must be removed from TPV cells to maintain the cell temperature below a certain level. If the heat is not dissipated effectively, the temperature of the cells will increase, resulting in a decrease in cell conversion efficiency. The hydronic loop return or circulating water driven by the pump is directed through a water jacket which dissipates excess heat from the TPV cells.
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Fig. 5. Thermal emitter made of SiC foam in the TPV burner.
Antireflection coating Front contacts Diffused p-InGaAsSb emitter (approx. 0.5micron) n-GaSb substrate Back contact
Fig. 6. Schematic representation of GaSb cell structure.
Fig. 4. Design of the TPV integrated residential boiler.
3. Experiments
External quantum efficiency
Fig. 4 illustrates the TPV integrated boiler configuration. The TPV burner assembly replaced the existing burner in the residential boiler and it was thus upgraded to a micro-CHP boiler. This system is capable of providing a home’s water and space heating needs as well as some independent power generation. Consequently, the value of the system to the end-user will be both heating system reliability and electricity (energy) consumption and cost reduction.
1 0.8 0.6 0.4 0.2 0 0.4
0.8
1.2
1.6
2
Wavelength, micron 3.1. Prototype system Fig. 7. Typical external quantum efficiency spectrum of standard GaSb cells.
A prototype TPV integrated boiler system was built and tested in order to obtain experimental performance. The TPV burner assembly (Fig. 3) replaced the old gas burner in the precursor boiler. The new burner design was based on the combustion theory of a fully stirred reactor. Natural gas flows at high velocities from the plenum into the combustion chamber to prevent flashback, where the combustion of the premixed natural gas and air occurs rapidly with high intensity, producing very hot flue gases. The hot flue gases then flow through the porous emitter, which is thus heated to incandescence (Fig. 5). The fuel to air ratio is adjusted to a near stoichiometric level in order to attain the highest temperature on the emitter. The thermal emitter is made of SiC foam, which is capable of withstanding the sufficiently high temperatures, as shown in Fig. 5. TPV cells employed in the prototype system are binary semiconductor GaSb cells whose bandgap is 0.72 eV at 300 K. This corresponds to a wavelength of 1.73 lm. The cell structure is depicted in Fig. 6. These cells are made from the wafers of p-GaSb while their p–n junctions are formed by the process of Zn diffusion. Fig. 7 illustrates the external quantum efficiency of GaSb cells. In practice, the cells are incorporated into a module or array. In this study, one GaSb cell module consists of seventy-two cells and each cell has an area of 2 cm2. The cells are electrically connected in series and parallel combinations. Four TPV cell modules have
Fig. 8. Two GaSb cell modules on water cooling device.
been installed in the present burner assembly. By symmetry, there are two cell modules on each side. Fig. 8 shows the two TPV cell modules (from JX Crystals Inc.) soldered onto the water cooling
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jacket. It is noted that since a broadband emitter was used in the TPV system, an optical control filter was needed to increase the spectral efficiency and minimise the level of the waste heat produced in the cell modules. The filter transmitted the desired portion of the overall thermal radiation energy while reflecting the non-convertible radiation. The filter also reduced direct convective heat transfer to the cell modules by preventing flue gases from being in contact with the cell modules. Fig. 9 shows the prototype TPV integrated boiler. Measurement and control devices were installed with the prototype system, including thermocouples, flow meters, digital voltage/current/power analyzers, gas analyzers and a data acquisition system. Thermocouples were used to measure the temperatures of the flue gas and the emitter. Eight thermocouples were installed on the emitter to obtain the average emitter temperature. Flue gas composition (O2, CO2, CO and NOx) was measured using paramagnetic, infrared, and chemiluminescent flue gas analyzers. The fuel flow rate was controlled and measured using a gas flow meter/controller. The air flow rate was monitored to calculate the excess air levels. The radiant power output from the emitter was measured using a radiometer (Land 2p Ellipsoidal Radiometer, Land Combustion Limited, UK). The data acquisition system (Campbell Scientific Inc., Canada) collected the signals from the measurement devices and monitored the output from the TPV modules. The readings were scanned and recorded every 2 s. The radiant efficiency was calculated from the measured radiant output and fuel input rate (Eq. (4)). There were uncertainties and factors that may affect the accuracy of the measurements in the experiments. The major errors arose from the ambient air temperature uncertainty, the temperature measurements using thermocouples, air and fuel flow rate measurements and radiant power output measurements. The maximum error or inaccuracy in the measurement results and calculated results, obtained from the method of error propagation, is presented in Table 1. The relationship between the heat dissipation by the water cooling jacket, the rise in cooling water temperature and the water flowrate is expressed as:
Table 1 Maximum error in measurement results and calculated results. Measurement parameter and calculated result
Maximum error
Fuel flow rate Air flow rate Temperature Radiant power density Cell electric power density Radiant efficiency
±2.5% ±3.0% ±4.0 °C ±3.5% ±2.0% ±4.0%
uw ¼
Q wh
ð6Þ
qw C p ðT out T in Þ
where Qwh is the dissipated heat (transferred to circulating water), uw is the volume flowrate of water through the cooling system, qw is the density of water, Cp is the specific heat of water, and Tout and Tin are the water outlet and inlet temperatures. The required heat dissipation can be estimated as follows.
