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MICRO-CHP SYSTEMS
For general application, energy appears in different forms. These forms typically include some combination of: heating, ventilation, and air conditioning, mechanical energy and electric power. Often, these forms of energy are produced by a heat engine. However, according to the second law of thermodynamics, heat engine’s maximum efficiency is limited by Carnot’s principle. It always produces a surplus of low-temperature heat which is commonly referred to as a « waste heat ».
To make efficient use of energy, the « waste heat » must be used purposefully. And it can be put to a good use by a system called “Combined Heat and Power (CHP)” system or sometimes called “Cogeneration”.
Micro-CHP system is an energy producing system involving the simultaneous generation of thermal (steam or hot water) energy between 3kW and 18 kW and electric energy between 1kW and 5kW by using a single primary heat source (Disenco Energy plc, 2006). By this way, Micro-CHP systems are able to increase the total energy utilization of primary energy sources. Most of the time, this system is used for home and small commercial building operations.
In traditional power plant, delivering electricity to consumers, about 30% of the heat content of the primary heat energy source reaches the consumer, although the efficiency can be 20% for very old plants and 45% for newer gas plants. In contrast, a CHP system converts 15%–42% of the primary heat to electricity, and most of the remaining heat is captured for hot water or space heating. Totally, as much as 90% of the heat from the primary energy source goes to useful purposes when heat production does not exceed the demand (Wikipedia, 2010a). However, when heat production exceeds the demand, a way to avoid it is to reduce the fuel input to the CHP plant. Moreover, CHP is most efficient when the heat can be used on site or very close to it.
Micro-CHP engine systems are currently based on several different technologies:
Internal Combustion Engines Stirling Engines
Steam Engines Microturbines
Microturbine
Definition
A Microturbine is a gas turbine which incorporates all the components and represents all the characteristics of a gas turbine. It is becoming widespread for CHP application. And it is used for generating electricity, primarily, but having smaller power output when compared with a gas turbine. Typical microturbines produce between 30 kW and 250 kW of electric power (PERC, 2010). The exhaust heat from the microturbine can be used for water heating, space heating, drying processes or absorption chillers, which create cold for air conditioning from heat energy instead of electric energy, Microturbine-based CHP system. Microturbines can also be used in automotive applications such as buses. Moreover, microturbine is a recent development and is still being developed in many cases.
Microturbine operates with different fuels, but mostly with natural gases delivered at pressures exceeding 55 psi (379 kpa), may go as high as 90 psi (629 kpa), for driving the small turbine that powers the electric generator (Kolanowski, 2004). However, due to some reasons like: unstable natural gas prices, action toward carbon emissions regulation, and excess risk from dependence on a single fuel sources, we are forced to look for an alternatives (Capstone Turbine Corporation, 2010). An alternative fuel source, attractive from both environmental and economic perspective is a renewable fuel source, biomass. Though, biomass is a low-pressure gas, it can be boosted with centrifugal or scroll-type compressors and used (Kolanowski, 2004).
The electrical output of the microturbine is a high-frequency AC (1500–4000 Hz, 3-phase). The voltage is rectified and inverted to a normal 3-phase 50 or 60 Hz. Virtually, all microturbines are installed with recuperators to achieve 28–30% electric efficiency. Unrecuperated microturbines generally run at 14–17% efficiency (LHV) (Kolanowski, 2004). However, the efficiency of the microturbine can also be increased by utilizing the waste heat, either in a combined-cycle with a waste heat boiler and a steam turbine or in a combined-heat-and-power system, though; a further possibility is the recuperative air heating in the gas turbine itself.
Of the many others, one advantage of a microturbine is the ultra-low emissions that it emits. The disadvantage is that the small size of the compressor and turbine wheels limits the component efficiency, holds down the pressure ratio and prevents the turbine wheel from being internally cooled. Thus, the efficiency of a microturbine is well below that of a reciprocating engine, 14% vs. 40% (Kolanowski, 2004).
