Hybrid Energy Systems for Fossil Fuel-based Power Generation

Hybrid energy system from fossil fuel-based power generation processes (both static and mobile) can be generated by converting either (a) waste heat or (b) CO, to power or useful fuels. Here we describe both of these in some details.

Hybrid Energy System Created by Conversion of Waste Heat to Power by Thermoelectricity

As we pushed toward less fossil fuel consumption for power generation, the conversion efficiency becomes an important issue. For example, the total energy efficiency of a conventional thermal power plant using steam turbines is approximately 40%; the best modern combined cycle plant using a gas turbine and a steam turbine is between 50% and 60%. In vehicles using gasoline-powered combustion engines, the conversion efficiency is about 30%, and diesel-powered combustion engines achieve about 40% efficiency. Similarly the use of thermal energy in industrial processes is also highly inefficient and these processes, in general, generate a large amount of waste heat. While these different sources of waste heat can be used for local heating and cooling needs by cogeneration, thermoelectric generators (TEG) have the potential to recover them as power and to make a major contribution to reducing fossil fuel consumption [6-12]. As a consequence of lower energy consumption and higher total energy efficiency, TEG also can help reduce CO, and other greenhouse gas emissions.

A thermoelectric power generator is a solid-state device that provides direct energy conversion from thermal energy (heat) due to a temperature gradient into electrical energy based on “Seebeck effect” [6-12]. The thermoelectric power cycle, with charge carriers (electrons) serving as the working fluid, follows the fundamental laws of thermodynamics and intimately resembles the power cycle of a conventional heat engine. Thermoelectric power generators offer several distinct advantages over other technologies [6-12]:

• • They are extremely reliable (typically exceed 100,000 hours of steady-state operation) and silent in operation since they have no mechanical moving parts and require considerably less maintenance.
• • They are simple, compact, and safe.
• • They are very small in size and virtually weightless.
• • They are capable of operating at elevated temperatures.
• • They are suited for small-scale and remote applications typical of rural power supply, where there is limited or no electricity.
• • They are environmentally friendly.
• • They are not position dependent.
• • They are flexible power sources.
• • They produce no chemical substance, operate in silence, and are reliable.

On the other hand, the most significant disadvantage is the low-energy efficiency of TEG (typically 5%) [7]. In addition, the energy efficiency and the released output power are temperature dependent. Modern automated combustion systems need to be connected to the electricity grid. Boilers with an integrated thermoelectric generator can utilize waste heat from the furnace. The CHP (combined heat and power) units that are created provide an independent source of electric energy and enable efficient use of fuel. Low-conversion efficiency has been a major cause in restricting their use in electrical power generation to specialized fields with extensive applications where reliability is a major consideration and cost is not.

In general, the cost of a thermoelectric power generator essentially consists of the device cost and operating cost. The operating cost is governed by the generator’s conversion efficiency, while the device cost is determined by the cost of its construction to produce the desired electrical power output [6-12]. Since the conversion efficiency of a module is comparatively low, thermoelectric generation using waste heat energy is an ideal application. In this case, the operating cost is negligible compared to the module cost because the energy input (fuel) cost is cheap or free. Therefore, an important objective in thermoelectric power generation using waste heat energy is to reduce the cost-per-watt of the devices. Moreover, cost-per-watt can be reduced by optimizing the device geometry, improving the manufacture quality, and simply by operating the device at a larger temperature difference [7]. In addition, in designing high-performance thermoelectric power generators, the improvement of thermoelectric properties of materials and system optimization has attracted the attention of many research activities [6-12]. Their performance and economic competitiveness appear to depend on successful development of more advanced thermoelectric materials and thermoelectric power module designs.

Thermoelectric generators are semiconductor devices based on thermoelectric effects that can convert thermal energy directly into electricity. When a temperature gradient is established between junctions of materials, e.g., one junction is heated and the other cooled, a voltage (Seebeck voltage) is generated. The thermocouple that is created can be connected to a load to provide electric power. Thus, based on this Seebeck effect, thermoelectric devices can act as electrical power generators, as shown in the literature [6-12]. The equation that dictates the performance of TEG can be expressed as