Q wh ¼ Acell ½ð1 qcell Þptf pel
ð7Þ
where Acell is the cell area, qcell is the reflectivity of cell surface, ptf is the radiant power density incident on the cell modules and pel is the electricity density generated by cell modules. Our calculations show that the required heat dissipation or cooling load for one cell module is approximately 800 W. 3.2. Experimental results and discussion The burner assembly was first investigated with regard to its combustion performance. Fuel and air flow rates were regulated by mass flow controllers and the excess air was calculated. The temperature of the thermal emitter was changed by regulating gas flow rate and adjusting the excess air level. The emitter converts the heat released from combustion into thermal radiation. The emitter temperature was measured to be between 1078 °C and 1265 °C at the combustion heat input loads of 8.5–12.3 kWt, yielding radiant power outputs of 4.61 kWt to 5.90 W/cm2. The electric outputs of the TPV cell modules were measured in the prototype system at varying operating conditions and characterized. Fig. 10 shows the variation of short circuit current (ISC) and open circuit voltage (VOC) with emitter temperature for the four cell modules installed in the prototype boiler system. The four modules were connected in series. It was observed that the short circuit current increased significantly with increasing radiant
39
10
38.5
9
ISC , A
37.5
VOC, V
38 8
7 37 6
5 1050
36.5
1100
1150
1200
1250
36 1300
Emitter temperature, °C Fig. 9. Prototype TPV integrated boiler.
Fig. 10. Short circuit current and open circuit voltage of TPV cell modules vs. emitter temperature.
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Table 2 Power output characteristics for four TPV cell modules connected in series at different emitter temperatures. Emitter temperature, °C
Voc, V
Isc, A
Jsc, A/cm2
Vmpp, V
Impp, A
Impp, A
FF
PTPV, W
1078 1135 1231 1265
37.20 37.48 37.84 38.16
6.20 6.73 7.94 9.12
1.032 1.104 1.320 1.512
28.68 28.92 29.28 29.44
5.51 6.06 7.15 8.37
0.912 1.008 1.200 1.392
0.69 0.70 0.71 0.71
158.0 175.2 209.6 246.4
movers such as Sterling engines and internal combustion engines that have recently received attention for use in residential microCHP applications.
Table 3 Experimental and calculation results of TPV boiler performance. Fuel input, kW
Emitter surface temp, °C
Radiant efficiency, %
Electric power output, W
System electrical efficiency, %
System thermal efficiency, %
8.5 10.1 11.7 12.3
1078 1135 1231 1265
22.6 21.2 21.5 22.1
158.0 175.2 209.6 246.4
1.86 1.73 1.79 2.00
82.9 83.2 83.7 83.4
power density or emitter temperature whereas the open circuit voltage changed slightly with emitter temperature. Table 2 presents the electric power output characteristics from the four cell modules at different testing conditions. As expected, the electric power generated by the cell modules was increased substantially with radiant power density or emitter temperature. The TPV cell modules successfully generated 246.4 W of electric power at an emitter temperature of 1265 °C, which would be more than adequate to power the electrical components of the whole heating system. Surplus electricity was used to charge the starting battery. This is a realistic and reliable design for the prototype at this point of time, although TPV cells have the potential to produce much more electricity at higher emitter temperatures. Table 3 presents the experimental results of TPV boiler performance and the calculated efficiencies of the system. It has been shown that the electrical efficiency reached 2.0%. This was obtained without heat recuperation in the TPV system. The electrical efficiency was limited by the low conversion of fuel energy into suitable IR radiation since the radiant efficiency was only about 20% (see Table 2). An effective approach to increasing the efficiency of heat to electricity conversion is heat recuperation in a combustion-driven TPV system where a recuperator is employed to recover exhaust heat and used to preheat combustion air, thus returning a portion of exhaust heat back into the emitter heating cycle. This approach could significantly increase TPV system efficiency [15–20], however, the heat recuperation would make the system more complicated. Experimental results show that TPV power conversion was achieved in a gas-fired residential boiler and it formed a cogeneration process where unconverted heat was recovered. Note that the integration of TPV was observed to have little effect on thermal efficiency of the boiler. A thermal efficiency of 