Design of a 5 KW Residential Fuel Cell System Using

Anaerobic Digestion of Animal Waste

Kalyan Rapolu

Villanova University

ABSTRACT

This paper reports on the combination of solid oxide fuel cell generators fueled with biogas as renewable energy source, recoverable from wastes. The solid oxide fuel cells have gained much importance in the recent years for residential fuel systems. SOFCs could improve and promote the exploitation of biogas on manifold generation sites as small combined heat and power especially for farm and sewage installations raising the electrical conversion efficiency on such reduced power level. The design of an independent stationary residential fuel cell system with a generation capacity of 5 KW along with the process of producing biogas from animal waste is studied. This document compiles and estimates the biogas data that is required for the production of hydrogen to supply the fuel cells and presents the thermodynamics and electrochemical conversion processes. This paper also presents the power conditioning system of the fuel cell system to provide the required voltage and power to the application.

INTRODUCTION

The interest in the distributed generation has increased significantly in the recent years. It is believed that the distributed generation market will be between US $10 and $30 billion by 2010. Due to environmental concerns, more effort is now being put into the clean distributed power like geothermal, solar thermal, photovoltaic, and wind generation, as well as fuel cells that use hydrogen, propane, natural gas or other fuels to generate electricity without increasing pollution.

There are five major types of the fuel cells in current technology. Among these five, Alkaline Fuel Cells (AFC) have been used in the NASA space program since 1960s. Polymer Electrolyte Membrane (PEM) fuel cells have very fast slew rates and low operating temperatures and are being used in electric vehicles. Phosphoric Acid Fuel Cells (PAFC) are very tolerant to impurities in the fuel steam and by far are the most mature in terms of system development and commercialization. Over 200 stationary units with typical capacity of 200 kW have been installed in the United States. Molten Carbonate Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC) both operate at high temperature 600-1,0000C, and are targeted at medium- and large-scale stationary power generation. In SOFC, a solid ceramic material is used for the electrolyte, and viable fuels can be used without a separate reformer. The byproduct: hot water and heat can be used for heating. Much research has been done towards the residential application of SOFC. One of the major obstacles of its commercialization is the high cost of installation. In recent years, the production costs of fuel cells keep decreasing.

Biomass offers worldwide large exploitation potential among renewable energy sources. Biogas fuel feeding presents an attractive option among emerging application for fuel cells, especially for the high temperature ceramic type solid oxide fuel cells (SOFCs). Compared to natural gas it shows advantages of being indigenous and renewable, free of non methane hydrocarbon, with the exception of landfill gas and containing a large fraction of methane reforming agent CO2. Biogas fabrication inherently is a friendly and zestful way to process waste streams of variable nature (sewage sludge, liquid organic industrial effluents, farm residues, and animal waste, and landfill, municipal and industrial solid organic residues).

Rising energy prices, broader regulatory requirements, and increased competition in the marketplace are causing many in American agriculture's livestock sector to consider anaerobic digestion of animal wastes. They view the technology as a way to cut costs, address environmental concerns, and sometimes generate new revenues. While hundreds of anaerobic-digestion projects have been installed in Europe and the U.S. since the 1970s, it was not until the 1990s that better designed, more successful projects started to come on line in the U.S. Today, there are an estimated 40 farm-scale projects in operation on swine, dairy, and poultry farms across the country. This paper studies the process of production of biogas through anaerobic digestion and the conversion of biogas to hydrogen and the production of electricity from hydrogen by fuel cells. This paper calculates the amount of biogas needed and the amount of manure required for producing 5 KW power from the fuel cells. The total study can be briefly given as below.

Animal Waste (Biomass) ΰ Methane (Biogas- along with other gases)

Methane ΰ Hydrogen (fuel for fuel cells)

Hydrogen ΰ Electricity (through fuel cells)

 

ANAEROBIC DIGESTION

Anaerobic digestion works in a two-stage process to decompose organic material (i.e., volatile solids) in the absence of oxygen, producing bio-gas as a waste product. In the first stage, the volatile solids in manure are converted into fatty acids by anaerobic bacteria known as "acid formers." In the second stage, these acids are further converted into bio-gas by more specialized bacteria known as "methane formers." With proper planning and design, this anaerobic-digestion process, which has been at work in nature for millions of years, can be managed to convert a farmer's often problematic waste-stream into an asset. The Figure 1 shows the basic components of the anaerobic digestion process to produce biogas.

The key by-products of anaerobic digestion include digested solids ( useful as a soil amendment) and methane, the primary component of "bio-gas," which can be used to fuel a variety of cooking, heating, cooling, and lighting applications, as well as to generate electricity. Capturing and using the methane also precludes its release to the atmosphere, where it is 20 times more damaging to the ozone layer than carbon dioxide.