While the thermoelectric materials’ dimensionless figure of merit ZT is a well- defined metric to evaluate thermoelectric materials, it can be a poor metric for maximum thermoelectric device efficiency because of the temperature dependence of the Seebeck coefficient “S,” the electrical resistivity “k,” and the thermal conductivity “p,” where “T” is the absolute temperature. Historically the field has used a thermoelectric device dimensionless figure of merit ZT to characterize a device operating between a hot side temperature “T_h” and cold side temperature “T_c.” While there are many approximate methods to calculate ZT from temperature-dependent materials properties, an exact method uses a simple algorithm that can be performed on a spreadsheet calculator [6-12]. The conversion efficiency from heat to power is a function of operating temperature difference. An increase in the temperature difference provides an increase in heat available for conversion, so large temperature differences are desirable [7]. Only materials which possess ZT > 0.5 are regarded as thermoelectric materials [7]. Established thermoelectric materials can be divided into groups depending upon the temperature range of operation [6-12]:

• • Low-temperature materials, up to around 450 К
• • Medium-temperature materials, from 450 К up to around 850 К
• • High-temperature materials, from 850 К up to around 1,300 К

Alloys based on bismuth in combinations with antimony, tellurium, and selenium are low-temperature materials. Medium-temperature materials are based on lead tel- luride and its alloys. High-temperature materials are fabricated from silicon germanium alloys [7].

The TEG device is composed of one or more thermoelectric couples. The simplest TEG consists of a thermocouple, comprising a pair of P-type and N-type thermoelements or legs connected electrically in series and thermally in parallel. The differentiation between N- and P-doped materials is important. The single and multistage configurations of TEG are described in Figures 2.2a, and 2.2b respectively. The details on these configurations are given in numerous references [6-12].

The sizes of conventional thermoelectric devices vary from 3 mm² by 4 mm thick to 75 mm² by 5 mm thick. Most of thermoelectric modules are not larger than 50 mm in length due to mechanical consideration. The height of single-stage thermoelectric modules ranges from 1 to 5 mm. The modules contain from 3 to 127 thermocouples [7]. There are multistage thermoelectric devices designed to meet requirements for large temperature differentials. Multistage thermoelectric modules can be up to 20 mm in height, depending on the number of stages. The power output for most of the commercially available thermoelectric power generators ranges from microwatts to multi-kilowatts [7]. For example, a standard thermoelectric device consists of 71 thermocouples with a size of 75 mm² can deliver electrical power of approximately 19 W [6-12]. The maximum output power from a thermoelectric power generator typically varies depending on temperature difference between hot and cold plates and module specifications, such as module geometry (i.e., cross-sectional area and thermoelement length), thermoelectric materials, and contact properties. The maximum power output increases parabolically with an increase in temperature difference. For a given temperature difference, there is a significant variation in maximum power output for different modules due to variation in thermoelectric materials, module geometry, and contact properties. The maximum power output follows a clear trend and increases with decrease in the thermoelement length for a given module cross- sectional area. As demonstrated in my previous book [11], the performance of TEG depends on the materials and TEG configurations. Significant literature [6-10, 12], along with my previous book [11], describes the progress made on materials and TEG configurations over the past several decades. Here we will mainly focus on the applications of TEG for various types of waste heat.

A. Applications

About 70% of energy in the world is wasted as heat and is released into the environment with a significant influence on global warming [7, 13]. The waste heat energy released into the environment is one of the most significant sources of clean, fuel- free, and cheap energy available. The unfavorable effects of global warming can be

FIGURE 2.2 Schematic diagram showing components and arrangement of a typical single- stage and multiple stages thermoelectric power generator [7].

diminished using the TEG system by harvesting waste heat from residential, industrial, and commercial fields [6-12].TEG is substantially used to recover waste heat in different applications ranging from pW to MW (see Figure 2.3). Different waste heat sources and temperature ranges for thermoelectric energy harvesting are shown in Table 2.1 [7]. What makes thermoelectricity most interesting with regard to hybrid energy is that here not only processes generating power generate additional power from waste heat, but processes using heat as a source of energy can also generate power from the waste heat. This is more like a reverse cogeneration form of hybrid energy system.

FIGURE 2.3 Energy conversion applications [7].

TABLE 2.1

Different Waste Heat Sources and Temperature Ranges for Thermoelectric Harvesting Technology [7]

Enormous quantities of waste heat generated from various sources are continuously discharged into the earth’s environment, much of it at temperatures which are too low to recover using conventional electrical power generators. Thermoelectric power generation, which presents itself as a promising alternative green technology, has been successfully used to produce electrical power in a range of scales directly from various sources of waste heat energy. Some of them are described here. The applications can be divided into microscale and macroscale. At microscale, waste heat can be converted to electric power with a micro thermoelectric generator. Waste human body heat can be used to power a thermoelectric “watch battery”. It is estimated that 2,875 thermoelements connected in series would be required to obtain the 2 V required to operate the watch [7, 14, 15]. The TEG devices are especially suitable for waste heat harvesting for low-power generation to supply electric energy for microelectronic applications [6-12, 16-31]. Glatz et al. [16] presented a novel polymer-based wafer level fabrication process for micro thermoelectric power generators for the application on nonplanar surfaces. Furthermore, it can be used for various microelectronic devices, like wireless sensor networks, mobile devices (e.g., MP3 player, smartphones, and iPod), and biomedical devices. The thermoelectric energy harvesters are microelectronic devices made of inorganic thermoelectric materials, at different dimensions, with a lifetime of about five years [7, 14, 15]. A TEG to be applied in a network of body sensors has been presented in several references [7, 14, 15].