82.9–83.7% was attained for space/water heating needs (see Table 3). The TPV integrated boiler has a high overall energy efficiency since the waste heat can be efficiently utilized for space and water heating. Primary energy is saved by displacing the electricity generated by the power utility, that would have been used to power the boiler, where a large portion of fuel energy inevitably becomes waste heat. Note that the primary interest in this work was to develop selfpowered heating equipment in which the electricity load is much lower than thermal load. This is the first step, yet technically it would be feasible to further increase electrical power output to realize micro-CHP concept in residential buildings through heat recuperation or more efficient spectral control. In this case, a TPV generator could be an alternative to other micro-CHP prime
4. Conclusions A full size TPV integrated boiler for potential micro-CHP applications in residential buildings was developed and researched in this paper. A commercially available boiler was used as a precursor to construct the prototype system where a burner converts fuel energy into thermal radiation and sensible heat of flue gases and recently-developed TPV cell modules were integrated. The exhaust flue gases flowed through a heat exchanger to recover heat for home space and water heating. The performance of the burner was noticeably affected by combustion variables. The emitter temperature reached 1078–1265 °C at the combustion heat input loads of 8.5–12.3 kWt, resulting in the radiant power outputs ranging from 4.61 W/cm2 to 5.90 W/cm2 and radiant efficiencies varying from 21.2% to 22.6%. The GaSb TPV cell modules installed in the boiler generated 246.4 W of electric power at an emitter temperature of 1265 °C. The generated electricity was capable of powering the entire heating system, thus realizing self-powering for the first time by TPV. Surplus electricity was used to recharge the starting battery but could be supplied to other electrical loads. The integration of TPV with the boiler was found to have little effect on the thermal efficiency, with the boiler attaining a thermal efficiency of 82.9–83.7% for heating needs. In the present research, the cogeneration of electricity and heat using TPV as the source was confirmed to be feasible, as demonstrated by the self-powered boiler prototype. The TPV integrated boiler could be further redesigned to a micro-CHP system in residential homes. This provides an effective means for primary energy savings and on-site power and heat security. Experiments with the prototype system show that the TPV generator could be a viable alternative to other micro-CHP prime movers such as Sterling engines and internal combustion engines (or reciprocating engines) that have found applications in residential CHP systems. The advantages of TPV power generation include no moving parts, relatively low maintenance and high overall system efficiency due to the fact that the waste heat can be recovered efficiently on-site, for the space and water heating needs of a residential home. The experimental data presented provides the basis for the designing of commercially viable TPV boiler units. Acknowledgements Funding for this work was provided by Natural Resources Canada through the Program of Energy Research and Development (PERD). Parts of this work were based on the design practice of a TPV stove made by JX Crystals Inc. We appreciate their cooperative efforts in developing TPV. References [1] Schubnell M, Benz P, Mayor JC. Design of a thermophotovoltaic residential heating system. Sol Energy Mater Sol Cells 1998;52:1–9. [2] Horne E, Morgan M, Butcher T. Microcogeneration concepts using thermophotovoltaics. In: Proceedings of ACEEE’s 11th biennial summer study on energy efficiency in buildings, vol. 10; 2000. p. 123.
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Implementation of a TPV integrated boiler for micro-CHP in residential buildings K. Qiu ⇑, A.C.S. Hayden CanmetENERGY-Ottawa, Natural Resources Canada, 1 Haanel Drive Ottawa, Ontario K1A 1M1, Canada
h i g h l i g h t s A TPV integrated boiler for micro-CHP application is designed, tested and demonstrated. Thermal radiation was emitted by a porous emitter in the TPV unit. The electric output of four TPV cell modules connected in series is measured and characterized under various conditions. 246.4 Electricity is generated at the emitter temperature of 1265 °C. This study shows that TPV generation in boilers/furnaces is feasible for micro-CHP application in residential buildings.
