Thermoelectric coupling effect and energy harvesting
The thermoelectric effaect indicates that the thermal diffusion of the charge carriers in the thermoelectric material can generate electricity while an applied flow of charge carriers can influence the thermal balance in material.
When a temperature difference is achieved on the thermoelectric material, the charge carriers on the hot side possess larger kinetic energy. They diffuse to the cold side and accumulate at that place. They diffusion causes difference in the quantity of charge carriers between the hot and cold side. Consequently an internal electrical field is achieved. The generated internal field will decrease the diffusion of the rest charge carriers. When the electrical potential reaches thermal potential, the diffusion of charge carriers will stop.
When an electrical current passes through the thermoelectric material, the charge carriers move in a fixed direction. It takes away the kinetic energy of the vibrated molecule on one side and releases them to another side. This causes temperature difference between the two sides in the material.
A typical structure of a thermoelectric device is shown in Figure 2. 1. There are many thermoelectric couples in this device. They are placed in parallel thermally. From the electrical point of view, the p type and the n type semiconductors work as a group and all the groups are connected in series. This structure can help to increase the total voltage output when it works as generator. Besides, it helps to match the input impedance of the current source when it aims to generate temperature difference.
It is found that the variation of ηm with r is significant when the ZTavg is large. It is noteworthy that the optimized value of external resistor for power generation is a little higher than the internal resistor of the TEG, as highlighted with the dashed line in this figure. This characteristic is achieved when the incoming heat flow of the TEG is given. Nowadays, by manipulating the nanostructure of the materials, the reported ZTavg has reached 5.8 [NI 2009]. In actual situation, the Seebeck coefficient, electrical resistivity and thermal conductivity of the thermoelectric material are all associated with temperature [YAM 2008]. Consequently ZTavg associates with the temperature. The material with ZTavg=1 is currently available and it possess maximum material energy conversion efficiency around 17%.
Review of literature
Thermoelectric material is a promising candidate for power generation, being highly reliable, free of moving parts and environmentally friendly. Much scientific research has been devoted to the application of this material for energy. Categorized by species of heat source, varieties of temperature difference are available for this energy conversion process. We present here a brief review of recent progress of thermoelectric application with fossil fuel, waste heat and living being.
Due to the energy conversion efficiency of a thermoelectric device which is closely associated with the applied temperature difference, it is favorable with power generation with high temperature difference. A self-powered residential heating system operating entirely on fuel combustion is developed [QIU 2008]. The structure and performance of this system are shown in Figure 2. 3.
The thermoelectric module in this system is a radial type that fits in well with the burner of the stove. The thermoelectric elements in the module are made from PbSnTe doped to have either p- or n-type semiconductor properties. The module has 325 couples with each couple consisting of a p-type element and an n-type element. It is found that such a TEG can provide a power generation capacity of 550W at a temperature difference of 552°C. A similar system [JIA 2011] is introduced after but with much smaller dimension. Both of these two systems can work with an energy conversion efficiency as high as 1% since a large temperature difference is achieved by the combustion of fuel and water cooling. When the thermal loading on the cold face of the thermoelectric device changes into natural convection as reported in [NUW 2005], the maximum steady-state temperature difference achieved is 150°C which leads to a much lower efficiency. The difference between the water cooling method and the convection cooling method of a stove based thermoelectric power generator is addressed in [CHA 2010].
The power generation with thermoelectric device in stove consumes fossil fuel and it can achieve high efficiency. However, it is far from the scope of this study which aims to analyze the power generation when the intensity of the heat source is much lower. Such as those thermoelectric device works with waste heat. A typical application is to harvest energy from exhaust pipe of a motorcycle [SCH 2008]. The experimental prototype and performance of such a system is shown in Figure 2. 4.
The thermoelectric module used in this study is Melcor HT3-12-30. It is 1.18 inch wide by 1.34 inch long and 0.126 inch tall, consisting of 127 Bismuth Telluride thermocouples soldered together with 271°C SnSb solder and enclosed in Alumina ceramic plates. With different driving speed of the motorcycle (from 25 to 65 mph), the generated temperature difference on the thermoelectric devices varied from 47°C to 73°C). It produced an average of 0.4694W from an average temperature difference of 48.73°C. A similar study is presented by [HSU 2010]. Their focus is on the optimization of the heat sink. A mathematic model of thermoelectric module with applications on waste heat recovery from automobile engine is introduced by [HSI 2010]. The maximum power density produced from their prototype thermoelectric module is 51.13mWcm-2 at 290°C temperature difference. The model also shows that TE module presents better performance on the exhaust pipe than on the radiator.