At macroscale, TEG can be used for domestic waste heat applications and industrial waste heat applications. Rowe [6, 17] reported that a waste heat-based thermoelectric power generator is used in a domestic central heating system with the modules located between the heat source and the water jacket. It was concluded that two modules based on PbTe technology when operated at hot and cold side temperatures of 550°C and 50°C, respectively, would generate the 50 W required to power the circulating pump [6, 17]. Waste heat energy can also be utilized proportionally from 20 kW to 50 kW wood- or diesel-heated stoves [6-12, 18], especially during the winter months in rural regions where electric power supply is unreliable or intermittent to power thermoelectric generators.

The most amounts of heat are emitted and released into the atmosphere in the form of flue gases and radiant heat energy with a negative impact to the environmental pollution (emissions of C0₂) by industrial processes [6-12]. For this reason, thermoelectric harvesters are good candidates to recover waste heat from industries [6-12] and convert it into useful power (e.g., to supply small sensing electronic device in a plant).

Utilization of TEGs in the industrial field is beneficial from two points of view':

• • In the industrial applications where recoverability of the waste heat by the conventional system (radiated heat energy) is very difficult to be done.
• • In the industrial applications where the use of thermoelectric materials reduces the need for maintenance of the systems and the price of the electric power is low, even if the efficiency is low [6,6-12, 19].

The results of a test carried out on a TEG system attached at a carburizing furnace (made of 16 Bi₂Te, modules and a heat exchanger) are shown by Kaibe et al. [20]. The system harvested about 20% of the heat (P=4 kW). The maximum electrical output power generated by TEG w'as approximately 214 W, leading to thermoelectric conversion efficiency of 5%. Aranguren et al. [32] built a TEG prototype in which TEG w'as attached at the exhaust of a combustion chamber, w'ith 48 modules connected in series and two different kinds of finned heat sinks, heat exchangers, and heat pipes. TEGs are also useful for recovery of w'aste heat from the cement rotary kiln to generate electricity, considering that the rotary kiln is the main equipment used for large-scale industrial cement production [21]. The electric output power evaluation of a TEG system attached to an industrial thermal oil heater is presented by

Barma et al. [22]. The impact of different design and flow parameters were assessed to maximize the electrical output power. The estimated annual electrical power generation from the proposed system was about 181,209 kWh. The thermal efficiency of the TEG based on recently developed thermoelectric materials (N-type hot forged Bi₂Te, and P-type (Bi,Sb)₂Te, used for the temperature range of 300 K-573 K) was enhanced up to 8.18%.

Most of the recent research activities on applications of thermoelectric power generation have been directed toward utilization of industrial waste heat [6-12]. Vast amounts of heat are rejected from industry, manufacturing plants, and power utilities as gases or liquids at temperatures which are too low to be used in conventional generating units (<450 K). In this large-scale application, thermoelectric power generators offer a potential alternative of electricity generation powered by waste heat energy that would contribute to solving the worldwide energy crisis, and at the same time, help reduce environmental global warming. In particular, the replacement of by-heat boiler and gas turbine by thermoelectric power generators makes it capable of largely reducing capital cost, increasing stability, saving energy source, and protecting environment [7].

Min and Rowe [23, 24] reported that New Energy and Industrial Technology Development Organization (NEDO) designed low-temperature waste heat power generator that consisted of an array of modules sandwiched between hot and cold water-carrying channels. When operated using hot water at a temperature of approximately 90°C and cold flow at ambient temperature, Watt-100 generates 100 W at a power density approaching 80 kW/m³. In this application, the system was scalable, enabling 1.5 kW of electrical power to be generated [6-12]. Thermoelectric power generators have also been successfully applied in recovering waste heat from steel manufacturing plants. In this application, large amounts of cooling water are typically discharged at constant temperatures of around 90°C when used for cooling ingots in steel plants. When operating in its continuous steel casting mode, the furnace provides a steady-state source of convenient piped water which can be readily converted by thermoelectric power generators into electricity. It was reported [6-12] that total electrical power of around 8 MW would be produced employing currently available modules fabricated, using Bi₂Te, thermoelectric modules technology.