 

Figure 1. Basic components of an anaerobic-digestion system

Source: ATTRA - National Sustainable Agriculture Information Service,

http://attra.ncat.org/attra-pub/anaerobic.html

The plug flow type of anaerobic digester is the most commonly used digester. This consists of a cylindrical tank in which the gas and other by-products are pushed out one end by new manure being fed into the other end. This design handles 11-13% solids and typically employs hot-water piping through the tank to maintain the necessary temperature. Most appropriate for livestock operations that remove manure mechanically rather than washing it out.

Procedure

The digestion process is not difficult but requires long period of time. The digester tank is filled with water and then heated to the desired temperature. "Seed" sludge from a municipal sewage treatment plant is then added to about 15% of the tank's volume, followed by gradually increasing amounts of fresh manure over a three-week period until the desired loading rate is reached. Assuming that the temperature within the system remains relatively constant, steady gas production should occur in the fourth week after start-up. The bacteria may require two to three months to multiply to an efficient population.

Temperature within the digester is critical, with maximum conversion occurring at approximately 95°F in conventional mesophilic digesters. For each 20°F decrease in temperature, gas production will fall by approximately 50 percent. Even more significant is the need to keep the temperature steady. Optimal operation occurs when the methane formers use all the acids at approximately the same rate that the acid formers produce them. Variations of as little as 5°F can inhibit methane formers enough to tip the balance of the process and possibly cause system failure.

Bio-gas produced in an anaerobic digester contains methane (60-70%), carbon dioxide (30-40%), and various toxic gases, including hydrogen sulfide, ammonia, and mercaptans. Bio-gas also typically contains 1-2% water vapor. At roughly 60% methane, bio-gas possesses an energy content of 600 Btu/ft3.

Uses of Bio-gas


Because of the extreme cost and difficulty of liquefying bio-gas, it is not feasible for use as a tractor fuel. Bio-gas has many other on-farm applications, like cooking, heating (space heating, water heating, and grain drying), cooling, and lighting. Bio-gas can also be used to fuel generators for producing steam and electricity.

While methane is a very promising energy resource, the non-methane components of bio-gas tend to inhibit methane production and, with the exception of the water vapor, are harmful to humans and the environment. For these reasons, the bio-gas produced should be properly "cleaned" using appropriate scrubbing and separation techniques.

Digester Design Factors

Digesters are installed primarily for economic and environmental reasons. Digesters represent a way for the farmer to convert a waste product into an economic asset, while simultaneously solving an environmental problem. Under ideal conditions, an anaerobic-digestion system can convert a livestock operation's steady accumulation of manure into a fuel for heating or cooling a portion of the farm operation or for further conversion into electricity. The solids remaining after the digestion process can be used as a soil amendment, applicable on-farm or made available for sale to other markets.

Anaerobic digestion requires careful consideration of many factors. They can be quite costly to install. A straightforward batch-loading design will involve an air-tight tank, means of mixing the contents of the tank and maintaining a constant temperature, and a means of collecting the gas with appropriate safety precautions. Additional hardware will include regulators, flame traps, pressure gauges and relief valves, a hydrogen-sulfide scrubber, and means of removing the carbon dioxide. The size of the system is determined primarily by the number and type of animals served by the operation, the amount of dilution water to be added, and the desired retention time. The most manageable of these factors is retention time; longer retention times mean more complete breakdown of the manure contents, but require a larger tank.

BIOGAS TO HYDROGEN CONVERSION

Methane steam reforming (MSR) is a major route for the industrial production of H2. The three main reactions in a MSR reactor are represented by following equations.

CH4 +H2O ΰ CO + 3H2; ∆H298 = 206.2 Χ 103 kJ/kmol; ………………. (1)

CO+H2O ΰ CO2 +H2; ∆H298 = 41.1 Χ 103 kJ/kmol; …………………… (2)

CH4 + 2H2O ΰ CO2 + 4H2; ∆H298 = 164.9 Χ 103 kJ/kmol; …………….. (3)

 

Fig 2: Steam Methane Reforming Process in a Fuel Processor

Source: [3]"Hydrogen from Methane in a single step process", Chemical Engineering Science

 

Reforming reactions (1) and (3) are highly endothermic and thermodynamically favored by high temperature and low pressure. On the other hand, the water–gas shift (WGS) reaction given by (2) is favored at low temperature, but it has no pressure dependence. MSR is generally operated at a temperature of 750–900oC due to the overall endothermic nature of the reactions. Although high-temperature operation is indispensable for a substantial conversion of CH4, it facilitates the reverse WGS reaction, giving the product gas containing 8–10% CO on a dry basis. For the purpose of obtaining the product gas with less CO and more H2, it is conventional that the MSR product gas is fed to another reactor where the temperature is kept as low as 300–400oC for the WGS reaction to take place prior. To obtain the H2 product stream, the effluent is then cooled and fed to a multicolumn pressure swing adsorption (PSA) process.