Another typical study of harvesting waste heat in liquid is reported [NIU 2009]. The focus of this study is to examine experimentally the influences of the hot and cold fluid inlet temperatures, flow rates and the load resistance, on the power output and conversion efficiency. The experimental prototype is shown in Figure 2. 5. The thermoelectric material adopted in their study is Bi2Te3. Their experiments are conducted for a range of operating conditions as: the hot fluid inlet temperature between 50°C and 150°C, the cold fluid inlet temperature between 20°C and 30°C, and the ranges of both cold and hot fluid flow rate between 0.2m2h-1 and 0.6m2h-1.
When the hot and cold fluid inlet temperature are respectively 150°C and 30°C, the system produce a maximum power output of 146.5W which correspond to a conversion efficiency of 4.44%. A numerical model of such an energy harvesting system [YU 2007] is presented by them two years ago which predict the actual performance well.
With the similar thermoelectric device, the performance of this system is distinct with the foregoing prototype [NIU 2009]. When the temperature of the heat source is 90°C, the temperature difference achieved is 17.5°C with forced convection and only 4°C with natural convection. It indicates that the thermal boundary condition is essential with the performance of a waste heat recovery system which utilize thermoelectric device.
The thermoelectric energy harvesting system with waste heat is still large in dimension. Another attractive research on thermoelectric energy harvesting with much smaller dimension aims to utilize the heat from the body of living being.
Figure 2. 7: Varieties of wearable TEG prototype system: (a) [GYS 2005] (b)(c) [TOR 2006] (d) [PEN 2008] (e) [TOR 2008] (f) [PEN 2009]
According to [STA 2006], the warmness of a human body (and also an animal body) can be used as steady energy source, and the amount of energy released by the metabolism (traditionally measured in Met; 1 Met = 58.15 W/m² of body surface) mainly depends on the amount of muscular activity. A normal adult has a surface area in average of 1.7 m², so that such a person in thermal comfort with an activity level of 1 Met will have a heat loss of about 100 W. The metabolism can range from 0.8 Met (46 W/m²) while sleeping up to about 9.5Met (550W/m²) during sports activities as running with 15 km/h. A Met rate commonly used is 1.2 (70 W/m²), corresponding to normal work when sitting in an office, which leads to a person’s power dissipation of about 119 W, burning about 10.3 MJ a day.
The first body powered TEG [GYS 2005] reported in literature is 6 years ago. According to [LEO 2007], this wearable TEG serves as power supply for wireless sensor nodes on a wrist. It had been fabricated in 2004 as shown in Figure 2. 7(a). With ambient temperature at 22 °C, it produced a power of 100µW. In 2006, a wireless pulse oximeter has been designed, fabricated and tested on people, Figure 2. 7(b). The power generation the thermoelectric device in such a system is shown in Figure 2. 7(c). The idea of wrist watch progressed in 2007 as reported in [LEO2 2007]. The human++ project keeps on progress in the topic of wearable thermoelectric power generation [PEN 2008][TOR 2008][ PEN 2009].
Mateu et al. [MAT 2007] proposed to harvest energy between a human hand and the ambient environment. With a Peltier element PKE 128 A 1030, the gradient between the hand and the heat sink in ambient temperature can provide a maximum output current of 18mA and output voltage between 150mV and 250mV. The corresponding maximum output power is approximately 3mW. Similar research includes also [LOS 2010]. It distinguishes the problem of optimization in power generation and efficiency when the energy source is weak.
A medical healthcare system [HOA 2009] which is based on thermoelectric device is introduced most recently. Based on their experimental test results, the accumulated energy is around 1.369 mJ which is able to power the loads comprising of sensor, RF transmitter and its associated electronic circuits. The sensed information is transmitted in 5 digital words of 12-bit data across a transmission period of 120 msec. The receiver platform displays the patient identification number and sounds out an alarm buzzer for aid if a fall event is detected. Another medical care application of thermoelectric device is reported by [LAY 2009] which aims to extract warmth from body tissue to supply a hearing aid.