Components of Microturbine
A microturbine comprises components like: a combustion chamber (combustor), a heat exchanger, a turbocompressor (turbocharger), a generator and a lubrication system, with recuperator as an optional component. In addition, for biomass to be used as a fuel source, an integration of the microturbine with biomass gasifier, atmospheric gasifier, is required.
Here are some of the major components of a microturbine:
Combustor
A combustor or combustion chamber is one of the components of the microturbine, where combustion of the fuel-air mixture takes place to elevate the temperature of the exhaust air from the compressor, which finally expands in the turbine to drive the compressor and the electrical generator. The heat transfer, heating of the compressed exhaust air from the compressor, takes place through different mechanisms, accordingly with the type of the microturbine.
For a directly fired microturbine, the heat transfer takes place by burning the fuel with the compressed air in the combustor directly, the combustion gases are in direct contact with the moving parts of the machine. However, for an externally fired microturbine, the combustor is replaced by a heat exchanger and a burner; in case of using recuperation, the recuperator becomes part of the heat exchanger, the combustion gases do not pass through the moving parts of the machine (Kautz et al., 2009).
Heat Exchanger
The high temperature heat exchanger is the key to the success in the externally fired gas turbine. It is required to transfer heat from the heat source, combustion chamber, to the working fluid of the microturbine. The higher temperature the heat exchanger can provide the higher the system efficiency will be (Al-attaba et al., 2006).
Though, there are different types of heat exchangers, the main issue is how to build or select a heat exchanger that can withstand the stresses caused by the working conditions, considering a reasonable building cost. In addition, issues like: ability and ease of future expansion, Clean-ability, maintenance and repair and others are also considered.
With the nickel-based super alloys heat exchanger, the turbine inlet temperature could reach 800-825°C and in many projects they are using an additional natural gas combustor to raise the temperature up to around 1100°C to increase the cycle efficiency (Al-attaba et al., 2006). However, ceramic heat exchangers can reach such a high turbine inlet temperature with a long operational life time, but the production cost of these heat exchangers are very high which takes a very long payback time, but this option might be economical in the future with further developments for the ceramic heat exchangers technology to reduce its costs (Al-attaba et al., 2006). We can calculate the power transferred to the air through the heat exchanger using the equation below:
Q = ma * Cp * (Δ Ta) (3.1)
Turbocharger
Turbocharger is a name given for a compressor and turbine mounted on a single shaft as an electrical generator. The main construction of the turbocharger is the impeller of turbine and the impeller of compressor which are placed on the same shaft, supported with two bearings. The compressor is taking the rotating torque from radial flow turbine which is placed in the exhaust of the engine, expanding the high speed exhaust gases. This main construction of the turbocharger is typically the same with the main construction of one shaft micro gas turbine, or we can call it the heart of the microturbine.
A truck turbocharger can be used to build a microturbine, for the advantage that a turbocharger is very cheap comparing with the microturbine engine.
In truck turbochargers, the compressor air pressure is about 3 bar absolute and the air flow rate is assumed to be low because the turbocharger is not expected to run in full speed, due to the low turbine inlet temperature, so air flow rate will be around 0.12 kg/s (Al-attaba et al., 2006).
Generator
Generator is a component of the microturbine that converts mechanical energy obtained from the rotating shaft of the turbine to electric current, usually by rotating a conductor in a magnetic field, thereby generating current through electromagnetic induction. This sort of generator produces an alternating current (AC).
Thought, generators are made in a wide range of sizes, from very small machines with a few watts of power output to very large central-station generators providing 1000 MW or more, generators used for microturbines typically produce between 30 kW and 250 kW of electrical power (PERC, 2010).
Lubrication System
Lubrication system is a secondary system used to lubricate and cool the turbocharger’s bearings. However, this doesn’t mean that it is not important. Our turbocharger depends totally on the lubrication system, because the rotating shaft is totally floating on the oil film which is also acting like a damping system to eliminate the vibrations. The modern microturbine engines are using air bearings with no oil.