DESIGN OF COMPLETE FUEL CELL SYSTEM

Numerous calculations are involved in the design of a 5 KW residential fuel cell system. It is desired to find the number of cows (cattle) required to produce 5KW power, the size of the digester tank, the amount of manure and biogas produced, the retention time of the manure in the digester tank, the heat energy involved in the conversion of methane to hydrogen, the volume of methane required to produce sufficient amount of hydrogen to produce 5 KW electricity.

This paper focuses on the design of a 5 KW solid oxide fuel cell (SOFC) power generation system to provide non utility and ultra clean residential electricity. The aimed fuel cell stack outputs 22 V to 41 V dc. For residential applications, the needed output is two split phase 60 Hz, 120V ac, the 5 KW fuel cell stack is supplemented by a 5 KW battery pack to meet peak power demand of 10 KW.

The generating capacity of a typical hydrogen fuel cell is 167 watts at a voltage output of 1 volt. For higher output and terminal voltage, cascades of series- parallel connections of fuel cells- will meet the required electric load demands.

For a 5 KW residential cell system, the number of fuel cells required in a stack is

No. of fuel cells = 5000/167≈ 30 fuel cells in series.

The output voltage of the fuel cell stack is Vo = 30 * 1 = 30 V …………….. [1]

The current passing through the fuel cell stack is I = 5000/30 ≈ 167 A …….. [2]

It is known that ‘current is the rate of charge’. Hence

I = (n * e-)/t …………………………………………………………………. [3]

where n: no. of electrons passing through the external load connected to the fuel cell.

e-: charge of an electron = 1.602*10-19 coul

t: time of discharge of the electron

*Note: The time of discharge of the electron is the time taken by the electron to travel from one electrode of the fuel cell to the other. The time of discharge depends on the distance between the electrodes. The time of discharge of a typical hydrogen fuel cell is

t = 1.0813 * 10-17 sec.

Hence the number of moles of electrons (from [3])

n = I *t / e- = (166.67 * 1.0813*10-17)/ 1.602*10-19

n = 11249.70 moles of electrons

The electrochemical reactions taking place in the fuel cell are

At anode A, 2H2 = 4H+ + 4e- …………………………………………… [4]

At cathode K, 4H+ + 4e- + O2 ΰ 2H2O …………………………………. [5]

And the overall reaction is

2H2 + O2 ΰ 2H2O ………………………………………... [6]

Hence from [4] it can be taken that the molar ratio of H2 to the electron is 1: 2.

Hence the number of moles of H2 ,

nH2 = 11249.70/2 = 5624.85 moles of H2

The reactions occurring in the conversion of methane to hydrogen are

 

CH4 +H2O ΰ CO + 3H2;

CO+H2O ΰ CO2 +H2;

The overall reaction is

CH4 + 2H2O ΰ CO2 + 4H2;

From this it can be observed that the molar ratio of methane to hydrogen is 1: 4.

*Note: It is taken that the complete chemical reactions take place in the fuel reformation i.e., conversion of methane to hydrogen. The rates of chemical reactions are considered when the dynamic model of the fuel cell system is designed. The rates of reactions depend on the partial pressures of the reactants and the products. The dynamic model of the fuel cell system is required to analyze the transient changes in the electrical load. To study how fast the fuel cell can respond to the sudden changes in the load can be determined by the dynamic model. The time of response of the fuel cell system depends on the rate of change in the input to the fuel cell (i.e., hydrogen). This in turn depends on the rate of reactions of the fuel processor.

Hence the number of moles of methane

nCH4 = 5624.85/4 = 1406.21 moles of methane

Ideal Gas Equation

PV = nRT ……………………………………………………………… [7]

Where P: pressure of the processor

V: volume of methane

n: no. of moles of methane

R: Universal Gas Constant

T: temperature of the reaction

P = 1 atm , n = 1406.213, R = 0.0826 litre/atm/mol-K, T = 800+273 = 1073 K

From [7] , volume of methane required is

V = nRT/P = 1406.213*0.0826*1073/1 = 124632.386 liters of methane

According to ATTRA -National Sustainable Agriculture Information Service,

One cow yields 12.5 gallons of manure per day. This amount of manure produces 46 cubic feet of biogas through anaerobic digestion.

Hence 1 cow produces 12.5 gal/day manure ΰ 46 cubic feet of biogas ΰ 1302.574 liters of methane.