a r t i c l e
i n f o
Article history: Received 10 January 2014 Received in revised form 14 July 2014 Accepted 4 August 2014 Available online 23 August 2014 Keywords: TPV Boiler Power generation Micro-CHP
a b s t r a c t There is a growing interest in direct thermal-to-electric energy conversion using solid state devices such as thermophotovoltaic (TPV) generators. TPV devices convert thermal radiation from heat sources into electricity without involving any moving parts. TPV opens up possibility for efficient and stand-alone power generation in boilers and furnaces. In this paper, a TPV integrated boiler was designed, built and investigated for micro combined heat and power (micro-CHP) application in residential buildings. A full size gas fired residential boiler was used as a precursor for integration with TPV devices. Experiments were conducted with the prototype TPV boiler so as to evaluate various issues related to this new technology. The electric output of TPV modules installed in the boiler was characterized under different operating conditions. The TPV cell modules generated 246.4 W at an emitter temperature of 1265 °C, which would be enough to power the electrical components of the whole heating system. Moreover, such a TPV integrated boiler could be employed to form a micro-CHP system in residential homes, providing an effective means for primary energy savings, on-site power and energy security. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
1. Introduction Thermophotovoltaic (TPV) generators convert radiant energy from heat sources such as combustion into electricity (Fig. 1). This energy conversion is achieved by means of photovoltaic (PV) cells. Low bandgap PV cells are preferred for TPV generators since the photons emitted from heat sources at temperatures of practical interest are distributed at lower energies in comparison to solar radiation. Low bandgap PV cells are referred to as TPV cells. The advantages of TPV systems include: (a) a high fuel utilization efficiency is achievable due to that the fact that the heat dissipated can be recovered on-site for space conditioning and water heating needs, (b) no moving parts are involved in the power generation,
⇑ Corresponding author. Tel.: +1 613 996 9516; fax: +1 613 947 0291. E-mail address: [email protected] (K. Qiu). http://dx.doi.org/10.1016/j.apenergy.2014.08.016 0306-2619/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
and (c) this is a relatively low maintenance technology. Based the above features, the TPV generator is well suited for micro combined heat and power (micro-CHP) applications in residential buildings. A combustion driven TPV generator usually consists of a heat source, a thermal emitter, a spectral control filter and TPV cell modules. The thermal emitter converts the combustion heat into radiation energy (photons) at appropriate wavelengths. The emitter could be a solid surface heated by flame impingememt, or a porous medium burner with combustion taking placing inside the porous medium. The TPV cells play a pivotal role in the heat energy to electricity energy conversion process. Cells made of low bandgap semiconductor materials, such as GaSb, are favored because these cells can convert more infrared radiation (low energy photons) from the emitter to electricity. To maximize thermal to electric energy conversion efficiency, a spectral control filter should be used. The filter is capable of passing the desired portion of broadband thermal radiation and reflects the non-convertible
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Emitter
PV cells
Heat sources
(a) Hydrocarbon combustion (b) Concentrated solar radiation (c) Waste heat (d) Nuclear sources
Infrared radiation
Filter
Infrared radiation
(a) Silicon carbide (b) Tungsten (c) Ceramics (d) Metal alloy
(a) Si (b) GaSb (c) InGaAs (d) Ge (e) InGaAsSb
(a) Selective emitters (b) Quartz shield (c) Dielectric films (d) Back-surface reflector
Fig. 1. Illustration of TPV energy conversion.
radiation back to the emitter to increase the overall system efficiency. A number of research groups have been developing cogeneration systems based on TPV devices [1–4]. Schubnell et al. [1] have analyzed the influence of various parameters including cell bandgap and flue gas temperature on the electricity conversion efficiency of a TPV central heating unit, however, they did not provide their experimental results. Horne et al. [2] presented a liquid fuel-fired TPV system for portable power applications where a resonant metal mesh infrared band pass filter was used, yet a TPV system with such a filter would be difficult to implement in practical applications. The investigators at JX Crystals Inc. reported on the development of a prototypical propane-fueled TPV stove, generating up to 100 W of electric power [3]. In a study by Nelson [4], a self-powered, gas-fired, warm air furnace was evaluated as a candidate for the autonomous generation of electric power. In addition to the above work, TPV systems were shown to have the potential for application in high temperature industries [5,6]. There have been other investigations that have examined the combination of TPV with other power generation approaches such as thermoelectrics [7] and low temperature Rankine cycles [8,9]. The reviews of TPV research [10–12] have shown that TPV systems have many advantages, including no moving parts, high reliability and the capability of converting radiation from a variety of heat sources directly into electricity. The same reviews have also shown that TPV systems have certain limitations, for example, the electrical efficiency of combustion-driven TPV systems developed to-date has been relatively low [3,12–14]. The major energy losses in combustion-driven TPV systems arise from the limited conversion of fuel to radiation energy and low fraction of in-band radiation. It has been suggested that heat recovery or recuperation from the flue gas exhaust could be an effective means of increasing TPV system efficiency [15–20]. TPV devices can suitably be integrated with heating equipment to form micro-CHP systems for use in residential buildings. Residential combined space/water heating equipment, such as fuel fired boilers, use both fuel for heat production and electric power to drive its electrical components. If a boiler utilises a TPV generator to convert part of the combustion heat energy to electricity that then operates its electrical components including a combustion fan/blower, pumps, valves, and controls, it becomes a so-called self-powered heating system. Any surplus electricity generated can be provided to other electrical loads within the home, realizing