The main components of the lubrication system are:
High pressure automotive oil gear pump Driving electrical motor
Oil cooling unit.
Recuperator
The recuperator is a heat exchanger, transferring heat from the hot turbine exhaust gas to the colder compressed air in a heat exchanger between compressor and combustor. Almost all microturbines require recuperators to achieve desirable system thermodynamic efficiency. Before the compressed air enters the combustor, the exhaust gas is reduced to near compressor discharge temperature and the compressor discharge air is heated to near turbine exhaust gas temperature. The heat added to the compressed air reduces the amount of fuel required to raise the temperature to that required by the turbine and the thermal efficiency increases (Kolanowski, 2004). Recuperators are difficult to design and manufacture because they operate under high pressure and temperature differentials.
Atmospheric gasifier
While using biomass as a fuel source, an integration of the microturbine with biomass gasifier, atmospheric gasifier, is required. And the type of an atmospheric gasifier used varies with: the types of fuels, the efficiency level we want to operate in, expenses and others. Below, some of the gasifiers used are discussed.
Generally, two principal types of gasifiers have come into view, namely (Naik-Dhungel, 2007):
Fixed Bed Gasifiers and Fluidized Bed Gasifiers
Major characteristic of these gasifiers is discussed below:
Fixed Bed Gasifiers
Fixed Bed Gasifiers typically have a fixed grate inside a refractory-lined shaft. The fresh biomass fuel is typically placed on top of the pile of fuel, char, and ash inside the gasifier. A further classification is based on the direction of air, oxygen, flows (Naik-Dhungel, 2007). Based on the direction of air flow, fixed bed gasifiers are classified as:
Downdraft Gasifier
Updraft Gasifier and
Crossflow Gasifier
In Downdraft Gasifier: air flows down through the bed and leaves as biogas under the grate. Whereas, in Updraft Gasifier: air flows up through the grate and biogas are collected above the bed and for the case of Crossflow Gasifier: air flows across the bed, exiting as biogas. Schematics of the primary section of the fixed bed gasifier types are shown in Figure 3.1.
Figure 3.1: Fixed Bed Gasifier Types (Bain, 2006)
Fixed bed Gasifiers are usually limited in capacity, typically used for generation systems that are able to produce less than 5 MW (Naik-Dhungel, 2007). The physics of the refractory-lined shaft reactor vessel limits the diameter and thus the throughput. Though, developers have identified a good match between fixed bed gasifiers and small-scale distributed power generation equipment, the variable economics of biomass collection and feeding, coupled with the gasifier’s low efficiency, make the economic viability of the technology particularly site-specific. The typical physical characteristics of this Fixed Bed Gasifier are clearly shown in Table 3.1.
Due to the difference in the direction of air flow of the different types of Fixed Bed Gasifiers, their methods of operation differs too, and this results in different advantages and disadvantages. A major comparison between these different types of Fixed Bed Gasifiers is clearly shown in Table 3.2.
Fluidized Bed Gasifiers
Fluidized bed gasifiers utilize the same gasification processes and offer higher performance than fixed bed systems, but with greater complexity and cost. Similar to fluidized bed boilers, the primary gasification process takes place in a bed of hot inert materials suspended by an upward motion of oxygen deprived gas. As the amount of gas is augmented to achieve greater throughput, the bed will begin to levitate and become fluidized. Sand or alumina is often used to further improve the heat transfer. Notable benefits of fluidized bed devices are their high productivity (per area of bed) and flexibility. Fluidized bed gasifiers can also handle a wider range of biomass feedstocks with moisture contents up to 30 % on average (Naik-Dhungel, 2007). The schematic of this Fluidized Bed Gasifier is shown in Figure 3.2.