Another application where thermoelectric power generators using waste heat energy have potential use is in industrial cogeneration systems [19]. For example, Yodovard et al. [25] assessed the potential of waste heat thermoelectric power generation for diesel cycle and gas turbine cogeneration in the manufacturing industrial sector in Thailand. The data from more than 27,000 factories from different sectors, namely, chemical product, food processing, oil refining, palm oil mills petrochemical, pulp and paper rice mills, sugar mills, and textiles, were used. It is reported that gas turbine and diesel cycle cogeneration systems produced electricity estimated at 33% and 40% of fuel input, respectively. The useful waste heat from stack exhaust of cogeneration systems was estimated at approximately 20% for a gas turbine and 10% for the diesel cycle. The corresponding net power generation was about 100 MW. The potential for thermoelectric power from various industrial processes is depicted in Table 2.2 [11].

TABLE 2.2

Potential of Thermoelectric Power from Various Manufacturing and Process Operations [11]

Source: © Waste Heat from Incineration of Solid Waste Applications

Recently, the possibility of utilizing the heat from incinerated municipal solid waste has also been considered. For example, in Japan, the solid waste per capita is around 1 kg per day and the amount of energy in equivalent oil is estimated at 18 million kJ by the end of the 21st century. It was reported by [6-12, 19] that an on-site experiment using a 60 W thermoelectric module, installed near the boiler section of an incinerator plant, achieved an estimated conversion efficiency of approximately 4.4%. The incinerator waste gas temperature varied between 823 К and 973 K, and with forced air cooling on the cold side, an estimated conversion efficiency of approximately 4.5% was achieved. An analysis of a conceptual large-scale system burning 100 ton of solid waste during a 16-hour day indicated that around 426 kW could be delivered [6-12, 19]. In the management of waste heat in incineration applications, the thermoelectric modules are typically placed on walls of the furnace’s funnels. This construction can eliminate the by-heat furnace, gas turbine, and other appending parts of steam recycle [19].

B. TEGs attached to the solar systems

Solar TEG (STEG) systems, PV systems, and concentrating solar power plants can generate electricity by using the solar heat. A STEG is composed of a TEG system sandwiched between a solar absorber and a heat sink as shown in Figure 2.4. The

FIGURE 2.4 Components of STEG system [7].

solar flux is absorbed by the solar absorber and is concentrated into one point. Then, the heat is transferred through TEG by using a pipeline and is partially converted into electrical power by the TEG. A heat sink rejects the excess heat at the cold junction of the TEG to keep a proper AT across the TEG [26, 27].

Due to the development of the thermoelectric materials, a solar TEG with an incident flux of 100 kW/m² and a hot side temperature of 1,000°C could obtain 15.9% conversion efficiency. The solar TEG is very attractive for stand-alone power conversion. The efficiency of a solar TEG depends on both the efficiency with which sunlight is absorbed and converted into heat and the basic TEG efficiency. Furthermore, the total efficiency of a solar TEG is also influenced by the heat lost from the surface. The efficiency of solar TEG systems is relatively small due to the low Carnot efficiency provoked by the reduced temperature difference across the TEG and the reduced ZT [28]. Its improvement needs to rise temperature differences and to develop new materials w'ith high ZTlike nanostructured and complex bulk materials (e.g., a device with ZT= 2 and a temperature of 1,500°C would lead to obtaining a conversion efficiency about 30.6%) [29]. According to the literature survey, both residential and commercial applications gain much more interest in the regions of incident solar radiation of solar TEGs. This can be explained by the fact that most of the heat released at the cold side of the TEG can be used for domestic hot water and space heating [27].

C. Grid integration of TEG

Most TEG applications have been designed for autonomous operation within a local system. In general, a TEG can be seen as a renewable energy pow'er generation source that supplies an autonomous system or a grid-connected system. To be suitable for grid connection, the TEG needs an appropriate power conditioning system. This power conditioning system has to be a pow'er electronic system, with specific regulation capabilities, different with respect to the ones used for solar photovoltaic and wind power systems [30], because the TEG operating conditions are different with respect to the other renewable energy sources. Molina et al. [31] proposed a control strategy to perform energy conversion from DC to AC output voltage, which maintains the operation of the thermoelectric device at the maximum power point (MPP). In the same proposal, active and reactive power controls were addressed by using a dedicated power conditioning system.