the CHP concept in a residential setting. This provides a possible means for power supply, energy security for the home, and overall primary energy savings as well. Although a number of TPV systems were investigated [3,7,13–28], no work on practical or full-size TPV systems has been reported in the literature and little attention has been paid to an integrated system in the earlier investigations. In this paper, a TPV integrated boiler was designed, built and investigated for micro-CHP application in residential buildings. A full size gas fired residential boiler was used as a precursor to integrate with TPV. Recently-developed GaSb TPV cell modules were employed in the direct thermal-to-electric energy conversion system. Experiments were conducted with the prototype TPV boiler in order to address certain issues related to this technology, which must be resolved, before TPV can be applied in practice. While the development of a self-powered boiler was considered as a first target, upgrading the equipment to a micro-CHP system was also addressed. 2. TPV integrated boiler for micro-CHP in residential buildings 2.1. Concept description Fig. 2 shows the schematic diagram of the TPV power generation process in a gas-fired boiler. Natural gas is converted by a radiant burner into thermal radiation and the sensible heat of flue
DHW heat exchanger
Hydronic space heating
Diverter valve
Flue exhaust
Domestic hot water Electrical power output
Flue-water heat exchanger
Cold water Pump
Power supply Cooling system
Control system Electricity TPV converter
Heat
Photons Optical filters In-band photons
Gas burner
Emitter
Out-of-band photons
Blower Air Gas
Heat
Fig. 2. Schematic diagram of the TPV power generation process in gas-fired residential boiler.
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gases. A TPV converter generates electricity to power electrical auxiliaries of the boiler, including an air blower, a fan, water pumps, an igniter, valves and a control unit and surplus electricity is supplied to other electrical loads or is exported to the grid. The exhaust flue gases flow through a heat exchanger and heat is thus recovered for home space and/or water heating needs. A microprocessor is used in the system control unit, which is capable of initiating the air blower, the pump, the igniter, and the gas valve from a battery. The battery supplies the required electric power for a short period during the start-up process until the TPV generator is functioning. The battery will also power the pump to run for a certain period to avoid overheating of the TPV converter during the system shut-down process. The control unit controls the battery charging circuit, flame sensing and safety shutoff. The working procedure of the control system is as follows: (1) When the control unit receives a signal for hot water or space heating needs from a thermostat, it will activate switches to turn on the combustion air blower and the water pump. (2) The igniter will start and the fuel solenoid valve will be switched on. (3) Upon successful burner ignition and when the electricity being generated by the TPV exceeds the power being consumed by the boiler auxiliaries, the power supply will be switched to the TPV generator. (4) The surplus power generated can recharge the battery or be conveyed to other electric loads, and/or exported to the grid. (5) If the control unit detects any demand to shut-off the boiler, e.g. from the thermostat or safety controls, the fuel solenoid valve will be first closed, the power switched immediately from the TPV generator to the battery, and then the blower will be turned off. The water pump will continue running for enough time to prevent the TPV modules from being overheated.
(a) Burner assembly Exhaust gases
Heat Shield Emitter Combustion chamber Filter
Water jacket
TPV cells
2.2. Development of TPV integrated boiler design
Combustible
A full size TPV integrated boiler was designed and developed during the course of this work. A commercially available gas fired residential boiler was selected as a precursor to integrate a TPV generator. The boiler selected was a wall-mounted combi-boiler that serves as both a home space heater and domestic hot water provider. The power consumption of the boiler’s electrical auxiliaries was 150 W. The highest electrical consumption in the boiler was from the pump that circulates the water and the fan that evacuates the exhaust gases. The balance of the power consumed was due to the electronics that control the boiler system. A TPV burner assembly, as shown in Fig. 3, was devised for the integrated boiler. The TPV burner assembly consists of an air/gas premixing tube, a natural gas/air mixture plenum, a combustion chamber, porous thermal emitters, optical filters, TPV cell modules and a cell cooling system. The natural gas and air are well mixed prior to entering the combustion chamber and the gas mixture burns within the combustion chamber. The high temperature combustion products flow through the porous emitters, which are heated and produce thermal radiation. The optical filters reflects the non-convertible radiation, helping to keep the TPV cells cooler. The TPV cells then produce electricity from the infra-red radiation incident upon them. The electrical efficiency of a combustion driven TPV system is defined as the ratio of electric output power, Pel, to fuel chemical energy input, Qchem:
gTPV
Pel ¼ Q chem
ð1Þ
or
gTPV ¼
pel Acell Q chem
ð2Þ
(b) Cross section of the TPV burner Fig. 3. Design of the TPV burner assembly.