There are three stages of fluidization that can occur on the gasifier depending on the design: bubbling, recirculating, and entrained flow. At the lower end of fluidization, the bed expands and begins to act as a fluid. As the velocity is increased, the bed will begin to “bubble.” With a further increase in airflow, the bed material begins to lift off the bed. This material is typically separated in a cyclone and “recirculated” to the bed. With still higher velocities, the bed material is entrained. The typical physical characteristics of this Fluidized Bed Gasifier are clearly shown in Table 3.3.
Generally speaking, Fixed Bed Gasifiers are typically simpler, less expensive, and produce a lower heat content syngas. Whereas, Fluidized Bed Gasifiers are more complicated, more expensive, and produce a syngas with a higher heating value (Naik-Dhungel, 2007).
Types of Microturbine
Based on the firing of the gas turbine, microturbine can be of two types. These two categories are:
Externally or Indirectly fired Microturbine Directly fired Microturbine
Externally or Indirectly fired Microturbine
The externally or indirectly firing of the gas turbine means that the combustion chamber is not directly connected to the gas turbine, therefore, the combustion exhausted gases are not inserted directly to the turbine and not in direct contact with the turbine’s impeller. Rather, the combustion process here is used for heating up a compressed fluid, commonly air, using a high temperature heat exchanger, and then the fluid is expanded in the turbine producing a high rotary speed shaft power (Al-attaba et al, 2006).
Both the indirectly and directly fired microturbines are similar in concept and both are explained thermodynamically by the Brayton cycle. In the indirect fired open Brayton cycle; first, the working fluid is drawn by the compressor and compressed. Then, the working fluid passes through a heat exchanger for heating up. After heating up, it expands through the turbine, and finally, either discharged directly to the environment from the turbine or returned back to the compressor after a cooling process (Al-attaba et al., 2006).
The externally fired gas turbine, EFGT, has the advantage of freedom in choosing the fuel source. The fuel sources could be liquid, gas or even solid types, like: coal and biomass fuels. Though, this advantage is also available in the Rankine steam cycle, the rankine steam cycle has a lower thermodynamic efficiency compared with the Brayton cycle which has a higher temperature of the working fluid in the inlet of the turbine with a lower pressure compared with steam temperature and pressure in Rankine cycle (Al-attaba et al., 2006).
The externally fired gas turbine cycle can be divided in to two types:
The open cycle externally fired gas turbine. The closed cycle externally fired gas turbine.
In the open cycle, the working fluid is discharged to the combustion chamber as a heat and oxygen supply for the combustion process, or discharged to the environment after decreasing its temperature in a heating or drying process. However, in the closed cycle, the working fluid is returned back to the compressor after a cooling process (Al-attaba et al., 2006).
Directly fired Microturbine
The direct fired gas turbine, DFGT, has a higher thermodynamic efficiency compared with the externally fired gas turbine because of the higher temperature of the combustion gases in the turbine’s inlet. However, on the other hand, direct fired gas turbine can only deal with the clean liquid or gas fuels along with the fuel compressing and injecting equipments which is not necessary in the externally fired gas turbine. The direct fired gas turbine can use the solid fuels like biomass or coal after gasification process only after an intensive cleaning process for the producer gas (Anheden, 2000).
Generally, in comparison with the directly fired gas turbine, the externally fired gas turbine sets less stringent requirements with respect to composition and cleaning of the combustion gas, as far as, the combustion gases do not pass through the turbine. In addition, EFGT also has the advantage of the preheated air, the utilization of the waste heat from the turbine in a recuperative process, and the advantage of allowing burning alternative, non-standard fuels, for instance biogenic fuels. The smaller unit size also enables decentralized units appropriate for the biomass output from farms and agricultural processing units.
Sources of Energy for Microturbines
There are different sources of energy, source of fuel, for microturbine to drive the small turbine that powers the electric generator. Broadly, we can divide these energy sources in to two. These are:
Non-Renewable energy sources, and Renewable energy sources
Non-Renewable energy sources
Non-Renewable energy sources are sources of energy which are found naturally, not produced or generated by human beings (Wikipedia, 2010b). These energy sources have the highest consumption rate in our day to day activities. Most of our systems or machines today, industrialized society, are designed to fit for these energy sources, as well as microturbine. Quantitatively speaking, over 85% of the energy used in the world is from non-renewable sources (Connexions, 2009). Non-Renewable energy sources are also known for their higher heating values.