Unlike the wearable thermoelectric application, the thermoelectric device is also proposed to be planted under the skin of a human body [YAN 2007] to drive a medical device. It is found that a stabilized temperature difference of 1.3°C is achieved with a thermoelectric module TEC1-00706T125 planted in the muscle tissue of a rabbit. The corresponding voltage generation is 5mV. The temperature difference can increase to 5.5°C when an ice water is put on the skin of the rabbit.
The most amazing work is introduced by [GHA 2008] which design, fabricate and test an implantable micro-scale thermoelectric power scavenging device that utilizes the body temperature of beetles as an energy source, as shown in Figure 2. 8. The brief study shows that the maximum body-ambient temperature difference was about 11 °C, measured on the back of the beetle close to the wings base.
Figure 2. 8: Experiments on beetles (a) Temperature distribution over beetle’s body before and after flight (b) Implanted dummy chip inside beetle’s pupa [GHA 2008]
The foregoing review in power generation and energy harvesting with thermoelectric device shows clearly the popularity of this effect in scientific research. However, it is not the only direction of progress in thermoelectric effect. More effort is devoted to the structure design [WEB 2006][YAD 2008][WHA 2008] and fabrication technique [SUD 2005][WAN 2009][CAR 2010] of TEG. Besides, the power management circuit of a TEG [KIM 2008][YU 2009][CAR2 2010] is also an important topic in the realm of thermoelectric.
The scope of this study which focuses on those applications in buildings is in the range of harvesting waste heat. The intensity of energy source is neither as strong as the fossil fuel nor as weak as body heat. There is also no strict requirement in the dimensions of the energy harvesting system in this study. A more similar research on the scope of building applications will be addressed in section 3.
Table of contents :
1. General introduction
2. Energy harvesting and thermal energy storage with some well known effect
2.1 Thermoelectric coupling effect and energy harvesting
2.1.1 Thermoelectric effect
2.1.2 Review of literature
2.2 Pyroelectric coupling effect and energy harvesting
2.2.1 Pyroelectric effect
2.2.2 Review of literature
2.3 Piezoelectric coupling effect and energy harvesting
2.3.1 Piezoelectric effect
2.3.2 Review of literature
2.3.3 Enhanced energy conversion efficiency with SSHI technique
2.4 Electromagnetic and electrostatic effects for energy harvesting
2.4.1 Electromagetic energy harvesting
2.4.2 Electrostatic energy harvesting
2.5 Thermal energy storage with phase change material
2.6 Sectional summary
3. Ambient energy harvesting
3.1 Characteristics of ambient energy source
3.1.1 Analysis of typical case
3.1.2 Modeling of solar thermal energy
3.2 Literature review
3.2.1 Direct Solar thermal energy harvesting
3.2.2 Wind energy harvesting
3.2.3 On harvesting other ambient energy
3.3 Sectional summary
4. Solar energy harvesting through thermoelectric effect
4.1 Design of the thermoelectric energy harvesting system
4.1.1 Strategy for ambient thermal energy harvesting
4.1.2 Thermoelectric device
4.1.3 Phase change material
4.2 Experimental study
4.2.1 Fabrication of the prototype TEG system
4.2.2 In lab test and results
4.2.3 Test outside and results
4.3 Modeling of the prototype system
4.3.1 Electrical analogy method
4.3.2 Finite element method
4.3.3 Simulation and results
4.4 Sectional summary
5. Solar energy harvesting through pyroelectric effect
5.1 Design of the pyroelectric energy harvesting system
5.2 Experimental study
5.2.1 In lab test and results
5.2.2 Test outside and results
5.3 Modeling of the prototype system
5.3.1 Equivalent electrical model
5.3.2 Numerical simulation
5.3.3 Results of the simulation
5.4 Sectional summary
6. Wind (or airfow) energy harvesting through piezoelectric effect
6.1 Design of the piezoelectric energy harvesting system
6.2 Experimental study
6.2.1 In lab test and results
6.2.2 Test outside and resutls
6.3 Modeling of a self-exctied energy harvester
6.3.1 Fluid structure interaction analysis with dynamic pressure
6.3.2 Lumped parameter model with SSHI technique
6.4 Sectional summary
7. Typical application of thermoelectric generator in building
7.1 Architecture of the application
7.2 Experimental study
7.2.1 Performance of an improved TEG
7.2.2 Configuration and performance of the self-powered system
7.2.3 Configuration and performance of the PV with water cooling
7.3 Sectional summary
8. General conclusion and perspective
List of Publication
List of figures
List of tables