where pel is the cell power density and Acell is the cell area. Since the TPV cell modules were arranged very close to the emitter, the view factor can be assumed to be one. In this case, the system electrical efficiency may also be calculated from the efficiencies of thermal emitter, spectral control and TPV cells:
gTPV ¼ grad gspe gcell
ð3Þ
where grad is the radiant efficiency, gspe is the spectral efficiency and gcell is the cell efficiency. The radiant efficiency is defined by [13]:
grad ¼
Q rad Q rad ¼ Q chem mfuel DHL
ð4Þ
where Qrad is the total radiant power, mfuel is the fuel mass flow rate and DHL is the natural gas lower heating value. The thermal efficiency of the system is defined by:
gth ¼
Q heat Q chem
ð5Þ
where Qheat is the thermal output of the system. During TPV operation, unconverted heat must be removed from TPV cells to maintain the cell temperature below a certain level. If the heat is not dissipated effectively, the temperature of the cells will increase, resulting in a decrease in cell conversion efficiency. The hydronic loop return or circulating water driven by the pump is directed through a water jacket which dissipates excess heat from the TPV cells.
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Fig. 5. Thermal emitter made of SiC foam in the TPV burner.
Antireflection coating Front contacts Diffused p-InGaAsSb emitter (approx. 0.5micron) n-GaSb substrate Back contact
Fig. 6. Schematic representation of GaSb cell structure.
Fig. 4. Design of the TPV integrated residential boiler.
3. Experiments
External quantum efficiency
Fig. 4 illustrates the TPV integrated boiler configuration. The TPV burner assembly replaced the existing burner in the residential boiler and it was thus upgraded to a micro-CHP boiler. This system is capable of providing a home’s water and space heating needs as well as some independent power generation. Consequently, the value of the system to the end-user will be both heating system reliability and electricity (energy) consumption and cost reduction.
1 0.8 0.6 0.4 0.2 0 0.4
0.8
1.2
1.6
2
Wavelength, micron 3.1. Prototype system Fig. 7. Typical external quantum efficiency spectrum of standard GaSb cells.
A prototype TPV integrated boiler system was built and tested in order to obtain experimental performance. The TPV burner assembly (Fig. 3) replaced the old gas burner in the precursor boiler. The new burner design was based on the combustion theory of a fully stirred reactor. Natural gas flows at high velocities from the plenum into the combustion chamber to prevent flashback, where the combustion of the premixed natural gas and air occurs rapidly with high intensity, producing very hot flue gases. The hot flue gases then flow through the porous emitter, which is thus heated to incandescence (Fig. 5). The fuel to air ratio is adjusted to a near stoichiometric level in order to attain the highest temperature on the emitter. The thermal emitter is made of SiC foam, which is capable of withstanding the sufficiently high temperatures, as shown in Fig. 5. TPV cells employed in the prototype system are binary semiconductor GaSb cells whose bandgap is 0.72 eV at 300 K. This corresponds to a wavelength of 1.73 lm. The cell structure is depicted in Fig. 6. These cells are made from the wafers of p-GaSb while their p–n junctions are formed by the process of Zn diffusion. Fig. 7 illustrates the external quantum efficiency of GaSb cells. In practice, the cells are incorporated into a module or array. In this study, one GaSb cell module consists of seventy-two cells and each cell has an area of 2 cm2. The cells are electrically connected in series and parallel combinations. Four TPV cell modules have
Fig. 8. Two GaSb cell modules on water cooling device.