However, since these energy sources exist in a fixed amount and due to their high consumption rate, they are being consumed much faster than nature can create them and they also have higher environmental effect, climate change. From fuels that are grouped under this energy source, fossil fuels and nuclear power are the major ones.
Fossil fuels are compounds of the chemical elements carbon and hydrogen. Fossil fuels were formed millions of years ago, during the Carboniferous Period, from the remains of plants and animals. As the plants and animals that inhabited the swamps died, they were buried under sand and mud which stopped them from decaying. Over time, more sediment covered the remains and pressure, together with heat, turned them into natural gas, coal and oil deposits. These three are the main types of fossil fuels. However, there are also other like: oil shale and tar sands, but less-used. Fossil fuels are mostly found deep underground. Since the Industrial Revolution, we have come to rely on fossil fuels as our main source of energy. Today, fossil fuels are used in power stations to generate electricity.
Natural gas, one of the fossil fuels, is a gas that occurs naturally underground and piped to the surface through wells drilled into the underground rock. It is a mixture of gases, the most common being methane (CH4) and it can be processed into propane and other types of fuels. It is usually not contaminated with sulfur and is therefore the cleanest burning fossil fuel. All natural gas fuels are highly flammable and odorless, so for safety reason, natural gases are mixed with chemicals to give a noticeable smell before being sent to consumers. It can be stored and shipped in pressurized containers.
The use of natural gas is growing rapidly and is commonly used in homes to cook food and heat water, and compressed natural gas can power specially designed vehicles. Natural gas is the main energy source for microturbines. It can also be used as a source of hydrogen gas by reforming process.
The other fossil fuel is coal, which is a solid hydrocarbon that we excavate from underground, just as we mine for minerals. During the formation of coal, carbonaceous matter was first compressed into a spongy material called « peat, » which is about 90% water (Connexions, 2009). As the peat became more deeply buried, the increased pressure and temperature turns it into coal. Coal is easy to transport, usually in large containers aboard ships or in special cars on trains. Coal is the most abundant fossil fuel in the world with an estimated reserve of one trillion metric tons.
Coal is mainly used for electricity production and sometimes for heating and cooking in less developed countries and in rural areas of developed countries. The burning of coal results in significant atmospheric pollution. The sulfur contained in coal forms sulfur dioxide when burned. Harmful nitrogen oxides, heavy metals, and carbon dioxide are also released into the air during coal burning. The harmful emissions can be reduced by installing scrubbers and electrostatic precipitators in the smokestacks of power plants.
The third one is oil, also known as petroleum or crude oil. It is a thick black liquid hydrocarbon found in reservoirs, hundreds to thousands of feet below the surface, and extracted by drilling wells deep into the underground rock and then inserting pipes. Once extracted, crude oil can be refined to various products. These include gasoline, diesel, jet fuel, home heating oil, asphalt, and oil burned for electrical power. Oil products are sent from refineries through pipelines directly to their consumers, or are delivered in large tanks aboard trains, trucks, or tanker ships.
Oil is the main source of power for vehicles, in the form of petrol or diesel. The burning of oil releases atmospheric pollutants such as sulfur dioxide, nitrogen oxides, carbon dioxide and carbon monoxide. These gases are smog-precursors that pollute the air and greenhouse gases that contribute to global warming. However, it is a preferred fuel source over coal. An equivalent amount of oil produces more kilowatts of energy than coal. It also burns cleaner, producing about 50 % less sulfur dioxide than coal (Connexions, 2009).
The other non-renewable source of energy is Nuclear power. Nuclear power originates from nuclear reactions which results in mass reduction that is converted to energy. Specifically speaking, from the atoms of the chemical element uranium, which is found in certain types of rock.