To produce 124632.386 liters of methane the number of cows required is

No. of cows = 124632.386/1302.574 ≈ 96 cows

Hence with 96 cows the amount of manure produced is 1200 gal/day.

According to ATTRA, the dimensions of the digester tank for 100 cow dairy herd is

18 ft diameter*18 ft height (cylindrical tank). The capacity of the tank is 32,250 gal.

The retention time is 15 days in the tank.

35 % of the biogas produced is used to maintain the temperature in the tank.

 

POWER CONDITIONING SYSTEM

This paper uses an inverter system that supports the commercialization of a 5 kW solid-oxide fuel cell (SOFC) power generation system to provide non-utility and ultra-clean residential electricity. The aimed fuel cell outputs 22 V to 41 V dc. For residential applications, the needed output is two split-phase 60 Hz, 120 V ac, the 5 kW SOFC is supplemented with a 5 kW battery pack to meet peak power-demand of 10 kW. The general inverter system configuration is shown in Fig. 3. The dc voltage from the fuel cell is first boost up to 350-450 V by a dc-dc converter then a dc-ac inverter with output filter is cascade-connected to produce ac voltage. The battery can be added to either the low voltage side or the high voltage side of the converter.

 

Fig 3: Components of Power Conditioning System of a Fuel Cell system

Source: [5] A New Low Cost Inverter System for a 5 KW fuel cell

The dc-dc converter uses phase shifting to control power flow through a transformer with a full bridge on the low voltage side and a controlled voltage doubler on the high voltage side. The transformer provides voltage isolation between the fuel cell and the ac output voltage improving overall safety of the system. A voltage doubler on the high voltage side decreases the turn’s ratio of the transformer, which reduces leakage inductance and makes the system more efficient and easier to control. And at the same time, the voltage and current stresses on the low voltage side are also minimized. A high voltage battery pack is added after the voltage doubler as transient power for load dynamics. Thus the capacitance of the high voltage side capacitors are minimized, which will significantly reduce the total cost of the system.

The overall residential fuel cell system is shown in the figure 4.

CONCLUSION

This paper mainly concentrated on the design issues of an independent residential fuel cell system. The power generation system is not connected to the grid but it supplies power to the farm house. A typical 5 KW fuel cell system is taken into consideration. It is calculated that 96 cows are required to yield 1200 gallons of manure per day. This manure when subjected to anaerobic digestion for 15 days in a cylindrical 18 feet diameter , 18 feet tall tank, it produces biogas which consists of 60 -70 % of methane, the rest consisting of carbon dioxide, oxides of nitrogen and other gases. This biogas is sent to the fuel processing system which converts methane into hydrogen. It is noted that 99% of methane is converted into hydrogen. This is a highly endothermic reaction which requires a temperature of 800o C. The hydrogen obtained is passed into a fuel cell stack consisting of 30 fuel cells in series. The output voltage of the fuel cell stack is 30 V with a power of 5 KW. The obtained power is in DC form. This is converted into 120V ac split phase power by a power conditioning system consisting of a dc-dc boost converter followed by an inverter. An auxiliary power supply of 5KW is taken by a battery connected to the low voltage side of the dc-dc converter.

 

96 Cows are required in the Herd

photo

 Cylindrical Anaerobic digestion Tank, 18ft diam * 18 ft tall

Fuel processor, Methane to Hydrogen conversion at 800oC

Fuel cell stack, 30 fuel cells in series

 Power conditioning system, DC – DC Converter and Inverter

  Residential Farm House

Figure 4: Complete Residential fuel cell system

REFERENCES

 

[1] ATTRA - National Sustainable Agriculture Information Service,

http://attra.ncat.org/attra-pub/anaerobic.html

[2] B. Balasubramanian, A. Lopez Ortiz, S. Kaytakoglu, D.P. Harrison, "Hydrogen from methane in a single-step process", Chemical Engineering Science, Journal of Power Sources

[3] K. Denno, "Power System Design and Applications for Alternative Energy Sources", Prentice Hall Advanced Reference Series Engineering

[4] Jan Van herle, Yves Membrez, Olivier Bucheli, "Biogas as a fuel source for SOFC co-generators", Journal of Power Sources 127 (2004) 300–312

[5] Jin Wang, Fang Z. Peng, Joel Anderson, Alan Joseph and Ryan Buffenbarger, "A New Low Cost Inverter System for 5 kW Fuel Cell", 2003 Fuel Cell Seminar Special Session on fuel Cell Power Conditioning and International Future Energy Challenge Eden Roc Hotel, Miami Beach, Florida

[6] Aashish Mehta, "The Economics and Feasibility of Electricity Generation using Manure Digesters on Small and Mid-size Dairy Farms"