been installed in the present burner assembly. By symmetry, there are two cell modules on each side. Fig. 8 shows the two TPV cell modules (from JX Crystals Inc.) soldered onto the water cooling
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jacket. It is noted that since a broadband emitter was used in the TPV system, an optical control filter was needed to increase the spectral efficiency and minimise the level of the waste heat produced in the cell modules. The filter transmitted the desired portion of the overall thermal radiation energy while reflecting the non-convertible radiation. The filter also reduced direct convective heat transfer to the cell modules by preventing flue gases from being in contact with the cell modules. Fig. 9 shows the prototype TPV integrated boiler. Measurement and control devices were installed with the prototype system, including thermocouples, flow meters, digital voltage/current/power analyzers, gas analyzers and a data acquisition system. Thermocouples were used to measure the temperatures of the flue gas and the emitter. Eight thermocouples were installed on the emitter to obtain the average emitter temperature. Flue gas composition (O2, CO2, CO and NOx) was measured using paramagnetic, infrared, and chemiluminescent flue gas analyzers. The fuel flow rate was controlled and measured using a gas flow meter/controller. The air flow rate was monitored to calculate the excess air levels. The radiant power output from the emitter was measured using a radiometer (Land 2p Ellipsoidal Radiometer, Land Combustion Limited, UK). The data acquisition system (Campbell Scientific Inc., Canada) collected the signals from the measurement devices and monitored the output from the TPV modules. The readings were scanned and recorded every 2 s. The radiant efficiency was calculated from the measured radiant output and fuel input rate (Eq. (4)). There were uncertainties and factors that may affect the accuracy of the measurements in the experiments. The major errors arose from the ambient air temperature uncertainty, the temperature measurements using thermocouples, air and fuel flow rate measurements and radiant power output measurements. The maximum error or inaccuracy in the measurement results and calculated results, obtained from the method of error propagation, is presented in Table 1. The relationship between the heat dissipation by the water cooling jacket, the rise in cooling water temperature and the water flowrate is expressed as:
Table 1 Maximum error in measurement results and calculated results. Measurement parameter and calculated result
Maximum error
Fuel flow rate Air flow rate Temperature Radiant power density Cell electric power density Radiant efficiency
±2.5% ±3.0% ±4.0 °C ±3.5% ±2.0% ±4.0%
uw ¼
Q wh
ð6Þ
qw C p ðT out T in Þ
where Qwh is the dissipated heat (transferred to circulating water), uw is the volume flowrate of water through the cooling system, qw is the density of water, Cp is the specific heat of water, and Tout and Tin are the water outlet and inlet temperatures. The required heat dissipation can be estimated as follows.
Q wh ¼ Acell ½ð1 qcell Þptf pel
ð7Þ
where Acell is the cell area, qcell is the reflectivity of cell surface, ptf is the radiant power density incident on the cell modules and pel is the electricity density generated by cell modules. Our calculations show that the required heat dissipation or cooling load for one cell module is approximately 800 W. 3.2. Experimental results and discussion The burner assembly was first investigated with regard to its combustion performance. Fuel and air flow rates were regulated by mass flow controllers and the excess air was calculated. The temperature of the thermal emitter was changed by regulating gas flow rate and adjusting the excess air level. The emitter converts the heat released from combustion into thermal radiation. The emitter temperature was measured to be between 1078 °C and 1265 °C at the combustion heat input loads of 8.5–12.3 kWt, yielding radiant power outputs of 4.61 kWt to 5.90 W/cm2. The electric outputs of the TPV cell modules were measured in the prototype system at varying operating conditions and characterized. Fig. 10 shows the variation of short circuit current (ISC) and open circuit voltage (VOC) with emitter temperature for the four cell modules installed in the prototype boiler system. The four modules were connected in series. It was observed that the short circuit current increased significantly with increasing radiant
39
10
38.5
9
ISC , A
37.5
VOC, V
38 8
7 37 6
5 1050
36.5
1100
1150
1200
1250
36 1300
Emitter temperature, °C Fig. 9. Prototype TPV integrated boiler.
Fig. 10. Short circuit current and open circuit voltage of TPV cell modules vs. emitter temperature.
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Table 2 Power output characteristics for four TPV cell modules connected in series at different emitter temperatures. Emitter temperature, °C
Voc, V
Isc, A
Jsc, A/cm2
Vmpp, V
Impp, A
Impp, A
FF
PTPV, W
1078 1135 1231 1265
37.20 37.48 37.84 38.16
6.20 6.73 7.94 9.12
1.032 1.104 1.320 1.512
28.68 28.92 29.28 29.44
5.51 6.06 7.15 8.37
0.912 1.008 1.200 1.392
0.69 0.70 0.71 0.71
158.0 175.2 209.6 246.4
movers such as Sterling engines and internal combustion engines that have recently received attention for use in residential microCHP applications.