Renewable energy sources
Renewable energy sources are sources of energy that describe a wide range of naturally occurring, replenishable within a short period of time and infinite energy sources (Mourelatou, 2001). In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Renewable energy sources include: Solar energy, Wind energy, Hydropower, as well as geothermal energy and energy from plants and animals, Biomass energy or biofuel. Though, there are many others like: Hydrogen and Ocean energy, the previous five are sources used most often.
In 2008, around 19% of the global total energy consumption was from renewable energy, with 13% coming from traditional biomass, mainly used for heating, and 3.2% from hydroelectricity. New renewable (small hydro, modern biomass, wind, solar, geothermal, and biofuels) accounted for another 2.7% and are growing very rapidly (Wikipedia, 2010c). Renewable energy sources are also seen as an alternative source of energy that are renewable, will never run out , and will not contribute to the climate change as that of non-renewable energy sources, produce little or no pollution or greenhouse gases. In other word, they overcome the problems of non-renewable energy.
Table of contents :
1 INTRODUCTION
1.1 Objectives
2 PERFORMANCE OF WORK
3 MICRO-CHP SYSTEMS
3.1 Microturbine
3.1.1 Definition
3.1.2 Components of Microturbine
3.1.2.1 Combustor
3.1.2.2 Heat Exchanger
3.1.2.3 Turbocharger
3.1.2.4 Generator
3.1.2.5 Lubrication System
3.1.2.6 Recuperator
3.1.2.7 Atmospheric gasifier
3.1.3 Types of Microturbine
3.1.3.1 Externally or Indirectly fired Microturbine
3.1.3.2 Directly fired Microturbine
3.2 Sources of Energy for Microturbines
3.2.1 Non-Renewable energy sources
3.2.2 Renewable energy sources
3.2.2.1 Solar Energy
3.2.2.2 Biomass Energy
4 GENERAL COMPANY PROFILE
4.1 Company overview
4.2 Mission and Vision
4.3 Products and Byproducts
4.4 Energy supply and management
4.5 The potential quantities in energy amounts of biomass and use
4.6 The total supply of poultry litter, manure from cattle and pig and its use
5 EXPLORE POLYGENERATION
5.1 Introduction
5.1.1 The Explore Vision
5.1.2 Explore polygeneration demonstration unit
5.1.2.1 Objectives
5.1.3 Approach
5.2 Biomass-Powered Gasturbine Based Ploygeneration
5.2.1 Design
5.2.2 Fuel supply
5.2.2.1 Biogas supply
5.2.2.2 Gas mixing panel from bottles
5.2.2.3 Downdraft atmospheric gasifier
5.2.2.4 Fluidized bed atmospheric gasifier
5.2.3 Prime mover – Compower Unit (CPU)
5.2.3.1 Fuel valve
5.2.3.2 Burner
5.2.3.3 Control systems
5.2.4 Energy services
5.2.5 Connections, logistics and safety
5.2.5.1 Connection fuel source to prime mover
5.2.5.2 Connection prime mover to downstream services
5.2.5.3 Floor occupation and service availability
5.2.5.4 Safety procedures
5.2.6 System modeling and analysis
6 METHODOLOGY
7 INVESTIGATION AND DETAILED STUDY OF FUEL SOURCES
7.1 Introduction
7.2 Syngas production
7.2.1 Biomass gasifiers available in the laboratory
7.2.2 Selection of the raw biomass
7.3 Production of syngas from poultry litter
7.4 Alternative approach to energy generation from poultry litter
7.4.1 Anaerobic digestion
7.5 Alternative approach to utilization of poultry litter
7.6 Problems related to poultry litter gasification
7.7 Solutions for the problems related to poultry litter gasification
8 CONCLUSION
9 FUTURE WORK
10 ACKNOWLEDGMENTS
REFERENCES
APPENDIX 1: LIST OF THE 39 TRIZ CONTRADICTION MATRIXES
APPENDIX 2: LIST OF THE 40 PRINCIPLES OF TRIZ