Table 3 Experimental and calculation results of TPV boiler performance. Fuel input, kW
Emitter surface temp, °C
Radiant efficiency, %
Electric power output, W
System electrical efficiency, %
System thermal efficiency, %
8.5 10.1 11.7 12.3
1078 1135 1231 1265
22.6 21.2 21.5 22.1
158.0 175.2 209.6 246.4
1.86 1.73 1.79 2.00
82.9 83.2 83.7 83.4
power density or emitter temperature whereas the open circuit voltage changed slightly with emitter temperature. Table 2 presents the electric power output characteristics from the four cell modules at different testing conditions. As expected, the electric power generated by the cell modules was increased substantially with radiant power density or emitter temperature. The TPV cell modules successfully generated 246.4 W of electric power at an emitter temperature of 1265 °C, which would be more than adequate to power the electrical components of the whole heating system. Surplus electricity was used to charge the starting battery. This is a realistic and reliable design for the prototype at this point of time, although TPV cells have the potential to produce much more electricity at higher emitter temperatures. Table 3 presents the experimental results of TPV boiler performance and the calculated efficiencies of the system. It has been shown that the electrical efficiency reached 2.0%. This was obtained without heat recuperation in the TPV system. The electrical efficiency was limited by the low conversion of fuel energy into suitable IR radiation since the radiant efficiency was only about 20% (see Table 2). An effective approach to increasing the efficiency of heat to electricity conversion is heat recuperation in a combustion-driven TPV system where a recuperator is employed to recover exhaust heat and used to preheat combustion air, thus returning a portion of exhaust heat back into the emitter heating cycle. This approach could significantly increase TPV system efficiency [15–20], however, the heat recuperation would make the system more complicated. Experimental results show that TPV power conversion was achieved in a gas-fired residential boiler and it formed a cogeneration process where unconverted heat was recovered. Note that the integration of TPV was observed to have little effect on thermal efficiency of the boiler. A thermal efficiency of 82.9–83.7% was attained for space/water heating needs (see Table 3). The TPV integrated boiler has a high overall energy efficiency since the waste heat can be efficiently utilized for space and water heating. Primary energy is saved by displacing the electricity generated by the power utility, that would have been used to power the boiler, where a large portion of fuel energy inevitably becomes waste heat. Note that the primary interest in this work was to develop selfpowered heating equipment in which the electricity load is much lower than thermal load. This is the first step, yet technically it would be feasible to further increase electrical power output to realize micro-CHP concept in residential buildings through heat recuperation or more efficient spectral control. In this case, a TPV generator could be an alternative to other micro-CHP prime
4. Conclusions A full size TPV integrated boiler for potential micro-CHP applications in residential buildings was developed and researched in this paper. A commercially available boiler was used as a precursor to construct the prototype system where a burner converts fuel energy into thermal radiation and sensible heat of flue gases and recently-developed TPV cell modules were integrated. The exhaust flue gases flowed through a heat exchanger to recover heat for home space and water heating. The performance of the burner was noticeably affected by combustion variables. The emitter temperature reached 1078–1265 °C at the combustion heat input loads of 8.5–12.3 kWt, resulting in the radiant power outputs ranging from 4.61 W/cm2 to 5.90 W/cm2 and radiant efficiencies varying from 21.2% to 22.6%. The GaSb TPV cell modules installed in the boiler generated 246.4 W of electric power at an emitter temperature of 1265 °C. The generated electricity was capable of powering the entire heating system, thus realizing self-powering for the first time by TPV. Surplus electricity was used to recharge the starting battery but could be supplied to other electrical loads. The integration of TPV with the boiler was found to have little effect on the thermal efficiency, with the boiler attaining a thermal efficiency of 82.9–83.7% for heating needs. In the present research, the cogeneration of electricity and heat using TPV as the source was confirmed to be feasible, as demonstrated by the self-powered boiler prototype. The TPV integrated boiler could be further redesigned to a micro-CHP system in residential homes. This provides an effective means for primary energy savings and on-site power and heat security. Experiments with the prototype system show that the TPV generator could be a viable alternative to other micro-CHP prime movers such as Sterling engines and internal combustion engines (or reciprocating engines) that have found applications in residential CHP systems. The advantages of TPV power generation include no moving parts, relatively low maintenance and high overall system efficiency due to the fact that the waste heat can be recovered efficiently on-site, for the space and water heating needs of a residential home. The experimental data presented provides the basis for the designing of commercially viable TPV boiler units. Acknowledgements Funding for this work was provided by Natural Resources Canada through the Program of Energy Research and Development (PERD). Parts of this work were based on the design practice of a TPV stove made by JX Crystals Inc. We appreciate their cooperative efforts in developing TPV. References [1] Schubnell M, Benz P, Mayor JC. Design of a thermophotovoltaic residential heating system. Sol Energy Mater Sol Cells 1998;52:1–9. [2] Horne E, Morgan M, Butcher T. Microcogeneration concepts using thermophotovoltaics. In: Proceedings of ACEEE’s 11th biennial summer study on energy efficiency in buildings, vol. 10; 2000. p. 123.
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